Merced County is sinking; researchers blame over-pumping of groundwater
11/21/2013 9:40 AM
04/20/2014 9:09 PM
So much groundwater is being pumped from the San Joaquin Valley that it’s causing a massive swath of Merced County’s surface to sink at an alarming rate, U.S. Geological Survey researchers revealed Thursday.
Parts of Merced south of El Nido dropped more than 21 inches in just two years. That area – often called Red Top by locals – appears to be continuing to sink at a rate of nearly 1 foot per year.
Researchers warn that the area that’s sinking is gradually spreading across 1,200 square miles – from the cities of Merced on the north, to Los Banos on the west, Madera on the east and Mendota on the south.
That’s a much larger region than previous studies had ever documented.
USGS officials said they fear sinking ground levels will wreak havoc on economically vital man-made structures like the Delta-Mendota Canal, the California Aqueduct and irrigation canals that serve Merced and Madera counties.
The sinking soil – called subsidence – also could damage dams, roads, railroads, pipes and bridges.
The problem area includes part of the San Joaquin River and most of the Eastside Bypass, which is the primary flood control channel east of the river.
“A foot a year of subsidence (near El Nido) is a very rapid rate,” said Michelle Sneed, the USGS hydrologist who was the lead author of the new report. “I think that’s alarming.”
Sneed said that’s “among the fastest subsidence rates ever measured in the San Joaquin Valley.”
The land closer to Merced and Los Banos hasn’t sunk as much. Sneed said it may be sinking by about one-half inch per year, so people may not have realized it’s happening.
Subsidence moving north
Back in the 1950s, there was dramatic subsidence in parts of Madera County, but that stopped once the California Aqueduct went in.
That aqueduct and the Delta-Mendota Canal were built to supply farmers surface water, which was supposed to reduce groundwater pumping.
But pumping apparently has increased so much that groundwater levels have fallen to new lows in Merced County. Sneed said that’s causing layers of clay to collapse beneath the surface, which is compressing the land above. Once that happens, the aquifers can never be refilled.
“The subsidence is permanent,” Sneed warned.
That’s bad news for future groundwater reserves. It’s also bad for surface water supplies.
USGS researchers warn that sinking ground is reducing the capacity of canals that transport floodwater and deliver water to agriculture, cities, industry and wildlife refuges. They predict falling surface levels could cause infrastructure damage in local communities, too.
Solution: Stop pumping
“To stop subsidence, groundwater levels have to stop being lowered,” Sneed said. “If I was queen bee, I would say that would be the thing to do.”
Among the things that seems to be causing overdraft of Merced’s aquifer, is that farmers have changed what they grow.
“We are finding that row crops are decreasing and more permanent crops are being planted,” Sneed noted.
That includes trees – especially almonds and pistachios – and grape vineyards, all of which need water whether it’s a wet year or dry year. Sneed said land used for row crops, by contrast, can be allowed to go fallow during droughts.
“There are a lot more permanent crops” in that Red Top area, confirmed Chase Hurley, general manager of the San Luis Canal Co. in Dos Palos. “They’re pumping too much water from the deep aquifer and causing subsidence.”
Hurley’s privately owned mutual water company supplies San Joaquin River water to 45,000 acres of Merced County farmland west of El Nido. But the land under his company’s diversion dam has sunk so much the last five years that it soon may be too low for gravity to work to supply his canals.
“If we don’t get this subsidence under control,” Hurley warned, “I’m going to have to build a $10 million pumping plant (to move the river water).”
Hurley’s company has started working with Red Top landowners who use groundwater to nurture nearly 30,000 acres of farmland. He’s trying to find alternatives for them so they’ll reduce pumping from the deep aquifer.
Hurley said those farmers, most of whom are in Madera County, don’t currently have access to surface water, but that might be one option to stop them from draining down the groundwater basin and triggering subsidence.
Merced County farmer Cannon Michael questioned the wisdom of growers who plant permanent crops, like nut trees, on land that has no access to river or canal water. With no surface water, Michael said, growers must pump groundwater to keep trees alive.
