Time Domain Reflectometry Probes Research Articles

Modelling DUALEM-2 measured soil conductivity as a function of measuring depth to correlate with soil moisture content and potato tuber yield

Electromagnetic induction (EMI) is a non-destructive technique to measure the apparent electrical conductivity (ECa) of agricultural soils. It has become popular in precision agriculture applications to estimate crop yield from ECa. This study was conducted to relate ECa with soil moisture content (θv) and potato tuber yield under Prince Edward Island (PEI), Canada growing conditions and to establish a two-layered soil model for ECa. During year one, horizontal co-planar (HCP), perpendicular co-planar (PRP), and θv were measured with DUALEM-2 sensor and time domain reflectometry (TDR) probe and correlated with yield measured from 12 m2 grids at 40 locations in each of the two potato fields on PEI. During 2018, vertical electrical sounding was undertaken by raising the DUALEM-2 from ground-level at 0.2 m intervals to a height of 1.6 m to investigate variations in ECa with soil depth. A two-layered soil model was developed using generalized reduction gradients to determine the surficial conductivity and thickness, and subsurface conductivity. ECa variables (HCP and PRP), θv, and yield were significantly correlated and remained consistent throughout the study period, indicating their potential in predicting potato tuber yield. ECa in the fields varied both spatially and laterally with depth. GIS interpolated maps of a representative set of data for HCP and tuber yield (2017 data) showed structured spatial patterns and thus feasibility for delineating management zones. Further studies should examine the potato root zone (0.15–0.3 m) ECa specifically for developing higher layered models, which may be useful for accurate prediction of tuber yield. Non-destructive and accurate yield mapping using DUALEM-2 sensor combined with mathematical modelling of soil ECa layering has potential for use in developing management zones in potato fields.

Laboratory Calibration of TDR Probes for Simultaneous of Measurements Soil Water Content and Electrical Conductivity

ABSTRACTAccurate measurement of water content (θ), apparent electrical conductivity (ECa) of soils, and soil solution electrical conductivity (ECw) is critical for a better management of irrigation water and the effective monitoring and control of soil salinity. In this paper, we demonstrate the design and validation of an identical time domain reflectometry (TDRWO) probe for accurate and non-destructive measurements of ECw. Two calibration functions were used to determine the soil-water content by TDR. Theses relations were verified by laboratory experiment over a wide range of θ and bulk density (BD) for dune sand. The exact length and the impedance of all TDR-probes were calibrated using the WinTDR procedure. Estimates of ECw were based on θ and the bulk ECa using a special modeling approach which applies long-time TDR waveform analysis. The geometrical disposition of the matrix phase factors (τ and β) was calculated by immersing the TDR probe in sodium chloride (NaCl) solutions of different electrical conductivities in pressure cells wetted and drained with each of these NaCl solutions. The reliability of the TDRWO was validated/proven under laboratory conditions. The laboratory experiment consisted in inserting the TDR probe into a pressure cell packed with mixed sand and 2-mm sieved loam soil that was subsequently wetted and drained with different NaCl solutions at various pressure heads. The ECw estimated by TDRWO was compared to the ECw measured in the draining solutions after they stabilized in the soil porous system. The τ and β factors calculated for the ceramic disks sets were 1.991 and 4.312, respectively. In the laboratory experiments, high and therefore significant correlations (R2 = 0.986; P < .001) were found between the ECw estimated by the TDRWO and the corresponding ECw values measured in the column-drainage. These results demonstrate that the TDRWO is an instrument which allows researchers to accurately estimate soil solution salinity independently of the soil water content and the porous medium in which the TDR probe is installed. Test results showed that the TDR method has high accuracy for monitoring soil water content but is less accurate in monitoring soil electrical conductivity across a water content range of about ≤0.03cm3/cm3. Regression analysis between TDR-Volumetric water content (ϴv) and weighing method – ϴv showed good correlation with an R2 of 0.97 and 0.98 during two laboratory experiments.

