High-Resolution Estimation of Methane Emissions from Boreal and Pan-Arctic Wetlands Using Advanced Satellite Data

Round 1

Reviewer 1 Report

Overview:
Albuhaisi et al. study the incorporation of high resolution soil moisture (SM) data from satellite into a simple model of methane flux from wetland. The fluxes are calibrated with, then validated against, data from the FLUXNET network. The simple model is then extended with a hydrological model to estimate methane fluxes across a large range (north of 50N). The bulk result agrees well with a number of other model estimates of northern wetland fluxes for recent years.

The paper is logically sequenced, with adequate references to recent published work in the area. This reader has a couple of concerns regarding the use of data and the tracking of uncertainty through the results. These are elaborated upon below, along with a thorough list of typos and corrections referenced by line number in the manuscript.

The use of satellite SM data for estimating methane fluxes is certainly of interest, and improvements to the methods of doing so, particularly those that aim to reduce the uncertainty in the estimate. This study does not go into data assimilation or inversion of fluxes, but relies on a simplified model that relates soil moisture and other site-level variables to methane flux. A future study might extend this to parameter estimation or inversion using the same high resolution data.

Main issues:
In describing the methods used in this paper, the authors spend very little time describing the validation data (FLUXNET), other than to state which sites are used. A brief paragraph on the measurements being taken, and which variables are used in the present study, would go a long way in elucidating what we are looking at. Later, in the discussion section, it is revealed that where SM data were absent, WTD was used as a proxy. Neither of these measurements are mentioned in the description of the FLUXNET data. Up to the discussion, this reader assumed that only methane fluxes were used from FLUXNET. So, some of the discussion can be brought forward and the description of the input data to the study expanded.

The prime issue of scientific concern is the reuse of calibration data for the purposes of validation. Usually, one tries to separate these. If the MeSMOD model is calibrated to match the measurements at half the FLUXNET sites being used here, is it any surprise that the model then performs well when validated against the same FLUXNET sites? I understand that the tuning is a single parameter that matches in a "mean" sense, and that the variability plays a big role here, but it should at least be mentioned. The small number of available sites is likely to blame for needing to "double-dip" like this, but it does weaken the conclusions.

There is a missed opportunity to investigate the cause of differences between MeSMOD and LPJ/CAMS geospatial fluxes as the authors stop short after realizing the wetland maps are different. Both maps are available (they're in Figure A3) - why not try a control run where the map is the same for both (i.e. use the CAMS wetland map instead of CLCC inside of MeSMOD and see where the differences remain)? This could give additional insights that are germane to the conclusions of the paper.

Another oversight is the lack of discussion of uncertainty in the results. There is a sense that each of the terms in the MeSMOD equation (1) has some degree of uncertainty; however, this is not tracked through to the results. Therefore, it is difficult to know whether the difference in flux obtained with the higher resolution data is in fact significant, or whether we are within the range of the uncertainty on the coarse estimates. I suspect some of this information exists, but it is reported inconsistently throughout the paper (sometimes results have uncertainty, other times no). This undercuts the conclusion that incorporating higher resolution data will lead to lower uncertainty in flux estimates, since to do that we would need to see a reduction in the uncertainty.

Some of the conclusions drawn are beyond what the evidence presented in the paper provides. Specifically, there doesn't seem to be evidence that the higher resolution SM data yields a better temporal match in the fluxes at validation sites, just a better overall correlation, which could be due to matching the individual site means more reliably. More evidence needs to be provided for this conclusion, or the conclusion removed.

Some of the body text and figure captions seem like the paper was rushed into the review process. In multiple places, the same paragraph is repeated in different words. Figure captions are too terse to fully explain what is happening in the figure, or figures are borrowed wholesale from other works. These are elucidated in the "Typos" section below.

