Spatiotemporal variations in surface velocity of the Gangotri glacier, Garhwal Himalaya, India: Study using synthetic aperture radar data

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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. Here, I have characterized the spatiotemporal variations of surface velocity of the Gangotri glacier using synthetic aperture radar (SAR) data spanning nearly two decades (1991-2011). The main findings are as follows: (1) The glacier undergoes seasonal fluctuations in surface velocities: there was a clear summer speedup of similar to 57%-126% compared to winter velocities between 0 and 12.6 km from the terminus, with peak summer speeds of 63.1 +/- 5.4 m/yr in 1992, 66.6 +/- 6.0 m/yr in 1999, 58.2 +/- 4.5 m/yr in 2004 and 42.8 +/- 4.2 m/yr in 2007 whereas winter speeds were relatively stable (25-30 m/yr) during the same period. This summer speedup is indicative of increased basal sliding, known to occur when melt-waters penetrate to the glacier bed resulting in reduced friction. (2) These summer velocities exhibited an inter-annual reduction, which was manifest as a reduction in summer speed up from >120% in 2004 to <60% in 2007. This pattern continued into 2011, despite the availability of abundant melt-water during those years. This inter-annual reduction indicates the formation of increasingly efficient melt-water channels year on year resulting in lesser lubrication at the glacier-bed interface. My results show that the Gangotri glacier is dynamic throughout its length with systematic spatiotemporal variations in surface velocity that elucidate its subglacial processes. Based on my results and other evidence in the literature, I propose that the subglacial drainage system of the Gangotri glacier is subject to seasonal and inter annual hydrodynamic coupling between winters and melt-seasons akin to other glaciers. (C) 2016 Elsevier Inc. All rights reserved.