This study was conducted for Ribb Catchment in Tana sub-basin, to assess the impact of climate change and irrigation expansion on current and future water use practices of Ribb Reservoir release for its downstream irrigation, domestic and livestock water use using a WEAP model. Records of hydrology, meteorology, irrigation and water supply for the study area have been statistically checked and arranged as an input data source to fit the model. The study also presents the results of downscaling large-scale atmospheric variables simulated with general circulation models to meteorological variables at the local scale to investigate the hydrological impact of possible future climate change in study area. Statistical downscaling model (SDSM) was used to convert the GCM output into daily meteorological variables appropriate for hydrological impact studies. Statistically downscaled daily maximum / minimum temperature, and precipitation in 30-year intervals from the second generation of the Earth System Model (CanESM2) under two RCP Scenarios (RCP4.5 and RCP8.5) to examine future changes and their extremes. Compared to the base period Maximum temperature increases by 0.5 oc and 1.6 oc under RCP4.5 scenario and 0.6 oc and 1.8 oc under RCP8.5 scenario for the years 2036 and 2066 respectively. The demand and the supply for the baseline and the future development activities of the study area were compared in climate change and irrigation expansion scenarios. For the climate change scenario, the volume of net reservoir evaporation in the baseline period was 10.9 Mm3 and for the future period was 13.7 Mm3. The study also shows, in the case of irrigation expansion scenario, the total water demand for irrigation was 135.22 Mm³ at the base period and 350.69 Mm³ on the future projected period. Also domestic water demand shows 0.6 Mm3 and 1.01 Mm3 for current and future projection scenario respectively. Livestock water demand value shows 1.82Mm3 in both scenario. Based on planned irrigation expansions demand and future climate change for the future time periods 189.42Mm3 unmet demand were estimated. Keywords: Ribb reservoir, climate change, irrigation expansion, SDSM and WEAP model.