Review Reports - Characteristics of the Sediment Transport Process in Vegetation Hillslopes under Different Flow Rates

Round 1

Reviewer 1 Report

The contribution and novelty of this work is not clear. The linear model does not show higher correlation than power law application in this study. The authors should explain the reasons for low correlation coefficient in tables.

Literature review needs more critical investigation showing the necessity of the present study.

Limitations of this study are not discussed in the text.

Conclusions do not improve our understanding from the subject.

This study needs more physical explanations and discussion about the contribution of vegetation in the results.

Turbulence rather than laminar flow developed by vegetation should be discussed. Low velocity inside the vegetation may not be measured in practice. This may limit the extension of this study.

References should extend and report the effect of vegetation on the sediment transport.

Author Response

Response: Thank you very much for your comments.

The novelty of this study is mainly to reveal the sediment trapping characteristics of vegetation hillslopes under different inflow rates through the movement processes of sediment particles.

Both linear and exponential models are empirical regression models based on experimental data, which were used to explore possible relationship expressions. We found that both models can better express this relationship, but the linear model has more stable performance. This can provide a helpful reference when building a water-sediment process model. Here, R² is the goodness of fit rather than the correlation coefficient. The low values of R² in some tests (S15Q15SC120 and S15Q45SC120) may be due to the large size of the experimental plot, which is 10 m in length. It is difficult to ensure that the optimal state can be achieved during each experimental process, which may result in some experimental data being biased. However, under the same data conditions, linear models exhibit better stability.

Please see the Page 3 Lines 120-122 and Page 12 Lines 363-372 in the revised manuscript.

Literature review needs more critical investigation showing the necessity of the present study.

Response: Thank you for your comments. We have added some critical studies.

Generally, coarse particles are more prone to deposition, while small particles are more easily transported by overland flow. However, some experimental results indicated that this is not absolute. Ma et al. (2013) conducted experiments on simulated grass strips under three slopes (3°, 9°, and 15°), two flow rates (20 and 60 L min⁻¹ m⁻¹), and sediment concentrations in the range of 100 − 300 g L⁻¹. The results indicate that the proportion of coarse particles > 25 μm was greater, while the deposition efficiency of particles < 1 μm and 10 – 25 μm was greater than that of particles 1 – 10 μm. This study explained some particle selectivity characteristics of silt-laden overland flow passing through VFS but did not further explore the differences in the process caused by different flow rates.

Please see the Pages 2-3 Lines 89-98 in the revised manuscript.

Limitations of this study are not discussed in the text.

Response: Thank you for your comments. We have added limitations of this study。

This study still has some limitations, mainly including the inability to distinguish the differences in sediment particle movement caused by rigid and flexible vegetation and the inability to identify the impact of the vegetation growth cycle on its processes. When modeling, the problem is finally simplified with different particle size thresholds, which cannot yet be provided in this study.

Please see the Page 15 Lines 473-477 in the revised manuscript.

Conclusions do not improve our understanding from the subject.

Response: Thank you for your comments. We have carefully revised the conclusions to make the theme more prominent.

Please see the chapter 4 conclusions, Pages 15-16, Lines 489-520 in the revised manuscript.

This study needs more physical explanations and discussion about the contribution of vegetation in the results.

Response: Thank you for your suggestions. We have provided more physical explanations and discussions on the contribution of vegetation in the results.

Overall, the variation process of sediment particle size composition at the outlet shows that in the early stage of sediment trapping by the VFS, all particle sizes exhibit high deposition rates (Figure 6). However, as the sediment-trapping process continues, the sediment particles of each size at the outlet gradually increase. From Figure 2, as the flow rate increases and the average flow velocity increases, the resistance f of the VFS to overland flow increases. The grass used in this study was a rigid plant, and its resistance to overland flow was mainly composed of soil bed surface, litter, and stems. This is because the sediment cover weakened the resistance of the VFS to overland flow, and enhanced the overland flow dynamics, and more sediment particles were exported from the VFS. In the later stage, the output sediment volume remains stable (Figure 4), and the overall particle size level and composition of sediment also remain relatively stable (Figure 5 and Figure 6).

