Field Evaluation of Micro Irrigation
Water Application Uniformity
A.G. Smajstrla, B.J. Boman, G.A. Clark, D.Z. Haman,
D.J. Pitts and F.S. Zazueta
Florida Cooperative Extension Service
Institute of Food and Agricultural Sciences
University of Florida, Gainesville
John T. Woeste, Dean for Extension
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.
One of the primary objectives of a micro irrigation system is to
apply water uniformly. Approximately the same amount of water
should be applied to all portions of the area with each irrigation.
This is especially important when the irrigation system is used to
apply chemicals along with the water.
The uniformity of water application from a micro irrigation system
is affected both by the hydraulics of the pipe network of the system
and by the hydraulic properties of the emitters used for water distri-
bution. The emitter hydraulic properties include the effects of pres-
sure, temperature, water quality and other factors on emitter flow
rate. With age, the effects of emitter plugging and wear of emitter
components on hydraulics become more important.
The uniformity of water application can be estimated from the
statistical distribution of emitter flow rates using the following
Us = 100% (1 Vqs) (1)
where Us = statistical uniformity of the emitter discharge rates,
Vqs = statistical coefficient of variation of emitter discharge
In Equation (1), the coefficient of variation is the statistical defini-
tion of the sample standard deviation divided by the mean. Thus,
when emitter flow rates are measured in the field, Vqs includes the
effects of variability in emitter flow rate from all causes, including
both the effects of the pipe network and of the emitter hydraulic
properties (including plugging).
From Equation (1), when the variation in emitter flow rates in-
creases, the uniformity of water application decreases. Table 1 lists
five micro irrigation uniformity classifications, ranging from excel-
lent to unacceptable, recognized by the American Society of Agricul-
tural Engineers (ASAE, 1989a, 1989b).
Table 1. Micro irrigation system uniformity, Us, classifications for emitter
Class Uniformity, Us (%)
Excellent above 90
Unacceptable below 60
The uniformity of water application can be evaluated by measuring
emitter flow rates at several locations throughout each irrigated zone
and calculating Vqs and Us. The uniformity should be evaluated for
each individual irrigated zone. Then the overall system uniformity
is considered to be the lowest zone uniformity measured (See ASAE,
1989a). Although this procedure permits the uniformity to be esti-
mated, it does not allow the evaluator to determine the cause of low
uniformities which may be observed.
The flow rates of micro irrigation emitters have different responses
to pressure variations. The response of a specific emitter depends
on its design and construction. Variations in flow rates between emit-
ters of the same type, which are operated at the same pressure, also
occur because of manufacturing variations in the tiny plastic compo-
nents. Because their orifice diameters are very small, micro irrigation
emitters are also subject to partial or complete plugging from particu-
late matter, chemical precipitates, and organic growths. For these
reasons, water application uniformity may be greatly affected by the
The manufacturing coefficient of variation (Vm) is defined as the
statistical coefficient of variation in emitter discharge rates when
new emitters of the same type are operated under identical conditions
(same pressure and water temperature). Under these conditions,
differences in flow rates observed are assumed to be due to variations
in emitter components. Table 2 classifies point source (drip emitters
and microsprinklers) and line source (drip tubing) emitters based
on manufacturing variation. To achieve highest uniformities of water
application, emitters with small manufacturing variations should
be installed in micro irrigation systems.
Table 2. Classifications of manufacturer's coefficient of variation, Vm, for
Emitter type Vm range Classification
Point source below 0.05 Excellent
(drip emitters and 0.05 to 0.07 Average
microsprinklers) 0.07to0.11 Marginal
0.11 to 0.15 Poor
above 0.15 Unacceptable
Line Source below 0.10 Good
(drip tube) 0.10to 0.20 Average
above 0.20 Unacceptable
Pressure losses qccur when water flows through a pipe network
because of friction losses in the pipes and fittings. Pressure changes
also occur as water flows uphill (pressure loss) or downhill (pressure
gain) in a pipe network. If a micro irrigation system is poorly designed
or improperly installed, pressure losses may be excessive because
components are too small for the design flow rates or slopes are too
steep for the components selected. For these reasons, water applica-
tion uniformity may be greatly affected by the design of the pipe
This bulletin presents procedures to separately evaluate the uni-
formity of water application due to pressure variations in the pipe
network (hydraulic uniformity) and the variations due to the emitter
characteristics (emitter performance variation). Knowing both of
these factors will help an irrigation system manager identify the
causes of low application uniformities and the type of corrective
action that may be required to improve the uniformity of water appli-
These procedures should be performed on newly installed micro
irrigation systems to verify the quality of designs and installations
and to provide a reference for future evaluations. Also, evaluations
should be conducted at least annually to determine the effects of
emitter plugging or changes in other system components on system
Hydraulic (pressure) Uniformity
The hydraulic uniformity refers to the effects on uniformity of
pressure variations which occur in a micro irrigation system. Hy-
draulic uniformity, Ush, is defined similar to Equation (1):
Ush = 100% (1 Vqh) (2)
where Us = hydraulic uniformity based on pressure distributions,
Vqh = statistical coefficient of variation of pressures.
