Investigating seasonal and multi-decadal water/ice storage changes in the Murtèl rock glacier using time-lapse gravimetry

This article describes a novel use of time lapse gravity to assess water storage changes from the active layer of a rock glacier in Switzerland. The paper is well written, has a comprehensive description of methodologies, and cites appropriate literature.

General comments for authors:

1. Only 8 gravity stations were reoccupied on and around the rock glacier for this study. Having more measurements would have provided more statistical significance to the interpretations of storage changes in different areas. Can the authors provide an explanation as to why only 8 stations? Can they recommend including more stations in future studies?
2. Are there historical repeat lidar surveys that could help determine rates of subsidence or uplift of the rock glacier over time?
3. Figure 2 and Figure 3 could be improved by annotating the station numbers

Detailed comments:

1. Page 2, line 46. I don’t understand why there are 2 numbers for ice storage changes. Are these minimum and maximum changes over a period of years? The authors just need to clarify this a bit better.
2. Page 2, Line 54. Seismic is a broad category. Do the authors mean seismic refraction, seismic reflection, passive seismic monitoring? Should clarify.
3. Page 3, line 88. Is “slightly continental” the correct technical term? Consider rephrasing for accuracy
4. Page 3, Line 91. I had trouble understanding what “permafrost-underlain” meant. Consider rephrasing to “Surficial material within the small (30 ha), non-glaciated Murtel catchment consists of unvegetated debris (on rock glaciers and talus slopes) and bedrock, with occurrences of permafrost”.
5. Page 4, Line 94. What was measured in the bedrock depression? Consider rephrasing to “…depression and the buried bedrock topography has been mapped with a multi-geophysical…”
6. Page 4, line 97. For clarity, consider changing “…that its thickness…” to “…that the AL thickness…”
7. Page 4, Line 105. For clarity, consider changing “… distance, share this three-part stratigraphy” to “…distance, intercept this three-layer stratigraphy”
8. Page 6, Line 135. How were the gravity observation points marked? With nails into rock? Spray paint? Please include this detail to help others trying this.
9. Figure 2. Label the black dots with the station numbers.
10. Page 10, line 237. Could the authors not calculate the snow volume change by subtracting the July DSM from the September DSM?
11. Page 10, line 262. Change “changes in the” to “changes were in the”
12. Page 10, line 263. Add the detail that the -15.2 uGal change was for MUERTEL05
13. Page 10, line 264. Would “terminus” be a better word than “front”?
14. Figure 3. Add directions “S” and “N” to the plot in (a) to make it easier for the reader and label the 2024 points with the station numbers in both (a) and (b).
15. Page 14, Line 295. Change “comparisons on a relative” to “comparisons to the 2024 data on a relative”
16. Page 14, line 296. Change “with an offset” to “with a DC offset”
17. Page 14, line 302. I did not see the ~200 uGal increase as described. Can this be annotated on Figure 3? Does it increase between the July and September surveys or from the smoothed trend of the 1991 points? Some clarification is needed here.
18. Figure 5. What are the dashed blue lines? Can you add this extra detail in the figure caption?
19. Page 18, line 377. Change “aproportion” to “proportion”
20. Page 19, line 389. Instead of neighbouring ‘regions’, can you be more specific and say neighbouring ‘catchments’?
21. Caption to Table 3. Change “environment potentially affecting” to “ environments that potentially affect”
22. Page 22, Line 464. Do the authors mean elevation instead of ‘altitude’? Altitude is height above the local ground surface

The paper concerns the novel use of time-lapse microgravimetry to investigate changes in water/ice storage in an already well studied rock glacier. The paper reads well, the topic is well introduced, and somehow becomes a review paper, thanks to the relevant and correct reference list. Some parts are perhaps too extended, as in Chapter 6.1, where the discussion is processed. I think that one of the main stability constraints remains in the gravimetric reference point near the site. This strengthens the results of the authors, but on the other hand may strongly limit the application of such a methodology elsewhere. This aspect is mentioned, but not emphasised as it should be, especially in chap. 6.3.

