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Small-scale pumped irrigation - energy and cost iii
PREFACE
This manual is about reducing the costs involved in small-scale pumped irrigation schemes.
Too often, schemes are designed and constructed with thought given only to the immediate cost
of constructing the scheme and of buying and installing equipment. Little or no attention is
given to operating costs, with the result that some schemes may well be cheap to install but very
costly to run. When water is pumped, every litre has a real cost because of the energy needed.
If more water is pumped than is needed or is pumped inefficiently, then operating costs can rise
significantly because of the additional energy which is wasted.
Ways of approaching scheme design and equipment selection are described so as to take
account of the operating costs. Simple examples are used to show how this can be done, andhow true comparisons can be made between different designs. Guidelines are given, based on
experience in many developing countries, so that sound practical choices can be made.
The manual is not just for those starting a new scheme. It is also for those who wish to
evaluate and improve existing schemes, and practical ways of reducing operating costs by
improving the efficiency of water use and pumping are described.
The readership envisioned is that group of people with some practical experience in
small-scale irrigation but who have little or no technical or engineering knowledge and wish to
be able to advise farmers on appropriate equipment selection and its proper and efficient use.
Although not numbered in the same series as the FAO/ILRIIrrigation Water Management
Training Manuals, this particular publication is seen as being complementary to that series,and, as a consequence, numerous cross-references are made in the text to the various volumes
of the Training Manuals series.
The text is substantially the work of Dr Melvyn Kay, of Silsoe College, UK, with additional
technical input from N. Hatcho of the Land and Water Development Division, FAO, Rome.
The text was edited and prepared by Thorgeir Lawrence for publication by FAO.
Any comments on the text as it stands and any suggestions for potential improvements
that could be included in subsequent editions are welcomed, and should be addressed to:
Water Resources, Development and Management Service , AGLW
FAO
Viale delle Terme di CaracallaI-00100 ROMA,
Italy
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Small-scale pumped irrigation - energy and cost v
CONTENTS
Page
1.INTRODUCTION 1
1.1 Small-scale irrigation 1
1.2 Problems 1
1.3 Solutions 2
1.4 Making choices 2
2.SOME BASIC CONCEPTS 5
2.1 Introduction 5
2.2 Pressure 5
2.3 Discharge 7
2.4 Energy 9
2.5 Power11
2.6 Efficiency 13
3.CHOOSING A NEW IRRIGATION SYSTEM 15
3.1 Introduction 15
3.2 Water sources 18
3.3 Pumps and power units 18
3.3.1 Pump types 18
3.3.2 Pump Characteristics 22
3.3.3 Pump selection 233.3.4 Power units 24
3.3.5 Efficiency 26
3.4 Distribution systems 29
3.4.1 Open channels 29
3.4.2 Pipelines 31
3.4.3 Distribution efficiency 35
3.5 Methods of irrigation 37
3.5.1 Surface irrigation 37
3.5.2 Sprinkler irrigation 39
3.5.3 Trickle irrigation 40
3.5.4 Selecting an irrigation method 403.6 System capacity 41
3.6.1 Crop water requirements 42
3.6.2 Peak scheme water demand 44
3.6.3 Seasonal scheme water demand 45
3.7 Peak power and energy demand 45
3.8 Costs 46
3.8.1 Capital cost 47
3.8.2 Operating cost 47
3.8.3 Overall cost 49
3.8.4 Effects of changes 52
3.8.5 Some general conclusions 533.8.6 Some practical considerations 54
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4.CASE STUDY-1 55Introduction 55
4.1 Options available 55
4.2 Scheme water demand 56
4.3 Peak power and energy demand 57
4.4 Overall costs 59
4.5 Conclusions 59
4.6 Guidelines 62
5.CASE STUDY-2 63
5.1 Options available 63
5.2 Scheme water demand 63
5.3 Overall power and energy demand 64
5.4 Overall costs 65
5.5 Conclusions 67
5.6 Guidelines 70
6.IMPROVING EXISTING SCHEMES 71
6.1 Introduction 71
6.2 Inefficient water use 72
6.3 Inefficient equipment 73
6.4 Effect of inefficiency 74
6.5 Evaluating a scheme 74
6.6 Obtaining data 766.6.1Observing and questioning 76
6.6.2Some basic data 76
ANNEX 79
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TABLES
Page
1. Energy content of fuels and foods 10
2. A guide to selecting centrifugal pumps 21
3. Pump selection for small-scale schemes 23
4. Indicative values of distribution efficiency (%) 35
5. Typical field application efficiencies for irrigation methods 37
6. Typical sprinkler data 39
7. Factors affecting selection of irrigation method 41
8. Indicative values for crop water needs and growing periods 43
9. Useful life of irrigation system components 47
10. Indicative maintenance and repair costs 48
11. Capital recovery factors (CRF) 50
12. EAC values for pumps at various discount rates 51
13. EAC values for pumps for different life expectancies 51
14. Changing the distribution system and its effects on energy and cost 52
15. Calculating scheme water demand 56
16. Overall power and energy demands 57
17. Overall cost comparisons 58
18. Calculating scheme water demand 64
19. Overall power and energy demands 64
20. Overall cost comparisons 6521. Efficiency of surface irrigation methods 73
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FIGURES
Page
1. Making choices , the design process 4
2. Relationship between force and pressure 5
3. Measuring pressure in a pipe 6
4. Calculating discharge 7
5. Measuring discharge 8
6. Energy conversion - analagous systems in people and machines 10
7. Illustration of the problem considered in Example 3 11
8. Relationship between rate of energy use and power 12
9. Graph relating flow, static head and power 13
10. Choosing irrigation system components 15
11. Components of a typical irrigation scheme 16
12. Typical axial flow pump 19
13. The radial flow (centrifugal) pump 20
14. Typical mixed flow pump 21
15. Pump characteristics of the three pump types 22
16. Pump selection based on head and discharge parameters 24
17. Manufacturers data for a centrifugal pump 25
18. Efficiency of components of pumping plant 26
19. Suction lift limitations 28
20. Energy demand for open channel distribution 29
21. Channel design: dimensions and drop structures 3022. Pipe system and its energy demand 32
23. Hydraulic gradient 33
24. Nomograph relating pipe diameter, discharge, head loss and velocity 34
25. Basin, border and furrow irrigation 36
26. Sprinkler irrigation 38
27. Hose-pull sprinkler system 39
28. Trickle irrigation 40
29. Peak and seasonal scheme water demands 42
30. The concept of water requirements in mm 43
31. Relationship between pipe size and seasonal energy cost 53
32. Effect on EAC values of reducing pump efficiency 6033. Effect on EAC values of changing interest rate 60
34. Effect on EAC values of a greater depth to the groundwater table 61
35. Effect of reducing pumping efficiency on EAC values 66
36. Effect of greater depth to groundwater on EAC values 66
37. Effect of increasing scheme size on capital and operating costs 68
38. Evaluating irrigation scheme performance 71
39. System efficiency value ranges 74
40. The relationship between efficiency and seasonal operating costs 75
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Small-scale pumped irrigation - energy and cost 1
Chapter 1
Introduction
1.1 SMALL-SCALE IRRIGATION
Small-scale irrigation is an important aspect of irrigation development in many countries.
Approximately half of the irrigated area in Sub-Saharan Africa, for example, is irrigated in this
way. It involves individual or small groups of farms, organized and managed by farmers,
usually independent of government resources. This type of development has often proved
successful in places where the larger-scale, primarily government controlled, projects havenot. This is not to say that small-scale is therefore better than large-scale farming, or indeed
that small-scale is more simple to develop. It is a different approach to irrigated farming, with
its own challenges. Irigation development requires careful design, construction and management
to be successful. It is, perhaps, in the management element that the key difference lies. In a
small system there are no tiers of management, as in the large-scale schemes. Farmers alone
decide when to irrigate and how much water to apply; start and stop the pumps; and generally
run the entire scheme with the help of the family or local community.
Small-scale farming can be highly productive in terms of yield per hectare of land. The
energy input into large-scale schemes can be up to 15 times greater than that required for small-
scale farming for the same output of crops produced. This is in sharp contrast to large-scale
schemes where the ratio is normally less than 4. Thus, on a national or regional scale, whenconsidering the use of commercial fuel in agriculture, which in many countries is both scarce
and expensive, the small-scale approach can be an attractive one.
