Delta Electronics SS1-UM-1.05 user manual Modelling the canopy transmission

Diffuse light transmission (cosine corrected sensor)

		Transmission	1
		fraction
Leaf Angle Distribution			0.1			vertical
	τ	diff( 0 , L )
vertical
spherical	τ	diff( 1 , L )
horizontal	τ diff( 1000 , L )					spherical
			0.01
					horizontal
		0.001		2	4	6	8	10
			0
					L	Leaf Area Index

Modelling the canopy transmission

Accounting for incomplete absorption of PAR by the canopy elements, and scattering of light within the canopy is complicated. It is no longer sufficient to consider only the vertical component of PAR (i.e. as measured by a cosine-corrected sensor) because scattering effectively transfers power between the horizontal and vertical components, so the canopy changes the spatial distribution of power in the PAR flux as it is transmitted down through the canopy and reflected back up.

A multi-stream computer model was set up to calculate these effects as follows.

Assumptions

•The canopy is an infinite, uniform, horizontal slab, with leaf elements randomly distributed in proportion to the surface area of an ellipsoid, as described by Campbell.

•The incident light consists of a component from a point source at a given zenith angle (the Direct Beam); and a Diffuse component of equal intensity from every point in the sky (Uniform Overcast Sky).

•The canopy either has sufficiently high LAI that light reflected back from the ground below is negligible, or the reflectance of the ground is similar to that of the canopy.

•Of the light intercepted by the leaf element, a fraction a (absorption) is totally absorbed. The remainder is re-emitted uniformly in all directions.

58 • LAI theory Document code: SS1-UM-1.05