Media playback is unsupported on your device Media caption During California's worst drought on record, some farmers are using precious water to grow hay that is then shipped to China

While historic winter storms have battered much of the US, California is suffering its worst drought on record. So why is America's most valuable farming state using billions of gallons of water to grow hay - specifically alfalfa - which is then shipped to China?

The reservoirs of California are just a fraction of capacity amid the worst drought in the state's history.

"This should be like Eden right now," farmer John Dofflemyer says, looking out over a brutally dry, brown valley as his remaining cows feed on the hay he's had to buy in to keep them healthy.

In the dried-up fields of California's Central Valley, farmers like Dofflemyer are selling their cattle. Others have to choose which crops get the scarce irrigation water and which will wither.

"These dry times, this drought, has a far-reaching impact well beyond California," he said as the cattle fell in line behind his small tractor following the single hay bale on the back.

"We have never seen anything like this before - it's new ground for everybody."

Image copyright unknown Image caption The drought-stricken Central Valley (left) contrasts with the lush Imperial Valley, which gets water through a canal from the Colorado river

California is the biggest agricultural state in the US - half the nation's fruit and vegetables are grown here.

Farmers are calling for urgent help, people in cities are being told to conserve water and the governor is warning of record drought.

But at the other end of the state the water is flowing as the sprinklers are making it rain in at least one part of southern California.

The farmers are making hay while the year-round sun shines, and they are exporting cattle-feed to China.

The southern Imperial Valley, which borders Mexico, draws its water from the Colorado river along the blue liquid lifeline of the All American Canal.

It brings the desert alive with hundreds of hectares of lush green fields - much of it alfalfa hay, a water-hungry but nutritious animal feed which once propped up the dairy industry here, and is now doing a similar job in China.

"A hundred billion gallons of water per year is being exported in the form of alfalfa from California," argues Professor Robert Glennon from Arizona College of Law.

"It's a huge amount. It's enough for a year's supply for a million families - it's a lot of water, particularly when you're looking at the dreadful drought throughout the south-west."

Manuel Ramirez from K&M Press is an exporter in the Imperial Valley, and his barns are full of hay to be compressed, plastic-wrapped, packed directly into containers and driven straight to port where they are shipped to Asia and the Middle East.

Image copyright NOAA

"The last few years there has been an increase in exports to China. We started five years back and the demand for alfalfa hay has increased," he says.

"It's cost effective. We have abundance of water here which allows us to grow hay for the foreign market."

Cheap water rights and America's trade imbalance with China make this not just viable, but profitable.

"We have more imports than exports so a lot of the steamship lines are looking to take something back," Glennon says. "And hay is one of the products which they take back."

It's now cheaper to send alfalfa from LA to Beijing than it is to send it from the Imperial Valley to the Central Valley.

"We need to treat the resource as finite, which it is," he says. "Instead, most of us in the states, we think of water like the air, it's infinite and inexhaustible, when for all practical purposes it's finite and it's exhaustible."

Alfalfa farmer Ronnie Langrueber believes he's doing his bit to help the American economy out of recession.

"In my opinion it's part of the global economy," he says, adding that only a fraction of the hay goes to China.

"We have to do something to balance that trade imbalance, and alfalfa is a small part we can do in the Imperial Valley to help that."

He believes the whole "exporting water" argument is nonsense - that all agricultural exports contain water - and that there are few better uses for it.

"Is it more efficient to use water for a golf course for the movie stars?" Langrueber said.

"Or is it more efficient for farmers to use it to grow a crop and export it and create this mass economic engine that drives the country?"

Japan, Korea and the United Arab Emirates all buy Californian hay. The price is now so high that many local dairy farmers and cattle ranchers can't afford the cost when the rains fail and their usual supplies are insufficient.

But they have to buy what they can.

Cattle rancher John Dofflemyer certainly sees it as exporting water abroad - he resents the fact hay is sent overseas.

Hay trucks are a common sight heading north up the road from the Imperial Valley - despite the high prices, the cattle farmers have to buy what they can.

Even with recent rains in northern California there's still a critical shortage of water.

Drought is often an excuse for politicians to build dams or reduce environmental controls, but it's no long-term fix.

In those places awash with water - where global trade distorts the local market - decisions need to be made by those without something to gain.

That's where it gets even more complicated.