CHAPTER II
 BASICS IN IRRIGATION ENGINEERING
2.1. Planning Irrigation
systems
2.2. soil-plant-water relation –
over view
2.3. Crop water requirement
2.4. Base, delta and duty
2.2: Soil-Plant-Water Relationships
• Soil- Plant Water relationships relate the
properties of soil that affect the
movement, retention and use of water.
• It can be divided & treated as:
• Soil-water relation
• Soil-plant relation
• Plant-water relations
– Movement
– Retention
– utilization
2.2: Soil-Plant-Water Relationships cont…
• Simple analogy:
•
Soil – Water Reservoir
•
Plant Roots – pump with many inlets
• Rate of pumping = f(character of the pump)
• Rate of extraction = f(character of soil)
• The physical and chemical properties of soil
are very important in irrigation.
Soil Suitability for agricultural practices
• Soil is a very important agricultural complement with
out which no agricultural is possible.
• It is important to study the soil characteristics to say
a particular soil type is suitable for agriculture or
not.
• The process whereby the suitability of land for
different uses such as agriculture is assessed is
known as land evaluation.
• Land evaluation for agricultural purpose provides
information for deciding ‘which crops to grow where’
and other related crops.
• Hence, before a land is put certain land uses, its
suitability for that particular land use should be
evaluated.
Soil Suitability for agriculture cont..
• Soil map provides us with detailed information
on soils that are utilized for land capability
classification.
• This indicates the suitability or unsuitability of
the soil for growing crops.
• Land capability classification is an interpretive
grouping of soils based on inherent soil
characteristics, external land features and
environmental factors that may restrict the
use of the land for growing varieties of crops.
Soil Suitability for agriculture cont..
• For land capability classification we need
information on:
• 1) The susceptibility of the soil to various factors
that cause soil damage & decrease in its
productivity.(we get this from soil map)
• 2) Its potential for crop production (obtained by
growing crops).
• Lands are first tentatively placed in different land
capability groups on the basis of slope of the land,
erosion and depth of the soil.
• The suitability of soil for agricultural practices may
be affected by physical and chemical soil
characteristics.
Physical characteristics
1. Effective soil depth:• The depth of the soil which can be exploited
by crops is very important in selecting soils
for agricultural purpose.
• Experience has shown that many irrigated
crops produce excellent yields with a well
drained effective root depth of 90 cm.
Physical characteristics cont…
2. Water Holding capacity:• This refers to the depth of water that can be held in
the soil and available for plants.
• A good soil from agricultural point of view should
have a very good water holding capacity.
• Clay soils have large water holding capacity,
because drainage water is low in these soils.
• Ideally loam soils are the best in this regard.
• Sandy soils application losses are high and
• in clay soils drainage and aeration is difficult.
Physical characteristics cont…
3. Non – capillary porosity:• High values of non-capillary porosity is desirable,
• lower values of porosity and high values of bulk
density hinders root development and expansion.
4. Topography: • A leveled land is the most suitable for agriculture.
• Because, the water for irrigation can easily be
conveyed and less conservation and management
practices are required.
• Where as, in sloppy soils, the more is the land wasted
in bunds and channels in surface irrigation and there
fore that cost for land development per unit area will
be high
Physical characteristics cont…
5. Texture:• It is the weight percentage of the mineral matter
that occurs in each of the specified size fractions of
the soil.
• It is the particle size of the soil
• It is the relative proportions of sand silt and clay,
(Particles sized groups smaller than gravel i.e. < 2 mm
in diameter).
• It is the number and size of its mechanical particles
after all compounds holding them together have been
destroyed.
• Loamy soils are the best texture for agriculture.
• Deviation either into sandy or clayey texture will
reduce the value of the land for agriculture.
Physical characteristics cont…
6. Soil Structure:
• It refers to the manner in which primary soil
particles are arranged into, secondary particles
or peds or aggregates.
