Integrating Eco-Index and Hydropower Optimization for Cascade Reservoir Operations in the Lancang–Mekong River Basin

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript integrated Eco-Index (EI) and Hydropower Optimization for Cascade Reservoir Operations in the Lancang-Mekong Basin,even key parameters were selected and a multi-objective optimization model was developed that explicitly integrates the EI as an ecological constraint, enabling a balanced trade-off between energy production and ecosystem health. It is interesting and important.however it needs major revision.

 

Suggestions:

1. The EI construction relies solely on statistical correlations among hydrological indicators, lacking validation through biological or ecological field data.What’s fish? Incorporate field observations (e.g., fish migration, riparian vegetation) to ensure the EI reflects real ecosystem responses to validate the EI with ecological data:
2. Weight assignment in the EI is based solely on PCA contribution rates, without sensitivity or uncertainty analysis.Pls evaluate how different weighting schemes affect optimization outcomes to strengthen robustness.
3. important factors such as sediment transport, water quality, and temperature are not considered.pls expand ecological objectives,Include additional factors like sediment, water quality, and thermal regimes to better represent ecosystem health.
4. The vulnerability (Vul) formula in the RRV framework is ambiguously defined (e.g., usage of the Heaviside function), affecting interpretability.
5. Where should be where after equations
6. Quantify improvements by comparing optimized results with current reservoir operation strategies.

Author Response

Responses to Comments of Reviewer 1

We sincerely appreciate the reviewer’s thoughtful and constructive comments on our manuscript. We have addressed each point raised, and where appropriate, revised the manuscript accordingly to clarify and strengthen our work. Our responses to the reviewer’s comments are provided below.

 

Comment 1: The EI construction relies solely on statistical correlations among hydrological indicators, lacking validation through biological or ecological field data. What’s fish? Incorporate field observations (e.g., fish migration, riparian vegetation) to ensure the EI reflects real ecosystem responses to validate the EI with ecological data:

Response: We thank the reviewer for this valuable comment. While direct field validation of EI with local biological data was limited in our study due to data availability and the systems optimization nature of our study, we note the following:

• Indeed, the ecological relevance of the EI relies on the underlying indicators of hydrologic alteration (IHA), which have been widely recognized to reflect riverine ecosystem responses.

Ecological interpretation of IHA metrics: In the study The changing hydrology of a dammed Amazon [1], the authors explicitly explain the ecological implications of each IHA group, showing how metrics such as high-flow magnitude, low-flow duration, and frequency of flow reversals correspond to key ecological processes like fish migration, spawning, and habitat connectivity.

• Empirical evidence linking IHA and fish communities:

Ziegeweid et al. [2] demonstrated that hydrologic metrics including flow magnitude, variability, and rate-of-change are significantly linearly related to fish community biological metrics, with tolerance-based metrics showing the strongest associations. A small set of hydrologic variables can explain over 70% of the variation in fish community responses.

 

Sinnathamby et al. [3] analyzed 50 years of streamflow data at 34 streamgages in the Kansas River Basin and found that changes in IHA metrics were significantly negatively correlated with the presence and distribution of two native fish species (Plains Minnow and Common Shiner).

 

These studies from different river systems worldwide provide strong evidence that IHA metrics are closely associated with fish community health and ecological integrity.

• Justification for EI:

The EI in our study is derived from IHA metrics through principal component analysis to select representative indicators. Consequently, EI retains the ecological relevance of IHA metrics. In the Lancang–Mekong River Basin, which supports numerous migratory fish species, EI therefore reflects key aspects of fish habitat and ecosystem health, consistent with its derivation from ecologically meaningful IHA variables.

 

In summary, although EI is a simplified and aggregated index, it is grounded in IHA metrics that have been empirically shown to respond to fish community dynamics and other ecological conditions, ensuring that EI provides a valid representation of riverine ecosystem health.

 

Comment 2: Weight assignment in the EI is based solely on PCA contribution rates, without sensitivity or uncertainty analysis. Pls evaluate how different weighting schemes affect optimization outcomes to strengthen robustness.

