Stormwater is water that originates during precipitation events. It may also be used to apply to water that originates with snowmelt that enters the stormwater system. Stormwater that does not soak into the ground becomes surface runoff, which either flows directly into surface waterways or is channeled into storm sewers, which eventually discharge to surface waters.
Stormwater is of concern for two main issues: one related to the volume and timing of runoff water (flood control and water supplies) and the other related to potential contaminants that the water is carrying, i.e. water pollution.
Stormwater is also a resource and ever growing in importance as the world's human population demand exceeds the availability of readily available water. Techniques of stormwater harvesting with point source water management and purification can potentially make urban environments self-sustaining in terms of water.
Since humans began living in concentrated village or urban settings, stormwater runoff has been an issue. During the Bronze Age, housing took a more concentrated form, and impervious surfaces emerged as a factor in the design of early human settlements. Some of the early incorporation of stormwater engineering is evidenced in ancient Greece.
Stormwater pollution 
Because impervious surfaces (parking lots, roads, buildings, compacted soil) do not allow rain to infiltrate into the ground, more runoff is generated than in the undeveloped condition. This additional runoff can erode watercourses (streams and rivers) as well as cause flooding after the stormwater collection system is overwhelmed by the additional flow. Because the water is flushed out of the watershed during the storm event, little infiltrates the soil, replenishes groundwater, or supplies stream baseflow in dry weather.
Pollutants entering surface waters during precipitation events is termed polluted runoff. Daily human activities result in deposition of pollutants on roads, lawns, roofs, farm fields, etc. When it rains or there is irrigation, water runs off and ultimately makes its way to a river, lake, or the ocean. While there is some attenuation of these pollutants before entering the receiving waters, the quantity of human activity results in large enough quantities of pollutants to impair these receiving waters.
Stormwater runoff as a source of pollution 
In addition to the pollutants carried in stormwater runoff, urban runoff is being recognized as a cause of pollution in its own right. In natural catchments (watersheds) surface runoff entering waterways is a relatively rare event, occurring only a few times each year and generally after larger storm events. Before development occurred most rainfall soaked into the ground and contributed to groundwater recharge or was recycled into the atmosphere by vegetation through evapotranspiration.
Modern drainage systems which collect runoff from impervious surfaces (e.g., roofs and roads) ensure that water is efficiently conveyed to waterways through pipe networks, meaning that even small storm events result in increased waterway flows.
In addition to delivering higher pollutants from the urban catchment, increased stormwater flow can lead to stream erosion, encourage weed invasion, and alter natural flow regimes. Native species often rely on such flow regimes for spawning, juvenile development, and migration.
In some areas, especially along the U.S. coast, polluted runoff from roads and highways may be the largest source of water pollution. For example, about 75 percent of the toxic chemicals getting to Seattle, Washington's Puget Sound are carried by stormwater that runs off paved roads and driveways, rooftops, yards, and other developed land.
Stormwater management 
Managing the quantity and quality of stormwater is termed, "Stormwater Management." The term Best Management Practice (BMP) is often used to refer to both structural or engineered control devices and systems (e.g. retention ponds) to treat polluted stormwater, as well as operational or procedural practices. Stormwater management includes both technical and institutional aspects, including:
• manage stormwater to control flooding and erosion;
• manage and control hazardous materials to prevent release of pollutants into the environment (source control);
• plan and construct stormwater systems so contaminants are removed before they pollute surface waters or groundwater resources;
• acquire and protect natural waterways where they still exist or can be rehabilitated;
• build "soft" structures such as ponds, swales or wetlands or newer Green Infrastructure solutions to work with existing or "hard" drainage structures, such as pipes and concrete channels;
• revise current stormwater regulations to address comprehensive stormwater needs;
• develop funding approaches to stormwater programs potentially including stormwater user fees;
• develop long-term asset management programs to repair and replace aging infrastructure;
• enhance and enforce existing ordinances to make sure property owners consider the effects of stormwater before, during and after development of their land;
• educate a community about how its actions affect water quality, and about what it can do to improve water quality; and
• plan carefully to create solutions before problems become too great.
Integrated water management 
Integrated water management (IWM) of stormwater has the potential to address many of the issues affecting the health of waterways and water supply challenges facing the modern urban city.
