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Air Encapsulation During Infiltration

U.S. Geological Survey, Water Resources Div., 345 Middlefield Road, MS/496, Menlo Park, CA 94025

* Corresponding author.

ABSTRACT

A series of field and laboratory experiments were performed to measure the effects of air encapsulation within the soil's transmission zone upon several infiltration properties. In the field, infiltration rates were measured using a double-cap infiltrometer (DCI), and soil-water contents were measured using time-domain reflectometry (TDR). Before half of the infiltration experiments, CO₂ was injected through the DCI into the soil to reduce the amount of air encapsulation in the soil's transmission zone. For a gravelly loam as steady infiltration rates were approached, the average volumetric water content was 0.38 cm³ cm^–3 for control experiments and 0.43 cm³ cm^–3 for CO₂ experiments. The average steady infiltration rate was 0.42 cm min^–1 for the control experiments compared to 4.40 cm min^–1 for the CO₂ experiments. For a sandy loam as steady infiltration rates were approached, the average volumetric water content was 0.43 cm³ cm^–3 for control experiments compared with 0.45 cm³ cm^–3 for CO₂ experiments. The average final infiltration rate was 0.09 cm min^–1 for the control experiments compared with 0.42 cm min^–1 for the CO₂ experiments. In the laboratory, infiltration experiments were performed using repacked soil columns (15-cm i.d. by 140 cm long), again using TDR and CO₂ flooding. For a medium sand as steady infiltration rates were approached, the average volumetric water content was 0.29 cm³ cm^–3 for the control experiments and 0.36 cm³ cm^–3 for the CO₂ experiments. The average steady infiltration rate was 0.25 cm min^–1 for the control experiments and 1.23 cm min^–1 for the CO₂ experiments. For a loam as steady infiltration rates were approached, the average volumetric water content was 0.45 cm³ cm^–3 for the control experiments and 0.50 cm³ cm^–3 for the CO₂ experiments. The average steady infiltration rate was 0.02 cm min^–1 for the control experiments and 0.10 cm min^–1 for the CO₂ experiments. These results suggest that a significant portion of the total encapsulated air resided in interconnected pores within the soil's transmission zone. For the time scale considered, this residual air caused the effective hydraulic conductivity of the transmission zone to remain at a level no greater than 20% of the saturated hydraulic conductivity of the soil.

Contribution from the U.S. Geological Survey, Menlo Park, CA.

Received for publication March 23, 1987.

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