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Enhancing the Glacial Lake Inventory in Bhutan through Semi-Automatic Techniques using Google Earth Engine and QGIS for the Phochhu Basin

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23 June 2023

Posted:

26 June 2023

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Abstract
Bhutan located in the Hindu Kush Himalayan (HKH) region consists of several glaciers and glacial lakes at higher elevations.  With the rapid change in global temperature, glaciers are found to melt at an accelerated rate. This rapid melt of glaciers gets accumulated in weak moraine walls forming a glacial lake, posing a major threat to the downstream communities. As per the Bhutan Glacial Lake Inventory 2021, Bhutan has 567 glacial lakes. Furthermore, the Phochhu basin has the maximum glacial lakes (0.05%) and of which 9 are PDGL. Hence a need for a time monitoring system is imminent. With the availability of free High-resolution Satellite Imageries and Advanced Remote Sensing tools, it has been a sine qua no for monitoring glacial lakes in high areas. Therefore, using the Google Earth Engine and Qgis Platform a semi-automated technique was used to generate glacial lake inventories for Phochhu Sub-basin for the year 2021.  We found out that there are 166 glacial lakes covering an area of 24.051 km2.
Keywords: 
Subject: Environmental and Earth Sciences  -   Geography

1. Introduction

When the surface temperatures in the high altitudes are rising due to climate change, the rising temperatures and higher precipitation potentially trigger glacial lakes to burst and cause catastrophic flood downstream (Gyeltsen & Wangdi, 2021). In Bhutan 17 glacial lakes are identified as potentially dangerous lakes but Rapstreng Lake and Thorthomi lakes remain an imminent threat to western Bhutan in Punakha along the Phochhu River. Rapstreng is a fully formed lake with an area of 1.31 km2. It is separated from the bigger Thorthomi Lake by a narrow moraine wall, which is made up of ice and water blocks and stretches for an area of 3.62 km2. If the ice block in Thorthomi melts with increasing pressure, it will cause the moraine wall to collapse, considered one of the most dangerous glacial lakes in Bhutan (Wangchuk & Bolch, 2020).
The tiny size of glacial lakes, cloud cover in optical satellite imageries, shadows from mountains and clouds, seasonal snow in satellite photos, varying degrees of turbidity among glacial lakes, and frozen glacial lake surfaces all make mapping of glacial lakes in the Phochhu basin difficult. We used a cloud-based tool called "Google Earth Engine" to construct an automated method for mapping glacier lakes in the Phochhu basin for this work.

2. Material and Methods

2.1. Objectives

The main aims of this report are to
Study the spatial distribution of glacial lakes in Phochhu basin using geospatial techniques.
Helps in the development of glacial lake maps.
To monitor potentially dangerous glacial lakes in Phochhu basin.

2.2. Study Area

The main emphasis of this Glacial Lake Inventory is the Phochhu Basin, which contains the majority of glacial lakes as well as potentially dangerous lakes such as Thorthomi Tsho, Rapstreng Tsho, and Luggye Tsho. The Figure 1 shows the study area map of Phochhu basin.

2.3. Data Resources

Because of their spatial resolution and accessibility, Landsat satellite images have been frequently utilized to track the extent of glaciers and glacial lakes around the world in the past. Sentinel 2 images have been used in this study due to their consistent spatial coverage with a high spatial resolution of 10m and temporal resolution which is free accessibility on cloud-based (Google Earth Engine) and web portals (https://scihub.copernicus.eu/). We have used the Sentinel 2 MSI to prepare the glacial lake inventory of the Phochhu basin for 2021. Images taken between September and December were used primarily because there is a lesser likelihood of snow or cloud cover during this period compared to other months of the year.
For this study, SRTM DEM 30m resolution was used for the extraction of Phochhu Basin. The Advanced Land Observing Satellite (ALOS) DEMs of a 10m resolution was used to carry out terrain analysis in order to correct shadows as misclassified lakes

