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Soil Air Pressure and Water Infiltration Under Border Irrigation¹

ABSTRACT

Soil air pressure and water infiltration were measured during actual and simulated border irrigation of a uniform loam soil having a water table about 2 m beneath the surface. These measurements were made with instrumentation specifically designed to distinguish between displaced and entrapped air; to separate hydraulic and pneumatic pressure sources; and to isolate infiltration effects of displaced air pressure. Displaced air pressure decreased downslope and across slope from center to edge of the border strips. A maximum displaced air pressure of 21 cm of water was observed in the upslope central part of the border strip. Air entrapment commonly occurred at a 50-cm depth next to border dikes but rarely occurred midway between dikes, which implies that water penetration was greatest in the region of lowest displaced air pressure. Infiltration measurements, made under actual and simulated border irrigation, indicated that displaced air pressure, building to a maximum of about 19 cm, reduced total infiltration by about . Such pressure appears to impede infiltration mainly by preventing or retarding direct flow of surface water into and within open macropores. In the central region of the border strip where displaced air pressure exceeds the surface water head, macropores vent displaced soil air upward; whereas along the border dikes where surface head exceeds air pressure, macropores conduct free surface water downward. A leaking-system form of Boyle's Law is used to interpret typical displaced air pressure-time curves and to suggest possible ways of controlling displaced air pressure. This research implies that soil air pressure and its infiltration effects are not negligible as is commonly assumed by Darcy-based flow theory and that soil air can be a useful tool for controlling infiltration in some important situations.

¹ Contribution of the Agricultural Research Service, USDA, in cooperation with the Nevada Agr. Exp. Sta., Univ. of Nevada, Reno, Journal series no. 206. Presented before Div. S-6 and S-1, Soil Science Society of America, New York City, NY, Aug. 18, 1971.

² Soil Scientists, USDA, University of Nevada, Reno, Nevada 89507.

Received for publication May 19, 1972. Accepted for publication July 13, 1972.