BIDIRECTIONAL REFLECTANCES OF SELECTED DESERT SURFACES AND THEIR 3-PARAMETER SOIL CHARACTERIZATION

Spectral bidirectional reflectances were measured over three natural alkali soil sites using a specially designed radiometer called the Portable Apparatus for Rapid Acquisition of Bidirectional Observations of Land and Atmosphere (PARABOLA). Two of the sites were void of vegetation with different surface characteristics (alkali flat and dune sand flat), while the third had a sparse cover of desert scrub. The reflectances were strongly non-Lambertian for all three surfaces, but with markedly different patterns. The measured data were fitted with a quasi-physical reflectance model in which the surface backscattering and forwardscattering are separately formulated. A soil reflectance characterization was obtained by assessing the fractional contributions of the forward, backward and Lambertian components. For the desert scrub surface the backscattering component was larger by a factor of 10 or more than the forwardscattering component and by a factor of about two larger than the Lambertian component. The alkali flat backscatter component was little more than half the magnitude of the Lambertian component, and the dune sand was characterized by a forwardscatter component more than double the backscatter component. This three-parameter characterization produced a satisfactory fit to the measured reflectances. Since the component values remain relatively constant at different sun angles, this approach appears very promising as a basis for soil patterns or terrain categorization based on their anisotropic reflectance patterns. © 1990.