2.4. Methodology

The SRTM 30m DEM was used to extract the Phochhu basin. The Phochhu basin was used in the Google Earth Engine for sentinel-2 imageries with false color composite. Then NDWI was also calculated for automatic delineation of Glacial Lake and converted NDWI raster into NDWI vector format (shp). The vectorized NDWI mask was then exported to Google Drive and downloaded. Later it was imported to QGIS for further analysis. An area threshold of 0.01km was utilized for this inventory, misclassified lakes were manually removed using slope map generated from the ALOS DEM classifying. A slope threshold of 10 degrees was the glacial lake and glacial lake detected on slope degree more than 10 is removed because the lake surface of the area has to be plain. The delineation of the glacial lake using sentinel-2 imageries can be carried out using automatic and semi-automatic techniques. The glacial lake which is not detected by NDWI was manually delineated. The semi-automatic techniques can solve the problems faced in automatic techniques. Therefore, in this current preparation of the Glacial Lake Inventory of Phochhu basin the semi-automatic techniques were used which is shown below;
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Figure 2. Workflow for semi-automatic delineation of the glacial lake of Phochhu basin using Sentinel-2 MSI.
Figure 2. Workflow for semi-automatic delineation of the glacial lake of Phochhu basin using Sentinel-2 MSI.
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3. Results

The formation of glacial lakes is a continuous and dynamic process that requires constant monitoring and updating of glacial lake data. During the preparation of this inventory, 4463 water bodies (lakes) were discovered. Following the use of a separate set of criteria to designate a glacier lake, in total we found out that there are 166 glacial lakes in the Phochhu basin meet the criteria covering an area of 24.051 km2 accounting for 0.06 percent of the total area. The Table 1 shows the area of glacial lakes.
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Figure 3. Glacial lake classification using NDWI on sentinel-2 with false color composite.
Figure 3. Glacial lake classification using NDWI on sentinel-2 with false color composite.
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The threshold criteria for glacial lake identification using NDWI is kept at 0.2 since it detects most of the glacial lake perfectly. The ALOS DEM of 10m resolution was used to delete misclassified lakes. The lakes located on slope above 10° indicate hills and steep slopes are removed (Khadka et al., 2018). The automatic delineation of the lakes did provide glacial lake boundary for larger glaciers, however it also generated many misclassified lakes polygons as shown in Figure 4. The manual delineation is done for misclassified lakes and undetected lakes to have accurate lake polygons. Interestingly while doing automatic delineation of the glacial lake few portions of lakes are detected since the surface of the lake is mostly covered with glaciers. In total, we found that there are 166 glacial lakes located in Phochhu basin covering an area of 24.051 km2 accounting for 0.06 percent of the total area of the Bhutan.
Among 166 glacial lakes in Phochhu basin Thorthomi glacial lake has highest area coverage. It has an area of 3.62 km2 followed by Luggye and Rapstreng covering an area 1.53 and 1.31 sq.km respectively. Figure 5 depicts the general distribution of glacial lakes in Phochhu basin

3.1. Hypsography

The hypsometric shows a clear picture of glacial lake distribution patterns in different elevations. The glacial lake in Phochhu basin has located a range between 4130 m.a.s.l to 5330 m.a.s.l shown in Figure 6. Most of the glacial lakes are located in higher elevations since most of the glacial lakes are fed by glaciers. The maximum number of the glacial lake is located at the elevation of 5030 m.a.s.l.

3.2. Discussions

The method for Glacial Lake Inventory was built upon the existing studies and knowledge gained through a decade of research. The most used technique is manual editing of lakes after NDWI masking. The semi-automated techniques for the removal of misclassified lakes are used. The most widely used data for glacial lake mapping were Landsat data. The Bhutan glacial lake inventory was initially published by ICIMOD in 2001 using Landsat data and also published a research article on Bhutan Glacial lake inventory by Wangchuk & Bolch (2020), using Sentinel 1 and 2 data. The National Centre for Hydrology and Meteorology published a Bhutan Glacial lake inventory 2021 using Sentinel 1 and 2 data (Karma et al., (2021). Different inventories published by different author/organization has kept different threshold for lake identification (e.g., ICIMOD inventories 2021 for Nepal is -0.6 to -0.9 for Landsat data (Bajracharya et al., 2020) and NCHM Bhutan Glacial Lake Inventories 2021 for Bhutan is less than 0.3 for Sentinel 2 MSI imageries. Similarly, the criteria for the area are different since ICIMOD (2021) kept the area above 0.003 km2 and NCHM (2021) kept the area above 0.01 km2.

3.3. Comparison with earlier inventory

In 2001 ICIMOD prepared a glacial lake inventory for Bhutan and recorded 549 glacial lakes in Phochhu basin (Mool et.al, 2002) and 157 glacial lakes in 2021 inventory published by NCHM. However, the present inventory of the Phochhu basin has 166 glacial lakes which are 9 more than the NCHM inventory. For our study, the non-glaciers fed lakes are considered. That will be the reason for the increasing glacial lake at Phochhu basin. The Table 2 shows the comparison among different inventories.

