Published online 16 November 2005
Published in Vadose Zone J 4:1030-1036 (2005)
DOI: 10.2136/vzj2004.0144
© 2005 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
SPECIAL SECTION: SOIL WATER SENSING
Monitoring Soil Water Content Profiles with a Commercial TDR System
Comparative Field Tests and Laboratory Calibration
Jean-Paul Laurenta,*,
Pierre Ruelleb,
Laurent Delageb,
Abdelaziz Zaïric,
Béchir Ben Nounac and
Tarek Adjmic
a Laboratoire d'étude des Transferts en Hydrologie et Environnement (CNRS-INPG-IRD-UJF), BP53 F-38041 Grenoble-Cedex 9, France
b 361, Unité de Recherche, "Irrigation," CEMAGREF, Rue J-F Breton BP 5095 F-34033 Montpellier-Cedex 1, France
c INRGREF, Rue Hédi Karray BP10 Ariana 2080 Tunis, Tunisia
* Corresponding author (jean-paul.laurent{at}hmg.inpg.fr)
1 Mention of trade names does not constitute a guarantee or warranty of the product by the French CNRS, IRD, or CEMAGREF and does not imply its approval to the exclusion of other products that may be available. 
Received 30 September 2004.
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ABSTRACT
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The TRIME-FM3 (Imko GmbH, Ettlingen, Germany) time domain reflectometry (TDR) handheld instrument combined with the T3 access tube probe is a commercial soil water content (SWC) profiling system. The main purpose of this study was to evaluate its performance under real field conditions. The evaluation was performed on four sites in Tunisia and France where several measuring campaigns were organized between 2000 and 2002. Reference SWC measurements were also taken systematically. Direct comparisons of the corresponding data showed that a linear correction was generally sufficient to locally calibrate the TRIME (time domain reflectometry with intelligent microelements) measurements. Nevertheless, because of the influence of other interfering factors, a significant error (0.01 < RMSE < 0.07) still remained after this correction. As a possible alternative, the internal standard TRIME-FM3 calibration was also examined. On the basis of our large set of data, new calibration coefficients were derived to convert the TRIME-FM3 internal measurement (a so-called pseudo transit time) into the displayed SWC. However, its general validity should be tested with other independent data. Several laboratory experiments were also performed: TRIME-tube measurements were taken on reference media and on a 10-L sample of soil that was monitored twice during drying from total water saturation to an equilibrium dry state. It was thus possible to quantify the sensitivity of the TRIME-tube measurement to the surrounding medium permittivity. An empirical formula was also established for inferring the soil permittivity from the TRIME-tube measured pseudo transit time.
Abbreviations: SMNP, soil moisture neutron probe • SWC, soil water content • TDR, time domain reflectometry • TRIME, time domain reflectometry with intelligent microelements
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INTRODUCTION
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TIME DOMAIN REFLECTOMETRY is now widely used for monitoring SWC. Several types of equipment are commercially available (Charlesworth, 2000; Robinson et al., 2003) and numerous scientific papers have been devoted to this technique during these last twenty years. An up-to-date and state-of-the-art review on TDR can be found in (Robinson et al., 2003) or (Topp et Ferré, 2002). However, profiling SWC with TDR is a problem that remains partially unsolved. A straightforward solution consists of installing a set of probes at different depths, but this is cumbersome, time-consuming, and rather destructive. An ideal alternative would be to inverse a single TDR waveform taken on a vertical probe covering the whole depth of interest to obtain directly the dielectric constant profile. Although some theoretical works aiming at this objective were already published (Schlaeger et al., 2001; Oswald et al., 2003; Heimovaara et al., 2004), their practical applications are apparently not yet completely finalized so far. The TRIME-tube system investigated here gives an interesting possibility for profiling SWC (Laurent et al., 2001; Laurent, 2000). Operated on a probe moved inside a plastic access tube, TRIME's TDR measurement in this condition is similar to that taken on a coated probe. Therefore, an additional step (relationship between the soil dielectric constant and the measured effective electrical permittivity) must be used in the calibration process (Ferré et al., 1996, 1998; Knight et al., 1997). The purpose of this work was to test the performance of the TRIME-tube system under real field conditions and to characterize its calibration through well-controlled laboratory experiments.
