the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 18 Dec 2025
Regulatory role of permanent gully in runoff dissolved nitrogen and phosphorus transport across rainfall types
Abstract. Tracking the transport of runoff-dissolved nitrogen (N) and phosphorus (P) from upslope farmland to the catchment outlet is vital for controlling non-point source pollution in agroecosystems. However, the hydrological and regulatory roles of permanent gully within catchment in modulating dissolved N and P losses dynamics under natural rainfall conditions remain poorly understood. In this study, runoff and associated losses of dissolved NH4+, NO3-, and P were measured at both the gully head and the outlet from 2022 to 2023. The results are as follows: (1) Gully significantly enhanced runoff generation, contributing 36.1 % of total runoff despite occupying only 12.4 % of the area. This contribution varied across rainfall types (Type A, frequent, low-depth, low erosivity; Type B, short duration, high intensity; Type C, long duration, high erosivity) and was highest under Type A (43.2 %) and lowest under Type C (33.8 %). (2) Gully exerted a pronounced dilution effect on the concentrations of dissolved NH4+, NO3-, and P, particularly for dissolved NO3- (dilution ratio: 0.65). Consequently, gully contributed less to dissolved nitrogen and phosphorus fluxes relative to its contribution to runoff volume, accounting for 31.4 %, 22.4 %, and 31.1 % of dissolved NH4+, NO3-, and P fluxes, respectively. (3) Type C rainfall dominated the loss of dissolved N and P. Only 10.2 % of events contributed over 68 % of dissolved N and P fluxes at the catchment scale and markedly increased their loss sensitivity to rainfall compared to Type A and Type B. These sensitivities were also intensified by gully. The study provides new insights into runoff dissolved nutrient interactions within gully systems and offers a foundation for improving nutrient management in gully-dominated agricultural landscapes.
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RC1: 'Comment on egusphere-2025-5839', Anonymous Referee #1, 14 Jan 2026
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-5839/egusphere-2025-5839-RC1-supplement.pdfCitation: https://doi.org/
10.5194/egusphere-2025-5839-RC1 -
AC2: 'Reply on RC1', Zhuoxin Chen, 13 Feb 2026
We have provided two versions of our responses: one for online viewing and one as an attachment. The content of both versions is identical.
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Dear Reviewer#1,
We appreciate your thoughtful and constructive feedback, which has greatly improved the quality and clarity of our manuscript. Below, we provide detailed point-by-point responses to all comments. Your comments are presented in bold, and our responses are given in regular text.
Sincerely,
Zhuoxin Chen (on behalf of all authors)
General comments
1. Overall summary: This study by Chen et al. aims to understand how gullies influence nutrient transport under different rainfall conditions. This research is ethically sound, scientifically valid, and technically accurate. Additionally, the methods are clear, making this reproducible. Overall, the paper is well-written and not too long, and the authors do a good job of describing why this research is important. I recommend professional English-language editing to improve clarity and consistency, especially in the abstract. For example, the term gully is repeatedly used as a plural noun; this should be corrected to gullies throughout the manuscript. Consequently, I have restricted my language-related comments and focus on the scientific content. Minor comments are listed below.
Response: We sincerely thank you for your positive and constructive evaluation of our study. We appreciate your suggestion regarding professional English-language editing and will carefully revise the manuscript to improve clarity and consistency. Regarding the use of “gully” versus “gullies,” our original intention was to use the singular form when referring to the influence of an individual monitored gully on dissolved nitrogen and phosphorus transport, and the plural form when discussing broader, regional-scale implications. However, we acknowledge that this distinction may not have been sufficiently clear and could potentially cause confusion. We will therefore carefully review and standardize the terminology throughout the manuscript to ensure grammatical accuracy and consistency. We appreciate your helpful suggestion and will further refine the manuscript to enhance its clarity and rigor.
Specific comments
1. Minor Comments: L17: Define ammonium and nitrate before acronym use.
Response: We thank you for this helpful comment. We will define ammonium and nitrate at their first occurrence before introducing the corresponding acronyms in the revised manuscript.
2. L42: Perhaps a photo (could be supplemental) of a gully could be useful here.
Response: Thank you for this constructive suggestion. We agree that field photographs can provide a more intuitive understanding of gully morphology and activity. Following your advice, we will include an unmanned aerial vehicle (UAV) image of the gully in the Supplementary Materials. We have accumulated extensive visual materials from previous field surveys. For example, one of our earlier studies on gully morphological characteristics and their controlling factors, published in Earth Surface Processes and Landforms, was selected as a cover article, and the cover image clearly illustrates the destructive nature of gullies and their hydrological significance (https://onlinelibrary.wiley.com/doi/10.1002/esp.70002). In addition, based on comments from the community, we recognize the need to further clarify the activity status of the gully in Lines 101–110. Field photographs provide direct and convincing evidence of ongoing headcut retreat and exposed gully slopes. Therefore, we will also add two UAV photographs to Fig. 1 in the revised manuscript to better demonstrate the gully’s active condition and geomorphic features. We sincerely thank you again for this valuable suggestion. For readers who are not directly engaged in gully erosion research, the inclusion of field photographs will serve as an important and helpful supplement, facilitating a clearer and more intuitive understanding of gully function.
3. L79-80: Define ammonium and nitrate before acronym use.
Response: We completely agree with you. We will define ammonium and nitrate at their first occurrence before introducing the corresponding acronyms in Lines 79–80. Based on this, we will check the abbreviation usage throughout the text.
4. L118: How did you determine how much erosion occurred? Is this using the equation below, or did you ever measure suspended sediments? And what was the threshold for “significant” here?
Response: We thank you for pointing out this issue. We intended to indicate that only rainfall events under natural conditions that generated clearly observable surface runoff at both the gully head and outlet were included in the monitoring dataset. We agree that the previous wording was not sufficiently precise and will replace “significant” with a more appropriate term (such as measurable or clearly observable) to improve clarity.
5. L157-163: I recommend putting the sentences that describe the types of rain (A/B/C) first, followed by the type A was dominant, followed by B and C sentence.
