Citation
Energy requirements for drip irrigation of tomatoes in North Florida

Material Information

Title:
Energy requirements for drip irrigation of tomatoes in North Florida
Series Title:
Florida Cooperative Extension Service bulletin 289
Creator:
Smajstrla, A. G. (Allen George)
Castro, B. F. (Ben F.)
Clark, Gary A.
Affiliation:
University of Florida -- Florida Cooperative Extension Service -- Institute of Food and Agricultural Sciences
Place of Publication:
Gainesville, Fla.
Publisher:
Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida
Publication Date:
Language:
English
Physical Description:
6 p. ; 28 cm.

Subjects

Subjects / Keywords:
Agriculture ( LCSH )
Farm life ( LCSH )
Farming ( LCSH )
University of Florida. ( LCSH )
Agriculture -- Florida ( LCSH )
Farm life -- Florida ( LCSH )
Microirrigation -- Energy consumption -- Florida ( LCSH )
Tomatoes -- Irrigation -- Energy consumption -- Florida ( LCSH )
Irrigation pumps -- Florida ( LCSH )
North Florida ( local )
Pumps ( jstor )
Irrigation systems ( jstor )
Crops ( jstor )
Spatial Coverage:
North America -- United States of America -- Florida

Notes

Funding:
Florida Historical Agriculture and Rural Life

Record Information

Source Institution:
Marston Science Library, George A. Smathers Libraries, University of Florida
Holding Location:
Florida Agricultural Experiment Station, Florida Cooperative Extension Service, Florida Department of Agriculture and Consumer Services, and the Engineering and Industrial Experiment Station; Institute for Food and Agricultural Services (IFAS), University of Florida
Rights Management:
All rights reserved, Board of Trustees of the University of Florida
Resource Identifier:
30324211 ( OCLC )
026329734 ( ALEPH )
AJY7921 ( NOTIS )

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used only to trace the historic work of
the Institute and its staff. Current IFAS
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site maintained by the Florida
Cooperative Extension Service.






Copyright 2005, Board of Trustees, University
of Florida




/0/

.12R
UNIVERSITY OF

F FLORIDA


Bulletin 289
February 1994


Florida Cooperative Extension Service


Energy Requirements for Drip Irrigation of Tomatoes in
North Florida'
A.G. Smajstrla, B.F. Castro and G.A. Clark2


INTRODUCTION
The energy required to pump irrigation water for
crop production is measured in terms of fuel use or
electric power use. Energy use depends on the
amount of water pumped and on the fuel or electric
power required to pump each unit of water.

For example, if an irrigation system is used to
apply 20 acre-inches (ac-in) per acre per year and
uses 2 gallons (gal) of diesel fuel per ac-in, then the
annual energy use per acre is the energy contained in
40 gal of diesel fuel. Likewise, if an electric-powered
irrigation pump is used to apply 10 ac-in per acre per
year and uses 25 kilowatt-hrs (kwh) per ac-in, then its
annual energy requirement is 250 kwh per acre.

This publication discusses the factors that affect
energy requirements for irrigation pumping. It
emphasizes ways that irrigation systems can be
designed and managed to minimize energy
requirements for drip irrigation of tomatoes in north
Florida.

AMOUNT OF WATER PUMPED
The amount of irrigation water pumped depends
on several irrigation system factors, and on crop,
climate, and management factors that are independent
of the irrigation system.


Irrigation System Factors

Potential Irrigation System Efficiency
An important irrigation system factor is the
potential irrigation system efficiency. Efficiency is a
measure of the fraction of the water pumped that is
available for plants to use. The potential irrigation
system efficiency is the maximum efficiency that can
be obtained with an irrigation system, assuming
perfect management. It depends on the type of
irrigation system and how well the system is designed.

Microirrigation systems, including drip irrigation
systems, have high potential application efficiencies.
Because water is applied very near or directly into the
crop root zone, evaporation and wind drift losses are
minimized. Water applications can be limited to the
amounts that are needed by the crop, and efficiencies
can be as high as 85 percent. Conversely, seepage
(subirrigation) systems apply water to establish and
maintain a water table just below the crop root zone.
Larger fractions of the water pumped are not used by
the crop, thus application efficiencies are lower,
typically in the range of 50 percent. Efficiencies of
sprinkler and surface irrigation systems are normally
greater than seepage efficiencies but less than
microirrigation efficiencies.


