TIME DOMAIN REFLECTOMETRY MEASUREMENTS OF WATER-CONTENT AND ELECTRICAL-CONDUCTIVITY OF LAYERED SOIL COLUMNS

The major argument for measuring the soil salt concentration and water content at the same position is their high correlation and spatial variability. Time domain reflectometry (TDR) was used for simultaneous measurement of soil water content, theta (derived from the soil dielectric constant epsilon), and bulk soil electrical conductivity, sigma-a (from the attenuation of a transmitted pulse), for uniform and layered soil profiles in the laboratory. The purpose of the study was to test the different concepts of travel time and attenuation by using independent estimates of theta and sigma-a. Both parallel (two-rod) and simulated coaxial (three-rod) probes resulted in essentially the same epsilon. The three-rod probe measurements were easier to interpret and did not necessitate an impedance-matching transformer. The TDR measurements of theta for layered profiles were not always accurate, especially when wet soil was overlying dry soil, due to erroneous interpretation of the TDR trace. In this case, the pulse travel time should be measured at the inflection point, which is sometimes difficult to identify, and not at the minimum of the TDR trace. This study reports a new method of calculating sigma-a, which is based on a direct measurement of the transmission-line load by TDR. This new method, which is simpler than the previous ones and independent of multiple reflections, was found to correlate better than the previously published methods with another, independent method for sigma-a measurement (four-electrode technique).