Repeated electromagnetic induction measurements for mapping soil moisture at the field scale: validation with data from a wireless soil moisture monitoring network

Martini, Edoardo; Werban, Ulrike; Zacharias, Steffen; Pohle, Marco; Dietrich, Peter; Wollschläger, Ute

doi:https://doi.org/10.5194/hess-21-495-2017

Articles | Volume 21, issue 1

https://doi.org/10.5194/hess-21-495-2017

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

https://doi.org/10.5194/hess-21-495-2017

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

Articles | Volume 21, issue 1

Research article

|

26 Jan 2017

Research article |

| 26 Jan 2017

Repeated electromagnetic induction measurements for mapping soil moisture at the field scale: validation with data from a wireless soil moisture monitoring network

Edoardo Martini, Ulrike Werban, Steffen Zacharias, Marco Pohle, Peter Dietrich, and Ute Wollschläger

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Final revised paper (published on 26 Jan 2017)
Preprint (discussion started on 04 Mar 2016)

Interactive discussion

Status: closed

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

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RC1: 'Referee review', Anonymous Referee #1, 08 Apr 2016
- AC1: 'Authors' response to the review comment by Anonymous Referee #1', Edoardo Martini, 09 May 2016
RC2: 'Review comments', Anonymous Referee #2, 17 Apr 2016
- AC2: 'Authors' comment to the review comment by Anonymous Referee #2', Edoardo Martini, 09 May 2016
RC3: 'Review', Anonymous Referee #3, 18 Apr 2016
- AC3: 'Authors' response to review comment by Anonymous Referee #3', Edoardo Martini, 09 May 2016
SC1: 'Review: Repeated electromagnetic induction measurements for mapping soil moisture at the field scale: validation with data from a wireless soil moisture monitoring network', Beo Hjaldrgegnir, 28 Apr 2016
EC1: 'Start interacting', Nunzio Romano, 29 Apr 2016
AC4: 'Final authors' comment', Edoardo Martini, 10 Jun 2016

Peer-review completion

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

ED: Reconsider after major revisions (03 Jul 2016) by Nunzio Romano

AR by Edoardo Martini on behalf of the Authors (10 Aug 2016) Author's response Manuscript

ED: Referee Nomination & Report Request started (17 Aug 2016) by Nunzio Romano

RR by Anonymous Referee #4 (12 Sep 2016)

Suggestions for revision or reasons for rejection

This paper investigates whether electromagnetic induction (EMI) measurements of the soil electrical conductivity (ECb) can be used to estimate and monitor soil water content. This is not trivial since there is a range of factors that influence ECb. The results from a hillslope transect equipped with a wireless soil moisture network showed poor correlations between ECb and water content for a range of measurement days. It is concluded that there are too many confounding factors to determine soil water content using EMI at this site.

Overall, I am supportive of the general direction of the manuscript, and I think that it conveys an important message that is not well represented in recent literature on EMI measurements for soil moisture characterization. However, I do have three general comments and a wide range of specific comments that need to be addressed before this paper can be accepted for publication. Moderate to major revisions are still required to address these comments.

GENERAL COMMENTS
1. As already indicated by other reviewers in round 1, the introduction is long and not really focused. At several places, I could not really follow the point that the authors were trying to make. I think this section still needs more focus. I have provided some specific comments below that could help.

2. I was not able to follow the presentation in the results and discussion at all times. The line of argumentation remained unclear in several places. Again, I hope that the specific comments below will show you where I got confused. The authors seem to have a lot of supporting data for the field site, but it is not used in this manuscript. For example, it is stated that soil texture is the dominant control (Page 14, Line 14) but this is not illustrated with supporting data. Now it is argued that soil water content is not a key factor, but no alternative data are presented to support the narrative explanation for this conclusion. It would great if this could be addressed.

3. The balance between results and discussion seems inappropriate. There is a lot of discussion for a relatively small amount of results. I would prefer a more concise discussion that makes some key points. I felt that the current version was quite repetitive, which was partly also due to long replies to reviewer comments that were included in the manuscript. Also, section 4 comes after a long discussion in section 3. Shortening seems appropriate to me.

SPECIFIC COMMENTS
Page 1, Line 24-27. Although I agree with the general stance of this manuscript, I believe that these conclusions are too general. They should be formulated in a more site-specific manner. I am also not so happy with the formulation of point ii. Of course, this effect can be measured. However, changes in water content may co-vary with changes in pore water electrical conductivity and the relationship may be obscured. However, this is not valid in general. In short: reformulation is required here.

Page 2, Line 4. Not sure whether they were developed for soil salinity. However, it was a first main area of application. Consider rewriting.

