GOVERNMENT OF ETHIOPIA
WATER RESOURCES DEVELOPMENT AUTHORITY
AMIBARA IRRIGATION PROJECT
WATER MANAGEMENT MANUAL
SIR WILLIAM HALCROW & PARTNERS
Consulting Engineers
Resident Engineer's Office
P.O. Box 2306
Addis Ababa
Ethiopia
Sir William Halcrow & Partners
Consulting Engineers
Burderop Park
Swindon
Wiltshire SN4 OQD
United KingdomGOVERNMENT OF ETHIOPIA
WATER RESOURCES DEVELOPMENT AUTHORITY
AMIBARA IRRIGATION     PROJECT WATER MANAGEMENT MANUAL
July 1983
SIR WILLIAM HALCROW & PARTNERS
Consulting Engineers
Resident Engineer's Office
P.O. Box 2306
Addis Ababa
Ethiopia
Sir William Halcrow & Partners
Consulting Engineers
Burderop Park
Swindon
Wiltshire SN4 OQD
United KingdomAMIBARA IRRIGATION PROJECT
2         Project Works
3         Soils
4          Crop Water Requireinents
5         Irrigation Requirements and Scheduling
6         Furrow irrigation
7         Conveyance System
0        Distribution System
9        Salinity and Drainage
10       Efficiencies
APPENDIX I -         Moisture      Characteristics      of      the      soils      in      the      Amibara      Irrigation Project Area and Melka Warer Research Station.
APPENDIX II     - References
APPENDIX III - GlossarySect ion I
Irrigation and Water Management
l.l        Introduction
1.2      Water Managenent
1.3      [rrigation System
1.4      Drainage System Figure I.I
Page
1
I
2
2Section 2
Project Works
2.1        Diversion Weir
2.2         Intake
2.3        Head Regulator
2.^        Ma in Cana1
2.5        Main Canal Check Drops 2.6        Rejection Spillways
2.7        Offtakes
2.8        Main Drain
2.9         Flood Protection Dykes
Page
3
3
3
4
4
4
4
4
5
2.10
Distributary Networks
5
2.10.1 Irrigation Water Duties
6
2.11
Drainage Networks
7
2.11.1 Drainage Water Duties
7
2.12 Roads
2i13 Flow Chart
F igure 2.1 Drawing 2.I
9
9Section 3
Soils
3. I        Soil Texture
3.2         Soil Structure
3.3        Pore Space
3.4        Saturat ion
3.5        Field Capacity
3.6        Permanent Wilting Point
3.7        Readily Available Soil Moisture
3.8        Measuring Soil Moisture
3.8.1          Tensiometer
3.8.2          Gypsum Blocks
3.8.3 Neutron Probe Method
3.9        Infiltration Rates
3.9.1          Determining Infiltration Rates
Tables    3.1    and    3.2
Forma    3.I    and    3.2
Figures    3-1    to    3.5
Drawing 3.1
Page
11
11
11
11
12
12
12
13
13
13
13
14
14Section 4
Crop Water Requirements
4. 1        I nt reduct ion
4.2 Sechods of Estimating Crop Water Requirements
4.3         Definicions
4.3. 1         Evapotranspiration
4.3.2         Reference Crop Evapotranspiration 4.3.3         Crop Evapotranspiration
4.3-4         Actual Evapotranspiration
4.4
Calculation of Reference Crop Evapotranspiration
Pages
21
21
21
21
21
22
22
22
4.4.1
Climatic Data
22
4.4.2
Reference Crop Evapotranspiration by
the
R laney
22
Criddle Method
4.4. 3
Reference Crop Evapotranspirat ion by
the
23
Radiation Method
4.4.4
Reference Crop Evapotranspiration by
the
Class  A'
r
24
Pan Evaporation Method
4.4.5
Reference Crop Evapotranspiration by
the
Penman
24
Method
4.5         Crop Coefficients
4.6         Some Crops grown in project area
4.6.1         Bananas
4.6.2         Cot ton
4.6.3         Haire
4.6.4         Onions
4.6.5         Peppers
4.6.6         Haricot Beans
4.7 Determining Crop Water Requirements
4.7.1         Crop Water Requirements for Bananas
27
28
28
28
29
30
30
31
31
31
4.7.2
"
'* 
"
11 Cotton
32
4.7.3
"
” 
” 
’’ Peppers
33
4.7.4
,r
11
'* 
11 Haricot Beans
33
4.7.5
"
"
"
" Maize
34
4.7.6
"
"
"
" Onions
35
4.7.7
"
” 
"
" Sesame
36
Tables 4.1, 4.2, and 4.4 to 4.LISect ion 5
Irrigation Requiregents and Scheduling
5.1         Irrigation Requirement
5.2         Irrigation Schedules
5.3        Soil Moisture Balance
5.3.1           Groundwater contribution (Ce) 5.3.2           Stored Soil Water (Wb and We)
5-4       Effective Rainfall
5-4,1 Definition of Effective Rainfall
5-4.2         Effect of Soil Characteristics on Effective Rainfall 5.4.3           Effect of Crop Characteristics on Effective Rainfall 5.4.4          Estimation of Dependable Rainfall
5.4.5          Method of Determining Effective Rainfall
Pages
42
42
43
43
43
44
44
44
44
44
45
5.5 
Leaching Requirement
47
5.6 
Net Irrigation Requirement
48
5.7 
Depth of Application
48
5.8 
Irrigation Interval
49
5.9 
Calculation of Irrigation Schedules
5.10 
Establishment of Rain Gauge Stations
^2
Tables 5*1 to 5.4
Form             5.1
Figures 5.1 to 5.15
Drawings 5 - 1 and 5-2Section 6
Furrow Irrigation
6.1 Scope of Furrow Irrigation
6.2         Furrows
6.2.1         Character is tics and Types of Furrow Irrigation 6.2.2         Furrow Spacing
6.2.3         Furrow Slopes
6.2.4         Furrow Lengths
6.2-5         Flow Races
6.2.6         Regulation of Stream Size
6.2.7         Average Depth of Water Applied
6.3         Furrow  Evaluation
6.3.1         Objective
6.3.2         Initial Collection of Dacn
6.3.3         Furrow Intake Rates
6.3.4         Water Needed to Refill Soil Moisture Reservoir
6.3.5         Furrow Trials
6.3.6         Time Required to Refill the Soil Moisture Reservoir
6.4 Analysis of Results
6.4. L Objective of Analysis
6.4.2          Maximum allowable time for getting water through the furrow
6.4.3          Maximum allowable furrow           stream 6.4.4          Opportunity Time
6.5       Temporary Field Canal Checks
Forms 6.1 and 6.2
Figures 6 . L to 6.11
Page
54
54
54
54
54
55
55
56
56
56
56|
56
57
58
58
59
59
59
60
6®
60
61Section
Conveyance System
7*1      Weir
7.2         Intake
7.2-1 Scour Sluice
7.3        Head Regulator
7.4        Main Canal
7-5      Controlled Check Drops
7.fe     Constant Head Orifice Offtakes
7.7      Rejection Spillways
Tables     7.1     to     7.3,.and     7.5     to     7.7 Figured 7.1 to 7.7
Pages
64
64
65
^5
68
69Sect lull 8
Distribution System
8.1        Description of System
8.2        Irrigation
8.2.1          Water  Application 8.2.2          Field    Canal Inlets
8.2.1          Syphon Operation
Pages
73
75
75
76
76
0.2.4 
Canal
Parameters
78
8.2.5 
Drain
Parameters
79
8.3         Rejection Spillways
8.4        Furrows
8.4.1          Land Grading for       Furrow Irrigation
8.4.2          Restricting Sail            Layer
8.5        Field Canals-
8.6        Fa rm Units
8.6.1          Requests for Water
8.6.2          Monitoring Irrigation Schedules and Application Efficiencies
8.7         24 hour Irrigation
Table  8.1
Foras  8.1 to 8.4
Figures 8.1 to 8.6
Drawings 8.1 and 8.2
81
81
82
82
83
83
83
83
84Section 9
Salinity and Drainage
9.1 Saline and Alkali Soils
Pages
90
9.1.1
Saline Soils
90
9.1.2
Saline-Alkali Soils
90
9.1.3
Non-saline Alkali Soils
91
9.2         Effect      of Salts on Crops
9.2.1           Boron
9.3 Qua 1 icy of Irrigation Water
9.3.1
9.3.2
Salinity Problem Permeability Problem
9.3.3           Toxicity Problem
9.4 Drainage
9.4.1           Necessity for Drainage
9.4.2           Surface Drainage
9.4.3           Subsurface Drainage
9.4.4           Irrigation Regime Adopted by MSAE
9.5        Groundwater
9.5.1           Investigations
9.5.2          Well Hydrographs and Groundwater Maps
Tables 9.1 ta 9.3
Form 9 .1
Figures 9.1 to 9.16
91
91
91
92
92
93
93
95
95
96Section 10
Efficiencies
10.1 Types of Efficiencies
10.1.1       Conveyance Efficiency
10.1.2        Distribution Efficiency
13.1.3        Field Application Efficiency
10.1.4       Overall or Project Efficiency
10.2 Water Measuring Devices
10.2.1 Water Measurement
10.3 Methods of Water Measurement
10.3.1       Volume Method of Water Measurement
10.3.2       Velocity - Area Method of Water Measurement 10.3.3       Structures for Water Measurement
10.4 Parshall Flume
Tables 10.1 to 10.3
Figures 10.1 to 10.3
Forms 10.1 to 10.3
Pages
101
102
103
103
103
104
104
104
104
104
104
105SECTION 1
Irrigation and Water ManagementAMIBARA IRRIGATION PROJECT 11
WATER MANAGEMENT MANUAL
Section I
Irrigation and Water Management
I.! Int roduct ion
Irrigation was practised in some form since the early civilizations. These civilizations depended on irrigation for their economies* Irrigated farming increases productivity and allows for a changeover from a few dryland crops to several other crops in greater demand. Irrigation creates more employment opportunities strengthening a region’s economy.
Irrigation increases crop production but maintaining increased crop production over a long period of time is difficult without proper rnanagement. Agricultural productivity in an irrigated area requires that a salt balance is achieved in
the root tone and that excess water ls not applied such that the groundwater levels rise till the water table nears the ground surface resulting in
waterlogging and Increased salinity in the root zone* The proper amount of water should be applied to the root zone at proper times. Timing and quantity
of irrigation water are related to climate, soil type and stage of growth-
proper water management to minimise diversion to meet crop water requirements must be instituted and improved with new lands coming under cultivation.
Water is considered a scarce resource. In the past irrigated agricultural development paid little or no attention to management aspect* of water resource utilization. However, with scarcity of water, water management has assumed prime importance for the operation of or to modify existing systems to meet new and changing demands.
The Amihara Irrigation Project, in the Awash Valley has increased the total area under irrigated crops substantially. The project area covers the Helka Sadi and Amibara plains in the middle valley. The net irrigable area of the project is
10,300 ha of which 4.600 ha are in Melka Sadi and 5.700 ha are in Amibara. Prior to project implementation 2,800 ha were served by pumps and 7*500 ha were undeveloped.
According to meteorological data available, at the Melka Merer Research Station, the rainfall is an average of 533 tnm per year. Long rains occurring from July to September account far 49 Z of this rainfall (264 am) and the short rains from February to April (156 am) account for 291. The mean minimum temperature is 15.2*C in December and 23°C in June, while the mean maximum temperature is 32.5   C in December and 38“C in June. Mean relative humidity is lowest in June at 36Z and highest in August at 561. The average daily sunshine hours is 8.5 with an average solar radiation of 536 cal/cm^/day. The climate is semi-arid.
The main irrigated crop under the project is cotton, followed by bananas, peppers, maize and onions in that order. The entities benefitted by gravity irrigation are the Institute of Agricultural Research (300 ha), the Relief and Rehabilitation Commission (The Awash Valley, Amibara Settlement Project), C2000 ha), and Ministry of State Farms Development (8,000 ha).
1.2     Water Management
Water Management, which forms an essential part af any irrigation project includes diversion, conveyance, regulation, measurement, distribution and application at irrigation water tn soil as well as drainage of excess water.FLOW CHART
FIGURE l-lImproved water management requires implementation of technological solutions with institutional changes for optimum water use restrictions. Water use
regulation combined with flow measurement devices are pre-requisite for
effective water management.
Figure 1.1 is a flow chart applicable for water management of the Amibara Irrigation Project.
1. 3 Irrigation Sys tern
The core of an irrigation system is the on-farm distribution and drainage
system. The distribution system should be capable of providing quantities of
water at the time required by the crop to meet evapotranspiration determined by climatic factors. The application of this water to the root zone is dependent
on the efficiency with which it is appLied. At the same time, the leaching requirement for maintaining a salt balance should be supplied. Therefore the distribution system must supply not only the crop water requirement but also the leaching requirement.
Potential evapotranspiration for various crops grown in the project area can be calculated using the meteorological data available at the Research Station*
Melka Merer. These computations will provide irrigation water requirements with time for providing sufficient moisture to the root zone. The replenishment of
soil moisture in the root tone varies with the water holding capacity of the two predominant types of soils in the project area.
One of the essential requirements for water management is an adequate number of flow control and measurement devices. Without knowing the quantity of water to be managed there cannot be good water management. Flow measurement structures should be provided at least at the beginning of each tertiary and at
intermediate points on long tertiaries. All types of farm inlets should be calibrated. By calibrating the farm inlets the amount of water delivered to
meet any particular demand can be measured. The quality of personnel operating the distribution system is an important factor in the proper management of water at farm level. Provision of trained personnel for the operation and maintenance of irrigation systems, although understood as necessary, is often difficult to accomplish. Attention should be paid to training those working directly and indirectly with the "on-farm" distribution and drainage system.
1.4 Drainage System
An efficient drainage system removes surplus surface water caused by excess rainfall or over irrigation. If the distribution system is properly managed drainage is minimised and problems may be reduced. Water quality of the drainage water is an important consideration as canal seepage losses and deep percolation losses will result in degradation of the ground water supply. Return Elows to the river may affect downstream users.
2SECTION 2 Project Works2. Project Works (See Drawing No 2,1)
2,I Diversion Weir
The diversion weir at the Headworks is a rockfill structure founded on a blanket
of graded filter material and having a synthetic impermeable membrane. The weir
3
is 4 m high. The design flood is 700 m /s (100 year flood) represented by an upstream river stage of approximately 751-6 m above mal Che design weir crest elevation being 749,0. The 100 year flood is expected to pass over the weir
without major damage to the rackfill- In the region of the toe of the weir the
rockfill was expected naturally to adjust itself to adopt a stabLe profile in
the form of a convex curve. However on 30 March 19flI> a 1200 cumec flood (return period 1 in 750 years) passed safely over the weir causing adjustment of
the rockfill in its entire overflow area in the form of a convex curve without naajor damage. The river level upstream of the weir rose to El 752.63 nasi.
The synthetic impermeable membrane was designed co provide a water barrier for the first few years of operation, after which the area immediately upstream of
the weir would have silted up and acted as a barrier. tn fact the reservoir
silted up within six months of operation.
Staff gauges upstream and downstream of the weir, on the right bank give direct readings of the river stage. However, the racing curves for the river discharge
over the weir are at present determined theoretically using the weir formula.
But the weir formula is strictly not applicable due to the presence of silt
behind the weir up'to crest level. Accurate calibration will be carried out by
the Water Resources Development Authority by discharge measurements combined with stage measurements at suitable stations upstream and downstream of the
weir.
2-2 Intake
The intake is sited on the right bank of the weir. It is a structure for
drawing off irrigation water into twin barrels (each 1650 x 1500 am) with the entrance to the silt excluder or scour sluice barrel( (1500 x 1500 mm) passing through the weir, placed directly below the entrance Co the twin barrels. The entrance to the twin barrels is provided with a trashrack.
The intake has been a regular source of worry during the rainy season. Bedload, silt and debris have blocked the intake resulting in water not being available
to the entities for a few days at a time. The scour sluice stopped functioning
when the differential head across the weir was less chan 0.5 □.
Steps have been taken to re-model the entrance to the intake by installing
special screens to deflect large trees, logs and debris. This in no way is a
deterrent to the deposition of silt in the intake when the scour sluice scops functioning.
2.3     Head Regulator
The Head Regulator is fed by the twin supply culverts to discharge 13 cumec into the main canal for day and night watering with a minimum upstream river stage of 749.0 m (ie the weir crest Level). Control of the discharge is effected by two vertical lift gates (1500 x 1500 mm) installed in separate chambers with
separate headstocks. Based on empirical formulae, discharge curves have been constructed for a range of gate openings and river stages. Calibration of the
33
gates will be carried out by the Water Resources Development Authority co relate gate openings, canal depths and river stages. Water discharges through the vertical lift gates into twin barrels (1500 x 1500 mm) terminating in a common stilling basin.
2.4      Main Canal
The main canal is 25.5 km Long, and is designed for a discharge of 13.5 m /s
with a velocity of 0.70 m/s and freeboard of 450 mm. Ten offtakes combined with controlled check drops, and three others sited at the tail end without a
controlled check, serve the entities. In addition six drop structures in the
Melka Sadi area are provided where the natural slope of the land exceed the
slope of the canal- The design capacity of the main canal in its final reach is
2.80 m3/s.
The possibility of bringing one or more of the old State Farms, such as Algeta
and Ambash. under gravity irrigation ls under consideration.
2.5      Main Canal Check Drops
The main canal check drops have overshot weir gates designed to pass the
required downstream discharge whilst maintaining a constant upstream full supply level, At low flows the gates act as clear overfalL weirs and in four cases as submerged weirs at high flows. The gates have to be calibrated to determine the discharge passing over each structure point and thereby calculate canal losses
for each reach.
2.6      Rejection Spillways
The rejection spillways are each situated a short distance upstream of a check drop and its offtakes. There are four rejection spillways. The crest level of
the rejection spillways are generally 150 mm above the canal full supply level-
It is necessary to raise the check drop gates to increase the full supply depth
to ensure rejection flows are diverted over the spillways.
2.7      Offtakes
The main canal offtakes are located immediately upstream of each check drop and comprise of an intake structure, discharge pipe and outlet structure. Each
intake structure has been fitted with a rectangular orifice gate providing a means for measuring water issues co entities. The intake end of the pipe is
fitted with a vertical lift gate and outfall structure incorporates a baffle
beam to ensure energy dissipation.
2.8      Main Drain
The main drain which starts west of the Melka Sadi new development area cannot be drained into the river at Melka Werer due to high river stages when drainage would be required. The main drain 29 km long crosses the link road at A km and
t
flows northwards along the eastern dyke, picking up surplus water from the newly developed area in Amibara. The rejection flowy from the 4 main canal spilLways also lead into the main drain. At present flows in the main drain spread out
over undeveloped area in the northern sector of the project reaching the
4vicinity of Dehitali Stat*? Far»- A suitable gravity outlet for these drainage
flows away from the project area back into the Awash River is available to minimise the danger of raising the ground water table and creating health 
hazards.
2.9     Flood Protection Dykes
Flood protection dykes have been constructed along the eastern and western boundaries of the project area. The total length of the western dyke is 43 km
with an average height of 3 m above the natural ground level forming the river bank. The eastern dyke 34 km long is an average 1.5 n high. The western dyke protects the project area from floods tn the Awash River and the eastern dyke excludes surface run-off from the escarpment bordering the Awash/Mille highway* 2.10   Distributary Networks
The      secondary      canals
(30 km long) are fed directly by offtakes
canaL. The offtakes Offtake
No
1
2
3
4
5                Tert iary
6
7
8                Tertiary
9
10
11
secondary
canals and areas              served are as
on    the    main fol lows:-
Se condaty
Area                            Entity
Canal No
Se rved
1
1229                           Melka
2
1 320                          MSAE
3
1009                           Melka
-4
640                          Melka
5
203                           Melka
6
526                           Melka
7
575                           Me L ka
8
300                          Melka
-
1060
-
Farm
940 Awash
F
10)
arm
Me 1 ka
11)
12
13 Tertiary 13)
2491                           Melka
Sadi State Farm I
Sadi State Farm
Sadi State Farm
Merer Merer Farm Merer State Farm Merer State Farm Merer Research Station
Awash   Settlement Project Settlement Project
Merer State Farm Merer State Farm
Melka   Merer State Farm
The secondary canal
d ischarges        are based on a            design irrigation duty at the
field gate of 1,00 l/s/ha (with distribution losses at approximately 1,52 per kilometre} and continuous operation at this discharge will provide the required water application in a 60 days linear planting season. To allow for possible rejection flows the secondary canals are designed for the maximum peak discharge over their entire length. Secondary canals reject into a drain or tertiary 
canal at their tail end.
The tertiary canals (93 km Long) are 0*4 m^/s with offtakes to the field
designed for a maximum discharge of canals (270 kra) having a maximum discharge
of 0.2 
In ____1         special cases such as Tl/2 T3/1 and T5B/2 the design capacity I
r
t
exceeds      0.4      m^/s      being      1*0,      0.5      and      0*6      atysec      respectively.      Tertiary      canals reject into a drain at their taiL end or into a drain through field canals.
The secondary canal checks or cross regulators are Long crested masonry duckbill weirs with sloping rubble aprons. The crests are at full supply level and of varying length from 12.85 m to 17.65 tn designed to reduce the canal freeboard normally by a maximum of 0.| t. A cross regulator serves one offtake upstream*
5The offtakes to tertiary canals are semi-modular (ie the discharge is
independent of taiL water level provided the minimum head is maintained) and are gate controlled either on the offtake crest or, in the case of pipe offtakes at
the pipe entrance. Tertiary canal checks are also duckbill weirs with a lop Lug aprons. The crests are at full supply level and of length 12 * JO m designed to reduce rhe canal free-board by a maximum of 0.09 nj for full discharge. A cross check serves one to four offtakes upstream. Offtakes to field canals from a tertiary and occasionally from a secondary are rectangular orifices controlled by metal slide gates. A long crested duckbill check at the end of a tertiary 
acts as the rejection spillway, discharging into □ tertiary drain.
2.10.1 Irrigation Water Duties
Irrigation water duty or water requirement for irrigation without pre~irrigation can be given as (a) the total quantity of application per given crop per unit area or (b) the discharge capacity of the distribution system at peak demand per unit area served.
Referring         to is seen that
Chapter 5 "Irrigation the irrigation water
Requirement s duty for case
and       Scheduling”       Section       5.6*1       it (a) above for seven selected
crops is        as
fallows:’
Banana                             2344 1000
Cot ton           =                 730 1000
□r                            720 1000
X 10,000
X 10,000
x 10,000
=
-
=s
Pepper
Haricot Beans
-
79 0
1000
x 10.000
-
490
1000
X    10,000
■
Mai ze            =                 632 1000
Onions            ua                600 1000
X   10,000
=t
X  10,000
-
Sesame           -
470     X    10,000 1000
=
Case (b)
depends on case
(a) as the required "wa
23,400 nP/ha
7,300 m3/ha
7,200 m3/ha
7,900 m3/ha
4.900  m3/ha
3
6,320  m /ha
6,000  m3/ha
3
4,700  m /ha
ter duty” as total application
should be applied ac appropriate times'to conform to aiiowabirsoil“mo i^ture depletes for any particular crop. Figure 2.1 gives the application of
7.390 a /ha for cotton in the project area for 3 May planting on a 24 hour basis. Thfj duty is divided into 8 equal applications of 910 cn /ha net and 
3
follows a 60 day linear planting* A 60 day linear planting allows for planting to commence during the first half of May and finish before mid-July as cotton planted after mid-July shows a marked reduction in yleLd and an increase in pests.
6910  m^/hn  is a net application  of 91  ^in  and  taking  the application  efficiency  as 70X, the gross application  will be 130  The flow rate required  at field  gate for this application  is as follows
where q = flow rate in fflVs
t ■ time in seconds
A w area in tn^
d - depth of application ra
q -       13Q x 10,000 
1000            36400x60 - 0.25 1/s/ha
x 1000 l/s per ha
Froa      21st      July      to      5th      September      there      is      an      overlap      of      4      irrigation      applications which can be considered as the peak demand. Hence the peak demand is
4 x 0.25 1/s/ha - 1-00 l/s/ha at field gate.
In the absence of any data on efficiencies for the project, the efficiencies suggested by the International Commission on Irrigation and Drainage and the United State Soil Conservation Service could be used to obtain the duty at divers ion.
Application Efficiency for furrow
=         0.7
Tertiary Canal Efficiency 
a
0.35
Secondary Canal Efficiency 
=
0.90
Conveyance Efficiency 
=
0.92
Project Efficiency 
■
0.7 x 0.85 x 0.90 xx 0-92 0.49
Water duty at diversion for cotton 2.11    Drainage Networks
■             14,878 ni^/ha or              =             1.43 L/s/ha
Secondary and tertiary drains (13.4 km and 74 kra long respectively) remove surplus water due to excess rainfall or excess irrigation from Che field drains
to the main drain. Drains are designed for a drainage coefficient of 63 
mm/24 hrs {10X probability). RemovaL of this 24 hours rainfall will be effected in 18 hours. On Chis basis field drains should have a capacity of 1.54 l/s/ha, tertiary drains a capacity of 0.31 l/s/ha, and main drains the sum of tertiary drain inflows less 15Z to alLou for canal storage.
2.11.1 Drainage Water Duties
Surface drainage removes Che runoff from excess rainfall produced by a design rainstorm in time to prevent damage to crops. The rate of removal is called the drainage coefficient and is the rate of water removal per unit area to provide a certain degree of crop protection.
The drains in the projecC area have been designed using storms with ,i 10 year return period. A 24 hours s to rm magnitude of 63 min with a corresponding drainage period of 13 hours has been used.
7Infiltration losses over a 24 hours period have been taken is 2 mm/hr and evaporation from bare soil or evapotrnnspiration from a cropped surface is taken as 5 mm/day.
Hence, if design 24   hour rainfall infiItration   losses
evaporation         losses runoff
A field drain serves 10 to 20 ha.
=              63
H'             48  mm =                5 rum =              10  mm
Runoff per unit area ■
10_ x 1000
10,000 
18 x 3600
-      1.54 l/a/ha
However since this drainage coefficient requires only a small field drain which cannot be handled by earth moving equipment , field drains are designed for a discharge of about 100 1/s which is one half the design capacity of a field canal.
A tertiary drain serves an area of 200 to 300 ha* There will be a certain amount of storage and time lag in the entire storm runoff from the field drains in reaching the tertiary drain. If 20Z of the total runoff is taken as the peak discharge, then:-
Runoff per unit area -
10 x 1000
0-2 x 10,000 18 x 3600
a 0.3 L/s/ha
This drainage coefficient gives a discharge of 93 1/s for 300 ha. However, since it is possible for at least two field canals to be discharging into field drains and the two field drains into a tertiary or a tertiary canal to reject directly into a tertiary drain the tertiary drains are designed for a minimum discharge of 400 1/s.
Secondary drains are designed for the sum of tertiary drain inflows.
The main drain is designed to carry the maximum rejection discharges from the four rejection spillways sited along the main canal* The main drain discharge
3
is 2,0 m /s at its upper reach and 7.8 o /a at the outfall.
The drains could also be designed using the Cyprus Creek Formula
Q -         OI5/< -----.......................................................................(2) where: Q = run-off in cu ft per second
C ■        16.39 + 14.75 Re
Re * rainfall excess in inches
M - drainage area in square miles For a field drain serving 20 ha the runoff
Q ’         (16.39 + 14.75 x 10 )
the drainage coefficient ■
10,000 x 20 5/6 cu ft/sec 25.4         2?59 x 10h
=    22.2 x ,118 cu ft/sec 3.72 l/s/ha
8Hence      it      is      evident      chat      the     field      drains      have     sufficient     capacity     even      if     this method were to be used.
2*12 Roads
About 90 km of gravel surfaced roads have been constructed of which 40 km are principal roads and 50 km service roads. Unsurfaced farm roads extend to approximately 140 km.
2.13 Flow Chart
The flow chart given in Figure 1.1 indicates schematically how water once divorced □: the Melka Sadi weir finds its way through a network of canals and appurtenant structures to the ultimate field unit.
■ill
9PLANTING
0 25 1/5/ho
0 25 M/tw
I
0 25Jt/s/ha
0 25 I/a/ha
0 25lA/ho |
0 25 t/»/ha
2?
I
0 251/3/ho
I-------- 1
I 0 25lA/ba
I'll                                                                                  1                 1            1            1
0 25
I 0 50 1
0 75
1 00
075
□ 50
0 25
|
r
ltSECTION 3
Soils3, Soil
Action of climate on parent rack at Che earth1 s surface produces soil. The wide variety of soil depends on the landscape, vegetation and time-factor, but the parent material may be the predominant factor. For example clay soils are formed from basalt.
Soil can be regarded as a porous medium in which solid liquid and gaseous material are present providing plants with essential nutrients.
The principal soil types in the project area are recent alluvia and vertisols which can be classified as clay, clay loam, silty clay Luaia, siLty loam and silty clay. Drawing 3.1 gives the general distribution of these soils in the
new area developed under the project.
The alluvial soils are coarser in texture than vertisols and range between sandy Loams And silty loams. The sails are brown in colour and turn to dark brown when moist. They are slightly sticky when taoist and are weak to moderate resistant to pressure. Alluvial soils are found in the backswamps of river floodplains *
The vertisols are silty clay to clay in texture with rhe clay content decreasing with depth. The colour changes from very dark grey brown to black when wet. The soil is hard when dry and very sticky when wet. It develops large cracks when dry. They are generally found on sedimentary plains, both on level laud and depressions.
The soil moisture characteristics of the two soils on a dry weight basis according to available data from the Institute of Agricultural Research are as follows
Alluvia
I
Field Capacity Permanent Wilting Point Available Soil Moisture Bulk Density
Pore Space X Available
35.75 
19.55
16.20
1-335 
44.25
soil moisture in depth value is given by
Vertisols I
46.00 
30.40
15.60
1.175
50.20
FC - PWP     X Bulk Density x 100 cm/ra 100
□ r      Sfc - Sw x V X D.......................— . ..............(3) 100
where Sfc
= soil moisture percentage   at   Field Capacity
Sw      a soil moisture percentage V         - bulk density
at   Wilting Point
D
* depth of soil in centimetres
Hence      available      soil      moisture      expressed      in      depth      value      for      alluvia      and      vertisols are 21.63 cm/ra and 18.33 era/m respectively.
Representative soil moisture characteristics for the whole project area will be determined using the new pressure plate apparatus to be installed at the Institute of Agricultural Research. The pressure plate apparatus subjects saturated soil samples to a pressure of l/3rd atmosphere and 15 atmosphere for 48 hours in two separate chambers designed to withstand these pressures. Once
10the excess water has drained oft, the sampler are gravinjetrically analysed far
the water content. These results will give the field capacity and permanent
wilting point on a dry weight basis. The bulk density of the soils at the
representative sices should also be determined,
3.I      Soil Texture
Texture of a given soil determines its water holding capacity which in turn 
influences the depth of water to be appLied as well as the frequency of
irrigation *
The particle size distribution of a soil determines its texture according to the
percentage of coarse, radium and fine particles. Coarse particles are termed 
sand, radium particles silt and fine particLes clay. Figure 3.1 gives the
texturaL triangle which classifies soils on the basis of mixed proportions of
sand, silt and clay available in a soil. The use of this textural triangle
entails laboratory analysis of soil samples to determine the particle size
distribution. For example if a soil sample has 65Z silt, 30Z clay and 5Z sand 
it can be classified as a silty clay loam, and if the silt-content is I2Z, clay 
28Z and sand 60Z the classification would be sandy clay.
Frequently in the field, the textural class of a soil has to be determined.
This is possible with experience by the feel of the soil when rubbed between 
thumb and fingers or in the palm of the hand.
3,2     Soil Structure
The arrangement of soil particles and  adhesion  of smalter particles to  form larger particles is known  as soil structure. It is the soil structure that influences permeability for water and air and also penetration of roots.
Soil structure can be maintained ar improved by good cultural practices or
destroyed by cultivation when the moisture content is too high or Coo dry. The
presence of sodium salts in the soil causing alkalinity, deteriorates the soil
structure by dispersion of clay particles resulting in low infiltration rates.
3.3     Pore Space
Soil, being  a porous material has many  void  spaces between  the soil particles. These spaces, known  as pore space, occupy  40  to  60  percent of the soil volume. Water and air scored in the pore space fs used by crops.
3.3.1               Bulk Density
Bulk density is the mass (grams) of dry soil per unit volume of soil (1 including pore spaces. Hence Bulk density ’ grams oven-dry soil
volume cnJ soil 3,4     Saturation
When the pore space of a soil is almost completely filled by water, with little or no air in the pore space, the soil is said to be saturated. With time, some
of this water called free water will drain downward by gravity, same will move upward due to capillarity, some will travel laterally and some will be taken up by the crop roots for plant growth.
133-5 Field Capacity
If a soil, not cultivated, is laturated and left to drainT the moisture stored 
in the pore space after drainage is known as field capacity. This moisture is
held against gravity by a force known as moisture tension. The tension is
expressed in equivalent atmosphere.
One atmosphere is equal to a suction or negative pressure of 1 kg/cra^. At field 
capacity soil retains moisture at about l/3rd atmosphere. The field capacity of
soil must necessarily depend on the soil texture or soil particle si ep. A fine
textured soil will retain more water chan a coarse textured soil.
3.6 Permanent Kilting Point
Removal of soil moisture by crop roots reaches a stage when the soil particles
exert a greater tension on the soil moisture than the crop roots can exert to 
extract the remaining moisture. When this condition is reached, the soil is at
the permanent wilting point. The corresponding soil moisture tension is about
15 atmospheres.
The best method of determining the field capacity and permanent wilting point of
a soil is to subject a saturated soil sample to a suction pressure of l/3rd 
atmosphere and 15 atmosphere respectively in a special pressure plate apparatus.
Data obtained from two soil samples in the project areas are given in Table 3.2-
Further data collection is in progress to verify the data in Table 3-2. Vide
Appendix I.
3- 7 Readily Available Soil Moisture
Readily available soil moisture is a percentage of the total soil moisture
available in mra/m between the field capacity and permanent wilting point of the
soil, permitting unrestricted evapotranspiration. The percentage depletion of
the total soil moisture is dependent on the crop, soil and climate. A rule of
thumb is that the readily available soil moisture is about two thirds of the
total available soil moisture between field capacity and permanent wilting 
point.
Fine textured soils, such as clay or clay loam, have high field capacity and 
permanent wilting point and they contain a large amount of readily available
moisture. For all practical purposes rhe field capacity and permanent wilting 
point are constant for any given soil.
Table      3.3      below      gives      the      average      physical      alues      and      textural      relationships      for different soils.
Table 3.3
Total Pore
Space
Bulk
Density
Wilt ing
Point
Field
Capacity
Total Available Moisture
Vol T
gm/ c c
Vol I
Vol 1
Vol Z
Loamy Sand
42
1.55
9
21
12
Sandy Loam
44
1.50
9
21
12
Loam
46
1.40
14
30
16
Clay Loara
49
1-35
17
36
19
Silty Clay
52
1.30
20
41
21
Clay
53
1.25
23
43
20
123.8 Measuring Soil Moisture
Sampling of soil collected from various depths and locations in the field is probably the best method for measuring soil moisture. The moist samples arc weighed, dried to constant weight in an oven at 105 C and reweighpd. The difference in weight is due to the loss of water. The weight of water lost divided by the dry weight of the soil gives the percent of moisture on a dry weight basis. This method is laborious and time consuming. Form 3.1 is used for gravimetric soil moisture determination.
ft
There are various instruments to measure soil moisture which have the advantage of giving immediate readings. They have their advantages and disadvantages too. The fastest and most modern method of soil moisture determination is the Neutron Probe method. The use of gypsum blocks and tensiometers is not recommended on a large scale as they cannot be safeguarded against vandalism.
3.8.1              Tensiometer
A tensiometer under equilibrium conditions indicates the ambient soil moisture tension. The best way to position a tensiometer in the soil is to use .i small auger of approximately the same diameter as the porous cup of the tensiometer, to bore a hole to the required depth. The soil used for backfiLling the hole round the tubing should be well compacted. It is advisable to shield the tensiometer against radiation of the sun and to read the gauge regularly at the same hour of the day.
Tensiometer should be calibrated against soiL moisture content.
3.8.2             Gypsum Blocks
The gypsum block works on the principle of the change in electrical resistance in a porous material due to a change in moisture content. When gypsum blocks which have two parallel electrodes embedded in them are placed in soil, the resistance to an electrical current is dependent on the moisture condition of the unit which is in equilibrium with the surrounding soil.
The electrical resistance of the unit should be calibrated against Che soil moisture content.
3.8.3               Neutron Probe        Method
[n     this     method     neutrons the moisture content of
emitted from a radio active source are slowed down by the soil. The slowed neutrons are received by detector
and     registered     on     a     digital     counter.     The     count     of     slowed     neutrons     is proportional to the moisture content.
The neutron probe method is not widely used in developing countries due to its high cost, maintenance requirements and necessity for skilled technicians.
The average soil moisture loss in depth is determined for periods between irrigations by the formula d “ PVD ---------------------------- ---------------------------- (4)
100
4
13where d = Equivalent depth of water in centimeters
P ■ Chans** in soil moisture percentage from field capacity
V = Bulk density
D ■ Depth of soil in centimetres.
Soil moisture should be determined at regular intervals to determine the consumptive use, frequency of irrigation and depth of application required for each irrigation.
Irrigation water requirement at field Level is therefore determined by the equation
d ■ p.Sa.D mm ---------------------------*----- —---------------- —----- (5) Ea
where p - fraction of readily available soil water
permitting unrestricted evapotranspiration
Sa ■ total available soil moisture in rara/m soil depth
D ■ rooting depth, in m
Ea ■ field application efficiency
Frequency of irrigation, or irrigation intervals are worked out using the formula:-
where i
i 3 p. Sa. D ETc
- irrigation
days
interval in days
(6)
ETc       crop water        requirements, in tnm/day
3.9 Infiltration Rates
The time required to wet soil and distribute the moisture within the soil
profile is, to some entent, determined by the water intake rate. The entry of
water at Che soil surface lk measured as infiltration rates.
Infiltration      takes      place      most      rapidly      when      water      ia      first      a()lied      and      gradually decreases to an almost constant rate.
Infiltration rates should be determined at least once a week for the two types of soil in the project area hb the structure of the soils may change with cultural practices.
3.9.1 Determining Infiltration Races
Infiltration in the field can be determined by measuring the rate of fall of ponded water. The rate of fall of ponded water can be determined by using concentric cylinders or furrows. The method used for measuring soil infiltration rate should simulate the mechanism of water intake during appl[cation.
14The most common method is the use at a "double ring*' i nf i 11rotne ter which is in 
fact two concentric metal cylinders. The metal cylinders are driven into the
soil up to 5 cm using a level wooden batten and a hammer* A scale reading 
graduated in centimeters and millimeters is fixed on to the inside of the inner
ring with its zero at the soil surface inside, A polythene sheet is then used 
to line the sides arid bottom of the inner cylinder 50 that when it is filled 
with water no infiltration takes place till the polythene is quickly extracted.
Hie space between the outer ring and rhe inside ring is filled with water to the
same level as the water in the inner ring. The polythene lining is now
extracted, gently and quickly. The initial reading of the water level on the
scale and subsequent readings ar regular intervals as the water level drops are
recorded on Form 3,2.
Infiltration  data is then  plotted  with  the intake rate in  cm/hr versus time in minutes on  ordinary  graph  paper. The final infiltration  rate is the flat part of the curve of ter two or three hours.
Examples:- Initially the depth of water in the infiItrometer is 19 cm. After
60 seconds the level drops to 18.8 cm and after two minutes it drops to 18.7 cm. The difference in reading for the first 60 seconds is
0*2 cm and for the second interval of 60 seconds it is 0.1 cm.
Hence the initial intake rate * and the second intake rate
0.2 x 60 x 60 - 12 cm/hr 60
=    0,1 x 60 x 60 = 60
6 cm/hr
Hie     initial     moisture     content     of     the     soil     profile     ar     30     cm     intervals     up     to     100     era should be determined.
Figures 3.2 to 3.5 show typical curves for the measured infiltration rates for
Melka Sadi and Amibara areas, It wil be seen that when the surface soil is dry 
the basic infiltration rates are high compared to those when the surface soil is
moist. This is a result of the moist top soil and lower strata reaching 
saturation point faster than when the soil is initially drier.
As an  alternative to  the ’’double ring” infiItrometer method, infiltration  can  be determined  by  measuring  the run-off from real or simulated  rainfall. When  using the real or simulated  rainfall system, the rainfall as well as the run-off for a known  area and  given  period  of time are recorded. The infiltration  will hen  be rainfall minus run-off in mm divided by the time in hours.
The above methods are suitable for determining infiltration rates for basin irrigation. Determination of infiltration rates for furrow irrigation are given in Chapters 6.3.2 and 6.3.3. It is essential to use three furrows, the external furrows acting as buffers for the centre furrow in which all rhe measurments are taken. Inflow and outflow of water, for a pre-determined length of furrow, is measured at regular intervals. The soil intake at any time in litres/sec is
then converted into an infiltration rate in cm/hr for rhe width nf soil covered by the furrow.
15Example:-           Two 3 inch parshall flumes are installed in a furrow 25 m apart.
The initial rending of the upstream flume is 4*5 cm and that of the downstream flume 3.9 cm. After 10 minuLes the readings are 3.7 and 3.2 cm respectively. The corresponding flow rates are 1.45 and 
1*07 l/s for the upstream flume anti 1.16 and 0.R5 1/s for the downstream flume. The differences in inflow and outflow are
□.29 1/s and 0.22 l/s per 25 m. The furrow width is 90 cm.
Hence furrow intake rate
initially
-        0.29 10
X   60 x 60 0.9 x 25
= 4.64   cm/h r
and
= 0.22
X
60 x 60
= 3.52
cm/h r
after 10 minutes
10
0.9 x 25
eum
ula t ive
time on
log-log
paper.
Intake rates in cra/h are plotted versus
This gives 4 straight line with the equation I » lc T --------------------------------------------------------------------
n
(7)
where I = intake rate of soil in cm/h
k « intake rate intercept at unit time
T ■ time water is on surface of soil
n - slope of line (scaled vertical height divided by scaled horizontal length)TABLE 3.1
Textural Guide for Readily Available Moisture
Available Soil
Moisture Remaining
0 -     25       percent 25 -     50      percent 50 -     75      percent
75 - 100        percent
Silt Loam
(Medium Textured)
Powdery,       sometimes       slightly       crusted       but easily broken into powdery condition..
Somewhat      crumbly      but      will      hold      together from         pressure» 
Forms     a     ball, slightly with
somewhat       plastic       will       slick pressure,
100 - percent
(field capacity)
Forms a ball very pliable, slicks readily.
Upon squeezing, no free water appears on soil, but wet outline of ball is left on hand, soil will stick to thumb when rolled between thumb and forefinger.
Silty Clay Loams and Clay
Loam jfine textured)
Hard,       baked,       cracked:       difficult       to       break down into powdery condition.
Somewhat pliable, will ball under pressure.
Forms     a     ball,     will     ribbon     out     between     thumb and forefinger, has a slick feeling.
Easily     ribbons     out     between     thumb     and forefinger, has a slick feeling.
Same as si Lt loum.
Saturated
Free water appears on soil when squeezed.
Same‘as silt LoamTABLE 3.2 AlP Farm
1  Field  Wo [Soil Type
1
i
1
1
[ 6/4/A
1 Alluvial
[
1 Depth 1 1             1 1
1             1 1
~r
1 0-30 1
Sample No
T"
1I i1
Water Content
on Dry Wei
ght Basis
I
1
l/3rd Bar
1. Bar
3 Bar            5 Bar
8 Bar             15 Bar |
1
1!111
1
1
71
1
11
15.68      |  14.79 I
[ 1
L
1
1 27.50 20.84 1 16.23
