As drought-weary Californians watched trillions of gallons of runoff wash into the Pacific Ocean during recent storms, it underscored a nagging question: Why can’t we save more of that water for not-so-rainy days to come?

But even the rare opportunity to stock up on the precious resource isn’t proving enough to unite a state divided on a contentious idea to siphon water from the north and tunnel it southward, an attempt to combat the Southwest’s worst drought in more than a millennium.