GOVERNMENT 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 0 Box 2306 Addis Etftiopn
Ser William Halcrow & Partners
Consulting Engineers
Burderop Park
Swindon
Wiltshire SN4 0QD
United KingdomAXIWU lUlOATlOW F1CJXCT
warn wucgxtrr mmtal
xmmmy cownrrs un
1 IrrigatiM Water Kautrntt
2 Project WorUa
J Ulla
« Crop Hater levicrw^t*
Uri|tClM toqoirrwnCt a»4 ScheHuiing
a Furrow Xrtigetioe
7 CoavoyaQce Byetew
< 9ietribvttoe Syetew
* Utility Oraioage
;0 tfflctiaoea
aPPUDIX : - Woiatore Charactertatica of the ao&la in the Am bare Project Area aaC MeiU Harer Beaearrh Station.
APPtXDlX XX - leferencea
aPPEXDIX ::: - Cloaaary
Irrigat ionSection 1
[rn^icuo Mt<r
1.1 Intro4>icCiM
1.2 Water Haaageea-st l.J Irrigative STUM
1.4 Drainage $ytc«e Figura 1. 1E
110a
Frojact Woraa
2.1 Oivertioa Wair
2.2 Satai#
2. ) Ha a 4 lagMlator
2Jb Hail Canal
2.5 Hakn Canal Chad Dro^a 2.H 1# jace too SpiHvava
2.7 Offtafcae
2.< Ha.a Drain
2.9 Flood Frotaction Dy la a
2.10 Diacri^utary Utaorn
2.10.1 Irrigation Wafer Outlet
2.11 
Drai^aga Watvoraa
I
2.11.1 Drainage Water Outlet
7
2.12
2.13
foa l a
0
I
Float Chart
9
I
1
I
I
I
I
Figure 2.1
Drawing 2.1
'••
I.Section J
Soils
3. 1 Soil Tenure
3.2 Soil Structure
3.3 Fore Space
3.4 Sat eratioa
3. 5 Field Capacity
3.6 FffMMr.t Wilting Poiat
3.7 leadily Available Soil Moisture J.* Measuring Soil Moisture
11
:i n
H
12
12
12
13
3.5.1 
Teasiowter
13
3.5.2 
Srpidi Blocks
13
3.3.3 
Meutron Probe Method
13
3.» Infiltration Bates
3.5.1 Deterwiaiag Infiltrstion Bates
14
14
Tablas 3.1 
3.2
Foma 3.1 and 3.2 Figures 3.1 co 3.3
□raving 3.11
WCC1004
Crap Water •equi rrwat t
4.1 1stroduct
4.2 Methods of Kat Crop Water Requires anta
4.3 Deflations
4. 3.1 Evapotransptration
4.3.2 Reference Crop Evapotranapiratlon
4.3.3 Crop Evapotranapirat 101
4. J.4 Actual Evapotranapirat ion
4.4 Calculation of Reference Crop Evapotranapirat ion 4.4.1 Climatic Data
4.4.2 Reference Crop Evapotranapirat ion by the 8 laney Criddle Method
4.4.3 Reference Crop Evapotranapiret ion by the Radiation Hetbod
Pa get
21
21
21
21
21
22
22
22
22
22
23
4.4.4 Reference Crop Evapotranapiration by the Class ’A* 24
Pan Evaporation Method
4.4.5 Reference Crop Evapotranapirat ion by the Pvnman
Method
4.5 Crop Coefficients
«.6 Some Crops grown in project area
4.6.1 Bananas
24
27
28
28
4.6.2
Cotton
28
4.6.3
Haire
29
4.6.4
Onions
30
4.6.5
Peppers
30
4.6.6
Haricot Beans
31
► .7 Determining Crop Water Requxremi mt a
4.7.1 Crop Water Requirement! for Beninas
31
31
4.7.2
n
<t
••
M
Cotton
32
4.7. J
»•
n
n
ft
Peppers
33
4. 7.4
M
H
»•
»<
Haricot Beans
33
4.7.5
ft
ft
M
tt
Ma ire
34
4.7.6
ft
H
ft
ft
Oniona
35
4.7.7
tt
H
»t
ft
Sesame
36
Tablet 4,1, 4.2. and 4.4 to 4.11Se< cion 5
Irrigation Requirement a aa4 ScHedulmg
5.1
_
irrigation Requirement
5.2 Irrigation Schedules
5. J Soil Moiat are Balance
5.5.1 Gr.medvater contribution (Ge)
5.5.2 Stored Soil Water Wb and Ve)
5.4 Effective Rainfall
5.4.1 Definition of effective Rainfall
5.4.2 Effect of Soil Cbarectertetice on Effective Rainfall 5.4.5 Effect of Crop Charact eriatica on Effective Rainfall 5.4.4 Eatiaation of Dependable Rainfall
5.4.5 Method of Deteraia^ng Effective Rainfall
42
42
45
45
45
5.5
LeacMmg Requirement
47
5.4
Bet Irrigatioe Requirement
44
5.7
Depth of Application
a
5.a
Irrigation Interval
49
5.4
Calculation of Irrigation Schedules
49
5.10 Eatabi .ahoent of Ram Gauge Station*
52
Tablet
Pore
Figuree
Dreeing*
5.1 to 5.4
5.1
5.1 co 5.15
5.1 and 5.2t ICQ h
Furrow Irrigation
6.1 Scope of Furrow Irrigation
6.2 Furrows
6.2.1 Characterlatica and Type* of Furrow Irrigation
6.2.2 Furrow Spacing
6.2.) Furrow flop**
6.2.6 Furrow Lengths
6.2.5 Flow Batea
6.2.6 BeguioCloe of Screen Site
6.2.1 Average Septa of Veter Applied 6. ) Fur r ow tvaluacion
54
54
54
54
54
55
55
56
56
56
6.1.1
Object l re
56|
6.).2
initial Collect toe of Seta
56
6.).)
Furrow Intahe Bate*
57
6 ).4
Voter Weeded to Befill Soil *toi*ture
Reservoir
SB
6.1.5
Furrow Trial*
5S
6.).6
Tine Bequired to tefill the Soil Moisture Reservoir
59
6.4 Analyaia of leeulta
6.4.1 Object ire of Aaalrti*
6.6.2 Mox&mb allowable tine for getting water through the furrow
6.4.) Haaicuc allowable furrow itrean 6.4.4 Opportunity Tine
6.5 7«*npcrary Field Cecal ObecM
Force 6.1 tad 6.2
Figure* 6.1 Co 6.11
59
59
60
ho
60
61Section 5
1 rr xEac 100 Repair went > a nJ Sc* edui tag
5.1 Irrigation Iqyirm<B(
' 5.2 Irrigation Schedules
5.) Soil Moisture Balance
5.1.1 QrouaJvarar contribution (Ge) 5.3.2 Stored Soil Water ( Wb and We)
5.* Etfeetive RamfalI
5.4.1 Definition of Effective Rainfall
5.4.2 Effect of Soil Char act er 1 at 1ce oe Effective 5-4.3 Effect of Crop Characteristics on Effective
5.4.4 Eat vast ion of Dependable Rainfall
5.4.5 Method at Deternming Effective Rainfall
Rainfall
Rainfall
F Urt 4*
4*
43
43
44
44
44
44
44 4$
5.5
Leaching teqeirtarnt
47
5*
Wet Irrigation Re^aireaent
4g
5.7
Depth of Application
a
5.3
Irrigacion toteruel
49
5 .♦
Catcwlacioa of Irrigation Schedules
49
5.10
Eetablishoent of lain Gauge Station*
52
Tatilee
Fora
Figure*
Drawing«
5.1 to 5.*
5. 1
5.1 to 5.15
5.1 l 5.2Swctioo 6
Furrow Irrigation
6.1 Scope ai Furrar Irrigation
6.2 Furrows
6.2.1 Characteristics and Type* Farrow Irrigation
6.2.2 Furrow Spacing 
6.2.) Furrow Slopes
l!S2±
54
54
54
54
54
6.2.4 
Furrow t«eogrbs
55
6.2.5 
Flow laces
55
6.2.6 
Begulatioa of Str«an Site
56
6.2.7 
Awerage Depth at Water Applied
56
6.) Furrow Evaluation
6.1.1 Objective
6.1.2 Initial Collection of Onto 
6.1.) Furrow Intake Bates
6.1.4 Utter needed to lefill Soil Nllfturr Aeservoir 6.1.5 Furrow Trials
6.).6 Tine Inquired to Infill the Soil Moisture Beaervoir 6.4 Analysis of lasalta 
56
56 l
56
6.4.1 Objective of Analysis
6.4.2 Hiiioji allowable C.ar for getting water through the farrow
6.4.) «tai:a\ia allowable farrow street 6.4.4 Dpportuatty T.ne
6.5 Tenpcrary Field Canal Chads
Foma 6.! and 6.2
Figures 6.1 to 6.21
59
59
60
6d
60
61Section 7
Conveyance Sy*tew
7.1 W«ir
7.2 Inc«*« 
7.2.1 Scour Sluice
7.3 Kea4 Regulator
7 . «♦ Mata Cana I
7.5 Controlled Chee a 3rcpe
?.b Conataat Read Orifice Offtakes
7.7 Rejection Spillway*
Tab lee 7.1 to 7. J, and 7.5 to 7.7 F igur ea 7.1 to 7.7
M
M
6*xctava *
Out r ijktt ion Ipf
8.1 Description of $?•€*• 1.2 Irrigation
Pages
73
75
1.2.1 
Water application
75
8.2.2 
FioN Cana. Inlet* 
76
8.2.3 
Srp^on Operation
76
1.2.4 
Canal Foranerera 
78
6.2.3 
Drain Far oxters
79
4.) Injection SpiiIvaya 
6.4 Furrows
1.4.1 Land Crad.ng for Furrow Irrigation
8.4.2 toatrictmg toil Layer
6.5 Field Canola 
8.6 Fara Units
9.6.1 Inquests for Mater
8.6.2 Monitoring Irrigation ScMnMulee and Application efficiencies
8.7 24 hour Irrigation
Tabla 9.1
Fom 9.1 to 8.4
Figures 8.1 to 8.6
Drawings 8.1 and 8.2
81
81
82
82
83
83
83
83
MSect iqq 9
Sal toxcy an4 drainage
9.1 Sei iM 4ad Alkali Beils
9 . i . I tol i no toi 1 a 
9 1.2 Salino-Alkali Soils
9-1.J too-saline Alkali Soils 9.2 If feet of Salts on Crops
♦ .1.1 toroa 
*21"
90
90
90
91
91
91
9.3 
Quality 
or Irrigation Water
91
9.3.1 
Salinity Problee
92
9.5.2 
Perer ability Prob lee
92
9.3.3 
Toaieity Problee
93
9.4 
>i inage
93
9.4.1 
Oecsss&ty for drainage
93
9.4.2 
Surface drainage
93
9.4.)
Subsurface drainage
94
9.4.4 
Irrigation Begin* 4dopted by MSAE
94
9.5 Groundwater
9.5.1 laeestigat ions
9.5.2 Mell Hydrographs and Groundwater Maps
Tables 9.1 to 9.3
Fore 9.1
Figures 9.1 to 9.14
95
95leCClizfll .0
t!!; »£c;gI
10.1 Tvpet $t efficiencies
10.1.1 Conveyance efficiency 
10.1.2 dietribetion efficiency 
10.1.3 Field AppH cation Ifficiencv 
’0.1.4 Overall or Project efficiency 
10.2 Water Meaeuriag devices
10.2.1 Mater MeaaureBeat
10.) Methode of Water NnaauraBMC
10.3.1 Vo lune Method of Water MeasureBent
10.). 1 Velocity - Area Method of Water Mean*rase nt 10.).) Itrwctwree for Water Maaearoseat
10.4 Parahall Flunv 
Tablet 10.1 to 10.3
Figaros 10.1 to 10.)
bna 10.• to 20.3
Pages
101
102
103
103
103
104
104
104
104
104
104
105SlCTiON 1SICTlOh 1amiwu ;u:catxq» rtojtcr n WATQ MAhALEMUrr HAJCAL
Wet loh I
Irrigaijag and Water Hanagemeat
1.! >>( r oducUot
irrigation wus practised ta 
form since the early civi line ioua . These
civiligatiMS depended mi irrigation for their economies. Irrigated fining
increases projectivity iM elluee for a changeover from a few dryland crops to several other trope in greater demand. Irrigation creates more employment
4
opportunities atrewgcheuiag a region a economy.
Irrigation increases crop production but maintaining increased crop production
.ver a Song period of c.me it difficult without proper nanageesent. Agricultural productivity ia an irrigated area requires that a salt balance ia achieved in
the root tone and that excess water is not applied such that the groundwater
•evals rise till the water table nears the ground surface resulting in
waterlogging and increased salinity ia rhe root sone. The proper amount of
water should be applied to the root tone at proper times. Timing and quantity
if irrigation water are related to cl.mate. toil type and stage of growth.
Proper water wane gement to minimise diversion to meet crop water requirements must bo metltuted end 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 aspects of water resource utilisation. However, with scarcity of water, water aaaagemant has assumed
prime .mportance for the operation of or to modify ousting systems to meet new and changing demands.
The Ami bare Irrigation Project, tn the Awash Valley has increased the total area under irrigated crops substantially. The project area covers the Heike Sadi and Amibara plains in the middle valley. The ^et irrigable eras of the project is
10. XX) ha of which A.bOC he are in Wtlka Sadi sod 5,700 ha are in Anibara. Prior to project implementation 2,SOO ha were served by pumps and 7,500 ha were uodeve 1 oped.
According to stteorological data aval labia, at the Malta Merer Research Station, the rainfall is an average of 53) am per year. Long reins occurring from July to September account for At* of this rainfall (2b* mm) and the short rams from February to April (154 *) account for 2tX. The mean minimum temperature is 15'2’C in December and 2)*C in June, while the mean maximum temperature is 12.5*C in December and M*C in June. Mean relative humidity is lowest in June at HZ and highest tn August at 5SZ. The average daily sunshine hours is 8.5 with so average solar radiation of 5H cnl/cm*.'<ay. The climate is semi-arid.
The ma.n irrigated crop under the project ia cotton, followed by bananas, peppers, maiae and onions m that jrder. The entities benefitted bv gravity irrigation are the Institute of Agricultural Research (500 ha), the Relief and Rehabilitation Commission (The Awash Valiev Ambers Settlement Project).
2000 ha), and Ministry of State Farms Development '8.000 ha).
j,2 Water Management
Water Management, which forma an essential part of any irrigation project includes diversion, conveyance, regulation, measurement, distribution and application of irrigation water to soil as we.1 as drainage of excess water.new CHAPT
Ft GuRE I-1
T
/•
—"•------------* !-• MAIN CAMA1
'SI
X</
SE£X*P* J 
“L
V
I FIELD
' DRAiNI I I I I I I I I I I I u
»
I I I I I I
I
FLOW Ch APT
FfGjRE i-l
FOR WEP CONVEYANCE and
r.»Ais
S£C0*O^»Improved watvr management requires imp I ♦wnt at ion of technological solutions with inecit ellomal Jmgn (or opt im water use reet r let ions. Water use regulation ;^mbiwd with (low msatwremnnt devices are pre-requisite for
effective water nanagesseat .
Figure 1.1 it 4 flow chart applicable for water management of the Aaubara Irrigation Project.
5.1 Irrigation Sy t tew
The core of <a irrigation ay a ten u the on-fam diactibution and drainage
system. T>ie distribution system should be capable of providing quantities of water at the ties required by the crop CO wet evapotranspiration determined bv climatic factors. The application of thia water to the root zone is dependent
an the efficiency with which it is applied. At the same time, the leaching requirewat tor nemtaiaiM a salt balance should be supplied. Therefore the distribution spate* m-at supply not onlv the crop water requirement but also the
• •aching requirement .
Potential evapotranapirat ion for various crops grown in the project area can be calculated aiisg the wteorologicnI data available at the Research Station,
WlM Werer. These computations will provide irrigation water requirements with time for providing sufficient moisture to the root gone. The repleni shwnt of
soil moisture m the root tow varies with the water holding capacity of the two predominant types soils is the project area.
Jw of the essential requirement e for water management la an adequate number of flow contril and measurement devices. Without anowmg the quantity of water to be wnagod there cannot be good water wnagewot. Flow measurement structures should be provided st least at the beginning of each tertiary and at
intermediate points on lang tertiaries. All tv pea it far* inlets should be calibrated. Sy calibrating the (am inlets the amount of water delivered to
wet any particular desmM can be mtaeared. TN* quality of personnel operating the distribution system n an important factor in the proper nanagewnt of water at fam level. Provision of trained personnel for the operation and maintenance of irrigation systems, although understood ss necessary, it often difficult to accomplish. Attention should be pa.d to training those working directly and indirectly with the "os-firs* distribution and drainage system.
1.4 drainage System
An efficient drainage system removes surplus surface water caused bv eicess ramfsll or over irrigation. If the distribution system is properly manege*! drainage is minimised and prob.ems wy be reduced. Water quality of the drainage water is an .mportaat consideration as canal seepage losses and deep percolation losses will result in degradation of the ground water supply. Return flows to the river may affect downstream users.SECTION 2 IMyoct *orti2. FrojeCC Borka (See Drawing Bo 2.1)
2. • Diversion inir
The diversion weir at the Meedworks is a rockfill structure founded on a blanket
of graded filter material and having a synthetic imperwable membrane. The weir
la 4 a high. THe design flood is TOO m^/s (100 year flood) represented bv an
upetroea river stage of approximately 751.6 ■ above ma I the design weir crest
elevation being 749.0. The 100 year flood ia expected to pees over the weir
without major damage to tbe rocfcfill. In the region of the toe of the weir the
rockfill was espected naturally to adjust itself to adopt a stable profile in
tbe tan at a convex curve. Iknsever on 30 March 1911. a 1200 cumtc flood
'return period 1 in 750 years) paseed safely over the weir causing adjustment of
the rockfill in its entire overflow area ia the form of a convex curve without
major damage. Th* river level upstream of the weir rote to El 752.63 eel.
The synthetic impermeable membrane was designed to provide a water barrier for
the first tew years of operation, after ^ich the area immediately upstream of
tbe wwir would have silted up sad acted as a barrier. In fact the reservoir
silted up within six months of *per<tiom.
Staff gauges upstream and downstream of the weir, on the right bank give direct
readings at the river stage. However, tbe rat log curves for the river discharge
mt the wwir are at present determined theoretically using the weir formula.
ImC the wwir 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 Mater Eeaources Development Authority by discharge mseeurementa combined
with stage measurements at suitable stations jpstreaa and downs trees* of the
weir.
2.2 Intake
The intake is sited on tbe right bank of the weir. It it a structure for
drawing off irrigation water into twin barrels (each 1650 < 1500 ma) with the
entrance co the silt excluder or scour sluice barrel, (1500 x 1500 am) passing
through tbe weir, placed directly below the entrance to 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
when the differential
few days at
head across
a tiwr. The
the weir was
scour sluice stopped functioning
lees than 0.5 m.
Steps have been taken to re-medal 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 tn tbe intake when the scour sluice stops
funct lonxag.
2.3 Head Eegulator
The Head Eegulator is fed by the twin supply culverts to discharge 13 cumec into the main canal for day and night watering with a minimum jpetream river stage of 749.0 m (le the weir crest level). Control of the discharge is effected by two vertical lift gates (1500 x 1500 am) installed in separare chambers with
separate headstocks. Based on empirical formulae, discharge curves have been constructed for a range of gate openings and river stages. Calibration of Che
3J. Frmject
2.• Steer slow tair
( See &rawt ng go 2.1)
The httfiiai weir at the Headworks it > rockfill structure founded on a blanket
of graded filter materiel and having a synthetic itptrwible OMNSbrane. The vtn is 4 • high. The design flood is '00 m’/a (100 year flood; represented by an upstream river stage of approximately 751.6 • above mil the design weir crest elevation being 766.0. The JOO year flood is expected to pets over the weir without major damage to the rcxkfill. In the region of the toe of the weir the rochtill vet expected naturally to adjwet itself to adopt a stable profile io
the fora of s comvex curve. tawevyr on X) Kerch 16B 1 , a 1200 cunsc flood return period 1 io 250 years) passed safely over the weir causing adjustment of
the rockfill m its entire overflow area in the fora >f a convti curve without major daemge. TSe river level upstream of the weir rone to El '52.63 asl.
The evethetic impermeable membrane wee designed to provide a water barrier for the first few years of ->per st a on, after <*ich the ares immediately upstream of the weir would have silted jp smd acted as a barrier. In fact the reservoir
silted ap within m months ot ^perdt torn.
ttaff gauges upstream sad downstrsaw at the weir, on the right beak give direct feedings of the river stage. however, the rating curves for the river discharge
*ver the weir are at present determined theoretically using the weir formula,
tat the weir formula is strictly not applicable due to the presence of tilt
behind !M weir up to crest level. Accurate calibration will be carried out by
the Water tneources development Authority by discharge wasurement s combined with stage measurements at suitable stations upstream end dcvnatream of the weir.
2.2 Ifttaxe
The intake it sited on the right bank of the weir. It is a structure for drawing off irrigation water into twin barrels (each 1650 i 1500 am< with the entrance to the silt excluder or scour sluice barrel, (1500 i 1500 mm) passing through the weir. placed directly below the entrance to the twin barrels. The entrance to the twin barrels xs provided with a trashrack.
The intake Has been a regular source of worry during the rainy season. Bedluad silt and debris have blocked the intake resulting in water not being available
to the entities for a few days at • time. The scour sluice stopped functioning
when the differential
head acmes the weir was less than 0.5 a.
Steps haw been taken to re-model the entrance to the intake by installing
special screens to deflect large trees, logs sod debris. This in no wey is «
deterrent to the deposition of silt in the intake when the scour sluice stops functioning.
2.3 Head kegulator
The Mead Regulator is fed by the twin supply culverts to discharge 13 cumec into the mam canal for day end night watering with a minimum jpstresm river stage of ?4$.O n (it the weir crest level). Control of the discharge is effected by two vertical lift gates (1500 x 1500 am installed m 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
3gates vi 11 be carried
by the Hater heoovrcee bevolopwat hethorite to ratal*
gate opening*, :aaal depths and river atages. Hater discharge* through the vertical lift gates nu twin barrels (IMX> i 1500 en laminating »• a c<*»
stilling baaia.
2 •* Iteat Caaal
The mis canal is 25.1 <• lang, is designed far • discharge of IJ.5 •
ailb a veiecity of 0.^0 n-a and freeboard of *50 <■> Tea offtakes c<aehino4 vltb controlled check drops. aa^ three athera sited at tbe tail and without •
cjatrol 1*4 chock, servo the entitles. la additiee aii dreg structures in the Halaa ladl area are provided where the natural slope of I ba I a *4 eaceed the slope jt the canal- The deaigu capacitv of tbe win canal le ltd final reach la
2.40 •’ a.
•
The possibility of briog.ag '»• ar nor* of tbe old Stat* Ferna. web aa Algeta 4a4 uabaah . under gravity irrigation la under coeai 4* rat :oa.
• • * "*M* Chech Dr.? pa
The mli canal check Oropa have overabet wir galea designed to pa a a the
required teiMCrtaa discharge wilet wiatotniTig a coeat ant vgatreae fall aupply
level. At I<rv flow tbe gates act aa clear kartell weirs and in fear cases • •
aubwirged weirs at high flows. The galea here to be calibrated to detemin* th*
iiecharge paasisg over each structure peine tad thereto? calculate canal losses
for each reach.
2 . * le e c 11 o a Sp 11 I va ? a
fhe rejection apillvawa are each situated a abort distance apetreae of a check
drop ««*t itt offtakes. There are four reject ion apilleave. Th* crest level of
the rejection spillvara ere generally 150 ee above the canal full supply level.
It la Mceeeory to raise tbe check drop petes to increase tbe full •□pply depth
to ensure rejectioe flows ere diverted over the apilloeve.
2.7 Of trade•
The mi« canal afftahee are located iOB»d lately epatreae >f each check drop and
googriee >f a* intake structure. discharge pipe and outlet structure. lech
intake itructara baa been fitted with a rectangular orifice gate providing a
wane for waeurihg eater leaves to eetltioo. The intake end of the pipe is
fitted vi ch a vertical lift gate and outfall structure incorporates a baffle
beae to ensure energy dieeipatioe.
2 .A Hein Dram
TH* wm Irani -me* .carta Wat at tba *!U Udi w. 4»wlo*«a.t area cannot
b* 4ram*d into t*a ri**r at Malta Merer
to high rirer ategea -h»n drainag.
wo.U ** re«-vr»4.
T** <r.ie 2* ka loe«, cro..*. th* l1M ro<4 at n aft<J
f north-ar*. a I '** ea.tere 4y*e. ricking aer»l.a *. .
t r froe th. n wlf
.
l
WTeloM* * ** “ **«»•'••
r
1.0 lead i«a 
re -nderelo^
Th ' rejectee f;<w. fro. [K. 4, .... t4(-J ,pUiWav,
At »»••••’ t’<^' ‘* »*• win dram ..
r..4
tfc<
<*• project reaching th.
4vicieaty at Whitalt State Fem. A evitable gravity outlet tor these drainage fiwi km* Iron tU project area back late the Ave eb liver is available to viitiH the 4aM« >f raising the ground wear table mt creating health
r
hagarda.
• * *
Protection lyses
Flood protection event have been ceeitrvtUt along the eaatern and westarn hNUtnti of (be project area. The Intel length of the westeru dvkv IB <3 kn
with an overage Wight of ) • above the natural grayed level foming the river bonk. The eastern 4yW 3a an 1 ng ie as average 1.5 n high. The western dyke protects tw project area Iran flood a is the Awash liver and (he eastern dyke
•eclwdoa surface ruo-«ff free the eacarpooat bordering the Awash Wille highway. 2 • i0 •<***»<• *2 *t?VC?M
TW H<‘Jtdtrr canola X) •• .
are ted directly bv offtakes on the earn
anal . TW offtake*, secondary sm.s end areas served are as follows-
f f t aka
Wc oodary
Canal hr
tatitr
•a
♦1
Area
M: ved
122M
Wika lad; State Fam I
2
1
4
2
3
4
1321 MSAF.
IMO
Wika Sad: State Fam
WO Melia Sadi State Fam
5 Tartisry 5 203 Mil ha Merer Merer Fam
6
•
♦ 524
Wika Merer State Fam
y 575 Malka Merer State Fam
a Tertiary a
s
10
li
12
—
•
10)
i•3
M»
IMO
MO
2M1
Wika Merer Seaearch Station
Awash Settlement Project
Pam
Awash
Fin
Sett least nt Project
13 Tertiary 13)
Ms! ha Merer State Fam
Malka Merer State Fam
Mr " Merer State Fam
The secondary canal di echarge • am based on a des ign irrigation duty st the field gate of 1.00 1 t ba with distribution losses at approaieutely 1.52 per
tiiuwtrt' and contiMowe operation at this discharge will provide the required water application is a 60 dart linear planting season. To allow for possible rejection flows the wcoedary canals are des;gued for the *iii»ua peas discharge over their entire length. Secondary canals reject into a dram or tertiary
canal at their tail end.
The tertiary canals (S3 an long* are designed for a Mi.eua discharge of
0.4 a s with offtakes to the field canals <270 m) having a mbisms discharge
of 3.2 •' s. tn special cases such as Tl/2. T3/1 sod T5B/2 the design capacity eaceede 0.4 o^/s being 1.0. 0.5 and 0.6 n^ sec respectively. Tertiary canals reject into a dram at their tail end or into a dram through field canals.
The secondary ;anal checks ar cross regulators are long crested aasonrv ducsbill weirs with sloping rubble aprons. The crests are at full supply level and of varying length frov 12.e to 17.65 e deigned to reduce the canal freeboard ftcrnally bv a mum of 0.1 u. A cross regulator serves one offtake upstream.
5eaten wiU be carried Ml by the deter toeaurcea >veiapmewt Utberity *• relate <at« .> peeing* camel deptha a*4 river atagee. deter iitcMrpi cbtxgh the vertical Lift gates into turn barrels ilSOC t I MW » tarwaatiM *• • ; *•*>fl
•tilling mm.
2.4 Hnxn Caul
Th# ba in canal ib 15.5 Ba bag, a»d !• designed tar a dixWrge of U.S • •
• ich a velocity at 0.*0 a • and freeboard of a50 w Tee efftahea combined vith ceatrelled (had drops. and three ethers sited at the tail and vitbout a
control Led check. wrva the entities. Is addition ait drap structures i® C Ba Unlha iadi area are provided Ubere the natural tl^po at the land oaceed 'Na alepe at the <aaal. The design capacity at the aa.a <aaal in its tiM. reach la
Z.«0 a1..
The possibility at Iri&fi^ xe nr wu nt tbe >14 State Fame* tach aa Algeta
and kiahaaA. «edtr gravity irrigative is a nd er canal durst iaa.
2 • * Canal Check 3rope
The mu canal chock drops have seerabot vair gat at designed to part the rtyuirad SaaMtraaa discharge wilat aviate I mag a coeataot upstream full supply level. at tom ftava the gates act aa clear overfall aeira and in four caoea aa tubwrged wira at high fixe. T^e gates have to be calibrated to determine Che iiK*ar|< H,,lU ever each atructure point and therebr calculate canal loaaee
far each reach.
2 -4 Ujactiot dpi I.us* a
The rejacCLM spillways are each situated a abort distance upstream of a check drop and its offtases There are four re)octLM epi I Ivave. The treat level of the rejectix spillways are generally 150 m above the canal full eupplv level,
[t ia aacaeaarp co raiee the cheek drop gates to increase the full supply depth to ensure rejection flxa are diverted over the spillways.
2 . ’ of t taxa
The a«u caaal afftaXe are located nmrd.ately upatreaa of each check drop and comprise of an tetaha atructure. diecharge pipe iM outlet atructure. Fach mraX itmctar* hoe bee® fitted with a rectangular orifice gate providing e nrana for eaaturiM eater laaeao to entatiea. The intake end of the pipe it fitted eith a vertical lift gate and outfall atructure incorporatee > baffle
to enour* eeergv di a a ipet i o®.
2 .• earn Praia
v h^ imiii dram ubicb atarta wet of the *»lta Udi new development area cannot be drained into tU river at «olka Werer doe to high river atggea when drainage
t>* r.««»r*4- TW 
low. ^rthw.rd. .U»« t *♦
ir*« i« AmMr.,
.i.o L..4 i.to !«»•
<*•*• * *■
. cro.... th. llM roa4 ae e aft4
OVV
i1 n
a.wvei'>»*4 
Ok.. >«<»!□« up Mr»U. ..t.r fro. th. <wwlr r.j.ctt.. fl~. fro. th. 4 w., e.Ml o,Uwy,
*’••* ** Fr.M.t fl w. .. th. a.,B o.t. .,f..d out l * **• *»rth.rw Hct»r of th. proj.ct raachia. th.
4• iUiHf et iMuteli State Fam. A suitable gravity outlet far thee* drainage
flaws ewav fr«e the project area beck into the Awash Liver is available co
maims* tM dodger f raising the ground water tabla and creating health
baaarde
• F;^wd Protective Oyaeo
Flood protection dvkos hove beos CMStmcted along Cha eastern and western boundaries of fho project area. The ratal length ef the western dyke is a3 »» with a* average be. got at ) • tboss the natural <r wed level fomia< the river >eeh. ?%• eastern dvk* 34 • • l^og it an average 1.5 a h. gh . The wetter* dyke protects the project area free flooda u the Awash Liver a ad the eastern dvke esclodoe earfade rve-^ft fr^e tbe •KURMt bordering (he Awash Rille highway
• • ** bistribwterv Ihvhu
The awcoeOary ceeale 30 se long) are fed directly bv offtakes on the eam
anal. The offtakes, seconder r
canals end areas served ere as follows-
Offtake
Be
ke c ondery
Cana* Bn
Area
Served
Lotitv
•
1
l«t
Helsa
Sadi State Fare I
♦
•
2
iirt
HSAf
3
3
:oo«
Hr 1 ha
Sadi State Fam
4
4
M
Heike
Sadi State Fare
5
Tart isry
5
203
Heike
Merer Werer Fare
4
w
4
521
Nel ha
Merer State Fam
7
V5
*iU
M»r.r Stat. Far*
•
Tert i arv
•
300
Heike
Merer to asar ch Station
4
10M
Awash
Sertleeoet Project
Pam
10
•
440
Awash
Fare
Settlement Froject
11
10)
Belka
Merer State Fam
12
U)
2441
Heiko
Merer State Fam
13
Tert i ary
13)
Nr Iks
Merer State Fare
The secondary canal discharges are based on * design irrigation duty at the field gate of 1.00 . • ba with distribution losers at epproxmer ely 1.5X per lulomtre sod coatitvose operation st this discharge will provide the required water application is a kJ Lave lieear planting season. To allow for possible rejection flews the secondary canala are designed for the asiieue peak discharge over their entire length. Secondary canals reject into a drain or tertiary
canal at their tail end.
The tertiary canals 43 a* long) are designed for a nmsus discharge of
0.4 ■' • with offtakes to the field canals (2?0 fee, having a miibio diecharge of 0.2 •’ a. In special cases such as T1'2, TJ/l and T5L/2 the design capacity exceeds 0.-» s'/s being 1.0, 0.5 and 0.6 n^ Sec respectively. Tertiary canals reject into a drain at their tail end or into a dram through field canals.
The ittjftdsrs canal checks or cross regulators are long created uionrv duckbill weirs with sloping rabble iprjas. The crests are at full supp.v level and of varving length frm 12.15 • to )7.e5 ■ designed to reduce the canal freeboard noma'.ly by a -wnwun of 9,1 u. a cross regulator serves one offtake jpsrrean.
5The vfttaaee te tertiary «aaala ere tea i-«edelar * •• the die chart* • •
ia4«^«(W4nE of tail eater lew! pceriaed the titiaa heed ie eetatdiaed eed ere d*c« coetrelled either ja the efftaBe treat *f» to IW <*•• ef
the pip« eecraa^e Tertiary easel ahecha are ait* bathhiil eeira with • lepiag epcjsa. TVe .react ate at fell aapply lerel aad at leag|b
atftabae at
• deti<aea re
reduce the easel free-beara >▼ • aaaia»a at 0.04 • far fel. discharge* < <rae»
-be ch aereee iee ta fxr
epetveaa Offtehea <a (arid caaala trow •
tertiary ecu xciiuaaUy free • eeceadery are recteagular etifkaee cast railed by wo cal elide gates. a '.^eg created dw<ml) ahead a< the **d at a tertiary acta ae the re jetties spillway, diarhargiM iatd * tertiary draia.
2.10.1 Irrigative deter titles
Irrigative voter duty ar eater reyeirearet ter irritative without pre*lrrigat law
•aa he giwaw at lol the tat al guaatlty at app.^cativa per giraa crop per wait area ar ‘hl the discharge capacity at the dietributiaa ryatoe at peal Uaeed per
aait area eorrod.
deterring U Chapter 5 ~lrrigatiM itpiirfwalt ead 1 chads 1 **< Beetles 5.4.1 it it roes that the ungatioa water date far case (al those far seres aelocted
crepe it at fol 1 see-
Banatra •
Cwt tea •
B 10.000
•
T?S
B IO.OOC •
n.eOO .J ha
V 100 ha
or
'}«
755c
a to 300
•
7 .*00
■ ' ha
Pepper
•
"»0
. M
a 10.000
•
7,000
ha
Barica< Betas
•
*♦0.000
•
4. toe
•>
ba
riR
Wai ae
•
a 10.000
•
4. VO
•>
ha
^aivaa
•
POO
a 10,30c
•
4 300
ha
iW
dee a er
•
4*0
a 10.300
•
4, *00
•>
Too?
ha
Case th' depewde >• «•*<
ofc«.l4 *•
•>.•• tw r*<-‘'*4 -»♦» mc .. t.t.i wiu.IWB
•« Wtri.l. <!■.« t. coafor. ce olUw^lo M>ll W1l,lur<
far ««v 
’ *0 «3 fc- far cotiJ. i» :*• >r ;««t .r»* for J May ,Um.
F«r»r. 1.1 
(lr
.. th. .Mhcotio. of
9
M «• • {« h<Mir
**CT ‘* 4»*‘4*4 lM* * *’*al •»»“«»••• a( tjo WJ »»*•» 
•**’/•* ,,B‘* 
.. cottoe
.U-^lT **" • ~rka<
»• »»•!< .M „ lacto... i.>10 o* he it > mi application at fl sb and tahmg tU application efficiency <• MX. the greed •nlkc«t»^t v* I • *« 1 M> «• T>e fine rata required at field gate ter th*a aaaUcat»a« 4a aa fallow •
gt • ad ■ ■■**■■■ ■ ■»■■■■
dhere a • tloo rate m •' a
t • ties IS Mta»44
a • area »a ••
4 • depth at appt met mo n

