Review Reports - Simulation Study on the Impact of South–North Water Transfer Central Line Recharge on the Water Environment of Bai River

Round 1

Reviewer 1 Report

Paper "Simulation study on the impact of South-North Water Transfer Central Line recharge on the water environment of Bai River" presents research related to the water management, in general. The paper also presents a real case study. Several suggestions will be emphasized.

- Keywords should not be identical words from the title of the paper. Changes are suggested.

- The results should not be commented on in the abstract. It should be concise. Changes are suggested.

- The Introduction is not adequately written. In addition to the presented details, the authors are suggested to add text and references related to the previous application of the mentioned methodology and similar researches.

- Within the chapter where the study area is described, an appendix related to the hydrogeological settings and description of aquifers is proposed. Since it is a hydrodynamic model, it is necessary to display the values of the hydraulic parameters of the porous medium.

- There are no references in the methodology section - the third chapter should contain only a description of the methodology, and not a presentation of the input parameters. On the other hand, why was a 2D model applied and not a 3D model?

- Figure 6 is very simplified and does not have much significance.

- It is not necessary to show general formulas on page 16. It is suggested that they be thrown out, because it is a discussion of the results. Possibly transfer them in the section where the methodology is described.

- Chapters 4 and 5 should be harmonized, because there are results in both chapters.

- The conclusion is in the form of a summary. It is necessary to write directly what is the scientific contribution of this research?

Author Response

Dear Editor and Reviewers,

We gratefully thank the editor and all reviewers for their time spend making their constructive remarks and useful suggestions, which has significantly raised the quality of the manuscript and has enable us to improve the manuscript. Each suggested revision and comment, brought forward by the reviewers was accurately incorporated and considered. Below the comments of the reviewers are response point by point and the revisions are indicated.

With best regards!

Yours sincerely

Your comments and my reply are as follows：

Reviewer 1#

1 Comment: Keywords should not be identical words from the title of the paper. Changes are suggested.

1 Reply: We have modified the keywords to words that are different from the title of the paper. (Lines39-40)

2 Comment: The results should not be commented on in the abstract. It should be concise. Changes are suggested.

2 Reply: In response to this comment, the last sentence of the abstract has been modified to achieve brevity. We have changed the phrase “This study provides valuable insights for policymakers and water managers, offering guidance for the integrated allocation of water resources to achieve sustainable water management in the Bai River basin.” to “The results of this study may provide theoretical and technical support for the future management of river water environments”. (Lines36-38)

3 Comment: The Introduction is not adequately written. In addition to the presented details, the authors are suggested to add text and references related to the previous application of the mentioned methodology and similar researches.

3 Reply: We have included in the introduction two recent papers and some text whose research is closely related to our approach and content. The two references are:

Chen, J. (2023). Dynamic and loss analysis of flood inundation in the floodplain area of the lower Yellow River considering ecological impact. Journal of Engineering and Applied Science, 70(1), 1-18.

Zuo, Q.N., Lin, D.C., & Wang, X.L. (2022). Study on the Dilution and Diffusion of Liquid Effluent from Coastal Nuclear Facility and Discharging Optimization Based on MIKE21 Simulation. Journal of Isotopes, 35(2), 75. (Lines94-101)

4 Comment: Within the chapter where the study area is described, an appendix related to the hydrogeological settings and description of aquifers is proposed. Since it is a hydrodynamic model, it is necessary to display the values of the hydraulic parameters of the porous medium.

4 Reply: Our study used MIKE21 to build a hydrodynamic model, mainly for the simulation of the water environment in surface water. values of the hydraulic parameters for groundwater related porous media are not available in the MIKE21 hydrodynamic module.

5 Comment: There are no references in the methodology section - the third chapter should contain only a description of the methodology, and not a presentation of the input parameters. On the other hand, why was a 2D model applied and not a 3D model?

5 Reply: We have cited two new references in the introductory method section of the third chapter for the formulae used to introduce the method.

