MODESTO -- Parts of the Central Valley overdraw groundwater so heavily that future farming could be at risk, according to a new report.
The situation is relatively stable in and near Stanislaus County, thanks to river supplies and other advantages, but the area has groundwater concerns nonetheless.
The report says the problem is most acute near the southern end of the San Joaquin Valley, where the overuse of aquifers was compounded by the 2007-09 drought.
"Potential impacts of future droughts in the state will depend on the ability to store more water in groundwater banks," said Bridget Scanlon, lead researcher in the study and senior research scientist at the University of Texas. "Per-capita water storage has generally been decreasing in most places."
Much of the Northern San Joaquin Valley is in better shape because many farmers and city residents rely on river supplies. They are subject to drought, too, but reservoirs help carry them through.
Some places, such as the western end of the Modesto Irrigation District, have water tables so high that they have to be pumped to keep crops from getting waterlogged.
But on the east side of the north valley, where less river water is available, groundwater has been dropping as the hilly land is turned into fruit and nut orchards watered by wells, said Joe Marcotte, a consultant for the Eastside Water District.
"The groundwater seems to be declining," he said. "We've observed that for the last three to five years."
The district has been working on plans for recharging this aquifer via basins supplied with water from the adjacent Turlock Irrigation District in wet years. A full-scale effort would require a reliable water supply and several hundred acres with soil that drains well, Marcotte said.
Phillip Stine, who grows almonds and walnuts near Modesto Reservoir and founded Waterford Irrigation Supply, has seen the same problems.
"It just seems like it's getting harder and harder to find water, and if we do find it, many of the wells are not producing well," he said.
The groundwater concern is among the reasons cited by opponents of MID's proposed water sales to San Francisco. They say this Tuolumne River supply, most of which would be freed up by canal system conservation projects, could instead seep underground and serve as a savings account for dry years.
Stine said he would like to see the MID provide water to farms near its boundaries in years when it has some to spare. As an added benefit, he said, it would be lower in salts that can harm crops.
"It's pure water," Stine said. "The snowmelt water is much better than the groundwater."
Domestic water users in the Modesto area have reduced their reliance on wells thanks to the treatment plant the MID built for river water in the mid- 1990s. The plant expansion is mired in a controversy over design and construction flaws, but once it's done, city customers will get even less from underground.
The new report looked at the Central Valley and the High Plains region, which stretches from Texas to South Dakota. The researchers used data from NASA satellites and thousands of wells.
Jay Famiglietti, director of hydrologic modeling at the University of California at Irvine, said wet years ease the groundwater problems only temporarily.
"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," said Famiglietti, whose 2009 study laid the foundation for the new report.
Modesto Bee staff writer John Holland can be reached at email@example.com or (209) 578-2385.