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Vadose Zone Journal November 01, 2003, Vol.2, 443. doi:https://doi.org/10.2113/2.4.443

Time domain reflectometry has become a standard method of estimating soil water content and soil bulk electrical conductivity in both laboratory and field applications. Topics covered in this review include general theory of operation, waveform analysis, signal propagation in layered and dispersive media, and probe design and construction.

Vadose Zone Journal November 01, 2003, Vol.2, 444-475. doi:https://doi.org/10.2113/2.4.444

This comprehensive review distinguishes four methodologies for measuring soil water content with ground penetrating radar: reflected wave velocity, ground wave velocity, transmitted wave velocity between boreholes, and surface reflection coefficient. The basic principles underlying each of these methods are reviewed, and the quality of data acquired from each is illustrated with field examples. The review also evaluates the potential and the limitations of each of the methods and outlines future research needs.

Vadose Zone Journal November 01, 2003, Vol.2, 476-491. doi:https://doi.org/10.2113/2.4.476

The early development of time domain reflectometry (TDR) for measuring water content in soils began somewhat by chance, but supportive relationships quickly developed among the authors to sustain the research and development program through the early difficult stages. By 1987, TDR in soil had “come of age” with the presentation of five TDR papers at the Utah State University Centennial Symposium.

Vadose Zone Journal November 01, 2003, Vol.2, 492-499. doi:https://doi.org/10.2113/2.4.492

Ground penetrating radar with a suspended 1-GHz horn antenna was used for continual measurement of water content over bare and vegetated soils using surface reflection (SR) and signal propagation time (PT). We measured marked differences in water content dynamics for different soil textures, including the influence of soil temperature on SR diurnal variations. Wheat canopy development caused a decrease in SR-measured water content but did not affect PT. Key features of wheat canopy structure were deduced from radar reflections.

Vadose Zone Journal November 01, 2003, Vol.2, 500-510. doi:https://doi.org/10.2113/2.4.500

Borehole ground penetrating radar (BPGR) may allow us to monitor water content rapidly to great depths with high spatial resolution over large sample volumes and with minimal need for medium-specific calibration. We demonstrate that BGPR can be used to monitor water content with a vertical sampling interval that is smaller than the antenna length. However, while there is good agreement between the patterns of maximum depth of drainage and water table depth during pumping and recovery, the maximum depth of drainage, referenced to the middle of the BGPR antennae, is consistently 50 cm deeper than the water table.

Vadose Zone Journal November 01, 2003, Vol.2, 511-518. doi:https://doi.org/10.2113/2.4.511

The capabilities of ground penetrating radar (GPR) and time domain reflectometry (TDR) to assess the temporal evolution of surface volumetric water content spatial variation are compared. The results show that the larger number of GPR measurements that can be acquired in the same time span resulted in more reliable estimates of spatial variation, both in terms of variogram model parameters and water content maps. It was concluded that GPR is an attractive alternative to other measurement techniques for monitoring spatial and temporal water content variation at the field scale.

Vadose Zone Journal November 01, 2003, Vol.2, 519-529. doi:https://doi.org/10.2113/2.4.519

Outflow from a tension disc infiltrometer, TDR measurements of soil water content with probes inserted diagonally from the soil surface, and a branch of the water characteristic curve were simultaneously utilized to inversely estimate hydraulic parameters of a repacked soil. The procedure permitted concurrent estimation of at least four hydraulic parameters and simulation of transient water contents. Parameter uniqueness was improved by use of volume averaged TDR water contents in optimizations.

Vadose Zone Journal November 01, 2003, Vol.2, 530-538. doi:https://doi.org/10.2113/2.4.530

Surface soil solute transport properties measured with a time domain reflectometry (TDR) technique were used to predict subsurface leaching in a greenhouse soil pit. The centers of mass of measured and predicted chemical distributions were 13.8 and 13.5 cm, respectively. The TDR technique has potential for use in fields where surface and subsurface conditions are similar.

Vadose Zone Journal November 01, 2003, Vol.2, 539-543. doi:https://doi.org/10.2113/2.4.539

Heat pulse and time domain reflectometry technologies have been combined to produce a thermo-TDR sensor capable of measuring soil thermal and electrical properties. The thermo-TDR sensor performed well in determining soil water content, temperature, electrical conductivity, thermal conductivity, heat capacity, air-filled porosity, and degree of saturation. Further developments are needed in order for it to make accurate determinations of soil bulk density.

