Monitoring Lakes in Mountains with Sentinel-1 SAR Imagery

Lakes are crucial markers of climate fluctuations, yet identifying water bodies in mountainous regions remains a persistent challenge for satellite remote sensing due to terrain-induced obstacles and dynamic surface conditions of ice and snow. We present a novel approach based solely on Sentinel-1 Synthetic Aperture Radar (SAR) data that allows for the detection of lakes as well as the tracking of their surface area and conditions in mountainous terrain. Our methodology is built around a new index called OASIS, which uses pairs of SAR images to detect and monitor lake surface conditions. We show that this index provides critical insights into lake surface conditions, including the onset and duration of ice cover and the observation of frozen lakes concealed beneath snow. We evaluate this capacity using in situ lake temperature measurements at Arpont Lake. At this lake, the estimated open-water onset typically occurs in mid- to late July, with an ice-free season lasting roughly three to three and a half months. The years 2017 and especially 2022 stand out, showing an earlier break-up (2 July and 24 June, respectively) and longer open-water periods (114 and 132 days; revisit 6 days in 2017–2021 and 12 days in 2016 and 2022–2024). When combined with a segmentation technique, the index allows for accurate quantification and tracking of temporal changes in lake surface area, highlighting lakes that experienced severe drought from 2016 to 2024 in the French Alps. We successfully validated our lake surface area estimates using state-of-the-art lake databases derived from optical sensors.