Assessing Impacts of Climate Change and Human Activities on Streamflow and Sediment Discharge in the Ganjiang River Basin (1964–2013)

Round 1

Reviewer 1 Report

This manuscript focused on estimating the impacts of changing climate and human activities on flow and sediment discharge in the Ganjiang River basin.

Overall, this is a well written and clearly designed manuscript with seemly innovative approaches to estimating potential impacts of climate and human activity impacts on sediment discharges and steamflow. There are however a few items that should be addressed before publication is considered. Why did you choose ‘Breakpoint analysis’, ‘Double mass curve’ and ‘Pettitt’s’ tests? I am sure there are other methods. This needs to be substantiated early on in the manuscript. Second, based on your description of the Double mass curve method, I don’t see how you isolate the effects of climate from human activities (e.g. construction, land use changes, etc…). This should be better described in section 2.2.3. In Figure 3, based on your description it’s not clear how the changpoint year is identified. This should be described in the text and in Figure 3. In Table 5 it is not clear how the percentages of human and precipitation are calculated. This should be clearly described as these results appear to be major points in this manuscript. Lastly, the authors predict that impacts from human activities outweigh climate impacts. Describe how decreased sediment loading from human activities could cause ecosystem problems? Suggestions, e.g., shift in ecosystem balance from decreased nutrients to estuaries, increased photosynthesis from increased water clarity, changes geochemical ionic balances, etc…

Specific Points:

Lines 43-44: Say “human impacted to varying degrees”

Line 57: Provide citation after Colorado and Nile.

Line 65-68: This is an important point that human activities could outweigh climate and should be supported with similar findings, for example, see https://www.sciencedirect.com/science/article/pii/S2214581817303439

Line 73: Say “….Basin also has the most…”

Figure 1: It is not clear how/where the watershed in the right fits into the Jiangxi watershed.

Lines 146: Say 95% confidence level, not 5%.

Line 247: Say “The black arrow…”

Lines 255 to 257: It sounds convincing but I do not see how you isolated human impacts from precipitation.

Table 5: What does a negative (-) percentage mean, i.e. -13.31, -8.35?

Lines 353-359: I assume what you describe in these lines is what you suspect is causing sediment decreases. Correct?

Author Response

Responses to the comments of the first reviewer

Why did you choose ‘Breakpoint analysis’, ‘Double mass curve’ and ‘Pettitt’s’ tests? I am sure there are other methods. This needs to be substantiated early on in the manuscript.

Many thanks for your advice. We think the “Breakpoint analysis” is an effective method to determine if human activity is one of major driving factors that caused a marked upward or downward change of streamflow and sediment discharge. And we had gave the detailed description in the methodologies part 2.2. Moreover, the combination methods the MK test, Pettitt and DMC is a progressive method of mutual verification to assess impacts of climate change and human activities on streamflow and sediment discharge. Which was used by our group in many river basin of China, like the Yellow River, Gongshui River, Songhuajiang River catchment etc. For more details, please see these studies:

Gao P.; Mu X.; Wang F.; Li R. Changes in streamflow and sediment discharge and the response to human activities in the middle reaches of the Yellow River. Hydrol. Earth Syst. Sci. 2011, 15, 1-10.

Mu X.; Li Y.; Gao P.; Shao H.; Wang F. The runoff declining process and water quality in Songhuajiang River catchment, China under global climatic change. CLEAN–Soil, Air, Water 2012, 40, 394-401.

Gao P.; Li P.; Zhao B.; Xu R.; Zhao G.; Sun W.; Mu X. Use of double mass curves in hydrologic benefit evaluations. Hydrological processes 2017, 31, 4639-4646.

Guo L.; Yu Q.; Gao P.; Nie X.; Liao K.; Chen X.; Hu J.; Mu X. Trend and change-point analysis of streamflow and sediment discharge of the Gongshui River in China during the last 60 years. Water 2018, 10, 1273.

 

Second, based on your description of the Double mass curve method, I don’t see how you isolate the effects of climate from human activities (e.g. construction, land use changes, etc…). This should be better described in section 2.2.3.

Thank you for your question. In the equation7, the calculated sediment discharge was only connect with rainfall, so that we calculated the contribution of rainfall by the equation7-9, and then used the equation10 to get the contribution of human activities under the hypothesis that climate change (mainly changes in regional precipitation) and local human activity as the two main factors impacting basin hydrology.

In Figure 3, based on your description it’s not clear how the changpoint year is identified. This should be described in the text and in Figure 3.

Thank you for your question. We had gave the calculation formula in equation 5 and 6, and had explained the detailed determination method in line 151-157, please have a look.

In Table 5 it is not clear how the percentages of human and precipitation are calculated. This should be clearly described as these results appear to be major points in this manuscript. Many thanks for your question. We had gave the calculation formula in equation 8-10, and had explained how the percentages of human and precipitation are calculated in line 168-177, please have a look. Lastly, the authors predict that impacts from human activities outweigh climate impacts. Describe how decreased sediment loading from human activities could cause ecosystem problems? Suggestions, e.g., shift in ecosystem balance from decreased nutrients to estuaries, increased photosynthesis from increased water clarity, changes geochemical ionic balances, etc…

Thank you for your question. At the end of the paper, we added these words: Specifically, the reduced sediment discharge would cause land degradation and river regime evolution like river channel slices downwards, which influence the ecological balance of terrestrial and river ecosystems, and maybe even exacerbate the seasonal drought and ecological hazards in Poyang Lake Basin.