A low value of Ush is most often due to improper design. However,
improper installation of components or the installation of the wrong
components can also reduce Ush. Low values of Ush may be due to
pipe sizes that are too small, laterals that are too long, laterals that
are incorrectly oriented with respect to slope, improper emitter selec-
tion, or other characteristics of the hydraulic network. All of these
system characteristics must be properly selected and sized based on
the system flow requirements.
The hydraulic uniformity of a micro irrigation system is estimated
by measuring pressures at points distributed throughout each irri-
gated zone. Measure pressures to the nearest pound per square inch
Pressures can be measured using a portable pressure gauge con-
nected with a flexible tube. Gauges are manufactured with a needle
on a flexible tube for direct insertion into the lateral pipe. Alterna-
tively, some emitters are constructed so that a flexible tube can be
slipped over the emitter, allowing the pressure to be measured with
the emitter in place. Note that pressure distributions in lateral pipes
with a large number of emitters (more than 10) will not be signifi-
cantly affected by blocking one emitter while the others continue to
Some microsprinkler emitters are not mounted directly on the
laterals. Rather, they are connected to the lateral using small diame-
ter flexible tubing with a barbed insertion fitting. Because of pressure
losses in these connecting tubes, pressure should be measured at
the end of the tube near the emitter while the emitter is operating.
This can be done by using a small barbed tee in the connecting tube.
Care should be taken to distribute the measurement points
throughout the irrigated zone. Specifically, some points should be
located near the inlet, some near the center, and some at the distant
end. Also, some should be located at points of high elevation and
some at points of low elevation. However, the specific emitters to be
tested should be randomly selected at each location. Do not visually
inspect the emitters to select those with certain flow characteristics
before making measurements. A minimum of 18 points should be
measured. Computations will be simplified if the number of points
measured is a multiple of 6.
The hydraulic uniformity can be read from Figure 1. The following
six steps are required:
Step 1. Calculate 1/6 of the number of data points measured. That
is, divide the number of data points by 6. For example, if 18 points
were measured, this number will be 3.
Step 2. Look at the set of data measured to locate and then add
the lowest 1/6 of the pressures measured. For 18 data points this
will be the sum of the 3 lowest pressures.
Step 3. Look at the data set again to locate and then add the
highest 1/6 of the pressures measured. For 18 data points this will
be the sum of the 3 highest pressures.
Step 4. Locate the sum of the high pressures on the vertical axis
in Figure 1. Draw a horizontal line across the graph from that point.
If this sum does not fit on the scale, or if the value is very small
so that the scale is difficult to read, the sums calculated in Steps 2
and 3 can both be multiplied or divided by a common factor. This
can be done because their absolute values are not important, but
only the relative differences between the high and low values are
Step 5. Locate the sum of the low pressures on the horizontal axis
in Figure 1. Draw a vertical line up the graph from that point.
Again, if necessary, modify the values from Steps 2 and 3 as dis-
cussed in Step 4. However, be sure to change the sums of both the
lowest afd highest values at the same time.
Step 6. Read the hydraulic uniformity at the intersection of the
two lines drawn.
Figure 1. Statistical uniformity nomograph for the determination of
hydraulic and water application uniformity coefficients.
STATISTICAL UNIFORMITY NOMOGRAPH
Sum of the Lowest One-Sixth of
Times, Tmin, or Pressures, Pmin
As an example of the use of Figure 1, consider the data set in Table
3. Assume that the 18 pressures shown in Table 3 were read at
random locations throughout an irrigated zone. Then, following the
above six-step procedure:
Table 3. Sample data set for Figure 1 example.
27 (high #2)
22 (low #3)
21 (low #1)
28 (high #1)
21 (low #2)
27 (high #3)
Step 1. 1/6 of 18 = 3
Step 2. 21 + 21 + 22 = 64 psi (lowest 3 values)
Step 3. 28 + 27 + 27 = 82 psi (highest 3 values)
Step 4. Because the data from Steps 2 and 3 would be located in
the lower left-hand corner of Figure 1, and the graph would be difficult
to read, multiply both values by a factor of 5:
(5)(64 psi) = 320 psi
(5)(82 psi) = 410 psi
Then locate 410 psi on the vertical axis in Figure 1 and draw a
horizontal line across the graph from that point.
