Estimation of Water Yield under Baseline and Future Climate Change Scenarios in Genale Watershed, Genale Dawa River Basin, Ethiopia, Using SWAT Model

It is important to estimate the quantity and quality of water resources in terms of spatial and temporal variability to utilize such resources sustainably. Change in future climate conditions affects the availability of water resources by modifying the magnitude of precipitation, groundwater recharge, surface runoff, actual evapotranspiration, lateral flow, water yield, and river flows, and provokes water stress in the downstream. Local government authorities around the globe have also emphasized water resource project exploration, design, planning, and management aspects within river basins. To assist such decisions, knowledge and understanding of water yield and water balance at basin and subbasin levels are extremely important. Water yield and water balance components of the Genale Watershed of Ethiopia are analyzed using the Soil Water Assessment Tool (SWAT) model under future climate change scenarios for the understanding of water resources status and to assist decision makers in adopting a sustainable management strategy. Potential areas of high water yield were identified to recommend water resources project planning and management. For detailed analysis, 25 subbasins and 464 hydrologic response units (HRUs) were created covering the Genale River Basin, a ${\mathrm{54,942}\mathrm{km}}^2$ area. The soil conservation service (SCS) curve number (CN2.mgt), available water capacity of the soil layer (SOL_AWC.sol), saturated hydraulic conductivity (SOL_K.sol), and base-flow alpha factor (days) (ALPHA_BF.gw) were the most sensitive parameters to flow. Nash-Sutcliffe efficiency (NSE) for calibration and validation period was 0.81 and 0.78, and the coefficient of determination ($R^2$) was obtained as 0.87 and 0.85 during calibration and validation, respectively, monthly, shows satisfactory performance in both the cases. Hydrological analysis of the Genale Watershed was revealed a high potential value of water yield at Subbasin 8 and Subbasin 12 under all scenarios. The assessment was done for the whole watershed, and the variation ranges from 7 to $\mathrm{2,124}\mathrm{mm}$. Average values of 421.17, 543.5, and 358.1 mm were found under baseline conditions, representatives concentration path (RCP)4.5, and RCP8.5, respectively. Under bias-corrected regional climate model (RCM)-coordinated regional climate downscaling experiment (CORDEX) data, the result shows there is a decline in precipitation and an increase in future temperature under representative concentration pathways (RCP8.5) and likely reduces the future production of water yield in the basin, which shows the RCP8.5 projection is warmer than RCP4.5. Based on this estimate, the regional governmental authority can prioritize projects to solve water-related problems of the community.