U.S. Department of the Interior

U.S. Geological Survey

Prepared in cooperation with the

ARIZONA DEPARTMENT OF WATER RESOURCES, CITY OF TUCSON WATER DEPARTMENT, 

PIMA COUNTY, THE TOWN OF ORO VALLEY, THE TOWN OF MARANA, and the 

METROPOLITAN DOMESTIC WATER IMPROVEMENT DISTRICT

Land Subsidence and Aquifer-System Compaction

in the Tucson Active Management Area, South-Central 

Arizona, 1987–2005

Marana

Casa Grande

Green

Valley

Tucson

Oro

Valley

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ARIZONA

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Scientific Investigations Report 2007-5190 MEXICO
Other than this note, this page intentionally left blank.
Land Subsidence and Aquifer-System 

Compaction in the Tucson Active 

Management Area, South-Central Arizona, 

1987–2005

By Robert L. Carruth, Donald R. Pool, and Carl E. Anderson

Prepared in cooperation with the Arizona Department of Water Resources, 

City of Tucson Water Department, Pima County, the Town of Oro Valley, the Town 

of Marana, and the Metropolitan Domestic Water Improvement District

Scientific Investigations Report 2007–5190

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2007

This report and any updates to it are available at:

http://pubs.usgs.gov/sir/2007/5190/

For product and ordering information:

World Wide Web: http://www.usgs.gov/pubprod

Telephone: 1-888-ASK-USGS

For more information on the USGS—the Federal source for science about the Earth, 

its natural and living resources, natural hazards, and the environment:

World Wide Web: http://www.usgs.gov

Telephone: 1-888-ASK-USGS

Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the 

U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Cataloging-in-publication data are on file with the Library of Congress (http://www.loc.gov/).

Manuscript approved for publication, August 31, 2007

Text edited by Tracey Suzuki

Layout by David R. Jones

Suggested citation:

Carruth, R.L., Pool, D.R., Anderson, C.E., 2007, Land subsidence and aquifer-system compaction in the Tucson 

Active Management Area, south-central Arizona, 1987–2005: U.S. Geological Survey Scientific Investigations Report 

2007-5190, 27 p.

FRONT COVER

Left, diagrammatic sketch of a borehole extensometer used in the Tucson Active Management Area. 

Right, map of the Tucson Active Management Area in south-central Arizona.
iii

Contents

Introduction.....................................................................................................................................................1

Background............................................................................................................................................3

Purpose and Scope ..............................................................................................................................3

Previous Investigations........................................................................................................................4

Acknowledgments................................................................................................................................4

Description of the Study Area ............................................................................................................4

Methods of Data Collection .........................................................................................................................4

Hydrogeology................................................................................................................................................10

Potential for Land Subsidence .........................................................................................................12

Land Subsidence and Aquifer-System Compaction ..............................................................................13

Summary........................................................................................................................................................24

References Cited..........................................................................................................................................25

Figures

1. Tucson Active Management Area in south-central Arizona ...........................................................2

2. Long-term elevation-change monitoring stations in the Tucson Active Management Area.....5

3. Photograph of a typical setup of a geodetic GPS receiver in the Tucson 

Active Management Area.....................................................................................................................6

4. Tucson Active Management Area borehole extensometer stations .............................................7

5. Diagrammatic sketch of a borehole extensometer in the Tucson Active 

Management Area................................................................................................................................10

6. Generalized geology of the Tucson Active Management Area. ...................................................11

7. Water level and compaction readings at extensometers in the 

Tucson Basin, 1989 to 2005..................................................................................................................14

8. Water level and compaction readings at extensometers in the 

Avra Valley, 1989 to 2005......................................................................................................................16

9. Land-surface elevation change in the Tucson Active Management 

Area from 1987 to 1998.........................................................................................................................19

10. Land-surface elevation change in the Tucson Active Management Area 

from 1998 to 2002...................................................................................................................................20

11. Land-surface elevation change in the Tucson Active Management Area 

from 2002 to 2005...................................................................................................................................21

12. Land-surface elevation change in the Tucson Active Management Area 

from 1987 to 2004 and 2005..................................................................................................................22

13. Land-surface elevation change in the Tucson Active Management Area based 

on a 3-year, 8-month interferogram, February 2003 to October 2006...........................................23

Tables

1. Positions and elevations of monitoring stations in the Tucson Active Management Area and 

changes in vertical position from 1987 to 2005....................................................................................8

2. Aquifer-system compaction and water-level data for the Tucson Active 

Management Area, 1989-2005..............................................................................................................24
iv 

Conversion Factors 

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

yard (yd) 0.9144 meter (m)

Area

acre 4,047 square meter (m2)

acre 0.004047 square kilometer (km2)

square foot (ft2) 929.0 square centimeter (cm2)

square foot (ft2) 0.09290 square meter (m2)

square inch (in2) 6.452 square centimeter (cm2)

square mile (mi2) 2.590 square kilometer (km2) 

Any use of trade, product, or firm names is for descriptive purposes 

only and does not imply endorsement by the U.S. Government.

