The High Plains aquifer, seen from northern Weld County, Colo., where oil and gas operators are using increasing amounts of water. A wet year has given Greeley a bonanza of water to lease out, but sharing could be a challenge in a dry year. Photo courtesy of U.S. Geological Survey.

When a large tanker truck pulls up to a fire hydrant in Greeley, Colo. -- a midsize town on the semiarid Eastern Slope of the Rocky Mountains -- and starts siphoning water from the city's supply, passersby start looking nervous.

Chances are good the tanker is on its way to an oil or natural gas field, where the water will either go to drilling or be injected underground to reach previously inaccessible resources via a process known as hydraulic fracturing, or "fracking."

Such water transfers are becoming an increasingly common sight. But in a region that has battled drought on and off for decades, and where state officials predict there won't be enough water to sustain expected population and agriculture levels in the not-too-distant future, the tankers spark questions about how water is being used.