Runoff

Runoff is water from precipitation or snowmelt that flows over the land surface and into waterways when infiltration is insufficient, or as direct channel flow. It is a key component of the hydrological cycle, varying with rainfall intensity, soil properties, land slope, and vegetation. It contrasts with infiltration, evaporation, and groundwater recharge.

In hydrology, runoff generation occurs by infiltration-excess (where rainfall rate exceeds infiltration capacity) and saturation-excess (soil becomes saturated). The amount of runoff is estimated with rainfall-runoff models and metrics such as runoff coefficient and peak discharge; common methods include the rational method for small catchments and the SCS-CN method for broader areas.

Human activities influence runoff. Urban areas with impervious surfaces increase runoff and transport pollutants; green infrastructure can mitigate it by promoting infiltration and storage. Agricultural runoff can carry nutrients and pesticides; best management practices reduce environmental impact and protect water quality. Climate variations and land use changes also affect runoff magnitude and timing, influencing flood risk and ecosystem health.

In politics, runoff refers to a second election held when no candidate meets a required threshold in the first round. In a two-round system, the top candidates from the initial round compete in the runoff; some jurisdictions use instant runoff or ranked-choice voting to determine a winner without a separate election. The presence of a runoff shape campaign strategies and voter participation.