The Influence of Thermo-Gradients Upon Water Movement and Retention in Unsaturated Soils

Temperature differences of about 5, 10, and 15 V, were applied to uniformly packed and insulated horizontal column of Millville silt loam soil using mean temperatures near 13, 14, 15, 16, 17, 18, 19, 20 and 23.5 C. The temperature distribution along the soil column was determined using copper constantan thermocouples. Soil samples were taken to determine volumetric moisture content, and soil water potential, using the vapor pressure psychometer technique. The thermodynamic flow equation of Taylor and Cary, (1964) was used to calculate the hydro-thermal transfer coefficient (β) as given by: -β=(d(μW)Τ)/dlnT where (μW)Τ is the constant temperature water potential and Τ the temperature in °K. The net transfer of moisture from the warm to the cold face of the soil column was found to depend upon the temperature difference, the mean temperature of the system, the water potential distribution that resulted and the average soil water potential as predicted by the equation. The hydro-thermal-transfer coefficient (B) was found to depend primarily on the average water potential with secondary dependence on the mean temperature. The hydro-thermal transfer coefficient was found to be small and nearly constant at high soil moisture potential where water was moving mainly in the liquid phase. At low soil water potentials the hydro-thermal transfer coefficient was again nearly constant and reached high values. In this situation water is moving mostly in the vapor phase. In the zone of water potential where significant water movement is taking place, in both liquid and vapor phases, the influence of the coefficient on the movement was not determined because of the rapidly changing relations