In figure 4, we suggest reviewing the analysis of the decay of sediments from the "ROAD" source, which has an increase between S2 and S3, and only then has a decrease as mentioned. It was also not very clear why S2 and S4 have little or no sediment contribution from the "road" source.

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

Reply and revision: Thank you for your comments and suggestions. We have added more description about the trend of related sites in L365-367: “except for the high contribution rate of S3, which is located at the intersection of the road and the river. On the contrary, S2 and S4, which were connected to low gradient road segments, showed little road sediment contribution.”

Reviewer 2 Report

Comments and Suggestions for Authors

This manuscript presents an evaluation of the impact of road construction on sediment deposition within a typical watershed in southeastern Tibet. Field sampling was conducted within a watershed affected by road construction, complemented by a composite fingerprint analysis to identify the sources of sediment. The findings offer valuable insights for assessing the environmental impact of human activities, particularly concerning the identification of sediment contributions from road construction zones and their spatial distribution within the watershed. This understanding is pivotal for guiding conservation efforts to more effectively target areas where interventions are most needed, thereby underscoring the significance of this research. However, this manuscript is considerable further improvement. There are some general comments below that aim to strengthen this work in future.

(1) Fig. 4 shows the presence of extensive bare rock areas (34.78 km²) in the upper reaches of the watershed, which are highly susceptible to erosion due to the absence of vegetative cover. It is perplexing that these areas have not been considered as potential sources of sediment.

(2) Line 163: Sediment samples in this manuscript was the collection of surficial samples in river to reflect the recently deposited sediments in late September 2021. Thus the samples primarily reflect erosion phenomena in recent years, predominantly from the year 2021. However, the specific details regarding the concurrent road construction within the watershed during this period remain unclear. The text in line 126 mentions that the construction project of this section began in August 2014, marking a considerable duration to date. Hence, it is crucial to ascertain how many areas are still under construction at present. This information is vital for understanding the magnitude of erosion triggered by the roads.

(3) Line 145: This paper should explicitly state that the gullies and roads are the specific locations for sampling. The description of the sampling process in lines 144-145 is too brief, and it is recommended to supplement with detailed sampling steps for each sampling source.

(4) Line 182: The data on particle size of source and sediment samples should be provide, particularly the content of particles with a diameter less than 63 μm, to demonstrate the representativeness of the selected particle size range in fingerprint analysis.

(5) Line 194: The conservative behavior of fingerprint factors (i.e., tracers) plays a crucial role in fingerprint analysis and significantly impacts the results. It is recommended to incorporate a test of the conservative characteristics of potential fingerprint factors. For specific references, one may consult the description of this procedure by Wilkinson et al. (Agriculture, Ecosystems & Environment, 2013, 180: 90-102). Additionally, Collins et al. (J Environ Manage, 2017, 194: 86-108.) have also made important discussions on the conservatism of tracers in a review of the current state of knowledge sediment source fingerprinting.

(5) Line 249: I am puzzled about what "rill erosion" specifically refers to in this paper.

(6) What do the horizontal lines in the middle and at both ends of the box plot (Fig. 2) represent? Clear labeling and explanation of each component is recommended to enable accurate interpretation of the chart and comprehension of the statistical properties of the presented data.

(7) Fig. 3 shows that the distinction between the road and the grassland & shrubland sources is not pronounced. It is recommended to provide a brief explanation for this outcome, such as the possibility that the road tends to pass through grassland areas, among other reasons. The authors should determine the specifics based on actual conditions.

(8) Fig.1 shows that sediment sampling was carried out along the main river channel in the study area, collecting a total of 16 sediment samples from upstream to downstream. It is noteworthy that the catchment area controlled by each sediment sampling site varies within the basin, and it gradually increases from upstream to downstream. In other words, each sediment sampling point controls a different sub-watershed, and within these sub-watersheds, the conditions of the four sources are distinct. Therefore, I am puzzled about how the authors processed the source samples within the sub-watersheds controlled by each sediment sampling point. One of the assumptions of sediment source fingerprinting is that the identified sediment sources entirely from upstream sources. Consequently, sediment sampling sites are typically located at the outlet of watershed, ensuring that erosion from each source area upstream has the potential to be transported to this outlet.

