California's great Central Valley aquifer and the rivers that feed it, already losing water in the changing climate, are now being drained because of the drought, leaving water levels at their lowest in nearly a decade.

Water experts say many farmers who depend on the huge water source beneath the valley for irrigation will have to resort to pumping water from ever deeper levels at greater costs, even as they plant crops on fewer and fewer acres as more of their land is gobbled up for development.

"The combination of climate change, growth and groundwater depletion spells a train wreck," said James Famiglietti, a water resource expert and director of the UC Center for Hydrologic Modeling at UC Irvine.

The aquifer holds water that runs into the valley from three great river systems - the Sacramento, the San Joaquin and the Tule Lake basins. It is the state's major source of stored water and is primarily used for agriculture. But over the past two years, it has lost nearly 8 million acre-feet of the precious resource, Famiglietti's research center reported last week.

"That's equivalent to virtually all of California's urban and household water use each year," he said.

The Water Advisory report, which was transmitted to the California Department of Water Resources, updates a similar report published last year.

Famiglietti noted that the new estimates of drought-induced losses cover only the two years between November 2011 and November 2013, but they do not take into account this winter's lack of rain and snowfall.

If the drought continues, Famiglietti said, "Central Valley groundwater levels will fall to all-time lows."

Jeanine Jones, deputy drought manager for the Water Resources Department, said her agency's records indicate that this year "may end up as the driest year of our measured record - edging out 1923-24 for the Sacramento and San Joaquin River basins."

As water has been sucked more deeply from the aquifer, it has caused severe new land subsidence in many parts of the valley, Famiglietti's report notes. The shifting land has caused sidewalks in many small valley towns to crack and casings to buckle on the wells that are being dug for more pumping.

The report comes just as the Department of Water Resources announced last week that it will be cutting off all deliveries from the State Water Project to local agencies and contractors serving 25 million people and 750,000 acres of farmland, a decision that is bound to put more pressure on pumping water from the Central Valley aquifer.

To estimate the aquifer's changing water levels, the Irvine center receives monthly gravity maps from a pair of twin orbiting satellites named Grace, for Gravity Recovery and Climate Experiment.

The satellites carry instruments that can sense minute variations in Earth's gravitational pull caused by changing features of the land below. They detect - at least roughly - changes in the gravity of the Central Valley aquifer as its water content diminishes - both from pumping and from decreases in water from the great river basins that feed it.

Launched in 2002, the satellites are expected to end their working lives within a year or two, and NASA and its international partners are planning a new mission to succeed the Grace spacecraft.