AGEN-PU-208

10/2021

Center
Pivot
Irrigation
The essential guide to the
selection, optimization, and
management of center pivots

Guy Fipps
Professor and Extension
Agricultural Engineer
Contents
Design Choices 1 Components and Other
Corner-arm and Linear Systems Important Considerations 13
Costs Flow Meter
Types of Drive Systems Pressure Gauges
Wheel and Gear Options Outlet Spacing
The Pivot Design Printout Variable-rate Irrigation (VRI)
System Capacity Control Panels and Systems
Mainline Pipe Sizing Communication Technology
End Guns
Pressure Regulators 8
Pivot Management 15
Water Applicators 9 Runoff Management

Terminology Controlling Wheel Rutting

Pads
Impact Sprinklers Irrigation Scheduling 17
Low-pressure Applicators ET-based
Above-canopy and In-canopy Soil Moisture-based
Water Application

Chemigation 18
Close Drop Spacing 11 Pesticides
LESA Fertigation
LEPA

Center Pivot
Converting Existing Buyer’s Checklist 21
Pivots to Close Drop Spacing 12
Conversion Tips
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The center pivot is the agricultural irrigation system Center pivot machines revolve around the pivot point.
of choice for large acreage because of its low labor The amount of land they can irrigate is based upon the
and maintenance requirements, convenience, length of the pivot or the number of towers. Spans are
flexibility, performance, and easy operation. available in several length options. Additional towers
Center pivot systems, when properly designed and and spans are added to expand the length of the
equipped, conserve valuable resources such as system and the total area irrigated. The flow rate of the
water, energy, money, and time. water applicators changes as they move out from the
pivot point, since each water applicator covers a larger
The first center pivot systems were produced in
area. Nozzle sizes are designed with computer software.
the 1950s and were water-drive systems. These
pivots operated at high pressures and had low
water application efficiencies, resulting in significant Corner-arm and Linear Systems
evaporation losses and high energy use. Today, pivots
In rectangular fields, a pivot leaves the corners dry.
are primarily driven by electric or oil hydraulic motors.
Some growers either leave the corners dry or install
Energy requirements have decreased, and high
a separate sprinkler or drip irrigation system in these
application efficiency is now possible with the use
areas. Pivots may also be equipped with a corner-arm
of close drop spacings equipped with low-elevation
system. This span automatically swings out at the
spray application (LESA) and low-energy precision
corners and activates flow to the water applicators.
application (LEPA) water applicators. Operating
The drive mechanisms are more complex for corner-
pressures as low as 6 pounds per square inch (psi) are
arm systems, which may require more maintenance.
achievable.
Achieving a uniform precipitation rate and good water
distribution in the corner has been a major challenge
Design Choices with these systems. However, variable-rate water
application technology is now commercially available
Wise selection of a center pivot system will result and can help address these problems.
in good water management and conservation, low
operating costs, and future flexibility. When ordering a Another option for rectangular fields is a linear-move
pivot, there are many choices and decisions that need system (also referred to as a lateral-move system),
to be made, including: which moves in a straight line across the field. Water
is usually supplied to the machine from a small canal
Mainline size and outlet spacing
ƒ
that runs along one side of the field. Alternatively,
Pivot height: low profile, standard, high profile, or
ƒ water is supplied by an underground pipeline, which is
ultra-high profile equipped with quick-connect outlets installed along the
Pipe material: galvanized, aluminum, stainless
ƒ pipeline. A flexible hose is attached and repositioned
steel, or poly-lined for adverse water quality to the next outlet as the machine moves. Linear-move
conditions machines are more expensive than pivots due to the
Length, including the number of towers costs of the control and water supply systems. One
ƒ
major difference from pivots is that the flow rate of
Drive mechanisms
ƒ each water applicator is the same for the entire system.
Water application rate of the pivot
ƒ Otherwise, the design of linear-move systems is like
Type of water applicator, operating pressure, and
ƒ that of pivots.
whether pressure regulators are needed
Sizes of motors, driveshafts, and wheels
ƒ Costs
Types of tires and configuration
ƒ The cost of pivots has fluctuated greatly during recent
Type of control panel, associated sensors, and
ƒ years due to the price and availability of materials
communication options for their manufacturing and changing energy and
transportation costs. The pivot system commonly used
As pivots have long life spans (25+ years), taking
for general pricing purposes is a “quarter-mile system,”
the time to carefully consider the options and
which is about 1,300 feet long and irrigates 120 acres.
associated advantages/disadvantages will make a
In the United States, a quarter-mile system may cost
huge difference in the cost and level of management
anywhere from $650 to $750 on a per-acre basis,
required for years to come.

Center Pivot Irrigation  1

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excluding shipping, installation, and the additional normally located at the pivot point. At the 100 percent
expense of running power and water to the pivot. setting, the end tower moves continuously. At the
Longer systems usually cost less on a per-acre basis. 50 percent setting, for each minute of operation, the
For example, a half-mile pivot system (2,600 feet) outer tower moves 30 seconds and stops 30 seconds.
irrigates about 500 acres and costs $500 to $550 per The speed options on most control panels range from
acre. 2 to 100 percent.

The relatively high cost of a center pivot system often

can be offset by advantages over other types of Hydraulic
irrigation systems, such as: Unlike electric-drive pivots, all oil-hydraulic-drive
Reduced labor and tillage towers remain in continuous motion (Fig. 2). Each
ƒ
tower moves continuously at a proportionally reduced
Improved water distribution
ƒ speed compared to that of the outermost tower. Travel
More efficient water use
ƒ speed is selected at a central master control valve
More timely irrigation
ƒ that increases or decreases oil flow to the hydraulic
motors. Two types of hydraulic drives are available:
Flexibility and convenience, with such options as:
ƒ
worm and planetary. The planetary option provides
– Remote and automatic control to start or stop greater strength and efficiency and uses two motors
irrigation, increase or decrease travel speed, per tower—one for each wheel. One motor per tower
reverse direction, and send alarm warning powers an optional worm-drive assembly. Required
messages to the user hydraulic oil pressure usually is 1,500 to 1,800 psi,
– Application of chemicals and fertilizers maintained by a central pump, most often located
– Towable pivot machines that can irrigate near the pivot pad. This central pump may be powered
additional tracts of land by natural gas, diesel, or electricity.

Types of Drive Systems Water-drive Systems

The first pivots manufactured in the early 1950s were
Electric water-drive systems. These tended to operate at very
For electric-drive pivots, individual electric motors high pressures (90+ psi) and had very poor efficiencies.
(usually 0.6 to 1.5 horsepower [hp]) power the two Modern water-drive pivots can operate at pressures
wheels at each tower (Fig. 1). Typically, the outermost as low as 45 psi and come in a towable option so the
tower moves to its next position and stops. Then, same pivot can be shared among fields. Water-drive
each succeeding tower moves into alignment. pivots may be particularly applicable to fields without
Rotation speed (or travel time) of the pivot depends power. Maximum pivot length is limited to less than
on the speed of the outermost tower and controls 290 feet or field sizes less than 7.5 acres. Water-driven
the amount of water applied. The system operator pivots are continuous-move systems and share the
can select the tower speed using the control panel, same relative advantages of hydraulic-move systems.

Figure 1a. Electric drive. Figure 1b. Electric drive. Figure 2. Hydraulic drive.

Center Pivot Irrigation  2

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Which is Better? Hydraulic-drive pivots have only one gear reduction.

