How WRIA 8 Cost Share Allocation was Calculated

Data Sources Used:

  • KingCounty and SnohomishCountyCity Boundaries
  • Unincorporated King County Boundary
  • Unincorporated SnohomishCounty Boundary
  • WRIA Watershed Boundaries
  • Parcels with 2008 Assessment data
  • 2000 Census Tracts
  • Water Bodies
  • Puget Sound Regional Council (PSRC) 2007 Population Estimates by Census Tract(2008 estimate is not yet available)
  • Washington Office of Financial Management (OFM)2007 Population Estimates by City. Although 2008 population estimates are available, 2007 was used to be consistent with the PSRC data.

Preprocessing Steps

  • Parcels are converted into points in the center of each parcel. Whether or not the point falls within the watershed boundary determines whether or not it is included in the calculations.
  • Some census tracts and city boundaries extend into the water. Use shoreline of water bodies to remove areas in Puget Sound, Lake Washington, LakeSammamish,LakeUnion, and the Ship Canal.
  • ExcludeUpperCedarBasin from WRIA 8 analysis.

To Calculate Area:

  • Combine WRIA boundaries, city boundaries, and county boundary
  • Sum area by WRIA and jurisdiction

Note: Some jurisdiction’s areas may have changed since 2006 recalculation. The watershed boundaries were redrawn using much more accurate source data. Also, some census tracts and city boundaries extend into water bodies. So to calculate the area of jurisdictions adjacent to Puget Sound, Lake Washington, LakeSammamish, LakeUnion, and the Ship Canal, the GIS staff uses the shorelines of the water bodies to determine the area. Water body shorelines were updated using 2007 digital orthophotography. Therefore there may be a slight difference in area calculations from the last recalculation.

To Calculate Assessed Values:

  • Combine WRIA boundaries, city boundaries, county boundary, and parcel center points
  • Sum assessment by WRIA and jurisdiction

To Calculate Population:

  • For cities that are wholly within the WRIA boundary, use the population estimate from the OFM
  • For jurisdictions that cross the WRIA boundary, combine WRIA boundaries, city boundaries, county boundary, and Census tracts
  • Find the ratio of the area of tract in each jurisdiction to the area of the whole tract (Example: area of tract in Renton = 2.85 acres, area of whole tract = 6.26 acres, Ratio = 2.85/ 6.26 = 0.455)
  • Multiply the ratio by the PSRC2007 population estimate for each tract
  • Sum population by WRIA and jurisdiction

Note: Using Census tracts and PSRC datainstead of city boundaries and OFM data for jurisdictions crossing the WRIA boundary gives a better estimate of the population in each WRIA. Each method assumes the population is evenly distributed over the tract or city when the proportion in the WRIA is determined. Using Census tracts means proportioning a smaller unit and it was felt that this would result in a more equitable number.