1. This document is Bulletin 289, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida.
Publication date: February 1994.
2. A.G. Smajstria, Professor, Agricultural Engineering Department; B.F. Castro, Extension Agent in Gadsden County Extension Office, Quincy,
FL; G.A. Clark, Professor, Gulf Coast Research and Education Center, Bradenton, FL, Cooperative Extension Service, Institute of Food and
Agricultural Sciences, University of Florida, Gainesville FL 32611.
The Institute of Food and Agricultural Sciences is an equal opportunity/affirmative action employer authorized to provide research,
educational information and other services only to individuals and institutions that function without regard to race, color, sex, age, handicap,
or national origin. For information on obtaining other extension publications, contact your county Cooperative Extension Service office.
Florida Cooperative Extension Service / Institute of Food and Agricultural Sciences / University of Florida I John T. Woeste, Dean


.: T ITY OF FLORIDA LIBRARIES


--




/0/

.12R
UNIVERSITY OF

F FLORIDA


Bulletin 289
February 1994


Florida Cooperative Extension Service


Energy Requirements for Drip Irrigation of Tomatoes in
North Florida'
A.G. Smajstrla, B.F. Castro and G.A. Clark2


INTRODUCTION
The energy required to pump irrigation water for
crop production is measured in terms of fuel use or
electric power use. Energy use depends on the
amount of water pumped and on the fuel or electric
power required to pump each unit of water.

For example, if an irrigation system is used to
apply 20 acre-inches (ac-in) per acre per year and
uses 2 gallons (gal) of diesel fuel per ac-in, then the
annual energy use per acre is the energy contained in
40 gal of diesel fuel. Likewise, if an electric-powered
irrigation pump is used to apply 10 ac-in per acre per
year and uses 25 kilowatt-hrs (kwh) per ac-in, then its
annual energy requirement is 250 kwh per acre.

This publication discusses the factors that affect
energy requirements for irrigation pumping. It
emphasizes ways that irrigation systems can be
designed and managed to minimize energy
requirements for drip irrigation of tomatoes in north
Florida.

AMOUNT OF WATER PUMPED
The amount of irrigation water pumped depends
on several irrigation system factors, and on crop,
climate, and management factors that are independent
of the irrigation system.


Irrigation System Factors

Potential Irrigation System Efficiency
An important irrigation system factor is the
potential irrigation system efficiency. Efficiency is a
measure of the fraction of the water pumped that is
available for plants to use. The potential irrigation
system efficiency is the maximum efficiency that can
be obtained with an irrigation system, assuming
perfect management. It depends on the type of
irrigation system and how well the system is designed.

Microirrigation systems, including drip irrigation
systems, have high potential application efficiencies.
Because water is applied very near or directly into the
crop root zone, evaporation and wind drift losses are
minimized. Water applications can be limited to the
amounts that are needed by the crop, and efficiencies
can be as high as 85 percent. Conversely, seepage
(subirrigation) systems apply water to establish and
maintain a water table just below the crop root zone.
Larger fractions of the water pumped are not used by
the crop, thus application efficiencies are lower,
typically in the range of 50 percent. Efficiencies of
sprinkler and surface irrigation systems are normally
greater than seepage efficiencies but less than
microirrigation efficiencies.


1. This document is Bulletin 289, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida.
Publication date: February 1994.
2. A.G. Smajstria, Professor, Agricultural Engineering Department; B.F. Castro, Extension Agent in Gadsden County Extension Office, Quincy,
FL; G.A. Clark, Professor, Gulf Coast Research and Education Center, Bradenton, FL, Cooperative Extension Service, Institute of Food and
Agricultural Sciences, University of Florida, Gainesville FL 32611.
The Institute of Food and Agricultural Sciences is an equal opportunity/affirmative action employer authorized to provide research,
educational information and other services only to individuals and institutions that function without regard to race, color, sex, age, handicap,
or national origin. For information on obtaining other extension publications, contact your county Cooperative Extension Service office.
Florida Cooperative Extension Service / Institute of Food and Agricultural Sciences / University of Florida I John T. Woeste, Dean