In field tests, both electric- and hydraulic-drive Tables 1a and 1b list examples of electric- and
systems worked well. Choice of pivot type usually hydraulic-drive systems and the end-tower speed
is guided by the power source available, personal based upon the given specifications.
preferences, required system maintenance, local
dealer service history, local-market product The Pivot Design Printout
availability, purchase price, and dependability. In
principle, continuous-move systems provide better The design computer printout (commonly referred
irrigation and water uniformity, which are particularly to as the “pivot printout” or just “printout”) provides
beneficial for chemigation, and are less likely to get information required for the operation and
stuck. However, uniformity also is influenced by other management of a center pivot and details how it will
factors, including travel speed, system design, type perform on a specific tract of land. A portion of a
of water applicator, and operator management. Timer typical design printout is shown in Figure 3. Printouts
settings for electric machines have a greater range of include the following:
adjustment. Pivot-design flow rate
ƒ
Irrigated acreage under the pivot
ƒ
Wheel and Gear Options Maximum elevation change in the field, as
ƒ
The speed of the pivot controls the amount of water measured from the pivot point
applied. Pivot travel speed depends on both the wheel Operating pressure and mainline friction loss
ƒ
size and the power-drive mechanisms. Electric power- Pressure regulator rating in psi
ƒ
drive systems have two gear reductions: one in the
Type of water applicator, spacing, and position on
ƒ
driveshaft and one in the gearbox driving each wheel.
the mainline
Thus, maximum center-pivot travel speed depends on:
Nozzle size for each applicator
ƒ
Electric motor speed or rotation in revolutions per
ƒ
minute (rpm) Water applicator nozzle pressure
ƒ
Speed reduction rotation in both the center Maximum travel speed
ƒ
ƒ
driveshaft and the gearboxes Precipitation chart
ƒ
Wheel size
ƒ

Table 1a. Typical gear reduction, wheel drive rpm, and maximum end tower travel speed of electric-move pivots.

Wheel dimension (in)

Rim and tire
Center drive Gearbox Rim and circumference Last wheel End tower
motor rpm ratio Ratio Rim tire (ft) drive (rpm) (ft/hr)
1,740 58:1 52:1 24 40 10.47 0.5769 362
1,800 40:1 50:1 24 40 10.47 0.8700 546
3,450 40:1 52:1 38 54 14.13 1.6586 1,406

Table 1b. Typical gear reduction, wheel drive rpm and maximum end tower travel speed for hydraulic drives.

Hydraulic Rim and tire

Number of pump drive Tire size circumference Last wheel End tower
Drive type towers (hp) (in) (ft) drive (rpm) (ft/hr)
Hydraulic 8 10 16.9 × 24 10.47 0.5730 360
Hydraulic 8 15 14.9 × 24 10.47 0.9312 585
Hydraulic High-Speed 8 25 11.2 × 38 14.13 1.5723 1,333
Hydraulic High-Speed 18 25 11.2 × 38 14.13 0.6286 533

Center Pivot Irrigation  3

 Figure 3. Sample design computer printout.
Pivot identification J & J Farms Overall length 1309.00 ft
Pivot location Section 130 Drop tube length 12.50 ft
Design flow rate 625.00 gpm Regulator position (from mainline) 12.00 ft
Design Pressure at the end 4.00 psi Design elevation of end tower +7.0 ft, -8.0 ft
Pressure at pivot 13.67 psi End gun gpm 0

SPAN SPAN MAINLINE NUMBER OF DROP DROP 1st DROP REGULATOR

NO. LENGTH DIAMETER DROPS SPACING DIAMETER POSITION SIZE ACRES
(ft) (inches) (ft) (inches) (ft) (psi)
1 160 6.38 19 6.67 0.75 36.60 6 1.84
2 160 6.38 24 6.67 0.75 3.335 6 5.53
3 160 6.38 24 6.67 0.75 3.335 6 9.23
4 160 6.38 24 6.67 0.75 3.335 6 12.92
5 160 6.38 24 6.67 0.75 3.335 6 16.61
6 160 6.38 24 6.67 0.75 3.335 6 20.30
7 160 6.38 24 6.67 0.75 3.335 6 23.99
8 160 6.38 24 6.67 0.75 3.335 6 27.68
9 29 5.78 5 6.67 0.75 3.335 6 5.41
Total 1309 192 123.51

1. Mainline outlet number from pivot point

2. Distances in feet between outlets or span length between towers
3. Distance in feet from pivot point to outlet or tower
4. The gpm needed based on the area covered by the applicator
5. Actual gpm delivered by the applicator based on the applicator’s nozzle size and operating pressure
6. Pressure in psi in the mainline at the outlet
7. Pressure at the nozzle (when pressure regulators are used, the pressure at the nozzle should
be no less than the psi of the regulator’s rating)
8. Brand name and/or type of applicator and nozzle size (nozzle size is reported
either by number or actual size in inches)
9. Applicator number or position on mainline
10. Pressure regulator’s brand name, psi rating, and flow
capacity (gpm) often expressed as LF (low flow), HF
(high flow), etc.
11. Plug number, if outlet is plugged
12. Distance from furrow
arm to applicator, inches

1 2 3 4 5 6 7 8 9 10 11 12
OUTLET LAST DISTANCE GPM GPM PIPE NOZZLE SPRINKLER LABEL SPRK REG PLUG DROP
NO. OUTLET TO PIVOT NEED DEL. PSI PSI & NOZZLE SIZE NO. SIZE NO. LENGTH
1 6.08 1
2 36.60 36.60 0.18 0.29 13.27 6.66 4.0 1 6LF 150
3 6.67 43.27 0.21 0.29 13.20 6.66 4.0 2 6LF 156
4 6.67 49.94 0.24 0.29 13.13 6.66 4.0 3 6LF 156
5 6.67 56.61 0.27 0.29 13.05 6.66 4.0 4 6LF 162

Center Pivot Irrigation 

20 6.67 156.66 0.76 0.76 11.86 6.66 6.5 19 6LF 144

4
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 Figure 3. Sample design computer printout (continued).

Tower 1 160.00 160.00

21 6.67 163.33 0.79 0.76 11.79 6.66 6.5 20 6LF 144
22 6.67 170.00 0.82 0.88 11.72 6.66 7.0 21 6LF 144
23 6.67 176.67 0.85 0.88 11.65 6.66 7.0 22 6LF 150
24 6.67 183.84 0.89 0.88 11.58 6.66 7.0 23 6LF 150
25 6.67 190.01 0.92 0.88 11.50 6.66 7.0 24 6LF 156

44 6.67 316.67 1.53 1.61 10.20 6.66 9.5 43 6LF 144

Tower 2 160.00 320.00

45 6.67 323.33 1.56 1.61 10.03 6.66 9.5 44 6LF 144
46 6.67 330.00 1.59 1.61 9.96 6.66 9.5 45 6LF 144

Figure 4. Sample precipitation chart.

IRRIGATOR – XXXXX
MOTOR SIZE (HP) = 1 WHEEL GEAR BOX RATIO = 50T01
LOADED MOTOR RPM = 1745 TIRE SIZE = 11.2 X 24.0
CENTER GEAR BOX RATIO = 58T01 LAST TOWER MAX. SPEED (FPM) = 5.90

**Irrigation Precipitation Chart**

****************************************************************************************************************************************
1. Total amount of water applied 2. Timer (or speed) setting on the control 3. Time in hours to make a complete
in inches at this speed setting usually indicated as a percentage of circle at this speed setting
the maximum speed

1 2 3
****************************************************************************************************************************************
PRECIPITATION – INCHES TIMER SETTING – % TIME – HOURS
0.25 100.00 22.70
0.32 80 28.38
0.36 70 32.44
0.42 60 37.84
0.51 50 45.41
0.64 40 56.76
0.85 30 75.68
1.02 25 90.82
1.27 20 113.53
1.42 18 126.14
1.70 15 151.37
2.12 12 189.22
2.55 10 227.06

