Oct 7 2015

Will El Niño ‘solve’ drought? Not if the rain falls in Southern California

Lake Shasta and Northern California’s other largest reservoirs, Oroville and Trinity, account for almost a quarter of the state’s surface water supplies. Combined, they can hold more than 10.5 million acre feet – or 3.4 trillion gallons – of rainwater and snowmelt. To put that in perspective, the city of Sacramento in 2014 used just 94,000 acre-feet. Greg Barnette Redding Record Searchlight file

In recent weeks, conditions have gelled for what forecasters say could be one of the strongest El Niño weather patterns in recorded history. Will it substantially ease California’s historic drought? If the storms center on Southern California, the answer is probably not.

Experts stress that El Niño is notoriously unpredictable, and when its storms do hit the state, they’re prone to soaking the southern third of California. While more than 75 percent of the demand for irrigation and drinking water is in the south state, the backbone of California’s water supply and delivery system – and most of its reservoir capacity – is in the north.

“We’re much better off if it rains in the north than in the south,” said Peter Gleick, president of the Pacific Institute, an environmental policy group based in Oakland.

In a typical year, about 75 percent of the state’s annual precipitation falls north of Sacramento, in the form of rain and mountain snow.

Four years into the drought, conditions have been far from typical. On April 1, when California snowpack generally has reached its greatest depths, the Sierra snowpack was at just 5 percent of normal. Researchers said it was the lowest it had been in more than 500 years. State officials say the 2015 “water year” that ended Sept. 30 recorded the warmest high-elevation temperatures in the 120 years people have been keeping track.

Those conditions have strained California’s massive water-delivery system, a series of reservoirs and canals operated by the state and federal governments. The infrastructure was built to take advantage of historic weather patterns, with a focus on regulating flows to prevent downstream flooding in heavy storms and capturing snowmelt to buoy the state through summer and fall.

“If you only get a series of early spring and early summer rainstorms, we’re not really designed to capture that runoff,” said Noah Garrison, a water-law expert and geologist at the UCLA Institute of the Environment and Sustainability.

The state has approximately 1,500 reservoirs, which portion out water over the year to meet demand for farm and landscape irrigation, drinking water, and fish and wildlife habitat. The vast man-made conveyance network is capable of funneling Mount Shasta snowmelt 700 miles south to San Diego.

The trick is storing the right amounts at the right times to ensure there is adequate water to meet yearlong demand in a state with enormous regions of developed land that get minimal precipitation or have just one wet season a year.

We’re much better off if it rains in the north than in the south.

Peter Gleick, Pacific Institute

In all, California has 43 million acre-feet of reservoir storage space, almost three-quarters of it north of Fresno. The largest of these reservoirs, Shasta, Oroville and Trinity in far Northern California, account for almost a quarter of the state’s surface water supplies.

Jay Lund is a civil and environmental engineering professor at UC Davis and heads the university’s Center for Watershed Sciences. During a recent interview, Lund held up a chart that showed a seemingly random scattering of points on a graph. The dots represented Sacramento River runoff during El Niño years, he said, underscoring the uncertainty of whether this year’s El Niño will substantially raise water levels in the northern reservoirs.

“It looks like a shotgun blast,” he said. “You wouldn’t want to bet on this. Maybe we will get a lot of water. Maybe we won’t.”

El Niño conditions occur when ocean temperatures warm along a stretch of the equatorial Pacific roughly twice the size of the United States. The warming leads to a shift in weather patterns that typically cause West Coast storm systems to move south.

During weak or moderate El Niño events, in which Pacific water temperatures rise by a modest amount, it’s hard to find a consistent rain pattern in Sacramento, according to a Sacramento Bee review of data back to 1950. The average precipitation in those years was 18 inches – about normal for the city. Stronger El Niño years – when ocean temperatures rise by a significant amount as they have this year – are more encouraging. During those years, rainfall in Sacramento averaged 24 inches, roughly 130 percent of normal.

If that happens, and El Niño douses central California as far north as Sacramento, it would substantially ease the burden on the state’s water supply – even if the storms don’t dump deep snow in the northern mountains, said Maury Roos, an hydrologist with the state Department of Water Resources.

Roos said there are a number of smaller reservoirs south of Sacramento that help supply the state’s Central Valley farm belt. Crop irrigation, most of it in the Valley, accounts for about 80 percent of the “developed” water in California, meaning water that people put to use.

Map of precipitation forecast and reservoir levels

“If it gets as far north as where we are, then it will help a lot more,” said Roos from his office in Sacramento. “Then you can help to refill some of the major reservoirs around the rim of the Valley.”

If the bulk of the heaviest rains stay further south, a wetter Southern California will help, but not nearly as much.

“We’re just not set up to handle the capacity, the total volume of water that we’re really dealing with,” said Garrison, the UCLA geologist. “A 1-inch rainstorm in L.A. can produce 10 billion gallons of runoff … most of which ultimately will end up flowing down the L.A. River and out to the ocean. We don’t have capacity to capture large events like that and really put them to use yet.”

Still, the situation has improved since the state’s last deep drought in the early 1990s. Several major Southern California cities and irrigation districts have made strides in recent years to capture more stormwater, reduce local use and make imported reserves last longer.

