UNIT 1

CROP WATER REQUIREMENT

 Topics to be discussed…
• Need of irrigation
• Classification of irrigation
• Historical development of irrigation
• Merits and demerits of irrigation
• Types of crop
• Crop season
• Duty,delta,base period
• Consumptive use of crop
• Estimation of evapo transiration
 IRRIGATION
• Irrigation is the artificial application of water
to the land or soil.
• It is used to assist in the growing of
agricultural crops, maintenance of landscapes,
and revegetation of disturbed soils in dry
areas and during periods of inadequate rainfall
 IRRIGATION ENGINEERING
• It is the science dealing with planning,
designing, construction, operation and
maintenance of various irrigation works
• Irrigation engineering is a field of engineering
which would discuss the uses of irrigation in
crop production and how new principles and
technologies can be used and applied for
irrigational practices.
• Irrigation Engineering is very important for
designing and planning of cost-effective and
efficient irrigation systems
• Irrigation projects are somewhat complicated and
expensive. 
• The primary responsibility of an irrigation engineer is
not only related to the application of the water
– it deals with various other aspects.
– It deals with the designing and planning of the irrigation
systems.
– It deals with the problems that may arise in the watershed
or the agricultural fields.
– The study of irrigation engineering deals with the rivers and
the hydrology.
– It is the study of water, soil, and crop relationship.
– It deals with the designs and structure of dams, canals,
other hydraulic and irrigation structures.
 Importance of irrigation
Importance of water to plants
1. Plants contain 90% water which gives turgidity
and keeps them erect
2. Water is an essential part of protoplasm
3. It regulates the temperature of the plant system
4. It is essential to meet the transpiration
requirements
5. It serves as a medium for dissolving the
nutrients present in the soil
6. It is an important ingredient in photosynthesis
 Necessity of irrigation
• Inadequate rainfall
• Non-uniform rainfall
• crop area
• Growing number of crops during year
• Growing perennial crops
• Growing superior crops
• Increasing the yield of crops
 Scope of irrigation engineering
• Engineering aspect
• Agriculture aspect
• Management aspect
 Engineering aspect
• It involves the development of source of water
for irrigation and arrangement for the
conveyance of water from the source right up
to agricultural fields.
a) Storage ,diversion or lifting of water.
b) Conveyance of water to the agricultural fields.
c) Application of water to agricultural fields.
d) Drainage and reliving water – logging.
e) Development of hydropower
 Agriculture aspect
• It involves the timely and systematic application of
irrigation water to the agricultural fields. It deals with
following points:
1) Proper leveling and shaping of the agricultural fields.
2) Soil investigation and classification of the agriculural land.
3) Provision of field channel.
4) Distribution of water uniformly and periodically.
5) Capacities of different soil.
6) Conservation of soil against erosion.
7) Choosing proper crop pattern.
8) Reclamation of waste and alkaline lands.
 Management aspect
• It deals with the successful implementation and efficient
management of both the engineering as well as the
agricultural aspects of the project.
Following points are considered in this aspect:-
1) The farmers should be trained and educated.
2) The cultivation should be carried out in a scientific
manner with due control on all the inputs so as to obtain
the maximum yield.
3) The distribution of water to the farmers should be
managed properly.
4) The agricultural land holding of the small farmers should
be consolidated to increase the efficiency of irrigation.
5) Suitable measures are to be adopted to eliminate the ill
effect of irrigation.
 Irrigation system

Lift irrigation

Flow
irrigation
By man or By mechanical or
animal Inundation
electric power Perennial irrigation
irrigation