1.2 PROBLEMS
Despite their apparent attractiveness in terms of potential productivity, small-scale schemes
are, however, not always as efficiently run as they could be. Most schemes rely on pumping to
supply their water needs and are often designed on the basis of minimum investment cost, with
little or no thought given to the effect that this might have on operating costs over many years.
For example, a farmer may purchase a cheap pump which runs at a very low level of efficiency.
The energy cost may be considerable and it may require much servicing and spare parts. If thefarmer were to purchase a better and more appropriate pump then more money might be spent
initially but there should be much more money saved over the years through reduced fuel
(energy) costs and maintenance. Similar issues arise when selecting other components of an
irrigation system.
An equally important issue to consider is how well the scheme is managed once it is operating.
The most appropriate system design and selection will be of little use in the hands of an
inexperienced or unskilled irrigator. Good equipment is no substitute for good management
and, here too, considerable savings in energy and operating costs can be made by ensuring
good equipment and water management practices.
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Introduction2
1.3 SOLUTIONS
This manual describes ways of approaching scheme design and equipment selection which
take account of both investment and operating costs, and, in particular, emphasise the significance
of energy costs.
Some basic concepts need to be understood about water flow, energy and power, and, for
those who have little or no knowledge of these, they are described in Chapter 2.
In Chapter 3 the basic components of an irrigation scheme are described together with ways
of choosing between different pumps, distribution systems and methods of irrigation. There
may be many different ways of irrigating a farm and a basis for comparison and selection is
needed. Cost is often the dominant factor. Thus the idea ofcost effectiveness is introduced,
showing that both capital costs and operating costs must be considered when selecting equipment,
and that the one affects the other. This is demonstrated in Chapters 4 and 5, where two contrasting
case studies show how the principles and practices of Chapter 3 can be applied.
Many small-scale irrigation schemes are already in operation, and one question here might
be how to get the best results from what is already there. Chapter 6 examines ways of looking
at existing schemes to determine energy use and operating costs, and to find ways of reducing
them through improved efficiency of equipment and water use.
1.4 MAKING CHOICES
Much of this manual is about the process ofdesign the process of making logical choices
between systems of irrigation and equipment (Figure 1). It is important to realize at the outset
that there is unlikely to be just one ideal choice; there may be many alternatives, any one of
which might be quite appropriate. The job of the designer is to present the options available in
relation to good irrigation practice, water availability, equipment, its reliability and cost. The
farmer can then choose the system which he or she feels is most appropriate.
The design process
Apreliminary design is usually done first. This is often done quickly in order to establish the
options available. Once a choice has been made, work then proceeds to a detailed design
which details every nut and bolt to be purchased and every canal and structure to be constructed.
To undertake a preliminary design, basic information is needed about the land and crops to
be irrigated. However, accurate details about land areas and crops may not be necessary at this
stage. To understand this it is important to realize what preliminary design is about. It is to
establish the maximum capacity or size of the system to be constructed and the choices available
to the farmer. The system capacity must be enough to satisfy the maximum amount of water
needed by the crops, and there are simple ways of assessing this without detailed knowledge of
the cropping. Clearly the answer will not be exact but great accuracy is not needed at this stage.
Remember that when a scheme is operating it will run for most of the time at well below its
maximum capacity. It may only run at full capacity for a very short period when the crops are
maturing and need most water. It is very much like designing and using a car. It may be
designed to operate at a maximum speed of 150km/h, but most drivers would travel well below
this speed and only use the maximum speed occasionally. Thus whether the maximum speed is
150 or 160km/h is not really very critical to the overall use of the car if it otherwise meets allthe demands made upon it by the driver. If the actual maximum performance is less than
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Small-scale pumped irrigation - energy and cost 3
150km/h, then the car will still get there it will just take a little longer. In the same way the
capacity of an irrigation system need not be determined with great accuracy as long as thecapacity will meet most, if not all, of the operating demands that the farmer will make. If the
capacity falls a little short of demand then the difference can be made up by running the system
for a longer period.
A further aspect of design is considering How will the final cost of the scheme be affected
by the decisions made during the design process?. If, for example, the crop water requirement
is changed by 10%, or a channel is increased in size by 20%, does this significantly affect the
overall cost of the scheme? If it does, then this figure needs to be chosen with considerable
care. If it does not, then such accuracy is not needed. A good designer will concentrate on the
important factors which will have significant effects on the outcome. The inexperienced designer
will need to experiment a little to determine which are the critical factors in the design process.
A final aspect of design, which the inexperienced designer may not realize at first, is that
there are no formulae which can help with the initial decision making. For example, there is no
formula which would show that a pipe should be used instead of an open channel. This is a
matter of choice, which may eventually be decided by cost or some other constraint. The
designer would thus consider both options, prepare a preliminary design for each one, and then
see which was best. Several designs may be done in this way before the best one can be chosen.
In other words, the designer will often choose what seems to be appropriate and then set about
proving that the choices made are indeed the best. This is where an experienced designer can
be invaluable. On the basis of past experience of similar situations the designer may well be
able to greatly simplify the design process because he or she may have a very good idea of what
will be the best solution. Unfortunately, the inexperienced designer must go through a morerigorous process to arrive at the best solution. This manual is to help the inexperienced designer,
and to try and pass on some of the experience of others in order to shorten and simplify the
design process.
Cost
Cost will be an important factor when making choices. In this manual typical costs are used to
demonstrate the selection process, but the reader must take great care when using conclusions
drawn from this because local costs may vary considerably from those shown. The reader is
thus encouraged to go through the design process using local costs and to make judgements
based on local solutions. Throughout the text the unit of currency used is the United States
dollar ($US).
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Introduction4
FIGURE 1Making choices - the design process
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Small-scale pumped irrigation - energy and cost 5
Chapter 2
Some basic concepts
2.1 INTRODUCTION
This chapter provides a guide to some of the basic principles which affect energy needs in
small-scale irrigation. SI units the International Metric System are used throughout the
text. Reference is made to other units where appropriate, because it is an unfortunate fact of
life that many different systems are in use in irrigation, and sometimes it can be confusing and
lead to serious mistakes.
The fundamental units in the SI systems are:
Measurement Unit Symbol
Length metre m
Volume cubic metre m3
Mass kilogramme kg
Force newton N
2.2 PRESSURE
Pressure is a commonly used term, but it does have a special meaning in hydraulics. It describes
the force exerted by water on each square metre of some object submerged in water. It may be
the bottom of a tank, the side of a dam, or a pipeline.
Pressure is normally measured in kilonewtons per square metre (kN/m2). An alternative to
this in irrigation is the bar, where 1 bar is equal to 100kN/m2. Pressure is calculated by:
Thus pressure is force per unit area (Figure 2).
Pressure (kN/m2) =force (kN)
area (m2)
FIGURE 2
Relationship between force and pressure
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Pressure measurement
Pressure in pipes can be measured using a
bourdon gauge (Figure 3). Inside the gauge isa curved tube of oval section, which tries to
straighten out when the system is under
pressure. The tube is linked to a pointer which
moves across a graduated scale and indicates
pressure. Irrigators normally measure pressure
in the field using these gauges as they are robust
and simple to use.
However, engineers often refer to pressure
as a head of waterin metres (m) rather than a
pressure in kN/m2. If the bourdon gauge was
replaced with a long vertical tube, the waterpressure in the pipe would cause water to rise
up the tube. The height of this water column
is a measure of the pressure in the pipe. For
example, a pressure of 3bar in the pipe would
result in water rising to a height of 30m in the
tube. Thus, engineers may refer to the pressure
as 3bar or 30m head of water.
EXAMPLE 1
Calculate the pressure when a force of 10 kN is applied to an area of 2 m2.
- We know that Pressure = force / area, so P= 10 / 2 = 5 kN/m2.
If the area is increased to 4 m2, what will be the nre pressure?
- P= 10 / 4 = 2.5 kN/m2.
Thus the force has remained the same but the pressure is reduced by spreading the force over
A typical operating pressure for a sprinkler system is 3bar pressure, or 300kN/m2. This
means that every square metre of the inside of the pipes and pump has a uniform force of
300kN acting on it. Other common units of pressure are kilogrammes-force per squarecentimetre (kgf/cm2) and pounds-force per square inch (lbf/in2).
For conversion from one unit to another:
1 bar = 14.7 lbf/in2 = 1 kgf/cm2 = 100 kN/m2
FIGURE 3
Measuring pressure in a pipe
In this manual both the termspressure and headare used to mean the same thing.