4. Conclusions

In conclusion to the glacial lake inventory of the Phochhu basin, it has the most potentially dangerous lake as compared to other basins in Bhutan namely Rapstreng, Thorthomi, and Luggye Lake. Phochhu basin has 166 glacial lakes in Phochhu basin covering an area of 24.051 sq.km accounting for 0.06%. Most of the studies reveal that the glaciers are retreating at an alarming rate with the rising temperature due to climate change. If preventive measures are not carried out, then there will be a high chance of glacial lake outburst flood in the downstream causing huge damage to life and properties. The glacial lakes are dynamic in nature therefore; glacial lake inventory has to update from time to time.

Author Contributions

Conceptualization, R.N.; methodology, R.N.; software, R.N.; validation, C.S.; and C.S.; formal analysis, R.N., C.S., and C.S.; investigation, R.N., and C.S.; resources, R.N., C.S., and C.S.; data curation, R.N.; writing—original draft preparation, R.N.; writing—review and editing, C.S; and C.S.; visualization, R.N.; supervision, R.N.; project administration, R.N. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

The completion of the Phochhu glacial lake inventory would not have been possible without the cooperation and assistance of the institute/agencies/organizations and individuals for their direct, and indirect contributions to the report's successful completion and publication. I would like to thank Sherubtse College’s Head of Social Science Department and Programme Leader of Geography Department for writing a reference letter for me to complete internships at NCHM. With their help, I was able to enroll in short-term internships. I would like to express my gratitude to the management of the National Centre for Hydrology and Meteorology (NCHM) for allowing me to perform my internship. I appreciate the Chief of Cryosphere Division’s approval of our intern request, and my work would not have been possible without the help of my mentor, who deserves my appreciation as well. This report on the Phochhu glacial lake inventory was successful because of his supervision and recommendations. Sentinel-2 and SRTM DEM are publicly, and thanks to the European Space Agency (ESA) and the Consultative Group on International Agricultural Research-Consortium for Spatial Information (CGIAR-CSI). Lastly, I would like to acknowledge my intern mates and other friends for rendering support while doing my report.

Conflicts of Interest

The authors declare no conflict of interest.

