Response of the Thick and Thin Debris-Covered Glaciers between 1971 and 2019 in Ladakh Himalaya, India—A Case Study from Pensilungpa and Durung-Drung Glaciers

Round 1

Reviewer 1 Report

Dear authors,

It is a well-written study, however, additional information is needed regarding the methodology (calculation of errors and uncertainties), results and discussion on the impact of debris cover on glacier changes as this aspect is not well-researched and is speculative due to other factors which strongly influence the mentioned glacier changes. It is of course quite a complex goal to evaluate glacier changes, main drivers and contributing factors, thus your study is important.

I have all my comments in pdf file. Here I mention only the most important.

The core part of the abstract consist of just too much of numbers. These are important results of course, but the aim of the abstract is to encourage the reader to read it, thus such details are redundant. Please shorten and give some summary, not all the numbers.

For this study, the distribution and thickness of debris cover if very important but here and further I miss more detailed information about the debris cover. For example in Table 8 you have data on ice thickness change at various elevation ranges, it would be beneficial to add information in this table of debris cover, are glaciers on these elevation ranges covered with debris?

Also what about debris thickness, are there any data on this? How the debris thickness change with the altitude?

You could provide information on the change of debris cover (at least area) through study years from remote sensing data.

You used the data of various resolutions to obtain the ice margin retreat, what about uncertainties, especially for Google Earth imagery, which was not co-registered with your satellite scenes?

You do not provide the methods for the error estimations in your paper.

Why you did not use also the position of the snow line which could be delineated from satellite images?

Glacier volume estimation: you present several formulas on how to determine the ice thickness, there are also several parameters  - what value of factor c and Y did you use, and how did you choose it? Also, what is the uncertainty of all these estimates? Do you have any data like GPR measurements to validate them?

Do you have glaciological mass balance measurements from stakes to validate the proposed empirical relationship of mass balance? How accurate are the used relationships?

Please provide information on the uncertainties after the registration of DEMs.

It seems that there is quite a large error of terminus retreat field measurements compared to remote sensing, why is that? How did you estimate the errors?

There is quite a large difference between the mass balance estimated with different approaches, which rises questions about the validity of these approaches for these glaciers. Can you validate these results against field measurements and geodetic mass balance? Why did not you calculate the geodetic mass balance if you have at least two DEMs?

I did not understand why you estimate surface elevation change only along longitudinal profiles but did not produce DEMs of difference and did not calculate the geodetic mass balance.

Line 409: Many other studies have concluded this in other regions of the world as well, you should cite them as well and discuss the relation to debris cover as here you have only one reference. It is also interesting, is this phenomenon similar in all glaciated regions or do glaciers of the Himalayas differ somehow?

There is also still the question of how the geometry affects glacier changes, one glacier is much bigger, so is it its geometry or debris cover that plays the major role? What about other factors - slope, subglacial topography, thermal structure?

I suggest extending the discussion about all the mentioned issues.

Lines 430-458: in the paragraph, you have listed many results of studies but what are the broader consequences and causes of all these changes?

Lines 459-471: again you have listed many numbers about the changes of other glaciers. Is there a need to list them all in such detail, because it is not a discussion but a listing of results, you could shorten this part and discuss more the reasons for these changes and similarities with your studied glaciers. Are the mentioned glaciers from other studies debris covered? How important is the debris cover of your mentioned glaciers to their changes?

Line 515: the proglacial lake is very important factor that drives the frontal retreat and it again suggests that there are many other factors promoting the faster retreat of the largest glacier, thus the existence of debris cover and its impact on glacier change is very speculative in this study. When this lake appeared and did it change substantially during the study period?

Comments for author File: Comments.pdf

Author Response

Detailed Response to the Reviewer-1

Manuscript ID: sustainability-2084190,

Title: “Response of the thick and thin debris-covered glaciers between 1971 and 2019 in Ladakh Himalaya, India; a case study from Pensilungpa and Durung-Drung glaciers”.

Authored by: Manish Mehta*, Vinit Kumar*, Pankaj Kunmar and Kalachand Sain

Please note that the comments from the reviewer’s main concerns are stated as “RC” in black color, and our detailed responses are stated as author’s response “AR” in blue color.

