Solar radiation absorption

After the calculation of the coefficients for solar radiation reflection and extinction (see topic Solar radiation reflection and extinction), for each canopy layer the following equation is applied for the computation of absorbed diffuse radiation:

where PARdiff,abs is photosynthetically active absorbed diffuse radiation by layer (W m-2),  Rc is reflection coefficient of a green leaf canopy (unitless), PARdiff is photosynthetically active diffused radiation (W m-2), Extdir is extinction coefficient of direct light (unitless) and LAIl is leaf area index for the layer l (m2 m-2).

Then, the photosynthetically active absorbed radiation from direct origin is calculated:

where PARori,abs is photosynthetically active absorbed radiation from direct origin by layer (W m-2),  Rc is reflection coefficient of a green leaf canopy (unitless), PARdir is photosynthetically active direct radiation (W m-2), Extori is extinction coefficient of light from direct origine (unitless) and  LAIl  is leaf area index for the layer l (m2 m-2).

Finally the amount of PAR absorbed from direct light is calculated:

where PARdir,abs is photosynthetically active absorbed radiation from direct origin by layer (W m-2),  Sc is scattering coefficient fraction (transmission and reflection) of single leaves for visible radiation (unitless), PARdir is photosynthetically active direct radiation (W m-2), Extdir is extinction coefficient of direct light (unitless) and  LAIl  is leaf area index for the layer l (m2 m-2).

Starting from these equations the absorbed radiation fluxes for shaded and sunlit leaves are calculated with the following equations:

where Fluxsh is absorbed radiation flux for shaded leaves (W m-2), PARdiff,abs is photosynthetically active absorbed diffuse radiation by layer (W m-2), PARori,abs  is photosynthetically active absorbed radiation from direct origin by layer (W m-2) and PARdir,abs is photosynthetically active absorbed radiation from direct origin by layer (W m-2).

where Fluxsun is absorbed radiation flux for sunlit leaves (W m-2), Sc is scattering coefficient fraction (transmission and reflection) of single leaves for visible radiation (unitless), PARdir is photosynthetically active direct radiation (W m-2) and sinß is sine of solar elevation (unitless).

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