Basics of Crop Water Requirements in Irrigation Engineering

2

2

.

.

3

3

.

.

C

C

R

R

O

O

P

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W

W

A

A

T

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E

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R

R

R

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E

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Q

Q

U

U

I

I

R

R

E

E

M

M

E

E

N

N

T

T

S

S

•

It is defined as 

“the depth of water 

needed to

meet the water loss 

through 

evapotranspiration

(ETcrop) 

of a disease free crop growing in large

fields under non-restricting soil conditions

including soil water and fertility and 

achieving

full production potential

 under the given growing

environment”.

•

It is the 

quantity of water required 

by the crop 

in

a given period of time to 

meet its normal growth

under a given set of environmental & field

conditions.

C

C

R

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O

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P

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.

.

.

.

•

The determination of 

water requirements 

is the

main part of the 

design and planning 

of an

irrigation system.

•

The 

water requirement

 is the water required to

meet the 

water losses

 through

–

Evapotranspiration (ET)

–

Unavoidable application losses

–

Other needs such as leaving & land

preparation

C

C

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N

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c

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n

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t

.

.

.

.

•

The 

water requirement of crops 

may be

contributed from 

different sources 

such as

i

rrigation

, 

Effective rainfall

, 

Soil moisture storage

and 

ground water contributions.

•

Hence, WR = IR + ER + S + GW

•

Where,

     IR = Irrigation requirement

    ER = Effective rainfall

     S   = carry over soil moisture in the crop root

zone

   GW  = ground water contribution

I

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p

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•

Irrigation water requirement of crops 

is defined

as the 

part of water requirement of crops that

should be fulfilled by irrigation

•

In other words, it is the 

water requirement of

crops excluding effective rain fall, 

carry over soil

moisture and 

ground water contributions

.

  WR=IR +ER + S +GW

IR= WR-(ER+S+GW)

E

f

f

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i

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R

a

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f

a

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l

(

E

R

)

•

Effective rainfall 

can be defined as the 

rainfall

that is stored in the root zone 

and can be

utilized by crops.

•

All the rainfall that falls is not useful or effective.

•

As the total amount of rainfall varies, so does

the amount of useful or effective rainfall.

•

Some of the seasonal rainfall

 that falls will 

be

lost 

as unnecessary 

deep percolation

; 

surface

runoff 

and 

some water may remain in the soil

after the crop is harvested.

•

From the water requirement of crops point of

view, this water, which is lost, is ineffective.

E

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(

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)

c

o

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…

•

People in 

different disciplines

 define effective

rainfall in different ways.

•

To a 

canal irrigation engineer

, it is the rainfall

that 

reaches the storage reservoir

,

•

to a 

hydropower engineer

, it is the rain fall

that is useful for 

running the turbines 

and

•

for 

Ground water engineers 

or Geo –

hydrologists, it is that 

portion of the rainfall

that contributes to the 

ground water reservoir

E

f

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•

The effective rainfall is taken as a fixed

percentage of the monthly rainfall;

•

Effective Rainfall = % of Total Rainfall

D

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•

An empirical formula developed by

FAO/AGLW based on analysis for different

arid and sub-humid climates.

•

Effective Rainfall = 0.6 * Total Rainfall - 10 ...

(Total Rainfall < 70 mm)

•

Effective Rainfall = 0.8 * Total Rainfall - 24 ...

(Total Rainfall > 70 mm)

E

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This formula is similar to FAO/AGLW formula

(see Dependable Rain method above) with

some parameters left to the user to define.

•

Effective Rainfall = a * Total Rainfall - b ...

(Total Rainfall < z mm)

•

Effective Rainfall = c * Total Rainfall - d ...

(Total Rainfall > z mm)

•

where a, b, c, and z are the variables to be

defined by the user.

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)

•

The effective rainfall is calculated according to

the formula developed by the USDA Soil

Conservation Service:

•

Effective Rainfall = Total Rainfall / 125 * (125 -

0.2 * Total Rainfall) …(Total Rainfall < 250 mm)

•

Effective Rainfall = 125 + 0.1 * Total Rainfall ..