Response

RC: It is a well-written study, however, additional information is needed regarding the methodology (calculation of errors and uncertainties), results and discussion on the impact of debris cover on glacier changes as this aspect is not well-researched and is speculative due to other factors which strongly influence the mentioned glacier changes. It is of course quite a complex goal to evaluate glacier changes, main drivers and contributing factors, thus your study is important.

AR: We sincerely thank the reviewer for identifying the manuscript potential and for appreciation for our work. We also thank the reviewer for his/her thorough review, constructive comments, and remarks on the manuscript. We concur with the reviewers and appreciate their efforts and compliment on this study. Such a review and editing, followed by comments and suggestions, is highly encouraging for young researchers. We also concur with the comments and suggestions provided by the reviewers. I hope the detailed response below will help present our understanding much better than earlier.

RC: Also, the name of the manuscript is already long, it asks for clarification - what kind of response? Response to climate change?

AR: Thanks for the comment. Yes, the name of the manuscript is quite long, but in this manuscript, we compared the response of two glaciers located in the same climatic zone that behaved differently between 1971 and 2019. The factors (debris cover, snout morphology, proglacial lake, and ice cliffs) responsible for the glaciers' variable response are assessed in this study. Therefore, the title of the manuscript was chosen to reveal the broad gist of the manuscript. However, we know that the climate is changing, and glaciers are affected due to the ongoing climate change. Still, here we are talking about the “Response of the glaciers in relation to the debris cover, snout morphology, and other factors.” Therefore, in this manuscript, we have tried to determine the role of multiple factors (debris cover, snout morphology, proglacial lake, etc.) influencing the frontal retreat and surface lowering of the glaciers.

RC: The core part of the abstract is just too much of numbers. These are important results of course, but the aim of the abstract is to encourage the reader to read it, thus such details are redundant. Please shorten and give some summary, not all numbers.

AR: Thanks for the constructive comments. We have reduced the abstract and simplified it as suggested by the reviewer.

RC: for this study, the distribution and thickness of debris if very important but here and further I miss more detailed information about the debris cover. for example in Table 8 you have data on ice thickness change at various elevation ranges, it would be beneficial to add information in this table of debris cover, are these elevation ranges or are not covered with debris? Also what about debris thickness, are there any data on this? How the debris thickness changes with the altitude? Also you could provide information on the change of debris cover (at least area) through these years from remote sensing data.

AR: Thanks for your comment. Yes, we know that the distribution and thickness of debris over the glacier are essential. In this regard, we have included one para (in the study area section) about the characteristic of debris cover over the PG and DDG. Table 8 shows the elevation-wise thickness lost by the glaciers estimated using SRTM and ASTER DEMs, so it is impossible to insert debris thickness data in this Table. Yes, the ablation of the PG glacier is covered by thick debris (30% of the ablation area), while the ablation zone of DDG is partially covered by debris or almost clean (5.5% of the ablation area). The debris thickness data is mentioned in the text in the study area section. The debris thickness distribution, though heterogeneous, largely follows a common pattern such that the debris thickness (a) increases from the central flowline toward the margins and (b) decreases with increasing distance from the snout (Anderson and Anderson, 2018). The debris thickness of DDG increases from the central flowline toward the margins, and the debris thickness of PG decreases with increasing distance from the snout. Yes, we agree with the reviewer that we could provide information on the change of debris cover from remote sensing data, but in this manuscript, we are talking about glacier change. If we focus on debris, there is a possibility to change the objectives of the manuscript. So we don't want to make significant changes to the manuscript.

RC: any references to this except your analyses?

AR: We have included the references.

RC: what is meant by hyphen (-) in the Table? Why is there no source for data except the first row and the same for purpose? Please wrote them even if they are the same.

AR: Thanks for the suggestion. We have corrected the table as suggested by the reviewer.

RC: In fig. 3. you have reference points, I suppose you mean these, please correct.

AR: Thanks for your comment and suggestion. We have corrected and replaced reference points instead of references station.

