Benchmarking soil moisture and its relationship to ecohydrologic variables in Earth System Models

Abstract

Soil moisture (SM) plays a crucial role in regulating ecosystem biogeophysics by influencing processes such as plant water relations, evapotranspiration (ET), and surface water/energy cycles, among others. This study evaluates the representation of SM in Earth System Models (ESMs) using the International Land Model Benchmarking (ILAMB) framework, focusing on both surface (0-10 cm) and rootzone (0-100 cm) SM. By incorporating multiple observational and assimilated datasets, we benchmark the performance of Coupled Model Intercomparison Project Phase 6 (CMIP6) models in capturing SM across various soil depths, as well as examine the relationships between SM and ecohydrological processes and traits such as gross primary productivity (GPP), leaf area index (LAI), and ET. Results indicate that while models often capture surface SM patterns reasonably well, they tend to overestimate variability in rootzone SM and display consistent biases in deeper soil layers. Simulated ET shows strong agreement with observations, but larger discrepancies persist in GPP and LAI. The strength of SM–ecohydrology relationships varies depending on both model structure and observational reference, with improved agreement when using assimilated SM datasets. Regional evaluations using Köppen classifications reveal distinct model behaviors across different environmental zones, with stronger performance in tropical regions and reduced skill in high-latitude areas. These findings provide quantitative insights into model strengths and limitations, helping to guide targeted improvements in the representation of SM and its coupling to vegetation and hydrological processes in future ESM development.