(Total Rainfall > 250 mm)

G

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:

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•

Some times there is a 

contribution from the

groundwater reservoir for water requirement of

crops.

•

The 

actual contribution from the groundwater table

is 

dependent on the depth of ground water table

below the root zone 

& 

capillary characteristics 

of

soil.

•

For 

clayey soils 

the 

rate of movement is low 

and

distance of upward movement is high 

while

•

for 

light textured soils 

the 

rate is high

and the

distance of movement is low.

•

For 

practical purposes the GW contribution

 when

the 

ground water table is below 3m

 is assumed to

be 

nil.

C

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(

(

S

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)

:

:

•

This is the 

moisture retained in the crop root

zone 

b/n cropping seasons 

or 

before the crop

is planted.

•

The 

source

 of this moisture is either from the

rainfall

 that man occurs before sowing or it

may be the 

moisture that remained 

in the soil

from past irrigation

.

•

This moisture also 

contributes

 to the

consumptive use of water 

and should be

deducted from the water requirement of crops

in determining irrigation requirements.

N

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•

After the exact evapotranspiration of crops have been

determined the NIR should be determined.

•

This is 

the net amount of water applied to the crop by

irrigation exclusive of ER, S and GW.

•

NIR = WR – ER –S –GW

•

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NIR is determined during different stages of the crop 

by

dividing the whole growing season into suitable intervals.

•

The growing season is more preferably divided into

decades.

•

The ETcrop during each decade is determined by

subtracting these contributions from the ETcrop.

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Usually more amount of water than the NIR

is applied during irrigation to compensate for

the unavoidable losses.

•

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•

GIR =

NIR

             Ea

•

Where Ea =application efficiency

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This includes the water lose through

evaporation and transpiration.

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•

This loss of water 

includes

 the 

quantity of

water transpired by the plant

 and 

that

retained in the plant tissue.

•

That is, 

the water entering plant roots 

and

used to build plant tissue 

or 

being passed

through leaves of the plant into the

atmosphere.

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:

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•

This is 

also called reference crop

evapotranspiration

•

it is the 

rate of evapotranspiration 

from an

extensive surface 8 to 15 cm tall

, 

green grass

cover of uniform height,

actively growing,

completely shading the ground 

and 

not short of

water”.

•

Under normal field conditions, 

the 

potential

evapotranspiration does not occur

 and thus

suitable crop coefficients

 are used to 

change

ETo 

to actual evapotranspiration of the crops

.

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Consumptive use (CU) 

is 

synonymous to

evapotranspiration (ETcrop).

•

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.

•

Note: CU= ET + water used by the plants in their

metabolic process for building plant tissues

(insignificant)

•

It involves:

–

Problems of water supply

–

Problems of water management

–

Economics of irrigation projects

•

CU use can apply to water requirements of a crop, a

farm, a field and a project.

•

However, when the CU of the crop is known, the

water use of larger units can be calculated.

C

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•

Prediction methods 

for crop water requirements are

used owing to the 

difficulty of obtaining accurate

field measurements.

•

The 

methods

 often need to be applied under

climatic and agronomic conditions vary different

from those under which they were originally

developed.

•

To calculate 

ETcrop a three-stage procedure 

is

recommended

1. The effect of climate given by the reference crop

1. The effect of climate given by the reference crop

evapotranspiration (ETo).

evapotranspiration (ETo).

•

The methods to 

calculate ETo 

presented here in are

–

the Blaney-Criddle method,

–

Thornthwaite method, the

–

Hargeaves class A evaporation method and

–

the penman method.

•

These methods are modified 

to calculate ETo 

using

the mean daily climatic data for 30 or 10 days periods.

•

 The 

choice of the method 

must be based on:

–

 the type of climatic data available and

–

on the accuracy required in determining water needs.

2. The effect of crop characteristics.

2. The effect of crop characteristics.

•

This is given by the 

crop coefficient (Kc)

which 

presents the relationship between ETo

and ETcrop.

