Unit 1 Crop Water Requirement

CE8603- Irrigation Engineering
A.Leema Margret,
Assistant Professor
(Civil),
Ramco Institute of
Technology,
Rajapalayam

UNIT I CROP WATER REQUIREMENT
Need and classification of irrigation- historical development and merits and demerits of
irrigation- types of crops-crop season-duty, delta and base period- consumptive use of
crops- estimation of Evapotranspiration using experimental and theoretical methods.
UNIT II IRRIGATION METHODS
Tank irrigation – Well irrigation – Irrigation methods: Surface and Sub-Surface and
Micro Irrigation – design of drip and sprinkler irrigation – ridge and furrow irrigation-
Irrigation scheduling – Water distribution system- Irrigation efficiencies.
UNIT III DIVERSION AND IMPOUNDING STRUCTURES
Types of Impounding structures - Gravity dam – Forces on a dam -Design of Gravity
dams; Earth dams, Arch dams- Diversion Head works - Weirs and Barrages.

UNIT IV CANAL IRRIGATION
Canal regulations – direct sluice - Canal drop – Cross drainage works-
Canal outlets – Design of prismatic canal-canal alignments-Canal lining
- Kennedy’s and Lacey’s Regime theory-Design of unlined canal
UNIT V WATER MANAGEMENT IN IRRIGATION
Modernization techniques- Rehabilitation – Optimization of water use-
Minimizing water losses- On form development works-Participatory
irrigation management- Water resources associations- Changing
paradigms in water management-Performance evaluation-Economic
aspects of irrigation.
TOTAL :45 PERIODS

TEXTBOOKS:
1. Dilip Kumar Majumdar, “Irrigation Water Management”, Prentice-Hall of India, New Delhi,
2008.
2. Punmia B.C., et. al; Irrigation and water power Engineering, Laxmi Publications, 16 th
Edition, New Delhi, 2009
3. Garg S. K., “Irrigation Engineering and Hydraulic structures”, Khanna Publishers, 23 rd
Revised Edition, New Delhi, 2009
REFERENCES:
1. Duggal, K.N. and Soni, J.P., “Elements of Water Resources Engineering”, New Age
International Publishers, 2005
2. Linsley R.K. and Franzini J.B, “Water Resources Engineering”, McGraw-Hill Inc, 2000
3. Chaturvedi M.C., “Water Resources Systems Planning and Management”, Tata McGraw-
Hill Inc., New Delhi, 1997.
4. Sharma R.K.. "Irrigation Engineering", S.Chand & Co. 2007.
5. Michael A.M., Irrigation Theory and Practice, 2nd Edition, Vikas Publishing House Pvt.
Ltd., Noida, Up, 2008
6. Asawa, G.L., “Irrigation Engineering”, NewAge International Publishers, New Delhi, 2000.

A.Leema Margret,
Assistant Professor (Civil),
Ramco Institute of Technology,
Rajapalayam
UNIT-1
Crop Water
Requirement

UNIT-1
CROP WATER REQUIREMENT
Need and classification of irrigation- historical
development and merits and demerits of irrigation-
types of crops-crop season-duty, delta and base
period- consumptive use of crops- estimation of
Evapotranspiration using experimental and
theoretical methods.

Irrigation- Definition
Irrigation is defined as the process of
artificial application of water to the soil for the
growth of agricultural crops.
It is practically a science of planning and
designing a water supply system for the
agricultural land to protect the crops from bad
effect of drought or low rainfall.
Includes the construction of weirs, dams,
barrages and canal systems for the regular
supply of water to the cultivable crops

Irrigation Engineering
It is the science dealing with planning, designing,
construction, operation and maintenance of
various irrigation works.

NECESSITY OF IRRIGATION
Insufficient rainfall
Total rainfall is less than needed for the crop,
artificial supply is necessary.
Non-uniform rainfall
rainfall in a particular area may not be uniform
over the crop period.
 Growing a number of crops during a year
Growing perennial crops
Commercial crops with additional water
Controlled water supply

Benefits of Irrigation
 Increase in food production
 Protection from famine
 Cultivation of cash crop ( Sugarcane, Tobacco)
 Elimination of mixed cropping
 Addition to the wealth of the country
 Increase the prosperity of people
 Generation of hydro-electric power
 Domestic & industrial water supply
 Inland navigation
 Improvement of communication
 Canal plantations
 Improvement in the ground water storage
 General development of the country.

