PRESCOTT, Ariz. — The Verde River is one of the largest remaining perennial rivers in Arizona and a lifeline for multiple species of desert flora and fauna in the Prescott area.
Scientists say the first major use of a federal computer model for the Verde Watershed shows that groundwater pumping has reduced the river’s flow.
The Verde River Basin Partnership commissioned the model runs and report done by the U.S. Geological Survey, according to the Prescott Daily Courier.
The USGS model estimates the river’s annual base flow dropped by about 4,900 acre-feet between 1910 and 2005 at the upper end of the Verde Valley 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 because of 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 water demands increase, even using highly conservative demand estimates.
“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 USGS hydrologist and groundwater model expert who heads the hydrology subcommittee for the Verde River Basin Partnership.
People throughout the basin say they want to preserve the Verde River and “we need to work together to find a way to do that,” said Basin Partnership chairman Tom O’Halleran, a former state legislator.
“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,” USGS hydrologist Bradley Garner said.
The conservative future hypothetical scenario shows a decrease of 2,700 to 3,800 acre-feet at the upper end of the Verde Valley, and a decrease of 5,400 to 8,600 acre-feet at the lower end of the Verde Valley between 2005 and 2110.
It uses just a 3 percent increase in human water use every decade for 50 years, and then zero growth for the next half-century.
The future growth scenario includes only half of the water needs projected in an ongoing study by the U.S. Bureau of Reclamation.
The Verde Valley’s population actually grew 13 percent between 2000 and 2010, the USGS said.
Prescott-area communities have refused to join the Basin Partnership because officials fear the public could misinterpret what the model concludes about their plan to pump as much as 8,000 acre-feet 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 base flow.
Copyright 2013 The Associated Press.