In Arizona vs California (1968), the Supreme Court ruled in favor of Arizona, which forced California to reduce its over withdrawals from the Colorado River, which largely were used to supply southern California. A major driver for Arizona in this court case was the City of Tucson, Arizona. Tucson in the 1960's was running out of water, having withdrawn and then pumped the Santa Cruz river basin dry. In the mid-to-late 1800s, the Santa Cruz River was a continuously flowing perennial river, by the early 1970s groundwater levels in the southern part of the basin were as much as 1,000 feet below the riverbed, and waters being pumped were on the order of 10,000 years old. The Court ruling meant Arizona could initiate the Central Arizona Project, which could bring water from the Colorado via a canal across the arid lands to the Tucson Valley. Because withdrawals these withdrawals were so great, the city was being sued for water level declines in tribal (Tohono OOdham) lands to the south.  
Enacting a statewide management plan in Arizona for both ground and surface water was considered an insurmountable task. Historically, as everywhere else under appropriation rights, water was a battleground issue among Arizona agricultural, urban, tribal, and mining interests. These battles were fought almost exclusively in the courts, and often decided on the basis of personalities and politics, and very little hydrological data. But in the early 1970s we had an effective governor, Bruce Babbitt, and a premier groundwater hydrology program at the University of Arizona in Tucson. Governor Babbitt was able to get the legislature to agree to a statewide management plan that had real teeth, which included mandatory water restrictions when needed, defining sustainable yields for every groundwater basin, and permits that required information on well construction and quarterly to annual reporting of groundwater water use on a per well basis. However, the legislation came with a poison pill--if the state lost one case at the Arizona State Supreme Court or higher level the entire management plan would be considered void and null. Governor Babbitt relied heavily on the UA Hydrology Program faculty and graduate students who utilized the available and incoming data and their unique groundwater dating and modeling skills to effectively help develop the state's management plan. When I left Tucson, thirteen cases had been filed against this plan and had gone to the State Supreme Court, and the state won them all. Arizona now has one of the more effective management plans in the country. Governor Babbitt went on to become one of the most highly recognized and praised Directors of the Department of Interior--except maybe in the west where most states still haven't managed their resources well.  
Turn now to California almost fifty years after that Supreme Court decision. Users still battling water issues in the courts. A statewide management plan that still produces no effective data to determine sustainable yields, or reuires detailed well characteristics, and actual groundwater pumping per well. A state government that spends most of efforts simply grabbing any water it can from the north to feed the southern interests, while imposing the highest water restrictions in the north. Led by Governor Brown who served his first term soon after Arizona versus California decision and is currently serving his fourth term. A governor, who despite all his claims toward progress, apparently learned nothing about how to effectively manage the most precious resource of this state. Too bad the US Supreme Court as part of their 1968 decision didn't require California to develop a statewide ground and surface water management plan. It's now almost a half century since that court decision, and California now risks creating what Tucson did, mining it's groundwater water resources until there is no water left. Hydrologists have a saying that is largely true--surface water is nothing more than rejected groundwater. Mine the groundwater, and watch your rivers and streams disappear.  
Given the above, by the time this state does pass legislation to develop a sustainable conservation based and adequate data driven program to do what's needed, it will be too late. We could send all the water in northern California to southern California and it wouldn't be enough. The agricultural production of almonds alone equals the entire population's actual annual consumptive use. The failure here is this state's governance, which considers passing the "SGMA is unprecedented, hugely impactful, and a work in progress." The very last part of this statement may be true ("a work in progress"), but "unprecedented" and "impactful", is the truly effective work Arizona began fifty years ago. I would highly recommend California's Governor and Legislature take a serious look at Arizona's groundwater and surface water management plan as the immediate next step to evolve the SGMA into an effective sustainable, conservation based, data driven management plan. Now that would be "totally unprecedented", "impactful", and less of a work in progress. But you have to have real grit to do it.