1
1
1             1         3              1 27.53                 20.83      | 16.31                  15.74 I 14.57 I         11-86 |
1 1   2              , 26.41                 120.54       | 16.11          1 15.94     |
11.88 14.64 I 12.11
1
1
11
1 1  mean       [ 27.15                I  20.74       1 16-22
11
1.
| 15.79  ;
1
14.67  |
1
11.95  1
[
i
ri1
i1
1 1 1 2 | 30.00  I 24.41 1 19.31 1 18.92 1 17.65 I 14.94 I
1 1 1 3 131.15  i 24.33 ! 19.31 1 18.35 I 17.78  1 14.87 I
i 30-60
1             [ 30.25                124-26      ! 19.37           1 18.42      1 17-52      1 55.06
1
1
1 6/l/B
1 Vertisol
1
1
1
1 1 me.in       1 30.47 
|
24.33       [19.33
1              I          F"                                            1                                 1
1
1
1
■ 16.56  |
1
17.65  1
1
1 0-30 I  1             i 49.91
37.52       1 32.84          1 30.74 I    28.80 !  2ft. 84        I
14.9 6        | 1
i
1 1 2              1 47.03
1 1 3             147.70 
37.73       1 33. 14           1 30.9? 1  29.05       1 27.51
|
38.80         32.49              131-13 J    30.09 I  26.35
1
r
1 1 mean 1 48. 18 
11
1 1                          “T          1                                1           1
|
1
38.02 32.82 1 30.95 I 29.31 I 26.90  |
1:         1            1
i              1 30-60 1  1             142.89 
i                    1 1 2             143.57 
131.44      1 27.41              '26.12 I 24.62       122.39
I 31.02      1 27.33           f 25.77 I    24.87 i 22.94         ■
!                   1 1 3              ’43.84 
i 1 1 1 43.43
mean
i 31.63       J 27.18        1 26.22 1    24.99 I 23.22         )
i                       1                       11
31,36      1 27.31            ‘ 26.04 I    24.83 I      23.01 f
i
, Soil
11
1 Depth I         i     Water Content    X Dry  Wt Basis              ! Bulk
1                 11         1
1 1
i Type                       1 1        1
1
1
1
1
1
1 Alluvial
1
1
0-30 I 1 30-60 [
1
11
1 0-30 '
1
FC 1PWP
1
1
27.15 1 11.95
I ASM 1 Density gm/cc: [ ccn/m
1 5 1 AS) l)
1
1
1 15. 20
1
30.47 1
14.96
1
1
48. !S 26,90
1
1 1.35
i1
1
1             1
T[ 1 20.50
1 6/4/A
1
1
| Vert Lsol i 6/L/B
1
1
1
1 15.
1 21. 28
1 20- 42
51 1 1.35
1
1
1 30-60 '
1
43.43 i 1
23.01
1
1.33
1.33
1 20.94 1
1 1 28.30 ’ 1 27.16 1
1 1
1)                      FC = Field Capacity
PWP ■ Permanent Wilting Point ASM =■ Available Soil Moisture
lbFORK J J
GRAVIMETRIC SOIL MOISTURE DETERMINATION
Date .• ••...*.... ................ ,,.................
Name of Officer Sampling Soil:
Field:
Sampling Time: ........................*•»»
Crap: -
i
I Location
II
I
Depth   I Can | Can . Wet Soil           I Dry Soil I Soil Moisture |
I---------- 1--------------- i
I---------- I
I        ID         |     km)   I No I Wt I + can Wt I           I + can Wt I
Z
Sulk |1         Soil Dens i ty   1 Moisture [
I
I
I
I
II
II
I
I
I
I
I
I
!
g/cc I
I' 1
!M3       |
Comments
19FORK 3.2
INFILTROMETER TEST DATA
Dace:
Location:
Sol L Type:
Any other comments
Sheet Nd:
I SOIL MOISTURE SAMPLE I Can No;
r
i              _______
Wt of can
Wt of can + wet soil
Wt of can * dry solI
Wt of moisture
Wt of dry soil
I Moisture content I
i
I TIME
I
I____
I
I 0.00
r~
( 15 sec
I
I 30 sec
I
I 1 min I
I 2 min
I
| 5 min
i
I 10 rain
I
I  20  mm I
I 30 min
I
I  45  min I
I 60 min
I
I 90 min I
| 120 min I
I 150 min
I
j ELAPSED f TIME (2)
I
I
I 15 Sec*
1
READING (3) cm
I
I
I
I
r
l
r
i
r
'I
I
1
I DIFFERENCE I INTAKE RATE
( 4) cm
I (5) Gtii/hr
I
I
I I________
I
I CUMULATIVE INTAKE I I (6) era
T—-----------------------------
I
I
I
1
i
I
r
I
15 sec
30 sec
1 min
3 min
I-------------------- i
’i
1
f------------------- 1 I I_________
i I
1
I
r
5 min
i
J----------------I ____________l
r
1
I
I
10 min
1
i ------- --------- —I 1___________ I
)
10 min
15 min
-
I
r ---------------- —
I
I
15 min
30 rain
■_ J 
I___________ I
I
I
30 min
1 30 min
I
I
r
I 30 min
I 180 min
I
I
1 30 min
_____ __
I 210 min
I
I
■ 30 min [
Name dE observer ............................. .. .................. .. ............................................................ .. 20FIGURE 3 I
PERCENT SAND
PHYSICAL CHARACTERISTICS OF SOILSIntake rate cm/hr
/
1
70 €
10    90 l<
do no (2
>0 130 140 150 i€
>0 170        1
301
INFILTROMETER TEST MELKA SADI HELD 4/6B LOCATION (!) initial SURFACE moisture content^%
Time in m inu les
_vj
\Intake rote cm/hr
Cumulative intake
Dote 12 I 81
Time in minutes
INFILTRATION    TESTAMI0ARA    FIELD    5A/6A    LOCATION (I)
"
JNTTIAL MOISTURE CONTENT 6■ 6       %Inioke rote cm/ fir
Time in minutes
2
iNITIAT 'SURFACE MOISTURE CO
INFILTRATION TEST MELK A SADI
FIELO 4/1
2
1,11 * ■ 1
NTENT 25 8 %
”1
— 
ftIntake rale cm/hr
Cumulaiiwe intake cm
INFILTRATION TEST AMIBARA FIELD 5A/2B
INITIAL SURFACE MOISTURE CONTENT 37 50%SECTION 4
Crop Water Requirements4. Crop Water Requirements
4.1       Introduction
The fundamental objective of irrigation is co supply plants with water at the
correct time and in correct amounts for optimum yield and quality of a desired 
plant constituent. Irrigation scheduling should be designed so that irrigation 
takes place when the soil moisture is high enough to meet crop water
requirements of the plants without placing them unde r stress that would reduce
yield. Other considerations should be control of groundwater Cable, leaching,
slow infiltration of the soils and coat of irrigation water. A knowledge of
crop water requirements is therefore essential for successful irrigated 
agriculture giving high water use efficiencies, optimum crop production and 
maximum profits..
4.2       Methods for Estimating Crop Water Requirements
There are three methods available to determine crop water requirements, namely;
1)       Direct field measurements using drainage or weighing lysimeters
2)              Irrigation       trials       with       various       combinations       of       application       depths       and frequency       correlating       the       treatment       combinations       with       yield       and       other pararaeters
3)        Prediction methods based on climatic parameters.
The first method is very accurate and reliable but needs high technical skill,
is costly and requires accurate measurements. The Institute of Agricultural Research does not have a lysimeter. The second method is time consuming as experiments have to be conducted for many years before reliable data can be obained. The third method is useful as fairly reliable information can be obtained at short notice for on-going irrigated cultivation, for new irrigation projects and project planning.
4.3      Definitions
4.3.1               Evapotranspiration
Evapotranspiration is the combined effect of transpiration of water from a growing crop and evaporation of water from moist soil and crop surface.
Temperature, irrigation practices, length of growing season, rainfall and other factors influence evapotranspiration. Transpiration of water by plants depends on the water at their disposal, temperature, humidity of the air, wind movement intensity and duration of sunlight, development stage, foLiage type and nature of leaves.
4.3.2               Reference Crop Evapotranspiration (ETo)
Referen
ce
crop 
evapotran
spirati
on 
is
from
an 
extensive
surface
of
grass
8 
actively
growing,
complet
ely 
sha
ding 
reference crop in this case is short
defined as *’the rate of evapotranspiration to 15 cm tall, green, of uniform height, the ground and not short of water”. The gras s*
214.3.3               Crop Evapotranspiration (ETc)
The loss through evapotranspiration from a crop has to he made good by supplying the water lost and hence the crop water requirement is defined as the depth of water required by j crop for evapotranspiration during a given period.
The effects of crop characteristics on evapotranspiration are given by a crop coefficient (Kch Factors affecting the selection of a crop coefficient (Ke)
are mainly the crop characteristics* crop planting date, rate of development, length of growing period and climatic conditions. Crop evapotranspiration refers to conditions of adequate water supply for unrestricted growth and development under the given environment. The relationship between reference crop evapotranspiration (ETo) and crop evapotranspiration (ET ) is therefore
C
ETc = KcETo.
4.3.4              Actual Evapotranspiration (ETa)
Actual evapotranspiration, aa the tera denotes is the actual amount of vapour transferred to the atmosphere depending on existing climatic conditions as well as the availability of water to meet the atmospheric demand and the ability of the plants to extract moisture from the soil. Actual evapotranspiration equals crop evapotranspiration, when the available soil water to the crop is adequate. When the available soil water is Limited ETa<ETc. Available soil water can be defined as the fraction (p) to which the total available soil water can be depleted without causing ETa to become less than ETc<
4.4      Calculation of Reference Crop Evapotranspiration
4.4*1 Climatic Data
Climatic data available at the Institute of Agricultural Research, Mel^a Warer are presented in Table 4.1. These data are used to estimate reference evapotranspiration by four different methods.
The    number    of    years be low:-
Eor which      different         climatic data are available is given
Rainfall          Temperature         Relative Humidity
Sunshine Hours
Wind Veloc i ty
Pan Evaporation
Kadia tian
Years 17
u                17                   13                   13                   13                            12
4.4.2               Reference Crop Evapotranspiration by
31aney-Criddle Method
This     method     is     generally     used     where     the     climatic     data     available     covers     only     air temperature data.
The original 5Laney-Criddle formula was based on a consumptive use factor (f) dependent on the mean monthly air temperature (T) and percentage (P) of the total annual daylight hours for the month considered. This was expressed as
v - kf
22where             v * monthly consumptive use
k - empirical coefficient for growing season f = monthly consumptive use factor txp
t ■ mean temperature in degrees Fahrenheit p • percentage of day-time hours of the year
occurring during the month.
100 
The empirical coefficient k varies with the crop and climatic conditions. FAO in their Irrigation and Drainage paper No 24 page 3 recommends the following 
equation
ETO -
c(p(0.46T + 8)) mm/day
where
ETo =
reference evapotranspiration for month considered in mm/d.iy 
3
p*  m
ean daily percentage of total annual daytime hours for given 
month and latitude
c       -       adjustment       factor       depending       on       minimum       relative       humidity sunshine hours and daytime wind estimates.
The values of          ETo for      twelve months.          using the above            presented formula are given below:-
Month
January February March April May June
ETo rani / day
5.02
5.47
5.67
6.19
6.45
7,87
Month
July August September October November December
ETo ram/day 6.28
5.00
6.19
5.80
5.8?
5.58
4.4.3              Reference Evapotranspiration by the Radiation Method
FAO in its Irrigation and Drainage paper No 24 has suggested that this method be used for areas where the available climatic data includes measured air temperature, sunshine and radiation only. General levels of humidity and wind are to be estimated from published weather descriptions.
The Radiation Method is normally more reliable than the Blaney-Criddle Method. The recommended equation is as follows:-
KT-, ■ c(W.R ) on/day
s
where             ETo ■ reference evapotranspiration for period considered in inm/day
R s         = solar radiation in equivalent mm/day
W      » Weighting factor depending on temperature and altitude
c      •      adjustment      factor      depending      on      mean      humidity      and      day-time wind conditions*
Monthly values of ETo based on the above method are given below:-
13Month
ETo min/day
Month
ETd
mm/day
January
5.29
July
7.48
February
5.78
August
5.57
March
6.11
September
6.11
April
6.56
October
6.31
May
6. .'<8
November
6.06
June
6.82
December
5.49
4.4.4
Reference Evapotranspiration by the Class
"A" Pan
Evaporation Method
Evaporation pans provide a quick estimate of reference evapotranspiration from the measured evaporation of a free water surface. Evaporation from a free water surface is affected by radiation, wind temperature and humidity. Planrs respond to those game climatic variables, but the loss of water depends on the stage of growth and plant characteristics. The coefficient of reflectivity of solar radiation from a free water surface is 5 to 3 percent and from green vegetative surface 20 to 25 percent. 5ence a correction factor has to be applied to the evaporation from a free water surface to relate it to vegetative growth.
Evaporation pans store heat during the day and evaporation may be equal ai daytime and night time. This is not so with plants which transpire only during the day.
FAO    in    its    Irrigation    and    Drainage    paper    No    24 equation:-
recommends the following
where
ETo ETo ^pan Kp
Kp . ^pan
■ reference evapotranspiration in nm/day
3 pan  evaporation 
■ pan coefficient wind.
in mm/day depending on location
Monthly values          of ETo using the above formula are given
of pan, humidity and below:-
Month
ETo mm/day
Month
ETo cmu/day
January
4.47
July
5.68
February
4.88
August
March
4.47
5.08
September
Apri 1
5.54
5.17
October
May
6.22
5.58
November
June
4.88
6.49
December
4.47
4.4.5 Reference Evapotranspiration by the Penman Method
Penman made rhe .most complete theoretical approach to show that reference evapotranspiration is inseparably connected to incoming solar energy. The
formula developed by him predicted the evaporation looses from a smooth free
water surface. The basic formula is given below:—
, E "            ___(•£)«„* LE,----------------------------------------------------- (13)
**W)
where E = evaporation frota a free water surface (mm/day)
24A-
*=
L
E’ [J2  x  - e aat’
«2 "
net radiation (cal/cm 2/day)
isothermal evaporation as aerodynamic term
slope of the temperature - vapour pressure
relationship at temperature T (ram Hg/’C)
psychrometric constant (■ 0.457 mm Hg/°C) dependent on average station barometric pressure and latent heat of vaporization 
latent heat of evaporation of 0.1 
0.35 (0.5 * 0.54 U ) <*
wind velocity at 2 m (m/aec) saturated vapour pressure (mm Hg) actual vapour pressure at 2 m (ram Hg)
(• 59 cal)
2         sat “ «2}
The     equation     consists     of     two     terms;     the     radiation     term     and     the     aerodynamic     term depending on wind and humidity-
FAO in its Irrigation and Drainage paper 24 page 15 gives a slightly modified 
Penman equation related to evapotranspiration from a green crop. The equation 
is:-
ETo - c (W.R  + (1-W) «f (u) ■ (ea-ed))----------------------------------—----------------- — (14)
n
radiation 
aerodynamic
term
term
where ETo ■ temperature - altitude related weighting factor R  - net radiation ram/day
n
f(u) ■ wind related function
(ea-ed) ■ difference between saturated vapour pressure at mean air
temperature and the mean actual vapour pressure of the air
in m bar
c     -     adjustment     factor     to     compensate     for     the     effect     of     day     and 
night weather conditions.
The climatic data available at the Institute of Agricultural Research makes this
method of estimating reference evapotranspiration the most reliable in the
absence of actual research data on crop coefficients and crop
evapotranspiration.
a)       Net Radiation Rn
Net     radiation     as     the     term     implies     is     the     difference     between     all     incoming     and outgoing radiation which includes visible 1 ght.
The total radiation Ra received at the tup of the atmosphere depends on the
latitude and time of year only. Table 4.5 gives the values of this extra
terrestrial radiation for each month of the year in northern latitudes of 10“tn 
H“. The latitude of Amibara could be taken as 20 H. Ra for the month of
July ■ 15.23 iw/day of evaporation.
While     passing     through     the     atmosphere     part     of     Ra     is     absorbed     and     scattered,     and the balance inclusive of some of the scattered radiacion reaches the earth’s
surface and  is termed  ’’solar radiation" Rs. Part of Rs is reflected  back  by  the
soil and crops as lung wave radiation shortwave radiation Ros. Reflection coefficient the surface cover. For a tree water surface oc for a green vegetative surface between 20 to 2
and that which remains is net
depends on the nature of hntrifg" c to 8 percent and
V * C
\
25
- *4#J,
A,n
Total net radiation Rn - Rns • Rnl
The reflected longwave radiation is normally greater than the downcoming longwave radiation from the atmosphere. The difference is known as net longwave radiation R £ and is a loss of energy.
------------------- (15)
Where Rs • Ha (0.25 +• 0.5 n ) ——--------------------------- -----------------  (16) N
R
ns
=             (1 "          ) Kb
--------------------------------- H7)
n     is     the     actual     duration     of     sunshine     hours     and     N     the     maximum     possible     duration of sunshine hours given in table 4.5.
Measured n for the month of July cable 4.1 ■ 7.38 hrs.
N    =    12.60    hours,    c*    *    0.25.    Hence    Rns      -    (1    -    0.25)    (0.25    *    0.5    x    7.38    )    15.20 12.60
= 6.20 mtn /day
R [ is determined from available data, temperature (I) vapour pressure (ed) and 
n
ratio n/N. Values for f(T), f(ed) and f(n/N) are given in tables 4.7, 4.8 and 
4.9.
b)        Vapour Pressure
The vapour pressure at a free water surface is the vapour pressure at saturation for the existing  temperature at the surface and  is given  in  table 4.3. For July 
when the mean temperature (T max ♦ T min) « 
2
corresponding vapour pressure Ln the air ed - ea x R mean ra bar.
28.37’C, ea«38.60 m bar. The
H
For July R  mean from table 4.4 is 49.71Z  hence ed 
H
>
100
a 19.19 □ bar and the
saturation vapour pressure deficit (ea - ed) • 19.41 m bar
Rnl - 16.4 x 0.145 x .63
w 1.49 mm/day
Rn =
6.20 - 1*49 = 4-75 mm/day
c)        Weighting factor (W)
W is the weighting factor for the effect of radiation on reference evapotranspiration ETo and its values are related to temperature T max ► T min
and altitude. From table 4.10 for July W ■ 0.987.
Hence WRn - 4.71 x 0.987 - 3.71 om/day
d)        Wind function f(u)
The effect of wind on reference evapotranspiration ETa has been studied for different climates and is defined as f(u) = 0.27 fl + U \
t 100/ where U H wind run in km/day at 2 m height.
From table 4.1, for July U 3 9.24 km/day f(u) is therefore 0.87. Hence (1-w) . f(u) . (ea * ed) ■ 3.58 nm/day
2
26e)      Adjustment factor (c)
The most common conditions assumed in the Penman equation are medium to high radiation and maximum relative humidity and moderate day time wind double the night time wind. These conditions are not always met. For instance in the
Project area daytime and night time winds are equal and maximum relative humidity is about 502 more than the mean relative humidity. A correction factor is required for such conditions and table 4. U gives the value of c for
different conditions of R  max, R , U day and U day/U night for the project
Hs
H
1 Hence c c 0.97*
Therefore July ETo “ 0.97 (3.71 * 3.58)
= 7.07 min/day
Values for ETo on a monthly basis are given, together with the calculations, in 
Table 4.2.
Penman  values of ETo  combined  with  suitable crop  coefficients were used  in determining  irrigation  regimes for different crops, especially  bananas and cotton grown in the project area.
4.5     Crop Coefficient
Kc values for crops grown in the project area compiled from FAO Irrigation and Drainage papers 24 and 33 and those for bananas adjusted to suit observed ninthly water demands, are given in Table 4.3. The growing season for each crop has been divided into five stages, namely seedling (initial), active vegetative growth (crop development), reproductive (mid-season), fruit formation (late season), and maturity (harvest). There is no data available at the Institute of Agricultural Research on actual crop coefficients applicable to crops grown in the project area. See also section 4.3.3.
Table 4.3
area.
For July R max  75Z, Rs « 8.27 nun/day, u day ’ 2.57 m/sec and rj day/U night •
s
Crop
Seedling              Vegetative              Re prod lie live
Fruit Formation
Maturity
Bananas         0.45 Peppers          0.4 Cotton            0.45 Haricot)         0.35 Bi*ans )
Maize 0*40 Onions 0.50 Sesame 0*45
0.85
0.80
0.75
0.75
0.30
□ .75
0.75
1.10
1.00
1.15
1.10
1.10
1.05
1.00
LOO
0.90
0.85
0*70
0.85
0.85
0.70
0.80
0.80
0.65
0.30
0.60
0.80
0,30
The corresponding days
for each stage            of development          is given for            the seven
crops in        Sections 4.7.]         L to 4.7.7.
274.6      Some Crops Grawn in Project Area
4.6*1               3anana
Banana can be grown on well drained fertile land with a wide range of soils.
Optimum pH is between 5 and 7. Bananas are very sensitive to salinity and soils
having an electrical conductivity of less than 1 uimhos/cni are required tor good 
growth.
Th* total crop water requirements for bananas are high, it being a long duration 
crop. Annual water requirements vary from 1200 mm to 2200 mci - the former in 
humid tropics and the latter in the dry topics.
Water Uptake
The banana plant's root system is sparse and shallow, with the majority of
feeding root spread laterally near the surface* The rooting depth generally 
does nor exceed 0.75 ra. Normally 1002 of the water is obtained from the first
0.5     to     0.80     m     soil     depth     with     602     from     the     first     0*3     m.     Generally     J5X     of     the total available soil water should not be depleted.
Irrigation Scheduling
Frequent irrigation is important if ch* available soil moisture depletion is Co 
be limited to 352, Irrigation interval will depend on the soil water holding 
capacity in the rooting depth and the maximum evapotranspiration.
4.6*2 Cotton
Cotton can be grown on a wide range of soils but preferably on medium .qnd heavy 
textured deep soils with good water holding characteristics. Root penetration 
is restricted by acid or dense soils. The pH range is 5.5 to fl with 7 to 8 as
optimum. Cotton is tolerant tn soil salinity with no percentage yield decrease
if ECe ’ 7-7 mmhos/cra.
Cotton     needs     700     to     1300     win     of    water     depending     on    the     climate    and    length     of growing period.
Water Supply
Cotton requires idequate water for vigorous growth and formation of healthy bolls. Root and crop development is restricted by excess water during early stages of growth. Just prior to and during bud formation an adequate water supply is necessary. However, if this water supply is continued during flowering and yield formation prolonged and excessive growth will result. Water stress on cotton is noticeable by the bluish green colour on the leaves and discolouring of the stem.
The entire root zone of 1.8 a should be fully wetted before sowing and subsequent irrigations should wet the upper part of the root zone from 0.50 to 1.0 ci* A 702 depletion of the total available soil water during flowering will
be a saving on irrigation water without impairing yield. During the vegetative period, soil water content over the first 0.75 m of root depth should not fall
28
zbelow 501. Depletion of available soil over 502 will restrict vegetative growth but when followed by adequate supply, vegetative growth will be excessive causing lace flowering and boll shedding.
During yield formation there should be an adequate supply of water which should be discontinued at a certain time before ripening depending on the water holding capacity of the soil, development of root system and ETc* Irrigation after the first harvest i.s sousetitaes necessary to obtain a second yield*
The following table is indicative of the possible rooting depths at four different dates from planting-
Rooting Depth cm
40 120 160 180
Days       after planting
20 40 95 120
However, in the project area the rooting depth does not appear to exceed 120 cm after 120 days from planting.
It is hoped the Institute of Agricultural Research will be able to provide data
on rooting depths of crops grown in the project area.
Water Uptake
Th? tap root could extend in deep soils to a depth of 1.8 m. Frequent, light irrigations during early growth tends to cause shallow root systems. Generally 70 to 802 of the total water uptake occurs over the first 0.75 m depth, where 902 of the total root weight is found. When the crop is fulLy grown, 100 percent of water extracted is from 1.0 to 1.2 * soil depth.
Irrigation         Scheduling
Pre-irrigation is required when stored soil water is not available to enhance
root development. In the vegetative period irrigation can he scheduled when 602 of available soil water in the first 0.75 m has been depleted. During flowering 702 depletion will check excessive vegetative growth but delayed irrigation may cause considerable flower an'. bud shedding. During boll filling and ripenir’ depletion can increase from 602 to higher values with irrigation being terminated 4 to 5 weeks before final picking.
4.6.3 Maize
Maite grows well on most soils but to a lesser extent on very heavy dense clay and very sandy soils. Soils should be well-aerated and well-drained. Maize is moderately sensitive to salinity with 1002 yields at Ec = I.7 mmhos/cm.
Maize needs 500 to 800 :nra of water depending on climate.
29Water Uptake
Soil water depletion of 55Z has a small effect on yield. For rapid and deep 
root growth a greater depletion during early stages is advantageous. Depletion 
of BOZ or more is possible during ripening.
In deep soils roots can teach a depth of 2 tn. but the highLy branched sysrem is
located in the upper 0.8 co i.O m where BOZ of the soil water uptake occurs.
Generally 100Z of the water is extracted from the first 0.8 to 1.2 ra soil depth.
Irrigation Scheduling
Irrigation scheduling should be based on avoiding water deficits during the
flowering period followed by the yield formation period. Water depletion is
about 40Z during establishment, 55 to 65Z during vegetative, flowering and yield 
formation and up to BOZ during ripening.
4*6.4 Onions
Onions can be grown on many soils but preferably on medium textured soils,
optimum pH being in the range of 6 to 7. Onions are sensitive to soil salinity 
with no decrease in yield when ECe » 1.2 mmhos/cm. Onions require 350 to 550 zjoi
of water.
Water Supply
Soil water depletion should not exceed 25Z of available soil water. Relatively 
wet soil reduces root growth and therefore favours bulb enlargement. Irrigation 
is discontinued before maturity.
Water deficits during yield formation should be avoided to achieve large bulb 
site.
Water Uptake
Onion has a shallow rooc system concentrated in the upper 0.3 m soil depth.
Usually 100Z of water uptake occurs in the first 0.3 to 0-5 m soil depth*
Irrigation Scheduling
Onions require frequent Light ir igations when  25X of available water in  first 0*3  ra soil depth  is depleted. Irrigation  is discontinued  15  to  25  days before harvest.
4.6.5 Peppers
Peppers prefer light-textured soils with adequate water holding capacity and drainage with an optimum pH of 5.5 to 7.0. Water Logging even for short periods causes leaf shedding* Pepper is moderately sensitive to soil salinity with 
ECs = 1.5 ;nmhos/cm for no decrease in yield* Pepper requires 600 to 900 imn nnd 1250 am for Long growing periods and several pickings.
3GWater Uptake
The pepper tap root is generally broken at the time of transplanting, but a profusely branched lateral root system develops subsequently. Root depth can extend to 1 m but under irrigation, roots concentrate in the upper 0.3 m soil depth. 1002 of water uptake normally occurs in the first 0.5 to 1.0 o soil
depth. 25 to 302 of total available soil water can be depleted.
4.6.6                Haricot Beans
Haricot beans do not have a specific soil requirement but friable, deep soils with pH 5.5 to 6.0 are preferred. The plants are sensitive to soil salinity and soil having an electrical conductivity of less than 1.0 aanhos/cm is required for good growth.
The total crop water requirement for Haricot Beans is between 300 and 500 ann depending on climate^
The bean plant has a tap root that may reach a depth of 1 to 1.5 m. The extensive lateral root system is concentrated in the first 0.3 ra. At emergence the rooting depth is about 0.07 m, at flowering 0.3 to 0.4 m and at maturity 1 to 1.5 m. Water uptake is mainly from the first 0.5 to 0.7 m depth, 402 to 502 depletion of Che total available soil moisture is possible when ETc is 5 to 6 mm/day.
Irrigation Scheduling
Under full irrigation, the soil water depletion daring flowering and yield formation should not exceed 40 to 50 percent.
4.7 Determining Crop Water Requirements
Crop water requirements for seven selected crops grown in the project area are given below. The aquation used is: —
ET
k       ETo
crop *  c -
4.7.1                Crop Water Requirements far Ban.iTi.3S
Growing Period:
Planting date:
Development stag*:
Rooting depth (cm):
365 days
June       -       perennial A mixed stand of
75
3 years old
ckers actively growing
31Month
ETo inn/day
Days
K
c
ETc mm/month
January
5.52
31
0.75
I 28,34
Feb runry
6,19
28
0.90
155,99
March
6.60
31
1,10
225,06
April
6.Hu
JO
L-25
256.50
May
7.33
31
1.25
284.04
June
R.ia
30
1.25
306.75
July
7.07
31
1.20
263.00
Alignst
6.38
31
1.20
237.34
September
6.49
30
1.15
223,34
October
6. 70
31
0/70
145.39
November
5.94
30
0.75
133.65
December
5.68
31
0.70
173-26
Total
2432,66 mm/year
4.7.2  Crop Water Requirements for Cotton
Growing Period:                                   145    days
Planting Dates:
U)   3rd    May  and (b)
15 May
Development Stages:
15/25/60/30715 days
- 145 days
Rooting Depth (
tn) :
40/80/120/120/120
(a)       3rd  May   P lane mg
Month
ETo enn/day
Days
Kc
ETc so/month
May
7.33
15
0.45
13)
0,75
120.95
June
8.18
12)
0.75
IB)
1.10
235.58
July
7,07
August
31)
1.10
241.09
6.38
111
1.10
20)
September
0.80
1 79.28
6.49
10)
0.80
15
0.65
115,20
Total
89 2,10 mm/sea son
32I
I
I
I(b) L5th May Planting
Month
May
June
July
August
ETo mm/day
K
c
0.45
0.75
1.10
1.10
1.10
0.00
0.80
0.65
0.65 Total
ETc month
7.33
0.18
7.07
6.38
15
25
5)
31)
24)
7)
23)
7)
0)
49.48
September
6.49
October
6.70
198.37
241.09
204-16
148.95
34.84
876.89 m/season
4.7.3               Crop Water      Requirements for Peppers
Growing Period:
Date of Planting:
Development Stage:
160 days
15th August
20/40/50/30/20 days ’ 160 days
Rooting Depth (cm):
20/50/80/80/80
Month
ETo mm/day
Days
K
c
ETc mm/month
Angus t
43.38
Septemb cr
6.38
6.49
17)
3)
0.40
0.40
October
6.70
27)
13)
0.80
0.80
147.97
18)
1.00 
190.28
November
Decembe r
5.94
5.68
30)
2)
I.00 
178.20
29)
1)
20
1.00
0.90 
159.61
January
5.52
0.90
0.80 
93.29
812.73  Tin/season
4.7.4
Crop Water Requirements    for
Total LCot Beans
Growing Period: Date of Planting;
Development Stages:
Routing depth (cm):
110 days
1st November
15/20/30/35/10 days = 110 days 10/3D/B0/80/80
33Month November December January
February
ETo  mm/day 5.94
5.68 5.52
Days 15 15)
5) 26)
4) 27
8 10
Kc
0.35
0.75
0.75
1.10 1.10
ETc so/month
98.02
183.75
6. 19
0.70                    139.66 0.70
0.30 Total
53.23
474.66 mm/season
4.7.5               Crop Water Requirement a             for MaizE
Growing Period       ; Date of Planting:
140 days
(a) 1st   November (b)         15th July        (c) 1st August
Development Stages: 35/35/30/35/15 days
Rooting depth ( cm) : 40/B0/100/120/120
a 140 daya
(a) 1st: November Planting
Month
ETo mm/day
Days
K
c
ETc inm/month
November
5.94
25
.40
5)
.80
83.16
December
5.68
30)
.80
1)
I. 10
142.57
January
5.52
29)
1.10
2)
.75
184.37
Feb ruary
6- 19
28)
.75
129 .99
March
6.60
5)
.75
15
.60
04.15
Total
624.24 m/ s? a son
34(b) 15 th JuLy Planting
Month
ETo a m/day
Days
K
c
ETc uEa/monch
July
7.07
16)
0.4
45*55
August
6.38
9)
0.4
22)
0.8
135.26
September
6,49
13)
0.8
17)
1.1
198.86
October
6.70
13)
1.1
ia)
0.75
186.26
November
5.94
17)
0.75
13)
0.60
122.07
December
5.69
2)
0.60
6.82
Total
604.52 mm/season
(e) lat
August
Planting
Month
ETo ma/day
Pay 5
*c
ETr mm/month
Angust
6. 38
25
0.4
6)
0.8
94.42
September
6.49
29)
0.8
1)
1.1
157.7!
October
6.70
29)
la
2)
0.75
223.70
November
5.94
30)
0.75
133.65
December
5.68
3) 15
0.75
0.60
Total
63-90
673.467 aa/season
4.7.6               Crop Water            Requirements   for Onions
Growing Period:
Dace of Planting;
Dcvelopmenc Stages:
Rooting depth (cd):
120 days
1st October
20/35/30/20/15 days - 120 days 10/30/40/50/50
35Month
October
ETo mm/day
Days
Ke
.5
ETc mm/month
6*70                                 20
November
5.94
11)
24)
.75 
.75
122.28
December
5.68
6)
24)
1.05
1.05
144.34
176.94
January
5.52
7)
13)
.85
.85
15
.80
127.24
Total
570-80 mm/season
4*7.7              Crop   Wa ter  Requirements for Sesame
Growing Period:
110 days
Date of Planting:
1st January 
Development Stages:
Rooting Depth (cm):
20/30/30/20/10 days 15/30/60/80/80
= 110 days
Month
ETa am/day
Days
«g
ETc nun/month
January
5.52
20
.45
February
6. 19
11)
19)
.75
.75
95.22
143.92
March
6.60
9)
21)
1.00
1.00
April
6.84
10)
10)
.70
.70
104.80
ID
, 30
68.40
Total
492.34 mm/season
16TABLE 4,1
SUMMARY OF CLIMATIC DATA FUR MtLKA MEKER RESEARCH STATION
I
i Month
| January
| Rainfall I Min Temp I Max Temp I Mean Temp I Mean Sun- I Rel Hum [ Wind Vel | Pan Evap
| mm.
' T* C
1 I’ c
| T* C
1
1 whine hrs
1X
1
1
1
1
1
| Km/hr             1 mm/dav 11
1
: 17.79
1
1 15.32
1
T
T
T"
I February
I
| March
| April
I
I May
I
| June
I
| July
I
I August
1
I Septenjb-er I
I October
I
| November I
| December
I
I 31-47
1 23.40
1
1 a. 42
1
1 47.22
1 1
I1I
1 ’
1 50.47
1
1 5.23              1   6.97 |11
i
1 
| 16.60
J1
1 32.32
11
1 24,46
1
1 H .57
11
1 52.41
1 
1 5.53             1   7.50 111
|  58.52
i
1
I 18.65
1
1 
1 34.00
1 26.33
1
1 8.12
1
; 49,71
1
l
F1
1 5.74             1 8.00
l
1l
i
1  50.42
Ir
| 19.81
I1
1 35.57
1 27,70
I
L
1 a.53 
1 5.50             1 8,93
1
1 37,23
11
1 49.35
1
11
11
I 24,00
11
1 20.52
1 1
1 28.88
1 9.00
|
|  44.88 
1
1
1 6.36              1 9 .98
1   28.89
i
! 23.11
11
1 37,98 
1
1
1 30.55
I 8.69
1 36.24
1
11
1 9.02  1 11-33
1
1 28.37
11
1i
1 l!
l1
I 110.84
1 21.50
1 35.23
1 7.38 
1 49,71
1 9.24             I 9 .02
1
i
1 33.52
1
11
I1
11
1 U4.ua
! 20-31
1 26.92
. 7.47 
1
1 57.88
1
1 20.60
11
1 1
11
1
11
1 7.46 1  7.37
1
1 38/17
11
Ii
1 35 .19
1
1 34 .90
i1
1 27.90
[ 7.87
i1
1 51.65
11
1 6.32              | 8.21
I1
11
I 21.40
I1
1 17.44
i1
i1
1 26.17
1 9.05
1 42.76
I 1 5.70             1 8.88
11
I1
i1
1I 1                     1 
1 23-43
1 15.24
I
1 32 .52
1 23.80 
11
I1
iI
1 9.62
11
1 45.12
11
1 5.52             |    7*92
i1
l1
1     2,65
1 13.52
1 31.40
i 22.50
I 9.37
147.23
i 5.00              1   7.04
Radia Cal/ cm Per day
478 .J3
516.23
514.83
560-69 569 -65 561.00 527.59 558.20 550.90
544.43
522.53
49^.67& win union
lAfitC *.2
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2-45 |
kTable 4.4 Saturatiun Vapour Proaaura (ca) in abar au Function af Menn Air Teepetature (1) in  C 1/
a
1
1 temper - J Bture *C
15         14        15         16        17         IB         19        za      21        22        23        24        25        26        27          29          29          30        31         32
ea mLaar          15.0     16.1     17.0     19.2      19.4    20.6     ?z.o   Zl.4     24*9     26.4   ZB. I     29.B     31.7     55.6    >5.7-      n.e*    40.1*      42.4      44.9     47.6
1/ Also actual vapour preaauw (ed) eon be obtained from thin tabla using available Tdewpoint data. (Emu*p1o: 1 dewpoint ka lfl"C; od is 20.6 mbor) I nb 1 e it* 5      Lxtrn-terrestrial Radiation (Ra) expressed in equivalent evaporation in ma/day
[
1 Northern Hceioptioro
1
i Jan Feb Hur Apr
Hay            Juno           July          Almj                 Sept           Oct             Nq*        Dec
1 1J.2
1 13.6
14.2
14*5
15.3
15.3
15.7
15.6
15.5
15*3
15.3
15.0
15,3
15.1
15.5
15.4
15.3
15*3
14,7
14.Q
13.6
13.9
12.9
13.3
I
1
1
1 ot
ID0
G°
Table 4.6          Haan  Daily    Duration of HbkimilM            Fcoeiblo Sungluriu Hauro (N)        Tur Different Mentha end Latitudes
Northern
Latitudes
IQ
5
0
11.9            11.9            12.0            12.2            12.3            12.4            12.3           12.5            12 4            12.0           11.9         11.0 1 12. D          IZ.O           12.0           12.0           12*0          12 .□           12.0            12.             12*0           12,0            12.0                12.0 I
I
Jbd Fob Hur Apr May .June July Aug Sept □ct Nov
11.6 11*0 12.0 12.3 12.6 12.7 12.6 12*4 12.1 11.6. 11.6
--------------- 1
Dec         |
1
--------------- 1
11.5         |[»ble 4.7 Effnct gf Temperature f{ 1) an Longwave Radiation (Rnl)
r
r
1
1 I*C
14           16          IS         20          22          Z4          Z6          ZU           30          32
i nn = 8 ru*
i
13.5        u.e        14.2       14.6       15.0       15.4       15 .9     16   J*    16,7       17.2
+
Tnble 4.0 Effect of Vapour PresaurB f(ed) on Lonifrave Rudi at ion (flul)
I
I ed rebar
J
I
I F(ad) = 0.54 - O.Q44(ad)
.18
16 10
.16
.15
20 ZZ
.14
Z4          26 20
.13- .12 .12 .11Table 4.9 Effect of the Ratio Actual and Maximum Bright Sunshine Hours f(n/N) on Longwave Radiation (Rn1)
I n/N
I f(n/N) “ Q.i + 0.9 n/N I .60 
I .55  .6
.64
.65            .7 
.75             .8 
.85
.69            .73            .78             .82*            .87
-9 I
■ 91 I
Tabla 4.10 Values of Weighting Factor (W) for the Effect of Radiation an ETo at Different Temperatures
Altitudes
r
I Temperature  C
a
1
14
16
18
20
22
24
26
28
30
32 
1
1
1
1
I W at alt itude m
1
1
1 0
.61
.64
.66
.69
.71
.73
.75
-77*
.78
,80 |
i
500
.62
.65
.67
.70
.72
.74
.76
.78
.79
.81 
I
| 1000
1
.64
.66
.69
.71
.73
.75
.77
-79
.80
.82
1
Table 4.11 Adjuatment Factor (cl in Presented Pennan Equation
!'
1
1
r
| RS nan./day 1 1
r
| U day ra/sec
1 —
1 Rlimax * 60Z 1
1 3                6                9                   12                   3                   6                  9                    12
1-------------------------- 1 RHniax -901
1
r
1
1
11
| U day/Unight         ■ 1.0
’
1
1 I
“I 1 1
~n
i
1
r
1
i
1
f 0 
1 .96 
.93
1.05
1.05
1 .02
I -06
i
1 -10
1.10
!
1J
1 .78 
.86
-94*
.99- 11 
.85
.92
1-01
1.05*
1
1 6 1
| .62 
.70
.84
.93     1          .72
.82
.95              1.00        1SECTION 5
Irrigation Requirement) and Scheduling5. Irrigation Requirements and Scheduling 
5. 1 Irrigation Requirement
irrigation requiretnent for a crop is the depth of application of water over the entire growing season to supplement rainfall, stored soil water and contribution From groundwater, if any. This implies that there is a water balance between demand (etc) and supply (Ig+Pe*Ge). The quantity of application over unit area for any particular crop is known as irrigation duty for that crop.
For example the crop water requirements for cotton, estimated according to Penman for 3rd May planting and a growing period of 145 days is a depth of 391.62 mm. Effective rainfall during this period is 167,67 (vide para 5*4.5), and the groundwater contribution is nil as the water table is below 6.0 metres. Hence the net irrigation requirement is 391.62 - 167.67 ■ 723.95 mm. If this depth of water is applied to one hectare the duty is 723.95 x 10,000 = 7240 m^/ha.
1000
Therefor? we say the net irrigation duty for cotton is 7240 m^/ha.
The cotton crop has to receive this quantity of water spread out over the entire growing season to keep the soil moisture in the root zone either at field 
capacity or within an allowable depletion level of readily available soiL moisture between irrigations. This involves what is termed scheduling.
5-2 Irrigation Schedules
For the operation of an irrigation distribution system, a supply schedule is required for a block of fields in extent from 300 ha to 500 ha which is the area supervised by a Unit Manager. Drawing 5.2 shows the Farm Units for both Melka Sadi and Melka War er Farms. Irrigation schedules should be flexible to cater for changes in soil type, allowable depletion of soil moisture based on rooting depth and effective rainfall.
An irrigation schedule gives the depth of application (d) in uini and th? irrigation interval (i) in days. The readily available soil moisture in the
root rone (0) in mm must meet the crop water requirements ETc ram/day. The actual evapotranspiration ETa of a crop is determined by whether the readily available soil moisture p.SA.D over the root zone is adequate or whether the crop will be stressed inducing a water deficit when ETa will be equal to or less than ETc.
The totat available soil moisture Sa mm/m which is th? soil moisture between field capacity Sfc and permanent wilting point Sv, varies widely for soils having different texture. In addition, most soils are layered and an integrated value of Sa over the root zone should be selected.
tn general, Sa in nm/in for different soil texture is: —
Fine textured Medium fine textured 
200 mm/m co 280 nni/n
140 rm/rn to 200 mm/m
However, the field capacity and permanent wilting point for representative samples oE soil covering the whole project area as development proceeds should be undertaken by che Water Management Unit with the help of the Institute of Agricultural Research and made available to Unit Managers of the Ministry of State Farm* Development.
425.1     Soil Moisture Balance The running soil moisture balance
for any period is given byi-
We    =    Wb + Ge * Pe - LAC
We tjoi 1 moisture available at the end
---------- ----------------           ■
Wb    ■        soil     moisture     at     beginning     of     each previous irrigation or rain
Ge     s        groundwater contribution ETc     a        crop water requirements
Pc     ■        effective raintail
5.3.1              Groundwater Contribution (Ge)
of each period period from
The groundwater contribution is determined by how far below the rooting depth the groundwater table is located to affect the soil moisture content in Che root zone by the capillary properties of the soil* In heavy soils the distance of movement is high and the rate low, whereas for coarse textured soils the distance of movement is small and the rate high. Detailed experiments are required to determine the groundwater contribution under field conditions for areas where the groundwater table is within 2 metres from the surface.
For example if the water table is 2 m below the soil surface the groundwater' contribution for different soils is given below:-
Soil Type
Ge tan/day
Organic
Clay
Sandy Loams
Fine Sandy Loam
0.3
0.B
1.5
4.5
5.3.2              Stored Soil Moisture
The stored soil moisture is primarily affected by evapotranspiration. Whether thia storage is within the total available sail moisture range fur a given soil,
or whether it is below this range depends on the moisture left over from previous rainfall, the Last irrigation or groundwater contribution. On the
basis of Laboratory experiments, Penman stated under summer conditions evaporation from a wet soil surface is equal to open - water evaporation until a cer“ain critical deficit is reached. In areas with high radiation conditions, a surtace mulch of dry soil may develop acting as a capillary break, and, hence, greatly reduce the evaporation rate. Experiments under actual field conditions will be conducted in 1982 to determine the soil moisture lost due to evaporation from the soil after pre-irrigation up to planting of cotton seedn. Water is
Lost also from the root zone due to deep percolation when rhe groundwater table is deep.
The stored soil moisture in the root zone can be determined gravimetrically.
435.4 Effective Rainfall (Pe)
5.4.1             Definition of Effective Rainfall
Effective rainfall (Pe) is defined as Chat portion of mean (or dependable)
rainfall Char is utilised for meeting crop water requirements during the growing season of a crop. Mot all rainfall that is recorded during the growing season 
can be effective and part of it is lost by surface runoff, deep percolation or evaporation. The effectiveness of rainfall is a function of its intensity and 
the extent of crop cover. Light rains on a fully vegetated area can be 100Z effective whereas intensive rains (25 to 30 nun in a short period) may be only 
60Z effective on an area with intermittent crop cover.
Due to the variability of rainfall both seasonal and annual, especially in 
semi-arid and arid conditions, rather than using mean rainfall data ( i* one year drier, the next year wetter), a dependable level of rainfall should be selected 
in estimating Pe. The level of dependability can be determined statistically 
(see Section 5.4.4) co derive rainfall depth that can be expected in 3 yearn out
□f 4 (75X exceedance probability) or 4 years out of 5 (801 exceedance
probability).
The method that is commonly used for estimating Pe is based on dependable rainfall and mean evapotranspiration and was developed by US Soil Conservation Service. It is also described in FAO Irrigation and Drainage Paper No 24 (see Section 5.4.5).
5.4.2              Effect of Soil Characteristics on Effective Rainfall
Soil acts as a reservoir for moisture to the crops. Therefore its properties of absorption, retention, release and movements of water have a direct bearing on effective rainfall.
Infiltration rate and permeability of a soil are its intake and water movement characteristics. With an increase in the water holding capacity of a soil, the fraction of effective rainfall increases. The greater the soil depth, the
higher the proportion of effective to total rainfall. Initial moisture content
in a soil also has a bearing on the extent of effective rainfall. If there is
rainfall 9oon after an irrigation the soil cannot store any more water and the rainfall is Lost to surface runoff.
5.4.3              Effect of Crop Characteristics on Effective Rainfall
Crop characteristics influencing intake of water are the degree of ground cover, rooting depth and growth stage, Soil moisture in deeper layers is capped only when the crop matures and roots penetrate to these depths. Therefore deep rooting crops will increase the extent of effective rainfall.
5.4.4             Estimation of Dependable Rainfall
Monthly rainfalL data for 17 years, collected from the Institute of Agriculture Research, Melka Warer, were ranked in descending order and the plotting position of each ranked value was determined using the WeibuLl plotting formula.
The ranked monthly rainfalls were plotted against respective plotting position on a normal probability paper for individual months (Figure 5.3 to 5.13) and the 75Z probability or 3 years out of 4 rainfall was computed as follows:-
*14m x 100-------------------------------------------------------------------------------- ----------------- - (20)
N+l
wh ere P =  plotting position
N =   total number of events m »   rank of each event
Month
Mean Rainfall
Probable      Rainfal
1
mtn/'non th
3 out of
4 years
mm/month
January
17.8
0
February
47.2
0
March
58.5
30.0
Apri I
50.4
26.5
May
24.1
6.5
J une
23.9
3.5
July
1 10.8
85.0
August
114.fi
78.5
September
38.2
24.0
October
21.4
0
November
23.4
0
December
2-7
0
The 751 probable rainfall is used in section 5.4.5 in computing the effective
rainfall for seven selected crops, with storage factors relative to the readily 
available soil moisture in rhe root zone at progressive stages of growth.
5.4.5              Method of Determining Effective Rainfall (Pe)
By processing long term climatic and soil moisture data, the Unites States
Department of Agriculture Soil Conservation Service has developed a procedure
for estimating effective rainfall. This is termed the evapotranspiration/
precipitation ratio method. (FAO Irrigation and Drainage Papers 24e and 25)-
Table 5.1 gives the relationship between average monthly effective rainfall and 
