Volumetric Water Content Measurements Research Articles

Volumetric water content measurement probes in earth-dam construction

Two frequency domain reflectometry (FDR) probes have been used. They were used on compacted soils both in the laboratory and in the field. Measurements in the laboratory were intended for calibration. The range of densities and types of materials where insertion of the probes can be achieved was investigated first. The effect of sporadic presence of coarser grains and density on these calibrations, once insertion could be achieved, were investigated second. Measurements on laboratory prepared samples with the same moisture content were different when the sample was kept in the mould from when it was extruded from it. Also both these measurements were different from that in a sample of the same density but significantly larger in diameter. It was found that measurements with these probes are affected by dilation exhibited by soil around the rods of the probes during insertion. Readings immediately after insertion of the sensors on samples extruded from their moulds were the ones closer to measured values. These readings combined with total volume and mass obtained from sand-cone tests during the construction of an earth-dam allowed fairly accurate estimation of the dry unit weight but not the gravimetric water content.

Soil heat flux calculation for sunlit and shaded surfaces under row crops: 1. Model development and sensitivity analysis

Soil heat flux at the surface (G0) is strongly influenced by whether the soil is shaded or sunlit, and therefore can have large spatial variability for incomplete vegetation cover, such as across the interrows of row crops. Most practical soil–plant–atmosphere energy balance models calculate G0 as a function of either total (RN) or soil net radiation (RN,S). Even though RN,S includes sunlit and shaded conditions, this is seldom considered, even at spatial scales of a few m. In order to improve the utility of surface energy balance models designed for row crops at relatively small spatial scales, a method was developed to calculate G0 as a function of shaded, partially sunlit, or fully sunlit RN,S. Calculation of RN,S was derived using a geometric approach, and G0 was derived by the calorimetric method using measurements of soil temperature and volumetric soil water content under upland cotton (Gossypium hirsutum L.) over a wide range of canopy cover conditions. Calorimetric G0 and calculated RN,S were related by assuming their normalized values were equal at 24h time steps and in phase. The method required only a single empirical parameter, which related the 24h minimum G0 (G0,MIN) to the 24h maximum RN,S (RN,S,MAX) as G0,MIN=a×RN,S,MAX, and a=−0.31 was found by simple linear regression (p<0.01). Model sensitivity (SM) of calculated G0 was calculated for sparse, medium, and full canopy cover; nighttime, shaded, partially sunlit, and fully sunlit surface conditions; and north–south and east–west row orientations, where input values of agronomic, shortwave, and longwave input variables were varied ±25% of their base values. The method was most sensitive (1.0<SM<36) to canopy width, canopy height, leaf area index, row spacing, canopy and soil emittances, and canopy and soil temperatures for medium to full canopy cover. Also, there was generally greater sensitivity for shaded and partially sunlit surfaces compared with sunlit and nighttime surfaces, and north–south rows compared with east–west rows. However, the method had little sensitivity (SM usually <0.50) to input variables used to calculate the shortwave components of RN,S.

Density-dependent calibration of multisensor capacitance probes in coarse soil

Zettl, J. D., Huang, M., Barbour, S. L. and Si, B. C. 2015. Density-dependent calibration of multisensor capacitance probes in coarse soil. Can. J. Soil Sci. 95: 331–336. Coarse-textured reconstructed soils are utilized extensively in the reclamation of mining waste. Accurate and continuous sensing of soil water content is required to understand soil water dynamics and evaluate the hydraulic characteristics of these soils. The EnviroSCAN (Sentek Pty. Ltd, Australia) is a semi-permanent multisensor capacitance probe (MCP) capable of continuous measurement of volumetric water content (θv) and has been used to monitor reclamation soil cover performance. Calibration of these probes is required to improve the accuracy of field measurements. In this study, field and laboratory measurements were undertaken over a range of water contents and bulk densities to refine the relationship between θvand scaled frequency (SF) measured by the MCP. The manufacturer's calibration equation tended to underestimate θvunder wet conditions (θv&gt;0.35 cm3cm–3). Our experimental data showed that bulk density (ρb) did affect the MCP calibration and consequently a new calibration equation that includes the effect of ρbis developed using laboratory measurements and validated using field measurements. This equation provided the highest degree of correlation and the smallest standard deviation of prediction to measured values of θvfor laboratory and field measurements, respectively. This calibration improves the application of the EnviroSCAN for coarse-textured soils such as those utilized in this study.

