CHAPTER THREE
Crop Water Requirements
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Crop Water Requirements
It is the total amount of water required by the crop in a given
period of time for normal growth, under field conditions.
It includes evapotranspiration, water used by crops for metabolic
growth, water lost during application of water and the water
required for special operations such as land preparation, tillage
and salt leaching etc.
it is expressed as the surface depth of water in mm, cm or inches
per unit cropped area.
2
Crop period and Base period
The time period that elapses from the instant of its sowing to the
instant of its harvesting is called the crop period.
The time between the first watering of a crop at the time of its
sowing to its last watering before harvesting is called the base
period.
 Duty and Delta of Crops
Duty (D): is defined as the number of hectares of land irrigated
for full growth of a given crop supply of one (1 m3/sec) of
water continuously during the entire base period of that crop.
It is expressed in hectares / cumecs.
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Cont…..
 Delta (∆) ; is the total depth of water required by a
crop during the entire base period.
∆ = (Total quantity of water in (ha-m)/Total area of
land in (ha))
 The relation between duty, base period and delta,
can be obtained as follows:Let there be a crop base period of B days, let one
cumec of water be applied to this crop on the field for B
days.
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Now, the volume of water applied to this crop during B days
•V= (1*B*24*60*60*, m3),V=86400B cubic meter
 Different forms of Duty
1. Flow duty: the duty of water in hectares /cumec is convenient in the case of
flow,irrigation from canals and duty and the area of land to be irrigated are
known, therequired discharge in the canal can be determined.
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2. Quantity of Duty: For Tank /pond irrigation, the duty is usually
expressed as the totalarea of land which can be irrigated per million
m3 of water stored in the tank. If the duty and the area to be irrigated
are known, the volume of water to be stored in the tank can be
determined.
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3. Duty in the form of total depth (Delta): It can be expressed in
terms of the total depth(i.e. delta) of water required for a crop.
It is another form of the quantity duty because thetotal depth is
equal to the volume divided by the area of land.
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Factors affecting Duty
 Duty of water depends upon different factors. Some of the
factors are listed below.
 Type of soil: - if the permeability of the soil under the irrigated
crop is high, the water lost due to percolation will be more and
hence the duty will be less.
 Type of crop and base period: - different types of crops
require different amount of water and hence their duties for
them are different.
 Climatic and season:-duty incudes he water lost in
evaporation and percolation, the losses will vary with season
and climate condition. Hence duties vary from season to
season and from time to time in the same season.
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 Method of cultivation:-If the cultivation method (including
tillage and irrigation) is faulty and less efficient resulting in the
wastage of water, the duty of water will be less. If the
irrigation of water is used economically then the duty of water
will improve.
 Useful rainfall:- if some rain falling directly over the irrigated
land is useful for the growth of the crop, then so much les
irrigation water will be required to mature the crop. And hence
more will be the duty.
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Importance of duty
It helps us in designing an efficient canal irrigation system.
Knowing the total available water at the head of main canal,
and the overall duty for all the crops to be irrigated in different
season of the year the area which can be irrigated can be
worked out.
Measures for improving duty of water
The duty of canal water can certainly be improved by effecting
economy in the use of water by resorting to the following
precaution and practices.
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 Evapotranspiration Process
The combination of two separate processes whereby water is lost
on the one hand from the soil surface by evaporation and on the
other hand from the crop by transpiration are referred to as
evapotranspiration (ET).
 Evaporation: - Evaporation is the process whereby liquid
water is converted to water vapor (vaporization) and removed
from the evaporating surface (vapor removal).
 Transpiration:-Transpiration consists of the vaporization of
liquid water contained in plant tissues and the vapor removal
to the atmosphere.
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Factors affecting evapotranspiration
• Weather parameters
The principal weather parameters affecting evapotranspiration are
radiation,temperature,humidity and wind speed. Several
procedures have been developed to assess the evaporation rate
from these parameters.
• Crop factors
The crop type, variety and development stage should be considered
when assessing the evapotranspiration from crops grown in large,
well-managed fields. Differences in resistance to transpiration, crop
height, crop roughness, reflection, ground cover and crop rooting
characteristics result in different ET levels in different types of crops
under identical environmental conditions.
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 Determination of Reference Crop Evapotranspiration (ETo)
The evapotranspiration rate from a reference surface, not short of
water, is called the reference crop evapotranspiration or reference
evapotranspiration and is denoted as Eto.
 Evapotranspiration is not easy to measure. Specific devices
and accurate measurements of various physical parameters or
the soil water balance in lysimeters are required to determine
evapotranspiration.
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Direct Measurement includes:
A.
