the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 05 May 2025
Hysteresis between groundwater and surface water levels indicates the states of hydrological turnover affecting solute transport and redox processes
Abstract. Small streams are highly sensitive to variations in discharge, a sensitivity predicted to increase in future climate scenarios, impacting ecological health of streams and water management practices. Prolonged low-flow conditions alter groundwater-surface water (GW-SW) exchange patterns, leading to extended losing phases and a reduced duration of gaining periods. This study examines the relationship between hydrological turnover (HT) and hysteresis patterns under various system states in a third-order tributary of the River Mosel in Trier, Germany, using high-resolution hydrological and chemical data collected over two years.
Our results reveal distinct seasonal dynamics in GW-SW exchange. Counterclockwise hysteresis, prevalent during summer and drought conditions, was linked to the expansion of the hyporheic zone and bank storage, which reorganizes flow paths and influences redox dynamics. We established a strong correlation between HT and hysteresis characteristics, identifying the h-index as a valuable diagnostic tool for tracking seasonal changes in GW-SW connectivity, storage and hyporheic zone behavior based on hydraulic preconditions.
As climate change intensifies drought conditions, the hyporheic zone will play a vital role in solute cycling and GW-SW connectivity. The h-index, combined with chemical and hydrological monitoring, provides a robust framework for understanding and predicting these dynamics in small stream ecosystems.
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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.
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We investigated the interplay between stream stage and near-stream groundwater levels using hysteresis patterns to characterize hydrological turnover (HT) and exchange dynamics. Based on two years of high-resolution data, hysteresis direction and magnitude track seasonal system states. The hysteresis index (h-index) is related to HT and serves as a diagnostic indicator of connectivity, storage, and hyporheic exchange, with implications for biogeochemical processes under drought.
Interactive discussion
Status: closed
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CC1: 'Comment on egusphere-2025-1674', Nima Zafarmomen, 08 May 2025
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AC1: 'Reply on CC1', Lars Bäthke, 14 May 2025
We thank the commenter Nima Zafarmomen for the detailed and constructive feedback. Below we respond to the points raised:
- Objectives and hypotheses
The Introduction motivates the study but never states concrete research questions or hypotheses. Add a short paragraph (or numbered list) at the end of the Introduction that spells out exactly what you test or demonstrate.
We agree that explicit objectives improve clarity. We will add a paragraph at the end of the Introduction section stating our specific hypotheses regarding hysteresis direction and hydrological turnover.
“The objective of this study is to explore how seasonal changes in hydraulic conditions influence groundwater–surface water interactions in a headwater stream. We hypothesise that (1) the direction and magnitude of hysteresis between groundwater and stream water levels are indicative of seasonal hydrological states, and (2) the hysteresis index is related to hydrological turnover and reflects the degree of hyporheic exchange, consequently affecting water chemistry.”
- Causality vs. correlation
Several conclusions—such as counter-clockwise hysteresis proving hyporheic expansion—are inferred from correlations. Either soften the causal language or supply additional evidence (e.g., time-lag analysis, hydraulic modelling) that directly links loop direction to flow-path reversal or bank-storage volume.
We acknowledge the reviewer’s concern and have revised the manuscript to avoid overstating causality where only correlative evidence is available. We replaced causal terms such as “proves” or “demonstrates” with “suggests,” “indicates,” or “is associated with” where appropriate. These changes were made in the Abstract (lines 12–13),
“Counterclockwise hysteresis, prevalent during summer and drought conditions, coincides with conditions indicative of hyporheic zone expansion and bank storage, potentially affecting flow paths and redox dynamics.”
Discussion (lines 316–317, 327),
“This may be indicative of a hyporheic response occurring during early infiltration phases.”
“Counterclockwise hysteresis and high hydrological turnover are associated with enhanced riparian bank storage.”
and Conclusion (line 370),
“Our findings suggest that hysteresis behavior can serve as an indicator of changes in hyporheic zone dynamics associated with hydrological turnover, particularly during summer droughts.”
where the connection between hysteresis direction and hyporheic expansion was previously framed in causal terms.
- Drought sample size
Only nine drought events are analysed, yet they underpin strong statements. Provide confidence intervals (bootstrapping would suffice) or, alternatively, fold drought events into a broader “low-flow” class and discuss the limitation.
Within our sampling campaign, nine events were conducted under exceptionally low-flow conditions. We classified these as “drought” events, as discharge fell below the 5th percentile of the five-year streamflow record at our observation site. This threshold-based classification aligns with standard hydrological drought definitions and was supported by concurrent low-flow observations at the downstream catchment gauging station (>10 a). To investigate differences in system behavior across hydrological states, we grouped events into winter, summer, and drought categories and tested for statistical significance using non-parametric methods. We applied a 95 % confidence level and will clarify this in the Methods section. Figure 5 will be updated to reflect these confidence intervals.
- Statistics
Pearson, Spearman, and Wilcoxon tests are referenced, but p-values and effect sizes are scattered, and no correction for multiple comparisons is mentioned. Consolidate the statistical results in one concise section, report effect sizes, and apply a correction such as Holm–Bonferroni where you test several correlations simultaneously.
We applied a Holm–Bonferroni correction where multiple comparisons were made. We will refer to the correction method in the manuscript accordingly.
- Figures
Figures 2, 4, 6, and 7 pack in many axes, symbols and colours. Some panels duplicate information already shown elsewhere. Split oversized figures, move supporting plots to the Supplement, use a single colour palette across all figures (e.g., winter = blue, summer = orange, drought = red) and increase font sizes.
We appreciate the feedback regarding figure complexity. While we aim to retain the current number of figures to preserve coherence in the main text, we will revise Figures 2, 4, 6, and 7 to improve readability. This includes increasing font sizes, unifying the color palette, and removing redundant visual elements where possible. We believe these adjustments will improve clarity without compromising the completeness of the data presentation.
- Chemical Interpretation Depth
The simultaneous presence of nitrate, manganese and iron is intriguing but only briefly noted. Expand the Discussion to explore residence times, lateral mixing, and implications for nitrate removal or metal mobilisation under overlapping redox zones.
Thank you for highlighting this. We will expand the discussion section to better interpret the simultaneous presence of nitrate, manganese, and iron. We will explore how overlapping redox zones may arise from lateral mixing and variable residence times and discuss the implications for nitrate removal and metal mobilization. We will refere in the manuscript to additional studies, such as Kaufman, M. H., M. B. Cardenas, J. Buttles, A. J. Kessler, and P. L. M. Cook (2017), Hyporheic hot moments: Dissolved oxygen dynamics in the hyporheic zone in response to surface flow perturbations, Water Resour. Res., 53, 6642–6662, doi:10.1002/2016WR020296 and Briggs, M. A., F. D. Day-Lewis, J. P. Zarnetske, and J. W. Harvey (2015), A physical explanation for the development of redox microzones in hyporheic flow, Geophys. Res. Lett., 42, 4402–4410, doi:10.1002/2015GL064200, discussing the dynamic nature of typical streams and rivers driving equally dynamic redox conditions in the hyporheic zone.
These additions help clarify the biogeochemical significance of the observed solute patterns under different hydrological conditions.
