Small Area Estimates program, Office of Financial Management, State of Washington.

High Resolution Change Detection, Department of Fish and Wildlife, State of Washington.

Housing units estimates for 2011 and 2017, Washington Office of Financial Management’s Small Area Estimates program.

Impervious surface change polygons for 2011 and 2017, Washington Department of Fish and Wildlife’s High Resolution Change Detection program.

GIS data about general land use, including areas zoned to allow residential, 2012. Puget Sound Mapping Project.

Jurisdiction boundaries (cities, towns, urban growth areas, and counties), 2017. Washington Office of Financial Management.

This Progress Indicator relies on aerial imagery data from the Washington Department of Fish and Wildlife (WDFW) High Resolution Change Detection (HRCD) program to evaluate impervious surface area gains over time.

The indicator is calculated between 2011 and 2017. It is calculated by summarizing the total gain in impervious surface area in 2011 and 2017 in each jurisdiction and dividing that value by the estimated net new housing units for 2011 and 2017 in that jurisdiction. Due to data availability, results cannot be broken out by year from 2011 to 2017.

The indicator is calculated for every city, town, unincorporated UGAs, and rural areas. Results can be rolled up to the county or regional scale.

Baseline period: change between 2011 and 2017.

We do not currently have enough data to evaluate trends in this indicator. We thus apply the “No Trend” designation. As we collect more information on this indicator, we will compare new data to the baseline reporting period (2011-2017) and evaluate the trend of the indicator. 

Limitations

This Progress Indicator helps assess progress minimizing new impervious surface areas with new housing development. But it has limitations:

The indicator is not a direct measure of infill development.

•  Data is not readily available to assess the specific location of each newly constructed home and its proximity to established urban areas. Instead, this indicator serves as a proxy for infill development as we assume that infill development would result in less new impervious surface areas per new housing unit developed.

There is local variation in zoning practices that impacts the ability to identify areas that allow for residential growth.

• This indicator focuses only on residential and mixed-use development zones. Some jurisdictions have local zones that allow for either residential or other development (commercial, etc.). Jurisdictions thus vary in how much non-residential development is captured in this indicator.
• Counties also vary in how rural areas are zoned. In some counties, residential development is clearly allowed in some rural areas, whereas other rural zones appear to limit residential development. For rural areas only, this indicator considers all impervious surface change instead of focusing exclusively on rural residential zones.

Zoning classes used by this indicator represent a simplified picture of zoning practices.

• The Puget Sound Mapping Project (PSMP) developed a novel approach to review and standardize zoning classes across the Puget Sound in 2012. 
• This indicator considers all development between 2011 and 2017. If a jurisdiction changes its zoning practices to allow residential growth in a new area, this indicator will not capture impervious surface area changes in that area.
• The PSMP dataset is currently being updated and when complete, new information can be integrated into this indicator. 

Not all impervious surfaces have the same environmental impacts.

• This indicator does not track different types of impervious surfaces or account for impervious surfaces with stormwater retrofits.

The indicator does not specify where impervious surface gain is occurring in relation to key natural areas. 

•  This indicator should be evaluated alongside the Farmland Conversion, Forestland Conversion Progress Indicators (in development), and Habitat Function Vital Sign Indicators to better understand how land cover change is impacting the environment.

View a detailed report of the methods used to calculate this Progress Indicator here. 

Impervious surface area: the area of man-made structures (pavement, sidewalks, roofs, driveways, parking lots) that are covered by water-resistant materials like asphalt, concrete, brick, etc.[i]

Infill development: New construction built within established urban areas. In many cases, infill development is built in locations that are already impervious, such as through redevelopment of a strip mall or parking lot. Infill development typically results in much more limited expansion of impervious surface area because much of the necessary infrastructure (roads, parking lots, etc.) already exists nearby.

Urban Growth Area (UGA): Areas designated by counties within which urban growth shall be encouraged and outside of which growth can occur only if it is not urban in nature (RCW 36.70A.110).