Although this report is in the public domain, permission must be 

secured from the individual copyright owners to reproduce any copyrighted materials contained within this report.
Abstract 

The U.S. Geological Survey monitors land subsidence and 

aquifer-system compaction caused by ground-water depletion in 

Tucson Basin and Avra Valley—two of the three alluvial basins 

within the Tucson Active Management Area. In spring 1987, 

the Global Positioning System was used to measure horizontal 

and vertical positions for bench marks at 43 sites to establish 

a network for monitoring land subsidence in Tucson Basin 

and Avra Valley. Between 1987 and 2005, the original number 

of subsidence monitoring stations was gradually increased to 

more than 100 stations to meet the need for information in the 

growing metropolitan area. Data from approximately 60 stations 

common to the Global Positioning System surveys done after an 

initial survey in 1987 are used to document land subsidence. For 

the periods of comparison, average land-surface deformation 

generally is less than the maximum subsidence at an individual 

station and takes into account land-surface recovery from elastic 

aquifer-system compaction. Between 1987 and 1998, as much 

as 3.2 inches of subsidence occurred in Tucson Basin and as 

much as 4 inches of subsidence occurred in Avra Valley. For the 

31 stations that are common to both the 1987 and 1998 Global 

Positioning System surveys, the average subsidence during the 

11-year period was about 0.5 inch in Tucson Basin and about 

1.2 inches in Avra Valley.

For the approximately 60 stations that are common to 

both the 1998 and 2002 Global Positioning System surveys, the 

data indicate that as much as 3.5 inches of subsidence occurred 

in Tucson Basin and as much as 1.1 inches of subsidence 

occurred in Avra Valley. The average subsidence for the 4-year 

period is about 0.4 inch in Tucson Basin and 0.6 inch in Avra 

Valley. Between the 2002 and the 2005 Global Positioning 

System surveys, the data indicate that as much as 0.2 inch of 

subsidence occurred in Tucson Basin and as much as 2.2 inches 

of subsidence occurred in Avra Valley. The average subsidence 

for the 3-year period is about 0.7 inch in Avra Valley.

Between 1987 and 2004-05, land subsidence was greater 

in Avra Valley than in Tucson Basin on the basis of the average 

cumulative subsidence for the stations that were common to the 

original Global Positioning System survey in 1987. The average total subsidence during the 17- to 18-year period was about 

1.3 inches in Tucson Basin and about 2.8 inches in Avra Valley. 

Three stations in Tucson Basin showed subsidence greater than 

4 inches for the period—5 inches at stations C45 and X419 and 

4.1 inches at station PA4. In Avra Valley, two stations showed 

subsidence for the 17- to 18-year period greater than 4 inches—4.3 inches at station AV25 and 4.8 inches at station SA105. 

In 1983, fourteen wells were fitted with borehole extensometers to monitor water-level fluctuations and aquifersystem compaction. Continuous records of water level and 

aquifer-system compaction indicate that as much as 45 feet of 

water-level decline and 4 inches of aquifer-system compaction occurred in Tucson Basin from January 1989 through 

December, 2005. In Avra Valley, extensometer data indicate 

that as much as 55 feet of water-level decline and 1.7 inches 

of aquifer-system compaction occurred during the same time 

period. Rates of compaction vary throughout the extensometer 

network, with the greater rates of compaction being associated 

with areas of greater water-level decline and more compressible sediments. In Avra Valley, data from the Global Positioning System surveys indicate that more than half of the total 

subsidence of the land surface may be the result of aquifersystem compaction below the portion of the aquifer instrumented with the vertical extensometers.

For the area in the northern part of Tucson Basin between 

the Rillito and Santa Cruz rivers, an Interferometric Synthetic 

Aperture Radar interferogram indicates that about 1.65 inches 

of subsidence occurred between 2003 and 2006. Between 

2002 and 2004, the Global Positioning System station at C45, 

in the same northern area of Tucson basin, shows subsidence 

of 1.2 inches, indicating a good correlation between the Global 

Positioning System data and the Interferometric Synthetic 

Aperture Radar data.

