Reviews and syntheses: Isotopic approaches to quantify root water uptake: a review and comparison of methods

Rothfuss, Youri; Javaux, Mathieu

doi:https://doi.org/10.5194/bg-14-2199-2017

Articles | Volume 14, issue 8

https://doi.org/10.5194/bg-14-2199-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/bg-14-2199-2017

© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 14, issue 8

Reviews and syntheses

|

02 May 2017

Reviews and syntheses |

| 02 May 2017

Reviews and syntheses: Isotopic approaches to quantify root water uptake: a review and comparison of methods

Youri Rothfuss and Mathieu Javaux

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Final revised paper (published on 02 May 2017)
Supplement to the final revised paper
Preprint (discussion started on 05 Oct 2016)

Interactive discussion

Status: closed

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

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- Supplement

RC1: 'Review of Review on RWU with isotope approaches', Matthias Sprenger, 07 Nov 2016
- AC1: 'Anwer to review of Matthias Sprenger', Youri Rothfuss, 24 Nov 2016
  - RC3: 'Comment to dual isotope approach', Matthias Sprenger, 25 Nov 2016
    - AC3: 'Answer to Comment#2 from Matthias Sprenger', Youri Rothfuss, 25 Nov 2016
      - RC4: 'Soil water isotopes do not necessarily all plot along an evaporation line', Matthias Sprenger, 25 Nov 2016
        
        AC4: 'Answer to Comment#3 from Matthias Sprenger', Youri Rothfuss, 25 Nov 2016
RC2: 'Review: “Isotopic approaches to quantifying root water uptake and redistribution: a review and comparison of methods” by Rothfuss and Javaux', Anonymous Referee #2, 08 Nov 2016
- AC2: 'Anwer to review of referee 2', Youri Rothfuss, 24 Nov 2016

Peer-review completion

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

ED: Reconsider after major revisions (02 Dec 2016) by Michael Bahn

AR by Youri Rothfuss on behalf of the Authors (06 Dec 2016) Author's response Manuscript

ED: Referee Nomination & Report Request started (08 Dec 2016) by Michael Bahn

RR by Anonymous Referee #3 (09 Dec 2016)

Suggestions for revision or reasons for rejection

Here I am reviewing 'only' the revised version. The manuscript is basically split into two parts: 1. testing three different isotopic methods for determining root water uptake with artifical data, and 2. recommendations for sampling water isotopes.

I was expecting something very different reading the title: 1. redistribution was not quantified; 2. methods were not reviewed or compared for redistribution; and 3. there was no full review of all available methods using isotopes for root water uptake but rather a review on the three selected methods.
The title should hence be more specific.

I think the paper would gain a lot by concentrating on a single task, which would be the comparison of three methods to determine root water uptake. I would hence delete section 5 and the paragraph page 4, lines 6-9. This is corroborated that in the conclusions 17 lines talk about root water uptake and there are only three lines for redistribution.

As written above, I lso think that the manuscript is no review of methods. It describes three methods and aplies them.

The authors argue against the use of soil water content alone because it can be redistributed: "Rather, the full soil water flow equation accounting for root uptake and soil water redistribution must be solved in an inverse mode [...]". This should be true for isotopes as well and is not included in the presented methods. I have to admit that I did not fully follow the Couvreur et al. (2012) model but from what I got, isotopes are only diagnostic so if the model works for isotopes than it should work for water content as well.
It is not well described how the virtual experiments were produced. The delta-values in the tables 2 and 3 suddenly appear from nowhere. I have thought that the authors have at least two highly sophisticated models at hand that can produce isotope profiles in the soil. Why do I need Couvreur et al. (2012)?

More specific comments are:
1. Use less abbreviations in the text. Up to the end, I could not remember what meant DeDr_IT. Not very talkative. All the MSPG, MSPa, etc. makes the manuscript hard to read.

2. Figures 4 and 6 have no legends, which makes it hard to follow. I have to read the very, very, lengthy captions to find the meaning of a certain line. One line was only explained after 15 lines of caption.

3. The figure captions include too much superfluous information that is or should be in the text.

4. Figure 1 would benefit from the root and soil moisture profiles. In the current state, it does not really help me understanding anything.