« • 44 •
:ooo
• 0.25 l a ha
42*** .
tOhdOehO
a 1000 l/a per ha
Proa flat Jala to Itn U^MtWr there ta aa overlap at a irrigation appl i cat i o«i Ohl ph can w considered <• the peal teased. Wewce tbo peed lewad la
4 a d.25 I a'We • 2.00 l'a.ho at field gate.
la (bo absence at ear Uta eo efficiencies ter the preject , the efficiencies euggoatod by (fee ItfreaMiMMl QooifiM on Irrigation and drainage as4 the boiled State toil GoooervotiM Service could be ueed to obtain the duty at
4i ver a tea .
Application tfficieecy tor furrow
Xertmrv Canal Sfficmoay
lee ><4arf Canal tfficmecy
Coeeepeace tfficieecv
Project tfflcieecr
eater Oetv at 4Ivor a ion far tat tea
•c
2*11 drainage feet ocrea
o * y• »
0.45
0.00
o.tx
0.? i 0.05 ■ 0.00 ks 0.02
O.eO
U.«U «J/h.
1.*) l/./ba
Secoe4arv sod tertiary drama <13.4 as and U an long reepec 11 ve I y) meow surplus voter tee la oncooo rainfall er wceoo irrigation froo the field trains to the earn dram. Brama are hi.|%el for a drainage coefficient of 4)
m 24 hra (10X probability*. taw vol of thia 24 hours rainfall oil] •w effected in Id hanra. >a thia beeit field drams should have a capacity of i.SA 1/o/ha tertiary drama a capacity of 0.31 l/s U, and asm drama the »ua of tertiary dram inflow .»•• 15* to olloa for tonal storage.
2.11.1 dramage deter Ont i as
Surface drainage rsnmi the rebuff frjo excess ramfall produced by a design ramatom m t me to prevent laoage to crops. The rate of reervai is called the dramage coefficient and is the rate of eater removal per unit area to provide a certain degree of crop protection.
The drama m the project area haw been designed uiiag atoms with a 10 year return period. A 24 hours stars eagn.tude of 61 wr with a cor ret ponding dramage period of IS hours has been used.f±•
* • •»
• • ft ft 5 • S
**
: -i
ft A ft 9
8
ft
•* <
•* OTft 3
•99
ft ft
si"
** ** %
::
9 • ft •* 
•» ft a i
1
ft ft ft
•• 9 m •
9 • •»
ft — 9
:-h
* ft ft 4
9 • • • p7?
ft
HR
ft ar
f5* 9 «% *%
ft ft ft ft
ft •— *W
2 I II• A ft •-
•* ft
ft ft ••
• ft o •* •-•
•* ft ft
9
ft ft
a * 
**
V
• • «a ft ft
•* a
•* •* •»
• • 9
ft « 
•* 9
ft
? *9
ft
i;
ft ft
9
m ft
‘.5
•9
9•oa<a If u eoidoaf CMf tW field draiao Mh •nfticionc capacity mi if this ears to M »ood.
2.11
Uc«( tO •• <rerol
r»ad» Mr* Mrs xooat reefed of vfcicta <0 ba are
pfiac;pol r*od* aM $0 aa **rvi<* roado. Vae«rfac*d fara nrda *ic*m to approaiaatoly loO • •
1 •. > F* w Obert
TM tlw cfcart <«*•* ia f»<*r* 1.) leditoteo xbeasit ical lp b«w voter oac* diverted *( tM **«»* ladt *<ir fa ads Its war tbf'VMgh * Mtoorfc of canal* and opp*"*wot Hrxttrt* to tM alfiaate field *ait.2LA
AMIBARA IRRIGATION PROJECT
WATER MANAGEMENT MANUAL
PROJECT L
if, MELKA SADj
DRAWING No 2 ISECTION 3 Soils3 Soil
Action of climate on parent rock at the earth’s surface produces soil. The vide 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 loam, 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 soils are brown in colour and turn to dark brown when moist. They are slightly sticky when moist 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 the 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 land
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
Z
Vertisols
Z
Field Capacity
35.75
46.00
Permanent Wilting Point
Available Soil Moisture
Bulk Density
Pore Space Z
19.55
16.20
1.335
44.25
30.40
15.60
1.175
50.20
Available soil moisture in depth value is given by
FC - PWP x Bulk Density x 100 cm/m or Sfc - Sw x V X D---------------------------------------------------------- (3) 100 100
where Sfc 3 soil moisture percentage at Field Capacity Sy 3 soil moisture percentage at Wilting Point V 3 bulk density
D 3 depth of soil
in centimetres
Hence available soil moisture expressed in depth value for alluvia and vertisols are 21.63 cm/m and 18.33 cm/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
10e excess water has drained off, the samples are gravimetrically analysed for
y ter content. These results will give the field capacity and permanent
a
W1 point on a dry weight basis. The bulk density of the soils at the
representative sites should also be determined.
3.1 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, medium and fine particles. Coarse particles are termed
sand, medium 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, 302 clay and 52 sand
it can be classified as a silty clay loam, and if the silt-content is 122, clay
282 and sand 602 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 smaller 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 or improved by good cultural practices or
destroyed by cultivation when the moisture content is too high or too 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 Co 60 percent of Che soil volume. Wacer and air scored in Che pore space is used by crops.
3.3.1 Bulk Density
Bulk density is Che mass (grams) of dry soil per unit volume of soil (1 cm )
including pore spaces. Hence Bulk densicy = grams oven-dry soil
3
volume cmJ soil
3.4 Saturation
When Che pore space of a soil is almosc completely filled by water, with little o air the Pore 3Pace> che soil sald t0 be sacurated. With time, some
°f this water called free water will drain downward by gravity, some will move Ot C d due to capillarity, some will travel laterally and some will be taken up
by^the
rO °CS
113.5 Field Capacity
If a soil, not cultivated, is saturated and left to drain, 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/cm^. 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 size. A fine textured soil will retain more water than a coarse textured soil.
3.6 Permanent Wilting 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 mm/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 the field capacity and permanent wilting point are constant for any given soil.
Table 3.3
different
Table 3.3
below gives the average
soils.
physical values and textural relationships for
Total Pore
Space
Bulk
Density
gm/ cc
Wilting
Point
Vol Z
Field
Capacity
Total Available Moisture
Vol Z
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
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 are weighed, dried to constant weight in an oven at 105°C and reweighed. 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.
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 modem 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 a 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 tne unit should be calibrated against the soil moisture content.
3.8.3 Neutron Probe Method
In this method neutrons emitted from a radio active source are slowed down by the moisture content of 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 co 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)
13where d « Equivalent depth of water in centimeters
P ■ Change 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 mm/m soil depth
D ■ rooting depth, in m
Ea » field application efficiency
Frequency of irrigation or irrigation intervals are worked out using the
formula: -
i » p.Sa.D days-------------------------------------------------------------------- (6)
ETc
where i = irrigation•interval in days
ETc ■ crop water requirements, in mm/day
3.9 Infiltration Rates
The time required to wet soil and distribute the moisture within the soil
profile is, to some extent, determined by the water intake rate. The entry of
water at the soil surface is measured as infiltration rates.
Infiltration takes place most rapidly when water is first applied 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 as the structure of the soils may change with
cultural practices.
3.9.1 Determining Infiltration Rates
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
application.
14e most common method is the use of a "double ring" infiltrometer which is in act two concentric metal cylinders. The metal cylinders are driven into the
801
to
.
cm
a level wooden batten and a hammer. A scale reading
gra uated 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 ine the sides and bottom of the inner cylinder so that when it is filled
with water no infiltration takes place till the polythene is quickly extracted.
e 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 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 race « 
and the second intake rate
0.2 x 60 x 60 s 12 cm/hr
60
* 0.1 x 60 x 60 * 
60
6 cm/hr
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
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" inf il trometer method, infiltration can be determined by measuring the run-off from real or simulated rainfall. When using che 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 Chen be rainfall minus run-off in mm divided by che time in hours.
The above methods are suitable for determining infiltration races for basin irrigation. Determination of infiltration races for furrow irrigation are given
. Chapters 6.3.2 and 6.3.3. It is essential to use three furrows, che external frrowa acting as buffers for the centre furrow in which all che measurments are taken Inflow and outflow of water, for a pre-determined length of furrow, is
easured at regular intervals. The soil intake at any time in litres/sec is
then converted into an infiltration race in cm/hr for the width of soil covered
by the furrow.
15Example:- Two 3 inch parshall flumes are installed in a furrow 25 m apart.
The initial reading of the upstream flume is 4.5 cn 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 rates are 1.45 and
1.07 1/8 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 tn.
The furrow width is 90 cm.
Hence furrow intake rate
initially
- 0.29 10
x 60 x 60 
.9 x 25
3 4.64 cm/hr
and
3 0.22
x 60 x 60
3 3.52 cm/hr
after 10 minutes
10
.9 x 25
Intake rates in cm/h are plotted versus cumulative time on log-log paper.
This gives a straight line with the equation I 3 kT°------------------------------------------------------------- (7)
where I 3 intake rate of soil in cm/h
k 3 intake rate intercept at unit time
T 3 time water is on surface of soil
n 3 slope of line (scaled vertical height divided by scaled horizontal length)
16TABLE 3.1
Textural Guide for Readily Available Moisture
Available Soil Moisture Remaining
0 - 25 percent 25 - 50 percenl 50 - 75 percent 75 - 100 percent
100 - percent
(field capacity)
Silt Loam
(Med ium Textured)
Powdery, sometimes slightly crusted but easily broken into powdery condition.
Somewhat crumbly but will hold together from pressure.
Forms a ball, somewhat plastic will slick slightly with pressure.
Forms a ball, very pliable, slicks readily.
Upon squeezing, no free waler appears on soil, but wet outline of ball is left on hand, soil will slick to thumb when rolled between thumb and forefinger.
Silty Clay Loams 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, has a slick feeling.
Same as silt loam.
Sat urat ed
Free water appears on soil when squeezed.
Same as silt loam.TABLE 3.2 AIP Farm
r
I Field No I Soil Type I
I
I
I 6/4/A
I Alluvial I
I
I
I
I
I
I
I
I
I
I 6/1/B
1---------- 1--------------r
-
I Depth 1
1
1
0-30
I Sample 1 Water Content on Dry Weight Basis 1 No | 
1 1
i
1
I
1 1
| 
l/3rd Bar
n
1 5 Bar 1
1
r
8 Bar 1 15 Bar |
i
1 1 1
1
I 27.50
1 Bar 1 3 Bar 1
1
1
20.84
1 16.23
I 15.68
r
14.79
1 1
1 1
I 11.88 |
1 2 1 26.41
20.54 I 16.11 1 15.94 14.64 1 12.11 1
1 3 I 27.53 20.83 1 16.31 1 15.74 14.57 1 11.86 I
1
1
1
I mean 1
1
I 27.15 1
1
1 30.25
1
1
20.74 1 16.22 1
1 15.79 1
1
14.67 1 11.95 1
1 1
I 30-60 11
1
1
1 24.26
1 19.37
1 1
1 18.42 17.52 1 15.06 1
1 2 I 30.00 1 24.41 1 19.31 1 18.92 17.65 1 14.94 | 3 1 31.15 1 24.33 1 19.31 1 18.35 17.78 1 14.87 1
mean 1 30.47 1
1
1
1
24.33 1 19.33 1
I 18.56
1 1
17.65 ! 14.96 1
1 1
0-30 1
1 49.81
1
1
1 37.52
1 32.84
1 ' 1
1 30.74 I 28.80
1 1
1 26.84 I
I Vertisol I 2
1 47.03 1 37.73 1 33.14 1 30.97 I 29.05 127.51 I
1 1
1 1
1
1
1
1
1
1
1
1 T I Soil
1 Type 1 1
1 1
1 1
1 1
3 I 47.70 1 38.80 1 32.49 1 31.13 1 30.09 1 26.35 1
1 mean I 48.18
1 30.95 1 29.31 1 26.90 1
1
1 30-60 | 1
1
I
I 42.89
1 38.02
1 32.82 1
1
1
1
1 31.02
1 27.33
1 1
1 1
125.77 | 24.87
1 
I
1 1 ! 22.94 I
1 2 1 43.57 1 31.44 1 27.41 126.12 1 24.62 1 22.88 I
1 3 1 43.84 1 31.63 1 27.18 1 26.22 1 24.99 1 23.22 1
1 mean I 43.43 1 31.36 1 27.31
1
“T Depth |
1 1 1
1
1
1
26.04 I 1
I
I Water Concent X Dry Wt Basis
“T"
I Bulk
24.83 I 23.01 | 1 1
i1 1 ASM 1
1 
" 11------------- I PWP
1
—1------
1
1 Density gm/ cc 1 cm/m 1 i
1 1
i
1
1
FC
I ASM
1
1)
1
1
1
1
1
1
1
1 11.95
1
| Alluvial 1
0-30 |
27.15
1 15. 20
1 1.35
1 1
1 20.50 I
1 6/4/A 1 30-60 1 30.47 1 14.96
1 
1 
| Vertisol 1 1 6/1/B
1 1
1
I
1
1
0-30 | 48.18
1
1
1 26.90
30-60 | 1
43.43 1 23.01 1
1 15. 51 1
1
1 21. 28 1 20. 42
1
1 1.35 1
1
1 1.33 1 1.33
1
1 1
1 1
1 20.94 1
1 1
1 1
1 28.30 | 1 27.16 1 1 1
1) FC = Field Capacity
PWP = Permanent Wilting Point ASM “ Available Soil Moisture
18FORM 3.1
gravimetric soil moisture determination
Dace
Name of Officer Sampling Soil: .....................................................................................................................................................
Field: ........................................................ ................................................................. Sampling Time: ................................
Crop:
| Location I Depth | Can I ID | (cm) | No
Can | Wet Soil | Dry Soil i Soil Moisture | Wt I + can Wt , * can Wt
Bulk I
Soil I
I
Z
I Density I Moisture I
| 
g/cc I mm
Comments
19FORM 3.2
INFILTROMETER TEST DATA
Date:
Location:
Sheet No:
I
I SOIL MOISTURE SAMPLE
Soil Type:
| Can No:
Any other comments .......................................... ..............
1 1
I
gms
1
I Wt of can
| Wt of can + wet soil
I Wt of can * dry soil
| Wt of moisture
I Wt of dry soil
I Moisture content Z
Name of observer
204. Crop Water Requirements
4.1 Introduction
The fundamental objective of irrigation is to 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 under stress that would reduce
yield. Other considerations should be control of groundwater table, leaching,
slow infiltration of the soils and cost 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 parameters
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)
Reference crop evapotranspiration is defined as "the rate of evapotranspiration from an extensive surface of grass 8 to 15 cm tall, green, of uniform height, actively growing, completely shading the ground and not short of water". The reference crop in this case is short grass.
214.3.3 Crop Evapotranspiration (ETc)
The loss through evapotranspiration from a crop has to be made good by supplying the water lost and hence the crop water requirement is defined as the depth of water required by a crop for evapotranspiration during a given period.
The effects of crop characteristics on evapotranspiration are given by a crop coefficient (Kc). Factors affecting the selection of a crop coefficient (Kc) 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, as the term 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 availaDle soil water can be depleted without causing ETa to become less than ETc.
4.4 Calculation of Reference Crop Evapotranspiration
4.4.1 Climacic Data
Climatic data available at the Institute of Agricultural Research, Melka Warer are presented in Table 4.1. These data are used to estimate reference evapotranspiration by four different methods.
The number of years for which different below:-
climatic data are available is given
Rainfall Temperature Relative Sunshine Wind
Pan
Radia-
Humidity
Hours
Velocity
Evaporation
tion
Years 17 17 
17
13
13
13
12
4.4.2
Reference Crop Evapotranspi
ration by
Blaney-Criddle Me
thod
This method is generally used where the climatic data available covers only air temperature data.
The original Blaney-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 3 kf
223
where v monthly consumptive use
k 3 empirical coefficient for growing season
f * monthly consumptive use factor txp
100
t 3 mean temperature in degrees fahrenheic
p 3 percentage of day-time hours of the year occurring during the month.
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 3
c(p(0.46T + 8)) mm/day
where
ETo 3 refer
ence evapotranspiration for month considered in mm/day
p 3 mean daily percentage of total annual daytime hours for given month and latitude
c
3 adjustment factor depending on minimum relative humidity sunshine hours and daytime wind estimates.
The values of ETo for twelve months, using the above be low:-
presented formula are given
Month
ETo mm/day
Month
ETo i
January
5.02
July
6.28
February
5.47
August
5.00
March
5.67
September
6.19
April
6.19
October
5.80
May
6.45
November
5.87
June
7.87
December
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:-
ETO = c(W.R ) mm/day
s
where
ETo 3 reference evapotranspiration for period considered in mm/day 
R
s 3 solar radiation in equivalent
W 3 Weighting factor depending on 
mm/day
temperature and altitude
c - adjustment factor depending on mean humidity and wind conditions.
Monthly values of ETo based on the above method are given below:-
day-time
23Month
January February March April May June
5.29
5.78
6.11
6.56
6.48
6.82
Month
July August September October November December
4.4.4 Reference Evapotranspiration by the Class
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. Plants respond to these same 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 8 percent and from green vegetative surface 20 co 25 percent. Hence 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 at daytime and night time. This is not so with plants which transpire only during the day.
ETo mm/day
7.48
5.57
6.11
6.31
6.06
5.49
"A” Pan Evaporation Method
FAO in its Irrigation and Drainage paper No 24 equation:-
recommends the following
where
ETo = Kp . Epan
ETo = reference evapotranspiration in nnn/day
Epan = Pan evaporation in mm/day
Kp = pan coefficient depending on location of pan, humidity and wind.
Monthly values of ETo using the above formula are given below:-
Month
January February March April May June
ETo mm/day
4.47 4.88
5.08 5.54
6.22 6.49
Month
July August September October November December
ETo mm/day 5.68
4.47
5.17
5.58 
4.88
4.47
4.4.5 Reference Evapotranspiration by the Penman Method
Penman made the most complete theoretical approach to show that reference evapotranspiration is inseparably connected to incoming solar energy. The formula developed by him predicted the evaporation losses from a smooth free
water surface. The basic formula is given below:-
E»
LE,---------------------------------------------------(13)
*•»)
where
E ■ evaporation from a fr
ee water surface (mm/day)
24SECTION 4
Crop Water RequirementsR
n 
3
E * 
x
A=
* =
L
E-
x
U2 -
e sat3 e2 =
net radiation (cal/cm^/day )
isothermal evaporation as aerodynamic term (inn,
slope of the temperature - vapour pressure
relationship at temperature T (tnm Hg/°C)
psychrometric constant (» 0.457 mm Hg/°C) dependent on
station barometric pressure and latent heat of vaporization latent heat of evaporation of 0.1 cm^ (■ 59 cal)
0.35 (0.5 * 0.54 U2) (esaC
-e)
2
wind velocity at 2 m (m/sec) saturated vapour pressure (mm Hg) actual vapour pressure at 2 m (mm Hg)
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 n ’
f(u) « 
net radiation unn/day
wind related function
(ea-ed) 3
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 light.
The total radiation Ra received at the top 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°to 8°. The latitude of Amibara could be taken as 9° 20 N. Ra for the month of July » 15.23 mm/day of evaporation.
While passing through the atmosphere part of Ra is absorbed and scattered, and the balance inclusive of some of the scattered radiation reaches the earth’s surface and is termed ’’solar radiation” Rs. Part of Rs is reflected back by the soil and crops as long wave radiation R j and tnat which remains is net
n
shortwave radiation Rns. Reflection coefficient (o<) depends on the nature of the surface cover. For a free water surface o< ranges between 5 to 8 percent and for a green vegetative surface between 20 to 25 percent.
25ct
P le re^l® ®d longwave radiacion is normally greater than the downcoming
n
Total net radiation Rn = Rns - Rnl
ongwave radiation from the atmosphere. The difference is known as net longwave radiation R ^ and is a loss of energy.
---------------------------------(15)
Where Rs = R (0.25 ♦ 0.5 n ) ------------------------------------------------------(16)
a
ns = (1 - « ) Rs
N
R
-------------------------------- (17)
n is tne 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 table 4.1 » 7.38 hrs.
N = 12.60 hours,
0.25. Hence Rns » (1 - 0.25) (0.25 * 0.5 x 7.38 ) 15.20 12.60
M 6.20 mm /day
R^
n
is determined from available data,
ratio n/N. Values for f(T), f(ed) and 4.9.
b) Vapour Pressure
temperature (T) vapour pressure (ed) and f(n/N) are given in tables 4.7, 4.8 and
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 in the air ed « ea x R mean m bar..
28.37°C, ea=38.60 m bar. The
For July R^ mean from table 4.4 is saturation vapour pressure deficit
H
100
<♦9.712, hence ed = 19.19 m bar and the (ea - ed) = 19.41 m bar
Rnl = 16.4 x 0.145 x .63 
=
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 mm
and altitude. From table 4.10 for July W = 0.987.
Hence WRn = 4.71 x 0.987 » 3.71 mm/day
d) Wind function f(u)
The effect of wind on reference evapotranspiration ETo nas been studied for different climates and is defined as f(u) = 0.27 Zl <• JjJ
\ ±00/ where 0 = wind run in km/day at 2 m height.
2
From table 4.1, for July u “ Hence (1-w) •
9 24 km/da
- y < )
• (ea ~ ed) = 3,58 day
f u is therefore 0.87.
26s
area.
For July RH max = 75Z, Rs - 8.27 mm/day, u day =» 2.57 m/sec and U day/U night - 1 Hence c = 0.97.
Therefore July ETo = 0.97 (3-71 * 3.58)
■ 7.07 mm/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 monthly 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
e) 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 50Z 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 max, R , U day and U day/U night for the project
Crop
Seedling
Vegetative
Reproductive
Fruit
Maturity
Formation
Bananas
0.45
0.85
1.10
1.00
0.80
Peppers
0.4
0.80
1.00
0.90
0.80
Cotton
0.45
0.75
1.15
0.85
0.65
Haricot)
0.35
0.75
1.10
0.70
0.30
Beans )
Maize
0.40
0.80
1.10
0.85
0.60
Onions
0.50
0.75
1.05
0.85
0.80
Sesame
0.45
0.75
1.00
0.70
0.30
The corresponding days for each stage of development is given for the seven crops in Sections 4.7.1 to 4.7.7.
276 Some Crops Grown in Project Area
^•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 nunhos/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 mm 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 to 0.80 m soil depth with 60Z from the first 0.3 m. Generally 35Z 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 optimum. Cotton is tolerant to soil salinity with no percentage yield decrease if ECe ■ 7.7 mmnos/cm.
Cotton needs 700 to 1300 mm of water depending on growing period.
Water Supply
the climate and length of
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 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 m should be fully wetted before sowing and subsequent irrigations should wet the upper part of the root zone from 0.50 to 1 0 m A 70Z depletion of the total available soil water during flowering will
be a saving on irrigation water without impairing yield. During the vegetative riod soil water content over the first 0.75 m of root depth should not fall
^low 50Z. Depletion of available soil over 50Z will restrict vegetative growth //when followed by adequate supply, vegetative growth will be excessive causing late flowering and boll shedding.
28During 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 is sometimes necessary to obtain a second yield.
The following table is indicative of the possible rooting depths at four different dates from planting.
Rooting Depth 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
The 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 m 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 be 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 and bud shedding. During boll filling and ripening depletion can increase from 602 to higher values with irrigation being terminated 4 to 5 weeks before final picking.
4.6.3 Maize
Maize 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 » 1.7 mmhos/cm.
Maize needs 500 to 800 turn of water depending on climate.
29Water Uptake
oxi 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 80Z or more is possible during ripening.
In deep soils roots can reach a depth of 2 m, but the highly branched system is located in the upper 0.8 to 1.0 m where 80Z of the soil water uptake occurs. Generally 100Z of the water is extracted from the first 0.8 to 1.2 m soil depth.
Irrigation Scheduling
Irrigation scheduling should De based on avoiding water deficits during the flowering period followed by the yield formation period. Water depletion is
about <*0Z during establishment, 55 to 65Z during vegetative, flowering and yield formation and up to 80Z 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 mm 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 oe avoided to achieve large bulb size.
Water Uptake
Onion has a shallow root system concentrated in the upper 0.3 m soil depth.
Usu^xy 100Z of water uptake occurs in the first 0.3 to 0.5 m soil depth.
Irrigation Scheduling
Onions require frequent light irrigations when 25Z of available water in first
0.3 m soil depth is depleted. Irrigation is discontinued 15 to 25 days before harvest.
4.6.5 Peppers
Peppers prefer lignt-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 aminos/cm for no decrease in yield. Pepper requires 600 to 900 mm and 125C am for long growing periods and several pickings.
30Water 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. 100Z of water uptake normally occurs in the first 0.5 to 1.0 m 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 mmhos/cm is required for good growth.
The total crop water requirement for Haricot Beans is between 300 and 500 mm 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 m. Ac emergence Che 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 co 0.7 m depth, 402 to 502 depletion of the total available soil moisture is possible when ETc is 5 to 6 mm/day.
Irrigation Scheduling
Under full irrigation, the soil water depletion during 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 equation used is:-
k ET°
ETcrop = c •
4.7.1 Crop Water Requirements for Bananas
Growing Period: 365 days
Planting date: June - perennial 3 years old
------- (18)
Development stage:
Rooting depth (cm):
A mixed stand of suckers actively growing 75
31Month
ETo mm/day
Days
K
c
ETc mm/month
January
5.52
31
0.75
128.34
February
6.19
28
0.90
155.99
March
6.60
31
1.10
225.06
April
6.84
30
1.25
256.50
May
7.33
June
31
1.25
284.04
8.18
30
1.25
306.75
July
7.07
31
1.20
263.00
August
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
123.26
Total
2482.66
4.7.2 Crop Water Requirements for Cotton
Growing Period: 145 days
Planting Dates: (a) 3rd May and (b) 15 May
Development Stages:
15/25/60/30/15 days » 145 days
Rooting Depth (cm):
40/80/120/120/120
(a) 3rd May Planting
Month
ETo nmi/ day
Days
Kc
ETc mm/month
May
7.33
15
0.45
13)
0.75
120.95
June
8.18
12)
0.75
18)
1.10
235.58
July
7.07
31)
1.10
241.09
August
6.38
11)
1.10
20)
0.80
179.28
September
6.49
10)
15
0.80
0.65
115.20
Total
892.10 mm/season
32(b) 15th May Planting
Month
ETo mm/day
Days
K
c
ETc mm/month
May
7.33
15
0.45
49.48
June
8.18
25
0.75
5)
1.10
198.37
July
7.07
31)
1.10
241.09
August
6.38
24)
1.10
7)
0.80
204.16
September
6.49
23)
0.80
October
6.70
7)
0.65
148.95
8)
0.65
34.84
Total
876.89 mm/season
4.7.3 Crop Water Requirements for Peppers
Growing Period:
160 days
Date of Planting:
15th August
Development Stage:
20/40/50/30/20 days
’ 160 <
Rooting Depth (
cm):
20/50/80/80/80
Month
ETo mm/day 
Days
K
c
ETc mm/mo nth
August
6.38
17)
0.40
43.38
September
6.49
3)
0.40
27)
0.80
October
6.70
13)
0.80
18)
1.00
November
5.94
30)
1.00
December
5.68
2)
1.00
29)
0.90
January
5.52
1)
0.90
20
0.80
147.97
190.28
178.20
159.61
93.29
Total
4.7.4 Crop Water Requirements for Haricot Beans
812.73 mm/season
Growing Period:
Date of Planting:
Development Stages:
Rooting depth (cm):
110 days
1st November
15/20/30/35/10 days = 110 days 10/30/80/80/80
33Month
ETo mm/day
Days
K
c
ETc mm/month
November
5.94
15
0.35
December
5.68
15)
5)
0.75
0.75
98.02
January
5.52
26)
4)
1.10
1.10
183.75
27
0.70
139.66
February
6.19
8
10
0.70 0.30 Total
53.23
474.66 inm/ season
4.7.5 Crop Water Requirements for Maize
Growing Period:
Date of Planting:
Development Stages:
Rooting depth (cm):
140 days
(a) 1st November (b) 15th July (c) 1st August 25/35/30/35/15 days = 140 days
40/80/100/120/120
(a) 1st November Planting
Month
ETo mm/day
Days
K
c
ETc mm/month
November
5.94
25
.40
5)
.80
83.16
DecemDer
5.68
30)
.80
1)
1.10
142.57
January
5.52
29)
1.10
2)
.75
184.37
February
6.19
28)
.75
129.99
March
6.60
5)
.75
15
.60
84.15
Total
624.24 mm/season
34(b) 15th July Planting
Month
ETo mm/day
Days
Kc
ETc mm/month
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
188.86
October
6.70
13)
1.1
18)
0.75
186.26
November
5.94
17)
0.75
13)
0.60
122.07
December
5.68
2)
0.60
6.82
r
Total
684.52 mm/season
(c) 1st Aug
ust Planting
Month
ETo 
mm/day
Days
K
c
ETc mm/month
August
6.38
25
0.4
6)
0.8
94.42
September
6.49
29)
0.8
1)
1.1
157.71
October
6.70
29)
1.1
2)
0.75
223.78
November
5.94
30)
0.75
133.65
December
5.68
3)
0.75
15
0.60
63.90
Total
673.467 mm/season
4.7.6
Crop Water
Requirements
for Onions
Growing Period:
Date of Planting:
Development Stages:
Rooting depth (cm):
120 days
1st October
20/35/30/20/15 days = 120 davs 10/30/40/50/50
35Month October
November December January
ETo bzm/ day
6.70 5.94 5.68
5.52
Days 20 11) 24)
6) 24)
7) 13) 15
c
.5
.75
.75 1.05 1.05
.85
.85
.80 Total
ETc nm/month
122.28
144.34
176.94
127.24
570.80 mm/season
4.7.7 Crop Water Requirements for Sesame
Growing Period:
110 days
Date of Planting:
1st January
Development Stages:
20/30/30/20/10 days
’ 110 days
Rooting Depth
(cm):
15/30/60/80/80
Month
ETo lnm/day
Days
K
c
ETc mm/month
January
5.52
20
.45
11)
.75
95.22
February
6.19
19)
.75
9)
1.00
143.92
March
6.60
21)
1.00
10)
.70
184.80
April
6.84
10)
.70
10
.30
68.40
Total
492.34 mm/season
36TABLE 4.1
SUMMARY OF CLIMATIC DATA FOR MELKA WERER RESEARCH STATION
I
I Month
I
I
r
i
i
I Rainfall I
|T‘C
| Km/hr |mm/day | Per day
■]-------- I--------- !--------- !---------- !!!!i
IT°
Ishine hr. | X
I I I I T------- J--------- I----------J--------
Min Tcemp | I MT°ax CTemp | Mean Temp | Mean Sun- | Rel Hum | Wind Vel | Pan Evap | Radia Cal/
January
i
i
i
i
i
i
i
i
i
i
i
i
i
i
i
i
i
i
i
i
i Febr uary
Mar ch
i Apri1
May
June
July Augus t September October November December
I
I
I
I
I
I
I
I
I
I
I
I
I
II
II
I
I
17.79
15.32
31 .47 23.40 8.42
5.23
6.97
47.22 16.60
32.32
24.46
8.57
5.53
58.52
18.65
34 .00 26.33 8.12
50.42 19 .83 35.57
27.70
