the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 24 Sep 2025
Reframing gullies as recharge zones in dryland landscapes of the Loess Plateau, China
Abstract. Large gullies in dryland landscapes are often viewed as indicators of land degradation, yet in some settings they may serve critical ecohydrological functions—supporting groundwater recharge and subsurface connectivity. In China’s Loess Plateau, we assess these functions in the Nianzhuang Catchment using a multi-indicator approach that integrates stable isotopes (δ²H, δ¹⁸O), chloride concentrations, and groundwater level fluctuations. Our results show that precipitation is the dominant source of recharge for shallow pore water within gully zones, while deeper fissure water is replenished more slowly through percolation from the upper layers. Restoration interventions—particularly check dams and ponds—act as focal points for groundwater infiltration, enhancing recharge in otherwise limited dryland systems. Estimated annual recharge (238–241 mm) accounts for over 43 % of annual precipitation, far exceeding typical rates observed in nearby tableland and hilly areas. These findings revise prevailing assumptions by positioning gullies not simply as degraded features, but as hydrologically active zones that can buffer seasonal variability and support ecosystem resilience. The study advances a conceptual framework for using isotopic damping, chloride accumulation, and recharge partitioning as indicators of landscape function in semi-arid regions, offering valuable tools for dryland monitoring and restoration planning.
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Notice on discussion status
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Preprint
(2963 KB)
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(2963 KB) - Metadata XML
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- Final revised paper
Journal article(s) based on this preprint
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2025-4065', Anonymous Referee #1, 09 Dec 2025
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AC1: 'Reply on RC1', Li Wang, 30 Dec 2025
We are grateful for your thoughtful and constructive comments, which have provided invaluable guidance in strengthening our work. In response to your feedback, we have thoroughly revised the manuscript. For specific responses, please refer to the Supplement (HESS-2025-4065-RC1).
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AC4: 'Reply on RC1', Li Wang, 21 Jan 2026
We are grateful for your thoughtful and constructive comments, which have provided invaluable guidance in strengthening our work. In response to your feedback and the comments from reviewers #RC2 and #CC1, we have thoroughly revised the manuscript, and this is our second reply to your valuable comments. For specific responses, please refer to the Supplement.
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AC1: 'Reply on RC1', Li Wang, 30 Dec 2025
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CC1: 'Comment on egusphere-2025-4065', Laiming Huang, 14 Dec 2025
General Comments:
This manuscript presents a timely and important study that challenges the conventional view of gullies as purely erosional, degraded features by positioning them as significant zones for groundwater recharge in the semi-arid Loess Plateau. The research employs an integrated multidisciplinary approach, combining stable isotope analysis, chloride concentration measurements, water-level fluctuation analysis, and hydro-statistical modelling to trace moisture flow paths among surface water, pore water, fissure water, and spring water at a high resolution. Based on this evidence, the authors redefine the hydrological role of gullies in arid ecosystems, directly challenging the traditional view of gullies as symbols of land degradation. The findings reveal that reframing gullies are not merely degraded geomorphic units but rather critical groundwater recharge zones and subsurface connectivity hubs. Precipitation primarily replenishes shallow pore water, while deep fissure water is supplemented by slow, top-down percolation. This understanding overturns the long-standing negative perception of gullies on the Loess Plateau, highlighting their capacity to buffer seasonal hydrological variability and enhance ecosystem resilience. Overall, this study addresses a key knowledge gap regarding groundwater dynamics in gully systems and holds significant practical implications for sustainable water resource management on the Loess Plateau. The manuscript is generally well-written and structured. However, some moderate revisions are needed.
Major Concerns:
- The manuscript sets up a contrast with “piston flow” and “preferential flow” models from tableland studies but does not clearly define what process is dominant in the gullies. The proposed “gully-dominated preferential recharge mechanism” (Line 779) is not well-defined. Is the “preferential” aspect the topographic focusing of runoff into the gully, or are there actual preferential flow paths (macropores, cracks) within the gully soils?
