Submitted to: Meeting Abstract
Publication Type: Abstract only
Publication Acceptance Date: 6/10/2002
Publication Date: 6/10/2002
Citation: CHOPPING, M.J., BOREL, C., SU, L., RANGO, A., MAXWELL, C.J. BRDF RECONSTRUCTION IN CHIHUAHUAN DESERT GRASS-SHRUB TRANSITION CANOPY-SOIL COMPLEXES: VALIDATION WITH AN AIRBORNE MULTIANGULAR DATASET. INTERNATIONAL WORKSHOP ON MULTIANGULAR MEASUREMENTS AND MODELS (IWMMM-3). 2002. ABSTRACT P. 8. Interpretive Summary:
Technical Abstract: The objective of this study was to evaluate the importance of the different elements (overstory, understory, soil) in the bidirectional reflectance distribution function (BRDF) of a Chihuahuan Desert grass-shrub transition zone by simulations using the POVRAY and RGM raytracing packages with parameters provided by detailed explicit measurement of plant locations, dimensions and type surveyed over plots of 25 m2 (comparable to the intrinsic length scale of the albedo of the landscape at approximately 33 m). The simulated reflectance values were assessed against samples of the BRDF at 650 nm acquired from the air at six view zenith angles and three solar zenith angles by a tilting, radiometrically calibrated digital camera. The 2 m ground resolution multiangular images were used to simulate the spatial response of a sensor with a 50 m footprint and were then mapped at a 25 m2 interval. The results shed light on the potential and limitations of a simplified geometrical-optics/radiative transfer model (SGM) with a limited parameterization developed for inversion with limited multiangular observations. In particular, the understory of small forbs and subshrubs, such as snakeweed, play an extremely 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 large numbers of small plants relative to large shrubs and also to a positive feedback: where there is a higher density of snakeweed there is also a higher proportion of (senescent) black grama grass and prone grass litter, both of which darken the scene.