(9) Line 308-322: A significant portion of this section is dedicated to comparing the differences in sediment contributions from various sources upstream and downstream. I am somewhat confused about the significance of these comparisons and what issues they might elucidate.

(10) Line 382-404: It remains challenging to comprehend why the contribution from roads is so substantial. The sediment sampled in this study predominantly represents the eroded sediment from the rainy season of 2021, as well as some sediment from previous years. What was the progress of road construction within the study area during this period? Were the construction progresses consistent between upstream and downstream areas? Given that it is a national highway, it is likely that once construction is completed, the road surface will be hardened, and measures such as slope protection may be implemented, which would subsequently lead to a significant reduction in the erosion and transportation of sediment from the roads. Erosion from road construction is likely most concentrated during the construction phase, such as erosion caused by excavation and the disposal of earth materials. For example, a recent study (Shu et al., Fingerprinting sediment sources in mountainous catchments disturbed by plantation and construction activities in southern China. Land Degradation & Development, 2024) used fingerprint analysis to evaluate soil erosion caused by construction activities, emphasizing the impact of roads and similar infrastructure during the construction process on magnitude of sediment downstream. I believe that analyzing from these perspectives can help explain the high contribution rate of road sediment.

(11) Line 386: This statement may not be entirely accurate, as roads only occupy a small area within a watershed. However, the erosion processes associated with roads include the erosion of road slopes and the collapse of road surfaces, which are partly attributed to gravitational erosion and are largely similar to erosion within major gullies. For specific details, one can refer to the papers by Zhang et al. (Rill and gully erosion on unpaved roads under heavy rainfall in agricultural watersheds on China’s Loess Plateau. 2019, Agriculture, Ecosystems & Environment, 284: 106580.).

(11) It is suggested to include a few paragraphs of discussion, primarily elucidating how the findings or conclusions of this study can guide the soil and water conservation efforts within basins where road construction or related activities are taking place.

Author Response

Dear Editor,

With kind regards,

The authors

Issues raised by Reviewer 2

Reply and revision: Thanks for your comments. The bare rock areas in the Doxiong River watershed are predominantly located above 4000 meters or on mountaintops, which are challenging to access due to the steep terrain and dense primary forest without roads. Moreover, these bare rock areas are primarily composed of rock rather than soil. In previous studies assessing soil erosion on the Tibetan Plateau, bare rock areas were excluded, similar to other categories such as water bodies and permanent glaciers (Teng et al., 2018, “Current and future assessments of soil erosion by water on the Tibetan Plateau based on RUSLE and CMIP5 climate models”, Science of the Total Environment, 635, 673–686). Considering the above reasons, the bare rock areas are not considered as potential sources of sediment along with the permanent snow cover areas in the study watershed. To prevent any possible misunderstanding among readers, we have added statements “It is important to note that bare rock areas were generally excluded when assessing soil erosion on the Tibetan Plateau [37] and were therefore not considered as potential sediment sources.” in L153-155 to clarify this issue.

Reply and revision: Thank you for your comments and suggestions. This information is crucial, and we apologize for the insufficient description of road construction in the study watershed. The construction project of the G219 national highway began in August 2014, with the starting point approximately 7 km west of the Doxiong River watershed on the west side of Doxiongla Mountain. The construction took about four years to build the road and a tunnel that passes through the mountain, reaching the northwest side of the Doxiong River watershed in 2018. The 30.8 km road span within the study area was constructed from November 2018 to November 2020. After completing the road embankment, most of the road remained unpaved throughout 2021, except for a small 2.5 km section near the starting point, which was paved. During this period, construction efforts were primarily focused on the tunnel on the southeast side of the Doxiong River watershed, with no further work on the road embankment. In fact, the road surface was not paved until the tunnel was completed in 2022 (This may be due to people trying to avoid some heavy vehicles causing damage to the paved road surface). During the research stage of this study, the highway should be considered an unpaved road, with the road surface generally consistent throughout the watershed.

