Spatial estimation of long-term seasonal and annual groundwater resources: application of WetSpass model in the Werii watershed of the Tekeze River Basin, Ethiopia

Abstract

A spatially distributed water balance model, WetSpass, was used to estimate seasonal and annual average surface runoff, evapotranspiration, and groundwater recharge in the Werii watershed (1797 km2), Ethiopia. Groundwater resources were estimated based on spatial variations of land use, soil texture, topography, slope, groundwater level, and hydro-meteorological conditions. Average annual long-term groundwater recharge is estimated as 30.06 mm, of which 19.51 mm occur in the wet and 9.55 mm in the dry season. Although 77% of annual rainfall is received in the rainy season, only 65% of total recharge occurs in the rainy season. The long-term average annual precipitation (717 mm) is distributed as 90.7% (650.16 mm) evapotranspiration, 6% (44.06 mm) runoff, and 4.2% (30.06 mm) recharge. Model results were verified against river flow observations and showed a reliable agreement, with R2 = 0.98. To fulfill irrigation water deficits, a safe yield groundwater abstraction rate up to 10.7 m3 d−1 ha−1 can be used without affecting the sustainability of groundwater. We conclude that the WetSpass model could be used to simulate water balance components in semi-arid regions provided that the model input parameters are fitted to local conditions.