Appendix A Phochhu Glacial Lake Inventory

Total number of Glacial lakes: 166 Total area: 24.051 km2
Object
_ID 		PGLI_ID GLIMS_ID Basin Latitude Longitude Area (Km2) Elevation (m.a.s.l) Local_
Name 		Image
_Year
1 Pho_gl 70 G090084E28069N Phochhu 28.06936 90.08410 0.090 4660.7330 2021
2 Pho_gl 69 G090083E28077N Phochhu 28.07661 90.08325 0.070 4711.2170 2021
3 Pho_gl 68 G090082E28082N Phochhu 28.08186 90.08153 0.036 4706.6381 2021
4 Pho_gl 62 G090043E28096N Phochhu 28.09584 90.04284 0.064 5104.8016 2021
5 Pho_gl 61 G090042E28092N Phochhu 28.09165 90.04164 0.089 5039.0335 2021
6 Pho_gl 66 G090063E28092N Phochhu 28.09189 90.06346 0.023 4903.2414 2021
7 Pho_gl 67 G090075E28088N Phochhu 28.08846 90.07520 0.023 4764.9234 2021
8 Pho_gl 17 G089886E27923N Phochhu 27.92267 89.88593 0.063 4841.1497 2021
9 Pho_gl 18 G089891E27925N Phochhu 27.92526 89.89148 0.032 4874.4438 2021
10 Pho_gl 21 G089883E27931N Phochhu 27.93065 89.88337 0.018 4935.2167 2021
11 Pho_gl 19 G089900E27929N Phochhu 27.92933 89.90026 0.227 4945.0899 2021
12 Pho_gl 24 G089906E27941N Phochhu 27.94106 89.90605 0.135 5029.0532 2021
13 Pho_gl 27 G089923E27948N Phochhu 27.94826 89.92341 0.018 5071.5389 2021
14 Pho_gl 26 G089910E27948N Phochhu 27.94785 89.91006 0.017 5079.7225 2021
15 Pho_gl 12 G089898E27904N Phochhu 27.90401 89.89812 0.118 4784.0404 2021
16 Pho_gl 8 G089885E27896N Phochhu 27.89601 89.88461 0.112 4563.9039 2021
17 Pho_gl 14 G089897E27909N Phochhu 27.90939 89.89733 0.061 4868.7374 2021
18 Pho_gl 16 G089861E27915N Phochhu 27.91480 89.86114 0.078 4742.5543 2021
19 Pho_gl 5 G089846E27880N Phochhu 27.87968 89.84642 0.073 4584.6740 2021
20 Pho_gl 4 G089845E27877N Phochhu 27.87722 89.84464 0.061 4587.1431 2021
21 Pho_gl 3 G089843E27870N Phochhu 27.87034 89.84255 0.214 4505.5084 2021
22 Pho_gl 2 G089835E27866N Phochhu 27.86554 89.83464 0.041 4555.4504 2021
23 Pho_gl 7 G089856E27897N Phochhu 27.89731 89.85630 0.025 4574.9224 2021
24 Pho_gl 11 G089866E27903N Phochhu 27.90256 89.86648 0.028 4573.3249 2021
25 Pho_gl 13 G089862E27909N Phochhu 27.90861 89.86173 0.046 4687.1247 2021
26 Pho_gl 15 G089872E27910N Phochhu 27.90972 89.87166 0.017 4741.1617 2021
27 Pho_gl 10 G089946E27897N Phochhu 27.89654 89.94557 0.079 4860.8232 2021
28 Pho_gl 9 G089943E27893N Phochhu 27.89290 89.94280 0.085 4845.9766 2021
29 Pho_gl 22 G089922E27933N Phochhu 27.93331 89.92209 0.020 4978.0503 2021
30 Pho_gl 44 G089887E28097N Phochhu 28.09692 89.88690 0.030 4739.2259 2021
31 Pho_gl 51 G089934E28139N Phochhu 28.13936 89.93370 0.035 5069.8736 2021
32 Pho_gl 97 G090310E28072N Phochhu 28.07196 90.31040 0.055 5042.1324 2021
33 Pho_gl 91 G090232E28066N Phochhu 28.06621 90.23161 0.115 4728.6070 2021
34 Pho_gl 102 G090249E28057N Phochhu 28.05733 90.24939 0.081 5128.3918 2021
35 Pho_gl 115 G090234E27997N Phochhu 27.99727 90.23355 0.112 4978.4924 2021
36 Pho_gl 129 G090222E27995N Phochhu 27.99545 90.22206 0.213 4934.2824 2021
37 Pho_gl 130 G090216E27992N Phochhu 27.99203 90.21579 0.173 4934.4375 2021
38 Pho_gl 90 G090211E28016N Phochhu 28.01611 90.21059 0.546 5128.7748 Tsho Chena 2021
39 Pho_gl 89 G090187E28012N Phochhu 28.01163 90.18683 0.208 5141.4725 2021
40 Pho_gl 88 G090179E28023N Phochhu 28.02280 90.17934 0.162 5074.1911 2021
