Precipitation

1
Precipitation
2
Definition

• All types of moisture reaching the surface of
  earth from atmosphere.Precipitation is the basic
  input to the hydrology.

• Factors determining precipitation or the amount
  of atmospheric moisture over a region
• Climate
• Geography
• Ocean surfaces is the chief source of moisture
  for precipitation

3
Forms of precipitation
4
Rain

• Rain is the most common type of precipitation in
  our atmosphere. Rain is when liquid droplets fall
  to the surface of the Earth.
• There are two different forms of rain, either in
  the form of
• showers
• drizzles
• Showers are heavy, large drops of rain and
  usually only last a period of time.
• Drizzles however usually last longer and are made
  up of smaller droplets of water.
• Rain can either be formed as ice crystals melt or
  it can be smaller water droplets.

Light I 2.5mm/hr Moderate I
2.8-7.6mm/hr Heavy I gt 7.6 mm/hr
5
Snow

• Snow is the second most common precipitation in
  the North East.
• Snow forms when water vapor turns directly into
  ice without ever passing through a liquid state.
  This happens as water condenses around an ice
  crystal.

Density of freshly fallen snow varies between
125-500mm of snow required to equal 25mm of
liquid water Average density (specific gravity)
0.1
6
Hail

• Hail is created when moisture and wind are
  together. Inside the cumulonimbus clouds ice
  crystals form, and begin to fall towards the
  surface of Earth. When this starts to happen wind
  gusts start to pick up the ice crystals pushing
  them up high into the clouds. As they start to
  fall down again they continue to grow in size. A
  wind gust might catch the hail stone again which
  will push it back up into the cloud. This whole
  process gets repeated several times before the
  hail stone becomes so big that it is too heavy
  for the wind to carry so it must fall towards
  Earth.

• Shapes of hail particles
• Spherical
• Conical
• Irregular
• Diameter range 5 to 125 mm
• Specific gravity 0.8
• Average density (specific gravity) 0.1

7
Fog

• There is really no different between fog and the
  clouds that are high in the sky. In simple terms
  fog is a cloud that has formed near the surface
  of the Earth.

• There are four main types of fog, 
• radiation fog
• advection fog
• upslope fog
• evaporation fog

8
Dew

• The small drops of water which can be found on
  cool surfaces like grass in the morning.
• This is the result of atmospheric vapor
  condensing on the surface in the colder night
  air.
• Dew Point is the temperature in which
  condensation starts to take place or when dew is
  created.

9
Mist

• Mist is a bunch of small droplets of water which
  are in the air. This occurs with cold air when it
  is above a warm surface, for example water.
• Fog and mist are very similar, the only
  difference is their visibility.
• If you cannot see 1 kilometer or less you know
  you're dealing with fog.
• You can see visuals through mist and it is
  more haze looking than a thicker substance.

Diameter range between 0.1 and 0.5 mm
10
Glaze

• Glaze is the ice coating, generally clear and
  smooth, formed on exposed surfaces by the
  freezing of super cooled water deposited by rain
  or drizzle.

Specific gravity may be as high as 0.8-0.9
11
Rime

• Rime is the white opaque deposit of ice granules
  more or less separated by trapped air and formed
  by rapid freezing of super cooled water drops
  impinging on exposed objects.

Specific gravity may be as low as 0.2-0.3
12
Sleet

• Sleet consists of transparent, globular, solid
  grains of ice formed by the freezing of raindrops
  or freezing of largely melted ice crystals
  falling through a layer of sub freezing air near
  the earths surface.

13
(No Transcript)
14
Lapse rate

• The lapse rate is defined as the rate of decrease
  with height for an atmospheric variable. The
  variable involved is temperature unless specified
  otherwise.
• The terminology arises from the word lapse in the
  sense of a decrease or decline thus, the lapse
  rate is the rate of decrease with height and not
  simply the rate of change. While most often
  applied to Earth's atmosphere.