Vadose Zone Journal November 01, 2003, Vol.2, 544-551. doi:https://doi.org/10.2113/2.4.544

In a field evaluation of the dual-probe heat-pulse method, measurements of water content taken near the soil surface were found to have substantial offset error in addition to slight bias. Offset error was reduced considerably by using the heat-pulse sensors to measure change in water content and coupling the results with an independent (gravimetric) measurement of water content. Using a published calibration relationship to eliminate bias reduced the offset error.

Vadose Zone Journal November 01, 2003, Vol.2, 552-560. doi:https://doi.org/10.2113/2.4.552

A multi-functional heat pulse probe (MFHPP), consisting of a single central heater, four thermistors, and four electrodes (Wenner array), was developed. Tests with the MFHPP were performed on Tottori Dune sand for which the soil water content and bulk soil thermal properties were determined. Simultaneously, the bulk soil electrical conductivity was measured using the Wenner array, from which soil solution concentration can be obtained after calibration. With the MFHPP it was possible to estimate soil water fluxes at rates larger than 0.7 m/d.

Vadose Zone Journal November 01, 2003, Vol.2, 561-571. doi:https://doi.org/10.2113/2.4.561

Soil water content in a soybean field was monitored using dual-probe heat-pulse (DPHP) sensors to evaluate the effectiveness of the DPHP technique. A simple in situ calibration of the sensors reduced the difference between water content estimated by the sensors and that estimated by soil sampling and reduced the average standard deviation of water content estimates between sensors. The results demonstrate that the DPHP technique can be used to accurately monitor soil water content in the vadose zone.

Vadose Zone Journal November 01, 2003, Vol.2, 572-579. doi:https://doi.org/10.2113/2.4.572

Controlled laboratory experiments and a field study were used to evaluate the accuracy of six soil heat flux plate designs. Plate estimates of soil heat flux were consistently in error by 10 to 70%, and a proposed procedure for correction did not always reduce the errors to acceptable levels. Significant theoretical development and auxiliary measurements will be required to improve the correction procedure, suggesting that research initiatives would be better directed to developing soil heat flux sensors that avoid the errors inherent in flux plate measurements.

Vadose Zone Journal November 01, 2003, Vol.2, 580-588. doi:https://doi.org/10.2113/2.4.580

Tri-needle probes were used to measure soil heat flux with the gradient technique where the heat flux is equal to the product of the temperature gradient and the soil thermal conductivity measured with a transient heated needle technique. Laboratory investigations indicated that the tri-needle probes are more accurate than commonly used heat flux plates in coarse media, but spatial effects in the field confounded meaningful comparison. The application of the gradient technique using tri-needle probes appears to be a viable alternative to the heat flux plate technique.

Vadose Zone Journal November 01, 2003, Vol.2, 589-594. doi:https://doi.org/10.2113/2.4.589

A constant flux infiltration experiment was conducted to determine whether downhole temperature measurements, made within cased boreholes located within ephemeral streambeds, can be used to estimate streambed infiltration flux. Numerical sensitivity analyses showed that the temperature profile was most sensitive to infiltrating water temperature, the infiltration flux, and the specific heat capacity of dry soil, thereby providing the opportunity to numerically invert temperature measurements to infer water flux.

Vadose Zone Journal November 01, 2003, Vol.2, 595-601. doi:https://doi.org/10.2113/2.4.595

Hydrostatically induced variations in water content produce severe limitations for the use of conventional gas diffusion measurement methods for coarse-textured and aggregated porous media. To overcome this problem, a fully automated dual-chamber cell was developed using a thin profile design to provide gas diffusion measurements in a closed chamber system. Diffusion coefficients were measured in 0.25- to 2-mm aggregated porous medium at fixed points along both wetting and draining water retention curves, showing good correlation with two widely used gas diffusion models.

Vadose Zone Journal November 01, 2003, Vol.2, 602-610. doi:https://doi.org/10.2113/2.4.602

We developed a rugged handheld device to measure air permeability in the field using either of two interchangeable probes, a contact probe or an insertion probe. The entire device, excluding the probes, consists of a power supply, voltage regulators, an air pump, a pressure transducer, and a voltmeter; is contained in a 25 by 19 by 8 cm rectangular box; and weighs approximately 1700 g. Values obtained with the probe were found to compare favorably with other published data and results obtained in the laboratory.

