Impact of extreme atmospheric heat events on river thermal dynamics and heatwaves

Atmospheric hot temperature extremes (HTEs) can impact river water temperatures, but how HTEs affect river thermal dynamics and heatwaves is not well understood, especially at a regional scale. In this study, we used numerical modelling and field observations to quantify the contribution of HTEs to variations in river water temperatures and river heatwaves in 70 Polish rivers covering 125 gauges between 1966 and 2020. During the study time period, HTEs duration and both cumulative and maximum intensities over the studied rivers increased significantly, at average rates of 0.379 days per year, 0.582°C per year, and 0.037 °C per year, respectively. Our results showed that HTEs can accelerate river warming; despite only accounting for 4.9 % of the total days, HTEs are responsible for 25.8 %, 16.9 %, 23.7 %, 32.8 %, and 38.3 % of river warming trends at annual, spring, summer, autumn, and winter time scales. Moreover, HTEs are important drivers of both duration and severe heatwave events on the studied rivers. The results showed that though HTEs contribute to 17.8 % of the occurrence of river heatwaves, they contribute greatly (84.3 %) to the occurrence of severe heatwave events. As the first study on this topic, our findings underscore the critical role played by short-term extreme atmospheric heat events in shaping long-term river thermal dynamics.