Managing stormwater is one of the biggest and most expensive problems facing cities across the United States. Stormwater is in part a water quantity problem, and for decades the focus of local governments and public works officials was on how to engineer solutions to move rainwater rapidly away from urban areas to avoid the economic damages of flooding. Stormwater also is a pollution problem. As it moves across the surface of the land, stormwater picks up toxic contaminants, oil and grease, organic material, and other substances, which may be directly discharged into streams, thus delivering pollutants into nearby waterways. Or, it may enter the public sewer system through storm drains, and then the water quantity and water quality problems are joined in the water infrastructure system. Cities face dual challenges in managing stormwater—how to prevent or minimize stormwater entering sewers in the first place, thus preventing overflows from the start, and how to remediate overflows that occur. For a variety of reasons, many communities are exploring the use of so-called "green infrastructure" to address both types of challenges. Green infrastructure systems and practices use or mimic natural processes to infiltrate, evapotranspire, and/or harvest stormwater on or near the site where it is generated in order to reduce flows to municipal sewers. There are many success stories in communities around the country, each different, but there also are a number of issues about feasibility, sustainability, and cost-effectiveness. When rainwater falls, some of the water is absorbed into the ground, and the rest flows along the surface as runoff into rivers and streams. In forested areas, with porous and varied terrain, about half of rainfall infiltrates into the ground, where it recharges groundwater. About 40% returns to the atmosphere through evapotranspiration, and the remaining 10% flows along the surface as runoff. Unlike forested areas, urbanized areas often have around 45% or more of land surface that is impervious to rainfall, due to hard surfaces such as parking lots, roads, and rooftops. When rain hits impervious cover, it is unable to absorb into the ground and instead flows quickly into sewers and ditches and directly into rivers and streams. The Environmental Protection Agency (EPA) estimates that because of impervious surfaces such as pavement and rooftops, a typical city block generates five times more runoff than a woodland area of the same size, while only about 15% infiltrates into the ground for groundwater recharge. (See Figure 1 .) Further, the increased amount of runoff also increases pollutant loads that are harmful to aquatic life and public health into streams, rivers, and lakes. Pollutants can include sediment; oil, grease, and toxic chemicals from motor vehicles; road salts; pesticides and nutrients from lawns and gardens; bacteria and pathogens from pet waste and failing septic systems; and heavy metals from roofs, cars, and other sources. Stormwater runoff that does not directly enter streams and rivers instead enters public sewer systems. Two types of public sewer systems predominate in the United States: combined sewer systems and sanitary sewer systems. Combined sewers convey domestic and industrial wastewaters and stormwater runoff through a single pipe system to a wastewater treatment facility. Nationally, about 750 cities operate combined sewer systems. Most of these are located in the upper Midwest, mid-Atlantic states, and New England. During wet weather events, such as rainfall and snowmelt, the combined volume of wastewater and stormwater runoff entering combined sewers often exceeds conveyance capacity. Most combined systems are designed to discharge when the capacity of the sewer is exceeded. When this occurs, the untreated overflow is discharged directly to nearby surface waters, onto city streets, or as backups in basements prior to the wastewater treatment plant. Overflows pose particularly significant risks to human health when the discharges occur near sources of drinking water. Some combined sewer systems discharge infrequently, while others discharge every time that it rains. Combined sewer overflows (CSOs) are subject to permit requirements under the Clean Water Act (CWA). Permits authorizing discharges from CSO outfalls must include technology-based effluent limits. Since the beginning of the 20 th century, U.S. municipalities have generally constructed sanitary sewer systems, rather than combined sewer systems. Sanitary sewer systems convey domestic and industrial wastewater, but not large amounts of stormwater runoff, to a wastewater treatment works. Separate sanitary sewers are located in all 50 states, but are concentrated in the eastern half of the United States and on the West Coast. Areas served by sanitary sewer systems often have a municipal separate storm sewer system (MS4) to collect and convey runoff from rainfall and snowmelt. Overflows from sanitary sewers and separate storm sewers also can occur, as a result of blockages, line breaks, or sewer defects that allow excess stormwater and groundwater to overload the system. Discharges from MS4s serving more than 100,000 persons and smaller MS4 systems in urbanized areas also are subject to CWA permit requirements. Operators of these systems must implement stormwater best management practices that include public education, eliminating illicit