Principles of irrigation by Dr Thomas Abraham_Course Code_Chapters 1 to 5__26-3-2014

Irrigation can be defined as the artificial application of water
to the crop root zone to meet its consumptive use which can
not be provided by rainfall.
Therefore, need for irrigation is as follows :
 Where, Rainfall is not sufficient to supply crop water
needs
 Where rainfall is not uniformly distributed to supplement
the crop requirement
 If there is variable crop water requirement
1. Introduction

Irrigation maximizes crop productivity and thereby
ensures food security
It makes agricultural industry profitable and competitive
 Irrigation improves land productivity and value of land
Promotes employment generation and lively hood of society
Main conveyance/irrigation canals can be used for flood
protection, recreation and navigation.
It promotes fish and wildlife preservation
Rapid development of irrigation facilities through construction
of multipurpose dams enhances hydroelectric power
generation.
Irrigation promotes a whole array of agro-based industries
which are drive engines of economy.
Direct/indirect benefits of Irrigation

1. Irrigation supplies moisture to the soil essential for,
germination of seeds and various growth processes of
crop.
2. Cools the soil and the surroundings thus making the
environment more favorable for crop growth.
3. Washes out or dilutes salts present in the soil.
4. Softens the clods and thus helps in tillage operations.
5. Enables application of fertilizers.
6. Reduces the adverse effects of frost on crops.
7. Ensures crop success against short duration drought.
Functions of Irrigation

 Stabilized Yield of Crops
 Protection from Famine
 Improvement of Cash Crops
 Prosperity of Farmers
 Source of revenue
 Navigation
 Hydroelectric Power Generation.
 Water Supply
 General Communication
 Development of Fishery
Advantages of Irrigation

At the micro level:
 Irrigation leads to an increase in yield per hectare
 Subsequent increases in income, consumption and
food security.
 Irrigation enables smallholders to diversify cropping
patterns
 To switch from low- value market subsistence
production to high-value market –oriented production.
Contribution of Irrigation to Country’s Economy

 Irrigation benefits through :
higher production, higher yields, lower risks of crop failure,
and higher and all year round farm as well as non-farm
employment.
 In Ethiopia during dry season, farmers grow Cereals and a
variety of Vegetables including Onions, Tomatoes, and
Leafy Green Vegetables like Lettuce under full irrigation.
 Farmers also grow perennial crops like Mango, Banana,
Chat, Sugarcane etc. which are sometimes intercropped
with seasonal crops
Benefits of Irrigation in Ethiopian Scenario

Therefore, we can summarize the
To ensure enough moisture essential for plant life.
To provide crop insurance against short duration
drought.
To cool the soil and atmosphere to provide a
favorable environment for plant growth.
To wash out or dilute harmful salts in the soil
To soften the tillage pans.
Objectives of irrigation

Full Irrigation :
Under adequate water supply (i.e., full irrigation), the crop
water requirement is fully met.
Crop water consumption (ETa) in this case is equal to the
maximum evapotranspiration (ETm),
i.e., ETa = ETm.
No water stress develops under such conditions, and crop
yield (Ya) is expected to be potential yield (Ym), i.e.
Ya = Ym (if other production factors are not limiting).
1.2. Types of Irrigation

Under limited water supply (i.e., water stress or water
deficit) :
- the level of soil water status within the plant root zone is
less than what would be under full irrigation.
Crop water consumption in this case (ETa) falls below
maximum evapotranspiration (ETm), i.e. ETa < ETm.
Under such conditions, water stress will develop in the
plant, which will adversely affect crop growth, and
therefore the expected yield (i.e., actual yield, Ya) will be
less than the potential yield, i.e. Ya < Ym.
Deficit Irrigation

Supplemental irrigation (SI) can be defined as the
addition of small amounts of water to essentially rainfed
crops during times when rainfall fails to provide sufficient
moisture for normal plant growth, in order to improve and
stabilize yields. SI in areas with limited water resources is
based on the following three grounds:
Water is applied to a rainfed crop which would normally
produce some yield without irrigation.
SI is only applied when precipitation fails to provide
essential moisture for improved and stabilized production.
Supplemental Irrigation

The amount and timing of SI are not scheduled to provide
moisture-stress-free conditions throughout the growing
season, but to ensure that the minimum amount of water
required for optimal (not maximum) yield is available
during the critical stages of crop growth.
Supplemental irrigation is the opposite of full or
conventional irrigation (FI). SI is dependent on the
precipitation of a basic source of water for the crop.
Supplemental Irrigation (cont..)

Soil can be regarded as a porous medium, consisting of ;
Solid,
Liquid and
 Gaseous phases.
Solid phase comprises :
 Mineral material,
 Organic Matter (OM) and
 living Micro Organisms which decomposes various
residues into beneficial Humus.
Organic Matter content in a mineral soil is usually
between 1 and 4% by dry weight.
Soil and Its Constituents

Organic soil is clay soil with more than 30% OM or a
sandy soil with more than 20% OM.
If a soil contains more than 65% OM it is called a peat
soil.
Mineral and organic compounds of the soil form the soil
matrix or 'skeleton' of the soil.
Part of the soil not occupied by the matrix is the pore
space or voids.
For most agricultural crops the ideal soil consists of
approximately 50% solid material and 50% pores, half of
which are filled with water.
Soil and Its Constituents (cont..)

Many of the physical and chemical properties of soil
are affected by soil texture.
 Soil texture is described by classifications which are
determined by the particle size, distribution of sand,
silt, and clay within the soil.
Particle size distribution of soil can be measured in
the laboratory.
After pretreatment of the soil sample, the sand
fractions are measured using mesh sieves of various-
sized openings.
Soil texture
Soil PropertiesSoil Properties

Soil Texture
Definition: relative proportions of various sizes of
individual soil particles
USDA classifications
 Sand: 0.05 – 2.0 mm
 Silt: 0.002 - 0.05 mm
 Clay: <0.002 mm
 Textural triangle: USDA Textural Classes (previous slide)
Affects water movement and storage
Soil Structure
Definition: how soil particles are grouped or arranged
Affects root penetration and water intake and
movement
Soil PropertiesSoil Properties (cont…)(cont…)

Bulk density is the ratio of the mass of solids to the
total soil volume.
It can be used to estimate the degree of compaction
It is needed to calculate soil moisture content and
porosity.
Bulk density of soil is influenced by soil structure due
to its looseness or degree of compaction and by its
swelling and shrinking characteristics.
Bulk density of Soil

Bulk Density (ρb)
◦ ρb = soil bulk density, g/cm3
◦ Ms = mass of dry soil, g
◦ Vb = volume of soil sample, cm3
Typical values: 1.1 - 1.6 g/cm3
Particle Density (ρp)
◦ ρP = soil particle density, g/cm3
◦ Ms = mass of dry soil, g
◦ Vs = volume of solids, cm3
Typical values: 2.6 - 2.7 g/cm3
b
s
b
V
M
=ρ
s
s
p
V
M
=ρ
Bulk density of Soil

Pore spaces in the soil matrix vary in amount, size,
shape, and continuity.
Porosity is an important physical property of the soil,
especially with regard to the retention of soil water.
Porosity is important because it is a measure of the
soil’s capacity to infiltrate water.
Porosity is the volume fraction of pores and ranges
from 0.3 to 0.6 for most soils.
Porosity is equal to the pore volume divided by the
original soil volume.
Porosity

Porosity (φ)
Typical values: 30 - 60%
soilofvolume
poresofvolume
φ =
PorosityPorosity

Development of sustainable irrigation practices will
require that we understand better the biophysical
processes of root-water uptake in soil, and transpiration to
plant canopies.
Solar energy is the driving force for most of the
biophysical processes in the plant system and water
movement from soil to the atmosphere.

