Time Domain Reflectometry Probes Research Articles

NOTA TÉCNICA: CONSTRUÇÃO DE SONDAS TDR E AVALIAÇÃO EM DIFERENTES SOFTWARES DE APLICAÇÃO TÉCNICA

A aplicação eficiente de água na agricultura envolve equipamentos modernos e manejo adequado deste recurso natural. A quantidade de água absorvida pela cultura pode ser determinada pelo fluxo de seiva xilemática e a técnica da Reflectometria no Domínio do Tempo (TDR) tem potencial para efetuar essas medições por ser precisa e praticamente livre de interferências. O objetivo deste trabalho foi construir e avaliar sondas TDR de diferentes tamanhos que pudessem ser lidas por três diferentes softwares com a finalidade de mensuração do fluxo de seiva em plantas lenhosas.Foram avaliados seis tamanhos de sondas (200, 150, 100, 70, 50 e 20 mm) sendo construídas e avaliadas características físicas em água deionizada por três programas de aplicação da técnica, PCTDR da Campebell Scientifc, WinTDR da Utah State University e o TDR-Lab da CSIC. Os softwares demonstraram dificuldades na leitura automática, sendo WinTDR e PCTDR mais limitados quanto à leitura manual.O PCTDR não conseguiu realizar leituras em sondas de 1,6 mm de diâmetro de haste. Já o software TDRLab se mostrou promissor na análise de sondas de tamanhos menores, porém ainda são necessários estudos da constituição física das sondas para maior precisão nas leituras.

Effects of forest evapotranspiration on soil water budget and energy flux partitioning in a subalpine valley of China

Jiuzhaigou Valley (JZG) has been rated as World Heritage, National Nature Reserves and world-class places of interests, due to its fantastic beauty of water. In recent years, however, JZG is losing its beauty with runoff decreasing and stream drying up. An increase of evapotranspiration (ET) over the rehabilitating forest is assumed to be the main reason for runoff decreasing, however, this assumption remained untested for lack of long-term observing data. To test this assumption, a long-term field experiment was carried out from August 2013 to December 2015 in JZG over a typical secondary forest. Meteorological parameters and energy fluxes were measured by eddy covariance system, soil moisture by time-domain reflectometry probes, and ET by eddy covariance method and soil water budget. Results showed that (1) despite the limitation of high elevation, high atmospheric humidity and low annual air temperature, annual ET was quite large. It was 700mm and 739mm, respectively for 2014 and 2015, while the corresponding precipitation (P) was 1003mm and 782mm. ET/P was close or over 1, which meant that P was mostly used for forest ET and only a very small proportion of P contributed to runoff. (2) The ratio of latent heat flux (LE) to net radiation (Rn) was 0.69 and 0.75 in the growing season of 2014 and 2015, respectively. These high LE/Rn ratio values were considerably higher in JZG than in other areas of the Tibetan Plateau, which could be attributed to better forest rehabilitation in JZG. Our results also showed that ET at the forest site was limited by available energy. We conclude that ET in JZG was energy-limited, whereas forest ET consumed most precipitation. (3) The above assumption is verified to be true. The high ET rate after forest rehabilitation in JZG comes at a cost of decreasing runoff and leads to streams drying up. These results will be useful for sustainable managements of water and forest in JZG.

EVALUATION OF APPLIED REMEDIATION WORKS ON HISTORICAL MASONRY WALL EXPOSED TO LONG-TERM DETERIORATION

The exposure of massive historical masonry walls to varying climatic conditions represents a crucial factor for long-term damage related to moisture and soluble salts increase. Moisture movement in the porous structure of massive historical masonry wall driven by capillary forces is usually connected with transport of salt solutions, which can be responsible for a severe damage. Because of missing damp-proof insulation layers, the moisture ingress carrying soluble salt from the ground especially during winter periods is not prevented from. This study is aimed on the evaluation of the effect of commonly used restoration works on the conservation of an exposed building. The influence of applied precautions was continuously monitored by time-domain reflectometry probes in various depths and heights of the studied masonry wall located in the basement of a residential house in Prague, Czech Republic. Based on the obtained results, taking into account also the influence of indoor and outdoor temperature and relative humidity variations monitored by combined temperature/relative humidity sensors, the wall drying process and the success of applied methods is analyzed.

