Spatiotemporal Variations in Fingerprinting Sediment Sources in a Watershed Disturbed by Construction

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The research is useful for hydrological research.  I have two observations:

1) I recommend rearranging photos in figure 1, line 116. The photographs should be linked to a specific site in the map with a line.  In that way, readers could easily identify site characteristics in the map. The map could be in the center, three photos at the right and three photos at the left.  In that way, linking lines could not interfere with each other.

2) By reading the text at the beginning, it is not clear which samples come from undisturbed areas and which don't.  Although, there is description of natural sources in line 294, and this type of sources are shown in figure 1, line 116. However, one question remains: Is the drainage area at a sample site completely disturbed or undisturbed? If a sample site apparently locates in a natural site, it does not necessarily mean that upstream drainage area could not be completely undisturbed.  Sites with a completely undisturbed upstream drainage areas should be treated as control sites, and the paper does not mention explicitly which sample sites are control sites.  This should be clarified in the paper.

On the other hand, lines 290 and 291 states that natural sources cannot be clearly differentiate among each other.  If these natural source's sampling sites are really representing undisturbed drainage areas, these sites should be grouped as UNDISTURBED AREAS and the rest of sampling sites grouped as DISTURBED AREAS.  Apparently, figure 3 tries to do this showing three classifications (natural, exposed slopes and spoil heaps).  As stated before, it is important to clarify whether natural sources represent completely undisturbed drainage areas.

Author Response

Dear Editor,

 

We are very grateful for your and the reviewer’s comments and suggestions on this paper. These comments have been very valuable and helpful for revising and improving our manuscript, and they have provided important guidance for our research. According to these suggestions, we have carefully addressed the comments and made detailed revisions, as discussed below.

 

With kind regards,

The authors

 

 

Issues raised by Reviewer 1

 

1) I recommend rearranging photos in figure 1, line 116. The photographs should be linked to a specific site in the map with a line. In that way, readers could easily identify site characteristics in the map. The map could be in the center, three photos at the right and three photos at the left.  In that way, linking lines could not interfere with each other.

Reply and revision: Thank you for your suggestion to link the images to the sampling points. It is important to highlight the specific location of the disturbance and the natural source sites. We have modified Figure 1 according to your and other reviewers' suggestions. Meanwhile, we have also revised the footnote to ensure that the content matches the new order of the photos.

 

2) By reading the text at the beginning, it is not clear which samples come from undisturbed areas and which don't. Although, there is description of natural sources in line 294, and this type of sources are shown in figure 1, line 116. However, one question remains: Is the drainage area at a sample site completely disturbed or undisturbed? If a sample site apparently locates in a natural site, it does not necessarily mean that upstream drainage area could not be completely undisturbed. Sites with a completely undisturbed upstream drainage area should be treated as control sites, and the paper does not mention explicitly which sample sites are control sites. This should be clarified in the paper.

On the other hand, lines 290 and 291 states that natural sources cannot be clearly differentiate among each other. If these natural source's sampling sites are really representing undisturbed drainage areas, these sites should be grouped as UNDISTURBED AREAS and the rest of sampling sites grouped as DISTURBED AREAS. Apparently, figure 3 tries to do this showing three classifications (natural, exposed slopes and spoil heaps). As stated before, it is important to clarify whether natural sources represent completely undisturbed drainage areas.

Reply and revision: Thank you for your valuable comments and suggestions. We agree with you that a sampling site located within the boundaries of a natural source does not necessarily mean that it is undisturbed. In this study, we have collected two types of samples, that are, the surface samples in different sediment sources and the deposited sediment samples along the channel. Prior to field sampling, we obtained information on land use and engineering disturbance in the watershed based on field surveys and 2022 and 2023 Sentinel imagery. In turn, we grouped the potential sediment sources into the natural land use types (forests, grasslands, shrublands and the channel bank) and the engineering disturbance types (spoil heaps and exposed slopes). As shown in Figure 1, most of the engineering activity and disturbance is distributed in the downstream of the watershed and close to the channel.

