Why California’s atmospheric rivers can’t break the megadrought

Why California’s atmospheric rivers can’t break the megadrought

A “river” more than 100 miles wide is flowing through the air high above California, bringing with it heavy rain, winds and snow. It is the third in a series of weather systems known as atmospheric rivers — long, heavy columns of water vapor in the sky — to hit the state in the past two weeks.

It has already proven deadly: Two people have died as a result of the storms, including an infant; roads are flooded or hit by mudslides, forcing evacuation; and more than 180,000 Californians lost power. On Wednesday, California Gov. Gavin Newsom declared a state of emergency ahead of the storm’s arrival, and the city of San Francisco ran out of sandbags for the second day in a row as residents rushed to protect their homes from possible flooding.

After the storm passes, there will be some time: Another storm surge is forecast to hit the state this weekend and next week, bringing even more flooding.

California is looking sunk at the moment, but for the past two decades, it has been suffering from a mega-drought of the kind not seen in more than 1,000 years. The drought threatens the region’s agricultural industry and ordinary citizens, putting livelihoods at risk and raising concerns about what the future of life in the West might look like.

Which, of course, can raise a simple question: Can all this rain, despite the suffering it brings, help alleviate the drought?

The simple answer: Unfortunately not. A flood during a time of drought is a double disaster.

Reason 1: Too much water at once

As we wrote last August, droughts and floods are something of a vicious circle. It takes time for water to soak into the ground, and multiple storms hitting each other is like overwatering a potted plant: The ground simply can’t take any more water. Eventually the rain turns into floods, which further erode the soil and bring the risk of falling trees, which can take down power lines and damage buildings; A 2-year-old child was killed this week when a redwood fell on a mobile home in Sonoma County.

“We’re in the middle of a flood emergency and also in the middle of a drought emergency,” California Department of Water Resources (DWR) Director Karla Nemeth said at a news conference Wednesday. “This is an extreme weather event and we are going from extreme drought to extreme flooding. This means that many of our trees are stressed, after three years of intense drought, the ground is saturated and there is a significant chance of trees falling which will create significant problems.”

In non-drought conditions, tree roots act a bit like sponges, absorbing water from the soil. But droughts make tree roots less sponge-like, meaning they can’t absorb as much water at once. This also makes roots weaker and trees more susceptible to falling during extreme flooding.

If the rain had been spread over a series of months, it could have helped with the drought by filling reservoirs over time, said Noah Diffenbaugh, a climate scientist at Stanford University’s Water in the West program. The ground would also be less saturated, allowing more water to soak in more slowly, recharging groundwater wells and reducing the chance of flooding.

Instead of collecting in reservoirs or sinking into the ground, the water has nowhere to go productively. So it floods.

Reason 2: Too little water at all

Expecting these extreme rain events to relieve drought is a bit like racking up thousands of dollars in debt over months and only getting one or two paychecks at the end of the year.

“Most people wouldn’t say the problem is solved because of a normal monthly payment,” Diffenbaugh said. “A normal year with rainfall would not break the drought. In fact, even a wet year would not necessarily break the drought.”

California’s “mega Drought” designation is a recognition that the state has gone through a series of drought years with relatively few wet seasons. Breaking the drought would require many years of above-average rainfall and snowfall.

As things stand, the atmospheric rivers hitting the state have filled smaller reservoirs to capacity, while the big reservoirs still remain largely empty.

Filling smaller reservoirs to capacity isn’t exactly good news: those reservoirs are used for flood control as well as storage, meaning the risk of flooding increases as there’s nowhere else for the water to go. Water cannot be diverted into large reservoirs either, as the system used to move water across the state is not designed for fast and severe events like these atmospheric rivers, and building a system that can do so will take investment massive in time and money.

Reason 3: Climate change is causing the snow to melt earlier

Reservoirs are also only one piece of the California water puzzle. Equally important is the snowpack, or snow accumulated in the mountains, which acts as a natural water storage system and provides about 30 percent of the state’s water.

Atmospheric rivers are bringing snow to the Sierra Nevada mountains, but the snow line is moving higher and higher as climate change intensifies, meaning there is less snow overall and the snow doesn’t last as long as it used to.

A snow survey conducted by the DWR earlier in the week showed that the storms that hit California in December brought a significant amount of snow with them, but the question is whether that snow will last all year. A January 2022 snow survey returned the seventh-highest readings on record for that location, but by April 1, most of that snow was gone, leading to the third-lowest readings on record for the same place. Losing that snow early means it won’t be available during the summer months, when water is most needed.

Storm surges like those hitting California this winter will continue to hit the state. As climate change continues to transform the West’s water realities, Diffenbaugh said, the state will likely retool its infrastructure to capture more stormwater from those events and reduce its reliance on the ever-disappearing snowpack.

“We have a lot of water infrastructure systems that are really sophisticated,” Diffenbaugh said. “Part of the challenge is updating those really well-developed systems to be resilient and prepared for the challenges this new climate presents.”

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