Updated study still says Prescott pumping will impact river
PRESCOTT VALLEY – A geologist and hydrologist who analyzed the potential impacts of Prescott's plan to pump groundwater from the Big Chino Sub-basin conclude that the pumping would reduce the flow of the Verde River.
U.S. Geological Survey Scientist Emeritus Ed Wolfe, a geologist, and retired U.S.G.S. hydrologist Bill Meyer previously made the same conclusion when they conducted a free analysis of the impacts of Prescott's plan to pump from the CV Ranch northwest of Paulden. They talked about their CV Ranch findings to the Citizens Water Advocacy Group in February.
Now that the city is considering buying another nearby ranch, the JWK Ranch, as early as December, the scientists re-evaluated their conclusion and reached the same one, using updated Prescott pumping estimates.
"This is not as much of an attack or commentary on the City of Prescott as much as it is a call to attention about the concern about pumping in the Big Chino Valley," Wolfe told the Yavapai County Water Advisory Committee during a presentation at its monthly meeting in Prescott Valley Wednesday.
The two scientists dispute some previous claims that an underground clay plug blocks Big Chino groundwater from flowing into the river's headwater area. A region of low permeability exists, but it doesn't completely block the groundwater flow, they said.
If Prescott follows through with its plan to pump 8,717 acre-feet of groundwater annually from the neighboring Big Chino Sub-basin into its Little Chino Sub-basin, the city will reduce the flow of the river by 6 percent in five years and at least 36 percent (8.9 cubic feet per second) in 100 years, the scientists concluded.
The Water Advisory Committee's technical advisory group concluded that the two scientists were using too many simplifying assumptions and did not provide a margin of error for their conclusion, Water Advisory Committee John Munderloh told the committee Wednesday.
The two used a simplified analytic model instead of a numeric model because the latter is complicated and requires lots of data they didn't have access to, said Meyer, who serves on the technical advisory group.
The men purposely underestimated the pumping's impacts because they knew the model wasn't perfect, Meyer said.
The "potential error … could be significant," the technical advisory group wrote. The technical group "did not believe that the method employed was a valid approach to determining the impacts of future pumping on the flows in the Upper Verde River, but that it does provide an idea of what possible impacts might be."
Munderloh noted that the committee's contract with the U.S.G.S. for long-term studies of the Verde River Basin will produce a numeric model within three or four years.
"We already know that pumpage from the Big Chino is going to affect the Verde River, I think," Meyer said. "The only thing we're discussing is timing."
"It's really a question of how much, how fast," Munderloh said.
An abstract of the Wolfe and Meyer study, along with brief information about similar studies, is available on the Yavapai County government's web site at www.co.yavapai.az.us via the water committee's link.