Seattle Public Utilities

Long-Range Water Demand Forecast and Firm Yield Estimate

Seattle Public Utilities (SPU) updates its official water supply yield estimate and long-range water demand forecast when it updates its Water System Plan or when significant new information becomes available.

The official forecast (pdf) was most recently updated for the 2019 Water System Plan. Highlights from the official forecast include:

  • Water Supply: The yield estimate—172 million gallons per day (mgd)—remains the same as described in the 2016 update of the Conjunctive Use Evaluation (CUE) model.
  • Water Demand: Total demand is forecast to increase gradually to 137 mgd by 2039 before dropping off slightly as the declining block portion of the Cascade Water Alliance contract goes into effect. After 2043, demand is projected to remain relatively flat at about 133 mgd through the forecast period as the gradually declining Cascade block continues to offset demand growth.
  • Supply/Demand Implications: Water demand on SPU’s system is not projected to exceed the current firm yield of existing sources until well beyond 2060. Taking demand uncertainty into consideration, there’s still more than a 90% probability that demand will remain below current firm yield through 2060.

Contact information

Joan Kersnar
Drinking Water Planning Manager
(206) 684-0839

Bruce Flory
Principal Economist
(206) 684-5859