Continuous, near-real-time observations of water stable isotope ratios during rainfall and throughfall events

Herbstritt, Barbara; Gralher, Benjamin; Weiler, Markus

doi:https://doi.org/10.5194/hess-23-3007-2019

Articles | Volume 23, issue 7

https://doi.org/10.5194/hess-23-3007-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/hess-23-3007-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 23, issue 7

Research article

|

17 Jul 2019

Research article |

| 17 Jul 2019

Continuous, near-real-time observations of water stable isotope ratios during rainfall and throughfall events

Barbara Herbstritt, Benjamin Gralher, and Markus Weiler

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Final revised paper (published on 17 Jul 2019)
Preprint (discussion started on 26 Jun 2018)

Interactive discussion

Status: closed

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

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RC1: 'Review of the manuscript “Real-time observations of stable isotope dynamics during rainfall and throughfall events“ by Herbstritt et al. submitted to the Hydrology and Earth System Sciences', Anonymous Referee #1, 19 Jul 2018
- AC1: 'Reply to reviewer comment RC1', Barbara Herbstritt, 21 Aug 2018
RC2: 'Review of Herbstritt et al.', Anonymous Referee #2, 20 Jul 2018
- AC2: 'Reply to reviewer comment RC2', Barbara Herbstritt, 21 Aug 2018
RC3: 'Review of the manuscript “Real-time observations of stable isotope dynamics during rainfall and throughfall events“ by Herbstritt et al.', Anonymous Referee #3, 26 Jul 2018
- AC3: 'Reply to reviewer comment RC3', Barbara Herbstritt, 21 Aug 2018

Peer-review completion

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

ED: Reconsider after major revisions (further review by editor and referees) (14 Sep 2018) by Erwin Zehe

AR by Barbara Herbstritt on behalf of the Authors (26 Oct 2018) Author's response Manuscript

ED: Referee Nomination & Report Request started (02 Nov 2018) by Erwin Zehe

RR by Anonymous Referee #1 (21 Nov 2018)

Suggestions for revision or reasons for rejection

Re-review of the manuscript “Real-time observations of water stable isotope dynamics during rainfall and throughfall events“ by Herbstritt et al. submitted to Hydrology and Earth System Sciences

I acknowledge that the authors have improved the manuscript, which now reads better than the previous version. I furthermore think that this is a valuable proof of concept of a novel method that finally could merit publication. Generally, I have the impression that the authors try to oversell their results with the help of claims that are not true. This devaluates the paper unnecessarily. I also recognize that several of my previous suggestions were not followed, which is of course always the right of the authors if they have convincing arguments to do so. Unfortunately, I am not convinced by their arguments concerning the following critics I have raised previously:

1. The title is misleading, because the presented method is not real time but has a delay of four minutes. Therefore, I repeat my suggestion to change the title to “Temporally highly resolved measurement of stable hydrogen and oxygen isotope ratios in water of rainfall and throughfall with a novel infrared laser-based method”.
2. The issue of the spatial representativeness of throughfall measurements, which frequently require a larger number of samplers, particularly if a high temporal resolution is envisaged, is not mentioned at all. The first conclusion stating that the proposed method is suitable for continuously observing the stable water isotope dynamics in precipitation and throughfall is not true, because the method in the current setting can only measure at a single, likely non-representative point, while all hydrological models require an area-representative measure. It is disappointing that the authors were not ready to simply discuss this restriction, which is another attempt to oversell their results.
3. The authors show in Fig. 6 that the “deuterium excess” value changes with time in both rainfall and throughfall. This finding is just mentioned but not discussed. What do we learn from this change? You should at least present a speculation or not show the result. The cited reference of Gat (1996) suggests e.g., that a change of the deuterium excess is related with different water sources or with a changing contribution of re-evaporated water (from isotopically distinct water pools). Which role do (bio)chemical reactions play? Mass-dependent kinetic fractionation of water during phase transition – which is generally non-equilibrium! - will always change the isotope ratios of both, H and O in a related way. To explain a changing deuterium excess, you need a specific deuterium (or 1H) source (free of 18O), e.g., from organic and mineral acids.