- Consistency in terminology and units
The manuscript alternates between “hydrological turnover,” “turnover” and “HT [% m⁻¹].” Likewise, hysteresis direction is described as clockwise/anticlockwise in some places and positive/negative h in others. Define the terms once, specify units clearly, and stick to a single vocabulary throughout.
We agree that consistent terminology improves readability. Throughout the manuscript, we will consistently refer to “hydrological turnover (HT)” and specify it with the unit [% m⁻¹] upon first mention. For hysteresis, we will define the relationship to positive/negative h-index values early in the Methods section.
- Suggested citation of Sentinel-based study by Nima Zafarmomen
Please also consider citing “Assimilation of Sentinel-based Leaf Area Index for Modeling Surface-Groundwater Interactions in Irrigation Districts”. That study demonstrates how remotely sensed vegetation parameters—specifically Sentinel-derived LAI—can be assimilated into coupled surface–groundwater models to improve estimates of evapotranspiration and return flows in irrigated landscapes. Referencing it would (i) situate your h-index/HT framework within the broader move toward multi-sensor data integration and (ii) underscore the relevance of vegetation dynamics when interpreting seasonal GW–SW connectivity, especially under drought or low-flow conditions.
While we appreciate the suggestion and recognize the relevance of remote sensing for hydrological studies, the focus of Zafarmomen et al. (2024) on model-data assimilation in irrigated systems differs substantially from our process-based approach in a non-irrigated headwater stream. As such, we chose not to include this reference to maintain focus on directly related work.
Thank you again for the helpful suggestions, which significantly improved the clarity and robustness of our manuscript.
Citation: https://doi.org/10.5194/egusphere-2025-1674-AC1 -
AC2: 'Reply on AC1', Lars Bäthke, 20 May 2025
In addition to our previous comment, we would like to offer two brief supplementary points regarding our hypotheses and the classification of drought conditions:
- Objectives and hypotheses
As outlined earlier, we will state our hypotheses explicitly at the end of the Introduction. To improve, we refined the formulation of hypothesis (2):
“The objective of this study is to explore how seasonal changes in hydraulic conditions influence groundwater–surface water interactions in a headwater stream. We hypothesise that (1) the direction and magnitude of hysteresis between groundwater and stream water levels are indicative of seasonal hydrological states, and (2) the hysteresis index is related to hydrological turnover and reflects the degree of hyporheic exchange, consequently affecting water chemistry.”
- Drought sample size
Within our sampling campaign, nine HT measurements were conducted during exceptionally low-flow conditions. We classified these as “drought” events, as they fell below the 5th percentile of the five-year streamflow record at our observation site. This threshold-based approach follows established hydrological drought definitions (Stahl et al., 2020) and is supported by simultaneous low-flow observations at a downstream gauging station with a >10-year record. To evaluate differences in system behavior across hydrological conditions, we grouped events into winter, summer, and drought categories. Statistical differences were assessed using non-parametric tests with a 95 % confidence level. We will clarify this methodology in the revised Methods section, and update Figure 5 to include the corresponding confidence intervals.
Stahl, K., Vidal, J.-P., Hannaford, J., Tijdeman, E., Laaha, G., Gauster, T., and Tallaksen, L. M. (2020): The challenges of hydrological drought definition, quantification and communication: an interdisciplinary perspective, Proc. IAHS, 383, 291–295, https://doi.org/10.5194/piahs-383-291-2020.
Citation: https://doi.org/10.5194/egusphere-2025-1674-AC2
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AC1: 'Reply on CC1', Lars Bäthke, 14 May 2025
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RC1: 'Comment on egusphere-2025-1674', Anonymous Referee #1, 04 Jul 2025
The interaction between groundwater and surface water affects the dynamics of streamflow, solute transport, and nutrient cycling in the hyporheic zone, with significant implications for water resource management and ecological health. This study, using high-resolution hydrological and chemical data, examines the relationship between hydrological turnover (HT) and hysteresis patterns. The results highlight the h-index as a diagnostic tool for quantifying seasonal shifts in GW-SW interactions, particularly its strong correlation with HT. The research methodology provides a valuable reference for similar regions. However, the manuscript still requires minor revisions. The following are specific comments:
- Section 2.2.2: The description of how to calculate the h-index is not clear. It may be beneficial to add a schematic diagram.
- What are the valence states of iron and manganese measured in this study? How can the concentrations of these ions and DOC be used to determine changes in redox conditions? Why was dissolved oxygen (DO) concentration not measured?
- The hydraulic gradient is used to determine losing and gaining conditions, and these results should be compared with the analysis of the h-index.
- Before the section “3.1 Hysteresis Index in relation to HT,” please add a results section heading. Change 3.1 to 3.2.
- In Figure 3, for panel c, should the y-axis label “Field Capacity” be “Ratio of soil moisture to field capacity”?
- In Figure 4, panel f is difficult to interpret. Please provide additional explanation. What do the vertical line and cyan filled circle represent?
- Lines 271-272: This sentence is difficult to understand; please elaborate to make it more convincing: “The redox sensitive species were observed in all samples in varying concentrations, with the highest concentrations in the near stream groundwater.”
- Lines 277-279: Please check for logical inconsistencies in these two sentences: “During summer and the corresponding system states GW1 exceeds the GW2 concentration median showing increased redox potential/activity directly in the riparian zone of the stream. During summer, losing conditions and with counterclockwise hysteresis manganese and iron concentrations are highest at GW1.”
- Lines 337-339: Please elaborate to make this statement more convincing: “This promotes biogeochemical activity, as fluctuating water stages create alternating oxic and anoxic conditions, evident by the ions present during such conditions, especially at GW1 (Figure 8).”
- Lines 344-346: “We observed increased microbial activity and oxygen depletion within the riparian zone, likely driven by enhanced inflow of labile organic carbon from the stream into nearstream sediments.” Please provide the data or evidence to support this statement.
Citation: https://doi.org/10.5194/egusphere-2025-1674-RC1 -
AC3: 'Reply on RC1', Lars Bäthke, 16 Jul 2025
We would like to thank the reviewer for their careful reading and valuable comments, which helped to improve the clarity and quality of our manuscript. Below, we address each point raised in detail.
- “Section 2.2.2: The description of how to calculate the h-index is not clear. It may be beneficial to add a schematic diagram.”
Response:
Thank you for this suggestion. We have revised the description in Section 2.2.2 to improve clarity and we will have a schematic diagram illustrating the calculation steps of the h-index following Zuecco et al. (2016). The diagram will show the rising and falling limbs of a sample hysteresis loop, normalized variables, and how the area differences are computed. We believe this will help readers understand the calculation more intuitively.- “What are the valence states of iron and manganese measured in this study? How can the concentrations of these ions and DOC be used to determine changes in redox conditions? Why was dissolved oxygen (DO) concentration not measured?”
Response:
In this study, total dissolved iron and manganese were measured using atomic absorption spectroscopy (AAS) after filtration, without speciation of individual oxidation states. Therefore, both Fe(II)/Fe(III) and Mn(II)/Mn(IV) may be present, but due to their solubility characteristics, we interpret the dissolved fractions to largely represent the reduced forms: Fe²⁺ and Mn²⁺.We used the presence and relative concentrations of DOC, NO3⁻ Fe, and Mn as indirect indicators of redox conditions, following the redox ladder in oxygen-depleted environments, microbial respiration first consumes oxygen, then nitrate, followed by manganese and iron oxides (Zehnder & Stumm, 1988). Hence, the co-occurrence of elevated DOC and dissolved Fe²⁺/Mn²⁺ in riparian groundwater, especially during low-flow summer conditions, suggests partly reducing environments with potentially high microbial activity.
