Simulation of grassland-shrubland transition zone landscape images at 650nm using a simple BRDF model

TitleSimulation of grassland-shrubland transition zone landscape images at 650nm using a simple BRDF model
Publication TypeConference Proceedings
Year of Publication2002
AuthorsChopping M., Rango A., Goslee S.C., Schmugge T.J., Ritchie J.
Conference NameProceedings of the International Geoscience and Remote Sensing Symposium
Date PublishedJune 24-28, 2002
Conference LocationToronto, Canada
ARIS Log Number141609
AbstractGrassland-shrubland transition zone landscapes are usually a complex of soils; biotic and abiotic crusts; floristic lifeforms, including C4 grasses, C3 shrubs and semi-shrubs; forbs; cactii; and plant litter. The three-dimensional physical structure of the various elements results in reflectance anisotropy which is described by the surface bidirectional reflectance distribution function (BRDF). The objective of this study was to assess the capability of a simple BRDF model to simulate angular 650nm images of these transition landscapes when driven by a small number of spatially-varying structural parameters (shrub density and width from aerial photogaphy), together with two static parameters (spectral reflectance of leaves and a parametric soil/understory BRDF adjusted against field observations). The model is based on previous work in geometric-optics and radiative transfer codes. Shrub width and number density were estimated for honey mesquite (Prosopis glandulosa), acacia (Acacia spp.) and Mormon tea (Ephedra torreyana); shrub height was estimated from width, while crown height:width ratio was set to 0.75 (oblate). It was hypothesized that this simple parameterization would lead to important errors in simulation of directional (off-nadir) images, by reference to airborne multiangular images acquired at six different viewing directions and three different solar zenith angles close to the solar principal plane. The results show that, when small plants such as broom snakeweed (Gutierrezia sarothrae) are not included in plant density calculation, their large numbers result in an important aggregate effect which can overwhelm the contribution from large plants. The implication is that angular image simulations, using simple models, require estimates of plant densities at both large and small sizes.