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Precipitation and rain gauges

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Precipitation and rain gauges

  1. 1. GOVT.COLLEGE OF ENGINEERIG AURANGABAD. Topic :- MESUREMENT OF PRECIPITATION GUIDED BY:SUBMITED BY:- PROF .K. A.PATIL 1) Nikhil Holsamudrkar (BE11F01F017) 2)Suresh Hatkar(BE11F01F015) 3) Swapnil Dhakane(BE11F01F013) 4) Mamta Ingole(BE11F01F018)
  2. 2. PRECIPITATION Introduction
  3. 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.
  4. 4. PRECIPITATION   1. 2. 3. 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
  5. 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 that is warmer than it’s surroundings. Generally this kind of precipitation occurs in tropics.
  6. 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.
  7. 7. RAINGAUGES 1. Recording type 2. Non recording type  Most rain gauges used in India are recording type i.e. Symon's raingauge.  Recordings are taken at 8:30 am  And if rainfall is more then intermediate readings are taken at 5:30pm
  8. 8. PRECIPITATION Measurement, Estimation and Probability
  9. 9. PRECIPITATION DATA  Necessary for various fields  Municipal  Industrial  Agricultural  Forestry  Flood prevention  Recreation
  10. 10. 1) Nonrecording gauge:Symons’ Raingauge Extensively use in India Accuracy At 0.1mm 8.30am Capacity Incase rainfall is10cm of Heavy
  11. 11. 2)RECORDING RAIN GAGES Weighing Tipping bucket type bucket type Natural-syphons type
  12. 12. TIPPING BUCKET TYPE 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  Electric circuit 
  13. 13. TIPPING BUCKET TYPE
  14. 14. WEIGHING BUCKET TYPE
  15. 15. Weighing bucket type 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.
  16. 16. FLOAT RECORDING GAUGES
  17. 17. FLOAT RECORDING GAUGES
  18. 18. RAINGAUGE NETWORK Since the catching area of the raingauge is very small as compared to the areal extent of the storm, to get representative picture of a storm over a catchment the number of raingauges should be as large as possible, i.e. the catchment area per gauge should be small.  There are several factors to be considered to restrict the number of gauge:  Like economic considerations to a large extent  Topography & accessibility to some extent. 
  19. 19. MINIMUM DENSITY OF RAINGAUGES 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   10% of total should be self recording raingauges
  20. 20. ADEQUACY OF RAINGAUGE STATIONS
  21. 21. RAINFALL ON A WATERSHED SCALE  3 common methods for estimating average rainfall. 1. Arithmetic Mean 2. Thiesson polygon method 3. Isohyetal method ∑ Wi Ri R= ∑ Wi
  22. 22. Measured Rainfall at Six Rainfall Gages Watershed boundary P6 = 1.81” P4 = 2.26” P2 = 2.15” P1 = 1.62” P5 = 2.18” P3 = 1.80”
  23. 23. ARITHMETIC MEAN METHOD  Pavg = [Σ Wi x Pi ] / Σ Wi  All gages given equal weight  Weight = 1 Pavg = (1.82 + 2.15 + 2.26 + 2.18 + 1.62 + 1.8) / 6  Pavg = 1.97 in. 
  24. 24. THIESSEN POLYGON METHOD   First: Draw straight dashed lines between each rainfall gage Second: Draw solid perpendicular bisectors to these lines so that watershed area associated with each gage is enclosed by bisector lines  These enclosed areas are known as Thiessen Polygons  The area within each polygon is closer to the rain gage enclosed than any other rain gage.  The rainfall measured in the polygon is assumed to be representative of the rainfall in the entire polygon
  25. 25. THIESSEN POLYGON METHOD  Third: Determine the area of each polygon   The rain gage weight is the area of the polygon it is located in Fourth: Calculate the average rainfall using:  Pavg = [Σ Wi x Pi ] / Σ Wi
  26. 26. Step #1: Dashed Lines Between Each Rain Gauge Watershed boundary P6 = 1.81” P2 = 2.15” P4 = 2.26” P1 = 1.62” P5 = 2.18” P3 = 1.80”
  27. 27. Step #2: Draw the Perpendicular Bisector Lines Watershed boundary
  28. 28. Step #3: Determine the Area of Each Polygon Watershed boundary A6= 65 ac A4= 269 ac A2= 150 ac A1= 56 ac A5= 216 ac A3= 136 ac
  29. 29. STEP #4: CALCULATE THE AVERAGE RAINFALL  Pavg = [Σ Wi x Pi ] / Σ Wi   Pavg = [(65x1.81)+(150x2.15)+(269x2.26)+ (216x2.18)+(56x1.62)+(136x1.8)] / [65+150+269+ 216+56+136] Pavg = 2.08 in.
  30. 30. ISOHYTAL METHOD  Plot gauge locations on map;  Subjectively interpolate between rain amounts between gauges at a selected interval;  Connect points of equal rain depth to produce lines of equal rainfall amounts (isohyets);  
  31. 31. CALCULATION OF AVERAGE RAINFALL OVER CATCHMENT
  32. 32. 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 gages are not uniformly distributed  Can use gages outside of watershed Isohyetal method  Theoretically the most accurate  Most time consuming method  Can use gages outside of the watershed
  33. 33. DAD CURVES  DAD stands for Depth Area Duration curve.  DAD curves exhibit the depth and the area covered by the rainfall with a particular duration.
  34. 34.    There is a definite relation among depth, area and duration of rainfall. The longer duration rainfall covers a wider area. Short time rainfalls normally cover small areas. Rainfall rarely occurs uniformly over a large area.
  35. 35.  A depth-area-duration curve expresses graphically the relation between progressively decreasing average depth of rainfall over a progressively increasing area from the center of the storm outward to its edges for a given duration of rainfall.
  36. 36.  Purpose of DAD analysis of a particular storm is to determine the largest average depth of rainfall that fell over various sizes of area during the standard passage of time.  hydrologists and engineers require techniques whereby point rainfall amounts can be transformed to average rainfall amounts over a specified area
  37. 37. DAD CURVE FOR ONE DAY RAINFALL OVER THE AREA 5000 KM2
  38. 38. 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-h maximum rainfall ) is of extreme important to determination of the design flood.
  39. 39. The probability of an event bring equaled by the following formulae •California •Hazen formula : Pro = m/N formula : Pro = 2m-1/2N •Weibull formula : Pro = m/N+1 Where N= no of years of record Pro = probability
  40. 40. REFERENCES • • • Introduction to Physical Hydrology, Martin R. Hendricks Hydrology and Floodplain Analysis, Bedient, Huber and Vieux National Geographic Magazine
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