Step 5. Locate 320 psi on the horizontal axis in Figure 1 and draw
a vertical line from that point.
Step 6. The two lines drawn intersect at Ush = 90%, indicating
a hydraulic uniformity that falls between the "Very Good" and "Excel-
From the above analysis, it can be concluded that the irrigation
system represented by the pressure data in Table 3 was designed
and constructed to achieve a high degree of uniformity in pressures
throughout the zone that was analyzed.
Measured Time (sec)
62 (low #1)
90 (high #1)
64 (low #2)
64 (low #3)
86 (high #3)
88 (high #2)
Water Application Uniformity
The water application uniformity is determined by measuring emit-
ter flow rates at a minimum of 18 points throughout each irrigated
zone. The emitter discharge rates are analyzed just as the previously
discussed pressure data were analyzed. Flow rates can be determined
by measuring the times required to fill a container of known volume.
A stop watch can be used to measure times.
In Figure 1, the sums of the high 1/6 and the low 1/6 of the times
measured should be used to determine the water application uni-
formity. Again, as discussed for pressure measurements, the high
and low 1/6 times can be adjusted by multiplying both by the same
constant. This is required if the data do not fit the scales in Figure
1 well. Alternatively, Figure 1 can be used with flow rates (calculated
by dividing the volumes measured by the times) instead of the time
or pressure axis scales currently used.
As an example of the use of Figure 1, assume that water from 18
emitters was collected to fill a small container, and the times required
to fill the container was recorded (in seconds). The data measured
are recorded in the third column of Table 3.
From the six-step procedure used for pressure analyses:
Step 1. 1/6 of 18 = 3
Step 2. 62 + 64 + 64 = 190 sec (lowest 3 values)
Step 3. 90 + 88 + 86 = 264 sec (highest 3 values)
Step 4. Locate 264 sec on the vertical axis in Figure 1 and draw
a horizontal line across the graph from that point.
Step 5. Locate 190 sec on the horizontal axis in Figure 1 and draw
a vertical line from that point.
Step 6. The intersection of these two lines occurs between the 80%
and 90% lines. This indicates a "Very Good" water application (statis-
tical) uniformity coefficient of about 88%.
From this analysis, it can be concluded that the irrigation system
represented by the flow rate data in Table 3 was designed and con-
structed to achieve a high degree of uniformity of water application
throughout the zone that was analyzed. Also, because the 88% water
application uniformity was only slightly less than the 90% hydraulic
uniformity, it can be concluded that the variability among emitters
used in this irrigation system is very low. No emitter plugging is
Emitter Performance Variation
Emitter performance variation, Vpf, refers to non-uniformity in
water application caused by the emitters. If the emitter performance
variation is high, this is normally due to emitter plugging or to
manufacturing variation among emitters. It may also be due to other
factors which affect emitter flow rates, such as temperature.
Emitter performance variation can be evaluated by measuring
emitter flow rates at known pressures. This requires removing the
emitters and testing them in a laboratory. Alternatively, both pres-
sures and flow rates can be measured at individual emitters, but
then flow rates must be corrected to a common pressure by using
the manufacturer's data for that emitter. Neither of these procedures
are desirable because of the amount of labor involved for each.
The nomograph in Figure 2 simplifies the procedure for determin-
ing the emitter performance variation from the hydraulic and water
application uniformities. The emitter performance variation can be
estimated in the following three steps:
Step 1. Locate the previously measured hydraulic uniformity co-
efficient (for pressure, from Figure 1) on the upper bar of the nomo-
Step 2. Locate the previously measured water application (statis-
tical) uniformity coefficient on the center bar of the nomograph.
Step 3. Draw a straight line from the measured hydraulic uni-
formity, through the statistical uniformity, and extend it down to the
lower bar. Read the emitter performance variation, Vpf, on the lower
As an example, assume that the uniformity of a new micro irriga-
tion system was measured immediately after installation, and that
the hydraulic uniformity determined from pressure measurements
was 95% (or coefficient of variation due to hydraulics = 0.05). Also,
the statistical uniformity of water application from emitter flow rate
measurements was 93% (or coefficient of discharge variation = 0.07).
From Figure 2, a straight line drawn through Ush = 95% and Us
= 93% intersects the lower bar at an emitter performance variation
of 5% (or coefficient of variation due to emitter performance = 0.05).
