57
FEDERAL DEMOCRATIC REPUBLIC OF ETHIOPIA MINISTRY OF WATER RESOURCE
fe \sibii in sum and detail design of R\I FGVDl I \ IRRIGATION PROJEC T
V OK ME 4 ANNEX 3 HYDRO-GEOLOGICAL
STUDY
FEASIBILITY FINAL REPORT
MAY 2010
in association with
WjUU Woilt
41 Ml
Supci v UKMJ i iMcipiIm:
IWWIM)
I ntcfcont menial Consultants
and Technocrats
PVT LTD (ICT)Federal Democratic Republic of Ethiopia-Ministry of Water Resources feasibility Study and Detail Design of Bale Gadula Irrigation Project
VOL 4- Annex 3
Hydro-geological Study
FEDERAL DEMOCRATIC REPUBLIC OF ETHIOPIA
MINISTRY OF WATER RESOURCES
FEASIBILITY STUDY AND DETAIL DESIGN OF
BALE GADULA IRRIGATION PROJECT
FINAL FEASIBILTY REPORT
VOLUME 4-ANNEX 3
HYDRO-GEOLOGICAL STUDY
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Feasibility Study and Detail Design of Bale Gadula Irrigation ProjectHydro-geological Study
List of Volumes in the Final Feasibility Report
VOL 1
Executive Summary
VOL 2
Main Report
VOL 3
Annex 1
Meteorological and Hydrological Study
VOL 4
Annex 2
Geological and Geotechnical Investigations
VOL 4
Annex 3
Hydro-geological Study
VOL 5
Annex 4
Soil Survey
VOL 5
Annex 5
Land Evaluation
VOL 6
Annex 6
Socio Economic Study
VOL 6
Annex 7
Settlement Study
VOL 7
Annex 8
Irrigation Agronomy
VOL 7
Annex 9
Farm Mechanisation
VOL 7
Annex 10
Agricultural Marketing
VOL 8
Annex 11
Livestock Study
VOL 9
Annex 12
Environmental Impact Assessment
VOL 9
Annex 13
Watershed Management
VOL 10
Annex 14
Institutional Dev.
VOL 11
Annex 15
Financial & Economic Analysis
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VOL 4- Annex 3 Hydro geological Study
TABLE OF CONTENTS
LIST OF TABLES....................................................................................................................................................................................H
LIST or FIGURTS..................................................................................................................................................................................... II
1. INTRODUCTION................................................................................................................................................................................ 1
1 1 Bac kground to the project...................................................................................................................................... 1
1 2 Objectives of the Hydrogeological investigation.................................................................................................... 1
2. METHODOLOGY............................................................................................................................................................................... 3
2 1 Field survey, approach and data acquisitions...........................................................................................................3
2 2 Previous works.......................................................................................................................................................  5
2.2.1. Summary from Scientific Literature...................................................................................................5
2.2.2. Summary from previous projects...................................................................................................... 6
2.3                Hydrogeological data.............................................................................................................................. 7
3. GEOLOGY, CLIMATE, HYDROGRAPHY AND GEOMORPHOLOGY..........................................................................................10
3.1  Regional Geology................................................................................................................................................ 10
3.2   Local Geology..................................................................................................................................................... 11
3.3  Topography and geomorphology......................................................................................................................... 13
3.4   Climate................................................................................................................................................................. 16
3.5   Hydrography....................................................................................................................................................... 18
4.  HYDROGEOLOGY.........................................................................................................................................................................19
4.1   Regional Hydrogeology...................................................................................................................................... 19
4.2  Groundwater Potential of the Bale Gadula Plain and Surrounding Plateau..................................................... 22
4.3. Water supply for settlement areas once irrigation starts................................................................................ 29
4.4   Predicting the groundwater level rise under irrigation.................................................................................... 31
4.5   Water logging and water level rise management option.....................................................................................34
5. CONCLUSIONS AND RECOMMENDATIONS............................................................................................................................. 36
5.1  Specific Hydrogeological Recommendations......................................................................................................36
5.2   Recommendation on the way foreword.............................................................................................................. 36
REFERENCES................................................................................................................................................................................... 37
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LIST OF TABLES
Table 2.1 Water points within the Bale Gadula Project...................................................................................... ......... 7
Table 2. 2 Physico chemical characteristics of the Weyib River water........................................................ ......... 9
Table 3. 1a Mean monthly rainfall data....................................................................................................................... 17
Table 3.1 b Mean monthly, mean minimum and mean maximum temperatures at the Goro
Metrological Station......................................................................................................................................27
Table 4.1: Basis for classification of aquifers in the Genale Dawa basin.........................................................20
Table 4. 2: Estimation of potential evapo-transpiration, actual evapo-transpiration and
MONTHLY AND ANNUAL GROUNDWATER RECHARGE RATES IN THE BALE GADULA IRRIGATION
COMMAND AREA.....................................................................................................................................................27
Table 5.1 Geographic coordinates of proposed pieozmeters.................................................................................. 36
LIST OF FIGURES
Figure 2.1 Location map of the Bale Gaduux Irrigation Project...............................................................................4
Figure 2.2 Water points in the project area...................................................................................................................... 8
Figure 3.1 Geological map of the Bale Gardula project site and surrounding area.................................... 12
Figure 3.2 Topographic features of the Bale Gadula Command area and its bounds...................................1 5
Figure 3.3 Typical North South profile’ of across the Bale Gadula plain.........................................................16
Figure 3.4 Mean monthly rainfall distribution of the Goro Station....................................................................17
Figure 4.1 Groundwater potential zonation of the Genale Dawa Basin............................................................. 21
Figure 4.2 Hydrogeological map of the Bale Gadula Plain and surrounding areas..................................... 23