According to your and other reviewer’s suggestions, we have added specific details about road construction and associated erosion problems in the revised manuscript to clarify this issue in L131-143: “The construction of the highway span in the Doxiong River watershed occurred from November 2018 to November 2020. After completing the road embankment, most of the road remained unpaved throughout 2021, except for a small span (about 2.5 km long) near the starting point, which was paved. At this stage, construction efforts were focused on the tunnel on the southeast side of the Doxiong River watershed, with no further work on the road embankment. Therefore, the highway should be treated as an unpaved road, with the road surface generally consistent throughout the watershed. Additionally, highway construction has disrupted the natural terrain, resulting in engineering cutslopes (average slope = 42.6°) and fillslopes (average slope = 38.4°) (Figure 1e, f). Most of these slopes have been treated with simple conservation measures, such as sowing grass seed to create sparse ground cover, but soil erosion remains intense, with rill erosion frequently observed (Figure 1e). Road drainage system consisting of ditches and culverts have been constructed to allow the overland flow to pass through.” Meanwhile, we have also revised Figure 1 and Figure 5 to show the paved and unpaved road segments.

Reply and revision: Thank you for your comments and suggestions. The authors apologize for the unclear description of field sampling. We have revised the methods section and added more detailed information about the sampling procedures in L155-167: “In natural forest and grassland areas, sample sites were selected on relatively gentle slopes representative of the type of vegetation coverage. For each sampling site, three subsamples were collected within a 2×2 m area. The litter layer was removed to collect the top layer (0–5 cm) samples of the mineral soil. (We apologize for confusing the depths of litter and the mineral soil in the previous manuscript) For channel bank sources, sampling sites were chosen in tributaries or gullies along the main channel. Samples were taken by collecting the surface layer of 2cm thickness from the sidewall of channel bank. Three replicates would be collected to form a composite sample. For road-related sources, road surface sites were selected in areas where significant erosion was evident. Topsoil samples (0–2 cm) were taken along the transverse line of the road surface. Sampling for cut-slope sources was similar to those for channel banks and gullies, with a 2 cm surface layer collected. On fillslopes, the soil is relatively loose and therefore 0–5 cm topsoil was sampled to reflect the depth of rill erosion. All road-related samples were composite samples, each consisting of three subsamples per site.”

Reply and revision: Thank you for your comments and suggestions. We have added the grain size curves of different sources and sediment samples in Figure 2 in accordance with your and the other reviewer’s recommendations. The reason we chose a particle diameter of less than 63 μm because previous studies have reported that the < 63 μm fraction appears to be the most commonly used particle size fraction for sediment tracing (Laceby et al., 2017). We have made revision in L205-206 to clarify this issue.

Reply and revision: Thank you for your comments and suggestions. In the previous version of the manuscript, we have performed a range test to demonstrate the conservative characteristics of potential fingerprints and stated in Section 3.1: “Figure 2 also shows that the minimum SOC and TN levels in the deposited sediments were lower than those in the source area sediments and were therefore excluded from further analysis due to their lack of conservative characteristics.” To provide readers with more comprehensive information, we have added content in Section 2.4 about the procedures of the conservatism test in L223-231: “The application of sediment fingerprinting techniques is based on the assumptions that tracers follow conservatism and that the selected tracers behave conservatively during movement and migration within the catchment [14]. There is a direct link between the source and the sediment through the tracer; therefore, before applying the fingerprinting technique, a conservatism test should be carried out to select a tracer with conservative behavior. A commonly used conservatism test is the range test: (1) The range of tracer concentrations in all sediment samples must be within the range of all source samples; (2) the average tracer concentration in all sediment samples must be within the range of the average source sample concentration [41].” Meanwhile, we have also added the specific values of the minimum SOC and TN in L308-311 to support the conservatism test results.