During the natural source sampling process, most of the forest, grassland, shrubland and channel bank sites were selected upstream of the engineering activities or at higher elevation positions to avoid the influence of disturbance. Therefore, the natural source samples have not been disturbed by engineering activities and can represent completely undisturbed drainage areas in the watershed. We have made revisions in Section 2.2, Line 142-143: undisturbed natural land use types (forests, grasslands, shrublands and channel banks), and Line 155-157: To avoid the influence of engineering disturbance, the above natural sources sites have been selected upstream of the spoil heaps and exposed slopes, or at higher elevations. It should be noted that the main focus of this study is on the different types of engineering disturbance contributing to the deposited sediment. The two main types, spoil heaps and exposed slopes, differ greatly in spatial scale, geometric pattern and sediment characteristics. Therefore, three classifications (natural, exposed slopes and spoil heaps) are made in Figure 3.

When collecting deposited sediment along channels, most sites are within the disturbed area to represent the spatial variation in sediment contribution rate from different disturbed sources. Meanwhile, we also sampled a site that within the undisturbed channel span to show the natural sediment as a control point. According to your valuable suggestions regarding the control area, we have made revisions in Section 2.2 at line 188-191: According to the disturbance areas shown in Figure 1a for 2022 and 2023, the upstream drainage area of sediment sampling site S03 was completely undisturbed in 2022; therefore, S03 was used as the undisturbed control site.

 

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

 

The Article „Spatiotemporal variations in fingerprinting sediment sources in a watershed disturbed by construction” presents an application of the sediment fingerprinting methodology in a mesoscale catchment in China that is prone to high soil erosion both from natural sources and from disturbed surfaces.

The topic of the study is very interesting for research as well as for soil and water resource management. It is a good example of an application of the sediment fingerprinting methodology as the authors thoroughly addressed most of the challenges and pitfalls of the methodology. The presentation and discussion of the results as well as the structure of the paper are very good. The paper is very well written and a pleasure to read. I congratulate the authors on this nice work!

The authors took a lot of source samples and sediment samples in order to be able to assess the spatial and temporal variability of source contributions and performed detailed field surveys as well as an analysis of satellite images to identify potential sediment sources based on the best available data. They rigorously assessed the impact of sediment source classification and clarified the choice of tracers. They used a common procedure to identify conservative tracers and to eliminate tracers that did not pass the test. The results showed that the methodology was suitable to answer the research question because the tracers were able to discriminate the sources of sediment that are of interest for the study and the differences in the source contributions in time and space were clearly visible.

Thus, I have only one concern about the methodology (see below) and a few minor comments that will hopefully help to improve the quality of this manuscript which is already good.

Best regards,

 

 

Main concern:

In this study, the source samples (possibly also the sediment samples) were sieved to 2 mm meaning that a broad range of particle sizes was used for the further analysis. One of the key challenges for sediment fingerprinting is the fact that the processes of soil detachment, transport and deposition are highly size-selective (Laceby et al., 2017). Thus, sediments samples are usually finer than source samples. Within the fraction <2mm that contains sand as well as silt and clay, tracer concentrations often depend on particle sizes and the particle-size selectivity of the erosion processes violates the assumption of tracer conservativeness. This problem is most often addressed by fractionation to the size range <63 µm or even smaller fractions (Collins et al., 2020). Can you comment on whether you did any analysis on the effects of particle size? I strongly recommend sieving to <63 µm or addressing the particle size effects for future work.

Minor remarks:

Is the supplement referenced in the paper? I didn’t find a mention.

p.1 l.37 and l. 43: References [3, 5,6]: I recommend citing more general literature instead of case studies

p.2 l.73-75: I suggest rephrasing. The second part is clear, but the first part of the sentence is unclear. Why do composite samples taken at the outlet not represent the overall watershed contribution?

p.2 l.81 “researches”: do you mean “research”? “studies”?

p.2 l.85 “…were selected as research objects.” I suggest rephrasing. The research object is still the entire catchment isn’t it?

p.3 l.99-100: Consider adding the information that Zheduo is a tributary/headwater river of the Yangtze River. This would be helpful for international readers.

p.3 l. 108-110: I would place the sentence on vegetation types behind the sentence on land use (l.115).

