Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

A plugin to estimate the level of water stress (SDG 6.4.2) by season and by sub-basin

Review

The paper presents a new methodology (both conceptual and practical) for quantifying SDG Indicator 6.4.2. at the subbasin scale. This research addresses an important knowledge gap that is highly relevant for SDG monitoring by the global community. The quality and substance of the material and insights presented are in principle sufficient for warranting publication, although the overall structure and presentation of the methodology and results should be improved first.

Main comments

• An important comment relates to the structure of the paper. There is not a clear hierarchy to the section headings. In total, 14 sections (preceded by headings in bold) are comprising the paper, including commonly distinguished sections (introduction, methodology, etc.) but also others that should probably be integrated as subsections into the former. The overall structure needs to be revised for clarity to the reader. An option is to take the key variables from the basic equations given in the paper (e.g. Eq. 4) and dedicate subsequent subsections to each of the variables.
• The title of the paper suggests that the plugin is the main topic and novelty of the research. However, the plugin (and WEAP in general) does not appear until page 15. It is strongly recommended to strengthen the introduction section with further background and information on the scope of the research, merging the introduction and “background and bottlenecks” section which are now separate. If the plugin is kept as the main focus of the title and paper, the introduction should also introduce WEAP and the rationale for developing a plugin in there. I would also expect a more explicit reference in the introduction to the SDG 6.4.2 monitoring guide (https://www.unwater.org/sites/default/files/app/uploads/2020/02/EN-Step-by-step-for-indicator-6-4-2-V20190204_rev.pdf) which supposedly is the current state of art for organizations interested to estimate indicator 6.4.2.
• I would assume that part of the added value of this method, and WEAP in general, is the opportunity for doing scenario analysis in addition to monitoring. If so, this can be more explicitly part of the key messages from the paper.
• As mentioned above, the actual plugin is introduced at a rather late stage of the paper, and a lot of the manuscript is made up of conceptual discussion based on hypothetical examples. To keep the paper more concise and help the reader understand the added value of this new methodology, it would be good to present the added value of the method/plugin based on insights from the SRB application. For example, the need for having SB disaggregation rather than basin/national monitoring, can be demonstrated by showing the additional level of insights gained from the spatial disaggregation in Figure 11. In other words, what would be the conclusions for the SRB water stress if only basin and national monitoring was done, and how is it improved based on the plugin? That would likely prove the points made earlier in the paper about double counting and SB disaggregation, and could enable some earlier parts to be shortened / removed.

Specific / minor comments

• p3, lines 76-84: this section would benefit from highlighting a few examples from the map to illustrate
• Figures 2,3,4: the subbasins are separated by continuous lines, similar to the overall basin boundaries. More correct would be to indicate boundaries between subbasins in a different way (dashed lines?) to highlight that the SBs together make up a hydrological unit.
• p6, line 163: the result should be 1110 rather than 1100
• p9, lines 238-239: this sentence needs to be rephrased for better understanding
• p9, line 264: here the “riverbed” is introduced as the only means of water flow between SBs. Later on in the paper, fossil GW and canals etc. are also mentioned. But how about groundwater available from recharge in an upstream SB and withdrawn (or accessed e.g. through a gw-dependent ecosystem) in a downstream SB?
• p15, line 469: what is an “annual basin”?
• p15, line 479: “WPS” -> WSP
• p16, lines 513-519: Is the plugin also compatible with inflows of water from beyond the basin boundaries? E.g. interbasin transfers or desalination?
• p17, line 538: “integrate” -> “integrates”
• p17, line 541: “Applications”-> only one application is provided, so the title may need to be changed. But why is only one application discussed, can this be justified? Previously five different test sites are mentioned, so supposedly there are interesting takeaways / lessons learned from the others as well?
• P18, line 599: “SDAGE” – I think this acronym is never introduced?
• P22, line 711: I don’t think it is the indicator itself that is disaggregated

Comments for author File: Comments.pdf

Author Response

Main Comments, replies in red

Please consider that lines numbers refer to " All Markup" visualisation in track changes

Comment 1: An important comment relates to the structure of the paper. There is not a clear hierarchy to the section headings. In total, 14 sections (preceded by headings in bold) are comprising the paper, including commonly distinguished sections (introduction, methodology, etc.) but also others that should probably be integrated as subsections into the former. The overall structure needs to be revised for clarity to the reader. An option is to take the key variables from the basic equations given in the paper (e.g. Eq. 4) and dedicate subsequent subsections to each of the variables.

