The Colorado River, part of it seen here in southeastern Utah, is the lifeline for the Southwest, providing drinking water for Tucson, Phoenix, Los Angeles, San Diego and Las Vegas.

President Trump on Tuesday signed the plan for how Arizona and other Colorado River basin states will divide up the limited river water that’s now available.

It buys Arizona time to deal with the fact that the state is headed into a hotter and drier future with probably even less water.

The deal, formally known as the Drought Contingency Plan, recognizes that existing allocations among basin states were drawn up during periods that measured unusually heavy flows on the river.

Projections showed Lake Mead would drop next year to a point where there would be mandatory reductions in water allocations. Arizona, having the lowest priority claim to the river water, would be the hardest hit.

Until now, Arizona has been taking 2.8 million acre-feet per year from the river. Under the drought plan, its draw will drop by at least 18 percent between now and 2026. An acre-foot is considered enough water to serve four Tucson households for a year.

That reduction, coupled with other deals, like the Colorado River Indian Tribe agreeing to leave some of its allocation in Lake Mead, should prevent the lake from dropping to a point where even deeper cuts are necessary.

Still, there is general agreement there will have to be yet another deal reached between now and 2026 for how to divvy up the Colorado River water, which serves about 40 million people in the West.

And Arizona at this point has no specific plan for how and where it might either cut usage or find more water.

In a prepared statement, Gov. Doug Ducey acknowledged that the plan the president signed is not the last word.

“We’re one step closer to protecting our water supplies and securing Arizona’s water future,” he said.

What’s next — after all the formal paperwork is signed later this year — is starting work on what happens after 2026.

“This isn’t the end,” agreed gubernatorial press aide Patrick Ptak. “There’s a lot more work to do to ensure that Arizona’s prepared for a drier future.”

Some environmental interests have objected to the fact that Arizona’s part of the just-signed Drought Contingency Plan does little to promote conservation and reduce water use.

Ducey has acknowledged such complaints, saying conservation remains a priority.

He signed an executive order forming a council “to analyze and recommend opportunities for water augmentation, innovation and conservation.”

Its first report is not due until July 1, 2020.

Ptak said Ducey recognizes the problem and wants to create “a culture of conservation.”

The governor has acknowledged that agriculture is likely to be the focus of future cuts, as it uses 70 percent of all the water consumed in Arizona.

Ducey believes the fact that Arizona and the other states were able to get this far and get the president’s signature “is nothing short of historic,” Ptak said.

It is also a sign that there can be future deals, he said: “It should be looked to as the model for how we go forward and how we address the really big issues in front of us.”