Numerical modelling of permafrost spring discharge and open-system pingo formation induced by basal permafrost aggradation

Hornum, Mikkel Toft; Hodson, Andrew Jonathan; Jessen, Søren; Bense, Victor; Senger, Kim

doi:https://doi.org/10.5194/tc-14-4627-2020

Articles | Volume 14, issue 12

https://doi.org/10.5194/tc-14-4627-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/tc-14-4627-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 14, issue 12

Research article

|

21 Dec 2020

Research article |

| 21 Dec 2020

Numerical modelling of permafrost spring discharge and open-system pingo formation induced by basal permafrost aggradation

Mikkel Toft Hornum, Andrew Jonathan Hodson, Søren Jessen, Victor Bense, and Kim Senger

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Final revised paper (published on 21 Dec 2020)
Supplement to the final revised paper
Preprint (discussion started on 12 Feb 2020)
Supplement to the preprint

Interactive discussion

Status: closed

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

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- Supplement

RC1: 'Comment on ``Numerical modelling of permafrost spring discharge and open-system pingo formation induced by basal permafrost aggradation''.', Melissa Bunn, 11 Mar 2020
- AC1: 'Answer to comments on ms by Referee 1', Mikkel Toft Hornum, 22 May 2020
RC2: 'Review', Anonymous Referee #2, 08 Jun 2020
- AC2: 'Answer to comments on ms by Referee #2', Mikkel Toft Hornum, 09 Jun 2020

Peer-review completion

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

ED: Publish subject to revisions (further review by editor and referees) (11 Jun 2020) by Peter Morse

AR by Mikkel Toft Hornum on behalf of the Authors (15 Jul 2020) Author's response Manuscript

ED: Publish subject to revisions (further review by editor and referees) (29 Jul 2020) by Peter Morse

ED: Referee Nomination & Report Request started (30 Jul 2020) by Peter Morse

RR by Melissa Bunn (11 Aug 2020)

Suggestions for revision or reasons for rejection

The following minor revisions are offered for the consideration of the authors:
Line 53-54: Heat transport and groundwater flow
Table 1 – Does the Asterisk in Site/*Event have a purpose, is there a note missing?
Line 258: In discussing the RMS error for the heat flow model validation, opposed to reasonable accuracy, could it be stated that it is an acceptable level of accuracy for the given purpose.
Line 333: The properties were that of THE Janusfjellet Group.
Line 353: would tidal flat be more appropriate relative to Fjord, given the extent of the model boundary?
Line 360: were assigned no-flow conditions
Line 447 to 448: Partial sentence left over?
Line 449: Two exceptions to this result were the minimum…
Line 468: and the lack of discharge at the up-valley….
Line 479: all of this outflow discharge to the fjord.
Line 479: Instead of referring to the water flux that is redistributed to subpermafrost liquid water as “outflow to permafrost thaw” perhaps it would be clearer to explain this as a phase change and redistribution to storage opposed to an outflow, as it does not leave the model domain.
Line 480: Larger proportions of outflow to the fjord were simulated…..
Line 520: low-permeability
Line 590: Negelecting this effect represents an uncertainty in the simulation results,….
Line 600: These conditions should be taken into account….
Supplement S2 – Figure S5: The higher measured hydraulic conductivity values do not seem to fit the Weibull distribution presented. Were other distributions presented, with this distribution yielding the best fit? If so, it may be useful to include a statement indicating such. Was a log-normal distribution (using the negative log of hydraulic conductivity) attempted?

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RR by Anonymous Referee #2 (02 Oct 2020)

Suggestions for revision or reasons for rejection

The authors have made many edits to address my points in the previous review. I have the following suggestions for the revised manuscript.

L25-27: I suggest rewriting/replacing the last two sentences and try and make it more impactful. I think the interesting implication is how methane emissions are affected by the processes investigated in this paper, or I suppose what can be inferred about methane from the processes. The introduction also set up this implication. Also, give that this is an unusal hydraulic system, where would you expect to find other sites like it?

L210: Maybe clarify “by the eye” a bit. Was any quantitative measurement involved, or is this just a guess? Perhaps add an uncertainty range to indicate a potential range, since this probably is an unprecise method of estimation.

L431: When using terms such as “surprising” it should also be clarified why the results are “surprising”.

L418: explain what labels were used to identify the pingo springs in panel 1c.

L473: I would rephrase: It is the model that shows that outflow only take place if hydraulic pressure is artesian, the colour fill and figure 8 is only visualizing the model result. So the figure text says that red is artesian flow, and blue is not. But several figures have both (1a, 2a, 3a). Clarify that the pressure needs to be artesian at the pingos. I guess this is the case. I think converting the pie charts into bar charts where the data is grouped based on simulation and the artesian conditions may be more effective, but perhaps this is an issue of taste so I leave it for the authors to consider.

L487: Clarify what distribution in the term “outflow distribution” refer to

Figure 8: I still think there is too much information in one figure, but I guess the authors disagree. Pie charts are typically not recommended for scientific data, since it is difficult to discern magnitudes. Bar charts are typically more effective than pie charts. Of course, the authors have provided the actual data in the pie charts, but that adds clutter and information overload.

Figure 10. Add a line that the figure is using the same structure, legend etc. as Figure 8.

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ED: Publish subject to minor revisions (review by editor) (05 Oct 2020) by Peter Morse

AR by Mikkel Toft Hornum on behalf of the Authors (26 Oct 2020) Author's response Manuscript

ED: Publish subject to minor revisions (review by editor) (28 Oct 2020) by Peter Morse

AR by Mikkel Toft Hornum on behalf of the Authors (03 Nov 2020) Author's response Manuscript

ED: Publish subject to technical corrections (05 Nov 2020) by Peter Morse

AR by Mikkel Toft Hornum on behalf of the Authors (09 Nov 2020) Author's response Manuscript

Short summary

In Arctic fjord valleys, considerable amounts of methane may be stored below the permafrost and escape directly to the atmosphere through springs. A new conceptual model of how such springs form and persist is presented and confirmed by numerical modelling experiments: in uplifted Arctic valleys, freezing pressure induced at the permafrost base can drive the flow of groundwater to the surface through vents in frozen ground. This deserves attention as an emission pathway for greenhouse gasses.