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Indicator: Urbanization and Impervious Surface Changes

Data and Data Discussion provided by Communities Count

Figure 2: Percent of Total Land Cover by Different Categories

Figure 3: Numbers of Acres of Land Cover

Figure 1: Impervious Surface Along Stream Banks

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Upstream Indicators
  •   Land Use Mix --- The data for this indicator was taken from a snapshot study of King County.  
  •   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)
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Downstream Indicators
  •   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)
  •   Prevalence of Overweight & Obese Adults --- The proportion of King County adults who are overweight and obese continues to climb. • The combined percent of overweight and obese adults has increased in each of the four regions of King County since 1994.  (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)
  •   Work Commute Choices --- The proportion of commuters in King County who drive alone decreased slightly from 1990 to 2007. The proportion who carpool, use public transportation, bike, walk or use other means to commute to work increased slightly during this period.   (view data graph)
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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 number of roads, parking lots, sidewalks, 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

Between 1991 and 2001 impervious surface along stream banks expanded in King County, while forested lands decreased, as seen in Figure 1.

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 Figure 2.

Acres of urban land, clear-cut, and regenerating forest have increased in King County since 1986. (Figure 3) 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.

Approximately 1,000 linear miles of roads were built in King County in the past 26 years, creating access to undeveloped lands for new home building and related development (data not shown).

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 obtained from the Urban Ecology Research Laboratory, University of Washington. They are based on Landsat Thematic Mapper satellite imagery for the Central Puget Sound. Data using image analysis of satellite data may differ from other data based on land sales and other sources.

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)