Dynamics of directional reflectance factor distributions for vegetation canopies

Directional reflectance factors that span the entire exitance hemisphere were measured for vegetation canopies and bare soils with different geometric structures. Two spectral bands were used—NOAA 6/7 AVHRR bands 1 (0.58–0.68 μm) and 2 (0.73–1.1 μm). Geometric measurements of leaf orientation distributions were taken when possible, and other structural and agronomic measurements were collected. For each cover type, these data were taken several different times on a clear day. Polar coordinate system plots of directional reflectance factors, along with 3-D computer graphic plots of scattered flux, were created. These field data were used in conjunction with literature data to study the dynamics of the directional reflectance factor distribution as a function of the geometric structure of the scene, solar zenith angle, and optical properties of the leaves and soil. Physical mechanisms causing the observed dynamics were proposed and were supported by a number of field and modeling studies. For complete homogeneous vegetation canopies, the major trend observed at all sun angles and spectral bands was a minimum reflectance near nadir and increasing reflectance with increasing off-nadir view angle for all azimuth directions. This trend is well known in the experimental and theoretical literature and is caused by the shading of lower canopy layers by components in the upper layers and by viewing different proportions of the layer components as the sensor view angle changes. In some cases the reflectance minimum was shifted slightly off-nadir in the foward scattering direction. The reflectance distributions tended to be azimuthally symmetric because the leaf transmittance was nearly equal to the leaf reflectance for most wavelengths. For sparse homogeneous canopies the anisotropic scattering properties of the soil significantly influenced the observed directional reflectance in the visible band. Soils have strong backscattering characteristics which can dominate the observed reflectance distribution for sparse canopies and small solar zenith angles. This knowledge is important in interpreting aircraft and satellite data, where the scan angle varies widely and can have different orientations with respect to the sun. Finally, the measured data and knowledge of the mechanics of the observed dynamics of the data can provide rigorous validation and verification tests for theoretical radiative transfer models.