LOS ANGELES, FEB. 12 -- The Metropolitan Water District (MWD) today reduced by 31 percent overall water supplies to its customers in a move designed to see Southern Californians through a fifth year of devastating drought.

The board also appropriated $30 million to buy water from rice growers in the San Joaquin and Sacramento valleys, if any excess is available. The state is encouraging growers to reduce crop acreage and sell the water instead.

The MWD, a water wholesaler, supplies 27 agencies and 15 million water users in Southern California, nearly half the state's population.

Today's action was prompted by a sharp reduction in supplies from the State Water Project, which had been expected to supply 1.4 million acre feet of water to the MWD during the current water year. But the driest winter ever in California and a critically low mountain snowpack have reduced State Water Project supplies to only 15 percent for residential users and to none for farmers.

The board's decision to reduce overall supplies to its customers by 31 percent effective March 1 was based on what MWD spokesman Robert Gomperz called "a worst-case scenario" that assumes a continued dry winter. While these reductions will guarantee most Southern Californians sufficient water this year, MWD general manager Carl Boronkay has said that further and more drastic cuts will be necessary if the drought persists into a sixth year.

The MWD action, like earlier ones taken by the state and federal governments, fell especially heavily on agricultural users. The action was taken on a vote of 46 to 1.

To reach its 31 percent overall goal, the MWD decreed reductions of 50 percent for agricultural users and 20 percent for residential and municipal customers. But some MWD customers, notably the San Diego Water Authority, have been distributing the water reductions evenly among all customers in an effort to protect high-value crops such as avocados and citrus fruit, which in San Diego County generally escaped the severe December freeze that devastated these crops in other regions of California.

Agencies that exceed their allotment of water will be penalized with triple costs; those that conserve more water than required will receive a 50 percent rebate for the unused allotment.

In voting for the 31 percent reduction, the MWD board moved immediately from the third stage of a previously approved five-step conservation plan to the final stage. Water supplies had been reduced 17 percent during the third stage. A fourth stage in which supplies would have been cut 24 percent was skipped altogether.

By moving up the timetable, the MWD hopes to encourage agencies that the district supplies with water to speed their own conservation plans.

Mandatory water rationing is scheduled to start with a 10 percent cut March 1 in Los Angeles and increase to 15 percent by May 1. The last time water was rationed here -- a 10 percent cut in a fourth drought year of 1977 -- the city was able to reduce actual use by 18 percent through additional voluntary conservation.

Compared to most other cities in California, Los Angeles is amply supplied with water. In addition to its allotment from the MWD, Los Angeles also receives water from the Colorado River, Mono Lake and the Owens Valley and has stored a reserve of ground water.

For this reason, a drought that has caused havoc in the inland valleys, the central coast region and much of Northern California has until now been only a mild nuisance in Los Angeles.

But far steeper cutbacks will be required here and almost everywhere else in California if the drought continues. By acting today, instead of waiting until the end of the winter to go to the fifth step of their conservation plan, MWD officials said they hoped to preserve some water reserve in case there is a sixth year of drought.

Meanwhile, in Sacramento, a drought action team appointed by Gov. Pete Wilson (R) neared the end of its work on a report that will be presented later this week.

Bill Livingstone, the governor's press secretary, said Wilson is expected to receive the report Thursday and make recommendations Friday on "responsible ways to mitigate the impact" of a drought that has become a serious threat to California's already recession-battered economy.