Center Pivot Irrigation 

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Water applicators come with stickers Table 2. Daily and seasonal irrigation capacity for irrigation
with numbers, which correspond to the systems operating 24 hours per day.
number of the drop on the pivot printout, Inches in irrigation days
making it easy to install applicators in GPM/ Inch/ Inch/
the right location. Printouts include a acre day week 30 45 60 80 100
precipitation chart, as shown in Figure 1.5 .08 .55 2.4 3.8 4.8 6.4 8.0
4. The chart is often located at the end
2.0 .11 .75 3.2 4.8 6.4 8.5 10.6
of the printout and shows the irrigation
amounts (in inches of water applied) for 3.0 .16 1.10 4.8 7.2 9.5 12.7 15.9
each speed setting, based upon the gear 4.0 .21 1.50 6.4 9.5 12.7 17.0 21.2
reduction ratios and tire size of the pivot.
5.0 .27 1.85 8.0 11.9 15.9 21.2 26.5
When ordering a pivot, it is essential 6.0 .32 2.25 9.5 14.3 19.1 25.4 31.8
to specify the actual/measured (not
7.0 .37 2.60 11.1 16.7 22.6 29.7 37.1
estimated) available water supply (in
gallons per minute [gpm]), along with
the changes in field elevation, from the
pivot point to the lowest and highest points in the between adjoining towers). Typical span length
field. This information is critical to determine accurate options are:
irrigation amounts, operating pressure requirements, 113 to 140 feet for 10-inch mainline
ƒ
and whether pressure regulators are needed.
113 to 180 feet for 8⅝- and 8-inch mainlines
ƒ
160 to 205 feet for 6⅝-, 6-, and 59/16-inch mainlines
ƒ
System Capacity
Selecting the correct mainline pipe size will reduce the
A pivot’s irrigation-system capacity is determined by energy costs of operating the pivot over its lifespan.
the flow rate in gpm and the number of acres irrigated. While smaller-diameter pipes are less expensive to
System capacity is often expressed in terms of: purchase, they may have higher friction pressure loss
Total flow rate of the pivot in gpm
ƒ from the flow of water. Friction losses cause a loss in
Application rate per acre in gpm pressure, resulting in higher energy costs. For a flow
ƒ
rate of 1,000 gpm, rules of thumb are as follows for
Knowing a system’s capacity in gpm/acre is pivots in Texas with the water supplied from wells:
necessary for proper irrigation water management
Each additional 10 psi of pivot pressure requires
ƒ
and in determining if the pivot can meet the water
an increase of approximately 10 hp.
requirements of specific crops. Table 2 shows the
relationship between gpm per acre and irrigation Each additional 10 psi of pivot pressure increases
ƒ
amounts. This table applies to pivots and all other fuel costs by about $0.35 per hour (or $0.16
irrigation systems that have the same capacity in per acre-inch) for natural gas costs of $3.00 per
gpm per acre. The amounts do not include application thousand cubic feet (mcf).
losses and assume that the pivot operates 24 hours a At $0.07 per kilowatt hour, electricity costs $0.60
ƒ
day. For example, if one irrigates 120 acres with 480 per hour ($0.27 per acre-inch) for each additional
gpm, and it takes 24 hours to complete a circle, then 10 psi of pressure.
the capacity is 4 gpm/acre per day. From Table 2, this For diesel fuel priced at $2.00 per gallon, it costs
ƒ
system would apply 0.21 inches of water in one 24- $1.20 per hour ($0.56 per acre-inch) for each
hour day, 1.5 inches each week, and 6.4 inches in 30 additional 10 psi of pressure.
days.
For diesel fuel priced at $3.00 per gallon, the cost
ƒ
for each additional 10 psi increases to $1.80 per
Mainline Pipe Sizing hour ($0.84 per acre-inch).

Typical mainline sizes are 10, 8⅝, 8, 6⅝, 6, and 59/16 Table 3 lists friction losses for different mainline sizes
inches. Short and small (mini) pivots often use a and flow rates. Total pressure lost due to friction
4½-inch pipe. The size of the mainline determines should not exceed 10 psi in quarter-mile systems on
available options for span lengths (the distance flat to moderately sloping fields. Therefore:

Center Pivot Irrigation  6

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Friction loss exceeds 10 psi when more than 600

ƒ For center pivots 1,500 feet long, 6⅝-inch
ƒ
gpm is distributed through 6-inch mainlines. mainline can be used for 700 gpm, while keeping
For flow rates less than 800 gpm, 6⅝-inch
ƒ friction-pressure loss under 10 psi.
diameter mainline can be used. Friction and operating pressure for half-mile
ƒ
Some 8-inch spans should be used when more
ƒ systems can be greatly reduced by including
than 800 gpm are delivered by a quarter- mile some spans of 10-inch mainline pipe.
system. Saving money on the initial
purchase price often means paying
Table 3. Approximate friction loss (psi) in center pivot mainlines.* more in energy costs over the life
Mainline pipe diameter (nominal size)**, inches of the system. Some dealers may
undersize the mainline to reduce
6 65/8 8 10
their bids, especially when pushed
Flow rate, gpm Mainline pressure loss, psi to give the best price. Check the
A. Quarter-mile system proposed design printout. If the
(1,300 ft): operating pressure appears high,
500 6.5 4 ask the dealer to provide another
600 9 5.7 design using proportional lengths
700 12 7.6 of larger pipe, referred to as
800 15.5 10 4 telescoping, to reduce operating
900 19 12 4.6 pressure.
1,000 23.5 14.7 5.5
1,100 28 18 7 Telescoping
1,200 33 20.8 7.9
Telescoping involves using a larger
B. 1,500-ft system: diameter mainline pipe at the
600 10 6.7 2.5 beginning of the pivot, followed
700 14 9 3.3 by smaller sizes as the flow rate
800 18 11 4.2 decreases away from the pivot
900 23 14 5.2 point. Telescoping the mainline is
C. Half-mile system used to obtain the lowest friction
(2,600 ft): losses and operating pressure
1,600 111 70 27 9 possible for the given flow rate
2,000 105 41 13.5 and length of the machine, which
2,400 57 19 in turn, will lower energy costs
2,800 25 over the life of the pivot. Pipe
* C-factor = 140 used for friction loss calculations sizing is based on the velocity of
** Actual internal pipe diameters used for these calculations are 6 in, 6.375 in, 7.79 in, and 9.79 in.
the water flowing through the
mainline in each span. Computer
Table 4. Telescoping to reduce mainline friction loss with outlets spaced at software is used to examine
60 inches.
multiple pipe-size options to
Feet of mainline size (nominal dimensions) achieve the lowest purchase price
Total Friction
GPM 10-in 81/2-in 8-in 65/8-in feet loss – psi and operating costs.

Example 1 Table 4 illustrates how telescoping

1,100 0 0 0 1,316 1,316 18 can be used to reduce friction
1,100 0 0 640 676 1,316 9.4 losses.
Example 2 In Example 1, total friction
ƒ
2,500 0 0 1,697 927 2,624 70.5 loss is reduced from 18 to 9.4
2,500 0 897 800 927 2,624 60 psi by using 640 feet of 8-inch
2,500 897 0 800 927 2,624 47 mainline rather than all 6⅝-
2,500 1,057 640 540 387 2,624 30.1 inch pipe in a 1,316-feet-long
2,500 1,697 0 540 387 2,624 25 pivot.

Center Pivot Irrigation  7

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Example 2 lists friction losses for various lengths

ƒ function properly at operating pressures less than
and combinations of mainline pipe size for a their rating plus about 5 psi.
2,624-feet-long pivot nozzled at 2,500 gpm and
Table 5 shows how variations in terrain elevations
irrigating 496 acres. Total friction loss is reduced
influence mainline operating pressures. Elevation
from 70.5 to 25 psi by using more 10- and 8-inch
changes in the field have the largest impact on center
mainline pipe and less 6⅝-inch pipe.
pivots with lower design pressures. From the first to
When selecting a system, compare the higher initial the last drop on a pivot, operating pressure at the
purchase cost of larger mainline pipe sizes to the nozzle should not vary more than 20 percent from
increased pumping costs associated with smaller pipe design operating pressure. Pressure regulators
sizes. Higher operating pressure requirements result in usually are not necessary if the elevation change from
higher costs for pumping. The total operation pressure the pivot point to the end of the machine is 5 feet or
requirement is the sum of friction loss, system design less. The best approach is to have the irrigation dealer
pressures, and changes in field elevation. For existing run pivot printouts with and without regulators. As
pivots, pressure gauges installed near the pivot point shown in Table 5, every additional 2.3 feet of elevation
and on the last applicator drop will identify system requires an additional 1 psi of operating pressure.
operating pressure. When including pressure regulators, the smallest
psi-rated pressure regulator as required by elevation
changes should be used.
Pressure Regulators
Special attention is required for situations where
Some dealers automatically include pressure the flow rate and the operating pressure vary
regulators regardless of whether they are actually significantly during the growing season. This is often
needed. This often occurs in situations where there is due to seasonal variations in groundwater pumping
uncertainty about the available flow rate and pressure levels. As groundwater levels decline, the available
that will be provided to the pivot, and in situations pressure may fall below that required to operate the
where there is no data on elevation changes in the regulators, resulting in insufficient water application
field where the pivot will be located. However, pressure and poor uniformity. Using variable frequency drive
regulators impact the design pressure of the pivot, pumps is one strategy for this situation, which may be
and in turn, the design pressure defines the energy effective depending on site-specific conditions. The
requirements to pump water and the long-term cost of more common solution is to renozzle the pivot for the
pivot irrigation. Pressure regulators are not needed for reduced flow rate. Nozzles are inexpensive and can
all sites, and their use should be carefully considered.

