Future water temperature of rivers in Switzerland under climate change investigated with physics-based models

River ecosystems are highly sensitive to cli-mate change and projected future increase in air tempera-ture is expected to increase the stress for these ecosystems. Rivers are also an important socio-economic factor impact- ing, amongst others, agriculture, tourism, electricity produc- tion, and drinking water supply and quality. In addition to changes in water availability, climate change will impact river temperature. This study presents a detailed analysis of river temperature and discharge evolution over the 21st cen- tury in Switzerland. In total, 12 catchments are studied, situ- ated both on the lowland Swiss Plateau and in the Alpine re- gions. The impact of climate change is assessed using a chain of physics-based models forced with the most recent climate change scenarios for Switzerland including low-, mid-,and high-emission pathways. The suitability of such models is discussed in detail and recommendations for future improvements are provided. The model chain is shown to provide robust results, while remaining limitations are identified. These are mechanisms missing in the model to correctly simulate water temperature in Alpine catchments during the summer season. A clear warming of river water is modelled during the 21st century. At the end of the century (2080– 2090), the median annual river temperature increase ranges between +0.9 ◦C for low-emission and +3.5 ◦C for high- emission scenarios for both lowland and Alpine catchments. At the seasonal scale, the warming on the lowland and in the Alpine regions exhibits different patterns. For the lowland the summer warming is stronger than the one in winter but is still moderate. In Alpine catchments, only a very limited warming is expected in winter. The period of maximum discharge in Alpine catchments, currently occurring during mid-summer, will shift to earlier in the year by a few weeks (low emission) or almost 2 months (high emission) by the end of the century. In addition, a noticeable soil warming is expected in Alpine regions due to glacier and snow cover decrease. All results of this study are provided with the corresponding source code used for this paper.