Introduction

The Tucson Active Management Area (TAMA) encompasses about 3,900 mi2

 in south-central Arizona (fig. 1). The 

TAMA is one of five Active Management Areas established 

Land Subsidence and Aquifer-System Compaction in the 

Tucson Active Management Area, South-Central Arizona, 

1987–2005

By Robert L. Carruth, Donald R. Pool, and Carl E. Anderson
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2 Land Subsidence and Aquifer-System Compaction in the Tucson Active Management Area, South-Central Arizona, 1987–2005

Figure 1. Tucson Active Management Area in south-central Arizona.
Introduction 3

pursuant to Arizona’s 1980 Groundwater Management Act 

and is administered by the Arizona Department of Water 

Resources. There are three alluvial basins within the TAMA: 

Tucson Basin, Avra Valley, and Altar Valley. Land use varies 

significantly within the three basins. Tucson Basin is heavily 

urbanized with a rapidly growing population. Avra Valley is 

mostly rural, having been primarily agricultural in the past. 

Land use in the Avra Valley basin presently is mixed use, 

as much of the agricultural land has been retired for use in 

housing development and for use by the City of Tucson Water 

Department for artificial recharge of Central Arizona Project 

water and other municipal water-supply projects. Altar Valley, 

located in the southeastern portion of the TAMA, has many 

ranches and a wildlife refuge. Land subsidence and aquifersystem compaction is not monitored presently in Altar Valley 

owing to the largely undeveloped nature of the area, 

The TAMA has been dependent upon ground water for 

urban and agricultural consumption, and ground-water withdrawal has exceeded natural recharge for many decades. As 

a result, between 1989 and 2005, water levels in the TAMA 

declined by as much as 45 ft in Tucson Basin, and 55 ft in 

Avra Valley. Water levels in Altar Valley, where residential and 

commercial development are minimal, have remained largely 

unchanged.

Background

Ground water is a critical resource in the TAMA, providing drinking water to urban and rural communities, supporting 

irrigation, mining, and industry, and sustaining baseflow in 

small streams along mountain fronts that support riparian ecosystems. Land subsidence and aquifer-system compaction can 

occur when water is removed from alluvial-aquifer systems. 

Land subsidence is the loss of surface elevation as the result 

of the removal of subsurface support (Galloway and others, 

(1999). Decisions by stakeholders concerning the sustainable 

development of land and water resources within the TAMA 

could benefit from improved scientific understanding, detection, and monitoring of aquifer-system compaction and land 

subsidence. 

According to Galloway and others (2000), more than 

80 percent of the identified subsidence in the United States 

is a consequence of human impact on subsurface water and 

is dominated by three distinct processes—compaction of 

aquifer systems, drainage and subsequent oxidation of organic 

soils, and dissolution and collapse of susceptible rocks. In the 

TAMA, ground-water pumping in excess of natural recharge is 

the major cause of aquifer-system compaction and associated 

land subsidence. 

The focus of this report is on compaction of sediments 

within the saturated ground-water system. It is important to 

note, however, that other processes can cause land-surface 

movement in the area. In particular, near-surface compaction 

or expansion of soils from wetting and drying has been known 

to cause damage to houses and other structures in the TAMA, 

particularly in areas that are underlain by soils with a high 

clay content. The Natural Resources Conservation Service has 

produced maps of soil-shrink/swell potential for the greater 

Tucson area (http://www.az.nrcs.usda.gov/technical/soils/

shrinkswell.html). Shrink/swell potential is the relative change 

in volume to be expected with changes in moisture content. 

Platt (1963) found that mapped zones with a high frequency of 

building fractures in the Tucson area had a positive correlation 

with the Tucson Loam soil type when superimposed on the 

county soil map. In the Tucson area, urban wetting and drying 

of soils with a high shrink/swell potential is an important issue 

in terms of realized and potential damage to buildings and 

foundations; however, the study of this process is beyond the 

scope of this report.

As early as the 1940s, water-level declines of up to several feet per year have resulted in aquifer-system compaction 

and measurable land subsidence in Tucson Basin and Avra 

Valley (Evans and Pool, 2000). Conventional first-order leveling surveys showed that ground-water pumping resulted in 

about 0.5 ft of land subsidence in Tucson Basin between 1952 

and 1980 and about 1 ft of land subsidence in the northwestern 

part of Avra Valley between 1948 and 1980 (Schumann and 

Anderson, 1988). 

In 1979, the U.S. Geological Survey (USGS), in cooperation with the City of Tucson, began an investigation to 

determine the potential for aquifer-system compaction, land 

subsidence, and earth fissures in Tucson Basin. This USGSCity of Tucson study lead to the construction of a network of 

14 vertical extensometers, seven in Tucson Basin and seven 

in Avra Valley. In 1987, a network of vertical control stations 

was established in cooperation with the City of Tucson and 

the National Geodetic Survey to use the Global Positioning 

System (GPS). The vertical-control network consisted of 

43 benchmarks, and the network was designed to map land 

subsidence in Tucson Basin and Avra Valley.

