Improved field probes for soil water content and electrical conductivity measurement using time domain reflectometry

Abstract

Volumetric water content θ and soil electrical conductivity σ may be measured in situ using time domain reflectometry (TDR). The parallel‐wire or two‐wire transmission line TDR probes currently in field use suffer from unwanted noise and information loss due to impedance mismatch between the probe and the coaxial connecting cable. Here we describe symmetric, multiwire probes designed to minimize these problems and eliminate the need for a balancing transformer between probe and TDR device. Analysis of the electric field distributions around these new probes shows that they emulate a coaxial transmission line, and their measured characteristic probe impedances approach that of coaxial probes. Signals from the new probes permit more reliable and accurate θ and σ measurement and are superior to those of two‐wire probes with balancing transformer. The enhanced signal clarity of the new probes extends to sample diameters of at least 0.2 m. We show that electrical conductivity determined with the new probes is identical to that found with a coaxial cell and substantially different from that measured by a two‐wire probe. Our results indicate that values of σ, determined using the Giese‐Tiemann thin sample approach and measured characteristic probe impedances of coaxial or multiwire probes, agree with values of σ measured using an ac bridge for both electrolyte solutions and soil samples to within ±10%, provided σ exceeds 10 mS m−1. Finally, we give an example of the use of multiplexed three‐wire probes in following rainfall infiltration and redistribution during and after a simulated rainfall event in the field. Infiltrated quantities of water estimated from the TDR water content profiles agreed within ±10% with the amount applied.