“Once you’ve made the investment, you have a hard demand for water,” Michael explained. “It’s not sustainable.”
Michael said sinking ground levels is damaging Merced County canals and dams. He said subsiding land has lowered the government-owned Sack Dam on the San Joaquin River near Dos Palos, making it necessary to rebuild.
Public agencies also pumping
But individual farmers are not the only ones pumping groundwater: Cities like Merced and the Merced Irrigation District do it, too.
And despite this week’s rain, the valley has been in a drought for two years. That’s reduced runoff from the Sierra Nevada, which normally provides surface water to farmers in the Merced Irrigation District.
To compensate for reduced runoff, that irrigation district – which recently annexed El Nido – has substantially increased pumping from its 180 wells.
Hicham El Tal, the Merced Irrigation District’s deputy general manager, said his district normally pumps about 6,000 acre-feet of groundwater per year. But this year it pumped nearly 55,000 acre-feet.
“The MID cannot fix the groundwater basin alone,” said El Tal, noting how his district only serves about 20 percent of the Merced basin. “The only thing we could do is stop pumping groundwater … and that would cause even more damage (to the aquifer).”
El Tal predicted that if Merced farmers cannot buy enough water from their irrigation district, they will drill their own wells instead. And once they start pumping, El Tal said they’ll pump far more than his district ever has.
“Once they drill their own wells … they’ll not come back to the MID for water even during wet years,” said El Tal, who fears that would further deplete Merced County’s aquifer.
Over-pumping of San Joaquin Valley aquifers has caused subsidence for decades, but that trouble primarily had been south of Merced County.
Sneed said she was surprised to discover the problem’s epicenter has shifted north toward El Nido.
Falling ground levels between 1926 and 1970 in the Madera County community of Mendota convinced state and federal agencies to start importing surface water to that region. Initially that helped groundwater levels there recover.
Court-mandated and drought-related reductions in surface-water deliveries since 1976, however, have led to increased groundwater pumping, which the USGS report said has caused historic low groundwater levels in some areas.
Additional land sinking
Researchers found the subsidence rate doubled in 2008 in some areas around the Delta-Mendota Canal, which weaves through Merced and Madera counties. They said that’s when water levels in many of that region’s deep wells started hitting historic lows.
So the USGS used satellite technology to measure the declines.
Comparing images and data from 2008 with 2010, they measured the subsidence and discovered the bowl of depression is much larger than originally believed. Sneed said additional research and observations indicate the land continued to compact at alarming rates in 2011 and 2012.
“It seems to be occurring during non-drought years, too,” Sneed said.
That sinking land is buckling and damaging the canals built on the surface, according to the federally funded researchers.
They warn that such large-scale and rapid subsidence could cause significant operational and structural challenges for California’s water delivery infrastructure, which brings water from the north to the south to nurture thirsty cropland and cities.
Reliable water deliveries may be jeopardized because of it, the researchers concluded.
“The USGS report was commissioned by the U.S. Bureau of Reclamation to learn more about the challenges we face due to subsidence. It will help us take additional proactive measures to ensure efficient delivery of water to the San Joaquin Valley,” explained David Murillo, the Bureau of Reclamation’s Mid-Pacific regional director.
To help public agencies and resource managers minimize risk and damage to California’s infrastructure, the USGS is studying and providing information on groundwater conditions and land subsidence in the San Joaquin Valley.
Researchers said availability of surface water remains uncertain, so the potential for future subsidence is high.
The USGS uses a range of monitoring techniques to continually measure ground displacement, groundwater levels and aquifer compaction. That information, they said, can be used to develop simulation models of groundwater flow and land subsidence, which can be used to help manage the groundwater and limit future subsidence in the Valley.
The new USGS report is called “Land subsidence along the Delta-Mendota Canal in the northern part of the San Joaquin Valley, California, 2003-10: USGS Scientific Investigations Report 2013–5142,” by Michelle Sneed, Justin Brandt and Mike Solt.
A copy of it is posted at www.modbee.com/groundwater.
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