ASSESSING THE IMPACT OF CLIMATE CHANGE ON STREAMFLOW OF THE RIBB WATERSHED, ABAY BASIN, ETHIOPIA

Abstract

Among many challenges, climate change is one of the world's most pressing issues today. Changing climate may have a major effect on water availability. It would have an adverse effect on food productivity, socio-economic processes, and environmental sustainability. These effects can put their main influence, especially over developing countries whose economies rely heavily on agricultural production. By considering those occurring issues, this study intended to assess the impact of climate change on the streamflow of the Ribb Watershed. Hydrological data have taken from 1990-2017, and meteorological data (Rainfall and Temperature) was from 1990-2019. Spatial data used were Soil, Land use, and digital elevation model of the Ribb watershed. The performance of three regional climate models (KNMI-RACMO22T, SMHI-RCA4, and CLMcom-CCLM4-8-17) from GCM MOHC HadGEM2-ES with their ENSEMBLE model checked and by comparing their performance, KNMI-RACMO22T was the one which performing best and selected for further simulation and impact assessment. Rainfall and Temperature data of KNMI-RACMO22T RCM for RCP4.5 and RCP8.5 were bias-corrected by power transformation and linear transformation techniques respectively. Trend analysis was analyzed for two-time horizons, near-term from 2025-2054 and long-term 2055-2084 by using Mann-Kendal trend test. The result shows that in the future, for both near-term and long-term, mean month rainfall is increased on December to April and decreasing from May-September. Summer and autumn show rainfall decrease while spring and winter are increasing on both RCP4.5 and RCP8.5. Annual rainfall shows decreasing trend on both RCP4.5 and RCP8.5 for near and long term. Near and long term, mean month maximum and minimum temperature show mostly an increasing trend on both scenarios. Annual temperature tends increasing on both scenarios in near and long term. HEC-HMS version 4.7.1 calibration (1990-2010) and validation (2011-2017) result of the model shows satisfactory agreement between observed and simulated streamflow in which coefficient of determination (R2) and Nash-Sutcliffe Efficiency (NSE) was 0.89 and 0.86 for calibration and 0.87 and 0.84 for validation respectively. The near term RCP4.5 and RCP8.5 show the highest decrease in streamflow in August (-46.52m3 /s and -49.43m3/s), respectively. Long-term streamflow also shows the highest decrease in August (- 42.85m3/s and -43.86m3/s), respectively. On seasonal time-step, summer shows a decrease in streamflow; the winter season tends to increase on both RCP4.5 and RCP8.5 over near-term and long-term time horizons. Annually the stream flows appear decreased on RCP4.5 and RCP8.5 in near-term and long-term time horizons. Flow Duration curve used to examine the mean monthly extreme flows, and the result was increasing in low flow and decreasing in high flow. Future streamflow of the Ribb watershed have decreased compared to the base period. This will occur because of decrease in rainfall and increase in temperature. Therefore, there should be proper water resource planning and management in the area for the better operation and usage of stream water of the Ribb watershed