The Impact Mechanism of Climate and Vegetation Changes on the Blue and Green Water Flow in the Main Ecosystems of the Hanjiang River Basin, China

Round 1

Reviewer 1 Report

This is a paper with a full workload. At present, the following problems may exist. Based on the importance of these issues, I recommend minor revisions. The detailed comments are as follows, hoping to further improve the quality of the paper.

1. The abstract can be simplified. Firstly, the background section of the abstract is too long. For example, “Line 12-14” can be considered for deletion. Secondly, the first result in the abstract provides too little information, while the third result is somewhat redundant. Finally, the research significance is too broad and can be further clarified.

2. The content of the introduction is very substantial, but there is some redundancy, especially in the second paragraph. It is recommended to simplify it.

3. In the discussion, most of the content is not an explanation of the results. For example, the first paragraph can be deleted or inserted into the introduction. Suggest modifying the discussion section to focus on explaining the results.

4. Line 267-268. “even the water resources of grassland ecosystem in some sub-basins are close to 0.” Why is it close to 0? It is recommended to explain in the discussion.

5. Line 324-348. This part is not closely related to the main results, and it can be considered to be moved to the appendix.

6. Line 445. “micro scale of local area.” What exactly does it mean?

7. Line 448-450. “Furthermore, this study shows a synergy between climate change and blue and green water flow that is in accordance with the law of global basin water resources change.” What is the purpose of this sentence here?

 

8. Line 450-451. It is recommended to be more specific about the benefits that can be obtained from this study.

Author Response

Responses to the comments from Review on

Authors’ response:

Thank you very much for your comments and suggestions. Those comments are all valuable and very helpful for revising and improving our paper. We have addressed your comments one by one below and revised our paper based on your suggestions. The detailed responses are listed below. We sincerely hope that our revisions will be satisfactory.

Reviewer 1:

This is a paper with a full workload. At present, the following problems may exist. Based on the importance of these issues, I recommend minor revisions. The detailed comments are as follows, hoping to further improve the quality of the paper.

Authors’ response:

Thank you very much for your comments. Below is a revision-by-revision and response to your review comments. Please review.

1. The abstract can be simplified. Firstly, the background section of the abstract is too long. For example, “Line 12-14” can be considered for deletion. Secondly, the first result in the abstract provides too little information, while the third result is somewhat redundant. Finally, the research significance is too broad and can be further clarified.

Authors’ response:

We appreciate your careful reading. Based on your comments, we've revised the abstract.

Water resources management and planning traditionally focus on visible liquid or blue water. However, green water also maintains social development and ecosystem services. Therefore, Blue and green water should be incorporated into the watershed management system for evaluating water resources. To analyze the water resources of the Hanjiang River Basin, the SWAT model was set up using long-term and high-precision geographic data. The methods of wavelet analysis and Pearson correlation analysis were used to explore the influence mechanism of climate and vegetation changes on the blue and green water flow (BWF and GWF) of the main ecosystems in the basin. Results showed that: (1) The spatial-temporal distribution of BWF and GWF in the main ecosystems of the basin over the past 50 years was uneven. Forest ecosystems and farmland ecosystems have a greater concentration of water resources in the south, while grassland ecosystems have a greater concentration of water resources in the east. (2) Climate plays a leading role in the change of BWF and GWF in the main ecosystems of the basin. BWF and the precipitation change cycle are synergistic, and GWF and the temperature change cycle are synergistic. (3) The correlation between vegetation and BWF and GWF in farmland ecosystem is significant. Vegetation affects the hydrological change process of BWF and GWF at the micro-scale. This study can provide data support and scientific rules for ecosystem water resource management in the Hanjiang River Basin.

2. The content of the introduction is very substantial, but there is some redundancy, especially in the second paragraph. It is recommended to simplify it.

Authors’ response:

We appreciate your careful reading. Based on your suggestions, we have revised and simplified some of the introduction. Since the introduction is long, we invite you to review it in the manuscript.

3. In the discussion, most of the content is not an explanation of the results. For example, the first paragraph can be deleted or inserted into the introduction. Suggest modifying the discussion section to focus on explaining the results.

Authors’ response:

Thank you for your suggestions. We have deleted the first paragraph of the discussion and revised the discussion section. Please review it in the manuscript.

4. Line 267-268. “Even the water resources of grassland ecosystem in some sub-basins are close to 0.” Why is it close to 0? It is recommended to explain in the discussion.

Authors’ response:

Thank you for your question. We have added an explanation of this issue to the discussion in Line 387-392.

