Page 164 - Hydrogeology Principles and Practice

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Groundwater investigation techniques 147

necessary to house the gauge in a pit and to surround
it with an antisplash grid. The measurement of snow-
fall is also possible with a standard rain gauge, although
subject to error due to turbulence around the rim of
the gauge. The snow is caught and melted, and the
equivalent amount of water recorded.
Recording gauges (or autographic gauges) are able
to automatically measure or weigh precipitation and
are useful at remote, rarely visited sites. An example
is the tipping-bucket rain gauge in which the number
of times a small bucket of known volume fills and
tips is recorded. Each tip activates a reed switch
which sends an electrical pulse to a logger. The tilting
siphon autographic gauge has a chart pen that floats
up as rainfall fills a chamber, which tips when full,
thus returning the pen to the bottom of the chart.
Recording gauges are more expensive and more prone
to error (for example, very low rainfall amounts are
not recorded) but have the advantage of measuring
rainfall intensity as well as rainfall total.
The number of rain gauges required to give a reli-
Fig. 5.7 The copper 5-inch standard rain gauge used by the
able estimate of catchment rainfall increases where
UK Meteorological Office. The rain gauge consists of a 5-inch
rainfall gradients are marked. A minimum density of
(127-mm) diameter funnel with a sharp rim, the spout of the
2
1 gauge per 25 km is recommended, considering that
funnel being inserted into a glass collecting jar. The jar is in an
inner copper can and the two are contained in the main body of large thunderstorm systems may only cover an area
2
the gauge, the lower part of which is sunk into the ground with of about 20 km . In hilly terrain, where orographic
the rim 12 inches (305 mm) above the surrounding short grass or
effects may cause large and consistent rainfall vari-
gravel, this height being chosen so that no rain splashes from the
ations over short distances, higher rain gauge dens-
surroundings into the funnel. The funnel has a narrow spout in
ities are necessary in the first years of measurement
order to reduce evaporation loss. Normally, the gauge is sited
such that its distance from any obstructions (trees, houses, etc.) (Table 5.1). In tropical areas there is large spatial vari-
is at least four times the height of the obstruction. ation in daily rainfall, but only a small gradient in
annual totals. In such areas, the rain gauge densities in
Table 5.1 will be excessive and higher priority should
5.3.1 Precipitation measurement be given to obtaining homogeneous records of long
duration at a few reliable sites.
Precipitation falls mainly as rain but may also occur To be able to assess a representative value of
as hail, sleet, snow, fog or dew. The design of the rainfall over large areas, it is necessary to employ a
standard rain gauge used in the United Kingdom is method of averaging the individual gauge measure-
shown in Fig. 5.7 with a cylinder diameter of 5 inches ments. The simplest method is to take the arithmetic
(127 mm). In the United States and Canada, standard
rain gauges have diameters of 8 and 9 inches (203 and
Table 5.1 Density of rain gauges required in a hill area.
229 mm), respectively. Standard rain gauges are read
daily, for example at 0900 h in the United Kingdom. Catchment area (km ) Number of gauges
2
In exposed locations, the rain catch of the gauge is
affected by high winds and it is generally accepted 4 6
that more accurate results will be obtained from a 20 10
80 20
rain gauge set with its rim at ground level. Although
160 30
more expensive, with a ground-level installation it is