.: T ITY OF FLORIDA LIBRARIES


--






Energy Requirements for Drip Irrigation of Tomatoes in North Florida


System Design

To achieve high efficiencies, the irrigation
distribution system must be well-designed. Good
design means that pipe diameters, lengths, and flow
rates are compatible so that water can be uniformly
applied. If uniformity is poor, water will be wasted
and crop production may decline because excess water
will be applied in some areas, while too little will be
applied in other areas.

Area of Coverage

For many agricultural crops, plants are very
closely spaced. Their roots overlap and extend
throughout the production area. These crops can be
very efficiently irrigated with sprinkler systems
because sprinklers distribute water over the entire
production area. An example is the use of center
pivot systems to irrigate field crops such as corn in
north Florida.

For many other crops, plants are widely spaced
and roots are concentrated near the individual plants.
There may be areas between plants where no roots
grow. Examples are mulched-bed vegetable
production systems such as tomatoes where roots are
primarily located in the beds and few roots extend to
the row middles. In these systems, water use can be
reduced if water is not applied to the row middles,
but is limited to the production bed. Drip irrigation
systems minimize water use by avoiding applications
to row middles where losses would occur due to deep
percolation, evaporation from bare soil, and weed
growth.

Crop Factors
The crop production system is the type of crop
and the associated production practices such as row
spacing and the use of plastic mulch. Crop water use
depends on the type of plants, size of plants, and
number of plants per acre. It also depends strongly
on climate factors as discussed under the following
heading of this publication.

In Florida, the irrigation requirement for most
crops depends heavily on rainfall because a large part
of the crop water use is supplied by rainfall. It also
depends on soil water-holding capacity and root
distributions because these factors affect the amount
of rain that can be held in the soil and extracted by
plant roots. For tomatoes and other crops grown on
plasticmulched beds, the irrigation requirement is a
o101
F (,3


large fraction of the crop water use because much of
the rainfall runs off of the mulched beds.

Climate Factors

Climate factors affect the amount of water
required by a crop. Climatic demand depends on
solar radiation, temperature, humidity and wind
speed. Climatic demand is high on hot, dry, clear sky
days, and it is low on cool, humid, overcast days.

Management Factors

Management practices greatly affect the amount
of irrigation applied. If well-designed and well-
installed systems are poorly managed, both water and
energy will be wasted. Drip-irrigated tomatoes
require small, frequent water applications, normally
daily or more than once a day, except when
interrupted by significant rainfall. Smaller irrigation
durations (amounts) are required early in the growing
season when plants are small. Larger amounts are
required as plants and fruit develop and grow to
maturity.

This publication emphasizes irrigation system
characteristics which minimize energy requirements
for drip-irrigated tomato production. For detailed
discussions of irrigation system management and
irrigation scheduling, see IFAS Extension Bulletin
245, Microinigation on Mulched Bed Systems:
Components, System Capacities, and Management, and
IFAS Extension Circular 872, Inigation Scheduling and
Management of Micro-Inigated Tomatoes.

ENERGY REQUIRED PER UNIT OF WATER

The energy required per unit of irrigation water
pumped depends on the total dynamic head that the
pump is operating against and the efficiency of the
pumping system. The total dynamic head depends on:

the vertical distance that the water is lifted,

the pressure required to operate the drip
emitters, and

the friction losses that must be overcome as water
is pumped from its source through filters, valves,
and pipelines to the emitters.

The efficiency of the pumping system depends on the
efficiencies of the pump, power unit, and connecting
drive units.
SCIUcjt
L7*dW


Page 2






Energy Requirements for Drip Irrigation of Tomatoes in North Florida


Total Dynamic Head

The total dynamic head is the sum of the
pumping lift, operating pressure, and friction losses
within the irrigation system. The total dynamic head
is defined for each of the irrigation subunits. In a
well-designed irrigation system, flow rate and total
dynamic head should be approximately the same for
each subunit so that the pumping system can operate
as efficiently as possible.

