83rd Annual

Thursday, 13 February 2003: 3:30 PM
Runoff sensitivity to climate warming: a process study with a coupled climate-runoff model
Jan Kleinn, ETH, Zurich, Switzerland; and C. Frei, J. Gurtz, P. L. Vidale, and C. Schär
Poster PDF (662.9 kB)
Two possible effects of global climate change could affect the runoff statistics and flood frequency of large mid-latitude river basins: the shift from snowfall to rain as a consequence of higher temperatures and the increase in mean precipitation and shifts in the frequency distribution associated with the intensification of the hydrological cycle. Here we study the combined effect of these processes for the river Rhine in Central Europe, covering more than 140'000 square-kilometers, and its major sub-basins of 10'000 to 25'000 square-kilometers. To this end sensitivity experiments are performed using a coupled model chain including a regional climate model and a distributed runoff model. The boundary conditions for the climate simulations are taken from the ECMWF reanalysis. 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 results from these climate simulations with modified boundary conditions are then used for the hydrologic modelling. The regional climate model CHRM is used in a nested mode with horizontal resolutions of 56 km and 14 km and the distributed runoff model (WaSiM) is used at a horizontal resolution of 1 km. The coupling of the models is provided by a downscaling of the climate model fields (precipitation, temperature, radiation, humidity, and wind) to the resolution of the distributed runoff model. The simulations cover multiple years for CHRM with 56 km horizontal resolution and multiple winter seasons for CHRM with 14 km horizontal resolution. The presentation includes a validation of the control simulation and results from the warm simulations for the whole basin as well as for Alpine and lowland sub-basins. In winter-time mean precipitation increases in the warm simulations by about 10%, mainly due to an increase in the frequency of strong precipitation events. The increase in liquid precipitation is of more than 20% in all of the basins for the winter months, as the fraction of snowfall decreases with increasing temperature. Similar results can be observed in the runoff simulations with an increase in winterly discharge of about 10% and an increase in the frequency of strong runoff events.

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