A Comparison of Second‐Order Derivative Based Models for Time Domain Reflectometry Waveform Analysis

Core Ideas Two methods, AWIGF and second‐order BMO, were evaluated on long‐ and short‐probe TDR waveforms. A corner‐preserving filter was designed to improve the performance of AWIGF. With the new filter, AWIGF performance was consistent with second‐order BMO analysis. Adaptive waveform interpretation with Gaussian filtering (AWIGF) and second‐order bounded mean oscillation Z2(u,t,r) are time domain reflectometry (TDR) analysis models. The AWIGF was originally designed for relatively long‐probe (>150 mm) TDR waveforms, while Z2(u,t,r) was originally designed for relatively short‐probe (<50 mm) TDR waveforms. The performances of AWIGF and Z2(u,t,r) on both long and short TDR probes have not been evaluated. The main objective of this study was to evaluate theoretically and experimentally the AWIGF and Z2(u,t,r) performances on long and short TDR probes. The evaluations are performed via mathematical analysis, and measurements of long probe and short probe waveforms in CaCl2 solutions with various electrical conductivities (EC), adding Gaussian noise, and testing the stability of Z2(u,t,r) and AWIGF. A corner‐preserving filter (CPF) is proposed to improve the stability of AWIGF on short‐probe TDR waveforms. The CPF preserves the second order derivatives of the waveforms, and emphasizes the reflection positions (t2) compared to the original Gaussian filter. Both theoretical and experimental tests illustrate the consistency of Z2(u,t,r) and AWIGF. The standard deviations of relative permittivity (εr) are <5% for all noise levels. In conclusion, Z2(u,t,r) and AWIGF can provide stable analysis for long and short probe TDR waveforms. The AWIGF with CPF is capable of stably analyzing challenging short probe TDR waveforms. The original AWIGF exhibits the lowest standard deviation of εr at a given EC, whereas AWIGF with CPF filter exhibits the lowest bias of εr across solutions varying in EC.

In-Situ Measurement of Soil Permittivity at Various Depths for the Calibration and Validation of Low-Frequency SAR Soil Moisture Models by Using GPR

At radar frequencies below 2 GHz, the mismatch between the 5 to 15 cm sensing depth of classical time domain reflectometry (TDR) probe soil moisture measurements and the radar penetration depth can easily lead to unreliable in situ data. Accurate quantitative measurements of soil water contents at various depths by classical methods are cumbersome and usually highly invasive. We propose an improved method for the estimation of vertical soil moisture profiles from multi-offset ground penetrating radar (GPR) data. A semi-automated data acquisition technique allows for very fast and robust measurements in the field. Advanced common mid-point (CMP) processing is applied to obtain quantitative estimates of the permittivity and depth of the reflecting soil layers. The method is validated against TDR measurements using data acquired in different environments. Depth and soil moisture contents of the reflecting layers were estimated with root mean square errors (RMSE) on the order of 5 cm and 1.9 Vol.-%, respectively. Application of the proposed technique for the validation of synthetic aperture radar (SAR) soil moisture estimates is demonstrated based on a case study using airborne L-band data and ground-based P-band data. For the L-band case we found good agreement between the near-surface GPR estimates and extended integral equation model (I2EM) based SAR retrievals, comparable to those obtained by TDR. At the P-band, the GPR based method significantly outperformed the TDR method when using soil moisture estimates at depths below 30 cm.

