Using Time Domain Reflectometry to Measure

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Using Time Domain Reflectometry to Measure
Williams, Jeff
Dukes, Michael ( Mentor )
Place of Publication:
Gainesville, Fla.
University of Florida
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University of Florida
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Using Time Domain Reflectometry to Measure

Jeff Williams


The purpose of this project was to determine whether the use of soil moisture content measured by time

domain reflectometry (TDR) could be used to estimate microirrigation uniformity based on traditional

volumetric measurements. Experimental methods were designed such that various levels of uniformity were

tested for both volumetric discharge and soil moisture content at three distances from the emitter. Despite

volumetric statistical uniformities ranging from 0.64 to 0.91, soil moisture uniformity values only ranged from 0.80

to 0.88. These results indicate that redistribution of soil water leads to a relatively uniform soil moisture

condition within the range of system uniformities tested.


Microirrigation (sometimes referred to as drip or trickle irrigation) has the potential for increased efficiency in

both water and energy use. According to the 2000 Irrigation Survey (Irrigation Journal, 2001),

microirrigation accounts for nearly 5% of all irrigated acreage nationwide. The high efficiency achieved by

these systems is due to reduced flow rate and pressure requirements. Water is directly applied to the root

zone, which lessens the total volume of irrigation water needed to sustain the crop. However, the efficiency of

this method is largely dependent on the uniformity of the water distribution and management.

Irrigation uniformity is a measure of how evenly water is distributed to different areas of the field. At high

uniformity, water can be applied adequately with little excess. However, at low uniformity, some portions of the

field will be deprived of water while other locations will be over-irrigated. Therefore, the effects of high uniformity

are improved yield, reduced water and energy use, and decreased environmental impacts. These benefits

become more significant when chemicals are being injected through the irrigation lines.

In drip irrigation, distribution uniformity is affected by emitter conditions and hydraulic factors within the

system. Emitter conditions include manufacturing variation, clogging (due to sediment or biological growth),

and general deterioration. Because pressure variation will impact the flow rate of the emitters, hydraulic conditions

in the system also influence uniformity. These conditions include pressure variation in the laterals, topography,

and wear inside the pipes (Clemmens, 1987). Uniformity should be tested as part of the system

maintenance. Problems within the system such as clogging and pressure variations can be diagnosed using

the results from the uniformity test.

Irrigation uniformity has been quantified in several different equations. Low-quarter distribution uniformity (DUIq)

is defined as the ratio of the average of the lowest quarter of samples to the average of all samples:

DU d = (Kruse, 1978) [1]

where diq = the average of the lowest-quarter depths (or volume)
d=g = the average depth (or volume)

Another measure of uniformity that is defined with the coefficient of variation is statistical uniformity.

U, = (1- CV) (ASAE, 1997) [2]

where CV = - [3]

CV = the coefficient of variation
s = standard deviation
x= sample average

Current guidelines for uniformity determination are based on volumetric flow from the emitters. The water

discharged over a constant time is caught from several emitters throughout the sample area. These data

are analyzed according to the above equations to determine the uniformity of the irrigation system or subunit

(ASAE, 1997).

It is becoming increasingly popular for drip irrigation to be installed below the soil surface or under plastic mulch

in vegetable production. As such, it is inconvenient and destructive to evaluate these systems in the field using

the existing procedures described above. Additionally, these results only represent the distribution from the

emitter without accounting for the water's movement through the soil.

A method of determining the soil volumetric moisture content could provide a more practical method for

conducting uniformity tests. Time domain reflectometry, which indicates this soil property, could provide a solution.

Time domain reflectometry (TDR) was initially used to find breaks in coaxial transmission lines, but has been used

in the last two decades to determine the soil moisture content of soils. Electromagnetic waves are propagated along

a wave guide. By timing their return, the dielectric constant of the soil is determined. It has been shown that

the dielectric constant is strongly correlated to soil moisture content, with little effects from texture,

material, temperature or salinity. Thus, water content in the surrounding media can be determined (Topp et.

al., 1980).

The objective of this study was to determine if TDR could be used to accurately quantify the uniformity of
drip irrigation systems. Uniformity was calculated using both soil moisture and volumetric measurements, then
the results were compared.


Field Preparation

This project was conducted at the Irrigation Research and Education Park on the University of Florida campus
in Gainesville, FL. The test area was first tilled, disked, and leveled. Landscaping fabric was installed to
prevent weeds from growing, which could disturb the results. A plumbing setup was constructed that consisted
of pressure regulators to maintain the desired head in the drip tape, pressure gauges before and after the
manifold, and an injection point for clogging the lines (figure 1). Four 30 m sections of drip tape (RoDrip 16mm,
300 Iph/100m at 55 kPa, 30cm emitter spacing, Roberts Irrigation, San Marcos, CA) were installed for
each assembly. Typical operating pressures for the system varied between 55 to 69 kPa.

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Figure 1. Typical microirrigation system layout.

