Hydro-Climatic Variability and Water Balance of Lake Fitri, Sahel (Chad)

Round 1

Reviewer 1 Report (New Reviewer)

Comments and Suggestions for Authors

The paper investigates the hydro-climatic functioning of the Lake Fitri, Sahel(Chad) by integrating rainfall records, hydrological observations, water balance analysis, and the GR2M hydrological model. In the data-scarce Sahel region, combining in-situ observations with modeling to evaluate the water balance of arid-zone lakes is of significant scientific value and holds practical importance for local water resource management.

However, while the general framework is reasonable, several aspects require substantial revisions to meet publication standards—particularly regarding model parameterization, interpretation of data consistency, and the technical quality of figures. Detailed comments are as follows:

1. [Introduction section] The introduction provides a good background on the Sahelian climate, but it lacks a focused review of recent hydrological modeling studies in similar endorheic lake systems. Please consider adding 2-3 recent references to better outline how this work advances beyond existing regional studies.
2. [Lines 87-88, Figure 1] The description of the GR2M model is currently missing from the Methodology section. It is recommended to add a dedicated subsection to introduce the GR2M model, including its operational framework, the specific parameters involved, and the explicit role the model plays in this study (e.g., simulating runoff dynamics and supporting water balance calculations).
3. [Methodology section] The description of the GR2M model is currently missing from the Methodology section. It is recommended to add a dedicated subsection to introduce the GR2M model, including its operational framework, the specific parameters involved, and the explicit role the model plays in this study (e.g., simulating runoff dynamics and supporting water balance calculations).
4. [Lines 229-230] There is an inconsistency between the mathematical symbols used in the equations and the descriptive text (e.g., " "). Please ensure that all variables are defined consistently throughout the manuscript to avoid confusion.
5. [Results section, Figure 4] The labels on the horizontal axis (time) are overly dense and difficult to interpret. It is recommended to rotate these labels (e.g., a slanted orientation) to improve readability. Additionally, the label "Evap." on the vertical axis should be replaced with the abbreviation "ETP." to ensure consistency with Table 1.
6. [Lines 295-296, Figure 6] The abbreviations for months on the horizontal axis are non-standard; please use conventional formats (e.g., "Jan."). Additionally, the temperature scale on the vertical axis (0–150°C) is unrealistic for environmental data; it is recommended to adjust the range (e.g., 20–40°C) to better reflect the actual observed values and improve the visual clarity of the plot.
7. [Discussion section] While the paper mentions the "chronic water deficit," it fails to link these hydrological findings back to the socio-economic context mentioned in the introduction (e.g., impacts on berbere agriculture and pastoralism). Expanding on how episodic recharge patterns affect local livelihoods would strengthen the impact of the study.
8. [Conclusion section] The current conclusions are somewhat generic, stating that "integrated management" is needed. The authors should provide more specific, actionable policy or management recommendations based on their findings—for example, how the episodic nature of recharge should influence local water conservation strategies or the placement of monitoring stations.

Author Response

We sincerely thank the reviewer 1 for their thorough, constructive, and encouraging evaluations. We are particularly grateful for the recognition given to the scientific relevance of the study, its innovative nature in a Sahelian context that is still poorly documented, and the integrated approach combining climatic, hydrological and satellite.

We have taken into account all of the general and specific comments and have carried out a substantial and thorough revision of the manuscript, both in terms of scientific content and editorial form, in order to fully meet the requirements of MDPI Water.

We have responded to the comments in the following lines.

 

Comments 1: [Introduction section] The introduction provides a good background on the Sahelian climate, but it lacks a focused review of recent hydrological modeling studies in similar endorheic lake systems. Please consider adding 2-3 recent references to better outline how this work advances beyond existing regional studies.   Response 1: We thank the reviewer for this pertinent comment. The introduction has been revised to better position this study in relation to recent work on endorheic lake systems in Sahelian and semi-arid contexts.

Comments 2: [Lines 87-88, Figure 1] The description of the GR2M model is currently missing from the Methodology section. It is recommended to add a dedicated subsection to introduce the GR2M model, including its operational framework, the specific parameters involved, and the explicit role the model plays in this study (e.g., simulating runoff dynamics and supporting water balance calculations).   Response 2: In accordance with reviewer 2's suggestion to refocus the article on a robust hydroclimatic diagnosis, we have decided to remove the GR2M modeling from the study. Consequently, the addition of a methodological subsection on GR2M is no longer necessary.

