SG WATER– Smart Growth Water Assessment Tool for Estimating Runoff

How and where development occurs can significantly affect stormwater runoff and pollution levels. Development patterns can change sedimentation and pollutant loadings and alter stormwater runoff rates. Extensive studies suggest ways to minimize water quality impacts from development by taking into account the site and design characteristics of particular developments.To help states and local governments better understand development-related water impacts in order to protect water quality while planning for growth, the U.S. Environmental Protection Agency (EPA) has developed the Smart Growth Water Assessment Tool for Estimating Runoff (SG WATER). The model can estimate levels of stormwater runoff and associated pollutants (nitrogen, phosphorus, and total suspended solids) from specific development sites or regions. SG WATER is a sketch tool for measuring and comparing water quality impacts from different development patterns, site designs, and site locations.SG WATER can be used to compare the water quality impacts from development scenarios defined by the user. As such, it can aid local governments, planners, developers, and water quality professionals who need preliminary information on the water quality impacts of various possible site designs, development patterns, development locations, or even alternative regional growth projections. The model also allows users to choose a variety of best management practices to reduce the projected runoff and pollutant loading.SG WATER uses a four-part methodology:It calculates stormwater runoff volume as a function of rainfall and land use, using curve numbers developed by the Natural Resources Conservation Service. The user can override these default land use curve numbers by assigning percent impervious cover to a land use. In that case, the model uses a different set of curve numbers to more accurately model runoff from the specific land use. This function allows the user to capture potential subtle land use differences between scenarios, such as street width, rooftop gardens, shared driveways, etc.It calculates event mean concentrations (EMC) for total suspended solids, nitrogen, and phosphates as a function of land use. The default EMCs are based on numerous national studies. However, the user has the option of overriding these national default values and using local or other preferred values.It calculates pollutant loading as the product of runoff volume and EMC.It applies user-selected best-management practices in order to reduce pollutant loads by a specific amount.SG WATER's outputs can take the form of specific information on one scenario, or a comparative analysis of up to 10 scenarios. Also included are comparative graphs and tables that can be exported to a spreadsheet program for further comparison and analysis. This paper provides an overview of EPA's proposed technical methodology for the SG WATER model. EPA is seeking comments on the selected approach, usefulness and efficacy of the model.