Jon M. Wraith

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

Improved biomass productivity and water use efficiency under water deficit conditions in transgenic wheat constitutively expressing the barley HVA1 gene.

The ABA-responsive barley gene HVA1, a member of group 3 late embryogenesis abundant (LEA) protein genes, was introduced into spring wheat (Triticum aestivum L.) cv. Hi-Line using the biolistic bombardment method. High levels of expression of the HVA1 gene, regulated by the maize ubi1 promoter, were observed in leaves and roots of independent transgenic wheat plants and were inherited by offspring generations. T(3) progenies of four selected transgenic wheat lines were tested under greenhouse conditions for tolerance of soil water deficit. Potted plants were grown under moderate water deficit and well-watered conditions, respectively. Two homozygous and one heterozygous transgenic lines expressing the HVA1 gene had significantly (P<0.01) higher water use efficiency values, 0.66-0.68 g kg(-1), as compared to 0.57 and 0.53 g kg(-1), respectively, for the non-expressing transgenic and non-transgenic controls under moderate water deficit conditions. The two homozygous transgenic plant lines also had significantly greater total dry mass, root fresh and dry weights, and shoot dry weight compared to the two controls under soil water deficit conditions. Results of this study indicate that growth characteristics were improved in transgenic wheat plants constitutively expressing the barley HVA1 gene in response to soil water deficit.

Temperature effects on soil bulk dielectric permittivity measured by time domain reflectometry: A physical model

Near-surface measurements of soil water content (θ) using time domain reflectometry (TDR) may exhibit anomalous behavior in the presence of diurnal temperature (T) fluctuations. Experimental results obtained in a companion paper led to the hypothesis that the observed bulk dielectric permittivity (ϵb) is determined by an interplay between two competing phenomena: (1) the reduction in the dielectric permittivity of bulk water with increased T; and (2) the increase in TDR-measured ϵb with increased T due to release of bound water. In this study we develop a physically based model for the temperature dependency of TDR-measured soil bulk dielectric permittivity and propose practical correction factors. The model considers the modified properties of water near solid surfaces to define a layer of rotationally hindered water (within the TDR frequency bandwidth) having a temperature dependent thickness. Changes in measured ϵb(T) are thus attributed to variations in the thickness of the rotationally hindered layer which has a lower dielectric permittivity than free water and hence is less “visible” to travel-time-based TDR waveform analyses. The model is sensitive to the soil specific surface area and the water content, both of which determine the ratio of bound to bulk soil water. Comparisons with experimental data covering a wide range of soils, water contents, and temperatures showed good agreement. Further studies are needed to evaluate some of the models critical parameters such as the cutoff frequency below which water is considered bound. A temperature correction approximation is based on analytical expressions for TDR-measured bulk dielectric permitivity and requires estimates of soil specific surface area and bulk density, which may be estimated from soil texture. The thermodielectric sensitivity of TDR-measured bulk dielectric permittivity and water content may serve as a basis for estimating soil specific surface area.

Temperature effects on soil bulk dielectric permittivity measured by time domain reflectometry: Experimental evidence and hypothesis development

Reports on temperature (T) effects on time domain reflectometry (TDR) measurements of soil water content (θ) are contradictory and often exhibit conflicting trends. We imposed step T changes on sealed columns of four soils having variable θ, while monitoring bulk apparent dielectric constant (or permittivity ϵb) and bulk electrical conductivity (σb) using TDR. Measured ϵb increased substantially with increasing temperature for one silt loam soil, for all θ. For another silt loam soil and for an Oxisol, measured ϵb increased with increasing T at relatively low θ but decreased with increasingT at higher θ. For a sandy loam soil, measured ϵb decreased with increasing T for all θ. The experimental results led to the hypothesis that TDR-measured ϵb is determined by an interplay between two competing phenomena: (1) the reduction in the dielectric constant of bulk water with increased T; and (2) the increase in TDR-measured ϵb with increased T due to release of bound water. TDRmeasured ϵb is thus dependent on solid surface area and wetness. Our results have implications for routine use of TDR in fine-textured and organic soils and potentially for microwave remote sensing of soil water status.

