Emerging Signal of Englacial Debris on One Clean Surface Glacier Based on High Spatial Resolution Remote Sensing Data in Northeastern Tibetan Plateau

Round 1

Reviewer 1 Report

This manuscript has important reference value for the study of the alpine glacier mass balance and its influence on river runoff. This manuscript introduces the Qiyi Glacier, northern Tibetan Plateau. It monitored the conversion of glacial terminal from clean glacier to debris covered glacier, and their effects on glacier mass balance and downstream river runoff. It proposed a conceptual model of the Qiyi Glacier which showed the transformation from a clean-surface to debris-covered glacier due to climate warming, and its effect on glacial runoff. The following are some suggestions.

1. The influence of glacier and debris-covered glacier changes on runoff is unclear and obscure in Section 3.3 from Line 164. Please describe the computational procedure in detail.

2. It is suggested that the author provide more images of UAV or satellite of the debris coverd glacier, so that we can compare with the field pictures. Maybe we can find out more features of the debris coverd glacier in UAV or high-resolution satellite images, with the purpose of provide more reference information for monitoring the large scale of debris covered glaciers.

3. Only one glacier experiments are too simplistic to propose a conceptual model. It is better to provide more glaciers or evidence to prove the importance, objectivity and feasibility of this model.

4. The title shows “the Tibetan Plateau”, but the manuscript only has a single Qiyi glacier, the content and the title do not match.

5. The manuscript has more pictures but less text. Please add text and descriptive content to improve the readability of the manuscript.

Author Response

Reviewer #1：

Comments and Suggestions for Authors:

This manuscript has important reference value for the study of the alpine glacier mass balance and its influence on river runoff. This manuscript introduces the Qiyi Glacier, northern Tibetan Plateau. It monitored the conversion of glacial terminal from clean glacier to debris covered glacier, and their effects on glacier mass balance and downstream river runoff. It proposed a conceptual model of the Qiyi Glacier which showed the transformation from a clean-surface to debris-covered glacier due to climate warming, and its effect on glacial runoff. The following are some suggestions.

Reply: Thank you for your fruitful comments and suggestions in improving our manuscript. In accordance with your comments and suggestions, we have revised the manuscript and we hope the revised version will be satisfactory to you.

1. The influence of glacier and debris-covered glacier changes on runoff is unclear and obscure in Section 3.3 from Line 164. Please describe the computational procedure in detail.

Reply: Thank you for your suggestion. In the revised text, we have supplemented descriptions and relevant equations to show the detailed computation procedure on the influence of glacier and debris-covered glacier changes on ice mass loss.

The relative ice mass loss (RIML) of a given single pixel can be expressed as:

RIML = SEC× A × 900

where, the SEC is surface elevation change (Unit: m), A refers to the area of the glacier pixel (m²), and 900 represents the density of glacier ice (900kg/m³). By using the equation mentioned above, the ice mass loss of all the bare ice pixels and debris-covered pixels can be obtained (including A, A_c, B, B_c, C, C_c, D, D_c, E, and E_c). Moreover, field investigation found evident retreat in channel shoulders where debris emerges along rivers, which is because the higher surface temperature of the debris melts more ice of surrounding slope. This extra part of mass loss was also taken into account for this type of debris. Therefore, for the total ice mass loss of three debris cover along river channels, the ice mass losses within the entire channel (zones Ar–Cr encircled by thick dashed lines) were compared with those bare ice at the same elevation band (i.e., Ar_c, Br_c, Cr_c) (Fig. 4a). This part of descriptions has been placed in Section 3.3.

2. It is suggested that the author provide more images of UAV or satellite of the debris covered glacier, so that we can compare with the field pictures. Maybe we can find out more features of the debris covered glacier in UAV or high-resolution satellite images, with the purpose of provide more reference information for monitoring the large scale of debris covered glaciers.

Reply: Thank you for your suggestion. In the revised Figure 3, we have added a new subfigure (Fig. 3c) to show the UAV or satellite of the debris covered glacier from 1972 to 2020.

Fig.3c Qiyi Glacier surface condition based on satellites and UAV orthoimage data in 1972-2020.

