Long-term mass balance, runoff and area change in Stok group of glaciers, Ladakh, India, between 1969 to 2019.

An analysis of mass balance (MB), runoff and area of a group of small glaciers (< 1 km²) in Stok region, Ladakh, India, was conducted to investigate their behaviour in past few decades. These glaciers are essential to the downstream village of ~300 households as they are entirely dependent on the ice and snow meltwater for domestic and agricultural use. The study presents an in-situ (2014-2019) and historical modelled MB (1978-2019) of Stok glacier, which is first of its kind in Ladakh region. The observed MB was found to be negative since 2014 with an average MB of -0.4 m w. e. a^-1. However, modelled MB showed a balanced condition during 1980s, followed by a severe retreat in the first decade of 21^st century. MB sensitivity analysis suggests that the winter precipitation and summer temperature are almost equally significant in driving mass balance of the glacier and water resources. Around 27% increase in precipitation is required to compensate the melt due to 1°C rise in temperature. Net changes of glacier extent were determined from a detailed manual comparison of remotely sensed imagery acquired between 1969 to 2019 by the high-resolution declassified Corona mission, Landsat ETM+/OLI and PlanetScope satellites. All the glaciers in this region retreated with different rates during different periods. Overall, the reduction in glacier extent was found to be around -0.73 km² (-0.016 km² a^-1) equivalent to ~15% of the total glacier extent, in the past five decades. Runoff from the catchment was also modelled with the help of available temperature, precipitation and remote sensing data. The runoff model was calibrated and validated using daily in-situ discharge data of two summers (2018 and 2019). It was found that the runoff was highest during July and August months due to both increased snow and ice melt. Winter precipitation in this region is essential not only for glacier health but for early spring sowing season when the demand for the water is highest, and snowmelt water is the only source of early streamflow. Thus, this study assumes greater significance in light of an perceptible shift in precipitation from winter to summer in past two decades, which needs further investigation.