Pressure regulators have specific ratings such as 6

Table 5. Percent variation in system operating pressure
psi, 10 psi, 15 psi, 25 psi, etc. The rated delivery psi
created by changes in land elevation for a quarter-mile
of the pressure regulators governs the size of the pivot. Maintain less than 20 percent variation.
nozzle selected for each water applicator. For the same
application rate, nozzles used with 10 psi regulators System design pressure (psi)*
elevation
will be smaller than those used with 6 psi regulators.
Lower-rated (low psi) pressure regulators, if used, allow 6 10 20 30 40
Change
the center pivot to be designed for minimum operating in feet psi % Variation
pressures, which lowers energy costs.
2.3 1 16.5 10.0 5.0 3.3 2.5
Pressure regulators are “pressure killers.” They reduce 4.6 2 33.0 20.0 10.0 6.6 5.0
pressure at the nozzle so that the appropriate amount
6.9 3 50.0 30.0 15.0 10.0 7.5
of water is applied by each applicator. Pressure
regulators require energy to function properly. Water- 9.2 4 40.0 20.0 13.3 10.0
pressure losses within the regulator may be 5 psi or 11.5 5 50.0 25.0 16.6 12.5
more. Thus, the entrance (or inlet) water pressure 13.9 6 30.0 20.0 15.0
needs to be about 5 psi higher than the regulator
16.2 7 23.3 17.5
pressure rating. Six-psi regulators require 10 psi at the
inlet; 10-psi regulators, 15 psi; 15-psi regulators, 20 18.5 8 26.6 20.0
psi; and 20-psi regulators, 25 psi. Regulators do not *pressure at the nozzle

Center Pivot Irrigation  8

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be quickly swapped out when the pressure becomes water applicators are positioned within 1½ feet off the
too low. Dealers will be glad to produce a new pivot ground surface.
printout when ordering a new nozzle package.
Precision Application,
Water Applicators Residue Managed (PARM)
Included in the close-drop-spacing category is PARM,
Terminology an approved conservation practice by the United States
Department of Agriculture (USDA) Natural Resources
Figure 5 shows the Conservation Service (NRCS). PARM includes residue
typical configuration Mainline
Gooseneck
management using a no-till or strip-till tillage system
outlet Slip couplings
of a drop and the or hose clamps and “flat planting.” It normally consists of a LEPA drop
terminology used Pivot in each row, with applicators discharging water in
to refer to each mainline
a bubble pattern, positioned within 1½ feet of the
Flexible drop
component. The ground surface (Fig. 6). Drops in every other row are
drop may be a rigid also allowable under the NRCS practice.
metal tube, a semi-
rigid plastic tube, or
a poly drop. Shown Weight

is a generic water
Couplings
applicator positioned
Pressure
close to the ground. regulator

Water applicators Applicator

and the terminology

used to describe water Figure 5. Drop arrangement.
applicator systems
on pivots continue to evolve. Since the early 1990s,
common terminology consisted of the following:
Applicators located on top of the mainline,
ƒ
Mid-elevation spray application (MESA),
ƒ
Figure 6a. A PARM center pivot, equipped with LEPA
Low-elevation spray application (LESA), and
ƒ drops in every row. Photo provided by the USDA-NRCS.
Low-energy precision application (LEPA).
ƒ
Both LESA and LEPA are also classified as low-pressure
systems, with operating pressures as low as 6 psi.
MESA systems typically have operating pressures in
the 20 to 25 psi range.

Today in the irrigation industry, water applicator

systems are commonly referred to as:
Over-canopy,
ƒ
In-canopy, and
ƒ
Close drop spacing.
ƒ
a. LESA
b. LEPA

Close drop spacing systems include both LESA and

LEPA. The spacing of drops is based upon the row
spacing, with drop spacing equal to twice the row
Figure 6b. LEPA drop with applicator discharging water
spacing (or a drop in every other row) for LESA, and
in a bubble along with residue management.
either every row or every other row for LEPA. These Photo provided by the USDA-NRCS.

Center Pivot Irrigation  9

 Return to Contents page

Pads current center pivot designs. Low-pressure applicators

require less energy and, when appropriately
There are different types of spray water applicators positioned, ensure that most of the water pumped
available, each with several pad options, including flat, gets to the crop.
concave, and convex pad designs that direct the water
spray pattern horizontally, upward, or downward at Generally, the lower the operating pressure
minimum angles. Spray applicator pads also vary in requirements, the better. With close drop spacing
the number and depth of grooves, which affects the (i.e., applicators spaced no wider than 60 to 80
size of water droplets produced. Fine droplets may inches apart), nozzle operating pressure can be
reduce erosion, runoff, and soil compaction, and they as low as 6 psi. Water application is most efficient
promote better infiltration in heavy soils but are less when applicators are positioned within 18 inches of
efficient because of their susceptibility to evaporation ground level. Water applicators have interchangeable
and wind drift. “deflector” pads, which allow various spray patterns
and bubble modes, enabling quick and easy switching
Some growers prefer to use coarse pads that produce from LESA to LEPA water application modes.
large droplets and to control runoff and erosion with
agronomic and other management practices. Little Field testing has shown that when there is no wind,
university-based research data is available on the low-pressure applicators positioned 5 to 7 feet
performance of various pad arrangements. In the above ground can apply water with up to 90 percent
absence of personal experience and local information, efficiency. However, as the wind speed increases, the
following the manufacturer’s recommendations is amount of water lost to evaporation increases rapidly.
likely the best strategy for choosing pad configuration. In one study, wind speeds of 15 and 20 miles per hour
Pads are inexpensive, and some growers purchase created evaporative losses of 17 and 30+ percent,
several groove configurations and experiment to respectively. In another study on the Southern High
determine which works best in their operations. Plains of Texas, water loss from a linear-move system
was as high as 94 percent when wind speed averaged
22 miles per hour with gusts of 34 miles per hour.
Impact Sprinklers When applicators are located near the ground surface,
efficiencies of 95 to 98 percent are possible. However,
High-pressure impact sprinklers mounted on top of
evaporation loss is influenced by multiple factors such
the center pivot mainline were prevalent in the 1960s
as wind speed, relative humidity, and temperature.
when energy prices were low and water conservation
did not seem so important. Now, such sprinklers are
recommended only for special situations, such as land Above-canopy and In-canopy
application of wastewater, where high evaporation can Water Application
be beneficial. Impact sprinklers usually are installed
directly on the mainline and release water upward at With in-canopy systems, water applicators are located
15 to 27 degrees. below the truss rods, often about midway between
the mainline and ground level. However, shorter drops
High-pressure impact sprinklers normally produce
may be used so that water is applied above the crop
water pattern diameters in the range of 50 to more
canopy, even on tall crops such as corn and sugarcane.
than 100 feet. Water application losses average 25
Rigid drops or flexible drop hoses are attached to the
to 35 percent or more, depending on how much
mainline gooseneck or furrow arm and extend down
wind there is. Low-angle (i.e., 7 degrees) sprinklers
to the water applicator (Fig. 5). Weights are needed
somewhat reduce water loss and pattern diameter but
with flexible drop hoses. Typically, these systems
do not significantly decrease operating pressure.
operate at 25 to 45 psi. However, improved designs
are available, which only require 10 to 15 psi with
Low-pressure Applicators conventional 8½- to 10-foot drop spacing.

Very few center pivots in Texas are now equipped Nozzle pressure requirements depend on both the
with impact sprinklers, as improved water applicator type of water applicator and the height at which it is
designs are available, resulting in more responsible positioned. For a given pressure, the radius of throw
irrigation water management. These applicators (i.e., the distance that water is thrown from the nozzle,
operate at low water pressure and work well with also referred to as the wetted area) depends upon

Center Pivot Irrigation  10

 Return to Contents page

how high the applicator is above the ground. Sufficient

overlap is required for good distribution of water.
Consult with a dealer and have a new printout run
before changing the height of the water applicators.

