Geoscience Reference
In-Depth Information
leaves above you, the more likely it is that you will
avoid getting wet from direct throughfall.
The amount of
indirect throughfall
is also
controlled by the LAI, in addition to the
canopy
storage capacity
and the rainfall characteristics.
Canopy storage capacity is the volume of water that
can be held by the canopy before water starts
dripping as indirect throughfall. The canopy storage
capacity is controlled by the size of trees, plus the
area and water-holding capacity of individual leaves.
Rainfall characteristics are an important control on
indirect throughfall as they dictate how quickly
the canopy storage capacity is filled. Experience of
standing under trees during a rainstorm should
tell you that intensive rainfall quickly turns into
indirect throughfall (i.e. you get wet!), whereas light
showers frequently do not reach the ground surface
at all. In reality canopy storage capacity is a rather
nebulous concept. Canopy characteristics are con-
stantly changing and it is rare for water on a canopy
to fill up completely before creating indirect
Forest rainfall partitioning
Once rain falls onto a vegetation canopy it effectively
partitions the water into separate modes of move-
ment:
throughfall
,
stemflow
and
interception
loss
. This is illustrated in Figure 2.3.
Throughfall
This is the water that falls to the ground either
directly, through gaps in the canopy, or indirectly,
having dripped off leaves, stems or branches. The
amount of
direct throughfall
is controlled by the
canopy coverage for an area, a measure of which is
the leaf area index (LAI). LAI is actually the ratio of
leaf area to ground surface area and consequently
has a value greater than one when there is more than
one layer of leaf above the ground. When the LAI is
less than one you would expect some direct through-
fall to occur. When you shelter under a tree during
a rainstorm you are trying to avoid the rainfall and
direct throughfall. The greater the surface area of
Figure 2.3
Rainfall above and below a canopy. Indicated on the diagram are stemflow (white arrow
on trunk); direct and indirect throughfall (lightly hatched arrow); and interception loss (upward-
facing darker arrow).
