Growing cold-season dominance of European streamflow

Seasonal variations in streamflow govern hydrological extremes and water availability for both society and ecosystems. In snow-influenced catchments, climate warming commonly shifts streamflow toward winter, whereas trends in streamflow seasonality in rain-fed catchments are more heterogeneous and often remain poorly quantified. Here, we reveal pan-European trends in streamflow seasonality across both rain- and snow-fed catchments using mass centers derived from directional statistics for 8,911 catchments spanning 1980–2023. Streamflow in rain-fed catchments is concentrated in the cold season, with recent decades exhibiting a strengthening of this cold-season dominance. In contrast, snow-influenced catchments (typically characterized by late-spring and summer-centered flows) have experienced a recent weakening of streamflow seasonality. This systematic attenuation aligns with declining snow fraction, reduced snow storage, and rising evaporative demand. The increasing seasonality observed in rain-fed catchments is driven primarily by enhanced evaporative demand and reduced annual precipitation, rather than changes in precipitation seasonality. Collectively, these trends indicate that across Europe, water availability is increasingly constrained during the warm season, when societal and ecosystem demands are generally highest.