Assessment of climate change impacts on flood hazard potential in the Alpine Lech watershed

The present study attempts to (i) analyse potential impacts of climate change on floods in an Alpine catchment in general and (ii) assess the range of uncertainty in the projection of flood events in particular. The study was performed on the Lech basin (∼1000 km2), located in the Northern Limestone Alps. Two General Circulation Models (GCMs) with multiple ensemble integrations were used as a basis for assessing uncertainty in the climate projections. A downscaling method which attempts to counteract variance reduction, called Expanded Downscaling (XDS), was applied to statistically downscale large-scale atmospheric data to local stations. The resulting data was in turn used as forcing for the hydrological model HQsim in order to simulate runoff for present (1971–2000) and future (2071–2100) climate conditions. The results show that both the single models as well as the modelling chain performed well in reproducing the overall statistics and also single events for present climate conditions. Thus, the modelling chain seems to be a valid tool to assess hydrological extremes on local scales. The future scenarios indicate a decrease in the intensity of mean annual floods and considerable shifts in seasonal floods, including a decrease of summer floods and significant increases of winter floods. Floods with return periods up to around 10 years are projected to decrease in future, whereas no clear signals were obtained for floods with higher return periods as these projections are highly uncertain. An analysis of the timing of annual maximum floods indicates a substantial extension of the potential flood period from around 7 months under current climate conditions up to the whole year in future scenarios.