Despite the Puget Sound region’s reputation for abundant water, communities across western Washington are increasingly confronting summer water stress. Hotter, drier summers, declining snowpack, and rapid population growth are reshaping the timing and reliability of freshwater availability in the Puget Sound Region. 

Over the next 40 years, the region is expected to add roughly five million residents, increasing pressure on water infrastructure, freshwater ecosystems, and instream flows that support salmon and other species.

Understanding both how future regional water demand may change and the potential for alternative supplies are critical pieces of the region’s broader climate resilience strategy. To help address this challenge, the Climate Impacts Group partnered with Puget Sound Partnership on a multi-year project examining how future water management actions can support broader Puget Sound Recovery goals, both past and future. 

This project focuses on two key questions: how regional residential water demand may change in the future, and how alternative water supplies such as reclaimed water could contribute to long-term water resilience. This article focuses on the first portion of the work: developing a regionally consistent assessment of current and future residential water demand under different development, water efficiency, and climate change scenarios. A follow-up article will explore opportunities for water reuse within the region. 

Together, these efforts aim to support utilities, local governments, planners, and regional agencies working to balance future growth, water reliability, and ecosystem protection by providing insights into where and how communities can reduce water demand and diversify supplies. By linking water use to land use and growth decisions, the project offers practical tools for integrating water management options into regional climate resilience planning. 

“This work directly addresses one of the 2020-2024 Science Work Plan priority actions by applying, expanding, and leveraging the Puget Sound Partnership’s Future Scenarios project to understand the drivers and dynamics of future human use of water in Puget Sound. It shows us that there are actions we can take to meet future water demand in the face of expected climate change and population growth. By exploring different future scenarios, the Climate Impacts Group is informing and empowering specific audiences like legislators, water utilities, and urban planners to make choices that ensure a water-resilient future.” Laura Rivas, assistant science director, Puget Sound Partnership

Exploring Multiple Possible Water Futures

Local water demand forecasts provide valuable information for individual water systems, but are difficult to compare across jurisdictions and typically only extend a few decades into the future. To better understand long-term regional trends, the project used a scenario-based approach to estimate residential water demand across the Puget Sound region through 2080.

Rather than predicting a single future, our study explored multiple plausible futures to understand which factors most strongly influence residential water demand at the regional scale. The analysis combined parcel-scale land use projections, demographic growth scenarios, climate projections, and residential water use estimates to model how water demand may change over time. 

Why residential water use?

Residential water use is one of the largest components of total water demand across the Puget Sound region, particularly during the summer months when outdoor irrigation increases significantly (USGS 2015). Residential water use includes water used by households for drinking, bathing, irrigation, and other uses, differing from other classes of use such as agricultural and industrial use.

The scenarios examined three major drivers of future demand:

Patterns of Urban Growth:  The study compared a  “Business-as-Usual” growth scenario characterized by lower-density outward growth with a more compact  “Hybrid” scenario focused on more more compact growth within existing urban growth areas 

Adoption of Water Efficiency Measures: Researchers evaluated three levels of indoor and outdoor water efficiency ranging from continuation of current practices  (Status quo) through partial (Efficient) and full (Highly Efficient) adoption of current best available technologies and practices.

Climate Change Impacts: The study also examined how changes in temperature, precipitation, and evapotranspiration may impact outdoor water use under moderate (RCP 4.5) and high (RCP 8.5) emission scenarios. 

Key Finding #1: How and Where We Grow Matters 

While total residential water use in the region is similar under both growth scenarios, differences in patterns of growth impact which watersheds experience changes in demand. 

Current residential water use is concentrated in the region’s most populous watersheds. In the business-as-usual (BAU) – status quo (SQ) scenario nearly all watersheds are projected to experience significant increases in residential demand, with many increasing by more than 100% relative to current levels of use (Figure 1). In contrast, in the compact growth (hybrid (HYB)) – highly efficient (HE) scenario, increases in demand were more muted with many watersheds remaining at or below current levels of residential demand. Land use patterns influence where and how residential water use occurs. 

The maps in Figure 1 highlight that future demand is not distributed evenly across the region. Watersheds on the urban fringe generally experience the largest percentage-based increases in demand under outward, BAU growth scenarios while compact growth and efficiency substantially reduce demand increases across much of the region. Compact development patterns can reduce the extent of irrigated landscapes, lower outdoor water demand, and help limit infrastructure expansion into outlying areas.

Figure 1: Percent change in residential demand relative to baseline across urban growth and efficiency scenarios.

Key Finding #2: Water Efficiency Has the Largest Influence on Future Residential Water Demand

Across all scenarios, efficiency improvements had a larger influence on total future residential water demand than either patterns of urban growth or climate change impacts. 

While patterns of urban growth impact the local distribution of demand and hotter, drier summers push outdoor demand higher, the most influential variable across all modeled scenarios was efficiency. In the status quo efficiency scenario, projected residential water demand hovered just under 300,000 MGY, a >90% increase over current residential water use (Figure 2). However, widespread adoption of highly efficient indoor devices and outdoor landscape practices could constrain this growth, saving roughly 130,000 MGY. These findings suggest that modest efficiency measures could nearly offset the impacts of significant population growth on residential water demand.

Figure 2: Current and future water taking into account two growth scenarios (business as usual “outward growth”, and hybrid “compact, within existing UGA”), three efficiency scenarios (status quo, efficient, highly efficient), and two climate scenarios (RCP 4.5, “moderate emissions”, and RCP 8.5, “high emissions”).

Key Finding #3: Outdoor Efficiency Plays a Critical Role in Managing Summer Demand Peaks

The analysis also highlights the importance of outdoor water use. Outdoor use accounts for roughly 38% of future annual residential demand, but upwards of 65% of projected use during peak summer months. Periods of peak outdoor demand coincide with times when water supplies are often most constrained and ecological water needs are greatest. Reducing summer outdoor demand through efficient irrigation, drought-tolerant landscaping, and conservation measures may provide outsized resilience benefits for both communities and ecosystems.

Climate change is expected to intensify seasonal water management challenges across the Puget Sound region. Warmer temperatures and drier summers increase outdoor irrigation demand at the same time that summer streamflow declines and ecosystem water needs increase. This seasonal mismatch between water availability and demand is particularly important for regional resilience planning. While impacts are context specific, these findings suggest that strategies focused on reducing seasonal outdoor demand may be especially valuable in helping communities adapt to future climate conditions.

Planning for a More Resilient Water Future

The study findings suggest that future residential demand in the Puget Sound region is not determined by population growth alone. Decisions around housing growth, landscape practices, water efficiency, and long-term planning priorities can significantly influence the trajectory of future residential demand.

These choices have implications not only for water supply reliability, but also infrastructure costs, ecosystem health, and the region’s ability to adapt to increasingly warm and dry summers. As communities across the region plan for long-term climate resilience, water efficiency and land use planning may provide important opportunities to reduce pressure on freshwater resources while supporting continued population growth.

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