Thursday, 13 February 2003
Coupling a distributed hydrological model to regional climate model output: An evaluation of experiments for the Rhine basin in Europe
Driving hydrologic runoff models with climate model output provides a promising tool for climate impact assessment studies. To this end, the scale gap between the climate model and the hydrologic model, which is usually of about one order of magnitude, has to be overcome. In this study, downscaling techniques are applied to the output of a perfect boundary regional climate model to drive a distributed hydrological model. The presentation includes a detailed validation of daily precipitation fields provided by the regional climate model, the downscaling techniques and the hydrologic simulations. Precipitation and runoff are analyzed with respect to their frequency distributions of daily values as well as their yearly cycle of monthly means. 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 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 and including catchments with different topographical complexity. 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. A detailed validation of the control simulation shows a good correspondence 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 in the high resolution simulations at the scale of single subcatchements and include precipitation biases of up to 20% compared to the observed precipitation, an overestimation in the frequency of strong precipitation events exceeding about 15 mm/day, and an overestimation of precipitation by about 20% for altitudes between 1000 and 2000 meters above sea level. The errors in the climate simulation partly compensate in the hydrologic simulations and the simulated runoff shows good correspondence with observed runoff for the whole Rhine basin as well as for the bigger lowland sub-basins. Simulated runoff in the more complex Alpine sub-basins shows stronger disagreement with observations.