• It is the arrangement of soil particles
• Soil structure determines the total porosity, the
shape of individual pores and their size distribution,
Hence it affects: • - Retention & transmission of fluids in the soil
• - Germination, root growth,
• - Tillage, Erosion etc.
Physical characteristics cont…
7. Soil Consistence:
• Is the resistance of the soil to deformation
or rupture.
• It is determined by the cohesive and adhesive
properties of the entire soil mass.
• Structure deals with size, shape and
distinctness of natural soil aggregates, and
consistence deals with strength and nature of
the force between particles.
• It is important for tillage or traffic
consideration.
Physical characteristics cont…
• Soil Consistence Terms:
• - Consistence is described for three moisture
levels: wet, moist & dry.
• For instance, a given soils may be sticky when
wet, firm when moist and hard when dry.
• The terms to describe soil consistency include: 1) Wet soil - non sticky, sticky, non plastic, plastic
2) Moist soil - loose, friable, firm
3) Dry soil - loose, soft, and hard.
Physical characteristics cont…
8. Soil Permeability and Hydraulic Conductivity
• Permeability - is the ease with which liquids,
gases and roots pass through the soil.
• Hydraulic conductivity- is the permeability of
the soil for water. I.e. the ease with which the
soil pores permit water movement.
• It controls the soil water movement.
Physical characteristics cont…
• The major factors affecting hydraulic conductivity
are
1. texture and structure of soils.
• Sandy soils have higher saturated conductivity than
finer textured soils.
• Soils with stable granular structure conduct water
rapidly than those with unstable structural units,
since they will not break down when get wetted.
• Fine textured soils during dry weather because of
their cracks allow water rapidly then the cracks
swell shut, and drastically reduce water movement
USDA Textural
Triangle
Soil structure types and their effect on downward movement of water (Source:
USDA, 1997)
Single grain
Blocky
Plate-like
Rapid
Granular
Rapid
Moderate
Prismatic
Moderate
Slow
Massive
Slow
Physical characteristics cont…
2. Salinity (soluble salt content) – hydraulic
conductivity
• When the quantity of salts in irrigated land is
too high, the salts accumulate in the crop root
zone.
• These salts create difficulty to crops in
extracting enough water from the salty
solution. Osmotic effect
• Thus, for the land to be of high value for
irrigation, the soluble salt content should be
low as much as possible.
Physical characteristics cont…
3. Amount of Exchangeable sodium:- hydraulic
conductivity
• When the amount of exchangeable sodium is high
in the soil, the soil will have large amount of Na+ in
the form of colloid. Soil dispersion
• This results in tremendous reduction of the
permeability of the soil.
• This in turn makes it difficult to the cop to get
sufficient water and causes crusting of seedbeds.
• Such a soil is called Black alkali soil.
• Hence, the amount of exchangeable sodium should
be low in agricultural lands.
Physical characteristics cont…
4. Soil Reaction (PH) :
• PH of a soil is a measure of its acidity or
alkalinity.
• It is a measure of the concentration of hydrogen
ion in a soil.
• Mathematically,
• Excessively low or high pH values are not good for
proper growth and adequate yield production as
they bring about acidity or alkalinity in the soil.
Key roles of soils in any ecosystem
• In general, in any ecosystem, (a farm, forest,
regional water shed etc.) soils have five key roles
1. Medium for plant growth:
• It supports the growth of higher plants by providing
a medium for plant roots and supplying nutrient
elements that are essential to the entire plant.
2. Regulator of water supplies:
• Its properties are the principal factor controlling the
fate of water in the hydrologic system.
• Water loss, utilization, contamination, and
purification are all affected by the soil.
Key roles of soils in any ecosystem cont..
3. Recycler of raw materials:
• With in the soil, waste products and dead bodies of
plants, animals and people are assimilated, and their
basic elements are made available for reuse by the next
generation of life.
4. Habitat for soil organisms:
• It provides habitats for living organism, from small
mammals and reptiles to tiny insects to microscopic
cells.
5. Engineering medium: In human - built ecosystem, soil
plays an important role as an engineering medium.