DO was not measured due to field limitations and the focus on solute-based proxies. We acknowledge that including DO data would have strengthened the redox interpretation and have now added this as a limitation in the discussion (line 340-345):
Revised: “This sequence is influenced by factors such as redox potential, pH, electron acceptor availability, and the presence of bioavailable organic matter. While our study lacks direct measurements of dissolved oxygen (DO), we infer redox conditions based on observed concentrations of DOC, Fe²⁺, and Mn²⁺. During summer and under counterclockwise hysteresis conditions, when potentially increased microbial activity and oxygen depletion within the riparian zone occur, these are likely sustained by enhanced inflow of labile organic carbon from the stream into near-stream sediments (Smith & Arah, 1986).”
- “The hydraulic gradient is used to determine losing and gaining conditions, and these results should be compared with the analysis of the h-index.”
Response:
We agree with the reviewer that this connection strengthens the interpretation. We revised the Discussion to clarify the comparison of hydraulic gradients with h-index values. We emphasize that steep negative gradients correlate with counterclockwise hysteresis (losing conditions), while positive or flat gradients often coincide with clockwise hysteresis (gaining conditions) (lines 300-304).Revised: “A distinct seasonal pattern emerges, steeper hydraulic gradients during summer events correspond with counterclockwise hysteresis, while gentler gradients in autumn and winter are associated with clockwise hysteresis (Figure 7). This relationship is reflected in our results, where counterclockwise hysteresis (negative h-index) predominantly occurs under steep negative hydraulic gradients, indicating losing conditions, whereas positive or near-zero gradients are typically associated with clockwise hysteresis and gaining conditions (Figure 7 and corresponding h-index values in Section 3.2).”
Additionally, we will add a short paragraph/sentence in the discussion section relating to the non-exclusive co-occurrence of H-Index and gaining and losing conditions to literature:
“Additionally, our results show that gaining and losing conditions are not exclusively associated with one hysteresis direction. Similar observations were reported by Gelmini et al. (2022), who found that hysteresis behavior—captured by the h-index—can vary depending on antecedent conditions and event timing, rather than being strictly determined by the direction of exchange.”
4 “Before the section ‘3.1 Hysteresis Index in relation to HT,’ please add a results section heading. Change 3.1 to 3.2.”
Response:
Thank you for pointing this out. We have added the missing section heading as “3 Results” before the previous subsection 3.1, which is now correctly labeled as 3.2 Hysteresis Index in relation to HT. We changed all the following section titles.5: “In Figure 3, for panel c, should the y-axis label ‘Field Capacity’ be ‘Ratio of soil moisture to field capacity’?”
Response:
Yes, the reviewer is correct. The label was imprecise. We will update the y-axis label in Figure 3c to: “Soil moisture as ratio to field capacity (–)” to clarify that the values are normalized.6: “In Figure 4, panel f is difficult to interpret. Please provide additional explanation. What do the vertical line and cyan filled circle represent?”
Response:
We agree that this panel needed clarification. The vertical line indicates the transition point between summer and winter periods. The cyan filled circle marks the streamflow net change over the observed stream section. We have revised the figure captions:“(f) Hydrological turnover (HT) in %/m point measurements (n= 28) during the observation period. Grey bars indicating net losses and gains, cyan dots net changes”
7: “Lines 271–272: This sentence is difficult to understand; please elaborate to make it more convincing.”
“The redox sensitive species were observed in all samples in varying concentrations, with the highest concentrations in the near stream groundwater.”
Response:
We revised the sentence for clarity as follows:Revised:
“We detected redox-sensitive species (Fe, Mn, NO3⁻) in all samples, with the highest concentrations of iron and manganese consistently observed in near-stream groundwater wells. This suggests heterogeny redox conditions in the riparian groundwater compared to the stream.”8: “Lines 277–279: Please check for logical inconsistencies in these two sentences.”
“During summer and the corresponding system states GW1 exceeds the GW2 concentration median showing increased redox potential/activity directly in the riparian zone of the stream. During summer, losing conditions and with counterclockwise hysteresis manganese and iron concentrations are highest at GW1.”
Response:
We understand the confusion and revised these sentences for clarity and internal consistency:Revised:
“During summer, manganese and iron concentrations were higher at GW1 compared to GW2, suggesting increased redox activity closer to the stream. These conditions coincided with losing hydraulic gradients and counterclockwise hysteresis, indicating stream-derived import may stimulate microbial activity in the near-stream sediments.”9: “Lines 337–339: Please elaborate to make this statement more convincing.”
“This promotes biogeochemical activity, as fluctuating water stages create alternating oxic and anoxic conditions, evident by the ions present during such conditions, especially at GW1 (Figure 8).”Response:
We have elaborated this point to be more specific and evidence-based. We introduced the hydrological framework of redox-active compounds in aquatic systems by Peiffer et al., (2021) and the inherent redox variability of wetlands shown by Frei et al., (2012) as micro-topographic hot-spots together with Knorr et al., (2009), to show that heterogeneity in redox environment at the riparian zone impacting biochemical activity:Revised:
“Short-time alternating water level gradients during and after stream flow events create oxic-anoxic transition zones within the riparian zone that could promote microbial processes (Knorr et al., 2009). These redox hot spots are variable in time and space across the riparian zone and facilitate biogeochemical processes (Frei et al., 2012). This is supported by elevated concentrations of DOC, Fe²⁺, and Mn²⁺ at GW1 during summer losing conditions, fitting with the concept of the hydrological framework of redox-active compounds in aquatic systems (Peiffer et al., 2021) explaining the simultaneous occurrence of ions along the redox ladder. Thus, enhanced biogeochemical activity is likely to be driven by the observed redox fluctuations (Figure 8).”10: “Lines 344–346: Please provide the data or evidence to support this statement.”
“We observed increased microbial activity and oxygen depletion within the riparian zone, likely driven by enhanced inflow of labile organic carbon from the stream into near stream sediments.”Response:
We recognize the need for clearer support. We revised the statement according to the changes in the Discussion.Revised:
“During summer and under counterclockwise hysteresis conditions, we interpret increased microbial activity and oxygen depletion based on the co-occurrence of DOC concentrations, elevated Fe²⁺ and Mn²⁺ in GW1 during losing conditions, likely driven by enhanced inflow of labile organic carbon from the stream into near-stream sediments (Smith & Arah, 1986). This is evidenced by elevated DOC concentrations and redox-sensitive solutes at GW1 (Figure 8).”We added relevant citations at the Conclusion (Peiffer et al., 2021; Frei et al., 2012 and Knorr et al., 2009) line 360-365.
Peiffer, S., Kappler, A., Haderlein, S. B., Schmidt, C., Byrne, J. M., Kleindienst, S., ... & Planer-Friedrich, B. (2021). A biogeochemical–hydrological framework for the role of redox-active compounds in aquatic systems. Nature Geoscience, 14(5), 264-272.
