•     California’s Central Valley lost approximately 60 million acre-feet (maf) of groundwater since 1961.  Though a large amount of water, this is a small portion of the amount of groundwater stored in the Central Valley, estimated at 800 maf.  About 90 percent of annual groundwater extractions occur in the spring-summer growing season.  On average, 68 percent of groundwater extractions in the Central Valley are from the San Joaquin Valley, 57 percent of which are from the Tulare Lake basin.  Total flow through the aquifer system increased from about 2 maf to 12 maf between 1962 and 2003, principally from increased pumping and increased recharge from imported surface water.  

•    The effects of groundwater pumping since 1961 vary across the Central Valley.  For example, while some groundwater levels within the Central Valley dropped 400 feet, some areas saw 300-foot increases to the local groundwater levels, and the groundwater levels in other Central Valley areas remained stable.  Specifically, groundwater levels in the Delta increased.  In the Sacramento Valley, groundwater levels declined only slightly in specific local areas, primarily in the Sacramento and Stockton urban areas. The Tulare Lake Basin experienced and continues to experience dramatic declines in groundwater levels.

•    Climatic conditions significantly affect groundwater use.  Dry periods result in increased groundwater use as available surface water supplies decrease.  For example, during the drought of 1976-1977, recharge into the groundwater system was at a low of 3.4 maf, agricultural pumping was at a high of 15.6 maf, surface water deliveries were at a low, and 69 percent of irrigation water came from groundwater.  In contrast, in the wetter years of 1982-1983, water recharged to groundwater basins exceeded the amount removed by 14.8 maf.
 
•    Historically, increased surface water deliveries reverse declining groundwater levels.  In the 1960s, groundwater pumping in the San Joaquin Valley exceeded recharge, causing groundwater levels to decline.  As surface water delivery systems brought water from Northern California to the San Joaquin Valley, the amount of groundwater extracted decreased and groundwater levels recovered in the northern and western parts of the San Joaquin Valley.  Between 1962 and 2003, overall groundwater extractions decreased as a result of increased surface water deliveries.

•    Groundwater pumping significantly contributes to land subsidence in the Central Valley.  The principle areas of subsidence are the Los Banos-Kettleman City area, the Tulare-Wasco area, and the Arvin-Maricopa area.  Some land subsidence has also occurred in the Sacramento Valley.  Subsidence causes a permanent loss of storage capacity in the affected aquifer.  Subsidence may also cause the California Aqueduct to sink in certain stretches, decreasing its efficiency. 

•    Groundwater extractions for urban consumption increased from less than 5 percent in 1962 to 30 percent in the early 2000s.  On average about 12 percent of groundwater extractions within the Central Valley are for urban uses.