25 8
The Light Environment of Plant Canopies
while the beam radiation intercepted by the canopy is
Clearly the absorptivity of the canopy depends on wavelength but the
interception does not depend on wavelength.
15.7 Daily Integration
Equation (14.13) requires estimates of the fraction of radiation intercepted
by the canopy, averaged over whole days. Fuchs et al. (1976) suggested
that the interception of beam and diffuse radiation, averaged over whole
days, can be approximated by the intercepted function for diffuse radiation because the sun traverses the whole sky over the period of the day.
Tests with detailed models have shown this to be correct. Therefore the
average transmission of canopies can be modelled over whole days using
Eq. (1 5.6), with Kbe (+) replaced by K d (from Fig. 15.4). '
Based on these observations, the daily fractional interception can be
computed from
Absorption of PAR is about equal to interception, while absorption of
total solar radiation is about 80 percent of interception (Campbell and
van Evert, 1994).
15.8 Calculating the Flux Density of Radiation
on Leaves in a Canopy
The equations we have just derived can be used to compute the flux density
of radiation on leaves within the canopy. Knowing the flux density on
leaves is important for the purpose of computing photosynthesis and for
calculating the radiation viewed by a remote sensor.
Let Qob be the flux density of beam radiation on a horizontal surface
at the top of the canopy and Qod be the flux density of diffuse radiation on
the horizontal above the canopy. At a depth L in the canopy, three different
flux densities can be calculated: the total beam, Qbt(+) (unintercepted
beam plus down scattered beam); beam, Q b ( + ) (unintercepted beam)
and the diffuse flux, Q d . These are given by
Qd = Tdr Qod.
(15.17)
Here, Tbr(@) and t d t are given by Eq. (15.6) with the appropriate K for
beam or diffuse radiation, and rb (+) = exp(- Kbe ($) L ) .
At depth L in the canopy some leaves are sunlit and some leaves are
in the shade. The flux density on a horizontal surface at the position of
a sunlit leaf is Qbt (+) + Qd. The flux density on the leaves themselves
The Light Environment of Plant Canopies
while the beam radiation intercepted by the canopy is
Clearly the absorptivity of the canopy depends on wavelength but the
interception does not depend on wavelength.
15.7 Daily Integration
Equation (14.13) requires estimates of the fraction of radiation intercepted
by the canopy, averaged over whole days. Fuchs et al. (1976) suggested
that the interception of beam and diffuse radiation, averaged over whole
days, can be approximated by the intercepted function for diffuse radiation because the sun traverses the whole sky over the period of the day.
Tests with detailed models have shown this to be correct. Therefore the
average transmission of canopies can be modelled over whole days using
Eq. (1 5.6), with Kbe (+) replaced by K d (from Fig. 15.4). '
Based on these observations, the daily fractional interception can be
computed from
Absorption of PAR is about equal to interception, while absorption of
total solar radiation is about 80 percent of interception (Campbell and
van Evert, 1994).
15.8 Calculating the Flux Density of Radiation
on Leaves in a Canopy
The equations we have just derived can be used to compute the flux density
of radiation on leaves within the canopy. Knowing the flux density on
leaves is important for the purpose of computing photosynthesis and for
calculating the radiation viewed by a remote sensor.
Let Qob be the flux density of beam radiation on a horizontal surface
at the top of the canopy and Qod be the flux density of diffuse radiation on
the horizontal above the canopy. At a depth L in the canopy, three different
flux densities can be calculated: the total beam, Qbt(+) (unintercepted
beam plus down scattered beam); beam, Q b ( + ) (unintercepted beam)
and the diffuse flux, Q d . These are given by
Qd = Tdr Qod.
(15.17)
Here, Tbr(@) and t d t are given by Eq. (15.6) with the appropriate K for
beam or diffuse radiation, and rb (+) = exp(- Kbe ($) L ) .
At depth L in the canopy some leaves are sunlit and some leaves are
in the shade. The flux density on a horizontal surface at the position of
a sunlit leaf is Qbt (+) + Qd. The flux density on the leaves themselves