Aug. 19--Farmland near Corcoran in the southern San Joaquin Valley sank 13 inches in just eight months last year. To the north, near El Nido, the land surface dropped about 10 inches.
Groundwater overpumping is causing some parts of the San Joaquin Valley to sink at faster-than-ever rates, according to a new NASA report.
The survey, released Wednesday, documents the toll the drought is taking on the Central Valley's water-savings account, its vast aquifer.
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As surface water deliveries have been slashed, growers have drilled new and deeper wells and ramped up pumping to irrigate their fields.
That has accelerated land subsidence, a historic problem in the San Joaquin Valley that has grown worse in the past, mostly dry decade.
The sinking is so subtle it is imperceptible on the ground, save for the damage it causes to infrastructure. Aqueducts and irrigation canals buckle. Roads crack, causing millions of dollars worth of damage.
The NASA report, based on radar data from satellites and aircraft, underscores the unsustainable levels of groundwater pumping in the San Joaquin Valley. Even in wet years, growers in parts of the valley pump more from the region's vast aquifer than nature or man puts back.
California's new groundwater law calls for local agencies to end chronic overpumping and balance aquifer withdrawals with recharge. But the requirements won't take full effect for more than two decades.
"There's no doubt these sorts of effects are going to continue to some extent," said Mark Cowin, director of the state Department of Water Resources, which commissioned the NASA report. "I don't think we can end overdraft and subsidence overnight."
Cowin nonetheless cited the NASA figures as reason for counties to move quickly and adopt local pumping limits in overdrafted basins. "There's more to be done in the near term," he said. "We need to press for action ahead of the schedules."
He added that the state was "not ready to prescribe to counties exactly what they should do at this point."
The NASA results are based on comparisons of surface land changes recorded by satellite and aircraft radar equipment.
They show main areas of recent subsidence in the San Joaquin Valley, one bowl near El Nido that is 25 miles in diameter and a larger one that extends for 60 miles in the Corcoran area. From 2006 to 2010, the land surface south of El Nido sank about two feet, and the ground near Corcoran dropped about three feet.
The subsidence accelerated last year, when in just eight months, the land around Corcoran fell 13 inches, and the ground south of El Nido dropped 10 inches.
Groundwater levels have dropped to new lows in some parts of the San Joaquin Valley as agriculture has turned to wells to make up for drought-related cuts in irrigation deliveries. "We're pumping at record levels," said Cowin, adding that he was not surprised by the subsidence.
Not all of it is caused by drought-related pumping. Near El Nido, scientists with the U.S. Geological Survey who are monitoring the area say much of the groundwater is being used to irrigate new nut orchards expanding into areas that weren't previously irrigated and have no access to surface deliveries.
The Central Valley aquifer extends for about 400 miles under the Sacramento and San Joaquin valleys. The subterranean water, some of which seeped into the ground 10,000 to 20,000 years ago, is California's biggest reservoir. Yet it has been largely unregulated and unmonitored. Most of the pumping is by agriculture, but the state doesn't have good figures on how much is being withdrawn or by whom.
Subsidence occurs when water withdrawals compact the soil, permanently shrinking the pore space between clay particles. Even if the water table recovers, subsided basins can't hold as much water as they did, reducing the aquifer's capacity.
When the land sinks unevenly, roads and irrigation canals can buckle, causing millions of dollars worth of damage.
The satellite data showed surface drops along short portions of the California Aqueduct, which conveys supplies from Northern California to the San Joaquin Valley and Southern California. The subsidence increased dramatically between June and October 2014, when the area fell eight inches.
The state has repaired the aqueduct's berms in the past, Cowin said, but he did not have up-to-date figures on the cost.
Even if such drops don't buckle canals, they can slow water movement.
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UPDATE
2:47 p.m.: This article has been updated with information from the NASA studey, about the Central Valley aquifer and subsidence.
1:53 p.m.: This article has been updated with additional information, including the amounts the land has sunk in the past years.
This article was originally published at 12:25 p.m.
