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and stage of development, which should be well characterized by leaf area index;
and (2) the intensity, duration, frequency and form of precipitation. The interception
loss ranges from 10-40% of gross precipitation, depending on vegetation and climate
(Dingman,
2002
; Muzylo et al.,
2009
; Gerrits,
2010
). Therefore, to simulate evapo-
transpiration and rainfall iniltration into the soil, we should quantify properly the
amount of rainfall interception.
Rutter et al. (
1975
) presented a conceptual, physically based model for forests that
proved to be very useful. Their model represents the interception process by a running
water balance of rainfall input, storage and output in the form of drainage and evap-
oration (
Figure 6.26
). The canopy structure is described by the free throughfall coef-
icient
r
(-), the stemlow partitioning coeficient
r
t
(-), the canopy storage capacity
S
(mm) and the trunk storage capacity
S
t
(mm). The Rutter model estimates throughfall,
stemlow and interception loss from input rainfall and evapotranspiration data. Essen-
tially, it is based on the dynamic calculation of the water balance for the canopy and
for the trunks through the equations:
(
)
=
∫
∫
∫ ∫ ∫
∫
1−−
rr Pt
d
Dt
d
+
Et
d
+
∆
∆
C
t
int, c
(6.43)
rPt
d
=
I
d
t
+
E t
d
+
C
t
s
int, t
t
where
P
is the intensity of gross rainfall (mm d
-1
),
D
is the drainage rate from the
canopy (mm d
-1
),
E
int, c
is the evaporation rate of water intercepted by the canopy
(mm d
-1
),
ΔC
is the change in canopy storage (mm),
I
s
is the stemlow (mm d
-1
),
E
int, t
is the evaporation rate of water intercepted by the trunks (mm d
-1
) and
ΔC
t
is the
change of trunk storage (mm).
The evaporation rate from a saturated canopy
E
pot
is calculated using the Penman-
Monteith equation with the canopy resistance set to zero (
Chapter 7
). When actual can-
opy storage
C
(mm) is less than canopy storage capacity
S
, evaporation rate is reduced
in proportion to
C
/
S
. The rate of drainage from the canopy is usually calculated as:
DD
=
s
exp ( (
bC
−
S
))if
if
C
≥
S
(6.44)
D
=
0
CS
<
where
D
s
is the drainage rate when
C
=
S
and
b
is an empirical coeficient.
Modelling of stemlow and trunk evaporation follows closely the procedure previ-
ously used for the canopy. Evaporation from trunks is calculated as:
E
=
ε
E
if
CS
≥
int, t
pot
t
t
C
S
(6.45)
E
=
ε
E
t
if
CS
<
int, t
pot
t
t
t
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