California has been blessed with a wet winter this year. At the time of publishing, most of the state is at or well above the historical average precipitation to date for this time of year and Sierra Nevada snowpack is at more than 140% of historical average. That’s been good news for the California plants, animals, and humans that rely on water to survive and recreate. But lots of precipitation now doesn’t necessarily mean that California will have lots of water when it needs it. That’s because what matters is not only how much water we get, but when and how we get it.
The “bathtub” of California storage doesn’t capture most water from extreme rainfall
Consider California’s water system as a bathtub. That bathtub has a faucet pouring water in and a drain pulling water out. The faucet in this analogy is our precipitation, the drain is our water use, and the bathtub is our storage capacity—that is, how much water we have available to use in our three main types of storage: surface reservoirs, groundwater aquifers, and snowpack. There’s only so much water California’s natural and human systems are using at any given time – only so fast that the drain pulls water out. If water (snow and rain) were to flow in through the faucet at the same rate as the drain pulls water out, then we wouldn’t need the bathtub or the storage system. But this almost never happens in California’s highly variable hydroclimate. In reality, the faucet often flows fast, the bathtub fills up quickly (two-thirds of California’s yearly precipitation falls in less than 4 months – a shorter wet season than anywhere else in the United States), and for the rest of the year—or year-round during a drought—we’re mostly draining water out.
Now imagine that we point a firehose at our bathtub for a few minutes—that’s more or less what the atmospheric rivers that bring California’s extreme precipitation events do. It’s what happened to the state in recent storms and is happening this week. Some of these atmospheric rivers can carry an amount of water vapor into California that’s up to 15 times what flows out of the mouth of the Mississippi River. Because our proverbial bathtub isn’t big enough or located in all the right places to hold that much water, most of that water – some experts estimate up to 80% – goes over the tub’s edge, which in our analogy would be into the Pacific Ocean. So just because the faucet is turned on full blast over the state, it does not mean that people, plants, and animals will have all of that water available to use during the dry summer months. Add to that the risks to people and property that come with getting our water in this extreme form, including the flooding, evacuations and mudslides along the Russian River that have come with this week’s storms.
Extreme precipitation will likely intensify with climate change
Atmospheric rivers are a natural part of California’s water cycle, and it’s long been the case that we get most of our water in just a few storms. Our recent storms reflect this: in just the last month, California went from average year-to-date precipitation to more than 20% above average. Across most of Southern California, 20-30% of precipitation comes from just one atmospheric river event.
Climate change is likely to intensify our storms, and further compress California’s precipitation into extreme events, as a number of studies have shown. In the next 50 years, another 25-30% of California’s annual rainfall could come during these extreme events. That’s a third more of our precipitation coming out of the faucet in a way that our bathtub currently isn’t well equipped to hold. Oroville Dam was almost exactly 50 years old when its spillway failed during an extreme rainfall event in 2017, raising concern that the dam itself would fail and leading to the mandatory evacuation of nearly 200,000 downstream residents. These increasing extremes brought about by climate change will happen well within the lifespan of our current infrastructure.
So what do we do? Build bigger, stronger bathtubs? Put our bathtubs in different places? Use other peoples’ bathtubs? Change the size of the drain?
We need a smarter bathtub
We could cope with these changes in California’s hydroclimate if our bathtub were smarter. At the moment, our snowpack is storing a lot of the deluge we’ve gotten over the last few months. But climate change is already jeopardizing that part of our storage. As temperatures rise, more precipitation will fall as rain rather than snow; the snow that does fall will melt and run off more quickly; and there will be increased risk of rain-on-snow events, in which warm rainfall washes down huge amounts of snowpack at once.
We can pick up some of that slack with our dams and reservoirs. There is valuable work happening now to improve the way we forecast atmospheric river events and incorporate those forecasts into reservoir operations, so that we can store more winter water while still keeping our rivers and wetlands healthy and buffering against floods. But it’s hard to imagine where and how we would build enough surface reservoir capacity to capture and store the massive precipitation and runoff events of the future. Meanwhile, the consequences of our reservoirs failing to hold these flood events is only too fresh, as the newly reconstructed Oroville Dam spillway awaits its first use.
Luckily, the state of California has over 850 million acre feet of available groundwater storage – more than 15 times the available surface reservoir capacity – that could replace snowpack as our natural reservoir system as we get more of our precipitation in liquid rather than solid form. The 2014 Sustainable Groundwater Management Act, which creates a statewide framework for stabilizing California’s groundwater resources, could open the door to using groundwater to sustainably manage California water under a changing climate. There’s plenty of work left to do in figuring out where and how to recharge those floodwaters into our enormous underground storage, but there are lots of promising leads. Urban areas are also increasingly figuring out strategies to capture and use stormwater locally.
Profound changes are already underway in California’s climate and our water supply. As the climate changes, how and when we get our water in California is changing and so must how we manage it. If we’re going to be able to keep using what we get, we need to need to start creating a smarter bathtub now.