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Climate and Environment

LA Is Capturing More Rain, But Increasingly Extreme Storms Present A Challenge

Photo of Tujunga Dam, surrounded by tree-covered hills, and a river winding through the center.
Tujunga Dam is one of the largest of 14 dams in the San Gabriel Mountains that serve as a local water supply source for communities across the L.A. County.
Flickr Creative Commons)
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It seems like we’re always in a drought in Southern California, so when it rains, the question becomes: Where did all that precious water go?

What's in an acre-foot?
  • One acre-foot is about the amount of water it would take to fill a football field a foot deep. The city of L.A. uses about 500,000 acre-feet of water every year. 

Well, not all of it ends up in the ocean. According to Art Castro, watershed manager at the L.A. Department of Water and Power, in the last four days alone, the city of L.A. captured enough water to fill about 8,900 Olympic-sized swimming pools.

And the county’s system of dams and spreading grounds was boosted by 213,200 acre-feet (69.5 billion gallons); enough water for more than 1.7 million people for a year, according to Steve Frasher, a spokesperson with L.A. County Public Works. For comparison, the city of L.A. uses about 500,000 acre-feet of water per year.

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Most of that water fell into dams in the San Gabriel Mountains, the largest of which are the Tujunga and Pacoima dams. That water will then be slowly released into spreading grounds so it can seep into underground basins that store much of our local water.

Why catching more local rain matters

The city of L.A. currently pipes in about 90% of its water from reservoirs fed by Sierra Nevada snowmelt and the Colorado River. As the climate crisis and overuse threaten those traditional supplies, the city’s goal is to be able to capture 150,000 acre-feet of stormwater in a year by 2035.

L.A.'s Stormwater Goals
  • The city’s goal is to capture a max of 150,000 acre-feet of stormwater in a year by 2035. That’ll require cleaning contaminated groundwater basins like those in the San Fernando Valley and dramatically expanding permeable infrastructure at schools and parks and in streets and alleyways to help capture more water as well as alleviate flooding.

A quick history of water in the city

Southern California has long been a place of drought and deluge. When L.A. was developing in the early 20th century, “water management” was primarily focused on controlling flooding and getting stormwater to the ocean as quickly as possible. That’s why the L.A. River was paved and our asphalt city streets are lined with storm drains.

As the city grew, local groundwater supplies were quickly stretched thin (and contaminated by a booming oil and auto industry), so the city started importing water from the Sierra Nevada and Colorado River.

That led to what we now view as the norm: a heavily engineered system that helps us perform the delicate act of boosting water supply and controlling flooding. But that system is being challenged by the climate crisis, said Castro of the L.A. Department of Water and Power.

Where we go from here

The climate crisis is driving increasingly unpredictable and extreme weather, but climate projections show Southern California is still expected to get similar total amounts of rainfall every year. The difference is that rain is increasingly likely to come in really intense storms instead of consistent precipitation throughout the winter months. That’s a big challenge for flood and mudslide danger, as well as water supply management.

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“In the last 20, 30, 40 years, our storms in Southern California have been pretty boring. They’re not very intense,” Castro said. “We're looking at a dramatic increase in intensity, which our systems are not built for.”

This weekend’s storm alone brought a glimpse of that. Castro said that, for a three-hour period, rainfall was a third of an inch per hour, far above the average of 0.1 inches per hour.

“If climate change is going to bring us more intense storms, we're going to have to change the way we design stormwater capture projects,” Castro said.

But he added there’s little room left in our concrete metropolis to add new dams or expand spreading grounds, like the Tujunga spreading basin in Sun Valley.

“So now we're looking at what kind of open spaces are there in the city of L.A.,” Castro said.

He said that means a significant piece of adapting to the new climate reality requires making our concrete metropolis more permeable as a whole. That looks like converting flood-prone streets and alleys into “green ways," retrofitting parks and school campuses to catch stormwater, as well as adding lots of new park spaces that can help alleviate flooding and capture more rain when it comes.

Castro said the lowest-hanging fruit includes existing parks and school campuses. But a recent report by watchdog group L.A. Waterkeeper found those efforts are moving very slowly.

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