the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Changes in flowing drainage network and stream chemistry during rainfall events for two pre-Alpine catchments
Abstract. Many headwater catchments embed non-perennial streams that flow only during wet conditions or in response to rainfall events. The onset and cessation of flow results in a dynamic stream network that periodically expands and contracts. The onset of flow can flush sediment and nutrients from previously dry streambeds and enhance carbon processing rates. The expansion of the flowing drainage network also increases hydrologic connectivity between hillslopes and streams because it decreases travel distances to the stream. However, datasets on flowing drainage network dynamics during rainfall events and short-term changes in stream chemistry are rare. This limits our interpretation of hydrological processes and changes in stream chemistry during events.
Here, we present joint hourly measurements of solute concentrations and stable isotopes from precipitation and streamflow at the outlets of two 5-ha catchments in the Swiss pre-Alps during seven rainfall-runoff events in the snow-free season of 2021. Relevant samples were also collected from soil- and groundwater across the catchments before and after rainfall events. In addition, 10-min frequency information was collected on the flowing drainage network length. We used these synoptic measurements to infer the dominant runoff-generating mechanisms for the two experimental catchments.
Despite their proximity and similar size, soil and bedrock features, the flowing drainage network dynamics proved very different for the two catchments. In the flatter catchment (average slope: 15°), the stream network was more dynamic and expanded rapidly, up to 10-fold, while in the steeper catchment (average slope: 24°), it remained relatively stable (only a 2-fold change). The event water contributions were also higher for the flatter catchment. The dilution of calcium at the time of the rapid expansion of the network and increase in discharge suggested that the contribution of rainfall falling directly on the stream channels is important, especially for the smaller events during dry conditions. In wet conditions, unchanneled areas must have contributed event water as well. In the flatter catchment endowed with the more dynamic stream network, a “first flush” of nitrate was detectable, possibly attributed to the transport of material from previously dry stream segments. In the catchment characterized by a more stable flowing drainage network, such flush was not observed and nitrate concentrations decreased, suggesting enhanced contributions from riparian groundwater with reducing conditions during rainfall events. Our experimental study not only highlights the large differences observable in stream network dynamics and stream chemical responses for neighboring, nearly equal-size catchments but also shows the value of fine-scale observations on both the channel network dynamics and stream chemistry to fully understand runoff generation mechanisms.
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RC1: 'Comment on hess-2024-67', Anonymous Referee #1, 03 May 2024
General comments
This manuscript presents a very interesting study, linking dynamics of expansion and contraction of intermittent stream networks in a pre-alpine setting to resulting streamflow chemistry and streamflow generation mechanisms during several rainfall events. The hydrochemistry and hydrometric datasets are impressive in their extent and temporal resolution.
Datasets like this are rare, and they allow addressing the relevance of hydrologic connectivity (either between landscape elements and streams or along the stream) for the patterns and dynamics of the export of solutes. This is a very important question where we still have a lack of understanding.
In general, I think, this study is a valuable contribution to this field of research. It is well written and describes clearly the research objectives, the study sites and experiments, and the results. I only have a few specific comments. What I think is problematic is that results and discussion are presented together and not separately. There is this constant mixing of description of results and explanations and interpretations, and the text keeps jumping between the different aspects of the study. In my opinion, the focus on the answers to the (important!) research questions posed at the end of the introduction gets lost by this setup. I would strongly recommend splitting results and discussion. After describing the results in a concise way with the very informative figures the authors used, they can come back to the research questions in the discussion and answer them. At the moment, the reader has to look for the answers to the research questions in the various parts of the results and discussion sections. I think that the authors could elaborate more on their second research question, i.e., the comparison between the two topographically different catchments. Is the main impact of topography the influence on the flow network and the “state of connectedness”? Also, the link of the research findings particularly to intermittent streams is missing in the interpretation. At the end of the discussion, the conceptual model of the flowing drainage network and solute export could be presented as a summary of the new understanding that was gained, instead of mentioning it in the conclusions only. By presenting results and discussion separately, the main findings of this very interesting study and the interpretations and implications could be presented in a more focused and clearer way.