Comments 3: [Methodology section] The description of the GR2M model is currently missing from the Methodology section. It is recommended to add a dedicated subsection to introduce the GR2M model, including its operational framework, the specific parameters involved, and the explicit role the model plays in this study (e.g., simulating runoff dynamics and supporting water balance calculations).   Response 3: In accordance with reviewer 2's suggestion to refocus the article on a robust hydroclimatic diagnosis, we have decided to remove the GR2M modeling from the study. Consequently, the addition of a methodological subsection on GR2M is no longer necessary

Comments 4: [Lines 229-230] There is an inconsistency between the mathematical symbols used in the equations and the descriptive text (e.g., " "). Please ensure that all variables are defined consistently throughout the manuscript to avoid confusion.   Response 4: It’s done!

Comments 5: [Results section, Figure 4] The labels on the horizontal axis (time) are overly dense and difficult to interpret. It is recommended to rotate these labels (e.g., a slanted orientation) to improve readability. Additionally, the label "Evap." on the vertical axis   Response 5: It’s done! (see Fig. 4)

Comments 6: [Lines 295-296, Figure 6] The abbreviations for months on the horizontal axis are non-standard; please use conventional formats (e.g., "Jan."). Additionally, the temperature scale on the vertical axis (0–150°C) is unrealistic for environmental data; it is recommended to adjust the range (e.g., 20–40°C) to better reflect the actual observed values and improve the visual clarity of the plot.   Response 6: The abbreviations for the months on the horizontal axis have been harmonized and standardized in accordance with standard conventions (Jan., Feb., March, etc.). The temperature scale is deliberately imposed by the construction of the Bagnouls-Gaussen type ombrothermic diagram, which is based on the conventional relationship P = 2T in order to identify wet and dry periods. This graphical constraint implies a common scaling of precipitation and temperature, which can lead to a wider apparent range of values than the temperatures actually observed.

Comments 7: Discussion section] While the paper mentions the "chronic water deficit," it fails to link these hydrological findings back to the socio-economic context mentioned in the introduction (e.g., impacts on berbere agriculture and pastoralism). Expanding on how episodic recharge patterns affect local livelihoods would strengthen the impact of the study.     Response 7: We have added a section at the end of the Discussion (5.6. Socio-hydrological implications: recharge variability and resilience of rural livelihoods). This text now establishes an explicit link between the identified episodic recharge regime and the specific constraints faced by agriculture and pastoralism in Berber areas.

Comments 8: [Conclusion section] The current conclusions are somewhat generic, stating that "integrated management" is needed. The authors should provide more specific, actionable policy or management recommendations based on their findings—for example, how the episodic nature of recharge should influence local water conservation strategies or the placement of monitoring stations.   Response 8: The conclusion has been substantially revised to go beyond a general formulation of “integrated management” and to make more specific and operational recommendations based directly on our findings.

 

 

 

Author Response File: Author Response.pdf

Reviewer 2 Report (New Reviewer)

Comments and Suggestions for Authors

General Comments

The manuscript is focused on an important and understudied hydro-system in the central Sahel region (eastern Chad). Lake Fitri and Batha River (its single tributary) are clearly of regional socio-ecological relevance, and the authors compile a substantial amount of observational, satellite, and modelled information. The manuscript's main strength lies in assembling different datasets in a data-scarce region, while providing a first-order hydroclimatic characterisation of the target basin. Nonetheless, the manuscript is overly descriptive and insufficiently accurate in several key aspects, namely in hydrological modelling, water balance assumptions, and uncertainty treatment. In its current form, it reads more like a technical report than a mature scientific article. Therefore, from my viewpoint, major revisions are required before it can be considered for publication.

 

Major Comments

1. The authors claim that this work constitutes a “first reference” for Lake Fitri. While the compilation effort is valuable, the scientific novelty is not sufficiently demonstrated. Most results confirm well-known Sahelian natural processes (e.g., Sahelian climatic control mechanisms related to short wet monsoons, high PET). The manuscript would benefit from a clearer articulation of what is new beyond site-specific description. For instance, the comparison with Lake Chad is largely qualitative and based on secondary sources, not adding strong analytical insight. Please narrow the scope and clearly frame the paper as either a baseline hydroclimatic diagnosis with quantified constraints, or (ii) a methodological demonstration for data-scarce Sahelian lakes.

 

2. The use of the GR2M model to model the Batha River flow rate is the weakest component of the paper. The application of suitable performance metrics (NSE, KGE, BIAS, RMSE) are critical to evaluate the actual model performance, particularly owing to the shortage of available observational data for calibration and validation. Please either remove the GR2M section and refocus the paper on observed hydroclimatic variability and water balance, or substantially strengthen the modelling analysis.