•

ETcrop= Kc . ETo

•

Values of Kc vary with the

-  type of crop

-

its stage of growth

-

growing season and

-

the prevailing weather conditions

3. Effect of local conditions and agricultural practices

3. Effect of local conditions and agricultural practices

•

This includes:

-

the variation in climate over time

-

size of field

-

distance and altitude

-

soil water availability

- Irrigation and cultivation methods and

practices.

F

F

a

a

c

c

t

t

o

o

r

r

s

s

A

A

f

f

f

f

e

e

c

c

t

t

i

i

n

n

g

g

C

C

o

o

n

n

s

s

u

u

m

m

p

p

t

t

i

i

v

v

e

e

U

U

s

s

e

e

o

o

f

f

W

W

a

a

t

t

e

e

r

r

:

:

-

-

•

The 

consumptive use of water

–

is not constant throughout the stages of the crop

and also

–

varies for different types of crops.

•

Generally the factors affecting consumptive

use of water can be classified as

–

climatic factors.

–

crop factors

A

A

.

.

C

C

l

l

i

i

m

m

a

a

t

t

i

i

c

c

f

f

a

a

c

c

t

t

o

o

r

r

s

s

B

B

.

.

C

C

r

r

o

o

p

p

f

f

a

a

c

c

t

t

o

o

r

r

s

s

•

The agronomic feature of the crops is

variable, 

some crops completely shade the

ground while others shade only some part of

the ground.

•

To account these variations 

in the nature of

the crop 

suitable values of crop coefficient

are used 

to convert the PET to actual

evapotranspiration.

•

So 

for the same climatic conditions 

different

crops have different rates of consumptive

uses

D

D

e

e

t

t

e

e

r

r

m

m

i

i

n

n

a

a

t

t

i

i

o

o

n

n

o

o

f

f

C

C

o

o

n

n

s

s

u

u

m

m

p

p

t

t

i

i

v

v

e

e

U

U

s

s

e

e

o

o

f

f

w

w

a

a

t

t

e

e

r

r

•

Under normal field conditions PET (ETo) will

not occur and thus consumptive use

(ETcrop) can be determined by determining

the ETo and multiplying with suitable crop

coefficients (Kc).

•

Alternatively it can be determined by 

direct

measurements of soil moisture.

1

1

.

.

D

D

i

i

r

r

e

e

c

c

t

t

M

M

e

e

a

a

s

s

u

u

r

r

e

e

m

m

e

e

n

n

t

t

o

o

f

f

C

C

o

o

n

n

s

s

u

u

m

m

p

p

t

t

i

i

v

v

e

e

U

U

s

s

e

e

:

:

•

A) Lysimeter experiment

•

B) Field experimental plots

•

C) Soil moisture studies

•

D) Water balance method

a

a

.

.

L

L

y

y

s

s

i

i

m

m

e

e

t

t

e

e

r

r

E

E

x

x

p

p

e

e

r

r

i

i

m

m

e

e

n

n

t

t

•

Lysimeters

 are 

large containers 

having

pervious bottom

.

•

This experiment involves 

growing crops in

lysimeters

 there by 

measuring the water

added to it and the water loss 

(water draining)

through the pervious bottom.

•

Consumptive use 

is determined by

subtracting the water draining through the

bottom

 from the 

total amount of water needed

to maintain proper growth.

•

ETc = IR + Eff.P +or –soil moisture- Drainage

b

b

.

.

F

F

i

i

e

e

l

l

d

d

E

E

x

x

p

p

e

e

r

r

i

i

m

m

e

e

n

n

t

t

a

a

l

l

P

P

l

l

o

o

t

t

s

s

•

This is most 

suitable for determination of

seasonal water requirements.

•

Water is added to selected field plots

, 

yield

obtained from different fields

 are plotted

against the total amount of water used.

•

The 

yield increases as the water used

increases 

for some limit and 

then decreases

with further increase in water.

•

Production function

•

The 

break in the curve indicates 

the amount

of 

consumptive use of water.

C

C

.

.

S

S

o

o

i

i

l

l

M

M

o

o

i

i

s

s

t

t

u

u

r

r

e

e

S

S

t

t

u

u

d

d

i

i

e

e

s

s

:

:

•

In this method 

soil moisture measurements

are done 

before and after each irrigation

application.

