Influence of solar illumination angle on soybean canopy reflectance.

Remotely sensed spectral measurements taken over croplands are influenced not only by the cover type and condition, cultural practices, and environmental factors, but also by the solar illumination angle in relation to the scene. The reflectance of soybean canopies of differing row directions were measured at 15-min intervals during three clear days with a Landsat band radiometer to determine the interaction of the solar illumination angle and row azimuth angle on the measured reflectance factor of soybean canopies. Diurnal reflectance changes of as much as 140%, which were related to the fraction of shadow, were observed in the red wavelength region. As soil cover approached 100% the diurnal changes diminished. A function that describes the solar illumination angle with respect to the row azimuth explained most of the diurnal variation in the measured reflectance. Variation in near-infrared reflectance was much less and did not appear to be as strongly related to sun-row angle interactions as the visible region.