A plant grows in soil and opens to atmosphere. About
99% of all the water that enters the roots leaves the plant’s
leaves via the stomata without taking part in metabolism.
On a dry, warm, sunny day, a leaf can evaporate 100% of
its water weight in just an hour. Water loss from the
leaves must be compensated for by the uptake of water
from the soil.
Water movement is due to differences in potential
between soil, root, stem, leaf, and atmosphere

Under normal conditions, the water potential in soil is
higher than that in root saps or fluids.
 - (Negative potential)
Typical moist soil might have a water potential of about
0.3 to 1.0 Bar,
- root tissue about 4.0 Bar, stem about 7.0 Bar,
 - leaf about 10.0 to 12.0 Bar,
- dry atmosphere about 400 to 600 Bar.
These differences in water potential (and hence potential
gradient) are the driving force for causing water
movement.
Water Potential
Chapter - 3

Water potential of the atmosphere is computed as:
Water Potential of
Atmosphere

Water movement in soil–plant–atmospheric
continuum

Soil water content
Mass water content (θm)
θm = mass water content (fraction)
Mw = mass of water evaporated, g
(≥24 hours @ 105o
C)
Ms = mass of dry soil, g
s
w
m
M
M
=θ
Water in SoilsWater in Soils

θV = volumetric water content (fraction)
Vw = volume of water
Vb = volume of soil sample
θV = As θm
As = apparent soil specific gravity = ρb/ρw
(ρw = density of water = 1 g/cm3
)
As = ρb numerically when units of g/cm3
are used
Equivalent depth of water (d)
d = volume of water per unit land area =
(θv A L) / A = θv L
d = equivalent depth of water in a soil layer
L = depth (thickness) of the soil layer
b
w
v
V
V
=θ
Volumetric water content (θv)

(g) (g)
(cm3
)
(cm3
)
Equivalent DepthEquivalent Depth
Volumetric water content &
Equivalent DepthEquivalent Depth

1 in.1 in.
0.50 in.0.50 in.
0.15 in.0.15 in.
0.20 in.0.20 in.
0.15 in.0.15 in.
Soil Solids (Particles): 50%Soil Solids (Particles): 50%
Total PoreTotal Pore
Space: 50%Space: 50%
Very Large Pores: 15%Very Large Pores: 15%
(Gravitational Water)(Gravitational Water)
Medium-sized Pores: 20%Medium-sized Pores: 20%
(Plant Available Water)(Plant Available Water)
Very Small Pores: 15%Very Small Pores: 15%
(Unavailable Water)(Unavailable Water)
Volumetric Water Content &Volumetric Water Content &
Equivalent DepthEquivalent Depth :: Typical Values for Agricultural SoilsTypical Values for Agricultural Soils

Coarse SandCoarse Sand Silty Clay LoamSilty Clay Loam
Gravitational WaterGravitational Water
Water Holding CapacityWater Holding Capacity
Available WaterAvailable Water
Unavailable WaterUnavailable Water
Dry SoilDry Soil
Water-Holding Capacity of Soil :Water-Holding Capacity of Soil :
Effect of Soil TextureEffect of Soil Texture

• Soil-water content refers to the amount of water present in
the soil. The soil-water content can be expressed in two
ways:
• on the basis of its quantity
• on the basis of its energy or potential
• 2.2. Forces acting on soil-water
• Several forces at the molecular level interact :
• Molecular-level cohesive and adhesive forces-
• Cohesion – attraction between like molecules
• Adhesion – attraction between differing molecules
• - hold water between soil particles and on the surfaces of
soil particles.
• Cohesive forces hold capillary water and adhesive forces
hold the hygroscopic water.
Soil-water content

• Hydrogen bonding
• Van der Waals - London forces
Cohesive forces

Water molecules
(+ve charged) is
attracted to the
negatively
charged surfaces
of soil solids
(adheision force)
Negatively
charged soil
solid
surfaces
Figure: Adsorbed (hygroscopic) water
adheres so tightly to soil particles that it can
be removed only by oven-drying the
soil. It is not available to plants.

Adsorbed water
Capillary water
Adhesion results from double-layer forces.
Adsorbed (hygroscopic) water adheres so tightly to soil
particles; it can be removed only by oven-drying the soil.
Capillary water coheres to adsorbed water and to itself. (Cohesion force)
•Gravitational force – acts on soil-water in the macro pores under saturated
conditions and tends to move water downward.
•Osmotic forces – are very important in saline soils and move water across
plant membranes.

Some points about potential concept :
• The concept of energy is the most important concept
in nature.
• This is because energy is a fundamental entity
common to all forms of matter.
• Water retention in the soil is a consequence of energy
effects.
• Capacity of doing work is called energy.
• The concept of energy is, therefore, closely linked to
the concept of work.

• Depending on how work is done, two kinds of mechanical
energy are distinguished:
• Potential energy – if by virtue of its position or state a body
is able to do work.
• Kinetic energy- if a body is able to do work by virtue of its
motion.
• Since the movement of water in the soil is very slow, the
contribution of kinetic energy to flow of water in soil is
negligible compared to that of potential energy.
• In the soil system, the state and movement of water is
determined by its potential energy.
• The amount of work done, or potential energy stored, per
unit mass in bringing any mass, m, from the reference to the
point in question is called potential.

• Expression in terms of energy makes it more easy to
compare availability of the moisture in soils of
different textures.
• Most commonly accepted unit at present is bars of
suction.
• Suction is negative pressure, the higher the numerical
value - the lower the energy status of the water.
• Soils at field capacity - 0.1 - 0.3 bars of suction
• When soils at wilting point - 15 bars of suction
• CONCEPT Managing soil moisture is the critical
management component of irrigated agriculture.

• Water will move in a soil from one point to
another if the water at the first point has higher
energy status than the water at the second.
• The rapidity of movement depends on the size of
the energy difference and soil characteristics.
• If water is applied to the surface by rain or
irrigation is much faster than it can enter soil and
be transmitted downward, the excess water
accumulates on the surface.
• If the slope is great, erosion will likely result
(unless surface stable or protected by plant
residues).

Measure of the energy status of the soil water
Important because it reflects how hard plants must
work to extract water
Units of measure are normally bars or atmospheres
Soil water potentials are negative pressures (tension
or suction)
Water flows from a higher (less negative) potential
to a lower (more negative) potential

• Gravity is the dominant force acting on soil
water when soils are wet.
• Water tends to flow downward- from a region
of more positive potential to one of less
positive potential- under the influence of
gravity, until the force of gravity is balanced
by that of capillarity.

• Matric potential is due to the attraction of soil
surfaces for water as well as to the influence
of soil pores.
• Matric potential is, therefore, the result of
two phenomena
• Adsorption
• Capillarity
• The soil solids tightly adsorb water, whereas
capillary forces are responsible for the water
being held in the capillary pores.