Grasslands are scattered throughout the watershed, so water resources in grassland ecosystems are very low. There is almost no grassland distribution in the west-central part of the watershed (Figure 1), so the water resources of grassland ecosystems in these sub-watersheds are close to 0. Changes in water resources in grassland ecosystems are most sensitive to land-use change due to the fragmented nature of grassland distribution [45].

5. Line 324-348. This part is not closely related to the main results, and it can be considered to be moved to the appendix.

Authors’ response:

Thank you very much for your advice. We believe that this section is useful for discussing the mechanisms of vegetation influence on blue and green water flow, so we decided to keep it in the main text. Thanks again for your comments.

6. Line 445. “Micro scale of local area.” What exactly does it mean?

Authors’ response:

Authors’ response:

Thank you for your careful reading. The sentence here should be “vegetation will affect the hydrological change process of BWF and GWF on the micro scale”. This refers to the fact that the blue-green water transition mechanism also exists at the scale of single vegetation, which is something we would like to investigate further in future studies.

7. Line 448-450. “Furthermore, this study shows a synergy between climate change and blue and green water flow that is in accordance with the law of global basin water resources change.” What is the purpose of this sentence here?

Authors’ response:

Thank you for your question. The purpose of this sentence is to show that the findings of this study are generalizable, and hopefully provide some basis for research of blue and green water flows in other basins.

8. Line 450-451. It is recommended to be more specific about the benefits that can be obtained from this study.

Authors’ response:

Thank you for your suggestion. Based on your suggestion, we have been more specific about the research implications of this paper in Line 447-449.

Water resource managers in the Hanjiang River Basin can obtain the pattern of change of water resources in the ecosystems from this study to better plan land use and allocate water resources.

Author Response File: Author Response.docx

Reviewer 2 Report

The paper employed the SWAT model to explore the spatio-temporal change in BWF and GWF. It is an interesting study. However, the analysis of the influence mechanism of climate and vegetation on the BWF and GWF is insufficient. And there are some minor comments as following.

How to identify the 48 sub-basins? Please introduce the method in detail.

In the formula (1), what the data sources of the parameters, such as SW0, Pday, Qsurf, Ea, Wseep and Qgw. Please introduce them in detail.

In different ecosystems, the change patterns of BWR and GWF are quite similar. Please supply adequate discussion for it.

Line 302 “3.2.2. The Correlation between Vegetation Change and BWF and GWF and its Driving Mechanism”, If the paper uses the vegetation change rate to be correlated to BWF and GWF, or use static NDVI? Please check if it is appropriate to use the term“Vegetation Change and BWF and GWF”？

Author Response

Responses to the comments from Review on

Authors’ response:

Thank you very much for your comments and suggestions. Those comments are all valuable and very helpful for revising and improving our paper. We have addressed your comments one by one below and revised our paper based on your suggestions. The detailed responses are listed below. We sincerely hope that our revisions will be satisfactory.

Reviewer 2:

The paper employed the SWAT model to explore the spatio-temporal change in BWF and GWF. It is an interesting study. However, the analysis of the influence mechanism of climate and vegetation on the BWF and GWF is insufficient. And there are some minor comments as following.

Thank you for your comments and recognition. We have revised the article based on your comments. Please see below.

1. How to identify the 48 sub-basins? Please introduce the method in detail.

Authors’ response:

Thank you for your comment. We have introduced the method of identification in Line 150-154.

The SWAT model's watershed delineation tool automatically calculates and generates the stream network based on the input DEM. The area thresholds are matched with the actual river network to get the most suitable one. Once the watershed outlets are manually entered, the model automatically generates 48 sub-basins.

2. In the formula (1), what the data sources of the parameters, such as SW₀, P_day, Q_surf, E_a, W_seep and Q_gw. Please introduce them in detail.

Authors’ response:

We are very grateful for your comment. Formula (1) is derived from the Theoretical Basis of SWAT2009, and it is the water balance equation for the hydrologic cycle. The parameters in this equation are not specific to the inputs used to build the SWAT model. The SWAT model contains many variables and the relationships between the numerous input and output variables follow the hydrologic balance equation.

3. In different ecosystems, the change patterns of BWF and GWF are quite similar. Please supply adequate discussion for it.

Authors’ response:

Thank you for your comment. We had provided discussion about it. Please see in Line 352-355.

The patterns of change in BWF and GWF are very similar in different ecosystems (Figure 10, Figure 11). That is because climate change dominates water resource changes in both climate change and land use change (including vegetation). The pattern of blue and green water changes among different ecosystems will remain relatively consistent[23].