Specific Points:  

 

Lines 43-44: Say “human impacted to varying degrees” Many thanks for your suggestion. Line 57: Provide citation after Colorado and Nile. Many thanks for your suggestion. Line 65-68: This is an important point that human activities could outweigh climate and should be supported with similar findings, for example, see https://www.sciencedirect.com/science/article/pii/S2214581817303439 Many thanks for your suggestion. Line 73: Say “….Basin also has the most…” Many thanks for your suggestion. Figure 1: It is not clear how/where the watershed in the right fits into the Jiangxi watershed. Thank you for your question. As we know, the topographic characteristic of Jiangxi Province is south-high and north-low. So that, water flows from the upper reaches of Ganjiang River (right map in figure1) through the middle reaches to the lower reaches, and flow through Waizhou Station into Poyang Lake(The dark blue area in the lower left), which is the largest freshwater lake in China. And the watershed area of Poyang Lake is 162,200 km², accounting for 97% of the watershed area of Jiangxi Province. Lines 146: Say 95% confidence level, not 5%. Many thanks for your suggestion. We had corrected several mistakes in the manuscript. Line 247: Say “The black arrow…” Thank you for your question, we distinguished the graph title from note. Lines 255 to 257: It sounds convincing but I do not see how you isolated human impacts from precipitation.

Thank you for your question, you are right. This part work is under the hypothesis that climate change (mainly changes in regional precipitation) and local human activity as the two main factors impacting basin hydrology. So that, C_r+ C_h=100%.

Table 5: What does a negative (-) percentage mean, i.e. -13.31, -8.35? Thank you for your question, if the difference of observed annual sediment discharge before the change-point() and calculated annual sediment discharge after the change-point() is negative, and the is positive, the contribution of precipitation is negative. In our study, both Xiajiang and Waizhou station are the case. Lines 353-359: I assume what you describe in these lines is what you suspect is causing sediment decreases. Correct?

Thank you for your question, firstly, we calculated the contribution rate of rainfall and human activities to the variation of streamflow and sediment change. And then, we inferred the main driving factors according to the historical and literature data, as well as the provincial water resources census information, the deduction is not a baseless guess.

 

Author Response File: Author Response.docx

Reviewer 2 Report

The authors identified trends and abrupt change in precipitation, runoff, and sediment discharge in the Ganjiang River basin. They used methods such as Mann-Kendall test, Pettitt test, and double mass curve to find a significant results on hydrological changes. Therefore, I think that the authors tackled the important issue and suggested the interesting results. But I think that this study should have provided more results beyond some change point analysis. The followings are comment that the authors can revise for readers.

The plain characters such as n, E, D, Z... (see page 5, 6) should be revised as italic. The authors have to provide descriptive statistics (mean, standard deviation, skewness, kurtosis) of precipitation, runoff, and sediment discharge. The dams can impact significantly on runoff and sediment discharge. In the selected basin, the authors have to describe the existence of dam. Finally, I think that the authors should have provided the amount of changes in runoff or sediment discharge before and after on change point. Is it possible to provide amount of changes ? 

  

Author Response

Responses to the comments of the second reviewer

The plain characters such as n, E, D, Z... (see page 5, 6) should be revised as italic.

Many thanks for your advice. We have amended these characters as italic.

The authors have to provide descriptive statistics (mean, standard deviation, skewness, kurtosis) of precipitation, runoff, and sediment discharge.

Thank you for your question, we have analyzed the descriptive statistics of Bashang station (as an example), and because of streamlining and habits, we did not put these results in the manuscript.

Table. Descriptive statistics of Bashang station
Item	N	Minimum	Maximum	Mean	Standard deviation	skewness		kurtosis
						Value	Standard error	Value	Standard error
Precipitation(mm)	50	898.00	1936.00	1383.31	259.94	0.415	.337	-0.46	0.66
Streamflow(108 m3)	50	29.45	116.29	62.49	18.79	0.768	.337	0.44	0.66
Sediment Discharge(104 t)	50	11.51	268.09	95.63	57.61	0.974	.337	0.54	0.66

The dams can impact significantly on runoff and sediment discharge. In the selected basin, the authors have to describe the existence of dam. Finally, I think that the authors should have provided the amount of changes in runoff or sediment discharge before and after on change point. Is it possible to provide amount of changes?

Many thanks for your question. In this paper, we confirmed the importance of dam, and we calculated the characteristics of large and medium-sized reservoirs constructed in the past decades in the Ganjiang River Basin, more detail was given in Table7. Moreover, in our study, the change-point was only detected in sediment discharge, while no change-point year could be detected for annual streamflow at any station (Figure 3). And we gave the relative reduction rate in sediment discharge before and after on change point of every station in Table4, and in line 257-262, we had wrote: Results showed that compared with the calculated cumulative sediment discharge (Sc), observed cumulative sediment was 21.55%, 13.33%, 13.12%, 27.23%, 30.07%, 30.17%, and 31.10% lower at Bashang, Xiashan, Julongtan, Hanlinqiao, Ji’an, Xiajiang and Waizhou stations, respectively. In other words, after the transition year, the sediment discharge of these hydrological stations had declined at varying degrees, and the average degree of decline at stations of midstream and downstream were almost the same(30.1%), and which were larger than that of upstream(18.8%).

 

Author Response File: Author Response.docx

Reviewer 3 Report

On account of the manuscript WATER-564749, entitled “Assessing impacts of climate change and human activities on streamflow and sediment discharge in the Ganjiang River basin (1964-2013)” by Li-Ping Guo et al., the authors evaluated changing trends and detected transition years for both streamflow and sediment discharge using long term historical records at seven hydrological stations in the Ganjiang River basin, where is the most important major river in the Poyang Lake River basin, and is also one of the eight major tributaries of the Yangtze River, over the past 50 years. The topic is important to conduct effective management of water resources, and the authors got interesting results. The manuscript was well written and designed. After careful consideration, I made a decision that the manuscript is acceptable for publication in its present form.

Author Response

Thank you very much for your affirmation of our work.