It was a perfect storm of, well, pretty perfect storms.

There was a lot of rain and snow during California’s just completed “water year,” from Oct. 1, 2022 to Sept. 30 — nearly double the historical average in the southern half of the state. But all of that rain didn’t fall too fast, and snowpack-melting temperatures didn’t spike too high, making it possible for most areas to avoid major flooding.

The agencies that capture and store stormwater also have become better at finding ways to keep more of that precipitation in Southern California rather than letting it all run out to the ocean.

Recent projects by the Chino Basin Watermaster, for example, which manages the aquifer that sits under much of northwestern Inland Empire, allow the agency to capture an additional 4,000 acre feet of stormwater. (Each acre foot is enough to serve two households for a year.) And given how much rain fell, Justin Nakanowater, who serves as the agency’s manager of technical resources, said the Chino Basin was able to hold onto 20,000 acre feet of water this year — two and half times more than last water year.

That’s helping to replenish reservoirs and groundwater basins that had been depleted by persistent drought, with lakes and rivers also looking much better than this time last fall.

It’ll take many more perfect storms to make up for past deficits, though, water experts caution.

“Just one rain year does not get us out of a drought,” said Kelly Gardner, assistant executive officer for the Main San Gabriel Basin Watermaster.

Forecasters are cautiously optimistic that the El Niño season shaping up offshore might mean another wet winter ahead.

That’s actually putting pressure on water agencies to ready systems that, in some cases, haven’t fully dried out from the past year’s storms. And they’re using technology and getting creative to further boost stormwater capture and storage options, since much more water still heads to the ocean during big storms than agencies are able to divert and hold onto for the inevitable dry years to come.

Rainfall in context

This time last year, most weather experts were predicting another dry winter ahead. Instead, 33.56 inches of rain fell statewide in the most recent water year, which is 141% of the historical average. And the South Coast region, which includes most of non-desert Southern California, did even better, with 33.62 inches of rain for 192% of the historical average.

It was a one in 50-year event, according to Dennis Lettenmaier, a UCLA professor focused on hydrology.

It was also the most rain the area has seen since the 2004-05 water year, when the Department of Water Resources recorded 39.96 inches. But that year, Gardner said most of the rain fell in a 30-day window. The U.S. Army Corps of Engineers, which controls her region’s Santa Anita Dam and many others in the area, had to fill and empty reservoirs above the dams as quickly as possible to prevent flooding, which meant releasing much of that year’s stormwater out to the ocean.

Just two years later, we saw the least amount of precipitation since the agency started tracking these figures in 1981, with just 5.8 inches of rain during the entire 2006-07 water year. Most years since have been pretty dry, with 2011, 2017, 2019 and now 2023 as exceptions.

“California does have the highest annual variation in precipitation of any state in the U.S,” said Jeanine Jones, interstate resources manager for the Department of Water Resources. “So for us, these wild swings from wet to dry are pretty normal.”

But when you look at the historic record, Jones noted, this century overall has been hotter and drier in California than any other 23-year stretch.

“One of the expectations with climate change is that extremes become more extreme,” Jones said. “So while we experience warming and drying overall, and more drought risk, the flip side of that is some of the big winter storms we have are expected to become worse or more extreme.”

For water agencies, Gardner said that means planning for the possibility of another wet winter while keeping the overall trend toward more frequent and severe droughts front of mind when managing local supplies. And they’re urging residents to think the same way, making conservation practices — such as reduced outdoor watering, which is typically mandated only during dry years — a permanent routine.

“We really can’t say what we’re going to have this winter,” Jones said. “And that’s always why we have to prepare for either extreme.”

Surface water solid, with caveats

The good news is that even if it turns out to be a dry El Niño year, California’s 17 major water reservoirs are at 127% of average levels. That means residents who depend on those surface reservoirs for large portions of their water supply probably won’t face serious restrictions next spring or summer, even if we return to drought conditions.

Southern Californians, however, don’t get much water from reservoirs; we have just four in the region. Among those, the Cachuma reservoir near Santa Barbara and Castaic reservoir in northwest Los Angeles County saw the biggest jumps during the most recent water year, with both now full to 92% of their capacities. The Diamond Valley reservoir in Hemet is at 83% capacity, while Casitas in Ventura is 72% full.

Those reservoirs are always required to leave capacity for quick surges in stormwater, Jones said. So even if we do have another wet winter, she said there shouldn’t be concerns about flooding around those reservoirs, even as agencies try to hold as much water in them as they can.

Lakes that don’t serve as reservoirs — but are important for everything from wildlife to recreation to firefighting — also are looking good.

