Future Shifts in European River Flow Regimes under Climate Change

River flow regimes across Europe are increasingly influenced by climate change in addition to direct human interventions. Rising air temperatures, shifts in the spatiotemporal patterns of precipitation, and changes in terrestrial and snow water storage are expected to modify hydrological processes across European river basins, with direct implications for water availability and the frequency and magnitude of hydrological extremes. Alterations of flow regimes represent a further direct consequence of climate change. Such changes can have profound consequences for riverine ecosystems, as aquatic species composition and biodiversity have evolved under relatively stable natural flow conditions and are sensitive to climate-driven hydrological change. This study assesses climate change induced shifts in natural river flow regimes at the continental scale using the Soil and Water Integrated Model (SWIM). The model is calibrated against observed discharge from the Global Runoff Data Centre (GRDC) and remotely sensed evapotranspiration from MODIS using multi-objective optimization, with meteorological forcing from the E-OBS dataset. Future projections are driven by bias-adjusted climate forcing from ISIMIP3b, based on an ensemble of ten global climate models and three socioeconomic pathways (SSP1-2.6, SSP3-7.0, and SSP5-8.5). Hydrological alterations are quantified using the Indicators of Hydrologic Alteration (IHA) framework and Range of Variability Analysis (RVA), capturing changes in the magnitude, timing, frequency, and duration of high- and low-flow events. To explore potential ecological implications, changes in selected IHA metrics are linked to the Shannon diversity index using a previously derived empirical relationship, allowing estimation of biodiversity responses at hydrological stations lacking ecological observations. Natural flow conditions support higher ecological diversity and provide a reference for assessing climate induced deviations. Results are analyzed at fine spatial resolution across major European river basins, enabling the identification of sub-basins experiencing particularly strong hydrological alterations. The study aims to provide a comprehensive assessment of future flow regime shifts across Europe and to improve understanding of their potential hydrological and ecological consequences under climate change.