CARSON CITY, Nev. — The white rings that wrap around two massive lakes in the U.S. West are a stark reminder of how water levels are dropping and a warning that the 40 million people who rely on the Colorado River face a much drier future.

Amid prolonged drought and climate change in a region that’s only getting thirstier, when that reckoning will arrive — and how much time remains to prepare for it — is still a guess.

The U.S. Bureau of Reclamation released projections Friday that suggest Lake Powell and Lake Mead will dip 16 feet (5 meters) and 5 feet (1.5 meters), respectively, in January from levels recorded a year earlier. Despite the dip, Lake Mead would stay above the threshold that triggers severe water cuts to cities and farms, giving officials throughout the Southwest more time to prepare for the future when the flow will slow.

“It’s at least a couple of decades until we’re saying, ‘We don’t have one more drop for the next person that comes here,’” said Ted Cooke, general manager of Central Arizona Project, the canal system that delivers river water. “But people certainly ought to be aware that water — the importation of a scarce commodity into a desert environment — is expensive and, with climate change, going to get even more expensive.”

The Colorado River supplies Arizona, California, Nevada, Colorado, New Mexico, Utah, Wyoming and Mexico. Its water pours out of faucets in growing cities like Denver, Las Vegas and Phoenix and nourishes enough farmland to yield 15% of total U.S. crop output and 13% of livestock production.

Last year, with increasingly less water flowing to Lake Mead and Lake Powell — the two largest man-made reservoirs in the United States — Arizona, California and Nevada agreed to a drought contingency plan that built in voluntary cuts to prevent the reservoirs from dropping to dangerous levels. The other states historically haven’t used their full allocation of water and focus on keeping Lake Powell full enough to generate hydropower.

Since 1990, the Population Has More Than Tripled in the Las Vegas Area

Nevada and Arizona will make those voluntary cuts under the new projections, which they also made last year for the first time. But because neither state is using its full share of water, the impact has been minimal and hasn’t trickled down to homes. Mexico also is facing another round of cuts.

Lake Mead’s expected level of 1,089 feet (332 meters) is almost identical to last year’s projections because conservation efforts and a snowy winter prevented an expected drop, said Michael Bernardo, Bureau of Reclamation river operations manager. The wet weather didn’t last, prompting engineers to forecast the lakes will keep receding.

When projections drop below 1,075 feet (328 meters), Nevada and Arizona will face deeper cuts mandated by agreements between the seven states and Mexico.

“The future of the river is going to be drier than the past. All the climate models and the current drought suggest that,” said Colby Pellegrino, Southern Nevada Water Authority’s deputy general manager of resources. “Every sector is going to have to learn how to do more with less.”

Since 1990, the population has more than tripled in the Las Vegas area, which gets nearly 90% of its water from the Colorado River. But by treating and recycling almost all water used indoors — for flushing toilets and running dishwashers, for example — and replacing nearly 305,000 square miles (790,000 square kilometers) of grass with desert-friendly landscaping, the area has consumed far less than it’s allocated.

Elsewhere, officials are scrambling to find alternative water supplies to sustain growing cities and farms. Agricultural areas can’t replicate Las Vegas’ turf removal program. And Nevada’s ability to restore treated wastewater to Lake Mead, which is about 30 miles (48 kilometers) east of Las Vegas, can’t be done in places with less storage capacity, like Southern California, where wastewater runs into the Pacific Ocean.

Tribes Within the Colorado River Basin Also Have at Least 785,000 Acre-Feet of Water Each Year

In Arizona, where nearly 40% of water comes from the Colorado River, officials need to aggressively pursue alternative sources, ranging from underground aquifers to ocean water desalination, to keep serving customers long term, said Cooke of the Central Arizona Project.

For now, he said, people can take comfort in progress made to secure the river’s future with last year’s drought contingency plan. But once Lake Mead dips low enough, Arizona will endure the most painful cuts of any state based on an agreed-upon priority list — first rural farmers, then eventually cities.

Tribes within the Colorado River basin also have at least 785,000 acre-feet of water each year that they have claimed but haven’t legally settled, or enough to fill about 3.2 billion average-sized bathtubs, according to a federal study. Arizona pays two tribes for their unused water, relying heavily on it to fulfill the state’s obligations in the drought contingency plan. At some point, that arrangement could change as supplies shrink and tribes need more of their share.

John Fleck, director of University of New Mexico’s Water Resources Program, said that unlike conservation, costs hinder most proposals to bring in new water.

“What you’re seeing is these expensive projects are dying because of this conservation trend,” he said. “They’re just super expensive, and we’re seeing communities successfully conserving without too much trouble. Without them, it doesn’t feel particularly painful.”

Cooke acknowledged the costs of alternative supply projects but said conservation-minded academics like Fleck have a different perspective because they aren’t accountable to customers and constituents.

“We’re working on both of those things — both to reduce consumption and to increase supply — and we don’t have to make a choice between one or the other,” Cooke said.