- In my opinion, the manuscript could benefit from clearer articulation of the broader implications of the key findings. For example, how can this insight change land management practices or ecological restoration strategies in other dryland regions globally?
Specific Comments:
1. It is recommended to simplify long sentences to improve readability. For example, lines 53–56: “In these ‘fragile’ and diverse landscapes, understanding the processes that govern when, where, and how groundwater is replenished—including the countervailing influences of vegetation dynamics, geomorphology, and engineered features—is essential for sustaining ecosystems, securing water resources, and informing land restoration and catchment management.” This sentence is structurally complex and could be simplified by breaking it into shorter clauses or highlighting the core information more clearly.- The text categorises groundwater into “pore water, spring water, fissure water”, and further suggested that the criteria for classification be clarified, such as medium type, storage space, and relationship with aquifer structure, to help readers understand the logical framework.The definition of “piston flow”(Line 145-146) is helpful but could be more concise. Consider: “Piston flow describes the displacement of pre-existing water by newly infiltrating water, moving frontally through the pore spaces.”
- The manuscript lists permeability for Neogene coarse sandstone and conglomerate as 7.5–36.19 m/d (lines 213–214). These magnitudes are unusually high for such lithologies; I suspect a units or conversion mistake and recommend the authors re-examine the original data and report corrected values if necessary.
- The text indicates that a low ITTP represents a long residence time, but the high ITTP of ponds (1.5±0.7) is interpreted as "rapid turnover + evaporation dominance,”seemingly overlooking the effect of evaporation on increasing variance. Could this be due to the small sample size for ponds/springs affecting the reliability of the analysis? Additionally, what is the reason for the small sample size for ponds/springs?
- The high recharge rate of gully groundwater, accounting for 43% of precipitation—significantly higher than that in hill areas (<20%)—is a core conclusion of this paper and key evidence supporting the claim that “gullies are critical groundwater recharge zones and subsurface connectivity hubs.”While this conclusion is important, its robustness and uncertainties require further discussion, such as the assumptions underlying the recharge rate estimation method, spatial representativeness, and the impact of extreme events.
- 9 shows that significant rises in groundwater levels and the main recharge period occur during the drier autumn and winter seasons (October to April), while recharge during the summer monsoon rainfall peak is minimal. The authors explain this as effective infiltration during the "cool, low-evaporation period" (Lines 601-604). Are there other potential reasons? For example, freeze-thaw processes, soil water reservoir effects, antecedent moisture conditions, or the competition between rainfall intensity and infiltration capacity?
- The conceptual model (Fig. 10) emphasises the “restructuring”role of the gully system but does not discuss the potential risks of associated pollutant transport. Given that related issues are mentioned in the introduction, it is recommended to include a discussion on this aspect to present a more comprehensive perspective.
- The conclusion section (Section 7) provides a good summary of the study's core findings. However, some statements appear slightly absolute, such as the claim to be “the first to quantitatively identify the unique cascading recharge processes in a thin loess gully catchment” (Lines 781-782). While the research is innovative, caution is advised with phrases like “the first.” It would be preferable to provide supporting literature references or adopt a more measured description.
- The manuscript is largely well-written, but some sections contain complex or awkward sentence structures that could be improved for readability. For instance, the introductory and results sections sometimes use dense scientific language, which might be simplified without losing technical precision. Additionally, the formatting of the references section could be revisited for consistency.
- Some important references are missing from the introduction and discussion sections:
De Vries, J. J., & Simmers, I. (2002). Groundwater recharge: an overview of processes and challenges. Hydrogeology Journal, 10(1), 5-17.
Huang L.M., Shao M.A., Advances and perspectives on soil water research in China’s Loess Plateau. Earth-Science Reviews, 2019: 102962.