23. Li, Y.; Deng, J.; Zang, C.; Kong, M.; Zhao, J. Spatial and temporal evolution characteristics of water resources in the Hanjiang River Basin of China over 50 years under a changing environment. Frontiers in Environmental Science 2022, 10, doi:10.3389/fenvs.2022.968693.
24. Line 302 “3.2.2. The Correlation between Vegetation Change and BWF and GWF and its Driving Mechanism”, If the paper uses the vegetation change rate to be correlated to BWF and GWF, or use static NDVI? Please check if it is appropriate to use the term “Vegetation Change and BWF and GWF”?

Authors’ response:

Thank you for your careful reading. This paper uses static NDVI to be correlated to BWF and GWF. We've revised the heading in Line 300.

3.2.2. The Correlation between NDVI and BWF and GWF and its Driving Mechanism

Author Response File: Author Response.docx

Reviewer 3 Report

This manuscript measured dynamics in the blue and green water flow of Hanjiang River Basin in China during 1970-2020 using the SWAT model, and then revealed the impacts of Climate and Vegetation Changes on it. This work is significant to deeply understand the relationships between the blue and green water flow and environment changes at a watershed scale. The result is expected to support the optimization of water resource management locally. However, there are some problems with writing in this manuscript.

 

Comments and suggestions are as follow,

1. In the Introduction, reviews on the recent studies of the changes in blue and green water and its underlying processes should be expanded to fully demonstrate the scientific problem proposed by the authors.

2. In Lines 45, 52, 76-77, the related citation should be added to show the evidence for this sentence.

3. In the Materials and methods, the authors should briefly describe the source of these statistical values of the section 2.1.

4. The usage of these data in Table 1 should be supplemented to enhance readability. Additionally, the spatial resolutions of some data are inconsistent. The authors should explain how dealt with these inconsistent data.

5. The method section is too long and unclear, please simplify it. If the author did not improve these conventional methods, a brief description is sufficient.

6. Quantification of the green water flow is crucial for this study. The authors used the evapotranspiration to measure the green water flow. How feasible is it? How did the authors ensure the reliability and accuracy of this measurement?

7. In the Results, there is a lack of the validation for calculating the BWF and GWF.

In the lines 330-331, did climate change contribute to the NDVI reduction?

8. The correlation analysis in Figure 9 merely showed the statistical relationships between the NDVI and BWF/GWF. It is difficult to reveal the mechanism of the BWF/GWF changes based on ecosystem processes as canopy interception and soil infiltration. It is a good option to explore this mechanism using a process-based eco-hydrological model and assimilated data.

9. In the Discussion, an uncertainty of this study caused by data and methods being used should be analyzed further.

10. In Table 4, the meaning of these abbreviations should be explained in notes.

11. Figure 1 should show the locations of runoff observation stations for the model validation.

12. The language of this manuscript needs careful editing by a native English speaker.

The language of this manuscript needs careful editing by a native English speaker.

Author Response

Responses to the comments from Review on

Authors’ response:

Thank you very much for your comments and suggestions. Those comments are all valuable and very helpful for revising and improving our paper. We have addressed your comments one by one below and revised our paper based on your suggestions. The detailed responses are listed below. We sincerely hope that our revisions will be satisfactory.

Reviewer 3:

Comments and Suggestions for Authors

This manuscript measured dynamics in the blue and green water flow of Hanjiang River Basin in China during 1970-2020 using the SWAT model, and then revealed the impacts of Climate and Vegetation Changes on it. This work is significant to deeply understand the relationships between the blue and green water flow and environment changes at a watershed scale. The result is expected to support the optimization of water resource management locally. However, there are some problems with writing in this manuscript.

Thank you for pointing out the shortcomings in our writing. We have revised the manuscript based on your comments and would appreciate reviewing it.

Comments and suggestions are as follow.

1. In the Introduction, reviews on the recent studies of the changes in blue and green water and its underlying processes should be expanded to fully demonstrate the scientific problem proposed by the authors.

Authors’ response:

We appreciate your comments. We've expanded our overview of the latest research on blue and green water in Line 76-83.

Currently in the Hanjiang River Basin, there is no study on the blue and green water in the basin. Among the studies on blue and green water in other basins, Zhang et al. (2020) studied the effects of climate and land use changes on blue and green water in the Ganjiang River Basin[24]. Zang et al. (2019) studied the distribution of green and blue water flows in typical ecosystems in arid basins and the spatio-temporal study of their ecosystem service functions[21]. The impact of climate change on blue-green water has also been studied[25,26]. However, there are fewer studies on the mechanism of climate and vegetation changes on blue-green water at the ecosystem scale.

