## Infiltration Terms

**Infiltration rate** is the amount of water able to enter the soil in a specified time period. It is expressed as depth per time; for example, 10 millimeters per hour.

**Infiltration capacity** is the upper limit of infiltration rate. It includes surface infiltration and percolation and is expressed in depth per time; for example, 15 millimeters per hour.

If precipitation rate is less than or equal to infiltration capacity, no surface runoff occurs.

If precipitation rate is greater than infiltration capacity, surface runoff occurs.

Surface runoff equals rainfall or snowmelt rate minus infiltration capacity.

For example, if our rainfall rate increases to 25 millimeters per hour but the infiltration capacity remains at 15 millimeters per hour, then the rainfall rate is 10 millimeters per hour greater than the infiltration capacity. The 10 millimeters per hour that does not infiltrate becomes surface runoff.