Moreover, I still think that the “double delta” notation used by the authors to express differences between two delta values is highly unusual and that Figs. 2 and 6 should be combined.

Further to these major concerns, I again offer a number of more minor suggestions to improve the manuscript.
p. 1, l. 3: water isotopic composition
p. 1, l. 15: Delete “and thereby,”.
p. 1, l. 18: This comparison might be possible with respect to the temporal resolution but not to the spatial representativeness.
p. 1, l. 23: Define for what the tracer is ideal. Usually, the stable isotope ratios of H and O are far from ideal, because differences among various sources are frequently (too) small and the signal is prone to changes as a consequence of kinetic isotope fractionation or mixing and thus not stable.
p. 1, l. 25-26: This is mostly true for the long-term mean isotopic signature of rainfall water (>1 yr) but hardly if short time periods are considered as is done in this study.
p. 2, l. 3: “amount” was changed to “depth” but not consistently. Please also replace the remaining “amount”.
p. 2, l. 11-13: Who does this? Seems trivial.
p. 2, l. 7-8: I did not understand this sentence.
p. 2, l. 9-11: What is the volume-weighted mean of the relative interception loss? Usually, the cumulative interception loss in percent of rainfall during a defined period is given.
p. 2, l. 17-19: The patterns of throughfall are indeed heterogeneous but I didn’t understand what you mean by “when related to leaf area index as well as to spatial variability in general”.
p. 2, l. 22-24: The method of roving throughfall collectors is used to improve the representativeness of the throughfall measurement with a limited number of samplers. How could it help to measure the size of spatial variability? The latter is usually reached with the help of a sufficiently high number of samplers, which are not roved.
p. 3, l. 17: Replace two times “amount”.
p. 4, l. 19: Unclear. Did you collect volume-proportional subsamples, which you combined to a bulk sample? I know "volume-weighted" mainly in the context of solute concentrations. Perhaps volume-proportional is the better term at all places, where you use “volume-weighted”?
p. 4, l. 26: leaves
p. 4, l. 28: 15 m is a large distance in a forest. There are possibly several other trees in between.
p. 6, l. 6-7: This can be seen in the figure and should therefore usually not be repeated in the text.
p. 6, l. 15 (and throughout the manuscript): I suggest to consistently report results in the past tense.
p. 7, l. 18-p. 8, l. 2: Switch to past tense.
p. 8, l. 16-18: I did not understand this statement. What did you expect and why?
p. 8, l. 29: detectable
p. 9, l. 16: Who prefers “large and long surfaces”? I would prefer small and many separate surfaces. Large and long surfaces get quickly too dirty in forests so that their use is just not feasible in the practice.
p. 10, l. 6: Why should a mere flow have an isotope effect? In fact, it does not!
Fig. 2: Add the abbreviation “vapour_mov” after “moving average” in the legend. From where does the temperature originate? From the station at 15 or at 250 m from the tree?
Fig. 3: I already recommended not to show regression lines if there is no significant correlation, because they are misleading. You should explain all abbreviations used in the figure in the legend.
Fig. 6: The term “vapour sampling” is likely misleading, because I assume that you converted the vapour measurement to liquid values. Otherwise the measurements of the discrete liquid samples would likely not match so well. Where were temperature and vapour pressure deficit measured (15 vs. 250 m away from the tree)? Anyway, I think that this figure is not needed.

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RR by Anonymous Referee #3 (04 Dec 2018)

RR by Anonymous Referee #2 (05 Dec 2018)

Suggestions for revision or reasons for rejection

General comments:
I thank the authors for their efforts to improve the manuscript following the suggestions of the reviewers.
The authors satisfactorily answered many of my specific comments and made the corresponding changes in the manuscript. Yet, some answers to my general comments have been omitted and a few of my specific comments remained unanswered, which I thought were among the most interesting aspects to be discussed.
Moreover I think that the manuscript would still benefit from a clearer presentation of the ideas currently presented in the introduction and in the discussion. More context about the hydrological applications of stable isotopes (e.g. hydrograph separation, end-member mixing analysis, and transit time calculations) would improve the introduction and make the manuscript and its importance more compelling. The discussion also still needs a clearer explanation to why the presented advances in isotope measurement techniques are important for such applications and/or for stimulating the creation of new applications currently challenged by limited isotopic data. Right now the manuscript stops right before making the step. The readers need to be completely convinced that it is not enough to simply take bulk samples in TF and in Pg.
Therefore, I recommend an additional round of revisions. I believe this work will be very useful, yet it contains only little interpretation of the data in terms of interception processes and the added value of the approach is not convincingly presented. Everything is there, but the authors cannot leave this to the reader.