21. Zang, C.; Mao, G. A Spatial and Temporal Study of the Green and Blue Water Flow Distribution in Typical Ecosystems and its Ecosystem Services Function in an Arid Basin. Water 2019, 11, doi:10.3390/w11010097.
22. Zhang, Y.; Tang, C.; Ye, A.; Zheng, T.; Nie, X.; Tu, A.; Zhu, H.; Zhang, S. Impacts of climate and land-use change on blue and green water: a case study of the Upper Ganjiang River Basin, China. Water 2020, 12, 2661.
23. Yuan, Z.; Xu, J.; Meng, X.; Wang, Y.; Yan, B.; Hong, X. Impact of climate variability on blue and green water flows in the Erhai Lake Basin of Southwest China. Water 2019, 11, 424.
24. Jiang, J.; Lyu, L.; Han, Y.; Sun, C. Effect of climate variability on green and blue water resources in a temperate monsoon watershed, northeastern China. Sustainability 2021, 13, 2193.
25. In Lines 45, 52, 76-77, the related citation should be added to show the evidence for this sentence.

Authors’ response:

Thank you for your comment. We have added related citations to the sentences being referred to. Please see in Line 43, 50, 66-68.

It is important to note that blue water dominated water perception until recently, representing only one-third of actual freshwater resources today[6,7].

At the same time, green water is an important water source to maintain the landscape coordination and balance of terrestrial ecosystems[5].

Despite numerous studies, there are surprisingly few driving mechanisms behind blue and green water change at the ecosystem scale[5].

5. Liu, C.; Liu, X.; Yu, J.; Yang, S.; Zhao, C.; Men, B.; Zhao, Z.; Wang, H. The Rise of Eco-hydrology: A Review of Theoretical and Practical Issues. Journal of Beijing Normal University (Natural Science) 2022, 58, 412-423, doi:10.12202/j.0476-0301.2022124.
6. Liu, J.; Zehnder, A.J.B.; Yang, H. Global consumptive water use for crop production: The importance of green water and virtual water. Water Resources Research 2009, 45, doi:10.1029/2007wr006051.
7. Zang, C.; Liu, J. Trend analysis for the flows of green and blue water in the Heihe River basin, northwestern China. Journal of Hydrology 2013, 502, 27-36, doi: 10.1016/j.jhydrol.2013.08.022.
8. In the Materials and methods, the authors should briefly describe the source of these statistical values of the section 2.1.

Authors’ response:

Thank you for your comment. We have added the source of the statistical values in section 2.1. Please see in Line 109-111, 116-120.

The basin area is 30112 km², the mainstream length is 470km, and the annual runoff is 24.5 billion m³ (Hanjiang River Basin Comprehensive Plan (2021)).

About land use types in the basin, forest land accounts for about 67.2% of the area; Approximately 18.7% of the land is arable, mainly distributed near the water system; water accounts for about 8.8%; urban land is about 3.6%, and urban land is concentrated in the southeast of the basin. The grassland area accounts for only about 1.6% (2020) (Hanjiang River Basin Comprehensive Plan (2021)).

4. The usage of these data in Table 1 should be supplemented to enhance readability. Additionally, the spatial resolutions of some data are inconsistent. The authors should explain how dealt with these inconsistent data.

Authors’ response:

Agreed, thanks for the comment. We have added the data usage and processing method in Table 1. Please see in Line 130-136.

The model used DEM data to generate the river network and delineate sub-basins by developing a vector file of the river network in the basin. Land use, soil, and meteorological data were used to simulate water resources for the past 50 years. The runoff data were used for model rate calibration and validation. Whereas the spatial resolution of land use and NDVI were the same, they were used to analyze the land use and vegetation changes. So, the different spatial resolution of the data does not affect this study.

5. The method section is too long and unclear, please simplify it. If the author did not improve these conventional methods, a brief description is sufficient.

Authors’ response:

Thank you for your comment. We have simplified our research method section. Please see in section 2.3 in the manuscript.

6. Quantification of the green water flow is crucial for this study. The authors used the evapotranspiration to measure the green water flow. How feasible is it? How did the authors ensure the reliability and accuracy of this measurement?

Authors’ response:

Thank you for your careful consideration and question. We refer to the definitions of the scholars who first proposed the concept of blue-green water (The concept of green water was first introduced by Falkenmark (1995) referring to the total crop evaporation during crop growth. Later, green water resource has been generally used to refer to the water that comes from precipitation, is stored in the soil, and subsequently fed back to the atmosphere (Falkenmark and Rockström, 2006; Savenije, 2000) through crop evaporation) as well as the formulas in existing articles that study blue-green water flows (The flow of green water refers to actual evapotranspiration (Zang, 2013)). It is ensured that the use of evapotranspiration to measure the green water flow is reliable and accurate.