This value would be expected to be approximately the coefficient of
manufacturing variation for the emitter because this system is newly
installed, and emitter plugging has not yet occurred. For this exam-
ple, both the hydraulic and water application uniformities are above
90% and would be classified as excellent, indicating that the system
was well-designed and properly installed.
As a second example, assume that the same irrigation system was
again evaluated after operating for 6 months, and that the hydraulic
uniformity was again found to be 95%, but the water application
uniformity was now 88%. From Figure 2, a straight line drawn
through these two points shows that the emitter performance vari-
ation increased to 11%. These results demonstrate that the cause of
the lower water application uniformity measured is a change in emit-
ter performance, probably emitter plugging. This suggests that chem-
ical water treatment or flushing of lines may be required to restore
the system to its original high uniformity. Because the Ush remained
unchanged, this demonstrates that changes in the hydraulics of the
system was not a cause of the lower application uniformity measured.
Thus, these tests not only indicate whether a problem with water
application uniformity has occurred, but also demonstrate whether
the changes were due to changes in the system hydraulics or changes
in the emitter performance.
Figure 2. Emitter performance nomograph for the determination of emitter
EMITTER PERFORMANCE NOMOGRAPH
Ush 100 96 94 92 90 88 86 84 82 80 78 74
0 .04 N, 08 J 12 14 16 8 2 22 .24
0 2 8 WATER APPLICATION UNIFORMITY
i 90s 84 0 78 76 74 2 70
0 04 1081 12'N 14 .16 18 .2 22 24 .26 28 3
EMITTER PERFORMANCE VARIATION
02 4 56 7 8 9 10' 11I 12 13 14 15 16 17 18 19 2
II s ; I I I I I I I
002.040506 07 .08 .09 .1 II 12 13 .14 15 16 .17 18 19
Accuracy of Estimates
The estimates of uniformities and performance variations made
using the methods presented in this bulletin are based on statistical
samples of pressures and flow rates measured in the field. As with
any statistical estimate, the results will not be completely accurate
unless all emitters are sampled. Thus it is necessary to consider
confidence limits on the estimates made. Table 4 gives confidence
limits on the uniformities (or variabilities) measured.
From Table 4, the confidence limits are smaller if the uniformity
is greater. For example, for 18 samples, the confidence limit is 3.5%
if the uniformity measured was 90%, while the confidence limit was
+ 16.2% if the uniformity measured was 60%. This means that the
Table 4. Confidence limits (95% level) on statistical uniformity estimates.
Number of Samples
US(%) 18 36 72 144 Vqs
90 3.5 2.4 1.7 1.2 0.1
80 7.3 5.0 3.4 2.4 0.2
70 11.2 7.8 5.4 3.8 0.3
60 16.2 10.9 7.6 5.4 0.4
SI 1 1 IIBI 11 II 1 III
UI UNIVERSITY OF FLORIDA
Ui''-- ., 3 1262 05252 1225
actual uniformity would be expected to be in the range of 86.5% to
93.5% if the estimated value was 90%, while it could be expected to
range as much as 43.8% to 76.2% if the estimated value was 60%.
The smaller confidence limits occur at the higher uniformities be-
cause it is very improbable that samples would be randomly selected
that would indicate a high uniformity if the uniformity was actually
From Table 4, the confidence limits decrease as more samples are
taken. This indicates that we are more confident in the results if
more measurement are made. In fact, the confidence limits decrease
by a factor of two when the number of samples is multiplied by four.
If the uniformity estimates are low when only 18 samples are taken,
the need to take more data is indicated in order to improve the
confidence in the estimate. Thus, Table 4 can be used to determine
the number of samples that must be taken in order to estimate the
actual uniformity with the desired accuracy.
In summary, a method was presented to evaluate micro irrigation
uniformity of water application under field conditions. Both pressure
and flow rate data are required for these evaluations. These tests
not only indicate whether a problem with water application uni-
formity exists, but also demonstrate whether the problem is due to
the system hydraulics or to the emitter performance.
ASAE. 1989a. Field evaluation of micro irrigation systems. EP405.1.
ASAE Standards. Amer. Soc. Agric. Engr., St. Joseph, MI.
ASAE. 1989b. Design and installation of micro irrigation systems.
EP409. ASAE Standards. Amer. Soc. Agric. Engr., St. Joseph, MI.
Smajstrla, A.G., D.S. Harrison and ES. Zazueta. 1984. Field evalua-
tion of trickle irrigation systems: Uniformity of water application.
IFAS Ext. Bul. 195. Univ. Fla., Gainesville, FL.
This publication was produced at a cost of $672.30, or 22.4 cents
per copy, to present information on techniques that can be used
to evaluate the performance of micro irrigation systems under field
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