Vineyard owners are pruning earlier than usual and on a shorter schedule, Domenick Bianco of Renteria Vineyard Management said.
If the Valley does not see late winter or spring rains, 2014 will yield a smaller crop.
"Water amount determines yield. If you use 80 per cent less water than last year, you could see 80 per cent of the crop," Bianco said.
Still, unlike other areas, some Napa growers say access to water in underground aquifers will help them irrigate crops even if rains are light through this year.
While it will not make up for rainfall, it can help mitigate the loss of yield to dry weather in the short-term, said Hal Huffsmith, director of vineyard operations at Trinchero Family Estates.
Because rains may still come, it is too early to tell if the drought will affect the retail price of Napa wines, said Jennifer Putnam, executive director of Napa Valley Grapegrowers.
Meantime, growers will reduce the amount of irrigation they do to conserve water.
They will also not plant so-called "ground cover," or smaller plants that live between vines to help with erosion, and the fields of lush green vines so many are used to seeing in the valley will be pared down.
"They're going to be more precise about irrigation," Putnam said. "We're not going to grow those lush canopies."