Performance Analysis of Capacitance and Electrical Resistance-Type Soil Moisture Sensors in a Silt Loam Soil

The performance of the Irrometer 200SS Watermark granular matrix sensor (WM), John Deere Field Connect (JD-v2) probe, and Delta-T PR1-capacitance (PR1-C) probe were evaluated against a Troxler 4302 neutron gauge (NG) for in-season field volumetric water content (θv) measurements at two soil depths in a Hastings silt loam soil at the University of Nebraska-Lincoln/Institute of Agriculture and Natural Resources South Central Agricultural Laboratory (SCAL) near Clay Center, Nebraska. The performances of the sensors were investigated over three years (2011-2013) under various water, nutrient, and crop management practices. The WM sensors performed best when using a field-calibrated soil water retention curve (SWRC) [root mean square difference (RMSD) = 0.024 m3 m-3] as compared to a SWRC developed from a pedotransfer function (RMSD = 0.070 m3 m-3). The WM sensors using a previously developed SWRC for the experimental field resulted in RMSD values less than 0.05 m3 m-3 when compared to the NG-measured θv at all depths and years. The JD-v2 probes underestimated θv in the dry range and overestimated θv in the wet range, which resulted in regression slopes and intercepts for the 0.30 and 1.0 m soil depths that were significantly different from unity (i.e., 1.0) and zero (p0.05

Impact of sensor failure on the observability of flow dynamics at the Biosphere 2 LEO hillslopes

The Biosphere 2 Landscape Evolution Observatory (LEO) has been developed to investigate hydrological, chemical, biological, and geological processes in a large-scale, controlled infrastructure. The experimental hillslopes at LEO are instrumented with a large number of different sensors that allow detailed monitoring of local and global dynamics and changes in the hydrological state and structure of the landscapes. Sensor failure, i.e., a progressive reduction in the number of active or working sensors, in such an evolving system can have a dramatic impact on observability of flow dynamics and estimation of the model parameters that characterize the soil properties. In this study we assess the retrieval of the spatial distributions of soil water content and saturated hydraulic conductivity under different scenarios of heterogeneity (different values of correlation length of the random field describing the hydraulic conductivity) and a variable number of active sensors. To avoid the influence of model structural errors and measurement bias, the analysis is based on a synthetic representation of the first hydrological experiment at LEO simulated with the physically-based hydrological model CATHY. We assume that the true hydraulic conductivity is a particular random realization of a stochastic field with lognormal distribution and exponential correlation length. During the true run, we collect volumetric water content measurements at an hourly interval. Perturbed observations are then used to estimate the total water storage via linear interpolation and to retrieve the conductivity field via the ensemble Kalman filter technique. The results show that when less than 100 out of 496 total sensors are active, the reconstruction of volumetric water content may introduce large errors in the estimation of total water storage. In contrast, retrieval of the saturated hydraulic conductivity distribution allows the CATHY model to reproduce the integrated hydrological response of LEO for all sensor configurations investigated.

Spatio-temporal interpolation of soil water, temperature, and electrical conductivity in 3D + T: The Cook Agronomy Farm data set