Lysimeter experiment
B.
Field experimental plots
C.
Soil moisture studies
D.
Water balance method
A. Lysimeter experiment: By isolating the crop root zone from its environment
and controlling the processes that are difficult to measure, the different terms
in the soil water balance equation can be determined with greater accuracy.
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B. Field experimental plots
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.
C. Soil moisture studies
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.
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D Water balance method
Evapotranspiration can also be determined by measuring the
various components of the soil water balance. The method
consists of assessing the incoming and outgoing water flux
into the crop root zone over some time period.
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 The following methods are the combination of some empirical,
analytical and theoretical approach.
 FAO Balnney-Criddle Method
 FAO Radiation Method
 FAO Penman Method
 Hargreaves’s Class A Pan Evaporation Method
 FAO Pan Evaporation Method
 FAO Penman-Monteith Method
 Thornthwaite Method
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A. FAO Balney-Criddle Method
Blaney and Criddle (1962) proposed an empirical relation which
is used largely by irrigation engineers to calculate crop water
requirement of various crops. Estimation of potential
evapotranspiration (consumptive use) is carried out by
correlating it with sunshine temperature.
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Here K is the monthly crop coefficient to be determined from
experimental data, F the monthly consumptive use factor, ETo
the potential evapotranspiration in cm; Tm the mean monthly
temperature in 0C, P is the monthly percentage of hours of bright
sunshine in the year. i.e is extensively used for seasonal water
requirement.
•EAMPLE
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B Hargrave's Class A Pan Evaporation Method
In this method evapotranspiration (consumptive use) is related to
pan evaporation (EP) by a constant Kc, called consumptive use
coefficient.
ETo or Cu = Kc * Ep
Consumptive use coefficient Kc is different for different crops
and different for the same crops at different places. It also
varies with crop growth, and is different at different crop
stages for the same crops.
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 Ep can be determined experimentally by directly measuring
the quantity of water evaporated from the standard class A pan.
 Ep = 0.459R * Ct*Cw*Ch*Cs *Ce
Where R= extra-terrestrial radiation in the same unit as Ep in cm
or mm
Ct = Coefficient for temperature, is given by
Ct = 0.393 +0.02796Tc+0.0001189Tc2
Tc= mean temperature, 0c
Cw = Coefficient for wind velocity, is given by
Cw= 0.708+0.0034w-0.0000038w2
w=mean wind velocity at 0.5m above the ground, km/day.
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Ch= Coefficient for relative humidity, is given by
Ch= 1.250-0.0087H-0.75*104H2 –0.85*10-8H4
H= mean percentage relative humidity at noon
Cs= Coefficient for percent of possible sunshine, is given by
Cs= 0.542+0.008S-0.78*10-4S2+0.62*10-6S3
S= mean sunshine percentage
Ce= Coefficient of elevation, is given by
Ce= 0.97+ 0.00984E
E= elevation in 100 meters
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C. Thorn Thwaite Method
Thorn Thwaite (1948) developed an exponential relationship between
mean monthly temperature and mean monthly consumptive, given as.
Where Rf is the reduction factor, Tm the mean monthly temperature in
0C, a is a constant which can be computed from the relation.
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4 Penman method
Developed the formula using important climatic parameters such
as solar radiation, temperature, vapour pressure and wind
velocity to compute the evaporation from open free water
surface
o Determination of Crop Evapotranspiration (ETc) Under
Standard Condition
This part examines crop evapotranspiration under standard
conditions (ETc). This is the evapotranspiration from diseasefree, well-fertilized crops, grown in large fields, under
optimum soil water conditions and achieving full production
under the given climatic conditions.
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Calculation procedure by the crop coefficient approach:
In the crop coefficient approach the crop evapotranspiration, ETc,
is
calculated
by
evapotranspiration,
multiplying
ETo,
by
a
the
reference
crop
crop
coefficient,
Kc:
ETc = Kc * ETo
Where: ETc crop evapotranspiration [mm d-1],Kc crop
coefficient [dimensionless],
ETo reference crop evapotranspiration [mm d-1]
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 Most of the effects of the varios weather conditions are
incorporated into the ETo estimate.
Therefore, as ETo represents an index of climatic demand, Kc
varies predominately with the specific crop characteristics and
only to a limited extent with climate. The crop coefficient, Kc,
is basically the ratio of the crop ETc to the reference ETo, and
it represents an integration of the effects of four primary
characteristics that distinguish the crop from reference grass.
