Soil Science Society of America Journal 66:735-743 (2002)
© 2002 Soil Science Society of America
DIVISION S-1—SOIL PHYSICS
Horizontal and Vertical TDR Measurements of Soil Water Content and Electrical Conductivity
Arie Nadler*,a,
S. R. Greenb,
I. Vogelerb and
B. E. Clothierb
a Soil and Water Institute, Agricultural Research Organization, Ministry of Agriculture, State of Israel, POB 6 Bet Dagan, Israel, 50250
b Environmental group, HortResearch, Private bag 11-030 Palmerston North, New Zealand
* Corresponding author (vwnad{at}agri.gov.il)
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ABSTRACT
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When time domain reflectometry (TDR) is used to measure soil water content (
) and salinity, the probes can be installed either horizontally or vertically. We tested the common convention that values averaged from horizontal probes will be equal to a vertical direct measurement. In a laboratory experiment, a sandy loam soil, packed uniformly (0.02-m layers to 0.18-m depth) into a box, was gradually wetted to saturation by CaCl2 solutions of 0, 0.5, 1.1, 2.1, 3.1, 4.5, 5.9, and 8.4 dS m-1. Horizontal and vertical TDR probes for measuring and electrical conductivity of the bulk soil (
a) were installed during soil packing. A comparison between values averaged from three horizontal probes and from two vertical ones showed deviations of 0.02 (L L-1). A simple water redistribution model was used to attribute this deviation to the process of averaging the horizontal results. The best practical attainable reproducibility, under our experimental conditions, were close to the theoretical limit 0.005 (L L-1) but the average experimental reproducibility of was 0.01 to 0.02 (L L-1). Width of the soil layer affecting the moisture measurement was reconfirmed to be close to 30 mm. The resistors-in-series model was found to be a good approximation to describe the soil profile a from separately measured horizontal a. Final values of the electrical conductivity of the soil solution (w) after sufficient leaching were in good agreement with w values calculated by an empirical protocol that uses a, , and a soil texture property.
Abbreviations: DW, distilled water • OM, organic matter • TDR, time domain reflectometry • , soil water content • 
, dielectric constant • a, electrical conductivity of bulk soil • w, electrical conductivity of soil solution
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INTRODUCTION
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USEABLE GRADE WATER is a precious and extinguishing commodity in many semi-arid and arid parts of the world. Efficient use of this resource may be achieved through optimizing irrigation by closely monitoring the . The TDR technique is suitable for accurate and automatic monitoring of the soil moisture.
Basically, there are two ways to install a TDR probe in the soil, horizontally or vertically. It is commonly accepted that the horizontal option should be preferred where one is after a detailed profile of the and salinity of the w. This orientation takes advantage of the fine depth resolution characteristic to this technology. On the other hand, a vertical installation is recommended, for integrating the usually nonuniform soil moisture distribution.
Vertical probes inserted at the ground surface and extending to the depth of interest can be used to construct a water storage profile (Ferre et al., 1998) and for monitoring solute transport (Kachanoski et al., 1992, Baker and Spaans, 1994; Young et al., 1997b). Horizontally installed probes will interrogate a small region whose dimensions will be dependent on the rods diameters, spacing, and length. The ability of these probes to give a true description of water content and residence concentration distributions will be dependent on the number of probes one is willing to install horizontally. Field installation of horizontal probes is destructive and one has to weigh the advantage of data resulting from such an installation with the disruption of flow fields. Discrepancies may be observed when natural systems get more complicated because of layering (of texture, , or salinity), when higher accuracy is required (like in sandy soils), or in exceptional soils (rich in Fe oxides or organic matter [OM]). Deviations from Topp's ‘universal’ calibration were attributed to several factors: (i) presence of rare clay minerals (Dasberg and Hopmans, 1992), (ii) high OM contents (Roth et al., 1992), (iii) experimental errors like Gregory et al. (1995) stemming from electronics and software (contributed to the error ±0.0022 [v/v]), positioning of the probes (±0.015), and forming a cavity at the rods tip (contributed ±0.012), (iv) variability in texture and nonuniform density (contributing to the error in the dielectric constant [] and estimated by Perdok et al. [1996] to be ±1 dielectric unit for sand, up to ±2.5 for a sandy loam, and ±5 dielectric units for a clay), and (v) variability in bulk density (a 0.1 kg L-1 change Db cause a ±1–2 dielectric units in ).