Also known as low impact development in the United States, or Water Sensitive Urban Design (WSUD) in Australia, IWM has the potential to improve runoff quality, reduce the risk and impact of flooding and deliver an additional water resource to augment potable supply.
The development of the modern city often results in increased demands for water supply due to population growth, while at the same time altered runoff predicted by climate change has the potential to increase the volume of stormwater that can contribute to drainage and flooding problems. IWM offers several techniques including stormwater harvest (to reduce the amount of water that can cause flooding), infiltration (to restore the natural recharge of groundwater), biofiltration or bioretention (e.g., rain gardens) to store and treat runoff and release it at a controlled rate to reduce impact on streams and wetland treatments (to store and control runoff rates and provide habitat in urban areas).
There are many ways of achieving low impact development (LID). The most popular is to incorporate land-based solutions to reduce stormwater runoff through the use of retention ponds, bioswales, infiltration trenches, sustainable pavements (such as pervious concrete), and others noted above. LID can also be achieved by utilizing engineered, manufactured products to achieve similar, or potentially better, results as land-based systems (underground storage tanks, stormwater treatment systems, biofilters, etc.). The proper LID solution is one that balances the desired results (controlling runoff and pollution) with the associated costs (loss of usable land for land-based systems versus capital cost of manufactured solution). Green (vegetated) roofs are also another low cost solution.
IWM as a movement can be regarded as being in its infancy and brings together elements of drainage science, ecology and a realization that traditional drainage solutions transfer problems further downstream to the detriment of our environment and precious water resources.
Regulation in the United States 
Federal requirements 
In the United States, the Environmental Protection Agency (EPA) is charged with regulating stormwater pursuant to the Clean Water Act (CWA). The goal of the CWA is to restore all "Waters of the United States" to their "fishable" and "swimmable" conditions. Point source discharges, which originate mostly from municipal wastewater (sewage) and industrial wastewater discharges, have been regulated since enactment of the CWA in 1972. Pollutant loadings from these sources are tightly controlled and limited. However, despite these controls, thousands of water bodies in the U.S. remain classified as "impaired," meaning that they contain pollutants at levels higher than is considered safe by EPA for the intended beneficial use of the water. Much of this impairment is due to polluted runoff.
Under the CWA, point source discharges to "Waters of the United States" require National Pollution Discharge Elimination System (NPDES) permits. To address the nationwide problem of stormwater pollution, Congress broadened the CWA definition of "point source" in 1987 to include industrial stormwater discharges and municipal separate storm sewer systems ("MS4"). These facilities were required to obtain NPDES permits. This 1987 expansion was promulgated in two phases: Phase I and Phase II. Phase I required that all municipalities of 100,000 persons or more, industrial dischargers, and construction sites of 5 acres (20,000 m2) or more have NPDES permits for their stormwater discharges. Phase I permits were issued in much of the U.S. in 1991. Phase II required that all municipalities, industrial dischargers, construction sites of 1 acre (4,000 m2) or more, and other large property owners (such as school districts) have NPDES permits for their stormwater discharges. Phase II rules came into effect in 2003.
EPA issued a new Construction General Permit (CGP) in July 2008. This permit expires in 2011 and continues the provisions of the previous permit. In December 2009 EPA issued new discharge standards, called effluent guidelines, for construction sites. These requirements set a new national minimum standard for erosion controls and sediment controls, and pollution prevention measures. The effluent guideline provisions will be incorporated into the next round of EPA and state general permits.
State and local requirements 
EPA has authorized 46 states to issue NPDES permits. In addition to implementing the NPDES requirements, many states and local governments have enacted their own stormwater management laws and ordinances, and some have published stormwater treatment design manuals. Some of these state and local requirements have expanded coverage beyond the federal requirements. For example, the State of Maryland requires erosion and sediment controls on construction sites of 5,000 sq ft (460 m2) or more. It is not uncommon for state agencies to revise their requirements and impose them upon counties and cities; daily fines ranging as high as $25,000 can be imposed for failure to modify their local stormwater permitting for construction sites, for instance. REF:fines REF: CASQ
Nonpoint source pollution management 
Agricultural runoff (except for concentrated animal feeding operations, or "CAFO") is considered by the CWA to be nonpoint source pollution. It is not included in the CWA definition of "point source" and therefore not subject to NPDES permit requirements. The 1987 CWA amendments established a non-regulatory program at EPA for nonpoint source pollution management consisting of research and demonstration projects. Related programs are conducted by the Natural Resources Conservation Service (NRCS) in the U.S. Department of Agriculture.
