Assessing hydropower production potential with multisite hydrological modeling in the Koshi river Basin, Nepal

• Multi-site SWAT calibration with CMIP6 scenarios assesses future hydropower in Koshi. • Climate scenarios SSP2-4.5 and SSP5-8.5 project 25–45% rise in energy generation. • Modified Design Discharge approach enhances hydropower production beyond baseline. • Adaptive discharge strategies strengthen long-term energy security in mountain basins. Hydropower is one of the major sources of energy for the Himalayan country Nepal which has abundant water resources. Several hydropower projects are being constructed and planned which envisage to meet the present and future energy demand, however, their production behavior under future climate conditions is not sufficiently documented. The Koshi River Basin (KRB) is located in eastern Nepal which is one of the hotspots for hydropower development and is particularly sensitive to climatic fluctuations in river discharge. To evaluate the potential impact of climate variations on river discharge and corresponding hydropower generation in KRB, a multi-site Soil and Water Assessment Tool (SWAT) model was applied in this study. The model was calibrated and validated for 1991–2009 and driven by downscaled CMIP6 climate projections (SSP2-4.5 and SSP5-8.5) for the future period 2031–2060, to simulate future discharge and hydro energy production. Results indicate that baseline energy production was projected to increase modestly under future scenarios, while adopting a Modified Design Discharge (MDD) could substantially enhance generation. Specifically, increases of 35–45% under SSP2-4.5 and 25–30% under SSP5-8.5 are identified from two independent CMIP6 GCM projections, highlighting additional hydrological opportunities beyond those achievable with conventional baseline discharges. These findings indicate additional hydrological opportunities but also emphasize that static design assumptions underestimate future variability. Adaptive discharge strategies, such as MDD, provide actionable guidance for aligning hydropower planning and infrastructure design with projected climatic variability, thereby strengthening long-term energy security and resilience in vulnerable mountain basins.