Surface runoff

Runoff flowing into a stormwater drain

Surface runoff (also known as overland flow) is the flow of water that occurs when excess stormwater, meltwater, or other sources flow over the Earth's surface. This can occur when the soil is saturated to full capacity, and rain arrives more quickly than soil can absorb it. Surface runoff often occurs because impervious areas (such as roofs and pavement) do not allow water to soak into the ground. Surface runoff is a major component of the water cycle. It is the primary agent of soil erosion by water.[1][2] The land area producing runoff that drains to a common point is called a drainage basin.

Runoff that occurs on the ground surface before reaching a channel can be a nonpoint source of pollution, as it can carry man-made contaminants or natural forms of pollution (such as rotting leaves). Man-made contaminants in runoff include petroleum, pesticides, fertilizers and others.[3]

In addition to causing water erosion and pollution, surface runoff in urban areas is a primary cause of urban flooding, which can result in property damage, damp and mold in basements, and street flooding.


Surface runoff from a hillside after soil is saturated

Surface runoff can be generated either by rainfall, snowfall or by the melting of snow, or glaciers.



Infiltration excess overland flowEdit

runoff and filter soxx

This occurs when the rate of rainfall on a surface exceeds the rate at which water can infiltrate the ground, and any depression storage has already been filled. This is called flooding excess overland flow, Hortonian overland flow (after Robert E. Horton), or unsaturated overland flow. This more commonly occurs in arid and semi-arid regions, where rainfall intensities are high and the soil infiltration capacity is reduced because of surface sealing, or in paved areas. This occurs largely in city areas where pavements prevent water from infiltrating.

Saturation excess overland flowEdit


Antecedent soil moistureEdit


Subsurface return flowEdit


As it flows, the amount of runoff may be reduced in a number of possible ways: a small portion of it may evapotranspire; water may become temporarily stored in microtopographic depressions; and a portion of it may infiltrate as it flows overland. Any remaining surface water eventually flows into a receiving water body such as a river, lake, estuary or ocean.[5]

Human influenceEdit

Precipitation washing contaminates into local streams
Urban surface water runoff

Urbanization increases surface runoff by creating more impervious surfaces such as pavement and buildings that do not allow percolation of the water down through the soil to the aquifer. It is instead forced directly into streams or storm water runoff drains, where erosion and siltation can be major problems, even when flooding is not. Increased runoff reduces groundwater recharge, thus lowering the water table and making droughts worse, especially for agricultural farmers and others who depend on the water wells.



As humans continue to alter the climate through the addition of greenhouse gases to the atmosphere, precipitation patterns are expected to change as the atmospheric capacity for water vapor increases. This will have direct consequences on runoff amounts.[7]

Effects of surface runoffEdit

Erosion and depositionEdit

Surface runoff can cause erosion of the Earth's surface; eroded material may be deposited a considerable distance away. There are four main types of soil erosion by water: splash erosion, sheet erosion, rill erosion and gully erosion. Splash erosion is the result of mechanical collision of raindrops with the soil surface: soil particles which are dislodged by the impact then move with the surface runoff. Sheet erosion is the overland transport of sediment by runoff without a well defined channel. Soil surface roughness causes may cause runoff to become concentrated into narrower flow paths: as these incise, the small but well-defined channels which are formed are known as rills. These channels can be as small as one centimeter wide or as large as several meters. If runoff continue to incise and enlarge rills, they may eventually grow to become gullies. Gully erosion can transport large amounts of eroded material in a small time period.

Soil erosion by water on intensively-tilled farmland.




Environmental effectsEdit


Surface runoff occurring within forests can supply lakes with high loads of mineral nitrogen and phosphorus leading to eutrophication. Runoff waters within coniferous forests are also enriched with humic acids and can lead to humification of water bodies [10] Additionally, high standing and young islands in the tropics and subtropics can undergo high soil erosion rates and also contribute large material fluxes to the coastal ocean. Such land derived runoff of sediment nutrients, carbon, and contaminants can have large impacts on global biogeochemical cycles and marine and coastal ecosystems.[11]


Agricultural issuesEdit

The other context of agricultural issues involves the transport of agricultural chemicals (nitrates, phosphates, pesticides, herbicides, etc.) via surface runoff. This result occurs when chemical use is excessive or poorly timed with respect to high precipitation. The resulting contaminated runoff represents not only a waste of agricultural chemicals, but also an environmental threat to downstream ecosystems.



Mitigation and treatmentEdit

Runoff holding ponds (Uplands neighborhood of North Bend, Washington)

Mitigation of adverse impacts of runoff can take several forms:



Flood control programs as early as the first half of the twentieth century became quantitative in predicting peak flows of riverine systems. Progressively strategies have been developed to minimize peak flows and also to reduce channel velocities. Some of the techniques commonly applied are: provision of holding ponds (also called detention basins or balancing lakes) to buffer riverine peak flows, use of energy dissipators in channels to reduce stream velocity and land use controls to minimize runoff.[16]


The U.S. Clean Water Act (CWA) requires that local governments in urbanized areas (as defined by the Census Bureau) obtain stormwater discharge permits for their drainage systems.[17][18] Essentially this means that the locality must operate a stormwater management program for all surface runoff that enters the municipal separate storm sewer system ("MS4"). EPA and state regulations and related publications outline six basic components that each local program must contain:

  • Post-construction (i.e. permanent) stormwater management controls


Measurement and mathematical modelingEdit




See alsoEdit


  1. ^ Ronnie Wilson, The Horton Papers (1933)
  7. ^ Wigley T.M.L & Jones P.D (1985). "Influences of precipitation changes and direct CO2 effects on streamflow". Letters to Nature. 314 (6007): 149–152. doi:10.1038/314149a0.
  11. ^ Renee K. Takesue,Curt D. Storlazzi. Sources and dispersal of land-based runoff from small Hawaiian drainages to a coral reef: Insights from geochemical signatures. Estuarine, Coastal and Shelf Science Journal. 2/13/17
  13. ^ U.S. Environmental Protection Agency (EPA). "Impervious Cover." Ecosystems Research Division, Athens, GA. 2009-02-24. Archived May 9, 2009, at the Wayback Machine
  14. ^ Urban Runoff, City of Santa Monica website. Retrieved 29 July 2007.
  15. ^ Maryland Department of Environment. Baltimore, MD. "Erosion and Sediment Control and Stormwater Management in Maryland." 2007. Archived September 12, 2008, at the Wayback Machine
  17. ^ United States. Code of Federal Regulations, 40 CFR 122.26

Further readingEdit


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