Panicle contribution to bidirectional reflectance factors of a wheat canopy

Bidirectional reflectance factors (BRFs) of crop stands are strongly influenced by canopy architecture. In wheat, as well as in many other crops, canopy architecture changes dramatically with the phenological development of the plant community.

A ground-based experiment was performed to examine the effect of panicles of winter wheat (Triticum aestivum L.) at the flowering stage on canopy BRFs. Reflectance factors were measured in the field with a portable radiometer in the red (0-63-0-69 μm) and near-infrared (0-76-0-90 μm) wavelength intervals. Observations were made at three viewing angles and 14 solar zenith angles during two consecutive days on a control target and on a target where panicles had been removed.

Panicles did not contribute significantly to the red nor to the near-infrared (NIR) reflectance factors computed from nadir observations. Off-nadir NIR reflectance was also not altered by the presence of panicles, but was moderately sensitive to illumination angle. Off-nadir red reflectance in the backscattcring direction was higher in the canopy with panicles than in the canopy without panicles: at a solar zenith angle of about 50° the difference in the reflectance of the two targets reached a maximum of about 39 per cent.

These findings imply a potential to identify crops and their phenological development by more fully exploiting reflectance at several different viewing and solar angles.