Thermal Conductivity of Sands under Varying Moisture and Porosity in Drainage–Wetting Cycles

A class of problems in hydrology and remote sensing applications requires an understanding of how water and heat flux boundary conditions affect the soil moisture in the shallow subsurface near the land surface. This requires a fundamental understanding of how soil moisture content that depends on variations in soil drainage–wetting paths, porosity, and grain size affect the soil's thermal behavior. Very few experimental data showing the effects of these variations are available and the relationships are only empirical. Thermal conductivity (λ) was measured for four sands under varied porosity and water content (θ) in drainage–wetting–drying cycles. Several sensors were integrated into a Tempe cell modified to continuously monitor water saturation, capillary pressure, temperature, and λ. The data showed that λ increased with an increase in θ in three distinct regimes including a drastic decrease in λ below the residual water content. The λ–θ relationship was independent of whether the soil was going through wetting or drainage. The effect of porosity on λ was more pronounced above residual θ; however, the difference in λ as a function of porosity at a given θ was very consistent. The grain size effect was relatively insignificant for the range (0.36–1.04 mm) tested. Measured thermal properties were then compared with independent estimates made using available empirical models. In general, good agreement was observed for most λ–θ data except for intermediate θ values. These experimental results highlight the importance of a fundamental understanding of the various factors that affect the λ behavior of soils and the need for accurate experimental data sets to improve modeling.