mean monthly rainfall for different values of average monthly consumptive use.
The values of effective rainfall obtained from the table have to be multiplied 
by a storage factor which adjusts for che effective depth of storage. These
factors are presented in Table 5.2-
For exampLe, from Section 4.7.2 the mean monthly consumptive use for cotton 
during August is L79.3 emo and from Section 5.4.4, the 75% probable rainfall is
70.5       nan*    Assuming    the    storage    of    moisture    per    metre    of    root    zone    is    220    mmt      the soil moisture storage tor a rooting depth of 120 cm with 50% depletion is
50% x 220 x 120           ■ 132 ram/month. 100
From Table 5.1 the monthly effective rainfall is 63.5 mm and rhe storage factor from Table 5.2 is 1.04. The mean monthly effective rainfall far August is therefore * 63.5 x 1.04 - ££.0 mtn.
45TABLE $.1
Monthly
Mean
Ruin
ful i m
Average Monthly Iffectivc Rainfall as Related !□ Muon Monthly Raitifal] end Mean Monthly Tonsinptive Uiir (USDA SC5)
25               50               75                 ino
Mean Honthly Consumptive U^e rnm
125                I5O
Hi? ™«i
175
Monthly
200
L r feet 1 vu
225
Rainfall mm
2SLI
275         JOO          325          35U
12,5
7.5 
s*a
a.7
9.0
9.2
10. Li
25.0
15.0 
lb.2
19.5
10.5
11.2
17 .5
10. u
37.5
22.5 
24.0
27.5
19.7
20.5
11,1
22.0
24.5
12.5
12/5
12.5
12 .5
12 5
26.2
29.2
25 
32.2
34.5
35.7
56.7
!?-!
JO .5
50.0
33.0
25.0
36,2
37.5
25.0
25 J)
25.0
37.5
37.5
25 [I
39.0
40.5
45.7
37 5
17 S
62,5
«t UTT
».7
4? 3
A4.5
£>6.0
4fl.5
50.5
47.0
53.7
57,5
50.0
50,0
62.5
50 .0
62.5
50 .0
50 3
75,0
46.2
49.7
52.7
55.0
57.5
60.2
65.7
67.5
62,5
62 .5
62 *5
B7.5
50.0
56.7
60.2
63.7
66.0
69.7
73.7
77,7
73.7
04.5
75,0
75.0
75.0
75.0
100. a
ut on.7
6J.7
67.7
70.5
72,0
74.2
70.7
B5.0
75.0
Ml.2
02.5
B7.I
97.7
112-5
95.0
07.5
1ULJ
loo
07.5
07.5
07 5
92.7
9B.0
LQO
inn
125.0
al
75.0
Hl .5
97.7
90.5
95.7
89.7
102
105
10B
115
111
112
112
1 17
137.5
95.2
98.7
104
111
121
125
125
125
150.0
95.2
102
106
112
120
ua
127
126
132
137
[37
137
162.5
100
109
113
120
136
120
135
143
150
150
150
175.0
at
160 *
115
120
127
135
143
145
153
160
162
154
164
170
175
162
107.5
121
126
134
142
151
175
znn.a
125
133
140
225
el
144
hfl
150
161
170
105
166
17B
179
160
196
107
151
160
171
102
200
250
I5O
161
170
275
nt JZS
193
171
101
194
MIO
175
19D
194
205
325
203
it 2FF“
199
215
350
ZU
224
375
200
220
ut 331
232
400
225
240
425
.t jfJT"
247
25U
at      m- fl 50                    25                50                   75               100            125                 150               175               200               225               250
l.nhlu 5.2
Lffeclive Stnrnqe
20
25
37.5
50
62.5
SI nrago Factor
0.73
0,77
75
too
125
150
175
0.66
0,93
0.97
i.tJO
200
1.02
1.U4
1.06
1,07
1.00Table 5.3
Bananas
Cot ton Cotton
P e p pe r s Haricot Beans Maize
Maize Maize Onions Sesame
Effective Rainfall for seven selected Crops Effective Rainfall
rum
253.91
167.67 - 3rd May planting 362.97 - I5ch May planting
61.52
0
25.59
133.06 - 1st July planting 70.90 - 1st August pLanting
0 45-31
5-5 Leaching Requirements
Salinity of the soil is mainly affected by irrigation water quality, irrigation 
practices and methods, soil conditions and rainfall. Leaching requirement (LR)
is therefore the minimum amount of irrigation water that must be drained through 
Che root tone to control soil salinity at a given specific level. Figure 5-1 
shows the effect of saline soils on the germination of cotton seed and height of
seedLings according to data available at the Melka Warer Institute of
Agricultural Research
For clay  loam soils with  good  drainage and  low rainfall the leaching  requirement
is given by the equation:-
ECw
” (21)
5 ECe - ECw
where ECw =
EC of irrigation water mmhos/cm is an average of
0.35 mhos/cn for Awash River water.
ECe =
ECe of the soiL saturation extract for a given crop 
appropriate to a tolerabLe degree of yield reduction 
m.mhos/cm.
For a 100X yield potential the value of ECe
Crop
Bananas Cotton Peppers Onions
Maize
Beans Sesame
ECe mmhos/cm
1 .0 7.7 1.5 1.2 1.7 1.0 1.5
The    average    value    of    ECe    is grown in the project area.
LR ■    _______0.3 5
5 K 1.5 - 0.35
taken as 1.5 mmhos/cm for crops other than cotton * 0.049
47For cotton LR
0.35 
5 x 7.7 - 0.35
0.009
The efficiency of leaching varies with the soil type and its internal drainage.
5.6 Net Irrigation Requirements
The net irrigation  requirement (In) for a crop  is the depth  of application  of water over the entire growing  season  to  supplement rainfall stored  soil water and contribution from groundwater if any. This can be represented by: —
In   ETcrop - Pe - Ge - Wb ------------------------------- "* 
a
-------
-      (22)
where     In     ■     net     irrigation     requirement.     The     other     terms     in     the     equation     are defined in para 5.3.
5.6.1              Gross Irrigation Requirements
Other than for meeting the net irrigation requirements, water is needed for Leaching and application losses. Hence gross irrigation requirement is given by the equfltioa:-
In        x 1
L-LR           Ea where lg ■  gross irrigation
---- (23)
requirement
Ea -               application effi ciency
LR -   leaching requirement
The gross       irrigation requirements for the seven selected crops soil moisture and application efficiency but including leaching
only are .15 follows and may in the project area.
be considered as a net requirement
Pe
neglecting stored rcqu irements for crops grown
Crop
ETC
nan/season
nun/ season
mm/season
Bananas
2483-23
253.91
2 344
Cotton
891.62
167.67
730
Cotton
876.89
162.97
720
Peppers
812.73
61.52
790
H.ficot
474.66
0
499
Haize
624.24
23.59
632
Maize
684.52
133.06
580
Maize
673.46
70.90
634
Onions
570.80
0
600
Sesame
492.34
45.31
470
5-7 Depth
of Application
Dcpth of application d, ig the depth of water required to replenish the readily available soil moisture after the allowable depletion and is given by p.Sa.D., where p is the fraction of the total available soil moisture which can be used by the crop without causing stress. The allowable depletion level varies with the crop development stage and evaporative demand.
43Therefore d = p.Sa. D
--------------------------------------------
(24)
Ea
where d ’ depth of application in ram
p = allowable depletion fraction
Sa - total available soil moisture in mm/m D ■ rooting depth in ra
£□ » application efficiency Z
Table 5.4 ETcrop.
Table 5.4
gives the soil             water depletion
fraction (p)
for cotton at different
ETc rn/day
2
3
4
5
6
7
fi
9
10
P
.875
.80
.70
.60
.55
.50
.45
.425
.40
5.8 Irrigation interval
Timing of irrigation application is of utmost importance as delayed irrigations could affect yields- Timing of irrigation should conform to soil water depletion requirements of the crop. Therefore, during the crop growth, flexibility in time and depth of irrigation should be maintained to accommodate differences in crop water needs. This method is preferable to having fixed schedules. However, in the absence of data on soil characteristics for the whole project area, a partially fixed schedule was adopted based on estimated crop water requirements and stages of growth. The schedule used for cotton grown in the project area appears to be well suited to overall soil
characteristics           as           presently   known and given below:-
climatic        cond irions             encountered   and    is
Days from planting
Gross depth of application cm
- 12 2 30 51  65 79 93 114 135
28 .6 13 13 13  13 13 13 13 13
Interval (days)                                                              14          28       21        14       14       14       21          21
Yield response to water during the 1980 and 1981 cotton season was more than expected (over 35q/ha) being well above the yields predicted in the appraisal of April 1977.
The irrigation interval is given by the equationi-
i - p»Sa» D
-------------------- --------------------- ------------------------------------ (25)
ETc
5.9 Calculation of Irrigation Schedules
An     irrigation     schedule     should     be     made     for     the     crop     using     the     monthly     soil     water balance and allowable soil water depletion data.
Example I (Alluvia Soil)
The cotton growing season in the project area is May through October. The planting date is 15th May and the growing period 145 days. A net pre-irrigation of 220 ram is applied 8 days before planting to fill the soil moisture reservoir of the root zone to field capacity, for efficient weed control prior to planting and to flush down rhe salts which may have been carried up by capillary action during the dry season following harvest of the previous crop, and better yield
49according to data available at the
Research.
Melka Warer Institute o£ Agricultural
May
June
July
August
Sept
Oct
1.        ETo nni/day
7.33
8.18
7.07
6.38
6.40
6.70
2.        Davs
15
254/5
31
24/7
23/7
8
3.       Kc
.45
.75/1.1
1.1
1.1/.80
.80/.65
.65
4.        ETc ram/month
49.48
198.37
241.04
204.16
148.95
34.84
5.        Probable rainfall
81.56
68.48
12.93
May
June
July
August
Sept
Oct
6*       Wb
*220
+140.52
-57.85
-217.38
-353.06
-489.08
7.        Pe
0
0
>81.56
+68.48
*12.93
0
8.        ETc
-79.48
-198.37
-241.09
-204.16
-148,95
-34.84
9.        We
+140.52
-57.85
-217.38
-353.06
-489.0
-532.92
10.     Total available soil moisture
for Che
soils in
the project
area is
taken h.h
220 ™/m
11.      Rooting depth D ram
300
750
1000
1000
1000
1000
12.     Sa.D
66
165
220
220
220
220
13.      p for cotton
.50
.50
.50
.55
.60
.65
14.     p.Sa .0
33
83
110
121
132
143
15.      (I - p) Sa.D +
33
82
110
99
88
77
16.      plot Sa.D and (I - p)                  Sa.D at end of month as 17.      plot We as given.
given,      figure 5.2.
18.     When the soil moisture balance curve (We) meets the soil moisture depletion curve (I - p) Sa.D the soil moisture is replenished by drawing vertical lines between Sa.D and (I - p) Sa.D = pSa.D.
19.      Plot new soil moisture balance curve starting from Line Sa.D and parallel to line We for that period to meet line (I - p) Sa.D. Repeat this
process.
20.      The number of vertical lines in a month gives the number of irrigations fur chat month.
May 
June
July 
August
Sept
Mil
3 
2 
1 
I
21.      The length of the vertical lines between Sa.D and (l - p_) Sa.D gives the depth of net application required in iron.
June 50
60 73
August 115
5022* The horizontal spacing of the vertical lines gives the irrigation intervals fro® planting.
June
July 
August
Sept
36 
H
22 
27
6 
15
7
23. The net depths of application should be corrected for leaching and 
application efficiency for gross water requirements at field level.
24* Application efficiency Ea Water directly available to crop over root zone
Water received at field inlet
= Average depth infiltrated
Depth of water actually applied
5,9.1              Partially Fixed Schedule
The     irrigation     schedule     in     operation     at     present     for     cotton     growing     on     new     lands in the project area .is calculated as follows:-
The depth of gross application per irrigation ls 130 nm and the manageable flow rate in a field canal is 150 1/s although the canal is designed for a maximum of 200 1/s. Hence the time taken to apply 130 nnn over an area of A hectares is given by:-
t •       130  x A x 10,000
’     2-41A   hours
1000
3600 x 0.15
Applying
this equation
to part of
Unit I Amibara the following data is obtained
Field
Area
t
T
No
(ha)
hrs
days
I
11.39
60.4
2i
2
22.78
120.7
5
3
11.39
60.4
2i
4
12.02
63*7
3
5
17.56
93.1
4
6
24.56
130.6
Si
7
21.63
114.6
5
8
11.45
60.7
2»
9
14.42
76*4
3
10
20*90
110.8
5
11
32.98
174.8
H
The commencement of the eight post-planting irrigations ls fixed as follows:- Days from planting 2                                 30            51            55            79 g} [ l4
13 $
Hence     the     schedule     given     below     is     obtained     assuming     planting     takes     place     on 3rd May*
51
✓Field
No
Area
Post
Planting Irrigatio
n Dates
Irrigation
ha
1
2
3
4
5
6
7
8
Time days
1
11.39
5*5
2.6
23.6
7.7
21-7
4.8
25.8
15.9
2i
2
22.70
3.5
5.6
26*6
10.7
24.7
7.8
28.3
18*9
5
3
11. 39
13.5
10*6
1.7
15.7
29.7
12.8
1.9
23*9
2i
4
12*02
15.5
12.6
3-7
17.7
31.7
14.8
3*9
25*9
3
5
17.56
18.5
15.6
6.7
20*7
3-8
17.8
6.9
28*9
4
6
24.56
22.5
19.6
10.7
24*7
7.8
21.8
10*9
2* 10
si
7
21.63
28.5
25.6
16.7
30.7
13.8
27.8
16.9
8*10
5
8
11.45
1.6
29*6
20*7
3.3
17*8
31*8
20.9
12. 10
2i
9
14*42
4.6
2.7
23*7
6.3
20.8
3.9
23.9
15. 10
3
10
20.90
6.6
4.7
25.7
8.8
22.8
5.9
25.9
17*10
5
11
32.98
11.6
9*7
30.7
13.0
27-8
10.9
30.9
22-10
7i
Interva1
. days
28
21
14
14
14
21
21
5.10 Establishment of Rain Gauge Stations
[t has been observed that rainfall is uneven over the project area. Although 
theoretical crop water requirements have been determined using data from the
Institute of Agricultural Research. Helka Warcr, the rainfall and evaporation 
data in particular may not apply to the whole project area due to the variations
in rainfall. Hence it is imperative that rain gauge stations should be
established approximately for every 600 ha. A proposed network is given in 
Drawing 5*1. Some of these rain gauge stations have now been established and a
Class I weather station near check drop 3 for the Melka Sadi area.
There should be an up to date record of meteorological data for the project
area, with the Water Management Unit.
The State Farm, Irrigation Manager and Irrigation Supervisors, could use Form
5.1        for     scheduling     irrigations     on     a     daily     basis     for     each     field     where     rain     gauge stations are established.
Figure 5.14 gives the regime worked out on the above basis and adjusted for
application over the entire project using dailv rainfall data for 1900, 1981 and 
1982.
Some     fields     are     dry     planted     without     pre-irrigat     ion     due     to     lack     of     time.     Figure 5*15 is the regime adopted for dry planting.FORM 5 . i
SOIL WATER BALANCE METHOD OF SCHEDULING
Unit No:                                                                                    Soil Type; ................................... Field No:  Water Holding Capacity:
Month; ...................................................................................... Planting Date:
r1
1
i                 Depth    1
I Date Rai a fa 11
i
Irrig      1
Soil Water         1
at begin
of day mtn i Kc
Crop CoeEf
1 nun
1
1
11
Ref        J                              1 Soil Water Evapa | Crop Evapo               at end of ETo | ETC aim                        dav nun
1
1
1
2
■1
3
1
'5
6
1 7
8
9
10
! 11
12
13
—                       —--- -----------
14
15                   '"1
1
1
16
1
17
1
ia
1
1
19
1
20
1
21
1
22 1
1
23
f
24 1
1
25
1
26
27
1
20
1
29
---- ---------- -
1
30
31  1
1
!
53FIGURE 5
EFFECT OF SOIL SOLUTION CONCENTRATION ON THE HEIGHT OF COTTON SEEDLINGS ONE WEEK
AFTER GERMINATIONFIGURE 5 2
*400
*500
+200
+ 100
(l-p)Sot
JUL
AUG
SEP
OCT
-100
-200
-300
-400
-500
-6001Lienee 3 3
LOHJ401| tu LULUMean rainfall in mm95 99                99 9         &              99       90            95          90             00___ 70
__
50     40       30       20             IQ 5
2i05
02 04 0 05               0 Of
ni
Probobdily o
1 “T T 1
n or 1
1L.W
%
\
X
-
-
+
—
—
—
—
—
&
E
c
o
*-
C
o
5
-
—
— -
—
---------
120
110
100
90
80
70
60
50
40
30
20
10
0
OCX 006 d 02
-
—
—
■—
\x
L
1
J
Pro
t>abiJ
ity of —1
exci »dence c
Zo
-
05 I 2                              5            O             20        30
-
1
50 6Q 70 BO 90
95 SB 99
996 999
L
9999
MARCH
CCMean ran fa It in mm
99 39                999 99-H                  99      98            95          90             SO 70           60     50     40
*
1
1' ’
—
-
i
i?n
1 IikJm 00 90
80
70
60
50 40
F~ —
J
-
—
r-
1
-
■
■ 1—
ImU
0
Pro
babiliFy oi       earsdonee *
7e
0 01 OOS 0' 02
05 I1 2                             5            >0           20         30       ^0 50 60
i
J i
APRIL
FIGURE 5 6■ ■■
FIGURE 57mm ni IlDtnioi nMM
innE
—r
b»-oj|cpiji
05 OS I S                             2
eo 10 so do a? as aa aae aaa aaaa
—
------------1
j
__
|X qi cxctqeucc c
L
1
1
1
■
1
1
I
-
1
•
.-
*
1
-
1
1
[.
1
r
•
biopop
^0 20 SO 10 2 ’ S I 0-9
!||P o
I UOl
j-eKG
__
05 Ol 002
cqti
L_
J
LI
Cfl—-
0 01
WV«iso r lao
170 160 150 140 130 130
no 100
90 90 70 60 50
40 O
i
i
3    40      30       20
S>2
02 Oi qpS _______OOI
I
Probability of non-encodence % ___ _
I
I
H
—•—■— 
■
Pro
babili
ly ol
L excedence ZVo
3
20        30       40
60      70 BO
90          95
96      99              99 fi 999                   99S9
FIGURE 5 9Meon rainfall in
* IB * * * * M
99 99 220
210 200
-Probabdiiyof non-excedence %
160 £150
60.
Probability of excedence %
02       05 I 2                              5
4 0    50     60 TO 00                        90          95            90      99              995 999                   9999
AUGUST
FIGURE 5i01
I20             10          5             2 I               0 5      02 CH 0 05             001
_______                               ______
Pm hn hili 1 unft nr’ip wrnrtAnrs
1
1   T] '
%
•
-L.
■—
-■
I, __ 1.
r
*--------
■
10
Probability of excedence %
5
2D 30              40     50     60
60           90          95            90 99                   99 9 999                     9999
SEPTEMBER
------------i
I
1
IIC                      -50     40      50 2Q                   io 5
2I05
02 Ol 005 OOl
T’l i
fi/
/c
<40 130 120 110 100
90
80
70 80 50
40
30 20 10
0
—1
<
--------- ---
-
r
-1
—' -
--------- F        ■
______
1
______ I
Pro
Isabrij
1
1
ty of
e*ci
idadce e
4)
1
00*         005 01 02             05 I 2                              5
20 SO
50     60      70         80            90          95            98      99              998 999                     9999
OCTOBERrainfall in mm
FIGURE 513APPL DEPTH
m mj
GROSS
MAY
JUN         1              JUL
'
AUG              1 .           SEP             [            OCT       J
NOV
!
Ntl
220 314
9 1 130
1
1
BLACK cracking SOILS
9 1 130 1
1
PLANTING
1
1
1
1
1
1
1
1
I 1
Pre - irrigation Post ptarrf irrigation
19 hr 30 mt
6 hr.
Q1
31
91
iIV
<30
1
1
1
1
1
II
11 1
9 1 1 30
9 1 130
1
1
11
11
! 26 9 <                130             | DAYS
i 21                   1 14          1      14      1   14     1           21 1              21
| DAYS            I DAYS | DAYS | DAYS |
DAYS |          DAYS
220 314
77 1 1 0
1
1
1
PLANTING
1
77 1 10 1
1
77               5 <0            1
1
|
1
1I
i:
ii
11
11
11
11
LIGHT BROWN SOILS Pre ^Irrigation
post plan! irrigation
20 hr
3 he   30 mt
77               i 10
77               1 10
77                f 10
77               1 10
1
1
1
1
1
1 21
1
1
1
1
1 1 •  “
11
1                11
1
15       |     15       1 15        1 15          | 15            1 '5 7                    21
77 1 10 |_DAYS [ DAYS | DAYS | DAYS 1 DAYS i DAYS | DAYS , days
IRRIGATION REGIMES
FIGURE 514APPL DEPTH
(mm)
NET GROSS
9 1                l 30
9 .                 130
9 1                130
9 1                130
9 J                130
9 1                130
9 1                 130
9 1                 130
|______ MAY
JUN
PLANTING
JUL
AUG
SEP
OCT
NOV
BLACK CRACKING SOILS
Post plant irrigation 6 nr
I
14              14
21
I 21
n days _ [_days_10ay2LdaLs-L dJ¥S_ 1_°Av1 J
planting
77
l 10
LIGHT BROWN SOILS
77                1 10
77                1 10
77                1 10
77                 110
77                 I 10
77                1 10
77                1 10
77 1 1 0
Post plant irrigation 3 hr 30 mt.
f
13 'S                    15            15          15                15
L^OAYSJ DAYS j_ DAYS DAYS
DAYS
’is | rfl 2J
J_DAYS ]_DAYSj _DAYS_
REGIMES FOR DRY PLANTINGSECTION 6
Furrow Irrigation6           Furrow Irrigation
6.I       Scope of Furrow Irrigation
Furrow irrigation is rhe method of irrigation used for row crops. The equipment
used and the spacing between rows de tsnsint? the sise and the shape of furrows
for Che crop. Irrigation is accomplished by running water into the furrows
construe ted down or across the slope of a field. Water infiltrates into the
soil and spreads laterally to irrigate the areas between furrows. The duration
of irrigation or the Length of time the water is to flow in the furrows depends
on the amount of water required to replenish the root zone> the infiltration
rate of the soil, and the rate of lateral spread of water in the soil.
Labour input is generally more for furrow irrigation than any other methods of
surface irrigation. Skilled irrigators arc needed to divide the water in the
field canal into a number of furrow streams and maintain the correct rates of
flow until irrigation is complete.
6.2       Furrows
6.2.1                  Characteristics and TypejJ of Furrow Irrigation
Compared to other methods of surface irrigation, the furrow method has distinct
advantages: water in the furrows is in contact with only half to one fifth of
the land surface, this reduces evaporation losses, crusting of the soil and
puddling; earlier cultivation is possible.
Furrow       irrigation       requires       proper       land       grading       so       chat       water       can       travel       the entire length of the row without ponding.
Adequacy and efficiency of irrigation depends considerably on the shape of the furrows. The cross-section of the furrows should be sufficient to carry a discharge needed to give uniform distribution throughout the furrow.
The most common furrow is V-shaped, in depth 15 to 20 centimetres and 25 to 30 centimetres wide at rhe top. Furrow shapes are modified by the movement of water down the slope; a narrow channel being formed for steep slopes and a broad channel for flatter slopes.
6.2.2                  Furrow Spacing
In general furr_iv spacing is determined by the crop, machinery used and cultural practices adopted. However in terms of irrigation practice^ selection of furrow spacing is to ensure that the lateral movement of water between adjacent furrows will wet the entire root zone of the crop. Wetting patterns can be determined
by digging a trench across a furrow after it is irrigated. The examination of
Che wetting pattern for a number of furrow? in which water has run for different lengths of time is the best way to decide furrow spacing for any particular
soil.
6.2.3                  Furrow Slopes
Furrows should have a uniform slope for efficient application of water. Uneven slopes cause uneven depths of irrigation along the furrows, with erosion in steeper areas and deposition in flatter areas. The danger of ponding and injuring the crop is also present.
54The principles governing the flow of water in an, open channel apply to tha f Low
of water in a furrow, which is a smaLl open channel. The velocity of water
flowing in the channel is therefore proportional, to the square root of the slope
of the furrow, to the shape and roughness of the furrow and flow rate. The
velocity should not be so high as to cause soil movement.
The slope of the bottom of the furrow indicates the steepness of the furrow and
is expressed as a percentage. The slope causes the water to flow and also
provides drainage for excess rainfall or irrigation water. The optimum slope
should be O.20Z to 0*601 far furrows in the project area. The maxiraum
permissible slope is around 0.751.
The maximum aon-errosive flow rate in furrows is given by the equation.”
Q -       0.60 i/s —-------------------------- -----------------------------------------------------------------  C26>
S
where S - slope of furrow bottom in percentage,
6.2.4             Furrow Lengths
Cost of irrigation increases as furrows become shorter and uniformity of water application decreases as furrows become longer. Therefore farrow length should permit a reasonable application efficiency.
If too long a furrow is used* excessive water enters the soil at the upper end before the lower end is adequately irrigated. This is pronounced in soils with high infiltration rates. Farrow Lengths should be adjusted to account for changes in soil type where soils in the sani« field have different intake rates. Factors considered in determining maximum furrow lengths are the type of soil, time of irrigation, slope, and the stream size.
Furrows wiLl necessarily be shorter for sandy soils having low water holding capacities than for clay soils with high water holding capacities.
Furrow lengths in the project area are fixed to conform with economical land levelling and configuration of fields. They are generally 200m to 225m in
length.
6.2.5                  Flow Rates
The size of stream delivere 1 into each furrow is ealled the flow rate and generally measured in litres per second. Once furrow lengths and slopes are fixed, the flow rate is the only variable.
Uniform distribution is obtained by starting the irrigation with the largest flaw that the furrow can safely carry. For the desired wetting of the soil the water should reach the end of the furrow within a quarter of the total time that water muse be carried in the furrow to refill the root zone to field capacity. Thus the opportunity time for the soil to absorb water will be about 25 percent greater at the upper than at the lower end of the field. Figure 6.1.
55When the stream has reached the lower end of the furrow the flow rate is
adjusted or cut back to maintain the flow without losing water by surface
run-off at the end of the furrow. This flow rate ia equal to the rate water is
infiltrating into the bed and sides of thr* furrow. The Lntakt* rate decreases
with the length of time water runs in the furrow, therefore the flow rate has to 
be decreased accordingly using one’s judgement.
6.2.6               Regulation of Stream Size
In practice approxiniarely ^qual flows are discharged into a number of furrows at
the same time using an equal, number of 4maLl syphons p®r furrow*. Rates of flow
are altered either by increasing or decreasing the number of syphons per furrow
or by changing the pressure head. The pressure head is the difference in 
elevation between rhe water surface in the field canal and centre of syphon 
outlet if flowing free or the water surface above Che outlet if it is submerged.
The designed pressure head Eor the project area is 25cm and varies by + 5 cm.
6.2.7 Average Depth of Water Applied
If     the     furrow     spacing,     furrow     Length,     flow     rate     and     duration     of     irrigation     are known, then:
Water application m/hr. = litres/a x 3600 
where W ■ spacing of furrow in metres L * length of furrow in metres
----------------------------- --------------- (27)
W x L
Soil     sampling     should     be     used     co     check     rhe     uniformity     of     waiter     penetration     at different points along the furrow, two or three days after an irrigation.
6. J Furrow Evaluation
6.3.1              Objective
The overall objective of evaluating a furrow irrigation system is to determine
whether the system provider the required amount of water to the crops uniformly 
throughout the furrow Length with optimum water application efficiency. An 
evaluation will give Che following information:-
a)        Furrow slope
b)        Time of    irrigation
c)        Optimum  furrow       length
d)        Optimum  stream      size
6.3.2              Initial Collection of Data
Inspect the area to be evaluated and select seven alternate furrows for six 
different flow rates the furrows can safely carry and one furrow for determining 
the furrow intake rate using two 3" Parshall flumes.
Two  days before the trial, stakes .should  be driven  into  the ground  on  the furrow ridge at 25m intervals for each  of six  furrows, starting  at the field  canal end. A line of levels are taken  along  the bed  of each  furrow to  determine the average slope along  its entire length*  as well as individual slopes between  stakpn. The Levels could  be related  to  an  arbitrary  datum. The furrow spacing  and  depth  of
56furrow should also be measured at each stake. Examples of this data collected 
during a furrow evaluation trial in Melka Sadi, and Anri bar a area are given in 
Figures 6.2 and 6.3.
Integrated soil sampling using an auger should then be made at 30cm intervals
up to a depth of 100cm to determine the initial soil moisture status of the root
iane. Sampling is done at the top end, middle and bottom end of each furrow.
Form 3.J is used for this purpose. Figure 6.4 gives the soil moisture status
prior to irrigation at the middle of a furrow during actual furrow evaluation 
trials in Melka Sadi, At the top end of each selected furrow a pit of adequate
size should be constructed to receive a plastic bucket for periodic volumetric
calibration of the stream entering rhe furrow. This information is required to 
estimate the actual depth of water applied to each furrow during the trials.
A fairly high embankment is built at Che tail end of each selected furrow,
beyond th* last stake, to prevent any water escaping to waste,
A canvas or plastic check dara across the field cana1 should be placed just
downstream of the last furrow to maintain a constant head for the syphons
discharging into the furrows. An adequate number of 2.4cm internal diameter and 
2.20m long syphons should be available. This is the standard syphon in use for
new Lands under cultivation in the project area and lends itself to easy 
adjustment of flow rates.
A day before the trial, install two 3” Parshall flumes, 25m apart, in the
seventh furrow. Using a furrow stream given by 3 or 4 syphons, note the two 
Parshall flume readings instantaneously at regular intervals for four hours on 
Form 6.1 Calculate the furrow intake rate from these readings. A discharge
curve for a 3" Parshall flume under free flow conditions is given in Figure
6.11,
6.3,3               Furrow Intake Rate
The rate at which water is absorbed by the soil decreases with time. After a
few hours it becomes constant. The furrow intake data which will be in 
litres/sec for a length of 25m should be converted to cm/hr.
Intake I » I/sec x 3600 x 1QQ - cm/hr. ----------------------------------------------- ------------------------—— (28) W x 25 x 1000
where W M furrow spacing
0..9'0 m for the project area
Plot this intake rate versus time in minutes on log—Log paper. The resultant
curve is given by the formula;
where
I « intake rate of the soil (cm/hr)
T •“ time that water is on the surface of the soil (minutes)
K ■ intake rate intercept at unit time
n       slope       of       the       line       (vertical       scaled       distance       divided       by       horizontal scaled distance)
57A typical curve for .illuvia soil in Melkn Sadi area is given in Figure 6.5 and a curve for vertisol in Ami bar a area is given in Figure 6.6. Both these curves were obtained frcra actual, furrow intake datn.
6.3.4             Water Needed to Refill the Soil Moisture Reservoir
The amount of water needed to refill the soil moisture reservoir to field capacity, is obtained by subtracting the amount of moisture in the root zone from the field capacity. Therefore the water required to refill the root zone is given by:-
where
d - (Sfc - S) x V x D cm ------------------------------------------------------------------------------------- (30) LOO
SFc = soil moisture percentage at field capacity
SS = soil moisture percentage at time of soilsampling using an auger VV - dry bulk density of soil (refer to 3.3.1)
DD < depth to be filled
Example:- The average initial soil moisture is found to be 27.342 upto a depth 
of 100cm and the field capacity 42.02. The bulk density is 1.2 gm/cc.
Therefore water required to refill the 100cm root zone.
= 42.0D - 27,34 x 1.2 x 100 - 17.59 on/m.
100 •
6.3.5              Furrow Trials
Maintaining  a constant head  in  the field  canal, constant flow streams using  an increasing  number of standard  syphons (2.4  era internal diameter and  2.2m long) are Let into the six furrows in sequence.
The time when water starts to flow into each furrow and when the stream reaches the stakes should be recorded on Form 6.2. This is known as the advance time.
Volumetric measurements should be taken of each stream periodically.
Inspect the furrows for breaching or overtopping to ensure that each individual stream irrigates only its allotted furrow. Cut back the streams to prevent overflow at the tall end of each furrow, but equal to the intake rate of the entire furrow Length. Measure woluraetrically th? cut™back stream sizes.
After some hours of irrigation given by the furrow intake curve, and assuming chat the required depth of water has been applied to each furrow the streams are turned off. The time when water in the furrows disappeared at each stake is
then recorded on Form 6*2. This is known as "recession time”.
The wetting pattern along each furrow should then be checked by auger sampling soils at 30 cm intervals up to a depth of 100cm at ridges and bed of each furrow at three sections namely:- the upper end, middle and tail end using Form 3.1. Figure 6.4 gives the wetting pattern after irrigation al the middle section of a furrow referred to at 6.3.2.
566.3.6               Time Required to Refill Soil Moisture Reservoir
The     intake     rate     equation     I     ’     KTn        cra/hr     when     integrated     gives     the     cumulative depth of infiltration- The infiltration depth is given by
(n ♦ 1)
60d - K
n * 1 1 in cm/hr.
x T                    d is multiplied by 60 as T is in minutes and J
From the above T -
60d (n + 1) n + I in minutes- --------- K------ --
Hence     using     this     equation     and     the     data     given     in     figures     6.5     and     6.6,     the     time required for a pre’irrigation and a post planting irrigation can be calculated.
The gross application per post-plant irrigation is a depth of 13cm and the pre-irrigation is a depth of 31.43cm.
Time required to apply                 31-43cm depth
1
on Alluvia              T  -      60 x 31.43(1 - 24
=     1130 minutes on Vertisol            T  =■  60 x 31.43(1 -
11.5
■     1027 minutes Time required to apply                 13cm depth
-47)          I -   .47
1
.32)         1 -   .32
1
on Alluvia
T         60 x 13(1 - .47) 24
215 minutes
1 - .47
on Vert'sol
T
60 x 13(1 - -32) 1
LUS
280 minutes
I___
.32
6.4 Analysis of Results from Evaluation Trials
6.4/1               Objectives of Analysis
The     objectives     in     analysing     the     furrow     evaluation     data     are     six     fold     and     are     as follows:-
a)             Knowing     the     soil     moisture     content     prior     to     irrigation,     th**     water     needed     to refill the soil moisture reservoir to field capacity can be determined.
b)        Determining the infiltration or furrow intake rate.
59c)        Calculating the time required to refill the soil moisture reservoir.
d)       Determining the maximum allowable tine to pass water through Che furrow.
e)       Determining the optimum stream aize
f)        Determining the optimum furrow length 
6.4,2 Maximum Allowable Time for getting Water through the Furrows.
If the intake rate is uniform throughout the length of the furrow absorbtion 
will be uniform along each segment of furrow for equal intervals of time and depth infiltrated is approximately proportional to the square root of time. However, water advances more slowly down a furrow when it is dry. Therefore th water is in the upper end of the furrow longer than at the lower end.
For nearly uniform irrigation, the stream front should reach the end of the furrow within V^th of the total time needed to refill the soil moisture reservoir. Thus the "opportunity time" for the soil to absorb water will be about 251 greater at the upper end than at the lower end. But the intake decreases with time. So, the amount of water absorbed during this extra time will be less than 25Z of the total. See Figure 6.1.
Advance time being l/4th irrigation time, total irrigation rimes are shown below.
Total irrigation time for post plant
ire igation         for
Alluvia
Vertisol
215 * 54 ■       269    minutes    ■    4    hrs    29    minutes 280 + 70 =       350 minutes - 5 hrs 50 minutes
Total irrigation time for pre-irrigacion for:-
Alluvia Vert isol
1138   +   54   •   1192   minutes   a     19   hrs   52 minutes
1027 + 70 » 1097 minutes ■ 18 hrs
17 minutes
6.4.3              Maximum Allowable Furrow Stream
The maximum allowable furrow stream is the scream that doea not cause erosion. In cases where the furrow length is fixed as in the project area this can be determined by exrmining the advance rate curves, figure* 6.7 and 6.9, and selecting the stream which reaches the furrow length within the ' advance time”
(1/4ch the time required to refill rhe soil nboigture reservoir),
6.4.4              Maximum Furrow Length
Th«* maximum length of furrow is indicated by the point where the line for one fourth the time required to refill the soil moisture reservoir intersects the
1
plotted "advance curve ’ of the maximum allowable furrow scream. Figure 6.7 and 6.8.
6.4.5            Opportunity Time
Stream advance curves and stream recession curves of time in minutes versus distance are plotted on the same paper for each furrow Figure 6.9 and 6.10. The ordinate at any particular furrow stake, between the advance and recessioncurves       gives       the       opportunity       time.       Ideal       distribution       of       water       along       the       furrow will he obtained when opportunity tine is equal co irrigation time.
The stream advance and recession curves are a good indication of the uniformity of furrow slopes and/or variation in infiltration rates along the furrows. The recession curves in Figure 6.9 and 6.10 confirm the non—uniformity of furrow slopes shown in Figures 6.1 and 6.2. When there Is a low spot along a furrow, water ponds at this spot for a longer Length of time than at a high spot which will drain faster. A dip in the recession curve indicates a high spot and a
hump a low spot.
Won-uniformity of furrow slopes are due to improper Land grading or land smoothing and may also be due co ridging against the natural slope of the land. Very often furrow slopes could be made uniform by manually adjusting high spots in furrows prior to or during irrigation. Ridging should also, if possible, be carried out with gauge wheels fitted to the ridger allowing it Co float and excavate a uniform depth of furrow. This has been found to be effective in the project area.
Each field should have a minimum of two diagonal passes of a land plane to correct irregularities in land surface. Ridging should be done according to directions shown on design drawings for each field.
6-5 Temporary Field Canal Checks
The use of heavy duty plastic or canvas overflow cross checks for fieLd canals is very necessary to maintain a constant level of water up-stream so that the syphons discharge into furrows under a constant head. The use of non-overflow earth dams is not advocated as the large syphons used to discharge water over these dams cannot maintain a constant level upstream and often water overflows the earth dam breaching it*
61
iFORM fi -1
FL3H0W IRRIGATION SYSTEM EVALUATION FURROW INTAKE DATA SHEET
Hane of Recorder
.......... *.................           Date
. . . ....... *
Field ........................................................................................           Furrow No ■ . . ............................................................. ...................
Furrow width ...«•*•*
Condi cion of furrow: New
Average Furrow depth ..............
Soil Moisture: a) before irrigation
Old 6 Reshaped
Old
Depth 
Z
Distance between inflow
b) atter
irrigation
& outflow stations ................
..............................................
Depth 
................ *Z
Parshall Fluan size
Stream size ................................... . ..•..................... ..
11
1 Clock 1
11
Elapsed j Cua-j 1 - 1 tNF LOW
OUTFLOW
| Tiifii               Time       1   a Live                  PF
Flaw
1
1 Mins Time | Reading Rate
1
1
PF               Flow Reading          Rate
1
Intake Intake 1 1 /sec/ ram/hr 1
25 a
11
i
1I1
11I!1
1
1
Illi
Illi
r
1
1
Illi
III
1
1
1
11
11
1
1
1I
1
1
1
1
1
1
1
i
11
1i
11
1
1
1111I
1 I . J  1 1
1
1
1I1i
1 1-1 1
ri
i
1
1
1i
1 1 - !- 1
l”“
‘
1
11
1
1
.
Illi
------------ i---------------
i
1
1
C^an’rtts:FORM 6.2
FURROW IRRIGATION SYSTEM EVALUATION FURROW ADVANCE AND RECESSION DATA SHEET Nine of Recorder
Da t e ..........
Field ...................................................................................
Furrow Slope (Average) I , .'J* *. t
Furrow condition New
Old & reshaped Old
■
Furr ov Nc . » • Uniformity of 3 lope Good
Distance between
Scat ions (in) .......... — 
» . .  .
Fair
Poor
Si phon size
Raced Discharge (1/a)
.......... ..
Siphon Head .. . *....................... Measured discharge (1/s)
STREAM ADVANCE
| 
STREAM RECESSION
1 
1
| STATION   CLOCK   ELAPSED 1                TIME       TIME        | 1                                      min)
1
!
CUMULATIVE J         STATION TIME
1
J
CLOCK
TIME
ELAPSED TIME
mi ha
1
CUMULATIVE TIME
mine
1
1
I
1
1
1’1
11
!
|
nr
ii
1
1
1
1
1
1
m
[i
iii
111
1
1
iii
iii
i
i
1
1
iii
i
i
i
1
1
1
1
CALIBRATION OF SIPHON                     COMMENTS
I Time
1 Sec
Vol ume     Flaw   rate   I           * Lit              Lit/sec I
Clock time at which the stream reached
Che end of furrow * * * ............ ........... ............ .. .............. ..
1
—~r ---------------- 1       *   Clock time the stream was cut off ............................ .............. ....
1
i                   Did the stream over How? YES/NO
*
1
I      tAf          W33 the stream cut back? YES/NO
r
1        *   Stream cut back time .......................... ..............
11
*
1         *   Stream size after cut hack (L/s) ...................................................
-              Did significant erosion occur? YES/NG
1 1
Any other c anime nr
IPERCOLATION
Time required to opply Dem of water *. Advance flow = hours
4
T hours
TIME
1 DEPTH INFILTEREO
U/S                           o/s
Infiltration D is proportional
approximately to square roof
T/4
2T
4
3T
4
4T
4
5T
4
dl
d l * d2
d +d  +d
f23
d | t d d-^>
d( + d^+dj+d^dg
0
of time
D ex t^2
d|
t
or
M7
r fl ^2
dr
d,  d
+2
d I + d2 ♦ d 3 d i*d2+d3+d4
LOSS BY DEEP PERCOLATION
= [A - 2] d,
- —-------- ——t 100%
[ 2 +75]d|_
2
53%
• df ♦ dg
*'♦ d2 z
d|
Wd3
d3
dl
d|4 d24d3+d4
Ad4
fa
(2t )'/2
[72 - l]d(
fa
(3 1
= [^3-/2] d.
d|4-dj+ d^t-d^+d^    (5t }'/2
■d5
5
If  -2] «l
■ ■ d| + d^+d^t-d^+dg - yj~5
d 4 d2 + d Hj
t
3     4 = 2 dr
.'.Average depth absorbed = (2 +-/S)
depth of water absorbed during an irrigationFurrow depth - Cm
99
97
FURROW No I
L
AV SLOPE 0.14%
99!
97
FURROW No. 2
AV SLOPE 0.(5%
»
99
97
FURROW No 3
AV SLOPE O 15%
FURROW No 4
av. slope o is %
FURROW No 5
AV SLOPE 0 14%
FURROW No 6
AV SLOPE 0 08%
MELKA SADI FURROW SLOPES IN EVALUATION TRIALSFurrow depth
95
94
93
95
FURROW No I
av. Slope - -
94
FURROW No 2
93
AV SLOPE____
FURROW Nd.3
AV SLOPL--
FURROW No.4 AV SLOPE _
FURROW No 5 AV SLOPE
FURROW No.6 AV. SLOPE
MELKA WARER FURROW SLOPES IN EVALUATION TRIALSDepth in cm
Depth in cm
LOCATION MELKA SADI FIELD 47, 61Intoke rcfe cm/ hr
FURROW INTAKEIntake rale cm/fir
FURROW INTAKE
AGURE 6 6Furrow Distance
STREAM ADVANCE CURVESTime in Minutes
Furrow Distance
STREAM ADVANCE CURVES200
LOCATION MEJlKA SADI
—------ Irrigation time
- ------- Time required for iScmdepth
STREAM advance-recession curvesLo co ti on - A mibaro
---------- Time required for irrigation
—---------irrigation time.
STREAM ADVANCE-RECESSION CURVES
FIGURE 6'IOFREE FLOW DISCHARGE l/s
FIGURE 611
CALIBRATION CURVE FOR 3" PARSHALL FLUMESECTION?
Conveyance System7. Conveyance System
7.1      Weir
Thp diversion weir is a rackfill structure founded on a blanket of graded filter materia! and having a synthetic i rope roe able membrane on the upstream side. The membrane was designed to provide a water barrier for the first few years of operation after which the area upstream of the weir was expected to silt up and
act as a barrier. In fact the area immediately upstream of the weir silted up to crest level (el 749.00) during the first six months of operation commencing 27ch February 1980, and formed a barrier*
The weir was designed for a 100 year flood expected to give a flow depth of 2.8 m (el 751-60) over the crest for a design discharge of 700 n3/i, However, this design flood is now found to hive a return period of 10 years. The crest Length of the weir at el 749*00 is 110 th.
On 31st March 1981, the river rose approximately 3*63 m (el 752.63) above the crest level of the weir. The estimated discharge was 1200 m /s with a return period of 750 years. However according to updated data, the return period is now estimated approximately to be 200 years for a flood of this magnitude, Na appreciable damage to the rockfill weir was evident. The downstream profile stabilised itself to a convex curve and the boulders that were above the crest
3
level were adjusted.
With rhe silting up of rhe weir crest the normal weir formula is not applicable
for estimating discharges. The Water Resources Development Authority has
established an automatic river level recorder 800 m upstream of the weir. By
gauging the river flows at this point a relationship will be established with
the gauge just upstream of the weir.
The right bank of the river at the weir is at el 753.0Q.
7.2      Intake
The common intake to  the scour culvert and head regulator is sited close to  the right bank of the river and parallel to  the direction of flow or what is termed as "frontal".
3
The intake was designed for a 500 year flood with a peak discharge of 1000 m /s corresponding to a rivei stage of el 752* 3 ( 3.30 m depth over weir cm ). The return period ia now estimated as 200 years. A higher design flood for the
intake was adopted as the consequence of an intake failure would be more serious than that of a weir failure.
The intake is provided with a trashrack on its river face and triangular shaped screens at its entrance. The screens act as a first defence for the trees,
branches and debris carried down by the Awash from blocking the trashrack/ At least twice a day it is necessary for the scour sluice and head regulator to be closed for a few minutes, while the gate operators or divers dislodge the material deposited on the screens. Any debris passing through the screens and blocking the trashrack should be either prodded clear with steel bars and dislodged or divers employed to manually clean the trashrack care efficiently. The period during which the river carries a heavy load of debris is soon after a storm in the catchment area, throughout the year, or during the big rains from rnid-JuLy to end September when the river is in spate.
64The waterway between the operating tower for the scour sluice and head regulator wells on the right bank should be kept free of boulders to provide a high
velocity to carry downstream the debris dislodged trots the screens. Any interference with this waterway causes a reduction in velocity and deposition of debris which ultimately will find its way onto the trashrack and restrict the
flow of water to the head regulator.
7.2*1             Scour Sluice
The scour sluice removes the bedload and heavy ailt that enters the intake by discharging them back into the river downstream of the weir. The trashrack and screens do not interfere with the functioning of the scour sluice. The Scour
sluice is provided with a radial gate 1.5 tn wide to completely shut-off the
scour culvert which is 1.5 m x 1.5 ffl. Operation of the radial gate is from the
Cop of the tower located halfway between the intake and exit of the scour culvert. The tower is connected to the headregulator by an access bridge.
For the efficient functioning of the scour sluice, the river levels upstream and downstream of the weir should have a difference of more Chan 1 .1) m. As the differential head decreases below 1.0 m, the velocity of flow through the scour culvert decreases. At this time the bedload and heavy silts carried by the