• It is not only an important building material (earth fill,
bricks) but provides the foundation for virtually every
road, airport, and house we build.
Key roles of soils in any ecosystem cont..
6. Soils In relation to irrigation:
• The capacity of the soil to accept, transmit or retain
relatively large amounts of water (Water holding capacity
of the soil) in a relatively short time should be measured.
• The surface infiltration rates and the case of water
movement through unsaturated and saturated layers
(hydraulic conductivity) need to be measured
quantitatively.
• The amount, kind and distribution of clay minerals (Soil
chemical properties) are specially important to water
movement, relation and availability of plants.
• Studies of cracking and structural changes under
different management practices (helps surface sealing
or a need of pre irrigation)
Soil- water relations
• It means that physical properties of soil in
relation to water
•
•
•
•
The rate of entry of water in to the soil
its retention,
movement
availability to plant roots are all physical
phenomena.
• Hence it is important to know the physical
properties of soil in relation to water.
Soil- water relations cont…
• Quantity of water in a soil as determined by its
moisture content does not give a true indication of
the soil ‘wetness’.
• A clay soil, which on handling feels dry, can be at
the same moisture content as a sandy soil, which
feels wet.
• A plant will have less difficulty extracting water from
a sandy soil than from a clay soil at the same
moisture content.
• The Concept of Soil Water Potential is therefore
used in Soil/Plant/Water Relations
Soil- water relations cont…
• The flow of water in any hydraulic system, including
the soil-plant-water system, takes place from a state
of higher to one of lower potential energy.
• Soil moisture/water: the relative water content in the
soil.
• Due to the complex nature of the soil system, soil
water can be best explained by taking the soil
systems as a simple reservoir which contains solids
(soil particles), liquid (water), and gas (air).
• There is need for a soil ‘wetness’ which reflects the
ease or difficulty of extraction of water from the soil
by the plant.
Volume
Relationship
Mass
Relationship
Va
Ma
Vw
Mw
Vs
Ms
Fig. 2.1 Illustration of volume and mass relationships of the soil system
Thus, Soil moisture can be expressed in the form of:
a) Dry -weight basis
MC = sample wet wt. - sample dry wt.
sample dry wt.
MC (% ) =
wt. of water (Mw) x 100
wt. of solid (Ms)
b) Volumetric basis
V = Vw =
Vw
x 100
Vt
Vs + Va + Vw
V = ρb x MC
Where, ρb = bulk density = ρb = Ms
Vt
c) Equivalent Depth: This is expressed in depth e.g. in mm.
normally
used in irrigation engineering.
This is
d = ρb x MC x D
where: d is the equivalent depth of water applied (mm); D is the root zone
depth (mm).
Volumetric Water Content & Equivalent Depth
(cm3)
Equivalent Depth
(g)
(g)
(cm3
)
Volumetric Water Content &
Equivalent Depth
Typical Values for Agricultural Soils
Soil Solids (Particles): 50%
0.50 in.
1 in.
Very Large Pores:
(Gravitational Water)
Total Pore
Space: 50%
15%
0.15 in.
Medium-sized Pores: 20%
(Plant Available Water)
0.20 in.
Very Small Pores:
(Unavailable Water)
0.15 in.
15%
Measurement of Soil moisture
• Determination of moisture content of a given
soil is important in irrigation for the following
three main points.
• For suitable scheduling of irrigation. i.e. to
estimate the amount and when to apply.
• To estimate evapotranspiration needs of the
crop
• For proper interpretation of soil-plant-water
relationships
Method of soil moisture determination
A) By Feel and touch: This is by far the
easiest method.
• Assessment by feel is good for experienced
people who have sort of calibrated their
hands.
Gravimetric Method:
• known volume of soil samples are taken from the field,
weighed, and then dried in an oven for 24hours at an
average temperature of 105 0C.
• After dried, the samples will be taken out from the oven
and weighed again.
• The difference in weight before and after drying is the
amount of moisture present in the soil.
