A multi-year analysis of forest snow and its contribution to the water cycle of Switzerland

Switzerland is covered 30% by forests, a considerable part of which receives snow in winter. Accurate information about where, when, and how much snow is stored across Swiss forests remains scarce due to the extensive area, complex terrain, and intricate forest snow processes leading to substantial spatiotemporal variability across scales. The Swiss Operational Snow-Hydrological Service (OSHD) runs a physics-based model system that includes detailed forest snow routines, providing daily nationwide snow distribution and snow melt grids at 250m resolution. While these routines have been validated on multiple occasions and at various research sites within and outside forests, the forest simulations have not yet been evaluated over large areas. As a first step, we therefore validated the model results against remotely sensed snow cover information from 3m Planet Labs RGB imagery. The evaluation revealed a very good match throughout the winter season, across regions and years, and within aspect classes, expressed by an overall mean absolute error in snow cover fraction of only 0.14. These results motivated us to analyze a multi-year dataset from the OSHD model system with regards to the relevance of forest snow for the hydrology of Switzerland. In the period investigated, hydrological years 2017-2024, Swiss forests stored, on average, a fifth of the total snow water equivalent during peak SWE. Yet, if hypothetically all forests in Switzerland were removed or lost, this would increase SWE storage by approximately 5% only. However, this does not render the impact of forests on snow water resources irrelevant. At smaller spatial scales and between years, the differences can be considerable for both the amount and timing of snowmelt runoff. In the Swiss Alps, on average, snow remains longer on the ground in the open, reaching its maximum storage later in winter and having significantly higher ablation rates than in adjacent forests. However, aspect matters as snow in south-facing slopes often deviates from the above with prolonged snow cover durations in the forest due to later melt-out. In summary, this study provides a detailed view of the effects of forests on snow water resources and quantifies how these differ with region, topography, season, and between years.