Measurement of soil bulk electrical conductivity in saline soils and solutions using TDR probes partially coated with high-dielectric material

The definitive version is available at: http://vzj.scijournals.org/Time Domain Reflectometry (TDR) is a valuable technique that allows simultaneous estimation of the apparent permittivity (a) and the bulk electrical conductivity (a). However, in highly conductive media the signal is completely attenuated which precludes permittivity measurements. This paper shows that a can be estimated in conductive media by applying the long-time TDR waveform analysis to a TDR probe partially insulated with a high-dielectric coating. Four 10-cm-long three-rod TDR probes with identical geometry but different percentages of rod-coating were tested: an uncoated sensor (P0) and probes with 50% (P50), 75% (P75) and 95% (P95) of the rod-length, respectively, coated with an insulator with a relative permittivity (r) equal to 32.3. A good relationship (r2 = 0.99) was found between the a estimated, if possible, with P0 immersed in several conductive and nonconductive media and the corresponding values estimated with P50, P75 and P95. The cell constant value (m-1) for P0 (5.10), P50 (9.60), P75 (16.92) and P95 (51.31), which were experimentally determined in different NaCl-water solutions, were on average 10% greater than the corresponding values calculated using a numerical model. The results show that, for a values ranging between 0.4 and 0.75 S m-1, simultaneous measurements of a and a were only possible using the partially coated probes. A good correlation was found between the a estimated with P0 inserted in different soil columns wetted with highly saline solutions (i.e. a > 0.2 S m-1) and those values estimated with P50, P75 and P95 (R2 = 0.96, RMSE = 1.08 and SD = 0.38). For a values lower than 0.2 S m-1 the accuracy of the partially coated TDR probes for estimations of a decreases as increases the percentage of the rod-coating, with errors up to 292% when a determined by P95 was compared to that estimated by P0.This research was supported by the Comisión Interministerial de Ciencia y Tecnología of Spain (grant AGL2007-66320-CO2-02/AGR) and the European Union (FEDER funds).Peer reviewe