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Determining Impervious Surfaces for Watershed Modeling Applications

The Nonpoint Education for Municipal Officials (NEMO) Project uses a simple GIS-based model to estimate existing nonpoint source pollution (NPS) impacts on water quality within the state’s watersheds. The model requires only two geographic information system (GIS) data sets, watershed boundaries and satellite derived land use and land cover (LULC), and a set of LULCspecific impervious surface (IS) coefficients. The model estimates overall watershed imperviousness and uses this value to generate maps that depict estimated watershed water quality. The maps help to educate municipal land-use officials about how their land-use practices and policies can and do affect water resources. To improve the application of the model to Connecticut, research was conducted to generate Connecticut-specific LULC IS coefficients. Accurate, large-scale and up-to-date planimetric data were used from four Connecticut municipalities to create a digital GIS database of impervious surface features that included buildings, roads, driveways, sidewalks and other constructed landscape features. These data were analyzed with a digital 1995 satellite derived 28-category Connecticut LULC database to produce LULC-specific IS coefficients. There was a strong correlation among the LULC IS coefficients for the three suburban towns, however; the IS coefficients for one urban town were significantly higher. The results suggest that there may be a need to develop two sets of IS coefficients, one for rural/suburban towns and the other for urban cities.