ASSESSMENT OF SURFACE IRRIGATION POTENTIAL IN UPPER WEYIB WATERSHED, GENALE DAWA BASIN BALE ZONE OROMIYA REGION, ETHIOPIA

Abstract

Planning and decision-making for new irrigation development projects require the systematic assessment of irrigable land together with available water resources. This study was started with the objectives of assessing the water and land resources potential of the upper Weyib river watershed for irrigation development and generating a geo-referenced map of these resources by using a Geographic information system. Watershed delineation, identification of potentially irrigable land, and estimation of irrigation water requirement and surface water availability of river watershed were the steps followed to evaluate this irrigation potential. GIS was used to map the irrigation potential of the watershed; the ArcSWAT model; was used to estimate the water availability in the watershed; AHP and pairwise comparison was used to land suitability assessment and a CROPWAT model was used to determine the major crop water requirement. To identify potentially irrigable land, irrigation suitability factors such as soil physical properties, slope, land use/ land cover, and distances from the water supply(sources) are taken into account. The irrigation suitability analysis if these factors indicate that 98.59% of soil 75.72% LULC and 41.62%slope in the study area are in the range of highly suitable to marginally suitable for the surface irrigation system. The final results of irrigation land suitability indicate that 14.90% high suitable (S1), 55.08% moderately suitable (S2), 5.62% marginally suitable (S3), whereas 24.38% not suitable (N) for surface irrigation development. After calibration and validation, the observed monthly streamflow values with a coefficient of determination (R) and Nash-Sutcliffe Coefficient (NSE) of 0.81 and 0.77 for the calibration period and 0.77and 0.76 for the validation period, respectively. The Irrigation water demand of the four major crops which are grown in the study area through the growing season was found to be 1170.1mm. Based on the Model Builder of ArcGIS, the SWAT estimated available water can potentially irrigate an area of 10,564.15ha for highly suitable areas