Page 2, Line 12. Awkward sentence. I do not understand your link between EMI theory and the work of Rhoades here. Please formulate more carefully.

Page 2, Line 23-24. I agree that soils with high CEC usually show higher ECw. However, the causality provided here is too simplified. Please improve by reformulating together with the next sentence that already goes in the right direction.

Page 2, Line 29. Looking at this sentence, you seem to make a difference between soil properties and states. Perhaps it is better to avoid this. It is my experience that most people do not really make this distinction (e.g. they consider water content to be a soil property too). Removing this distinction will require some rewriting.

Page 3, Line 4-5. Perhaps emphasize here already that depth-weighting is complicated. Your formulation seems to imply homogeneous depth weighting.

Page 3, Line 5-15. This is very detailed for an introduction. Perhaps this can be moved to a more appropriate position later in the paper?

Page 3, Line 17. You write “…the sensitivity of any EMI sensor to soil ECa…”. This is physically incorrect. ECa is a property of the device, not of the soil. There are multiple occasions of this error within the manuscript. I think you need to additionally use the bulk electrical conductivity at specific points in the manuscript.

Page 3, Line 19. I am not sure that this is a “physico-chemical” measurement. Consider rewriting.

Page 3, Line 15-20. I am not so convinced that the discussion of Callegary et al. is really necessary here. Do you expect significant first-order effects here because of violation of the low-induction number approximation? Here I see some potential to shorten the introduction, which is not really focused and perhaps picks up too many threads at the same time.

Page 5, Line 13. You state “…specific attention to the physical principles controlling the EMI measurements...”. However, it is not clear from the introduction what you mean with this. I would have expected that you wanted to focus on physical principles affecting soil bulk electrical conductivity after reading the introduction. Needs clarification.

Page 5, Line 30. You write “… with a sub-continental superimposed.”. It is not clear to me what you mean with this. Please clarify.

Page 6, line 25. Consider using two significant digits for the porosity values. I think this better reflects the measurement accuracy for this soil property.

Page 8, line 13. More quantitative information needs to be provided about this drift correction. Is the correction a function of time? How appropriate is the linear drift assumption? How strong is the correction as compared to the measured values?

Page 8, Line 24. Perhaps also state here that you will be using rank correlations later.

Page 8, Line 26. Also the corrections based on the reference transect should be discussed in more quantitative terms. How much shift was applied, and how does this relate to the mean measured ECa?

Page 9, Line 8. Not sure that spatialization is the correct choice of word here. Why not interpolation or regionalization?

Page 9, Eq. 1. I am not convinced that this is the correct way to obtain the profile water content. I think the weights should add up to one, so a normalization is missing in the equation. Also, it may be a good idea to consider the measurement depths that the sensors represent. You mention that you associate layers to each sensor. Would it not be better to use layer depth instead of sensor depth here, and the associated cumulative sensitivity?

Page 12, Line 5-7. I do not really agree with this statement here. What other factors do you want to blame? This should be immediately discussed when this statement is made. I guess the main point is that the soil water content and the pore water electrical conductivity co-vary in different manners for different landscape positions, but this point could be made in a clearer manner.

Page 12. Line 8 onwards. I found the discussion presented here difficult to follow. What are you trying to say? It seems that you are building up towards an explanation of the different patterns derived from the rank stability, but you need to help the reader better here.

Page 14, Line 10. Soil texture is not a factor that affects bulk soil electrical conductivity directly. What physical process are you thinking of here? If it is porosity, can the results be improved when the soil saturation instead of the water content is used in Figure 4?

Page 14, Line 14. Here you state that spatio-temporal variations in ECw are of secondary importance. However, this seems to be in contrast with the previous text where different hydrological regimes are discussed. If ECa is primarily affected by static soil properties as suggested here, you should be able to find correlations that are persistent.

Page 16, Line 15-25. A lot of words to say that it is likely not important for this site. I think that this can be shortened.

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RR by Anonymous Referee #5 (11 Dec 2016)

ED: Publish subject to minor revisions (further review by Editor) (28 Dec 2016) by Nunzio Romano

AR by Edoardo Martini on behalf of the Authors (06 Jan 2017) Author's response Manuscript

ED: Publish as is (09 Jan 2017) by Nunzio Romano

Short summary

With a process-based interpretation of electromagnetic induction measurements, we discussed the potential and limitations of such a method for soil moisture mapping. Results will help clarify the complex and time-varying effect of stable soil properties and dynamic state variables on the physical parameters measured, with implications for future studies. We highlighted the importance of time-series data and the need for a multidisciplinary approach for proper interpretation.