• The osmotic potential is attributable to the
presence of solutes in the soil.
• It results from dissolved solutes lowering the
free energy of soil-water and is always
negative.

Components :Components :
◦ ψt = total soil water potential
◦ ψg = gravitational potential
◦ ψm = matric potential
◦ ψo = osmotic potential
◦ Matric potential, ψm, normally has the greatest
effect on release of water from soil to plants
omgt ψψψψ ++=

Curve of matric potential (tension) vs. water content
Less water → more tension
At a given tension, finer-textured soils retain more water
(larger number of small pores)
Soil Water Release Curve

Height of capillary
rise inversely related
to tube diameter
Tension or suction created by small capillary tubes
(small soil pores) is greater that that created by large
tubes (large soil pores).
At any given matric potential coarse soils hold less
water than fine-textured soils..
Matric Potential and Soil Texture
Small soil pores
Large soil pores

–Soil water content where gravity drainage becomes
negligible
–Soil is not saturated but still a very wet condition
–Defined as the water content corresponding to a soil water
potential of -1/10 to -1/3 bar
–Soil water content beyond which plants cannot recover
from water stress (dead)
– (Plants can no longer absorb water)
–Still some water in the soil but not enough to be of use to
plants
–Traditionally defined as the water content corresponding
to -15 bars of suction
Field Capacity (FC or θθfcfc)
Permanent Wilting Point (WP or
θθwpwp)

Definition :
Water held in the soil between field capacity and
permanent wilting point
“Available” for plant use
Available Water Capacity (AWC)
AWC = θfc - θwp
Units: depth of available water per unit depth of
soil (in/in, or mm/mm)
Measured using field or laboratory methods
Available WaterAvailable Water

(θfc - θv) = Soil Water Deficit (SWD)
θv = current soil volumetric water content
Fraction available water remaining (fr)
(θv - θwp) = soil water balance (SWB)
=
wpfc
vfc
df
θθ
θθ
=
wpfc
wpv
rf
θθ
θθ
Fraction available water depleted (fd)

TAW = (AWC) (Rd)
TAW = total available water capacity within the
plant root zone(mm).
AWC = available water capacity of the soil,
(mm of H2O/mm of soil)
Rd = depth of the plant root zone, (mm)
If different soil layers have different AWC’s, need
to sum up the layer-by-layer TAW’s
TAW = (AWC1) (L1) + (AWC2) (L2) + . . . (AWCN) (LN)
- L = thickness of soil layer, (mm)
- 1,2,N: subscripts represent each successive soil layer
Total Available Water (TAW)

Horizontal movementHorizontal movement
due to capillaritydue to capillarityVertical movementVertical movement
due largely to gravitydue largely to gravity
Gravity vs. CapillarityGravity vs. Capillarity

Influencing FactorsInfluencing Factors
Soil texture
Initial soil water content
Surface sealing (structure, etc.)
Soil cracking
Tillage practices
Method of application (e.g., Basin vs. Furrow)
Water temperature
Water Infiltration -Water Infiltration -
Entry of water into the soilEntry of water into the soil

Cumulative Infiltration Depth vs. Time
For Different Soil Textures

Infiltration Rate vs.Infiltration Rate vs.
TimeTime for Different Soil Texturesfor Different Soil Textures

Gravimetric
Measures mass water content (θm)
Take field samples → weigh → oven dry → weigh
Advantages: accurate; Multiple locations
Disadvantages: labor; Time delay
Feel and appearance
Take field samples and feel them by hand
Advantages: low cost; Multiple locations
Disadvantages: experience required; Not highly accurate
Soil Water MeasurementSoil Water Measurement

Neutron scattering (attenuation)
Measures volumetric water content (θv)
Attenuation of high-energy neutrons by hydrogen nucleus
Advantages:
 samples a relatively large soil sphere
 repeatedly sample same site and several depths
 accurate
Disadvantages:
 high cost instrument
 radioactive licensing and safety
 not reliable for shallow measurements near the soil surface
Dielectric constant
A soil’s dielectric constant is dependent on soil moisture
Time domain reflectometry (TDR)
Frequency domain reflectometry (FDR)
Primarily used for research purposes at this time
Soil Water MeasurementSoil Water Measurement (cont..)(cont..)

Neutron AttenuationNeutron Attenuation

Tensiometers
Measure soil water potential (tension)
Practical operating range is about 0 to 0.75 bar of
tension (this can be a limitation on medium- and fine-
textured soils)
Electrical resistance blocks
Tend to work better at higher tensions (lower water
contents)
Thermal dissipation blocks
Require individual calibration

Porous Ceramic Tip
Vacuum Gauge (0-100 centibar)Vacuum Gauge (0-100 centibar)
Water ReservoirWater Reservoir
Variable Tube Length (12 in- 48 in)
Based on Root Zone Depth
Tensiometer for Measuring Soil Water PotentialTensiometer for Measuring Soil Water Potential

Electrical Resistance Blocks & MetersElectrical Resistance Blocks & Meters

Chapter-4Chapter-4
4. Irrigation water requirement and scheduling4. Irrigation water requirement and scheduling
4.1. Introduction
• The irrigation system is usually not expected to supply all of the
moisture required for maximum crop production. To do so would
ignore the valuable contribution of other water sources such as rain
and thereby force the irrigation system to be larger and more
expensive than necessary.
• In arriving at the contribution an irrigation system will make to an
irrigated area, particularly a surface irrigation system, four major
factors require consideration. These are:
 The concept of water balance in the region encompassing the plant
environment
 The body of soil supplying moisture, nutrient, and anchorage for the
crop and the associated characteristics of this porous medium
 The crop water requirements, including drainage for aeration and
salt leaching
 The efficiency and uniformity of the irrigation system

4.2. Water balance
• The employment of a water balance is a useful concept for
characterizing, evaluating, or monitoring any surface irrigation system.
A schematic of the water balance parameters used for characterizing a
surface-irrigated field is shown in Fig. 4.1. The terms are defined as:
• Da = depth of applied irrigation water
• D∆s = depth of change in soil moisture storage in the root zone where
D∆s is positive for increasing soil moisture storage
• Ddp = depth of deep percolation
• De = depth of evaporation from soil surface or ponded water surface
• Det = depth of evapotranspiration
• Dgw = depth of capillary rise from the groundwater table entering the
root zone
• Dp = depth of precipitation
• Dpl = depth of precipitation intercepted by the plants (crop)

• Dpz = depth of precipitation that infiltrates into the soil
• Dt = depth of transpiration from plants
• Dtw = depth of tail water (surface) runoff resulting from
overland flow of the irrigation water supply
• Dz = depth of infiltrated water resulting from overland flow
of the irrigation water supply
• There are two additional terms that are useful to define at this
point:
• Dpn = depth of net precipitation, or the depth of precipitation
that is made available to the plant system
• Dd = depth of drainage requirement for maintaining a salt
balance in the root zone

• Figure 4.1: The water balance parameters for a surface-irrigated field.

