Estimating Water Content from Electrical Conductivity Measurements with Short Time‐Domain Reflectometry Probes

Abstract

Time domain reflectometry (TDR) is a widely used technique for measuring the dielectric constant (Ka) and bulk electrical conductivity (σa) of soil. The Ka measurement can be converted to water content (θ) by means of a (soil specific) calibration. Since the accuracy of the Ka measurement is dependent on the TDR probe length, probes longer than about 0.1 m are preferred. However, shorter probes are desired for many applications. The possible use of the σa measurement of short TDR probes for estimating θ under conditions with constant soil solution electrical conductivity (σw) is investigated and the accuracy of the Ka and σa measurements of two reference TDR probes (0.20 m long) and four miniature probes (0.02 m long) is determined. The standard deviation of the Ka measured by the miniature probes was found to be ten times higher compared to the reference probes. The standard deviation of the σa measured by the miniature probes was only slightly larger compared with the reference probes. A calibration experiment in sand using the reference TDR probes showed that when σw is constant, the θ estimations from Ka and σa measurements have the same accuracy. TDR measurements were taken with the miniature probes in small sand samples. From the Ka–θ and σa–θ relationships determined in the calibration experiment, the Ka and σa measurements of the miniature probes could be converted to θ. The root mean square error (RMSE) of the θ estimated by the Ka measurements was 10 to 20 times higher compared with the reference probe measurement. The RMSEs of the θ estimated by the σa measurements was only two to three times higher compared with the reference probes. The results presented in this study clearly show that the σa measurement made with short TDR probes can give accurate θ estimations under conditions of constant σw