Heavy rain is still hitting California. A few reservoirs figured out how to capture more for drought

Despite several weeks of torrential rain and flooding, California is still facing a severe multi-year drought. That has many people thinking about how to better capture winter floodwaters to last through the dry season.

An innovative approach at two California reservoirs could help boost the state's water supply, potentially marking a larger shift from decades-old water management approaches to a system that can quickly adapt to precipitation in a changing climate.

At issue are rules that, at face value, seem perplexing to many Californians. Even in a chronically dry state, reservoirs are not allowed to fill up in the winter.

Throughout the late fall and winter, most are required to release water if they get too full, sometimes emptying out almost by half. That's because the empty space is crucial if an intense storm hits. Reservoirs collect runoff and prevent it from flooding downstream cities.

Still, in some years, reservoirs preemptively empty out with little need if no major storms materialize. That means valuable water is lost for potentially drier months ahead.

Two sites, Folsom Reservoir and Lake Mendocino, are rethinking this by using weather forecasts to guide their operations. Instead of sticking to set rules, they only empty out if a major storm is forecasted for the days ahead.

The parade of major storms that have hit California, known as atmospheric rivers, is providing a key test for these systems. Water experts say it's showing that "forecast-informed" reservoir operations have the potential to reshape how water is stored across the West.

"We have to use every drop of water that much more effectively," says Marty Ralph, director for the Center for Western Weather and Water Extremes at UC San Diego's Scripps Institution of Oceanography. "There's not much to spare, and we need to do the best we can to use that water efficiently."

Making water decisions in real-time

Most reservoirs have two jobs that are completely at odds with each other.

On the one hand, reservoirs need to be as full as possible to provide water for people and wildlife. On the other, staying empty ensures they can safely handle the runoff from major storms.

The stakes are huge for walking that line. If a dam is overwhelmed, potentially hundreds of thousands of people risk being flooded downstream. Stay too empty, and cities and agriculture run short of water when a drought hits.

Historically, reservoirs used fixed rules to guide those decisions, most created decades ago before human-induced climate change began fueling extreme weather. At Folsom Reservoir outside Sacramento, California, the water level could only reach 60 percent full in the winter. If more water flowed in, it had to be released. Some winters, where major storms stopped arriving, that water could have been safely stored and used later during the long, dry summer months.

After many years of study, water managers remade that system in 2019, working with the federal Army Corps of Engineers which is responsible for flood safety. Now, the reservoir can stay 20 percent fuller in the winter, though not completely full. Then, if a major storm appears, the reservoir makes space by releasing water three to five days ahead of time.

"Back when the dams were built, it was a pretty wise choice in my opinion not to use weather forecasts because they weren't very good," Ralph says. "But now with satellites and radars and models and science, there's been a lot of improvements so it seems sensible to give it a try."

The key is spotting atmospheric rivers, massive plumes of moisture that stretch hundreds of miles across the Pacific. Predicting where they'll land in California is crucial for forecasting how much runoff a reservoir will see. The relentless storms hitting the state this winter means water managers are continually recalibrating how much water Folsom Reservoir can hold.

"They're constantly rerunning these ensemble forecasts for river flows," says Drew Lessard, who manages Folsom Reservoir at the Central California office of the Bureau of Reclamation. "So it's working as intended, but it's certainly pretty dynamic."

Other Western reservoirs looking at dynamic methods

Closer to the Bay Area, Lake Mendocino is also using forecast-informed operations. A handful of other California reservoirs are in the process of studying it as well. The federal Bureau of Reclamation, the largest provider of water for utilities in the country, says it's looking into other places where it might be a good fit.

"The climate is changing, hydrology is changing, weather patterns are changing," says David Raff, chief engineer at the Bureau of Reclamation. "In addition to that, the demand for water is increasing in the Western United States. When you put those things together, there is a significant interest to optimize operations in all of our reservoirs."

The method may not be a good fit in all Western reservoirs, however. Some are affected by other weather patterns or melting snow that's harder to predict than California's weather. Other reservoirs, like on the Colorado River, have the capacity to hold so much water that releasing water during the flood season isn't much of an issue.

Water experts say as the climate gets hotter, Western water managers will need to use real-time data to be more responsive to the changing conditions. California is expected to see more "weather whiplash," the abrupt swings from extreme dry periods to extreme floods.

"Longer droughts, deeper droughts and bigger storms between them," Ralph says. "That's what Mother Nature is going to deliver us under a warmer climate. So we need to prepare. There's a lot at stake and these are methods that could really help us with climate adaptation."