1 Section of Biology and Environmental Science, The Faculty of Engineering and Science, Aalborg University, VBN2 Department of Chemistry and Bioscience, The Faculty of Engineering and Science, Aalborg University, VBN3 The Faculty of Engineering and Science (ENG), Aalborg University, VBN4 Purbanchal University5 Saitama University6 Peradeniya University7 Department of Civil Engineering, The Faculty of Engineering and Science, Aalborg University, VBN8 Institut for Agroøkologi - Jordfysik og Hydropedologi9 Saitama University
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