“The physical laws of thermodynamics raise the ceiling on the intensity of these precipitation events in a really robust, reliable way,” says Swain. “It doesn’t mean it will always be raining, but it does mean that eventually all that water has to fall out somewhere.”

It’s fairly normal for all of California’s yearly water to fall in just a few days; the difference between a drought year and wet year could be just one or two storms. So western water policy wonks have long been doing a balancing act. The trick is to have enough storage space to capture the sky’s bounty when it arrives, so it doesn’t charge unbound into populated areas. For a long time water managers have been able to rely on the mountains to help out—locking up much of the winter’s precipitation as snowpack and gently doling it out over the summer months. But warmer temps are making those reserves less reliable. Adding bigger, more frequent rain storms only makes management more challenging.

“Increased variability just kicks everything up a notch,” says Ellen Hanak, water policy director of the Public Policy Institute for California. “You’ve got to think about being able to store more for dry seasons and keep more water out of harm’s way in really wet years.” Those things can be compatible, she says, but it will take building new infrastructure and changes in the way cities and rural areas develop their lands.

Rather than fighting the coming flood waters by building bigger levees, California water managers are looking at other strategies to allow water to spread out on the land and recharge underground aquifers. Particularly porous soils can resorb up to a foot of water a day. Keeping those areas undeveloped should be a priority for communities in the future, says Hanak. “It can’t just be about concrete and rocks anymore. Keeping space in reservoirs for winter flood flows and having levees keep water out of harm’s way as it moves through river systems has worked pretty well for decades. But going forward we’re going to have to rethink all that.”

Swain says there’s still a lot of work to be done to resolve his models down to a scale that could actually be useful for local water managers, but that’s where his research is headed next. Still, it’s not obvious any amount of planning will make a difference when that 200 (soon to be 50)-year storm comes. “It’s hard to exaggerate how large an impact it would have on the entire state,” says Swain. The best he can compare it to? A 7.5 magnitude earthquake hitting downtown Los Angeles. The US Geological survey even has a catastrophically appropriate nickname for California’s impending megaflood: “the other big one.”

Our Warming Climate