On the behaviour of a stressed cotton canopy in a direct air stream

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Abstract
Reflectance variations of a stressed cotton canopy were conducted in the presence of a fan-generated air stream to investigate the effects of air movement and the resulting temperature changes on remotely-sensed data. The initial drop in reflectance after application of the air stream was found to be greatest in the morning because leaf turgor was at a maximum, enabling leaves on the windward side of the canopy to assume surprisingly stable vertical positions. This transient configuration in the morning was responsible for greater stem displacement (relative to that of the afternoon), because the leaf area normal to the air stream was maximized, thus increasing the capacity of the canopy to trap light. By afternoon, a reduction in leaf turgor was responsible for less stem displacement and consequently a reduction in light-trapping capability. However, reflectance oscillations were greater because the leaves had become sufficiently limp to flutter at the edges and about the petioles exposing both adaxial and abaxial surfaces to the incident light. The optical and thermal radiometric manifestations of this behaviour were modelled using the damped harmonic oscillator approximation by considering the canopy as a transducer for converting a mechanical response to an optical response. These relationships were combined with temperature information to derive a simple relationship for the relative entropy per radian employing thematic mapper bands 3 and 4. For a constant temperature situation the relative entropy was found to be nearly linear for both bands.

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