Comparison of Synthetic Aperture Radar Sentinel-1 and ALOS-2 observations for lake monitoring

This work investigates the efficacy of L -band and C -band Synthetic Aperture Radar (SAR) sensors onboard ALOS-2 and Sentinel -1 satellites, as compared to optical sensors onboard Sentinel -2 satellite, for mapping open water of the Tri An reservoir, one of the largest artificial reservoirs in South Vietnam, during the 2016-2023 period. The Google Earth Engine (GEE) was the primary computing platform to pre-process all satellite observations. The Otsu threshold algorithm was employed to generate water/non-water maps derived from the VH- and HH-polarized backscatter coefficient data acquired by Sentinel -1 and ALOS-2 satellites and from the Modified Normalized Difference Water Index (MNDWI) data acquired by Sentinel -2 satellite, respectively. The findings reveal the stability of Tri An reservoir's surface water extent from 2017 to 2022, followed by a significant decline of nearly 70% during the dry season of 2023 to approximately 100 km2. This substantial decrease can be explained by the impact of a robust El Nino phase occurring in the region simultaneously. Overall, there is a high consistency between results derived from SAR and optical sensors, but the correlation between Sentinel -1 and Sentinel -2 (R = 0.9774) was higher than that between ALOS-2 and Sentinel -2 (R = 0.9145). During the drought period, both C -band and L -band SAR sensors overestimate the reservoir's surface water extent due to the similarity in their backscatter coefficient between water and dry flat soil surfaces. This misclassification is more pronounced in ALOS-2 data than Sentinel -1 data, suggesting that the C -band sensor is more suitable than the L -band sensor for mapping the lake's open water areas.