Ill-Effects of Irrigation
 Creation of damp climate
 Formation of water-logged area
 Formation of marshy land
 Water pollution problem: Seepage of nitrate applied in the
soil as fertilizer may pollute underground water.
 Raising of the water table
 Soil erosion
 Loss of soil fertility
 Loss of valuable land

Attention to Students……
 Put Separate note for Irrigation Engg
 Paste or Write neatly the syllabus on
the first page
 Take regular class notes on the note
with Date
 After completion of each topic the
notes will be checked and Signed
 At the end of each Unit an assignment
will given. Do the Assignment sincerely.
 Study all the notes daily
 Score High marks and Enjoy the class

Types of Irrigation OR Classification of
Irrigation

I. Surface irrigation
In this technique, water flows and spreads
over the surface of the land. Varied quantities
of water are allowed on the fields at different
times.

1) Flow irrigation
If the water is available at higher elevation
and it is supplied to lower level under action of
gravity Is called flow irrigation.
2) Lift or well Irrigation:
 Water is lifted up by mechanical such as
pump etc or manual to supply for irrigation .
 Lift irrigation is adopted when the water
source is lower than the level of lands to be
irrigated.

1) Flow irrigation
a) Perennial irrigation :
• Continuous supply of water to the crop is assured
throughout the crop period accordance to
requirement.
• This system of may be obtained from any perennial
source of water.

i). Direct irrigation :
 Water is directly diverted from the river into the
canal by the construction of diversion weir or
barrage across the river without attempting to
store water is called Direct irrigation.
 Direct irrigation can be adopted only where there
is enough flow in the river to provide sufficient
quantity of water required for irrigation
throughout the crop period.

ii). Storage Irrigation:
 River flow is not perennial or insufficient during crop period,
Storage Irrigation is adopted.
 A dam is construction across the river to store water in the
reservoir.
 In some area rain water that run off from a catchment area is
stored in tanks and is used for irrigation during the crop period.

1) Flow irrigation
b) Inundation/Flood irrigation:
 In this type of irrigation, soil is kept submerged and fully
flooded with water for saturation of land.
 Usually practiced in delta regions wherever the stream water
level during the flood is sufficiently high to provide water to the
land by flow, or partly by flow and lift.
 Lands are allowed to drain off & the crop are sown.
 Now the soil retains sufficient moisture for the crops to grow.

II) Sub-surface irrigation
In this type of irrigation water does not actually wet
the surface of soil rather it flows underground and nourishes
the plant root by capillarity.
a) Natural Sub-surface irrigation:
• Water seeping through channels and water bodies may
irrigate crops grown on the lower area of capillarity.
• When underground irrigation is achieved, simply by
natural process, without any additional extra efforts, it is
called natural sub-surface irrigation.
b) Artificial Sub-surface irrigation:
Water is directly supplied to the root zone of the
plants by providing a network of open jointed drains laid
below the soil surface.

a) Natural Sub-surface irrigation

b) Artificial Sub-surface irrigation

Historical development of Irrigation
 Indus Civilization flourished on the banks of rivers
and the water was harnessed for sustenance of
life.
 Irrigation technologies during the Indus Valley
Civilization were in the form of small and minor
works like digging wells.

Irrigation during Medieval India
 Rapid advances took place in the construction of canals.
 Water was blocked by constructing bunds across steams
 Ghiyasuddin Tughluq is credited to be the first ruler who
encouraged digging canals. Fruz Tughlug is considered
to be the greatest canal builder.
 Irrigation is said to be one of the major reasons for the
growth and expansion of the Vijayanagar Empire in
southern India in the fifteenth century.
 As agricultural income was the pillar of the economy,
irrigation systems were paid special attention during this
period.

 Babur, in his memoirs called ‘Baburnamah’ gave a
vivid description of prevalent modes of irrigation
practices in India at that time.
 The Gabar Bunds, presently in Sindh, Pakistan,
captured and stored annual runoff from surrounding
mountains and river Sindhu (Indus) to be made
available to tracts under cultivation.

Irrigation in British India
 Renovation, improvement extension of existing
works and
 New projects, like the Upper Ganga Canal, the
Upper Bari Doab Canal and Krishna and Godavari
Delta Systems.
 Major canal works like the Sirhind, the Lower
Ganga, the Agra and the Mutha Canals, and the
Periyar Dam and canals.

Irrigation development after
Independence
 To achieve set targets of economic development,
the responsibility of irrigation development was
given to the Union Ministry of Water Resources
 It took initiatives from time to time on water
resources development and for technical
assistance to the states on irrigation, multipurpose
projects, ground water exploration and exploitation,
drainage, flood control, water logging, sea erosion
problems, dam safety and hydraulic structures for
navigation and hydropower.

