Aquifer-Storage Change and Land-Surface Elevation Change Monitoring in the Tucson Active Management Area

Science Center Objects

The Arizona Water Science Center is monitoring aquifer-storage changes and land subsidence within the Tucson Active Management Area (AMA). Land-surface elevation change is monitored at a network of benchmarks throughout the Tucson AMA by measuring changes in land surface elevation over time (approximately annually) with Interferometric Synthetic Aperture Radar (InSAR).  Aquifer-storage change is monitored by measuring changes in gravity over time at the same network of benchmarks.

Aquifer-storage change has been monitored by the U.S. Geological Survey (USGS) within the Tucson Active Management Area (AMA) since 1996 (Pool and Schmidt, 1997) . The USGS began a cooperative study with Metropolitan Domestic Water Improvement District and the town of Oro Valley in 1996 to monitor aquifer-storage change in the Lower Cañada del Oro sub-basin. In 1998, the USGS began a cooperative study with the Arizona Department of Water Resources (ADWR), Pima County, and the City of Tucson to monitor land-surface elevation change and aquifer-storage change in the Tucson AMA. In 2003, these two monitoring studies were combined, and the town of Marana joined the study. 

Objectives

The objectives of this project are to monitor aquifer-storage changes and land subsidence within the Tucson AMA.

USGS A10 absolute gravimeter collecting data near Tucson, Arizona

USGS A10 absolute gravimeter set up at station ASDM2 in the Tucson Active Management Area.(Credit: Rob Carruth, USGS Arizona Water Science Center)

Person using meter to collect relative gravity data near Tucson, Arizona

USGS hydrologist Libby Kahler collecting relative gravity data in the Tucson Active Management Area at station ASDM(Credit: Rob Carruth, USGS Arizona Water Science Center)

Approach

Land-surface elevation change is monitored at a network of benchmarks throughout the Tucson AMA by measuring changes in land surface elevation over time (approximately annually) with Interferometric Synthetic Aperture Radar (InSAR).  The Arizona Department of Water Resources (ADWR) has an InSAR program in the Tucson AMA. InSAR is a technique that utilizes interferometric processing to compare the amplitude and phase signals received during one pass of the satellite-based SAR platform over the Tucson AMA with the amplitude and phase signals received during a second pass of the platform over the same area but at a different time.

The InSAR data are used by ADWR to produce a land-surface elevation-change map over the same time period as the gravity surveys conducted by the USGS in the Tucson AMA. The ADWR provides the elevation-change maps to the USGS as an in-kind contribution to the project.

Aquifer-storage change is monitored by measuring changes in gravity over time at the same network of benchmarks. Gravity is affected by mass and distance; a change in mass or a change in elevation will cause a change in gravity. Groundwater depletion is a mass change and land-surface elevation change is a distance change. By removing the effect of change in distance, changes in gravity are used to determine changes in aquifer-storage.

Temporal-gravity surveys are used in the Tucson AMA to detect local changes in the gravitational field of the Earth attributed to water mass change. The method is readily applied to measurement of aquifer-storage change in the Tucson AMA because of the occurrence of significant variations in pore-space storage that result from ground-water withdrawal, periodic natural recharge events, and focused artificial recharge. 

Two instruments are used at the network of benchmarks: the relative gravity meter and the absolute gravity meter. The relative meter is the primary instrument by which differences in gravity are monitored at stable monuments. Much as control benchmarks are used in conventional land surveying, repeated relative gravity surveys for ground-water storage monitoring should include reference stations where the absolute acceleration of gravity is monitored. The USGS uses a Micro-g LaCoste A-10 field-portable absolute gravity meter to establish these reference stations as needed. This is particularly valuable in a hydrologic context where a number of absolute stations may be located throughout a basin, thereby serving to constrain and adjust the gravity differences from relative gravity surveys.

Relevance and Benefits

This study addresses the science of aquifer-storage change and land-surface elevation change within the Tucson AMA, specifically related to groundwater withdrawal and natural and artificial recharge. The study contributes to the goals of the USGS strategic science direction “A Water Census of the United States,” as identified and described in the Strategic Science Plan of the USGS (U.S. Geological Survey, 2007).

Additional Resources

Pool, D. R., and W. Schmidt, 1997, Measurements of groundwater storage change and specific yield using the temporal gravity method near Rillito Creek, Tucson, Arizona: U. S. Geological Survey,Water-Resources Investigations Report 97-4125. https://pubs.usgs.gov/wri/1997/4125/report.pdf

D. R. Pool; The utility of gravity and water-level monitoring at alluvial aquifer wells in southern Arizona. Geophysics ; 73 (6): WA49–WA59. doi: https://doi.org/10.1190/1.2980395

Kennedy, J., T. P. A. Ferré, and B. Creutzfeldt (2016), Time‐lapse gravity data for monitoring and modeling artificial recharge through a thick unsaturated zone, Water Resour. Res., 52, 7244–7261, https://doi.org/10.1002/2016WR018770.

Kennedy, J.R., 2018, Changes in Earth’s gravity reveal changes in groundwater storage: U.S. Geological Survey Fact Sheet 2018–3032, 4 p., https://doi.org/10.3133/fs20183032.

More information on the Tucson AMA gravity project and other projects by the U.S. Geological Survey Southwest Gravity Program (including complete bibliography) is also at https://go.usa.gov/xqBnQ

Monitoring Results in the Tucson AMA

Increases in gravity and water-levels occurred between 1997 and 1999, due to above-average natural recharge, but overall declining gravity and water-level trends from 1999 to 2002 in Tucson Basin reflected general declining groundwater conditions and redistribution of the recent recharge throughout a larger region of the aquifer. Increases in gravity and water levels occurred from summer 2006 to summer 2008 in central Tucson following a large natural recharge event during the summer of 2006. Overall however, declining gravity and water-level trends from spring 2003 through fall 2014 in Tucson Basin reflected general overdraft conditions and redistribution of recent recharge throughout a larger region of the aquifer. From fall 2014 to spring 2018, water-level rise and corresponding increases in gravity occurred following higher than average precipitation in 2014 and a trend toward a reduction in groundwater withdrawal in the Tucson Basin.

Groundwater storage in Avra Valley has increased throughout the entire monitored period, largely as a result of managed recharge of Central Arizona Project water in the monitored region