In winter-time, two possible effects of global climate change could affect the runoff statistics and flood frequency of large Central European rivers: the shift from snowfall to rain as a consequence of higher temperatures and the increase of heavy precipitation events due to an intensification of the hydrological cycle. Here we study the combined effect of these processes for the river Rhine in Central Europe. To this end sensitivity experiments are performed using a coupled model chain including a regional climate model and a distributed runoff model. The sensitivity experiments are based on an idealized surrogate climate change scenario which stipulates a uniform increase in temperature by 2 Kelvin and an increase in atmospheric specific humidity by 15% (resulting from unchanged relative humidity) in the forcing fields for the regional climate model.

The regional climate model CHRM is used in a nested mode with horizontal resolutions of 56 km and 14 km. The boundary conditions are taken from the original (CTRL) and modified (WARM) ECMWF reanalysis. The distributed runoff model (WaSiM) is used at a horizontal resolution of 1 km for the whole Rhine basin (gauge Cologne), covering more than 140'000 square-kilometers. The coupling of the models is provided by a downscaling of the climate model fields (precipitaion, temperature, radiation, humidity, and wind) to the resolution of the distributed runoff model. The simulations cover the five winter seasons 1989/90 till 1993/94, each from November until January.

A detailed validation of the control simulation shows a good correspondance of the precipitation fields from the regional climate model with measured fields regarding the distribution of precipitation at the scale of the Rhine basin. Systematic errors are visible at the scale of single subcatchements, in the altitudinal distribution and in the frequency distribution of precipitation. These errors only marginally affect the runoff simulations, which show good correspondance with runoff observations.

The presentation includes results from the scenario simulations for the whole basin as well as for Alpine and lowland subcatchements. The change in the runoff statistics is being analyzed and discussed with respect to the changes in snowfall and to the fequency distribution of precipitation.