Rich Atwater, executive director of the Southern California Water Committee, said the region, on average, now gets about 12 percent of its water supply from locally captured stormwater. Southern California is investing in infrastructure improvements that should increase this capacity 5 percent, he said.

The region also gets about 10 percent of its water from recycled sewage, he said. He expects that figure to double in the next 25 years. That’s based, in part, on an ambitious plan for what would easily be the largest wastewater recycling effort in the state. The Metropolitan Water District of Southern California is discussing a project that would produce 168,000 acre-feet of potable water using treated sewage to replenish groundwater supplies.

The water district, which serves 19 million customers, recently spent close to $3 billion on a reservoir and tunnel project at Diamond Valley Lake near Hemet in Riverside County. The reservoir can capture 800,000 acre-feet – or about 260 billion gallons – of water. But for now, there’s no ready infrastructure for funneling in storm runoff from around the region. The reservoir will capture rain that falls directly in its walls, but it is only plumbed to receive water piped from Northern California.

A 1-inch rainstorm in L.A. can produce 10 billion gallons of runoff … most of which ultimately will end up flowing down the L.A. River and out to the ocean. We don’t have capacity to capture large events like that and really put them to use yet.

Noah Garrison, UCLA Institute of the Environment and Sustainability

The biggest benefit to Southern California from El Niño storms could be replenishment of groundwater supplies. In the drought, government surface deliveries have been slashed to a fraction of what they have been in average rainfall years. Central and Southern California cities and farms have been furiously pumping groundwater to make up for the loss.

Over the decades, several Southern California water districts have invested in groundwater banks and groundwater recharging projects to offset the unreliability of imports from the north and the Colorado River. These projects make use of imported water or natural flows that are channeled into swampy or porous areas where the water can seep into the ground for later pumping.

Kern County has created the state’s largest water bank, primarily to help irrigate its $7.5 billion agricultural industry. The storage network, spread across numerous irrigation districts, can hold 5.7 million acre-feet of water.

In the drought, even this massive system has been depleted. Jon Parker, general manager of the Kern Water Bank Authority, said his district alone can store up to 1.5 million acre-feet. In the drought, pumping has lowered that level to 500,000 acre-feet.

Robb Whitaker, general manager of the Water Replenishment District of Southern California, said drought-related pumping has similarly drained the groundwater stored under his district, which supplies about 40 percent of the water for 4 million people in southern Los Angeles County. He said a single wet El Niño year could put more than 150,000 acre-feet of water back into the ground.

“The basins are very, very dry. … They’re ready to capture water,” Whitaker said. “It’s like a dry sponge, and we’re hopeful we’d be able to get about twice the normal capture, if not more. In that case, we could be caught up in two or three wet seasons.”

The challenge with water banks and groundwater recharging is that too much rain too fast can overwhelm the system. Unlike a traditional reservoir, the basins that capture groundwater need time for that water to seep in.

“If the engineers and the water managers could control all the knobs like the great and powerful wizard of Oz, they would like it to come down at a moderate pace for a long time, so the system could sort of absorb it as it happens,” said Kelly Redmond, deputy director and regional climatologist for the federal government’s Western Regional Climate Center in Reno.

“If you overwhelm the system, some of it will go into groundwater recharge, but a lot of it will just go out to the ocean, and I guess your perspective on whether that’s wasted water or not might depend on if you’re a water manager or a fish.”

At the most basic level, a prolonged soaking would keep Southern California residential landscapes green longer without sprinklers. Some residents are hoping to extend that run with rain barrels, which while not widespread, have gained some traction through rebate programs.

Geri Cicero, a retired administrative assistant from Costa Mesa, is ahead of the curve on that front. She said that even before the drought, she installed rain barrels and other water-catching devices around her property. She’s anxious for an El Niño to fill them up for later landscape use.

“If you walked around my house, you’d see bucket after bucket and barrel after barrel,” she said. “It’s almost like a game for me. I really enjoy it.”

Atwater, with the Southern California Water Committee, has rain barrels, too. While they’re helpful in getting people to think about how much water they use on their landscape, he said, they can only do so much in solving the state’s water storage needs. He said he uses larger rain barrels than most people, and they’re empty within a week or two after a storm.

“A 50-gallon rain barrel doesn’t go very far,” he said.

Water storage

Shasta Lake was 35 percent full on Saturday. That’s 58 percent of the amount of water it would normally have this time of year. How the state’s largest reservoirs compare:

Reservoir Size (acre-feet) Pct. full Pct. of normal
Shasta Lake 4,552,000 35% 58%
Lake Oroville 3,537,577 30 49
Trinity Lake 2,447,650 22 32
New Melones Lake 2,400,000 11 20
San Luis Reservoir 2,041,000 19 40
Don Pedro Reservoir 2,030,000 31 47
Lake Berryessa 1,602,000 52 70
Lake Almanor 1,308,000 55 96
Lake McClure 1,024,600 8 19
Pine Flat 1,000,000 12 35
Folsom Lake 977,000 18 31

Source: California Department of Water Resources

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