Potential Benefits of Water Supply Regionalization: A Case Study the Seattle and Everett Water Systems

Central Puget Sound includes three of the four largest and fastest growing counties in Washington state: Snohomish, King, and Pierce. Three city public utilities (Everett, Seattle, and Tacoma) supply water from four water reservoirs to most of the population in these counties. These agencies face common challenges, which include meeting increasing instream flow requirements, providing water to growing wholesaler populations and developing new water supply sources. One alternative to costly new water supply development is regionalization: connecting the supply systems, promoting the transfer of water rights and making more efficient use of existing sources. A regional water supply model is presented that evaluates the benefits from interconnecting the three utilities and managing the supply system from a regional perspective. The results indicate that sufficient storage exists in the Everett system to supply the region through 2040 demand projections. An Everett— Seattle intertie (ESI) improves long-term supply availability, provides needed supply redundancy for the Everett system, and can enhance instream flows in the Cedar and South Fork Tolt Rivers. When the intertie is operated only during low flow years, the same water supply benefits are derived, but the enhancement of instream flows is decreased.