Recent summer soil moisture drying in Switzerland based on measurements from the SwissSMEX network

Abstract. Notably drier summers and more frequent droughts were reported in Switzerland in the last decades. Here, we present curated timeseries of in situ soil moisture measurements from the Swiss Soil Moisture Experiment (SwissSMEX) network, which as of now cover 15 years. We demonstrate the potential of this comprehensive network for analysing the documented drying trends. At 12 grassland stations, SwissSMEX provides data on the volumetric soil water content at various depths in the soil profile, which can be used to calculate integrated soil water content down to 50 cm depth as an indicator of root zone water. 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. These timeseries are used to analyse trends in summer and summer half-year anomalies of integrated soil water content, and to investigate 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 remote sensing 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, which vary in their temporal coverage due to sensor failures. Comparing the in situ timeseries from stations with best temporal coverage with ERA5-Land and ESA CCI PASSIVE soil moisture from the corresponding grid cells also reveals a good agreement between these three independent data sources, with correlations of 0.85 or higher for the median timeseries calculated across stations. Based on these stations, the drying over the common 2010–2023 period amounts to ‑2.0 mm yr^-1 for the absolute summer soil moisture anomalies of SwissSMEX and ERA5-Land (or ‑1.2 % yr^‑1 for the percentage anomalies), and to -0.9 mm yr^-1 (or ‑0.6 % yr^‑1) for ESA CCI PASSIVE. The summer half-year trends are about half of those from the summer values for SwissSMEX (-1.0 mm yr^-1, p < 0.05 in this case) and ERA5-Land (‑1.1 mm yr^-1), while ESA CCI PASSIVE shows similar summer and summer half-year trends. Although most of the trends are not significant over the short 2010–2023 period, trends in summer half-year soil moisture anomalies from ERA5-Land become significant for certain time frames when the period for the trend test calculation is extended to years before 2010. Although the SwissSMEX network indicates that summer soil drying has increased in recent years, the 15 years of in situ data currently available are in many cases not yet sufficient to robustly estimate a significant trend. This highlights the importance of sustaining ongoing measurements to ensure a seamless continuation of soil moisture monitoring in Switzerland.