How the North-South Disparity in California's Rain Is Good for the Water Supply

A remarkably wet kickoff to Northern California’s rainy season has coincided with a desperately dry fall in Southern California — a huge disparity, perhaps unprecedented, between the haves and have-nots of rainfall.

Los Angeles usually gets several inches of rain by now, halfway into the rainy season, but it’s only recorded a fifth of an inch downtown since July, its second driest period in almost 150 years of record-keeping. The rest of Southern California is just as bone-dry.

At the same time, much of the northern third of the state has weathered nearly two months of storms, flooding and even tornadoes. Santa Rosa, north of San Francisco, has received more rain than nearly any other city in California — nearly two times its average rainfall to date. At the city’s airport, almost 7 inches fell on Nov. 20 alone, an all-time daily record.

Northern California is always wetter than the semi-arid southern half. But the scale of the north-south gap that has persisted for several months has stunned experts.

“There have been few if any years since 1895 … that have been so much above-normal in the northern part of the state while simultaneously so dry in the south,” Daniel Swain, a UCLA climate scientist, wrote in his blog Weather West. He added, in an email exchange with CalMatters, that “it is likely that the current north-south disparity is record-breaking in magnitude by at least some metrics.”

This season’s stark imbalance isn’t bad in terms of water supply. That’s because Northern California’s rain and snow feed major reservoirs, which provide much of the water used by Californians. If this occurred in reverse — a wet south and a dry north — most of the water would remain uncaptured, providing little benefit for supplies.

Snowfall to date (PDF) has followed a similar pattern, with relatively heavy snowpack accumulating in the northern Sierra Nevada and substantially below-average levels in the southern Sierra.

All of Southern California, between Kern County and the borders with Arizona and Mexico, is experiencing moderate to extreme drought so far this season. The dryness, along with fierce Santa Ana winds, ignited two major fast-moving wildfires in Los Angeles County on Tuesday.

But while the latest forecasts predict dry weather for at least the next 12 days, water suppliers aren’t panicking. They say thanks to ample precipitation two years in a row, their reservoirs and groundwater basins are brimming with water to supply Southern Californians.

“In terms of water reliability and water supply at this moment in Southern California, things are looking pretty solid,” said Mike McNutt of the Las Virgenes Municipal Water District, which serves 75,000 people in northwest Los Angeles County.

In addition, demand in the district has declined since the 2020 drought year by 23%, McNutt said.

Statewide, average per capita water use has declined sharply since the 1990s in spite of a growing population. Last July, state officials approved controversial and costly mandatory conservation rules that force 405 cities and other urban suppliers serving about 95% of Californians to meet individualized water budgets that decline over time.

Orange County Water District Chief Hydrogeologist Roy Herndon said his agency, which supplies groundwater to the north half of the county, has enough supply to carry its 2.5 million customers through the worst of any potential droughts  — multiple years in a row with minimal rainfall. “Three years, no problem. I’d say even five years,” Herndon said.

The Metropolitan Water District of Southern California — which serves 19 million people mostly with imported water — also has an abundance, “with a record 3.8 million acre-feet of water in storage,” according to Interim General Manager Deven Upadhyay, who issued a statement last week. That’s enough water to supply 40 million people for a year.

Last year was the eighth wettest year statewide in a century, and 2024 was about average. Now, the state’s largest reservoirs are well above their average levels for this time of year. Lake Oroville, the State Water Project’s largest, contained 128% of its average to-date storage as of Jan. 6. Lake Shasta was at 130%. Casitas and Diamond Valley reservoirs, key holding facilities for Southern California’s supply, are both almost full.

Moreover, snowpack conditions in the Sierra Nevada are promising.

As of Monday, the Sierra Nevada contained a snow water equivalent (PDF) of 103% of average. The northern end of the mountain range is at 150% — a reassuring status since it’s the source of the bulk of the reservoirs. The southern end, however, now holds just two-thirds of its average for early January.

“Off to a better start this water year than we were last year, but much remains to be seen in terms of how this water year actually ends up,” said David Rizzardo, hydrology section manager of the California Department of Water Resources.

The agency announced in early December that its State Water Project — which serves 750,000 acres of farmland and 27 million people — would be delivering 5% of requested water supplies to local agencies. Later in the month, after November’s atmospheric rivers pummeled Northern California, the department boosted the initial allocation to 15%. The allocation will likely jump again before the rainy season peters out — usually in April or May.

What’s in store for the next three months?

In California climatology, only one prediction is downright easy to make: Summers will be dry across most of the state. But California’s winters are extremely variable, and how the next three months — historically the wettest of most years — will unwind is anyone’s guess.

Michael Anderson, a climatologist with the California Department of Water Resources, said a single large storm could change the outlook for Southern California’s water year. But he also observed that most years that have started out so exceptionally dry have stayed dry.

Of the ten times on record when Southern California’s coastal region had received less than an inch of rain by New Year’s Day, only once did the water year’s precipitation tally come out above normal, he said. Three of those times, precipitation levels reached almost average.

John Abatzoglou, a climatologist at UC Merced, said an abrupt shift in precipitation patterns into a full-on drought statewide, though not necessarily probable, would not be surprising, either.

