Evaluation of Koka Reservoir Operation under Sedimentation: Implications for Irrigation Water Demands in Upper Awash Valley, Awash River Basin

Abstract

Water scarcity has been a major issue in the Awash River Basin of Ethiopia. The Koka Reservoir is the sole water storage source supporting extensive irrigation activities in the Upper Awash Valley, a key part of the basin. However, sedimentation has reduced the reservoir’s active storage capacity, posing serious risks to its ability to meet irrigation demands in the valley. This study aims to evaluate the reservoir’s operational performance by assessing irrigation water demand and analyzing the water supply-demand balance under recent sedimentation conditions. Key datasets include crops, census, digital elevation model, hydro-meteorological, and reservoir operation data. The primary software tools applied were CROPWAT, ArcMap, and HEC-ResSim. Water resource system performance evaluation criteria such as reliability, vulnerability, and resilience were employed.  The annual estimated gross irrigation water requirement for the study region was 1,525 Mm³. Reservoir operation has been simulated using three decades of daily inflow from 1985 to 2015 by considering different alternatives of the irrigation water demands. When the model was simulated for Alternative 1, the reservoir met full demand less than half the time (time reliability 42.7%), supplied most of the total demand volume despite shortages (volumetric reliability 73.7%), experienced severe deficits when failures occurred (mean vulnerability 25.4 m³/s, maximum 80.0 m³/s), and had limited ability to recover (resilience 27.7%); even in Alternative 2, reliability and resilience improved and vulnerability was moderate, but full demand was still not consistently met. The results indicate that the reservoir capacity under recent sedimentation conditions is insufficient to store high inflows and meet high demand during rainy and dry seasons, respectively. This study does not account for the potential impacts of land use/land cover and climate change, which could influence the hydrological balance and reservoir performance.

Keywords: Awash River; CROPWAT; HEC-ResSim; Performance Evaluation; Reservoir Operation; Water Scarcity.