Frei, S., K. H. Knorr, S. Peiffer, and J. H. Fleckenstein (2012), Surface micro-topography causes hot spots of biogeochemical activity in wetland systems: A virtual modeling experiment, J. Geophys. Res., 117, G00N12,doi:10.1029/2012JG002012.
Knorr, K. H., Lischeid, G. & Blodau, C. Dynamics of redox processes in a minerotrophic fen exposed to a water table manipulation. Geoderma 153, 379–392 (2009).
Zehnder, A. J. B., & Stumm, W. (1988). Geochemistry and biogeochemistry of anaerobic habitats. In Biology of anaerobic microorganisms, 1-38.
Citation: https://doi.org/10.5194/egusphere-2025-1674-AC3
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RC2: 'Comment on egusphere-2025-1674', Anonymous Referee #2, 03 Aug 2025
General comment
This manuscript presents an interesting investigation of the connectivity between groundwater and surface waters, examined through an analysis of the hysteresis between water levels carried out during different runoff events. The authors also analysed the hydrological turnover and related it (along with a hysteresis index) to solute transport. Overall, this is a valuable study, but it presents a key limitation that is the very limited number of groundwater wells, which may not be representative of the hydrological conditions in other locations in the catchment. Despite the interesting dataset and findings, such limitation should be discussed in the manuscript. In addition, a characterization of the runoff events is currently missing. The main features of the runoff events (e.g., precipitation characteristics, magnitude of the runoff response, antecedent wetness conditions) should be reported and shown in a table and/or figures. Finally, the specific objectives should be clarified at the end of the introduction, and results and discussion should be organized based on these objectives.
Specific comments
- Line 12: Please explain which variables are used for hysteresis analysis.
- Introduction: The specific objectives of this study are not clearly presented. Furthermore, the authors should consider reporting their research hypotheses, which are later mentioned at Line 306 (Discussion).
- Materials and methods: In this section, the authors should give details about the selected events, starting with their definition. The monitored characteristics should be clarified to understand the magnitude of the events, at least in terms of rainfall amounts and intensities, duration, peak discharge, runoff coefficients, and antecedent wetness conditions. Descriptive statistics of these key characteristics can be reported in a table (or figures) as well.
- Lines 114-123: I think these sentences belong to other sections, such as the experimental setup or a section about data analysis.
- Lines 139-141: Please provide here details about where NaCl was injected (exact location with coordinates and distances from the groundwater wells) and where electrical conductivity was measured. Based on the current text, I do not understand where the injection site is and when the injection started (e.g., during rainfall events, after their end, etc.).
- Lines 155-156: Did you carry out the samplings before, during, or after the events? This detail is important for understanding the results reported in Section 3.3.
- Lines 157-159: What do the values after ± indicate? Is the instrumental precision?
- Equation 3: How is Qgain determined?
- Lines 191-195: A scheme with two examples of hysteretic loops between GW and SW levels would be very helpful to understand the direction of the loops and the computation of the index. Please also explain what clockwise and counterclockwise loops indicate.
- Results: I think this section would benefit from a clear structure, which should reflect the specific objectives of the manuscript.
- Section 3.3: This section about the link between hysteretic behaviour and exchange of solutes between the stream and the groundwater is the most interesting one. Other results, such as the temporal variability of the hysteresis depending on the season, are not that novel, but this section deserves to be expanded. For instance, you could relate tracer concentrations to the characteristics of the different events. I also recommend explaining why you grouped the tracers in the mixing and redox zone; is it due to different research hypotheses?
- Figure 8: Are the observed differences significant or not?
- Line 306: What are these hypotheses? They should be reported at the end of the introduction, and in relation to the specific objectives of the manuscript.
- Discussion: I think this section lacks a structure and, more importantly, does not address the limitations of this study. For instance, findings are based on data collected at one stream section and two groundwater wells, with a limited number of events for HT analysis (just 28 out of 68 events). Therefore, it is impossible to assess whether the observed processes and the conceptual framework are representative of the entire riparian zone-stream system.
Technical corrections
- Lines 77-78: Phrasing is not very clear.
- Line 105: Please replace ‘mouth’ with ‘outlet’.
- Figure 3: Explain in the caption what ‘mQ’ means.
- Figure 5: Please add the number of samples used for each violin plot. In the caption, please use ‘violin plots’ instead of ‘violine charts’.
- Line 241: Use a comma before ‘a comparable’.
- Line 253: ‘divided’ instead of ‘deivided’.
- Figure 8: Please add the number of samples used for each boxplot.
- Line 267: I think you should add ‘for’ before ‘the winter’.
- Line 330: ‘with’ is missing after ‘associated’.
Citation: https://doi.org/10.5194/egusphere-2025-1674-RC2 - AC4: 'Reply on RC2', Lars Bäthke, 02 Oct 2025
Peer review completion
Post-review adjustments
Interactive discussion
Status: closed
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CC1: 'Comment on egusphere-2025-1674', Nima Zafarmomen, 08 May 2025
The manuscript presents a valuable, two-year data set that links hysteresis behaviour (h-index) to hydrological turnover (HT) and redox-sensitive chemistry in a third-order stream. The topic is timely, and the field evidence is strong. With clearer framing, tighter statistics and leaner figures, the study could make a solid contribution to hydrology and eco-hydrology journals.
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Explicit objectives.
The Introduction motivates the study but never states concrete research questions or hypotheses. Add a short paragraph (or numbered list) at the end of the Introduction that spells out exactly what you test or demonstrate. -
Causality vs correlation.
Several conclusions—such as counter-clockwise hysteresis proving hyporheic expansion—are inferred from correlations. Either soften the causal language or supply additional evidence (e.g., time-lag analysis, hydraulic modelling) that directly links loop direction to flow-path reversal or bank-storage volume. -
Drought sample size and treatment.
Only nine drought events are analysed, yet they underpin strong statements. Provide confidence intervals (bootstrapping would suffice) or, alternatively, fold drought events into a broader “low-flow” class and discuss the limitation. -
Statistics and multiple testing.
Pearson, Spearman, and Wilcoxon tests are referenced, but p-values and effect sizes are scattered, and no correction for multiple comparisons is mentioned. Consolidate the statistical results in one concise section, report effect sizes, and apply a correction such as Holm–Bonferroni where you test several correlations simultaneously. -
Figure overload and clarity.
Figures 2, 4, 6, and 7 pack in many axes, symbols and colours. Some panels duplicate information already shown elsewhere. Split oversized figures, move supporting plots to the Supplement, use a single colour palette across all figures (e.g., winter = blue, summer = orange, drought = red) and increase font sizes. -
Chemical interpretation depth.
The simultaneous presence of nitrate, manganese and iron is intriguing but only briefly noted. Expand the Discussion to explore residence times, lateral mixing, and implications for nitrate removal or metal mobilisation under overlapping redox zones. -
Consistency in terminology and units.
The manuscript alternates between “hydrological turnover,” “turnover” and “HT [% m⁻¹].” Likewise, hysteresis direction is described as clockwise/anticlockwise in some places and positive/negative h in others. Define the terms once, specify units clearly, and stick to a single vocabulary throughout.
