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BUL265

Field Evaluation of Microirrigation Water Application

Uniformity 1

A.G. Smajstrla, B.J. Boman, D.Z. Haman, D.J. Pitts, and F.S. Zazueta2

1. This document is BUL265, one o a series o the Agricultural and Biological Engineering Department, Florida Cooperative Extension Service, Institute

o Food and Agricultural Sciences, University o Florida. Original publication date March 1990. Revised February 1997. Reviewed January 2012. Visit the

EDIS website at http://edis.ias.u.edu.

2. A.G. Smajstrla, Water Management Specialist, Agricultural and Biological Engineering Department; D.J. Boman, Citrus Irrigation Specialist, IRREC Ft.

Pierce, FL; D.Z. Haman, Water Management Specialist, Agricultural and Biological Engineering Department; D.J. Pitts, Water Management Specialist,

SWFREC Immokalee, FL; and F.S. Zazueta, Water Management Specialist, Agricultural and Biological Engineering Department; Cooperative Extension

Service, Institute o Food and Agricultural Sciences, University o Florida, Gainesville, 32611.

The Institute o Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational inormation and other services only toindividuals and institutions that unction with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national

origin, political opinions or aliations. U.S. Department o Agriculture, Cooperative Extension Service, University o Florida, IFAS, Florida A&M University CooperativeExtension Program, and Boards o County Commissioners Cooperating. Millie Ferrer-Chancy, Interim Dean

A microirrigation system has the potential to be a very

ecient way to irrigate. However, to be eciently applied,

irrigation water must be uniormly applied. Tat is, with

each irrigation, approximately the same amount o water

must be applied to all o the plants irrigated. I irrigation is

not uniormly applied, some areas will get too much water

and others will get too little. As a result, plant growth willalso be nonuniorm, and water will be wasted where too

much is applied. Uniormity is especially important when

the irrigation system is used to apply chemicals along with

the irrigation water because the chemicals will only be

applied as uniormly as the irrigation water.

Te uniormity o water application rom a microirrigation

system is aected both by the water pressure distribution

in the pipe network and by the hydraulic properties o the

emitters used. Te emitter hydraulic properties include the

eects o emitter design, water quality, water temperature,and other actors on emitter ow rate. Factors such as

emitter plugging and wear o emitter components will aect

water distribution as emitters age.

Tis publication presents procedures to separately evaluate

the eects o pressure variations in the pipe network

(hydraulic uniormity) and variations due to the emitter

characteristics (emitter perormance variation) on unior-

mity o water application. Knowing both o these actors

will help an irrigation system manager identiy the causes o

low application uniormities and the corrections that may

be required to improve the uniormity o water application.

Tese procedures should be used on newly installed

microirrigation systems to veriy that they were properly

designed and installed, and to provide a reerence or utureevaluations. Also, evaluations should be done each year

to determine the eects o emitter plugging or changes in

other components on system perormance. More requent

evaluations may be required to diagnose and treat emitter

plugging problems.

Flow Rate Variation and

UniformityTe uniormity o water application can be calculated rom

the statistical distribution o emitter ow rates that aremeasured in the eld.

where US

= statistical uniormity o the emitter discharge

rates, and Vqs

= statistical coecient o variation o emitter

discharge rates.

In Equation (1) , the coecient o variation is the standard

statistical denition o the sample standard deviation

divided by the mean. Tus, when emitter ow rates are

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measured in the eld, Vqs

includes the eects o variability

in emitter ow rate rom all causes, including both water

pressure distributions and emitter hydraulic properties,

emitter plugging.

From Equation (1), when the variation in emitter ow rates

increases, the uniormity o water application decreases.

able 1 lists ve microirrigation uniormity classications,

ranging rom excellent to unacceptable, recognized by the

American Society o Agricultural Engineers (ASAE, 1996a;

1996b).

