QUANTIFYING REFLECTANCE ANISOTROPY OF PHOTOSYNTHETICALLY ACTIVE RADIATION IN GRASSLANDS

The fraction of photosynthetically active radiation (PAR) (lambda = 0.4-0.7 mum) absorbed (F(APAR)) is a critical parameter in canopy processes and in land surface-atmosphere interactions. F(APAR) is typically inferred indirectly through empirical relationships established between hemispheric measurements of PAR and either (1) canopy bidirectional, usually nadir, reflectances in the visible wavelengths or (2) spectral vegetation indices. However, the bidirectional reflectance distribution function (BRDF) of vegetated surfaces has been shown to exhibit angular and spectral anisotropy, complicating the estimation of average surface reflectances, including that of PAR, from remote observations. Quantifying the surface's reflectance anisotropy was an important element of the First ISLSCP Field Experiment (FIFE), since its major objectives focused on retrieval of surface parameters from satellite-derived reflectances. Prior to the experiment reported here, the explicit remote measurements for approximating the BRDF of PAR had not been undertaken. The proper expression of reflectance for BRDFs for retrieval of canopy parameters is assessed. A rationale is given for the expression of reflectances as bidirectional reflectance fractions (RFRs), instead of the conventional bidirectional reflectance factors (RFs), when relating spectral data tc canopy states or processes. The characterization of the grassland anisotropy in the solar principal plane is markedly different when expressed as RFRs versus RFs. Whereas RFs are extremely useful in addressing important "remote sensing" issues, the "surface only" BRDF is obscured by pixel-size effects since the projected viewing area increases by a factor of cos(theta(V))-1 with view angle (theta(V)) for off-nadir views. The BRDF for RFRs exhibited increased absorption by the canopy in the forward scatter directions; the anisotropy is most pronounced at midday, and more isotropic at intermediate sun angles, at which time the nadir observation was adequate for estimating the mean surface PAR reflectance.