Bulletin of Hydrogeological Association, Vol. 5, September 2020 Chand MB, Watanabe T, 2020 High-resolution inventory of the glacial lakes in the Ngozumpa Glacier basin, Everest Region, Nepal

Mohan Bahadur Chand1* and Teiji Watanabe1 1 Faculty of Environmental Earth Science, Hokkaido University, Sapporo, Hokkaido, Japan *Corresponding e-mail: [email protected]

Received: 15 April 2020/Accepted: 4 September 2020 ABSTRACT

Several glacial lakes are developing and increasing in their size with the warming temperature and glacier melt. Some of the glacial lakes are potential for glacial lake outburst floods (GLOFs) in the Himalayan region. This study presents the inventory of the glacial lakes of the Ngozumpa Glacier basin, a source of River using 2-m resolution satellite imageries of 2016. Index based approach with manual correction was applied to the WorldView and GeoEye images for mapping all the glacial lakes with size >20 m2. The inventory revealed the 1,022 glacial lakes with total area of 2.67 ± 0.14 km2. The supraglacial lakes were found most frequently among all types of glacial lakes, and they accounted for 96% of total number of the lakes, however, unconnected glacial lakes accounted for 75% of total area of the glacial lakes. The distribution of size of the lakes was not normal, and dominated by the small size lakes. The number of lakes with a size of one pixel (0.0009 km2) and four pixels (0.0036 km2) of Landsat image accounted for 86%, 94%, respectively showed the usefulness of high-resolution dataset to observe small features. Similarly, inventory exhibited the presence of relatively large glacial lakes at terminus of the glacier, which indicates the potentiality for the development of large glacial lakes. This inventory presented the highly accurate map of the glacial lakes of the Ngozumpa Glacier basin, which can be used as reference for the future study.

Keywords: glacial lakes, remote sensing, high-resolution, , Ngozumpa Glacier

INTRODUCTION increases (Östrem, 1959; Chand et al., 2015). Consistent loss of ice observed across 2,000-km However, debris-covered glaciers have also lost transect of the Himalaya and doubling of the average significant mass despite the presence of thick debris loss rate have been observed during 2000 and 2016 (Bolch et al., 2008a; Kääb et al., 2012; Basnett et al., compared to 1975 and 2000 (Maurer et al. 2019). 2013). The loss of glacier mass in the Himalayas leads The wastage of glaciers in this region is spatially to the formation of glacial lakes and dramatic changes heterogeneous (Fujita and Nuimura 2011). Bolch et in total runoff, affect the irrigation and hydropower, al. (2011, 2012) also indicated the loss of Himalayan and alter hazards (Bolch et al. 2012). glaciers at a significant rate. Mass wastage in the

Himalaya resulted in increasing debris cover (Bolch King et al. (2017) estimated the more negative et al., 2008a; Azam et al., 2018), and the elevation of mass balance of the glaciers that border on a lake the debris-covered glaciers is changing (Lamsal et al. compared to land terminating glaciers. Similarly, 2011), which depend on their scale, slope and the Watanabe et al. (1995) also observed a very high melt existence of glacial lakes (Nuimura et al. 2012). The rate (5.0 m y-1) where the ice surface was submerged area of the glaciers had been observed to be lost by by lake water and ice-cored moraines can become more than 20% from the 1980s to 2010 in the Hidden extremely unstable due to undercutting action of lake Valley, Nepal Himalaya (Lama et al. 2015), and had water (Watanabe et al. 1994). Glacier recession driven retreated by 30-60 m in terminus elevation over 20 by climate change produces glacial lakes and some of years between 1974 and 1994 (Fujita et al. 1997). which are hazardous (Richardson and Reynolds, 2000; The melt rate is retarded where debris thickness Haritashya et al., 2018), and the number and area of 61

