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Precepitation

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Precepitation

  1. 1. 1 Section : Precipitation  Forms of precipitation  Measurement of precipitation  Gauges  Recording  Non-recording  Errors in measurement of rainfall  Location of rain gauge stations  Assignment  Locate rainfall stations using the internet  Collect data in a spreadsheet
  2. 2. 2 Introduction  All forms of water that reach the earth from the atmosphere is called Precipitation.  The usual forms are rainfall, snowfall, frost, hail, dew. Of all these, the first two contribute significant amounts of water.  Rainfall being the predominant form of precipitation causing stream flow, especially the flood flow in majority of rivers. Thus, in this context, rainfall is used synonymously with precipitation.
  3. 3. 3 Introduction….  In nature water is present in three aggregation states:  solid: snow and ice;  liquid: pure water and solutions;  gaseous: vapors under different grades of pressure and saturation  The water exists in the atmosphere in these three aggregation states.
  4. 4. 4 Introduction….  Types of precipitation  Rain, snow, hail, drizzle, glaze, sleet  Rain:  Is precipitation in the form of water drops of size larger than 0.5 mm to 6mm  The rainfall is classified in to  Light rain – if intensity is trace to 2.5 mm/h  Moderate – if intensity is 2.5 mm/hr to 7.5 mm/hr  Heavy rain – above 7.5 mm/hr
  5. 5. 5 Introduction….  Snow:  Snow is formed from ice crystal masses, which usually combine to form flakes  Hail (violent thunderstorm)  precipitation in the form of small balls or lumps usually consisting of concentric layers of clear ice and compact snow.  Hail varies from 0.5 to 5 cm in diameter and can be damaging crops and small buildings.
  6. 6. 6 Mechanisms for air lifting 1. Frontal lifting 2. Orographic lifting 3. Convective lifting
  7. 7. 7 Definitions  Air mass : A large body of air with similar temperature and moisture characteristics over its horizontal extent.  Front: Boundary between contrasting air masses.  Cold front: Leading edge of the cold air when it is advancing towards warm air.  Warm front: leading edge of the warm air when advancing towards cold air.
  8. 8. 8 Frontal Lifting  Boundary between air masses with different properties is called a front  Cold front occurs when cold air advances towards warm air  Warm front occurs when warm air overrides cold air Cold front (produces cumulus cloud) Cold front (produces stratus cloud)
  9. 9. 9
  10. 10. 10 Orographic lifting Orographic uplift occurs when air is forced to rise because of the physical presence of elevated land.
  11. 11. 11 Convective lifting Hot earth surface Convective precipitation occurs when the air near the ground is heated by the earth’s warm surface. This warm air rises, cools and creates precipitation.
  12. 12. 12 Condensation  Condensation is the change of water vapor into a liquid. For condensation to occur, the air must be at or near saturation in the presence of condensation nuclei.  Condensation nuclei are small particles or aerosol upon which water vapor attaches to initiate condensation. Dust particulates, sea salt, sulfur and nitrogen oxide aerosols serve as common condensation nuclei.  Size of aerosols range from 10-3 to 10 mm.
