The present study evaluates the hydrological effects of climate change on the change in precipitation and temperature over the Andassa catchment in the upper Blue Nile Basin based on downscaled RCP 4.5 and RCP 8.5 scenarios over the Africa Coordinated Regional Climate Downscaling Experiment (CORDEX). The chosen approach was used to choose the best-fit RCMs (MPI-ESM-LR) model for the study area to test the performance of RCMs. The bias correction method of linear and power transformation was performed for the selected model to increase the simulation performance of the RCA4 regional climate model with a high correlation to the data observed. The bias-corrected data was then used to simulate the future runoff in the Andassa catchment as an input to the HEC-HMS model. For the two-time horizon, future forecasts were done; (2021-2040 and 2041-2075). 1990-2005 was taken as baseline period against which comparison was made. The performance of the model HEC-HMS based on R2, NSE, RMSE were 0.863/0.943, 0.833/0.84 and 0.13/0.1 during calibration and validation process respectively. As a result, in both seasonal and annual scenarios, the maximum and minimum temperature increased in RCP 4.5 and decreased in RCP 8.5 scenarios, while precipitation decreased in RCP 4.5 and increased annually in RCP 8.5 scenarios and Kiremt season increased and Bega and Belg season decreased in the future in two baseline cycles. The model output shows that there may be an annual decrease in runoff depth up to 16.1% and seasonally 14.01%, 28.15%, and 7.6% decreased in Kiremt, Bega, and Belg season respectively for both scenarios in two benchmark periods in the future. The decreased runoff in all seasons and annuals is based on the findings of the present report. In particular, the Belg season (irrigation period) should also provide an adjustment option for water shortages in the small-scale irrigation activities carried out by local farmers.