I 24.00 20.52 37.23 28.88
8.53
9.00
5.74
5.50
6.36
7.50 516.23
8.00 514.83
8.93 560.69
9.98 569.65
I 28.89 23.11 37.98 30.55
11.33
561.00
110.84 21.50
35.23
28.37
114 .80 20.31 33.52 26.92
38.17 20.60
21 .40
17.44
23.43 15.24
35.19
34.90
32.52
I 2.65 13.52 31 .48 I
27.90
26.17
23.88
22.50
36.24
7.38 49.71
7.47 57.88 7.46
7.87 51.65 6.32 42.76 5.70
5.52
5.00
9 .02 527.59 7.37 558.20 8.21 550.90
8.88 544.43 7.92 522.53
7.048$6Si»g***3*
NORTH
KI
READ TEMPERATURE T0C
issssassBasr
B
? 8 £ £ * 5 jr t 1 t a «cif ‘&B2-i!BB2i±
**
o
»
w
RELATIVE HURIDIT* HHX
sSSSSSrSSSH
8C8S88St8*S
s
**
Vi
M
NIRO RUA AT ?i U KR/DAY
siseseiisBsns
c
REAR ACTUAL SUISHINE HOURS ®ER DAY b
2C2SS35gSgl!
»
RAXIRUP RELATIVE HURIDI’* RH RAX X
***jwwww»wwww
8&E8BE88B8B
M
•
B
SATURATION VAPOUf PRESSURE AT TaC
aa • a bar
MwX«e«v»wa~»
skkefekkirBk
k
ACTUAL VAPOUR PRESSURE
ad • aa AX/l00 ■ bar
ekseie’iikisk
»
k
• a - ad
kekbasabkie;
£
u
VINO 'UNCTION *(U) - 0.27 lUlUU*
keiskBakse*
w
r (U) > (aa - ad
ekakecBekisB
w
Vi
EXTRA TERRESTRIAL RADIATION •■/day Ra
kaBsisekske
RAXIRUP POSSIBLE SUNSHINE HOURS PER DA* R
s
BB3tfiXfc25t»
a
w
n/N
kkkkkisesska
k
INCONING SHOR’ NAVE RADIATIOk
Rs - (0.25 • 0.5 • /N) Ra
eskkkeekkks
NE' .ONG NAVE RADIATION
Rfil - MT) F (ad) Fh/«
8
kkaBisakkes
s
total net radiation
Rn . 0.75 Rs - Ra.
aara^lBas^a
a
«c
HEIGHTIIG FACTOR V
JSSskaSSaSSTk
2
N. Rn aa/da»
sseEBesKreB
K
(1 - N)
BtBtskaekse;
e
(1 - ■) F (U' (ta-edi aa/day
BksektttfKS£3
VI
V Rn * (I - V) F(U) (aa-ad
kskkesskskk
c
U DA* /
B aec
^v.v-v-v-0©**-^*-
EgBE esiteeefi
k
CORRECTION FACTOR C
fcfSSSSKSfS-
Vi
K*
REFERENCE EVAPOTRANSPIRATION ETo aa/day
^cEaSseEisxt
betssteoeiss
X
a
REAR RAINFALL aa/dav
MODIFIED HNMAN Ilo = C (W.Rn ♦ (1 W)f(U)(ta ad)) IAlIIUtt - 9«2U'N
LONG 11 Utt
- (LtVAllUN 7J7 ■ A.S.Ltable 4.4 Saturation Vapour Pressure (ea) in mbar aa Function of Mean Air Temperature (T) in °C 1/
1 empei
ature °C
13 14 15 16 17 18 19 20 2! 22 23 24 25 26 27 28 29 30 31 32
ea mbar 15.0 16.1 17.0 18.2 19.4 20.6 22.0 23.4 24.9 26.4 28.1 29.8 31.7 33.6 35.7* 37.8* 40.1* 42.4 44.9 47.6
1/ Also actual vapour pressure (ed) can be obtained from thia table using available Idewpoint data. (Example: 1 dewpoint ia 18°Cj od is 20.6 mbar) I ab le 4.5 Extra-terrestrial Radiation (Ra) expressed in equivalent evaporation in ww/day
Northern Hemisphere
Jan F eb Mai Apr May June July Aug
Sept Oct Nov Dec Let
13.2 14.2 15.3 15.7 15.5 15.3 15.3 15.5 15.3 14.7 13.6 12.9 10°
13.6 14.5 15.3 15.6 15.3 15.0 15.1 15.4 15.3 14.8 13.9 13.3 8°
lable 4.6 Mean Daily Duration of Maximum Possible Sunshine Hours (N) for Different Months and Latitudes
Northern 1 at itudes
Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec
10 5 0
11.6
11.8
12.0
11.8
11.9
12.0
12.0
12.0
12.0
12.3
12.2
12.0
12.6
12.3
12.0
12.7
12.4
12.0
12.6
12.3
12.0
12.4
12.3
12.0
12.1
12.1
12.0
11.8.
12.0
12.0
11.6
11.9
12.0
11.5
11.8
12.0lable A.7 Effect of Temperature f(1) on Longwave Radiation (Rnl)
PC
14 16 18 20 22 24 26 28 30 32
F(I) = 8 Tk*
13.5 13.8 14.2 14.6 15.0 15.4 15.9 16.3* 16.7 17.2
1able 4.8 Effect of Vapoui Preaaure f(ed) on Longwave Radiation (Rnl)
ed mbai
14 16 18 20 22 24 26 28 30
f(ed) = 0.X - 0.0«(ed)
.18 .16 .15 .14 .13* .12 .12 .11 .10Table 4.9 Effect of the Ratio Actual and Maximum Bright Sunshine Hours f(n/N) on Longwave Radiation (Rnl)
11
I n/N
I .55
.6
.65
.7
.75
.8
.85
.9
| f(n/N)
=
0.1 + 0.9 n/N | .60 
.64
.69
.73
.78
.82*
.87
.91
11
1
1 1
Table 4.10 Values of Weighting Factor (W) for the Effect of Radiation on
Alt itudes
1
ETo at Different Temperatures and
| Temperature °C 14 16 18 20 22 24 26 28 30 1
r
32 
r
| W at alt i tude m
| 0 .61
| 500 
| 1000 
.64 .66 .69 .71 .73 .75 .77* .78 .80 |
.62 .65 .67 .70 .72 .74 .76 .78 .79 .81 |
1
|
1 •
“I 1
.64
1
Table 4.11 Adjustment Factor
.66 .69 .71 .73 .75 .77 .79 .80 .82 |
(c) in Presented Penman Equation
1
i
r
1
| 
| RHmax 
= 60X
I RHmax =
90Z
1 1
i
| RS mm/day | 3 6 9 12 1 3
r
r
1
1
1
1
n
69
1 
1
1
12 | 1
1
i
| U day m/sec | U day/Unight =
r
1 .0
1
1 1
i
r
1.05 1 1 .02
r
I 0 
I .96 
.98
1.05
1.06
1.10
1.10
1
1
|3 
1 .78 
.86
.94*
.99* 1 
.85
.92
1.01
1.05*
1
|6 
| .62 
.70
.84
.93 1 
.72
.82
.95
1.00
1 
1 -
1
1
15• Irrigation Requirements and Scheduling
5.1 Irrigation Requirement
Irrigation requirement 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 891.62 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 891.62 - 167.67 » 723.95 mm. If this
depth of water is applied to one hectare the duty is 723.95 x 10,000 • 7240
1000
m^/ha.
Therefore we say the net irrigation duty for cotton is 7240
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 Warer 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 mm and the irrigation interval (i) in days. The readily available soil moisture in the
root zone (D) in mm must meet the crop water requirements ETc mm/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 total available soil moisture Sa mm/m which is the soil moisture between field capacity Sfc and permanent wilting point Sw, 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.
In general, Sa in mm/m for different soil texture is:-
Fine textured Medium fine textured 
200 mm/m co 280 nnn/m
140 mm/m to 200 mm/m
However, the field capacity and permanent wilting point for representative samples of soil covering the whole project area as development proceeds should be undertaken by the Water Management Unit with the help of the Institute of Agricultural Research and made available to Unit Managers of the Ministry of State Farms Development.
425.3 Soil Moisture Balance
The running soil moisture balance for any period is given by:-
We - Wb ♦ Ge ♦ Pe - ETc
------------------------------------------------------ (19)
where We « soil moisture available at the end of each period
Wb ’■ soil moisture at beginning of each period from
previous irrigation or rain
Ge =■ groundwater contribution
ETc ■ crop water requirements
Pe = effective rainfall
5.3.1 Groundwater Contribution (Ge)
The groundwater contribution is determined by how far below the rooting depth the groundwater table is located co affect the soil moisture content in the 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 mm/day
Organic
Clay 
Sandy Loams
Fine Sandy Loam
0.3
0.8
1.5
4.5
5.3.2 Stored Soil Moisture
The scored soil moisture is primarily affected by evapotranspiration. Whether this storage is within the total available soil moisture range for 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 certain critical deficit is reached. In areas with high radiation conditions, a surface 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-irngation up to planting of cotton seeds. Water is
lost also from the root zone due co deep percolation when the groundwater table is deep.
The stored soil moisture in the root zone can be determined gravimetrically.
43SECTION 5
Irrigation Requirements and SchedulingAMIBARA IRRIGATION PROJECTINFILTRATION TEST AMIBARA FIELD 5A/2B INITIAL SURFACE MOISTURE CONTENT 37 50% 'Intake rate cm/ hr
■■
Time in minutes
INFILTRATION TEST MELK A SADI FIELD 4/1
INITIAL SURFACE MOISTURE CONTENT 25 8 %
a
c
X
m
OiIntake rate cm/hr
Date 12 I 81
• m-..—Time in minutes
TyTAM.™5A/6A LQCAJIOfJIU
FIGURE 3 3Intake '■ate cm/hrFIGURE 3
PERCENT SAND
PHYSICAL CHARACTERISTICS OF SOILS5-4 Effective Rainfall (Pe)
5.4.1 Definition of Effective Rainfall
Effective rainfall (Pe) is defined as that portion of mean (or dependable)
rainfall that is utilised for meeting crop water requirements during the growing season of a crop. Not 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 mm 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 (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 that can be expected in 3 years out of 4 (75Z exceedance probability) or 4 years out of 5 (80Z 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 soon 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 tapped 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:-
445.4 Effective Rainfall (Pe)
5.4.1 Definition of Effective Rainfall
Effective rainfall (Pe) is defined as that portion of mean (or dependable)
rainfall that is utilised for meeting crop water requirements during the growing season of a crop. Not 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 mm 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 (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 (75Z exceedance probability) or 4 years out of 5 (80Z exceedance
probability).
The method chat 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 co total rainfall. Initial moisture content
in a soil also has a bearing on the extent of effective rainfall. If there is
rainfall soon 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 tapped 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:-
44(20)
wnere
Month
P = m x 100 — 
N+l
P « plotting position 
N = total number of events
m - rank of each event
Mean Rainfall
Probable Rainfall
mm/month
3 out of 4 years
January
17.8
February
47.2
March
58.5
April
50.4
May
24.1
June
23.9
July
110.8
August
114.8
September
38.2
October
21.4
November
23.4
December
2.7
ram/month
0
0
30.0
26.5
6.5
3.5
85.0
78.5
24.0
0
0
0
The 75Z probable rainfall is usea 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 mm and from Section 5.4.4, the 75Z 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/monzh. 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 mm.
45
/TABLE 5 J Average Monthly Effective Rainfall aa Related to Mean Monthly Rainfall and Mean Monthly Conaunptive Use (USDA, SCS)
Monthly
Mean
Mean Monthly Consumptive Use mm
Rain- 25 50 75 100 125 150 175 200 225 250 275 300 325
350
fall mn
Mean
Monthly
E ffective
Rainfall mm
12.5
7.5
8.0
8.7
9.0
9.2
10.0
10.5
11.2
11.1
12.5
12.5
12.5
12.5
12.5
25.0
15.0
16.2
17.5
18.0
18.5
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
50.0
25
32.2
34.5
35.7
36.7
39.0
40.5
43.7
47.0
50.0
50.0
50.0
50.0
50.0
62.5
at 41.7
39.7
42.5
44.5
46.0
48.5
50.5
53.7
57.5
62.5
62.5
62.5
62.5
62.5
75.0
46.2
49.7
52.7
55.0
57.5
60.2
63.7
67.5
73.7
75.0
75.0
75.0
75.0
87.5
50.0
56.7
60.2
63.7
66.0
69.7
73.7
77.7
84.5
87.5
87.5
87.5
87.5
100.0
at 80.7
63.7
67.7
72.0
74.2
78.7
83.0
87.7
95.0
100
100
100
100
112.5
70.5
75.0
80.2
82.5
87.2
92.7
98.0
105
111
112
112
112
125.0
75.0
81.5
87.7
90.5
95.7
102
108
115
121
125
125
125
137.5
al l2T"
88.7
95.2
98.7
104
111
118
126
132
137
137
137
150.0
95.2
102
106
112
120
127
136
143
150
150
150
162.5
100
109
113
120
128
135
145
153
160
162
162
175.0
at
160
115
120
127
135
143
154
164
170
175
175
187.5
121
126
134
142
151
161
170
179
185
187
200.0
125
133
140
148
158
166
178
188
196
200
225
at
I9T
144
151
160
171
182
250
150
161
170
183
194
275
al 240
171
161
194
205
300
175
190
203
215
325
al 2B7-
198
213
224
350
200
220
232
375
at 331
225
240
400
at 372
247
425
250
at
414
450
25 >0 75 100 125 150 175 200 225 250
lable 5.2
Effective Storage
20
25
37.5
50
62.5
75
iqo
125
150
175
200
Storage Factoi
0.73
0.77
0.86
0.93
0.97
1.00
1.02
1.04
1.06
1.07
1.08Table 5.3 Effective Rainfall for seven selected Crops
Crop
Bananas
Cotton
Cotton
Peppers
Haricot Beans
Maize
Maize
Maize
Onions
Sesame
5.5 Leaching Requirements
Effective Rainfall
mm
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 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 
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 the leaching requirement is given by the equation:-
LR
where ECw =
ECe
ECw -------------------------------------------------------------------- (21) 5 ECe - ECw
EC of irrigation water mmhos/cm is an average of 0.35 mmnos/cm for Awash River water.
ECe of the soil saturation extract for a given crop appropriate to a toleraole degree of yield reduction mmhos/ cm.
For a 100Z yield potential the value of ECe is given below:-
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 taken as 1.5 mmhos/cm for crops other than cotton grown in the project area.
LR -
0-35 _ 5 x 1.5 - 0.35
= 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 ------------------------------------------------------------------------- (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 neede^ for leaching and application losses. Hence gross irrigation requirement is given by the equation:-
Ig « In x 1 
1-LR Ea
where Ig = gross irrigation requirement Ea » application efficiency LR = leaching requirement
------------------------------------------------------ (23)
The gross irrigation requirements for the seven selected crops neglecting stored 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.
Crop
ETc
Pe
mm/season
mm/season
Is
mm/season
Bananas
2483.23
253.91
2344
Cotton
891.62
167.67
730
Cotton
876.89
162.97
720
Peppers
812.73
61.52
790
Haricot Beans
474.66
0
499
Maize
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
Depth of application d, is 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.
43Table 5.3
Effective Rainfall for seven selected Crops
Crop
Effective Rainfall
nim
Bananas
253.91
Cotton
167.67 - 3rd May planting
Cotton
162.97 - 15th May planting
Peppers
61.52
Haricot Beans
0
Maize
25.59
Maize
133.06 - 1st July planting
Maize
70.90 - 1st August planting
Onions
0
Sesame
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 
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 the leaching requirement is given by the equation:-
LR where ECw =
ECe -
ECw ------------------------------------------------------------------ (21) 5 ECe - ECw
EC of irrigation water mmhos/cm is an average of 0.35 mmhos/cm for Awash River water.
ECe of the soil saturation extract for a given crop appropriate to a toleraDle degree of yield reduction mmhos/ cm .
For a 100Z yield potential the value of ECe is given below:- ECe mmhos/cm
Bananas
Cotton
Peppers
Onions
Maize
Beans
Sesame
1.0
7.7
1.5
1.2
1.7
1.0
1.5
The average value of ECe is taken as 1.5 mmhos/cm for crops other than cotton grown in the project area.
IB
=
0.35 5 , 1.5 - 0~35
= 0.049
47For cotton LR 3
0.35_____________ 5 x 7.7 - 0.35
0.009
3
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 --------------------------------------------------------------------------- (22)
where In 3 net irrigation requirement. The other terms in the equation are defined in para 5.3.
5.6.1 Gross Irrigation Requirements
Other chan for meeting the net irrigation requirements, water is neede^ for leaching and application losses. Hence gross irrigation requirement is given by the equation:-
Ig 3 In x 1 
1-LR Ea
where Ig 3 gross irrigation requirement Ea 3 application efficiency LR 3 leaching requirement
-------------------------------------------------------(23)
The gross irrigation requirements for the seven selected crops neglecting stored 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.
Crop
ETc
Pe
mm/season
mm/season
mm/season
Bananas
2483.23
253.91
2344
Cotton
891.62
167.67
730
Cotton
876.89
162.97
720
Peppers
812.73
61.52
790
Haricot Beans
474.66
0
499
Maize
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
Depth of application d, is the depth of water required co 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.
48Therefore where
d » p.Sa.D Ea
----------------------------------------------
-------- (24)
d « depth of application in mm p = allowable depletion fraction
Sa » total available soil moisture in mm/’m D « rooting depth in m
Ea - application efficiency Z
Table 5.4 gives the soil water depletion fraction (p) for cotton at different ETcrop.
Table 5.4
ETc mm/day
2
3
4
5
6
7
8
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 co 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 (days)
- 12
28.6
14
2 30 51 65 79 13 13 13 13 13
28 21 14 14
93 114 135 13 13 13
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 equation:-
i = p.Sa. D
-------------------------------------------------------------------------------- (25)
ETc
5.9 Calculation of Irrigation Schedules
An irrigation schedule should De 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 dace is 15th May and the growing period 145 days. a net pre-irrigation of 220 mm is applied 8 days before planting co fill the soil moisture reservoir of the root zone co field capacity, for efficient weed control prior to planting and to flush down che salts which may have been carried up by capillary accion during che dry season following harvest of che previous crop, and becter yield
49according to data available at the
Melka Warer Institute of Agricultural
Research.
,
May
June
July
August
Sept
1. ETo mm/day
7.33
8.18
7.07
6.38
6.40
2. Days
15
254/5
31
24/7
23/7
3. Kc
.45
.75/1.1
1.1
1.1/.80
.80/.65
4. ETc mm/month
49.48
198.37
241.09
204.16
148.95
Oc
6. 8
.6 34
5. Probable rainfall
81.56 68.48 12.93
May
>220
June
July
August
Sept Oct
6. Wb
+140.52
-57.85
-217.38
-353.06 -489.08
7. Pe
8. ETc
0 
-79.48
0 
-198.37
>81.56 
-241.09
>68.48 
-204.16
>12.93 0 
-148.95 -34.84
9. We
+140.52
-57.85
-217.38
-353.06
-489.0 -532.92
10. Total available soil moisture
for the
soils in
the project
area is taken as
220 mm/m
11. Rooting depth D mm
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.D
33
83
110
121
132 143
15. (I - p) Sa.D
33
82
no
99
88 77
16. plot Sa.D and (I - p) Sa.D at end of month as given, figure 5.2.
17. plot We as given.
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 for that month.
May 
June
July 
August
Sept
Nil
3 
2 
1 
1
21. The length of the vertical lines between Sa.D and (I - p_) Sa.D gives the depth of net application required in ram.
June
50 
93
August 115
60 
HO
73
50Therefore where
d = p.Sa.D Ea
---------------------------------------------
- (24)
d « depch of application in mm p ■ allowable depletion fraction
Sa = total available soil moisture in mm/m D = rooting depth in m
Ea 3 application efficiency Z
Table 5.4 gives the soil water depletion fraction (p) for cotton at different ETcrop.
Table 5.4
ETc mm/day 2 3 4 5 6 7 8 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 aDsence 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 of application cm Interval (days)
- 12
28.6
14
2 30 51 65 79 13 13 13 13 13
28 21 14 14
93 114 135 13 13 13
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 equation:-
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 througn October. The planting date is 15th May and the growing period 145 days, a net pre-irrigacion of 220 mm 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 the salts which may nave been carried up by capillary action during the dry season following narvest of the previous crop, and better yield
49
.
✓according to data available at the Melka Warer Institute of Agricultural
Research.
,
May
June
July
August
Sept
Oct
1. ETo mm/day
7.33
8.18
7.07
6.38
6.40
6.70
2. Days
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 tnm/month
49.48
198.37
241.09
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 the soils in 
the
• project
area is
taken as
220 mm/m
11. Rooting depth D mm
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. D
33
83
no
121
132
143
15. (I - p) Sa.D
33
82
no
99
88
77
16. plot Sa.D and (I - p) Sa.D at end of month as given, figure 17. plot We as given.
! 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 for that month.
May 
June
July 
August
Sept
Nil
3 
2 
1 
1
21. The length of the vertical lines between Sa.D and (I - p__) Sa.D gives the depth of net application required in mm.
June
50 
93
August
115
60 
110
73
5022.
The
horizont
al
spa
cing 
of
the
vertical
lines
give
s
the
irri
gation 
intervals
from pl
anting.
June
July 
Augus
t
Sept
36
6
7
11
15
22
27
23.
The
net
depths
of
applicati
on 
should 
be
corre
cted 
for
le
aching 
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 is 130 mm and the manageable flow race 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 mm over an area of A hectares is given by:-
t = 130 1000
x A x 10,000 = 2 .41A hours 3600 x 0.15
Applying this equation to part of
Field Area
No
1
2
3
4
5
6
7
8
9
10
11
(ha)
11.39
22.78
11.39
12.02
17.56
24.56
21.63
11.45
14.42
20.90
32.98
t hrs
Unit I Amibara the following data is obtained T
days
60.4 2i 120.7 5
60.4 2i 63.7 3 93.1 4
130.6
5i
114.6 5
60.7 2i 76.4 3
110.8 5
174.8
7t
The commencement of the eight pose -planting irrigations is fixed as follows:-
Days from planting 2 30 51 65 79 93 114 135
Hence the schedule given below is obtained assuming planting takes place on 3rd May.
51Field
No
Area
Post Plan
ting Irrigat
ion 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
22.78
8.5
5.6
26.6
10.7
24.7
7.8
28.8
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
5i
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
3.8
17.8
31.8
20.9
12.10
2i
9
14.42
4.6
2.7
23.7
6.8
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
13.8
27.8
10.9
30.9
22.10
7i
Interval da
ys
28
21 
14
14
14
21
21
5.10 Establishment of Rain Gauge Stations
It 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, Melka Warer, 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 daily rainfall data for 1980, 1981 and 
1982.
Some fields are dry planted without pre-irrigation due to lack of time. Figure 5.15 is the regime adopted for dry planting.
52FORM 5.1
SOIL WATER BALANCE METHOD OF SCHEDULING
Unit No: ..............................................
Field No: .......................................................
Month: ..........................................................
53AGURE 51
EFFECT OF SOIL SOLUTION CONCENTRATION ON THE HEIGHT OF COTTON SEEDLINGS ONE WEEK
AFTER GERMINATIONFIGURE 5 250 40 30 20 •0 5 2 I 0 5 02 01 0 05 O0I
Probability of non-excedence %-----------
50 60 70 to 90 95 96 99 996 999 9999
JANUARYMeon romfall in mrr
£ c awnou
I 0 5 02 01 0 05 0
Probability of non
IT
-excedence %--------
50 SO 70 SO
JANUARY
95 98 99 998 999 9999Mean rotntoil in mm
w
J
W C.
01
Pa
2i
11
0 5 02 Ol 005 0
Probab lily o f noi l-exa Iderice %------
1 Til—I
-
—
-
---.
—
_
-
-
--•
-—
— -
— — — - - - ------
-
L
\
-
3 10 20 40 50 40 70 t0 50 55 95 99 999 999 9999
FEBRUARY
L
<
FIGURE 5 499 99
99 9 998
99 98 95 90 80
60 50 40 30 20 10 5 2 I 05 02 Ol 0 05 OOl
1
*
riuuuui
'~
Ider
l
iny u
11
I1UI
)-exci
ice
%—
140 130 120
no
-
--
—
—
—
—
---------- ----
--- .
100
E 90
E
c
80 70 60
50
40
30 20
10 _
0_
OOl
-
—
-
■
-
-
Pro
bobili
♦y of
9XC
Bdenc* ‘
7.
005 01 02 05 I 2 5 10 20 40 50 60 TO 50 90 95 99 99 999 999 9999
FEBRUARY
FIGURE 5 4
•
■
1FIGURE 5-599 99
I
FIGURE 5 6FIGURE 5-7FIGURE 5-7Mean romfoll in mm
FIGURE 5*8L
I I
*
nFIGURE 5- 9romfoll m mm
FIGURE 5 10FIGURE 5 IIFIGURE 5 IIMean rainfall in mm
OCTOBER
FIGURE 5 12Mean rainfall m mm
NOVEMBER
FIGURE 513APPL DEPTH
NET <m "GROSS I MAY I JUN 1 JUL 1 AUG L SEP J OCT j NOV ]
220 314
|
PLANTING
BLACK CRACKING SOILS
9 1 130
1
I
|
Pre- irrigation
18 hr 30 mt
91
130
1
1
1
1
Post plant irrigation
6 hr
91
130
1
91
130
1
1
1
1'
1
1
91
1 30
1
|
:i
1
91
130
1
1
1
ii
11
28 1 21 1 14 1 14 14 1 21 I 21
9 1 130 | DAYS_ | DAYS. 1 DAYSJ DAYSj DAYSJ _DAYS J) AYS
220 314
PLANTING
77 110
77 1 10
77 1 10
77 1 10
77 1 10
77 1 10
77 1 10
77 1 10
LIGHT BROWN SOILS
Pr«-Irrigation 20 hr Post plant irrigation 3 hr. 30 mt
I
I
I 21 15 | 15 | 15 L_days_ L°^s_l days_1 9^
1
1
15 1 15 1 15 , 21
1 2-AL9 J_ DAYS__l DAYSj _days
IRRIGATION REGIMES
FIGURE 514APPL DEPTH
(mm)
NET
2 20
GROSS
314
MAY | JUN 1 JUL | AUG | SEP | OCT | NOV
PLANTING
BLACK CRACKING SOILS
9 1 130
9 1 130
9 1 130
9 1 130
9 1 1 30
9 1 130
9 1 130
|
1
1
1
1
11
1
1
1
' 28
| DAYS
1
Pre- irrigation
I8hr
30 ml
11
11
Post plant irrigation 6 hr.
111
111
1
ill
11111
1 21 1 14 * 14 1 14 1 21 | 21
| DAYS I DAYS_[ DAYS_[ DAYS_[ _DAYS _1
220 314
77 1 10
|_____________PLANTING____________
||
77 1 10 1
LIGHT BROWN SOILS Pre “Irrigation
20 hr
111
77 1 10 1 1 -
Post plant irrigation 3 hr. 30 mt
77 1 10
1
11
III
77 1 10
77 1 10
77 1 10
77 1 10
11111
1
11111
11111
1111
till
1'1
1
1
I 21 1 15 | 15 | 15 1 15
11
1 15 1 15
1
21
|_DAYS 1 DAYS [ DAYS | OAYS_j DAYS DAYS_j
DAYS , DAYS
IRRIGATION REGIMES
FIGURE 514APPL DEPTH (mm)
NET GROSS
[
MAY
I
JUN 1
JUL
| AUG
I
‘ NOV
planting
]
9 1 130
9 1 130
9 1 130
9 1 130
9 1 130
9 1 130
9 1 130
9 1 130
SEP | OCT
BLACK CRACKING SOILS Post plon1 irrigation 6 hr
I I I
21
DAYS
14 14 I 1144I 14 14 1 21
I
I
21 | 21
|_DAYS_l DAYS^ |_DAYS
I OAYS
L days j_ DAYS-
PLANTING
77 1 10
77 1 10
77 1 10
77 1 10
77 1 10
77 1 10
77 1 10
77 1 10
77 1 1 0
V
LIGHT BROWN SOILS
Post plant irrigation 3 hr 30 mt
13 15
lLDAYSJ days] DAYSJ^DAYS
15 15 15
I
I
I '5 15 .
I '5 I _21.
.
[ DAYS | DAYS 1_DAYSJ. _2AYS_
REGIMES FOR DRY PLANTING
FIGURE 5-15AMIBARA IRRIGATION PROJECT
WATER MANAGEMENT MANUAL
PROPOSED RAIN GAUGE STATIONS
MELKA SApI
MELKA WAPE-r
no a \ An Mr* Ai_ e iAMIBARA IRRIGATION PROJECT
WATER MANAGEMENT MANUAL
PROJECT LAYOUT
/ Unit 7(406 94Hos
Unif5 (379 00 Ha.)
Unit 3(597 18 HaF
HDAUUIMO Ki- c Ob
I 1
■
B
B
IB
:< Petree Jtr
i . i.< f ! »t > wjf nirl* j *
»*»tw irr.g.'ie* »• »w w<*hM •» »rrkgati«* «»•< tof '•• <»«»• TW
*•*4 *«4 iu
wi»m« »••• 4»t«rwa« Jh. •»•• ••< **»• •*'••’• »<f fwrrewa
far th. «re». Irtlgat. — »• MtaepJieWd hr nui q ester i«t» tbe twraea
lM HtWlr4
•* •<»»•• tw •(«*• »f • f»e|4. Water laHitr.t.. leto th.
M«; »v< »^f.*4. laterellr r. irrigat. «*• erega ba tee.a ferreea. Th. deration
• f Un^UM M !W ;»M<* •• **•• ,V*
»• <• <*«• •• ’**• <’»»■•• 4.p.n<l«
<m iU
f eater reg*.red te ir».»«i»* th. root «OM. <be »•( i It rat »<>«•
«ata «.f the M«i. *«4 <*■• rat. • ( lateral .»r.ed •»/ eater io the Mil.
f-*-—t* i«gMt >• g.aar.l.r
f«r ferrew irrigeliaa than .at ether oethoda t
.»*(•<• ttftgatiaa. Mil.ed »rt»get.»e ata •e.4.4 ta dirt da the eater i« th. f,..4 ■awl »at. • eetaber ■< f ar r *e .tree*. ..4 an.eta^a the correct rate* ot f . *• wet 4 1 irrigate** i« *ow>ef«e
♦ 1 Fjjrr
n.J,; dhereater lttice a*d • FW« *< Per row Irrigatken
H^aref te HWr wthwda ef eaffade irrigative, < be twrrw
hat dietmcf
wdeaai •<• • *•**' i» the fartwa it is ext art with otif Half (9 one fifth of
tba .and e*rfe<e. thia redwea eeeperefiee hiwi. emoting et th# a^tl and earlier cwltleetiea it petetble.
Pert** irrigative ripairri
•ncite length ef ’ba r w
• and grading an rhgt enter can travel (ha
pMdiag*
bVagnacy atficienry »< IP* .get inn depend* Mi*kdarab1r on ebo shape *»f (he f^rrw*. 7Sw <tMt«M((laa af !U f *r r wa e4kml4 be ewfticieat (a carry * diarbtrge we-4e4 te <» *• *•» t .re 4«*trtbwtieo thrflR»gbo*r the furrow.
Th. w*t :aa»*n Jtftw • • V«Aa*e4« te depth IS t® 20 ceatieetree and M t<> X;
r<«!.wtr«* e»da at the tap. Par row ahepaa are notified be the emeut of water *<wa (be alepe, a aarrw rhaneei being feme* for steep elope* and a broad
tiaw. for f lat tar *lepoe.
d.2.2 Pbrrw Spacing
ta general (iff** *po<ing *• ieCsnained by tbe crop. nachinery seed and cultural practices adopted. Kwveer ta farx af irritattm practice, aelectiM of farrov
•pa..<a It fa ooare that <w lateral weewa* af eater befwea adjacent farrowa eill eet the eat I re raet one et the crap, btttiai patteraa caa be derenamed be <*«>M • treach acraea a farrov attar it la irrigated. The eaamnatiea of
(he eettiag patter* tar • eiah ir ef farreve ia ah; ch wear bat rua for different length* ot tiw ie the beat way to decide ferrw apeciag for any particular
eoil.
a, 2. ) Par r jw Slapea
Farrawt aboaJ Mee a waitoro alepe far efficient application of water. Vaewea
•
•
lajuring the crop ia alec preeeet.
eaewea depth* of irrigative dloag the furrow with erosion in
t m (latter ar.aa. Th. danger M ponding and
•'••• ••« 4«w«U ««
M"He pt l ip lea gweraaeg the f«ev at wrier le • • epee theMel epplt 1* **♦ f«w
>f water n < Urrw ehk.h It a
tluvi4< n the chaaee: re therefore
epee <**•**>. ^he nl^Ht *1 water
ta th* »<»<'« • >**< •< <ha •’♦ r*
3t Che furtw, te the taape a*4 rawgheeaa e( the fertw and flee <•<•- *h* velocity aiu>wL4 w< >e re big* aa la <»ae aeiJ aw-rww*!.
The tufa ->f the talti* at the ftirw lalHilei the eteapweaa at the (•<(« aa4
la eipreeaed <• a /aGaaiH*
T%e tiape teweee the water la flaw a»4 alee
;r;vLlai drat vegw tee rum raiefall ar irrigative water The oftiiwa • •<»>*
•iu>u 14 x JJ$l fa 4*01 tar krt^ti la IM prefect area. The MUM
ttnutaiiu il/rt ae if/*W 0*M.
The whim we*earxiw (<** ut< «• far feet u (ieve Itf !U egwatiea
Q*
I
after* 1 • a l ege it terrw Wtt.ae i« per realag*«
4.2.4 Ferrw *e*|<be
•nt >f irvtgMbt* iBcetiMi at fetrwe W<<» ehart ar eat *a l f a mi! v ef eater i^ph atiea hiMiwi aa (trftet beta* Iaeger. The re far a (er raw length thould ^rtii a reaeeaabla arplicaliee etficiexv.
tf • m L>fig « tecrwe it reed eireaaiwe water eetere the eeil at the tapper ee4 xt ire *.he lever eed it edegwet a 3 y irrigafaf Th. • It prMMMed »• M»Jl with
;>)
>i<n tafitiretkae relet. Grr-a
ehenld W adverted te actcwet (er
hangee m titii • rpe Vhera eeiJi >• the eave fia'.d have different aetata rater, ’etui xeidered IB detereieitg niiM f»ticw ^eagtta are the t »p< af roll,
live ft irrtpCiMi, a;»pe. rW the etrree »«te.
r arrive vui xreeeariiv w eherter far
eparitiee rhea far Her eetle nth Mg* water held)ng lapectllea.
a-lie having I «w velar hr; a«rg
Fsir roe lengths ie iw
area are tiaed la <oef«rw with «< *»»•. >; .a&4
levelling red Mf ig*t at kte at fieida. Thar are geeere.lt ?OOe t« STMl l« Ieogth.
4.2.5 f iae lac er
’X tire at itriaa delivered «ate each farrev it <4lJed the (;<•> rate im
<e«erallr watered »a litres per x< wed.
fleet he fix rare te fhe K If ver i at; a
C*<< Ferree .eegthe «W
«r»
r
ait>ra iirtribetiM .« tteaiwd hr etertiM the irrigetiM with I he largeat
f;w U! the farr v aa eafeif /arrf. Fee ’he 4ee«rv4 vetfiag ef the eeH th*
water ahewi4 rear* the vw4 at the terra* eilhift a eaercer ef the total tier that water ever x carried ie cw farrw te refill the reel r*M (• field capacity.
Thee the ^ppartaeirv nee far the rail t-i ebeerh eater et 11 he abw^t n perceet greater et the »pp*r cbaa ar rhe iwer <ed af the field* Figure e.|.i be •<•«« bee rae< w< tw Start >f the furrow the flew vate it
»4>e<etf or *wi bath La watataie <w flaw vtiWvf ImIm water by whit
r we-eft e< it* <*4 af ’be f«trtt TMi flaw rate •• e^ual t« tW rate water it lafHlretia* **•* «M *WO • jwdgewaft
♦.J.a
ef Kr«a Itea
:* H*IM* eeprewi eat e |y ♦<**: flaw are 4tt<*arga4 ta<* • wabat at ferrate at
the mw hw
*• *r*e< **wbet at ta*n etpbeaa
farrow. lataa of flaw
are altered either W IMraaafttg x aarroaeie* the
at W *a*a.«4 the preeewre toad. ?W pretowre head it the differeece it.
• loot law Wiw«« IM water warfare ia the field 'anal and centre ef eyphue
•»>< .<< if flowing (few at the water warfare th-re the oat la t if it it tvtaaarged.
e< vypheet jwr tarrow
The beetgawd eteaewto Mrf far the prefect
<• Hew a*d venae bp ♦ J
<.« f
4«wre«o
ef Meter applied
19 the f«rrw aparaag. fvrrw length, fie* rata eM dwrattoa of tmptiat are a uctrt. < Ua
Meter epglMatlee aiit. • {llrO/4 4 MOI — ■- ■
we;
Where e ♦ »M'IH at feftw .• •efret t • twagtlb at far tew m aotfee
— • ■■■.■» ■■■■- (g?)
W«: •*•*** W
la Ma the «•&toreify at water pewtratio* at
t.ffereat pn*<* a.awg !err»W. let ar litre <aye after aa irrigative.
* • J hnw fra.•at s a*
5*4 11 ••
TW ewriH aW)w«iwe ef eoaleatiag a t«rrw inijtUM tv a tea it fa de terwane
*•«*♦* tM arete* pr.wtde* tm repair ad oerwat ef water ta the croyr waiforwly <** t»n» >«M<* *‘»» >»tlM »at«r
