Diurnal Patterns of Wheat Spectral Reflectances

Spectral reflectances of Produra wheat were measured at 13 different times of the day at Phoenix, Arizona, on April 14, 1979 using a nadir-oriented hand-held 4-band radiometer which had bandpass characteristics similar to those on current LANDSAT satellites. Different sun altitude and azimuth angles caused significant diurnal changes in radiant return in both visible and near-IR regions of the spectrum and in several vegetation indices derived from them. The magnitude of these changes were related to different canopy architecture, percent cover, and green leaf area conditions. Spectral measurements taken at each time period were well correlated with green leaf area index but the nature of the relationship changed significantly with time of day. Thus a significant bias in the estimation of green leaf area index (GLAI) from remotely sensed spectral data could occur if sun angles are not properly accounted for.