Figure 4.3 Conceptual groundwater flow and recharge model for the Bale Gadula Plain..................... 25
Figure  4.4  Assumed  sites  of  discharge  for  ground  waters  of  the  Bale  Gadula  Irrigation
Command Area................................................................................................................................................... 33
Figure 4. 5 Potential site of water logging..................................................................................................................... 35
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1.          INTRODUCTION
1.1 BACKGROUND TO THE PROJECT
The WWDSE has  entered to an agreement with MoWR to render consultant services  for Yadot Feasibility  Irrigation Project. One of among the study component is  the investigation of hydrogeology, which assist the design of Irrigated agricultural development of the area. The feasibility irrigation project is planned to be conducted in Mena-Angetu woreda, Bale zone, Oromia Regional State following the course of Yadot River, which stretches from west to east in the zone. The Bale Gadula irrigation project covers and areas of 12,000 ha..
1.2 OBJECTIVES OF THE HYDROGEOLOGICAL INVESTIGATION
The purpose of this investigation principally it to address the objectives set in the TOR of the Bale Gadula Irrigation Project as regard to hydrogeology. The principal objectives of the hydrogeological study are
•    To assess the groundwater potential and its quality for the water supply system of the irrigation project
o Assess the extent of the available groundwater resources with possible average water table yield and quality,
o Prepare hydro geological map to the scale of 1:250,000
o Select appropriate location of well fields and estimate its yield to be used for water supply to settlement areas
o Design borehole to be drilled for water supply to the settlement areas
•    To predict the groundwater level rise and change of water quality on the irrigable land under the impact of irrigation, and
o Work out long term forecast of probable changes in quality and quantity
o Assess the effect of abstraction on other elements of the hydrologic cycles and adjacent aquifers
o Evaluate time of impact of irrigation on the groundwater level and quality
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•  To  determine  the  requirement  of  subsurface  drainage  and  to  predict  time for  the  construction  of  the  subsurface  drainage  facilities  to  avoid  water logging during irrigation development.
o  Recommend  shallow  piezometers  as  required  for  future  water  level and water quality monitoring purposes
o   Analyze   the   possibility   of   advanced   groundwater   management schemes   such   as   conjunctive   use   with   surface   water   as   a supplementary use in irrigation
o Predict groundwater level rise and the time of construction of
drainage systems
In order to address these objectives a number of activities have been carried out. The activities include collection, review and analysis of available reports, data and maps. Collection of relevant information related to borehole, hand dug well, springs yield, location, and geology. Field survey has also been carried out for the purpose of sampling water points, mapping the terrain and landscape. Physico chemical analyses has been conducted on selected water points for the purpose
of    determining    water    quality,    water    quality    impacts,    and    water    quality determination for different water uses.
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2. METHODOLOGY
2.1 FIELD SURVEY, APPROACH AND DATA ACQUISITIONS
Accessibility
The Bale Gudula Irrigation project (Figure 2.1) is bounded by of 7°00’ and 7°12’ North and 40°18 and 40°38 E coordinates. The project site can be accessed by driving through Robe- Ginnir road and turning left after reaching Gorro town on the way to Alem Kerem town. The project site is at a distance of about 20km from the main road. The main project site (irrigation command area) is situated on the left bank of the Weyib River, bordered by the river from the southwest and, Kubsah village from the west and the Asendabo dry  stream from North and north east. The altitude of the proposed irrigation area is between 1800m and 1900m. Goro town is located at elevation of 2000 mask
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Feasibility Study and Detail Design^f Bale^GadulaJrrigation ProjectHydro-geological Study
Field Work, Data Acquisition and Methodology
A one week  field survey  has  been conducted during the month of July  2009 to survey  the geology, groundwater occurrence and map field characteristics  of the lithologies underlying the Bale Gadula Plain Existing data has been acquired from the previous works and from the current study. The fact that the groundwater table is deep in the area did not allow extensive drilling Only limited data exist on well and spring discharge data, well logs  or geophysical surveys. The most important data source is  the water resources  inventory  report of the Genale Dawa master clan study. Hydrometrological data from the nearest station (Goro) has  been acquired from the Ethiopian Metrological Agency. Geologic  mapping and hydrogeologic maps has been produced based on the field acquired observations, oreviously  acquired regional data and from combination on indirect evidences 
such as  topographic  and hydroggrphic  informations. Digital elevation model of
I
'esolution 30 m by 30 meter has been used to help in the mapping process. The
DEM has been used in mapping tectonic features and regional lineaments. Water
I
samples  have  been  collected  in  polyethelyn  plastic  bottles  from  existing  water
sources  and  have  been  analyzed  for  their  physic-chemical  characteristics  in  the
I
laboratory of Water Works Design and Supervision Enterprise.
Prior to going to the field the consultant has revisited existing information from the
prevous works. The most prominent previous work used in this study is the
Genale Dawa Master Plan Study. Published works on the Sof Omr Cave and the
aquifer properties of basalts of Ethiopian highlands has also been used for the
purpose of attaining the objectives of this work.
2.2 PREVIOUS WORKS
2.2.1. Summary from Scientific Literature
The Bale Gaduai Irrigation Project Site makes a small part of the Bale Highlands. The Bale Highlands  are known for their massive volcanic  chains, intervening plateau and depressions. The basaltic  highlands  of southern Eastern Ethiopia are scarcely  investigated compared to other volcanic  terrains  of Ethiopia. However a few geological studies  exist on the Mesozoic  sediments  bordering the bale highlands from East and South. The volcanic products are mostly basaltic
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rigation Project
Hydro-geolog
ical St
udy
ryholitic  and
trachyte.
The
volcanic
missives  are
do
minated
by
tracytic 
with
typical
characteristics 
domina
nt
large
phenocrysts 
of
feldspar
s.
Some
inte
rvening
depression and intermountain valleys  are filled sites  of alluvio lacustrine sediment accumulation. Glaciations  of the Bale Mountains  during the last glacial maximum have also led to formation of glacial deposits, moraines  and has  left glacial landscape in the highest places  of the Bale Mountains. The details  of the finding from previeous studies has been presented in sections 3 and 4.
2.2.2. Summary from previous projects
Existing    hydrological,    hydrgeological,    climatological,    geological    data    has    been gathered. These include:
•    BALE GUDULA IRRIGATION PROJECT- PHASE I- MAIN REPORT, By 
Water Resource Development authority, Addis Ababa, 1992
•    GENALE -DAWA RIVER BASIN Integrated resources development
master plan study report, Ministry of Water Resources, 2007
•    EGS, 1996. Geological map of Ethiopia at 1:2000000 scale. Ethiopian
Geological Survey, Addis Ababa, Ethiopia
•     EGS, 1993. Hydrogeological map of Ethiopia at 1:2000000 scale. Ethiopian Geological Survey, Addis Ababa, Ethiopia
•     EGS, 1972. Geological map of Ethiopia at 1:2000000 scale. Ethiopian
Geological Survey, Addis Ababa, Ethiopia
In these reports  the geological and hydrogeological results  were depicted at the scale of 1:2000000. There reports  provide general information as  to the lithologies, age relationships, and groundwater occurrences. However there are not at the scale required in this  particular study. Furthermore this  is  a significant mismatch between the maps  geological or hydrogeological maps  produced by these investigations. This  necessitates  a detailed field investigation in the project site to as  to attain the objectives. It should be noted that the Ganale Dawa River Basin integrated master plan study  report provide the most accurate geological map of the region.