Response: We fully agree with your suggested revision. After comparing it with our original structure, we found that your proposed logic presents the ideas more clearly and improves the overall flow of the text. We sincerely appreciate this helpful suggestion.
6. Results (general): I am finding it difficult to remember the differences in the types of rainfall (A, B, C). Perhaps you can call them something different or redefine them in the captions of the figures?
Response: We sincerely thank you for this valuable suggestion. We agree that using simple alphabetical labels (e.g., A, B, C) may reduce clarity and make it difficult for readers to distinguish among rainfall types. Following your comment, we considered adopting more descriptive abbreviations, such as LER (Low-Erosivity Rainfall), SHI (Short-duration High-Intensity Rainfall), and LDR (Long-duration High-depth Rainfall). However, we recognize that even these abbreviations may not be sufficiently intuitive. To further enhance clarity and readability, we plan to revise the relevant figures by introducing distinct graphical symbols for each rainfall type. Specifically, the size and number of raindrops within the symbols will visually represent differences in rainfall intensity and erosivity. We believe this visual approach will make the classification more intuitive and improve overall readability.
7. Figures (general): In any figure with multiple panels, please define each letter in the caption. This is also a lot of figures, and a lot of information. Some of the figures only have a couple of sentences describing their results. I would recommend moving a couple of these to the supplemental, in order to make this an easier read and reduce redundancy.
Response: We sincerely thank you for this constructive comment. We fully agree that some subfigures lack sufficient description. In the revised manuscript, we will add clearer explanations in the figure captions to improve clarity. In addition, we agree that some figures (e.g., Figs. 8 and 9) could be moved to the Supplementary Material, as they provide supporting analyses rather than the core findings of the study. We will revise the figure organization accordingly.
8. Figure 11: Type C relationships seem to be driven by a single point. Did you check if this is an outlier? Also, these all have regression lines on them. Are they all significant relationships?
Response: We thank you for this careful observation. In this study, Type C rainfall corresponds to high-erosivity events. During the two-year monitoring period, one relatively extreme rainfall event (100–200 mm) was recorded, which appears as the high-value point in Fig. 11. Based on long-term local rainfall characteristics, such events occur roughly once every three years. Therefore, rather than representing a spurious outlier, this data point reflects a naturally occurring high-magnitude rainfall event and enhances the representativeness of our dataset by capturing both regular (Type A and B) and relatively extreme (Type C) conditions. In Fig. 11, the regression patterns differ substantially among rainfall types. As commonly observed, the relationship between nutrient flux and rainfall amount follows linear or power-law forms. The notably steeper slope under Type C rainfall indicates a disproportionately stronger response of nutrient loss to incremental rainfall, highlighting its enhanced erosive and transport capacity compared to the more frequent rainfall types. We will clarify this interpretation in the revised manuscript and explicitly report the statistical significance of all regression relationships in the figure caption.
9. Discussion (general): I think the discussion could be a bit longer. There are so many results, and I think you could pull more from the literature to place this into context and provide suggestions for future research.
Response: We fully agree with your suggestion that the Discussion section can be further strengthened. In the revised manuscript, we will expand the discussion by incorporating additional literature to better contextualize our findings. For example, we will include comparisons with artificial drainage ditches to clarify the distinct hydrological and geomorphic characteristics of naturally formed gullies. In addition, we will broaden the literature review to include studies from other regions worldwide addressing gully-mediated nutrient transport, not limited to dissolved forms, in order to enhance the breadth of the discussion. Furthermore, we will add a dedicated paragraph in Section 4.3 outlining potential directions for future research based on the findings of this study. We appreciate this valuable suggestion and will revise the Discussion accordingly.
10. Conclusion: Most of the conclusion is just repeated from results. I think you could cut it down to just the last couple of sentences.
Response: We fully agree with your suggestion. In the revised manuscript, we will streamline the Conclusion section by reducing repetitive descriptions of the results and focusing on synthesizing the key findings. We appreciate this helpful comment.
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AC2: 'Reply on RC1', Zhuoxin Chen, 13 Feb 2026
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CC1: 'Comment on egusphere-2025-5839', hongqiang shi, 24 Jan 2026
I came across the preprint of this paper on ResearchGate and found it very interesting, so I would like to share a few thoughts and comments here.
Overall, this is a nice piece of work with rare field data, Most existing studies rely on large-scale remote sensing for water quality analysis, but field monitoring at the gully scale is still very limited. This is understandable, because gully flow is intermittent and hard to capture, and remote sensing basically cannot do this. So any field data at this scale are rare and valuable.
Gully development rate, driving factors, and modeling have already been well studied. But as the authors also mention, gullies are important hydrological pathways, not only erosion features. The problem is that monitoring gullies under natural rainfall is very challenging. Because of this, studies on how gullies regulate dissolved N and P are almost absent. At least, I have not seen many papers doing this with real field data. So I think this work fits well in HESS, and it also fills a gap in current gully research.
Below are some questions and suggestions that may help improve the manuscript.
- Line 14: I think there is a small issue with the abbreviations. N is used first, but then NH₄⁺ and NO₃⁻ appear directly later. It might be clearer to define NH₄⁺ and NO₃⁻ directly at the beginning, to avoid confusion between abbreviations. If similar issues appear elsewhere, I suggest checking and revising them as well. In addition, I think lines 14-17 could be merged into one sentence, which may help emphasize the role of the gully more clearly.
- Lines 68–78: In my opinion, this part may not need too much description about the proportion of grain production, although it is important. After that, it might be better to further emphasize that more fertilizer input is needed to maintain high crop yields, which increases the risk of agricultural non-point source pollution in this region. Given that gullies act as important transport pathways but are rarely studied, this logic may make the motivation of the study clearer. Just a suggestion for the authors.
- Lines 91–92: For the soil properties mentioned here, it would be helpful to clarify which soil layer they refer to (e.g., topsoil, ploughed layer, etc.).
- Lines 95–96: I think this sentence is repetitive, as similar information has already been mentioned before. It could be simplified to make the text more concise.
