Infiltration

This has two meanings. Applied to the rainfall-runoff process, it is the passing of rainwater and other forms of precipitation into the soil. Under the action of capillary and gravity forces, rainwater is absorbed into the soil to be held as soil moisture, to percolate downwards through the soil to become groundwater, or to travel laterally emerging as interflow, flowing out of the partly-saturated soil layers located above the water table, and forming seepages or springs.

Most of the water that does not infiltrate becomes surface runoff.

The second meaning refers to the movement of water into pipes through cracks or faulty joints. The opposite process is exfiltration. Entry of stormwater and groundwater through inflow/infiltration is a major problem in sanitary sewers.

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Stormwater infiltration has become a favoured means of reducing stormwater runoff in locations where suitable soils, with medium to high infiltration capacities, are available. This has advantages in preventing flooding, particularly at lower average recurrence intervals, and of limiting the exports of pollutants in stormwater runoff.

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