LOS ANGELES, DEC. 11 -- The Metropolitan Water District (MWD) of Southern California voted 49 to 2 today to approve the first mandatory water conservation plan imposed here since the drought of 1976-77.

The start of a fifth year of drought conditions, "frightening information" from weather forecasters and reports on reservoir levels persuaded the board of directors to move into the second stage of the district's five-phase program to save water, according to its spokesman, Bob Gomperz.

The second stage, designed to produce water savings of 10 percent, will take effect Feb. 1 and require a 20 percent reduction in agricultural use and ground-water replenishment and 5 percent in residential and industrial service. Penalties, not yet determined, would be imposed on violators.

The first phase called for 10 percent voluntary conservation. Because it took effect only 10 days ago, results are not known. The program's third through fifth stages are designed to increase savings by 18, 24 and 31 percent.

Stored water is at "roughly half of what we normally have at this time," said Larry Joyce, resource specialist at the state Department of Water Resources drought center.

By this time each year, the state usually has had about 19 percent of total annual rainfall. "So far this year, we've had about 5 percent," Joyce said. The water year extends from October through September.

Gomperz said the agency has been told to expect a "minimum of 15 percent in reduction of our order." Colorado River water has been cut by 25 percent, and a 15 percent reduction in water from Northern California is expected.

Districts statewide are choosing individual water conservation plans to cope with drought. Santa Barbara, which imposed a mandatory 45 percent water rationing program, has the state's most stringent plan, according to Joyce.

"We were at 10 percent," a mayoral spokesman said of Los Angeles, "but for the last two to three months, since the weather has gotten cooler, we've been maintaining a 5 to 6 percent conservation rate."