Huang, L.M., Wang, Z.W., Pei, Y.W., Zhu, X.C., Jia, X.X., Shao, M.A., Adaptive water use strategies of artificially revegetated plants in a water-limited desert: A case study from the Mu Us Sandy Land. Journal of Hydrology, 2024, 644: 132103.
Xiang, W., Si, B. C., Biswas, A., & Li, Z. (2019). Quantifying dual recharge mechanisms in deep unsaturated zone of Chinese Loess Plateau using stable isotopes. Geoderma, 337, 773-781.
Citation: https://doi.org/10.5194/egusphere-2025-4065-CC1 - AC2: 'Reply on CC1', Li Wang, 30 Dec 2025
- AC5: 'Reply on CC1', Li Wang, 21 Jan 2026
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RC2: 'Comment on egusphere-2025-4065', Anonymous Referee #2, 03 Jan 2026
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-4065/egusphere-2025-4065-RC2-supplement.pdf
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AC3: 'Reply on RC2', Li Wang, 21 Jan 2026
We sincerely thank you for taking the time to review our manuscript and for providing valuable, professional, and rigorous feedback. Your constructive comments have been crucial in improving the quality of our manuscript. For specific responses, please refer to the Supplement.
-
AC3: 'Reply on RC2', Li Wang, 21 Jan 2026
-
CC2: 'Comment on egusphere-2025-4065', Laiming Huang, 04 Jan 2026
The authors have thoroughly addressed all the reviewers' comments they have received, making detailed revisions, clarifications, and additional analyses. These changes notably improve the manuscript’s clarity, rigor, and broader implications, especially in defining key mechanisms, addressing uncertainties, and incorporating relevant literature. I therefore recommend acceptance for publication in HESS with minor revisions.
- There are too many dashes in the text; it is recommended to revise and simplify.
- I suggest replacing the keyword "connectivity" with "hydrological connectivity", and if possiple add a keyword "groundwarer recharge".
- It is unclear why the legend colors in Figure 6 cover the text instead of being right-aligned.
- Redrawing Figure 10 to include the main findings of this study would be beneficial.
Citation: https://doi.org/10.5194/egusphere-2025-4065-CC2 -
AC6: 'Reply on CC2', Li Wang, 21 Jan 2026
Thank you for taking the time to review our manuscript again. We have thoroughly considered your feedback and made further refinements to the manuscript. This is our reply to your second comments. For specific responses, please refer to the Supplement.
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2025-4065', Anonymous Referee #1, 09 Dec 2025
This multidisciplinary study on the Loess Plateau centers on surface–groundwater interactions and fits well within the scope of the Journal-HESS. Based on extensive field observations, the manuscript investigates groundwater recharge processes within gully systems on the Loess Plateau, aiming to reframe gullies as hydrologically active recharge zones rather than merely erosional landforms. The study uses an integrated, multi-method approach—including stable isotopes, chloride concentrations, water table fluctuation (WTF) analysis, and structural equation modeling (SEM)—to examine the linkages among precipitation, surface water, and different groundwater bodies. The authors have invested substantial effort in data collection, fieldwork, and laboratory analyses. Given the increasing importance of groundwater sustainability in arid regions, the study carries clear novelty and relevance, and makes several notable contributions: (1) Reframing the hydrological role of gullies in the loess hilly region (core innovation); (2) Identifying the key mechanisms and process chain of groundwater recharge within gully systems; (3) Demonstrating the significant enhancement of groundwater recharge by engineering interventions (check dams and ponds). Overall, the manuscript is of good quality but still lacks certain details. The following specific comments may help strengthen the paper. I recommend publication after moderate revision.
- Line 85-90: Clearly state the research goals to fully encompass the study content. It is recommended to include a goal specifically addressing the analysis of isotopic characteristics, which will ensure alignment with your methodology and results.
- Line 85-90: Rearrange the research goals to align with the structure of the results section, as the order of goals 1 and 2 appears to be reversed. The goals should follow the sequence in which the results are presented.