 

3. The water balance calculations (Tables 6 and 7) are informative but over-interpreted. Recharge estimates rely on highly simplified assumptions (fixed soil water holding capacity, single-year analysis), whereas only one year (2020) is analysed. Please revise to tone down claims regarding recharge rates and explicitly state that findings are scenario-based and non-generalizable.

 

4. The manuscript is too long and repetitive. The Results and Discussion sections overlap in several parts, often repeating the same interpretations.

 

5. The number of figures is too high (e.g., multiple meteorological plots), providing redundant or secondary information. Several tables are also redundant with figures. Please move some figures and tables to the supplementary material.

 

6. Some methodological descriptions (e.g., SPI justification) are overly detailed for a specialist audience.

 

7. Figures, in general, can be improved in quality and readability. Axis, grids, labels and font sizes are not always clear. Figure 1 is of very low resolution. Please improve the general quality of the figures.

 

8. The Supplementary material file is not correct. A response letter is provided instead.

Author Response

We sincerely thank the reviewer 2 for their thorough, constructive, and encouraging evaluations. We are particularly grateful for the recognition given to the scientific relevance of the study, its innovative nature in a Sahelian context that is still poorly documented, and the integrated approach combining climatic, hydrological and satellite.

We have taken into account all of the general and specific comments and have carried out a substantial and thorough revision of the manuscript, both in terms of scientific content and editorial form, in order to fully meet the requirements of MDPI Water.

We have responded to the comments in the following lines.

 

Comments 1: The authors claim that this work constitutes a “first reference” for Lake Fitri. While the compilation effort is valuable, the scientific novelty is not sufficiently demonstrated. Most results confirm well-known Sahelian natural processes (e.g., Sahelian climatic control mechanisms related to short wet monsoons, high PET). The manuscript would benefit from a clearer articulation of what is new beyond site-specific description. For instance, the comparison with Lake Chad is largely qualitative and based on secondary sources, not adding strong analytical insight. Please narrow the scope and clearly frame the paper as either a baseline hydroclimatic diagnosis with quantified constraints, or (ii) a methodological demonstration for data-scarce Sahelian lakes.   Response 1: We thank the reviewer for this insightful comment, which has led to a substantial clarification of the scientific positioning of the article. In order to remove any ambiguity, the manuscript has been refocused and explicitly defined as a reference hydroclimatic diagnosis with quantitative constraints, addressing the points suggested.

Comments 2: The use of the GR2M model to model the Batha River flow rate is the weakest component of the paper. The application of suitable performance metrics (NSE, KGE, BIAS, RMSE) are critical to evaluate the actual model performance, particularly owing to the shortage of available observational data for calibration and validation. Please either remove the GR2M section and refocus the paper on observed hydroclimatic variability and water balance, or substantially strengthen the modelling analysis.   Response 2: We followed the reviewer's recommendation by removing the entire GR2M modeling section. We refocused the article on a reference hydroclimatic diagnosis based on the analysis of in situ and satellite observation data.

Comments 3: The water balance calculations (Tables 6 and 7) are informative but over-interpreted. Recharge estimates rely on highly simplified assumptions (fixed soil water holding capacity, single-year analysis), whereas only one year (2020) is analysed. Please revise to tone down claims regarding recharge rates and explicitly state that findings are scenario-based and non-generalizable.   Response 3: The sections relating to Tables 6 and 7 have been revised and the necessary nuances added.

Comments 4: The manuscript is too long and repetitive. The Results and Discussion sections overlap in several parts, often repeating the same interpretations. Response 4: We have taken note of the comment regarding redundancies between the Results and Discussion sections. We have deleted the former section 5.3 (Hydroclimatic Seasonality). Descriptive information on climate has been grouped together in the Results section (4.1), while the consequences on water deficit have been incorporated into the discussion on recharge (5.5). This makes for smoother reading and avoids repetition of meteorological parameters already presented in the tables.

Comments 5: The number of figures is too high (e.g., multiple meteorological plots), providing redundant or secondary information. Several tables are also redundant with figures. Please move some figures and tables to the supplementary material. Response 5: We followed this recommendation in order to streamline the manuscript and improve its readability. Figures and tables providing routine or secondary data (Ati station, ombrothermic diagram, SPI indices) have been renamed with the prefix ‘S’ and moved to the ‘Supplementary Materials’ document. The text has been updated to include the corresponding references (e.g., Figure S1, Table S1).

