This paper is concerned with the changes of vertical water movement procsses in unsaturated sand under nonponding condition during and after a steady rain. In order to reveal the mechanism of vertical water movement, experiments were carried out by, using a large rainfall simulator, which can generate artificial rains with various intensities and durations, and two iron boxes filled with fine and coarse sands, repectively.
Water contents were measured by using a neutron scattering moisture meter, and capillary potentials were obtained simultaneously from eight porous cups buried vertically at different drpths, each one being connected to a tensiometer.
The mechanism of vertical water movement was studied by analyzing the changes of water content and capillary potential at selected depths. This analysis showed that the processes of vertical water movement (infiltration, percolation, redistribution, drainage) can be determined individually with water gradient and total potential gradient, that vertical water movemnt is affected by the presence of entrapped air near the ground surface during a heavy rain, that the early outflow begins as a result of the propagation of the air and water prssures in the suspended water and the upward transition zones before the wetting front reaches an upward infiltration zone, and that water and total potential profiles of water gradient G-≈ 0 and total potential gradient G° ≈ 1 (coarse sand) andC° ≈ 0.75 (fine sand) move paralle to each initial profile with time during drainage, respectively.
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