The effect of soil electrical conductivity on moisture determination using time-domain reflectometry in sandy soil

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1 The effect of soil electrical conductivity on moisture determination using time-domain reflectometry in sandy soil Z. J. Sun, G. D. Young, R. A. McFarlane, and B.M. Chambers E.S.I. Environmental Sensors Inc., Glanford Avenue, Victoria, British Columbia, Canada V8Z 4B9. Received 1 December 1998, accepted 4 September Can. J. Soil. Sci. Downloaded from by on 01/09/18 Sun, Z. J., Young, G. D., McFarlane, R. A. and Chambers, B. M The effect of soil electrical conductivity on moisture determination using time-domain reflectometry in sandy soil. Can. J. Soil Sci. 80: 13. A series of laboratory experiments was conducted, in order to systematically explore the effect of soil electrical conductivity on soil moisture determination using time domain reflectometry (TDR). A Moisture Point MP-917 soil moisture instrument (E.S.I. Environmental Sensors Inc., Victoria, BC, Canada) was used to measure propagation time (time delay) of a step function along a probe imbedded in fine sand with different moisture and salinity. The volumetric soil water content was independently determined using a balance. With the help of the diode-switching technique, MP-917 could detect the reflection from the end of the probe as the electrical conductivity of saturated soil extract (EC e ) increased to 15.9 ds m 1. However, the relationship between volumetric soil water content and propagation time expressed as T/T air (the ratio of propagation time in soil to that in air over the same distance) deviated from a linear relationship as the conductivity exceeded 3.7 ds m 1. At the same water content, the time delay in a saline soil was longer than that in a non-saline soil. This leads to an over-estimation of volumetric soil water content when the linear calibration was applied. A logarithmic relationship between volumetric soil water content and T/T air has been developed and this relation includes soil electrical conductivity as a parameter. With this new calibration, it is possible to precisely determine the volumetric water content of highly saline soil using TDR. Key words: Time domain reflectometry, time delay, bulk electrical conductivity (σ), volumetric soil water content (θ), relative permittivity or dielectric constant (ε r ), propagation velocity V p Sun, Z. J., Young, G. D., McFarlane, R. A. et Chambers, B. M Effets de la conductivité électrique des sols sablonneux sur les measures d humidité par la méthode de reflectométrie en domain temporel. Can. J. Soil Sci. 80: 13. Une série d expériences en laboratoire a été conduite dans le but d explorer de façon systématique l effet de la conductivité électrique du sol sur la détermination du taux d humidité du sol par Réflectométrie en Domaine Temporel (TDR). Un instrument de mesure d humidité du sol, Moisture Point MP-917 (E.S.I. Environmental Sensors Inc., Victoria, CB, Canada) était utilisé pour mesurer les temps de propagation d une impulsion carrée le long d une sonde (ligne de transmission) immergée dans du sable fin, à différents niveaux d humidité et de salinité. L humidité volumétrique était déterminée de facon indépendante par mesures gravimétriques à l aide d une balance. A l aide de la technique brevetée MoisturePoint utilisant des rupteurs à diodes, MP-917 pouvait détecter la réflexion à la fin de la sonde, jusqu à un niveau de conductivité électrique de 15.9 ds m 1. Cependant, la relation entre humidité volumétrique et temps de propagation (exprimé sous la forme de T/T air : le rapport du temps de propagation dans le sol sur le temps de propagation dans l air, pour la même distance et le même signal) s écartait d une relation linéaire pour les valeurs de conductivité supérieures à 3.7 ds m 1. A teneur en humidité égale, le temps de propagation dans une eau saline était plus long que dans une eau non saline. En conséquence, l humidité volumétrique était sur-estimée quand l équation linéaire habituelle était appliquée. Une relation logarithmique entre teneur en eau et T/Tair a été établie, et cette relation varie avec la conductivité électrique. Avec cette nouvelle calibration, il est possible de déterminer avec précision l humidité volumétrique de sols relativement salins par la méthode de TDR. Mots clés: Refléctométrie en Domaine Temporel, temps de propagation, conductivité electrique apparente (σ), humidité volumétrique (θ), permittivité relative ou constante, diélectrique (ε r ), vitesse de propagation V p 13 Time-domain reflectometry has been used to measure volumetric water content of soil and other porous materials since 1980 (Topp et al. 1980, 198, 1994; Topp and Davis 1985; Heimovaara and Bouten 1990; Wraith and Baker 1991; Whalley 