Spatial variability of soil thermal properties and their relationships with physical properties at field scale

Characterizing the spatial variability of soil thermal properties and quantifying their relationships with commonly measured soil physical properties is important for successful implementation of precision agriculture practices. However, such information at field scale is scarce. This study aimed to characterize the spatial variability of soil thermal properties and to quantify their spatial relationships with soil physical properties at a field scale. Measurements of soil water content (SWC) and thermal properties were carried out at 80 locations using a time domain reflectometry (TDR) probe and a heat pulse probe (Decagon KD2 Pro), respectively in a cultivated cornfield in southwestern QC, Canada. Soil texture and total organic carbon (TOC) at those locations were also determined in the laboratory. Both coefficient of variation (CV%) and variogram range showed a weak to moderate variability and spatial dependency for the soil properties which was attributed to the homogeneity of the soil, crop, and land management practices. Results of principle component analysis (PCA) indicated that the soil thermal properties and soil water content (SWC) had strong correlations with principle component 2 (PC2). The variogram ranges indicated that the soil thermal properties varied on a similar spatial scale as the SWC in the field. Cross variograms indicated strong spatial correlations between SWC and thermal properties, particularly thermal conductivity (k) and thermal diffusivity (D). Overall, results of this study showed that the SWC had a stronger influence on the spatial variability of soil thermal properties than the other properties such as TOC and soil texture at the field scale. Therefore, the results of this study could be useful for accurate modeling of the soil thermal regimes and mass-energy exchange processes at the field scale as well as developing successful agricultural management strategies such as precision irrigation. Future studies should explore the relationships between the thermal properties and other soil properties such as bulk density under various cropping systems.

Modelling Soil Water Dynamics from Soil Hydraulic Parameters Estimated by an Alternative Method in a Tropical Experimental Basin

Knowledge about soil moisture dynamics and their relation with rainfall, evapotranspiration, and soil physical properties is fundamental for understanding the hydrological processes in a region. Given the difficulties of measurement and the scarcity of surface soil moisture data in some places such as Northeast Brazil, modelling has become a robust tool to overcome such limitations. This study investigated the dynamics of soil water content in two plots in the Gameleira Experimental River Basin, Northeast Brazil. For this, Time Domain Reflectometry (TDR) probes and Hydrus-1D for modelling one-dimensional flow were used in two stages: with hydraulic parameters estimated with the Beerkan Estimation of Soil Transfer Parameters (BEST) method and optimized by inverse modelling. The results showed that the soil water content in the plots is strongly influenced by rainfall, with the greatest variability in the dry–wet–dry transition periods. The modelling results were considered satisfactory with the data estimated by the BEST method (Root Mean Square Errors, RMSE = 0.023 and 0.022 and coefficients of determination, R2 = 0.72 and 0.81) and after the optimization (RMSE = 0.012 and 0.020 and R2 = 0.83 and 0.72). The performance analysis of the simulations provided strong indications of the efficiency of parameters estimated by BEST to predict the soil moisture variability in the studied river basin without the need for calibration or complex numerical approaches.

Improving traditional leafy vegetables production through water use efficiency and fertilizer microdosing technology in Benin Republic

In most developing countries, water and fertilizer management pose serious problems to vegetables production. This study aimed at determining the fresh leaf yields and Water Use Efficiency (WUE) under microdosed fertilizer technology of two main traditional leafy vegetables (TLVs) (Amaranthus cruentus L. and Solanum macrocarpon L.) that are consumed in Benin. A field experiment was carried out at Centre de Recherche Agricole d'INA, Northern Benin, under sprinkler irrigation at maximum evapotranspiration (MET) calculated from Cropwat 8.0. The study was laid out in a randomized complete block design with four replications and two factors: 1) urea (46% N) applied at D0, D20, D40, D60, D80 (kg ha-1) and 2) urea application timing at d1: early application (10 days after transplanting, DAT) and d2: later application (15 DAT). Following the MET (6.6 mm day-1), plants were irrigated twice a day (morning and evening) at 3.3 mm per watering. This irrigation rate was applied by operating the sprinklers with 0.078 mm min-1 of water flow for 44 min. Soil moisture and drainage were measured every week using TDR (time domain reflectometry) probe and micro-lysimeters respectively. The water balance was calculated using the trapezoidal method. WUE was determined using the ratio of dry biomass yields and real crop evapotranspiration (RET). Data were analyzed with R software following analysis of variance (ANOVA) and Pairewise t test at 0.05 level of probability. Quadratic regression was used to model TLVs response to fertilizer rates. At D60d1 and D40d2 we recorded the highest RET (35-42.45 mm week(-1)), highest fresh biomass yields (4.5-4.7 t ha(-1) for eggplant and 9.9-10.5 t ha(-1) for amaranth) and highest WUE (2.5-4 kg ha(-1) mm(-1)), respectively. Microdosed fertilizer technology may be therefore used to improve TLVs production and WUE because it may contribute to reduce fertilizer use by 2.5-10 times for urea and to 50% irrigation water while increase fresh leaf yield by 30-64%.

