Environmental Engineering Reference
In-Depth Information
within an open space in a fairly uniform enclosure of trees, shrubs, or fences, so that
wind effects (see below) are minimized. None of the surrounding objects should
extend into the volume of an imaginary inverted cone with the apex positioned
directly above the sensor and the sides extending from the sensor orifice at a 45
angle (Dingman
2002
:112). The gage orifice should be horizontal even on a sloping
ground surface. The gage height should be as low as possible to minimize wind
effects, which increase with height, but high enough to prevent splash of rain drops
from the ground. If the gage is used to measure snowfall, the orifice should be
located above the maximum snow height. An orifice height of 0.3 m is used in many
countries in areas that receive little snow. A standard of 1 m is suggested for most
areas that accumulate larger amounts of snow during winter (WMO
1994
:92).
Orifice size needs to be sufficiently large to minimize edge effects and should be
known to the nearest 0.5 % for an accurate conversion of volume of water collected
(m
3
) to equivalent depth of precipitation (mm). An orifice area of 200-500 cm
2
is
common (WMO
1994
:94). The collection cylinder should be deep enough and the
slope of the funnel steep enough to prevent rain from splashing out of the gage. For
storage-type gages (see below), a smaller-diameter restrictor should be positioned
between the orifice and the collection cylinder, and the cylinder should be covered
with a highly reflecting material to minimize loss of water by evaporation. Adding a
layer of non-volatile immiscible oil floating on the collected water also minimizes
evaporation. Low viscosity, non-detergent motor oils are recommended for this
purpose; transformer and silicone oils have been found to be unsuitable (WMO
1994
:95).
3.4.2 Type of Precipitation Gages
Precipitation gages can be classified into non-recording and recording types.
Non-recording gages generally consist of an open receptacle with vertical sides or
a funnel, and a reservoir that stores the collected water. Precipitation is determined by
weighing or measuring the volume of water collected in the reservoir, or by measur-
ing the depth of water using a calibrated measuring stick or scale. Care must be taken
to minimize observation errors for both graduated-cylinder and weighing-device
measurements (see WMO
1994
:96-100 for detailed discussion). If measurements
are made infrequently, evaporation loss can cause substantial negative bias in the
data, or overflow of the collector may occur as a result of unusually heavy storm
events. Despite these potential sources of errors, carefully operated non-recording
gages present a useful alternative to recording gages because they are simple,
accurate, and relatively inexpensive. They are particularly useful for applications
that require a large number of points to capture spatial variability of rainfall at a low
temporal resolution (e.g., weekly or monthly).
The most commonly used recording devices are the weighing gage and the
tipping-bucket gage. Weighing gages measure the weight of the storage reservoir
and its contents using electronic sensors, such as a load cell or strain gage, or a