•

Knowing the time gap 

b/n the two

consecutive irrigations

, the 

quantity of water

extracted per day

 can be computed by

dividing the total moisture depletion

 b/n the

two successive irrigations by the interval of

irrigation.

•

Then a curve is drawn by 

plotting 

the 

rate of

use of water

against the time 

from this curve,

seasonal water use of crops is determined

d

d

.

.

W

W

a

a

t

t

e

e

r

r

b

b

a

a

l

l

a

a

n

n

c

c

e

e

m

m

e

e

t

t

h

h

o

o

d

d

•

This method 

is used for determination of

consumptive use of large areas

.

•

It is expressed by the following equation.

•

Precipitation = Evapotranspiration + surface

runoff + deep percolation + change in soil

water contents

•

Except evapotranspiration

, 

all the factors in

the above equation are measured.

•

Evapotranspiration is determined from the

above equation

2

2

.

.

D

D

e

e

t

t

e

e

r

r

m

m

i

i

n

n

a

a

t

t

i

i

o

o

n

n

o

o

f

f

E

E

v

v

a

a

p

p

o

o

t

t

r

r

a

a

n

n

s

s

p

p

i

i

r

r

a

a

t

t

i

i

o

o

n

n

u

u

s

s

i

i

n

n

g

g

e

e

q

q

u

u

a

a

t

t

i

i

o

o

n

n

s

s

•

B

l

a

n

e

y

-

C

r

i

d

d

l

e

m

e

t

h

o

d

•

This method is suggested where 

only temperature data

are available.

•

ETo = C[ P (0.46T+8)] mm/day

•

Where

•

ETo= reference crop evapotranspiration in mm/day for

the month considered.

•

T= mean daily temperature 

in 

o

c over the month

•

P= mean daily percentage of total annual day time

hours

 obtained from table 1 for a given month and

latitude.

•

C = adjustment factor 

which depends on minimum relative

humidity, sunshine hours and daytime wind estimates

B

B

l

l

a

a

n

n

e

e

y

y

-

-

C

C

r

r

i

i

d

d

d

d

l

l

e

e

m

m

e

e

t

t

h

h

o

o

d

d

•

Figure 1 can be used to estimate ETo using

calculated values of 

p(0.46T+8)

 for

•

i) three levels of minimum humidity (RH min)

•

ii) three levels of the ratio of actual to maximum

possible sunshine  hours (n/N) and

•

iii) three ranges of daytime wind conditions at 2m

height (Uday).

B

B

l

l

a

a

n

n

e

e

y

y

-

-

C

C

r

r

i

i

d

d

d

d

l

l

e

e

m

m

e

e

t

t

h

h

o

o

d

d

•

Note: Minimum humidity refers to minimum

daytime humidity

•

wind refers to daytime wind.

•

Generally Uday/Unight =2 and mean 24 hr

wind data should be multiplied by 1.33 to

obtain mean daytime wind.

•

After determining ETo, ETcrop can be

predicted using the appropriate crop

coefficient (Kc).

•

ETcrop= Kc * ETo

simplified form of Blaney- Criddle

simplified form of Blaney- Criddle

•

A more simplified form of Blaney- Criddle

equation in which the potential

evapotranspiration ( consumptive use )

depends only in the mean monthly

temperature and monthly day light hours is

given as :

  u = Kf

•

Where u= monthly consumptive use ,m

•

K = empirical crop coefficient

•

F = monthly consumptive use factor

simplified form of Blaney- Criddle

simplified form of Blaney- Criddle

•

The monthly consumptive use factor

•

Where p is monthly day light hours

expressed as a percentage of the total day

light hours of the year .

•

I

t

d

e

p

e

n

d

s

o

n

t

h

e

l

a

t

i

t

u

d

e

o

f

t

h

e

l

o

c

a

t

i

o

n

.

•

T

m

i

s

m

e

a

n

m

o

n

t

h

l

y

t

e

m

p

e

r

a

t

u

r

e

i

n

o

C

.

