Department of Civil Engineering

University of Engineering and Technology Peshawar

CE-402: Irrigation Engineering and

Water Management

Lecture 3
Crop water requirements and its
measurement

8th Semester (4th Year)

Civil Engineering
Spring 2022

Lecturer: Alamgir Khalil

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Water Requirements of Crops

➢ The term ‘Water requirements of crops’ means the total quantity and the
way in which a crop requires water from the time it is sown to the time it is
harvested.
➢ Different crops will have different water requirements, and the same crop
may have different water requirements at different places of the same
country depending upon the variations in climate, type of soils, methods of
cultivation, and useful rainfalls etc.

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Irrigation requirements of certain crops

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Irrigation requirements of certain crops (cont.)

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Crop Period or 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.

✓ Crop period is slightly more than the base period, but for all
practical purposes, they are taken as one and the same thing,
and generally expressed in B days.

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Delta of a Crop

✓ Each crop requires a certain amount of water after a certain

fixed interval of time, throughout its period of growth.

✓ The depth of water required every time, generally varies from 5

to 10 cm depending upon the type of the crop, climate and soil.

✓ The time interval between two consecutive watering is called

the frequency of irrigation or rotation period.

✓ This total depth of water (in cm) required by a crop to come to

maturity is called its delta (Δ).

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Example

If rice requires about 10 cm depth of water at an average interval of about

10 days and the crop period for rice is 120 days, find out the delta for rice.

Solution

Water is required at an interval of 10 days for a period of 120 days.

Hence, No. of required waterings = 120/10 = 12

Therefore, Total depth of water required = No. of waterings x Depth of watering

= 12 x 10 cm = 120 cm

Hence, delta (Δ) for rice = 120 cm

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Average Approximate Values of Δ for Certain Important Crops

S.No Crop Delta on field

1 Sugarcane 120 cm (48”)
2 Rice 120 cm (48”)
3 Garden fruits 60 cm (24”)
4 Cotton 50 cm (22”)
5 Vegetables 45 cm (18”)
6 Wheat 40 cm (16”)
7 Maize 25 cm (10”)
8 Fodder 22.5 cm (9”)
9 Peas 15 cm (6”)

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Important Units

1 hectare = 104 m2
1 acre = 43,560 ft2
1 acre-foot = 43,560 ft3
1 acre = 0.4047 hectare
1 acre/cusec = 14.3 hectare/cumec

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Duty of Water

✓ The duty of water is the relationship between the volume of

water and the area of the crop it matures.

✓ It may be defined as “the number of hectares of land irrigated

for full growth of a given crop by supply of 1 m3/s of water
continuously during the entire base period (B) of that crop”.

✓ If water flowing at a rate of unit cumecs, runs continuously for B

days, and matures 200 hectares, then duty of water for that crop
will be defined as 200 hectares/cumec.

✓ The duty is represented by D.

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Relation between Duty, Delta and Base period

Let, base period of the crop be B days, and one cumec of water be
applied to this crop on the field for B days.

Now, volume of water applied to this crop during B days

= V = (1 x 60 x 60 x 24 x B) m3
= 86,400 B m3

By definition of duty (D), one cubic meter supplied for B days matures D
hectares of land.

:. This quantity of water (V) matures D hectares of land or 104 D m2 of

area.

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Relation between Duty, Delta and Base period (cont.)

Total depth of water applied on this land

= Volume/area = 86400 B / 104 D = 8.64 B / D meters

By definition, this total depth of water is called delta (Δ),

𝐵
∆ = 8.64 (m)
𝐷

𝐵
∆ = 864 (cm)
𝐷

where, Δ is in cm, B is in days, and D is duty in hectares/cumec

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Relation between Duty, Delta and Base period (cont.)

In FPS system

𝐵 𝐵
∆ = 1.985 ≈2 (ft)
𝐷 𝐷

where
Δ is in ft
B is in days
D is duty in acres/cusec

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Factors affecting Duty

Type Coarse
Percolation
losses are Low
of soil grained soil duty
high

Fine grained Percolation High

soil losses are less duty

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Factors affecting Duty (cont.)