- From the survey results in lines 95–100, about 90% of the gullies are active, which explains why F1 and F2 were selected. However, in lines 101–110, the development status of these two selected gullies is not clearly described. I think the authors should add some explanation here to better justify the selection.
- Line 115: For the instruments used in the study, I suggest adding manufacturer information to improve reproducibility. For example, Specord M40 (VEB Carl Zeiss, Jena, Germany).
- Line 130: Similarly, the previous sentence seems redundant and could be removed. Also, in line 131, adding instrument details would be helpful.
- One question I have is whether the monitored gully has baseflow during non-rainfall periods. If so, should the baseflow-related nutrient flux be excluded when calculating rainfall-event N and P losses?
- I noticed that correlation analysis and some other statistical analyses were used. Should these methods be briefly described in Section 2.4 as well?
- The Results and analysis section is clear and easy to follow. The key findings are well presented, and the figures are also nicely prepared.
- The Discussion could possibly be strengthened by comparing the function of natural gullies with artificial drainage ditches, and by referencing relevant studies. This may help further highlight the uniqueness and importance of erosional gullies, and also better connect with the points raised in the Introduction.
- Overall, I think this is a high-quality and innovative study. After addressing the issues mentioned above, the manuscript can be considered for publication in HESS.
Citation: https://doi.org/10.5194/egusphere-2025-5839-CC1 -
AC1: 'Reply on CC1', Zhuoxin Chen, 13 Feb 2026
We have provided two versions of our responses: one for online viewing and one as an attachment. The content of both versions is identical.
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Dear hongqiang shi,
We sincerely thank you for your constructive and insightful comments, which have greatly improved the quality and clarity of our manuscript. Below, we provide detailed point-by-point responses to all comments. Your comments are presented in bold, and our responses are given in regular text.
Sincerely,
Zhuoxin Chen (on behalf of all authors)
General comments
1. I came across the preprint of this paper on ResearchGate and found it very interesting, so I would like to share a few thoughts and comments here. Overall, this is a nice piece of work with rare field data, Most existing studies rely on large-scale remote sensing for water quality analysis, but field monitoring at the gully scale is still very limited. This is understandable, because gully flow is intermittent and hard to capture, and remote sensing basically cannot do this. So any field data at this scale are rare and valuable.
Response: We sincerely thank the commenter for the positive and encouraging remarks. We appreciate the recognition of the challenges associated with field monitoring at the gully scale, particularly given the intermittent nature of gully flow and the limitations of remote sensing approaches. We are pleased that the value of these rare field observations is acknowledged and hope that this study contributes to improving process-based understanding of gully-scale hydrological and nutrient transport dynamics.
2. Gully development rate, driving factors, and modeling have already been well studied. But as the authors also mention, gullies are important hydrological pathways, not only erosion features. The problem is that monitoring gullies under natural rainfall is very challenging. Because of this, studies on how gullies regulate dissolved N and P are almost absent. At least, I have not seen many papers doing this with real field data. So I think this work fits well in HESS, and it also fills a gap in current gully research.
Response: We sincerely thank the commenter for the thoughtful and encouraging remarks. We fully agree that, although gully development processes and driving mechanisms have been extensively investigated, their hydrological and biogeochemical roles remain insufficiently explored, particularly under natural rainfall conditions. As noted, the intermittent nature of gully flow makes continuous field monitoring extremely challenging, which partly explains the limited availability of field-based studies on dissolved nitrogen and phosphorus transport. We appreciate the recognition that this work helps address this research gap and contributes to advancing the understanding of gullies as functional hydrological pathways rather than solely erosion features.
Specific comments
Below are some questions and suggestions that may help improve the manuscript.
1. Line 14: I think there is a small issue with the abbreviations. N is used first, but then NH₄⁺ and NO₃⁻ appear directly later. It might be clearer to define NH₄⁺ and NO₃⁻ directly at the beginning, to avoid confusion between abbreviations. If similar issues appear elsewhere, I suggest checking and revising them as well. In addition, I think lines 14-17 could be merged into one sentence, which may help emphasize the role of the gully more clearly.
Response: We agree that defining NH₄⁺ and NO₃⁻ at their first occurrence would improve clarity. In the revised manuscript, we will introduce these terms explicitly and ensure consistent abbreviation usage throughout. We also agree that merging Lines 14–17 into one sentence will better emphasize the role of gully in regulating dissolved nitrogen and phosphorus losses, and we will revise this section accordingly.
2. Lines 68–78: In my opinion, this part may not need too much description about the proportion of grain production, although it is important. After that, it might be better to further emphasize that more fertilizer input is needed to maintain high crop yields, which increases the risk of agricultural non-point source pollution in this region. Given that gullies act as important transport pathways but are rarely studied, this logic may make the motivation of the study clearer. Just a suggestion for the authors.
Response: We fully agree with this suggestion. Since the focus of our study is dissolved nitrogen and phosphorus pollution, greater emphasis should be placed on fertilizer inputs required to sustain crop production and the associated environmental risks. Given that gullies serve as important hydrological connectors between hillslopes and downstream waters, their role becomes particularly relevant in this context. We will revise and expand this part accordingly in the revised manuscript.
3. Lines 91–92: For the soil properties mentioned here, it would be helpful to clarify which soil layer they refer to (e.g., topsoil, ploughed layer, etc.).
Response: We appreciate your careful observation. The statement refers specifically to the plough layer, and we will clarify this explicitly in the revised manuscript.
4. Lines 95–96: I think this sentence is repetitive, as similar information has already been mentioned before. It could be simplified to make the text more concise.
Response: We thank you for this helpful comment. We agree that this sentence is repetitive and will simplify it in the revised manuscript to improve conciseness. We will also carefully review the manuscript to remove any additional redundant expressions.
5. From the survey results in lines 95–100, about 90% of the gullies are active, which explains why F1 and F2 were selected. However, in lines 101–110, the development status of these two selected gullies is not clearly described. I think the authors should add some explanation here to better justify the selection.