Potential of soil moisture retrieval for tropical peatlands in Indonesia using ALOS-2 L-band full-polarimetric SAR data

ABSTRACTIn this paper, a soil moisture retrieval from full-polarimetric synthetic aperture radar (SAR) data is investigated for sparsely vegetated soil surfaces. An improved retrieval method adapting the variations in vegetation is proposed by incorporating the generalized volume model into the polarimetric two-scale two-component model (PTSTCM). The feasibility of the method, termed as the adaptive PTSTCM, has been tested for tropical peatland sites in Indonesia which exhibit a variety of sparse vegetation cover on soil after land clearing activities. The data were collected in March and August 2017 with the time domain reflectometry (TDR) probe for a total of 18 sample points over 11 regions. The method was applied to ALOS-2 L-band quad-pol SAR data that were acquired simultaneously with field measurements. We compared the results between the proposed adaptive PTSTCM and the original PTSTCM that utilizes specific types of volume model (i.e., randomly, horizontally, and vertically oriented volume models). Scatterplots of estimated versus measured results reveal that the adaptive PTSTCM yields a root-mean-square error (RMSE) of 5.1vol. and inversion rate of 35.0 and 58.5 for March and August data, respectively, which are found to be superior to those of the original PTSTCM.

Application of TDR Test Probe for Determination of Moisture Changes of Railway Substructure Materials

Abstract The initial part of the paper characterizes a non-destructive TDR (time domain reflectometry) method for measuring moisture. This method is used to identify the moisture changes in the materials incorporated in the structural layers of the railway models built at a 1:1 scale (experimental stand DRETM). The subsequent part specifies the procedure for calibrating the TDR probe for selected building materials composing the measuring profiles of the experimental stand (Liapor fr. 1/8 mm, sand and crushed aggregate fr. 0/31.5 mm). The result of the TDR probe calibration is the calibration curves for the tested building materials, which can be used to identify the real moisture values in the railway line structure, in any season. The specified moisture values represent the necessary inputs for the numerical modeling of the non-traffic load (climatic factors) on the railway line using the SoilVision software.

Suitability of time domain reflectometry (TDR) for moisture content monitoring in historic building walls

Measuring moisture content in building materials is crucial for the correct diagnosis of buildings pathologies, for the adoption of appropriate intervention measures and for the efficiency evaluation of the treatment solutions applied. There are several different techniques available to measure and monitoring the moisture content in building materials. However, it still remains a great challenge to perform those studies in historical buildings, since minor-destructive techniques are required. Furthermore, if continuous moisture content readings in space are desired, in order to study the moisture transfer phenomenon along the wall thickness, the challenge is even greater. The TDR technique is a relatively new method for measurement of moisture content in building materials with a set of unexplored potentialities capable of satisfying the two mentioned requirements. In this paper, the suitability of the TDR technique to obtain continuous cross-section moisture content profiles has started to be tested on two limestone prototype walls. Each wall was equipped with four TDR probes, designed practically with the same length of the wall thickness and placed at different heights. All the necessary equipment, installation steps, and difficulties are here presented. The preliminary results suggest that the TDR technique is suitable for moisture content monitoring in consolidated porous building materials.

