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Group Title: Bulletin
Title: Field evaluations of irrigation systems
Full Citation
Permanent Link: http://ufdc.ufl.edu/UF00008533/00001
 Material Information
Title: Field evaluations of irrigation systems solid set or portable sprinkler systems
Series Title: Bulletin
Physical Description: 9 p. : ill. ; 23 cm.
Language: English
Creator: Smajstrla, A. G ( Allen George )
Publisher: Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida
Place of Publication: Gainesville Fla
Publication Date: 1990
Subject: Irrigation farming -- Florida   ( lcsh )
Sprinkler irrigation -- Florida   ( lcsh )
Irrigation -- Florida   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
non-fiction   ( marcgt )
Statement of Responsibility: A.G. Smajstrla ... et al..
General Note: Cover title.
General Note: "March 1990."
 Record Information
Bibliographic ID: UF00008533
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: ltqf - AAA6795
ltuf - AHD1039
oclc - 21988014
alephbibnum - 001517927

Table of Contents
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Full Text

March 1990

Field Evaluations
of Irrigation Systems:
Solid Set or Portable Sprinkler

A.G. Smajstrla, B.J. Boman, G.A. Clark, D.Z. Haman,
D.J. Pitts and FS. Zazueta

Florida Cooperative Extension Service
Institute of Food and Agricultural Sciences
University of Florida, Gainesville
John T. Woeste, Dean for Extension

Bulletin 266

A.G. Smajstrla, B.J. Boman, G.A. Clark, D.Z. Haman, D.J. Pitts and F.S. Zazueta
are Water Management Specialist, Agricultural Engineering Department,
Gainesville; Citrus Irrigation Specialist, Agricultural Research & Education
Center, Ft. Pierce; Water Management Specialist, Gulf Coast Research & Edu-
cation Center, Bradenton; Water Management Specialist, Agricultural Engineer-
ing Department, Gainesville; Water Management Specialist, Southwest Florida
Research & Education Center, Immokalee; and Water Management Specialist,
Agricultural Engineering Department, Gainesville, respectively.

This bulletin describes techniques for measuring operating pres-
sures, water application rates and uniformity during field evaluations
of solid set or portable sprinkler irrigation systems. These irrigation
systems typically use groups of impact or gear-driven sprinklers
which operate at the same time to sprinkle water onto the soil or
crop canopy. Sprinkler spacings are relatively close so that overlap
between them increases the uniformity of water application.
The techniques presented do not apply to self-propelled irrigation
systems such as center pivot, linear move, or traveling gun systems.
Nor do they address single sprinkler systems such as large guns or
small individual lawn sprinklers. The unique geometries of self-
propelled and individual sprinkler systems require other procedures
to measure application rates and uniformities of water application.
Solid set sprinkler irrigation systems are those in which sprinklers,
with their associated riser, lateral, and manifold pipes, are placed
in a regular pattern over the entire irrigated area. All of the sprinklers
may be operated at once, or the crop may be irrigated in zones by
operating only a portion of the sprinkler laterals at a time.
Solid set sprinkler systems may be permanent, in which case lat-
erals and manifolds are typically constructed of buried PVC plastic
pipe. This is common in many Florida citrus, nursery, strawberry,
and ornamental fern production systems and in lawn and landscape
irrigation systems. Alternatively, solid set sprinklers may be set in
place only during a crop growing season. Sprinklers are then typically
mounted on risers above portable aluminum pipelines which are
placed on the surface. Laterals may be fed by either portable (typically
aluminum) manifolds placed on the soil surface or permanent (typi-
cally PVC) buried manifolds. These systems are common for many
Florida vegetable, tobacco, and turf crops.
In portable set sprinkler irrigation systems, the sprinklers and
associated pipelines are temporarily set up and operated for each
irrigated zone. They are then moved to a new zone for another irriga-
tion. These systems are used to irrigate several zones; thus, they are
designed so that all zones can be irrigated before the first zone needs
to be re-irrigated. Because these systems are portable, less pipe and
fewer sprinklers must be purchased as compared to solid set systems.
However, labor requirements are normally much greater than for
solid set systems. The specific system used normally depends on the
relative availability of capital versus labor.
Both the solid and portable set sprinkler systems described here
use sprinklers that are regularly spaced, typically in square, rectan-
gular, or triangular patterns. The individual sprinkler spacings and