Emitter Hydraulic CharacteristicsEmitter Flow EquationMicroirrigation emitter ow rates have dierent

responses to pressure variations. Te response o a specic

emitter depends on its design and construction. Te

relationship between emitter operating pressure and ow

rate is given by:

where

q= emitter ow rate (gal/hr),

Kd= emitter discharge coecient,

P= operating pressure (psi), and

x= emitter discharge exponent.

Te coecient and exponent or this equation will

normally be given by the emitter manuacturer or rom an

independent testing laboratory such as the Caliornia State

University Center or Irrigation echnology (CI).

Te emitter discharge exponent, x, is a measure o the sensi-

tivity o the emitter ow rate to changes in pressure. Tis

exponent is dimensionless and it is independent o the units

used to measure ow rate and pressure. Values o x rangerom 0 to 1, with values around 0.5 being very common or

turbulent ow emitters. Low values o x (below 0.5) indicate

emitters that are relatively insensitive to changes in pressure

(pressure compensating emitters), while large values o x

indicate emitters that have larger changes in ow rate as

pressures change (laminar ow emitters).

Emitter Manufacturing VariationIn addition to ow variations due to pressure, variations

between emitters o the same type also occur due to

manuacturing variations in the tiny plastic components.

Because their orice diameters are very small, microir-

rigation emitters are easily plugged or partially plugged

rom particulate matter, chemical precipitates, and organic

growths. For these reasons, water application uniormity

may be greatly aected by the emitter perormance.

Te manuacturing coecient o variation (Vm

) is dened

as the statistical coecient o variation (standard deviation

divided by the mean discharge rate) in emitter discharge

rates when new emitters o the same type are operated at

identical pressures and water temperatures. Under these

identical operating conditions, dierences in ow rates

observed are assumed to be due to variations in emitter

components. able 2 classies point source (drip emitters

and microsprinkiers) and line source (drip tubing) emitters

based on manuacturing variation.

Because manuacturing variation reduces uniormity o

water application, choose emitters with low values o Vm

. When comparing emitters with similar ow properties

(i.e., similar values o Kd

and x in Equation 1 ), the highest

uniormity will be obtained by selecting the emitter with

the smallest manuacturing variation.

Pressure Variation and UniformityWhen water ows through a pipe network, pressure losses

occur because o riction losses in the pipes and ttings.Pressure changes also occur as water ows uphill (pressure

loss) or downhill (pressure gain) in a pipe network. I a

microirrigation system is poorly designed or improperly

installed, pressure losses may be excessive because compo-

nents are too small or design ow rates or slopes are too

steep or the components selected. For these reasons, water

application uniormity may be greatly aected by the design

o the pipe network.

Hydraulic uniormity reers to the eects o pressure

variations on the uniormity o water application rom

a microirrigation system. Hydraulic uniormity, Ush

, isdened similar to water application uniormity in Equation

(1) , except that the emitter discharge exponent, x, must

also be considered. Tis exponent shows the relationship

between emitter operating pressure and ow rate. Because

x is dierent or dierent types o emitters, the allowable

pressure variation is also dierent or each emitter type.

Figure 1.

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where

Ush

= hydraulic uniormity based on pressure

distributions,

x= emitter discharge exponent, and

Vh= hydraulic variation, which is the statistical coecient

o variation o pressures.

A low value o Ush

is most ofen due to improper design.

However, improper installation o components or the

installation o the wrong components can also reduce Ush

. Low values o 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 causes. All o these items must be properly

designed and installed in order to obtain an acceptable

uniormity o water application.

Uniformity TestsTis publication presents procedures or making threeseparate tests that are useul to evaluate the perormance

o a microirrigation system: (1) overall water application

uniormity, (2) hydraulic uniormity or pressure variation,

and (3) emitter perormance variation. Tese tests should

be perormed in the order indicated because, i the overall

water application uniormity is high, there is no need to

perorm urther tests. I the water application uniormity

is low, then hydraulic uniormity tests should be conducted

in order to determine the cause o the low uniormity. Te

hydraulic uniormity test will indicate whether the cause o

the low water application uniormity is excessive pressure

variation in the system or emitter perormance problems

such as plugging.

o simpliy computations, the statistical uniormity

nomograph shown in Figure 1 was developed. Tis graph is

used to determine both water application uniormity rom

emitter ow rate data and hydraulic variation rom pressure

distribution data.

