Time Domain Reflectometry Field Measurements of Soil Water Content andElectrical Conductivity1

S. DASBERG AND F. N. DALTON2

ABSTRACTSimultaneous measurements of volumetric water content and bulk

electrical conductivity were made using time domain reflectometry(TDR) with a single parallel transmission line (PTL) imbedded inthe soil. Sixty PTL's were installed at five depths in 12 existinglysimeter plots, irrigated with different amounts of water at two saltconcentrations (1.3 and 3.1 dS/m). The water content measurementsobtained with TDR showed a good relationship to gravimetric de-terminations (r2 = 0.84) and were comparable to neutron probe mea-surements. The TDR measurements of bulk soil electrical conduc-tivity were similar to the measurements of the same soil physicalproperty measured with the four-probe electrode technique. The re-lation between bulk soil electrical conductivity and the conductivityof the soil solution is discussed. The importance of measuring bothsoil water content and salinity in a nondestructive manner on thesame sampling volume, enabling repeated in situ measurements isstressed.

Additional Index Words: soil salinity, neutron scattering, spatialvariability, soil solution conductivity, four-electrode probe.

Dasberg, S., and F.N. Dalton. 1985. Time domain reflectometryfield measurements of soil water content and electrical conductivity.Soil Sci. Soc. Am. J. 49:293-297.

THE CONVENTIONAL METHOD for measuring soilsalinity is by taking soil samples and determining

the electrical conductivity of the extract of a saturatedsoil paste (Rhpades, 1982). These measurements canbe converted into soil solution salt concentration bycorrecting for the soil water content at the time ofsampling. The soil solution can be sampled directlyby porous suction cups. This method, however, is lim-ited to a narrow range of soil moisture between (ap-proximately) field capacity and saturation and thesmall sample volume tends to make the measure-ments variable (Broadbent, 1981). Soil water contentcan be measured by destructive sampling and gravi-metric determination or by the in situ neutron scat-tering method which has the advantage of measuringsoil water content on a volume basis. The limitations

1 Contribution from the U.S. Salinity Laboratory, USDA-ARS,4500 Glenwood Drive, Riverside, CA 92501.

2 Visiting Soil Scientist and Soil Physicist. The permanent addressof the first author is Institute of Soils and Water, ARO, The VolcaniCenter, Bet Dagan 50-250, Israel.

of the neutron method are its relatively large samplingvolume, the inability to measure close to the soil sur-face, the radiation hazard involved, and the need forindividual soil calibration (Graecen, 1981). The factthat water content and salinity are usually obtainedfrom separate samples with different geometry intro-duces an additional error in soil salinity assessment.

Recently, Dalton et al. (1984) proposed the simul-taneous measurement of both soil water content andsalinity for studies on water and salt management, us-ing time domain reflectometry (TDR). The relativedielectric constant of soil is primarily related to itswater content. Measurement of the dielectric constantin the time domain, by measuring the propagationvelocity of a voltage pulse was introduced by Fellner-Feldegg (1969).

Topp et al. (1980) showed a unique relationship be-tween the relative dielectric constant« and the volu-metric water content 6 for a large range of soils andsoil-like materials. In later work, parallel transmissionlines (PTL) constructed of metal rods were used tomeasure water content in the field with TDR (Toppet al., 1982). The relative dielectric constant t is cal-culated from the measured transit time / of a voltagepulse through the length C of soil material as given bythe electrode length according to

= [ct/29] [1]where c = velocity of light in free space (3 X 108 m/s). Theempirical relationship between relative dielectric constant eand soil volumetric water content 6 was found to be (Toppet al., 1980)0 = -5.3 X 1CT2 + 2.92 X I0~2e

- 5.5 X 10-V + 4.3 X 10~6 [2]It was found that this relationship was nearly independentof soil texture, soil density, temperature and salt content. Itwas noted, however, that soil salinity did influence the TDRsignal in the soil medium (Topp et al., 1980).

