Add to that, so much of what ends up in Lake Mead ends up evaporating (the main problem with these manmade desert “lakes”).
I once talked with a hydrologist looking in central Tanzania for enough water for the new capitol, Dodoma. He said the problem was that rainfall averaged less than one meter per year while evaporation was 2 meters per year.
OK, I looked it up. Here are the numbers for Lake Mead:
"The average monthly rates for the Lake Mead open-water evaporation stations were computed for 1998 and 1999. For open-water stations, the sum of the average monthly rates for 1998 was 88.9 in. (7.4 ft) and the sum for 1999 was 90.7 in. (7.6 ft; table 7). For these 2 years, the average annual Lake Mead evaporation rate was 89.8 in. (7.5 ft). "