Indirect measurements of water content using TDR-inferred dielectric permittivity and electrical resistivity

The measurement of moisture distribution in Engineered Barrier Systems (EBS) in salt mines and deep geological disposals is essential in order to monitor fluid ingress and record data for long-term security analyses. Additionally, soil moisture content has influence over the mechanical properties of the soil as well as plant growth, soil stability and contaminant transport to cite some. Therefore, finding affordable and reliable ways to determine moisture content, quickly and in the field without sampling, is of great interested among people in different subject areas. Time-domain reflectometry (TDR) has become a recognized electromagnetic method for non-destructive measurement of dielectric permittivity and electrical conductivity of moist porous materials. It turns out that both these measurements depend on the material moisture content, among other things. This paper presents a series of calibration tests performed on soil samples. TDR probes were used to obtain the dielectric permittivity and electrical conductivity of the samples. As a consequence, relationships between these measurements and the samples' volumetric water content were later established. These relationships can then be used to indirectly determine that important information of water content on similar soil material using cheap, quick and non-destructive TDR probes.

Stress Effects on Soil Freezing Characteristic Curve: Equipment Development and Experimental Results

Core IdeasA triaxial apparatus is developed to measure stress‐dependent SFCC.Unfrozen water content of clay and sand increases with increasing confining stress.Stress effects on the SFCC of clay are more significant than sand.The soil freezing characteristic curve (SFCC) defines the relationship between soil temperature and unfrozen water content. This curve is important for predicting water flow and heat conduction in frozen soils, as well as freezing heave and thawing settlement. So far, SFCCs reported in the literature were usually determined at zero stress. To investigate stress effects on SFCC, a stress‐ and temperature‐controlled triaxial apparatus was developed in this study. The unfrozen volumetric water content was measured using a newly developed noninvasive time domain reflectometry (TDR) probe. The new apparatus was used to measure SFCCs of two typical soils (i.e., clay and sand) at different stress conditions (i.e., 30, 100, and 200 kPa). Each specimen was subjected to compression, and then a cycle of freezing and thawing. As expected, for both soils, the saturated water content prior to freezing was smaller at higher stresses because of compression. During the subsequent freezing and thawing, the soil specimen at a higher stress was able to retain more liquid water than that at a lower stress. The higher unfrozen water retention capacity at higher stresses is mainly because the pore size of a soil specimen becomes smaller during compression. Hence, more water can retain a liquid state due to the enhanced capillarity. On the other hand, stress effects on the SFCC were found to be more significant for clay than for sand. This is likely because the stress‐induced change in pore size distribution is larger in clay due to its higher compressibility.

Assessing AquaCrop model to simulate soil water contents under semi arid climate of Central Tunisia

The objective of the present study was to preliminary calibrate and validate AquaCrop model based on crop conservative parameters from the literature for plant growth and water stress thresholds. In addition, physical soil characteristics, root growth, duration of plant stages and atmospheric demands were introduced according to field measurements. Based on this preliminary calibration, simulated water contents were compared to a measured data set of water contents retrieved from deficit and full irrigation treatments on a potato cropped field during an experimental year of 2015. Statistical indexes were computed and finally this performance in simulating water contents were validated under independent measurements carried out during an experiments campaign on the same field on 2014. Moreover, the paper presents the experimental protocol followed for soil characterization, considered as a milestone component for this soil water contents prediction. Results showed, that under the followed preliminary calibration, the model was able to simulate water contents (0v). In general, values of Root Mean Square Error were lower than 0.03 cm3.cm-3 representing the magnitude of error of the time domain reflectometry probe. Moreover values of Nash coffecients were close to 1 confirming the goodness of fit between measured and estimated water contents. Once assessed, the model could be used to study effects of different irrigation strategies on dynamic of water contents aiming to increase water use efficiency.

An Innovative Use of TDR Probes: First Numerical Validations with a Coaxial Cable

In this paper we propose a study regarding some possibilities that can be offered by a time domain reflectometry (TDR) probe in retrieving both dielectric and magnetic properties of materials. This technique can be of interest for several applications, among which the characterization of soil in some situations. In particular, here we propose an extension of the paper “Retrieving electric and magnetic propetries of the soil in situ: New possibilities”, presented at the IWAGPR, held in Edinburgh in 2017, and as a new contribution we will validate a transmission line model with numerical data simulated by the CST code.