- Line 180. How many wells in this catchement were monitored? Please show their positions in Figure 2.
- It is recommended to unify the units in Line 235 ('m/day') and Line 214 ('m/d') for consistency. In addition, Line 213-214, the permeability of Neogene coarse sandstone and conglomerate here couldn’t possibly be this (7.5–36.19 m/d) high. I suspect the authors might have made a mistake with the units. Please double-check.
- Line 494: The numbers in the global meteoric water line equation need to be superscripted.
- Line 614-629: When explaining the phenomenon that 'the isotopic values of most groundwater in the gully areas are more depleted compared to those of rainfall and pond water', ensure the logical connection between 'the thin unsaturated zone' and 'direct recharge from intense rainfall events' is fully articulated, and consider including a discussion on the 'seasonal precipitation isotope effect.
- Line 657-666: When describing the differences between previous studies and this research, it is essential to explicitly highlight the fundamental distinctions in 'spatial scale' and 'hydrological units' to more precisely define the original contribution of your work.
- Line 807-815: When addressing the limitations of isotopes and structural equation modeling, the advantages of the Water Table Fluctuation (WTF) method should be articulated more precisely. Emphasize that these methods are 'complementary' rather than 'contradictory', thus presenting a more balanced argument.
Figures and tables
- 3a and 3c lack units on the x-axis. Additionally, the directions of profiles Line 1 and Line 2 should be clearly indicated in Fig. 3b.
- The terms 'Pore water table' and 'Porous water table' in Fig. 9a should be standardized for consistency.
- In Fig. 9b, the overlap of 'pore water recharge' and 'precipitation' affects the visibility of the recharge results. It is recommended to display precipitation separately or on the upper axis of the figure.
Citation: https://doi.org/10.5194/egusphere-2025-4065-RC1 -
AC1: 'Reply on RC1', Li Wang, 30 Dec 2025
We are grateful for your thoughtful and constructive comments, which have provided invaluable guidance in strengthening our work. In response to your feedback, we have thoroughly revised the manuscript. For specific responses, please refer to the Supplement (HESS-2025-4065-RC1).
-
AC4: 'Reply on RC1', Li Wang, 21 Jan 2026
We are grateful for your thoughtful and constructive comments, which have provided invaluable guidance in strengthening our work. In response to your feedback and the comments from reviewers #RC2 and #CC1, we have thoroughly revised the manuscript, and this is our second reply to your valuable comments. For specific responses, please refer to the Supplement.
-
CC1: 'Comment on egusphere-2025-4065', Laiming Huang, 14 Dec 2025
General Comments:
This manuscript presents a timely and important study that challenges the conventional view of gullies as purely erosional, degraded features by positioning them as significant zones for groundwater recharge in the semi-arid Loess Plateau. The research employs an integrated multidisciplinary approach, combining stable isotope analysis, chloride concentration measurements, water-level fluctuation analysis, and hydro-statistical modelling to trace moisture flow paths among surface water, pore water, fissure water, and spring water at a high resolution. Based on this evidence, the authors redefine the hydrological role of gullies in arid ecosystems, directly challenging the traditional view of gullies as symbols of land degradation. The findings reveal that reframing gullies are not merely degraded geomorphic units but rather critical groundwater recharge zones and subsurface connectivity hubs. Precipitation primarily replenishes shallow pore water, while deep fissure water is supplemented by slow, top-down percolation. This understanding overturns the long-standing negative perception of gullies on the Loess Plateau, highlighting their capacity to buffer seasonal hydrological variability and enhance ecosystem resilience. Overall, this study addresses a key knowledge gap regarding groundwater dynamics in gully systems and holds significant practical implications for sustainable water resource management on the Loess Plateau. The manuscript is generally well-written and structured. However, some moderate revisions are needed.
