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

The goal of the research presented here is to assess the factors controlling the remotely sensed signal returned in the solar wavelengths from Chihuahuan Desert grass-shrub transition canopy-soil complexes. The specific objectives were twofold: to evaluate the importance of the different elements (overstorey, understorey, soil) in the bidirectional reflectance distribution function (BRDF) of a Chihuahuan Desert grass-shrub transition zone; and to explore the behaviour 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 obtained over two contrasting 25 m² plots. The second was approached through simulations of bidirectional reflectance factors (BRFs) by both the RGM and a Simplified Geometric Model (SGM) developed for inversion purposes. The modelled BRFs were assessed against multi-angle observations (MAO) - samples of the BRDF at a wavelength of 650 nm acquired from the air at up to six view zenith angles and three solar zenith angles. The results show that the understorey of small forbs and sub-shrubs plays an important role in determining the brightness and reflectance anisotropy of grass-shrub transition landscapes in relation to that of larger shrubs such as mesquite and ephedra. This is owing to the potentially high density of these plants and to the fact that there is also a varying proportion of black grama grass and prone grass litter associated with snakeweed abundance. Both of these components darken the scene. The SGM performed well measured against both the RGM and the MAO at the MAO acquisition angles (R² of 0.98 and 0.92, respectively) and good correlations were obtained between RGM and SGM when modelling was performed at a wider range of angular configurations (R²≈0.90). The SGM was shown to be highly sensitive to its adjustable parameters. Both models underestimated BRF magnitude with respect to the MAO by a small amount (<6%), showing increasing divergence from the backscattering into the forward-scattering direction. A remaining problem for operational model inversions using MAO is the a priori estimation of understorey and grass abundance.