Unveiling spatiotemporal patterns of compound hydrological droughts and river heatwaves in Poland

Study region 64 Polish rivers in Central Europe. Study focus Despite their profound ecological and societal impacts, the dynamics of compound droughts and heatwaves (CDHWs) in river systems remain a largely uncharted territory. This study provides the first comprehensive assessment of riverine CDHWs, utilizing long-term river discharge and water temperature datasets from 64 Polish rivers during the period 1966–2020. New hydrological insights for the region Our analysis reveals a marked increase in the severity and frequency of CDHWs across Poland. Specifically, 58 stations exhibit significant upward trends in CDHWs frequency (0.38 times per decade, p < 0.05). Notably, more than half of all stations demonstrate substantial changes in CDHWs characteristics: duration (4.27 days per decade, p < 0.05), maximum CDHWs-related heatwave intensity (0.50 °C per decade, p < 0.05), and maximum CDHWs-related drought intensity (-0.10 in Standardized Streamflow Index (SSI) per decade, p < 0.05). Moreover, CDHWs become severe with more high-level categories. Distinct regional patterns reveal that CDHWs are more frequent and severe in western Poland, while extreme CDHWs are prevalent in eastern Poland. These insights underline the necessity for integrating riverine CDHWs dynamics into global water management and climate adaptation frameworks. As the pioneering study on riverine CDHWs, this work provides a foundational reference for future investigations.