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 (theta(v)). The dual-probe technique uses a transient heat pulse and a temperature sensor to determine the volumetric heat capacity of the soil (rho c(p)), which can be subsequently converted to theta(v) 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 rho c(p) 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 theta(v) were compared using the Tempe pressure cell technique to desorb soil columns. The two methods agreed to within 0.03 m(3) m(-3) and 0.04 m(3) m(-3) in two soil types, over a range of theta(v) from 0.45 m(3) m(-3) to 0.10 m(3) m(-3); estimates of the change in theta(v) (Delta theta(v)) between desorption steps agreed to within 0.01 m(3) m(-3), In the field, beneath bare soil, estimates of theta(v) among three dual-probes installed at the same depth agreed, on average, to within 0.02 m(3) m(-3), The probes detected Delta theta(v) within 30 min during the 21-d field trial, Dual-probe and gamma-attenuation measurements of theta(v) agreed to within 0.05 m(3) 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 theta(v) of approximate to 0.10 m(3) m(-3) between mulch-covered and bare soil beds was detected during the 25-d trial, The dual-probe heat-pulse technique for measuring theta(v) has direct applicability in agronomic and horticultural production, where small-stale, automated measurements of theta(v) are needed.