Snow in transition: 80 Years of SWE data for the Wägital, Switzerland

Temporary storage of precipitation in mountainous regions as snow is crucial for downstream water users, including agriculture and hydroelectric power generation. However, the amount and duration of this storage are changing over time. We used snow water equivalent (SWE) data from the pre-alpine Wägital in Switzerland, one of the longest datasets of SWE globally, to analyze trends in SWE. Measurements have been made annually on April 1st at multiple locations in the Wägital catchment since 1943. To overcome the limitations of relying solely on April 1st SWE measurements, we applied a degree-day model to reconstruct daily SWE values, and identified the annual maximum SWE (maxSWE) and duration of snow cover. Here, we present trends in meteorological parameters, April 1st SWE, maxSWE, and the duration of snow cover. In addition, we describe the spatial variation in maxSWE with respect to elevation, aspect, and slope.

The model results revealed that April 1st measurements often fail to capture maxSWE, highlighting the importance of using maxSWE for trend analyses. There was a general decline in maxSWE across the catchment during the study period and maxSWE now occurs earlier than in the past. Positive trends for maxSWE dominated from the 1940s to the 1980s, followed by stronger negative trends from the 1980s onward. The strength of these negative trends depended strongly on the chosen start year. Similar patterns were observed for the duration of snow cover. Not surprisingly, "cold and wet" years resulted in the most snow, whereas "warm and dry" years resulted in the lowest maxSWE. The variability in maxSWE was larger for higher elevations sites. Spatially, maxSWE increased with elevation, was lowest on south-facing slopes and highest on west-facing locations, and was lower for steeper than flat slopes. While uncertainties in input data and modeling limitations exist, this study underscores the value of long-term datasets like those from the Wägital monitoring program for understanding trends in snow cover and storage, and anticipating future challenges related to a reduced snow cover due to climate change.