Time Domain Transmissiometry for Measurement of Soil Moisture and Bulk Electrical Conductivity.

Time domain transmissiometry (TDT) is a viable and economical alternative to time domain reflectometry (TDR). Digital TDT sensors that communicate via the SDI-12 protocol and have the measurement electronics embedded directly within the sensor head are gaining increased popularity for monitoring soil moisture and bulk soil electrical conductivity. To evaluate the performance of commercially available TDT sensors, the travel time and amplitude of the broad band microwave frequency step-pulse was measured in sands with varying moisture contents and bulk ECs. While the sensors performed very well in moderately conductive soils, limitations for highly conductive soils as well as for soils with very low water contents and electrolyte concentrations have been experienced. An empirical relationship between the maximum slope of the TDT waveform and the bulk EC was developed to improve measurement capabilities in soils with low moisture contents and electrolyte concentrations.