Geoscience Reference
In-Depth Information
is to do with the particular circumstances of the
catchment. Fog from the cold North Pacific, with
no accompanying rain, is a common feature and it
is believed that the trees intercept fog particles,
creating 'fog drip' which is a significant part of the
water balance. Fog droplets are extremely small
and Ingwersen (1985) has suggested that the sharp
ends of needles on pine trees act as condensation
nuclei, promoting the growth of larger droplets
that fall to the ground (see an example of fogdrip
from tussock leaves in Plate 3). When the trees are
removed there are no condensation nuclei (or far
fewer) on the resultant vegetation so the water
remains in the atmosphere and is 'lost' in terms of
water yield. Equally important is the influence
of vegetation roughness. The turbulent mixing of
air as wind passes over a rough canopy promotes
rapid deposition of condensing water (directly
converse to interception loss, see Chapter 3). The
overall result of this is that the removal of trees leads
to less water in the river; this runs counter to the
evidence provided in the Case Study in Chapter 4.
point to this argument is that all measurement
techniques are in fact precipitation estimation
techniques. For the sake of clarity in this text
precipitation measurement techniques refer to the
methods used to quantify the volume of water
present, as opposed to estimation techniques where
another variable is used as a surrogate for the water
volume.
Rainfall measurement
The instrument for measuring rainfall is called a
rain
gauge
. A rain gauge measures the volume of water
that falls onto a horizontal surface delineated by
the rain gauge rim (see Figure 2.5). The volume is
converted into a rainfall depth through division by
the rain gauge surface area. The design of a rain
gauge is not as simple as it may seem at first glance
as there are many errors and inaccuracies that need
to be minimised or eliminated.
MEASUREMENT
For hydrological analysis it is important to know
how much precipitation has fallen and when this
occurred. The usual expression of precipitation is as
a vertical depth of liquid water. Rainfall is measured
by millimetres or inches depth, rather than by
volume such as litres or cubic metres. The measure-
ment is the depth of water that would accumulate
on the surface if all the rain remained where it had
fallen (Shaw, 1994). Snowfall may also be expressed
as a depth, although for hydrological purposes it is
most usefully described in water equivalent depth
(i.e. the depth of water that would be present if the
snow melted). This is a recognition that snow takes
up a greater volume (as much as 90 per cent more)
for the same amount of liquid water.
There is a strong argument that can be made to
say that there is no such thing as precipitation
measurement at the catchment scale as it varies so
tremendously over a small area. The logical end-
Figure 2.5
A rain gauge sitting above the surface to
avoid splash.