1993; Zegelin and White 1994; Nielsen et al. 1995). Time-domain reflectometry measures round-trip propagation time (time delay) of a step function propagating down and back along a probe buried in the medium. The time delay is converted to volumetric water content. The propagation velocity of a step function is related to the permittivity of the medium. Since water has a larger relative permittivity or dielectric constant (ε r ), which is close to 80 at 0 C, than that of other soil constituents (ε r = 1 for air and ε r = to 5 for dry soil), the time delay will be extremely sensitive to the amount of water present. Generally, soil texture and composition do not have large influences on time delay (Topp et al. 1980). Time-domain reflectometry has been widely accepted as an accurate, quick and non-destructive method to measure soil moisture. It has been found that there was a linear relationship between volumetric soil

2 Can. J. Soil. Sci. Downloaded from by on 01/09/18 14 CANADIAN JOURNAL OF SOIL SCIENCE water content and time delay when the latter is expressed as T/T air, where T is the time delay in soil and T air is that in air with the same distance (Hook and Livingston 1996). Soil salinity, commonly expressed in terms of electrical conductivity of soil solution (EC w ), or electrical conductivity of saturated soil extract (EC e ), or bulk soil electrical conductivity (σ) will affect TDR moisture determination in several ways. First, it attenuates the signal due to energy dissipation by current flow; second, it causes a signal dispersion, resulting in a longer rise time; third, it will increase the apparent dielectric constant (Topp et al. 1994), resulting in decrease of propagation velocity, and consequently, a longer time delay. These effects will cause a considerable measurement error if the soil electrical conductivity exceeds a certain threshold. Malicki et al. (1994) reported that the relationship between volumetric water content and permittivity of a silty loam, which was determined by time delay measurement, departs from normal when the EC w of the silty loam was above 10 ds m 1. The frequency bandwidth of the TDR instrument also has a profound influence on measurement error caused by soil electrical conductivity. Shaun et al. (1995) used a high-bandwidth (0 GHz) TDR system, and found that moisture measurements were independent of bulk electrical conductivity of the media, which was wetted with up to 0.5 M concentration of KBr solution. However, the common TDR systems have a bandwidth less than.5 GHz, and EC w of soil in many areas can be higher than 10 ds m 1 because of prolonged drought and/or abusive irrigation practice, such as in a greenhouse environment. The paper presented here explores the effect of soil electrical conductivity on water content determination for sandy soil using TDR. The study of the same effect in clayey soil is currently under investigation. THEORY 1. Propagation velocity The description of the propagation of the TDR step pulse along the parallel-conductor transmission line of the probe embedded in a material medium may be approached analytically with Maxwell s equations (Rao 1987). By using the phasor technique, the solution of Maxwell s equation is given in terms of exponential functions whose complex exponent the propagation constant captures the signal behaviors of interest for TDR purposes: the attenuation and velocity of propagation. The attenuation can be determined by using so-called attenuation constant α: ω µε σ ωε α = 1+ 1 ωε and velocity can be determined by using so-called phase constant β: ω µε σ ωε β = ωε (1) () and the phase velocity of the wave along the direction of propagation is given by V p = ω/β (3) From the propagation velocity of sinusoidal signals, the time taken for a single-frequency sinusoid to travel a length L is given by L Lβ L t = = = Vp ω µε σ ωε ωε where all of the factors are specific electrical properties of the medium: µ, permeability, it equals µ o µ r, where µ o is the permeability of free space and µ r is the relative permeability. For non-ferromagnetic such as sandy soil, µ r = 1. ε, real part of permittivity, it equals ε o ε r, where ε 0 is the permittivity of free space and ε r is the relative dielectric constant. ε, imaginary part of permittivity, σ, bulk electrical conductivity And ω is the angular frequency of the sinusoid (ω = πf, where f is the frequency).. The Effect of Electrical Conductivity on Time Delay (a) For non-conductive medium (σ = 0) The term ε in Eq. 4 represents the energy dissipation due to the relaxation of the molecules of a medium. Free water molecule has a relaxation frequency of approximately 15 GHz; however, bound water molecules have a much lower relaxation frequency in the range of several hundred MHz depending on the binding