• The principle of continuity requiresThe principle of continuity requires that inflow (I) minus
outflow (O) equals the change in storage (∆S) within the
defined boundaries of a system:
• Of primary concern in surface irrigation are boundaries A, B,
C, and D as shown in Fig. 4.1, for which the continuity
equation can be written as
• in which
• And
I O S− = ∆
( ) ( )a gw p et pr tw dp pl s
Inflows Outflows
D D D D D D D D D∆+ + − + + + + =
a z twD D D= + p pz pl prD D D D= + +
et e tD D D= +

4.3. Crop water requirement
• Crop water requirement is defined as the depth of waterthe depth of water
(mm) needed to raise crop in a given period.(mm) needed to raise crop in a given period.
• It comprises of the water lost through evaporation from crop
field, water transpired and metabolically used by crop
plants, water lost during application which is economically
unavailable and the water used for salt leaching and so on.
• Determination of crop water requirement is used for the
planning, design, and operation of both rain fed and
irrigated agriculture.

4.4. Evapotranspiration4.4. Evapotranspiration
• The concept of crop evapotranspiration (ETc) is intimately
connected with CWR.
• It is defined as the rate of ET (mm/day) of a given crop as
influenced by its growth stages, environmental conditions, and
crop management.
• To determine the crop ET, the reference evapotranspiration
(ET0) is used and it refers to a reference crop cultivated in
reference conditions such that its rate of ET (mm/day) reflects
the climatic conditions characterizing the local environment.
• The transfer from ET0 to ETc is done by adopting the crop
coefficients (Kc), which represents the ratio between the rates
of ET of the cultivated crop and of the reference crop, that is,
Kc =ETc/ET0.

• For irrigated crops, another main concept is the irrigation
water requirement (IWR), defined as the net depth of water
(mm) that is required to be applied to a crop to fully satisfy its
specific CWR. The IWR is the fraction of CWR not satisfied
by rainfall, soil water storage, and groundwater contribution.
• The rate and amount of ET is the core information needed to
design irrigation projects, and it is also essential for managing
water quality and other environmental concerns.
• Anyone involved with resource management will likely need
to understand the methods available for estimating
evapotranspiration.

Figure 4.2: Sub-processes in evapotranspiration
Evaporation essentially occurs on the surfaces of open water
such as lakes, reservoirs, or puddles, or from vegetation and
ground surfaces. Transpiration involves the removal of water
from the soil by plant roots, transport of the water through the
plant into the leaf, and evaporation of the water from the leaf
interior into the atmosphere.
Evaporation Transpiratio
n
Open water Soil Vegetation surfaces Plants

Transpiration Ratio and Consumptive UseTranspiration Ratio and Consumptive UseTranspiration Ratio and Consumptive UseTranspiration Ratio and Consumptive Use
• Two terms frequently used relating to transpiration are
transpiration ratio and consumptive use.
Transpiration ratio is the ratio of the weight of water
transpired to the weight of dry matter produced by the plant.
This ratio is a measure of how efficiently crops use water.
For example, the transpiration ratios are approximately 900 for
alfalfa, 640 for potatoes, 500 for wheat, 450 for red clover, 350
for corn and 250 for sorghum.
The least efficient crop, in terms of water use, would be alfalfa
because it uses 900 kilograms of water for every kilogram of
dry alfalfa it produces. Sorghum is the most efficient crop
listed because it uses only 250 kg of water for every kg of dry
matter produced.

• Consumptive use is the total amount of water needed
to grow a crop (the sum of the water used in
evapotranspiration plus the water stored in the plant's
tissues). The term consumptive use is generally used
interchangeably with evapotranspiration because the
amount of water retained in plant tissue is negligible
compared to the amount of evapotranspiration.

4.54.5 Potential EvapotranspirationPotential Evapotranspiration4.54.5 Potential EvapotranspirationPotential Evapotranspiration
• Evaporation and especially evapotranspiration are complex
processes because the rate of water vapor loss depends on the
amount of solar radiation reaching the surface, the amount of wind,
the opening of the stomates, the soil water content, the soil type and
the type of plant.
• In order to simplify the situation, researchers have attempted to
remove all the unknowns such as aperture of the stomates and soil
water content, and focus on climatic conditions.
• The simplified calculations are termed potential evaporation and
potential evapotranspiration. The definition given by Jensen et al.
(1990) for potential evaporation (Ep) is the "...evaporation from a
surface when all surface-atmosphere interfaces are wet so there is no
restriction on the rate of evaporation from the surface.

• The magnitude of Ep depends primarily on atmospheric
conditions and surface albedo but will vary with the surface
geometry characteristics such as aerodynamic roughness".
Surface albedo is the proportion of solar radiation which is
reflected from a soil and crop and crop surface.
• Potential evapotranspiration (ETp)
was originally defined as " the amount of water transpired in
unit time by a short green crop, completely shading the
ground, of uniform height and never short of water."
• The crop is assumed to be short and uniform, and completely
shading the ground so that no soil is exposed. The crop is
never short of water, so soil water content is no longer a
variable and presumably (most probably) the stomates would
always be fully open. These conditions theoretically provide
the maximum evapotranspiration rate based on the given
climatic conditions.

• Many authors treat potential evapotranspiration and potential
evaporation as synonymous but the original intent was that potential
evapotranspiration involved on actively growing crop and potential
evaporation did not.
4.6. Actual Evapotranspiration4.6. Actual Evapotranspiration
• Potential evapotranspiration and evaporation may be easier to estimate
but do not represent reality. In general, watersheds are not entirely
covered by well-watered short green crops.
• Actual evapotranspiration (ET) or actual evaporation (E) is the amount
or rate of ET occurring from a place of interest and it is the value we
want to estimate. In practice, actual ET is obtained by first calculating
potential evapotranspiration and then multiplying by suitable crop
coefficients to estimate the actual crop evapotranspiration.
• Crop coefficients are usually residual terms from a statistical analysis
of field data, so it is essential that the methods for estimating potential
evapotranspiration be consistent with the crop coefficients.

• some studies have used alfalfa and some have used grasses to
measure potential evapotranspiration. Other methods correlate
evaporation from free water surfaces to actual ET. The result is
that the methods for determining actual ET are variable and
confusing. Scientists have attempted to remedy this problem by
introducing reference crop evapotranspiration.
4.7. Reference Crop Evapotranspiration
• Reference crop evapotranspiration (ETr) is defined as "...the rate
at which water, if available, would be removed from the soil and
plant surface of a specific crop, arbitrarily called a reference
crop”.
• Typical reference crops are grasses and alfalfa. The crop is
assumed to be well-watered with a full canopy cover (foliage
completely shading the ground). The major advantage of relating
ETr to a specific crop is that it is easier to select consistent crop
coefficients and to calibrate reference equations in new areas.

4.8. Measuring Evaporation or Evapotranspiration4.8. Measuring Evaporation or Evapotranspiration4.8. Measuring Evaporation or Evapotranspiration4.8. Measuring Evaporation or Evapotranspiration
• There are several methods available for measuring evaporation or
evapotranspiration.
• Since vapor flux is difficult to measure directly, most methods measure
the change of water in the system. Figure 4.3 shows schematically
options available for measuring potential evaporation, potential
evapotranspiration, or actual evapotranspiration.
• An evaporation pan or ET gage can be used to measure potential
evapotranspiration. Actual evapotranspiration can be measured in
several ways.
• If you need a simpler, less expensive technique, measuring soil water
depletion or using a water balance would be possibilities. More precise,
but also more complex methods include lysimeters, either weighing or
non-weighing, or using an energy balance or mass transfer technique.

