Scientists: Past California droughts have lasted 200 years

Sunday, February 2, 2014

Scientists: Past California droughts have lasted 200 years

MCT: Aric Crabb, Bay Area News Group

Police officers Eric Baade, left, and Daren Prociw ride across the bed of Folsom Lake.

5 hr ago By Paul Rogers of San Jose Mercury News

Researchers have documented multiple droughts in California that lasted 10 or 20 years in a row during the past 1,000 years.

SAN JOSE, Calif. — California's current drought is being billed as the driest period in the state's recorded rainfall history. But scientists who study the West's long-term climate patterns say the state has been parched for much longer stretches before that 163-year historical period began.
And they worry that the "megadroughts" typical of California's earlier history could come again.
Related: California says it won't be able to fill water demand
Through studies of tree rings, sediment and other natural evidence, researchers have documented multiple droughts in California that lasted 10 or 20 years in a row during the past 1,000 years — compared to the mere three-year duration of the current dry spell. The two most severe megadroughts make the Dust Bowl of the 1930s look tame: a 240-year-long drought that started in 850 and, 50 years after the conclusion of that one, another that stretched at least 180 years.
"We continue to run California as if the longest drought we are ever going to encounter is about seven years," said Scott Stine, a professor of geography and environmental studies at Cal State East Bay. "We're living in a dream world."
California in 2013 received less rain than in any year since it became a state in 1850. And at least one Bay Area scientist says that based on tree ring data, the current rainfall season is on pace to be the driest since 1580 — more than 150 years before George Washington was born. The question is: How much longer will it last?
A megadrought today would have catastrophic effects.
California, the nation's most populous state with 38 million residents, has built a massive economy, Silicon Valley, Hollywood and millions of acres of farmland, all in a semiarid area. The state's dams, canals and reservoirs have never been tested by the kind of prolonged drought that experts say will almost certainly occur again.
Related: Water war fought underground
Stine, who has spent decades studying tree stumps in Mono Lake, Tenaya Lake, the Walker River and other parts of the Sierra Nevada, said that the past century has been among the wettest of the last 7,000 years.
Looking back, the long-term record also shows some staggeringly wet periods. The decades between the two medieval megadroughts, for example, delivered years of above-normal rainfall — the kind that would cause devastating floods today.
The longest droughts of the 20th century, what Californians think of as severe, occurred from 1987 to 1992 and from 1928 to 1934. Both, Stine said, are minor compared to the ancient droughts of 850 to 1090 and 1140 to 1320.
What would happen if the current drought continued for another 10 years or more?
Without question, longtime water experts say, farmers would bear the brunt. Cities would suffer but adapt.
The reason: Although many Californians think that population growth is the main driver of water demand statewide, it actually is agriculture. In an average year, farmers use 80 percent of the water consumed by people and businesses — 34 million of 43 million acre-feet diverted from rivers, lakes and groundwater, according to the state Department of Water Resources.
"Cities would be inconvenienced greatly and suffer some. Smaller cities would get it worse, but farmers would take the biggest hit," said Maurice Roos, the department's chief hydrologist. "Cities can always afford to spend a lot of money to buy what water is left."
Roos, who has worked at the department since 1957, said the prospect of megadroughts is another reason to build more storage — both underground and in reservoirs — to catch rain in wet years.
In a megadrought, there would be much less water in the Delta to pump. Farmers' allotments would shrink to nothing. Large reservoirs like Shasta, Oroville and San Luis would eventually go dry after five or more years of little or no rain.
Farmers would fallow millions of acres, letting row crops die first. They'd pump massive amounts of groundwater to keep orchards alive, but eventually those wells would go dry. And although deeper wells could be dug, the costs could exceed the value of their crops. Banks would refuse to loan the farmers money.

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Sunday, February 2, 2014