the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 30 Apr 2026
Hydrological and hydrochemical drought responses across ten solutes in a pre-alpine headwater catchment
Abstract. There is growing evidence that droughts affect stream water quality in multiple ways, often degrading it and thereby exacerbating water scarcity. However, our understanding of the hydrological and biochemical processes driving these changes is limited, due to a lack of high-frequency measurements across solutes covering pre-drought, drought and post-drought conditions. In this study, we analyzed the hydrological and hydrochemical responses to drought as compared to pre- and post-drought, in a forested pre-alpine catchment in Switzerland using high-frequency concentration data for ten different solutes. During the dry summer of 2018, discharge and groundwater table depth continued to decrease. The decrease in discharge slowed with increasing dryness, and flow never ceased entirely. Compared to normal summer conditions, a smaller fraction of rainfall converted into discharge, further illustrating the depletion of catchment storages. All solute concentrations exhibited significant breakpoints in their relationships with discharge and groundwater table depth. Mostly, they exhibited more chemostatic patterns at lower discharge (i.e., during drought) than during normal summer conditions. Groundwater table depth served as a complementary indicator for the disconnection of the hydrological and hydrochemical drought response. Overall, this observed divergence can be attributed to the fact that old groundwater was the only source of stream water during the drought, while shallower source areas, such as the catchment soils, were hydrologically disconnected from stream discharge. Our results also highlight the role of biochemical processes that alter the overall availability and mobility of different solutes, such as changes in redox conditions and nutrient uptake rates. In summary, our findings confirm the impact of drought on catchment water quality and demonstrate that the catchment's water quality response to drought cannot be explained by discharge dynamics alone. Rather, a detailed assessment of both hydrological and biochemical processes is necessary to identify the underlying drivers.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Hydrology and Earth System Sciences.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
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Status: open (until 13 Jun 2026)
- RC1: 'Comment on egusphere-2026-2218', Anonymous Referee #1, 11 May 2026 reply
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RC2: 'Comment on egusphere-2026-2218', Anonymous Referee #2, 20 May 2026
reply
I have reviewed the revised manuscript, “Hydrological and hydrochemical drought responses across ten solutes in a pre-alpine headwater catchment” by Winter et al. (2026). The study investigates the hydrological and hydrochemical responses to the extraordinary 2018 drought, including pre- and post-drought summer conditions, in comparison to normal summer conditions (2017, 2019, and 2020) in a forested pre-alpine catchment in Switzerland. Using high-frequency measurements of ten different solutes and discharge, as well as groundwater table depth, the authors demonstrate that severe drought conditions exert clear effects on stream water quality across ten different solutes.
A key contribution of the study is the identification of distinct breakpoints in the relationships between solute concentrations, discharge, and groundwater table depth, revealing a divergence between hydrological and hydrochemical catchment responses under exceptional dry conditions. The results suggest that, during drought, groundwater becomes the dominant source of streamflow, while shallower soil water sources become hydrologically disconnected. Consequently, stream solute concentrations increasingly approach groundwater concentrations and exhibit more chemostatic behavior despite continued declines in discharge and groundwater levels. In particular, the identification of breakpoints in the concentration–discharge and discharge–groundwater table relationships provides valuable insights into the linkages between stream water quality and changes in hydrological connectivity and disconnection during drought conditions. By combining hydrological and biogeochemical perspectives, the study provides important insights into the processes controlling water quality during drought, as well as during pre- and post-drought conditions.
However, I have some recommendation before publication. A few comments are detailed below.
General comments:
The manuscript characterizes concentration–discharge (C–Q) relationships using the slope parameter b, where positive values indicate enrichment patterns, negative values indicate dilution patterns, and values close to zero are interpreted as constant pattern. While this framework is appropriate, the interpretation of hydrochemical drought responses should not rely solely on the condition b ≈ 0 to define constant concentration patterns. In the context of drought analyses, the distinction between chemostatic and chemodynamic behavior is particularly important and should be addressed more explicitly throughout the manuscript, especially since these concepts later take an important role in the discussion. A near-zero C–Q slope alone does not necessarily imply chemostatic behavior, as low slopes may still coincide with substantial concentration variability. To better support the interpretation of constant or chemostatic behavior during drought conditions, I recommend complementing the C–Q slope analysis with variability-based metrics such as the ratio of the coefficient of variation of concentrations to discharge (CVC/CVQ). Including these metrics would provide a more robust quantitative basis for distinguishing chemostatic from chemodynamic responses and would strengthen the central thesis of the manuscript.
