Water requirement of crops refers to the amount of water needed for successful crop growth over a given period. It is met through rainfall, irrigation, soil water, and groundwater. Determining the water requirement is essential for crop planning, irrigation project design, and water resource management. Methods to calculate water requirement include the water balance method, field experiments, and climatological methods. Effective rainfall refers to the portion of precipitation available for crop use, while the rest is lost through runoff and deep percolation.

1. Water Requirement of Crops
Refers to the amount of water required to raise a successful crop in
a given period.
Comprises the water lost as evaporation, water transpired and
metabolically used by crop plants, water lost during application,
water used for special operations.
Usually expressed as surface depth of water in mm or cm.
WR = E + T + IP + Wm + Wu + Ws
(or) WR = ET + Wm + Wu + Ws
(or) WR = CU + Wu + Ws
Where,
WR = water requirement of crop, cm
E = Evaporation from crop field, cm
T = Transpiration by crop plants, cm
IP = Intercepted precipitation by the crop that gets evaporated, cm
Wm = water metabolically used by crop plants to make their body weight, cm
Wu = economically unavoidable water losses during application, cm
Ws = water applied for special operations, cm
ET = evapotranspiration from crop field, cm
CU = consumptive use of water by the crop, cm

2. Water required by crops is met from rainfall, irrigation, soil
water and ground water.
Hence WR = P + IRg + Δ SW + Δ GW – (R + PW)
(or) WR = ER + IRg + Δ SW + Δ GW
Where,
P = precipitation, cm
IRg = gross irrigation requirement of crop, cm
Δ SW = soil water contribution for crop use, cm.( Difference of soil
water contents at sowing and at harvesting of the crop that
may be positive or negative.
Δ GW = ground water contribution(usually from shallow water table),
cm.
R = run-off, cm
PW = deep percolation, cm
ER = effective rainfall, cm
ER = P –( R + PW)
Rainfall – ( run-off + deep percolation)

3. Gross irrigation Requirement
• Refers to the amount of water applied to the field from the start
of the land preparation to harvest of the crop together with the
water lost in conveyance thro’ distributaries and field channels
and during irrigation to the crop field.
IRg = WR – (ER + Δ SW + Δ GW)
where
IRg = Gross irrigation requirement, cm
Δ SW = ∑ Msi – Mhi Asi x Di
i=1 to n 100
Δ SW = soil water contribution, cm
Msi = soil water content at the time of sowing in the i-th layer,
percent w/w
Mhi = soil water content at harvest in the i-th layer, percent w/w
Asi = apparent specific gravity of soil
Di = depth of i-th layer of the root zone soil D, cm

4. Why determination of WR essential?
To decide the possible cropping pattern in a
farm or in an area.
To make effective use of available water supplies
during any season
To plan and design an irrigation project
To plan water resource development in an area
To assess the irrigation requirement of the area
and
To manage the water supply from sources

5. Net Irrigation Requirement
Refers to the amount of water needed to replenish the
soil water deficit in the crop field
It is amount of water needed to bring the soil water
content just before irrigation in the crop root zone to
field capacity
IRn = ∑ Fci – Mbi Asi x Di
i=1 to n 100
Where,
Fci = field capacity(percent w/w) in the i-th layer of the soil
Mbi = soil water content just before irrigation(percent w/w) in
the i-th layer of soil
Asi = apparent specific gravity of soil or bulk density of
soil(dimensionless)
Di = depth of the i-th layer of soil, cm
n = number of soil layers in crop root zone D

6. Duty of water
• Refers to the relationship between the
quantity of water made available and the area
irrigated with it.
• Expressed as the no. of hectares of a crop
successfully raised with a constant flow of one
cusec of water thro’out the growth
period(base period)

7. Water Table Vs Irrigation Requirement
Water table is the surface of free water standing in a
hole dug in the soil profile.
Can be determined by auguring out an observation
well to some depth in the soil profile or laying out
piezometer
The surface level of water in the well or the piezometer
representing the point of one atmospheric pressure
gives the position of water table.
Water table maintained at a suitable depth near the
effective crop root zone can be very helpful to meet
the crop needs and cut down the irrigation
requirement of crops.