Typos:
- "Arctic" should be capitalized when referring to the geographic region (e.g. "Arctic wetlands"). Same is true for "pan-Arctic".
- line 13: unneeded "'s" on MeSMOD
- abbreviations in abstract should be spelled out (e.g. LPJ-wsl, possibly also CAMS and FLUXNET), or replaced by more descriptive non-technical language
- is there a need to distinguish between "Arctic" and "pan-Arctic" here? The difference isn't defined and I think the terms are being used interchangeably. If no distinction, stick to one or the other
- lines 91-101: same paragraph repeats twice.
- lines 117-129: same paragraph repeats twice.
- line 131: the Arctic Circle is a specific thing, and is not located at 50N. It is roughly 66N (the latitude where you get 24h darkness at least once a year). It is fine to scale the results to north of 50N, just don't call it the Arctic Circle.
- line 131 and elsewhere: the degree symbol for latitude comes before the "N", and it should not be underlined.
- line 167: CH4 needs a subscript.
- line 207: refers to equation (2), which does not exist. Assume you mean eqn 1.
- line 214: here is the first time you spell out the PCR-GLOBWB acronym. You should spell it out the first time you use it (and in the abstract).
- line 229: an acronym of an acronym? If you're going to do this, do it earlier and use it consistently throughout the paper. To be truthful, I don't see PCRG much elsewhere, so just remove.
- line 237: adopted, or adapted? This paper should be original work!
- line 238: missing citation number [21].
- line 244-245: is this a citation? Why is it different to all the other citations?
- line 248: claim is based on what?
- lines 254-255: figure caption should be in full sentences, not just a definition of the acronyms it uses. What is this figure telling the reader?
- line 259: do you mean Table 1?
- lines 258-61: same paragraph repeats twice.
- section 2.4.1: you need to introduce the FLUXNET network and sites. How are the measurements taken? Why are you choosing these specific sites? Were some other sites excluded and why? What measurements are being used (I assume CH4 flux, but are there others)? What is the footprint of the sites? Why are they grouped into two groups? Don't assume the reader is already familiar.
- section 2.4.2: the title of this section is misleading. You are describing other models that you use to compare against. Some are process models, one is an inversion model. Maybe title "model intercomparison data sources"?
- section 2.4.2: you list 4 models used for validation and then have two paragraphs describing a couple of them in more detail. Describe all of them in detail. What is CAMS? What data are incorporated into the CAMS inversion? What is the RF model you are using? Why use them? Why not choose other methods?
- line 294: later on, you use IA, not AI. (AI would be confusing, because it usually reads as "Artificial Intelligence").
- equations 2-4: bold font in math often indicates a vector/matrix quantity, which is not the case here. Please remove the bold font. Also, the notation can be made more professional looking by using an actual variable rather than "Obs" and "sim". (e.g. x_o for observations, x_s for simulations). Just be sure to define any of the symbols you use.
- equations 3-4: is the overbar to indicate a mean value? Please define in the text.
- line 335-336: over what range of time? The calibration was over 2015-2018, so the validation should be from a separate time period.
- line 343: indicate which figure.
- line 345: "assuming all fluxes represent a single series." Does this mean all sites for all years? All group 2 sites? All group 1? A single year?
- lines 348-353: these values can be succinctly summarized in a table. It will be more readable and draw attention to the results that you spent a whole subsection setting up in the methods. (I see a partial table in Appendix B, just pull that forward and include the other metrics too).
- Figure 4: what is indicated by the red quarter circle? I would interpret it as "a process on this line has the same standard deviation as the observations, but a correlation coefficient less than one".
- line 378: I think this refers to Figure 5, not 4.
- line 383: refer the reader to the figure in the appendix. Also, there should be at least one reference in the body text to Table 2.
- line 384: extreme weather, you need to provide some evidence of this - was there extreme weather at all of the FLUXNET sites in group 2?
- Figure 5: Units on right hand axis should be "K", not "k"
- Figure 5: it is very difficult to distinguish the different SSM/HSM data points in the timeseries - which one is performing the best? We really can't tell without a lot of squinting. The symbol shapes (circle, triangle, x, etc) are all lost because the individual points are so small and overlapping. The lines connecting the observations should definitely be removed (these are discrete measurements).
- Table 3: values should be reported with some form of uncertainty (based on Fig 7, you have at least some of this information available)
- line 420: missing capital for "Planet" (I gather this is a corporation)
- lines 436-439: presumably there is some assumption in Treat, RF, and MeSMOD that differs from the way WETCHARTS, CAMS, and LPJ treat boreal winter fluxes - that is a big, consistent difference across half the year. Can you elaborate?
- line 474: if the wetland map is the main cause of disagreement, can't you include that as a control simulation? (i.e. try a run of MeSMOD using the CAMS wetland map to eliminate that cause of difference then examine how things are still different)
- Figures 8 and 9: flip order, to match the order they are introduced in the body text.
- Table 4: caption text is confusing: "mean seasonal interannual and variability". My guess is that the first number is the mean, and the bracketed range indicates the variability (interannual? seasonal? both?)
- Figure 9: the caption is reversed to what is shown (HB is in (b) and WSL in (a)).
- lines 529-537: this paragraph really descibes what you are drawing in from the FLUXNET dataset, and should be in section 2.4.1, as should Figure 10. It points to a weakness, in that WTD isn't a particularly good measure of SM (r ~ 0.25 at some sites!)
- line 561: "consensus" is incorrect here because it implies that those other models agree that MeSMOD is a good estimate. Maybe use "conclusion" instead.
- line 571: "our findings from LPJ-wsl ... and CAMS"... It is my impression that these results are taken from elsewhere, not models run by you. As such, these are not "your" findings, the MeSMOD ones are.
- lines 582-583: I do not see any evidence in this paper that the higher resolution SM data leads to a better temporal estimate. I see that you get a higher correlaation coefficient against the FLUXNET validation data. That is not the same thing. If I look in detail at Figure A2, the MeSMOD results with 10km input look like a scaled version of the 100km input. It doesn't do much for capturing temporal changes that the model was already missing. If I am incorrect, then please demonstrate this finding more clearly with an example.
- line 587: don't need "Zhang et al." here, the [18] suffices.
- Figure A1: axes labels are too small to read (well, if I squint)
- Figure A3: suggest using a colour that isn't in your colorbar as a means of highlighting those areas (blue on blue is too hard to see)
- Table B1: isn't this one of the main results? That the higher resolution does better than the lower? Why is it buried in the second appendix?
- references: chemical compounds should still have subscripts in journal titles (if Latex, use more {})
- reference 6: missing a capital in second author name
- reference 11: first author name is out of order (Bloom, A. A.)
- reference 12: if text is talking about changes since 2006, why refer to the 2007 IPCC report? Why not AR5 or AR6? Also, this isn't the right form to cite those reports
- reference 26: typo in "Uncertainties"
- reference 38: de Jeu in author list is incorrect
- reference 43: duplicate of reference 21
- references 46, 53: consult journal style guide for referencing datasets
- reference 47: accents in author names are improperly formatted (if Latex, use more {})
- reference 55: is this a manual? it may be missing publisher info
- references 59, 60: author lists both start with de Jeu but one is capitalized and the other isn't
- reference 59: this is not IEEE format for a patent reference. You need the country of origin and the patent number, at least.
- reference 61: is this a book? it may be missing publisher info
- reference 83: journal title has an extra "(80-.)"
- reference 87: incorrect journal title and missing volume, page, doi

