Thermal Property Measurement Errors with Heat-Pulse Sensors Positioned near a Soil-Air Interface

Heat-pulse sensor measurements analyzed with pulsed infinite line source (PILS) theory have been widely used to measure soil properties. The PILS theory assumes that the measured soil medium is uniform and infinite. When the sensors are positioned near the soil surface, the effects of the heterogeneity associated with the soil-air interface should not be ignored. In 1999, Philip and Kluitenberg (PK99) proposed an analytical solution using an instantaneous heating model to analyze the effects of the soil-air interface on soil thermal property measurements with heat-pulse sensors. The purpose of this study is to test the PK99 instantaneous heat source solution under controlled laboratory conditions. Soil thermal properties including volumetric heat capacity, thermal diffusivity, and thermal conductivity were measured with a commercially available dual-needle heat-pulse sensor buried at different depths beneath the soil surface. Three soil materials, sand, loamy sand, and sandy clay loam, were tested at both air-dry and saturated moisture conditions. With shallow sensor burial, measured thermal properties were underestimated by up to 50%, similar to the predicted thermal properties from the PK99 analytical solution, due to the effects of the soil-air interface. Using PK99 to adjust thermal property values obtained from shallow sensors has potential to improve estimates of water content, evaporation, and other soil measurements derived from heat-pulse sensors.