Aggregation in environmental systems &ndash; Part 2: Catchment mean transit times and young water fractions under hydrologic nonstationarity

Kirchner, J. W.

doi:https://doi.org/10.5194/hess-20-299-2016

Articles | Volume 20, issue 1

https://doi.org/10.5194/hess-20-299-2016

© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/hess-20-299-2016

© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 20, issue 1

Research article

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19 Jan 2016

Research article | Highlight paper |

| 19 Jan 2016

Aggregation in environmental systems – Part 2: Catchment mean transit times and young water fractions under hydrologic nonstationarity

J. W. Kirchner

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Final revised paper (published on 19 Jan 2016)
Preprint (discussion started on 18 Mar 2015)

Interactive discussion

Status: closed

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

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RC C1458: 'review', Anonymous Referee #1, 13 May 2015
- AC C3400: 'Author's response to anonymous referee #1', James Kirchner, 31 Aug 2015
RC C2277: 'Review: Aggregation in environmental systems 2: catchment mean transit times and young water fractions under hydrologic nonstationarity by J. W. Kirchner', Markus Weiler, 28 Jun 2015
- AC C3401: 'Author's response to referee Markus Weiler', James Kirchner, 31 Aug 2015
EC C3474: 'Decision', Thom Bogaard, 05 Sep 2015

Peer-review completion

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

ED: Reconsider after major revisions (05 Sep 2015) by Thom Bogaard

Dear author,

you received two in-depth and detailed reviews for which I thank the reviewers. The paper is commented as a significant contribution to catchment hydrology and I agree. The paper logically follows on the companion paper (part 1). Compared to the first paper, this one is more difficult to follow due to its large amount of results presented and discussed.

The paper contains very interesting results (reviewers do not dispute any of the findings being worth discussing, although have some suggestions for extra explanation), however, I do think this paper is quite 'full'. So the focus of the paper becomes less clear and discussions more widespread. This can be solved in different ways: selecting the most interesting results for the paper, moving some minor findings to an appendix or splitting the paper in two (adding a third paper). I prefer the first option. This does not shorten the length of the paper but it does help the reader to stay focused. So I agree with the suggestions of Markus Weiler, especially moving sections 2.1-2.2, 3.3, shortening 3.6 and the summary and conclusion section. I agree with the author the extensive figure captions are useful and a good communication strategy. It is the final choice of the author to split in two papers or shorten the main text by moving some material to an appendix.

It is not necessary for this paper to add an analysis on evaporation influence nor more benchmarking analysis or test the approach on real-world data, I agree these both can (and most likely will) be done in follow-up analysis and discussions.

[As a sidestep, I agree with having the sensitivity analysis on the age threshold of Fyw in paper 1]

The paper(s) will undergo a second round of review.

I look forward receiving your resubmission in due time.

Kind regards,
Thom Bogaard

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AR by James Kirchner on behalf of the Authors (26 Oct 2015) Author's response Manuscript

ED: Referee Nomination & Report Request started (27 Oct 2015) by Thom Bogaard

RR by Anonymous Referee #1 (20 Nov 2015)

Suggestions for revision or reasons for rejection

REVIEW of “hess-2015-63-manuscript-version2” by J.W. Kirchner
GENERAL COMMENTS
I have carefully read the revised version of the article (and the companion paper 1, as some material has been moved therein), together with the response of the author to my comments. I see that the author put great effort in improving the paper(s). All the parts that were insufficiently clear in the previous version have now been improved. In particular, I believe that the new sensitivity analysis included in Paper 1 section 4.2 is very important and beneficial to both the manuscripts.
Therefore, I consider the manuscript suitable for publications as is. I just include a few additional comments below, which the author may take as a personal feedback.
• As I said, I consider this approach very smart because it allows thorough analyses of catchment behavior without heavy data requirements. So I like this “countertrend” that the method is innovative not because of technological or data innovation, but rather because of the deeper comprehension of a process.
• The use of young water fractions Fyw is very intuitive, which is another strength of the approach. But the concept of Fyw, which is well defined in a stationary system, gets more abstract in real systems, where the TTD continuously change with time (I still have a hard time trying to express in words a clear definition of Fyw and τyw for a general system). Indeed, the author needs to assume the young water threshold for the boxes in the thought experiment of Paper 2. The sensitivity analysis helps showing that the indefiniteness of τyw and the following uncertainty in Fyw is overall quite limited (especially if compared to typical transport time-scales). Still, I think that it is at least unusual to have a good estimator of something which is not easy to define.
• As I already wrote in my previous review, I think the results and methodologies introduced by the author are extremely interesting and I foresee they will easily spread within the hydrologic community (and beyond?).

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ED: Publish as is (04 Dec 2015) by Thom Bogaard

AR by James Kirchner on behalf of the Authors (04 Dec 2015)

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Short summary

Here I show that seasonal tracer cycles yield strongly biased estimates of mean transit times in nonstationary catchments (and, by implication, in real-world catchments). However, they can be used to reliably estimate the fraction of "young" water in streamflow, meaning water that fell as precipitation less than roughly 2–3 months ago. This young water fraction varies systematically between high and low flows and may help in characterizing controls on stream chemistry.