The English itself is fine, but it does need a thorough review to get rid of typos and repetitions. I've given a list of corrections above, but it is likely that I missed some.

Author Response

Dear Reviewer #1,

We would like to extend our sincere gratitude to you for dedicating your time and expertise to review our manuscript. Your valuable feedback and thoughtful comments have greatly contributed to the improvement of our work, and we are truly appreciative of your efforts.

We have carefully reviewed and considered each of your comments, suggestions, and concerns. In response, we have made revisions and provided detailed explanations and clarifications in the attached document. Our aim was to address all of your points comprehensively and ensure that the manuscript is now more clear, accurate, and scientifically robust.

We recognize the importance of the peer review process in maintaining the integrity and quality of scientific publications. Your expertise and critical evaluation have been instrumental in refining our manuscript. We sincerely value your input and the time you have dedicated to this review.

We kindly request you to re-evaluate our revised manuscript based on the updates we have made. We believe that the changes have significantly strengthened the paper and addressed any issues raised during the review. Your final assessment and recommendation regarding the suitability of our manuscript for publication in your esteemed journal will be highly valued.

Once again, we express our heartfelt appreciation for your contribution to our work. We eagerly await your response and the opportunity to share our research with the wider scientific community through your esteemed journal.

Thank you for your time, consideration, and continued support.

Best regards,

Yousef A. Y. Albuahisi

Author Response File: Author Response.pdf

Reviewer 2 Report

The presented study attempted to improve the accuracy of model-simulated methane emissions from wetlands in the northern regions using high-resolution satellite-derived soil moisture, which may provide an alternative to the conventional approach of using coarser resolution biogeochemical models. The topic is interesting, and the intend of the study is really good. Meanwhile, the approach of this study may improve our understanding in upscaling CH4 emissions from wetland.

 

Specific comments:

 

L11-12 Model ass inputs?

 

L19 subscript

 

L39-40 Why?

 

L91-101 They are should be merged

 

L134-139 It is hard to understand these. Please change to improve clarity

 

L140-146 Removed

 

L148 What does class number 180?