RC: You used the data of various resolution to obtain ice margin retreat, what about uncertainties, especially for Google Earth imagery which was not co-registered with your satellite scenes.

AR: Thanks for the comment. The data sets we have used are from different platforms, i.e. (i) remote sensing-based total terminus retreat from 1971 to 2019 was ~270 ±27 m at an average rate of 5.6 ±0.57m a^-1, (ii) Google Earth-based total terminus retreat from 2004 to 2017 was ~110.5 ±18.25 m at the average rate of 7.4 ±1.23 m a^-1 and (iii) field observations from 2015 to 2019 with glacier retreat of ~27 ±11.5 m at an average rate of 6.7 ±3 m a^-1. Yes, Google Earth images are not co-registered with satellite scenes because this work has been done with an online platform.

RC: Figure 4. it would be better to put years on these figures not only in the description. Figure 4. What is untitled map and text below? Please remove. And it would be better to zoom in glacier margin in figs. G and J.

AR: Thanks for the constructive comment. We have modified Figure 4 and removed all the discrepancies in Figures 4 G and J.

RC: Any details about the validation procedure, years? Not all reconstructed ELAs you could validated in the field. Why you did not use also the position of snow line which could be delineated form satellite images?

AR: Yes, we used 2016 to 2019 in-situ or calculated the ELA of PG for validation. The in-situ study of PG suggested that the ELA of the glacier fluctuated between 5215 and 5223 m asl during the year between 2016 and 2019. The mean ELA of the glacier was 5221 m asl [Mehta et al., 2021]. Whereas, in 2019, the estimated ELA of PG is well correlated with the in-situ (calculated) ELA. The annual snow line separates the snow-covered upstream (accumulation zone) from the (downstream) ablation zone. Still, in the case of Himalayan valley glaciers, the snow line gives a false illusion because avalanches cover the accumulation zone of the glaciers and the upper ablation zone. This area is covered with fresh snow or avalanches triggered during the monsoon in September and October. Hence estimated ELA will give a more accurate result than the snowline, so we calculated the ELA instead of the snowline.

 RC: you present several formulas how to determine the ice thickness, there are also several parameters - what value of factor c and Y did you use, how did you choose it? Also what is uncertainty of all these estimates? Do you have any data like GPR to validate them?

AR: Thanks for the comment. We have presented the value of factors c and Y in Table 10. We choose three sets of scaling parameters, which have been applied by Cogley (2011) and Frey et al. (2014) for the Himalaya–Karakoram region. This equation is also used for all ICIMOD glacier inventories. This equation was derived by Chen and Ohmura (1990), who used measurements from 63 glaciers to determine the related scaling parameters. Bahr and others (1997) derived the parameters in a theoretical study, and LIGG et al. (1988) established a thickness–area relation based on ice-thickness measurements on 15 glaciers. This method ultimately depends on the area of the glaciers; an uncertainty of 3% was shown for glacier inventory data of the Suru River basin by Shukla et al. (2020). Therefore, we assume a 3% uncertainty in the glacier area. No, we have no data for GPR to validate.

RC: Did you use all three or only your developed empirical relationship? It is not clear here. How do you know that the mentioned formulas work for your glaciers? You also have not shown any data on which you based your empirical relationship. Do you have glaciological mass balance measurements from stakes to validate this relationship? How accurate are these methods?

AR: Thanks for the constructive comment. We have used all three empirical relationships to calculate the mass balance of PG and DDG (Table 7). We have developed the empirical relationship between ELA and specific mass balance derived from the in-situ measurements of the 11 glaciers in the Indian Himalayas to estimate the glacier mass balance. The details of the glaciers are given in Table below.

Table Annual mass balance of the Himalayan glaciers in different regions of India.