(6) Line 249: I am puzzled about what "rill erosion" specifically refers to in this paper.

Reply and revision: Thank you for your comments. We apologize for the incorrect statement. It should be the road sediment contribution rate. We have made the necessary revision in L287 of the revised manuscript.

(7) What do the horizontal lines in the middle and at both ends of the box plot (Fig. 2) represent? Clear labeling and explanation of each component is recommended to enable accurate interpretation of the chart and comprehension of the statistical properties of the presented data.

Reply and revision: Thank you for your comments and suggestions. The horizontal lines at the upper end, middle, and lower end of the box plot represent the maximum, median, and minimum values, respectively. We have added a label in the figure (Figure 3 in the revised manuscript) to clarify this.

(8) Fig. 3 shows that the distinction between the road and the grassland & shrubland sources is not pronounced. It is recommended to provide a brief explanation for this outcome, such as the possibility that the road tends to pass through grassland areas, among other reasons. The authors should determine the specifics based on actual conditions.

Reply and revision: Thank you for your comments and valuable suggestions. We have added an explanation in L320-325 to address this issue: “It should be mentioned that the distinction between the road and the grassland & shrubland sources is not pronounced. This may be partly due to the fact that the road passes through grassland and shrubland areas. Additionally, the vegetation restoration measures carried out on the roadside slopes may also contribute to the insignificant differences in the indices between the road and grassland sources.”

(9) Fig.1 shows that sediment sampling was carried out along the main river channel in the study area, collecting a total of 16 sediment samples from upstream to downstream. It is noteworthy that the catchment area controlled by each sediment sampling site varies within the basin, and it gradually increases from upstream to downstream. In other words, each sediment sampling point controls a different sub-watershed, and within these sub-watersheds, the conditions of the four sources are distinct. Therefore, I am puzzled about how the authors processed the source samples within the sub-watersheds controlled by each sediment sampling point. One of the assumptions of sediment source fingerprinting is that the identified sediment sources entirely from upstream sources. Consequently, sediment sampling sites are typically located at the outlet of watershed, ensuring that erosion from each source area upstream has the potential to be transported to this outlet.

Reply and revision: Thank you very much for your comments. We completely agree with your opinions regarding the application of sediment source fingerprinting. Initially, we aimed to divide the watershed into different sub-watersheds and explore the influence of road construction by collecting source soils and sediment samples at the outlets of these sub-watersheds. However, due to the narrow and elongated shape of the Doxiong River watershed, most tributaries intersect the main channel vertically, resulting in small sub-watersheds. These sub-watersheds contain only short spans of road segments, which are insufficient to reflect the disturbance caused by road construction. Therefore, we decided to sample the deposited sediment along the main channel, which runs parallel to the road throughout the watershed. Additionally, since most parts of the highway remained unpaved, the road erosion conditions were generally consistent between upstream and downstream areas. This consistency allowed us to discuss the spatial variation of road-related sediment contributions to the sediment deposition along the main channel. Considering the studied watershed is relatively small and the channel structure is simple, we did not divide it into sub-watersheds and used all source samples to represent the fingerprints of the whole catchment. It should be noted that previous sediment source fingerprinting research has demonstrated that average values can be representative of the entire catchment if the variation in topsoil is minimal across the catchment (Franz et al., 2014, "Sediments in urban river basins: Identification of sediment sources within the Lago Paranoá catchment, Brasilia DF, Brazil–using the fingerprint approach," Science of the Total Environment). Furthermore, using all sources for the fingerprint analysis ensures the comparability of contribution rates between different sites along the main channel. To clarify our methodology, we have added content in L221-222 explaining how we processed the source samples. We also look forward to receiving further valuable suggestions from you to improve this issue.

