Vegas takes 440,000 acre feet on a typical year and returns just over half that via the Las Vegas wash the total net consumption is 243,000 acre feet per year well under the allowed 300,000 and a drop in the bucket to California’s 4,400,000 per year.
Vegas recycles every drop that just a drain in the basin back to Lake Mead or uses it as reclaimed water for fountains, AC cooling towers, golf courses or athletic fields. Since those uses are consumptive they are in the above total use. Vegas gets credit for every gallon put back into Lake Mead and has banked water for ten years plus now.
With the third intake online at 860’ MSL they have access to 35 feet of water inside the dead pool of Lake Mead where no water would flow down stream once 895’ MSL is reached those 35 feet hold over two million acre feet of water about 12 years worth for Vegas this doesn’t include the inflows from rivers down stream from Lake Powell that cannot be held by the upper basin should Lake Powell also go dead pool. Those flows are in the 300,000 to 500,000 acre feet per year alone even in severe drought so Vegas has by drilling and tunneling to the deep water ensured a water supply effectively for a millennium or more which is the lifespan of Hoover dam at its current sedimentation rate.
https://m.lasvegassun.com/news/2019/sep/22/las-vegas-water-use-dropped-prominent-residents/
As a point of comparison my last water use was 12,000 gallons last month for two people half that was well water but it is tracked due to our ground water District. We got North Texas Council of governments water at the same time atmos piped in natural gas down our gravel road. NTCG water is so much better tasting than the crap well water we get from the aquifer here.
That may all be true, but sometime in the next year or two all the usable stored water in Lake Mead will be gone and nothing will flow out (with the exception of the LV usage) that didn’t flow in that year.
And that’s going to be a lot less than what’s flowing out now.