Experimental evaluation of the measurement errors of soil water content due to interference of two adjacent TDR probes

ABSTRACTThe objective of this experimental study was to determine the measurement errors caused by the interference effects when two TDR probes operate nearby in various distance apart for different soil types and soil water contents. The results will help to guide the design of an experimental apparatus which aims to study the coupled heat and moisture transfer in soils at high temperatures of higher than 40°C with the highest possible spatial resolution. The interference effect between two adjacent time domain reflectometry (TDR) probes has been evaluated and analyzed experimentally. It was observed that the distance between the probes, soil texture, and the water content of the testing soil is the key parameters which affect the interference and TDR probe capability of measurement. In this study, three types of soils were examined, namely FSJ#1 (silty clay), Matilda (loamy sand), and Ottawa sand C-190 (quartz sand). Three different volumetric water contents were evaluated for each soil. The interference effect was noticeable in all soil samples when the distances between the probes were 1 cm. For the samples with higher moisture content, the effect of interference on the TDRs’ electromagnetic waveform signals is more prominent. Interference was found in Matilda samples with degrees of saturations of 0.5 and higher even though the distances between the probes were greater than 1 cm; nevertheless, the determination of volumetric water content was not affected by the interference, because the interference only caused the two TDRs’ electromagnetic waveform signals to shift by the same length. No interference was observed in cases of FSJ#1 and Ottawa sand when the distances between the probes were greater than 1 cm. From the results, it was found that the sensitivity zone of our TDR equipment is a radius of about 10 mm surrounding the central needle of the TDR probe. In case of FSJ#1, when interference occurs, the measurement error of water content significantly increases from 9.3% to 15.5%, corresponding to the degree of saturation from 0.25 to 1.0.

A TDR Array Probe for Monitoring Near‐Surface Soil Moisture Distribution

Core Ideas A TDR array has been developed for soil moisture profiling. The sensor provides eight incremental measurements at centimeter‐depth resolution. Permittivity, evaporation rate, and soil moisture profile were determined. Near‐surface soil conditions (i.e., moisture and temperature) moderate mass and energy exchange at the soil–atmosphere interface. While remote sensing offers an effective means for mapping near‐surface moisture content across large areas, in situ measurements, targeting those specific remotely sensed soil depths, are poorly understood and high‐resolution near‐surface measurement capabilities are lacking. Time domain reflectometry (TDR) is a well‐established, accurate measurement method for soil dielectric permittivity and moisture content. A TDR array was designed to provide centimeter‐resolution measurements of near‐surface soil moisture. The array consists of nine stainless steel TDR rods spaced 1 cm apart, acting as waveguide pairs to form eight two‐rod TDR probes in series. A critical aspect of the design was matching the spacing of the coaxial cable–TDR rod transition to avoid unwanted reflections in the waveforms. The accuracy of the TDR array permittivity measurement (±1 permittivity unit) was similar to that of conventional TDR as verified in dielectric liquids. Electric field numerical simulations showed minimal influence of adjacent rods during a given rod‐pair measurement. The evaporation rate determined by the TDR array compared well with mass balance data in a laboratory test. Near‐surface soil moisture profile dynamics were monitored at centimeter‐depth resolution using the TDR array in a field experiment where volumetric moisture content estimates (0–8 cm) were within 2% of conventional three‐rod TDR probes averaging across 0 to 8 cm and from 1‐ to 3‐cm depths.

Assessing Soil Moisture Patterns Using a Soil Topographic Index in a Humid Region

