Measurement of thermal properties and water content of unsaturated sandy soil using dual-probe heat-pulse probes

Soil thermal property data, especially as a function of water content, are currently not readily available. Demand for these data is, however, on the increase because of improvements in and wider applications of soil heat and water transport models. Small dual-probe sensors have been developed that will assist to overcome this shortage in soil thermal property data, and in this paper, we demonstrate their capability through discussion of measurements carried out on unsaturated sandy soil which was subjected to a wetting and drying cycle. The dual-probes employ heat-pulse methodology and yield the soil thermal diffusivity, heat capacity and conductivity from a single heat-pulse measurement. Thermal properties measured in this study are compared with independent estimates made using standard procedures from the literature. These standard procedures require knowledge of the soil mineralogy, and our dual-probe measurements highlight the fact that we cannot always rely on particle size data for accurate mineralogical information. The consequences of using inappropriate mineralogical data and hence, incorrect thermal properties in soil physical analyses, are illustrated. We also show how volumetric water content can be determined from dual-probe heat capacity measurements and other basic soil data (bulk density and specific heat). These data showed the presence of strong hysteresis in the water retention of the material used in this study, and highlight the fact that the dual-probes have an important role to play in monitoring soil water content as well as providing soil thermal property data.