It's the proverbial drop in the bucket.

Despite heavy showers that delayed flights, felled trees and caused some flooding around Northern California, the region is not out of drought territory. Many water watchers say that unless the next six weeks deliver rains of biblical proportions, Californians will face major shortages this summer.

"When you look at the totality of the inflow, it's pretty insignificant when you see where we are in three years of drought," said Pete Lucero, spokesman for the U.S. Bureau of Reclamation, which operates pumps and canals that water much of the Central Valley's agricultural land and some urban areas, including parts of the East Bay.

The Contra Costa Water District, which serves 550,000 residents in central and eastern Contra Costa County, is considering mandatory rationing of 50 percent after the federal agency said it could supply only half of the normal water allotment.

"Our customers have been very responsive, with 7 to 8 percent conservation in peak months," said Jennifer Allen, spokeswoman for the district. "But it's likely we're going to have to ask for more."

Water districts across Central and Northern California are suffering from a host of problems. In addition to crumbling infrastructure and environmental disputes limiting water pumping through the critical Sacramento-San Joaquin River Delta, a third dry winter has sucked reservoirs to rock-bottom levels.

Parched soil has soaked up much of the precipitation so far, and snowpack in the Sierra Nevada - the source of most of California's supply - is 76 percent of normal to date.

That doesn't sound bad - but considering that the last two years were well below average, it would take a massive snowpack well above 100 percent of normal to alleviate the drought.

"It's like a big ship: It takes a lot of time to swing it around," said Elissa Lynn, chief meteorologist with the state Department of Water Resources. "Even if the next 10 days are good, I doubt we could declare the drought over because the reservoirs are so, so low."

Officials at the Bureau Reclamation, which made the staggering announcement Friday that many state farmers would receive no water at all from its system this year unless conditions improve substantially, hope the recent storms point to a trend. That might allow them to turn the spigot on - slightly - for those Central Valley growers of almonds, avocados, pears and alfalfa.

"We'd be able to create a much better forecast - something higher than zero," Lucero said. "But those farmers won't see 75 percent (allotment) unless we have some epic, Noah-type storms."

Spring deluges have occurred. In March 1993, more than 33 inches of rain fell, helping to end the prolonged drought of the late 1980s and early 1990s. Such instances are so rare, however, that water planners call it the "March miracle."

The status of each water district is different. Southern California's rainfall total, for example, is closer to normal for the year than the rainfall totals in Central and Northern California. And within each region, Mother Nature favored certain watersheds more than others.

San Francisco, for example, draws its water from the Hetch Hetchy reservoir in Yosemite. Normally, the reservoir runs at about 70 percent of capacity at this time of year - currently it's 67 percent. Shasta Lake, Lake Oroville and Folsom Lake each are about one-third full.

The recent inundation has put the probability of enacting mandatory rationing this summer at 21 percent, down from 53 percent probability two weeks ago, said Michael Carlin, assistant general manager of the San Francisco Public Utilities Commission.

"These past couple of storms have been good news," he said. "Things are looking better than we originally thought."

Not so for other Bay Area districts. At the East Bay Municipal Utility District, which serves 1.3 million in Alameda and Contra Costa counties, managers estimate they would need 40 more inches of rain by April 1 to come out of the drought. So far, the district has received just below 25 inches since the rainy season began on Oct. 1.

In total, the agency has about 400,000 acre-feet in storage - two-thirds of the ideal of 600,000 acre-feet.

"With all this (recent) rain, people are thinking, 'We don't have to worry about conservation anymore,' " said agency spokesman Charles Hardy. "But that's just not true. Things could get worse, and we could possibly have half of our system storage. At that level, the red lights go on."

E-mail Kelly Zito at kzito@sfchronicle.com.

This article appeared on page A - 1 of the San Francisco Chronicle