Mean transit times in headwater catchments: insights from the Otway Ranges, Australia

Howcroft, William; Cartwright, Ian; Morgenstern, Uwe

doi:https://doi.org/10.5194/hess-22-635-2018

Articles | Volume 22, issue 1

https://doi.org/10.5194/hess-22-635-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/hess-22-635-2018

© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 22, issue 1

Research article

|

25 Jan 2018

Research article |

| 25 Jan 2018

Mean transit times in headwater catchments: insights from the Otway Ranges, Australia

William Howcroft, Ian Cartwright, and Uwe Morgenstern

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Final revised paper (published on 25 Jan 2018)
Supplement to the final revised paper
Preprint (discussion started on 09 May 2017)
Supplement to the preprint

Interactive discussion

Status: closed

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

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RC1: 'Referee Comment', Anonymous Referee #1, 04 Aug 2017
- AC2: 'Response to Reviewers', William Howcroft, 20 Sep 2017
RC2: 'Review on “Mean Transit Times in Headwater Catchments: Insights from the Otway Ranges, Australia”', Anonymous Referee #2, 30 Aug 2017
- AC1: 'Response to Reviewers', William Howcroft, 20 Sep 2017

Peer-review completion

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

ED: Publish subject to revisions (further review by Editor and Referees) (20 Sep 2017) by Markus Hrachowitz

Dear authors,

thank you for your replies to the comments of the two reviewers. As you have seen, both reviewers do in general appreciate your manuscript, but they also highlight several points that need some more work. I would be glad if you addressed all reviewer comments in detail and incorporated the required changes in a revised manuscript. Both reviewers raised several questions. Providing clarifications/answers to these questions in the revised manuscript will tremendously help the reader to fully appreciate your work.

From my perspective, I would be glad if you could give special attention to the following points:

(1) in the objectives section it would be good to provide an explicit science question together with a research hypothesis. As it stands now, the objectives remain somewhat vague and unspecific.

(2) expand the discussion of your results to tie them more into the context of existing work, to better highlight the local and general relevance and to provide a more detailed overview of the limitations of this study.

(3) as noted by the reviewers, several parts of the results/discussion sections should actually go either into the study site section (catchment attributes - no matter if they were only derived for this study, only the results/discussion derived from these attributes need to go into the results/discussion sections) or into to methods section (e.g. analytical uncertainty).

(4) Much is, rightfully, made of aggregation errors. However, I would urgently encourage you to reflect and eventually re-think the concept of "true" MTTs. Of course, catchments do have true MTTs. However, in the presence of aggregation errors we can by no means meaningfully establish what this true MTT is. I understand your intention, but even by assessing the MTTS of smaller sub-catchments of a given catchment, these are not true MTTs, as smaller catchments are very likely also characterized by heterogeneity. What your approach does, it helps to quantify some incremental aggregation error between catchments at different scales, and which may (or may not) give some idea into which direction the true MTT may converge. But it will never provide you a actual true MTT.

I am looking forward to receiving your revised manuscript!
Best regards, Markus Hrachowitz

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AR by Ian Cartwright on behalf of the Authors (13 Nov 2017) Author's response Manuscript

ED: Referee Nomination & Report Request started (13 Nov 2017) by Markus Hrachowitz

RR by Anonymous Referee #2 (13 Dec 2017)

Suggestions for revision or reasons for rejection

Review on “Mean Transit Times in Headwater Catchments: Insights from the Otway Ranges, Australia”
The paper estimates mean transit times in 6 headwater catchments in southeast Australia using two methods with varying parameters and radioactive 3H tracers. The study assesses as well the possible aggregation effect on the estimated Mean Transit Times (MTT). The authors have written a very interesting story and provide with an overview that stable isotope tracers cannot provide. It was a pleasure to read it, the structure and the figures were improved; and I would only have a few minor comments and one concern.