41 Pho_gl 80 G090147E27964N Phochhu 27.96425 90.14673 0.219 4880.4836 2021
42 Pho_gl 150 G090174E27901N Phochhu 27.90053 90.17374 0.487 4674.6552 2021
43 Pho_gl 149 G090219E27893N Phochhu 27.89336 90.21891 0.162 4973.8740 2021
44 Pho_gl 159 G090157E27829N Phochhu 27.82900 90.15708 0.234 4429.4764 2021
45 Pho_gl 155 G090191E27850N Phochhu 27.85041 90.19132 0.232 4763.6804 2021
46 Pho_gl 158 G090222E27827N Phochhu 27.82736 90.22180 0.293 4658.7205 2021
47 Pho_gl 154 G090191E27857N Phochhu 27.85700 90.19133 0.160 4722.0031 2021
48 Pho_gl 152 G090185E27878N Phochhu 27.87765 90.18535 0.146 4824.0541 2021
49 Pho_gl 151 G090179E27877N Phochhu 27.87736 90.17858 0.094 4838.5798 2021
50 Pho_gl 145 G090254E27863N Phochhu 27.86275 90.25391 0.071 4946.2053 2021
51 Pho_gl 157 G090216E27845N Phochhu 27.84508 90.21596 0.065 4799.7703 2021
52 Pho_gl 156 G090213E27850N Phochhu 27.84990 90.21267 0.095 4852.7941 2021
53 Pho_gl 164 G090190E27942N Phochhu 27.94156 90.18973 0.163 4622.3388 2021
54 Pho_gl 146 G090209E27932N Phochhu 27.93186 90.20925 0.116 4985.8593 2021
55 Pho_gl 140 G090226E27961N Phochhu 27.96071 90.22613 0.051 5047.0098 2021
56 Pho_gl 78 G090139E28002N Phochhu 28.00242 90.13903 0.151 5056.8298 2021
57 Pho_gl 76 G090131E27988N Phochhu 27.98782 90.13056 0.195 5032.7758 2021
58 Pho_gl 83 G090149E27986N Phochhu 27.98554 90.14934 0.165 4903.7797 2021
59 Pho_gl 79 G090142E27992N Phochhu 27.99163 90.14224 0.092 4970.9389 2021
60 Pho_gl 77 G090139E28007N Phochhu 28.00666 90.13917 0.088 5047.9245 2021
61 Pho_gl 84 G090150E28004N Phochhu 28.00401 90.15006 0.135 5000.7430 2021
62 Pho_gl 32 G089928E28008N Phochhu 28.00806 89.92816 0.033 5030.6798 2021
63 Pho_gl 33 G089893E28030N Phochhu 28.02963 89.89327 0.104 5029.1856 2021
64 Pho_gl 34 G089895E28036N Phochhu 28.03561 89.89454 0.055 5188.2178 2021
65 Pho_gl 35 G089901E28038N Phochhu 28.03776 89.90051 0.037 5208.0676 2021
66 Pho_gl 43 G089871E28059N Phochhu 28.05943 89.87113 0.035 5117.5788 2021
67 Pho_gl 37 G089900E28047N Phochhu 28.04705 89.90027 0.045 5007.4079 2021
68 Pho_gl 38 G089889E28061N Phochhu 28.06076 89.88922 0.026 4895.5057 2021
69 Pho_gl 39 G089892E28064N Phochhu 28.06436 89.89228 0.027 4854.1798 2021
70 Pho_gl 81 G090150E27957N Phochhu 27.95679 90.15011 0.055 4818.6933 2021
71 Pho_gl 82 G090155E27939N Phochhu 27.93929 90.15491 0.077 4504.9961 2021
72 Pho_gl 36 G090029E28013N Phochhu 28.01287 90.02947 0.037 4417.0276 2021
73 Pho_gl 63 G090078E27954N Phochhu 27.95445 90.07845 0.084 4769.3636 2021
74 Pho_gl 64 G090091E27931N Phochhu 27.93112 90.09062 0.145 4515.7660 2021
75 Pho_gl 65 G090095E27942N Phochhu 27.94158 90.09522 0.096 4633.6293 2021
76 Pho_gl 31 G089956E28005N Phochhu 28.00531 89.95574 0.117 4606.2117 2021
77 Pho_gl 6 G089840E27888N Phochhu 27.88762 89.84028 0.032 4656.9502 2021
78 Pho_gl 114 G090231E28005N Phochhu 28.00547 90.23145 0.049 5067.9616 2021
79 Pho_gl 87 G090161E28010N Phochhu 28.01001 90.16127 0.078 5044.1990 2021
80 Pho_gl 1 G089926E27864N Phochhu 27.86385 89.92625 0.351 4522.7971 2021
81 Pho_gl 116 G089933E27908N Phochhu 27.90785 89.93339 0.037 4839.1771 2021
82 Pho_gl 94 G090270E28106N Phochhu 28.10628 90.26953 3.615 4450.1922 Thorthomi 2021
83 Pho_gl 52 G089896E28146N Phochhu 28.14567 89.89644 0.037 5107.0987 2021