Vadose Zone Journal November 01, 2003, Vol.2, 611-617. doi:https://doi.org/10.2113/2.4.611

This study involved the testing of a portable insertion-type air permeameter as a tool for rapid quantification of the spatial variability of saturated hydraulic conductivity in field conditions. Results showed the existence of a linear log-log relationship between air permeability measured at near field capacity and measured saturated hydraulic conductivity. Prediction of saturated hydraulic conductivity with this log-log relationship was found to be far superior to predictions based on pedotransfer functions using soil texture alone.

Vadose Zone Journal November 01, 2003, Vol.2, 618-626. doi:https://doi.org/10.2113/2.4.618

A water fluxmeter with divergence control, developed in previous work, was modified to measure solute concentrations and thereby facilitate the estimation of solute flux. The accuracy of the modified fluxmeter was proved by measuring water flow and solute fluxes simultaneously in experiments on coarse and fine sands in a laboratory setting. At low flow (621 mm/yr), the dispersion coefficient for nitrate in the coarse sand was about 40% lower than in the fine sand material.

Vadose Zone Journal November 01, 2003, Vol.2, 627-632. doi:https://doi.org/10.2113/2.4.627

Relationships between cone penetrometer resistance (PR) and water content (WC) were developed for conventional tilled and no-till fields where PR and WC data were collected concurrently and coincidentally along 300-m plots. WC measurements were collected with time domain transmissiometry, while a piezoelectric force sensor was used to measure PR. Calibration in the laboratory along with validations in the field were used to assure the quality of the acquired data. The PR vs. WC relationships developed for depths below cultivation were similar to that for the 0.10-m depth in the no-till plot and were not impacted by tillage, traffic, and cropping treatments.

Vadose Zone Journal November 01, 2003, Vol.2, 633-641. doi:https://doi.org/10.2113/2.4.633

A neutron moisture meter stand was designed to fit over access tubes, providing a constant elevation difference between the meter body and soil surface, thus ensuring that the probe is placed at the same depth(s) for each access tube and for each reading date. The stand also elevates the meter sufficiently above the soil surface to avoid errors in standard counts due to surface moisture changes. Improved calibration and reading accuracies for near-surface water content estimates were achieved, with less operator exposure to repetitive strain injuries and without additional exposure to radiation.

Vadose Zone Journal November 01, 2003, Vol.2, 642-649. doi:https://doi.org/10.2113/2.4.642

Two soil series and a potting material were used to evaluate a commercial soil moisture sensor over a range of moisture contents and temperatures. The sensors were found to be sensitive to changes in soil temperature in the range of 15.9 to 39.1 degrees Celsius and to produce different response curves depending on the soil type. The results thus point to a need for calibration of the sensors for each soil type encountered.

Vadose Zone Journal November 01, 2003, Vol.2, 650-654. doi:https://doi.org/10.2113/2.4.650

Diffusion and advection are defined to correspond with associated resistance functions. The paper shows that diffusion as usually defined (as a flux relative to the mean flux) cannot be evaluated with a constitutive equation. Both diffusion and advection are shown to contribute independently to the net transport of a solution constituent.

Vadose Zone Journal November 01, 2003, Vol.2, 655-663. doi:https://doi.org/10.2113/2.4.655

Multiple-tracer technique and undisturbed core technology were used to quantify hydrologic processes controlling the fate and transport of contaminants in the Hanford formation. Separation of multiple nonreactive tracers, early breakthrough of tracers, and a change in breakthrough curve symmetry indicated that preferential flow, media bypass, and the formation of immobile water regimes influence transport at various water contents. Our results demonstrate that original sedimentary depositional processes may be important in governing contaminant transport in porous media.

Vadose Zone Journal November 01, 2003, Vol.2, 664-676. doi:https://doi.org/10.2113/2.4.664

To assess aquifer vulnerability in southern New Jersey, point estimates of groundwater recharge were made at 48 locations using pedotransfer functions based on measured sediment properties. It was found that the estimated recharge is significantly related to soil texture and drainage characteristic. Nitrate concentration and percentage detection of atrazine in samples of shallow groundwater were higher for low recharge sites than for high recharge sites in developed areas, indicating possible dilution by infiltrating water.

