Wednesday, September 27, 2023

Why can’t California just store all of its rainwater?

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

The western United States remains parched in a massive dry spell, the worst in over 1200 years. Spurred on by multiple atmospheric rivers, California’s torrential downpours in recent weeks have quenched some of that thirst. More rain and snow is expected this week. But on its own, this epic fallout can’t handle decades of hot, dry weather.

Throughout the west, rainfall is measured over the “water yearspread from October 1 to September 30 the following year. Cities like Sacramento have already received more than twice as much rain this winter than is typical for the entire water year. The rain has filled reservoirs and waterways that were only a fraction of their capacity last year. Reservoirs usually help to carry water year round. However, the relentless storms have overwhelmed the drainage, leading to hazardous and dangerous situations deadly flood.

A boy and girl ride their legs through foot-deep water that floods a street and intrudes into porches and garages.

Most of the heavy rain that has fallen in California in recent weeks is pouring into the ocean, doing little to alleviate the massive drought.
Bronte Wittpenn/The San Francisco Chronicle via Getty Images

Much of the water delivered to the Golden State during this month’s storms is now flowing back into the ocean instead of being saved 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 is 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 mega-drought and the urgency of giant rainstorms “puts an exclamation mark on the need to be creative in finding ways to squirrel away some of this water that’s coming at us fast and furiously,” said Thomas Hartera professor of land, air and water resources at the University of California Davis.

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 faces the possibility of even more frequent and extreme weather events between wet and dry, putting even more strain on water infrastructure.

Drought and downpours put pressure on water storage in California

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 mega-drought and recent atmospheric rivers have put pressure on them all, according to Harter.

Years of drought have dried out and compacted soil deposits, making it paradoxically more difficult for them to absorb water. Then, during heavy rainfall, dry streambeds and creeks turn into chutes that quickly funnel water downstream. This in turn leads to flooding. Meanwhile, the grasses and forests that used to anchor the soil are also extinct in many parts of the state, and as massive wildfires in recent years have left burns in pine forests and chaparral, that heavy rain plus hard, exposed soil is a recipe for mudslides.

Snowpack, on the other hand, stores massive amounts of water during the winter and slowly drains it off during the warmer seasons as it melts. Until recently, the Sierra Nevada snowpack – which usually converges 30 percent of California’s water needs – faced winters with warmer temperatures that led to a higher proportion of precipitation falling in the form of rain rather than snow. Last year, the Sierra Nevada stood up 38 percent of its capacitythe lowest levels in seven years.

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

Drought and hot weather have also led to lower water levels in reservoirs. California’s great reservoirs can be collectively stored 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. This equates to the annual water needs of two households.

Currently, large water bodies such as Shasta and Oroville remain below their historical average and at half their total capacity. That’s because reservoirs have two missions that can conflict with each other. One is to store and supply water for drinking water and farms, and the other is to help prevent flooding. Water managers purposely leave some free space in reservoirs, sometimes up to half their capacity, to hold runoff from potential storms later in the season.

Water flows through the spillway at Nicasio Reservoir in California after continuous rain. Many reservoirs release water rather than store it to leave room to collect runoff water from future storms.
Justin Sullivan/Getty Images

That leaves the groundwater, which functions as a savings account for water. In a typical year, groundwater supplies about 40 percent of the state’s water supply. During droughts, that share can rise to 60 percent. Groundwater holds up 1,300 million acre-feet of water. “We have a lot of space there to store that water,” Harter said.

The problem is that the water on the surface needs time to infiltrate underground into the groundwater supplies. And with more paved surfaces and farmland, there are fewer surfaces in California to replenish its reserves. With the mega-drought, Californians are increasingly drawing groundwater faster than it can replenish, and until a few years ago, that process was gone unchecked.

Exceeding groundwater reserves also entails a whole series Environmental problems. Streams and other water streams fed from groundwater can dry up. Salt water can enter and contaminate stores. The water table is falling lower, requiring deeper wells to access. In some parts of the state, cities and farms are drilling more than thousand feet deep reach water. Currently, 64 percent of groundwater monitoring wells are below normal levels, while 10 percent are above normal.

All of this adds up to a situation where despite an overabundance of rain, the west coast is still struggling to store it. “What has done so far this year is put a lot of money in our pockets,” 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 hold more water, but it’s getting harder

Improving water infrastructure is a slow and expensive process, but a lot of work is being done. Many reservoirs operate under U.S. Army Corps of Engineer guidelines that specify how much water they can hold at any given time of the season. That means some reservoirs are preemptively releasing water to allow room for runoff from storms that never arrived.

Now there is a push to make these stores adaptive. Water managers benefit from this at reservoirs such as Lake Mendocino improvements in weather forecasts. If they do not expect major storms in the coming weeks, they will allow the reservoir to hold more water in the winter. If rain is coming, they can already release part of their property.

“This is advanced, state-of-the-art, and it’s being tested in several basins in California,” Hatchett said. “We consider it one of the promising potential adaptation strategies for greater 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 such as the Central Valley to protect farmland and development. Now California’s water supply department is coming up with strategies to keep floodwaters from accumulating, sometimes calling managed replenishment of the aquifer. For example, farmers can flood fallow fields. There are also new rules determine how much groundwater communities can extract.

In an aerial photo, a car drives through floodwaters in Planada, California on Jan. 11, 2023.

Many parts of California have already received twice as much rain as normal for this time of year.
Justin Sullivan/Getty Images

The state is also chasing new reservoirs, but most of the ideal sites are already occupied, land values ​​have risen, and construction costs have risen, so it ends up being more expensive. “We have reached our limits in expanding our surface water reservoirs,” Harter said. California approved it seven water storage projectsbut they have languished in the planning stages for nearly a decade and none have been built.

All of these measures — increasing reservoir storage, building new infrastructure, restoring floodplains — would still capture only a small portion of recent rainfall and alleviate a small portion of the mega-drought.

California also needs to consider how its water levels play a role in concerns such as wildfires. Heavy rainfall early in the year can fuel a huge crop of fast-growing vegetation. “If those grow and then dry out quickly, early spring, then we have a big, long wildfire problem,” Hatchett said. “That’s why we want to keep rainfall in the spring to keep those plants and grasses happy.”

The climate is also changing. Severe rainfall events will become more common as the average temperature rises. That means California could see even more intense precipitation periods in the coming years, followed in many cases by dry spells.

So no matter how tired Californians are of the wet weather, the state will still need more rain year-round to meet its water needs and solve other problems. Floods and droughts remain pressing concerns and the state will need to prepare for both extremes.


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