Precipitation Comparison between methods for calculating average rainfall What is Rain gauge History of Rain Gauge Principles of rain gauge Uses of rain gauge Types of rain gauge Other type of rain gauge Recording of Rain from gauge Calculation of Rainfall Adequacy of rain gauge stations frequency of the rainfall References

1. MEASUREMENT OF
PRECIPITATION
Prepared by
Shashikant Verma
(Lecturer)
Department of Civil Engineering
Pietech,Raipur (C.G.)

2. Contents
• Precipitation
• Comparison between methods for calculating average rainfall
• What is Rain gauge
• History of Rain Gauge
• Principles of rain gauge
• Uses of rain gauge
• Types of rain gauge
• Other type of rain gauge
• Recording of Rain from gauge
• Calculation of Rainfall
• Adequacy of rain gauge stations
• frequency of the rainfall
• References
30-08-2017 2Dept. of Civil Engg,Pietech,Raipur

3. Precipitation
 All forms of water that reaches earth’s surface is known as
precipitation.
 It is expressed in terms of depth to which rainfall water
would stand on an area if all the rain were collected on it.
 In case of snowfall equivalent depth of water is considered
as depth of precipitation.
 Rain gauges are used for measurement of precipitation.
30-08-2017 3Dept. of Civil Engg,Pietech,Raipur

4. Precipitation
 In India ‘Indian Meteorological Department (IMD)’
is responsible for all weather and rainfall
predictions.
 It occurs due to:
1. Lifting of air mass
2. Cooling of warm air
3. Condensation
30-08-2017 4Dept. of Civil Engg,Pietech,Raipur

5. Precipitation
Lifting of air occurs mainly due to three causes:
1. Cyclonic precipitation:- It is caused by lifting of
an air mass due to pressure difference.
2. Convective precipitation:- It is caused due to the
upward movement of air. Generally this kind of
precipitation occurs in tropics.
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Dept. of Civil Engg,Pietech,Raipur

6. Precipitation
3. Orographic precipitation:- It is most important
precipitation and responsible for most heavy rains in
India. It is caused by air masses which strike some
natural topographic barriers such as mountains and
can’t move forward hence rise up, causing
condensation and precipitation.
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Dept. of Civil Engg,Pietech,Raipur

7. Comparison between methods for
calculating average rainfall
• Arithmetic mean method
– Assumes uniform rainfall distribution
– Very seldom occurs
– Easiest to use but least accurate
• Thiessen polygon method
– Assumes linear variation
– Use when gauges are not uniformly distributed
– Can use gauges outside of watershed
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8. Comparison between methods for
calculating average rainfall
• Isohyetal method
– Theoretically the most accurate
– Most time consuming method
– Can use gauges outside of the watershed
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9. Rain Gauges
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10. What is Rain gauge
• A Rain gauge is a type of instrument used
by meteorologists and hydrologists to gather and
measure the amount of liquid precipitation over a set
of time.
30-08-2017 10Dept. of Civil Engg,Pietech,Raipur

11. History of Rain Gauge
• The first known rainfall records were kept by the Ancient
Greeks, about 500 B.C. This was followed 100 years later
by people in India using bowls to record the rainfall.
• While some sources state that the much later cheugugi of
Korea was the world's first gauge, other sources say that
Jang Yeong-sil developed or refined an existing gauge. In
1662, Christopher Wren created the first tipping-bucket rain
gauge in Britain.
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12. Principles of it
• Most rain gauges generally measure the precipitation
in millimeters. The level of rainfall is sometimes reported
as inches or centimeters.
• Rain gauge amounts are read either manually or
by automatic weather station (AWS).
• Another problem encountered is when the temperature is
close to or below freezing. Rain may fall on the funnel and
ice or snow may collect in the gauge and not permit any
subsequent rain to pass through.
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13. Use of Rain Gauge
• Study of water resources.
• Used to design water harvesting structures.
• Measure intensity of rainfall.
• Helps collecting information of rainfall with respect
to time.
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14. Minimum density of rain gauges
according to IS 4987-1968
• In plains : 1 station per 520 km2
• In regions of avg. elevation of 1000m : 1 station per 260-
390 km2
• In predominantly hilly areas with heavy rainfall : 1 station
per 130 km2
• In India, usually it is 630 sq km between 2 rain gauges
• Data collected are sent to weather station
• 10% of total should be self recording rain gauges
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15. Where to place rain gauge
• Collector area parallel to ground.
• Open surface area where there are no big trees or
building nearby.
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16. Types of rain gauge
1. Non - Recording Type Rain Gauge
• Most Commonly used rain gauge having collector
surface
2. Recording Type Rain Gauge
• A well calibrated system having equipments to record
on its own
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17. • Most rain gauges used in India are non-recording
type i.e. Symon's rain gauge.
• Recordings or readings are taken at 8:30 am
• And if rainfall is more then intermediate readings are
taken at 5:30pm or in case of heavy rainfall.
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17Dept. of Civil Engg,Pietech,Raipur