Thank you for pointing it out. We agree with this comment. The structure of the paper has been modified, following the suggestion of creating sections for each variable which appears in the SDG 6.4.2 formula

Comment 2: The title of the paper suggests that the plugin is the main topic and novelty of the research. However, the plugin (and WEAP in general) does not appear until page 15. It is strongly recommended to strengthen the introduction section with further background and information on the scope of the research, merging the introduction and “background and bottlenecks” section which are now separate. If the plugin is kept as the main focus of the title and paper, the introduction should also introduce WEAP and the rationale for developing a plugin in there. I would also expect a more explicit reference in the introduction to the SDG 6.4.2 monitoring guide (https://www.unwater.org/sites/default/files/app/uploads/2020/02/EN-Step-by-step-for-indicator-6-4-2-V20190204_rev.pdf) which supposedly is the current state of art for organizations interested to estimate indicator 6.4.2.

Thank you for pointing it out. We agree with this comment. The title of the paper has been changed in " A new methodology to estimate the level of water stress (SDG 6.4.2) by season and by sub-basin avoiding the double counting of water resources" . About the Step by Step methodology of SDG 6.4.2, we have added references in the Introduction (see lines 32 and 39)

Comment 3: I would assume that part of the added value of this method, and WEAP in general, is the opportunity for doing scenario analysis in addition to monitoring. If so, this can be more explicitly part of the key messages from the paper.

Thanks for the comment. Section 3.1.2 shows explicitly the scenarios analysed in the Senegal River Basin WEAP model. The capability of WEAP of allowing simulating different scenarios has been highlighted also in the Conclusions (see line 784 and lines 807-809)

Comment 3: As mentioned above, the actual plugin is introduced at a rather late stage of the paper, and a lot of the manuscript is made up of conceptual discussion based on hypothetical examples. To keep the paper more concise and help the reader understand the added value of this new methodology, it would be good to present the added value of the method/plugin based on insights from the SRB application. For example, the need for having SB disaggregation rather than basin/national monitoring, can be demonstrated by showing the additional level of insights gained from the spatial disaggregation in Figure 11. In other words, what would be the conclusions for the SRB water stress if only basin and national monitoring was done, and how is it improved based on the plugin? That would likely prove the points made earlier in the paper about double counting and SB disaggregation, and could enable some earlier parts to be shortened / removed.

Thanks for the comment. Taking into account the complexity of the methodology, we have considered more appropriate to present it through simple schematic examples, rather than a practical real life case, such as the Senegal river basin, which could add further to the complexity. Nonetheless we have added a paragraph to section 3.1.4 to highlight the relevance of the disaggregation to increase the information provided by the water stress indicator (see lines  764-768)

Specific comments: replies in red

Comment 1: p3, lines 76-84: this section would benefit from highlighting a few examples from the map to illustrate

Thanks, done

Comment 2: Figures 2,3,4: the subbasins are separated by continuous lines, similar to the overall basin boundaries. More correct would be to indicate boundaries between subbasins in a different way (dashed lines?) to highlight that the SBs together make up a hydrological unit.

Thanks. We have redone the figures 2-3-4-5-6-7 from scratch including the dashed line for sub-basins, when needed.