Irrigation development programs undertaken
Command Area Development & Water
Management (CADWM):
 To provide central assistance for development of
infrastructure to facilitate use of sprinkler / drip
irrigation systems as an alternative to construction
of field channels.
 The assistance is limited to construction of stilling
tank, pump house and laying of conveyance pipes
up to farmer’s fields.

Accelerated Irrigation Benefits Program (AIBP):
 The AIBP was conceived in the year 1996 by the
Government of India in order to provide financial
assistance to States to complete various ongoing
projects in the country so that envisaged irrigation
potential of the project could be created and
thereby extend irrigation to more areas

Some older methods of irrigation

Some modern methods of irrigation

Advantages of irrigation
I.Direct Benefits
 Multiple cropping for cultivation
 Productivity is high
 Quality of the crop is improved
 Higher economic return and employment
opportunities.
 Development of pisciculture and afforestation
 Prevention of damage through flood

Indirect Benefits
1.Increase in gross domestic product of the country, revenue,
employment, land value, higher wages to farm labour, agro-
based industries and groundwater storage.
2.General development of other sectors and development of
the country
3.Increase of food production.
4.Modify soil or climate environment – leaching.
5.Lessen risk of catastrophic damage caused by drought.
6.Increase income & national cash flow.
7.Increase labor employment.

Indirect Benefits
1)Increase standard of living.
2)Increase value of land.
3)National security thus self sufficiency.
4)Improve communication and navigation
facilities.
5)Domestic and industrial water supply.
6)Improve ground water storage.
7)Generation of hydro-electric power.

Disadvantages of Irrigation
 Water logging.
 Salinity and alkalinity of land.
 Ill aeration of soil.
 Pollution of underground water.
 Results in colder and damper climate causing
outbreak of diseases like malaria.

IMPORTANT TERMINOLOGY IN IRRIGATION:
Water requirement of crops:
The total quantity and the way in which
a crop requires water, from the time it is sown to
the time it is harvested.
Crop period:
The time period that elapses from the
instant of its sowing to the instant of its harvesting
is called the crop period.
Base period (B) :
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. It is
expressed in days.

Mostly, the crop period is greater than the base period.
Practically, both terms are considered same and are expressed in
days. In the calculation of water requirements of crops, the crop
period, base period and the growth period are considered same
and represented by ‘B’ in days
Delta (∆):
The total depth of water in (cm) required by a crop to
come to its maturity.

Problem –1: 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:
No. of watering required = 120/10 = 12
Total depth of water required in 120 days = 10 × 12 =
120 cm
∆ for rice = 120 cm

Problem –2: If wheat requires about 7.5 cm of
water after every 28 days, and the base period for
wheat is 140 days, find out the value of delta for
wheat.
Solution:
No. of watering required = 140/28 = 5
Total depth of water required in 140 days = 7.5 × 5
= 37.5 cm
∆ for wheat = 37.5 cm

Duty of Water (D)
• Duty is defined as the number of hectares
of land irrigated for full growth of a given
crop by supply of 1m3/sec of water
continuously during the entire base period ‘B’
of the respective crop.
• Duty gives a relation between the volume of
water and the area of the crops that are
harvested.
• For example, 200 (hectares per cumec) to the
base of B days means that the water flowing at a
rate of 1 cubic meter per second running
continuously for B days, matures 200 hectares of
crop.
• The duty is generally represented by the letter D.

1.Find the delta for a crop if the duty for a
base period of 110 days is 1400
hectares/cumec.
2.A crop requires a total depth of 92 cm of
water for a base period of 120 days.
Find the duty of water.

Flow duty and Quantity duty:
In direct irrigation, duty is always expressed in
hectares/cumec. It is then called as flow duty or
duty.
In storage irrigation, duty may, sometimes be
expressed in hectares/millions cubic metre of water
available in the reservoir. It eventually means that
every million cubic metre of water available in the
reservoir will mature so many hectares of a particular
crop. When duty is expressed in this manner, it is
called Quantity duty or Storage duty.

FACTORS AFFECTING DUTY(Cont..)

Importance of Duty
 It helps us in designing an efficient canal irrigation
system.
 Knowing the total available water at the head of a
main canal, and the overall duty for all the crops
required to be irrigated in different seasons of the
year, the area which can be irrigated can be worked
out.
 Inversely, if we know the crops area required to be
irrigated and their duties, we can work out the
discharge required for designing the channel.

