Most of us think about climate change in terms of drier, hotter summers, wildfires and rising seas. But drier seasons can also mean that water bills skyrocket.

A new Stanford-led study suggests that hotter, drier weather could force some cities to spend heavily on developing new water supplies — and that cost would eventually show up in the water bills in household mailboxes.

Not surprisingly, the people likely to feel it most are those with the least money to spare.

The average cost of U.S. tap water has risen three times faster than inflation over the past 20 years, largely because of aging systems and postponed maintenance.

The California research team examined what could happen when climate change, the cost of new infrastructure and household water use collide. The study focused on Santa Cruz, California, a small coastal city that depends almost entirely on local surface water and one reservoir.

Santa Cruz residents had already done a remarkable job of conserving water. Water use there had fallen by nearly two-thirds over two decades. That meant that many of the cheaper fixes, such as installing water-saving appliances and cutting back on irrigation, had already been put in place. If the city needed more water in a hotter, drier future, more expensive projects would likely be next.

Using data from the Santa Cruz Water Department, lead author Jennifer Skerker, a PhD student in civil and environmental engineering at the Stanford Doerr School of Sustainability and Stanford School of Engineering, and her colleagues built a computer modeling framework. They combined possible changes in temperature and rainfall over the next two decades, along with decisions a water utility might make, including whether, and when, to build a wastewater reuse facility, how to price water and how household demand might change.

Santa Cruz residents had already done a remarkable job of conserving water. Water use there had fallen by nearly two-thirds over two decades.

Then they tested different approaches to keeping the water supply in line with demand. One strategy called for building major desalination capacity in coastal Santa Cruz. This would remove dissolved salts and minerals from seawater or brackish groundwater to produce fresh water suitable for human consumption, agriculture and industrial use. It would make the water supply more reliable but send costs sharply higher.

Another model delayed big investments and kept bills lower but left the city much more vulnerable to drought. Under that approach, the system provided reliable water in only six out of 10 years, on average.

The financial projections were sobering. Measures intended to keep water flowing could double the median water bill of Santa Cruz residents by mid-century.

Under a dry-climate scenario, the share of households spending more than the Environmental Protection Agency's recommended affordability threshold could climb from 19 percent to 35 percent. The poorest residents could see monthly bills rise from about $60 to $111 in today's dollars. More than 5 percent of households might have to spend as much as one-third of their income on water, potentially forcing choices between water, food, medical care and other essentials.

Drought-proofing a water system costs money. Measures intended to keep water flowing could double the median water bill of Santa Cruz residents by mid-century.

“Climate change stresses water supplies, and forces utilities to build expensive new infrastructure to maintain reliability,” Skerker said in a press release. “In cities already struggling with affordability due to aging infrastructure, the additional costs passed on to ratepayers to pay for additional infrastructure and reliability measures can push a substantial share of households into crisis.”

The problem is not limited to Santa Cruz. The researchers say their model can be adapted for other cities facing similar pressures, including Los Angeles, San Diego, San Francisco, Cape Town and Melbourne. Cities with larger reservoirs, interconnected systems or cheaper water sources may fare better, but the basic tension remains: drought-proofing a water system costs money.

And water bills are already moving in the wrong direction. The average cost of U.S. tap water has risen three times faster than inflation over the past 20 years, largely because of aging systems and postponed maintenance.

The study's message is not that households should start stockpiling bottled water. It is that keeping water both dependable and affordable may require state and federal help, rather than asking individual utilities and their customers to shoulder the full cost. As climate measures grow, the price of turning on the tap may become one of the most personal ways people experience a warming world.

The study should be seen as a warning rather than a prediction for every city. One thing is clear: preparing water systems for a hotter, drier future will be expensive. The question is whether families already struggling to pay their bills will be asked to pick up the tab.

The study is published in Nature Sustainability.