Please see Page 15, Lines 461-472 in the revised manuscript.

Turbulence rather than laminar flow developed by vegetation should be discussed. Low velocity inside the vegetation may not be measured in practice. This may limit the extension of this study.

Response: Thank you very much for your comments. Please see Page 9, Lines 283-287, 289-290, Page 5, Lines 177-181 in the revised manuscript.

As proposed in the manuscript, there is currently no widely accepted parameter for qualitative analysis of the pattern of shallow flow on hillslopes. Re is still used in relevant studies. Even slight disturbances caused by raindrops can result in almost no laminar flow on the hillslope (Liu et al., 2012). Therefore, the disturbance of vegetation cannot maintain laminar flow on the hillslope. However, there are still some Re < 500 in the calculation of overland flow on hillslopes, so this study refers to the term pseudo laminar flow.

When using the dye tracer (KMnO4) method to measure the flow velocity on vegetation hillslopes, the leaves will be pushed aside at the measurement location to prevent obstruction of the view. The experimental vegetation in this study was rigid, and the leaves do not directly contact the overland flow, which does not affect the flow velocity. We have measured the velocity of overland flow well in other similar series of experiments.

Liu, G., Xu, W., Zhang Q., et al. (2012). Interrill and rill erosion on hillslope. Applied Mechanics and Materials. Trans Tech Publications Ltd. 170: 1344-1347.

References should extend and report the effect of vegetation on the sediment transport.

Response: Thank you for your suggestions. We have added some relevant references. Please see Page 2, Lines 47-51 in the revised manuscript.

The results of Ghadiri et al. showed that the sediment transport capacity of overland flow increased by more than 60 times as the slope increased from 1.5% to 5.2%. This indicates that on steep slopes, the overland flow on the hillslope has better sediment transport capacity, which corresponds to a worse sediment trapping effect.

Ghadiri, H., Rose, C. W., & Hogarth, W. L. (2001). The influence of grass and porous barrier strips on runoff hydrology and sediment transport. Transactions of the ASAE, 44(2), 259-268. https://doi.org/10.13031/2013.4687

Reviewer 2 Report

The paper presents results of research of sediment transport characteristics in hillslope covered with vegetation under different flow rate conditions. In my opinion, the Authors made considerable efforts to carry out the analyses, and the experiment was well designed. The study results are valuable, as they contribute to a better understanding of the effect of flow rate on sediment trapping by hillslope vegetation, which is one of key problems in erosion-prone areas. As such the manuscript can be published in the journal. However, I would suggest the following improvements prior to the final acceptance of the manuscript for publication:

1. In the chapter “Materials and Methods” the Authors wrote: “The experimental soil was obtained in Jixian County, Shanxi Province, China. The soil of this area is vulnerable to erosion, as it is loose, and the particles are fine. It is representative of the hill and gully region of the Loess Plateau.” (p. 3, l. 117-119). I strongly suggest supplementing this description with information on the climate conditions of the Loess Plateau with special regard to the seasonal occurrence of rainfall, related to the monsoon circulation in that part of China.

2. In the experiment the influence of vegetation on sediment transport was analyzed. From Figure 1 (photos) it can be guessed that the experiment was carried out in summer, during the growing season, and it was also confirmed by your statement that “Vegetation cover rate was higher than 90% (p. 7, l. 207). So, while the results of your research are applicable to the summer (growing) season, what about their representativeness for winter, spring or autumn, when the vegetation density is lower? Please explain in the manuscript.

3. In relation to the aforementioned comment No. 1, please justify the need to carry out your research; please refer to the high vulnerability of the Loess Plateau to erosion caused by rainfall.

4. Please discuss limitations of your study. You may refer to the aforementioned comment No. 2. on the representativeness of your results in seasons different than summer.

