BIDIRECTIONAL REFLECTANCE OF SELECTED BOREAS SITES FROM MULTIANGLE AIRBORNE DATA

Hyperspectral, multiangle reflected radiances were obtained using the airborne advanced solid-state array spectroradiometer (ASAS) over Bore al Ecosystem-Atmosphere Study (BOREAS) sites in Canada during four fie ld campaigns in 1994. Atmospherically corrected bidirectional reflecta nce factors and estimates of spectral hemispherical reflectance for th ree canopies in the BOREAS southern study area (old aspen, old black s pruce, and old jack pine) are presented. The multiangle spectral refle ctance factors derived from data acquired July 21, 1994 (for a limited solar zenith angle range of 34 degrees-37 degrees) for the forested s ites showed distributions of high backscatter and consistently low for ward scatter due to shadowing. Position and shape of the retrosolar ma ximums for the three sites varied. Spectral distinction among the site s was evident in the red, where reflectance factors for the jack pine exceeded those for spruce and aspen. and in the near infrared (NIR), w here the aspen reflectance factors were twice those for the conifers. Data presented here suggest that spectral reflectances acquired at 26 degrees backscatter in the principal plane would provide better discri mination 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 a nd red-reflecting materials observed as a function of view azimuth and zenith. At the aspen site the red reflectance displayed much less var iation with changing view zenith and azimuth. Accordingly, angular eff ects 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 radi ation (PAR) (0.4-0.7 mu m), red (0.63-0.69 mu m), and NIR (0.83-0.87 m u m) calculated using various combinations of azimuthal data sets as i nput to the Walthall et al. [1985] model showed the following trends: maximum estimates were generated using data from the solar principal p lane (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 + perpen dicular + oblique) produced intermediate values totaling 73-91% (relat ive) of the spp result. A preliminary review of ASAS-derived and indep endent ground-based measures of PAR hemispherical reflectance revealed a sizable range in this parameter.