•
M= Mwet – Mdry x 100
Mdry
• Gravimetric method is an accurate method but time
consuming.
• Moreover, the method is not practical for farm use, as the
oven cannot ordinarily be owned by farmers.
• But it is a standard against which other methods of
moisture determination are compared.
•
•
•
•
•
•
Neutron Probe:
It consists of a probe lowered down a hole in
the soil
A box (rate meter or rate scalar) is at the top.
Within the probe is a radioactive source e.g.
beryllium (435 years life span).
Close to the source is a detector.
The source emits fast neutrons,
some of which are slowed down when they
collide with water molecules (due to
hydrogen molecules).
Neutron Probe cont…
• A cloud of slow neutrons (thermal neutrons)
build up near the probe and are registered by
the rate meter or rate scalar which measures
the number of slowed down neutrons.
• The method is quick but very expensive.
• It is also dangerous since it is radioactive and
must be used with care.
NEUTRON PROBE
Fig. 2.1: Diagram and Photograph of Neutron Probe in Use
Soil Water Measurement
Neutron Attenuation
Electrical Resistance Unit:
• This consists of a porous body with two
electrodes embedded into it.
• The porous body when buried equilibrates
with the soil water and the readings are
obtained through the resistance meters
attached to the electrodes.
• Resistance units are measured and the
instrument needs to be calibrated against
matric suction or volumetric moisture content
(V).
•
•
•
•
Electrical Resistance Unit:
Various porous bodies needed are gypsum,
nylon or fiber glass.
The instrument is relatively cheap but it takes
a long time to equilibrate or react e.g.
48hours.
The method is insensitive in wet soils <0.5
bars.
It measures from 0.5 to 15 bars and more.
– Measure soil water potential (tension)
– Tend to work better at higher tensions (lower
water contents)
Electrical Resistance Blocks & Meters
ELECTRICAL RESISTANCE UNIT
Figure 2.2 Portable meter and resistance blocks used to measure
soil moisture.
•
•
•
•
•
Tensiometer:
Tensiometer operates on the principle that a
partial vacuum is developed
in a closed chamber when water moves out
through the porous ceramic tip to the
surrounding.
A vacuum gauge or a water or mercury
manometer can measure the tension.
The gauge is usually calibrated in centi-bars
or milli-bars.
After the porous cup is put in the soil, the
tensiometer is filled with water.
Tensiometer cont…
• Water moves out from the porous tip to the
surrounding soil (as suction is more in the
soil).
• A point is reached when the water in the
tensiometer is at equilibrium with the soil
water.
• The reading of the gauge is then taken and
correlated to moisture content using a
calibration curve.
• Practical operating range is about 0 to 0.75 bar of
tension (this can be a limitation on medium-and
fine-textured soils)
Tensiometer for Measuring Soil Water Potential
Water Reservoir
Variable Tube Length (12 in- 48 in)
Based on Root Zone Depth
Porous Ceramic Tip
Vacuum Gauge (0-100 centibar)
Tensiometer
•
•
•
•
Infiltration and soil Water movement
Infiltration is the entry of water into the soil.
It is a very important variable in irrigation
design since it shows the rate at which water
can move into the soil mass to replenish the
root zone.
Infiltration rate of a soil is the rate at which
water will enter the soil mass through the
surface.
Infiltration rates into soils depend on soil
texture and structure, density, organic matter
content, hydraulic conductivity (permeability)
and porosity.
Infiltration and soil Water movement cont.
• As wetting time increases, the infiltration rate
decreases and usually approaches a
constant value, which in the case of heavy
clays may be zero.
• A general equation for the Infiltration rate (I)
is the Modified Kostiakov equation:
I = (a tn )+ b
mm/hr
• Where:
• a and n are constants; t is the elapsed
wetting time and
• b is the basic infiltration rate
Methods of Measuring Infiltration
• For Flooded irrigation (border strip and basin), a
double infiltrometer is normally used.