river are deposited in the intake gradually blocking the supply of water to the head regulator. In September 1991, Che intake was blocked completely due to Che above reason. An excavator was used to clean the intake mid-Decober when the differential head .started to increase.
Table 7.1 and 7.2 give rhe upstream river levels during the rainy months of 19 BO and 1981 and also indicate the dates on which the intake of water to the
head regulator was affected by debris and silt deposition in the intake itself.
The scout sluice should not be opened fully during low river stages in order to reduce the amount of debris being attracted an to the screens and trashrack. Generally an opening of 30 cm has been found sufficient for removal of bedload.
7.3     Head Regulator
The head regulator is "supplied by twin supply culverts from the intake. They ate 1.65 m wide x 1.5 m high. Control of the discharge is by two vertical slide gates installed in separate chambers. The two slide gates discharge into twin barrels 1*5 m x 1.5 m which end in a common stilling basin.
When water flows through the screens into the intake and then through the trashrack into the twin culverts serving the head regulator there is a head
loss. There is a further head loss when water flows through the twin supply culverts before it reaches the two slide gates. This head loss is clearly
visible when one looks down into the two gate chambers. When the gates are closed the water level in the river is equal to the water level in the gate chambers. But when the gates ire open, the river level is higher than the water level in the gate chambers. This difference in level is the combined head loss through the screens, trashrack and supply culverts.
Dischargee through Che head regulator slide gates are governed by the depth of water in gate chambers themselves. However, as it is not convenient to install staff guages in the dark gate chambers where visibility is poor, the water level has co be related to the river level where .n gauge post has been installedTable 7.1
1--------------
1 Day
1
1
1 1
'2
Table HI Upstreaw River Levels - 1980
1
1 JUL
1
T
1 AL'G
1
1
1 SEP
1
1
1 750.05 1 749.95
1
1
1
i
| 749.91
1
1 750.10              j 749.90               1 749.88              I
1 3                      1 749 .85            I 750.00               I 749.85               1
,4
1 5
1 6
1 7
1 8
750.55             1 750.03*           1 749.92               1
f 749.9 2 I 750.31* 1 749.89 J
! 750.04
1 750.10              1 749.89              1
1 749.88              1 750.25*            1 750.10*             1
1 749.73              1 750.18*            | 750.04*              1
1 9                      1 750.20             1 750.08             1 750. 10*           1
1 10
1 11
1 750.15 1 750.05 | 750.02* [
1 750.34 1 749.97 1 750.12 1
1 12 1 750.20 1 749.87* 1 750.13  |1
1 13 1 749.96 1 750.05* 1 749.82 i
1 14
1 15
1 16
1 17
18
1 19
1 20
| 749-09
I 750.02               1 749.95             I1
I 749.85 1 750.10 1 749.95*  |1
1 749.78 1 750.10* 1 749.95*  i1
I 749.75              1 750.11
| 749.95*              1
I 750-85              f 750.23               1 749.88              1       * Days on which intake
’ 749.92*            1 750.05*            1 750.90
I 749.95*            1 750.03              1 749.01              1               silt and bedload
was blocked with debris
1 21                     1 749.94*           I 750.06               I 749.02               1
1 22 1 749.94*
1 23 [ 750-05*
1 24                    1 750.00
1 750.06
1 749.02
1 750.12
[
1 749.76
1 750.60
[
1 749.74
1 25
| 750.00*
1 750.05
1
1 26 1 749.99
1 27 1 749.98
1 28 1 750.05
1 749.60
1
1 750.33
1 749.62
1 750.25
i
1 750.02*
1 749.95
1
1 749-94
1 29
| 750.13
1 750.12
1
| 30                    1 750.00
t 749.92
1 750.04
1
1 749.85
1
1 31
1
| 750.00
1
i 749.99
1
1
1
1
66Table 7.2
Table Til Upstream River Levels - 1981
1
“T
n
1
n
i
1 Dav
[ JUN
1 JUL
1 AUG
1 SEP
1 OCT
1
1
I
1
1
1
1
1----------- 1 1
J
1 749.75*
n
TT
1 750.58
1 1
I 749.70
i 750.21
| 750.43*
1 2
[ 749.65
1 749.80
[ 750.50
i 750.74
| 750-42* 
I
1 3
1 749.65
I 749.78
1 750.58
1 750.62
1 750.41*
1 4
f 749.70
I 749.78
! 750.38
I 751.0 *
i 750.45* 
1
1 5
1 749.65
1 749.76
I 750.18
1 750.93*
1 750.47* 
1
1 6
[ 749.80
1 749.86
1 751.70
j 750.80*
] 750.52*
J 7
| 749.70
1 749.78*
I 750.81
1 750.90*
[ 750-50*
1 8
1 749.75
1 749-79
I 750.63
1 750.79*
1 750.46* 
1
1 9
| 749.70
1 749.84
[ 750.30
I 750.90*
1 7 50.50*
1 10
| 749.75
I 749-82
1 750. 17
[ 750.90*
1 750.36*
1 H
I 749.70
1 749.77
1 750.75
1 750.82*
1 750.28* 
1
1 12
| 749.65
1 749.84
I 750.30
I 750.80*
1 750.17 
1
1 B
1 740.70
1 749.94
I 751.10
[ 750.74*
1 1
1 14
1 749.70
i 749 .91
1 750.55
| 750.67*
1 1
1 15
1 749.70
1 750.66*
1 750.49
1 750.73*
1 1
1 16
I 749.65
1 750.10*
I 750.45
1 750.68*
1 
I
1 17
1 749-65*
1 750.10
1 750.43
1 750.78*
1 1
[ 18
1 749.80*
1 749.91
I 750.57
1 750.83*
1 1
1 19
I 749.65
1 751.10*
1 750.35
1 751.12*
1 1
1 20
1 749.70
1 751.40*
1 750.43
« 750.97*
1 !
1 21
1 749.80
[ 750.80*
! 750.52
! 750.97*
i
1
1 22
| 749,90
1 750.51*
1 750.90
I 750.80*
1 1
23
1 749.80
I 750.66
1 750.28
1 750.60*
1 1
24
1 749.75
1 750.48
1 750.25
1 750.69*
1 1
1 25
I 749.75
1 750.78
1 750.18
1 750.55*
1 1
1 26
' 749.70
1 750.47
1 750.22
1 750.78*
1 1
1 27
749.85
! 750.34
1 750.30
1 750.77*
1
1 28
749.85*
' 750.25
1 750.31
1 750.64*
1 1
| 29
1 750.00
( 750. 10
1 750.43
1 750.49*
1 1
1 30
i 749-80
[ 750.20
I 750.60
1 750.45*
1
1 ”
1 
i
1 31
1 -
1 750.38
I 750.60
1 1
1____
1
1
1
1
1 1
* Days on which intake wag blocked with debris, silt and bedload.
67upstream of the intake. This ls achieved by adding the combined head loss for different gate openings with varying water depths in the gate chambers.
The discharge through the slide gates is estimated using the equation:-
Q * CA ^2gy [ m^/s
-*—7
where Q • discharge through each gate in m /a
C - 0.65 
A ~ area of gate opening w x 1.5 m w = variable height of gate opening yj ■ depth of water upstream of gate
Entrance losses he * Ke V_ metres --
2g
where Ke =        0.25
—      *   -----                  (32)
2
Trashrack and Screen losses hr ■ Kt. V n metres —-------------------------------------- --—- t 33) 2g
where Kt =         1.45 “ 0,45 An - An 2
Ag Ag
------- --------------------------------------- (34)
where Vn = velocity through trashrack or screen m/sec
An * net area of trashrack or screen square metres Ag * gross area of trashrack or screen square metres
2
Velocity Head ho - Vo  metres
2R
Iota! head loss = he * ht * ho (mJ
------------------------*------------ - ------  (35)
2
It has been found during operation of the head regulator since 19B0 the total head loss can be approximated to Q.O92 V  where V is the velocity of flow through each supply culvert from the intake to the gate chamber. On this basis stage discharge curves were determined and are given in Figure 7.1. The curves have given fairly accurate results when monitored at Check Drop No 1, However, it is necessary for the Water Resources Development Authority to calibrate these gates during an entire cotton season when there will be a wide variation in gate openings and upstream river levels.
7.4    Hain Canal
The main canal is 25.5 km long with six controlled check drops and six uncontrolled drops provided where the natural slope of the land exceeds the slope of the canal.
Hail Canal Parameters are given in Table 7,3,
6BTable 7.3 Main Canal Parameters
Canal
Reach
Bed
Width
Length
m
Grad
ient
Side
Slopes
Full
Supply
Depth
01
Free
Board
Velo- [>dign
CD
city
m/ 5
Discharge g»J/a
HR^to
CDl^)
GDI to
CD2
CD2 to
CD3
CD3 to
C»4
Ct>4 to
C05
CD5 to
CD6
8,0               1934
0.227           1:2            1.7             0.45         0.70         13.50
7.0 5245 0,2 1:2 ! .6 0.45 0.70 10.15
6.0 4525 0.2 1:2 1.5 0.45 0.5? 7.95
6.0 5465 0.2 1:2 1.35 0.45 0.56 6.60
4.50 2095 0.2 1:2 1.35 0.45 0*54 5*21
3.50 14 52 0.2 1:2 1.31 0.45 0.51 4.09
CD6 to
24000 m            3.IQ           1784                 0.2               1:2             1.13           0.45         0.46            2 ,50 24000 m  to
27000 tn            5.62            3000                 0.2               1:2            0.85           0*45        0.43            2.80
1)       HR means head.   regulator      sti1 Ling         basin whetie the main canal starts
2) CD  means check     drop with       overshut          gates for  maintaining full su            pply level i n  main canal      in upstream reach
Referring, co section 2.10,1 Che peak water duty at diversion for cotton is
1.41 l/s/ha. The total project area is 10,300 ha net. Using the peak water
duty for cotton for the total area the peak discharge in che main canal should be 10,300 x 1*43 = 14.73 m^/s. This means the canal has to be overloaded by 14.73 - 13,50 x 100 = 9. IX.
13.50
Normally irrigation canals should cater for * 10X of the design discharge. For a discharge of LOZ Q « 14.8$ uJ/i. The depth of flow in this case will bs
1.80 ra as compared to 1.7 m for 13.50 s . This entails a redaction in the freeboard of 10 cm which is acceptable.
At different *iraes of the year, the Awash river carries varying percentages of suspended solxds which enter the main canal with diverted water.
This has been rhe subject of intensive study and is written up in the siltation Study Report dated May 1983.
7.5    Controlled Check Drops
The six controlled check drops are provided with overshot gate* to maintain the irrigation water in the main canal upstream of a gate at full supply level,
while passing the required discharge over the gates for the downstream users.
A staff guage is provided in the main canal, upstream of the drop, to indicate Che water level.
69The     controlled     check     drops     have     a     concrete     crest     as     shown     schematically     on Figure 7.2.
The overshot gates are installed on the downstream face of the crest. On top of
each gate is welded a steel painter which gives a direct reading on a scale,
fixed on top of the gate frame, of the height of the gate above the crest of the
drop.
The formula used for calculating the discharge over the drop gates is given by:-
Q     -   1.84     <L ~ 0.2H)               rJ/s ---------------------------------------------------------- --------------- (36) where L        •   length of gate over               which water flows
H     •   Full     Supply Depth -         (crest       height +       gate height)
Check drops I to  5 have two drop gates
has only one gate of length 3 m.
of total         length       6  m while check drop 6
Table    7*5    below    gives    the    maximum    discharge    aver    the    checks    when    the    gates    are fully lowered to crest level.
Table 7.5            Gate height above crest for design discharge
Full
Check
Supply
Crest
Drop
Depth
iii
Height
a
Possible
Discharge
Design Discharge
No
1
m-Vs
m-l/s
1.7
0.70
9-45
10.15
2
1.7
0.84
7.13
7.95
3
1.5
0*67
8*11
6.60
4
1.55
0.71
5.53
5.21
5
1.35
0.73
5*28
4.09
6
1*31
0.69
2.58
2.80
It is evident from
the above     that the upstream full supply level has to be
increased   tor Check       Drops It       2 and 6 to obtain design discharges over the
Check    Drops    2    and    6    will    operate    under    free    How    conditions    always    but    the others will operate under drowned conditions at full development.
See Figure 7.3 for corrections to be applied,
7.6     Constant Head Orifice Offtakes
The offtakes to the eleven secondaries and one tertiary fed from the main canal have been fitted with constant head orifices to measure water delivered to entities. The offtakes are located junt upstream of the check drops.
The constant head orifice la a combination of a regulating and measuring device. It has a submerged rectangular orifice for the measurement of the discharge. The race of flow is estimated by using the principle that a submerged orifice of
a gi/en size requires a differential head across the orifice and this is maintained at a constant value by adjusting Che downstream regulating gate. Figure 7.4 is a schematic drawing af an offtake.
70The discharge is estimated by the formula: —
where Q » discharge in
C -        0-65
A 3 area of orifice
= W (height of opening) X L (width of opening 1.7 rn)
H * differential head in ra.
A value of 0.65 for C was adopted after calibration of some offtakes with a current meter. Discharge graphs for offtakes I to 11 (L = 1.7 m) and offtake 11 and 12 (L - 2.0 ml when H = 10 cm and 8 cm are given in figures 7.5, 7.6 and 7.7.
7■7 Rejection Spillways
There are four rejection spillways along the main canal *ach one situated upstream of a control structure. The spiLLways arc designed to reject excess water in case of an emergency shutdown of a main canal offtake due to a breaching of canals In the distributary system or due co excessive rainfall in the irrigable area. Rejection takes place during the time the headworks gates are being adjusted for a reduction in flow or complete closure.
Rejection spillway crests are located above the full supply level of the main canal. By raising the drop gates when the canal indicates full supply level, the water level will rise above the full supply level till it starts rejecting over the Spillways.
Table 7-6 gives the location of
Table 7.6 Rejection Spillways
the rejection spillways and their
Spillway
Ho
1
Locac ion
F.S.L.
Crest
Level
2
3
CD2
CD4
745-ao
737.92
4
CD6
Tail end
745 .90
737.92
736.10
732.98
736.20
733.93
parameters * Length
m
11
19
15
fl
Rejection spillways 1, 3 and 4 reject into secondary drains 2, 4 and 5 while
rejection spillway 2 rejects directly into the main drain where the main cans1 crosses the main drain.
f
71Table 7.7            Discharge   Over   Check   Drops   with   Two   Cates Q • 1.84 (L - O.2HJ H ‘‘
J
H(m) Q
L/s
i H(ra)
1
1
Q
1/ s 1
1
H(tn)
qi
1 H(m)
l/s f                    1 j1
Q
l/s
1
.01 11 1 .23
1
.02 31 I .24
1
.0] 57 1 -25
1
.04 88 1 .26
1
.05 123 1 .27
1
.06 162 1 .28
1
1
1208 |
I
1287 |
1
1369 1
1
1451 I
i
1535 i
1620 j
1
.45
.46
.47
.48
.49
•50
I1 3283                          I  .67
11r 3391 1                      | .68
11
3501 1                      1 .69 1t
i1 3612 1                      1  -70
1It
3724 1
|I
1 .71
3838 1
1
1
1 .29     |
1707 |
i1
’ 1 .30 1 796 |
j
1|
1 .72
5832
6038
6146
6362
6469
6469
.07 204 I
.08          249
.5! 39 52 1
1
1
1  .73 6674
1
1
.52
4060 I
»
.09 29 7 1 1 .31
1
.10          348
.11          401
.12         457
.13          515
.14          576
.15          638
! 886 |
.53        4184 1 |1
1 .32 1977 I
i|
.54
! .74
i1
1  .75
1
1
1  .76
i
1
6889
699 7
7092
1 -33
I
2070 1
|
.55
1 .34
[
1 -35
1
2164 1
1
1
2256 1
I
.56
.57
■
1 .36 2356 I
i1
1 .37 2454 |
1  .77       7307
i
1 .78 7414
1
1
1 -79 7522
1
1 .80 773 7
i
.16         70 3
.17          770
.18          838
.18         906
.20          981
.21        1055
.22     , 1131
i1
1 -38
1
2553 1
I
.58
.59
.60
.6!
.62
1
1 .81
1
1
7844
1
1 .39
[
2653 I
|
2755 I
1 .82 1 7952
1
i |.83         8153
1 .40
1
1
t 8260
1 .41 2858 1
|
4302 1
i
1
4420 1
l
1
4539 [
i
1
4661 I
■
1
4782 1
,i
1
4804 1
i
1
5028 1
i
1
5152 |
i
1
5278 |
1
1
5405 I
1  i
1  1
i 5531 |
ii
1  1
1
1 -84
1
! .85      1 8367
1 .42
I
2963 [
1
.63
-64
1
l
1 .65
1
1
1 -86 1 9582
i
1 .43 3069 j
1
1 .44 1 3175 |
jI
5659 1
1
I .87
1
I 86*9
I
1 .66
’ 5789 1
ii                                    1
1 .83      1 d988
72Openings
7519
Discharge m/s
0 45
055
FIGURE 7 IStaff gauge indicating gote
height above crest
U/S Full supply
.i
level
Staff Gauge
—
Gate height
above crest
Crest
height
T
Overshot gate
D  1/S Water level
U/S Bed level
”7
*4?
i
Drop height
1
D/S Bed level
r
i
Q = I 84 Ql=O2H]H% m’/s
H = Full supply depth-£cresf height* gate height above crest]
FIGURE 7 2FIGURE 7 3
DISCHARGE CORRECTION C ' FOR EFFECT OF WEIR SUBMERGENCE c'= Q|_         = (Q H) 3/a
Q~ H %
d - downstream heod over crest H = upstream head over crest
d
7T
OOI
01
02
03
• 04
- 05
06
-07
-08
■09
0
1 ■ 00
t 006
1 009
1 009
1 011
(Oil
1011
1-009
1 009
1-007
i
1 07
1 005
1 003
1-00
997
994
-991
980
• 983
■ 981
2
970
•973
9 70
■966
963
-958
-955
-951
■946
-942
3
939
■ 935
931
■926
■921
•917
• 91 3
-909
903
■900
4
09 5
•891 .
885
■ 081
-875
-877
-865
-859
■ 85 4
848
5
• 042
■837
835
82 5
- 819
■ 812
•806
-799
■ 792
-785
6
■ 778
• 771
-764
• 75 6
.748
•740
■733
■724
-715
-707
7
■690
■609
-600
-670
■ 660
•649
■639
•626
- 61 5
- 603
a
• 589
■576
-562
-547
-531
-517
.501
• 486
• 469
-45 3
9
■435
.41 6
■396
■ 375
-351
■323
•2 93
•255
- 209
-14 4SCHEMATIC VIEW OF A CONSTANT HEAD ORIFICE TURNOUT
HUUWt ( 4I
I B
t l
i
I
l
I
I
i
I
l
I
I
l
l
l
I
FIGURE 7 5FIGURE 7 7
Dischorge i/sSECTION 8 Distribution System8,       Digtribution System
fl. I Description of system
The      distribution      system      comprises      the      network      of      secondarya         tertiary      and      field canals tor the movement of water from offtakes on the main canal to the field*
Each     main     canal     offtake     feeds     a     secondary     canal,     except     offtakes     5     and     13     which feed tertiary canals directly.
A secondary canal has two or more tertiaries and rejects into a secondary drain or tertiary in an emergency or when irrigation water is in excess of
requi remant s.
Tertiary canals are designed to feed a minimnini of two field canals at peak water demand. In exceptional cases three to six field canals are served by a tertiary
at peak demand. These particular tertiary canals are Tertiary 3 and Tertiary 2 and 2B from Secondary 5 and 1*
Field canals are designed for a maximum discharge of 200 litres per second. Field canals are usually fed by tertiary offtakes. But there are four offtakes
on secondary canal S5; from main canal offtake 6. which directly serve four
field canals.
Secondary and tertiary canals are provided with duckbill weirs, with a varying crest length. These duckbill weirs help to maintain the upstream length of
canal at a pre-determined elevation to provide the necessary head at offtakes
for designed discharges. A duckbill weir serves one offtake on a secondary and one to four offtakes on a tertiary.
The maximum afflux over a secondary canal duckbill weir is 17cm while that for a tertiary canal duckbill weir is 9cm. Duckbill weirs at the end of secondary or tertiary canals serve as rejection spillways.
It is proposed to use duckbill weirs as measuring devices in the absence of any other form nf metering- The flow over a duckbill weir is generally given by the
formula»
where
Q     =  1*05 LH
Q     ■  discharges in ta^/s
L     »  length of           crest  (m) H         head over          crest  (m)
(33)
Discharge curves for two common lengths of weir are given in Figure 8.1,
The    rate    of    flow    over    a    duckbill    weir    being    known,    the    discharge    through    an
iinxtream offtake can be estimated hv 
™ 
:r *u^ j
rate     over     the     duckbill     weir     reduces     to     200     l/s.     By     deduction,     therefore,     400
l/s should be passing through the offtake.
73
T------------Table 8.1
Discharge        through Field Inlets
Q ■ f 0.60 A ^/2g yj
where A =  area of gate opening
*1   a  head upstream of gate
A-
£
!
1
0.65 x w           where w - height of gate opening 0.65 • width of gate opening
I
‘ Height of
1 gate opening 1 Cffl
1
1
I 7^47.5
1 cm
1 Q 1/s
1 1
r
| yj-45.0 1 Cm 1 Q 1/s
11
r
! y 42.5
1 cm
1 Q 1/5
1
J 
| y i^40.0 1 cm
1 Q 1/3
i--------------- 1
I Jj-37.5 I yi = 35.0
1 cm 1 Q l/s
11
1 cm
1 Q j/s
11
1  71-32.5   I 1 cm
J Q 1/s
f                  —           1                1                1 ■ ■■■                              1                 1                 1                 “I
i
1
12
1 24
1 23
1 22
1 22
1 21
1 20
1 20
14
1 48
1 46
1 45
1 44
1 42
1 41
: 39 
:
16
1 71
1 7D
1 I 68
1 66
1 63
I 61
1   59 
I
1 8
i 95 
11 93 
:  f 90
1 a?
I as
1 a?
1   79 
[
1 10
I 119 
i
| 116 1         ill
1 109 
11 106
1 102
[   98 
[
1 12
1 143
: i39
1 135
1 131
I 127 
I1 123
1 1.18 1
1u
1 16?
1 162 |   158
1 153
| 148 
|1 143
! 138 
I
1 16
1 190
' 18 5 
I  180
1 175
1 169
1 163
i 158
1 ia
[ 214 
|1 209 
|   202
1 19 7 
I1 190 
|1 184
1 177 
|
! 20
1-1
1 '
1 -
1 218
i 212 
I1 204
1 22
1 196 
1
i-
1[-
1-
1-
1’
1“
1 216 
1
1
1
1
1
i1 1
1 
i
7*1thickbill wlrs should always be fctpt free of meds and silt deposits around the
crest, if they are to be used as measuring devices. The Awash water at times
carries an appreciable percentage of suspended solids. These soLids are
deposited at structures along the canals, where the velocity io the lower depths
is almost «ro, Therefor* regular maintenance is a must for such structures,
especially, if they are to be used as measuring devices. All duckbill weirs
should be calibrated if they are to be used as measuring devices.
Secondary, tertiary, and field canals have uniform bed-widths» gradients, full
supply Levels and freeboards throughout their lengths .
Offtakes from secondary canals to tertiaries are either gated pipe culverts
under roads or gated turn outs. Offtakes from tertiary canals to field canals
ace rectangular orifices with a steel flashboard feeding either a pipe crossing
under a road-way or directly into a field canal* Ln Melka Sadi Stage lit
’’constant head” orifice offtakes have been provided on Secondary Canal SI and
Tertiary Canal 2 for accurate water weaaurement. See section 7 .6 for the
operation of these turnouts. Lt is essential that measuring devices arc
provided to calibrate all the different types of offtakes under varying field
conditions,
8 -2 Irrigation
8.2*1              Water Application
The Amibara Irri gat ion Project has been designed an the basis of a 24 hour
operation* For the success and efficient water management of the project, it is
necessary to reduce losses during conveyance, distribution and application. One
important contribution to reducing losses is to operate main, secondary and
tertiary canals at constant flows for a minimum of five days before an increase
or decrease in flow rate la next scheduled.
In determining irrigation schedules the basic rate of flow in a field canal,
either for pre-irrigat ion or a post-planting irrigation, has been taken as 150
litres per second net. To this should be added the distribution losses and
conveyance losses to arrive at the gross irrigation requirement at diversion.
The    150    Litres    per    second    in    a    field    canal    wi    LI    be    applied    to    the    furrows    using 2.4cm internal diameter plastic syphons.
The basic equation for water application is given by;
qt - Ad x 1000 
36
where q - litres per second (150 1/a)
t ® time in hours
A ■ area of field in hectares
d = depth of application in ems
Then:
t » 2-4 x A hours
when;             d    -
13cm (post-planting-irrigation)
and ;
t    “        5*30 x A hours
when;             d    “              28.6 cm (pro-irrigation )
759.2.2                  Field Canal Inlets
Field canal inlets are rectangular orifices The steel flashboard Ls provided tfith slots vertical edges. Pins on the gate frame can the flashboard at any desired opening corre
with         an         undershot         steel         flashboard
at 2cm spacing on both
be inserted into the slots to keep
spending to the 2cm slots.
The opening in Cha flashboard wilt function as an orifice operating under
free-flow or drowned conditions depending on the water level in the field canal or pipe culvert downstream of the opening. The upstream head varies from a minimum of 37 cm to a maximum of 45 cm. A field canal inlet discharges (a) directly into a field canal where there is no farm access road (b) into a 60 cm diameter pipe culvert under a farm access road (c) into a 43 cm diameter pipe culvert under a farm access road. Farm inlets were calibrated using a 9 inch throat width Parshall flume.
Figure 8.4 is the calibration curve for tertiary offtakes discharging under free flow conditions directly into u field canal or a 60 cm diameter pipe culvert under a farm access road. Figure 8.5 is the calibration curve for tertiary offtakes discharging freely into a 45 cm diameter pipe culvert under a farm access road. These curves have been issued to State Farms for measurement of water. A variation in the upstream head from 37 cm to 45 cm makes a difference in actuaL discharges from those given by the calibration curves of less than
+ 6Z.
8.2.3                  Syphon Operation
Field canals are designed co give an average discharge head of 25cm to syphons delivering water to individual furrows from the fieLd canal. However, in practice it has been found due to the formation of the field canal banks and difference in elevation of the two banks, the average operating head achieved is 20cm. giving an average discharge of 0.47 litres per second per syphon of 2.40cm internal diameter. A calibrated syphon discharge curve is given in Figure 8.2.
Therefore the number of syphons required to handle 150 l/s is 150 
These 319 syphons have to be utilised to obtain an "advance flow" that will reach the end of each furrow in one hour approximately. This figure of one hour was obtained from furrow evaluation trials both at Melka Sadi and Acaibara. Initially 5 syphons per furrow should be used for the advance flow when the furrow length is 200m. This means 64 furrows or S3 furrows, depending on the furrow I Tigth wil t be operative at the beginning of pre- rrlgati on. During actual operation it is observed that more syphons art* required than calculated. This is because the syphon length is not long enough to obtain even a minimum
head of 20cm.
At the enJ of one hour, the advance flow in each furrow has to be cut-back once or twice so that the final rate of flow will be equal to the rate of
infiltration throughout the furrow length. The cut-back rate of flow should continue for 19 hours and 17 hours to apply 31.43 cm gross pre-irrigation on alluvia and vertisol respectively and for 3j hours and 5 hours respectively to apply i3cm gross post-planting irrigation. After "cut-back" the number oE syphona in use has been found to he 2 per furrow or 3 per furrow for long furrows ie 128 for the 64 furrows or 159 for the 53 furrows.
= 319.
(T47
76The syphons that become available .ifter ”cut-back” of the "advance flew  should
1
be moved to new furrows and started i mediate Ly bringing them coo under irrigation with water overflowing into the second section of field canal* This process involving leap—froging of syphons will continue until the* entire field served by the field canal has been irrigated* In the case of furrows over 200n in length, as stated above, it is necessary to use 6 syphons per furrow for the
"advance flow" and 3 syphons after ’’cut-back”*
Syphons should be laid out along the banks of the next field canal to be commissioned. When the first field is nearing completion, the inlet could be partially closed and part of the water diverted co the next field canal inlet*
This will ensure a smooth switch over from one field canal to another and save valuable time on a tight irrigation schedule. It will also avoid the necessity
to reject water while the second field canal is being made ready for operation.
Cross bunds on field canals are sited at convenient intervals along the length
of the field canal to cater for aa many syphons as possible and will ensure syphon discharges remain within + 101 of the average 0.47 l/s. Cross bunds can be fixed earth dans, or plastic or canvas moveable dams. The advantage of plastic or canvas moveabLe dams is that they can also serve as spills for excess water to flow into the next section maintaining a constant head for the syphons in operation. In the case of earth dams Large syphons of 6.5cm diameter have to be used to pass water from an upper reach to a lower reach. The water level in the upper reach is seldom maintained at a constant leveL - causing the syphons to operate at fluctuating heads. This in turn affects the uniform application
of water to each furrow.
The number of furrows per hectare is 55 when the furrow length is 200a and 44 when the furrow length is 25Chn. The quantity of water applied per irrigation per hectare = 130 Ora and the total quantity for one pre-irrigation and eight post plant irrigations - I3.26Qm /ha per season*
A
3
778.2.4                  Canal Parameters
Canal
Length          Bed            Side              Full              Free           Bank         Grad lent
(ta)
Width      Slopes          Supply         Board        top
Discharge V J1
1/s
(n)
Depth           (a)             width
(cn)
(m)
Si
j
1500
1*4
1:2
0.68
0.30
1.00
1 x IO"3
1000
19 30
1,4
1:2
0.73
0.30
1.00
0.5 x IO"3
■j
1000
1227
1.00
1:2
0.76
0.30
1.00
0.6 x 10 3
1080
T3/1
2480.94
1.25
1:1.5
0.78
0.25
0.50
0.5 x 10“ 3
550
1988.98
1.25
1:1.5
0.78
0.25
0.50
0.15 x 10"3
550
T3/2
3600.02
1.25
1:1*5
0.70
0.25
0.50
0.22 x 10-3
440
TV 3
3176.60
1.25
1:1-5
0.70
0.25
0.50
0.22 x IO"3
440
T3/4
2 300
1.25
1: 1.5
0.70
0.25
0.50
0.2 x ICT3
440
317.82
1.25
1:1.5
0.70
0.25
0*50
0.45 x 10“3
440
262.40
1.25
1:1.5
0.56
0.25
0.50
1.06 x 10-3
440
S4
998*57
1.0
1:2
0.60
0.30
1.0
1.3 x IO-3
1000
T4/1
3238
1.0
1: 1.5
0.56
0.25
0.50 ‘
1.0 x 10“3
440
T4/2
2213.5
1.0
1:1.5
0.78
0.25
0,50
0.50 x 10"3
440
T4/3
3904.34
1.0
1:1.5
0.78
0.25
0.50
.15 x IO"3
440
T5
4018.97
1.0
1: 1.5
0.78
0.25
O. 50
.15 x IO"3
440
S5
1565.6
1.50
1:2
0.92
0.30
1 *0
0.4 x IO'3
800
T5B/I
200.0
1.0
1:1.5
1.02
0.25
0.5
0.15 x IO-3
440
T5B/2
7185.19
1.60
1: 1.5
0.76
0.25
0.5
0.20 x IO-3
660
T5B/3
480.0
1.50
1:1.5
0.78
0.25
0.5
0.15 x 10“3
640
1527
uo
1.2
0*75
0.30
1.0
0.427 x ID3
720
S6
2473.0
1.0
1:2
0.75
0.30
1.0
0.2857 x 1D“3
720
16/I
2232.88
1.0
1:1.5
0.75
0.25
0.5
o. ifi x 10“3
440
T6/2
480.0
1.0
1:1.5
0.73
0.25
O.5
0,2 x 10“3
440
T6/3
720.0
1.0
1:1.5
0.78
0.25
0.5
0.15 x IO-3
440
T6/4
687.0
1.0
1:1.5
0.78
0.25
0.5
0.15 x 1O“3
440
T6/5
920.0
t.o
1:1.5
0.78
0.25
0.5
0.15 x 10~3
440
T6/6
3760.8
1*0
1:1*5
0.78
0.25
0.5
0.15 x 10“3
440
Sil
7063
1*00
1:2
1.20
.35
1.00
0.2 x 10“3
1700
Tll/1
799.98
1.00
1:1.5
0.78
.35
0.50
0.15 x 10“3
440
Tll/2
497.39
1*00
1:1.5
0.78
.35
0.50
0. 15 x 10“3
440
Tl 1/3
732.11
1.00
1:1.5
0.78
.35
0*50
0. 15 x IO-3
440
Tll/4
1019
1.00
1:1.5
0.78
.35
0.50
0. 15 x 10“ 3
440
T9/1
1888
1.00
1:1.5
0.78
0.35
0.50
0.15 x IO-3
440
T9/2
1395
1.00
1:1.5
0.73
0.35
0.50
0.2 x IO-3
440
T9/3
6050
L.00
1:1.5
0.90
0.35
0,50
0.2 x IO"3
650
T9/3-!
687
1.00
1:1.5
0.78
0.35
0.50
0.15 x 10-3
440
Field
-
0.5
1:1.5
0.45
0.25
0.3
200
Canal
788.2.5 Drain Parameters
Drain
Length
(m)
Bed
Width
(ra)
Side
Slopes
Full
Supply
Depth
Free
Soard
(ro)
Gradient
Discharge
1/a
U)
SD3
750
0.60
1:1.5
0.45
0.30
3.5 x IO"3
400
1000
0.80
1:1.5
0.75
0.25
0.8 x 10“3
640
300
1.00
1:1.5
0.79
0.25
0.8 x IO-3
800
1100
1.20
1:1.5
0.83
0.25
0.8 x IO-3
1000
TD3-1
642
1 .25
1:1.5
0.50
0.25
0.5 x 10-3
500
297 5
1.25
1:1.5
0.72
0.25
0.2 x IO-3
500
TD3-2
2910
1.25
1:1.5
0.70
0.25
0.3 x IO-3
500
TD3-3
2881
1.25
1:1.5
0.73
0.25
0.15 x 10“3
500
TD3-4
1573
1.25
1:1.5
0.74
0.25
0.14 x IO'3
500
TD4-1
450
0.50
L: 1.5
0.50
0.25
1 x 10"3
220
550
1.20
1:1.5
0.50
0.25
1 x IO-3
440
1869
1.20
1:1.5
0.40
0.25
0.5 x IO-3
440
TD4-2
1000
1.25
1:1.5
0.78
0.25
0.7 x IO-3
800
TD5-1
fill
0.90
1:1.5
0.70
0.25
0.4 x 10~3
400
69 3
0.90
1:1.5
0.6
0.25
0.83 x IO-3
400
671
1.15
1:1.5
0.6
0.25
0.83 x IO-3
1000
TD5-2
526
0.90
1:1.5
0.78
0.25
0.4 x IO-3
400
TD5-3
1000
0.75
1:1.5
0.76
0.25
1.36 x IO-3
840
965
1.25
1:1.5
0.78
0.25
0.67 x 10“3
840
TD5-4
740
0.70
1:1.5
0.78
0.25
0.61 x 10“3
400
2660
1.00
1:1.5
0.93
0.25
0.49 x IO-3
780
435
1.40
1:1.5
1.00
0.25
0.40 x IO-3
1180
TD5-5
2288
0.78
1:1.5
0.70
0.25
0.4 x L0“3
89
0.70
400
1:1,5
0.70
0.25
1.07 x IO-3
610
TD6-1
1600
0.80
1:1.5
0.00
0.25
0.375 x IO-3
TD6-2
805
0.80
510
1:1.5
0.72
0.25
0.40 x IO-3
400
775
0.75
1:1.5
0.98
0.25
0.75 x IO-3
132
0.75
550
1: 1.5
1.23
0.25
0.75 x 10-3
436
0.75
9 60
1:1.5
1.15
0.25
0.75 x IO-3
795
1040
0.75
1:1.5
L.OO
0.25
0.70 x IQ-3
TD6-3
918
0.75
1:1.5
0.83
1100
0.25
0.5 x 1C-3
TD6-4
1000
0.75
1:1.5
400
0.77
0.25
0.40 x IO-3
TD6-5
480
0.75
1:1.5
400
0.72
0.25
0.40 x 10-3
400        *
RC T6-1
65
0.75
1:1.5
0.58
0.25
1.0 x IO-3
RC T6-3
130
0.75
400
1:1.5
0.67
0.25
RC T6-5
715
0.75
0.46 x 1O~3
400
1:1.5
0.55
0.25
1.48 x IO-3
400
TD U-l
3009
1.00
1:1.5
0.82
765
1.50
0.25
TD 11-2
0.57 x IO-3
1: 1.5
620
0.75
1290
l.oo
0*25
0.60 x IO-3
TD 11-3
1:1.5
830
0.82
0.25
1.20 x IO-3
900
RC TU-2
1235
1.50
1:1.5
0.75
0.25
0.6 x 10-3
RC TU-4
1010
1.50
1:1.5
830
0.75
0.25
0.63 x 10-3
850
79
1Drain
Length
(n)
Bed
Width
(m)
Side
Slopes
Full
Supply
Depth
Free
Board
(o)
Gradient
Discharg
l/s
(m)
TD9-1
242
0.75
1:1.5
0.49
0.25
4.2 x 10"3
400
TD9-Z
1178
1.20
1:1.5
1.10
0.25
0.125 x I0"3
700
TD9-3
607
O.SO
1:1.5
0.85
0.25
0.4 x IO'3
550
TD9-4
713
0.80
1:1.5
1.00
2.25
0,14 x I0’3
440
TD9-5
1708
0.80
1:1.5
0.78
0.25
0.62 x 10'3
400
1152
3.00
1:1.5
0.78
0.25
0.125 x 10’3
800
TO9-6
15 59
0.85
1:1-5
0.79
0.25
0.4 x IO-3
400
1819
3.00
1:1.5
0.85
0.25
0.1 x IO"3
690
RC T9-1
200
0.75
1:1.5
0.4
0.25
4.5 x IO"3
400
RC T9-2
100
0.75
1:1.5
0.37
0.25
5.a x i<r3
400
RC T9-3/1
294
0.75
1:1.5
0.46
0.25
2.5 x LO"3
400
Field Drains
0.50                1:1.5           0.50                                                                            100
80fl - 3 Rejection Spillways
During irrigation it may sometimes be necessary to reject all or some of the water flowing along secondary and tertiary canala. The reasons for rejection are (a) Requesting note water than required at any particular interval of time
(b)     Delay in setting out syphons thereby delaying the opening of field inlets (c)     Breaches in canal banks due to overtopping or animal burrow (d) Not informing the Project Control Centre ^3 hours in advance when flow rates should
be reduced.
Rejection spillways
have been provided in the distribution network as
Rejection   Spi1Iway
follavs:
Canal
S3
T3/1
T3/2
T3/1
T3/4
54
T4/1
T4/2
14/3
T5
55
T5B/1
T5B/2
T5B/3
Rejection   at     tail end
Rejection   at    tail end
Rejection   at    tail end
Rejection   at    tail end
Rejection   at    tail end
Rejection into T4/2 and T4/3
Rejection   into field canals
Rejection   at    tail end
Rejection   at     tail end
Rejection   at    tail end
Rejection   into T5B/3
Reject ion   into field canals
Reject ion   at    tail end
Reject ion   at    tail end
56                     Spillway at Cail end
T6/I
n
ii
ii
o
T6/2
11
pi
ir
n
T6/3
n
ri
IT
ip
T6/4
IP
PI
pi
ii
T6/5
it
IP
11
T6/6
11
PI
ii
Ii
ii
SI I
Reject ion
at
tail and
Tl l/l
■■
M
ri
Tl1/2
ii
ii
ii
TlL/3
ii
ii
II
111/4
ii
H
ii
ti
Ii
ti
II
8Furrows
Prior to pre
-irrigat ion
as close as p
ossible to
the
Che
will have its own independent syphon
to      another      at      the      upper      end      does      not      portEit      proper
upper      end      □£      all      furrows      should      be      buiLt      up      manually field canal banks so as co ensure that each furrow
— ’      1 stream. Spreading of water from one furrow
furrow.
,
‘ water application along each
SIVisual inspection of furrows is essential and any Localised high spots
smoothened out or the ridges strengthened at localised depressions that wi 1 L
cause ponding in individual furrows or across several furrows. Figure 8.3 shows
the unevenness of furrow slopes which lead to difficulties tn uniform water
application. Ponding is also evident at 150m from the field canal end.
A fairly high embankment should be built at the tail end of each furrow to
prevent escape of irrigation water into drains, in case the cut-back stream
exceeds the furrow Intake rate.
The furrow method of irrigation necessitates the wetting of only a port of the
surface (from one-half to one-fifth) thus reducing evaporation losses, lessening
the puddling of heavy soils, and making it possible to cultivate the soil sooner
after irrigation than with basin irrigation.
A required depth of application is achieved by the time water is allowed to
infiltrate into the soil along each individual furrow.
It was shown in section 6.3.6 that the time required to apply a gross depth of
31.43 cm C pre-irrigat ion) on a vertisol was 1027 minutes. The irrigators are
not provided with pocket watches. Hence they can only guess if water has been
applied for 1027 minutes. Supposing they apply water for only 800 minutes, then
the depth of application will be 26.55cm gross or 15.59cm net and not the
required 31.43 cm gross. This proves that every irrigation crew should be
provided with a pocket watch to record the total time of irrigation per furrow
and therefore of the whole irea for every irrigation. This will appreciably
reduce either deficit irrigation, or an over irrigation which causes excessive
losses due to surface run-off and deep percolation below the effective root
zone.
8.4.1                  Land grading for furrow irrigation
Land grading, or land levelling as it is commonly known, is the process of
improving the surface by means of earth movement for the purpose of better water
control. The surface created by land grading lends itself to a more even
distribution of a required amount of water over the irrigated area. This
reduces the labour required for irrigation and above all increases the efficient
application of water. Leaching of saline soils also requires a smooth surface
for an even penetration of water and removal of salts. Figure 8.6 shows how two
diagonal planing operations could be carried out to cover the entire field
with 2 passes of the land plane in the minimum amount of tixoe.
It has been found that certain areas under cultivation had Low spots that were
water logged causing an aeration problem in the root zone and restricting
growth. The fields where low spots are detected during one season should bn
ploughed and graded before the next season to eliminate these low spots.
8.4.2                 Restricting soil Layer
A        restricting        soil        layer        causes        a        concentration        of        roots        above        chat        layer water         logging.         Growth         is         retarded         by         excessive         soil         moisture.         Often         a compacted        lay^r        occurs        at        shallow        depths        due        to        faulty        tillage        practices, use        of        rubber        tyred        vehicles,        discs        and        vibrating        tillage        implements        can bring about a reduction in soil voids which in turn prevents root growth.
and
The easily
82A reprice inR sail layer ax 30em depth has been found in several fields. The
cotton in these areas showed symptom* of low fertility, These areas should be
subjected to deep sub-soiling using a single tine. However, if it i *? not a
compacted layer, which normally develops after several year., but a silty clay
layer with a very low infiltration rate it la not easily avoided or eliminated
without very deep sub-soiling using crawler tractors with long tines.
8.5       Field Canals
Manual labour should be used for trimming the outer slopes, heights and top
widths of field canal embankments to design parameters for efficient functioning
of standard syphon lengths. High and wide bunds with flat slopes contribute to
a reduction in Che designed average operational syphon head of 25cm.
8.6       Fama Units
8.6.1                 Requests for water
Individual fields in any unit are at different distances from Che intake- Water
when released from the intake will take varying times to reach different
fields. This means that field* nearer Eo the intake wi 1 L be served faster than
those at the tail end.
Drawings 8.1 and 8*2 serve us a guide to the approximate time 150 1/s would take
to reach any given point on the conveyance and distribution system. The main
canal has controlled checks ju*t downstream of the main offtakes. Similarly
secondaries and terriaries have duckbill weirs acting as checks serving one to
four offtakes upstream. The system has been divided into offtakes served by
individual checks co compute the time taken by 150 1/s to travel from one check
to the next. During operation the main, secondary, and tertiary canals will be
functioning at full supply level.
Unit Managers may use these drawings to estimate when they should request for
water* When they are nearing completion of irrigation along a tertiary, it
should be burnt in nind that there is 150 1/s available in rhe wain canal,
secondary and tertiary reach which win be adequate to irrigate at lease ana
hectare of * fiel_d. To avoid excessive rejection of irrigation water on
completion of irrigation or when a reduction in demand is requited, Unit
Managers and Section Managers should give at least 48 hours notice of the
anticipated time shut-down ar reduction in flew. Form 8.1 should be used by the
Irrigation Manager when requeuing the Project Control Centra (or water and Form
8.2           used      by      Section      Managers      to      notify      the      Itri-ation      Manager      oE      their      water requirements.
8.6.2
Monitoring Irrigation
Schedules and -Application Efficiency
field canals are designed fur a raaxirauta dischargemaofx200  iminl/sd. iHsowcheavergreitohfas200 l/a. However, it has
bean observed that irrigator, can bandU only 15Q l/s fairly efficiently for
ayp ayphhonon opopeerrat atiionon.. Therefore irrigation schedules have heet/determined on the
basis      of      150      1/s      per      field      canal.      Fona      3.3      h irrigation unit of State Farms or RRC can monitor how much ttpie
as      be
en      devised      so
taking to complete the application of a given depth of water to any field _ - _ J h       I"  1=1 faji k Vi, jl i i ■— ■ ■ F  h ■   * 1           — 
compared       to       the       theoretical       tin*       allowed       in
1
1
1’1l the
■
B
schedule.       From
-
that th<*
Irrigators at^
.
this       info
it       is       possible       co       daterraine       th*       average       field       canal       discharge       used       by       the irrigators and adjust Future scheduling where necessary.
83Fora     8.4     is     for     recording     data     necessary efficiency for any specific irrigation*
8.7      24 hour irrigation
to determine the field application
The canal reticulation system for the project has been designed on the basis o 24 hours irrigation and 60 days linear planting, each field canal having a capacity of 200 L/s serving 10 to 50ha-
Supposing the system is to be changed from 24 hours to 12 hours irrigation . (daylight hours). The capacity of the distribution canals (secondary, tertiary and field canals) have to be doubled to keep a 60 day linear planting period.
f
[f    a    secondary    canal    serves    IQOOha    and on a 24hrs basis to apply 28.6cm depth
q • 1000 X 10,000 X 28.6 x 1 
this     area     has     to     be     irrigated     in     60     days of water, the Elow rate required would be
°>3/$
100                3600 x 24 X 6(1
_ 0.55m3/a
On a 12 hours basis
q 2 2 x 0.55 1 1.lOm-^Z s
(neglecting efficiency)
For a post-plant irrigation of
q * 1000 x 10,000 x 13 x 
■ 0.25tp/s
and on a 12 hour basis
q = 0.50 m^/s
L3cnj on a 24 hours basis
1_______  m^/s
100        3600 x 24 x 60
At peak J e ma nd there will be an overlap of four post-plant irrigations as shown in section 2.10.1 figure 2.1. This means the secondary canal will discharge 4 x 0.25 = 1.00m3/s net on a 24 hours basis and 4 x 0.50 ■ 2.00ra3/a net on a 12 houii basis. The quantity of water required per da?1 either on a 24 hours basis Dr 12 hour basis remains the same and is equal Co:-
□r
1.00 x 3600 x 24 - 86,400 m3
2.00 x 3600 x 12  - 86,400 m3
me main cfllaL capacity remaining the aaw, storage reservoirs should be
provided tor each secondary canal if a n hour irrigaLion is t0 be pr;icci3ed t
augment the main canal supply and provide a total Qf 86,400 niVday,
During  a  12  hour irrigation the storage reservoir 43,200-ra^/day.
fill the storage reservoir with
main canal will supply 43,200 m /day and the
3
During the night the main canal discharge will 43,200aJ for use the following day.This is the basic difference between 24 hours and 12 hours irrigation. It raeans additional expenditure in constructing storage reservoirs, doubling the capacity of the distribution system, and doubling the number of syphons. Saving on man-power is tninicul.
85FORM 3.1
ID CONTROL CENTRE
AMIBARA IRRIGATION PROJECT [I
REQUEST FORM FOR WATER REQUIREMENTS
Nome of Firm: ..................... .................................. ............................. ...................         Datil:      ................. .. ....................................
Reque*ted by: ...................... .. ............... .................. .. ...........................................
1
1 OFFTAKE
1
1
1 
1
| DISCHARGE
| SEVEN
DAY PERIOD
I
1 (m^/gec or 1/see)
1 
t
1
1
1
1
1
1
1
f Froo 
| To 
|
1 
1 1
I
I
I
B6FORM ft.2
REQUEST rORH FOR WATER REQUIREMENTS
AMI3ARA IRRIGATION PROJECT FARMS
SECTION MANAGER .......................................................... ........................       UNIT
DATE ..................................................................................
I TERTIARY I FIELD NOS I IRRIG
I CANAL NO I BEING IRflIG I NO
WATER
REQD L/S
REMARKS
1
I I
I
i
I
i
!I
TOTAL THROUGH MAIN
OFFTAKE ............................................................................. .. . SIGNATURE ...........................................................................
87FUHM 8.3
FIELD IRRIGATION DATA
LOCATION
UNIT ................................ .. .................................................. ,
TERTIARY
No
FIELD I No I
I
I
area IRRIGATED
Ha
DAY
STARTED
GLOCK
TIME
IRRIGATION
DAY I
completed I
CLOCK I
TIME |
I i
SCHEDULE
TOTAL TLME I IRRIGATION | IRRIGATION HRS | NUMBER I TIME
s
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
IFDflH a.4
IRRIGATION DMA FDR A SPECIFIC IRRIGATION
location........ ........................... . .................... ........
IMT ....................................................................
TERIIARY
Mo
1 FIELD Mo
1       KKfA. 1 IRRIGATED
H*
IRRIGATION DATA
GAIL OPENING
H
OPLRAIING KT AD
H
DISCHARGE
HJ/b
1
DAY STARTED
d
1
fl* | STARTED I
DAY               | TIME COMPLETED I COK’LETED
I
1
1 IDTAL 1 TIME 1 ms I
QUANTITY
mJ
_________
r
i
i
i
i
i
i
I
•
1 11
1
i
I
Gc
■-0
1
1
1
I
1
1
1
1
1
1
1
1
1—
1
1
1
1
1I
I
I
I
I
I
I
I
I
I
I
I
I
I
■
a a a
a
a
figure 81
DISCHARGE CURVES FOR DUCK BILL WEIRSFIGURE S2
■I
■I
■I pr
*
*
*
*99 65
99.5
99.6
99.5
SLOPE =015%
FURROW No I
SLOPE =        .0 4%
FURROW NO. 2
SLOPE =       07%
SLOPE =      .0 4 %
SLOPE = 055%
MELKA SADI FIELD 5-1/1 22 6 82
LONGITUDINAL PROFILE ALONG FURROW
F IGURE 8 3Tertiary Gate Opening (cm)
I
I
I
I
I
I
FIGURE 8 4
■
■
H
ft M
«
I
I
calibration CURVE for tertiary offtakes1I I I
I I I I I I
I
I
fl
I
fl
fl
I
CALIBRATION    CURVE    FOR    TERTIARY    OFFTAKES DISCHARGING INTO A 45cm. DIA. CULVERTMETHOD OF PERFORMING diagonal planing
FIGURE 8 6I I I I I I I I I
I I
I
M lUNlf •
tWTUCJ
CABAL M*O>    ■
TIME ItMATM Ju~
CANAL AtACM
O1TAM
:C-RtaiiT
■
crrrue
viLoan
7INl
in
1»TM
w •••
CANAL MAW
pVANCt
a
C1N71’
a/M<
rrart 1 aarM
an
) mtau to cot
| SI TO SXAC* 1 J<CB1TOO«O(» 1 CO * TO txa 5 1 cua i re txs • | fj fO C«*CM >
|ckw> rocoi | r.«anTOO«ai> | ;>.(Oi»roo«xA 1 Cxa« rocxcAi 1 r  TO CKCA 1
7,ir«
>0
I.A70
1,110
1,117
’*0
»10
A10
Ml
All
•oo
111
MA
• 71
f.if      i*        0 ss r.ioi       T,         0 11
T                0 >1
»
>0,401 ’» 0 11 fl. IM ’* 0 IS
1170 M   o so
1.0*»    17 mi     0 >0
U.B4B 40TON    0 10
11,10*      4         0.10
11,714 41TO44 o io 11.10* ••to.o’ io
ll,»O*   •*ro«l     0 10
•3S47      to        0 10
•4041    ■ Sil*     0 10
14/14 U»M 0 >0
1 Al
o.oj
1 11
0 17
Oil
0 47
0 44
0 »■
0 01
oo*
■ >'
t »»
0 41
0 At
1 !•
1w
• <•
A 11 7 IB
7 01
7 *1
• 01
I 10
• 17
7 11 1
• ••
A 54
I 11
• 31
i*TAAA TO C01 cot 70 col
54 <0 CMICX< f, TO WECA 1
Mnirooo i
r,nt 4,111
1»
til
u>
Mir
54 r
Ml
505
—
1,171 11 II,*04 I*
7
<1.440       1
JJ05        1
ii.no      170*
...
» ••
oei
. .. 1
• 41 |
INTAIt 30 CHI 7,171 col ’0 C01 4.111
1A TO <"0        1.000
|  to  CMC*  1  I.SO0
ooitooed 11
“ °”
11.404 14 | 0 10
■1404  r,  ’j  |  o  is >M04 | iiton I □ »o
•4,n I | IB TO-<^ 0 10
V41 i J >1
in i ss
0 71  | 1 44 1  00  .  7  •• 1 OS | 1 41
11*14    |    •»    |  ®
Oi
M’7
•
C
KCAlTO CHINA
cxc* 4 TO ENO
.1,131
14,54. f *01
.4.M1 • TOC
0 31
0 50
o IS
0 >0
0 >0
