IRRIGATION AND DRAINAGE
ENGINEERING (10 CREDITS)
By: Ir. NAHAYO Déogratias
Lecturer, Civil Engineering Department
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.
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(ii) To develop analytical skills relevant to the
areas mentioned in (i) above, particularly the
design of irrigation and drainage projects.
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Course Outline
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Introduction to irrigation and drainage
Basic Soil-Plant-Water Relations.
Irrigation Water Requirements,
Sources, quantity and quality of irrigation
water:
Irrigation
planning,
scheduling
and
efficiencies.
Design of irrigation structures.
Design of drainage structures.
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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
iii. Be able to plan and design irrigation and drainage
projects.
iv. Design channels and other irrigation structures
required for irrigation, drainage, soil conservation,
flood control and other water-management
projects.
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Course Assessment
i.
ii.
iii.
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Assignments
CAT
Final Exam
: 20 %
: 40%
: 40%
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Reading Materials
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(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.
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Prerequisites
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Basic Soil
Science/Physics
Plants
Water
Plant/Soil/Water
Relations
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Hydraulics
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Hydrology
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General Engineering
Principles
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Lecture I: Introduction to Irrigation and
Drainage Engineering
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Three basic requirements of agricultural production are
soil, seeds and water.
In addition, fertilizers, insecticides, sunshine, suitable
atmospheric temperature, and human labour are also
needed.
Among all of them water appears to be the most important
requirement of agricultural production.
The application of water to soil is essential for plant
growth and it serves the following functions:
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Functions of application of water to the soil for
plant growth
i.
ii.
iii.
iv.
v.
vi.
vii.
It supplies moisture to the soil essential for the
germination of seeds, and chemical and bacterial
processes during plant growth.
It cools the soil and the surroundings thus making the
environment more favorable for plant growth.
It washes out or dilutes salts in the soil.
It softens clods and thus helps in cultivation operations.
It enables application of fertilizers.
It reduces the adverse effects of frost on crops.
It ensures crop success against short-duration droughts.
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Definitions of Irrigation and Drainage
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In several parts of the world, the moisture available in the
root-zone soil, either from rain or from underground
waters, may not be sufficient for the requirements of the
plant life.
This deficiency may be either for the entire crop season or
for only part of the crop season.
Irrigation: the application of water to the soil to
supplement natural precipitation
and provide an
environment that is optimum for crop production.
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Definitions of Irrigation and Drainage……
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Irrigation water delivered into the soil is always
more than the requirement of the crop for building
plant tissues, evaporation, and transpiration.
In some cases the soil may be naturally saturated
with water or has more water than is required for
healthy growth of the plant.
This excess water is as harmful to the growth of
the plant as lack of water during critical stages of
the plant life.
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Definitions of Irrigation and Drainage……
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This excess water can be naturally disposed of
only if the natural drainage facilities exist in or
around the irrigated area.
In the absence of natural drainage, the excess
water has to be removed artificially.
Drainage: Artificial removal of the excess water
is termed drainage which, in general, is
complementary to irrigation.
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Methods of Irrigation
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In general, there are many methods of
applying water to the field. However, in
irrigation practice there are three basic
methods namely:
Surface irrigation:
basin irrigation
furrow irrigation
border irrigation
Sprinkler irrigation
Drip irrigation
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OBJECTIVES OF IRRIGATION
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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
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To Facilitate
continuous cropping
To Enhance Fertilizer
ApplicationFertigation
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Impact of irrigation on human environment
Impact
Positive
Negative
Engineering
Improvement of the water regime of irrigated Danger of waterlogging and salination of soils, rise in
soils.
ground water table.
Improvement of the micro climate.
Changing properties of water in reservoirs. Deforestation
Possibility provided for waste water use and of area which is to be irrigated and with it a change of the
disposal
water regime in the area.
Retention of water in reservoirs and possible Reservoir bank abrasion.
multipurpose use thereof.
Health
Securing increased agricultural production and Possible spread of diseases ensuing from certain types of
thus improving the nutrition of the population. surface irrigation.
Recreation facilities in irrigation canals and Danger of the pollution of water resources by return
reservoirs.
runoff from irrigation. Possible infection by wastewater
irrigation, new diseases caused by retention of water in large
reservoir.
Social
and Culturing the area. Increasing the social and Colonization of the irrigated area. Displacement of
Cultural
Aesthetic
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Political
cultural level of the population. Tourist interest population from retention area. Necessity of protecting
in the area of the newly-built reservoir.
cultural monuments in inundated areas.
