沙漠研究 19-1, 241- 244 (2009 )Journal of Arid Land Studies“Desert Technology IX ”
Refereed Paper

Trials on Hydro Meteorological Analysis for Spate Irrigation Developmentin Oromia, Ethiopia
Girma NEGUSSIE1, Hussien AMAN1, Kozo INADA2, Shinji SUZUKI3, Hiromichi TOYODA3
Abstract: The current report discusses issues of the spate irrigation project executed in theOromia Regional state of Ethiopia in collaboration with Japan International Cooperation
Agency (JICA) and Tokyo University of Agriculture (TUA), particularly addressing to necessityof watershed analysis in Boro Dodota. Spate irrigation is newly introduced in the region inmodern and scientific approach since 2004, and many challenges are observed in planning,
design, and management of the system. Among these challenges estimation of amount andperiod of occurrence of spate flood is principal. The project is located at altitude of 1,500 m
that can be categorized as lowland area with frequent crop failure due to erratic rainfall and highevaporations prevailing. Automatic hydrological and meteorological observation systems withshort data logging interval have been established in the divide line of Boro Dodota stream since
March 2008, and data from these stations will be analyzed using GIS and compared with theactual conditions of the project. The result of the analysis will be used in water managementof this scheme, and useful recommendations for other similar schemes will be forwarded.
Keywords: Hydro-meteorological data, Land use, Semi-arid region, Spate irrigation, Watershed

1. Introduction
The spate irrigation is a unique water management system in arid and semi-arid region which collects
flush flood of ephemeral/intermittent streams originating from highland areas and distributes to areas oflittle and uneven rainfall (i. e. low land areas) resulting in increased soil moisture. The State of Oromia in
Ethiopia is approximately 36 million ha accounting for almost 32% of the country, and consists of threeagro-ecological zones, namely: ‘high lands’ with an altitude above 2,000 m, ‘mid-altitude lands’ withaltitudes ranging from 1,500 to 2,000 m, and ‘lowlands’ with altitudes below 1,500 m. The lowlands arecharacterized by few and erratic distribution of rainfall resulting in difficulty to sustain the crop production.
The food insecurity problem is even worst in lowland areas, which cover about 30% of the total land areaof Oromia. Therefore, traditional spate irrigation had been practiced using simple diversions made fromstone, wood, or sand in those dry lands of the country. Since 2004, the State of Oromia has implemented
to advance the spate irrigation with modern and scientific approaches. The Boro Dodota spate irrigation
project is the first case of the modern spate irrigation in the State having 5,000 ha of command area, where
there is frequent crop failure due to the erratic rainfall condition. The current report discusses contexts,constraint, and prospects of the project, particularly addressing to necessity of watershed analysis.
2. Material and Method
The Boro Dodota spate irrigation project is located at 8˚11’ N and 39˚22’ E. The area is at boundaryof tropical humid climate. The average altitude of irrigable area is 1,500 m. The slope of the area
ranges from 1 to 5%. The mother material of the soil in the area is volcanic ejecta (mainly ash cindersand pumice) with light brown and gray colors. The soil has low strength hence easily disturbed anderoded by wind and water. Further, the soil in the area has high infiltration and rapid drainage.
Dominant rain fed crops over the area are wheat, barley, maize, teff and haricot bean (80% is cereals) withyield ranging from 4 to 10 quintals per hectare, which is very low yield due to limited rain compared to thepotential evaporation (see below). The main target of the project is to undertake supplementary irrigation
during the rainy season from June to September.
The Boro River is characterized by intermittent stream located under the Chilalo Mountains with4,000 m asl. The project plan is to establish a diversion structure at the Boro River at 1,840 m in altitude
1Oromia Water Resources Bureau, Adama, Ethiopia. email: eth.girma@yahoo.com
Japan International Cooperation Agency, P. O. Box 436, Adama, Ethiopia. email: kmtskinada@yahoo.co.jp
3Tokyo University of Agriculture, 1-1-1, Sakuragaoka, Setagaya, Tokyo, 156-8502, Japan.
E-mail: s4suzuki@nodai.ac.jp, h1toyoda@nodai.ac.jp2