Figure 1: Very nice figure! It would still be helpful to show the location of the basin in China.

p.3 l. 123: unit “um”: should it be µm (micrometer)?

p.4 l. 171-172: How do you know that the samples are recently deposited samples?

Section 2.5: Can you give some additional details on the mixing model? What kind of model is used to solve eq.  3-5? As you have more fingerprints than sediment sources, the system of equations 3-5 is overdetermined. How did you solve it? The last sentence suggests that you used an optimization procedure. Please explain how you optimized R_es.

Section 2.7: I was missing this information earlier. Consider integrating it into sections 2.4 and 2.5.

p.7 l. 281: In the bottom left subfigure of Fig. 2 I don’t see a difference in Sn concentrations between the sources. Do you mean Sr?

Section 3.1: can you comment on the channel banks as a natural subsoil source? I think that it is an interesting result that there are tracers that discriminate well between the natural subsoil sample "banks" and the disturbed subsoil samples "spoil" and "slope".

p.8 l.305-309: Tracers not passing the range test could also hint at missing sources. I would also mention particle size selectivity of the erosion and sediment transport processes (at least in the discussion). See my main concern above.

p.9 l.338: Consider changing the title of the section. "different stages" made me think of water level, i.e. during floods or during low-flow periods.

Table 4: I would be careful about giving growth rates for contributions in %. For example, the negative growth rate of the natural sources leads to the assumption that the mass of sediment originating from this source is decreasing, which is probably not the case because the decrease in contribution in % is likely to be a result of the increase of the other sources. Also, the table as it is presented at the moment, suggests that the growth rates for area can be compared to the growth rates of the contributions in %. This is not really the case, it should be compared to changes in absolute mass of sediment which you don’t have.  

Figure 4: The figure is not very easy to read. Can you increase the size of the pie charts? Also please change the colors, red-green is not color-blind-friendly and the yellow is hard to differentiate from the green in the small pie charts.

p.10 l.378-379 However, this equation is not valid on 2023 anymore, right? Can you comment on that in the discussion?

p.11 l 422: I would replace “modulus” by “rate” as it is more common.

p.12 l.434 Why did you use conjunctive here? I suggest rephrasing.

p.13 l.489 See my comment above on table 4. You cannot compare growth rates in area directly to growth rates in the contributions in %. Assuming that the mass of spoil heap material that is entering the river is proportional to the increase in area, and that the contributions of the other sources (also in mass, not in %) remain constant, the growth rate in mass would be equal the growth rate in area but the growth rate in % would be smaller. As you don’t have information about changes in mass, I would avoid this kind of comparisons.

p. 14 l.565-566 avoid the conjunctive (“would”, “could”)

 

References:

Laceby, J. P., Evrard, O., Smith, H. G., Blake, W. H., Olley, J. M., Minella, J. P., & Owens, P. N. (2017). The challenges and opportunities of addressing particle size effects in sediment source fingerprinting: a review. Earth-Science Reviews, 169, 85-103.

Collins, A. L., Blackwell, M., Boeckx, P., Chivers, C. A., Emelko, M., Evrard, O., ... & Zhang, Y. (2020). Sediment source fingerprinting: benchmarking recent outputs, remaining challenges and emerging themes. Journal of Soils and Sediments, 20, 4160-4193.

Comments on the Quality of English Language

The English sounds good to me. I suggest rephrasing a few sentences (see comments above).

Author Response

Dear Editor,

 

We are very grateful for your and the reviewer’s comments and suggestions on this paper. These comments have been very valuable and helpful for revising and improving our manuscript, and they have provided important guidance for our research. According to these suggestions, we have carefully addressed the comments and made detailed revisions, as discussed below.