5. I would put only equations 2 and 4b. 3 is only a unit conversion and 4a is the same as 2.

6. page 7, line 24: this is wrong. delta=0 is an isotope ratio of V-SMOW so not nothing. Or put it like this J_Ti must include S<0, otherwise you mass balance is not right.

7. The passage about Zimmermann is too lengthy. Does not add much. I guess it is Barnes and Allison who first included the importance of vapour transport.

8. page 3, line 25: I have read as well on Wikipedia that the Richards equation should be attributed first to Richardson. However, it is called Richards equations and not Richardson equation. Otherwise, nobody knows what you are talking about. And the review of Vereecken et al. (2016) is not mentioning it so remove the reference.

9. The authors cite themselves that the evaporative front is not at the point of maximal isotope value but at the point of maximal derivative. This is a hypothesis in their earlier paper but it was not shown theoretically. This could be done, probably, with one of their isotope in soil models.

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ED: Reconsider after major revisions (13 Dec 2016) by Michael Bahn

AR by Youri Rothfuss on behalf of the Authors (23 Jan 2017) Author's response Manuscript

ED: Referee Nomination & Report Request started (30 Jan 2017) by Michael Bahn

RR by Anonymous Referee #2 (13 Feb 2017)

RR by Anonymous Referee #3 (08 Mar 2017)

Suggestions for revision or reasons for rejection

The manuscript has improved tremendously. Well done.

There are basically two items, which I would ask to consider:
1. I would delete sections 5.1-5.3. They are not adding to the manuscript. This would also delete the last paragraph of the conclusions and te last sentence of the abstract.

If experimental precision were an issue than it should be treated in the results section with a virtual experiment, just as the two disjoint soil layers. Otherwise this is just the usual muttering that we need higher resolution observations.

Section 5.4, however, is interesting and not enough elaborated. If a model were inverted, one needs to identify quite a few extra variables such as hydraulic conductivities, etc. The information content in the data is probably not enough to infer all required information. So the method, which quite a few people would find best, might not be superior to the Bayesian approaches presented here.

It is unclear to what extent this plays into the presented methods. For example, the preferred Bayesian inversions depend on strongly on the priors and hence to some assumed soil properties. Did the authors try different priors, e.g. flat priors, Jeffreys prior, etc.?

2. I still do not understand the use of Couvreur et al. (2012). These are virtual experiments so running a full isotope-enabled soil model gives all the information for the virtual experiments.

Here, I have to believe that Couvreur et al. (2012) gives the right answer. Is this so?

Or I do not understand the method. It is NOT well described:
"Mean RWU depths (provided by the GI method) and xj distribution (provided by the two end-member and multi-source mixing models) were determined from soil and xylem water oxygen isotopic composition distributions. While the former information was prescribed to the different methods, the latter was
20 calculated with the physically based analytical RWU model (referred to as “Couv”) of Couvreur et al. (2012)."
How is GI entering in this? Where are the "soil and xylem water oxygen isotopic composition distributions" coming from? What is "the former" and what "the latter"? What are you reffering to?

How I understood virtual experiments: you run the most advanced physical model and add some noise on the output. The you use this noisy output to infer properties, states or fluxes, which were calculated in the physical model.
It seems to be different here.

Minor comments:
a) page 1, lines 17-20: I cannot understand the sentence. Please reformulate.

b) page 4, 20-23: theory says something different and Rothfuss et al. (2015) only proposed that it might be the point of the steepest gradient. Please reformulate.

c) page 9, 8: i should be superscript and A, B subscript.

d) page 14, 31: you call Figure 6 still Figure 5 from this point on.

e) page 15, 32: 8 layers were not shown but only mentionned. If you claim that it might be a good way forward then it might warrant a figure or including it in Fig. 6.

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ED: Publish subject to minor revisions (Editor review) (14 Mar 2017) by Michael Bahn

AR by Youri Rothfuss on behalf of the Authors (24 Mar 2017) Author's response Manuscript

ED: Publish subject to technical corrections (04 Apr 2017) by Michael Bahn

AR by Youri Rothfuss on behalf of the Authors (04 Apr 2017) Author's response Manuscript

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Short summary

Plant root water uptake (RWU) has been documented for the past 5 decades from water stable isotopic analysis. In this paper, we review the different methods for reconstructing RWU profiles on the basis of isotopic information and confront them with each other during a series of virtual experiments. Finally, we call for a development of approaches coupling physically based RWU models with controlled condition experimental setups.