Research has shown that in corn production, 10 to 12

percent of water applied by above-canopy irrigation is
lost by wetting the foliage. More is lost to evaporation.
Field comparisons indicate 20 to 25 percent more
water is lost in above-canopy irrigation than from
LESA and LEPA systems.
Figure 8. An example of a water applicator used for
close drop spacing systems that can be easily switched
Close Drop Spacing to the bubble mode (LEPA) and spray mode (LESA).

LESA It is best to plant corn, sorghum, and other high-

profile crops in circle rows, with the water sprayed
LESA applicators are positioned 12 to 18 inches above
underneath primary foliage. Growers have
ground level or high enough to allow space for wheel
successfully used LESA irrigation in straight rows
tracking (Fig. 7). Less crop foliage is wetted, especially
using a flat, coarse pad that sprays water horizontally.
when crops are planted in a circle, and less water
Heavier weights help keep the drops in position as
is lost to evaporation. LESA applicators usually are
they drag through high-profile crops. Truss rod sling
spaced 60 to 80 inches apart, corresponding to two
clamps (Fig. 9) can be used to raise the drops when
crop rows. The usual arrangement is illustrated in
growing wheat and other densely planted crops.
Figure 5. Each applicator is attached to a flexible drop
hose, which is connected to a gooseneck or furrow
arm on the mainline. Weights made of metal or plastic
are available for each type of applicator to help
stabilize the applicator in winds and allow it to work
itself through crops planted in straight rows. Nozzle
pressure as low as 6 psi is best with a correctly chosen
water applicator. Water application efficiency usually
averages 90 to 95 percent. LESA center pivots can be
converted easily to LEPA with an applicator adapter
pad (Fig. 8).

Figure 9. Truss rod hose clamps/slings.

LEPA
In the original LEPA prototypes developed in the
1980s, water was applied in every row using a drag
sock/hose or an applicator that discharged water in a
“bubble” pattern. Furrow diking was also used. Double-
ended socks were developed to prevent these dikes
from washing out during irrigation. To reduce the
cost of converting existing pivots to LEPA, research
was undertaken to evaluate the use of a drop in every
other row. Results showed that in many situations,
Figure 7. Close drop spacing with LESA water
every-other-row irrigation was just as effective.
applicators.

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Figure 10 shows the original multi-functional LEPA gets to the crop. Water application is precise and
“quad” applicator, which could deliver a bubble concentrated, requiring a higher degree of planning
water pattern, spray mode (useful for germination), and management, especially in clay soils. Center pivots
and chemigation mode for applying the water/ equipped with LEPA applicators provide maximum
chemical mixture to the underside of leaves. The water application efficiency at minimum operating
quad applicator is now replaced with low-pressure pressure. LEPA can be used successfully in circles or in
applicators with interchangeable adapter pads to straight rows and is even more beneficial where water
achieve the original LEPA modes, as well as LESA spray is limited.
modes (Fig. 8). Socks help reduce furrow erosion when
discharging water directly into the furrow (Fig. 11). If
desired, drag-sock and hose adapters can be removed Converting Existing Pivots
from applicators, and a spray or chemigation adaptor to Close Drop Spacing
pad can be attached in their place.
When ordering a new pivot, there are several options
Drops in every other row and planting in a circle for water outlet spacing. However, some dealers
result in alternate wet and dry furrows. Dry middles default to a wide outlet spacing (8½ to 10 feet
allow more rainfall to be stored. Applicators are apart) when ordering pivots. For pivots with wide
arranged to maintain dry rows for the pivot wheels outlet spacing, additional outlets are needed when
when the crop is planted in a circle, thereby reducing converting to close drop spacing. For example, for
the potential for rutting. Field tests show that with row spacing of 30 inches, drops are needed every
LEPA, 95 to 98 percent of the pumped irrigation water 60 inches (5 feet). Likewise, for 36-inch row spacing,
drops are placed every 72 inches (6 feet). Additional
outlets can easily be added using truss rod hose
slings and double goosenecks, as shown in Figures 9
and 12. Installation is quicker if a platform is placed
underneath the pivot mainline. Planks placed across
the truss rods or the sideboards of a flat-bed truck
work but are not safe and may lead to accidents and
injury. A tractor equipped with a front-end loader
provides a more secure platform. A boom lift or a
telehandler with a work basket are readily available
and provide the safest platform.

Bubble Spray Chemigate

Conversion Tips
Figure 10. Original multi-functional LEPA head.
First, use stakes or other items to mark the location
for each drop on the ground. Begin by measuring
from the center of the tower wheel. The first drop
is positioned one row spacing over from the wheel.

Figure 11. LEPA sock and furrow diking. Figure 12. Double barb gooseneck.

Center Pivot Irrigation  12

 Return to Contents page

Switch out the existing gooseneck with a double warning of water deficiency
gooseneck where needed. Attach the drop hose to and other system failures.
the gooseneck and cut to the appropriate length. The Two pressure gauges are
plastic double gooseneck requires that the drop be needed on the center pivot:
slung over the truss rods. Attach the truss rod hose one at the end of the system,
clamp to the drop hose before attaching to the truss usually in the last drop, and
rod. Some existing outlets can be used by switching one at the pivot point. A
out the existing gooseneck with a furrow arm. third pressure gauge in the
first drop of span one will
When water is pumped into a center pivot, it fills
monitor operating pressure
the mainline and then drops. The weight of the
when the machine is down
water causes the pivot to lower or “squat.” With
slope in relation to the pivot
160-foot spans, the pivot mainline will be lowered
point. The pressure gauge
approximately 5 inches at the center of the span.
is positioned above the
Likewise, when filled with water, a 185-foot span
water applicator or regulator.
will be about 7 inches lower at its center. Length of
Special pressure gauge drops
the hose drops should account for this change so
(Fig. 13) are available to
that when the system is running, all applicators are
simplify gauge installation.
about the same height above the ground. The new
drops that are attached to the double goosenecks To ensure the pivot is
are installed alternately on each side of the mainline operating properly, check
to help equalize stresses on the pivot structure for the pressure in the last Figure 13. Pressure
gauge drip.
high-profile crops. Also, when crops are not planted drop when the pivot is at its
in circles, having drops on both sides of the mainline up-slope position (or at the
helps prevent all the water from being dumped into highest elevation in relation to the pivot point). If the
the same furrows as the system parallels crop rows. field has downward sloping areas, check the pivot
pressure in the first drop when the pivot is located at
its down-slope position (lowest elevation in relation to
Components and Other the pivot point).
Important Considerations
Outlet Spacing
Flow Meter
On older equipment, conventional mainline outlets
All pivots should have a permanently installed,
are spaced every 8½ to 10 feet. When ordering a new
continuously functioning flow meter to measure the
pivot, specify close outlet spacing, even if this reduced
actual amount of irrigation water applied. A flow meter
spacing is not required by the water applicator initially
is needed to troubleshoot problems and for proper
selected. Manufacturers continue to develop more
irrigation water management in conjunction with
efficient applicators designed to be spaced closer
the design printout. Flow meters need to be located
together to achieve maximum irrigation efficiency
within a straight pipeline section that is sufficiently
and pumping economy. Ordering a pivot with closer
long. The straight pipe section needs to extend 10
mainline outlet spacing will ensure that in the future,
pipe diameters upstream and 5 pipe diameters
it can be quickly and inexpensively equipped with new
downstream from the flow meter. This is to reduce
applicator designs.
water turbulence in the pipe, which decreases the
meter’s accuracy. Due to water turbulence, small
insertion meters located at the pivot point are Variable-rate Irrigation (VRI)
problematic.
VRI, also referred to as “precision irrigation,” has been
successfully demonstrated in research settings over
Pressure Gauges the past 30 years. In one early system, each drop was
equipped with a solenoid valve, allowing each drop
Pressure gauges monitor pivot performance and,
to be turned on and off as needed based upon soil
combined with the flow meter, provide immediate
conditions, as the pivot moved around the circle. Such