force. For TDR instruments, the operation frequency is below.5 GHz. Therefore, for sandy soil or other soils with low clay contents, ε can be neglected. However, if the soil contains large amounts of clay, there are numerous water molecules being bound to the surface of clay particles, then, the effect of ε has to be taken into consideration. For non-conductive sandy soils, both ε and σ equal zero and µ r = 1, then Eq. 4 can be simplified as: t = ( L/ c) εr where c is the propagation velocity in free space, and it equals 1/ µ 0ε0. This is the equation that has been commonly used in TDRbased moisture meters to determine the dielectric constant of the medium from measured time delay. To convert the dielectric constant to volumetric water content of the medium, an empirical relationship is used. In non-conductive medium, there is no signal attenuation since α = 0 when σ = 0 according to Eq. 1. (b) For conductive medium (σ 0) In conductive medium where σ cannot be neglected, then (4) (5)

3 SUN ET AL. ELECTRICAL CONDUCTIVITY AND TDR MEASUREMENT 15 Can. J. Soil. Sci. Downloaded from by on 01/09/18 the term σ/ωε in Eq. 4 must be taken into account. The time delay increases as the conductivity σ increases. Only in a high-frequency range, where σ/ωε << 1, the effect of σ on time delay can be neglected. The propagation time (t) of an electromagnetic wave propagating along a transmission line with length L buried in a conductive medium is given by (where ε is negligible): t L/ c εr = ( ) 1+ ( σ/ ωε0εr ) + 1 By comparing Eqs. 5 and 6, one may expect that the propagation time would increase at least by a correction factor of 1+ ( σ/ ωε0εr ) + 1. There are two challenges of directly using Eq. 6. First, TDR instruments send a step function that consists of a band of frequencies rather than a single one; second, the electrical conductivity σ might also be a function of frequency (Hasted 1973). In determining the propagation time using diode difference functions (Hook et al. 199), the intercept of the difference function base line and the line of the steepest slope of the reflection are used. The steepest slope corresponds to the maximum frequency component in a frequency band. The maximum frequency (f m ) in the frequency band can be estimated by the rise time of the reflection signal using the simple relation (Oliver and Cage 1971) (6) f m (MHz) = 0.35/rise time (microsecond) (7) As a first order approximation, the maximum frequency in a band calculated by the measured rise time can be used in Eq. 6 to estimate the time delay error caused by electrical conductivity of the medium. The effect of σ on wave propagation properties is governed by the expression (σ/ωε ) ; consequently, at high frequencies, the effect declines and is more important at low frequencies. It is found (Nyfors and Vainikainen 1989) that σ is independent of frequency in the range where its effects are observable. Therefore, it is reasonable to consider that σ is a constant with respect to frequency. 3. Signal Dispersion in Conductive Medium For a signal comprising a band of frequencies, in non-conductive medium (σ = 0), different frequency components travel at the same velocity according to Eqs. and 3, therefore, the waveform shape is maintained during propagation. However, in a conductive medium different frequency components do not maintain the same phase relationships as they propagate in the medium according to Eqs. and 3. Different frequency components travel at different velocities. This phenomenon is known as dispersion, causing distortion of the composite wave shape. The distortion causes the reflected signal to have an increased rise time, which has been shown to be a cause of increased time delay measurement error (Hook and Livingston 1996). MATERIALS AND METHODS The sand used was collected from Cordova Bay area of Victoria, BC, Canada, and passed through a -mm sieve before a particle size analysis was conducted using hydrometer method. The particle size distribution for sand was: sand, 90%; silt, 8.0% and clay.0%. The electrical conductivity of its saturated extract is 0.34 ds m 1 measured using K30 Microcomputer Conductometer (Consort pvba, Turnhout, Belgium), and the ph value for the saturated extract was 6.3. The TDR instrument used was Moisture Point MP-917 (E.S.I. Environmental Sensors Inc., Victoria, BC, Canada). MP-917 utilises TDR as its baseline technology, but it also employs switching diode technique (Hook et al. 199) to produce non-ambiguous time marks. This innovation enhanced the ability of MP-917 to detect the highly attenuated signal in high saline medium when other standard TDR instruments failed to show the reflection signal (Hook et al. 199). The probe used in this experiment consisted of two 30-cm rectangular stainless steel bars (1.3 cm in width) separated 