EFFECTS OF LEAF-TRANSMITTANCE VERSUS LEAF-REFLECTANCE ON BIDIRECTIONAL SCATTERING FROM CANOPY SOIL SURFACE - AN ANALYTICAL STUDY

A simple single-scattering model for a surface with sparse erectophile plants Is developed as a plane-parallel canopy consisting of small le aves (relative to leaf-to-leaf spacing) with a spherical-shell distrib ution of the leaf area. The contributions to the overall surface refle ctance in the visible spectral bands by the soil-reflectance, leaf-ref lection, and leaf-transmission (which are assumed isotropic) are analy zed under different view/illumination geometries. High values of leaf reflectance r, relative to leaf transmittance t, produce significantly different patterns of bidirectional reflectance, because r and t cont rol the backscattering and the forward scattering, respectively. These two effects, especially strong at large solar zenith angles, produce high canopy reflectance at large view zenith angles around the princip al plane. Model inversion with the PARABOLA bidirectional reflectance measurements over the Konza Prairie yield values of r and t for grassb lades of this grassland canopy. The inversion results point to a possi bility of assessing canopy condition from its bidirectional reflectanc es, as both r and t are sensitive to plant vigor and phenology. In an inversion with satellite measurements over a desert-scrub surface in t he northern Sinai, the optical thickness of these dark plants (inferre d in the visible band) and the near-infrared reflectance of the plant elements were inferred. The value of the optical thickness of this spa rse canopy essentially did not depend on the assumed plant-element tra nsmittance, but the inferred infrared reflectances of the plant elemen ts were appreciably dependent. The canopy structure representation (th e spherical-shell distribution of the planar leaf area) constitutes a rotation-invariant reflectance model. It allows formulation of the lon gwave exchanges identical to the conventional radiative transfer calcu lation through a layer of molecules or particles with a specified opti cal thickness.