CEE 4420

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Introduction to Precipitation

• CEE 4420 Engineering Hydrology
• (Prepared by Abebe Gebregiorgis)

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2.1 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.

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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.

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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

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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.

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2.2 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)

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2.3. 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 raingauge.

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Rainfall measurement
Precipitation gauge 1 - pole 2 - collector 3 -
support- galvanized metal sheet 4
funnel 5 - steel ring
1. Non recording gauge
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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 needle5-drum with
diagram 6-clock mechanism
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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

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Tele-rain gauge

The tele-rain-gauge 1 - collecting funnel 2 -
tilting baskets 3 - electric signal 4 - evacuation
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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.

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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.

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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

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2.4 Preparation of Data

• Before using rainfall data, it is necessary to
  check the data for continuing and consistency
• Missing data
• Record errors

Estimation of Missing Data

• 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

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Test for consistency record
(Double mass curve techniques)

• 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

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Test for consistency record.
a
c
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
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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

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Arithmetic Mean Method

• 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

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Method of Thiessen polygons

• 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

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Thiessen polygons .
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Thiessen polygons .
P7
P6
A7
A6
P2
A2
A1
A8
A5
P1
P5
P8
A4
A3
P3
P4
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Thiessen polygons .
Generally for M station
The ratio is called the weightage factor
of station i
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Isohyetal Method

• An isohyet is a line joining points of equal
  rainfall magnitude.

10.0
8
D
a5
C
6
12
9.2
12
a4
a3
7.0
B
4
7.2
A
E
10.0
a2
9.1
4.0
a1
a1
F
8
6
4
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Isohyetal Method

• 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

NOTE
The isohyet method is superior to the other two
methods especially when the stations are large
in number.
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2.6 Intensity Duration Frequency (IDF)
Relationship
Mass Curve of Rainfall
1st storm, 16 mm
2nd storm, 16 mm
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IDF .
Hyetograph

• is a plot of the accumulated precipitation
  against time, plotted in chronological order

Total depth 10.6 cm Duration 46 hr
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IDF .

• 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

i Intensity (cm/hr) D Duration (hours) K, x,
a, n are constant for a given catchment
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IDF .

• k 6.93
• x 0.189
• a 0.5
• n 0.878