Significance of Heat Loss and Improved Calibration for Soil Property Estimates Using a Heat-Pulse Probe.

A multi-functional heat pulse probe (MFHPP) is an instrument that simultaneously measures soil thermal properties and water content.  Previous studies discussed limitations and measurement inaccuracies due to oversimplified heat transport models and poor performance of calibration techniques. The objective of this study was to determine the significance of heater output losses within measurement error, through comparison of soil property estimates using newly developed calibrations.  An improved MFHPP was constructed and two new calibration approaches were developed to estimate the thermal properties and water content of six different textured soils.  In general, the newly developed calibrations: 1) reduced average estimation error of heat capacity and water content for different soil textures by half, to 11.4 and 15.3%, respectively, 2) was unilaterally applicable across soil textures, 3) decreased overall variability between individual sensor estimates (minimum RMSE values 266.0 and 0.063 as compared to 316.9 and 0.076 for heat capacity and water content, respectively) and 4) was more forgiving in manufacturing induced imprecision (e.g. poor thermistor seating).  Even so, the improved MFHPP and all the calibrations overestimated soil properties by up to 30% on average.  The results of this study further confirm and build upon findings of previous studies which suggest inherent limitations of the currently applied methods and the possibility for future improvements.