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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.

This is one of the planimetric coverages developed as part of the aerial survey project of 1996 and updated using new aerial photography collected between 25 March 2004 and 23 April 2004.

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.