Type
of crop

Large quantity of
Crop A Low duty
water

Less quantity of
Crop B High duty
water

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Factors affecting Duty (cont.)

Structure
of Soil
Good structure is called Good Tilth of Soil

Evaporation
Good structure High duty
losses are less

Properly aerated Yield of

Good structure because of large crop
voids increases

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Factors affecting Duty (cont.)

Slope of d/s portion

u/s portion Low
Steep slope get more
Ground water
remain drier duty

Equal
Properly
distribution of High duty
prepared field
water

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Factors affecting Duty (cont.)

Climatic
Conditions

Temperature and
more evaporation
wind velocity are Low duty
losses
high

Rainfall during base Less irrigation

requirement High duty
period

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Factors affecting Duty (cont.)

Method of
Cultivation

Not properly ploughed &

method of cultivation is Low duty
faulty & less efficient

number of
Properly ploughed & water retention
watering High duty
made quite loose capacity increase
reduced

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Factors affecting Duty (cont.)

System of
Only during wasteful use of
Irrigation Non-perennial
flood season water Low duty

Water
application soil remains
less quantity High
Perennial throughout continuously
of water duty
the growth wet
period
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Factors affecting Duty (cont.)

Application Very high

Drip Irrigation Most efficient
of Water duty

Sprinkler Seepage losses

Irrigation are reduced High duty

More waste of
Surface irrigation Low duty
water

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Consumptive Use or Evapotranspiration

➢ Consumptive use may be defined as “the total amount of water used

by the plant in transpiration (building of plant tissues etc.) and
evaporation from adjacent soil or from plant leaves in any specified
time”.
➢ The values of consumptive use may be different for different crops
and may be different for the same crop at different times and places.
➢ Factors affecting consumptive use
✓ Temperature ✓ Soil topography
✓ Sunlight ✓ Precipitation
✓ Humidity ✓ Method of irrigation
✓ Wind Movement ✓ Depth of water applied for irrigation

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Effective Rainfall (Re)

➢ Precipitation falling during the growth period of a crop that is available

to meet the evapotranspiration needs of the crop is called effective
rainfall.
✓ It does not include precipitation lost through deep percolation below
the root zone or the water lost as surface runoff.

Consumptive Irrigation Requirement (CIR)

➢ It is the amount of irrigation water required in order to meet the
evapotranspiration needs of the crop during its full growth. It is,
therefore, consumptive use exclusive of effective precipitation, stored soil
moisture, or groundwater.
𝐶𝐼𝑅 = 𝐶𝑢 − 𝑅𝑒
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Net Irrigation Requirement (NIR)

➢ It is the amount of irrigation water required in order to meet the

evapotranspiration needs of the crop as well as other needs such
as leaching of salts.

NIR = Cu – Re + water lost in deep percolation for leaching

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Field Irrigation Requirement (FIR)

➢ It is the amount of water required to meet net irrigation requirements plus

the water lost in percolation in the field water courses, field channels and
in the field application of water. It is given by

𝑁𝐼𝑅
𝐹𝐼𝑅 = Where 𝜂𝑎 is the water application efficiency
𝜂𝑎

Gross Irrigation Requirement (GIR)

➢ It is the amount of water required to satisfy the field irrigation requirement
and the water lost as conveyance losses in distributions up to the field.

𝐹𝐼𝑅
𝐺𝐼𝑅 = Where 𝜂𝑐 is the water conveyance efficiency
𝜂𝑐
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Potential Evapotranspiration (PET) & Actual Evapotranspiration (AET)

➢ We know that evapotranspiration

is the total loss of water from
farmland as evaporation and from
plants grown on it as transpiration.
If sufficient moisture is always
available to completely meet the
needs of the plants, the resulting
evapotranspiration is called
potential evapotranspiration (PET).