Yes, we have glacier mass balance measurements data from stakes of one glacier (PG), and this data is correlated with modeled mass balance data. The average modeled mass balance of the PG glacier was -0.4 m w.e., which is near about in-situ measurement i.e., -0.36 m w.e. and the standard deviation (SD) of both values is 0.04 (10%). The equation (5) given in the text used by several workers (Kulkarni et al., 2004; Brahmbhatt et al., 2012; Singh et al., 2018, etc.) was derived from the in-situ measurements of two glaciers (Shaune Garang and Gor Garang). We modified this equation (eq. 6 and 7) and used in-situ data from 11 glaciers to reduce the uncertainty. We have validated the modeled results of each glacier, and the data difference was 10 to 20%. The in-situ measurements of PG show the difference of model data (10%), which is valid for glacier mass balance study in Himalaya (Wagnon et al. 2007; Dobhal et al. 2008, 2013; Azam et al. 2012, 2016; Mehta et al., 2021).

 RC: please provide the information on the uncertainties after the registration of DEMs

AR: Thanks for the comments. For this study, we have used the method of Garg and others (2022b) for DEM co-registration (horizontal and vertical adjustments) and surface elevation change. Therefore, to account for this error, here we have employed penetration depth correction of 2.3 ± 0.6 m for snow, 1.7 ± 0.6 m for clean ice, and 0.4 ± 0.8 m for debris-covered glacier parts, which is equal to an average penetration depth of ±1.46 m (Kääb and others, 2012; Gardelle and others, 2013; Zhou and others, 2018).

RC: it seems that there is quite a large error of field measurements compared to remote sensing, why is that? How did you estimate the errors?

AR: Thank you very much for this comment. Yes, we agree that the error is large for field measurements compared to remote sensing because the changes represent the short period between 2015 and 2019. Due to the short period, the glacier's snout does not retreat uniformly, and it oscillates, sometimes the right, sometimes the left, and sometimes the middle part retreats. So there is more error in field base measurements. Remote sensing images show the long periods of glacier recession, which minimizes the error. The measurement was carried out by field surveys using chain tape and DGPS.

RC: Please reformat the table as something is in bold, something is not.

AR: Thanks for your comment and suggestion. We have corrected the Table as per the suggestion.

RC: there is quite a large difference between the mass balance estimated with different approaches, which rises question about the validity of these approaches for these glaciers. Can you validate these results against field measurements and geodetic mass balance? Why did not you calculate the geodetic mass balance if you have at least two DEMs?

AR: Thanks for your comment. Yes, we agree that there is a significant difference between the mass balances estimated with different approaches. But we averaged these results and found that the mass balance value is near the field-based study. The glacier mass balance measurements data from stakes of one glacier (Pensilungpa) and this data is correlated with modeled data. The average modeled mass balance of the PG was -0.4 m w.e., which is near in-situ measurement, i.e., -0.36 m w.e., and the standard deviation (SD) of both values is 0.04 (10%). The in-situ measurements of PG show the difference of model data (10%), which is valid for glacier mass balance study in Himalaya (Wagnon et al. 2007; Dobhal et al. 2008, 2013; Azam et al. 2012, 2016; Mehta et al., 2021). We did not calculate the geodetic mass balance because we have only two DEMs (2000 and 2017). Still, we have more accurate data on the field-based mass balance of 11 glaciers representing the whole Himalaya (NE to NW). We estimated more accurately ELA of the glaciers using different methods, which gave more accurate results. We must try to use new approaches to estimate the mass balance of the Himalayan glaciers.

RC: can you show these in-situ measurements?

AR: Thanks for the comments. We have already mentioned the result of in-situ measurements in the text (Last section of “Glacier mass balance and thickness change”). A recent study by Mehta et al. (2021) based on glaciological mass balance suggested that the average specific balance of PG was -0.36 m w.e. a^-1 during the periods 2016/17-2018/19. The mass wastage rate in the ablation zone is about 0.82 m w.e. a^-1, with the mass gain in the accumulation area being about 0.27 m w.e. a^-1 between 2016/17 and 2018/19. The modeled value of the specific mass balance of PG is near to in-situ measurement.

RC: I did not understand why you estimate surface elevation change only along longitudinal profiles but did not produce DEMs of difference and did not calculate geodetic mass balance?

AR: Thanks for the constructive comment. The longitudinal profile and geodetic mass balance depend on the glacier surface elevation. The accumulation zone of the valley glacier of the Himalaya is mainly covered by an avalanche throughout the year and gives the wrong interpretation of glacier mass balance. But the longitudinal profile below ELA only gives surface lowering results, which is more reliable in comparison to geodetic mass balance for surface lowering.

