LARS Tech Report Number
An experiment was conducted at West Lafayette, Indiana in 1979 to quantitatively describe the interaction of the solar illumination angle and row azimuth angle on the measured reflectance factor (RF) of soybean canopies consisting of 11 plots. Nine of the plots were planted in 71 cm wide rows; the other plots were of bare soil and soybeans with 100 percent soil cover. Reflectance factor data in four spectral bands, 0.5-0.6, 0.6-0.7, 0.7-0.8, and 0.8-1.1 pm, were taken at 15 minute intervals during three clear days, August 12 and 31 and September 19 over nine plots of differing azimuthal direction with a Landsat-band radiometer (Exotech Model 100) at 5.2 meters above the soil.
Diurnal changes of nearly 140 percent were observed in the red wavelength region when canopies covered 64 percent of the soil. The amount of shadow observed was a function of the plant geometry and row width. As soil cover approached 100 percent, 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 RF. Variation in near infrared response was much less and did not appear to be as strongly related to sun-row angle interactions. The ratio, near infrared/red, was highly sensitive to sun angle-row direction interactions, whereas the greenness function, utilizing all four spectral bands, was not.
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