This is part 2 of our series on the Upper Verde River. Part 1, “What is the source of the Upper Verde Springs?” was the subject of our August 2015 Hydro Note.
The debate continues over the potential effects of groundwater pumping in the Big Chino basin on base flows in the Upper Verde River. Garner and others (2013) used the Northern Arizona Regional Groundwater Flow Model, which was developed by the USGS, to estimate the effects of pumping on Verde River base flows. The model simulated a reduction of about 7 cfs in base flow at the Clarkdale gage in response to pumping since 1910.
However, a comparison of measured streamflow data for the Clarkdale and Paulden gages tells a different story. If this 7-cfs reduction at Clarkdale gage were caused by Big Chino pumping, we would expect to see a similar decrease at the gage near Paulden, which is located farther upstream and monitors groundwater discharge from the basin. However, this is not the case; in fact, the variations measured at the Paulden gage have been smaller than those at Clarkdale.
Furthermore, the ratio of base flows at the two gages also suggests that the base flow variations are unrelated to pumping. Since 1940, groundwater has been pumped at an average rate of 10,000 AF/yr from the Big Chino basin and at a negligible rate from the discrete basin between the Paulden and Clarkdale gages. If the base flow at both gages were unaffected by pumping, there should be a consistent relationship between them — and there is. The ratio of base flows (both mean annual and annual minimum) at Paulden to those at Clarkdale has had a consistent trend of 1:3.2, from 1965 (the beginning of concurrent records) through 2014, a period of nearly 50 years. If pumping from Big Chino had caused a measurable decrease in base flows at the Paulden gage, this ratio would surely decrease over time.
The measured streamflow data show that Big Chino pumping has not yet affected base flows in the Upper Verde River. Instead, variations in base flows can be explained by changes in climate and precipitation.
Ed McGavock, a hydrologist with M&A since 1994, is a 31-year veteran of the USGS, where he served Assistant District Chief of the Water Resources Division in both Arizona and Washington. In 2000, Ed was appointed by Governor Jane Dee Hull to a commission to review the 21-year-old Groundwater Management Act and recommend changes. He is a frequent presenter at conferences, particularly those focusing on the use of brackish water resources and the potential impact of climate change on water supplies.