[TigeriusKirk]

Can't say about the 1970s, but there's a chart in this article showing that total water usage for southern Nevada has indeed decreased since 2002.

https://www.watereducation.org/western-water/climate-change-...

[finfinfin]

According to that graph 2020 usage is almost identical to 2003. So the only significant drop happened between 2002 and 2003. Not to nitpick, just to emphasize that the missing data from 1970 to 2002 could contain numerous large changes too.