Understanding Terrestrial Water Storage Changes Derived from the GRACE/GRACE-FO in the Inner Niger Delta in West Africa

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The current manuscript provided an analysis of terrestrial water storage for the Inner Niger Delta based on remotely sensed GRACE/GRACE-FO data. The writing is generally good, and the reviewer has a few comments:

1) The citation format needs revision. MDPI journals have not adopted the current citation format. Please see the author’s guidelines for details.

2) Line 112: Why is the reference in boldface? The same question applies to other places in the manuscript.

3) The introduction already provided a comprehensive review of GRACE/GRACE-FO. The reviewer suggests including a review of attempts in the past on terrestrial water storage estimation for the research region.

4) What delta_GWSa needs to be explained on page 9.

5) Figures 2-3: Please put the units in the figures.

6) Figure 4: What is TWSA? From what I can find, only TWS is mentioned in the text.

7) Lines 557-572: Why is a different font used?

8) The conclusion should provide more analysis of the practical implications of the results.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

This study examines terrestrial water storage (TWS) changes in the Inner Niger Delta (IND) in Mali from April 2002 to September 2022 using GRACE/GRACE-FO and GLDAS data. TWS variations show strong seasonal patterns (-170 mm to 330 mm) with a high correlation (r = 0.92, RMSE = 35 mm). The TWS trend is positive (7.3–9.5 mm/year). Wavelet analysis identifies dominant seasonal cycles (8–16 months). Precipitation (P) and potential evapotranspiration (PE) influence TWS only in humid regions (r = 0.18–0.26, p < 10⁻²). Surface water bodies (SWB) exhibit a trend reversal in 2014, with correlations exceeding 0.50 in most regions. Groundwater changes, estimated from GRACE/GRACE-FO and GLDAS, show strong agreement (r > 0.60, RMSE < 120 mm), with recharge rates increasing in semi-arid and Sahelian regions (r > 0.70, p < 10⁻³). This study highlights the regional significance of P, SWB, and groundwater recharge in driving TWS changes. I have no severe academic or technical issues with any in this manuscript, but I just left a series of specific commenting points for quality improvement. Therefore, a major revision is needed.

1. Lines 8-40: Your abstract is too lengthy. Would you mind shortening your abstract to be as concise and informative as mine above?
2. Your citation format does not accord with the MDPI publishing standards. All citation insertions should be in numbers (e.g., [1], [2-4], etc.) instead of the author followed by the date. Also, Lines 107-110: Regarding the drivers of water storage changes, such as land use changes, high-quality research could be reviewed and cited here to support this statement.

Lu, Y., Dai, L., Yan, G., Huo, Z., Chen, W., Lan, J., ... & Chen, J. (2023). Effects of various land utilisation types on groundwater at different temporal scales: a case study of Huocheng plain, Xinjiang, China. Frontiers in Environmental Science, 11, 1225916.

3. Lines 119-120: It would be better to concentratedly highlight your research gap after the literature review and before your research aims. Ending your literature review without a clear gap identification undermines your research significance, although you have those attempts very discretely ahead of the end of your literature review (I can spot them where they are). A clear gap identification will help the reader quickly capture what has never been done so as to pursue this goal.
4. A series of quality issues: Lines 143, 162 and 164: square kilometres -> km². Line 188: You can summarise specifications, configurations, and modelling settings in a series of tables. Tables are more straightforward for professionals to read. Line 286: no indentation ahead of notation starting with “where”. Line 322: “where” is missed. Line 346: Notation nearby, please. Alternatively, you should provide a nomenclature in an alphabetic order for all abbreviations and mathematical symbols. I have either memorised all abbreviations or scrolled upward to check the meaning of each. Very inconvenient.
5. Line 223: Can you summarise those in a nice and concise flow chat that can illustrate your informative and concise methodology in a very logical way? It will help you save many literal descriptions.
6. Figure quality issues: Figure 2 looks a little blurry, and Figure 3 was not perfectly cut; e.g., 16-10 in the contoured bar were cut off on the last digits. You may want to put a unit into both figures. Figure 4: The datasets between 2017 and 2019 are missing. Why? Lost of logging or other reasons? Please explain or defend if possible. Figure 6: Please clarify the y-labels of both line and contour plots in the caption of Figure 6. The y-label, “mm”, should be a millimetre, isn’t it? What is the label (unit)? Also, how do you define “Period”, and what is the unit of values in the contour bar? Figures 7-10: Y-labels are all missing, but only units are given.
7. Formating issue: Line 557-572: Font style and size mismatch the rest of the content.
8. Some academic challenges: Regarding Figure 7 (Precipitation > Evapotranspiration), what do you think is the water storage in the vadose zone (unsaturated zone above the groundwater table)? Any discussion and defence in this regard? Some in-depth discussions regarding land use, e.g., Lu et al. (2023) and other aspects, could be expanded with proper citations.
9. The conclusion should follow a structured format:

Paragraph 1: (I) Summarise the research gaps identified in the literature review and state the research objectives. (II) Briefly outline the methodology, highlighting key experimental settings.

Paragraph 2: (III) Present the key findings concisely in a point-by-point format.

Paragraph 3: (IV) Discuss the study’s limitations and provide reflections on its implications. (V) Emphasise the significance of the research and its potential engineering applications.

10. Please update your reference style using the MDPI Endnote style, which is downloadable from their author instructions. Once accepted, the proofreading editor will match the ones you provided against their databases. However, they will then ask you to manually change insertions of citation numbers throughout the entire manuscript, which will be something you never want to do manually (very exhausting). Instead, as I recommended, you may spend some effort reinserting all citations, but the entire reference list will be automatically updated per publishing standards.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

This paper deals with an important aspect it studies “Understanding Terrestrial Water Storage Changes derived from 2 GRACE/GRACE-FO in the Inner Niger Delta in West Africa”. However the following comments can improve the paper:

• The work needs to be connected to sustainable development Goals
• In Figure:1 north arrow and scale missing
• Why are relationships poor between P-PE and TWS
• Figure 8 shows similar trend in all cases needs to be explained
• In table 2 y all P-value is same needs to be checked
• Add Taylor diagram to show the variation over the years
• Following papers can be used as references and cited for better understanding and cited:
  • Surface water estimation at regional scale using hybrid techniques in GEE environment-A case study on Punjab State of India
  • Remote sensing-based monitoring and evaluation of the basin wise dynamics of terrestrial water and groundwater storage fluctuations
  • A comprehensive evaluation of GRACE‐like terrestrial water storage (TWS) reconstruction products at an interannual scale during 1981–2019
  • Understanding global groundwater-climate interactions

 

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have made great efforts to improve the quality of the manuscript from the last submission substantially. I have no more severe issues with the current version. The only academic concern is whether preferential flow paths (faults, fissures, fractures, and any other geological formations) have ever been considered in modelling analysis. If not, you can simply ignore this request. If yes, please provide a concise expansion anywhere in the relevant discussion session. Recommended work: Liu, B., Yan, G., Ma, Y., & Scheuermann, A. (2023). Measurement of in-situ flow rate in borehole by heat pulse flowmeter: Field-case study and reflection. Geosciences, 13(5), 146.

Author Response

We sincerely appreciate the reviewer’s thoughtful comments and his/her recognition of the improvements made in the manuscript. Regarding the consideration of preferential flow paths (faults, fissures, fractures, and other geological formations) in our modeling analysis, we would like to clarify that this aspect is beyond the scope of our study. Our research focuses on hydrological processes and does not specifically address subsurface flow pathways.

Therefore, we respectfully choose not to incorporate this aspect into our discussion. We also acknowledge the suggested reference but believe it falls outside the direct relevance of our study. Nonetheless, we appreciate the reviewer’s suggestion and his/her engagement with our work.

Thank you again for your valuable feedback and for helping us refine our manuscript.