Reservoir performance assessment and operations in Lake Tana, upper Blue Nile Basin, Ethiopia, in response to climate change and water management.

Climate change significantly impacts the hydrological system, river flows, and available water resources. In this study, we have investigated the potential impact of climate change on the Lake Tana water resources by using data generated from two Global Circulation Models (GCMs) (i.e., MIROC5 and MPI) under two Representative Concentration Pathways (RCPs), i.e., RCP4.5 and RCP8.5. Climate data, mainly precipitation and temperature, was generated for the two future time horizons (the 2040s (2020-2049) and 2070s (2050-2079). The lake temperature is increasing for all RCP scenarios and time domains. The result confirmed that lake evaporation and rainfall increased for all future scenarios. The ungauged surface water inflow also increased in the 2040s time domain, while gauged watershed surface inflow increased for RCP4.5 (2070s) and RCP8.5 (2040s) and decreased for RCP4.5 (2040s) and RCP8.5 (2070s). Performance indices such as reliability, resilience, and vulnerability were used to assess the performance of the Lake Tana reservoir under climate change. The time-based and volumetric reliability have an average value of less than 80% in both the 2040s and 2070s under all scenarios. The resilience values are below 50%, which indicates that the reservoir will take a long time to recover from the shortage. The dimensionless vulnerability has also a value of less than 50%, indicating that the reservoir will be discharged by sufficient inflow to satisfy the demands. From the performance measures, the reservoir will not somehow have good performance due to the increase in upstream abstractions (small and large-scale irrigation). Facing future climate and according to hydrological changes, reservoir rule curves have been developed that can help for the sustainable use of the resources.