Impact of water background on canopy reflectance anisotropy of a paddy rice field from multi-angle measurements

Water background significantly affects the reflectance of partially flooded vegetation. However, the flooded background effects on the reflectance anisotropy have not been fully understood. This paper explores the water background effects on the anisotropic reflectance of rice canopy through multi-angle measurements. Seasonal multi-angle reflectance was measured for different view zenith angles (VZA) (−60°-60° with 15° interval) in a flooded paddy rice site. Wavelengths of 550 nm, 660 nm, 850 nm and 1640 nm were extracted to show the canopy anisotropic characteristics. Two canopy anisotropic indices, the anisotropic factor (ANIF) and the anisotropy index (ANIX), were examined for both non-flooded and flooded canopies in key phenology phases. Results show that the reflectance anisotropy of rice canopy is significantly weakened by a shallow layer (≈10 cm) of water. For VZA > 15°, background water decreases the canopy reflectance by up to 28% and weakens the anisotropic properties when LAI < 4.0 and the fraction of vegetation cover (fCover) <80%. The background water shows barely no effect on canopy reflectance when LAI > 5.0. 1640 nm shows the strongest anisotropy when LAI < 2.0, whereas 660 nm shows more significant anisotropy when LAI > 3.0. The backward observations show stronger scattering than the forward observations for the same VZA. The data from this study can be used as input for biophysical models and as validation data for remote sensing products.