1. Water Application Uniformity TestTe water application uniormity is a measure o how

evenly the volumes o water are applied rom each emitter.

Tis uniormity can be determined by measuring emitter

ow rates or the times required to ll a container o known

volume. I some emitters tested are completely plugged,

then ow rates must be measured since a container will

never be lled. o measure emitter ow rates, use a gradu-

ated cylinder to measure the volume collected or a given

time, such as one or two minutes. A stop watch or wrist

watch with a second hand can be used to measure times.

o accurately determine uniormity, measurements shouldbe made at a minimum o 18 points located throughout

each irrigated zone. More may be required or greater

accuracy. Computations will be simplied i the number o

points measured is a multiple o 6. Te statistical coecient

o variation is then calculated rom these data points.

Care should be taken to distribute the measurement points

throughout the irrigated zone. 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 o high

elevation and some at points o low elevation. However, the

specic emitters to be tested should be randomly selected ateach location. Do not visually inspect the emitters to select

those with certain ow characteristics

beore making measurements. In act, to avoid being

inuenced by the appearance o an emitter as it operates,

emitters could be selected and agged beore the irrigation

system is turned on. Te water application uniormity can

be read rom Figure 1. Te ollowing 6 steps are required:

1. Calculate 1/6 o the number o data points measured.

Tat is, divide the number o data points by 6. For

example, i 18 points were measured, this number will be

3.

2. Look at the set o data measured to locate and then add

the lowest 1/6 o the ow rates (or times or volumes)

measured. For 18 data points this will be the sum o the 3

lowest ow rates, times or volumes measured.

Figure 2.

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3. Look at the data set again to locate and then add the

highest 1/6 o the ow rates, times, or volumes measured.

For 18 data points this will be the sum o the 3 highest

values.

4. Locate the sum o the high ow rates, times, or volumes

on the vertical axis in Figure 1. Draw a horizontal line

across the graph rom that point. I this sum does not t

on the scale, or i the value is very small so that the scale

is dicult to read, the sums calculated in Steps 2 and 3

can both be multiplied or divided by a common actor.

Tis can be done because their absolute values are not

important, but only the relative dierences between the

high and low values are critical.

5. Locate the sum o the low ow rates, times or volumes

on the horizontal axis in Figure 1. Draw a vertical line up

the graph rom that point. Again, i necessary, modiy the

values rom Steps 2 and 3 as discussed in Step 4. However,

i changes are made, be sure to change the sums o both

the lowest and highest values by multiplying or dividingboth by the same actor.

6. Read the water application uniormity at the intersection

o the two lines drawn.

As an example o the use o Figure 1 , assume that water

rom 18 emitters was collected at random locations

throughout an irrigated zone. Assume that the method used

was to record the times required to ll a small container,

and that the times were recorded in seconds. An example

set o data is shown in able 3.

From the above 6-step procedure:

Step 1.1/6 o l8 data points = 3

Step 2.62 + 64 + 64 = l90 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 l and

draw a horizontal line across the graph rom that point.

Step 5.Locate 190 sec on the horizontal axis in Figure 1 and

draw a vertical line rom that point.Step 6.Te intersection o these two lines occurs between

the 80% and 90% lines. Tis indicates a Very Good water

application uniormity coecient o about 88%.

From this analysis, it can be concluded that the irrigation

system represented by the ow rate data in able 3 was

designed and constructed to achieve a high degree o

uniormity o water application throughout the zone that

was analyzed. Also, because o the high water application

uniormity, it can be concluded that the variability among

emitters used in this irrigation system is low. No signicant

emitter plugging is indicated.

