GROUNDWATER IRRIGATION POTENTIAL ASSESSMENT OF THE LAKE HARAMAYA WATERSHED, OROMIYA, EASTERN ETHIOPIA

Abstract

There is growing concerned about food security in Africa and especially in sub-Saharan Africa, Ethiopia. Even in good years, Ethiopia cannot meet its large food-deficit through rain fed production. Irrigation in Ethiopia is considered a basic strategy to alleviate poverty and hence food security. Most of the traditional irrigated lands in Ethiopia are dominantly supplied by surface water sources; while groundwater is smaller, this means that there is significant potential to increase irrigation sources from the groundwater. This paper focuses on further developing groundwater irrigation potential in the Lake Haramaya watershed. The concept of the approach is to identify the irrigation potential of the groundwater by comparing monthly and annual groundwater availability and crop irrigation requirements after satisfying the human and environmental groundwater needs. A GIS-based MCE technique was used to identify potential land suitable for surface irrigation using groundwater. Key factors that significantly affect irrigation suitability evaluated in this study include the slope, land use, soil depth, soil texture, soil drainage, and road proximity and town proximity factors. The suitability of individual factors was analyzed separately and assigning the weight to each factor then overlaid using the weighted overlay tool in Arc GIS to get the overall suitability based on the criteria set by FAO for surface irrigation using groundwater. The available groundwater potential for irrigation was quantified by allocating some fraction of groundwater recharge that is in excess after satisfying other human needs and environmental requirements. The groundwater recharge was estimated using spatially distributed physical based models called WetSpass. WetSpass-M model has been simulated the monthly and annual water balance components of Lake Haramaya watershed successfully. The key components WetSpass-M model analyzing include rainfall, groundwater depth, wind, temperature, PET, land use, soil, slope, topography, soil look up table, and land use lookup table data. The human water demand was estimated by multiplying the total projected population of three towns (Harar, Haramaya, and Bate) with 20l per capita per day. Due to the high uncertainty of groundwater environmental needs, three scenarios, leaving 30, 50, and 70 % of recharge for the environment, were implemented. And finally, crop irrigation water requirements were computed using the CROPWAT model. The key components of the CROPWAT model input include climate data, crop data, and soil data. The irrigation potential of the groundwater was identified by comparing groundwater availability and crop irrigation demand after satisfying the human and environmental groundwater needs. The result of the suitability analysis revealed that around 56.33% (7031ha) of the total watershed area (12,481.3 ha) was in the range of high to marginally suitable for surface irrigation development using groundwater. Results of the analysis show that average annual renewable groundwater availability for irrigation ranges from 174165268.8 to 321036480m3depending on the scenario. The CROPWAT8.0 model result revealed that crop irrigation requirements vary due to climatic station and type of major crops selected. The total crop irrigation requirements irrigable with groundwater ranges from 15531349.6 to 2274841.40 m3/year over the watershed. By comparing monthly groundwater available in m3/s at all scenarios with monthly crop gross irrigation demand in m3/s, the obtained total irrigation potential of the study area was 12,012.57ha. To conclude, this indicates that there is significant potential to increase irrigation sources from the groundwate