Specific comments:
Page 1
L6: Please add “with the water vapor” after “diffusive exchange” to make it more specific.
L15: Please add “on the stable isotopes of water” after “the temporal effects of interception processes” to make it more specific.
L23: One could add “from the use of stable isotopes of water” after “events” to make it clearer that the advances presented in the approach are about isotope measurement techniques.
Introduction: Until now, it is still difficult to follow the argumentation in the introduction. I think all ideas are there, but they are not well organized. For me, the introduction should be organized along these key points (just a suggestion):
• The role of stable isotopes as the input for many hydrological applications (with more references to these specific applications) and as a tool to better understand interception processes (refs)
• The often neglected differences between Pg and TF amounts and isotopic composition (refs).
• The consequences of this neglected difference for the mentioned applications in isotope hydrology (refs + personal interpretation).
• The possible interception mechanisms causing these differences and the current research need to better understand these mechanisms (refs).
• Despite recent progress in measurement techniques (refs), the current lack of high-resolution isotopes in both TF and Pg to correct the estimation of the true isotopic inputs and to shed light on interception mechanisms.
• The obvious goal of the paper.

L25: You changed “signatures” to “ratios” but my point was that nothing should follow “stable isotopes” to be exact. For example you would not say that chloride concentration is a tracer. Chloride itself is a tracer.
L26: adding “paths” after “flow” would make more sense.
Page 2
L4-6: This sentence is still not clear, and it is still too long. Don’t use so many commas. This is not the only long sentence that needs to be restructured.
L25-27: This sentence is not clear, please reformulate it. “spatial variability in general” was not clarified as requested, please change it.
Page 4
Line 15-16: That was not what I meant. I meant that the instrument setup could induce a moving average with time window up to 36 s long applied to the isotopes being pumped to the membrane modules. This smoothing happens before the 90 s moving average is applied to the isotope readings. This 36 s moving average is the result of the physical configuration of the setup because some water could accumulate in the smaller 3 mL funnel during rainfall intensities larger than 5 mL/min. Of course the 90 sec moving average discards the variability of the data happening during periods shorter than 90 sec. But from the measurement protocol, it seems that during rainfall events with large intensities (i.e. when Vd is full with water for some time), the isotope readings every 4 sec will actually correspond to a moving average of 36 sec. Please mention it in the discussion.
Page 6
L28: I know one reviewer asked to change “compared” to “correlated”, but it now sounds like there is a correlation between all the variables in the plot. Why not change it to “variables […] are plotted against each other to check for correlations between them”?
Page 8
Discussion: Why not use subsections in the discussion? That would make it clearer.
Page 9
L6: “mixing processes”: where?
L7: “indicates” is better than “indicative for”
L14: Why did this last another two days? Do you mean that the analysis was completed only two days after the samples were brought back to the lab?
L23-27: These two sentences are a repetition of what is written in the results section. Either rewrite, shorten and add interpretations, or remove. These sentences would actually fit much better in the conclusion which is missing these relevant results!
Page 12
Conclusion: A sentence or two about the key results of the study as from the measurements is missing.
L7: Please add “and stable isotope measurements” after “characteristics”. You looked at the stable isotopes in rainfall as well.
Figure 3: Please make the symbol associated with p<0.05 larger. Why is there a symbol for p<0.01? It is not used. Please add “of precipitation and throughfall” after “bulk samples” in the caption.
Figure 5: “deviations” is not consistent with previous occurrences of “differences” for this metric. I think you should use only “differences” throughout the manuscript. Please make sure this is done consistently.