3. Only one glacier experiments are too simplistic to propose a conceptual model. It is better to provide more glaciers or evidence to prove the importance, objectivity and feasibility of this model.

Reply: Thank you for your comment and suggestion. According to our field observation on Qiyi Glacier over the past ~20 years, this clean-surface glacier was found to expose its englacial moraine at small spatial scale, which need to be identified by remote sensing images at a spatial resolution of meter or sub-meter that are not affected by snowfall. However, the existing remote sensing data appears hardly meet that requirement on the Tibetan Plateau. In accordance to your suggestion, we would attempt to find more similar glaciers from field trip and remote sensing images (e.g., KH-9) to support our conclusions in the future.

4. The title shows “the Tibetan Plateau”, but the manuscript only has a single Qiyi glacier, the content and the title do not match.

Reply: Thank you for your suggestion. The title has been changed to “… northeastern Tibetan Plateau” to make it more specific for the study area.

5. The manuscript has more pictures but less text. Please add text and descriptive content to improve the readability of the manuscript.

Reply: Thank you for your suggestion. In the revised manuscript, we have supplemented relevant descriptive content in the Section 3 and 4 so as to improve the readability of the manuscript.

Author Response File: Author Response.docx

Reviewer 2 Report

I had strong interesting to review the paper “Emerging signal of englacial debris on one clean surface glacier based on high spatial resolution remote sensing data in Tibetan Plateau”. This paper investigate the change of debris-cover and their influences on the Qiyi Glacier, northeastern Tibetan Plateau by using in-situ measurement, satellite images and UAV survey. The topic is very interesting and the Figure quality is good. The author address that the emergence of englacial debris will significantly change the surface melting and the consequent discharge in this region. I have listed several main concerns for improving the paper.

Main comments

1. The high-resolution images taken in 1975, 2005, 2012, 2020 and 2021 were used in this study. The boundaries of the debris cover region are roughly shown in Figure 3. However, there are no any numbers showing their total area at different periods in the text. Has the area of debris cover region increased or decreased over the last 50 years? I only found that debris area increase by 1095.8m2 from 2020 to 2021 in Line 211-212. A Table should be provided to show their areas, annual changing rates. And the possible satellite images and UAV images should be also provided to show the surface condition in different periods. From Figure3 and the corresponding scale bar, I estimated that the percentage of debris-covered region is only 0.5% of the total glacierized area (2.6 km2). The contribution of debris-cover is therefore negligible. Figure 5 shows the conceptual model of Qiyi Glacier showing the evolution from a clear-surface glacier to debris-cover glacier. However, this evolution pattern is completely different from that shown in Figure 3. I suggest that the author should re-design Figure 5 according to the exact area of debris cover in 1975, 2005, and 2020. In contrast to Figure 3, the present pattern in Figure 5 exaggerated the coverage and percentage of surface debris. In fact, another important question was naturally raised about the formation of debris-cover glacier in the Qilian Mountain in northeastern Tibetan Plateau. To my knowledge, there are few debris-covered glaciers in this region due to the distinctive glacier dynamics and topography. During the past centuries (e.g. since little Ice Age), few new debris-covered glaciers have formed with the melting and retreat in this region. Why did you conclude that the Qiyi Glacier will change from a debris-free condition to debris-covered condition? The author should provide more background on the formation of debris-covered type glaciers.

2. I am very interested in the exposure process of the englacial debris in Figure 3b. The debris exposed in the river channel is easy to understand. Could you please provide some field evidence of melt-out debris along the channels? From Figure 1 and Figure 3 I cannot see the location and distribution area of such melt-out debris. If it is only occasional, how could you guarantee the popular pattern of such debris development?

3. .In the results section, the author analyses the changes in terminus, debris cover area and surface elevation using in-situ measurements, UAV and satellite imagery. However, there are few quantitative numbers showing their changes. For example, the area of the debris-covered region, the mean surface elevation in the debris-covered and debris-free regions. In addition, Figure 4b shows the volume of ice mass loss (m3 w.e.). How did you delineate the boundary of the clear surface ice at the same height with A-E debris? Could you show its boundary?

