Surface runoff (also known asoverland flow orterrestrial runoff) is the unconfined flow ofwater over the ground surface, in contrast tochannel runoff (orstream flow). It occurs when excessrainwater,stormwater,meltwater, or other sources, can no longer sufficiently rapidly infiltrate in thesoil. This can occur when the soil issaturated by water to its full capacity, and the rain arrivesmore quickly than the soil can absorb it. Surface runoff often occurs becauseimpervious areas (such asroofs andpavement) do not allow water to soak into the ground. Furthermore, runoff can occur either through natural or human-made processes.[1]
Surface runoff is a major component of thewater cycle. It is the primary agent ofsoil erosion by water.[2][3] The land area producing runoff that drains to a common point is called adrainage basin.
Runoff that occurs on the ground surface before reaching achannel can be anonpoint source of pollution, as it can carry human-made contaminants or natural forms of pollution (such as rotting leaves). Human-made contaminants in runoff includepetroleum,pesticides,fertilizers and others.[4] Muchagricultural pollution is exacerbated by surface runoff, leading to a number of down stream impacts, includingnutrient pollution that causeseutrophication.
In addition to causing water erosion and pollution, surfacerunoff in urban areas is a primary cause ofurban flooding, which can result in property damage, damp and mold inbasements, and street flooding.
Surface runoff is defined asprecipitation (rain, snow, sleet, or hail[5]) that reaches a surface stream without ever passing below the soil surface.[6] It is distinct fromdirect runoff, which is runoff that reaches surface streams immediately after rainfall or melting snowfall and excludes runoff generated by the melting ofsnowpack or glaciers.[7]
Snow andglacier melt occur only in areas cold enough for these to form permanently. Typicallysnowmelt will peak in the spring[8] and glacier melt in the summer,[9] leading to pronounced flow maxima in rivers affected by them.[10] The determining factor of the rate of melting of snow or glaciers is both air temperature and the duration of sunlight.[11] 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 ofAustralia andSouthern Africa,[12]proteoid roots 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 infertilecracking clay soils, high amounts of rainfall and potential evaporation are needed to generate any surface runoff, leading to specialised adaptations to extremely variable (usually ephemeral) streams.
This occurs when the rate ofrainfall on a surface exceeds the rate at which water caninfiltrate the ground, and any depression storage has already been filled. This is also called Hortonian overland flow (afterRobert E. Horton),[13] or unsaturated overland flow.[14] This more commonly occurs inarid andsemi-arid regions, where rainfall intensities are high and thesoilinfiltration capacity is reduced because ofsurface sealing, or in urban areas where pavements prevent water from infiltrating.[15]
When thesoil is saturated and the depression storage filled, and rain continues to fall, the rainfall will immediately produce surface runoff. The level of antecedent soil moisture is one factor affecting the time until soil becomes saturated. This runoff is called saturation excess overland flow,[15] saturated overland flow,[16] or Dunne runoff.[17]
Soil retains a degree of moisture after arainfall. This residual water moisture affects the soil'sinfiltration capacity. During the next rainfall event, the infiltration capacity will cause the soil to be saturated at a different rate. The higher the level of antecedent soil moisture, the more quickly the soil becomes saturated. Once the soil is saturated, runoff occurs. Therefore, surface runoff is a significantly factor in the controlling of soil moisture after medium and low intensity storms.[18]
After water infiltrates the soil on an up-slope portion of a hill, the water may flow laterally through the soil, and exfiltrate (flow out of the soil) closer to a channel. This is called subsurface return flow orthroughflow.
As it flows, the amount of runoff may be reduced in a number of possible ways: a small portion of it mayevapotranspire; water may become temporarily stored in microtopographic depressions; and a portion of it mayinfiltrate as it flows overland. Any remaining surface water eventually flows into areceiving water body such as ariver,lake,estuary orocean.[19]
Urbanization increases surface runoff by creating moreimpervious surfaces such aspavement and buildings that do not allowpercolation of the water down through the soil to theaquifer. It is instead forced directly into streams orstorm water runoff drains, whereerosion andsiltation can be major problems, even when flooding is not. Increased runoff reducesgroundwater recharge, thus lowering thewater table and makingdroughts worse, especially for agricultural farmers and others who depend on thewater wells.[20]
When anthropogenic contaminants are dissolved or suspended in runoff, the human impact is expanded to createwater pollution. Thispollutant 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.[21]
As humans continue to alter the climate through the addition ofgreenhouse 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.[22]
Urban runoff is surface runoff of rainwater, landscape irrigation, and car washing[23] created byurbanization.Impervious surfaces (roads,parking lots andsidewalks) are constructed duringland development. Duringrain, storms, and otherprecipitation events, these surfaces (built from materials such asasphalt andconcrete), along withrooftops, carry pollutedstormwater tostorm drains, instead of allowing the water topercolate throughsoil.[24]
This causes lowering of thewater table (becausegroundwater recharge is lessened) andflooding since the amount of water that remains on the surface is greater.[25][26] Most municipal storm sewer systems discharge untreated stormwater tostreams,rivers, andbays. This excess water can also make its way into people's properties through basement backups and seepage through building wall and floors.
