Volumetric Water Content Measurements Research Articles

Modelling the effect of geo-matrix conduction on the bulk and pore water resistivity in hydrogeological sedimentary beddings

Generally, the electrical method of geophysics is potentially valuable in typifying the subsurface conductivity and its surrounding medium. Sixteen datasets of 1-D electrical resistivity survey was integrated with 8 core samples, which were taken in the neighbourhood of eight of the sixteen VES datasets. The two sets of data were intertwined with geological and hydrogeological datasets to enhance the realization of unique results used in assessing the aquifer geo-matrix and pore water geo-resistivity in four counties in the coastal province of Akwa Ibom State, southern Nigeria. The current electrode separations of 1-D resistivity data were extended up to 300 m to ensure that the prolifically exploited water-bearing units were assessed. The VES data were manually and electronically modeled and each of them showed characteristic four geo-electric layers with KH, HK, KQ, HA and A group of curves. The core samples, which cut across the local Government Areas under survey, included sandy clay (12.5%), fine-grained sand (50.0%), medium-grained sand (12.5%) and coarse sand (25.0%). Bulk resistivities of water-bearing units and other primary geo-electrical indices were measured in all sixteen locations. However, in the eight aquifer core sample locations, water resistivities were measured in situ. The core samples were taken to the laboratory for measurements of fractional porosity and volumetric water content. The measured fractional porosity ranged from 0.189 to 0.267 with an average of 0.232, whilst volumetric water content varied from 0.23 to 0.31 with an average value of 0.268. The transmission coefficients were computed from volumetric water content and other parameters in the work. The values ranged from 0.4135 to 0.5612 with an average value of 0.4945. Fractional porosity was also modeled from field parameters and the results showed a good match with laboratory measurement as the root—mean square error was just 1.3%. Intrinsic specific resistance and conductivity of soil aquifer matrix were found to be $$3.333 \times 10^{3} \;\Omega {\text{m}}$$ and $$3.000 \times 10^{ - 4} \;{\text{Sm}}^{ - 1}$$ respectively. The study has provided useful information about the geo-electrical and geo-pore properties of the prolifically exploited aquifers in the study area. The aquifer parameter contour maps have been delineated and these, together with other parameter relations generated in this study could help in predicting the volumetric conductivity between pore water and the matrix texture of hydrogeological units in the study area and other areas with similar geology.

Evaluation of a multi‐sensor for measuring solution electrical conductivity in coir

AbstractThe simultaneous measurements of both volumetric water content (θ) and electrical conductivity of pore solutions (ECw) are essential for nutrient solution management within soilless culture systems. The present study was carried out in order to evaluate a capacitance sensor (METER 5TE) for the measurement θ and ECw within coir. Following the evaluation of the spatial sensitivity of the 5TE sensor, a calibration experiment was executed with six levels of θ (0.30, 0.35, 0.40, 0.45, 0.50, and 0.55 m3 m−3) and four levels of KCl solutions (1.0, 3.0, 5.0, and 7.0 dS m−1). The measured relationship between θ and dielectric permittivity (εa) for coir significantly deviated from Topp's equation for organic soil, but was similar to the calibration equation for growing media from the sensor manufacturer and for peat soils. The Hilhorst (2000) model recommended by the manufacturer overestimated ECw within coir. On the other hand, accounting for EC dependency on the real part of the complex dielectric permittivity of the solution within the Hilhorst model will greatly improve ECw prediction. In contrast, the Rhoades (1976) model was more capable of describing the dependence of the apparent soil electrical conductivity (ECa) on θ and ECw for the coir samples for ECa < 2.9 dS m−1. Hence, when ECw−ECa−θ relations are known, the capacitance sensor is a powerful tool for horticultural practices because a single measurement can yield both θ and ECw.

