Abstract
Non-destructive monitoring of soil water content (W) and the electrical conductivity of the soil solution (ECw) has been desired for environmental evaluation and sustainable agriculture. Dielectric probes and four-electrode probes are widely used for the non-destructive determination of W and the soil bulk electrical conductivity (ECb), respectively. Since the output of dielectric probes is affected by soil salinity, the calibration for the effect is indispensable for accurate determination of W. Meanwhile, four-electrode probes require the W value for determination of ECw from ECb. We present an empirical calibration method for the salinity dependence of commercial capacitance moisture probes. A four-electrode probe was also calibrated to investigate the possibility of simultaneous monitoring of W and ECw by combining each calibration equation for capacitance and four-electrode probes. A laboratory experiment was conducted using a sandy soil to obtain probe outputs at various W (air-dry-near-saturation) and ECw (0-31.9 dS m-1). The output of the capacitance probe exhibited strong, nonlinear dependence on ECw. The root mean square error (RMSE) between actual W and calculated W using the linear functions provided by the manufacturer was at a maximum of 0.162 m3 m-3. A calibration equation, describing the probe output as a function of W and ECw, was developed using curve fitting approach. The RMSE between the actual and calibrated W by this equation was at a maximum of 0.011 m3 m-3. The output of the four-electrode probe (ECb) was also expressed as a function of W and ECw. The calibration equations for each probe were combined and solved for W and ECw. Although both W and ECw were determined with acceptable accuracy, the combined calibration equation had multiple solutions for W. Development of the method to select optimal solutions will be needed for the practical application of this probe combination.
Highlights
Soil water content (θ, m3 m−3) is one of the most important hydrologic variables that affects surface runoff, infiltration, evaporation and transpiration
An empirical calibration method for the salinity dependence of the ECHO10/20 probe was presented in this study
A calibration equation of a four-electrode probe helpful for the selection of optimal θc because σwc was developed to investigate the possibility of obtained by substituting inappropriate θc in Eq 1 can simultaneous monitoring of θ and σw by combining take an inappropriate value (Fig. 10)
Summary
Soil water content (θ, m3 m−3) is one of the most important hydrologic variables that affects surface runoff, infiltration, evaporation and transpiration. Every type of dielectric moisture sensor outputs an electrical signal depending on the apparent permittivity of the soil. The value of θ is empirically determined from relationships between the soil water content and electrical signals, or theoretically determined by dielectric mixing models. These dielectric moisture sensors enable nondestructive and real-time monitoring of θ. The outputs of sensors are usually affected by soil type, salinity and temperature[3,29,30]. Calibration for these effects is essential for accurate determination of θ. We focus on the dependence of the output of dielectric probes on soil salinity
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