It would be a great premise for a Hollywood apocalyptic disaster thriller. Imagine that after several years of devastating drought, California's supply of water gradually vanished. As the reservoirs went bone dry, in Los Angeles water would stop flowing from faucets, while in California's Central Valley, crops would wither as irrigation ceased.

To the north, in Silicon Valley, the clean rooms that produce computer chips would shut down. Eventually, as the populace grew implacably thirsty, civil order would break down, and dehydrated zombies would rampage through the streets, fighting for the last few remaining bottles of Arrowhead bottled spring water.

That movie might be fun to watch  at the multiplex in Peoria, but for Californians, the scenario is a bit too close to their actual dilemma. After several winters of low rainfall, the state is in the third year of a brutal drought that has some of the state's 12 major reservoirs dipping to less than 50 percent of their historic average water levels.

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To make matters worse, a just-released study by University of California-Irvine and NASA scientists shows that the Colorado River -- a source of water for California and six other states, in addition to parts of Mexico -- is becoming dangerously depleted as well.

A handful of small towns are already experiencing drinking water shortages, and 428,000 acres of irrigated farm fields -- about 5 percent of the total cropland -- has gone out of production in the Central Valley, Central Coast and Southern California due to the drought. On July 28, state officials were forced to impose a list of water conservation measures, such as banning residents from washing their cars in their driveways, and prohibiting the use of potable water in decorative fountains.

But the questions remain. If the drought continues, could California's water supply run out? And what really would happen if it did?

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While the current extended drought is worrisome, water experts say that a California water apocalypse isn't yet imminent. Doug Parker, director of the California Institute for Water Resources at the University of California, says that the state typically experiences cyclical droughts lasting up to three years, so this one isn't all that unusual. And there's still enough more than enough water in the reservoirs to supply Californians until the winter.

"If we knew for sure it was going to rain a lot from November to March, we wouldn't have to worry," he explains.

But if the rain is again sparse and the drought continues, that's a different matter. Parker says that tree-ring analysis shows that centuries ago, before the European colonization of California, the state experienced much longer parched periods of 30 to 50 years. Back then, of course, California didn't have sprawling cities, industry and vast farm fields that needed water. Now, it does.

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"There is an apocalyptic side to it," says Jay Famiglietti, a University of California-Irvine professor of earth science and civil and environmental engineering, who also works as a water scientist for NASA's Jet Propulsion Laboratory in Pasadena. "What happens if the drought continues for two or three more years? It starts to become an emergency, disaster scenario."

But it probably would be a slow-motion apocalypse, experts say. Currently, the state gets about a third of its water supply from groundwater, but if the drought continued and the reservoirs started to bottom out, officials would start pumping greater quantities of water from the aquifers to take its place.

The problem is that nobody's quite sure how much groundwater is down there. The best available estimate, according to Famiglietti, is that there's enough to last 50 years at current usage levels. "But if we start draining it at three times the rate, that changes everything," he says. "We'd be looking at running out of groundwater in maybe 15 years."

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Before then, though, the difficulty in meeting water demand would probably force state officials to divert water from California's farms to quench the cities' thirst, Parker says. That potentially could be ruinous to farmers, who currently use about 80 percent of the state's water.

"They're a lot more efficient in their water use than in the past, because they've been breeding crops for drought tolerance for years," he says. But it's not clear how much more they could cut back, without going broke and/or having dire effects on food availability and prices.

A severe water shortage also would require cities to import even more potable water long distances, which could lead to conflicts with other parched western states that also lay claim to it. But if that water turned out to be unavailable, Californians might have trouble finding enough to drink -- or to flush their toilets.

While some communities have turned to recycled "gray water" for use in sanitation systems, current building codes and other regulations are hindering that transition, says Famiglietti. That means a water shortage could interfere with the movement of sewage, causing a potentially serious disease risk to Californians.

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The horizon for a water apocalypse is still far enough off that California would have time to act. Parker says that coastal cities, for example, could embark upon rush programs to build desalinization plants and recycle more water to reduce demand. Famiglietti says that the aging, mostly low-tech infrastructure for supplying irrigation water to farms could also be upgraded, so that scientists could use satellite data and sensors to more efficiently manage water distribution.

Even so, Famiglietti and others are hoping for intervention from the heavens. They can take some comfort in history: A previous severe drought in 1976-77, after all, was followed by a winter of heavy rains that quickly restored depleted reservoirs.

"If we get lucky and have above average rainfall next few winters everything will be fine," he says. "If we don't have that, then things are going to get really tight."