Physics‐Informed Neural Networks With Monotonicity Constraints for Richardson‐Richards Equation: Estimation of Constitutive Relationships and Soil Water Flux Density From Volumetric Water Content Measurements

AbstractWater retention curves (WRCs) and hydraulic conductivity functions (HCFs) are critical soil‐specific characteristics necessary for modeling the movement of water in soils using the Richardson‐Richards equation (RRE). Well‐established laboratory measurement methods of WRCs and HCFs are not usually suitable for simulating field‐scale soil moisture dynamics because of the scale mismatch. Hence, the inverse solution of the RRE must be used to estimate WRCs and HCFs from field measured data. Here, we propose a physics‐informed neural network (PINN) framework for the inverse solution of the RRE and the estimation of WRCs and HCFs from only volumetric water content (VWC) measurements. The proposed framework does not need initial and boundary conditions, which are rarely available in real applications. The PINNs consist of three linked feedforward neural networks, two of which were constrained to be monotonic functions to reflect the monotonicity of WRCs and HCFs. Alternatively, we also tested PINNs without monotonicity constraints. We trained the PINNs using synthetic VWC data with artificial noise, derived by a numerical solution of the RRE for three soil textures. The monotonicity constraints regularized the inverse problem. The PINNs were able to reconstruct the true VWC dynamics. We demonstrated that the PINNs could recover the underlying WRCs and HCFs in nonparametric form. However, the reconstructed WRCs and HCFs at wet and dry ends were unsatisfactory because of the strong nonlinearity. We additionally showed that the trained PINNs could estimate soil water flux density with a broader range of estimation than the currently available methods.

Evaluation of mine site reclamation performance using physical models: Case of Ity mine (Ivory coast)

The exploitation of the Ity gold mine in Ivory Coast will generate tailings that can be considered as potential generators of acid mine drainage (AMD). Various materials such as marble and clay are available around the mine site that could be used as components of an engineered cover for the reclamation of the reactive tailings impoundment. However, it is important to assess the properties of these potential cover materials and test different reclamation scenarios in the laboratory to select the best scenario for mine site reclamation. For this purpose, physical modeling was performed using experimental instrumented column tests simulating covers with capillary barrier effects (CCBEs) and monolayer covers. The columns were instrumented with sensors for measurement of volumetric water content (VWC) and suction. Leaching tests were performed over many cycles and physical and chemical analyses were carried out on the leachates. The observed hydrogeological behavior shows that either a CCBE or a monolayer cover with clayey silt materials can be used for the Ity mine site reclamation. However, hydrochemical results allow confirming that the monolayer cover made with clayey silt material can be considered as the best scenario for the Ity mine site reclamation.

A Novel Sensor for In Situ Detection of Freeze‐Thaw Characteristics in Plants from Stem Temperature and Water Content Measurements

Freezing is a typical abiotic stress on plants, which can induce physiological damages of plants. A better understanding of plant freeze‐thaw characteristics contributes to solving some hot issues in plant physiology, such as cold resistance and cold acclimation. This article presents a novel sensor for in situ detection of freeze‐thaw characteristics in plants based on stem temperature and water content. The measuring circuit of stem temperature was designed based on constant current source and platinum resistance. The measuring circuit of stem water content was designed based on standing wave ratio and the dielectric properties of stem tissue. The temperature resolution of the compound sensor is less than 0.1°C. The MAE and RMSE of temperature measurement are approximately 0.57°C and 0.65°C, respectively. The volumetric water content resolution of the compound sensor is less than 0.05%. The MAE and RMSE of volumetric water content measurement are approximately 1.59% and 1.81%, respectively. Moreover, a mathematical model for describing the freeze‐thaw characteristics of plant stem was established and solved based on the compound sensor. Then, some freeze‐thaw indicators including stem water content, ice content, freezing depth, freezing velocity, thawing depth, and thawing velocity were solved and used to interpret the freeze‐thaw rules of plant stem. It can be concluded that the freeze‐thaw velocity is closely related to the physicochemical properties of plant stem which also change dynamically in the freeze‐thaw cycle.

Development of a Pilot Smart Irrigation System for Peruvian Highlands

AbstractWith growing developments in the technology of cloud storage and the Internet of Things, smart systems have become the latest trend in major agricultural regions of the world. The Arequipa and Caylloma provinces of Peru are highly productive agricultural areas that could benefit from these technologies. This region has low precipitation, generally less than 100 mm per year. Electricity is not available in most of the agricultural fields, limiting the types of irrigation methods and technologies that can be supported. Currently, 20 ponds supplied by water runoff from the Andean glaciers are used for irrigating approximately 545 hectares of land in the Majes district (Caylloma province). In order to develop optimal techniques for water irrigation in Arequipa and improve the infrastructure, there is a need for development of a smart water irrigation system applicable to the existing conditions in the region. The current study proposes a pilot smart water irrigation framework comprised of a drip irrigation module, wireless communication module, and a sensor network for intelligently regulating water flow from the cloud. In this study, a TEROS 12 soil moisture sensor is connected to a Digi XBee wireless module for collecting measurements of volumetric water content, temperature, and electrical conductivity, which are sent through a secure IP gateway to the cloud. A user‐friendly web interface is available for end‐users to access and analyze real‐time data. The proposed framework is easily implementable, low‐cost, and is predicted to conserve water through optimization of irrigation cycles based on a set moisture threshold.