Response: We agree that the development status of the selected gullies should be more clearly described to justify their selection. In the revised manuscript, we will provide additional information on their activity and geomorphic characteristics, including photographs of active gully heads and descriptions of vegetation cover, to strengthen the rationale for selecting F1 and F2.
6. Line 115: For the instruments used in the study, I suggest adding manufacturer information to improve reproducibility. For example, Specord M40 (VEB Carl Zeiss, Jena, Germany).
Response: We agree that providing manufacturer information improves reproducibility. In the revised manuscript, we will add the manufacturer details for all instruments used.
7. Line 130: Similarly, the previous sentence seems redundant and could be removed. Also, in line 131, adding instrument details would be helpful.
Response: We agree that the previous sentence is redundant and will remove it in the revised manuscript. We will also add the relevant instrument details in Line 131 to improve clarity and reproducibility.
8. One question I have is whether the monitored gully has baseflow during non-rainfall periods. If so, should the baseflow-related nutrient flux be excluded when calculating rainfall-event N and P losses?
Response: We thank you for raising this important point. Gullies differ substantially from rivers in that they generally do not sustain baseflow in the absence of rainfall, particularly for hillslope gullies. Therefore, baseflow effects are not relevant for the two selected gullies in this study. To avoid potential misunderstanding, we will clarify this point explicitly in the revised manuscript.
9. I noticed that correlation analysis and some other statistical analyses were used. Should these methods be briefly described in Section 2.4 as well?
Response: We agree that the statistical methods, including correlation analysis and related approaches, should be briefly described in Section 2.4. We will add a concise description to improve methodological clarity.
10. The Results and analysis section is clear and easy to follow. The key findings are well presented, and the figures are also nicely prepared.
Response: We appreciate your positive comments and are glad that the Results section and figures are clear and easy to follow.
11. The Discussion could possibly be strengthened by comparing the function of natural gullies with artificial drainage ditches, and by referencing relevant studies. This may help further highlight the uniqueness and importance of erosional gullies, and also better connect with the points raised in the Introduction.
Response: We agree that comparing artificial drainage ditches with naturally formed gullies is an interesting and meaningful perspective. Following your suggestion, we will review relevant literature on drainage ditches and strengthen the comparison and discussion in the revised manuscript to better highlight the distinct characteristics of gullies and the significance of this study.
12. Overall, I think this is a high-quality and innovative study. After addressing the issues mentioned above, the manuscript can be considered for publication in HESS.
Response: We appreciate your positive assessment and constructive comments. We will carefully revise the manuscript to further improve its quality.
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RC2: 'Comment on egusphere-2025-5839', Anonymous Referee #2, 26 Jan 2026
General comments
This is my first review of the manuscript “Regulatory role of permanent gully in runoff dissolved nitrogen and phosphorus transport across rainfall types” by Chen et al.
This paper investigated the influence of gullies on runoff and nutrient transport (nitrogen and phosphorous) in two small catchments in Northeast China, and how this influence changed under three different rainfall types. The authors found that gullies contributed to total runoff mostly during frequent, low-depth, low erosivity rainfall events, and they diluted dissolved NH4+, NO3-, and P concentrations. Loss of dissolved N and P were mainly caused by long duration, high erosivity rainfall events.
The topic of this manuscript is relevant and important, it is of interest for the community. A few major comments, as well as minor technical corrections are listed below.
Specific comments
-The study presents two gullies close to each other in Northeast China. Please provide information on the generalizability of the findings.
-The authors should identify clear research gaps in literature in the Introduction section. After listing several references and literature on gullies, the sentence in the Introduction saying “Nevertheless, the role of gullies in modulating dissolved nutrient losses under varying rainfall conditions remains insufficiently investigated.” does not seem to be justified.
-The results should be reproducible. The methodological description is not complete, often it is hard to understand how the results were obtained (e.g., events selection, calculation of volumes and masses, statistical analyses, etc.). The methods section should be extended.
-The Discussion section should be strengthened by more data-based process understanding and interpretation of the results and referring to more and relevant studies in literature.
-Readers should be able to understand each figure without reading the entire methods and results section, figures with their captions should be interpretable alone, without the entire manuscript. Please extend the figure captions by explaining each subfigure (a, b, c, etc.), and add legends where they are missing.
-Can the findings (e.g., dilution effect of the gully, etc.) be attributed really to the influence of the gully or dilution could be caused by e.g., subsurface contributions? On a similar note, were the catchment characteristics (land use, subsurface, etc.) the same/similar between the upstream catchment area (until the inlet of the gully) and the rest of the catchment?
-In certain parts of the Results section, the text does not seem to be justified by the figures (please see details in technical corrections later).
-It is good that the Results section is compact, but using one single sentence to describe a complex plot seems to be not enough. If a figure contains the same message as another figure and not much text can be added, then such a figure should be removed or added to the Appendix.
-The authors refer to the area of the gullies (i.e., 12.4% of the catchment area; still, they contribute so much runoff to the total runoff, “36.1% of total runoff”)? Would it not be more meaningful to refer to the drainage area of the gullies in this context, i.e., the catchment area where they collect the water from? Did I understand correctly that at the “outlet” (red points in Figure 1) the catchment area contributing to runoff is the entire catchment area, i.e., 100% of the catchment? Then what do the two percentages 12.4% and 36.1% refer to? But more generally: the authors should better explain in the study area or methods section: what do “UDGH”, “Gully” and “Outlet” refer to, when (permanent/ephemeral?) and where (drainage areas?) does the water flow? On a similar note, some photos (of the gauges, gullies, etc.) might help the readers to imagine the study area and instrumentations.
-The manuscript would greatly benefit from thorough English language editing, please find below some technical corrections, but the list is not complete.
Technical corrections
-title, line 14, etc.: “runoff-“dissolved nitrogen? Is this term correct/does this term exist? Or dissolved nitrogen in runoff?
-Line 16: permanent gully in a catchment
-Line 16: losses or dynamics?
-Line 17: please define NH4+, etc. before using the term
-Abstract: please also describe in 2-3 sentences which methods were used.
-Line 27: by the gullies?
-Line 28: improving how? Please be specific
-Lines 42-43: repetition of lines 37-38
-Line 49: riparian buffers? Do you mean riparian zone?
