Measurements of soil water content, heat capacity, and thermal conductivity with a single TDR probe
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Soil heat capacity c and thermal conductivity determine how a soil warms or cools with exchanges of energy by conduction, convection, and radiation. Both c and depend on water content . The ability to monitor soil c, , and is an important tool in managing the soil temperature regime to affect seed germination and crop growth. We developed a time domain reflectometry (TDR) probe whose electrodes enclosed thermocouple junctions and a heater. The probe served as a dual-probe heat-pulse device to estimate c and and as a three-electrode-type TDR probe for . The probe was tested under laboratory conditions in loamy sand at s from 0.005 (air dry) to 0.36 m3 m-3. First, a TDR trace was recorded from a probe, then power was applied to a heater in the center electrode and temperature in an outer electrode 10 mm from the heater was recorded. Recorded TDR traces and temperatures were analyzed to estimate , c, and . Water contents estimated by TDR agreed well with gravimetrically determined values. Measured c and agreed with models proposed by De Vries, but there was considerable difference between measured and modeled values at high water contents. Water contents estimated from c showed more variability than those estimated from TDR when compared with gravimetrically determined values.
