Wente Family Estates, Livermore Valley, basks in spring sunshine. The vineyards are on sloping hillsides, on ancient stony riverbeds, along fertile valley floors and at the base of steep sandstone cliffs. The microclimate of each of these locations is well suited to specific grape varieties. [Photo provided to China Daily]

"Even in the low season you get at least 10 Chinese visitors coming in here every day," she says.

The winery has offered the Chinese-language tours and tastings since 2013, and the number of Chinese visitors has gradually increased, she says.

A family-friendly walking tour with a tasting of two wines on the Mondavi estate costs about $20 a person. A walk through vineyards and cellars, and a sit-down guided tasting costs $35. A wide variety of tours that include tastings of food and wine and inspections of wine-making workshops is available.

The winery's website, robertmondaviwinery.com, has details of tastings, tours and other events, including details of whether a booking is required, Gong says.

"At one stage so many were popping in unannounced that we just could not cope with the numbers."

International visitors

California's wine country is home to about 4,100 wineries that make about 90 percent of all US wines. Overall, more than 21 million tourists－that equates to the population of Beijing－visit Californian wine regions each year, according to the Californian Wine Institute, the largest wine advocacy organization in California, which represents more than 1,000 wineries throughout the state.

Napa has become so popular that many Chinese know of Napa Valley before knowing anything about American wines. A report of Visit Napa Valley, the official marketing bureau of Napa County, says that last year the area received 60,928 visitors from China, accounting for 13.6 percent of visitors out of the US, second only to those from Canada, at 24.6 percent.

Overall, 448,800 international visitors called in on Napa Valley, 88 percent more than in 2012.