Bulletin of Nepal Hydrogeological Association, Vol. 5, September 2020 Chand MB, Watanabe T, 2020 the glacial lakes is increasing in recent decades 2019). This glacier is a source of the Dudh Koshi (Gardelle et al. 2011; Nie et al. 2013). Gardelle et al. River. The glaciers of the NGB are characterized by (2011) reported the increase in area by the presence of debris in the ablation zone, which 20% between 1990 and 2009 in the extended Everest accounted for 32% of the total area of glaciers. The region, Nepal. A recent study by Khadka et al. (2018) elevation of the NGB ranges from 3,338 m a.s.l. at the revealed an increase in the number of glacial lakes by confluence of the Dudh Koshi and to more than 150% and an increase in area by about 8,201 m a.s.l. at the peak of Mt. Choyu. 45% from 1977 to 2017 in the Nepal Himalaya. NGB region have the three debris-covered A continuous retreat of the glaciers and increase glaciers, which have relatively gentle slope in in the area and number of glacial lakes lead to the comparison to clean glaciers, and some of these have changes in water storage and formation of potentially the potential to form glacial lakes at their terminus. hazardous glacial lakes. Past studies in the Everest region were used the coarse resolution satellite images, i.e., Landsat (30 m) (Shrestha et al. 2017; DATASET AND METHODS Khadka et al. 2018), Advanced Visible and Near Dataset Infrared Radiometer type 2 (AVNIR-2, 10 m) (Salerno et al. 2012) to map the glacial lakes. High-resolution WorldView and GeoEye images from Additionally, studies were lake specific (Benn et al., 2016 were obtained from the DigitalGlobe Foundation 2000, 2001; Bolch et al., 2008b; Thompson et al., and were used to prepare the inventory of glacial 2012). Recently, a study was carried out using 30-m lakes. This study employed the multispectral images Landsat images to understand the development of WorldView-02, WorldView-03 and GeoEye-01 of supraglacial lakes on the surface of debris-covered Basic 1B (Level 1) and Standard 2A (Level 2) glaciers in the Everest region from 1989–2017 by imagery products of 2-m spatial resolution, which Chand and Watanabe (2019), which showed the most were originally taken at much higher spatial resolution rapid increase in area and persistent lake on the (Table 1). The WorldView-2 and 3 comprise eight Ngozumpa Glacier. They also indicated the trajectory multispectral bands, and the GeoEye-1 sensor has four of glacial lake at terminus towards large lake. bands with slight difference in wavelength. Therefore, it is necessary to monitor the glacial lakes WorldView-3 has additional 8 SWIR bands and 12 to understand their status with higher accuracy. CAVIS bands. Level 1 images are radiometrically and Studies that focused on glacier catchment scale using sensor corrected but not projected to a plane using a very high-resolution satellite images are not available map projection or datum. Level 2 products are in the Ngozumpa Glacier basin (NGB). The aim of projected to the plane using the map projection. Level this study is to prepare high-resolution inventory of 1 products have different ground sampling distance the glacial lakes for the NGB, Everest region, Nepal (GSD), while Level 2 products have uniform GSD by using 2-m spatial resolution images of and corrected for topographic relief using coarse WorldView and GeoEye for the year 2016. DEM (Terrain corrected). Similarly, the Shuttle Radar Topography Mission (SRTM) DEM based on data 2000 obtained from USGS was used in this study. STUDY AREA The Ngozumpa Glacier basin is located in eastern Methods Nepal in Sagarmatha National Park (SNP), Everest region (Figure 1). This region includes the upper Pre-processing of the satellite images catchment of the Dudh Koshi River basin, one of the Level 1 products of WorldView and GeoEye are for seven major tributaries of the Koshi River basin. This customers with advanced images processing region covers the centre part of the SNP, and capabilities, and suitable for advanced 2 encompasses an area of 279 km and exhibits the 20 photogrammetric processing. Firstly, a rapid glaciers with 31% of total area of the NGB (Chand atmospheric correction tool available in ERDAS and Watanabe 2019). The Ngozumpa Glacier is IMAGINE was applied to convert sensor raw Digital largest glacier of Nepal (Bajracharya et al. 2014) Number (DN) values to ground reflectance values. 2 with an area of 80 km , which is located in this Rapid atmospheric correction uses information from region, and contributes to 30% of the total area of the companion metadata files (*.IMD), and normalized glaciers in the Everest region (Chand and Watanabe the top-of atmosphere (TOA) reflectance to ground 62