• In general, a lapse rate is the negative of the
  rate of temperature change with altitude change,
  thus
• where ? is the lapse rate given in units of
  temperature divided by units of altitude, T 
  temperature, and z  altitude. Average lapse rate
  is about 2C/1000ft

15
Formation of precipitation

• Moisture is always present in the atmosphere,
  even on the cloudless day.
• Saturation however does not necessarily lead to
  precipitation.

16
Necessary mechanism to form Precipitation

• 1. Lifting mechanism to cool the air
• 2. Formation of cloud elements
• (Droplets/Ice crystals)
• 3. Growth of cloud elements
• 4. Sufficient accumulation of cloud elements

17
1. Lifting mechanism to cool the air

• Lifting mechanism gives the three main types
  of Precipitation.
• Cyclonic Precipitation (Frontal /non Frontal)
• Convective Precipitation
• Orographic Precipitation

18
Cyclonic Precipitation(Frontal/Non
frontal)Frontal precipitation results when the
leading edge( front) of  a warm air mass meets a
cool air mass. The warmer air mass is forced up
over the cool air. As it rises the warm air
cools, moisture in the air condenses, clouds and
precipitation result.
19
Convective Precipitation Convectional
precipitation results from the heating of the
earth's surface that causes air to rise rapidly.
As the air rises, it cools and moisture condenses
into clouds and precipitation
20
Orographic Precipitation It results when warm
moist air of the ocean is forced to rise by large
mountains. As the air rises it cools, moisture in
the air condenses and clouds and precipitation
result on the windward side of the mountain while
the leeward side receives very little. This is
common in British Columbia.       
21
Formation of cloud elements(Droplets/Ice
crystals)

• For droplets, hygroscopic nuclei ,small particles
  (0.1-10µm) having affinity for water must be
  available in upper troposphere.
• For ice crystals, Freezing Nuclei are required.
• Source of condensation nuclei are particles of
  sea salts, products of sulphurous and nitric acid
• Source of freezing nuclei are clay minerals,
  usually kaolin, silver iodide etc

22
Growth of cloud elements

• For occurrence of precipitation over an area
  it is necessary that cloud elements must be grown
  in size to over come
• Coalescence of cloud droplets
• Cloud droplets are usually smaller than 50µm
  in diameter, due to different diameters of
  droplets they fall with varying fall velocities.
  As the bigger cloud elements are heavier , having
  more fall velocity, hence they collide with
  smaller droplets. Smaller droplets join the
  bigger droplets and in this way the size of cloud
  droplets increases.
• Co-existence of cloud droplets ice crystals
• If in a layer of clouds there is mixture of
  water droplets and ice crystals. As the
  saturation vapour pressure over ice is lesser
  than over water. As a result of this difference ,
  there results evaporation of water drops and
  condensation of much of this water on ice
  crystals. Causing their growth and ultimate fall
  through clouds. The ice crystals will further
  grow as they fall and collide with water drops.

23
Growth of droplets and ice crystals

• For the occurrence of precipitation over an
  area necessary conditions are
• Cloud elements must increase in size until their
  falling speeds exceed the ascending rate of air
• Cloud elements should be large enough in size not
  to get evaporated completely before reaching the
  ground

24
Measurement of Precipitation

• 1. Amount of precipitation
• 2. Intensity of precipitation
• 3. Duration of precipitation
• 4. Arial extent of precipitation

25
Measurement Methods

• Measurement of precipitation (Rain and Snow) can
  be done by various devices. These measuring
  devices and techniques are
• Rain Gauges
• Snow Gauges
• Radars
• Satellites
• Scratching of snow packs
• Water equivalent in snow packs

26
RAIN GAGES

• Rain gages are most commonly used for the
  measurement of precipitation, both in terms of
  rain fall and snow.

27
Types of rain gages

• There are two main types of rain gages which are
  used to measure the precipitation. These are
• 1. Non recording rain gages
• 2. Recording rain gages

28
Non recording rain gauges

• It is a rain gage which does not provide the
  distribution of amount of precipitation in a day.
  It simply gives the amount of precipitation after
  24 hours (daily precipitation).

