Measuring Soil Thermal Properties Research Articles

High‐Resolution Field Measurement of Soil Heat Capacity and Changes in Soil Moisture Using a Dual‐Probe Heat‐Pulse Distributed Temperature Sensing Approach

AbstractIncreasing interest in studying the variability of soil water content and its spatial scale dependency necessitates the development of new techniques to accurately monitor soil water over a wide range of spatial scales. Distributed Temperature Sensing (DTS) techniques offer unprecedented opportunities to measure temperature with a spatial resolution of a few centimeters over several kilometers, which can be used to measure soil moisture. This study is the first of its kind that investigates under field conditions the feasibility of combining the Dual‐Probe Heat‐Pulse (DPHP) technique with the DTS technology to measure soil thermal properties and variation in soil moisture. A field experiment was conducted over a 30 m transect in the Lake Wheeler Field laboratory in Raleigh, NC. Three different DPHP sensors were constructed from combinations of different fiber optic cables and heating elements and were tested to assess their performance and the effect of their construction characteristics on their accuracy. Measurements were taken over different soil moisture conditions and the system performance was compared against independent soil water content sensors. The system was able to track changes in soil water content with a mean RMSE of 0.02 m3 m−3 using the optimal DPHP sensor. The key advantage of the tested system is that it does not need any site‐specific calibrations typically required for other DTS‐based systems. The findings of this study provide some practical information and measures that need to be taken for successful DTS‐DPHP construction and application under field conditions.

Integral form of the cylindrical perfect conductors solution for the dual‐probe heat‐pulse method

AbstractThe dual‐probe heat‐pulse (DPHP) method is useful for measuring soil thermal properties. Measurements are made with a sensor that has two parallel rods: one for introducing a pulse of heat and one for measuring change in temperature. Thermal properties are obtained by fitting a solution to the temperature data. The cylindrical perfect conductors solution of Knight et al. (https://doi.org/10.2136/vzj2011.0112) allows for accurate determination of those properties because it accounts for the finite radius and finite heat capacity of the rods. In this note, we show how their solution can be written in integral form. Specifically, we rearrange their Laplace‐domain solution and perform contour integration in the complex plane to express their solution as the integral of a real function over the positive real axis. This form of their solution is easier to evaluate and thus improves its accessibility for routine use in determining thermal properties with the DPHP method.

A dual-probe heat pulse-based sensor that simultaneously determines soil thermal properties, soil water content and soil water matric potential

A dual-probe heat pulse (DPHP) sensor is a robust tool that can measure soil thermal properties and associated soil physical properties and processes. A new DPHP based sensor was developed to simultaneously measure soil thermal properties, soil water content and soil water matric potential. A series of experiments were performed to evaluate the sensor performance. The new sensor was able to measure each soil property with acceptable accuracy. The accuracy of the matric potential determinations was approximately 10% in log scale. The effective range for ψ measurements was −1000 to −2.5 m of water, and the accuracy of the ψ determinations was best between −350 and −2.5 m of water, which included values of field capacity and wilting point for most soils and crops. The sensor design included a single heater wire to provide simultaneous heat inputs to two different materials, i.e., the soil and the sensor porous medium. The single heater wire did not influence sensor accuracy as long as the soil and the sensor porous medium thermal conductivity values were not significantly different. The new DPHP based sensor effectively measured various soil properties, including soil water matric potential, soil water content, and soil thermal property values.

Advances in the heat‐pulse technique: Improvements in measuring soil thermal properties

AbstractSoil thermal properties, including thermal diffusivity (α), thermal conductivity (λ), and volumetric heat capacity (C), are basic parameters describing the ability and efficiency of a soil to store and transfer heat. Soil thermal properties influence heat and mass transfer in soils and therefore have fundamental effects on the energy balance at the ground surface, water exchange between the soil and atmosphere, and engineered structures. Accurate determination of soil thermal properties is essential for describing thermal regimes that influence the rates of physical, chemical, and biological reactions and processes occurring in soil.

