How worried is this city getting about water? Tom Warden answers the question by driving the streets of a sprawling community called Summerlin.

He passes a gleaming row of suburban-style homes abiding by a strict new rule: no lawns.

He points to sculpted fountains that were built to give the place a touch of class. They have been turned off indefinitely.

He stops by a traffic circle carpeted with about 100,000 square feet of lush green grass. Landscapers are rushing to replace it with rocks and desert plants.

"This problem is coming at us like a freight train," said Warden, a vice president for the developer of Summerlin, a community of 34,000 homes along the western edge of the city. "No one here has ever seen it this dry."

In the throes of a prolonged drought, and growing at runaway speed across barren desert, Las Vegas is having a hard time these days reconciling its big dreams with an unforgiving new fact of life: It is running out of water.

The rest of the West, which has been afflicted by the drought for about five years, is watching its predicament with anxiety.

Some scientists say the drought may be the worst to strike the region in centuries. If it persists for several more years, the seven western states whose shared lifeline for water is the Colorado River could see their supplies threatened, too -- and face some of the same pressures that Las Vegas is feeling. The river's water level is at its lowest point in at least 100 years.

Lake Mead, the reservoir that collects some of the river's flow and that holds nearly all of the city's supply, also keeps evaporating amid the drought. The lake, which has dropped about 75 feet in the past few years, is now only half-full.

If its water level declines two more feet, which is almost certain to occur by the end of the year, Las Vegas is likely to declare its first drought emergency.

It already feels as though one exists. The Southern Nevada Water Authority plans to spend $32 million over the next year offering residents rebates to give up their grass. It is paying $1 per square foot -- double the price it set two years ago. On average, a football field of turf is being ripped up every day.

That is not enough. Here and in other cities around the Las Vegas Valley, authorities desperate to improve conservation are raising water rates on homeowners and businesses, restricting the use of lawn sprinklers, cracking down on car washes and threatening water-guzzling golf courses with fines.

Authorities also are launching elaborate advertising campaigns, in English and Spanish, describing the drought in dire terms and urging residents to curtail their water use. On some bus stop signs, the plea is blunt: "Join the Crusade."

Some communities are even contemplating covering parks and playgrounds with artificial turf -- but that idea is provoking intense debate. Some parents fear their children are bound to get burns from fake grass during the scorching days of summer in Las Vegas. Others dread cleaning up after dogs.

"We've left the era of abundance and entered the era of shortages," said Patricia Mulroy, the general manager of the Southern Nevada Water Authority. "It's an ugly, ugly period."

Managing water in the arid West has never been easy. But there always has been enough of it to go around. In fact, California, the only colossus in the region, has long been taking more than its allotted share from the Colorado River because no other neighboring state needed it. And when Lake Mead was filled decades ago, officials figured it would quench southern Nevada's thirst for centuries.

Then the population boom began. Today, Nevada and Arizona are the fastest-growing states in the country. Colorado and Utah are not far behind.

The pace of development around Las Vegas is dizzying. In the first six months of this year, officials here approved permits for the construction of 20,300 new homes -- a 67 percent increase from the same period last year. The population has doubled in the past decade, to 1.6 million. It grew by 60,000 people last year. About 37,000 came from other states.

"We can't build housing fast enough," said Monica Caruso, a spokeswoman for the Southern Nevada Home Builders Association.

The city and its suburbs have long welcomed the mass migration. But it is plainly beginning to strain the water supply.

That Las Vegas gets almost no rain is hardly news. It is always one of the driest places in the country. But researchers examining geological patterns here over hundreds of years say that the past four decades may have been wetter than usual.

Since the 1960s, the region has occasionally averaged about seven inches of rain a year. The tally during the past few years -- about four inches -- is closer to its historical average.

So the drought may not be an anomaly. It may be normal.

And if that is true, growth has been guided in part by mistaken climate assumptions. An extended drought could profoundly affect how development proceeds.

"People are not facing reality," said Robert Ferraro, the mayor of nearby Boulder City. "We have to change our priorities. But I think it's going to be a hard sell. I don't see anything happening fast enough to thwart the problems we're going to have if we're headed into a major drought cycle. Every step being taken to conserve water is being overtaken by the number of people who keep coming."

But the mood here may be changing. A poll conducted last month by the Las Vegas Review-Journal found that 75 percent of residents favored limits on home construction until the drought ends. Some community leaders are urging the federal government to stop auctioning to builders the large tracts of land that it owns around the valley.

So far, those requests have been ignored. Growth is as much an addiction in Las Vegas as gambling. It is not likely to stop.

"How do you just raise the drawbridge to a community that is attracting thousands of new people every month?" Caruso asked.

Mulroy warned that slowing or stopping home construction could harm the region's economy. "To say you can just cut it all off is naive," she said. "The issue is not whether we grow, but how we grow."

For now, she is preaching the urgent need for water conservation. And for the first time, it appears that message is taking hold.

Last year, after doubling their offer for turf, water officials hastily had to add $8 million to the rebate program to accommodate the number of residents volunteering to give up grass in their yards. Every square foot removed saves about 80 gallons of water a year.

Developers are responding to the emerging crisis, too. Some have started building homes on smaller lots, which lessens the need for landscaping -- the main drain on the local water supply. Others have restricted the size of swimming pools or have stopped planting grass along sidewalks and medians.

At Summerlin, which eventually will include 64,000 homes, residents have to comply with an array of new water-saving edicts. Grass is banned in front yards and restricted in back yards. Only trees and shrubs suited for desert climates can be planted. And plans for golf courses are being scaled back -- in part because managers at Summerlin have discovered that most residents who want to live beside beautiful fairways do not play the game. They just like the view.

Now, in some areas they will have to learn to love looking at desert rocks and plants instead.

"People are really waking up on this issue," Warden said.

The consequences of Las Vegas declaring a drought emergency are not yet clear. But they are bound to be severe.

Water rates may be raised again, and more restrictions may be imposed on lawns, golf courses and the use of sprinklers. Some residents sound panicked; they fear they could eventually be required to use plastic plants in their yards.

"We're the first urban community in the West that has to come to grips with these questions," Mulroy said.

Hal Bloch, a retired corporate executive, is among the homeowners at Summerlin ready for change. He moved to Las Vegas from Pittsburgh in 1996 and had grass in his yard. Earlier this year, he decided to help the city save water by pulling up all of it. He was rewarded with nearly a $1,000 rebate.

But he said friends and neighbors are divided on the issue. Some are showing new interest in conservation, but some are still skeptical of the need for it.

"They say, 'If it's really as bad as everyone is portraying it, why are they continuing to hook up homes willy-nilly?' " Bloch said.

One other old and formidable obstacle, he said, might prevent Las Vegas from saving more water.

"Some people here," Bloch said, "just don't like the desert look."

The gateway at Promontory, part of the Las Vegas development called Summerlin, features a converted fountain. A photo held below shows the fountain when it had water. Conservation measures during the drought prohibit decorative fountains outside the tourist corridor. Lake Mead, the reservoir that holds most of Las Vegas's water, has dropped 75 feet, revealing a water clarifying tank built during the construction of Hoover Dam in the 1930s.