M. Verbunt

The hydrological role of snow and glaciers in alpine river basins and their distributed modeling

Abstract A temperature index approach including incoming solar radiation was used as a sub-model in the gridded hydrological catchment model WaSiM-ETH to simulate the melt rate of glacierized areas. Melt water and rainfall are transformed into glacier discharge by using linear reservoir approaches. The complex WaSiM model was applied to three Swiss high-alpine river catchments with different portions of glacierized areas to simulate the discharges of the whole catchments. Gridded data sets of elevation, soil type, and land-use were used including meteorological input data from the network of MeteoSwiss. These data were spatially and temporally interpolated and modified according to exposition, slope and topographic shading. Continuous discharge simulations for the catchment areas were performed in a spatial resolution of 100 m and a temporal resolution of 1 h for the period 1981–2000 and compared with hourly discharge observations measured at the catchment outlets. To improve the calculation of glacier runoff, a seasonal varying radiation factor has been implemented in the glacier melt equation. The pronounced diurnal and seasonal fluctuations in discharge, which are typical of partly glacierized catchment areas, were simulated in a good agreement with the observed values. For the investigated catchments mean annual values of precipitation, evapotranspiration, snowmelt, runoff, and components of glacier runoff are discussed and compared in their altitudinal dependence. Further, the temporal and the spatial distribution of snowmelt and runoff generation in these glacierized catchment areas are discussed.

The Use of Hydrological Models for the Simulation of Climate Change Impacts on Mountain Hydrology

According to the Second and Third Assessment Reports of the Intergovernmental Panel on Climate Change (IPCC 1996; 2001) the increase in mean surface air temperature of the northern hemisphere was larger in the 20th century than in any other period of the last 1000 years. The decade 1990–1999 was the warmest of this time period. It is also believed that this increase in air temperature will be accompanied by intensification of the global hydrological cycle and, in the same chain of cause and effect, by enhanced evaporation and precipitation (Schar and Frei, this volume). However, the scientific community needs to gain a better understanding of the biosphere-atmosphere system before being confident on the predictions of hvdrolodcal processes in a future climate (Frei et al. 2000: Ohmura and Wild 2002).