New man-made lakes in the area.
Project architecture may not blend with the area.
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Increased self-sufficiency in food, thus lesser
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SOIL CONSTITUENTS
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Mineral Material: Sand, clay and silt
Organic matter - very valuable
Water
Air
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MINERAL COMPONENTS
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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.
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MINERAL COMPONENT CONTD.
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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.
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MINERAL CONSTITUENTS
AFNOR
DIN
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 to 0.02 mm
CLAY
< 0.002 mm
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< 0.002 mm
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SAND COMPONENT
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Visible to the Naked Eye and Vary in
Size.
They are tenacious when rubbed between
Fingers.
Sand Particles do not Adhere to one
another and are therefore not Sticky.
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SILT AND CLAY COMPONENTS
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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.
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Clays are the smallest class of mineral
particles. They adhere together to form a sticky
mass when wet and form hard clods when dry.
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SOIL TEXTURE
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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.
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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.
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COLLOIDAL MATERIAL
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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.
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WATER
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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.
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SOIL WATER CONTD.
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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
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Mechanism of Soil Water Movement
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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.
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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:
 Gravitational Potential: Reflects
gravitational forces on the soil water.
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Components of Soil Water
Potential Contd.
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Pressure Potential: This is positive when
greater than atmospheric pressure, and
negative when below atmospheric.
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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.
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Components of Soil Water
Potential Contd.
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Osmotic Potential: reflects the effect of
solutes in soil water, in the presence of a semipermeable membrane
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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.
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Soil Water Potential and Soil Water Content
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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.
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Soil Water Potential and Soil Water Content….
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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.
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Methods of Measuring Soil Water Content
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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.
Gravimetric Method: This is equal to:
Mw
Mass of Water
Pm 

M s Mass of Dry Solids
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Gravimetric Method Contd.
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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%.
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Methods of Measuring Soil Water Content ……
(iii) Volumetric water content, Pv. This is
equal to:
Vw
Volume of Water
Pv 

Vs Va Vw
Total Volume of Undisturbed Soil Sample
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Recall that volume = mass/density i.e.
Mw
Dw
Mw
Pv 
and Pv 
x D sin ce Dw 1
b
Ms
Ms
D
b
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Pv  Pm x D Irrigation
where
D is the bulk density of the soil
b
b
37
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.
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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.
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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.
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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.
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ELECTRICAL RESISTANCE UNIT
Figure 1.4 Portable meter and resistance blocks used to measure
soil moisture.
(Courtesy Industrial Instrument, Inc.)
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Methods of Measuring Soil
Water Suction…
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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.
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Soil Water Equilibrium Points
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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.
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Soil Water Equilibrium Points…..
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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.
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Soil Water Equilibrium Points…..
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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.
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Soil Water Equilibrium Points…..
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Available Water (AW): This is the water
available to crops. It is the water content at
field capacity minus that at permanent wilting
point.
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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.
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Typical Soil Water Equilibrium
Points
Field
Permanent
Capacity (FC) Wilting Point
(PWP)
(By Weight)
(By Weight)
Available
Readily
Water (AW) Available Water
= 0.5AW
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
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Available Water in the Soil
Saturated
•Excess water
100% available
Field Capacity
Readily Available Water
Available
Water
Wilting Point
Oven dry
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•Little reserve available
and plants stressed
0% Available
•No water available
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1.5.7 DEFINITION OF SOIL WETNESS
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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
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d is the equivalent depth of water applied (mm);
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Pm is the moisture content by mass (fraction or decimal);
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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
in g/cm3.
The unit of d51 is
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therefore determined by the unit of the root zone depth, D
Table: Effective Rooting Depth (mm) of Some
Crops
Crops
Fruits
Effective Rooting
Depth(mm)
750
Lucerne
1200
Cotton
900
Maize, small grains, wheat
600
Most Vegetables
300
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Source: Hudson’s Field Engineering
52
Infiltration of water
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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.
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Infiltration of water….
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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 )
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
mm/hr.
Where: a and n are constants and T is the elapsed
wetting time
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Methods of Measuring Infiltration
Irrigation is practiced mainly in three ways:
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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.
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Infiltration Measurement For
Flooded Irrigation
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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.
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Double Ring Infiltrometer
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Infiltration measurement for Furrow Irrigation
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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.
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Infiltration measurement for Sprinkler Irrigation
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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.
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Thank You For Your
Attention
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