Measurement of rainfall

This presentation will provide complete knowledge on forms of precipitation, recording and non recording type rain gauges.

1. Measurement of Rainfall Vijitha Vikneshwaran Lecturer(Temporary) Faculty of Technology University of Jaffna

2. Introduction Rainfall is an important natural resource which provides water for different usages such as domestic, industry, agriculture and public requirement. Accurate measurement of rainfall helps in conserving and managing water resources efficiently. Measuring the rain is the most fundamental part of the water budget. Harvesting , conservation and utilization of rainwater are important for undertaking agriculture and other allied activities. 2

3. Hydrological cycle Figure 01: Hydrological cycle 3

4. Precipitation Precipitation is defined as the liquid or solid products of the condensation of water vapor falling from clouds or deposited from air onto the ground. It includes, Rain Hail Snow Dew Rime Hoar frost Fog precipitation The total amount of precipitation which reaches the ground in a stated period is expressed in terms of the vertical depth of water. 4

5. Precipitation, cont …. Rain Most common type of precipitation Liquid droplets Two different form Showers : Heavy, Large drops, Last for a period of time Drizzles : small droplets, Last longer Figure 02: Rain 5

6. Snow Second most common type of precipitation Forms when water vapor turns directly into ice without ever passing through liquid sate Average density : 0.1 Figure 03: Snow 6 Precipitation, cont ….

7. Hail Created when moisture and wind are together Ice crystals form inside the cumulonimbus clouds Shapes : Spherical, Conical and Irregular Average density : 0.1 Figure 04: Hail 7 Precipitation, cont ….

8. Dew Small drops of water that appears on cool surfaces(grass) in the morning Forms due to condensation of atmospheric vapor in the colder night air Figure 05: Dew 8 Precipitation, cont ….

9. Rime White opaque deposit of ice granule Forms by rapid freezing of super cooled water drops impinging on exposed surface. Figure 06: Rime 9 Precipitation, cont ….

10. Sleet Consists of transparent, globular, solid grains of ice Forms by the freezing of raindrops or largely melted ice crystals falling through a layer of sub freezing air near the earth surface. Figure 07: Sleet 10 Precipitation, cont ….

11. Fog Same as cloud This is a cloud forms near the earth surface There are 4 types: Radiation fog Advection fog Upslope fog Evaporation fog Figure 08: Fog 11 Precipitation, cont ….

12. Mist Bunch of small water droplets in the air Forms when the cold air occurred above a warm surface Very similar to fog and only differ in their visibility Figure 09: Mist 12 Precipitation, cont ….

13. Glaze Clear and smooth ice coating Forms on exposed surfaces by the freezing of super cooled water deposited by rain or drizzle. Figure 10: Glaze 13 Precipitation, cont ….

14. Measurement of Precipitation Precipitation measuring devices and techniques are listed below. Rain gauges Snow gauges Radars Satellites Scratching of snow packs Water equivalent in snow packs Rain gauges are most commonly used for the measurement of precipitation both in terms of rainfall and snow. 14

15. Consideration for location selection Airflow around the rain gauges should be horizontal. Avoid concave, elevated or tilted sites. The site should be far from mountain ridges, where local winds are strongly distorted. Site should be away from trees, building and other instruments. The rain gauge should be installed at a distance from any objects equivalent to at least 2-4 times of their height. The ground surface around the rain gauge should be flat and covered with short grass. 15

16. Types of rain gauges 16 Rain gauges Non recording type Recording type Tipping bucket Weighing type Floating type

17. Non recording rain gauge It does not provide the distribution of amount of precipitation in a day. It gives the amount of accumulated rainfall after 24 hrs. It consists of a funnel, body, receiver and base. Figure 11: Non-recording rain gauge 17

18. Procedure: Remove the funnel Take out the receiver Pour the rainwater in to the measuring cylinder Measure the rainfall Reset the funnel and receiver If snow or other solid precipitation accumulates in the funnel, Pour in a known amount of warm water to melt it Measure the total amount of water Subtract the amount of added water from the total 18 Non recording rain gauge, cont ….

