Solar radiation reflection and extinction

WOFOST considers the part of the total incoming photosynthetically active radiation flux reflected by the canopy.

The reflection coefficient is defined as fraction of the downward radiation flux that is reflected by the whole canopy. According to Goudriaan (1977), the reflection coefficient of a green leaf canopy with a random spherical leaf angle is calculated with the equation:

where Rc is reflection coefficient of a green leaf canopy (unitless), Sc is scattering coefficient fraction (transmission and reflection) of single leaves for visible radiation (=0.2; Goudriaan, cited by Spitters, 1986) and sinß is sine of solar elevation (unitless). In this equation the first term indicates the canopy reflection of horizontal leaves and the second term stands for the approximate correction factor for a spherical leaf angle distribution. A fraction (1-Sc) of incoming visible radiation is potentially available for absorption.

An AFGEN table is used for deriving the extinction coefficient of diffused light (Extdiff) in function of development stage code. From this calculation, the extinction coefficient of direct light is derived with the following equation:

where Extdir is extinction coefficient of direct light (unitless), sinß is sine of solar elevation (unitless), Extdiff is the extinction coefficient of diffused light (unitless) and Sc is scattering coefficient fraction (transmission and reflection) of single leaves for visible radiation.

Then, the extinction coefficient of light from direct origine is calculated with the equation:

where Extori is extinction coefficient of light from direct origine (unitless), Extdir is extinction coefficient of direct light (unitless) and Sc is scattering coefficient fraction (transmission and reflection) of single leaves for visible radiation.

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