eeelwetaew will give tw fallwwaag laferwet iaai •
at F«trw a lap*
Bl ?iwr at irrigatiM
OptMMt farrw .eugch
4) 3pti»*a etreaa tiaa
b.).t laitial CallettAaa of 3eta
!•»?•<' <»• •«•• ta *» «*ai«4'«4 m1m( ••«•• altcractt ferrovs for • !«
«Uf«r«»< tl — rat«« th. f«rr«M cm t.f.ly carry «M -,w furrow for Jctaratainf
«ff>ct«»<y. An
th. t«rr*w
rat. ..ihc n« y- p«r»Mll
Twc lata MUfe t^e trial, atatea Miwm.4 W Oiwi i"a the groaM oa rhe farrow „4ge at J* lateroala for ea<h of ei. ferret. atarnM at rhe field canal end
A
Cm 4
•ewale are . ear a.oag the Wd of each farrow to daterwiae the average 4*«ira »gth. a. - U >f iMietdwal .;oFet between ataMea. The
.•vw.t eaw.d ba related (a <• arbitrary darwe. Tba farrow tracing and depth of
$4Hi* priaciplea jwaritJif tit flaw at eater is oa -*foe •booool •Ffri* t* *K> C . .w >f voter io 4 furrow oft i eft io a mm . I <^oa iOmmwoI. The wla<H» «f voter flawing til tfto .'MM41 it IWtrftH proper I * eriai 1a t bo »<*<>• iw< at l*a alofo
jt tee furrow. o tu toope avd
eelocitr tftouL4 wi bo w hid* ae <a <>•*♦ aa»i eeewWMl •
at (fee forrw ood flaw rate, TV#
Hee elope of the
of (M farrow lediraloe fho atoapeoaa of the Carew aod
u caproaood at < porevetaet. TVa aJepe <o«aoa |W rtf«f to <«<w a»4 aloe provides Oreioepo for oaroat rata/all ar irrigative valor The opl le»«o alepo thuoid he 0 Z01 to t -Wt far Hire* ie IW pre • << ••••’ The wiiw
perwitoihla tljpo la aratot i.flt.
M<<au«o aoe~orfvei«v flaw rata la
Q■
1
Mftoro S • a:»eo of farrow W< tw >• torr root 040.
h.l.o Furw wewftha
u giooa >r tbs oftoet »** *
ixwt >f imoac&M looreeeot oa fvrrwo WtiMM ofeorier aod aoifamcv of voter
appi i < at i it dooreaooo at (octave Wkw 100401. Tberof rt far tew langth >h<wl4 jeruc a rtiKoalti oppliaolioe official*?
It rjo leap a farrow 10 aow4 eaceooftvo varor eotora the ooil at the appor «od retire ?fto wor aed 10 odo^wotolr irrigated. Thia it proeewocod le eoi 1 a with *><h l at 1 1 er at io* rate* Forrw length a ofetw.d W adjooiod ta accrwet tar
nangea
factor*
tLW if
M ooil ”tee Wtero eeila iO the
field Mn IkfCere*! latate rate*,
.leaidered io de tonal a 1 ftg wtiM ferrov leetcha era the : tn • tail, irrigatidw aLoom aed the tireas e>re.
F ar r wa vill eezeeoarllr ho eboeter for aovdr x «'. a U«i|| Jw valor h^ldreg apacUiot thee for tier voila vilb high valor held, eg <opo<<tlo»
Furrow laagtfti ia the project area era fised tr coefore rid ocwvwmoI . aftd lore. Hag <**d <«»<f 1 r«r ar ioa sf fields. Thv » are gowrallv XKMi to 2*N» i«
Iongth.
ft. 2. S F itjv tor o«
*he iiao if otreea toHeerod itta art* fecrw <t caHed the flow ret* «ad (•atrillf eeaiwred io Iifrae per served Oei* (otrov oogfM and a) »pa» >fc fieet, the fI w rate it rho aelr oar.aft.t
'aif.irw 4t it r ibwt tow ta >0<a>eed M atari teg the irrigative vith the largest f. w hat the forrw ea u'i»« arfp tor the desired v«!t»eg of ft, ><H j
r
voter should roach rhe ted if tw forrw vifftto a poor ter of the loidi t * w that voter wet be rarried 10 the fwrraw to refill the roof two (< field <ape<i!« Theo the >pperr«aito tier tor rhe oe«l tft oho'-rh voter vi 11 be about J% Hr eat greater ar the wwr the* at rhe iwor rod *t *bo field. Figure g.|.
Mton (to 
*.<• rta^wi
I veer *>f t to* forrw tM f!«w fate >•
»4;toe<wC or cw< feacto <♦ Mitotaio tW f I** eittoxt loaiog ••*•* eorface
rwe-eft •< rto «< of (tor Mrw. 
flw rate *• ogoal t« **♦ r*c< water i»
<to to4 ••< ♦ of tto fortoo. TVa letatoe rata decrease* <Ht> tto ;«ng‘V of doo osier fwoa it tto tvrrw, tfeorafare
to <*cre«»*4 •«« <4.t^.* «»t.M '*• • jeggOWOt . t.l.a Wfw«e<»«* *t ><#«•• Stea
f<^r rale toes to
So 
?U »*«* f.w
<>♦» .a««to’*»r *t«bt3 flows ar* <»x^arg*4 iota ♦
•• ogool wa*e< ef *w il rypfeooe per fwtrw. Meet of flow
•/• altered «.ftof toy ♦ to tooting ’M ga<r*aa»M
r d <te«g*M 1®*# ><••»<<• tone. TM pfsaeore towel la tto giffereoce in eleoadoo w^w«t iw vator aorfe^a <• ftoi fielg im. e*4 ceotre of trpfeoa
rnaatw r of
< •! torrowa at
p* r farrow
-.f d w»M Cree ?f 1 ba water evffeca e^ewe <W w< let if if it aotontrged. 
*W toa»ge»4 freeware w>4 for <W peofoet ores it X5na a>4 eariee toy • 5 co.
a < • a*ata<a torptto at Water aptdeg
H <t* f-vtrw »po<«M farrow tw^we iWt
f»w rate *M goratioo of irrigative era
Wafer appldodoo mlf. ♦ ..tree a a WKi — — — -------------------------------------------------------------------------- < 27 J a a U
wv«r* a ♦ spacing <f farms *> wirti
1 ♦ leogtto at f arrow ii net rea
etoow.l to .awl t* (to a < tow natttrmty at wafer perxtradoQ at toll
iiftereac poiote el*Og tto terms, two »r ttor*e dopa altar i« irrigation. a• * I****— tn. at . •*
to.). I
Stojocdss
?to reared *;o<tiee *< ewalwating a fwrrjw irrigation ayamn is to deiemme bettor ttoe teat** prweitot (fee repaired aa*i< of water to the crape eoifomly throogtonat tw forme towgtfe mtto opt.w^s eater application efficiency. ton eeoloode* ml. glee ffe* fdlwiif teferwetioe *
a) Far row e.spw
to) Tine of irdgadoe
□pt itokuB (straw .eegfto 4> □prints screen (law
O.f.X
Itofie. Gel'.active of tota
lMP*
4Ut.r««t I'.w r«t«. t»« f.rrwi
;as
aat«l
r c.rrp an4 «. (.rrw for iatanaiai^
( »« 
taraa. rat. »>in tw> J* FaraMH fl«..
(
Tva lav* »«for. t». trial, atakaa tkou.a w driven into th. trownd on th. furrow r.a,. ,t :y .at.r,.;. for .a<h of at. f rrw.. at.rti. at th. fi.14 canal .nd. 
4
t
.«»•.» ar. t.a.a alo«g th. b.a nt *ach farrow to d.c.raiM th. av.r.f* 
*' *** 
*** ••<*.. «•*• h, «• 
. I a. ia4i*.4ua> alo^.a i>.tw*m atak.a. TW
row.4 4. r. .tod to aa arh.tr.r» 4atM. Th. farrow apacist and depth of
%furrow should also be measured at each stake. Examples of this data collected 
during a furrow evaluation trial in Melka Sadi and Amibara 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
zone. Sampling is done at the top end, middle and bottom end of each furrow. 
Form 3.1 is used for this purpose. Figure 6.4 gives the soil moisture status
prior co 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 the 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 the tail end of each selected furrow, 
beyond the last stake, to prevent any water escaping to waste.
A canvas or plastic check dam across the field canal 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 = 1/sec x 3600 x 100 - cm/hr. -------------------------------------------------------------------------------------- (28) W x 25 x 1000
where W = furrow spacing
= 0.90 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;
j . KTn ----------------------------------------------------------------------------------------------------------------------------- (29) where
I » intake rate of the soil (cm/hr)
T 3 time that water is on the surface of the soil (minutes) K ■ intake rate intercept at unit time
n 
3 slope of the line (vertical scaled distance divided by horizontal scaled distance)
57A. typical curve for alluvia soil in Melka Sadi area is given in Figure 6.5 and a
curve for vertisol in Amibara area is given in Figure C.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 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:-
d =■ (Sfc - S) x V x D cm ------------------------------------------------------------- -----------------------------------(30)
100
where SFc « soil moisture percentage at field capacity
S = soil moisture percentage at time of soilsampling using an auger 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 27.34Z upto a depth 
of 100cm and the field capacity 42.0Z. 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 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 tail end of each furrow, but equal to the intake rate of the
entire furrow length. Measure volumetrically the cut-back stream 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 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 at the middle section of a furrow referred to at 6.3.2.
586.3.6 Tine Required co Refill Soil Moisture Reservoir
The intake race equation I 3 KTa cm/hr when integrated gives the cumulative depth of infiltration. The infiltration depth is given by
(n ♦ 1)
60d - K x T
n * 1
I in cm/hr. From the above T 3
d is multiplied by 60 60d (n ♦ 1)
K
as T is in minutes and
1
n * 1 in minutes.
Hence using Chis equation and required for a pre-irrigatjiou
Che data given in figures 6.5 and 6.6, Che time 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 -
.47)
1 -
.47
24
3
1138 minutes
1
on Vertisol
T
• 
60 x 31.43(1 -
.32)
1 -
.32
11.5
3
1027 minutes
Time required to apply 13cm deptn
on Alluvia
T 3 60 x 13(1 - .47) 24
3 215 minutes 60 x 13(1 - .32)
11.5 3 280 minutes
1
1 - .47
on Vertisol T 3
1
1 - .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 followa:-
a) Knowing the soil moisture content prior co irrigacion, the 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 time to pass water through the furrow.
e) Determining the optimum stream size
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 the
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 25Z 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 times are shown below.
Total irrigation time for post plant irrigation
Alluvia = 215 + 54 = 269 minutes » 4 hrs 29 minutes Vertisol « 280 ♦ 70 3 350 minutes - 5 hrs 50 minutes
Alluvia * 1138 ♦ 54 3 1192 minutes ■ 19 hrs 52 minutes Vertisol 3 1027 + 70 = 1097 minutes « 18 hrs 17 minutes
6.4.3 Maximum Allowable Furrow Stream
for:-
The maximum allowable furrow stream is the stream that does not cause erosion. In cases where the furrow length is fixed as in the project area this can be determined by examining the advance rate curves, figures 6.7 and 6.8, and selecting the stream which reaches the furrow length within the "advance time"
(l/4th the time required to refill the soil moisture reservoir).
6.4.4 Maximum Furrow Length
The 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
plotted "advance curve" of the maximum allowable furrow stream. 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 recession 
60curves gives the opportunity time. Ideal distribution of water along the furrow will be obtained when opportunity time is equal to 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.
Non-uniformity of furrow slopes are due to improper land grading or land smoothing and may also be due to 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 to 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.
61FORM 6.1
FURROW IRRIGATION SYSTEM EVALUATION FURROW INTAKE DATA SHEET
Name of Recorder ....................................................... Field 
Furrow width ...............................................................
Condition of furrow: New
Old 
Old
&
Reshaped 
Distance between 
inflow
& outflow stations
Parshall Flume size Stream size
Date
Furrow No 
Average furrow depth 
Soil Moisture: a) before irrigation Depth Z
b) after irrigation
Depth ..................... ...............Z
Comments:
62FORM 6.2
FURROW IRRIGATION SYSTEM EVALUATION FURROW ADVANCE AND RECESSION DATA SHEET Name of Recorder
Dace
Field 
Furrow Slope (Average) Z Furrow condition New
Furrow No 
Uniformity of slope Good
Old & reshaped Old
Siphon size
Raced Discharge (1/s)
I
STREAM ADVANCE
Fair
Poor
Distance becween 
Stations (m) .............................................. ................. Siphon Head 
Measured discharge (1/s) STREAM RECESSION
1
i
r
i
i
| STATION | CLOCK
ELAPSED 1 CUMULATIVE | 
| STATION | CLOCK
| 
| TIME
TIME
1 TIME
I TIME
1 1 ELAPSED I CUMULATIVE 1
11
11
mins 1 1
TIME mins
TIME
mins
1 
1
11
11
11
11
1 II
1111111
I 11 11
II 1 1
|
|
1
1 i111i
1 11 1 1 1 1
i
11
11
“111 1111111
11i
111 11
r 11 1 1 | |
II II 1
iri
iii
11 111
11
11
!I
"1 1 1 1 i------------- 1I1
!!1
ii
iii
i iI
i
ii
ii
i-------------
i
1
1
1
1
1
i
CALIBRATION OF SIPHON
COMMENTS
| Time
1 Sec |
★
X
Clock time
the end of
Clock time
i------ 1
r
I Volume 1 Lit
| Flow rate I I Lit/sec I
1
n
at which the stream reached furrow
Che stream was cut off ....
i
ii
r
1
i
Did Che stream overflow? YES/NO
Was the scream cut back? YES/NO
Stream cut back time
“I
n
i
Scream size after cut back (1/s)
r
i
i
★
★
★
*
★
i
Did significant erosion occur? YES/NO
Any other comments:UPSTREAM DEPTH
INFILTERED
FIGURE 6 I
LOSS BY
DEEP
PERCOLATION
FURROW LENGTH
Time required to apply Dem of water Advance flow = X hours
4
T hours
TIME
DEPTH INFILTERED
u/s
D/S
Infiltration 0 is proportional approximately to square root
of time t
T/4
XL
4
3T
4
4T
4
5T
4
dl
d l ♦ d2
dl+d +d
23
d| t d >d d
2 3+ 4
d| + d +d Hi +d
2345
0
dl
dj + d2
d| >d + d
23
d l*d2+d3+d4
0 ex t^z or
• - d| *d2
d2
D oc
f'/2
(2t )>2
[/2-l]d,
r'/z
(3t )'/2
*3 =Gf3-/2] <»l
LOSS BY DEEP PERCOLATION
= [7~5 - 2] d|
a, * d2*^3**^4
t'/2
(4t )‘/2
.-. 100 7.
[Z5 " 2J T
f
[ 2 ♦ J5] IL
t>2
2
53%
• d5
Va2+d
d
3* 4 = 2 d,
. . Average dept
h absorbed = (2 ♦>/5)d|
2
DEPTH of water absorbed during an irrigationFurrow depth - Cm
99
97
FURROW No I AV SLOPE 0.14 %
FURROW No. 2 AV SLOPE O 15%
FURROW No 3 AV SLOPE 0 15%
FURROW No 4 AV. SLOPE 0.12 %
FURROW No 5 AV SLOPE 0 14%
FURROW No 6 AV. SLOPE 0 08%
MELKA SADI FURROW SLOPES IN EVALUATION TRIALS
FIGURE 6Furrow depth
95
FURROW No I
AV SLOPE - -
FURROW No 2 AV SLOPE
FURROW No 3 AV SLOPE --
FURROW No 4
AV SLOPE-------
FURROW No 5
AV SLOPE---------
FURROW No 6 AV SLOPE
MELKA WARER FURROW SLOPES IN EVALUATION TRIALS
FIGURE 6.3FIGURE 6 4
Soil Moisture %
LOCATION MELKA SADI FIELD 47. 61Intone rate cm
FURROW INTAKE
FIGURE 6-5Intake rote cm
100
FURROW INTAKE
FIGURE 6 6FIGURE 6 7
I
I
I
I
Furrow Distance
STR EAM ADVANCE CURVESTime in Minules
FIGURE 6 0
STREAM ADVANCE CURVETime in minutes
Time in minutes
Time in minutes
/
FIGURE 6 9
LOCATION MELKA SADI
-----------Irrigation time
---------- Time required far !3cm4<MhTime in minutes
Time in minutes
Time in minutes
FIGURE 6 9
LOCATION MELKA SADI
-----------Irrigation time
---------- Time required far I3cmdep*hLocation-Amibara
----------Time required for irrigation ---------- Irrigation time
FURROW NO 3
FURROW NQ4
STREAM ADVANCE-RECESSION CURVES
FIGURE 6 IOFREE FLOW DISCHARGE l/s
FIGURE 611
CALIBRATION CURVE FOR 3" PARSHALL FLUMESECTION 7 Conveyance System1. Conveyance System
7.1 Weir
The diversion weir is a rockfill structure founded on a blanket of graded filter
material and having a synthetic impermeable 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 
27th February 1980, and formed a barrier.
The weir was designed for a 100 year flood expected to give a flow depth of 2.6 
m (el 751.60) over the crest for a design discharge of 700 m^/s. However, this
design flood is now found to have a return period of 10 years. The crest length 
of the weir at el 749.00 is 110 m.
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. No 
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
level were adjusted.
With the silting up of the 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 753.00.
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'*.
The intake was designed for a 500 year flood with a peak discharge of 1000 m^/s corresponding co a river stage of el 752.3 (3.30 m depth over weir crest). The return period is 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 more 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 mid-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 from 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 silt 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 m wide to completely shut-off the
scour culvert which is 1.5 m x 1.5 m. Operation of the radial gate is from the
top 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 than 1.0 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 1981, the intake was blocked completely due to the
above reason. An excavator was used to clean the intake mid-October when the
differential head started to increase.
Table 7.1 and 7.2 give the upstream river levels during the rainy months of 1980 
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 scour sluice should not be opened fully during low river stages in order to 
reduce the amount of debris being attracted on 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 
are 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 are 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, crashrack and supply culverts.
Discharges through the head regulator slide gaces are governed by the depth of
water in gate chambers themselves. However, as it is not convenient co install staff guages in Che dark gate chambers where visibility is poor, the water level
has co be related to the river level where a gauge post has been installed 
I
I ITable 7.1
1
1 Day 
1
Table III Upstream River Levels - 1980
-------1
1 JUL
I
“1 
I AUG
1
n
I SEP
1 1
1
1 1
1 2
1 3
1 4
1 5
1 6
I7
1 8
1 9
1 io
1 U
1 12
1 13
1 14
1 15
1 16
1 17
1 18
1 19
1 20
1 21
1 22
1 23
1 24
1 25
1 26
1 27
1 28
1 29
------ 1
n
I 750.05
I 750.10
1 749.85
I 750.55
I 749.92
| 750.04
| 749.88
I 749.73
| 750.20
| 750.15
I 750.34
I 750.20
I 749.96
I 749.89
I 749.85
I 749.78
I 749.75
I 750.85
| 749.92* I 749.95* I 749.94* I 749.94* | 750.05*
| 750.00
I 750.00*
I 749.99
I 749.98
I 750.05
I 750.13
I 749.95
I 749.90
I 750.00
I 750.03* I 750.31*
| 750.10
I 750.25* I 750.18*
| 750.08
I 750.05
I 749.97
I 749.87* | 750.05*
1 750.02
I 750.10
| 750.10*
I 750.11
I 750.23
| 750.05*
I 750.03
I 750.06
I 750.06
I 750.12
I 750.60
1 750.05
I 750.33
I 750.25
I 750.02*
| 750.12
1
I 749.91 I 749.88 I 749.85 I 749.92 I 749.89 I 749.89 I 750.10* | 750.04* I 750.10* | 750.02* 
I 750.12
1750.13 
I 749.82 
I 749.95
I 749.95* 1749.95* I 749.95* I 749.88 1 750.90 
I 749.01 
I 749.82 
1 749.82 
I 749.76 I 749.74
I 749.60
I 749.62
I 749.95 
1 749.94 I 749.92 
1
1
1
1
1
1
1
1
1
I
1
1
1
i
1
1
1
* Days on which intake
was blocked with debris silt and bedload
1
1
1
1 30
| 750.00
| 750.04
1749.85 
1
1 31
1
| 750.00
1
I 749.99
1 - 1
1
1 1
66Table 7.2 Table III Upstream River Levels - 1981
1-------------- ------ 1-----------------
1 Day 1
1
1 JUN 1
T
1 JUL 1
“1---------------- “1-------------------
n 
1 AUG 1
1 SEP 1
i
| OCT 1
1 
1
1
------ 1----------------- n--------------------n------------------ 1 '
1 1
1 749.70
1 749.75*
1 750.21
1 750.58
I 750.43* 
1
1 2
1 749.65
1 749.80
1 750.50
1 750.74
I 750.42*
1 3
I 749.65
| 749.78
1 750.58
1 750.62
I 750.41*
1 4
I 749.70
I 749.78
1 750.38
1 751.0 *
I 750.45*
1 5
I 749.65
1 749.76
I 750.18
1 750.93*
1 750.47*
1 6
I 749.80
I 749.86
1 751.70
I 750.80*
1 750.52*
1 7
I 749.70
I 749.78*
I 750.81
I 750.90*
1 750.50*
1 8
I 749.75
I 749.79
1 750.63
I 750.79*
I 750.46*
1 9
1 749.70
1 749.84
1 750.30
1 750.90*
I 750.50*
1 10
I 749.75
I 749.82
1 750.17
1 750.90*
I 750.36*
1 11
I 749.70
1 749.77
1 750.75
i 750.82*
1 750.28*
1 12
1 749.65
1 749.84
I 750.30
1 750.80*
1 750.17
1 13
I 740.70
I 749.94
1 751.10
I 750.74*
1
1 14
1 749.70
1 749.91
1 750.55
1 750.67*
1
1 15
I 749.70
1 750.66*
1 750.49
1 750.73*
1
1 16
I 749.65
I 750.10*
1 750.45
1 750.68*
1
1 17
1 749.65*
I 750.10
1 750.43
1 750.78*
1
1 18
I 749.80*
I 749.91
1 750.57
1 750.83*
1
1 19
I 749.65
I 751.10*
1 750.35
1 751.12*
1
1 20
I 749.70
1 751.<+0*
1 750.43
1 750.97*
1
1 21
1 749.80
I 750.80*
I 750.52
1 750.97*
1
1 22
I 749.90
I 750.51*
1 750.90
I 750.80*
1
1 23
I 749.80
I 750.66
I 750.28
I 750.60*
1
1 24
I 749.75
I 750.48
I 750.25
I 750.69*
1
1 25
I 749.75
I 750.78
| 750.18
1 750.55*
1
1 26
1 749.70
1 750.47
I 750.22
I 750.78*
1
1 27
I 749.85
1 750.34
1 750.30
1 750.77*
1
I 28
I 749.85*
1 750.25
I 750.31
1 750.64*
1
1 29
1 750.00
1 750.10
1 750.43
I 750.49*
1
1 30
1 749.80
I 750.20
I 750.60
1 750.45*
1
1 31
1 -
I 750.38
1 750.60
1
1
* Days on which incake was blocked with debris, silc and bedload.
67upstream of the intake. This is 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 ^2gyi m^/s
----------------------- - (31)
where Q = discharge through each gate in m /s
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 2 metres ---------------------------------------------------------------------------- (32) 2g
where Ke - 0.25
Trashrack and Screen losses ht « Kt. V^n metres ------------------------------------------------------------ (33) 2g
where Kt = 1.45 - 0.45 An - An 2 
Ag Ag
where Vn ■ velocity through trashrack or screen m/sec
-------------------------------------------------(34)
An = net area of trashrack or screen square metres Ag = gross area of trashrack or screen square metres
Velocity Head ho = Vo^ metres
2g Total head loss = he + ht ♦ ho (m)
------------------------------------------------ (35)
2
It has been found during operation of the head regulator since 1980 the total head loss can be approximated to 0.092 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 Main 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.
Main Canal Parameters are given in Table 7.3.
68Table 7.3 Main Canal Parameters
Canal
Reach
Bed
Width
Length 
m
Grad
ient
Side
Slopes
Full Supply Depth 
m
Free Board 
m
Velo- city
m/ s
Design Discharge nP/s
HrDco
CDl2) CD1 to CD2 CD2 to CD3 CD3 CO CD4 CD4 to CD5 CD5 to CD6 CD6 to
8.0 1934 0.227 1:2 1.7 0.45 0.70 13.50
7.0 5245 0.2 1:2 1.6 0.45 0.70 10.15
6.0 4525 0.2 1:2 1.5 0.45 0.59 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 1452 0.2 1:2 1.31 0.45 0.51 4.09
24000 m 3.10 1784 0.2 1:2 1.13 0.45 0.46 2.80 24000 m to
27000 m 5.62 3000 0.2 1:2 0.85 0.45 0.43 2.80
1) HR means head regulator stilling basin where the main canal
starts
2) CD means check drop with overshot gates for maintaining full supply level in main canal in upstream reach
Referring to section 2.10.1 the peak water duty at diversion for cotton is
1.43 1/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 the main canal should be 10,300 x 1.43 = 14.73 ra^/s. This means the canal has to be overloaded by 14.73 - 13.50 x 100 = 9.1Z.
13.50
Normally irrigation canals should cater for ♦ 10Z of the design discharge. For a discharge of 10Z Q « 14.85 m3/s. The depth of flow in this case will be
1.80 m as compared to 1.7 m for 13.50 m^/s. This entails a reduction in the freeboard of 10 cm which is acceptable.
At different times of the year, the Awash river carries varying percentages of suspended solids which enter the main canal with diverted water.
This has been the 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 gates 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 the 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 pointer 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) H3/2 m3/s
where L ■ length of gate over which water flows
----------------------------------------------------------------(36)
H • Full Supply Depth - (crest height * gate height)
Check drops 1 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 over the checks when the gates are fully lowered to crest level.
Table 7.5 Gate height above crest for design discharge
Check
Full
Supply
Crest
Possible
Discharge
Design
Drop
Depth
Height
Discharge
No
m
m
m^/s
m^/s
1
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 fr
om
the above
that the upstream
i full supply level has to be
increased
for Check
Drops 1,
2 and 6 to obtain
design discharges over the drops.
Check Drops 2 and 6 will operate under free flow 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 just upstream of the check drops.
The constant head orifice is a combination of a regulating and measuring device. It nas a submerged rectangular orifice for the measurement of the discharge. The rate of flow is estimated by using the principle that a submerged orifice of
a given size requires a differential head across the orifice and this is maintained at a constant value by adjusting the downstream regulating gate. Figure 7.4 is a schematic drawing of an offtake.
70The discharge is estimated by the formula:-
Q - CA
where Q = discharge in m^/j C - 0.65
A * area of orifice
---------------------------------------------------------------------------
— (37)
■ W (height of opening) X L (width of opening 1.7 n)
H « differential head in m.
A value of 0.65 for C was adopted after calibration of some offtakes with a current meter. Discharge graphs for offtakes 1 to 11 (L ■ 1.7 m) and offtake 11 and 12 (L * 2.0 m) when H » 10 cm and 6 cm are given in figures 7.5, 7.6 and 7.7.
7.7 Rej ection Spillways
There are four rejection spillways along the main canal each one situated upstream of a control structure. The spillways are 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 to 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 the rejection Table 7.6 Rejection Spillways
spillways and their
parameters.
Spillway No
1
2
3
4
Crest
Location CD2 CD4 CD6 Tail end
Length
F.S.L.
Level m
745.80
745.90 11
737.92
737.92 19
736.10
736.20 15
732.88
733.93 8
Rejection spillways 1, 3 and rejection spillway 2 rejects crosses the main drain.
4 reject into secondary drains 2, 4 and 5, while directly into the main drain where the main canal
71Table 7.7 Discharge Over Q =■ 1.84 (L -
Check Drops with Two Gates 0.2H)
H(m)
Q
1/s
H(m)
Q 1/s
H(m)
1 1
1 Q
1 1/s 1 1 1
.01 11
.02 1 31
1
.03 1 57 1 i|
1
.47 1 3501 1 
11
.05 1 1 1
.06 1 1 1
.07 | 1
1
.08 | 1 1
.09 1 I 1
.10 1 1
.11 1 1
.12 1 1
.13 1 1
.14 1 1
.15 1 1 1
.16 1 1
1
.17 1 i 1
.18 1
1
.19 1
I 1
1
.04 1 88 I
| 
|
| 123 1 
1 1
162 I |
204 | 1
249 1 1
.23
.24
.25
.26
.27 1
.28 | I
1
.29 I 1 1
.30 | 1
1208
1287
1369
1451
1535
1620
1707
1796
1 1
.45 I 3283 I 1 1 1
.46 1 3391 I
1 |
1 
.48 1 3612 I 1 1
.49 1 3724 | | |
.50 I 3838 1
| 
|
.51 I 3952 1 1 I 1 1
.52 1 4068 I 1 1
297 1
1
348 I 1
401 1 |
457 I 1
515 1 576 1 
| 638 I
1 703 1 
| 770 I
I i
838 I I
906 1 1
.31 1 1886 i
1
1 
.53 1 4184 | l
1 
1
i
1
.32 I 1 1
1977
.54
I 4302 | 1 | 1 1
.33 1 2070 1
.34 I 1
2164
.55 1 4420 I 1 1| 1 
.56 1 4539 1 
I 1 
I
1
.35 1 2256 1
1
.57 1 4661 1
1 
1
1
.36 I l
.37 | 1
1
2356
2454
.58 1 4782 1 1 |
.59 1 4804 1 1 |
1 1
H(m) I Q I I 1/s I
1 i
.67 1 5832 1 1
1
.68 1 6038 | 1
1
.69 1 6146 1 1
1
.70 1 6362 I
1
.71 i 6469 1
1
.72 1 6469 1
1
.73 1 6674 |
.74 I 6889 1
1
.75 1 6997 1
1
.76 1 7092 l
1
.77 1 7307 1
1
.78 1 7414 1
1
.79 1 7522 1
1
.80 1 7737 i
1
.81
I 7844 1
1
.38 | 2553 I
.39 1 2653 1
1
.60
| 
1 5028 | |
.61 1 5152 1
1 
1 
1
1
.82 1 7952
|
.83 1 8153 1
1
.40 | 1
I 8260 l
1
.20 1 981 I
|
.21 1 1055 1 1
.41 | 1
.42 I
1
2755
2858
2963
.62
1 5278 |
.84
1 
i 1 1
.63 1 5405 1 |
.64 1 5531 1
.65 1 5659 I | |
.85 1 8367 i
1
.86 1 8582
l 1
.22 I
1
1131 1
1
.43 1 3068
1
1
.44 | 3175 1
1 .66 1
1 5789 I 1 1
.87
.88
I 8689 i
1
I 8888 1
72.0 45
9 10
II
12 13 14
FIGURE 7 1Staff gauge indicating gate
height above crest
Staff Gauge
U/S Full supply level
t------------- ------------------
Pointer
UZS Bed level
T
Gate height
above crest
cT
height
L
D/S Water level
i
o
p/s Bed level
Q = I 84 [L = 0 2H]H% m’/s
H = Full supply depth-[crest height* gate height above crest]
FIGURE 7 2FIGURE 7 3
I
I
I
I
I
DISCHARGE CORRECTION C ' FOR EFFECT OF WEIR SUBMERGENCE
c'= Qi = (n h) 3/z
Q H 3/2
d = downstream head over crest H = upstream head over crest
_d_
I
H
001
•01
•02
• 03
•04 • 05
06
•07
•08
•09
0
1 ■ 00
1006
1 009
1-009
1011 1 011 1
■Oil
1009
1 009
I- 007
I
■1
1 07
1 005
1003
100
997 994
991
988
• 983
-981
I
2
978
•973
970
■966
•963 -958
•955
•951
-946
-942
•3
-939
-935
-931
•926
•921 •917
•913
•909
•903
-900
I
I
I
I
4
• 895
891
885
•881
• 875 - 8 77
-865
•859
-854
•848
•5
- 842
837
835
- 825
-819 -812
•806
• 799
• 792
-785
•6
■ 778
• 771
-764
- 75 6
•748 -740
•733
•724
- 71 5
-707
• 7
•698
•689
•680
• 670
• 660 -649
•639
•62 6
615
• 603
•8
- 589
•576
• 562
-547
• 531 • 517
-501
-486
469
•453
9
•4 35
.416
•396
-375
-351 •323
-293
•255
• 209
-14 4
I
I
I
I
I
I
ISCHEMATIC VIEW OF A CONSTANT HEAD ORIFICE TURNOUT
FIGURE 7 4Height of Orifice Opening cm
FIGURE 7 5FIGURE 7 5FIGURE 7.6/
FIGURE 7 7
ISECTION 8
Distribution System8. Distribution System
8.1 Description of system
The distribution system comprises the network of secondary, tertiary and field canals for 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 xrrigation 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 to six field canals are served by a tertiary at peak demand. These particular tertiary canals are Tertiary 2 and Tertiary 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 of metering. The flow over a duckbill weir is generally given by the formula.
Q = 1.05 LH 1-383 where Q 3 discharges in m3/s
L = length of crest (m) H = head over crest (m)
------------------------------------------------------------------------- (38)
Discharge curves for two common lengths of weir are given in Figure 8.1.
The rate of flow over a duckbill reir being known, the discharge through an upstream offtake can be estimated by deduction. For instance if the discharge over a secondary canal duckbill weir is 600 1/s and it is necessary to pass 400 1/s through the upstream offtake the offtake will be regulated so that the flow rate over the duckbill weir reduces to 200 1/s. By deduction, therefore, 400 1/s should be passing through the offtake.
73Table 8.1
Discharge through Field Inlets
Q ■■ 0.60 A ^2g yi where A •■ area of gate opening
71 1■
head upstream of gate
A■
■ 0.65 x w where w ■ height of gate opening 0.65 ■ width of gate opening
1------------------- 1-----------------
Height of I =47.5 I -45.0
yi 7l
y y yi
ir
1*42.5 | ]*40.0 | *37.5
------------------ 1
*35.0 | -32.5
cm cm
7l yi
gate opening 1 cm cm cm I cm 1 cm
cm 1 Q 1/3 1 Q 1/s
1 1
Q 1/3 1 Q 1/3 1 Q 1/3 Q 1/s
11
Q 1/3
2
4
6
8
1 1
1 24 I 23
1 48 | 46
1 71 I 70
1 95 1 93
22
11
1 22 1 21 20 20
10 1 119 1 116
12 1 143 1 139
14 1 167 I 162
16 1 190 I 185
18 1 214 I 209
45 I 44 1 42 41 39
68 1 66 1 63 61 59
90 1 87 1 85 82 79
113 1 109 1 106 102 98
135 1 131 1 127 123 118
158 1 153 I 148 143 138
180 1 175 1 169 163 158 I 202 1 197 1 190 184 177 |
20
1 - 1
-
-
1 218
1 212
204
196 
1
22
1 - 1
-
-
1 1
1 -
1
1 -
1
216 1
1
74Duckbill weirs should always be kept free of weeds 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 in the lower depths
is almost zero. Therefore 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
are rectangular orifices with a steel flashboard feeding either a pipe crossing
under a road-way or directly into a field canal. In Melka Sadi Stage III
"constant head" orifice offtakes have been provided on Secondary Canal SI and
Tertiary Canal 2 for accurate water measurement. See section 7.6 for the
operation of these turnouts. It is essential that measuring devices are
provided to calibrate all the different types of offtakes under varying field
conditions.
8.2 Irrigation
8.2.1 Water Application
The Amibara Irrigation Project has been designed on 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 is 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 will 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/s)
t
=
time in hours
A
=
area of field in hectares
d
a
depth of application in cms
Then:
t
a
2.4 x A hours
when;
d
a
13cm (post-planting-irrigation)
and;
t
=
5.30 x A hours
when;
d
=
28.6cm (pre-irrigat ion)
75Table 8,1
Discharge through Field Inlets
Q-
where A ”
71 ’
A-
0.60
area
head
0.65
upstream of gate
x w where w ■
0.65 -
height of gate opening width of gate opening
r
