Infiltration with controlled air escape

Infiltration into the soil is restricted if the movement of displaced air is hampered. In this study, separate infiltration experiments were performed where the air could escape only through capillary glass tubes (three different diameters were used). Control experiments, where the air movement was not restricted, were performed also. During the experiments both air pressure and the cumulative infiltration were measured. Air pressure measurements showed a rapid rise to a maximum after the water was ponded, followed by a gradual decrease. A first aim of this study was to show that even for a small increase in pressure relative to the case where the air was free to escape, e.g., <1 cm of water, there was a small but measurable reduction in infiltration. The air movement was obtained as a function of the air pressure via Poiseuille's law. By equating air and water movement (both fluids assumed incompressible) it was shown that the air pressure measurements could be used in predicting the water flux into the column and hence the cumulative infiltration. Then, by using the precise air pressure measurements for the various capillary tubes we were able to assess the sensitivity of hydraulic conductivity and sorptivity to minor increases in air pressure.