Application of a spatially distributed water balance model for assessing surface water and groundwater resources in the Geba basin, Tigray, Ethiopia

Abstract

Summary The Geba basin is one of the most water-stressed areas of Ethiopia, with only a short rainy period from mid-June to mid-September. Because rainfall in this region has been consistently erratic in the last decades, both in time and space, rain-fed agriculture has become problematic. Hence, in order to supplement rain-fed agriculture by irrigation, a detailed understanding of local and regional surface water and groundwater resources is important. The main objective of this study is to assess the available water resources in the Geba basin using a spatially distributed water balance model (WetSpass). Relevant input data for the model is prepared in the form of digital maps using remote sensing images, GIS tools, FAO and NASA databases, field reconnaissance and processing of meteorological and hydrological observations. The model produces digital maps of long-term average, seasonal and annual surface runoff, evapotranspiration and groundwater recharge. Results of the model show that 76% of the precipitation in the basin is lost through evapotranspiration, 18% becomes surface runoff and only 6% recharges the groundwater system. Model predictions are verified against river flow observations and are shown to be reliable. Additional maps are derived of accumulated surface runoff, safe yield for groundwater abstraction and water deficit for crop growth. Comparison of existing reservoirs with the accumulated runoff map shows that many reservoirs have failed because their design capacity is much higher than the actual inflow. Comparison of the safe yield map with the crop water deficit map shows that in most areas groundwater can be safely abstracted to supplement the water deficit for crop growth during the wet summer season. However, in the dry winter season the crop water deficit is too high to be supplemented by groundwater abstraction in a sustainable way.