Testing Procedure

The goal of this project was to determine the relationship between soil moisture content uniformity
and conventional volumetrically measured uniformity. In a given soil, the volumetric soil
moisture content due to drip tape is a function of the irrigation time and the distance from the emitter.
To properly assess the usefulness of soil moisture content in measuring distribution uniformity, both

of these variables must be taken into account. Soil moisture measurements were taken at irrigation

times of 0, 10, 20, 40, 80, and 120 minutes, at distances of 5, 10, and 15 centimeters from the

emitter being tested. Treatments were arranged in a completely randomized block design with

four replicates. These data were collected using the Spectrum Field Scout soil moisture sensor

(Spectrum Technologies, Inc., Plainfield, Illinois), which is equipped with a CS620 water

content reflectometer (Campbell Scientific, Inc., Logan, UT). A wooden template was created to

make TDR measurements accurately and uniformly (fig. 2). There is also a gentle slope in the test

area. To eliminate the effect of the slope, the orientation of the template was rotated 90- for each

tape per irrigation time.

Figure 2. Taking soil moisture measurements with the TDR probe during a test.

It was necessary to test the TDR method for a range of DU values. To accomplish this, the drip

tapes were artificially clogged by injecting a solution of bentonite and soil mixture at varying levels.

The procedure above was then repeated four times at subsequently higher clogging intensities. The

five tests resulted in volumetric DUIq of 0.87, 0.82, 0.82, 0.64, and 0.42. At each emitter tested,

the volumetric discharge was also measured to compare with the soil moisture uniformity.


22500 - //
A m

0' 2000 - / A *

> 1500- A /*/ *
_ o-o / -/


o 500 ~ 0

0 2 4 6 8 10 12 14

Change in Soil Moisture Content (%)
Figure 3. Relationship between volume emitted and the change in soil moisture at 3 distances from
the emitter.


Statistical analysis of the data resulted in no correlation between the TDR values and the
volumetric measurements. As seen in figure 3, there is a good correlation between the volume

applied and the change in soil moisture content (average r2 of 0.872). However, variability in the
data prevented prediction of a DUIq equivalent to that calculated from volumetric data. Also,

the regression has less resolution at the high and low ends of the spectrum.

Distribution uniformity based on soil moisture data and volumetric data was calculated according
to equation 1 and graphed for each time and distance combination for a total of 18 graphs. According
to figure 4, which shows all of these graphs, there is no consistent relationship between the
DUIq calculated from soil moisture content and the DUIq based on volumetric data. Although

the volumetric DUIq varied between 0.42 and 0.87, there was no predictable variation from the

DUIq calculated from TDR data.

Time 0 Time 10 Time 20 Time 40 Time 60 Time 120

(TDR DU j) and volumetrically (Vol. DU q).
moisture based statistical uniformity Ivalues ranged from 0.80 to 0.88, except one value of 0.58. This is

Uniformity results a measure of the error in the irrigation application. By trying to calcuStatise ethical

uniformity using sol properties, additional error is introducvlumetric data ranged duefrom 0.91 to the error associated withe soil

sol oisture based statistical uniformity values ranged variation of0.80 theo 0.88, except one value of 0.58. This is

The TDR probe manufacturer provides an equipment error of �3% soil moisture content. This

is especially noteworthy since the majority of the measurements were between 7-22% soil

moisture content. Even in a perfect system, the inherent variability of the probe will produce

flawed values for uniformity. Additionally, the probe only measures the volume of soil close to

it (approximately 1100 cm3 with the 20cm probe). The portion of this volume influenced by the

irrigation water is small, especially at the 15 centimeter distance with short irrigation times (figure

5). All of these errors are then reflected in the soil moisture uniformity.

Figure 5. Wetting front of the drip emitter compared to the

measurement area of the TDR probe at the 15 cm distance.

Also, soil water will redistribute during and after an irrigation event such that non-uniformity of

water application will not necessarily result in the same magnitude of non-uniform soil

moisture, particularly at longer irrigation intervals.

The statistical uniformity calculated from the TDR values should not be used to judge the performance

of the microirrigation system, but it might still be useful. TDR Us is consistently in the 0.8-0.9

range, even when volumetric Us is a nearly unacceptable 0.64 (figure 6). The simplest conclusion is

that even in a clogged or poorly designed system, water redistribution within the soil can improve

the uniformity of the irrigation application. To take full advantage of this effect, the proper

emitter spacing should be selected based on the soil type and the planting density.

*5cm TOR 010cm TDR 15em TDR

Test I Test 2 Test 3 Test 4 Test 5
Figure 6. Statistical uniformity calculated by equation 4 for volumetric measurements

and soil moisture measurements at three distances from the emitter.

Further studies in this direction could produce beneficial results. A similar experiment could be

conducted with various soil types and various emitter spacings. Also, plants could be introduced in

a future study to determine the effects of uniformity of microirrigation systems on yield. It may be that

a new uniformity classification system needs to be developed for microirrigation that takes soil type

into account.


ASAE Standards, 44th Ed. 1997. EP458. Field evaluation of microirrigation systems. St. Joseph,

Mich.: ASAE

Clemmens, A.J. 1987. A statistical analysis of trickle irrigation uniformity. TRANSACTIONS of the ASAE


Hart, W.E., and W.E. Reynolds. 1965. Analytical design of sprinkler system. TRANSACTIONS of the ASAE

8(1): 83-89

Irrigation Journal. 2001. 2000 Irrigation survey. 51(1): 12-41

Kruse, E.G. 1978. Describing irrigation efficiency and uniformity. Journal of Irrigation and Drainage




ASCE 104(IR): 35-41

Topp, G.C., J.L.Davis, and A.P.Annan. 1980. Electromagnetic determination of soil water
content: Measurements in coaxial transmission lines. Water Resources Research 16: 574-582


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