Scott B. Jones

A Review of Advances in Dielectric and Electrical Conductivity Measurement in Soils Using Time Domain Reflectometry

constant) of a material emerged as an elegant method of estimating water content in porous materials. For the Substantial advances in the measurement of water content and first time the same physical property (permittivity) could bulk soil electrical conductivity (EC) using time domain reflectometry be measured for a range of scales and used to estimate (TDR) have been made in the last two decades. The key to TDR’s success is its ability to accurately measure the permittivity of a material water content. Electromagnetic methods, whether TDR and the fact that there is a good relationship between the permittivity (localized measurement), ground penetrating radar of a material and its water content. A further advantage is the ability (two-dimensional profile), or active microwave remote to estimate water content and measure bulk soil EC simultaneously sensing (land surface), all estimate water content based using TDR. The aim of this review is to summarize and examine on the permittivity of the target medium. A further advances that have been made in terms of measuring permittivity and advance was the development of analysis methods using bulk EC. The review examines issues such as the effective frequency TDR. Time domain reflectometry was adapted to estiof the TDR measurement and waveform analysis in dispersive dielecmate both soil water content (Hoekstra and Delaney, trics. The growing importance of both waveform simulation and in1974; Topp et al., 1980) and soil bulk EC simultaneously verse analysis of waveforms is highlighted. Such methods hold great (Dalton et al., 1984). In spite of decades of research, potential for obtaining far more information from TDR waveform analysis. Probe design is considered in some detail and practical guidwe are only beginning to efficiently utilize electrical ance is given for probe construction. The importance of TDR measuretechnology that ranges from satellite and airborne radar ment sampling volume is considered and the relative energy storage to ground penetrating radar and localized sensors such density is modeled for a range of probe designs. Tables are provided as TDR and impedance probes. that compare some of the different aspects of commercial TDR equipThe underlying success of these techniques can be ment, and the units are discussed in terms of their performance and considered in two parts, the first of which is the equiptheir advantages and disadvantages. It is hoped that the review will ment’s ability to accurately measure the bulk dielectric provide an informative guide to the more technical aspects of permitpermittivity and EC of a material. The second is the close tivity and EC measurement using TDR for the novice and expert alike. relationship between the measured permittivity and the volumetric water content, or the ionic concentration and the bulk EC of the material. This review concentrates W is required in some way by all living things; on the first stage, the accurate measurement of bulk perit is a fundamental constituent of life on our mittivity and EC, and we confine ourselves to the use planet. Our survival as well as that of other organisms of TDR but acknowledge that other devices such as depends on a supply of water both to our own bodies impedance probes (Dean et al., 1987; Hilhorst et al., and to the flora and fauna on which we live. One of the 1993; Gaskin and Miller, 1996; Paltineanu and Starr, best ways to regulate water consumption is to know 1997) may also be used for this purpose. Time domain the quantity available and to manage the resource with reflectometry has become a large topic in soil physics, prudence and stewardship (Hillel, 1991). To achieve this primarily because of its adaptability and the continued aim, techniques are preferred that can be used to meadevelopment of novel applications. The focus of this sure a physical quantity closely related to the amount review is on the measurement of bulk permittivity and of water contained in a porous material, be it rock, soil, EC, and thus some topics are dealt with only briefly or or an artificial medium. omitted. One of the strengths of the TDR measurement The revolution in electronics in the latter half of the method is that many probes can be monitored almost last century made the measurement of the electrical simultaneously using a multiplexer (Baker and Allproperties of materials more accessible than ever before. maras, 1990; Heimovaara and Bouten, 1990; Herkelrath Measurement of the dielectric permittivity (dielectric et al., 1991). This review discusses the measurement of bulk EC; however, we don’t go any further to examine D.A. Robinson, U.S. Salinity Laboratory, USDA-ARS, 450 W. Big the interpretation of this in terms of soil solution conSprings Road, Riverside, CA 92507; S.B. Jones, Dep. Plants, Soils ductivity. The literature on this aspect of TDR applicaand Biometeorology, Utah State University, Logan, UT 84322-4820; tion is large, and the reader is referred to a recent publiJ.M. Wraith, Land Resources & Environmental Sciences Dep., Moncation for further reference (Dane and Topp, 2002). A tana State University, Bozeman, MT 59717-3120; D. Or, Dep. of Civil and Environmental Engineering, University of Connecticut, 261 further topic omitted from this review is the use of Glenbrook Road, Unit 2037, Storrs, CT 06269; S.P. Friedman, Institute coated TDR probes. Coated probes have been proposed of Soil, Water and Environmental Sciences, The Volcani Center as a way to extend the working range of TDR in saline (ARO), Bet Dagan 50250, Israel. Received 21 Nov. 2002. Special soils (Kelly et al., 1995; Nichol et al., 2002). However, Section—Advances in Measurement and Monitoring Methods. *Corresponding author (darobinson001@yahoo.co.uk). the studies of Ferre et al. (1996) and Knight et al. (1997) Published in Vadose Zone Journal 2:444–475 (2003).  Soil Science Society of America Abbreviations: EC, electrical conductivity; TDR, time domain reflectometry. 677 S. Segoe Rd., Madison, WI 53711 USA 444 Published November, 2003

