Soil spectral response in relation to viewing angle, soil moisture and surface roughness

The objective of this study was to characterize the bi-directional reflectance factor (BRF) of three soil series (McAllister, Stronghold, and Epitaph) located at the Walnut Gulch Experimental Watershed (Arizona, USA) as a function of the viewing angle, soil moisture and surface roughness. Soil spectra were taken in the visible, near and mid-infrared regions, convolved to match the Landsat-TM bands, and the results were normalized to the Nadir response and expressed as relative BRF The anisotropic behavior varies from soil to soil and it was higher when the following conditions were taken combined or individually: shorter wavelengths, higher viewing and solar zenith angles, in the backscattering direction, soil in the dry condition as opposed to wet condition, and in the rough surface as compared to the smooth surface of Epitaph soil series (the only soil tested for the effect of roughness). Rough and smooth surfaces of Epitaph soil, however, were better discriminated in the forward scattering direction. Differences in scale and methods used to obtain the spectral curves were pointed out as responsible for the enhancement of the anisotropic behavior of the soils for lab results as compared to field results.