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

� 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.

�

� (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

Application-

Fertigation

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

45%

5%

30%

20%

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 to 0.02

mm

CLAY < 0.002 mm < 0.002 mm

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.

� 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:

wm

s

M Mass ofWaterP

M 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:

� Recall that volume = mass/density i.e.

SampleSoildUndisturbeofVolumeTotalWaterofVolume

wVaVsVwV

vP =++

=

soiltheofdensitybulktheisbDwhere

bDxmPvP

wDcebDx

sMwM

vPand

bDsMwD

wM

vP

=

=== 1sin

Soil Bulk Density

� 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.

� (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 w ith care.

1.5.5 Methods of Measuring

Soil Water Suction � 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. � 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. � 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

Wilting Point

(PWP)

(By Weight)

Available

Water (AW)

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

Field Capacity

Wilting Point

Available

Water

•Excess water

100% available

Readily Available Water

•Little reserve available

and plants stressed

0% Available

Oven dry •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

600

Most Vegetables 300

Source: Hudson’s Field Engineering

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.