Pumping Lift
The pumping lift is the vertical distance from the
water source to the entrance to the subunits. This is
the height that water must be lifted to deliver it from
its source to the irrigation distribution system.
Energy is required to lift the water, and the amount
of energy required is the same for each unit of water:
1 foot-pound (ft-lb) of energy must be expended to
lift each pound of water a vertical distance of 1 foot.
This is equivalent to 1 horsepower (hp) to lift 30 gpm
a distance of 100 feet. It is not possible to avoid this
energy use, because it depends on the location of the
water source and the field elevation and slope.

Because the elevation of the water source may
change during the irrigation season, a pumping system
should have sufficient capacity to lift water from its
lowest anticipated level. In wells, drawdown during
pumping reduces the water level below the static
(non-pumping) level, and a greater amount of energy
is required to lift water from this lower level.

Operating Pressure

The operating pressure is the pressure required
at the entrance to each subunit for the emitters to
operate effectively and water to be uniformly
distributed. The required pressure is defined by the
choice of emitter and the subunit pipe network
design. Pipelines are designed to distribute water to
the emitters with controlled pressure losses so that
water can be uniformly applied throughout the
subunit.

Operating pressures can be minimized by
selecting emitters that operate at low pressures. Drip
systems for tomato production typically use drip tape
laterals that operate at about 10 psi. It is not feasible
to reduce operating pressures much below this level.
Only about 0.8 hp is required to pump 100 gpm at 10
psi.


Friction Losses

Friction losses must be minimized in order to
minimize the energy requirements for irrigation
pumping. Energy must be provided to overcome
friction losses which occur as water flows through all
components from the water source and throughout
the irrigation system. Some friction losses are
unavoidable, even in well-designed, well-constructed,
and properly-maintained irrigation systems. However,
excessive losses waste energy and should not be
tolerated.

Proper selection of irrigation system components
requires that the cost of energy lost to friction be
compared against the cost of larger components with
lower friction losses. Then components with the
overall lowest cost throughout the expected life of the
irrigation system should be selected.

In general, friction losses can be minimized by
selecting pipe sizes to limit the velocity of flow to 5
feet per second (fps) in all mainlines and submains,
and selecting valves and fittings compatible with the
pipe sizes. Proper maintenance is essential to prevent
excessive friction losses as water flows through an
irrigation system, especially at points where large
pressure losses can easily occur, such as filters and
intake strainers on pumps.

For a detailed discussion of component selection,
installation, and maintenance to minimize friction
losses in irrigation systems, see Agricultural
Engineering Department Extension Report 93-7,
Improving Energy Efficiency for Drip-Irigated Tomato
Production: II. Conserving Energy by Reducing Friction
Losses.

Pumping System Efficiency

The overall efficiency of the pumping system is
the multiple of the individual efficiencies of the
pump, power unit, and connecting drive units. Energy
losses can be minimized by properly selecting,
installing, and maintaining each of these components.

Pump Efficiency
The efficiency of irrigation pumps typically ranges
from 60 to 90 percent, with values of 75 to 80 percent
being very common. If a pump has an efficiency of
75 percent, this means that 75 percent of the energy
that is delivered to the pump from the power unit is
transmitted to the water being pumped, where it


Page 3






Energy Requirements for Drip Irrigation of Tomatoes in North Florida


produces the flow rate and pressure delivered by the
pump. The remaining 25 percent of the energy that
was input into the pump is lost due to friction and
turbulence in the pump.

Pump efficiencies can be much lower than 75 to
80 percent if a pump is not properly selected for a
specific application, if is not operated at the proper
speed, or if the impellers or other components are
worn or damaged.

Pump Selection

Irrigation pumps operate near peak efficiency
over a fairly narrow range of discharge rates and
pressures. If an irrigation system requires that a
pump discharge 300 gpm at 50 psi, but the pump
installed is too large, its peak efficiency will occur at
another operating point, for example, 500 gpm and 80
psi. Then this large pump will operate at a low
efficiency and waste energy because it is being applied
at the wrong operating point, even though the pump
may be in good repair.

