Natural Landcover in Floodplains (Southeast Blueprint Indicator)

Reason for Selection Habitat near rivers and streams is strongly linked to water quality and instream flow (Naiman 1997). Intact vegetated buffers within the floodplain of rivers and streams provide aquatic habitat, improve water quality, reduce erosion and flooding, recharge groundwater, and more (WeConservePA 2014). Natural floodplain landcover is often described as “the first line of defense” for aquatic systems. Input Data * [2021 National Land Cover Database] (NLCD): Land cover * [Southeast Blueprint 2024 extent] * Estimated Floodplain Map of the Conterminous U.S. from the [Environmental Protection Agency’s (EPA) EnviroAtlas]; [see this factsheet] for more information; [download the data] The EPA Estimated Floodplain Map of the Conterminous U.S. displays “...areas estimated to be inundated by a 100-year flood (also known as the 1% annual chance flood). These data are based on the Federal Emergency Management Agency (FEMA) 100-year flood inundation maps with the goal of creating a seamless floodplain map at 30-m resolution for the conterminous United States. This map identifies a given pixel’s membership in the 100-year floodplain and completes areas that FEMA has not yet mapped” (EPA 2018). * [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 * Clip the 2021 NLCD to the EPA estimated floodplain layer. * Reclassify the clipped NLCD to identify natural landcover using the following classes: open water, barren land, deciduous forest, evergreen forest, mixed forest, scrub/shrub, grassland/herbaceous, woody wetlands, and emergent wetlands. * Calculate the percent of riparian natural landcover inside each NHDPlus catchment using ArcPy Spatial Analyst Zonal Statistics “MEAN” function. * Take the resulting raster times 100 to convert from a decimal to whole number percent. * Reclassify the above raster into the 1-5 classes seen in the final indicator values below. * Clip the resulting raster back to the EPA estimated floodplain layer. It is necessary to do this again since the Zonal Statistics function outputs pixel values for the entire catchment. During this step, assign a value of 0 to areas outside the EPA floodplain. Zero values are intended to help users better understand the extent of this indicator and make it perform better in online tools. * 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: 5 = >90% natural landcover within the estimated floodplain, by catchment 4 = >80-90% 3 = >70-80% 2 = >60-70% 1 = ≤60% natural landcover within the estimated floodplain, by catchment 0 = Not identified as a floodplain Known Issues * Small headwaters and creeks are not included in this indicator because the EPA estimated floodplain dataset does not include them. * This indicator does not account for the accumulated impacts of upstream riparian buffers. Buffers at the headwaters are treated the same as those downstream. * This indicator does not consider the river or stream size in relation to the estimated floodplain. Aquatic habitat needs may differ based on the river size class. For example, smaller headwater streams may need more natural landcover than larger rivers to maintain aquatic health. It also does not account for variation in buffer quality within the floodplain at a scale below the catchment. This means that within the estimated floodplain, loss of natural habitat adjacent to the river is treated the same as loss farther away. * While this indicator generally includes the open water area of reservoirs, some open water portions of reservoirs (e.g., Kerr Lake in NC/VA) are missing from the estimated floodplain dataset. * 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 the area just south of Guadalupe Mountains National Park in West Texas, this indicator depicts the floodplain as a series of straight lines that poorly match the actual floodplain. This is due to an error in the EPA floodplain map used in this indicator. * 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. 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 Dewitz, J., 2023, National Land Cover Database (NLCD) 2021 Products: U.S. Geological Survey data release. [[https://doi.org/10.5066/P9JZ7AO3]]. EPA EnviroAtlas. 2018. Estimated Floodplain Map of the Conterminous U.S. [[https://enviroatlas.epa.gov/enviroatlas/DataFactSheets/pdf/Supplemental/EstimatedFloodplains.pdf]]. Naiman, Robert J., and Henri Decamps. “The Ecology of Interfaces: Riparian Zones.” Annual Review of Ecology and Systematics 28 (1997): 621–58. [[https://www.nativefishlab.net/library/textpdf/19487.pdf]]. U.S. Environmental Protection Agency (USEPA) and the U.S. Geological Survey (USGS). 2012. National Hydrography Dataset Plus 2. [[https://www.horizon-systems.com/nhdplus/]]. WeConservePA. 2014. ConservationTools.org: The Science Behind the Need for Riparian Buffer Protection. [[https://conservationtools.org/guides/131-the-science-behind-the-need-for-riparian-buffer-protection]]. Yang, L., Jin, S., Danielson, P., Homer, C., Gass, L., Case, A., Costello, C., Dewitz, J., Fry, J., Funk, M., Grannemann, B., Rigge, M. and G. Xian. 2018. A New Generation of the United States National Land Cover Database: Requirements, Research Priorities, Design, and Implementation Strategies, ISPRS Journal of Photogrammetry and Remote Sensing, 146, pp.108-123. [[https://doi.org/10.1016/j.isprsjprs.2018.09.006]].