Millions of people have moved to Arizona, New Mexico, Colorado and surrounding states- and many more are expected to follow, adding to the demand for potable water.

Sharon Megdal, director of the Water Resources Research Center at the University of Arizona, said water harvesting efforts by Tucson residents and other communities are conserving water, but the demand has outstripped the supply.

Much of Arizona's water comes from the Colorado River via the Central Arizona Project, but the river is having trouble, she told AZ Illustrated Nature.

"We now have the U.S. Bureau of Reclamation, the manager of the Colorado River saying there's a 2 percent chance of a shortage in 2015, but a 50 percent chance in 2016," Megdal said. "This is based on the river flows, based on precipitation up at the Colorado River, in the headwaters and what they're releasing from Lake Powell to Lake Mead."

Megdal said she doesn't want to alarm people, but she added that our water situation could be serious.

"I don't want to get people worried. If there is a shortage in 2016, it won't really affect the Colorado River water to the cities like Tucson and Oro Valley and others with allocation, it will affect some of the users we have but it's getting more real," she said. "And I think that the reality of a shortage is resonating with people."

In the future, Megdal said she believes Tucson and other cities will have to implement new systems for recycling water, including cleaning waste water for use again by customers.

This method is commonly referred to as "toilet to tap" and it's already underway in other cities, including Santa Clara, California.