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Hydro-geological Study
2.3 HYDROGEOLOGICAL DATA
Previous    hydrogeological    investig
ation    in    the
Bale
Gadula    area    and    its 
surroundings  is  scarse  It  cannot
be  denied  that
this 
has  affected  the  detailed
hydrogeological  knowledge  of  the
Bale  Gadula
plain.
Hydrogeoloical  data  (well
logs, spring yields, well depth, yield, SWL, etc) are scarce. For a site of thousands of sq km the number of identified water points does not exceed 10 Table 1 shows the water points  and associated hydrogeological data. The data has  been compiled from previous  works  and current field investigation. Figure 2.2 show locations  of water points  on which hydrogeological data has  been retrieved. It is noted that out of the drilled wells all turn out to be not functional. This probably is related to deeper water table than the drilled depth. Table 2 shows  the physic chemical characteristics  of the Weyib River analyzed under this  particular field investigation and project. The typical characteristics  of the Weyib River water is its  very  low total dissolved solids  (less  than 100 mg/L) and low salt content in general. This is make the river water the most favorable for irrigation practices and reduce significantly the salinity risk.
Table 2. 1 Water points within the Bale Gadula Project
UTM N   UTM E   Water         Point Name
Depth Jm)
SWL
Yield
(US)
Number of
Users
Remark
667575 784572 Bale Dureni
Spring
2.5
Population of
1 Kebele
664178    780630    Bale Alem Kerem No 1
93
NF
663729    780088    Bale Alem Kerem No 2
NF
658012 782490 Waltai Negaya 92
No 3
NF
657319 782613 Waltai Negaya
No 2
657086 782985 Waltai Negaya
No 1
650924 787142 Sinana Dinsho 650580 787352 Sinana Dinsho
89
NF
89
93
85
NF
NF
NF
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It should be noted that all 7 drilled wells in the Bale Gadula command area are non functional because they turn out to be dry during drilling.
Table 2. 2
Weyib at
Gadula
chemical characteris   ics of the          ib River water
pH
EC
TDS
Na
Mg
Ca
K
HCO3
Cl
SO4
NO3 P04
6.92
97
66
7.9
3.6
8.4
2.5
61.49
3.09
0.38
0 05
(Samples collected at Weyib bridge on July 2009)
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3. GEOLOGY, CLIMATE, HYDROGRAPHY AND
GEOMORPHOLOGY
3.1 REGIONAL GEOLOGY
The Genale Dawa basin in which the Gadula irrigation areas belongs is underlain by  Proterozoic  crystalline basement complexes, covered by  marine Jurassic  to early fluviatile Caenozoic and successions and Tertiary volcanic rocks.
The basement complexes  (gneissic  terrane and narrow low grade belts) are designated as  parts  of the Mozambique belt and the Arabian-Nubian Shield, respectively. These rocks  are then intruded by  syn-and post-tectonic  organic Dlutons.
The crystalline basement rocks are suggested to be affected by the late
D roterozoic deformation, metamorphism, and magmatism and are contemporaneously intruded by syn and post tectonic basic to acid intrusive rocks.
The late Triassic  is  a time of regional subsidence during which rifting begins. During this period the progression in the Karroo rifting allowed the deposition of a thick clastic rocks of continental origin thicker and ticker toward the central part of the rift. After a long period of subsidence, in the Callovian early Oxfordian the sea floor spreading began (separation of east Gondwana from west Gondwana). The floor spreading ended in the early Hauterivian (121-120Ma). According to Dainelli, the Jurassic transgression came from the southeast, reaching its maximum limit in western Ethiopia and Eritrea during the Kimmeridgian. This  transgression deposited a sandy  formation (Adigrat Sandstone), followed by  neritic  sediments composed mainly by thick limestones.
In cenozoic crustal uplifting lead to consecutive formation of the Upper Sandstone due to a forced regression of the sea. Tertiary uplift of the Arabian-Ethiopian swell was accompanied by laterization processes and followed by eruption of trap volcanics In some localities quaternary volcanics has continued to erupt.
In literature, the Mesozoic sedimentary rocks have been divided into two different successions on the base of their presumable age and limits. The lower carbonate succession is referred to be Jurassic (Kimmeridgian) while the upper carbonate
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Hydro-geological_SLjdy__
succession is  considered to be of Jurassic  to Cretaceous age (Kimmeridgian to Cenomanian). Between the two, a very slight unconformity v/as referred to exist by Kazmin. Although this, no field evidences were noted and being the limit iota y transitional and arbitrary, the two successions  will be treated as  a sing e geological unit of Mesozoic age in general.
The Tertiary basaltic flows of Oligocene to Miocene age outcrop at the Northern part of the Genale Dawa basin including the areas surrounding the Bale Gadula plain and upper Weyb river basin. The basaltic  flow are mainly  constituted by olivine basalt very dark grey in colour and reddish brown when weathered. The volcanic rocks belonging to the trap basalts are known for rarity of paleosol layers as the whole succession is emplaced in very short (1 million year) span of time (Kiffer et al.. 2004).
3.2 LOCAL GEOLOGY
Three principal lithologies underlie the Gadula plain and its sholders (figure 3.1). These are Mesozoic sediments at the outlet of the Wayibe River from the Gaduala plain, basalts belonging to the trap series, and younger basalts belonging to the quaternary. Aerially  less  extensive alluvio - colluvial materials  mantling the topography  particularly  along the foot hills  of the escarpments  bounding the Gadula plain are also notable.
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succession is  considered to be of Jurassic  to Cretaceous  age (Kimmeridgian to Cenomanian). Between the two, a very  slight unconformity  was  referred to exist by Kazmin. Although this, no field evidences were noted and being the limit totally’ transitional and arbitrary, the two successions  will be treated as  a single geological unit of Mesozoic age in general.
The Tertiary basaltic flows of Oligocene to Miocene age outcrop at the Northern part of the Genale Dawa basin including the areas surrounding the Bale Gadula plain and upper Weyb river basin. The basaltic  flow are mainly  constituted by olivine basalt very dark  grey  in colour and reddish brown when weathered. The volcanic rocks belonging to the trap basalts are known for rarity of paleosol layers as the whole succession is emplaced in very short (1 million year) span of time (Kiffer et al., 2004).
3.2   LOCAL GEOLOGY
Three principal lithologies underlie the Gadula plain and its sholders (figure 3.1). These are Mesozoic sediments at the outlet of the Wayibe River from the Gaduala plain, basalts belonging to the trap series, and younger basalts belonging to the quaternary. Aerially  less  extensive alluvio - colluvial materials  mantling the topography  particularly  along the foot hills  of the escarpments  bounding the Gadula plain are also notable.
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Geological Map of Bale Gadula Project Site and Its Surroundings
—i—
672000
Quaternary Volcanics              Geologic StructuresFederal Democratic Republic of Ethiopia-Ministry of Water Resources Feasibility Study and Detail Design of Bale Gadula Irrigation Project
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The Gadula plain and the irrigation command area is  entirely  underlain by Quaternary  basalt lithology. Overlaying the quaternary  basalts  rests  layers  of colluvial deposits, black  cotton soils. The Weyib River flows  along a fault line, which has a prevalent direction N100o W or 80°E. At the field outcrop both basalt units  are s  dense black  with vesicular textures. In general, Weyib River flows along a fault line, which has  a prevalent direction N100°W or 80°E. Most of the catchment area is covered with dense forest, which account for the low sediments in the river. The project area is surrounded by hard formations of the Trap series of the basaltic rock, outcropping on both sides of the valley and the river stream. In the valley there is thick layer of colluvial deposits of black cotton soil and slopes gently  decrease toward the river stream. The colluvial material thickness  is  low and therefore is not mappable at the scale in the geologic map. The basaltic rock which is  dense black  with vesicular texture is  outcropping on both sides  of the valley. This  basalt was  formed in tertiary  period. The Gadula plain itself is underlain by the quaternary basalts. Regardless of the age difference between the Quaternary  and the Cenozoic  trap series  basalts, the appearance in the field (degree of fracturing, flow bed thickness of joint settings) of the two lithologies is similar.
