The Effects of Salinity on the Accuracy and Uncertainty of Water Content Measurement

We used an automatic network analyzer (ANA) operated in both time and frequency domain modes to investigate the measurement accuracy of metallic time domain reflectometry (TDR) probes operated in sands saturated with NaCl solutions of varying electrical conductivity (EC). We chose to use time domain transmission (TDT) measurements for this investigation to separate the effect of the bulk soil-probe interaction from the effect of the large reflection typically found at the air-soil boundary for a TDR configuration. Pulse travel times and their variability increase with increasing pore-water EC. The source of travel time variability arises from the extreme variability in pulse shape, thereby introducing a high degree of uncertainty to curve fitting routines used to determine travel times. Pulse shape distortion is due primarily to attenuation of high frequency components through conductive loss rather than by dispersion. There is generally good correspondence between pulse rise time and the average water content measurement error over a 0- to 40-dS m^-1 EC range. For rise times <6 ns, measurement errors are <0.1 m³ m^-3. Given that rise times can be determined easily, we recommend that they be reported routinely as indicators of data quality. Because the variability of travel time measurements at a given EC is on the same order as the average difference between the travel time measured at that EC and that measured at zero salinity, pulse rise times cannot be used to correct for individual travel time measurements at high salinity. Similarly, knowledge of the EC of the medium may allow for identification of erroneous water content measurements, but does not allow for error correction.