Arizona is located in an arid to semiarid region in the southwestern United States and is one of the fastest growing States in the country. Population in Arizona surpassed 6.5 million people in 2008, an increase of 140 percent since 1980, when the last regional U.S. Geological Survey groundwater study was done as part of the Regional Aquifer System analysis program.

Water is one of the most important natural resources of the United States and is essential to our health and economic well-being. Increasing competition for water to meet the needs of a growing population, municipalities, agriculture, industry, ecosystems, and recreation weighs on the sustainability of this resource. The sustained use of water resources is contingent on understanding the hydrologic implications of various alternative development strategies and properly evaluating their short- and long-term implications at scales that make sense. Compounding this complexity are unforeseen factors such as climate variability and change, which can further exacerbate an already challenging situation.

The U.S. Geological Survey, at the request of Congress, is assessing the availability and use of the Nation's water resources to help characterize how much water is available now, how water availability is changing, and how much water can be expected to be available in the future. The Great Lakes Basin Pilot project of the U.S. Geological Survey national assessment of water availability and use focused on the Great Lakes Basin and included detailed studies of the processes governing water availability in the Great Lakes Basin.