PETALUMA, Calif. - Solar panels have sprouted on countless rooftops, carports and fields in Northern California. Now, several start-up companies see potential for solar panels that float on water.

Already, 144 solar panels sit atop pontoons moored on a three-acre irrigation pond surrounded by vineyards in Petaluma in Sonoma County. Some 35 miles to the north, in the heart of the Napa Valley, another array of 994 solar panels covers the surface of a pond at the Far Niente Winery.

"Vineyard land in this part of the Napa Valley runs somewhere between $200,000 and $300,000 an acre," said Larry Maguire, Far Niente's chief executive. "We wanted to go solar but we didn't want to pull out vines."

The company that installed the two arrays, SPG Solar of Novato, Calif., as well as Sunengy of Australia and Solaris Synergy of Israel, are among the companies trying to develop a market for solar panels on agricultural and mining ponds, hydroelectric reservoirs and canals. While it is a niche market, it is potentially a large one globally. The solar panel aqua farms have drawn interest from municipal water agencies, farmers and mining companies enticed by the prospect of finding a new use for - and new revenue from - their liquid assets, solar executives said.

Sunengy, for example, is courting markets in developing countries that are plagued by electricity shortages but have abundant water resources and intense sunshine, according to Philip Connor, the company's co-founder and chief technology officer.

Chris Robine, SPG Solar's chief executive, said he had heard from potential customers as far away as India, Australia and the Middle East. When your land is precious, he said, "There's a great benefit in that you have clean power coming from solar, and it doesn't take up resources for farming or mining."

Sunengy, based in Sydney, said it had signed a deal with Tata Power, India's largest private utility, to build a small pilot project on a hydroelectric reservoir near Mumbai. Solaris Synergy, meanwhile, said it planned to float a solar array on a reservoir in the south of France in a trial with the French utility EDF.

David Fuller's view The electricity generating potential of floating solar panels on agricultural and mining ponds, hydroelectric reservoirs and canals is obvious, although I wonder how they will hold up in adverse weather conditions such as very high winds.