Ground-Water Storage

Ground water in the TAMA (fig. 1) is

stored in the pore spaces between grains

of sediment in the subsurface (fig. 2).

Some pore spaces are filled with water

and others are dry or partially filled with

water. The amount of ground water that

is stored in the TAMA changes as pores

drain or fill with water and compact or

expand as water levels in the aquifer fall

and rise. Under current conditions of

overdraft, ground water is derived from

two storage sources—drainage of pore

spaces between grains of sediment and

reduction in pore volume in compressible parts of the aquifer system.

Prepared in cooperation with the

ARIZONA DEPARTMENT OF WATER RESOURCES,

CITY OF TUCSON, PIMA COUNTY, and

NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION NATIONAL GEODETIC SURVEY

32°30'

32°

111°30'

31°30'

111°

EXPLANATION

UNITED STATES

MEXICO

VALLEY

AVRA

TUCSON

MOUNTAINS

Tucson

PIMA CO.

SANTA CRUZ CO.

PINAL CO.

Oro PIMA CO. Valley SANTA CATALINA

TUCSON

BASIN

MOUNTAINS

RINCON

MOUNTAINS

TORTOLITA

MOUNTAINS

SIERRITA

MOUNTAINS

ta

an

S

Cruz

er vi R

0 10 KILOMETERS

0 10 MILES

QUATERNARY ALLUVIAL

AND TERTIARY

SEDIMENTS

CRYSTALLINE AND SEDIMENTARY ROCKS

LAND-SUBSIDENCE AND

GROUND-WATER

STORAGE MONITORING

STATION

ABSOLUTE-GRAVITY

STATION

FOOTHILLS AND RILLITO

CREEK GROUND-WATER

STORAGE MONITORING

STATION

CAÑADA DEL ORO SUBBASIN GROUND-WATER

STORAGE MONITORING

STATION

Figure 1. The Tucson Active Management Area

and land-subsidence and ground-water storage

monitoring stations.

U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet 084-00

April 2000

Land-Subsidence and Ground-Water Storage Monitoring in

the Tucson Active Management Area, Arizona

Figure 2. Saturated, partially saturated,

and dry spaces between grains of

sediment.

The Tucson Active Management

Area (TAMA) comprises two

basins—Tucson Basin and Avra

Valley. The TAMA has been directed

by Arizona ground-water law to attain

an annual balance between groundwater withdrawals and recharge by the

year 2025. This balance is defined by

the statute as "safe yield." Current

ground-water withdrawals exceed

recharge, resulting in conditions of

ground-water overdraft, which causes

removal of water from ground-water

storage and subsidence of the land

surface. Depletion of storage and

associated land subsidence will not be

halted until all discharge from the

system, both natural and human

induced, is balanced by recharge.

The amount of the ground-water

overdraft has been difficult to estimate

until recently because it could not be

directly measured. Overdraft has been

estimated using indirect water-budget

methods that rely on uncertain

estimates of recharge. As a result, the

status of the ground-water budget

could not be known with great

certainty.

Gravity methods offer a means to

directly measure ground-water overdraft through measurement of changes

in the gravitational field of the Earth

that are caused by changes in the

amount of water stored in the

subsurface. Changes in vertical

position also affect the measured

gravity value and thus subsidence also

must be monitored. The combination

of periodic observations of gravity and

vertical positions provide direct

measures of changes in stored ground

water and land subsidence.
Global Positioning System (GPS) equipment.

Land subsidence is being monitored on

an annual basis using GPS equipment.

The surveys are done over about 2 to 3

weeks using geodetic GPS receivers; four

are operated by Pima County and two are

operated by the City of Tucson. Data

collected in the field are downloaded to a

computer and processed using software

that calculates the differential positions

among the stations. A network adjustment

is done using stations that are located on

bedrock as control points with known

positions that are assumed not to change.

Results of the network adjustments from

previous surveys can then be compared to

determine changes in position of the

stations. Surveyed positions of stations

generally are accurate to within about 1in.

in the vertical dimension.

