EPA's Report on the Environment: External Review Draft
Note to reviewers of this draft revised ROE: This indicator reflects data from the 2006 edition of the National Land Cover Database.
Land cover represents the actual or physical presence of vegetation (or other materials where vegetation is nonexistent) on the land surface. Land cover is also often described as what can be seen on land when viewed from above. Land cover is one means to describe landscape patterns and characteristics which are critical in understanding aspects of the environment, including the availability of and changes in habitat, the potential for dispersion of chemicals and other pollutants, and potential contributors to climate change, such as reflectivity of the land.
In 1992, several federal agencies interested in land cover agreed to operate as a consortium, known as the Multi-Resolution Land Characteristics (MRLC) Consortium. The goal of the MRLC Consortium is to generate consistent and relevant land cover information at the national scale for a wide variety of environmental, land management, and modeling applications (MRLC Consortium, 2013). The MRLC Consortium has supported the publication of three national land cover products: the 1992 National Land Cover Dataset (NLCD), the 2001 National Land Cover Database (Homer et al., 2007), and the 2006 National Land Cover Database (Fry et al., 2011). Publication of the 2006 NLCD represents a shift in emphasis by the MRLC Consortium from characterizing land cover to monitoring land cover change over time. The Consortium also implemented a change in the mapping interval from 10 years to 5 years (Fry et al., 2011).
This indicator provides summary-level land cover information from the 2006 NLCD, as well as land cover trend data from the comparison of NLCD 2001 and NLCD 2006. NLCD 2006 covers the contiguous U.S.; Alaska and Hawaii are expected to be included in NLCD 2011, scheduled to be released in late 2013.
The 2006 NLCD provides a synoptic nationwide classification of land cover into 16 classes. For this indicator, the 16 land cover classes were aggregated into seven major land cover types: forest, herbaceous/grassland, shrubland, developed, agriculture, wetlands, and other (includes ice/snow, barren areas, and open water). See the definitions page for a detailed description of each land cover category. Open water is shown on the land cover map (Exhibit 1) but is otherwise not discussed as a land cover type in this indicator. See the Water theme area for more information on trends related to water. More information about forests can be found in the Forest Extent and Type indicator, and wetland acreage is discussed in greater detail in the Wetlands indicator.
What the Data Show
The 2006 NLCD for the contiguous 48 states shows approximately 498 million acres of forest cover, 444 million acres of agriculture, 426 million acres of shrub, 289 million acres of grass, 109 million acres of developed land, and 102 million acres of wetland cover (Exhibits 1 and 2).
Comparison of NLCD 2001 and 2006 data shows that 98.3 percent of land cover remained unchanged across the two databases. Exhibit 3 reflects the 1.7 percent of land cover that did change, and shows net changes between 2001 and 2006 by acreage and by percent change relative to the total amount of land for each land cover class in 2001 (Exhibit 3). Grassland, developed land, and shrubland showed the largest increases in acreage, while forest cover had the largest loss of acreage during this 5-year period (Fry et al., 2011). The increase in developed land cover was second largest by acreage, and largest in terms of percent change from its 2001 baseline. The data also show a 7.3 percent loss of ice/snow between 2001 and 2006.
NLCD 2006 data show variation in land cover types by EPA Region (Exhibit 4), with forest dominating in Regions 1, 2, 3, and 4; agriculture in Regions 5 and 7; grassland in Region 8; and shrubland in Region 6 and 9. Region 10, excluding Alaska, is dominated by roughly equal areas of forest and shrubland. Two-thirds of the grassland acreage in the nation is located in Regions 6 and 8; more than one-third of the wetland acreage is in Region 4; more than 80 percent of shrubland acreage is in Regions 6, 8, and 9; and more than half the developed acreage occurs in Regions 4, 5, and 6. Forest coverage ranges from 67 percent of Region 1 to 12 percent of Region 7.
- Land cover data for the entire nation at adequate resolution to support the indicator have been produced for three nominal dates (1992, 2001, and 2006). Differences in methodology create some limits on comparability. NLCD 2006 was designed to be comparable to NLCD 2001; following corrections needed to NLCD 2001, it is comparable to NLCD 2001 version 2.0. While NLCD 1992 and NLCD 2001 initially were not comparable, the MRLC Consortium developed a Land Cover Change Retrofit Product (Fry et al., 2009) that enabled valid comparisons between the two. However, NLCD 1992 and NLCD 2006 are not comparable.
- The overall accuracy of NLCD land cover classifications at the aggregated level reported in this indicator is 85 percent for NLCD 2001 and 84 percent for NLCD 2006. Overall accuracy at the level of the 16 underlying classifications is 79 percent for NLCD 2001 and 78 percent for NLCD 2006 (Wickham et al., 2013).
- While the nominal date of the 2001 NLCD implies that data were collected during 2001, in fact the data were collected from satellite imagery captured between 1999 and 2003, and for NLCD 2006, from imagery captured between 2005 and 2007 (Fry et al., 2013).
National estimates of land cover vary, depending on the survey approach, data sources, classification, timing, etc. The interaction of these variables will result in different estimates of the extent of any given land cover category depending on the data set used. Techniques relying on satellite data to generate land cover estimates classify what is visible from above, meaning they may underestimate developed cover in heavily treed urban areas and underestimate forest cover where trees have been harvested.
- No standardized land cover classification system is currently used among federal agencies. As a result of this limitation, there is no consistency in the assessment of land cover trends across agencies.
- NLCD 2006 land cover data for the contiguous 48 states were obtained from Fry et al. (2011) and the Multi-Resolution Land Characterization (MRLC) Consortium (2012).
- Land cover trend data, comparing NLCD 2001 and NLCD 2006, are from Fry et al. (2011).
For More Information
- USGS Land Cover Institute
- Multi-Resolution Land Characteristics Consortium's National Land Cover Database
- MRLC: An Innovative Partnership for National Environmental Assessment
- Learn how this indicator fits into conceptual diagrams for: Nutrient Impacts, Wetland Loss
- This indicator relates to: Extent and Distribution, Land Cover
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