 

With kind regards,

The authors

 

 

 

Issues raised by Reviewer 2

Main concern:

In this study, the source samples (possibly also the sediment samples) were sieved to 2 mm meaning that a broad range of particle sizes was used for the further analysis. One of the key challenges for sediment fingerprinting is the fact that the processes of soil detachment, transport and deposition are highly size-selective (Laceby et al., 2017). Thus, sediments samples are usually finer than source samples. Within the fraction <2mm that contains sand as well as silt and clay, tracer concentrations often depend on particle sizes and the particle-size selectivity of the erosion processes violates the assumption of tracer conservativeness. This problem is most often addressed by fractionation to the size range <63 µm or even smaller fractions (Collins et al., 2020). Can you comment on whether you did any analysis on the effects of particle size? I strongly recommend sieving to <63 µm or addressing the particle size effects for future work.

Reply and revision: Thank you for your valuable comments and suggestions. We fully agree with your opinion that particle size is highly selective during soil detachment, transportation, and deposition processes. Therefore, the selection of specific particle sizes for tracer concentration measurements is critical. In this study, the particle size range of soils from natural sources (forests, grasslands, shrublands and channel banks) and disturbed sources (spoil heaps and exposed slopes) varied widely. The disturbed sources are characterized by large particle sizes and are close to the channel and sediment sampling sites. Sediment from these sources would easily enter the channel and contribute to the riverbed deposition. This means that the coarse sediment may account for a large proportion of the deposition along the sampled channels. As this study quantifies the sediment source contribution based on the deposited sediment, we selected a relatively wide range of particle sizes (<2 mm) to represent the sources and deposited sediment conditions in the study area. It should be noted that there are some researchers who have also used this particle size range to trace the sources of sediments (Evrard et al., 2011 and Mukundan et al., 2010). Koiter et al. (2013) believe that the use of samples smaller than 2 mm for tracing is beneficial for quantifying the variability of larger particles during sediment transport. We have added content in Section 2.3 Line 194-196 to support the particle size of this study: “This range of particle sizes could represent the sources and deposited sediment conditions in the study area, especially for the disturbed sources which characterized by large particle sizes”. A new reference citation (Koiter et al., 2013) has been added.

Furthermore, we have also made revisions to the Discussion section 4.1 (Line 433-447) regarding the potential impact of particle size influence on tracer conservativeness and the future work according to your suggestions: It is important to note that the particle size selectivity of the erosion and sediment transport processes may affect the conservativeness of certain tracers and influence the results of the fingerprint analysis (Laceby et al., 2017). In this study, source samples were sieved to 2mm, which means that a relatively high proportion of coarse soil particles were used in the measurement of trace indices. Considering that the deposited sediment sample sites are close to spoil heaps and exposed slopes which are characterized by large particle size, the coarse disturbed sediments are more likely to enter channels and contribute to deposition than those from natural sources. The particle size selectivity of coarser materials may lead to tracer accumulation and exceed the highest concentrations at sources, resulting in tracer non-conservatism (e.g. the CaO in Figure 2 and Table 1). In previous research, the particle size range <63 µm has been most commonly used in sediment source fingerprinting (Laceby et al., 2017). The use of this particle size range, or even smaller fractions, could address the aforementioned influence on tracer conservativeness (Collins et al., 2020). Future research will be needed to further explore the range of particle sizes that are suitable to use in engineering disturbed watersheds.

Related references:

Evrard, O.; Navratil, O.; Ayrault, S.; Ahmadi, M.; Némery, J.; Legout, C.; Lefèvre, I.; Poirel, A.; Bonté, P.; Esteves, M. Combining suspended sediment monitoring and fingerprinting to determine the spatial origin of fine sediment in a mountainous river catchment. Earth Surf. Process. Landf. 2011, 36, 1072-1089. https://doi.org/10.1002/esp.2133.

Mukundan, R.; Radcliffe, D.E.; Ritchie, J.C.; Risse, L.M.; McKinley, R.A. Sediment Fingerprinting to Determine the Source of Suspended Sediment in a Southern Piedmont Stream. J. Environ. Qual. 2010, 39, 1328-1337. https://doi.org/10.2134/jeq2009.0405.