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MATERIALS AND METHODS
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Description of the Tested Soil Water Content TDR Profiling System
The "TRIME-tube system" (Fig. 1)
refers to the association of a commercial TDR moisture meter (TRIME-FM3, Imko GmbH, Ettlingen, Germany) with a cylindrical probe (T3 probe also from Imko) designed to take SWC measurements inside plastic access tubes.1
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Fig. 1. Overview of the TRIME-tube system: (top) the TRIME-FM3 moisture meter, (bottom) the T3 access tube probe.
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The T3 access tube probe is equivalent to a TDR waveguide with two parallel 18-cm-long rods, each made of a set of four metal plates mounted on springs to ensure good contact with the cylindrical inner wall of the access tube and to compensate for small variations of its diameter. The T3 tube probe has a standard 2.5-m-long cable terminated by the DIN connector that attaches to the TRIME-FM3 case.
The TRIME-FM3 is a battery powered hand-held TDR instrument. Its robust, waterproof case exhibits only a very simple user interface on its front panel (a one press-button to start the measurement and a four line LCD display) and three DIN connectors for connecting: the probe, an external 12-V power supply and/or a serial cable to an external PC, the battery charger or a specific connector that allows the basic calibration to be redone. An analog 0.1-V output is also available on this last connector. The TRIME-FM3 internal 75-
TDR electronics is based on an original voltage comparator circuitry (Imko GmbH, 2000) that is designed to accurately measure an overall transit time t1. To compensate for the length of the cable and probe characteristics, the measured transit time is transformed into a "pseudo-transit-time," t2, by a simple linear relationship:
 | [1] |
where Offset and Divisor are two parameters adjusted when operating the basic calibration of the TRIME-FM3 with its associated T3-probe (Imko GmbH, 2001). A first SWC value, 
1, is then calculated using a fifth degree empirical polynomial fitted to measurements taken in several soils at various water contents (Stacheder, 1996):
 | [2] |
The final displayed SWC value, 2, is subsequently evaluated from 1 by applying a second fifth degree polynomial correction presenting an option for future calibration:
 | [3] |
All parameters appearing in Eq. [1], [2], and [3] are stored inside the TRIME-FM electronics and in the T3 probe connector that incorporates a memory chip. They can be visualized and/or modified using the utilities provided by Imko. A TRIME-tube system is delivered factory calibrated, and the corresponding TRIME-FM and T3 probe are identified by the same, unique serial number. In this initial state, C1' = 1 and the other coefficients in Eq. [3] are set to 0. With these defaults, the TRIME-FM will display "standard" SWC values, 1. All the measurements reported hereafter were taken in this way.
The 42-mm (i.d.) Polycarbonate (Tecanat, Ensinger GmbH et Co., Nufringen, Germany) cylindrical access tubes manufactured specially for Imko have a 1-mm wall thickness to maximize the measurement sensitivity. Since that makes them also fragile, a specific toolkit has to be used for their installation into the soils (Imko GmbH, 2001). Standard lengths of 1, 2, and 3 m are available. Other plastic tubes with similar diameters could also be used. Fiberglas tubes were previously distributed by Imko and, in this study, commercially available PVC tubes were also tested.
With the TRIME-tube system, a SWC measurement is performed in three steps. First, the probe is manually positioned inside the access tube at the desired depth. Second, the user presses the only button of the TRIME-FM3 to start the measurement procedure. Third, after approximately 30 s, the measured SWC is displayed on the LCD panel. Since there is no internal memory, the measurements must be manually recorded. This sequence is repeated for each depth.
Comparative Field Tests
To evaluate the performance of the TRIME-tube SWC profiling system under real field conditions, four sites were selected in France and Tunisia (Table 1).