Figure 4.3: Options available for measuring potential or actual
evapotranspiration
Actual ET
Simpler, less
expensive
More complex
and expensive
Soil water
depletion
Water
balance
Lysimeters Energy
balance
Mass
transfer
Weighing Non-weighing
Percolation Constant water table
Potential ET
Pan
evaporation
ET Gage

Soil Water Depletion
Lysimeters
Empirical methods
Soil conservation service modified Blaney-Criddle
FAO Modified Blaney-Criddle
Pan Evaporation
Combination Method
Penman Method
FAO Modified Penman Method

4.9. Depth of Irrigation4.9. Depth of Irrigation
• In planning and managing irrigation, the soil’s capacity to store
available water can be thought of as the soil water reservoir, which
must be filled periodically by irrigation or rainfall.
It is slowly depleted by evapotranspiration. Water application in
excess of the reservoir capacity is wasted unless it is required for
leaching or to meet a specific management need.
Irrigation must be scheduled to prevent the soil water reservoir from
becoming so low as to inhibit plant growth.
• The difference between FC and PWP is the available water (AW)
and can be estimated from
• Where.
AW = (FCv–PWPv)Dr/100

FCv and PWPv= the volumetric field capacity and permanent wilting
point percentages, respectively,
Dr = depth of the root zone or depth a layer of soil with in the root
zone (L)
AW = depth of water available to plants (L)
Plants can remove only a portion of the available water before
growth and yield are affected.
This portion is readily available water (RAW) and for most crops
ranges between 40 and 65 percent of AW in the root zone. Readily
available water can be estimated from
RAW = (MAD)(AW)
In which MAD is the management allowed deficiency or the portion
(decimal) of the available water that management determines can be
removed from the root zone without adversely affecting yield and/or
economic return.

• The supply requirements at the field level are determined by the
depth and interval of irrigation. These data can be obtained from the
soil water balance and are primarily determined by:
i. the total available soil water (Sa = Sfc – Sw where Sfc and Sw are the
soil water content in mm/m soil depth at field capacity and
wilting point, respectively);
ii. the fraction of available soil water permitting unrestricted
evapotranspiration and/or optimal crop growth; and
iii. the rooting depth (D). The depth of irrigation application (d)
including application losses is:
and frequency of irrigation expressed as irrigation interval of the
individual field (i) is:
( ). .
.a
a
p s D
d mm
E
=
( ). ap s D
i days
ET
=

• where:
p = fraction of available soil water permitting unrestricted
evapotranspiration, fraction
Sa = total avalable soil water, mm/m soil depth
D = rooting depth, m
Ea = application efficiency, fraction
• Since p, D and ET will vary over the growing season, the depth in
mm and interval of irrigation in days will vary.
• For design and operation of the water distribution system, the
requirements of the individual fields will need to be expressed in
flow rates or stream size (q in m3
/sec) and supply duration (t in
seconds, hours or days). The field supply (q.t) is:
( ) 310
. . . .a
a
q t p s D A m
E
=

where:
q = stream size, m3
/sec
t = supply duration, seconds
Ea = application efficiency, fraction
p = fraction of available soil water permitting unrestricted
evapotranspiration, fraction
Sa = total available soil water, mm/m soil depth
D = rooting depth, m
A = acreage, ha
In determining the relative values of q and t, the soil intake rate and
method of irrigation must be taken into account. For instance, t will
be greater for heavy as compared to light soil and also sprinkler and
furrow irrigation as compared to basin. Furthermore, the stream size
(q) must be handled easily by the irrigator.

• Field irrigation requirements will vary for each crop during the
growing season and the supply must follow those changes over area
and time. Analysis of the system and selection of the method of
supply (continuous, rotation or demand) should therefore start with
an evaluation of the field variables.
• Example: A clay soil having an average field capacity (FC) and
permanent wilting point (PWP) of 47.2 and 30.1%, respectively at
the first 45 cm of the soil profile is planted to corn. If actual
evapotranspiration (AET) is 6.2 mm/d, irrigation interval is 1 week
and application efficiency is 65%, find the following:
A, Available water in the root zone (AW) if apparent specific gravity
(As) is 1.2,
B, Depth of irrigation (DI) based on allowable depletion (MAD) of
50% and application efficiency (Ea) of 65%
C, Depth of irrigation based on consumptive use (AET) and irrigation
interval

• Solution:
A) Available water in the root zone (AW),
Aw = (FC-PWP)/100 *As*Dr
= (47.2-30.1)/100 *1.2*0.45
= 0.092m
= 92 mm
b) Depth of irrigation (DI) based on allowable depletion (MAD)
Readily available water in the root zone (RAW)
RAW = AW*MAD
= 92 mm*0.5
= 46mm
DI = RAW/Ea
= 46mm/0.65
= 71 mm
• c) Depth of irrigation based on consumptive use (AET) and irrigation
interval
DI = AET*I/Ea
= 6.2mm/d*7d/0.65
= 67mm

4.10. Crop Coefficient Curves4.10. Crop Coefficient Curves
• FAO Crop Coefficient Method
• The FAO methods, all modifications of the Penman equation, and many
other evapotranspiration estimating methods result in an
evapotranspiration estimate for a reference surface of water or
reference crop of grass or alfalfa. To determine water use for a crop
other than the reference, crop coefficients must be applied. This is
demonstrated by the following equation:
ETc = KcETr
where
ETc = crop evapotranspiration
Kc = crop coefficient
ETr = reference evapotranspiration

• the Kc values have to be with respect to the same crop as the ETr
values used in the equation.
• The FAO method divides the crop coefficient curve into four linear
line segments which approximate the curve. The first step is to
divide the growth season into the following growth periods:
1. Initial period1. Initial period:: Time of planting to time of 10 percent ground cover.
2. Crop development period2. Crop development period:: From end of initial period to time of
effective full cover – that is, 70 to 80 percent ground cover.
3. Mid-season period3. Mid-season period:: From end of crop development period to start of
plant maturity as indicated by leaf discoloration (e.g., beans, maize),
leaves falling off (e.g., cotton), or leaves curling and discolouring
(e.g., tomatoes, potatoes).
4. Late season period4. Late season period:: From end of mid-season period to time of full
maturity or harvest.

• After the season has been divided into the four growth periods,
coefficients must be determined corresponding to the initial, mid-
and late season periods to allow plotting of the total seasonal Kc
function. The initial period crop coefficient, Kci, is related to the
evaporation from basically a bare soil. This coefficient is a function
of soil wetness and the reference evapotranspiration rate during the
initial period.
Fig. Generalised crop
coefficient curve

Irrigation Methods are mainly classified into
:
1. Surface Irrigation or Gravity Irrigation
2. Subsurface Irrigation or Sub-irrigation
3. Sprinkler or overhead irrigation
4. Drip or Trickle irrigation
◦

1) SURFACE IRRIGATION
 Irrigation water flows across the field to the point of
infiltration
 Primarily used for field crops and orchards
 Water is applied to the soil surface and the water flows by
gravity either through furrows, strips or basins.
 Water is applied from a channel located at the upper reach
of the field.
 Loss of water by conveyance and deep percolation is high
and the efficiency of irrigation is only 40-50% at field level
in surface method of irrigation.
 Properly constructed water distribution systems to give
sufficient control of water to the fields
 And effective land preparation to permit uniform
distribution of water over the field are very important.