Head of water in metres (m) = 0.1 x pressure (kN/m2
) = 10 x pressure (bar)
It is simple to change from pressure to head of water:
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Small-scale pumped irrigation - energy and cost 7
Atmospheric pressure is important to the understanding of suction when pumping water
(Section 3.3.3) and particularly its effects on the efficiency of pumping (Section 3.3.5).
2.3 DISCHARGE
The speed at which water flows in a pipe or channel is called the velocity and is measured inmetres per second (m/s). The discharge is the volume of water flowing along the pipe or
channel each second, and is measured in cubic metres per second (m3/s). To understand this,
consider the case of water flowing in a 100mm diameter pipe at a velocity of 1.5 m/s (Figure4).
In one second the quantity of water moving past some point in the pipe will be equal to the
shaded volume shown. This volume is numerically equal to the water velocity multiplied by
the cross-sectional area of the pipe, i.e., 1.5 0.008 = 0.012m3/s.
Importance of Pressure
Pressure is important to the successful operation of both sprinkler and trickle irrigation.
Sprinklers must be operated at the right pressure so that the water jet breaks up properly and a
uniform water application is achieved (Section 3.5.2.). The right pressure is also required in
trickle systems so that each emitter gives the same discharge throughout the scheme
(Section3.5.3).
Atmospheric pressur e
Atmospheric pressure is the pressure of the atmosphere around us, pressing down on our bodies
and the surface of the earth. Although air seems very light, when there is a large depth, as at the
earths surface, it creates a pressure of approximately 100kN/m2. This is equivalent to lbar or
10m head of water.
Atmospheric pressure = 100 kN/m2 = 1 bar = 10 m head of water
In general terms:
Discharge (m3/s) = cross-sectional area of pipe (m2) x velocity of water (m/s)
FIGURE 4
Calculating discharge
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Some basic concepts8
For most small irrigation systems the unit of discharge (m3/s) is much too large and so litres
per second (l/s) is very often used. The conversion is made by multiplying by 1000.
FIGURE 5
Measuring discharge
Figure 5-A
Figure 5-C
Figure 5-B
Discharge (l/s) = discharge (m3/s) x 1000
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Small-scale pumped irrigation - energy and cost 9
2.4 ENERGY
Energy is another word commonly used in everyday language, but in hydraulics and irrigation
it has a very specific meaning: Energy enables useful workto be done
People and animals require energy to do work. This is obtained by eating food and converting
it into useful energy for work through the muscles of the body.
In irrigation, energy is needed to lift or pump water. Water energy is supplied by a pumping
device driven by human or animal power, or a motor using solar, wind or fossil fuel energy.
Energy measurement
Energy is normally measured in units of watt-hours. One watt-hour is a very small amount of
energy and so engineers tend to use a larger unit, the kilowatt-hour (kWh) instead, where
1kilowatt-hour = 1000watt-hours.
Here are some examples of energy use which may be familiar to the reader and which will
provide some practical indication of energy use:
A farmer working in the field uses 0.2-0.3kWh every day.
An electric desk fan uses 0.3kWh every hour.
An air-conditioner uses 1kWh every hour.
Notice how a time period (e.g., every hour, every day) is always given when describing the
amount of energy needed. The farmer using 0.2 kWh every day, for example, indicates that thisenergy must be supplied from food each day otherwise he or she would not be able to work
properly. In irrigation, energy requirements may be determined on a daily, monthly or seasonal
basis.
Discharge measurement
Discharge in a pipeline can be measured using a flow meter (Figure 5-A). The meter indicates
the volume of water passing through the pipeline. By noting the time for a given volume of
water to pass the discharge can be determined using the formula:
Discharge (m3/s) = volume of water (m3) / time (s)
A simple way of measuring discharge from a pipe or sprinkler is to catch the flow in a
bucket of known volume, measuring how long it takes to fill (Figure 5-C). The discharge is
calculated using the above formula. See Example 2.
Discharge in open channels can be measured using a weir or flume measuring structure(Figure 5-B). If no measuring structure is available, a rough guide can be obtained by estimating
the velocity of flow using a float; measuring the cross-sectional area of the channel; and
multiplying the velocity and the area together. (See Training Manual 7: Canals)
EXAMPLE 2
A small plastic tube is connected to a sprinkler nozzle to collect water in a bucket. If the bucket
holds 5 litres and it takes 15 seconds to fill, calculate the sprinkler discharge.
Discharge = volume / time = 5 / 15 = 0.33 l/s
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Some basic concepts10
Energy sour ces
Energy comes from food, in the case of animals and people, and from fossil fuel, wind and
sunshine in the case of engines and motors.
Foods have energy values which our bodies convert into useful energy so that we can do
useful work. In the same way fossil fuels, wind and sunshine have energy which can be converted
into useful energy to pump water.
Table 1 gives some indication of energy values for typical foods, fossil fuels and energy
sources.
TABLE 1
Energy content of fuels and foods
Changing energy
An important aspect of energy is that it can be changed from one form of energy to another.
People and animals can convert food into useful energy to drive their muscles (Figure 6). In a
typical pumping system powered by a diesel engine, the energy is changed several times beforeit is usefully used by the water. Chemical energy contained within the fuel (diesel oil in this
case) is burnt in a diesel engine to produce mechanical energy. This is passed to the pump via
Fuel or
food
Energy Indicative
efficiency (1)Comment
Maize
Wood
Diesel
Petrol
Wind
Solar
1 kWh/kg
4 kWh/kg
11 kWh/l
9 kWh/l
0.01-41 kWh/m2
1 kWh/m2
10%
10%
20%
10%
20%
5%
As animal and human consumption
Sometimes also expressed as fuel consumption
(0.09 l/kWh for diesel and 0.11 l/kWh for petrol)
For wind speeds from 2.5 to 40 m/s respectively
Maximum solar energy at sea level
Note: 1. Approximate efficiency when converted to mechanical power.
FIGURE 6
Energy conversion - analogous systems in people (top) and machines (bottom)
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Small-scale pumped irrigation - energy and cost 11
a drive shaft, and finally to the water. Thus the discharge, pressure or both can be increased. A
pump can be thought of as a device for putting additional energy into a water system.
The system of energy transfer is not perfect and energy losses occur through friction between
the moving parts and are usually lost as heat energy (the human body temperature rises when
work hard; an engine heats as fuel is burnt to provide power). Energy losses can be significant
in pumping systems, and so can be costly in terms of fuel use. This concept is discussed further
in Section 2.6.
Calculating energy requi rement
The amount of energy needed to pump water depends on the volume of water to be pumped and
the head required and can be calculated using the formula:
Increasing either the volume of water or the head will directly increase the energy required
for pumping.
2.5 POWER
Poweris often confused with the term energy. They are related, but they have different meanings.
Energy is the capacity to do useful work whereas power is the rate at which the energy is used.
Water energy (kWh) =volume of water (m3) x head (m)
367
EXAMPLE 3
600 m3 of water is pumped each day to a tank 10 m above ground (Figure 7). Calculate the
amount of energy reguired to do this.
Water energy (kWh) = (600 x 10) / 367 = 16.3 kWh.
This is the energy required each day.
FIGURE 7
Illustration of the problem considered in Example 3
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Some basic concepts12
Power is the rate of using energy and is commonly measured in kilowatts (kW). The
power needed to pump water is called water power.
Another commonly used measure of power is horse power (HP). As it is not part of the
metric units system it will not be used in this manual. However, if comparison is needed the
relationship is 1kW = 1.36HP.
An air conditioner may have a power rating of 3kW. This means that it uses 3kWh of
energy every hour. In 24 hours it will consume 72kWh (3kW 24h) of energy at the rate of
3kW every hour. Thus, power is describing the rate at which the energy is used. The greater
the energy use rate the greater is the power need (Figure 8).
Another way of calculating power and energy is to use the pump discharge rather than
the volume of water to be pumped.
In this case the water power required can be calculated by first using the formula:
Figure 9 is a graph of this formula and from which water power can be obtained.Energy can then be calculated from power. It is the amount of power used in a given
time period and so:
Power (kW) =energy (kWh)
time (h)
EXAMPLE 4
In Example 3 it was calculated that the water energy required each day to lift 600 m 3 of water
through 10 m was 16.3 kWh. Calculate the water power needed to do this.To calculate water power from water energy it is necessary to know the time over which pumping
takes place.