South wards  the Weyib River leaves  the Bale gadula plain and enters  into the Mesozoic sediments which underlie the basalt formation (see geologic map). The Mesozoic  sediment particulary  the Antalo formation at the outlet of the Weyib River from the Baslatic  highlands  is  known for extensive karst geomorphic development. The Sof Omar Cave maze forms  part of this  karst. At 15.1 kilometres  long, Sof Omar Cave is  the longest cave in Ethiopia and ranks as the 2nd longest in the World. The Weyib River flows through the cave system. It sinks  at the Ayiew Maco entrance and reappears  at the Holuca resurgence 1 kilo meter away.
3.3   TOPOGRAPHY AND GEOMORPHOLOGY
Topography, climate, hydrogaphy, and land use are important elements  that determine the groundwater availability, groundwater circulation, recharge and discharge processes. These factors obviously are determinants of impacts of
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Note : That the dissected gored running in the middle of the figure is the Waibe River and the command area is located the plain bound in the central depression.
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Regardless of the fact that the Gadula plain is bounded by highlands from north and south, the plain itself is an isolated plateau secluded between the highlands (figure 3.3). This  should make the plain site of recharge rather than site for groundwater discharge. Any regional groundwater flow emerging from the north should discharge at the foot of highland (eg. Bale Dureni spring is manifestation of such a process), while in the southern part of the plain any groundwater emerging from the south should discharge to the Waibe River before reaching the Gadula plain. The fact that the northern and eastern boundary of the Gadula plain is bounded by the Asendabo dry stream is an indication of minimal groundwater inflow to the Gadula plain from the-North.
‘rom Pos. 668272.399. 77B077.163
To Pos: 666137.496. 789414.234
2.5 km
5.0 km
7.5 km
10.0 km                11.61 km
Figure 3. 3 ‘Typical North South profile’ of across the Bale Gadula plain.
The Gadula plain is the flat laying area in the middle of the profile.
3.4 CLIMATE
Table  3.1a  and  figure  3.4  show  the  metrological  data  for  the  Goro  town,  the nearest metrological station to the project site. As recorded in the station mean monthly annual temperature of the region is 17.3 °C (table 3.1b). Air temperature at  Gorro  is  uniform  throughout  the  year.  On  the  basis  of  the  three  years  of available data, mean monthly temperature vary between 17°C (June) and 20 °C in January.  Mean  monthly  maximum  temperature  vary  between  24°C  (June)  and 28°C (February).
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Table 3.1a Mean monthly rainfall data
VOL 4- Annex 3 Hydro-geological Study
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Annual
15
21
111
137
135
46
12
32
112
77
21
5
724
Table 3.1b Mean monthly, mean minimum and mean maximum temperatures at the Goro Metrological Station
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Annual
Min
12
11
10
10
10
10
10
10
9
7
10
10
10
Mean
20
19
19
18
18
17
18
18
18
17
19
19
18
Max
28
28
27
26
26
24
25
27
27
27
27
28
27
Rainfall in mm
Figure 3. 4 Mean monthly rainfall distribution of the Goro Station
Between 1973 and 2002 the maximum rainfall recorded was 1383 mm/yr while the minimum record shows rainfall amount as low as 732.9 mm. In an average year, more than 50% of the total annual rain falls during Beige (March, April), which is the principal cropping season. Meher rainfall (July, August) falls account for more than 25% of the annual total. The other seven months  are relatively  dry  having only 25% of the total annual rain fall.
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The mean monthly  pan evaporation varies  from 128mm/month (Sep) to 180mm (march) with annual mean of 1732mm. The mean daily  wind velocity  varies between 1.5m/s  and 4m/s  with the high wind speeds  occurring from June till August.
3.5  HYDROGRAPHY
The hydrography  of the regions  straddling the Gadula plain are characterized by dendritic, parallel and rectangular pattern which are manifestations  of tectonics, geology processes in the region (figure 3.2).
The Waibe River, the River south of the Waibe River, the tributaries of Waibe form a set of parallel pattern which is manifestation of control of the drainable pattern by  regional parallel faults. The Gadula Plain has  two separate hydrographic patterns whereby in the North Western Sector the plain is drained southward while in the South Eastern Sector, the plain is drained north east ward.
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4. HYDROGEOLOGY
As depicted in the TOR the hydrogeological investigation has three principal objectives. These are
•    Assessing groundwater potential
•    Prediction of water level rise and water logging effects related to irrigation activities,
•    Proposing approaches of irrigation water management so as to manage or reduce effects of irrigation activities.
4.1 REGIONAL HYDROGEOLOGY
Generally, the principal aquifers the Genale Dawa basin including the Weyib River basins  are basalts  belonging to the trap series. The basement lighologies  are known for their poor potential. The Mezozoic sediments are known for their good potential but the fact that they  are dissected by  Major River decrease their importance unless otherwise favorable geologic structural conditions allow.
In  the  highlands  particularly  close  to  the  Weyib  river  basin  the  limestones belonging to the Mesozoic sediments show extensive features of karstification and cave formation. This make the Antalo limestone sequence in the upper part of the Genale Dawa basin attractive sites for groundwater development. Regardless of the enhanced porosity by karstification the groundwater has short residence time and fast flow velocities in the karst features. This makes the karstified iimestone aquifers  succeptible  to  variation  in  climate  and  landuse.  Generally  the  hydraulic conductivity and yield of wells increases from the highlands surrounding the Bale massif to the lowlands bordering Kenya from South.
The region surrounding the foothills of the Bale massive are known for their high groundwater potential. The Genale Dawa Master Plan Study classify the aquifers of the basin into four major groups  (figure 4.1) depending on the type of permeability and the extent of the aquifer as set in tables 4.1. These are:
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r
evaporate . marble)
•      Extensive  aquifers  with  fracture  permeability  (volcanic  rocks,basalts, rhyolites, trachytes and ignimbrites)
•       Localized   aquifers   with   fractured   and   intergranular   permeability ( noncarbonated metamorphic rocks . granitic intrusive, dolerites)
General permeability and productivity classification are made for regional surveys the basis of field observations and borehole data.
’able 4 1: Basis for classification of aquifers in the Genale Dawa basin
•       Extensive    aquifers    with    intergranular    permeability(unconsolidateci sediments,  alluvim.  eluvim.colluviums,  lacustrine  sediments  and  poorly cemented sand stons
•    Extensive aquifers with fracture and/or karastic
pe meability(consolidated sediments, limestone, sand stone, shale marl,
v/yyyi i I hi Abi.u‘-idliuii mlh IL I
I null I easibility Study Repoit
204°<H»
Figure 4.1 Ground Water Potential Zonation map of Genale Dawa Basin
38'00 E
39;0'0"E
40-00" E
41-0 0' E
42-0'0'E
43=0-0 'E
Legend
Goro
Gale Gadula Command Area
Aquifer productivity map
Productivity
| High to Moderate
.ov, Aquifuge
Moderate
Moderate to _o ;.
