overlandflow

Overland flow, or surface runoff, is water that moves over the land surface during rainfall or snowmelt without infiltrating into the soil or entering a stream immediately. It occurs when the rainfall rate or duration exceeds the soil’s infiltration capacity, or when the soil is saturated. The water can travel as sheet flow and may concentrate into small channels.

Two main mechanisms generate overland flow. Infiltration-excess runoff (Hortonian) happens when rainfall exceeds the soil’s infiltration rate, common on compacted or crusted soils. Saturation-excess runoff (Dunne) occurs when soils are saturated and cannot accept more water, producing runoff even at moderate rain, especially in humid or poorly drained areas.

Factors influencing overland flow include soil infiltration rate, antecedent moisture, rainfall characteristics, land cover and management, surface roughness, slope, and urbanization with impervious surfaces. Vegetation and soil structure can enhance infiltration and roughness, reducing runoff, while bare, compacted, or crusted surfaces increase it.

Overland flow affects hydrology by contributing to flood peaks, sediment and pollutant transport, and erosion. It starts as sheet flow and can develop into localized channels, supplying stream networks during storms and shaping small-scale hydrological response.

Measurement and modeling of overland flow rely on rainfall–runoff relationships, infiltration models, and surface routing. Velocity is often estimated with roughness-based equations such as Manning’s, while runoff generation uses Horton-type or Dunne-type formulations depending on the dominant mechanism.

Management approaches aim to reduce adverse effects by promoting infiltration and reducing peak discharge. Techniques include vegetation cover, soil conservation, contour farming, buffers, terracing, and permeable pavements or infiltration basins in urban areas.