Geoscience Reference
In-Depth Information
Fig. 4.6 Sketch of a typical cross
section through the
underside of a leaf.
Photosynthesis occurs in
the mesophyll cells in the
interior. The epidermis
consists usually of a
single layer of cells,
covered by the protective
cuticle; some of these
cells are guard cells,
which surround and
control the size of the
stomata by shrinking
and swelling. The
stomata are slit-like
openings through which
gas exchange and water
vapor loss take place.
Mesophyll
cells
Sub-stomatal
cavity
CO
2
Epidermis
Guard cell
of stoma
H
2
O
Cuticle
Guard cell
10
μ
m
Fig. 4.7 Schematic diagram showing the
resistance parameters, which can be
used to describe transfer to and from a
vegetational surface. The subscripts 1
and 2 refer to the levels of the wind speed
and specific humidity measurements,
respectively, in the atmospheric surface
layer. In this context, Equation (4.3) is
sometimes writ
te
n in resistance notation
as
E
Level
zz
2
where
qq
=
2
−
1
r
=
(C )
u
1
av
Surface
of
fo
li
age
where
qq
=
s
r
s
r
av
, which defines
the aerodynamic resistance parameter
for water vapor
r
av
in terms of the mass
transfer coefficient Ce, as shown.
=
ρ
(
q
s
−
q
2
)
/
Stoma
ta
l cavity
where
*
q qT
=
()
s
defined by
r
s
=
ρ
(
q
s
−
q
s
)
/
E
(4.37)
in which
r
s
is the surface resistance, and
q
s
is the actual (not saturated) mean specific
humidity at the evaporating surface; the basic concept is illustrated inFigure 4.7. For
practical use there have generally been two types of evaporation equation based on the
resistance concept. In the derivation of the first type,
q
s
,which is unknown, iseliminated
between Equation (4.37) and the standard mass transfer equation (4.3) to yield the expression
Ce
u
1
(1
+
r
s
Ce
u
1
)
ρ
(
q
s
E
=
−
q
2
)
(4.38)
In the derivation of the second type, Equation (4.38) is used (instead of (4.3)) to obtainan
expression in a way analogous to (4.23), namely
Q
ne
+
γ
Ce
u
1
ρ
(
q
2
−
q
2
)
E
=
(4.39)
[
+
γ
(1
+
r
s
Ce
u
1
)]
