As Silicon Valley voters fill out their ballots over the next two weeks, more than half a billion dollars for flood control, environmental cleanup and other water projects is riding on a basic question: Does the public trust Santa Clara County's largest water provider to spend it wisely?

Hoping the answer is "yes" after a decade of controversies, the Santa Clara Valley Water District has placed on the ballot Measure B, an extension of an existing parcel tax that would raise $548 million over the next 15 years.

If Santa Clara County voters approve the "Safe, Clean Water and Natural Flood Protection Program" -- $56 a year for the average home -- the tax would continue through 2028, increasing at 3 percent a year to about $87 per home, rather than expiring in 2016.

The largest funding measure on the Nov. 6 ballot in Santa Clara County, Measure B has a prominent list of supporters. Among them: the Silicon Valley Leadership Group, the South Bay Labor Council, Save the Bay, the San Jose Silicon Valley Chamber of Commerce, San Jose Mayor Chuck Reed and U.S. Rep. Mike Honda, D-San Jose.

Supporters say the first parcel tax, approved by voters in 2000 at $39 per home, has provided important benefits to Silicon Valley, including flood control, new trails and restored wetlands, trash removal along creeks and improved drinking water quality.

"A clean, reliable source of water is integral to the success of Silicon Valley's innovation economy," said Carl Guardino, CEO of the Silicon Valley Leadership Group, a nonprofit representing major employers. "Our businesses are a lot less successful if they are flooded, as are the homes of our employees and utilities. Silicon Valley views Measure B as vital to our success."

But the measure faces several hurdles.

First, it needs a two-thirds majority. When it passed the first time in 2000, during a better economy, it won by less than 1 percentage point.

Since then, the water district, a government agency based in San Jose that provides flood control and drinking water to 1.8 million people, has been the subject of seven critical grand jury reports. The reports cited project delays, high salaries and questionable spending, such as the district's decision in 2008 to spend $1.4 million building a gazebo and "outdoor education center" on a vacant lot in Alviso.

The district has doubled water rates in the past decade, yet San Jose remains the largest city in the nation without fluoridated water. And six of the agency's 10 dams can't be filled to the top because they need earthquake repairs. The district has nearly 30 top managers who earn between $150,000 and $230,000 a year and are provided full medical and dental benefits, car allowances, life insurance, 28 vacation days, 7 days of personal leave and 12 paid holidays at public expense.

"We feel that the water district spends far too much money," said John Roeder, president of the Silicon Valley Taxpayers Association, which opposes Measure B. "They haven't spent money wisely in the past. They don't deserve to be given more."

Some environmentalists also are refusing to endorse the measure, including the Loma Prieta chapter of the Sierra Club. Their concern is that the district moved $16 million in funding earmarked in the original tax for fish restoration and other conservation work into reserves that may be used to pay for flood control projects instead.

The district's leaders say significant reforms have taken place under new CEO Beau Goldie, including reducing the number of jobs from 853 to 746, limiting practices such as paying employees for accumulated sick leave, and better oversight of projects. Also, three new board members elected in 2010 -- Linda LeZotte, Brian Schmidt and Don Gage -- have worked to make changes.

Goldie said that when the original measure passed in 2000, the water district was able to become a state environmental leader in green flood control and other projects, rather than just turning creeks into concrete channels.

"The trails, trash removal, wetlands restoration, removing mercury from the Guadalupe watershed -- those are the kinds of things we've accomplished," Goldie said. "In 2016, when the measure expires, funding for those will dry up."

Goldie said if the measure -- which provides 12 percent of the agency's $285 million annual budget -- fails, the district will probably go to voters again in November 2016. But the current measure expires in June 2016, so the district would face a cash crunch.

Audits have found that the original tax has achieved most of its goals, including opening nearly 70 miles of creekside trails, restoring more than 500 acres of tidal marsh and wetlands, and removing thousands of tons of sediment and non-native vegetation that was reducing the flood-carrying capacity of local streams. But the district fell short in one key area: It set a goal of protecting 16,000 homes and other parcels from floods, but is on pace for only 9,500 by 2016 because it overestimated the federal funding that was going to be available.

Has the district made enough reforms to win the votes of Silicon Valley residents who tend to support environmental and public works projects?

"If people feel they are getting their money's worth they'll vote for it," said Larry Gerston, a professor of political science at San Jose State University. "But if they feel the bad press is justified, the district may be in for a rude awakening."

Paul Rogers covers resources and environmental issues. Contact him at 408-920-5045. Follow him at Twitter.com/paulrogerssjmn.