沙漠研究 19-1, 69 - 72 (2009 )Journal of Arid Land Studies“Desert Technology IX ”
Refereed Paper

Conveyance and Drainage System in Spate Irrigation-A case of Boro Dodota Spate IrrigationYohannes GELETA1, Chali EDESSA1, Kozo INADA2, Yoshiaki OTSUBO2,Shinji SUZUKI3, Hiromichi TOYODA3
Abstract: Total area of Oromia State of Ethiopia is 36 million ha, out of which 1.7 million ha
the potential area for surface irrigation, while only 0.2 million ha had been developed.
Previously, the irrigation development was mainly focusing on diversion from perennial riverflow. However, it is required to promote agricultural development utilizing seasonal wadi
floods for supplemental irrigation (called spate irrigation) in regions where rainfall isinsufficient for crop production. The Boro Dodota spate irrigation project is the first case of themodern medium scale spate irrigation in the State initiated in 2004 having 5,000 ha of thecommand area. The current report discusses contexts, constraint, and prospects of the project,in particular focusing on designing and function of the system. Currently assessment conductedby the Irrigation Farming Improvement (IFI) indicates that the project had positive impact on
crop production, while several challenges remained concerning conveyance and drainage. Theresults obtained in the current study indicate that establishment of the silt elimination and thedrainage system is strongly recommended to enhance the benefit of the project.
Keywords: Central Ethiopia, Flood, silt sedimentation, Spate irrigation, Watershed analysis

1. Introduction
Total area of Oromia State of Ethiopia is 36 million ha, out of which 1.7 million ha is suggested tobe suitable for surface irrigation with a slope ranging 0 to 8%, while only 0.2 million ha has beendeveloped (Oromia Irrigation Development Authority, 2006). Previously, the irrigation development wasmainly focusing on diversion from perennial river flow. However, it is required to promote agriculturaldevelopment utilizing seasonal wadi floods for supplemental irrigation in regions where rainfall isinsufficient for crop production. This is called spate irrigation. Currently, many spate irrigation
development schemes are underway particularly in eastern part of the Oromia State where there arelimited perennial river flows. The Boro Dodota spate irrigation project located at 8˚11’N and 39˚22’E, isthe first case of the modern medium scale spate irrigation in the State initiated in 2004 having 5,000 haof the command area.
The paper discusses the context, constraint, and prospects of the project, in particular focusing on
designing and function of the system in limited scope.
2. Material and Method
The Boro Dodota spate irrigation was projected to intake floods maximum up to 6 m3/s from theseasonal flow of Boro river into the command area for supplemental irrigation (Oromia Water Resources
Bureau, 2008). The watershed consists of arable land (70 %), high land covered by bushes and trees(20 %), and grazing land (10 %). The flood taken is due to be distributed to each command field through17.4 km of the main canal (out of which 14.7 km is earthen canal), 7 branch canals (total length is 61 km
with trapezoid section), secondary and tertiary canals. Mean slope of the earthen main canal is 2/1,000
and the maximum fluid velocity is 1.2 m/s. Further, 11 super passages and level crossings are provided
to the main canal in order to drain the excess runoff from the upper command area.
3. Results and Discussion
3.1. Positive Impact of Spate at Boro1

Oromia Water Resources Bureau, Addis Ababa, Ethiopia, email: yohketi@yahoo.com, je_chali@yahoo.com
Japan International Cooperation Agency (JICA), Addis Ababa, Ethiopia, E-mail: kmtskinada@yahoo.co.jp,
yoshiaki_otsubo@hotmail.com
3
Tokyo University of Agriculture, Setagaya, Tokyo, Japan, email: s4suzuki@nodai.ac.jp, h1toyoda@nodai.ac.jp
2

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35
Irrigated (2007)30
Yield (Qn/ha)

In order to investigate theimpact of the project on crop
production, the Irrigation Farming
Improvement (IFI) organized byJapan International Cooperation
Agency (JICA) and Oromia Water
Resource
Bureau(OWRB)
undertook field visit and datacollection in 2007. Out of thecommand area 280 ha of the land
was subjected to the analysis. It isrevealed that the production withspate irrigation was significantly
higher than that with non-irrigatedcondition, and it was comparable
to the yield record in 2006 which
had sufficient rain (Fig. 1,Kebebew et al., 2008).

Non Irrigated(2007)

25

Good Year (2006)

20151050Wheat

BarelyMaize
Crop type

TeffH.Bean

Fig. 1. The effect of spate irrigation on crop production.
(After Kebebew et al., 2008)

3.2. Watershed and Flood
MicroTo scale up the previous
Micro- watershed G
verification result to all over the watershed F 12%
Boroproject command area, constraints
watershed6%
were also observed. The project30%
encounterselevennaturaldrainages which collect floodfrom the sub and micro watershed,
MicroMicroout of which five of them had watershed E
watershed C
severe impact on the main canal,
20%
14%
the command area, and theMicroKaleta subdrainage system.
The micro
watershed D
watershedcatchment area of these eleven12%26%
natural drainages covers 163 km
area which is greater than that ofFig. 2. Component of watershed of the Boro dodota spate
irrigation.
Boro watershed, 69.3 km2
retrieved by Arc GIS (Fig. 2).
Assuming the same land use and rainfall for the main rivercatchment and sub and micro watershed of the commandarea, one would judge that the flood yield from sub andmicro watershed could have not only positive impact but
also considerable negative impact when treatedinadequately. During the past two years, flood from thesemicro catchments brought significant damage on farming
areas and some residences (Fig. 3).
3.3. Drainage System
Initially, the project did not include drainage systems,
especially cutting drain to protect the main canal from floodof the micro watershed. Flood from the watershed wasallowed to pass through the command area across canals,
and the flood was not led to the main natural drain. Theinterference on the natural drain system during construction

Fig. 3. Flood over the hydraulicallyregulated structure.