242

and 66.5 km below the extreme upstream source of the river, and to design the intake floods maximum up
to 6 m3/s into 5,000 ha of command area. Mean annual rainfall in the command and the catchment areas
are 804 and 1,298 mm, respectively. Mean annual pan evaporation is 1,600 mm. The catchment area ofBoro River is 50 km2 and its shape is elongated slender. Based on visual observation and estimation
during project study, the land use cover of the watershed was found that 20% is high land bushes andscattered trees, 70% is cultivated land, and 10 %is grazing lands.
Discharge analysis of Boro River was conducted according to flood trace, farmer's empiricalknowledge, and visual observations. One automatic meteorological observation system was establishedin March 2008 in the command area (HOBO Weather station, Onset Computer Co., Bourne, MA, USA)
(Fig. 1). The system records rainfall data with 10 minutes intervals. From the rainfall data observed,intensity of rainfall can be determined to use for catchment analysis and computation of peak flood whichis expected at the head structure. It is quite true that long year data is necessary to come up with goodresults and recommendations but the data obtained from the stations established can be used as anindicator.
In order to evaluate the physical discharges of the river, pressure gauges (HOBO U20-001, Onset
Computer Co., Bourne, MA, USA) were installed on Boro stream (since September 2008) and at intake
point of headwork (since July 2008) (Fig. 2, Fig. 3). The pressure gauges record the absolute pressure in
the water and the atmospheric pressure at the same point with an interval of 20 minutes. A hydrostaticpressure is derived from the absolute pressure in the water and the atmospheric pressure, namely; thedifference in the two measurements is equivalent to the water level. Due to delay of installation of thepressure gauges on Boro stream, only the water level monitored at the intake point of the headwork wasused for the evaluation in this report.
3. Results and Discussion
According to the discharge analysis, mean flow in each month from June to September were
estimated to be 7, 12, 15, and 7 m3/s, respectively (Table 1), while the peak flow from flood marks
estimated to be 100 m3/s. However, the amount of intake flow was insufficient to moisten the cropping
fields at the end of the command area. This is in part due to under estimation of the discharge analysis
derived from the flood trace. The result indicates the importance and necessity of a physical hydrologicalobservation of the watershed. Therefore, it is indispensable to install multipoint hydro-meteorologicalobservation systems having short data logging intervals(less than an hour) in the watershed.
As shown in Figure 4, the command area had 436 mm
of rainfall in the rainy season of 2008. Usually, daily
rainfall was less than 20 mm with exceptional event over
40 mm (the end of July). Although the water level recordin June was not detected due to delay of installation of theCap
PVC pipe
WirePressure gauge(Atmosphericcalibration)

Fig. 1. Automatic meteorological observation
system in the command area.

Hole
Pressuregauge(Hydraulic)Fig. 2. Schematic diagram of a pressure gauge.

Fig. 3. Pressure gauge installed at intakepoint.

243

Table 1 Monthly spate flow as transferred from the flood marks.
JFM
AMJJASONMonths
3

Discharge (m /s)

0068171215720DMean05

Daily rainfall (mm)50403020100JunJulAug

Sep

Fig. 4. Rainfall record from June to September, 2008.

Daily mean water level (m)

0.4
0.3
0.2
0.10JunJulAug

Sep

Fig. 5. Water level record derived from pressure gauge in June, 2008.

pressure gauges, the water level of intake flow was ranging from 0 to 0.4 m with an average of 0.13 m
overall (Fig. 5). Actually, the location of water level observation was several km away from the point
where the rainfall monitoring was undertaken, changes in the water level was linked with the rainfall
record (Figs. 4 and 3). The authors will undertake topographic survey of the Boro river in near future.
The water level record will be analyzed to obtain the amount of discharge by combining with the result of
the topographic survey.

244

Considering
thosechallenges,
thefollowing attempts must be made, namely; aprecise survey of the command area using theGround Positioning System (GPS) andwatershedanalysisusing
Geographic
Information System (GIS). The attempts willbe also made to extent meteorological (i.e. rain)
and hydrological (i.e. discharge) monitoring
systems as well as collecting land use data overthe catchment area of the Boro river. Out ofthe divide line of the Boro catchment, there areseveral meteorological stations, for example, theAsella Meteorological station which is at 30 km
air distance and 2,420 m alt. and the Wonji
Meteorological station which is 28 km air
distance and 1,545m alt (Fig. 6).
Themeteorological data recorded at those stations
are also able to be set in the GIS analysis todetermine discharge of the Boro river and thewater demand of the command area.

Adama station

Wonji Station
Awash Melkasa station

Newly established inthe current study

Kulumsa station

Boro catchment

Asela station

Fig. 6. Location of meteorological stations andcatchment.

4. Conclusions
After obtaining necessary data from the automatic observation systems, and based on the result of theobservations and analyses, recommendations useful for estimation of amount and period of flood suitablefor diversion and distribution will be made. The results will be used as an input for water management,
particularly supply and distribution of water on the Boro Dodota spate irrigation scheme and other similar
schemes. Wide area analysis tools such as GIS have high potential for the watershed analysis in thesemi-arid regions with limited scientific information such as the central Ethiopia. The current project of
the spate irrigation development will be one of valuable case study for wise use of water resources in thesemi-arid region.