the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 14 Oct 2025
Retrieving root-zone soil moisture from land surface modelling and GRACE/-FO and validating its dynamics with in-situ data over West Africa
Abstract. Rainfall variability in West Africa, driven by the West African Monsoon, poses significant challenges to agricultural productivity and livelihoods. In this context, understanding root-zone soil moisture (RZSM) dynamics is crucial since it serves as the primary water source for crops. While surface soil moisture (SSM) has been widely studied, research on RZSM remains limited. This study investigates RZSM dynamics across West Africa from 2003 to 2019 using multiple satellite-derived and model-based datasets, including ESA CCI v0.81, GLWS2.0, WaterGAP, CLM5.0, and in-situ observations. Results indicate that ESA CCI exhibits the strongest temporal and spatial alignment with ground measurements, whereas CLM5.0 and GLWS2.0 effectively capture latitudinal soil moisture gradients associated with climatic zones. A novel application of an analytical solution to Richards' equation was employed to translate surface moisture signals to deeper soil layers, demonstrating GLWS2.0’s superior ability to reproduce seasonal patterns at various depths, notably in Benin and Niger. Despite challenges posed by sparse in-situ data and vegetation-induced signal attenuation, the study highlights the significant benefits of GRACE/-FO data assimilation in enhancing model accuracy. The proposed depth-projection methodology improves the vertical representation of soil moisture, offering new insights into the dynamics of surface and subsurface water storage. These findings have important implications for agricultural forecasting, sustainable water resource management, and climate adaptation strategies in regions where accurate soil moisture data are essential for resilience planning.
Competing interests: Harrie-Jan Hendricks Franssen is a member of the editorial board of Hydrology and Earth System Sciences. The authors declare that they have no other competing interests.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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Status: open (until 02 Dec 2025)
- RC1: 'Comment on egusphere-2025-4600', Anonymous Referee #1, 29 Oct 2025 reply
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RC2: 'Comment on egusphere-2025-4600', Anonymous Referee #2, 18 Nov 2025
reply
This study offers a valuable integration of GRACE/-FO-based water storage anomalies with land surface and hydrological model soil moisture products to estimate root-zone soil moisture. The fusion of these data sources is a significant contribution to understanding soil moisture dynamics. However, the methodological novelty, in comparison to previous GRACE assimilation or analytical approaches, is not fully explained. The paper would benefit from a clearer explanation of how the adopted analytical solution extends beyond existing methods, and what specific improvements in accuracy or process representation have been achieved by this approach.
While the results are well presented, they remain largely descriptive. The discussion could be expanded to explore the physical reasons behind the observed differences between products, particularly in relation to regional hydro-climatic conditions. Interpreting how these differences manifest in the context of the West African climate would significantly enhance the scientific insight and generalizability of the conclusions.
The workflow is logically structured, but certain analytical steps, particularly the parameterization of the Richards equation, require deeper justification. A more thorough explanation of why specific parameter values were chosen would add robustness to the methodology. Additionally, a stronger uncertainty analysis is needed to make the validation results more convincing. The current approach could benefit from more detailed discussion of the limitations and sources of uncertainty, especially in relation to data assimilation.
The use of the GRACE/-FO-based global assimilation model GLWS2.0, which is based on WaterGAP, is an interesting aspect of the study. However, the paper lacks sufficient details on how GRACE data, which includes various components like groundwater, soil moisture, and vegetation weight, is assimilated into the model. It would be helpful to know how the integration of these different components improves soil moisture predictions specifically. For example, how do variations in groundwater levels influence soil moisture estimates? Are these variations significant enough to warrant inclusion in the model?
The application of the analytical solution to Richards’ equation, as proposed by Sadeghi et al. (2020), is an interesting approach, but the assumptions underlying the Gardner model need to be more thoroughly examined. This model assumes homogeneous soil and neglects plant water uptake, which may not align with the assumptions of the WaterGAP model. The paper does not sufficiently address whether these assumptions are compatible or if they introduce contradictions. A more detailed discussion of the theoretical framework, particularly how the assumptions in both models align or diverge, would strengthen the overall argument.