“That is certainly the California story — bouts of very wet to bouts of very dry, flipping within a single season, flipping year to year, hoping that it evens out in the end,” he said.

Though it’s essentially a tossup how the rest of the year shapes up, there is a caveat: the onset of La Niña. The counterpart to the more prestigious El Niño, La Niña often features dry conditions in Southern California and wetter conditions farther north.

Alexander Gershunov, a research meteorologist at UC San Diego’s Scripps Institution of Oceanography, said the precipitation patterns seen so far this year, while somewhat consistent with La Niña, are more the “outcome of natural variability.”

“La Niña signals typically don’t kick in until January,” he said.

A classic La Niña could draw out current patterns through the remainder of the winter, though Gershunov also leaned toward the possibility that the year would finish up a wet one. Citing 2011, 2017 and 2023, he said, “three of the wettest years this century were La Niña years.”

Southern California’s dry conditions primed the region to burn. The largest wildfire, the Palisades Fire, in Los Angeles County’s Pacific Palisades, grew to more than 3,000 acres within hours, forcing evacuation of tens of thousands of residences and destroying homes. The Eaton Fire ignited in Altadena, near the San Gabriel Mountains, on Tuesday night.

Wetter wets and drier dries

Increasingly pronounced weather extremes have been a growing area of interest among scientists and state officials, who frequently cite “weather whiplash” — “wetter wets” and “drier dries” — as one of the most pressing climate-related challenges facing California.

Mid-winter wildfires are just one potential outcome of this climate pattern. Increasingly erratic and unpredictable water cycles are another. California’s intricate system of reservoirs, canals and pumping stations was built to accommodate winter rainfall followed by a steady flow of snowmelt in the summer.

“Our reservoir system was predicated on the notion that much of our water would stay frozen in the mountains for months,” said Jeffrey Mount, a senior fellow and water supply expert at the Public Policy Institute of California.

But the climate that has historically supported this pattern is changing.

“Hots are getting hotter, dries are getting drier, the wets are getting a lot wetter,” said Gov. Gavin Newsom in a recent press briefing. “You have simultaneous droughts and simultaneous rain bombs.”

One trend frequently described is that of warmer and shorter winters, compressed from both ends. This causes more precipitation to fall in a shorter period of time, more of it to fall as rain, and more of the snow that does fall to melt earlier in the season.

Climate models suggest that the state’s snowpack — a resource that on average has provided 15 million acre-feet of water annually, or nearly four times the capacity of the state’s largest reservoir — could mostly disappear by the end of the century, when winters may be too warm to support long-lasting snow on all but the highest peaks and ridges in the state.

“The outlooks for California are a warmer future, where we see that snow line … moving up the hill, towards the end of the century, really leaving that seasonal snowpack to the higher elevations,” Anderson, at the Department of Water Resources, said.

Mount said the decline of snowpack is happening slowly on average, but with extreme snow drought periods increasingly falling into the bar charts of California history. Notable gaps are seen from 2012 to 2015 and 2020 to 2023 — low-snow periods that could reflect future norms.

“We’re facing a future without snowpack, and we’re not ready for that,” Mount said.

This shifting hydrology has prompted calls from many water supply advocates for more reservoirs and enlargement of existing ones to capture more water at once. The long-delayed proposed Sites Reservoir — one of Newsom’s frequent rallying points — is well into planning stages, and other reservoirs are being enlarged.

Mount said relying on such concrete infrastructure enhancements will eventually prove much too costly. The solution, he says, lies underground.

“Our best hope is to aggressively start using groundwater for our main storage for the future,” he said.

Underground solutions

That’s not so easy in agricultural areas. Farms use such great volumes of water that it’s impossible to recharge many groundwater basins faster than growers pump it out, especially in the San Joaquin Valley. Water agencies and state officials are seeking ways to channel floodwaters into gravelly basins where water can rapidly sink in, but logistical and regulatory barriers continue to stifle progress.

Because urban areas use relatively little water compared to farmland, local stormwater capture has made more of a splash. The Los Angeles Department of Water and Power captured more than 108,000 acre-feet of local stormwater in 2023 — about 20% of its annual demands — and diverted it into groundwater basins. Last year, the agency captured 41,000 acre-feet of stormwater in February alone.

Delon Kwan, the department’s assistant director of water resources, said city plans are afoot to increase stormwater capture to 150,000 acre-feet per year by 2035, in part by using urban parks as infiltration grounds.

Water recycling systems are another way to help drought-proof California communities. In his 2022 water supply strategy (PDF), Newsom set a target of 800,000 acre-feet of recycled water by 2030 and 1.8 million acre-feet by 2040. Some cities already reuse wastewater for irrigation or treat it after storing it in aquifers. Now, direct potable reuse — sometimes erroneously called toilet-to-tap — has been approved in California.

Mount said Southern California has made great steps toward improving local water supplies, but climate change could limit the region’s potential for self-sufficiency.

“As you see increasing aridity in Southern California, with a climate more and more like the Southwest, you’re going to see increasing reliance on water from other areas,” he said. “If you haven’t got water to recycle, it becomes a real problem. If you haven’t got stormwater to capture, it’s a problem.”