Comment 3: p6, line 163: the result should be 1110 rather than 1100

Thanks. Corrected (see line 185)

Comment 4: p9, lines 238-239: this sentence needs to be rephrased for better understanding

Thanks. Corrected (see lines 272-273)

Comment 5: p9, line 264: here the “riverbed” is introduced as the only means of water flow between SBs. Later on in the paper, fossil GW and canals etc. are also mentioned. But how about groundwater available from recharge in an upstream SB and withdrawn (or accessed e.g. through a gw-dependent ecosystem) in a downstream SB?

Thanks. We have clarified the concept following your suggestion (see lines 298-299)

Comment 6: p15, line 469: what is an “annual basin”?

Thanks. Corrected, it was a typo (see line 526)

Comment 7: p15, line 479: “WPS” -> WSP

Thanks. Corrected, it was a typo (see line 536)

Comment 8: p16, lines 513-519: Is the plugin also compatible with inflows of water from beyond the basin boundaries? E.g. interbasin transfers or desalination?

Yes. The plugin is compatible with inflows of water from beyond the basin boundaries (see lines 580-582). In principle WEAP manages also water coming from desalination, however, in the SDG 6.4.2 this portion cannot be accounted in the TRWR. 

Comment 9: p17, line 538: “integrate” -> “integrates”

Thanks. Corrected (see line 602)

Comment 10: p17, line 541: “Applications”-> only one application is provided, so the title may need to be changed. But why is only one application discussed, can this be justified? Previously five different test sites are mentioned, so supposedly there are interesting takeaways / lessons learned from the others as well?

Many thanks for point it out. As the paper is already very long, we suggest to leave only the application of the Senegal river basin also because it includes both the spatial and temporal disaggregation of the indicator

Comment 11: P18, line 599: “SDAGE” – I think this acronym is never introduced?

Done. Thanks (see line 645)

Comment 12: P22, line 711: I don’t think it is the indicator itself that is disaggregated

Ok thanks. we have rephrased the sentence (see line 810)

Reviewer 2 Report

Comments and Suggestions for Authors

This paper looks at the freshwater resources at basin and sub-basin level and argues that disaggregating water stress at basin and sub-basin level is fundamental to provide a finer view of both its causes and effects, allowing to target interventions at areas with high water stress and sectors with high water use. They further argue that when the disaggregation is done at sub-basin level, there is the possibility that the same amount of water is counted twice or even more (double counting), as it flows from one sub-basin to the neighboring ones and this causes an underestimation of water stress, and an overestimation of the water resources available for human use in a given area. They develop a new plugin to assess the the double counting of the water resources is addressed by assessing the water supplies of each sub-basin taking into consideration the water demands of their neighboring sub-basins. They tested the plugin in Tunisia, Algeria, Rwanda, Peru and in the countries sharing the Senegal river basin and it is available online with its user manual in English, French and Spanish. They further argue that this plugin fills a gap in the information provided by the previous assessment of the indicator at major river basin, offering a better insight to practitioners and decision maker alike.

Although the idea is interesting and relevant, there is a need for the authors to first identify what methods have been used currently and what are the weaknesses. The review of the literature is mostly dated and does not clearly underlie the concern for the study. The methodology section needs more details in terms of the development of the new plug in and why this is better that anything already available. Figures and tables can be more professionally presented.

The paper is technically well documented but unfortunately it fell flat in the discussion and conclusion section. What was found interesting and relevant to the issues and which stakeholders will benefit. What would be the way forward?

The abstract was missing for the main paper and references are very limited and dated with not many references to good and impactful journals in the area of study. The authors should check the reference for format and completeness.

Author Response

Replies to comments are in red

Please consider that lines numbers refer to " All Markup" visualisation in track changes

Comment 1: Although the idea is interesting and relevant, there is a need for the authors to first identify what methods have been used currently and what are the weaknesses. The review of the literature is mostly dated and does not clearly underlie the concern for the study. 

Many thanks for this comment. Indeed there is no relevant literature on this subject. However we have underlined the need of looking for a new approach considering the methodology applied in the current available water accounting frameworks (see lines 90-98)

Comment 2: The methodology section needs more details in terms of the development of the new plug in and why this is better that anything already available.