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In the Genale Dawa basin recharge generally, decrease from the highlands to the lowlands. Baseflow separation method (Cherent. 1993) show recharge is  in the order of exceeding 150 mm in the highlands and less than 50 mm in the lowlands bordering Kenya In the Bale Gadula plain and surrounding regions recharge has been estimated to be around 100 mm/year.
4.2 GROUNDWATER POTENTIAL OF THE BALE GADULA PLAIN AND SURROUNDING PLATEAU
Groundwater potential is  measured by  a) rate and sustainability  of recharge, b) storage and tramsimisivity  characteristics  of the aquifers and c) the suitability  of the groundwater for different uses. From existing literature recharge rate in the Bale plateau and upper Weyib catchement is estimated at 100 mm/yr. Regardless of the high recharge rate a number of wells  drilled into the quaternary  basaltic volcanics and in the trap series basalts turn out to be dry (table 2.1). Generally the topography around the Gadula plain favors formation of springs and presence of wetlands  in the depression. However the number of springs  emerging in the project area does  not exceed 5 in number and discharge rate of the only prominent springs are low (2.5 l/s).
It should be noted that the low groundwater potential of the Bale Gadula plain is an exception in a region which is otherwise known as high groundwater potential zone. In the regional hydrogeological classification the region is classified as high potential zone. The low groundwater potential of the Bale Gadula region is related to the presence of hydrogeologic  shadow whereby  groundwaters  emerging from the highlands  surrounding the plain discharge into the Weyib River (in south) or into the Asendabo Stream (in north and north east). Therefore the Bale Gadula plain is a local plateu secluded between two river gorges- this put.
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4.2
Hydrogeological Map of Bale Gadula Command Area and Its Surroundings
a-rannn 
|
Legend
*
Water Points and their yield
Major rivers
Major rivers
Geological Structures
Infered groundwater flow direction
I Extensive aquifer in dual porosity medium
'associated with fractured and karstrfied limestone
The Sof Omor Cave is assoicated with this lithlology
Medium to high productivity aquifer with fracture porosi Lrthologies are dominantly basalts groundwaters are
contained in bedding plains and joints.
Low productivity aquifers composed of tryacytes and
ryholites. Groundwater mainly discharge to springs at
the contact betweem the Tryachytes and the underlying
trap series basalts.
High productivity and extensive aquifers composed of qutemary basalts. Groundwater occures both in fractures and is co re a beds associated with the basalt flows
10 Kilometers
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the gadula palin as  a 'local groundwater shadow’. Figure 4.2 given the detailed hydrogeological map and aquifer classification of the region. In the classification criteria set in table 4.1 has  been used. Groundwater flow map inferred from the general topographic  feature show flow towards  the south west and discharge of groundwater takes place to the Weyib and Asendabo Rivers.
Measurement of EC value of the Waibe River during the dry  season show a value less  than 500 micro simens/cm. Measurement of EC value of the Yadot River during the dry  season show a value less  than 200 micro simens/cm. Because of scarcity  of water points  intensive measurement of EC and water quality  was  not attained. However the combination of geology, topography  and drainage should result in low TDS of the groundwater and surface waters. This  is  confirmed by  the regional observation under the Genale Dawa Master Plan study  which reveals  that groundwaters  of the volcanic  highlands  has  low TDS in the order of less  than 500 mg/L. The few measurement sin the irrigation command areas  and their surroundings  confirm this  low TDS of surface and groundwaters. Table 2.2 show the river water is suitable for irrigation and domestic water supply.
Combination of geological information, recharge, and water table depth and water quality  has  been used to draw a conceptual model for groundwater occurrence and flow in the Bale Gadula plain and surrounding high grounds  (figure 4.3). As shown in the figure the Mesozoic  sediments  are assumed to exist beneth the basaltic  cap. However the exact depth to the Mesozoic  sediments  is  unknown because of absence of exposure of such a lithology in the Bale Gadula Plain.
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Hydro qculoqiGal Study
Z-onprlly OCCU' in frac'.rP'. .vsa-sandcorn?. •
be- jer- a a fl? -.s grounr'we emeroin; 4r.: - rhe highlands ir the north a xl set d schtrge in the
'■■•epl: R ve- before reaching theGadula iNam Gmundweter flow IM th is gene' ? . shod ano residence trie c'g rod" ct.-xue isalsoshort
lr the genet, a ouifer classification
o' the Gena C av* basin th $ sector s ciass •>. as high potential agu for
-
Depth to wsfor tabe >s m onler of excelling
Ilin peters. Recharge ta*«e« place from ■ imfai
is estimated at 1 AO romp Groun f.-fitei xurs 
m fi actureel quaternary basalts. Loca ly cJluvrum and al u\ ial materials of small latered and vertical aquifers hold and transmit water, Since regional groundwaters get discharged into the alleys bounding the plain from south north and north east, the Gadula plain is considered as groundwater shadow zone. Total dissolved solids is less than 400 mg-L.
GroutKl'.'jflter ienedly occur n fractured ha salts and contact
ler.’^n lava II0/.5. gr'Tinrlw'f emerging from the highlands in te north and south discharge in rhe Weyib River before reaching
the Gadula plain. Groundwater flow path is general/ short and residence time of groundwater is also short.
In the general aquifer classification of theGenal Dawe basin this sector is classified as high potential aquifer
5 K~t
Figure 4.3 Conceptual groundwater flow and recharge model for the Bale Gadula Plain
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Recharge estimation in the Bale Gadua Plain
VOL 4- Annex 3 Hydro-geological Study
In order to estimate groundwater recharge in the Gadula Plain first estimate has been made on the potential evapotranspiration and actual eveportranspiration using the Thornthwaite Matter Soil Water Balance Method. The method is based on the consideration of monthly  soil water balance accounting where by  the Potential evapo-transpiration is  estimated from the classical relation of Thornthwaite and the Actual evapotranspiration is  estimated from soil water holding capacity  and rooting depth of the soils, monthly  soil water deficit and accounting for monthly potential accumulated water losses.
Thornwaite formula for calculating PET is  based on temperature with an adjustment being made for the number of daylight hours. The equation for calculating PET on monthly base is given by:-
(
a
mm
k
Where wis the month 1, 2, 3... 12, and
-A 10 T
I
/Vm
7
is the
monthly adjustment factor
related   to   hours   of   daylight,   is   the   monthly   mean   temperature   0C   ), a = 6.7x10"7 Z3 - 7.7x1 O’512 +1.8x1 O’21 + 0.49
I is the heat index for the year,
given by:
m = 1, 2, 3             12
The result of the estimation of the potential evapotranspiration is  given in table 4.2. The actual evapo-transpiration is then calculated from the Thornwaite- Mattar soil water balance approach as depicted in table 4.2. Recharge to groundwater is then estimated by considering that half of the monthly surplus goes to recharging groundwater and the remaining half leaves  the terrain as  runoff. The recharge water of any given month leaves the aquifer in the next month. The bulk annual recharge is therefore the sum of the monthly recharge of each month. By doing so the recharge for the Gadula plain irrigation area is estimated at half of 106 mm of surplus water which is equal to 53 mm/yr of recharge. It should be noted that the
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results  obtained from the soil water balance method is  lower than previous estimates  from the base flow separation method taking large rivers  only. The discrepancy  lies  in the fact that the regional recharge estimates  by  Tesfaye Chernet (1994) does not take into consideration the local soil water balancing and is  based on regional assessment of river discharges. Given that the deep water table and the general groundwater scarcity  in the Bale Gadula plain, the lower -echarge rate estimated by the current study should be the most valid.
