Various: When the planners of Las Vegas peered into the future in 1950, they projected that the desert city’s population–then 25,000–would be lucky to break 100,000 by the end of the century. As it turned out, they were off by a factor of 19, and as you leave the sizzling Strip–the iconic center of this metropolis of 1.9 million people–for the Lake Mead reservoir, 65 miles to the northwest, you can see the source of all that growth. In a city that receives just 4 in. of rain a year, residents in …