Time domain reflectometry (TDR) is used widely for measuring soil-water content. New TDR coil probe technology facilitates the development of small, nondestructive probes for simultaneous measurement of soil-water content (θ) and soil-water potential (ψ). In this study we developed mini tensiometer-time domain reflectometry (T-TDR) coil probes, 6-mm wide and 32-mm long. The coil probes were calibrated against a conventional three-rod probe and were used for measuring θ for a aggregated volcanic ash soil (VAS) and a uniform sand. A commonly-used dielectric mixing model did not accurately describe the measured relation between the dielectric constant of the T-TDR coil probe (εcoil) and θ, and a new calibration model for εcoil (θ) was proposed instead. The new model assumes single-region behavior for sand and two-region behavior for aggregated VAS, when plotting the normalized dielectric constant of the coil probe (εcoil–ε dry; where εdry is the dielectric constant of the T-TDR coil probe for air-dried material) as a function of θ. The new calibration model accurately described the (εcoil–ε dry)-θ relations measured by 7 T-TDR coil probes on both sand and VAS. Additionally, there was a good agreement between measured soil-water retention curves (ψ > –100 cm H2O) by the new T-TDR coil probes and independent measurements by the hanging water column method.
Soil Science Society of America. Journal, 2013, Vol 77, Issue 5, p. 1517-1528