Please also consider citing “Assimilation of Sentinel-based Leaf Area Index for Modeling Surface-Groundwater Interactions in Irrigation Districts”. That study demonstrates how remotely sensed vegetation parameters—specifically Sentinel-derived LAI—can be assimilated into coupled surface–groundwater models to improve estimates of evapotranspiration and return flows in irrigated landscapes. Referencing it would (i) situate your h-index/HT framework within the broader move toward multi-sensor data integration and (ii) underscore the relevance of vegetation dynamics when interpreting seasonal GW–SW connectivity, especially under drought or low-flow conditions
Citation: https://doi.org/10.5194/egusphere-2025-1674-CC1 -
AC1: 'Reply on CC1', Lars Bäthke, 14 May 2025
We thank the commenter Nima Zafarmomen for the detailed and constructive feedback. Below we respond to the points raised:
- Objectives and hypotheses
The Introduction motivates the study but never states concrete research questions or hypotheses. Add a short paragraph (or numbered list) at the end of the Introduction that spells out exactly what you test or demonstrate.
We agree that explicit objectives improve clarity. We will add a paragraph at the end of the Introduction section stating our specific hypotheses regarding hysteresis direction and hydrological turnover.
“The objective of this study is to explore how seasonal changes in hydraulic conditions influence groundwater–surface water interactions in a headwater stream. We hypothesise that (1) the direction and magnitude of hysteresis between groundwater and stream water levels are indicative of seasonal hydrological states, and (2) the hysteresis index is related to hydrological turnover and reflects the degree of hyporheic exchange, consequently affecting water chemistry.”
- Causality vs. correlation
Several conclusions—such as counter-clockwise hysteresis proving hyporheic expansion—are inferred from correlations. Either soften the causal language or supply additional evidence (e.g., time-lag analysis, hydraulic modelling) that directly links loop direction to flow-path reversal or bank-storage volume.
We acknowledge the reviewer’s concern and have revised the manuscript to avoid overstating causality where only correlative evidence is available. We replaced causal terms such as “proves” or “demonstrates” with “suggests,” “indicates,” or “is associated with” where appropriate. These changes were made in the Abstract (lines 12–13),
“Counterclockwise hysteresis, prevalent during summer and drought conditions, coincides with conditions indicative of hyporheic zone expansion and bank storage, potentially affecting flow paths and redox dynamics.”
Discussion (lines 316–317, 327),
“This may be indicative of a hyporheic response occurring during early infiltration phases.”
“Counterclockwise hysteresis and high hydrological turnover are associated with enhanced riparian bank storage.”
and Conclusion (line 370),
“Our findings suggest that hysteresis behavior can serve as an indicator of changes in hyporheic zone dynamics associated with hydrological turnover, particularly during summer droughts.”
where the connection between hysteresis direction and hyporheic expansion was previously framed in causal terms.
- Drought sample size
Only nine drought events are analysed, yet they underpin strong statements. Provide confidence intervals (bootstrapping would suffice) or, alternatively, fold drought events into a broader “low-flow” class and discuss the limitation.
Within our sampling campaign, nine events were conducted under exceptionally low-flow conditions. We classified these as “drought” events, as discharge fell below the 5th percentile of the five-year streamflow record at our observation site. This threshold-based classification aligns with standard hydrological drought definitions and was supported by concurrent low-flow observations at the downstream catchment gauging station (>10 a). To investigate differences in system behavior across hydrological states, we grouped events into winter, summer, and drought categories and tested for statistical significance using non-parametric methods. We applied a 95 % confidence level and will clarify this in the Methods section. Figure 5 will be updated to reflect these confidence intervals.
- Statistics
Pearson, Spearman, and Wilcoxon tests are referenced, but p-values and effect sizes are scattered, and no correction for multiple comparisons is mentioned. Consolidate the statistical results in one concise section, report effect sizes, and apply a correction such as Holm–Bonferroni where you test several correlations simultaneously.
We applied a Holm–Bonferroni correction where multiple comparisons were made. We will refer to the correction method in the manuscript accordingly.
- Figures
Figures 2, 4, 6, and 7 pack in many axes, symbols and colours. Some panels duplicate information already shown elsewhere. Split oversized figures, move supporting plots to the Supplement, use a single colour palette across all figures (e.g., winter = blue, summer = orange, drought = red) and increase font sizes.
We appreciate the feedback regarding figure complexity. While we aim to retain the current number of figures to preserve coherence in the main text, we will revise Figures 2, 4, 6, and 7 to improve readability. This includes increasing font sizes, unifying the color palette, and removing redundant visual elements where possible. We believe these adjustments will improve clarity without compromising the completeness of the data presentation.
- Chemical Interpretation Depth
The simultaneous presence of nitrate, manganese and iron is intriguing but only briefly noted. Expand the Discussion to explore residence times, lateral mixing, and implications for nitrate removal or metal mobilisation under overlapping redox zones.
Thank you for highlighting this. We will expand the discussion section to better interpret the simultaneous presence of nitrate, manganese, and iron. We will explore how overlapping redox zones may arise from lateral mixing and variable residence times and discuss the implications for nitrate removal and metal mobilization. We will refere in the manuscript to additional studies, such as Kaufman, M. H., M. B. Cardenas, J. Buttles, A. J. Kessler, and P. L. M. Cook (2017), Hyporheic hot moments: Dissolved oxygen dynamics in the hyporheic zone in response to surface flow perturbations, Water Resour. Res., 53, 6642–6662, doi:10.1002/2016WR020296 and Briggs, M. A., F. D. Day-Lewis, J. P. Zarnetske, and J. W. Harvey (2015), A physical explanation for the development of redox microzones in hyporheic flow, Geophys. Res. Lett., 42, 4402–4410, doi:10.1002/2015GL064200, discussing the dynamic nature of typical streams and rivers driving equally dynamic redox conditions in the hyporheic zone.
These additions help clarify the biogeochemical significance of the observed solute patterns under different hydrological conditions.
- Consistency in terminology and units
The manuscript alternates between “hydrological turnover,” “turnover” and “HT [% m⁻¹].” Likewise, hysteresis direction is described as clockwise/anticlockwise in some places and positive/negative h in others. Define the terms once, specify units clearly, and stick to a single vocabulary throughout.
We agree that consistent terminology improves readability. Throughout the manuscript, we will consistently refer to “hydrological turnover (HT)” and specify it with the unit [% m⁻¹] upon first mention. For hysteresis, we will define the relationship to positive/negative h-index values early in the Methods section.
- Suggested citation of Sentinel-based study by Nima Zafarmomen
Please also consider citing “Assimilation of Sentinel-based Leaf Area Index for Modeling Surface-Groundwater Interactions in Irrigation Districts”. That study demonstrates how remotely sensed vegetation parameters—specifically Sentinel-derived LAI—can be assimilated into coupled surface–groundwater models to improve estimates of evapotranspiration and return flows in irrigated landscapes. Referencing it would (i) situate your h-index/HT framework within the broader move toward multi-sensor data integration and (ii) underscore the relevance of vegetation dynamics when interpreting seasonal GW–SW connectivity, especially under drought or low-flow conditions.
