Calibration of Time Domain Reflectometry (TDR) Using Suction Cups for Measuring Electrical Conductivity in Loamy and Sandy Loam Soils

Time domain reflectometry (TDR) is increasingly employed in soil science for measuring soil water content as well as soil bulk electrical conductivity (ECa). However, TDR calibration especially for solute measurements at large field scales is not an easy task. A series of calibrations might be required for TDR probes used in solute transport measurements to ristly relate the impedance (Z) to ECa and then the latter to electrical conductivity of soil solution (ECw). In this paper, these TDR calibration steps are discussed for measuring solute concentrations in large undisturbed soil monoliths. Six undisturbed soil experimental monoliths, each having roughly 0.5 m3, were collected from two different fields, being loamy and sandy loam in texture. The monoliths were subsequently equipped with such measuring devices as TDR probes for measuring both water content and as well bulk electrical conductivity, temperature sensors intended for soil temperature measurements, suction cups (corresponding in depths and position to the TDR probes) for extracting soil water solution from different depths, as well as tipping buckets equipped with EC metres at the outlet. One tracer experiment was conducted on each monolith. Automated TDR measurements were made during the experiments whereas soil solutions (ECw) were manually collected fallowing the pulse application. The TDR measurements were subsequently transformed into ECa using obtained probe properties in known solutions. The ECa–ECw relationships were characterized by hysteresis. This behaviour was clearly observed in both of the soils studied as well as for nearly all probes. To calibrate the relationship between ECa-ECw, a linear model was fitted to the selected data close to equilibrium conditions. The accuracy of the calibration was verified at a few points on the breakthrough curves (BTCs) of known ECw.