Infiltration Excess Flow

Infiltration excess occurs when the rate of rainfall or snowmelt is greater than the infiltration capacity. The water that cannot infiltrate becomes surface runoff.

For example, if the infiltration capacity is 15 millimeters per hour and the rainfall rate is 25 millimeters per hour, then the rainfall rate exceeds the infiltration capacity by 10 millimeters per hour. That 10 millimeters per hour becomes infiltration excess overland flow, even if the underlying soil is dry.

Infiltration excess is most commonly observed with short-duration intense rainfall. It also occurs most often in areas with high clay content or where the surface has been altered by soil compaction, urbanization, or fire.

Infiltration excess overland flow is sometimes called Hortonian flow.