1 ------------------ 1-----------------
------- r
| Height of
I yj-47.5
y^-45.0 | yj-42.5
| gate opening I cm
cm
yj-40.0 I yi=“37.5
cm cm
1
71-35.0 71-32.5 I
cm
cm 1
|1
I cm
1 Q l/s Q 1/3 I Q l/s
1
Q l/s Q l/s
r
Q l/s Q l/s ------------------ 1
1 2
1 4
1 6
1 8
1 io
1 12
1 14
1 16
1 18
1
1 24 1 23
1
1 22
1 22
1
21
I ^8
I 46 1 45
I 44 42
1 71 1 70 I 68 66 63
1 95
I 119
I 143
I 167
93 I 90 1 87 85
I 116
I 139
I 162
I 113 109 106
1 135 1 131 127
1 158 153 148
1 190 185 I 180 175 169
I 214
I 209
20 I - 1 -
1 22 1 -
1 -
I 202
1 -
1 -
1
197 190
218 212
--
1
1
1
20 20 1
41 39 1
61 59 1
82 79 1
102 98 I
123 118 I
143 138 I
163 158 I
184 177 I 204 196 1
- 216 I
1
74Duckbill weirs should always be kept free of weeds 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 in the lower depths
is almost zero. Therefore 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
are rectangular orifices with a steel flashboard feeding either a pipe crossing
under a road-way or directly into a field canal. In Melka Sadi Stage III
"constant head" orifice offtakes have been provided on Secondary Canal SI and
Tertiary Canal 2 for accurate water measurement. See section 7.6 for the
operation of these turnouts. It is essential that measuring devices are
provided to calibrate all the different types of offtakes under varying field
conditions.
8.2 Irrigation
8.2.1 Water Application
The Amibara Irrigation Project has been designed on 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 is 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 will be applied to the furrows using 2.4cm internal diameter plastic syphons.
The basic equation for water application is given by:
----------------------
where
Then:
when;
qt = Ad x 1000 
36
q = litres per second (150 1/s)
t » time in hours
A = area of field in hectares
d - depth of application in cms
t s 2.4 x A hours
d ■ 13cm (post-planting-irrigation)
-------------(39)
and; t s 5.30 x A hours
when; d » 28.6cm (pre-irrigat ion)
758.2.2 Field Canal Inlets
Field canal inlets are rectangular orifices with an undershot steel flashboard. The steel flashboard is provided with slots at 2cm spacing on both
vertical edges. Pins on the gate frame can be inserted into the slots to keep
the flashboard at any desired opening corresponding to the 2cm slots.
The opening in the flashboard will 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 45 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 a 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 to 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 1/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 Amibara. Initially 5 syphons per furrow should be used for the advance flow when the furrow length is 200m. This means 64 furrows or 53 furrows, depending on the furrow length will be operative at the beginning of pre-irrigation. During actual operation it is observed that more syphons are required than calculated. This is because the syphon length is not long enough to obtain even a minimum head of 20cm.
At the end 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 3i hours and 5 hours respectively to apply 13cm gross post-planting irrigation. After "cut-back" the number of syphons in use has been found to be 2 per furrow or 3 per furrow for long furrows ie 128 for the 64 furrows or 159 for the 53 furrows.
« 319. 0.47
76The syphons that become available after "cut-back" of the "advance flow" should be moved to new furrows and started immediately bringing them too 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 200m
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 to 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 as many syphons as possible and will ensure syphon discharges remain within ♦ 10Z of the average 0.47 1/s. Cross bunds can be fixed earth dams, 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 200m and 44 when the furrow length is 250m. The quantity of water applied per irrigation per hectare = 1300m and the total quantity for one pre-irrigat ion and eight
3
post plant irrigations 3
13,260m /ha per season.
3
778.2.4 Canal Parameters
Canal
Length
(m)
Bed
Width
(m)
Side
Slopes
Full
Free
Board
(m)
Bank
Gradient
Discharge
Supply
Depth
(m)
top
l/s
width
(m)
s3
1500
1.4
1:2
0.68
0.30
1.00
1 x IO"3
1080
19 30
1.4
1:2
0.73
0.30
1.00
0.5 x IO"3
1080
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 IO"3
550
1988.98
1.25
1:1.5
0.78
0.25
0.50
0.15 x IO"3
550
T3/2
3600.02
1.25
1:1.5
0.70
0.25
0.50
0.22 x 10“3
440
T3/3
3176.60
1.25
1:1.5
0.70
0.25
0.50
0.22 x 10“3
440
T3/4
2300
1.25
1:1.5
0.70
0.25
0.50
0.2 x 10“3
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 10“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 IO-3
440
T4/3
39 04.34
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
0.50
.15 x 10“3
440
S5
1565.6
1.50
1:2
0.92
0.30
1.0
0.4 x 10“3
800
T5B/1
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 10“3
660
T5B/3
480.0
1.50
1:1.5
0.78
0.25
0.5
0.15 x IO-3
640
1527
1.0
1.2
0.75
0.30
1.0
0.427 x IO3
720
S6
2473.0
1.0
1:2
0.75
0.30
1.0
0.2857 x 10“
3 720
T6/1
2232.88
1.0
1:1.5
0.75
0.25
0.5
0.18 x 10“3
440
T6/2
480.0
1.0
1:1.5
0.73
0.25
0.5
0.2 x 10“3
440
T6/3
720.0
1.0
1:1.5
0.78
0.25
0.5
0.15 x 10“3
440
T6/4
687.0
1.0
1:1.5
0.78
0.25
0.5
0.15 x 10“3
440
T6/5
920.0
1.0
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
Til/ 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
Tll/3
732.11
1.00
1:1.5
0.78
.35
0.50
0.15 x 10“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 10“3
440
T9/2
1395
1.00
1:1.5
0.73
0.35
0.50
0.2 x 10“3
440
T9/3
6050
1.00
1:1.5
0.90
0.35
0.50
0.2 x 10“3
650
T9/3-1
687
1.00
1:1.5
0.78
0.35
0.50
0.15 x 10“3
440
Field
Canal
-
0.5
1:1.5
0.45
0.25
0.3
-
200
788.2.5 Drain Parameters
Drain
Length
(m)
Bed
Width
Side
Slopes
Full
Free
Gradient
Discharge
Supply
Depth
(m)
Board
(m)
l/s
(m)
SD3
750
0.60
1:1.5
0.45
0.30
3.5 x 10"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. 10“3
1000
TD3-1
642
1.25
1:1.5
0.50
0.25
0.5 x 10~3
500
2975
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 10-3
500
TD3-3
2881
1.25
1:1.5
0.73
0.25
0.15 x IO"3
500
TD3-4
1573
1.25
1:1.5
0.74
0.25
0.14 x 10“ 3
500
TD4-1
450
0.50
1:1.5
0.50
0.25
1 x 10-3
220
550
1.20
1:1.5
0.50
0.25
1 x 10-3
440
1869
1.20
1:1.5
0.40
0.25
0.5 x 10~3
440
TD4-2
1000
1.25
1:1.5
0.78
0.25
0.7 x IO"3
800
TD5-1
811
0.90
1:1.5
0.70
0.25
0.4 x IO-3
400
693
0.90
1:1.5
0.6
0.25
0.83 x 10-3
400
671
1.15
1:1.5
0.6
0.25
0.83 x 10~3
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
965
1.25
840
1:1.5
0.78
0.25
0.67 x IO"3
TD5-4
740
0.70
840
1:1.5
0.78
0.25
0.61 x 10-3
2660
1.00
400
1:1.5
0.93
0.25
435
1.40
0.49 x IO"3
1:1.5
1.00
780
0.25
0.40 x 10-3
TD5-5
2288
0.78
1:1.5
1180
0.70
0.25
89
0.70
0.4 x IO-3
1:1.5
400
0.70
0.25
1.07 x IO-3
610
TD6-1
1600
0.80
1:1.5
0.80
0.25
TD6-2
805
0.80
0.375 x IO-3
1:1.5
0.72
510
0.25
775
0.75
0.40 x 10-3
1:1.5
0.98
400
0.25
132
0.75
0.75 x IO-3
1:1.5
1.23
550
0.25
436
0.75
0.75 x IO-3
1:1.5
1.15
960
0.25
79 5
0.75
0.75 x 10~3
1:1.5
1.00
1040
TD6-3
918
0.25
0.75
1:1.5
0.70 x IO"3
0.83
1100
0.25
TD6-4
1000
0.75
0.5 x 10~3
1:1.5
0.77
400
480
0.25
TD6-5
0.75
0.40 x 10~3
1:1.5
0.72
400
0.25
0.40 x IO-3
400
RC T6-1
65
0.75
1:1.5
0.58
0.25
RC T6-3
130
0.75
1.0 x IO-3
1:1.5
0.67
400
0.25
RC T6-5
715
0.75
1:1.5
0.46 x IO-3
0.55
400
0.25
1.48 x IO-3
400
TD 11-1
TD 11-2
3009
765
1290
1.00
1.50
1.00
1:1.5
1:1.5
TD 11-3
1:1.5
0.82
0.75
0.82
0.25
0.25
0.25
0.57 x IO-3
0.60 x 10-3
1.20 x IO-3
620
830
900
RC Tll-2
RC Tll-4
1235
1010
1.50
1.50
1:1.5
1:1.5
0.75
0.75
0.25
0.25
0.6 x IO-3
0.63 x 10~3
830
850
79Drain Length
(m)
Bed
Width
Side
Slopes
Full
Supply
Free
Board
Gradient
Discharge
1/s
(m)
0.75
1.20
0.80
0.80
0.80
3.00
0.85
3.00
0.75
0.75
0.75
0.50
Depth
(m)
(m)
242
1178
607
713
1708
1152
1559
1819
1:1.5
1:1.5
0.49
0.25
4.2 x 10~3
400
TD9-1
TD9-2
TD9-3
TD9-4
TD9-5
TD9-6
1.10
0.25
0.125 x IO-3
700
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
0.85
0.25
0.4 x 10-3
550
1.00
2.25
0.14 x 10-3
440
0.78
0.25
0.62 x IO'3
400
0.78
0.25
0.125 x IO-3
800
0.79
0.25
0.4 x 10“3
400
0.85
0.25
0.1 x 10"3
690
RC T9-1
200
0.4
0.25
4.5 x 10" 3
400
RC T9-2 
100
0.37
0.25
5.8 x 10~3
400
RC T9-3/1 294
0.46
0.25
2.5 x IO-3
400
Field
0.50
100
Drains
808.3 Rejection Spillways
During irrigation it may sometimes be necessary to reject all or some of the
water flowing along secondary and tertiary canals. The reasons for rejection
are (a) Requesting more 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 48 hours in advance when flow rates should
be reduced.
Rejection spillways have been provided in the distribution network as follows: Canal Rejection Spillway
S3
T3/1
T3/2
T3/3
T3/4
S4
T4/1
T4/2
T4/3
T5
S5
T5B/1
T5B/2
T5B/3
S6
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 Rejection into field canals Rejection at tail end Rejection at tail end
Spillway at i:ail end
T6/1
fi
iii
it
ii
T6/2
it
iii
ii
ii
T6/3
n
ii i
ii
ti
T6/4
ii
ii i
ii
ii
T6/5
ii
ni
ii
it
T6/6
n
ii i
ii
it
Sil
Rejection
at
tail
end
Tll/1
ii
ii
i»
ii
Tll/2
ii
ii
ii
ii
Tll/3
ii
ii
ii
ii
Tll/4
ii
ii
ii
ii
8.4 Furrows
Prior co pre-irrigat ion the upper end of all furrows should be built up manually as close as possible to the field canal banks so as to ensure that each furrow
will have its own independent syphon stream. Spreading of water from one furrow to anocher at the upper end does not permit proper water application along each furrow.Visual inspection of furrows is essential and any localised high spots
smoothened out or the ridges strengthened at localised depressions that will
cause ponding in individual furrows or across several furrows. Figure 8.3 shows the unevenness of furrow slopes which lead to difficulties in 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 part of the surface (from one-naif 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 (pre-irrigation) 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 18.59cm 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 area 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 time.
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 be 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 that layer and water logging. Growth is retarded by excessive soil moisture. Often, a compacted layer occurs at shallow depths due to faulty tillage practices. The use of rubber tyred vehicles, discs and vibrating tillage implements can'easily bring about a reduction in soil voids which in turn prevents root growth.
82A restricting soil layer at 30cm depth has been found in several fields. The
cotton in these areas showed symptoms of low fertility. ese areas s ou
subjected to deep sub-soiling using a single tine. However, i it is no a
compacted layer, which normally develops after several years but a silty clay
layer with a very low infiltration rate it is 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 the designed average operational syphon head of 25cm.
8.6 Farm Units
8.6.1 Requests for water
Individual fields in any unit are at different distances from the intake. Water
when released from the intake will take varying times to reach different
fields. This means that fields nearer to the intake will be served faster than
those at the tail end.
Drawings 8.1 and 8.2 serve as 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 just downstream of the main offtakes. Similarly secondaries and tertiaries have duckbill weirs acting as checks serving one to
four offtakes upstream. The system has been divided into offtakes served by individual checks to compute the time taken by 150 1/s to travel from one check
to the next. During operation the main, secondary, and tertiary canal9 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 borne in mind that there is 150 1/s available in the main canal,
secondary and tertiary reach which will be adequate to irrigate at least one
hectare of a field. To avoid excessive rejection of irrigation water on
completion of irrigation or when a reduction in demand is required, Unit
Managers and Section Managers should give at least 48 hours notice of the anticipated time shut-down or reduction in flow. Form 8.1 should be used by the Irrigation Manager when requesting the Project Control Centre for water and Form 8.2 used by Section Managers to notify the Irrigation Manager of their water requirements •
8.6.2 Monitoring Irrigation Schedules and Application Efficiency
Field canals are designed for a maximum discharge of 200 1/s. However, it has been observed that irrigators can handle only 150 1/s fairly efficiently for syphon operation. Therefore irrigation schedules have been determined on the basis of 150 1/s per field canal. Form 8.3 has been devised so that the
irrigation unit of State Farms or RRC can monitor how much cime irrigators are taking to complete the application of a given depth of water to any field compared to the theoretical time allowed in the schedule. From this information it is possible to determine the average field canal discharge used by the irrigators and adjust future scheduling where necessary.
83Visual inspection of furrows is essential and any localised high spots
smoothened out or the ridges strengthened at localised depressions that will
cause ponding in individual furrows or across several furrows. Figure 8.3 shows the unevenness of furrow slopes which lead to difficulties in 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 part 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 (pre-irrigation) 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 Che depth of application will be 26.55cm gross or 18.59cm net and not the required 31.43 cm gross. This proves chat every irrigation crew should be provided with a pocket watch co record the total time of irrigation per furrow and therefore of the whole area for every irrigation. This will appreciably
reduce either deficit irrigation, or an over irrigation which causes excessive
losses due co 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 Che irrigated area. This
reduces Che 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 time.
It has been found chat certain areas under cultivation had low spots chat 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 be 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 that layer and water logging. Growth is retarded by excessive soil moisture. Often a compacted layer occurs at shallow depths due Co faulty tillage practices. The use of rubber tyred vehicles, discs and vibrating tillage implements can easily bring about a reduction in soil voids which in turn prevents root growth.
82A restricting soil layer at 30cm depth has been
cotton in these areas showed symptoms of low fertility. tie areas s ou
subjected to deep sub-soiling using a single tine. However, i it is no a
compacted layer, which normally develops after several years but a silty clay
layer with a very low infiltration rate it is not easily avoided or elimina e
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 co
a reduction in the designed average operational syphon head of 25cm.
8.6 Farm Units
8.6.1 Requests for water
Individual fields in any unit are at different distances from the intake. Water
when released from the intake will take varying times to reach different
fields. This means that fields nearer to the intake will be served faster than
those at the tail end.
Drawings 8.1 and 8.2 serve as 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 just downstream of the main offtakes. Similarly secondaries and tertiaries have duckbill weirs acting as checks serving one Co
four offtakes upstream. The system has been divided into offtakes served by individual checks to 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 borne in mind that there is 150 1/s available in the main canal,
secondary and tertiary reach which will be adequate to irrigate at least one
hectare of a field. To avoid excessive rejection of irrigation water on
completion of irrigation or when a reduction in demand is required, Unit
Managers and Section Managers should give at least 48 hours notice of the anticipated time shut-down or reduction in flow. Form 8.1 should be used by the Irrigation Manager when requesting the Project Control Centre for water and Form 8.2 used by Section Managers to notify the Irrigation Manager of their water requirements.
8.6.2 Monitoring Irrigation Schedules and Application Efficiency
Field canals are designed for a maximum discharge of 200 1/s. However, it has been observed that irrigators can handle only 150 1/s fairly efficiently for syphon operation. Therefore irrigation schedules have been determined on the basis of 150 1/s per field canal. Form 8.3 has been devised so that the
irrigation unit of State Farms or RRC can monitor how much time irrigators are taking to complete the application of a given depth of water to any field compared to the theoretical time allowed in the schedule. From this information it is possible to determine the average field canal discharge used by the irrigators and adjust future scheduling where necessary.
in ®®vera 16
.
83Form 8.4 is for recording data necessary to determine the field application efficiency for any specific irrigation.
8.7 24 hour irrigation
The canal reticulation system for the project has been designed on the basis of 24 hours irrigation and 60 days linear planting, each field canal having a capacity of 200 1/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.
If a secondary canal serves lOOOha and this area has on a 24hrs basis to apply 28.6cm depth of water, the
to be irrigated in 60 days
flow rate required would be
q = 1000 x 10,000 x 28.6 x 1_________________________________ m /s
3
3
100 3600 x 24 x 60
= 0.55m3/9
On a 12 hours basis
q » 2 x 0.55 8 1.10m /s
(neglecting efficiency)
For a post-plant irrigation of 13cm on a 24 hours basis
q = 1000 x 10,000 x 13 x 1 
100 3600 x 24 x 60
= 0.25m3/s
and on a 12 hour basis
q = 0.50 m /s
m3/s
3
At peak demand 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 » 2.00m3/s net on a 12 hours basis. The quantity of water required per day either on a 24 hours basis or 12 hour basis remains the same and is equal to:-
1.00 x 3600 x 24 
2.00 x 3600 x 12 = 86,400 m3
- 86,400 m3
or
The main canal capacity remaining the same storage reservoirs should be provided for each secondary canal if a 12 hour irrigation is to be practised to augment the main canal supply and provide a total of 86,400 m /day.
3
During a 12 hour irrigation the main canal will supply 43,200 m /day and the
3
storage reservoir 43,200m /day. During the night the main canal discharge will fill the storage reservoir with 43,200m3 for use the following day.
3
84This is the basic difference between 24 hours and 12 hours irrigation. It means additional expenditure in constructing storage reservoirs, doubling the capacity of the distribution system, and doubling the number of syphons. Saving on man-power is minimal.
85Form 8.4 is for recording data necessary to determine the field application efficiency for any specific irrigation.
8.7 24 hour irrigation
The canal reticulation system for the project has been designed on the basis of 24 hours irrigation and 60 days linear planting, each field canal having a capacity of 200 1/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.
If a secondary canal serves lOOOha on a 24hrs basis co apply 28.6cm depth
and this area has to be irrigated in 60 days
of water, the flow rate required
would be
q = 1000 x 10,000 x 28.6 x 1
100 3600 x 24 x 60
= 0.55m3/s
On a 12 hours basis
q = 2 x 0.55 = 1.10m /s
(neglecting efficiency)
m3/ s
3
For a post-plant irrigation of
13cm on a 24 hours basis
q = 1000 x 10,000 x 13 x 1________________________________ m /s
3
3
100 3600 x 24 x 60
- 0.25m3/s
and on a 12 hour basis
q = 0.50 m /s
At peak demand 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 = 2.00m3/s net on a 12 hours basis. The quantity of water required per day either on a 24 hours basis or 12 hour basis remains the same and is equal to:-
1.00 x 3600 x 24 = 86,400 m3
or
2.00 x 3600 x 12 - 86,400 m3
The main canal capacity remaining the same, storage reservoirs should be provided for each secondary canal if a 12 hour irrigation is to be practised to augment the main canal supply and provide a total of 86,400 m /day.
3
During a 12 hour irrigation the
mam canal will supply 43,200 m /day and the
3
storage
reservoir
43,200m3/day.
fill the storage reservoir with
During the night the main canal discharge will 43,200m3 for use the following day.
84This is the basic difference between 24 hour9 and 12 hours irrigation. It means additional expenditure in constructing storage reservoirs, doubling the capacity of the distribution system, and doubling the number of syphons. Saving on man-power is minimal.
85FORM 8.1
TO CONTROL CENTRE
AMIBARA IRRIGATION PROJECT II REQUEST FORM FOR WATER REQUIREMENTS
Name of Farm: Date:
Requested by:
86FORM 8.2
SECTION MANAGER
REQUEST FORM FOR WATER REQUIREMENTS AMIBARA IRRIGATION PROJECT FARMS
UNIT
DATE
I TERTIARY I FIELD NOS I IRRIG I WATER I FROM I TO I REMARKS I CANAL NO | BEING IRRIG I NO I REQD L/S I I I
TOTAL THROUGH MAIN
OFFTAKE
SIGNATURE
87FORM 8.1
TO CONTROL CENTRE
AMIBARA IRRIGATION PROJECT II
REQUEST FORM FOR WATER REQUIREMENTS
Name of Farm: Date:
Requested by:
1
I OFFTAKE
------------- 1---------------------------------------------------------------1-----------
| DISCHARGE
I (m^/sec or l/'sec)
I SEVEN
1
DAY PERIOD
1
1
1
1
1
I
1
1
1
1
1
I From
1 
1 To 
1
|
1
1 
1
86FORM 8.2
SECTION MANAGER
REQUEST FORM FOR WATER REQUIREMENTS AMIBARA IRRIGATION PROJECT FARMS
UNIT
DATE
I TERTIARY I FIELD NOS I IRRIG I WATER I FROM I TO I REMARKS
I CANAL NO I BEING IRRIG I NO I REQD L/S |
II
I
I
I
I
TOTAL THROUGH MAIN
OFFTAKE SIGNATURE
87FORM 8.3
FIELD IRRIGATION DATA
LOCATION
UNIT
r
I TERTIARY |
I
I
I
No
FIELD
No
AREA IRRIGATED
Ha
I
IIRRRRIIGGAATTIIOONN
|
SSCCHHEEDDUULLEE
|
1 ------1--------- I----------- 1---------- 1
I
I
I I 1
I i m? »XED | 
| TOT^ST,ME | 
I ,RBZ‘°N ISECTION 9
Salinity and Drainage11
MflKA WfRtR UNIT I
MILKA WtRtR UNIT I
MILKA WIRIR UNIT I
Ml LJtA WCTIR UNIT 4
1 — L„.
ttaor
orruaa
da ntit
■OUXTO
IM
CUM la<
4r» a4*
CAM* 4(401
9CiaaU
cm* oar
OHM
00
44C444C
»UDOrr
•Aw
N»
CUM II4C
»»• ■»
CAMS* OS4CM
Muacs
CUM Mi
vriaa*
ob in*
avtSAAC
tcxoon
nu«
u»
CMITM
CAMS* CCSCM
OJIONT
arnuB
M*«L>
■«iu<
xiocnr
Ma
"R
ST RS
1 t no
* no
4.444
l Ito
1. 401
t in
HO
LU
5
LaUat io CO* CO* IO cot col IO (S‘
too ro cmc*i
OO > IO O«i*a
CM® 1 IO CMCa) at O OCC4 (<<UI TO OC Car octal ro 04® 1 CMC4* »O CaCCa« | H) O«La ■ l, TO OC1» •
■O
4«
»no >1 404 <044
■ o»»
i» M
*0 04' 1* (*l « ro* 4 4M
■ 404
• ABO
ti
*4
134 33 1 10 4 HO 1 * 1® I'
(
1KXI. 0 ft. 4 STO*
0*3
0*0
0 30
0 *0
0 30
0 *0
0 IS
0 *1
0M
0■
0M
0 10
141
*15
104
I or
i II
in
0 04
C 10
0 10
c •*
0■
0 ft
1 >0 IM * 01 «o ua II tt 1* *4
* 01
• IT
C 04
c Il
io is
KJ 14
6
Ml**( 10 co*
cot ro co*
COl 10 C04
C04 ro coi
13 TO 1,
1, 10 OOC4I oca ■ ID cxrai
MOtIO o®> cxatro oral
M ID 0(01 cr4ta.ro octal
04® >10 UCO4 OCCA4ID OCCal 1. ro occa ■
uccaiio tea. 1, TO OCA. 1, TO octa 1. TO OCTO' OCCBHD CaCCa. 1, 10 octal
r ITO 4. no 4.444
I 041 1. 34*
*43
1. *40 1. If*
0*4
ri
i.m
IN
1. IT* 1,001
4CD
’K>
■
414
0*0
T. 1*0 K.4O4 MSA* 10 44*
• 4*4
i*^N
K> V7
• 1.404
urn
« an
•CL144
■l.M*
■ /4 4
Kjhi
•tjno
■£44
IIJ04
• I A**
DP4I
110*4
*>
*4
I*
00
T*
10
li B II Q IDi3
■ K)i»
1,
«4
’•
1 10 I
4
i ID *
4 iOS
*
C IO II urao
0 S*
0 40
0 40
0 4*
0 33
0 30
0 >0
0 *0
0 10
0 14
0 *4
0M
0M
0 30
0 40
0 IO
0 10
0 10
0 ID
0 30
1 *1
1M
1 04
I SO
1 II
0 rs
1 10
1 04
o so
0 OS
1 II
0 41
0 Cl
1 0*
0 44
0 44
0 4’
0 0*
0 Si
0 43
I >* * 14 0 01
10 I* o II
II a* « ■> IS M 14 04
«u
II 44
■ II 1* 10 N *r
■■
ti Oi
a •4
aM a SO
•> 41
■14*1 10 CO* CO* TO COI COI ID C04 CO* 10 coo SS ID 1,
* , ID OCOA etc ail ID CM CAS SS * 04X44 *• io osai 0*4 1 10 CaCCal O«M ro acta J octal ID O®H ONCA4 ID OCCM3 o«m ro ocoi*
T.ITO A 1*1
A 4C3
* 041
1. SM
4 II*
A It*
II
•TO
no
m
s*i
• IT*
II 404
■AMS
■4 44* 1*413 11400 IL MO
H.H4
UM*
a r>
*•*»
nur
IS
14
15
**
’*
■ TO 10 B TO SB
’*
I*
■ ro a
■OK) 11
•*■>*>
•0
0 IS
0 10
0 IO
0 44
0 IS
0 so
0 10
0 SI
0 10
0 40
0M
0 10
0 10
1 SI
1 14
> 04
1 10
1 tl
4 It
1 *0
I to
o O*
0 41
0 01
0n
0M
> SO I 40 4 01
■0 10
« II
II 00
■M <1 10 a i* MM
• 0*
-mac TO cot
cot 10 CDA
COO 10 til
til TO O«UI CMOS' TO ocrai
ocxaiiD 04 Cal OCCalTO CMCH4 1, io ocn. CMCA HD CMLa* i, io octal
T, 10 CXTXI 1 TO OCO l
4
AHI
■ *4
• 4*
Creta. to oca*
exeat io OCCal
octal io 0*0.4
I4tl
3 343
IS
l 010
» no
■ 030
•SO
*40
401
14T
■
• 40
4S0
*40
• 441
■3.4*4
■L004
M 044
*» 034 M.WI
II *10
ItXM
M.444 II OH
Mtro *1.000
M.OOO M...
M *M
00
A**
OU
L
’I
h
»4
• *0 1 1 TO 4 1 10 0
■ TOI 1 TO*
1
4 rot * TO 1
0 40
0 40
0 33
0 it
0 >0
0 35
0 >0
0 10
0 SO
0 SO
0M
0 10
0 >0
0 >0
10 M
0 40
3 4*
0 0*
o *0
* to
1 41
0 14
C 4*
0 4*
0 *4
0 OI
0 11
0 44
O ft
C 1*
Il 01
e os
is or
n it
•0 u*
It 45
• 0 II
•0 to
I* •(
>0 41 10 u 10 44 ID .*
•O »•
1
AMMAMA IRRlOSTlON RCOJCCT [|
APPROXIMATE TIME REQUIRED FOR WATER TO BE RELEASED FROM INTAKE ANO REACH VARIOUS FIELO CANALS IN
AM I BAH A
STAGES £ B II
■AMTWO *• 0 I
I OCALlOSTAGES I a 0
ai
•i
i.
*
*' ~ ’ »-*•X *►«
«• • • jui t as
I— 
k<METHOD OF PERFORMING DIAGONAL PLANING
FIGURE 8 6FIGURE 8.5
I
I
I
160
CALIBRATION CURVE FOR TERTIARY OFFTAKES DISCHARGING INTO A 45cm. DIA CULVERTFIGURE 8 4
I
I
I
I
I
I
I
I
I
I
I
CALIBRATION CURVEFOR TERTIARY OFFTAKES99 65
SLOPE = 0.15%
SLOPE = 04%
FURROW No 2
SLOPE = .055%
MELKA SADI FIELD 3-1/1 22 6 82
LONGITUDINAL PROFILE ALONG FURROW
FIGURE 8 3FIGURE 8 2
I
IFIGURE 31
Discharge l/s
DISCHARGE CURVES FOR DUCK BILL WEIRSI
1
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
9
FIGURE W
DISCHARGE CURVES FOR DUCK BILL WEIRSFORM 8.4
IRRIGATION DATA FOR A SPECIFIC IRRIGATION
LOCATION
UNIT
111
I TERTIARY
No
1
1
1
—
FIELD
No
1
1
—
AREA
IRRIGATED
HA
1
IRRIGATION DATA
GATE OPENING
1M
OPERATING HEAD
M
DISCHARGE
M3/e
DAY | STARTED
1
TIME | STARTED
r
DAY COMPLETED
TIME COMPLETED
TOTAL TIME
KTS
QUANTITY
IP
1
1
1
1
1
19 Salinity and Drainage
9.1 Saline and Alkaline Soils
Saline and alkaline soils are characteristic 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 semi-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 problems soils can be grouped under three headings, namely:- Saline, Saline-Alkaline, and non-saline-alkaline.
9.1.1 Saline Soils
Saline soils are those soils having an electrical conductivity of the saturation
extract of more than 4mmhoa/cm and an exchangeable-sodium-percentage less than 15. Normally the pH is less than 8.5. These soils are sometimes 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 normality.
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 normally a minor constituent or sometimes a major constituent.
The chief anions are chloride, sulfate and nitrate. Bicarbonate may occur in
small 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
is greater than 4 mrnhos /cm and the exchangeable 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 leaching 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 Salinity and Drainage
9.1 Saline and Alkaline Soils
Saline and alkaline soils are characteristic of arid and semi-arid
Reclamation of these soils, and prevention of excess soluble salts in fertile areas under irrigation is of paramount importance in the projec area 
be called semi-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 p an 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 problems soils can be grouped under three headings, namely: - Saline, Saline-Alkaline, and non-saline-alkaline.
9.1.1 Saline Soils
.
Saline soils are those soils having an electrical conductivity of the saturation extract of more than 4mmhos/cm and an exchangeable-sodium-percentage less than
15. Normally the pH is less than 8.5. These soils are sometimes 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
normality.
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 normally a minor constituent or sometimes a major constituent.
The chief anions are chloride, sulfate and nitrate. Bicarbonate may occur in
small 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
is greater than 4 nmhos /cm and the exchangeable 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 leaching 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 Soils
The electrical conductivity of the saturation extract of non-saline alkaline
soils is less than 4 mmhos/cm and the exchangeable sodium percentage greater than 15. The pH ranges from 8.5 to 10. Non-saline-alkaline soils are generally known as "black alkaline" soils. Leaching of saline-alkaline soils in the
absence of gypsum either in the soil or in the irrigation water, leads to
formation of non-saline-alkaline soils. With increase in the exchangeable
sodium the soil tends to become more dispersed. The anions present are mostly chloride, sulfate, and bicarbonate with small quantities of carbonates. When pH is high in the presence of carbonations, calcium and magnesium are
precipitated.
9.2 Effect of Salt on Crops
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 is called "osmotic pressure". The flow through the membrane continues until equilibrium is established.
Plant roots have a semipermeable 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 moisture 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 saline or alkaline soils by providing subsurface drainage, leaching water, and chemical amendments, the feasibile 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 1.0 ppm. It is found
in scattered areas of arid and semi-arid regions under irrigation. High levels
of boron can usually be reduced by leaching and requires three to four times more water than for 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 of 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 long term irrigated agriculture to be successful. Salts accumulate in the upper portions of a water table 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
root zone rapidly.
9.3.1 Salinity Problems
be expected to deposit salts in the
The presence or absence of a salinity problem is evaluated from the conductivity of the irrigation water. The table be ow ls a gu
salinity problem:-
Degree of Problem
Increasing
Severe
No problem
problem
Problem
EC^ (mmhos/cm)
Less than 0.75
0.75 - 3.0
More than 3.0
The irrigation water diverted from the Awash River has an electrical
conductivity on an average of 0.35 mmhos/cm. Under an appropriate water
management practice there should not be a serious salinity problem by using the
Awash water for irrigation.
Table 9.1 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 moisture 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 sodium content in the irrigation water also can cause permeability
problem. In addition, such things as germination, soil aeration, disease and
weed control due to ponding and stagnation need to be considered.
The most commonly used method for evaluating the potential permeability problem is the sodium adsorption ratio (SAR) given by:-
