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."
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.