Prediction of profile soil moisture for one land use using measurements at a soil depth of other land uses in a karst depression

Information on root-zone soil water content (SWC) is essential for vegetation restoration, irrigation scheduling, and hydrological modeling. However, measurements of SWC within a variety of land uses may be time-consuming and labor-costing. This study tested whether SWC at a depth of a land use can be used to predict profile SWC of other land uses in terms of temporal stability analysis at a karst depression area in southwest China. A total of 30 datasets of root-zone SWC from 0.1- to 0.5-m depths were collected by time domain reflectometry probes for three typical land uses from March 12 to November 8, 2015. Results showed that the profile mean SWC and its associated standard deviation (SDP) and coefficient of variation (CVP) differed significantly (P < 0.05) among the grassland, farmland, and forestland. The profile SWC was more temporally stable according to the apparently lower CVT in comparison with CVP. The similarities of the vertical patterns of SWC were strong for the same land uses, while were relatively weak between the different land uses. The SWC measurements of the most temporally stable depth can be used to accurately predict profile SWC for both the same land use and other land uses. This study further expands the application of the temporal stability analysis and can aid water resource management in areas with diverse land uses.

Anaerobic biodegradation of dissolved ethanol in a pilot-scale sand aquifer: Gas phase dynamics

Groundwater contamination from ethanol (e.g., alternative fuels) can support vigorous biodegradation, with many possible reactions producing dissolved gases. The objective of this study was to improve the understanding of the development and evolution of trapped gas phase changes occurring within an ethanol plume undergoing biodegradation. The experiment performed involved highly detailed spatial and temporal monitoring of gas phase saturations using Time Domain Reflectometry probes embedded in a 2-dimensional (175 cm high × 525 cm long) synthetic aquifer (homogeneous sand tank with horizontal groundwater flow). Ethanol injection immediately promoted gas-producing reactions, including: fermentation, denitrification, sulphate-reduction and iron(III)-reduction, with methanogenesis developing between 69 and 109 days. Substantial in situ increases in trapped gas were observed over ~330 days, with maximum gas saturations reaching 27% of the pore volume. Despite sustained gas production, this maximum was never exceeded, likely due to the onset of gas phase mobilization (i.e., ebullition) upon reaching a buoyancy-capillarity threshold. Reductions in the quasi-saturated hydraulic conductivity, resulting from the gas phase accumulation, were restricted by ebullition to a factor of ≤2; but still appeared to alter the groundwater flow field. Overall, trapped gas saturations exhibited high spatial and temporal variability, including declines within the plume and increases outside of the plume. Influential factors included vertically-shifting ethanol inputs and resultant secondary redox reactions, microbial controls on redox zonation, ebullition, and altered groundwater flows. These observations have implications for the transport of gases and volatile compounds within plumes and above the water table at sites with groundwater contamination from ethanol or other highly degradable organics.

Silt fraction effects of frozen soils on frozen water content, strength, and stiffness

The strength and stiffness of soil mixtures vary with regard to the particle composition and the properties of soil mixtures that critically change when water, which is a component in soils, is frozen. The objective of this study is to evaluate silt fraction effects of frozen sand-silt mixtures on the frozen water content, shear strength, and stiffness. The sand-silt mixtures are prepared in a shear box at a fixed relative density of 60% and a fixed degree of saturation of 15% with various silt fractions ranging from 0 to 100% in weight (Wsilt/Wsand × 100%). A time domain reflectometry (TDR) probe, bender elements, and thermocouple are installed in the shear box, and the direct shear apparatus is placed in the freezing chamber. After the sand-silt mixtures are frozen at −5 °C, the direct shear tests are conducted. The TDR signals and shear waves are monitored before and after the freezing phases for the estimation of the volumetric water content and stiffness, respectively. Test results show that the void ratio, volumetric frozen water content, shear strength, and shear wave velocity are at a minimum near a silt fraction of 30%. As the relationships between the volumetric frozen water content and peak shear strength, as well as between the volumetric frozen water content and shear wave velocity after freezing, are linear, the peak shear strength correlates well with the shear wave velocity after freezing. Furthermore, as the residual shear strength and shear wave velocity before freezing are related to the silt fraction, the residual shear strength is bi-linearly proportional to the shear wave velocity before freezing. This study suggests that the void ratio and frozen water content of sand-silt mixtures are affected by the silt fraction. Thus, the silt fraction effects should be considered for the characterization of the strength and stiffness of frozen soils.

