Time Domain Reflectometry Readings Research Articles

Soil electrical conductivity estimated by time domain reflectometry and electromagnetic induction sensors: Accounting for the different sensor observation volumes

This paper dealt with the calibration of an EMI sensor for monitoring the time dynamics of root zone salinity under irrigation with saline water. Calibration was based on an empirical multiple regression approach largely adopted in the past and still applied in practice for its relative simplicity. Compared to the more complex inversion approaches, it requires an independent dataset of local σb measured within discrete depth intervals, to be compared to horizontal and vertical electrical conductivity (ECaH and ECaV) readings for estimating the parameters of the empirical regression equations. In this paper, we used time domain reflectometry (TDR) readings to replace direct sampling for these local σb measurements. When using this approach, there is the important issue of taking into account the effect of the different sensor observation volumes, making the readings not immediately comparable for empirical calibration. Accordingly, a classical Fourier’s filtering technique was applied to remove the high frequency part (at small spatial scale) of the original data variability, which, due to the different observation volume, was the main source of dissimilarity between the two datasets. Thus, calibration focused only on the lower frequency information, that is, the information at a spatial scale larger than the observation volume of the sensors. By this analysis, we showed and quantified the degree to which the information of the set of TDR readings came from a combination of local and larger scale heterogeneities and how they have to be manipulated for use in EMI electromagnetic induction sensor calibration.

Shape of Time Domain Reflectometry Signals during the Passing of Wetting Fronts

Core Ideas Precise wetting front arrival times can be determined from local TDR readings. Wetting shock fronts due to preferential flow are sharp. This approach uses the control volume with two‐rod TDR probes as the longitudinal axis. Estimates of preferential flow rates follow from this approach. Viscous flow theory expects sharp wetting shock fronts during infiltration in permeable media, but time domain reflectometry (TDR) measurements, using horizontally installed two‐rod probes, reveal concave and convex increases at the early and late stages during the passing of the front, i.e., the TDR signals are S‐shaped. Wetting front dispersal was initially considered as the cause, due to variations in the flow path lengths at the profile scale. However, later studies favored processes that are closer to the scale of the TDR control volume. In this study, an approach was developed that quantifies the shape of TDR signals exclusively with local features. It improves the determination of the arrival time of a wetting shock front at the depth of a TDR probe, and it ultimately supports the notion of sharp wetting shock fronts at the scale of the probe's length that evolve during preferential infiltration.

Metrological assessment of TDR performance for measurement of potassium concentration in soil solution

ABSTRACT Despite the growing use of the time domain reflectometry (TDR) technique to monitoring ions in the soil solution, there are few studies that provide insight into measurement error. To overcome this lack of information, a methodology, based on the central limit theorem error, was used to quantify the uncertainty associated with using the technique to estimate potassium ion concentration in two soil types. Mathematical models based on electrical conductivity and soil moisture derived from TDR readings were used to estimate potassium concentration, and the results were compared to potassium concentration determined by flame spectrophotometry. It was possible to correct for random and systematic errors associated with TDR readings, significantly increasing the accuracy of the potassium estimation methodology. However, a single TDR reading can lead to an error of up to ± 18.84 mg L-1 K+ in soil solution (0 to 3 dS m-1), with a 95.42% degree of confidence, for a loamy sand soil; and an error of up to ± 12.50 mg L-1 of K+ (0 to 2.5 dS m-1) in soil solution, with a 95.06% degree of confidence, for a sandy clay soil.

Evaluation of the FieldScout TDR300 for Determining Volumetric Water Content in Sandy South Georgia Soils

Although time domain refl ectometry (TDR) probes have been found to be accurate in most soils, the strength of the relationship between TDR values and soil water content can vary considerably from one soil to the next. This study examined the accuracy of water content measurements taken with a FieldScout TDR300 as compared to volumetric water content determined from samples of known volume. The primary soils sampled in the study are of the Albany, Chipley and Ousley series which have sand or loamy sand textures. Simple linear regression analysis was used to evaluate the relationship between TDR readings and volumetric water content. The slope of the regression line was 1.06 and the R 2 value was 0.70. These values lead to the conclusion that the FieldScout TDR300 provided reasonable estimations of soil water content in the soils investigated and would be appropriate to use in certain situations. However, the FieldScout TDR300 may not be appropriate for use in these soils if a high degree of water content accuracy is essential or if readings are needed from depth within the soil.