While the results are presented in detail, there is a need for deeper analysis of the underlying processes that lead to the observed patterns in the data. For instance, how do soil moisture and flux relationships differ across various depths, and how do these differences relate to regional hydro-climatic variability? Expanding this part of the discussion would help connect the empirical findings with broader environmental processes.
Minor Comments:
- The paper would benefit from a clearer structure in terms of presenting the key innovations of the study. It is important to ensure that the reader can easily identify how this work advances current understanding, particularly in relation to model improvements and new methodological contributions.
- Additional references to recent work in the field of soil moisture modeling and data assimilation, especially focusing on studies that use GRACE data, would provide more context and help justify the novel aspects of this study.
Citation: https://doi.org/10.5194/egusphere-2025-4600-RC2
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- 1
Review of
Retrieving root-zone soil moisture from land surface modelling and GRACE/-FO and validating its dynamics with in-situ data over West Africa
by Yap et al.
General comments:
This study benchmarked a number of modelling approaches and gridded datasets in West Africa, using in situ observations. The paper is not well written and cannot be published in its current form. In particular, a separate discussion section should be added. The table and figure captions are often incomplete. There is also no Data Availability section at the end of the paper.
Recommendation: major revisions.
Particular comments:
- L. 42: 'SM' is undefined.
- L. 162: The title of Section 2 is inaccurate. This section also includes a model description.
- L. 186 (Figure 1): I think that Figure 2 should be merged with Figure 1. The unknown satellite image in Figure 1 should be replaced by the land cover map in Figure 2.
- L. 187 (section 2.2): Neither Section 2.2.1 nor Section 2.2.2 provides a description of a dataset. I suggest replacing the title of Section 2.2 with 'Data and Models'. A short introduction to Section 2.2 should mention which atmospheric forcing data are used to perform the model simulations, rather than including this important information in the model description sections. The titles of sections 2.2.1 and 2.2.2 should also make it clear that these sections deal with models.
- L. 223: CORDEX was not defined before.
- L. 238: CRUNCEP was not defined before.
- L. 248: Only papers by Dorigo et al. are cited here. Dorigo et al. did not take these measurements. Please cite the papers describing the data and the sites here.
- L. 397: In the description of Eq. 4, you should indicate that theta_wt and theta_fc correspond to wilting point and field capacity, respectively. What you call "wt" has been called SWI (Soil Westness Index) during the last 3 decades. It seems that Tian et al. did not use the standard term 'SWI'. Could you switch to using the term SWI and cite a paper that uses this concept? It should be mentioned that the main purpose of SWI is to highlight the drought signal rather than mapping soil texture-driven volumetric soil moisture. Drought maps can only be produced using SWI.
- L. 414: 'The episodic and seasonal variations': these should be clearly defined, as should the method used to obtain them.
- L. 425: The results and discussion sections should be separate. The current organisation of the paper is unclear.
- L. 432: The methods used to produce these time series should be clearly presented in the 'Methods' section.
- L. 437: The quality of Fig. 3 could be improved. Could each year on the x-axis be clearly identified? Indicating months is not useful. The caption is incomplete.
- L. 442-444: Why have you suddenly moved from Africa to the Tibetan Plateau? This is confusing. Move this part to a discussion section.
- L. 444-448: Move to a discussion section.
- L. 464-466: This sentence should be moved to the Methods section or to the Discussion section.
- P. 22, Table 2: Use the correct symbols as defined in the main text. 'Corr' should be replaced by 'R', 'RMSE' by 'RMSE', and so on. To improve readability, use fewer digits. For example, replace 0.894 with 0.89.
- P. 22, Table 3: Table 3 cannot have the same title as Table 2.
- L. 491: The Sahel region should be labelled on either Fig. 5 or Fig. 1.
- L. 497-506: This part should be moved to a discussion section.
- L. 513-523: This part should be moved to a discussion section.
- L. 531: "these models": is ESA CCI a model?
- L. 534-554: This part should be moved to a discussion section.
- L. 555: Section 4.3 should be moved to a discussion section.
- l. 608: The conclusion section is far too long. Some parts could be moved to a discussion section. This section should focus on the main take-home messages and possible areas for further research.
Editorial comments:
- L. 223: “?”
- L. 479: Figure 4 or Figure 5?
- L. 525: Figure 5 or Figure 6?
- L. 563: Figure 6?