1.5 cm by epoxy between them. Switching diodes were mounted at both ends of the probe. The probe was previously calibrated in air and distilled water. The length of cable between MP-917 and probe was.0 m. The sand was air dried for more than 1 mo and placed in a half cylinder (5.5 cm in length and 15. cm in diameter PVC pipe) (Fig. 1). The total volume of sand was 4.76 L. The probe was installed in the middle of the half cylinder. With this configuration, all of the sand was included in the sensitive range of the probe. This was further demonstrated by the observation of only m 3 m 3 to m 3 m 3 increase in indicated volumetric water content when a beaker of water was placed on the top of the half-cylinder. That is, bringing a volume of water next to the volume occupied by the experimental sand had negligible effect on the measurement. The whole apparatus was set horizontally on a balance (Ohaus, Florham Park, NJ, USA). The probe was connected to an MP-917 and a computer with the balance connected to a second computer. This set-up supported continuous measurement and logging of the time delay through the moist sand and its changes in weight. At the beginning of the experiment, 1.43 L of distilled water was added to the sand and the volumetric soil water content measured at 0.30 m 3 m 3 using the balance. The cylinder was then covered with a plastic sheet to prevent water loss through evaporation. Continuous monitoring of the time delay measurement showed that 5 to 6 h after adding water the reading from MP-917 became stable, indicating that a stable water distribution had been reached. After an additional equilibrium interval of several hours, about 00 sequential time delay readings were obtained and the average calculated. The plastic sheet was then removed to expose the sand to the air, and a fan was used to blow the air across it to facilitate evaporation. Under these conditions, the moisture content of the sand decreased by approximately 0.03 m 3 m 3 in 8 h. The cylinder was then re-covered and the automatic acquisition and logging of the propagation time and weight readings re-started. After an equilibrium interval of more

4 16 CANADIAN JOURNAL OF SOIL SCIENCE Can. J. Soil. Sci. Downloaded from by on 01/09/18 Fig. 1. The set up of the experiment. Fig.. The relation between T/T air and volumetric soil water content when the electrical conductivity of saturated soil extract is equal to or less than 3.7 ds m 1. Fig. 3. Volumetric soil water content measured using balance (Y) vs. that measured using MP-917 (X) when the electrical conductivity of saturated soil extract is equal to or less than 3.7 ds m 1. than 4 h, about 00 data points were collected and averaged. The volumetric soil water content was then calculated using the appropriate equation (Hook et al. 199) from the time delay readings obtained from the MP-917 and the weight readings obtained from the balance. This procedure was repeated until the volumetric soil water content reached approximately 0.05 m 3 m 3. The time to equilibrium increased as the soil became drier because of the decrease of

5 SUN ET AL. ELECTRICAL CONDUCTIVITY AND TDR MEASUREMENT 17 Can. J. Soil. Sci. Downloaded from by on 01/09/18 Table 1. The overestimation of water content determined by TDR as soil water content and salinity increase Electrical conductivity of saturated extract 6.37 ds m ds m ds m ds m ds m m 3 m 3 (0.004) z (0.007) (0.009) (0.007) (0.013) 0.08 m 3 m 3 (0.01) (0.017) (0.016) (0.017) (0.048) 0.10 m 3 m 3 NA NA NA (0.079) (0.115) 0.16 m 3 m 3 (0.01) (0.019) (0.03) NA NA m 3 m 3 NA NA NA (0.17) (0.184) m3m 3 (0.06) (0.057) (0.066) NA NA 0.00 m 3 m 3 NA NA NA (0.183) (0.14) 0.10 m 3 m 3 (0.04) (0.058) (0.067) NA NA 0.50 m 3 m 3 (0.09) (0.043) (0.071) (0.185) (0.334) 0.70 m 3 m 3 NA NA NA NA (0.396) m 3 m 3 (0.084) (0.13) (0.148) (0.33) NA z The numbers in parentheses are the overestimations of volumetric water content in unit of m 3 m 3 unsaturated soil hydraulic conductivity. For example, when the soil moisture level was below 0.10 m 3 m 3, the time to equilibrium gradually increased to more than 7 h. In a sand, pore water hysteresis is greater than in most soils, and this may result in differing spatial moisture distribution. In order to remove the effect of the hysteresis, the above procedure was repeated but in the opposite direction, from dry to wet. The same sand column was then oven dried at 105 C for 48 h and the soil moisture was gradually increased at increments of 0.05 m 3 m 3 to 0.30 m 3 m 3 by adding distilled water. At each moisture level, about 00 propagation time readings were taken after a stable moisture distribution had been reached. The difference in measured time delay for these two procedures was approximately 170 ps, which translated into 0.01 to 0.0 m 3 m 3 in soil