  13. 13. 13 Precipitation formation  Lifting cools air masses so moisture condenses  Condensation nuclei  Aerosols  water molecules attach  Rising & growing  0.5 cm/s sufficient to carry 10 mm droplet  Critical size (~0.1 mm)  Gravity overcomes and drop falls
  14. 14. 14 Forces acting on rain drop FdFd Fb Fg D  Three forces acting on rain drop  Gravity force due to weight  Buoyancy force due to displacement of air  Drag force due to friction with surrounding air 3 6 DVolume   2 4 DArea   3 6 DgF wg   3 6 DgF ab   242 2 2 2 V DC V ACF adadd   
  15. 15. Terminal Velocity  Terminal velocity: velocity at which the forces acting on the raindrop are in equilibrium.  If released from rest, the raindrop will accelerate until it reaches its terminal velocity 3 2 23 6246 0 Dg V DCDg WFFF wada DBvert         33 2 2 6624 DgDg V DC WFF wa t ad BD                1 3 4 a w d t C gD V    Raindrops are spherical up to a diameter of 1 mm  For tiny drops up to 0.1 mm diameter, the drag force is specified by Stokes law Fd Fd Fb Fg D V Re 24 dC a aVD m  Re At standard atmospheric pressure (101.3 kpa) and temperature (20oC), w = 998 kg/m3 and a = 1.20 kg/m3
  16. 16. 16 Temporal and Spatial Variation of Rainfall  Rainfall varies greatly both in time and space  With respect to time – temporal variation  With space – Spatial variation  The temporal variation may be defined as hourly, daily, monthly, seasonal variations and annual variation (long-term variation of precipitation)
  17. 17. 17 Temporal Variation of rainfall at a particular site Total Rainfall amount = 6.17 cm 0 2 4 6 8 10 12 14 0 20 40 60 80 100 120 140 Time, min RainfallIntensity,cm/hr
  18. 18. 18 Long term Precipitation 0 5 10 15 20 25 30 35 40 45 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 Years Annualrainfall,mm Annual Precipitation average precipitation
  19. 19. 19 Global precipitation pattern
  20. 20. 20 Measurement of Rainfall  Rainfall and other forms of precipitation are measured in terms of depth, the values being expressed in millimeters.  One millimeter of precipitation represents the quantity of water needed to cover the land with a 1mm layer of water, taking into account that nothing is lost through drainage, evaporation or absorption.  Instrument used to collect and measure the precipitation is called rain gauge.
  21. 21. 21 Rainfall measurement… Precipitation gauge 1 - pole 2 - collector 3 - support- galvanized metal sheet 4 – funnel 5 - steel ring 1. Non recording gauge
  22. 22. 22 2. Recording gauge / graphic raingauge  The instrument records the graphical variation of the fallen precipitation, the total fallen quantity in a certain time interval and the intensity of the rainfall (mm/hour).  It allows continuous measurement of the rainfall. The graphic rain gauge 1-receiver 2-floater 3-siphon 4-recording needle 5-drum with diagram 6-clock mechanism
  23. 23. 23 3. Tele-rain gauge with tilting baskets  The tele-rain gauge is used to transmit measurements of precipitation through electric or radio signals.  The sensor device consists of a system with two tilting baskets, which fill alternatively with water from the collecting funnel, establishing the electric contact.  The number of tilting is proportional to the quantity of precipitation hp
  24. 24. 24 Tele-rain gauge …… The tele-rain-gauge 1 - collecting funnel 2 - tilting baskets 3 - electric signal 4 - evacuation
  25. 25. 25 4. Radar measurement of rainfall  The meteorological radar is the powerful instrument for measuring the area extent, location and movement of rainstorm.  The amount of rainfall overlarge area can be determined through the radar with a good degree of accuracy  The radar emits a regular succession of pulse of electromagnetic radiation in a narrow beam so that when the raindrops intercept a radar beam, its intensity can easily be known.
  26. 26. 26 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  Topographic & accessibility to some extent.
  27. 27. 27
  28. 28. 28 Raingauge Network…..  World Meteorological Organization (WMO) recommendation:  In flat regions of temperate, Mediterranean and tropical zones  Ideal  1 station for 600 – 900 km2  Acceptable 1 station for 900 – 3000 km2  In mountainous regions of temperate , Mediterranean and tropical zones  Ideal  1 station for 100 – 250 km2  Acceptable  1 station for 250 – 1000 km2  In arid and polar zone  1 station for 1500 – 10,000 km2  10 % of the raingauges should be self recording to know the intensity of the rainfall
  29. 29. 29 2.4 Preparation of Data  Before using rainfall data, it is necessary to check the data for continuing and consistency  Missing data  Record errors • Given annual precipitation values – P1, P2, P3,… Pm at neighboring M stations of station X 1, 2, 3 & m respectively • The normal annual precipitation given by N1, N2, N3,…, Nm, Ni… (including station X) • To find the missing precipitation, Px , of station X Estimation of Missing Data        m mx x N P N P N P M N P ... 2 2 1 1