The newly cited references are as follows：

Ahn, J., Na, Y., & Park, S. W. (2019). Development of two-dimensional inundation modelling process using MIKE21 model. KSCE Journal of Civil Engineering, 23, 3968-3977. (Line194)

Liu, J. C., Hu, Z. Q., & Zhu, M. Y. (2021). Comparison and analysis of calculation of Bridge backwater based on Mike21 hydrodynamic model. In E3S Web of Conferences (Vol. 233, p. 03043). EDP Sciences. (Line217)

We have transferred the third section 3.2.2 'Water quality assessment criteria and results' to the fourth section 4.2, leaving only the description of the methodology in the third section. (Line307)

Our insights as to why we used a 2D model instead of a 3D model for this study are as follows：The main difference between 2D and 3D models in the MIKE software is the dimensionality of the model. 2D models are limited to the horizontal plane, whereas 3D models can be simulated in both the horizontal and vertical planes. In this paper we are looking at surface water bodies. 2D models simulate the behaviour of water and other materials (e.g. pollutants, sediments) in the horizontal plane. 2D models are suitable for simulating water flow and transport processes that occur mainly in the horizontal direction, such as water quality modelling. Three-dimensional models are suitable for simulating complex hydrodynamic processes such as stratification, mixing and circulation in lakes and estuaries, and the interaction of surface and groundwater. We have therefore chosen a two-dimensional model for our study.

6 Comment: Figure 6 is very simplified and does not have much significance.

6 Reply: We have removed Figure 6.

7 Comment: It is not necessary to show general formulas on page 16. It is suggested that they be thrown out, because it is a discussion of the results. Possibly transfer them in the section where the methodology is described.

7 Reply: We have removed the generic formula on page 16 and revised the sentences to make them smooth. (Lines428-429)

8 Comment: Chapters 4 and 5 should be harmonized, because there are results in both chapters.

8 Reply: We have changed Chapter 4 'Model Validation and Results' to ' Model construction' and moved the 'Model Validation' section to Chapter 5.（Line260）（Line334）

9 Comment: The conclusion is in the form of a summary. It is necessary to write directly what is the scientific contribution of this research?

9 Reply: We have added the scientific contribution of this study in the third conclusion section.

“This study provides valuable insights for policymakers and water managers, offering guidance for the integrated allocation of water resources to achieve sustainable water management in the Bai River basin.”（Lines671-674）

Author Response File: Author Response.docx

Reviewer 2 Report

In this study, authors evaluate the current water quality of the Bai River, and simulates three recharge period scenarios on this basis constructs the Mike21 model, which reveal the mechanism and response law of ecological recharge on the water environment of the Bai River. This is a work of practical significance. The manuscript needs to be improved to be published in Water.

Abstract: What does Scenario1 stand for? It is suggested that the author should use the improvement effect of water quality in the optimal scenario, such a similar description. Meanwhile, what type of simulation software to choose should also be given.

Introduction：1. The author finally chooses the MIKE model, why to choose this software and what are the advantages compared with other simulation softwares? this part is not enough. It is suggested that combined with the literature, the author should increase the discussion on the feasibility or advantages of MIKE model. 2. The purpose of this paper is suggested to be listed in items at the end.

Study area overview: There are too many introductions to the the South-North Water Transfer Central Project, and it is recommended to focus on the research area - Bai River.

Model principles: It feels like a research report in this part. It is recommended to choose the key points to illustrate. In addition, in table1, such simple water quality index limit do not need to be placed, and it is recommended to delete them.

Conclusion: The suggestion is divided into three parts: 1) the change of spatial pattern of water quality in Bai River, the main pollutants and their possible sources; 2) the changes of water quality parameters, the accuracy of the model and possible deficiencies under the optimal scenario by MIKE model; 3) the future changes of water quality in Bai River and suggestions for water environment management.

Overall, I think the paper can be accepted after major revision.

Comments for author File: Comments.pdf

Author Response

Dear Editor and Reviewers,

With best regards!

Yours sincerely

Your comments and my reply are as follows：

Reviewer 2#

1 Comment: Abstract: What does Scenario1 stand for? It is suggested that the author should use the improvement effect of water quality in the optimal scenario, such a similar description. Meanwhile, what type of simulation software to choose should also be given.

