A group of California farmers, in a surprising turnaround, is volunteering to give up a fourth of its available water this year, sharing a resource all but guaranteed to them for more than a century.

It could become one of the most important concessions yet forced by California's record four-year drought.

The offer was made as California's "senior water rights holders" face an imminent threat of being included in the mandatory cutbacks that apply to most other water users in the state. It was being presented to state Water Board officials on Wednesday. George Kostyrko, a spokesman for the board, said none of the officials involved could comment.

The farmers in the Sacramento-San Joaquin River Delta are offering to either take less river water for irrigation, or leave a quarter of their crops unplanted, in exchange for protecting the remaining 75 percent of their water rights.

Delta water managers say it could become a model for farmers throughout California who are facing curtailments. It also could impact food prices: California is the country's top food producing state, and agriculture uses 80 percent of the water drawn from the land.

State officials had threatened to take action as early as this week against senior water rights, some dating to claims made during the Gold Rush era, long before industrialization in the 20th Century led to global warming. The rights give nearly 4,000 landowners the strongest claims in California to this precious and increasingly limited resource.

With California's drought showing no signs of easing, the state already has ordered mandatory, 25 percent cutbacks in water use by cities and towns, and greatly curtailed water available to other farmers and others whose rights are less than a century old, and therefore less iron-clad.

It is difficult to predict how many farmers elsewhere in California will participate, said attorney Jennifer Spaletta, who represents several Delta growers, but if the water board agrees to the deal, they could plan growing seasons with more certainty.

"From a business standpoint, it makes a lot of sense to do our part and to help in the emergency," Spaletta said. "At this point, obviously we're in an absolute drought emergency."

Gov. Jerry Brown has been criticized for sparing many farmers from the tightening regulations forcing cutbacks in communities throughout the state. This is the second straight year that thousands of "junior water-rights holders," whose claims were staked after 1914, have been ordered to stop pumping river water for irrigation.

The Delta proposal is being made by so-called riparian water rights holders, who have the oldest and most secure access to California rivers and streams. A coalition of these farmers is working out details with the State Water Resources Control Board, and Spaletta said officials have responded positively.

Legally, Delta farmers with the most senior water rights say the state can't stop them from irrigating their crops with river water, said John Herrick, manager of the South Delta Water Agency. He called their proposal a "safe harbor" for both sides, which he said would likely be adopted by other senior rights holders in the San Joaquin and Sacramento River watersheds.

Thomas Howard, the water board's executive director, would ultimately decide any deal.

Michael George, who works for the water board as the Delta Water Master, said the proposal is a classic example of risk assessment and reduction.

"It is my personal opinion that a certain 25 percent reduction is a reasonable trade-off for regulatory uncertainty," George said. "Nobody benefits if uncertainty persists."