It means that one gallon of water that could've been used elsewhere in California during the period of growth went into the clouds and is likely going to rain d

It means that one gallon of water that could've been used elsewhere in California during the period of growth went into the clouds and is likely going to rain down a few thousand miles away. The point of the "n gallons of water per x fruit" is saying that one certain crop is incredibly inefficient with its water consumption, and it'd be better to use it on something else. It's like saying a bloated web browser consumes 500 MB of memory/tab. Your memory isn't gone forever after you use it, but so long as you're using that browser or growing that crop, it's draining that resource away from other applications. You probably only have 8-16 GB of memory to use at once, just like a region only has N gallons of water it can use at any one time.