• For furrow irrigation, the difference between inflows
and outflows of water flowing through hydraulic
flumes placed at different distances of test furrows
represent the total infiltration
• Ring infilterometer consists of two concentric
cylinders, the inner about 0.3m diameter, the outer
0.6m.
• Water is maintained at the same level in each
cylinder, 25 mm above the soil surface, or more if
the water level is likely to be higher during irrigation.
Methods of Measuring Infiltration cont..
• The water infiltrating from the outer ring
prevents lateral seepage by the water from
the center cylinder.
• By measuring the rate at which the water is
added to the center cylinder, the infiltration
rate can be found.
Double ring infiltrometer
Infiltration Rate vs. Time
For Different Soil Textures
Cumulative Infiltration Depth vs. Time
For Different Soil Textures
Soil Water Classification
• Gravitational water:
– It is the water in the large pores that moves downward
freely under the influence of gravity
– It drains out so fast that it is not available to the crops.
– The time of draining out varies from one day in sandy soils
to three days in clay soils.
• Capillary Water:
– It is the amount of water retained by the soil after
gravitational water has drained out.
– It is the water in the small pores which moves because of
capillary forces and is called capillary water.
– Capillary water is the major source of water available for
the plant
• Hygroscopic Water
– Soil moisture further reduced by ET until no longer moves
because of capillary forces. The remaining water which is
held on particle surfaces so tightly is called hygroscopic
water.
– the water is held by adhesive force. And therefore, it is
unavailable to the plant.
Available water
• Water may also be classified as unavailable,
available and gravitational or superfluous.
• Such a grouping refers to the availability of soil
water to plants.
• Gravitational water drains quickly from the root zone
under normal drainage conditions.
• Unavailable water is held too tightly by capillary
forces and is generally not accessible to plant roots.
• Available water is the difference
gravitational and unavailable water.
between
Available water cont..
• Sandy soils drain readily,
• clay soils drain very slowly.
• one day after irrigating a sandy soil, most of
the gravitational water has drained out of the
soil,
• where as clay may require 4 or more days for
gravitational water to drain.
2.3.4 Soil Water Classification
Fully Saturated
Gravitational
Water
Field Capacity
Capillary Water
Hygroscopic Water
Permanent
Wilting Point
Complete dry
• Fig. Soil moisture levels and available water ranges
Soil Water Constants
• The following soil moisture contents are of
significance importance in agriculture and
are termed soil moisture constants.
• Saturation Capacity: – When all the micro and macro pore spaces are
filled with water, the soil is said to have reached
its saturation capacity.
– At field capacity the water is held loosely and
tensions are almost nil.
• Thus, plants will not have any difficulty in
extracting moisture from soil.
Field Capacity (FC):
– This is the amount of water a well-drained soil
contains after gravitational water movement has
materially ceased.
– It is taken as the water content after 48 hours the
soil has been subjected to heavy rainfall or
irrigation sufficient to cause saturation.
– Field capacity can also be determined by finding
the moisture content when suction is 1/3 bar for
clay and 1/10 bar for sand.
– At field capacity, the macro pores are filled with
air & capillary pores filled with water.
– Field capacity is the upper limit of available soil
moisture.
Field Capacity (FC):
• It is often defined as:
• moisture content in a soil two days (light sandy soil)
• or three (heavy soil), days after having been
saturated and after drainage of gravitational water
becomes slow or negligible and moisture content
has become stable.
Some factors which influence FC.
- Soil texture
- Presence of impending layer (soil profile),
arise from plaguing the same depth yearly
- hard pan.
• The volumetric moisture content at field
capacity is given by
•
•
•
•
Field Capacity determination
Field capacity can be determined by:
ponding water on a soil surface
in an area of about 2 to 5 m2 and
allowing it to drain for one to three days
preventing surface evaporation.
• Then soil samples are taken from different
depths and the moisture content is
determined as usual, which gives the field
capacity.
Permanent Wilting Point (PWP):
• Permanent Wilting Point: - is the moisture content
beyond which plants can no longer extract enough
moisture and remain witted unless water is added
to the soil.