In‐situ probe spacing calibration improves the heat pulse method for measuring soil heat capacity and water content

AbstractHeat pulse (HP) sensors have been used extensively to measure soil thermal properties and volumetric water content (θ) simultaneously. Probe spacing (R), the distance between heater probe and temperature probe, is routinely calibrated in agar solution before making HP measurements. In reality, R may change due to probe deflection at insertion into soil, causing uncertainties in HP measured volumetric heat capacity (C) and θ. In this paper, we present a simple on‐site calibration method for determining in‐situ R by using theoretical C values estimated from soil bulk density and θ values of intact soil cores collected near the sensor locations at the end of experiment. The proposed approach was tested on two field soils where C and θ were monitored continuously. Inconsistent results were observed between agar‐calibrated R values (Ragar) and R values obtained from in‐situ calibration (Rin‐situ). Maximum difference between Ragar and Rin‐situ reached 20% of Ragar. Compared to TDR measured θ, using Ragar led to root mean square errors (RMSEs) of 0.26 MJ m−3 K−1 in C and 0.06 m3 m−3 in θ on average. For Rin‐situ, the RMSE of C was less than 0.16 MJ m−3 K−1 and RMSE of θ was within 0.04 m3 m−3. Thus, the use of in‐situ probe spacing calibration improved the accuracies of HP measured C and θ.

Assessing soil water content variability through active heat distributed fiber optic temperature sensing

Soil spatial variability is a key point for the sustainable water management in agriculture. Fractal techniques provide proper tools to analyze soil spatial variability searching for statistical self-similarity patterns among different scales. Although they have been extensively applied to study the soil properties variability, its applicability for the soil water content (SWC) distribution is complicated because requires many data difficult to obtain with the typical point soil water sensors. Recently, a fiber optic distributed temperature sensor has been used to measure soil thermal properties which relate to SWC. These sensors provide large amount of data with high spatial and temporal resolution, thus filling the gap of point soil water sensors. In the present work, soil temperature was measured with a Distributed Temperature Sensing (DTS) and SWC was estimated by different fitting functions which have been studied with focus on spatial variability.Temperature was measured in a 133 m fiber optic cable laid in a sandy soil field plot. A The Active Heat Fiber Optic (AHFO) method was used, with 12 cm sampling resolution, and heat pulses (19,4 W/m during 2 min) were applied. The temperature data were correlated to SWC, considering the integration of temperature during the heat pulse Tcum, and then the datasets Tcum-SWC were fitted to the best fit statistical function (exponential, potential and polynomial). The results showed that the Tcum distribution presented a non-Gaussian pattern. Additionally, highly anti-persistent patterns have been detected for the larger spatial scaling lags. The function`s performance was different thus, the exponential function reproduced better the absolute moments of the temperature profile but it failed reproducing the non-Gaussian behavior.

Summary of Advances in the Heat‐Pulse Technique: Improvements in Measuring Soil Thermal Properties

Core Ideas The dual probe heat‐pulse (DPHP) method is used to measure soil thermal properties. Special care is needed to obtain accurate DPHP measurements in field soil. A review of theory, instrumentation and procedures needed is presented for DPHP. This essay provides a summary of “Advances in the Heat‐Pulse Technique: Improvements in Measuring Soil Thermal Properties” recently appearing in Methods of Soil Analysis. Series.

Advances in the Heat-Pulse Technique: Improvements in Measuring Soil Thermal Properties

Advances in the Heat-Pulse Technique: Improvements in Measuring Soil Thermal Properties

Open hardware portable dual-probe heat-pulse sensor for measuring soil thermal properties and water content

A portable probe based on open hardware architecture for the assessment of soil thermal properties and water content using the DPHP method is presented. The mean percentage errors for assessment of volumetric heat capacity and thermal conductivity were 4.6% and 8.9% respectively, computed after sensor spacing calibration in agar stabilized water.The DPHP probe has been tested for soil thermal properties assessment and compared to TDR probe for the water content estimation in four different soils. The DPHP probe showed an accuracy comparable to TDR in estimating water content, with DPHP showing lower values of error index in 3 samples out of 4. The DPHP can be used to investigate smaller volume of soil than TDR probe can do however, TDR is much faster than DPHP in measure acquisition.

Application of a thermo-time domain reflectometry probe in sand-kaolin clay mixtures

Simultaneous measurements of soil thermal properties, moisture content and dry density are quite challenging, but very significant to geothermal related engineering applications. Thermo-time domain reflectometry (TDR) probe is able to measure soil thermal properties by the thermos probe, and predict soil moisture content and dry density by the TDR technique. This research studies the application of a new thermo-TDR probe in measuring thermal property, moisture content and dry density of sand-kaolin clay mixtures. Laboratory experiments were performed on five sand-kaolin clay mixtures with clay content ranging from 0% to 30% by dry weight. Measured thermal properties by the thermo-TDR probe were compared with a KD2 Pro. The moisture content and dry density were predicted by four alternative methods: Topp's equation, heat capacity method, one-step TDR method and Jung's method. The results revealed that Jung's method exhibited the highest prediction accuracy among the four methods, being the relative deviations between the predicted and measured values within 1% and 5% for gravimetric moisture content and dry density, respectively. It is concluded that the thermo-TDR probe can be applied in sand-kaolin clay mixtures successfully to accurately measure their thermal and geotechnical properties.

