Changes in summertime soil water patterns in complex terrain due to climatic change

Climate change is expected to have a profound influence on soil moisture. According to that the present study aimed at the investigation of future soil water evolution by considering the relationship between soil moisture changes resulting from climate change projections, soil, and terrain characteristics, with special emphasis on slope and groundwater depth. The distributed hydrological model WaSiM-ETH was used to simulate root zone soil water content at a 1 km grid resolution in the 1700 km2 Thur river basin (NE Switzerland) during the 6-month period from April to September. The model was driven by baseline climate data from national data records for 1981–2000, or by scenario data for 2081–2100. The latter were based on grid point projections of two global climate models (CSIRO [B2] and HadCM3 [A2]). The datasets were completed with observations and simulations for the exceptionally warm and dry summer of 2003. The results suggested that a warmer climate with less summertime precipitation may significantly lower the seasonal mean soil water content in many parts of this basin, leading to frequent water stress conditions. Strongest effects occurred with HadCM3, and to a similar extent with 2003 weather data. The magnitude of soil moisture changes were related to land use, soil texture, and slope. In relative terms, reductions in soil water were largest for sloping soils with low water storage capacity, and also larger for forests than for cropland and grasslands. In absolute (volumetric) units, most pronounced reductions were indicated for flat areas with good water supply (mostly dominated by cropland). Here, the rooting zone was often connected to the groundwater, and capillary rise counteracted soil water depletion under current climate during the first half of the season, but was disconnected much earlier under climate change conditions. In steeper areas, groundwater had no influence and thus soil water content changed mainly in response to decreased precipitation and increased evapotranspiration. It is concluded that soil water contents generally decline in this pre-alpine river basin with climate change, but that the degree of soil water depletion varies with climate scenario, land use, soil texture, and topographic conditions. Thus, realistic soil water projections require reliable predictions of summertime climate, and the use of a full representation of biophysical processes that control evapotranspiration, including vertical and lateral subsurface water flows.