Comprehensive assessment of the long-term mass balance (40 years) has been performed for Patsio glacier located in Bhaga river basin in the Great Himalayan region. The temperature-index approach has been employed for generating the time-series of annual specific mass balance of 25 years (1993–2018) using Snow and Avalanche Study Establishment (SASE) field observatory data in Patsio region. Results based on observational data showed nearly balanced condition until the year 1999 (+0.03 m.w.e/yr), followed by an accelerated rate of mass loss during the period 2000–2007 (−0.17 m.w.e./yr). This was evident due to the annual rise (+0.7$^{\circ}$C) in the temperature and drop (−122 mm) in the amount of precipitation during 2000–2007. A slight drop in the mass balance (−0.14 m.w.e/yr) was observed during the past decade (2008–2018) which was well supported by in-situ climate observations and published geodetic results in the region. The altitudinal distribution of modelled mass balance was observed well in correlation with the glaciological measurements for the periods 2010–2011 and 2011–2012. Further, seasonal sensitivity characteristic (SSC) was developed for the glacier illustrating the dependence of specific mass balance on monthly anomalies in temperature and precipitation. The developed SSC was used with bias-corrected temperature and precipitation estimates from recently released ERA5 reanalysis to reconstruct annual specific mass balance during since 1979s.

$\bf{Highlights}$

$\bullet$ Field based temperature lapse rate, precipitation gradient and snow melt factor established for the Patsio glacier.

$\bullet$ Seasonal sensitivity of the glacier mass balance was studied.

$\bullet$ Past reconstruction of the glacier mass balance using bias corrected ERA5 reanalysis.

$\bullet$ Comparison of modeled mass balance with published geodetic and glaciological results.

The Himalayan cryosphere system is a major source of freshwater supply for the Indian subcontinent. The rivers originating from the Himalaya are strongly influenced by the behaviour of glaciers depending upon the regional climatic variables. Therefore, understanding the mass budget of glaciers is important. In the present study, we discuss the hydrological mass balance of the largest Siachen glacier in the Karakoram range. The computations of runoff and other parameters on such a large glacier are challenging due to data scarcity, difficult accessibility and complex topography of the region. The snowmelt runoff model (SRM) coupled with in-situ and remotely sensed information was used to simulate the glacier runoff for the past three decades (1986–2018). The accumulation in the form of snowfall and ablation in the form of snowmelt and evaporation were used as inputs for the estimation of hydrological mass balance. The SRM has well estimated the discharge of Siachen glacier with the coefficient of determination (R$^2$) 0.92 and average volume difference (D$_v$) 4.4% for the validation period. The study revealed that the Siachen glacier experienced few marginally mass gain episodes before 2000 but after that, a rapid and continuous mass loss has been observed. The average mass balance during the past three decades was found to be -0.11 ± 0.24 m w.e./year. This suggests that glaciers of East Karakoram have started losing mass; still, it is not as significant as generally observed in other parts of the Hindu Kush Himalaya.