Climate change impact on rain, snow and glacier melt components of streamflow for the river Rhine: synthesis of a model experiment and relevance for water use

Streamflow of the river Rhine and its tributaries consists of rain, snowmelt and glacier ice melt components. The amounts of these components have already changed in the past years due to climate warming. Hydrological modelling until the year 2100 was carried out for the Rhine catchment using an ensemble of downscaled and bias-corrected climate projections and a chain of hydrological models considering cryosphere changes. The modelled daily streamflow components provide unique insight into the hydrological processes of a warmer future at different spatial and temporal scales down to individual events. In the Rhine basin, projected precipitation for the RCP8.5suggest wetter winters and drier summers, but annual net precipitation change differs in the up- and downstream regions with a net increase projected only in the lower basin. The model experiments suggest that the rain component of streamflow will dominate the seasonal variability in the future more than in the past. Snow will provide less seasonal water storage and melt earlier in winter and spring. Glaciers will continue their retreat with differences among individual glaciers and the ice melt component in the main river Rhine is projected to retreat fast with almost no ice melt component left at the end of the century. As a consequence, in particular low flows in downstream reaches will exacerbate due to the lack of buffering snow and ice melt; esp. during hot summer drought years. This change will affect environmental flows, water use for energy production, navigation and other water uses, changes of which can be estimated from the modelled scenarios. Overall, streamflow variability and extremes will increase. Despite propagated uncertainties from a range in the downscaled and bias-corrected climate model input, the projected changes are substantial and are a clear mandate to reconsider water uses and enhance river protection goals.