0 30
0 10
0 10
0 10
0 >0
047
0 51
0 t*
0*7
Dll
1. .11
111 00
|Tj TO CM Cl l      t»l oco.tooaoi            500 IcXXlTOCMOI       •»
lo^OlTOOMNA I..11 CKCOTOO®. '.OO'
■ L------------------- ---------
I1JS4 1 4 |0 1O
■MOI ! '7 TOIli 0 *0 | 11101 | n TOKlI O 50 1 I. MM 1 11 iow| 0 30 iLJ
tn i » oo
04.  1  7  !• obi ' a io
1 10     4 14 01. |A> .1
4
'oO> TOOOt CKCX1TOOKM5
•
,        »n> ono • CHEA. TOORCT,
ANKARA iRBICtr.QN MAOlCCT Q
I
APPROXIMATE time REOUREO FOR WATER TO 0E RELEASED
PROM INTAKE ANO REACH VARIOUS FIELD CANALS IN
I
MELKA SADI
STAGES I ® HSECTION 9
Salinity and Drainage9 Salinity and Drainage
9.1    Saline and Alkaline Soils
Saline arid alkaline soils are characCeria tic of arid and semi 'arid regions.
Reclamation of these soils, and prevention of excess soluble salts in fertile
areas under irrigation is of paramount importance in the project area which can
be called ««ai-arid. The scanty rains in the area do not penetrate the soils
deeply enough for appreciable leaching. Low infiltration combined with
excessive evaporation causes accumulation of soluble salts injurious to plant
growth. In areas where there is a shallow water table either by transport of
groundwater or excessive irrigation and poor drainage, the upward flow of
groundwater results in continuing accumulation of salt in the surface soil.
This is particularly so in large extents of the Melka Sadi Agricultural Estate.
To evaluate salt probleas soils can be grouped under three headings, namely
Saline, Saline-Alkaline, and rton-sa I me-al kaline.
9.1.1            Saline Soi Is
Saline     soils     are     chose     soils     having     an     electrical     conductivity     of     the     saturation extract of more than Ananhos/cni and an exchangeable-sodium-percentage Less than
15.   Normally   the   pH   is   less   than   8.5.   These   soils   are   some   times   known   as "white alkaline” soils.
If there is adequate drainage or when proper drainage is established the
excessive soluble salts can be removed by leaching and the soils restored to
norma Iity.
Saline soils are easily recognisable by the presence of white salt crusts on the
surface. Sodium seldom is more than half of the soluble cations and therefore
not absorbed to any significant extent. Relatively the amounts of soluble and exchangeable calcium and magnesium may vary considerably while that of potassium can be either noraally a minor constituent or sometimes a major constituent.
The chief anions are chloride, sulfate and nitrate. Bicarbonate may occur in
^nall amounts but soluble carbonates are absent. Saline soils may also contain salts of low solubility such as calcium sulfate and calcium and magnesium
carbonates.
9,1.2           Saline-Alkaline Soils
The electrical conductivity of the saturation extract for saline-alkaline soils
i. greater than 4 mrahos /co and the exchai geable sodium percentage greater than 15. In the presence of excess salts, the appearance and properties of these
soils are similar to saline soils and the pH seldom higher than 8.5 with soil particles remaining flocculated. After leuching the pH is greater than 8.5 and the soil particles disperse. This causes a low infiltration rate and
permeability. The excess soluble salts when leached and the exchangeable sodium removed from the root zone, these soils reach a favourable condition.
Gypsum is often found in saline-alkaline soils and when leaching takes place the calcium dissolves and the exchangeable sodium is replaced by calcium together with the removal of excess salts.
909.1.3
Non-Saline Alkaline* Sails
The      electrical      conductivity      of      the      saturation      extract      ot      nan      sa      me
soils is
than 15.
known as
absence
less chan 4 mtihoi/™ 
th.' eKch.ng--.ble
P^cen
The pH ranges Era® 8.5 to 10. flon-s.l ih -.IkiHae i'il are general
jL-
a
"black Ilka line11 sails. Le*thing of ja I Liie-.-i I ka lin*? -alii in Of gypiLim either in the .oil or in the irrigation w.ter, lead, 0
.
formation of non-si Lina-alkaline soils.
sodium the soil tends to become more dispersed.
With increase in the exchangeable
The anions present ar? mostly
chloride, sulfate
, flod bi«rbOMte with ™ll quantity. of urbo^tel. When pH
is high in the presence or
p re c i pitate d.
9.2  Effect of Salt on Crops
carbonations, calcium and mag lies ium are
When two solutions of different strength are separated by a semipermeable
membrane, the weaker solution moves through the membrane to the stronger .
solution in proportion to the difference in pressure between, the two solutions.
This difference in pressure la called "osnMJtic pressure". The flow through the
membrane continues until equilibrium is established.
Plant roots have a semip^rneable membrane separating the fluid in the roots from
the soil moisture- With normal soil the fluid in plant roots is at a higher
pressure than the soil moiiCure and osmosis takes place. Soil moisture tension
also affects the net pressure differential. As soil moisture becomes saline and
approaches the concentration in the plant fluid, inflow of water to the plant
roots decreases and the plant wilts.
Where it is not economic to reclaim salina or alkaline soils by providing
subsurface drainage, leaching water, and chemical amendments, the feaaibile
thing to do would be to plant salt tolerant crops or forage,
9,2.1              Boron
Boron is essential to plant growth in quantities less than HO ppm. It is found
in scattered areas □£ arid and serai-arid regions under irrigation. High levels
of boron can usually be reduced by leaching and requires three to four times
mure water than fur saline and alkaline soils.
As in all cases, good subsurface drainage is essential for efficient leaching.
9,3     Quality of Irrigation Water
Irrigation water always contains measurable quantities of dissolved salts. The
quality is determined by the amount and nature or salts present. With poor
quality water, various soil and cropping problems can develop, while with good
quality water problems affecting productivity are infrequent*
problems      resulting      from      poor      quality      water      are      commonly      salinity,      permeability and toxicity.
Water quality and drainage problems are inter related and good control of a potentially damaging water table is an essential requirement for a Loni? irrigated agriculture to be successful Salts accumulate in the upper portions
a
f a water tahle and when the water table is within 2m from the soil surface it
can contribute additional salts to the root zone* Even with good quality
91irrigation water , a  high water table can be expec
root zone rapidly.
9.3.1          Salinity Problems
ted to deposit saLts io the
The presence or absence of a salinity
conductivity of the irrigation water.
• salinity problcm:-
problena is evaluated from the electrical The table below is a guide to a potential
Degree of Prob Lem
ECW (mmhos/cm)
No problem
Leaa than 0.75
Increas ing
prob Leia
0,75 - 3.0
Severe
Problem
More than 3.0
The irrigation waler diverted from the Awash River has iQ electrical
conductivity on an average of 0.35 mmhos/cm. Under an appropriate water
man agorae nt practice there should not be a serious salinity problem by using the
Awash water for irrigation.
Table 9.[ gives the quality of Awash river water at the Melka Sadi weir site.
At least once a year during May or June the Awash water at the weir site should
be analysed.
9.3*2              Permeability problem
A permeability problem occurs when the irrigation water does not enter the soil
fast enough to replenish the soil nwisture before the next irrigation.
Permeability reduces the quantity of water placed in the root zone while
saLinity reduces the availability of water in storage. Low salinity waters are
corrosive, tending to deplete surface soils of easiLy soluble minerals and
salts. They tend to dissolve all sources of calcium from the surface soil
causing the finer soil particles to disperse, to fill pore space and to seal the
soil surface. This causes a permeability problem.
A high sod 1 urn content in the irrigation water also can cause permeability
problem. In addition, such things as germination, soil aeration, disease and
weed control due co ponding and stagnation need to be considered.
The     most     commonly     u.ed     nethocJ     for     evaluating     the     potential     permeability     problem is the sodium adsorption ratio (SAR) given by:-
SAR ’
-------------------- (40)
and ESP =            100 (-0.0126 * 0.Q1475 SAR)
1 +(-0.0126 + 0.01475 SAR)
Where     all     ionic     concentrations     are     expressed     in     aeq/1     if     the     ionic concentrations are in mg/1 then meq/l = tug/1
Equivalent Weight
For   SAR   values
greater than 6 to 9 , irrigation water could be
a a ppeerrmmeeababiilliittyy problem.
The diagram used for classifying irrigation and groundwater is
expected to cause
Riven in FiRure9*1.
Salinity     (C)     and     Sodium     (S)     are     classed     in     four     groups     giving     “"c*ert
• - --
types of 3is ti nguishable water
varying from excellent (C| S[1 where ids
0.25 mtnhos/cm
SAR 26.
Figure 9-2  is
irrigation or
atiJ SAR 10 to very poor (C4 S4) when EC is 2.25 mmhos, cm an
a nomogram for determining the Sodium - adsorption ratio °f
ground water and the corresponding exchangeable sodium percentage
of the soil in equilibrium with the groundwater E’™*  ?
le!
December 1970. Na - 1.40 meq/l and Ca * Mg meq/L - 2.66 cneq 71 . On figure join 1.4 on the Na axis to 2.66 on the Ca + Mg aeq/l axis. This line intercepts the SAR diagonal at 1.21 and the corresponding ESP = 0.52-
9.1.3
Toxicity Problem
A toxicity problem is different from a salinity or permeability problem. The toxic constituents of irrigation water are sodium, chloride and boron, which can reduce yields and cause crop failure. To evaluate the potential for a toxicity problem a water analysis is required to determine B, Na, Ca, Mg. Cl. CO^ and
HCO} Levels*
9.4      Drainage
9*4.1              Necessity for Drainage
In agriculture, irrigation and drainage are complement ary practices. Excess water due to rainfall, irrigation, or lateral seepage through canal hanks has to
be removed from the surface of the land where there is slow infiltration, low permeability or a restricting Layer at shallow depth in the soil profile. If
there is insufficient surface drainage due to inadequate or improper land shaping, ponding with prolonged saturation will occur to affect plant growth.
Excess groundwater is removed by subsurface drainage. Subsurface drainage also lowers high groundwater tables which damage crops Co varying degrees. Soluble salts in the soil profile can also be leached by re-establishing a downward percolation of water. Subsurface drainage should be adequate to hold the water table at a sufficient depth to prevent capillary water from reaching the root
zone.
9,4.2              Surface Drainage
Sur face
project
drainage is a combination of collector and disposal drains In the area wLere rhe land is Elat, levelled and in regular sL.ape, field drains
are established in n parallel pattern,
orientated to the slope of the land for
collecting water within a field. Disposal drains commonly known as tertiary '
and      secondary      drains,      transport      Cha      collected      water      in      field      drains      to      the      aain dr-iina .
Land forming by smoothing, grading or levelling results in better surface drainage. Land smoothing eliminates minor differences in field elevation including low spots. Land grading and levelling involves cut and fill followed by smoothing. Continuous surface grades ensure run-off without pondin-
Surface drains lose their effectiveness unless properly maintenance progr.nnnr is necessary to prevent the drains with vegetation and partially filled with silt and soil slopes. Unfortunately Little or no attention is paid by
maintained. a good
from becoming clogged
from collapsed side
entities in the project
93The
depth of irrig
ation
application • p.Sa»P
Ea
,35 x 200 x >75 - 75n=i
.70
The
irrigation int
erval
in days = p » Sa ■ D
ETc
For
January
i =
.35 x 200 x .75
* 14
days
4.14
For
June
i =
.35 x 220 x .75
=* 6
days
10.22
Far
November
i =
.35 * 220 x .75
- 13
days
4.46
Am it is not
practical on a large farm to alter the irrigation u
month to month, a
regime of 90mm application depth every 15 days
21609mm/year is recommended.
trials carried out over two years.
9.5     Ground^ Water
9.5.1               Investigations
IAR has recommended I00mm every 15
Investigations of the groundwater cable are necessary to gather information on its position, fluctuation and total salt content at various points in the
project area and its perimeter. From this, the extent and severity of the drainage problem could be determined and the general type and location of subsurface drain# indicated.
Observation wells, for periodic observations of the groundwater table were established throughout the project area in 1970/80. They were set out on □ 2km grid pattern and spaced to give a close approximation of the groundwater surface. There is no set rule for spacing observation wells but during monitoring it may be deemed necessary to drill additional wells to investigate the presence of a perched water table or , sharp departure from the general ' water table surface. Ten more wells are reauired
th. .....a..... fl-., 4 io ft.m s.di .„d7£ tai^o?
■
"
°<
The      depth      specified      for      observation      wells      in      the      proiect      area      sh^.ld      h      k
>. J.l„ th. ..... t.hl.
„ hil.
“X
Amrbara       area       driller       established       four       observation       wells       with       depth       above water coble and therefore these wells cannot be monitored.
The elevation of observation wells should be established
groundwater levels with groundwater surface levels to contour -caps (b) depth to groundwater maps and
Periodic measurements of once a month during off 8eaaon anH during the cotton season are made on form 9 1 ok .
t “P1 le groundw' eatleecrtrical cOnductivity mapfi
, ce a fortnight
of      the      well      casing      to      the      water      table      level      in      eid/ln'      d“tance      from      the      top
wells should function at least for two years after rh cultivation to observe the fluctuation of -n-
saline soils during the growing season. The
lhea* observation
_> C vhole
comes under
V”? leaChi"* of
ry wens in Amtbara should be
95area  to  maintain the drainage system in working  order should be worked out with the Project Control Centre, control of vegetation and timely  removal of sediment» consolidating  large erosions at out fallsr and repairs effective surface drainage system.
A maintenance programme
Maintenance work iticludfis
backfilling and
to structures to retain an
9.4.3             Subsurface Drainage
Subsurface drainage is the removal of excess groundwater and Lowering of the groundwater table to acceptable levels for crop growth. The optimum height o: the water table varies with soil texture, depth of soil and subsoil layers,
sa Unity and crops grown. The capillary fringe in fine textured soils and subsoils may be a controlling factor, especially when the ground water is high, saline as in certain areas of MSAE. Subsurface drainage should permit in addition to crop water requirements the necessary leaching to maintain a a salt balance in the soil-
In arid and semi-arid areas excess water is fractionally due sometimes to
rainfall and often to excess irrigation water, and seepage losses from canals traversing the area. When the total quantity of water applied to the soil is
more than the quantity of water removed by natural drainage, the groundwater table will rise. It is then that submergence drains are necessary to remove the surplus water to maintain the groundwater table at a predetermined level.
A classic example of the above is the groundwater table in MSAE around packing centres 3 and 4- Before the establishment of the banana plantation, Italconsult drilled an observation well Pll in this area recording the groundwater table at
6.2m below the soil surface. The soil and subsoil encountered was silty sand,
siltj and fine silt having permeability coefficients in m/stc decreasing rapidly with depth (0 - S-Sra.k - lx ID’S- 5.3 _ 11.Dm, k - 1.6 :< 10“^; 11,0 - 15.6m,
k => 5.5 xlO" )* The water after seven or eight years of irrigation now stands at 1.25m below the soil surface with an EC of 10 mmhos/cm. The electrical conductivity of the groundwater when the water table was at 6.2m below surface level was not known.
7
9.4,4               Irrigation ReginiE Adopted by NSAE
MSAE has adopted an irrigation regime of 13cm depth applied every 15 days. This gives a total gross irrigation application of 3 UOtam/year. According to the gross irrigation requirement estimated in para 5.6.1. banana requires
2344 « 4x «•« " 251 LiWvpor where the appLication efficiency is taken ,■ 7QZ and the effective rooting depth of banana as T5cm. From this it i5 seen that a Mftnra depth of water i» applied in excm of jjross irr igat Lon requirements every
year. This
fends its way to the perched groundwater table and in
seven years would have contributed 42 JOOnl/ha to the raiain^ of the water
table.
The groundwater table in MSAE in certain areas is within Im from the ground surface and tn sows areas within 2m. Hence the groundwater Contribution in the water balance equation has to be considered. According to figure 9.3 this contribution is an average of laun/day far a groundwater table O.Jm below the root sone reducing the gross irrigation requirement by 36£tem/year . This «iVM gjnH irrigation requirement of 215lan/year.
"a
94
A ‘ALU, ■-re-drilled to specifications and a further sik new wells constructed to forra a
better grid. Four new wells ar** also required in the Melka Sadi area to
identify perched water cables.
In stratified soils excess water in the root zone is some times held up by a
layer of low-permeability so that the perched water Is disconnected from the
main body of groundwater. Perched water tables in irrigated areas may be controlled by reducing seepage from canals* by improving irrigation practices or by providing efficient surface drainage.
9.5.2 WeLl Hydrographs and Groundwater Maps
A well hydrograph is obtained by plotting the depth to groundwater table from
the ground surface against time and the corresponding value of electrical conductivity of the groundwater in srohos/cm. These hydrographs will show the rate of rise and fall of the water level, periods when critical water tables
occur* and the increase in total soluble salts due to leaching or some other
factor.
Well hydrographs for 25 observation wells in Melka Sadi and 26 in Amibara are available at the Project Control Centre. Hydrographs for 13 observation wells
in and around MSAE are given in figures 9.4 to 9-16. The locations of 58 wells
are shown on Drawing No 9.1.
Tables 9*2 and 9.3. indicate the depth to groundwater table in metres and the electrical conductivity of the groundwater in mmhos/cm at 24 locations in the Melka Sadi area and at 22 locations in the Auiibara area recorded in August 1980 and September 1982. Readings were taken at Least once a month from August 1980 tn September 1982, which covers a period of 25 months.Table 9. A Quality of Awash River Hater at Helka Sadi Weir Si te
[Table 9-
I       Qualjity
-J-----------
"T"               1                1            “T--------     T“      T"       “1----------- I—
1 Month
1
1
1
PH          I EC
I m ration
1 /cm
1
HC0 3
1 Cl           I C03        1 S04
1 B
1 Jia          1 Ca          1 Mg          1 K            I EAR
THkjq/1 1 meq/i      1 meq/l
I tneq/ 1
1 ppm       1 meq/i
1    wq/l     1 TIM-q/1 1 meq/1     1
111111111
11111
“T                 1
| 0.40         1 ni 1
T~              n
T~“           T
111
T                ~T~            T
1
1 |DEC/70
1
1 JAN/71
i
1
I FEB/71 |
■|
8.1           1 0.27
1
1
8.5          10.33 
I
1             1 7.8          I 0.32      |
Ii
2.9
I ni 1         1 nil           | 1 .40
11j|
|i11111
1 1.33       1 1.33        1 0.19
11.21
1 0.40         1 0-43
I ni I         | ni I          1 ni 1         1 1.54         1 1.12       1 0-24        1 0.18
1|(j1I|j
I 2-00
2.85        1 0.41
| ni 1          1 0.01        1 ni 1         1 1.70         1 1.24
«
1 JUL/79 1 7.85
1
1 0.42  | 2.90
1
1  0.64
j
1iI|
i
| Trace I 0.04
|
i
1 0.25
|
1 0.17 | 1.97
|1
1 
f
1
11 I
1 1.20
I
|
1 2-88
j
1 2.52
1
1
1 0.40 1 0. w 1  2.38
1
1
1
1
f AUG/79 1
i
i
7.95
1               1
10.25  1
1  f
J .so I 0.4!
j
1 0.81
j
j
I Trace
I 0.08
|
1 1.89
|
1 2.40
j
I
1 0.14
1
1 0.13
|
I NOV/79 f
8.10        10.22        1  1.20        f   0.28
1 0.05        1 1.58
l
i
f
i
1
1
1
/
I
J .80 J  0.32
j
[0.60
i
1 0.80
1
1
Ii
| Trace
J
I
(1
( 1.20
1
1 0.09         1 0.09
1
l
1 DEC/79 1 d. 15
J               1
1 JAN/80 [8.45
II
f 0.28      1
1J 1J
10.30  1
11
1 Trace     I 0.12         1 2,34
|
! 2.01
]
1 0.14
1
1
1 0.12
1
1
1 1.68
1
1 1.97
1
1
1 2.26
1
1
<
I
1.70 1 0.4!
|
l
i
I Trace
1
1
1
1 0.03
j
1 2.59
j
1 1.35
1
1 0.10
1
1
I  Fts/80  r  e*  io       10.33         |  1.70        | 0.87
j
J 
i
r
r
i
iI
| MAR/80 | 7.60
J 0.33        / 1.80
i
i
i
i
1
1
I 0.55
1
r
1 0.80
iJ
1 0.60
11
I
i
1 Trace     1 0.01
1 2.80
1
t 1.30
1
1 0.13
1
1
1 0.15
1
1
1 0.80
1
I Trace
1
1
1
1
1 0.0]
1
1
1 2.59 1 1.80
i
i
1 0.16
1
1 0.19
I
1
1 0.16
1
1 APR/80 7 ♦ 80      10-36         12 .60
1 2.60
1
[0.60
0.05
i
r
1r
ti
ir
1 0. 10
1
1 1.36
|
t 1.20
1
1
1 13.16      1 0.14
1 3-04
1
1 3.28
1
1
’2.60
1
1  1.65
i
1
1 MAY/BO | 7* JO
f
1 1
f
/ 0.38         ] 2
i
i
.40 1  0.60
I
J
1
1 0.60
I
1
0.07         1 0.25        1 1.32
1.50
i
1
I 0.20
i
I
1 0,21
1
1 0.14
i
1
1 JUN/BO 1 8 .10        10.5a          |2.90
1 0.65
I 0.60            0.08           0.35
1
I 1.92 [1.20
1 0.16
1 1 .43
i
1
1 2.29TARLE 9.2 5ADT
1 1 1
1          1 
1 HEIL L
*4jGusr i9w                 1
____________ L_
1              1 SEPTEMBER 1902             1              1
1 USE  J
1 1 1
REMARKS !
I  NO |
I 1  Depth to
I|
:l
I
il
EE mmhos/c* 1
Im
1
1
1
*at er table err
i/l
1 
1
1
1
1 
1
|
1 <-a
1
1
Depth to          |  EC whau/cn | itertable (nJ 1
L
1 
1
1
1 
1 
1 
f
1 1 1
s.a
1
1
3.7
r
i
1
1
2.a
1.55
1
1         4.8 11
i1
1  1.711 1
1
1
]
1  2  1
ii
2.8
I
1
1
D.58
1
.1
■
1
J
1
0.96
1   1,25 I
'tear Canal
1
11
1
1
3 1
■1
4 i
11
1 1
11
6 [
r1
7 I
1 1
a l
i
i 91
1 1
:o 1
ii
11 1
11
12 I
I
13 1
1
14 I
1
15 |
1
is 1
i
17 1
1
1
15 J
1
J.25
Fi
1
1
l* 
■i
0.78
1
2.75
11
1
0.92
I 0.50 I
Near CiHol
1
Vr
ii
11
4.1
1
id.a
I
2.03
1
1
0.60
1   1,27 I
Near Drain
1
■
1
1»
11
ii
ii
11
7.2
1
2.5
1
5*51
|
1
11
11
11
1
l.M
|    1.69 
Lr
Jl
ii
3.55
1
2.3
1
3.25
1
3.0
I    0.30 
I
11
11
1I
II
ii
1
1
7.02
1
w.a
1
5*71
10.0
11
11
[
11
t    1.31 
1
1
1 1 
1
■
1
1
2.5
1
0*56
1
1.95
i
0.71
I    □.55 
f
Nnar Canal
11
J1
li
i1
1
10.55
1
10.0
1
B. 39
11
1
II
1
I1
io. a
1    Z.U
I
i1,
iI
1
1
2.9
1
1
4.4
2.09
1
1
1
1
1
2.4
I   0.01 
1
Banana Saline
l
1 1
i
Area
|
9.75
1
1
2.2
1
8*67
11
1
2.4
11
1 l.t» 1
1
1
5.6
1
1
5.6
1 
1
1
1
4.59
1
3.7
1 1.01 |
11
i
1 
i
1
**&nr Drain
1
1
8.15
1
1
4.4
1
6.28
1
1
1
io.a
1
1   1.87 
4,2
i1 
1
Ii
Near Drain
1
1
1
2.0
1
3.04
1
2.3
1
1
1
1   1.16 I
Cotton Saline
Area
|
3.1
0.99
'1
1
7.22
1 
1 
i11
1
1
1
0.86
11
| 0.88 |
Air Strip
1
1
9.1
1
1
4.2
I
1 1
i
1
1
1
9.05
I
4.00
11
1   0.05 J   1
1
7.15
1
1
1,6
1
1
5.92
1
1
5.B0
1
1 l.U |
1
ST Cotton Saline Area |
9.45
1
1
1
1
i
10.0
1
7.85
1
1
(
I
id.a
11
1
T l.«3
1 ST Cotton
20 1           10.40           1           7.1         1            >.J1 I11
Il              1i
1
1
1
I
1
1
1
I
1 Sf Cotton
1
21 1
|
11.0 1 7.6 1
11
10*0 1  2.09 |
I
■1
11.12
1
1
8.4 1  -0.32  |
i
ii
c1
I ST Cotton Near Drain 1
22 |          1D.0            1           3.6         1            8.35
1
4.6           1   1.65     1 ST Cotton
1
1i                                    1                          1
II
1'
i
J1
1
P10I          JD-7            1         10.0         1          9.10
1
10.0           1 0 52       1
1
1111
11
.
11
1
pill
2.3            1           9.0         1          1.41
111
I
1
1
Area |
______
9.8 1 D.B? 1 Soriano Saline
1
Nnt
.:                   N«. 1> 2, i,« a «nd lfl
t                     Bn3 xithjn the ttou rater y of Hfllra 5adi Agricultural Eitdtctable ?,i wibma
1 1
«LL |
No |
11 1
AUUU5T 19BO                 1I
1  ' 1■L
1
SEPTEMBER 1982  1 1
RISE 1
REMARKS
1
1 
1
*1
1         Depth to
I EC mhna/cfli 1
1
Depth to  |
1
EC emhna/c. |             1
1  rtltartati 1b m     1                      1
intertable (a) I                          1
1
I
1
I
I
I
I
|
I
r
L
1
1
J
1
32 1
I
1
I
1
1
0 1
I
2.78
1 
5.3 
1
2.78 
1
4.B
1I|
lAfl Pasture
i
i
1 1
r
g1
11
35 1
9.5
1 
1.4 
|
7.90 
|
1.7 
I
1.6 1
Motka Harer Village
11
1 
■
11
1i
11
14 |
9.55
1 
2.6 
I
7.72 
I
0.72 
|
1.63 |
5F Cotton
Ii
I1 
r1
11
r
II
t
II
I
1
35 I1 
fl.7
|
6.0 
I
H.69 
|
8.00 |
0,01 I
5F Cotton
11
■Il
I1 
1|
1I
1I
j1i
36 ]!
9.95
■I
ii
|
4.3 
1
6.16 1
6.3 
I
I
2.79 |
Nner Drain A Rottan
1 1 
i1
11
11
37 !I
9.75
1 1.0 1
9.76 
I
2.2 |
0.99 |
SF Cotton
1
l1 
I|1 
1i
1i
11
I
30 ! 5.2
1
39    1 1.6
l|
1 1*0 |
1 
1 
3.05 
iI1
|
IIt
1.4  |
1
1
1        2.5           |i         1.&5 
|
2.0 1
1 
lI                                 1r                        1
2.15 | ASP Cotton
i
1
0.25 r ASP Cotton
i
1
Al
I 8.5
1
1        3.0           :!
5.17 
42 I 15.0
1
I
44 I 9.0
1
45 Jal
|
1
1 1
1 10.0
1
1 10.0
1
1 1.2
I
1 
1 
i1 
|
f
1
1 11.86 |
I1I
|
11
|
I
2.8 |
i
1
1  8.52 
1 
i
1  2.52 
1 
10.0 ]
1
10.0 1
3.13 | ASP Cot tan
1
1.14 I    5F Cotton
1
-0.52 I
1         15.1                  1 10.0             1         13.00 
1
U2  |
i
1
47   1         14.0
|
40 | 6.45
1
1
1  7.0
I
I1
1  13.44 
1 
|
l!
10*0  I
1
1
|
|
I 1.6
|
1           5*64               I
49   I           3.0              1         7.4
t1
!
5.02 
i
I1 
|
1 10.0 
10.0 1
1
1
1 1.6 1
1i
1
0.58 | A legeta Cotton
1
2.10 |    Near Drain
1
0.56 |    SF Cotton
1
.81 |
1
I
1
|                                   1 
1 1
I
I     2.1a f    AabivRh Cotton
50   I         12-45            1        5-4
I         12.73
1
1
51 | 14.25
|
52 | 13.06
r
51 | 13.95
1
r
1      7.4           I 1 -0.28 |
J 
i
1 1
1
1 10.0 1  13.79
1
9.4
|
| 4.0
|
1  12,7a
1
1 12.41
1
54   J         15.77             1 1.6                   1         15.25
1  10.0  1
r1
1 10.
11
1        s.a
1
1
1         4.4
1
1
|
1
1
I 0,46 |
1 1
1 0.30 1
1 1
1 1.54 |
1 1
1  0*52 I
I I t
i
I I I I
I I I
f I 1 I
t I I I 1
55
7.5
1 
1.2
1 
6.29
1
i
1      l.a
1 1
1 1.21 |
Sublale Cotton
During early WD ob.ervation *11 Noa. 2T-M arKj Ho,_ >8_„ -Oro constructed in the Malka Sadi area, *il„ observation -ell Noa. 6Q 67 ^ro ronatructcd
.
in the AeibtM area. The qropndaater table over the project are, 1# rising at the rate nf Q.> to G.75« per year as « evident from t(lo aLflve
90FORM 9.1
AMISARA IRRIGATION PROJECT II Piezometer Records for Drainage Investigations
Name of Observer
Date;
1 Piezometer J Bench Mark.
Depth to
1 Water Table
1 Elec Conductivity I Remarks
I Number
1 
J or
1 Ref Point ’
1 (m)
I (mmhos/cni)
11
III
i
Illi
I
1
1
f
1
1
1
1
1
1
1
1
111 1
1
1
1
1
1
1
III
111
1
Illi
11
Illi
Illi
1
I
1
1
1
I
1
1
1
1
1
I
1
'1
1
1
1
'1■1
III
111
1
1
1
1
1
1
111
111
1
11
Illi
Illi
1
1
1
1
1
1
I
1
1
1
1
I
1
1
1
1
1
1
I-
11
111
11
1
1
1
i
111
Illi
11
1
1
1
I
1
1
1
1
1
I
!
1
1
I
Illi
1
i
1
•
1
1
1
111
1
1
1
l
1|
1
11
«
1
1
11
1
Illi
I'll
1
1
I
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
I
1
1
1
.1
JOOSALINITY HAZARDI" lUUKt » t
No *
meq/l
Co^ + Mg*4-
meq/l
NOMOGRAM FOR DETERMINING THE SAR VALUE OF IRRIGATION WATER ANO FOR ESTIMATING THE CORRESPONDING ESP VALUE OF A SOIL THAT IS AT EQUILIBRIUM WITH THE WATER (USDA, 1954)FIGURE 9-3
CONTRIBUTION OF GROUNDWATER TO MOIST ROOT ZONEI9G0
1901
E.C. mmhos /cm
J     u
u,               -M
cn
>
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A.I.R3
Depth to ground waterE ,C mm hos/ cm
Depth ic groundwater
A.I.P 4E-G mmhoi/cm
A.I.P 5
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FIGURE 9 8E C mm hoe /cm
Depth 10 groundwoter
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I
1981
1982SECTION 10
Efficiencies10 Efficiencies
10.1  Types gf Efficiencies
Movement of water through an irrigation system frora its source t« the crop can be spLit into three successive stages; conveyance, distrLbution, and field
application.
Conveying mo &c regarded a* the
ujin canal to the primary offtakes,.
of wat^r fron diversion through the
Distribution     will     then     be     the     movement     of     water     through     secondary     and     tertiary canals to field canals.
Application is the movement of water from the field canals through syphons to
the land to be irrigated to maintain the soil moisture above a minimum level
required far the crop.
When water flows through a canal1 furrow or drain some of the water i< lost due
to evaporation from the free water surface. Water is also lost due to seepage
Laterally through canal banks and vertically due to percolation- Excessive
lateral -seepage la often due to faulty construction of canal banks or due to
animal burrow. The seepage water is partly evaporated from the moist soiL at
the outer slope of the banka where it accumulates and the rest percolates
vertically to the groundwater table. The percolating water through the surface
area in contact with the flow of water either in canals or drains also finds its
way to the groundwater* In addition to evaporation and seepage losses there are
operational losses due to rejection into drains.
Water losses from the irrigated crop are due to evapotranspiration which is the
combined effect of evaporation from the moist soil surrounding the crop and
transpiration from the growing crop itself* Excess water applied at the top end
of furrows which is inevitable in a furrow syatem ( see figure 6.1) or water if
applied at a rate higher than the infiltration rate to cause ponding and surface
runoff will be lost to percolation or to drains from which evaporation and
percolation losses take place. No lateral seepage losses take place from drains
as they arc in cutting.
Water     in     the     main     drain     that     is     not     lost     due     to     evaporation     or     percolation     is discharged back into the river.
for an estimate of operaticnal losses, intensive flow measurement a in canals and drains* This can only be done with time on a large thia nature* However trained technicians are required to monitor
analyse the operations.
?
are necessary
project of
the losses and
Figure 10.1  is a  flow chart depicting and application losses due to  seepage
phaue..
anschd emevapaticoralatly.ion fromtheeach indicvionduvealyance           distribution
Seepage Usses according  to  Poiree and Ollier,
J2
soils and 0,18 ai /en /day for Loam-clay.
1968  are 0.09nP/c!2/d v  for cl,
a
a
101Table 10.1 below gives the possible seepage losses in Che eosin canal at full
capacity.
Table 10.1
3
Losses m /day
Wetted
Clay
0.09a^/a|2/dny
Loam Clay
Per ime ter
Length
0.18m^/m^/day
15.60
1933
2.714
5,428
14.16
5245
6.684
13,368
12.71
4225
4.831
9.666
12.04
544 5
5.900
11,800
10.54
1820
1.726
3.452
9.36
1445
1.217
2.434
9.15
3387
2.484
4,968
9.42
3000
2.543
5,066
TOTAL 28.101
56,303
At full capacity  the flow rate is I3^5ra^/s in the first equal co  a  discharge of 13.5  x  86400  - 1  . 166,400m^/day  . 2.41 for a clay surround and 4.8Z for loara clay.
reach of canal. This
The Seepage logs is
is
If    rhe    average    evaporation    from    a Su/day, the losses from the iiain
free water surface during
canal are as follows:
the     season ls
Surface
Area
14.8
13.4
12.0
11.4
Length
E3
1933
5245
4225
5445
Los s
w ^7 day
143
351
254
310
9.9
1820
72
8.74
1445
63
7.62
336 7
129
9.02
3000
135
1457
The evaporation loss is therefore Q.12X
it follows from the above,that all operational leases nl...
percolation losses plus evaporation losses fronl field *■aeepa*e losses plus main canals and surface runoff from the field.
evapotranspiration to obtain the gross irrigation rlqui rereTTA? g ■ 10.1.1            Conveyance Efficiency
This is the efficiency of main canal from the head regulator to the dis tributary system. The intake rates of flaw
co the distribution system can be Treasured.
u      lary ’jFcondary and
L Primary offtakes
quantities delivered
102The conveyance efficiency of the canal system can be expressed as:-*
P C = Vd * V2 ---------------------- “
V  * Vj
c
where V, « Volume diverted
V(j = Volume delivered to the distribution syste®B
VJ = Inflow from other sources
V2 — Hon”irri ga c ion. deliveries front conveyancn
10*1.2 Distribution Efficiency
This is the efficiency of the distribution system supplying water from the conveyance network to field canal inlets or tertiary offtakes. The inta e water quantities from the conveyance net-work are known and the output water quantities to the field canal inlets should be measured.
The distribution efficiency can be expressed:-
(42)
e  = y + y  —--------------------------------------------------------------------------------------------------------------- (43)
df?
Vd
where Vj • Volume delivered to the distribution system
Vf = Volume of water furnished to the field cartal inlets
V3 » Non-irrigation deliveries from the distributary system
Form 10.1 should be used to calculate the distribution efficiency from commencement of irrigation up to any specific date or the total growing period.
10.t,3 Field Application Efficiency
This is the relation between the quantity of water delivered at the field canal inlet and the quantity of water needed to maintain the soil moisture above a nLinimiLm level required for the crop.
Evaluation of the field application efficiency requires the measurements of water deliveries to each field and also measurements of soil moisture before and after each application of irrigation water.
Field application efficiency can be expressed as: -
where:       Vf « Volume of water furnished to the fields through field canal inlets
V
m
- Volume of water needed to maintain the soil moisture above a minimum level required for the crop
Form     10.2     and     10.3     shouLd     be     used     f
efficiencies.
10.1.4 Overall or Project Efficiency
or
determining     field
* eia application
i       ,•
a0D ir
Separate assessments of conveyance, distribution and fi.ia efficiencies wilL indicate if and where reoedi.l n.
improve the efficiency of water use in the project
H apPllcst»on
rc 1ULCf’<« to
103The overill (or project) efficiency can be expressed x,:-
(45)
e p - Va, *- V? + Vi ----------------------------------------------------------------------------------------- v c   Vj
+
If v , V  and v
t2               3
are negligible with respect td V  and V„, e  = e -e ‘«
c
p         cda
10.2    Water Measuring Devices
10.2.1           Water Measurement
Irrigation water must be measured i£ it is to be used efficiently. Moat water
measuring devices indicate the rate of flaw as litres per second or cubic metres
per second. These flow rates can be conveyed, distributed and applied.
10.3   Method of Water Measurement
Methods of water measurement can be placed in three categories 1) volumetric;
2) velocity-area; 3) Twafluring structures*
10.3.1            Volume Method of Water Measurement
The     Elow     in     small     irrigation     streams     can     be     collected     into     a     container     of     known volume for a laeasured period of time using a stop watch.
D i s ch a rge rate 1 /s . = Vol use of container, litres
Time required to fill, seconds
10.3.2            Velocity Area Method of Water Measurement
The race of flow in a  canal passing  a  point is determined by  multiplying  the cross-sectional area  of the flow section nt right angles to  the direction nf flow by the average velocity of flow.
Q ' A x v —--------------------------- ---------------------------------------------------------------------------------------- U6)
where
3
Q • I is or ra /s
A = Cross-sectional area t-
V = Velocity of flow ®/s
The cross-sectional area is measured directly w.iereas the average velocity of flow is usually flashed as 0.85 times the surface velocity. The surface velocity is measured by using a float and recording the time it take, to travel a known distance. The cross-sectional area of the canal at the beginning and end of this distance should he determined before hand.
10.3.3             Structures for Water Measurement
The coitrooa measuring devices for measuring flow are the w«ir orifice Parshall flue*. Figure 10.2 shows diagrsnraatical ly four different tvoes weirs used as measuring structures.
rThe basic formula for calculating discharge through a weir is
(47)
3
Q - CLHn ----------------------------------------------------------------------------------------------- Q - discharge in m /s
C     •     coefficient     depending     on     nature     of     crest     and approach conditions
L » length of crest
H = head on crest
n * an exponent, depending on weir opening
Francis' formula for a weir
with no end contract ions is given by: Q -        1.84 LH 3^2 mVs
with one end contraction Q
1*84 (1 - O.IH) LH
3 /2
-
with two end contraction Q - 1.84 (I * 0.2H) Lfp/2
(48)
(49)
(50)
The discharge through a 90° V-notch weir is given by Q w 1.38 U^/3 The discharge through an orifice is given by: ~
Q-
where: Q -
----- (51)
A ’ cross-sectional area of orifice cu^
g = acceleration due to gravity 
H ’ depth of water over centre of orifice on the upstream side in case of free flow orifices, or the difference in elevation between the water surface
(9.81 m/sec2)
at the upstream and downstream faces of Che orifice plate in case of a submerged orifice,®.
Cd - coefficient of discharge varies between 0.85
and 0.7.
10.4 Parshall Flumes.Di a charge
det ermine
paintfl at
through Parshall flumea  can either ba  frtffl the rate of flow, two  depth gauges, %  and the mean elevation of crest.
flow or submerged flow. To
Hjj are provided with zero
Under free flow conditions, the rate of discharge is solely dependent on the length of cre c. W. and the depth of water Ha In the converging section. The
fl    (
discharge may be read directly from, tables using the upstream head Ha tor the particular throat width W*
A Parshall flume can wichntatid a high J^grep u£ submergence
* wide rangv 01
downstream     water     Levels     without     a     reduction     in     the     free     flow     rate.     The^     free
flow limit is governed by the ratio of
An arbitrary classification at
Parshall     flumes     according     to     throat     width     is     (a)     I     loch     to     3     inch     very     ‘small     (b) 6 inch through B feet small and (c) 10 feet to 50 feet large.
Table 10.1 gives the dimensions for Parshall flumes from 1 inch to 8 feet with reference to figure 10.3. The upstream head—discharge relationship to Parshnll flumes of various sizes > calibrated empirically, is givan by the equation*-
where    K
Q "
- a  dimens . tuna  1  factor rwhich is a  function of the throat width
u 31 varies between 1.522 and 1.60 Table 10.2 gives the values of K and □
The selection of flume size depends on
for each size of
F1une,
the maximum and minimum flows to be
measured.,.     the     corresponding     depths     of     flow,     and     channel     parameters     at     the     site where the flume is to be installed.
Example!
A    discharge    of    up    to    440    I/s    is    to    be    measured    in    a    channel    whose    gradient    is 0,7 x LO’i and depth of flow 7Rcm. The bedwidth is 1.25m and freeboard 25cm
with
1:1.5 side slopes. This gives a width
of 3.49m between canal banks.
From  table
10.J    it    is    seen    chat    a    2ft    flume
with
a miniaiuTn upstream head of 46mm at 702
has a  maximum discharge of 937  1  / <t
su btner gence .
Ha is found to be 47cm for discharge of 440 The level at which the flume crest is to be (downstream water level)), minus Kb (32.9cni)
l/s  Hence Hb - 0.7 x 47 = 32.9cm
A
will   give   4   na*   nva   diseh*rSt   of   44D   I   /   s   with   70Z   Submergence downstream water depth at 7Hcm as designed.
Installed is therefore 78cm
“ 35.1 cm above the bed level.
Th I a
- —! maintaining the
106Table 10»2 Parshall Flume Dimensions as shown in Figure 10,1
mn
r“
Iw
I
IA
IF
IG
I K
I
[N
[
I
I
1
r—
1-----
1--------
11"
I 242
356
f 93
1 229
r   76
I 203
i  io
I   29
I 2"
i    276
406
I 135
1 254
I 114
I 254
I 22
t 43
I 3"
I   311
457
|    178
I 457
[ 152
I 305
I 25
I 57
f 6"
t    414
610
1 394
] 610
I 305
!  610
I 76
I 114
] 9"
J 587
864
[    301
1 762
I 305
I 45?
I 76
I 114
ir
I    914
1343
I 610
[ 914
I 610
I 914
I 76
I 229
[   1'6"
[ 965
1419
1 762
I 914
1 610
[ 914
1 76
I 229
I 2'
I  1016
1495
I 914
1 914
I 610
I 914
I 76
I 229
I 3'
I  1118
1645
I 1219
I 914
I 610
I 914
[ 76
I 229
I 4'
I 1219
1794
! 1524
1 914
I 610
I 914
I 76
I 229
I 5’
I 1321
1943
1 1829
1 914
I 610
I 914
I 76
I 229
I 6'
I  1422
2092
1 2134
1 914
I 610
I 914
I 76
I 229
2242
I 2438
| 914
I 610
I 914
I 7'
I  1524
I 76
| 229
| 1626
2391
I 2743
I 914
' 610
[ 914
I 76
! 229
I 8’
I
I
■ u-+»w—
2Ji1
Table 10.3 Discharge characteri»tics of PirahallJlimea
1
----- r
Threat
Width
' Dili charge Range I
| in tn  x 10-3 
3
| Q = KH
3
| ra /a
au
| Head Range
| in (■)
|
I------r— —i
:
I Submergence I
| Li a mi bt
I M /H
Minimum I Maximum
1
I Minimum | Maximum I
I--------- T--------
1"
2”
3M
6°
I 0.09
I 0.18
5.4    | 0.0604
I 13.2 I 0.1207
1 0.77 | 32.1 I 0.1771
I 1.5D I 111 | 0.3812
9
n                   I 2.50         | 251             I 0.5354
I1
1'6°
1 J.32         1 457           I 0.6909
| 4.80         I 695
| 1.056
2'
3*
41
5*
6'
7'
81
1 12-10 1 937 I 1.428
| 17.60 1 1427 I 2.184
| 35.80 I 1923 I 2.953
I 44.10 1 2424 I 3.732
I 74.10 | 2929 I 4.519
I 85.80 | 3438 1 5.312
I 97.20 1 3949 I 6.112
1.55
1.55
1.55
1.58
1-53
1.522
1.538
1.550
1.566
1.578
1.587
1.595
1.601
1.607
0.015        1 0.21            I 0.50
0.015        1 0.24            1 0.50
0.03           1 0.33            I 0.50
0.03           1 0.45            I 0.60
0.03           1 0.61            1 0.60
0.03           1 0.76            1 0.70
0.03          1 0.76            1 0.70
0.046        1 0.76            1 0.70
0.046         I 0.76             1 0.70
0.06           1 0.76            1 0.70
0.06           1 0.76            1 0.70
0.076        1 0.76             10.70
0.076        1 0.76            1 0.70
0.076         1 0.76             1 0.70FIGURE 10 1
FLOW CHART DCP/CT/WC WAT£Fl LOSSES DURING CONVEYANCE. DISTRIBUTION f APPLICATION
1
I I 1
I I I I I I I I I I
Ibroad-crested rectangular profile weir                     RECTANGULAR SHARP-CRESTED WEIR
DEFINITION SKETCH OF A ClPOLETTl WEIR
V-NOTCH SHARP-CRESTED WEIRSECTION L-L
PLAN AND ELEVATION OF A CONCRETE PARSHALL MEASURING FLUME SHOWING COMPONENT PARTSForm 10.1
Irrigation Efficiency Evaluation
Calculation      of      distribution      efficiency      for      a      Farm      unit served from rhe Main Canal
Date .a
Unit
Farm * +
1         Crop grown
2         Grass a re a
3 Net cultivated area
4         Period
*
Name
ha
ha
days
5
fatal flow delivered to
distribution system (Vd)
6
Total flow delivered to field canal
inlets (Vf)
m3
7
Total non^irrigation deliveries from
distribution System (Vj)
8
Distribution efficiency
1
7)            Distribution efficiencyForm 10.2
Irrigation Efficiency Eva[nation
Calculation of jhe Field Application Efficiency for an Individual Field for a Specific Irrigation
Farm ....................... .. ■ ... ..................
1         Name of field
2         Grass area
I         Net cultivated area
4  Type of soil of the field
5         Bulk density
6         Field capacity
7  Flow rate at field canal
inlet
8         Method of measurement of             flow rate
9         Field application method
10      Duration of flow in field canal
II       Total flow delivered to the field Vf 12     Rooting depth of crop
Id Soil moisture status before irrigation 14  Soil moisture status after irrigation
15      Volume of water added to root zone, 16      Crop grown
17       Stage of developcnent
IB     Irrigation number
19     Field application efficiency
Unit . *..................................
Designat      ion
ha
ha
Name
gra/ cc
z
i/»
Name
Name
Hours
in 3
cm
I
X
No
Z
18      Refer to event number in irrigation schedule
19       Field application efficiency = Available soil moisture over root zone Wat pt delivered to field
noFotb IQ. 3
Irrigation Efficiency EvaLuiition
Calculation of field application efficiency for an individual field during a cropping seaaon
Finn .
1         Name of field
2         Soil type
3         Crop grown
4         Gross srflji
3       Net cultivated area
6         Date of planting
7         Total growing period
3  Total flow per post planting irrigation
9         Number of post planting irrigations
Darn ......***** - - - - .
Unit
Designst ion
Name
Name
ha
ha
Date
Days
No
10 
Total flow for pra-irrigation
11 
Total ■seasonal applicat lent Vc)
12 
Actual seasonal consumptive use (W)
13 
Leaching requirement (LR)
14 
Total precipitation during the season(P)
15 
Effective precipitation during the season
CPeJ
16 
Actual irrigation requirement
c'j
n>3
I
m3
17       Field application efficiency during
the season
I
Note:
13) Leaching requirement ■
ECN
5-ECe - ECw
15 USDA SCS method
16) Actual irrigation requirement Vm ■ W - Pc 17} Field. applicMtion efficiency = Actual ir i
r Rdt on
<1 - LR)
i   egqtlLrecent (16)
Total ^ftaaonar application. Ml)
111APPENDIX I
MOISTURE CHARACTERISTICS OF THS SOTLS IN
THE AMIBARA IRRIGATION PROJECT AREA
ARD HELKA WERER RESEARCH STATION
1.      Introduction
water retention characteristics of the soils arc discussed.
Water retention properties to a very great extent determine irrigation schedules and depth of application. Thus these properties should be known Cor efficient water use and management.
The following soil water properties, viz Field capacity, Permanent wilting point, Available Soil Moisture and Readily Available Soil Moisture are defined and discussed below.
1.1     Field Capacity
The amount of water retained after drainage of a saturated soil is called field capacity (1). It is the moisture held in rhe soil by surface tensional forces against gravitational pull. The tension at which thia water is held is about l/3rd atmosphere (pF - 2.3) for fine textured soils, and about 1/lOth atmosphere (pF ■ 2-01) for coarse textured soils.
Recent investigations (6) have shown that the field capacity concept is more applicable to coarse than to fine textured soils. However for practical irrigation scheduling this soil maistate characteristic is still used
irrespective of texture.
1.2     Permanent Wilting Point
Permanent wilting point is the soil water content below which plants growing in that soil remain permanently wilting (6). For dost soils the tension at permanent wilting point is about 15 atmospheres (pF « 4.2),
Wilting or drooping of the leaves is the common symptom that Che permanent wilting point has been reached. Some plants will not wilt but show other signs such as decreased growth or a change in appearance such as leaf colour,
|.3 Available Soil Moisture
The amount of water released by a soil between Field capacity and peraananr wilting point is called available soil moisture.
1.4 Readily Available Soil Moisture
Tt wai once generally accepted that the soil water w»« 
......... . sthe soil vater content approaches permanent wilting  point*  ii'i t e t»  tm readily  available soil moisture is defined as a  fraction of the aval a  e moisture that is easily  absorbed by  plants. For most crops this rai-ion between 0*5  to  0*75  of the available moisture*
2, Soil MoisEure on Volumetric and Depth Basis
For general purposes soil nwisture is expressed ai percentage on weight basis. It is determined by the following equation:-
soil
lies
Soil Moisture on
weight basis
’ Wet weight of soil - Dry weight x 10DZ
DrF Weight
However, Chia cannot be used in comparing soil types ns the actual water content will vary depending on bulk, density which is related to pore space- Thus Co express water content realistically, soil moisture on weight basis should be converted either into volun^cric percentage or depth of water per unit depth of soil. The following formula show these conversions*
