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| 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 studies in the
TAMA began in 1992 as part of a cooperative study by the
USGS and the Pima County (Arizona) Department of
Transportation and Flood Control District; 50 stations were
established in a 6-square-mile area centered on Rillito
Creek along the north edge of the city of Tucson. During the
winter of 1992-93, unusually high precipitation occurred
that was associated with El Niņo (El Niņo-Southern
Oscillation), a warm water anomaly in the central to east
equatorial Pacific Ocean. That wetter-than- normal winter
caused ground-water recharge that provided the opportunity
to demonstrate the use of microgravity to make detailed
measurements of changes in water stored in aquifers. Late in
1997, the USGS established a network of about 60
microgravity stations within the TAMA to measure basinwide
changes in ground-water storage. A return of El Niņo in the
winter of 1997-98 afforded another opportunity to use
microgravity to measure the effects of such conditions on
ground-water recharge in the basin.
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.
The usefulness of microgravity
measurements for monitoring ground-water change was
demonstrated in the winter of 1992-93 when El Niņo
conditions contributed to sustained, high streamflows
throughout southern Arizona. Rillito Creek, which is
normally dry, had substantial streamflow from December 1992
to March 1993. Infiltration of streamflow through channel
sediments recharged the underlying aquifer, causing water
levels and microgravity readings to rise. Repeated gravity
measurements at 50 stations within the study area (12
stations along a 2.5-mile stretch of Swan Road) between
December 1992 and January 1994 enabled scientists to
calculate change in ground-water storage and to estimate
recharge for each measurement period.
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).
In 1997, strong El Niņo conditions
returned to the Pacific Ocean increasing the possibility of
higher-than-average precipitation in southern Arizona. In
advance of the winter-storm season, microgravity stations in
the network for the 1992-93 study were surveyed to establish
new baseline-gravity levels. In addition, a basinwide
network of about 60 microgravity stations was installed and
surveyed. Widespread flooding did not occur in the winter of
1997-98 as the result of the El Niņo, but normally dry
washes flowed for days to weeks as the result of
above-average precipitation. Although data collection has
not been completed, a preliminary evaluation indicates that
gravity changes adjacent to Rillito Creek were similar to
changes in 1992-93. The same pattern of recharge was seen
with the large bulge in gravity values at Rillito Creek
caused by saturation of surficial deposits adjacent to the
channel.
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.
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District Chief
U.S. Geological Survey
520 N. Park Avenue, Suite 221
Tucson, AZ 85719-5035
(520) 670-6671
http://az.water.usgs.gov
Arizona Department
of Water Resources
Metropolitan
Domestic Water Improvement District
Tucson Water
Town of Oro Valley
Pima County
Flood Control District
fs-060-98.pdf
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