Table 4. 2: Estimation of potential evapo-transpiration, actual evapo transpiration and monthly and annual groundwater recharge rates in the Bale Gadula Irrigation command area.
'' sir
Fsnterer
Esti~3t;cn for 3 silt\ clay soil with v.-ater caoacit'. ct 350 -"m
'an Feb N2ar Apr Mav Jun Jul Aus Sep Oct Nov Dec Annual
?
15.00      21.10    111.20  13 ".00    135.00    16.OO     12.00     32.00 1 12.00
PIT 
P PIT 
120.22 95 Cl
45.00
129.22   122.22    123.22     $5 20       '2.22
-3           --
'll
1091
Accumulated Potential Water lo -25S.22  -335.22
Sciwoattre Sm
135.1"
2.22      3*22      32.22    -39.20    -50.00     -1 00        35 00
-39.22    -99.20   • 1-2 02
13" 1"         r-.r    226.1’   313.29      253.”       231 61  2:9 51
r
is-
-32.32
2.22       3".c:      32 22    12c 93      -9.32     -29 16      35 20
”00 21.00       5.0) 90.22 93 00      93 02 
-13.20 "2 00 -SS 2? -13.00 45.20 -13 22 33’ 2- 2’: 53 213 52
$’.53 -62’0 ■cl 23
zrual F  anozanspuauon                      15 22     21.22
Sc-2 ?-2ctsrure Deficit                          S5.CC         22
C.cc
Total A-.-alahle Water for P.unof 2.19 2.22
-
22                   135 22           22      12.22      52 22      ”22 2.22       2.22        2.22      39.22      60.00      -1.02 0.20 2.22     3".2C      32.20       0.22        0.00        2 02     35.00
2.22    3S.2C      51.20
>>
11.29               5^  35 20
r
”.00 21.20              522                       650 13 00 '2.00          SS 22
0.00 0.00              202 106 l'.5O       S'5           -3S
Peano?
1-29      2 22
19.22      25 52      1! 29        5.5-        2.S2     1’.52
106
Detention 
l->         2.22        1.22     19.22
$.’5       - 3$         2 19 
11.29 5.6- 2.S2  1-52 S'5  -3S 2 19
■    P  -  Average  monthly  rain  fall  (obtained  from  rainfall  data  from  Goro station)
•    PET- Potential Evapotranspiration (obtained from Thornwaite formula)
•      P-PET  -  difference  between  rainfall  and  potential  evapotranspiration, positive  values  indicate  wet  season  and  negative  values  indicate  dry season  Positive  values  are  representing  additions  of  moisture  to  the  soil while  the  negative  values  are  showing  the  monthly  demand  of  moisture by the vegetation which is not satisfied by the monthly rainfall.
■    Lam   -  accumulated  potential  water  loss,  is  obtained  by  cumulating  the negative  values  of  the  differences  between  monthly  precipitation  and evapotranspiration  for  dry  season  only;  and  the  summation  begins  with the first month of dry season; for our catchment, October and November for lowlands and highlands respectively.
■    Sm—  Soil moisture. Accumulated potential water loss is used to obtain the soil moisture during the dry months using the following formula:
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(Water balance calculation of the Thornthwaite type, 1997)
Where, Sm - Soil moisture during the month M (mm), Lam - Accumulated potential water loss at month M (mm), ana W- available water capacity of the root zone (mm)
Soil moisture values for each wet month are obtained by adding the excess of rain of the current month to the soil moisture of the month before. However, this sum may not exceed the water capacity and excess is booked as moisture surplus.
•      AET   -   Actual   evapotranspiration.   Monthly   actual   evapotranspiration (AETm), is found as:
AET = PET                   if Pm > PETm
Otherwise, AETm = Pm + Sm -1 - Sm, , m stands for month.
Where,   Sm.i   and   Sm    are   soil   moisture   during   the   month   m-1   and   m respectively.
■    SMD  -  Soil  moisture  deficit.  Monthly  SMD  is  the  difference  between PETm & AETm
■   S - Soil moisture surplus, is the excess of soil moisture values(S  ).
m
•    TARO - total available for runoff. The value is determined, starting from the  first  month  of  the  water  surplus  period.  For  our  catchment,  May  in highlands and June in lowlands. The first months surplus is the TARO for that month, and from the surplus, 50% value is detained (D) and carried over  to  the  next  month  TARO,  and  50%  is  river  discharged  (RO). Therefore, the TARO of preceding month is given by the surplus of that
month (S  ) plus the detention of
M
last month (D  _ j).
m
Therefore the total recharge for the 12000 ha of the irrigation command area is estimated at 6 4 x 10  m3/yr of water.
6
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4.3. WATER SUPPLY FOR SETTLEMENT AREAS ONCE IRRIGATION
STARTS
The hydrogeological investigation envisages  to propose best alternative water supply  source in line with the settlement or resettlement options  that is  proposed by  the Socio economic  survey. The socio economic  survey  indicates  the need for resettling or relocating the population is minimal or insignificant. See excerpt
below:
One of the expected adverse effects  of the Bale- Gadula irrigation project is  the displacement of families  and population from their homes. Nonetheless, during the fieldwork  it was  observed that, the proposed irrigation command area is  very sparsely  populated. In addition almost all of the residential houses  are concentrated around the Kebele administration offices  forming well patterned rural village town, where social services like school, health post, grain mills, farmers' training center, mosques  and other infrastructures  are relatively available. Five Kebeles  fall in the irrigation command area. The total population living in five Kebeles falling in the command area was estimated at 16,472 people in 2008 of which 8068 are male and 8404 are female.
Weltai Nagaya Kebele is located along the bank of Web River, which is  technically  not feasible for irrigation. The other Kebeles (Bale Anole and Bale- Gadula) are located along the gravel road that connects Robe town, the capital of Bale Zone with Ginir Woreda traversing Goro town. Moreover, in all of the villages  found in the proposed irrigation command area, there is  at least one health post, one primary school, two mosques, three grain mills,
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one      framers'      training      center      and      Kebele
administration office.
Thus,   the   relocation   of   these   villages   to   other
places   could   not   be   technically,   economically   as
well   as   socially   feasible   so   as   to   realize   the
implementation of the proposed irrigation project.
Thus,   the   project   engineering   design   should   be
carefully   undertaken   so   as   not   to   disturb   the
existing villages. However, 230 households and
1,610 populations who are haphazardly settled in
the   proposed   irrigation   command   area   may   be
relocated    during    project   implementation.   In   this
scenario,  the  number  of  households  to  be  displaced
from  their  residential  area  will  be  insignificant.  As
agreed  with  engineering design team of the project,
these villages will be excluded from command area.
They should not be relocated so as to minimize the
adverse  impact  of  the  project  on  local  community.
Only    those    households    which    are    haphazardly
settled  will  be  relocated  during  the  implementation
of the project.
Under  this  condition  of  no  major  resettlement  need  water  supply  for  domestic water use for the community need to consider the following options:
•      Under  the  present  condition  depth  to  water  table  exceeds  90  meter.  But once  the  irrigation  starts  option  may  exist  in  the  command  area  to  drill shallow  wells  and  hand  dug  wells  and  tap  infiltrating  irrigation  water.  The exact  location  of  this  wells  should  be  selected  based  on  post  irrigation practice site investigation
•      Use  of  community  based  surface  water  purification  system-  this  can  be achieved  by  installing  disilting  containers  accompanied  by  some  treatment for chemical quality by chlorination techniques
•      Option  to  strike  the  underlying  Mesozoic  sediments.  It  is  known  that  the Mesozoic sediments particularly the Antalo limestone which is found
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downstream of the Weyb River Valley  is  karstified because of the high rainfall and vegetation cover of the area. Deeper drilling in the order of 20D meters  may  penetrate the high potential limestone aquifers  (as  shown in figure 4.2 and 4.3). However as  to the judgment of the consultant, deep drilling may  appear costly  for the purpose of merely  community  water supply. Prior to drilling appropriate geophysical survey  has  to be conducted to delineate the depth to the Mesozoic  sediments  underneath the basalt overburden.