The hypothesis that vertically and horizontally installed probes give the same results was tested both, theoretically and experimentally. Ferre et al. (1996) stated that the theoretical model of a square root of the soil conforms to the relationship determined by Topp et al. (1980). Recently it was experimentally shown by Young et al. (1997a), (while introducing an easy and faster – calibration procedure, based on upward infiltration) that vertically installed TDR probes accurately integrated (compared with water mass measurements by weight) distribution along the probe rods, for three soil types, even when embedded in extremely different moisture levels. Furthermore, vertical 800-mm long TDR probes, installed in a weighting lysimeter to measure water storage, accurately followed water losses as long as the probe was long enough to sample the dynamic depth (Young et al., 1997b). Also Baker and Spaans (1994) successfully measured water storage with vertical TDR probes in microlysimeters.
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Crossing over of Soil Water Content Curves
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Several experimental studies have presented averaged TDR horz curves intersecting the curves of obtained gravimetrically. In most cases, being secondary in magnitude, this mismatch was mostly ignored: horizontally installed TDR probes (Wraith and Baker, 1991) at five depth intervals were used to calculate water loss by plant consumption which was compared with changes in water weight. Time domain reflectometry-determined water changes were alternately below (the maximum values) and above (the minimum values), crossing daily the balance-obtained values (Wraith and Baker, 1991, their Fig. 5).
Jones and Friedman (2000) measured in glass beads the effect of probe orientation on the values and showed that when plotting the horizontal and vertical data as a function of , the curves cross each other. The same happened in the experimentally measured eff and the predicted by effective medium approximation (EMA) model for three mica flakes differing in diameters. Curves crossing of - were obtained for different soil types (Wang and Schmugge, 1980) and could not be explained by several models of soil/water mixing. We will suggest that the alternating deviations to be because of the averaging process.
Considering that the TDR pulse, advancing along the probe, integrates linearly the values along the rods, the vertically measured values (vert) will be used as reference. The present study compares vertically and horizontally obtained and is not comparing TDR with gravimetric sampling.
Having the alternative installations, we would like to know how close are water storage values obtained from these two options. Namely, will an average of values from horizontally installed probes be the same as a single vertical one sampling the same layers, to the same depth.
A TDR probe can simultaneously measure also the load impedance of the medium in which it is embedded. This impedance is the total opposition to the flow of electrical energy in the transmission line. It is composed partly of the reactance (opposition to alternating current) and partly of the resistance (opposition to the direct current). The impedance is a useful parameter because from it, by knowing the length, spacing, and diameter of the probe's rods, the bulk soil electrical conductivity can be measured (Ward et al., 1994). When the measured soil is not uniform it can be assumed to be made up of narrow uniform layers, one on top the other. For the propagating electromagnetic pulse, these layers may be represented as an array of resistors. Two basic arrangements for an array of resistors are the series or the parallel models (Shainberg and Levy, 1975).
Our objectives were: (i) to verify if the horizontally installed probes can accurately integrate the variable water content down the medium in which they are embedded, (ii) to find which of the two resistors combination models (series or parallel) results in values closer to the vertically measured a, and (iii) to evaluate the practical reproducibility of and a measurements under optimal conditions of a controlled, uniformly packed disturbed soil, where the dimensions of the sampling probe and total soil volume are similar. The quality of the data thus obtained may serve as the upper limit of accuracy for field applications.
