Modeling the effects of forest stand characteristics on the water dynamics of mountain forests

Mountain forests are essential for reducing runoff, sediment transport, and risk of natural hazards. In this analysis we address the protection function of mountain forests by assessing the interactions among the forest structure and the water dynamics. We are specifically interested in the forest's ability to reduce the outflow during a 10-day rainfall period according to the leaf area index (LAI) of the forested areas. The study was conducted using 31 Norway spruce (Picea abies) forest stands covering a wide range of LAI located in the Rindbach watershed in Austria. The elevation ranges from 446 m to 1379 m, and the predominant soil type is the Orthic Rendzina. From 1960 to 2022, the mean average annual precipitation was 1498 mm, and the mean average annual temperature was 6.6 °C. We use the biogeochemical ecosystem model Biome-BGC with its parameter settings for European tree species to simulate the daily carbon, nitrogen, water, and energy flux dynamics and assess the relative proportion of the daily water balance parameters during the 10-day rain period grouped according to the leaf area index (LAI). Our results for the 10-day rain period with a total accumulation of 135.3 mm (about 9.7 % of the annual rainfall in the area) suggest: (i) Norway spruce forest areas with an LAI < than 1 m²/m², outflow was evident on the first day of rainfall while Norway spruce forests with an LAI ≥ 7 m²/m^-2 exhibited the first outflow on the ninth day of rainfall. (ii) This resulted in a 4 to 5 times lower outflow compared to forest stands with an LAI < 1m²/m² (e.g. 51.3 versus 11.1 mm, within 10 days). This emphasizes the importance of forest vegetation coverage in reducing runoff, avoiding flooding, mudslides, and sediment transport, and improving the protection function of mountain forests.