Geoscience Reference
In-Depth Information
throughfall. This means that indirect throughfall
occurs before the amount of rainfall equals the
canopy storage capacity, making it difficult to gauge
exactly what the storage capacity is.
is possible for the water to rapidly enter the soil
through flow along roots and other macropores
surrounding the root structure. This can act as a
rapid conduit of water sending a significant pulse
into the
soil water
.
Stemflow
Interception loss
Stemflow is the rainfall that is intercepted by stems
and branches and flows down the tree trunk into the
soil. Although measurements of stemflow show that
it is a small part of the hydrological cycle (normally
2-10 per cent of above canopy rainfall; Lee, 1980)
it can have a much more significant role. Durocher
(1990) found that trees with smoother bark such
as beech (
Fagus
) had higher rates of stemflow as
the smoothness of bark tends to enhance drainage
towards stemflow.
Stemflow acts like a funnel (see Figure 2.4),
collecting water from a large area of canopy but
delivering it to the soil in a much smaller area: the
surface of the trunk at the base of a tree. This is
most obvious for the deciduous oak-like tree illus-
trated in Figure 2.4, but it still applies for other
structures (e.g. conifers) where the area of stemflow
entry into the soil is far smaller than the canopy
catchment area for rainfall. At the base of a tree it
While water sits on the canopy, prior to indirect
throughfall or stemflow, it is available for evap-
oration, referred to as
interception loss
. This is an
evaporation process and it is discussed further in the
following chapter.
Interception gain
In some circumstances it is possible that there is
an interception gain from vegetation. In the Bull
Run catchment, Oregon, USA it has been shown
that the water yield after timber harvesting was sig-
nificantly less than prior to the trees being logged
(Harr, 1982; Ingwersen, 1985). This is counter to
the majority of catchment studies reported by Bosch
and Hewlett (1982) which show an increase in water
yield as forests are logged. The reason for the loss of
water with the corresponding loss of trees in Oregon
Figure 2.4
The funnelling effect of a tree canopy on stemflow.
