Geoscience Reference
In-Depth Information
l
E
P
l
E
T
l
E
I
S
S
t
Figure 22.8
The Rutter
model of rainfall interception
and evaporation.
which is illustrated in Fig. 22.8, assumes there is a depth
S
of water, called
the
canopy capacity
, which is the minimum necessary to saturate the canopy.
The model makes a running water balance of water storage on the canopy (and
in some versions of the model, also the stems) from the difference between the
incoming precipitation intercepted by the canopy (and stems), the drip rate of
water from the canopy (or flow of water from the stems), and the intercepted water
that is evaporated.
The rate of evaporation of intercepted water,
E
I
, is calculated using the Penman-
Monteith equation with the surface resistance set to zero. When the calculated
amount of water stored on the canopy,
C
, is less than S, the evaporation is weighted
by the fractional fill of the canopy store. Thus, the calculated rate of evaporation of
intercepted water is:
l
(
)
Δ+
Ac Dr
ρ
C
⎛⎞
λ=
⎜
⎝⎠
ap f a
(22.14)
E
I
S
Δ+γ
The water balance of the canopy is calculated from the equation:
dC
pf
(22.15)
dt
=−−
(1
f ED
)
λ−
t
s
I
where
p
is the incoming precipitation rate,
D
is the rate of canopy drainage,
f
c
is the
fraction of precipitation falling through holes in the canopy, and (assuming a stem
storage balance is also calculated)
f
s
is the fraction of rain diverted to the stems of
