Submitted to: International Journal of Remote Sensing
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/11/2003
Publication Date: 7/20/2004
Citation: Chopping, M.J., Su, L., Rango, A., Maxwell, C.J. 2004. Modelling the reflectance anisotropy of Chihuahuan Desert grass-shrub transition canopy-soil complexes. International Journal of Remote Sensing. 25(14):2725-2745. Interpretive Summary: An end of wet season multiangle remotely-sensed data set collected from the air over a grass-shrub transition zone in the Jornada Experimental Range near Las Cruces, New Mexico, was used together with intensive plant surveys over 1,250 sq. m. to model light scattering in the red wavelengths. The 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 the multiangle observations. The models were driven by plant maps derived from the field survey data and from aerial photography and, unlike previous modeling efforts, the understory of small sub-shrubs and forbs was accounted for. Over 1,315 plants were measured, including over 1000 broom snakeweed (Gutierrezia sarothrae). The research shows the understory cannot be ignored in modeling or model inversions and remnant black grama grass (Bouteloua eriopoda) and its litter play a non-negligible role. A simple hybrid geometric-optical/radiative transfer model designed with inversion in mind closely matched a more sophisticated 3-D radiosity model. The research promotes understanding the factors controlling the remotely sensed signal returned in red wavelengths from grass-shrub transition complexes, particularly with respect to reflectance anisotropy. This knowledge is important in the interpretation of remote sensing data from satellites and aircraft and, in inverse modeling of these data, to retrieve important canopy attributes, such as fractional vegetation cover, fraction of intercepted photosynthetically-active radiation, canopy plant density, plant height and radius distributions, and leaf area index.
Technical Abstract: The 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.