|The manuscript deals with the modelling of rock glacier ice content based on InSAR-derived surface velocity in 5 active rock glaciers of Khumbu and Lhotse Valleys, northeastern Nepal. These estimates rely on empirical ice content and kinematic data drawn from three rock glaciers in the Swiss Alps. Modelled ice content in the five rock glaciers of interest are then applied to an existing inventory of active rock glaciers in Nepalese Himalaya.|
I have read with interest the reviewers’ comments, the authors’ responses and the revised manuscript. The authors have done an excellent job in their point-by-point replies. The revised manuscript shows an extensive effort made to address all of the reviewers’ concerns. Although some of the main objections raised by Dr. Arenson remain unsolved, the revised/rewritten discussion acknowledges most of the limitations adequately. In this regard, the upscaling procedure to estimate water storage from the five study rock glaciers to the entire Nepalese Himalaya represents quite a leap, and therefore inherent uncertainties could be described in a more explicit and systematic way.
In particular, the authors could enrich their state-of-the-art by adding reference to recent work on the uncertainties involved in the compilation of rock glacier inventories on optical imagery, and on Google Earth (GE) in particular: (e.g., Schmid et al., 2015; Jones et al., 2018b; Brardinoni et al., 2019; Way et al., 2021). Uncertainty derives from: (i) the spatial resolution of optical imagery and cloud cover, which in GE vary greatly across a given region; (ii) the mapper (experience, training and personal interpretation); (iii) rock glacier typology (e.g., lobate, tongue-shaped, and multilobe polymorphic).
Uncertainty applies to: (1) identification of rock glaciers; (2) delineation of rock glacier outline, whose inter-operator variability will affect the rock glacier area, hence the estimated ice/water content; and (3) dynamic classification of the rock glacier (active, inactive and relict), which will affect the number of rock glaciers for which ice/water content is estimated (Brardinoni et al., 2019; Way et al., 2021). Variability in point 2 between mappers has been shown to vary greatly depending on rock glacier type. Uncertainty in point 3, including inter-operator variability, can be reduced greatly by incorporation of InSAR-based kinematic attribute, following a protocol tested on 11 regions across the globe (Bertone et al., 2021).
Bertone, A, Barboux, C, Bodin, X, Bolch, T, Brardinoni, F, Caduff, R, Christiansen, H H, Darrow, M, Delaloye, R, Etzelmüller, B, Humlum, O, Lambiel, C, Lilleøren, K S, Mair, V, Pellegrinon, G, Rouyet, L, Ruiz, L, and Strozzi, T. Incorporating kinematic attributes into rock glacier inventories exploiting InSAR data: preliminary results in eleven regions worldwide. The Cryosphere Discuss. [preprint], https://doi.org/10.5194/tc-2021-342
Brardinoni F, Scotti R, Sailer R, and Mair V. 2019. Sources of uncertainty and variability in rock glacier inventories. Earth Surface Processes and Landforms, 44, 2450-2466.
Jones et al 2018b (already in reference list)
Schmid MO, Baral P, Gruber S, Shahi S, Shrestha T, Stumm D,Wester P. 2015. Assessment of permafrost distribution maps in the Hindu Kush Himalayan region using rock glaciers mapped in Google Earth. The Cryosphere 9(6): 2089–2099.
Way RG et al., 2021 Consensus-Based Rock Glacier Inventorying in the Torngat Mountains, Northern Labrador. American Society of Civil Engineers Proceedings. Regional Conference on Permafrost and the 19th International Conference on Cold Regions Engineering. https://doi.org/10.31223/X5C60W
With reference to the five rock glaciers in Khumbu and Lhotse Valleys, and the three rock glaciers from Switzerland, please consider adding an attribute in Tables 1 and 4 to characterize rock glacier typology (e.g., talus lobate, debris tongue-shaped, or others) so that the reader can compare area, width, slope, but also typology. Perhaps you could acknowledge briefly that the three rock glaciers in Switzerland (lines 60-61) are substantially smaller than the five selected in Nepal.
Since you are extrapolating your modelling results to Nepalese Himalaya, please consider: (i) justifying briefly the selection of those valleys and the five rock glaciers in particular; (ii) describing where the average size of your five rock glaciers plots (percentile) within the size distributions of rock glaciers across Nepalese Himalaya. The latter would allow the reader to understand where the five sample rock glaciers stand compared to the regional population.
InSAR methodology: please describe how movement along LOS was projected to the line of maximum slope, adding relevant reference (e.g., Bechor, NB and Zebker, HA. 2006. Measuring two‐dimensional movements using a single InSAR pair. Geophysical Research Letters, 33(16)). Please specify whether the projection was conducted systematically or was limited to pixels with slope below a given threshold. When the angle between LOS and line of maximum slope is high (>60°), projecting may amplify InSAR related errors.
Other minor comments:
Line 434: please consider adding the following reference:
Scotti R, Crosta G B, and Villa A. 2017. Destabilisation of Creeping Permafrost: The Plator Rock Glacier Case Study (Central Italian Alps): The Destabilised Plator Rock Glacier. Permafrost and Periglacial Processes, 28(1), 224–236.
Lines 439-440: please consider removing the following sentence: “Rock glaciers showing strong subsidence indicators from optical images, such as surface depressions or cracks, are not suitable for the current method”. It defeats the purpose of using InSAR data. Interpretation of vertical surface deformation (e.g., subsidence) based on morphologic features observed on optical images is unreliable and potentially misleading.
Line 450: please consider modifying the citation of the IPA report, currently referred to as “Delaloye & Echelard, 2020”, with the “How to cite” indication contained in the updated version of the document: “RGIK, 2021” and in the reference list as: “RGIK (2021). Towards standard guidelines for inventorying rock glaciers: baseline concepts (version 4.2.1). IPA Action Group Rock glacier inventories and kinematics (Ed.), 13 pp.”. This effort involved a broad international working group.
I enjoyed reading the thread of revisions and look forward to seeing the paper published.