Rain Gauges
Rain Gauges
Rain Gauges
Introduction
Figure 1.1: The National Weather Service Standard Rain Gauge, SRG (source:
http://www.srh.noaa.gov/tlh/cpm/srg_page.html
Recording gauges
Unlike non-recording gauges, a recording gauge is designed to automatically
record the amount of rainfall reaching the surface as a function of time during the
lifespan of a storm. The most common types of recording gauges are:
Tipping-bucket rain gauge
The tipping bucket rain gauge consists of a cylinder (typically 8 or 12-inch
diameter) with a funnel that drains into a pair of buckets that are balanced on a
horizontal axis. When a predetermined amount of rainwater (commonly 0.01
inches) has been collected into one of the buckets, the bucket tips causing the
other bucket to move quickly into position and catch the incoming rain. With each
tip, an electronic signal is sent to a data logger that records the time of tip
occurrence. The known amount of each tip and its occurrence time makes it
possible to calculate the incremental amounts of rain over variable or fixed
intervals of time. These measurements can also be used to estimate the rainfall
rates during the life of the storm.
Shielded gauges:
0.58
0.69
(2.1)
(2.2)
In the above formulae (Dingman, 2002) u is the daily average wind speed at the
elevation in m/s and K is the wind correction factor. To adjust for wind undercatch, the calculated correction factors are multiplied by the measured daily
rainfall values. The following figure shows a graphical representation of such
formulae.
Gauge catch due to wind effect for gauges without wind-shield (solid line) and
with wind-shield (dotted line).
Research has shown that for the US 8-inch standard rain gauge (SRG), wind
under-catch can be in the order of 5 to 10% on an annual basis but can be
relatively larger on individual storm scales. Therefore, it is recommended to
perform wind corrections on a monthly or daily basis.
Evaporation and Wetting Losses
These losses are encountered in storage-type non-recording gauges, gauges
with small orifices, and gauges recording at long intervals (several days). The
magnitude of these losses depends on temperature, humidity, and time between
rain and collection of the measurement. However, such errors are usually small
and can be often neglected except for low-intensity rainfall events.
Calibration Errors
This error is encountered in tipping-bucket rain gauges. These gauges require
calibration and adjustment of the tipping mechanism which is mostly done at a
fixed small or intermediate rain rate (usually referred to as static calibration).
However, at high rain rates a tipping-bucket gauge may suffer from
underestimation problems due to the fact that the tipping buckets cannot keep up
with heavy rain during a severe thunderstorm. To correct for such problems, rain
rate-dependent calibration procedure (Humphery, 1997) should be developed;
however, this can be time consuming and only static calibration is developed and
provided by the gauge manufacturer. The user of such gauges should be aware
of possible underestimation at high rainfall intensities (>50 mm/h).
Other sources of errors in gauge measurements
Other sources of errors include rainfall splashing, possible electronic and
mechanical breakdown of gauges, clogging of gauge orifices and funnels, and
observer mistakes in recording, processing and publishing rainfall
measurements. Also, improper sitting configuration of rain gauges near trees or
building can cause significant losses of rainfall amounts. As a general rule, an
obstruction object should not be closer to the gauge than twice its height above
the ground (Dingman, 2002).
How to deal with missing rainfall measurements
Missing records in rainfall measurements is not an uncommon problem. This is
due to the high probability of gauge mechanical and electrical failures and is also
caused by erroneous recording and publishing of rainfall measurements.
Engineers often have to work with rainfall data from stations where rainfall
records might be missing for a day or several days. This will limit most types of
rainfall analyses such as calculation of water budgets, determining maximum
rainfall intensities, and estimation of area-average rainfall intensities. Several
methods have been developed to estimate missing records at a certain station
from concurrent measurements at nearby stations based on a certain weighting
scheme (Dingman, 2002):
Uniform weighting
This is the simplest weighting scheme where the same weight is assigned to
each of the available n nearby station:
Rg =
i=n
1
n
(2.3)
i =1
Rg =
1
n
i =n
ARg
i =1
AR
(2.4)
Ri
1
i =n
2
i =1 d i
i =n
(d
i =1
2
i
Ri )
(2.5)
The user of these methods is cautioned against over weighting that may be
caused by station clustering. When several of the nearby gauges are clustered
together, they may bias the estimate of missing data. In such situation, only one
out of the clustered stations (the closest to the missing station) should be
included in the analysis.
