Heat Pulse Technique Research Articles

Overlooked biocrust impacts on surface soil thermal properties: Evidence from heat‐pulse sensing on large volume samples

AbstractBiocrusts are a critical surface cover in global drylands, but knowledge about their influences on surface soil thermal properties are still lacking because it is quite challenging to make accurate thermal property measurements for biocrust layers, which are only millimetres thick. In this study, we repacked biocrust layers (moss‐ and cyanobacteria‐dominated, respectively) that had the same material as the original intact biocrusts but was more homogeneous and thicker. The thermal conductivity (λ), heat capacity (C) and thermal diffusivity (k) of the repacked and intact biocrusts were measured by the heat pulse (HP) technique at different mass water contents (θm) and mass ratios (Wt), and the differences between repacked and intact biocrusts were analysed. Our results show that biocrusts substantially alter the thermal properties of the soil surface. The average λ of moss (0.37 W m−1 K−1) and cyanobacteria biocrusts (0.90 W m−1 K−1) were reduced by 63.0% and 10.3% compared with bare soil (1.00 W m−1 K−1), respectively. Edge effects including heat loss and water evaporation caused the λ and k of the biocrusts to be underestimated, but the C to be overestimated. The differences in thermal properties were significant (p <0.001), except for the differences in thermal conductivity between repacked and intact cyanobacteria biocrusts, which were not significant (p = 0.379). Specifically, in the volumetric water content (θv) range of 0 to 20%, the λ and k of the repacked moss biocrusts were underestimated by 59.1% and 61.8%, respectively, and the C was overestimated by 23.9% compared with the intact moss biocrusts. The λ and k of the repacked cyanobacteria biocrusts were underestimated by 15.8% and 79.2%, respectively, and the C was overestimated by 34.8% compared with the intact cyanobacteria biocrusts at the θv range of 0 to 30%. Typically, this difference increased as the θv rises between repacked and intact biocrusts. Our new measurements provide evidence that the thermal properties of biocrusts were previously misjudged due to the measurement limitations imposed by their limited thickness when measured in situ. Biocrusts are likely more significant in regulating soil heat and temperature in drylands than was previously assumed.

Optimization of the number and locations of the calibration stations needed to monitor soil moisture using distributed temperature sensing systems: A proof-of-concept study

The single-probe heat-pulse (SPHP) technique combined with the Fiber-optic Distributed Temperature Sensing (DTS) technology can offer novel high-resolution measurements of soil moisture (θ) over spatial scales ranging from several centimeters to several kilometers. However, the key limitation of this method is in obtaining the calibration relationship between θ and soil thermal conductivity (λ) across a specific field. In a previous study, a new methodology using a Gaussian processes model was presented to account for the spatial variability in the λ-θ relationship. The model aggregated θ measurements from soil moisture sensors scattered over the SPHP transect with the corresponding DTS λ measurements at their locations. In this study, a novel methodology is tested to optimize the number and locations of soil moisture sensors required to account for the spatial variability of the λ-θ relationship to achieve higher accuracy from the SPHP technique. The proposed methodology utilizes hierarchical clustering to analyze the information contained in the spatial structure of the SPHP measurements as the soil dries from a nearly-saturated condition. The proposed methodology was tested using data from a field in Oklahoma. Monte-Carlo simulation was performed to validate the performance of the proposed methodology. The predictions obtained from the proposed methodology resulted in θ measurements accuracy comparable to those obtained from the 10% best Monte-Carlo iterations of randomly assigned soil moisture locations. This study demonstrates that the proposed methodology is more efficient than the traditional practice of randomly spreading calibration soil moisture sensors along the SPHP transect.

Characterizing soil water content variability across spatial scales from optimized high-resolution distributed temperature sensing technique

Fiber-optic Distributed Temperature Sensing, when combined with the Single-probe Heat-pulse technique can measure soil moisture (θ) across spatial scales. The key limitation of this system is in obtaining the relationship between soil thermal conductivity (λ) and θ for a specific field. Using the Department of Energy Atmospheric Radiation Measurement (ARM) site, this study tested a new methodology to account for the spatial variability in the λ-θ relationship using a Gaussian processes model. The resulting accurate θ measurements (RMSE = 0.03 m3m−3) were used to characterize the spatial variability of θ across scales and to develop an empirical equation that can correct for the changes in the θ spatial variability observed at different spatial resolutions. In addition, the number of required samples to accurately characterize θ and its variability over scales ranging from 5 m and 350 m were estimated. These findings provide key information to scale soil moisture from centimeters to hundreds of meters for process understanding.