Many thanks for this comment. We believe that the methodology is well described and it includes many details. About the plugin, we have explained better the value added of developing it in WEAP. Advantages are for example:

• WEAP is widely used as the planning and management platform in many countries.
• WEAP IS SUITABLE to integrate data on TRWR, TFWW and EFR to produce a practical “water accounting framework” for planners and decision makers  (see section 3)
• WEAP allows to run the WSP in different climate and policy scenarios 

Comment 3: Figures and tables can be more professionally presented.

Many thanks for the comment. We have re-done basically all the figures and we have improved also the design of the tables. 

Comment 4: The paper is technically well documented but unfortunately it fell flat in the discussion and conclusion section. What was found interesting and relevant to the issues and which stakeholders will benefit. What would be the way forward?

Many thanks for the comment. Unfortunately we don't agree with you and we hope you will find the paper interesting considering the changes made after the reviewers' comments. 

In this regard, we have modified also the conclusions (see section 4) specifying that the methodology presented implies that some sub-basins (SBs) may experience water deficits while appearing to have plentiful available water supplies, because the calculation of deficits accounts for only the water resources generated within the SBs. In fact, these SBs are dependent on water flowing in from upstream SBs, which makes them vulnerable to any changes in water management upstream. Consequently, while covering their needs, they will always have 100% water stress, which is to be considered correct considering their strong dependency on the water generated in SBs upstream with surplus.

The stakeholders who will benefit of this methodology are water managers and policy makers. 

Comment 5: The abstract was missing for the main paper and references are very limited and dated with not many references to good and impactful journals in the area of study. The authors should check the reference for format and completeness.

The abstract is available on the online platform, as requested. We have updated the references, however the subject is quite new, therefore there aren't many articles to be cited

 

Reviewer 3 Report

Comments and Suggestions for Authors

My decision: Reject

The manuscript introduced a plugin to estimate the level of water stress (SDG 6.4.2) by season and sub-basin. However, the manuscript lacks essential components such as an abstract and keywords. Its structural organization is improper, there is no data verification, and the descriptions of the methods and tools used are ambiguous. Solving these problems may make the article better:

Firstly, the manuscript lacks an abstract and keywords. Secondly, the manuscript should be discussed in separate chapters. Each formula in the manuscript and every character in each formula require a detailed explanation. In the Materials and Methods section, the methods and tools used should be added; for instance, on page 18, an explanation of what the WEAP - SRB model is must be provided. The format of the main text and some units in the manuscript are not standardized and need to be revised, and the aesthetics of the figures and tables should be further enhanced. The data source for the content in lines 155 and 156 needs to be specified. Regarding the results from the plugin calculation, it is necessary to prove the reliability of the obtained data by supplementing data verification. Also, the number of references is relatively small, so more new literature should be cited, and all citations must adhere to journal requirements.

Comments on the Quality of English Language

The writing of the paper needs improvement with the assistance of English editing.

Author Response

Replies in red:

Comment 1:Firstly, the manuscript lacks an abstract and keywords.

We have uploaded the Abstract and the keywords to the MDPI platform, as requested

 Comment 2: Secondly, the manuscript should be discussed in separate chapters. 

We have updated the structure of the paper. We have changed also the title.

Comment 3: Each formula in the manuscript and every character in each formula require a detailed explanation.

Each formula and each variable is properly defined in the paper. Unit measures are all defined as well.

Comment 4: In the Materials and Methods section, the methods and tools used should be added; for instance, on page 18, an explanation of what the WEAP - SRB model is must be provided. 

The paper includes a long and detailed section related to the methodology. About the tool, there is a section explaining the plugin that we have developed. We have defined better also what is WEAP and why we have decided to develop the Water Stress plugin in WEAP. Then we have dedicated a section specifically to the plugin and another section to the WEAP - SRB model

Comment 5: The format of the main text and some units in the manuscript are not standardized and need to be revised, and the aesthetics of the figures and tables should be further enhanced.

