Influence of Cascade Hydropower Development on Water Quality in the Middle Jinsha River on the Upper Reach of the Yangtze River

Round 1

Reviewer 1 Report

Comments for author File: Comments.pdf

Author Response

Dear Editors and Reviewers:

Thank you very much for your letter and reviewers’ comments concerning our manuscript entitled “Influence of cascade hydropower development on water quality in the middle Jinsha River on the upper reach of the Yangtze River” (ID: water-1749293). We appreciate your efforts and valuable comments very much and that are very helpful for revising and improving our paper. We have considered the comments carefully and revised our ms according your comments and suggestions as following.

Reviewer #1:

1. Comment: The comparative table shall be prepared on the status of various such projects reported in the literature, and also current work. A table could be as follows:

It can be part of the discussion section. Authors can modify the table format if it is needed.

Response: We added Table 6 that includes study area, water quality indicators, change of water quality, authors and publication year.

2. Comment: It is mentioned in the research outcome that the maximum concentration of CODMn and NH3-N are 3.5 mg/L and 0.44 mg/L, respectively while the standard limit of CODMn and NH3-N are 4 mg/L and 0.5 mg/L. So, there is a risk that the above two indicators will exceed the standard values in the future. What are your very specific suggestions to the concerned authority to either minimize or retain the same level of quality in the reach?

Response: We modified Subsection “4.4 Suggestions on water quality protection measures” and suggest that both local governments and operating divisions of hydropower stations should pay special attention to the water quality monitoring in the middle reach of the Jinsha River, as the COD, TN and TP contents in some reservoirs are already show accumulation.

3. Comment: It is mentioned in the study that “The air temperatures are higher from May to November and lower from December to April of the following year, so the concentrations of pollutants are affected by the air temperatures.” How is that air temperature affects the concentration of pollutants? Is it directly or indirectly? Authors shall properly interpret this aspect before writing such a statement. Or authors can support this statement through the results of other studies.

Response: we agree the Reviewer’s comment and these sentences were deleted.

4. Comment: Authors can add the stepwise procedure adopted for the regression discontinuity method for the present case study. This will provide better readability of the manuscript.

Response: We added the stepwise procedure adopted for the regression discontinuity method for the present case study.

5. Comment: How was the cut-off value being set in the present study for transforming the pretest?

Response: In subsection “Methods and Models”, we added two sentences to explain the cut-off value. t is the running variable and t₀ is the time of the impoundment of the power station. When t<t₀, D_t is 0, indicating that the power station does not store water; When t≥t₀, D_t equals to 1, indicating that the power station has stored water.

6. Comment: The data from 2004 to 2019 might have included before construction, after construction, and during construction. How did you handle the data belonging during construction?

Response: We modified the subsection “Methods and Models”. We think that the impact of hydropower development on the hydrological and hydraulic conditions of the river starts from the impoundment of the hydropower station. The data during construction belongs to one before hydropower development.

7. Comment: It is found that most of the referred papers belong to China. What about such a study at the international level? What are their observations? How is it vary with your study area?

Response: In introduction section, more references on the international level have been added. In discussion section, we added Table 6. Due to different geographical location, climate factors and so on, there are obvious differences in different watersheds.

Thank you very much again and your efforts are appreciated very much!

With the best wishes!

Author Response File: Author Response.pdf

Reviewer 2 Report

The paper „Influence of cascade hydropower development on water quality in the middle Jinsha River on the upper reach of the Yangtze River” could be interesting for many scientists who are working with environmental impact assessment of activities of hydropower plants or cascades of HPPs. The paper is interesting, the discussion part is described clearly and the conclusions are in line with the results of the work. Therefore, the main question for the authors is related to the used data of water quality parameters. It is not clear what time series were used for analysis before and after the construction of HPPs. As I understand – there were used the time series of different lengths comparing the periods before and after the construction of HPP. Are the all results of statistical analysis reliable? More detailed comments are below.

My evaluation is “Publication after major changes”. I have some remarks for improvements of the paper quality:

11.     The references are not cited in the order. For example: [2-5], [41-44] … (Page 2).

22.     Page 2. The table of abbreviations could be moved to the end of the paper.

33.   Numerations of subsections in Chapter 2 and Chapter 3 could be checked.

44.    Lines 141-144. This sentence is not clear.

55.     Subsection “Water Quality Indicators and Data Sources”. It is not clear what time series were used for analysis before and after the construction of HPPs. I recommend that the authors create a new Table that could be: the list of HPPs, duration (in years) of water quality measurements before and after HPPs construction, and quantities of water quality measurements before and after HPPs construction.

66.   Subsection “Methods and Models”. I ask the authors to explain the nature and unit of parameter “hydropower development” (Line 159).

77.     What is the term “the treatment variable” (Line 167)? What are the nature and unit of this parameter?

88.    D_t is described in formula (2). The authors used parameter D in the section “Results”. I ask the authors to describe parameter D in the section “Methodology”.