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ED: Reconsider after major revisions (further review by editor and referees) (07 Dec 2018) by Erwin Zehe

AR by Barbara Herbstritt on behalf of the Authors (31 Mar 2019) Author's response Manuscript

ED: Referee Nomination & Report Request started (09 Apr 2019) by Erwin Zehe

RR by Anonymous Referee #2 (08 May 2019)

Suggestions for revision or reasons for rejection

I thank the authors for their efforts to improve the manuscript. I am satisfied with the changes to the manuscript that tackled my suggestions/demands. Changing the order of the ideas in the discussion would eventually give the work a better readability and impact. I also have a few more minor comments. These changes can probably be made without an additional round of revisions.
Page 1
L26: “superior” may be too strong and start a debate with geochemists. Please consider another adjective. Also note that while isotopes are not affected by chemical reactions like solutes, stable isotopes nevertheless go through fractionation processes. These make it hard to link isotopic ratios to other processes (e.g. flows) in the absence of controlled conditions (e.g. temperature and humidity).
Page 2
L6: De Walle is not an appropriate citation, cite rather Liu et al. (2004), https://doi.org/10.1029/2004WR003076.
L7: I think it is “von Freyberg” et al.
L14: “catchment residence times”
L15-16: This sentence is really important. Splitting it and adding more details would give it more impact.
L27: I would replace “precipitation input” with “the water effectively recharging the catchment”
L30: “decreasing TTDs” does not make much sense. Write “lower travel times” instead.
L32: “data” -> “isotopes”
Discussion:
The discussion has almost all the ideas needed to give the work enough impact. However, these ideas are presented in a way that makes it challenging to follow and makes the reading a bit painful. I recommend that the authors really focus on their technological progress offering the benefits of continuous measurements against bulk samples at the start of the discussion. Then, they should move on to interpretations of their correlations and plots in terms of processes and applications to models/isotope hydrology applications, and then only technical aspects + limitations/way forward. Therefore, I suggest the following order (using the current section numbers) and advice for improvement:
(1) 4.1: Another title would describe the contents better. Paragraph II seems to fit better in 4.3.
L25: You could mention that this finding about dTF too is possible only with continuous data and that further investigations about this will need such a measurement setup.
L31: You could mention that this also dispenses the transport/storage of many samples. Also, avoid using “obviously” (found also in other sentences). What is obvious for someone may not be for someone else.
(2) 4.6: the sentences lines 9-14 and the first sentence of paragraph IX seem to fit better in 4.5. Paragraph X would be better directly after VI.
(3) 4.7: Make it clear here that only the continuous measurements in isotopes can validate such a mechanistic model. A word about the isotope applications in hydrology to echo the contents of the intro?
L14: Not clear what simulation is meant. “needed” -> “needs”?
L14-16: Not clear, break down / reformulate.
(4) 4.2
L15: “meteorological variable” seems unnecessary
(5) 4.4
L5: Similar but less distinct than what?
L8: But why would these uncertainties be higher for initially wet conditions?
L14-15: Really? For me this is not obvious. This needs further explanation (show the equations allowing this conclusion?)
(6) 4.3
L24: The first sentence is repetitive w.r.t. the last sentence of 4.2. This may be irrelevant after rearranging the order of the paragraphs.
L27: Alterations in the isotopic signal evidenced by interception losses? That does not make sense. Please explain further.
(7) 4.5
Conclusion: Currently this section lacks enough detail about the impact/consequences for isotope hydrology. It is nice approach, but how will it impact the field. Does it really matter?
Page 14:
L8-10: More details about this and applications in isotope hydrology.

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ED: Publish subject to minor revisions (review by editor) (16 May 2019) by Erwin Zehe

AR by Barbara Herbstritt on behalf of the Authors (05 Jun 2019) Author's response Manuscript

ED: Publish as is (07 Jun 2019) by Erwin Zehe

Short summary

We describe a novel technique for the precise, quasi real-time observation of water-stable isotopes in gross precipitation and throughfall from tree canopies in parallel. Various processes (e.g. rainfall intensity, evapotranspiration, exchange with ambient vapour) thereby control throughfall intensity and isotopic composition. The achieved temporal resolution now competes with common meteorological measurements, thus enabling new ways to employ water-stable isotopes in forested catchments.