moisture. Normally, the propagation time measurement in the drying procedure was longer than that in the wetting one. The mean of time delay was used in analysis. The different soil salinity was generated by adding different concentrations of salt solution (NaCl) instead of distilled Fig. 4. The relationship between T/T air vs. volumetric water content when the electrical conductivity of saturated soil extract (EC e ) exceeds 3.7 ds m 1. water to the soil. In order to maintain a constant salt quantity in soil at different water contents, at the beginning of the experiment approximate g of NaCl was added to 149 ml of distilled water, which would bring the volumetric soil water content to 0.30 m 3 m 3 (close to its saturation) with 1.54 ds m 1 electrical conductivity for saturated soil extract (EC e ). As soil dried the bulk soil electrical conductivity decreased; however, the amount of salt in the soil remained unchanged as well as the electrical conductivity of soil-saturated extract. As the experiment progressed, the same soil sample had additional salt added to produce higher EC e levels. For a desired EC e for an increased salinity series, the total salt was calculated, then an amount required in addition to that already present from the previous series was added. The whole database included eight EC e levels and soil moisture ranged from dry to 0.30 m 3 m 3 at each EC e level. The saturated extract was collected by first wetting 0.5 L saline soil to saturation, the soil was then placed in a funnel and filtered by a quantitative 1 filter paper. A vacuum pump

6 18 CANADIAN JOURNAL OF SOIL SCIENCE Table. The comparison of calculated time delay using Eq. 6 and measured one at 0.30 m 3 m 3 volumetric soil water content with different electrical conductivity Difference Calculated time between f MAX delay (ns) Measured measured and σ (ds m 1 ) Rise time (ns) (MHz) using Eq. 9 time delay (ns) calculated Can. J. Soil. Sci. Downloaded from by on 01/09/18 was used to facilitate the extraction. The electrical conductivities of the extract EC e were measured using a K30 Microcomputer Conductometer (Consort pvba, Turnhout, Belgium). They were (at 0.30 m 3 m 3 volumetric soil water content): 0.34 ds m 1, 1.54 ds m 1, 3.7 ds m 1, 6.37 ds m 1, 9.7 ds m 1, ds m 1, ds m 1 and 15.9 ds m 1 generated by adding an appropriate amount of salt. The time delay in the saline soil was measured using the same MP-917 and probe as mentioned before. All the above experiments were conducted in ESI research laboratory at room temperature of to 3 C. RESULTS AND DISCUSSION 1. For Soils with Low Electrical Conductivity (ECe 3.7 ds m 1 ) The results of time delay (T/T air ) and volumetric soil water content measurements for soil with low EC e (0.34 ds m 1, 1.54 ds m 1, and 3.7 ds m 1 ) were pooled together, because the relationship between T/T air and volumetric water content all appeared linear in this electrical conductivity range (Fig. ). At next electrical conductivity level where EC e = 6.37dS m 1, this relation obviously deviated from linear. The slope was smaller than the published (Sun 1995; Hook and Livingston 1996). However, if the point at the upper right corner was excluded, then the equation became y = 0.133x The slope was very close to the published one. The intercept of was very close to from another experiment by Sun (Sun 1995). The point at the upper right corner represented the measurement at 0.30 m 3 m 3 water content and 3.7 ds m 1 EC e conditions. TDR overestimated approximately 0.05 m 3 m 3 in water content at this condition. This indicated that salt started affecting the measurement at relatively low concentration if the water content was high. In general the volumetric water content determined using MP-917 agreed well with that measured using the balance (Fig. 3). If the upper right point was excluded, then the published linear calibration of time delay vs. volumetric water can be used for saline soil when EC e is no more than 3.7 ds m 1. As soil dries, the concentration of the soil solution increased; however, the impedance (tortuosity) associated with current flow through the water phase increases as well, and this offset the effect of the increase in ionic concentration on bulk soil electrical conductivity. The lack of influence of electrical conductivity on time delay measurement using TDR when EC e was smaller than 3.7 ds m 1 could be further demonstrated by the following calculation. Assuming that the volumetric soil water content θ v is 0.30 m 3 m 3, EC e was 1.54 ds m 1, then the calculated bulk soil electrical conductivity σ using Archie s formula (Ferré at al.1998) would be 0. ds m 1. The measured rise time was.783 ns and the calculated f m according to Eq. 7 is 16 MHz. This caused 14 ps longer in time delay (calculated using Eq. 9) than