Spatio-temporal evolution of glacial lakes in the Hindu Kush Himalaya

Abstract

The glacial lakes are an important part of the cryosphere, originating from the deposition of glacial meltwater in the depressions created mainly by the glacial movements. Under the ongoing trend of rising annual mean temperatures, glacial lakes around the world are observing an increase in both number and area. An increase in the annual mean temperature is responsible for the rapid melting of glaciers. However, at the regional scale, the evolution of glacial lakes is a complex process due to the interplay between factors like the differential rate of glacial melting, change in annual mean temperature, change in annual mean precipitation, local topography, etc. Hindu Kush Himalaya (HKH), being no exception, has experienced rapid growth in both the number and area of glacial lakes in the recent past. This rapid expansion of glacial lakes has some serious consequences, such as an increase in the risk of Glacial Lake Outburst Floods (GLOFs) in the region. In fact, the population of HKH has the highest exposure to the risk of GLOFs in the world. Furthermore, due to harsh topography and a large number of glacial lakes, it is very difficult to monitor each and every one based on in-situ information. The recent development of remote sensing techniques and computer-aided mapping has helped researchers immensely monitor the evolution of glacial lakes in the HKH. Therefore, this doctoral thesis systematically investigates the spatio-temporal evolution of glacial lakes at the sub-basin level in the HKH, factors affecting the growth of glacial lakes, and associated risks with the continuous expansion of glacial lakes. Firstly, glacial lake inventories were developed for four decades (1990, 2000, 2010, and 2020) using Landsat satellite imagery (TM/OLI) and the Shuttle Radar Topography Mission's (SRTM) Digital Elevation Model (DEM) at the subbasin level for the Indus, Ganga, and Brahmaputra (IGB) river basins. We then employed spatial analysis tools to comprehend the distribution, growth, and factors influencing the growth of glacial lakes in the IGB river basins. We found that the distribution of glacial lakes is uneven in the IGB river basins. The Brahmaputra River basin had the highest concentration of glacial lakes in the HKH, followed by the Indus basin and Ganga basin (Article I). However, the Ganga basin shows the highest growth rate in both the number and area of glacial lakes, followed by the Indus and Brahmaputra river basins (Articles I and IV). The main cause of the expansion is believed to be the rapid melting of glaciers. The mean distance between glaciers and glacial lakes also saw a reduction between 1990 and 2020 (Article I). The Ganga and Indus river basins have significantly lower mean distances as compared to the Brahmaputra river basin, which helps explain the above-average expansion of glacial lakes in the Ganga and Indus river basins as compared to the Brahmaputra river basin (Article I). Furthermore, end-moraine dammed lakes, especially those within 10 km of the nearby glaciers, are the most dominant type of glacial lakes, and supraglacial lakes show the highest increase in growth among different types of glacial lakes. This again highlights the contribution of the rapid melting of glaciers and the aggravating risk of future GLOF events in the region. Secondly, new empirical equations were developed to estimate the mean depth and volume of glacial lakes using primary field-based and secondary literature-based bathymetry data to further enhance the understanding of glacial lake evolution and facilitate the modelling of GLOF scenarios and risk assessments (Article II). We developed the empirical equations using the area-scaling method. It was observed that regional lake characteristics play a crucial role in the development of empirical equations. Factors like lake shape, dam material, glacier movement, sedimentation rate, and lake bottom topography play a crucial role in determining the depth and volume of glacial lakes. The findings of present research highlight the rapid expansion of glacial lakes, with subregional heterogeneities in glacial lake characteristics and changes in them. The study recommends continuous monitoring of glacial lakes with high-resolution satellite data and, wherever possible, by field-based observations, to better understand the evolution of glacial lakes, factors affecting the evolution, identification of potential hazardous glacial lakes, employment of early warning systems at potentially hazardous glacial lakes, and development of GLOF mitigation strategies.