Furthermore, precipitation is a hydrological driver in this study, yet its temporal dynamics and role during the drought are currently insufficiently visualized. To better support the conclusions regarding hydrological connectivity and storage depletion, I recommend adding precipitation to the main hydrometeorological time series (e.g., Figure 2), together with discharge and groundwater table depth. A combined visualization of precipitation, discharge, and groundwater dynamics would substantially improve the reader’s ability to follow the temporal evolution of drought development and recovery.
Specific comments:
Lines 94–96: The authors list the investigated solutes as elemental species (e.g., Ca, Mg, Fe, Mn). Since the study focuses on dissolved constituents in stream water, it would be scientifically more precise and chemically consistent to use ionic notation where appropriate (e.g., Ca2+, Mg2+, K+, Fe2+/3+, Mn2+, Sr2+, Cl-, NO3-, SO42-). This would improve clarity regarding the actual dissolved species analyzed and align the terminology with standard hydrochemical conventions.
Lines 119–121: The definition of the recovery period and the criterion used to determine the end of the drought should be clarified further. In particular, the term “normal” discharge levels is currently too vague and should be quantitatively defined. Please specify which hydrological threshold, reference period, or statistical criterion was used to determine when discharge conditions had returned to “normal” levels. This clarification is important for the reproducibility and interpretation of the drought and recovery period delineation.
Line 123 (Figure 1b): The readability and interpretability of the figure could be improved by revising the legend to explicitly indicate which years correspond to the respective displayed data points. In addition, it may be helpful to label or number the five identified precipitation events directly within the figure. This would facilitate a clearer comparison of the different discharge and groundwater table responses to specific precipitation events during drought conditions.
Lines 130–131: Figure 1a does not appear to show discharge on a logarithmic scale; this likely refers instead to Figure 1b. Please check and correct the figure reference accordingly.
Lines 142–143: The manuscript refers to Table S1 for the overview of discarded events. However, this table could not be found in the Supplementary Material. Please include the missing table for completeness and transparency of the data selection procedure. In addition, the sentence could be reformulated for clarity as follows: “Overall, we identified 63 events, out of which, depending on the respective solute species, 6–27 events had no or incomplete concentration measurements and were therefore discarded (Table S1).”
Lines 145–149: A brief clarification that precipitation rates were normalized within the ERRA framework would improve the methodological description and further highlight that this approach enables a consistent comparison between precipitation events and the corresponding rainfall–runoff responses across the different hydrological periods.
Lines 155–157: For clarity, please specify explicitly that the analysis refers to changes in the slope of the Q–gw relationship. This would make it easier to follow which relationship is being referred to, particularly since the manuscript frequently discusses C–Q relationships. In this context, simply writing “slope of Q–gw” or “Q-gw slope” would already be sufficient.
Lines 162–164: The interpretation of b ≈ 0 as a constant concentration pattern should be phrased more carefully, as a near-zero C–Q slope does not necessarily indicate a truly chemostatic response. Changes in concentration variability decoupled from discharge may still occur despite b ≈ 0, which can reflect chemodynamic behavior. It may therefore be helpful to mention that constant or chemostatic patterns can be further substantiated using variability-based metrics such as CVC/CVQ (e.g. Thompson, S. E., Basu, N. B., Lascurain, J., Aubeneau, A., andRao, P. S. C.: Relative dominance of hydrologic versus biogeochemical factors on solute export across impact gradients, Water Resour. Res., 47, 10, https://doi.org/10.1029/2010wr009605,2011.)
Lines 166–186: As breakpoint determination plays a central role in this study, please clarify whether breakpoint detection was performed systematically and, if so, describe the specific method, statistical criterion, or threshold used to identify the breakpoints.
Line 173: Please add the index i to C in Equation 2 (i.e., Ci) for consistency with Equation 1 and to clearly indicate that concentrations are evaluated separately for each solute species.
Lines 186–188: The wording “groundwater table gradually declines while discharge levels do not drop much further” may imply that discharge no longer decreases after the breakpoint. However, discharge still appears to decline, although at a substantially lower recession rate once discharge fall below the breakpoint. Consider revising the wording to reflect this more clearly. Furthermore, the statement that “discharge continues to respond to precipitation events, while the groundwater table lacks such short-term responses and appears to be largely decoupled from the discharge dynamics” could be supported more convincingly by adding a time series of precipitation, discharge, and groundwater table depth in Figure 2. Such a visualization would help illustrate the differing temporal responses and the inferred decoupling during drought conditions. Alternatively, a direct reference to Figure 1b at this point in the text would help support and contextualize this interpretation.