Particle shape effects on the effective permittivity of anisotropic or isotropic media consisting of aligned or randomly oriented ellipsoidal particles

The effective permittivity (dielectric constant) of anisotropic or isotropic porous media is affected by the shape of particles composing the mixture. Directional permittivities are influenced by extreme aspect ratio particles, often found aligned with the bedding plane of rock or soil. Our objectives were to determine the effects of particle shape and preferential orientation on the effective permittivity of porous media. Confocal spheroids (ellipsoids of revolution) were used to mathematically describe a range of particle shapes from disks to spheres to needles. Dielectric mixing models which account for the polarization due to inclusion shape and axial alignment were used to estimate the shape effect. Permittivity measurements in an anisotropic packing of disk-shaped mica particles using time domain reflectometry showed an alteration of the permittivity due to the shape effect. Two- and three-phase predictions based on Maxwell-Garnett (1904) showed trends similar to measurements in anisotropic packings of mica. Particle shape effects can be a significant factor in dielectric permittivity measurements and should be a consideration especially where particle aspect ratio deviates by more than an order of magnitude from that of a sphere (unity). As the particle shape is less spherical, the resulting effective permittivity of the mixture is more similar to the inclusion permittivity and differs more from the permittivity of the background. Ellipsoid size and surface area provide an estimate of the combined effects of bound water and particle shape on the effective mixture permittivity. For high aspect ratio particles, shape effects on the effective permittivity appear to be comparable in magnitude to those of bound water prevalent in clay-sized media.

Geophysical imaging of watershed subsurface patterns and prediction of soil texture and water holding capacity

work we show how EMI can be used to image the subsurface of a � 38 ha watershed. We present an imaging approach using kriging to interpolate and sequential Gaussian simulation to estimate the uncertainty in the maps. We also explore the idea of difference ECa mapping to try to exploit changes in soil moisture to identify more hydrologically active locations. In addition, we use a digital elevation model to identify flow paths and compare these with the ECa measurement as a function of distance. Finally, we perform a more traditional calibration of ECa with clay percentage across the watershed and determine soil water holding capacity (SWHC). The values of SWHC range from 0.07 to 0.22 m 3 m � 3 across the watershed, which contrast with the uniform value of 0.13 derived from the traditional soil survey maps. Additional work is needed to appropriately interpret and incorporate EMI data into hydrological studies; however, we argue that there is considerable merit in identifying subsurface soil patterns from these geophysical images.

Microgravity effects on water flow and distribution in unsaturated porous media: Analyses of flight experiments

Plants grown in porous media are part of a bioregenerative life support system designed for long-duration space missions. Reduced gravity conditions of orbiting spacecraft (microgravity) alter several aspects of liquid flow and distribution within partially saturated porous media. The objectives of this study were to evaluate the suitability of conventional capillary flow theory in simulating water distribution in porous media measured in a microgravity environment. Data from experiments aboard the Russian space station Mir and a U.S. space shuttle were simulated by elimination of the gravitational term from the Richards equation. Qualitative comparisons with media hydraulic parameters measured on Earth suggest narrower pore size distributions and inactive or nonparticipating large pores in microgravity. Evidence of accentuated hysteresis, altered soil-water characteristic, and reduced unsaturated hydraulic conductivity from microgravity simulations may be attributable to a number of proposed secondary mechanisms. These are likely spawned by enhanced and modified paths of interfacial flows and an altered force ratio of capillary to body forces in microgravity.

