(Adds reaction from industry and expert in paragraphs 6, 11)

By Daniel Trotta

June 1 (Reuters) - The state of Arizona on Thursday restricted future home-building in the Phoenix area due to a lack of groundwater, based on projections showing that wells will run dry under existing conditions.

The action by the Arizona Department of Water Resources stands to slow population growth for the Phoenix Active Management Area, home to 4.6 million people and one of the most rapidly expanding areas of the United States.

The state's recently concluded analysis projected a water shortfall of 4.86 million acre feet (6 billion cubic meters) in the Phoenix area over the next 100 years.

In response, the state said it will deny new certificates of Assured Water Supply, which enable home construction.

Arizona has imposed such restrictions on other areas, and not all of greater Phoenix requires a certificate, but experts said the measure was certain to slow home-building in an area representing over half the state's population.

"It's a reality check. We need to have the water supplies in order to grow," said Sharon Megdal, director of the University of Arizona's Water Resources Research Center.

The Department of Water Resources said developers would need to find other sources to build.

Those sources could include officially designated entities that have excess water to sell, or farmers or Native American tribes with water rights, but all are facing short supplies given overuse and a historic drought this century.

Recycled water or desalinated brackish groundwater could also increase future supplies, Megdal said.

A home builders trade association said theirs is the only industry required to meet 100 years of demand for groundwater use, even though new homes have doubled their water efficiency in recent years and already restock the groundwater they consume through the Central Arizona Groundwater Replenishment District.

"We have struggled with the fact that we're the only one that ultimately is stopped when groundwater issues arise," said Spencer Kamps, vice president of the Home Builders Association of Central Arizona.

Arizona's other main water source, the Colorado River, is also under strain.

Arizona along with partner states in the Colorado River Compact last week agreed to reduce their intake from the river by 13% over the next three years as part of a seven-state plan to save a river that provides drinking water for 40 million people, including Phoenix. (Reporting by Daniel Trotta; Editing by Richard Chang and Kim Coghill)