Responses of Sap Flow of Deciduous and Conifer Trees to Soil Drying in a Subalpine Forest

Co-occurring species may adopt different water-use strategies to adapt to limited soil water. In Jiuzhaigou Valley, a continuous decline in soil water after an initial recharge from the thawing of snow and frozen soil in early spring was observed, but its effects on the sap flow dynamics of co-occurring species are not well understood. To clarify the species-specific water-use strategy, variations in sap flow and environmental conditions were investigated for two co-occurring species (Betula albosinensis Burk. and Pinus tabuliaeformis Carr.) in a mixed forest during a transition from the wet to dry period in 2014. Sap flow was measured using Granier-type thermal dissipation probes, and the soil-water content was measured using time-domain reflectometry probes for a successive period. Our study showed that B. albosinensis maintained relatively high transpiration until late into the season regardless of soil moisture, while the transpiration of P. tabuliformis showed a continuous decrease in response to seasonal soil drying. Sap flow for both species exhibited a marked hysteresis in response to meteorological factors and it was conditioned by the soil-water status, especially in the afternoon. We found that P. tabuliformis was sensitive to soil-water conditions, while for B. albosinensis, the sap flow was not very sensitive to changes in soil-water conditions. These results indicate that B. albosinensis could manage the water consumption conservatively under both dry and wet conditions. These results may have implications for evaluating the species-specific water-use strategy and carrying out proper reforestation practices.

Validation of a Thermo–Time Domain Reflectometry Probe for Sand Thermal Conductivity Measurement in Drainage and Drying Processes

Abstract The thermo–time domain reflectometry (TDR) probe is a promising tool for simultaneous measurements of the thermal and geotechnical properties of soil under both laboratory and field conditions for geothermal applications. This study presents the validation of a thermo-TDR probe for sand thermal conductivity measurement during drainage and drying processes. Laboratory experiments were performed on four quartz sands with different particle sizes using a modified hanging column device. A KD2 Standard Pro was used to measure sand thermal conductivity, which was then compared with the results obtained by the thermo-TDR probe. A series of statistical analyses, including root mean square error, mean absolute error, and variance account for, was conducted to further evaluate the performance of the thermo-TDR probe for sand thermal conductivity measurement during the dryout process. It was concluded that the thermo-TDR probe can measure sand thermal conductivity accurately with at least 85 % confidence interval during the drainage and drying processes. The measurement accuracy of the probe increases as the grain size of the sand decreases.

Evaluating Commercial Moisture Probes in Reference Solutions Covering Mineral to Peat Soil Conditions

Core Ideas Two commercially available moisture probes were tested on reference solutions. Dielectric permittivity was measured across the range from 1 to about 80. Uncertainties increased with increasing dielectric permittivity. Electric conductivity influenced dielectric permittivity measurements. Capability of the probes to measure electric conductivities differed among the probe types. Capacitance and time domain reflectometry (TDR) probes are frequently used for measurements of the volumetric soil water content. The measurement concept is based on the correlation between volumetric water content and dielectric permittivity (ε). While considerable effort has been made to accurately measure ε in the typical range of mineral soils (&lt;40), little attention has been paid to the capability of moisture probes to measure high ε (&gt;40), typical for highly porous media like organic soils. We evaluated the capability of two moisture probe types (TRIME‐PICO 64 and GS3) to measure ε across the range from 1 to 80. In the case of the TRIME probes, different equations to calculate ε from transit times were tested. Measuring in a set of reference solutions, the TRIME probes had an RMSE of 18.73 for ε values derived using the manufacturer's calibration. With a new calibration, the RMSE was decreased to 3.55. The GS3 probes had an RMSE of 3.96. For both probes, uncertainties increased with increasing ε. We also tested the performance for different electrical conductivities of the reference solutions. Accuracy of ε values was unaffected by increasing conductivities for the TRIME probes but decreased for the GS3 probes. The GS3 probes, however, were able to determine electric conductivities accurately, while TRIME probes failed for electrical conductivity although indicated differently by the manufacturer.