O

b

t

a

i

n

v

a

l

u

e

s

o

f

P

f

r

o

m

s

t

a

n

d

a

r

d

t

a

b

l

e

s

.

simplified form of Blaney- Criddle

simplified form of Blaney- Criddle

•

The crop coefficient 

K depends 

on the

location and type of crop 

.

•

Values varies according to the different stage

of crop growth period.

•

This method gives good results if the value of

K is selected judiciously after field test.

•

Where n= number of months in crop period

Blaney- Criddle

Blaney- Criddle

Example on Blaney- Criddle on your lectrure

Example on Blaney- Criddle on your lectrure

Note

Note

Assignment

Assignment

T

T

h

h

o

o

r

r

n

n

t

t

h

h

w

w

a

a

i

i

t

t

e

e

m

m

e

e

t

t

h

h

o

o

d

d

•

According to the Thornthwaite equation ,

based on the data from the eastern U.S.A ,

the monthly consumptive use or the potential

evapotranspiration is given by

•

Where ,

Tm = mean monthly temperature in oC.

I = annual heat index , obtained from monthly

heat index I of the year

T

T

h

h

o

o

r

r

n

n

t

t

h

h

w

w

a

a

i

i

t

t

e

e

m

m

e

e

t

t

h

h

o

o

d

d

T

T

h

h

o

o

r

r

n

n

t

t

h

h

w

w

a

a

i

i

t

t

e

e

m

m

e

e

t

t

h

h

o

o

d

d

Example on Thornthwaite on your lecture

Note

Assignment

H

H

a

a

r

r

g

g

r

r

e

e

a

a

v

v

e

e

s

s

c

c

l

l

a

a

s

s

s

s

A

A

p

p

a

a

n

n

E

E

v

v

a

a

p

p

o

o

r

r

a

a

t

t

i

i

o

o

n

n

•

ET or CU is related to pan evaporation (EP)

by a constant 

Kc, called consumptive use

coefficient.

•

ET = Kc * Ep

•

Determination of Ep

•

(a.) Experimentally

•

(b.) Christiansen formula

•

Ep = 0.459R * Ct*Cw*Ch*Cs*Ce

H

H

a

a

r

r

g

g

r

r

e

e

v

v

e

e

s

s

m

m

e

e

t

t

h

h

o

o

d

d

M

M

o

o

d

d

i

i

f

f

i

i

e

e

d

d

P

P

e

e

n

n

m

m

a

a

n

n

M

M

e

e

t

t

h

h

o

o

d

d

•

A slightly modified penman equation from the

original (1948) is suggested here to determine

ETo involving a revised wind function term.

•

The method uses mean daily climatic data,

since day and night time weather conditions

considerably affect level of ET; an adjustment

for this is included.

•

The 

modified penman equation 

is ,

•

ETo = c ( W.Rn + (1 – W) * f(u). (ea – ed))

              Radiation      Aerodynamic term

                Term

M

M

o

o

d

d

i

i

f

f

i

i

e

e

d

d

P

P

e

e

n

n

m

m

a

a

n

n

M

M

e

e

t

t

h

h

o

o

d

d

•

Where:

•

ETo = reference crop evapotranspiration ,mm/day

•

W = temperature – related weighting factor

•

Rn = net radiation in equivalent evaporation in ,

mm/day

•

F(u) = Wind – related function

•

(ea-ed) 

= difference between the saturation vapor

pressure at mean air temp. and the mean actual

vapor pressure of the air in mbar.

•

C = adjustment factor to compensate for the effect

of day and night weather conditions.

M

M

o

o

d

d

i

i

f

f

i

i

e

e

d

d

P

P

e

e

n

n

m

m

a

a

n

n

M

M

e

e

t

t

h

h

o

o

d

d

•

For areas where 

measured data on temperature

,

humidity, wind and sunshine duration or radiation 

are

available, the penman method is suggested.

The penman equation consists of 

two terms

:

- 

the energy (radiation) term and

- 

The aerodynamic (wind and humidity) term.

•

The relative importance of each term varies with climatic

conditions.

