SIMULATION OF WATER ALLOCATION AND UTILIZATION FOR BORKENA RIVER SUB-BASIN UNDER GLOBAL CHANGE

Abstract

Water resources in the Borkena River Sub-basin, part of the Awash River basin, play a vital role in serving diverse users. This study aims to simulate the water allocation and utilization in the sub-basin under the influence of global change using the Water Evaluation and Planning Model (WEAP). Initially, the study evaluated the current surface water potential in the area through station observations and ENACTs gridded data. To assess the impact of climate change on surface water availability, data from two General Circulation Models (GCMs) representing the SSP2-4.5 and SSP5-8.5 scenarios were analyzed. Future projections of surface water potential, demand, and allocation in the sub-basin were made using medium term development scenarios. Four key scenarios—irrigation expansion, population growth, livestock growth, and a combined scenario—were developed to predict water demand for the year 2050. Before integrating future climate data into the WEAP model, bias correction techniques were applied to ensure accurate simulations. The model's performance was evaluated using various metrics, demonstrating its capability for streamflow simulation. Projections indicate an increase in mean temperature for the 2050s under different scenarios (1.8℃ for SSP2-4.5 and 2.9℃ for SSP5-8.5). The sub-basins average annual rainfall of the baseline period is about 1026.43 mm per annum. Under both the SSP2-4.5 and SSP5-8.5 scenarios, the sub-basin's expected mean annual rainfall for the 2050s will increase by 280.51mm and 410.64 mm, respectively. The baseline period's (1991-2020) surface water availability was assessed at 395.2 million cubic meters (MCM). Future estimates of surface water potential will rise by 106.7 MCM under SSP2-4.5 and 210.1 MCM under SSP5-8.5. The WEAP model was instrumental in analyzing current and future water supply and demand, considering irrigation, domestic, livestock, and industrial sites. The study revealed that the current average water demand stood at 68.83 MCM, with projected increases under different scenarios for the year 2050. In 2050, water demand will increase under different scenarios: 168.939 (145.44%), 131.229 (90.66%), and 102.082 MCM (48.31%) for irrigation expansion, population and livestock growth respectively in SSP2-4.5. Whereas, 184.229 (167.66%), 136.318 (98.05%), and 107.178 MCM (55.71%) in SSP5-8.5. The combined scenario will see demand rise to 238.669 (246.75%) and 253.959 MCM (268.97%) for SSP2-4.5 and SSP5-8.5, respectively. This study had identified unmet demands under all demand scenarios, especially during dry months, underscore potential challenges in meeting water demands under all development scenarios. This study recommends more research on the impact of changes in land use, groundwater, and soil on Sub-basin surface water resource. Policymakers can use the findings as a reference when developing effective plans for water resources