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MATERIALS AND METHODS
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The 0.15- to 0.30-m layer of the local soil (Manawatu, fine sandy loam a Dystric Eutochrept) was sampled, air-dried, sieved through 2 mm, and packed into a box of 0.24 by 0.12 by 0.20 m to its natural density (1.39 Mg kg-1). The experimental box had holes at the bottom that enabled free water drainage but was not in contact with any soil. A plastic sheet covered the boxes and its contents to minimize evaporation. A fresh soil sample was used for each run. Five TDR probes, constructed in our laboratory (three 2-mm rods, 150 mm long, 12.5-mm rod spacing) were installed in each box during soil packing: Three were installed horizontally, at the center of the layers 0 to 0.05, 0.05 to 0.10, and 0.10 to 0.15 m, and two were installed vertically between the later and the partition, far enough not to be in each other's sphere of influence. A coarse filter paper was placed over the soil surface to maintain a uniform wetting and avoid crust formation.
The geometric factors (Kc = solution x Impedance, Ward et al., 1994) of the self-produced TDR probes were determined separately for each probe by immersing them in three solutions of known (0.212, 0.245, 1.08 dS m-1). The average value for the 20 probes used was 36.6 ± 0.74 dS (m x ohm)-1. Calcium chloride solutions of eight different concentrations (Distilled water [DW], 0.5, 1.1, 2.1, 3.1, 4.5, 5.9, and 8.4 dS m-1) were prepared and spiked with chloroform to reduce biological activity. Generally, every morning 0.100 L of each solution was evenly applied over the packed soil surface. At high levels, towards the end of the run, 0.200 L portions were used, totaling to 
2.500 L per run.
Soil water content and the a were measured three times daily: Just before the solution was applied, an hour later, and about 6 to 8 h afterwards. This schedule supplied the extreme values (possible under the experimental conditions) and an intermediate one.
Semi-automatic measurement technique was used. The probes were manually connected consecutively via a 50 
coaxial cable to a cable tester, and a computer automatically analyzed the traces for and a. The a was calculated from the ratio between reflection coefficient values determined at two locations on the TDR trace; one, where the coaxial cable is connected to the probe's rods and the second at an infinite distance relative to it (usually six to eight times the probe's apparent length). Room temperature was measured and data results were adjusted to 25°C.
During the first runs, (using solutions 0.5, 1.1, 2.1, and 4.5 dS m-1) and a were measured both, manually and semi-automatically. When it was found that the difference between the two is <0.01 (m3 m-3) we dropped the manual option. The advantages of the semi-automatic procedure are speed, repeatability, and the fact that each trace is recorded, enables recalculation. The software driving the semi-auto measurement was locally written (Dr. S. Green, Palmerston North, New Zealand). The probes' starting point is considered constant and is determined once, as accurately as possible. The probe's end (reflection point) is obtained by interpreting the trace's inflection point.
Measuring Electrical Conductivity of Bulk Soil
At low frequencies the load impedance (Z) is dependent only on the direct current component and is equal to the resistance (R). Since the lowest frequency of the TDR signal is associated with the longest travel time, Z should be measured at the longest travel time (Ward et al., 1994). The a is easily calculated from the linear relations between R and a which depend on the probe's rods spacing (d) and cross-sectional surface area (A) expressed as by R = d/(a x A).
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RESULTS
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Water content
Water content changes with time during a typical run where the solution was sequentially added, is shown in Fig. 1
. The final (saturation) value, 0.42 ± 0.01 (m3 m-3) was common to all runs and similar to known value for the soil. As expected, changes were sharper for the 0.025-m probe and smoothed out with depth. The increase in measured daily values (0.06–0.07 m3 m-3) matches qualitatively the calculated values (by dividing 0.10 L [added] by 1.40 L [the layer's volume]). For > 0.20 the daily intermediate values (usually 6–8 h after water application) can be clearly seen. From the match between both (i) the daily increase after each water addition (0.038 for 0.025-m probe plus 0.017 for the 0.075-m) and (ii) the final values (0.42) it can be estimated that the practical reproducibility of measurement is ±0.005 (m3 m-3). Accumulated water amount added to two boxes at two separate runs (solutions were DW and 3.1 dS m-1, Fig. 2)
calculated from the vertical probes and compared with the actually added volumes, shows the same reproducibility.
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Fig. 1. Changes in volumetric water content with time of Manwatu soil monitored by horizontally (shallow: 0.025 m, middle: 0.075 m, and deep: 0.125 m) and vertically (average of two) installed TDR probes during wetting with a 1 dS m-1 CaCl2 solution (see text).
