Is California’s drought finally over? Here’s the impact of the latest storms

UPDATE: In stunning improvement, half of California now out of drought

If there’s concern about California’s wet winter turning dry, consider it shushed.

The heaps of snow over the past week on top of the parade of deluges in early January have been extraordinary and left much of the state with well-above-average precipitation for the season. The winter storms, which account for the bulk of the state’s rain and snow, are forecast to continue into next month, virtually ensuring a good water year for California.

• California reservoir levels: Charts show water supply across the state

But just how far one year will go to relieving what has been one of the West’s most excruciating droughts is less clear. While many parts of the state are benefiting from brimming rivers and reservoirs, the three previous years, which saw record low precipitation, as well as several painfully dry years over the past two decades, have burdened the state with a gaping water deficit.

Many aquifers are far from recharging, forests remain parched and often in very bad shape and fire danger lingers in part because of the widespread tree die-offs. Adding to the problem, rising temperatures, the result of the warming, fossil-fuel-charged atmosphere, makes drought recovery that much harder.

“The definition of drought and where we stand sort of depends on what you care about,” said LeRoy Westerling, a climate scientist at UC Merced. “Yes, reservoirs are filling up, (but) you can look at it from the point of view of water storage, from the point of view of agriculture, from the point of view of ecosystems, from the point of view of wildfire. All these things have slightly different profiles.”

Climate experts and water managers agree that winter has been exceptional from many perspectives, far exceeding expectations for the season. Initial outlooks were based largely on the La Niña weather pattern’s drying potential and augured another year of punishing drought.

As of Monday, cumulative snowpack across the Sierra Nevada and southern Cascades measured 181% of average, according to state data. This amount of snow is already 50% greater than the usual peak on April 1. Nearly a third of the state’s drinking and irrigation water comes from snow.

Precipitation in the state’s north, which is most important for filling California’s biggest reservoirs, measured 116% of average since Oct. 1 . Rain is generally measured over the October-to-September water year.

San Francisco, meanwhile, has already received a year’s worth of water since Oct. 1. The rainfall downtown measured 24.45 inches this week since the start of the water year. The city averages roughly 23 inches annually.

Jeanine Jones, drought manager for the California Department of Water Resources, says much of the state will likely see its water supplies replenished by this year’s wet weather. But not everywhere.

“Drought is really a local phenomenon and it depends on an individual water district’s or irrigation district’s supply,” she said.

Communities that depend on groundwater, particularly those reliant on the depleted aquifers of the San Joaquin Valley, will continue to struggle to provide water for customers. Some are still delivering bottled water to households.

Depending on the hydrology, an aquifer can require years, if not decades, of wet weather for enough water to soak into the ground so that the supply is recharged. The state’s groundwater basins have notoriously been overpumped to meet water demand during dry times, particularly in California’s farm belt.

Farmers that get water from the Central Valley Project also will likely not get as much supply as they want this year, though they will get more than last year. Lake Shasta, the centerpiece of the federal project, is much fuller than it has been but was still just 84% of its historical average this week.

Central Valley Project managers have estimated that most customers will receive just 35% of the water they requested in coming months.

On the flip side, many water agencies with supplies in the southern and central Sierra are in good shape, including San Francisco and the suburbs that share the city’s mountain water. Officials at the San Francisco Public Utilities Commission say they expect their water system, which includes Hetch Hetchy Reservoir in Yosemite, to completely fill this spring.

Most water agencies, including the SFPUC, have not yet stopped asking their customers for cutbacks, but suppliers are expected to revisit drought policies soon. Several communities put in place steep water reductions during the past two years.

“I think the local supplies in a number of locations are probably adequate to where they would pull back restrictions,” Jones said.

The state is yet to lift its requirement that water agencies plan for drought contingencies and residents voluntarily conserve 15%.

By most accounts, California lost about a year’s worth of water over the past three years of drought. The loss, though, was not shared equally.

The U.S. Drought Monitor, a rough gauge of water deficits, estimates that about a third of California remains in “severe” drought — but none in the more troubling categories of “extreme” or “exceptional” drought. The report was last updated before the weekend storms.

The continuing drought is part of what scientists have called a “megadrought.” Studies have shown that the period since 2000 has been the driest in the West in at least 1,200 years, a plight that makes it difficult for a single wet year to improve.

Beyond the lack of precipitation, scientists cite the warming climate as the reason for the relentless aridity.

Reach Kurtis Alexander: alexander@sfchronicle.com Twitter: @kurtisalexander