Characterizing plant root parameters with deep learning-based heat pulse method

Accurate quantification of root structure is essential for understanding plant ecological system. Nevertheless, the lack of in-situ root characterization methods has limited further research of the relevant field. Heat pulse (HP) technique has emerged as a promising, cost-effective in situ monitoring approach. The existing HP technique relies on the analytical solution of heat transfer model with the assumption that the medium under test is homogeneous. Thus, its performance is compromised in dealing with heterogeneous formations. The objective of this study is to establish a novel in-situ HP method to characterize root parameters based on a deep learning technique. With the fully connected neural networks (FCNN), the number of root fragments was firstly estimated by solving a classification problem, and then the size and location of each fragment were estimated by solving regression problems. All FCNNs were firstly pre-trained using synthetic dataset generated by the numerical heat transfer model. Although FCNNs showed promising accuracy on the synthetic testing dataset, they failed to provide satisfactory results on real-world experimental data due to the model error and observation noise, i.e., the deviations between the numerical model and actual situation. To this end, FCNNs were fine-tuned with some experimental data through transfer learning for performance improvement. Under controlled indoor experiment in a sandy soil, the proposed method provided satisfactory results for the estimation of root fragment number and diameters with 83.3% accuracy and RMSE of 1.87 × 10−4 m, respectively. The position estimation had relatively larger error, i.e., RMSE 2.41 × 10−3 m. This study is the first step in translating heat pulse signals to root parameters, which indicates the potential of combining HP technique and deep learning in studying root system.

Measuring and modelling crop water use of sweet pepper crops grown in screenhouses and greenhouses in an arid region

This research studied microclimate and evapotranspiration (ET) of pepper in high tunnel screenhouses and greenhouses. A study was carried out during three consecutive seasons, 2015–2016 (Season I), 2016–2017 (Season II) and 2017–2018 (Season III), in two high-tunnels. One was covered by a plastic sheet (greenhouse) and the other by a porous screen (17 mesh insect net; screenhouse). Other than the covers, the structures were identical. In both houses, microclimate variables were measured simultaneously above the canopy. Evapotranspiration was estimated using six different variations of the Penman–Monteith model. Plant transpiration was measured by the heat pulse technique. Measured transpiration was compared with model estimates of ET to identify the model in best agreement with measurements. Linear regressions between internal and external climatic conditions resulted in R2 values between 0.003 (greenhouse temperature) and 0.99 (greenhouse global radiation). Generally, higher R2 values were obtained for the screenhouse because internal microclimate interacted with the outside more than in the greenhouse. Mean daily evapotranspiration estimated by the six Penman–Monteith models tested was 1.61–4.23 and 1.42–4.43 mm day−1 for the greenhouse and screenhouse, respectively. Mean daily ratio between ET models and transpiration measurements varied between 0.81 and 2.01 with a mean value of 1.20 ± 0.34 (where ± is standard deviation). The comparison between transpiration measurements and the PM models showed that in most cases models based on internal climatic conditions were in better agreement with measurements than those based on the external conditions.

Measurement and prediction of thermal properties of pantoa during deep-fat frying

Thermal properties of pantoa were determined during deep-fat frying in sunflower oil at 125-145°C. Thermal conductivity (k: 0.357 to 0.199 Wm -1 K -1 ), thermal diffusivity (α: 0.105x10 -6 to 0.140x10 -6 m 2 s -1 ) and volumetric specific heat (C p : 3.275 to 1.401 MJm -3 K -1 ) were measured simultaneously using dual probe heat pulse technique. Thermal conductivity and volumetric specific heat of pantoa decreased as the moisture content decreased, while thermal diffusivity increased with the decrease in moisture content. Empirical models were also developed as a function of moisture content, fat content and temperature of frying of pantoa . It was found that moisture content was inversely related to fat content during the progress of frying. The thermal properties and the developed models would be useful in the mathematical modeling of heat transfer in pantoa during DFF.