that in nonsaline soil with the same water content. The difference was within the accuracy of MP For Soils with High Electrical Conductivity (ECe 6.37 ds m 1 ) There was an overestimation of volumetric soil water content using TDR when the electrical conductivity of soil solution at saturation was above 6.37 ds m 1 and the magnitude of overestimation increased as EC e increased at moisture saturation (Table 1). Two general patterns are shown in Table 1. First, at each EC e level, the overestimation increased with increase of soil water content. Second, at each water content, the overestimation increased with increase of electrical conductivity. Both patterns could be explained by an increase in ionic conductivity in soil either by increasing the amount of conductive carriers, and/or by increasing the facility of migration of the ions as soil moisture increases. Both mechanisms would increase σ, resulting in a longer time delay according to Eq. 6. The overestimation was as large as 0.40 m 3 m 3 at 0.70 m 3 m 3 moisture level (balance measured) when the EC e reached 15.9 ds m 1. There were several data in Table1 out of the expected descent/ascent pattern. For example, at 9.7 ds m 1 conductivity, the overestimation at 0.5 m 3 m 3 moisture level was less that at 0.1 m 3 m 3. At 0.08 m 3 m 3 moisture level, the overestimation at ds m 1 conductivity was less than that at 9.7 ds m 1 level. The cause of the inconsistency might be the results of uneven distribution of water and ions, and/or different soil compacting in different measurements since the same soil sample had been repacked at each salinity level. However, the inconsistency was not detectable when the soil water content was above 0.08 m 3 m 3 with EC e above ds m 1. This indicates that at this moisture and salinity levels, the overestimation in water content due to electrical conductivity overwhelmed the errors caused by other reasons. Clearly, TDR cannot measure volumetric soil water content correct-

7 SUN ET AL. ELECTRICAL CONDUCTIVITY AND TDR MEASUREMENT 19 Can. J. Soil. Sci. Downloaded from by on 01/09/18 Fig. 6. The comparison of reflected signal between EC e = 15.7 ds m 1 and 0.34 ds m 1 at 0.1 m 3 m 3 water content. ly without a suitable calibration when the soil electrical conductivity and moisture exceed a certain threshold. When the EC e reached 6.37 ds m 1 or higher, the relation between T/T air vs. θ v deviated from linear, and the deviation increased as the soil electrical conductivity elevated (Fig. 4). As conductivity increased, time delay increased significantly. For example, at 0.0 m 3 m 3 volumetric water content, the measured time delay expressed as T/T air increased from when EC e = 6.37 ds m 1 to when EC e = 15.9 ds m 1, and this would translate into approximately 0.18 m 3 m 3 increase in displayed moisture reading. The reason for a non-linear relation at high soil electrical conductivity can be explained by the fact that the effect of addition of salt on time delay increases with increasing soil water as the ions become more mobile. In this situation, the Fig. 5. The relationship between the normalized rise time (the ratio of rise in saline soil to that for no saline dry soil), soil water content and electrical conductivity. increase in time delay (T/T air ) is no longer directly proportional to the increase in water content. A large portion of the increase of time delay is attributed to the soil electrical conductivity. Figure 4 shows that at the same soil water content, the time delay increases with the increase in soil electrical conductivity. On the other hand, soils with different water content may correspond to the same time delay when their electrical conductivity is different. It is interesting to see whether Eq. 6 is adequate to explain the increased time delay in highly conductive soils when using TDR. Calculated and measured time delays for soil with 0.30 m 3 m 3 water content but different electrical conductivity are summarized in Table. The ε r in Eq. 6 can be determined by using the relationship between volumetric water content θ V and travel time T for non-saline soils, if θ V

