The portion of precipitation that flows over the land surface or through the shallow subsurface to reach streams and rivers. It is the primary mechanism by which water leaves a catchment.
Runoff is the component of the water cycle in which water from precipitation, snowmelt, or irrigation flows over the land surface (surface runoff) or through shallow subsurface pathways (subsurface runoff, interflow) to eventually reach stream channels, rivers, lakes, or oceans. It is calculated as the difference between precipitation and the combined losses from evapotranspiration, infiltration to deep groundwater, and changes in surface storage. Runoff generation mechanisms include Hortonian overland flow (infiltration excess), saturation overland flow (saturation excess), and subsurface stormflow through macropores and soil pipes. The runoff ratio (total runoff divided by total precipitation) varies from less than 5% in arid regions with permeable soils to over 90% in impervious urban areas. Factors affecting runoff include rainfall characteristics, land use, soil type, slope, antecedent moisture, and vegetation cover. Runoff carries sediments, nutrients, pesticides, and other pollutants to receiving waters, making it a primary concern for water quality management. Accurate runoff estimation is essential for water supply planning, flood prediction, erosion assessment, and stormwater management.