Wetland Water Level Monitoring Based on Hydrological Unit Division Using SBAS-InSAR: A Case Study in Louisiana, USA

As one of the most important ecological indicators of wetlands, water level directly reflects hydrological processes and ecological patterns. Therefore, efficient and accurate monitoring of water level is critical for wetland conservation and restoration. Interferometric Synthetic Aperture Radar (InSAR), with its advantages of wide coverage, all-day/all-weather observation, and high measurement precision, has been successfully applied to wetland water level monitoring. However, due to pronounced heterogeneity in internal hydrological connectivity within wetlands, conventional InSAR techniques often suffer from phase discontinuities and error propagation across hydrological boundaries, making it difficult to accurately characterize water level variations over large and complex wetland systems. To address this limitation, we propose an absolute wetland water level monitoring method based on hydrological unit division using the Small Baseline Subset InSAR (SBAS-InSAR) framework, aiming to improve the reliability of InSAR-derived water level estimates under complex hydrological conditions. Taking the floodplain of Louisiana, USA, as a case study, multi-temporal Sentinel-1 SAR imagery combined with global land cover data is used to analyze hydrological connectivity and partition the study area into multiple relatively independent hydrological units. Within each hydrological unit, a small-baseline interferometric network is constructed to retrieve relative water level change time series, which are subsequently calibrated using in situ observations from United States Geological Survey (USGS) hydrological stations. Finally, least-squares estimation is applied to derive the spatiotemporal distribution of absolute water level changes. The experimental results demonstrate that: (1) hydrological unit division significantly improves the reliability of time-series inversion, reducing the overall root mean square error (RMSE) from 13.20 cm to 4.03 cm; (2) hydraulic barriers such as levees and urban infrastructure substantially disrupt the spatial continuity of wetland water level variations; and (3) C-band coherence in wetlands exhibits pronounced seasonal variability, with the highest coherence observed from late winter to early spring and the lowest from late summer to early autumn, mainly influenced by vegetation phenology and inundation conditions. Overall, the proposed method enables centimeter-level, large-scale monitoring of wetland water level changes, providing technical reference and data support for wetland water resource management and ecological protection.