U.S. Fish & Wildlife ServiceReason for Selection Impervious cover is strongly linked to water quality, estuary condition, eutrophication, and freshwater inflow. Impervious surface affects not only aquatic habitats and biodiversity, but also human communities. High levels of impervious surface cause more frequent flooding by increasing the volume of stormwater runoff, reduce the amount of available drinking water by preventing groundwater recharge, and pollute waterways where people swim and fish (Chesapeake 2023, USGS 2018, EPA 2018). The 90% permeable surface threshold (i.e., 10% impervious) is a well-documented signal of major negative changes to aquatic ecosystems (Schueler et al. 2009). The 95% permeable surfac e threshold (i.e., 5% impervious) has been documented to impact Piedmont fish [tricolor shiner (Cyprinella trichroistia) , bronze darter (Percina palmaris) , Etowah darter (Etheostoma etowahae) ] (Wenger et al. 2008) and estuarine species [blue crab (Callinectes sapidus) , white perch (Morone americana) , striped bass (M. Saxatilis) and spot (Leiostomus xanthurus) ] (Uphoff Jr. et al. 2011). Input Data 2021 National Land Cover Database (NLCD): Percent developed imperviousness Southeast Blueprint 2024 extent National Hydrography Dataset Plus (NHDPlus) Version 2.1 medium resolution catchments (note: V2.1 is just the current sub-version of the dataset generally called NHDPlusV2); view the user guide Catchments A catchment is the local drainage area of a specific stream segment based on the surrounding elevation. Catchments are defined based on surface water features, watershed boundaries, and elevation data. It can be difficult to conceptualize the size of a catchment because they vary significantly in size based on the length of a particular stream segment and its surrounding topography—as well as the level of detail used to map those characteristics. To learn more about catchments and how they're defined, check out these resources: An article from USGS explaining the differences between various NHD products The glossary at the bottom of this tutorial for an EPA water resources viewer , which defines some key terms Mapping Steps The NLCD impervious surface raster uses a value of 127 as NoData. Use the Set Null function to change these values to NoData so they won't impact the analysis. Calculate percent impervious for each NHDPlus catchment using the NLCD 2021 impervious surface layer and the ArcPy Spatial Analyst Zonal Statistics "MEAN" function. The Zonal Statistics Mean calculates the average of the impervious surface values in each catchment and assigns that value to all the cells inside that catchment. Convert percent impervious to percent permeable using the formula [percent permeable = 100 - percent impervious] to maintain consistent scoring across Southeast indicators (high values indicate better ecological condition). Reclassify the above raster into 4 classes, seen in the final indicator values below. As a final step, clip to the spatial extent of Southeast Blueprint 2024. Note: For more details on the mapping steps, code used to create this layer is available in the Southeast Blueprint Data Download under > 6_Code. Final indicator values Indicator values are assigned as follows: 4 = >95% of catchment permeable (likely high water quality and supporting most sensitive aquatic species) 3 = >90-95% of catchment permeable (likely declining water quality and supporting most aquatic species) 2 = >70-90% of catchment permeable (likely degraded water quality and not supporting many aquatic species) 1 = ≤70% of catchment permeable (likely degraded instream flow, water quality, and aquatic species communities) Known Issues This indicator may not account for differences in permeability between different types of soils and land uses. The catchment boundaries are inconsistent in how far they extend toward the ocean. As a result, this indicator does not consistently apply to estuaries, coastal areas, and barrier islands in different parts of the Southeast. The catchment boundaries cross the United States/Mexico border, but the NLCD impervious data does not; as a result, the values along the United States/Mexico border are only based on the portion of the catchment where there are NLCD impervious values. The NLCD percent impervious layer contains classification inaccuracies that may cause this indicator to overestimate or underestimate the amount of permeable surface in some catchments. Disclaimer: Comparing with Older Indicator Versions There are numerous problems with using Southeast Blueprint indicators for change analysis. Please consult Blueprint staff if you would like to do this (email hilary_morris@fws.gov ). Literature Cited Chesapeake Bay Program. 2023. Stormwater Runoff. Accessed September 7, 2023. [ https://www.chesapeakebay.net/issues/threats-to-the-bay/stormwater-runoff ]. Dewitz, J., 2023, National Land Cover Database (NLCD) 2021 Products: U.S. Geological Survey data release. [ https://doi.org/10.5066/P9JZ7AO3 ]. Environmental Protection Agency. EnviroAtlas. Data Fact Sheet. January 2018. Percent of Stream and Shoreline with 15% or More Impervious Cover within 30 Meters. Accessed September 7, 2023. [ https://enviroatlas.epa.gov/enviroatlas/DataFactSheets/pdf/ESN/Percstreamw15percentimperviousin30meters.pdf ].ArcGIS ImageMapLayerhttps://hub.arcgis.com/datasets/0563907e124d4a07b60436063442a867engU.S. Fish and Wildlife Service Open DataPublicPermeable Surface (Southeast Blueprint Indicator) [United States]Web servicesUnited StatesLast Modified: 2025-12-04