Geoscience Reference
In-Depth Information
3.4
Interception
3.4.1 Definition and observed magnitudes
Interception is the part of precipitation that moistens the different surface elements, mainly
vegetation, and is temporarily stored on them. When the surface elements are fully saturated,
so that they have reached their full
interception storage capacity
, any excess intercepted
water on them flows or drips down to the ground. In practice, the interception storage
capacity is usually defined more specifically as the amount of water left on the canopy at
the end of a storm, under conditions of zero evaporation and after all drip has ceased; thus
during a storm the stored depth of water can exceed the storage capacity. The precipitation
that reaches the ground is often called
net precipitation
. In the case of vegetation, most
of the net precipitation filters through the canopy as
throughfall
; a small part flows down
along major branches and stems as
stemflow
, and tends to concentrate over the roots. The
interception of precipitation by a vegetation canopy can greatly affect the hydrologic budget
at the ground surface. The water held by the foliage elements that evaporates before it can
reach the ground is thus no longer available for infiltration and runoff. Therefore, the amount
of intercepted precipitation, that returns to the atmosphere by evaporation, is often called
the
interception loss
.
Most interception studies have focused on forested surfaces, where the largest values
occur. Both the type of vegetation and the type of precipitation appear to play a role.
Indeed, tall or dense vegetation tends to incur larger interception losses than short or sparse
vegetation. Also, interception losses as a fraction of precipitation are usually larger when the
precipitation events are of moderate intensity and longer duration, than when they occur in
the form of short intense bursts and downpours. For example, in tall dense forest vegetation
at temperate latitudes interception losses have been observed that are as large as 30% to 40%
of the gross precipitation (Gash
et al
., 1980). In tropical forests with high intensity rainfall,
however, the observed losses have tended to be more of the order of 10% to 15% (see Lloyd
and Marques, 1988; Lloyd
et al
., 1988; Ubarana, 1996), even though the evaporation rates
during rainfall were not very different. Similarly, sparse forests also tend to have lower
values of interception, namely around 10% to 20% of the precipitation (see Gash
et al
.,
1995; Valente
et al
., 1997). Interception losses in heather and shrub covered terrain are
smaller than one third of the values in dense forest (Calder, 1990) under the same climatic
conditions.
3.4.2 Interception loss mechanisms in vegetation
For a single precipitation event the total interception loss is the sum of the evaporation
from the wet vegetation during the event and the evaporation of the water remaining on
the vegetation after the precipitation has ceased. Horton (1919) was probably the first to
formulate this, for a storm duration
D
, which is long enough to saturate the vegetation, as
follows
D
L
i
=
E
i
dt
+
S
ic
(3.6)
0
where
E
i
is the rate of evaporation of intercepted water [L
/
T], and
S
ic
the interception
storage capacity of the vegetation [L]. When the precipitation ceases before the vegetation
