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Digital Elevation Model, SWAT, runoff, Sensitive parameters


Attempt has been made to analyze runoff generation in two agricultural watersheds of Rift Valley Lakes basin of Ethiopia. Watershed physiographic parameters are extracted from 90m SRTM and 30m ASTER DEM data using ArcHydro and TOPAZ watershed delineation algorithms. Climatic, soil and land use data are obtained from historical observation and satellite acquired information to simulate runoff. It has been observed that 30m ASTER DEM resulted in spurious flow accumulation path that subsequently reduced the watershed area by 29% and affected other basin parameters at Hare watershed. However, 90m DEM generated well patterned flow accumulation path. ArcHydro and TOPAZ watershed delineation algorithms failed to capture the actual flow path accurately in flat terrain of Bilate watershed. The mainstream flow path deviates from surrogate Landsat (acquired for the year 2000) and Google Earth (2005/2006 image) extracted stream path by an average distance of 7.5 km in west-east direction for a total stream length of 30km at the lower foot. The 90m resolution SRTM DEM could able to generate well patterned streamflow paths that eventually delineated the watershed boundaries conforming to the existing manually delineated area. SWAT hydrologic model has been applied to simulate runoff in two watersheds. The model successfully simulated the runoff magnitude in the watersheds. Initial curve number for average soil moisture condition, deep aquifer fraction, minimum water depth in the shallow aquifer for flow and available soil water holding capacity parameters found to either attenuate or accentuate the resulting runoff more significantly than others in the watersheds. The overall Nash-Sutcliffe and R2 model performance indices range between 0.79 and 0.96 during calibration and validation period at both watersheds. Results obtained from present analysis can be utilized for sustainable management of the lake ecosystem in the basin.