Design and Field Tests of an Access-Tube Soil Water Sensor

Accurate soil profile water content monitoring at multiple depths has heretofore been possible only using the neutron probe (NP) but with great effort and at infrequent time intervals. Despite the existence of several frequency domain electromagnetic (EM) sensor systems for profile water content measurements, accuracy and spatial representativeness has been precluded by fundamental problems related to soil conductivity and structure effects on the volume explored by the EM field of these sensors, which causes nonrealistic spatial variation in reported profile water contents. Time domain reflectometry (TDR) methods have the distinct advantage of employing a moving EM field that must pass through and be affected by both the drier and wetter soil structures in which the TDR electrodes are embedded. This article describes a profiling water content system based on TDR. The design, laboratory calibration, and field testing is detailed. The sensor system provides unattended, real-time, data acquisition. And, it can be installed without disturbing the soil around the access tube on the outside of which the TDR electrodes are embedded. The correlation coefficient between neutron probe and TDR measured soil water content was 0.94 with a slope of 1.40 and an intercept of -0.08 m3m-3. Bias between TDR and NP readings (TDR-NP) was positive at all depths below 10 cm, ranging from 0.021 and 0.096 m3 m-3. Uncertainty in data of ~0.012 m3 m-3 for soil water, and uncertainty in bulk electrical conductivity of 0.030 S m-1 (both partly due to unreliable mechanical electrical connections) shows that improvements must be made before such a system is acceptable for widespread use.

Measuring the Moisture Content of Green Wood Using Time Domain Reflectometry

The responsible usage of water by facilities that rely on wet log storage in the southern United States has become an issue of great importance as restrictions on water usage have grown in recent years. In order to learn about the dynamics of moisture content in wet-stored logs over time, it is necessary to conduct continuous monitoring of log piles. Time domain reflectometry (TDR) is a method that current research has shown to have potential for use in this area. In this study, TDR probes of three lengths (75, 100, and 125 mm) were systematically inserted into 39 saturated bolts of Pinus taeda L., and both TDR and moisture content measurements were taken nine times over a period of 16 days as the bolts air dried. The samples were then oven dried, and measurements were taken three more times during that process. TDR readings from the 125-mm probes had the strongest relationship (R2 = 0.9426) with moisture content measurements. This result indicates TDR readings are sufficiently correlated with moisture content to accurately predict moisture variation over time and can be used to learn how water application and other factors affect the moisture content of wet-stored logs.

CALIBRATION OF TIME DOMAIN REFLECTOMETRY (TDR) SOIL MOISTURE POINT PROBE FOR TWO SOILS

Maintaining adequate soil moisture in the root zone is crucial in achieving good plant growth. Accurate measurement of soil moisture is essential to keep the right level of soil moisture. Many studies have reported the successful application of Time Domain Reflectometry (TDR) for soil moisture measurement. This study was initiated to obtain calibration curves for soil water content determinations by TDR for two soil types. Measurements were taken in the laboratory for a silt loam and a sandy loam soils, using TDR Soil Moisture Measurement Instrument, Moisture PointTM Model MP-917, and Moisture Point Probe type-K. TDR probe calibration was performed for two soil types contained in wooden boxes (100 cm x 100 cm x 80 cm). The calibration was accomplished by comparing the volumetric moisture content (qTDR) and time delay (tTDR) response of TDR probe to that of the gravimetric volumetric moisture content (qgrav). The TDR measurements were taken, in triplicates, at four depths (0-15 cm, 15-30 cm, 30-45 cm, and 45-60 cm) for 38 days after wetting the soil. Soil samples for the gravimetric moisture content measurements were collected from the same locations from where TDR readings were taken. The study has demonstrated that the TDR technique is a reliable alternative method for measuring soil moisture content. The moisture content measurements obtained with TDR were comparable to that of the gravimetric method and showed a good relationship to gravimetric determinations (r 2 =0.85 for silt loam and 0.89 for sandy loam).