Knowledge of soil moisture is essential for soil conservation and efficient water resources management especially related to control nonpoint-source pollution. Soil topographic indices (STI) are often used to understand the soil moisture patterns in landscapes and make effective landscape management decisions. This study assessed the relationships between soil moisture measurements and STI values in two study sites in North-central New Jersey, USA. The soil moisture measurements were taken in these study sites using a time domain reflectometry probe during thirteen sampling events between April 2013 and July 2015. The STI values at the sampling points were derived from a 3-m LiDAR digital elevation model and SSURGO soil database. The Spearman’s correlation analysis based on these data in all sampling points identified a significant positive correlation between soil moisture and STI. Strong positive relationships between soil moisture and STI were also identified when using binned data to eliminate the impacts of unevenness in data distribution and the impacts of micro-variations in topography, vegetation, soil compaction, and instrumental errors. The linear mixed modeling results revealed significant and positive impacts of STI and precipitation, and significant but negative impacts of temperature on soil moisture. The degrees of these effects vary across two study sites, which reflect the complex and dynamic interactions among soils, topography and climate in landscapes that affect soil moisture. Given the stochastic nature of climate factors such as precipitation and temperature, the static STI would be a reliable factor to predict soil moisture patterns in the landscape. The findings support various STI-based conservation planning efforts in New Jersey and beyond such as targeting hydrologically sensitive areas for natural resources protection and preservation and best management practice implementation.

Impact of Groundwater Level on Nitrate Nitrogen Accumulation in the Vadose Zone Beneath a Cotton Field

In this study, the impacts of groundwater level on nitrate nitrogen accumulation in the vadose zone of a cotton field were investigated. Experiments were conducted in a cotton field at the CAS Ecological Agricultural Experiment Station in Nanpi from 2008 to 2010. A vertical observation well was drilled, and time-domain reflectometry probes and soil solution extractors were installed every 50 cm in the walls of the well to a depth of 5 m. The soil water content was monitored, and soil solution samples were obtained and analyzed every six days throughout the growing seasons during the three studied years. Additionally, a water consumption experiment was conducted, and the topsoil water content and leaf area index were measured in the cotton field. The resulting data were used to estimate parameters for use in a soil hydraulic and nitrate nitrogen movement model, and cotton evapotranspiration was calculated using the Penman–Monteith method. Groundwater level increases and decreases of ±4 m were simulated during a ten-year period using HYDRUS-1D. The results showed significant nitrate nitrogen accumulation in the vadose zone when the groundwater level remained unchanged or decreased, with increased accumulation as the groundwater depth increased. Additionally, increased precipitation and a deeper groundwater level resulted in greater nitrate nitrogen leaching in the cotton root zone. Therefore, irrigation and fertilization strategies should be adjusted based on precipitation conditions and groundwater depth.

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.

Evaluations of the effects of soil properties and electrical conductivity on the water content reflectometer calibration for landfill cover soils

This study presents soil-moisture calibrations using low-frequency (15–40 MHz) time domain reflectometry (TDR) probe, referred to as water content reflectometer (WCR), for measuring the volumetric water content of landfill cover soils, developing calibrations for 28 different soils, and evaluating how WCR calibrations are affected by soil properties and electrical conductivity. A 150-mm-diameter PVC cell was used for the initial WCR calibration. Linear and polynomial calibrations were developed for each soil. Although the correlation coefficients (R<sup>2</sup>) for the polynomial calibration are slightly higher, the linear calibrations are accurate and pragmatic to use. The effects of soil electrical conductivity and index properties were investigated using the slopes of linear WCR calibrations. Soils with higher electrical conductivity had lower calibration slopes due to greater attenuation of the signal during transmission in the soil. Soils with higher electrical conductivity tended to have higher clay content, organic matter, liquid limit, and plasticity index. The effects of temperature and dry unit weight on WCR calibrations were assessed in clayey and silty soils. The sensor period was found to increase with the temperature and density increase, with greater sensitivity in fine-textured plastic soils. For typical variations in temperature, errors in volumetric water content on the order of 0.04 can be expected for wet soils and 0.01 for drier soils if temperature corrections are not applied. Errors on the order of 0.03 (clays) and 0.01 (silts) can be expected for typical variations in dry unit weight (± 2 kN/m<sup>3</sup>).

Estimation of Soil-Water Characteristic Curves in Multiple-Cycles Using Membrane and TDR System.