Koiter, A.J.; Lobb, D.A.; Owens, P.N.; Petticrew, E.L.; Tiessen, K.H.D.; Li, S. Investigating the role of connectivity and scale in assessing the sources of sediment in an agricultural watershed in the Canadian prairies using sediment source fingerprinting. J. Soils Sediments 2013, 13, 1676-1691. https://doi.org/10.1007/s11368-013-0762-7.

Laceby, J.P.; Evrard, O.; Smith, H.G.; Blake, W.H.; Olley, J.M.; Minella, J.P.G.; Owens, P.N. The challenges and opportunities of addressing particle size effects in sediment source fingerprinting: A review. Earth-Science Reviews 2017, 169, 85-103. https://doi.org/10.1016/j.earscirev.2017.04.009.

Collins, A.L.; Blackwell, M.; Boeckx, P.; Chivers, C.A.; Emelko, M.; Evrard, O.; Foster, I.; Gellis, A.; Gholami, H.; Granger, S.; et al. Sediment source fingerprinting: benchmarking recent outputs, remaining challenges and emerging themes. J. Soils Sediments 2020, 20, 4160-4193. https://doi.org/10.1007/s11368-020-02755-4.

 

Minor remarks:

1. Is the supplement referenced in the paper? I didn’t find a mention.

Reply and revision: Thanks for your comments. The authors apologize for mistakenly selecting the type of supplement when uploading the minimal dataset file. The uploaded file is the minimal dataset for review purposes, not a supplement.

 

2. p.1 l.37 and l. 43: References [3, 5,6]: I recommend citing more general literature instead of case studies

Reply and revision: Thanks for your suggestions. We have replaced the original literature (Nosrati et al., 2019) (Reference 3) by Li et al. (2016) in Line 37. The original fourth and fifth references (Guo et al., 2023 and Shivhare et al., 2018) were replaced by Xu et al. (2022) in Line 43.

New references:

Li, Z.Y.; Fang, H.Y. Impacts of climate change on water erosion: A review. Earth-Science Reviews 2016, 163, 94-117. https://doi.org/10.1016/j.earscirev.2016.10.004.

Xu, Z.; Belmont, P.; Brahney, J.; Gellis, A.C. Sediment source fingerprinting as an aid to large-scale landscape conservation and restoration: A review for the Mississippi River Basin. J. Environ. Manage. 2022, 324, 116260. https://doi.org/10.1016/j.jenvman.2022.116260.

 

3. p.2 l.73-75: I suggest rephrasing. The second part is clear, but the first part of the sentence is unclear. Why do composite samples taken at the outlet not represent the overall watershed contribution?

Reply and revision: Thank you for your valuable comments and suggestions. The authors apologize for any confusion caused by the unclear expression. Our intended meaning was: Composite sediment samples collected at the watershed outlet can represent the sediment sources of the entire watershed contribution. However, it may not reflect the spatial variation of the sediment contribution within the watershed, where the engineering disturbed sediment sources exhibit both temporal and spatial variability. To make the statements more clearly, we have revised the sentence (Line 73-75) into “Composite sediment samples collected at watershed outlets can represent the overall watershed sediment contribution, but may not necessarily explain the spatial differentiation characteristics of the impact of engineering disturbances [34].”

 

4. p.2 l.81 “researches”: do you mean “research”? “studies”?

Reply and revision: Thank you for your comments. We have changed the word “researches” into “studies” according to your suggestion in Line 81 of the revised manuscript.

 

5. p.2 l.85 “…were selected as research objects.” I suggest rephrasing. The research object is still the entire catchment isn’t it?

Reply and revision: Thanks for the suggestion. We have revised the sentence (Line 84-86) as “Different types of disturbance, such as spoil heaps and exposed slopes within the watershed during various periods, were selected as sediment sources along with the natural lands”.

 

6. p.3 l.99-100: Consider adding the information that Zheduo is a tributary/headwater river of the Yangtze River. This would be helpful for international readers.