Ground-Water Storage

Changes in ground-water storage are

monitored using the gravity method. The

method is based on the principles of

Newton's Law of Gravitation, which

states that the acceleration of gravity

within an object's gravitational field is

directly related to the mass of the object

and inversely related to the square of the

distance to the center of the object. In

simple terms, an object's gravitational

field increases with the mass of the object

and decreases with distance from the

center of the object. The object that

produces the gravitational field in this

case is the Earth. Change in the Earth's

gravitational field at a location on the

surface can be caused by change in the

amount of water stored in the subsurface,

mass movement in magma and geothermal reservoirs, subsurface faulting,

and change in the distance from the

center of the Earth. Fortunately, changes

in ground-water storage and subsidence

are the only likely causes of

gravity change in the TAMA.

as part of the USGS Southwest GroundWater Resources Program.

Land Subsidence

TAMA during the late-1980's (Cole,

1990; Hatch, 1991). In addition, the NGS

established two stations in the Santa

Catalina Mountains where the absolute

value of gravitational acceleration was

measured in February 1989. The stations

are part of a national absolute-gravity

station network (Peter and others, 1991)

and serve as local references for gravity

surveys in the Tucson area. The current

investigation incorporates most of the

stations established during the late1980's by the UA and NGS.

Current Monitoring Program

Begining in December 1997, the U.S.

Geological Survey (USGS) began a

3-year land-subsidence and groundwater storage monitoring program in

cooperation with the Arizona

Department of Water Resources, City of

Tucson, and Pima County. The goal of

the program is to measure land

subsidence and ground-water storage on

an annual basis at a network of

monitoring stations throughout the

TAMA. The current network includes 75

stations—43 in the Tucson Basin and 32

in Avra Valley (fig.1). In addition, 64

stations are being monitored in the

northwestern part of the Tucson Basin in

cooperation with the Metropolitan

Domestic Water Improvement District

and the Town of Oro Valley (Pool,

1999). Information from these programs

should result in a better understanding of

recharge rates, ground-water overdraft,

and the overall status of the water supply

in the TAMA. Thirty-seven stations also

are being measured on a seasonal

basis near Rillito

Creek and the

foothills of the

Santa Catalina

M o u n t a i n s

Sediments that contain a significant

amount of silt and clay are the most

compressible. Declining water levels

result in compaction of pore spaces and

subsidence of the land surface.

Losses in ground-water storage in the

TAMA have been difficult to estimate

using previously existing methods. Storage

loss that results from pore-space

compaction across the upper part of the

aquifer system is monitored at 14 vertical

extensometers in the TAMA, but extensive

areal surveys, which are required to

measure compaction of the entire aquifer,

have not been done on a routine basis.

Storage loss that is caused by drainage of

pore spaces also has not been monitored.

Pore-space drainage can be estimated

indirectly by multiplying estimated specific

yield of the aquifer and water-level decline

in wells. The specific yield of the aquifer

is the amount of water that is released from

storage per unit decline in the water table.

The specific-yield method requires

knowledge of: (1) specific yield and (2)

change in position of the water table. This

information is rarely available over large

areas for aquifers in the Southwest. Values

of specific yield can be determined, but

long-term and costly tests are required.

Values estimated from the tests are only

applicable to the small volume of aquifer

that was tested. Futhermore, water-level

changes in deep wells seldom reflect

changes in the position of the water table

because water levels vary with depth in

alluvial aquifers of the Southwest. Most

wells in the TAMA probably do not have

water levels that are representative of the

water table because the wells derive water

from multiple deep aquifers. Monitoring

of land subsidence using the Global

Positioning System (GPS) and monitoring

drainage of pore spaces using gravity

methods will provide direct measurement

of the total ground-water storage change.

Previous Investigations

The current investigation builds on

previous work done by the University of

Arizona (UA) and the National Geodetic

Survey (NGS). NGS and John Sumner of

the Geosciences Department at the UA

began a program during the 1980's to

assess the utility of GPS and gravity

measurements for regional monitoring of

land subsidence and ground-water

storage. Their efforts resulted in GPS and

gravity surveys of many stations in the
PINAL CO.

PIMA CO.

