Soil water retention measurements using a combined tensiometer-coiled time domain reflectometry probe

The objective of the presented study was to develop a single probe that can be used to determine soil water retention curves in both laboratory and field conditions, by including a coiled time domain reflectometry (TDR) probe around the porous cup of a standard tensiometer. The combined tensiometer-coiled TDR probe was constructed by wrapping two copper wires (0.8 mm diam. and 35.5 cm long) along a 5-cm long porous cup of a standard tensiometer. The dielectric constant of five different soils (Oso Flaco [coarse-loamy, mixed Typic Cryorthod-fine-loamy, mixed, mesic Ustollic Haplargid], Ottawa sand [F-50-silica sand], Columbia [Coarse-loamy, mixed, superactive, nonacid, thermic Oxyaquic Xerofluvents], Lincoln sandy loam (sandy, mixed, thermic Typic Ustifluvents), and a washed sand - SRI30) was measured with the combined tensiometer-coiled TDR probe (coil) as a function of the soil water content (theta) and soil water matric potential (h). The measured dielectric constant (epsilon(coil)) as a function of water content was empirically fitted with a third-order polynomial equation, allowing estimation of theta(h)-curves from the combined tensiometer-coiled TDR probe measurements, with R2 values larger than 0.98. In addition, the mixing model approach, adapted for the tensiometer-coiled TDR probe, was successful in explaining the functional form of the coiled TDR data with about 30% of the coiled-TDR probe measurement explained by the bulk soil dielectric constant. This new TDR development provides in situ soil water retention data from simultaneous soil water matric potential and water content measurements within approximately the same small soil volume around the combined probe, but requires soil specific calibration because of slight desaturation of the porous cup of the tensiometer.