Soil Moisture
on Volume Basis
Soil Moisture
□n Depth Basis
2* 1 Bulk Density
Soil Moisture
an Weight Basis
- Soil caoisture
on Weight Basis
x Bulk Density Z
x Bulk Density x 1/10 cm/ra
Bulk      density      is      defined      as      the      density      of      undisturbed      soil      including      pore      space. It is determined by the following equation:-
Bulk   density 
- Weight of Dry Soil st/cc
Undisturbed Volume
3*       determination of Field Capacity and Permanent Wil tine Poinr
3.1      Field Capacity (F.C*)
Field expxcity tan^be determined in the field by flooding the soil so ns to saturate a depth of about 1.0 meter and monitoring the soil regular intervals from the time of flooding up Co**l 2o“hours . “°i c ' i s ’ i
cover the soil surface to prevent
3'D
.
’ evaporation*
Table AJ.l below shows the change
two soil types, dark vertisol and
in soil moisture content after flooding fOr
brown alluvial soil*
T13Table AC. 1 Change* in soil awisture content after flooding- Moisture X on a dry weight basis (MWHS Progress Report 196
Time after
Vertisol
Alluvia
flooding hrs
24
4fl
64.2
55. 8
48.7
40.0
60
72
48.5
45.4
36.2
35.3
96
45.2
45.1
35.2
35.2
120
from the above table it con be concluded that the field capacity value for
vertisol is. about. 4 5. LZ whili! Chat for alluvij, i.5 35-2.2. ■
Field capacity can also be determined in rhe Laboratory using a pressure plate
apparatus* Ln this niethod the soil sample is air dried, crushed and sieved
using a 2nmi sieve. The sieved sample is then poured into a rubber ring placed
on a porouH plate and saturated with water for 24 hours. The porous plate with
the saturated soil is loaded into a pressure chamber and pressurised to l/3rdl
atmosphere until drainage has ceased. The sample is then unloaded and its soil
moisture determined gravitmetrical ly .
3-2 Permanent Wilting Point (P.W.P.)
Permanent wilting point can be determined by using a sunflower plant in a pot*
The sunflower when it haa reached a 5 leaf stage is subject to stress without
irrigation. The soil surface is covered with polythene to prevent evaporation
from the soil. When the plant shows permanent wilting, the water content of the
soil is determined gravitmetries 1ly ,
The permanent wilting point can also be determined in the laboratory using the
pressure plate apparatus. The procedure is the s-ime as that described in
section 3.1 except that the soil sample in this case is subject to a pressure of
15 atmospheres.
4        Factors influencing F.C. and P.W.P,
+
F.ctor* that influence F.C. and P.W.P. are texture, structure, porosity and
organic nutter content, then F.C. and P.W.P. are expressed in volumetric basis
the effect of itructure and porosity is minimised. For low-organic soils such
’« the .oils of the Middle Awash, the influence of organic matter   neglkiblo
is
(organic matter varies between 0.34 to 0.76Z (3)). Thus the most signiH™
factor that influences F.C. and P.W.P. is ,oil texture. Soil texture is
determined by particle sire distribution and particle
i determines the
3 te
diameter. Smaller the particle lire, smaller will be the pore
versa. Pore diameter or the capillary  pore size la related to tensions! forte with which water is held in the soil. Surface is related to capillary diameter by the equation given below--
P - 2T
r
pore
diameter and vice
a surface
t e n s il o n a L force.
114From the above equation it is evident that the sew Iler the dianaeter of the capillary greater will be the capillary tensioa with which the moisture is e
in the soil. Thus in fine texture soils, soil moisture is held with a greater force than in coarse textured soils. This explains why coarse textured soi s are easily drained.. Also for a given soil moisture tension a fine textured sot will have more water content than i coarse textured soil* It should also be noted that a fine textured soil has more pore space than a coarse textured soil. Table Al.2 shows the relationship between texture and pore space, F.C- and
P.W.P.
Table Al.2 Influence of texture on pore space,
(ILLHl Publication 16 Vol 1, (4))
F* C. and P.W.P.
Soi 1
Pore
Bulk
Wilting
Fie Id
Available
Texture
Space
Density
Point
Capacity
Soil Moisture
Vol I
g/cc
Vol 1
Vol Z
Vol Z
Loamy Sand
42
1.55
9
21
12
Sandy Loam
44
1.50
9
21
12
Loam
46
1.40
14
30
16
Clay Loam
49
1.35
17
36
19
Silty Clay
52
1.30
20
41
21
Clay
53
1.25
23
43
20
It is interesting to note that the available soil moisture is the lowest in
sandy soils and increases steadily ,1S the clay content increases up to a point
where the clay content is around 60Z at which composition the texture is clay
loam to silty clay loam. Further increase in clay content causes a slight
reduction in available soil moisture. This phenomenon has been reported by
Sweeney (B) and the data is presented in Table AL.3.
Table     Al.     3     Available     Soil     Moisture     for     Various     Soil     Texture     Classes     (From Sweeney, 1970)
Textural
Class
Coarse Sand and Gravel
Sand
Available
Moisture
cm/m
2 -■ 6
4 -- 9
Loamy Sand
6 -- 12
Sandy Loam
11 -- [5
Fine Sandy Loam
14 -- 18
Loam and Silt Loam
17 -- 23
Clay Loam and Silty Clay Loam
14 -- 21
Silty Clay and Clay
13 -- 185         Soils of the Amibara Irrigation Project Area
The ItalconsuItants (5) in cldSSLfying the soils of the Amibara Irrigation Project (AIP) including the Melk* Werer Research Station identifted eleven soil types, but all of them could be broadly grouped under alluvia and vertisols. Table Al.4 given below indicates the eleven soil types identified by
Ita 1 consultants and grouped under the two taajor types. For most practical purposes of irrigation scheduling and Water Management the grouping of the eleven soil types into two major soil types is adequately justified. Further at the Me Iks Werer Research Station where more detailed soil surveys have been done, only two soil types have been identified and their properties determined.
Table Al.4 Soils of the ATP Area
Alluvia
I Soils on very recent alluvium.
2       Undulating phase of (1)
6  Soils on recent alluvium with
alterations in grain size.
7         Hydromorphic phase of (6).
8         Sandy phase of <6).
9         Arid brown soili on terrace
alLuvi ura.
10       Saline and alkali soils on
recent alluvium.
11       Calcareous soils on pebbly
terrace alluvium.
Vertisols
3  Vertisols on recent alluvium
4  Non-caIcareous vertisols on
recent alluvium
5       Hydromorphic vertisols
According  re a  Soil Survey  of the Awash Riv-r Basin conducted predominant soil types in the Middle Awash are again Alluvial Vertisols as shown in Table Al.5.
by FAO.(2)
Soi L s and
the
Table Al. 5 Distribution of Soil Type* in the Middle Awash
Soil Group
X
a) Soil on recent_AJ Lnvia
Alluvial Soils
16.6
Vert iaol
14.4
Other types
2.0
X
33
116Soil Group
b) Soils on old Alluvia
Z
9.9
15-5
9.3
3*3
Z
38
Vertisol
Serai-Arid Brown Soils
Regasols
Other types
e) Lithosols and Rock outcrops
29
100
6 Experimental Procedure
Thirty one sail sampLes in the A£F areas and MWRS were collected from zero to
30cm. depth for determining soil moisture characteristics. Pressure plate
apparatus was used in the determination of F.C. and P.W.P. the procedure of
which, is described in Sect.ion 3.
A portion of each sample was used for textural analysis using hydrometer method-
At each sampling sire bulk density of rhe soil was determined using a core
sampler* Using the bulk density values water contents obtained on weight basis
were converted into volume basis as described in Section 2*
Volumetric water content of F.C. and P.W.P. were linearly correlated ta clay
content using the built-in computer programme of Tt-58c. Using the regression
equations straight line graphs of the type y ” mx + C were drawn,
7 Results and Discussions
Table     Al     .6     presents     the     results     of     this     study*     The     following     conclusions     can     be made from Chis table.
a)       All the samples analysed irrespective of whether they were alluvia or vertisols have clay contents about 32Z with a maximum value of 84t* The mean
t
clay content of 31 samples is 52.48Z* This means that the soils of the project area are essentially fine grained.
b)       Classification of the soils into textural triangle, indicates that the soils Lie within a grouping viz: clay loam to clay. Out of the 31 2 silty clay loams* 17 silty clays and 9 clays,
are silly clay in texture.
groups, using the textural
narrow range of textural
samples, there are 3 clay looms
Thus predominantly the* soils
■
It      is      also      interesting      to      note      that      silty      clay      texture alluvial and vertisol types.
is observed both in the
c ) Bulk density values ranged between 1.35 g/cc and L. 15 g/cc and is ba related to the clay content. With higher clay content low bulk density values are observed which is reported by other workers (4 fc)
.
°
y
117d) Both F.C. and P.W.P. are found to be highly correlate a Regression analysis between clay content and F.C-/P.W-P- «ave
equations;-
clay dontunt
che following
F.C. P.W.P.
-
=
O.38C
0.42C
+     14.98     (r     =     0.83)
+ IQ.53 (r = 0.81
)
where
C
cliy content
in per rent.
e)     F.C.     and     P.W.P.     values     were compared    with    measured    values    of matched     reasonably     well     with     the
predicted and measured values of
predicted using the regression equations ami samples picked at random. The predicted values measured values. Table Al.7 shows the
F.C. and P.W.P.
f)        The available soil moisture for the 31 samples fell within a narrow range
irrespective of the soil type. The values vary between 22.0cm/m at 302 elay
content to 21 era/m at 841 clay content as shown in Figure Al. 5.
g)           It     should     be     noted     that     the     regression     equations     given     under     pari     (d)     above are valid under the following limiting conditions:-
clay content between 30t and 85X
textural group between elay loam to clay
organic matter content negligible
h)       The more or less equal available soil moisture within fine grained soils irrespective of variations in clay content os observed i a explained as follows. An increase in clay content increases the pore space and thus will result in increased water content at a given tension and vice versa. Thus a soil with high clay content will have a high moisture content value at F.C* as well as at P.W.P. Similarly a soil with relatively low clay content will have relatively low F.C. and P.W.P. values. This result is more or Less the same amount of available soil moisture which is the difference between F.C. and P.W.P. in this narrow range nt textural groups.
8 Summary and recommendations
Experimental       evidence       indicates       that       the       predicted       available
ssoioil lmoismtuoi restofure       of
the     two     soil     types     in     the     project     area     does     not     vary     very     much,
but     fall     within
, but fall within
a    range    of    22.0    cm/m    to    21.0    cm/m.    The    mean    of    the    measured    values    of    available soil     moisture     £■     22.17     cra/m.     Thus     it     can     be     summarised     that     irrespective     of     the soil type the available soil moisture in the project area aj well as the
research     station     Is     around     22     cm/m.     Thia     will     justify     the     use     of     one     irrigation regime far the whole project area.
Soil varies very greatly in texture, structure and soil water properties vertically (between protiles) and hcrixontally even in .i so called "unifn soil", Thue the soil parameters taken into consideration for irrigation an I other agricultural loanageuient practices are averaged values obtained f
representative samples.
to unavoidable losses caused by surface evaporation,
run-off acid losses through structured. Thus average values moisture are not likely to cause any serious error i
Water Management. It is concluded that the use of soil moisture for the two sail types is justified.
Further Lnjrrtgacion practice inaccuracies ariae due
d<’ep percolation, surface
- —J of available 30i]
tn irrigation scheduling and
a single value oE available
118Tabla Al,6
Soil Watt and Toxtijral Characteristics of Soil Sanplea
1
1
1
1 Ml
1
1
1
1
1 code
1
1
1
1
I 31 IL 1«
1 irPt
1 SAND
J1                       1
1
1
1«
1 Sill
J
1
1
11
| CLAY
1
1
1
1 TEXTURAL
I GROUP
•
1
I SULK
1
1
11
1 FIELO                   1                                1                 1
I DENSITY     1 CAPACITY
1 P.o.P.
| A. S.H.      |
I gt/cc
J
1 tt.S 1 Vol.S
1II1
1   Wt.X   |   Vai.3    1 c*/m         |
I1
L1
1
11
1
I
I HrtRS 102 1 A
1
1 4
1
I *0
1
1 46
1
1 SiC
1
1
1              t
1 40.30 I 52.49
1 1
1 23.17 I 50.12
1 
1
1 22.37          |
1 2         1            1C7 i A            1 4           I 46    1 40         1 SIC              1 1-3               1 42.70 | 55.51           1 27.47 | J5.71           1 19.00           1
1 3         1 HWffS 109 1 A           1 a           I 49         1 M          1 5K           i 1.3                1 37.te | 49.17           1 20.09 I 26.12          1 25.05           |
1 4         1 f*H5    202 | V             I -            I J6         1 64         1 c
f 5 1 200 | V 1 6 I 46 | AB 1 Sic
1 1.20 ( 47.11 | 56.33 1 20.53 I >4.24 122.29  |
1 1.3 | 41,45 | 53.09 ! 24.94 | 52.42 121.47  |
1 6         J *RS     212 1 A            1 J2         I 44          1 64         1 SIC              1 1.3               1 3S.CI 1 50.71         1 20,54 I 26.70          1 24.01           1
J 7         1 HHR5 2IQ 1 V           1 H          1 52         I 40          1 SIC              1 1.3               1 39.43 1 51.26          1 22.011 28.61           I 22.65            |
1 a 1 mwis 220 1 V 1 - 1 24 1 76 1 c
1 ? 1 225 I V 1 - 1 20 I 00 1 c
1 1.15             1 57.OB I 65.64         1 37.05 | 43.5}           122.11 1
1 1.15 1 55.B3 I 64.20 1 J6.10 I 41.52 1 22.68  I
1 10       t HUBS  201 I V            1 3           1 43         1 54         1 sic                1 1.25             I 48.32 1 60.40          1 32.51 | 40.64           1 19.76           |
jn         |
201 [ V            1 I            1 19         1 60         1 c
1 1.25              1 52.78 | 65.98           1 37.63 | 47.04           I 18.94            I
1 iz        1 Mvns  206 1 V            1 5           1 45         1 52         1 SIC              1 1.25             1 44.IS 1 55.48          1 26.10 I 32.73          1 22.75           I 1 u         I rtMS   206 1 V            1 5           1 37         1 SB        1 Sic                1 1.25             1 49.67 | 62.99           1 33.61 | 42.01           120.08 |
1 U        1 MWRS105 1 A            1 5           1 51         1 34         1 S1CL           1 1.30             I 40.40 I 52.52           1 23.7} ) 30.85           121.67            | 1 15       1 MWRS105 I A            1 17         |49           tu            I SiC               I 1.15              1 57.59 I 50.75          1 17.96 | 24.25           1 26.50           1
116        1 HkfftS noi a                   1 13         1 43         I 44          1 Sic                1 1.3               t 40,83 f 53.00           1 20.22 | 26.29           1 76.7?           1
1 17
1
I HWH5 110 I A
JI
I 15
L
1 4?
1
1 32
J
1 S1CL       1 1.35
J
J
1 35.85 ( 48.40
J                  1
1   14.07   I   16.99    1      29.41      |
J                 1-
L1
A s Alluvlii
V z Vertisollobio Al.6 Cont
11
1
1
1
1
1
1
I
1
1
1
1
1
1
I BULK
1
I FIELD
1
1
11
1 MJ I CQOf
I san        1«            1 X          1 5           I TEXTURAL I OEhSllY     1 CAPAC IT If        Lp.w.p.
1 SAFO  I SILT     1 CL AV 1 GHO.JP
I
I
1
1
1 hpl
I
1
1 A
1
1
1 2?
1
1
1 U
1
1
1 32
JU—
I IS        1
1
la
1 Ljm/cc
J
1
1 1,37
I Wt.X | Vol.I
1            J
1             1
| 35.30 | 44.96
1 «.K 1 Vol.X
1  t
1 1
I 13.88 | 18.77
1  1
I1
j A.S.H. I
1 »/m |
1II1
1 
1
1 26.19          1
1 19 1 A/SA/J 1 ¥ 1 5 1 47 I 1 sic
f 20 1 V5A/1 1 V 1 23 1 17 1 5B 1 Cl
1 1-30             1 40.41 I 52.53          | 22.60 1 19.38           123.15            1 1 1.15             1 31,3O| 42.as           1 15.52 j 20.95          in.n i
1 21       I A/5A/11       I V          1 2           1 42         1 56         r SiC                1 1.25             1 *3.til 1 53.76          1 34.90 I 33.63          J 2a. o i
1 22 1 A/58/14 1 V I 0.6 I 21,4 1 7A 1 c
1 23 1 A/6/JfJ J * 1 - 1 2B [ 72 1 c
1 1.15 1 54.56 I 64.04 1 35.55 | 40.88 124.16  |
1 1.15 1 49.U8 ) 54.44 | 30.85 I >5.48 1 20.96  I
1 24       | 4/6/Z6          I A           1 0           1 AZ       1 50         1 SiC                1 1.30             1 38.07 | 49.49           1 22.46 1 29.20          1 20.29           | 1 25       I A/6/6B         1 ¥           1 &          13a          1 56         1 C                   1 1.25             1 41.89 I 52.56          1 25.84 | 32.30           I 20.06 I
1 26       1 HS/4/15       1 V          1 7            1 47         1 *6         1 SiC                ( 1.30              1 44.62 I 58.01          | 27.37 I 35.58           1 22.43            | 1 27       | HS/4/li          1 V          1 5           1 43         1 52         1 SiC                1 1.25             1 46.05 1 57.56          1 >9.86 | 37.33           1 20.23           |
1 20       1 HS/3/1         1 A          1 12         1 32         1 36         1 a
1 29       1 >6/V 18       I V           1 6           J 32         1 62         1 c
J ?□       | MS/4/J1       1 V          1 0.6        1 153       1 64         1 c
1 1.35             | 34.49 I 46.56            1 18.95 ] 25.45           1 21.11 1
1 1.20             1 43.88 I 52.66           | 26.04 1 31.25           1 21.41           | 1 1,15              I 59.33 | 68.23            [ J7.83 | 53.50            124.73            I
1 JI       I Hs/4/ia         1 *           1 6           1 44         I 50          1 SiC                11.»                1 41.59 [ 54.07           1 29.00 1 38.74           1 15*3)           1
1
J
J
J
■1 -
L
1
J 
L
J 
1
J JI)
0ItLblB iUJ
Predicted and measured l~.C. mid P.W.P. ^■Iuhh
11              1              i
1 CL AV        1 HOICTEO I HLASUREO                     I
11
1 SOIL        I TEXTURAL        I CONTENT I
1 !
I M)
|
I CtKE
1
I ITFE
1
1 HASS
1
1
1 SIC
1X
1
1 F.C,
1
1 P.h.FJ F.C. 1 p.if.f. [
.1               1              I                 1
1
11
1
1 mts 102
1
1 A
1
i <6
1
1 52.46
Illi
1 77. TO | 52.« I JO. LL I
1 2       I ws ?n8              1 v                1 SIC 1 1       1 HWRS 225      1V                1 c
i 40                 1 51.ZZ                   1 ID.54 | J1.J7| M.*Z I
1 80                j 65.>8
| 4J.90 | 64.201 41.M I
1 4       1 >*HS !□*>       I A                1 SIC                       1 J*                1 47.[B                  I 24.64 I MJ.75 1 24.25 (
1 >      [ MUSS 210        J V               1 Sic
| 40
| 50. IB
1 6       1 V5A/11             1 A               1 SiC                       1 M                 1 56,26
1 7       | A/5A/14             1 V               1 c 1 a       1 M5/5/1             ( A                1 Cl 1 9       I rti/4/jJ              I V               1 c
< 7B r 64.62
1 16 1 46.66
1 27, IH | 51.26 I 28.61 F
1 JJ.9G [ 51.76 I H.6J I
1 41.14 I 65.04 ] 4Q.60 [
1 25.501 46.56 I 25.45 I
I 84
1 50
1 66.90                  1 45 .66 j 6S.2J | 43.KJ I
1 10
|
1  HS/4/10
r
I*
J
I SiC
J
J
( >1.98
1
| JJ .58 | 54.07 ( 16.74 |
J  I L-!
A = Alluvia
V s Vert molAPPENDIX II
References for Appendix t
FAO Irrigation and Drainage Paper No 1
Irrigation Practice and Water Management (1972)
FAO Irrigation and Drainage Paper No 24
Crop Water Requirements (1977)
FAO Irrigation and Drainage Paper No 25
Effective Rainfall (1974)
FAO Irrigation and Drainage Paper No 26
Small Hydraulic Structures (1975)
FAO Irrigation and Drainage Paper No 29
Water Quality for Agriculture (1976)
FAO Irrigation and Drainage Paper No 33
Yield Response CO Water (1979)
International Institute for Land Reclamation and loprovemenc
Drainage              Principles              and              Applications              Publication              16   (1974)
On                      Irrigation                      Efficiencies Discharge Men lureinc nt Structures Publication 20
Publication
19
(1978)
(1978)
United States Department of Agriculture, Soils Conservation Service
National Engineering Handbook Section 16 (1971)
122
v/APPENDIX II
References
Doneen I D 19/2 Irrigation Practice and Water Management
FAO Irrigation and Drainage Paper No I p S
FAO 1965 Report on Survey of the Awash River Basin Vol 1 p 65
IAR 19613/69 Progress Report of the Melka Water Research Station p 18
IILR.F 1974 Drainage Principles and Applications Publication 16 Vol 1 p 43
ItaIconsuit 1969 Melka Sadi - Aaibara Irrigation Project Feasibility Study*
Awash Valley Authority, Addis Ababa
Jensen    M    E    1901    Design    and    Operation    of    Farm    Irrigation    Systems    ASAE    Monograph No 3 p B5
Richards    L    A    and    Wadleigh    C    H    1952.    Soil    Water    and    Plant    Growth*    Agronnray Monograph No 2 p 73
Sweeney     M     D     19     70     Soil     Water     Characteriatics     Important     in     Irrigation.     North Dakota Agr Exp Sts Bulletin No 573 p 6
123APPENDIX UI
GLOSSARY
(EiccerpC from FAO I&D Paper 24. 1977)
ACTUAL CROP EVAPOTRANSPIRATION, ETatcrop): race of *va poErans p t;ta C J’"
j"
nr     seller     than     predicted     ET     crop     a,     affecced     by     the     level     of     available     .ail water, salinity, field lire or other causes; eam/day
ACTUAL    VAPOUR    PRESSURE,    ed:    pressure    averted    by    water    vapour    contained    in    th
air; millibar
ADVECTION,     horizontal     transport     of     sensible     heat     by     air     movement     as     for     instance frofl large, dry fallow surrounds into irrigated areas
ALLOWABLE SOIL WATER DEPLETION, p Sa: depth of soil water in the root zone
readily available to the crop for given soil and climate allowing unrestricted
evapotranspiration as the fraction p of Cotai available soil water between field
capacity (Sfc) and wilting point (Sw); mnj/ra soil depth
AVAILABLE    SOIL    WATER,    Sa:    depth    of    water    stored    in    Che    root    zone    between    field capacity (Sfc) and wilting point (Sw); hud/ m soil depth
AVERAGE INTAKE RATE: rate of infiltration of water into the soil obtained by
dividing the total depth of water infiltrated by the total time from start to
finish of water application; cnm/hour
BASIC    LJXTAKE    RATE:    rate    at    which    water    will    enter    the    soil    when    after    initial wetting of the soil the rate becomes essentially constant; nnn/hour
CANOPY INTERCEPTION: depth of precipitation caught and held by plant foliage and
lose be evaporation without reaching the ground surface; m or sometimes
percentage of rainfaLl
CLOTHESLINE EFFECT: horizontal heat transfer (advection) from warm and dry
upwind are* to a relatively cooler crop field resulting in increased ET crop;
particularly refer* to the field border effects or to patchwork of small
interspersed fields
CLOUDINESS:     degree     of     cloud     cover,     usually     seen     of     several     observations     per     day expressed in oktaa [in eighths) of sky covered, or in tenths of sky covered
CONTINUOUS    SUPPLY:    method    oE    water    delivery    with    continuous    but    often    variable dischetH': in «ter distribution syitan up to inlet of individual faro or field
CONVEYANCE    EFFICIENCY,    Ec:    ratio    between    received    at    the    inlet    to    a    block    of fields and that released at the projects headworks; fraction
CRITICAL    PERIOD;    periods    during    crop    growth    when    soil    water    stress    will    h lasting effect on crop growth and yields
avG 1
CROP COEFFICIENT, kc: ratio between crop evapotranspiration (ET cron) .nd rh reference crop evapotranspiration (ETo) when crop is grown in large fields^ optimum growing conditions, or ET crop • kc ETo; fraction
under
124CROP EVAPOTRANSPiRATlOH, tTcrop: rate oE evapotranspiration o£ a dKeaeeyft’''-
crop xrowing in a large field (one or more ha) under optimal ”°l co7 1 l“
including sufficient water and fertiliser and achieving full pro action
potential of that crop under the given growing environment; includes wate
through transpiration by the vegetation, and evaporation from the soi tu. ace
and wet leaves; ™/day
CROPPING INTENSITY: for a given period the percentage of the total scheme area
which is under a (irrigated) crop, percentage
CROPPING PATTERN: sequence of different crops grown in regular order on any
particular field or fields
CROP WATER REQUIREMENTS: depth of water required by a crop or a diversified
pattern of crops for evapotranspiration (ET crop) during a given period, ca/day
as average for given period
DA'f LENGTH FACTOR, p: percentage p of total annual daylight hours occurring
during the period being considered, percentage
DESIGN FACTOR,or i ratio betwen canal capacity or maximum discharge in a^/spc and
the maximum daily supply requirements during the peak water use period in
flp/day, or * 86400 Qmax/Vmax; fraction
DEPTH OF IRRIGATION, d: depth of irrigation, including application losses,
applied to the soil in one irrigation application and which is needed to bring
the soil water content of root zone to field capacity; mm
DEVELOPMENT STAGE: for a given crop the period between end of initial
(emergence) stage and full groundcover or when groundcover is between 10 and
80S; days
DEWPOINT TEMPERATURE. Tdewpoint: temperature to which the air needs to be cooled
in order to become saturated and at which water vapour starts to condense;
degree Celsius
DISTRIBUTION    EFFICIENCY.    Ed:    ratio    of    water    made    directly    available    to    the    crop and chat released at the inlet of a block of fields; Ed ■ Eb,Ea; fraction
EFFECTIVE   FULL   GROUNDCOVER:   percentage   of   groundcover   by   the   crop   when   ET   crop is approaching maximum - generally 70 to 801 of surface area; percentage
EFFECTIVE     RAINFALL,     Re:     rainfall     useful     for     meeting     crop     water     requirements;     it excludes deep percolation, surface runoff and interception; un/period
EFFECTIVE ROOTING DEPTH, D: soil depth from which the full grown crop extracts
most of the water needed for evapotranspiration; id
ELECTRICAL CONDUCTIVITY. EC: the property of a substance to transfer an
electrical charge (reciprocal of resistance). It is measured in ohms of a
2
conductor which is I cm long and I cm ; electrical conductivity is expressed
the reciprocal of ohms/cm (mhos/cm); 1q hos/cm = I 00Q muhos/cm; lrnci hoa/esi « '
l ODD ^ios/cra
ELECTRICAL  CONDUCTIVITY.  IRRIGATION  WATER.  ECw:  is  used  as  a  measure  of  rhe  s  1
content of the irrigation water; ramhos/cm
iL
125ELECTRICAL CONDUCTIVITY, MAXIMUM, EGmst lb concentration      of      the      soil      saturation      paste
(zero yield); ramhos/cm
used .is a Huie of the salt
(ECe) beyond which growth would stop
ELECTRICAL CONDUCTIVITY, SATURATION EXTRACTS, ECe: m               a* a
salt content of an extract from a soil when saturate « 
conditions ECe - 1.5 ECy and also is approximately half the salmi . water to which the crop is actually exposed in the soil, mhos, cn
_ i-lirw r the soil
ihos/cs
EVAPORATION,    E:    rate    of    water    loss    from    liquid    to    vapour    phase    from    an    open
water ar wet soil surface by physical proctfuca; am/day
EVAPOTRANSPIRATION:     rate     of     water     loss     through     transpiration     from     vegetation
plm evaporation from the soil, mm/day
EXTRA-TERRESTRIAL    RADIATION.    Ra:    amount    of    solar    radiation    received    on    a
horizontal at the top of the atmosphere; equivalent evaporation earn/day
FIELD    APPLICATION    EFFICIENCY,    Ea:    ratio    of    water    made    directly    available    to    Che crop and that received at the field inlet
FIELD    CANAL    EFFICIENCY,    Eb:    ratio    between    water    received    at    the    field    inlet    and that at the inlet of a block of fields; fraction
FIELD CAPACITY, Sfe: deth of water held in the soil after ample irrigation or
heavy rain when the rate of downward movement has substantially decreased,
usually 1 to 3 days after irrigation or rain: soil water content ac soil water
tension of 0*2 to 0*3 atmosphere; mm/re soil depth
FIELD    SUPPLY    SCHEDULE-    stream    size,    duration    and    interval    of    water    supply    to    the individual field or farm
FIELD   WATER   BALANCE   J   sub      of   all   gains   and   losses   of   water   over   a   given   period   of time; nun/period
FLEXIBILITY FACTOR, C. coefficient greater chan one to account for tluctuations
in water supply in excess of thoae determined for Jn assumed cropping 3flrtern
and cropping intensity
FULL GROUND COVER: soil covered by crops approaching IOOX when looking downwards
GROUND COVER: percentage of soil
directly overhead; percentage
surface shaded by the crop if the Oun vere
I
i
I
I
GROUNDWATER TABLE: upper boundary  of groundwater where to  atmosphere, ie depth of water level in borehole whe enter the borehole; cm below soil surface
GROWING SEASON:
for a given crop the time between planting or
harvest; day*
INITIAL   DEVELOPMENT   STAGE:   for   a   given   crop   the   time   during growth when ground cover is les* than 10Z; days
water pressure is equal
n groundwater can freely
sowing and
£ erainacion or early
INITIAL INTAKE RATE:
applied; ma/hour
126IRRIGATION   INTERVAL.   i:   tim*   between appILe»tidm an the sane field; days
the start of successive Field irti^Hon
IRRIGATION REQUIREMENTS: depth of water required tor netting •’**' minus contribution by effective precipitation, groundwater, stored soil wate . required far nornval crop production plus leaching require
and operational wastes, scmetiinea called gross irrigation
ram/period
LATE SEASON STAGE: time between the end of the raid-season
maturity; days
LEACHING     EFFICIENCY,     Lei     fraction     of     the    irrigation    water control (Leaching) which was effective
reqa £rente at 5;
atage and harvest or
applied for salt
LEACHING REQUIREMENTS, LR: fraction of the irrigation water entering the soil
that effectively must flow through and beyond the root zone in order to prevent
■salinity build-up, This value is the miniffliu'ra amrount of wat?r un’Ce-usnry to
control salts; fraction
LEVEL     OF     SUPPLY:     selected     water     supply     on     the     basis     of     probability     to     wet     crop irrigation requirements
MAXIMUM   NUMBER   OF   BRIGHT   SUNSHINE   HOURS,   N:   number   of   bright   sunshine   hours   for a 24 hour day wlth.no cloud cover; hours
METHOD OF SUPPLY: method of operating an irrigation system to convey water from
the source of supply and to distribute it according co crop requirements to each
field served by the syiteni
MID-SEASON     STAGE:     for     a     given     crop     the     period     between     effective     full     ground cover and the onset of maturity (ie leave* start to discolour or fall off); days
NET IRRIGATION REQUIREMENT, In: depth of water required for meeting
evapotranspiration minus contribution by precipitation, groundwater, stored soil
water; doe* not include operation losses and leaching requirements; ' mm/pariod
NET    LONGWAVE    RADIATION,    Rnl;    balance    between    all    outgoing    and    incoming    longwave radiation; almost always a negative value, equivalent evaporation mm/day
NET RADIATION, Rn: balance between all incoming  and ou-goinfl 3h<Jrt ;ifld lon^aVfl
radiation; Rn ■ Rna + Rnl; equivalent evaporation mu/dty 
NET SOLAR RADIATION. Rns: difference between gh earth1! surface and :hat reflected by  rhe soil, equivalent evaporation nm/day
fa
□rtwave radiation received on the
crop or water surface;
OASIS EFFECT; effect of dry fallow surrounds an the micro-climate of relatively small acreage of Land where an air nSS, moving into an Irnt. * a will give np sensible heat. For small fields this hm   result in ', h ® °d '‘res
z
crop .,s compared to predicted ET crop using climatic data collected insid L
irrigated area; conversely ET crop predictions baaed on
weather «
3a
outside the irrigated fields may aver-predict actual evapotranspirationOSMOTIC PRESSURE: equivalent negative pressure co whic
to bring the saline soil water through a serai-permeable equilibrium with pure water; atmosphere
PAW  COEFFICIENT, kp: ratio  between reference evapct
w^i
ter jflH
aLb
e
raeff
lbr’a.tld 
into
subjected static
ranapi
rat
run
ETo
A
nd water
loss    by    evaporation    from    an    open    water    surface    of    a    pan    or    ETa    °    *P
fraction
PAN    EVAPORATION,    Epan:    rate    of    water    loss    by    evaporation    from    an    open    WJ"
surface of a pan; mm/day
PEAK OR MAXIMUM SUPPLY, Vmx: average daily uupply requirement during the pi*ax
water use period for given crop or cropping pattern and climate; ra /day
PEAK    SUPPLY    PERIOD:    water    uae    period    for    a    given    crop    or    cropping    pattern    during the month or period thereof of highest water requirements; rm/day
PLANT POPULATION: number of plants per unit of crop area
PRECIPITATION: total amount of precipitation (rain, drizzle, snow, hail, fag,
condensation, hoar frost and rime) expressed in depth of water which would cover
a horizontal plane if there is no runoff, infiltration or evapotranspiration;
□m/day
PROJECT     EFFICIENCY,     Ep:     ratio     between     water     made     directly     available     to     the     crop arid that released at project headworks; Ep - Ea.Eb.Ec; traction
PSYCHROMETER: device to measure air humidity; normally conaiatin^ of two
standard the rmome tert, one of whose bulb is surrounded by a wet loufllin bag and
is called wet-bulb thermometer; both should normally be force-ventilated and
shielded against radiation (Asamann type)
READILY AVAILABLE SOIL WATER, p Sa: depth of soil water available for given
crop,       soil       and       climate       allowing       unrestricted       evapotranspiration       and       crop       growth; equals allowable soil water depletion; rnra/m soil depth
REFERENCE CROP EVAPOTRANSPIRATION, ETo: rate of evapotranspiration from an extended surface of 8 tu 15 co tall, green grass cover of uniform height
actively growing, completely shading the ground and not short of water; nt/dav
x 
Epan;
RE1LECIION COEFFICIENT, also called albedo,ot
ratio between the amount of
shortwave radiation received at the earth's surface and that reflated back
RELATIVE HUMIDITY, RH, RHmax: actual amount of water vapour in the air re I ■ to the amount of water vapour the air would hold when saturated at the temperature; RHntx: mean of nmximum RH of each day over the period considered;
ROTATIONAL SUPPLY’ supply of water rotated amongst laterals field inlets at varied intervals
«ub-laterals ar
SATURATION  VAPOUR   PRESSURE,  ea:  upper  limit;  of saturated at given air temperature; millibar
vapour pressure at Jr when air iSEASONAL IRRIGATION REQUIREMENTS: Sctal depth of water, minus contribution by precipitation, groundwater, stored soil water, surface rune require
nornwl crop growth during the crop growing season; um and period
SOIL HYDRAULIC CONDUCTIVITY, k; rate of water flow through a unit cross-sect loo of the soil under a unit hydraulic gradient; also called permeability or
transmi 9 a ion; ma/J ay
SOIL INTAKE (INFILTRATION) RATE: instantaneous rate at which water will enter
the woil
SOIL    SPECIFIC    GRAVITY,    As:    ratio    of also called bulk nensity; g/cm^
the weight of water-free
soil to           its vol□□>■;
SOIL   STRUCTURE:   arrangement   of   soil variety of recognised shapes, sizes
particles into  aggregates which occur
and strengths
in a
SOIL TEXTURE: characterisation of soil in respect of its particle size and
distribution
SOIL WATER CONTENT: depth of water held in the soil; to/ih soil depth
SOIL WATER DEPLETION FRACTION, p: fraction of available soil water (Sfc-5w) that
can be taken by the crop permitting unrestricted evapotranspiration and crop
growth; fraction
SOIL WATER STRESS: sura of soil water tension and osmotic pressure to which water
Bust be subjected to be in equilibrium with soil water; also called soil water
potential; atmosphere
SOIL WATER TENSION: force at which water is held by the soil or negative
pressure or suction that must be applied to bring the water in a porous cup into
static equilibrium with the water in the soil; soil water tension does not
include osmotic pressure; also called metric potential; atmosphere
SOLAR RADIATION, Rs: amount of shortwave radiation received on a horizontal
plane tit the earth’s surface; equivalent evaporation mm/day
STORED SOIL WATER, Wb: depth of water stored in the root zone from earlier
rains, snow or irrigation applications which partly or fully meets crop water
requirements in following periods; nun
STREAM SIZE, q: flow selected for supply to field inlet or irrigation block;
1/iac or m^/sec
SUNSHINE HOURS, n: number of houra  of bright lunshine per day, aL
ao
sonetiws
defined    at    Che    duration    of    traces    or    burns    made    an    a    chart    by    Campbell    Stokes recorder; hours
SUPPLY DURATION, t or I: Length of time during which a given stream size is delivered to the field or farm (t) or irrigation block (T) during any part of
the irrigation season; hour or day 
K
SUPPLY DURATION FACTOR, ft: it T/l or for a fixed or mscatit canal disehar
the ratio between uupply duration in dnyi and the supply interval in davS d^r
any part of the irrigation seaion; fraction 7
ln 2
129SUPPLY    FACTOR,    fsl    is    Q/qiwit    ar    ratio    between    actual    and    maxi    touts    possible    supply in mVsec; fraction
SUPPLY    INTERVAL,    i    or    I:    time    interval    between    the    start    of    successive
irrigation supplies at field or fnrm (i) irrigation block or sector (I), ays
SUPPLY    ON    DEMAND;    irrigation    water    supply    to    satisfy    need    for    irrigation    water at any stream size. duration and interval during Che growing season
SUPPLY REQUIREMENT FACTOR, ft: is V/Vmax or ratio between average daily supply
requirement over a. given period and maxi muni daily supply requirement during the
period of peak water use in m^/day; fraction
SUPPLY     SCHEDULE:     stream     size,     supply     duration     and     supply     interval     of     irrigation water supply to field or irrigation block, during the growing season
TENSIOMETER: a device for measuring the tension of soil water in the soil
consisting of a porous, permeable ceramic cup connected through a tube tn a
manometer or vacuum gauge
TOTAL AVAILABLE SOIL WATER, Sa « (Sfc - Sw): depth of soil water available in
the root zone to the crop; difference between field capacity and wilting point;
mm/m soil depth
TRANSPIRATION: rate □£ water loss through the plant which is regulated by
physical and physiological processes; mm/day
WET BULB TEMPERATURE, Twetbulb: temperature recorded on a thermometer whose bulb
is surrounded by a wet muslin bag. thus lowering the temperature by loss of
latent heat through evaporation; degree Celsius
WET    BULB    DEPRESSION:    difference    between    simultaneous    readings    of    wet    and    dry bulb thermometers; decree Celsius
WILTING POINT, Sw: depth of soil water below which the plant cannot effectively obtain water from the soil; soil water content at 15 atmospheres soil water
tension; min/m soil depth
WIXOSPEED, U : speed of air movement at 2 m above ground surface in unobstructed
surroundings; mean ltim/sec over the period considered, or total wind run in '
kn/day
2nGURE 5 2
*40Qi *300
SOD
•200
\
\
MAY
JUN \
JUL
AUG
SEP
OCT
X
-100
-200
-300
-400
-500
-600The moat common conditions assumed la the Penman equation are medium to high radiation and maximum relative humidity and moderate day time wind double the night time wind. These conditions are not always me". For instance in the
winds are equal and maximum relative Project area daytime and night time i------------------------
humidity is about 502 more than the mean relative humidity.
A correction factor
is     required     for     such     conditions     and     table     4.     11     gives     the     value     of     c     for
different conditions of RH twx, R ,
5
area.
□ day and 0 day/U
For July  Rh max  =*  75X, Rs ■ 8.27  mm/day, u day  «  2-57
1 Hence c ■ 0.97.
Therefore July ETo - 0.97 (3.71 ♦ 3.58)
■ 7.07 mm/day
night for the project
m/sec and U day/U night
Values     for     ETo     on     a     monthly     basis     are     given,     together     with     the     calculations,     in Table 4-2.
Penman values of ETo  combined with suitable crop coefficients were used in determining  irrigation regimes for different crops, especially  bananas and cotton grown in the project area.
4.5 Crop Coefficient
Kc values for crops grown in the project area compiled from FAO Irrigation and Drainage papers 24 and 33 and those for bananas adjusted to suit observed fwnthly water demands, are given in Table 4.3. The growing season for each crop has been divided into five stages, namely seedling (initial), active vegetative growth (crop development), reproductive (mid-season), fruit formation (late season), and maturity (harvest). There is no data available at the Institute of Agricultural Research on actual crop coefficients applicable to crops grown in the project area. See also section 4.3.3.
Table 4.3
Crop
Seedling                Vegetative Reproductive
Fruit
Formation
Maturi cy
Bananas
0.45
0.85 ‘
1.10
Peppers 0.4
0.80
1.00
Cotton
0.45
0.75
1.15
Haricot) 0.35
0.75
1.10
Beans )
Maize
0.40
0.80
1.10
Onions
0.50
0.75
1.05
Sesame
0.45
0.75
1.00
1-00
0.90
0.85
0.70
0.85
0.85
0.70
0.80
0.8Q
0.65
0.30
0.60
0.80
0-30
The     corresponding     days     for     each     stage     of crops'in Sections 4.7.1 to 4.7.7.
development is given fOr the
sfcven4-6 Some Crops Grown in Project Area
4.6.1                Banana
Banana can be grown on well drained fertile land with a wide range of soils.
Optimum pH is between 5 and 7. Bananas are very sensitive to salinity and soils
having an electrical conductivity of less than 1 amhas/cm are required for good
growth.
The total crop water requirements for bananas are high, it being a long duration
crop. Annual water requirements vary from 1200 uhd to 2200 mm * the former in
humid tropics and the latter in the dry topics.
Water Uptake
The banana plant's root system is sparse and shallow, with the majority of
feeding root spread laterally near the surface. The rooting depth generally
does not exceed 0.75 m. Normally 100Z of the water is obtained from the first
0.5 co 0.80 m soil depth with 60Z from the first 0,3 m. Generally 351 of the
total available soil water should not be depleted.
Irrigation Scheduling
Frequent irrigation is important if the available soil moisture depletion is to
be limited to 35Z. Irrigation interval will depend on the soil water holding
capacity in the rooting depth and the maximum evapotranspiration.
4.6-2                Cotton
Cotton can be grown on a wide range of soils but preferably on medium and heavy
textured deep soils with good water holding characteristics. Root penetration
is restricted by acid or dense soils. The pH range is 5,5 to 8 with 7 to 8 as
ootimun. Cotton is tolerant to soil salinity with no percentage yield decrease
if ECe ■ 7.7 mmhos/cm.
Cotton    needs    700    to    1300    mm    of    water    depending    on    the    climate    and    length    of growing period.
Water Supply
Cotton requires adequate water for vigorous growth and formation of healthy bolls. Root and crop development is restricted by excess water during early stages of growth. Just prior co and during bud formation an adequate water supply is necessary. However, if this water supply is continued during flowering and yield formation prolonged and excessive growth will result. Water stress on cotton is noticeable by the bluish green colour on the leaves and discolouring of the stem.
The entire root zone of 1,8 m should be fully wetted before sowing and subsequent irrigations should wet the upper part of the root zone from 0 50 co 1.0 «. A 70Z depletion of the total available soil water during Ei  werin« will
O
be a saving on irrigation water without iopairing yield. During the vegetative period, soil water content over the first 0.75 x of root depth should not fall
2&TABLE 3.1
Textural Quick; for Keedily Available Mai a Cure
Available Soil
Moisture Remaining
0 * 
25 percent
25 -
50 percent
50 -
75 percent
75 - 100 percent
100 “ percent
(field capacity)
Saturated
Silt Loan
1 Medioni Textured)
Powdery, sametinea slightly crusted but easily broken into powdery condition*
Somewhat crumbly but will hold together
from pressure.
Fore- a ball, somewhat plastic will slick slightly with pressure.
Forme a ball, very pliable, slicks readily.
Upon squeezing, no free water appears on
■oil! but wet outline of ball is left on hand, soil will stick to rhumb when rolled between thumb and forefinger.
Free water appears on soil when squeezed.
Silty Clay Uiaaa and Clay
Loam (fine textured)
Hard,      baked,      cracked:      difficult      to      break down into powdery condition.
Somewhat pliable, will ball under pressure.
Forms    a    ball,    will    ribbon    out    between    thumb and forefinger, has a slick feeling.
Easily     ribbons     out     between     thumb     and forefinger, baa a slick feeling.
Same as silt loam.
Same aa silt loam.Two J inch parshall flumes are installed in a furrow 25 ® apart. The initial reading of the upstream flume is 4.5 cm and that of the downstream flume 3.9 cm. After 10 minutes the readings are 3-7 and 3-2 cm respectively. The corresponding flow races are 1.45 and
1.07 l/s for the upstream flume and 1.16 and 0.85 l/s for the downstream flume. The differences in inflow and outflow are 0.29 l/s and 0.22 l/s per 25 m.
The furrow width is 90 aa.
Hence furraw intake rate
-        0.29           x 60 x 60        » 4.64      cm/hr
initially                                      10
0.9 x 25
and
- 0.22
x 60 x 60
= 3.52
cm/hr
after 10 minutes
10
0.9 x 25
Intake      rates      in      cm/h      are      plotted      versus      cumulative      time      on      log-log
This gives a  straight line with the equation I • kTn
where I a intake rate of soil in cm/h
k ■ intake rare intercept at unit time
T ■ time water is on surface of soil
n ■ slope of Line (scaled vertical height divided by scaled
horizontal length)
——
paper.
---------- (7)
16Hence      it      is      evident      that      the      field      drains      have      sufficient      capacity      aven      if      this method vert to be used.
2.12 Roads
About 90 km of gravel surfaced roads have been constructed of which 40 km are principal roads and 50 kn service roads. Unsurfaced farm roads extend to approximately 140 km.
2-13 Flow Chart
The flow chart given in Figure 1.1 indicates schematically how water once diverted at the Melka Sadi weir finds its way through a network of canals and appurtenant structures to the ultimate field unit.
9where d •* Equivalent depth of water in centimeters
P * Change in soil moiature percentage from field capacity
V - Bulk density
□ « Depth of soil ia centimetres.
Soil moisture should be determined at regular intervals to determine the consumptive uie, frequency of irrigation and depth of application required tor each irrigation.
Irrigation water requirement at field level is therefore determined by the
equation
d p.S-a.D tnm ------------------- ------------------------------------------------ (5)
Ea
where p - fraction of readily available soil water
permitting unrestricted evapotranspiration
Sa ■ total available soil moisture in mra/tn soil depth
D * rooting depth, in a
Ea - field application efficiency
Frequency of irrigation or irrigation intervals are worked out using the
formula;-
i ■ p. Sa»D
ETc
