In the usual analysis of the flow of water into a porous medium, such as infiltration into a soil, air resistance is neglected. Obviously as water moves into the air-filled pores air must be driven out. In most cases the air flows with a negligible resistance compared to the resistance of water to flow. However, if the air escape route deeper into the medium is blocked by an impermeable barrier, the air will be trapped and its pressure will increase as the combined action of gravity and capillary forces try to force the air from the pores. This air pressure build-up will decrease the infiltration rate below the rate predicted from measurements of the permeability of the medium and its capillary characteristics. The report presents a mathematical analysis of this effect.