Dalton et al. (1984) in attempting to account for the signalattenuation which occurs in saline soils, used results fromtransmission line theory to calculate the electrical conduc-tivity of moist soil. Figure 1 shows the assumed equivalentcircuit of a differential length of the parallel transmissionline. L and R are the inductance and series resistance of theelectrodes. C and G are the equivalent shunting capacitance

294 SOIL SCI. SOC. AM. J., VOL. 49, 1985

DISTANCE (m)1.20 150 1.80 2.40 2.70 3.00

(a)

-dz-

(b)

Fig. 1. (a) Parallel transmission line (PTL) of length / and (b) dis-tributed inductance L, resistance R and shunting conductance Gand capacitance C along a differential element of the PTL.

and conductance between the electrodes respectively. Thereis no inherent frequency dependence assumed for the incre-mental conductance component in Fig. 1. It is this com-ponent of the equivalent circuit which is later compared toanother method of measuring soil electrical conductivity. VTis the input voltage to the line, but due to losses, the voltagealong the line is assumed to decrease exponentially. Thevoltage at the end of the line is given by VL = VTe\p (-aL)where a is an attenuation coefficient. If perfect reflection atthe end of the line is assumed, the voltage returning to thesource will be

F rexp(-«2C). [3]For steady state sinusoidal conditions and for low loss linesan approximate expression for the attenuation coefficient isgiven from electromagnetic theory (Ramo and Whinnery,1959) as

a ~ R/[2(L/Q1'2] + [G(L/Ql/2]/2 . [4]Using values of L and C for parallel rod transmission lineand assuming the skin resistance R to be negligible,

a « 607r<T/(e)1/2 [5]where a is the bulk electrical conductivity. From Eq. [3] and[5], the bulk electrical conductivity is

a !=* [(e)1/2]/120irC In (VT/VR . [6]This equation, developed by Dalton et al. (1984), in con-junction with Eq. [1] and [2], allows for the simultaneousdetermination of soil water content 6 and soil electrical con-ductivity <r, based on the measured parameters t, VT, andV*

The relationship between <r and 6 was investigated byRhoades et al. (1976) in their development of the four-elec-trode probe measurement of a and was shown to be

T(S) + as [7]where aw is the electrical conductivity of the soil water, Tisthe transmission coefficient linearly dependent on 6 and a,is the solid phase conductivity.

Dalton et al. (1984) conducted an experiment where thedielectric constant and electrical conductivity were mea-sured in soil columns of equal water content, but highlyvarying salt content. The results showed a good linear re-lationship between TDR measured bulk electrical conduc-tivity (Eq. [6]) and soil water electrical conductivity. Fur-thermore, these results were in close agreement to calculateda values based on Eq. [7] using parameters from four-probeelectrode calibrations.

These results form the basis of the following experimentalstudy where we compare this TDR method under field con-ditions with the established methods: neutron and gravi-metric methods for soil water content, four-electrode probe

1.50

Fig. 2. Schematic cross section through lysimeter plot showing lo-cation and volume of soil samples.

and soil water extraction methods for soil electrical con-ductivity.

MATERIALS AND METHODSTwelve of the 24 rhizotron plots established in 1981 at

the U.S. Salinity Laboratory for measuring the influence ofirrigation frequency on the leaching requirement of alfalfawere used for evaluation of the TDR method. These plotswere described previously in detail (Hoffman et al., 1983).Each plot has dimensions of 3 by 3 by 1.5 m and includedone neutron probe access tube, and four-electrode burialprobes installed horizontally at five depths. The profile ofeach plot is horizontally accessible by plywood panels. Thesoil is Pachappa fine-sand loam topsoil (coarse, loamy, mixed,thermic Mollic Haploxeralfs). Transmission lines were in-stalled at five depths in four experimental treatments rep-licated three times; all irrigated twice per cutting as follows:

1. With water of 1.3 dS/m, at 0.12 leaching fraction2. With water of 1.3 dS/m, at 0.06 leaching fraction3. With water of 3.1 dS/m, at 0.24 leaching fraction4. With water of 3.1 dS/m, at 0.12 leaching fractionThe transmission lines were constructed from 0.32-cm di-

ameter parallel brass rods 5 cm apart at lengths of 20 cmfor the high salinity plots or 30 cm for the low salinity plots.Shielded TV wire embedded in 70-cm-long 2.16-cm od PVCtubing, which served as a handle, provided the connectionby means of an impendence matching transformer to a TDRmeasuring unit, consisting of oscilloscope, pulse generatorand sampling head (Tectronics model 7603).

Horizontal holes, 6.35-cm in diameter and 60 cm long,were made at depths of 15, 25, 45, 75 and 105 cm from thesoil surface through the side panels of 12 plots. Prior toinstallation of the transmission lines, in situ electrical con-ductivity was measured with a hand-held four-electrode probeinstrument (Rhoades and van Schilfgaarde, 1976) at 30 cmdistance from the side panels. Soil samples were taken atthis location. From half of each sample soil solution wasextracted by centrifuge and the electrical conductivity wasdetermined. The gravimetric water content was determinedby oven drying the other half of each sample and 6 wascalculated using the known bulk density of the soil (1.57 Mgm~3). The electrical conductivity of the saturated extract wasdetermined after adding 25% of water by weight (approxi-mately SP) to a dry soil sample and extracting by vacuum(Rhoades, 1982).

