Modeling the reflectance anisotropy of Chihuahuan Desert grass-shrub transition canopy-soil complexes

TitleModeling the reflectance anisotropy of Chihuahuan Desert grass-shrub transition canopy-soil complexes
Publication TypeJournal Article
Year of Publication2004
AuthorsChopping M., Su L., Rango A., Maxwell C.J.
JournalInternational Journal of Remote Sensing
Date Published2004
ARIS Log Number141607
KeywordsBRDF, canopy-soil complexes, Chihuahuan Desert, grass-shrub transition, reflectance anisotropy, remote sensing, RGM, SGM
AbstractThe goal of the research presented is to understand better the factors controlling the remotely sensed signal returned in the red wavelengths from grass-shrub transition canopy-soil complexes. The specific objectives were to evaluate the importance of the different elements (overstory, understory, soil) in the bi-directional reflectance distribution function (BRDF) and to explore the behavior of simple parametric and explicit scattering models with respect to observations. The first objective was approached by simulations using the Radiosity Graphics Method (RGM) with surface parameters provided by measurements of plant locations and dimensions surveyed over 25m plots. The second was approached through simulations of bi-directional reflectance by both the RGM and a Simplified Geometric Model (SGM). The simulated reflectance values were assessed against samples of the BRDF at a wavelength of 650nm acquired from the air at six view zenith angles and three solar zenith angles by a tilting, radiometrically-calibrated multispectral digital camera providing multi-angle observations (MAO). The results show that the understory of small forbs and sub-shrubs plays a very important role in determining the brightness and to some extent reflectance anisotropy of these landscapes. This is owing to the potential large number density of broom snakeweed and to a varying proportion of black grama grass and prone grass litter. Both of these components darken the scene. The SGM performed well measured against both the RGM and the MAO (r2 of 0.98 and 0.92 against the RGM and MAO, respectively). Both models underestimated reflectance by a small amount, less than 6% over all angles, with both showing increasing divergence from the backcsattering into the forward-scattering direction.