Modelling hydrological response under climate change scenarios using SWAT model: the case of Ilala watershed, Northern Ethiopia

This study evaluates surface runoff generation under climate change scenarios for Ilala watershed in Northern highlands of Ethiopia. The climate change scenarios were analyzed using delta based statistical downscaling approach of RCPs 4.5 and 8.5 in R software packages. Hydrological response to climate changes were evaluated using the Soil and Water Assessment Tool model. The Soil Water Analysis Calibration and Uncertainty Program of Sequential Uncertainty fitting version 2 algorithm was also used to compute the uncertainty analysis, calibration and validation process. The results show that the minimum and maximum temperature increases for the future of 1.7 and 4.7 °C respectively. However, the rainfall doesn't show any significant increase or decrease trend in the study area. The 95% prediction uncertainty brackets the average values of observation by 71 and 74% during the calibration and validation processes, respectively. Similarly, R-factor equals to 0.5 and 0.6 during calibration and validation periods. The simulated and observed hydrographs of the total river yield showed a good agreement during calibration (NSE = 0.51, R2 = 0.54) and validation (NSE = 0.54, R2 = 0.63). From the total rainfall received only 6.2% portion of the rainfall was changed into surface runoff. The rainfall-runoff relationship was strongly correlated with R2 = 0.97. Moreover, there had been also high evapotranspiration (ET) loss in the watershed; almost 75% of the total rainfall was lost as ET and 7.8% as ground water recharge. Due to an increase trend in temperature and evaporation loss for the future, the surface runoff also declined from 1.74% in RCP4.5 near-term to 0.36% in RCP8.5 end-term periods. This implies, proper planning and implementation of appropriate water management strategies is needed for sustainable water resources management in the region.