Back in Inspirada, Bradd Robison, a shiny-pated, aviator glasses-wearing KB Homes representative, escorts us into a model Renoir Garden home. On the next block are the Monets; then, the Van Goghs. The disconnect between the French Impressionists and stucco, vaguely Tuscan homes set in Nevada seems to elude most visitors, perhaps because less than 10 miles away sit miniature versions of Paris, Manhattan and Venice. Still, Monet would miss his lily ponds: They're illegal here.

We're accompanying two water cops, aka Southern Nevada Water Authority conservation program officers. Toby Bickmore and Hillery Francis inspect the homes to ensure they meet the criteria for the Water Smart label and Water Authority codes. They check for plumbing leaks, measure faucet flow, and even time how long it takes to produce hot water – each extra second or minute contributes to waste. And swamp coolers, though they use less energy than air conditioners, are forbidden because they use too much water.

But, interestingly enough, indoor water waste isn't that frowned upon. Every drop that goes back into the sewer system is treated, and much of it – 200,000 acre-feet per year – is released into the Las Vegas Wash, which empties into Lake Mead. There, by 1964 Supreme Court decree, it is credited against Nevada's Colorado River allotment, allowing the state to pull 439,000 acre-feet out of the river in 2012, far more than its share. The arrangement drives efficiency hardliners like Jeffrey Roberts, a lead architect of the Desert Living Center, nuts: The same water-recycling system he built into the Center is not allowed elsewhere in the city, because it would take precious water away from the wash. Once a mostly dry arroyo, it has been transformed into a green-robed stream that sparkles even in 118 degree July heat and sustains dozens of species. It's the closest Vegas will ever get to recreating the Garden of Eden.

"For us, outdoor use is most critical," says Bickmore, who resembles actor Peter Sarsgaard in a blue water-guy uniform. While we generally regard The Strip's giant resorts as Vegas' biggest water hogs, they actually account for just 7 percent of the urban area's use. Single-family residences, meanwhile, consume about 40 percent, and 70 percent of that is used outside, primarily to water grass. Replace just one square foot of turf with desert-friendly xeriscape, and you save 55 gallons per year, says Bickmore. That's why, in 2003, as drought tightened its grip, the Water Authority banned turf in the front yards of new homes and commercial buildings, and limited it to just half of backyards. Pools – which are no longer being installed because of the economy – count against that allowance.

The result is what I call the Vegas paradox. A 2010 survey found that new homes used an average of 115,000 gallons annually, while pre-2003 homes used 185,000 gallons. Altogether, the 120,000 homes built in the Las Vegas Valley since 2003 use about 8 billion gallons less per year than their predecessors. That means that, under current regulations, developers will have to keep building and building in order for the Water Authority to succeed in reducing per capita water-use to 199 gallons by 2035. The more the place grows, in other words, the more sustainable it appears.

The Water Authority currently offers homeowners $1.50 per square foot of grass torn out. As a result, 167.5 million square feet of grass have vanished, saving billions more gallons of water. Residents can't just put in gravel or concrete, though, because those contribute to the urban heat island effect. Instead, like Inspirada's Water Smart homes, yards need enough arid-friendly plants to shade at least half their area.

Still, plenty of Las Vegans cling to their grass. As we drive through the city, Davis and I traverse a verdant neighborhood near the old spring. I notice an early-'60s house that's so retro-cool I half expect to see Dean Martin stumble out grinning, martini in hand. The lawn is an eye-popping shade of green that looks less real than the artificial turf at newer homes, and most of the neighborhood's other homes are equally lush and boast tree-shaded pools. This means that each of them uses as much as 10 times the water that one of the Water Smart homes does. Davis winces at the sight of a sidewalk-spraying sprinkler system, but it occurs to me that this neighborhood has vast potential. If times get really dry, the Water Authority will have the water used here essentially in reserve and as a last resort could force even these residents to tear out their lawns.

Meanwhile, the Water Authority labors to remind these folks that they live in a desert, aggressively encouraging compliance with watering restrictions. In one series of ads, a man runs sprinklers indiscriminately, until a nun, old woman or cute little dog arrives at his door, glares at him, and eventually takes action: The old woman kicks him in the crotch, the nun whacks him with her nunchuck ruler, or the dog attacks. Davis says the ads have offended some, but are effective.

This education-over-regulation approach has drawn criticism, especially when it comes to water rates. Though Vegas' water-pricing structure is far stronger than it was in the '90s – a heavy water user pays about five times more than a water miser – rates are still far lower than in many other arid Western cities, such as Santa Fe and Tucson.

But Vegas' libertarian streak and strong labor movement mean heavy regulation isn't always politically feasible, says city sustainability director Perrigo. That's why the city government leads by example – switching out 40,000 streetlights with LEDs or installing solar arrays to save some $5 million on its yearly electric bill – or pushes voluntary efforts, like Green Chips, a public-private partnership that, among other things, created a sustainable leadership academy that fosters a greener organizational culture. In many cases, that's enough. "For the private side, it's a bottom-line thing," says Perrigo. "That's our big story." KB Homes, for example, deliberately chose to build 9,500 Water Smart homes.