Bulletin of Nepal Hydrogeological Association, Vol. 5, September 2020 Chand MB, Watanabe T, 2020 reflectance scaled by 10,000 (Hexagon Geospatial, attained by using third order 3D rational functions 2019). Secondly, orthorectification of each used with vendor’s Rational Polynomial Coefficients image was carried out in the ERDAS IMAGINE. The (RPCs) (Aguilar et al. 2013). best horizontal geopositioning accuracies can be

Table 1 List of satellite images used for inventory of glacial lakes. SN Image Catalogue ID Date Satellite Resolution at nadir (m) 1 1050010007140900 11/05/2016 GE01 1.84 2 1040010019841600 04/27/2016 WV-03 1.24 3 10300100531C0A00 04/03/2016 WV-02 1.84

Fig. 1: A study area map showing the location of the Ngozumpa Glacier basin. The lakes and glaciers in the map were obtained from 2-m spatial resolution WorldView and GeoEye imageries of year 2016.

Therefore, Level 1 images were orthorectified sensor model and the quality of the imagery. using RPCs and the 30-m SRTM DEM. The RPCs Orthorectification of images using RPC sensor model are type of sensor model, which is a mathematical also geometrically correct the data to remove transform that defines the physical relationship distortions that occur during image capture. In this between image coordinates and ground coordinates. study, orthorectification was carried out without Sensor models are different for each sensor, and the Ground Control Points (GCPs), which is sufficiently accuracy depends on the accuracy of the original accurate to map the features of single time. However, 63