Response: We thank the reviewer for this detailed comment. We would like to clarify that in our study, the weights of the ecological indicators (EI) were not assigned subjectively, but were objectively derived using Principal Component Analysis (PCA). Specifically, PCA transforms correlated hydrological indicators into a set of uncorrelated principal components, with the loadings determined by the eigenvectors of the covariance matrix and the importance of each component measured by its eigenvalue. The variance contribution of each principal component reflects its explanatory power for the overall variability and is used to determine the weights of the EI. Therefore, the weights in the EI represent the objective contribution of each indicator to the total information, rather than arbitrary assignment. We also acknowledge the reviewer’s concern that optimization results may be affected by the choice of weighting method; however, PCA-based weights were adopted in this study to ensure both objectivity and methodological simplicity.

 

Comment 3: important factors such as sediment transport, water quality, and temperature are not considered. pls expand ecological objectives, Include additional factors like sediment, water quality, and thermal regimes to better represent ecosystem health.

Response: Thank you for your valuable comment. Indeed, due to data limitations, sediment transport, water quality, and thermal regimes were not directly quantified in this study. However, the IHA metrics used here are closely related to these environmental factors and can serve as important references for ecological assessment. We have added a discussion on the role of IHA metrics in evaluating ecological conditions in Lines 231-242, highlighting their significance.

 

Specifically:

Sediment transport: Binh et al. [4] reported that dam operations in the Lancang–Mekong River Basin altered IHA metrics, characterized by reduced peak flows and increased low flows. This weakened sediment transport during flood pulses and exacerbated downstream channel incision and delta erosion, demonstrating a close coupling between hydrologic alterations and sediment dynamics.

Biotic integrity: DeGasperi et al. [5] demonstrated that in rapidly urbanizing basins of the Puget Lowland, hydrologic metrics such as High Pulse Count and High Pulse Range are strongly correlated with benthic macroinvertebrate integrity. Urbanization increased the frequency of high pulses in both winter and summer, highlighting the importance of these metrics for ecological management and stormwater control.

Water temperature: Vishwakarma et al.[6] examined the Yangtze River at Cuntan, where dam-induced changes in flow magnitude, timing, and frequency substantially altered downstream water temperature, with potential consequences for aquatic ecosystems and water resource management.

In summary, although sediment transport, water quality, and temperature were not directly considered in this study, the analysis based on IHA metrics can indirectly reflect changes in these factors and provide valuable references for evaluating ecological objectives.

 

Comment 4: The vulnerability (Vul) formula in the RRV framework is ambiguously defined (e.g., usage of the Heaviside function), affecting interpretability.

Response: Thank you for the valuable comment. To clarify the definition of Vulnerability (Vul), we have reorganized Equation (11) in the revised manuscript and added Equation (12) to explicitly illustrate the role of the Heaviside function, thereby providing a more precise and interpretable representation of the Vul calculation. This revision ensures that Vul clearly reflects the average magnitude of deviation from the ecological flow target during failure periods.

 

Comment 5: Where should be where after equations

Response: Thank you for your suggestion. We have standardized all occurrences of “where” following equations to “Where.”

 

Comment: 6: Quantify improvements by comparing optimized results with current reservoir operation strategies.

Response: Because the inflow to the reservoirs is fixed, the increase in hydropower generation due to optimization is negligible. The main improvement lies in the ecological performance of the cascaded reservoir operation. According to Li et al.[7], the long-term average EI resulted from operations of the Jinghong reservoir alone is 0.36, whereas in our study the long-term average EI under cascaded reservoir operation is 0.29, representing a reduction of approximately 19%. This indicates that cascaded reservoir operation has a smaller ecological impact and better ecological performance compared with single-reservoir operation. It is noteworthy that single-reservoir operation was analyzed in the cited literature due to the lack of actual reservoir operation data.

 

 

References

[1] Timpe K , Kaplan D .The changing hydrology of a dammed Amazon. Sci. Adv. 2017, 3(11):e1700611.DOI:10.1126/sciadv.1700611.