  30. 30. 30 • Let a group of 5 to 10 base stations in the neighbourhood of the problem station X is selected • Arrange the data of X stn rainfall and the average of the neighbouring stations in reverse chronological order (from recent to old record) • Accumulate the precipitation of station X and the average values of the group base stations starting from the latest record. • Plot the against as shown on the next figure • A decided break in the slope of the resulting plot is observed that indicates a change in precipitation regime of station X, i.e inconsistency. • Therefore, is should be corrected by the factor shon on the next slide Test for consistency record (Double mass curve techniques)   xP   avgP  xP   avgP
  31. 31. 31 Test for consistency record…. a c xcx M M PP  Pcx – corrected precipitation at any time period t1 at stationX Px – Original recorded precp. at time period t1 at station X Mc – corrected slope of the double mass curve Ma – original slope of the mass curve Double Mass Curve Analysis 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 0 0.5 1 1.5 2 2.5 Accumulated annual rainfall of neigbouring stns in 10^3 cm accumulatedannualrainfallofXstnin10^3cm c a a c M M a c 
  32. 32. 32 2.5 Mean Precipitation over an area  Raingauges rainfall represent only point sampling of the areal distribution of a storm  The important rainfall for hydrological analysis is a rainfall over an area, such as over the catchment  To convert the point rainfall values at various stations to in to average value over a catchment, the following methods are used:  arithmetic mean  the method of the Thiessen polygons  the isohyets method
  33. 33. 33 • When the area is physically and climatically homogenous and the required accuracy is small, the average rainfall ( ) for a basin can be obtained as the arithmetic mean of the hi values recorded at various stations. • Applicable rarely for practical purpose Arithmetic Mean Method     N i i ni P NN PPPP P 1 21 1.......... P
  34. 34. 34 • The method of Thiessen polygons consists of attributing to each station an influence zone in which it is considered that the rainfall is equivalent to that of the station. • The influence zones are represented by convex polygons. • These polygons are obtained using the mediators of the segments which link each station to the closest neighbouring stations Method of Thiessen polygons
  35. 35. 35 Thiessen polygons ……….
  36. 36. 36 Thiessen polygons ………. A1 A2 A3 A4 A5 A6 A7 A8 P1 P2 P3 P4 P5 P6 P7 P8
  37. 37. 37  m mm AAA APAPAP P    ..... ..... 21 2211      M i i i total i M i i A A P A AP P 1 1 Thiessen polygons ………. Generally for M station The ratio is called the weightage factor of station i A Ai
  38. 38. 38 • An isohyet is a line joining points of equal rainfall magnitude. Isohyetal Method F B E A C D 12 9.2 4.0 7.0 7.2 9.1 10.0 10.0 12 8 8 6 6 4 4 a1a1 a2 a3 a4 a5
  39. 39. 39 • P1, P2, P3, …. , Pn – the values of the isohytes • a1, a2, a3, …., a4 – are the inter isohytes area respectively • A – the total catchment area • - the mean precipitation over the catchment Isohyetal Method P A PP a PP a PP a P nn n                         2 ... 22 1 1 32 2 21 1 The isohyet method is superior to the other two methods especially when the stations are large in number. NOTE
  40. 40. 40 2.6 Intensity – Duration – Frequency (IDF) Relationship Mass Curve of Rainfall Mass curve of rainfall 0 10 20 30 40 50 60 0 20 40 60 80 100 120 Time, hour accumulatedprecipitation,mm 1st storm, 16 mm 2nd storm, 16 mm
  41. 41. 41 IDF ….Hyetograph - is a plot of the accumulated precipitation against time, plotted in chronological order Hyetograph of a storm 0 0.1 0.2 0.3 0.4 0.5 0 – 8 8 – 16 16 – 24 24 – 32 32 – 40 40 – 48 Time, hours Intensity,cm/hr Total depth = 10.6 cm Duration = 46 hr
  42. 42. 42  In many design problems related to watershed such as runoff disposal, erosion control, highway construction, culvert design, it is necessary to know the rainfall intensities of different durations and different return periods.  The curve that shows the inter-dependency between i (cm/hr), D (hour) and T (year) is called IDF curve.  The relation can be expressed in general form as: IDF ….  n x aD Tk i   i – Intensity (cm/hr) D – Duration (hours) K, x, a, n – are constant for a given catchment
  43. 43. 43 IDF …. Typical IDF Curve 0 2 4 6 8 10 12 14 0 1 2 3 4 5 6 Duration, hr Intesity,cm/hr T = 25 years T = 50 years T = 100 years k = 6.93 x = 0.189 a = 0.5 n = 0.878

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