Effects of climatic change on hydrological patterns of a forested catchment: a physically based modeling approach

Effects of climatic change on hydrological patterns on a catchment scale were evaluated using a semi-distributed, topographic TOPMODEL. The model was calibrated to a forested catchment for the period 1991–92 with a daily time step, using a model version with a snow subroutine. The main criteria in the model calibration were to fit simulated and observed runoff, to obtain an adequate water balance and to compare simulated and observed groundwater depths and water equivalents of snow. The model was tested with data from the years 1993–94, and long-term forecasts of up to 100 years were conducted. Precipitation and temperature input were obtained from the CLIGEN weather generator, which generates stochastic daily time series of these variables, according to the SILMU-policy oriented scenarios high (annual increase in precipitation 1.5%/decade and in temperature 0.6 °C/decade), and low (annual increase in precipitation 0.25%/decade and in temperature 0.1 °C/decade). Ten high scenarios and ten low scenarios were used as an input to TOPMODEL, in order to predict long-term changes of annual averages and interannual variability in runoff, evapotranspiration and average amount of soil water. According to the model runs conducted, first an increase of 4% (12 mm) and then a slight decrease (2–7%, i.e. 6–18 mm) in annual runoff was predicted. The most notable change in runoff was the change from the present spring high flow to a more evenly disributed winter flow. Soil moisture was predicted to increase during the autumn and winter periods and to decrease during summer. A strong reduction of 80–90% in the volume and duration of the snow cover was predicted by the high SILMU scenario during the next 100 years. The reduction by 2050 was predicted to be about 50%. Already at present, three years (1991–93) of the observation period could be considered as ‘future’ winters with very little snow and several distinct, short snow accumulation/melting periods.