In late August 2022, Big Bear Lake was 16.5 feet below its full mark, which put it at less than 50% capacity, leaving docks stranded and adjacent wetlands dry. This August, the lake was down just 6.9 feet and nearly three-quarters full.

Rivers and streams — including the Colorado River, which is critically important for Southern California’s imported water supply — can be more temperamental.

Since rivers tend to get at least some of their supply from baseflow, or flows fed by groundwater, a new study out of UC Riverside shows they don’t recover as quickly or easily from drought years as do some other types of surface water. It took the Arroyo Seco stream near Pasadena nearly a year to recover each time drought hit over three decades tracked in the study, while some of the other 350 sites included in the study took up to 3.5 years to bounce back.

Underground water tougher to gage

The lag in drought recovery for underground basins happens as surface water seeps through layers of sediment and rock that can run more than 1,000 feet deep. Couple that with decades of people in some parts of the state pumping too much from aquifers and it’s easy to see why groundwater numbers didn’t jump up the way reservoir and lake numbers did during the last wet water year.

Some 42% of groundwater monitoring wells still show as below normal, per the state.

But while groundwater basins in the Central Valley have been overdrafted for years, to the point that land in some areas is physically sinking, Jones noted that Southern California’s aquifers have been much better managed.

“Our groundwater table has come up about 50 feet,” Gardner said of the basin in eastern Los Angeles County. “That’s a big improvement over almost hitting a historic low.”

Since recent storms were more spread out than the rains that fell during the 2004-05 water year, Gardner said her agency was able to capture more water even though rainfall totals were a bit lower. They then let hundreds of thousands of gallons of water percolate back into the groundwater basin through spreading grounds near the interchange of the 605 and 210 freeways.

The Orange County Water District, which manages a large underground basin that supplies much of the water for northern Orange County, has made substantial investments to create similar recharge stations near the intersection of the 91 and 55 freeways.

John Kennedy, the district’s executive director of engineering and water resources, said in a typical year they capture 53,000 acre feet of water. Last year, they got 94,000 acre feet. They have so much water that even though wholesalers are offering imported water at good prices, to use now or to store for future dry years, Kennedy said his district isn’t buying.

The Chino Basin Watermaster is actually storing some of that imported water in its catch basins on behalf of the Metropolitan Water District of Southern California, Nakanowater said. They’re hoping more water agencies decide to buy that water and have places to store it. Otherwise, if more storms come and Chino needs space in those catch basins to store its own captured stormwater, large volumes of water could be released to the ocean.

There just aren’t enough reservoirs in Southern California to capture all or even most of the rain that falls in wet years, Kennedy noted. While his agency held on to 94,000 acre feet last year, for example, he said 140,000 acre feet got by them and went out to sea.

“For an average year, there might be six to 10 days where you see water get by us and go out the Santa Ana River to the ocean,” he said. “This year was so wet that there were more like 20 to 25 days where water went out to the ocean.”

But there also isn’t a lot of vacant land in low-lying portions of Southern California to add new water storage facilities, Kennedy and others said. Especially when that pricey project might only be needed once every several years, when rains are heavy.

Instead, water agencies are looking for ways to maximize the systems they have now.

In Chino, for example, Nakanowater said they’re working to pump water from one catch basin into another, where the geology allows the water to percolate into the underground basin more quickly. Many of those basins were created decades ago, he said, before technology allowed them to pinpoint more ideal locations.

The Orange County Water District and Main San Gabriel Basin Watermaster also have been asking the Army Corps to hold more water for a longer time above the Prado and Santa Anita dams. Since the Army Corps’ priority is preventing flooding, Kennedy said they’d typically release water out to the ocean when supplies behind the Prado Dam hit 498 feet in winter. Now, he said they’re letting water get to 505 feet year round.

More accurate and localized weather forecasts also can help. In the past, Kennedy said the Army Corps has started releasing water from local dams as soon as models showed a big storm building off the coast. But those storms often veer off and miss us entirely, and we dump water that could be saved.

That’s why California has partnered with federal and local water agencies to test a strategy called Forecast Informed Reservoir Operations, or FIRO, in the Santa Ana River watershed and other areas, Jones said. With FIRO, the Army Corps uses the latest forecasting technology to track storms and make decisions about water releases. Kennedy said they’ve been hoping the Army Corps would start testing this system locally this winter, but it he said it will be in place next winter for sure.

None of these changes are going to make Southern California drought proof or able to suddenly start capturing 100% of stormwater, Kennedy said. But he said they all add up to a water system that’s able to better withstand whiplash conditions in California that are being exacerbated by climate change.

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