1 Reply: We have replaced 'Scenario 1' in the abstract with a specific optimal scenario presentation to show what Scenario 1 represents. Scenario 1 represents the simulated conditions for the scenario with a recharge flow rate of 2Q and a recharge duration of 1/2T. We also give the choice of using the MIKE21 coupled hydrodynamic water quality model software for the simulation(Lines29-31)

2 Comment: Introduction：1. The author finally chooses the MIKE model, why to choose this software and what are the advantages compared with other simulation softwares? this part is not enough. It is suggested that combined with the literature, the author should increase the discussion on the feasibility or advantages of MIKE model. 2. The purpose of this paper is suggested to be listed in items at the end.

2 Reply: 1. We have added in the introduction why MIKE software was chosen for the simulation. (MIKE software has a wide range of applications and offers a wide range of tools for water resources and environmental modelling that can be used in a variety of applications. In addition to this, MIKE software has the advantage of high accuracy, flexibility and the ability to integrate with other software compared to other simulation software.) (Lines112-116)

In the final paragraph of the introduction we set out the aims of the study in the following three points.

（1）. To study the improvement of the water environment by ecological recharge in the Bai River region through modelling;

（2）. To simulate different recharge scenarios for a 35.46 km long section of the Bai River from the Upper Zhantou Barrage section to the Upper Fanying section in Nanyang using the MIKE21 coupled hydrodynamic-water quality model, and to study the changes in the hydrodynamic conditions and pollutant concentration fields in the Bai River under the three recharge scenarios.

（3）. To reveal the mechanisms and response patterns of ecological recharge The impact mechanisms and response patterns on the water environment of the Bai River. (Lines117-125)

3 Comment: Study area overview: There are too many introductions to the the South-North Water Transfer Central Project, and it is recommended to focus on the research area - Bai River.

3 Reply: We have removed some of the description of the South-North Water Transfer Central Project from the study area overview section and added a description of the White River in the study area.

The water system in which the Bai River is located is the Han River system, which is one of the main water systems in the Yangtze River basin and has an important historical status. The Bai River is located at the headwaters of the Han River and has a large overall slope drop. There are many hydrological stations on the main stream of the Bai River, including the basic water control station Xindianpu Hydrological Station, which has been responsible for the observation of water level, flow and sediment of the Bai River, providing a solid foundation for flood and drought prevention and rational development of water resources downstream. (Lines130-137)

4 Comment: Model principles: It feels like a research report in this part. It is recommended to choose the key points to illustrate. In addition, in table1, such simple water quality index limit do not need to be placed, and it is recommended to delete them.

4 Reply: We have re-enriched this section by re-citing some references. In addition, we have removed the original Table 1.(Line194 Line217)

5 Comment: Conclusion: The suggestion is divided into three parts: 1) the change of spatial pattern of water quality in Bai River, the main pollutants and their possible sources; 2) the changes of water quality parameters, the accuracy of the model and possible deficiencies under the optimal scenario by MIKE model; 3) the future changes of water quality in Bai River and suggestions for water environment management.

5 Reply: We have reworked the section on conclusions into the following three parts:

（1）Due to the lack of water resources during the dry season, the rivers have poor hydrodynamic conditions and the water quality does not meet the water quality objectives of the water function zones, so ecological replenishment needs to be implemented to improve the quality of the water environment of the rivers. Non-flood conditions tend to have better water quality conditions as they are less subject to confluence action. Water quality indicators show a spatial pattern of better upstream than downstream, with more dramatic changes in water quality near the outfall influenced by the discharge of sewage near the urban section of Nanyang.

（2）Based on the simulation results of the three scenarios during the recharge period, the hydrodynamic conditions of the three recharge scenarios are good, and Scenario 1 is the best of the three recharge scenarios, and the ecological recharge has a significant effect on the river water quality. According to the single factor evaluation method, all water quality indicators met the water quality objectives of the Bai River Water Function Area, and the river was upgraded from Grade IV water quality standards to Grade III water standards. Among them, the COD and TP indicators were raised to Grade II water standard, and the DO rate was the largest, 94.67%, among which the DO indicator had the best improvement effect, 94.67%, and the TP indicator had the best reduction effect, 66.67%, and the changes of each water quality indicator were in line with the law of natural evolution, and a good simulation effect was achieved. However, the model parameters such as eddy viscosity coefficient and Koch's force are set as constants. Future studies can consider setting these parameters as variables that can change from time to time to further improve the accuracy of the model. In addition, this study did not consider the confluence of many small tributaries of the Bai River, and the hydrological information of the study area is missing in some years.

