The effects of climate change on Swiss waters, including the impact on river temperatures and discharge, can already be observed today. As part of the research project "Future river temperatures in Switzerland under climate change – SwissFuRiTe", nationwide projections of future river temperatures were simulated for all 82 river measuring stations of the Federal Office for the Environment (FOEN). 

Future water temperatures in Swiss rivers were projected using a multi-fidelity modelling approach. To this end, we used two different semi-empirical models for surface water temperature, 22 coupled and downscaled general circulation and regional climate models, future projections of river discharges from four hydrological models, and three climate change scenarios (RCP2.6, 4.5 and 8.5). By grouping river sections, catchment areas and spring-fed watercourses under representative thermal regimes and applying hierarchical cluster-based thermal pattern recognition, an optimal model and model configuration were automatically selected, model performance was optimized and the estimation of the effects of climate change on river water temperatures was improved. 

The results show that the average water temperatures of rivers in Switzerland are likely to rise by 3.1+/-0.7 °C (or 0.36+/-0.1 °C per decade) by the end of the 21st century under RCP8.5, while under RCP2.6 the temperature increase could remain at 0.9+/-0.3 °C (0.12+/-0.1 °C per decade). Under RCP8.5, the temperatures of rivers associated with the Alpine heat regime will rise the most, namely by 3.5+/-0.5 °C, followed by rivers in the regime below lakes, which will rise by 3.4+/-0.5 °C. 

A general decline in runoff in summer (-10 to -40%) and an increase in winter (+10 to +30%), combined with a further increase in average near-surface air temperatures (0.5 °C per decade), may not only lead to warmer rivers overall, but also to longer periods of extreme summer river water temperatures.

Finally, a classification of hysteresis loops revealed that global warming stretches most thermal hysteresis loops diagonally upwards to the left (Figure: Example BAFU measuring station Thur – 2044). This stretching trend results from the general decrease in runoff and the increased seasonal near-surface air temperature and water warming in the summer months; together, these two processes increase the water temperature of flowing waters even in summer.

This drastically increases the thermal stress potential for temperature-sensitive aquatic animals such as brown trout. Increasingly frequent stress phases affect not only rivers where such situations have already been observed, but also rivers that have not had any problems to date. By providing information about future water temperatures, the results of this study can serve as a guide for management measures to combat climate change.