Developed
Developed Gains Developed Loss
What is an impervious surface?
An impervious surface is composed of any man-made material that impedes or prevents the natural infiltration of water into the soil mantle. Such surfaces include building roofs, patios, sidewalks, concrete or asphalt streets, parking lots, and gravel roads. This definition does not include rock outcroppings or compacted earthen materials that could also increase or divert water flow, thus preventing infiltration.
Why is the impervious surface area of the landscape relevant?
When impervious surfaces cover areas where water naturally seeps into underground water sources, or aquifers, they reduce the amount of water available to recharge wells and springs. During storms, excess water that could not seep into the ground flows across impervious surfaces where it can gather harmful pollutants (e.g., oil and fertilizer) and deposit them into surrounding waters and farther downstream. As impervious surfaces increase, so do stormwater runoff volumes, the velocity of stormwater flows, and pollutant levels in runoff (Schueler, 1994).
How is impervious different than development?
Developed features are made up of impervious surfaces, but also include the grasses (and potentially smaller amounts of other cover types, such as individual trees in yards) that surround these man-made impervious features.
What developed classes are mapped in NLCD?
In the Regional land cover data, produced for the Contiguous United States (CONUS), we map 4 categories of developed features. These include:
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Developed, High Intensity - Includes highly developed areas where people reside or work in high numbers. Examples include apartment complexes, row houses and commercial/industrial. Impervious surfaces account for 80 to 100 percent of the total cover.
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Developed, Medium Intensity - Includes areas with a mixture of constructed materials and vegetation. Impervious surfaces account for 50-79 percent of the total cover. These areas most commonly include single-family housing units.
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Developed, Low Intensity - Includes areas with a mixture of constructed materials and vegetation. Impervious surfaces account for 20-49 percent of total cover. These areas most commonly include single-family housing units.
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Developed, Open Space - Includes areas with a mixture of some constructed materials, but mostly vegetation in the form of lawn grasses. Impervious surfaces account for less than 20 percent of total cover. These areas most commonly include large-lot single-family housing units, parks, golf courses, and vegetation planted in developed settings for recreation, erosion control, or aesthetic purposes.
How is a pixel's percentage of imperviousness calculated?
In the Regional land cover data (CONUS area) coefficients were derived for each of the developed categories. These values were based on National Land Cover Database (NLCD) Imperviousness data (Coan et al., 2003).These coefficients represent the average impervious values for each category nationwide. For developed, high intensity, it is 0.8503;for developed, medium intensity, it is 0.5768; for developed, low intensity, it is 0.2929; and for developed, open space, it is 0.0941.
In the High Resolution land cover data (island geographies) no coefficients were necessary. The area of the impervious class was simply summed and divided by the total area of the appropriate watershed and/or county.
How are development "gains" and "losses" defined in NLCD?
With respect to the EVA Tool, a development "gain" occurs when either of the following criteria is met: (1) a pixel changed from a non-developed class in the early date to a developed class in the late date, or (2) a pixel that was mapped as a developed class in the early date changed to a greater intensity of development in the late date (e.g., developed, low intensity to developed, high intensity). Conversely, a development "loss" occurs when either of the following criteria is met: (1) a pixel changed from a developed class in the early date to a non-developed class in the late date, or (2) a pixel that was mapped as a developed class in the early date changed to a lower intensity of development in the late date.
References
Schueler, T. R., 1994. The importance of imperviousness. Watershed Protection Techniques, 1(3): 100-111.
Coan, M. J., B. K. Wylie, C. G. Homer, C. Huang, and L. Yang, 2003. An approach for mapping large-area impervious surfaces: synergistic use of Landsat-7 ETM+ and high spatial resolution imagery. Canadian Journal of Remote Sensing, 29(2): 230-240.