If pumping continues for 24 hours per day:Water power (kW) = energy used per day (kWh) / time (h) = 16.3 / 24 = 0.68 kW.
If the pump operates only 12 h/day:Water power = 16.3 / 12 = 1.35 kW.
If pumping is only 6 h/day:Water power = 16.3 / 6 = 2.7 kW.
Note that the water energy is the same in each case, but that the rate of using the energy - the
power - changes with the time period. More power is needed when less time is available for pumping
the same volume of water.
FIGURE 8
Relationship between rate of energy use and power
Water power (kW) = 9.81 x discharge (m3/s) x head (m)
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Small-scale pumped irrigation - energy and cost 13
Example 5 demonstrates this approach and shows that the results are the same whichever
method is used to calculate power and energy.
2.6 EFFICIENCY
When pumping irrigation water it is not enough just to meet the water power and energy
requirements. Additional energy and power must be provided because losses occur in transferring
fuel energy to water energy via the power unit and pump. The losses in the system are caused
by friction and water turbulence and are usually expressed as efficiency. This can be expressed
both in terms of energy use and of power use.
FIGURE 9Graph relating flow, static head and power
Water energy (kWh) = water power (kW) x operating time (h)
EXAMPLE 5
Referring to Example 4, if 600 m3 of water is pumped 10 m each day, calculate the water power
and energy required, using the pump discharge approach if pumping is for only 6 h/day.
Discharge (m3/s) = volume (m3) / time (s) = 600 / (6 x 3600) = 0.028 m 3/s.
Using the above equations:
Water power (kW) = 9.81 x discharge (m3/s) x head (m) = 9.81 x 0.028 x 10 = 2.7 kW.
Water energy (kWh) = water power (kW) x operating time (h) = 2.7 x 6 = 16.3 kWh.
These answers are the same as those obtained in the previous example, thus demonstrating
that water power and energy can be calculated using either approach.
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Some basic concepts14
Energy use eff iciency
This provides an overall indication of the way energy is used. It would usually be assessed on
a seasonal or annual basis.
Power use eff iciency
This provides an assessment of the efficiency with which power is converted from the fuel to
the water, but only at the moment of measurement. The efficiency may vary over time,
particularly if there is wear in the engine and pump.
A system with no friction would have an efficiency of 100% and all the energy and
power input would be transferred to the water. However, this is not the case in real life and
there are always friction losses in all the components of the power unit and pump. This is
discussed more fully in Section 3.3.5.
Sometimes efficiencies can be very low without pump users being aware of the problem.
This can result in excessive energy use and high pumping costs. This is an important aspect of
pumping and is discussed more fully in Chapter 5.
For the purposes of this manual, the efficiencies of energy and power use are assumed to
be the same. In practice this may not be the case. A seasonal assessment of energy use efficiency
may not always give the same value as power use efficiency measured only once or twice
during the season. Note that, in calculations using efficiencies, we always use the decimal
form [(efficiency in %)/100] of the value.
Pumping plant efficiency (%) = (water energy / actual energy) x 100
Pumping plant power efficiency (%) = (water power / power input) x 100
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Small-scale pumped irrigation - energy and cost 15
Chapter 3
Choosing a new irrigation system
3.1 INTRODUCTION
Choosing a new irrigation system is about choosing the various components which make up the
system. In this chapter the main components are listed, and guidance is given in how to choose,
for preliminary design purposes, between the various options and component configurations
available.
Figure 10 illustrates the process of preliminary design and the decisions to be made.
Each part of the process is described in this chapter.
Small-scale pumped irrigation systems are made up of the following components (Figure11):
Water source;
Pump and power unit;
Distribution system; and Method of irrigation.
FIGURE 10
Choosing irrigation system components
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Choosing a new irrigation system16
FIGURE 11
Components of a typical irrigation scheme
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Small-scale pumped irrigation - energy and cost 17
The water source, the distribution system and the method of irrigation determine
the energy demand.The pump and power unit provide the energy supply.
Water sour ce
The water source may be a river or lake (surface water) or a shallow well or borehole
(groundwater). In some cases, water can be abstracted from rivers by gravity, but in many
cases pumping will be needed. In the case of groundwater abstraction, pumping is essential.
(See also Training Manual 6: Scheme irrigation water needs and supply.)
The amount of water abstracted and the height through which it must be lifted from the
river or borehole add to the energy demand.
Pump and power uni t
The pump may be driven by a power unit such as a diesel or petrol engine, or an electric motor.
In some special cases solar or wind power, or even hand or animal power, may be used to
provide the power source for the pump, but they are not so common and are generally limited
to very small irrigated plots. In this manual the primary concern is with the use of pumps
driven by diesel or petrol engines, as these are usually the main sources ofenergy supply
available to most small-scale farmers.
Distr ibut ion system
The distribution system conveys water from the pump to the fields and may consist of pipes oropen channels. Some systems are a combination of both. The choice of distribution system has
a significant effect on the energy demand.
Method of ir ri gation
The method of irrigation may be surface, sprinkler or trickle irrigation. This may also affect
the choice of distribution system and is also significant in determining the energy demand.
Surface irrigation may be supplied by either pipe or open channel systems. Sprinkler and
trickle irrigation systems would normally use piped distribution systems. (See also Training
Manual 5:Irrigation Methods.)
Typi cal systems
The most common combinations of components for an irrigation system are:
Pump open channel surface irrigation.
Pump pipe supply surface irrigation.
Pump pipe supply sprinkler or trickle irrigation.
The first system is the most common for small-scale irrigation, although the advantages of
the second are now being more fully realized. Sprinkle, and especially trickle, irrigation are
growing in importance in some areas where soils are very sandy and water is scarce, or energy
costs are high, or both, but surface irrigation is the dominant method and is likely to remain so
in many countries for the foreseeable future.
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Choosing a new irrigation system18
3.2 WATER SOURCES
Rivers and lakes
Many small irrigation schemes are located close to natural river channels and lakes and obtain
water by pumping from these sources. They provide a supply which can be seen by the farmer
and be judged whether sufficient or not for the seasonal needs of the farm. Usually, the pumping
pressures , and hence energy requirements, needed to use such sources are small because the
difference in elevation between the source water level and the level of the field are usually not
large.
Shallow groundwater
This is an ideal source of supply for farms located some distance from a river or lake. Usually
the groundwater table is fed by seepage from a river or lake and may be only a few meters
below ground level. This source may be less reliable than surface water because except through
pumping experience there is no easy way of assessing whether there is a sufficient reserve of
water to ensure adequate irrigation. However, the farmer can save the cost of an expensive
canal or pipe system to bring water from a more distant surface supply.
As with surface supplies, the energy costs involved in pumping are relatively low.
Deep gr oundwater
This may be water which has permeated through the ground from a surface source many
kilometres away or water which has been trapped in the ground by impermeable soils for many
thousands of years (fossil water).
Pumping deep groundwater which may be 20 - 100m or more below ground level can be
expensive in terms of energy use, as well as in the cost of drilling the borehole, and requires
special, deep borehole, pumping equipment, which may also be expensive to buy.
3.3 PUMPS AND POWER UNITS
A pump is a machine which changes fuel energy into useful water energy and needs a petrol or
diesel engine or an electric motor to drive it. In special circumstances it may also be possible to
use wind or solar energy. For surface irrigation the pump lifts water from a river or groundwater
into a channel or pipe system. For sprinkler and trickle irrigation the pump provides the energyfor the pressure and discharge needed to distribute water in the pipes to the sprinklers and
emitters, in addition to the energy needed to lift water from the source.
3.3.1 Pump types
Although there are many types of pumps and water lifting devices, many are unsuited to irrigation
use. The most commonly used types are the axial flow (or propeller) pump, the radial flow (or
centrifugal) pump, and the mixed flow pump. These are looked at in detail below.
Axial flow pump
An axial flow pump consists of a propeller hence its alternative name housed inside atube which is located below the water level (Figure 12). The tube acts as the discharge pipe,
and the power unit turns the propeller by means of a long shaft running down the middle of the
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Small-scale pumped irrigation - energy and cost 19
pipe and this lifts the water up the pipe. This pump is very efficient for lifting large volumes of
water at low pressure and is ideally suited to lifting water from a river or lake to provide surface
irrigation water to a farm with open channel distribution. However, these pumps tend to be
very expensive because of the high cost of materials, particularly the drive shaft and bearings to
support the shafted propeller. For this reason there are no small axial flow pumps manufactured
of a size suitable for the small farm of 1 - 2ha. They tend only to be used on larger farms and
for communal schemes, where several small farms are irrigated from the same pump. They are
particularly suited to paddy rice schemes because of the large volumes of water usually needed
for this crop.
