The dual impact of climate change on irrigation water demand and reservoir performance: a case study of Koga irrigation scheme, Ethiopia

Climate change and variability are threatening the water demand and availability in irrigated agriculture. This study aims to see how climate change may affect future irrigation water demand and reservoir volume in Koga irrigation project. Two climate change scenarios (RCP4.5 and RCP8.5) and three climate models (GFDL-ESM2M, MIROC5, and HadGEM2-ES) were employed. The temperature and rainfall data were bias-corrected and served as input to HBV-96 and CROPWAT models to simulate reservoir inflow and crop water requirement. The HBV-96 hydrologic model was calibrated and validated for the future prediction of streamflow. According to the findings, the maximum and minimum temperatures are projected to rise, while rainfall is expected to reduce. Consequently, streamflow in the Koga watershed will shift significantly, ranging from 14.2 to - 29.3% for RCP4.5 and from 9 to - 37.4% for RCP8.5 scenarios, resulting in reduced reservoir inflow. There is an expected increase in irrigation water requirements under the two climate change scenarios between 2041 and 2070. Nearly 15 and 21 mm more irrigation water, respectively, is required under the RCP4.5 and RCP8.5 scenarios compared to the baseline period. This demonstrates how climate change can affect the Koga irrigation scheme in two ways: increased demand and decreased reservoir inflow. The reliability of the reservoir is better in meeting target demands, and the reservoir will not be significantly vulnerable to future climate perturbations despite it may face low resilience. Water abstraction rates and reservoir operation rules need to be in tandem with the future climate variability, changing demand, and reservoir sedimentation.