Air entrapment effects on infiltration rate and flow instability

Abstract

Abstract. Experiments were conducted to quantify the effects of entrapped air on water infiltration into a loamy sand. Transparent three-dimensional (3-D) and 2-D columns were used for experiments carried out for two infiltration conditions: (1) when air was free to move ahead of the wetting front and leave the bottom of the column (air draining) and (2) when air was confined ahead of the wetting front and hence could escape only through the soil surface (air confining). The measurement setup was composed of a tensionpressure infiltrometer, an air flowmeter, water manometers, and video-picture cameras. We applied both positive and negative water pressures at the soil surface and measured the simultaneous changes in the rates of water inflow and air outflow, the air pressure ahead of the wetting front, and the dynamic behavior and advance of the wetting front. The air pressure ahead of the wetting front for the air-confining condition was generally found to increase with time rather than reaching a constant level, as observed in other studies by other researchers. The air pressure fluctuated locally because of air escaping from the soil surface. On the basis of an analysis of the results we present two empirical equations to predict the maximum air pressure at which air begins to erupt from the soil surface and to predict the minimum air pressure at which air eruption stops. We found that the infiltration rate was always equal to, and controlled by, the rate of air outflow. The infiltration rate varied inversely with the air pressure ahead of the wetting front and with the ponding depth at the soil surface. The infiltration rate fluctuated with time rather than undergoing changes in a three-stage process, as is often characterized in the literature. The volume of residual entrapped air in the air-confining condition increased 7% on average, and the infiltration rate decreased threefold to tenfold as compared to the air-draining condition. Finally, it was shown that the air-confining infiltration flow is fingered and unstable, consistent with the predictions of an existing theory.

Two‐phase flow infiltration equations accounting for air entrapment effects

“…Results of (13) For the different input parameters, Table 2 compares the output of (10), (11), (13), (14), (22) Air compression ahead of the wetting front is a major cause of wetting front instability followed by fingering [Peck, 1965b;Raats, 1973;Philip, 1975;Wang et al, 1997]. These processes may substantially affect the rate of water infiltration.…”

High-resolution permeability determination and two-dimensional porewater flow in sandy sediment

“…Use of this method precludes fine control over which depths are going to be tested. However, it disallows changes to sample compaction, desaturation, and air bubble intrusion, the greatest sources of error in permeametry (Wang et al 1998;Klute and Dirksen 1986;Klute 1965). …”

“…This was done to ensure that water permeating through the sample had salinity as close as possible to the local porewater, and that no fine particulates present would eventually decrease the testsample's hydraulic conductivity by clogging of the natural pore structure with finer particles and organic colloids (Barrington et al 1987;Rowsell et al 1985). To avoid flow instability and air entrapment effects on flow rate (Wang et al 1998), cores were kept water saturated during tests (overlying water column in excess of 1 cm length). In addition, while waiting for experimental procedures, cores were sealed to preclude any evaporation and subsequent artifacts due to increased salinity (flow fingering, Wang et al 1998).…”

“…To avoid flow instability and air entrapment effects on flow rate (Wang et al 1998), cores were kept water saturated during tests (overlying water column in excess of 1 cm length). In addition, while waiting for experimental procedures, cores were sealed to preclude any evaporation and subsequent artifacts due to increased salinity (flow fingering, Wang et al 1998).…”

Preferential Flow Systems Amended with Biogeochemical Components: Imaging of a Two-Dimensional Study

“…Analytical and quasi-analytical solutions of the flow equation have been developed through the time expansion (Philip, 1969(Philip, , 1975 and integral approaches (Parlange, 1971b). Further Saffman and Taylor (1958) and Chouke et al, (1959) concluded that the development of wetting front instability resulted from unsaturated flow (Pendexter and Furbish, 1991;Raats, 1973;Selker et al, 1992c), an increase in hydraulic conductivity with depth (Hill and Parlange, 1972;van Ommen et al, 1989), a decrease in soil wettability with depth (Hill and Parlange, 1972;Hillel, 1987;Philip, 1975;Raats, 1973;Tamai et al, 1987), water repellency 25 (Dekker and Ritsema, 1994;Ritsema et al, 1993;Ritsema and Dekker, 1994), a redistribution of infiltration after the end of rainfall or irrigation (Philip, 1975), and air entrapment (Peck, 1965;Philip, 1975;Raats, 1973;Wang et al, 1998).…”