Arid region – the area where irrigation is a must for
agriculture.
Semi-arid region – the area in which inferior crops can
be grown without irrigation.
Types of Crops:
1) Wet crops- Wet crops are those crops which require
irrigation water
2) Dry crops-These are such crops which do not require
irrigation water.
3) Kharif crops (summer crops) - which are sown
during the south west monsoon & harvested in autumn.
The Kharif crops are rice, bajra, jowar, maize, cotton,
tobacco, groundnut etc.

5) Rabi crops (winter crops) -which are sown in
autumn & harvested in spring.
The Rabi crops are wheat, barley, gram,
linseed, mustard, potatoes, peas etc.,
6) Cash crop – which has to be encased in the
market. As it cannot be consumed directly by the
cultivators. Crops like jute, tea, cotton, tobacco,
sugarcane etc are therefore called cash crops.

Crop Seasons:
• In north India the crop season is divided as Rabi &
Kharif.
• Rabi crops are called as winter crops and kharif crops
are called as summer crops.
• Kharif crops requires about 2 to 3 times the quantity of
water required by the Rabi crops.
• Kharif season starts from 1st April and ends on 30th
September.
• Rabi season starts from 1st October and ends on 31st
March.
• In Tamil Nadu crops are classified as wet and dry crops.

7. Outlet factor
It is defined as the duty at the outlet.

8. Overlap allowance:
It may happen sometimes that the crop of some season
may overlap some period of the next crop season. When such
overlapping takes place, the crops of both the seasons require
water simultaneously. Thus the overlap allowance is the extra
discharge for this purpose.
9. Full supply coefficient:
Defined as the area estimated to be irrigated during the
base period divided by the design full supply discharge of the
channel at its head during maximum demand. This is also
known as duty on capacity.

10. Cumec-day:
The quantity of water flowing for one day at the rate of 1
cumec is known as cumec-day. It is equal to 8.64 hectare-
metres.
9. Nominal duty:
• In some areas farmers have to submit an application to the
irrigation authorities, stating their water demands in advance
of every crop season.
• After a permit is granted, they have to pay full amount for the
water irrespective of the area they actually irrigate.
• Nominal duty is the ratio of the area of which the permit
has been granted for the period divided by the mean
supply for the base period.

11. Open Discharge:
It is the ratio of the number of cumec-days to the
number of days the canal has actually been used for irrigation.
12. Root zone depth:
It is the maximum depth of soil strata in which the crop
spreads its root system and derives water from the soil.

Intensity of Irrigation:
It is defined as the percentage of culturable
commanded area proposed to be irrigated during either a
crop season or during a year.
If C.C.A of an irrigation field is 120 hectares, out of which 90
hectares of the land is cultivated during Kharif season and 60
hectares of land is cultivated during rabi season.
The intensity of irrigation during Kharif season will be
=(90/120)* 100= 75%
Intensity of irrigation during rabi crop will be = (60/120)*100
=50%

Crops rotation:
The method of growing different crops in rotation one after
the other in the same field is called crop rotation.
Necessity for rotation :
 Fertility of land gets reduced as the soil becomes
deficient.
 More balanced fooding.
 Rotation will reduce the diseases and wastage due to
insects.
 Increase nitrogen content of soil
 The soil will be better utilized
 Rotation of cash crops, fooder crops and soil
renovating crops.

General rotation of crops can be summarized as:
1. Wheat – Great millet – gram.
2. Rice – gram
3. Cotton – wheat – gram - Fallow
4. Cotton – wheat – sugarcane
5. Cotton – Great millet – gram

𝑊𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟 𝑡𝑟𝑎𝑛𝑠𝑝𝑖𝑟𝑒𝑑 𝑏𝑦 𝑡ℎ𝑒 𝑝𝑙𝑎𝑛𝑡 𝑑𝑢𝑟𝑖𝑛𝑔 𝑖𝑡𝑠 𝑔𝑟𝑜𝑤𝑡ℎ
𝑊𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑑𝑟𝑦 𝑚𝑎𝑡𝑡𝑒𝑟 𝑝𝑟𝑜𝑑𝑢𝑐𝑒𝑑 𝑏𝑦 𝑡ℎ𝑒 𝑝𝑙𝑎𝑛𝑡𝑠
Transpiration Ratio:
• Transpiration ratio of a crop will increase if the
water requirement of the crop is more.
• T.R of wheat is of the order of 300 – 600
(average = 450) and that of the rice is of the
order of 600 – 800 (average = 700)

