Soil moisture response as a tool to understand hydrological processes across forested catchments in different climates

Comparative analysis of the hydrological response at the catchment scale across different climates is critical to understand possible similarities in runoff generation processes. In this work, we relied on high-resolution soil moisture measurements in three European forested catchments to characterize hydrological responses during different wetness conditions. The study sites include Ressi, Italy (2.4 ha), Weierbach, Luxembourg (42 ha), and Can Vila, Spain (56 ha). We analyzed the seasonal variability in the difference between soil moisture at a relatively shallow (10–15 cm) and deep (45–60 cm) location within soil profiles in each catchment in the period 2017–2021, which included a wide range of meteorological conditions. We found contrasting soil moisture patterns across the investigated catchments. In the most humid site, Ressi, which receives over 2000 mm of precipitation per year, we often found similar soil moisture at the two soil depths, and soil moisture at the shallow depth was rarely higher than that at the deeper layer, suggesting very frequent vertical connectivity in this site. In Weierbach, which receives around 1000 mm of precipitation per year, soil moisture in the shallow sensor was consistently higher than in the deeper soil except during wet conditions when water content was similar across the entire soil profile. During dry conditions, evaporation of shallow water resulted in consistently higher soil moisture in the deeper layers. We infer that in Weierbach vertical connectivity between deep and shallow soil layers develops only during wet conditions. Despite similar total precipitation amount between Can Vila and Weierbach, soil moisture patterns were very different. In Can Vila, soil moisture was consistently higher in the deeper layer compared to the shallow one irrespectively of the season. This difference could be driven by very high evaporation of shallow water or a significant contribution of groundwater that promotes vertical connectivity. Our approach provides a relatively simple and inexpensive method to assess differences in hydrological behavior solely based on soil moisture data, opening the possibility for further analysis and comparisons across multiple catchments.