[Review on wetland water level monitoring using interferometric synthetic aperture radar]

Water level is an important indicator of wetland hydrological regime. Detection of wetland water levels through interferometric synthetic aperture radar (InSAR) has outstanding advantage, including high spatial resolution, high accuracy, low cost, and high efficiency. We introduced prerequisites for the monitoring of wetland water levels with InSAR, discussed the types of InSAR techniques, the influencing factors for monitoring wetland water levels and their advantages and disadvantages. There are three prerequisites for effectively detecting wetland water levels with InSAR techniques: 1) the presence of emergent aquatic plants; 2) the main backscattering mechanism is double bounce scattering; and 3) the interferometric coherence exceeds a certain threshold. Current water level monitoring techniques have been developed from traditional InSAR techniques to advanced InSAR techniques, such as STBAS, MM, and DSI. These techniques evolve from detecting relative water level changes to estimate absolute water level and water depth time series. The influencing factors of InSAR techniques for monitoring wetland water levels include operating para-meters of the synthetic aperture radar (SAR) and characteristics of the wetlands themselves. Finally, we proposed the key directions for future research in this field: i) investigating the potential use of specific water level monitoring techniques in other regions with different backscattering and interferometric coherence characteristics; ii) developing new algorithms to integrate multi-sensor, multi-track, multi-band, multi-polarization, and multi-temporal InSAR repeat-pass observation; iii) considering alternative sources of SAR data; and (iv) strengthening research on "by-products" of wetland water level monitoring with InSAR, such as wetland hydrological connectivity, flow direction, and flow regime.