Review of the manuscript “Pressurised water flow in fractured permafrost rocks revealed by joint electrical resistivity monitoring and borehole temperature analysis” submitted for publication in The Cryosphere by Maike Offer, Samuel Weber, Michael Krautblatter, Ingo Hartmeyer, and Markus Keuschnig.

This reviewer has expertise in permafrost field observations, numerical modelling of frozen soil, and (to a lesser degree) permafrost geophysics.   

This manuscript presents a unique dataset of repeated ERT, borehole temperature observations, and site characterization in steep permafrost rock. The combined dataset is beautifully presented and affords insights into the evolution of frozen, thawed, and wet zones in the rock. The careful design of temperature observations allowed detecting fast thermal events at depth that are attributed to water infiltration. These are important topics for research in the context of better understanding permafrost moderated climate control on rock instability. 

The manuscript did not convince me that the data revealed pressurized water as stated in the title. The authors support this inference by mentioning piezometric measurements from late summer 2023 (which are not shown or referenced) and the assumption (which is not developed in detail) that pressurised water flow explains the observed rapid electrical resistivity decline. While I am enthusiastic about the data and many of the analyses presented, a clearer focus, structure, and methodology are required for publication. I recommend encouraging resubmission of this manuscript after adjusting focus and conceptual clarity. 

• Water flow in fractured permafrost rock has been investigated, and detected with ERT, previously. This study adds to the body of knowledge incrementally. Confident detection of pressurized flow would indeed make it a novel and significant contribution. A more detailed analysis of the thermally detected flow events could likewise be interesting.
• The specific objectives of the research are not clearly articulated. Consequently, the exact state of the art is unclear, the approach and methods cannot be judged in their appropriateness, and the conclusions are not as compellingly underpinned by the evidence presented as they could be.
• Line 300: The cause of the thermal offset stated appears to be speculation. It can equally be explained by transient effects or lateral variation of surface temperature – and neither require invoking thermal effects of water flow.
• Figures 5 and 6: There are strong temporal trends that would provide important background information. Consider showing a depth profile of temporal trends in mean (and maximum?) temperatures over the entire measurement duration. This will help contextualize Figure 4 and its decadal gap.  
• Line 313: The authors state that obstacles exist for interpreting ERT profiles in fractured rock masses based on laboratory measurements on intact samples (an error). Section 5.1 argues that the differences between lab and field point to pressurized water flow (a signal). Can error and signal be distinguished with sufficient confidence? Explain how.
• Line 21: Is permafrost thaw a hazard?
• Section 4.2: What is the impact of using summer ERT that has been measured ten years after the profiles in other seasons? Are we interpreting the influence of seasons or a decade of atmospheric warming (see Figure 6)? This needs to be addressed clearly.
• Section 5.3: Some of the statements seem rather confident. They could be shortened and made specific to well supported conclusion and, as such, added as a short outlook paragraph to the conclusion. Some of the other text in the section is better suited for the introduction of a paper.
• The manuscript text should be shortened and edited for clarity in structure and arguments. Some of the referencing could be tightened, giving preference to one good reference backing up a particular argument instead of listing a handful of publications.

The paper entitled “Pressurised water flow in fractured permafrost rocks revealed by joint electrical resistivity monitoring and borehole temperature analysis” presents a combination of repeated electrical resistivity tomography (ERT) data and borehole temperature data on a high mountain rock wall site in Austria to discuss potential effect of pressurised water on rock wall temperature and destabilization.

The paper addresses an important topic in high alpine permafrost and geomorphology community that is the characterization of water flows and their impacts for permafrost dynamics and morphodynamics. Addressing this topic is challenging due to the difficulty to observe and measure water flow processes in high mountain and the non-linearity of the related processes. Geophysical approaches are for sure on eof the most promising method to investigate these processes.

Overall, the paper is well written and well structured. The ERT dataset is also quite unique and was gained through challenging field work. The figures are very nice and clear. However, I find some major limitations and I would recommend publication after major revisions.

GENERAL COMMENTS

One of the major issue is that some of the main findings that are reported are not appropriately demonstrated (see further comments). Furthermore, in the current state, I find it difficult to understand what is the novelty of the conclusions of this paper that echoes a former paper from Keushing et al. (2017). Therefore, I am not convinced by the last sentence of the abstract, especially by the expression “shows for the first time”.

INTRODUCTION/ STUDY SITE

Since the core of the paper is about water infiltration I would suggest to better introduce water infiltration in rock slopes (see Hasler et al., 2011a, Ben-Asher et al., 2023) and in mountain permafrost ground in general.

A climate and weather analysis during the measurement period would be highly welcome in the site description, especially for discussing the results afterwards (see further comments). An option would also be to make a general section about Study site and instrumentation as I missed some information about the boreholes (depth, available time series…) and the ERT system (length, number of electrods…).

I also wondered why the ERT data and temperature data are not directly compared and discussed since the resistivity values could be used to infer temperature values based on the lab results.

METHODS

Has tap water some implications on the freezing point? Duvillard et al. (2021) showed that it has a different freezing temperature than snowmelt water that is more representative of the natural environment. I would have first presented the field before the laboratory calibration approach as the latter completes the former.

The ground contact resistance is not presented while it is a major parameter of the measurement as explained by Herring et al. (2023). But some datasets have huge RMS and this could be partly due to poor contact resistance. This part of the work has to be described and addressed.

From L175, images of the rock discontinuities are mentioned but not displayed in any way. That is a pity because the paper attempts to link ERT data to rock discontinuity data. That would be interesting to better show these data.

