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CHAPTER TWO PRECIPITATION

CHAPTER TWO PRECIPITATION. Engineering Hydrology (ECIV 4323). Instructor: Dr. Yunes Mogheir 2015. Precipitation. The term precipitation denotes all forms of water that reach the earth from the atmosphere. The usual forms are rainfall, snowfall, hail, frost and dew.

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CHAPTER TWO PRECIPITATION

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  1. CHAPTER TWOPRECIPITATION Engineering Hydrology (ECIV 4323) Instructor: Dr. Yunes Mogheir 2015

  2. Precipitation The term precipitation denotes all forms of water that reach the earth from the atmosphere. The usual forms are rainfall, snowfall, hail, frost and dew

  3. For precipitation to form (i) the atmosphere must have moisture, (ii) there must be sufficient nucleii present to aid condensation, (iii) weather conditions must be good for condensation of water vapour to take place, and (iv) the products of condensation must reach the earth

  4. FORMS OF PRECIPITATION Rain The term rainfall is used to describe precipitations in the form of water drops of sizes larger than 0.5 mm. The maximum size of a raindrop is about 6 mm

  5. FORMS OF PRECIPITATION Snow Snow is another important form of precipitation. Snow consists of ice crystals which usually combine to form flakes. When new, snow has an initial density varying from 0.06 to 0.15 g/cm3 and it is usual to assume an average density of 0.1 g/cm3.

  6. FORMS OF PRECIPITATION Drizzle A fine sprinkle of numerous water droplets of size less than 0.5 mm and intensity less than 1 mm/h is known as drizzle. In this the drops are so small that they appear to float in the air.

  7. FORMS OF PRECIPITATION Glaze When rain or drizzle comes in contact with cold ground at around 00 C, the water drops freeze to form an ice coating called glaze or freezing rain.

  8. FORMS OF PRECIPITATION Sleet It is frozen raindrops of transparent grains which form when rain falls through air at subfreezing temperature. In Britain, sleet denotes precipitation of snow and rain simultaneously.

  9. FORMS OF PRECIPITATION Hail It is a showery precipitation in the form of irregular pellets or lumps of ice of size more than 8 mm. Hails occur in violent thunderstorms in which vertical currents are very strong.

  10. WEATHER SYSTEMS FOR PRECIPITATION Front A front is the interface between two distinct air masses. Under certain favorable conditions when a warm air mass and cold air mass meet, the warmer air mass is lifted over the colder one with the formation of a front. The ascending warmer air cools adiabatically with the consequent formation of clouds and precipitation.

  11. WEATHER SYSTEMS FOR PRECIPITATION Cyclone A cyclone is a large low pressure region with circular wind motion. Two types of cyclones are recognized: tropical cyclones and extratropical cyclones.

  12. WEATHER SYSTEMS FOR PRECIPITATION Convective Precipitation In this type of precipitation a packet of air which is warmer than the surrounding air due to localized heating rises because of its lesser density. Air from cooler surroundings flows to take up its place thus setting up a convective cell. The warm air continues to rise, undergoes cooling and results in precipitation.

  13. WEATHER SYSTEMS FOR PRECIPITATION Orographic Precipitation The moist air masses may get lifted-up to higher altitudes due to the presence of mountain barriers and consequently undergo cooling, condensation and precipitation. Such a precipitation is known as Orographic precipitation

  14. MEASUREMENT Precipitation is expressed in terms of the depth to which rainfall water would stand on an area if all the rain were collected on it. Thus 1 cm of rainfall over a catchment area of 1 km represents a volume of water equal to 104 m3 The precipitation is collected and measured in a raingauge

  15. Rain gauge Setting For setting a rain gauge the following considerations are important: 1.The ground must be level and in the open and the instrument must present a horizontal catch surface. 2. The gauge must be set as near the ground as possible to reduce wind effects. 3. The instrument must be surrounded by an open fenced area of at least 5.5 m x 5.5 m. No object should be nearer to the instrument than 30 m or twice the height of the obstruction.

  16. Non-recording Gauges

  17. Recording Gauges • Tipping—Bucket Type • Weighing—Bucket Type • Natural—Syphon Type

  18. Recording Gauges • Telermetering Raingauges • Radar Measurement of Rainfall where Pr = average echopower, Z = radar-echo factor, r= distance to target volume and C = a constant Generally the factor Z is related to the intensity of rainfall as Z=aIb

  19. RAINGAUGE NETWORK 1. In flat regions of temperate, Mediterranean and tropical zones: ideal – 1 station for 600 – 900 km2 acceptable – 1 station for 900 – 3000 km2 2. in mountainous regions of temperate, Mediterranean and topical zones: ideal - 1 station for 100—250 km2 acceptable - 1 station for 250—1000 km2 3. in arid and polar zones: I station for 1500—l0,000 km2 depending on the feasibility.

