Monitoring Aquifer-Storage Change and Land Subsidence Related to Groundwater Withdrawal in the Willcox and Douglas Groundwater Basins in Southeastern Arizona

Abstract:

Groundwater is the primary source of water in the Willcox and Douglas Basins in southeastern Arizona and about 90 percent of the groundwater withdrawal is for agriculture. It is estimated that current groundwater production exceeds recharge by at least a factor of two in the Douglas Basin and by a factor of three to eight in the Willcox Basin. The groundwater mining has resulted in groundwater declines of as much as 105 feet in the Willcox Basin between 2006 and the present and 52 feet in the Douglas Basin between 2005 and the present.

 

The U.S. Geological Survey collected repeat absolute gravity measurements at 8 sites in the Willcox Basin and at 4 sites in the Douglas Basin for the purpose of measuring the change in aquifer storage between 2008 and 2014. All sites in the Willcox Basin showed aquifer-storage loss between 2008 and 2014, with values ranging from 0.5 to 8.5 feet of water. In the Douglas Basin, two sites showed aquifer-storage loss of 0.8 feet and 3.0 feet, respectively. Additionally, two sites in the Douglas Basin showed storage increases of 2.0 feet and 2.9 feet of water in storage. The storage increases are attributed to the sites being close to ephemeral streams where two large precipitation and associated runoff events (remnant East Pacific hurricanes) occurred shortly before the gravity measurements in 2014.

 

The Arizona Department of Water Resources has identified three major areas of land subsidence using Interferometric Synthetic Aperture Radar (InSAR) data – two in the Willcox Basin and one in the Douglas Basin. Land subsidence of as much as 29 inches in the Willcox Basin and 18 inches in the Douglas Basin has occurred between 2006 and the present—the magnitude and rates of human-induced subsidence have caused earth fissures and impacted roads, a power generation facility, and a railway.

 

The declining groundwater levels, decrease in aquifer storage, and land subsidence are a challenge for future groundwater availability in the Willcox and Douglas Basins. Continued monitoring by the USGS and the ADWR will provide 1) the temporal data needed to understand and evaluate how the hydrogeologic systems in the two basins are responding through time to groundwater overdraft and 2) provide a scientific basis for future mitigation efforts such as redistribution of pumping and/or a reduction of groundwater withdrawal.