Gas Diffusion Measurement and Modeling in Coarse-Textured Porous Media

Conventional gas diffusion measurements in coarse-textured and aggregated porous media are severely limited due to hydrostatically induced variations in water content and air-filled porosity. Motivated by the need to measure gas diffusion in coarse-textured plant growth media designed for use in microgravity (e.g., aboard the International Space Station), our objectives were (i) to develop and test an automated diffusion measurement system on earth with water content adjustment capability and that minimizes hydrostatic effects, and (ii) to model characteristics of gas diffusion in partially saturated aggregated porous media. The horizontally oriented O 2 diffusion cell design for reducing the gravitational effect was based on a thin profile rectangular cell. Continuous measurement of O 2 in sealed dual-chamber diffusion cells provided concentration data for fitting diffusion coefficients where water content was controlled volumetrically using a porous membrane with an imposed pressure for incremental addition or removal of water. Gas diffusion was modeled as a function of air-filled porosity in millimeter-sized aggregated particles exhibiting a substantial difference between internal and external aggregate pore sizes. For this case, the internal aggregate porosity contribution to diffusion compared with external aggregate pore space was minor as described by a dual-porosity diffusion model. The measurement approach described can be used in other coarse-textured and structured porous media.

Measurement and approximate critical path analysis of the pore‐scale‐induced anisotropy factor of an unsaturated porous medium

Nonspherical particles aligned parallel to preferential bedding planes form sedimentary rocks and soils, which are anisotropic in their resistance to flow of fluids, solutes, heat, and electrical current at the microscopic scale. Understanding the extent of the anisotropy of an unsaturated medium, containing both water and air, and its quantification is of major importance for improving our prediction capability of pollutant transport occurring in those media. The anisotropy factor is defined as the ratio of the conductivities parallel to the bedding plane and perpendicular to it. Measurements of the anisotropy factor of the apparent electrical conductivity in initially saturated packings of platy mica particles resulted in an initial moderate increase of the anisotropy factor with desaturation, followed by its decrease when approaching the drier region of residual moisture contents. The anisotropy factor for the hydraulic conductivity is expected to follow a pattern similar to that found for the apparent electrical conductivity. This behavior is opposite to that predicted by previous theoretical models, which suggest a possible initial decrease of the anisotropy factor of the hydraulic conductivity with desaturation, followed by its divergence to infinity toward the dry end. A critical path analysis of a simplified three-dimensional pore network model, based on different characterization of the pores parallel and normal to the bedding plane, served to evaluate the saturation degree-dependent anisotropy factors. The critical path analysis reconstructed the experimentally determined pattern qualitatively and helped to explain the observed dependence of the apparent electrical conductivitys anisotropy factor on saturation degree. The critical path analysis also provided an indication of the anisotropy factor for hydraulic conductivity, which is expected to be of higher magnitude than that of the apparent electrical conductivity.

Sodic soil reclamation using cottage cheese (acid) whey

Abstract Cottage cheese production in the United States yielded approximately 3 × 106 Mg of cottage cheese (acid) whey in 1991. Unmarketable whey is disposed of in sewage treatment facilities or on land. Environmental concerns and new laws make disposal even more costly and difficult. While much whey is applied to land for fertilizer or disposal purposes, acid whey has recently been used as a sodic soil amendment. The objectives of this research were to determine the effects of whey application on chemical properties and infiltration rates of sodic soils. Four treatments of acid whey (0, 25, 50, and 100 mm) were applied to Freedom silt loam (fine‐silty, mixed, mesic, Xerollic Calciorthids) in greenhouse lysimeters and to De‐clo loam (coarse‐loamy, mixed, mesic, Xerollic Calciorthids) in field basins. Accumulative sodium removal at 0.5 pore volumes of leachate was 1.0, 1.1, 1.2, and 1.7 mol for the 0‐, 25‐, 50‐, and 100‐mm lysimeter treatments, respectively. Whey applications lowered sodium adsorption rati...