discharges, and control of construction site and post-construction runoff. Traditional, or "gray," infrastructure systems for managing stormwater consist of pipes, storm drains, and concrete storage tanks. These systems are expensive to construct and maintain. EPA estimates that funding needs for stormwater management and projects to correct sewers that overflow total $106 billion over the next 20 years. Thus, the high cost of construction is one challenge with gray infrastructure that has led to considering options that are less costly. As a result, technologies or practices called green infrastructure are receiving increased attention. Green infrastructure, also known as Low Impact Development (LID), generally refers to the use of the natural landscape, instead of engineered structures, to capture and treat rainwater where it falls. EPA has defined it as using "natural hydrologic features to manage water and provide environmental and community benefits." At its heart, green infrastructure is a demand management technique that eliminates a portion of the stormwater entering municipal sewers or waterways, thereby raising the capacity of the sewer system by lowering pressure on it. Green infrastructure includes, but is not limited to, green roofs, downspout disconnection, trees and tree boxes, rain gardens, vegetated swales, pocket wetlands, infiltration planters, vegetated median strips, curb extensions, permeable pavements, reforestation, and protection and enhancement of riparian buffers and floodplains. (For images of some of these practices, see Appendix A .) Proponents contend that cities can downsize their gray infrastructure, extend its lifetime, save money, create green jobs, and enhance livability. These technologies do not entirely eliminate the need for gray infrastructure, but they can complement or supplement conventional infrastructure. Green infrastructure can alleviate local urban flooding by minimizing runoff volume and peak discharges. In addition, the infiltration, evapotranspiration, and slow release associated with green infrastructure approaches can control flood flows throughout a watershed. The economic benefits are a combination of the decreased costs of damage resulting from flooding and the reduced cost of constructing stormwater management and drainage infrastructure. The growing interest in green infrastructure practices is driven to a great extent by arguments that it is a cost-effective way to manage urban stormwater problems, particularly compared with costs of gray infrastructure. Cities with combined sewer systems have documented that the use of green infrastructure practices to reduce runoff volume is cost-competitive with conventional stormwater and CSO controls. In general, recent examples indicate that properly scaled and sited green infrastructure can deliver equivalent hydrological management of runoff as conventional stormwater infrastructure at comparable or lower costs. It has been estimated that green infrastructure is 5%-30% less costly to construct and about 25% less costly over its life cycle than traditional infrastructure. Several examples are described in the following box. In addition to controlling stormwater volume, a number of other benefits are frequently cited by advocates. Reducing Energy Costs. Green roofs provide insulation and shade for buildings, thus reducing their need for both heating and cooling costs. Water harvesting and reuse reduce the energy consumption of water utilities for conveyance and treatment. Preventing Disease and Protecting Local Economies. Green infrastructure practices can reduce pollutant loadings to waterways, which can help to minimize illness from recreational contact or consuming contaminated drinking water. Other Non-Water Benefits. Other non-water benefits include improved air quality (trees and plants filter the air, capturing pollution in their leaves and on their surfaces), reduced atmospheric carbon dioxide (green roof vegetation sequesters carbon), and lowered air temperature (trees and plants cool the air through evapotranspiration). Green roofs and lighter-colored surfaces in urban areas reflect more sunlight and absorb less heat, thus reducing the heat island effect (an urban heat island is a metropolitan area with large amounts of impervious surfaces, which is warmer than nearby suburban and rural areas). Green infrastructure is believed to improve urban aesthetics, increase property values, and provide wildlife habitat and recreational space for urban residents. Despite growing enthusiasm for these practices, a number of obstacles and challenges to integrating green infrastructure into stormwater programs have been identified. Overall, when considering green infrastructure options, decisionmakers confront risk and uncertainty related to skepticism regarding the ability or consistency with which practices deliver the level of benefits expected, and uncertainty that investing in green infrastructure will deliver better returns than more traditional practices. Many observers believe that the biggest barriers are lack of information on performance and cost-effectiveness, and uncertainty whether green infrastructure will contribute to achieving water quality improvements. Some of the obstacles are technical. For example, green infrastructure is not suitable in areas where soils don't drain, slopes are too steep, or where there just is not enough space.