Environmental Engineering Reference
In-Depth Information
spring. Since these gages store water in a reservoir, similar to non-recording gauges,
they also are susceptible to evaporation loss or overflow resulting from infrequent site
visits. Weighing gages record cumulative precipitation data at a specified frequency
(e.g., hourly). Estimating the amount of rainfall or snowfall during specific time
intervals may not be straightforward due to instrument noise. In particular, electronic
sensors are sensitive to temperature; even after temperature compensation routines
are applied, the data may contain substantial temperature-related error. Therefore,
it is best to use weighing gages for recording precipitation over a relatively long
interval (e.g., weekly, in which case temperature-related effects can be integrated
over a longer time), and tipping-bucket gages for studies requiring greater temporal
resolution.
Tipping-bucket rain gages introduce the water received in a funnel to one of a pair
of identical vessels (buckets) balanced on a fulcrum. When one bucket is filled, it tips
and sends an electronic pulse to a recording device, and the other bucket is brought
into position for filling (Dingman 2002 :105). These instruments are useful for
collecting rainfall data at a high temporal resolution, but they also have some
disadvantages. For example, during high-intensity rainfall the sensor measures less
rainfall than actually occurs. The bucket does not tip instantly and during the first half
of its motion, rain is being fed into the compartment already filled with the designed
amount of rainfall (WMO 1994 :103). This delay results in systematic negative bias in
measured rainfall for high intensity events. Events with less than a minimum amount
of precipitation required to tip the bucket also are not recorded (these are called
“trace” events). Similarly, a small amount of water collected at the end of an event
commonly is left in the bucket and subsequently evaporates, resulting in undere-
stimation of precipitation. The sensitive balance of buckets requires periodic calibra-
tion of the amount of precipitation per each tip. Without such calibration, the data
may contain a substantial degree of positive or negative bias.
Optical devices are less commonly used, but represent promising new technology
(Nitu and Wong 2010 ). When water passes through an optical scintillation gage it
alters the frequency of an infrared beam, which can be analyzed to deduce the time,
amount, and intensity of precipitation. In the second type, the sensor measures the
extinction caused by precipitation droplets falling through a thin sheet of light, from
which precipitation type (rain or snow), amount, and intensity are deduced. The third
type measures the forward optical scattering by the particles, from which the
precipitation type, amount, and intensity are estimated. In comparison to conven-
tional gages, these optical devices tend to have a larger degree of measurement
uncertainty, but they provide useful alternatives when tipping-bucket or weighing
gages cannot be used, for example, on a ship or a floating platform affected by wave
motion (Nystuen et al. 1996 ).
Recording precipitation gages are commonly used with internal or external data-
logging devices that record the data at a fixed interval (e.g., every 30 min) or record
the time stamp of individual tips of a tipping bucket. Many data-logging devices
can also accommodate other environmental sensors, such as water-level or water-
quality sensors, and transmit the data via telephone, satellite, or wireless communi-
cation network.
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