Microgravity effects on water supply and substrate properties in porous matrix root support systems

The control of water content and water movement in granular substrate-based plant root systems in microgravity is a complex problem. Improper water and oxygen delivery to plant roots has delayed studies of the effects of microgravity on plant development and the use of plants in physical and mental life support systems. Our international effort (USA, Russia and Bulgaria) has upgraded the plant growth facilities on the Mir Orbital Station (OS) and used them to study the full life cycle of plants. The Bulgarian-Russian-developed Svet Space Greenhouse (SG) system was upgraded on the Mir OS in 1996. The US developed Gas Exchange Measurement System (GEMS) greatly extends the range of environmental parameters monitored. The Svet-GEMS complex was used to grow a fully developed wheat crop during 1996. The growth rate and development of these plants compared well with earth grown plants indicating that the root zone water and oxygen stresses that have limited plant development in previous long-duration experiments have been overcome. However, management of the root environment during this experiment involved several significant changes in control settings as the relationship between the water delivery system, water status sensors, and the substrate changed during the growth cycles.

Effects of precipitation pulses on water and carbon dioxide fluxes in two semiarid ecosystems: measurement and modeling

Modeling of soil–water, –heat and –carbon (C) fluxes provides an important tool for predicting mass and energy transfers based on a hydraulic-, thermal- and C-mass balance approach. Model predictions were evaluated using measured data from two water-limited study sites, one pasture and one supporting an alfalfa crop, to indentify differences between these ecosystems. Soil water content, temperature, and evapotranspiration (ET) data were used to validate soil water dynamics components of a process-based numerical model. Soil surface CO2 efflux estimates (i.e., fluxes from soil respiration) were also made to estimate soil CO2 emissions. The results show that the Hydrus-1D numerical model can be parameterized to simulate the soil hydrodynamics and CO2 fluxes measured at both locations. Rainfall and irrigation events triggering increases in plant root and microbial respiration rates were simulated to recreate observed pulsed CO2 fluxes. There were distinct differences in ET and soil CO2 effluxes between the ecosystems and watering events significantly modified the fluxes. Differences in potential evapotranspiration and soil texture could help explain these discrepancies. The results demonstrate that numerical modeling can be a useful tool for estimating soil surface fluxes in calibrated ecosystems when micrometeorological methods may not be suitable.

INV-WATFLX, a code for simultaneous estimation of soil properties and planar vector water flux from fully or partly functioning needles of a penta-needle heat-pulse probe

Soil thermal properties and water fluxes are fundamental for understanding water and heat transport phenomena in the vadose zone. Processes of interest include quantifying infiltration and runoff in addition to solute transport rates, which are of great interest in many scientific and engineering applications where water flux and temperature are key parameters. In this study, INV-WATFLX was developed for simultaneously fitting thermal diffusivity, thermal conductivity and heat velocities in a plane normal to a penta-needle heat-pulse probe (PHPP) using temperature rise measurements in a porous medium. The inverse problem is formulated as the minimization of a generalized least-squares criterion by means of a Gauss-Newton-Levenberg-Marquardt method. Fitted temperature measurements following a heat-pulse injection were calculated from an analytical solution of temperature rise derived at the four thermistor positions of the PHPP. The INV-WATFLX code was tested with a set of synthetic simulations using CORE^2^D V4. Relative errors of thermal diffusivity, conductivity, bulk volume heat capacity, and water fluxes estimated in INV-WATFLX to their prescribed values in the synthetic simulations were smaller than 3%. We also evaluated the ability of INV-WATFLX to provide estimation of thermal properties and fluxes from temperature rise measured by a sub-set of the four thermistors. INV-WAFLX was applied to laboratory column flow experiments for water flux estimation using a PHPP. Water fluxes estimated using INV-WATFLX was comparable to independently measured fluxes. The new code provides reliable estimation of soil thermal properties and water fluxes from temperature rise using heat-pulse measurements.