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Fig. 2. Cumulative water manually added, and calculated from measurements by vertically and horizontally installed TDR probes.
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Bulk Soil Electrical Conductivity
Daily a values measured with the horizontal and vertical probes (compared in Fig. 3)
for w = 3.1 follow the same pattern shown by regarding probes order of response to wetting, sharpness of daily values as a function of probe's depth, and the ability to detect the intermediate value. This is because of the dependence of on . For wetting solutions DW, 0.5, 1.1, 2.1, 3.1, 4.5, 5.9, and 8.4 dS m-1, the final a values were 0.22, 0.6, 0.5, 0.8, 1.05, 2.1, 1.9, and 2.55 dS m-1 (not shown).
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Fig. 3. Changes in bulk soil electrical conductivity (a) with time of Manwatu soil monitored by horizontally (0.025, 0.075, and 0.125 m) and vertically (average of two) installed TDR probes during wetting with a 3 dS m-1 CaCl2 solution (see text).
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In runs with dilute solutions (0.5, 1.1, and 2.1 dS m-1, not shown) a values for the shallow probes reached a temporal maximum value because of salt accumulation by leaching at the wetting front. During solution addition, and a distribution is expected to differ. While water is accumulating monotonously until reaching the maximal value (saturation), a value is determined by two parameters, the moisture level at each horizon and its salt load, and the original salts present in the natural soil and salts that are leached down by the wetting front. This phenomenon was most pronounced with DW (Fig. 4)
and was felt also by the 0.125-m probe. In the 0.025-m probe, a reached 0.14 dS m-1 and dropped to 0.12 dS m-1, in the 0.075-m probe, a reached 0.2 and dropped to 0.12, and in the 0.125-m probe reached 0.31 and later even 0.51 dS m-1 while the average a value of the vertical probes was 0.23 dS m-1.
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Fig. 4. Changes in bulk soil electical conductivity (a) with time of Manwatu soil monitored by horizontally (0.025, 0.075, and 0.125 m) and verically (average of two) installed TDR probes during wetting with a distilled water (see text).
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DISCUSSION
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Horizontal Water Content Measurements
The sensitivity of the TDR probe to water content is defined as the change in the measured travel time for a unit change in the average soil water content. Accordingly, the sensitivity of the probes to soil water content that vary along the rods was shown to be constant, regardless of the water content distribution.
Our averaged horz values were both above and below the vert (the selected reference), indicated graphically by crossings between the two curves (Fig. 5A)
. Data scatter is probably related to errors (see above) stemming partly from operator's errors in measuring and interpreting waveforms, and partly from the error in depth of placing the horizontal probes, such that the spacing is not exactly 0.050 m and also the integrated soil volume sampled by the two orientations do not fully overlap.
Expected changes during progressive wetting (in this study) were estimated from a simple numerical calculation that assumed (i) a piston flow, (ii) a step function of water transport, and (iii) that the rate of the vertical movement of water is proportional to the square root of time (t1/2) (Fig. 6B)
. Depth of box in the numerical calculation was 0.15 m (similar to the experimental box) and it was divided into virtual layers of 0.002 m. Two extreme values were chosen to represent : saturation (0.42 L L-1) and air-dry (0.05 L L-1), as was the case in reality. Depth of wetting front was chosen to be dependent on t1/2. For representing the averaged soil layer width of the 0.025, 0.075, and 0.125-m horizontal probes the 5, 10, 15, and 20, 0.002-m wide layers both, above and below the probes depth, were averages to create the influencing soil widths of 0.01, 0.02, 0.03, and 0.04 m respectively, above and below the probes' plane (Fig. 7) . The resulting distribution was averaged at 30 consecutive time occasions (Fig. 6B). Averaged experimental results of horz and vert for the same stages are presented in Fig. 6A. The similarity between these two figures supports the use of the same reasoning to explain the experimental results.