Na
SAR =
Ca * Mg
2
and ESP = 100 (-0.0126 ♦ 0.01475 SAR)
1 *(-0.0126 * 0.01475 SAR?
Where all ionic concentrations are expressed in meq/1 if the ionic
------ (41)
concentrations are in mg/1 then meq/1 =
For SAR values greater than 6 to 9 , irrieation
a permeability problem.
mg /1
Equivalent Weight
’ rrl6*cl°n
.
be expected to cue.
Th. diagram used for cla.axfytng irrigation and ground.at.r i, given i„ Figure
929.1. Salinity (C) and Sodium (S) are classed in four groups giving sixteen
types of distinguishable water varying from excellent (Cj S}) where EC is
0.25 mmhos/cm and SAR 10 co very poor (C4 S4) when EC is 2.25 ramhos/cm and
SAR 26.
Figure 9.2 is a nomogram for determining the Sodium - adsorption ratio of
irrigation or ground water and the corresponding exchangeable sodium percentage
of the soil in equilibrium with the groundwater. Example:- From Table 9.1 for
December 1970, Na - 1.40 meq/1 and Ca ♦ Mg meq/1 - 2.66 meq/1. On figure 9.2
join 1.4 on the Na axis to 2.66 on the Ca ♦ Mg meq/1 axis. This line intercepts
the SAR diagonal at 1.21 and Che corresponding ESP - 0.52.
9.3.3 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, CO3 and
HCO3 levels.
9.4 Drainage
9.4.1 Necessity for Drainage
In agriculture, irrigation and drainage are complementary practices. Excess
water due to rainfall, irrigation, or lateral seepage through canal banks 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 cables which damage crops to 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 Che water
cable at a sufficient depth to prevent capillary water from reaching the root
zone.
9.4.2 Surface Drainage
Surface drainage is a combination of collector and disposal drains. In the
project area where the land is flat, levelled and in regular shape, field drains
are established in a 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 the collected water in field drains to Che main
drains.
Land forming by smoothing, grading or levelling results in better surface drainage. Land smoothing eliminates minor differences in f la,race
including low spots. Land grading and levelling 
by smoothing. Continuous surface grades ensure run-off^ir^ *nd fl11 followed Surface drains lose their effectiveness unless properly ma.nr •
16 elevation
orr without ponding.
maintenance programme is necessary to prevent the dra' “ "
,
lncained. A good
with vegetation and partially filled with silt and soi^f r°m becomin8 clogged slopes. Unfortunately little or no attention is wi/k rom collapsed side
y entities in the project
93area to maintain the drainage system in working order. A maintenance programme should be worked out with the Project Control Centre. Maintenance wor inc control of vegetation and timely removal of sediment, backfilling an
consolidating large erosions at outfalls, and repairs to structures to retain an effective surface drainage system.
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 of the water table varies with soil texture, depth of soil and subsoil layers,
salinity 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 PII in this area recording the groundwater table at 6.2m below the soil surface. The soil and subsoil encountered was silty sand,
silt, and fine silt having permeability coefficients in m/sec decreasing rapidly with depth (0 - 5.3m,k = 1 x 10“5 5,3 11.0m, k » 1.6 x 10’ ; 11.0 - 15.6m,
;
6
k - 3.5 xlO’ ). The water after seven or eight years of irrigation now stands
7
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.
9.4.4 Irrigation Regime Adopted by MSAE
MSAE has adopted an irrigation regime of 13cm depth applied every 15 days. This gives a total gross irrigation application of 3120mm/year. According to the
gross irrigation requirement estimated in para 5.6.1. banana requires
2344 x _1 x 0.75 - 251 lmm/year where the application efficiency is taken as 70Z
•7
and the effective rooting depth of banana as 75cm. 610mm depth of water is applied in excess of gross year. This 6100m^/ha finds its way to the perched seven years would have contributed 42,700m /ha to
3
table.
From this it is seen chat a
irrigation requirements every
groundwater table and in
Che raising of the water
The groundwater table in MSAE in certain areas is within lm from the around
_____and in 
surface and in some areas within 2m. Hence the groundwater contribution in the
arpsQ wirhin 9™ u_____________________________.
6
water balance equation has to be considered. According co figure 9.3 this
contribution is an average of lrnnzday for a groundwater table 0.5m below the
root zone reducing the gross irrigation requirement oy 360mm/year. This gives a
gross irrigation requirement of 2151mm/year. 7
gives a
94The depth of irrigation application 3 p. Sa.D 3 .35 x 200 x .75 75mm
3
Ea 
.70
The irrigation interval in days 3 p.Sa.D
ETc
For January
i3
.35 x 200 x .75
=
14 days
4.14
For June
i3
.35 x 220 x .75
=
6 days
10.22
For November
i3
.35 x 220 x .75
3
13 days
4.46
As it is not practical on a large farm to alter the irrigation interval from
month to month, a regime of 90mm application depth every 15 days giving
21609mm/year is recommended. IAR has recommended 100mm every 15 days from field trials carried out over two years.
9.5 Ground Water
9.5.1 Investigations
Investigations of the groundwater table 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 drains indicated.
Observation wells, for periodic observations of the groundwater table, were established throughout the project area in 1970/80. They were set out on a 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 a sharp departure from the general water table surface. Ten more wells are required for a better interpretation of the groundwater flow, 4 in Melka Sadi and 6 in Amibara.
The depth specified for observation wells in the project area should have been
3m below the water table encountered while drilling. Unfortunately in the Amibara area the driller established four observation wells with depth above water table and therefore these wells cannot be monitored.
The elevation of observation wells should be established to correlate groundwater levels with groundwater surface levels to compile (a) groundwater contour maps (b) depth to groundwater maps and (c) electrical conductivity maps.
Periodic measurements of once a month during off season and once a fortnight during the cotton season are made on Form 9.1 to show the distance from the top of the well casing to the water table level in each well. These observation
wells should function at least for two years after the whole area comes under cultivation to observe the fluctuation of the groundwater table and leaching of saline soils during the growing season. The four dry wells in Amibara should be
95re-drilled to specifications and a further six new wells constructed to form a better grid. Four new wells are also required in the Melka Sadi area to
identify perched water tables.
In stratified soils excess water in the root zone is sometimes held up by a
layer of low-permeability so that the perched water is disconnected from the main body of groundwater. Perched water cables 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 mmhos/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 Cable in metres and Che electrical conductivity of the groundwater in nnnhos/cm at 24 locations in the Melka Sadi area and at 22 locations in the Amibara area recorded in August 1980 and September 1982. Readings were taken at least once a month from August 1980 to September 1982, which covers a period of 25 months.
96The depth of irrigation application = p. Sa.D = .35 x 200 x .75 ■ 75nnn
Ea 
.70
irrigation interval
The
in days = p.Sa.D
ETc
For
January
i=
.35 x 200 x .75
4.14
-
14 days
For
June
i=
.35 x 220 X .75
« 6
days
10.22
For
November
i=
.35 x 220 x .75
4.46
= 13
days
As it is not practical on a large farm to alter the irrigation interval from
month to month, a regime of 90mm application depth every 15 days giving
21609mm/year is recommended. IAR has recommended 100mm every 15 days from field trials carried out over two years.
9.5 Ground Water
9.5.1 Investigations
Investigations of the groundwater table 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 drains indicated.
Observation wells, for periodic observations of the groundwater table, were established throughout the project area in 1970/80. They were set out on a 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 a sharp departure from the general water table surface. Ten more wells are required for a better interpretation of the groundwater flow, 4 in Melka Sadi and 6 in Amibara.
The depth specified for observation wells in the project area should have been
3m below the water table encountered while drilling. Unfortunately in the Amibara area the driller established four observation wells with depth above water table and therefore these wells cannot be monitored.
The elevation of observation wells should be established to correlate groundwater levels with groundwater surface levels to compile (a) groundwater contour maps (b) depth to groundwater maps and (c) electrical conductivity ma pa.
Periodic measurements of once a month during off seaunn
during the cotton season are made on Form 9.1 to show th a- °Ce 3 ^ortnight of the well casing to the water table level in each well6
wells should function at least for two years after th h CSe °^servaC^on cultivation to observe the fluctuation of the groundwaterW?^* C°meS Q?der saline soils during the growing season. The four drv « i\ . e and leaching of
frOm C^e top
y wens m Amibara should be
95re-drilled to specifications and a further six new wells constructed to form a
better grid. Four new wells are also required in the Melka Sadi area to
identify perched water tables.
In stratified soils excess water in the root zone is sometimes 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 mmhos/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 Amibara area recorded in August 1980 and September 1982. Readings were taken at least once a month from August 1980 to September 1982, which covers a period of 25 months.
96—-1 - 9.1 Quality of Awash River Water at Melka Sadi Weir Site
(Table 9 .]1
Qualp. ty
1 Month | PH
1 1
1 1
1 1
1 EC
I m mil os
1 HC03
1 meq/ 1
1 1----------------- 1-------- 1------ 1----------1------- 1—
1 cl
1 C03 
1 S04 
IB
1 Na 
| Ca 
| Mg
7 
Ik
1---
| sah
I meq/1 | meq/1 | meq/1 | ppm
1 meq / 1 | meq / 1 | meq/ 1
1 / cm
1
1 meq/1 I
1
1
Illi
1 1 1
1 1 1
1 1 1
1 1
DEC/70 I1 8.1
|
1 0.27
|
1 2 .9
1
1 0.40
I1
1 ni 1
11
1 nil
11
1 ni 1
I1
1 1 .40
I1
1 1 .33
11
1 1 .33
1
1
1 1
1 0.19
1
1
I 1.21
1 0.33
|
1
1
JAN/71 18.5
|
FEB/71 1 7.8
|
JUL/79 1 7.85
1
1
1 0.40
|
1 0.43
1
1
1 ni 1
|
1 1.54 I
1
I 1.12 1
1
1 0.32
1
1
1 0.42
1
1
AUG/79
NOV/79
1 7.95
I
1
1 P. 10
1
I 0.25
1
I
1 0.22
11
1 2.85
I
1
1 2.90
1
1
I 1.50
1
1
I 1.20
11
I 0.41
i
1
1 0.64
1
1
1 0.41
1
1
I 0.28
I nil
I
1
1 0.01
1
1
I ni 1 1
1
I ni 1 1
1
1 1.70 1
1 0.24
1
1
1 0.25
i
1
I 0.40
1
1
1 0.14
1
1
I 0.18
l
1
I 0.17
1
1
1 0.19
1
I 2.00
1
1
1 1.97
|
1 0.13
j
I 2.38
1|
I 1.68
|
11
I ni 1
1
1
1 1.20
1
1
1 0.81
1
1
I 0.60
11
I Trace 1
1
I Trace I
1
I Trace
11
1 0.04 1
1
I 0.08 1
1
I 0.05
11
I 2 .88 1
1
I 1 .89 1
1
I 1.58
11
1 1.24 1
1
I 2.52 1
1
1 2.40 1
1
I 1.20
|
I 0.09 I 0.09 I 1 .97
j
|
|
DEC/79 1118.15
i
1
I 0.28
11
I 1 .80
I1
I 0.32
11
I 0.80
I Trace
I 0.12
1 2.34
1
I 2.01
I 0.14
| 0.12
I 2.26
I1
I1
11
j
|
|
|
JAN/80 | 8.45
1i
1
1 0.30
i
1 1.70
i
1
I 0.41
i
1
I 0.80
11
11
I Trace
1 0.03
11
1 2.59
I 1.35
I 0.10
I 0.13
I 3.04
j
1
|
|
FEB/80 I 8.10
I 1.70
1 0.87
1 0.60
I Trace
I 0.01
I 2 .80
I 1.30
I 0.16
I 0.15
1
I 0.33
i
1
i
1
1
1
11
11
1
|
|
|
|
1 3.28
|
MAR/80 | 7.60
I 0.33
I 1.80
i
1
I 0.55
i
1
I 0.80
| Trace
11
1 0.01
I 2.59
I 1 .80
I 0.19
I 0.16
1
APR/80 I 7.80
1
l
1
11
|
|
|
|
1 2.60
1
I 0.36
I 2.60
I 2.60
I 0.60
I 0.05
I 0.10
I 1.36
I 1.20
I 0.16
| 0.14
I 1 .65
1
1
1
1
i
1
11
11
|
|
|
I
1
MAY/80 II 7.30
I 0.38
I 2.40
I 0.60
1
1
1 0.58
1
1
| 0.60
i
1
I 0.07
I 0.25
I 1.32
I 1.50
| 0.14
11
j
|
|
1 0.20
|
1
1 1.43
1
I 0.60
I 0.08
1 0.35
I 1.92
i
| 1.20
I 0.21
| 0.16
I 2.29
JUN/80 I 8.10
I 2 .90
I 0.65
1
1
1
1TABLE 9.2 KLKA SADI
I
I AUGUST 1980
AUGUST 1960
I
I
SEPTEMBER 1982
«LL I_________________I---------------------
I
I
I RISE
No I
I
I
I
I Depth to I EC Mhoe/ca I Depth to I EC iwhos/ca I
I intertable c®/ I
I watertable (■) I
I
I
I
|
I
I
I
REMARKS
J_________ I_______ 1__________ I----------- L
J_________
I
I
I
I
I
I
J
1
2
3
4
5
I
l
I
I
I
l
l
I
I
I
I
61
i
1
71
1
1
3.7
2.8
3.25
4.1
7.2
5.0
0.58
0.78
10.0
2.3
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
4.8
0.96
0.92
8.60
2.50
I
I
I
I
I
I
I
I
I
I
I
1.70
1.25
I
I
I
I
I
0.50 1
1.27
1.69
Neer C®ial
Near Canal
Near Drain
3.55 1 2.3 |
7.02
11
1
1 10.0 |
1
1
1
1
1
8 1 2.5
I
1 0.56 |
Near Canal
91
i
1
10 I
i
1
11 1
1
1
12 I
I
1
13 1
i
1
14 I
i
1
15 1
i
1
10.55
2.9
1
1
1 10.0 |
1
1
i
1
1
1
1 4.4 |
I
I
I
I
I
I
1
I
I
1
I
I
I
I
I
|
I
I
I
1
1
1 2.2 |
1
1
I
1
9.75
5.6
I
1
1 5.6 |
2.0
1.55
2.75
2.83
5.51
3.25
5.71
1.95
8.39
2.09
8.67
4.59
3anana Saline Area |
I
I
I
1
l
|
1
8.15
1 4.4 |
I
11
Near Drain
Near Drain
4.2
1 2.0 |
1
1
|
I
|
I
i
8.1
1 0.99 |
1
1
|
1
16 1 9.1
1I
1 4.2 |
1
f
17 I
1
1
19 I
i
1
20 1
1
1
7.25 1 1.6 |
11
1
9.45
1 10.0 |
i
1
i
10.40 1 7.1 |
1
i
i
1
21 1 11.0
1i
1 7.6 |
1
i
22 1
1
1
P10I
1
1
10.0 1 3.6 |
1
10.7
I
i
1 10.0 |
1
i
6.28
3.04
7.22
9.05
5.92
7.85
8.31
11.32
8.35
9.18
I
1
Pll 1 2.3
1 9.0 | 1.41
I 3.0
I
I 10.0
I
I 0.71
I
I 10.0
I
I 2.4
I
I 2.4
I
I 3.7
I
I 10.0
I
I 2.3
I
I 0.86
I
I 4.80
I
I 5.80
I
I
I 10.0
I
I 10.0
I
I 8.4
I
I 4.6
I
I 10.0
I
I 9.8
I 0.30
I
I 1.31
I 0.55
I 2.16
I
I 0.81
I
I 1.08
I
I 1.01
I
I 1.87
I
I 1.16
I
I 0.88
I
I 0.05
I
I 1.33
I
I
I 1.60
I
I 2.09
I
I -0.32
I
I 1.65
I
I 0.52
I
I 0.89
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
Air Strip
Cotton Saline Area I
I
|
I
II
SF Cotton Saline Area I
I
I
SF Cotton
SF Cotton
|
I
|
I
SF Cotton Near Drain |
I
SF Cotton
|
I
I
I
Nets: Well Nos. 1,2,3,6.8 ano 10 are within the boundary of Melia Sadi
Banana Saline Area | Agricultural Estate.
98TABLE 9.3 AMIBARA
I
I
II
AUGUST 1980
I
I SEPTEMBER 1982 I
I
CLL |_____________________________ |______________________________ [ RISE I
REMARKS
No I
I
I
I
|m|
I Depth to I EC ramhos/cffl I Depth
to I EC lennos/cffi I
I
I watertable n> |
| watertable (a) I
II
II
I
1
1
1
1
I 32 | 2.78
II
I 33 I 9.5
II
I 34 I 9.35
I
I
I
I
I
5.3 1
i
2.78 |
ii
4.8
1 0I
1I
1.4 |
1
•
7.90
1
1
1
1.7
I
1 1.6 I
1
1
I
1
1
IAR PaBture
Melka Warer Village
2.6 I 7.72 1 0.72 1 1.63 1 ST Cotton
I
I
il
11
I1
i1
I
35
|
8.7
6.0 1
8.69
1
8.80
1 0.011
ST Cotton
I
I
i
11
I1
i1
I
36
I
8.95
4.3
I
6.16
1
6.3
1 2.79 1
Near Drain & Cotton
I
I
1i
11
11
11
I
37
|
9.75
1.8 |
8.76
1
2.2
1 0.99|
SF Cotton
I
I
1i
11
I1
I
38
|
5.2
1.0 |
3.05
1
1.4
1 2.15 1
ASP Cotton
I
I
I
I
I
I
I
I
I
I
I
11
Ii
11
11
11
39
|
1.6
2.3
i
1.85
1
2.0
1 0.25 1
ASP Cotton
I
1i
11
11
11
41
|
8.5
I
I
I
I
I
I
3.0
I
5.37
1
2.8
1 3.DI
ASP Cotton
I
1i
11
i1
i1
42
I
13.0
10.0
I
11.86
1
10.0
1 1.14 1
SF Cotton
I
11
11
1
1
1
1
44
|
8.0
I
I
I
10.0 |
8.52
1
10.0
1 -0.52 I
I
11
11
1I
II
45
I
3.1
1.2
I
2.52
1
1.2
| 0.58|
Alegeta Cotton
I
I
i
1
11
iI
|l
46
|
15.1
10.0 |
D.OD
1
10.0
1 2.101
Near Drain
I
11
11
11
11
47
|
14.0
7.0
I
D.44
1
10.0
1 0.56 1
SF Cotton
I
i
1
11
1
1
1
1
48
|
6.45
1.6 |
5.64
1
1.6
1 0.81 |
I
I
1
1
’1
11
|I
49
|
8.0
I
I
7.4
|
5.82
1
10.0
1 2.18 1
Ambash Cotton
I
i
1
11
1
1
50
|
12.45
5.4
1
12.73
1
7.4
1 -0.28 |
I
11
11
1
1
I
1
51
I
14.25
10.0 |
D.79
1
10.0
1 0.46 1
I
l
1
11
11
11
52
I
13.06
9.4
|
12.78
1
10.0
I 0.30 1
I
1
1
11
11
i1
53
I
13.95
I
I
I
I
I
4.0
|
12.41
1
5.8
1 1.54 1
I
1I
11
1
1
I
1
54
|
15.77
3.6
I
1
15.25
1
4.4
1 0.52 1
I
11
11
11
55
|
7.5
1.2
I
6.29
1
1.8
1 1.21
1
SiDlale Cotton
J
1
During early 1963 observation Well Noa. 23-30 and Nos. 58-59 were constructed in the Melka Sadi area, while observation well Nos. 60-67 were constructed
in the Amibara area. The groundwater table over the project area is
rising at the rate of 0.5m to 0.75m per year as is evident from the above tables.
99FORM 9.1
AMIBARA IRRIGATION PROJECT II
Piezometer Records for Drainage Investigations
Name of Observer
Date:
I------------- 1------------ 1-------------- 1--------------- --- I
I Piezometer I Bench Mark I Depth to I Elec Conductivity I Remarks
I Number | or
I Water Table I (mmhos/cm)
I
| Ref Point | (m)
|
ITABLE 9.3 AMI8ARA
II
II
AUGUST 1980
I
I SEPTEMBER 1982
II
I
I
I *LL I|___________________________________________________________________ | RISE I
REMARKS
I No |
I
|
I
I | Depth to I EC mmnos/cm I Depth to I EC ■nhos/ce I
ImI
II
I
I
I watertable m I
I watertable (a) |
II
I1L
11
1 32 I
11
11
2.78
11
1 0I
11
IAR Pasture
1 33 |
i
11
i
1
1
1 5.3
I
1
1 1.4
l
1
I 2.6
i
1
I 6.0
1
1 2.78
i
1
I 7.90
i
1
1 7.72
i
1
I 8.69
1
I 4.8
i
1
1 1.7
1
1
1 0.72
i
1
I 8.80
1
1
9.5
9.35
8.7
1 1.6 1
11
11
Melka Warer Village
1 34 |
11
1 35 |
1 1.63 I
11
11
1 0.Q1 |
SF Cotton
SF Cotton
I
I
I
I
I
I
1
I
1
I
I
I
i
I
1
I
1
11
I
1
l1
11
I1
|l
1
1 36 |
8.95
1
4.3
I
6.16
I
6.3
1 2.79 1
Near Drain & Cotton
I
1
I
i
1
i
1
i1
1
1
1i
1 37 |
9.75
I
1.8
I
8.76
I
2.2
1 0.99
1
SF Cotton
1
1
I
i
1
i
1
i
11
11
iI
1 38 |
5.2
| 1.0
I
3.05
1
1.4
1 2.15
1
ASP Cotton
I
1
I
i
1
i
1
1I
11
11
11
1 39 |
1.6
1
2.3
|
1.85
I
2.0
1 0.25
1
ASP Cotton
I
11
1
i
1
1i
11
11
iI
1 41
|
8.5
|
3.0
i
5.37
I
2.8
1 3.13
1
ASP Cotton
I
11
1
i
I
11
11
11
11
1 42 | D.O | 10.0
I 11.86
1
I
1 1.14 1 SF Cotton
11
1 44 | 8.0
11
11
1
I 10.0
11
1
1 1.2 1
i
1
1
11
I
I
1
1
|1
i
1
i
1 -0.52 1
1
1
1
|l 1
1
1 45 |
11
3.1
8.52
2.52
13.00
13.44
5.64
1 10.0
i
1
I 10.0
1
1
1 1.2
i
I 10.0
1
1
1 10.0
1
1
1 1.6
i
1
I 10.0
11
1 0.58 1
Alegeta Cotton
I
I
i1
I
1 46 | 15.1
11
1 47 | 14.0
11
1 | 6.45
11
| 10.0 |
1 2.101 Near Drain
1
1
1
1
I
l
|l
1 7.0 |
i
1
i
1 0.56 1
11
SF Cotton
1
1 1.6 1
i
1
i
1
1
1 0.81 |
1
I
1
|
1
1 49 |
1l
8.0
1 7.4 |
■
1
i
1
5.82
12.73
13.79
12.78
1 2.18|
1
l
Ambash Cotton
I
1
1
I
I
i
i
1
1
i
1
1
i
|
1
I
1 50 | 12.45
11
1 5.4 I
1 -0.28 | 1
i
1
i
I
|
l
1 51 I
11
1 52 I
11
1 53 1
11
14.25 11 10.0 |
1i
1
|
1
i
1
|
1 0.46|
I
11
13.08
I
9.4 |
1
1
4.0 |
i
i
1 7.4
1
I
1 10.0
1
1
1 10.0
1
1
1 5.8
1
I
1 0.30 1
13.95
1
11
1
12.41
I
I
I
1 1.54 1
1
11
i
i
|
1
I
1
1
1
1
1
1
1
1
1
1 54 |
15.77
I
3.6
1
15.25
1
4.4
|
0.52 I
1i
11
1I
|I
i
1
11
•
i
1 55 I
7.5
1
1.2 1
6.29
i1 1.8 |
1.21 | Suolale Cotton
|
i
L
—LL
—1
L
I
During early 1983 observation Well Noa. 23-30 and Nos. 58-59 were constructed in the Melka Sadi area, Oile observation well Nos. 60-67 were constructed
in the Anibara area. The groundwater table over the project area is
rising at the rate of 0.5a to 0.75m per year as is evident froa the above tables.
99FORM 9.1
AMIBARA IRRIGATION PROJECT II Piezometer Records for Drainage Investigations
Name of Observer .............................................................................. • •••
Date:
I Piezometer I Bench Mark | Depth to I Elec Conductivity I Remarks
I Number | or
I Water Table I
I
I Ref Point I (m)
I
(nnnhos/cm) |
I
I
I
I
100WATER QUALITY CRITERIA
•00 250 750 2250
CONDUCTIVITY- MICROMHOSX:m (ECx IQ6) AT 25°C
SODIUM (ALKAU) HAZARD
O
O
Q
c 
J)
IDSODIUM (ALKAU ) HAZARD
O
o
c
X
m
WATER QUALITY CRITERIAF IGURE 9 2
nomogram for determining the sar value of irrigation water AND FOR ESTIMATING THE CORRESPONDING ESP VALUE OF A SOIL THAT IS AT EQUILIBRIUM WITH THE WATER (USDA 1954)FIGURE 9 3
i I i
I I I I I I I 1
I B
B
I
BGround Levelhos /cm
Depth to groundwater
n
n
Giqu
ind Li
ikfil
------ k
f
Q
□ __
A
r
/
X
er *
r-
•a
--
X
*
✓
/
X
✓
*
7
X \
/
✓
V
------ <
3
J
)
E.C>
s
A
S
0
N
D
J
F
M
A
M
J
J
A
S
0
N
D
J
F
M
A
M
J
J
A
S
0
N
1980
1981
198 2n
Giou
nd L
<:xel
0
7
b
5
4
A
.—
X
7
/
X V
/
/
'•x
s
»
X
__ /
r
X ■■ \
s
/
J
">y
/
V
□
c
EC>
1-
a
s
0
L
N
D
J
F
M
A
M
J
J
A
s
0
N0
J
F
M
A
M
J
L
A
S
0
N
980
1981
1982
A. I P 2
Q
C X m
<0
Depth to ground waterA.I.P3
Depth to groundwoterE C mm hos /cm
Depth to groundwoter
0--------
u-------
ECs
9
8 -------
7-
Grou
id L
tve 1
c
X.
6
1
5--------
—
—
—
—
■
_
~-
—
„ —
r-
-—
.
—
— ■
■--
d
<“
—
A
S10
N
0
J
F
M
A
M
J
J
A
S
0
hl
D
J
F
M
A
M
J
J
- A l s
0
N
1 980
19 81
1982
A.I.P4T~r
Ground level
0
1
2
3
4
5
6
7
8
s
0
N
D
J
F
M
A
M
J
J
A
S
0
N
1982
A.I.P5
Depth to ground water mE C m m hos /cm
n
Grow
d n lev
f1l
•
—
0
1E
aj
S
3 I
&
6
—
I
4 ♦- o
.c
5*
Q
6
4
3
21
EC
—1
AL s
-------- !
°I N
0J
M1 A
M
J
J
A
s
\!3
0
N
D
J
F
M
A
M
J
J
A
S
0
N
Xg
1980
1981
1982
\X
n»
Al P6ios / cm
Depth to ground woter
■--------------
—
____
E
c
D
Gro
und If
vel
0
9 _
8
7
)
—
k
f
-
1 z*
A
s I0
N
D
J
F
M
A
M
J
J
A
s
0
N
D
J
F
M
A
M
J
J
A
S
0
N
I960
1981
982
A.I.P7E Cm mhos /cm
Depth to groundwoter m
Grou
ndilava
1
•
•
—
1
>
—
** “
*
__
X X
—-
EC.
v
_
1
A
S1 0
N1D
J 1F
Ml A
M
J
JAs
0
N
0
J
F
M
A
M
J
JA
s
0
N
1900
1981
1962
AIR 61
I
Depth to ground water mA . I P. 12
FIGURE 9 13E o
X. </)
o r
E E O
UJ
A.I.P.I3
Depth to groundwaterEC mm hos / cm
Depth to groundwoter
r
Gro
□nd L
evel
------- U
r\
------- 1
■a
_
_.
—_-
—
—
—
—
—
—
4
□
A1
□------------
4-------
3-------
C
L______ B
X
1A
s
0
N
0
J
F
M
A
M
J
J
A
s
0
N
D
J
F
M
A
M
J
J
A
S
0
N
1980
1 981
1982
A .I . P. 14E C rnmhos /cm
Depth to Groundwater m
A.I.P.III
I
I I I I I I I I 1
■
■
■
■
■
■
■
■
AMIBARA IRRIGATION PROJECT
WATER MANACEMEJil MANUAL PlEZOMET£RJ.pgATlQNs
Gc.» ,
aM
- MFLKA warep
C \ -------------------------s—
VvS V
SC .-««•
DRAWING No 9 I10 Efficienci e s
10.1 Types of Efficiencies
Movement of water through an irrigation system from its source co the crop can be split into three successive stages; conveyance, distribution, and field
application.
Conveyance can be regarded as the movement of water from diversion through the main canal co the primary offtakes.
Distribution will then be the movement of water through secondary and tertiary canals co field canals.
Application is the movement of water from Che field canals through syphons Co the land to be irrigated to maintain the soil moisture above a minimum level required for the crop.
When water flows through a canal, furrow or drain some of the water is 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 is 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 banks 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 cron and transpiration from the growing crop itself. Excess water applied at the to
of furrows which is inevitable in a furrow system ( see figure 6 1) or w t** applied at a rate higher than the infiltration rate to cause ponding and’anZ^ runoff will be lost to percolation or to drains from which evaporation and f percolation losses take place. No lateral seepage losses take place from a •
as they are in cutting.
Water in the main drain that is not lost due to evaporation or Dermic.™ • discharged back into the river.
For an estimate of operational losses, intensive flow me a
in canals and drains. This can only be done with time Sureinents are necessary this nature. However trained technicians are required * large P °ject of
m arains
p
lon 13
r
analyse the operations.
Co mOf>itor the losses and
Figure 10.1 is a flow chart depicting schematically
and application losses due to seepage and evaporation f COnveyance distribution
phase.
Seepage losses according
soils and 0.18 m^/m^/aay
rom each individual
to Poiree and Ollier,
for loam-clay.
101Table 10. 1 below gives Che possible seepage losses in Che main canal ac full
Losses m^/day
Length
Clay
0.09ml/m2/ day
Loam Clay 0.18*$/*%/day
_= H
1*33
5*45
2,714
5,428
6,684
13,368
— -
*223
5**5
4,833
9,666
5,900
11,800
JX ->
522
1,726
3,452
* Jfc
.4*5
1317
1,217
2,434
2,484
4,968
* —*1
3OOG
2,543
5,086
TOTAL
8,101
56,303
apacxty tie flw rate
is I3.5
m^/s in che
first
reach of canal. This
is
% t. of 13.5 z
86400 •
1, 166,400m
^/day.
The seepage loss is
< *La‘ t-rrcr-sc and 4.8Z fo
r
r loam clay.
L* emerge rrayzratzos fro* a free w
ater surface
during che season
is
tut .;ihi from the xa.o canal are as follows:
Length
Loss
s
m^/day
** - 4
.9 33
143
5245
351
.2.1 4225 254
)*
41
> 3G
5445
LI20
.445
n»7
y/z»
310
72
63
129
135
HE
•'Fa^rar^xi .oas is therefore 0.J2Z
fro* the above,that ell operational losses plus seepage losses plus ' sti*a losses plus evsporstion losses from field tertiary secondary and
•• ^eeels er,'* turfs/* runoff from th- fields should be summed up with crop "*b*>^reeopiraeioe to obtain the groat irrigation requirements at diversion.
i. >
''"•teyaece Efficiency
<* <-ua
tsncy of mam canal from the head regulator to primary offtakes
Cv C<w
* rieutiQn g --_
f
"'*ry sysfe*. Th- intake rates of flow and quantities delivered
Vttrr•aygi-km can b* measured.
.
102The conveyance efficiency of the canal system can be expressed as:-
V♦
c Vl
where V ■ Volume diverted
c
Vd " Volume delivered to the distribution system
Vj ■ Inflow from other sources
V2 ■ Non-irrigation deliveries from conveyance
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 intake 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:-
e d " vf ♦ V3 
Vd
where V - Volume delivered to the distribution system
d
Vf « Volume of water furnished to the field canal 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 10.1.3 Field Application Efficiency
----- (42)
---------- (43)
This is the relation between the quantity of water inlet and the quantity of water needed to maintain minimum level required for the crop.
delivered at the field canal
the soil moisture above a
Evaluation of the field application efficiency reo *
water deliveries to each field and also measurements^ measuremencs of
after each application of irrigation water. Field application efficiency can be expressed as
Ot 8011 moisture before and
where: Vc = Volume of water furnished to the fields
.
V_ = Volume of water needed to maintain the soil U*- 1Cld Canal lnleCs
” 
level required tor rhe Cop
ab °ve •
Form 10.2 and 10.3 should Oe used for
efficiencies.
^cennining field applicacioQ
l0 1 4 Overall °r PrOjCCt EffiCieOCy
ents of conveyance, distribution and field application
Separate [i indicate if .
aSSCSS
and wnere remedial measures are required to
efficiencies wi
LO prove the eff^ie
of waCer uSe in the project as a whole.
103The overall (or project) efficiency can be expressed as:-
e+
p = Vm - V? ^3 
v <■
c
------------------------------------------------------------------------------------------ (45)
Vl
If Vi, V2 and V3 are negligible with respect to V and V , e = ef/e^ea
c mp
* 
10.2 Water Measuring Devices
10.2.1 Water Measurement
Irrigation water must be measured if it is to be used efficiently. Most water
measuring devices indicate the rate of flow 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) measuring structures.
10.3.1 Volume Method of Water Measurement
The flow in small irrigation streams can be collected into a container of known volume for a measured period of time using a stop watch.
Discharge rate 1/s « Volume 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 at right angles to the direction of flow by the average velocity of flow.
Q = A x V ------------------------------------------------------------------------------------------------------------------------ (46) where
Q = 1/s or m^/s
A 38 Cross-sectional area m^
V = Velocity of flow m/s
The cross-sectional area is measured directly whereas the average velocity of flow is usually assumed as 0.85 times the surface velocity. The surface velocity is measured by using a float and recording the time it takes to travel a known distance. The cross-sectional area of Che canal at the beginning and end of this distance should be determined before hand.
10.3.3 Structures for Water Measurement
The common measuring devices for measuring flow are the weir, orifice and Parshall flume. Figure 10.2 shows diagrannnatica1ly four different types of weirs used as measuring structures.
104The basic formula for calculating discharge through a weir is
where:
Q • CLHn ------------------------------------------------- "
Q 3 discharge in m3/s
C 3 coefficient depending on nature of crest and approach conditions
L 3 length of crest
H 3 head on crest
Q 3 an exponent, depending on weir opening
Francis' formula for a weir
a) with no end contractions is given by:
Q = 1.84 LH 3/2 m3/s -------------------------------------------------------------------------- (48)
b) with one end contraction Q 3 1.84 (1 - 0.1H) LH 3/2 
------------------------ (49)
c) with two end contraction Q = 1.84 (1 - 0.2H) Lfl3/2 -------------------------------------------------(50)
The discharge through a cipoletti weir which is a contracted trapezoidal weir with 4 in 1 side slopes is given by Q « 1.86 zlh 5/2
The discharge through a 90° V-notch weir is given by Q • 1.38 H^/3 The discharge through an orifice is given by:-
where:
Q « CdA V2g H
Q « discharge in ar/s
---------------------------------------------------------------------- (5!)
A 3 cross-sectional area of orifice m2 g = acceleration due to gravity m/sec2
2
(9.81 m/sec )
H 3 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
at the upstream and downstream faces of the orifice plate in
case of a submerged orifice,m.
Cd 3 coefficient of discharge varies between 0.65
and 0.7.
10.4 Parshall Flumes.
Parshall flume is a critical depth measuring device which can be installed
canal, drain or furrow co measure the race of flow of water.
consists of three main sections: a converging section at its upstream T he flume che throaC anj a diverging section downstream. The width of the
end, Lea ate9 the s£ze of the flume. The flow of the converging section is
throat ^esi longitudinaliy and transversely. The floor of the diverging section slopes upward. Figure 10.3.
re with relatively small head loss, permitting its use in The flu®e can ?PC^£ h flat graces. The head loss through a Parshall flume is
ghallow channe s w
C
about one
conditions.
requ
£ d by a weir for a given discharge and free flow
re
^ 0Ujhe flume is negligibly affected by the velocity of approach. The
icy through
velocity
structure.
flume is high enough to prevenc silt deposition within the
105Discharge through Parshall flumes can either be free flow or submerged flow. To determine the rate of flow, two depth gauges, H and are provided with zero
a
points at the mean elevation of crest.
Under free flow conditions, the rate of discharge is solely dependent on the length of crest, W, and the depth of water Ha in the converging section. The discharge may be read directly from tables using the upstream head Ha for the particular throat width W.
A Parshall flume can withstand a high degree of submergence for a wide range of downstream water levels without a reduction in the free flow race. The free flow limit is governed by Che ratio of Hb:Ha. An arbitrary classification of Parshall flumes according to throat width is (a) 1 inch to 3 inch very small (b) 6 inch through 8 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 co figure 10.3. The upstream head-discharge relationship to Parshall flumes of various sizes, calibrated empirically, is given by Che equation:-
where K » a dimensional factor which is a function of the throat width