（3）The study takes the Bai River as the research object, based on the current situation of water resources and water environment in the study area, constructs the Mike21 coupled hydrodynamic-water quality model for the study section of the Bai River, analyzes the evolution of river hydrodynamics and water quality under different scenarios, and obtains relatively good simulation results. This study provides valuable insights for policymakers and water managers, offering guidance for the integrated allocation of water resources to achieve sustainable water management in the Bai River basin.(Lines643-674)

Author Response File: Author Response.docx

Reviewer 3 Report

This paper presents the combination of a two-dimensional hydraulic model and a mass dispersion model for the purpose of predicting river water quality parameters due to improve in water quantity. The accuracy of the analysis using the model is high and its reproducibility has been confirmed, which is commendable. The discussion based on the analysis results is also valid and provides useful information. However, the scenarios are set up in a way that is obvious even without analyzing the model. The reasons for setting up the scenarios and what authors want to demonstrate in this study, and its future application should be clearly stated. In addition, there are some points that are unclear due to lack of explanation, so some additional information should be added.

Comments on the overall content are as follows:

1) Reasons for setting scenarios and their rationale need to be explained. Section 5.1 should be explained in the Methods or other chapter. The scenario is the setting for the subject of this paper.

It is obvious that doubling the hydration flow rate and halving the hydration duration will improve water quality without model analysis.

(2) Please explain the specific measures to realize Scenario 1.

3) Ecological recharge
Definition of “ecological recharge” and specific countermeasures should be explained. Scenario 1 sets the flow rate at twice the current level. Please explain what kind of countermeasures and improvements are needed to secure a flow rate that can improve water quality.

The individual points raised are as follows:

1) Line52
Karimi A et al. Not in the list of references.

2) Line 213-214.
Please briefly explain why both COD and CODMn, which are similar water quality parameters, are set and information on the difference between COD and CODMn.

3) Table 2
Table 2 lacks information on measured values. Information such as number of measurements, mean, range, and standard deviation should be explained.

4）Lines 319-320, 338-339
The literature or statistical validity should be provided to show the validity of the error range.

5）Line 365-368
Does direct uptake of nutrients in river water by animals occur?

Explanation based on literature on the effect of water temperature change on nutrient uptake by algae and microorganisms should be needed.

6) Fig. 16, 17, 18
The text in the table is too small to be deciphered.

The text needs to be clearly written. Water quality items should be clearly indicated.

The location of M1, M2, and M3 should also be indicated for easier understanding.

7) Table 6

Make significant figures consistent.

Author Response

Dear Editor and Reviewers,

With best regards!

Yours sincerely

Your comments and my reply are as follows：

Reviewer 3#

The changes to the overall content are as follows:

1 Comment: Reasons for setting scenarios and their rationale need to be explained. Section 5.1 should be explained in the Methods or other chapter. The scenario is the setting for the subject of this paper.

It is obvious that doubling the hydration flow rate and halving the hydration duration will improve water quality without model analysis.

1 Reply:

We have an explanation of why we set up the scenarios:

Scenario analysis, also known as scenario description, is a more intuitive approach to forecasting, where certain parameters are adjusted based on existing scenarios, or certain premises are assumed based on existing scenarios, to simulate and predict the possible outcomes of these hypothetical scenarios.(Lines440-443)

However, different recharge options will have different degrees of improvement in water quality. We have consulted the following literature and found that the best recharge option requires numerical simulations to accurately investigate the degree of improvement in ecological recharge in order to find the best recharge option.

Li, R., Wu, Z., Li, L., Cai, D., Wang, F., & Huang, L. (2017). Pollution load and ecological replenishment plan of Lijiang River, China. Environmental Engineering & Management Journal (EEMJ), 16(11).

ZHANG, L., et al. Simulation of multiple water source ecological replenishment for Chagan Lake based on coupled hydrodynamic and water quality models. Water Science and Technology: Water Supply, 2017, 17.6: 1774-1784.