discharge rates determine the average irrigation application rate.
Many additional factors, including operating pressure, changes in
elevation, friction pressure losses, wind, and individual sprinkler
characteristics affect the uniformity of water application within an
irrigated zone.
The specific objectives of this publication are to present techniques
(1) to measure operating pressures, (2) to measure application rates,
and (3) to measure the uniformity of water application under field
conditions for existing solid set or portable sprinkler irrigation sys-
tems. Knowledge of these three factors and changes in their mag-
nitudes over time is important to determine the causes of deficiencies
in application rates or uniformities observed. This information is
also needed to efficiently and effectively manage sprinkler irrigation
systems. Field evaluations should be conducted at least annually to
reveal changes which require system maintenance or repair.

Measuring Operating Pressures
Always operate sprinklers within the manufacturer's specified
pressure ranges. Sprinkler effectiveness is reduced by operation at
either excessively high or low pressures. Pressures that are too high
produce fogging and irregular turning. Fogging produces too many
small droplets that fall too close to the sprinkler. Pressures that are
too low cause improper jet breakup, producing a doughnut-shaped
spray pattern. Under either condition, water is not uniformly distri-
Operating pressures should be within the range specified by the
irrigation system designer. Pressure gauges should be permanently
installed at the irrigation pump and at entrances to zones. Test
gauges periodically to verify that pressures are being measured accu-
rately. This can be done by substituting a test gauge for the field
gauges. Replace the field gauges if they are no longer accurate.
Pressures within zones can be measured at the sprinkler nozzles
using pitot tube pressure gauges. Position pitot tubes in the discharge
stream about 1/8-inch from the nozzle. Adjust the pitot tube by moving
it slowly within the stream until the highest constant pressure read-
ing is obtained.
Pressures recorded at critical points within the system, including
at the pump discharge, at the entrance to zones, at the distant end
of laterals, and at extreme high and low elevations, should be near
the pressures specified by the system designer. Extreme deviations
from the design pressures should be corrected before proceeding with
further system tests.

As examples, low pump discharge pressure may occur because of
pump wear, insufficient pump operating speed, insufficient water
supply, a broken pipe downstream, too many open valves downstream,
or eroded sprinkler nozzles that discharge excessive flow rates. Con-
versely, high pump pressures may indicate excessive pump speed,
valves that are closed or partially closed downstream, or components
that are clogged. Pump discharge rate measurements and visual
inspections will help to determine which problem may have occurred.
Similar flow rate measurements and visual inspections should be
used to determine causes of excessively low or high pressures at
other points in the system.

Measuring Sprinkler Application Rates
Sprinkler application rates must be known so that irrigation dura-
tions needed to apply specific depths of water can accurately be
determined. Measure application rates under field conditions (1) to
verify irrigation system designs and (2) to determine whether
changes in application rates have occurred with time. Measurements
to verify irrigation system design should be made soon after instal-
lation. Subsequent measurements should be made at least annually
to track changes in system performance and to schedule repairs.
Three techniques can be used to measure application rates:
1. Measure the flow rate and area of each irrigated zone. Measure
the flow rate with either a flow meter at the pump or at each zone.
Units are normally gallons per minute (gpm). To convert to acre-
inches per hour, divide the measured flow rate by 453. The average
application rate per zone can then be calculated from:
Rate = Q / Area (1)
where Rate = application rate in inches per hour (iph),
Q = total flow rate per zone in acre-inches per hour, and
Area = total irrigated zone area in acres.
For example, if the measured flow rate to a 10-acre zone is 906
gpm, this is equivalent to 906/453 = 2.0 acre-inches per hour. Then,
the average application rate is 2.0 acre-inches per hour / 10 acres =
0.20 iph.
2. Measure the average flow rate and area covered by each sprinkler.
For regularly spaced sprinklers, the application rate is then calcu-
lated from:
Rate = 96.3 q / [(S1) (Sm)] (2)
where Rate = application rate in inches per hour (iph),
q = sprinkler discharge rate in gallons per minute (gpm),
Sl = sprinkler spacing along the lateral in feet (ft), and
Sm = sprinkler spacing along the manifold between laterals
in ft

