Temperature and precipitation trend fluctuations influence the components of the hydrological cycle and the availability of water supplies and their resulting shifts in the balance of lake water (lake level). Quantile mapping was applied to correct temperature biases, and power transformation was applied for rainfall correction. The performance of the HBV model was evaluated through calibration and validation using objective functions (RVE, NSE) and provide RVE of 3.7%, -1.27%,1.05%, -0.72%,8.9% and -0.68 during calibration and RVE of -1.5%, 6.93%, -3.04%,8.796%, -5.89% and 8.5 % during validation for Gumara, Kiltie, Koga, Gilgel Abay, Megech and Rib respectively, While the model provided NS of 0.79,0.63,0.72,0.803,0.68 and 0.797 during calibration and NSE of 0.8,0.64,0.7,0.82,0.801 and 0.82 during validation for Gumara, Kiltie, Koga, Gilgel Abay, Megech, and Rib respectively. The simulated Lake level showed adequate agreement to the observed with NS and RVE of 0.7 and 6.44 % respectively. The result confirmed that over lake evaporation and rainfall increase for all future scenarios. The ungauged surface inflow is also increased shortly scenarios while gauged surface inflow increased for RCP4.5 (the 2070s) and RCP8.5 (2040s) and decreased for RCP4.5 (2040s) and RCP8.5 (2070s). The decreased in gauged surface water inflow is due to a decrease in inflow for Gilgel Abay, Koga and Gumara gauged catchments. Lake storage results showed a decrease in all future scenarios of all-time horizons.