Reply and revision: Thanks for your suggestions. We have added new text (a tertiary tributary of the Yangtze River) in Section 2.1 Line 100 accordingly.

 

7. p.3 l. 108-110: I would place the sentence on vegetation types behind the sentence on land use (l.115).

Reply and revision: Thank you for the suggestion and we have place the vegetation type sentence after land use (Section 2.1 Line 114-115).

 

8. Figure 1: Very nice figure! It would still be helpful to show the location of the basin in China.

Reply and revision: Thank you for the suggestion. We have revised the Figure 1 according to your and other reviewer’s suggestions.

 

9. p.3 l. 123: unit “um”: should it be µm (micrometer)?

Reply and revision: Thank you for pointing out the error, we have modified the unit in Section 2.1 Line 123.

 

10. p.4 l. 171-172: How do you know that the samples are recently deposited samples?

Reply and revision: Thank you very much for your comment. In this study, the deposited channel sediments were sampled in 2022 and 2023 to show the temporal variation of the source contribution. To make sure the samples can represent the sediment generation process during the sampled year, the sampling was conducted in October at the end of the rainy season. Considering the sediment generation and deposition within channel mainly accumulated in the wet season between May and September, we can treat the surface sediment as the recently deposited samples. In order to make this point more clearly, we have revised the sentence (Section 2.1 Line175-177) as “Multiple mixed samplings were conducted at each site, and surficial samples were collected. Since most sediment generation and deposition occurred during the rainy season which is just ended, the samples could reflect recently deposited sediments [32]”. A reference (Bravo-Linares et al., 2024) was cited to support this statement, in which the surficial deposition samples were also collected to assess the contemporary sediment source contributions similar to our study.

 

11. Section 2.5: Can you give some additional details on the mixing model? What kind of model is used to solve eq. 3-5? As you have more fingerprints than sediment sources, the system of equations 3-5 is overdetermined. How did you solve it? The last sentence suggests that you used an optimization procedure. Please explain how you optimized Res.

Reply and revision: Thank you for your comments and suggestions. The detailed information about the model can be found in Section 2.5, Line 247-255, where we discuss the model's underlying assumptions and equations. Our hybrid model is represented by Equation 3, with further details provided in Section 2.5, Line 264-267. We utilized the Sediment Source Assessment Tool to solve Equations 3-5, and revisions have been made to contain this information in Line 269-271: Based on the above steps, the source contribution rates were analyzed in the Sediment Source Assessment Tool to determine the source contributions to the sediment samples [44].

Regarding your concern that the number of fingerprints exceeds the number of sources, resulting in an overdetermined system, it is important to note that equations 4 and 5 impose constraints on the contribution rates. The solution is obtained by minimizing the least squares, which provides an approximate minimum value for Res. To improve the explanation regarding the optimization of Res, we have revised the content in Section 2.5, Line 267-269: Under the constraints of Equations 4 and 5, the relative contribution of each source can be determined by obtaining the optimal solution that minimizes Res using the least squares method.

 

12. Section 2.7: I was missing this information earlier. Consider integrating it into sections 2.4 and 2.5.

Reply and revision: Thank you very much for your suggestion. We have integrated the content in section 2.7 into section 2.4 and 2.5 accordingly. For example, Section 2.4.1, Line 209-210: Source reclassification was performed in RStudio using linear discriminant analysis (LDA) with the Fingerpro package. Section 2.4.2, Line 219-220: Range test was performed in RStudio using rangeTest with the Fingerpro package. Section 2.4.3, Line 233-235: Kruskal‒Wallis H test and DFA performed in RStudio using KWTest and DFATest with the Fingerpro package. Section 2.5, Line 269-271: Based on the above steps, the source contribution rates were analyzed in the Sediment Source Assessment Tool to determine the source contributions to the sediment samples.

 

13. p.7 l. 281: In the bottom left subfigure of Fig. 2 I don’t see a difference in Sn concentrations between the sources. Do you mean Sr?