Width of TDR Sampled Soil Layers
The width of the sensitivity region normal to the plane containing the probe's rods is 30 mm, indicating that the method should allow a fine depth resolution when installed horizontally. This value is similar to Baker and Luscano's (1989) who found in a laboratory experiment that the probe sensitivity is largely confined to a region with a cross sectional area of 1000 mm2 surrounding the rods and a limited sensitivity extends much further enclosing 3500 to 4000 mm2. The standards of our experiments were not high enough to enable an exact definition of the sampled-soil effective distance away from the TDR rods. Still, a rough estimate can be found (Fig. 7) by visually considering the increasing deviation between the calculated vert and horz when the width of the layer contributing to the measurement is decreased from 0.04 to 0.01 m. At least under our experimental conditions, Plot 8C is most similar to 6A implying that the soil is sampled not more than 0.015 to 0.020 m away from the rods plane.
Accuracy of Soil Water Content Measurement
The practical lower limit of accuracy of determining (of this study) is half the inter-pixel distance, in our case -0.005 L L-1. The average experimental lower limit of reproducibility is ±1 to 2%. That is because of the unavoidable nonuniform packing (even under careful dry soil packing), trace interpretation (visual or automatic), and the wetting process. It falls within the range reported by previous studies (Table 1).
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Table 1. Average and maximal experimental error reported from laboratory or field measurements, for uniform and layered soil profiles.
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Impedance can be measured with TDR if the attenuation of the electromagnetic waves is proportional to the total amount of solute within the volume of influence of the wave-guide. The attenuation of the electromagnetic waves is constant as long as the solute remains above the end of the probe, as demonstrated by Kachanoski et al. (1992) and Vanclooster et al. (1993) for vertically and horizontally inserted TDR probes, respectively. Horizontally installed TDR probes have been used to measure the temporal development in resident solute concentration and to evaluate transport processes (Ward et al., 1994, Vanderborght et al., 1996).
Validating Electrical Conductivity of Bulk Soil Integration
The resistors-in-series or -in-parallel (Shainberg and Levy, 1975) approach for calculating the combined values of several overlying soil horizons was experimentally tested (Fig. 8)
. The electrical conductivity of bulk soil horizontally calculated by the series model gave better predictions than the parallel model and were consistently 10% lower than the a vert, but under extremely nonuniform situations the difference is significantly higher (Fig. 8, Days 5 to 10).
Electrical Conductivity of Bulk Density of the Soil Solution
While collecting and a data for a soil wetted by solutions of known concentrations, we could not resist the temptation to apply an existing protocol (Nadler et al., 1984) that calculates the of the soil solution (w) from the available data.
The above protocol processed the experimental data into w values (Fig. 9)
using a single value (air-dry water content) to identify the soil texture. The relations between final w values for all eight experimental runs, wetted by the different solutions, are presented in Table 2. The agreement between the expected values (the w of the applied solutions), and those calculated using the approach of Nadler et al. (1984) is good. This should encourage the use of this simple approach for other mineral soils. For extremely different soils (e.g., high in OM content) the approach has yet to be tested.
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Fig. 9. Changes in salinity of the soil soltion (w) of Manwatu soil with time monitored by horizontally (0.025, 0.075, and 0.125 m) and two verically installed TDR probes during wetting with distilled water (see text).
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Table 2. Range of electrical conductivity of soil solution (w) (dS m-1) values calculated for: (i) each separate horizontal layer, (ii) average of separate horizontal values, and (iii) average of the two vertical probes.
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In conclusion, although the predicted lower limit of accuracy for measuring is practically attainable, it would be more realistic to expect measurement errors of at least 0.01 to 0.02 (L L-1). It is advisable to select the orientation of installing the TDR probe according to the purpose of the study.
The simple approach for calculating w developed by Nadler et al. (1984) could be used successfully to calculate w from TDR-measurements of and a in the local Manawatu fine sandy loam soil. Readers are thus encouraged to try the protocol for calculating w for their soils, keeping in mind that some site specific adjustments may be needed.
Received for publication January 19, 2001.
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TOP
ABSTRACT
INTRODUCTION
) and two vertical probes (
) monitored 
