Over the last 70 years, ground water in Arizona's alluvial basins was depleted by more than 74.5 million acre-feet, or approximately three times the maximum storage of Lake Powell.  

U.S. Geological Survey (USGS) scientists recently released a report evaluating ground water availability and use for all of Arizona's alluvial basins from 1940–2007. Arizona’s alluvial basins, located south of the Colorado Plateau, are aquifers composed of sand, silt and clay material that has eroded from mountains surrounding the basins.

These alluvial basins account for 95 percent of the state’s ground water use, with ground water providing about 43 percent of the entire state’s water supply.

"Arizona is one of the fastest-growing states in the nation, and ground water supplies will undergo increased demand as water needs for growing population are balanced with Arizona's agricultural sector," says USGS hydrologist Fred Tillman, who led this water availability and use pilot study. "This USGS report is intended to aid state and local agencies by providing them information about ground water to help better plan for the future."

Ground water withdrawals total about 2.4 million acre-feet per year in the study area, mainly for agriculture use and secondarily for municipal use. The primary source of ground water recharge, or replenishment, comes from mountain runoff that flows into the ground water basins. Ground water in these basins has been depleted because the amount of withdrawal has outpaced aquifer recharge.

The new report includes updated ground water budget information for each of the 45 individual basins or management areas, as well as a new ground water-flow model to test an approach for evaluating interconnected ground water basins. Several indicators of ground water conditions also are analyzed, building on previous work that resulted in the popular Arizona Groundwater Conditions Interactive Map.

"The new USGS report on ground water availability and use in Arizona is a valuable compilation of information and data," says Frank Corkhill, chief hydrologist of Arizona Department of Water Resources. "The report provides a significant analysis of recent water-level data and long-term trends and provides a well-balanced assessment of the strengths and weaknesses of various methods used to estimate ground water budget components."

The ground water assessment and methods development effort piloted in Arizona's Southwest Alluvial Basins is being conducted by the USGS Groundwater Resources Program. Information derived from this study in collaboration with more than 30 regional aquifer studies, once completed, will provide a collective assessment of America's ground water availability.

This pilot study of Arizona's Alluvial Basins was an important step because the ground water-flow model developed for several basins in southern Arizona demonstrated that it is possible to model the interconnected nature of adjoining basins and assess how changes to ground water supplies in one area might affect flow to or from adjacent areas.

The report can be found online by following this link.