4. The accuracy of UAV survey. The author claimed that the vertical accuracy is up to 0.021m. The detailed description of the UAV accuracy should be provided. And why did the abnormal positive elevation change in the proglacial region? The pattern of surface elevation change in such a water-channel region is different from other similar glaciers (e.g. Xue et al., 2021).

Xue, Y., Z. Jing, S. Kang, X. He, and C. Li (2021), Combining UAV and Landsat data to assess glacier changes on the central Tibetan Plateau, J. Glaciol., 1-13.

 

5. In the discussion, the author discussed the influence of climate warming and the hydrological impact of exposed debris. Such discussion disengage completely with the result section. This discussion can be suitable for any paper in other regions and is therefore a well-known fact. The author should pay more attention to the innovative finding on the Qiyi Glacier and discuss the possible mechanism and penitential influences.

Author Response

Reviewer #2

Comments and Suggestions for Authors:

I had strong interesting to review the paper “Emerging signal of englacial debris on one clean surface glacier based on high spatial resolution remote sensing data in Tibetan Plateau”. This paper investigate the change of debris-cover and their influences on the Qiyi Glacier, northeastern Tibetan Plateau by using in-situ measurement, satellite images and UAV survey. The topic is very interesting and the Figure quality is good. The author address that the emergence of englacial debris will significantly change the surface melting and the consequent discharge in this region. I have listed several main concerns for improving the paper.

Reply: Thank you for your favorable comments and suggestions in improving our manuscript. We really appreciate your time and help.

Main comments

1. The high-resolution images taken in 1975, 2005, 2012, 2020 and 2021 were used in this study. The boundaries of the debris cover region are roughly shown in Figure 3. However, there are no any numbers showing their total area at different periods in the text. Has the area of debris cover region increased or decreased over the last 50 years? I only found that debris area increase by 1095.8m² from 2020 to 2021 in Line 211-212. A Table should be provided to show their areas, annual changing rates. And the possible satellite images and UAV images should be also provided to show the surface condition in different periods.

Reply: Thank you for your suggestion. In the revised Fig. 3, we have added the orthoimage data of 1972-2020 to show the surface condition and the clear boundaries of the debris cover region in different periods (Fig.3c).

Fig.3c Qiyi Glacier surface condition based on satellites and UAV orthoimage data in 1972-2020.

 In addition, we have also supplemented a table to show the areas of debris cover of different years at various distance to the glacier terminal and their total areas, annual change rates in Section 3.2.

Table 1. The areas of debris cover of different years at various distance to the glacier terminal and their total areas/ annual change rates.

Distance to glacier terminal (m)	Debris cover area (m2)
	1972	2005	2012	2020	2021
50	1815.26	1355.98	458.46	828.67	571.57
100	1000.06	2932.51	36.88	1608.89	1796.35
150	0.00	586.28	424.14	1000.19	1225.05
200	0.00	0.00	223.37	1569.82	1890.57
250	0.00	0.00	161.62	493.53	549.43
300	0.00	0.00	118.42	693.17	762.75
350	0.00	0.00	284.43	243.01	272.46
400	0.00	0.00	0.00	8.18	15.64
450	0.00	72.24	0.00	15.06	19.85
500	0.00	54.46	0.00	0.00	0.00
Total debris cover area	2815.32	5001.47	1707.33	6460.52	7103.67
Annual change rate	--	66.25	-470.59	594.15	643.15

 

From Figure3 and the corresponding scale bar, I estimated that the percentage of debris-covered region is only 0.5% of the total glacierized area (2.6 km²). The contribution of debris-cover is therefore negligible.

 Reply: Thank you for your suggestion. Yes, the percentage of debris-covered regions is low, but it is abnormal that the englacial moraine became exposed within the glacier tongue for such a clean surface glacier because the Qiyi Glacier has long been categorized as a clean surface glacier since it was first observed in 1958. However, in 2005, a small pile of supraglacial debris was found on the glacier tongue, which has since expanded towards higher elevations. This anormal phenomenon makes us wonder whether it is the initiation into debris-covered glacier for Qiyi Glacier. Even though the percentage of surface debris in 2021 was still small, the emerging process along the mainstream line does reflect an important signal that should not be neglected that the glacier might eventually switch into a new era with rapid emergence of englacial debris under the sustained thinning of glacier.

