ME31D: DRAINAGE AND
IRRIGATION ENGINEERING
INTRODUCTION
COURSE GOALS

This course has two specific goals:
 (i) To introduce students to basic concepts of
soil, water, plants, their interactions, as well
as irrigation and drainage systems design,
planning and management.
 (ii) To develop analytical skills relevant to the
areas mentioned in (i) above, particularly the
design of irrigation and drainage projects.
Course Outline



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Basic Soil-Plant-Water Relations.
Irrigation Water Requirements,
Sources, quantity and quality of irrigation
water:
Irrigation
planning,
scheduling
and
efficiencies.
Design of irrigation systems and structures.
Design of drainage systems and structures.
Computer
applications
and
simulation
modeling.
Course Objectives

On Completion of this course, students
should be able to:
 (i) Understand the basic soil-plant-water
parameters related to irrigation

(ii) Understand how to estimate the quantity
of water required by crops using manual and
computer methods.

(iii) Be able to plan and design irrigation and
drainage projects.
COURSE OBJECTIVES
CONTD.

(iv) Understand the computer applications in
irrigation and drainage designs.

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(v) Design channels and other irrigation
structures required for irrigation, drainage,
soil conservation, flood control and other
water-management projects.
Course Assessment
(i) One (1) mid-semester test, 1-hour
duration counting for 10% of the total
course.
 (ii)One (1) Project Assignment counting
for 10%.
 One (1) end-of-semester examination, 2
hours duration counting for 80% of the
total course marks.

Reading Materials

(i) James, L.G. (1988). Principles of Farm
Irrigation System Design. John Wiley, New
York.
 (ii) Chin, D.A.. (2000).
Water Resources
Engineering, Prentice Hall, New Jersey.
 (iii) Journal of Irrigation and Drainage
Engineering, American Society of Civil
Engineers.
 (iv) Course comprehensive note book and
other handouts and tutorial sheets.
ME31D: DRAINAGE AND
IRRIGATION ENGINEERING
1.1. Irrigation is the application of water to
the soil to supplement natural precipitation
and provide an environment that is optimum
for crop production.
TYPES OF IRRIGATION

SUPPLEMENTARY IRRIGATION: IN
AREAS WITH RAINFALL FOR A PART
OF THE SEASON OR YEAR

TOTAL IRRIGATION: IN AREAS OF NO
RAINFALL
OBJECTIVES OF
IRRIGATION

To Supply Water
Partially or Totally for
Crop Need
 To Cool both the Soil
and the Plant
 To Leach Excess Salts
 To improve
Groundwater storage

To Facilitate
continuous cropping
 To Enhance Fertilizer
ApplicationFertigation
To Understand Irrigation, One
Needs Knowledge of:

Basic Soil
Science/Physics
 Plants
 Water
 Plant/Soil/Water
Relations

Hydraulics

Hydrology

General Engineering
Principles
1.2
SOIL CONSTITUENTS
Mineral Material: Sand, clay and silt
 Organic matter - very valuable
 Water
 Air

PROPORTIONS OF SOIL
CONSTITUENTS
20%
45%
30%
5%
MINERALS
OM
Water
Air
MINERAL COMPONENTS
Except in the case of organic soils, most
of a soil’s solid framework consists of
mineral particles.
 They
are variable in size and
composition. They can vary from small
rock particles to colloids.

MINERAL COMPONENT CONTD.

The mineral can be raw quartz and other
primary materials – coarse fractions which
have not changed from parent material)
 They can also be silicate clays and iron
oxides formed by the breakdown and
weathering of less resistant minerals as soil
formation progressed.
These are called
secondary minerals.
MINERAL CONSTITUENTS
USDA
ISSS
ROCKS
> 2 mm
> 2 mm
SAND
0.05 to 2 mm
0.02 to 2 mm
SILT
0.002 to 0.05
mm
< 0.002 mm
0.002 to 0.02
mm
< 0.002 mm
CLAY
SAND COMPONENT
Visible to the Naked Eye and Vary in
Size.
 They are Gritty when rubbed between
Fingers.
 Sand Particles do not Adhere to one
another and are therefore not Sticky.

SILT AND CLAY COMPONENTS
Silt Particles are smaller than sand. The
silt particles are too small to be seen without
a microscope. It feels smooth but not sticky,
even when wet.
Clays are the smallest class of mineral
particles. They adhere together to form a
sticky mass when wet and form hard clods
when dry.
SOIL TEXTURE
Relative proportions of the various soil
separates (sand, silt and clay) in a soil.
 Terms such as sandy loam, silty clay,
and clay loam are used to identify soil
texture.
 Soil Components are separated using
Mechanical Analysis, Sieving for Sand
and Rate of Settling in Pipette for Silt
and Clay.