2. Hydraulic Uniformity (Pressure

Variation)TestTe hydraulic uniormity o a microirrigation system is

estimated by measuring pressures at points distributedthroughout each irrigated zone. Measure pressures to the

nearest pound per square inch (psi). Although it is not

necessary to measure pressures at the same emitters where

ows were measured, it is normally convenient to do so

while ow rates are being measured.

Pressures can easily be measured using a portable pressure

gauge connected with a exible tube. Gauges are com-

mercially available with a needle on a exible tube or direct

insertion into the lateral pipe. Alternatively, some emitters

are constructed so that a exible tube can be slipped overthe emitter, allowing the pressure to be measured with the

emitter in place.

Some microsprinkler emitters are connected to the lateral

using small diameter exible tubing with a barbed insertion

tting. Because o pressure losses in these connecting tubes,

pressure should be measured at the end o the tube near the

emitter while the emitter is operating. Tis can be done by

installing a small barbed tee in the connecting tube.

Te pressure distribution data are analyzed in the same way

that the previously discussed ow rate data were analyzed.Te only dierence is that the hydraulic coecient o varia-

tion due to pressure, Vh, is read rom Figure 1 rather than

directly reading the statistical uniormity. Ten, in Step 7,

the hydraulic uniormity must be calculated rom Vh

and

the emitter discharge exponent, x, as shown in Equation

(3). Te hydraulic uniormity can be determined using the

ollowing 7-step procedure:

1.Calculate 1/6 o the number o data points measured. I

18 pressures were measured, this number is 3.

2.Look at the data measured to locate and then add

the lowest 1/6 o the pressures measured. For 18

pressure measurements, this is the sum o the 3

lowest pressures.

3.Look at the data again to locate and then add the

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highest 1/6 o the pressures measured. For 18

pressure measurements, this is the sum o the 3

highest pressures.

4.Locate the sum o the high pressures on the vertical axis

in Figure 1 . Draw a horizontal line across the graph rom

that point. I this sum does not t on the scale, or i the

value is very small so that the scale is dicult to read, thesums in Steps 2 and 3 can both be multiplied or divided by

a common actor.

5.Locate the sum o the low pressures on the horizontal axis

in Figure 1 . Draw a vertical line up the graph rom that

point. Again, i necessary, modiy the values rom Steps 2

and 3 as discussed in Step 4. However, i modications are

made, be sure to modiy the sums o both the lowest and

highest values by multiplying or dividing both by the same

actor.

6.Read the hydraulic variation, V h , at the intersection othe two lines drawn.

7.Calculate the hydraulic uniormity, Ush

, rom Equation

(3).

As an example o the use o Figure 1 to calculate the

hydraulic uniormity, consider the pressure data set in able

3. Assume that the 18 pressures shown in able 3 were read

at random locations throughout the irrigated zone. It will

normally be easiest to measure these pressures at the same

time and location while the ow rates were being measured,

but this is not necessary. For example, they could bemeasured later at 18 dierent locations, afer the ow rate

data indicated that there was a problem with water applica-

tion uniormity, and it is now important to determine the

cause o that low uniormity.

For this analysis, the emitter discharge exponent, x, must

be obtained rom the emitter manuacturer or the emitter

being used. For this example assume that a typical turbulent

ow emitter was used with x = 0.5. Ten, ollowing the

above 7-step procedure:

Step 1. 1/6 o 18 = 3

Step 2. 21 + 21 + 22 = 64 psi (1owest 3 values)

Step 3. 28 + 27 + 27 = 82 psi (highest 3 values)

Step 4. Because the data rom Steps 2 and 3 would be

located in the lower lef-hand corner o Figure 1, and the

graph would be dicult to read, multiply both values by 5

to expand the scale:

(5)(82 psi) = 410 psi

(5)(64 psi) = 320 psi.

