Spatio-temporal controls of C–N–P  dynamics across headwater catchments of a temperate agricultural region from public data analysis

Guillemot, Stella; Fovet, Ophelie; Gascuel-Odoux, Chantal; Gruau, Gérard; Casquin, Antoine; Curie, Florence; Minaudo, Camille; Strohmenger, Laurent; Moatar, Florentina

doi:https://doi.org/10.5194/hess-25-2491-2021

Articles | Volume 25, issue 5

https://doi.org/10.5194/hess-25-2491-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/hess-25-2491-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 25, issue 5

Research article

|

18 May 2021

Research article |

| 18 May 2021

Spatio-temporal controls of C–N–P dynamics across headwater catchments of a temperate agricultural region from public data analysis

Stella Guillemot, Ophelie Fovet, Chantal Gascuel-Odoux, Gérard Gruau, Antoine Casquin, Florence Curie, Camille Minaudo, Laurent Strohmenger, and Florentina Moatar

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Final revised paper (published on 18 May 2021)
Supplement to the final revised paper
Preprint (discussion started on 22 Jun 2020)
Supplement to the preprint

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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- Supplement

RC1: 'Referee comments on Guillemot et al.', Anonymous Referee #1, 18 Aug 2020
- AC1: 'Reply to Anonymous Referee #1', Ophelie Fovet, 21 Oct 2020
RC2: 'referee comments on hess-2020-257', Anonymous Referee #2, 12 Sep 2020
- AC2: 'Reply to Anonymous Referee #2', Ophelie Fovet, 21 Oct 2020

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Publish subject to revisions (further review by editor and referees) (30 Oct 2020) by Genevieve Ali

AR by Ophelie Fovet on behalf of the Authors (16 Nov 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (25 Nov 2020) by Genevieve Ali

RR by Anonymous Referee #1 (02 Dec 2020)

Suggestions for revision or reasons for rejection

General comment
The authors did a good job in presenting a much more balanced manuscript that now better discusses the processes that create the observed spatial and seasonal variability of nutrient concentrations. The issues I raised in the first review on the use of the PCA, and the GAM was only partially resolved. I am not arguing against these methods but suggest to better explain why they are used, what are underlying assumptions and what are implications for the interpretation.
More details can be found below.
Specific comments
L55: Homogenization of formerly (naturally) heterogenerous catchments is argued to be the main reason for nitrate chemostatic behavior and thus purely transport-limited nitrate exports. This does not fit to your description of homogeneity as a characteristic of natural catchments.
L66 and L87: You boil down seasonality to a hydrological control (controlling other biogeochemical processes). What about light and temperature. In-stream ecosystems but also shallow near stream systems may be also be seasonally controlled by that.
L103: This sentence does not make sense for me.
L181ff: For me this is still not resolved and need to be explicitly mentioned here. By GAM you cannot analyze catchments without seasonality. Your SI metric thus does not cover the whole range of analysed catchments. If you want it like that, its ok. But you need to mention it.
Table 1: You added the information on the Topo_i. I just don't see the point why you don't use the established name "topographic wetness index" here. Moreover it is not clear how you derive one value for the entire catchment. Usually the Beven-Kirkby approach derives this for each grid cell of the DEM. Finally, the unit seems to be wrong: log[ha]? Note that Beven and Kirkby use the local slope in radian not %.
L205: Again I ask for a bit more words on the idea of the PCA. Tell the reader why you use this technique, what it can tell you and what are potential drawbacks that have to be kept in mind. For instance, PCA is a linear method and explained variance may be misleading when relations between the elements are non-linear (e.g. the power law that Taylor and Townsend propose for C:N). Your rank correlation acknowledge that but this, again is not mentioned. It would help the paper elaborating on this to not leave the impression you randomly chose you methods which surely is not the case.
L233: I criticized the use of PCA in the first review. Now you moved it to the Supplement which does not solve the issue but rather complicates it. If you want to use it, it's ok but needs to be justified (see above comment) and then properly used in the main manuscript.
L237: This link to S4b is for C50 only - should be mentioned here. Wouldn't it make sense and be easier to have a correlation matrix for all concentration metrics across all constituents in the SI?
L242: Here you use SI the first time in the results. At this point it would be good to mention that this does not incorporate all catchments and give the number of cases?
L295f: If the classification of "in-phase" and "out-of-phase" refers to van Meter et al. 2019, it would be fair to mention it.
L414: The meaning of a synchronization of a contribution with maximum flow is not fully clear to me.
L415: Redox-processes may be mentioned explicitly as this seems solely refer to assimilation.
L422: I know what you mean but expression is not fully transporting that. Maybe: "decrease NO3 concentration more in shallower and younger groundwater than in deeper, older groundwater"?
L495ff: Without explicitly mentioning it the entire monitoring and management implications are boiled down to the discussion of C:N ratios which haven't been presented in the results at all. and actually no implications are mentioned in this section. I suggest to broaden the view here a bit and share with the reader how your result may influence water quality management and monitoring design.

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RR by Anonymous Referee #2 (11 Dec 2020)

Suggestions for revision or reasons for rejection

The authors have done a thorough job of addressing reviewer comments and revising this paper.

I appreciate the clarification provided on statistical methods and the potential challenges to implementing an expanded GAM model. As the authors suggest in their responses, the approach to statistical analysis being altered is unlikely to change the over conclusions of the descriptive study. Suggestions of an expanded GAM model based on reading the first draft of the manuscript were mainly out of interest as to whether an alternative statistical approach would be complementary in simplifying or integrating presentation of some of the results and exploring interactions between geographic variables, hydrology, and concentration. As currently presented the results clearly highlight seasonality very well. Also, where discharge and concentrations follow similar seasonality it is visually evident through the GAM results. However, I do wonder if there might still be some room to explore whether the geographic variables influencing seasonality of discharge are the same as those for concentrations. This could just be a simple addition of correlations between discharge seasonality metrics and geographic variables to Table 3. That would allow for a test of whether the controls are similar and may be useful in interpretation of results / identifying catchment types. The seasonal trends of discharge and connection between flow path and concentrations are visually evident with other results, with connections to base-flow or flashiness indicators (Figure 6), and in Figure 7 (a nice addition). This suggestion of exploring the correlations for discharge seasonality further if mainly to help further highlight geographic vs. weather vs. in-stream or point source processes as controls on the observed concentration patterns.

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ED: Publish subject to minor revisions (review by editor) (18 Dec 2020) by Genevieve Ali

AR by Ophelie Fovet on behalf of the Authors (08 Mar 2021) Author's response Manuscript

ED: Publish subject to minor revisions (review by editor) (10 Mar 2021) by Genevieve Ali

AR by Ophelie Fovet on behalf of the Authors (14 Mar 2021) Author's response Author's tracked changes Manuscript

ED: Publish subject to technical corrections (23 Mar 2021) by Genevieve Ali

AR by Ophelie Fovet on behalf of the Authors (25 Mar 2021) Manuscript

Short summary

This study investigates the drivers of spatial variations in stream water quality in poorly studied headwater catchments and includes multiple elements involved in major water quality issues, such as eutrophication. We used a regional public dataset of monthly stream water concentrations monitored for 10 years over 185 agricultural catchments. We found a spatial and seasonal opposition between carbon and nitrogen concentrations, while phosphorus concentrations showed another spatial pattern.