SOIL TEXTURE CONTD.
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From the mechanical analysis, the proportions of
sand, silt and clay are obtained.
The actual soil texture is determined using the Soil
Textural Triangle e.g. for a Soil with 50% sand, 20%
silt and 30% clay, the texture is Sandy Clay Loam.
Arranged in the increasing order of heaviness,
there are 12 soil textures namely: sand, loamy sand,
sandy loam, loam, silt loam, silt, sandy clay loam,
silty clay loam, clay loam, sandy clay, silty clay and
clay.
1.4 COLLOIDAL MATERIAL

The smaller particles (< 0.001 mm) of clay
and similar sized organic particles) have
colloidal properties and can be seen with an
electronic microscope.
 The colloidal particles have a very large area
per unit weight so there are enough surface
charges to which water and ions can be
attracted. These charges make them adhere
together. Humus improves the water holding
capacity of the soil.
1.5 WATER
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.

SOIL WATER CONTD.
There is need for a soil ‘wetness’ which
reflects the ease or difficulty of
extraction of water from the soil by the
plant.
 The Concept of Soil Water Potential is
therefore used in Soil/Plant/Water Relations

1.5.1. Mechanism of Soil
Water Movement
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.
 The steepness of the potential gradient
from one point in the system reflects the
ease with which water will flow down the
potential gradient between the points.

1.5.2 Components of Soil
Water Potential
As in any other hydraulic system, the
total potential (or total hydraulic head) in
the soil-water system is made up of a
number of distinguishable components.
Some of these are as follows:
 i ) Gravitational Potential: Reflects
gravitational forces on the soil water.
Components of Soil Water
Potential Contd.

ii) Pressure Potential: This is positive when
greater than atmospheric pressure, and
negative when below atmospheric.

A negative pressure potential (or tension, or
suction) is also known as the matric potential.
It is characteristic of soil water above a free
water surface.
Components of Soil Water
Potential Contd.

iii) Osmotic Potential: reflects the effect of
solutes in soil water, in the presence of a
semi-permeable membrane

The total potential of soil water at a point is
the sum of all the components of potential,
which are acting. Note that the movement of
water in the soil is slow, so kinetic energy is
neglected.
1.5.3 Soil Water Potential
and Soil Water Content:

If a water pressure less that atmospheric
(usually referred to as suction) is applied to a
saturated soil, some water will drain off until
equilibrium is reached.
 At this state of equilibrium, the total potential
of the soil water relative to a free water
surface at the same elevation will be
negative. Its value is known as the soil
suction or matric suction since it is equal to
the negative pressure potential of the soil
water.
Soil Water Potential and Soil
Water Content Contd.
As the pressure potential is reduced ( i.e.
suction increased) more water is removed
from the soil.
 The relationship between suction and actual
water content is referred to as soil water
characteristic.
 Soil Water Potential is normally measured by
tensiometers (matric potential), hanging water
column (sand box) and pressure chamber.

1.5.4 Methods of Measuring
Soil Water Content

i) By Feel: This is by far the easiest
method. Assessment by feel is good for
experienced people who have sort of
calibrated their hands. The type of soil is
important.
 ii) Gravimetric Method: This is equal to:
Pm 
Mw
Mass of Water

M s Mass of Dry Solids
Gravimetric Method Contd.

Weigh wet soil in a container, put in
oven at 105 oC for about 48 hours;
weigh again and obtain the weight of
water by subtraction. A good soil should
have moisture contents between 5 and
60% and for peat or organic soils, it can
be greater than 100%.
Methods of Measuring Soil
Water Content Contd.
(iii) Volumetric water content, Pv. This
is equal to:
Vw
Volumeof Water
Pv 

Vs Va Vw
TotalVolumeof Undisturbed Soil Sample

Recall that volume = mass/density i.e.
Mw
Dw
Mw
Pv 
and Pv 
x D sin ce Dw 1
b
Ms
Ms
D
b
Pv  Pm x D where D is the bulk densityof the soil
b
b
Soil Bulk Density
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Bulk Density, Db is defined as the mass of a unit
volume of dry soil.
This includes both solids and pores.
i.e. bulk density = Ms/V ;
Ms is the mass of dry soil and V is the total volume of
undisturbed soil.
The major method of measuring bulk density in the
field is to collect a known volume of undisturbed soil
(V) in a soil core, and drying it in the oven to remove
all the water to obtain Ms.
Methods of Measuring Soil
Water Content Contd.
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(iv) 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).
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.
NEUTRON PROBE
Fig. 1.3: Diagram and Photograph of Neutron Probe in Use
The method is quick but very expensive.
It is also dangerous since it is radioactive and must be used with care.
1.5.5 Methods of Measuring
Soil Water Suction
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i)
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
(Pv).
Various porous bodies needed are gypsum, nylon or fibreglass.
The instrument is relatively cheap but it takes a long time to
equilibrate or react e.g. 48 hours. The method is insensitive in
wet soils <0.5 bars. It measures from 0.5 to 15 bars and more.
ELECTRICAL RESISTANCE
UNIT
Figure 1.4 Portable meter and resistance blocks used to measure
soil moisture.
(Courtesy Industrial Instrument, Inc.)
Methods of Measuring Soil
Water Suction Contd.
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ii) 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
centibars or millibars.
After the porous cup is put in the soil, the tensiometer is
filled with water. 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.
1.5.6 Soil Water Equilibrium
Points
In a soil, which is completely saturated,
large pores are filled with what is called
gravitational water because it can drain
out under 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 four days in clay soils.