-Line 58: high-frequency? Do you mean more common/events that occur more often? Please try to rephrase
-Line 59: efficiencies? In what?
-Line 63: concentration gradients? Please explain
Lines 42-67: research gaps should be identified, what is missing in literature? This should be justified.
-Lines 68-78: how is this paragraph logically linked to the previous? Please keep a logical flow of thoughts/sentences/paragraphs – these need to be logically linked
-Line 79: please first define terms NH4+, etc
-Line 82: gullies?
-Introduction last sentence should be more specific
-Line 87: (1), later (2) etc. – are these numbers necessary? Why were they added?
-Fig 1: please add some photos of the gullies
-Line 102: land use proportions? Please explain here term
-Line 104: please remove “within catchment”
-Line 105: “F1 measured” please rephrase, F1 is a catchment?
-Please extend all figure captions
-Line 114: rainfall data (please remove capture)
-Line 115: resolution of 0.2 mm? please explain which type of gauge was used
-Line 116: there was one gauge within one catchment – spatially heterogenous rainfall could not be recorded in one catchment – differences between the catchments could be recorded
-section 2.2: please explain how exactly events were selected, which thresholds were used, how was the beginning and end of a rainfall event defined? And how was the beginning and end of the corresponding runoff event defined?
-Line 118: what is significant? And how was soil erosion measured?
-Line 120: usually rainfall erosivity is EI30 in literature
-Line 125: in which equation?
-is the dimension of rainfall kinetic energy correct? It is usually MJ/ha
-Line 126: what is a sub-event? Please explain
-Line 127: what does it mean “based on rainfall intensity”, how exactly?
-Line 129: Sample collection? Sampling strategy?
-Line 134: “post-runoff stages”? please explain or rephrase
-Line 136: what is sufficient? How many exactly? Please provide some statistics and numbers of the samples
-Line 145: please explain the methods from Chen et al briefly here
-2.4: please extend
-Line 156: were these the same events? Why different number of events?
-Please extend table captions
-Table 1: D might be more meaningful in hours
-Line 169: which figure shows this? Please provide references, not just at the end of the section
-Line 174: these percentages should be better explained, to what exactly they refer to?
-Figure 2: how were the volumes defined? Please add to methods
-3.3 title is unclear, effect on what? Please rephrase, also the English
-Line 186: belongs to methods. Why downstream divided by upstream?
-Lines 193-195: Fig 4 does not entirely show this – because the two catchments differ
-Line 202: transport fluxes: how were they estimated? Please explain in methods
-Line 205: please add reference to figure
Line 206: “similarly dominated” please rephrase
Line 208-209: which figure shows this? Where do these values stem from?
-Figure 5: what does “gully” refer to? To the outlet? What do “sites” refer to in the caption? What is 100%?
-Figure 6: methods should explain how cumulative transport fluxes were obtained?
-Figure 7: methods should explain the scales (event scale? etc)
-3.4.1: correlation analysis should be explained in methods
-Figure 8: legend is missing
-Lines 235-236: meaning not clear, please explain or rephrase
-Fig 9: should be explained in methods
-Lines 242-245: belongs to methods
-Fig 11: to how many points were these lines fitted? C: line would be flat without that one outlying point – why is that event so different than the others? Were the measurements correct?
-Line 270: mobilized?
-Line 272: gullies?
-Line 279: interception effects of gullies?
-Line 292: the methods do not mention information on land management practise? Was the timing of fertilizer application managed?
-Line 295: gullies
-Line 304: gullies in agricultural catchments play a dual role
Citation: https://doi.org/10.5194/egusphere-2025-5839-RC2 -
AC3: 'Reply on RC2', Zhuoxin Chen, 13 Feb 2026
We have provided two versions of our responses: one for online viewing and one as an attachment. The content of both versions is identical.
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Dear Reviewer#2,
We sincerely appreciate your careful review and insightful suggestions. We also thank you for your patience in identifying these issues and helping us improve the manuscript. The author team has discussed your comments in detail and will address them thoroughly in the revised manuscript to improve the clarity of the text and the presentation of the figures. Below, we provide detailed point-by-point responses to all comments. Your comments are presented in bold, and our responses are given in regular text.
Sincerely,
Zhuoxin Chen (on behalf of all authors)
General comments
1. This is my first review of the manuscript “Regulatory role of permanent gully in runoff dissolved nitrogen and phosphorus transport across rainfall types” by Chen et al. This paper investigated the influence of gullies on runoff and nutrient transport (nitrogen and phosphorous) in two small catchments in Northeast China, and how this influence changed under three different rainfall types. The authors found that gullies contributed to total runoff mostly during frequent, low-depth, low erosivity rainfall events, and they diluted dissolved NH4+, NO3-, and P concentrations. Loss of dissolved N and P were mainly caused by long duration, high erosivity rainfall events. The topic of this manuscript is relevant and important, it is of interest for the community. A few major comments, as well as minor technical corrections are listed below.
Response: We feel truly honored to have the opportunity to receive your guidance and sincerely appreciate your positive evaluation of our manuscript. Although the research topic addressed in our study has received relatively limited attention worldwide, your insightful comments have helped us recognize several shortcomings in our writing and presentation. In particular, certain textual expressions and figure presentations can be further refined and clarified. We will carefully consider each of your suggestions and revise the manuscript accordingly to improve its overall quality and rigor. Thank you again for your constructive and thoughtful comments. We will do our utmost to revise the manuscript into a version that meets your expectations.
Specific comments
1. The study presents two gullies close to each other in Northeast China. Please provide information on the generalizability of the findings.
Response: Thank you very much for your valuable suggestion. In Northeast China, approximately 667,000 gullies have been identified, of which more than 85% remain active and over 70% are developed in cropland. Prior to conducting this study, we carried out a comprehensive gully inventory survey in the study area (Lines 95–100). The selected gullies are representative of the average characteristics of active gullies in this region. Therefore, we conservatively state that our findings are at least representative for this region. It should also be noted that our previous study published in the Journal of Hydrology in 2025 demonstrated the significant sediment-enhancing effect of these gullies, which is generally consistent with observations reported worldwide. However, few studies to date have specifically examined how gullies regulate dissolved nitrogen and phosphorus losses under different rainfall types. Based on your suggestion, we will further clarify the broader significance and general applicability of this study in both the Abstract and Discussion sections to better highlight its wider relevance.
