In a normal year, 30% of California water use comes from ground water. In the current severe drought, 60% or more is ground water.
There are 850 million acre-feet in 450 ground water reservoirs. One half of that is unavailable because it is of poor quality or because of its cost. From 2011 to 2014, during the current drought, ground water withdrawal has averaged 12 million acre feet per year.
Of total water use, 51% is for environmental use to maintain rivers, lakes, and to block salt water intrusion. 39% is for agriculture, and 11% is urban. 75% of California water comes from North of Sacramento.
There is no accounting for ground water withdrawals, and the new call for and implementation of restrictions will not be fully in place for decades.
Ground Water Depletion Measure by the GRACE Satellite
The deepest drop of 160 mm is 6.3 inches averaged over an area that looks like half of the state of California. The same rate of drop has continued to the present.
Prof. James Famiglietti of UC Irvine’s Department of Earth System Science has written an editorial in the LA Times urging swifter action on ground water depletion. He works on the GRACE gravitational measurement experiment that detected the groundwater depletion.