The paper describes a framework for modeling dynamic soil properties in 3-dimensions and time (3D + T) using soil data collected with automated sensor networks as a case study. Two approaches to geostatistical modeling and spatio-temporal predictions are described: (1) 3D + T predictive modeling using random forests algorithms, and (2) 3D + T kriging model after detrending the observations for depth-dependent seasonal effects. All the analyses used data from the Cook Agronomy Farm (37 ha), which includes hourly measurements of soil volumetric water content, temperature, and bulk electrical conductivity at 42 stations and five depths (0.3, 0.6, 0.9, 1.2, and 1.5 m), collected over five years. This data set also includes 2- and 3-dimensional, temporal, and spatio-temporal covariates covering the same area. The results of (strict) leave-one-station-out cross-validation indicate that both models accurately predicted soil temperature, while predictive power was lower for water content, and lowest for electrical conductivity. The kriging model explained 37%, 96%, and 18% of the variability in water content, temperature, and electrical conductivity respectively versus 34%, 93%, and 5% explained by the random forests model. A less rigorous simple cross-validation of the random forests model indicated improved predictive power when at least some data were available for each station, explaining 86%, 97%, and 88% of the variability in water content, temperature, and electrical conductivity respectively. The high difference between the strict and simple cross-validation indicates high temporal auto-correlation of values at measurement stations. Temporal model components (i.e. day of the year and seasonal trends) explained most of the variability in observations in both models for all three variables. The seamless predictions of 3D + T data produced from this analysis can assist in understanding soil processes and how they change through a season, under different land management scenarios, and how they relate to other environmental processes.

Centrifuge Modeling of End-Restraint Effects in Energy Foundations

AbstractThis study presents the results from physical modeling experiments on centrifuge-scale energy foundations in dry sand and unsaturated silt layers. These experiments were performed to characterize end restraint effects on soil-structure interaction for energy foundations in different soils and include tests on foundations with semifloating and end-bearing toe boundary conditions and free-expansion and restrained-expansion head boundary conditions. Two scale-model energy foundations having different lengths were constructed from reinforced concrete to simulate end-bearing and semifloating conditions in soil layers having the same thickness. The foundations include embedded thermocouples and strain gauges, which were calibrated under applied mechanical loads and nonisothermal conditions before testing. The variables measured during the experiments include axial strain and temperature distributions in the foundation, temperature, and volumetric water content measurements in the soil, vertical displace...

Numerical modeling of water balance for temporary landfill cover in North Germany

AbstractAt the end of waste disposal in 2005, a temporary mineral system was constructed to cover the waste body of the municipal landfill in Rastorf (N Germany). The aim of this study was to evaluate the effectiveness of this temporary cover system in limiting the infiltration of surface water into the waste body. The numerical modeling for the hydrological year 2012 was performed in FEFLOW 6.0. The required input data for modeling were achieved by laboratory measurements of the saturated hydraulic conductivity and the water retention characteristics of the mineral layers. The assumed initial and boundary conditions were based on in‐situ measurements of matrix potential and volumetric water content at three different measuring points by tensiometers, FDR sensors and observed meteorological conditions. The model was applied according to Darcy's and Richards' equations for variably saturated flow. Our results allowed assessing the components of water balance for a temporary cover system, especially water seepage into the waste body. The correlation between the measured and modeled volumetric soil water content in the layered temporary cover system was observed (R2 = 0.56 to 0.90). Deviations between the measured and modeled values are superficially related to the soil´s heterogeneity. In conclusion, the measured and modeled water balance parameters indicate that the temporary coverage system remains effective in limiting the infiltration rate. However, our studies revealed that the calculated and measured water seepage exceeded the values allowable by the German regulations. Additional studies have to assess variations in the soil water characteristics caused by weather‐related wetting and drying cycles and influence of waste settlement changing the profile of slope.

Errors associated with estimating vegetation water content from radar for use in passive microwave brightness temperature algorithms

The Soil Moisture Active Passive Validation Experiment 2012 was conducted as a pre-launch validation campaign for the Soil Moisture Active Passive mission over 6 weeks in June and July 2012. During this campaign, the Passive Active L-Band System (PALS) was flown at a low altitude, providing radar and radiometer measurements that were contained within a single agricultural field. The campaign domain consisted of 55 agricultural fields, where soil moisture was measured coincident to the PALS flight times and measurements of vegetation volumetric water content (VWC) and leaf area index (LAI) were measured weekly. The low-altitude flights allowed for the comparison between measured VWC and LAI for 11 fields to radar parameters derived from the radar backscatter. Only the correlation between the HV backscatter and the soybean VWC was considered strong (|r| > 0.7). All other correlations between the radar parameters and the VWC (or LAI) were moderate (0.3 < |r| < 0.7) or weak (|r|< 0.3). The established relationships between radar parameters and VWC were used in a forward radiation transfer model to estimate H-pol brightness temperature. It was found that the RMSE between the brightness temperatures estimated using the measured VWC was lowest when using the relationship between VWC and LAI (3.9 K for soybeans, 6.8 K for spring wheat, and 9.3 K when all crop data are combined). Despite a lower correlation, the RMSE associated with using the radar vegetation index relationship with VWC was less than when HV was used (7.9 K) for soybeans, which would result in an error in soil moisture estimation of just over 4%. The RMSEs for all other VWC and radar parameter relationships were greater than 10 K.

