Space-based Water Level Observation of the Everglades From Dual Polarization InSAR Data

Monitoring water levels in South Florida's Everglades wetland is very important for understanding the hydrology and for the management of this valuable ecosystem. Space-based Synthetic Aperture Radar (SAR) observations processed with interferometric techniques (InSAR) successfully reveal high spatial resolution maps of water level changes in the Everglades and other wetlands. This technique utilizes radar pulses, which are double-bounced from the water and vegetation in emergent vegetated wetlands. The polarization characteristics of radar signal is a very important parameter affecting interferometric coherence over wetlands. In this study, we focus on the feasibility of dual polarization mode observations (HH-HV, VV- VH) for their usage in Wetland InSAR applications. The study is based on TerraSAR-X observations acquired over the Everglades wetland and near by urban area during six month period in 2008. Interferometric and coherence analyses of the TerraSAR-X co- and cross-polarization observations (HH-HV and VV-VH) reveal that interferometric phase maintains good coherences over both urban and wetland areas (all coherences > 0.40). The HH polarization yields the highest coherence (0.46) compared with other polarization interferograms (VV: 0.44, HV and VH: 0.42). These high coherence levels are surprising and indicative of the scattering mechanism occurring in wetlands. Insofar, co-polarization is attributed to surface and double bounce scattering mechanisms, whereas cross-polarization is attributed to volume scattering. For the 3.3 cm TerraSAR-X wavelength, volume scattering is assumed to occur mainly in tree's canopies. However, it seems impossible for canopies, which easily move by wind, to maintain a coherent phase over 11 days or longer time periods. Thus, we suggest that cross-polarization in wetlands is also dominated by the double bounce scattering mechanism, involving scattering from both vegetation and water surfaces. Our conclusion is supported by the very similar fringe pattern detected in the co- and cross-polarization interferograms, reflecting water level changes in the wetlands. Furthermore these inspiring results suggest that quad polarimetric data can be useful in discriminating the scattering behaviors over wetland areas.