I have a grape vine in southern California (only rains in winter).
How much water does it need?
I have seen advice ranging from nothing to several gallons a day. Does anyone have any concrete numbers?
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Do not give your grapes gallons of water. Think of them like weeds because they're really resilient.
Without specialized equipment for measuring the water content (water potentials), you won't know exactly how stressed your vines are. But, you can gauge how vigorously they are growing by looking at the canopy and specifically the the growing ends of the tendrils. Have they shriveled up and turned black? If so, you should put on some water.
The vines in my vineyard generally receive 2-4 hours of water a week, from 1gph drip embitters. They are watered throughout the growing season. Generally, I water them heavily one last time following harvest or preceding a frost. You really don't want them investing too much energy into growing a larger canopy at that point, and want to force the vines to invest in root formation. Though, this may not be much of an issue for you being in Southern Cali.