An experimental study of coupled heat and moisture transfer in soils at high temperature conditions for a medium coarse soil

A study of one-dimensional heat and moisture transfer within a vertical soil column was conducted experimentally. An experimental soil cell made of stainless-steel tube was exposed to differential heating by sandwiching it between two differentially-heated plates for studying heat and moisture transfer in the soil column at different temperature levels and temperature differences. The main objective of the experimental study was to investigate heat and moisture transfer characteristics in a medium coarse soil at temperatures greater than 40 °C up to 90 °C. In this paper, the results are divided into two parts. In the first part, the results of heat transfer in dry soil are presented and discussed. The purpose was to investigate temperature distributions and heat gains/losses at the steady-state conditions along the soil cell at the various temperature levels. In the second part, the results of heat and moisture transfer in wet soil are presented and discussed. In this part, the transient temperatures, moisture contents and thermal properties along the soil column were obtained using the heat pulse technique. A loamy sand with a porosity and an initial water content of 0.40 and 0.26 m3/m3 (or saturation degree of 65%), respectively, was used in the study. The results of the case with the largest temperature difference of 65 °C (or overall temperature gradient of 440 °C/m) and the mean temperature of 55 °C are presented in detail. The highest temperature gradient of 1450 °C/m was recorded at the top of the soil column during the test, driving downward a moisture flux as high as 3 g/s·m2 when the soil was at temperature of 51 °C and saturation degree of 40%. Under these conditions, the thermal vapor diffusion is the main mechanism for the moisture flux.

Leaf water relations in Diospyros kaki during a mild water deficit exposure

The resistance mechanisms (stress avoidance and stress tolerance) developed by persimmon plants (Diospyros kaki L. f. grafted on Diospyros lotus L.) in response to mild water stress and the sensitivity of continuously (on a whole-day basis) and discretely (at predawn and midday) measured indicators of the plant water status were investigated in 3-year old ‘Rojo Brillante’ persimmon plants. Control (T0) plants were drip irrigated in order to maintain soil water content at levels slightly above soil field capacity (102.3% of soil field capacity) and T1 plants were drip irrigated for 33 days in order to maintain the soil water content at around 80% of soil field capacity. The results indicated persimmon plants confront a mild water stress situation by gradually developing stomata control (stress avoidance mechanism) and exhibiting some xeromorphic characteristic such as high leaf relative apoplastic water content, which could contribute to the retention of water at low leaf water potentials. In addition, sap flow measurements made by the heat-pulse technique were seen to be the most suitable method for estimating persimmon water status, because it provided the highest signal intensity (actual value/reference value):noise (coefficient of variation) ratio in almost all intervals of time considered and provides continuous and automated registers of the persimmon water status in real time.

Measurement of water-use by <i>Jatropha curcas</i> L. using the heat-pulse velocity technique

In response to the proposed introduction of the potential bio-diesel species Jatropha curcas (Linnaeus) to South Africa, field experiments were conducted to investigate its likely water-use impacts relative to other forms of vegetative land use. As no existing water-use data could be found for this species worldwide, sap flow in Jatropha curcas trees was measured continuously for a 17-month period at two sites in eastern South Africa. These consisted of young (4-year-old) trees at a relatively wet site and mature (12-year-old) trees at a dry site. The heat-ratio method of the heat-pulse technique was utilised, together with measurements of meteorological variables and soil water. Sap- flow rates varied according to tree age, season, prevailing meteorological conditions, and soil moisture levels. Peak sap-flow rates occurred during the warm wet summer months, but due to the deciduous nature of the species, water use was negligible during winter. Scaled-up sap-flow measurements resulted in estimates of total annual transpiration of 1 983 l (147 mm) for a 4-year-old J. curcas tree, and 4 884 l (362 mm) for a 12-year-old J. curcas tree. The study concluded that the J. curcas trees studied were conservative in their water use, and were unlikely to transpire more water than indigenous vegetation types of the area.