•

Under calm weather conditions the aerodynamic term is

usually less important than the energy term.

•

It is more 

important under windy conditions 

and

particularly in the more arid regions.

M

M

o

o

d

d

i

i

f

f

i

i

e

e

d

d

P

P

e

e

n

n

m

m

a

a

n

n

M

M

e

e

t

t

h

h

o

o

d

d

•

Due to the interdependence of the variables

composing the equation, the correct use of units in

which variables need to be expressed is important

(see example below).

•

D

e

s

c

r

i

p

t

i

o

n

o

f

v

a

r

i

a

b

l

e

s

a

n

d

t

h

e

i

r

M

e

t

h

o

d

o

f

c

a

l

c

u

l

a

t

i

o

n

a

.

V

a

p

o

r

p

r

e

s

s

u

r

e

(

e

a

-

e

d

)

•

Air humidity affects ETo.

•

Humidity is expressed here as saturation vapor

pressure deficit (ea-ed),

•

(ea-ed) is the difference between 

mean saturation

water vapor pressure (ea)

 and the 

mean actual

vapor pressure (ed).

M

M

o

o

d

d

i

i

f

f

i

i

e

e

d

d

P

P

e

e

n

n

m

m

a

a

n

n

M

M

e

e

t

t

h

h

o

o

d

d

•

Air humidity data are reported as:

- 

Relative humidity (RH max ad RH min in percentage

)

- 

Psychometric readings 

(

T

o

C of dry and wet bulb

) from

wet and dry bulb thermometers, or as a dew point

temperature j (T dew point 

o

C)

•

Time of measurement is important, but is often not

given.

•

Fortunately actual vapor pressure (ed) is a fairly

constant element and even one measurement per day

may suffice.

•

Vapor pressure must be expressed in mbar. If ed is

given in mm Hg multiply by 1.33 to find mbar.

•

Tables 5 and 6 give values of ea and ed from available

climatic data.

e) Net radiation (Rn).

e) Net radiation (Rn).

•

Net radiation (Rn

) is the 

difference

 between

all 

incoming

 and 

out going radiation

.

•

It can be measured, but such data are rarely

available.

•

Rn can be calculated from solar radiation or

sunshine hours (or degree of cloud cover),

temperature and humidity data.

•

The amount of 

radiation received at the top

 of

the atmosphere (Ra) is dependent on

•

- 

latitude 

and

•

- 

time of the year

 (Table 10).

e) Net radiation (Rn).

e) Net radiation (Rn).

•

Part of 

Ra is absorbed and scattered

 when

passing through the atmosphere the

remainder, including some that is scattered

but reaches the earth’s surface is called the

solar radiation (Rs).

•

Rs is dependent on Ra 

and the transmission

through the atmosphere that is 

dependent on

cloud cover.

e) Net radiation (Rn).

e) Net radiation (Rn).

•

Part of Rs is reflected back directly by the soil

and crop and is lost to the atmosphere.

•

Reflection (

α

) depends

 on the nature of the

surface cover

 and is 

approximately 5 to 7%

for water 

and around 

15 to 25% for most

crops.

•

(i.e. it depends on crop cover and wetness of

the exposed soil surface).

•

That, which remains is 

net short-wave solar

radiation (Rns).

e) Net radiation (Rn).

e) Net radiation (Rn).

•

Additional loss at the earth’s surface occurs

since the 

earth radiates part of its absorbed

energy

 back 

through the atmosphere

 as 

long

wave radiation.

•

This is normally greater than the down

coming long wave atmospheric radiation.

e) Net radiation (Rn).

e) Net radiation (Rn).

•

The difference 

between out going

 and 

in coming

long wave radiation

 is called 

net long wave

radiation (R

n

 ℓ  ).

•

Since outgoing is greater than incoming, Rn 

represents net energy loss.

•

Total net radiation (R

n

 ) = R

ns

 – R

n

ɭ 

.

•

Radiation can be expressed in different units.

•

It can be given as the energy required to evaporate

water from an open surface and is given here as

equivalent evaporation in mm/day.

e) Net radiation (Rn).

e) Net radiation (Rn).

