Time Domain Reflectometry Probe for Simultaneous Measurement of Soil Matric Potential and Water Content

Simultaneous measurement of matric potential, ψ, and water content, θ, is demanded in many disciplines. Time domain reflectometry (TDR) has been routinely used to measure water content and electrical conductivity of soil. Previous efforts to combine TDR probes with porous materials functioning as tensiometers were successful, but these probes were still constrained by characteristics of tensiometers, such as the need to supply water and measuring ranges ψ 85 kPa. A new ψ-θ TDR probe was developed to overcome shortcomings of the previous work. A portion of the TDR rod was embedded in a dental plaster (gypsum), whose matric potential equilibrated with surrounding soil. The rest of the TDR rod was inserted into the soil. The TDR technique was used to determine dielectric constants, κ, of both the gypsum and the soil. The new ψ-θ TDR probes were tested in clay loam soil using a pressure-plate apparatus to produce κ-ψ relationships of the gypsum and the soil for -1000 ≤ ψ ≤ 10 kPa. Changes in κ of the gypsum corresponded well to applied pressures for - 1000 ≤ ψ ≤ 30 kPa, but K values did not noticeably change for ψ > -30 kPa. Values of κ of the soil corresponded well to the whole ψ range tested. The new probes accurately measure θ and ψ of soil when soil water content gradually decreases or increases. The newly developed ψ-θ TDR probe requires no more maintenance than ordinary TDR probes and requires no additional instrumentation.