32°

111°

111°30'

32°30'

VALLEY

0 10 KILOMETERS

0 10 MILES

Relative gravity meter.

Absolute gravity meter.

Gravity surveys are done annually at

most of the same stations where land

subsidence is measured. The relative

gravity meter used for this project

measures the difference in gravity

among stations in the network. Results

of the GPS surveys are used to correct

the observed gravity change for any

change in the vertical positions of

stations. Surveys of stations in the

Tucson Basin and Avra Valley are

completed over a period of about 2

months using several subsurveys of 5 to

8 stations, which include one or more

stations in common with another

subsurvey. The surveys generally are

capable of measuring differences in

gravity among stations of about 5

microGal, which is roughly equivalent to

the change in gravity resulting from the

addition or removal of an extensive layer

of water that is about 5 in. thick.

Use of relative-gravity surveys for the

purpose of estimating mass change

requires at least one gravity-reference

station at which the acceleration of

gravity does not change or where

changes can be monitored. Because of

this need, the monitoring network

includes stations in bedrock areas

surrounding the TAMA at which gravity

is expected to vary little. Any change that

occurs at the bedrock stations will result

in an inaccurate assessment of mass

change throughout the network. Annual

measurements of the absolute value of

gravitational acceleration are, therefore,

done at 6 stations in the network—4 of the

absolute-gravity stations are on bedrock

and 2 overlie aquifers. The relative-gravity

surveys are corrected for changes that

occur at the absolute-gravity stations.

The absolute-gravity measurements

are done by the NGS using a falling-mass

gravity meter. Current measurements are

accurate to within about ±2 microGal,

which is equivalent to the gravity change

Results, 1987–98

Results of the 1998 surveys were

compared with results of the surveys

completed during the late-1980’s. The

1998 survey included 31 subsidencemonitoring stations surveyed during 1987

by NGS in the Tucson Basin and Avra

Valley (Schumann and Anderson, 1988),

and 22 ground-water storage monitoring

stations surveyed in the Tucson Basin

during 1989 by the UA. The 1989

surveys did not include ground-water

storage monitoring stations in Avra

Valley. Many stations that were surveyed during the late-1980’s have been

destroyed by construction activities and

could not be resurveyed.

Land Subsidence

Land subsidence from 1987 to 1998

was greater in the Tucson Basin than in

Avra Valley (fig. 3). Most of the stations in

the Tucson Basin subsided by 1 in. or more

and the maximum subsidence of about 4

in. occurred in the central part of the basin.

Subsidence in Avra Valley generally was

less than 1 in., but several stations in the

northern part of the valley displayed uplift

of as much as 2 in. Maximum subsidence

of 2 to 3 in. occurred in the south-central

part of Avra Valley.

Ground-Water Storage

Ground-water storage losses occurred

throughout most of the Tucson Basin

from 1989 to 1998 (fig. 4). Storage

change is expressed as an equivalent

thickness of water that would result in

the measured change in gravity. Losses

of more than 2 ft of water occurred in the

north-central part of the basin and

included a maximum loss of more than 6

ft of water. Increases in storage of nearly

3 ft of water occurred in sediments near

the mountains along the north, east, and

south margins of the basin, which

resulted in increases in gravity at station

POST and absolute-gravity station

TUCSON AA that were equivalent to

1.6 ft and 0.5 ft of water respectively.

These stations are on bedrock near the

basin margin (fig. 4). Gravity change

equivalent to –0.1 ft of water at the

absolute-gravity station TUCSON AB,

located near the peak of the Santa

Catalina Mountains, was less

than the range of error of

the measurements.

resulting from the addition

or removal of an extensive

layer of water that is about

2 in. thick.

EXPLANATION

SUBSIDENCE,

IN INCHES:

AREA OF

UPLIFT—

0–2 inches

MONITORING

STATION

3 to 4

2 to 3

1 to 2

0 to 1

Figure 3. Land-subsidence in the Tucson Basin and Avra Valley, 1987–98.

AVRA

TUCSON

BASIN
Figure 5. Water-level change in the Tucson Basin, 1989–98.