29
Recording rain gauges

• These rain gauges are also called integrating
  rain gauges since they record cumulative
  rainfall. In addition to the total amount of
  rainfall at a station, it gives the times of
  onset and cessation of rains (thereby gives the
  duration of rainfall events)

30
Types of recording Rain gauges

• There are three main types of recording rain
  gauges
• 1. Float type rain gages
• 2. Tipping bucket type rain gages
• 3. Weighing type rain gages
  

31
1. Tipping bucket gauges

• A tipping bucket rain gauge is used for
  measurement of rainfall. It measures the rainfall
  with a least count of 1 mm and gives out one
  electrical pulse for every millimeter of rainfall

32
2. Weighing type gauges

• It consists of a storage bin, which is weighed to
  record the mass. It weighs 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.

33
3. Float recording gauges

• The rise of float with increasing catch of
  rainfall is recorded. Some gauges must be emptied
  manually while others are emptied automatically
  using self starting siphons. In most gauges oil
  or mercury is the float and is placed in the
  receiver, but in some cases the receiver rests on
  a bath of oil or mercury and the float measures
  the rise of oil or mercury displaced by the
  increasing weight of the receiver as the rainfall
  catch freezes. Float may get damaged by rainfall
  catch freezer

34
Errors in precipitation
measurement by Rain Gauges

• Instrumental errors
• Errors in scale reading
• Dent in receivers
• Dent in measuring cylinders
• About 0.25mm of water is initially required to
  wet the surface of gauge
• Rain gauges splash from collector
• Frictional effects
• Non verticality of measuring cylinders (10
  inclination gives 1.5 less precipitation)
• Loss of water by evaporation
• Leakage in measuring cylinder
• Wind speed reduces measured amount of rain in the
  rain gauges.

35
Measurement of snow

• In case of snow fall following two properties
  of more interest are measured.
• 1. Depth of snow at a particular place in
  mm/inches
• 2. Equivalent amount of water in mm

36
1. Depth of snow

• Depth of snow fall at a particular place can
  be measured by the following methods.
• a. Standard rain gauges without collectors
• b. Snow gauges
• c. By scratching snow packs

37
Depth of snow methods

• Standard rain gauges can also be used for
  measuring the snow depth, with some alterations
  in the arrangement of rain gauges, particularly,
  the collectors are not used
• On a paved surface with snow over it, scratching
  that snow layer with some scrapper helps to
  measure the depth of snow fall with a tape.
  Visual observation and with help of measuring
  tape helps to measure the depth of snow

38
Snow gauges

• A snow gauge is a type of instrument used to
  measure the solid form of precipitation.

39
2. Equivalent water in snow

• Snow Water Equivalent (SWE) is a common snow pack
  measurement. It is the amount of water contained
  within the snow pack. It can be thought of as the
  depth of water that would theoretically result if
  you melted the entire snow pack instantaneously.
• Equipment used is
• Standard rain gages without receivers
• Weighing type rain gages
• Snow gages

40
Measurement of equivalent amount of water in a
snow pack

• The equivalent amount of water in a snow pack can
  be measured by
• 1. Heating
• 2. Weighing
• 3. Adding measured amount of hot water

41
1. By Heating

• The equivalent amount in mm of water can be
  obtained by heating the cylinder. it will melt
  the snow and the depth of the liquid water can be
  measured with a measuring stick but this approach
  is adjustable because some water may get
  evaporated during the heating.

42
2. By Weighing

• Weight is measured either by weighing type rain
  gauges or by using a snow gauge.
• WW1-W2
•  
• W1 weight of snow empty cylinder
• W2 Weight of empty cylinder
• W Weight of snow
•  
• By using weight volume relationship
•  
• G Weight/ Volume
•  
• G W/ A.h
•  
• h W/A G
• Where,
•  
• h Equivalent amount of water in snow.