A general in situ probe spacing correction method for dual probe heat pulse sensor

The dual probe heat pulse (DPHP) method is gaining popularity for measuring soil thermal properties. However, because of the fact that the DPHP measured heat capacity (c) of soil is hyper-sensitive to probe spacing variation, probe deflection causes large error in the measured c. To deal with probe deflection, recently, Liu et al. (2013) has proposed an in situ probe spacing correction method, but, their method was based on the inline deflection assumption. Wen et al. (2015) found that for non-inline deflected DPHP sensors, the method of Liu et al. (2013) provided poor reduction of the error in c. To cope with the non-inline deflection, in this study, we introduce a new DPHP sensor design by using three thermistors within the same temperature probe. A related probe spacing correction method for non-inline deflected probes was also presented. Both numerical simulation and experiment were conducted to test the new model. We define θ as the inclination angle of the probe deviation from the vertical direction. For experiment of the outward deflection (5.8°<θ<6.8°), the error in c is in the range from −23% to −76%; after inline in situ correction, the error decreases to the range of 12% to 44%; after using our non-inline in situ correction, the error is between 1% and 8%. Our three-dimensional finite element numerical simulation also demonstrates that, compared with the method of Liu et al. (2013), the new sensor and correcting method can significantly eliminate errors in c caused by probe deflections. The new DPHP sensor design and the non-inline deflection model have the potential to replace the method of Liu et al. (2013), and become the standard method for correcting probe spacing in situ.

Determining Soil Ice Contents during Freezing and Thawing with Thermo‐Time Domain Reflectometry

Core Ideas An optimum heat application strategy was established for measuring thermal properties with the heat pulse method in partially frozen soils. At temperatures ≤ −5°C, T‐TDR probes were able to measure soil ice content changes with acceptable accuracy. At temperatures between −5 and 0°C, soil ice contents could be estimated from water content before freezing, TDR measured unfrozen water content, and T‐TDR measured total water content at temperatures below −5°C. Determining soil ice content during freezing and thawing is important and challenging for both engineering and environmental issues. The thermo‐time domain reflectometry (T‐TDR) probe, which can monitor unfrozen soil water content and soil thermal properties simultaneously, has the potential to measure ice content in partially frozen soils. The objective of this study was to identify an optimum heat application strategy for measuring soil thermal properties with T‐TDR probes in partially frozen soil while minimizing ice melting during the process. The optimized heating schemes were then applied for monitoring soil ice content dynamics during freezing and thawing. The results indicated that the heat pulse method failed at temperatures between −5 and 0°C because of temperature field disturbances from latent heat of fusion. When soil temperatures were ≤ −5°C, ice melting during heat pulse applications could be limited effectively with a combination of 60‐s heat‐pulse duration and 450 J m−1 heating strength, or a 90‐s heat‐pulse duration and heating strength of 450 to 900 J m−1. With the optimized heating scheme, T‐TDR probes were able to measure soil ice content changes at ≤ −5°C during freezing and thawing, and the errors were within ±0.05 m3 m−3 in sandy loams and within ±0.1 m3 m−3 in soils with high clay content. At temperatures between −5 and 0°C, soil ice contents could not be measured accurately with the heat‐pulse method directly, but they could be estimated coarsely from water content before freezing, TDR measured unfrozen water content, and T‐TDR measured total water content at temperatures below −5°C.

A Dual‐Probe Heat‐Pulse Sensor with Rigid Probes for Improved Soil Water Content Measurement

The dual-probe heat-pulse (DPHP) method is attractive for measuring soil thermal properties and volumetric water content. The purpose of this study was to develop and test a DPHP sensor having rigid probes made from thick-walled stainless steel tubing (2.38-mm outside diameter). The probes of this sensor are much more resistant to deflection than those of conventional DPHP sensors, decreasing measurement error caused by probe deflection during insertion into the soil. Laboratory experiments were conducted across a wide range of saturation levels with glass beads and three soils of different textures. For inferring soil properties from the proposed sensor, we applied the recently developed identical cylindrical perfect conductors (ICPC) model instead of the infinite line source (ILS) model that is typically used. The ICPC model improves solution for heat transport through the probe–soil system by accounting for the heat capacity and radius of the probes. Our results show a root mean square error of 1.4% volumetric water content and elimination of the measurement bias typically encountered with DPHP measurements. We conclude that the improved sensor, in combination with the ICPC model, provides a general, soil-independent water content estimate that is especially suitable for field soil water content monitoring because of its robust design with rigid probes. Because of its simplicity and measurements independent of soil type, we propose the presented DPHP method as an excellent alternative to other available measurement techniques for soil water content.