While we appreciate the suggestion and recognize the relevance of remote sensing for hydrological studies, the focus of Zafarmomen et al. (2024) on model-data assimilation in irrigated systems differs substantially from our process-based approach in a non-irrigated headwater stream. As such, we chose not to include this reference to maintain focus on directly related work.
Thank you again for the helpful suggestions, which significantly improved the clarity and robustness of our manuscript.
Citation: https://doi.org/10.5194/egusphere-2025-1674-AC1 -
AC2: 'Reply on AC1', Lars Bäthke, 20 May 2025
In addition to our previous comment, we would like to offer two brief supplementary points regarding our hypotheses and the classification of drought conditions:
- Objectives and hypotheses
As outlined earlier, we will state our hypotheses explicitly at the end of the Introduction. To improve, we refined the formulation of hypothesis (2):
“The objective of this study is to explore how seasonal changes in hydraulic conditions influence groundwater–surface water interactions in a headwater stream. We hypothesise that (1) the direction and magnitude of hysteresis between groundwater and stream water levels are indicative of seasonal hydrological states, and (2) the hysteresis index is related to hydrological turnover and reflects the degree of hyporheic exchange, consequently affecting water chemistry.”
- Drought sample size
Within our sampling campaign, nine HT measurements were conducted during exceptionally low-flow conditions. We classified these as “drought” events, as they fell below the 5th percentile of the five-year streamflow record at our observation site. This threshold-based approach follows established hydrological drought definitions (Stahl et al., 2020) and is supported by simultaneous low-flow observations at a downstream gauging station with a >10-year record. To evaluate differences in system behavior across hydrological conditions, we grouped events into winter, summer, and drought categories. Statistical differences were assessed using non-parametric tests with a 95 % confidence level. We will clarify this methodology in the revised Methods section, and update Figure 5 to include the corresponding confidence intervals.
Stahl, K., Vidal, J.-P., Hannaford, J., Tijdeman, E., Laaha, G., Gauster, T., and Tallaksen, L. M. (2020): The challenges of hydrological drought definition, quantification and communication: an interdisciplinary perspective, Proc. IAHS, 383, 291–295, https://doi.org/10.5194/piahs-383-291-2020.
Citation: https://doi.org/10.5194/egusphere-2025-1674-AC2
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RC1: 'Comment on egusphere-2025-1674', Anonymous Referee #1, 04 Jul 2025
The interaction between groundwater and surface water affects the dynamics of streamflow, solute transport, and nutrient cycling in the hyporheic zone, with significant implications for water resource management and ecological health. This study, using high-resolution hydrological and chemical data, examines the relationship between hydrological turnover (HT) and hysteresis patterns. The results highlight the h-index as a diagnostic tool for quantifying seasonal shifts in GW-SW interactions, particularly its strong correlation with HT. The research methodology provides a valuable reference for similar regions. However, the manuscript still requires minor revisions. The following are specific comments:
- Section 2.2.2: The description of how to calculate the h-index is not clear. It may be beneficial to add a schematic diagram.
- What are the valence states of iron and manganese measured in this study? How can the concentrations of these ions and DOC be used to determine changes in redox conditions? Why was dissolved oxygen (DO) concentration not measured?
- The hydraulic gradient is used to determine losing and gaining conditions, and these results should be compared with the analysis of the h-index.
- Before the section “3.1 Hysteresis Index in relation to HT,” please add a results section heading. Change 3.1 to 3.2.
- In Figure 3, for panel c, should the y-axis label “Field Capacity” be “Ratio of soil moisture to field capacity”?
- In Figure 4, panel f is difficult to interpret. Please provide additional explanation. What do the vertical line and cyan filled circle represent?
- Lines 271-272: This sentence is difficult to understand; please elaborate to make it more convincing: “The redox sensitive species were observed in all samples in varying concentrations, with the highest concentrations in the near stream groundwater.”
- Lines 277-279: Please check for logical inconsistencies in these two sentences: “During summer and the corresponding system states GW1 exceeds the GW2 concentration median showing increased redox potential/activity directly in the riparian zone of the stream. During summer, losing conditions and with counterclockwise hysteresis manganese and iron concentrations are highest at GW1.”
- Lines 337-339: Please elaborate to make this statement more convincing: “This promotes biogeochemical activity, as fluctuating water stages create alternating oxic and anoxic conditions, evident by the ions present during such conditions, especially at GW1 (Figure 8).”
- Lines 344-346: “We observed increased microbial activity and oxygen depletion within the riparian zone, likely driven by enhanced inflow of labile organic carbon from the stream into nearstream sediments.” Please provide the data or evidence to support this statement.
Citation: https://doi.org/10.5194/egusphere-2025-1674-RC1 -
AC3: 'Reply on RC1', Lars Bäthke, 16 Jul 2025
We would like to thank the reviewer for their careful reading and valuable comments, which helped to improve the clarity and quality of our manuscript. Below, we address each point raised in detail.
- “Section 2.2.2: The description of how to calculate the h-index is not clear. It may be beneficial to add a schematic diagram.”
Response:
Thank you for this suggestion. We have revised the description in Section 2.2.2 to improve clarity and we will have a schematic diagram illustrating the calculation steps of the h-index following Zuecco et al. (2016). The diagram will show the rising and falling limbs of a sample hysteresis loop, normalized variables, and how the area differences are computed. We believe this will help readers understand the calculation more intuitively.- “What are the valence states of iron and manganese measured in this study? How can the concentrations of these ions and DOC be used to determine changes in redox conditions? Why was dissolved oxygen (DO) concentration not measured?”
Response:
In this study, total dissolved iron and manganese were measured using atomic absorption spectroscopy (AAS) after filtration, without speciation of individual oxidation states. Therefore, both Fe(II)/Fe(III) and Mn(II)/Mn(IV) may be present, but due to their solubility characteristics, we interpret the dissolved fractions to largely represent the reduced forms: Fe²⁺ and Mn²⁺.We used the presence and relative concentrations of DOC, NO3⁻ Fe, and Mn as indirect indicators of redox conditions, following the redox ladder in oxygen-depleted environments, microbial respiration first consumes oxygen, then nitrate, followed by manganese and iron oxides (Zehnder & Stumm, 1988). Hence, the co-occurrence of elevated DOC and dissolved Fe²⁺/Mn²⁺ in riparian groundwater, especially during low-flow summer conditions, suggests partly reducing environments with potentially high microbial activity.
DO was not measured due to field limitations and the focus on solute-based proxies. We acknowledge that including DO data would have strengthened the redox interpretation and have now added this as a limitation in the discussion (line 340-345):
Revised: “This sequence is influenced by factors such as redox potential, pH, electron acceptor availability, and the presence of bioavailable organic matter. While our study lacks direct measurements of dissolved oxygen (DO), we infer redox conditions based on observed concentrations of DOC, Fe²⁺, and Mn²⁺. During summer and under counterclockwise hysteresis conditions, when potentially increased microbial activity and oxygen depletion within the riparian zone occur, these are likely sustained by enhanced inflow of labile organic carbon from the stream into near-stream sediments (Smith & Arah, 1986).”
- “The hydraulic gradient is used to determine losing and gaining conditions, and these results should be compared with the analysis of the h-index.”
