Estimating water content from electrical conductivity measurements with short time-domain reflectometry probes

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Abstract

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

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  • Water Engineering
Original languageEnglish
Pages (from-to)478-482
JournalSoil Science Society of America Journal
Volume67
Issue number2
Publication statusPublished - 2003
Publication categoryResearch
Peer-reviewedYes