Comparing Heat-Pulse and Time Domain Reflectometry Soil Water Contents from Thermo-Time Domain Reflectometry Probes

The thermo-time domain reflectometry (thermo-TDR) technique provides a valuable tool for monitoring coupled heat, water, and chemical transport in the vadose zone. This study evaluated the heat-pulse and the TDR methods for soil water content determination using the thermo-TDR probe. Laboratory measurements were conducted on repacked and undisturbed soil columns of different bulk densities and water contents. For the heat-pulse method, the bulk specific heats of soil solids were determined using the thermo-TDR probe on oven-dried soil samples, and volumetric soil water content (θHP) was estimated from the heat capacity and water content relationship. For TDR measurements, the first reflection point on the waveform was determined by shorting the probe in air and the apparent probe length was determined from calibration in distilled water. The Topp equation was applied to convert the apparent relative permittivity to soil water content (θTDR). The thermo-TDR probe is ideal for making the comparison between the heat pulse and TDR methods because the probe makes both measurements on nearly the same soil volume (approximate radius of 14 mm about the central heater for θHP and approximate radius of 11 mm about the central cylinder for θTDR). Experimental results on eight soils showed that both TDR and heat-pulse methods gave reliable soil water content data for repacked and undisturbed soil. Comparing with gravimetrically measured volumetric water content, the root mean square error (RMSE) of θTDR measurements was 0.023 m³ m⁻³ for repacked soils and 0.018 m³ m⁻³ for undisturbed soils. The RMSE of θHP measurements was 0.022 m³ m⁻³ for repacked soils and 0.021 m³ m⁻³ for undisturbed soils. The relatively large RMSE of the TDR measurements is attributed to the relatively short length (4-cm) of the thermo-TDR probe. The TDR method showed less sensitivity to spatial soil variability than did the heat-pulse method. The heat-pulse technique seemed better suited than TDR for water content measurements on soils with relatively high organic matter content.