SOIL MOISTURE MOVEMENT UNDER TEMPERATURE GRADIENTS

Temperature gradients of 0.5, 1.0, and 1.5 degree C/cm were imposed on sealed soil columns of a fine sandy loam at various moisture contents. When steady state was reached, heat flux, temperature distribution, and moisture content or moisture tension distribution were measured. The coupling between heat and moisture flux was calculated using the theory of irreversible thermodynamics. The coupling between heat and moisture flux was negligible for tensions less than 0.1 bar or higher than 15 bars. Coupling increased as the temperature increased. Within experimental error, Onsager's relation for the interaction between heat and moisture flow was valid. The agreement between the thermodynamic approach and the "series-parallel" theory was satisfactory between 0.3- and 15-bar tension. At tensions above 15 bars or below 0.1 bar, the series-parallel theory predicted more interaction between heat and moisture flow than was observed. It was concluded that the coupling between heat and moisture flow was significant only when the moisture flow occurred in the liquid phase and when soil–water interaction was pronounced.