California is no stranger to big swings between wet and dry weather. The “atmospheric river” storms that have battered the state this winter are part of a system that has long interrupted periods of drought with huge bursts of rain — indeed, they provide somewhere between 30 and 50 percent of all precipitation on the West Coast.

The parade of storms that has struck California in recent months has dropped more than 30 trillion gallons of water on the state, refilling reservoirs that had sat empty for years and burying mountain towns in snow.