84 Pho_gl 45 G089899E28106N Phochhu 28.10593 89.89863 0.236 4265.4842 2021
85 Pho_gl 46 G089909E28114N Phochhu 28.11396 89.90935 0.449 4338.2057 2021
86 Pho_gl 165 G089925E27951N Phochhu 27.95123 89.92478 0.039 5063.2564 2021
87 Pho_gl 50 G089924E28131N Phochhu 28.13063 89.92431 0.032 5065.6486 2021
88 Pho_gl 30 G089930E27973N Phochhu 27.97336 89.93019 0.628 5071.8259 2021
89 Pho_gl 48 G089925E28119N Phochhu 28.11899 89.92485 0.030 4612.7993 2021
90 Pho_gl 25 G089930E27941N Phochhu 27.94117 89.92996 0.705 4996.6530 2021
91 Pho_gl 29 G089931E27965N Phochhu 27.96455 89.93136 0.022 5125.7897 2021
92 Pho_gl 20 G089933E27924N Phochhu 27.92368 89.93307 0.064 4824.0174 2021
93 Pho_gl 28 G089933E27950N Phochhu 27.95009 89.93305 0.063 5035.0823 2021
94 Pho_gl 23 G089936E27932N Phochhu 27.93189 89.93619 0.018 4922.5932 2021
95 Pho_gl 49 G089945E28124N Phochhu 28.12398 89.94491 0.057 4747.0541 2021
96 Pho_gl 40 G089946E28078N Phochhu 28.07848 89.94623 0.033 4697.1487 2021
97 Pho_gl 42 G089956E28091N Phochhu 28.09080 89.95550 0.021 4925.1274 2021
98 Pho_gl 53 G089985E28104N Phochhu 28.10441 89.98488 0.027 5020.7684 2021
99 Pho_gl 41 G089990E28085N Phochhu 28.08485 89.98979 0.019 5005.4663 2021
100 Pho_gl 55 G090012E28108N Phochhu 28.10753 90.01158 0.017 4577.3046 2021
101 Pho_gl 58 G090030E28060N Phochhu 28.06045 90.02952 0.061 4380.6264 2021
102 Pho_gl 57 G090030E28070N Phochhu 28.07012 90.02979 0.027 4389.5129 2021
103 Pho_gl 59 G090035E28057N Phochhu 28.05714 90.03528 0.051 4366.5529 2021
104 Pho_gl 60 G090035E28042N Phochhu 28.04226 90.03458 0.025 4328.8780 2021
105 Pho_gl 56 G090028E28113N Phochhu 28.11293 90.02765 0.373 4684.1453 2021
106 Pho_gl 72 G090106E28007N Phochhu 28.00738 90.10642 0.085 4529.2313 2021
107 Pho_gl 73 G090113E28084N Phochhu 28.08445 90.11311 0.081 4132.4213 2021
108 Pho_gl 74 G090115E27969N Phochhu 27.96874 90.11520 0.063 4986.5692 2021
109 Pho_gl 75 G090119E27964N Phochhu 27.96372 90.11919 0.031 4872.5197 2021
110 Pho_gl 86 G090167E28090N Phochhu 28.09050 90.16700 0.046 4212.6409 2021
111 Pho_gl 85 G090167E28098N Phochhu 28.09825 90.16687 0.024 4259.4520 2021
112 Pho_gl 147 G090209E27917N Phochhu 27.91700 90.20934 0.070 4895.4657 2021
113 Pho_gl 153 G090207E27876N Phochhu 27.87590 90.20712 0.395 4908.4793 2021
114 Pho_gl 148 G090207E27902N Phochhu 27.90230 90.20723 0.018 4948.8182 2021
115 Pho_gl 166 G090211E27970N Phochhu 27.97011 90.21055 0.058 4894.8130 2021
116 Pho_gl 131 G090233E27978N Phochhu 27.97804 90.23284 0.684 5071.3220 2021
117 Pho_gl 132 G090240E27953N Phochhu 27.95308 90.23970 0.035 5126.1570 2021
118 Pho_gl 139 G090244E27915N Phochhu 27.91534 90.24391 0.227 5152.1527 2021
119 Pho_gl 133 G090244E27940N Phochhu 27.94049 90.24409 0.019 5101.2487 2021
120 Pho_gl 126 G090245E27954N Phochhu 27.95394 90.24546 0.027 5143.0296 2021
121 Pho_gl 138 G090244E27922N Phochhu 27.92245 90.24389 0.046 5106.5043 2021
122 Pho_gl 117 G090245E27983N Phochhu 27.98271 90.24467 0.175 5086.7060 2021
123 Pho_gl 125 G090245E27956N Phochhu 27.95642 90.24465 0.096 5138.5371 2021
124 Pho_gl 160 G090246E27848N Phochhu 27.84798 90.24605 0.092 4794.2659 2021
125 Pho_gl 128 G090249E27938N Phochhu 27.93848 90.24861 0.024 5110.2017 2021
126 Pho_gl 127 G090249E27941N Phochhu 27.94099 90.24922 0.016 5131.7925 2021