When an irrigation pump is considered for a given
application, its pump characteristic curves must be
studied to verify that it can operate efficiently at the
required discharge rate and pressure. If it cannot,
another pump which is efficient at the required
operating point should be selected. Pump
characteristic curves should always be provided by the
pump dealer and kept by the pump owner so that the
pump operating characteristics will be known if
operating conditions change.

Pump Speed

Irrigation pumps can be operated at a range of
speeds, and the proper speed is required to obtain the
required discharge rate and pressure. This is not a
problem when the power unit is an electric motor,
because the pump will be directly connected to the
motor, and the pump speed will be the same as the
speed of the electric motor.

When an irrigation pump is driven by an internal
combustion engine, the proper engine speed must be
set by throttling the engine to obtain the desired
pump output at high efficiency. The engine should be
equipped with a tachometer so that the engine and
pump speeds can be accurately set.


Worn or Damaged Impellers
With age, pump impellers may become worn or
damaged. Damage may occur due to the corrosive
nature of some water supplies or due to physical
damage by sand or gravel in the water. When this
occurs, impeller adjustment, repair, or replacement
may be needed. Adjustments should be made by
qualified, experienced individuals because improper
adjustment can damage a pump.

Pump characteristic curves should be used as a
reference when pump tests are conducted to
determine whether a loss in efficiency has occurred.
This reference is needed to help determine when
repairs should be scheduled. In general, repairs
should be scheduled when pump efficiency has
decreased to the point that the expected energy
savings from the pump repair would be greater than
the cost of the repairs.

Power Unit Efficiency

Power unit efficiency is the effectiveness of the
power unit in converting electric power or engine fuel
to mechanical power to drive an irrigation pump. A
convenient way to express this is the performance
standard for a specific type of power unit.
Performance standards are expressed in units of
horsepower-hours per kilowatt-hour (kwh) of electric
power or gallon (gal) of fuel. The performance
standards are:

* electric motors = 1.19 hp-hr/kwh

* diesel power units = 14.75 hp-hr/gal

" gasoline engines = 11.30 hp-hr/gal, and

* LP gas engines = 8.92 hp-hr/gal.

Consider the diesel performance standard data for
example. The standard of 14.75 hp-hr/gal means that
a 14.75 hp diesel engine would be expected to use 1
gal of diesel fuel per hour of operation. Likewise, a
60-hp diesel engine would be expected to use about
4 gal of diesel fuel per hour. If the measured fuel use
rate for a diesel engine is higher than these standards,
this could indicate problems with the power unit.

Power unit efficiencies will decline if the power
units are not maintained, not properly loaded, or if
they are worn with age. The solution to these
problems might be routine maintenance such as a


Page 4






Energy Requirements for Drip Irrigation of Tomatoes in North Florida


tune-up or adjustment, cleaning of fuel injectors, or
change of partially plugged filters. To make best use
of performance standard data, the pump owner
should measure fuel use rates of their systems when
they are in good repair, and record this information
as a reference for later comparisons. As a minimum,
comparisons should be made during equipment
preparation for each irrigation season.

Performance standards will probably not be met
if power units are either overloaded or underloaded.
Overloading will occur if the power unit is too small
for the power requirements of the pump. Also,
running an internal combustion engine at a too-high
speed can quickly overload the engine. Underloading
will occur if the power unit is significantly larger than
the power requirements of the pump. In this case,
the large power unit will waste fuel as compared to a
power unit of the proper size.

It is possible to exceed the previously given
performance standards if high efficiency power units
are used or if other innovations are used to reduce
the loads on internal combustion engines. For
example, the use of turbocharged diesel engines will
increase the efficiency of fuel conversion to
mechanical energy. Thus, a turbocharged diesel
engine would be expected exceed the performance
standard of 14.75 hp-hr/gal. Likewise, many of the
newer air-cooled internal combustion engines have
fuel conversion efficiencies that exceed the
performance standards described above.

The performance standards presented here were
determined for internal combustion engines using
standard accessories, including a water pump, fan, and
radiator. If a heat exchanger is used, replacing the
fan and radiator with cooling by the irrigation water
pumped, then the energy used to power the fan would
be saved. This could save 3 to 5 percent of the power
unit horsepower.

