Water content response of a profiling time domain reflectometry probe

We designed a profiling time domain reflectometry (TDR) probe. The probe includes two short rods that measure through two parallel access tubes. Small diameter wires lead to the top of the measurement interval, limiting the sensitivity of the probe to the medium above the region of interest. Larger diameter target rods span the sample interval, leading to greater sensitivity of the probe to the medium surrounding the access tubes within the measurement interval. The probe sensitivity is further increased by sealing the access tubes and filling them with water. Six prototype probes built using readily available components showed a consistent sensitivity to the soil, receiving ≃25% of the total probe response from the medium around the access tubes in the sample interval. We tested three methods of field calibration of the probe. The calibrated profiling-probe responses produce water-content profiles comparable to those determined with a neutron probe during the advance of a wetting front.