Delta SS1-UM-1.05 User Manual download pdf (Page 59)

SunScan User Manual v 1.05 LAI theory •

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• 59

In detail:

• The canopy is divided into horizontal layers of LAI 0.1

• Direct beam absorption by each layer is calculated using Campbell's equation. In

calculating the amount of absorbed light that is re-emitted, the total power in the

direct beam has to be used (i.e. as measured by an integrating hemisphere or a

cosine-corrected sensor perpendicular to the beam). The amount intercepted by

the layer is the difference between the absolute intensity above and below the

layer.

• Diffuse light intercepted by the layer is calculated in a similar way, taking into

account the incident Diffuse light, and the sum of re-emitted light from all other

layers, attenuated by the intervening layers. This is done for both downwelling

and upwelling Diffuse light. A fraction of the Diffuse light absorbed by the layer

is also re-emitted. Again, absolute rather than cosine-corrected intensity

measurements must be used.

• Both cosine-corrected and absolute light measurements are calculated for each

layer, and the model iterated until it converges. This has been done for a range of

different values of zenith angle, Direct/Diffuse ratios, Leaf Angle Distributions

and Absorptions.

Simpler functions have been found to approximate these results, and are used in the

SunData software when inverting transmitted fraction back to LAI. These are

described in detail in the next section.

The canopy model

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Delta SS1-UM-1.05 User Manual Page 59