Modeling climate change impact on Runoff in the Upper Birbir Sub Basin, Western Ethiopia

This study explores the potential effects of climate change on runoff in the upper Birbir sub-basin of Western Ethiopia. Utilizing the Coordinated Regional Climate Downscaling Experiment (CORDEX)-Africa data from the Hadley Global Environment Model 2-Earth System (HadGEM2-ES), we made future projections under two Representative Concentration Pathway scenarios, RCP4.5 and RCP8.5, for the mid-term (2041–2070) and long-term (2071–2100). To evaluate data trends at a 5% significance level, we employed the Mann-Kendall (MK) trend test alongside Sen's estimators. For simulating future stream flow based on projected climate variables, we used the Hydrologiska Byråns Vattenbalansavdelning (HBV) model. The simulations of future runoff under climate change scenarios reveal that rainfall, with the exception of at the Yubdo station, tends to increase, while temperatures exhibit a warming trend. Runoff is expected to decrease in winter and spring but rise during the summer and autumn, largely following rainfall patterns. In the mid-term, we project an annual average runoff decrease of 2.05% and 1.35% under RCP4.5 and RCP8.5, respectively. Long-term runoff projections indicate a decline of about 0.03% under RCP4.5 and 2.0% under RCP8.5. The findings of this study are intended to assist decision-makers in mitigating the impacts of climate change and in the sustainable use of water resources for irrigation projects and other applications, while also considering the health of riverine ecosystems.