About the units, we have revised all the unit measures and now they are all defined. Please specify which units are not standardised and where they are mentioned. About the figures we have improved them, and also the tables have been re-styled. In general, we decided to give more importance to clarity than to aesthetics. 

Comment 6:  The data source for the content in lines 155 and 156 needs to be specified.

There is no need to specify the source of this data, as it is fictitious data, invented to explain with an example how to account for external renewable water resources (ERWR) in a sub-basin in order to avoid the double counting.

Comment 7: Regarding the results from the plugin calculation, it is necessary to prove the reliability of the obtained data by supplementing data verification. 

The SRB WEAP model was developed by OMVS and updated by FAO in collaboration with OMVS (Senegal River Basin Development Authority) to support the SDAGE 2050, Masterplan for the Senegal River Basin, developed to inform priority-setting and decision-making for water resource management in the basin. All the data have been provided and approved by OMVS which has worked with us on this model for one year. The results have been validated by OMVS and presented to the basin authorithy of Niger, Volta and Gambia rivers at a workshop held in Noukchott (25-27 February 2025) and organised by FAO and OMVS. We have clarified this in the paper. More info here https://www.fao.org/in-action/integrated-monitoring-initiative-sdg6/news-and-events/news/details/workshop--environmental-flows-and-disaggregation-of-the-level-of-water-stress-by-basin-and-sub-basin---experiences-from-senegal-river-basin/en  

Comment 8: Also, the number of references is relatively small, so more new literature should be cited, and all citations must adhere to journal requirements. 

Literature should be cited when it is available and when it is relevant to the topic, we agree. Unfortunately there aren't many articles on this topic, as it is quite new. However we have added some more references.

Comment 9: The writing of the paper needs improvement with the assistance of English editing.

The text has been fully reviewed by one of the  authors who is mother tongue. 

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

In the manuscript, there are some abrupt solid red lines. Please adjust it.

Author Response

Responses, here below in red

Comment 1: Literature review: All three reviewers get down on the poor literature review . Reviewer 2 says: “The review of the literature is mostly dated and does not clearly underlie the concern for the study”. The authors suggest “Many thanks for this comment. Indeed there is no relevant literature on this subject.” What? I know of numerous articles off-hand on water scarcity, water stress, and basin management etc etc which they could have used to identify the/a problem. Also, there is UN documentation on the specific topic of the 6.4.2 indicator and even published literature on WEAP. The authors somehow insist that this is a new topic and that there is no literature. I ask: How can they be so sure if they have not done a literature review? It is tempting to ask them to actually do one, but that would of course change the whole paper. I’d settle for them to update slightly and – needless to say: ensure that the formatting of the reference list is correct and up to journal standards. (as it stands now, I’d say it is a bit ‘untidy’)

Response: Many thanks for this comment. Indeed there are many articles on the water stress but no relevant literature is available on the double counting of the freshwater resources which is the main focus of this methodology. In the introduction we refer to the water accounting methodology (reference 30) commonly used at basin level, where actually the double counting issue, in our opinion, is not solved. However, we agree that we should have included more references in the manuscript, and so we have added some to justify the need of improving the calculation of SDG 6.4.2, in particular the need of disaggregating the indicator spatially and temporally (see reference 33, 34).

Comment 2: Abstract. As requested by the reviewers, they added an abstract. Ok, solved, but the abstract is quite long and wordy and misses any conclusions. I would suggest they revise the abstract as follows:

a. Rows 11-20 are fine, but rows 21-28 are superfluous (details not suitable for an abstract). Suggestion: Delete rows 21-28. 

