View azimuth and zenith, and solar angle effects on wheat canopy reflectance

Abstract Since crop canopies are not lambertian reflectors, their reflectance varies with sun and view positions. It is not always possible or convenient to make reflectance measurements from the nadir position nor at the same time of day. Therefore, ways of estimating nadir reflectance from off-nadir views and for various solar zenith angles are needed. In this study, spectral measurements were made with a Mark II radiometer five times during the day on each of four dates from 15° interval zenith and 45° azimuth positions for wheat canopies during the development interval stem extension to watery ripeness of the grain. The ratio of off-nadir [ R ( Z v , A v )] to nadir [ R (0)] radiance in NIR band (0.76–0.90 μm) was described by the regression equation: R(Z v ,A v ) R(0) = 1.0 + [β 0 + β 1 sin ( A v 2 ) + β 2 (1/ cos Z s )] sin Z v where A v is view azimuth angle relative to the sum position, Z s is solar zenith angle, and Z v is view zenith angle. The coefficient of determination was 0.70 or higher. The equation describes the observations that 1) the ratio of off-nadir to nadir radiance increases or decreases as view zenith angle increases depending on view azimuth angle; backscattering is stronger than forwardscattering and the pattern is azimuthally symmetric about the principal plane of the sun; and 2) the rate of change in the radiance ratio increases with increasing solar zenith angle. The coefficients, β 0 , β 1 and β 2 , changed as the canopies grew. Although the equation needs to be more fully tested, it should help summarize and compare various angular observation data taken in crop fields.