Storage
Open Shallow Deep tube Direct irrigation irrigation
well tube well well

doon Swinging Wind Rati or

mote lass pulley
Persian basket
wheel
Doon
Persian wheel
mote
Wind lass
Raft or pulley
Swinging basket
Archimedian screw
Denkli
 Merits of irrigation
• Increase in crop yield
• Protection from famine
• Cultivation of crash crops
• Elimination of mixed cropping
• Increase in the wealth of country
• Increase in prosperity of people
• Generation of hydro-electric power
• Domestic and industrial water supply
• Inland navigation
• Communication facility
• Canal plantation
• Increase in ground water storage
• Overall development of the country
 Demerits of irrigation
• It is due to the excess irrigation and
unscientific use of irrigation water may give
rise to the ill-effects. some of the common ill-
effect of irrigation are:
1) Water logging
2) Mosquitoes nuisance
3) Damp climate
4) Pollution of ground water
 Definitions of some Common Important
Terms
•
• Gross Command Area (GCA)
It is the area up to which irrigation canals are capable of
supplying water for irrigation purpose.
Culturable Command Area (CCA)
It is the area on which crops can be grown satisfactorily.
Cash Crops
Crops like vegetables, fruits are cultivated by farmers to
sell in the market to meet the current financial
requirements and they are called cash crops.
Crop-ratio: The ratio of the areas under the two crop
seasons is called crop-ratio.
Crop Rotation
The process of changing type of crop to be grown in the same
field is known as crop rotation.
It has been found that if same crop is grown in the same land
every year, fertility of the land gets diminished and crop
production is reduced.
The necessary salt required by the same crop for growth is
exhausted. If crop rotation is adopted, fertility of soil is restored.
Crop Period
• It is the period required by a crop from the time of sowing to
the time of harvesting.
Intensity of Irrigation
Intensity of irrigation means the ratio of area under cultivation
to the total culturable area. If this intensity is more, more area is
under cultivation, hence water requirement is more.
• CROP RATIO:- It is defined as the ratio of the land
irrigated During the two main crop season rabi and
kharif.
• TIME FACTOR:- It is the ratio of number of days the
canal has actually run to the number of days of
irrigation Period.
• CAPACITY FACTOR:- It is the ratio of mean supply
to the Full supply of a canal.
• BASE PERIOD:- The base period is the relation
between he First watering and the last watering
supplied to the land.
• PALEO:- It is defined as the first watering before
sowing the Crop
• Saturation capacity is the amount of water
required to fill all the pore spaces between the
soil particles by replacing all the air held in the
pore spaces.
• Field capacity is the moisture content of the
soil after free drainage has removed most of
the gravity water.
Crop Rotation
 Types of crop
• Rabi Crops :- Rabi crops are sown in winter from
October to December.
• Harvested in summer from April to June
• Important Rabi crops – wheat, barley, peas, gram,
and mustard.
• Grown in – States from North and North western
parts such as Punjab, Haryana, Himachal Pradesh,
Jammu and Kashmir, Uttaranchal and Uttar Pradesh
• Availability of precipitation during winter months
due to western temperate cyclones helps in the
success of these crops.
• Kharif Crops :-
These crops are grown with the onset of
monsoon and harvested in September-October.
Important crops grown during this season are
rice (paddy), maize, jowar, bajra, tur (arhar),
moong, urad, cotton, jute, groundnut and
soyabean.
Some of the most important kharif regions
are Assam, West Bengal, coastal regions of
Orissa, the Konkan coast, Uttar Pradesh and
Bihar.
• Zaid Crops :-
• In between the rabi and the kharif seasons,
there is a short season during the summer
months known as the Zaid season.
• Zaid crops are grown mainly from March to
June Some of the crops produced during ‘zaid’
are watermelon, muskmelon, cucumber,
vegetables and fodder crops and Sugarcane.
DUTY:-
it is defined as number of hectares that can be irrigated by constant
supply of water @ rate of I cumec through the base period. It is
dentoted by Dit is expressed in hectares/cumec.Thus, duty gives the
relationship between the volume of water and area of the crop which
it matures,
For example:
If 5 cumec of water is required for a crop sown in an area of 5000
hectares, The duty will be 5000/5=1000 hectares/cumec.
DELTA:-
• It is total depth of water required for a crop during the entire period
of the crop in the field and is denoted by Δ. The unit of delta is days
Relation between Duty (D), Base (B) and Delta∆ (
)
Let, D= Duty of crop in ha / cumec
B= Base period of crop in days
∆ = Delta is depth of water in m.
Now 1 cumec of water running continuously for
a period of B days provides a volume of
[ (B x 24 x 60 x 60) x 1] m3
Amount of water required to flood 1ha of
land with a depth ∆ m = (1 x 10 4) m2 x ∆ m
•  Hence, the area in ha that can be irrigated by
1 m 3/sec running for the base period B days,
i.e.
• Duty= =
 Types of duty
There are four types of duty:
1) GROSS DUTY:-It is the duty of water
measured at The head of maincanal.
2) NOMINAL DUTY:-It is the duty sanctioned
as per Schedule of an irrigation department.
3) ECONOMIC WATER DUTY:-It is the duty of
water Which results in the maximum crop yield.
4) DESIGNATED DUTY:- It is the duty of water
assumed in an irrigation project for designing
capacities of the channel.
 Factors affecting duty of water
• 1. Method of irrigation
• 2) System of irrigation
• 3) Method of cultivation
• 4) Type of crop
• 5) Base period of crop
• 6) Climatic condition
• 7) Quality of water
• 8) Canal condition
• 9) Type of soil and sub soil
• 10) topography of irrigation.
 METHODS OF IMPROVING DUTY
• Proper ploughed and leveled crop land improves duty.
• The source of supply should provide good quality of
water.
• Crop rotation, if practiced, improves duty.
• Volumetric assessment of water with water tax compels
the farmers for economic use of water which improves
duty.
• The farmers must be trained to apply correct quantity of
water at right time.
• Maintenance of irrigation project from headworks to the
end of canal by the administrative should be adequate.
 Field capacity
• It means the water content retained by an
initially saturated soil against the force of
gravity.
• This stage is reached when the excess water
from a saturated soil after
