IRRIGATION ENGINEERING-P.A.Pandya Sr.Lecturer G.P. Himatnagar

HYDROLOGY

INTRODUCTIONIrrigation Engineer is not only concern

with the collection and distribution ofwater for irrigation , but it alsoessential for him to know about theoccurrence ,distribution andmovement of water on the earth.

HYDROLOGYWhat is Hydrology???

It is the science which deals with occurrence,distribution and movement of water on the earthincluding that in the atmosphere and below thesurface of the earth.

Water occurs in the atmosphere in the form of watervapour, on the surface as water, snow or ice and belowthe surface as ground water occupying all the voidswithin a geological stratum.

HYDROLOGYExcept for the deep ground water , the

total water supply of earth is in constant circulation from earth to atmosphere and back to earth.

The earth’s water circulatory system is known as hydrological cycle.

HYDROLOGICAL CYCLE Hydrological cycle is the process of transfer of

moisture from the atmosphere to the earth in theform of precipitation, conveyance of precipitedwater by streams and rivers to the oceans and lakesetc. and evaporation of water back to theatmosphere complete the hydrological cycle.

HYDROLOGICAL CYCLE

HYDROLOGICAL CYCLEEvaporation and Transpiration (E)

Precipitation(P)

Runoff (R)

Evaporation and TranspirationEvaporation: (E)

Surface Evaporation

Water Surface Evaporation From River

From Oceans

Evaporation from Plants and Leaves(Transpiration)

Atmospheric Evaporation.

PrecipitationIt may be defined as fall of moisture from theatmosphere to the earth’s surface in any form.

Precipitation may be in two forms:

(1)Liquid Precipitation: Dizzel –Size<0.5 mm

(2)Frozen precipitation: Rain- Size >0.5 mm

Snow-Precipitation in the form of ice crystal.

Hail – Lump of ice over 5 mm

Sleet- frozen rain drops cooled to the ice stage.

Snow Flakes-Number of ice crystal fused together.

Run offIt is that portion of precipitation that is notevaporated.

Run off = Precipitation – Evaporation

i.e. R = P – E or P= E + R

Run off ultimately runs to ocean through surface orsub –surface streams.

Thus it may be classified as

Surface run off ,Subsurface run off, Ground water flowor base flow.

Types of Run off Surface run off : Water flows over the land and is first

to reach the streams or rivers which ultimately discharge the water to the sea.

Sub Surface Run off: A portion of precipitation infiltrates in to surface soil and depending upon the geology of basin runs as sub-surface soil run off and reaches the streams and rivers.

Ground Water Flow: A portion of precipitation which after infiltrate percolate down and joins the ground water reservoir which is ultimately connected to ocean.

Terms related to Rainfall Intensity of Rainfall: It is rain fall in unit of time

e.g if 18 cm rain fall in 3 hours,

Intensity of rain fall= 6 cm

Daily rainfall:

Rain fall in 24 hours at any rain gauge St.

Annual rain fall:

Rain fall in 1 year at any rain gauge St.

Mean annual rainfall:

Rain fall in 35 year at any rain gauge St.

Rainfall Measurement Rain fall is source of all water used for irrigation

purpose and therefore the knowledge of its amountcharacter season or period the effects produced by it isof prime importance to irrigation engineer to design,carry out, improve or maintain irrigation works.

The amount of precipitation is expressed as the depthin cm or inches which fall on the level surface, and ismeasure by rain-gauge.

Types of Rain gauge Non –Automatic or Non-recording rain gauge.

e.g. Symon’s Rain-gauge.

Automatic or Recording rain gauge.

e.g. (1) Weighing bucket rain gauge.

(2) Tipping Bucket rain gauge.

(3) Float type rain gauge.

Symon’s Rain-gauge Figure to be attached

Symon’s Rain-gauge Used at all Govt. Rain gauge stations through out

India.

It consist of cylindrical vessel 127mm dia.with a base enlarged to 210mm dia.

Weighing Bucket Rain-gauge

Tipping Bucket Rain-gauge

Recording Rain-gaugeAdvantages over non recording rain-gauge:

Rainfall recorded automatically.

Also gives the intensity of rain fall at any time, while non recording type rain gauge gives total rain fall in particular interval of time.

As no attendant required can be installed in far-off places.

Possibilities of human error is obviated.

Recording Rain-gaugeDisadvantages:

Costly

Fault may be developed in electrical/mechanical mechanism/ recording rain fall.

Sources of Error in recording the Measurement

Mistake in reading the scale of gauge.

Some amount of water displaced by measuring stick.

(this may increase error by 1%)

Dent in the collector rim may change its receiving area.

Funnel and inside surface required about 2.5 mm of rain to get moistened when gauge is initially dry.

( this may extent of amt of 25mm/yr in some areas)

Deficiency of measurement due to wind.

Sources of Error in recording the Measurement

Inclination of Gauge may cause lesser collection.

(10 % inclination = 1.5 % low catch)

Tipping of bucket may be affected due to rusting or accumulation of dirt at the pivot.

No rainfall is recorded during tipping of the bucket.

Computation of Av.Rainfall Required no. of gauges in a catchment area:

Area in square km No. of rain gauge station

0 to 80 1

80 to 160 2

160 to 320 3

320 to 520 4

560 to 800 5

800 to 1200 6

Computation of Av.Rainfall If a basin or catchment area contain more than 1 rain gauge station the average rainfall may be done by the following methods:

Arithmatic Average method

Thiessen Polygon method

Isohytal Method

Arithmatic Average MethodIf P1 , P2, P3……etc are the Precipitation or rain fall values measured at n rain gauge stations then,

The average rain fall,

Pav= (P1 + P2 +P3+…………+Pn)/n

= ∑P/n

Thiessen Polygon Method

Pav= (P1A1 + P2A2+P3A3+…….PnAn)/(A1 + A2+…An)

= ∑(P x A)/∑A

Here A1, A2,A3……An= Area of Polygon

Isohytal Method

Pav = [∑A * {(P1 +P2)/2}]/∑A

Here A= Area enclosed by two successive isohytes.

P1 & P2 = Precipitation Values at two isohytes.

Run offThe runoff of a catchment area in any specified period is the total quantity of water draining in to a stream or in to a reservoir in that period.

It can be expressed as

(1) cm of water in a catchment area

(2) The total water in m3 or hectare- metre for a given catchment.

Factors affecting run off Precipitation Characteristics.

Shape and size of the catchment.

Topography

Geological Characteristics of the catchment.

Meteorological Characteristics.

Character of the catchment surface.

Storage Characteristics of the catchment

Precipitation CharacteristicsMore the rainfall more will be run off.

Duration of rainfall.

Type of rainfall

Intensity of rainfall-more intensity-more run off.

Shape and size of the catchment. Fan shape catchment Area- More run off

Fern leaf shape catchment-Less run off.