| 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.
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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