Why can’t California just save all its rain water?

Wedged between deluge and drought, most of California’s recent rain is washing away. But more of that water could be saved up for a sunny day.

The western United States remains parched in a massive dry spell, the worst in more than 1,200 years. Spurred by multiple atmospheric rivers, the torrential downpours over California in recent weeks have quenched some of that thirst. More rain and snow is expected this week. But on its own, this epic precipitation can’t unto decades of hot, dry weather.

Throughout the West, rainfall is measured across the “water year” spanning from October 1 until September 30 the following year. Cities like Sacramento have already received more than twice as much rainfall this winter than is typical for the entire water year. The rain has filled in reservoirs and waterways that were only at a fraction of their capacity last year. Reservoirs typically help dispatch water throughout the year. However, the relentless storms have overwhelmed drainage, leading to dangerous and deadly flooding.

Most of the torrential rain in California in recent weeks is flowing into the ocean and doing little to alleviate its massive drought.
Brontë Wittpenn/The San Francisco Chronicle via Getty Images

Much of the water delivered to the Golden State in this month’s storms is now flowing back into the ocean rather than being saved up for the rest of the year. That’s partly due to inadequate infrastructure and limitations in how quickly the landscape can absorb water. But it’s also due to water management decisions, including deliberately limiting water storage in reservoirs below capacity due to flood control requirements.

The combined stress of the megadrought and the urgency of gargantuan rain storms “puts an exclamation mark on the need for being creative around finding ways to squirrel away some of this water that’s coming fast and furious at us,” said Thomas Harter, a professor of land, air, and water resources at the University of California Davis.

There are several efforts are already underway to increase the state’s storage capacity, from improved forecasting to building new storage facilities to deliberate flooding to allow underground layers of water-permeable rock known as aquifers to refill. But as average temperatures rise, the West Coast is facing the possibility of even more frequent and extreme weather whiplash between wet and dry, further stressing water infrastructure.

Drought and downpours are stressing California’s water storage

There are four main places where California can store its water: in soil and vegetation, in mountain snow, in surface reservoirs, and in groundwater basins. The ongoing megadrought and the recent atmospheric rivers have stressed all of them, according to Harter.

Years of drought have led soil deposits to dry out and compact, paradoxically making it harder for them to absorb water. Then, during intense rainfall, dry streambeds and creeks turn into chutes that rapidly channel water downstream. That in turn leads to flooding. Meanwhile, the grasses and forests that used to anchor the soil have also died out in many regions of the state, and since massive wildfires in recent years have left burn scars across pine forest and chaparral, that heavy rain plus hard, denuded soil is a recipe for mudslides.

Snowpack, on the other hand, store huge amounts of water through the winter and discharge it slowly throughout the warmer seasons as it melts. Until recently, the Sierra Nevada snowpack — which usually meets 30 percent of California’s water needs — faced winters with warmer temperatures that led to a larger share of precipitation falling as rain rather than snow. Last year, the Sierra Nevada was at 38 percent of its capacity, the lowest levels in seven years.

This winter, parts of the Sierra Nevada mountains have snow at more than 260 percent of average levels for this time of year. That bodes well for water supplies in the West. But snow isn’t immediately accessible to drink, and changes in weather like an early-season heat wave could start to deplete these reserves before they can be used. “While some places have record snow on the ground for mid-January, there is still a long winter ahead and weather patterns can change,” Keith Musselman, a scientist at the Institute of Arctic and Alpine Research at the University of Colorado Boulder, said in an email.

Drought and hot weather have led to lower water levels in reservoirs, too. California’s major reservoirs can collectively store 45 million acre-feet of water. An acre-foot of water is enough to cover an acre of land 1 foot deep, about 326,000 gallons. That adds up to the annual water needs of two households.

Currently, water levels major reservoirs like Shasta and Oroville remain below their historical average and at half of their total capacity. That’s because reservoirs have two missions that can conflict. One is to store and provide water for drinking and for farms, and the other is to help prevent floods. Water managers deliberately leave some headroom in reservoirs, sometimes up to half their capacity, to hold onto runoff from potential storms later in the season.