Response:
We agree with the reviewer that this connection strengthens the interpretation. We revised the Discussion to clarify the comparison of hydraulic gradients with h-index values. We emphasize that steep negative gradients correlate with counterclockwise hysteresis (losing conditions), while positive or flat gradients often coincide with clockwise hysteresis (gaining conditions) (lines 300-304).Revised: “A distinct seasonal pattern emerges, steeper hydraulic gradients during summer events correspond with counterclockwise hysteresis, while gentler gradients in autumn and winter are associated with clockwise hysteresis (Figure 7). This relationship is reflected in our results, where counterclockwise hysteresis (negative h-index) predominantly occurs under steep negative hydraulic gradients, indicating losing conditions, whereas positive or near-zero gradients are typically associated with clockwise hysteresis and gaining conditions (Figure 7 and corresponding h-index values in Section 3.2).”
Additionally, we will add a short paragraph/sentence in the discussion section relating to the non-exclusive co-occurrence of H-Index and gaining and losing conditions to literature:
“Additionally, our results show that gaining and losing conditions are not exclusively associated with one hysteresis direction. Similar observations were reported by Gelmini et al. (2022), who found that hysteresis behavior—captured by the h-index—can vary depending on antecedent conditions and event timing, rather than being strictly determined by the direction of exchange.”
4 “Before the section ‘3.1 Hysteresis Index in relation to HT,’ please add a results section heading. Change 3.1 to 3.2.”
Response:
Thank you for pointing this out. We have added the missing section heading as “3 Results” before the previous subsection 3.1, which is now correctly labeled as 3.2 Hysteresis Index in relation to HT. We changed all the following section titles.5: “In Figure 3, for panel c, should the y-axis label ‘Field Capacity’ be ‘Ratio of soil moisture to field capacity’?”
Response:
Yes, the reviewer is correct. The label was imprecise. We will update the y-axis label in Figure 3c to: “Soil moisture as ratio to field capacity (–)” to clarify that the values are normalized.6: “In Figure 4, panel f is difficult to interpret. Please provide additional explanation. What do the vertical line and cyan filled circle represent?”
Response:
We agree that this panel needed clarification. The vertical line indicates the transition point between summer and winter periods. The cyan filled circle marks the streamflow net change over the observed stream section. We have revised the figure captions:“(f) Hydrological turnover (HT) in %/m point measurements (n= 28) during the observation period. Grey bars indicating net losses and gains, cyan dots net changes”
7: “Lines 271–272: This sentence is difficult to understand; please elaborate to make it more convincing.”
“The redox sensitive species were observed in all samples in varying concentrations, with the highest concentrations in the near stream groundwater.”
Response:
We revised the sentence for clarity as follows:Revised:
“We detected redox-sensitive species (Fe, Mn, NO3⁻) in all samples, with the highest concentrations of iron and manganese consistently observed in near-stream groundwater wells. This suggests heterogeny redox conditions in the riparian groundwater compared to the stream.”8: “Lines 277–279: Please check for logical inconsistencies in these two sentences.”
“During summer and the corresponding system states GW1 exceeds the GW2 concentration median showing increased redox potential/activity directly in the riparian zone of the stream. During summer, losing conditions and with counterclockwise hysteresis manganese and iron concentrations are highest at GW1.”
Response:
We understand the confusion and revised these sentences for clarity and internal consistency:Revised:
“During summer, manganese and iron concentrations were higher at GW1 compared to GW2, suggesting increased redox activity closer to the stream. These conditions coincided with losing hydraulic gradients and counterclockwise hysteresis, indicating stream-derived import may stimulate microbial activity in the near-stream sediments.”9: “Lines 337–339: Please elaborate to make this statement more convincing.”
“This promotes biogeochemical activity, as fluctuating water stages create alternating oxic and anoxic conditions, evident by the ions present during such conditions, especially at GW1 (Figure 8).”Response:
We have elaborated this point to be more specific and evidence-based. We introduced the hydrological framework of redox-active compounds in aquatic systems by Peiffer et al., (2021) and the inherent redox variability of wetlands shown by Frei et al., (2012) as micro-topographic hot-spots together with Knorr et al., (2009), to show that heterogeneity in redox environment at the riparian zone impacting biochemical activity:Revised:
“Short-time alternating water level gradients during and after stream flow events create oxic-anoxic transition zones within the riparian zone that could promote microbial processes (Knorr et al., 2009). These redox hot spots are variable in time and space across the riparian zone and facilitate biogeochemical processes (Frei et al., 2012). This is supported by elevated concentrations of DOC, Fe²⁺, and Mn²⁺ at GW1 during summer losing conditions, fitting with the concept of the hydrological framework of redox-active compounds in aquatic systems (Peiffer et al., 2021) explaining the simultaneous occurrence of ions along the redox ladder. Thus, enhanced biogeochemical activity is likely to be driven by the observed redox fluctuations (Figure 8).”10: “Lines 344–346: Please provide the data or evidence to support this statement.”
“We observed increased microbial activity and oxygen depletion within the riparian zone, likely driven by enhanced inflow of labile organic carbon from the stream into near stream sediments.”Response:
We recognize the need for clearer support. We revised the statement according to the changes in the Discussion.Revised:
“During summer and under counterclockwise hysteresis conditions, we interpret increased microbial activity and oxygen depletion based on the co-occurrence of DOC concentrations, elevated Fe²⁺ and Mn²⁺ in GW1 during losing conditions, likely driven by enhanced inflow of labile organic carbon from the stream into near-stream sediments (Smith & Arah, 1986). This is evidenced by elevated DOC concentrations and redox-sensitive solutes at GW1 (Figure 8).”We added relevant citations at the Conclusion (Peiffer et al., 2021; Frei et al., 2012 and Knorr et al., 2009) line 360-365.
Peiffer, S., Kappler, A., Haderlein, S. B., Schmidt, C., Byrne, J. M., Kleindienst, S., ... & Planer-Friedrich, B. (2021). A biogeochemical–hydrological framework for the role of redox-active compounds in aquatic systems. Nature Geoscience, 14(5), 264-272.
Frei, S., K. H. Knorr, S. Peiffer, and J. H. Fleckenstein (2012), Surface micro-topography causes hot spots of biogeochemical activity in wetland systems: A virtual modeling experiment, J. Geophys. Res., 117, G00N12,doi:10.1029/2012JG002012.
Knorr, K. H., Lischeid, G. & Blodau, C. Dynamics of redox processes in a minerotrophic fen exposed to a water table manipulation. Geoderma 153, 379–392 (2009).
Zehnder, A. J. B., & Stumm, W. (1988). Geochemistry and biogeochemistry of anaerobic habitats. In Biology of anaerobic microorganisms, 1-38.
Citation: https://doi.org/10.5194/egusphere-2025-1674-AC3
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RC2: 'Comment on egusphere-2025-1674', Anonymous Referee #2, 03 Aug 2025
General comment
This manuscript presents an interesting investigation of the connectivity between groundwater and surface waters, examined through an analysis of the hysteresis between water levels carried out during different runoff events. The authors also analysed the hydrological turnover and related it (along with a hysteresis index) to solute transport. Overall, this is a valuable study, but it presents a key limitation that is the very limited number of groundwater wells, which may not be representative of the hydrological conditions in other locations in the catchment. Despite the interesting dataset and findings, such limitation should be discussed in the manuscript. In addition, a characterization of the runoff events is currently missing. The main features of the runoff events (e.g., precipitation characteristics, magnitude of the runoff response, antecedent wetness conditions) should be reported and shown in a table and/or figures. Finally, the specific objectives should be clarified at the end of the introduction, and results and discussion should be organized based on these objectives.
