TEMPERATURE GRADIENTS: EVALUATION OF THE THEORY OF PHILIP AND DE VRIES (1957)

The theory of Philip and de Vries ( 1957 ) has been used extensively to describe the transfer of water in response to water-content and temperature gradients. The same theory was evaluated using vertical columns in which there were fluxes both into ( from a water table at 55-cm depth ) and out of the soil ( evaporation to the atmosphere ). The fluxes of water were differentiated. The isothermal vapour flux was negligibly small in all experiments. The thermal vapour flux was found to be negligibly small below a depth of 10 cm but its magnitude at the soil surface increased with decreasing water content and increasing temperature. The isothermal liquid flux contributed significantly to the net soil-water flux except at very low water contents. Thermal liquid flux contributed significantly to the net soil-water flux except at very low contents and was comparable in magnitude to both the isothermal liquid flux and net vapour flux. The downward liquid flux due to gravity was comparable in magnitude to those of the isothermal and thermal liquid fluxes, particularly at high water contents. The Philip and de Vries ( 1957 ) theory predicted close to the measured soil-water flux at very low water contents but over predicted upto a factor of 20 at intermediate and high water contents.