Water flows down the spillway at Nicasio Reservoir in California after relentless rain. Many reservoirs release water instead of saving it in order to leave room to collect runoff from future storms.
Justin Sullivan/Getty Images

That leaves groundwater, which functions like a savings account for water. In a normal year, groundwater provides about 40 percent of the state’s water supply. During droughts, that share can rise to 60 percent. Groundwater holds upward of 1,300 million acre-feet of water. “That’s where we have significant space to store that water,” Harter said.

The problem is that it takes time for water at the surface to infiltrate underground into groundwater stores. And with more paved surfaces and farmland, there are fewer surfaces in California to recharge its reserves. With the megadrought, Californians have increasingly drawn on groundwater faster than it can refill, and until a couple years ago, that process had gone unchecked.

Overdrawing from groundwater reserves also brings a host of its own environmental problems. Streams and other water flows fed from groundwater can dry out. Saltwater can intrude and contaminate stores. The water table falls lower, requiring deeper wells to access. In some parts of the state, cities and farms are drilling more than a thousand feet deep to reach water. Currently, 64 percent of groundwater monitoring wells are below their normal level, while 10 percent are above normal.

All this adds up to a situation where despite a surfeit of rain, the West Coast can still struggle to save it up. “What this year’s doing so far is putting a lot of money in our wallet,” said Benjamin Hatchett, an assistant research professor of atmospheric sciences at the Desert Research Institute in Reno, Nevada. “We want to hopefully move some of that back into our savings account.”

California could bank more water, but it’s getting harder

Enhancing water infrastructure is a slow, expensive process, but there are efforts underway. Many reservoirs operate according to guidelines from the US Army Corps of Engineers that specify how much water they can hold at a given time of the season. That means some reservoirs preemptively let out water to leave room for runoff from storms that never arrived.

Now there’s a push to make these stores adaptive. At reservoirs like Lake Mendocino, water managers are taking advantage of improvements in weather forecasting. If they don’t anticipate major storms in the weeks ahead, they allow the reservoir to bank more water in the winter. If there is rain on the horizon, they can release some of their holdings in advance.

“This is cutting-edge, state-of-the-art, and it’s being tested out in several basins in California,” Hatchett said. “We do view it as one of the very promising potential adaptation strategies to increased climate variability.”

Another strategy is to restore floodplains so that accumulated surface water can replenish groundwater. For decades, the state has tried to limit flooding in areas like the central valley to protect farmland and development. Now, California’s water resources department is coming up with strategies to let flood water accumulate, sometimes call managed aquifer recharge. Farmers, for instance, can allow flooding on fallow fields. There are also new rules governing how much groundwater communities can extract.

In an aerial view, a car drives through floodwaters on January 11, 2023 in Planada, California.
Many parts of California have already received double the amount of rain that is typical by this time of year.
Justin Sullivan/Getty Images

The state is pursuing new reservoirs as well, but most of the ideal sites are already taken up, land values have increased, and construction costs have risen, so it ends up being more expensive. “We have hit our limits on expanding our surface water reservoirs,” Harter said. California did approve seven water storage projects, but they’ve been languishing in planning stages for almost a decade and none have been built.

All these measures — increasing reservoir storage, building new infrastructure, restoring floodplains — would still only bank a tiny fraction of the recent rainfall and will alleviate a small portion of the megadrought.

California also has to consider how its water levels will play into concerns like wildfires. Heavy rain early in the year can fuel a bumper crop of fast-growing vegetation. “If those grow and then dry out soon, early in the spring, then we have a big, long wildfire problem,” Hatchett said. “That’s why we want to keep precipitation coming in the spring to keep those plants and grasses happy.”

The climate is also changing. Severe rainfall events are poised to become more common as average temperatures rise. That means California could face even more intense precipitation periods in years to come, in many cases followed by dry spells.

So as tired as Californians may be of the wet weather, the state will still need more rainfall throughout the year to meet its water needs and stem other problems. Flooding and drought remain urgent concerns and the state will have to prepare for both extremes.

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