Abstract Climate change is expected to affect hydropower generation by modifying river runoff and increasing reservoir evaporation. Anticipating the impact of climate change on hydropower generation is crucial to develop adaptation strategies and to efficiently plan a renewable energy transition. In this study, we assess the impact of climate change on hydropower generation using the Soil & Water Assessment Tool (SWAT) in Portuguese river basin with strategic importance, since it is responsible for 20% of the national hydropower generation. SWAT was calibrated against 6 reservoir flow-in and 1 river discharge, with a good agreement between simulated and observed values. Future climate projections were based on EURO-CORDEX climate simulations under RCP4.5 and 8.5 scenarios for 2031–2060 (short-term) and 2071–2100 (long-term), compared to 1976–2005. Results reveal that in the short-term, reservoir flow-in is expected to decrease up to 55% in the summer under RCP4.5, and up to 90% in the long-term under RCP8.5. Consequently, the hydropower plants may generate less 79 GWh per year in the short-term under RCP4.5, and less 272 GWh per year in the long-term under RCP8.5, which is equivalent to 11% and 38%, respectively, of the total electricity used in the study area in 2019. Our study shows that, at least in some regions, climate change can substantially reduce hydropower generation and thereby hamper the renewable energy transition. This is relevant for policymakers and water managers by allowing them to anticipate the impact of climate change on hydropower generation and better plan a renewable energy transition.
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