18. 1) Non - recording gauge:-
Symons’ Rain gauge  Extensively use in
India
 Accuracy 0.1mm
 Recording done at
8.30am
 Capacity is10cm
 Incase of Heavy
rainfall
 Low Maintenance
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19. Non-recording type
Symons' Rain gauge
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20. Non-recording type Symons' rain gauge
• It consists of a funnel with a circular rim of 12.7 cm
diameter and a glass bottle as a receiver.
• The cylindrical metal casing is fixed vertically to the
masonry foundation with the level rim 30cm above the
ground surface.
• The rain falling into the funnel is collected in the receiver
and is measured in a special measuring glass graduated in
mm of rainfall; when full it can measure 1.25 cm of rain.
• The rainfall is measured every day at 08.30 hours IST.
30-08-2017 20Dept. of Civil Engg,Pietech,Raipur

21. • During heavy rains, it must be measured three or four
times in the day, due to the receiver fill and overflow, but
the last measurement should be at 08.30 hours IST and the
sum total of all the measurements during the previous 24
hours entered as the rainfall of the day in the register.
• Usually, rainfall measurements are made at 08.30 hr IST
and sometimes at 17.30 hr IST also.
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22. • Non-recording or the Symon’s rain gauge gives
only the total depth of rainfall for the previous 24
hours (i.e., daily rainfall)
• It is often desirable to protect the gauge from being
damaged by cattle and for this purpose a barbed wire
fence may be erected around it.
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23. Recording type rain gauge
• This is also called self-recording, automatic or integrating
rain gauge.
• This type of rain gauge has an automatic mechanical
arrangement consisting of a clockwork, a drum with a graph
paper fixed around it and a pencil point, which draws the
mass curve of rainfall.
• From this mass curve, the depth of rainfall in a given time,
the rate or intensity of rainfall at any instant during a storm,
time of onset and cessation of rainfall can be determined.
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24. Types of recording rain gauges
1. Tipping bucket Rain gauge.
2. Weighing Rain gauge.
3. Float type Rain gauge.
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25. Tipping bucket Rain gauge
30.5 cm size as per US
weather bureau.
water collect from Tip
bucket to storage tank.
least count of 1 mm and gives
out one electrical pulse for
every millimeter of rainfall.
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26. Tipping bucket Rain gauge
30-08-2017 26Dept. of Civil Engg,Pietech,Raipur

27. Tipping bucket rain gauge:- This consists of a cylindrical
receiver 30 cm diameter with a funnel inside . Just below the
funnel a pair of tipping buckets is pivoted.
such that when one of the bucket receives a rainfall of 0.25
mm it tips and empties into a tank below, while the other
bucket takes its position and the process is repeated. The
tipping of the bucket actuates on electric circuit which causes
a pen to move on a chart wrapped round a drum which
revolves by a clock mechanism. This type cannot record
snow.
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27Dept. of Civil Engg,Pietech,Raipur

