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the vegetation. One expression for the drainage rate that has been used in the
Rutter model is:
[
]
Da bCS
=
exp
(
−
)
(22.16)
with
a
4 mm being typical values. The total evaporation
from the canopy includes that from the intercepted water,
=
0.002 mm min
−1
and
b
=
l
E
I
, and that from any
dry leaves,
l
E
T
, and is estimated from the weighted sum:
C
C
⎛⎞
⎛
⎞
λ=
E
λ + −
E
1
λ
E
(22.17)
⎜⎟
⎜
⎟
T
I
⎝⎠
S
⎝
S
⎠
where
(
)
Δ+
λ=
Δ+
Ac Dr
r
ap f a
E
(22.18)
(
)
T
rr
g
1
+
s a
where
r
s
is the surface resistance of the canopy were it all dry.
The Rutter model of canopy interceptions has been adopted in the land surface
schemes of General Circulation Models but it has sometimes been substantially
simplified when used in this application. A running canopy water balance is
still made, but evaporation is set equal to
l
E
I
whenever
C
> 0, and
D
=
0 whenever
C
<
S
, but
D
p
-
S
whenever
p
>
S
.
Recently there have been developments of the original Rutter model described
above to allow its use in sparse canopies (e.g., Valente
et al
., 1997). In essence the
approach used is to separate the landscape into two portions, a fraction
c
without
vegetation cover for which it is assumed there is no interception loss, and a frac-
tion (1-
c
) with vegetation cover for which evaporation is assumed calculated using
a version of the original Rutter model, perhaps with a simplified description of
drainage similar to that decribed in the last paragraph. This revised sparse canopy
version of the Rutter model is now accepted as being the preferred form because
as the fractional vegetation cover changes, it has appropriate asymptotic limits.
For a recent comprehensive review of models of wet canopy interception loss the
reader is referred to Muzylo
et al.
(2009).
=
Equilibrium evaporation
Consider the enclosed volume,
V
, of air above a water surface of area,
A
, inside a
thermally insulating box, shown in Fig. 22.9. Assume the air is well-mixed and has
reached saturation. Now assume that for a period
t
, a radiant energy input flux,
R
n
, is applied to the water surface but the air remains well-mixed. The incoming
energy will be used to both evaporate some of the water and to raise the temperature
δ
