Drought’s Impact On California Winegrapes

When it comes to the impact the drought is having on winegrapes in California, it all depends on location.

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Larry Williams, professor and plant physiologist-AES at the University of California-Davis, says that grapes in the San Joaquin Valley are OK for now, because growers have been able to pump groundwater for irrigation. There has also been minimal impact on coastal winegrapes in Napa and Sonoma counties, thanks to reservoirs and groundwater.

But the future is uncertain. “If the drought continues, then the state, I think, may start limiting how much water you can pump from the ground,” Williams says.

The Paso Robles region is already facing some problems.

“The groundwater there is somewhat saline. If they use that water, it may have a salt impact on the grapevines there,” Williams says. “Grapes don’t like salty water. They perform poorly.”

If water is saline, one approach is to apply more water to try to leach the salt out of the soil. But, if you have less water available to pump, then less salt will be leached out of the soil. “That’s what’s nice about winter rainfall – it will leach the salt to a deeper depth,” Williams says.

The other issue is that the groundwater table continues to get deeper. “Some growers I know have to drill deep wells so they can get down to where the water is,” Williams says.

And while some water stress is OK for grapevines, severe stress will negatively impact wine quality in the form of less sugar accumulated in the fruit and lower alcohol in the wine.

Varietal Considerations
Williams is currently working on a study looking at 16 different red wine varieties to see how they respond to water stress. “We’ve found, of the 16, some just produce more fruit than others with less water, so it’s not that they respond differently to water stress – they just have more fruit to start.”

Rootstock rather than variety could be the bigger determiner of water stress tolerance.  “Some rootstocks have roots that grow deeper, so if they grow deeper, they’re going to tap into water farther down in the soil profile,” Williams says.

He adds that grapevines can go a year without water and still survive. “The question is, if you don’t give them any water this year, and you do get rain next year, what impact will that have?” he says. “I’ll probably start looking at that next year.”

Minimizing The Impact Of Water Stress
There are some things growers can do to try to minimize the impact of water stress. Williams and his colleagues have found that if you stress vines between berry set and veraison, there will be a much greater negative impact on final yield than if you stress them between veraison and harvest. So for a better yield, be sure to apply water between berry set and veraison.