After weeks of rain, Northern California’s surface water systems have been pushed to the breaking point, even while the effects of a five-year drought continue to threaten the state’s long-term groundwater supplies.
More than 180,000 people were evacuated Sunday afternoon after spillways for the Oroville Dam in Northern California showed signs of possible collapse.
The potential flooding comes as California begins to emerge from its historic, drought. But despite the overabundance of water on the surface, drought conditions underground have likely had a permanent impact on the state. Even under the best conditions, it could take half a century for underground reservoirs to refill.
Recent analysis from the California Institute of Technology shows that drought — combined with water use — has caused parts of California to sink, perhaps permanently. Some places show up to two feet of subsidence over the past two years.
That’s because as surface water from rain and snowmelt has decreased, farmers have been using more groundwater from reservoirs.
“Subsidence caused by groundwater pumping in the Central Valley has been a problem for decades,” write the authors of a new study, which uses NASA data to track California’s sinking.
Pumping groundwater reduces the support below-ground aquifers give to the surface, causing it to sink. In recent years, that process has been sped up, as California’s drought has reduced the surface water available to farmers. While surface water can come in the form of rain, or snowmelt, groundwater sits in aquifers and is the result of a slow build up of water below ground.
In California, the makeup of the land adds to the risk of subsidence. There is a layer of fine-grained, water-retaining sand that lies above the deep aquifers, and the authors note that if too much water is withdrawn, “irreversible compaction” of that layer occurs. Even if water is returned to the ground, it “cannot recharge the layers, causing permanent subsidence and loss of some groundwater storage capacity.”
According to the U.S. Geological Survey, California’s Central Valley aquifers are so depleted right now that it would take 50 years of normal rainfall and, crucially, stopping water withdrawals, for them to achieve their historical norms.
How to deal with the California’s chronic water shortages — while keeping produce prices low and without destroying the state’s environment — has been a focus in Congress for years.
In December, President Obama signed a massive infrastructure bill, supported by Sen. Diane Feinstein (D-CA) but vehemently opposed by now-retired Sen. Barbara Boxer (D-CA), which added hundreds of millions of dollars for California water improvements, including for dams and levees around Sacramento. But the bill allows for even more groundwater withdrawals by farmers in California, potentially exacerbating drought impacts. It also allows for more water to be taken out of lakes and rivers, which environmentalists say puts local ecosystems at risk. California state law is more restrictive, and some say the new plans could be challenged in court.
The Central Valley is the most important fruit and vegetable-producing region in the United States. According to the California Water Science Center, $17 billion worth of crops are grown there each year, supplying 8 percent of the country’s agricultural output, including 40 percent of U.S. fruits, nuts, and “other table foods.”
It also hold 17 percent of the irrigated land in the United States, which means the Oroville Dam and surrounding infrastructure play a critical role in getting food to U.S. tables.
Ironically, the subsidence in California, caused by groundwater overuse, is putting the surface water infrastructure at risk.
Much of the subsidence has occurred close to the California Aqueduct, a massive water conveyance system that brings rain and snowmelt runoff through the Central Valley to Southern California. Subsidence around irrigation channels means the infrastructure can hold less water, which, in turn, affects how much water farmers and Southern California can use.
“The rates of San Joaquin Valley subsidence documented since 2014 by NASA are troubling and unsustainable,” Department of Water Resources director William Croyle told the Mercury News. “Subsidence has long plagued certain regions of California. But the current rates jeopardize infrastructure serving millions of people. Groundwater pumping now puts at risk the very system that brings water to the San Joaquin Valley. The situation is untenable.”