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

Time domain reflectometry (TDR) is a widely used technique for measuring the dielectric constant ( K a ) and bulk electrical conductivity (σ a ) of soil. The K a measurement can be converted to water content (θ) by means of a (soil specific) calibration. Since the accuracy of the K a 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 σ a measurement of short TDR probes for estimating θ under conditions with constant soil solution electrical conductivity (σ w ) is investigated and the accuracy of the K a and σ a 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 K a measured by the miniature probes was found to be ten times higher compared to the reference probes. The standard deviation of the σ a 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 σ w is constant, the θ estimations from K a and σ a measurements have the same accuracy. TDR measurements were taken with the miniature probes in small sand samples. From the K a –θ and σ a –θ relationships determined in the calibration experiment, the K a and σ a measurements of the miniature probes could be converted to θ. The root mean square error (RMSE) of the θ estimated by the K a measurements was 10 to 20 times higher compared with the reference probe measurement. The RMSEs of the θ estimated by the σ a measurements was only two to three times higher compared with the reference probes. The results presented in this study clearly show that the σ a measurement made with short TDR probes can give accurate θ estimations under conditions of constant σ w