The movement of water over the land surface toward stream channels when rainfall intensity exceeds the soil's infiltration capacity or when the soil becomes fully saturated.
Overland flow, also known as surface runoff, is the movement of water across the land surface as a thin sheet or in small rills before it concentrates into defined stream channels. Two primary mechanisms generate overland flow: Hortonian overland flow occurs when rainfall intensity exceeds the infiltration capacity of the soil, while saturation-excess overland flow occurs when the soil profile becomes completely saturated, typically in areas near streams, in topographic convergence zones, or where the water table is shallow. Overland flow is a major agent of soil erosion, transporting detached soil particles and associated contaminants to receiving water bodies. The velocity and depth of overland flow depend on rainfall intensity, slope gradient, surface roughness, and the distance from the drainage divide. In urban areas, impervious surfaces such as roads, rooftops, and parking lots generate overland flow from virtually all rainfall events, leading to increased peak flows and reduced groundwater recharge. Understanding overland flow mechanics is essential for erosion prediction models (such as USLE and WEPP), stormwater management design, and non-point source pollution assessment.