Water from the big rains soaked into what amounts to a subterranean reservoir. A study by Luhdorff & Scalmanini calculated the Napa Valley subbasin alluvial aquifer in 2017 held 219,000 acre-feet of water, compared to a 20-year low of 191,000 acre-feet in 2014. One acre-foot is the equivalent of about 326,000 gallons.

That 219,000-acre feet is the equivalent of one-seventh of the water in a full Lake Berryessa, 71 billion gallons, enough to fill 107,000 Olympic-size swimming pools. Water depth ranged from a few feet to 15 feet in most places beneath the Napa Valley floor in the heart of wine country.