The challenges of an in situ validation of a nonequilibrium  model of soil heat and moisture dynamics during fires

Massman, William J.

doi:https://doi.org/10.5194/hess-25-685-2021

Articles | Volume 25, issue 2

https://doi.org/10.5194/hess-25-685-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/hess-25-685-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 25, issue 2

Research article

|

17 Feb 2021

Research article |

| 17 Feb 2021

The challenges of an in situ validation of a nonequilibrium model of soil heat and moisture dynamics during fires

William J. Massman

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Final revised paper (published on 17 Feb 2021)
Preprint (discussion started on 11 May 2020)

Interactive discussion

Status: closed

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

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- Supplement

RC1: 'Brief comments on William J. Massman "the challenges of an in situ validation ...."', Yijian Zeng, 20 May 2020
- AC1: 'Response to Yijian Zeng', William Massman, 20 May 2020
- AC4: 'Full Response to Yijian Zeng', William Massman, 16 Jun 2020
RC2: 'Review Comments', Anonymous Referee #2, 08 Jun 2020
- AC2: 'Response to Reviewer #2', William Massman, 10 Jun 2020
- AC5: 'Full Response to Anonymous Referee #2', William Massman, 16 Jun 2020
  - RC3: 'Additional Comment Regarding the Assumption of Constant Bulk Density', Anonymous Referee #2, 06 Jul 2020
    - AC6: 'Response to second comment of Anonymous Referee #2', William Massman, 07 Jul 2020
SC1: 'Interactive comment on “The challenges of an in situ validation of a non-equilibrium model of soil heat and moisture dynamics during fires” by William J. Massman', Michael D. Novak, 15 Jun 2020
- AC3: 'Response to Michael Novak', William Massman, 15 Jun 2020
RC4: 'Review', Anonymous Referee #3, 12 Aug 2020
- AC7: 'Response to second anonymous reviewer (RC4)', William J Massman, 20 Aug 2020

Peer-review completion

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

ED: Reconsider after major revisions (further review by editor and referees) (27 Aug 2020) by Nunzio Romano

Dear Author:
Your original submission received appraisals from three reviewers and comments from one discussant. The three reviewers evaluated this study quite well and also make suggestions aimed at improving the original paper. The discussant, Dr. M.D. Novak, was a bit more critical on some points of this study and raised concerns about a few ways you have tackled the problem at hand. However, you did not respond to the discussant’s comments. Anyhow, allowing for the reviewers’ rating and your preliminary replies I inform you that your paper can pass on the subsequent step under moderate/major revisions.

As editor in charge, below I report some points that deserve clarifications in your next point-by-point reply:

1) L. 290-297. Please, make more explicit the type of lower boundary condition you used in this study. I disagree with your statement that, I quote, “for field-based applications the lower boundary condition will never be known (or knowable without extraordinary effort before hand)” since definitely there are ways to have at least an idea of the type of condition one has to set at the lowe end of the flow domain. For soil-water flow, one might at least superimpose a unit gradient of the total water potential head or a zero flux if a compacted clay soil is located at a depth. Moreover, setting the lower boundary condition at a soil depth of 0.60 m does not solve the problem that this condition might affect the outcomes of model simulation. As some modelers also do, I suggest you should put the lower boundary conditions well below the flow domain of interest, for example, you can set the lower boundary condition at a depth of, let’s say, 1.0 m or 1.5 m, and then show the model results for the first 0.60 m of the soil profile. I’d appreciate it if you discuss this point in your reply and justify your choices.

2) L. 323. It seems there is a mix-up with the concept of residual soil-water content. The literature on this subject is also showing that the volumetric soil-water content takes on the zero value when the suction head attains a value of about 10^7 cm, and also should also account for film flow (e.g. see J. Hydrol., 2020, vol. 588 #125041).

3) L. 334. I suggest you should title this section as follows: “2.7 Hydraulic conductivity function”. Please also change the name of this function accordingly.

Please, upload a revised version of your manuscript together with a detailed point-by-point reply to all of the comments received so far from both the reviewers and the discussant. Please be advised that a subsequent round of referees’ evaluation might be required.

