Severe drought plagues California, the Colorado River is perilously low, and yet billions of gallons of water are being "virtually exported" via one of the West's thirstiest crops: alfalfa.

Robert Glennon, author of Unquenchable: America's Water Crisis and What to Do About It, estimates about 50 billion gallons of Western water – enough to supply 500,000 families annually – were sent to China via alfalfa in 2012. China isn't the world's biggest hay buyer – land-poor Japan leads, followed by the United Arab Emirates – but its hunger is rapidly growing.

Water also leaves in the form of food exports such as rice, wheat or almonds, but alfalfa stands out as a low-value forage crop, Glennon says. China's consumption of milk and meat is skyrocketing, and it can't feed 42 million cattle on its own.

Half of all Chinese shipping containers bringing goods into Western ports return empty, and it's cheaper for farmers (and more lucrative for shippers) to fill them with alfalfa for China than it is for Imperial Valley farmers to truck alfalfa around the West. But Glennon suggests a more direct transaction: Perhaps the farmers should abandon alfalfa and have the option to sell the water to Las Vegas, which is looking to spend billions to supplement its paltry Colorado River allocation.

"They would pay anything for that water," Glennon says.