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Canal depth (1.2 m)

Depth (m)3.4. Silt
The main canal wasdesigned and constructed totake 6 m3/sec of flood fromthe wadi or the micro
catchments and to spill theexcess water at 6 different
hydraulicallyregulatedcrossing structures to thenatural drain. However, due
to high silt content of theflood, especially from thewatershed, the capacity of thecanal decreased resulting in
overtopping from the main
canal at different places.
Further, structures were filledwith silt (Fig. 4).
Silt
sediment
wasobserved for the lined main
canal and division box
structures ranging from 20 to
40% and from 30 to 45%,
respectively (Fig. 5). The
earthen main canal had alsosilt sediment up to 60% of
their capacity with single
rainy season flood event (Fig.
6).

1.2

0.8
Designed water depth (0.65 m)
0.4

00500

1,000
Distance (m)

1,500

2,000

Fig. 4. Silt sedimentation in the main canal.

1.2
Canal depth (1.2 m)

Depth (m)resulted disturbance on theprevious natural drain andcreated further inundation inother areas.

0.8

0.4

0D.Box 1

D.Box 2

D.Box 3Fig. 5. Silt sedimentation in the division boxes.

3.5 Discussion
Meteorology, watershed and soil data are essential for sustainable and efficient project. The project
was designed using FAO retrieved data at Asella meteorological station, 30 km apart from the head work to
the south. The head work and the command area are located between 1,500 and 1,800 m in altitude, while
the station is at 2,450m of altitude. Additional three stations are found around the project area at 20, 28,
and 34 km apart from the head work of the project. Recently, one station was established by Tokyo
University for Agriculture (TUA) at 6 km apart from the head work. The observation, particularly rain fall,of the newly established station and FAO retrieved data for important months of spate irrigation (June, July,and August) ensured adequate, relevant, and reliable data (Fig. 7).
Although silt load analysis was not conducted, considerable amount of silt accumulated in the canalresulting in severe negative impact in the system. Particle size of the sedimentation in the main canal weresand (0.06 mm< d< 2 mm) and silt (0.004 mm< d < 0.06 mm). Since the slope in the main canal designed
was 1/2,000, the designed system is in part responsible for the sedimentation as it generated insufficientvelocity to spill out silt.
The floods overtopped the super passage also caused eroding the downstream part of the structure.

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250

Rainfall (mm)

200
150
100
50

AsellaKuklumsa

Wonji

Koka

Hajii

0June

July

AugustFig. 7. Rainfall data observed at a meteorological station andFAO retrieved data.

4. Conclusion
By its natural characteristics of spate irrigation, it is true that
it is a high risk development. But when the development is notdone with reliable and relevant data and proper design approach
Fig. 6. Soil sedimentation in
the risk will be exponential. Especially, the flood and the silt canmain canal (above) andhinder the scale up of positive impacts of the project as intended.
division box (below).
Without proper hydrological analysis for the micro watershed andwadi Boro, silt load estimation, overall drainage system design and implementation the project, strong
water user association and better water and facility management the blessing can be reversed to curse.
Establishment of meteorological stations and silt measurement in selected areas of the watershedwould make planning and design more effective if combined with recent satellite images for land use
analysis with GIS and remote sensing.
Certain conservation measures in the watershed are required to minimize sedimentation. Designingand constructing water storage facilities like ponds in the command area and the watershed are also very
useful against severe draught, and the facilities would also function as silt reservoirs to protect the maincanal (Oromia Water Works and Supervision Enterprise, 2008). Effective drainage system design andimplementation are also essential measures to enhance the benefit of the spate irrigation and to mitigate
inundation. Design of main canal and hydraulic structure, and improvement of construction arerecommended to be conduct in the future to maximize the positive result of the project. The results andrecommendation obtained from the current project can be useful for other existing and new spate irrigation
projects in the country, Ethiopia.
References
Kebebew B., Bose M., Geleta Y. (2008): Effects of supplementary (spate) irrigation on field crop. Project for Irrigation FarmImprovement (IFI), Addis Ababa.
Oromia Irrigation Development Authority (2006): Oromia Region Water Resource Strategic Planning and Management. Oromia
Irrigation Development Authority, Addis Ababa.
Oromia Water Resources Bureau (2008): Boru Dodota Spate Irrigation Project. Oromia Water Resources Bureau, Addis Ababa.
Oromia Water Works and Supervision Enterprise (2008): Boru Dodota Spate Irrigation Project, Draft Report on Operation andMaintenance Manual. Oromia Water Works and Supervision Enterprise, Addis Ababa.