Forest-water dynamics within a mountainous catchment in Austria

In this study, we assessed the role of forests in the local water budget within a 7.3 km(2) catchment that has measured river runoff and a long history of forest exploitation and disastrous flooding and debris flow events. Forests retain and absorb water from turning into runoff which is a possible trigger for catastrophic events. Forest water budgets (i.e. interception, transpiration, evaporation, sublimation, soil water storage and outflow) interact with ecosystem processes that are related to the carbon, nutrient and energy cycles and consequently affect forest growth rates. Therefore, we employed a biogeochemical-mechanistic ecosystem model, Biome-BGC, as a diagnostic tool to evaluate the dynamic relationships between key forest ecosystem characteristics and the water cycle. Our study was conducted in the Schmittental catchment with about 70 % forest coverage, situated in the Greywacke Zone of the Austrian Alps. Using stand and site information from 21 Norway spruce stands from the region and 29 years of total catchment runoff data for model validation, we demonstrated that the process-based ecosystem model mimics the interaction of forest growth and the water budget realistically. The weekly catchment runoff calculations based on Biome-BGC grid simulations compared well with observed runoff data. The analysis of the forest-water dynamics/relations showed that the water budget is affected by the size of the canopy and the physiological canopy behaviour in response to daily weather. The results suggested that for fully stocked stands and with a standing volume of > 250 m(3) ha(-1), forest water outflow was minimised.