Figure 4. Ground-water storage change in the Tucson Basin, 1989–98.

margins of the basin, consistent with

increased ground-water storage near

stations TUCSON AA and POST.

Water-level data were not available for

the south margin of the basin to compare

to measured increases in storage.

Future Monitoring Program

Monitoring of land subsidence and

ground-water storage in the TAMA will

continue to provide water managers with

valuable information in the future. The

monitoring schedule probably will be

modified to accomodate changing

technology and data needs. Advancing

technology in the measurement of the

Earth’s gravitational field should allow

more accurate and more frequent

measurement of the absolute value of

gravity at more monitoring stations. The

existing network of monitoring stations

provides control for additional investigations of land subsidence and groundwater storage change near wells and

areas where natural and artificial

recharge occurs. More detailed

knowledge of the seasonal and areal

distributions of changes will allow better

evaluation of changes that are observed

over periods of a year or more.

—D.R. Pool (USGS), Daniel Winster

(National Oceanic and Atmospheric

Administration National Geodetic Survey), and K.C. Cole (Grand Valley State

University)

E-mail: drpool@usgs.gov

or visit home page

http://az.water.usgs.gov

References Cited

Cole, K.C., 1990, Estimation of mass flux and aquifer

properties using global-positioning system and microgravity in the Tucson Basin, southern Arizona: Tucson,

University of Arizona doctoral dissertation, 193 p.

Hatch, M.A., 1991, Global positioning system measurement

of subsidence in the Tucson Basin, Pima County,

Arizona: Tucson, University of Arizona doctoral

dissertation, 71 p.

Peter, G., Klopping, F.J., Carter, W.E., and Dewhurst, W.T.,

1991, Absolute gravity reference sites in the United

States: The Leading Edge, Society of Exploration

Geophysics, v. 10, no. 7, p. 43–45.

Pool, D.R., 1999, Aquifer-storage change in the Lower

Cañada del Oro subbasin, Pima County, Arizona,

1996–98: U.S. Geological Survey Water-Resources

Investigations Report 99–4067, 3 sheets.

Schumann, H.H., and Anderson, S.R., 1988, Landsubsidence measurements and aquifer-compaction

monitoring in Tucson Basin and Avra Valley,

Arizona: U.S. Geological Survey Water-Resources

Investigations Report 88–4167, 15 p.

EXPLANATION

GROUND-WATER

STORAGE CHANGE,

IN FEET OF WATER:

–6 to –8

–4 to –6

–2 to –4

0 to –2

0 to 3

EXPLANATION

–40 to –60

–30 to –40

–20 to –30

–10 to –20

0 to –10

0 to 40

0 5 KILOMETERS

0 5 MILES

MONITORING STATION

A point worth noting is that because the

aquifer includes both solids and void

spaces, change in position of the water

table is much greater than the storage

change expressed as an equivalent thickness of water. For example, assuming a

specific yield of 0.02, a thickness of water

storage loss of 1 ft might reflect a decline

in the water table of 5 ft or more.

Water Levels

Changes in water levels in wells in

the Tucson Basin from 1989 to 1998

WELL

WATER-LEVEL CHANGE,

IN FEET:

P 111° 110°45' INAL CO.

PIMA CO.

111°15' 32°30'

32°

15'

0 5 KILOMETERS

0 5 MILES

110°45'

TUCSON AA

POST

TUCSON AB

P 111° INAL CO.

PIMA CO.

111°15' 32°30'

32°

15'

For further information contact:

Don R. Pool

U.S. Geological Survey, WRD

520 N. Park Avenue, Suite 221

Tucson, Arizona 85719-5035

(fig. 5, City of Tucson, unpublished data)

generally compare favorably to groundwater storage change. Water levels

declined throughout most of the basin. The

maximum water-level decline of greater

than 40 ft occurred in the north-central

part of the basin near the area of maximum

loss in ground-water storage. More than

40 ft of decline also occurred near the

southeast extent of the Tucson Mountains

in an area lacking storage-monitoring

stations. More than 10 ft of water-level

decline occurred in two areas near the

south and north margins of the basin.

Water levels rose near the north and east