|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.
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.
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.