A simple bidirectional-reflectance model applied to a tallgrass canopy

Abstract

A simple bidirectional reflectance model suitable for homogeneous plant canopies is developed. The model was applied to simulate directional changes of spectral reflectances from a tallgrass canopy and compared with field measurements by Deering and Middleton (1990). A modified two-stream approach was used in the model by considering the transfers of upward reflected and diffuse radiation in different viewing directions, downward direct and diffuse radiation, and their dependences upon canopy leaf orientation. An analytical solution was derived to allow a fast computation of directional reflectances based on canopy parameters or a retrieval of canopy parameters based on reflectance measurements. Modeled spectral reflectances are generally in a good agreement with measurements, and effects of inhomogeneous canopy characteristics such as hot “spot” and of soil reflection were examined. Both modeled and observed reflectances show strong backscattering and relatively weak forward scattering in the red band, but a more symmetric distribution in the near-infrared band. The directional changes of reflectance in both red and near-infrared bands are dominated by primary scattering; the magnitudes of reflectance in the near-infrared band are largely influenced by secondary scattering. Model simulations for three theoretical leaf angle distributions (planophile, spherical, and erectophile) show that hemispherical reflectance or albedo could be overestimated by nadir reflectance at a small solar zenit angle and underestimated at a large solar zenith angle. The error, however, can be reduced to a minimum at an optimal solar zenith angle which has the same magnitude for the three leaf angle distributions but decreases with increasing canopy leaf area index. Modeled vegetation spectral index, NDVI, appears to vary by 10–24 % with viewing angle for a given leaf angle distribution and canopy leaf area index, and it generally has a larger value for forward scattering than for backscattering.