 

L167 subscript

 

L169 Please added reference for the formula

 

L195 How to obtain the daily CH4 flux? By eddy covariance?

 

L293-298 Please merged them in a paragraph

 

L390 Table 2: Pleased added the standard deviation, and same to the following tables (e.g. Table 3 and Table 4)

 

424-426 Previous study in subtropical climate (https://doi.org/10.1002/2017JG003805) also are consistent with patterns

 

L519 For the Discussion, I can’t follow/catch the main/key findings of the study. Please re-write the Discussion to improve the manuscript

The manuscript could benefit from language editing.

Author Response

Dear Reviewer#2,

We would like to express our gratitude for taking the time to review our manuscript and for providing us with valuable feedback. We appreciate your thoughtful comments and suggestions, which have helped us to improve the quality and clarity of our work.

We have carefully considered all of your comments and questions, and have provided detailed responses and explanations in the attached document. We hope that these responses address all of your concerns and provide you with the information you need to evaluate our manuscript.

We understand that the review process is an essential component of the scientific publishing process, and we value the insights and perspectives of our peers. We appreciate the effort and expertise that you have invested in reviewing our manuscript, and we hope that our responses are satisfactory.

Once again, thank you for your time and effort in reviewing our manuscript. We look forward to hearing back from you regarding the suitability of our manuscript for publication in your esteemed journal.

Kind Regards, 

Yousef A. Y. Albuhaisi

Author Response File: Author Response.pdf

Reviewer 3 Report

The study describes the model of CH4 emissions predictions based on soil moisture and temperature, calibrated with measured fluxes on eddy-covariance sites. The model's performance is demonstrated on the basis of different maps of wetland distribution from satellite-derived products. Given the simplicity of the model it produces the results similar to the more complicated ones, adequately reproducing the seasonality and interannual variability for the sites with measurement duration of more than 4 years. 

The study is written clearly, and apart of some textual corrections, which authors can follow from the file I attach (red marks mean valuable information for me, but yellows and greens - are the ones to be corrected). And, which is more important, I did not see in the methods the description of how was the extrapolation made. The model's K coefficient was derived based on several sites' measurements, and the variation of K is shown in the table B2. But how was the areas distributed in boreal and polar regions where models have been used with different K? Please clarify this in the method section.  

 

 

Comments for author File: Comments.pdf

Author Response

Dear Reviewer #3,

We would like to express our sincere gratitude for your thorough review of our manuscript and for providing valuable feedback. We have carefully considered each of your comments and suggestions, and we greatly appreciate the insights and perspectives you have shared.

We would like to inform you that several of the concerns and questions you raised have been addressed by the other reviewers and incorporated into our revisions. We have provided detailed responses and explanations in our previous correspondence and in the attached document, which we believe have adequately addressed your concerns.

We understand the importance of addressing all reviewer comments and ensuring that no concerns are left unattended. Therefore, we kindly request you to review the responses provided to the other reviewers, as we believe they have effectively covered most, if not all, of the concerns you have raised.

We highly value your expertise and input, and we are confident that the revisions we have made based on the feedback from all reviewers have significantly improved the quality and clarity of our manuscript. We sincerely hope that you find our responses satisfactory and that our revised manuscript meets the standards for publication in your esteemed journal.

Thank you once again for your time and effort in reviewing our manuscript. We look forward to hearing your final evaluation and recommendation.

Best regards,

Yousef A. Y. Albuhaisi

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

I think the manuscript still need to be improved due to the following reasons; First, the comments in my last round were not fully addressed; Second, the study should provide novel insights into relevant environmental mechanisms or processes, not only focusing the method!

Minor revision

Author Response

Dear Reviewer#2,

We would like to express our sincere gratitude for your diligent review of our manuscript and for providing us with valuable feedback. Your insightful comments and suggestions have significantly contributed to enhancing the quality and clarity of our work.

We have thoroughly considered each of your comments and questions and have addressed them comprehensively in the attached document. We believe that our responses effectively address your concerns and provide the necessary information for evaluating our manuscript.

We highly value the perspectives and expertise of our peers. Your meticulous examination of our manuscript is greatly appreciated, and we hope that our responses meet your expectations.

Thank you once again for your time and dedication in reviewing our manuscript. We eagerly await your assessment of the suitability of our work for publication in your esteemed journal.

Kind Regards,

Yousef A. Y. Albuhaisi

Author Response File: Author Response.docx