Bulletin of Nepal Hydrogeological Association, Vol. 5, September 2020 Chand MB, Watanabe T, 2020 horizontal distortion obtained by this method is type lakes), 2) proglacial lakes (P-type lakes), and 3) greater than sub pixel range and not suitable for unconnected lakes (U-type lakes), and their change detection with sub pixel accuracy. In this distribution over NGB is shown in Figure 1. The lakes study, these images were only used to make high- were classified into four size classes (<0.01 km2, 0.01- resolution inventory of the glacial lakes and glaciers 0.02 km2, 0.02-0.1 km2, and >0.1 km2) and for one time. distribution of lakes in each class is presented in Figure 3. Inventory of glacial lakes Normalized Difference Water Index (NDWI) is Supraglacial (S-Type) lakes widely used methods for the detection of water bodies from satellite images (Huggel et al. 2002; Xu S-type lakes, which formed at the surface of the 2006; Miles et al. 2017; Chand and Watanabe 2019). glacier, were most frequently occurring glacial lakes However, the possibility of misclassification and in the NGB in 2016. S-types lakes were observed on omission of lakes increases significantly with all three debris-covered glaciers; namely Ngozumpa, moderate resolution of the dataset, when we classify Choloste, and Taweche of the NGB. The total number the images automatically (Watson et al. 2016). of the S-type lakes observed to be 925, which was Therefore, manual editing is recommended to 91% of the total number of lakes in the NGB. The area covered by the S-type lakes was about 0.62 ± increase the accuracy of the mapping (Mergili et al. 2 2 2013; Shukla et al. 2018). In this study, we adopted 0.10 km with mean size of 0.0007 km , accounting the automatic method using NDWI and band metrics, for 23% of the total lake area in the NGB. There were more than 99% of the S-types lakes with size <0.01 and manual post editing after automatic classification 2 to improve the results. The detail method is km in 2016 (Figure 3), accounting for 56% of the total area of the S-type lakes. However, S-types lakes explained in Chand and Watanabe (2019). All the 2 glacial lakes with size >20 m2 were mapped in the with size >0.01 km were only seven, which NGB. contributed about 44% of the total area of the S-type lakes, while no lakes with size >0.1 km2 were observed. This result was similar to the results of Chand (2020), which presented the typical size of the RESULTS AND DISCUSSION S-type lakes <0.001 km2 in the Everest region. Mapping of the glacial lakes using an area threshold 2 of 20 m (5 pixels of 2-m images) had identified the About 97% of the S-type lakes were observed 1,022 glacial lakes covering an area of 2.67 ± 0.14 on the surface of the Ngozumpa Glacier, a largest 2 2 km in the NGB and a mean size of 0.0026 km in the glacier in the NGB, which contributed about 98% of year 2016. The distribution of glacial lakes in each the total area of the S-type lakes. There were only sub-basin is presented in Figure 1. The Shapiro-Wilk 29 S-types lakes on the surface of the Cholotse and distribution test at 95% confidence interval reveals Taweche glaciers. These glaciers were <2 km2 in that the distribution of lakes was not normal and size. Significant variability in the surface area of S- skewed positively with a factor of 15.76. The type lakes was observed among the studied glaciers, probability distribution of lakes reveals that 86% (n = ranging from 0.81% (Cholotse Glacier) to 2.33% 2 880) of lakes have an area of <0.0009 km (1 pixel of (Ngozumpa Glacier) of the debris-covered area in Landsat), which contribute approximately 5% of the 2016. Chand and Watanabe (2019) found strong total surface area of lakes (Figure 2). Similarly, about correlation between the number and area of the 94% of the lakes were found with the size <0.0036 lakes with the size, mean slope, mean glacier width. 2 km (4 pixels of Landsat). Only ⁓0.5% (n = 52) of They observed about 87% of the lake area on the 2 the glacial lakes of sizes >0.005 km contribute to glaciers with slopes <10°, of which 55% of the lake ⁓89% of the total lake area in the NGB. These area was observed on slopes from 2–6°, 17% on statistics showed that small sized lakes with size >20 slopes <2° and 15% on slopes from 6–10°. They 2 m were mapped accurately, which is difficult to map also demonstrated that S-type lakes could be found by using coarse-resolution images, i.e., Landsat (30 as high as ⁓5,560 m a.s.l., ⁓200 m lower than the m). The uncertainty for the coarse-resolution images upper extent of the debris portion of the glacier. To increases for the small features. Glacial lakes were understand the elevation wise distribution of the S- categorized into three types: 1) supraglacial lakes (S- type lakes, Ngozumpa Glacier was divided into 100 64

Bulletin of Nepal Hydrogeological Association, Vol. 5, September 2020 Chand MB, Watanabe T, 2020 m elevation difference zones (Figure 4). The largest area of the mostly consistent till 5,200 m, which was decreased lakes was observed at the first 100 m elevation after this elevation. zone (4600-4700 m) of the glacier, thereafter, it was found

Fig. 2: Probability distribution of glacial lake area in the Ngozumpa Glacier basin.

Fig. 3: Distribution of three types of glacial lakes on the Ngozumpa Glacier basin according to their different size classes: (a) number and (b) area.

The Ngozumpa Glacier is also characterized by which had an area of 0.25 km2, accounted 41% of the the presence of the complex of large supraglacial total area of the S-type lakes on this glacier. Series of lakes at the terminus of the glacier, a spillway lake, S-types lakes at the terminus of the glacier showed the 65

Bulletin of Nepal Hydrogeological Association, Vol. 5, September 2020 Chand MB, Watanabe T, 2020 trajectory toward the development of the large glacial lake (Chand and Watanabe 2019).

Fig. 4: Supraglacial lake area per 100-m elevation bins on surface of the Ngozumpa Glacier.