Real time measurements of volumetric water content in seafloor sediments by the Time Domain Reflectometry （TDR）with gravity coring in the surface methane hydrate survey

Real time measurements of volumetric water content in seafloor sediments by the Time Domain Reflectometry （TDR）with gravity coring in the surface methane hydrate survey

Simulation of vadose zone flow processes via inverse modeling of modified multistep outflow for fine‐grained soils

AbstractCharacterization and measurement of the hydraulic properties of unsaturated porous media is still a challenge in natural environments, although exact knowledge of the soil's hydraulic properties [unsaturated soil hydraulic conductivity (K), volumetric water content (θ), soil matric head (h)] is crucial for solving many soil, hydrological, and environmental issues. The main purpose of this study was to establish a modified multistep outflow technique that facilitates laboratory operations and reduces time and costs. We also focused on inverse parameter optimization of the Mualem–van Genuchten (MVG) model with laboratory‐provided data for three fine‐grained soils. This modified version was assessed by inverse modeling of the MVG model and the generalized reduced gradient algorithm. The results represent the best estimate of the adjusted parameters and the very low uncertainty associated with the fitted values. The soil hydraulic functions were simulated well up to pressures of 1,500 kPa and their uncertainties were extremely low. The R2 values were 88 to 97 and 81 to 96% for the soil water characteristics and the unsaturated hydraulic conductivity functions, respectively. The simultaneous optimization of the nonlinear hydraulic functions from a single transient flow experiment agreed well with their observed data for each soil examined. Besides, by eliminating the use of tensiometers and having no independent measurements of saturated soil hydraulic conductivity and saturated volumetric water content, the modified version was faster and easier than the conventional method. The proposed protocol appears to be suited for the vadose zone flow simulation of fine‐grained soils.

Impacts of warming winters on recharge in a seasonally frozen bedrock aquifer

Under conditions of a changing climate, winters are predicted to be warmer and wetter in the northern hemisphere. Yet, the impacts of increasing midwinter snowmelt and rain-on-snow (ROS) events on groundwater recharge are poorly understood, particularly in seasonally frozen shallow bedrock. To characterize the mechanisms and antecedent conditions inhibiting or enabling winter recharge in seasonally frozen bedrock, a field investigation was conducted in eastern Ontario, Canada over the winter of 2019–2020. Since rock outcrops are known areas of recharge in nonwinter months, a low-lying granitic outcrop and adjacent soils were heavily instrumented. Both hydrogeologic and cryospheric conditions of the surface, unsaturated and saturated zones were monitored at a high temporal resolution (10–15-minute intervals). Climate data collected during the study indicated that winter conditions were warmer and wetter than long-term (30 year) averages, which allows for parallels to be drawn between observations from this winter and what could be expected for future winter conditions under climate change. Hydraulic head and temperature measurements in two bedrock wells indicated rapid midwinter recharge occurred in response to most ROS and snowmelt events despite the presence of basal ice in the snowpack and shallow frozen ground. Volumetric water content and temperature measurements in two soil profiles revealed that where the soil-bedrock interface was unfrozen, this became the primary pathway permitting infiltration to bypass the frozen surface layer. A major midwinter ROS event generated ponding on surface which subsequently froze, ultimately reducing effective recharge from the event and inhibiting future snowmelt recharge. Estimates of net recharge per ROS and/or snowmelt event indicated that moderate events were more effective at recharging the bedrock aquifer than higher intensity events and events in winter were more effective than in fall. Implications of these findings suggest that rock outcrops provide a window for rapid localized recharge during midwinter warming or ROS, where basal ice or frozen soil would otherwise inhibit vertical infiltration. Study results provide field-based evidence for both enhanced and impeded winter recharge in seasonally frozen bedrock aquifers due to warming winters under climate change.