Averaging Performance of Capacitance and Time Domain Reflectometry Sensors in Nonuniform Wetted Sand Profiles

The averaging behavior of two capacitance water content sensors in nonuniform wetted sand samples was assessed and compared with the averaging behavior of a time domain reflectometry (TDR) water content sensor in identical samples. Four different nonuniform wetting situations were assessed. In one of the four experiments, the orientation of the capacitance sensor was altered, while in another the sensitivity along the length of the sensor was tested. For one of the tested sensor types, the overestimation of the volumetric water content (VWC) was 0.034 compared with the value determined by drying, while the corresponding TDR value deviated 0.009. The conclusion is that in a nonuniform wetted soil profile, the capacitance sensors tested weigh the wet parts more heavily than the dry parts, resulting in an overestimation of the average VWC. Therefore we conclude that the capacitance sensors tested are less suitable for applications in situations where both nonuniform wetted profiles are encountered and accurate VWC measurements are required.

Heated Fiber Optic Distributed Temperature Sensing: A Dual‐Probe Heat‐Pulse Approach

Implementation of the dual‐probe heat‐pulse (DPHP) approach for measurement of volumetric heat capacity (C) and water content (θ) with distributed temperature sensing heated fiber optic (FO) systems presents an unprecedented opportunity for environmental monitoring (e.g., simultaneous measurement at thousands of points). We applied uniform heat pulses along a FO cable and monitored the thermal response at adjacent cables. We tested the DPHP method in the laboratory using multiple FO cables at a range of spacings. The amplitude and phase shift in the heat signal with distance was found to be a function of the soil volumetric heat capacity. Estimations of C at a range of moisture contents (θ = 0.09– 0.34 m3 m−3) suggest the feasibility of measurement via responsiveness to the changes in θ, although we observed error with decreasing soil water contents (up to 26% at θ = 0.09 m3 m−3). Optimization will require further models to account for the finite radius and thermal influence of the FO cables. Although the results indicate that the method shows great promise, further study is needed to quantify the effects of soil type, cable spacing, and jacket configurations on accuracy.

Effect of fine particles on the hydraulic behavior of interlayer soil in railway substructure

The conventional railway substructure in France was built by emplacing ballast directly on subgrade. Over years of operation, the interpenetration of ballast and subgrade created a soil layer between them. Under different conditions, this naturally formed layer, namely interlayer, can contain different quantities of fine particles, becoming more or less sensitive to changes in water content. As the water content changes are governed by the hydraulic behavior of the interlayer soil, assessing the influence of fine particle content on the hydraulic behavior of interlayer soil is of importance. To this end, the hydraulic behavior of an interlayer soil taken from Sénissiat (near Lyon, France) was investigated using two infiltration columns, a large-scale column equipped with tensiometers and a time domain reflectometer (TDR) for suction and volumetric water content measurements, respectively, and a smaller column equipped with high-capacity tensiometers only. Different fines contents were considered and wetting–drying cycles were applied to the soil specimens. The hydraulic conductivity was determined by applying the instantaneous profile method. The results obtained showed that (i) hysteresis exists for both the soil water retention curve and the hydraulic conductivity changes with suction; (ii) the effect of wetting–drying cycles is insignificant; (iii) adding 10% fine particles to the natural interlayer soil gives rise to changes in the soil water retention curve but does not induce significant changes in hydraulic conductivity; (iv) the unsaturated hydraulic conductivity of interlayer soil with 10% fine particles added is close to that of soil sieved at 2 mm, suggesting that the unsaturated hydraulic conductivity of interlayer soil is mainly governed by fine particles through the suction effect. By contrast, in a saturated state, the value for the interlayer soil with 10% fine particles added was found to be higher, suggesting that in this case the hydraulic conductivity is mainly governed by the water transfer through macropores.

