Geoscience Reference
In-Depth Information
reasonable compromise. There is a strong argument
to be made against changing its design. Any change
in the measurement instrument would make an
analysis of past rainfall patterns difficult due to the
differing accuracy.
growth of trees so that at all times during the
rainfall record the distance apart is at least twice the
height of an obstacle.
Gauges for the continuous measurement
of rainfall
Siting of a rain gauge
The standard UK Meteorological Office rain gauge
collects water beneath its funnel and this volume
is read once a day. Often in hydrology the data needs
to be measured at a finer timescale than this,
particularly in the case of individual storms which
often last much less than a day. The most common
modern method for measuring continuous rainfall
uses a tipping-bucket rain gauge. These are very
simple devices that can be installed relatively
cheaply, although they do require a data-logging
device nearby. The principle behind the tipping-
bucket rain gauge is that as the rain falls it fills up
a small 'bucket' that is attached to another 'bucket'
on a balanced cross arm (see Figure 2.10). The
'buckets' are very small plastic containers at the
end of each cross arm. When the bucket is full it
tips the balance so that the full bucket is lowered
down and empties out. At the time of tipping a
magnet attached to the balance arm closes a small
reed switch which sends an electrical signal to a
data-logging device. This then records the exact
time of the tipped bucket. If the rain continues to
fall it fills the bucket on the other end of the cross
Once the best measurement device has been chosen
for a location there is still a considerable measure-
ment error that can occur through incorrect siting.
The major problem of rain gauge siting in hydrol-
ogy is that the scientist is trying to measure the
rainfall at a location that is representative of a far
greater area. It is extremely important that the
measurement location is an appropriate surrogate
for the larger area. If the area of interest is a forested
catchment then it is reasonable to place your rain
gauge beneath the forest canopy; likewise, within
an urban environment it is reasonable to expect
interference from buildings because this is what is
happening over the larger area. What is extremely
important is that there are enough rain gauges to
try and quantify the spatial and temporal variations.
The rule-of-thumb method for siting a rain gauge
is that the angle when drawn from the top of the
rain gauge to the top of the obstacle is less than 30°
(see Figure 2.9). This can be approximated as at
least twice the height of the obstacle away from the
gauge. Care needs to be taken to allow for the future
rain gauge
Figure 2.9
Siting of a rain gauge away from obstructions.
