Geoscience Reference
In-Depth Information
source in the canopy and the reference level.) In a whole-canopy representation,
the total energy available to support sensible and latent heat transfer is given by:
(21.32)
l
EHA
+=
where A, the available energy, in addition to net radiation, includes allowance for
soil heat flux,
G
, and the physical and biochemical storage in the canopy,
S
and
P
,
respectively (see Chapter 4).
Equations (21.30), (21.31), and (21.32) are analogous to Equations (21.21),
(21.22), and (21.23), respectively, and the derivation of the whole-canopy Penman-
Monteith Equation therefore follows by direct analogy with that for unit area of
leaf given above. Assuming
r
a
=
r
a
H
=
r
a
V
, the resulting equation takes the form:
r
cD
ap f
Δ+
A
r
(21.33)
l
E
=
a
⎛
⎞
r
r
Δ+
g
1
+
s
⎜
⎟
⎝
⎠
a
where
D
ref
is the observed VPD at the reference level above the canopy.
Important points in this chapter
Turbulent and non-turbulent controls
: within a vegetation canopy flux
exchange involves interplay between turbulent vertical diffusion of fluxes
and the divergence of these fluxes through non-turbulent interaction with
the vegetation elements that make up the canopy. Resistances associated with
molecular diffusion through boundary layers and stomata control the
dissipation or generation of portions of the fluxes at each level in the canopy.
●
Skin friction and bluff body transfer
: all fluxes from the canopy air stream to
leaves can occur by
skin friction
transfer (i.e. by molecular diffusion through
the non-turbulent boundary layers surrounding leaves), but momentum can
also be transferred more efficiently by pressure forces in
bluff body
transfer,
hence the boundary-layer resistance for momentum is about an order of
magnitude less than for other transfers, depending on the orientation of
the leaf.
●
Influence of molecular diffusion coefficient
: when boundary-layer resist-
ance is controlled by skin friction transfer the relevant molecular transfer
coefficient controls both rate of diffusion and thickness of the boundary
layer, hence the ratio of the boundary-layer resistances for two transfers is
inversely proportional to the ratio of their molecular diffusion coefficients
raised to the power 0.67.
●