• The plant will wilt and may die later if water is not
available.
• The water beyond the permanent wilting point is
tightly held to the solid particles that plants cannot
remove moisture at their normal rate to prevent
wilting of the plants.
• Water tension of soil at PWP is generally taken as
15 bars.
Permanent Wilting Point (PWP) cont..
• The soil moisture tension at PWP ranges from 7 to
32 atm. depending on the soil texture, kinds of
crops and salt content in the soil solution.
• For field estimation, a crop is planted and when it
wilts, the moisture content is the PWP.
• This technique requires personal judgment and
prone to mistakes.
• Since the change in moisture content (∆θ) is
insignificant for changes in SMT from 7 to 32 atm.
• Hence, 15 atm. is taken as SMT at PWP.
Permanent Wilting Point (PWP) cont…
• At PWP the plant starts wilting, and if no
water is given to the plant, then it will die.
N.B. Θv (PW)
=
ρb * θm(WP)
• (volumetric moisture content at Permanent
wilting point
Total Available Water (TAW): Soil moisture
range
– The soil moisture b/n field capacity and
permanent wilting point is called available
water.
– It is the water content between field capacity
and permanent wilting point.
TAW = FC – PWP
TAW = (Θv (FC) - θv(WP) ) D
– Where, D =root depth of the crop
Total Available Water (TAW): Soil moisture
range
• This is the water available to crops.
• Fine grained soils generally have a wider
range of available moisture than course
textured soil.
Moisture content (%) Available Depth of water
Soil type
per unit depth
water(%)
FC
PWP
(cm/m depth)
Fine sand
3 to 5
1 to 3
2
2 to 4
Sandy
loam
5 to 15
3 to 8
2 to 7
4 to 11
Silty loam
12 to 18
6 to 10
6 to 8
6 to 13
Clayey
Loam
15 to 30
7 to 16
8 to 14
10 to 18
Clay
25 to 40 12 to 20 13 to 20
16 to 30
Management allowed deficit, MAD.
• The degree to which the volume of water in
the soil is allowed to deplete before the next
irrigation is applied.
• That is portion of the available moisture which
is easily extracted by the plant roots.
• It is commonly 60 – 80 % of the available
water.
• P depends on type of crop and crop growing
stages
Soil moisture deficit, SMD
• The depletion of soil moisture below field
capacity at the time that particular soil
moisture content, θv , is measured.
• That is the amount of water required to bring
the soil moisture back to the field capacity.
• Deficit = Fc – soil moisture at that instant.
Readily Available Water (RAW):
– This is the level to which the available
water in the soil can be used up without
causing stress in the crop.
– For most crops, 50 to 60% available water
is taken as readily available.
Evaluation of Soil Water
TAW
Yield
RAW
PWP
θC
Soil Water Content
RAW = (FC – θC) = fraction of TAW
FC
As a rule of thumb, two-third of TAW is easily accessible to plants and on
the average, only three-fourth of the root zone is most effective. Thus,
RAW = 2/3 (FC - pwp). ¾ Drz
RAW = ½ (FC - pwp) . Drz
= ½ TAW
= 0.50 TAW
Or, MAD = 0.50 for most crops
Soil Water Movement
2.4.2 Soil Water Movement
Sandy Soil
 Vertical Movement is much
greater
(y>>x)
x
than
horizontal
 coarse textured, dominated
y
by large sized pores, and is
affected
more
by
gravitational force than
adhesive force
 Suitable for frequent and
low volume irrigation
 Suitable
crop
for
long
rooted
Clay Soil
 Horizontal movement is
much greater than vertical
(x>>y)
x
 dominated by large number
y
of small sized pores,
 have high suction potential,
and not very much affected
by gravitational potential.
 Suitable for flood irrigation
 Suitable for shallow rooted
crops
Loam Soil
 Vertical movement is more
or less equal to
horizontal (y  x)
x
y
the
 Has
properties
intermediate
between
sandy and clay soil
 Suitable for almost all kind
of crops and irrigation