It might seem odd to talk about a drought after a weeklong series of thundering storms drenched the Bay Area and heaped snow on the Sierra, but California's water lords know the state's lingering thirst cannot be quenched by one showy display of wet weather.
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The recent downpours have temporarily brought the state back from the dusty brink, but officials with the California Department of Water Resources claim much more rain and snow will have to fall if the Golden State is going to pull out of its drought.
"It took three years to get this dry," said David Rizzardo, chief of the department's snow surveys section. "Barring an extraordinary year, it's going to take more than one year to get out of it."
The next two months will tell the tale, according to Rizzardo. The state could emerge from drought if the storms caused by that tempestuous weather maker known as El Niño keep coming, he said. But they would have to be particularly ferocious.
The Weather Service predicts increasing clouds today leading to a chance of rain, with heavier downpours likely Monday. Drier weather is in the forecast for midweek, possibly followed by more rain next weekend. At least a couple of feet of snow should fall in the Sierra, but there will be less precipitation than last week and the storms will be less intense, a spokesman said.
Forecasters are predicting regular storms until spring, but it is difficult for meteorologists to predict what will happen a week ahead, let alone several months in the future.
Although rainfall is now above normal in the Bay Area, Lake Oroville, the State Water Project's primary source of drinking water, is only 31 percent of capacity. That's about 48 percent of average for this date.
Reservoirs statewide, including Hetch Hetchy, appear to be doing relatively well, hovering around 72 percent of average. But most of them are tiny compared with Lake Oroville, which has a capacity of 3.5 million acre-feet of water. An acre-foot is about 326,000 gallons, enough to cover an acre in a foot of water. Hetch Hetchy holds 360,400 acre-feet of water.
Rain and snow runoff in the northern Sierra feeds both Lake Oroville and the state's largest reservoir, Shasta Lake, which can hold 4.5 million acre-feet and is part of the U.S. Bureau of Reclamation's Central Valley Project. Shasta is only half full. The two dams provide water to millions of people in cities up and down the state and on farms in the Central Valley.
Rizzardo said one cannot adequately gauge how much water will be available during the year by measuring water in the state reservoirs in January. That's because dam operators are required to leave a certain amount of space - from 10 to 20 percent of capacity - for flood control purposes and to release water to help the fish.
The best gauges, he said, are precipitation and snowpack, which often fall in line with one another.
As of now, a little more than 23 inches of rain has fallen in the northern Sierra. On average, 50 inches of rain fall each year, meaning another 27 inches would have to fall this year just to reach normal. The 50-year average between Feb. 1 and the end of May is only 21 inches, Rizzardo said. The deficit so far is the result of dry weather in November and December.
The water content of snow in the Sierra, where the bulk of California's water supply is stored during the winter, is currently 107 percent of average statewide. But the telltale measurement, Rizzardo said, will be on April 1, which is considered the peak for snowpack.
"If we get up to average this year it will certainly help bring the reservoirs back to a respectable level, but if next year is another dry year then we will plunge back into it," Rizzardo said. "One good year would give us temporary relief. A couple of good years would pull us out of it."
Things are looking a lot better in areas that don't rely as much on the state and Central Valley water projects. San Francisco, for example, draws its water from the Hetch Hetchy Reservoir in Yosemite, which was at 77 percent of capacity after the big deluge Thursday.
"We're looking good for this year," said Tyrone Jue, spokesman for the San Francisco Public Utilities Commission. "If things continue along this trend, we are going to be in really good shape."
The East Bay Municipal Utility District had plenty of water even before the storms because its 1.3 million customers in Alameda and Contra Costa counties conserved so much water, according to spokesman Charles Hardy.
"We were one of the first water districts to declare a drought emergency in 2008 and since then our customers have saved nearly 40,000 acre-feet of water, enough to serve four or five small cities in our service area for a year," Hardy said.
Regardless of how things go this winter, water districts across Central and Northern California will not be resting easy. If not the vagaries of the weather or climate change, they have to worry about crumbling infrastructure and environmental disputes limiting water pumping through the critical Sacramento-San Joaquin River Delta.
"The reality is we have far more people in the state than we did in the 1970s and far more regulations, so the constraints are greater on what water is available," Rizzardo said. "The state's water has a lot of hands in it" even when there is plenty of rain and snow.
E-mail Peter Fimrite at pfimrite@sfchronicle.com.
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