States in the Colorado River basin are scrambling to propose steep cuts in the water they’ll use from the river next year, in response to a call by the federal government for immediate, drastic efforts to keep the river’s main storage reservoirs from reaching critically low levels.

The request comes with the Southwest still in the grip of a severe two-decade drought that shows no signs of letting up. And it comes on top of earlier, less desperate, efforts to keep more water in the two reservoirs, Lake Mead and Lake Powell, including a first-ever shortage declaration last year that cut water to farmers in Arizona.

The call to conserve up to an additional 4 million acre-feet of water, an amount equal to about one-third of the Colorado’s current annual flow, is just for 2023. But the long-term outlook for the Colorado is bleak, as climate change continues to affect runoff into the river and reduces the likelihood of a series of wet years that could end the drought.

The request for cuts has further exposed the fault lines between the upper basin states of Utah, Colorado, New Mexico and Wyoming and the lower basin states, California, Arizona and Nevada. The upper basin states note that they do not use all the water allocated to them, and that the most significant cuts will have to come from the lower basin states, which use more than their allocated share.

Apart from the immediate crisis regarding the two reservoirs, experts in Western water issues writing Thursday in the journal Science say significant policy changes could stabilize the river over the long term, even if the drought continues. But concessions that “may be unthinkable at the moment” must be implemented soon, they wrote.

Water managers from the states, irrigation districts, Native tribes and others are discussing proposals for steep 2023 cuts, which must be submitted to the Bureau of Reclamation next month. The reductions are expected to fall most heavily on agriculture, which uses about three-quarters of Colorado water, and on the lower basin states.

“The tough thing is we’ve had these incremental steps toward reducing water use on the river, a long runway for water users to adjust to the new normal,” said Sarah Porter, director of the Kyl Center for Water Use Policy at Arizona State University. “And now we suddenly have this unprecedented demand to leave more water in the system.”

In calling for the cuts at a Senate hearing last month, Camille C. Touton, the commissioner of the Bureau of Reclamation, warned that if negotiations failed, the government could act unilaterally. “We will protect the system,” she said.

“The challenges we are seeing today are unlike anything we have seen in our history,” Ms. Touton said.

Levels at both reservoirs are at historic lows, a result of declining flows and increasing withdrawals from a river that supplies water to 40 million people and more than 5.5 million acres of agricultural land. A major concern is that Lake Powell, behind Glen Canyon Dam near the Arizona-Utah border, could drop so low next year that it could no longer generate hydropower, and even water passage through the dam, downstream to the Grand Canyon and Lake Mead, could be affected.

Tina Shields, water manager for the Imperial Irrigation District in Southern California, which has rights to 3.1 million acre-feet of Colorado water, making it the largest user, said it was in talks “to determine the opportunities for participation in a voluntary program as well as the challenges necessary to move forward.”

The need to submit proposals by next month “doesn’t make it easy,” she said.

While few details of the talks throughout the region have been made public, some agricultural users are suggesting fallowing fields, in return for financial compensation. Depending on the acreage fallowed and the amount of water conserved, the compensation could reach into the billions of dollars. It’s not clear where the money would come from.

Chuck Cullom, executive director of the upper Colorado River Commission, which allocates water among the upper basin states, said there were basic differences between the two basins. Almost all upper basin users get their water directly from the river and its tributaries, so their supply is subject to the year-to-year hydrology, or flow. Lower basin users get almost all of their supply from water stored in Lake Mead. And most of Lake Mead’s water comes, in turn, from Lake Powell.

“Because they rely on storage, their water use patterns are out of sync with the hydrology,” Mr. Collum said, referring to the lower basin users.

Mr. Collum sent a letter to the reclamation bureau this week outlining the commission’s proposed actions to conserve an unspecified amount of additional water. But he noted that “our water users already suffer chronic shortages under current conditions” and that “additional efforts to protect critical reservoir elevations must include significant actions focused downstream of Lake Powell,” in the lower basin.

Maintaining or increasing storage is the goal of the strategies proposed in the Science paper. The researchers said their management approach would consider the storage in Mead and Powell together, rather than the current practice, in which only the level at Lake Mead is used to trigger cuts in the lower basin.

Treating river storage as a whole “opens up a lot more possibilities for better planning,” said Kevin Wheeler, the study’s lead author and a senior research fellow at the Environmental Change Institute at the University of Oxford in Britain.

The researchers used simulations incorporating river flow data over the past two dry decades to see how combinations of cuts in the lower basin and reduced growth in water use in the upper basin would affect the reservoirs.

Allowing very limited growth in the upper basin and reducing use by about 20 percent in the lower basin would maintain storage levels through the middle of the century, they found, as long as the measures were implemented soon.

Long-term reductions would be difficult for lower basin users to accept, the researchers noted, as would growth limitations in the upper basin, where there are plans for projects that would use more water.

“Although these concessions by both basins may be unthinkable at the moment,” the researchers wrote, “they will be necessary if recent conditions persist.”