2. The authors should identify clear research gaps in literature in the Introduction section. After listing several references and literature on gullies, the sentence in the Introduction saying “Nevertheless, the role of gullies in modulating dissolved nutrient losses under varying rainfall conditions remains insufficiently investigated.” does not seem to be justified.
Response: Thank you very much for pointing out this weakness in the Introduction. After carefully revisiting this section, we agree that the discussion of the research gaps was not sufficiently in-depth. In the revised manuscript, we will refine this part accordingly. Specifically, we will expand the review of previous studies worldwide that have examined the effects of gullies on dissolved nutrient transport. In fact, we have already begun to incorporate and synthesize these relevant studies. In addition, we will reorganize the logical structure of the Introduction to address several weaknesses in the flow of argumentation and to ensure a clearer progression from background to research gap and objectives. The literature review will also be strengthened to better position our study within the broader international context. We believe that these revisions will make the Introduction more rigorous, coherent, and persuasive.
3. The results should be reproducible. The methodological description is not complete, often it is hard to understand how the results were obtained (e.g., events selection, calculation of volumes and masses, statistical analyses, etc.). The methods section should be extended.
Response: Thank you very much for your valuable comments on the Methods. We fully agree that several aspects of the methods section were not described with sufficient clarity, particularly regarding the calculation of certain indices. With respect to rainfall event selection, we monitored all rainfall events during the two rainy seasons that generated observable surface runoff. However, our description was not sufficiently precise, especially in the use of terms such as “erosive rainfall.” In the revised manuscript, we will refine and clearly define these terms to avoid ambiguity and improve readability. Regarding hydrological monitoring and calculations, we will provide a more detailed description of the monitoring procedures and include the relevant calculation formulas to enhance transparency and reproducibility. In addition, we will further elaborate on the statistical analyses applied in this study, including correlation analysis and other relevant methods, to ensure that the analytical framework is clearly presented and scientifically rigorous.
4. The Discussion section should be strengthened by more data-based process understanding and interpretation of the results and referring to more and relevant studies in literature.
Response: We fully agree with this comment. In the revised manuscript, we will enhance the data-driven interpretation of the observed patterns and further discuss the hydrological and biogeochemical processes underlying nutrient transport. We will also incorporate additional relevant studies to strengthen the scientific context of our findings.
5. Readers should be able to understand each figure without reading the entire methods and results section, figures with their captions should be interpretable alone, without the entire manuscript. Please extend the figure captions by explaining each subfigure (a, b, c, etc.), and add legends where they are missing.
Response: Thank you very much for pointing out these issues. We fully agree that it is inappropriate to expect readers to search through the entire manuscript to understand the information presented in the figures. We have recognized this shortcoming. In the revised manuscript, we will provide more detailed and self-explanatory figure captions and supplement any missing legends, ensuring that each figure can be clearly understood on its own without requiring cross-reference to the main text.
6. Can the findings (e.g., dilution effect of the gully, etc.) be attributed really to the influence of the gully or dilution could be caused by e.g., subsurface contributions? On a similar note, were the catchment characteristics (land use, subsurface, etc.) the same/similar between the upstream catchment area (until the inlet of the gully) and the rest of the catchment?
Response: Thank you very much for your constructive suggestion. We will provide a more detailed description of subsurface runoff and land use conditions in the Materials and Methods section. In this study, no subsurface runoff was observed during dry periods. Even during rainfall events, runoff generation and recession occurred rapidly, indicating that subsurface flow contributed negligibly to the overall hydrological response. Therefore, its influence on runoff and associated sediment and nutrient losses can be considered minimal. Regarding land use, the catchments are predominantly cultivated farmland. Field observations showed that limited woodland areas are distributed along both sides of the gullies. However, during rainfall events, runoff from these woodland patches rarely entered the gully system, and surface runoff generation within these areas was minimal. As a result, their contribution to total runoff was very small, and we consider their impact on the overall results to be negligible. Based on your valuable comment, we will add a subsection in the third part of the Discussion to explicitly address the limitations of this study. We sincerely appreciate your thoughtful feedback, which has helped us improve the clarity and rigor of the manuscript.
7. In certain parts of the Results section, the text does not seem to be justified by the figures (please see details in technical corrections later).
Response: Thank you very much for your careful and detailed comment. We will thoroughly re-examine the correspondence between the figures and the text throughout the manuscript and ensure that all relevant figure citations are clearly indicated. This revision will improve the traceability of the results and make the sources of the presented findings more explicit and transparent.
8. It is good that the Results section is compact, but using one single sentence to describe a complex plot seems to be not enough. If a figure contains the same message as another figure and not much text can be added, then such a figure should be removed or added to the Appendix.
Response: We fully agree with your suggestion. Based on your comment, we plan to move Figures 8 and 9 to the Supplementary Materials so that the main results can be presented in a more concise and focused manner. The revised manuscript will provide clearer presentation and more streamlined descriptions of the key findings.
9. The authors refer to the area of the gullies (i.e., 12.4% of the catchment area; still, they contribute so much runoff to the total runoff, “36.1% of total runoff”)? Would it not be more meaningful to refer to the drainage area of the gullies in this context, i.e., the catchment area where they collect the water from? Did I understand correctly that at the “outlet” (red points in Figure 1) the catchment area contributing to runoff is the entire catchment area, i.e., 100% of the catchment? Then what do the two percentages 12.4% and 36.1% refer to? But more generally: the authors should better explain in the study area or methods section: what do “UDGH”, “Gully” and “Outlet” refer to, when (permanent/ephemeral?) and where (drainage areas?) does the water flow? On a similar note, some photos (of the gauges, gullies, etc.) might help the readers to imagine the study area and instrumentations.
