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Agriculture, growth blamed for Az's groundwater depletion

Study spans 67 years of aquifer levels around state

PHOENIX — Arizona has depleted its groundwater over the past 70 years enough to fill Lake Powell nearly three times, according to the first federal study of the state’s groundwater since the 1980s.

Knowing that fact and others contained in the U.S. Geological Survey’s report will help policy makers better understand the state’s limited water resources, said Fred Tillman, a Tucson-based hydrologist who served as the study’s lead author.

“It’s critical that we are vigilant about our groundwater use,” he said.

The study, which covers 1940 through 2007, establishes a baseline measurement of levels in the state’s aquifers prior to development, allowing researchers and policy makers to assess how population growth has affected the state’s groundwater.

The 45 alluvial basins in the state, which are composed of sand, silt and clay, account for 95 percent of Arizona’s groundwater usage.

Over nine years beginning in 1997, the study looked at wells in areas with the most groundwater usage.

Of the 1,279 wells studied, 53 percent were losing water at a rate of 1 foot or more per year. Twenty-one percent showed groundwater levels rising by 1 foot or more per year, while the remaining 26 percent were nearly stable – gaining or losing no more than 1 foot per year.

Many of the losses occurred in areas with heavy agricultural use, such as around Willcox, or groundwater-reliant cities like Prescott that have experienced large population growth.

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“There are very limited water supplies in very rural areas,” Tillman said. “It’s really hard to know what’s coming.”

The report analyzes the processes through which each aquifer is recharged, including mountain runoff, drawing on historical precipitation and other data to estimate how much each is recharged annually.

It also shows groundwater increases associated with efforts to recharge Central Arizona Project water northwest of Tucson, though wells within Tucson show losses.

Across the state, the balance between groundwater being extracted and recharged showed a net loss of 74.5 million acre-feet in the last 70 years.

Frank Corkhill, the chief hydrologist for the Arizona Department of Water Resources, said the study is useful for understanding long-term trends in water usage.

“It gives an idea of how long, in a very broad sense, the water might last,” he said.

Tillman said it’s up to Arizona’s leaders to decide what to do with the information.

“We need data, unbiased, scientific analysis, to help policy makers make decisions,” he said.

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Brittny Goodsell/Cronkite News Service