irrigation or rainfall has fully
percolated down.
• Field capacity is determined in
the field where the water table is below 2m.
 Factors influencing field capacity
• Previous soil history
• • Soil texture and structure
• • Type of clay
• • Organic matter
• • Temperature
• • Water table
• • Depth of wilting
• • evapotranspiration
 determination of field capacity
• Ponding water on surface in area of 2-5sq.m &allow
water to drain for few days upto layer fully saturated
• • Soil samples are taken from the desired layers
@short intervals for 4-6days and water content is
determined
• • This process is continued till the two successive
values of water content are nearly equal
• • The lower one of these equal values are taken as
field capacity
• • Field capacity=wt of water/wt of soil*100
 Depth of irrigation
• It is a function of the water retentive capacity
of the root zone soil & extent of soil water
depletion at the time of irrigation
• It refers to the depth to which the applied
water would covered an area Soil
 Frequency of irrigation
• It is defined as the ratio of available soil
moisture depletion to rate of consumptive use
• Frequency of irrigation = available soil
moisture depletion/ rate of consumptive use
 Permanent wilting point
• The moisture content at which plant can no
longer extract the water from the soil for their
growth and finally wilts up(dry off)
• It is also known as wilting coefficient
Determination of permanent wilting point
• The indicator plants are grown in containers ,usually small
cans about 600gms of soil contains
• • The most commonly used indicator plants is sunflower
plant
• • To ascertain weather the plant is wilted ,it is placed over
a night in a chamber having
saturated environment
• • These sample of soil is taken
out when the plant is wilted &
its water content is equal to
permanent wilting point.
• • PWP= field Capacity/2 to 2.4
• Temporary wilting point:
It is the condition of a plant when the soil water
stress causes wilting of the plant during the
hottest part of the day & the plant recovers during
night time under favorable soil water condition
without addition of water
Ultimate wilting point:
• The plants do not recover from wilting even
after the addition of water to the soil. The water
content of the soil when the ultimate wilting
occurs is termed as ultimate wilting point
 Consumptive use of water
• It is defined as the total quantity of water used for the
growth of the plants by transpiration and the amount
lost by evaporation
• Factors affecting consumptive use
– Evaporation
– • Mean monthly temperature
– • Growing rear of crop & crop requirements
– • Monthly precipitation in the area
– • Depth of irrigation
– • Wind velocity in the locality
– • Soil & topography
– • Irrigation practices & methods of irrigation
• C.U = E +T + 𝑊𝑚
>99% <1%
~ E+T
• C.U =consumptive use
• 𝑊𝑚=Water require for metabolic process
• E= evapouration
• T=evapouration
 Necessity of consumptive use
• Plan the crop pattern for a particular locality
depending upon availability of water .
• Require to design a irrigation project .
• Require to determine the quantity of water to
be released from reservoir or irrigation project
• In kharif season CU is less as compered to rabi
season .
 Types of consumptive use
• Daily consumptive use: Total amount of 𝐻2O used in
ET & metabolic activities by crop during a single day .
Expressed in – mm/day or cm/day
• Seasonal consumptive use: Total amount of 𝐻2O
used in ET & metabolic activities by crop during it’s
total growing season . Expressed in- mm/season or
cm/season
• Peak period of consumptive use :The average daily
𝐻2o use rate in term of ET & metabolic activities
during highest CU period of season .
 Evapo-traspiration
• It is a combination of two separate processes
• Evaporation:
Loss of water from the soil
surface or anyother open
water body
• Transpiration:
from plant surface
Difference between evaporation
& transpiration
 Types of evapotranspiration
• Evapotranspiration may be classified as:
– Potential Evapotranspiration
– Referential Evapotranspiration
– Actual Evapotranspiration
Potential Evapotranspiration
• concept given by Thornthwait.
• It is the highest rate of ET from an actively
growing short height vegetation completely
covering the ground with abundant water supply
at a given climatic condition.
• REFERENCE ET-:
• Concept given by Doorenboss and Pruit
• It is the rate of ET , not highest from an
extended area completely covered by grasses of
12-15cm high completely covering the ground .
It is the abundant soil moisture substance.
• ACTUAL ET-:
• It is the actual ET of a crop at particular locality
and at particular time .It depends upon crop ,
soil , climate , and management factors.
 Significance of et-:
• It is an important process in water cycle as it
responsible for 15% atmosphere water vapour.
• It maintains soil temperature.
• It helps in movement of nutrients in plant
• Optimizes temperature of plant.
• Make the plant cell turgid which provide a
proper shape to plant.
 Factors affecting ET-:
• Environmental factors
• Plant crop factor
• Geographical factor
• Soil factors
 Environmental factors
• Several factors affecting a plant's rate of
transpiration and therefore evapotranspiration.
1. Air temperature, as temperatures increase,
evapotranspiration also goes up.
2. Humidity, as the air becomes more and more
saturated, less water is able to evaporate into that
air.
3. Movement of wind and air increases,
evaporation and transpiration does as well because
moving air is less saturated than stagnant air.
• Moisture/ water available, less water
available means plants begin to transpire less
water in an effort to survive. This in turn
decreases evapotranspiration.
• Solar radiations, more solar radiations more
will be the rate of evapotranspiration
• Plant and crop factors-:
– Vegetative cover
– Leaf shape and size
– Type of plant
• 3. Geographical factors-:
– ET is also dependent upon an area’s geography,
latitude and climate.
– Regions on the globe with the most solar radiation
experience more evapotranspiration.
– Evapotranspiration rates are also highest in areas
with a hot and dry climate.
– ET is less in higher latitudes.
• Soil factors-:
– ET depend upon water table depth, soil moisture
and capillary character.
– When soil is lacking moisture, plants begin to
transpire less water in an effort to survive, this in
turn decreases evapotranspiration.
MEASUREMENT OF EVAPO TRANSPIRATION

