Summer drying of soils in Switzerland: Insights from the SwissSMEX network

Notably drier summers and more frequent droughts were reported in Switzerland in the last decades. We analyse these drying trends based on the comprehensive network of in situ soil moisture measurements from the Swiss Soil Moisture Experiment (SwissSMEX), which as of now covers 15 years. We document recent measures that have been taken to secure the SwissSMEX network and to ensure the continuity of its long-term soil moisture timeseries. The analysis focuses on trends in summer and summer half-year anomalies of vertically integrated soil water content and investigates the robustness of the recent drying based on different sets of Swiss Plateau stations. Furthermore, the SwissSMEX-based trends are compared with those from soil moisture of a widely used land reanalysis product (ERA5-Land) and of a merged passive microwave satellite product (European Space Agency Climate Change Initiative ESA CCI).

There is good agreement between the temporal evolution and the drying tendency of SwissSMEX in situ soil moisture based on different sets of Swiss Plateau stations. Comparisons with ERA5-Land and ESA CCI reveal a consistent evolution of soil moisture across the three independent datasets. Summer drying tendencies over the common 2010–2025 period amount to ‑11 mm/decade for ERA5-Land and ESA CCI, and to ‑14 mm/decade for SwissSMEX. While most drying trends are not statistically significant over this short span, ERA5-Land shows significance when extending the analysis period. The findings underscore the need for continued soil moisture monitoring in Switzerland for further investigation of long-term drying trends.