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Module-21

Irrigation & Drainage

Part-III

21.1 Definition of important terms

1. Gross Command Area (G.C.A)

The whole area enclosed between an imaginary boundary line which can be included in an

irrigation project for supplying water to agricultural land by the network of canals is known as

Gross Command Area (G.C.A). It includes both culturable and unculturable areas.

2. Unultura!le area

The area where the agriculture cannot be done and crops cannot be grown is known as

unculturable area. The marshy lands barren lands lakes ponds forests !illages etc. are

considered as unculturable area.

". Cultura!le area

The area where the agriculture can be done satisfactorily is known as culturable area.

#. Cultura!le ommand area (C.C.A)

The total area within an irrigation project where the culti!ation can be done and crops can be

grown is known as Culturable Command Area. Again C.C.A. may be of two categories.

(a) Culturable Culti!ated area" It is the area within C.C.A. where the culti!ation has been

actually done at present.

(b) Culturable unculti!ated area" it is the area within the C.C.A. where culti!ation is possible

but it is not being culti!ated at present due to some reasons.


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#igure $%.% Gross Command Area

$. Intensit% of irrigation

The total C.C.A. may not be culti!ated at the same time in a year due to !arious reasons.

&ome area may remain !acant e!ery year. Again !arious crops may be culti!ated in the

C.C.A. so the intensity of irrigation may be defined as a ratio of culti!ated land for a

particular crop to the total culturable command area. It is e'pressed as a percentage of C.C.A.

#or e'ample if total C.C.A is % hectares where wheat is culti!ated in $ hectares then

Intensity of irrigation for wheat *2501000x100=25%. &o

Area to be irrigated * C.C.A. ' Intensity of irrigation


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. Crop 'atio

It is defined as the ratio of the areas of the two main crop seasons e.g. +harif and ,abi.

#or e'ample if the area under kharif crop is $hectares and the area under ,abi crop is

hectares then crop ratio of +harif to ,abi is %"$ (i.e. C.,*25005000=1:2)

The crop ratio should be selected that the discharge of the canal for supplying water to kharif and

rabi may be nearly e-ual.

. Crop eason

The period during which some particular types of crops can be grown e!ery year on the same

land is known as crop season. The following are the main crop seasons.

(a) +harif &eason" This season ranges from une to /ctober. The crops are sown in the !ery

beginning of monsoon and har!ested at the end of the autumn. The major crops are ,ice

0illet ute Groundnut etc.

(b) ,abi &eason" This season ranges from /ctober to 0arch. The crops are sown in the !ery

beginning of winter and har!ested at the end of spring. The major ,abi crops are 1heat

Gram 0ustard ,apeseed 2inseed 3ulses /nion etc.

Again there are se!eral crops which are not included in +harif and ,abi as they re-uire more

time and they co!er both the main seasons.

#or e'ample cotton re-uires eight months to mature and sugarcane re-uires about whole

year to mature. 4ence they are designated as

(i) Cotton 5 eight month crop

(ii) &ugarcane 5 3erennial crop

*. Cas+ Crop

The crops which are culti!ated by the farmers to sell in the market to meet their current financial

re-uirements are known as cash crops. The crops like !egetables fruits etc are considered as

cash crops.


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,. Crop 'otation

The process of changing the type of crop for culti!ation on the same land is known as crop

rotation. It is found that if the same crop is culti!ated on the same land e!ery year the fertility of

the land gets reduced and the yield of crop also gradually reduces. This is so because the

necessary salts re-uired for the growth of a particular crop gets e'hausted. It is found by

e'periment that if the principle of crop rotation is practiced the fertility of the soil can be

restored.

#ew crop rotations possible are

(i) ,ice6Gram

(ii) 1heat60illet6Gram

(iii) ,ice6Gram61heat

1. Crop Period

The crop period is defined as the total period from the time of sowing a crop to the time of

har!esting it. That means it is the period in which the crop remains in the field.

11. ase Period

7ase period for a crop refers to the whole period of culti!ation from the time when irrigation

water is first issued for preparation of the ground for planting the crop to its last watering before

har!esting.

12. /0erlap Alloane

It may happen sometimes that the crop of some season may o!erlap some period of the ne't crop

season. 1hen such o!erlapping takes place the crops of both the seasons re-uire water

simultaneously. Thus the o!erlap allowance is the e'tra discharge for this purpose.

1". ime fator

The ratio of the number of days the canal has actually been kept open to the number of days the

canal was designed to remain open during the base period is known as time factor.


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#or e'ample a canal was designed to be kept open for % days but it was practically kept open

for % days for supplying water to the culturable area. Then the time factor is %8%.

&o Time factor * No. of days the canal practically kept openNo. of days the

canal was desined to keep open* act!al discharedesined dischare

1#. Capait% fator

Generally a canal is designed for a ma'imum discharge capacity. 7ut actually it is not re-uired

that the canal runs to that ma'imum capacity all the time of the base period. &o the ratio of the

a!erage discharge to the ma'imum discharge (designed discharge) is known as capacity factor.

#or e'ample a canal was designed for the ma'imum discharge of cumec but the a!erage

discharge is 9 cumec.

&o Capacity factor * 98 *.:

1$. 3um!er of atering

The total depth of water re-uired by a crop is not supplied at one time. 7ut it is supplied o!er the

base period by stages depending upon the re-uirement.

