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What to Do With All That Rain?

  1. Water Amount
    1. One inch of rain over one acre equals 27,154 gallons of water (USGS Rain and Precipitation Info).
    2. One acre is generally equivalent to 43,560 square feet, which is about 9/10ths the size of an American football field (Wikipedia). 
    3. Volume: According to the DAIReXNET, a national resource for dairy producers funded by the USDA National Institute of Food and Agriculture, "a typical straight chassis [milk] truck will have a 4,000 to 5,000 gallon tank" (Mark Stephenson, Cornell Program on Dairy Markets and Policy). A volume of 4,500 gallons was used in our calculations.
  2. Water Flow (runoff, infiltration, and evaporation rates):
    1. Parking lot: A parking lot will generally see evaporation rates of 12% and runoff rates of 88% (EPA National Stormwater Calculator, using 20-year historical precipitation and 0.1 event threshold). 
    2. Neighborhood: Small rural lots in Thurston County typically have 15% impervious cover (Thurston Regional Planning Council). Runoff estimates were generated using assumptions of clay loam soils with cover consisting of 75% lawn, 5% forested, and 5% meadow (EPA National Stormwater Calculator, using 20-year historical precipitation and 0.1 event threshold). 
    3. Forest: EPA estimates that 100% of rain is infiltrated on forest land (EPA National Stormwater Calculator, using 20-year historical precipitation and 0.1 event threshold). However, this does not accurately reflect evapotranspiration (similar to evaporation), which in our region is estimated to be between 40-50% (Thurston Regional Planning Council). Our calculations used a rate of 40%. 

County Water Trends & Impacts

  1. Stations: As of December 2020, Thurston County maintains 18 weather stations in addition to 14 stream monitoring stations and 41 groundwater monitoring stations (Nat Kale, Thurston County Community Planning & Economic Development). 
  2. Rainfall & Water Levels: As of December 2020, the Thurston County 2020 Water Year Report has not yet been published, but will include annual precipitation amounts for the locations shown, and analysis showing stream/river levels were at 10-year lows, groundwater levels were below average, and rainfall across the county was lowest since 2001 (Nat Kale, Thurston County Community Planning & Economic Development).


Thurston County is required to comply with federal and state water pollution control laws. The County was issued a municipal stormwater permit by the Washington Department of Ecology, which requires the County to manage where and how rainwater from storms, or “stormwater,” enters our streams, rivers, lakes, and groundwater.

The Thurston County stormwater utility works hand-in-hand with this community advisory board.

This organization is cooperatively sponsored and funded by the storm and surface water utilities of the cities of Lacey, Olympia and Tumwater and Thurston County and fills a niche in the South Sound for providing free quality environmental education programs and activities and hands-on action projects.

The County is building a stormwater pond in Rochester near US-12/Main Street, to drain a 30-acre area of nearby streets. Most construction work will occur at 18133 Albany Street SW between September 2019 through December 2019.

Storm drains and ponds are man-made features used to reduce erosion, flooding and the amount of pollution in stormwater by collecting and slowly releasing rainwater runoff. Thurston County works with the responsible facility owner, typically homeowners, business owners or HOAs, to keep them in good working order.


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​Learn about the stormwater utility and major accomplishments for 2019 in the latest edition of SPLASH! Want to know even more numbers, stats, and quick links for information or help? Check out our Splash page!


Learn about the stormwater utility and major accomplishments for 2019 in the latest edition of SPLASH! Want to know even more numbers, stats, and quick links for information or help? Check out our Splash page!