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DIVISION S-1—SOIL PHYSICS

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

Department of Water Resources Engineering, Lund University, Box 118, SE-221 00 Lund, Sweden

^* Corresponding author (magnus.persson{at}tvrl.lth.se)

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

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