Response: Ok. We have kept part of the rows, as we consider them needed for clarity

b. The substance of rows 29-39 should be abridged. Important to keep the mention about the WEAP plug-in (which is a key part of the paper, but only mentioned the next time on page 18 onwards). Keep the reference to the Senegal basin, which is indeed part of the paper, but do drop the reference to Tunisia, Algeria, Rwanda and Peru in the abstract! This testing of WEAP is totally irrelevant for the abstract since not part of the paper substance. (fine that now after revision it says in the body of the paper that the WEAP plug-in has been tested in other countries. Fine, but it should be kept there.)

Response: Many thanks. Done

c. Some conclusion: is this going to make water stress information available to water managers and decision makers? Or is the sub-basin plug-in suggested to be part of the global indicator 6.4.2 monitoring? Also, see point about conclusion below.

Response: Many thanks. Done

Comment 3. Introduction

a. The paper/intro starts with: “The inevitable rise in demand for water to meet the growing population…” – which is fine. Most people do see the rise in population as axiomatically linked to growing water demands. However, if these FAO colleagues working on indicator 6.4.2 (reported on in collaboration with UN-Water: Progress on Level of Water Stress – 2024 Update | UN-Water), would have looked at the results of colleagues working on indicator 6.4.1 (Progress on Change in Water-Use Efficiency – 2024 Update | UN-Water), they would have been aware that “Water-use efficiency rose from 17.4 USD/m3 in 2015 to 20.8 USD/m3 in 2021 worldwide, representing a 19.3 percent efficiency increase. Globally, less water is needed to generate economic output than back in 2015…” – hence, no INEVITABLE rise. In this context, this is an unfortunate opening of the paper.

Response: Many thanks for this comment, we have updated the sentence following your suggestion.

b. Then, the definition of “water scarcity” is really odd. Even wrong! This illustrates the need for a literature review to frame the topic. A paper like this should (adequately) define terms like water scarcity, water stress, water shortage, and why not aridity (which is indirectly discussed by way of the areas pointed to as water stressed). Then water stress and water scarcity is used interchangeably, it seems. Good to clarify.

Response: Many thanks. We have clarified the concept of water scarcity, according to the quoted FAO publication. About water stress, it is defined by the SDG indicator 6.4.2. We think that water stress helps understanding the water scarcity, but the two concepts are different and are not used interchangeably in this paper, which is focused on water stress. Finally water shortage and aridity have not been defined as they are not the focus of this publication, and we believe they could even be misleading.

c. The sentence on lines 49-50 - “Water scarcity, defined as an “imbalance of supply and demand” [8] is a global problem which can affect water security even in countries with ample water resources [1].” - has more problems: Is water scarcity a global problem? – like climate change? But the whole paper is about the point that you can have water scarcity/stress in one part of the basin while not in other parts, and at different times. Hence, IN THIS CONTEXT, it seems odd to suggest that water scarcity (however defined) is a GLOBAL problem. (The abstract also suggests that “… at the global level water stress does not seem to present a serious threat… Consistency needed.

Response: Ok. We have adjusted the sentence according to your comment.

 d. To satisfy the reviewers (and guest editor), a brief mention of the WEAP plug-in could be made also in the last bit of the intro.

Response: Many thanks. We have added a mention of the WEAP plugin also in the Intro.

4. Conclusion: Reviewer 2 suggested: “The paper is technically well documented but unfortunately it fell flat in the discussion and conclusion section. What was found interesting and relevant to the issues and which stakeholders will benefit. What would be the way forward?” Again, the authors challenge this comment, but suggest e.g. that “The stakeholders who will benefit of this methodology are water managers and policy makers.” I’d say. Great! – how about an inclusion in the final Conclusions section about HOW water managers and policy makers will benefit of this methodology  (I wonder, will they need to get the WEAP software and start doing the scenarios themselves? Or is there a suggestion that the sub-basin level is to become part of the UN exercise of 6.4.2 monitoring?). So, just a mention about the potential use of this new methodology would be good to have in the concluding section.

Response: Many thanks for this comment. We have clarified the potential use of this methodology in the conclusions.