|The availability of ground water is of
extreme importance in areas, such as southern Arizona,
where it is the main supply for agricultural,
industrial, or domestic purposes. Where ground-water
use exceeds recharge, monitoring is critical for
managing water supplies. Typically, monitoring has
been done by measuring water levels in wells; however,
this technique only partially describes ground-water
conditions in a basin. A new application of
geophysical technology is enabling U.S. Geological
Survey (USGS) scientists to measure changes in the
amount of water in an aquifer using a network of
microgravity stations. This technique enables a direct
measurement of ground-water depletion and recharge.
In Tucson, Arizona, residents have relied solely upon ground water for most of their needs since the 19th century. Water levels in some wells in the Tucson area have declined more than 200 ft in the past 50 years. Similar drops in water levels have occurred elsewhere in Arizona. In response to the overdrafting of ground water, the State of Arizona passed legislation designed to attain "safe yield," which is defined as a balance between ground-water withdrawals and annual recharge of aquifers. To monitor progress in complying with the legislation, ground-water withdrawals are measured and estimated, and annual recharge is estimated. The Tucson Basin and Avra Valley are two ground-water basins that form the Tucson Active Management Area (TAMA), which by State statute must attain "safe yield" by the year 2025.
Microgravity methods are based on the principles of Newton's Law of Gravitation that states that the acceleration due to gravity within an object's gravitational field is directly related to the mass of the object and inversely related to the distance to the center of the object. In simple terms, the greater an object's mass, the stronger its gravitational field. Differences in measured gravitational fields over the earth's surface have been used by geophysicists for years to map variations in crustal thickness, the presence of magma bodies, and the subsurface distributions of different rock types.
Ground water is stored within the pore spaces of aquifers. As an aquifer is drained by pumpage or filled by recharge, its mass changes, which results in changes in the strength of its gravitational field. Recent technological advances in geophysical techniques have made measurement of the extremely small gravitational changes caused by fluctuations of water volume practical. The standard unit of measurement for conventional gravity studies is the milligal, a unit equal to 10-3 cm/sec2 ; microgravity work uses microgals, or 10-6 cm/sec2. The USGS microgravity network is based on the University of Arizona network and is the first basinwide application of microgravity methods to the measurement of changes in ground-water storage.
Microgravity studies showed that the greatest initial recharge occurred in normally unsaturated surficial deposits along Rillito Creek as a direct result of the winter streamflows. Throughout the winter of 1992-93, gravity values increased over a 0.5-mile-wide strip of flood plain adjacent to Rillito Creek and reached their maximum by April 1993. As water flowed away from the saturated, near-channel surficial deposits and dispersed through the aquifer, gravity values declined steadily nearest Rillito Creek. By January 1994, gravity values had returned almost to those measured at the start of the study. Throughout the area south of Rillito Creek, however, a residual gravity increase that correlated with rising water levels in wells indicated an increase in ground-water storage resulting from the previous winter's streamflows.
By integrating the values of gravity change over the network of microgravity stations, USGS scientists estimated that about 10,900 acre-ft of recharge occurred along Rillito Creek during the winter of 1992-93, which was about 9 percent of the total streamflow recorded at the Dodge Boulevard streamflow-gaging station in that time (Pool and Schmidt, 1997).
The use of synoptic microgravity measurements to determine changes in ground-water volume was demonstrated by its successful application to a site-specific area during the El Niņo flooding of 1992-93. In 1997, application of the method was expanded in scale to a basinwide network that will be surveyed annually. The resultant data will enable water managers and scientists to monitor annual changes in ground-water conditions throughout the Tucson Basin and Avra Valley.
U.S. Geological Survey
520 N. Park Avenue, Suite 221
Tucson, AZ 85719-5035
of Water Resources
Metropolitan Domestic Water Improvement District
Town of Oro Valley
Pima County Flood Control District
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Last modified: Tuesday, February 18 2014, 12:44:11 PM