TDR法

As the fundamental study for the measurement of electrical conductivity, EC, or solute concentration of soil water using TDR method, the calibration experiment of the relationship between EC of soil water, ECw, volumetric water content, θ, and bulk soil EC, σ, was conducted on glass beads. The characteristics of the four functions for calculating σ were evaluated and the accuracy of the eye analysis of TDR waveform was examined through the calibration experiment. Furthermore the measurement of EC. using TDR method was done on sand to check the precision of the measuring procedure. The results of the calibration experiment on glass beads showed that the apparent dielectric constant, εa, of samples, to which NaCl solution applied, were smaller than that of samples, to which distilled water applied, on the same condition of volumetric water content. The underestimation of εa on the solution-applied samples might occur because of the unbecoming analysis technique, and the cause of this has not been solved Yet. While the comparisons of the σ using εa of solution-applied sample and that using εa of distilled water applied sample showed that the influence of the differences of εa on estimating σ was negligible. The bulk soil EC was calculated by using four functions in various conditions of θ and ECw. The values of σ evaluated by these models were compared each other, and the models were,examined whether they were suitable for this measuring system. Then the Topp model was selected for the measuring system in cpnsideration of the facts such that the TDR instrument would be operated manually. For the measurement of ECw on sand, the calibration experiment of the relation between ECw, θ and σ was conducted in advance. ln the calibration experiment NaCI solution were prepared in four levels of concentration, and the volumetric water content was set two or three levels for each solution level. The data of ECw, θ and σ were obtained from nine samples and used to determine the calibration function. The sample was prepared in the condition of known values of ECw and θ. Then the measurement of σ was done for the sample, and the measured σ was used to estimate ECw by using the calibration function. The fact that the estimated ECw was close to the actual value indicates that electrical conductivity of soil water would be measured accurately by this measuring procedure.