A generalized relationship between thermal conductivity and matric suction of soils

The soil thermal conductivity (λ) and matric suction of soil water (h, the negative of matric potential) relationship has been widely used in land surface models for estimating soil temperature and heat flux following the McCumber and Pielke (1981, MP81) λ-h model. However, few datasets are available for evaluating the accuracy and feasibility of the MP81 λ-h model under various soil and moisture conditions. In this study, we developed a new λ-h model and compared its performance with that of the MP81 model using measurements on 18 soils with a wide range of textures, water contents and bulk densities. The heat pulse technique was used to measure λ, and the suction table, micro-tensiometers, pressure plate device, and the dew point potentiometer were applied to obtain soil water retention curves at the appropriate suction ranges. In the range of pF (the common logarithm of h in cm) ≤ 3, the λ-h relationships were highly nonlinear and varied strongly with soil texture and bulk density. In the dry range (i.e., pF > 3), there existed a universal λ-h relationship for all soil textures and bulk densities, and an exponential function was established to describe the relationship. Independent evaluations using λ-h data on five intact soil samples showed that the new model produced accurate λ data from pF values with root mean square errors (RMSE) with the range of 0.03–0.18 W m−1 K−1. While, large errors (RMSEs within 0.17–0.36 W m−1 K−1) were observed with λ estimates from the MP81 model.

Effect of spatial variations and desiccation cracks on the DPHP and MPHP sensors

In sensing soil-moisture, there are inherent field related issues such as uncertainty and unpredictable distribution of minute amounts of trapped water and morphological changes of soil such as lumps and cracks due to desiccation. This poses problems for measurements done using a dual-probe heat-pulse (DPHP) technique that uses a narrow cylindrical sampling zone between heater and temperature probes. Thus, DPHP sensors are susceptible to errors. Hence, we studied and compared single-point and multi-point soil-moisture measurements using the heat-pulse technique. For this, we designed and developed a multi-point heat-pulse (MPHP) sensor with three temperature probes that enhanced the sampling zone to a volume of 840 mm3, which is three times that of the DPHP sensor. Sequentially deployed DPHP and MPHP sensors with spatial variations revealed that the maximum difference between the measured percentage of moisture from the DPHP sensor and standard oven-dried instrument was more than 10%, whereas for the MPHP sensor the discrepancy was only around 3%. Furthermore, when cracks appeared in the soil mass, it was observed that the discrepancy of DPHP sensor is around 16% while that for the MPHP sensor was still only 3%.

Critical Analysis of Dual-Probe Heat-Pulse Technique Applied to Measuring Thermal Diffusivity

The paper presents an analysis of the experimental parameters involved in application of the dual-probe heat pulse technique, followed by a critical review of methods for processing thermal response data (e.g., maximum detection and nonlinear least square regression) and the consequent obtainable uncertainty. Glycerol was selected as testing liquid, and its thermal diffusivity was evaluated over the temperature range from − 20 °C to 60 °C. In addition, Monte Carlo simulation was used to assess the uncertainty propagation for maximum detection. It was concluded that maximum detection approach to process thermal response data gives the closest results to the reference data inasmuch nonlinear regression results are affected by major uncertainties due to partial correlation between the evaluated parameters. Besides, the interpolation of temperature data with a polynomial to find the maximum leads to a systematic difference between measured and reference data, as put into evidence by the Monte Carlo simulations; through its correction, this systematic error can be reduced to a negligible value, about 0.8 %.

Influences of thermal dispersion on soil water flux estimates using heat pulse technique in saturated soils

The heat-pulse technique (HPT) shows good potentials for in situ determinations of soil water flux (J), but its applications are limited because of J underestimates. This study aims to experimentally investigate the influences of thermal dispersion on J estimates using HPT in packed sand, silt loam, and sandy clay loam with one-dimensional saturated water flow, hypothesizing that J underestimates are caused by neglecting heat dispersion in heat transport model that arises from the heterogeneity of water velocities within and between water-filled soil pores. The results indicated that J estimates exhibited good linearity with the measurements (R2 > 0.95). However, they were biased toward underestimates. The thermal dispersion's dependence on J was described by a power function. When heat dispersion was considered in the traditional heat conduction–advection model, the traditional ratio method yielded a higher accuracy with relative errors reduced by 45.0–74.3%, the bias reduced by 47.1–74.0% and the root mean square error reduced by 48.0–68.8%. In terms of the range of J herein, the maximum Keith Jirka Jan (KJJ) numbers obtained were 6.8%, 10.4%, and 13.4% for sand, silt loam, and sandy clay loam, respectively. Moreover, their corresponding J thresholds were 41.7, 21.1, and 12.3 μm s−1, respectively.

