Chapter 2 Precipitation

ENGINEERING HYDROLOGY :

PRECIPITATION

Prepared by:SUZILAWATIE BT ABDUL GHANI

P 60832

PRECIPITATION

Characteristics of Precipitation in India

Measurement of Precipitation

CHARACTERISTICS OF PRECIPITATION IN INDIA

CLIMATE

South-west monsoon

• June -September

Transition-I ( post

monsoon )

• October - November

Winter season

• December –

February

Transition-II ( summer )

• March - May

2 major seasons

2 transitional periods


South – West Monsoon ( June – September )

Popu

larl

y k

now

n

“MO

NSO

ON

”

Principal rainy season in INDIA

Principal source of rain

July - month with has maximum rain

75 % of the annual rainfall is received over a major portion of the country

Originates in the Indian ocean

Heralds its appearanc

e in the southern part or

Kerala (end of May)

High south-westerly winds at

speed 30-70 kmph

• Monsoon wind across the country in 2 branches : >> the Arabian sea

>> the Bay of Bengal

• 1st week of June >> Southern part of Kerala >> Latter at Assam

• The Bay >> north-eastern region >> westwards to Bihar and UP

• The Arabian >> northwards – Karnakata, Maharashta and Gujerat

• 4th week of June – both branches reach Delhi

• Monsoon Trough >> low pressure region

>>formed between 2 branches

• Monsoon wind increase from June to July and begin to weak in September



• Monsoon wind across the country in 2 branches : >> the Arabian sea

>> the Bay of Bengal

• 1st week of June >> Southern part of Kerala >> Latter at Assam

• The Bay >> north-eastern region >> westwards to Bihar and UP

• The Arabian >> northwards – Karnakata, Maharashta and Gujerat

• 4th week of June – both branches reach Delhi

• Monsoon Trough >> low pressure region

>>formed between 2 branches

• Monsoon wind increase from June to July and begin to weak in September

New Delhi


• Marked by substantial rainfall activity

• Start in September

• Northern part


MONSOON

Not a period with continuous rainfall

Generaly, cloudy with frequent spell of rainfall

Heavy rainfall >> passage of low pressure region

The bay of Bengal >>Depressions formed >>Frequency 2-3 per month >>Cause excessive precipitation

200–400cm >> Assam >> North-Eastern

200–300cm >> West Coast >> Western Ghats

120–160cm >> West Bengal

100–120cm >> UP,Haryana,Punjab


200–400cm >> Assam >> North-Eastern

200–300cm >> West Coast >> Western Ghats

120–160cm >> West Bengal

100–120cm >> UP,Haryana,Punjab


Post - Monsoon ( October – November )

Low pressure area form in the Bay of

Bengal

A North-Easterly flow of air that

pick up moisture in the Bay of

Bengal is formed

The air mass strikes the

east coast of the southern peninsula >>

cause rainfall

November

>> The Bay of Bengal >> The Arabian Sea

Tropical cyclone

Strike the coastal area cause : Intense rainfall Heavy damageLife

Property


Winter Season ( December – February )

Moderate to heavy rain and snowfall (about 25 cm)

in Himalaya, Jammu and Kashmir

Light rainfall occur in Northern plains

10-12cm of rainfall in the southern part of Tamil Nadu

Western Disturbance

>> Mid December>> Disturbances of extra tropical origin

travel eastwards


Summer – Pre Monsoon ( March – May )

Convective cells cause

thunderstorms

Cyclone

>> Kerala>> West Bengal>> Assam

>> East Coast

Very little rainfall

Annual Rainfall

High rainfall of the magnitude of 200 cm >> Assam >> north-

eastern part >> western ghat

Scanty rainfall >> eastern Rajasthan >> part of Gujerat,

Maharashta and Karnataka

Average annual rainfall = 117 cm

Coefficient of variation

Cv = 100 x standard deviation mean

Annual rainfall varies between 15 and 70 from place to place

Average value of about 30

MEASUREMENT OF PRECIPITATION

Non-recording Gauge

Recording Gauges

Telemetering Raingauges

Radar Measurement of Rainfall

Rainfall

Depth of Snowfall

Water Equivalent of Snow

Snowfall

Tipping-bucket type

Weighing-bucket type

Natural-syphon type

Snow Gauges

Snow tubes


Rainfall

Rain

gauge

Cylindrical-vessel assembly kept in open to collect rain

The rainfall catch of the raingauge is affected by it exposure condition

For setting a raingauge

Raingauge is used to collect and measure the precipitationPluviometer , ombrometer and hyetometer also used as a raingauge

