Surface runoff is a term used to describe when soil is infiltrated to full capacity and excess water, from rain, snowmelt, or other sources flow over the land. This is a major component of the water cycle or the hydrological cycle.
A land area that produces runoff draining to a common point is called a watershed. When runoff flows along the ground, it can pick up soil contaminants such as petroleum, pesticides (in particular herbicides and insecticides), or fertilizers that become discharge or nonpoint source pollution. Runoff that occurs on surfaces before reaching a channel is also called a nonpoint source. If a nonpoint source contains man-made contaminants, the runoff is called nonpoint source pollution.
In urban areas, the foremost concern about runoff is contamination and safety. Therefore, stormwater is removed from the surface quickly and efficiently through draining. However, once off the surface and out of sight, stormwater is overlooked; consequences of stormwater impacts on water quality further down the line is looked over. Some consequences include flooding and erosion.
Generation of Surface Runoff
Surface runoff can be generated either by rainfall or by the melting of snow or glaciers. Based on the hydrological cycle, runoff is flow from a drainage basin or watershed that appears in surface streams. The flow is made by parts of precipitation that falls directly into the stream, surface runoff, subsurface runoff, and groundwater runoff. Surface runoff has the greatest capacity to carry pollutants into the water flow because on the surface, it is in contact with the greatest amount of pollutants. Also, unlike subsurface and groundwater runoff, surface runoff does not undergo any filtering through soils. Urban stormwater runoff has been identified as a leading cause of waterway impairment for many pollutants.
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, proteoid roots with their extremely dense networks of root hairs can absorb so much rainwater so that runoff is prevented even with substantial amounts of rain fall. In these regions, even on relatively less infertile cracking clay soils, high amounts of rainfall and low potential evaporation are needed to generate any surface runoff, leading to specialized adaptations to extremely variable (usually ephemeral) streams.
Snow 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.
Infiltration excess overland flow
Infiltration excess overland flow 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 infiltration 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 heavily paved or urban areas.
Saturation excess overland flow
When the soil 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 saturation excess overland flow or saturated overland flow.
Subsurface return flow
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 or interflow.
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 become run-on, which is the infiltration of runoff as it flows overland. Any remaining surface water eventually flows into a receiving water body such as a river, lake, estuary or ocean.
Effects of surface runoff
Surface runoff causes erosion of the earth's surface. There are four principal types of erosion: splash erosion, gully erosion, sheet erosion and stream bed erosion. Splash erosion is the result of mechanical collision of raindrops with the soil surface. Dislodged soil particles become suspended in the surface runoff and are carried into streams and rivers. Gully erosion occurs when the power of runoff is strong enough to cut a well defined channel. These channels can be as small as one centimeter wide or as large as several meters. Sheet erosion is the overland transport of runoff without a well defined channel. In the case of gully erosion, large amounts of material can be transported in a small time period. Stream bed erosion is the attrition of stream banks or bottoms by rapidly flowing rivers or creeks.
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 .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 cause loss of the fertile top soil and reduces the its fertility and quality of the agricultural produce.
Human impact on surface 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 farmers and others who depend on water wells.
When anthropogenic contaminants are dissolved or suspended in runoff, the human impact is expanded in creating 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.
On the other hand, a contrarian could note there is considerable surface runoff in natural systems from animal wastes being entrained in runoff or from natural sediment loading in the absence of human alteration of the land. However, the most pernicious consequences to human health and ecosystems are from runoff issues related human intervention; however, in underdeveloped countries the proportion of runoff attributable to natural factors has greater dominance, principally due to the lack of isolation of water supplies from potential animal waste carrying runoff.
Environmental impacts of surface runoff on the biota
The principal environmental impacts on biota associated with runoff are the impacts to surface water, groundwater and soil 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 are petroleum substances, herbicides and fertilizers. Quantitative uptake by surface runoff of pesticides and other contaminants has been studied since the 1960s, and early on contact of pesticides with water was known to enhance phytotoxicity. 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 potable water supplies, they can be compromised regarding health risks 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 fish kills, or alter the balance of populations present. Other specific impacts are on animal mating, spawning, egg and larvae viability, juvenile survival and plant productivity. Some researches show surface runoff of pesticides, such as DDT, can alter the gender of fish species genetically, which transforms male into female fish .