28. Weighing Rain gauge
30-08-2017 28Dept. of Civil Engg,Pietech,Raipur

29. It consists of a storage bin, which is
weighed to record the mass. It
weights rain or snow which falls
into a bucket, set on a platform with
a spring or lever balance. The
increasing weight of the bucket and
its contents are recorded on a chart.
The record shows accumulation of
precipitation.
Weighing Rain gauge
30-08-2017 29Dept. of Civil Engg,Pietech,Raipur

30. • Weighing type rain gauge. In this type of rain-
gauge, when a certain weight of rainfall is collected in
a tank, which rests on a spring-lever balance, it makes
a pen to move on a chart wrapped round a clock driven
drum.
• The rotation of the drum sets the time scale while the
vertical motion of the pen records the cumulative
precipitation.
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31. Weighing type rain gauge
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32. Float type Rain gauge
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33. Float type Rain gauge
30-08-2017 33Dept. of Civil Engg,Pietech,Raipur

34. • Float type rain gauge. In this type, as the rain is
collected in a float chamber, the float moves up which
makes a pen to move on a chart wrapped round a clock
driven drum. When the float chamber fills up, the water
siphons out automatically.
• The clockwork revolves the drum once in 24hr.The
clock mechanism needs rewinding once in a week when
the chart wrapped round the drum is also replaced. This
type of gauge is used by IMD.
30-08-2017 34Dept. of Civil Engg,Pietech,Raipur

35. Other rain gauges
30-08-2017 35Dept. of Civil Engg,Pietech,Raipur

36. The standard rain gauge,
developed around the start of
the 20th century, consists of a
funnel attached to a
graduated cylinder (2 cm in
diameter) that fits inside a
larger outside container
(20 cm in diameter and
50 cm tall).
Standard rain gauge
30-08-2017 36Dept. of Civil Engg,Pietech,Raipur

37. These have a row of collection
funnels. In an enclosed space
below each is a laser diode and
a photo transistor detector.
When enough water is collected
to make a single drop, it drips
from the bottom, falling into the
laser beam path.
Optical rain gauge
30-08-2017 37Dept. of Civil Engg,Pietech,Raipur

38. The acoustic
disdrometer developed
by Stijn de Jong is an
acoustic rain gauge. The
big advantage of this
design is that it is a lot
less costly.
Acoustic rain gauge
30-08-2017 38Dept. of Civil Engg,Pietech,Raipur

39. Recording of Rain from gauge
• Collected Rainfall is measured around 8:30 am.
• The rainfall accumulated is put into a measuring
cylinder.
• This measuring cylinder is standard and
calibrated.
• Recording of rainfall is done daily.
30-08-2017 39Dept. of Civil Engg,Pietech,Raipur

40. Calculation of Rainfall
We know two things,
• Volume of rain water collected(Vol.)
• Area of the funnel/cup collecting surface(A)
Rainfall in mm = Vol. of water/Area of collector
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41. Adequacy of rain gauge stations
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42. frequency of the rainfall
 the frequency of the rainfall is the number of time
that a given magnitude of the rainfall may occur in a
given period.
 The study of the probability of the occurrence of a
particular extreme (such as 24-hr. maximum rainfall )
is of extreme important to determination of the design
flood.
30-08-2017 42Dept. of Civil Engg,Pietech,Raipur

43. frequency of the rainfall
The probability of an event bring equaled
by the following formulae
• California formula : Pro = m/N
• Hazen formula : Pro = 2m-1/2N
• Weibull formula : Pro = m/N+1
Where N = no of years of record
Pro = probability
30-08-2017 43Dept. of Civil Engg,Pietech,Raipur

44. REFERENCES
• Engineering Hydrology K.Subramanya, Mc Graw Hill Publication.
• Engineering Hydrology Rajesh Shrivastav and Ashu Jain, Mc Graw Hill
Publication.
• Introduction to Physical Hydrology, Martin R. Hendricks.
• Hydrology and Floodplain Analysis, Bedient, Huber and Vieux.
• National Geographic Magazine.
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45. 45
THANK YOU AND
HAVE A NICE DAY