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Plus it is recycled. They would only replace what is lost due to evaporation or after a drain and cleaning.
I looked it up (because the air is very dry in Nevada) and about 32,000 gallons of water per day are evaporated at the Bellagio fountains.
Source: The Las Vegas Sun
An average car wash uses 40 gallons per car and washes a hundred cars per day: Source
A car wash uses 80 gallons per car. The car wash by my office probably washes a thousand cars a day.
Car washes recycle water, too.