Multi-mode SAR observations of interannual frozen regimes of glacial lakes in central Himalaya

Study region The Koshi River Basin in central Himalaya. Study focus Many glacial lakes (GLs) in the Himalayas may freeze or become buried beneath seasonal snow cover during winter. These frozen or snow-covered lakes can persist through the cold season, altering lake thermal conditions, influencing ice thickness evolution, and affecting the timing and intensity of potential glacial lake outburst floods (GLOFs) in the following melt season. However, the spatial distribution characteristics and interannual dynamics of such lakes remain poorly understood, and quantitative assessments of the overall frozen percentage are still lacking. Here, we conduct a multi-mode synthetic aperture radar (SAR) investigation to automatically identify frozen/snow-covered GLs across the Koshi River Basin in central Himalaya from 2014 to 2024. To minimize misclassification from other low-backscatter or snow-covered areas, summer GLs were initially mapped from optical imagery to define their potential lake boundaries. Then an adaptive thresholding approach was developed to identify frozen/snow-covered GLs within the dilated extents of summer lakes, based on histogram edge detection on winter Sentinel-1 and ALOS PALSAR-2 imagery. This method enables the integration of SAR extraction results obtained from various radar configurations. The extracted results demonstrated high consistency with manual interpretation, yielding a precision of 0.84 and a Kappa coefficient of 0.78 for Sentinel-1 VH data, and a precision of 0.78 and a Kappa coefficient of 0.76 for ALOS PALSAR-2 HH data. New hydrological insights for the region From 2014–2024, frozen and snow-covered GLs covered an average total area of 69.28 km², primarily concentrated in central basin and southern high-relief regions. A notable expansion was detected in 2023–2024, particularly in the main mountain range and northern flat areas, where anomalously high temperatures and enhanced surface runoff were observed. In 2015 and 2021, a widespread occurrence of frozen lakes was identified in the western mountains, likely associated with above-average snowfall. Overall, approximately 81% of summer GLs in the Koshi River Basin remained frozen or snow-covered through winter, indicating a significant potential impact on spring melt dynamics and downstream flood hazards.