Liquid and vapour phase transport in soil

Measurements of the desorption soil water diffusivities for vapour and liquid phase flow in a loamy sand and a clay at water contents covering the range between air dry and permanent wilting point are reported. The liquid and vapour components were separated using soil columns maintained at various air pressures. Vapour diffusivities were also calculated from desorption isotherms. At some water contents the measured vapour diffusivities were up to an order of magnitude greater than the calculated values. Liquid phase flow was detected in soils nearly air dry, suggesting some solute transport can occur under such conditions. The liquid and vapour diffusivities were found to be of comparable magnitude over a range of water contents, rather than each cutting off sharply as has been suggested in the literature. The measured component diffusivities at 298 K were used to calculate the total diffusivities at other temperatures, using the temperature coefficients of the basic physical parameters governing liquid and vapour flow, and good agreement was found between measured and calculated values.