Given the sorry state of surface water regulation, I [Damian] am not sure statewide groundwater regulation will produce many benefits. I really am not sure. Peter Gleick is probably right that there will always be areas of overdraft as long as pumping levels are not capped.
The massive overdraft in the southern San Joaquin Valley stirs up the most interest in statewide regulation. However, the local districts are not sitting idly by while their users mine the water underneath. They have no authority to cap their farmers' wells, but they do affect the water table with conjunctive use programs, etc. Furthermore, in talking with West Kern Water District (south-western corner of the valley) for example, although the groundwater is not regulated, they have MOUs with neighboring districts which prohibit taking "too much."
Valley of the Moon Water District, an urban supplier in Sonoma County, said something similar. The Sonoma basin is not adjudicated, and the users prefer it that way. I asked, naively, why not adjudicate to perfect the rights to the source, and Krishna Kumar's (the GM) response was quite insightful - the legal hassle and cost of moving to an adjudicated aquifer is large, and their current goodwill towards each other is enough to prevent pernicious pumping.
So, despite a lack of statewide regulation, many local agencies do manage their groundwater. Clearly, they understand that it is better to pump from 300 feet than 400 feet as they bear the costs of falling water tables. If they prefer to live in a world where pumping is costly and mostly unrestricted, why should we say otherwise? Put differently, given that many sets of users in the state have developed ways of managing groundwater without state intrusion, why should we think that the San Joaquin Valley is any different? Could it be that overdraft is slightly beneficial for the Valley? It most certainly leads to grant money and gives credibility to the call for more surface supplies...
Another cost associated with statewide regulation - In passing a cap, the legislature may give landowners gifts. In years past, falling groundwater tables led sympathetic politicians to build massive surface projects to ameliorate the farmers' "woes". I cannot imagine the legislature imposing a cap without some giveaways to landowners, and if they include new surface storage, that would most certainly be a waste.
The benefits I see from a cap on groundwater use would be more efficient use of water. Suppose the state did impose a cap on each basin that is not currently capped. (The costs of establishing the cap would be very high and there would be many winners and losers - let's ignore this massive issue for now). If some farmers can no longer pump as much as they want, it would push up prices of surface water and / or lead to land fallowing. Put simply - reduce groundwater availability, increase the price of water. Higher prices usually eventually lead to more efficient use. Water markets may also get a kick in the pants. The interplay with water markets I think has the most potential for good,
Bottom Line: I am less convinced today that state-imposed groundwater caps are the right way to go than I was a couple years back...