Infiltration is the process by which water on the ground surface enters the soil.
Infiltration is governed by two forces, gravity, and capillary action.
While smaller pores offer greater resistance to gravity, very small pores pull water through capillary action in addition to and even against the force of gravity.
Infiltration rate in soil science is a measure of the rate at which a particular soil is able to absorb rainfall or irrigation.
It is measured in inches per hour or millimeters per hour.
The rate decreases as the soil becomes saturated.
If the precipitation rate exceeds the infiltration rate, runoff will usually occur unless there is some physical barrier.
It is related to the saturated hydraulic conductivity of the near-surface soil.