Residents of the Sacramento area have been on a meteorological seesaw this past week: After 212 days without rain, the area saw precipitation again — which, this weekend, turned into an all-out rainstorm.
Already, the rainfall this weekend broke records: The area got more than half of the rain it had all of the last wet season, which was particularly dry, in a single storm.
“This is a reminder that even in the middle of a drought, you can have a flood,” said Jay Lund, co-director of UC Davis’ Center for Watershed Sciences. “That's just the nature of California's hydrology, and with climate change, it's supposed to become more that way.”
Extreme, but 'not a disaster'
In the past, experts have said that Sacramento is at risk of severe flooding, especially since it’s nestled between two rivers. But overall, experts have seen this rainstorm as a net positive for the Sacramento region’s water supply.
A storm of this magnitude would typically be more concerning, but UCLA climate scientist Daniel Swain said that the region’s dry conditions made it safer than usual.
“It's pretty hard to get this much water falling that quickly without causing harm,” Swain said. “But the context, the fact that this event was the first big storm of the season, that it occurred in the middle of an extreme drought, that the landscape just kind of soaked a lot of it up … this was an interesting case where you can talk about an extreme weather event that was not a disaster.”
If soil in the Sacramento area had been wet, more flooding could have occurred. But after months without precipitation, the soil was particularly dry, which protected the Sacramento and American rivers from flooding over.
“You saw some local flooding, but you're not seeing the main rivers get in the flood stage, because that has to fill up all that soil moisture up in the watershed before it hits the river,” Lund said. “And then when it hits the river, it's hitting the empty reservoirs on its way downstream.”
With the soil now wet after this heavy rainstorm, Lund added that this helps set the stage for the rest of the season, helping ensure that “the rest of the storms this wet season will produce more runoff.”
State of variability
This rainstorm, following a dry period, isn’t completely surprising. California has always been a state known for its weather variability. There have been other instances of intense precipitation, like the heavy rainfall in 2017 that led to the Oroville dam crisis.
But Swain said that particularly intense precipitation during periods of dryness are expected to become the norm due to climate change.
“In a warming climate in California, we don't actually expect there to be a tremendous change in the overall average precipitation,” Swain said. “But we do expect there to be an increase in the severity of droughts and an increase in the intensity of precipitation events.”
This rainfall, he explained, came by way of atmospheric river, which is a stream of water vapor carried through an area that’s often accompanied by intense rainfall. In a warming climate, the atmosphere’s capacity for holding water vapor increases exponentially, so the threat of intense precipitation carried by atmospheric rivers spikes too.
But that doesn’t mean it will rain more all the time. Swain said that the state has actually been seeing drier conditions than ever, despite atmospheric river storms like this one occurring when it does rain.
That’s why Swain warns that metrics only looking at how much rain has fallen could be deceiving when it comes to preparing for drought or flooding.
“If you take the average precipitation now for the past few months, we're way above average just because of yesterday's rain,” he said. “But it doesn't really characterize the experience of the past few months very accurately.”
Not enough to end the drought
So, despite the storm’s positive impact on local water supply, preparations for drought conditions continue.
Ryan Ojakian, legislative and regulatory affairs manager for the Regional Water Authority, said Sacramento would need much more precipitation before it's in the clear — about 140% of what the area gets in a typical year in the wet season that goes from now until April, according to projections he’s seen.
“It's going to take a really wet year for us to feel like we can say the drought is over,” Ojakian said. “As water suppliers in the Greater Sacramento area, we're going to be preparing and behaving as if we have dry conditions until we can see otherwise.”
In the Sacramento area, water agencies are working to adapt the region’s resources to these concerns. Ojakian said that means building up our capacity to use groundwater during dry seasons and using surface water, like rivers and reservoirs, during wet seasons.
So far, the Sacramento region is only using about a quarter to a third of its identified groundwater capacity. In order to become more resilient in times of drought and uncertainty, when surface water sources like reservoirs are less reliable, Ojakian said the region needs to work on improving that use of capacity. Taking from both resources strategically, he said, is “how you will get to climate resilience.”
And while the region has already spent years building out this infrastructure, Ojakian said local water agencies need to continue to stay on track with the conditions climate change will only exacerbate.
“We were already swinging between extremes under ‘normal conditions,’ between wet and dry, he said. “This last year and yesterday [is] a prime example of that… that is climate change in a nutshell.”
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