Surface Soil Moisture–Vegetation Feedbacks in Water-Limited Regions across Land Surface Models

Soil moisture plays a critical role in water-limited regions through its strong coupling and feedbacks with vegetation. However, state-of-the-art Land Surface Models (LSMs) used in reanalysis and near-term prediction systems still lack a realistic coupling of vegetation, limiting their ability to properly account for the fundamental role of vegetation in modulating the feedback with soil–moisture.
In this study, we incorporate Leaf Area Index (LAI) variability from observations - derived from the latest-generation satellite products provided by the Copernicus Land Monitoring Service - into three different LSMs. The models perform a coordinated set of offline, land-only simulations forced by hourly atmospheric fields from the ERA5 reanalysis. An experiment using interannually varying LAI (SENS) is compared with a control simulation based on climatological LAI (CTRL) in order to quantify vegetation feedbacks and their impact on simulated near-surface soil moisture.
Our results show that interannually varying LAI substantially affects near-surface soil moisture anomalies across all three models and over the same water-limited regions. However, the response differs markedly among models. Compared with ESA-CCI observations, near-surface soil moisture anomalies significantly improve in one model (HTESSEL–LPJ-GUESS), whereas the other two models (ECLand and ISBA–CTRIP) exhibit a significant degradation in anomaly correlation. The improved performance in HTESSEL–LPJ-GUESS is attributed to the activation of a positive soil moisture–vegetation feedback enabled by its effective vegetation cover (EVC) parameterization. In HTESSEL–LPJ-GUESS, EVC varies dynamically with LAI following an exponential relationship constrained by satellite observations. Enhanced (reduced) soil moisture limitation during dry (wet) periods leads to negative (positive) LAI and EVC anomalies, which in turn generate a dominant positive feedback on near-surface soil moisture by increasing (decreasing) bare-soil exposure to direct evaporation from the surface. In contrast, ECLand and ISBA–CTRIP prescribe EVC as a fixed parameter that does not respond to LAI variability, preventing the activation of this positive feedback. In these models, the only active feedback on near-surface soil moisture anomalies is negative and arises from reduced (enhanced) transpiration associated with negative (positive) LAI anomalies.
Our findings demonstrate that simply prescribing observed vegetation properties in LSMs does not guarantee a realistic coupling between vegetation and soil moisture. Instead, it is shown that the explicit representation of the underlying vegetation processes is essential to activate the proper feedback and capture the correct soil moisture response.