Q “ 
ra U
a
.
u - varies between 1.522 and 1.60
Table 10.2 gives the values of K and u for each size of flume.
The selection of flume size depends on the maximum and minimum flows to be
measured, the corresponding depths of flow, and channel parameters at Che site
where the flume is to be installed.
Example:
A discharge of up to 440 1/s is to be measured in a channel whose gradient is
0.7 x 10”3 and depth of flow 78cm. 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 cable 10.3 it is seen that a 2ft flume has a maximum discharge of 937 1/s with a minimum upstream head of 46mm at 70Z submergence.
Ha is found co be 47cm for discharge of 440 1/s. Hence Hb « 0.7 x 47 = 32.9cm. The level at which the flume crest is to be installed is therefore 78cm (downstream water level), minus Hb (32.9cm) = 35.1cm above the bed level. This will give a maximum discharge of 440 1/s with 70Z submergence maintaining the downstream water depth at 78cm as designed.
106Table 10.2 Parahall Flume Dimensions as shown in Figure 10.1
mm
r
~r —1
1W 1A 1B
“T" -----1----------
1c 1E
11
---- 1---------
1 F 1 G 1 K
1---------- 1
1
11
1
N
“T" 111
11
r ~r
T
11-
1 2"
1 3"
1 242
1 276
T“ T~ T
1 356 1 93 1 229
I 406 1 135 1 254
1 76
1 203
1 114 1 254
1 311 1 457
I 178
1 457 1 152
1 305
1 6" 1 414
1 9" 1 587
1 610 1 394 1 610 1 305 I 610
I 864 1 381
1 762 1 305 1 457
1 1' 1 914 1 1343
1 1 6" 1 965 1 1419
,
I 610 1 914
1 762 1 914
1 2' 1 1016 1 1495 1 914 1 914
I 610 1 914
1 610 1 914
1 610 1 914
1 3’ I 1118 I 1645 1 1219
1 4 ' 1 1219
1 5* I 1321
1 6' 1 1422
1 7* I 1524
1 1794 1 1524
1 914 1 610 1 914
1 914 1 610 1 914
I 1943 I 1829 1 914
I 2092 I 2134 I 914
I 2242
1 8’ I 1626 I 2391
I 2438 I 914
I 2743 1 914
I 610 1 914
1 610 1 914
1 610 1 914
I 610 1 914
1 19
1 22
1 25
1 76
1 76
1 76
1 76
1 76
1 76
1 76
1 76
1 76
1 76
1 76
29
43
57
114
114
229
229
229
229
229
229
229
229
229
i
1111111FIGURE 10 1
FLOW CHART DEPICTING WATER LOSSES
DURING CONVEYANCE. DISTRIBUTION 2 fiPPLlC^TlQ/V
£VAPOHAT/O/V
GROUND
WATLRe
~ blC 10,3 gl?char8 Characteristics of Parshall Flumes
I Discharge Range j
I Throat
I Width
I W
I in m3 x 10~3 
| q „ u
1 _________ ________ I n>3/
~1 1
8
II
Submergence | Limit
I Minimum | Maximum |
H /H
ab
I 1"
I 2"
I 3"
I 6"
I 9"
I 1'
I 1'6"
I 2'
I 3'
I 4'
I 5'
I 6'
I 7*
I 8’
0.09
0.18
5.4 1 0.0604 H
a
1.55
0.77
1 13.2 1 0.1207 Ha 1.55
1.55
1.50 1 111 1 0.3812 H
1 32.1 1 0.1771 H
a
a
1.58
s
2.50 1 251 1 0.5354 H
a
1.53
4.80 1 695 1 1.056 Ha 1.538 12.10 1 937 1 1.428 Ha
1.550 17.60 1 1427 1 2.184 Ha
1.566 35.80 1 1923 1 2.953 Ha
1 .578 44.10 1 2424 1 3.732 Ha
1.587
3.32 1 457 1 0.6909 H
1.522
74.10 1 2929 1 4.519 Ha
1.595 85.80 1 3438 1 5.312 Ha
1.601
97.20 1 3949 1 6.112 Ha
1.607
a
I Head Range
I in (m)
I Minimum | Maximum |
0.015 1 0.21
0.015 1 0.24
0.03 1 0.33
0.03 1 0.45
0.03 1 0.61
0.03 1 0.76
0.03 1 0.76
0.046 1 0.76
0.046 1 0.76
0.06 1 0.76
0.06 1 0.76
0.076 1 0.76
0.076 1 0.76
0.076 1 0.76
1
________
0.50 
0.50 
0.50 
0.60 
0.60 
0.70 
0.70 
0.70 
0.70 
0.70 
0.70 
0.70 
0.70 
0.70
I I
I I
|
|
|
|
|
|
|
|
|
|
|
|
|FIGURE 10-I
FZQPP' CHART DEP/CT//VG WATER COSSES,
during convenance. distribut/o/v—s applicatiq/vRECTANGULAR SHARP-CRESTED WEIR
DEFINITION SKETCH OF A CIPOLETTI WEIR
FIGURE IO 2INLET TUBE CREST ELEVATION
SECTION L-L
PLAN AND ELEVATION OF A CONCRETE PARSHALL MEASURING FLUME SHOWING COMPONENT PARTS
Ui
FIGURE 10Form 10.1
Irrigation Efficiency Evaluation
Calculation of distribution efficiency for a farm unit served from the Main Cana1
Date
Unit
Farm
Crop grown
Grass area
3 Net cultivated area
4 Period
5 Total flow delivered to distribution system (Vd)
6 Total flow delivered to field canal inlets (V )
f
7 Total non-irrigarion deliveries from distribution system (V3)
8 Distribution efficiency
Name
7) Distribution efficiency
109Form 10.2
Irrigation Efficiency Evaluation
Calculation of the Field Application Efficiency for an Individual Field for a Specific Irrigation
Farm .....................................................
1 Name of field
2 Grass area
3 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
11 Total flow delivered to the field Vf
12 Rooting depth of crop
13 Soil moisture status before irrigation
Unit .................................... Designation 
ha
ha Name gm/cc Z
1/s Name Name Hours m^ cm
Z
14 
Soil moisture status after irrigation
15 
Volume of water added to root zone, Vm
16 
Crop grown
17 
Stage of development
18 
Irrigation number
19 
Field application efficiency
Z
m^
No
Z
18 Refer to event number in irrigation schedule
19 Field application efficiency Available soil moisture over root zone Water delivered to field
3
110Form 10.3
Irrigation Efficiency Evaluation
Calculation of field application efficiency for an individual field during a cropping season
Date
Farm
1 Name of field
2 Soil type
3 Crop grown
4 Gross area
5 Net cultivated area
6 Date of planting
7 Total growing period
8 Total flow per post planting irrigation
9 Number of post planting irrigations
10 Total flow for pre-irrigation
11 Total seasonal application(Vf)
12 Actual seasonal consumptive use
Unit Designation
Name Name
ha ha
(W)
13 
Leaching requirement (LR)
14 
Total precipitation during the season(P)
15 
Effective precipitation during the season
(Pe)
m3
16 
Actual irrigation requirement (V )
m
m^
17 
Field application efficiency during
the season
Z
Note:-
13) Leaching requirement » 
15 USDA SCS method
16) Actual irrigation requirement Vm 17) Field application efficiency
ECW
5.ECe - ECu
W - Pe
(1 " LR)
irrigation requirement (16) T °Ca 1 ^asonal ^ppUcadon ( 11"-----------------------------
111APPENDIX 1
MOISTURE CHARACTERISTICS OF THE SOILS IN
THE AMIBARA IRRIGATION PROJECT AREA
AND MELKA WERER RESEARCH STATION
1. Introduction
Soil moisture characteristics in general refer to water intake, water retention, water re-distribution and water movement in soils. In this report however the water retention characteristics of the soils are discussed.
Water retention properties to a very great extent determine irrigation schedules and depth of application. Thus these properties should be known for 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 the soil by surface tensional forces against gravitational pull. The tension at which this water is held is about
l/3rd atmosphere (pF ■ 2.3) for fine textured soils, and about l/10th atmosphere (pF 3 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 moisture 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 mo9t 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 the 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.
1.3 Available Soil Moisture
The amount of water released by a soil between field capacity and permanent wilting point is called available soil moisture.
1.4 Readily Available Soil Moisture
It was once generally accepted that the soil water was equally available to plants from field capacity to permanent wilting point. This was disputed (7) and it is now recognised that more energy must be exerted to extract water as
112the soil water content approaches permanent wilting point. us e
readily available soil moisture is defined as a fraction of the aval a e si moisture that is easily absorbed by plants. For most crops this raction les between 0.5 to 0.75 of the available moisture.
2. Soil Moisture on Volumetric and Depth Basis
For general purposes soil moisture is expressed as percentage on weight basis. It is determined by the following equation:-
Soil Moisture on weight basis
= Wet weight of soil - Dry weight x 100Z Dry Weight
However, this cannot be used in comparing soil types as the actual water content will vary depending on bulk density which is related to pore space. Thus to express water content realistically, soil moisture on weight basis should be converted either into volumetric percentage or depth of water per unit depth of soil. The following formula show these conversions.
Soil Moisture
on Volume Basis
Soil Moisture
on Depth Basis
2.1 Bulk Density
Bulk density is defined It is determined by the
■ Soil Moisture
on Weight Basis
Soil moisture on Weight Basis
x Bulk Density Z
x Bulk Density x 1/10 cm/m
Bulk density
as the density of undisturbed soil including pore
space
following equation:-
Weight of Dry Soil g/cc Undisturbed Volume
3. Determination of Field Capacity and Permanent Wilting Point 3.1 Field Capacity (F.C.)
Field capacity can be determined in the field by flooding the saturate a depth of about 1.0 meter and monitoring the regular intervals from the time of flooding up to 120 hou 
cover the soil surface to prevent
evaporation.
soil so as to soil moisture content at -ars. It is important t
Table Al.l below shows the change in soil moisture concent after two soil types, dark vertisol and brown alluvial soil.
flooding forTable Al.l Changes in soil moisture content after flooding.
Moisture X on a dry weight basis (MWRS Progress Report 1968/69, (3))
Time after
Vertisol
Alluvia
flooding hrs
64.2
48.7
24
55.8
40.0
48
48.5
36.2
60
45.4
35.3
72
45.2
35.2
96
45.1
35.2
120
From the above table it can be concluded that the field capacity value for vertisol is about 45.IX while that for alluvia is 35.2X.
Field capacity can also be determined in the laboratory using a pressure plate apparatus. In this method the soil sample is air dried, crushed and sieved using a 2mm sieve. The sieved sample is then poured into a rubber ring placed on a porous 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/3rd atmosphere until drainage has ceased. The sample is then unloaded and its soil moisture determined gravitmetrically.
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 has 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 gravitmetrically.
The permanent wilting point can also be determined in the laboratory using the pressure plate apparatus. The procedure is the same 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.
Factors that influence F.C. and P.W.P. are texture, structure, porosity and organic matter content. When F.C. and P.W.P. are expressed in volumetric basis the effect of structure and porosity is minimised. For low-organic soils, such as the soils of the Middle Awash, the influence of organic matter is negligible (organic matter varies between 0.34 to 0.76X (3)). Thus the most significant factor that influences F.C. and P.W.P. is soil texture. Soil texture is
determined by particle size distribution and particle size determines the pore diameter. Smaller the particle size, smaller will be the pore diameter and vice versa. Pore diameter or the capillary pore size is related to a surface tensional force with which water is held in the soil. Surface tensional force is related to capillary diameter by the equation given below:-
P = 2T r
114From the above equation it is evident that the smaller the diameter of t e capillary greater will be the capillary tension with which the moisture is he in the soil. Thus in fine texture soils, soil moisture is held with a greater force than in coarse textured 9oils. This explains why coarse textured soils are easily drained. Also for a given soil moisture tension a fine textured soil will have more water content than a 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,
(ILLRI Publication 16 Vol 1, (4))
F.C. and P.W.P.
Soil
Pore
Bulk
Wilting
Field
Available
Texture
Space
Density
Point
Capacity
Soil Moisture
Vol Z
g/cc
Vol Z
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 sandy where
interesting to
note that the
available
9oils and increases steadily
soil moisture
as the clay content increases up to
the clay content is around 60Z at which'composition'the'tZxture”i
loam to silty clay loam. Further increase in clay content
reduction in available soil moisture. — • Sweeney (8) and the data is presented in Table Al. 3.
is the lowest in
» a point ---------------- is clay 
causes a slight
This phenomenon has been reported bv n Ta h 1 o a 1 
-
Table Al.3 Available Soil Sweeney, 1970)
Textural
Class
Coarse Sand and Gravel Sand
Loamy Sand
Sandy Loam
Fine Sandy Loam
Loam and Silt Loam
Clay Loam and Silty Clay Silty Clay and Clay
Moisture for Various Soil
Available Moisture
cm/m
2 - 6
4-9
6-12
11 - 15
14 - 18
17 - 23
Loam 14-21
13 - 18
Texture Classes (From
1155 Soils of the Amibara Irrigation Project Area
The Italconsultants (5) in classifying the soils of the Amibara Irrigation Project (A1P) including the Melka Werer Research Station identified 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 Italconsultants and grouped under the two major 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 Melka 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 Al? Area
Alluvia
1 Soils on very recent alluvium. 2 Undulating phase of (1)
6 Soils on recent alluvium with
alterations in grain size.
Vertisols
3 Vertisols on recent alluvium
4 Non-calcareous vertisols on recent alluvium
5 Hydromorphic vertisols
Hydromorphic phas
e of (6).
8 
Sandy phase of (6).
9 
Arid brown soils
on terrace
alluvium.
10 
Saline and alkali
soils on 
recent alluvium.
11 
Calcareous soils
on pebbly 
terrace alluvium.
According to a Soil Survey of the Awash River Basin conducted by FAO,(2) the predominant soil types in the Middle Awash are again Alluvial Soils and Vertisols as shown in Table Al.5.
Table Al.5 Distribution of Soil Types in the Middle Awash Soil Group
a) Soil on recent Alluvia
Alluvial Soils
I 33
Vertisol
Other types
16.6
14.4
2.0
116Soil Group
b) Soils on old Alluvia Vercisol
Semi-Arid Brown Soils Regasols
Other types
c) Lithosols and Rock outcrops 6 Experimental Procedure
Z
38
9.9 15.5
9.3 3.3
29 100
Thirty one soil samples in the AIP 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 o-
which is described in Section 3.
A portion of each sample was used for textural analysis using hydrometer method.
At each sampling site bulk density of the 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 to clay 
content using the built-in computer programme of TI-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 this table.
a) All the samples analysed irrespective of whether they were alluvia or
vertisols have clay contents about 32Z, with a maximum value of 84Z. The mean 
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 groups, using the textural
triangle, indicates that the soils lie within a narrow range of textural
grouping viz: clay loam to clay. Out of the 31 samples, there are 3 clay lo.m>
2 silty clay loams. 17 silty clays and 9 clays. Thus predominantly the soils ’ are silty clay in texture.
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 1 IS a . a
be related to the clay content. With higher clay content low bulk density values are observed which is reported by other workers (4 6)
'°
tensity
117d) Both F.C. and P.W.P. are found co be highly correlated to clay content. Regression analysis between clay content and F.C./P.W.P. gave the following equations
F.C. P.W.P. where
- 0.38C ♦ 34.98 (r « 0.83)
- 0.42C * 10.53 (r = 0.81) C ■ clay concent in per cent.
e) F.C. and P.W.P. values were predicted using the regression equations and compared with measured values of samples picked at random. The predicted values matched reasonably well with the measured values. Table Al.7 shows the predicted and measured values of 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 30Z clay content to 21 cm/m at 84Z clay content as shown in figure Al.5.
g) It should be noted that the regression equations given under para (d) above are valid under the following limiting conditions:-
clay content between 30Z and 85Z
textural group between clay 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 as observed is explained as follows.
An increase in clay concent 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 of textural groups.
8 Summary and recommendations
Experimental evidence indicates that the predicted available soil moisture of
the two soil types in the project area does not vary very much, 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 is 22.17 cm/m. Thus it can be summarised that irrespective of the soil type the available soil moisture in the project area as well as the
research station is around 22 cm/m. This will justify the use of one irrigation regime for the whole project area.
Soil varies very greatly in texture, structure and soil water properties vertically (between profiles) and horizontally even in a so called "uniform soil". Thus the soil parameters taken into consideration for irrigation and other agricultural management practices are averaged values obtained from representative samples. Further in irrigation practice inaccuracies arise due to unavoidable losses caused by surface evaporation, deep percolation, surface run-off and losses through structures. Thus average values of available soil moisture are not likely to cause any serious error in irrigation scheduling and Water Management. It is concluded that the use of a single value of available soil moisture for the two soil types is justified.
118Table Al.6
Soil Water and Textural CharacteriseicB of 11 Soil Samples
1
1
1 1
1 1
1
1
1
1
1
1
1
1
1
I BULK
1
1 FIELD
1
1
1 1
1 1
1 1 
1 SOIL 1 % 1 % 1 X I TEXTURAL1 DENSITYI CAPACITY 1 P.W.P. JA.S.M. |
1 ND
1
I CODE I IYPE 1 1
1 SANDI SILT 1 a ay 1 GROUP 1 gn/cc
11
1
1
J
I Wl.X I Vol.« J1
I Wt.« I Vol.% 1 <*/■ 1
II
1 1
1 11
1 1
I MWRS 102 I A
1 1 4
1
1
1 50 1 46
1
I SIC
1
1 1.3
1 1
I 40.38 | $2.49
1 1
1 23.17 | 30.12
1 1
122.37 |
1 2 I MWRS 107 | A 1 6 1 46 I 48 I SIC 1 1.3 I 42.70 | SS.S1 I 27.47 | 55.71 1 19.80 | 1 3 I MWRS 109 I A 1 8 1 49 1 43 I SiC 1 1.3 I 37.82 I 49.17 1 20.09 | 26.12 1 23.05 | 1 4 I MWRS 202 | V 1 - 1 36 1 64 1 c I 1.20 I 47.11 | $6.33 1 28.$3 | 34.24 1 22.29 | 1 5 I MWRS 208 | V 1 6 1 46 1 48 1 SIC 1 1.3 I 41.4$ | $3.89 I 24.94 | 32.42 121.47 | 1 6 I MWRS 212 I A 1 12 | 44 I 44 1 SiC 1 1.3 | 59.01 | 50.71 1 20.54 | 26.70 124.01 | 1 I MWRS 210 I V 1 8 1 52 | 40 1 SiC 1 1.3 1 39.43 | $1.26 1 22.01 | 28.61 1 22.6$ | 1 8 | MWRS 220 | V 1 - 1 24 1 76 1 c 1 1.15 I $7.08 | 6$.64 1 37.8$ | 43.S3 122.11 | 1 9 I MWRS 225 I V 1 - | 20 | 80 1 c I 1.15 I $$.63 | 64.20 1 36.10 1 41.32 122.68 | 1 10 | MWRS 201 I V 1 3 143 1 54 1 SiC | 1.25 | 48.32 | 60.40 I 32.31 | 40.64 1 19.76 | 1 11 | MWRS 201 | V 1 1 1 39 1 60 1 c 1 1.2$ I $2.78 | 6$.98 I 37.63 | 47.04 1 18.94 | 1 12 | MWRS 206 | V 1 5 1 43 1 52 1 SiC 1 1.2$ I 44.38 | $$.48 I 26.18 | 32.73 122.75 | 1 13 | MWRS 206 1 V 1 5 1 37 1 58 I SiC I 1.2$ 1 49.67 | 62.89 I 33.61 | 42.01 1 20.08 | 1 14 | MWRS 10$ | A 1 5 151 1 34 I SiCL I 1.30 1 40.40 | $2.$2 I 23.73 | 30.8$ 121.67 | 1 15 | MWRS 10$ I A 1 17 I 49 I 44 1 SiC 1 1.3$ | 37.$9 | $0.7$ 1 17.96 | 24.2$ 126.50 | 1 16 | MWRS 110 | A 1 13 1 43 1 44 1 SiC 1 1.3 I 40.83 | $3.08 I 20.22 | 26.29 126.79 | 1 17 | MWRS 110 | A 1 19 I 49 1 32 1 SiCL 1 1.3$ | 3$.6$ | 48.40 1 14.07 | 18.99 1 29.41 |
1J
1J1JJ
J— J 1 J 1 J
1
A = Alluvia
V = Vertisol120
Table Al.6 Cont
ND
I
I
I
I CODE J_______
I
I
|
%
1 1 1 $
1
1
1
1
I
I FIELD
I
| 
1 % 1 TEXTURAL
I
I BULK
I OENSITY
SANDI SILT
II
1 CLAY1 GROUP I gm/cc
I CAPACITY | P.W.P.________ | A.S.M.
I Wt.X | Vol.S | Wt.S | Vol.S | on/n
1
1
I 18 I
A
V
V
V
V
V
A
V
V
V
A
V
V
A
27 •
1
1*1
1
1
I 19 
I 20 
I 21
I 22 
I 23 
I 24
I 25 
I 26 
I 27 
I 28 
I 29 
I 30 
I 31
I A/5A/1 
I A/5A/1 
I A/5A/11 
I A/5B/14 
I A/6/10 I A/6/2B I A/6/6B
I MS/4/15 I MS/4/15 
I MS/3/1 
I MS/3/18 
I MS/4/31 
I MS/4/18 J
1 32 1 a
5 1 47 1 48 1 sic 23 1 39 1 36 1 CL
2 1 42 1 56 1 Sic 0.6 1 21.4 1 78 1 c
- I 28 1 72 1 c
8 1 42 1 50 1 SiC 6 1 38 1 56 1 c
7 1 47 1 46 1 SiC 5 1 43 1 52 1 SiC
32 1 32 1 36 1 Cl 6 1 32 1 62 1 c
0.6 1 15.4 1 84 1 c
6 1 44 1 50 1 SiC
I
I 1.37 I 1.30 I 1.35 I 1.25 
I 1.15 
I 1.15 
I 1.30 
I 1.25 I 1.30 I 1.25 I 1.35 I 1.20 I 1.15 I 1.30
J
J______I
II
J______ L
II
I 33.30 | 44.96 I 13.88 | 18.77 I 26.19 
I 40.41 | 52.53 I 22.60 | 29.38 I 23.15 
I 31.30 | 42.26 I 15.52 | 20.95 I 21.31 
I 43.01 | 53.76 I 36.90 | 33.63 I 20.13 
I 56.56 | 66.04 I 35.55 | 40.88 I 24.16 
I 49.08 | 56.44 I 30.85 | 35.48 I 20.96 
I 38.07 | 49.49 I 22.46 | 29.20 I 20.29 
I 41.89 | 52.36 I 25.84 | 32.30 I 20.06 
I 44.62 | 58.01 I 27.37 | 35.58 I 22.43 
I 46.05 | 57.56 I 39.86 | 37.33 I 20.23 
I 34.49 | 46.56 I 18.85 | 25.45 I 21.11 
I 43.88 | 52.66 I 26.04 | 31.25 
I 59.33 | 68.23 I 41.59 | 54.07
I 37.83 | 53.50 
I 21.41 I 24.73 
I 29.80 | 38.74 I 15.33
JL
J
I
I
I
I
I
I
I
I
I
I
I
I
JTable Al.7
Predicted and measured F.C. and P.W.P. Values
11
1
1
1 NO I CODE
1
1 SOIL
1 TYPE
1
I TEXTURAL
I Cl ASS
1 CLAY
1 C0NTEN1
1*
1 PREDICTED I hCASURED
|
I__
1 F.C.
________ 1______________ 1
1 P.W.P.I F.C. I P.W.P. |
1___J
JJ
1
1
Illi
1
1
1
1
1
1
Illi
11
| MWRS 102
1A
I SiC
1 46
1 52.46
1 29.70 | 52.49 | 30.11 I
12
| MWRS 208
1v
1 SiC
I 48
I 53.22
1 JO.54 | 5J.89 | 32.42 I
13
| MWRS 225
1v
1c
1 80
1 65.38
I 43.98 | 64.20 | 41.52
|
14
| MWRS 105
1A
1 SiC
1 34
1 47.09
I 24.66 | 50.75 | ?4.25 |
15
I MWRS 210
1v
I SiC
I 40
I 50.18
I 27.18 | 51.26 | 28.61 |
16
1 A/5A/J1
1A
I SiC
1 56
I 56.26
I 33.90 | 53.76 | 33.63 |
17
1 A/5B/14
1v
1c
1 78
I 64.62
I 43.14 | 65.04 | 40.88 |
18
1 F6/3/1
1A
1 CL
1 36
I 48.66
I 25.50 | 46.56 I 25.45 |
19
I tE/4/13
1v
1C
I 84
I 66.90
I 45.66 | 68.23 I 43.50 |
1 io
I M5/4/18
1A
1 SiC
1 50
I 53.98
1 J1.J8 | 54.07 | 38.74
1
1__ J
A
V
J1 J JJ111
Alluvia
VertisolMoisture
WATER HOLDING CAPACITY OF SOILS IN PROJECT AREA
J
APPENDIX .1.APPENDIX II
References for Appendix I
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 to Water (1979)
International Institute for Land Reclamation and Improvement
Drainage Principles and Applications
Publication 16
(1974)
On Irrigation Efficiencies
Publication 19
(1978)
Discharge Measurement Structures
Publication 20
(1978)
United States Department of Agriculture, Soils Conservation Service National Engineering Handbook Section 16 (1971)APPENDIX II
References
Doneen I D 1972 Irrigation Practice and Water Management
FAO Irrigation and Drainage Paper No 1 p 8
FAO 1965 Report on Survey of the Awash River Basin Vol 1 p 45
1AR 1968/69 Progress Report of the Melka Warer Research Station p 18
IILRI 1974 Drainage Principles and Applications Publication 16 Vol 1 p 48
Italconsult 1969 Melka Sadi - Amibara Irrigation Project Feasibility Study,
Awash Valley Authority, Addis Ababa
Jensen M E 1981 Design and Operation of Farm Irrigation Systems ASAE Monograph No 3 p 85
Richards L A and Wadleigh C H 1952- Soil Water and Plant Growth. Agronomy Monograph No 2 p 73
Sweeney M D 1970 Soil Water Characteristics Important in Irrigation. North Dakota Agr Exp Sta Bulletin No 573 p 6
123APPENDIX III
GLOSSARY
(Excerpt from FAO I&D Paper 24, 1977)
ACTUAL CROP EVAPOTRANSPIRATION, ETa(crop): rate of evapotranspiration equal to 
or smaller than predicted ET crop as affected by the level o aval a e
water, salinity, field size or other causes; mm/day
ACTUAL VAPOUR PRESSURE, ed: pressure exerted by water vapour contained in the
air; millibar
ADVECTION, horizontal transport of sensible heat by air movement as for instance from 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 total available soil water between field 
capacity (Sfc) and wilting point (Sw); mm/m soil depth
AVAILABLE SOIL WATER, Sa: depth of water stored in the root zone between field capacity (Sfc) and wilting point (Sw); mm/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; mm/hour
BASIC INTAKE RATE: rate at which water will enter the soil when after initial wetting of the soil the rate becomes essentially constant; mm/hour
CANOPY INTERCEPTION: depth of precipitation caught and held by plant foliage and 
lost be evaporation without reaching the ground surface; ran or sometimes
percentage of rainfall
CLOTHESLINE EFFECT: horizontal heat transfer (advection) from warm and dry 
upwind area to a relatively cooler crop field resulting in increased ET crop;
particularly refers to the field border effects or to patchwork of small
interspersed fields
CLOUDINESS: degree of cloud cover, usually mean of several observations per day expressed in oktas (in eighths) of sky covered, or in tenths of sky covered
CONTINUOUS SUPPLY: method of water delivery with continuous but often variable discharge in water distribution system up to inlet of individual farm 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 have
lasting effect on crop growth and yields
CROP COEFFICIENT, kc: ratio between crop evapotranspiration (ET
reference crop evapotranspiration (ETo) when crop is grown in 1 
ea
optimum growing conditions, or ET crop = kc ETo; fraction
crop) and the
arge fields under
124CROP EVAPOTRANSPIRATION, ETcrop: rate of evapotranspiration of a disease-free crop growing in a large field (one or more ha) under optimal soil conditions,
including sufficient water and fertiliser and achieving full production 
potential of that crop under the given growing environment; includes water loss
through transpiration by the vegetation, and evaporation from the soil surface
and wet leaves; mm/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; mm/day 
as average for given period
DAY LENGTH FACTOR, p: percentage p of total annual daylight hours occurring 
during the period being considered, percentage
DESIGN FACTOR,of : ratio betwen canal capacity or maximum discharge in m^/sec and the maximum daily supply requirements during the peak water use period in 
m^/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) 3tage and full groundcover or when groundcover is between 10 and 
80Z; 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 that 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 80Z of surface area; percentage
EFFECTIVE RAINFALL, Pe: rainfall useful for meeting crop water requirements; it excludes deep percolation, surface runoff and interception; mm/period
EFFECTIVE ROOTING DEPTH, D: soil depth from which the full grown crop extracts most of the water needed for evapotranspiration; m
ELECTRICAL CONDUCTIVITY, EC: the property of a substance to transfer an electrical charge (reciprocal of resistance). It is measured in ohms of a
conductor wmch is 1 cm long and 1 cm2. electrical conductivity is expressed as the reciprocal of ohms/cm (mhos/cm); lm hos/cm = 1 000 nunhos/cm; 1mm hos/cn> = 1 000 ^unhos/cm
ELECTRICAL CONDUCTIVITY, IRRIGATION WATER ECw: content of the irrigation water; mmhos/cm
is used as a measure of the salt
125ELECTRICAL CONDUCTIVITY, MAXIMUM, ECmax: is concentration of the soil saturation paste
(zero yield); nmhos/cm
used as a limit of the salt
(ECe) beyond which growth would stop
ELECTRICAL CONDUCTIVITY, SATURATION EXTRACTS, ECe: is used as a measure o t e
salt content of an extract from a soil when saturated with water, un er average
conditions ECe - 1.5 EC* and also is approximately half the salinity f the soi
water to which the crop is actually exposed in the soil, nnnhos/cm
EVAPORATION, E: rate of water loss from liquid to vapour phase from an open 
water or wet soil surface by physical processes; mm/day
EVAPOTRANSPIRATION: rate of water loss through transpiration from vegetation 
plus 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 mm/day
FIELD APPLICATION EFFICIENCY, Ea: ratio of water made directly available to the 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, Sfc: 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 at soil water
tension of 0.2 to 0.3 atmosphere; nnn/m soil depth
FIELD SUPPLY SCHEDULE: stream size, duration and interval of water supply to the individual field or farm
FIELD WATER BALANCE: sum of all gains and losses of water over a given period of time; mm/period
FLEXIBILITY FACTOR, C: coefficient greater than one to account for fluctuations
in water supply in excess of those determined for an assumed cropping pattern 
and cropping intensity
FULL GROUND COVER: soil covered by crops approaching 100X when looking downwards
GROUND COVER: percentage of soil surface shaded by the crop if the sun were directly overhead; percentage
GROUNDWATER TABLE: upper boundary of groundwater where water pressure is equal to atmosphere, ie depth of water level in borehole when groundwater can freelv 
enter the borehole; cm below soil surface
GROWING SEASON: for a given crop the time between planting or sowing and harvest; days
7
INITIAL DEVELOPMENT STAGE: for a given crop the growth when ground cover is less than 10Z ; days
INITIAL INTAKE RATE: applied; mm/hour
rate at which water will enter the
time during germination or early soil when water is first
126IRRIGATION INTERVAL, i: time between Che scare of successive field irrigacion 
applications on Che same field; days
IRRIGATION REQUIREMENTS: depth of water required for meeting evapotranspiration 
minus contribution by effective precipitation, groundwater, stored soil water,
required for normal crop production plus leaching requirement and water losses
and operational wastes, sometimes called gross irrigation requirements,
ram/period
LATE SEASON STAGE: time between the end of the mid-season stage and harvest or
maturity; days
LEACHING EFFICIENCY, Le: fraction of the irrigacion wacer applied for salt control (leaching) which was effective
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 minimum amount of wacer necessary to 
control salts; fraction
LEVEL OF SUPPLY: selected water supply on the basis of probability Co meet crop irrigation requirements
MAXIMUM NUMBER OF BRIGHT SUNSHINE HOURS, N: number of bright sunshine hours for a 24 hour day with no cloud cover; hours
METHOD OF SUPPLY: method of operating an irrigacion system to convey water from
the source of supply and to distribute it according co crop requirements to each 
field served by the system
MID-SEASON STAGE: for a given crop the period between effective full ground cover and the onset of maturity (ie leaves start co discolour or fall off'% days
NET IRRIGATION REQUIREMENT, In: depth of wacer required for meeting 
evapotranspiration minus contribution by precipitation, groundwater sto-red soil
water; does not include operation losses and leaching requirementmm/period
NET LONGWAVE RADIATION, Rnl: balance between all outgoing snd Incoming longwave radiation; almost always a negative value, equivalent evaporation mm/day
NET RADIATION, Rn: balance between all incoming and outgoing short and longwave radiation; Rn - Rns ♦ Rnl; equivalent evaporation mm/day
NET SOLAR RADIATION, Rns: difference between shortwave radiation received on Che earth s surface and that reflected by the soil, crop or water surface;
equivalent evaporation mm/day
OASIS El FECT: effect of dry fallow surrounds on the micro-climate of a
r ? ^CLYe
acreage of land where an air mass moving into an irrigated area
outside the irrigated fields may over-nrodirr = i
wUl give up sensible heat. For small fields this may result in a higher ET
crop as compared to predicted ET crop using climatic data collected inside the irrigated area; conversely ET crop predictions based on weather data collected 
y er predict actual evapotranspiration losses
uaua uuiieuuc
127

