Understanding glacier retreat in the Bhutan Himalaya through regional inventory and influence of glacial lakes.

Abstract The present study demonstrates the importance of glacier inventories in understanding and managing glaciers in the Himalayan region, specifically in the Pho Chu Basin (PCB) of Bhutan Himalaya. The study utilised, Sentinel-2A imagery of 2021 to create a novel inventory of glaciers in the PCB, and further modified the boundaries of 35 (area > 1 km2) glaciers for the years 1991 and 2006. The results suggest, 184 glaciers covering an area of 192.07 + 15.50 km2 in the PCB, with the areal extent of the 34 glaciers (excluding Thorthormi Glacier) decreasing from 153.32 + 8.60 km2 to 139.97 + 8.44 km2 (1991-2021), indicating an area loss of 8.7 % in the basin. Further, we have observed the topographic parameters of each glacier. Interestingly, the glaciers retreated at a rate of -14.62 + 11.39 ma-1 (1991-2021), and small glaciers retreat much faster than large glaciers in PCB. Our observation reveals that glacier with lakes in this region are subjected to more retreat when compared to clean and debris covered glaciers. The alarming retreat of the Thorthormi Glacier has been the subject of multiple GLOF events and is analysed separately in the present study. Overall, the study demonstrates the importance of glacier inventories as a database for researchers, policymakers, and local communities about the location, extent, area, length, orientation, and other characteristics of glaciers.