Near-record rainfall has battered California for weeks, sparking floods and landslides as the state struggles to cope with so much water.

But scientists say even this much precipitation won't reverse the western US state's decades-long drought.

A parade of atmospheric rivers -- massive flows of moisture dragged through the skies from the oceans -- has unleashed staggering volumes of rain and snow since December.

San Francisco got more rain in the last two weeks than it has done in any similar period for 150 years, while the Sierra Nevada mountains have been buried in as much as 33 feet (10.5 meters) of snow.

Peter Gleick, co-founder of the Pacific Institute in Oakland, a research organization that specializes in water issues, says it's difficult to quantify exactly how much water has fallen from the sky.

"But we're definitely talking about trillions of gallons (liters)," he told AFP.

"There's no doubt that the water we're getting now is a great help in eliminating the drought... But it is too soon to say that the drought is over."

"2023 could be a wetter year than normal," he said, but we'll have to wait until the end of winter to know for sure.

The western United States is in its 23rd year of drought, with major rivers and reservoirs at a fraction of capacity.

California's largest reservoir, Lake Shashta, is only 42 percent full, official data shows, even after all this rain.

The monster snowpack in the Sierra Nevada mountains -- it is currently twice what it normally is in January -- is most helpful, because this provides the gradual run-off that drip-feeds reservoirs in drier months.

- Aquifers in deficit -

Beyond its reservoirs and snow reserves, the state faces a much deeper-seated problem with its depleted groundwater, says Nicholas Pinter, a geologist from University of California, Davis.

About half of measured wells in California have seen their water levels decline over the past 20 years, according to a report from the California Natural Resources Agency released in October.

And the torrents of rain that have fallen are not all that effective at recharging them -- the soil became saturated quickly, so instead of being absorbed, subsequent downpours just ran off.

"Groundwater is like our retirement account. It's slow to go in and we have to withdraw it very, very carefully," says Pinter.

"Only, a lot of California water users draw on that retirement account like if it was the savings for tomorrow."

California's vast agricultural sector -- which supplies a swathe of America's supermarkets -- is a heavy user of this underground water.

That intense need -- coupled with big cities like Los Angeles that have nowhere near enough of their own water to support their populations -- means almost no matter how much it rains, it will not be enough.

"We will never end the discussion of drought in California because drought in California is largely driven by water demand," says Pinter.

- Climate change -

Human-caused climate change is already making itself felt, with average global temperatures up 1.2C since pre-industrial times.

That contributes to longer term trends like the drought, as well as to the increasing ferocity of winter storms like those that are whipping the western US coast now.

Adapting to the climate that we have altered is going to be key, says Gleick.

That includes learning how to retain the water when it falls as rain, to tide us through the hotter, drier summers.

"California built levees to protect communities from flooding, but they prevent the aquifers from recharging," he said.

"Every major river would benefit from widening the floodplains, by moving the levees back from the river and letting the rivers flood more.

"Instead of thinking we can control all floods, we have to learn to live with them."

Crucially, that means not assuming we can just go back to the way things were before a disaster struck.

"When communities are hit by repeated flooding, we should not rebuild in the same place," he said.

US west coast girds for more damaging storms
Los Angeles (AFP) Jan 12, 2023 - Western US states were readying Thursday for yet more torrid weather as so-called atmospheric rivers lined up to dump heavy rain across the already soaked region.

California has been battered by weeks of downpours that have killed 19 people, flooding communities, toppling powerlines and threatening deadly mudslides.

Forecasters now say a cyclone churning in the Pacific Ocean will spread the rain further north, forming a band from northern California to Washington state.

"The relentless parade of cyclones that have been targeting California in the past week is forecast to shift focus," the National Weather Service said Thursday.

A strengthening ridge of high pressure will "keep the heaviest precipitation locked in place across northern California to coastal Pacific Northwest," which includes Oregon and Washington state.

"By Friday night, this somewhat stagnant pattern will show signs of breaking down, allowing the next Pacific cyclone to direct yet another surge of atmospheric river toward California by Saturday morning."

Forecasters said they expect up to six inches (15 centimeters) of rain to fall over 48 hours near Seattle.

Avalanche warnings were in effect for a tranche of Washington state, with the storm bringing wetter, heavier snow into the mountains.

"Dangerous, large avalanches are expected to run naturally during this snowy, wet weather event that will be impacting us Thursday," the Northwest Avalanche Center (NWAC) said.

So-called "wet slab avalanches" were a particular concern as the wetter snow piles up on top of fluffier, lighter snow, making the snowpack unstable.

"Wet slab avalanches are not something to tiptoe around, and this is a day where it's a good idea to stay off steep slopes and runouts beneath avalanche paths," the NWAC said.

- Sewage -

In northern California, up to six feet (1.8 meters) of snow was forecast over the mountains between Friday and Tuesday, with winds gusting up to 50 miles (80 kilometers) an hour.

"Mountain travel is highly discouraged this weekend! If you must travel, plan to be at your destination before 4pm Friday," the NWS said.

At lower elevations, a flood watch was in place in a vast area from around San Francisco and into Oregon.

The warnings come as the region picked up the pieces after repeated downpours.

San Francisco saw more rain over a two-week period than at any other time in 150 years, straining the city's drains where raw sewage was mixed with storm run off.

"Don't jump in puddles. Especially in San Francisco...there (could be) sewage in that," said Eileen White of the regional Water Quality Control Board, according to the San Francisco Chronicle.

The storms have left at least 19 people dead in California.

They include drivers who have been found in submerged cars, people struck by falling trees, a husband and wife killed in a rockfall, and people whose bodies were discovered in floodwaters.

In San Luis Obispo, members of the National Guard joined the search for 5-year-old Kyle Doan, who was swept away in floodwaters as his mother tried to pull him to safety from their car.

The sheriff of San Luis Obispo County said Wednesday that underwater teams were combing water courses.

"We will search until we find him," spokesman Tony Cipolla told The San Luis Obispo Tribune.

- Climate change -

California is no stranger to wild weather, with winter storms commonplace.

But scientists say climate change, supercharged by humanity's burning of fossil fuels, is making such storms more ferocious.

While it is causing short term misery, the rain is badly needed in the western United States, where more than two decades of drought have forced unprecedented restrictions on water usage.

However, climatologists warn that even the kind of monster downpours that have pummelled the region this month are not going to reverse 20-plus years of below-average rainfall.

Shasta Lake, the state's largest reservoir, was still only at two-thirds of its historical average for early January, water resources department data showed.

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