High-Resolution Glacier Inventory Update for the Bhagirathi Basin (NW Himalaya) Using Integrated Optical and SAR Coherence Analysis

Accurate monitoring of glacier extent at high spatial resolution is essential for understanding cryospheric responses to climate change, particularly in sensitive and data-sparse regions such as the Indian Himalaya. In this study, we present an updated glacier inventory for the Bhagirathi basin in the NW Himalaya using an open-source, reproducible workflow that integrates optical and Synthetic Aperture Radar (SAR) observations. The Randolph Glacier Inventory (RGI v7, ~2000) was used as the baseline dataset. Although RGI has enabled global-scale glacier assessments, its application in the Bhagirathi basin reveals spatial inaccuracies, particularly along ice divides and within debris-covered glacier zones, largely due to resolution limitations. These shortcomings were systematically addressed using high-resolution LISS-IV imagery (5.8 m) from Resourcesat-2/2A, allowing precise manual refinement of clean-ice margins and narrow glacier tongues. To improve delineation of debris-covered glaciers, we incorporated Sentinel-1A/B Single Look Complex (SLC) data from ascending and descending passes to generate interferometric coherence maps using open-source InSAR processing routines. SAR coherence, sensitive to surface motion and temporal change, proved effective in identifying actively flowing glacier ice beneath debris and in shaded terrain. Integration of coherence information with optical data significantly reduced misclassification associated with seasonal snow, surface debris, and shadow effects. The RGI baseline contained 124 glacier polygons within the Bhagirathi basin. Following detailed boundary correction and flow-divide refinement, the updated inventory identifies 103 distinct glaciers, reflecting the merging of previously mis-segmented units. The revised total glacierized area is 397.62 km², exceeding the RGI estimate due to improved detection of debris-covered ice. The updated inventory provides a robust baseline for multi-temporal glacier change analysis and demonstrates the potential of SAR-supported, high-resolution approaches for next-generation glacier inventories in the Himalaya and beyond.