A study of reflectance anisotropy and canopy structure using a simple empirical model
Walthall, C.L.
The coefficients of a simple three-term model describing the bidirectional reflectance of vegetation canopies as a function of leaf area index (LAI) were examined for potential information retrievable from bidirectional reflectance measurements. Simulations of principal plane spectral reflectances (0.5-0.6 micrometer, 0.6-0.7 micrometer, 0.7-0.8 micrometer, and 0.8-1.1 micrometer), for vegetated canopies with LAI values ranging from 0.1 to 5.0 under solar zenith angles of 20, 30, and 45 degrees, were generated with the use of a one-dimensional radiative transfer model. The simulated reflectances were then fit with a simple model by using a least squares procedure. Plots of the resulting coefficients with LAI showed that the degree of curvature of the simulated principal plane reflectances, as represented by one of the coefficients, and the coefficient expressing interaction of view zenith angle and view azimuth angle were both related to LAI. This relation holds in all four spectral bands, including the 0.7-0.8 micrometer band, despite a lack of relation between the constant coefficient (which is closely related to the nadir value) and LAI. Examination of the coefficients relating to solar zenith angle revealed a dependence on LAI. The coefficient for view zenith angle, or a related quantity, is suggested as a candidate for a bidirectional index that may be useful within the context of satellite-based vegetation classification and characterization schemes.
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