Does this include the suburbs of Las Vegas? A quick google search gives a report that the whole state of Nevada as a whole has increased their water use (edit: at least for domestic usage). This is in line with most of the population moving out of city centers as people moved into the suburbs that has happened for the last few decades.[0]

Anyway, the fact that the water use of the state as a whole has increased throws out any suggestion that "here, this one segment of the state has decreased their usage" as if that is a serious argument against the rise of water consumption being an issue.

[0] It's a report with data until 1995 but lucky for us it compares levels with 1970 and it did increase significantly EDIT: sorry, figure 1-3

http://water.nv.gov/programs/planning/stateplan/documents/pt...