The objective of this study is to estimate multiple-cycles of the soil-water characteristic curve (SWCC) using an innovative volumetric pressure plate extractor (VPPE), which is incorporated with a membrane and time domain reflectometry (TDR). The pressure cell includes the membrane to reduce the experimental time and the TDR probe to automatically estimate the volumetric water content. For the estimation of SWCC using the VPPE system, four specimens with different grain size and void ratio are prepared. The volumetric water contents of the specimens according to the matric suction are measured by the burette system and are estimated in the TDR system during five cycles of SWCC tests. The volumetric water contents estimated by the TDR system are almost identical to those determined by the burette system. The experimental time significantly decreases with the new VPPE. The hysteresis in the SWCC is largest in the first cycle and is nearly identical after 1.5 cycles. As the initial void ratio decreases, the air entry value increases. This study suggests that the new VPPE may effectively estimate multiple-cycles of the SWCC of unsaturated soils.

Volumetric Water Content Measurements as Obtained from Remote Sensing and In Situ Instrumentation

A single-polarization, vertical wave propagation transmitted-vertical wave propagation received (VV), terrestrial imaging radar and time domain reflectometry (TDR) equipment were used to monitor the fluctuations of water content within two thin (15.24 centimeters) compacted clay test sections (each 30.48-m long, by 15.34-m wide). Radar imagery was obtained in 2013 on March 22, June 26, July 7 and July 10 using a Gamma Remote Sensing Portable Radar Interferometer version II (GAMMA Remote Sensing and Consulting AG, Bern, Switzerland). In situ observations were acquired hourly using TDR probes. The background, methodology, and comparison of the results of the remote sensing and in situ measurements are presented. Specifically, two remote sensing data reduction algorithms were considered. The water content results obtained from these algorithms were compared with the water content values derived from correlations with the dielectric permittivity values measured by the TDR probes. Key findings include: 1) there were differences between the values obtained from the data reduction methods, 2) there were differences between the values obtained from the remote sensing methods and the values obtained from the in situ method, and 3) the identification of additional avenues of research. The remotely sensed measurements were, on average, higher than the in situ measurements (the average volumetric water content values were 0.2 and 0.5 depending on the method, as obtained via the remotely sensed methods; the average volumetric water content value in the compacted native soil test sections was 0.22, as obtained via the in situ method).

Analysis of the drought recovery of Andosols on southern Ecuadorian Andean páramos

Abstract. The Neotropical Andean grasslands above 3500 m a.s.l., known as páramo, offer remarkable ecological services for the Andean region. The most important of these is the water supply of excellent quality to many cities and villages in the inter-Andean valleys and along the coast. The páramo ecosystem and especially its soils are under constant and increased threat by human activities and climate change. In this study, the recovery speed of the páramo soils after drought periods are analysed. The observation period includes the droughts of 2009, 2010, 2011, and 2012 together with intermediate wet periods. Two experimental catchments – one with and one without páramo – were investigated. The Probability Distributed Moisture (PDM) model was calibrated and validated in both catchments. Drought periods and its characteristics were identified and quantified by a threshold level approach and complemented by means of a drought propagation analysis. At the plot scale in the páramo region, the soil water content measured by time domain reflectometry (TDR) probes dropped from a normal value of about 0.84 to ∼ 0.60 cm3 cm−3, while the recovery time was 2–3 months. This did not occur at lower altitudes (Cumbe) where the soils are mineral. Although the soil moisture depletion observed in these soils was similar to that of the Andosols (27 %), decreasing from a normal value of about 0.54 to ∼ 0.39 cm3 cm−3, the recovery was much slower and took about 8 months for the drought in 2010. At the catchment scale, however, the soil water storage simulated by the PDM model and the drought analysis was not as pronounced. Soil moisture droughts occurred mainly in the dry season in both catchments. The deficit for all cases is small and progressively reduced during the wet season. Vegetation stress periods correspond mainly to the months of September, October and November, which coincides with the dry season. The maximum number of consecutive dry days were reached during the drought of 2009 and 2010 (19 and 22 days), which can be considered to be a long period in the páramo. The main factor in the hydrological response of these experimental catchments is the precipitation relative to the potential evapotranspiration. As the soils never became extremely dry nor close to the wilting point, the soil water storage capacity had a secondary influence.

