As the drought bakes its way toward a fourth year, the state has a string of secret weapons in the works that could supply millions of gallons of new drinking water and help stave off disaster: desalination plants.
Seventeen plants are in planning stages along the coast to convert salt water from the ocean or bays, including one near Concord that would serve every major water agency in the Bay Area.
That plant is tentatively targeted to open in 2020, but could be kick-started earlier in an emergency, officials say - and once online, would gush at least 20 million gallons a day of drinkable water.
Starting up this string of desalination plants would be no easy skate, though.


Machines that filter salt out of water still face the same opposition they have for generations from critics who say they are too expensive to run, kill fish as they suck in briny water, and spew greenhouse gases into the air from the energy they require to run.
But in recent years, as technology and techniques for desalination have improved, such plants have gained momentum - enough so that in Carlsbad near San Diego, the biggest desalination facility in the Western Hemisphere is under construction and set to begin operation in two years.
The $1 billion plant will tap the biggest water tank around, the Pacific Ocean. It will produce 50 million gallons of potable water daily, supplying more than 110,000 customers throughout San Diego County.
Another large plant, with a potential price tag of $400 million, could begin construction in Monterey County by 2018. It would be near the only desalination plant in California that fills the needs of an entire municipality - the one that has been supplying water to Sand City, population 334, since 2010.
"It's a miracle how we managed to get this plant," said Sand City Mayor David Pendergrass. "If we didn't have it, the whole area would be in trouble. We're not under any rationing here, but then we've been practicing conservation for years already, so we are responsible about our water use.
"I would absolutely recommend desalination for other areas."
Bay Area project

Two hours north of Sand City, there is cautious enthusiasm for the $150 million Bay Area Regional Desalination Plant - as well as serious reservations.
The biggest water agencies in the area, including San Francisco's, have been developing the plant since 2003 and ran a successful small pilot version of it three years ago to make sure the location would work. The plant would sit in windswept Mallard Slough outside Bay Point and draw from delta waters flowing into Suisun Bay.
"Certainly, the project is years out from being done, but it could be in the back of people's minds as a 'what if' - and if we got into dire straits, money could be mobilized fast to finish it," said Steve Ritchie, assistant general manager for water for the San Francisco Public Utilities Commission.
San Francisco has been developing the plant with the East Bay Municipal Utility District, the Santa Clara Valley Water District, the Contra Costa Water District and the Zone 7 Water Agency, which serves the Livermore region. So far the consortium has spent $2.5 million in mostly state grant money on the plan.
Supplemental source

If built, the plant would be only a supplemental source for districts that collectively distribute about 750 million gallons of water a day. But that still makes it an important potential weapon in the fight for dwindling supply, proponents said.
The agencies' officials emphasized they would explore other options such as conservation, recycling and tapping new groundwater wells before turning to desalination. But even the prospect of the plant opening has some environmentalists concerned.
New plants require electricity that puts more greenhouse gases in the air, when simple conservation methods should be encouraged instead, some say. There is also the possibility that the pumps could suck in and kill small marine organisms and fish such as the endangered delta smelt, although the Concord-area plant's designers say that's unlikely because of its location at the side of a flowing channel.
Environmental fears

Also, though the delta water at Mallard Slough is brackish water rather than seawater - meaning it contains less salt and requires less energy to screen - the salinity level is expected to increase in coming decades as sea levels rise. And as the salinity goes up, so does the cost of screening the water. That cost would probably be passed on to water customers.
Similar environmental and cost concerns over the past couple of years have stalled plans to build desalination plants in Santa Cruz and Marin County.
"We actually support desalination when properly used, but you should look at the other options first," said Charlotte Allen, co-chairwoman of the Sierra Club Bay Chapter Water Committee.
The delta water plant - like the other 16 proposed along the coast and a handful of tiny plants already in use besides Sand City - would use a method called reverse osmosis, in which salty water is pulled in through filters. Typically, it takes two gallons of salty water to produce one gallon of potable water.
Mothballed plant

During the last major California drought, from 1985 to 1991, there was enough interest in desalination that a large plant was built to serve Santa Barbara. But it was promptly mothballed after being finished in 1992. By then, with the drought over, water from traditional sources was again about two-thirds cheaper than the $3,000 per acre-foot it cost to produce the plant's water.
An acre-foot is equivalent to one acre covered by water 1 foot deep, enough to supply two families of four for a year.
That cost gap has narrowed, however. With better screens and technology that helps the plants power themselves by recycling the energy used to suck in water - in a way, like a hybrid car regenerates power from its own motion - the typical cost of running desalination plants can dip below $2,000 an acre-foot. Because pulling up groundwater from wells and recycling water can now cost the same or more, desalination is suddenly relatively affordable for many areas - such as the Bay Area.
Surface water from reservoirs and mountain runoff, in plentiful years, can be as cheap as $100 an acre-foot. But that bargain has become scarce in the drought.
An expensive option

"In most areas of California we have exhausted a lot of the obvious water sources, and desalination is certainly an option - but it tends to be among the most expensive, even though the price has come down from what it was in 1991," said Heather Cooley, a senior water researcher with the Pacific Research Institute, a nonprofit in Oakland. "Certainly there are other options that can be looked at first."
She also noted that with no sizable desalination plants operating in California, there hasn't been much study on the full effect they could have on the coastline.
"I would argue there is a risk in building too early or too big," Cooley said. "Our understanding is improving. We know the technology works. But the challenge is that it is not appropriate in every location.
"It would be better to go forward very carefully."