Climate-driven transformations in the Western Himalaya: projecting glacier retreat, snow cover variability, and...

The high rate of warming in the Himalayan region is causing increased glacier melting and altering river flow patterns. This study focuses on the western Himalayan watershed, the Upper Ganga Basin (UGB), which is starting point of the Ganga River and a major contributor of snow/glacier melt to its streamflow. A coupled modeling framework was developed by integrating glacier thickness with the Spatial Processes in Hydrology (SPHY) model. The model was calibrated through a three-way hybrid approach using observed streamflow, MODIS-derived snow cover area (SCA), and historical glacier thinning records. The calibrated model reproduced hydrological processes well, achieving mean R², NSE, KGE, and RSR values of 0.815 (0.82), 0.715 (0.635), 0.64 (0.59), and 0.53 (0.595) for calibration (validation), respectively. It also reproduced an average winter snow cover of 47 % (2002-2005), closely matching the MODIS estimate of 46.5 %, and a mean glacier thinning of -3.24 m (2001-2015), compared with the observed -3.74 m. The model was then applied to project streamflow variation, SCA, and glacier thickness changes for 2001-2100 under two Shared Socioeconomic Pathway scenarios: high-emission (SSP585) and moderate-emission (SSP245). Projections indicate that the rainfall-runoff contribution to total streamflow will rise from 70.6 % (historical) to 84.7 % under SSP585 (2055-2100), while glacier melt and snowmelt contributions will decline from 9.9 % to 6.8 % and from 18.3 % to 7.7 %, respectively. The glacier area in the UGB is projected to shrink by approximately 40 % under SSP245 and 45 % under SSP585 by 2100. Greater dependence on rainfall runoff could lead to higher peak flows during the monsoon season, increasing flood risk, while reduced glacier and snowmelt contributions would increase drought risk during dry years. Under the high-emission scenario, snowfall is projected to start later and end earlier, with less snowfall in December and March and greater concentration in January-February.