Delta SS1-UM-1.05 User Manual download pdf (Page 61)

SunScan User Manual v 1.05 LAI theory •

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• 61

This was again calculated numerically and curves fitted to the data with similar

accuracy as above. The curves fitted are:

Given:

Diffuse light transmission (hemispherical response sensor)

Modelling incomplete PAR absorption and scattering

Radiation models have been used for many years to calculate the effects of scattering

in the canopy e.g. Norman & Jarvis (1975). Wood's model incorporates Campbell’s

ellipsoidal leaf angle distribution and the effects this has on transmission of both

Direct and Diffuse light.

The model splits the canopy into layers of LAI 0.1, extending to a sufficient depth to

absorb all of the incident light. Incident light above the top layer was a known

fraction of Direct (at a given zenith angle) and Diffuse light. The amount of light

absorbed by a layer, assuming completely black leaves, was calculated. The fraction

of this absorbed light re-emitted by the leaves was then assumed to be re-emitted in

all directions uniformly (see Monteith & Unsworth, 1990, p85 onwards) .

The light level at any point in the canopy is then the light calculated assuming

complete absorption, plus the sum of the light re-emitted by each canopy layer,

attenuated by the intervening layers.

These calculations had to take full account of both horizontal and vertical light

components. This involved an iterative solution and a lot of computer time. Finally,

the light intensity as measured by a cosine corrected sensor was calculated.

P( )x 1

0.4 exp( )

0.1 x ( )atan( )

0.9 x 0.95

Q( )x

0.255 atan( )x 0.6

R( )x exp( )x

spher

(),x L exp

P( )x L

Q( )x

R( )xln( )

0 2 4 6 8 10

0.001

0.01

0.1

spher

(),0L

spher

(),1L

spher

(),1000 L

Transmission

fraction

Vertical

herical

Horizontal

Leaf Angle

Distribution

Horizontal

herical

Vertical

Leaf Area Index

1 2 ... 56 57 58 59 60 61 62 63 64 65 66 ... 85 86

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Delta SS1-UM-1.05 User Manual Page 61

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