Land Subsidence in the Mekong Delta Derived From Advanced Persistent Scatterer Interferometry With an Infrastructural Reference Network

The Vietnamese Mekong Delta (VMD) has been affected by environmental challenges for several decades, including coastal erosion and land subsidence. Subsidence rates of several cm/yr have been reported in parts of the delta, which are one order larger than the regional sea level rise of 3.3 mm/yr. The precise monitoring of subsidence with high spatial and temporal resolution is essential to understand its causes and associated risks, and to support the development and monitoring of countermeasures. Here, we present subsidence estimates between 2017 and 2022 derived from advanced persistent scatterer interferometry (PSI) applied to Sentinel-1 data. The PSI approach fully integrates temporary persistent scatterers, in order to derive the best possible persistent scatterer network for long time series. Furthermore, we adapted a method to optimally integrate reference points with known displacement to suppress spatially correlated nuisance (SCN) in the derived displacement time series. Due to a lack of geodetic references, we built an infrastructural reference network based on large bridges with piled foundations of about 70 m depth, which are only affected by compaction in deeper soil layers. We compare the method with the standard referencing approach in PSI, and show that referencing to the selected reference bridges with our method enables to measure the largest part of the subsidence while reducing the SCN considerably. We find the highest subsidence rates of more than 7 cm/yr mainly in urban areas, and various kinds of temporal nonlinearities.