Assessment of Hydrological Regime Alteration in the Upper Tagus River Basin Under Climate Change and Dam Regulation Using the SWAT Model

The functioning of riverine ecosystems depends on the flow regime. This study investigated the effects of climate change and dam operation on the hydrological regime of the upper Tagus River basin and their individual and combined effects using SWAT, IHA indicators, and DHRAM under SSP2-4.5 and SSP5-8.5 for the periods 2021–2050 and 2051–2080. SWAT was calibrated and validated with appropriate accuracy, and future climate data from the ESD-RegBA method were used for streamflow simulation. In the baseline period 1991–2019, dams reduced wet-season flow, increased summer flow in the upper and middle basin, and altered IHA indicators; upstream stations were classified in the High Risk of Impact category. Under SSP5-8.5 in 2051–2080, precipitation decreases by up to 7.7% and temperature increases by 3.4 °C relative to baseline; flow decreases, especially in spring and autumn, and the timing of extreme events changes. Dams altered upstream flow and intensified downstream flow reduction. Hydrological alteration increased, and most stations were classified as high hydrological alteration risk according to DHRAM. Climate change was the dominant factor, and the moderating role of dams was weakened with increasing warming. The results emphasize adaptive reservoir management and climate-adapted environmental flows.