Public education campaigns 
Education is a key component of stormwater management. A number of agencies and organizations have launched campaigns to teach the public about the problem, and how they can contribute to solving it.
The West Michigan Environmental Action Council (WMEAC) has coined the term Hydrofilth to describe stormwater pollution. This is part of their 15 to the River campaign. WMEAC has taken an active role in preventing Hydrofilth from striking again. They have created a rain barrel distribution program in which they teach people how to have and use a rain barrel in their backyard and educate them on the positive effects it has on combating storm water runoff. Also they have created a website teaching others about how to make their own rain garden. WMEAC as well as many other organizations such as the United States Environmental Protection Agency and Clean Water Action are educating people and communities about the problems with storm water runoff and its effects and what they can do in order to prevent further pollution of their waterways from it.
See also 
- Antecedent soil moisture
- Certified Professional in Erosion and Sediment Control
- Detention basin and Retention basin
- First flush
- Line source
- Nationwide Urban Runoff Program (U.S. research program)
- Permeable paving
- Sanitary sewer overflow
- Stormwater detention vault
- Treatment wetlands
- Volumetric flow rate
- Stanley W. Trimble (2007) Encyclopedia of Water Science, CRC Press, 1586 pages ISBN 0-8493-9627-1
- C. Michael Hogan, "Phaistos Fieldnotes." The Modern Antiquarian (2007).
- Schueler, Thomas R. "The Importance of Imperviousness." Reprinted in The Practice of Watershed Protection. 2000. Center for Watershed Protection, Ellicott City, MD. Document no. ELC_PWP1.pdf
- Washington State Department of Ecology. “Control of Toxic Chemicals in Puget Sound, Phase 2: Development of Simple Numerical Models", 2008
- Washington State Department of Ecology (2005). Olympia, WA. "Stormwater Management Manual for Western Washington." Publication No. 05-10-029.
- Municipal Stormwater Management, Tom Debo and Andrew Reese (2nd ed. 2003). CRC Press, Boca Raton, FL.  ISBN 1-56670-584-3, 1141 pages
- "Water Sensitive Urban Design - Melbourne Water". Wsud.melbournewater.com.au. Retrieved 2011-12-05.
- Federal Water Pollution Control Amendments of 1972, P.L. 92-500.
- Water Quality Act of 1987, P.L. 100-4. Added CWA section 402(p), 33 U.S.C. § 1342(p).
- U.S. Environmental Protection Agency (EPA). Washington, DC. "Construction General Permit"
- EPA (2009-12-01). "Construction and Development: Final Effluent Guidelines."
- EPA (June 2009). "EPA Unveils Watershed Central." Nonpoint Source News-Notes. #87. 1-3.
- EPA (2008). "NPDES State Program Status." Accessed 2010-02-10.
- Maryland Department of the Environment (2009). Baltimore, MD. "Maryland Stormwater Design Manual."
- State of Maryland. Code of Maryland Regulations (COMAR). Activities for Which Approved Erosion and Sediment Control Plans are Required. Sec. 26.17.01.05.
- "RainGardens.org". RainGardens.org. Retrieved 2011-12-05.
Further reading 
- Ferguson, Bruce K. (1998). Introduction to Stormwater. New York: John Wiley and Sons. ISBN 978-0-471-16528-6.
- Debo and Andrew Reese, Tom (2003). Municipal Stormwater Management. Boca Raton FL: CRC Press. p. 1141. ISBN 1-56670-584-3.
- Stormwater at the Open Directory Project
- Center for Watershed Protection
- EPA Stormwater Permit Program
- Permit Requirements for Tribal Construction Projects
- Homeowner's Guide to Rainfall - Toronto *Stormwater Best Management Practice Maintenance, Kansas State University
- StormCon - World's Largest Stormwater Pollution Prevention Conference
- Stormwater Magazine - a stormwater trade journal
- Storm Water Solutions magazine - a stormwater trade publication
- Pervious Concrete Blog - Discussion on the latest in Pervious Concrete Technology
- Stormwater Harvesting Applications - Examples of Stormwater harvesting in large and small scale
- International Stormwater Best Management Practices (BMP) Database
- Minnesota Stormwater Manual (cold weather stormwater management)