Sensitivity Analysis of Soil Thermal Property Measurements with Heat Pulse Probes in Partially Frozen Soils.

Soil thermal properties are fundamental for understanding heat transfer in soils. The heat pulse method for measuring soil thermal properties has been used in a variety of studies. Measuring thermal properties of partially frozen soils with heat pulse method is difficult because ice around the heater needle melts during heating. Because of ice melting, heat capacity is overestimated, and thermal diffusivity is underestimated. There is a need to account for the effect of ice melting on needle temperature change. In this study, we aim to understand the sensitivity of needle temperature rise to soil type, initial temperature, total water content, and freezing characteristics of partially frozen soils.

A finite difference model was developed to solve the radial heat conduction equation taking thawing and re-freezing into account. Sand, silt loam, and sandy clay loam, were chosen for this study. Simulations were performed with a variety of initial temperatures between 0 °C and -10 °C. Five different total water contents for each soil were examined. Soil freezing characteristics were described by the van Genuchten model. Simulations with a variety of empirical parameters were performed. The impact of each parameter was evaluated by way of temperature change at the detecting needle.

More than 80 percent of the applied heat was separated into latent heat to melt ice when initial soil temperature was between -1 °C and 0 °C. The amount of heat partitioned into latent heat strongly impacted temperature changes at the detecting needle. Initial temperature and freezing characteristics were critical factors for determining latent and sensible heat partitioning. Special care is required for the initial temperature and freezing characteristics to accurately estimate thermal properties in partially frozen soil.