France
The two sites in France were instrumented for the experimental comparisons: one in Grenoble in an urban environment and the other at the Lavalette experimental station of CEMAGREF-Montpellier in a cultivated field. The experimental setup in Grenoble consisted of three 1.5-m access tubes made of Al, Tecanat, and PVC. During three periods of several months in 2000, 2001, and 2002, periodic measurements were taken both with a soil moisture neutron probe (SMNP) (Troxler 4300, Troxler Electronic Laboratories Inc., NC, USA), and with the TRIME S/N 7646. TRIME was operated in the two types of plastic access tubes. The SMNP measurements were taken in the plastics tube for further direct comparison with the corresponding TRIME measurements, and in the Al tube for which a classical SWC vs. count ratio calibration curve was established. In Montpellier, a 1-m-long Tecanat tube was installed before the 2000 cropping season, and two others were installed in 2001. The tubes were maintained until the end of the 2002 cropping season. Measurements were taken weekly with a SMNP (SOLO 25, Nardeux Humisol, French company no longer trading) and the TRIME S/N 9112.
Tunisia
As part of an existing collaborative study between the French CEMAGREF and the Tunisian INRGREF, two 1-m-long Tecanat tubes were installed in 2000 at Cherfech and Saïda, both located in the agricultural region of the lower Medjerda river valley (Ruelle et al., 2001). Profiles were monitored in 2000 and 2001 with the TRIME S/N 7491, and comparative SWC measurements were taken either with a SOLO 25 SMNP operated in Tecanat tubes or gravimetrically. For that purpose, samples were periodically taken at different depths in the vicinity of the tubes using an auger. Oven-dried mass SWC values thus obtained were converted into volume SWC values using available bulk density data. Direct permittivity measurements were also taken using a TRASE TDR instrument (Soilmoisture Equipment Corp., Santa Barbara, CA) connected directly on the T3 TRIME probe positioned inside the access tubes.
Laboratory Experiments
To investigate the physics of the TRIME-FM3's measurement technique along its tube access probe, several experiments were undertaken under laboratory controlled conditions.
Reference Materials
First, measurements were taken on a set of reference media: air, water, ethanol, and glass beads (either dry or saturated with ethanol or water). For each medium, three measurements where performed: (i) a classical TDR measurement of the medium permittivity using a 20-cm-long three-rod TDR waveguide (TRASE "buriable" probe, Soilmoisture Equipment Corp.), (ii) a normal TRIME-tube measurement with the T3 probe positioned inside a Tecanat access tube placed into the considered medium, and (iii) a permittivity measurement with another TDR instrument connected to the T3 probe still in the same conditions.
For alternative TDR measurements in the laboratory, two alternative TDR instruments (Tektronix 1502C, Tektronix, Beaverton, OR, USA or 86100A Infiniium DCA Wide-Bandwidth Oscilloscope with its 54754 TDR plug-in module, Agilent Technologies, Palo Alto, CA, USA) were used. The connection with the TRIME T3 probe required a specifically designed 50- BNC/DIN adaptor.
Natural Soil
Soil taken from the Grenoble site (main characteristics are shown in Table 1) was repacked into a 10-L plastic cylindrical container around a Tecanat access tube positioned along its central axis (Fig. 2)
. A buriable TDR probe was also installed into the soil between the Tecanat tube and the container wall. After careful saturation with water from the bottom to avoid air entrapment, the entire setup was placed on a weighing balance to monitor the mean SWC and then was allowed to dry naturally at 23°C in the air-conditioned laboratory atmosphere. During this drying process, which lasted approximately 1 mo, gravimetric, TDR, and TRIME measurements were taken and recorded each time a given amount of water evaporated. Finally, the container was oven dried at 105°C to obtain a reference dry weight that was used to recalculate precisely the successive measured mean SWC. The experiment was repeated once (after resaturation and again drying) to ensure that the soil structure had not changed during the first experiment.
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Fig. 2. Overview of the laboratory setup for the cylindrical 10-L container experiments. The top right close-up shows the TDR probe used for measuring the soil permittivity.
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RESULTS AND DISCUSSION
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Comparative Field Tests
As a first level of analysis, the TRIME measurements were directly compared with their respective reference SWC values determined either gravimetrically or with a calibrated SMNP. Figure 3
gives typical examples of such comparisons. For most cases studied here, it we found that a simple linear relationship of the form:
 | [4] |
was sufficient to accurately calibrate the TRIME-tube system for a particular access tube. This means that the polynomial expression (Eq. [3]) can be reduced to a straight line with only two non-zero coefficients C0' = a and C1' = b. Table 2 summarizes the results of linear calibrations (Eq. [4]) as applied to all field experiments. Results show the efficiency of the procedure as reflected by the RMSE values calculated before and after calibration.