• To finding consumptive use following methods

are used
• • Experimental methods[DIRECT METHODS]
– a) Lysimeter method
– b) Field experiment method
– c) Soil moisture study
• • Empirical methods[INDIRECT METHODS]
– a) Hargreaves class A pan evaporation method
– b) Blanney - criddle method
 LYSIMETER METHOD
• A water tight tank of cylindrical shape
having dia of 2m &depth of 3m is placed
vertically on ground
• The tank is filled with soil
• The bottom of the tank consists of
a sand layer & a pan for collecting
surplus water.
• The consumptive use of water is ascertained b
By measuring the amount of water
required for satisfactory growth of
the plants within the tank.
•  Consumptive use(Cu)=Wa-Wd
• Several plots of land are selected where irrigation
water is to be supplied.
• The soil samples are taken from diff. depths @ the root
zone of the plants just before & after the irrigation
• • Then the water contents of the soil samples are
ascertained by laboratory tests.
• • The depth of water removed from the soil is
determined by relation
• Dr=𝑝𝑤𝑑/100
• Consumptive use(Cu)= KPxEP
– EP=0.45RC1CwChCsCe
R=Radiation, Cs=sunshine,
C1=temperature, Ch=humidity,
Cw=wind velocity, Ce=elevation.

• Consumptive use(Cu)= K(𝑃/40)[1.8t+32]

K=crop factor
P=% of dried hours in a year
t=temparature