Specific comments
- Line 12: Please explain which variables are used for hysteresis analysis.
- Introduction: The specific objectives of this study are not clearly presented. Furthermore, the authors should consider reporting their research hypotheses, which are later mentioned at Line 306 (Discussion).
- Materials and methods: In this section, the authors should give details about the selected events, starting with their definition. The monitored characteristics should be clarified to understand the magnitude of the events, at least in terms of rainfall amounts and intensities, duration, peak discharge, runoff coefficients, and antecedent wetness conditions. Descriptive statistics of these key characteristics can be reported in a table (or figures) as well.
- Lines 114-123: I think these sentences belong to other sections, such as the experimental setup or a section about data analysis.
- Lines 139-141: Please provide here details about where NaCl was injected (exact location with coordinates and distances from the groundwater wells) and where electrical conductivity was measured. Based on the current text, I do not understand where the injection site is and when the injection started (e.g., during rainfall events, after their end, etc.).
- Lines 155-156: Did you carry out the samplings before, during, or after the events? This detail is important for understanding the results reported in Section 3.3.
- Lines 157-159: What do the values after ± indicate? Is the instrumental precision?
- Equation 3: How is Qgain determined?
- Lines 191-195: A scheme with two examples of hysteretic loops between GW and SW levels would be very helpful to understand the direction of the loops and the computation of the index. Please also explain what clockwise and counterclockwise loops indicate.
- Results: I think this section would benefit from a clear structure, which should reflect the specific objectives of the manuscript.
- Section 3.3: This section about the link between hysteretic behaviour and exchange of solutes between the stream and the groundwater is the most interesting one. Other results, such as the temporal variability of the hysteresis depending on the season, are not that novel, but this section deserves to be expanded. For instance, you could relate tracer concentrations to the characteristics of the different events. I also recommend explaining why you grouped the tracers in the mixing and redox zone; is it due to different research hypotheses?
- Figure 8: Are the observed differences significant or not?
- Line 306: What are these hypotheses? They should be reported at the end of the introduction, and in relation to the specific objectives of the manuscript.
- Discussion: I think this section lacks a structure and, more importantly, does not address the limitations of this study. For instance, findings are based on data collected at one stream section and two groundwater wells, with a limited number of events for HT analysis (just 28 out of 68 events). Therefore, it is impossible to assess whether the observed processes and the conceptual framework are representative of the entire riparian zone-stream system.
Technical corrections
- Lines 77-78: Phrasing is not very clear.
- Line 105: Please replace ‘mouth’ with ‘outlet’.
- Figure 3: Explain in the caption what ‘mQ’ means.
- Figure 5: Please add the number of samples used for each violin plot. In the caption, please use ‘violin plots’ instead of ‘violine charts’.
- Line 241: Use a comma before ‘a comparable’.
- Line 253: ‘divided’ instead of ‘deivided’.
- Figure 8: Please add the number of samples used for each boxplot.
- Line 267: I think you should add ‘for’ before ‘the winter’.
- Line 330: ‘with’ is missing after ‘associated’.
Citation: https://doi.org/10.5194/egusphere-2025-1674-RC2 - AC4: 'Reply on RC2', Lars Bäthke, 02 Oct 2025
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Journal article(s) based on this preprint
We investigated the interplay between stream stage and near-stream groundwater levels using hysteresis patterns to characterize hydrological turnover (HT) and exchange dynamics. Based on two years of high-resolution data, hysteresis direction and magnitude track seasonal system states. The hysteresis index (h-index) is related to HT and serves as a diagnostic indicator of connectivity, storage, and hyporheic exchange, with implications for biogeochemical processes under drought.
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Cited
2 citations as recorded by crossref.
- Seasonal river-level fluctuations regulate riparian nitrate removal through coupled hydrologic exchange and redox dynamics J. Chen et al. https://doi.org/10.1016/j.jhydrol.2026.136138
- A comparative analysis of machine learning models for predicting groundwater and surface water in a stressed semi-arid watershed: The Khanmirza case study Z. Ebrahimzadeh et al. https://doi.org/10.1007/s12665-025-12801-4
Tobias Schuetz
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
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The manuscript presents a valuable, two-year data set that links hysteresis behaviour (h-index) to hydrological turnover (HT) and redox-sensitive chemistry in a third-order stream. The topic is timely, and the field evidence is strong. With clearer framing, tighter statistics and leaner figures, the study could make a solid contribution to hydrology and eco-hydrology journals.
Explicit objectives.
The Introduction motivates the study but never states concrete research questions or hypotheses. Add a short paragraph (or numbered list) at the end of the Introduction that spells out exactly what you test or demonstrate.
Causality vs correlation.
Several conclusions—such as counter-clockwise hysteresis proving hyporheic expansion—are inferred from correlations. Either soften the causal language or supply additional evidence (e.g., time-lag analysis, hydraulic modelling) that directly links loop direction to flow-path reversal or bank-storage volume.
Drought sample size and treatment.
Only nine drought events are analysed, yet they underpin strong statements. Provide confidence intervals (bootstrapping would suffice) or, alternatively, fold drought events into a broader “low-flow” class and discuss the limitation.
Statistics and multiple testing.
Pearson, Spearman, and Wilcoxon tests are referenced, but p-values and effect sizes are scattered, and no correction for multiple comparisons is mentioned. Consolidate the statistical results in one concise section, report effect sizes, and apply a correction such as Holm–Bonferroni where you test several correlations simultaneously.
Figure overload and clarity.
Figures 2, 4, 6, and 7 pack in many axes, symbols and colours. Some panels duplicate information already shown elsewhere. Split oversized figures, move supporting plots to the Supplement, use a single colour palette across all figures (e.g., winter = blue, summer = orange, drought = red) and increase font sizes.
Chemical interpretation depth.
The simultaneous presence of nitrate, manganese and iron is intriguing but only briefly noted. Expand the Discussion to explore residence times, lateral mixing, and implications for nitrate removal or metal mobilisation under overlapping redox zones.
Consistency in terminology and units.
The manuscript alternates between “hydrological turnover,” “turnover” and “HT [% m⁻¹].” Likewise, hysteresis direction is described as clockwise/anticlockwise in some places and positive/negative h in others. Define the terms once, specify units clearly, and stick to a single vocabulary throughout.
Please also consider citing “Assimilation of Sentinel-based Leaf Area Index for Modeling Surface-Groundwater Interactions in Irrigation Districts”. That study demonstrates how remotely sensed vegetation parameters—specifically Sentinel-derived LAI—can be assimilated into coupled surface–groundwater models to improve estimates of evapotranspiration and return flows in irrigated landscapes. Referencing it would (i) situate your h-index/HT framework within the broader move toward multi-sensor data integration and (ii) underscore the relevance of vegetation dynamics when interpreting seasonal GW–SW connectivity, especially under drought or low-flow conditions