19. Causes of errors Some water is used to wet the surface of instrument. The rain recorded may be less than the actual rainfall due to the direction of the rainfall as affected by wind. Dents in the instrument may also cause error. Losses due to evaporation The volume of stick of funnel replaces some water. Note: The accuracy of precipitation measurement significantly depends on the environment and wind at the installation site rather than the performance of the instrument itself. 19

20. Ensure that rain gauge is leak proof Check the funnel for deformation Measuring cylinder should be kept clean Plant leaves and other foreign particles should be removed from the receiver The instrument should be kept 30-40 cm above the natural surface to prevents splashing of water into the funnel. 20 Precautions

21. Recording type rain gauges There are 3 kind of recording type rain gauges. Tipping bucket Weighing type Floating type These are named according to their principle of operation. This type of gauges continuously record rainfall and thus rainfall depth can be obtained for any period. It enables to determine rainfall characteristics such as, Intensity Frequency Duration 21

22. Floating type rain gauge It enables automatic, continuous measurement and recording of precipitation. The collected rainwater level is measured by the position of a float resting on the surface of the water. The recording pen   is connected to a float. As the rainwater gathered in the cylinder, the float rises along with the pen which records the rainfall on a clock-driven drum with chart. 22

23. When the rainwater in the storage tank reaches a level equivalent to a fixed amount, the entire water gets drained by the siphon. So that the recording pen drops to the zero position. When the rainfall stops, the pen traces a horizontal line. Figure 12: Floating type rain gauge 23 Floating type rain gauge, cont ……

24. Figure 13: Parts of Floating type rain gauge 24 Floating type rain gauge, cont ……

25. Figure 14: Principle of siphon out 25 Floating type rain gauge, cont ……

26. Number of rainfall events, number of siphon outs, total rainfall, maximum and minimum intensity and the duration of rainfall can be estimated by using this chart Figure 15: Rainfall chart 26 Floating type rain gauge, cont ….

27. Tipping bucket rain gauges It generates an electrical signal for each unit of precipitation collected. It allows automatic observation with a recorder. It consists of a receptacle that collects and channels the collected rainwater through the funnel into a tipping bucket. Tipping buckets are 2 triangular vessels attached to the left and right of the rotation shaft . 27

28. After a pre-set amount of precipitation falls, the lever tips and dumps the collected water in to the drain cylinder. This process causes generation of an electrical signal. The reed switch is connected to the tipping bucket, to generate the signal at each time the buckets tip. The subsequent rainwater is filled into the other bucket. Tipping bucket rain gauge recorder is responsible to count and record the signals. 28 Tipping bucket rain gauges, cont …

29. Eddy type cam drives the recording pen and mark a trace on the recording paper of a clock driven drum. When the pen tip reaches the maximum position on the recording paper, the cam will move the pen tip to the zero position. Figure 16: Tipping bucket rain gauge 29 Tipping bucket rain gauges, cont …

30. Figure 17: Parts of t ipping bucket rain gauge 30 Tipping bucket rain gauges, cont …

31. It is the most common self-recording rain gauge. It consists of a receiver bucket supported by a spring or lever balance or some other weighing mechanism. As the rainwater enters into the bucket, the weight will increase and the bucket will move down. The movement of bucket due to the raise in weight is transmitted to a pen which traces record on a clock driven chart. It gives a plot of the accumulated rainfall values against the elapsed time. 31 Weighing type rain gauge

32. Figure 18: Weighing type rain gauge 32 Weighing type rain gauge, cont …..

33. Instrument errors Errors in scale reading Dents in the collector Moistening of inner surface of the funnel and the tube Splashing of rain drops from the collector Residual moisture in the storage bottle and receiver Inclination of the gauge. Heavy wind Evaporation Absorption of rainwater by dirt, dust or by other inerts 33 Causes of errors

34. Precautions in recording type rain gauges Floating type rain gauge Check the siphon drains water when it reaches the predetermined amount. Check the clock driven drum for deformation Tipping bucket Verify the tipping buckets tips smoothly Check the contact points operates properly upon tipping Weighing type rain gauge Check the spring for deformation Ensure the proper function of the pen arm 34

35. THANK YOU 35