Huang, J., Zhao, L., & Sun, S. (2021). Optimization model of the ecological water replenishment scheme for Boluo Lake National Nature Reserve based on interval two-stage stochastic programming. Water, 13(8), 1007.

2 Comment: Please explain the specific measures to realize Scenario 1.

2 Reply: The specific parameters for Scenario 1 are set as follows:

The Bai River Retreat Sluice was selected as the recharge inlet for the simulated scenario during the recharge period.
The total amount of ecological recharge in 2020 was chosen as the base condition, and this total amount of recharge was used to control the total amount of water in the simulation scenario.
The flow rate of ecological recharge to the Bai River from the South-North Water Transfer Central Project in Scenario 1 is 42~46m³/s.
The duration of the ecological recharge of the Bai River by the South-North Water Transfer Project in Scenario 1 is from 9 May to 31 May, a total of 22 days, as the standard duration of this simulation.

3 Comment: Ecological recharge
Definition of “ecological recharge” and specific countermeasures should be explained. Scenario 1 sets the flow rate at twice the current level. Please explain what kind of countermeasures and improvements are needed to secure a flow rate that can improve water quality.

3 Reply:Ecological recharge refers to the process by which natural ecosystems are replenished and restored with water, nutrients, and other essential elements needed for their survival and function. The term typically refers to the restoration of groundwater levels and surface water flows, which are crucial for maintaining healthy ecosystems and supporting human communities that rely on them.

Some specific responses that can be taken to promote ecological recharge include:

Implementing sustainable water management practices: This can involve promoting efficient water use, reducing pollution, and protecting and restoring wetlands, rivers, and other natural systems that help to recharge groundwater and surface water resources.

Encouraging the use of green infrastructure: This involves using natural systems such as forests, wetlands, and green roofs to manage stormwater runoff and promote water infiltration into the ground.

Water recharge by transport through projects such as the South-North Water Transfer.

In this paper, the ecological recharge scenario developed by the scenario analysis method uses the total volume control method and only considers the influence of two key factors, namely the recharge duration and the recharge flow rate. From the simulation results, under the condition that the total volume of water is recharged, the scenario with high flow rate recharge has better hydrodynamic conditions and has a significant effect on scouring the silt at the bottom of the river, which has a better effect on improving the water quality of the receiving river. There is still room for optimisation of the existing ecological recharge scheme. The geography, meteorology, hydrology, ecological recharge volume, recharge pathway and recharge method of the receiving area should be taken into account to optimise the existing recharge scheme, so that ecological recharge can bring more ecological, economic and social benefits.

The individual amendments are as follows:

1 Comment: Karimi A et al. Not in the list of references.

1 Reply: The text has been changed to Nikoo et al. (Line63)

2 Comment: Please briefly explain why both COD and COD_Mn, which are similar water quality parameters, are set and information on the difference between COD and COD_Mn.

2 Reply: COD (chemical oxygen demand) and COD_Mn (chemical oxygen demand with permanganate as the oxidising agent) are both water quality parameters used to measure the amount of organic pollutants in water. The two parameters are similar in that they both provide an estimate of the amount of oxygen required to oxidise organic matter in water. However, there are some differences between them.

COD measures the amount of oxygen required to chemically oxidise all organic and inorganic compounds in water, including biodegradable and non-biodegradable compounds. COD is a more comprehensive and faster test, but it does not differentiate between different types of organics and some non-biodegradable compounds may contribute to the total COD value.

COD_Mn, on the other hand, measures the amount of oxygen required to oxidise only the biodegradable organic matter in the water, using potassium permanganate as the oxidising agent. This test is more specific and provides an estimate of the biodegradable organic matter in the water. COD_Mn is often used in conjunction with COD to provide a more complete picture of the organic matter present in the water.

Both COD and COD_Mn are important water quality parameters used to monitor and manage wastewater treatment processes, as well as ensuring compliance with environmental regulations.

3 Comment: Table 2 lacks information on measured values. Information such as number of measurements, mean, range, and standard deviation should be explained.

