USGS: groundwater pumping reduces Verde River flow
The first major use of a federal computer model for the Verde Watershed helps quantify how past and future human water use reduces the flow of the Verde River.
The high-tech model work puts a face on what scientists have been saying for years: that groundwater pumping affects the flow of the Verde River. It's one of the largest remaining perennial rivers in Arizona and a lifeline for multiple species of desert flora and fauna.
"For the first time, we really have a definitive look at the impact of past pumpage on the Verde River," said Bill Meyer, a retired U.S. Geological Survey (USGS) hydrologist and groundwater model expert who heads the hydrology subcommittee for the Verde River Basin Partnership. The VRBP commissioned the model runs and report.
The USGS released the results of the model runs and accompanying report on Tuesday, almost exactly two years after it originally created the Northern Arizona Regional Groundwater Flow Model.
USGS experts will present their new findings to the VRBP at 4-6 p.m. Thursday at the Camp Verde Multi-Use Complex Auditorium, 37 Camp Lincoln Rd. Meyer also will give a PowerPoint presentation, and people can ask questions. The public is invited to the free event. To RSVP and save a space, send an email to firstname.lastname@example.org.
The VRBP is calling it "one of the most pivotal meetings about water in the Verde Valley's history."
People throughout the basin say they want to preserve the Verde River, said VRBP Chair Tom O'Halleran, a former state legislator.
"Then we need to work together to find a way to do that," he said, and the USGS model will help.
The USGS model estimates the river's annual baseflow dropped by about 4,900 acre-feet between 1910 and 2005 at the upper end of the Verde Valley (Clarkdale gage) because of human use, mainly because of groundwater pumping in the Prescott region. The annual flow dropped about 10,000 acre-feet at the lower end of the valley (Camp Verde gage) because of human surface water and groundwater uses in both the Upper and Middle Verde.
The model also shows that the river's flow will continue to drop through 2110 as human water demands increase, even using highly conservative demand estimates.
USGS Hydrologist Bradley Garner helped with the work.
"This study is important because it allows us to examine human-caused stresses, namely groundwater pumping, independently from other factors that change over time, such as annual precipitation rates," Garner said in a USGS news release Tuesday.
The conservative future hypothetical scenario shows a decrease of 2,700 af to 3,800 af at the upper end of the Verde Valley, and a decrease of 5,400 af to 8,600 af at the lower end of the Verde Valley between 2005 and 2110. It uses just a three percent increase in human water use every decade for 50 years, and then zero growth for the next half-century.
The VRBP officials said they used such conservative numbers so no one could say the estimate was exaggerated.
The future growth scenario includes only half of the water needs projected in an ongoing study by the U.S. Bureau of Reclamation, said Ed Wolfe, chair of the VRBP technical advisory group and retired USGS geologist.
The Verde Valley's human population actually grew 13 percent between 2000 and 2010, the USGS noted.
The Yavapai County Water Advisory Committee was a partner in the effort to create the Northern Arizona Regional Groundwater Flow Model. It paid the USGS $40,000 to run future population growth scenarios on the model, but opposition from Prescott-area members has put a hold on that work.
Prescott-area communities have refused to join the VRBP.
Prescott area officials fear the public could misinterpret what the model concludes about their plan to pump as much as 8,000 af of groundwater each year from the Big Chino Valley to supplement their dwindling Little Chino Valley supplies. Scientists generally agree that the Big Chino Aquifer supplies more than 80 percent of the Upper Verde River's baseflow.
The VRBP model runs didn't include Prescott's Big Chino wells, but they did feature the same population growth estimates in the Prescott region as in the Verde Valley. The computer model runs had to include the Upper Verde to operate correctly.
The report focuses on the Verde Valley because of money constraints, O'Halleran said. The VRBP hopes to run more growth scenarios in the future.
VRBP Coordinating Committee Member Bob Luzius, a former Prescott City Council member, urged the Prescott Council Tuesday to attend Thursday's meeting. Mayor Marlin Kuykendall said he couldn't attend, but he hopes someone from the council will be there.
The partnership commissioned the $300,000 model runs and report with the help of money from the USGS and the Walton Family Foundation.
Verde communities will use the report to help plan for future water needs for humans as well as the environment.
"Groundwater flow models provide a sophisticated tool to help communities responsibly manage, develop and use their groundwater resources," said William Alley, director of science and technology for the National Ground Water Association, in a USGS news release about the new report.