Effects of Inundated Vegetation on X-Band HH–VV Backscatter and Phase Difference

TerraSAR-X HH and HH-VV polarization data of a wetland in Estonia are used to study the effects of HH-VV phase difference on detecting inundated vegetation. Patches with different tree species composition are selected from two images representing flooded and unflooded conditions. Contribution of the double bounce mechanism is studied via phase difference and backscatter analysis. An increase in both HH and HH-VV backscatter was observed for all study cases in flooded conditions, as compared to unflooded periods. Over the different patches, HH backscatter increased from 2.3 dB to 8.2 dB, while for HH-VV - from 3 dB to 9.8 dB. The HH-VV channel allowed for a better separation between flooded and un flooded forest than HH, with the difference between the HH-VV and HH backscatter increase in inundated conditions ranging from 0.2 dB for coniferous stands with a tree height of less than 10 m to 1.6 dB for deciduous stands with a tree height of less than 10 m. A larger separation on both channels was in general observed in deciduous stands as the leaf-off season allowed for a better penetration depth. A considerable phase shift between HH and VV channels ranging from 10° to 28° was observed due to inundation, suggesting a strong contribution from the double bounce mechanism. For unflooded patches, the difference in backscatter between the two acquisition dates ranged from -0.8 dB to 1 dB for HH, and from 0 dB to 2 dB for HH-VV channel. Phase shift for unflooded patches varied from 5° to 8°.