New capabilities of standard of volumetric water content of oil and oil products

The last decade has shown major changes in the field of oil production and processing: the necessity of commercial accounting of produced oil with high water content (water content over 90%); the development of on-stream moisture meters with new operating principles, moisture meters with measurement range of volumetric water content from 0 to 100% including meters of foreign manufacture; the measurement accuracy increase of on-stream moisture meters. The work features an overview of the existing methods and instruments for the measurement of volumetric water content of oil and oil products. It is shown that working measuring instruments operate in the volumetric water content range of 0 ÷ 100%, while the State verification schedule and State special standard GET 87-75 provide transfer of the unit up to 60% of the operating range only. Due to this, the necessity has arisen for the improvement of State primary special standard of unit of volumetric water content of oil and oil products. As a result of the performed works, a new process system has been developed, consisting of a hydraulic circuit, measuring instruments for parameter determination of original components of oil-water mixtures, a facility for deep drying of oil, devices and auxiliary equipment providing refinement of the created mixture and maintenance of internal environment in the standard room. Reproduction of the unit of volumetric water content by the standard is based on the method of creating a water-oil mixture with set water content by means of precise proportioning of components and creation of an equal-part mixture on their basis. The proportioning is performed in mass fractions; the transition from mass to volume fractions is performed on the basis of predetermined parameters of original components of the mixture (density, original water content, salt content). As a result of the conducted research, metrological characteristics of the new standard have significantly improved; the possible sources of additional error have been identified and minimized. The new standard GET 87-2011 approved by Rosstandart order No. 252 features improved metrological characteristics and provides reproduction of the unit of volumetric water content within the range of (0,01 ÷ 99,9%).

Continuous-time monitoring of liquid water content in snowpacks using capacitance probes: A preliminary feasibility study

Liquid water in snowpacks rules wet snow avalanche formation, surface albedo and snowmelt runoff timing. By now, volumetric liquid water content (LWC) measurements are collected mainly with destructive methods, while continuous-time and non-invasive measurements would be preferable to track its time dynamics. Here, we assess the feasibility of continuous-time monitoring of LWC using EnviroSMART® capacitance sensors. These were conceived to track liquid water dynamics in soils, and their use in snow is evaluated here for the first time, as far as we know. A field case study was settled up in April 2013 within an Italian Alpine valley. An instrumental set up with eight capacitance sensors was installed. Two time-domain reflectrometers were added to the aim of comparison. To assist in interpreting the signal of the capacitance sensors, two laboratory tests were run, and a FEM model was implemented. This preliminary study demonstrates that capacitance sensors are sensitive to increasing LWC, although their long-term installation in snow entails the development of an air gap around them, due to localized melting, air turbulence and solar radiation absorption, which hinders following LWC variations. As a result, capacitance sensors readings are challenging to be interpreted quantitatively. Perspectives on future investigation are discussed to bring the proposed procedure towards long-term applications in snowpacks.

Calibration equations for Diviner 2000 capacitance measurements of volumetric soil water content in salt-affected soils