Radial fl ow (centri fugal) pump
Centrifugal pumps are the most common type of pump used on small schemes because they are
much cheaper than axial pumps to buy and maintain. Small pump sets are often readily available
in most developing countries (Figure 13). They are best suited to sprinkler and trickle irrigation,
where a higher pressure is needed than for surface irrigation.
To understand how a centrifugal pump works, consider first how centrifugal forces occur.
Most readers will at some time have spun a bucket of water around at arms length and observed
that no water falls from the bucket even when it is upside down (Figure 13). Water is held in
the bucket by the centrifugal forces created by spinning the bucket. A centrifugal pump makes
use of this idea and can be thought of as many buckets all spinning around together. Thebuckets are replaced by an impellerwith blades or vanes which spin at high speed inside the
pump casing (Figure 13). Water is drawn into the pump from the source of supply through a
FIGURE 12
Typical axial flow pump
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Choosing a new irrigation system20
FIGURE 13
Radial flow (centrifugal) pump
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Small-scale pumped irrigation - energy and cost 21
short length of inlet pipe called thesuction pipe. As the impeller spins, water is thrown outwards
and is collected by the pump casing and guided towards the outlet. This is called thedelivery.
Some pumps have very simple impellers with
straight vanes. These tend to be inefficient because
they create a lot of turbulence in the flow and hence
energy losses. However they are cheap to make and
are used in cases where efficiency is not important.
Most irrigation pumps have curved vanes so that the
water enters and leaves the impeller smoothly. This
means lower energy losses and higher energy use
efficiency. Some impellers have side plates and are
called closed impellers. When there is debris in the
wateropen impellers are used to reduce the risk ofblockage.
Centrifugal pumps can be classified into two
types: volute pumps, and turbine (diffuser) pumps. The main difference between them is that
the turbine type has diffuser vanes, which provide diverging passages to direct the water flow.
Centrifugal pumps are often described by the diameter of the delivery connection pipe,
e.g., a 50mm pump. Table 2 is a guide to selecting centrifugal pump sizes for different flow
ranges.
Mixed flow
This pump is a mixture of the axial flow and the centrifugal pump and has the advantage of
combining the best features of both pump types (Figure 14). Mixed flow pumps are more
efficient at pumping larger quantities of water than centrifugal pumps and are more efficient at
pumping to higher pressures than axial flow pumps.
They can also operate as submersible pumps, i.e., being completely below the source
water surface.
FIGURE 14
Typical mixed flow pump
Pump size (mm) Discharge (l/s)
255075100125
0 - 55 - 1515 - 2525 - 3535 - 50
TABLE 2
A guide to selecting centrifugal pumps
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Choosing a new irrigation system22
3.3.2 Pump Characteristics
Axial flow, centrifugal and mixed flow pumpsare designed to run at a constant speed and
their performances are described by the fol-
lowing characteristics:
Head and discharge;
Power requirements; and
Efficiency of operation.
Typical characteristics for operating head
and discharge for the three pump types appear
as Figures 15-A, 15-B and 15-C. They show
how head, power and efficiency vary as the
discharge changes. For example, when the
head requirement is 120% of the design head
value, discharge is reduced to 60%, 80% and
90% of design discharge for centrifugal, mixed
flow and axial flow pumps respectively.
Head and discharge
Pumps can deliver a wide range of discharges depending on the pressure required and the speed
at which the pump is operated. However, there is a trade off between head and discharge. If
more discharge is needed the head drops, and if less discharge is needed, then the head rises. Adifferent set of curves would be obtained if the pump was running at a different speed. The
faster it runs the greater the head and the discharge.
FIGURE 15-A
Pump characteristics: discharge - head
FIGURE 15-B
Pump characteristics: discharge - power
FIGURE 15-C
Pump characteristics: discharge - efficiency
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Small-scale pumped irrigation - energy and cost 23
Power
All pumps need power to rotate the impeller. The amount of power needed depends on thespeed of the pump and the discharge that is required. The faster the pump rotates, the more
power is needed.
For axial flow pumps there is a very large power demand as the pump is starting because
there is a lot of water and a heavy pump impeller to get moving. Once the pump is under way
the power demand drops to its normal running level.
Centrifugal pumps behave quite differently. The power demand is very low when starting,
but as the discharge increases the power also gradually increases.
Mixed flow pumps operate in between these two contrasting conditions and have a more
uniform power demand over the discharge range.
Efficiency
The concept of efficiency was first developed in Section 2.6. It measures how well the mech-
anical energy and power from the power unit is converted into water energy and power in the
pump. The pump power efficiency is calculated by:
The efficiency generally increases to some maximum value and then falls again over the
discharge range. The maximum efficiency is usually between 30 - 80% and there is only a
limited range of discharges and heads over which the pumps operate at maximum efficiency.Outside this range the pump will be less efficient and so more power and energy will be needed
to operate the system. Smaller pumps tend to operate at lower efficiencies than larger ones
because they have more friction to overcome relative to their size.
3.3.3 Pump selection
There are many pumps on the market and the designer must try to select a pump which will
provide the discharge and head needed for the scheme while the pump is operating within its
maximum efficiency range.
Table 3 indicates the range of good operating conditions for different pump types.
TABLE 3Pump selection for small-scale schemes
Note: 1. The ideal pump type, but not usually available for small-scale farming.
A large number of irrigation schemes use surface irrigation and open channel distribution
pumping from shallow water supplies. This situation is ideal for axial flow pumps but
unfortunately few, if any, pumps are available at a reasonable price for the small discharges
Pump power efficiency (%) = (water power output / actual power output) x 100
Irrigation system Pressure or Head(bar) (m)
Discharge(l/s)
Pump type
Surface irrigation- open channel distribution- pipe distribution- deep tube wellSprinkler systemTrickle system
0.5 51.0 10
>2.0 >202 - 6 2 - 601 - 2 10 - 20
any dischargeany dischargeany dischargeany dischargeany discharge
axial1 or mixedaxial1 or mixed
mixed or centrifugalcentrifugalcentrifugal
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Choosing a new irrigation system24
required on many farms. The only alternative is to use centrifugal pumps instead and accept
that they will run at well below their peak efficiency (Figure 16).
For sprinkler and trickle irrigation much higher pressures are needed and so centrifugal
pumps are ideally suited to this use and will operate more efficiently.
A typical example of pump selection using the data supplied by a manufacturer would be
as follows:
3.3.4 Power units
There are two main types of power unit: internal combustion engines, and electric motors.
I nternal combustion engines
Many small irrigation schemes do not have access to electricity and so rely on petrol (spark
ignition) engines or diesel (compression ignition) engines to drive the pumps. These engines
have a good weight:power output ratio, and are compact in size and relatively cheap due tomass production techniques.
FIGURE 16
Pump selection based on head and discharge parameters
EXAMPLE 6
A centrifugal pump is required for a small sprinkler irrigation system. The discharge required is12 l/s, at a pressure of 2 bar. Using the information supplied by the manufacturer (see Figure 17),determine the pump efficiency.
If the same pump was to be used to pump water into an open channel and the pressure neededfor this was only 1 bar, show what effect this would have on the pump discharge and the efficiency.
From Figure 17, the efficiency of the pump at a discharge of 12 l/s and pressure of 2 bar (20 m ofhead) is 52%. This is within the high efficiency zone of the pump.
If the pressure required was only 1 bar (10 m of head) the discharge would increase to 18 l/s, butat the much reduced efficiency of only 12%.
Thus, using an inappropriate pump for the surface irrigation option has a significant effect on theefficiency of pumping.
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Small-scale pumped irrigation - energy and cost 25
Diesel engines tend to be heavier and more robust than petrol engines and are more
expensive to buy. However, they are also more efficient to run and if operated and maintained
properly they have a longer working life and are more reliable than petrol. In some countries
petrol-driven pumps have needed replacing after only 3 years of operation. Diesel pumps
operating in similar conditions could be expected to last at least 6 years. However, it must not
be forgotten that engine life is not just measured in years, it is measured in hours of operation
and its useful life depends on how well it is operated and serviced. There are cases in developing
countries where diesel pumps have been in continual use for 30 years and more.