Regarding groundwater, the foremost 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 relatively pristine soils, creating health or ecological consequences.
Surface Runoff of Mercury
One study specifically shows that urban stormwater runoff is a leading cause of waterway impairment for many pollutants. High frequency sampling was used to measure levels of mercury (Hg) export from a parking lot during individual rains. Results show that about 84% of mercury in the parking lot is flushed into the surroundings at just the first rain by surface runoff. Specifically, measuring Hg levels in Mimico Creek in Toronto, Canada (Lake Ontario), researchers discovered large external inputs of Hg. Although it is premature to conclude that the source of the mercury is from surface runoff, there is a significant relationship between surface runoff and the mercury levels in water.
Flooding occurs when a watercourse is unable to convey the quantity of runoff flowing downstream. The frequency with which this occurs is described by a return period. Flooding is a natural process, which maintains ecosystem composition and processes, but it can also be altered by land use changes such as river engineering. Floods can be both beneficial or harmful to societies. Agriculture along the Nile 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. Adverse impacts span loss of life, property damage, contamination of water supplies, loss of crops, and social dislocation and temporary homelessness. Floods can be among the most devastating of natural disasters.
A common topic with runoff is agriculture. When farmland is tilled and bare soil is revealed, rainwater carries billions of tons of topsoil into waterways each year, causing loss of valuable topsoil and adding sediment to produce turbidity in surface waters.
Another concern with 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. The alternative to conventional farming is organic farming which eliminates or greatly reduces chemical usage.
Mitigation and treatment
Mitigation of adverse impacts of runoff can take several forms:
- Land use development controls aimed at minimizing impervious surfaces in urban areas
- Erosion controls for farms and construction sites
- Flood control programs
- Chemical use and handling controls in agriculture, landscape maintenance, industrial use, etc.
Regarding Land use controls, the U.S. Environmental Protection Agency and others have encouraged research on methods of minimizing total surface runoff by avoiding unnecessary hardscape. Many municipalities have produced guidelines and codes for land developers that encourage minimum width sidewalks, use of pavers set in earth for driveways and walkways and other design techniques to allow maximum water infiltration in urban settings. An example land use control program can be viewed at seen in the city of 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 local governments began to focus their efforts on mitigation of construction runoff by requiring builders to implement erosion and sediment controls (ESCs). This included such techniques as: use of straw bales and barriers to slow runoff on slopes, installation of silt 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.
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 to buffer riverine peak flows, use of energy dissipators in channels to reduce stream velocity and land use controls to minimize runoff.
Chemical use and handling has become a focal point mainly since passage of NEPA in the U.S. 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 or stormwater sewers.
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. 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:
- Public education (informing individuals, households, businesses about ways to avoid stormwater pollution)
- Public involvement (support public participation in implementation of local programs)
- Illicit discharge detection & elimination (removing sanitary sewer or other non-stormwater connections to the MS4)
- Construction site runoff controls (i.e. erosion & sediment controls)
- Post-construction (i.e. permanent) stormwater management controls
- Pollution prevention and "good housekeeping" measures (e.g. system maintenance).
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.
- Arnold, Jr., Chester L., and C. J. Gibbons. "Impervious Surface Coverage: The Emergence of a Key Environmental Indicator." Journal of the American Planning Association 62 (1996): 243-58.
- Eckley, Chris S., and Brian Branfireun. "Mercury Mobilization in Urban Stormwater Runoff." Science of the Total Environment 403 (2008): 164-77.
- "Hydrological Cycle." National Oceanic and Atmospheric Administration. 8 Dec. 2005. NOAA. 7 Oct. 2008.
- Saey, Tina H. "DDT Treatment Turns Male Fish into Mothers." Science News 157 (2000): 87.
- Stages of Erosion. Digital image. Dartmoor National Park Authority. 9 Nov. 2007. Dartmoor National Park. 11 Dec. 2008.
- All text is available under the terms of the GNU Free Documentation License.