Sun Position and Cloud Effects on Reflectance and Vegetation Indices of Corn
2010
de Souza, Eduardo G. | Scharf, Peter C. | Sudduth, Kenneth A.
The reflectance characteristics of plants and plant canopies far from solar noon (i.e., at large solar zenith angles) or with cloudy skies are not well known. This is an obstacle to making real-time, variable-rate N fertilizer applications based on canopy reflectance because such a system must work under cloudy skies and at all times of day. Our objective was to develop spectral radiometer reflectance corrections for variations in incoming sunlight so that the same reflectance reading would be obtained (and the same N recommendation made) for the same plants regardless of time of day or cloud conditions. Passive spectral radiometers were mounted in a stationary position about 25 cm above the corn (Zea mays L.) canopy. Readings were taken from morning until night over several days with a range of sky conditions (sunny, overcast, and partly cloudy). Experiments were done in the field in April and May on greenhouse-grown corn ranging from V10 to R2 growth stages. Sun angle, time of day, and cloud cover all influenced reflectance measured from the corn canopy. When regression models were applied to correct reflectance values to reference conditions for these variables, coefficients of variation were reduced by 29 to 56% for vegetation indexes and by 43 to 56% for reflectance values. The near-infrared/green ratio and the green normalized difference vegetation index were the indices most sensitive to N deficiency among six analyzed indices.
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