Coiled Time Domain Reflectometry Matric Potential Sensor

Abstract

Matric potential is a measure of the combined capillary and adsorptive forces of soil particles and is imperative for determining both the direction and magnitude of water flow in unsaturated soils. Measuring soil matric potential in semiarid and arid environments is a challenge. Current methods cannot measure the entire range of values, from 0 to −1.5 MPa. The objective of this research was to develop a new sensor capable of measuring the relationship between the dielectric constant of a ceramic core and the range of field soil matric potential values. This instrument uses two Cu TDR rods wrapped around a dual‐threaded Plexiglas core, which is inserted into a porous ceramic block salvaged from damaged Campbell Scientific 229 heat dissipation sensors. The relationship of the ceramic's measured dielectric constant to soil matric potential is determined through a laboratory calibration curve. Calibration of the probe involved the use of a tension table and pressure plate apparatus. The dielectric constant of the probe was measured at a range of points between 0 and −1.5 MPa, and a modified van Genuchten water retention model was used to fit the data (R2 = 0.84). The proposed instrument provided an effective method of measuring soil matric potentials from 0 to −1.5 MPa.