To calculate Rn the steps are

i) If measured Rn is not available, select Ra value in

mm/day from Table 10 for given month and latitude.

ii) To obtain Rs , correct Ra value for n/N

iii) For most crops 

α

 = 0.25 Table 12 can be used to

calculate Ras from the ratio n/N and 

α 

 = 0.25.

iv) Not long wave radiation (R

n

ɘ) can be determined

from T, ed and n/N. Values for the function f (T),

f(ed) and f(n/N) are given in Tables 13, 14, and 15

respectively.

•

v) To obtain total net radiation (Rn), the algebraic

sum of Rns and Rnl is calculated.

•

Rnl always constitutes a net loss so Rn = Rns - Rnl.

f) Adjustment factor (C)

f) Adjustment factor (C)

•

The Penman equation given 

assumes

 the most

common conditions where

- 

radiation is medium to high

- 

RH max is medium to high

- Moderate daytime wind about double the night time

wined.

•

However, these conditions are not always met.

•

For other conditions the penman equation should

be corrected (Table 16 for values of C depending

on RHmax , Rs , U day and U day / U night )

Example on modified penman and Radiation

on your lecture Note

Assignment

I

I

r

r

r

r

i

i

g

g

a

a

t

t

i

i

o

o

n

n

E

E

f

f

f

f

i

i

c

c

i

i

e

e

n

n

c

c

i

i

e

e

s

s

•

i

)

W

a

t

e

r

s

t

o

r

a

g

e

e

f

f

i

c

i

e

n

c

y

•

is the 

amount of water actually stored

 in the

subject area expressed as a percentage of

the 

volume of water that can be stored.

•

The general form of the Es equation is given

as follows.

Where Z = amount infiltrated (m

3

 . m

-1

 )

L = channel length (m)

Lov = length of that part of the channel that received

an amount of 

water equal to or in excess of the

perceived requirements (m)

Zr = required amount of application 

(perceived

requirements ) (m

3

 . m

-1

 )

i

i

i

i

-

-

W

W

a

a

t

t

e

e

r

r

D

D

i

i

s

s

t

t

r

r

i

i

b

b

u

u

t

t

i

i

o

o

n

n

E

E

f

f

f

f

i

i

c

c

i

i

e

e

n

n

c

c

y

y

(

(

E

E

d

d

)

)

•

This shows 

how uniformly water is applied to

the field along the irrigation run.

•

In 

sandy soils 

there is generally

 over irrigation

at upper reaches of the run

 where as in

clayey soils, 

there is 

over- irrigation at the

lower reaches 

of the run.

i

i

i

i

i

i

-

-

F

F

i

i

e

e

l

l

d

d

C

C

a

a

n

n

a

a

l

l

E

E

f

f

f

f

i

i

c

c

i

i

e

e

n

n

c

c

y

y

(

(

E

E

f

f

)

)

•

This is a measure of the efficiency with which

the water is conveyed through 

the field

channels until it feeds the plots

i

i

v

v

-

-

W

W

a

a

t

t

e

e

r

r

U

U

s

s

e

e

E

E

f

f

f

f

i

i

c

c

i

i

e

e

n

n

c

c

y

y

•

This shows the 

yield of the crop per unit

volume of water used.

•

It may be expressed in 

Kg/ha.cm or q/ha.cm

•

A

.

C

r

o

p

W

a

t

e

r

U

s

e

E

f

f

i

c

i

e

n

c

y

:

i

s

t

h

e

r

a

t

i

o

o

f

t

h

e

c

r

o

p

y

i

e

l

d

(

Y

)

t

o

t

h

e

a

m

o

u

n

t

o

f

w

a

t

e

r

c

o

n

s

u

m

p

t

i

v

e

l

y

u

s

e

d

b

y

t

h

e

c

r

o

p

.

v

v

-

-

P

P

r

r

o

o

j

j

e

e

c

c

t

t

E

E

f

f

f

f

i

i

c

c

i

i

e

e

n

n

c

c

y

y

(

(

E

E

p

p

)

)

•

This shows how efficiently the water source

used in crop production.