The ground must be level and in the open and its instrument must present a horizontal catch surface

The gauge must be set as near the ground as possible to reduce wind effects but must be sufficiently high to prevent splashing or flooding

The instrument must be surround 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


` Rainfall (Non-Recording Gauges)Circular collecting area with diameter

12.7 cm

The rim of the collector is set in

a horizontal plane at a height of 30.5 cm above the ground level

Funnel disharge the rainfall catch into a receiving

vessel

Funnel and receiving vessel are housed in

metalic container

Measure by a suitable graduated

measuring glass (accuracy =

0.1mm)

Proper care, maintenance and

inspection :-Especially during dry weather to keep the instrument free from

dust and dirt


Rainfall (Recording Gauges)Produce a continuous plot of rainfall against timeProvide valuable data of intensity and duration of rainfall for hydrological analysis of storm

Tipping-bucket

A 30.5 cm size raingauge

Catch from funnel bucket Rainfall collect in a buckettip and bring the other one positioncollected in a storage can measure to provide the total rainfall and serve a checkThe tipping actuates an electricity driven pen to trace a record on clockwork-driven chart

Ideally suited for digitalizing of the output signal

Weighing-bucket

Catch from funnel empties into a bucket mounted on weighing scale

Weight of the bucket and its contents are recorded on a clock-work driven chart

The clockwork has the capability to run one week

This instrument gives a plot of the accumulated rainfall against the elapsed time the mass curve of rainfall

Natural-Syphon

Known as float type gauge

Collected by funnel-shaped collector is led into a float chamber causinga float to rise

A pen through lever system records the elevation of the elevation on a rotating drum driven by a clock-work mechanismA syphon arrangement empties the float chamber when the float has reached the pre-set max level


Rainfall (Recording Gauges)

Tipping-bucket Weighing-bucket Natural-Syphon


Rainfall (Recording Gauges)

Rainfall of 53.8 mm in 30 hr


Rainfall (Telemetering Raingauges)

Recording type and contain electronic unit to transmit the data of rainfall to a base station

Use in gathering rainfall data from >> mountainous >> inaccessible places

Rainfall (Radar Measurement of Rainfall)

Consider as a remote sensing super gauge

For measuring the areal extent, location and movement of rain storms

Advantages :- determine amount of rainfall over large areas

Operate with wavelenghts from 3 to 10 cmHeavy flood 10 cm radar Light rain and snow 5 cm radar


Snowfall

Form of precipitation accumulate over a surface for sometime before it melts and cause runoff

Snowfall (Depth of Snowfall)

Indicator : useful for seasonal

precipitation and long term- runoff forecast

A graduated stick or staff is

used to measure the

depth of snow at a selected

placeA snow stakes

are permanent posts used to measure the total depth of

snow at a place

Snow boards – 40 cm side

square boards >> collect

snow sample

SNOW BOARDS

>> Place horizontally on a previous accumulation of

snow>> After the snow event, the

snow samples are cut off >> The depth of snow and

water equivalent of snow are derived and record


Snowfall (Water Equivalent of Snow)

Depth of water that result in melting of a unit of snowImpotant : to assess the seasonal water resource of a catchment in estimates

>> stream flow >> flood

DENSITY OF SNOW

Freshly fallen snow = range of 0.07 to 0.15Compaction snow = 0.4 to 0.6

Specific data not avaiable = assume 0.10


Snowfall (Water Equivalent of Snow)

Snow

G

au

ges

A large cylindrical receiver - 203 mm in diameter - from 60 cm in height

Windshield is provided at the top

Melting agents or heating system to reduce the size of containerThe snow collected is brought in to a warm room and the snow melted by adding a pre-measured quantity of hot water

Through volume measurement, the result is ascertained and recorded

Snow

Tu

bes

A set of telescopic metal tube - normal size = 40 mm dia - higher size = up to 90 mm dia

Provided with a cutter edge for easy penetration to enable extracting of core sampleTo extract the sample, the sample is driven into the snow deposit till it reaches the bottom of deposit and twisted and turned to a cut a coreThe core is extracted and studied for its physical properties and melted to obtain water equivalent of the snow core

THANK YOU

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Chapter 2 Precipitation

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Transcript of Chapter 2 Precipitation