Assessing flooding and vegetation structure in forested wetlands using Radarsat SAR imagery

Abstract

We used 32 Radarsat SAR images collected between 1996 and 2001 over the Roanoke River floodplain, North Carolina (USA), to assess the utility of C-HH Radarsat SAR for operationally mapping flooding beneath forest canopies. Our objective was to test the sensitivity of standard-beam Radarsat to flooded forests under leaf-on and leaf-off conditions, and at multiple incidence angles (ranging from 10-46/spl deg/). We found that winter (leaf-off images) could be used to accurately map flooding (>95% accuracy) regardless of incidence angle. Mapping accuracy was acceptable for leaf-on images (>85%) but was nevertheless significantly lower than for leaf-off images. Images at shallow incidence angles (e.g. S6 mode, 41-46/spl deg/) were especially affected by speckle in the interpretations, whereas images acquired at very steep incidence angles (extended low beam, 10-23/spl deg/) exhibited a substantial amount of geometric feature displacement that limited their utility for practical applications. For mature forests, forest structure (independent of leaf-on or leaf-off status) did not appear to affect the ability to map flooding; however, separate interpretations were required to delineate flooding in very young, dense successional stands (<15 years). When controlling for flooding status, we found that Radarsat images were sensitive (in a statistical sense) to some components of forest structure (basal area, canopy height, crown depth), especially if both leaf-on and leaf-off images and images acquired at multiple angles were used for the analysis.