 Water is applied to the field in either the controlled or
uncontrolled manner.
 Controlled: Water is applied from the head ditch and
guided by corrugations, furrows, borders, or ridges.
 Uncontrolled: Wild flooding.
 Surface irrigation is entirely practised where water is
abundant.
 Low initial cost of development is later offset by high
labour cost of applying water.
 Deep percolation, runoff and drainage problems

 1. Furrow irrigation
 2. Border irrigation
 3. Basin irrigation

 Furrow irrigation - in which the water poured
on the field is directed to flow through narrow
channels dug between the rows of crops,
instead of distributing the water throughout
the whole field evenly.
 The furrows must all have equal dimensions,
in order to guarantee that the water is
distributed evenly.
 Like flood irrigation, furrow irrigation is rather
cheap in areas where water is inexpensive.

 In furrow irrigation, only a part of the land
surface (the furrow) is wetted thus minimizing
evaporation loss.
 Irrigation can be by corrugation using small
irrigation streams.
 Furrow irrigation is adapted for irrigating on
various slopes except on steep ones because of
erosion and bank overflow.

 There are different ways of applying water to the furrow.
 As shown in Fig 3.1, siphons are used to divert water
from the head ditch to the furrows.
 There can also be direct gravity flow whereby water is
delivered from the head ditch to the furrows by cutting
the ridge or levee separating the head ditch and the
furrows.
 Gated pipes can also be used. Large portable pipe(up
to 450 mm) with gate openings spaced to deliver water
to the furrows are used.
 Water is pumped from the water source in closed
conduits.
 The openings of the gated pipe can be regulated to
control the discharge rate into the furrows.

 The Specific Design Parameters of Furrow
Irrigation Are Aimed at Achieving the Above
Objectives and Include:
 a) Shape and Spacing of Furrows:
Heights of ridges vary between 15 cm and 40
cm and the distance between the ridges
should be based on the optimum crop
spacing modified, if necessary to obtain
adequate lateral wetting, and to
accommodate the track of mechanical
equipment.
 The range of spacing commonly used is from
0.3 to 1.8 m with 1.0 m as the average.

 In this, Parallel ridges are made to guide a sheet of flowing
water when the water moves down the slope.
 The field is divided into several long parallel strips called
borders that are separated by low ridges.
 Field should be even surface over which the water can flow
down the slope with a nearly uniform depth.
 Every strip is independently irrigated by turning a stream of
water at the upper end.
 Then water spreads and flows down the strip in a thin sheet.
 Water moves towards the lower end without erosion covering
the entire width of the border.
 Sufficient moisture is provided to the soil to entire length of the
border.
 Border method is suitable for most of the soils, while it is best
suited for soils having moderately low to high infiltration rates.
 However, it is not suitable for course sandy and clay textured
soils.

Border Irrigation SystemBorder Irrigation System
 In a border irrigation, controlled surface flooding is
practised whereby the field is divided up into strips by
parallel ridges or dykes and each strip is irrigated
separately by introducing water upstream and it
progressively covers the entire strip.
 Border irrigation is suited for crops that can withstand
flooding for a short time e.g. wheat.
 It can be used for all crops provided that the system
is designated to provide the needed water control for
irrigation of crops.
 It is suited to soils between extremely high and very
low infiltration rates.

Border Irrigation SystemBorder Irrigation System

Border IrrigationBorder Irrigation

Border Irrigation Contd.Border Irrigation Contd.
 In border irrigation, water is applied slowly.
 The root zone is applied water gradually
down the field.
 At a time, the application flow is cut-off to
reduce water loses.
 Ideally, there is no runoff and deep
percolation.
 The problem is that the time to cut off the
inflow is difficult to determine.


 Basin method of irrigation is adopted mainly in
orchards.
 Usually round basins are made for small trees and
square basin for large trees.
 These basins allow more water to be impounded as the
root zones of orchard plants are usually very deep.
 Each basin is flooded and water is allowed to infiltrate
into the soil.
 Based on type of crop and soil, nearly 5-10 cm depth of
water may be needed for every irrigation.
 The advantage of basin method is that unskilled labour
can be used as there is no risk of erosion.
 Disadvantages : there is difficulty in using modern
machinery and it is also labour intensive.

 Basin irrigation is suitable for many field crops.
 Rice grows best when its roots are submerged in water
and so basin irrigation is the best method to use for this
crop.
 Other crops which are suited to basin irrigation include:
 Pastures, e.g. alfalfa, clover;
 Citrus, banana;
 Crops which are broadcast, such as cereals, and
 To some extent row crops such as tobacco.

Basin Irrigation DiagramBasin Irrigation Diagram
I
rrigation time.

Size of BasinsSize of Basins
 The size of basin is related to stream size and soil type(See
Table below).
 Table : Suggested basin areas for different soil types and rates of water flow
 Flow rate Soil Type
 Sand Sandy loam Clay loam Clay
 l/s m3
/hr .................Hectares................................
 30 108 0.02 0.06 0.12
0.20
 60 216 0.04 0.12 0.24
0.40
 90 324 0.06 0.18 0.36
0.60
 120 432 0.08 0.24 0.48
0.80
 150 540 0.10 0.30 0.60
1.00
 180 648 0.12 0.36 0.72
1.20
 210 756 0.14 0.42 0.84
1.40
 240 864 0.16 0.48 0.96
1.60
 300 1080 0.20 0.60 1.20
2.00
 Note: The size of basin for clays is 10 times that of sand as the infiltration rate for clay is low
leading to higher irrigation time. The size of basin also increases as the flow rate increases. The
table is only a guide and practical values from an area should be relied upon. There is the need for
field evaluation.

 Most common among surface irrigation
 Suitable for close growing crops like groundnut,
wheat, finger millet, pearl millet, paragrass etc.
 In this method field is divided into small plots
surrounded by bunds on all four sides.
 Water from head channel is supplied into the field
channel one after the other.
 Each field channel supplies water to two rows of
check basins and water is applied to one basin after
other.

 In this, field is laid out into long, narrow,
strips, bordering with small bunds.
 Most common size of strips are 30-50 m
length and 3-5 m width.
 Borders are laid out along the general slope.
 Water from the channel is allowed into each
strip at a time.
 This method is suitable for close growing
crops and medium to heavy textured soils.
 Not suitable for sandy soils.

 It should be applied only to flat lands that do not
concave or slope downhill so that the water can
evenly flow to all parts of the field.
 Yet even so, about 50% of the water is wasted
and does not get used by the crops.
 Some of this wasted water accumulates at the
edges of a field and is called run-off.
 In order to conserve some of this water, growers
can trap the run-off in ponds and reuse it during
the next round of flood irrigation.

 In flood irrigation, a large amount of
water is brought to the field and flows
on the ground among the crops.
 In regions where water is abundant,
flood irrigation is the cheapest
method
 This low tech irrigation method is
commonly used by societies in
developing countries.