where l - irrigation
ETC ’ crop water
3.9 Infiltration Rates
day s
interval in days
requirements, in mm/day
The time required to vet soil and distribute the moisture within the soil
profile ii to some extent, determined by che W4Cer iatake raC£
water at the soil surface is measured as infiltration rates.
infiltration        takes        place        □osc        rapidly        when        water        i.        first decreases to an almost constant rate.
'
Infiltration rates should be determined at least once a week for the rwo twr,
of Kiln the project area as the structure of the soils may chan„
cultural practices.
3.9.1 Determining Infiltration Rates
Infiltration      in      che      rield      can      be      determined      by      measuring ponded water. The rate of fall of ponded waJr
y
applied and graduaUv
graauaiiy
s with
fate of
concentric cylinders or furrows. The Meho(1
*             determined by
application.
infiltration        rate        should        simulate        the        mechanic        c        *"        aeasurin8        soil
^chanism of water inCak(.
fall        of using
ng
14The most common method is the use of a "double ring" infilCrowe ter which is in fact two concentric metal cylinders. The metal cylinders are driven into the
soil up to 5 cm using a level wooden batten and a hammer.
A aca  e
j  reading
graduated in centimeters and millimeters is fixed on to the inside of Che inner ring with its zero at the soil surface inside. A polythene sheet is then used
to line the sides and bottom of the inner cylinder so that when it is filled
with water no infiltration cakes place till the polythene is quickly extracted. The space between the outer ring and the inside ring is filled with water to the same level as the water in the inner ring. The polythene lining is now extracted, gently and quickly. The initial reading of Che water level on the scale and subsequent readings at regular intervals as the water level drops are recorded on Form 3.2.
Infiltration data is then plotted with the intake rate in cm/hr versus time in minutes on ordinary graph paper. The final infiltration rate is the flat part
of the curve after two or three hours.
Examples:- Initially the depth of water in the infiltrometer is 19 cm. After
60 seconds the level drops to 18.8 cm and after two minutes it drops to 18.7 cm. The difference in reading for the first 60 seconds is
0.2 cm and for the second interval of 60 seconds it is 0.1 cm.
Hence the initial intake rate ■
and the second intake rate ■
0,2 x 60 x 60 » 12 cm/hr
60
0.1 x 60 x 60 ■ 6 cm/hr
60
The initial moisture content of the soil profile at 30 cm intervals up to 100 cm
should be determined.
Figures 3.2 to 3.5 show typical curves for the measured infiltration rates for
Melka Sadi and Amibara areas. It wil be seen that when the surface soil is dry
Che basic infiltration rates are high compared to those when the surface soil is
moist. This is a result of the moist top soil and lower strata reaching
saturation point faster than when the soil is initially drier.
As an alternative to  the "double ring" infi1trometer method, infiltration can be determined by  measuring  the run-off from real or simulated rainfall. When using the real or simulated rainfall system, the rainfall as well as the run-off for a known area  and given period of time are recorded, the infiltration will then be rainfall minus run-off in mm divided by the time in hours.
The above m
irrigation.
]
furrows acting  as buffers for the centre taken. Inflow and outflow of water, for measured at regular intervals. The soil then converted into  an infiltration rate
by the furrow.
in Chapters 6.3.2 and 6.3.3.
“ -- T* 
thods are suitable for determining infiltration races for basin Determination of infiltration rates for furrow irrigation are given
sential to use three furrows, the external
furrow in which all the measurments are a pre-determined length of furrow, is intake at any time in litres/sec is
in cm/hr for the width of soil covered
155.4      Effective Rainfall (Pe)
5.4.1
Definition of Effective Rainfall
Effective rainfall (Pe) is defined as that portion of mean
the strewing
rainfall       th"       ia       utilised       for       meeting       crop       water       r^uye^nrs       <hi       ng       t-       ,L„n lewn □£ a crop. Not all rainfall that is recorded during .he
can be effective and part of it is lo" by surface runoff, deep percoleti evaporation. The effectiveness of rainfall ii 3 function or its in e
the extent of crop cover. Light rains on a fully vegetated area can he 100Z effective whereas intensive rains (25 to 30 ran in a short period) may be only
60Z effective on an area with iatenaitcenc crop cover.
Due co the variability of rainfall both seasonal and annual, especially in
semi-arid and arid conditions, rather than using mean rainfall data ( ie one year drier, the next year wetter), a dependable level of rainfall should be selected
in estimating Pe. The leveL of dependability can be determined statistically
(see Section 5.4*4) to derive rainfall depth chat can be expected in 3 years out of 4 (75X exceedance probability) or 4 year? out of 5 (80S axceedance
probability),
The isethod that is conononly used for estimating Pe is based on dependable rainfall and taean evapotranspiration and was developed by US Soil Conservation Service. It is alac described in FAO Irrigation and Drainage Paper tfo 24 (sec
Section 5.4.5).
5.4*2              Effect of Soil Characteristics on Effective Rainfall
Soil acts as a  reservoir for moisture co  the absorption, retention, release and movements
effective rainfall.
Infiltration rate and permeability
characteristics. With an increase
of a soil j
crops.          Therefore          its          properties          of of water have a direct bearing an
are its intake and water movement
in the water holding capacity of
fraction of effective rainfall increases* ~
higher the proportion of effective co tocal rainfall*
in a soil also has a bearing on the extent of effective rainfall soon after an irrigation the aoil
rainfall is lost to surface runoff*
5*4.3 Effect of Crop Characteristics on Effective Rainfall
-
_ 
The greater the soil depth, the
■ a soil , the
Initial floisture content
rainfall* If there is
cannot store any more water and the
Crop characteristics influencing
rooting depth and growth stage*
when the crop matures and roots
rooting crops will increase the
intake of « „ are the degree
C
Qf ground covef
Soil moisture ln deeper layer, is Capped otQ penetrate to these depths. Therefore deep extent of effective rainfall.
5.4.4  Estimation of Dependable Rainfall
Monthly rainfall data for 17 years,
Research, Melka Warer, were ranked
of each ranked value was determined
i:r:h
tuce o£
using the Weihull plotting 
,
POSltlDn
The ranked monthly «infalls -ere plotted against respective olaef.. on a normal prob t>> Lity paper for individual months
fl
Pitting position
751 probability or 3 years out of 4 rainfall Ua3 computed i3 ^'1° 5'13) and thep
X 100
---------  (20)
where P ■ plotting position
N = total number of events
m « rank of each event
Month
Mean Rainfall
Probable Rainfall
nns/month
3 out of 4 years
□xn/month
January
17.a
0
February
47.2
0
March
58,5
30.0
April
50.4
26-5
May
24. L
6.5
June
23.9
3-5
July
110.8
85.0
August
114.8
78.5
September
38.2
24.0
October
21.4
0
November
23.4
0
December
2.7
0
The 75Z probable rainfall is used in section 5.4.5 in computing the effective rainfall for seven selected crops, with storage factors relative to the readily available soil moisture in the root zone at progressive stages of growth.
5.4.5               Method of Determining Effective Rainfall (Pe)
By processing long term climatic and soil moisture data, the Unites States Department of Agriculture Soil Conservation Service has developed a procedure for estimating effective rainfall. This is termed the evapotranspiration/ precipitation ratio method. (FAO Irrigation and Drainage Papers 24 and 25).
Table 5.1 gives the relationship between average monthly effective rainfall and mean monthly rainfall for different values of average monthly consumptive use. The values of effective rainfall obtained from the table have to be multiplied
by a storage factor which adjusts for the effective depth of storage- These
factors are presented in TabLe 5.2.
For exampLe, from Section 4.7,2 the mean monthly consumptive use for cotton during August is 179.3 am and from Section 5.4.4, the 751 probable rainfall is
78.5    mm. Assuming the storage of moisture per metre of root zone is 220 mm, the soil moisture storage for a rooting depth of 120 cm with 50Z depletion is
50Z x 220 x 120              ■ 132 mm/mcnth.
100
From Table 5-1 the monthly effective rainfall is 63.5 mm and the storage factor from Table 5-2 is 1.04. The mean monthly effective rainfall for August is therefore =■ 63.5 x 1.04 * 66-0 erm.
45
/lAHLt 5.1 AvcruQe Hwrillily effective Rainfall
Month ly
Mean
mb ReJutird to Mean Monthly Ha in Ta 11 and Mean btonthly Cunauoptive Hue (USD*. 5CS)
He Ji ii     Monthly Conant ivo Ua* mm
Rq in-                 25               50               75               iLMJ           125                 150               175              200              225               2 Ml             215         30U          325           350
fill m
12.5
25.0                  15.0             16,2             17.5             IB .0           16.5                19.7             20.5              22,0             24.5             25.0               25.0       25.0         25-0          25.0
37. 5                 22.5             24.0             26.2              27.5           28.2               29 .2             30.5              33.0             36-2             37.5              37.5       37.5          37.5          37.5
Mean  Monthly Effective            Rainfi 11 mm
7.5 a.Q 0.7 9.0 9.2 ia.o 10.5 11.2 11.1 12.5 12.5 12 .5 12 .5 12.5
5U Cl 25 32-2 >4 .5 15.7 36-7
£Z. 5 ul 4177 39-7 42-5 44.5 44.0
39.0 40.5 43.7 47.0 50.Q 5O.0 50.0 50,0 50-0
40.5 50.5  ' 53.7 57.5 62-5 62.5 62.5 62.5 62.5
75 .0
B? .5
IUD .0
112.5
125 .0
137.5
150.0
162,5
i?5.D
50.0              56.7              t£. 2           63.7               66.0             69.7               73.7             77.7             04.5              07,5       87.5         B7.5          07.5
■t S0T7            63.7              £7.7           72.0                74.2             70,7              83,0             07.7             95.0            100        100          1D0          100
70.5              75.0           00.2                02.5             87.Z              92.7             98.0           105               111        112          112           112
75.0              61.5           07.7                90.5             95.7            102              L0B              ID                121         125          125           125
46.2             45-7              52-7           55.0                57.5
61.7             47 .5             73-7             75 .0       75.0          25.0          75.0
ut 122
00.7           95.2                90.7           104               111               ns                126               132        137           137           137
JOO            109                    ID                120               128              135               145              D3          160                162           162 Bi    w      115                 120               127               135              143               154               164        17D               175           175
95.2        102                 Utt
ill                  DO              127               116              143         150          150           150
16?.5                                                                                                121                 126               134              142               151               161               DO         179          105           10?
?D0.11
225
250
275
WO
125                 133               M D             14B              150               l£B               178        180           196           200
at IM                m                151               160              171               182
150               161               170              103               194
ui ?40                 171               101               194                205
175              19U              203               2D
175
ait 2 07
m
213
224
350
200
220
232
375
nt
3 IT"
at M?~
225
240
40 D
247
425
250
ill
450 25 50 75 1DL1 125 150 175 200 225 250
TaMu 5.2
EftccLi'.e StofagE           20         25         37.5
5U            62-5             75         iqo         125         150          175         2LiU
Stcr&ge Factor            0.73        0.77           □ .86         0-93            0-9?          1.00         J.flZ       1.04        1.06         1.07        1.00Tibie 5.3           Effective Rainfall Eor seven selected Crops
Crop
Bananas
Cotton
Cot con
Peppers
Haricot Beans
Mai re
Maize
Maize
Onions
Sesame
5.5 Leaching Requirements
Effective Rainfall
253.91
167.67 - 3rd May planting 162*97 - 15th May planting
61.52
0
25.59
133.06 - 1st July planting
70,90 - 1st August planting
0
45,31
Salinity of che soil is mainly affected by irrigation water quality, irrigation
practices and methods, soil conditions and rainfall. Leaching requirement (LR)
is therefore the minimum amount of irrigation water that must be drained through
the root zone to control soil salinity at a given specific level. Figure 5.1
shows the effect of saline soils on the germination of cotton seed and height of
seedlings according to data available at the Melka Warer Institute of
Agricultural Research
For     clay     loam     soils     with     good     drainage     and     low     rainfall     rhe     leaching     requirement is given by the equation:-
LR
where ECw
ECe -
5 ECe - ECw
EC of irrigation water mmhos/cm is an average of 0-35 mnhos/cm for Awash River water.
ECe of the soil saturation extract for a given crop appropriate to a tolerable degree of yield reduction
©mhos/cm.
For a 1001 yield potential the value of ECe is given below:-
Crop
Bananas
Cotton
Peppers
Onions
Mai ze
Beans
Sesame
ECe mmhos/cm
1.0
7.7
1.5
1.2
1.7
1.0
1.5
The average value of ECe is
grown in the project area.
taken as 1.5 nunhos/CII1 for crop# other
=• 0.049
than cotton
LR ’
0.35
5 x 1.5 - 0.35
47Far eat ton LR -____________0+35
5 x 7+7 - 0+35
0.009
The efficiency of leaching varies with the soil type and its internal Irai R
5+6 Net Irrigation Requirements
The     net     irrigation     requirement     (tn)     for     a     crop     is     the     depth     of     application     of
water aver the entire growing season co supplement rainfall store soi
and contribution from groundwater if any. This can be represented y-’
In - ETcrop - Pe - Ge - Wb-------------------------- ------- -------------------------------------------<22?
where     In     *     net     irrigation     requirement-     The     ocher     terms     in     the     equation     are defined in para 5.3.
5.6.1 Cross Irrigation Requirements
Other than for meeting the net irrigation requirements, water ia needed for Leaching and application losses. Hence gross irrigation requirement is given by
the equation:-
L-LR Ea
where Ig =■ grass irrigation requirement
Ea ■ application efficiency
LR - leaching requirement
The gross irrigation requirements for the seven selected crops neglecting scored soil moisture and application efficiency but including leaching requirements only are as follows and may be considered as a net requirement for crops grown in the project area.
e
Crop
Bananas
Cotton
Cottan
Peppers
Haricot Beans
Maize
Maize
Maize
Onions
Sesame
ETc
ram/season
Pe
mm/sea:
2483.23
891.62
876.89
612.73
474.66
624 + 24
684.52
67 3.46
570 + 80
492.34
253*91
167.67
162-97
61.52
0
13. 59
133.06
70.90
0
45.31
5.7 Depth or Application
Depth      of      application      d,      ls        the      depth      of      watec      required      tq         reoienish available      *dl      cwnrure      after      the      allowable      depletion      and      is      zlvph        ' where     P        ia     the     fraction     of     rhe     total     available     soil     moisture     Chi     h by       the       crop       without       causing       stress.       The       allowable       depletion       level the crop development stage and evaporative demand.
sbb/ sea son
2344
7 30
720
790
499
6 32
580
6 34
600
470
the readily
P-Sa.D.      ,
can be used
varies with
48Therefore d = p.Sa.D
Ea
—---------------------------------------
(24)
wh
ere
d
’ depth of application in mm
P  *  allowable depletion fraction
Sa
■ total available soil moisture in ra/'m
D•
rooting depth in ®
Ea
= application efficiency X
Table
5-4 
gives      the      soil      water      depletion   fraction (p)            for cotton at different
ETcrop.
Table 5,4
ETc ma/day
P
5.8 Irrigation Interval
6
.55
7
.50
Timing of irrigation application is of utmost importance as delayed irrigations could affect yields. Timing of irrigation should conform to soil water depletion requirements of the crop. Therefore, during the crop growth, flexibility in time and depth of irrigation should be maintained to accommodate differences in crop water needs. This method is preferable to having fixed schedules. However, in the absence of data on soil characteristics for the
whole project area, a partially fixed schedule was adopted based on estimated crop water requirements and stages of growth. The schedule used for cotton grown in the project area appears to be well suited to overall soil characteristics as presently known and climatic conditions encountered and is
given below:-
Days from planting
Gross depth at application cm
Interval (days)
- 12          2 30 51
28.6 13 13 13
14          28        21
65        79
13        13
14        14
93       114          135
13           13        13
14        21           21
Yield response to water during the 1980 and 1981 cotton season was more chan expected (over 35q/ha) being well above the yields predicted in the appraisal of
April 1977.
The irrigation interval is given by the equation:-
i 21 p.Sa. D
ETc
5.9 Calculation of Irrigation Schedules
(25)
An     irrigation     schedule     should     be     made     for     the     crop     using     rhe     monthly     soil     water balance and allowable soil water depletion data.
Example I (Alluvia Soil)
The cotton growing season in the
planting date is 15th May and the growing period of** 220      mm is applied 8 days before planting to fill
project area is              May
145
of the
and to
through October. The
days. A net pre-irrigaeion
the soil moisture reservoir
--■-
root      -gone      to      field      capacity,      for      efficient      weed      control      prior      co      planting flush down the salts which may have been carried up by capillary action
during the dry season following harvest of the previous crop, and better yield
49according  to  data  available at the
Research.
1.        ETo no/ day
2.       Day s
3 + Kc
4.         ETc an/month
5.         Probable rainfall
Melka Warer Institute of Agricultural
O
L. 1/.80          .80/.65            .65
June
Jul
AugUBt
Sept
ct
7.33
8.18
7-07
6.38
6.40                6.70
15                    254/5
31
1.1
24/7                23/7
8
.45
+75/1-1
49.48                198-37
24U09
91.56
204.16
68.48
148.95
12.93
34.84
Hav June
__.a.
July
Auguft           SepC              Oct
6 +      Wb
7.         Pe
8.        ETc
9 .       We
10- Total available soil
+220                + 140 .52
-57.85
-217.38           -353.06           -489.08
0
0
*81 + 56        +68.48            + 12-9 3          0
-79.   48            -198 .37      ’241.09           -204.16           -148.95           -34.84
+140  .52            -57..85        *217+38         -353-06          -489-0
-532-92
220 Tnm/a)
11.
Rooting depth 0 on
12.
Si.D
ao la Cure     Ear the soils in the
300                   7 50                 1000               1000 1000 66                     165                220                 220 220
13.
p for cotton
+ 50
.50                 + 50                .55 
project area is taken as
1000
220
.60                   + 65
14.
p.Sa.D
15.
(I - p) Sa.D
16.
plot Sa.0 and (I - p)
33                    83                   110                 121                  132                  143 33                    82                   110                 99                   88                    77 Sa ■ D at       end of  month as given, figure 5.2.
plot We as given.
LB . When the soil aoisture
curve    (I    -    p)    Sa.D    the
lines between Sa . 0 and
balance curve (We) seats the soil moisture depletion soil moisture is replenished by drawing vertical
( I - p) Sa.D - pSa.D.
20.
The number of vertical lines in a month gives rhe
Chit month*
May
number of irrigations
far
June
July
3
2
August
1
Sept
I
21.
Rie      length      of      the
vertical
lines
between
depth of net application re
quired in ■».
Sa.D and
(1 - p_> Sa.D gives the
June
50
60
73
August
115
125
5022. The horizontal spacing of the vertical lines gi-ves Che irrigation intervals
from planting,
June
36 
6 
7
July 
11 
15
August
22 
Se£t
27
23.       The net depths of application should be corrected for leaching and
application efficiency for gross water requirements at field level.
24.       Application efficiency Ea = Water directly available to crop over root zone
Water received ar field inlet
* Average depth infiltrated
Depth or water actually applied
5.9*1 Partially Fixed Schedule
The     irrigation     schedule     in     operation     at     present     for     cotton     growing     on     new     lands in the project area is calculated as follows:-
The     depth     of     gross     application     per     irrigation     is     130     mn rate in a field canal is 150 l/s although the canal is i
i and the manageable flow
designed for a maximum of
200  l/s.
given by:
Hence    the time taken
to apply 130 mm over an ■
area of A hectares is
t=
130
x A x 10,000 
=     2.
LOGO
3600  x 0.
15
Applying
this equation
to part
is obtained.
Field
Area
t
No
(hI7
hrs
I
11.39
60.4
2
22.78
120.7
3
11. 39
60.4
4
12.02
63.7
5
17.56
93.1
6
24.56
130.6
7
21.63
114.6
8
11.45
60.7
9
14.42
76.4
10
20.90
110.8
11
32.98
174.8
41A hours
of Unit 1 Amibara the fo llowing data
T
days
21
5
21
3
4
51
5
21
3
5
71
The commen
cement
of the eight
post-planting irrigations
is fixed as
Days from planting                       2            30            51            65             79             93
114
f □ Hows : —
135
Hence the schedule given below is
3rd May.
obtamed aSSuming planting takes place on
51.
AMIBARA IRRIGATION PROJECT
/■ WATER MANAGEMENT MANUAL
PR0JE'CT LAY0 T
-
jT; .
',
'     / IMH 2(369-39Ho.
Unit *3(597 18 Hal"\
DRAWING No 5 ?A typical curve for alluvia soil in Melka Sadi area is given in Figure 6-5 and a
curve for vertisol in Aaibara area is given la Figure 6.6, Both these curves
were obtained from actual furrow intake data.
$•3.4              Water Needed to Refill the Soil Moisture Reservoir
The amount of water needed to refill the soil moisture reservoir to field
capacity, is obtained by subtracting the amount of moisture in the root sone
from the field capacity. Therefore Che water required to refill the root zone
is gLven by;-
d - (Sfc - S) x V x D em ------------------------------------------------------------------------------------------------(30)
100
where               SFc - soil itoisture percentage at field capacity
S      -      soil      moisture      percentage      at      time      of      aoilsampling      using      an      auger V =■ dry bulk density of soil (refer to 3*3*1)
D =■ depth to be tilled
Example: - The average initial soil moisture is found to be 27,145 upto a depth of 100cm and the field capacity 42.OX. The bulk density is 1.2 gm/cc.
Therefore water required to refill the 100cm root zone.
« 
42.00 - 27-34 x 1.2 x 100 - 17*59 cm/m*
100 *
6.3.5 
Furrow Trials
Maintaining  a  constant head in the field canal, constant flow streams using  an increasing  number of standard syphons (2.4  cm internal diameter and 2.2m long) are let into the six furrows in sequence.
The     time    when    water    starts     to    flow    into    each    furrow     and    when    the    stream    reaches Che stakes should be recorded on Form 6.2, This ia known as the advance time.
Volumetric measurements should be taken of each stream periodically.
Inspect     the     furrows     for     breaching     or     overtopping     to     en...r„     m.     ► stream irrigates only its allotted furrow. Cut back -he Ir * 
overflow at the tail end of each furrow, but equal to rh
lndlvid”al
Co Prev«nt
entire f urre / length. Measure volumetrical ly the cut-bac/str * 
After some hours of irrigation given by the furrow inct. char the required depth of water ha3 beeQ d R
udtK. scream sizeSi
® =urv«. and asking
° f 'he
turned      off.      The      time      when      water      in      the      furrow,      disappearJUrr°W         Che         serea®’      are
then recorded on Form 6-2. Thi3 i, known Jg »reCeaa^n The wetting pattern along each furrow should then be
eactl ’take i3
.u
soils    at    30    cm    intervals    up    to    a      depth    of    100cm    at    ridges    Ind    h at three sections namely:- the upper end. middle and tail 5 
° f each ^rrow
Figure       6.4       gives       the       wetting       pattern       after       irrigation furrow   referred   eo   at   6*3.2.                                                                    8
-I       US1°g           Fortn           3‘]-
a   4t    the     middle   section   of
a
508. Distribution _5yseem
8.1 Description gf system
The distribution system comprises Che network of secondary, tertiary and^field canals for the movement of water from offtakes on the main canal to 
Each main canal offtake feeds a secondary canal, except offtakes 5 and 13 which feed tertiary canals directly.
A secondary canal has two or more tertiaries and rejects into a secondary drain or tertiary io an emergency or when irrigation water is in excess of
requirements.
Tertiary canals are designed to feed a minimum of two field canals at peak water demand. In exceptional cases three co six field canals are served by a tertiary
■ t peak demand. These particular tertiary canals are Tertiary 3 and Tertiary 2 and 2B from Secondary 5 and 1.
Field canals are designed for a maximum discharge of 200 litres per second. Field canals are usually fed by tertiary offtakes. But there are four offtakes on secondary canal S5; from main canal offtake 6 which directly serve four
. Le
t
field canals.
Secondary and tertiary canals are provided with duckbill weirs, with a varying
crest length. These duckbill weirs help to maintain the upstream length of
canal at a pre-determined elevation to provide the necessary head at offtakes
for designed discharges. A duckbill weir serves one offtake on a secondary and
one to four offtakes on a tertiary.
The maximum afflux over a secondary canal duckbill weir is 17cm while that for j
tertiary canal duckbill weir is 9cm. Duckbill weirs at the end of secondary or
tertiary canals serve as rejection spillways.
Lt     is     proposed     to     use     duckbill     weirs     a3       measuring     devices     in     the     absence     of     any other form of Meeting. The flow over a duckbill weir is generally given bv the
formula.
where
q - 1.05 LU —
Q       ■ discharges in tn^/s
L = length of crest (m)
H - head over crest (m)
- (38)
Discharge curves tor two co tumen lengths of weir are given in Figure 3.1
The rate oE flow over a duckbill weir being known, the discharge through a upstream offtake can be estimated by deduction. For instance if the di over a secondary canal duckbill weir is 600 1/s and it is
' chj,r«e_
73
/APPENDIX 11
References for Appendix I
FAO Irrigation and Drainage Paper Ho 1 Irrigation Practice and Water Management (1972)
FAQ Irrigation and Drainage paper No 24
Crop Water Requirements (1977)
FAO Irrigation and Drainage Paper No 25
Effective Rainfall (1974)
FAO Irrigation and Drainage Paper No 26
Small Hydraulic Structures (1975)
FAO Irrigation and Drainage Paper No 29
Water Quality for Agriculture (1976)
FAO Irrigation and Drainage Paper No 33
Yield Response to Water (1979)
International Institute for Land Reclamation and Improvement
Drainage Principles and Applications
On Irrigation Efficiencies
Publication16
Publication19
Discharge Measurement Structures
Publication 20
(1974)
(1978)
(1978)
United States Department of Agriculture. Soils Conservation Service
National Engineering Handbook Section 16 (1971)
122APPENDIX II
References for Appendix L
FAO Irrigation and Drainage Paper Ho 1
Irrigation Practice and Water Management (1972)
FAO Irrigation and Drainage Paper No 24
Crop Water Requirements (1977)
FAO Irrigation and Drainage Paper No 25
Effective Rainfall (1974)
FAO Irrigation and Drainage Paper No 26
Small Hydraulic Structures (1975)
FAO Irrigation and Drainage Paper No 29
Water Quality for Agriculture (1976)
FAO Irrigation and Drainage Paper No 33
Yield Response to Water (1979)
International Institute for Land Reclanation and Improvnnient
Drainage Principles and Applications
On Irrigation Efficiencies
Publication 16
Publication 19
Discharge Measurement Structures
Publication 20
(1974)
(1978)
(1978)
United States Department of Agriculture. Soils Conservation Service
National Engineering Handbook Section 16 (1971)
122Hence it is evident that the field drains have sufficient capncity even if thi method were to be used.
2«12 Roads
About 90 km of gravel surfaced roads have been constructed of which 40 kta are principal roads and 50 ’<ra service roads. (Jnsurfaccd tarn roads extend to approximately 140 km.
2*13 Flow Chart
The flow chart given in Figure 2.1 indicates schematically how water once diverted at the MeLka Sadi weir finds its way through a network of canals and appurtenant structures to the ultimate field unit.The most eoumon method is the use ot a "double ring’1 infi 1 trotter which is in fact two concentric oet.l cylinders. The metal cylinders are driven into bhL wil up to 5 cm using □ lavel wooden tatren And a hansB**r. A scale tea ing. graduated in centimeters and millimeters is fixed on to the inside ol the inner ring with its zero at the soil surface inside- A polythene sheet is then used
to line the sides and bottom of the inner cylinder mo that when it is tilled with water no infiltration takes place till the polythene is quickly extracted. The space between the outer ring and the inside ring is filled with water to the same level as the water in the inner ring. The polythene Lining is now extracted, gently and quickly. The initial reading of the water level on -lie scale and subsequent readings at regular intervals as the water level drops are recorded on rora 3.2.
Infiltration data  ls then
limited an ordinary  graph
d£ the curve after two or
plotted with che intake race in cra/hr versus time in paper. The final infiltration rate is the tint part
three hours.
Examples:- Initially the depth of water in the infiltrometer is 19 cm. After
60 seconds the level drops to 18.8 ctn and after two minutes it drapa to 13.7 cm. The difference in reading for the first 60 seconds is
0.2 ch and for the second interval of 60 seconds it is 0.1 on.
Hence the initial intake rate
and rhe second intake rate
- 0.2
60
- 0.1
60
x 60   x 60 = 12 cm/hr
x 60 x 60 3 6 ctn/hr
The     initial     nwlsture     content     of     the     soil     profile     at     30     cm     intervals     up     to     100     cm should be determined.
Figures 3.2 to 3.5 show typical curves for the tne^sur-d infiltration rate
Melka Sadi and Annbara areas. It wil be seen that when the surface soil
s for
the basic infiltration rates are high compared to those when the surface
is dry
moist. This is a result of the moist top soil and lower strata reaching
soil is
saturation point faster than when the sail is initially drier.
As an alternative to the "doubLe ring" infi 1 trometer method infiltratl™ 
u
4«=™i..d b.
ch. ru^fi
„ ,c„ulat.dr,t^"
tb. rd or .cul.t.J ro.nf.ll sy.c, th. rainfall a, w.ll
the ron-off t 1 8
known      arei      and      given      period      of      time      are      recorded.      The      irfiltrati™      -in      1.      L3
rainfall minus run-off in nun divided by the time in hours.
11 th&n be
The      above      B^thods      are      suitable      for      determining      infiltration      rates      for      b-s      i
irrigation, Determination of
in Ghaptera 6-3.2 and 6,3*3.
furrows acting as buffers for rhe centre
taken. Inflow and outflow of water
n>easured at regular intervals,
then converted into an infiltration rate
by the furrow.
infiltration rates for furrow irrtohti
It
rhe
is      essential      to      use
f
______
:
... e arrows, the external
n
.
three furrows th dr' glWen
, for
The noil
15ExampLe:— Two 3 inch parshall flume* are installed in a furrow -25 a apart.
The initial reading of the upitrean flume is 4.5 cm and that oE the downstrean fluroe 1.9 cm. After 10 minutes the readings nre 1-7 ind 3.2 cm respectively. The corresponding flaw races are 1*45 nnd
1.07 l/s for the upstream flume and 1.16 and O.fi5 L/s for rhe downstream flume. The differences in inflow and outflow are 0.29 l/s and 0.22 l/s per 25 mw.
The furrow width is 90 era.
Hence furrow intake
rate
•
0.29 x
60 x
60
- 4.64
mm/hr
initially
1000
.9 x
25
and
-
0.22 x
60 x
60
- 3.52
Ecn/hr
after 10 minutes
1000
.9 x
25
Intake rates in cra/h are plotted     versus    cumn1j ti ve    time    on        log-log   paper.
This gives a straight line with the equation I » kTn ------------------------------------------------------------------ (7)
where I - intake rate of soil in cm/h
k » intake rate intercept at unit time
T = time water is on surface of soil
n * Slope of line (scaled vertical height divided by scaled
horizontal length)
164.6 Sone Crops Grown in Project Ata a
4.6.1 Banana
growth.
humid tropics and the Latter in the dry topics.
Water Uptake
The banana plant's root systf?ra is sparse and shallow, with the majority of feeding root spread laterally near Che surface. The rooting depth generally
does not exceed 0.75 m. Normally IDOZ of the water ia obtained from the first
0.5 to 0-00 m soil depth with 601 from rhe first 0.3 tn. Generally 35X of the
total available soil water should not be depleted.
Irrigation Scheduling
Frequent irrigation is important if the available soil moisture depletion is to
be limited to 35X-- Irrigation interval will depend on rhe soil water holding capacity in the rooting depth and the maximum evapotranspiration.
4.6.2 Cotton
Cotton can be grown on a wide range of soils but preferably on medium and he aw textured deep soils with good water holding characteristics. Root penetration
is restricted by acid or dense soils. The pH range is 5.5 to 8 with 7 to 8 as optimum. Cotton is totcrant to soil salinity with no percentage yield decrease if ECe ■ 7.7 mhos/cm.
Cotton needs 700 to 1300 mm of water depending on the climate and length of growing period-
Water Supply
flowering -ind yield formation prolonged end ex«»sive growth will stress on cotton is noticeable by the bluish green colour on the'leave?; discolouring of the stem.
Uaves and
23N+l
where ? =  platting position
N *   total number of events
=   rank of each event
Month
Mean      Rainfall
/
Probable Rainfall
o/inon th
3 out of 4 years
nn/month ■■
January
17.79
February
47.22
0
0
30.0
f
March
58.52
April
50 <42
26-5
May
24.09
S'. 5
June
23.89
3.5
July
110.84
August
114.80
September
38.17
October
21 .40
85.0
78.5
24.0
0
November
23.43
December
2.65
0
0
The 75Z probable rainfall is used in section 5.4.5 in computing Che effective rainfall for seven selected crops, with storage factors relative to the readily available soil moisture in the root zone at progressive stages of growth.
5.4.5               Method of Determining Effective Rainfall (Pe)
By processing long term climatic and soil moisture data, the Unites States Department o Agricultural Soil Conservation Service developed a procedure fnr estimating effective rainfall. This i8 known as the evapotranapi Lt i™ precipitation. method.
Table 5.1 gives the relationship between average monthly effective mean monthly rainfall for different values of average monthly con’
r
,,
fa l *nd
E T. .nd also ,i,.. .b.
effective root zone.
E .e.o„ fas «il*£
K Ln cn<?
For example uaing the values f ,r 75X probable rainfall
.
cotton      may      be      determined      from      the      above      Table      The      scorao      £ive      rainfall      fOl
— .1 root too. i. c.k.o a. 220
„  „ „„„ .XH
s
/
During August
the mean monthly consumptive use
rainfall 76.5  on. The rooting  depth is 120  cm. cm and soil noiitufe storage with 501  depletion 120 nm/m, from Table 5.1 the storage factor is
is 179.28 mm and the 752 probable
Since the rooting depth is 120
for both types of soil is
given as 1.04. From plotted
is 63-00 iDtiu Multiplying this by
effective rainfall for cottonTABLE 5,1 Monthly
Mean
R a In
ta IL no
Average Monthly ErhcUw Rainfall aw Related to Mean Monthly Rainfall and Hoan Monthly Conii^pU™ U.» (USDA SCS^
t
Mean  Monthly
25                50                 75                  100             125
150
175
125            350
Mtsan   Monthly
CcnauapLive Osa mm
200                 225                 25D                275          301
rrfociUa    HbLiJuII mm
7.5
a.o
B.7
9.0
9.2
10.0
10.5
11.2
11.1
12.5
12.5
12.5
12.5
12.5
12-5
25.0
37.5
50 .0
62.5  at
15 .0
16.2
17.5
10.0
13.5
19*7
20.5
22 .0
24.5
75.0
25.0
25.0
25.0
25.0
37.5
37.5
22.5
24.0
26*2
27.5
20.2
29.2
3U.5
33.0
36.2
37.5
17.5
37.5
2$
$4.5
35.7
36./
39.0
40.5
43.7
47.0
50.0
50.0
50.0
50.0
50.0
52*2
TT7
W.7
42.5
44.5
46.0
40.5
50.5
.
53.7
57.5
62.5
62.5
62 .5
62.5
62.5
75.0
75.0
75.0
75.0
75.0
87.5
100.0
112.5
46.2
49.7
52.7
55.0
57.5
60.2
63.7
67.5
73*7
60.2
77.7
B4.5
07.5
07.5
B7.5
50.0
M.7
66.0
69.7
71.7
07.5
at WT77
61.7
67.7
63.7
72.0
74.2
.7
BJ.O
87*7
95.0
100
100
100
100
70.5
75.0
60.2
02.5
B7.2
92.7
9B.0
105
111
112
112
112
75.0
Bl.5
07.7
90.5
95.7
102
10B
115
121
125
125
125
125.0
137.5
at
ill-
Hfl.7
95.2
98.?
104
111
llfl
126
152
137
137
137
95.2
102
106
112
120
127
156
14)
150
150
150
150.0
162.5
100
109
113
120
120
135
145
153
160
162
LAZ
at
rar
115
120
127
115
143
154
164
170
175
175
175.0
187.5
121
126
134
14?
151
161
170
179
1&5
10?
125
US
140
148
158
16B
170
Ida
196
200
200.0
225
at
KT"
144
151
160
171
162
250
150
161
170
103
194
at 240
171
175
101
194
205
275
500
325
190
203
215
U 2ft7
190
213
224
200
220
232
550
at nT"
225
240
175
ut 372
24 7
400
250
425
ui nr"
450
25 
50
75 
1D0 
125
150
175
200 
225
250
Uhlc 5.2
ZD
25
3? 5
J f r
50
62.5
75
100
125
150
175
2QD
Effective
Storage
.06
.93
.97
1*00
1.02
1.04
1.06
1.07
1.00
Storage Factor
.77 
.77Therefore
where
d 3 p.Sj.D ---------------- -------------------------■—---- - -------------------------------- (24)
Ej
d = dfpth of 4ppl ieat ion in ami
p « allowable depletion fraction
Sa 3 total available soil moisture in mm/a
D - rooting depth in a
Ea « application efficiency Z
Table 5.4  gives the soil
ETcrop.
Table 5.4
water    deplet ion       fraction (p)            for cotton at different
ETc mm/dev 
p
2
3
4
5
6
7
8
9
10
-875
.80
.70
.60
♦ 55
.50
.45
.425
.40
5.8 Irrigation Interval
Timing of irrigation application is of utmost importance as delayed irrigations could affect yields. Timing of irrigation should conform co soil water depletion requirements of the crop. Therefore, during the crop growth, flexibility in time and depth of irrigation should bf maintained to accommodatp differences in crop waLer needs. This method is preferable to having fixed schedules. However, in the absence of data on soil characteristics for the
whole! project area', a partially fixed schedule was adopted bated on estimated crop water requirement a and stages of growth. The schedule used for cotton grown in the project Area appears to be well suited co overall soil characteristics as presently known and climatic conditions encountered and is
given below:-
Days from planting
Gross depth of application cm
Interval
- 12
28.6
12
2        30        51        65        79         93 13        13        13        13        1]         13
2        28        21        14        14        14
114
13
21
135
13
21
Yield response
expected (over
April 1977-
to    water    during    the    1980
35q/ha) being well above
The irrigation interval 1S given by the
i ■ p.Sa ■ 0
ETc
5.9 Calculation of Irrigation Schedules
An irrigation schedule should be
balance and allowable .soil water
and 1981 cotton season was mote than the yields predicted in the appraisal of
equation:-
(25)
"XfurX"0'* USin8 tHe
—
Example I (Alluvia Soil)
The cotton growing season               in the
planting date is 15th May and the growing period
of 220 nun is applied 8 days before planting to fill r/ ”’' * 
^ hruM1^ October. The
of the root zone to field capacity, for efficient ^ed* S<>i L "°istUr
5            net PTe~irrlRat ;o
« r« p®
and to flush down the salts which tsay have bepn r '?d cqlnLr
^  prior to plane;
1
S       V(
,°
n
;
a„i„. cbe
W1 -, h„,J,
”P »r
° P’ and better yield
4922. The horizontal spacing of the vertical lines gives the irrigation interval^
frcira planting.
June
July 
August
Se P£
36 
II
6 
15
7
7
23.      The net depths of application should be corrected tor leaching and application efficiency for gross water requirements ar field level.
24.      Application efficiency Ea = Water directly available Co crop over root zone Water received at field inlet
= Av erage depth infiltrated_________________
Dept
h of
water actua
lly ap
plied
5.9.1 Partially Fixed S
chedule
The     irrigation
schedule
in
operation
at
present
for
cotton
growing 
on
new
lands
in the project area ls calculated as follows:-
The     depth     of     gross     application     per     irrigation     is     130     mm     and     the     manageable     flow rate in a field canal is 150 l/s although the canal is designed for a maxinum of
200    l/a.    Hence    the    time    taken    to    apply    130    am    over    an    area    of    A    hectares    is
given by:-
t-
130
x A x 10 ,000
=    2.41
1000
3600 x  0.15
A
pplying
this equation
to part of
a hour ■
Uni t
I Amibara the following data
is obtained.
Field              Area
No
I
2
J
4
5
6
7
9
9
10
11
(ha)
il-39
22.78
11.39
12.02
17.56
t
hrs
I
days
60.4                   24
120.7 5
60.4                   2i
63.7                   3
93.1                   4
?
.4.56                  130.6
5i
21-63
11.45
14.42
114.6                   5
60-7
2i
20.90
32.98
76.4                   3
110.8 5
174.8 7i
The ccramencement     af the eight post-pla.
nting irrigations is fixed as
fo1lows:-
Days from planting                    2            30            51              65             79             93 Hence the schedule given below is obtained assuming pUntine r.L,.. t
3rd May-
] 14
135
11 mKes place onA typical curve for alluvium soil in Melka Sadi area is given in Figure 6.5 and a curve for vertical in Amibara area is given in Figure 6,6. Both these curves were obtained from actual furrow intake data.
6.3.4
Water Needed to Refill the Soil Moisture Reservoir
The     amount     of     water     needed     to     refill     the     soil     moisture     reservoir     to     field
capacity, is obtained by
from the field capacity,
is given by: —
subtracting the amount of moisture in the root zone Therefore the water required to refill the root zone
■1
(Sfc - 5) x V 100
x D era
where               SFc - soil moisture percentage at field capacity 
s    -    soil    moisture    percentage    at    time    of    so    1    H.rap    11    nj?    using    an    aug«-r V • dry bulk density of soil (refer to 3.3.1)
D * depth to be filled
Example;- The average initial soil moisture is found to be 27.34* upto a depth of lOOcra and the field capacity 42.02. The bulk density n 1.2 gm/cc. Therefore water required to refill the 100cm root zone.
=    42 Q0 - 27,34 x 1.2 x 100 = 17.59 cm/ra.
( 30)
- ll<r„
±
LOO
b.3.5              Furrow Trials
Maintaining  a  constant head in the field canal, constant flow streams using  an increasing  number of standard syphons (2.4  cm internal diameter and 2.2m long) are let into the six furrows in sequence.
The    time    when    water    starts    to    flow    into    each    furrow    and    when    the    stream    reaches the stakes should be recorded on Form 6.2- This is known as the advance time.
Volumetric (measurements should be taken of each stream periodically.
Inspect the furrows fur breaching or overtopping to ensure that each individual stream irrigates only its allotted furrow. Cut back rhe streams to prevent overflow at the tail end of each furrow, but equal to the intake rate of the
entire furrow length. Measure volumetrically the cut-back strratn sizes.
After some hours of irrigation given by the furrow intake curve, and assuming that the required depth of water has been applied to each furrow the streams are turned off. The time when water in the furrows disappeared at each stake is then recorded on Form 6-2» This is known ai "recession ticae,r.
The wetting pattern along each furrow should then be checked by auger sampling soils at 30 cm intervals up to a depth of iOQcra at ridges and bed uF each furrow at three sections namely:- the upper end, middle and tail ^ d using Form 3 1
n
Figure 6-4 gives the wetting pattern after irrigation at the middle section of furrow referred to at 6.3.2*
58B. 1 Description of _systfQ
The      distribution      system      comprises      the      network      of      secondary canals for the movement of water from offtakes on the main
tertiary and field
canal to the field.
Each     sain     canal     offtake     feeds     a     secondary     canal,     except     offtakes     5     and     13     which feed tertiary canals directly.
A secondary canal has two or more tertiaries and rejects into a secondary dram or tertiary in an emergency or when irrigation water is in excess of
reqairerne nt s.
Tertiary canals are designed to feed a minimum of two field canals at
demand. Tn exceptional cases three to six field canals are served by
at peak deaacid. These particular tertiary canals are tertiary 3 and
from secondary 5.
Field canals are designed for a maximum discharge of 200 litres per second.
Field canals are usually fed by tertiary offtakes. But there are four offtakes
□n secondary canal S5; from main canal offtake 6, which directly serve four
field i A-iala.
Secondary and tertiary canal* are provided with duckbill weirs, with a varying crest length. These duckbill weirs help to milntaiQ the upstream length of
canal it a pre-deterrained elevation to provide the necessary head, at offtakes
for designed discharges. A duckbill weir serves one offtake on a secondary and one to four offtakes on a tertiary.
The maximum afflux over a secondary canal duckbill weir is 17cm while that for a tertiary canaL duckbill weir is 9cm. Duckbill weirs at the end of secondary or tertiary canals serve as rejection spillways.
It Is proposed to use duckbill weirs as lueaauring devices Ln the absence of any other fortn of metering. The flow over a duckbill weir is generally given by the
formula.
peak water
.i tertiary
tertiary 2
y
Q •        1.05 LH !-383
where            Q - discharges in
L ■ length of ci?st (m)
H = head over crest (m)
---------------------------- - -------------_-----------------------------
Discharge
curve, for two common lengths of weir are given in Figure 8.1.
The rate of flow over a duckbill weir being known, the discharge rh- u
upstrean offtake can be estimated by deduction. For instance if the^d^ J"
over a secondary canal duckbill weir is 600 1/s and it i<:                                                                         tscharge
1/s through the upstream offtake the offtake will be regulated
rate aver the duckbill weir reduces to  200  l/ .
s            Hy
P u’S 4°°
dXtion tLreft?^ nn^
1/s should be passing through the offtake.
’
tore, /4qo
73