The initial watering which is done on the land to pro!ide moisture to the soil just before sowing

any crop is known as paleo or pale!a.

The first watering is done when the crop has grown about three centimeters. This watering is

known asKorwatering and the period is known asKor period.

1. /utlet fator

It is defined as the duty at the outlet.

1. Cume Da%

The -uantity of water flowing continuously for one day at the rate of one cumec is known as

cumec6day.


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* :.;9 hectare6metre.

21.2 ase4 Delta. Dut%

ase5

The base is defined as the period from the first to the last watering of the crop just before its

maturity. It is also known as base period. It is denoted by e Ground nut 0illet

Delta in m %$ 9 ? ?

'a!i rop 3otato 0ustard Gram potato

Delta in m 9 9 ?


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Dut%5

The duty of water is defined as number of hectares that can be irrigated by constant supply of

water at the rate of one cumec throughout the base period. It is e'pressed in hectares8cumec and

is denoted by


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$. opograp+% of agriultural land5 If the agricultural land is une!en the water

re-uirement will be more and hence the duty will be low. If the land has slight slope the

duty will be high as water re-uirement is optimum. As the ground slope increases the

duty decreases because there is wastage of water.

. Met+od of ploug+ing53roper deep ploughing which is done by tractors re-uires o!erall

less -uantity of water and hence the duty is high. 7ut shallow ploughing with bullocks

re-uires o!erall more -uantity of water and hence the duty is low.

&. Met+ods of irrigation5the duty of water is high in case of perennial irrigation system as

compared to that in inundation irrigation system. It is so because in perennial system

head regulator is used whereas in inundation system there is no regulator.

'. 8ater ta95If some ta' is imposed on the basis of !olume of water consumption the

farmer will use the water economically and thus the duty will be high.

21.# Met+ods of impro0ing Dut%

The following points should be remembered for impro!ing the duty of water.

1. Proper ploug+ing53loughing should be done properly and deeply so that the moisture

retaining capacity of the soil is increased.

2. Met+ods of suppl%ing ater5The method of supplying water to the agricultural land

should be decided according to the field and soil conditions. #or e'ample

#urrow method 5 for crops sown in rows

Contour method 5 for hilly areas

7asin method 5 for orchards

#looding method 5 for plains

". Canal :ining5 To reduce percolation loss the canals should be lined according to site

condition.


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#. ransmission loss5 To reduce transmission loss the canals should be taken close to

irrigable lands as far as possible.

5. Crop 'otation5the principle of crop rotation should bre adopted to increase the moisture

retaining capacity and fertility of the soil.

$. Implementation a95The water ta' should be imposed on the basis of !olume of water

consumption.

21.$ 'elation !eteen ase4 Delta and Dut%5

2et * uty of water in hectares8cumec

7 * 7ase in days

B * elta in m

#rom the definition one cumec of water flowing continupusly for


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The amount of irrigation water supplied to the land is not fully utili>ed for the growth of the

crops. This is due to !arious losses. ow the ratio of the amount of water a!ailable (output) to

the amount of water supplied (input) is known as Irrigation @fficiency. It is e'pressed in

percentage. The following are the !arious types of irrigation efficiencies.

(a) 8ater on0e%ane effiien% (one

* Amount of water supplied applied to land.

(c) 8ater use effiien% (


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* water applied

(d) Consumpti0e use effiien% (one.

It is obtained from the e'pression

1here Hcu* Consumpti!e use efficiency

* Consumpti!e use of water

* Amount of water depleted from the root >one.

(e) 8ater storage effiien%

The concept of water storage efficiency gi!es an insight to how completely the re-uired water

has been stored in the root >one during irrigation. It is determined from the following e'pression.

1here Hs* water storage efficiency

* water stored in the root >one during irrigation

* water needed in the root >one prior to irrigation.

* (#ield capacity 5 A!ailable moisture)

(f) 8ater distri!ution effiien% (one. Jne!en distribution has many undesirable results. The more uniformly

the water is distributed the better will be the crop response.

It is determined by

1here Hd* water distribution efficiency

y * a!erage numerical deri!ation in depth of water stored from a!erage depth

stored for irrigation


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d * a!erage depth of water stored during irrigation.

21. Determination of irrigation re=uirements of rops

In order to determine the irrigation re-uirements of certain crop during its base period the

following terms are re-uired.

(i) ;ffeti0e rainfall('e)5@ffecti!e rainfall is that part of the precipitation falling during the

growing period of a crop that is a!ailable to meet the e!apotranspiration needs of the

crop.

(ii) Consumpti0e irrigation re=uirement (CI')5 Consumpti!e irrigation re-uirement is

defined as the amount of irrigation water that is re-uired to meet the e!apo6

transpiration needs of the crop during its full growth.

Therefore CI, * Cu6 ,e

(iii) 3et Irrigation 'e=uirement (3I')5et irrigation re-uirement is defined as the

amount of irrigation water re-uired at the plot to meet the e!apo6transpiration needs

of water as well as other needs such as leaching etc. thus

I, * Cu5 ,eK water lost in deep percolation for the purpose of leaching etc.

(i,) 7ield Irrigation 'e=uirement (7I')5#ield irrigation re-uirement is the amount of

water re-uired to meet

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