Soil Water Equilibrium Points
Contd.

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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.
There still remains the water held loosely between the soil
particles by surface tension at field capacity. This is called
capillary water and is the main source of water for plant growth.
Plants continuously take this up until there is no more water
available for crop growth and wilting occurs.
SOIL MOISTURE EQUILIBRIUM
POINTS CONTD.

Permanent Wilting Point (PWP): This is the
soil moisture content at which crops can no
longer obtain enough water to satisfy
evapotranspiration needs.
 The plant will wilt and may die later if water is
not available. Water tension of soil at PWP is
generally taken as 15 bars.
 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.
SOIL MOISTURE EQUILIBRIUM
POINTS CONTD.

Available Water (AW): This is the water
available to crops. It is the water content at
field capacity minus that at permanent wilting
point.

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.
Typical Soil Water Equilibrium
Points
Field
Capacity
(FC)
(By Weight)
Permanent
Available
Wilting Point Water (AW)
(PWP)
(By Weight)
Readily
Available
Water = 0.5
AW
Clay
45
30
15
7.5
Clay Loam
40
25
15
7.5
Fine Sand
15
8
7
3.5
Sand
8
4
4
2.0
Available Water in the Soil
Saturated
•Excess water
Field Capacity
100% available
Readily Available Water
Available
Water
Wilting Point
Oven dry
•Little reserve available
and plants stressed
0% Available
•No water available
1.5.7 DEFINITION OF SOIL WETNESS

Soil Wetness can be described as:
a)
By Mass (Pm):
This is the gravimetric system.
b) By Volume (Pv): This is the volumetric system. It is given as:
Pv = Pm x Dry bulk density ( Db).
c) By Equivalent Depth: This is expressed in depth eg. in mm.
This is normally used in irrigation engineering.
d = Pm . Db
.
D



where: d is the equivalent depth of water applied (mm);
Pm is the moisture content by mass (fraction or decimal);
D is the root zone depth (mm). In this case, Db is the specific
gravity of the soil, which is dimensionless. It has the same
units as bulk density when expressed in gm/cm3. The unit of
d is therefore determined by the unit of the root zone depth, D
Table: Effective Rooting Depth (mm) of
Some Crops
Crops
Effective Rooting Depth
Fruits
750
Lucerne
1200
Cotton
900
Maize, small grains,
wheat
Most Vegetables
600
Source: Hudson’s Field Engineering
300
1.5.8 INFILTRATION OF
WATER

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 maximum 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 CONTD.



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
Kostiakov (1932) equation:
I = (a Tn )
mm/hr.
Where: a and n are constants and T is the elapsed
wetting time
Methods of Measuring Infiltration
Irrigation is practiced mainly in three ways:



By flooding the whole surface of the soil surface;
By Flooding part of the surface and
By Sprinkling.
The method used influences the measured
intake rate of water into the soil. When designing
irrigation systems, the method used for measuring
the soil infiltration rate should simulate, as far as
possible, the mechanism of water intake during the
application.
Infiltration Measurement For
Flooded Irrigation

For Flooded irrigation (border strip and basin), a double
infiltrometer is normally used.

This consists of two concentric cylinders, the inner about 0.4 m
diameter, the outer 0.5 m.
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.
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 Measurement For
Furrow Irrigation

For flood irrigation (furrow), in addition to the
usual factors affecting infiltration, the intake of
water depends on the spacing and shape of
the furrow.
 The difference between inflows and outflows
of water flowing through hydraulic flumes
placed at different distances of test furrows
represent the total infiltration.
 Furrow dimensions are used to obtain the
infiltration rates. See Chapter 3
for test
calculations.
Infiltration Measurement For
Sprinkler Irrigation




The mechanism of infiltration under sprinkler
irrigation is different from the surface methods.
There is no head of water above the soil surface and
the effect of sprinkler drops on the soil tends to form
soil pans on the surface, reducing infiltration rate.
The ideal method of measuring infiltration rates for
sprinkler irrigation is to use sprinklers at various rates
of spraying.
Water could be sprayed into infiltrometers to obtain a
small head of water and the intake rate found as
described earlier.