8 0 CANADIAN JOURNAL OF SOIL SCIENCE Can. J. Soil. Sci. Downloaded from by on 01/09/18 is known. The first column is the bulk electrical conductivity σ calculated using Archie s formula. The second column is the rise time that can be measured using fine scan in Moisture.Point MP-917. The third column is the maximum frequency calculated using Eq. 7. Table shows that using Eq. 6 alone was not adequate to explain the increased time delay in highly conductive soils. For example, when σ = 1.1 ds m 1, there was still 1.91 ns increased time delay unexplainable by applying Eq. 6. The divergence between the calculated and measured time delay increased when soil electrical conductivity increased. The remaining overestimation might be explained by: (a) The rise time of the reflection signal increases significantly when soil electrical conductivity increases. The slope of the reflection signal becomes so low that identification of the portion of the waveform with the steepest slope becomes very difficult. The commonly used method to identify the time mark by intercepting the steepest slope and time base line might be not valid when the rise time becomes very large. There may be an increase of rise time related measurement error as mentioned before (Ferré et al. 1999). Figure 5 showed that the rise time of the reflected signal (estimated from the Fig. 7. (a) The relationship between electrical conductivity of saturated soil extract and coefficient C 1. (b). The relationship between electrical conductivity of soil solution and coefficient C. TDR trace displayed using MP-917 soil moisture instrument) increased with the increase of soil moisture and soil electrical conductivity. More research is needed to explore the relationship between the rise time and measurement error. (b) In TDR, the transmitted signal comprises a band of frequencies. According to Eq. 6, the time delay would be longer for low frequency components. In our calculation, only one frequency, the maximum frequency was used, and the effect of the low frequency components was not considered. (c) The relaxation related dielectric loss ε is neglected in the above calculation. The calculated time delay would have been be longer if this term had been included. Figure 6 shows the superimposed actual reflecting signals from sandy soil with EC e of 0.34 ds m 1 and 15.7 ds m 1 at 0.1 m 3 m 3 volumetric water content. It clearly shows increases in rise time, signal attenuation and time delay due to the raise of salinity, if the conventional steepest slope line method was applied. Even when the water content was the same, the time delay (T/T air ) increased by 0.91; that translates into m 3 m 3 of overestimation of water content.

9 SUN ET AL. ELECTRICAL CONDUCTIVITY AND TDR MEASUREMENT 1 Can. J. Soil. Sci. Downloaded from by on 01/09/18 3. The Logarithm Calibration The results indicate that using the linear relationship between T/T air and θ v plus theoretical correction using Eq. 6 is not adequate to explain all the increase in time delay in highly conductive media. Our experimental data suggest that it is better to use a logarithm calibration (Fig. 5). All the calibrations shown in Fig. 5 were conducted on sandy soil and the upper limit for EC e was 15.9 ds m 1. The general form of the logarithm calibration is given as θ v = C 1 ln(t/t air ) + C (8) where coefficient C 1 and C are functions of electrical conductivity of soil saturated extract. The dependence of C 1 and C on ΕC e is shown in Fig. 7. As the conductivity increases, C 1 declines and C increases (less negative). Using Eq. 8 to obtain correct moisture readings in saline soil requires the information of soil electrical conductivity. This can be achieved either by a direct measurement using salinity sensors or estimated from a TDR wave trace (Dalton et al. 1984; Topp et al. 1988). Nevertheless, Figs. 4 and 7 provide useful information that can be used to eliminate the discrepancy between the actual soil moisture and that measured using TDR in saline soil conditions. For the medium with conductivity above 15.9 ds m 1, the signal will be greatly attenuated according to Eq. 1 and final reflection is barely detectable unless the TDR probe is coated with electrical insulation material with the expense of reduced instrument sensitivity. It is necessary to conduct a separate calibration for coated probe. CONCLUSIONS The effect of soil electrical conductivity on volumetric soil water content determination using TDR has been studied. A very large amount of data for soil water content, soil electrical conductivity and time delay for sandy soil have been collected, and the empirical logarithmic calibrations among these three have been established. The ordinary linear relation between T/T air and soil water content θ is not valid as the electrical conductivity of saturated soil extract (EC e ) at saturation is above 6.37 ds m 1. The overestimation of soil water content is attributed to the increase in soil electrical conductivity and increase in rise time of the reflected signal, which lead to a longer time delay. The correction of the soil water content reading obtained using TDR in saline soils can be conducted by using the logarithmic calibration presented, but it requires an independent measurement of EC e or σ at the same time. ACKNOWLEDGEMENTS We thank Dr. Nigel Livingston, Biology Department, University of Victoria, for permitting the use of equipment. 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