Earlier impact studies have suggested that climate change may severely alter the hydrological cycle in alpine terrain. However, these studies were based on the use of a single or a few climate scenarios only, so that the uncertainties of the projections could not be quantified. The present study helps to remedy this deficiency. For 2 Alpine river basins, the Thur basin (1700 km 2

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Abstract:In mountainous catchments the quality of runoff modelling depends strongly on the assessment of the spatial differences in the generation of the various runoff components and of the flow paths as coupled with the amount and intensity of precipitation and/or the snow melting. These catchments are also suitable for the intercomparison of different kinds of hydrological models, particularly of different approaches for the simulation of runoff generation. Two differently structured catchment models were applied on the pre-alpine Rietholzbach research catchment (3Ð2 km 2 ) within the period 1981-98 and on the high-alpine Dischmabach catchment (43 km 2 ) within the period 1981-96 for the simulation of hydrological processes and of the runoff hydrographs. The models adopted are the more physically based WaSiM-ETH model, with grid-oriented computation of the water balance elements, and the rather conceptual PREVAH model, based on hydrological response units. The simulation results and the differences resulting from the application of the two models are discussed and compared with the observed catchment discharges, with measurements of evapotranspiration, soil moisture, outflow of a lysimeter, and of groundwater levels in three access tubes. The model intercomparison indicates that the two approaches for determining runoff generation with different degrees of complexity performed with similar statistical efficiency over a period longer than 15 years. The analysis of the simulated runoff components shows that the interflow is the main runoff component and that the portion of the runoff components depends strongly on the approach used. The snowmelt model component is of decisive importance in the snowmelt season and needs to take into account the role of air temperature and radiation for simulating runoff generation in a spatially distributed manner.

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There is growing evidence that, as a result of global climate change, some of the most severe weather events could become more frequent in Europe over the next 50 to 100 years. The paper aims to (i) describe observed trends and scenarios for summer heat waves, windstorms and heavy precipitation, based on results from simulations with global circulation models, regional climate models, and other downscaling procedures, and (ii) discuss potential impacts on agricultural systems and forests in Switzerland. Trends and scenarios project more frequent heavy precipitation during winter corresponding, for example, to a three-fold increase in the exceedance of today's 15-year extreme values by the end of the 21st century. This increases the risk of large-scale flooding and loss of topsoil due to erosion. In contrast, constraints in agricultural practice due to waterlogged soils may become less in a warmer climate. In summer, the most remarkable trend is a decrease in the frequency of wet days, and shorter return times of heat waves and droughts. This increases the risk of losses of crop yield and forage quality. In forests, the more frequent occurrence of dry years may accelerate the replacement of sensitive tree species and reduce carbon stocks, and the projected slight increase in the frequency of extreme storms by the end of the century could increase the risk of windthrow. Some possible measures to maintain goods and services of agricultural and forest ecosystems are mentioned, but it is suggested that more frequent extremes may have more severe consequences than progressive changes in means. In order to effectively decrease the risk for social and economic impacts, long-term adaptive strategies in agriculture and silviculture, investments for prevention, and new insurance concepts seem necessary.