Time-domain reflectometry probing systems for the monitoring of hydrological processes in the unsaturated zone

Precise measurements of the downward movement of precipitation through the unsaturated zone, as well as return flow of moisture to the atmosphere via evaporation, have always been challenging in regard to in-situ monitoring techniques. This study investigates the profile of volumetric water-content fluctuations within the unsaturated zone through a combination of field techniques, including in-situ measurements of the volumetric porewater content at different depths using specially designed time-domain reflectometry (TDR) probes. The probes are installed through direct-push vibro-coring methods, at significant depths within the unsaturated zone, providing continuous readings of the soil-moisture content throughout the unsaturated column. The measured waveform is analyzed by using the inverse modeling approach resulting in an apparent relative dielectric permittivity profile of the surrounding medium along the TDR probe length. The approach sufficiently analyzes the mechanisms of water fluxes through significant depths within the unsaturated zone, which in turn can be used to quantify groundwater recharge at areas where the unsaturated zone hydrology plays a key role in the recharge of the underlying aquifers (such as arid and hydrologically sensitive areas). The approach was applied at an experimental field site in the Upper Rhine Valley, Germany.

Transport of Solute from a Fine-Grained Unit to a Coarse-Grained Host Under Pulsed-Pumping Fluid Dynamics: An Experimental Investigation

Pulsed pumping has the potential to be an effective alternative to continuous pumping pump-and-treat remediation techniques in porous media that exhibit a sharp material interface and tailing phenomenon is observed. Solute transport experiments performed in a soil column filled with a uniform coarse sand containing a uniform silt cylinder were conducted. Concentration measurements using saltwater as a tracer were taken employing time-domain reflectometry probes located throughout the column. The laboratory trials employed continuous pumping, 24-h periods of pumping followed by 24-h periods of no pumping, and 4-h periods of pumping followed by 24-h periods of no pumping. The experimental results suggest that pulsed pumping can achieve similar contaminant removal for a given amount of time as continuous pumping while pumping less water.

A Layered Vegetation Model for GPR Full-Wave Inversion

The vegetation cover strongly affects microwave remote sensing backscattered signals and makes difficulties in retrieving the surface soil water content (SWC). In this paper, we developed a vegetation cover model for full-wave inversion of ground-penetrating radar (GPR) data assuming multiple layers with effective electrical properties. Measurements were performed in laboratory conditions over spelt wheat grown in a sandy soil. A stepped-frequency continuous-wave (SFCW) GPR system was set up using a vector network analyzer and a horn antenna operating in 1–4 GHz range in far-field conditions with different incidence angles and both VV and HH polarizations. In order to control the sand surface water content, a water table was fixed at 20 cm depth and 6 time-domain reflectometry (TDR) probes were horizontally placed at 2 and 20 cm depth. Measurements were performed and recorded hourly for about 1 month and the canopy height was measured daily. The effective electromagnetic properties of the vegetation layer were modeled using a complex refractive index model (CRIM) combined with a single Debye relaxation model. A scattering loss parameter was included in the electromagnetic full-wave GPR model for the vegetation layer. The proposed method was also applied in a $\sim 7$ ha agricultural field with a vegetation canopy height of $\sim 27$ cm (spring wheat). The laboratory measurements resulted in a strong link between the vegetation scattering losses parameter and canopy height (generally, the biomass structure), which cannot be retrieved directly from the GPR measurements. Applying this vegetation model to the GPR full-wave inversion led to a significant correction on GPR-derived SWC values from the field measurements.