  20. Adequacy of Rain gauge Stations where N = optimal number of stations, ε = allowable degree of error in the estimate of the mean rainfall and Cv = coefficient of variation of the rainfall values at the existing m stations (in per cent)

  21. Adequacy of Rain gauge Stations Pi = precipitation magnitude in the i4th station

  22. EXAMPLE A catchment has six rain gauge stations. In a year, the annual rainfall recorded by the gauges are as follows:- For a 10% error in the estimation of the mean rainfall, calculate the optimum number of stations in the catchment Solution:- from first data

  23. PREPARATION OF DATA Estimation of Missing Data Given the annual precipitation values, P1, P2, P3, . Pm at neighbouring M stations 1,2,3 M respectively, it is required to find the missing annual precipitation P. at a station X not included in the above M stations If the normal annual precipitations at various stations are within about 10% of the normal annual precipitation at station X:

  24. PREPARATION OF DATA Estimation of Missing Data If the normal precipitations vary considerably

  25. PREPARATION OF DATA Test for Consistency of Record Some of the common causes for inconsistency of record are: (i) shifting of a rain gauge station to a new location, (ii) the neighborhoods of the station undergoing a marked change, (iii) change in the ecosystem due to calamities, such as forest fires, land slides, and (iv) occurrence of observational error from a certain date

  26. PREPARATION OF DATA Test for Consistency of Record Accumulated Annual Rainfall at x ΣP in units of l03 cm Accumulated Annual Rainfall of 10 station Mean ΣP in units of l03 cm

  27. PRESENTATION OF RAINFALL DATA Mass Curve of Rainfall

  28. PRESENTATION OF RAINFALL DATA Hyetograph See also Example 2.9 Page 47

  29. MEAN PRECIPITATION OVER AN AREA Arithmetical—Mean Method

  30. MEAN PRECIPITATION OVER AN AREA Thiessen-Mean Method

  31. MEAN PRECIPITATION OVER AN AREA Isohyetal Method

  32. DEPTH- AREA—DURATION RELATIONSHIPS Depth-Area Relation where P= average depth in cms over an area A km2, Po= highest amount of rainfall in cm at the storm centre and K and n are constants for a given region

  33. DEPTH-AREA—DURATION RELATIONSHIPS Maximum Depth-Area-Duration Curves

  34. FREQUENCY OF POINT RAINFALL If the probability of an event occurring is P, the probability of the event not occurring in a given year is q= (1- P) where Pr,n = probability of a random hydrologic event (rainfall) of given magnitude and exceedence probability Poccurring rtimes in n successive years

  35. FREQUENCY OF POINT RAINFALL example, (a) The probability of an event of exceedence probability P occurring 2 times in n successive years is (b) The probability of the event not occurring at all in , successive years is (c) The probability of the event occurring at least once in n successive years

  36. FREQUENCY OF POINT RAINFALL 0.0323

  37. T : Return Period or Recurrence interval (Years) T is a characteristic time period called interval of occurrence or return period to be defined as the number of years until the considered Maximum Rainfall X equals or exceeds a specified value x only once. For example the return period of 280 mm rainfall is 50 years is by definition 280 mm rainfall may occur on average only once in 50 years. This does not imply necessarily that the above rainfall value (280 mm) will occur only after 50 years: it may occur next year or several times in the next 50 years or not at all for 100 years.

  38. FREQUENCY OF POINT RAINFALL Plotting Position

  39. FREQUENCY OF POINT RAINFALL T : Return Period or Recurrence interval (Years)

  40. For a station A, the recorded annual 24 h maximum rain fall are given below. (a) Estimate the 24h maximum rainfall with return period of 13 and50 year. (b) What would be the probability of a ran fall of magnitude equal to or exceeding 10cm occurring in 24 h at station A.

  41. ANNUAL MAXIMUM 24 h RAINFALL AT STATION A

  42. Probability Probability

  43. INTENSITY-DURATION-FREQUENCY RELATIONSHIP where K, x, a and n are constants for a given catchment

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