The transmission lines were installed carefully in order toensure good soil contact. The hole was backfilled with theoriginal soil and TDR readings were taken one day afterinstallation, measuring transit time /, transmitted voltage VTand reflected voltage VR. From these measurements, 0 anda were calculated using Eq. [1], [2], and [3]. Neutron mea-surements were taken at corresponding depths in existingaccess tubes, located about 1 m from the wave guides (see

DASBERG & DALTON: TIME DOMAIN REFLECTOMETRY FIELD MEASUREMENTS 295

Table 1. Water balance obtained during irrigation of experimentalplots by water metering and by soil water content

measurements before and after irrigation withneutron and TDK methods (mm/1.20 m

soil depth).

.30

Waterquality,

dS/m

1.3

1.3

3.1

3.1

Leachingfraction

0.06

0.12

0.12

0.24

Plot2

1320

Avg8

1719

Avg1

1022

Avg6

1621

Avg

Waterappliedf

110.2108.0110.4111.2135.0134.8133.3134.4134.6136.1136.7135.8158.7155.3159.7157.9

Originalneutron

159.5111.0118.7129.7114.682.2

116.4104.4121.3119.9161.5134.2125.7142.2111.1126.3

Additionalneutron

167.2..-

167.2157.9117.9

..137.7127.5-_

127.5

172.0150.5

161.3

TDK

121.564.8-

93.2128.5

_109.5119.0112.1128.2128.0122.8

_127.8174.5151.2

t Corrected for drainage and evapotranspiration.

Fig. 2). At a later stage, an additional neutron access tubewas installed in each plot and soil samples and four-probemeasurements were taken during installation (Fig. 2), usingthe procedure described previously. During irrigation of theexperimental plots, TDK and neutron measurements weretaken for all depths 1 d before irrigation and 2 d after irri-gation. The amount of water applied to each plot was mea-sured and corrected for drainage during that time period andfor evapotranspiration as estimated by measuring pan evap-oration. This was compared to the change in soil water con-tent as measured by both TDR and neutron methods.

RESULTS AND DISCUSSIONWater Content Measurement

Figure 3a shows the relationship between TDR andthe gravimetrically determined water contents ob-tained from subsamples taken while installing the PTLand the additional neutron access tubes. Figure 2bshows the relationship between the two sets of neutronprobe readings and the same gravimetric water con-tents as above. The calculated regression equationsand coefficients of determination (?) of 0TDR and 0neutr0grav for 102 data points are as follows:

0TDR = 1-02flneutr = 0.86

- 0.023 t3- = 0.84+ 0.031 I2 = 0.82

vs. 0neut r2 = 0.79These data show that the correlation between TDRand gravimetric determination is similar to that be-tween neutron and gravimetric determination, but itis somewhat closer to a 1:1 relationship.

The lower than expected correlation coefficient be-tween 0neutronand 0gKV may be due to spatial variabilityin the horizontal plane containing the sampling andmeasuring volumes. This may also be true for the 0TDR— 0p,v correlation. Some indication for this is foundin the following. Additional neutron probes were in-stalled in the plots as close to the TDR probes as pos-sible (Fig. 2). The comparison between these neutronprobe and gravimetric determinations of water con-

.20

.10

0 8.™ from TDR installationgrov

.10 ,zo .30

.20

.10

from TDR installationfrom neutron installation

.10 .20 .30

Fig. 3. (a) Soil water content as measured by TDR vs. gravimetricdetermination and (b) water content as measured by neutron scat-tering vs. gravimetric determination.

tent increased the correlation coefficient to r2 = 0.92for 56 data points.

Another test of the validity of the TDR determinedwater content measurements was carried out duringirrigation of the experimental plots. The water balancefor all plots was calculated using TDR, neutron mea-surements and applied water (see Table 1). The dis-crepancy between the water actually applied and thatmeasured by TDR and neutron probe seem to reflectsoil variability. In many cases the differences betweenthe two neutron sites per plot are as great or greaterthan the differences between TDR and neutron mea-surements. Both measurements seem to give a trueestimate of water content for their sampling volume,and no conclusion can be drawn which method givesa better estimate of water content. More rigorous test-ing is needed to reach such a conclusion.