Figure 5 shows the conceptual model of Qiyi Glacier showing the evolution from a clear-surface glacier to debris-cover glacier. However, this evolution pattern is completely different from that shown in Figure 3. I suggest that the author should re-design Figure 5 according to the exact area of debris cover in 1975, 2005, and 2020. In contrast to Figure 3, the present pattern in Figure 5 exaggerated the coverage and percentage of surface debris.

Reply: Thank you for your valuable suggestions and comments. In accordance to your suggestion, we have added a table in the revised Section 3.2 to show that the englacial debris did not expose at a linear mode but would witness a tipping point, beyond which, this clean surface glacier probably switch to a new type of state (i.e., debris-covered glacier), which is similar to the existing studies on climate change. In addition, according to the field observation on Qiyi glacier, beneath the tongue part, there are extensive englacial debris layers that are approximately parallel to the glacier surface. Therefore, although the speculation in Fig.5 appears to exaggerate the situation when compared to Fig3, the sustained thinning of glacier under climate warming might trigger a new phase under certain condition. Thank you for your fruitful suggestions, in our future field work, we will make some new measurements to quantify the depth between the englacial debris layer and glacier surface and the annual thinning rate of glacier surface, thereby better supporting our speculation.

In fact, another important question was naturally raised about the formation of debris-cover glacier in the Qilian Mountain in northeastern Tibetan Plateau. To my knowledge, there are few debris-covered glaciers in this region due to the distinctive glacier dynamics and topography. During the past centuries (e.g. since little Ice Age), few new debris-covered glaciers have formed with the melting and retreat in this region. Why did you conclude that the Qiyi Glacier will change from a debris-free condition to debris-covered condition? The author should provide more background on the formation of debris-covered type glaciers.

Reply: Thank you for your suggestion and very professional comments on the Qiyi glacier. Yes, there is very few debris-covered glaciers on Qilian Mountain for a long time, and there is no case that experienced evident change from debris-free condition to debris covered condition since the Little Ice Age. But in 2016, the top part of Qiyi Glacier had no accumulation zone for the first time since 1958 and the terminal part shrank substantially with a very evident expansion in the debris-cover area, which makes us rethink whether the Qiyi Glacier had entered a new stage (as shown in Fig.3b) that the sustained thinning in glacier surface cannot fully cover the englacial debris any more. In addition, our field investigation of the englacial debris of Qiyi Glacier can support our conjecture (Fig2).

2. I am very interested in the exposure process of the englacial debris in Figure 3b. The debris exposed in the river channel is easy to understand. Could you please provide some field evidence of melt-out debris along the channels? From Figure 1 and Figure 3 I cannot see the location and distribution area of such melt-out debris. If it is only occasional, how could you guarantee the popular pattern of such debris development?

Reply: Thank you for your suggestion. This Figure has been placed in the supplementary Material (Figure S1), and from Figure S1 you would see that the melt-out debris is mainly found in the D and E zone with relatively lower erosion from supraglacial river.

Fig. S1. Exposed englacial debris in five zones A-E (a-e, respectively; taken on summer 2022).

3. In the results section, the author analyses the changes in terminus, debris cover area and surface elevation using in-situ measurements, UAV and satellite imagery. However, there are few quantitative numbers showing their changes. For example, the area of the debris-covered region, the mean surface elevation in the debris-covered and debris-free regions. In addition, Figure 4b shows the volume of ice mass loss (m³ w.e.). How did you delineate the boundary of the clear surface ice at the same height with A-E debris? Could you show its boundary?

Reply: Thank you for your suggestion. In the revised text we have added a new table to show the quantitative change in the debris-covered area in Section 3. As for the boundaries of the clean surface ice at the same height with A-E debris, we delineate them using R coding. Taken the A-zone for example, the maximum and minimum DEM of A-zone are used to extract vector boundaries for the clear surface ice according to its altitude attribution table. within this boundary, the five debris zones had 10 corresponding iso-height lines, but they were not placed in the figure in order to keep the figure neat.

