California water law is a convoluted mess of senior and junior rights, special exemptions, and insanely, voluntary reporting of water use coupled with no regulation at all of groundwater pumping. The drought is now so severe that for the first time in decades, California is mandating senior water rights (pre-1914). This won’t help as much as you might expect because these  water users may well just pump more groundwater and then report voluntarily on how much they use. Aquifers will become further depleted.
California regulates water using a system that encourages selfishness to the point that it hurts everyone. It needs to create a system where the common good comes first and where, among other things, groundwater pumping is regulated, something all other western states already do.
Under California’s system, the cuts are based on seniority. Those who have younger claims have to stop taking water from rivers and streams so there is enough flow left to satisfy the demand of those with older claims.
The most senior rights holders in California are a mix of farmers, irrigation districts, utilities with hydropower stations, and cities, including San Francisco.
In yet another twist in the state’s convoluted water system, some large irrigation districts with senior rights have long-standing agreements with the big government projects and would not be subject to the curtailments.
So, senior water rights will be cut, except when they aren’t. Right.