The process by which water on the ground surface enters the soil. It is a key process controlling the partitioning of rainfall between surface runoff and subsurface flow.
Infiltration is the process by which water from precipitation, irrigation, or surface water bodies penetrates the ground surface and enters the soil. The rate at which infiltration occurs depends on soil properties (texture, structure, porosity), antecedent moisture content, surface conditions (vegetation, crusting, compaction), and the intensity of the water supply. Infiltration rates are typically highest at the beginning of a rainfall event when surface soil pores are unsaturated and decrease over time as the soil becomes saturated, approaching a constant rate equal to the saturated hydraulic conductivity. This behavior is described by empirical equations such as the Horton infiltration model, the Green-Ampt model, and the Philip equation. When rainfall intensity exceeds the infiltration capacity, excess water accumulates on the surface and may flow as overland runoff (Hortonian overland flow). Infiltration is the critical process that partitions precipitation between surface runoff and groundwater recharge. Land management practices, including tillage, mulching, terracing, and maintaining vegetative cover, can significantly enhance infiltration rates and reduce surface runoff and erosion.