Aquifer storage change and storage properties, 2010–2017, in the Big Chino Subbasin, Yavapai County, Arizona

The Big Chino Subbasin is a groundwater basin that includes the Verde River headwaters in Yavapai County in north-central Arizona. Groundwater in the southern part of the subbasin is found primarily in the Big Chino and Williamson Valleys. The former is a potential municipal water source for growing communities in Yavapai County, particularly groundwater from the Big Chino Water Ranch, about 15 miles northwest of the community of Paulden. Groundwater in the Big Chino Valley discharges to wells (by pumping), by evapotranspiration, and to the upper Verde River springs, which form the headwaters of the Verde River. Groundwater also discharges to short perennial reaches of Williamson Valley Wash, Walnut Creek, and a small number of small, ungaged springs and seeps. To monitor changes in groundwater storage and to identify aquifer-storage properties, a network of repeat microgravity stations and groundwater-level monitoring stations was established in the Big Chino and Williamson Valleys in 2010.

Small decreases in groundwater storage were observed throughout the study area from 2010 to 2017. Annual groundwater withdrawals for agricultural use varied between 2,800 and 4,000 acre-ft between 2013 and 2016, with an additional amount, probably less than 1,000 acre-ft, withdrawn for domestic use, primarily in the Paulden and Williamson Valley Wash areas. No local recharge events from sustained rainfall were observed during 2010 to 2017, and base-flow discharge in the Verde River near Paulden and Williamson Valley Wash near Paulden was consistently below the long-term average (for years 1964 to 2017 and 1966 to 2017, respectively) at each site. Relations between groundwater-level changes and aquifer-storage changes (determined from repeat microgravity data) indicate monitoring wells are representative primarily of semiconfined aquifer conditions in the Paulden area, the area west of Big Chino Wash, and the Big Chino Water Ranch area. Unconfined aquifer conditions are monitored in the Williamson Valley Wash area and at two sites in the Paulden area. Specific yield was estimated at five wells and ranged between 0.04 and 0.34, with a median value of 0.23. 

Negative groundwater-level trends (increasing depth to water) were observed between 2010 and 2017 at all sites where trends were identified using the Mann-Kendall trend test, except for the northernmost reaches of Big Chino Wash within and to the north of the Big Chino Water Ranch. Groundwater storage trends were negative at all sites where trends were identified except for one site in the foothills of the Santa Maria mountains west of Big Chino Wash. Declining storage in the Big Chino Water Ranch area, where water levels show no trend or are increasing, are likely the result of drying conditions in the unsaturated zone and (or) aquifers located above the aquifer(s) monitored by wells.