Ten locate 410 psi on the vertical axis in Figure 1 and draw

a horizontal line across the graph rom that point

Step 5. Locate 320 psi on the horizontal axis in Figure 1 and

draw a vertical line rom that point.

Step 6. Te two lines drawn intersect at Vh = 0.1, indicating

a hydraulic uniormity that alls between the Very Good

and Excellent categories.

Step 7. From Equation (3) , calculate Ush

:

Ush

= 100% (1.0 - (0.5)(0.1)) = 95%

From Figure 1, this hydraulic uniormity would be classied

in the Excellent category.

From the above analysis, it can be concluded that the

irrigation system represented by the pressure data in able3 was designed and constructed to achieve a high degree

o uniormity in pressures throughout the zone that was

analyzed. Tus, i a low water application uniormity was

previously indicated, the cause o that low uniormity is

not poor hydraulic uniormity. Rather, the cause is emitter

perormance, probably emitter plugging.

3. Emitter Performance Variation

EvaluationEmitter perormance variation, V

p, reers to non-unior-

mity in water application that is caused by the emitters. I

the emitter perormance variation is high, this is normally

due to emitter plugging or to manuacturing variation

among emitters. It may also be due to other actors which

aect emitter ow rates, such as temperature.

Emitter perormance variation can be evaluated by

measuring emitter ow rates at known pressures. Tis can

be done by removing the emitters and testing them in a

laboratory. Alternatively, both pressures and ow rates can

be measured at individual emitters, but then ow rates must

be corrected to a common pressure by using the manuac-

turers data or that emitter. Neither o these procedures are

Figure 4.

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recommended because o the amount o labor involved or

each.

Te nomograph in Figure 2 simplies the procedure or

determining the emitter perormance variation rom the

hydraulic and water application uniormities. Te emitter

perormance variation can be estimated in the ollowing

three steps:

1.Locate the previously calculated hydraulic (pressure)

uniormity coecient, USh

, on the upper bar o the

nomograph.

2.Locate the previously calculated water application unior-

mity coecient, Us, on the center bar o the nomograph.

3.Draw a straight line om the measured hydraulic unior-

mity (upper bar), through the water application uniormity

(center bar), and extend it down to the lower bar. Read the

emitter perormance variation Vp

on the lower bar.

As an example, assume that the uniormity o a new

microirrigation system was measured immediately afer

installation, and that the hydraulic uniormity determined

rom pressure measurements was 95%. Also, assume that

the statistical uniormity o water application rom emitter

ow rate measurements was 93%. From Figure 2, a straight

line drawn through Ush

=95% and US

=93% intersects the

lower bar at an emitter perormance variation o 5%. Tis

value would be expected to be approximately the coecient

o manuacturing variation or the emitter because this

system is newly installed, and it is assumed that no emitter

plugging has yet occurred. For this example, both the

hydraulic and water application uniormities are above 90%

and would be classied 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 afer operating or 6 months,

and that the hydraulic uniormity was again ound to

be 95%, but the water application uniormity was now

88%. From Figure 2, a straight line drawn through these

two pomts shows that the emitter perormance variation

increased to 11%. Tese results demonstrate that the cause

o the lower water application uniormity measured is a

change in emitter perormance, probably emitter plugging.

Tis suggests that chemical water treatment or ushing o

lines may be required to restore the system to its original

high uniormity.

Because the Ush

remained unchanged, this demonstrates

that a change in the hydraulics o the system was not the

cause o the lower application uniormity measured. Tus,these tests not only indicate whether a problem with water

application uniormity has occurred, but also whether the

changes were due to changes in the system hydraulics or

changes in the emitter perormance.

Accuracy of EstimatesTe estimates o uniormities and perormance variations

made using the methods presented in this publication are

based on statistical samples o pressures and ow rates

measured in the eld. As with any statistical estimate, the

results will not be completely accurate unless all emittersare sampled. Tus, it is necessary to consider condence

limits on the estimates made when only a ew emitters are

sampled. Tis is a method o estimating how accurate the

measured result is, and whether it is necessary to make

additional measurements to improve the accuracy. able 4

gives condence limits on the uniormities or variabilities

measured.

