Past and future impacts of climate change on swiss river temperature and discharge investigated with data analysis and numerical modelling

This thesis presents the first large-scale assessment of past, present and future impacts of climate change on river temperatures in Switzerland. Water temperature is a crucial factor influencing, among others, the health of aquatic ecosystems, the industrial use of water for cooling purposes and the quality of drinking water. River temperature is also interacting with lake and groundwater temperature.A comprehensive study of past measurements within Switzerland shows a clear warming of 0.33±0.03°C per decade over the last 40 years. Long-term variations in annual runoff are observed, but no clear trend can be attributed to climate change. The observed warming already has an impact on industrial water use for cooling purposes and on ecosystem health. The increase in river temperature is more pronounced in the Swiss Plateau than in the Alpine regions. The lower sensitivity of Alpine rivers to ongoing climate change is attributed to the advection of cold water from melting snow and glaciers. The analysis also shows that the lowest rate of warming of Alpine rivers disappears when they reach the lakes on the Swiss Plateau, and that the warming rate downstream of lake is similar to that observed for rivers on the Swiss Plateau.To assess the future evolution of river temperature, numerical models and climate change scenarios are needed. In addition, discharge has to be simulated in order to calculate the water temperature; consequently this variable is also studied in this work. Many improvements to the models chosen for this study are proposed, implemented and presented. The models have been optimised to allow for simulations over large areas and for long periods of time, the ground temperature modelling has been improved, and new tools to facilitate the use of the models have been developed. While these models achieve good results, they require input data at hourly resolution. However, the climate change scenarios available for Switzerland only offer data at daily resolution. An existing method for downscaling climate change scenarios to hourly resolution has been improved to increase the quality of the time series produced. In addition, statistical tools are used to extend the spatial coverage of the existing climate change scenarios to the Alpine regions. Finally, the evolution of river temperature and discharge throughout the 21st century is simulated and assessed for 12 largely undisturbed catchments using the developed numerical models and the newly produced downscaled climate change scenarios. For low emission scenarios, the water temperature is expected to stabilise in the near future and to remain at the same level until the end of the century. Although moderate, this warming will exacerbate the already observed effects of high water temperatures and adaptation will be necessary. However, compared to the situation expected under the high emission scenarios, the benefits of mitigating climate change are clear. Indeed, for the high emission scenarios and by the end of the century, the expected mean river warming in summer will be about 4°C in the Swiss Plateau and 6°C in the Alpine regions. Both regions will experience a mean annual warming of 3.3°C. Such an increase in water temperature will have serious impacts on ecosystems and human water use. This will be accompanied by significant changes in the discharge regime. The general conclusions of this work can be transposed to neighbouring regions with similar climatic conditions.