Geoscience Reference
In-Depth Information
and longevity of record, but also other issues, including those associated with the
intermittency of satellite observations. However, recognizing the vast oceans and
inaccessible land areas where ground-based measurements are difficult, remote
sensing is probably the only means by which precipitation observations might be
feasible with global coverage. In future, one alternative approach to providing
globally available precipitation estimates might be to merge all available data from
ground-based radar and satellite systems into a meteorological model by using
four-dimensional data assimilation.
Precipitation measurement using gauges
Measuring precipitation using a rain gauge is simple in principle. It requires a fun-
nel of known area to gather the rainfall, a collector to store the water gathered,
some means for measuring the amount of water stored in the collector (such as a
measuring cylinder), and an observer to write down the amount of water meas-
ured. This is the way most measurements of rainfall and some measurements of
snowfall are still made worldwide. Such manual measurements are made at regular
daily, weekly, or monthly intervals, often at 9:00 a.m. local time. The values are
given in equivalent rainfall depth over the sampling interval, in inches in the USA,
but in millimeters elsewhere, often quoted to an accuracy of 0.01 inch or 0.1 mm,
respectively, or designated a 'trace' if the rainfall depth is less than this amount.
Although the operators making such manual measurements are trained to make
measurements with care, clearly unquantifiable operator errors can occur from
time to time.
Gauges were used, and standards of gauge design were independently defined
in different countries, before errors associated with specific design and mounting
were properly appreciated. The result is that country-specific recommendations
on gauge design and site selection are not necessarily ideal. Examples of such spec-
ifications are given in Table 12.1. These recommendations remain in place despite
greater understanding of the weaknesses involved. The need for continuity of
record is an important inhibition on change, because long-term records are very
valuable in the context of hydrological design and when documenting precipita-
tion climate.
Table 12.1
Example of national gauge and gauge mounting recommendations.
Funnel
diameter
Mounting height
of funnel top
Country
USA
203.2 mm (8 in)
787 mm (31 in)
UK (since 1866)
127 mm (5 in)
300 ± 20 mm (12 ± 0.75 in)
Australia and Canada
203.2 mm (8 in)
305 mm (12 in)
