Geoscience Reference
In-Depth Information
where
g
0
is a constant;
g
c
is a canopy cover factor (to allow for gaps in the canopy
and/or patches of soil in the landscape);
g
R
is a radiation stress function, typically
parameterized in terms of the incoming solar radiation using the functional form:
S
K
(1000
+
)
(24.2)
gS
()
=
R
R
1000(
SK
+
)
R
where
S
is the incoming solar radiation (in W m
-2
);
g
D
is a vapor pressure deficit
stress function, typically parameterized in terms of the VPD using the functional
form:
1
2
2
(24.3)
gD KDKD
() 1
=+
+
D
D
D
where
D
is the vapor pressure deficit of the air (in kPa);
g
T
is a temperature stress
function, typically expressed in terms of the air temperature and specified tem-
perature parameters using the functional form:
α
TTT T
gT
TTTT
(
−
)(
−
)
T
(24.4)
()
=
L H
T
α
(
−
)(
−
)
T
o
L H
0
where
T
is the temperature of the air in K, and
a
T
is given by
(
TT
TT
−
)
(24.5)
a
=
H
o
T
(
−
)
o
L
and
g
M
is a soil moisture stress function, variously parameterized in terms of the
available soil moisture in the plant rooting zone, including in the functional form:
(24.6)
1
2
gSM K KSMSM
(
)
=−
1
(
[
−
])
SM
M
M
o
SM
is the soil moisture in a depth of soil accessible to the atmosphere via plant
roots, with maximum soil moisture holding capacity,
SM
o
. The several constants
that appear in equations (24.1) to (24.6) (i.e.,
K
R
, K
D
1
, etc.) are specific to the type
of plant represented and their values were sometimes calibrated against plot-
scale field data. Using observations made over several forest canopies, for
example, Shuttleworth (1989) demonstrated the functional forms for
g
T
, g
R
, and
g
D
shown in Fig. 24.5.
In some SVATS in this group, the aerodynamic roughness and zero plane
displacement of the vegetation were prescribed depending on the vegetation
represented, but in others (e.g., Sellers
et al.
, 1986) it was calculated from more basic
canopy characteristics, and in several, the difference in solar reflection coefficient
above and below 0.7
m was allowed for via the 'two stream' approximation.
Seasonal changes in the vegetation (especially leaf area index) were also often
recognized and prescribed through 'look up' tables.
μ