Therefore   the   irrigation   practice   itself   would   favor   easy   availability   of   water resources within the command area its surroundings.
4.4  PREDICTING  THE  GROUNDWATER  LEVEL  RISE  UNDER  IRRIGATION
Data requirement for prediction of groundwater level rise include transmisivity  of aquifers, hydraulic  gradient of groundwaters, and boundary  conditions  around the impacted aquifers. However obtaining the stated data under the current project may be costly or may not be necessary. This is because groundwater table is very deep, and the topography  favors  easy  discharge of the infiltrating irrigation water to adjacent gorges. Using a simple equation which relate water level rise to specific  capacity  of the aquifers, and Darcy  discharge a preliminary  prediction to water level rise is predicted as follows.
Assuming the porosity/specific  capacity  of aquifer 0.21 (this  is  appropriate for the basaltic  aquifers  underlying the area) assuming annul irrigation water application of 0.360 m/year, water level rise is  estimated at 1.8 meters/year. Assuming unsaturated thickness  of 100 meters  (estimated from the elevation difference between the Gadula Plain and the bottom or the Wybe River valley), it takes more than five decades  for the groundwater to rise to the surface. However, since part of this irrigation recharge water is discharged out of the irrigation area because of sub surface flows the water logging should take longer time that stated.
1 in absence of clear aquifer property  data this  value is  widely  accepted figure for porosity  of basaltic aquifers. It should be noted this  value is  also from experience of the consultant rather than from literature sources.
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However since all the infiltiatmq lechaigr w.tlei doos  not go Io rise Iho water lablr* alone and part ot it is  discharged out ol Iht' irrigation areas  as  groundwater out flow the predicted watei level list' should bn lower than natirriatod A slrnplf* calculation involving Darcy’s  law has  boon used Io calculate Iho siibsudar.r* discharge from the irrigation command area Because of lack  of hydraulic  gradient data and transmisivity  of aquifer preliminary  estimates  has  boon used Hydraulic gradient was  obtained from the slope of Iho irrigation aroa, cross  section along with discharge takes  place is  delineated from topographic  map, and rock oermeability  from literature has  been used Hie conceptual discharge takes  place is depicted in figure 4 4
Discharge Q = kdh/dx , whore k is permeability of the soils and dh/dx is hydraulic gradient
Assuming a k value of 0 1 m/day (this  is  high value) for the basaltic outcrops of the area and hydraulic  gradient of 0 1 for the Gadula plain, discharge cross section of 36000 meters (figure 10 ) and aquifer thickness of 100 motor?; annual discharge from the plain is  estimated at 36000 m3/day  It should bo noted that total annual recharge from irrigation and rainfall (0 36 rn/yr) is  far lower than Ihn total amount of subsurface discharge from the command aroa suggesting no major water logging or water level rise problems in the Gadula plain
Other contributing factors  to the water level rise could bo channel loss  during irrigation exercises  However considering the depth of the unsaturated /one. and the high infiltration the impact from such activity should bo minimum
1,1
i wilh IC I
• tlt.ll I luiniblllly
l'..|,.,,Feden
c Rcpub c c’ E’K cc <> V ' e’ * .»te- Resounrc*
Feas > S\ ?\ .- c ?cts Des c” c* Ba c va<k a
f>•’« c ' r’C c4
VOL 4 Annrw ?
Hydro ucv ouivdl S\uG»
11
-1
J
----
0   2        4               & Kilometers
I 1 1 1 I L_ 1 1 I
Figure 4. 4 Assumed sites of discharge for ground waters of the Bale Gadula Irrigation Command Area (The dark bold line shows this area)
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4.5 WATER LOGGING AND WATER LEVEL RISE MANAGEMENT OPTION
Management  option  for  water  logging  and  salinity  buildup  is  depends  whether such   problem   is   going   to   be   encountered   or   not.   From   the   simple   Darcy calculation in section 4.4 it is predicted that neither permanent water logging nor salinity  buildup  would  be  encountered  in  the  Bale  Gadula  irrigation  project  site. The  possibility  of  encountering  temporary  depression  storage  and  lower  grounds and formation of short lived water logging may not be avoided. This kind of short lived   surface   accumulation   of   irrigation   water   is   entirely   related   to   non groundwater  related  processes.  Presence  of  local  depression,  presence  of  clay soils, or other low permeability soils may increase the risk of temporary detention storage.  Based  on  topographic  evidence  the  north  eastern  sector  (figure  4.5)  of the Gadula plain is identified as the potential site of temporary water logging.
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Figure 4. 5
The northern
Potential site of water logging.
sector of the study area is identified as high likelihood zone of temporary water logging site
WWDSE in Association with ICT
Final Feasibility Study ReportFederal Democratic Republic of Ethlopla-MInlntry of Water Resources
VOL 4- Annex 3
Feasibility Study and Detail Design of Balo Gadula Irrigation Project Hydro-geological Study
5. CONCLUSIONS AND RECOMMENDATIONS
5.1 SPECIFIC HYDROGEOLOGICAL RECOMMENDATIONS
The Balo Gadula Irrigation project site because of its  favorable topography, geology  and hydrograpy  should not encounter a major water logging problem or salinity  buildup once irrigation started. Even under condition whereby  subsurface discharge is not taking place the plain, more than 10 decades has to pass befcre the deep groundwater rises  to the surface. Community  based surface water purification systems  are the best option for domestic  water supply  in Irrigable area, as  groundwater level is  deeper and experience show drilled wells  in the region fail to provide water. Another option for the community  water supply  may be to drilled shallow wells and hand dug wells once the irrigation starts so as to tap infiltrating water.
5.2 RECOMMENDATION ON THE WAY FOREWORD
Site visits, hydrogeological investigation, water quality  data all lead to the conclusion that groundwater level rise and salinity  are not major challenges  if irrigation starts  in the Bale Gadula plain. Furthermore the deep water table is indicative of limited option for use conjunctive surface water groundwater irrigation. The author recommends  that peizometirc  wells  be installed and drainage channels designed and constructed although the treat water logging and salinization is  minimal. Table 5.1 the recommended sites  for small diameter pizemeter well installation. Each piezometer should have a depth in the order of 10 to 20 meters.
Table 5.1: Geographic coordinates of proposed pieozmeters
E
N
Depth in diameter
667388
783819
10 to 20
670701
784334
10 to 20
665622
785218
10 to 20
666873
782420
10
662751
781684
10
658849
782935
10
672983
781684
10 to 20
VVWDSE in Association with ICT
Final Feasibility Study Report
36Federal Democratic Republic of Ethiopia-Ministry of Water Resources Feasibility Study and Detail Design of Balo Gadula Irrigation Project
VOL 4- Annex 3
Hydro-geological Study
REFERENCES
1.  Genale Dawa Basin Master Plan Study, Ministry of Water Resources, 1998, Addis Ababa Ethiopia
2.  BALE GUDULA IRRIGATION PROJECT- PHASE I- MAIN REPORT, By Water Resource Development authority, Addis Ababa, 1992
3.  GENALE -DAWA RIVER BASIN Integrated resources development master plan study report, Ministry of Water Resources, 2007
4.  EGS, 1996. Geological map of Ethiopia at 1:2000000 scale. Ethiopian Geological Survey, Addis Ababa, Ethiopia
5.  Tesfya Chernet (EGS, 1993). Hydrogeological map of Ethiopia at
1:2000000 scale. Ethiopian Geological Survey, Addis Ababa, Ethiopia
6.  EGS, 1972. Geological map of Ethiopia at 1:2000000 scale. Ethiopian Geological Survey, Addis Ababa, Ethiopia
7.  Kieffer, B., Arndt, N., Lapierre, H., Bastien, F., Bosch, D., Pecher, A., Yirgu, G., Ayalew, D.,Weiss, D., Jerram, D.A., Keller, F., Meugniot, C., 2004. Flood and shield basalts from Ethiopia: Magmas from the African superswell. J. Petrol. 45, 793-834.
yyWDSE in Association with ICT
Final Feasibility Study Report
37

