A new federal study shows that ground water levels are at historic lows. Three consecutive years of below-average rainfall and reduced surface water deliveries are placing more stress on ground water tables to provide irrigation water for the Central Valley, according to data gathered since 2003.

The study by the U.S. Geological Survey titled "Groundwater Availability in the Central Valley Aquifer," shows that the Central Valley has lost 60 million acre-feet of ground water since 1961. This loss affects not only the supply but also potentially the quality of the region’s ground water, and can lead to subsidence of the Valley floor.

The study states that ground water levels are particularly declining in the southern Tulare Basin portion of the San Joaquin Valley as more water is pumped out than recharges naturally. Ground water levels in the Sacramento Valley and northern San Joaquin Valley are generally stable.

The USGS study was developed in conjunction with a new three-dimensional model for assessing how ground water flows below ground and how it relates to surface water in canals and aqueducts in the Central Valley.