From able 4, the condence limits are smaller when the

uniormity is greater. For example, or 18 samples, the

condence limit is 3.5% i the uniormity measured was

90%, while the condence limit is 16.2% i the uniormity

measured was 60%. Tis means that the actual uniormity

would be expected to be in the range o 86.5% to 93.5%

(90% 3.5%) i the estimated value was 90%, while it could

be expected to range as much as 43.8% to 76.2% (60%

16.2%) i the estimated value was

60%. Te smaller condence limits occur at the higher

uniormities because it is not likely that samples would be

randomly selected that would indicate a high uniormity i

the uniormity was actually low.

From able 4, the condence limits decrease as more

samples are taken. Tis indicates that we are more condent

in the results i more measurements are made. In act, the

condence limits decrease by a actor o two when the

number o samples is multiplied by our. I the uniormity

estimates are low when only 18 samples are

Figure 3.

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taken, then more samples must be taken in order to

improve the condence in the estimate. Tus, able 4 can

be used to determine the number o samples that must be

taken in order to estimate the actual uniormity with the

desired accuracy.

SummaryA method was presented to evaluate microirrigation

uniormity o water application under eld conditions.As a minimum, emitter ow rate data are required or

these evaluations. Pressure data are also needed in order

to determine whether the cause o any low uniormity

observed is system hydraulic (pressure) problems or

emitter characteristics, including plugging. I both tests are

made, the results not only indicate whether a problem with

water application uniormity exists, but also demonstrate

whether the problem is due to system hydraulics or to

emitter perormance, so that appropriate corrective actions

can be taken.

ReferencesASAE. 1996a. Field evaluation o microirrigation systems.

EP405.1. ASAE Standards. Amer. Soc. Agric. Engr., St.

Joseph, MI. Pp. 756-759.

ASAE. 1996b. Design and installation o microirrigation

systems. EP409. ASAE Standards. Amer. Soc. Agric. Engr.

St. Joseph, MI. Pp.792-797.

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Table 1. Microirrigation system uniormity classifcations based

on emitter discharge rates1.

Classifcation Uniormity, U (%)

Excellent above 90%

Good 90%-80%

Fair 80%-70%

Poor 70% -60%

Unacceptable below 60%lAdopted rom ASAE (1996a).

Table 2. Classifcations o manuacturers coecient o

variation, Vm

, or emitters2.

Emitter Type Vm

Range (%) Classifcation

below 5% Excellent

Point Source 5% to 7% Average

(drip emitters 7% to 11% Marginal

and 11% to 15% Poor

microsprinklers) above 15% Unacceptable

Line Source below 10% Good

(drip tubes) 10% to 20% Average

above 20% Unacceptable

2Adopted rom ASAE (1996b).

Table 4. Confdence limits (90% level) on statistical uniormity

estimates3.

Uniormity Us

(%) Number o Samples Variability Vqs

(%)

18 36 72 144

90% 3.5% 2.4% 1.7% 1.2% 10%

80% 7.3% 5.0% 3.4% 2.4% 20%

70% 11.5% 7.8% 5.4% 3.8% 30%

60% 16.2% 10.9% 7.6% 5.4% 40%3Adopted rom ASAE (1996a).

Table 3. Data set or Figure 1 example.

Data Point Pressure (psi) Measured Time (sec)1 26 65

2 27 (high #2) 62 (low #1)

3 22 (low #3) 80

4 25 74

5 21 (low #1) 90 (high #1)

6 26 68

7 26 64 (low #2)

8 24 76

9 25 72

10 28 (high #1) 64 (low #3)

11 25 67

12 24 81

13 23 86 (high #3)

14 24 77

15 21 (low #2) 88 (high #2)

16 25 72

17 24 78

18 27 (high #3) 66