Specific comments
- L 147: I believe you mean the METER Group company
- L 176: “trickling” instead of ”tricking”
- L 228: Start new sentence after E2
- Section 2.5.2: If I understand correctly, the method of creating maps of flowing drainage networks and the indices does not allow a statement on connectedness or connectivity. If 50% of the stream reaches were classified as flowing, this could still mean a quite fragmented state (e.g. if every second stream reach was flowing). Maybe the authors could comment on this.
- Caption Fig. 4: I think there was a mix-up of how the panels a-f were described in the caption. Please check. E.g. “time-series of hydrologic variables…” are panels b and e and not b and c.
- L 473: redundant “from areas”
- L 689: Fig. 6! Not Fig. 8.
Citation: https://doi.org/10.5194/hess-2024-67-RC1 -
RC2: 'Comment on hess-2024-67', Anonymous Referee #2, 15 Jun 2024
In this manuscript the authors explore the relation between stream network extent and outlet stream chemistry during precipitation events. I really enjoyed reading the introduction, it serves as a nice overview of existing work and the importance of understanding how intermittency impacts chemistry, and highlights that it is relatively understudied. The posed research questions are important, and well stated. There has been extensive work on event water chemistry, but limited study of chemistry in intermittent streams, which makes this dataset unique.
I have one major comment regarding research questions in the results and discussion section. I appreciate that the sections are merged as there is lots of quantitative geochemistry in this paper and it is helpful to have discussion immediately following results to provide context for conclusions. However, the answers to the research questions are lost in the latter portion of the manuscript. Some reorganization of the results/discussion to clearly highlight the answers to the research questions would improve the manuscript, specifically in sections 3.2 and 3.3. One suggestion is to group analysis by solute type (i.e. weathering derived, nutrient, metal), since they seem to behave similarly, rather than jumping from one solute type to the next in each section. Another suggestion is to increase discussion of the conceptual figure, as detailed below.
The conceptual figure is an effective summary of the paper, but 1) I am confused by some of the flowpaths and colors, and 2) suggest including more text interpretation of the figure to tie it back to the research questions. Specifically, do the letters in the block diagrams correspond to color at all? The purple color is labeled groundwater flow, so I am not quite sure what the dashed purple ellipses are around the stream. Do the red stream lines represent a dry channel? Why then is panel a, spot A called streamflow if it is red, but spot E around a red stream in panel c is no flow? What does the light blue color in the subsurface of panel D represent? Similarly, why are some lines dashed and others soild? A detailed legend figure would help interpretation of this figure. It would also strengthen the paper to add a summary section discussing this conceptual figure in the context of the research questions. Much of the important conclusions are found at the end of paragraphs, and so discussion of them together would help.
Minor Comments:
60-61 – Please provide citation for this statement.
66 – Correct to “Warix et al. (2023) used CFC-12 & 3H”
209 – Are you reporting solute totals? Please specify.
309-323 – I suggest adding some mineral saturation indices to support the conclusions in this paragraph. The hypothesis that baseflow is saturated with respect to secondary calcium products seems reasonable but speculative and could be made more concrete by calculating the saturation index of calcite for groundwater samples.
314 – Do you have pH observations to support Lan being more acidic than Cha?
473 – “from areas” is repeated
469- 477 - I had to read these lines several times to understand the proposed flow mechanisms. Can some of these sentences be merged or shortened for conciseness?
510 – This idea could use some clarification. Where is the decrease in concentrations happening? If GW solute concentrations are higher, and the groundwater table elevation rises how does that lead to a decrease in concentration? I assume mixing with dilute soil waters, but this should be made explicit.
Figure 3 – The y-limits should be adjusted to the max observed data. Some plots (e.g. Fe) have lots of white space.Citation: https://doi.org/10.5194/hess-2024-67-RC2
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