Simulating the impact of climate change on drought in Swiss forest stands

The main objective of this study is to investigate and assess the possible effects of a changing climate on hydrological processes in Swiss forest stands. For this. sensitivity studies on temperature and precipitation as well as model runs with eight different climate scenarios were carried out using the hydrological model WAWAHAMO (from Waldwasserhaushaltsmodell, German). WAWAHAMO is designed to simulate the water balance for the entire forested area of Switzerland on a daily basis. It is a simple bucket model, which predicts soil moisture content, evapotranspiration. interception. snow cover and drainage, assuming a fixed bucket size defined by the reservoir capacity of the soil. Currently, the model runs at a spatial resolution of 1km. Emphasis is put on the occurence of drought as a limiting factor for forest condition. For this, an annual drought index is defined by the ratio of yearly totals of actual and potential evapotranspiration. Simulations of the annual drought index for the years 1969 to 1998 show a clear pattern of current drought over Switzerland. In dry forest stands as in the main valley of Valais and near the northern border the sensitivity studies and climate scenarios indicate a high sensitivity of the annual drought index to the possible climate change.