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Measurement of rainfall
- 1. Measurement of Rainfall VijithaVikneshwaran Lecturer(Temporary) Faculty of Technology University of Jaffna
- 2. Introduction • Rainfall isan 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 isdefined 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 commontype 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 mostcommon 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 whenmoisture 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 dropsof 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 opaquedeposit 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 oftransparent, 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 ascloud • 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 ofsmall 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 andsmooth 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 locationselection • 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 raingauges 16 Rain gauges Non recording type Recording type Tipping bucket Weighing type Floating type
- 17. Non recording raingauge • 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 thefunnel • 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 Nonrecordingraingauge,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 thatrain 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 raingauges • 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 raingauge • 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 therainwater 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 Floatingtyperaingauge,cont……
- 24. Figure 13: Partsof Floating type rain gauge 24 Floatingtyperaingauge,cont……
- 25. Figure 14: Principleof siphon out 25 Floatingtyperaingauge,cont……
- 26. • Number ofrainfall 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 Floatingtyperaingauge,cont….
- 27. Tipping bucket raingauges • 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 apre-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 Tippingbucketraingauges,cont…
- 29. • Eddy typecam 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 Tippingbucketraingauges,cont…
- 30. Figure 17: Partsof tipping bucket rain gauge 30 Tippingbucketraingauges,cont…
- 31. • It isthe 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: Weighingtype rain gauge 32 Weighingtyperaingauge,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 recordingtype 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.