Water levels in U.S. aquifers, the vast underground storage areas tapped for agriculture, energy and human consumption, between 2000 and 2008 dropped at a rate that was almost three times as great as any time during the 20th century, U.S. officials said on Monday.

The accelerated decline in the subterranean reservoirs is due to a combination of factors, most of them linked to rising population in the United States, according to Leonard Konikow, a research hydrologist at the U.S. Geological Survey.

The big rise in water use started in 1950, at the time of an economic boom and the spread of U.S. suburbs. However, the steep increase in water use and the drop in groundwater levels that followed World War 2 were eclipsed by the changes during the first years of the 21st century, the study showed.

As consumers, farms and industry used more water starting in 2000, aquifers were also affected by climate changes, with less rain and snow filtering underground to replenish what was being pumped out, Konikow said in a telephone interview from Reston, Virginia.

Depletion of groundwater can cause land to subside, cut yields from existing wells, and diminish the flow of water from springs and streams.

Agricultural irrigation is the biggest user of water from aquifers in the United States, though the energy industry, including oil and coal extraction, is also a big user.

The USGS study looked at 40 different aquifers from 1900 through 2008 and found that the historical average of groundwater depletion - the amount the underground reservoirs lost each year - was 7.5 million acre-feet (9.2 cubic kilometers).

From 2000 to 2008, the average was 20.2 million acre-feet (25 cubic kilometers) a year. (An acre-foot is the volume of water needed to cover an acre to the depth of one foot.)

One of the best-known aquifers, the High Plains Aquifer, also known as the Oglala, had the highest levels of groundwater depletion starting in the 1960s. It lies beneath parts of South Dakota, Nebraska, Wyoming, Colorado, Kansas, Oklahoma, Texas and New Mexico, where water demand from agriculture is high and where recent drought has hit hard.

Because it costs more to pump water from lower levels in an aquifer, some farmers may give up, or irrigate fewer fields, Konikow said. Another problem with low water levels underground is that water quality can deteriorate, ultimately becoming too salty to use for irrigation.

"That's a real limit on water," Konikow said. "You could always say that if we have enough money, you build a desalization plant and solve the problem, but that really is expensive."