Generally, not a lot; Vitis Vinifera is quite drought resistant, most vineyards are drip-irrigated to optimize, according to the desires and knowledge of the management, the delivery of water to the vine, and old dry-farmed vines have roots that go deep into soil and rock. We're having a different problem - our water tables are down, and older, shallow wells are drying out.
One extremely well-financed (vanity) winery on the west side of town dug big new wells and drenched their hillsides with two feet of water last summer (irrigation can be measured in hours applied, and/or depth of the water used if conceived as a horizontal sheet coming down all at once) - though a wine grapevine in this area needs no more than ten or twelve inches a year, these people were in a hurry, wanted to plant new vines in the heat of the summer, and poured on the water to soften up the rocky hillside soils. A neighboring small winery and vineyard, which has been there for decades, saw their well go dry soon afterwards.
Other vineyards on the east side of town, some very large, are pumping major gallonage to sustain big yields from the vines - more water, more fertilizer, more leaves, more grapes, less flavor, coming to a grocery store near you. First the Flood, then the Ziggurat of stacked cases!
As this is California, the water lawsuits and politics will go on for decades, with one certain outcome: there will be more attorneys and politicians planting vineyards and building wineries with their cut of the loot. So you need a consultant? Well, let me interview you first.