Major Concerns:
- The manuscript sets up a contrast with “piston flow” and “preferential flow” models from tableland studies but does not clearly define what process is dominant in the gullies. The proposed “gully-dominated preferential recharge mechanism” (Line 779) is not well-defined. Is the “preferential” aspect the topographic focusing of runoff into the gully, or are there actual preferential flow paths (macropores, cracks) within the gully soils?
- In my opinion, the manuscript could benefit from clearer articulation of the broader implications of the key findings. For example, how can this insight change land management practices or ecological restoration strategies in other dryland regions globally?
Specific Comments:
1. It is recommended to simplify long sentences to improve readability. For example, lines 53–56: “In these ‘fragile’ and diverse landscapes, understanding the processes that govern when, where, and how groundwater is replenished—including the countervailing influences of vegetation dynamics, geomorphology, and engineered features—is essential for sustaining ecosystems, securing water resources, and informing land restoration and catchment management.” This sentence is structurally complex and could be simplified by breaking it into shorter clauses or highlighting the core information more clearly.- The text categorises groundwater into “pore water, spring water, fissure water”, and further suggested that the criteria for classification be clarified, such as medium type, storage space, and relationship with aquifer structure, to help readers understand the logical framework.The definition of “piston flow”(Line 145-146) is helpful but could be more concise. Consider: “Piston flow describes the displacement of pre-existing water by newly infiltrating water, moving frontally through the pore spaces.”
- The manuscript lists permeability for Neogene coarse sandstone and conglomerate as 7.5–36.19 m/d (lines 213–214). These magnitudes are unusually high for such lithologies; I suspect a units or conversion mistake and recommend the authors re-examine the original data and report corrected values if necessary.
- The text indicates that a low ITTP represents a long residence time, but the high ITTP of ponds (1.5±0.7) is interpreted as "rapid turnover + evaporation dominance,”seemingly overlooking the effect of evaporation on increasing variance. Could this be due to the small sample size for ponds/springs affecting the reliability of the analysis? Additionally, what is the reason for the small sample size for ponds/springs?
- The high recharge rate of gully groundwater, accounting for 43% of precipitation—significantly higher than that in hill areas (<20%)—is a core conclusion of this paper and key evidence supporting the claim that “gullies are critical groundwater recharge zones and subsurface connectivity hubs.”While this conclusion is important, its robustness and uncertainties require further discussion, such as the assumptions underlying the recharge rate estimation method, spatial representativeness, and the impact of extreme events.
- 9 shows that significant rises in groundwater levels and the main recharge period occur during the drier autumn and winter seasons (October to April), while recharge during the summer monsoon rainfall peak is minimal. The authors explain this as effective infiltration during the "cool, low-evaporation period" (Lines 601-604). Are there other potential reasons? For example, freeze-thaw processes, soil water reservoir effects, antecedent moisture conditions, or the competition between rainfall intensity and infiltration capacity?
- The conceptual model (Fig. 10) emphasises the “restructuring”role of the gully system but does not discuss the potential risks of associated pollutant transport. Given that related issues are mentioned in the introduction, it is recommended to include a discussion on this aspect to present a more comprehensive perspective.
- The conclusion section (Section 7) provides a good summary of the study's core findings. However, some statements appear slightly absolute, such as the claim to be “the first to quantitatively identify the unique cascading recharge processes in a thin loess gully catchment” (Lines 781-782). While the research is innovative, caution is advised with phrases like “the first.” It would be preferable to provide supporting literature references or adopt a more measured description.
- The manuscript is largely well-written, but some sections contain complex or awkward sentence structures that could be improved for readability. For instance, the introductory and results sections sometimes use dense scientific language, which might be simplified without losing technical precision. Additionally, the formatting of the references section could be revisited for consistency.
- Some important references are missing from the introduction and discussion sections:
De Vries, J. J., & Simmers, I. (2002). Groundwater recharge: an overview of processes and challenges. Hydrogeology Journal, 10(1), 5-17.