Polymer tensiometers with ceramic cones: direct observations of matric pressures in drying soils

Abstract. Measuring soil water potentials is crucial to characterize vadose zone processes. Conventional tensiometers only measure until approximately −0.09 MPa, and indirect methods may suffer from the non-uniqueness in the relationship between matric potential and measured properties. Recently developed polymer tensiometers (POTs) are able to directly measure soil matric potentials until the theoretical wilting point (−1.6 MPa). By minimizing the volume of polymer solution inside the POT while maximizing the ceramic area in contact with that polymer solution, response times drop to acceptable ranges for laboratory and field conditions. Contact with the soil is drastically improved with the use of cone-shaped solid ceramics instead of flat ceramics. The comparison between measured potentials by polymer tensiometers and indirectly obtained potentials with time domain reflectometry highlights the risk of using the latter method at low water contents. By combining POT and time domain reflectometry readings in situ moisture retention curves can be measured over the range permitted by the measurement range of both POT and time domain reflectometry.

Efeito da instalação e da variabilidade entre sondas no uso da técnica de TDR em um Nitossolo Vermelho Distroférrico

Although soil moisture is defined by a single physical concept, its determination is not necessarily simple. Since such determination is necessary to soil-water management studies and practices, direct and indirect methods have been developed. Although the Time Domain Reflectometry (TDR) technique has been widely used in recent years, research has shown that prior calibration must be undertaken before the use of this technique. Scanty information exists for tropical soils. Research was undertaken on a Rhodic Kandiudalf, commonly found in the northeast region of the state of Parana, Brazil. Present study detects the effects of probe characteristics and of soil moisture conditions at the moment of probe installation on the TDR measurements. Installation effects was much more important than probe characteristics. Soil water content during probe installation process affects TDR readings, probably affecting soil-probe contact

TDR/DMT CHARACTERIZATION OF A RESERVOIR SEDIMENT UNDER WATER

The Shihmen Reservoir, completed in early 1960s, has been an important hydro project in Northern Taiwan. Soil erosion and sediment have been a major concern for the longevity of the reservoir. After a series of typhoons in 2004, the intake valve of the hydro power plant was covered by 10 m of sediment. The power generation has been halted since then. The intake valve was originally designed to be operated in clean water. In order to evaluate the feasibility of re-opening the power plant intake valve, it was necessary to know the density state of the sediment (referred to locally as the bottom mud) and the lateral pressure exerted on the intake valve. The center of the intake valve was at approximately 70 m below water. A testing device that consisted of a time domain reflectometry (TDR) probe placed on top of the Marchetti dilatometer (DMT) was developed by the authors with an original intention to determine simultaneously, the solid concentration, stiffness and stress state of the bottom mud. The TDR/DMT probe was attached to a string of 90 m long drill rods. A skid mount drill rig bolted to a barge was used to control the drill rods. The weight of the drill rods was sufficient to push the TDR/DMT probe into the bottom mud. TDR and DMT readings were taken from 60 to 80 m below water. The electrical conductivity measurement from the TDR probe was used to determine the solid concentration. The lateral stress was inferred from the DMT po readings. The difference between po and p1 was used to determine the density state of the bottom mud. Ten DMT profiles were taken five of them had TDR readings. The paper describes field set up of the TDR/DMT probe, its test procedure and interpretation of the test results.

Estimating unsaturated soil hydraulic properties from laboratory tension disc infiltrometer experiments

Four tension disc infiltration experiments were carried out on a loamy soil in the laboratory for the purpose of estimating the unsaturated soil hydraulic properties. Sixteen tensiometers were installed in pairs at the following coordinate (r, z) positions: (10, 2.5), (10, 5), (10, 10), (15, 5), (15, 10), (15, 15), (15, 20), and (15, 30), where r represents the distance from the axis of symmetry and z is the location below the soil surface. A time domain reflectometry (TDR) probe was used to measure water contents at a depth of 2 cm directly below the tension disc. The first three experiments involved supply pressure heads at the disc of −20, −10, −5, and −1 cm, with the experiment lasting for ∼5 hours. The same supply pressure heads were also used for the fourth experiment, which lasted 6.25 days so as to reach steady state at each applied tension. The measured data were analyzed using Wooding's [1968] analytical solution and by numerical inversion. The parameter estimation method combined a quasi three‐dimensional numerical solution of the Richards equation with the Marquardt‐Levenberg optimization scheme. The objective function for the parameter estimation analysis was defined using different combinations of the cumulative infiltrated volume, TDR readings, and tensiometer measurements. The estimated hydraulic properties were compared against results obtained with an evaporation experiment as analyzed with Wind's [1968] method. Water contents in the retention curves were underestimated when both transient and quasi steady state experiments were analyzed by parameter estimation. Unsaturated hydraulic conductivities obtained by parameter estimation and using Wooding's [1968] analysis corresponded well. Drying branches of the hydraulic conductivity function determined by parameter estimation also corresponded well with those obtained with the evaporation method.

