MEASUREMENT AND SIMULATION OF NONISOTHERM AL MOISTURE MOVEMENT IN WATER-REPELLENT MINERAL SOILS

Water repellency often occurs in surface soil horizons, where the temp erature map have a significant effect on water movement. Relatively fe w studies have been conducted to measure the effect of temperature gra dients on hydraulic processes in water-repellent soils. The objective of: this paper is to analyze the simultaneous impact of hydraulic and thermal gradients on water movement in unsaturated soils. Four sandy s oils with organic matter contents between 1.0 and 2.4 percent were use d in a laboratory column experiment. The soils had similar particle si ze distribution and contact angles between 25 and 111 degrees. The wat er repellency was measured with the ''sessile drop method''. The exper iments were conducted with open laboratory soil columns. A ceramic pla te was placed at the upper end of the soil column to allow water movem ent from a reservoir, the lower end was closed, The matric potential o f -175 hPa at the upper part of the column was held constant. Simultan eously a constant temperature gradient of approximatly 1.1 degrees C/c m was applied during the nonisothermal runs. Hydraulic properties of t he four soils were determined sequentially at different temperatures ( 5, 20 and 38 degrees C). The time dependent volumetric water content, matric potential and temperature were measured at five positions in th e column. It was found that after steady state was established the wat er content and matric potential profiles of the four soils differed co nsiderably. A computer code based on the theory of Philip and De Vries (1957) was developed to simulate water and heat transport. For three soils with contact angles of 25-35, 93-108 and 109-111 degrees, respec tively, the model performed well when the theoretically calculated the rmal vapor diffusivity, D-TV, was multiplied by a factor between two a nd three. For the slightly water repellent soil with a contact angle b etween 45 and 83 degrees, however, this factor was about seven to eigh t. This factor could be the result of a combined impact of the moderat e water repellency and the micromorphology of the humic substance.