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UPDATE: San Diego's regional water agency has voted to buy all of the proposed desalination plant's water for the next 30 years, which paves the way for construction to begin next year.

An effort to build the Western Hemisphere's largest seawater desalination plant faces a crucial test today, as San Diego's regional water agency votes on whether to buy all the water the plant would make.  

The plant, which builders say would come online in 2016, would be based in Carlsbad, California. Officials calculate that it would produce enough drinking water to supply 7 percent of the area's growing population by 2020. Currently the region imports almost all of its water, much from places like the Colorado River basin, where supplies have been strained by years of drought. Scientists say climate change could make that situation worse.  

While the prospective new water source is alluring, there are vocal critics of the proposed desalination plant. Some worry about harm to marine life from intake filters that suck water into the plant. Mostly, concerns center on the environmental and financial costs of the plant's massive electricy needs.  

A report out this week from the environmental research group Pacific Institute finds that ocean desalination is much more expensive than alternatives like conservation, or using "recycled water." A plant in Tampa, Fla., that came online in 2007 after long delays and cost overruns is now operating well below capacity, according to Cynthia Barnett, author of Blue Revolution: Unmaking America's Water Crisis. She says, in the lag time between planning and building the plant, "conservation really caught on" in the community, which was concerned about its dwindling groundwater supplies. At least for now, Tampa doesn't need to rely on the seawater desalination plant, says Barnett, "they were able to reduce their water use, without that costly desalinated water."

Still, with growing populations and shrinking fresh water sources across the U.S., there may still come a time when seawater desalination plants are needed, says Barnett. The trick is when to build them, because the technology is getting less expensive and energy intensive every year. Barnett says that means water managers don't want to pull the trigger on costly plants too soon, for fear the techonology will be outdated.  

"Of course they have to be thinking far out in the future," she says, "but this technology does seem to be getting better all the time, and cheaper, and hopefully less energy intensive."

In today's vote, the San Diego County Water Authority Board will decide whether to enter into a 30-year contract to buy water from private company, Poseidon Resources, which wants to build the plant. The vote may determine whether Poseidon, which plans to sell bonds to fund the billion dollar construction expense, can prove the plant's commercial viablity to investors.

Follow Krissy Clark at @@kristianiaclark