[2]   Ziegeweid, J. R., Johnson, G. D., Krall, A. L., Fitzpatrick, K., & Levin, S. B. Quantifying relations between altered hydrology and fish community responses for streams in Minnesota. Ecol. Process. 2022,11(1), 1-25. https://doi.org/10.1186/s13717-022-00383-z

[3]   Sinnathamby, S., Douglas-Mankin, K. R., Muche, M. E., Hutchinson, S. L., & Anandhi, A.. Ecohydrological index, native fish, and climate trends and relationships in the Kansas River basin. Ecohydrology, 2017, 11(1), e1909. https://doi.org/10.1002/eco.1909

[4]   Binh, D. V., Kantoush, S. A., Saber, M., Mai, N. P., Maskey, S., Phong, D. T., & Sumi, T. Long-term alterations of flow regimes of the Mekong River and adaptation strategies for the Vietnamese Mekong Delta. J. Hydrol. Reg. Stud.,  2020, 32, 100742. https://doi.org/10.1016/j.ejrh.2020.100742

[5]   Degasperi CL, Berge HB, Whiting KR, Burkey JJ, Cassin JL, Fuerstenberg RR. Linking Hydrologic Alteration to Biological Impairment in Urbanizing Streams of the Puget Lowland, Washington, USA. J. Am. Water Resour. Assoc., 2009, Apr;45(2):512-533. doi: 10.1111/j.1752-1688.2009.00306.x.

[6]   Vishwakarma, D.K., Ali, R., Bhat, S.A. et al. Pre- and post-dam river water temperature alteration prediction using advanced machine learning models. Environ. Sci. Pollut. 2022, 29, 83321–83346. https://doi.org/10.1007/s11356-022-21596-x

[7]   Li, D., Wan, W., & Zhao, J. (2018). Optimizing environmental flow operations based on explicit quantification of IHA parameters. J. Hydrol., 2018, 563, 510-522. https://doi.org/10.1016/j.jhydrol.2018.06.031

 

 

 

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

1. Please include the significant quantitative findings in the abstract.
2. Standardized Runoff Index was mentioned; more elaboration/ explanation is expected.
3. For the collected data, is there any process/ procedure to deal with missing data?
4. Can further discuss how climate projections can enhance the model's applicability?

Author Response

Responses to Comments of Reviewer 2

 

Comment 1: Please include the significant quantitative findings in the abstract.

Response: Thank you for your valuable suggestion. In the revised manuscript, we have updated the abstract to include key quantitative results, enabling readers to better understand the performance of the proposed model and its main findings.

 

Comment 2: Standardized Runoff Index was mentioned; more elaboration/ explanation is expected.

Response: Thank you for your valuable suggestion. We also recognized that the explanation of the Standardized Runoff Index (SRI) in the original manuscript was insufficient. In the revised version, we have carefully re-edited the relevant content in Section 4.2 and added Equations (15) and (16) to provide a more detailed and clearer explanation of the definition and calculation process of SRI, thereby enabling readers to better understand its application in this study.

 

Comment 3: For the collected data, is there any process/ procedure to deal with missing data?

Response: Thank you for your comment. The runoff data in this study were obtained from the Tsinghua Representative Elementary Watershed (THREW) model developed by Tsinghua University. Since the model generates continuous runoff series for all required hydrological sections, there are no missing data issues in our dataset. Moreover, the THREW model has been widely applied and validated in hydrological research, ensuring the reliability of the simulated runoff, such as Lancang-Mekong River[1] and Yarlung Tsangpo River[2].

 

Comment 4: Can further discuss how climate projections can enhance the model's applicability?

Response: We sincerely appreciate the reviewer’s valuable comment. We fully agree that incorporating climate scenarios is important for enhancing the model’s applicability. However, this study focuses on the coupling of historical runoff and multi-objective reservoir operation optimization, and currently, there is a lack of sufficiently consistent regional climate scenario data. Previous studies indicate that future climate change may increase runoff in the Mekong River Basin and alter its seasonal distribution, thereby increasing flood and drought risks[3] and intra-annual variability[4]. Based on the current HP–EI model analysis, this suggests that future hydropower benefits and ecological indicators (EI) could both rise, requiring more careful balancing of the HP–EI trade-off. Nevertheless, these are only rough estimates, and more precise quantification would require dedicated climate–hydrological modeling. Notably, the framework established in this study is highly extensible, allowing future integration with regional climate models (RCMs) or CMIP6 scenario data to systematically assess the potential impacts of climate change on runoff, environmental flows, and multi-objective trade-offs.