A new soil metric potential sensor based on time domain reflectometry

We developed and tested a new sensor based on time domain reflectometry (TDR) to measure soil matric potential (h). The TDR-matric (TM) sensor is constructed of porous disks having different known maximum pore sizes and stacked within a coaxial cage. The constant relationship between water content (θ) and h of the TM porous matrix is initially calibrated and subsequently used to infer matric potential of the surrounding soil, similar to existing porous heat dissipation and electrical resistance sensors. The θ of the sensors porous matrix in hydraulic equilibrium with the surrounding soil is measured by TDR travel time analysis. Prototype sensors were constructed using porous ceramic and plastic disks having maximum pore diameters between 120 μm(2.5 kPa) and about 0.6 μm (0.5 MPa). Calibration tests to evaluate sensor θ-h relationships were completed in a pressure chamber apparatus using four soils. These results and those from sensors installed in soil lysimeters in the presence of growing plants showed consistent θ-h relationships and synchronized responses of soils and TM sensors to changing water status. Pairing standard TDR probes with the new TM sensors facilitates in situ determination of soil θ(h) relationships, using conventional TDR instrumentation. The sensor design accommodates construction of media- or application-specific sensors using combinations of disks having different pore sizes. There is a trade-off between the TM sensors matric potential range and its sensitivity to changes in the surrounding soil. Additionally, a mismatch between the pore size distributions of the TM sensor and the soil (mostly relevant to coarse-textured soils) can lead to hydraulic decoupling of these and other porous sensors.

ADVANTAGES IN WATER RELATIONS CONTRIBUTE TO GREATER PHOTOSYNTHESIS IN CENTAUREA MACULOSA COMPARED WITH ESTABLISHED GRASSES

Semiarid steppe communities in North America appear particularly vulnerable to persistent infestations by exotic, taprooted forbs, such as European spotted knapweed (Centaurea maculosa). We determined whether species differences in ecophysiological response to water availability could help link traits of Centaurea with invasibility of steppe communities. Plant‐soil water relations and photosynthesis were measured under three water levels in a greenhouse and at two sites over two years in the field for Centaurea and dominant rangeland species of southwestern Montana: Pseudoregneria spicata, Pascopyron smithii, and Bromus inermis. Centaurea had greater and more seasonally persistent photosynthesis than the other species under field conditions but not in the greenhouse, where water availability was similar for the species. Centaurea had no greater water use efficiency, except under unusually dry conditions, but maintained greater water potentials despite greater transpiration than the grasses. Changes in soil water indicated uptake from deeper and wetter soils in Centaurea than in grasses. Greater photosynthesis in Centaurea compared with grasses may result from uptake of deeper soil water and corresponding drought avoidance. Interspecific differences in resource use may therefore contribute to the success of Centaurea, and Centaureas ecological requirement for water matches an available resource niche in the communities we examined.

Effects of the Invasive Forb Centaurea maculosa on grassland Carbon and Nitrogen Pools in Montana, USA

Invasions by exotic forbs are changing large areas of North American grasslands, but their biogeochemical impacts are not well characterized. Additionally, although many invasive plants may alter biogeochemistry, an invasive species’ effects have rarely been evaluated across physically diverse sites. We sampled nine sites containing the perennial Eurasian forb Centaurea maculosa to determine if this invasive species alters soil C and N pools in native grasslands in Montana, USA. We sampled surface soil in adjacent microsites with C. maculosa and native grasses and analyzed soil C and N pools with slow to rapid turnover. None of the pools evaluated in the laboratory showed significant differences between C. maculosa and grass microsites when analyzed across all sites. Some differences were found at individual sites, but they were infrequent and inconsistent: Four sites had no differences, four had differences in one or two pools with intermediate (particulate organic matter C or N) or rapid turnover rates (potentially mineralizable N), and just one site had differences encompassing pools with rapid, intermediate, and slow (total C and N, silt-and-clay-associated N) turnover rates. Where they differed, pools were usually smaller under C. maculosa plants than under native grasses, but the opposite was found at one site. In situ N availability, estimated using ion exchange resins, was significantly lower under C. maculosa than under grasses at one of three sites sampled. Results indicate that C. maculosa may sometimes reduce soil C and N pools, including those related to N availability, but they argue against generalizing about the impacts of C. maculosa in grasslands.