7. In the Results, there is a lack of the validation for calculating the BWF and GWF. In the lines 330-331, did climate change contribute to the NDVI reduction?

Authors’ response:

Thank you for your careful reading. The validation of the calculation has been added in the results. Please see in Line 233-238.

According to the research results, the basin BWF during the past 50 years averaged 367.90 billion m³, and GWF averaged 306.43 billion m³. The Hanjiang River Basin Comprehensive Plan (2021) states that the basin’s water resources amount are about 300 billion m³. The average annual evapotranspiration over the years is 959-1248 mm, or 288.77-375.80 billion m³. Considering the error between the model simulation and actual, the calculation results are within a reasonable range.

For the second question, Liu et al. (2015) noted that NDVI is positively correlated with both precipitation and temperature. However, the correlation between NDVI and precipitation was stronger than that with temperature. From 2015 to 2020, the precipitation in the Hanjiang River Basin was in normal fluctuation, while the temperature showed an increasing trend. And the NDVI value of the basin was decreasing. This indicates that the decrease of NDVI in the basin during this period is caused by urbanization rather than climate.

[1] Liu Y, Li Y, Li S, et al. Spatial and temporal patterns of global NDVI trends: correlations with climate and human factors[J]. Remote Sensing, 2015, 7(10): 13233-13250.

8. The correlation analysis in Figure 9 merely showed the statistical relationships between the NDVI and BWF/GWF. It is difficult to reveal the mechanism of the BWF/GWF changes based on ecosystem processes as canopy interception and soil infiltration. It is a good option to explore this mechanism using a process-based eco-hydrological model and assimilated data.

Authors’ response:

Agreed, thank you very much for your advice. Due to the limitations of the model in this study, we will use process-based eco-hydrological model in further studies. We had added these shortcomings and outlooks in our discussion in Line 424-427.

Finally, process-based ecohydrologic modeling and assimilation data should be used in future research to focus on process-based ecohydrologic changes to better explain the mechanisms of blue and green flow changes.

9. In the Discussion, an uncertainty of this study caused by data and methods being used should be analyzed further.

Authors’ response:

Thank you for your suggestion. We have further analyzed the uncertainties of this study caused by the data and methods in the discussion. Please see in Line 420-427.

At the same time, there are some shortcomings in this study. Firstly, the runoff data we used for model validation came from only one hydrological observatory, which may make the simulation results uncertain. Secondly, more sophisticated correlation analysis methods can be considered to study the correlation of climate and vegetation with BWF and GWF to improve the accuracy of the results. Finally, process-based eco-hydrologic modeling and assimilation data should be used in future research to focus on process-based eco-hydrologic changes to better explain the mechanisms of blue and green flow changes.

10. In Table 4, the meaning of these abbreviations should be explained in notes.

Authors’ response:

Thanks to your comment. We have added explanations of the meaning of the abbreviations in Table 4. Since Table 4 is quite long, please review it in the manuscript.

11. Figure 1 should show the locations of runoff observation stations for the model validation.

Authors’ response:

Thank you for your comment. In fact, the runoff observation station for the model validation is the Chaoan station, which is located in the southeastern part of the basin and has been represented by a blue pentagram symbol in Figure 1. We have added the names of the runoff observation stations in Table 1, in order to illustrate the runoff data sources more clearly.

Figure 1. The geographical location and general situation of the research region.

Table 1. Data information.

Type	Description	Sources
Digital Elevation Model (DEM)	90m resolution.	Science Data Center of Chinese Academy of Sciences
Land use data	Every ten years from 1980 to 2020 (30m resolution), obtained through remote sensing interpretation.	United States Geological Survey
Soil data	Soil types in China.	Harmonized World Soil Database (HWSD)
Meteorological data	Daily meteorological data of 32 meteorological stations in the Hanjiang River Basin from 1971 to 2020.	National Meteorological Science Data Center of China, Meteorology Bureaus of Guangdong, Fujian and Jiangxi Provinces
Runoff data	Monthly runoff data of Chaoan Station from 1980 to 2010.	Hanjiang River Basin Administration
Vegetation index data (NDVI)	Every five years from 1990 to 2020(30m resolution).	Resource and Environment Science and Data Center

 

 

12. The language of this manuscript needs careful editing by a native English speaker.

Authors’ response:

Thank you for pointing out the shortcomings of this manuscript. Our manuscript has now been carefully edited by a native English speaker. Thank you again for your careful review and guidance of the manuscript.

 

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

the authors have carefully revised the paper under my comments.

Reviewer 3 Report

All suggestions have been carefully addressed.