These characteristics are:
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 Crop height
 Albedo (reflectance) of the crop-soil surface
 Canopy resistance
 Evaporation from soil
 Factors determining the crop coefficient:
 Crop type
The crop coefficient integrates the effect of characteristics that
distinguish a typical field cropfrom the grass reference, which has
a constant appearance and a complete ground cover.
Consequently, different crops will have different Kc coefficients.
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 Climate
 The effect of the difference in aerodynamic properties
between the grass reference surface and agricultural crops is
not only crop specific.
 More arid climates and conditions of greater windspeed will
have higher values for Kc. More humid climates and
conditions of lower wind speed will have lower values for Kc.
 Crop growth stages
As the crop develops, the ground cover, crop height and the leaf
area change. Due to differences in evapotranspiration during
the various growth stages, the Kc for a given crop will vary
over the growing period.
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The growing period can be divided into four distinct growth stages:
Initial, crop development, mid-season and late season.
1. Initial stage:- The initial stage runs from planting date to
approximately 10% ground cover.
2. Crop development stage:- The crop development stage runs
from 10% ground cover to effective full cover. Effective full cover
for many crops occurs at the initiation of flowering.
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3. Mid-season stage:- The mid-season stage runs
from effective full cover to the start of maturity.
The start of maturity is often indicated by the
beginning of the ageing,
4. Late season stage:- The late season stage runs
from the start of maturity to harvest or full
senescence.
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Irrigation requirement of Crops
The 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.
Effective Rainfall (Peff)
Effective rainfall can be defined as the rainfall that is stored in the
root zone and can be utilized by crops.
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 There are four methods for calculating the effective rainfall
from entered monthly total rainfall data.
1. Fixed Percentage Effective Rainfall
2. Dependable Rain
3. Empirical Formula for Effective Rainfall
4. Method of USDA Soil Conservation Service (default)
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Irrigation Efficiencies
 Efficiency is the ratio of the water output to the water input,
and is usually expressed as percentage. Input minus output is
the losses, and hence the, if losses are more output is less and,
therefore efficiency is less. Hence efficiency is inversely
proportional to the losses. Water is lost during various
processes and, therefore there are different kinds of
efficiencies, as given below.
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1. Water Conveyance efficiency ( Ec)
 This term is used to measure the efficiency of water
conveyance system associated with the canal network, water
courses and field channels. It is also applicable where the
water is conveyed in channels from the well to the individual
fields.
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2. Water application Efficiency (Ea)
o
After the water reaches the field supply Channel, it is
important to apply the water as efficiently as possible. A
measure of how efficiently this is done is the water application
efficiency.
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3. Water storage efficiency (Es)
Small irrigation may lead to high water application efficiencies,
yet the irrigation practice maybe poor. The concept of water
storage efficiency is useful in evaluating this problem. This
concept relates how completely the water needed prior to
irrigation has been stored in the root zone during irrigation.
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4. Water use efficiency (Eb)
Water use efficiency is the ratio of the water beneficially used,
including leaching water to the quantity of water delivered.
Where Eb= Field Canal Efficiency
Wp = water received at the field inlet
Wf = water delivered to the field channel
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5. Water Distribution Efficiency (Ed)
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 when as in clayey soils, there is overirrigation at the lower reaches of the run.
Where Ed = water distribution efficiency, %
d = average depth of water penetration.
D = average deviation from d.
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Irrigation Scheduling
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.
The scheduling of irrigation can be field irrigation scheduling and
field irrigation supply schedules.
1. Field irrigation scheduling:
This scheduling of irrigation is done at field level.
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The two scheduling parameters of field irrigation scheduling are
the depth of irrigation and interval of irrigation.
I. Depth of irrigation (d):
This is the depth of irrigation water that is to be applied at once
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.
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The depth of irrigation (d) is given by :
d (net) = Gs *D *(FC – PWP)*P, m
Where Gs = Apparent specific gravity of soil
D = Effective root zone depth in m
Fc = water content of soil at FC
PWP = Water content of soil at PWP
P= depletion factor
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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.
Where Ea = Field application efficiency and other parameters are
as defined above
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II. Interval of irrigation (i):
The interval of irrigation is the time gap in days between two
successive irrigation applications. It depends on the type of the
crop, soil type and climate conditions.
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2.Field Irrigation Supply Schedules (Irrigation Scheduling in
a Command Area)
 This is the schedule of water supply to individual fields or
command area. This is a schedule of the total volume of water
to be applied to the soil during irrigation. It depends on crop
and soil characteristics.
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Where: q= Stream size (application rate) lit/sec
t = Application time in sec
Ea = Application efficiency
As = Apparent specific gravity
D = Effective root zone depth, m
P = Depletion factor
A = Area of the command (field) in ha
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