Canopy Reflectance, Photosynthesis, and Transpiration. II. The Role of Biophysics in the Linearity of Their Interdependence

A two-stream approximation model of radiative transfer is used to calculate values of hemispheric canopy reflectance in the visible and near-infrared wavelength intervals. Simple leaf models of photosynthesis and stomatal resistance are integrated over leat orientation and canopy depth to obtain estimates of canopy photosynthesis and bulk stomatal or canopy resistance. The simple ratio (SR) of the near-infrared and visible canopy reflectances has been found to be a near-linear indicator of the photosynthetically active radiation absorbed by the canopy, APAR, minimum canopy resistance, 1/r c, and photosynthetic capacity Pc, but a highly nonlinear and therefore less reliable predictor of leaf area index or biomass (Sellers, 1985). This paper extends previous work and investigates the biophysical processes giving rise to the near-linear dependence of APAR, Pc and l/r, , on SR. It is demonstrated that under normal field conditions, i.e., dark soft, the near-infrared reflectance term controls the variation of SR with leaf area index. As a result of this, near-linearity between SR and APAR, Pc or 1/r~ will occur if the lea[ scattering coefficient in the near-infrared region, toN, satisfies the following equality: ton = 1 [G(~) /2 / t ]2 (1 to,,), where to,, is the leaf's effective scattering coefficient for PAR, G(/~) is the average lea[ projection in the direction/x, and/x is the cosine of the zenith angle of the incoming flux. This condition is approximately met in nature. It is shown that a variety of satellite sensor combinations are well configured for the estimation of APAR, Pc and 1 / r C by responding to leaf scattering coefficients in bands that conform to the above expression. The relationships between SR and APAR, Pc or I/r, . becomes increasingly nonlinear as the soil reflectivity increases.