Geoscience Reference
In-Depth Information
Figure 2.6
Surface rain gauge with non-splash
surround.
Figure 2.8
Baffles surrounding a rain gauge to lessen
the impact of wind turbulence. The gauge is above
ground because of snow cover during the winter.
The optimum rain gauge design
There is no perfect rain gauge. The design of the
best gauge for a site will be influenced by the indi-
vidual conditions at the site (e.g. prevalence of
snowfall, exposure, etc.). A rain gauge with a non-
splash surround, such as in Figure 2.6, can give very
accurate measurement but it is prone to coverage
by heavy snowfall so cannot always be used. The
non-splash surround allows adjacent rainfall to pass
through (negating splash) but acts as an extended
soil surface for the wind, thereby eliminating the
turbulence problem from raised gauges. This may
be the closest that it is possible to get to measuring
the amount of rainfall that would have fallen on a
surface if the rain gauge were not there.
The standard UK Meteorological Office rain
gauge has been adopted around the world (although
not everywhere) as a compromise between the
factors influencing rain gauge accuracy. It is a brass-
rimmed rain gauge of 5 inches (127 mm) diameter
standing 1 foot (305 mm) above the ground. The
lack of height above ground level is a reflection of
the low incidence of snowfall in the UK; in coun-
tries such as Russia and Canada, where winter
snowfall is the norm, gauges may be raised as high
as 2 m above the surface. There is general recog-
nition that the UK standard rain gauge is not the
best design for hydrology, but it does represent a
Figure 2.7
The effect of wind turbulence on a raised
rain gauge. An area of reduced pressure (and uplift)
develops above the gauge in a similar manner to an
aircraft wing. This reduces the rain gauge catch.
both the wind speed and the raindrop diameter
(Neˇpor and Sevruk, 1999). At wind speeds of 20
km/hr (Beaufort scale 2) the loss could be up to 20
per cent, and in severe winds of 90 km/hr (Beaufort
scale 8) up to 40 per cent (Bruce and Clark, 1980;
Rodda and Smith, 1986). The higher a gauge is
from the surface the greater the loss of accuracy. This
creates a major problem for gauges in areas that
receive large snowfalls as they need to be raised to
avoid surface coverage.
One method of addressing these turbulence
difficulties is through the fitting of a shield to the
rain gauge (see Figure 2.8). A rain gauge shield can
take many forms but is often a series of batons
surrounding the gauge at its top height. The shield
acts as a calming measure for wind around the gauge
and has been shown to greatly improve rain gauge
accuracy.
