Representatives of water agencies from the three states said they are firming up the details of a deal that would leave an additional 500,000 acre-feet of water in the reservoir next year, and the same amount again in 2023 — about double the quantity of water used annually by Las Vegas and the rest of southern Nevada.

For California, the deal would mean participating in water reductions prior to Lake Mead reaching levels that would otherwise trigger mandatory cuts.

The talks took on urgency this summer after federal projections showed growing risks of Lake Mead falling to critically low levels, despite plans for mandatory cutbacks throughout the Southwest that the states agreed to in 2019.

With the reservoir in a first-ever shortage and those cuts still insufficient, water management officials settled on a goal of together leaving half a million acre-feet of additional water in the reservoir instead of sending it flowing to farms, cities and tribal lands. The stored water would be roughly as much as 1.5 million average single-family households use in a year.