 

4. The accuracy of UAV survey. The author claimed that the vertical accuracy is up to 0.021m. The detailed description of the UAV accuracy should be provided. And why did the abnormal positive elevation change in the proglacial region? The pattern of surface elevation change in such a water-channel region is different from other similar glaciers (e.g. Xue et al., 2021).

Xue, Y., Z. Jing, S. Kang, X. He, and C. Li (2021), Combining UAV and Landsat data to assess glacier changes on the central Tibetan Plateau, J. Glaciol., 1-13.

 Reply: Thank you for your suggestion. The vertical error of -0.021 is the sum of all the absolute errors of all 16 controlling points. Generally, the vertical accuracy of UAV data was estimated using the RMSE (0.137 m), and therefore, the ± 2 times of the RMSE (i.e., -0.28 to 0.28; or -2s, 2s) is considered as the vertical error with a significance level of 0.9544, as shown in the white in Fig. 4.

As for the anormal positive elevation changes in the proglacial region in Fig. 4, which is also one of the most interesting points in this study. As the proglacial moraines are loose being affected by the molten water of glacier, we find it difficult to find unmoved rocks that were identified as control points, and therefore there is no control points in the proglacial region, which might be the reason for the relatively lower accuracy of DEM over proglacial region than that of the tongue part. Theoretically, the abnormal positive elevation change can be explained as the result of the extrusion of moving glacier: when the glacier shrinks as a result of climate warming, the ice above the ground gets vanished with its englacial moraines exposed, but there is abundant hidden ice that is still being extruded by the moving glacier. This part of buried ice is covered by thick debris and therefore has little ablation, and might be forced to rise due to the extrusion of moving glacier. However, it needs further investigation in our coming field trip to support our conjecture of this abnormal positive SEC in the proglacial region. We have added these arguments in the discussion section.

5. In the discussion, the author discussed the influence of climate warming and the hydrological impact of exposed debris. Such discussion disengage completely with the result section. This discussion can be suitable for any paper in other regions and is therefore a well-known fact. The author should pay more attention to the innovative finding on the Qiyi Glacier and discuss the possible mechanism and penitential influences.

Reply: Thank you for your suggestion. In the revised manuscript, we have supplemented relevant statements and arguments in the Discussion section based on the results. When a glacier transfer from clean-surface condition to debris-covered condition, it is very important to capture this transfer signal at its initial stage, which is also a challenging subject. From field investigation and high-resolution UAV images, we find the emerging signal of englacial debris on the Qiyi Glacier and conjecture that such a clean surface glacier might switch to a heavy debris-covered one, but this bold speculation needs more observations to validate. For instance, based on more detailed measurement of englacial debris using the ground penetrating radar (GPR), combined with the glacier dynamical models, the development of englacial debris of the Qiyi Glacier might be analyzed to get a more solid results. Nevertheless, it is undeniable that the continuing climate warming has been substantially influencing the fate of the Qiyi glacier. According to our field observations over the past years, although the percentage of debris area is still low, the debris cover at the glacier terminal has been expanding upwards to the interior tongue part since 2005s, and if this change trend continues, it is eventually that the Qiyi Glacier might change to a debris-covered condition.

On the other hand, despite of the relatively higher uncertainties in SEC results over the proglacial region due to the absence of the control points, its abnormal positive changes still deserve further investigations. Theoretically, the abnormal positive elevation change can be explained as the result of the extrusion of moving glacier: when the glacier shrinks as a result of climate warming, the ice above the ground gets vanished with its englacial moraines exposed, but there is abundant hidden ice that is still being extruded by the moving glacier. This part of buried ice is covered by thick debris and therefore has little ablation, and might be forced to rise due to the extrusion of moving glacier. In our future field work, we plan to set control points with more accurate RTK to verify whether the SEC in the proglacial region increase or not. If we get a positive answer, it is direct evidence of the existence of hidden ice in the proglacial region, and it is also a breakthrough in the studies of ice loss estimation during the process of glacier shrinkage.

Round 2

Reviewer 1 Report

I agree to accept in present form.