43
3. By scratching snow packs

• A measured amount of hot water is added into
  the cylinder which will melt the snow. Now
  measure the total depth of water in the cylinder
  h1
•  
• h h1-h2
•  
• Where,
•  
• h2 measured amount of hot water
• h equivalent amount of water

44
Radar Measurements

• A weather radar is a type of radar used to locate
  precipitation, calculate its motion, estimate its
  type (rain, snow, hail, etc.), and forecast its
  future position and intensity. Weather radars are
  mostly Doppler radars, capable of detecting the
  motion of rain droplets in addition to intensity
  of the precipitation. Both types of data can be
  analyzed to determine the structure of storms and
  their potential to cause severe weather. 

45
Satellite Measurements

• A weather satellite is a type of satellite that
  is primarily used to monitor the weather and
  climate of the Earth.These meteorological
  satellites, however, see more than clouds and
  cloud systems, like other types of environmental
  information collected using weather satellites.

46
Interpretation Of Precipitation Data

• Interpretation of missing precipitation data
  includes
•  
• 1. Estimating missing precipitation data at a
  station
• 2. Checking inconsistency in particular data at a
  station
• 3. Averaging precipitation over an area

47
1- Estimating missing precipitation data at a
station

• Missing precipitation data is estimated by two
  commonly used methods.
• Arithmetic Mean Method
• Normal Ratio Method (NRM)

48
Arithmetic Mean Method

• Arithmetic mean method is used when normal annual
  precipitation is within 10 of the gauge for
  which data are being reconstructed. This method
  is least accurate however.

49
Example
50
Normal Ratio Method (NRM)

• Normal ratio method (NRM) is used when the normal
  annual precipitation at any of the index station
  differs from that of the interpolation station by
  more than 10. In this method, the precipitation
  amounts at the index stations are weighted by the
  ratios of their normal annual precipitation data
  in a relationship of the form
• Where
• Pm precipitation at the missing
  locationPi precipitation at index stationNm
  average annual rain at missing data gaugeNi
  average annual rain at gaugeN number of rain
  gauges

51
2- Checking inconsistency in a particular data
record at a station

• By a technique called Double Mass Curve Analysis.
• It is used to check the consistency of many kinds
  of hydrologic data by comparing date for a single
  station with that of a pattern composed of the
  data from several other stations in the area
• The double-mass curve can be used to adjust
  inconsistent precipitation data

52
Double Mass Curve Analysis

• The theory of the double-mass curve is based
  on the fact that a plot of the two cumulative
  quantities during the same period exhibits a
  straight line so long as the proportionality
  between the two remains unchanged, and the slope
  of the line represents the proportionality.
• This method can smooth a time series and
  suppress random elements in the series, and thus
  show the main trends of the time series.

53
3- Averaging precipitation over area

• It is the amount of precipitation which can
  be assumed uniform over an area. If the average
  precipitation over an area is known than total
  rain volume of water can be computed for that
  area.
• Rain volume Pavg A
•  

54
Methods for computing average precipitation

• There are some widely used methods to compute
  average precipitation over an area, but the most
  common of these used are
• Arithmetic mean method
• Theissen polygon method
• Isohytal method

55
Theissen Polygon Method

• Divide the region (area A) into sub-regions
  centred about each rain gauge
• Determine the area of each sub-region (Ai) and
  compute sub-region weightings (Wi) using
• Wi Ai/A
• Compute total aerial rainfall using Rainfall
  recorded at each station is given a weight age
  based on the area closest to the station.

56
Theissen Polygon Method

• Consider a catchment area with 3 rain gauge
  stations. Let there be 3 stations outside the
  catchment, but in its neighborhood.
• Catchment area is drawn to scale and position of
  these 6 stations is plotted on it. Stations are
  joined to get a network of triangles.
  Perpendicular bisectors are drawn to each of the
  sides of these triangles.
• These bisectors form a polygon around each
  station. If the boundary of catchment cuts the
  bisectors, then boundary is taken as outer limit
  of polygon. These bounding polygons are called
  Thiessen Polygons.
• The area of these polygons is measured with a
  planimeter or by grid overlay.

57
Isohytal Method

• Plot gauge locations on a 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)
•  

58
Isohytal Method

• Compute aerial rain using Isohyets. It is a line
  joining points of equal rainfall magnitude.
• The catchment area is drawn to scale and the rain
  gauge stations are marked on it. The recorded
  rainfall values for which aerial average is to
  determined are marked at the respective stations.
  