The calibration is based on an intact rock sample while the paper focuses on specific processes of fractured rock. The fractures might not be entirely filled with water or ice and this is not discussed. The signal of air and ice is the same, and this needs to be discussed and clarified. This means that the results must be considered with caution as well.

Rather than number of electrods, I would find it more convenient to speak in terms of distance along the profile (see also comment on the lack of information on the profile length).

The calculation of the thermal anomalies must be clearly detailed in the Methods section as this is a central part of the investigation.

RESULTS

Looking at Figure 4, I wonder how the results from Sep/June 2013 and Sep/June 2023 can be so different? Why don’t we see the summer signal reaching 10 m depth in early winter? Could the top part of the profile with relatively high resistivity values during the thawing season could be attributed to desiccation (see also comment on air signal)? The decadal permafrost change could be detailed and discussed to take full advantage of the presented data.

DISCUSSION

The contradiction with the Archie law is weak as the law is not presented nor discussed in the paper. The same is true with the piezometer data. That is a pity to mention such data without using them extensively nor showing them. L286-288: I do not fully agree with the statement “high impact … on thermal processes”. The study shows only short term and minor temperature changes, but great changes in the electrical resistivity that is by essence strongly sensitive to water changes. I would suggest using more balanced wording or to strengthen the demonstration.

Another point that comes to my mind is the effect of anisotropy in such type of rock with a high degree of schistosity. This could be at least discussed and ideally investigated through lab measurements.

CONCLUSION

The first point rather reminds the initial hypothesis than bringing a demonstration of its validation. In my opinion, the 3^rd and 4^th points are not demonstrated in the paper.

Detailed comments

• Abstract L1: failures do not occur from permafrost itself as permafrost is by definition a temperature, rather use permafrost ground or permafrost-affected slope.
• L 140: what “representative” means here?
• L 148: do you mean average values? (positive values for all days)
• L 195: here consider the comment about air and ice signal
• L218: where do we see the mentioned zero-curtain? This is crucial to see it and how long it lasts as it provides an information about the ice content.
• L 223: which construction activity are you talking about?
• L225: “thermal offset” is not an appropriate concept for rockwalls, see Hasler et al., 2011b
• L227-228: the explanation of the “thermal offsets” is not clear
• L230: calculation of these abrupt changes must be clearly explained in the method section
• L233: how is this threshold of values defined?
• L296-297: and what about air?

REFERENCE

I suggest to have a look at Hasler et al. 2011b when discussing “thermal offset” that is not an appropriate concept for steep mountain slopes.

I suggest also to consider Cathala et al., 2024 to link pressurized water to rock slope destabilization.

Ben-Asher, M., Magnin, F., Westermann, S., Bock, J., Malet, E., Berthet, J., Ravanel, L., and Deline, P.: Estimating surface water availability in high mountain rock slopes using a numerical energy balance model, Earth Surface Dynamics, 11, 899–915, https://doi.org/10.5194/esurf-11-899-2023, 2023.

Cathala, M., Magnin, F., Ravanel, L., Dorren, L., Zuanon, N., Berger, F., Bourrier, F., and Deline, P.: Mapping Release and Propagation Areas of Permafrost-Related Rock Slope Failures in the French Alps, https://doi.org/10.2139/ssrn.4522860, 27 July 2023.

Cathala, M., Bock, J., Magnin, F., Ravanel, L., Ben Asher, M., Astrade, L., Bodin, X., Chambon, G., Deline, P., Faug, T., Genuite, K., Jaillet, S., Josnin, J.-Y., Revil, A., and Richard, J.: Predisposing, triggering and runout processes at a permafrost-affected rock avalanche site in the French Alps (Étache, June 2020), Earth Surface Processes and Landforms, n/a, https://doi.org/10.1002/esp.5881, 2024.

Duvillard, P.-A., Magnin, F., Revil, A., Legay, A., Ravanel, L., Abdulsamad, F., and Coperey, A.: Temperature distribution in a permafrost-affected rock ridge from conductivity and induced polarization tomography, Geophysical Journal International, 225, 1207–1221, https://doi.org/10.1093/gji/ggaa597, 2021.

Hasler, A., Gruber, S., Font, M., and Dubois, A.: Advective Heat Transport in Frozen Rock Clefts: Conceptual Model, Laboratory Experiments and Numerical Simulation, Permafrost and Periglacial Processes, 22, 378–389, https://doi.org/10.1002/ppp.737, 2011a.

Hasler, A., Gruber, S., and Haeberli, W.: Temperature variability and offset in steep alpine rock and ice faces, The Cryosphere, 5, 977–988, https://doi.org/10.5194/tc-5-977-2011, 2011b.

Herring, T., Lewkowicz, A. G., Hauck, C., Hilbich, C., Mollaret, C., Oldenborger, G. A., Uhlemann, S., Farzamian, M., Calmels, F., and Scandroglio, R.: Best practices for using electrical resistivity tomography to investigate permafrost, Permafrost & Periglacial, 34, 494–512, https://doi.org/10.1002/ppp.2207, 2023.

Keuschnig, M., Krautblatter, M., Hartmeyer, I., Fuss, C., and Schrott, L.: Automated Electrical Resistivity Tomography Testing for Early Warning in Unstable Permafrost Rock Walls Around Alpine Infrastructure, Permafrost and Periglacial Processes, 28, 158–171, https://doi.org/10.1002/ppp.1916, 2017.