The capacitance sensor Diviner 2000 was used in an open field to monitor the volumetric soil water content (θν) in the Minqin Oasis of Northwest China. The objectives of the work were to evaluate the accuracy of the factory calibration equation; to obtain a soil-specific calibration as a linear and a power equation with both a saline and a tapwater reference; and to obtain a general equation for multiple soil layers. Calibration was done during an irrigation experiment by using water with different salinities. The soil was characterised by different salinities and textures across the plots and soil layers. The calibration was carried out using water of the same salinity and composition as the local irrigation water. This procedure would yield a better normalisation of the scaled frequency (SF) for each irrigation treatment. For the treatments under study, the accuracy of the factory calibration equation was not good (range of the root mean square error, RMSE: 0.048–0.171 cm3 cm–3). The accuracy of the soil-specific field calibration was higher. We therefore concluded that a soil-specific calibration is necessary. The RMSE shows no difference for the calibration equation with either tap or saline water reference. The linear and power equations, determined with a tap and saline water reference for each treatment and layer, gave larger differences in θν, Δθν, at high SF than at low SF values. The Δθv values increased with increasing salinity, i.e. from the control to the most saline water irrigation treatment. The linear equation had a significantly different slope for the 0.3–0.6 m layer, which has higher clay content. Pooling in a single equation of the data from multiple layers and/or treatments was satisfactory for the 0.3–0.6 m layer of the treatments W2 and W5, with RMSE of 0.026, with the higher clay content.

Evolution of soil wetting patterns preceding a hydrologically induced landslide inferred from electrical resistivity survey and point measurements of volumetric water content and pore water pressure

[1] The hydrological state of a hillslope prior to a sprinkling-induced shallow landslide was monitored using electrical resistivity tomography (ERT) along a 47 m long transect, supplemented by local time-domain reflectometry (TDR) and tensiometer measurements. The spatial and temporal evolution of wetting patterns in the soil material indicated attainment of a stationary fully saturated profile in a slope region underlain by shallow sandstone bedrock. The significant decrease in spatially averaged standard deviation of water saturation has not been observed during an earlier failed attempt to trigger a landslide by intense sprinkling. While for the “stable” experiment (no landslide was triggered) water saturation and soil moisture variability were still increasing with time, the “unstable” experiment reached a time-invariant state of high pore water pressures and saturations, until it finally failed. The results indicate that when large and interconnected regions of hillslope are saturated (as confirmed by high volumetric water content and low standard deviation of water saturation), additional water cannot be redistributed to empty drier regions and may eventually enhance local pore water pressure and seepage force, initiating large shear deformation and failure. Accordingly, a transition to such a critical steady state of high average water saturation, associated with low and constant spatial standard deviation, may serve as additional hydro-geophysical indicator for the imminence of a landslide release.

Seasonality of soil CO2 efflux in a temperate forest: Biophysical effects of snowpack and spring freeze–thaw cycles

Changes in characteristics of snowfall and spring freeze–thaw-cycle (FTC) events under the warming climate make it critical to understand biophysical controls on soil CO2 efflux (RS) in seasonally snow-covered ecosystems. We conducted a snow removal experiment and took year-round continuous automated measurements of RS, soil temperature (T5) and soil volumetric water content at the 5cm depth (W5) with a half-hour interval in a Chinese temperate forest in 2010–2011. Our objectives were to: (1) develop statistical models to describe the seasonality of RS in this forest; (2) quantify the contribution of seasonal RS to the annual budget; (3) examine biophysical effects of snowpack on RS; and (4) test the hypothesis that an FTC-induced enhancement of RS is jointly driven by biological and physical processes. Empirical RS–T5–W5 models explained 65.3–94.1% of the variability in the RS data, but the number of the regression terms and their coefficients varied with season. This indicates that the model should be fitted to the seasonal data sets separately to explicitly describe the seasonality of RS. The RS during the winter, spring FTC period, and growing season contributed 5.7%, 3.5%, and 91.1%, respectively, to the total annual RS. The relative enhancement of RS due to snowpack and FTCs averaged 3.4 and 2.5, respectively. The snowpack-induced enhancement of RS exponentially increased with T5 (R2=0.83) and snow depth (R2=0.16), while the FTC-induced enhancement of RS exponentially decreased with T5 (R2=0.45) and W5 (R2=0.67). These results suggest that the snowpack-induced enhancement mainly results from the snow-depth-dependent insulation of soil from low air temperatures, while the FTC-induced enhancement is dominantly driven by biological processes. Accumulatively, the snowpack and spring FTCs made a minor net contribution (2.3% and 1.2%, respectively) to the annual RS budget.