Calibration of TDR Test Probes for Measuring Moisture Changes in the Construction Layers of the Railway Line

In the introduction of the paper there is characterized a way of monitoring the moisture in the structure of the railway substructure and its subgrade in the Experimental stand DRETM II, which is a part of the experimental workplace of the Department of Railway Engineering and Track Management. The substantial part of the paper is devoted to the characteristics and calibration of TDR (Time domain reflectometry) test probes for selected rock materials, which are incorporated into the construction of Experimental stand DRETM II (crushed aggregate fr.0/31.5mm, crushed aggregate fr.0/63mm and clay blended with river gravel), as a basic prerequisite for determination of the actual moisture and its changes in the body of the railway substructure and subgrade during the year. Finally, this article outlines a comparison of individual TDR probes used in calibration and their suitability for monitoring moisture in various materials used in construction layers of the Experimental stand DRETM II.

Tangent Line/Second‐Order Bounded Mean Oscillation Waveform Analysis for Short TDR Probe

Core Ideas • A new method for analyzing TDR waveforms is presented. • The method provides consistent results on waveforms obtained from short TDR probes. • The method is stable enough for automatic implementation on large sets of TDR waveforms. Tangent line (TL) methods and the second‐order bounded mean oscillation (BMO) method have been proposed for determining the reflection positions of time domain reflectometry (TDR) waveforms, especially for short‐probe TDR sensors. However, the accuracy of TL methods is limited by the multi‐reflection effects of the short‐probe TDR sensor, and an automatic implementation of the second‐order BMO is challenging because of the superimposition of the TDR waveforms. In this study, we combined a TL method with second‐order BMO to develop a tangent line/second‐order bounded mean oscillation (TL‐BMO) method. Laboratory and field data were used to evaluate the TL‐BMO method. Separate tests were performed on laboratory data to compare the TL‐BMO method with the TL method and the second‐order BMO method. For selected waveforms, the TL‐BMO was more accurate than the TL method (the RMSE of TL‐BMO was 0.0197 m3 m−3 and the RMSE of the TL method was 0.1071 m3 m−3). The TL‐BMO was able to avoid calculation errors associated with automatic analysis by the second‐order BMO (RMSE of TL‐BMO automatic analysis was 0.0199 m3 m−3 and the RMSE of second‐order BMO automatic analysis was 0.1414 m3 m−3). For analyzing field measurements, the TL‐BMO method was able to determine soil water contents accurately during a 3‐wk‐long measurement period. Conclusively, the new TL‐BMO method was more accurate than the TL method, and it demonstrated the stability necessary for automatic analysis of short‐probe TDR sensors.

An Assimilated Time Domain Reflectometry Probe

Core Ideas We introduce a calibration procedure for a non‐penetrating TDR probe. The measured dielectric permittivity is a volume average of the soil and the measurement apparatus. We experimentally calibrated the TDR probe for various soils. Time domain reflectometry (TDR) is a simple and popular method for measuring soil water content. The TDR measures a weighted average of the dielectric permittivity of the soil along and around a probe. Under some experimental or technical restrictions, the penetration of the probe into the studied domain is undesirable or impossible. Here, we experimentally investigate the feasibility of measuring a material's water content by only touching it. Such a configuration, where the probe is not fully inserted into the measured domain, leads to a far‐from‐uniform distribution of the dielectric permittivity around the probe. A laboratory experiment was conducted, comparing wall‐embedded TDR (wall probe [WP]) measurements with those of a regular TDR probe (standard probe [SP]). Five different soils were examined (sand, “Hamra” [red loamy sand], loess, pure quartz, and clayey soil). For all soils, the WP measurements apparently underestimated water content. This is because it averages soil permittivity of the domain of interest (soil) with that of the probe host material, typically of lower dielectric permittivity. A linear relation between the water content as measured by the WP and the SP was found for all the soils. For most soils and for the non‐marginal water contents, the true soil permittivity (and hence the water content) was roughly twice that measured by the WP. However, the measurement was found to be highly sensitive to the exact location of the probe at the material interface. We conclude that the wall‐assimilated probe can be used with minimal calibration to measure water content without penetration into the measured domain, but high caution should be used with respect to the exact location of the probe at the wall–soil interface.