Center Pivot Irrigation  13

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systems may be particularly beneficial in fields with remote computers, and the internet. Communication
widely varying soil types and/or depths. technology options include cell service, short-haul
radios, other wireless technologies for short distances,
In the first commercial systems, groups of drops
microwave, satellite, and direct connect. Which
(often four drops) were connected to a common valve
technology is best depends on site-specific conditions
instead of a valve for each drop. The flow rate of each
and budgets. Some considerations are as follows:
nozzle does not change, with the flow remaining either
on or off. This reduces system costs and complexity. 1. Cell phone technology is widely used for remote
These systems are coupled to soil maps of the field so communications. A cellular internet modem and
that water application rates are matched to varying a subscription are required for each individual
soil types. Essentially, the irrigation prescription is set device. Check with the equipment manufacturer
and then implemented in subsequent irrigations. for a modem that is compatible with area cell
service. Modem size and power consumption are
The control systems for VRI continue to evolve,
considerations for the use of in-field sensors.
simplifying operation and management. True variable-
rate application systems are commercially available, 2. Local Wi-Fi technology is evolving rapidly. Off-
in which the actual flow rate of each nozzle can be the-shelf Wi-Fi repeaters, extenders, and mesh
varied. Challenges to these systems are costs and systems enable Wi-Fi to cover larger areas.
complexity. It is possible to achieve good uniformity Short-haul radio is a term that is also applied
of water application with proper design. The allowable to such systems, as Wi-Fi is, in essence, a radio
range in VRI flow rates depends upon characteristics communication technology. Radio components
of the pumping plant. Research is ongoing to develop are referred to as antennas, receivers, and
real-time plant sensors for use with VRI to detect plant transmitters. Line-of-sight and distance
stress, and insect and disease pressure, among others. limitations, and the number of units needed,
may make Wi-Fi impractical as the primary
Control Panels and Systems communication method. In-field sensors with Wi-
Fi connectivity have lower power requirements
Control panels and control systems continue to than modems. Combining a local Wi-Fi network
rapidly evolve. The major pivot manufacturers offer with other communication technologies that can
several control system options, ranging from simple cover larger distances is feasible.
panels that have limited functionality, such as turning
3. Direct connection is when a wire is run from
on/off the pivot and speed settings, to complete
the sensor to the control panel and/or from
automatic and remote control systems coupled to
the control panel to the computer. Direct
mobile device and computer apps, and cloud-based
connection is an attractive option, as there is no
tools for advance management and visualization.
cellular contract or other subscription required.
Control panels are also able to link with crop models
It eliminates the need for transmitters and
and real-time weather data for irrigation scheduling.
receivers, and it is very dependable. Devices do
Low-pressure shut-off sensors have become standard.
have maximum run lengths based on their power
Flow sensors shut off pivots if a leak or low-flow
levels, so be sure to check the manufacturer’s
conditions are detected. Soil moisture sensors and
guidance. Potential rodent and plow damage
evapotranspiration (ET) weather stations can be tied
should be evaluated with the use of buried lines.
into the control system as well. Research continues
on the development of other in-field sensors for 4. Short- and long-haul radios that use public bands
detecting water stress status, crop deficiencies, are relatively inexpensive, robust, have long
insects, among others. When these systems become line-of-sight distance capabilities, and require no
available, they will require more sophisticated control subscription.
systems.
End Guns
Communication Technology
End guns are not recommended due to their energy
Regardless of which control system is selected, one requirements to produce sufficient pressure for
important decision regards communications between the gun to operate properly and their low water
in-field sensors, the control panel, mobile devices and application efficiency. Pressure to operate the gun

Center Pivot Irrigation  14

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is provided by increasing the design pressure of the Eq 1. Inches applied =

pivot or by inclusion of a booster pump. The long-
Pivot gpm × hours to complete circle
term costs of end gun operation should be evaluated
carefully. End guns have poor water distribution 450 × acres in circle
efficiencies, and more water will be applied closer to Eq 2. Acres per hour =
the gun, decreasing as the water moves out to the end
of the water trajectory. Acres in circle
Hours to complete circle

Pivot Management Eq 3. End tower speed in feet per hour =

Pivot management is centered around knowing Distance from pivot to end tower in feet × 2 × 3.14
the number of inches of water being applied by the Hours to make circle
machine. The system design printout includes a
precipitation chart listing total inches applied for
various control panel speed settings. If a precipitation
Runoff Management
chart (Fig. 4) is missing, contact the dealer who first
Runoff from center pivot irrigation can often be
sold the pivot to obtain a copy of the pivot printout.
controlled by setting the speed control so that the
Dealers usually keep copies of design printouts
water application rate matches the soil infiltration
indefinitely. However, most dealers have the software
rate. Agronomic methods of runoff control include
to reproduce the pivot printout as well. When a
furrow diking (or “chain” diking for pastures), farming
precipitation chart is not available, use Table 6 to
in a circular pattern, deep chiseling of clay sub-soils,
determine irrigation amounts based on flow rate and
maintaining crop residue, adding organic matter, and
time required to complete a circle. For other sizes
using tillage practices that leave the soil “open.”
of pivots or travel speeds, irrigation inches can be
calculated using the equations listed below. Keep in Farming in the round is one of the best methods of
mind that the equations assume 100 percent water controlling runoff and improving water distribution.
application efficiency. Reduce the amounts by 2 to 5 When crops are planted in a circle, the pivot never
percent for LEPA, 5 to 10 percent for LESA, 20 percent dumps all the water in a few furrows, as it may when
for MESA, and 35 to 40 percent for impact sprinklers. it parallels straight rows. Circle farming begins by
marking the circular path of the pivot wheels
as the pivot makes a revolution without
Table 6. Inches of water applied by a 1,290-foot center pivot* on a water. The tower tire tracks then become
120-acre circle at 100 percent water application efficiency.
a guide for row layout and planting. If the
Hours to complete 120-acre circle mainline span length (distance between
Pivot gpm 12 24 48 72 96 120 towers) does not accommodate an even
number of crop rows, adjust the guide
400 0.09 0.18 0.36 0.53 0.71 0.89
marker so that the tower wheels travel
500 0.11 0.22 0.44 0.67 0.89 1.11 between crop rows.
600 0.13 0.27 0.53 0.80 1.06 1.33 Furrow diking is a mechanical tillage
700 0.16 0.31 0.62 0.93 1.24 1.55 operation that places mounds of soil at
selected intervals across the furrow between
800 0.18 0.36 0.71 1.07 1.42 1.78
crop rows to form small water storage
900 0.20 0.40 0.80 1.20 1.60 2.00 basins. Rather than running off, rainfall or
1,000 0.22 0.44 0.89 1.33 1.78 2.22 irrigation water is trapped and stored in the
basins until it soaks into the soil (Fig. 11).
1,100 0.24 0.49 0.98 1.47 1.95 2.44
Furrow diking reduces runoff and increases
End tower 667 334 167 111 83 67 yields in both dry land and irrigated crops.
speed (ft/hr) A similar practice for permanent pastures,
Coverage 10 5 2.5 1.7 1.3 1 called chain diking, involves dragging a
(acres/hr) chain-like implement that leaves water-
*1,275 feet from pivot to end tower + 15-foot overhang end section collecting depressions.

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Controlling Wheel Rutting

Wheel tracks develop
into ruts due the pivot
traversing wet soil
(Fig. 14). This is most
common in heavy
soils. Pivots are heavy
machines, and each
tower and the span
it supports when full
of water, can weigh
up to four tons or Figure 15. Wheels and pivot components catching and
concentrating irrigation water at the towers/wheels.
more. Often, wet soil
conditions are caused
wheel tracks (Fig. 15). Research has shown that up
by the irrigation
to three times the depth of water being applied in
water itself, which can
irrigation is applied to the wheel tracks through
be easily controlled.
interception of water by the tower.
However, it is important Figure 14. Wheel rut.
to investigate and To solve this problem, half-circle sprays and/or boom-
understand the cause of wet soil conditions. Surface backs are used. Half-sprays direct water away from
runoff that concentrates water in low-lying areas the wheels, which is often effective at reducing rut
may require land reforming. Pivots that transverse problems. Boom-backs are usually used along with
drainage or marshy areas may need special tires, half-circle sprays and move the discharge point
multiple tires, or other traction devices, which will also well behind the pivot. Boom-backs clamp onto the
require larger tower motors and stronger driveshafts. pivot mainline. There are several different designs
The best approach is to follow manufacturers’ for boom-backs. Figure 16 shows one type of boom-
guidance for such situations. back that suspends and extends the flexible drop

New Pivot Design

When purchasing a new pivot, the design of the pivot
can be adjusted in order to reduce the likelihood of
ruts. Begin by assembling a topographical and soil
map of the field where the new pivot will be installed.
Identify areas that may be prone to rutting, such as
areas consisting of clays and other soils with high
water-holding capacities and poor drainage, areas that
will likely receive surface runoff, low areas where water
tends to pond, etc. Working with a dealer using the
pivot design software, it may be possible to select span
lengths that will allow towers to avoid these areas.