Lines 265–267: It would be helpful to add a direct reference to Figure 1b in this section, as the figure supports the interpretation that precipitation events triggered short-term discharge responses while groundwater table depth remained largely unresponsive during drought conditions.
Lines 299–301: The discussion of chemodynamic patterns is scientifically relevant. However, this behavior is currently not explicitly demonstrated in the Results section. Since the interpretation of b ≈ 0 as a constant concentration pattern plays an important role in the manuscript, it would strengthen the analysis to include additional metrics that better distinguish chemostatic from chemodynamic behavior. In particular, the inclusion of the CVC/CVQ ratio (e.g., in a table similar to Table 1) would provide a more robust basis for supporting the assumption that low-flow conditions with b ≈ 0 indeed reflect chemostatic behavior. The CVC/CVQ metric would more clearly separate periods of low concentration variability (chemostatic behavior) from chemodynamic responses, as discussed here. Furthermore, the inclusion of the CVC/CVQ metric could help to sharpen the definition and interpretation of the term “chemodynamic” within the context of this study while also providing stronger quantitative support for the interpretations presented in the discussion.
Line 348 (Figure 5): The caption should describe the bottom row of Figure 5 in more detail to improve the interpretation of the graph. In particular, please clarify how the bottom panels differ from the upper panels and explicitly explain the meaning of the varying symbol sizes. A more detailed description of the symbol scaling and panel structure would make the figure easier to understand for the reader.
Lines 399–401: While the statement explains the observed enrichment behavior during rewetting, the chemodynamic component of the transport response could be described in more detail. In addition to the general reconnection of soil zones during rewetting, a chemodynamic enrichment behavior may result from the reconnection of heterogeneously distributed source zones within the catchment, leading to temporally variable enrichment patterns in solute mobilization. However, the term “chemodynamic” remains somewhat insufficiently defined throughout the manuscript and may therefore be interpreted differently in this context. In my view, chemodynamic behavior describe high concentration variability relative to discharge variability and thus indicate processes that are at least partly decoupled from discharge dynamics. If, in this section, the intention is primarily to describe an increase in solute concentrations coupled to increasing discharge during rewetting, the term “enrichment pattern” may already be more appropriate and precise. Lines 417–421: Since this section specifically discusses changes in redox conditions and associated mobilization processes, it may be more appropriate to replace the term “biochemical processes” with a more specific description related to redox-driven processes. For example, terms such as “redox-related processes” or “redox-controlled processes” may better reflect the mechanisms discussed here and provide a more precise description of the processes influencing metal mobility under reducing conditions.
Technical corrections:
Lines 94–96: The chemical notation throughout the manuscript should be checked carefully for consistency and formatting. In particular, the indices in chemical formulas such as NO3- and SO42- should be consistently formatted.
Line 115: Please adjust the font formatting of “Hisdal et al., 2024” to match the formatting style and font used consistently throughout the manuscript and reference list.
Line 542–543: The corresponding DOI could be added to this reference to improve completeness and accessibility of the cited source.
Citation: https://doi.org/10.5194/egusphere-2026-2218-RC2
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- 1
The manuscript investigates hydrological and biogeochemical drought responses across ten solutes in the intensively monitored Erlenbach catchment in Switzerland using high-frequency observations spanning pre-drought, drought, and recovery periods during the extreme 2018 European drought. The authors show that drought substantially altered stream chemistry and induced breakpoints in concentration–discharge (CQ) and concentration–groundwater depth relationships across multiple solutes. They interpret these shifts as evidence of hydrological disconnection between shallow soil water sources and streamflow during severe drought, with streamflow increasingly dominated by deeper groundwater sources.
One of the strengths is the dataset. High-frequency measurements across ten solutes covering pre-, during-, and post-drought conditions are rare and provide valuable process insight. The combined use of discharge, groundwater table depth, and multi solute CQ analysis is also a strong aspect of the study. In particular, the identification of consistent breakpoints across solutes and hydrologic variables is compelling and supports the interpretation of hydrological disconnection during drought. The manuscript is generally well written. Figures 3 and 4 are particularly effective in visualizing the breakpoint behavior across multiple solutes.
The conceptual novelty could be articulated more sharply in intro by identifying the relevant knowledge gap (beyond a cool dataset) and echo the intro in discussion. The strongest contribution of the paper is likely not the observation that drought alters water quality, which is already well established, but rather the identification of consistent hydrochemical breakpoints associated with hydrological disconnection and the complementary use of groundwater depth as an indicator of drought-state transitions. Breakpoint CQs are not commonly observed in low-frequency data (e.g., Kincaid et al., 2024). But they are very clearly showing up in the high frequency data. In other words, high frequency data enables the observation of patterns “missing” from low frequency data, and offer additional insights (hydrological connection / disconnection and related biogeochemical understanding). This aspect could be emphasized more clearly throughout the manuscript.
