The groundwater that is the lifeblood of many Central Valley farms is imperiled if farmers continue to use it at current levels, according to new research.

A recently released study warns that the current depletion rate of the Central Valley aquifer – the large store of underground water the region's farmers use for irrigation – is unsustainable, and will continue to be so despite the surety that wet years will eventually follow dry ones.

The study paints a stark picture of how much water has been removed from the aquifer. In the mostly drought-defined years of 2006-09, farmers in the southern end of the Central Valley used enough groundwater to fill the equivalent of Lake Mead, said Bridget Scanlon, lead researcher in the study and senior research scientist at the University of Texas. The study plumbed both the Central Valley and Texas aquifers and used data from NASA satellites and thousands of wells.

The study is timely given that California's recently ended rainfall year was very dry, and drought has seized a large portion of the country this summer. To date, nearly 1,300 counties in 29 states have been declared natural disaster areas by the U.S. Department of Agriculture, which qualifies area farmers for low-cost emergency loans.

In California, 21 counties have been given a primary designation as disaster areas, and all but a handful of the rest have been designated as contiguous to a disaster area. Farmers in all those counties qualify for the emergency aid.

The groundwater study says Central Valley agriculture will be affected by future droughts and current groundwater usage. In essence, the report suggests the Central Valley's crop output may look different in the near and distant future.

"During the drought years between 2007-09, between 9 (percent) and 14 percent of the state's cropland was fallowed," Scanlon said. In some cases, droughts and low water levels may force farmers to switch from a low-value crop like alfalfa to a much higher value crop.

"Urban expansion into irrigated areas will also be a problem," Scanlon said.

Demand for water is growing in the Central Valley. Since 1980, the Central Valley's population has nearly doubled to 3.8 million people and is expected to increase to 6 million by 2020.

In Scanlon's estimation, the time has come for the Central Valley to more actively and sustainably manage its groundwater.

"Potential impacts of future droughts in the state will depend on the ability to store more water in groundwater banks," Scanlon said. "Per capita water storage has generally been decreasing in most places."

During wet years, water stored in aboveground reservoirs, as snowpack and underground will create a short-term turnaround of groundwater depletion, but over time levels will not return to what they once were, said Jay Famiglietti, director of Hydrologic Modeling at the University of California, Irvine.

"We are depleting our groundwater at a rapid clip and at a pace much faster than it's being replaced, so the net impact is the dropping of water tables," he said.

"Unless we start doing very large-scale recycling, we will run out of groundwater in the Valley. It might be 50 years or 100 years, but it is going to happen."

It was a 2009 study by Famiglietti on the region's groundwater status that laid the foundation for Scanlon's study. Both studies identify the southern areas of the Central Valley – such as the Tulare basin – as facing the most dire groundwater issues.

"Our farmers are in an extremely tough position – they need to grow our food, and they won't have the water supply in the long term," Famiglietti said.

The situation in the southern Central Valley is in contrast to water usage in most of the north region of the Central Valley.

"There is not much of a problem regarding the quantity of groundwater in the Sacramento Valley. The water here is irrigated water diverted from rivers," said Sacramento Valley rice grower Jack DeWitt. "Not much groundwater is used for rice."

In general, farmers in the northern Central Valley have other concerns – like the amount of boron in the water – but there are some pockets of farmland with groundwater issues.

"There are local areas there that do have overdrafts where, generally, there has been more water taken out than has been put back in," said Thomas Harter, groundwater hydrologist at the University of California, Davis.

Those areas exist between Sacramento and Stockton east of Highway 99.

Harter said there are areas around the Cosumnes River, south in the San Joaquin Drainage District, and others on the east side of the Valley that don't have sufficient access to surface water.

"That's where we typically see groundwater levels dropping relatively deep," he said.

When surface water, such as water in rivers, grows scarce, farmers draw from underground. During drought years, lower groundwater levels cause rivers to flow underground, said Jay Lund, director of UC Davis' Watershed Sciences Center.

"In particularly dry years, the Cosumnes River will not flow in late summer and early fall – it will flow underground," Lund said.

That affects salmon populations that need to migrate upriver, Lund said. "And for farmers it means they will have to pump more. … Essentially the groundwater level will achieve a lower equilibrium," he said.

That creates a vexing situation for farmers.

"If you're a junior right holder to surface water, those rights have been taken out from under you because someone has pumped groundwater and induced less into your stream. … Most people don't realize this has happened," Lund said.

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