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AR by William J Massman on behalf of the Authors (09 Sep 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (02 Oct 2020) by Nunzio Romano

RR by Anonymous Referee #3 (16 Nov 2020)

Suggestions for revision or reasons for rejection

Thank you for the thoughtful revisions, they clearly improved the context of the manuscript. I only have a few smaller comments left and accept acceptance upon minor revisions.

Regarding fire types: thank you for the change, that looks good and I only have two small comments.

1. “On the other hand, prescribed slash pile burns, like the one studied here, are static fires that can heat the soil with 1-10 kWm􀀀2 for 10’s of hours (e.g., Massman and Frank 2004; Massman et al. 2008). Wildfires, and crown fires in particular, are dynamic and often fast moving and relatively brief by comparison to slash pile burns, but they can also be extraordinarily intense (10-100 kWm􀀀2/.”
This sentence may suggest that fire intensity directly translates into soil heating. This relationship between fire intensity and soil heating is however incredibly complex, and as the author probably knows a factor of not only residence time but also the degree of contact between the fuels and the soil, the presence of any wet litter layers, and the soil properties. While fire intensity may be positively related to soil heating in slash pile burns (I did not study this so I do not know whether that is indeed the case), it is certainly not the case for moving fire fronts. Please revise this sentence.

2. “This is the worst of all possible situations because whatever impacts the dynamic
portion of the fire may have had on the soil will only be made much worse (at least to the area surrounding the point of contact).”
This sentence suggests that fire has a negative impact on the soil, while this may not be the case. The sentence is also rather cryptic (what is it really about?). I suggest removing this sentence, or otherwise rephrasing it.
- Applicability to moving fire fronts, and the fact that the model was calibrated to moving fire fronts as well: good to hear that the model was calibrated to multiple fires. I think it would be valuable to mention that you did this and say something general about those findings, just so it is clear that your analysis went broader than just this single fire. I would be satisfied if you then include [data not shown], or if the editor prefers to include these data in supplementary material.
- Original line 513 and change made thereto. This looks very good. I only have one small comment about the crown fire sentence (“Intermediate between these two extremum case is the
- relatively fast moving, but extremely high intensity, crown fire”), which suggests that the degree of fire-soil contact in a crown fire is similar as in a grass or shrub fire. Yet while a sustained crown fire does need to be ‘fed’ by a moving surface fire, what I understood from fire scientist colleagues is that the heat of the fire is often more at a distance from the soil, so that the soil heating may actually be less. There is very little research on these relationships however, but I would argue this is enough information to be very careful in framing that a crown fire may have more impact on soil heating than a shrub fire.
- My compliments on getting the FAO soils description, I know how challenging soil conversions from USDA to FAO can be.
- Regarding information about duff: this would be great to add indeed (like the thickness, wetness, and whether it was litter (recognizable) or duff (unrecognizable plant parts))! Especially if you can indicate whether or not that affected your findings and what you did with that information in your model (was it a separate layer? I am sure the thermal properties are rather different from mineral soil).
- Measurement depth mystery (line Lines 520-523): I suggest adding that you checked the depth after taking the sensors out – I’ve installed many thermocouples and even with short ones the measurement depth can sometimes be really off (sometimes even by 8 cm for a very carefully inserted 10-cm long thermocouple – it got me totally puzzled but I think it was because of microtopography above the sensor and an unfortunate angle to the thermocouple). So if you can include that you ruled a measurement depth error out that would be valuable here.

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ED: Publish subject to minor revisions (review by editor) (28 Nov 2020) by Nunzio Romano

AR by William J Massman on behalf of the Authors (02 Dec 2020) Manuscript

ED: Publish as is (27 Dec 2020) by Nunzio Romano

AR by William J Massman on behalf of the Authors (31 Dec 2020) Manuscript

Short summary

Increasing fire frequency and severity now poses a threat to most of the world's wildlands and forested ecosystems and their benefits. The HMV (Heat–Moisture–Vapor) model is a tool to manage fuels to help mitigate the consequences of fire and promote soil and vegetation recovery after fire. The model's performance is surprisingly good, but it also provides insights into the existence of previously unobserved feedbacks and other physical processes that occur during fire.