BIDIRECTIONAL PROPERTIES AND UTILIZATIONS OF HIGH-RESOLUTION SPECTRA FROM A SEMIARID WATERSHED

A ground- and air-based high spectral resolution data set was collecte d during the summer Monsoon '90 experiment at the Walnut Gulch experim ental watershed in southeastern Arizona for the purpose of (1) charact erizing solar and view angle interactions on dry and wet season canopy spectra, and (2) exploring the use of multidirectional measurements t o infer vegetation properties for semiarid watershed studies. Bidirect ional reflectance factors were measured up to 40-degrees off nadir wit h a spectroradiometer over a semidesert grassland site. High-spectral resolution aircraft data were collected over grass and desert shrub si tes in order to investigate scaling effects. In this study, solar and view angle effects and interactions on canopy spectra varied with spec tral wavelength as well as between dry and wet seasons. The solar zeni th angle modified the view angle behavior of the bidirectional reflect ance factors. In general, view angle influences and spectral signature contrasts were greatest at the larger solar zenith angles and similar ly, Sun angle influences were more apparent at the larger view zenith angle. The scale dependency of the data was relatively minor. The bidi rectional measurements were sufficiently characterized by a physically based bidirectional reflectance distribution function (BRDF) model. T he parameters retrieved from the inversion of the BRDF model correspon ded with observed vegetation variations.