Geoscience Reference
In-Depth Information
Mesophyll
(Chloroplasts)
Stomatal
resistance
e
i
=e
sat
(
T
s
)
Sub-stomatal
cavity
Epidermis
Cuticle
T
s
e
s
Leaf surface
Boundary layer
Boundary
layer limit
T
e
Boundary layer
resistance
H
R
n
I
l
E
I
H
I
Figure 21.5
Cross-section of the surface of a leaf showing a stomata and the stomatal and boundary-layer resistance used
to represent the restrictions on the flows of latent heat and sensible heat.
molecular diffusion from the surface of the leaf to the air in the canopy. But it
applies only to the gaseous exchanges.
The flow of latent heat per unit area of leaf,
lE
l
, leaving from inside of leaves to
the surface of leaves is given by:
r
c
ee
(
r
−
)
ap
s
(21.18)
l
E
=
i
l
g
ST
where
e
s
and
e
i
are the vapor pressure in the stomatal cavity and at the surface of
the leaf outside the stomata, respectively, and
r
ST
is the stomatal resistance
per unit
area
of leaf. In practice it is usually assumed that the air inside the leaf is saturated
at the nearby surface temperature of the leaf,
T
s
, see Figure 21.5. Hence
e
s
=
e
sat
(
T
s
)
and Equation (21.18) becomes:
r
c
eTe
(() )
−
ap
sat
(21.19)
l
E
=
s
s
l
g
r
ST
As is the case for boundary-layer resistance (see Equation (21.16) for example), it
is the reciprocal of the stomatal resistance, i.e., the stomatal conductance, which
scales linearly with the area of leaf. Consequently the mean stomatal layer
resistance is calculated as the
reciprocal
of the area-weighted average of the