Drive Unit Efficiency

Drive units are the drive shafts and right-angle
gear drives or belt drives that are used to connect
internal combustion engines to irrigation pumps.
Normally, an electric motor does not require a gear
or belt drive because it is directly connected to the
pump or to the pump drive shaft.

Some energy is lost as power is transmitted
through gear or belt drives. This may amount to 5 to
10 percent of the transmitted energy because the


efficiency of these drive units typically ranges from 90
to 95 percent. However, energy losses can be greater
than this if these drives are worn or not in good
repair.

To ensure efficient operation, regularly check and
maintain the lubricating fluid level in a gear drive.
Change the fluid as recommended by the
manufacturer or at least annually.

The fluid in some large gear drives is water-
cooled using a cooling coil that is connected to the
irrigation water supply. Check that the required
amount of water is flowing through the coil so that
the lubricating fluid does not overheat.

Energy will be lost as gears and bearings wear.
Check regularly for excess "play" in the gears, leakage
around seals, or unusual noises and have these
repaired when needed.

Belt drives should be routinely inspected for
proper belt tightness and belt wear. Loose belts will
slip, causing excess energy loss and premature belt
failure. Worn belts may slip or fail during irrigation,
possibly damaging mechanical components. Worn or
damaged belts should be replaced. Belts should be
replaced in matched sets so that all belts in a set will
be uniformly loaded during use.

Some energy is required to spin a drive shaft and
overcome friction losses from drive shaft bearings or
universal joints. The energy required to spin a drive
shaft is unavoidable because it is a function of the
weight of the drive shaft. However, energy losses to
bearings can be minimized by keeping bearings in
good repair and properly lubricated.

Drive shaft bearings in wells may be either oil or
water lubricated. Inspect the lubricating oil reservoir
regularly and add oil as needed. Always pre-lubricate
water-lubricated bearings before starting pumps. Pre-
lubrication is done by pouring a few gallons of clean
water into the column pipe along the drive shaft
before pump start-up. This is very important for
water lubricated bearings because they will heat up
very quickly before the water flows through the
column pipe to lubricate them.

Universal joints are used on drive shafts between
internal combustion engines and gear drives or
pumps. These U-joints should be inspected regularly
for wear and proper lubrication. U-joints are used to
allow efficient power transmission even though the


Page 5







Energy Requirements for Drip Irrigation of Tomatoes in North Florida


drive shafts of the connected units are not in perfect
alignment. Engines and drives should be closely
aligned when installed to avoid excessive flexing of the
joints during operation. Excessive flexing will lead to
earlier failure and increased energy loss. Visually
inspect the drive shaft and U-joints during operation.
Excessive vibration indicates the need for repair.
Excessive vibration can also indicate a serious safety
problem, and thus should be repaired immediately.

SUMMARY

The energy required to pump irrigation water for
crop production is measured in terms of fuel use or
electric power use. Energy use depends on the
amount of water applied and on the fuel or electric
power required to apply each unit of water.

The amount of water applied depends on several
irrigation system factors and on crop, climate, and
management factors that are independent of the
irrigation system. Irrigation system factors include
specific system design factors, such as the potential
irrigation system efficiency, the system design
uniformity, and the relative area of coverage. Crop


factors include type of crop, size of plants, plant
density, and other production system factors such as
the use of plastic mulch. Climate factors include solar
radiation, temperature, humidity and wind speed.
Management factors include irrigation scheduling
decisions which affect irrigation frequencies and
durations.

The energy required per unit of water delivered
depends on the irrigation system design and on field
site characteristics. These factors can be summarized
as the total dynamic head that the pump is operating
against and the efficiency of the pumping system.
Total dynamic head depends on the vertical distance
that the water is lifted, the pressure required to
operate the drip emitters, and the friction losses that
must be overcome as water is pumped from its source
until it is delivered from the emitters. Efficiency of
the pumping system depends on the efficiencies of the
pump, power unit, and connecting drive units.
Recommendations were made for selecting, installing
and maintaining components to minimize energy loss
and maximize pumping efficiency.


Page 6