MEASUREMENTS OF SOIL WATER CONTENT, HEAT CAPACITY, AND THERMAL CONDUCTIVITY WITH A SINGLE TDR PROBE1

Abstract

Soil heat capacity ρc and thermal conductivity λ determine how a soil warms or cools with exchanges of energy by conduction, convection, and radiation. Both ρc and λ depend on water content θ. The ability to monitor soil ρc, λ, and θ 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 ρc and λ and as a three-electrode-type TDR probe for θ. The probe was tested under laboratory conditions in loamy sand at θs 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 θ, ρc, and λ. Water contents estimated by TDR agreed well with gravimetrically determined values. Measured ρc and λ 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 ρc showed more variability than those estimated from TDR when compared with gravimetrically determined values.