Flume-scale laboratory study of rainfall-runoff responses in Devon silt and capillary barrier profiles

The design of soil covers for mine waste reclamation requires comprehensive water balance calculations at the ground surface and the subsurface. To achieve a determinate water balance equation, rainfall-runoff quantities need to be predicted or measured. The parameters that control rainfall runoff were investigated by performing rainfall-runoff experiments on Devon silt and capillary barrier profiles at different saturation stages. A rainfall simulator apparatus was employed to measure water balance components at different rainfall intensities during each test; the apparatus’ design is presented. The rainfall intensity and the saturated hydraulic conductivity of the soil govern runoff volumes and rates in the saturated profiles. The rainfall intensity and infiltration capacity of the soil govern runoff volumes and rates in the unsaturated profiles. The capillary barrier effects are evaluated and discussed by comparing time-lapse photographs of the wetting front during each test with changes in matric suction and volumetric water content measurements.

Determining maize water stress through a remote sensing-based surface energy balance approach

Determining water stress levels of vegetated surfaces is crucial for irrigation scheduling. This paper aims to evaluate a new method for obtaining crop water stress index (CWSI) based on the estimation of sensible heat flux using an aerodynamic temperature gradient approach. Data were collected on a deficit irrigated maize field at a research farm located in Greeley, Colorado, USA, in 2017 and 2018. The irrigation treatment used subsurface drip. Weather data were measured on-site at 3.3 m above ground level. RED and NIR surface reflectance data were obtained on-site through multispectral radiometer measurements. Nadir surface temperature data were measured using infra-red thermometers at 1 m above canopy. CWSI estimated values were used to assess daily soil water stress index (SWSI), calculated from measurements of volumetric soil water content (VWC) and management allowed depletion (MAD) of 40%. Results show that SWSI is best represented through a non-linear rational CWSI function. Modeled CWSI estimates were compared to measured surface heat fluxes, resulting in a mean bias error of − 0.02 and a root mean square error of 0.09, while errors were 0.02 and 0.06 when compared with observed CWSI based on canopy transpiration measured with plant sap flow devices. Results seem to validate the proposed sensible heat flux-based CWSI model. The CWSI approach presented could be used to manage irrigation and conserve water resources for maize in semi-arid regions.

Slope Stability of a Scree Slope Based on Integrated Characterisation and Monitoring

Three years of geotechnical seasonal field monitoring including soil temperature, suction and volumetric water content plus geophysical measurements, lead to a preliminary ground model and assessment of slope stability for a steep scree slope in the Meretschibach catchment, near Agarn village in the Swiss Alps. Building on data reported in a previous paper, which focused on preliminary ground characterisation and seasonal field monitoring, this current research aims to understand whether a surficial failure in the scree slope, triggered by rainfall and depending on bedrock conditions, would represent a relevant natural hazard for Agarn village. A final year of field data is included as well as site-specific sensor calibration, a Ground Penetrating Radar (GPR) profile, and laboratory triaxial testing to provide strength parameters. A bedrock map is presented, based on GPR, with a realistic ground model of the entire scree slope. Furthermore, a preliminary numerical analysis, performed using SEEP-SLOPE/W, shows the influence of a bedrock outcrop observed in the field, for a specific soil thickness, strength parameters and rain intensity. The stability of a gravelly slope decreases with groundwater flow over a step in the bedrock, and the location of the failure will tend to move uphill of a bedrock outcrop at a shallow depth as groundwater flow increases.

Field monitoring of soil-moisture to understand the hydrological response of a road-cut slope

Rainfall and slope-cutting for road construction are two key landslide causative factors in Nepal, but how they interact to cause failures is poorly understood. To improve understanding of the effects of cut slopes during rainfall, geotechnical investigations and field monitoring were conducted in a mountainous district, Sindhupalchowk, located in central Nepal. This paper presents the results of the field-investigations and the measurements of volumetric water content obtained from the sensors installed in the study-site. Field-based evidence suggests that the slope that was cut for road construction during the dry period remained stable due to the presence of soil suction, which imparted additional strength to the soil. At the start of the monsoon, infiltration of rainwater caused saturation of the soil at shallow depth, consequently causing loss of suction and reduction of the soil strength. The presence of the road-cut in the hillslope resulting in steeper slopes then promoted the failure. These observations suggest that the presence of road-cuts in the hillslopes can cause landslides even during non-exceptional rainfall events.

