Simultaneous Measurement of Soil Water Content and Salinity Using a Frequency‐Response Method

Abstract

Laboratory tests were conducted to simultaneously measure soil water content and salinity using a four‐electrode Wenner array sensor. The sensor was modified to enhance the capacitive effect. Soil bulk density and the depth to which the electrodes penetrate into the soil were strictly controlled during the experiments. Sinusoidal current signals with a constant amplitude and frequencies ranging from 1 Hz to 15 MHz were sent to the outer electrodes of the Wenner array, whereas voltage outputs were measured from the inner electrodes. Frequency‐response data were analyzed using the partial least‐squares method to establish calibration models for simultaneously predicting water content and salinity from the frequency‐response patterns. The R2 values for predicting water content and salinity at the 30‐mm penetration depth reached 0.89 and 0.91, respectively, whereas the root‐mean‐square errors for the volumetric water content and salinity measurements were 0.019 m3 m−3 and 0.173 cmol kg−1, respectively. Test results showed that, in general, the calibration models predicted the water content more accurately than salinity. The depth to which the sensor penetrates into the soil has a strong effect on the measurement accuracy. This study has demonstrated that the modified Wenner array sensor and the frequency‐response method have a potential for simultaneously measuring soil capacitive and conductive properties. However, numerous difficulties, including contact resistance, depth control, and the effect of soil type, will need to be addressed to improve the measurement accuracy.