A diesel-engined pump can be up to four times as heavy as a petrol-engined pump of
equivalent power, and so if portability is important a petrol pump may be the answer.
Electri c motors
Electric motors are very efficient in energy use (75 - 85%) and can be used to drive all sizes and
types of pumps. The main drawback is the reliance on a power supply which is beyond the
control of the farmer, and which in many places is unreliable. Inevitably electrical power
supplies usually fail when they are most needed. Heavy demands occur when crops are needing
most water and so a power failure over several days can have disastrous consequences for a
crop. When using trickle irrigation on light sandy soils, serious crop losses may well occurafter only a few days without power.
FIGURE 17
Manufacturers data for a centrifugal pump
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Choosing a new irrigation system26
3.3.5 Efficiency
The efficiency of power units and pumps is very variable, and few data are available on actualfield performance of small-scale irrigation pumping installations. The data that are available
indicate that efficiencies are very low, in the range 0.5 to 8%, and that such poor levels are quite
common.
Many of the common causes of low efficiency can be corrected at little cost once the
problem is identified, but unfortunately it is easy to run an inefficient pumping system without
even realizing it. Any shortfall in output is simply made up by running the system for longer
than would otherwise be necessary.
Pumping efficiencies are likely to be much higher for sprinkler and trickle systems as the
head needs of these systems are more favourable to the hydraulic characteristics of centrifugal
pumps.Figure 18 shows the main components of a small pumping system and the poor efficiencies
that can commonly occur. The main reasons for inefficiency are listed below. Note that improved
efficiency can be achieved by rectifying the common faults.
Fuel efficiency 90-100%. Fuel is often spilt or leaks from tanks, or from joints in the
fuel pipeline.
Power unit efficiencySmall petrol engines (1kW) 10%.
Small diesel engines (1.5 - 2kW) 15-35%.
Large diesel engines 30-40%. (Text books normally quote 30-40% for engines
but these are optimistic. Ageing of engine, poor quality maintenance, excessive
power consumed by cooling fans, injectors, etc., all bring down efficiency.)
Electric motors have much higher efficiencies 75-85% but a reliable electricity
supply may be difficult to obtain in many locations.
FIGURE 18
Efficiency of components of pumping plant
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Small-scale pumped irrigation - energy and cost 27
Power unit to pump transmission If the engine and pump are direct coupled, then
transmission efficiency nears 100%.
Pump efficiency A pump running at optimum head and speed has an efficiency of between40% and 80%. Many pumps are not run at optimum head and speed, and so their efficiency
could be much lower. This is particularly true for small pumps where the frictional
losses are a higher proportion of the total power requirement.
The overall efficiency of the pumping system can be found by multiplying together the
efficiencies of each component:
Note that in any calculation of this type the decimal equivalent of the percentage is used,i.e., an efficiency of 10% becomes 0.1 in the calculation, 20% becomes 0.2, and so on.
Taking the worst and best possible combinations of all the above efficiencies provides
some indication of the most likely range of overall efficiencies:
This implies that the worst likely efficiency is around 3%. Even this seems good when
compared to the actual field measurements of 0.5% referred to earlier in this section!
Although an efficiency of 30% might be expected from a centrifugal pump operating a
sprinkler or trickle system, it is unlikely to reach this level of efficiency for surface irrigation.
The best that can be achieved would be around 10%.
Peak power demand
The water power and overall efficiency of the pumping plant are used to calculate the overall
power demand.
Developing the formula from Section 2.5:
Pumping plant efficiency (%)= fuel efficiency x power unit efficiency x transmission efficiency x pump efficiency x 100
EXAMPLE 7
Worst condition = 0.9 x 0.1 x 0.9 x 0.4 x 100 = 3%Best condition = 1.0 x 0.35 x 1.0 x 0.8 x 100 = 28%
Overall power demand = water power (kW) / pumping plant efficiency
Overall power demand (kW) =9.81 x discharge (m3/s) x head (m)
pumping plant efficiency
EXAMPLE 8
A small diesel-driven pump delivers a discharge of 2 l/s when lifting water 3 m from a river.
Calculate the peak power demand when the overall efficiency of pump and power unit is 10%.
Peak power demand = (9.81 x 0.002 x 3) / 0.1 = 0.59 kW
Note that the discharge of 2 l/s must be divided by 1000 to convert it into m3/s.
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Choosing a new irrigation system28
Pump suction
An aspect of using centrifugal and mixed flow pumps which is not always fully understood,
and which can seriously impair efficiency, is the suction side of the pump.
In cases of shallow groundwater or surface water pumping, the pump is located above
the water surface and water has to be sucked up a short length of pipe into the pump, as shown
in Figure19. The difference in height between the water surface and the pump is called the
suction lift.
When a pump is operating it draws in water in much the same way as a person sucks
water up through a drinking straw. There is a limit to how high water can be lifted in this way
and it depends on atmospheric pressure (Section 2.2). At sea level this is approximately 10m
head of water. Sucking creates a low pressure in the drinking straw and the outside pressure of
the atmosphere pushes down on the water surface and forces water up the straw. As atmospheric
pressure is the driving force, this puts a practical limit on the height to which water can be lifted
in this way.
Ideally it should be possible to lift water 10m, but because of friction losses in the pipe
and pump a practical limit is 7m. Even at this level many pumps will have difficulty sucking
water. Considerable energy will be needed to suck the water and the pump operator may have
difficulty keeping the pump primed (i.e., keeping the pump and suction pipes full of waterwhen starting the pump). For this reason, pumps should be located so that the suction lift is less
than 3m if possible. If the depth to the water is greater than 3m, then a small shelf can be
excavated and the pump located nearer to the water surface (Figure 19).
Note that these rules only apply when operating in areas close to sea level. Here the
atmospheric pressure is approximately 10m head of water. For schemes operating at higher
altitudes in mountainous regions the atmospheric pressure may be much lower than 10m and
so the suction lift will need to be reduced well below 3m to ensure proper pump operation.
However, not all pumps suffer from suction lift limitations. Pumps designed to work
below the water surface submersible pumps have no such problems.
An example of the effects of variations in suction lift on pump discharge is given by thecase of a small centrifugal pump, which delivered 6.5l/s when operating at 3m suction. When
the suction lift was increased to 8m the discharge dropped to 1.2l/s a loss in flow of 5.3l/
FIGURE 19
Suction lift limitations
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s, or a loss of 85% of the original discharge! Thus, at the greater suction lift the pump would
have to be operated considerably longer to meet water demand, and at such a low flow rate thepump may be well away from its best operating efficiency. This example was cited by Wagner
and Lanoix1 (1969).
3.4 DISTRIBUTION SYSTEMS
The distribution system conveys water from the pump to the fields. This may be by open
channels or through pipes. The choice of distribution system affects both the power and energy
requirements.
3.4.1 Open channels
The most common method of distribution is through open channels, which may be lined or
unlined. Channel design affects the energy demand of the system in three ways:
by determining the energy requirement to lift water from its source into the channels;
by influencing energy losses resulting from friction between the water and the canal; and
by influencing the extent of any additional energy required to pump water which is lostthrough seepage, canal breaches and misuse.
Water will only flow downhill in open channels and so the layout of canals should ensure
that the highest point in the canal system is near to the pump and water source. In this way
water will then flow downhill under the force of gravity and out onto the fields. Sufficient
power must be provided in this case to lift water from its source into the channels (Figure 20).
The head required is determined by the difference in level between the water source and the
water level in the channel. The water level in the channel at the source must be high enough toensure an adequate flow of water to the field, and must include adequate head to allow effective
flow from the channel to the field.
Large water losses can easily occur in open channels. This may be due to seepage
through the bed and sides of a channel. However, open channels, particularly unlined ones, are
prone to breaching, whereupon considerable amounts of water can be lost. They are also easily
FIGURE 20
Energy demand for open channel distribution
1 Wagner, E.G. & Lanoix, J.N. 1969. Water Supply for Rural Areas and Small Communities. Geneva:
World Health Organization.
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Choosing a new irrigation system30
misused. Channels may be left open, particularly when control gates are not working properly,
and water runs to waste. These features of open channels mean that considerable amounts ofwater may be pumped which are wasted, using additional energy and fuel for which there is no
benefit in terms of additional crops.
Of course channels can be lined to reduce seepage, but this requires additional capital
expenditure. A choice must then be made between the additional cost of lining and the cost of
pumping the water which would be lost through seepage. This involves a comparison between
capital expenditure and operating costs, which is discussed later in Section 3.8.3.
