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This dataset contains watershed means of estimated percent impervious surfaces for three time periods: 1992, 2002, and 2012. Estimates are based on coefficients derived from comparing land use of the 2012 NAWQA Wall-to-wall Anthropogenic Land-use Trends (NWALT) product to the 2011 National Land Cover Database (NLCD) imperviousness, then applying those coefficients to previous years (1974-2002) of the NWALT dataset.

Georeferenced raster image dataset representing City of Somerville, Massachusetts, impervious surface areas.

Anthropogenic impervious surfaces affect hydrology, water quality, and ecological health and are widely studied. Previous studies have been limited, however, by a lack of consistent representation of imperviousness nationally as a time series prior to 2001. This product presents estimated imperviousness at 60-meter spatial resolution, for the time periods 1974, 1982, 1992, 2002, and 2012. The mapping was derived by comparing imperviousness from the National Land Cover Database (NLCD) 2011 to national land use from the 2012 U.S. Geological Survey NAWQA Wall-to-wall Anthropogenic Land Use Trends (NWALT) product. The NWALT land use product includes a series of 60-m national rasters, containing 18 land use classes, for five time periods from 1974-2012. Coefficients of imperviousness were derived for each land use class for 2012, and then re-mapped back to the NWALT previous eras of 2002, 1992, 1982, and 1974. The impervious mapping is facilitated by the 7-class urban detail of the NWALT land use product. Coefficients were derived for each of 909 Census Core-Based Statistical Areas (CBSAs) in the conterminous U.S., and were thus tailored to the characteristics of specific metropolitan and rural areas. When measured over HUC-12 watershed areas, the coefficients provided very similar 2002 imperviousness estimates to those from the 2001 NLCD: r2 (RMSE) of 0.93 (0.6), and 96% of HUC-12s mapped to within 1% of their actual imperviousness value. The resultant rasters provide a tool from which consistent measurements of imperviousness can be made for zonal or watershed areas in the conterminous U.S. for the last 4 decades.

This product consists of two sets of imperviousness calculations for 3,535 urban areas and urban clusters in the conterminous United States. Imperviousness is given as the percent of the area covered by impervious surfaces, such as roads, buildings, or parking lots. One set of calculations is for the time periods 2001, 2006, and 2011, based on the U.S. Geological Survey (USGS) National Land Cover Dataset (NLCD) imperviousness data layer. The second set of calculations is for the time periods 1974, 1982, 1992, 2002, and 2012, based on landuse-based coefficients from the USGS NAWQA Wall-to-wall Anthropogenic Landuse Trends (NWALT) product. Urban area boundaries are based on the U.S. Census Bureau 2015 Urban Areas boundaries. The Census Bureau polygon shapefile on which the calculations were based is also provided.

This viewer shows impervious areas such as parking lots, driveways, and buildings in the District. DDOE makes this viewer available to help users identify potential sites for Stormwater Retention Credit (SRC) generation. The impervious areas have not been vetted for their suitability for SRC retrofits.