As examples of the use of Equation (2), if 5-gpm sprinklers are
spaced on a 40 ft x 40 ft square pattern, the application rate would
be 0.30 iph. If 6-gpm sprinklers were spaced on a 40 ft x 60 ft rectan-
gular pattern, the application rate would be 0.24 iph. If 4-gpm
sprinklers are spaced on a 30 ft x 30 ft triangular pattern, the appli-
cation rate would be 0.43 iph.
Sprinkler flow rate can be determined by either (a) measuring the
volume discharged from typical sprinklers per unit time, or (b)
measuring the sprinkler operating pressure with a pitot tube and
using the manufacturer's specifications to determine the flow rate.
Measuring the volume discharged is preferred because nozzle wear
can increase the flow rate over manufacturer's specifications.
Sprinkler discharge can be diverted to a graduated cylinder or
other volumetric container by slipping a flexible tube over the sprink-
ler nozzle. The tube should be large with respect to the nozzle diame-
ter to avoid restricting flow. Flow should only be measured while the
sprinkler is operating at its design pressure. A stopwatch can be
used to measure the sprinkler discharge collection time.
The pressure at the sprinkler nozzle can be measured by holding
a pitot tube connected to an accurate pressure gauge in the discharge
stream of the nozzle as shown in Figure 1. The nozzle size should be
checked for wear or distortion with a feeler gauge such as a drill bit
having the diameter specified for the nozzle. If the nozzle is worn or
misshapen, it should be replaced with a new one. The sprinkler flow
rate can then be determined by consulting the sprinkler manufac-
turer's specifications for the measured operating pressure and nozzle
size. If the sprinklers have more than one nozzle, the total sprinkler
flow rate can be determined by adding the flow rates of the individual



Figure 1. Using a pitot tube to measure pressure at a sprinkler nozzle.

To accurately determine the average sprinkler flow rate in an irri-
gated zone, measure several sprinklers (approximately 12 to 18).
Some of the sprinklers measured should be near the inlet ends of
the laterals, some near the center, and some at the distant ends. If
the measured values are highly variable (more than 15% from the
average), the number of sprinklers tested should be increased.
If different sizes of sprinklers or nozzles are used in a zone, such
as part-circle sprinklers at field boundaries, flow rates must be deter-
mined separately for each size. The total zone flow rate can then be
determined by adding the average flow rates for the total number of
sprinklers of each size in the zone. Finally, Equation (1) can be used
to calculate the average application rate for the zone.
3. Measure the application rate directly with catch cans or rain
gauges. The average application rate is then the average depth of
water measured divided by the time during which the data were
Because water is never applied with perfect uniformity under a
sprinkler irrigation system, several catch cans must be placed be-
tween adjacent sprinklers. Normally, at least 16 to 24 cans should
be used. To simplify later uniformity calculations, use a number of
cans that is a multiple of 4. Also, these tests should be conducted
under the same conditions as those during typical applications. Avoid
making tests during high wind conditions because wind distorts
sprinkler patterns.
Figure 2 shows a typical layout of catch cans for uniformity mea-
surements between the four sprinklers shown. The 16 cans are evenly
spaced between sprinklers so that each is centered within and repre-
sents equal land areas. The numbers shown adjacent to the catch
cans in Figure 2 are example catch can data which are used in later
example problems.
Catch cans should all be of the same size and type, and should be
placed upright so that their tops are level. Cans should be located
on or near the soil surface, but above any vegetation which might
obstruct access to the cans. For annual crops, schedule catch can
tests when plants are small so that they do not interfere with the
For large perennial plants such as citrus or other tree crops, catch
can tests may be very difficult to conduct because of the need to
elevate the cans above the canopies. Tests with cans under tree
canopies are not appropriate because the canopies will distort the
water distribution. If large unobstructed areas are available between
trees, these areas may be used to estimate uniformities. This might
be the case with young citrus trees. However, as trees grow, the tall

