Geoscience Reference
In-Depth Information
and encouraging local horizontal flow. Figure 12.2c shows the Nipher precipita-
tion gauge which is one example of such shielding. A second gauge, the Alter or
Tretyakov shield gauge, has slats mounted in a circle around the gauge which are
hinged at the top so they blow toward the gauge on the windward side.
Gauge designs
There are many designs for simple gauges that measure precipitation from the
depth of the water stored in a container between manual measurements. The
range is from simple, inexpensive, pole-mounted, graduated plastic cylinders to
more expensive brass or copper gauges with precision-engineered funnel in a
metal surround with adjusting feet to allow accurate leveling of the top of the fun-
nel. The former are preferred by the amateur on the basis of cost, the later by
professional organizations tasked with providing accurate and consistent gauge
measurements.
Over the last half century, there have been efforts to move away from manual
measurement toward automatic recording, and this transition is accelerating with
the advent of digital recording and remote data capture technology. The difficul-
ties involved in frequent manual sampling meant that automatic recording gauges
were used first in applications where measurement of precipitation timing and
intensity were required.
Siphon and chart recorders
, which date back to the nineteenth century, were the
first systems designed for automatic recording of precipitation (Fig. 12.3). Such
gauges are outwardly similar to professional standard, manually read gauges but,
instead of collecting the rain delivered by the funnel in a container, the water is
temporarily collected in a chamber whose volume is equivalent to a rainfall depth
typically of 10 mm. When the chamber is full it siphons and empties before again
being filled by the rain from the collector funnel. If the natural siphoning is used
to empty the chamber, drainage can take 10-20 seconds, which means that some
of the rain may not be measured over this period, this being especially important
in heavy rainfall.
In a natural siphon rainfall recorder, a float chamber on top of the water in the
collection chamber is mechanically connected to a pen that touches a chart which
rotates around a drum, see Fig. 12.3a. The chart typically records for either 1 or 7
days. It is removed after this time for interpretation and a replacement chart
installed. When there is no rain the chart has a level trace (Fig. 12.3b). When it is
raining the rate of upward movement of the pen on the chart is proportional to the
intensity of rainfall. When the storage chamber siphons the chart trace falls rapidly
to indicate an empty chamber and then starts to rise again if rainfall persists. Some
chart recording gauges incorporate a tilting mechanism which removes the pen
from the chart during the siphon, and some include a container which collects the
rainfall that would otherwise be lost during the siphon. The majority of historical
high time resolution precipitation data currently available were gathered using
chart recording rain gauges.