Water use and water-use efficiency of the invasive Centaurea maculosa and three native grasses

The Eurasian herb Centaurea maculosa Lam. has invaded millions of hectares of semi-arid grasslands in western North America. Its success may reflect that it may be more competitive than native species, it is not grazed by large herbivores, it was introduced without its native enemies, it may interfere with native species via allelopathy, or most likely some combination of these factors. Greater competitive ability could include greater use of limiting soil resources, such as water, or more efficient use of soil water, thereby inhibiting establishment, survival, and reproduction of native species. We measured water use and water-use efficiency of Centaurea and three native grasses, Pseudoroegneria spicata [Scribn. and Smith] A. Love, Pascopyrum smithii [Rybd.] A. Love, and Festuca idahoensis Elmer, in a glasshouse. Water-use efficiency was determined by the traditional measure of biomass produced per mass of water used, and by carbon-isotope discrimination (Δ). Centaurea did not use the most water, or use water more efficiently (based on biomass (g)/ water (kg) and carbon-isotope discrimination) than all three native grasses. We also determined carbon-isotope discrimination of Centaurea and dominant native grasses during the 1999 and 2000 growing seasons at three field sites. Centaurea rosettes had the lowest water-use efficiency (greatest carbon-isotope discrimination), followed by mature plants of Centaurea, and then native grasses. Water-use efficiency of mature Centaurea plants and native grasses was greater in late summer than early summer. Centaureas success as an invasive species in North America cannot be attributed to greater use of soil water or greater water-use efficiency than native grasses.

Monitoring soil water and ionic solute distributions using time-domain reflectometry

Abstract Improved ability to measure simultaneously volumetric soil water content ( θ ) and the distribution of ionic solutes would facilitate more intensive and efficient soil management. Time-domain reflectometry (TDR) exhibits a unique ability to measure θ and bulk electrical conductivity ( σ a ) in the same soil volume. Sensors at multiple locations may easily be multiplexed and continuously monitored. Although a number of simple and effective calibrations for θ are available, application of TDR to estimate soil solution conductivity ( σ w ) as an indirect measure of ionic concentration ( C i ) is more problematic, particularly under transient soil wetness conditions. We investigated the potential to monitor continuously soil θ , σ w , and nitrate concentrations in a production peppermint ( Mentha piperita L.) field during April through early August of 1996, using automated TDR. We evaluated two physical-conceptual TDR calibration models for σ w ( θ , σ a ), and compared results to σ w and nitrate concentrations as measured in soil solution extracts and soil cores from the same field area. TDR estimates were generally in reasonable agreement with soil cores, with measured core and TDR values exhibiting substantial variability among replicate samples. Our results suggest that heterogeneity resulting from spatial differences in soil properties, irrigation, fertilizer application, and plant uptake may need to be considered in calibration of TDR.

Shaft-mounted time domain reflectometry probe for water content and electrical conductivity measurements

A new shaft-mounted time domain reflectometry (TDR) probe design is described and evaluated. In contrast to previous shaft-mounted TDR probes (SMPs), the new design may be used to measure both dielectric constant ( K a) and bulk electrical conductivity (σa). Two SMP prototypes, 0.03 and 0.04 m long and having diameters of 0.006 m, were evaluated. The probes were calibrated in several fluids having different K a and σa. A primary advantage of the SMP is minimal physical probe length without sacrificing accuracy of K a readings. Accuracy of K a measurements for the new probes was similar to that of standard 0.20-m-long three-rod probes.