127 Pho_gl 93 G090247E28107N Phochhu 28.10705 90.24734 1.311 4369.6644 Rapstreng 2021
128 Pho_gl 163 G090253E27841N Phochhu 27.84140 90.25301 0.080 4987.1665 2021
129 Pho_gl 141 G090252E27897N Phochhu 27.89707 90.25218 0.084 5153.7910 2021
130 Pho_gl 120 G090252E27960N Phochhu 27.95973 90.25164 0.023 5174.0302 2021
131 Pho_gl 161 G090255E27847N Phochhu 27.84665 90.25489 0.027 4862.5148 2021
132 Pho_gl 121 G090253E27956N Phochhu 27.95636 90.25284 0.021 5172.7406 2021
133 Pho_gl 134 G090259E27925N Phochhu 27.92538 90.25896 0.112 5120.0668 2021
134 Pho_gl 123 G090255E27953N Phochhu 27.95289 90.25531 0.148 5208.4491 2021
135 Pho_gl 124 G090257E27943N Phochhu 27.94276 90.25671 0.058 5198.3799 2021
136 Pho_gl 162 G090259E27843N Phochhu 27.84346 90.25861 0.073 5017.6680 2021
137 Pho_gl 119 G090259E27957N Phochhu 27.95699 90.25913 0.019 5197.4656 2021
138 Pho_gl 112 G090261E27996N Phochhu 27.99603 90.26113 0.167 5157.2726 2021
139 Pho_gl 118 G090260E27964N Phochhu 27.96414 90.26003 0.470 5208.6979 2021
140 Pho_gl 101 G090263E28059N Phochhu 28.05880 90.26306 0.066 5111.7751 2021
141 Pho_gl 135 G090263E27920N Phochhu 27.91987 90.26252 0.229 5139.3498 2021
142 Pho_gl 113 G090263E27988N Phochhu 27.98796 90.26333 0.077 5200.6541 2021
143 Pho_gl 106 G090271E28029N Phochhu 28.02897 90.27058 0.219 5201.6249 2021
144 Pho_gl 108 G090277E28008N Phochhu 28.00838 90.27694 0.100 5281.4415 2021
145 Pho_gl 100 G090280E28050N Phochhu 28.04961 90.27962 0.045 5056.4580 2021
146 Pho_gl 107 G090280E28013N Phochhu 28.01288 90.27980 0.153 5283.3743 2021
147 Pho_gl 109 G090288E27997N Phochhu 27.99657 90.28835 0.426 5326.1661 2021
148 Pho_gl 95 G090302E28093N Phochhu 28.09280 90.30161 1.528 4509.6434 Luggye 2021
149 Pho_gl 104 G090295E28030N Phochhu 28.03021 90.29470 0.070 5339.5863 2021
150 Pho_gl 105 G090297E28027N Phochhu 28.02729 90.29672 0.020 5380.4000 2021
151 Pho_gl 110 G090299E27993N Phochhu 27.99257 90.29949 0.120 5359.3877 2021
152 Pho_gl 111 G090300E27989N Phochhu 27.98944 90.30030 0.015 5389.6118 2021
153 Pho_gl 103 G090301E28029N Phochhu 28.02902 90.30106 0.043 5363.6019 2021
154 Pho_gl 99 G090308E28043N Phochhu 28.04305 90.30834 0.017 5271.1628 2021
155 Pho_gl 98 G090313E28059N Phochhu 28.05944 90.31339 0.076 5098.1631 2021
156 Pho_gl 96 G090328E28086N Phochhu 28.08567 90.32751 0.045 4703.1031 Drukchung 2021
157 Pho_gl 92 G090231E28103N Phochhu 28.10344 90.23080 0.420 4345.5958 Bechung 2021
158 Pho_gl 144 G090238E27891N Phochhu 27.89129 90.23759 0.039 5009.9924 2021
159 Pho_gl 143 G090243E27897N Phochhu 27.89703 90.24284 0.023 5145.8363 2021
160 Pho_gl 142 G090241E27899N Phochhu 27.89909 90.24129 0.017 5169.9551 2021
161 Pho_gl 137 G090255E27916N Phochhu 27.91611 90.25481 0.014 5198.9343 2021
162 Pho_gl 136 G090257E27918N Phochhu 27.91842 90.25714 0.018 5189.4270 2021
163 Pho_gl 39 G089978E28056N Phochhu 28.05626 89.97796 0.030 4990.0199 2021
164 Pho_gl 71 G090089E28116N Phochhu 28.11593 90.08895 0.033 4491.4260 2021
165 Pho_gl 54 G090011E28123N Phochhu 28.12270 90.01102 0.010 4703.4857 2021
166 Pho_gl 47 G089955E28104N Phochhu 28.10429 89.95484 0.017 5133.7529 2021
Formula to Calculate GLIMS_ID in QGIS for geometry: 'G0'||to_int(x(centroid($geometry))*1000)||'E'||to_int(y(centroid($geometry))*1000)||'N'.

