Multi-decadal snow cover assessment in the upper alaknanda river basin: integration of spectral unmixing, uncertainty quantification, and trend analysis

This study presents a comprehensive 30-year (1994–2024) assessment of snow cover dynamics across 13 sub-basins of the Upper Alaknanda River Basin, Garhwal Himalaya, integrating multi-sensor satellite data (Landsat TM/ETM+/OLI and Sentinel-2 MSI) with Multiple Endmember Spectral Mixture Analysis (MESMA), rigorous uncertainty quantification (±4.5% to ±12.0%), and statistical trend detection. The analysis reveals a significant decline in total snow cover from 572.30 ± 42.92 km2 in 1994 to 325.84 ± 14.66 km2 in 2024, representing a loss of 226.5 ± 42.92 km2 (39.6%) at an average rate of 7.55 km2/year (p = 0.021). Six sub-basins exhibited statistically significant declining trends (p < 0.05), with Kagbhusand experiencing the most severe loss (83.1%). Change point detection identified 2010 as a critical threshold, after which snow cover decline accelerated markedly. Concurrent climatic analysis reveals consistent warming across all basins (average 0.9 °C, 92% significant) and widespread drying (46% significant), with basins experiencing both showing the most pronounced snow cover declines. MESMA substantially reduced mixed-pixel errors, achieving 93.7% overall accuracy for recent sensors. These findings provide robust evidence of rapid cryospheric transformation in the region, with profound implications for water security and ecosystem stability in this vulnerable Himalayan catchment.