 However a large part of the wasted water can not be
reused due to massive loss via evaporation and
transpiration.
 One of the advantages of flood irrigation is its ability to
flush salts out of the soil, which is important for many
saline intolerant crops.
 However, the flooding causes an anaerobic environment
around the crop which can increase microbial conversion
of nitrogen from the soil to atmospheric nitrogen, or
denitrification, thus creating low nitrogen soil.
 Surge flooding is an attempt at a more efficient version
of conventional flood irrigation in which water is released
onto a field at scheduled times, thus reducing excess
run-off.

 - Irrigation to crops by applying water from
beneath the soil surface either by
constructing trenches or installing
underground perforated pipe lines.
 In this system, water is discharged into
trenches.
 And allowed to stand during the whole
period of irrigation for lateral and upward
movement of water by capillarity to wet
the soil between the trenches.

 Conditions that favor subsurface irrigation
 An impervious subsoil at a depth of 2 m or more.
 A very permeable subsoil of reasonably uniform texture
permitting good lateral and upward movement of water.
 Permeable loam or sandy loam surface soil.
 Uniform topographic conditions and moderate slope.
 Existence of high water table.
 Irrigation water is scarce and costly.
 Soils should be free of any salinity problem.
 It must be ensured that no water is lost by deep percolation.
 Subsurface irrigation is made by constructing a series of
ditches or trenches 60 to 100 cm deep.
 Width of the trenches is about 30 cm and vertical.
 Spacing between the trenches varies between 15 to 30 m
depending on soil types and lateral movement of water in soils.

 Various types of crops, particularly with shallow root
systems are well adapted to subsurface irrigation
system.
 Wheat, potato, beet, peas, fodder crops etc.
 Advantages
 Maintenance of soil water at favorable tension
 Loss of water by evaporation is held at minimum
 Can be used for soils with low water holding
capacity and high infiltration rate where surface
irrigation methods cannot be adopted and sprinkler
irrigation is expensive.

 Presence of high water table.
 Poor quality irrigation water cannot be
used-good quality water must be available.
 Chances of saline and alkali conditions
being developed by upward movement of
salts with water.
 Soils should have a good hydraulic
conductivity for upward movement of
water.

 Sprinkler irrigation is a method of applying irrigation water
which is similar to natural rainfall.
 Water is distributed through a system of pipes usually by
pumping.
 Water under pressure is carried and sprayed into the air
above the crop through a system of:
 Overhead perforated pipes, nozzle lines, or through nozzles
fitted to riser pipes attached to a system of pipes laid on the
ground.
 Nozzles of fixed type or rotating under the pressure of water
are set at suitable intervals in the distribution pipes.
 Sprayed water wets both the crop and the soil and, hence,
has a refreshing effect.
 Water is applied at a rate less than the intake rate
of soil so that there is no runoff.
 Measured quantity of water is applied to meet the
soil water depletion.

 Sprinkler irrigation is suited for most row, field and tree
crops and water can be sprayed over or under the crop
canopy.
 Large sprinklers are not recommended for irrigation of
delicate crops such as lettuce because the large water
drops may damage the crop.
 Suitable slopes
 Sprinkler irrigation is adaptable to any farmable slope,
whether uniform or undulating.
 Lateral pipes supplying water to the sprinklers should
always be laid out along land contour.
 This will minimize the pressure changes at the sprinklers
and provide a uniform irrigation.

 Sprinklers are best suited to sandy soils
with high infiltration rates although they
are adaptable to most soils.
 Application rate from the sprinklers (in
mm/hour) is always chosen to be less
than the basic infiltration rate of the soil -
so that surface ponding and runoff can be
avoided.
 Sprinklers are not suitable for soils which
easily form a crust.

 A typical sprinkler irrigation system consists of the
following components:
 Pump unit
 Mainline
 Laterals
 Sprinklers

 Suitable irrigation water
 A good clean supply of water, free of suspended
sediments, to avoid problems of sprinkler nozzle
blockage and spoiling the crop by coating it with
sediment.

Components of Sprinkler IrrigationComponents of Sprinkler Irrigation

136
Sprinkler irrigation: Criteria
• Must permit cost recovery within one to two
years (and double investment in a short time)
• Must be suitable for use on small and irregular
shaped plots
• Must require only simple maintenance and tools
• Have a low risk of component failure
• Be simple to operate
• Be durable and reliable – able to withstand
rough and frequent handling without serious
damage

137
Sprinkler irrigation: System layout

139
Sprinkler irrigation: Drag hose system

141
Sprinkler irrigation: Spray pattern

142
Sprinkler irrigation: Spray pattern

143
Sprinkler Spray pattern: Variation in pressure

144
Sprinkler irrigation: Variation in pressure

145
Sprinkler irrigation: Hand move laterals

146
Sprinkler irrigation: Drag hose system

147
Sprinkler irrigation: Centre pivot system

148

149

150
Sprinkler irrigation: Linear move system

151

152

153
Sprinkler irrigation: Mobile Raingun

154
Sprinkler irrigation: Mobile Raingun

Raingun Irrigation SystemRaingun Irrigation System

Lateral DischargeLateral Discharge
The Discharge (QL) in a Lateral is defined
as the flow at the head of the lateral
where water is taken from the mainline
or submain.
Thus: QL = N. qL Where N is the number
of sprinklers on the lateral and qL is the
Sprinkler discharge (m3
/h)

Pressure at Head of LateralPressure at Head of Lateral
 The Pressure requirements (PL)where the
Lateral joins the Mainline or Submain are
determined as follows:
 PL = Pa + 0.75 Pf + Pr For laterals laid on
Flat land
 PL = Pa + 0.75 (Pf Pe) + Pr For Laterals
on gradient.
 The factor 0.75 is to provide for average
operating pressure (Pa) at the centre of the
Lateral rather than at the distal end. Pr is the
height of the riser.
±

Pumping RequirementsPumping Requirements
Maximum Discharge (Qp) = qs N
Where:
qs is the Sprinkler Discharge and
 N is the total number of Sprinklers operating at
one time during irrigation cycle.
 The Maximum Pressure to operate the system
(Total Dynamic Head, Pp) is given as shown in
Example.

DRIP OR TRICKLE IRRIGATIONDRIP OR TRICKLE IRRIGATION
 Introduction: In this irrigation system:
 i) Water is applied directly to the crop ie. entire field is
not wetted.
 ii) Water is conserved
 (iii) Weeds are controlled because only the places
getting water can grow weeds.
 (iv) There is a low pressure system.
 (v) There is a slow rate of water application somewhat
matching the consumptive use. Application rate can be
as low as 1 - 12 l/hr.
 (vi) There is reduced evaporation, only potential
transpiration is considered.
 vii) There is no need for a drainage system.

 Drip irrigation / trickle irrigation - involves dripping
water onto the soil at very low rates (2-20
litres/hour)
 -from a system of small diameter plastic pipes
fitted with outlets called emitters or drippers.
 Water is applied close to plants so that only part of
the soil in which the roots grow is wetted (Figure
60 in Notes).
 With drip irrigation water, applications are more
frequent (usually every 1-3 days).
 This provides a very favourable high moisture
level in the soil in which plants can flourish.

163
Water Use for Trickle Irrigation System
• The design of drip system is similar to that of
the sprinkler system except that the spacing of
emitters is much less than that of sprinklers
and that water must be filtered and treated to
prevent blockage of emitters.
• Another major difference is that not all areas
are irrigated.
• In design, the water use rate or the area
irrigated may be decreased to account for this
reduced area.

