High-resolution soil moisture reanalysis of Switzerland (2016-2023)

Soil moisture is a cornerstone variable in the interaction between the land and the atmosphere, controlling hydrological and vegetation processes. Soil moisture variations in space and time are a key input for various applications in hydrology, geomorphology, agriculture and soil sciences. The direct monitoring of soil moisture and upscaling to large areas is challenging, while satellite remote sensing is only possible for the top few centimetres of the soil column with considerable uncertainties. In this study, we present a new soil moisture reanalysis for the entire Switzerland, consisting of daily resolution soil moisture maps at six depths (from 0 to 2 m) at a horizontal resolution of 250 m during 2016-2023. The maps are generated as a part of a detailed numerical simulation of the hydrological cycle of Switzerland using the mechanistic eco-hydrological model Tethys-Chloris (T&C).

T&C represents essential components of the hydrological and carbon cycles, resolving exchanges of energy, water, and CO₂ between the land surface and the atmosphere. Soil moisture dynamics in saturated and unsaturated soils are solved using the one-dimensional Richards equation for vertical flow and the kinematic wave equation for lateral subsurface flow. The model was forced by hourly meteorological data from the SwissMetNet weather station network and a gridded precipitation product, alongside state-of-the-art land cover and soil characteristics. Results of T&C align well with independent in-situ and remote observations of soil moisture, as well as other eco-hydrological variables such as streamflow, snow depth, LAI, and fluxes of CO₂, water and energy which lend credibility to the soil moisture reanalysis.

The study period (2016-2023) includes two recent years of severe spring-summer droughts (2018 and 2022), which are used to showcase how soil moisture anomalies have been developing throughout these dry periods. Preliminary analyses show that during the spring and summer of 2018, which were preceded by a relatively wet winter, soil moisture anomalies were small except in the eastern areas of the Central Plateau where they reached approximately -35% compared to the 2016-2023 seasonal average. In contrast, the spring and summer of 2022, which were preceded by a dry winter, exhibited more widespread anomalies ranging from -15% to -35%, affecting the Jura Mountains, the Central Plateau, and the lower elevations of the Southern Alps. In general, results reveal a large spatial and temporal variability across the six biogeographical regions of Switzerland (Jura Mountains, Central Plateau, Northern Alps, Eastern Alps, Western Alps, and Southern Alps). The soil moisture reanalysis presented in this study is the first of its type, and can be used as a reference dataset and as input for studies looking at floods, landslides, crop productivity, tree water stress, wildfire risk and other applications, where knowledge of soil moisture is essential.