The influence of vegetation on the summertime evolution of European soil moisture

Four-months-long simulations using a regional climate model covering Europe are utilized to assess the role of continental-scale vegetation cover on the seasonal evolution of soil moisture. The seasonal simulations are driven by observed lateral boundary conditions from the ECMWF (European Centre for Medium-Range Weather Forecasts) analyses, and start in April, when the soil is close to saturation. Two sensitivity experiments are conducted in which the leaf area index is doubled and halved, respectively, and these are compared to the control run. In Middle and Northern Europe an increased leaf area index leads to additional evapotranspiration and soil moisture loss from April to July. In the Mediterranean regions the situation is more complex. Initially, but only until June, the same process operates and increased vegetation leads to increased evapotransporation. In early July, however, the soil moisture content reaches a critically low value, whereupon the run with increased vegetation yields reduced evapotranspiration amounts during July. The identified reversal in sign of the vegetation / evaporation feedback is of significant conceptual and practical interest since it has the potential to shift the maximum of evapotranspiration within the summer season, and might thereby affect the seasonal cycle of precipitation. (C) 1999 Elsevier Science Ltd. All rights reserved.