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

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;

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.

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.

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.

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.