Comments 6: Some methodological descriptions (e.g., SPI justification) are overly detailed for a specialist audience. Response 6: As suggested by the reviewer, we have simplified the description of the SPI. We have removed the general justifications and comparisons with other indices (PDSI, RAI), focusing solely on the technical parameters (12-month scale, reference period 1980-2020) relevant to our analysis of the Lake Fitri basin.

Comments 7: Figures, in general, can be improved in quality and readability. Axis, grids, labels and font sizes are not always clear. Figure 1 is of very low resolution. Please improve the general quality of the figures. Response 7: All figures have been carefully revised to improve their quality and readability. Axes, grids, labels, and font sizes have been standardized and enlarged where necessary to ensure better readability.

Comments 8: The Supplementary material file is not correct. A response letter is provided instead. Response 8: The supplementary documents file has been corrected and replaced with the appropriate version. The response letter to reviewers is now provided separately, in accordance with the journal's instructions.

 

 

 

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report (New Reviewer)

Comments and Suggestions for Authors

The authors have satisfactorily revised the manuscript, following my recommendations. Hence, I suggest accepting the manuscript.

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Dear Authors,
I added my thoughts and suggestions about the article to the system as a PDF file. 
Regards,

Comments for author File: Comments.pdf

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript is robust and well written. The language is fine.

The authors should follow the format of the journal, for instance, how the references should be representeed in the text (not full name, but with number). Also, there are parts in the text that figures are mentioned as Figure 1 and others as Fig.1. Please, schroll dow all the text and revise-correct all the mistakes according to the journal's instructions. Moreover, all the charts should be in english.

As far as concerns the introduction, it can be expanded.

What are the values of the metrics that are mentioned? KGI etc

In discussion, there are not mentioned the agriculture activity, the land use etc that are mentioned in introduction. How about an analysis among rainfall lake level and river discharge?

The authors can write a few things describing the model. 

Please, provide references for the data.

Line 199: correct the formula.

 

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The study tries to discuss water balance, but the only difference hydrological components are discussed. The study has no explain the balance which are input(rainfall), output (streamflow, evapotranspiration) and water storage (groundwater and flood plain). Several commands are as follows:

1. Line (53-56) several questions proposed, but that cited with Taylor et al., 2013; 56 MacDonald et al., 2012). It is not clear that these questions are important? But several references proposed that and what did they have deal with these question and conclusion?
2. Line 58~59, emphasizing the synergistic impacts of climate change and accelerated population expansion. I have no ideal about related the population. There is no water use discussion in study.
3. In Fig.1 No clear water balance boundary.
4. Fig. 2 is hydrogeological map. The language should be printed or redraw in English and cited reference. The profile of hydrogeology should be redrawn in Fig.1
5. Line 123, “Located” in Yao, Ati, and N’Djamena is not as Figure 3.
6. Line 162-163,” observations, satellite altimetry data were used to complement and extend the temporal coverage of hydrological records.” What is the data of altimetric is used for GR2M? Has any reference could be cited of these data?
7. Section 4.1.2 & 4.13 seem just description of rainfall and temperature. Table (1&2) and figure (5&6) are superfluous.
8. Line 277-278, “and the water level rises and falls quickly because of heavy rain upstream and the basin's low storage capacity”. No clear evidence is to prove “low storage capacity”; moreover, have rain in upstream is not clear. Because the study did not prove. Mabey discuss different upstream rainfalls compare with different groundwater storage.
9. The meaning of the upper and lower bars in Figure 9 is not explained. Please clarify whether they represent variability.
10. Line335-336, “The performance of the model was evaluated using the Kling-Gupta Efficiency Index (KGE), supplemented by NSE, PBIAS and RMSE measures. Calibration was performed for the period 1981–2000, while the validity of the model was assessed for the period 2001–2020.” There is no quantitative outcome to support that.
11. The col，” The flow of the Batha River and the fluctuations of Lake Fitri are mainly caused by variations in rainfall, which follow a cycle of wet and dry years.” However, since the hydrological data used in this study cover only a short three-year period, it is difficult to substantiate such a long-term cyclical interpretation. The conclusion seems overly general and not sufficiently supported by the presented data.
12. Throughout the manuscript, there are frequent descriptions of “changes” and “interactions” among hydrological components, yet few quantitative analyses are provided. This lack of quantitative validation is a major weakness of the study.

 

Author Response

Please see the attachment.

Author Response File: Author Response.pdf