Improved Calibration For Estimating Soil Properties With a Multi-Functional Heat Pulse Probe.

A multi-functional heat pulse probe (MFHPP) can simultaneously measure soil thermal and hydraulic properties.  Though its successful implementation has been well documented, previous studies have reported limitations and measurement inaccuracies. One specific cause of the limitations is the absorption of the generated heat pulse within the probe itself, and this unaccounted energy, not directly transmitted to the soil, creates error in the measurements. The objective of this study was to develop and evaluate a new calibration method to account for measurement error due to heat loss to the probe. A MFHPP was constructed and tested in six soil types using both a traditional and the newly-developed, improved calibration method.  The new calibration utilizes heat pulse response curves from real soils with thermal conductivities similar to that of the MFHPP rather than the traditional agar stabilized water solution.  This new approach termed “quasi-empirical” reduced average measurement error across multiple soil textures from 9.1% to 2.4% for heat capacity and 13.5% to 4.5% for volumetric water content. In addition, the new calibration decreased variation of readings between each individual sensor upon the probe head (heat capacity and volumetric water content RMSE reduced by 37.9%).