High-resolution time domain reflectometry coil probe for measuring soil water content

Time domain reflectometry (TDR) is becoming a widely used non-destructive method for measuring soil-water content (θ), but relatively little effort has been put into the development of TDR equipment and especially TDR probes. Therefore, TDR has been used with only limited success for measuring small spatial scale variations in θ. In this study a small-scale TDR probe (length = 15 mm, diameter = 3.6 mm), applicable with common TDR equipment and easy and inexpensive to produce, was developed. Travel time in the probe is prolonged by forcing the electromagnetic waves to travel in a coil surrounded by four grounded wires. The apparent relative dielectric permittivity (K_a) measured by the coil probe (K_{a, coil}) was calibrated against K_a measured by a standard two-rod TDR probe without a balun (K_{a, ref}) in (i) five soils during transient evaporation, and (ii) air and seven fluids. Calibration in air and fluids was fast and reliable compared with transient-state calibration in soil. A two-phase dielectric mixing model was used to describe the contributions of the probe materials and the surrounding media to K_{a, coil}. Soil volume of influence was found to be approximately 180 mm³, implying that the coil probe measurement can be considered close to a point measurement. By introducing the coil principle, a reduction in probe length of a factor of five was achieved without loss in the sensitivity of the probe to changes in the K_a of the surrounding soil.