Increasing cold-season dominance of European streamflow

Seasonal variations in streamflow shape hydrological extremes1-3 and water availability for society and the environment4,5. In snow-influenced catchments, climate warming often shifts streamflow towards winter6-9, whereas streamflow-seasonality trends in rain-fed catchments appear more diverse10-12 and often remain unquantified. Here, we expose pan-European trends in streamflow seasonality of both rain- and snow-fed catchments using mass centers derived from directional statistics across 7911 catchments13,14 from 1980 to 2023. Streamflow in rain-fed catchments is centered in the cold season, with recent decades showing an intensification of this cold-season dominance. Conversely, snow-influenced catchments, characterized by late-spring and summer-centered flows, have undergone a recent attenuation of streamflow seasonality. Their systematic weakening in flow seasonality aligns with reduced snow fraction, diminished snow storage, and increased evaporative demand. Increasing seasonality in rain-fed streamflow arises mainly from enhanced evaporative demand and reduced annual precipitation, rather than from human-induced flow modifications or shifts in the seasonality of precipitation. These trends expose that, across Europe, water is becoming increasingly scarce during the warm season when societal and ecosystem demands are generally highest15-16. Ongoing snow loss, temperature rise, and precipitation shifts17-19 could sustain the increasing cold-season dominance of European streamflow as the climate continues to warm.