RC: Many other studies have concluded this in other regions of the world as well, you should cite them as well and discuss the relation to debris cover as here you have only one reference. It is also interesting are this phenomenon similar in all glaciated regions or glaciers of the Himalayas differ somehow? There is also still the question how the geometry affects glacier changes, one glacier is much bigger, so is it its geometry or debris cover that plays the major role? What about other factors - slope, subglacial topography, thermal structure?

I suggest extending the discussion about all the mentioned issues.

AR: Thanks for the comment. We agree; many studies have concluded and discussed the relation to debris cover with glaciers. But here we cited Scherler and others (2011) because they suggested that the glacier retreat rate varies with >20% debris cover is zero, whereas it is high for the debris-free glacier. So, we cited this reference for the comparative study of Thick (PG) and Thin (DDG) debris cover glaciers. Yes, the debris-cover phenomenon is similar in all glaciated regions in the world, but most of the glaciers in the Himalaya is thickly debris-covered. The Himalayan glaciers are large debris-covered (70–80%) and have been receding since the end of the LIA (Mayewski and Jeschke 1979; Sakai et al. 2000; Casey et al. 2012; Dobhal et al. 2013; Pratap et al. 2015; Maurer et al. 2019). The supraglacial debris on the surface of the glacier is commonly found to have significant control on the rate of ice ablation and recession (Bozhinsky et al. 1986; Lundstrom et al. 1993; Schmidt and Nüsser 2012; Dobhal et al. 2013; Pratap et al. 2015; Kumar et al. 2017; 2020b; Mal et al. 2019; Shukla et al. 2020a). It has been observed that the supraglacial debris thickness significantly alters the glacier response to climate forcing (Scherler et al. 2011; Pratap et al. 2015).

                        We have discussed all the factors and morphology parameters in the last section of the Discussion. However, we cannot find a strong correlation between glacier retreat, meteorology, and topography factors, because both glaciers are located in the same climatic zone, aspect, and slope. Nevertheless, we expect that the glacier size, debris cover, and glacier lake probably influence the rate of glacier retreat in the Suru and Doda River basins.

RC: starting from here you have changes the reference style.

AR: Corrected and modified

RC: in the paragraph above you have listed many results of studies but what are the broader consequences and causes of all these changes?

AR: Thanks for the comment. In this part, we compared the present study results with other studies. We found that the ELA and AAR-based mass balance of DDG and PG are well correlated with in-situ (glaciological) and remote sensing (geodetic) based mass balance studies.  

RC: atmospheric warming?

AR: Yes, atmospheric warming. We have corrected.

RC: glaciers where?

AR: Himalayan Glacier. Corrected

RC: again you have listed many numbers about the changes of other glaciers? Is there a need to list them all in such details, because it is not discussion but listing of results, you could shorten this part and discuss more the reasons of these changed and similarities with your glaciers. Because I do not see how the mentioned glaciers are related to the ones you have studied as I do not know their location and parameters. Are they debris covered? How important is debris cover of your mentioned glaciers to heir changes?

AR: Thank you for your comment. All these listed glaciers show the changes of ELA and AAR of in-situ-based glaciers in a different part of the Himalaya. In this paragraph, we compared our results with other glaciers. We have cited the references of studies for readers' convenience to identify the location and characteristics of the glaciers. We have mentioned that the glaciers are debris-covered or clean. Here, we said that the ELA of the Central and Western Himalayan glaciers shifted upward, and the present ELA is located between 5000 and 5250 m asl, which shows that the glaciers are continuously losing their mass since the early twentieth century. 

RC: for what period?

AR: 1901 to 2018. Corrected

RC: These are important results which may be included in the Result section as well, maybe you present also maps of slope and aspect?

AR: Thanks for the encouragement, but this section is part of the discussion, so we include this section in the discussion part. We have modified Figure 1 and included the contour line representing the area's slope and aspect.

RC: in which years? Have these percentages changed through time?

AR: Between 2015 and 2019, the glacier is thick and debris-covered, so the minor changes are not visible in short periods.

