the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 10 Sep 2025
Benchmarking soil moisture and its relationship to ecohydrologic variables in Earth System Models
Abstract. Soil moisture (SM) is a key regulator of ecosystem biogeophysics, influencing plant water relations and land-atmosphere energy exchanges. This study evaluates the representation of SM in Earth System Models (ESMs) using the International Land Model Benchmarking (ILAMB) framework, focusing on both surface (0–5 cm, 0–10 cm) and rootzone (0–100 cm) depths. We benchmark Coupled Model Intercomparison Project Phase 6 (CMIP6) models against multiple observational and assimilated datasets to evaluate their performance in simulating SM, as well as their relationships with ecohydrological processes and vegetation traits such as gross primary productivity (GPP), leaf area index (LAI), and evapotranspiration (ET). Results show that while surface SM is generally well represented (r > 0.87), rootzone SM variability is overestimated (normalized standard deviation > 1). Simulated ET agrees strongly with observations (r > 0.9; normalized standard deviation 0.8–1.2), whereas GPP and LAI exhibit greater discrepancies (r > 0.7; normalized standard deviation mostly > 1). The strength of SM–ecohydrology relationships varies with model structure and observational dataset, with better consistency observed when assimilated SM products are used. Regional analyses using Köppen classifications reveal distinct model behaviors, with stronger performance in tropical zones and reduced skill in high-latitude regions, likely due to challenges in simulating freeze–thaw and permafrost dynamics. These findings offer quantitative benchmarks of model performance, highlighting specific areas for improving SM representation and its coupling with vegetation and hydrological processes in future ESM development.
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Status: open (until 26 Nov 2025)
- RC1: 'Comment on egusphere-2025-3517', Anonymous Referee #1, 05 Nov 2025 reply
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- 1
General comments
This manuscript extends the widely-used benchmarking tool ILAMB to include soil moisture and uses it to perform a comprehensive evaluation of soil moisture Earth System Models from CMIP6, both near the surface and to a depth of 1m. It represents a substantial contribution to the field. The evaluation is extensive and reproducible; of particular note is that a variety of different metrics are used, including the relationship between soil moisture and other variables. The presentation quality is high throughout.
Specific comments
Add some sentences mentioning that soil moisture below 1m can be important too (particularly in areas with deep roots), even though evaluating it is beyond the scope of this analysis.
Line 160: this is an important point you make here – refer back to it in section 3.2
Line 212-3 This statement needs citations. Even better would be to add a brief justification of this statement and mention the limitations of these datasets, so that the reader can bear these in mind when interpreting your results.
Line 249 Add a description of how Overall Score is calculated from Bias Score, RMSE Score, Seasonal Cycle Score, and Spatial Distribution Score. Add few words to clarify the “Seasonal Cycle Score” and “Spatial Distribution Score”.
Line 272: I couldn’t see anywhere what time resolution was used to calculate the Taylor plots (e.g. annual, monthly or daily?). Same for fig. 6 and 7.
Line 299: what do you mean by “parameters that mask deficiencies in SM representation”? Maybe add a clarifying phrase, or an example.
Line 300: I disagree with this statement because I would characterize transpiration as a vegetation-related process. The representation of water flux through the canopy and carbon flux through the canopy typically both rely structurally on very similar parts of the code.
Line 319: “However, it is important to note that the ILAMB spatial climatology used in Figures 6 and 7 may be affected by ESA-CCI’s inconsistent spatiotemporal coverage” I don’t understand this sentence – what does the phrase “ILAMB spatial climatology” mean here?
Technical corrections
Line 43: consider replacing “, most commonly relying” with “. The majority rely” (the existing sentence has an ambiguity about whether “most” refers to “ESMs” or “commonly” which hampers the sentence flow)
Line 45: consider replacing “by incorporating” with “using” or “incorporating it into”
Line 121: the minus sign in both units needs to be in the superscript
Consider putting mrsol and mrsos in monospace font, given that it is the name of a variable
Equation 1: It is more standard to not use italics if the variable contains more than one letter i.e. consider making all letters non-italic apart from n, l, z, w, ρ. Consider renaming mrsol in equation 1 with something briefer e.g. mSM, θm
Line 137: put the numbers in the volumetric SM units into superscript