Geoscience Reference
In-Depth Information
Fig. 3.18 Sketch illustrating the
water balance of a
vegetation canopy.
E
i
cP
S
O
is fully saturated, the precipitation amount lost by interception is
D
L
i
=
E
i
dt
+
S
(3.7)
0
in which
S
is the amount of water stored on the partly saturated vegetation. Before this
equation can be used to estimate the loss, the variables
S
and
E
i
must be known.
A common way to determine
S
is to treat the vegetation as one or more storage elements,
representing the canopy and the trunks, to which the lumped storage equation (1.10) can
be applied. In the simplest approach, when a single element is assumed to represent the
evolving canopy storage
S
, one can take the precipitation as the inflow, and the evaporation
and liquid drainage as the two outflow rates (Figure 3.18); thus,
dS
dt
=
cP
−
E
i
−
O
for 0
≤
S
≤
S
ic
(3.8)
where
c
is the horizontal density or fractional cover of the intercepting vegetation,
P
the
precipitation intensity, and
O
the liquid drainage outflow rate from the vegetation. Note that
Equation (3.8) should be applied only to stands of vegetation that are sufficiently uniform at
the scales under consideration; thus it would not be applicable, for example, in the case of
chessboard-like surfaces consisting of forest stands and clearings with different vegetation,
or to sparse stands of trees in a grassy or bare soil environment. In such situations the
analysis may have to be applied separately to each type of land cover and the results can
then be weighted according to the fraction of the area each one occupies.
Integration of Equation (3.8) yields for the storage
D
D
D
S
=
c
Pdt
−
E
i
dt
−
Odt
for
S
≤
S
ic
(3.9)
0
0
0
