More frequent and intense compound low-flow and heatwave events in European rivers since 1960

Water supply systems face their greatest challenge when stream flows decline while demand surges during extreme heat. When heatwaves coincide with low stream flows, low flows diminish thermal capacity and wetted surface area, amplifying thermal sensitivity to atmospheric forcing. The resulting impacts cascade across multiple sectors: elevated river water temperatures stress aquatic ecosystems beyond critical thresholds, thermal power plants struggle to access adequate cooling water during peak energy demand, concentrated pollutants in warm, stagnant water degrade drinking water quality, and irrigation withdrawals intensify competition among agricultural, industrial, and municipal users.

Low flow data traditionally inform how much water stakeholders can safely withdraw for domestic use, industry, agriculture, and energy production while preserving river ecosystems. However, examining low flows in isolation fails to capture how concurrent extreme heat amplifies these stresses and triggers cascading failures across interconnected water-dependent systems.

Here we quantify the spatial and temporal evolution of compound low-flow—heatwave events across European rivers from 1960–2020 using observed air temperature and high-resolution pan-European hydrological reanalysis data. We identify regional hotspots characterized by the most frequent, longest, and most intense compound events and assess changes in event frequency, duration, and intensity between historical (1961–1990) and recent (1991–2020) climatic periods. We further analyze the dominant drivers of these changes across different European regions, including increases in the number and duration of low-flow events and the frequency of heatwaves occurring during low-flow periods.

Our analysis reveals that Central and Eastern Europe exhibit the most pronounced increases in compound event frequency, duration and severity, potentially experiencing the largest impacts from these events. We find a marked escalation in compound event severity, with heatwaves increasingly coinciding with low-flow conditions in recent decades. Critically, the longest-duration compound events—which pose greatest risk to aquatic ecosystems and water-dependent economic activities—have become significantly more frequent in recent decades. These results reveal expanding spatial coverage of simultaneous low flow and heatwave hazards, with implications for water resource management under continued warming.