LOS ANGELES, FEB. 7 -- Californians, suffering through a fifth year of severe drought and record low amounts of rain and snow runoff, are facing further painful restrictions on agricultural and residential water use.

Farmers, reeling from loss of all state water supplies, will be told next week that, at a minimum, federal water deliveries will be halved and may even be reduced by 75 percent, Dennis B. Underwood, commissioner of the federal Bureau of Reclamation said today. This heavy blow to agriculture coincides with a new state proposal to reduce residential water deliveries statewide by 20 percent and with an alarming state report showing that runoff -- water reaching streams and reservoirs -- has never been lower.

"A bleaker picture I've never seen," said Doug Priest, manager of the California Drought Center.

The state's 1991 water year, which began Oct. 1, is the driest in history. California normally receives more than half its rainfall by the end of January and most of the rest in February and March. Through Jan. 31, statewide precipitation figures were 25 percent of normal and runoff figures only 15 percent because the dry ground had absorbed most of the rainfall.

In the driest California region, the Central Coast area around Santa Barbara, there was no measurable runoff. In the San Francisco area, the runoff was 1 percent of normal.

Farmers in the San Joaquin Valley, where a crippling freeze caused more than $800 million damage in December, would be hit hardest by reduction of federal water supplies from the Central Valley Project (CVP), which the Bureau of Reclamation manages.

An official bureau report on CVP supplies is due next Thursday, and Underwood said in an interview that federal deliveries of water to farmers will at least be halved and may be cut by 75 percent.

He said the bureau would develop "hardship criteria" for water users, giving priority to such permanent crops as fruit trees and vineyards and to preservation of wildlife habitat.

The federal government normally supplies 7 million acre feet of water to farmers through the CVP. An acre foot is 326,000 gallons of water. Underwood said that, as bad as conditions are now, the bureau must keep reserves for 1992 in case the drought continues for a sixth year.

"If you look at the figures, there are some alarming resemblances to the water-supply picture in the first five years of the longest California drought," Underwood said.

That drought, which ended in 1934, lasted seven years. California's population, about 6 million then, now is more than five times as large.

But the bureau's goal of a maintaining a carryover could be endangered if the drought persists through the winter, said Chet Bowlings, chief of water operations for the CVP in Sacramento. He said that, by the end of 1991, water reserves in the project could be close to the 1977 low of 1.3 million acre feet, the minimum needed to generate electric power.

The current water year has been drier than 1977, the last year in a four-year drought cycle and previously the driest single year in California history. Ground water supplies are far more depleted than then.

Underwood's warning of the impending reduction in federal water came two days after the California Department of Water Resources informed farmers that they would receive no deliveries of state water this year.

Acreage of cotton, the state's largest cash crop, has been reduced by one-third. State officials said today that they have encouraged growers of cotton and other high-water usage crops, such as rice and alfalfa, to idle land.

Meanwhile, residential and industrial users of water also face new cutbacks, although less severe than those confronting farmers. A proposal by the staff of the California Water Resources Control Board would require that 350 agencies and districts providing water to urban customers prepare conservation plans reducing water use 20 percent below the level in 1985, the last pre-drought year.

This plan would reward communities such as San Francisco and Santa Barbara that have used restrictions and rationing to bring water use below 1985 levels. In effect, it would penalize cities such as Los Angeles that have relied upon voluntary conservation and only recently imposed mandatory rationing.

As a courtesy, the board postponed a decision on the staff proposal until Feb. 26, pending a report by a drought action task force named last week by Gov. Pete Wilson (R). The board, which has independent authority, is expected to approve some version of the 20 percent cutback regardless of what Wilson decides.