Assessment of hydrology and optimal water allocation under changing climate conditions: the case of Megech river sub basin reservoir, Upper Blue Nile Basin, Ethiopia

The main aim of this study was to assess climate change impact on hydrology and water allocation 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. The HBV-light model was calibrated and validated to simulate upcoming Megech Reservoir inflow volume and had good performance. WEAP21 was used to assess the water demand requirements, supplied delivered, unmet demand and prepare rule curves for demand sites. The results indicated that the basin average temperature, potential evaporation and open water evaporation would all have increasing trends with pronounced increment in RCP8.5 than RCP4.5 and RCP2.6. However, 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 147.18 Mm(3), 179.14 Mm(3)and 300.44 Mm(3) for baseline, lower and higher growth scenarios, respectively. The supply delivered would be 146.92 Mm(3) for baseline period and maximum 155.49 Mm(3) and minimum 143.77 Mm(3) for 2055-2090 under RCP2.6 and RCP8.5, respectively. The annual sum of Megech reservoir capacity under baseline year was 1045.54 Mm(3). But it reduced to 570.1955 Mm(3), 570.66 Mm(3) and 587.59 Mm(3) for 2019-2054 period. And also reduced to 448.31 Mm(3), 395.88 Mm(3) and 383.61 Mm(3) for 20552090 demand projection periods under RCP2.6, RCP4.5 and RCP8.5 emission scenarios, respectively. So, in order to guarantee the water resource development project further studies should incorporate impact of land use, sedimentation, adaptation option of climate change, find other supply preference and use crops which have low water requirements.