Climatic and socioeconomic drivers of changing hydrological extremes in Europe

Abstract Hydrological extremes display long-term trends and natural oscillations in response to climatic and socio-economic factors. Knowing how and why these extremes are shifting is crucial for increasing water resilience in Europe. This work analyses trends in floods and droughts in European catchments between 1951 and 2020. We disentangle the effects of dynamics in climate, encompassing climate variability and climate change, land use, water abstraction and reservoirs. The assessment utilises the high-resolution European hydrological reanalysis (HERA) and counterfactual hydrological runs. Non-stationary extreme value analysis estimates time-varying return periods of floods and drought over the 70-year period. Our results reveal strong decadal fluctuations in magnitude and frequency of hydrological extremes. Since the 1950s, climate-driven changes have led to an average continent-wide increase in flood intensity of 3.2% and a 1.1% average decrease in drought intensity, though with considerable regional variation. Land use changes have generally increased flood and drought hazard, while water demand primarily intensified droughts. Reservoirs, conversely, have decelerated the hydrological cycle. Effects vary locally depending on reservoir volume and operation, degree of urbanization and reforestation. Our work delivers insights into the intricate connections between climate, society, and water in European river basins, enabling the development of effective strategies for enhancing resilience to extreme water events under global change.