Measuring soil water content, electrical conductivity, and thermal properties with a thermo-time domain reflectometry probe

Hydraulic and thermal regimes are coupled in surface soil. Accurate measurement of soil volumetric water content (θ) and thermal properties will improve our understanding of the hydraulic and thermal regimes. A thermo-time domain reflectometry (TDR) probe was developed to simultaneously measure θ, bulk electrical conductivity (σ), thermal conductivity (λ), heat capacity (ρc), and thermal diffusivity (α). Time domain reflectometry was used to measure θ and σ, and the dual-probe heat pulse (DPHP) method was used to determine λ, ρc, and α. Laboratory tests on two soils showed that the probe determined θ accurately and the measured σ values of saturated soil correlated well with values determined by a four-electrode probe. Measurements in agar-immobilized water produced values of λ and ρc that closely corresponded with standard values of these properties for water, an indication of the sensor's functionality in other media. Soil thermal properties as a function of θ also are presented. The results of these laboratory tests suggest that the thermo-TDR probe can be a valuable tool for simultaneously monitoring θ, σ, and soil thermal properties.