99.    Subsection “Monitoring Results of Water Quality Indicators”. There are comparisons of water quality parameters before and after HPPs development. Are the lengths of two time series (before and after HPPs) the same? Are the differences in water quality parameters (before and after HPPs) significant or not significant? For example (Figure 2): is it enough data for analysis of Guanyinyan HPP before construction (only 4 measurements)? The same situation in Figure 3 – could the authors compare the time series of different lengths?

110.  Subsection “Seasonal Variations in Water Quality Indicators”. What are the reasons for different patterns of monthly changes in pollutant concentrations for different HPPs (Figure 4)?

111.  Table 4. The author could add the denotation of “relationship” – is it a correlation coefficient?

112.  Figure 5. What data are used for this Figure - from all HPPs or from concrete HPP?

113.  Table 5. Why was chosen the air temperature as a covariate? There are many other important parameters as water temperature …

114.  Figure 6. Are the flow velocities equal to “0” in all reservoir areas?

115.  Conclusion (1). Is the analysis of these concentrations before the development of HPPs?

116.  Lines 477-478. This sentence is not clear.

Author Response

Dear Editors and Reviewers:

Thank you very much for your letter and reviewers’ comments concerning our manuscript entitled “Influence of cascade hydropower development on water quality in the middle Jinsha River on the upper reach of the Yangtze River” (ID: water-1749293). We appreciate your efforts and valuable comments very much and that are very helpful for revising and improving our paper. We have considered the comments carefully and revised our ms according your comments and suggestions as following.

Reviewer #2:

1. Comment: The references are not cited in the order. For example: [2-5], [41-44] … (Page 2).

Response: Thank you very much and sorry for such errors! We have adjusted the references in order.

2. Comment: Page 2. The table of abbreviations could be moved to the end of the paper.

Response: Thank you very much for your kindly suggestion. The table of abbreviations was moved to the end of the paper.

3. Comment: Numerations of subsections in Chapter 2 and Chapter 3 could be checked.

Response: Thank you very much! The numerations of subsections in Chapter 2 and Chapter 3 have been modified accordingly.

4. Comment:  Lines 141-144. This sentence is not clear.

Response: Thanks a lot. They have been modified accordingly.

5. Comment: Subsection “Water Quality Indicators and Data Sources”. It is not clear what time series were used for analysis before and after the construction of HPPs. I recommend that the authors create a new Table that could be: the list of HPPs, duration (in years) of water quality measurements before and after HPPs construction, and quantities of water quality measurements before and after HPPs construction.

Response: Thanks! In Subsection “Water Quality Indicators and Data Sources”, we created Table 2 that is overview of water quality monitoring data.

6. Comment: Subsection “Methods and Models”. I ask the authors to explain the nature and unit of parameter “hydropower development” (Line 159).

Response: Thanks! We added some sentences to explain the nature and unit of parameter “hydropower development”in Subsection “Methods and Models”.

7. Comment: What is the term “the treatment variable” (Line 167)? What are the nature and unit of this parameter?

Response: Thanks a lot! We added some sentences to explain the nature and unit of parameter “hydropower development”. Hydropower development (Dt) is the treatment variable.

8. Comment: D_tis described in formula (2). The authors used parameter D in the section “Results”. I ask the authors to describe parameter D in the section “Methodology”.

Response: Thank you very much for your comments. We modified Table 4. In this study, D_t and D is all the treatment variable (hydropower development).

9. Comment: Subsection “Monitoring Results of Water Quality Indicators”. There are comparisons of water quality parameters before and after HPPs development. Are the lengths of two time series (before and after HPPs) the same? Are the differences in water quality parameters (before and after HPPs) significant or not significant? For example (Figure 2): is it enough data for analysis of Guanyinyan HPP before construction (only 4 measurements)? The same situation in Figure 3 – could the authors compare the time series of different lengths?

Response: The lengths of two time series (before and after HPPs) are not same. We created Table 2 that show more details of data. According to Table 3, the differences in water quality parameters (before and after HPPs) are significant. In this study, we only collected 5 measurements of Guanyinyan, but we have 108 measurements of other plants in the middle Jinsha River, which could help us to analyze the water quality trend.

10. Comment: Subsection “Seasonal Variations in Water Quality Indicators”. What are the reasons for different patterns of monthly changes in pollutant concentrations for different HPPs (Figure 4)?

Response: We added a paragraph to explain different patterns of monthly changes in pollutant concentrations for different HPPs. We think that Residual pollutants in submerged cultivated land and forests caused the seasonal variation of COD_Mn and TP concentrations is not obvious at Ludila and Longkaikou.

11. Comment: Table 4. The author could add the denotation of “relationship” – is it a correlation coefficient?

Response: It is the correlation coefficient. We replaced the word “relationship” with the word” correlation coefficient”.

