Abstract

This study predicts future runoff conditions under changing climate using multi model outputs from Coupled Model Intercomparison Project Phase 5 (CMIP5) over Lake Ziway Catchment. The River system is located in the Central Rift Valley of Ethiopia which serves for wide range of socio-economic activity, but recently different water use sectors are increasing their pressure on the water balance of the catchment. Bias corrected precipitation, maximum and minimum temperature data from three climate models HadGEM2-ES, CSIRO-MK-3-6-0 and CCSM4 under representative concentration pathways RCP 8.5 and RCP 4.5 were used as input for the hydrologic model. A calibrated and validated HBV model is used to simulate the future inflow from Katar River and Meki River towards Lake Ziway. The result revealed that the maximum and minimum temperature increased under RCP 8.5 and RCP 4.5 scenarios. However, precipitation showed a decreasing trend. The percentage change in monthly average precipitation showed extremes for HadGEM2-ES model which range between -51.19% during January 2050s and +23.15% during February 2080s under RCP 8.5. The model output showed an annual decrement in runoff depth on Katar River up to 19.45% during RCP 8.5 on CSIRO MK-3-6-0 model and maximum reduction was recorded for RCP 4.5 at 17.49% for CCSM4 model. Meki River has shown maximum annual reduction of 20.28% during 2080s on RCP 8.5 for HadGEM2-ES model and seasonally during Bulg maximum increment was recorded for the same model which ranges up to 10.23% on 2050s for RCP 4.5. However seasonal maximum reduction is obtained from Bulg season by 40.27% on HadGEM2-ES model during 2050s. From the study, a reduction in rainfall has brought larger effects on runoff reduction than evapotranspiration components. Due to future reduction of River flow on the region optimal allocations for water use purposes at all levels of water resource development projects are crucial for future water planning and management.

Keywords: CMIP5; HBV model; Lake Ziway catchment; RCP; Runoff estimation