Interpreting TDR Signal Propagation through Soils with Distinct Layers of Nonaqueous‐Phase Liquid and Water Content

Core Ideas The TDR response in the presence of NAPL transport was investigated in a homogeneous soil. The experimental framework was designed to simulate sharp NAPL fronts that moved in the soil. The TDR waveforms were analyzed to deduce the influence of NAPLs on TDR signal propagation. A general procedure to determine the location of an NAPL‐contaminated front was developed. Several studies have demonstrated that time‐domain reflectometry (TDR) has an enormous potential to detect and monitor nonaqueous‐phase liquids (NAPLs) in uniformly contaminated soils with reference to different‐textured soils and under different saturation conditions. Few attempts have been proposed to describe NAPL distribution by using TDR when a contaminated front propagates through a soil. In this study, the TDR response in the presence of NAPL‐contaminant transport processes was investigated in a homogeneous soil under confined conditions with a series of laboratory‐controlled tests. The laboratory procedure involved measurements of dielectric permittivity (εb) and electrical conductivity (ECb) and the acquisition of the reflected TDR waveforms. The experimental framework was designed to simulate sharp NAPL fronts that moved in the soil columns and generated zones of contrasting permittivity. During the experiments, different initial conditions were assumed, as well as different degrees of contamination, by varying the volumetric water (θw) and NAPL (θNAPL) contents. The acquired TDR waveforms were systematically analyzed to deduce the influence of NAPLs on TDR signal propagation. A general procedure to determine the location of an NAPL‐contaminated front within a soil column was developed. Equipment calibration, measurement accuracy, and error sources were investigated in relation to the experimental procedure setup and the column preparation conditions. The results show that there may be some difficulties when interpreting TDR signals to locate the NAPL front for two main reasons: first, the expected additional reflection at the interface is not always distinguishable; second, there may be problems even locating both the first peak and the reflection point at the end of the TDR probe, especially when the signal shifts from a low to a high impedance layer. The suggested methodology provides a tool to overcome such intrinsic difficulties in NAPL front detection during the propagation of the contaminant.

Spatial variability of soil water content and soil electrical conductivity across scales derived from Electromagnetic Induction and Time Domain Reflectometry

Quick, reliable and accurate estimates of soil water content (SWC) at intermediate (slope) to larger scale (catchment) are important for understanding hydrological processes and may be provided by electromagnetic induction (EMI). EMI measures the apparent electrical conductivity of the subsurface (ECapp) which represents a depth weighted average value of the bulk soil electrical conductivity (ECb). The relation between ECb and SWC has generally been investigated in soil cores or using local measurements of SWC and ECb. Studies that investigated the relation between ECapp measured with EMI and SWC in considerably larger and internally more heterogeneous support volumes are far scarcer and cover a limited range of environments with a limited range of factors contributing to ECapp. This study developed a new calibration method to obtain quantitative estimates of SWC using EMI measured ECapp data in a sub-tropical region in Southern Brazil at sites with different soil properties. SWC and ECb were measured in soil pits with Time Domain Reflectometry (TDR) probes. Collocated ECapp was simultaneously measured with EMI using different coil separations and orientations to measure over increasing sensing volume. EMI measured ECapp data were first calibrated against calculated ECapp, which were derived from ECb profiles inserted in an exact EMI forward model. A depth averaged SWC (SWCavg) was calculated and different calibrations that relate ECapp to SWCavg were evaluated. ECapp measurements of the deeper sensing coil configurations could predict best the variability of SWCavg using a non-linear relation. Spatio-temporal variations of pore water electrical conductivity (ECw) were found to be an important cofounding factor. Temporal variations of ECw and the small temporal variability of SWCavg prevented the prediction of temporal variability of SWCavg using ECapp measurements. Overall, the combination of both calibration steps resulted in the description of 83% of the spatial variability of SWCavg from ECapp measurements.