5. Conclusions: Please underline the practical value of your analyses. “These results were helpful for enhancing the function of the vegetation module in erosion models.” (p. 13, 433-434) - this sentence closing your manuscript is not sufficient, taking into account the high vulnerability of the Loess Plateau to erosion and the importance of your study results.

To sum up, it is recommended to accept the paper for publication after major revision.

Minor linguistic improvements are required.

Author Response

Response: Thank you very much for your positive comments.

In the chapter “Materials and Methods” the Authors wrote: “The experimental soil was obtained in Jixian County, Shanxi Province, China. The soil of this area is vulnerable to erosion, as it is loose, and the particles are fine. It is representative of the hill and gully region of the Loess Plateau.” (p. 3, l. 117-119). I strongly suggest supplementing this description with information on the climate conditions of the Loess Plateau with special regard to the seasonal occurrence of rainfall, related to the monsoon circulation in that part of China.

Response: Thank you for your suggestions.

The Loess Plateau is located in the northern central part of China, with a warm temperate continental monsoon climate. The precipitation characteristics of summer and autumn are rainy, while winter and spring are dry and less rainy, with an annual precipitation of 150 - 750 millimeters. During the year, precipitation is mostly concentrated from July to September, accounting for 60-80% of the annual precipitation, while winter precipitation generally only accounts for approximately 5%.

Wang, W., Li, Z., Su, H., Xiao, J., Han, F., & Li, Z. (2022). Spatial and seasonal variability, control factors and health risk of fluoride in natural water in the Loess Plateau of China. Journal of Hazardous Materials, 434, 128897. https://doi.org/10.1016/j.jhazmat.2022.128897

Please see Page 4, Lines 141-146 in the revised manuscript.

In the experiment the influence of vegetation on sediment transport was analyzed. From Figure 1 (photos) it can be guessed that the experiment was carried out in summer, during the growing season, and it was also confirmed by your statement that “Vegetation cover rate was higher than 90% (p. 7, l. 207). So, while the results of your research are applicable to the summer (growing) season, what about their representativeness for winter, spring or autumn, when the vegetation density is lower? Please explain in the manuscript.

Response: Thank you very much for your suggestions.

The Loess Plateau is located in the northern central part of China, with a warm temperate continental monsoon climate. During the year, precipitation is mostly concentrated from July to September, accounting for 60-80% of the annual precipitation, while winter precipitation generally only accounts for approximately 5%. (Page 4, Lines 141-146)

Spring and winter have less rainfall and usually do not produce overland flow, making them inactive seasons for soil erosion. Therefore, this study has good representativeness in the Loess Plateau region in China.

Please see Page 3, Lines 131-134 in the revised manuscript.

In relation to the aforementioned comment No. 1, please justify the need to carry out your research; please refer to the high vulnerability of the Loess Plateau to erosion caused by rainfall.

Response: Thank you very much for your comments.

Loess has a loose structure, porosity, and rich vertical joints, making it susceptible to erosion by flowing water. The terrain in the loess hilly and gully areas is fragmented, with a high density of gullies and valleys. The proportion of sloping farmland is high, and the erosion of hillslope overland flow is active, resulting in a high risk of soil erosion.

Please see Page 4, Lines 149-153 in the revised manuscript.

Please discuss limitations of your study. You may refer to the aforementioned comment No. 2. on the representativeness of your results in seasons different than summer.

Response: Thank you for your comments.

This study still has some limitations, mainly including the inability to distinguish the differences in sediment particle movement caused by rigid and flexible vegetation and the inability to identify the impact of the vegetation growth cycle on its processes; When modeling, the problem is usually simplified with different particle size thresholds, which cannot yet be provided in this study.

Please see Page 15, Lines 473-477 in the revised manuscript.