3 Reply: Our measurements in Table 2 are annual averages for the years 2010-2020, and we have added the overall standard deviation. (Line318)

4 Comment: The literature or statistical validity should be provided to show the validity of the error range.

4 Reply: The absolute error (AE) is the absolute value between the measured value and the true value and can be expressed as AE=|x-x₀|, where x represents the measured value and x₀ represents the true value. The relative error (RE) is the ratio between the measured value and the true value. This can be expressed as RE=|x-x₀|/|x₀|, where x represents the measured value and x₀ represents the true value

5 Comment: Does direct uptake of nutrients in river water by animals occur? Explanation based on literature on the effect of water temperature change on nutrient uptake by algae and microorganisms should be needed.

5 Reply: We are very grateful for such constructive suggestions. In this study we have not considered the direct uptake of nutrients in river water by animals, which will be the focus of our next study.

6 Comment: The text in the table is too small to be deciphered.

The text needs to be clearly written. Water quality items should be clearly indicated.

The location of M1, M2, and M3 should also be indicated for easier understanding.

6 Reply: We have resized and reformatted the text in Figures 15, 16 and 17 to make it clearer. We have labelled the water quality items in the figures and in addition we have labelled M1, M2 and M3 in Figure 14.(Line489、520、552、477)

7 Comment: Make significant figures consistent.

7 Reply: We have modified the effective figures in Table 5. (Line583)

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The authors have tried their best to follow the suggestions. Adding a few more suggestions:

- Individual pictures should form a whole - eg. pictures 15, 16 and 17.

- Figure 4 is quite empty. Why isn't there even a background layer - eg. topographical map?

- In Figure 5, it is necessary to set a coordinate grid.

- The authors cited a large number of references from scientific meetings. It is recommended that they be based on scientific journals, because they are more representative.

Author Response

Dear Editor and Reviewers,

With best regards!

Yours sincerely

Your comments and my reply are as follows：

Reviewer 1#

1 Comment: Individual pictures should form a whole - eg. pictures 15, 16 and 17.

1 Reply: We have composited Figures 15, 16 and 17 separately. (Line487、517、548)

2 Comment: Figure 4 is quite empty. Why isn't there even a background layer - eg. topographical map?

2 Reply: We have added the terrain as a background layer in Figure 4. (Line272)

3 Comment: In Figure 5, it is necessary to set a coordinate grid.

3 Reply: We have added the coordinate grid in Figure 5. (Line291)

4 Comment: The authors cited a large number of references from scientific meetings. It is recommended that they be based on scientific journals, because they are more representative.

4 Reply: We have re-cited several scientific journals to replace the scientific conference literature, and the following are the scientific journals we have replaced: (Line46、191)

Yang, Y., Wang, H., Li, Y., Zhang, L., & Zhao, Y. (2023). New green development indicator of water resources system based on an improved water resources ecological footprint and its application. Ecological Indicators, 148, 110115.

20 Lai, Y., Lu, Y., Ding, T., Sun, H., Li, X., & Ge, X. (2022). Effects of Low-Impact Development Facilities (Water Systems of the Park) on Stormwater Runoff in Shallow Mountainous Areas Based on Dual-Model (SWMM and MIKE21) Simulations. International Journal of Environmental Research and Public Health, 19(21), 14349.

And we have added new literature from scientific journals, with the following additions: (Line624)

37 Yang, X., & Chen, Z. (2023). A hybrid approach based on Monte Carlo simulation-VIKOR method for water quality assessment. Ecological Indicators, 150, 110202.

Author Response File: Author Response.docx

Reviewer 2 Report

The authors have addressed most of the comments given in the previous review. The manuscript reads better now. The authors evaluates the current water quality of the Bai River, and on this basis constructs the Mike21 coupled hydrodynamic-water quality model for the study section of the Bai River, simulates three recharge period scenarios, analyses the laws of changes in hydrodynamic conditions and water quality indicators of the Bai River for different scenarios, which may provide theoretical and technical support for the future management of river water environments.

Overall, it is an excellent work and I do believe the manuscript is worth to be published.

Author Response

Dear Editor and Reviewers,

With best regards!

Yours sincerely

Author Response File: Author Response.docx

Reviewer 3 Report

The revised paper has been confirmed by the authors to have enhanced the significance and reliability of the objective. The response is also reasonable. Therefore, I recommend this paper as worthy of publication.

Author Response

Dear Editor and Reviewers,

With best regards!

Yours sincerely

Author Response File: Author Response.docx