Consumptive use of crops (Cu)
Definition:
 Consumptive use for a particular crop may be defined as
the total amount of water used by the plants in
transpiration (building of plant tissues, etc.) and
evaporation from adjacent soils, in any specific time.
 The values of consumptive use (Cu) may be different for
different crops and may be different for the same crop at
different times and places.
 It is expressed in terms of depth of water.
Mathematically,
Consumptive Use = Evapotranspiration =
Evaporation + transpiration

1. Evaporation which
depends on humidity
2. Mean Monthly
temperature
3. Growing season of
crops and cropping pattern
4. Monthly precipitation in
area
5. Wind velocity in locality
6. Soil and topography
7. Irrigation practices and
method of irrigation
8. Sunlight hours
Factors Affecting the Consumptive Use of Water

Types of Evapotranspiration
1.Potential Evapotranspiration (PET)
2.Actual Evapotranspiration (AET)
1) Potential Evapotranspiration (PET):
If sufficient moisture is always available to
completely meet the needs of the plants, the
resulting evapotranspiration is called PET.
2) Actual Evapotranspiration (AET):
The real evapotranspiration occurring in a
specific situation is called AET.

Methods of Determination of Evapotranspiration
1)Direct Methods/Field Methods
2)Empirical Methods
3)Pan evaporation method
1. Direct Methods:
In this method field observations are made and physical model is
used for this purpose. This includes,
Tanks and Lysimeter
Soil Moisture Studies
Field Experimental Plot Method
Integration Method/Summation Method
Inflow Outflow Method

i. Tanks and Lysimeter:
 In this method of measurement of consumptive use of
water, a watertight tank of cylindrical shape having
diameter about 60 to 90 cm and depth is about 180cm to
3m is placed vertically on the ground.
 The tank is filled with sample of soil. The bottom of the
tank consists of a sand layer and a pan for collecting the
surplus water.
 The plants grown in the Lysimeter should be the same as
in the surrounding field.

• The plants grown in the Lysimeter should be the
same as in the surrounding field.
• The water is applied at the surface of the Lysimeter
to maintain satisfactory growth of the plants.
• Water which is not used by the plants passes
through the soil column and is collected at the
bottom of Lysimeter.
• Lysimeter studies are time consuming and
expensive.

 The consumptive use of water is estimated by
measuring the amount of water required for the
satisfactory growth of the plants within the tanks.
Consumptive use of water is given by,
Cu = Wa – Wd
Where, Cu = Consumptive use of water
Wa = Water Applied
Wd = Water drained off
 The difference between the water applied at the
surface and that draining out through the
pervious bottom and collected in the pan gives the
consumptive use.

 Suited where soil is fairly uniform and ground
water 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.
 The depth of water removed from the soil is
determined by the relation :
ii. Soil Moisture Studies:

Dr =
𝒑𝜸 𝒅
𝟏𝟎𝟎
Where Dr = Depth of water removed in ‘m’ , p =
percentage of water content, γ = specific gravity of soil, d
= depth of soil in ‘m’.
 A curve is drawn by plotting the rate of use against time
and from this curve, the water consumption for any
period can be determined.

iii. Field Experimental Plot Method:
 More dependable than the tank and lysimeter method.
 It replicates the conditions of an actual sample field (field
plot). Irrigation water is applied to the selected field
experimental plots in such a way that there is neither
runoff nor deep percolation.
 Initially the yield increases rapidly with application of
water, then it decreases with further increase of water.
 Irrigation water vs Yield is plotted. At the ‘break in the
curve’ the amount of water used is considered as
Consumptive Use.

 The drawback in this method is that lateral
movement of water takes place although more
representative to field condition.
 Also some correction has to be applied for deep
percolation as it cannot be ascertained in the field.

iv. Integration Method:
 In this method, it is necessary to know the division of
total area, i.e. under irrigated crops, natural native
vegetation area, water surface area and bare land area.
 In this method, annual consumptive use for the whole
area is found in terms of volume.
 It is expressed in Acre feet or Hectare meter.
Mathematically,
Total Evapotranspiration = Total consumptive use

Total Area Annual Consumptive Use
= Total Evapotranspiration
= A+B+C+D
Where,
A = Unit consumptive use for each crop x its area
B = Unit consumptive use of native vegetation x its area
C = Water surface evaporation x its area
D = Bare land evaporation x its area

v. Inflow Outflow Method:
In this method 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
Where,
U = Valley consumptive use (in acre feet or hectare
meter)
I = Total inflow during a 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