< <

Summery

Amibara Irrigation Project - Summary

Amibara Irrigation Project Water Management Manual

Prepared by: Sir William Halcrow & Partners, Consulting Engineers

Date: July 1983

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). The semi-arid climate has average rainfall of 533mm/year with mean temperatures ranging from 15.2°C to 38°C.

Key Components

1. Project Works

Diversion Weir: 4m high rockfill structure designed for 700 m³/s flood capacity
Main Canal: 25.5 km long with 13.3 m³/s capacity
Drainage System: 29 km main drain with secondary/tertiary drains
Distributary Networks: 30 km secondary canals and 93 km tertiary canals

2. Soil Characteristics

Soil Type	Field Capacity (%)	Permanent Wilting Point (%)	Available Moisture (%)
Alluvial	35.75	19.55	16.20
Vertisol	46.00	30.40	15.60

3. Crop Water Requirements

Crop	Growing Period (days)	Water Requirement (mm/season)
Bananas	365	2,483
Cotton	145	891
Maize	140	624
Onions	120	571

4. Irrigation Methods

        Furrow Irrigation
        Primary method for row crops (cotton, maize, etc.)
Optimum furrow slope: 0.2-0.4%
Typical furrow length: 200-270m
Standard flow rate: 150 l/s per field canal

    

5. Water Management

The manual provides detailed guidance on:

Irrigation scheduling based on soil moisture balance
Calculation of effective rainfall
Leaching requirements for salinity control (LR = 0.049 for most crops)
Measurement techniques (tensiometers, neutron probes, etc.)
System efficiencies (conveyance, distribution, application)

Key Figures

Component	Specification
Total irrigable area	10,300 ha
Main canal capacity	13.3 m³/s
Field canal design capacity	1.00 l/s/ha
Drainage coefficient	63 mm/24 hrs (10% probability)

Appendices

The manual includes numerous technical appendices with:

Soil moisture characteristics
Reference tables for evapotranspiration calculations
Field data collection forms
Detailed drawings of irrigation structures