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Surface runoff (also known as overland flow or terrestrial runoff) is the unconfined flow of water over the ground surface, in contrast to channel runoff (or stream flow). It occurs when excess , stormwater, meltwater, or other sources, can no longer sufficiently rapidly infiltrate in the soil. This can occur when the soil is saturated by water to its full capacity, and the rain arrives more quickly than the soil can absorb it. Surface runoff often occurs because areas (such as and Road surface) do not allow water to soak into the ground. Furthermore, runoff can occur either through natural or human-made processes.
Surface runoff is a major component of the water cycle. It is the primary agent of soil erosion by water.Ronnie Wilson, The Horton Papers (1933)Keith Beven, Robert E. Horton's perceptual model of infiltration processes, Hydrological Processes, Wiley Intersciences DOI 10:1002 hyp 5740 (2004) 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 human-made contaminants or natural forms of pollution (such as rotting leaves). Human-made contaminants in runoff include petroleum, , and others.L. Davis Mackenzie and Susan J. Masten, Principles of Environmental Engineering and Science Much agricultural pollution is exacerbated by surface runoff, leading to a number of down stream impacts, including nutrient pollution that causes eutrophication.
In addition to causing water erosion and pollution, surface Urban runoff is a primary cause of urban flooding, which can result in property damage, damp and mold in basements, and street flooding.
Snowmelt and glacier melt occur only in areas cold enough for these to form permanently. Typically snowmelt will peak in the spring and glacier melt in the summer, leading to pronounced flow maxima in rivers affected by them. The determining factor of the rate of melting of snow or glaciers is both air temperature and the duration of sunlight. In high mountain regions, streams frequently rise on sunny days and fall on cloudy ones for this reason.
In areas where there is no snow, runoff will come from rainfall. However, not all rainfall will produce runoff because storage from soils can absorb light showers. On the extremely ancient soils of Australia and Southern Africa,McMahon T.A. and Finlayson, B.; Global Runoff: Continental Comparisons of Annual Flows and Peak Discharges with their extremely dense networks of root hairs can absorb so much rainwater as to prevent runoff even with substantial amounts of rainfall. In these regions, even on less infertile Vertisol, high amounts of rainfall and potential evaporation are needed to generate any surface runoff, leading to specialised adaptations to extremely variable (usually ephemeral) streams.
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 surface water body such as a river, lake, estuary or ocean.Nelson, R. (2004). The Water Cycle. Minneapolis: Lerner.
When anthropogenic contaminants are dissolved or suspended in runoff, the human impact is expanded to create water pollution. This pollutant load can reach various receiving waters such as streams, rivers, lakes, estuaries and oceans with resultant water chemistry changes to these water systems and their related ecosystems.
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.
Reduced crop productivity usually results from erosion, and these effects are studied in the field of soil conservation. The soil particles carried in runoff vary in size from about 0.001 millimeter to 1.0 millimeter in diameter. Larger particles precipitate over short transport distances, whereas small particles can be carried over long distances suspended in the water column. Erosion of silty soils that contain smaller particles generates turbidity and diminishes light transmission, which disrupts aquatic ecosystems.
Entire sections of countries have been rendered unproductive by erosion. On the high central plateau of Madagascar, approximately ten percent of that country's land area, virtually the entire landscape is devoid of vegetation, with erosive gully furrows typically in excess of 50 meters deep and one kilometer wide. Shifting cultivation is a farming system which sometimes incorporates the slash and burn method in some regions of the world. Erosion causes loss of the fertile top soil and reduces its fertility and quality of the agricultural produce.
Modern industrial farming is another major cause of erosion. Over a third of the U.S. Corn Belt has completely lost its topsoil. Switching to No-till farming practices would reduce soil erosion from U.S. agricultural fields by more than 70 percent.
In the case of surface waters, the impacts translate to water pollution, since the streams and rivers have received runoff carrying various chemicals or sediments. When surface waters are used as drinking water supplies, they can be compromised regarding risk assessment and drinking water aesthetics (that is, odor, color and turbidity effects). Contaminated surface waters risk altering the metabolic processes of the aquatic species that they host; these alterations can lead to death, such as , or alter the balance of populations present. Other specific impacts are on animal mating, spawning, egg and viability, juvenile survival and plant productivity. Some research shows surface runoff of pesticides, such as DDT, can alter the gender of fish species genetically, which transforms male into female fish. Science News. "DDT treatment turns male fish into mothers." 2000-02-05. (By subscription only.)
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 and can lead to Humus of water bodies Klimaszyk Piotr, Rzymski Piotr "Surface Runoff as a Factor Determining Trophic State of Midforest Lake" Polish Journal of Environmental Studies, 2011, 20(5), 1203-1210 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.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
In the case of groundwater, the main issue is contamination of drinking water, if the aquifer is abstracted for human use. Regarding soil contamination, runoff waters can have two important pathways of concern. Firstly, runoff water can extract soil contaminants and carry them in the form of water pollution to even more sensitive aquatic habitats. Secondly, runoff can deposit contaminants on pristine soils, creating health or ecological consequences.