Lining canals can often seem an attractive way of reducing seepage losses. It can also
reduce maintenance costs and improve irrigation system distribution efficiency. However, if
linings are to be successful they must be constructed with great care. A concrete lining, for
example, needs to be well vibrated as it is poured so as to be impermeable, and must be placed
on channel beds and banks that have been well compacted. If settlement occurs after construction
and the lining cracks, then not only will seepage losses be high but the cost of the specialist
repairs will also be significant.
Water losses in channels for typical irrigation schemes expressed in terms of efficiency
are shown in Table 4.
Channel hydraul ics
Most irrigation channels excavated in the natural soil are trapezoidal in shape and slope downhillaway from the water source. Channels usually follow the natural ground slope but if the land is
steep, then drop (or fall) structures may be needed to avoid serious erosion problems (Figure
21). Channels with longitudinal bed slopes of less than 1:1000 will usually avoid serious
erosion problems, but a minimum slope of greater than 1:5000 is needed to discourage siltation
and plant growth problems.
Channels which are lined may be trapezoidal but can also be rectangular or semi-circular.
The main aspect of channel design is choosing the bed width and depth of flow. This can
present some difficulties because choosing a value for one affects the other. Thus channel
design is a little more complicated than pipe design because pipes are always circular and so
only one value is chosen the pipe diameter. The reader must look to other texts for the
detailed design of channels, but as guidelines:
FIGURE 21
Channel design: dimensions and drop structures
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Unlined channels are designed so that the velocity is low and the bed and sides are not
eroded by the water. For this reason, unlined channels tend to be wide and shallow, spreadingthe flow over a large area to reduce the erosive power of the water.
Lined channels are expensive to construct. For this reason they tend to be narrow and deepwhich ensures the minimum area of lining for a given channel carrying capacity. The velocity
also tends to be high, but this is not usually a problem as the channel is protected from
erosion by the lining.
3.4.2 Pipelines
Pipelines are often thought to be too expensive for many small irrigation schemes except when
sprinkler or trickle irrigation is used, as then the use of pipes is essential. However, expensive
is a relative word and does not convey a specific meaning. It may well be that when the fulloperating advantages of pipes are considered they may be a viable alternative to open channels.
For small-scale surface irrigation schemes, recent research has shown many advantages
for piped distribution systems:
Very low distribution losses even less than lined channels, as it is much easier to closeoff the flow in a pipe than in an open channel (See Table 4 for water losses in pipelines
expressed as an efficiency).
Less land area is taken up by buried pipes. Channels can take up 0.5-2% of the commandarea.
Pipes can often be installed at lower cost than lined canals.
Pipe systems can provide a more flexible and reliable system of supply.
Reduced contact with water has potential health benefits.
Pipelines for surface irrigation usually operate at low pressures, typically around 0.5bar
(5m of head).
Pipelines are essential for the use of sprinkler and trickle irrigation, and they need to
operate at much higher pressures (typically 2 - 6bar for sprinkler and 1 - 2bar for trickle
systems) and need to be strong enough to withstand up to twice the working pressure. The
reason for this is that pressure surges which are much greater than the normal working pressure
can occur in pipes, and can cause bursts. It is thus important to install a pipe with the correctpressure rating to avoid the expense of repair or even replacement of a complete system.
Energy is needed in pipe systems not only to pump water from the source to the pipe but
also to overcome the energy losses due to friction as water flows down the pipe (Figure22). If
surface irrigation is used, then water can flow freely from the pipe into the field. If sprinkler or
trickle irrigation is used, then additional energy is needed to ensure the water sprays or drips
properly.
Predicting head losses in pipes is not an exact science, and it easy to make mistakes
when calculating them. In addition, losses can increase as the pipe ages and becomes rougher
inside through continued use. For these reasons the losses in the distribution system should be
kept low at the design stage by choosing pipe diameters that are large enough for friction to notdominate the operation of the system at some later date. As a guideline, energy losses in the
pipes should be less than 30% of the total pumping head.
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Choosing a new irrigation system32
Pipeli ne hydraul ics
Energy is lost when water flows along a pipe. This is due to friction between the flowing water
and the pipe wall. This energy loss means that the pressure near the pump will always be
greater than at the far end of the pipe. The change in pressure is called thehydraulic gradient
(Figure 23). Additional power and energy must be supplied by the pump to overcome that
friction so that sufficient water is still delivered to the scheme at the right pressures.
Energy loss in pipelines can be measured as a head loss in metres (m). It depends on the
following factors:
Discharge small changes in discharge can cause very large changes in head loss.
Pipe diameter small changes in pipe diameter can cause very large changes in head loss.
Pipe length changes in pipe length cause similar changes in head loss. Increasing a pipe
length from 100m to 200m will double the head loss.
Pipe layoutthe kinds and numbers of bends and junctions.
FIGURE 22Pipe system and its energy demand energy neededto pressurize sprinklers
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Small-scale pumped irrigation - energy and cost 33
Pipe materialit determines frictional resistance by its smoothness or otherwise.
Energy loss in pipes can be determined from information supplied by pipe manufacturers.
A typical nomograph for PVC pipes is shown in Figure 24. The following examples will
demonstrate effects of discharge, pipe diameter and pipe length on the head loss.
A good guide to selecting the right pipe diameter is to keep the velocity below 1.6m/s. This
is good engineering practice. It ensures that head losses are low and it will help to avoid the
surge and water hammer (sudden oscillations in water pressure) problems which can cause
pipes to burst.
Practical considerations
Different pipe materials have different friction characteristics. The example used in thistext is PVC. If other pipes are used, then values for friction head losses must be obtained
from the supplier.
The smallest diameter pipe may be the cheapest, but it is not always the best choice. Pressurelosses can be very high and so can the cost of providing the extra energy to overcome the
losses. It may be cheaper in the long term to use a larger pipe size, which may have a higher
capital cost but requires less energy in use and so has a much lower operating cost. This
issue is discussed in detail in Section 3.8.5.
Think long term when selecting pipes. Will more water be needed in the future? Will thesystem be extended? If so, investment now in a larger pipe size may save high energy costs
later when trying to pump an increased discharge down a pipe which is too small. A common
problem across the world is that farmers install pipelines which are too small. Many regret
the decision later when they see the potential for irrigation and wish to expand their system.
It is not necessary to use a pipe size which is the same diameter as the pump delivery pipe.For example, a 50mm diameter pump does not mean the farmer must use a 50mm diameter
FIGURE 23
Hydraulic gradient
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Choosing a new irrigation system34
pipe. The diameter is selected according to the above guidelines and if it is different from
the pump diameter then a special section of pipe with a varying diameter (a reducer) is
simply used to connect the pump to the pipeline.
It is important to see what pipe sizes and pumps are available in the local market and to
design around this equipment. This may not always give the most efficient system from anenergy use point of view but it will mean that local support for servicing, maintenance and
repair will be available. Such an advantage may far outweigh any fuel efficiency use issues.
FIGURE 24
Nomograph relating pipe diameter, discharge, head loss and velocity
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Small-scale pumped irrigation - energy and cost 35
3.4.3 Distribution efficiency
Water is not always distributed efficiently, and losses may occur from channels through seepage,
evaporation and mismanagement of the system. In the case of open channels this may involve
gates being left open when no one is irrigating, and canal banks breaching through poormaintenance. For pipe systems, there may be leakage from the joints because of poor sealing
and, again, valves may not always be closed properly. However, it is likely that pipelines have
a potentially higher efficiency than open channels. For design purposes, Table 4 indicates
typical values of distribution efficiency.
EXAMPLE 9
An irrigation scheme uses a 100 mm diameter pipeline, 130 m long, to deliver a discharge of 8 l/s.Determine the head loss.From Figure 24:
When discharge is 8 l/s through a pipe of 100 mm, head loss is 10 m/km.Therefore, over 130 m [= 0.13 km] head loss will be 10 x 0.13 = 1.3 m.
What will be the increase in head loss is the discharge is increased to 16 l/s?From Figure 24:
When discharge is 16 l/s through a pipe of 100 mm, head loss is 37 m/km.Therefore, over 130 m [= 0.13 km] head loss will be 37 x 0.13 = 4.8 m.
The increase in head loss is 4.8 - 1.3 = 3.5 m.Increasing discharge causes a large increase in head loss.