Reply and revision: Thank you for pointing out the error, we have revised Sn into Sr. (Section 3.1 Line 292)

 

14. Section 3.1: can you comment on the channel banks as a natural subsoil source? I think that it is an interesting result that there are tracers that discriminate well between the natural subsoil sample "banks" and the disturbed subsoil samples "spoil" and "slope".

Reply and revision: Thank you for the valuable suggestion, it will greatly help refine the content of Section 3.1. We have added a comment on the differences in tracer concentration between natural and disturbed subsoil in Line 307-312 of Section 3.1, following your suggestion and the tracer concentration characteristics: “It should be noted that the channel bank, generally recognized as a natural subsoil source, could be discriminated by tracers. This may be due to the fact that the channel bank, mainly at the lowest elevation, contains the deposited material from the upstream slopes, whereas the disturbed subsoil sources consist of tunnel rock material (the spoil heap) or the deep soil profile layer (the exposed slope), which are different from the channel material.”

 

15. p.8 l.305-309: Tracers not passing the range test could also hint at missing sources. I would also mention particle size selectivity of the erosion and sediment transport processes (at least in the discussion). See my main concern above.

Reply and revision: Thank you for the excellent suggestion. We fully agree with you that the effect of particle size on tracer concentration is significant, especially when tracer concentrations vary over different particle size ranges. In this study, the samples were sieved to 2 mm and thus a broad range of particle sizes were used in the measurement of trace indices for fingerprint analysis. Some of the tracers that are related to coarse particles may be accumulated due to the particle size selectivity of the erosion and sediment transport processes. This could potentially alter the conservativeness of these tracers and affect the results of the fingerprint analysis.

To make this point clearer, we have added a discussion according to your suggestion in Section 4.1, Line 433-443: “It is important to note that the particle size selectivity of the erosion and sediment transport processes may affect the conservativeness of certain tracers and influence the results of the fingerprint analysis (Laceby et al., 2017). In this study, source samples were sieved to 2mm, which means that a relatively high proportion of coarse soil particles were used in the measurement of trace indices. Considering that the deposited sediment sample sites are close to spoil heaps and exposed slopes which are characterized by large particle size, the coarse disturbed sediments are more likely to enter channels and contribute to deposition than those from natural sources. The particle size selectivity of coarser materials may lead to tracer accumulation and exceed the highest concentrations at sources, resulting in tracer non-conservatism (e.g. the CaO in Figure 2 and Table 1).”

References added:

Laceby, J.P.; Evrard, O.; Smith, H.G.; Blake, W.H.; Olley, J.M.; Minella, J.P.G.; Owens, P.N. The challenges and opportunities of addressing particle size effects in sediment source fingerprinting: A review. Earth-Science Reviews 2017, 169, 85-103. https://doi.org/10.1016/j.earscirev.2017.04.009.

Collins, A.L.; Blackwell, M.; Boeckx, P.; Chivers, C.A.; Emelko, M.; Evrard, O.; Foster, I.; Gellis, A.; Gholami, H.; Granger, S.; et al. Sediment source fingerprinting: benchmarking recent outputs, remaining challenges and emerging themes. J. Soils Sediments 2020, 20, 4160-4193. https://doi.org/10.1007/s11368-020-02755-4.

 

16. p.9 l.338: Consider changing the title of the section. "different stages" made me think of water level, i.e. during floods or during low-flow periods.

Reply and revision: Thanks to your suggestion, we have changed the title into “Sediment source contribution in 2022 and 2023”. (Section 3.3.1 Line 353)

 

17. Table 4: I would be careful about giving growth rates for contributions in %. For example, the negative growth rate of the natural sources leads to the assumption that the mass of sediment originating from this source is decreasing, which is probably not the case because the decrease in contribution in % is likely to be a result of the increase of the other sources. Also, the table as it is presented at the moment, suggests that the growth rates for area can be compared to the growth rates of the contributions in %. This is not really the case, it should be compared to changes in absolute mass of sediment which you don’t have.