Evaluation of a self-correcting dual probe heat pulse sensor

The dual probe heat pulse method (DPHP) can be used to measure soil thermal properties. However, in situ probe spacing is prone to change resulting in large errors of measured soil heat capacity c. A newly designed DPHP sensor has two thermistors in the temperature probe which can be used to calculate in situ changes in probe spacing. This new sensor has been tested under the limited condition of linear coplanar probe deflection. i.e., probes were deflected in-line but were not bent in different directions. However, actual probe deflections may not always be coplanar linear. In this study the applicability of the new DPHP sensor is evaluated in fine sand and in silica sand for a variety of probe deflections. Probes are deflected at the bottom end (bottom) or top end (top) of probes. Results indicate that the new sensor can significantly eliminate errors in c caused by coplanar and non-coplanar probe deflections. Compared with bottom deflections, top deflections have larger errors in c. Thus, theory used to correct for deflections is more applicable to bottom deflections than to top deflections. The errors in c for saturated soil materials are larger than the errors in c for dry soil materials, because of the finite response times of the thermistors used in the probes. Clearly, the newly designed DPHP sensors have the potential for reducing errors caused by various kinds of probe deflections that may be encountered in the field.

Field Test and Sensitivity Analysis of a Sensible Heat Balance Method to Determine Soil Ice Contents

Soil ice content impacts winter vadose zone hydrology. It may be possible to estimate changes in soil ice content with a sensible heat balance (SHB) method, using measurements from heat pulse (HP) sensors. Feasibility of the SHB method is unknown because of difficulties in measuring soil thermal properties in partially frozen soils. The objectives of this study were (i) to examine the SHB method for determining in situ ice content, and (ii) to evaluate the required accuracy of HP sensors for use in the SHB method. Heat pulse sensors were installed in a bare field to measure soil temperatures and thermal properties during freezing and thawing events. In situ soil ice contents were determined at 60‐min intervals with SHB theory. Sensitivity of the SHB method to temperature, heat capacity, thermal conductivity, and time step size was analyzed based on numerically produced soil freezing and thawing events. The in situ ice contents determined with the SHB method were sometimes unrealistically large or even negative. Thermal conductivity accuracy and time step size were the key factors contributing to SHB errors, while temperature and heat capacity accuracy had less influence. Ice content estimated with a 15‐min SHB time step was more accurate than that estimated with a 60‐min time step. Sensitivity analysis indicated that measurement errors in soil temperature and thermal conductivity should be less than ±0.05°C and ±20%, respectively, but the error in the soil heat capacity could vary by ±50%. Thus, improving the accuracy of thermal conductivity measurements and using short time steps are required to accurately estimate soil ice contents with the SHB method.

Using Late Time Data Improves the Heat‐Pulse Method for Estimating Soil Thermal Properties with the Pulsed Infinite Line Source Theory

Soil‐probe contact resistance and finite radius and heat capacity of the heat pulse (HP) probe produce significant errors in thermal property estimates. In this study, we demonstrated that estimating soil thermal properties from late‐time data of the temperature change‐by‐time (ΔT(t)) curve reduces these errors effectively. The weighted nonlinear curve fitting method was applied to estimate soil thermal properties following the pulsed infinite line source (PILS) theory using ΔT(t) data from the complete (PILS‐Complete), peak‐time (PILS‐Peak), and late‐time (PILS‐Late) ranges. Three experiments on specific heat of soil solids (cs), soil thermal properties, and soil water content (θHP) were conducted to examine the performance of these approaches. The results showed that the PILS‐Complete and PILS‐Peak methods overestimatedcsby 16.6% and 13.0% respectively, and the error from the PILS‐Late method reduced to 3.2%. Soil thermal conductivity measurements from the PILS‐Late method agreed well with those from the identical‐cylindrical‐perfect‐conductors theory and with the estimates from the heat flux plate data. The PILS‐Late method also effectively reduced the overestimation of soil heat capacity and underestimation of soil thermal diffusivity. In comparing to the PILS‐Complete method, the PILS‐Late method reduced the root mean square error (RMSE) of θHPfrom 0.039 to 0.021 m3m−3on a sand soil, and from 0.032 to 0.018 m3m−3on a clay loam soil. Thus, using late‐time data improved the accuracy of HP method for measuring soil thermal properties.