(10) Line 308-322: A significant portion of this section is dedicated to comparing the differences in sediment contributions from various sources upstream and downstream. I am somewhat confused about the significance of these comparisons and what issues they might elucidate.

Reply and revision: Thank you for your comments. This section of the results primarily demonstrates the spatial variation in sediment contribution from different sources along the main channel. This information supports the main objective of this paper by highlighting the spatial variation in road-related sediment contribution rates and provides a basis for further analysis in Section 3.3. To provide a comprehensive understanding of the deposited sediment sources, we also included the spatial variation of contributions from other land uses. For example, we found that the contribution rate from forest and grassland sources changes slightly as their respective areas change from the upper to the lower part of the watershed. Additionally, we observed that the contribution rate of channel bank sediment decreases with the decreasing number of tributaries from the upper to the lower part of the watershed. To explain the results more clearly, we have revised the manuscript in L356-360 as follows: “There is a slight decrease (from 55.3% to 49.8%) in the amount of bank-related sediment with the decreasing number of tributaries from the upper to the lower part of the watershed. On the other hand, the average sediment contribution from forest sources increases from 2.5% to 9.8% with the increasing forest area when comparing the upper and lower parts.”

(11) Line 382-404: It remains challenging to comprehend why the contribution from roads is so substantial. The sediment sampled in this study predominantly represents the eroded sediment from the rainy season of 2021, as well as some sediment from previous years. What was the progress of road construction within the study area during this period? Were the construction progresses consistent between upstream and downstream areas? Given that it is a national highway, it is likely that once construction is completed, the road surface will be hardened, and measures such as slope protection may be implemented, which would subsequently lead to a significant reduction in the erosion and transportation of sediment from the roads. Erosion from road construction is likely most concentrated during the construction phase, such as erosion caused by excavation and the disposal of earth materials. For example, a recent study (Shu et al., Fingerprinting sediment sources in mountainous catchments disturbed by plantation and construction activities in southern China. Land Degradation & Development, 2024) used fingerprint analysis to evaluate soil erosion caused by construction activities, emphasizing the impact of roads and similar infrastructure during the construction process on magnitude of sediment downstream. I believe that analyzing from these perspectives can help explain the high contribution rate of road sediment.

Reply and revision: Thank you for your comments and suggestions. We have added more information about the road construction progress within the study area in the method section (L131-143) according to your suggestions raised in Issue 2. After the roadbed construction was completed in late 2020, most of the highway span within the Doxiong River watershed remained unsealed, and no further construction work was carried out on the road embankment during 2021. Therefore, the construction progress can be considered consistent in both upstream and downstream areas of the watershed. The unpaved road surface, cutslopes, and fillslopes are the main sediment sources created by construction activities. Although simple vegetation restoration measures were adopted on roadside slopes, soil erosion remained intensive for most road segments. The eroded sediment would transport downstream through the road drainage system, including ditches and culverts, contributing to the nearby channel. To clarify why road sediment contributed so significantly to the deposited sediment in the watershed, we have added more explanation in L445-459 of the discussion section: “As most of the highway span within the Doxiong River watershed was unpaved during our study period, the compacted road surface is characterized by low infiltration and high runoff generation capacity [49]. This would increase runoff and lead to a higher risk of soil erosion on the unpaved road surface [50]. Additionally, heavy vehicles passing through for construction activities disturbed the unpaved road surface, resulting in highly erodible material (Figure 1g) [51], thus accelerating the soil erosion process. The road cutslopes also suffer from soil erosion due to intercepted upstream runoff and gravitational erosion [52,53]. Beside the abovementioned sediment source perspectives, the road drainage system of ditches and culverts would increase the sediment connectivity between the road and the river. This in turn would allow more sediment from road-related sources to be transported to the nearby channel [54]. It should be noted that the high contribution rate of road-related sediment mainly reflected the situation that the highway is still under construction. In fact, with the road surface are harden and construction activities are completed, the road-related erosion would be reduced and so is the sediment that entering nearby channel [31].”

