Projected river water temperatures in Poland under climate change scenarios

Study region: 70 Polish rivers in Central Europe. Study focus: This study projects river water temperatures (RWTs) of 70 Polish rivers (125 gauges) under two Shared Socioeconomic Pathway (SSP245 and SSP585) using the ensembled data of 10 CMIP-6 climate models till 2100. New hydrological insights for the region: The results show that choice of the climate models significantly impacts the projection of RWTs, suggesting the use of ensembles to reduce the uncertainty brought by the individual climate models. For both scenarios, the projected annual averaged RWTs in all gauges increase significantly, and rivers warm at an average decadal rate of 0.14 and 0.36 degrees C for SSP245 and SSP585, respectively. Irrespective of which scenario, in the future, autumn RWTs tend to increase the fastest, followed by summer and winter, then spring. As for one of the most important biologically relevant metrics, namely the annual number of days when RWTs exceed 20 degrees C (D20), the results show that for both scenarios, D20 increases for majority of river stations, with an average decadal rate of 3.58 and 7.53 days for SSP245 and SSP585, respectively. Our results suggest that climate protection measures play an important role in mitigating the impact of climate change on river warming and should be taken immediately.