The first thing you notice in North Dakota's oil patch are trucks. They dominate a landscape defined not long ago by cattle and wheat, and not long before that by bison and grass. Trucks groan through Watford City all night. They pile up traffic on highways designed for the occasional car or combine and whip dirt roads into dust storms that locals mistake for prairie fires. They're the first thing anyone mentions when you ask what has happened here in western North Dakota.

Thanks to the Bakken shale, the state has become the country's second-biggest oil-producer practically overnight. And while the world still runs on oil, with the rise of hydraulic fracturing, or fracking, oil increasingly runs on water. Drillers inject 1 million to 3.5 million gallons of pressurized water into each well to shatter the rock and free the oil. More of the trucks you see are carrying water than anything else, some 400 to 800 truckloads per well.

In the low, early-morning light one fall day, the trucks are already lined up eight deep at a water depot outside Watford City. In the time it takes to smoke a cigarette, one trucker from Minnesota pumps 4,200 gallons from the ground into his chrome tanker. The job is good enough that, after two years of living away from home, he says his wife will soon be joining him here.

The water business is good for locals, too. Several dozen farmers and ranchers with access to water and $150,000 to spend have built water depots like this one -- trailer-sized aluminum pump-sheds with eight-inch pipes sticking out of the sides. These private water sellers pulled in $25 million to $30 million last year, according to Steve Mortenson, who heads the Independent Water Providers, a group that represents the industry in the state capital. Several local towns have built depots to sell excess municipal water, pulling in another $10 million or so last year, Mortenson estimates, a substantial sum given their average population of a few thousand people.

The sales are raising uncomfortable questions in a region where fewer than 15 inches of rain falls each year. In many places, the nearest water is 1,000 feet down in a large aquifer that flows freely to the surface in low-lying areas. But it recharges slowly, and the level at which it flows without pumping is dropping more than a foot per year from overuse. Meanwhile, most of the fracking water comes from a series of smaller, shallower aquifers, some of which are already stretched to meet drinking and irrigation needs. The Missouri River has begun to provide some relief, though federal agencies are already tussling over the possible negative effects of withdrawals. To make matters worse, the fracking water ends up contaminated and must be injected thousands of feet underground, removing it from the hydrologic cycle.

There's plenty to supply the oil companies for now, says Bob Shaver, director of the water appropriations division for the Water Commission, which monitors the state's aquifers and regulates all surface and groundwater withdrawals. The best estimate for oil-field use is about 3 billion gallons, based on last year's activity, with demand projected to double over the next decade. But it's only a matter of time before the state's water is fully appropriated, he says, and any new use will have noticeable effects. That day is nearing as the oil rush drives population growth in rural areas with little infrastructure, further straining water supplies. McKenzie County, in the heart of the boom, has grown 20 percent in two years.

Each new depot draws more opposition from neighbors and other interests, Shaver says, highlighting the resource's increasing value -- and scarcity. "To me, water is going to be the oil of the 21st century."

Nationwide, energy companies have been using more and more water for hydraulic fracturing over the past decade, spurring worries about impacts, from the arid West, where many rivers are fully appropriated, to relatively water-wealthy Pennsylvania. The EPA estimates that fracking uses between 70 billion and 140 billion gallons of water total each year. That's small compared to irrigation, which uses about 128 billion gallons every day. But much of the irrigation water runs into the ground or streams, and a lot of the rest evaporates to rain down elsewhere. In contrast, water used for oil and gas is usually disposed of or otherwise removed from the system. Even relatively small withdrawals can have local impacts.

"Will oil and gas use limit other uses?" asks Reagan Waskom, director of Colorado State University's Colorado Water Institute. In dry years such as this, he says, the answer is sometimes yes. So far, it's seldom happened. But Colorado officials project the industry's water consumption for fracking will grow 20 percent over the next three years, to more than 6 billion gallons per year.

Meanwhile, during last year's drought in Texas, drilling hardly slowed even as farmers and ranchers lost crops and cattle, and towns restricted water use. Regulators predict the annual fracking-water use there will triple over the next decade to 39 billion gallons. Environmental groups in the state have begun pushing  the  Legislature  to  enact conservation requirements for drillers and other industries.

While North Dakota has produced oil since the '50s, this boom's water use is unprecedented and cuts to an ongoing concern. Delegates writing the State Constitution in 1889 enshrined water as a public resource, held in trust by the state. But anyone with physical access to an aquifer or surface water can apply to the Water Commission for a withdrawal permit. (You don't need a permit for most domestic or livestock use.) Other than a few hundred dollars in fees, the water is free.

Between 1980 and 2007, when the current oil boom started, the state issued just 10 permits for water depots. It's awarded at least six times as many since then. The process isn't onerous so long as the new use won't "unduly" affect existing permits, which have priority. The commission has even urged farmers to temporarily convert irrigation permits to help drillers; nearly two dozen have done so. For the most part, oil companies here have not begun reusing wastewater, nor have regulators pushed them to. With the Missouri River bisecting the oil fields, the prevailing view is that nothing, least of all water, should slow drilling. That some individuals have reaped millions selling this water is simply a bonus.