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California uses a dual doctrine of water rights: Riparian and Prior Appropriation.

In the eastern half of the nation(wet zone), the states use Riparian water rights.

In the west(dry zone), the states use Prior Appropriation water rights.

But, those states that have both a wet and dry zone use the dual doctrine. The dual doctrine states are those that straddle the 98th meridian(TX, OK, KS, etc) and those states that straddle the Cascades/Sierra Nevada(WA, OR, & CA)

Prior Appropriation is characterized as:
First in time, first right. Rights are in a hierarchy based on when they were first used or appropriated. The older the right, the higher the right. The older or more senior your right, the more likely you are to get your water in any given year. The younger or more junior your right, the less likely you will get your water in any given year.
Use it or lose it. If you don't use your water, someone else can, and under certain conditions, that someone else can end up owning the water right(prescriptive water rights).
Highest beneficial use. The water must be put to the highest beneficial use. Traditionally agriculture was the highest beneficial use but in modern times, water that attracts tourist/recreational dollars generates more money(economic output) than some types of agriculture, like growing a low value crop such as hay or potatoes. Tourists will pay big bucks to golf on bent grass greens and fish for trout and salmon.

A third water right in play in CA and other dry states is Federal and Indian Lands Reserved Water Rights, often called Reserved Water Rights. Tribes with Treaty rights to farm and fish have water rights that enable them to farm and fish, that trump prior appropriation water rights. Likewise, federal lands and the flora/fauna have water rights that trump prior appropriation water rights. Often times this takes the form of a minimum instream flow rate, which means you can't take all the water out of the stream to satisfy someone's prior appropriation water right. Most notably, the Delta Smelt(an indicator species) were entitled to a minimum flow, that trumped a farmer's prior appropriation water right.