Reply and revision: Thanks for your comments and suggestions. We fully agree that a comparison of growth rate between source areas and sediment contributions in % may not reflect the actual sediment generation. To avoid any possible misunderstanding of the results of this study, we have deleted the growth rate data in Table 4. Meanwhile, we have also made revisions in the Discussion section (Line 519-520) according to other suggestions from you.

 

18. Figure 4: The figure is not very easy to read. Can you increase the size of the pie charts? Also please change the colors, red-green is not color-blind-friendly and the yellow is hard to differentiate from the green in the small pie charts.

Reply and revision: Thanks for your comments and suggestions. We have revised Figure 4 accordingly.

 

19. p.10 l.378-379 However, this equation is not valid on 2023 anymore, right? Can you comment on that in the discussion?

Reply and revision: Thanks for your comments. It is true that the spatial variation of the spoil heap sediment contribution rate in 2023 did not follow the equation developed on the basis of the 2022 data. This is mainly due to the spoil heap being the dominant type of disturbance in 2022. The spatial variation of the contribution rate is mainly determined by the sediment generation in the spoil heap and the transport of spoil sediment to the channel. In the case of 2023, sediment from exposed slopes upstream of the spoil heap and the use of spoil material for construction may disrupt the spoil sediment contribution rate. Therefore, the 2022 equation may not be valid on 2023. To make this point more clearly, we have added the following content in Section 4.2 Line 541-546 of the discussion section: On the other hand, the sediment contribution from exposed slopes may disrupt the spatial variation trend of the spoil sediment contribution rate. This is one of the reasons why the parabolic equation developed based on 2022 data is not valid in 2023. It implies that the spatial variation of sediment contribution may be complicated in the case of sediments from different types of engineering disturbed sources overlapping with each other.

 

20. p.11 l 422: I would replace “modulus” by “rate” as it is more common.

Reply and revision: Thanks for your suggestion, we have changed the word “modulus” into “rate”. (Section 4.1 Line 451)

 

21. p.12 l.434 Why did you use conjunctive here? I suggest rephrasing.

Reply and revision: Thanks for your suggestion. We have revised the sentence as “This would provide a tracer selection strategy for classifying natural and disturbed sources. In other words, reducing the number of sources could reduce model uncertainty and enhance the classification accuracy.” (Section 4.1 Line 463-466)

 

22. p.13 l.489 See my comment above on table 4. You cannot compare growth rates in area directly to growth rates in the contributions in %. Assuming that the mass of spoil heap material that is entering the river is proportional to the increase in area, and that the contributions of the other sources (also in mass, not in %) remain constant, the growth rate in mass would be equal the growth rate in area but the growth rate in % would be smaller. As you don’t have information about changes in mass, I would avoid this kind of comparisons.

Reply and revision: Thanks for your comments and suggestion. We have revised this statement as “This may be one reason why the sediment contribution rate of the spoil heap increased relatively slowly in 2023 compared to that of the exposed slope.” in Line 519-520, Section 4.2.

 

23. p. 14 l.565-566 avoid the conjunctive (“would”, “could”)

Reply and revision: Thanks for your suggestion. We have revised the sentence as “A quadratic function was developed to describe the variation of the spoil heap sediment contribution rate along the channel”. (Line 602-603)

 

 

References:

Laceby, J. P., Evrard, O., Smith, H. G., Blake, W. H., Olley, J. M., Minella, J. P., & Owens, P. N. (2017). The challenges and opportunities of addressing particle size effects in sediment source fingerprinting: a review. Earth-Science Reviews, 169, 85-103.

Collins, A. L., Blackwell, M., Boeckx, P., Chivers, C. A., Emelko, M., Evrard, O., ... & Zhang, Y. (2020). Sediment source fingerprinting: benchmarking recent outputs, remaining challenges and emerging themes. Journal of Soils and Sediments, 20, 4160-4193.

Reply and revision: Thanks for your suggestion. The references have been cited in section 4.1.

 

Comments on the Quality of English Language: The English sounds good to me. I suggest rephrasing a few sentences (see comments above).

Reply and revision: Thank you for your valuable suggestions for improving the language of the manuscript. We have made revisions accordingly.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The paper was really improved.