Conclusions: Please underline the practical value of your analyses. “These results were helpful for enhancing the function of the vegetation module in erosion models.” (p. 13, 433-434) - this sentence closing your manuscript is not sufficient, taking into account the high vulnerability of the Loess Plateau to erosion and the importance of your study results.

Response: Thank you very much for your suggestions. We have added some new descriptions.

This study deepens the understanding of the mechanism of water and sediment processes on vegetation hillslopes, has important significance for the erosion control on the Loess Plateau, helps to enhance the functionality of vegetation modules in soil and water models, and promotes the widespread and efficient application of VFS management technology.

Please see Page 16, Lines 516-520 in the revised manuscript.

To sum up, it is recommended to accept the paper for publication after major revision.

Response: Thank you very much again for your positive feedback.

Round 2

Reviewer 1 Report

The manuscript needs more justification and it is not ready to be accepted.

Reynolds number is not definitely a good criteria for any justification in this manuscript.

What is the application of "quasi laminar flow" in practice?

The authors should clarify how they could estimate shear stress for a quasi-laminar flow.

Also, the interpretation of negative constants in table 4 is required.

Please discuss the results of the last line in table 4. For linear and power models, R2 is = 0.38 with b =-4.04. Also, the value of b= -48.24 in this table and a =0.009 should be explained.

No where in the text is observed the effect of vegetation on the results directly. This is partly stated by the authors in lines 177-181.

Please explain the novelty of this work. Lines 121-123 is not enough. The role of vegetation in this study is not clear as the conclusion shows in lines 489-490.

Lines 473-475 show a major limitation of this study, showing little progress with this study in our knowledge from the subject.

Conclusion numbers 1 and 3 are too superficial to draw any valuable progress in our understanding.

The authors may check the writing and meaning of each sentence.

For example, in line 487 (revised version) the sentence begins with "It"? Does it address Reynolds number?

Author Response

The manuscript needs more justification and it is not ready to be accepted.

Reynolds number is not definitely a good criteria for any justification in this manuscript.

Response: Thank you for your comments. The Reynolds number is usually used to determine the flow pattern and hydraulics. The focus of this study is the water-sediment process on vegetated hillslopes. The Re, as one of the parameters calculated in this study, was discussed in conjunction with the overland flow on vegetation hillslopes. Dense vegetation stems can cause significant disturbance in overland flow, making it difficult to form laminar flow. Therefore, refer to the pseudo laminar flow term used in similar situations in previous studies. At present, there is no widely accepted parameter for better judgment of overland flow patterns on vegetation hillslopes, so it cannot be demonstrated quantitatively and can only be discussed from the perspective of phenomena.

What is the application of "quasi laminar flow" in practice?

Response: The “pseudo laminar flow” has only been mentioned in relatively few studies and has not yet been applied to practical cases. This may require some time and more research to drive it forward.

The authors should clarify how they could estimate shear stress for a quasi-laminar flow.

Response: Thank you for your comments. The commonly used method for calculating the shear stress of overland flow is τ = ρgRJ, where R is the hydraulic radius of shallow flow and usually replaced by the depth of water flow h. J is the hillslope gradient. The shear stress and stream power are often used in calculating sediment transport capacity and can be used for sediment transport modeling, but they were not modeled in this study, so shear forces were not discussed.

Also, the interpretation of negative constants in table 4 is required.

Response: Thank you for your comments. The particle size composition of sediment is approximately a nonstandard normal distribution. The horizontal axis here represents the median particle size d₅₀ and the peak particle size d_p in units of μm. Even if there is very little sediment at the outlet, when detected, d₅₀ and d_p are still values greater than 0. Therefore, the intercept of the vertical axis should theoretically be negative. Most of the results are consistent with this situation, with only S15Q15SC40 showing a positive ordinate intercept, 1.94 for d₅₀ and 1.66 for d_p. Therefore, what must be explained here is mainly the case where the intercept is a positive constant for S15Q15SC40. This may be due to the high initial sampling particle size and measurement results.

