Sensitivity of radar backscatter to mangrove forest structure and AIRSAR imaging parameters

NASA JPL AIRSAR data, acquired over the Alligator Rivers of Kakadu National Park (KNP) in Australia’s Northern Territory, were used to investigate the potential of SAR data for retrieving forest structural and biomass information. The area was imaged during two independent PACRIM missions (in November 1996 and September 2000), with fully polarimetric data acquired at P- (0.44 GHz), L- (1.25 GHz) and C- (5.3 GHz) bands at a mean incidence angle of 42° and 69.8° respectively. Mangroves within KNP are most prevalent along the intrusive and tidally dominated creeks that dissect the northern coastline. The focus of this study was on the West Alligator River, where extensive mangrove forest was encountered. Notable differences in radar backscatter at all frequencies and polarisations were also observed between forest of differing height and species composition, as determined using a combination of stereo aerial photography and hyperspectral data. Enhanced backscatter on the seaward margin was associated with tidal inundation beneath the canopy on both dates. To better understand the interaction of microwaves with mangrove forest structures, field data collected in October 2002 were used to parameterize the wave scattering model of Karam et al. (1995) and decompose the backscatter in different channels. The decomposition showed a greater return from the canopy rather than trunks with increasing biomass, which questions the use of SAR data for biomass retrieval in mangrove forests. Differences in backscatter with incidence angle were also observed. The study emphasized the benefits of using finer spatial resolution AIRSAR data and the results of scattering analysis to better interpret and understand the complex scattering response over mangrove forest, and has implications for the future characterisation, mapping and monitoring of mangroves from both airborne and spaceborne SAR data.