(12) Line 386: This statement may not be entirely accurate, as roads only occupy a small area within a watershed. However, the erosion processes associated with roads include the erosion of road slopes and the collapse of road surfaces, which are partly attributed to gravitational erosion and are largely similar to erosion within major gullies. For specific details, one can refer to the papers by Zhang et al. (Rill and gully erosion on unpaved roads under heavy rainfall in agricultural watersheds on China’s Loess Plateau. 2019, Agriculture, Ecosystems & Environment, 284: 106580.).

Reply and revision: Thank you for your comments and suggestions. In this sentence we mentioned the road area and its ratio comparing with the watershed to stress the importance of road as a sediment source. To make sure the statement is more accurate, we have revised the sentence in L436 as: “although the total road surface area only account for approximately 0.21% of the total watershed area (0.3 km²).” In the Loess Plateau and the hilly red soil region of southeast China, the construction of low-grade unpaved roads is typically carried out using crawler tractors to remove the topsoil downhill and compact the underlying soil as a road surface. Due to the lack of strict engineering construction standards, these roads have very few culverts or cross drains to allow surface runoff to pass through. Consequently, overland flow accumulates along road segments, causing severe soil losses such as gullies or roadbed collapse as reported by Zhang et al. (2019) and Shu et al. (2024). However, the road in this study is a national highway constructed to relatively high engineering standards, despite remaining unpaved during 2021. A drainage system consisting of ditches and culverts was built along the road, preventing the formation of road gullies or the collapse of road surfaces. To explain how road construction affects soil erosion, we have added content (L445-459) highlighting the impact of the compacted and disturbed road surface, which enhances runoff generation capacity and reduces the erosion resistance of surface materials. Additionally, the drainage system increases the sediment connectivity between the road and the river, contributing to the transportation of road-related sediment to the nearby channel.

(13) It is suggested to include a few paragraphs of discussion, primarily elucidating how the findings or conclusions of this study can guide the soil and water conservation efforts within basins where road construction or related activities are taking place.

Reply and revision: Thank you for your comments and suggestions. We have made revisions and added content about soil and water conservation in L465-473: “In order to mitigate soil erosion and sediment generation occurring on cutslopes and roadside slopes, vegetation restoration combined with soil stabilization measures such as the application of geotextiles and blankets could be employed [55]. Additionally, a well-designed road drainage system incorporating conservation measures, such as armored ditches and sediment pits, would help protect the roadbed from erosion [7]. During the process of sediment transport from roads to nearby channels, sediment basins, energy dissipation structures and in-channel grade control structures [56] may be beneficial in controlling channel erosion and reducing sediment transport to the main channel.”

Reviewer 3 Report

Comments and Suggestions for Authors

This study reports a composite fingerprint analysis of the spatial variation of road sediment contribution to channel sediment deposition. The manuscript is generally well written and the methodology is sound. I suggest that it could be considered for publication after addressing the following issues:

(1) The abstract section, I suggest to add one sentence or two at the end of the abstract to describe the main innovation of this study.

(2) L131-132, How about the average height and slope gradient for the highway cutslope and fillslope? Generally, some conservation measures would be applied to control soil erosion from these slopes during the construction process. I suggest the authors to provide more information about this point.

(3) The sediment sources (forest, grassland, channel bank and roads) seems differ greatly from each other as demonstrated by the photos in Figure 1. I recommend the authors to clarify whether these sources were sampled using the same method or not.

(4) L160-162, “The above reason for choosing the sampling site ensures that one sampling site represents the sediment transport process within the area between it and the upstream adjacent site”. This design is critical to analysis the deposited sediment and maybe more references are needed to support it.

(5) L167, How many road segments have been surveyed in this study?

(6) L183-184, The selection of chemical soil indices is very important for applying the composite fingerprint analysis. Although the author has mentioned that it was based on the geological and chemical background of the study area. I recommended that a reference should be added.

