Department of Water Resources
A daily compilation for DWR personnel of significant news articles and comment
April 23, 2007
1. Top Item
Sinking on the west side; Because of dry conditions, farmers have had to pump water from underground, causing the ground to deflate
By Mark Grossi, staff writer
World-class farmland is sinking on
The land dipped 30 feet between 1925 and 1977 near Mendota -- and it's still going down in what the U.S. Geological Survey calls "the largest human alteration of the Earth's surface." Ever.
The soil has deflated as deep-water pumping for farm irrigation drains away ground water. The dropping ground level is responsible for millions of dollars in damage to irrigation canals and could threaten such landmarks as the California Aqueduct and Interstate 5.
The Central California Irrigation District already faces the need to spend up to $6 million to fix one of its main canals.
Eventually, the district's Mendota Dam, a key feature of the west side's irrigation network along the
"We have to look seriously at how to deal with this problem in the future," said Chris White, general manager of the irrigation district.
Indeed, land sinking underfoot is an issue all over the globe, scientists said.
In
"It has been very expensive to rebuild the system," he said.
There are two kinds of subsidence on the west side, said soil scientist Kerry Arroues of the U.S. Department of Agriculture. One is the landscape-sized drop near Mendota resulting from pumping water from deep underground. The other, many miles south of Mendota, results from the weight of irrigation water on the surface, which creates rolling hills by causing the ground to sink in places.
The effects of near-surface sinking can be felt while driving on Interstate 5 in gentle, rolling undulations of the freeway.
Near the west
"This is not a criticism of agriculture, which I think works very well here," said Arroues, but "people need to be aware of ground subsidence. It affects many kinds of structures."
The other kind of sinking, known as deep-land subsidence, is seen across the Valley's west side. It happens when soils several hundred feet underground crunch down as water is pumped out of the surrounding sediments.
This has contributed to a drop in the ground level that has been measured for decades.
A well-circulated photograph of a utility pole near Mendota drives the point home. The 1977 shot features USGS scientist Joseph Poland, who discovered the sinking ground, standing next to the pole. High above
The pole has since been replaced, and there are no signs marking the previous land levels. The sinking is too subtle to see, but Arroues -- who has studied soil in
The near-surface sinking near Three Rocks is hard to miss. There are up-and-down slopes on roads, leaning utility poles and rolling hills in the pistachio orchards.
The land's surface sank years ago as agriculture expanded onto soil that had not been irrigated before. The irrigation water made some areas sink, while others didn't.
In Three Rocks, it means living in a tilted world.
Bernadette Martinez said it's no problem at the Three Rocks Cafe, which she said is the only building in town on a wooden foundation.
"We love it here. I've been running this place for 17 years," she said. "We don't have any problem with the subsidence in my place."
In the 1970s, several families were forced to leave because the ground movement caused flooding in their homes. Strolling though Three Rocks, it is clear that many houses lean in response to ripples in the ground.
In surrounding orchards, pistachio trees flourish, supported by highly efficient irrigation systems that drip water at the base of the trees. Soil scientists Arroues and Edd Russell pointed out 20-foot dips in the rows of trees.
Said Russell, "You don't see any stream coming through here. This subsidence is from irrigating the land many years ago."
The land continues to settle in places around Three Rocks, but not the way it did when water was first poured onto the old sediments from the nearby
Closer to Mendota, the broader, deeper subsidence is likely to pick up slightly again this year because of the dry winter, scientists say.
A dry winter means a smaller Sierra snowpack and less spring runoff in rivers. With less river water available, growers must increase pumping from aquifers to support the $1 billion west-side farm economy.
Before the federal Central Valley Project began delivering
"But deep-land subsidence basically ceases when [river] water is used," said Steve Chedester, executive director of the San Joaquin River Exchange Contractors Water Authority, which represents several farm water districts.
"That's what happened when the federal project came in. The subsidence started again when we had a drought in the late 1980s and early 1990s."
Scientists do not usually give firm estimates of how much water is underground on the west side, but most agree billions of gallons can be pumped out. Some water comes from the end of the last Ice Age about 10,000 years ago, said USGS scientist Devin Galloway.
Water has percolated into the Valley floor for hundreds of thousands of years as this inland region has accumulated sediment from the Sierra Nevada and the
It has been settling slowly and sinking inch by inch naturally for centuries.
"But that's nothing compared to what has happened over the last 80 years," said
Why does the land sink when water is removed? The
When the state built the California Aqueduct -- a 444-mile canal carrying Northern California water to millions of
Engineers compacted the soil as much as possible, and construction on the canal began in the 1960s. Authorities today continue to closely monitor and maintain the vital north-south waterway.
Similarly, Interstate 5, a key north-south transportation corridor built mostly in the 1960s and 1970s through the Valley's west side, is closely watched for problems, officials said. Arroues said ground ripples caused by subsidence can be felt while driving on the freeway, though state officials said the phenomenon has not yet caused any significant problems.
To the east, the dam at the Mendota Pool -- a natural wide spot on the
District general manager White also said a 70-mile canal, called the
"We probably need to reconstruct about 20 miles of it," White said. "We can't send as much water through it as we could before. If we tried to push more water through, it would flood. It's something we have to deal with out here." #
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