Mandatory water rationing began Friday in California's two most populous areas amid a downpour of irony. Storms drenched the state but fell far short of the rain needed to make up for nearly five years of drought.

"Give us 40 days and 40 nights and we'll take care of the problem," Doug Priest, director of the state Drought Center, said in assessing how much rain it would take.Under the new rules, Los Angeles' 1 million homeowners must cut water use by 10 percent or face fines. Residents of parched Marin County north of San Francisco are limited to a scant 50 gallons of water per day per person, or 200 gallons per average four-person household.

The average four-person household usage in the state was 540 gallons daily before rationing began, Priest said in a telephone interview from Sacramento. Two five-minute showers could use up the 50-gallon per person limit.

"You can pretty well say the landscaping is gone" under such tight restrictions, he said. "You stop washing cars, stop hosing off cement areas. All that type of thing comes to a very quick halt."

Rationing is now in place throughout much of California, Priest said. On April 1, the city of Santa Monica, west of Los Angeles, plans to mandate a 25 percent cutback for residents and businesses.

"It's going to be very difficult," said Terry Muldoon, director of plant engineering at St. John's Hospital. "A hospital is a very water-intense place. We have sanitary conditions to meet."

"Today, I can't tell you that I'm going to meet that 25 percent. I'm going to try," he said.

In a speech Thursday, Los Angeles Mayor Tom Bradley called the drought "a natural disaster, an environmental crisis and an economic calamity." The mayor called on residents to prevail over the "great drought of 1991" with devices such as low-flow shower heads.

The stringent mandatory rationing in the Marin water district's service area also requires businesses to cut back 45 percent and institutions such as hospitals to cut back 50 percent from 1986-87 water usage levels. Irrigation accounts will have to reduce usage by 85 percent.

Los Angeles residents will have to cut water use an additional 5 percent by May 1. Industrial users only have to cut 10 percent.

Both water systems plan to fine anyone exceeding allotments.

A first violation for Los Angeles customers would result in a $3 charge per extra 100 cubic feet, or 748 gallons, plus a 15 percent fine of the total bimonthly water bill; the Marin district is charging $25 for every extra 748 gallons, and excess usage may bump customers into a higher per-unit rate.