Amid punishing drought, federal water managers projected Tuesday that – by a very narrow margin- the crucial Lake Mead reservoir on the Colorado River won’t have enough water to make full deliveries to Nevada and Arizona in 2018.

A 24-month projection, issued on a day the largest reservoir on the closely controlled and monitored river was 36 percent full, showed the surface level of the lake behind Hoover Dam is expected to clear the trigger point this year to avoid a shortage declaration in 2017.

The margin is expected to be just under 4 feet, or almost 228 billion gallons of water.

For 2018, the U.S. Bureau of Reclamation projects missing the mark by under a foot, which would trigger a shortage declaration, cuts to Arizona’s water allocation by 11.4 percent, and cuts to Nevada’s water flow by 4.3 percent.

That would be enough water to serve more than 600,000 homes a year in Arizona, and about 26,000 in Nevada.

Conservationists said a close call should be a wake-up to water-users to be careful.

“We avoided a shortage by the skin of our teeth,” said Bart Miller, of Western Resource Advocates. “If no action is taken … a shortage declaration is not far off. We need to step up water conservation, reuse and innovative water management.”

Public water managers in Nevada, California and Arizona said they hoped the shortage projection wasn’t a certainty.

Most pointed to swaps and storage programs that have propped up the lake level by reducing the amount of water drawn for use elsewhere.

“It’s good to know we won’t be in shortage in 2017. We’re hopeful we can again avoid shortage in 2018,” said Chuck Cullom, Colorado River programs chief for the Central Arizona Project.

If water cuts begin, the agency, based in Phoenix, would drain underground storage supplies and cut water supplies to farmers by half. Tribal and municipal customers wouldn’t immediately be affected, Cullom said.

The Colorado River provides crucial water supplies to about 40 million residents, farms, businesses and tribal entities in seven Southwest states, also including Colorado, New Mexico, Utah and Wyoming.

– By Ken Ritter, AP