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 -
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 -
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
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