Vadose Zone Journal November 01, 2003, Vol.2, 677-691. doi:https://doi.org/10.2113/2.4.677

Volatilization of organic compounds from multicomponent organic liquids and their transport in the unsaturated zone are modeled based on the combination of Fick's Second Law and Raoult's Law. The model predicts the diffusion process and the aging of the organic mixtures very well.

Vadose Zone Journal November 01, 2003, Vol.2, 692-701. doi:https://doi.org/10.2113/2.4.692

Chromate and tritium migration through the vadose zone was evaluated using vacuum and centrifuge techniques to obtain a steady state unsaturated flow. Tritium breakthrough curves indicated that mobile water was >90% regardless of saturation level, with dispersivity increasing nonlinearly with decreasing water content. Despite the difference in solute residence time, the average apparent distribution coefficient of all Cr(VI) leaching experiments was very similar to the distribution coefficient derived from batch equilibrations.

Vadose Zone Journal November 01, 2003, Vol.2, 702-714. doi:https://doi.org/10.2113/2.4.702

We measured the variability of transport among 90 undisturbed soil columns. Significant preferential flow effects were evident as lithium and chloride rapidly appeared in the outflow. Preferential flow model parameters determined from the chloride tracer were normally distributed, but lognormally distributed for the lithium tracer. Probabilistic approaches can be used to determine the number of columns in an experimental set required to assure a meaningful range of transport velocities will likely be represented.

Vadose Zone Journal November 01, 2003, Vol.2, 715-727. doi:

The (de)sorption, leaching and decay of atrazine in a Brazilian Oxisol were investigated using batch sorption and column scale leaching experiments. “Irreversible” sorption of atrazine was observed in the leaching experiment but not in the batch experiment. The atrazine breakthrough and the atrazine remaining in the column and bound to irreversible sorption sites were successfully modeled using a three-site chemical nonequilibrium model in which irreversible sorption was described as a first-order rate decay process.

Vadose Zone Journal November 01, 2003, Vol.2, 728-738. doi:https://doi.org/10.2113/2.4.728

A physically based model that conceptualizes the three-dimensional, axisymmetric problem of diffusion controlled release of an agrochemical from a spherical capsule and convective-dispersive vertical transport of the released chemical is presented. The governing equations are solved with a finite element numerical method. An analysis based on dimensionless variables is presented to quantify the sensitivity of release rate to the factors of capsule membrane conductance, capsule radius, capsule chemical concentration, water flux, and the longitudinal and transverse dispersivities.

Vadose Zone Journal November 01, 2003, Vol.2, 739-750. doi:https://doi.org/10.2113/2.4.739

Plant response functions to matric and osmotic stresses, commonly used in hydrochemical models, are formulated following a potential flow or a transpiration apportioning approach. An alternative formulation was proposed that used potential flow for response to matric stress and transpiration apportioning for response to osmotic stress. The proposed formulation better predicted yield, from a field experiment of melon response to water and salt, than a formulation based exclusively on either a potential flow approach or a transpiration apportioning approach.

Vadose Zone Journal November 01, 2003, Vol.2, 751-758. doi:https://doi.org/10.2113/2.4.751

A previous investigation demonstrated that dry-end deviations from fractal scaling of the water retention of USDOE Hanford site soils set on at a moisture content closely related to the moisture content at which solute diffusion vanishes. This result was interpreted in terms of a minimum water content for percolation of filled capillaries. We now find that wet-end deviations from fractal scaling set on at approximately the same air content, inviting an explanation in terms of air percolation, at least in these relatively coarse soils.

Vadose Zone Journal November 01, 2003, Vol.2, 759-765. doi:https://doi.org/10.2113/2.4.759

Book review

Vadose Zone Journal November 01, 2003, Vol.2, 766. doi:https://doi.org/10.2113/2.4.766
Vadose Zone Journal November 01, 2003, Vol.2, 767. doi:https://doi.org/10.2113/2.4.767
Vadose Zone Journal November 01, 2003, Vol.2, 768. doi:https://doi.org/10.2113/2.4.768
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