Calibration of a TDR instrument for simultaneous measurements of soil water and soil electrical conductivity

Time domain reflectometry (TDR) can be used to simultaneously measure in-situ water content and electrical conductivity (EC(TDR)). The method used to determine soil electrical conductivity from TDR waveforms requires measurement of the reflection voltages from a special part of the waveform. However, not all TDR instruments have this ability. The purpose of our research was to extend the ability of the MP917 Moisture Point instrument (ESI Environmental Sensors, Inc.) such that bulk soil electrical conductivity could be obtained. Our method involved use of resistors of known resistance to establish a relationship between arbitrary voltage and the reflection coefficient. As we did not have a standard of comparison (another TDR system that could measure both soil moisture content and EC), we used two probes of different lengths (150 and 220 mm). If our method worked, then the two probes should calculate the same EC values. Using our method, we achieved the following: determination of geometric constants for solution EC values between 0.7 and 25.7 dS m(-1), ability to determine reasonably accurate KCl solution EC values for each of the two probes, and ability to determine sand EC(TDR) that had a 1:1 agreement between the 150 and 220 mm probes for moisture contents less than 0.17 m3 m(-3) for non-saline and saline conditions, but not for saline conditions near saturation (0.28 to 0.35 m3 m(-3)). For such conditions, the long probe produced EC(TDR) values that were 1.4 times that of the short probe. Although the EC(TDR) values obtained from the MP917 could not be confirmed with an independent measure, there appears to be sufficient response to a range of saline conditions that the relative values of EC(TDR) provide suitable accuracy such that the usefulness of the MP917 can be extended to relative measurements of EC.