canopies will distort water distributions, and the catch cans will
need to be elevated to avoid the canopies.
In some citrus groves, sprinklers are located at about the same
height or just above the tree canopy. In these cases, catch can tests
may not be appropriate because the cans cannot be elevated suffi-
ciently to clear the canopies and still be positioned sufficiently below
the sprinklers to accurately measure water applications.
To avoid evaporation losses during data collection, place a few
drops of lightweight oil in the cans. The oil will disperse over the
water surface and restrict evaporation. This is especially important
for tests that require several hours to conduct.
In Figure 2, the depth of water collected in each can is given in
inches. The average of the 16 depths is 0.31 inches. If the test was
conducted for a 1-hr period of operation, then the application rate
was 0.31 iph.


LATERAL Q 010 !0
PIPE 0.32" 0.34" 0.32" 0.34"
(.01) 3.L __ .0___(3)

o o o o
0 30" 0.28" (.03)1 0.25" 0.30"
( 01) CATCH CANS -L-0 __ ._01)

0 -0 0
0.33" 0.30" 0.27" 0.33"
(.02) (.01) 10' (.04) (.02)

0 0 I -T
0.36" 0.24" 0.31" 5'0.37"
(.05) (.07) (.00) (.06)


Figure 2. Typical layout of catch cans for uniformity measurements be-
tweep four sprinklers.

Because application rates may vary throughout a large irrigated
field, measurements should be made at several locations as shown
in Figure 3. Test locations should be selected over the entire range
of pressures that might be encountered in the irrigation system.
That is, locations should be selected both near and distant from the
irrigation pump. Locations should also be selected at points of both
high and low elevation.



Figure 3. Example distribution of locations of catch can tests in a large
irrigated field.

Measuring Uniformity of Water Application
Uniformity of water application is a measure of the variability in
depths of water applied at different points throughout an irrigated
zone. Uniformity of water application can be measured using catch
cans set on or near the soil surface. Follow the procedures previously
described in this bulletin for application rate measurements.
Uniformities are normally measured under no-wind conditions.
Under no-wind conditions, the maximum possible uniformity is mea-
sured for the existing system hydraulic characteristics and sprinkler
Uniformity will be lower when sprinkler systems are operated dur-
ing windy conditions. However, that uniformity may be more repre-
sentative of the long-term average uniformity if the sprinklers are
normally operated under windy conditions. Where prevailing winds

are consistently strong, such as along the coasts in Florida, sprinklers
must be spaced closer together than under no-wind conditions. For
no-wind conditions, sprinklers are typically spaced at 55% to 60%
of their diameters of coverage. This should be reduced to 50% for low
wind speeds (less than 5 mph) and to 30% for wind speeds above 10
Uniformity of water application with sprinkler irrigation systems
is usually reported as either the Distribution Uniformity (DU) or
Christiansen's Uniformity Coefficient (UC).

Distribution Uniformity
DU is calculated as the ratio of the depth measured in the low
quarter of the irrigated area to the overall average depth applied.
Average Low Quarter Depth of Application
DU = 100% (3)
Overall Average Depth of Application
where DU is expressed as a percent. The average low quarter depth
is determined by inspecting the data collected and calculating the
average of the smallest 1/4 of the measured depths. The overall aver-
age is the arithmetic average of all of the catch can data. The compu-
tations are simplified if the total number of data are a multiple of 4.
DU can be calculated using the data shown in Figure 2. The low
quarter of the 16 data points are the four values: 0.24, 0.25, 0.27,
and 0.28 inches, shown underlined in Figure 2. The average of these
four low quarter values is 0.26 inches. The overall average of all 16
points is 0.31 inches. Then, from Equation (3):
DU = 100% (0.26 inches) / 0.31 inches = 83.9%