8. Depth of water table at which Irrigation is not
necessary
Crops Depth of water
table(cm)
Wheat 50
Peas 75
Maize 100
Pearl millet 125
Soybean 125

9. Effective Rainfall
Effective rainfall or precipitation comprises the
portions of the precipitation which are intercepted
by the vegetation, used to replenish the soil water
deficit and used for cultural operations and leaching
salts.
Part of the precipitation that gets lost thro’ surface
run off and deep percolation is termed as ineffective
precipitation.
Methods determining the ER involve measurement
of RF, losses thro’ surface run off and percolation
beyond root zone and soil-water used by crops

10. Methods of determining ER
1. Direct methods
2. Drum culture technique
3. Empirical methods

11. Direct methods of determining ER
Measurement of rainfall by standard rain
gauge.
Different components of effective and
ineffective rainfall can be measured by
weighing type lysimeters.
Soil water contents in the root zone before
and after the rainfall are estimated for
knowing the ER

12. Drum culture technique
Oil drums used for measuring the ER in rice fields by
measuring the ET, percolation and surface run-off
(Dastane et al. 1966)
A set of three drums with a portion of the drums raised
above the soil surface is installed in the field.
One drum with bottom(A) is used to assess ET.
2nd drum without bottom(B) to measure the
percolation.
3rd drum without bottom(C ) has an overflow device at
a suitable height above the ground to measure surface
run-off.
Well suited for rice fields and is cheap.

13. Effective precipitation based on
increments of monthly rainfall
• According to Doorenbos and Pruitt(1975)
Monthly rainfall
increment(cm)
Effective precipitation
Per cent Accumulated(cm)
2.5 90 2.25
5.0 85 4.38
7.5 75 6.25
10.0 50 7.50
12.5 30 8.25
15.0 10 8.50
Over 15.0 0 8.50

14. Percolation Loss
Ramdas(1960) suggested the method involving
setting up several cylinders filled with a soil column
of depth equal to the effective root zone of the crop.
Cylinders are irrigated whenever the field is irrigated.
Percolation water is collected in a receiver placed
below the soil column.
The crop in the cylinders is the same as in the field.
The water accumulates in the receiver after each
rainfall or irrigation is measured and considered as
the percolation loss

15. Irrigation Efficiency
Project Irrigation Efficiency : IE is usually expressed as
the percentage ratio of the amount of water stored
in crop root zone for crop use in the project
command area to the amount of water diverted from
the project source.
Ep = 100 Ws/Wd
where
Ep = project irrigation efficiency in per cent
Ws = amount of water stored in crop root zone soil
Wd = amount of water diverted or pumped from the source

16. Components of Project Irrigation
Efficiency
IE may be considered in stages from the point of
diversion of water from a source to its actual use by
crops.
The components of PIE are (i) water conveyance
efficiency (ii) water application efficiency. When
these components are known, the PIE is determined
as
Ep = 100 [ Ec/100 x Ea/100]
Where, Ep = PIE, per cent
Ec = water conveyance efficiency, per cent
Ea = water application efficiency, per cent

17. Water conveyance efficiency
defined as the percentage ratio of the amount
of water delivered to fields or farms to the
amount of water diverted from sources.
Ec = 100( Wf/Wd)
where Ec = water conveyance efficiency, per cent
Wf = amount of water delivered to fields or
farms(at the head of field supply channel
or farm distribution system)
Wd = amount of water diverted from sources

18. Water application efficiency
Refers to the efficiency of water application to
fields.
Water is applied to fields in many methods
such as surface, sub surface, sprinkler or drip
irrigation methods.
WAE is the percentage ratio of the amount of
water stored in the crop root zone to the
amount of water delivered to fields.
Ea = 100 ( Ws/Wf)

19. Efficiency of irrigation practices, water use and
operation of irrigation system
Water storage efficiency: Percentage ratio of
the amount of water stored in effective root
zone soil to the amount of water needed to
make up the soil water depleted in crop root
zone prior to irrigation.
Es = 100 [ Ws/We]

20. Water distribution efficiency
• Measures the extent to which water is uniformly
distributed and stored in effective root zone soil
along the irrigation run.
Ed = 100 [ 1- y/d]
where
Ed = water distribution efficiency in per cent
y = average numerical deviation in depth of water stored in
root zone soil along the irrigation run from the average
depth of water stored during irrigation
d = average depth of water stored during irrigation along the
water run.

21. Water use efficiency
(i) Field water use efficiency : ratio of the amount of
economic crop yield to the amount of water
required for crop growing.
Eu = Y/WR
where ,
Eu = field water use efficiency expressed in kg of
economic yield per ha-cm or ha-mm of water.
Y = economic crop yield in kg/ha
WR = water requirement of the crop in ha-cm or
ha-mm

22. (ii) crop water use efficiency : ratio of the amount
of economic yield of a crop to the amount of water
consumptively used by the crop.
Ecu(or WUE) = Y/CU or ET
Where,
Ecu = crop water use efficiency in kg of economic
yield per ha-cm or ha-mm of water
Y = economic yield of crop in kg/ha
CU = consumptive use of water in ha-cm or ha-mm
ET = evapotranspiration in ha-cm or ha-mm

23. Methods for determining water
requirements
• Transpiration Ratio method
• Depth-interval-yield method
• Water Balance method
• Field Experiments
• Climatological methods
• Drum culture technique for Rice

24. Transpiration Ratio method
• The amount of water transpired by a crop to
produce a unit of dry matter is called
transpiration ratio.
• Transpiration ratio measured in green house in
potted plants is different from field grown
plants.
• So, knowledge on transpiration ratio is of little
value in irrigation planning for field conditions.

