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.