Assessing long-term variability in daily river water temperature in the Seine Basin, including the Paris metropolitan area

Surface water temperature (SWT) is a key factor of aquatic ecosystem balance but also of domestic,industrial and agricultural water uses, particularly in densely populated regions with intense humanactivities. SWT is affected by multiple drivers, including meteorological conditions (e.g. airtemperature, precipitation, solar radiation, wind speed) and anthropogenic activities (e.g. wastewatertreatment plant effluents, nuclear reactor cooling, datacenter cooling, and urban cooling systems).These drivers exhibit strong spatial and temporal variability. Consequently, SWT exhibits pronouncedvariability at short-term (e.g. day-to-day) timescales, influencing the structure of aquatic communities(Bonacina et al., 2023) and human uses, such as energy production (Du et al., 2026), as well as atlong-term timescales, particularly since the Industrial Revolution, in response to ongoing globalchange. Characterizing these dynamics at the scale of large river basins is essential for understandingclimate-change impacts, assessing risks of critical thermal thresholds, and informing adaptive waterand energy resources management strategies. Conducting such characterization is particularlychallenging in the Seine River Basin (France), a 76,238 km² basin that includes the Paris metropolitanarea, hosts 17 million inhabitants, and faces competing water and energy uses. This study aimed tocharacterize long-term daily river water temperature dynamics across the Seine River Basin from1958 to 2025. Continuous daily SWT time series from 1958 to 2025 were reconstructed at nearly 80monitoring stations using an LSTM model. The time series were subsequently analysed with respectto critical thermal thresholds identified for different water uses (drinking water supply, industrialcooling, irrigation, and ecosystem preservation), in order to assess the risks of reaching or exceedingthese thresholds during the study period. This approach enables the analysis of long-term thermaltrends and paves the way for identifying adaptation levers supporting sustainable, multi-use water-resource management.