Bidirectional reflectance of selected BOREAS sites from multiangle airborne data

Abstract

Hyperspectral, multiangle reflected radiances were obtained using the airborne advanced solid‐state array spectroradiometer (ASAS)over Boreal Ecosystem‐Atmosphere Study (BOREAS) sites in Canada during four field campaigns in 1994. Atmospherically corrected bidirectional reflectance factors and estimates of spectral hemispherical reflectance for three canopies in the BOREAS southern study area (old aspen, old black spruce, and old jack pine) are presented. The multiangle spectral reflectance factors derived from data acquired July 21, 1994 (for a limited solar zenith angle range of 34°–37°) for the forested sites showed distributions of high backscatter and consistently low forward scatter due to shadowing. Position and shape of the retrosolar maximums for the three sites varied. Spectral distinction among the sites was evident in the red, where reflectance factors for the jack pine exceeded those for spruce and aspen, and in the near infrared (NIR), where the aspen reflectance factors were twice those for the conifers. Data presented here suggest that spectral reflectances acquired at 26° backscatter in the principal plane would provide better discrimination among the major cover types than those obtained from a nadir view. Red reflectance was strongly dependent on view geometry for the spruce and jack pine sites due to the varying amounts of deep shadow and red‐reflecting materials observed as a function of view azimuth and zenith. At the aspen site the red reflectance displayed much less variation with changing view zenith and azimuth. Accordingly, angular effects on the normalized difference vegetation index were large for the spruce and jack pine canopies but small for the aspen site. Estimated spectral hemispherical reflectances for photosynthetically active radiation (PAR) (0.4–0.7 μm), red (0.63–0.69 μm), and NIR (0.83–0.87 μm) calculated using various combinations of azimuthal data sets as input to the Walthall et al. [1985] model showed the following trends: maximum estimates were generated using data from the solar principal plane (spp) only; minimum values were derived from perpendicular plane data and amounted to 50–83% (relative) of the corresponding spp value; and inclusion of data from three view azimuths together (spp + perpendicular + oblique) produced intermediate values totaling 73–91% (relative) of the spp result. A preliminary review of ASAS‐derived and independent ground‐based measures of PAR hemispherical reflectance revealed a sizable range in this parameter.