Geoscience Reference
In-Depth Information
discuss the functions as given by Chen and Dudhia (
2001
).
1
Note that the functions
have similar shapes as the dependence of stomatal resistance on environmental fac-
tors as sketched in
Figure 6.15
).
The radiation response function
f
1
is formulated as follows:
↓
1
+
f
K
2
,
where
↓
fK
()
=
f
=
055
.
PAR AI
(9.25)
1
r
/
r
+
f
s,min
s,max
limit
where
r
s,max
is the cuticular resistance of the leaves (i.e., the resistance when the sto-
mata are fully closed, of the order of 5000 s m
-1
). The factor 0.55 in
f
is the fraction
of global radiation that is photosynthetically active (somewhat higher than the values
given in
Chapter 6
).
PAR
limit
is the level of PAR at which the resistance is roughly
doubled (equal to 30 W m
-2
for trees and 100 W m
-2
for low vegetation). Finally,
the LAI is included in
f
because the positive effect of radiation on stomatal open-
ing decreases below the top of the canopy due to the extinction of radiation.
2
At low
values of global radiation
f
1
tends to
r r
s,maxs,mi
/
(so that the actual canopy resistance
becomes
r
s,max
LA/ ) whereas for high levels of radiation the function tends to one.
The response of the canopy resistance to speciic humidity deicit (or in other
models: VPD) is taken as a linear dependence:
(
)
fD
(
)
=+ =+
1
hD
1
hq Tq
(
)
−
(9.26)
2
q
,
a
s
q
,
a
s
sat
a
a
where
h
s
depends on the vegetation type but has a typical value of 40 to 50 (kg kg
-1
)
-1
.
In some land-surface models
h
s
is nonzero only for high vegetation. The dependence
of canopy resistance on vapour deicit is in fact a dependence on transpiration rate:
plants limit their water loss if atmospheric demand for water vapour becomes too
large (Leuning,
1995
; Monteith,
1995
; and
Chapter 6
).
The temperature dependence of the canopy resistance is parameterized as a para-
bolic function around the reference temperature
T
ref
:
−
1
2
−
12
/
−
12
/
fT
()
=− −
1
aT
(
T
)
for
T
−
a
<<
T
T
+
a
(9.27)
3
a
T
ref
a
ref
T
a
ref
T
The value for the reference temperature commonly applied is 25
°
C, but in reality this
should depend on plant species. The usual value for the parameter
a
T
is 0.0016 K
-2
which corresponds with a temperature range where Eq. (
9.27
) can be used from
T
ref
-
25 K to
T
ref
+ 25 K. Beyond this range
f
3
needs to be set to an arbitrary large value. Eq.
(
9.27
) with the given value for
a
T
implies that the canopy resistance doubles when the
temperature differs from the reference temperature by around 18 K.
1
Note that here we present the reciprocal of the functions of Chen and Dudhia (
2001
), due to the fact that in Eq.
(
9.24
) we multiply with the response functions rather than divide by them.
2
The factor 2 in
f
seems to be inconsistent with the original two-layer formulation of Dickinson et al. (
1986
) which
would give a factor close to 1.
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