Calibration of Capacitance Probe Sensors in a Saline Silty Clay Soil

Abstract

Capacitance probe sensors are a popular electromagnetic method of measuring soil water content. However, there is concern about the influence of soil salinity on the sensor readings. In this study capacitance sensors are calibrated for a saline silty clay soil. An electric circuit model is used to relate the sensor's resonant frequency F to the permittivity (ε) of the soil. The circuit model is able to account for the effect of dielectric losses on the resonant frequency. Dielectric mixing models and empirical models are used to relate the permittivity to the soil water content (θ). The results show that the electric circuit model does not fit the F–ε(θ) data if the calibrated bulk electrical conductivity (EC) model is used. The dielectric losses are overestimated. Increasing the exponent c in the tortuosity factor of the bulk EC model and thereby lowering the bulk EC and the dielectric losses improves the performance of the model. Measured and calculated volumetric water contents compare reasonably well (R2 = 0.884). However, only 73 out of 88 data points can be described. The rejected points are invariably at high water contents where the high dielectric losses result in the sensor frequency being insensitive to ε(θ).

Accuracy of Two Electromagnetic Soil Water Content Sensors in Saline Soils

“…3 and 4), where sensors overestimated the VWC even beyond the degree suggested by the intercept. Consistent with these findings are those of Kelleners et al (2004), who reported that the effect of soil salinity on the readings of capacitance probe sensors became apparent at higher VWC. In their study, when capacitance probe sensors were calibrated in a saline silty clay soil, the effect of dielectric losses became apparent at a VWC of 0.31 m 3 m -3 or higher.…”

“…However, in both loamy sand and loam soils, the degree of linearity was greater at low salinity levels than at higher ones, where a quadratic term produced a better fit (not shown) than a linear relationship. As reported by Kelleners et al (2004), at salinity levels from 3 to 12 dS m -1 the fitted calibration equations for a capacitance sensor were linear; however, at 18 dS m -1 the equation required a higher order polynomial regression. Our findings also corroborate those of Inoue et al (2008), who found that the fitted equations obtained for two FDR sensors in a saline sandy soil were linear at low (<4 dS m -1 ) and medium salinity levels (4-10 dS m -1 ), but nonlinear at high salinity levels (>10 dS m -1 ).…”

“…In their study, when capacitance probe sensors were calibrated in a saline silty clay soil, the effect of dielectric losses became apparent at a VWC of 0.31 m 3 m -3 or higher. Kelleners et al (2004) concluded that using capacitance probe sensors in wet saline soils might result in the loss of data because frequencies cannot always be converted into reliable VWC values in wet soils. Thompson et al (2007) also observed that the capacitance sensor readings of VWC were 30% higher in soils (ranging from 0.23 to 0.28 m 3 m -3 ) to which a saline nutrient solution (EC of 6.5 dS m -1 ) had been added.…”

“…The EM sensors are generally classified as either time domain reflectometry (TDR; Topp et al, 1980;Topp and Ferré, 2002) and TDT (Topp et al, 2001;Harlow et al, 2003;Hook et al, 2004) or FDR (or capacitance) sensors (Dean et al, 1987;Paltineanu and Starr, 1997;Kelleners et al, 2004). Time domain transmissometry and particularly its precursor TDR, which are generally known to be of higher accuracy, measure the time required for an EM pulse to propagate along a transmission line that is surrounded by the soil.…”

“…As reported by Muñoz-Carpena et al (2005), the operating frequency of FDR sensors below 100 MHz changed the bulk permittivity of soil minerals, and consequently, soil water content measurements using FDR sensors were affected by temperature, salinity, bulk density, and clay content. Kelleners et al (2004Kelleners et al ( , 2005 suggested that empirical calibration of capacitance probe and TDR sensors remains the only option to overcome the effects of salinity when measuring soil water content in saline soils. Recently, CardenasLailhacar and Dukes (2014) reported that the TDT sensor was not sensitive to changes in salinity, particularly with no reading variations in estimating soil water content when exposed to a combination of three temperatures (10, 25, or 35°C) and three levels of EC of the saturated paste extract (0, 0.7, or 5 dS m -1 ).…”