By ELLIOT SPAGAT

Associated Press

SAN DIEGO (AP) -- The San Diego County Water Authority announced a tentative agreement Thursday to buy the entire output of what will be the Western hemisphere's largest seawater desalination plant, clearing a major hurdle for construction to begin.

The plant in Carlsbad will produce 50 million gallons a day, enough to supply about 7 percent of the San Diego region in 2020.

The agreement is subject to approval by the water authority board. Upon the board's approval, the developer -- Poseidon Resources LLC -- would sell bonds to finance 82 percent of construction of the project, which is estimated to cost about $900 million.

The water authority expects the plant to be completed in 2016.

Under the pact, San Diego would pay Poseidon $2,042 to $2,290 for an acre-foot of water, more than twice what it pays to import water from outside the region. For supporters, the premium is well worth paying to make the region less dependent on imported water from the Los Angeles-based Metropolitan Water District of Southern California, which supplied almost all its water in the 1990s and still provides nearly half.

"The story of San Diego has always been about the quest for reliable water," said Dennis Cushman, the San Diego agency's assistant general manager. "The history is drought and water supply shortage and being subject to decisions made by a board of directors in downtown Los Angeles ... This is about water reliability."

The San Diego agency estimates the cost is comparable to other new, local sources of drinking water, like treated toilet water or briny groundwater.

Poseidon, based in Stamford, Conn., was plagued with delays at its desalination plant in Tampa, Fla., which produces 25 million gallons a day.

Ken Weinberg, the San Diego agency's water resources director, said Poseidon underestimated costs on the Tampa plant and had problems with its contractor that he is confident will not be repeated.

"In the front of our minds was to Tampa-proof this project," Weinberg said.

Copyright 2012 The Associated Press.