Christiansen's Uniformity Coefficient
Christiansen's uniformity coefficient (UC) is another widely-used
method of calculating the uniformity of water application from
sprinkler irrigation systems:
Average Deviation from the
Average Depth of Application
UC = 100% ( 1 )(4)
Overall Average Depth of Application
where UC is expressed as a percent. The average deviation from the
average depth of application is calculated by averaging the absolute
values of the differences between each of the individual depths and
the average depth, and the overall average depth of application is
defined as before.

For the data given in Figure 2, the overall average depth-measured
was 0.31 inches. In Figure 2, the absolute values of the differences
between each of the individual depths and the average depth is shown
in parenthesis below each of the depths measured. The sum of the
absolute values of these differences for each of the 16 data points is
0.46 inches. The average deviation is then 0.46/16 = 0.029 inches, and
UC = 100% [1.0 (0.029/0.31)] = 92.7%

Acceptable Uniformity Coefficients
Acceptable values of uniformity coefficients vary with the type of
crop being grown and the specific uniformity equation used. Both
equations result in approximately the same values when uniformity
is high. However, DU values are normally much lower than UC values
when uniformities are low.
For high cash value crops, especially shallow rooted crops, the
uniformities should be high (DU values greater than 80%, or UC
values greater than 87%). For typical field crops, DU values should
be greater than 70% (UC values greater than 81%). For deep rooted
orchard and forage crops, uniformities may be fairly low if chemicals
are not injected (DU values above 55% and UC values above 72%).
Uniformity coefficients should be high (DU values greater than
80% or UC values greater than 87%) whenever fertilizers or other
chemicals are injected into the irrigation systems. If uniformity coef-
ficients are lower than these values, system repair, adjustment or
modification may be required. If uniformity coefficients are periodi-
cally measured (at least annually), system repairs or adjustments
can be scheduled when coefficients fall below the above values.

Runoff will reduce the amount of water applied to high areas and
may increase the amount applied in low areas where the water may
collect and infiltrate. During system tests and during normal sprink-
ler operation, runoff should not occur. This is normally not a problem
on typical Florida sandy soils, but if runoff occurs, design or manage-
ment changes should be made to eliminate it. Shorter, more frequent
irrigations may be scheduled to reduce runoff, or it may be necessary
to reduce nozzle sizes to reduce application rates. If the system oper-
ation must result in runoff (such as in some strawberry and nursery
operations), recovery ponds can be used to collect runoff for future use.

This publication described techniques which can be used to
evaluate sprinkler irrigation systems under field conditions.
Techniques were presented (1) to measure operating pressures, (2)
to measure application rates, (3) to measure the uniformity of water
application, and (4) to avoid runoff under field conditions for existing
solid or portable set sprinkler irrigation systems. Critical values of
uniformity coefficients for various crops and production systems were

Merriam, J.L. and J. Keller. 1978. Farm Irrigation System Evalu-
ation: A Guide for Management. Utah State University. Logan, Utah.
271 pages.

This publication was produced at a cost of $628.23, or 28.0 cents
per copy, to present information on techniques that can be used
to measure the uniformity of water application from sprinkler irriga-
tion systems under field conditions. 4-2.25M-90

in cooperation with the United States Department "f Agriculture, publishes this
information to further the purpose of the M- 'a-nA 30, 1914 Acts of
Congress; and is authorized to provide r ...... .,rmation and
other services only to individuals rid institutions that function wv.iout regard to
race, color, sex, age, handicap or national origin. Single copies ot extension publications (excluding
4-H and youth publications) are available free to Florida residents from county extension offices.
Information on bulk rates or copies for out-of-state purchasers is available .from C.M. Hinton,
Publications Distribution Center, IFAS Building 664, University of Florida, Gainesville, Florida 32611.
Before publicizing this publication, editors should contact this address to determine availability.

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