Seasonal discharge response to temperature-driven changes in evaporation and snow processes in the Rhine Basin

Abstract. This study analyses how temperature-driven changes in evaporation and snow processes influence the discharge in the Rhine Basin. Using an efficient distributed hydrological model at high spatio-temporal resolution, we performed two experiments to understand how changes in temperature affect the discharge. In the first experiment, we compared two 10-year periods (1980s and 2010s) to determine how changes in discharge can be related to changes in evaporation, snowfall, melt from snow and ice, and precipitation. By simulating these periods, we can exchange the forcing components (evaporation, temperature for snowfall and melt, and precipitation), to quantify their individual and combined effects on the discharge. Around half of the observed changes could be explained by the changes induced by temperature effects on snowfall and melt (10 %), temperature effects on evaporation (16 %), and precipitation (19 %), showing that temperature-driven changes in evaporation and snow (26 %) are larger than the precipitation-driven changes (19 %). The remaining 55 % was driven by the interaction of these variables: e.g. the type of precipitation (interaction between temperature and precipitation) or the amount of generated runoff (interaction between evaporation and precipitation). In the second experiment we exclude the effect of precipitation and run scenarios with realistically increased temperatures. These simulations show that discharge is generally expected to decrease due to the positive effect of temperature on (potential) evaporation. However, more liquid precipitation and different melt dynamics from snow and ice can slightly offset this reduction in discharge. Earlier snowmelt leaves less snowpack available to melt during spring, when it historically melts, and amplifies the discharge reduction caused by the enhanced evaporation. These results are tested over a range of rooting depths. This study shows how the combined effects of temperature-driven changes affect discharge. With many basins around the world depending on meltwater, a correct understanding of these changes and their interaction is vital.