This study describes the development, validation and application of the hydrological model WAWAHAMO (WaldWasser- HaushaltsModell, German). WAWAHAMOis 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, transpiration, evaporation, interception, snow cover and drainage, assuming a fixed bucket size defined by the reservoir capacity of the soil. Emphasis is put on the occurrence of drought as a limiting factor for forest condition. Therefore, an annual drought index is defined by the ratio of actual to potential transpiration.
The model was validated at several observation sites and then applied to the entire forested area of Switzerland with a horizontal resolution of 1 km resembling the resolution of the currently available forest data. Mean values and standard deviation of the different drought indices were derived for the time period from 1969 to 1998. Both show a clear pattern within Switzerland with dry conditions in the Rhone valley of Valais and near the northern border towards Germany.
Correlation analyses show that the occurrence of drought in forested ecosystems is mainly controlled by climatic conditions. However, if soil water availability is reduced due to climatic conditions, vegetation and soil properties play a substantial role in triggering the occurrence of drought. Furthermore, it showed that the daily distribution of precipitation is of great importance when modelling drought. On this account, water balance models which neglect the influence off vegetation and use a rather large time step have to be judged as to be not capable of adequately simulating drought.
Therefore, complex hydrological models like WAWAHAMO are necessary to successfully compute the occurrence of drought in forested ecosystems over a large geographical domain