Summery

Bale Gadula Irrigation Project Summary

Bale Gadula Irrigation Project - Hydro-Geological Study Summary

Project Overview

The Bale Gadula Irrigation Project is a 12,000 hectare irrigation development in Mena-Angetu woreda, Bale zone, Oromia Regional State, Ethiopia. The feasibility study was conducted by Water Works Design and Supervision Enterprise (WWDSE) in association with Intercontinental Consultants and Technocrats PVT LTD (ICT), with the final report completed in May 2010.

Location: Between 7°00' and 7°12' North latitude and 40°18' and 40°38' East longitude

Altitude: 1800m to 1900m above sea level

Nearest Town: Goro (elevation 2000m), approximately 20km from the project site

Study Objectives

Assess groundwater potential and quality for water supply
Predict groundwater level rise and quality changes under irrigation
Determine subsurface drainage requirements to prevent waterlogging

Key Findings

Geology

The project area is underlain by three principal lithologies:

Quaternary basalts (command area)
Tertiary trap series basalts (surrounding areas)
Mesozoic sediments (downstream of Weyib River)

Hydrogeology

The Bale Gadula plain is a "local groundwater shadow" with deeper water tables (>90m) compared to surrounding areas due to:

Discharge to Weyib River (south) and Asendabo Stream (north)
Limited groundwater inflow from surrounding highlands
Quaternary basalts with fracture-controlled groundwater flow

Recharge Estimate: Approximately 53 mm/year (6.4 million m³/year for 12,000 ha)

Water Quality: Surface waters show very low TDS (<100 mg/L), making them suitable for irrigation

Water Supply Options

For settlement areas (total population ~16,472):

Shallow wells/hand-dug wells tapping infiltrating irrigation water (post-implementation)
Community-based surface water purification systems
Potential deep drilling (200m) to reach Mesozoic limestone aquifers (costly option)

Groundwater Level Rise Predictions

Parameter	Value
Estimated annual irrigation application	0.360 m/year
Aquifer porosity/specific capacity	0.2
Predicted water level rise	1.8 meters/year
Time for groundwater to reach surface	>50 years (likely longer due to subsurface discharge)

Waterlogging Risk

The study predicts minimal waterlogging risk due to:

Deep initial water table (>90m)
Significant subsurface discharge (estimated 36,000 m³/day)
Favorable topography allowing drainage

Potential temporary waterlogging may occur in the northeastern sector of the plain.

Conclusions and Recommendations

Groundwater level rise and salinity are not expected to be major challenges
Community-based surface water purification is recommended as primary water supply
Installation of piezometers is recommended for monitoring (7 locations specified)
Drainage channels should be designed despite low waterlogging risk
Conjunctive surface-groundwater irrigation is not feasible due to deep water table