On the Diffusivity of Water Vapor Under Thermal Gradients in Soils

The isothermal transport of liquid water and water vapor in soils has been well understood for some time. Conversely, the thermally-driven transport of these two components is less well characterized, and depends on a number of unobserved hypothesized transport mechanisms. In field soils, the combination of the four components of isothermal liquid and vapor and thermal liquid and vapor into one measured flux makes it difficult to isolate the magnitude and direction of any individual component flux. An experimental system was designed to isolate and measure only the water profile in nonisothermal soils caused by vapor transport. The thermal gradient was kept large so that the vapor transport would be dominated by the thermally-driven flow instead of the isothermal vapor flow. Moisture content profile measurements over the temperature range of 20oC to 70oC were taken. A basic framework of explanation for the findings is presented.