Infiltrationexcess

Infiltration-excess, or Hortonian overland flow, is a type of surface runoff produced when rainfall intensity exceeds the soil's infiltration capacity. Under such conditions water cannot infiltrate as fast as it is supplied, so excess rainfall becomes overland flow before infiltration occurs. The phenomenon is most common when soils are dry, crusted, or compacted, and when rainfall is intense relative to soil properties and antecedent moisture.

The infiltration capacity is governed by soil texture, structure, moisture state, organic matter, and surface characteristics such as cover and roughness, as well as slope and land use. Infiltration-excess runoff tends to occur on bare or disturbed ground, urban impervious surfaces, and crusted soils in arid and semi-arid regions, particularly during heavy storms. It can be influenced by antecedent moisture; very dry soils may experience higher initial infiltration capacity until infiltration capacity declines, but if rainfall is heavy enough, the capacity is exceeded immediately.

Hydrologically, infiltration-excess runoff contributes to rapid surface runoff and can drive flash floods and erosion; it is often modeled with infiltration models such as the Horton-type decay or Green–Ampt infiltration equations, with rainfall intensity used to determine when infiltration capacity is exceeded. In watershed management, reducing infiltration-excess runoff involves increasing infiltration capacity through soil conservation, mulching, vegetation, and reducing crusting or compaction, as well as controlling impervious surface areas and improving drainage.