A computer controlled 36-channel time domain reflectometry system for monitoring soil water contents

Heimovaara, T.J.

doi:10.1029/90wr01239

Cited by 85 publications

(110 citation statements)

References 2 publications

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“…Many conventional applications use this kind of probe for point-wise measurements of the water content. One advantage of the use of noninsulated metallic rods is the possibility of the direct calculation of the permittivity of the material surrounding the waveguide from the wave velocity [Whalley, 1993;Heimovaara and Bouten, 1990], which can be determined using simple travel time analysis. However, a serious limitation in the use of uncoated rods is the restricted length, which is limited to a maximum of about 1 m for use in soils because of losses caused by the electric conductivity of the moist soil [Dalton and van Genuchten, 1986].…”

Section: Transmission Line Designmentioning

confidence: 99%

Spatial time domain reflectometry and its application for the measurement of water content distributions along flat ribbon cables in a full‐scale levee model

Scheuermann

Huebner

Schlaeger³

et al. 2009

Water Resources Research

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[1] Spatial time domain reflectometry (spatial TDR) is a new measurement method for determining water content profiles along elongated probes (transmission lines). The method is based on the inverse modeling of TDR reflectograms using an optimization algorithm. By means of using flat ribbon cables it is possible to take two independent TDR measurements from both ends of the probe, which are used to improve the spatial information content of the optimization results and to consider effects caused by electrical conductivity. The method has been used for monitoring water content distributions on a full-scale levee model made of well-graded clean sand. Flood simulation tests, irrigation tests, and long-term observations were carried out on the model. The results show that spatial TDR is able to determine water content distributions with an accuracy of the spatial resolution of about ±3 cm compared to pore pressure measurements and an average deviation of ±2 vol % compared to measurements made using another independent TDR measurement system.

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Section: Transmission Line Designmentioning

confidence: 99%

Spatial time domain reflectometry and its application for the measurement of water content distributions along flat ribbon cables in a full‐scale levee model

Scheuermann

Huebner

Schlaeger³

et al. 2009

Water Resources Research

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“…The specific location of the end reflection point can be determined by published methods [Toppet al, 1980;Heimovaara and Bouten 1990]. The end reflection point is somewhat more difficult to determine for layered soil systems as will be discussed below.…”

Section: Simulation Of Waveforms For Two-layered Specimensmentioning

confidence: 99%

Theoretical model of a multisection time domain reflectometry measurement system

Feng

Lin²,

Deschamps

et al. 1999

Water Resources Research

122

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Abstract. A multisection model is presented to simulate electromagnetic wave propagation in an unmatched time domain reflectometry (TDR) probe and layered soil system. The model uses a linear-time-invariant feedback system to model each section and links each section in a bottom-up fashion. Multiple sections can be incorporated in this model by a simple extension of a single-section system. An unmatched TDR probe system is modeled by dividing it into equivalent sections and matching the simulated waveform with the actual waveform. The excellent match between the simulated and the recorded waveforms verifies the model. This model eliminates the requirement of a matched 50-12 probe handler in probe design for dielectric measurement. It can also be used to assist probe design and TDR data interpretation. The reflected waveforms in layered soil specimens are modeled, and the simulated waveforms are compared to those obtained from experiments. The method of using apparent dielectric constant to obtain average volumetric water content is examined for the layered soil system. The results show that apparent dielectric constant is applicable for the measurement of average volumetric water content for a layered soil, but caution has to be used when interpreting the waveforms. The TDR method consists of two parts: (1) the physical measurement system that includes the TDR apparatus, probe, and other equipment leading to the generation of a consistent TDR waveform and (2) the interpretation of the TDR waveform including its rclationshi• to the desired material propcrty. The success of the interpretation is the key to applying this technology. Tooeoe et al. [1980] The electromagnetic properties of a material are characterized by three parameters: (1) dielectric permittivity e, (2) electrical conductivity tr, and (3) magnetic permeability/x. In general, these parameters are functions of frequency. However, for materials like soils t•e magnetic permeability differs from magnetic permeability of free space/x o by a negligible fraction, and the frequency dependency of conductivity can be neglected. Owing to the dipole polarization mechanism of water, wet soil is a dielectric material that h as a frequency-dependent complex relative permitfivity:2,n-f 6'r(f) 6r ( Analysis of a Typical TDR Measurement SystemA typical TDR measurement system consists of a cable test device, which sends out a step (or other shape) EM wave through a pulse generator and records the incident and reflection signal with an oscilloscope, and a probe that acts as a waveguide to transmit the EM wave into the soil specimen. A step wave or impulse wave is usually used in order to obtain recognizable reflections at the soil surface and the end of the probe.Transmission line theory is used to model the TDR measurement system in which the field structure in the transmission line is assumed to be of transverse electromagnetic mode (TEM). Owing to the special structure of TEM, (5) Scatter Function for Multisection TDR Measurement SystemDuring field measurement i...

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“…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 Time-domain reflectometry has been used to measure volumetric water content of soil and other porous materials since 1980 (Topp et al 1980(Topp et al , 1982(Topp et al , 1994Topp 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.…”

Section: Mots Clésmentioning

confidence: 99%

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

Sun¹,

Young²,

McFarlane³

et al. 2000

Can. J. Soil. Sci.

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The effect of soil electrical conductivity on moisture determination using time-domain reflectometry in sandy soil. Can. J. Soil Sci. 80: [13][14][15][16][17][18][19][20][21][22]. 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 15.29 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 3.72 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. 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 15.29 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 3.72 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 Time-domain reflectometry has been used to measure volumetric water content of soil and other porous materials since 19...

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A computer controlled 36-channel time domain reflectometry system for monitoring soil water contents

Cited by 85 publications

References 2 publications

Spatial time domain reflectometry and its application for the measurement of water content distributions along flat ribbon cables in a full‐scale levee model

Spatial time domain reflectometry and its application for the measurement of water content distributions along flat ribbon cables in a full‐scale levee model

Theoretical model of a multisection time domain reflectometry measurement system

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

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