Environmental Engineering Reference
In-Depth Information
The main reason for these differences in gauge height is snow. We have already
mentioned that snowfall is difficult to measure because of its lightness and tendency to
drift. In the United States the same gauge is used to measure both snowfall and rain, so it
has to be well above the level of drifting snow. In Canada separate gauges are used, while
in Britain snowfall is a relatively small component of the annual precipitation. Shields are
often provided around gauges to reduce the impact of wind, as seen in Figure 5.2.
In normal operation the amount of rainfall collected in a gauge is measured once a
day. In the United States an appropriately calibrated stick is used to measure the depth of
water which has accumulated in the gauge to obtain the quantity of rainfall. In Canada
and the United Kingdom the rainwater or melted snow in the gauge is poured into a glass
measuring cylinder, where the rainfall equivalent can be read directly. A standard rain
gauge will record only the total rain which has fallen between readings.
In many cases it is important to know when the rain fell and at what intensity. For this
purpose recording rain gauges are used. Recent systems use a tipping bucket of known
capacity which electronically records the number of times the bucket tips. This
information can be stored by a data logger and downloaded directly on to a computer.
Some can even be interrogated via telephone or satellite. This is particularly useful in
remote areas where heavy rainstorms may lead to flooding lower in the river basin.
Rain gauges are not the only means of measuring rainfall. Weather radar systems have
been developed which can provide quantitative estimates of the rates of rainfall. The
method is based on the amount of reflection of the radar signal from falling precipitation.
Although there are problems of interpretation of the reflected signal because of scattering
from local buildings or hills and when snowflakes melt, it is now possible to produce
maps of areas of precipitation and their intensity, as shown in Figures 5.3
Table 5.3 Variation of rainfall catch with gauge height
Height of gauge mouth above ground (cm)
5
10
15
20
30
46
76
152
610
Catch as % of that at 30 cm
105
103
102
101
100
99·2
97·7
95·0 90·0
Source
: After Bruce and Clarke (1966).