“The particle size composition of sediment is….” [lines 364-371]

Please discuss the results of the last line in table 4. For linear and power models, R² is = 0.38 with b =-4.04. Also, the value of b= -48.24 in this table and a =0.009 should be explained.

Response: Thank you for your comments. b = − 48.24 and k = 3.722, i.e., T = 3.722 d₅₀ − 48.24 = 3.722(d₅₀ −12.96). This indicates the initial moment of sediment output, with a d₅₀ of approximately 12.96 μm. The difference in coefficients is mainly due to the differences in conditions such as flow rate and sediment concentration, as well as some random errors in sampling measurements.

In the power function formula, α is a coefficient, β is the index, and d₅₀ and d_p are the base numbers. The method of evaluating parameters in regression used the least squares method, and the corresponding parameters (α and β) were calculated based on experimental data when the goodness of fit R² was maximized. The d₅₀ and d_p were measured through experiments, and their slight fluctuations can cause a significant change in the α value.

No where in the text is observed the effect of vegetation on the results directly. This is partly stated by the authors in lines 177-181.

Response: Thank you for your comments. According to the formula given by Zhang et al. (2011) in a flat organic glass bed without vegetation cover experiment to calculate the sediment transport capacity of overland flow based on slope gradient, flow rate, and median particle size, the flow rate Q = 7.5, 15, 30, and 45 L min⁻¹ m⁻¹, and the median particle size d₅₀ = 39.9 μm. When the slope S = 15°, the minimum sediment transport capacity of the overland flow is > 5739 g min ⁻¹, which is still greater than the maximum sediment inflow rate of 5400 g min⁻¹ in the design of the experiments of this study (45 L min⁻¹ m⁻¹ with 120 g L⁻¹). That is, on a smooth and flat 15° slope (without the effect of VFS), under the conditions of a maximum inflow sediment transport rate (5400 g min ⁻¹ m ^{− 1}) in the design of the experiments of this study, deposition will not occur.

VFSs are composed of vegetation stems, litter, bed soil surface, etc. All our experimental results here are directly related to the VFS effect, which is different from the experimental design scheme that compares vegetation and non-vegetation conditions to highlight the differences in vegetation effects. This study presents the differences in the flow velocity, particle size, and other processes during sediment trapping by the VFS under 4 flow rates and 2 sediment concentration treatments on the same vegetation hillslope, all of which are discussed and studied under vegetation hillslope conditions.

Zhang G H, Wang L L, Tang K M, et al. Effects of sediment size on transport capacity of overland flow on steep slopes [J]. Hydrological Sciences Journal, 2011, 56(7): 1289-1299.

Please explain the novelty of this work. Lines 121-123 is not enough. The role of vegetation in this study is not clear as the conclusion shows in lines 489-490.

Response: Thank you for your comments. We have rewritten this sentence. Please see Lines 120-123 in the revised manuscript.

‘The novelty of this study is mainly…’

The prerequisite for these conclusions is under the condition of sediment trapping by the VFS. We have already explained this again in the conclusion.

‘During the process of sediment trapping by VFS …’. Please see Line 500 in the revised manuscript.

Lines 473-475 show a major limitation of this study, showing little progress with this study in our knowledge from the subject.

Response: Thank you for your comments. The progress of research needs to be gradually promoted.

Conclusion numbers 1 and 3 are too superficial to draw any valuable progress in our understanding.

Response: Thank you for your comments. We have tried our best to present our research findings and believe that our research is still valuable.

The authors may check the writing and meaning of each sentence. For example, in line 487 (revised version) the sentence begins with "It"? Does it address Reynolds number?

Response: Thank you for your comments. We have corrected this sentence and checked other sentences.

“The calculation results showed that…” [Line 506]

Reviewer 2 Report

In my opinion, the Authors have made satisfactory improvements in the manuscript, in accordance with the Reviewer’s comments. Thus, it is recommended to accept the paper for publication in present form.

Author Response

Response: Thank you very much for your positive feedback.