Example:
Monthly rainfall measurements were recorded at six rain gauge stations as
shown in Table 1.1. The coordinates of each gauge are given in terms of Easting
(x) and Northing (y) values (Table 1.2). If the June reading of gauge 5 (G5) is
missing, calculate an estimate for this value using two different methods: uniform
weighting, and inverse-distance weighting.
Table 1.1: Monthly rainfall measurements (inch) at six stations
YEAR
1994
1994
1994
1994
1994
1994
1994
1994
1994
1994
1994
1994
MONTH
1
2
3
4
5
6
7
8
9
10
11
12
G1
4.4
1.5
2.1
7.0
3.4
10.0
9.2
3.3
3.6
9.5
1.3
5.8
G2
3.4
0.6
4.3
1.1
7.8
8.8
7.7
3.0
10.3
3.3
4.1
2.6
G3
4.7
2.8
5.1
5.5
4.6
6.1
10.8
2.8
3.6
4.4
2.3
3.5
G4
7.6
4.5
4.7
7.0
8.0
5.9
6.0
3.3
4.6
3.8
1.2
3.2
G5
4.0
1.3
2.4
5.2
3.8
?
10.7
1.7
6.2
2.7
1.9
3.1
G6
3.8
1.0
2.9
6.1
5.0
8.8
12.0
7.2
9.8
6.5
0.8
4.5
Easting (m)
761257
719842
807849
748650
714621
765282
Northing(m)
3262310
3275040
3276620
3298200
3296010
3321970
Solution:
According to the uniform weighting average method, the estimated missing
rainfall value is calculated as the simple arithmetic average of available
measurements at the five other stations:
G5= (10.0+8.8+6.1+5.9+8.8)/5 = 7.9 inches
In the inverse-distance weighting method we have to use weights that are based
on distance between the missing gauge and each of the other five gauges.
Therefore, we first need to calculate such distances as follows:
d1-5 = (761257 - 714621) 2 + (3262310 - 3296010) 2 = 57538 m
1
1
1
1
1
+
+
+
+
2
2
2
2
2
57538 21610 95223 34099 56925
1
1
1
1
1
10 +
8 .8 +
6 .1 +
5 .9 +
8.8 = 8.1 inches
2
2
2
2
2
21610
95223
34099
56925
57538
R A = wi Ri
(2.6)
i =1
1 i =n
Ri
n i=1
(2.7)
straight line to form a set of triangles that cover the whole area. Then,
perpendicular bi-sectors are drawn through the sides of each triangle. The
bisectors are extended until they interest with each other and with the boundaries
n
1 i =n
ai Ri
A i =1
(2.8)
Federal and State Wild-land Fire Programs operate more than 2,400 rain
gages over the contiguous United States.
The United States Geological Survey (USGS) operate about 1,734 rain
gauges.
The United States Army Corps of Engineers (USACE) operate more about
1,637 rain gages
The National Weather Service (NWS) operate about 222 recording gauges
(known as first-order stations) that provide rainfall measurements at an hourly
scale.
The National Weather Service (NWS) operate more than 8,000 nationwide
stations that record rainfall measurements (also known as cooperative
stations or cooperative network COOP; http://www.weather.gov/om/coop/).
Unlike the first-order stations which are served by NWS staff and technicians,
the COOP gauges are operated mostly by volunteers. Most COOP stations
are of the non-recording gauge type which requires that the volunteer
observer take daily visual readings of accumulated rainfall and record them in
standard charts and tables. Some COOP stations are of the recording
(weighing) gauges.
The National Weather Service (NWS) supports other regional networks such
as the Automated Local Evaluation in Real Time (ALERT;
Other networks are also operated by agencies such as the United States
Bureau of Reclamation (USBR) and Tennessee Valley Authority (TVA).