Future Climate change scenarios of precipitation and potential evaporation were developed using output of dynamically downscaled data of ECHAM5 (GCM) under A1B emission scenario condition for 2030 s (2031-2040) and 2090 s (2091-2100). The projected climate variable showed an increasing trend from the 1991-2000(base period) level. The monthly mean minimum and maximum temperature shows an increasing trend. It is estimated that the average seasonal and annual potential evaporation in the watershed for 2030 s might increase up to 5.2% and 4% respectively and in 2090 s the average potential evaporation might increase up to 15.85% seasonally and 12.66% annually. Besides, at 2030 s it is exhibited that the average seasonal precipitation might increase from 12.14% up to 62.79% and annually 30.22%. The simulation results obtained from the investigation indicated that there was a significant variation in the seasonal and monthly flow in both future period scenarios. Hence, in Didessa watershed, runoff is likely to increase in the future.