My main concern relates to the comparison with data from the 1990s. Although the authors correctly point out the great uncertainty of such an operation, I believe that this approach remains a great danger. Even though preliminary corrections are well explained, a simple but crucial argument like atmospheric pressure conditions is not considered. The authors rely on the atmospheric isolation of the CG-6 for their time series, but the same cannot be said for the data of the 90s. This aspect can strongly influence the resulting comparison and should at least be mentioned in the text. My suggestion is to limit the relevant results to the 2024 double surveys, avoiding an ambitious multi-decadal approach. This would also significantly shorten the paper and reinforce the message of model TLG potentials by focusing on the authors' relevant results. 

Minor comments below:

Chap.2 the description of the site stratigraphy 91-95 is not clear and should be rewritten.

Chap 3.1 statement on ice melt/gravity reduction needs to be better introduced, as you correctly state that several processes can explain the same results. I would avoid presenting this aspect here.

Chap 3.2 ln 133 <5h20m ?

Ln 135 what it means manually measured ?

Chap. 3.3 Atmospheric pressure is not given, neither for the historical nor for the recent measurements. I suppose the authors rely on the fact that cg-6 should be compensated, but they discuss temperature and tilt drift in detail without ever mentioning barometric issues. This should be clarified.

Chap 3.4;3.5;5.2;6.2 As explained, I find this temporal comparison too ambitious for quantitative estimation due to the large unresolved uncertainties. 

Chap 5 Before presenting the results, the authors should logically present how they installed the gravimeter, as this is a crucial part of gravity measurements. For example, was the gravimeter installed on blocks? Digging snow? How was it fixed? Some pictures might help the readers, especially the non-experts.

LN270-280 Again, no corrections for atmospheric changes are given.

Ln 313 typo (-)

Ln 328 double brackets

Chap 6.4 In this clear hydrological presentation, talking about permafrost aquitards, I would also add https://doi.org/10.5194/tc-17-1601-2023

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GENERAL COMMENTS

The manuscript presents an application of microgravimetry to estimate seasonal and long-term changes of water mass within a rock glacier. Using a case-study example, the authors present creative and innovative methods of data acquisition and analysis, which have the potential to provide a new tool for alpine permafrost research in broader regions around the world. The study will make significant contribution to advancing our understanding of rock glaciers and other permafrost landforms. The manuscript is very well written and organized. The data analysis is rigorous and figures are of high quality. I have a few minor suggestions to improve the clarity (please see below for specific comments). One important issue that needs a bit more careful attention is the attribution of observed mass changes to solid and liquid water. I feel that the authors dismiss the contribution of liquid water too casually. I will elaborate more on this in my specific comments below.

SPECIFIC COMMENTS

Line 97. Its thickness. Does this refer to the thickness of the rock glacier or the active layer? I had to stop and think for a moment. Please explicitly state it.

Line 113. The reader expects the ‘Method’ section here. I later discovered that both 3 and 4 are describing methods. Please change the section title to ‘3  Methods: Gravimetry’ and ‘4  Methods: Snow ….’.

Line 216-218.  Usually, there is a significant correlation between depth and density of snowpack. Therefore, depth-dependent snow density function is commonly used in hydrological studies. It becomes evident later that the effect of snow is secondary to gravity measurements, but at this point the reader does not know how (in)significant it is. Please add a sentence to justify why a single value of density is used in calculation.

Figure 2. Please annotate ‘01’, ’02, … in Figure 2a, so the reader does not have to go back and forth between Figure 1, Figure 2, and Table 1.

Figure 3. Please annotate ’05’, ‘06’, … in Figure 3b.

Line 314. The authors attribute the ‘dominant’ gravity-change signal to ice storage. This is not unreasonable, but the justification is not convincing. Changes in the water table of supra-permafrost groundwater has been observed at numerous locations around the world, both in alpine and subarctic environments. Also, a quick glance of Figure 3 gives the impression that 08 is not likely underlain by ground ice. Please add a few sentences here to present a convincing justification for dismissing liquid water storage. The authors can also refer the reader to Figure 5h.

Line 374-376. Groundwater export … is expected to be rather limited. From an objective reader’s viewpoint, this sentence contradicts with Figure 5h, which clearly shows the drainage of groundwater. It is true that unweathered granodiorite has low hydraulic conductivity, but overlying sediments may have high enough conductivity, or the top few meters of granodiorite may be weathered. Please present more careful discussion of solid vs. liquid storage of water in both supra- and sub-permafrost zones.