• Neighboring stations outside the catchment are
  also considered. Taking point rainfall values as
  the guide, isohyets of different rainfall values
  are drawn (similar to drawing contours based on
  spot levels.
• The area between adjacent isohyets is measured
  using a planimeter. If isohyets go out of the
  catchment, the catchment boundary is used as the
  bounding line.
• It is assumed that the average value of rainfall
  indicated by two isohyets acts over the inter
  isohytal area

59
Intensity of precipitation

• It is the total amount of precipitation falling
  on a particular area per unit time
• OR
• Intensity (I) is defined as the rate of change of
  precipitation per unit time
• mm per hour, mm per year, etc.

60
Duration

• we need to specify the length of time over which
  the rainfall occurred one year - in the case of
  annual rainfall one month (for many climate
  purposes) or so many days, hours or minutes.
  This period of time over which the rain is
  measured is called the duration .

61
Frequency (f)

• The number of times, during a specified period of
  years, that precipitation of a certain magnitude
  or greater occurs or will occur at a station
  numerically, the reciprocal of the frequency is
  usually given .
• What is the rainfall depth over one hour
  exceeded, on average, once in ten years?". This
  "once in ten years" is a FREQUENCY

62
(No Transcript)
63
IDF Curves
64
TC

• The Tc is generally defined as the time required
  for a drop of water to travel from the most
  hydrologically remote point in the sub catchment
  to the point of collection
• It is defined as the time needed for water to
  flow from the most remote point in a watershed to
  the watershed outlet.
• The time of concentration equals the summation of
  the travel times for each flow regime.  There are
  numerous methods used to calculate the travel
  time for each of the flow regimes.  Here, we will
  discuss a few of the most prevalent methods. E.g.
  NRCS Method (Tc  Lo  Lsc  Lc )
• Overland Flow Lo
• Shallow Concentrated Flow Lsc
• Channel Flow - Lc

65
Overland Flow Lo

• Seelye Method  Travel time for overland flow can
  be determined by using the Seelye chart
• Kinematic Wave Method  This method allows for
  the input of rainfall intensity values.
• Where              Tt  travel time           
     L    length of overland flow in
  feet                n    Manning's roughness
  coefficient               i     rainfall
  intensity                S    slope in
  feet/foot
• Manning kinematic formula
• Where  Tt    travel time (hr.) n   
  Manning's roughness coefficient (Table 3) L   
  flow length (ft.) P2    2-year, 24-hour
  rainfall (in.) (Diagram 5)s    slope of
  hydraulic grade line (feet/foot) 

66
Shallow Concentrated Flow - Lsc

• Where                 Tt   travel time
  (minutes)               L    length of shallow
  concentrated flow (feet)               V   
  velocity (feet per second)

67
Channel Flow - Lc

• Kirpitch Method  Tc 0.0078
  (L³/h)³85
• Where
• L length of the channel in (ft)
• hrelief along main channel
• Manning's equation
• Where 
• V    average velocity (ft./sec.) r    
  hydraulic radius (ft.) and is equal to a/Pw a 
    cross sectional flow area (ft.2)Pw   wetted
  perimeter (ft.) s    slope of the hydraulic
  grade line (ft./ft.) n    Manning's roughness
  coefficient for open channel flow

68
Rational Equation for flow estimaton

• When runoff is computed using the rational method
  tc is the appropriate storm duration and in turn
  determines the appropriate precipitation
  intensity for use in the rational method
  equation.
• When runoff is computed using the hydrograph
  method, tc is used to compute rainfall-runoff
  parameters for the watershed. tc is also used as
  an input to define the appropriate storm
  duration.
• Rational method is used to estimate the surface
  runoff in small watersheds.
• Q CIA
• Where
• Q Discharge (m³/sec)
• C Surface runoff coefficient
• I Rainfal Intensity (mm/hr)
• A Area (ha)

69
Runoff Coefficients