Unconnected (U-Type) lakes lake in the future (Chand and Watanabe 2019), which need a detailed study to understand the possible The unconnected lakes are not connected with hazards GLOF form the lake. glaciers, which are either glacier-fed or non-glacier- fed. The inventory of glacial lakes plotted the 95 U- type lakes in the NGB with a total area of 2.0 ± 0.04 CONCLUSION km2, and mean size of 0.0211 km2, about 31 times This study aimed to prepare the highest accurate larger than S-type lakes. These lakes contributed the inventory of the glacial lakes in the Ngozumpa largest lakes area (75%) among the three types of the Glacier basin in the Everest region using very high- lakes in NGB. The size distribution of the U-type resolution satellite images. The WorldView and lakes showed that about 64% if the total area of lakes GeoEye images of 2-m spatial resolution were was contributed by the lake with size >0.1 km2, and employed to map the three types of lakes for the year only 9% with size <0.02 km2 (Figure 3). Two largest 2016. All the glacial lakes with size >20 m2 were U-type lakes, i.e., Dudh Pokhari (Second Gokyo mapped using semi-automatic methods. The results Lake) and Tonak Pokhari (Third Gokyo Lake) with showed the largest number of S-type lakes (91%), and the surface area of 0.42 ± 0.7% km2 and 0.39 ± 0.7% largest area contributed by the unconnected lakes km2, respectively contributed about 42% of total area (75%). The number and area of the S-type lakes were of U-type lakes. found strongly correlated with size, mean slope, mean

width of the glacier (Chand and Watanabe 2019). The Proglacial (P-Type) lakes Ngozumpa Glacier exhibited the presence of large P-type lakes directly connected with glacier near complex S-type lakes at its terminus, which may terminus. In this study, we mapped the only two P- develop into large glacial lakes if the outlet channel type lakes with a total area of 0.05 ± 0.001 km2, remains at the same level. accounting for 2% of total area of the glacial lakes in the NGB. These two lakes were observed near the The U-type lakes showed the largest area of lakes, terminus of the clean type of the glacier. It can be which indicated the loss of the glacier area from the expected that number of P-type lakes are lake basin. These results complement the current transforming to U-type lakes with the retreat of the warming trend of temperature. No proglacial lakes glaciers in response to warming temperature. The P- were observed at the terminus of the debris-covered type lakes are usually large in size in comparison to glacier with gentle slope. other types of the lakes (Chand 2020), which is exceptional in case of NGB, where relatively small- This study advances the understanding of the status sized lakes were detected (Figure 3). P-type lakes of the glacial lakes in the NGB, a source of the Dudh could not be observed at the terminus of gently Koshi River. The result of this study could be the sloping debris-covered glaciers, where these lakes baseline for future study, development projects in the might develop into large lakes. However, complex of downstream region, which is essential for the spillway lakes at the terminus of the Ngozumpa implementation of GLOF risk management in the Glacier has shown a trajectory towards a large glacial region. 66