A Self-Calibrated Dual Probe Heat Pulse Sensor for In Situ Calibrating the Probe Spacing

Dual probe heat pulse method (DPHP) is a popular method for measuring soil thermal properties. However when this method is applied in the field, the distance between the temperature probe and the heating probe (probe spacing) may change due to probe deflection, and thus cause error in measured heat capacity (c) of soil. We developed a self-calibrated DPHP for in situ calibration of probe spacing, using a temperature probe with two imbedded thermistors (10 mm apart in the longitudinal direction). Experimental results for sand and fine sand samples indicate that this method can significantly reduce error caused by the probe deflection. If we define σ as the inclination angle of probe deviation from the vertical direction, for inward deflection with 0.8° < θ < 0.9°, the error in c is between –8.1 and –14.3%; after in situ calibration, it is between –0.1 and –4.3%. For outward deflection with 3.2° < θ < 3.5°, the error in c is in the range from 22.9 to 38.8%; after in situ calibration, the error reduces to from –3.8 to –7.8%. This new DPHP design may be applied in the field for reducing error caused by probe deflection.

Short, Multineedle Frequency Domain Reflectometry Sensor Suitable for Measuring Soil Water Content

Time domain reflectometry (TDR) is a well‐established electromagnetic technique used to measure soil water content. Time domain reflectometry sensors have been combined with heat pulse sensors to produce thermo‐TDR sensors. Thermo‐TDR sensors are restricted to having relatively short needles to accurately measure soil thermal properties. Short needle lengths, however, can limit the accuracy of the TDR measurement of soil water content. Frequency domain reflectometry (FDR) sensors are an alternative to TDR sensors that can provide an inexpensive measurement of soil water content. The objective of this study was to determine whether short FDR sensors can accurately measure soil water content. We designed and constructed a short FDR sensor. For four soil types across a range of water contents, temperatures, and salt contents, we measured soil dielectric spectra with the short FDR sensor. A vector network analyzer was used to obtain soil dielectric spectra in the 1‐MHz to 3‐GHz frequency range. The ideal frequency of a short FDR sensor is the frequency at which the permittivity is not altered by changing temperature or salt content. The 47‐ to 200‐MHz range was an ideal frequency range for measuring soil water content, and 70 MHz was the frequency least influenced by temperature and salt content. The short FDR sensor provided quick, continuous, stable, and cheap measurements of soil water content. Because of the promising performance of the short thermo‐FDR sensor in laboratory studies, sensors should be evaluated in future field studies.

Semianalytical Solution for Dual‐Probe Heat‐Pulse Applications that Accounts for Probe Radius and Heat Capacity

The dual‐probe heat‐pulse (DPHP) method is useful for measuring soil thermal properties. Measurements are made with a sensor that has two parallel cylindrical probes: one for introducing a pulse of heat into the soil (heater probe) and one for measuring change in temperature (temperature probe). We present a semianalytical solution that accounts for the finite radius and finite heat capacity of the heater and temperature probes. A closed‐form expression for the Laplace transform of the solution is obtained by considering the probes to be cylindrical perfect conductors. The Laplace‐domain solution is inverted numerically. For the case where both probes have the same radius and heat capacity, we show that their finite properties have equal influence on the heat‐pulse signal received by the temperature probe. The finite radius of the probes causes the heat‐pulse signal to arrive earlier in time. This time shift increases in magnitude as the probe radius increases. The effect of the finite heat capacity of the probes depends on the ratio of the heat capacity of the probes (C0) and the heat capacity of the soil (C). Compared with the case where C0/C = 1, the magnitude of the heat‐pulse signal decreases (i.e., smaller change in temperature) and the maximum temperature rise occurs later when C0/C &gt; 1. When C0/C &lt; 1, the magnitude of the signal increases and the maximum temperature rise occurs earlier. The semianalytical solution is appropriate for use in DPHP applications where the ratio of probe radius (a0) and probe spacing (L) satisfies the condition that a0/L ≤ 0.11.

Effect of Probe Deflection on Dual‐Probe Heat‐Pulse Thermal Conductivity Measurements

The dual‐probe heat‐pulse (DPHP) method is useful for measuring soil thermal properties; however, the probes of a DPHP sensor can deflect when inserted into the soil. Theoretical analysis has shown that measurements of thermal conductivity (λ) should be unaffected by deflection‐induced changes in probe spacing. To verify this result, the conductivities of water, dry sand, and saturated sand were measured using DPHP sensors with probes subject to inward deflection, no deflection, and outward deflection. No error in λ was detected when probes were deflected inward by an amount that caused a 14% reduction in probe spacing. Outward deflection (15% increase in spacing) caused error in λ estimates, but the errors were small (≤0.04 W m−1 K−1) and likely to be of little consequence in most applications. We conclude that estimates of λ obtained with the DPHP method are largely unaffected by changes in probe spacing caused by deflection.