25. Depth-interval-yield method
• Field expts. conducted with different depths of irrigation for each irrigation and
various intervals between irrigations.
• The total water used for obtaining max.yield is considered as water requirement of
crop.
• Variation in climate in different seasons has considerable influence on water
requirement of crops
• Consumptive use of irrigated crops grown on fairly uniform soil can be estimated
by measuring soil moisture from various depths at periodical intervals during the
crop growth period.
• Precondition is the ground water should be deeper and should not influence the
root zone soil moisture.
• Consumptive use is calculated from the change in soil water content in successive
samples using the formula
n
u= ∑Mxi-Mzi x BDi x Di
i=1 100
Cu = ∑ u
Seasonal consumptive use is obtained by adding cu for each sampling interval.

26. Water Balance method
• ET from a large area like a watershed can be estimated
by water balance method.
• It is the estimation of incoming and outgoing water
from an area.
P+I + S = E + T + D + R
P = precipitation
I = Irrigation
S = GW contribution
E = Evaporation
T = Transpiration
D = Deep percolation
R = Surface run off

27. Field Experiments
• Crops grown with best management practices
and with best known irrigation schedule in
different seasons.
• Water applied, ER and soil contribution are
measured.
• Water needed during the entire crop season is
the water requirement of the crop
n
WR = IR + ER + ∑ Mbi – Mei x BDi x Di
i= 1 100

28. Climatological methods
• Using empirical formulae PET is estimated
• PET multiplied by crop co-efficients for
obtaining ETc
• Application losses and water for special needs
are then added.

29. Drum culture technique for Rice
• Four metallic square drums of size 50cmx50cm and 120 cm
height, two with bottoms and two without bottoms
embedded in rice field leaving 20cm above the soil.
• In tanks A and B with bottoms, excavated soil is replaced
layer wise to simulate the soil conditions outside.
• Tanks C and D embedded in soil without disturbing the
inner core.
• Tanks A,C and D are planted with rice seedlings on the
same day as the surrounding plot.
• In Tank B, rice seeding are planted after removing roots.
The rootless plants of same age and size are replaced every
week with roots cut off.
• Water level maintained at 5cm as outside plot.
• Water level measured with point gauge daily at 8 AM and
losses due to ET and percolation are replenished to
maintain 5 cm continuous submergence.

30. Water requirement of different crops
Crop WR(mm) range
Rice 1100- 1250
Wheat 450-650
Sorghum 450-650
Maize 500-800
Sugarcane 1500-2500
Groundnut 500-700
Cotton 700-1300
Tomato 400-600
Onion 350-550
Banana 1200-2200
Sunflower 600 -1000

31. Effective Rainfall
Factors influencing ER are amount and intensity of
rainfall, consumptive use rate, moisture storage
capacity of soil which depends on moisture
holding capacity, initial moisture content and
infiltration rate of the soil.
Methods of computing ER
1. Evapotranspiration/Precipitation Ratio method
2. PET/Precipitation Ratio method
3. Soil Moisture changes
4. Soil moisture balance method

32. Evapotranspiration/Precipitation Ratio
method
• Average monthly ER is calculated based on the
average monthly values of ETc and mean
monthly rainfall.
• When the soil storage at the time of irrigation
is greater or smaller than 75mm, the
correction factor is used(0.73 to 1.08)

33. PET/Precipitation Ratio method
• A ratio of PET to the total RF is computed.
• Ratios expressed in percentage for each
period.
• Max value of the ratio can’t exceed 100.
• E.g
Period PET(mm) RF(mm) Percentage
ratio
Mean
ratio for
the
month
Aug 1-10 60 150 40
11-20 65 100 65 60
21-31 60 80 75

34. Soil Moisture changes
• Water in the root zone is determined by
gravimetric method before and after every
shower of rain.
• The increase in soil moisture plus ET from the
time the rain starts until the soil is sampled, is
the amount of effective rainfall.
ER = (M2 - M1) + Kc.ETo
M1, M2 = MC before and after rain respectively

35. Soil Moisture Balance method
• Like a Bank account
• Rainfall and irrigation are on the credit side.
• Soil moisture depletion on the debit side.
• Any amount in excess of the FC is a surplus and is
lost as deep percolation or surface run off or
both.
• RF and irrigation is directly measured and ET by
any empirical formulae.
• Rainfall –Water surplus = ER