➢ The real evapotranspiration

occurring in a specific situation is
called actual evapotranspiration
(AET).

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Reference Evapotranspiration (ETo) & Crop Evapotranspiration (ETC)

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Crop Coefficient (KC)

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Estimation of Consumptive Use

A) Direct measurement of Consumptive Use

1) Tank and Lysimeters method

2) Field experimental plots
3) Soil moisture studies
4) Integration method
5) Inflow and Outflow studies for large areas

B) Consumptive Use determination by use of Equations

1) Blaney-Criddle Equation
2) Hargreaves class A pan evaporation
3) Penman’s equation

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A) Direct measurement of Consumptive Use

1) Tank and Lysimeters method

➢ Tanks are containers set flush with the ground level having area of 10
m square and 3 m deep. Larger the size of the tank greater is the
resemblance to root development.

➢ Consumptive use is determined by measuring the quantity of water

required to maintain constant moisture conditions within the tank for
satisfactory proper growth.

➢ In Lysimeters, the bottom is pervious. Consumptive use is the

difference of water applied and that draining through pervious bottom
and collected in a pan.
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A) Direct measurement of Consumptive Use (cont.)

1) Tank and Lysimeters method

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A) Direct measurement of Consumptive Use (cont.)

1) Lysimeters method

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A) Direct measurement of Consumptive Use (cont.)

2) Field experimental plots

➢ In this method, irrigation water is applied to the selected field

experimental plots in such a way that there is neither runoff nor deep
percolation.
➢ Yield obtained from different fields are plotted against the total water
used and as basis for arriving at the consumptive use, those yields are
selected which appear to be most profitable.
➢ It is seen from observations that for every type of crop, the yield
increases rapidly with an increase of water used to a certain point and
then decrease with further increase in water. At the ‘break in the
curve’, the amount of water used is considered as the consumptive
use.
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A) Direct measurement of Consumptive Use (cont.)

3) Soil moisture studies

➢ This method is especially suitable to those areas where soil is fairly

uniform and groundwater is deep enough so that it does not affect
the fluctuations in soil moisture within the root zone of the soil.
➢ Soil moisture measurements are done before and after each
irrigation.
➢ The quantity of water extracted per day from soil is computed for
each period.
➢ A curve is drawn by plotting the rate of use against time and from this
curve, seasonal use can be determined.

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A) Direct measurement of Consumptive Use (cont.)

3) Soil moisture studies

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A) Direct measurement of Consumptive Use (cont.)

4) Integration method

➢ In this method, it is necessary to know the division of total area under

irrigation crops, natural vegetation, water surface area and baren land
area.

➢ The integration method is summation of the products of

a) Unit consumptive use for each crop times its area
b) Unit consumptive use of native vegetation times its area
c) Water surface evaporation times the water surface area, and
d) Evaporation from the bare land times its area.

➢ Thus, in this method, annual consumptive use for the whole of the
area is found, in acre-feet or hectare-meter units.
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A) Direct measurement of Consumptive Use (cont.)

5) Inflow-outflow Studies for Large Areas

➢ In this method also, annual consumptive use is found for large areas. If
U is the valley consumptive use, its value is given by;

𝑈 = 𝐼 + 𝑃 + 𝐺𝑠 − 𝐺𝑒 − 𝑅 All measurements in acre-feet

or hectare-meter

Where
𝑈 = Valley consumptive use (in acre-feet or hectare-meter)
𝐼 = Total inflow during 12-months year
𝑃 = Yearly precipitation on valley floor
𝐺𝑠 = Ground storage at the beginning of the year
𝐺𝑒 = Ground storage at the end of the year
𝑅 = Yearly outflow 37
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B) Consumptive Use determination by use of Equations

1) Blaney-Criddle Equation

➢ It states that monthly consumptive use is given by

𝑘∙𝑝
𝐶𝑢 = 1.8𝑡 + 32
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Where
𝐶𝑢 = Monthly consumptive use in cm
𝑘 = Crop factor determined by experiments for each crop under
the environmental conditions of the area
𝑡 = Mean monthly temperature in oC
𝑝 = Monthly percent of annual day light hours that occur during
the period
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Blaney-Criddle Equation (cont.)