Determine the change in head loss if a pipe of 80 mm is used to deliver the same discharge [8 l/s]over the same distance [130 m].FromFigure 24:
When discharge is 8 l/s through a pipe of 80 mm, head loss is 29 m/km.Therefore, over 130 m [= 0.13 km] head loss will be 29 x 0.13 = 3.8 m.
Difference is 3.8 - 1.3 = 2.5 m, i.e. anincrease in head loss.A decrease in pipe diameter causes an increase head loss.
Determine the change in head loss if the 100 mm pipe is used to deliver the same discharge [8 l/s] over twice the distance [260 m].FromFigure 24:
When discharge is 8 l/s through a pipe of 100 mm, head loss is 10 m/km.Therefore, over 260 m [= 0.26 km] head loss will be 10 x 0.26 = 2.6 m.
Difference is 2.6 - 1.3 = 1.3 m, anincrease in head loss.An increase in pipe length causes a corresponding increase in head loss.
TABLE 4
Indicative values of distribution efficiency (%)
Scheme size (ha)
Earth canals Lined canals Pipes
sand loam clay
Large: >2 000 haMedium: 200 - 2 000 haSmall:
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Choosing a new irrigation system36
FIGURE 25
Basin, forder and furrow irrigation
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Small-scale pumped irrigation - energy and cost 37
3.5 METHODS OF IRRIGATION
There are three methods of irrigation commonly used on small schemes (See also Training
Manual 5:Irrigation Methods):
Surface irrigation
Sprinkler irrigation
Trickle irrigation
The main objectives of these methods are to:
Apply an adequate amount of water to meet crop needs
Apply water uniformly across the field
Ensure there are no long-term problems (e.g., soil erosion, salinization).
3.5.1 Surface irrigation
This is the most common method used on small schemes and involves flooding water across
the soil surface so that it can infiltrate into the root zone and be used by the crop. Basin
irrigation, border irrigation and furrow irrigation are all surface methods (Figure 25). The
choice of surface method depends on the crop, cultivation practices, soils and topography, and
farmer preferences.
Surface irrigation is a labour-intensive method but generally requires less energy than
other methods because of the low head required for distribution.
Although surface irrigation is considered to be a simple method of irrigation this can bevery misleading. Surface irrigation design and construction is relatively simple and little or no
imported specialist materials are needed. However, the proper management of the method is
very complex. The efficient use of irrigation water all depends on the skill of the farmer, who
must decide when to irrigate and how much to apply, and then provide the right discharge into
the field so that water infiltrates adequately and uniformly into the root zone. This is not an
easy task, as the soil and topographic conditions can be very variable and the farmer may not
have the necessary degree of control over the discharge and timing of the application
Potentially, surface irrigation can be very efficient
if all the factors involved are under the careful control
of an experienced irrigator. More often however, the
water management skills are lacking and efficiency tendsto be low. As the designer will not know the level of
field application efficiency that the farmer will achieve
once the scheme is built, a typical value is used for design
purposes (Table 5). If the actual efficiency is less than
the typical value once the scheme is operating, then the
farmer will need to operate the system for longer each
day, or to reduce the cropped area to compensate. This
fall in efficiency will increase the energy demand
(Section 5.2).
For additional information on surface irrigation see Kay (1986)1.
TABLE 5
Typical field application efficiencies
for irrigation methods
Irrigationmethod
Efficiency (%)
SurfaceSprinklerTrickle
607590
1. Kay, M. 1986. Surface Irrigation: Systems and Practice. Cranfield, UK: Cranfield Press.
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Choosing a new irrigation system38
3.5.2 Sprinkler irrigation
Sprinkler irrigation involves distributing water in pipes under pressure and spraying it into the
air so that it breaks up into small droplets and falls to the ground like natural rainfall. Sprinkler
systems are generally more efficient and use less labour than surface irrigation and can be
adapted more easily to sandy and erodible soils on undulating ground. There are many types of
sprinkler system available, but the most common is a system using portable pipes (aluminium
or plastic) supplying rotary impact sprinklers (Figure 26).
An individual rotary impact sprinkler produces a circular wetting pattern with poor
uniformity. To obtain good uniformity, several sprinklers are always operated close together
so that the patterns overlap.
Pressure is an important factor in successful sprinkler operation. Typical operating
pressures range from 2 to 6bar, and so energy requirements can be much greater than for
surface irrigation. If sprinklers are working at the pressure recommended by the manufacturer
then the distribution will be good. If the pressure is above or below this value then the distribution
will be adversely affected. The most common problem is when pressure is too low and this
happens when pump and pipes wear, increasing friction and so reducing pressure.
Typical data for rotary impact sprinklers are shown in Table 6.
It is usually assumed that sprinkler irrigation is more efficient than surface irrigation.
Potentially this is the case, but it largely depends on how well the system is operated and
maintained. If pipe seals leak or burst, and if sprinklers are left running for longer than necessary,
then wastage is inevitable. For design purposes, a field application efficiency of 75% is generally
used.
Traditional sprinkler irrigation is not so well suited to small farms. Typical spacings forsprinklers are 18m 18m, and so they are not so flexible and adaptable to the multitude of
small plots usually found on many farms. An alternative which may be more applicable to
FIGURE 26
Sprinkler irrigation
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Small-scale pumped irrigation - energy and cost 39
small farms is the use of smaller sprinklers connected to the mainline by flexible hoses (Figure27). This is often called a hose-pull system. These sprinklers have great flexibility in operation
and are easily re-located around the farm.
For fuller details of the methods, their design and management the reader should refer to
standard text books and other publications3.
3.5.3 Trickle irrigation
Trickle irrigation involves dripping water onto the soil at very low flow rates (2-20l/h) from asystem of small diameter plastic pipes fitted with outlets calledemitters. Water is applied close
to the plants so that only the part of the soil volume in which the roots develop is wetted.
Applications are usually frequent (every 2-3 days) and this can provide a favourable high moisture
level condition in which the plants can flourish. Many other claims are made about the method,
including increased crop yields, greater efficiency of water use, possible use of saline water,
reduced labour requirements and its adaptability to poor soils. An important advantage is the
ease with which nutrients can be applied with the irrigation water. The relative importance of
each of these attributes will vary depending on the situation.
A typical trickle irrigation system is shown in Figure 28.
TABLE 6
Typical sprinkler data
FIGURE 27
Hose-pull sprinkler system
1. Two publications for further reading are: FAO/ILRI. [1988]. Irrigation methods. Irrigation Water
Management Training Manual5. Kay, M. 1983. Sprinkler Irrigation: Equipment and Practice.
London: Batsford.
Nozzlediameter
(mm)
Pressure(bar)
Diameter ofwetted circle
(m)
Flow(m3/h)
Application rate (mm/h) for spacings:
18 x 18 m 18 x 24 m 24 x 24 m
456810
3.03.03.04.04.5
2932354348
1.021.672.444.968.13
3.25.27.515.325.1
..3.85.711.418.9
..
..4.28.614.0
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Choosing a new irrigation system40
Trickle irrigation is potentially a very efficient method of applying water to crops. Field
application efficiency can be as high as 90%, but like any other method it relies very much on
the skill of the irrigator to achieve this. Field measurements on trickle systems have shownapplication efficiencies as low as 25%. This was the result of poor system management rather
than design. The farmers had not fully understood the concept of partial wetting of the root
zone and so they wasted a lot of water trying to wet up the entire area.
Because of the potentially higher efficiency and the operating pressure of only 1-2bar
this method can use less energy than sprinkler irrigation and in some cases less than surface
irrigation.
Trickle irrigation is very adaptable to small-scale irrigation. It can be ideal for small
plots of trees and row crops requiring different amounts of water. Trickle laterals may also be
moved from one crop row to another to reduce the cost of the system.
Many claims are made about trickle irrigation, such as that it saves irrigation water,increases yield, etc., but care should be taken in accepting such claims. Crops need a certain
amount of water to grow (Section 3.6.1) and generally they are not aware of where the water is
coming from. If it comes from surface flooding, sprinkling or trickle, it makes little difference
to the plants they respond to water. The saving in water comes from the efficiency with
which the water can be applied and it is here that trickle has a distinct advantage. Some yield
increases have been shown with trickle and this may be due to the favourable soil water conditions
and the nutrients added to the water.
For further detailed information reference should be made to specialist publications1.
3.5.4 Selecting an irrigation method
The selection of an appropriate irrigation method depends on a wide range of t