•

It shows the percentage of the total water that

is stored in the soil and available for

consumptive requirements of the crop.

•

It 

indicates the overall efficiency

 of the

systems

 from the 

head work to the final use

by plants for Cu.

•

It is given as

•

Ep = Ec * Eb * Ea

I

I

R

R

R

R

I

I

G

G

A

A

T

T

I

I

O

O

N

N

S

S

C

C

H

H

E

E

D

D

U

U

L

L

I

I

N

N

G

G

•

Scheduling of irrigation application is very important

for successive plant growth and maturity.

•

Water is not applied randomly at any time and in any

quantity.

•

Irrigation scheduling is the schedule in which water is

applied to the field.

•

If in an important aspect of an efficient operation of an

irrigation system.

•

The scheduling of irrigation can be field irrigation

scheduling and field irrigation supply schedules.

•

Field irrigation Scheduling is done at field level.

•

The 

two scheduling parameters

 of field irrigation

scheduling are the 

depth of irrigation

 and 

interval of

irrigation.

1

1

.

.

D

D

e

e

p

p

t

t

h

h

o

o

f

f

i

i

r

r

r

r

i

i

g

g

a

a

t

t

i

i

o

o

n

n

(

(

d

d

)

)

•

This is the 

depth of irrigation water that is to be

applied at one irrigation.

•

It is the depth of water that can be retained in the

crop root zone b/n the field capacity and the given

depletion of the available moisture content.

•

All the water retained in the soil b/n 

FC and PWP

 is

not readily available to crops.

•

The 

readily available moisture is only some

percentage of the total available moisture.

•

Thus, depth of irrigation is the readily available

portion of the soil moisture.

•

it is the 

depth of irrigation water required to

replenish the soil moisture to field capacity.

•

D

e

p

t

h

o

f

i

r

r

i

g

a

t

i

o

n

(

d

)

i

s

g

i

v

e

n

b

y

•

d(net) = As * D (FC – PWP) * P , m

•

As = Apparent specific gravity of soil

•

D = Effective root zone depth in m

•

FC = water content of soil at F.C

•

PWP = Water content of soil at PWP

•

P = depletion factor

•

Because of 

application losses

 such as

 deep percolation

and 

runoff losses

, the total depth of water to be applied

will be greater than the net depth of water.

•

G

r

o

s

s

d

e

p

t

h

o

f

a

p

p

l

i

c

a

t

i

o

n

2

2

.

.

I

I

n

n

t

t

e

e

r

r

v

v

a

a

l

l

o

o

f

f

I

I

r

r

r

r

i

i

g

g

a

a

t

t

i

i

o

o

n

n

(

(

i

i

)

)

•

The 

interval of irrigation

 is the 

time gap in

days b/n two successive irrigation

applications.

•

It depends on the type of the crop, soil type

and climate conditions.

•

Thus interval of irrigation depends on the

consumptive use rate of the crop and the

amount of readily available moisture in the

crop root zone.

•

The consumptive use rate of the crop varies

from crop to crop and also

•

during different stages of the crop.

•

The RAM moisture also varies from soil to soil

depending on soil water constants.

•

The interval (frequency) of irrigation is given by :

F

F

i

i

e

e

l

l

d

d

I

I

r

r

r

r

i

i

g

g

a

a

t

t

i

i

o

o

n

n

S

S

u

u

p

p

p

p

l

l

y

y

S

S

c

c

h

h

e

e

d

d

u

u

l

l

e

e

s

s

(

(

I

I

r

r

r

r

i

i

g

g

a

a

t

t

i

i

o

o

n

n

S

S

c

c

h

h

e

e

d

d

u

u

l

l

i

i

n

n

g

g

i

i

n

n

a

a

C

C

o

o

m

m

m

m

a

a

n

n

d

d

A

A

r

r

e

e

a

a

)

)

•

This is the 

schedule of water supply to

individual fields or command area.

•

is a schedule of the total volume of water to

be applied to the soil during irrigation.

•

It depends on crop and soil characteristics.

•