Bulletin of Nepal Hydrogeological Association, Vol. 5, September 2020 Chand MB, Watanabe T, 2020 Bolch T, Buchroithner MF, Peters J, et al (2008b) Identification of glacier motion and potentially dangerous glacial lakes in the Mt. Everest Acknowledgments region/Nepal using spaceborne imagery. Nat Hazards Earth Syst Sci 8:1329–1340. We express thanks to DigitalGlobe Foundation for https://doi.org/10.5194/nhess-8-1329-2008 providing the WorldView and GeoEye imagery and Bolch T, Kulkarni A, Kaab A, et al (2012) The State and Hexagon Geospatial for providing license for the Fate of Himalayan Glaciers. Science (80- ) ERDAS Imagine. We express our sincere thanks to 336:310–314. the National Geographic Society for providing early https://doi.org/10.1126/science.1215828 career grants during this study. Similarly, we also Bolch T, Pieczonka T, Benn DI (2011) Multi-decadal acknowledge the Ministry of Education, Culture, mass loss of glaciers in the Everest area (Nepal Sports, Science and Technology of Japan (MEXT) Himalaya) derived from stereo imagery. and the Kubota Fund, Japan for partially supporting Cryosphere 5:349–358. https://doi.org/10.5194/tc- this study. 5-349-2011 Chand MB (2020) Development of glacial lakes in the REFERENCES Everest and Kanchenjunga regions, Nepal Himalaya. Hokkaido University Chand MB, Kayastha RB, Parajuli A, Mool PK (2015) Aguilar MA, Saldaña M del M, Aguilar FJ (2013) Seasonal variation of ice melting on varying layers Assessing geometric accuracy of the of debris of Lirung Glacier, Langtang Valley, orthorectification process from GeoEye-1 and Nepal. Proc Int Assoc Hydrol Sci 368:21–26. WorldView-2 panchromatic images. Int J Appl https://doi.org/10.5194/piahs-368-21-2015 Earth Obs Geoinf 21:427–435. Chand MB, Watanabe T (2019) Development of https://doi.org/10.1016/J.JAG.2012.06.004 Supraglacial Ponds in the Everest Region, Nepal, Azam MF, Wagnon P, Berthier E, et al (2018) Review between 1989 and 2018. Remote Sens 2019, Vol of the status and mass changes of Himalayan- 11, Page 1058 11:1058. Karakoram glaciers. J Glaciol 64:61–74. https://doi.org/10.3390/RS11091058 https://doi.org/10.1017/jog.2017.86 Fujita K, Nakawo M, Fujii Y, Paudyal P (1997) Changes Bajracharya, Maharjan S, Shrestha F, et al (2014) in glaciers in Hidden Valley, Mukut Himal, Nepal Glacier Status in Nepal and Decadal Change from Himalayas, from 1974 to 1994. J Glaciol 43:583– 1980 to 2010 Based on Landsat Data. 588. https://doi.org/10.3189/S002214300003519X Kathmandu, ICIMOD. Fujita K, Nuimura T (2011) Spatially heterogeneous Basnett S, Kulkarni A V., Bolch T (2013) The influence wastage of Himalayan glaciers. Proc Natl Acad Sci of debris cover and glacial lakes on the recession U S A 108:14011–14014. of glaciers in Sikkim Himalaya, India. J Glaciol https://doi.org/10.1073/pnas.1106242108 59:1035–1046. Gardelle J, Arnaud Y, Berthier E (2011) Contrasted https://doi.org/10.3189/2013JoG12J184 evolution of glacial lakes along the Hindu Kush Benn DI, Wiseman S, Hands KA (2001) Growth and Himalaya mountain range between 1990 and 2009. drainage of supraglacial lakes on debris-mantled Glob Planet Change 75:47–55. Ngozumpa Glacier, Khumbu Himal, Nepal. J https://doi.org/10.1016/j.gloplacha.2010.10.003 Glaciol 47:626–638. Haritashya U, Kargel J, Shugar D, et al (2018) Evolution https://doi.org/10.3189/172756501781831729 and Controls of Large Glacial Lakes in the Nepal Benn DI, Wiseman S, Warren CR (2000) Rapid growth Himalaya. Remote Sens 10:798. of a supraglacial lake, Ngozumpa Glacier, https://doi.org/10.3390/rs10050798 Khumbu Himal, Nepal. Int Assoc Hydrol Sci Publ Hexagon Geospatial (2019) Rapid Atmospsheric 264 (Symposium Seattle 2000 — Debris-Covered Correction Glaciers) 177–185 Huggel C, Kääb A, Haeberli W, et al (2002) Remote Bolch T, Buchroithner M, Pieczonka T, Kunert A sensing based assessment of hazards from glacier (2008a) Planimetric and volumetric glacier lake outbursts: a case study in the Swiss Alps. Can changes in the Khumbu Himal, Nepal, since 1962 Geotech J 39:316–330. using Corona, Landsat TM and ASTER data. J https://doi.org/10.1139/t01-099 Glaciol 54:592–600. Kääb A, Berthier E, Nuth C, et al (2012) Contrasting https://doi.org/10.3189/002214308786570782 67

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