| 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.
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?