Huang L.M., Shao M.A., Advances and perspectives on soil water research in China’s Loess Plateau. Earth-Science Reviews, 2019: 102962.
Huang, L.M., Wang, Z.W., Pei, Y.W., Zhu, X.C., Jia, X.X., Shao, M.A., Adaptive water use strategies of artificially revegetated plants in a water-limited desert: A case study from the Mu Us Sandy Land. Journal of Hydrology, 2024, 644: 132103.
Xiang, W., Si, B. C., Biswas, A., & Li, Z. (2019). Quantifying dual recharge mechanisms in deep unsaturated zone of Chinese Loess Plateau using stable isotopes. Geoderma, 337, 773-781.
Citation: https://doi.org/10.5194/egusphere-2025-4065-CC1 - AC2: 'Reply on CC1', Li Wang, 30 Dec 2025
- AC5: 'Reply on CC1', Li Wang, 21 Jan 2026
-
RC2: 'Comment on egusphere-2025-4065', Anonymous Referee #2, 03 Jan 2026
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-4065/egusphere-2025-4065-RC2-supplement.pdf
-
AC3: 'Reply on RC2', Li Wang, 21 Jan 2026
We sincerely thank you for taking the time to review our manuscript and for providing valuable, professional, and rigorous feedback. Your constructive comments have been crucial in improving the quality of our manuscript. For specific responses, please refer to the Supplement.
-
AC3: 'Reply on RC2', Li Wang, 21 Jan 2026
-
CC2: 'Comment on egusphere-2025-4065', Laiming Huang, 04 Jan 2026
The authors have thoroughly addressed all the reviewers' comments they have received, making detailed revisions, clarifications, and additional analyses. These changes notably improve the manuscript’s clarity, rigor, and broader implications, especially in defining key mechanisms, addressing uncertainties, and incorporating relevant literature. I therefore recommend acceptance for publication in HESS with minor revisions.
- There are too many dashes in the text; it is recommended to revise and simplify.
- I suggest replacing the keyword "connectivity" with "hydrological connectivity", and if possiple add a keyword "groundwarer recharge".
- It is unclear why the legend colors in Figure 6 cover the text instead of being right-aligned.
- Redrawing Figure 10 to include the main findings of this study would be beneficial.
Citation: https://doi.org/10.5194/egusphere-2025-4065-CC2 -
AC6: 'Reply on CC2', Li Wang, 21 Jan 2026
Thank you for taking the time to review our manuscript again. We have thoroughly considered your feedback and made further refinements to the manuscript. This is our reply to your second comments. For specific responses, please refer to the Supplement.
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Zhenxia Ji
Alan D. Ziegler
Li Wang
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
(2963 KB) - Metadata XML
This multidisciplinary study on the Loess Plateau centers on surface–groundwater interactions and fits well within the scope of the Journal-HESS. Based on extensive field observations, the manuscript investigates groundwater recharge processes within gully systems on the Loess Plateau, aiming to reframe gullies as hydrologically active recharge zones rather than merely erosional landforms. The study uses an integrated, multi-method approach—including stable isotopes, chloride concentrations, water table fluctuation (WTF) analysis, and structural equation modeling (SEM)—to examine the linkages among precipitation, surface water, and different groundwater bodies. The authors have invested substantial effort in data collection, fieldwork, and laboratory analyses. Given the increasing importance of groundwater sustainability in arid regions, the study carries clear novelty and relevance, and makes several notable contributions: (1) Reframing the hydrological role of gullies in the loess hilly region (core innovation); (2) Identifying the key mechanisms and process chain of groundwater recharge within gully systems; (3) Demonstrating the significant enhancement of groundwater recharge by engineering interventions (check dams and ponds). Overall, the manuscript is of good quality but still lacks certain details. The following specific comments may help strengthen the paper. I recommend publication after moderate revision.
Figures and tables