References

  1. Bajracharya, S.R.; Maharjan, S.B.; Shrestha, F.; Sherpa, T.C.; Wagle, N.; Shrestha, A.B. (2020). Inventory of glacial lakes and identification of potentially dangerous glacial lakes in the Koshi, Gandaki, and Karnali river basins of Nepal, the Tibet Autonomous Region of China, and India. International Centre for Integrated Mountain Development and United Nations Development Programme.
  2. Global warming triggers glacial lakes flood threat. (2002, April 17). The United Nations. Retrieved January 3, 2022. Available online: https://www.un.org/press/en/2002/envdev634.doc.
  3. Gyeltsen, N.; Wangdi, P. (2021, September 4). Climate change casts shadow on Bhutan with glacial lakes at risk. NikkeiAsia. Available online: https://asia.nikkei.com/Spotlight/Environment/Climate-Change/Climate-change-casts-shadow-on-Bhutan-with-glacial-lakes-at-risk.
  4. Karma Yangzom, D.; Duba, T.; Namgay, W.; Lhamo, S.; Tshering, P.; Tashi, T.; Namgay, T. (2021). Bhutan Glacial Lake Inventory (BGLI). National Center for Hydrology and Meteorology.
  5. Khadka, N.; Zhang, G.; Thakuri, S. (2018, November 29). Glacial Lakes in the Nepal Himalaya: Inventory and Decadal Dynamics (1977–2017). Remote Sensing. Available online: https://www.mdpi.com/2072-4292/10/12/1913/pdf#:~:text=The%20mapping%20of%20glacial%20lakes,calculated%20to%20be%20~0.05%20km2.
  6. Mool, P. K.; Wangda, D.; Bajracharya, S.R.; Kunzang, K.; Gurung, D.R.; Joshi, S.P. Chapter 8: The Inventory of Glacial Lakes. In Inventory of Glaciers, Glacial Lakes, and Glacial Lake Outburst Floods: Monitoring and Early Warning Systems in the Hindu Kush-Himalayan Region, Bhutan; International Centre for Integrated Mountain Development, Mountain Environment and Natural Resources' Information Systems, 2002; pp. 65–80. [Google Scholar]
  7. Wangchuk, S.; Bolch, T. (2020, July 29). Mapping of glacial lakes using Sentinel-1 and Sentinel-2 data and a random forest classifier: Strengths and challenges. Science of Remote Sensing. [CrossRef]
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Figure 1. Study area map of Phochhu Basin.
Figure 1. Study area map of Phochhu Basin.
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Figure 4. 4a) Misclassified glacial lake & 4b) Misclassified shadow as glacial lake using NDWI on sentinel-2 with false color composite.
Figure 4. 4a) Misclassified glacial lake & 4b) Misclassified shadow as glacial lake using NDWI on sentinel-2 with false color composite.
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Figure 5. Distribution of glacial lakes in Phochhu basin.
Figure 5. Distribution of glacial lakes in Phochhu basin.
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Figure 6. Hypsometry of Phochhu glacial lake distributions.
Figure 6. Hypsometry of Phochhu glacial lake distributions.
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Table 1. Glacial lake area.
Table 1. Glacial lake area.
Sl.no Types of area calculated Total Features Area (Sq.km) Area (%)
1 Area with all features 4463 features 18.843 0.05
2 Area with shadows ( after Criteria) 233 features 16.344 0.04
3 Area without/removal of shadows 131 features 12.574 0.03
4 The final glacial lake area 166 features 24.051 0.06
Table 2. Comparison of number and changes in glacial lakes.
Table 2. Comparison of number and changes in glacial lakes.
ICIMOD 2001
(For Phochhu basin)		 BGLI 2021
(For Phochhu basin)		 PGLI 2021
(Phochhu Basin)
Number 549 157 166
Area (km2) 23.49 20.98 24.051
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