(7) Figure 6, Looks like one point is missing when building the equations between the sediment contribution rate and the channel gradient and flow length to road respectively. It is necessary to mention this issue in the footnote of the figure.

(8) The authors have established regression equations and proposed some critical values such as the critical road slope gradient and critical distance. Will these equations still suitable in watershed which has a complicate road networks composed of different road grades? I suggest to demonstrate the application conditions of these quantitative findings, as well as the limitation of this study and potential future researches in section 4.2.

Author Response

Dear Editor,

With kind regards,

The authors

Issues raised by Reviewer 3

(1) The abstract section, I suggest to add one sentence or two at the end of the abstract to describe the main innovation of this study.

Reply and revision: Thank you for your comments and suggestions. We have added a sentence “These results have clarified how road construction spatially affects sediment at the watershed scale.” in L25-26 to show the innovation.

Reply and revision: Thanks for your comments and suggestions. We have measured the gradient of cutslope and fillslope and therefore we added the information in L139. Meanwhile, we also provided the information about roadside slope conservation measures in L140-142: “Most of these slopes have been treated with simple conservation measures, such as sowing grass seed to create sparse ground cover, but soil erosion remains intense, with rill erosion frequently observed.” according to your and other reviewer’s suggestion.

Reply and revision: Thanks for your comments and suggestions. In this study, soil samples were taken from different sources using different methods. To clarify this issue, we have revised the methods section and added more detailed information about the sampling procedures in L155-167: “In natural forest and grassland areas, sample sites were selected on relatively gentle slopes representative of the type of vegetation coverage. For each sampling site, three subsamples were collected within a 2×2 m area. The litter layer was removed to collect the top layer (0–5 cm) samples of the mineral soil. For channel bank sources, sampling sites were chosen in tributaries or gullies along the main channel. Samples were taken by collecting the surface layer of 2cm thickness from the sidewall of channel bank. Three replicates would be collected to form a composite sample. For road-related sources, road surface sites were selected in areas where significant erosion was evident. Topsoil samples (0–2 cm) were taken along the transverse line of the road surface. Sampling for cut-slope sources was similar to those for channel banks and gullies, with a 2 cm surface layer collected. On fillslopes, the soil is relatively loose and therefore 0–5 cm topsoil was sampled to reflect the depth of rill erosion. All road-related samples were composite samples, each consisting of three subsamples per site.”

Reply and revision: Thanks for your comments and suggestions. We have added reference (36) to support this point in L184.

(5) L167, How many road segments have been surveyed in this study?

Reply and revision: Thanks for your comments. We have added the road segment number in L189 accordingly.

Reply and revision: Thanks for your comments and suggestions. We have added the reference (41) to support this point in L209.

Reply and revision: Thanks for your comments and suggestions. We have added a Note in Figure 7 about this issue: one site has been removed to build a better equation with channel gradient or flow length.

Reply and revision: Thanks for your comments and suggestions. We have added more discussion about this issue in L531-539: “It should be noted that the above equations and critical values were mainly represent the situation of the studied watershed, where the road network structure is relatively simple. In the case of more complex road networks comprising different road grades, further research is required to elucidate the spatial variation mechanism of road-related sediment contribution. For example, the results of the fingerprint analysis could be related to the sediment connectivity that is altered by the intersection between the road network and the channel network. In addition, the spatiotemporal variations of sediment sources should also be taken into account when different constructions are carried out at different stages.”

Reviewer 4 Report

Comments and Suggestions for Authors

Introduction written interestingly, factually correct and has a correct structure.

In my opinion, there is a definite lack of ground material grain size curves.

The sampling process is always problematic, especially in field conditions. From a geotechnical point of view, it would be appropriate to address the inadequacies of the presented method of sample collection. For example, in Europe, Eurocode 7 also indicates five quality classes of soil samples for laboratory testing. The quality classes of soil samples are as follows: (1) intact, (2) disturbed, (3) compacted, (4) reworked and (5) reconstructed. What are the local considerations for such testing?