Estimation of water consumption of riparian forest in the lower reaches of Tarim River, northwest China

The aim of this study is to estimate the water consumption of a Populus euphratica forest. Findings provide a scientific basis for water allocation to the lower reaches of the Tarim River, in Northwest China. In this paper, several different methods were used to gather data. The heat-pulse technique was applied to determine the stem sap flow of P. euphratica sample trees from May to September of 2015. Then, the P. euphratica sapwood area estimation model was established based on the correlation between the sapwood area and the tree biometric parameters. Finally, the water consumption of P. euphratica (n = 3000) trees was calculated. The results showed that the estimated sapwood area of P. euphratica (n = 3000) was 677,000 cm2, and the total water consumed by these trees during the measurement period was 23,000,000 L. The P. euphratica sap flow varies due to changing environmental factors. The average daily sap flow velocities were 814.3, 1461.75, 1907.27, 1657.51 and 1173.65 mL h−1 from May to September, respectively. The average daily accumulated sap flow was 22.44, 34.76, 46.98, 40.59 and 32.90 L from May to September, respectively. Pearson correlation analysis showed that variation of sap flow was positively related to solar radiation (R s) (R 2 = 0.715), air temperature (T a) (R 2 = 0.744), soil temperature (T s) (R 2 = 0.806), wind speed (W s) (R 2 = 0.104). At the same time, sap flow was negatively related to air humidity (H) (R 2 = −0.305). Stepwise regression analysis showed that solar radiation, air temperature, soil temperature and air humidity are the main factors influencing the P. euphratica sap flow variation.

A single-probe heat pulse method for estimating sap velocity intrees.

Available sap flow methods are still far from being simple, cheap and reliable enough to be used beyond very specific research purposes. This study presents and tests a new single-probe heat pulse (SPHP) method for monitoring sap velocity in trees using a single-probe sensor, rather than the multi-probe arrangements used up to now. Based on the fundamental conduction-convection principles of heat transport in sapwood, convective velocity (Vh ) is estimated from the temperature increase in the heater after the application of a heat pulse (ΔT). The method was validated against measurements performed with the compensation heat pulse (CHP) technique in field trees of six different species. To do so, a dedicated three-probe sensor capable of simultaneously applying both methods was produced and used. Experimental measurements in the six species showed an excellent agreement between SPHP and CHP outputs for moderate to high flow rates, confirming the applicability of the method. In relation to other sap flow methods, SPHP presents several significant advantages: it requires low power inputs, it uses technically simpler and potentially cheaper instrumentation, the physical damage to the tree is minimal and artefacts caused by incorrect probe spacing and alignment are removed.

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

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

Evaluation of water flux prediction with heat-pulse technique in saturated soils

ABSTRACTHeat-pulse technique (HPT) has shown promise for predicting soil water flux (Jw). This study evaluated the accuracy of HPT in predicting Jw in packed saturated columns of quartz, sand, silt loam, and sandy clay loam. Jw was predicted using the maximum dimensionless temperature differences (MDTD), ratio of downstream to upstream temperature increases (Td/Tu), and an improved Td/Tu method. Results indicated that Jw predictions had a good linear relationship with measurements (R2 > 0.93). The HPT underestimated Jw to varying degrees, and the underestimations increased as Jw increased and soil texture became fine. The Td/Tu method outperformed the MDTD and the improved Td /Tu because of its higher accuracy, fewer parameters, and simpler calculations. The MDTD exhibited the poorest performance. In coarse-textured soil materials (e.g. quartz and sand), Jw predictions by the Td/Tu method were most accurate, and even with high Jw (up to 72.4 μm s−1), relative errors still remained within 9.8%. However, in fine-textured soil materials, Jw was underestimated significantly by 16.9% in silt loam and by 23.3% in sandy clay loam. The lower Jw limits were 1.0, 2.3, 2.4, and 4.0 μm s−1 for quartz, sand, silt loam, and sandy clay loam, respectively (P > 0.05).