12. Comment: Figure 5. What data are used for this Figure - from all HPPs or from concrete HPP?

Response: In Figure 5, we used all data of six hydropower stations.

13. Comment: Table 5. Why was chosen the air temperature as a covariate? There are many other important parameters as water temperature …

Response: In the Subsection “Methods and Models”, we give the reason. At higher water temperatures, most chemical reactions and bacteriological processes run faster, which would change the water self-clarification ability. The water temperature of research area is affected by hydropower development and air temperature. Because the effect of hydropower development is reflected in the Dt variable, air temperature is selected as a covariate in the regression discontinuity model used in this study, which also reflects seasonality.

14. Comment: Figure 6. Are the flow velocities equal to “0” in all reservoir areas?

Response: We added two sentences to explain. The flow velocities are 0.03 m/s to 0.08 m/s after cascade hydropower development.

15. Comment: Conclusion (1). Is the analysis of these concentrations before the development of HPPs?

Response: Conclusion (1) includes the data of the water quality before and after cascade hydropower development.

16. Comment: Lines 477-478. This sentence is not clear.

Response: Thank you very much for your kindly comment. We have modified this sentence.

Thank you very much again and your efforts are appreciated very much!

With the best wishes!

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Thank you for corrections according to my remarks.

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

This paper demonstrates a statistical analysis to analyze the effect of the development of hydropower infrastructures on water quality. The method used in this research is interesting, but I don’t think the overall quality of this paper is publishable at this stage, given the weakness in method, analysis, and writing. Please see the detailed comments below.

1. The study area description should not be put in the Introduction. Please start a new subsection in your Section 2 and put your second paragraph in Introduction along with Figure 1 and Table 1 there.
2. Please provide more literature reviews regarding the application of regression discontinuity analysis in this research field.
3. Please justify why COD_Mn, NH3-N and TP are selected as indicators. Introduce more about their harm to environment.
4. Please provide more information about your data, such as the collection frequency of three indicators, temporal resolution of temperature data.
5. Please rewrite your Methods section. Your current description is long, vague, and redundant. Try using formulas to structurize your methods.
6. It’s improper to say water quality is affected by air temperature. Temperature could affect chemical solubility for sure but if you want to bring in temperature, you should provide more information that how temperature affect the three chemicals. Isn’t water temperature a more straightforward factor than air temperature in affecting the three indices? Why choose air temperature?
7. Replot Figure 3. Put the same indices before and after impoundment in the same figure. Use boxplot instead of line plot.
8. From my understanding, the treatment variables were set as 1 after impoundment and 0 before impoundment when doing correlation analysis. I don’t think simply assigning 1 and 0 to form a time series is statistically correct for correlation analysis.
9. Figure 4, are the curves fitted for dots? Show more details about how they were fitted.
10. I don’t see a marked increase for CODMn after impoundment in Figure 4, Line 251.
11. Section 3.4 is just a description of raw data without statical analysis. This should be moved before section 3.2.
12. From Figure 5, there is an obvious seasonality in some sites. How does this influence regression discontinuity analysis?
13. Figure 6, since you have water level and flow velocity data, why don’t try including them as covariates in regression discontinuity analysis to see its influence?
14. Please add a plot showing comparison between your observed concentration variable and modeled using regression discontinuity.
15. What’s the advantage of your method comparing with other commonly used methods, such as Mann-Kendall test, in addressing this specific problem?
16. The English writing is poor. Please improve your language.

Reviewer 2 Report

This is an interesting study. Nevertheless, it needs some further improvements. In general, there are still some occasional grammar errors throughout the manuscript, especially the article "the," "a," and "an" is missing in many places; please make a spellchecking in addition to these minor issues. The reviewer has listed some specific comments that might help the authors further enhance the manuscript's quality.

1. Specific Comments

• A list of acronyms is needed

Introduction

• The objectives should be more explicitly stated.
• Please elaborate on the introduction section regarding how hydropower impact the aquatic ecosystem and importance of the the e-flows. In this regard, the following literature may be helpful to <<Influence of hydrologically based environmental flow methods on flow alteration and energy production in a run-of-river hydropower plant>>, << Ecological impacts of run-of-river hydropower plants—Current status and future prospects on the brink of energy transition>>, << Influence of meteorological drought on environmental flows in mountain catchments>> , and << Water-energy-ecosystem nexus: Balancing competing interests at a run-of-river hydropower plant coupling a hydrologic–ecohydraulic approach>> you may consider additional references as well.
• What is the novelty of this work?
• Please move all figures and tables to another section perphaps called ‘’study cases’’

• Methods
• The methodology limitation should be mentioned.
• All variables should be explained.

• Results
• This section is well written.
• Please improve the quality of all figurës.

• Discussion

• Overall, the discussion part is week. The Discussion should summarize the manuscript's main finding(s) in the context of the broader scientific literature and address any limitations of the study or results that conflict with other published work.