Evaluating time domain reflectometry and coaxial impedance sensors for soil observations by the U.S. Climate Reference Network

AbstractThe objective of this study was to evaluate the performance of two commercial‐grade electromagnetic sensors in measuring soil water content for the U.S. Climate Reference Network (USCRN). One sensor was a 50‐MHz coaxial impedance dielectric sensor (model HydraProbe, Stevens Water Monitoring Systems). The second sensor was a 1‐GHz time domain reflectometry (TDR) sensor (model TDR‐315L, Acclima). There was no substantial difference between both probes in daily mean volumetric soil water content and soil temperature measurements at a 0.1‐m depth in a customized uniform loamy soil testbed in Oak Ridge, TN, from 2016 to 2018, and at select USCRN stations with a variety of soils. However, the TDR‐315L provided more representative soil water content measurements for stations with high‐clay‐content soils than the HydraProbe. These results confirm the necessity of using site‐specific soil properties for soil sensors like the HydraProbe for estimating soil water content in different soil environments. They also suggest that TDR sensors may provide an opportunity to improve USCRN soil moisture measurements in higher‐clay‐content soils.

Measurement of Volumetric Water Content by Gravimetric and Time Domain Reflectometry Methods at Field Experiment with Biochar and N Fertilizer

Abstract There are many methods used for soil water content measurement which we can divide into direct gravimetric methods from using soil samples or indirect methods that are based on the measurement of another soil property which is dependent on soil moisture. The paper presents the findings of volumetric water content measurements with gravimetric and time domain reflectometry (TDR) methods. We focused on four variants in the field experiment in Dolná Malanta (Slovakia): control variant (B0+N0), variant with biochar at dose 20 t.ha−1 without N fertilizer (B20+N0), variant with biochar 20 t.ha−1 and N fertilizer 160 kg.ha−1 (B20+N160) and variant with biochar 20 t.ha−1 and N fertilizer 240 kg.ha−1 (B20+N240). TDR is nowadays a well-established dielectric technique to measure volumetric water content; however, its accuracy is influenced by high concentration of salts in soil. In this paper, we evaluated the effect of added N fertilizer on the measuring accuracy of HydroSense II device that is operating under the TDR principle.

Comparison of a stand-alone surface renewal method to weighing lysimetry and eddy covariance for determining vineyard evapotranspiration and vine water stress

Surface renewal (SR) is a biometeorological technique that uses high-frequency air temperature measurements above a plant canopy to estimate sensible heat flux. The sensible heat flux is then used to estimate latent heat flux as the residual of a surface energy balance equation. SR previously relied on calibration against other methods (e.g., eddy covariance) to obtain accurate measurements of sensible heat flux, and this need for calibration limited the use of SR to research applications. Our group recently showed that compensating for the frequency response characteristics of SR thermocouples causes the calibration factor to converge near the theoretically predicted value of 0.5 (Shapland et al., Agric For Meteorol 189:36–47, 2014). This led to the development of an inexpensive, stand-alone SR system to measure sensible heat flux without the need for calibration, and here we evaluated the SR system in a mature vineyard containing a weighing lysimeter. Vineyard evapotranspiration (ET) measured with SR was strongly and positively correlated with that from the lysimeter, eddy covariance, and a soil water budget approach. ET measured with the various techniques responded similarly to changes in the microclimatic conditions (i.e., day to day variability) and when water was withheld from the entire vineyard for an extended period. A stress index, calculated using reference and actual ET from SR and lysimetry, was correlated to leaf water potential, stomatal conductance, and volumetric soil water content measurements, but some of these relationships were more variable than others. Our results suggest that the new SR method could potentially be used as a low-cost tool to provide growers with field-specific estimates of crop water use and stress for irrigation management in vineyards.

Study on influence and compensation for soil compactness on volumetric water content measurement

Volumetric water content measurements based on soil dielectric properties are affected by soil compaction. Today’s commercial products produce substantial measurement error as they do not account for the influence of soil compaction. This paper presents a portable soil volumetric moisture content sensor based on the principle of standing wave rate and simultaneous soil compactness measurement. The coupling relationship between soil compactness and soil volumetric moisture content can be easily observed by converting them into polar coordinates. A modified soil volumetric moisture content model based on the soil compactness is also established to support the operation of the proposed sensor.