Water Applicators
In situations where irrigation water is the primary
cause of the wet soil conditions, the basic approach is
to keep the applied water off the wheel tracks. Using
LEPA bubble applicators and planting in a circle are
effective methods of keeping the wheel tracks dry.
With pivots equipped with spray applicators, the
towers and wheels intercept some of the throw from
Figure 16. An example of a simple suspension boom-
the sprinklers, which runs down the towers into the
back design.

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away from the mainline. Other designs include a Some center pivots, such as many on the Texas High
combination flexible/galvanized pipe that functions Plains, are planned and designed for insufficient
similarly, and a U-shaped aluminum extension that capacity (gpm) to supply peak daily crop water
holds and extends the flexible drop. New designs requirements. Growers with insufficient center pivot
are expected in the coming years as boom-back use capacity should use a high-water management
becomes more commonplace. strategy to ensure that the soil root zone is filled
with water by rainfall, pre-watering, or early-season
Tires irrigation before daily crop water use exceeds
irrigation capacity. The county soil survey available
Pivot manufacturers now offer a variety of different
from the NRCS lists available water storage capacity for
types and sizes of tires. Little university-based
most soils. Be sure to use the value for the soil at the
research information is available on the effectiveness
actual center pivot site.
of different types of tires as related to rutting
potential. The best approach is probably to follow the
manufacturer’s recommendations in absence of local Soil Moisture-based
experience. Be aware that retrofitting pivots with
different tires may increase the stress on driveshafts Soil moisture monitoring is recommended and
and motors or require larger motors. For airless tires, complements ET-based scheduling, particularly
a neutral tread design is recommended, in which the when rainfall occurs during the irrigation season.
treads run horizontally and do not push the soil away Soil moisture sensors can identify existing soil
from the tires. moisture, monitor moisture changes, locate depth of
water penetration, and indicate crop rooting depths.
It is important to ensure that the wheel toe alignment Watermark sensors are widely used by growers to
is adjusted correctly on each tower. Since the pivot manage pivot irrigations. These are classified as
travels in a circle, the wheels are not parallel to the resistance sensors that absorb and release moisture,
driveshaft, but offset at a small angle, referred to as similarly to that of the surrounding soil.
the toe alignment. The toe alignment angle is larger
in the first few towers and decreases in outer towers. Dielectric sensors are often used in situations where
The first few spans are the most critical since they the soil moisture sensors are integrated into pivot
must turn the shortest circles. Improper toe alignment control systems. The most common dielectric sensors
can result in the tires being dragged around the circle, used for this purpose are water content reflectometer
worsening ruts. and capacitance sensors. These cost more than
resistance sensors but last longer, are more accurate,
and are resistant to the effects of salts. Time domain
Irrigation Scheduling reflectometer sensors are widely used in research
applications but require more skill and expertise and
ET-based are more expensive than the other options.

Maximum crop production and quality are achieved Watermark sensors are read using resistance
when crops are irrigated regularly with amounts that meters. Readings may be taken weekly during the
match their water use or evapotranspiration (ET), such early growing season. During the crop’s peak water-
as twice weekly during the peak crop growth period. use period, readings should be taken two or three
The TexasET Network (http://TexasET.tamu.edu) reports times each week for more timely irrigation water
reference evapotranspiration (ETo) and has tools that management. Plotting sensor readings with computer
allow the calculation of crop water requirements (ET) spreadsheets or on graph paper helps track and
from ETo. One strategy is to sum the daily crop water interpret readings to manage irrigations. An example
use (ET) reported for the previous three to four days, is shown in Figure 17.
then set the pivot speed setting to apply that amount
A single sensor installed at a depth of 12 to 18 inches
of water. Another strategy is to apply water in larger
measures moisture in the upper root zone. Another
volumes once per week to reduce evaporative losses
installed at 36 inches measures deep moisture.
and to better utilize any rainfall that occurs. This
Sensors usually are installed at three depths: 12, 24,
method has proven to be very successful in PARM
and 36 inches, and at a representative location in
systems.
the field where soil is uniform. They should not be

Center Pivot Irrigation  17

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Mahaltic Farms - Corn Economics. Chemigation is

ƒ
0 3.5 usually less expensive than
other application methods
10
3 and often requires smaller
20 amounts of chemicals.
2.5 Reduced soil compaction
ƒ
30
and crop damage. Because

IRRIGATION & RAINFALL, (INCHES)

conventional in-field spray
SOIL MOISTURE, (CBARS)

40
2
equipment may not be needed,
50
chemigation may reduce
60
1.5
tractor-wheel soil compaction
and crop damage.
70
1
Operator safety. Because
ƒ
80 an operator need not be
0.5 continuously present in a
90
field during applications,
100 0
chemigation reduces human
9-May 12-May 15-May 18-May 21-May 24-May 27-May 30-May 2-Jun 5-Jun 8-Jun 11-Jun 14-Jun 17-Jun 20-Jun contact with chemical drift
Rainfall Irrigation 1 FT 2 FT 3 FT 50%
and reduces exposure during
Figure 17. Soil moisture measurements in a corn field on a heavy clay soil. Soil frequent tank fillings and
moisture levels should not fall below a reading of 85 for this particular soil.
other tasks.

placed on extreme slopes or in low areas where Chemigation does have disadvantages, however,
water may pond. Select a location within the next-to- including:
the-last center pivot span but away from the wheel Skill and knowledge required. Chemicals
ƒ
tracks. Locate sensors within the crop row so they must always be applied correctly and safely.
do not interfere with tractor equipment. Follow the Chemigation requires skill in calibration,
manufacturer’s guidelines on preparing sensors. To knowledge of irrigation and chemigation
obtain accurate readings, the sensing tip must make equipment, an understanding of chemical and
good and complete soil contact. The soil auger used to irrigation scheduling concepts, and calculation of
install sensors should be no more than ⅛ inch larger mixing and injection rates.
than the sensor. How-to preparation and installation
Additional equipment. Proper injection and
ƒ
videos are available at http://irrigation.tamu.edu.
safety devices are essential. Growers must comply
with these legal requirements.
Chemigation
Chemigation uses irrigation water to apply an
Pesticides
approved chemical (fertilizer, herbicide, insecticide,
The use of pesticides and herbicides is highly
fungicide, or nematicide) through the center pivot.
regulated by the United States Environmental
Labels of pesticides must state whether a product is
Protection Agency (EPA), who has enacted the
approved for application in this way. If so, application
minimum regulations that all states must adopt and
instructions are provided on the label.
enforce. However, states may enact more stringent
The advantages of chemigation include: regulations than those required by the EPA. In Texas,
pesticide use in irrigation systems is regulated by the
Uniformity of application. With a properly
ƒ
Texas Department of Agriculture (TDA) and the Texas
designed irrigation system, both water and
Commission on Environmental Quality (TCEQ). A Texas
chemicals can be applied uniformly, resulting
Pesticide Applicators License is required. Pesticide
in excellent distribution of the water-chemical
labels must state if the chemical is allowed to be
mixture.
injected through an irrigation system. If so, the label
Precise application. Chemicals can be applied in
ƒ specifies requirements for the use of the chemical,
correct concentrations where they are needed. including mixing and application rates. The label also