Much of the interpretation in discussion is already largely expected from established hydrologic export theory and classic concentration–discharge relationships. The finding that solutes increasingly reflect groundwater chemistry as shallow flowpaths disconnect during drought is intuitive and consistent with long-standing ideas regarding hydrological connectivity and source-area activation (Pinder & Jones, 1969; Sklash & Farvolden, 1979; Hooper et al., 1990). It would be useful to cite existing literature to provide context of this idea (e.g., The manuscript sometimes presents these patterns as more mechanistically novel than they are. Relatedly, the study relies heavily on qualitative interpretations of “hydrological versus biochemical controls,” but these controls are not rigorously separated quantitatively.
The discussion could use some streamlining. It is repetitive in various places, where similar explanations regarding groundwater dominance and shallow flowpath disconnection are reiterated across multiple solutes. The paper could likely be shortened substantially without losing its central message. In addition, while the distinction between hydrological and biochemical controls is a central framing of the paper, the evidence for biochemical mechanisms is often indirect and mostly speculative, particularly regarding redox processes and nutrient uptake dynamics. For example, interpretations regarding sulfate oxidation and nitrate uptake reduction are plausible but not directly supported by measurements of redox conditions, microbial activity, or process rates. I think it is important to b be explicit about that the interpretations are speculations / hypotheses.
Specific comments
Line 38ff: nice review of water chemistry under droughts literature
Line 43: “can alter …” can you state how can they alter, like what was detailed for the Mosley 2015 paper. it would be more informative this way.
Suggest changing “biochemical” to “biogeochemical” throughout the manuscript. They mean very different things
Line 83, before this paragraph, I expect a knowledge gap about water chemistry during drought, in addition to the data limitations, as one major goal of introduction is to introduce knowledge gap. What is the current status of our understanding about water quality during drought?
Line 85; Hypothesis, can you also state why that is the case in the hypothesis?
Section 4.1. Much of the hydrology and biogeochemistry literature has talked about the fact that during dry conditions, most stream water comes from groundwater (e.g., Carroll et al., 2018, 2019). For example, Stewart et al. (2024) and Kerins et al. (2024) quantified the flow fractions of stream water from surface, shallow, and deep waters using streamflow and chemistry data, and they found flow fraction of groundwater is essentially 100%.
Section 4.3. explanation of the CQ patterns. Alternative mechanisms. In addition to the possible reactions and processes mention, Could they reflect the depth distribution of solutes in groundwater. As groundwater table become lower and lower during droughts, groundwater from different depths contribute to stream water, such that they reflect gw chemistry at different depths.
Section 4.6 on transferability. This is good discussion. But I found myself longing for some broader discussion echoing the review on water chemistry under droughts in Introduction. Are the findings here similar to or different from what was reported in literature? How do they resemble or differ?
References:
Pinder, G. F., & Jones, J. F. (1969). Determination of the groundwater component of peak discharge from the chemistry of total runoff. Water Resources Research, 5, 438–445.
Sklash, M. G., & Farvolden, R. N. (1979). The role of groundwater in storm runoff. Journal of Hydrology, 43, 45–65.
Hooper, R. P., Christophersen, N., & Peters, N. E. (1990). Modelling streamwater chemistry as a mixture of soil water end-members. Journal of Hydrology, 116, 321–343.
https://doi.org/10.1016/0022-1694(90)90131-G
Carroll, R. W. H., Bearup, L. A., Brown, W., Dong, W., Bill, M., & Willliams, K. H. (2018). Factors controlling seasonal groundwater and solute flux from snow‐dominated basins. Hydrological Processes, 32(14), 2187–2202. https://doi.org/10.1002/hyp.13151
Carroll, R. W. H., Deems, J. S., Niswonger, R., Schumer, R., & Williams, K. H. (2019). The importance of interflow to groundwater recharge in a snowmelt‐dominated headwater basin. Geophysical Research Letters, 46(11), 5899–5908. https://doi.org/10.1029/2019gl082447
Stewart et al., 2024. Illuminating the “Invisible”: Substantial Deep Respiration and Lateral Export of Dissolved Carbon From Beneath Soil. Water Resources Research 60 (6), e2023WR035940
Kerins et al., 2024. Hydrology outweighs temperature in driving production and export of dissolved carbon in a snowy mountain catchment. Water Resources Research 60 (7), e2023WR03607