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With La Niña around the corner, dry weather in Southern California could return

POSTED:

6.06.2016

Last month, state water officials eased conservation mandates in response to slightly above-average winter rain and snow in much of California, leading many to speculate that the state’s long-running drought has tapered off.

If only.

The El Niño winter that forecasters said could drench the state with rain and snow veered north instead, striking mostly the Pacific Northwest. The amount of rain and snow that hit Northern California was a tick above average and looked impressive mostly because it contrasted sharply with the extreme drought of the previous four years. Southern California was wetter than in previous years, but not by much.

Now, conditions are shifting, and El Niño’s counterpart, La Niña — a seasonal period marked by lower Pacific temperatures that shrivel rainfall in California — is expected to arrive around early fall and could prolong the dry times in California.

“I would be concerned about the drought continuing,” said Dave Pierce, who does El Niño and La Niña forecasts at the Climate Research Division of the Scripps Institution of Oceanography in La Jolla.

Another dry winter could hit at a time when the sources that provide Southern California with imported water — the Colorado River and the Sacramento–San Joaquin Delta region — face existential threats.

The Colorado River is over-allocated, meaning there are more demands on the river’s water than there is water. Levels at the river’s biggest reservoir, Lake Mead, hit a record low last month, after dropping in 14 of the past 17 years.

While Nevada and Arizona face cutbacks on river water before California, that might change. Negotiations are underway to distribute cuts more evenly among the three states early on to avoid more severe restrictions later.

Meanwhile, in Northern California’s delta region, environmental protections are increasing. This could mean healthier populations of fish and better water quality, while restricting the volume of water that can be pumped south and the time periods it can be sent.

Without a pair of tunnels to withdraw water from the delta in a less ecologically damaging way, Southern California could lose 440,000 acre-feet of water supply annually.

A decision on the tunnels, championed by Gov. Jerry Brown, is expected before the end of 2017.

“That (will have) a huge impact on Southern California in terms of water supply and the cost of water,” Hunter said.

‘RIDICULOUSLY RESILIENT’

During the drought, a persistent high-pressure ridge off the coast of the Pacific Northwest has bent the track of storms away from Southern California.

Forecasters predicted that El Niño-related, record-breaking warm water temperatures in the central Pacific would collapse that ridge. Instead, it re-formed intermittently during the winter and blocked the sort of parade of storms that hit Southern California during past El Niño winters. Only the strongest storms were able to break through.

The rainy season was “a pretty modest response” to the unusually warm ocean, said Pierce, the Scripps forecaster.

Now, conditions in the Pacific are cooling and shifting again. Forecasters at the Climate Prediction Center say the odds of a La Niña hitting this winter are about 75 percent.

That means more dry times and a potential revival of the high-pressure system that weather experts call the “Ridiculously Resilient Ridge.”

A La Niña year in Southern California is typically 25 percent drier than an average year, and La Niña has a similar effect on the Colorado River basin. In Northern California, the difference is just 15 percent, and further north the condition produces above-average rainfall.

Scientists are unsure why the local high-pressure ridge is so stubborn. Pierce said it could have to do with melting Arctic sea ice, or it could be a result of disruptions in weather patterns in the tropical Pacific, near Japan and Indonesia.

FIRE STARTER

A La Niña, even as it builds up for winter, figures to bring an extra threat — fire.

Four years of dry times have left the region with a buildup of dead, highly combustible shrubs and trees. As a result, forest managers say the summer could be a particularly difficult fire season.

“Even if the El Niño had brought us normal rain, or even twice as much rain as we normally get, it’s still a cumulative effect; the dead stuff is still dead,” said Gordon Martin, the fire management officer at the Trabuco Ranger District of the Cleveland National Forest.

“To get out of four years of drought, it takes four years of above-normal rainfall,” Martin said. “We didn’t get that.”

FALLING GROUND

For years, farmers and others in the Central Valley have pumped groundwater to supplement the rain and river water. But between 2014 and 2015, they pumped an unprecedented 11.5 million acre-feet of water out of the earth, causing the ground surface in some parts of the valley to collapse by as much as a foot per year.

“Groundwater has been overpumped for decades, and the recent acceleration of pumping has only made things worse,” said Tom Farr, a research scientist at NASA’s Jet Propulsion Laboratory in Pasadena, who uses satellites to measure how much the ground rises and falls as a result of extraction and recharge of groundwater.

The water is still down there, but it’s getting harder to pull out, with both the number and depth of drills increasing. “If we have another dry winter, there may be some hard choices to be made,” Farr said.

In Orange and Los Angeles counties, the groundwater basins are closely managed and regularly refilled after they’ve been drawn down. Farr said there isn’t much risk of ground surface collapsing here.

California can and should rely on groundwater during droughts, said Jeffrey Mount, a senior fellow at the Water Policy Center at the Public Policy Institute of California. But the amount of water withdrawn during the current drought has surpassed what can be recharged, he said.

“We still have all the hangovers of drought. We have had a dramatic lowering of groundwater basins,” Mount said. “We likely mined that water. We went to the bank and we wrote a big check and drained our bank account, which is groundwater. There’s no putting that back.”

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