Urban runoff can be a major source ofurban flooding andwater pollution in urban communities worldwide.
Industrial stormwater is runoff from precipitation (rain, snow, sleet, freezing rain, or hail) that lands on industrial sites (e.g. manufacturing facilities, mines, airports). This runoff is oftenpolluted by materials that are handled or stored on the sites, and the facilities are subject to regulations to control the discharges.[27][28]
To manage industrial stormwater effectively, facilities use best management practices (BMPs) that aim to both prevent pollutants from entering the runoff and treat water before it's released from the site. Common preventive steps include maintaining clean workspaces, conducting routine equipment checks, storing materials properly, preventing spills, and training staff on pollution prevention techniques.[29]
To treat stormwater, facilities may install structural controls such as detention and retention ponds, constructed wetlands, filter systems, or oil-water separators. These systems help reduce pollution by settling out solids, filtering water, or supporting natural treatment processes before the water is discharged.[30]Surface runoff can causeerosion of the Earth's surface;eroded material may bedeposited a considerable distance away. There are four main types ofsoil erosion by water: splash erosion, sheet erosion,rill erosion andgully 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 ofsediment 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.
Reduced crop productivity usually results from erosion, and these effects are studied in the field ofsoil conservation. The soil particles carried in runoff vary in size from about 0.001 millimeter to 1.0 millimeter in diameter. Larger particlessettle over short transport distances, whereas small particles can be carried over long distances suspended in thewater column. Erosion of silty soils that contain smaller particles generatesturbidity and diminishes light transmission, which disruptsaquatic ecosystems.
Entire sections of countries have been rendered unproductive by erosion. On the high centralplateau ofMadagascar, approximately ten percent of that country's land area, virtually the entire landscape is devoid ofvegetation, with erosive gully furrows typically in excess of 50 meters deep and one kilometer wide.Shifting cultivation is a farming system which sometimes incorporates theslash 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 industrialfarming is another major cause of erosion. Over a third of the U.S.Corn Belt has completely lost itstopsoil.[31] Switching tono-till practices would reduce soil erosion from U.S. agricultural fields by more than 70 percent.[32]
The principal environmental issues associated with runoff are the impacts to surface water,groundwater andsoil through transport of water pollutants to these systems. Ultimately these consequences translate into human health risk, ecosystem disturbance and aesthetic impact to water resources. Some of the contaminants that create the greatest impact to surface waters arising from runoff arepetroleum substances,herbicides andfertilizers. Quantitative uptake by surface runoff ofpesticides and other contaminants has been studied since the 1960s, and early on contact of pesticides with water was known to enhancephytotoxicity.[33]
In the case of surface waters, the impacts translate towater pollution, since the streams and rivers have received runoff carrying various chemicals or sediments. When surface waters are used aspotable water supplies, they can be compromised regardinghealth risks and clean drinking water aesthetics (that is, odor, color andturbidity effects). Contaminated surface waters risk altering the metabolic processes of the aquaticspecies that they host; these alterations can lead to death, such asfish kills, or alter the balance of populations present. Other specific impacts are on animal mating, spawning,egg andlarvae viability, juvenile survival and plant productivity. Some research shows surface runoff of pesticides, such asDDT, can alter the gender of fish species genetically, which transforms male into female fish.[34]
Surface runoff occurring within forests can supply lakes with high loads of mineral nitrogen and phosphorus leading toeutrophication. Runoff waters withinconiferous forests are also enriched withhumic acids and can lead tohumification of water bodies[35] 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 globalbiogeochemical cycles and marine and coastal ecosystems.[36]
In the case of groundwater, the main issue is contamination of drinking water, if theaquifer is abstracted for human use. Regardingsoil 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.
