California's San Joaquin Valley has lost 60 million acre-feet of groundwater since 1961, according to a federal study. That's enough water for 60 Folsom reservoirs.
This is among the findings in a massive study of groundwater in the Central Valley by the U.S. Geological Survey. It helps shed light on the mysteries and dangers of the state's groundwater consumption, which is mostly unregulated.
According to the study, groundwater pumping continues to cause the valley floor to sink, a problem known as subsidence. This threatens stability of surface structures such as the California Aqueduct, which delivers drinking water to more than 20 million people.
The Central Valley is the nation's largest farming region; it's also the single-largest zone of groundwater pumping. About 20 percent of groundwater pumped in the United States comes from under the Central Valley, said Claudia Faunt, the study's project chief.
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In the Sacramento Valley, the study found groundwater levels have remained stable. Virtually all of the groundwater loss has occurred to the south, in the San Joaquin Valley, where aquifer levels have dropped nearly 400 feet since 1961, she said.
The current drought has aggravated this problem.
"In most years, especially in the San Joaquin Valley, the groundwater pumping exceeds the recharge," said Faunt, a USGS hydrologist. "With recent times, those groundwater levels have dropped back down close to historical lows."
The study is part of a project by the USGS to update groundwater data around the country that dates to the 1980s. USGS chose to begin in the Central Valley because the region is so important to the nation's food supply. The study took five years and cost $1 million.
Water records checked
California is the only state in which groundwater use is almost completely unregulated. California well owners are not required to report pumping or consumption patterns.
The study relied, in part, on indirect measurements. State monitoring wells provide a peek at regional groundwater behavior. Researchers also tapped into more than 8,500 well-drilling records dating to 1900, as well as land-use patterns and surface water recharge data.
After 1900, when large-scale farming began in the Central Valley, water tables dropped significantly as wells were drilled to irrigate crops. Aquifers eventually dropped about 400 feet compared with pre-1900 levels. This was part of the impetus to build the state and federal canal systems in the 1960s. Switching farms to this new surface water supply allowed aquifers to recover.
Then drought came in the late 1970s, and surface water diversions were cut back, as they have been in the current three-year drought. In both periods, farmers relied more heavily on groundwater, and aquifers declined again.
From 1961 to 2003, the period covered by the new study, groundwater levels in the San Joaquin Valley fluctuated depending on drought, Faunt said. The current drought has caused aquifers to drop again by nearly 400 feet, to near the historic low.
Overall, Flaunt said, there's a loss in groundwater amounting to about 60 million acre-feet since 1961.
An acre-foot of water is enough to serve two average California households for a year. That groundwater lost from the San Joaquin Valley was enough for every California household for 10 years.
One consequence has been land subsidence over vast areas of the San Joaquin Valley. The most severe drop is about 29 feet near Mendota, which occurred before construction of the water canals, said Al Steele, an engineering geologist at the state Department of Water Resources in Fresno.
"That's a three-story building, almost," he said.
Sinking hasn't stopped
The land generally does not recover from this subsidence, and often the compacted aquifer loses its ability to store water.
It was assumed that subsidence had stopped after about 1970. But Steele and Faunt said it has continued because of periodic droughts.
Ironically, this threatens the 444-mile California Aqueduct, built in part to address groundwater shortages in the San Joaquin Valley.
Officials recently learned that the canal may be subsiding because of modern groundwater pumping. As land subsides, the canal drops with it. This slashes the canal's water capacity by creating low spots, which reduce flow rate. It also could crack the structure.
"There's incomplete data that shows subsidence during periods when there is increased groundwater pumpage," Steele said. "It's still occurring."
He said California Department of Transportation land survey data shows Highways 198 and 152 near Fresno have subsided "a number of feet" in four decades. How much the canal has subsided is unclear.
To find out, DWR hired USGS to monitor the canal by satellite.
The new USGS study includes a mathematical modeling tool that can help water officials manage groundwater. This could help target the best locations for new groundwater banking projects and prevent land subsidence.
Officials could use the model to determine where and when groundwater pumping most threatens the canal. The state could then manipulate surface water delivery in those areas to prevent groundwater pumping.
One option might be to stop farming in threatened areas.