Basically all Aquifer's are used faster than the recharge which only works for so long. The Ogallala Aquifer is one example, but most of California pumps out water vastly faster than the recharge rate. This is not only used directly but also adds water to the water cycle which for example increases rainfall. Without that extra water things are going to get a lot dryer.
https://ballotpedia.org/California_Proposition_1,_Water_Bond...
The legislature also passed laws regarding the use of water and monitoring:
http://leginfo.legislature.ca.gov/faces/billNavClient.xhtml?...
Water recycling plants are coming online that can recycle actual wastewater:
http://www.mercurynews.com/science/ci_26160300/california-dr...
Less than a year ago, the largest desalination plant in the country went online:
https://en.wikipedia.org/wiki/Carlsbad_desalination_plant
But please, keep telling people how little we're investing in to this.
Sure there are good reasons for this, but US water use is still incredibly wasteful.
The key form CA is not to replace all water sources, but rather close the gap between rainfall and water needs. Right now, we are doing this by pumping ancient water from aquifers, but as solar power and desalination techs mature, the price will come down and will likely be sufficient to cover that gap. The recycled water will also factor in.
http://israelnewtech.com/2016/07/california-shifts-gears-wat...
http://www.jpost.com/Business-and-Innovation/Environment/Isr...