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Fig. 3. Examples of comparison between TRIME measurements and corresponding reference soil water content (SWC), determined by soil moisture neutron probe (SMNP) for the four sites considered in this study.
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Table 2. Selected results from the field experiments, including soil water content (SWC), RMSE values before TRIME calibration, the obtained linear correction coefficients, and corresponding remaining RMSE values.
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Again plotting the TRIME measurements vs. reference SWC data, Fig. 4
includes all of our field experiment results. The data show some general trends. For intermediate SWC (0.2–0.4), the TRIME-tube system overestimated the SWC. In contrast, for low (<0.2) or high (>0.4) SWCs, the TRIME "standard" measurements were fairly close to the "true" reference SWC. For these last cases, applying a linear calibration yields only slightly improved RMSE values as can be seen in Table 2 for the experiments in Tunisia (Fig. 3, bottom) and for Grenoble in 2001 and 2002 (Fig. 3, top left).
Although application of Eq. [4], which is actually more of an empirical correction than a true calibration, may improve locally the accuracy of the TRIME-tube system measurements, this method has been shown to also have some severe limitations:- One limit is that its a and b coefficients can be determined only once a sufficient number of reference data covering a significant SWC range are acquired. Moreover, the method requires regular reference SWC measurements taken by another technique during the calibration period, which can be quite time-consuming.
- Another limitation of Eq. [4] is the fact that the SWC range observed along a particular access tube is strongly depending on such external factors as climatic conditions and cultural practices. In our experiments, for example, it was clear that there is a link between these parameters and the covered SWC ranges (see Table 2 and Fig. 4): high SWCs were mainly observed at Grenoble, whereas dry conditions were common in Tunisia with Montpellier corresponding to an intermediate situation. This limitation may prevent the extrapolation of the calibrations proposed here.
- A third limitation of the use of Eq. [4] is the considerable scattering in the TRIME-tube data that are compared with reference data in Fig. 3. Even after calibration, the corresponding RMSE values remained quite high, as can be seen in Table 2. This means that other factors, such as the presence of variable soil horizons, soil salinity, temperature, or a lack of significance of the reference SWC (e.g., due to strong soil spatial variability or to uncertainties about the bulk density profile) may also interfere.
For these reasons, we tried to analyze our results differently by considering more carefully the TRIME-FM's internal calibration given by Eq. [2]. For this purpose, we recalculated the pseudo transit times t2 (unfortunately not displayed at the end of the measurement procedure) from our TRIME measured SWCs using an inverse form of Eq. [2] with factory calibration values for C0, C1, ... C5 (Table 3):
 | [5] |
It was then possible to plot all of our 670 data points (t2, reference SWC) on a graph to evaluate the overall behavior of the TRIME-FM3 standard calibration (Fig. 5)
. The same trends as mentioned above (i.e., excessive scattering, overestimation in the range 400 < t2 < 600, good fit of the standard calibration outside and limited t2 variation for each experiment) are again clearly visible in this figure. To compensate for the observed overestimation of the SWC in the medium range of pseudo transit times, our results suggest a modified internal calibration (Fig. 5, Table 3). If this calibration is used rather than the standard Imko calibration, the overall RMSE calculated for our data decreases to 0.048 with no extra calibration, which is approximately the same as application of the linear calibration (Eq. [4]) using the coefficients in Table 2. Nevertheless, we emphasize that this newly proposed internal calibration should be validated more thoroughly before generalizing the result because we did not have points with pseudo transit time below 275. Also, it would be useful to replicate experiments yielding measurements in the intermediate SWC range.
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Fig. 5. Plot of field reference soil water content (SWC) values vs. their respective TRIME measured pseudo transit times. Dotted line: TRIME-FM3 factory standard internal calibration (Eq. [2]). Solid line: suggested new calibration curve.