The other context ofagricultural issues involves the transport ofagricultural 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. Pine straws are often used to protect soil fromsoil erosion and weed growth.[37] However, harvesting these crops may result in the increase of soil erosion.
Surface run-off results in a significant amount of economic effects. Pine straws are cost effective ways of dealing with surface run-off. Moreover, Surface run-off can be reused through the growth of elephant mass. InNigeria,elephant grass is considered to be an economical way in which surface run-off anderosion can be reduced.[38] Also,China has suffered significant impact from surface run-off to most of their economical crops such as vegetables. Therefore, they are known to have implemented a system which reduced loss of nutrients (nitrogen and phosphorus) in soil.[39]
Flooding occurs when a watercourse is unable to convey the quantity of runoff flowing downstream. The frequency with which this occurs is described by areturn period. Flooding is a natural process, which maintains ecosystem composition and processes, but it can also be altered byland use changes such as river engineering. Floods can be both beneficial to societies or cause damage. Agriculture along theNile floodplain took advantage of the seasonal flooding that deposited nutrients beneficial for crops. However, as the number and susceptibility of settlements increase, flooding increasingly becomes a natural hazard. In urban areas, surface runoff is the primary cause ofurban flooding, known for its repetitive and costly impact on communities.[40] Adverse impacts span loss of life, property damage, contamination of water supplies, loss of crops, and social dislocation and temporary homelessness. Floods are among the most devastating of natural disasters. The use ofsupplemental irrigation is also recognized as a significant way in which crops such as maize can retainnitrogen fertilizers in soil, resulting in improvement of crop water availability.[41]
Mitigation of adverse impacts of runoff can take several forms:
Land use controls. Many world regulatory agencies have encouraged research on methods of minimizing total surface runoff by avoiding unnecessaryhardscape.[42] Many municipalities have produced guidelines and codes (zoning and relatedordinances) forland developers that encourage minimum width sidewalks, use ofpavers set in earth fordriveways andwalkways and other design techniques to allow maximum waterinfiltration in urban settings. An example of a local program specifying design requirements, construction practices and maintenance requirements for buildings and properties is inSanta Monica, California.[43]
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 somestate andlocal governments began to focus their efforts on mitigation of construction runoff by requiring builders to implementerosion andsediment controls (ESCs). This included such techniques as: use ofstraw bales and barriers to slow runoff on slopes, installation ofsilt fences, 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.[44]
Flood control programs as early as the first half of the twentieth century became quantitative in predicting peak flows ofriverine 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 calleddetention basins orbalancing lakes) to buffer riverine peak flows, use of energy dissipators in channels to reduce stream velocity and land use controls to minimize runoff.[45]
Chemical use and handling. Following enactment of the U.S.Resource Conservation and Recovery Act (RCRA) in 1976, and later theWater 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 ofunderground storage tanks, registration ofhazardous 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 intosanitary orstormwater sewers.
The U.S.Clean Water Act (CWA) requires that local governments inurbanized areas (as defined by theCensus Bureau) obtainstormwater discharge permits for their drainage systems.[46][47] Essentially this means that the locality must operate astormwater management program for all surface runoff that enters the municipal separatestorm sewer system ("MS4"). EPA and state regulations and related publications outline six basic components that each local program must contain:
Other property owners which operate storm drain systems similar to municipalities, such as state highway systems, universities, military bases and prisons, are also subject to the MS4 permit requirements.
Runoff is analyzed by usingmathematical models in combination with variouswater quality sampling methods. Measurements can be made using continuous automated water quality analysis instruments targeted on pollutants such as specificorganic orinorganic chemicals,pH, turbidity, etc., or targeted on secondary indicators such asdissolved oxygen. Measurements can also be made in batch form by extracting a single water sample and conducting chemical or physical tests on that sample.
In the 1950s or earlier,hydrology transport models appeared to calculate quantities of runoff, primarily forflood forecasting. Beginning in the early 1970s, computer models were developed to analyze the transport of runoff carrying water pollutants. These models considereddissolution rates of various chemicals,infiltration into soils, and the ultimate pollutant load delivered toreceiving waters.One of the earliest models addressing chemical dissolution in runoff and resulting transport was developed in the early 1970s under contract to theUnited States Environmental Protection Agency (EPA).[48] Thiscomputer model formed the basis of much of the mitigation study that led to strategies forland 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 thestochastic empirical loading and dilution model (SELDM)[49][50] is astormwater 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.
Other computer models have been developed (such as theDSSAM 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 aline source ofwater pollution to the receiving waters.[51]
