Impacts of Climate Change and Growth on Water Demands in the Puget Sound Region

This study incorporates climate change and urban development scenarios into regional water demand projections. A disaggregated water demand model is coupled with an advanced urban simulation model (UrbanSim) to generate demands at detailed spatial resolution. The difference between including changes in variables such as density, income, and other demographic indicators is evaluated. The region in which this study takes place is the Puget Sound Region of the United States, which is located in the Pacific Northwest (Figure 1). This area has a population of over 4.2 million people and includes the cities and suburbs of Seattle, Tacoma, Everett, and Bellevue, Washington. The Puget Sound Region has experienced rapid growth during the past decade, with a 10% change in population from 2000 to 2008 (357,000 additional residents). This rate of growth is slower but similar to that which occurred in the 1990's when 525,000 residents were added to the region (a twenty percent increase in population). In 2000, two-thirds of the population resided within incorporated areas, in 1990 that ratio was 50/50. The growth from 2000–2008 continues the migration to incorporated areas, with growth concentrated primarily in existing dense urban areas but some new growth in the urban fringe. Seattle's center has experienced the development of condominiums creating residential space along with increasing redevelopment of single-family lots into multiple townhome style single-family homes. The development along the urban fringe has increased the density of housing, but large lots and residences are still the norm. Population within the region is projected to continue to increase, with an additional 1.7 million residents from 2000 to 2040 (roughly a million households). Most population increases are in the one and two resident households, suggesting large increases in multifamily populations and townhomes. This estimate agrees with the current trends of growth in the major urban areas (Seattle, Tacoma, Bellevue, and Everett) and is an important factor in future urban growth patterns. The climate is considered Mediterranean, in that the majority of precipitation falls during winter months, and the summers are unusually dry. The vast majority of the drinking water for this region is supplied by surface water. The lack of over-year storage capacities in the reservoirs results in a delicate balancing act between meeting summer water demands and maintaining environmental flows for ESA listed species. Accurately projecting water demands ensures that this balance is maintained and future water supply projects come online as needed.