2. Empirical Methods:
Empirical equations are given for the estimation of water
requirement. These are,
1)Blaney-Criddle method
2)Hargreave’s class A pan evaporation method
3)Penman’s Equation

1) Blaney-Criddle method:
 Blaney and Criddle (1962) proposed an empirical
relation which is largely used by irrigation engineers.
Blaney-Criddle equation expresses consumptive use in
terms of mean monthly temperatures and daylight hours.
If Cu is monthly consumptive use, its value is given by

Where,
Cu = Monthly consumptive use in cm.
k = Consumptive use coefficient or crop
factor determined by experiments for each crop
under the environmental conditions of particular
area.
t = mean monthly temperature in o F or mean
monthly temperature in o C
p = monthly percentage of annual day time
hours that occurs during the period.

f = Monthly consumptive use factor
Cu = Seasonal consumptive use, i.e consumptive use
during the period of growth for a given crop in an
given area.

• The above formula involves the use of crop factor, the
values of which is to be determined for each crop and
for different places. At present this information is not
available in India.
• Moreover this formula does not take into
consideration the factors such as humidity, wind
velocity, elevation, etc. on which the consumptive use
depends.
• Hargreaves class A Pan evaporation method is
therefore generally used in India.

2. Hargreave’s Class A Pan Evaporation Method:
 It is a very simple method.
 The pan is circular with a diameter of 1.2 m and depth of 25
cm which gives it a volume of about 0.3 m3.
 The basin is put on a 150 mm high wooden frame due to air
circulation around the basin.
 The water depth of 5 to 7.5 cm was maintained in this pan.
 The water level is measured every day, either you measure the
difference between the present and the origin water level or if
you have chosen to obtain the water level in the pan, you
measure the amount of water you have put into the pan.

Values of Hargreaves monthly consumptive use
coefficient K
Group A : Potato, Cotton, Maize, Bean, Peas, Jowar, Beat
Group B : Tomato, Olive, Plumes and some delicious fruits
Group C : Onions, Grapes, Melons, Carrots, Hope
Group D : Wheat, Barley, Celery and other grass type plants
Group E : Pesters, Plantain, Orchard crops etc.
Group F : Oranges, Fruits, Citrus crops
Group G : Sugarcane, Alfalfa etc.
Paddy (Rice)

The pan evaporation (Ep) can also be determined by
using the Christiansen formula which states,
Where
R – Extra-Terrestrial Radiation in cm
Ct - Coefficient for temperature
Cw – Coefficient for wind velocity
Ch – Coefficient for relative humidity
Cs – Coefficient for percent of possible sunshine
Ce – Coefficient for elevation

c. Penman’s Equation:
• Penman(1998) proposed an equation which has been
recently introduced for determining consumptive use of
different areas depending upon the type of vegetation
covering each area.
• Derived by energy balance and mass transfer
approaches of the computation of transpiration and
evaporation respectively.

Irrigation Requirements of crops
(1) Consumptive Irrigation Requirement (CIR)
CIR = Cu- Re
Where, Cu= total consumptive use requirement
Re= Effective rainfall.
(2) Net Irrigation Requirement (NIR)
NIR = CIR + Leaching requirement
(3) Field irrigation requirement (FIR)
(4) Gross irrigation requirement, (GIR)

Soil moisture and crop irrigation Relationship
Forms of soil water:
Water present in the soil may be to classified under three
heads
1.Gravitational water
2.Capillary water
3.Hygroscopic water

1. Gravitational water
 This water moves down from the soil under
gravity.
 When water falls over an area, a part of it get
absorbed in the root zone and the rest flows
downward under the action of gravity is called
gravity water.
 Not available for plants as it drains off rapidly from
the root zone.

2. Capillary water
 Water is attached to the soil molecules by surface
tension against gravitational forces and can be
extracted by the plants by capillarity.
3. Hygroscopic water
 The water that an oven dry sample of soil absorbs
when it exposed to moist air is termed as
Hygroscopic water.
 Hygroscopic water is a thin layer of water, in a vapor
form, held tightly to soil particles by surface forces.
Hygroscopic water is not available for plants.

Soil – Moisture deficiency :
The water required to bring the soil moisture content
of a given soil to its field capacity is called the field
moisture deficiency or soil moisture deficiency.

Unit 1 Crop Water Requirement

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Unit 1 Crop Water Requirement