Unraveling location-specific and time-dependent interactions between soil water content and environmental factors in cropped sandy soils using Sentinel-1 and moisture probes

Advances in sensor technologies provide the potential for real-time monitoring of soil moisture status. Little work has been conducted to study the spatial and temporal variations of soil water content at the field scale and the interactions between soil water content and environmental controlling factors in cropped fields during the growing season. In this study, we assessed the spatial and temporal variations of surface soil water content (0–0.1 m) using seven in-situ soil moisture probes and Sentinel-1 satellite data over the 2016 and 2017 cropping seasons in irrigated fields of the Central Sands Plains, Wisconsin, USA. An empirical multiple linear regression model was developed between the soil water content with Sentinel-1 backscatter data. The model was evaluated with leave-one-group-out cross-validation, which showed an R2 of 0.60, mean error of –0.00 m3 m−3, root mean square error of 0.02 m3 m−3, and Lin’s concordance correlation coefficient of 0.75. Soil volumetric water content (VWC) and soil water deficit (SWD) were mapped at 30-m resolution every 6–12 days across one field (DU field at the Wallendal farm). Using two statistical metrics (“first metric” and “last metric”) and a bootstrapping approach, the location-specific and time-dependent interactions between estimated soil water content and environmental controlling factors across the DU field during the cropping season were evaluated, and it was found that such interactions were affected by initial soil water content and plant stress status. The robustness of the model needs to be improved in the future by collecting more VWC measurements in different soil and crop conditions. The method has the potential to provide a baseline soil moisture map and can be coupled with mechanistic models and subsoil VWC measurements for improved irrigation management and water use efficiency under a changing climate.

Performance Evaluation of 5TM Sensor for Real-Time Monitoring of Volumetric Water Content in Landfill Cover System

AbstractAccurate measurement of volumetric water content (θ) is essential for studying soil-water interaction efficiently. The accuracy of various types of θ measuring sensors needs to be ascertained before deploying in specific projects. The objective of this study is to evaluate the performance of electromagnetic 5TM sensor for real-time monitoring of θ in a cover system (CS). This CS is provided on top of the hazardous waste containment to isolate it from the surrounding environment and minimize interaction with rain water. The CS consists of different layers of soil and geosynthetics that interact with the atmosphere and undergo changes in θ with changing local weather conditions. Because θ governs the hydraulic and mechanical characteristics of geomaterials, its variation is monitored as a function of space and time for evaluating the performance of CS. Performance assessment of 5TM sensor was carried out under controlled conditions for ten different soil materials that may find application in CS. This study indicates the importance of material specific calibration of a 5TM sensor for improving its measurement accuracy before deploying it for field monitoring. The accuracy of 5TM measurement was marginally better when the polynomial calibration equation was adopted as compared to linear calibration. In the absence of soil-specific calibration, the new set of calibration parameters proposed in this study can be used for the generic soils used in CS. With improved accuracy of 5TM, it was shown that the error in the determination of soil water storage for different layers of a trial CS reduced from 15 to 0.4 %.

Verification of soil salinity index model based on 0.02–3 GHz complex dielectric permittivity spectrum measurements

Determination of electrical conductivity of soil pore-water from measurements of soil bulk electrical conductivity and volumetric water content or dielectric permittivity, requires theoretical or empirical models connecting these quantities. One such model is the salinity index model, which in the original formulation requires the measurement of soil bulk electrical conductivity and apparent dielectric permittivity determined with the time-domain-reflectometry (TDR) technique. Currently, many popular soil moisture sensors are operated at a single frequency, usually in the low-frequency part of the typical TDR bandwidth. Yet, no systematic verification of the salinity index model applied with the use of dielectric permittivity measured at specific frequencies in a broad frequency range has been performed. The aim of this paper is to evaluate the salinity index model with the use of dielectric spectra of 277 samples of 14 soils obtained in a 20 MHz–3 GHz frequency range with the use of a coaxial transmission-line cell vector-network-analyzer system. A three-pole Debye model was fitted to the spectra in order to determine bulk electrical conductivity and the real part of dielectric permittivity at chosen frequencies in order to diminish the influence of measurement errors. Then, the salinity index model was applied at each examined frequency and its soil-specific parameters were calculated. The performance of the model was evaluated with the use of the leave-one-out cross-validation scheme, and the frequencies at which the model performed best were found within 0.5–2 GHz range. Finally, the salinity index model parameters obtained at the optimal frequencies were correlated with other soil properties.