Presentation for unit 2 precipitation and its measurement

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) D epartment o f C i v il E n g ineering 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 8/30/2017 2 Dept. 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. 8/30/2017 3 Dept. 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: Lifting of air mass Cooling of warm air Condensation 8/30/2017 4 Dept. of Civil Engg,Pietech,Raipur

5. Precipitation Lifting of air occurs mainly due to three causes: Cyclonic precipitation:- It is caused by lifting of an air mass due to pressure difference. Convective precipitation:- It is caused due to the upward movement of air. Generally this kind of precipitation occurs in tropics. 8/30/2017 5 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. 8/30/2017 6 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 8/30/2017 7 Dept. of Civil Engg,Pietech,Raipur

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 8/30/2017 8 Dept. of Civil Engg,Pietech,Raipur

9. Rain Gauges 8/30/2017 9 Dept. of Civil Engg,Pietech,Raipur

10. What is Rain gauge A  R ain 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. 8/30/2017 10 Dept. of Civil Engg,Pietech,Raipur

11. History of Rain G auge 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. 8/30/2017 11 Dept. of Civil Engg,Pietech,Raipur

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. 8/30/2017 12 Dept. of Civil Engg,Pietech,Raipur

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. 8/30/2017 13 Dept. of Civil Engg,Pietech,Raipur

14. Minimum density of rain gauges according to IS 4987-1968 In plains : 1 station per 520 km 2 In regions of avg. elevation of 1000m : 1 station per 260-390 km 2 In predominantly hilly areas with heavy rainfall : 1 station per 130 km 2 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 8/30/2017 14 Dept. of Civil Engg,Pietech,Raipur

15. Where to place rain gauge Collector area parallel to ground. Open surface area where there are no big trees or building nearby. 8/30/2017 15 Dept. of Civil Engg,Pietech,Raipur

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 8/30/2017 16 Dept. of Civil Engg,Pietech,Raipur

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. 8/30/2017 17 Dept. of Civil Engg,Pietech,Raipur

18. 1) Non - recording gauge:-Symons’ Rain gauge Extensively use in India Accuracy 0.1mm Recording done a t 8.30am Capacity is10cm Incase of Heavy rainfall Low Maintenance 8/30/2017 18 Dept. of Civil Engg,Pietech,Raipur

19. Non-recording type Symons' Rain gauge 8/30/2017 19 Dept. of Civil Engg,Pietech,Raipur

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. 8/30/2017 20 Dept. 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. 8/30/2017 21 Dept. of Civil Engg,Pietech,Raipur

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. 8/30/2017 22 Dept. of Civil Engg,Pietech,Raipur

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. 8/30/2017 23 Dept. of Civil Engg,Pietech,Raipur

24. T ypes of recording rain gauges T ipping bucket Rain gauge. Weighing Rain gauge. Float type Rain gauge. 8/30/2017 24 Dept. of Civil Engg,Pietech,Raipur

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. 8/30/2017 25 Dept. of Civil Engg,Pietech,Raipur

26. Tipping bucket Rain gauge 8/30/2017 26 Dept. 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. 8/30/2017 27 Dept. of Civil Engg,Pietech,Raipur

28. Weighing Rain gauge 8/30/2017 28 Dept. 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 8/30/2017 29 Dept. 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. 8/30/2017 30 Dept. of Civil Engg,Pietech,Raipur

31. Weighing type rain gauge 8/30/2017 31 Dept. of Civil Engg,Pietech,Raipur

32. Float type Rain gauge 8/30/2017 32 Dept. of Civil Engg,Pietech,Raipur

33. Float type Rain gauge 8/30/2017 33 Dept. 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. 8/30/2017 34 Dept. of Civil Engg,Pietech,Raipur

35. Other rain gauges 8/30/2017 35 Dept. 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 8/30/2017 36 Dept. 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 8/30/2017 37 Dept. 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 8/30/2017 38 Dept. 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. 8/30/2017 39 Dept. 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 8/30/2017 40 Dept. of Civil Engg,Pietech,Raipur

41. Adequacy of rain gauge stations 8/30/2017 41 Dept. of Civil Engg,Pietech,Raipur

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. 8/30/2017 42 Dept. of Civil Engg,Pietech,Raipur

43. frequency of the rainfall The probability of an event bring equaled by the following formulae California formula : P ro = m/N Hazen formula : P ro = 2m-1/2N Weibull formula : P ro = m/N+1 Where N = no of years of record P ro = probability 8/30/2017 43 Dept. 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. 8/30/2017 44 Dept. of Civil Engg,Pietech,Raipur

45. 45 THANK YOU AND HAVE A NICE DAY