Sun Position and Cloud Effects on Reflectance and Vegetation Indices of Corn

Abstract

Th e refl ectance 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. Th is is an obstacle to making real-time, variable-rate N fertilizer applications based on canopy refl ectance because such a system must work under cloudy skies and at all times of day. Our objective was to develop spectral radiometer refl ectance corrections for variations in incoming sunlight so that the same refl ectance 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 fi eld in April and May on greenhouse-grown corn ranging from V10 to R2 growth stages. Sun angle, time of day, and cloud cover all infl uenced refl ectance measured from the corn canopy. When regression models were applied to correct refl ectance values to reference conditions for these variables, coeffi cients of variation were reduced by 29 to 56% for vegetation indexes and by 43 to 56% for refl ectance values. Th e near-infrared/green ratio and the green normalized diff erence vegetation index were the indices most sensitive to N defi ciency among six analyzed indices.

Diurnal Variability in Reflectance Measurements from Cotton

“…The relatively consistent values for passive reflectance measured with the Cropscan from 0800 to 1800 h are in contrast with the arcing passive reflectance values observed during these hours byde Souza et al (2010). deSouza et al (2010) used Crop Circle passive reflectance sensors which had a long tube over the downward-facing sensor, and this may be the reason for the discrepancy. Other than the tube, the appearance and operational principles of Cropscan and Crop Circle passive sensors appear to be very similar.…”

Development of an android app to estimate chlorophyll content of corn leaves based on contact imaging

“…Lower influence of different ambient conditions: Cloudy or sunny lighting conditions, shadows, and wind conditions can affect the success of using cameras in field conditions. Sun position also affects the reflectance of vegetation indices of plants (de Souza et al, 2010). But in contact imaging, only low lighting condition has some influence on the resulting images; as described below, we reduce this effect by adding luminance factor to the features used in computing the SPAD estimates.…”

Temporal Variability in Active Reflectance Sensor-Measured Ndvi in Soybean and Wheat Crops

“…Several studies have already shown that passive sensors are influenced by the time of day, since their measurements depend on the level of solar radiation (SOUZA et al, 2004;SOUZA et al, 2010;ERDLE et al, 2011). On the other hand, active sensors use their own light source and so have the ability to work at any time of day without interference from solar radiation or atmospheric variations (TRIMBLE, 2013).…”