What's happened?
The state of California is enduring its fourth year of drought an arid spell that some scientists believe is the region's worst for 1,200 years. So severe is the lack of rain that geographers are talking of an unprecedented "megadrought" in the western US. Some say the Golden State home to 39 million people faces a permanent shift in its climate which poses a huge challenge to its economy and society. Last month California introduced its first state-wide mandatory water rationing for cities, requiring that municipal authorities enforce cuts in usage of 25% this year.
Who are the biggest guzzlers?
The water used by California's thriving agricultural sector (it produces half the fruit and vegetables in the US) is a big political and strategic issue. For all its success, farming accounts for only 2% of the state's GDP and 3% of jobs, but its thirsty crops (such as almonds, walnuts and grapes) account for 80% of water use.
Urban "almond-shaming" critics note that the nut uses enough water to support 75% of the state's population. In the long run, they argue, it makes no sense growing alfalfa (which consumes about 20% of the state's irrigation water) or raising cattle in a region with so little rainfall. "And by exporting alfalfa and other thirsty crops, the state is essentially shipping precious water to China," says The New York Times.
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How much water is left?
The reservoirs haven't run dry, but it's getting close. The Lake McClure reservoir, fed by the near-dried-up Merced River as it flows out of Yosemite Park, is at 10% of capacity, with almost no snow-melt on the way. New Melones Lake, another of the state's biggest man-made lakes, is at 20% of capacity, and could be drained within months after another dry summer. Overall, say analysts, California has about a year's water left dangerously low, given that last month scientists measured a record low Sierra snowpack in April, just 5% of normal, after the driest winter on record.
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And groundwater?
The depletion of California's groundwater aquifers is less visible than surface water (see box), but even more worrying. Farmers have got through three years of drought by drilling wells and pumping billions of gallons of water. As a result, in some places water tables have dropped 50ft or more in just a few years, with consequent effects on the surface: in some spots the land is sinking as much as a foot a year, causing roads to buckle and bridges to crack. "Climate conditions have exposed our house of cards," says Nasa scientist Jay Famiglietti. "The withdrawals far outstrip the replenishment. We can't keep doing this."
Can anyone pump groundwater?
Not anymore. Last year state governor Jerry Brown pulled off a political miracle, overcoming long-term resistance from farmers to push through the state's first meaningful groundwater law (the laststate in the western US to do so). Even so, critics say the law lacks urgency: its aim is to achieve sustainability (a long-term balance between water going into the ground and water coming out) by the 2040s. Yet scientists have no real idea if supplies can last that long at current depletion rates.
Couldn't sea water help?
A desalination plant is being built at Carlsbad, north of San Diego. However, other projects have stalled, and the example of Australia is not encouraging. Plants built during that country's epic drought in the 2000s were so expensive to run that four were mothballed, leaving billions spent without producing a drop of clean water. In Israel, which is 60% desert, desalination helped with a seven-year drought between 2004 and 2010 and the driest winter on record in 2013-14. But in California, desalination is harder because electricity is far more costly, and green rules make building anything slow.
What will happen?
"Mother Nature didn't intend for 40 million people to live here," reckons Kevin Starr, historian at the University of Southern California. "This is literally a culture that since the 1880s has progressively invented and reinvented itself" today, the state has a $2.2trn economy, the seventh-largest in the world. But "at what point does this invention begin to hit limits"? Brown has expressed similar views.
"For over 10,000 years, people lived in California, but the number of those people was never more than 300,000 or 400,000. Now we are embarked upon an experiment that no one has ever tried: 38 million people [up from 15 million in 1960] with 32 million vehicles, living at the level of comfort that we all strive to attain. This will require adjustment." What exactly that will look like, no one yet knows.
Depleting "fossil water" reserves
Vanishing groundwater is a crisis that is underestimatedbecause it is invisible, says Dennis Dimick in NationalGeographic. "Groundwater comes from aquifers sponge-likegravel and sand-filled underground reservoirs and we seethis water only when it flows from springs and wells."
The USrelies on this to meet half its need a proportion set to rise asdrought shrinks surface water in lakes, rivers, and reservoirs."Some shallow aquifers recharge from surface water, butdeeper aquifers contain ancient water locked in the earth bychanges in geology thousands or millions of years ago." Oncethis "fossil" water is gone, it is gone for good potentiallychanging how and where we can live and grow food.