Salinity MeasurementThe relationship between (TTDR and o-(4-pr) is given

in Fig. 4. These are two independent estimates of thesame physical property, based on completely differentmeasurement principles. Although the correlation be-

296 SOIL SCI. SOC. AM. J., VOL. 49, 1985

0.1 0.2 Q3 0.4 O5 0.6 0.7 0.8 0.9

Fig. 4. Bulk soil electrical conductivity measured by TDR vs. mea-surement with four-electrode probe and 1:1 line.

tween the two is high (r2 = 0.68 for 102 data points)many points are not close to the 1:1 line. The questiontherefore remains, which of the two measurementsgives a better estimate of the bulk soil electrical con-ductivity?

The purpose of measuring a is to obtain an estimateof (rm the electrical conductivity of the soil solution.An estimate of aw was obtained in our experiment bycentrifuging the soil samples obtained during instal-lation of the transmission lines and measuring theelectrical conductivity of the nitrate, <rex. Since ac-cording to Eq. [4] the relation between a and aw isdependent on the water content 6, we normalized ourrjex data by calculating <rsat = 0-^(0/0^,), taking <?«,, =0.40 (SP X bulk density). This procedure is preferableto the determination of aw on the saturated extract,since by drying the soil and adding distilled water, ionexchange and solution of additional salts occur. Com-parisons between a and <rsat are shown in Fig. 5a whencr is measured by TDR and in Fig. 5b when a is mea-sured by four-electrode probe. In each case the dataconsists of those taken while installing the transmis-sion lines and later while installing additional neutron

I

1.0

0.8

0.6

0.4

0.2

0

TDR installationneutron installation

1.20.2 0.4 0.6 0.8 1.0O~ calculated from (Tex , dS/m

Fig. 6. Bulk soil electrical conductivity measured by TDR as com-pared to same calculated from centrifuge extract electrical con-ductivity.

0.91

Q8

0.7

0.6

0.5

0.4

03

0.2

O.ITDR installationneutron installation

C/)•a

a.b'

U.3

OS

0.7

0.6

0.5

0.4

03

0.2

0.1n

b ° %o

o

Oo0 D

0°°° ° DO o° °O O Q

°0«,0fioD°°afl' #>

°S> n o § DDcp ° TDR installation

Sjaa°D o a neutron installation •cw\ . . . . . . .

0 1 2 3 4 5 6 7 8 9O r . , dS/m.

Fig. 5. Relationship between electrical conductivity of soil solutionat saturation and bulk soil electrical conductivity measured (a) byTDR or (b) by four-electrode probe method.

access tubes. Again no conclusion can be drawn onwhich measurement gives a better relationship be-tween ff and (jsat. The TDR measured value of a canalso be compared with the calculated value of cr ac-cording to equation (4). Previously published param-eters for Pachappa fsl (Rhoades et al., 1976) were usedfor this calculation: vs = 0.18 dS/m, T = 1.3820 -0.093). Figure 6 shows this comparison. According toequation (4), no a values below <rs were possible. How-ever with both types of measurement of or we obtainedvalues < 0.18 dS/m. This is in accordance with theconclusions of Shainberg et al. (1980) and Nadler andFrenkel (1980) who showed the non-linearity of the a/aw relationship in the low range of soil salinity.

The preliminary conclusion reached by Dalton etal. (1984) that "time domain reflectometry, in con-junction with known relationships between relativeelectrical conductivity and soil water conductivity,provides a new and powerful tool in soil water re-search in that a single measurement can yield both thesoil water content and the soil water salinity"—hasbeen borne out by the data presented in this paper.The water content measurements obtained with TDRshow a good relationship to the standard gravimetricmethod and are comparable to the neutron measure-ments, as has been demonstrated previously by Topp(1980). The simplicity of the measurement and thereported universal calibrations, independent of soildensity, temperature and salinity are additional ad-vantages. But the main advantage of the method isthat on the same sampling volume an estimate of thebulk soil electrical conductivity can be obtained. Theresults of this measurement seem to be comparable tothe data obtained with the four-electrode probemethod. Traditionally, soil salinity is evaluated by theelectrical conductivity of the saturation extract. "Ide-

KEREN ET AL.: PLANT UPTAKE OF BORON AS AFFECTED BY BORON DISTRIBUTION 297

ally, it would be desirable to know the solute concen-tration in the soil water over the entire range of fieldwater contents and to obtain this information directlyin the field, without need for collection of soil samplesor laboratory analysis" (Rhoades, 1982). The pro-posed measurement of a, together with the measure-ment of 6 at the time of sampling, may pave the wayin that direction. There seems to be no valid reasonwhy guides to crop tolerance and salinity managementcould not be developed based on a measurementsrather than on determinations of the conductivity ofthe saturated extracts. More research is needed toevaluate the effects of environmental conditions onthe a measurements and to develop less expensivemeasuring instruments.