Laboratory Measurements of Solute Transport Using Time Domain Reflectometry

AbstractMeasurement of solute transport at different depths in layered soil columns provides more information than traditional outflow measurements. The objective of this study was to extend the time domain reflectometry (TDR) method of measuring soil water content, θ, and bulk electrical conductivity, σT, to the nondestructive measurement of resident concentration, CR, of an added electrolytic tracer. Calibration curves of CR vs. θ and σT were obtained directly, by mixing a sandy soil with different amounts of tracer and water and taking TDR readings of impedance. Similar soil was packed into a Plexiglas box fitted with curved TDR probes at different radial distances from one corner. A constant flux of water was applied at the corner of the box and a pulse of KCl tracer added. Measurements of θ and CR as a function of time and radial distance were made during the three‐dimensional flux experiment. In a one‐dimensional flux experiment, undisturbed soil columns (1.5 m long, 0.15‐m diam.) were instrumented with straight TDR probes installed horizontally at 0.10‐m increments. Indirect calibration of CR vs. θ and σT were obtained under steady‐flow conditions by using a step increase in tracer, and by numerically convolving measurements from a pulse application of tracer. The final value of σT was equated to the input concentration of the tracer to obtain the calibration relationship. The method overcomes many of the limitations associated with soil solution samplers, allowing frequent measurements at close spacings and low cost.

Parameter Estimates of Field Solute Transport Models Based on Time Domain Reflectometry Measurements

AbstractA new technique using vertically installed time domain reflectometry (TDR) probes can be used to measure the mass flux of solute past the ends of the TDR probes and characterize the probability density functions of travel times. The procedure is applicable for a conservative tracer under steady‐state leaching conditions of constant surface volume flux density of water. The procedure is, however, limited to soils low in salts where the addition of a solute spike will cause a measurable reduction in the TDR signal. We developed the transport equations used to analyze the breakthrough data and obtain the appropriate parameters of two steady‐state models, the convective dispersion equation and the convective lognormal transfer function, as well as a moment analysis. Other models could also have been used. The TDR measurements (reduced in terms of the maximum measured value) for a Dirac‐delta function (spike) input of solute are shown to be equivalent to the solution of a step function input of pure water at the surface to a soil containing unit concentration to infinite depth, i.e., the complement of the flux‐averaged reduced concentration for a step function input of solute. Nonlinear curve‐fitting techniques were used to obtain the transport parameters. Fitting the measured TDR readings to the two models for both field and laboratory data gave good estimates of the transport parameters. The moment analysis, however, reflected the early breakthrough and tailing of the solute, resulting in a doubling of the variance. In addition, variances at the averaged field scale were found to be slightly more than double those at the local scale. The field TDR procedure is fast, inexpensive and nondestructive; the analysis presented yields needed parameters for model predictions.

Time Domain Reflectometry Response to Lateral Soil Water Content Heterogeneities

AbstractWater‐content heterogeneities can occur between the parallel rods of the time domain reflectometry (TDR) probe if TDR is used in cracked soils or in laboratory studies with probes oriented in a variety of directions. The accuracy of water‐content measurements using TDR under such heterogeneous conditions was investigated in a series of laboratory experiments. Two cases were studied: (i) one TDR rod in dry soil and the other in wet soil; and (ii) an artificial soil crack filled with water or air between the two TDR rods. In Case 1, TDR readings were heavily biased toward the water content of the dry soil. This effect occurs because the electromagnetic wave (EMW) reaches the end of the waveguide quicker in dry than in wet material. Because the time delay of the earliest reflection signal is read from the TDR, the travel time of the dry material is measured. In Case 2, the air‐filled crack between the waveguide rods had only a small effect on measured water content when the soil was dry; however, water‐content measurements were reduced by as much as 46% in wetter soils. The effect of water‐filled cracks was small in both dry and wet soils. The effect of soil cracks is explained by a static capacitance model. Results from the experiments do not contradict previous work that demonstrated that TDR measures the average water content of heterogeneous layers oriented perpendicular to the TDR probe axis. In the two cases examined here, knowledge of the orientation of soil water heterogeneities relative to the TDR waveguide rods is necessary for correct data interpretation.