Measuring Soil Water Content in the Laboratory and Field with Dual-Probe Heat-Capacity Sensors

Dual-probe heat-capacity sensors were constructed and tested in laboratory and field conditions to determine their application for measuring volumetric soil water content (θᵥ). The dual-probe technique uses a transient heat pulse and a temperature sensor to determine the volumetric heat capacity of the soil (pcₚ), which can be subsequently converted to θᵥ for a nonswelling soil if bulk density is known. In 1994, 24 dual-probes were simultaneously tested using a multiplexed data acquisition system. In the laboratory, probes were calibrated to the known heat capacity of water. Following calibration, the probes correctly estimated pcₚ of glass beads and water-saturated glass beads to within 2.2 and 0.2% of their calculated values, respectively. Gravimetric and dual-probe estimates of θᵥ were compared using the Tempe pressure cell technique to desorb soil columns. The two methods agreed to within 0.03 m³ m⁻³ and 0.04 m³ m⁻³ in two soil types, over a range of θᵥ from 0.45 m³ m⁻³ to 0.10 m³ m⁻³ estimates of the change in θᵥ (Δθᵥ) between desorption steps agreed to within 0.01 m³ m⁻³. In the field, beneath bare soil, estimates of θᵥ among three dual-probes installed at the same depth agreed, on average, to within 0.02 m³ m⁻³. The probes detected Δθᵥ within 30 min during the 21-d field trial. Dual-probe and gamma-attenuation measurements of θᵥ agreed to within 0.05 m³ m⁻³. In 1995, probe design was improved to produce more robust, reliable instruments. Sixteen probes were installed 10 cm beneath a drip-irrigated row crop. A consistent difference in θᵥ of ≈0.10 m³ m⁻³ between mulch-covered and bare soil beds was detected during the 25-d trial. The dual-probe heat-pulse technique for measuring θᵥ has direct applicability in agronomic and horticultural production, where small-scale, automated measurements of θᵥ are needed. Contribution no. 96-470-J from the Kansas Agric. Exp. Stn., Manhattan, KS.