A low-power, low-cost soil-moisture sensor using dual-probe heat-pulse technique

This paper presents the development and testing of an integrated low-power and low-cost dual-probe heat-pulse (DPHP) soil-moisture sensor in view of the electrical power consumed and affordability in developing countries. A DPHP sensor has two probes: a heater and a temperature sensor probe spaced 3mm apart from the heater probe. Supply voltage of 3.3V is given to the heater-coil having resistance of 33Ω power consumption of 330mW, which is among the lowest in this category of sensors. The heater probe is 40mm long with 2mm diameter and hence is stiff enough to be inserted into the soil. The parametric finite element simulation study was performed to ensure that the maximum temperature rise is between 1°C and 5°C for wet and dry soils, respectively. The discrepancy between the simulation and experiment is less than 3.2%. The sensor was validated with white clay and tested with red soil samples to detect volumetric water-content ranging from 0% to 30%. The sensor element is integrated with low-power electronics for amplifying the output from thermocouple sensor and TelosB mote for wireless communication. A 3.7V lithium ion battery with capacity of 1150mAh is used to power the system. The battery is charged by a 6V and 300mA solar cell array. Readings were taken in 30min intervals. The life-time of DPHP sensor node is around 3.6 days. The sensor, encased in 30mm×20mm×10mm sized box, and integrated with electronics was tested independently in two separate laboratories for validating as well as investigating the dependence of the measurement of soil-moisture on the density of the soil. The difference in the readings while repeating the experiments was found out to be less than 0.01%. Furthermore, the effect of ambient temperature on the measurement of soil-moisture is studied experimentally and computationally.

Phonon thermal transport and phonon–magnon coupling in polycrystalline BiFeO3 systems

Temperature-dependent thermal conductivity of polycrystalline BiFeO3, Bi0.9Ba0.05Ca0.05FeO2.95 and Bi0.9Ca0.1FeO2.95 materials was measured using a direct heat pulse technique. Thermal conductivity of the BiFeO3-based materials is analyzed using a phonon model to probe the thermal transport mechanisms in these ferrites. It is found that the calculated thermal conductivity of the BiFeO3-based compounds is in good agreement with experimental data. The suppression of the low-temperature phonon peak in the thermal conductivity of the doped BiFeO3 materials is mainly attributed to the phonon-point-defect scattering. In addition, the contribution of optical phonon–magnon resonance scattering to optical phonon thermal transport reveals the presence of phonon–magnon coupling in these BiFeO3 materials. Finally, magneto-thermal conductivity measurements show the magnon thermal transport in pure and doped BiFeO3 systems.

Thermal properties of sapwood of fruit trees as affected by anatomy and water potential: errors in sap flux density measurements based on heat pulse methods

The accuracy of heat pulse methods is compromised both by changes in sapwood water content and by the methodology applied to determine its thermal properties. Sapwood thermal properties and water content (F w) natural variations affect the accuracy of heat pulse sap flow methods, as they are typically set as constants or calculated during zero flow conditions. In a first experiment, a characterisation of both thermal properties and some of their determining anatomical and functional factors was conducted on several fruit tree species assessing the reliability of different methodologies. Besides that, a second experiment was carried out to evaluate the errors of heat pulse methods arising from ignoring F w variations. To do so, desorption curves were constructed, allowing the substitution of F w changes with those of water potential (Ψ). Results of the first experiment showed considerable differences between species in both the thermal, anatomical and functional properties of sapwood. Apart from that, discrepancies between the methods applied to determine thermal properties were also found and their implications for some heat pulse methods are discussed. The analysis conducted for the second experiment indicated that large errors in sap flux density (J) determinations might occur when daily and seasonal variations of Ψ (and hence of F w) are disregarded. The extent of these errors was influenced by the species and heat pulse technique. Thus, the heat ratio and T max were, respectively, the least and most vulnerable methods to errors in J determinations associated with changes in F w.