Investigating xylem embolism formation, refilling and water storage in tree trunks using frequency domain reflectometry

Trunks of large trees play an important role in whole-plant water balance but technical difficulties have limited most hydraulic research to small stems, leaves, and roots. To investigate the dynamics of water-related processes in tree trunks, such as winter embolism refilling, xylem hydraulic vulnerability, and water storage, volumetric water content (VWC) in the main stem was monitored continuously using frequency domain moisture sensors in adult Betula papyrifera trees from early spring through the beginning of winter. An air injection technique was developed to estimate hydraulic vulnerability of the trunk xylem. Trunk VWC increased in early spring and again in autumn, concurrently with root pressure during both seasons. Diurnal fluctuations and a gradual decrease in trunk VWC through the growing season were observed, which, in combination with VWC increase after significant rainfall events and depletion during periods of high water demand, indicate the importance of stem water storage in both short- and long-term water balance. Comparisons between the trunk air injection results and conventional branch hydraulic vulnerability curves showed no evidence of ‘vulnerability segmentation’ between the main stem and small branches in B. papyrifera. Measurements of VWC following air injection, together with evidence from air injection and xylem dye perfusion, indicate that embolized vessels can be refilled by active root pressure but not in the absence of root pressure. The precise, continuous, and non-destructive measurement of wood water content using frequency domain sensors provides an ideal way to probe many hydraulic processes in large tree trunks that are otherwise difficult to investigate.

Off- and on-ground GPR techniques for field-scale soil moisture mapping

Precise measurement of shallow volumetric soil water content (VSWC) is essential in different research fields, including agriculture, hydrology and meteorology. In this research three different approaches were applied for mapping the shallow VSWC of a bare soil at the field-scale, namely, (1) the direct ground wave (DGW) method using on-ground ground-penetrating radar (GPR), (2) the surface reflection coefficient (SRC) method also using on-ground GPR, and (3) the surface reflection inversion using off-ground GPR. Measurements were conducted in an agricultural field in both relatively dry and wet conditions. Soil sampling was performed at different depths (5, 10, 20, 30, 40cm) for determining the reference VSWC. The on-ground GPR was setup using two transmitting (Tx) and receiving (Rx) 400MHz bowtie antennas with Tx and Rx offsets of 0.16 and 1.26m, respectively, thereby setting up a multi-offset system. A vector network analyzer combined with a 0.2–2GHz TEM-horn antenna located at 1.1m above the ground was used as off-ground GPR. GPR measurements were performed following more than 10 transects in the entire field and positioning was performed using a dGPS. In addition, on-ground GPR measurements were conducted in the laboratory over a sandy soil layer including 10 different VSWC, ranging 0.02–0.18cm3·cm−3, in order to examine the SRC method for the on-ground GPR configurations. The Topp's model (1980) was used in order to transform the relative dielectric permittivity of the soil to the VSWC. Laboratory results showed that SRC method can be confidently used also for the on-ground GPR configurations when the air direct-coupling wave is subtracted from all signals. For the field data, the validation graphs using the ground-truths of the shallowest layer showed relatively reliable VSWC-maps with the RMSE values of 0.042, 0.044, and 0.059 in terms of VSWC for the SRC, DGW, and full-wave methods, respectively. In addition, the existence of a very shallow bedrock made a confusion on the detection of the DGW from the GPR data in the dry condition, but had no effect in the wet condition.

Salinity Effects on the Fieldscout TDR 300 Soil Moisture Meter Readings

While studying changes in volumetric water content (VWC) of soilless culture (sand and tuff), the need for a quick and easy-to-operate moisture measurement device with accuracy better than ±0.01 has risen. Before full scale application, we performed some basic tests with the Fieldscout TDR 300 soil moisture meter to evaluate the dependency of the VWC measurements on the salinity of the medium. The TDR300 was used to measure VWC in (i) sand and tuff wetted by deionized water or a 0.02 M NaCl solution to different VWC levels, (ii) sand or tuff wetted by solutions of different salinities to a given VWC level, and (iii) aqueous solutions of different salinities. Medium salinity had a significant effect (up to 60%) on the VWC readings of the TDR 300. Our observations suggest that the technology used by the TDR 300 is similar to that of low frequency water content reflectometers.