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

The objective of this study was to assess the capability of a simple bidirectional reflectance distribution function (BRDF) model to simulate angular 650 nm (red wavelength) images of shrubland landscapes when driven by a small number of spatially-varying structural parameters (mean shrub density and width, together with derived mean canopy height and overall brightness), together with two static parameters (spectral reflectance of leaves and a parametric soil/understory BRDF). It was hypothesized that this simple parameterization will lead to important errors in reconstruction of directional (off-nadir) images, as acquired by a tilting multispectral digital camera at six different viewing directions and three different solar zenith angles in the principal plane. The results show that great care is needed in parameterizing the BRDF in this way and that variations in the brightness of the understory - here represented by grasses, forbs, subshrubs, biotic crusts and bare soil - has an important effect on modeled estimates of bidirectional reflectance in the red wavelengths.