Sitting amid the redwoods and Japanese maples in the new park atop the Transbay Transit Center, visitors can’t hear or see the buses on the ramp one floor down. Instead, they see intermittent jets of water that shoot up along a bed of white and gray granite, tracing the path of vehicles passing directly below.

It takes a trainspotter to puzzle out that the geysers are triggered by a bus, looping into the terminal from the Bay Bridge. But that’s the catch to “Bus Fountain,” one of four pieces of public art for the long-awaited terminal.

“The fountain is like a huge musical instrument that is played by the bus drivers,” said artist Ned Kahn of Sebastopol during a recent test.

As the creator behind “Bus Fountain,” Kahn, 58, explained that there are nozzles planted along the granite stream bed and that each nozzle is attached to a sensor attached to the ceiling of the bus deck below. The water shoots up in time with the buses passing underneath — as many as 100 an hour during the commute rush — for an orchestrated water show.

There are 247 nozzles spaced four feet apart that meander like a stream for 1,000 feet along the northern edge of the park. When the transit center and park open to regular bus service on Sunday, “Bus Fountain” will debut as one of the longest water artworks in the world, said Kahn, who researched that fact.

“There is nothing like this anywhere,” he said.

It’s one of four permanent public art works in the terminal, at a cost of $4.75 million paid for by the Transbay Joint Powers Authority. Kahn’s fee was only $100,000, the least expensive project of the four art pieces, because the water works were built into the construction budget.

Kahn has calculated his hourly rate to $11, far less than San Francisco minimum wage. The project took him 11 years, with all the starts and stops, just like the fountain. He prepared for that by spending 12 years as an artist-in-residence at the Exploratorium. He works on a grand scale and likes to incorporate unreliable elements like wind and rain into his art.

“I am drawn to unpredictability, the uncontrolled, the potential to be surprised,” he said.

He’s worked all over the world. These include Brisbane, Australia, where he built a wind facade at the airport. It is the largest artwork Down Under, he said. And London, where he wrapped a department store in a skin that moved with the wind.

In the city, his main installation is “Firefly,” a shimmering screen that covers the north wall of the 13-story San Francisco Public Utilities Commission headquarters on Golden Gate Avenue. The screen shifts with the weather and the sun. From across the street the imagery looks completely different from the side.

Online extra Watch a video of Ned Kahn testing “Bus Fountain” at: http://bit.ly/busfountain

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And Kahn likes to think big. “Firefly” is 200 feet tall. “Bus Fountain” is twice as long as the Ferry Building is wide.

To convince the new terminal’s architects and engineers and members of the San Francisco Arts Commission that the fountain would work, Kahn brought them up to his Santa Rosa studio and ran a primitive test involving hoses and trucks.

This earned him a more elaborate test run in the large parking lot at Project Artaud, the arts complex in the Mission. He made a video of this test and sent it around to sell “Bus Fountain” once and for all.

“Ned’s work appeals to everyone, because it is a mixture of art, invention and technology,” said Jill Manton, director of special initiatives for the San Francisco Arts Commission, which is overseeing the public art in the terminal. “He brings this sense of boyish wonder to his artwork.”

If park visitors wanted to, they could stand on the park’s restaurant deck and watch a bus exit the bridge and pull into the station. Then they could walk down to the artwork and stand atop a nozzle, waiting for that bus to trigger a jolt of water and a good soak.

There is neither railing nor “Keep Out” sign, by design. During a recent tour, a test bus sneaked into the station and the fountain nailed a Transbay official standing in the stream bed.

“It is surprisingly quiet up there,” said Kahn. “You can’t hear traffic noise on the street or hear the buses. You just hear the water coming. It can surprise you if you are not paying attention.”

During quiet midday hours, the fountain could be dry, waiting for activation by the random arrival of an AC Transit, Golden Gate Transit, SamTrans or Greyhound bus. During commute hours, the buses will be constant and so will the water in the fountain, leaping from 3 to 5 feet, according to the speed of the bus beneath.

Only extremely high winds will shut it down, to prevent water from blowing across the park. Otherwise it will be open during regular park hours, 6 a.m. to 9 p.m. in summer, and 6 a.m. to 8 p.m. in winter.

“In the span of a day, like a piece of music, the fountain will have moments of calm punctuated by moments of exuberance,” Kahn said. “The calm moments set up the exuberant ones. They need each other.”

Sam Whiting is a San Francisco Chronicle staff writer. Email: swhiting@sfchronicle.com Instagram: @sfchronicle_art