Extensive groundwater pumping is causing a huge swath of central California to sink, in some spots at an alarming rate, the U.S. Geological Survey reports.

With California in the throes of a major drought and demand for groundwater rising, officials and landowners are racing to respond to the process known as subsidence. Some areas of the San Joaquin Valley, the backbone of California's vast agricultural industry, are subsiding at the fastest rates ever measured, said Michelle Sneed, a U.S. Geological Survey hydrologist and lead author of the recent report.

While the bulk of the sinking 1,200-square-mile (3,108-square-kilometer) area in central California is subsiding only about an inch (2.5 centimeters) a year, one 2-square-mile (5-square-kilometer) area Sneed studied is subsiding almost a foot (0.3 meters) annually. At that pace, "lots of infrastructure can't handle such rapid subsidence," Sneed said, including roads, water canals, and pipelines. The drought is likely to exacerbate the situation, as less rain drives more pumping.

Sinking Lands Raise Flood Risk

The worst subsidence has already increased the risk of flooding in the sparsely populated region, including to the low-lying town of Dos Palos, population 5,400, said Christopher White, manager of the Central California Irrigation District.

That's because portions of the area's flood control system have sunk, reducing their ability to contain floodwater. Local flood officials are crafting emergency plans for where to place sandbags when big rains return.

"We've got some serious issues," said Reggie Hill, manager of the Lower San Joaquin Levee District, which maintains part of the flood canal.

Other canals and dams that deliver water to irrigate the fields of hundreds of growers are also losing capacity as parts of them sink.

White oversees the local effort to respond to the subsidence. His irrigation district, which serves 1,900 growers, spent $5 million in recent years to raise canals and dams.

The federal Delta-Mendota Canal, which delivers water from northern California to growers and cities in the Central Valley, runs near the edge of the subsidence bowl and was the focus of the USGS study.

In 1969 the canal's banks were raised four feet (1.2 meters) along a 15-mile (24-kilometer) stretch in response to subsidence. More renovations—including the raising of several two-lane bridges over the canal—will be needed in 20 years if the sinking in the area doesn't slow, said Bob Martin, an engineer with the agency that oversees the canal.

Sneed said more research is needed to assess the impact of subsidence on cities around the Delta-Mendota Canal.

One permanent impact to the region may be lost groundwater storage. As groundwater levels drop, clay deposits move closer together and space for groundwater is lost. "You can never get the deposits to go back," Sneed said. Groundwater provides about one-third of the area’s total water supply, even more in drought years, officials said.

View Images An irrigation channel delivers water to farm fields in the Sacramento-San Joaquin River Delta near Isleton, California. PHOTOGRAPH BY ROBERT GALBRAITH, REUTERS/CORBIS

Surprising Find

The rapid subsidence was first noted several years ago when the U.S. Bureau of Reclamation did survey work as part of an $800-million restoration of the San Joaquin River.

The land had settled so much "we thought our data was wrong," said Rick Woodley, a bureau resource manager. That led to further study by the USGS. Because of the subsidence, some construction tied to the restoration has been delayed. Anything built "needs firm footing," Woodley said.

The California High-Speed Rail Authority, however, said subsidence will not have a significant impact on plans for a new high-speed rail through the area. The system will run from San Francisco to Los Angeles and can be engineered to deal with sinking land, said Frank Vacca, chief program manager.

Long-Standing Issue

Sinking land is not new to the San Joaquin Valley. In the four decades prior to 1970, portions of the valley sank 28 feet (8.5 meters), the USGS reported. Other states also suffer subsidence, and groundwater extraction is often the cause.

In the San Joaquin Valley, subsidence largely abated when growers began pumping from large federal and state water projects built in the 1950s and 1970s that are fed by Sierra Nevada snowpack.

But growers say they're now getting less of that water as the snowpack has diminished and more water goes to sustain critical habitat for endangered species. That combination has renewed growers' demand for groundwater, especially in drought years when surface water supplies dwindle, said Timothy Quinn, executive director of the Association of California Water Agencies.

Landowners near the heart of the subsidence bowl are totally reliant on groundwater to irrigate crops, including almonds and grapes. More tree crops have also been planted in the valley, and they will die if left dry in drought years.

Local officials are working with landowners to reduce the deep groundwater pumping that causes subsidence, and to secure future surface-water resources and recharge shallow groundwater reservoirs, the Central California Irrigation District's White said.

California's groundwater is largely managed locally, but the renewed subsidence may spur more state oversight, Quinn said. "The groundwater situation in California will be a crisis long after the drought."