Estimating Big Bluestem Albedo from Directional Reflectance Measurements

Total hemispherical shortwave reflectance (albedo) is a major component of the Earth's radiation budget. To develop capabilities for the remote determination of prairie grass albedo, the determination of albedo from spectral bidirectional reflectance data was explored. Estimates of total shortwave albedo were derived from multidirectional reflectance factor measurements of big bluestem (Andropogon gerardii) grass acquired during the summer of 1986. The data were analyzed to evaluate the variation of albedo with changes in solar zenith angle and phenology. On any one day, albedo was observed to increase by at least 19% (relative) as solar zenith angle increased. Over the growing season, changes in big bluestem albedo corresponded to changes in the green leaf area index of the grass canopy. The ability to estimate albedo using reflectance factor data acquired within only one or two azimuthal planes and at a restricted range of view zenith angles was also evaluated. Estimates were compared to "true" albedos derived from all available reflectance factor data. Although differences increased slightly with solar zenith angle, estimates derived from data from two azimuthal planes oriented 45 ° to the solar principal plane were always within 4% (relative) of the " t rue" albedos even when view zenith angles were restricted to 50 ° or less. Albedo estimates derived from data along a single azimuthal plane were closest to the "true" albedos when the plane was either the principal or a 45 ° plane mad were always within 10% of the true albedos. Albe, lo estimates derived from nadir reflectance factor measurements differed by up to 18% from the '" true" albedos wath tke differences increasing with solar zenith angle. Even a limited amount of multiple direction reflectance data proved preferable to a single nadir reflectance factor for the estimation of big bluestem albedo.