U.S. Department of the Interior

U.S. Geological Survey

Prepared in cooperation with the

Arizona Department of Water Resources,

Metropolitan Domestic Water Improvement District,

City of Tucson, Town of Oro Valley, and Pima County

USGS Fact Sheet 060-98

June 1998

Network of Microgravity 

Stations

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 Niño (El 

Niño–Southern Oscillation), a warm 

water anomaly in the central to east 

equatorial Pacific Ocean. That wetterthan-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 Niño 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 groundwater storage. 

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. 

Use of Microgravity to Assess

the Effects of El Niño on Ground-Water

Storage in Southern Arizona

San

Cruz

ta

R

Ri

Creek

llito

iver

111°15' 111° 45' 110°30'

 32°15'

 32°

SANTA CATALINA

MOUNTAINS

RINCON

MOUNTAINS

TUCSON

MOUNTAINS

Gravity meter setup at a microgravity 

station.

0 5 MILES

0 5 KILOMETERS

TUCSON

Network of existing microgravity (aquiferstorage monitoring) stations in the Tucson 

Basin area.
El Niño 1992–93, the First 

Measurements

The usefulness of microgravity 

measurements for monitoring groundwater change was demonstrated in the 

winter of 1992–93 when El Niño 

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.

— J.T.C. Parker and D.R. Pool

District Chief

U.S. Geological Survey

520 N. Park Avenue, Suite 221

Tucson, AZ 85719-5035

(520) 670-6671

http://wwwdaztcn.usgs.gov

For more information contact:

USGS Fact Sheet

June 1998 FS-060-98

The use of synoptic microgravity 

measurements to determine changes in 

ground-water volume was demonstrated 

by its successful application to a sitespecific area during the El Niño 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.

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).

El Niño 1997–98, Network in 

Place

In 1997, strong El Niño 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 Niño, 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.

Discharge of Rillito Creek at Dodge 

Boulevard, December 1992–March 1993.

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 

Gravity changes since December 1992 

along Swan Road, April and December 

1993.

80

60

40

20

–20

0

GRAVITY CHANGE, IN MICROGALS

0.0 0.5 RECHARGE

April. 3,

1993

Dec. 31,

1993

S N

GRAVITY CHANGE, IN MICROGALS

S N

1.0 1.5

DISTANCE, IN MILES

2.0 2.5

Microgravity stations along Swan Road.

Gravity changes since December 1992 

along Swan Road, December 1997 and 

March 1998.

0.0 0.5 1.0 1.5

DISTANCE, IN MILES

2.0 2.5

40

20

0

–20

–40

RECHARGE

–60

December

18–19

1997

March 13,

1998

0

0

1 KILOMETER

1 MILE

FLOOD PLAIN

RIVER ROAD

FLOOD PLAIN

N

S

SWAN ROAD

DODGE

BOULEVARD RIVE RO R AD

Ri

Creek

llito

FLOOD

PLAIN

110°56' 54'

 32°15'

16'

 32°17' 55' 53' 110°52'

DISCHARGE, IN CUBIC

FEET PER SECOND

RillitoCreek

RIVER ROAD

RillitoCreek

0

12,000

9,000

6,000

3,000

1992 1993

0

7,000

1997 1998

Discharge of Rillito Creek at Dodge 

Boulevard, December 1997–March 1998. DISCHARGE, IN CUBIC FEET PER SECOND

5,000

3,000

1,000

References Cited

Pool, D.R. and Schmidt, Werner, 1997, 

Measurement of ground–water 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, 30 p.