As large amounts of rain and snow soaked California last week, all eyes turned toward the threat of a dam failure at Lake Oroville, a reservoir that supplies much of the state’s drinking water. But what’s getting less attention is the fact that despite the easing of drought conditions in California, the situation below ground is still dry.
Thus far, approximately 188,000 people who live near Lake Oroville remain under evacuation orders put in place when the lake’s water levels began to rise, reports the Sacramento Bee. The earthen dam, which holds 3.5 million acre-feet of water, is the tallest in the United States. But when heavy storms hit the Sierra Nevadas, the reservoir filled to its highest level ever. Such excess forced officials to use an emergency spillway that has started to erode, creating the possibility of a collapse.
The Sierra Nevadas aren’t the only place in California hit with ample precipitation this winter. Earlier this year, a series of severe storms struck Southern California, driven by a temperature anomaly in the Pacific Ocean that shifted the jet stream from its usual position. Although that’s good news for the drought-parched state, it doesn’t mean that the drought is over.
Drought conditions continue to prevail throughout much of the state. According to the United States Drought Monitor, which tracks drought conditions throughout the country, a large portion of central and southern California is still in the midst of a drought. Though much of the state’s dry segment is at a “moderate” drought level, pockets of “severe” and “extreme” drought remain despite extensive snowfall that has put statewide snowpack at 176 percent of normal.
The water below the surface, known as groundwater, also remains in crisis. According to Thomas Harter, a groundwater expert and Robert M. Hagan Endowed Chair for Water Resources Management and Policy at the University of California, Davis, this deep water offers a kind of liquid insurance for the state.
“Our groundwater is an endowment of nature,” Harter tells Smithsonian.com. Since the 1920s, water has been pumped up from beneath the surface to supplement snowpack and surface reservoirs. But during dry years, more water is pumped out of the ground than is put back in by precipitation—and the recent drought has overdrawn the groundwater account. “We have a large deficit,” explains Harter. “It will take up to six average to wet years to make up for the losses we’ve incurred over the last 15 years of groundwater storage.”
Even if it rained constantly this year, says Harter, it wouldn’t make up for the loss, especially since groundwater takes longer to accumulate underground than it does to build up in above-ground reservoirs like Lake Oroville.
In the meantime, some areas of the Central Valley are experiencing a phenomenon known as subsidence, in which rock settles in on itself and becomes more compact due to excessive groundwater pumping. Last year, scientists at the Jet Propulsion Laboratory found that some parts of central and southern California have sunk as much as 6.5 inches from subsidence.
California is working on preventing such overpumping; in 2014, the state enacted a law that regulates the practice. But it will take plenty of precipitation to bring aquifers to the right levels—and ongoing depletion can contribute to decreases in water quality, habitat loss and even a higher risk of earthquakes.
It all goes to show that the drought above is only part of the story—and California’s water crisis is far from over.