This study present evaluating climate change impact on water resource availability and allocation for existing and future demands of Megech river sub basin reservoir, Upper Blue Nile Basin, Ethiopia by integrating rainfall runoff HBV-light and Water Evaluation and Planning (WEAP21) model. GCM of the CMIP5 output for RCP2.6, RCP4.5 and RCP8.5 scenarios were obtained from CORDEX-Africa database. Downscaling of the GCM was done statistically with bias correction method. Trend test with MK and climate change impact analysis were made by comparison the two future (2019-2054) and (20552090) with a baseline (1970-2005) period. The HBV-light model was calibrated and validated to simulate upcoming Megech reservoir inflow volume and had well performance. WEAP21 was used to assess the water demand requirements, supplied delivered, coverage, unmet demand and reliability for demand sites. The results indicated that the basin average temperature, potential evaporation and open water evaporation would all have increasing trends with pronounce increment in RCP8.5 than RCP4.5 and RCP2.6 but the inflow volume to the upcoming Megech reservoir would have decreasing and increasing trend and consistent with precipitation patterns. The total annual water demand requirement was estimated at 147.18Mm3, 179.14 Mm3 and 300.44 Mm3 for baseline, lower and higher growth scenarios respectively. The supply delivered would be 146.92Mm3 for baseline period and maximum 155.49Mm3 and minimum 143.77Mm3 was for 2055-2090 under RCP2.6 and RCP8.5 respectively. The annual Megech reservoir capacity to meet the target demand site requirements was 1028.2Mm3 for baseline period and it would be reduced to 570.54Mm3,570.20Mm3 and 587.59Mm3 for (2019-2054) and 444.31Mm3, 395.88Mm3 and 383.61Mm3 for (2055-2090) demand projection periods under RCP2.6,RCP4.5 and RCP8.5 respectively. The total demand coverage reduced from 99.89% in baseline period to 70.915% in higher growth rate scenarios and reliability also reduced from 99.42% to 67.36%. So, in order to guarantee the water resource development project further studies incorporate impact of land use, sedimentation, adaptation option of climate change and find other supply preference and advisable to use crops which have low water requirements.