Response: Thank you very much for pointing out these potentially confusing issues. In the revised manuscript, we will provide clearer definitions of UDGH and Outlet in the Materials and Methods section. In this study, the 12.4% refers to the proportion of the gully area relative to the total catchment area defined at the outlet. The 36.1% represents the runoff generated within the gully itself, calculated as the total runoff at the outlet minus the runoff produced from the upslope drainage area above the gully head. We acknowledge that this proportion may be slightly overestimated because a small fraction of runoff from the gully banks could not be directly monitored. However, according to our field observations, runoff from the gully banks rarely enters the gully due to the presence of wheel tracks and vegetated buffer strips (Lines 107–110), and the volume of such runoff is minimal. Therefore, its influence on the overall results is likely negligible. We will explicitly discuss this issue in the limitations section of the revised manuscript. In addition, as noted earlier, the gullies developed on hillslopes in this region receive no groundwater contribution during dry periods, and groundwater effects can also be disregarded during rainfall events. To improve clarity and transparency, we will also add photographs of the monitoring equipment and representative images of gully development to help readers better understand the study setting and methodology.
10. The manuscript would greatly benefit from thorough English language editing, please find below some technical corrections, but the list is not complete.
Response: We sincerely appreciate your detailed and highly professional comments. It is a great honor for us to receive guidance from such a respected reviewer. In response to your suggestions, we will carefully discuss the manuscript with several senior members of our research team and conduct a thorough re-examination of the entire paper. We will revise sections where the expression is unclear, the logic is not sufficiently coherent, or the figures and tables lack clarity. Particular attention will be given to improving the overall structure, precision of language, and the presentation of results. We look forward to submitting a substantially improved version of the manuscript. With your comprehensive and constructive feedback, we are confident that we can significantly enhance the quality and rigor of this work.
As the remaining comments mainly involve specific technical corrections, we have organized our responses in a tabular format to facilitate a clearer and more direct comparison between each comment and our corresponding response. The left column presents your comments, and the right column provides our replies.
Please note that due to formatting limitations in the online table display, some related comments have been addressed in a combined response, which may only appear in the first corresponding row. Responding to each row separately would result in unnecessary repetition. The attached PDF version presents the responses in full and displays correctly.
Technical corrections
Response
title, line 14, etc.: “runoff-“dissolved nitrogen? Is this term correct/does this term exist? Or dissolved nitrogen in runoff?
Thank you for your suggestion. We will correct it throughout the entire text to “dissolved nitrogen / phosphorus in runoff”.
Line 16: permanent gully in a catchment
We will carefully revise and correct these details in the revised manuscript.
Line 16: losses or dynamics?
Line 17: please define NH4+, etc. before using the term
Abstract: please also describe in 23 sentences which methods were used.
Line 27: by the gullies?
Line 28: improving how? Please be specific
We will further refine and elaborate on the research implications based on the discussion presented in Section 3.
Lines 42-43: repetition of lines 3738
We will rewrite this section to improve its clarity and coherence.
Line 49: riparian buffers? Do you mean riparian zone?
Thank you for this important clarification. The area referred to here is not a typical riparian buffer or riparian zone. Instead, it is a vegetated gully bank adjacent to cropland. In this region, the gully bank serves as a tractor-turning area, where wheel tracks frequently develop. During rainfall events, these surface irregularities, together with relatively dense grass cover, can partially impede direct runoff entering the gully. To avoid conceptual confusion, we will replace “riparian buffer” with “vegetated gully bank” and clarify its functional role in the revised manuscript.
Line 58: high frequency? Do you mean more common/events that occur more often? Please try to rephrase
Thank you for this clarification. We acknowledge that the previous wording was not sufficiently precise. We will revise the expression to ensure more accurate and unambiguous communication in the revised manuscript.
Line 59: efficiencies? In what?
We will remove this ambiguous word.
Line 63: concentration gradients? Please explain
Lines 42-67: research gaps should be identified, what is missing in literature? This should be justified.
Please refer to the answer to Question 2.
Lines 68-78: how is this paragraph logically linked to the previous? Please keep a logical flow of thoughts/sentences/paragraphs – these need to be logically linked
Line 79: please first define terms NH4+, etc
We will correct it throughout the entire text.
Line 82: gullies?
Introduction last sentence should be more specific
We will further refine and clarify our research methods and study scope in the final paragraph.
Line 87: (1), later (2) etc. – are these numbers necessary? Why were they added?
Yes, we don't really need them. We'll remove these serial numbers.
Fig 1: please add some photos of the gullies
We will add a land-use distribution map and include UAV photographs in the revised manuscript to better illustrate the spatial context and further clarify the activity status of the gullies.
Line 102: land use proportions? Please explain here term
Line 104: please remove “within catchment”
We will delete it.
Line 105: “F1 measured” please rephrase, F1 is a catchment?
Yes, F1 is a catchment area. What we mean by this is the total area of the catchment area.
Please extend all figure captions
We will correct it throughout the entire text.
Line 114: rainfall data (please remove capture)
Line 115: resolution of 0.2 mm? please explain which type of gauge was used
We used a tipping-bucket rain gauge in this study. In the revised manuscript, we will include a photograph of the instrument in Fig. 1 and provide a corresponding description to improve clarity.
Line 116: there was one gauge within one catchment – spatially heterogenous rainfall could not be recorded in one catchment – differences between the catchments could be recorded
Yes, we will rewrite this section and reframe it to focus on rainfall variability.
section 2.2: please explain how exactly events were selected, which thresholds were used, how was the beginning and end of a rainfall event defined? And how was the beginning and end of the corresponding runoff event defined?
Thank you for this important comment. We would like to clarify that no rainfall events were selectively chosen. During the two-year monitoring period, all rainfall events that generated clearly observable runoff in the catchments were included in the dataset. The previous wording referring to “erosive rainfall” may have caused misunderstanding, and we will revise this section to describe the event selection criteria more clearly. Regarding the definition of runoff initiation and termination, the monitored catchments are hillslope systems without baseflow under dry conditions. Runoff occurred only during rainfall events and ceased shortly after rainfall stopped. Therefore, the onset and end of runoff were clearly identifiable during the monitoring period. We will clarify this methodological detail in the revised manuscript.
