California officials have begun drawing up a contingency plan for mandatory statewide water rationing while Gov. Edmund G. Brown Jr. warned here today that, unless the current drought eases, the state faces "a disaster of unmeasurable magnitude."

Brown acknowledge that the contingency plan would impose mandatory rationing on all of the state's 21.5 million residents. But he said he would be reluctant to implement such a drastic measure except as a last resort.

"The specter of the drought is gathering momentum," the governor told nearly 1,000 state and local officials gathered here for the start of a two day conference on the two-year Western drought. Other state officials predicted that water shortages this year could cost California up to $6 billion in agricultural losses alone.

James G. Youde, acting director of the state Agriculture Department, said cattlement lost $500 million in the last year from the drought and this year the loss could be far worse.

Twenty-nine counties have been declared disaster areas as a result of livestock and other farming losses.

Officials have estimated that in adition to agricultural losses the drought could increases the unemployment rate here by nearly three percentage points because of layoffs in water-using industries. Officials also worry that water in some rivers has dropped so low that their sewage proportion has reached dangerously high levels.

The sewage content of the American River, near the Capital at Sacramento, has reached 12 per cent because of reduce water levels, officials said last week.

California has been one of the states hit the hardest by the drought, in part because it has lasted the longest here and also because so much of the agricultural output is dependent on water from mountain snowpacks. This year, said Ronald B. Robie, director of the Water Resources Department, snowpacks feeding water supplies are only 20 per cent of what they should be.

Already about 30 communties in Northern California have imposed mandatory water rationing on their residents because of the drought. Attorney General Evelle J. Younger ruled last Friday that the governor has the power to declare mandatory rationing throughout the state and some legislators have been calling on Brown to exercise that power soon.

Several weeks ago Brown ordered telegrams sent to about 1,000 of the larger water districts asking that they tell the Water resources Department how badly they had been affected and what their absolute minimum needs were.

Brown said today that this was part of a plan for mandatory rationing statewide if needed. The governor noted, however, that different portions of the state have different requirements and that a blanket rationing plan could be difficult if not impossible to enforce.

Instead Brown said it is more likely that as the drought continues the five members Water Quality Control Board - which he called California's "water supreme court" - would tour districts where the situation is most critical and oder mandatory cutbacks where required.

"Persuasive power is always better than coericive power," said the governor.

Brown has called for a statewide voluntary cut of 25 per cent in water use. Officials said today it is too early to tell how effective the request has been, but in areas like southern California, which has not been directly affected by the drought, there are indications that is is not working.