RC: glaciers in general or both your studied glaciers? are there ponds on the surface, crevasses - this is not specified.

AR: Glaciers, in general. There are many crevasses and supraglacial lakes over both glaciers, but we have not studied them in detail as that was not the aim of the manuscript.

RC: proglacial lake is very important factor which drives the frontal retreat and it again suggests that there are many other factors promoting the faster retreat of the largest glacier, thus the existence of debris cover and its impact on glacier change is very speculative in this study. When this lake appeared and did it change substantially during the study period?

AR: Thanks for the comment. Yes, we agree that not only is debris cover the factor to enhance the retreat of the glaciers, but also many other factors promote the faster retreat of the glaciers. That's why we have described all factors in the discussion section. We expect that the glacier size, debris cover, and glacier lake probably influence the rate of glacier retreat in the Suru and Doda River basins.

RC: it may be worth to mention the methods very shortly as well cause impact the results.

AR: Thanks for the comment. We have modified the text.

RC: this is too strong and speculative conclusion as above you mentioned many other factors as " we expect that the glacier size, debris cover, and glacier lake probably influence the rate of glacier retreat in the Suru and Doda River basins."

AR: Thanks for the comment. We have incorporated and modified the text as suggested by the reviewer.

RC: I also miss discussion above and conclusion here how the debris cover affects the changes of other glaciers you have referenced.

AR: Thanks for the comment. We have included the importance of debris cover on glacier recession in the discussion part.

RC: studied glaciers?

AR: Incorporated.

Kind regards,

Manish Mehta

Reviewer 2 Report

Author Response

Detailed Response to the Reviewer-2

Manuscript ID: sustainability-2084190,

Title: “Response of the thick and thin debris-covered glaciers between 1971 and 2019 in Ladakh Himalaya, India; a case study from Pensilungpa and Durung-Drung glaciers”.

Authored by: Manish Mehta*, Vinit Kumar*, Pankaj Kunmar and Kalachand Sain

Please note that the comments from the reviewer’s main concerns are stated as “RC” in black color, and our detailed responses are stated as author’s response “AR” in blue color.

Response

RC: This proposal paper shows a brilliant and rigorous research. The paper has a suitable theme for the journal. The method and techniques are adequate, and specially I must highlight the quality of the expressive figures and the tables. The references are appropriate and relevant. Below, I suggest some references linked to the same/similar theme or/and the same/similar geographical area. Also, I could see some several similarities with other references. Specially, between this paper and this other paper: 5. Mehta, M.; Kumar, V.; Garg, S.; Shukla, A. Little ice age glacier extent and temporal changes in annual mass balance (2016–2019) of Pensilungpa Glacier, Zanskar Himalaya. Regional Environment Change, 2021, 21, 38. doi: 10.1007/s10113-021-01766 567 This reference is mentioned several times in the page 2. However, the figure 2 shows the same results and this paper does not refer to this figure. Also, the table (1 in the paper) is partialed to the table (2 of reference 5). Some photographs (figure 3, and 4 both in the paper and reference 5) are similar scenes and evidence.  I suggest a expressive reference 5 in the similar figures and tables.

And also, the authors, could you separate the contributions in this paper and the reference (n. 5)? I understand this research shows a large area and an analysis successive... And then, I suggest that this should be clear with recent papers published in such a short time (2021 and 2023)

AR: Thank you very much for your encouragement and appreciation. We have removed some references which were not relevant. Table 1 is partially related to Table 2 of Mehta et al., 2021, but we can use the same satellite images for other studies. We have incorporated all the suggestions given by the reviewer. The paper by Mehta et al. (2021) is based on in-situ mass balance and glacier recession from Little Ice Age to the present day. Still, in the current manuscript, we compared two glaciers in the same climatic regime. In the present manuscript, we have to try to determine the behavior of both glaciers using field, remote sensing, and model-based results.

Kind regards,

Manish Mehta

Round 2

Reviewer 1 Report

Dear authors, thank you for the comprehensive study in general and all the constructive answers to my comments. I have no other concerns, great work!

Reviewer 2 Report

Thank you for considering the little remarks. It’s a great research!