Line 118: what is significant? And how was soil erosion measured?
Line 120: usually rainfall erosivity is EI30 in literature
We thank you for this professional and insightful comment. We acknowledge that EI₃₀ is commonly used to characterize rainfall erosivity in many studies. In our study, however, we used I₃₀ to represent rainfall intensity characteristics and calculated event kinetic energy (Eᵣ) separately to describe rainfall energy. This approach allowed us to examine the individual contributions of rainfall intensity and kinetic energy to runoff and nutrient transport. To avoid confusion, we will further clarify the rationale for using I₃₀ and Eᵣ in the revised manuscript.
Line 125: in which equation?
We will correct it.
is the dimension of rainfall kinetic energy correct? It is usually MJ/ha
We agree with your comment. Although MJ ha-1 and MJ hm-2 are equivalent, MJ ha-1 is more commonly used. We will therefore adopt MJ ha⁻¹ consistently throughout the manuscript.
Line 126: what is a subevent? Please explain
We will further refine the methodology by providing more detailed descriptions of the monitoring approach and sampling design.
Line 127: what does it mean “based on rainfall intensity”, how exactly?
Line 129: Sample collection? Sampling strategy?
Line 134: “post runoff stages”? please explain or rephrase
Line 136: what is sufficient? How many exactly? Please provide some statistics and numbers of the samples
Line 145: please explain the methods from Chen et al briefly here
2.4: please extend
We will further refine and clarify the entire analytical framework, including the procedures for difference testing, correlation analysis, and redundancy analysis (RDA), to improve methodological transparency.
Line 156: were these the same events? Why different number of events?
Although the two catchments are located relatively close to each other, convective summer rainfall events can exhibit substantial spatial heterogeneity. In fact, during one storm event, significant runoff was generated in the F1 catchment, whereas no measurable runoff occurred in F2. Such spatial variability in storm intensity is common for localized summer rainfall and can lead to differences in hydrological response between nearby catchments.
Please extend table captions
We will provide a more detailed explanation of this aspect in the revised manuscript.
Table 1: D might be more meaningful in hours
Line 169: which figure shows this? Please provide references, not just at the end of the section
We will ensure that all figures are correctly cited and referenced at the appropriate locations in the revised manuscript.
Line 174: these percentages should be better explained, to what exactly they refer to?
We will further clarify and elaborate on this aspect in the revised manuscript.
Figure 2: how were the volumes defined? Please add to methods
3.3 title is unclear, effect on what? Please rephrase, also the English
We will revise the section title to: Gully-mediated transport of dissolved NH₄⁺, NO₃⁻, and P
Line 186: belongs to methods. Why downstream divided by upstream?
We fully agree with your suggestion and will clarify this point in the Materials and Methods section.
Lines 193-195: Fig 4 does not entirely show this – because the two catchments differ
We will remove the inaccurate statement in the revised manuscript.
Line 202: transport fluxes: how were they estimated? Please explain in methods
We thank you for this helpful suggestion. We will provide a more detailed description in the Materials and Methods section in the revised manuscript.
Line 205: please add reference to figure
Line 206: “similarly dominated” please rephrase
Line 208-209: which figure shows this? Where do these values stem from?
Figure 5: what does “gully” refer to? To the outlet? What do “sites” refer to in the caption? What is 100%?
Figure 6: methods should explain how cumulative transport fluxes were obtained?
Figure 7: methods should explain the scales (event scale? etc)
3.4.1: correlation analysis should be explained in methods
Figure 8: legend is missing
We will add a legend explaining the symbol sizes in the revised figure.
Lines 235-236: meaning not clear, please explain or rephrase
We thank you for pointing this out. We agree that the meaning of this sentence is not sufficiently clear. We will rephrase and clarify this part in the revised manuscript to improve readability.
Fig 9: should be explained in methods
We will further clarify the purpose and interpretation of applying RDA in Section 2.4.
Lines 242-245: belongs to methods
We will move this part to the Materials and Methods section in the revised manuscript.
Fig 11: to how many points were these lines fitted? C: line would be flat without that one outlying point – why is that event so different than the others? Were the measurements correct?
Response: We thank you for this careful observation. In this study, Type C rainfall corresponds to high-erosivity events. During the two-year monitoring period, one relatively extreme rainfall event (100–200 mm) was recorded, which appears as the high-value point in Fig. 11. Based on long-term local rainfall characteristics, such events occur roughly once every three years. Therefore, rather than representing a spurious outlier, this data point reflects a naturally occurring high-magnitude rainfall event and enhances the representativeness of our dataset by capturing both regular (Type A and B) and relatively extreme (Type C) conditions. In Fig. 11, the regression patterns differ substantially among rainfall types. As commonly observed, the relationship between nutrient flux and rainfall amount follows linear or power-law forms. The notably steeper slope under Type C rainfall indicates a disproportionately stronger response of nutrient loss to incremental rainfall, highlighting its enhanced erosive and transport capacity compared to the more frequent rainfall types.
Line 270: mobilized?
The typical Mollisols in this region are slightly alkaline, under which NH₄⁺ is more likely to be retained through cation exchange and adsorption onto negatively charged soil colloids.
Line 272: gullies?
We will thoroughly review the manuscript to ensure grammatical accuracy and improve language clarity where necessary.
Line 279: interception effects of gullies?
We will rewrite this sentence to make the statement more specific and support it with additional relevant literature to better substantiate the role of gullies.
Line 292: the methods do not mention information on land management practise? Was the timing of fertilizer application managed?
We thank you for this helpful reminder. In the revised manuscript, we will describe fertilizer application practices in this region in Section 2.1. Based on our findings and existing literature, we will also discuss potential implications for agricultural management.
Line 295: gullies
We will correct it throughout the entire text.
Line 304: gullies in agricultural catchments play a dual role
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AC3: 'Reply on RC2', Zhuoxin Chen, 13 Feb 2026
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