164
Micro irrigation: Root zone

165
Micro irrigation: Emitters

166
Micro irrigation: Thick walled drip hose

 While drip irrigation may be the most expensive method of irrigation, it is
also the most advanced and efficient method in respect to effective
water use.
 Usually used to irrigate fruits and vegetables
 System consists of perforated pipes that are placed by rows of crops or
buried along their root lines and emit water directly onto the crops that
need it.
 As a result, evaporation is drastically reduced and 25% irrigation water
is conserved in comparison to flood irrigation.
 Drip irrigation also allows the grower to customize an irrigation program
most beneficial to each crop.
 Fertigation is possible.
 Caution : Water high in salts / sediments should be filtered - otherwise
they may clog the emitters and create a local buildup of high salinity soil
around the plants if the irrigation water contains soluble salts.

 Drip irrigation is most suitable for row
crops (vegetables, soft fruit), tree and
vine crops where one or more emitters
can be provided for each plant.
 Generally only high value crops are
considered because of the high capital
costs of installing a drip system.

 Drip irrigation is adaptable to any
farmable slope.
 Normally the crop would be planted
along contour lines and the water
supply pipes (laterals) would be laid
along the contour also.
 This is done to minimize changes in
emitter discharge as a result of land
elevation changes.

 Drip irrigation is suitable for most soils.
 On clay soils water must be applied slowly
to avoid surface water ponding and runoff.
 On sandy soils higher emitter discharge
rates will be needed to ensure adequate
lateral wetting of the soil.

 One of the main problems with drip irrigation is blockage of
the emitters.
 All emitters have very small waterways ranging from 0.2-
2.0 mm in diameter and these can become blocked if the
water is not clean.
 Thus it is essential for irrigation water to be free of
sediments.
 ]If this is not so then filtration of the irrigation water will be
needed.
 Blockage may also occur if the water contains algae,
fertilizer deposits and dissolved chemicals which precipitate
such as Ca and Fe.
 Filtration may remove some of the materials but the
problem may be complex to solve and requires an
experienced professional.

SOIL TYPE AND WATER MOVEMENT.
THE APPLICATION OF WATER IS BY
DRIPPERS
172

 A typical drip irrigation system is
shown in Figure 61 and consists of
the following components:
 Pump unit
 Control head
 Main line
 Laterals
 Emitters or drippers.

 Pump unit takes water from the source and provides
the right pressure for delivery into the pipe system.
 The control head consists of valves to control the
discharge and pressure in the entire system.
 It may also have filters to clear the water.
 Common types of filter include screen filters and graded
sand filters which remove fine material suspended in the
water.
 Some control head units contain a fertilizer or nutrient
tank.
 These slowly add a measured dose of fertilizer into the
water during irrigation.
 This is one of the major advantages of drip irrigation over
other methods.

 Supply water from the control head into the fields.
 They are usually made from PVC or polyethylene hose and
should be buried below ground because they easily degrade
when exposed to direct solar radiation.
 Lateral pipes are usually 13-32 mm diameter.
 Emitters or drippers are devices used to control the discharge of
water from the lateral to the plants.
 They are usually spaced more than 1 metre apart with one or
more emitters used for a single plant such as a tree.
 For row crops more closely spaced emitters may be used to wet
a strip of soil.
 Many different emitter designs have been produced in recent
years.
 The basis of design is to produce an emitter which will provide a
specified constant discharge which does not vary much with
pressure changes, and does not block easily.

 The water savings that can be made using drip
irrigation are the reductions in deep percolation, in
surface runoff and in evaporation from the soil.
 These savings, it must be remembered, depend as
much on the user of the equipment as on the
equipment itself.
 Drip irrigation is not a substitute for other proven
methods of irrigation.
 It is just another way of applying water.
 It is best suited to areas where water quality is
marginal, land is steeply sloping or undulating and of
poor quality, where water or labour are expensive, or
where high value crops require frequent water
applications.

Water Use for Trickle IrrigationWater Use for Trickle Irrigation
SystemSystem Contd.Contd.
 Karmeli and Keller (1975) suggested the
 following water use rate for trickle irrigation design
 ETt
= ET x P/85

 Where: ETt
is average evapotranspiration rate for crops under
trickle irrigation;
 P is the percentage of the total area shaded by crops;
 ET is the conventional evapotranspiration rate for the crop. E.g.
If a mature orchard shades 70% of the area and the
conventional ET is 7 mm/day, the trickle irrigation design rate is:
 7/1 x 70/85 = 5.8 mm/day
 OR use potential transpiration, Tp
= 0.7 Epan
where Epan
is the
evaporation from the United States Class A pan.

EmittersEmitters
 Consist of fixed type and variable size types.
The fixed size emitters do not have a
mechanism to compensate for the friction
induced pressure drop along the lateral while
the variable size types have it.
 Emitter discharge may be described by:
 q = K h x
 Where: q is the emitter discharge; K is
constant for each emitter ; h is pressure head
at which the emitter operates and x is the
exponent characterized by the flow regime.

Water Distribution from EmittersWater Distribution from Emitters
 Emitter discharge variability is greater than that of
sprinkler nozzles because of smaller openings(lower
flow) and lower design pressures.
 Eu = 1 - (0.8 Cv/ n 0.5
)
 Where Eu is emitter uniformity; Cv is manufacturer's
coefficient of variation(s/x ); n is the number of
emitters per plant.
 Application efficiency for trickle irrigation is
defined as:
 Eea
= Eu x Ea x 100
 Where Eea
is the trickle irrigation efficiency; Ea is the
application efficiency as defined earlier.

Pressure Head at Manifold InletPressure Head at Manifold Inlet
 Like Sprinklers, the pressure head at inlet to
the manifold:
 = Average Operating Head = 8.9 m
 + 75% of Lateral and Manifold head Loss =
0.75 (0.51 + 0.68)
 + Riser Height = Zero for Trickle since no risers
exist.
 + Elevation difference = Zero , since the field is
Level
 = 9.79 m

Solution ConcludedSolution Concluded
 Total Head for Pump
 = Manifold Pressure = 9.79 m
 + Pressure loss at Sub-main = 6.59 m
 + Pressure loss at Main = 2.90 m
 + Suction Lift = 20 m
 + Net Positive Suction head for pump = 4 m
(assumed)
 = 43.28 m
 i.e. The Pump must deliver 3.23 L/s at a head of
about 43 m.

SUB-SURFACE IRRIGATIONSUB-SURFACE IRRIGATION
 Applied in places where natural soil and
topographic condition favour water
application to the soil under the surface, a
practice called sub-surface irrigation.
These conditions include:
 a) Impervious layer at 15 cm depth or
more
 b) Pervious soil underlying the restricting
layer.
 c) Uniform topographic condition
 d) Moderate slopes.

SUB-SURFACE IRRIGATIONSUB-SURFACE IRRIGATION (Contd…)(Contd…)
 The operation of the system involves a
huge reservoir of water and level is
controlled by inflow and outflow.
 The inflow is water application and rainfall
while the outflow is evapotranspiration
and deep percolation.
 It does not disturb normal farm
operations. Excess water can be removed
by pumping.

Principles of irrigation by Dr Thomas Abraham_Course Code_Chapters 1 to 5__26-3-2014

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