References

[1]      K. Zhang, K. Morovati, F. Tian, L. Yu, B. Liu, and M. A. Olivares, Regional contributions of climate change and human activities to altered flow of the Lancang-mekong river, J. Hydrol. Reg. Stud., 2023,vol. 50, no. October, https://doi.org/10.1016/j.ejrh.2023.101535

[2]      M. Zhang, Y. Nan, and F. Tian, Runoff component quantification and future streamflow projection in a large mountainous basin based on a multidata-constrained cryospheric – hydrological model, 2025. pp. 1033–1060, https://doi.org/10.5194/hess-29-1033-2025

[3]      C. Wang,S. Leisz, L. Li et al., Historical and projected future runoff over the Mekong River basin, Earth Syst. Dynam., 2024. vol. 1, pp. 75–90, https://doi.org/10.5194/esd-15-75-2024

[4]      and S. J. Lee, Daeeop, Giha Lee, Seongwon Kim, Future Runoff Analysis in the Mekong River Basin under a Climate Change Scenario Using Deep Learning, Water (Switzerland), 2020.pp. 1–19,

 

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Dear Authors

I read with great interest your research report on an interesting and important aspect of water management within some ecologically viable framework incorporating multiparameter values from a diverse range of variables. I think this is a very good approach to deal with the issues like electricity generation through hydropower stations in a multiple network along a river that is impacted by extreme climate events on a decadal (or human lifespan) scale. ped, and the variables you considered are those that similar researchers deal with. Your data is robust, the method you applied is valid, and the overall research design fits well with the expected level for significant research. beside my positive observations, I find actually room to develop nearly in every part of the manuscript. The Introduction is good, but a bit territorial and not so exciting as to offer limited options to connect the site-dependent research here with other similar researcharch elsewhere. This problem propagates throughout he entire text and results in a complete lack of discussion points where the research is compared with, say, major river systems elsewhere, where the hydropower generation is significant. In this respect, the study could be compared with dams on Alpine environments or big flat alluvial regions, as well as in extreme arid to extreme humid base climate conditions. Without such comparative aspects, the manuscript remains fairly local or regional and unlikely to have a global impact. Also, I find that the manuscript is not an easy read. Too many acronyms, too many not fully explained in it. All those need to be rechecked and wherever you can provide a basic, often plain English explanation of their meaning. This is a valid request to explain the results. 

Also, the figure captions are too simple and do not help the reader get the message right. 

Overall, I think the manuscript requires major revision to be fully aligned with the journal Water expectations for publication.

Please note, an annotated PDF is provided with my comments.

Best regards,

Comments for author File: Comments.pdf

Author Response

We sincerely thank the reviewer for the careful reading of our manuscript and for the constructive comments and suggestions, which have been very helpful in improving the quality of our work. We carefully went through the annotated PDF and have revised the manuscript accordingly by addressing each comment point-by-point. Specifically, we have:

• Revised the Introduction to highlight broader comparative perspectives with other major river systems worldwide, beyond the Mekong Basin.
• Reduced the uses of acronyms and provided plain English explanations where appropriate.
• Expanded figure captions to ensure that the main message of each figure is clearly conveyed.

In addition, we have carefully modified the manuscript following all detailed annotations provided in the PDF and believe that these revisions have significantly improved the clarity, readability, and broader relevance of the paper.

 

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

according to the revised manuscript, it can be accepted.

Author Response

We sincerely thank the editor and reviewers for their time, effort, and valuable comments throughout the review process.   We greatly appreciate your consideration and support.

Reviewer 3 Report

Comments and Suggestions for Authors

Dear Authors,

Thank you for the detailed revision of your manuscript. The manuscript after the revision evolved significantly and now it is in the level to be ready for acceptance. I have no further questions or suggestions and have not provided extra files. I recommend the manuscript to be accepted as it is.

 

Best regards

Author Response

Dear Reviewer,

We sincerely appreciate your positive evaluation and kind recommendation for the acceptance of our manuscript. Thank you very much for your valuable comments and time devoted throughout the review process.

Best regards