Climate change impacts on runoff in Sweden - assessments by global climatemodels, dynamical downscaling and hydrological modelling

The Swedish regional climate modelling programme, SWECLIM, started in 1997 with the main goal being to produce regional climate change scenarios over the Nordic area on a time scale of 50 to 100 yr. An additional goal is to p roduce water resources scenarios with a focus on hydropower production, dam safety, water supply and environmental aspects of water resources. The sce narios are produced by a combination of global climate models (GCMs), regio nal climate models and hydrological runoff models. The GCM simulations used thus far are 10 yr time slices from 2 different GCMs, UKMO HadCM2 from the Hadley Centre and the ECHAM4/OPYC3 of the Max Planck Institute for Meteoro logy. The regional climate model is a modified version of the international HIRLAM forecast model and the hydrological model is the HBV model develope d at the Swedish Meteorological and Hydrological Institute. Scenarios of ri ver runoff have been simulated for 6 selected basins covering the major cli mate regions in Sweden. Changes in runoff totals, runoff regimes and extrem e values have been analysed with a focus on the uncertainties introduced by the choice of GCM and routines for estimation of evapotranspiration in the hydrological model. It is further shown how these choices affect the stati stical return periods of future extremes in a design situation.