Glacier change at the ISM-MLW transition: multi-decadal evidence of spatial heterogeneity from the Sara Umga watershed, western Himalaya

This study presents a multi-decadal assessment of glacier change in the Sara Umga Watershed (SUW), between the Parvati and Chandra basins. The region receives a majority of its annual precipitation from Indian Summer Monsoon (ISM) and a comparatively lower share contributed by winter Mid Latitude Westerlies (MLW). Using multi-temporal satellite data, we quantify changes in glacier length, area, surface velocity, elevation, debris cover, ice cliff dynamics, and high-altitude lakes, supplemented by long-term climate trend analysis. A total of 54 glaciers (>0.05 km 2 ) were mapped, covering 128.38 ± 2.98 km 2 in 2024, of which 24.84 ± 1.15 km 2 comprised debris-covered ice. Between 1994 and 2024, total glacier area declined by 4.54 ± 0.04 km 2 (0.15 ± 0.001 km 2 yr −1 ), while debris cover expanded to 24.84 ± 1.15 km 2 (0.37 ± 0.09 km 2 yr −1 ). Frontal retreat rates for the five major glaciers ranged from ∼12 to 26.2 m yr −1 . The largest glacier, Sara Umga (GL01), exhibited a mean surface velocity of ∼23.4 m yr −1 . Elevation-change analysis reveals pronounced spatial heterogeneity in glacier thinning, with maximum surface lowering concentrated in the lower, debris-covered ablation zone (3900–4400 m a.s.l.; −1.22 ± 0.95 to −1.67 ± 0.39 m yr −1 , locally exceeding −3.0 m yr −1 ), diminishing thinning at mid-elevations, and near-equilibrium conditions across the accumulation area of GL01 (−0.07 ± 0.54 to 0.06 ± 0.33 m yr −1 ). Zones of higher thinning and flow attenuation coincide with dense ice-cliff and lake development, indicating strong melt-dynamics coupling. Climate analysis (1901–2024) shows significant winter warming (0.01 °C yr −1 ) and declining Mid-Latitude Westerly (MLW) dominated winter precipitation (−0.160 mm yr −1 ), alongside increasing pre- (0.354 mm yr −1 ) and post-monsoon (0.070 mm yr −1 ) precipitation. These findings demonstrate that changes in debris-covered Himalayan glaciers cannot be inferred from frontal retreat alone. Instead, coupled changes in thickness, velocity, surface morphology, and climate trends govern long-term glacier response and hence should be studied alongside.