𝑝
Let 𝑓= 1.8𝑡 + 32 then 𝐶𝑢 = 𝑘 ∙ 𝑓
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It was found that 𝑘 was too low for the short periods between irrigations. So,
the formula was modified as;

𝐶𝑢 = 𝑘 ෍ 𝑓

Where
𝐶𝑢 = Seasonal consumptive use i.e. consumptive use during the period of
growth for a given crop in a given area.

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Blaney-Criddle Equation (cont.)

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Example 2.9 (Garg)

Wheat is to be grown at a certain place, the useful climatological conditions of

which are tabulated below in Table. Determine the evapotranspiration and
consumptive irrigation requirement of wheat crop. Also, determine the field
irrigation requirement if the water application efficiency is 80%. Make use of
Blaney-Criddle equation.
Month Monthly temperature Monthly percent of Useful rainfall in
in oC averaged over daytime hr. of the year cm, averaged
the last 5 years computed from sun-shine over the last 5
Tables years
November 18 7.20 1.7
December 15 7.15 1.42
January 13.5 7.30 3.01
February 14.5 7.10 2.25

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2) Hargreaves class A pan evaporation

➢ In this method evapotranspiration (consumptive use) is related to pan

evaporation by a constant K.

𝐸𝑡 𝑜𝑟 𝐶𝑢 = 𝐾𝐸𝑃
Where
𝐸𝑡 = Evapotranspiration or 𝐶𝑢 = Consumptive use
𝐸𝑃 = Pan evaporation
𝐾 = Consumptive use coefficient

✓ Consumptive use coefficient is different for different crops and is different for the
same crop at different places. It also varies with the crop growth and is different
at different crop stages for the same crop.

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2) Hargreaves class A pan evaporation (cont.)

In the absence of local figures for specific crops, the data given in Table 3.13,
recommended by Hargreaves, can be used. In the table various crops have been
divided into the following groups;

Group A Potato, Cotton, Maize, Bean, Peas, Jowar, Beat

Group B Tomato, Olive, Plumes, and some delicious fruits
Group C Onions, Grapes, Melons, Carrots, Hops
Group D Wheat, Barley, Celery and other grass type plants
Group E Pasters, Plantain, Orchard crops etc.
Group F Oranges, Fruits, Citrus crops
Group G Sugarcane, Alfalfa
Paddy (Rice)

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Class A pan evaporation measurement

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Class A pan evaporation measurement

✓ 𝐸𝑃 can be experimentally determined by directly measuring the quantity of

water evaporated from the standard class A pan.
✓ This pan is 1.2 m in diameter, 25 cm deep, and bottom is raised 15 cm above
the ground surface.
✓ The depth of water is to be kept in a fixed range such that the water surface
is at least 5 cm, and never more than 7.5 cm, below the top of the pan.

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Class A pan evaporation measurement (cont.)

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Class A pan evaporation measurement (cont.)

➢ The pan evaporation 𝐸𝑃 can also be determined by using the

Christiansen formula

𝐸𝑃 = 0.459 𝑅 ∙ 𝐶𝑡 ∙ 𝐶𝑤 ∙ 𝐶ℎ ∙ 𝐶𝑠 ∙ 𝐶𝑒
Where
𝐸𝑃 = Pan evaporation
𝑅 = Extra-terrestrial radiation in the same units as 𝐸𝑃 in cm or mm
𝐶𝑡 = Coefficient of temperature
= 0.393 + 0.02796𝑇𝑐 + 0.0001189𝑇𝑐2
where 𝑇𝑐 is the mean temperature in oC
𝐶𝑤 = Coefficient of wind velocity
= 0.708 + 0.0034𝑊 − 0.0000038𝑊 2
where W is mean wind velocity at 0.5 m above the ground in km/day

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Class A pan evaporation measurement (cont.)