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Laboratory Experiments
Table 4 shows the results of our measurements on a set of reference media. The results demonstrate that the parameter measured with the TRIME-FM3 (i.e., the pseudo transit time t2) is highly sensitive to the electrical permittivity KMaterial of the surrounding medium. However, the permittivity KTRIME that can be measured directly with the T3 probe in its Tecanat access tube increases from approximately 2 to 10 only when the surrounding medium permittivity shifts from 1 (air) to 80 (water). This confirms the well-known fact that taking measurements with coated TDR probes (in this case the coating was the 1-mm-thick plastic wall of the access tube itself) considerably reduces the sensitivity of the TDR measurement to the external permittivity and, consequently, to the corresponding SWC. Moreover, plotting permittivities measured by the TRIME vs. the three-rod TDR values shows that the higher the surrounding medium permittivity, the lower the sensitivity of the TRIME measurement (Fig. 6)
.
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Table 4. Pseudo transit times t2 measured with the TRIME-tube system on a set a reference media; corresponding permittivities KTRIME and KMaterial measured with a traditional TDR instrument on a T3 probe inside its access tube and on a three-rod 20-cm TDR probe placed directly into the medium.
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Fig. 6. Results of laboratory experiments relating the permittivity as measured with a T3 probe and that of the surrounding medium.
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Finally, examining our laboratory data as represented in Fig. 7 , an empirical formula can be suggested to infer the soil permittivity, Ksoil, from the TRIME measured pseudo transit time t2:
 | [6] |
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Fig. 7. Results of laboratory experiments showing the relationship between the surrounding medium permittivity K and the TRIME measured pseudo transit time t2.
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This equation may simplify further TRIME calibration approaches since permittivity–SWC relationships classically used for interpreting TDR measurements (Jacobsen and Schjønning, 1994; Robinson et al., 2003) could be then applied.
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CONCLUSIONS
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The TRIME-tube system for profiling SWC was extensively tested under various natural field conditions. Our comparisons with independent SWC measurements, indicate that: (i) the TRIME-tube system may be an operationally useful device for profiling SWC and (ii) the measurements must be calibrated to obtain more accurate SWC data. For that purpose, a simple field calibration similar to that of a neutron probe could be applied, or its internal calibration could be improved.
Complementary laboratory tests on reference materials and on a drying natural soil were also conducted to clarify several points about the TRIME internal calibration and signal processing. These data allowed us to establish empirical formulas relating soil and TRIME measured electrical permittivities and values of the soil dielectric constant as function of the TRIME measured transit time.
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ACKNOWLEDGMENTS
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This work was funded both by the "Programme National de Recherche en Hydrologie" from the French INSU-CNRS and by a contract with the FAO/IAEA joint section "Soil and Water Management et Crop Nutrition", Vienna, Austria.
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REFERENCES
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- Charlesworth, P. 2000. Soil water monitoring. Available at www.npird.gov.au/projects/finalrep_pdf/index.html (verified 12 Sept. 2005). Landlinks Press, Collingwood, Australia.
- Ferré, P.A., D.L. Rudolph, and R.G. Kachanoski. 1996. Spatial averaging of water content by time domain reflectometry: Indications for twin rod probes with and without dielectric coatings. Water Resour. Res. 32:271–279.[CrossRef]
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- Ruelle, P., A. Zaïri, B. Ben Nouna, J.P. Laurent, L. Delage, H. Quinones, and T. Ajmi. 2001. Métrologie TDR des sols de la Basse Vallée de la Médjerda et suivi de l'état hydrique sous culture. p. 77–92. In Séminaire INRGREF "Economie de l'eau en agriculture", Hammamet, Tunisia. 14–16 Nov. 2000. INGREF, Ariana, Tunisia.
- Schlaeger, S., C. Huebner, A. Scheuermann, and J. Gottlieb. 2001. Development and application of TDR inversion algorithms with high spatial resolution for moisture profile determination. p. 236–248. In TDR'2001: Second International Symposium and Workshop on Time Domain Reflectometry for Innovative Geotechnical Applications, Evanston, IL. 5–7 Sept. 2001. Available at http://www.iti.northwestern.edu/tdr/tdr2001/proceedings/final/TDR2001.pdf (verified 12 Sept. 2005). Seeley G. Mudd Library for Science & Engineering, Northwestern University, Evanston, IL.
- Stacheder, M. 1996. Die Time Domain Reflectometry in der Geotechnik, Messung von Wassergehalt, elektrischer Leitfähigkeit und Stofftransport. Ph.D. diss. Schriftenreihe angewandte Geologie, Karlsruhe, Germany.
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TOP
ABSTRACT
INTRODUCTION
