Large-scale study of spatiotemporal variations in surface velocity of the Gangotri glacier region (Garhwal Himalaya) using ERS-SAR, Envisat-ASAR and ALOS-PALSAR data spanning nearly two decades (1991-2011)
03-Apr-2020 UT
The Gangotri glacier is the largest in the Garhwal Himalaya, India and its melt water forms the main source stream of the Ganga River, yet its dynamics are poorly understood. Its long record of terminus retreat measurements (1800–present) shows multi-decadal oscillations and more recently a slowing down of the retreat. Its complex dynamics are also indicated by studies of proglacial melt–water at the terminus. However, there have been no systematic measurements of its surface velocity or how it changes seasonally or over longer-term time scales.

This is the first systematic, large-scale study of spatiotemporal variations in surface velocity of the Gangotri glacier region using Synthetic Aperture Radar (SAR) data spanning nearly two decades (1991-2011). The main finding is that the Gangotri glacier is dynamic throughout its length with both seasonal and inter-annual variations in surface velocity.

The region studied included the Gangotri glacier as well as ten other glaciers (Raktavarn, Meru, Chaturangi and its two tributaries, Kirti, Ganohim, Swatchand, Sumeru and Maiandi glaciers). Surface velocity maps were analyzed to reveal temporal and spatial dynamics of the Gangotri glacier and characterized them based on (1) spatial & temporal variation of surface velocities; (2) inter-annual (seasonal) and intra-annual surface velocity fluctuations; (3) characteristics of the dynamics and processes at work within the glacier, basal sliding and water (e.g. channelized or distributed drainage).

The results show that the glacier velocity vectors are oriented parallel to the glacier sides / centre-line of the glacier and increase in magnitude with the height of the glacier. These data also show consistent summer speed-up in velocity of the Gancotri glacier, which occurs throughout the length of the glacier up to ~20-25 km from the snout, compared to winter velocities. In addition, the Principal Investigator (PI) Scientist observed long-term changes in summer speed (Figure 4 & 5), where glacier velocity has reduced from >60-80% (1991-1999 mean winter/summer speeds: 30/55 m/year) to <30% (2010-2011 mean winter/summer speeds: 28/36 m/year) over two decades within the ablation region (up to ~15km), possibly even at higher elevations/accumulation region). In contrast, winter velocities have remained relatively stable throughout this period. Despite the reduction in summer velocities from 1991-2011, the Gangotri glacier is dynamic throughout its length with glacier velocities of ~15-35m/year in the ablation region and reaching ~35-70 m/year in the accumulation region. The velocity maps also suggest predominant and increased basal sliding during summer. Based on these results and other evidence in the literature,PI propose that the subglacial drainage system of the Gangotri glacier is subject to seasonal and interannual hydrodynamic coupling between winters and melt-seasons akin to other glaciers.

This study has been also presented in the paper "Spatiotemporal Variations in Surface Velocity of the Gangotri Glacier, Garhwal Himalaya, India: Study using Synthetic Aperture Radar Data" , authored by S.P. Satyabala and published on Remote Sensing of Environment Volume 181, Pages 151-161 in August2016.