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states which types of irrigation systems can use the Table 8. Amount of fertilizers needed to apply specific
chemical. Regulations require the use of specific safety amounts of nitrogen (N).
equipment and devices designed to prevent accidental Pounds of N per acre
spills and contamination of water supplies (Table 7),
which also are listed on the chemical’s label. Kind of fertilizer 20 40 60 80 100
Pounds per acre of fertilizer
Table 7. Summary of Chemigation Safety Equipment needed for rate of N listed above
Requirements for Pesticides and Herbicides. Check Solid
the chemical label for specifics on equipment
requirements for that substance. Alternative safety Ammonium sulfate
equipment is allowed. Contact your county Extension (21% nitrogen) 98 196 294 392 488
office for details.
Urea (45% nitrogen) 44 89 133 177 222
Components must include: Gallons per acre of fertilizer
1. Irrigation Pipeline needed for rate of N listed above
a. Check valve between well and injection points. Solutions
b. Vacuum relief valve between check valve and Urea-ammonium nitrate 6.7 13.4 20 26.8 33.4
well. (28% nitrogen)
c. Low pressure cut-off.
Urea-ammonium nitrate 5.7 11.4 17 22.8 28.5
d. Low pressure drain. (32% nitrogen)

2. Injection Hose
a. Anti-back flow injection valve—10 psi. that an appropriate backflow prevention device is
used. More strict requirements exist for systems
b. Normally closed solenoid valve between
injection pump and chemical tank. connected to a public water supply.
c. A metering type of injection pump. Nitrogen is the most common fertilizer used in
3. Power Interlock chemigation because crops need large amounts of
it. Keep in mind that nitrogen is highly water soluble
a. Interlock injection pump and water pump power.
and has the potential to leach, so its application
b. Interlock normally-closed solenoid valve and must be managed carefully. Table 8 lists several
injection pump power.
different nitrogen formulations that are often used for
fertigation. Be sure solid formulations are dissolved
Using proper chemigation safety equipment and completely in water before injecting them into the
procedures also aids the grower by providing irrigation system. Complete mixing may require
consistent, precise, and continuous chemical injection, initially agitating the mixture for several hours and
thus reducing the amounts (and costs) of chemicals then throughout the injection process.
applied. For more information, contact your county
The advantages of fertigation include:
Extension office.
Nutrients can be applied based on crop needs at
ƒ
any time during the growing season.
Fertigation
Mobile nutrients such as nitrogen can be
ƒ
Application of fertilizers through the irrigation system regulated with the amount of water applied so
(fertigation) often is referred to as “spoon-feeding” the that they are available for rapid use by crops.
crop. Fertigation is common and has many benefits. If the irrigation system distributes water
ƒ
Most fertigation uses soluble or liquid formulations uniformly, nutrients can be applied uniformly over
of nitrogen, phosphorus, potassium, magnesium, the field.
calcium, sulfur, and boron.
Some tillage operations may be eliminated,
ƒ
Fertigation is not regulated by the EPA or TDA. The especially if fertilization coincides with the
TCEQ does have rules designed to protect water application of herbicides or insecticides. However,
supplies from pollution, and in some instances, the do not simultaneously inject two chemicals
user is liable for any groundwater or surface water without knowing whether they are compatible
pollution that may result. These regulations require with each other and with the irrigation water.

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Groundwater contamination is less likely with

ƒ ƒ Ammonia solutions are not recommended for
fertigation because less fertilizer is applied at any fertigation because ammonia is volatile and too
given time. Application can correspond to periods much will be lost during the application process.
of maximum crop need. Also, ammonia solutions may precipitate lime and
There is minimal crop damage during fertilizer
ƒ magnesium salts, which are common in irrigation
application. water. Resulting precipitates can build up on the
inside of irrigation pipelines and clog nozzles.
Fertigation does have some disadvantages. These Various polyphosphates (e.g., 10-34-0) and
ƒ
include: iron carriers can react with soluble calcium,
Injectors must be calibrated.
ƒ magnesium, and sulfate salts to form precipitates.
Injectors must be sized appropriately for the
ƒ The quality of irrigation water should be
large volume of fertilizer solution typically used evaluated before using fertilizers that may create
(pesticide injectors cannot be used). precipitates.

Fertilizer distribution is only as uniform as

ƒ Many fertilizer solutions are corrosive.
ƒ
irrigation water distribution. Use pressure gauges Know the materials contained in all pump, mixing,
to ensure that the center pivot maintains proper and injector components in direct contact with
pressures. concentrated fertilizer solutions. Table 9 describes the
Lower-cost fertilizer materials, such as anhydrous
ƒ corrosion potential of various metals when they come
ammonia, often cannot be applied using fertigation. into direct contact with common commercial fertilizer
Fertilizer placement cannot be localized, as in
ƒ solutions.
banding.

Table 9. Relative corrosion of various metals after 4 days of immersion in solutions of commercial fertilizers.*

Type of metal
pH of Galvanized Sheet Stainless
Fertilizer solution iron aluminum steel Bronze Yellow brass
.................................................Relative corrosion.................................................
Calcium nitrate 5.6 Moderate None None Slight Slight
Sodium nitrate 8.6 Slight Moderate None None None
Ammonium nitrate 5.9 Severe Slight None High High
Ammonium sulfate 5.0 High Slight None High Moderate
Urea 7.6 Slight None None None None
Phosphoric acid 0.4 Severe Moderate Slight Moderate Moderate
Diammonium
phosphate 8.0 Slight Moderate None Severe Severe
Complete fertilizer 17-
17-10 7.3 Moderate Slight None Severe Severe
*Solutions of 100 pounds of material in 100 gallons of water.

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Center Pivot Buyer’s Checklist

Pivot Design
_______ Actual lowest and highest field elevations in relation to the pivot point were used in the computer design
printout.
_______ Actual measured flow rate and pressure available from pump or water source were used in the computer
design printout.
_______ Friction loss in pivot mainline is no greater than 10 psi for quarter-mile-long systems.
_______ Mainline outlets are spaced a maximum of 60 to 80 inches apart or, alternately, no farther apart than two
times the crop row spacing.
_______ For non-leveled fields, less than 20 percent pressure variation in system-design operating pressure is
maintained when the pivot is positioned at the highest and lowest points in the field (computer design
printout provided for each case).
_______ Pressure regulators were evaluated for fields with more than 5 feet of elevation change from pad to the
highest or the lowest points in the field.
_______ Tower wheels and motor sizes were selected based on soil type and slope, following manufacturers’
recommendations.
_______ Dealer has provided a copy of the pivot design printout.

Applicators
_______ Design has no end gun.
_______ Consideration was given to equipping the pivot with either LEPA or LESA applicators as follows:
Water application designed for an operating pressure requirement of 6 psi, positioned 1 to 1.5 feet above
ƒ
the ground, spaced at two times the crop row spacing. Flexible drop hose from gooseneck or furrow
arm on mainline to applicator, equipped with a plastic or metal weight. The applicator can accommodate
adaptor pads to allow for bubble and spray water modes and attachment of drag hose or double-ended
sock.

Installation and Water and Power Supply

_______ Pivot pad has been constructed to manufacturer’s specifications.
_______ Subsurface water-supply pipeline to pivot point is sized to keep water velocity at or below 5 feet per second.
_______ Power supply has been connected to pivot following manufacturer’s specifications. Power supply may be a
power unit alone, a power unit and generator, or subsurface power lines.

Accessories
_______ System includes propeller flow meter or other type of flow measurement device, having accuracy to +3
percent and instantaneous flow rate (i.e., gpm) and totalizer (acre-ft, ft3, etc.) indicators installed in the
water-supply pipeline near pivot point. The flow meter is installed in a straight section that extends 10 pipe
diameters upstream and 5 diameters downstream from the meter.
_______ System includes two pressure gauges, one on the mainline near the pivot point and one in the last drop,
located just above the applicator or pressure regulator.
_______ System includes a computer control panel for fields with soil changes and/or multi-crop situations.
_______ System has a remote control/monitoring system (optional).
_______ System includes a chemigation unit meeting state regulations, which is tied into the computer control
panel or power shut-off system with a positive displacement injector pump (for pesticides and herbicides)
sized for the pivot flow rate and injection volumes.

Texas A&M AgriLife Extension provides equal opportunities in its programs and employment to all persons, regardless of race, Center Pivot Irrigation  21
color, sex, religion, national origin, disability, age, genetic information, veteran status, sexual orientation, or gender identity.