< <

Summery

Amibara Irrigation Project Summary

Amibara Irrigation Project Water Management Manual Summary

Project Overview

The Amibara Irrigation Project in Ethiopia's Awash Valley covers 10,300 hectares (4,600 ha in Melka Sadi and 5,700 ha in Amibara) with gravity irrigation serving various agricultural entities. The project includes a comprehensive water management system for irrigation and drainage.

Key Components:

Diversion weir (4m high rockfill structure)
25.5 km main canal with 13.5 m³/s capacity
Secondary (30 km) and tertiary (93 km) canal networks
29 km main drain with secondary and tertiary drainage systems
Flood protection dykes (43 km western, 34 km eastern)

Water Management System

The manual provides detailed guidance on all aspects of water management including:

Key Sections:

Section	Content Highlights
1. Irrigation and Water Management	Project introduction, water management principles, system components
2. Project Works	Infrastructure details, irrigation water duties, drainage networks
3. Soils	Soil characteristics, moisture measurement, infiltration rates
4. Crop Water Requirements	Evapotranspiration calculations, crop coefficients, water needs for key crops
5. Irrigation Requirements	Scheduling methods, soil moisture balance, effective rainfall
6. Furrow Irrigation	Furrow design, evaluation methods, flow rate determination
7-10. System Components	Conveyance, distribution, salinity/drainage, and efficiency systems

Key Technical Data

Soil Characteristics:

Two main soil types in the project area:

Alluvial soils: Coarser texture (sandy loams to silty loams), brown color
Vertisols: Silty clay to clay texture, dark grey-brown to black when wet

Water Requirements for Major Crops:

Crop	Seasonal Water Need (mm)	Key Irrigation Considerations
Bananas	1,200-2,200	Frequent irrigation, sensitive to salinity
Cotton	700-1,300	Tolerant to salinity, critical water needs during bud formation
Maize	500-800	Avoid water deficits during flowering
Onions	350-550	Shallow root system, frequent light irrigations

Irrigation Scheduling:

The manual provides detailed methods for calculating irrigation schedules based on:

Soil moisture balance (Wb + Ge + Pe - ETC = We)
Crop evapotranspiration (ETc = Kc × ETo)
Effective rainfall (75% probability values used)
Soil water depletion fractions

Operational Guidelines

Furrow Irrigation:

Optimum slope: 0.20% to 0.60%
Typical length: 200-225m
Flow rate calculation: Q = 0.60/S (l/s) where S is slope percentage
Evaluation methods using Parshall flumes and intake rate curves

System Maintenance:

Regular cleaning of intake screens and trashracks
Scour sluice operation during high sediment loads
Monitoring of gate operations and canal flows
Establishment of rain gauge network (1 per 600ha)

Conclusion

The Amibara Irrigation Project Water Management Manual provides comprehensive technical guidance for operating and maintaining this large-scale irrigation system. It combines theoretical calculations with practical field methods to optimize water use for agricultural production while addressing challenges like soil salinity, drainage, and system efficiency.

The manual emphasizes the importance of proper scheduling, measurement, and maintenance to ensure long-term sustainability of the irrigation system and high crop yields.