General comments:

1. Throughout the manuscript there is an emphasis on using 2.4-3.2 as the TU for precipitation. However, in Tadros et al. (2014) these values are given as a rather coarse annual average activity values for the precipitation, which is very different to the intra-annual range of precipitation tritium activity. For example, in Tadros et al. (2014) they show the monthly variability in Alice Springs and in Brisbane, where the activity ranges were 7.5-20.6 TU and 3.6-8.2 TU respectively averaging all data collected for these two sites which include older data as well. The important point here is to emphasize that the high values were during autumn-winter in September, whereas the low activities were observed during the summer season (January to April). The high values were more than 2 times larger than the low activities. The 2.4-3.2 TU values are a good reference to have, but it is acceptable for this to be off the real values, considering the large area this map was covering, having to compromise in resolution. Summing up, the values 2.4-2.8 TU and 2.8-3.2 TU are not the annual range rather than just the annual average, missing the full range of the sampling. Additionally, observing the map with the precipitation 3H activities would also show that the values for Alice Springs and Brisbane are not covering the whole range they showed in the monthly values.

2. In this study the precipitation sampled was taken for 78 days during the period of the year of highest activity in the rain and it was 2.45 TU, leading to assume that the precipitation preceding the sampling of March 2015 could be ranging in activities close to half the sampled 2.45 TU, as discussed in the previous point. Since this study attempts to estimate the MTT at different times of the year, it is crucial to consider the correct end members, it is understood that the authors do not have such data but it is clear to me that the precipitation end-member used for March 2015 will not be the same as the precipitation for the other samplings. Even if the samplings are done during recession, the previous precipitations could have an influence that was not considered in this study because of the assumption of having such a high value on the precipitation activity. I do not believe this would change the overall conclusion of the study, but yes a significant change for the March 2015 sampling.

Specific comments:

1. Page 1 Line 8: it should say “rainfall average”, as it is not the range.

2. P6 L129: I think you meant to say “Identifying”.

3. P10 L228: It called my attention the second p-value, is that correct? 10^(-195)?

4. P10 L236: When in September was it sampled? September is the month with highest tritium activity, if the sampling included this month can give an idea on how high on the spectrum is this sample.

5. P15 L354: “The lower than expected 3H… …representing rainfall of only part of the year”. From the discussion above this is not a good reason, since the sampling was taken during the part of the year were precipitation activity is highest. This just tells that your range is lower than what you are assuming.

6. P17 L406-407: “Additionally the 3H activities plateau at ~2.0 TU, which is significantly lower than those of modern rainfall (Fig. 4)”. Again, I disagree with how this is written, it is correct in some way, but not how it is written because the modern rainfall is not what you show in Fig. 4. I do believe that the activities in the streams always plateau at lower values than the precipitation; however, you have to treat each time of the year as separate cases. March 2015 3H activities plateau at X TU, which is lower than the perhaps ~1 TU on precipitation at this time of the year, whereas the samplings taken during autumn plateau at ~2.0 TU, which is lower than the modern precipitation at this time of the year. Though somehow, Gellibrand River at JA and both Lardners have a larger than 1 TU at the March sampling, maybe something that differentiates them from the other catchments?

7. P19 L474: I think you meant Eq. (2).

8. P20 L485: Again Eq. (2).

9. P21 L520: “…estimate likely controls the MTTs.” I think it should be control instead of controls.

10. P22 Section 5.5: This paragraph shows results rather than analysis. Placed where it is it looks more like a "fun fact", since it not used later and it does not add to the overall conclusion of the paper.

11. P22 L559: “…between catchments in important…” I think you meant to say is.

12. Fig 4: Text ‘in Figure’ says “Expected range of 3H…”, change to expected annual average, since it is not the range.

13. Fig 7 was called in the manuscript after figures 8, 9 and 10. Change the order or call it earlier.

14. P-values were stated throughout the text but were missing in all figures, I would like to see the value in the figure as well, rather than having to look for it in the text when I am studying a figure.

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ED: Publish subject to minor revisions (review by editor) (13 Dec 2017) by Markus Hrachowitz

AR by Ian Cartwright on behalf of the Authors (19 Dec 2017) Author's response Manuscript

ED: Publish as is (19 Dec 2017) by Markus Hrachowitz

AR by Ian Cartwright on behalf of the Authors (20 Dec 2017)

Short summary

Documenting mean transit times is critical for understanding and managing catchments. Mean transit times in six headwater catchments of the Otway Ranges, Australia, determined using tritium, range from 7 to 230 years. Tritium activities correlate well with streamflow but are difficult to predict from catchment attributes or major ion geochemistry. The long mean transit times suggest that the catchments are buffered from short-term rainfall variations.