A STUDY OF THE ANGULAR REFLECTANCE CHARACTERISTICS OF CORN AND SOYBEAN CANOPIES
Abstract
Understanding the characteristics of vegetation canopy reflectance is important if remotely sensed data is to be fully exploited to monitor the amount and status of agricultural resources. Since vegetation canopies are not simple Lambertian reflectors the effects of illumination and viewing geometry must be understood to enhance the interpretation of the data. In this study the diurnal and seasonal directional reflectance characteristics of two important agricultural crops: corn (Zea mays L.) and soybean (Glycine max (L.) Merr.) were examined. Spectral reflectance factor (RF), agronomic and biophysical measurements were acquired on three dates during 1980 for soybeans and on five dates in 1982 for corn over a wide range of sun and view angles. A pronounced sun-view angle effect on soybean canopy RF was observed, especially in the 0.6 - 0.7(mu)m (red) wavelength band for canopies with well defined row structure. Near infrared (near-IR) RF and the transformed variable greenness showed a less noticeable dependence for both incomplete and complete canopies. The effect of sun angle was greatest for view directions perpendicular to the canopy rows. Analysis of the data for apparent Lambertian behavior yielded very few off- nadir view angles that approximated straight down measurements for the red band, while near-IR and greenness had extended off-nadir view angle ranges. A geometric optics model was developed to examine the effect of shadows on the red band RF for incomplete soybean canopies. Comparison of modeled and measured results indicated that for nadir view the model explained up to 95% of the variation in the data. A solar zenith angle dependence for visible, near-IR and middle-IR RFs was noted for corn canopies with a low leaf area index (LAI). A decrease in the spectral contrast between vegetation and soil due to shadows was cited as the cause of the dependence. Sun angle dependence was least for canopies with higher LAIs. RFs were maximized for coincident sun and view angles and minimized when the sensor view was towards the sun. Evaluation of three linear transformations (greenness, near-IR/red ratio and normalized difference) suggested they may be useful for correcting the sun angle dependence for nadir acquired data. A strong dependence with view angle was noted for all three transformations for low LAI canopies. Greenness and near-IR/red ratio varied with view zenith angle, normalized difference did not. The asymptotic nature of normalized difference as LAI increases was cited as the reason for the lack of variation with view angle.
Degree
Ph.D.
Subject Area
Agronomy
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