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You are here:   Home → Built Environment → Urbanization and Impervious Surface Changes
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Indicator: Urbanization and Impervious Surface Changes

Data and Data Discussion provided by Sustainable Seattle

Figure 1: Land Cover Change in King County, 1986-2002

Figure 2: Percent of Total Land Cover by Different Categories

Figure 3: Numbers of Acres of Land Cover

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Upstream Indicators
  •   Land Use Mix --- The data for this indicator was taken from a snapshot study of King County.  
  •   Green Building Stock --- The number of green buildings in all categories are increasing: LEED Certified, Silver, Gold, and Platinum, and BuiltGreen Homes to the 3, 4, and 5 star level in King County.  (view data graph)
  •   Planned Density --- From 1996-2000 to 2002-2004, residential densities increased in King County. The 2004 density of 6.4 lots per acre for King County as a whole exceeds the benchmark of six lots per acre.  (view data graph)
  •   Shoreline Armoring --- There are increasing levels of riprap and sea walls being installed on Puget Sound shorelines. Conclusions from a baseline survey for shoreline armoring in 2005 show that many beach-feeding sediment sources have been locked up behind armoring.   (view data graph)
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Downstream Indicators
  •   Air Quality --- The number of good and moderate air quality days has remained steady since 2001. Before then, the number of moderate days declined as good days increased. people most at risk to exposure to pollutants include children, the elderly, people with weakened immune systems and people with respiratory problems. Poor air quality can also impair scenic visibility, affect vegetation and cause damage to the built environment.   (view data graph)
  •   Carbon Footprint --- Since 2004, the state's and King County's emissions have been on the rise. In particular, transportation related GHG emissions and total energy use in King County continue to rise.  (view data graph)
  •   303d Listed Watercourses --- Although approximately 50% of watercourses included on the 1998 Washington State 303(d) list have been de-listed, the total number of listed water segments has increased in the past decade. The most significant increase in listings has occurred with regards to temperature.   (view data graph)
  •   Housing Affordability by City --- Homes prices have increased at a greater rate than incomes over the last two decades, making home ownership less affordable. However, rents have increased at a slower rate, generally in line with median household incomes.  (view data graph)
  •   Marine Sediment Quality --- Marine sediment quality is fairly steady in Puget Sound. Sediment contamination increases and are reduced only over many years.  
  •   Prevalence of Overweight & Obese Adults --- The proportion of King County adults who are overweight and obese continues to climb. In 2006, almost one in five adults were obese and more than half were overweight. Nearly nine percent of school age children were overweight.  (view data graph)
  •   Freshwater Habitat Index --- Overall freshwater quality in King County is below standards except for lakes. These findings are based on 2007 monitoring results for rivers, streams, groundwater, and lakes. Of the total sampled stream sites in King County in 2006-07, 45 percent were considered moderate to high water quality, and 55 percent were rated to be of high concern. This was a decrease in the number of sites considered moderate to high water quality from previous years.   (view data graph)
  •   State and Federal Listed Threatened and Endangered Species --- Limited historical data exists. However, the rate of global species extinction has accelerated over the past century. Steps are being taken to curtail the rate of species decline in our region, but populations of species intolerant to human development are expected to decline as our region grows.  
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Related Goals

As the amount of impervious surface increase in our region's watersheds, the ability of Nature to provide some of its air and water filtering and purification services decreases.

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Sustainability Snapshot:

Urbanization, defined as the transition from a native landscape to a built environment, increases the impervious surface coverage of roads, parking areas, sidewalks, landscaping and rooftops. Research has shown that once watersheds have developed roughly 10 percent of their drainage area into an impervious or paved condition, there is a high potential for physical, chemical, and biological impairments to both water quality conditions and other aquatic resources, resulting in scoured and smothered fish eggs, erosion, flooding and extreme peak storm events. More impervious surfaces quickly carry pollutants into Puget Sound and “reduce, disrupt or entirely eliminate native vegetation, upper soil layers, shallow depressions, and native drainage patterns that intercept, evaporate, store, slowly convey and infiltrate storm water." (1)

Sustainability Trend:

The number of acres of urban land, clear-cut, and regenerating forest have increased in King County since 1986. Acres of grass and deciduous, mixed and coniferous forest have decreased. Acres of rural forests and privately owned working forests have declined in recent years

Data Discussion

The Indicator Defined

Acres of impervious surface and forest cover based on land use analysis.

Data Interpretation/Evaluation

Historically, King County was completely covered by lush old growth Douglas fir forest, from mountains to tide water. Most of the old growth is gone now; however, forest cover still remains the dominant cover characteristic, as seen in the graph above. (Figure 2)

The table to the right gives figures showing that acres of urban land, clear-cut, and regenerating forest have increased in King County since 1986. Acres of grass and deciduous, mixed and coniferous forest have decreased. Acres of rural forests and privately owned working forests have declined in recent years due to increased development and declines in the timber industry. The table and the map were developed using image analysis of satellite data and may differ from other data based on land sales and other sources. (Figures 1 and 3)

There are 15 acres of parks and open space per one thousand urban residents in King County and numerous programs are designed to protect these important landscapes.

Through acquisition of development rights and other forest conservation programs throughout King County, including forest land in the Snoqualmie Forest east of Carnation, 92,852 acres of managed or working forests are protected.

There are 31,000 acres of public lands including parks, open space, and resource lands protected and managed by King County.

Data Source and Limitations

The data were contributed by Communities Count (www.communitiescount.org) and were obtained from the Urban Ecology Research Laboratory, University of Washington. They are based on Landsat Thematic Mapper satellite imagery for the Central Puget Sound. The results were grouped into the following seven categories:

  • Change from Open to Urban: open land was developed into urban land
  • Change from Forest to Urban: forested areas were developed into urban land
  • Change from Forest to Open: forested areas were developed into open land
  • Urban: ranges from heavy to low density
  • Forest and Other: includes forest cover, wetlands, shorelines, snow and bare rock
  • Water: includes all water bodies

There is some level of uncertainty between acres of grass, agriculture, and light urban areas; these numbers are deemed to be 85% to 88% accurate.

Data for land cover information not based on satellite images are available from the King County Department of Natural Resources and Parks, Office of Agriculture and the King County Benchmark Program.

Data on total number of farm acres (as opposed to combined grass/agriculture acres derived from the source above) are from the 2002 US Department of Agriculture, National Agricultural Statistics Service.

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Additional Resources

  • Urbanization and Forest Change Indicator
    This indicator describes changes in land use that include loss of forest land, as well as increase in urbanization and accompanying impervious surface cover. Patterns of land use and land cover are one of the driving forces behind overall ecosystem health. (Puget Sound Georgia Basin Ecosystem Indicators, U.S. Environmental Protection agency, Region X)