𝐸𝑃 = 0.459 𝑅 ∙ 𝐶𝑡 ∙ 𝐶𝑤 ∙ 𝐶ℎ ∙ 𝐶𝑠 ∙ 𝐶𝑒

Where,
𝐶ℎ = Coefficient of relative humidity
= 1.250 − 0.0087𝐻 + 0.75 × 104 𝐻 2 − 0.85 × 10−8 𝐻 4
where H is mean percentage relative humidity at noon or
average relative humidity for 11 and 18 hours.
𝐶𝑠 = Coefficient of percent of possible sunshine
= 0.542 + 0.008𝑆 − 0.78 × 10−4 𝑆 2 + 0.62 × 10−6 𝑆 3
where S is the mean sunshine percentage
𝐶𝑒 = Coefficient of elevation
= 0.97 + 0.00984𝐸
where E is the elevation in 100 meters.
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3) Penman Equation

➢ Penman developed a theoretical formula based on principles of both

energy budget and mass-transfer approaches to compute potential
evapotranspiration in the following form;

𝐴 ∙ 𝐻 + 𝛼𝐸𝑎
𝐸𝑡 = Penman
𝐴+𝛼
(1909-1984)
Where,
𝐸𝑡 = Evapotranspiration, mm/day
𝛼 = psychromatic constant = 0.49 mm Hg/oC
𝐴 = Slope of the curve between saturated vapor pressure and temperature
at mean air temperature
𝐸𝑎 = drying power of air which includes wind velocity and saturation deficit
and is given by
𝐸𝑎 = 0.002187(160 + 𝑢2 )(𝑒𝑠 − 𝑒𝑎 )
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Penman Equation (cont.)

𝑢2 = mean wind speed in km/day measured 2 m above the ground

𝑒𝑠 = Saturation vapor pressure at mean air temperature, in mm
𝑒𝑎 = Actual vapor pressure in the air, in mm of Hg = 𝑒𝑠 𝑅𝐻 where 𝑅𝐻 is the
relative humidity (%)
𝐻 = daily net radiation in mm of H2O given by equation

0.55𝑛 0.9𝑛
𝐻 = 𝑅𝐴 1 − 𝑟 0.29𝑐𝑜𝑠𝜑 + − 𝜎𝑇𝑎4 0.56 − 0.092 𝑒𝑎 0.10 +
𝑁 𝑁

𝑅𝐴 = mean monthly extra-terrestrial radiation in mm of H2O/day

𝜑 = latitude of the place where 𝐸𝑡 is being computed
𝑟 = reflection coefficient of the surface
= 0.15 to 0.25 for close crops
= 0.05 to 0.45 for barren land
= 0.05 for water surface
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Penman Equation (cont.)

𝑛 = actual duration of bright sunshine which is a function of latitude of the place

and is observed data at that place. Alternatively, it can be taken equal to p
𝑁 = Maximum possible hours of bright sunshine available at different locations.
𝜎 = Stefan-Boltzman constant = 2.01 x 10-9 mm/day
𝑇𝑎 = mean air temperature in K=273+ oC

✓ The wind speed measured at any other height can be reduced to 2 m height
by the relation (known as 1/7th power law)

1/7 0.143
2 2
𝑢2 = 𝑢 =𝑢
𝑧 𝑧

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Penman Equation (cont.)

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Penman Equation (cont.)

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Penman Equation (cont.)

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Example 3.19 (Punmia)

Compute the consumptive use of rice for the month of January by using
Penman’s formula, for the following data available at the field;
1) Latitude of place : 20o N
2) Mean monthly temperature : 15 oC
3) Relative humidity in January : 50%
4) Elevation of area : 250 m
5) Wind velocity at 2 m height : 25 km/day

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