As the nation’s driest big city, Las Vegas knows a thing or two about water. With a metro population of more than 2 million people and just 4 inches of rain a year, the city has learned how to be very, very frugal. Now it’s taking that water consciousness — and all the years of experimentation it’s driven — and using it to transform itself into a hub for new and innovative water technologies. Long a leader in water conservation, Las Vegas has kept its water use down through a combination of fines, enforcement, incentives, education and innovation.