California farmland is sinking  

Ground water drained away by irrigation for pumping

Scripps-Howard News Service    

By MARK GROSSI
April 23, 2007

FRESNO , Calif. —  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 $6million 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 San Joaquin River , will need to be replaced as a result of the sinking soil.

"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 Mexico City , for example, some land sank so much that the sewage system couldn't carry sewage away from the city anymore, said USGS scientist Steve Ingebritsen, water resources supervisor for nine western states.

"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 Fresno County town of Three Rocks, south of Mendota, the ground has gone from flat to rolling hills, a shift that forced some people out of their homes in the 1970s.

"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.

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 Coastal Range . The sediment can be several miles deep in some places.

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 USGA scientist Devin Galloway.

(Contact Mark Grossi at mgrossi(at)fresnobee.com.)

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