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Thu, May 23

USGS model will undergo more work before use

PRESCOTT - The Yavapai County Water Advisory Committee agreed Wednesday to do more work on the U.S. Geological Survey computer model of the Big Chino Sub-basin geology, before using it to see how people might affect local water supplies in the future.

The work they agreed upon could take approximately six months and cost approximately $36,000.

The decision came almost exactly a year after the USGS released the Northern Arizona Regional Groundwater Flow Model and report on April 12, 2011.

"Let's rock and roll and get moving" on the extra work, Yavapai County Supervisor and Committee Member Chip Davis said. "We've been sitting on it too long."

The model was the culmination of 12 years of data collection and studies that cost more than $1 million, with major contributions from the Water Advisory Committee.

While computer models of parts of the study area already existed, this new model connects several river basins to show how they interact with each other.

Before the model and report even became public, Prescott-area communities were criticizing it after obtaining an advance copy that Verde Valley communities did not get.

Prescott-area officials on the Water Committee and their employees who serve on the technical advisory group have expressed fears that citizens will think the model is the final word if the model shows their plans to pump Big Chino groundwater into their depleted Little Chino Sub-basin could hurt the Upper Verde River.

Experts generally agree that the Big Chino supplies more than 80 percent of the Upper Verde River's baseflow. But they don't know how much groundwater could be pumped from the aquifer before the groundwater level drops below the river's headwaters and leaves it dry, which already happened to other Arizona streams such as the Santa Cruz River.

So environmental groups and downstream users such as the Verde Valley communities have expressed concerns about Prescott-area plans to use the Big Chino water.

The first part of the model the USGS will change and test for accuracy is a clay-like formation between Prescott's Big Chino Water Ranch and the Upper Verde River. Prescott-area officials contend the formation is so deep that it acts as a barrier to the flow of groundwater toward the Upper Verde, thereby separating the areas or at least slowing down any potential Water Ranch pumping effects on the river. So the USGS will make the formation deeper in the model and see how it works.

The USGS and Water Committee technical advisors agreed to a list of as many as six items that they will test one at a time for a month each, prompting Water Committee Coordinator John Rasmussen to call it an "adaptive investigation approach." The Water Committee approved the plan Wednesday.

Rasmussen noted that the technical advisors representing the Verde Valley thought the model was fine to use now, but they agreed to more study so Prescott-area communities would have more confidence in the model.

Since the technical experts recommend it, "I don't see how we can move forward without doing something like this," said Brent Bitz, a Water Committee alternate and chair of Sedona's Sustainability Commission.

The model exercises could lead to changes in the model or a decision to gather more on-the-ground data, Water Committee officials said.

"That might in some cases be quite expensive," Rasmussen said.

Yavapai County Supervisor and Water Committee Member Carol Springer wondered out loud whether the USGS scientists really understand how the groundwater system works.

"A lot of this is what I might call 'educated guesses,'" Springer said, so she worries that the public will take the future growth scenario information as fact.

Clarkdale Mayor and Water Committee Member Doug Von Gausig said Springer's comments concerned him.

"You're never going to have a perfect picture of what's going on underground," he said, but he relies on the expertise of the USGS scientists.

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