Land use controls. Many world regulatory agencies have encouraged research on methods of minimizing total surface runoff by avoiding unnecessary hardscape.U.S. Environmental Protection Agency (EPA). "Impervious Cover." Ecosystems Research Division, Athens, GA. 2009-02-24. Many municipalities have produced guidelines and codes (zoning and related Local ordinance) for that encourage minimum width sidewalks, use of pavers set in earth for and and other design techniques to allow maximum water infiltration in urban settings. An example of a local program specifying design requirements, construction practices and maintenance requirements for buildings and properties is in Santa Monica, California.
Erosion controls have appeared since medieval times when farmers realized the importance of contour farming to protect soil resources. Beginning in the 1950s these agricultural methods became increasingly more sophisticated. In the 1960s some state and began to focus their efforts on mitigation of construction runoff by requiring builders to implement erosion control and (ESCs). This included such techniques as: use of and barriers to slow runoff on slopes, installation of , programming construction for months that have less rainfall and minimizing extent and duration of exposed graded areas. Montgomery County, Maryland implemented the first local government sediment control program in 1965, and this was followed by a statewide program in Maryland in 1970.Maryland Department of Environment. Baltimore, MD. "Erosion and Sediment Control and Stormwater Management in Maryland." 2007.
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 or ) to buffer riverine peak flows, use of energy dissipators in channels to reduce stream velocity and land use controls to minimize runoff. Channel Stability Assessment for Flood Control Projects U.S. Army Corps of Engineers, (1996)
Chemical use and handling. Following enactment of the U.S. Resource Conservation and Recovery Act (RCRA) in 1976, and later the Water Quality Act of 1987, states and cities have become more vigilant in controlling the containment and storage of toxic chemicals, thus preventing releases and leakage. Methods commonly applied are: requirements for double containment of underground storage tanks, registration of hazardous materials usage, reduction in numbers of allowed pesticides and more stringent regulation of fertilizers and herbicides in landscape maintenance. In many industrial cases, pretreatment of wastes is required, to minimize escape of pollutants into sanitary sewer or storm sewer.
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.United States. Code of Federal Regulations, 40 CFR 122.26 EPA. Washington, D.C. "Stormwater Discharges From Municipal Separate Storm Sewer Systems (MS4s)." 2009-03-11. 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:
In the 1950s or earlier, hydrology transport models appeared to calculate quantities of runoff, primarily for flood forecasting. Beginning in the early 1970s, computer models were developed to analyze the transport of runoff carrying water pollutants. These models considered Solvation rates of various chemicals, infiltration into soils, and the ultimate pollutant load delivered to surface water. One of the earliest models addressing chemical dissolution in runoff and resulting transport was developed in the early 1970s under contract to the United States Environmental Protection Agency (EPA).C.M. Hogan, Leda Patmore, Gary Latshaw, Harry Seidman et al. Computer modeling of pesticide transport in soil for five instrumented watersheds, U.S. Environmental Protection Agency Southeast Water laboratory, Athens, Ga. by ESL Inc., Sunnyvale, California (1973) This computer model formed the basis of much of the mitigation study that led to strategies for land use and chemical handling controls.
Increasingly, stormwater practitioners have recognized the need for Monte Carlo models to simulate stormwater processes because of natural variations in multiple variables affecting runoff quality and quantity. The benefit of the Monte Carlo analysis is not to decrease uncertainty in the input statistics but to represent the different combinations of the variables that determine potential risks of water-quality excursions. One example of this type of stormwater model is the stochastic empirical loading and dilution model (SELDM)
Other computer models have been developed (such as the DSSAM Model) that allow surface runoff to be tracked through a river course as reactive water pollutants. In this case, the surface runoff may be considered to be a line source of water pollution to the receiving waters.C.M.Hogan, Marc Papineau et al. Development of a dynamic water quality simulation model for the Truckee River, Earth Metrics Inc., Environmental Protection Agency Technology Series, Washington D.C. (1987)
/ref> is a stormwater quality model. SELDM is designed to transform complex scientific data into meaningful information about the risk of adverse effects of runoff on receiving waters, the potential need for mitigation measures, and the effectiveness of such management measures for reducing these risks. SELDM provides a method for rapid assessment of information that is otherwise difficult or impossible to obtain because it models the interactions among hydrologic variables (with different probability distributions), resulting in a population of values representing likely long-term outcomes from runoff processes and the potential effects of various mitigation measures. SELDM also provides the means for rapidly doing sensitivity analyses to determine the possible effects of varying input assumptions on the risks for water-quality excursions.
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