It is understandable why the methodology does not allow testing the density of an intact sample or its porosity, however, given the importance of these parameters in geotechnical testing, utors should make this clear.

Statements about some correlations are unclear, e.g. “the correlation coefficient is not significant” does not provide exact values, making it difficult to assess how weak or strong these correlations are. It would have been better to provide the exact values of the correlation coefficients and their confidence intervals, which would have allowed a more precise interpretation of the study results.

In the conclusion of the text, the authors recommend more attention to channel management and sediment generation, but there are no specific suggestions for actions or strategies that could be applied. Pointing out specific countermeasures or practical recommendations would be more valuable to decision-makers and environmental management practitioners.

Author Response

Dear Editor,

With kind regards,

The authors

Issues raised by Reviewer 4

Introduction written interestingly, factually correct and has a correct structure.

Reply: Thank you very much for your comment.

In my opinion, there is a definite lack of ground material grain size curves.

Reply and revision: Thank you for your comments. We have added the grain size curves of different sources and sediment samples in Figure 2, according to your and the other reviewer’s suggestions.

Reply and revision: Thank you for your comments and suggestions. The soil samples collected in this study should be classified as disturbed according to Eurocode 7. The sampling procedure follows the most commonly used method adopted for fingerprint analysis. It should be noted that composite fingerprint analysis is mainly based on soil chemical indices as source fingerprints. Therefore, the density of an intact sample or its porosity were not considered in this study. According to the suggestions from you and the other reviewers, we have added more detailed information about the sampling steps in L155-167: “In natural forest and grassland areas, sample sites were selected on relatively gentle slopes representative of the type of vegetation coverage. For each sampling site, three subsamples were collected within a 2×2 m area. The litter layer was removed to collect the top layer (0–5 cm) samples of the mineral soil. For channel bank sources, sampling sites were chosen in tributaries or gullies along the main channel. Samples were taken by collecting the surface layer of 2cm thickness from the sidewall of channel bank. Three replicates would be collected to form a composite sample. For road-related sources, road surface sites were selected in areas where significant erosion was evident. Topsoil samples (0–2 cm) were taken along the transverse line of the road surface. Sampling for cut-slope sources was similar to those for channel banks and gullies, with a 2 cm surface layer collected. On fillslopes, the soil is relatively loose and therefore 0–5 cm topsoil was sampled to reflect the depth of rill erosion. All road-related samples were composite samples, each consisting of three subsamples per site.”

Reply and revision: Thanks for your comments and suggestions. We have checked the related contents and added the specific correlation coefficients “R=0.31 and 0.19 for CRS and FLS respectively” in L398 and “R=-0.27” in L403.

Reply and revision: Thanks for your comments and suggestions. We have made revisions and added contents about soil and water conservation in L465-473: “In order to mitigate soil erosion and sediment generation occurring on cutslopes and roadside slopes, vegetation restoration combined with soil stabilization measures such as the application of geotextiles and blankets could be employed [55]. Additionally, a well-designed road drainage system incorporating conservation measures, such as armored ditches and sediment pits, would help protect the roadbed from erosion [7]. During the process of sediment transport from roads to nearby channels, sediment basins, energy dissipation structures and in-channel grade control structures [56] may be beneficial in controlling channel erosion and reducing sediment transport to the main channel.”

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

Reviewer 4 Report

Comments and Suggestions for Authors

After analyzing the entire text, I believe that it is highly polished in content, addresses an important topic and is a valuable scientific contribution.

For further investigations the authors could discuss how climate change, through increased frequency and intensity of extreme weather events, could exacerbate soil erosion, especially in areas already disturbed by human activities such as road construction. Also, the broader economic and social impacts of soil erosion caused by road construction could be discussed, based on literature studies.

As a additional future advise the soil should be classified, despite the presence of a grain size curve.

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Composite Fingerprint Analysis of Sediment Sources in a Watershed Disturbed by Road Construction in Southeastern Tibet

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