In 1996, the USGS began a cooperative study with Metropolitan Domestic Water Improvement District and the town 

of Oro Valley to monitor aquifer-storage change in the Lower 

Cañada del Oro subbasin. 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 subsidence and aquifer-storage change in the TAMA. In 

2003, these two monitoring studies were combined, and the 

town of Marana joined the study. This report documents the 

results of analyses made from measurements at a vertical-control network and a vertical-extensometer network established 

to determine the relation between ground-water depletion, 

aquifer-system compaction and land subsidence in Tucson 

Basin and Avra Valley. 

Purpose and Scope

The purpose of this report is to describe aquifer-system 

compaction and land subsidence caused by ground-water 

depletion in Tucson Basin and Avra Valley between 1987 and 

2005. Land-surface elevation was measured approximately 

annually by using GPS-survey methods at a network of stable-
4 Land Subsidence and Aquifer-System Compaction in the Tucson Active Management Area, South-Central Arizona, 1987–2005

benchmark monitoring stations in Tucson Basin and Avra 

Valley. Rates and magnitude of aquifer-system compaction 

and water-level change were measured by using a network of 

vertical extensometers.

Previous Investigations

Evans and Pool (2000), provide a comprehensive summary 

of Federal, State, county, municipal, and university studies that 

have focused on various aspects of the hydrogeologic framework 

and water resources of alluvial basins in southern Arizona. In the 

TAMA, the hydrogeology and water resources were described 

by Davidson (1973), Pool (1984), and Schmidt (1985), and stratigraphy was described by Allen (1981) and Anderson (1987a). 

Models of ground-water flow were developed by Anderson 

(1972), Moosburner (1972), Clifton (1981), Travers and Mock 

(1984), Mock and others (1985), and Mason and Bota (2006). 

The potential for aquifer-system compaction, land subsidence, 

and earth fissures was evaluated by Platt (1963), Caito and 

Sogge (1982), Anderson (1987a, 1989), Carpenter (1988, 1993), 

Hanson (1989), Hanson and others (1990), and Hanson and 

Benedict (1994). General ground-water conditions were defined 

by White and others (1966), Reeter and Cady (1982), Whallon 

(1983), and Cuff and Anderson (1987). Hydrologic and geologic 

terms used in this report are summarized by Poland and others 

(1972), and Laney and Davidson (1986). Holzer and others 

(1979) and Jachens and Holzer (1979) describe fissuring and 

subsidence related to ground-water withdrawal.

Using GPS methods, Schumann and Anderson (1988) 

established the original land-subsidence monitoring network in 

Tucson Basin and Avra Valley. A ground-water storage monitoring project in the Tucson Basin, incorporating stations from 

the original land-subsidence monitoring network, was also 

begun in the late-1980s by Dr. John Sumner of the University 

of Arizona Geosciences Department. 

Acknowledgments

This investigation and report is the result of cooperation 

among several individuals and agencies. Cooperating agencies 

include the Tucson Active Management Area of the Arizona 

Department of Water Resources, the City of Tucson Water 

Department, Pima County Department of Transportation, and 

the Pima County Regional Flood Control District. The Pima 

County Surveyors office organized and conducted the majority 

of the GPS surveys from 1998 to 2002. Support from several other agencies, including Metropolitan Domestic Water 

Improvement District, the Town of Oro Valley, and the Town 

of Marana was helpful in the development of this project.

Description of the Study Area

The alluvial basins of Tucson and Avra Valley are in the 

Basin and Range physiographic province of Arizona. The 

basins are partly surrounded by mountains that include ranges 

that are more than 9,000 ft above sea level. Tucson Basin and 

Avra Valley, the focus of this study, lie to the southeast of the 

Eloy Basin within the boundaries of the TAMA, except for a 

small area near Picacho Peak (fig. 1). The watershed area for 

the Tucson Basin extends beyond the study area and the southern boundary of the TAMA and encompasses about 2,870 mi2

in northern Sonora, Mexico, and in Santa Cruz, Pima, and 

Pinal, Counties, Arizona. Tucson Basin is bounded on the 

west by the Tucson and Sierrita Mountains, on the north by 

the Tortolita and Santa Catalina Mountains, on the east by the 

Rincon and Empire Mountains, and on the south by the Santa 

Rita Mountains. The southern drainage-area boundary is south 

of the study area in Mexico. The mountains range in altitude 

from about 3,000 ft to about 9,500 ft above sea level. Within 

the basin, the valley floor ranges in altitude from 2,000 ft above 

sea level near Rillito and the northwestern edge of the basin to 

3,500 ft near the international boundary with Mexico. Annual 

precipitation ranges from about 10 in. to 12 in. on the valley 

floor to as much as 30 in. in the surrounding mountains. 

Avra Valley encompasses about 520 mi2

 and is bounded 

on the south by the Sierrita Mountains and Altar Valley, on 

the west by Silverbell, Waterman, and Roskruge Mountains, 

on the northwest by Picacho Peak, and on the northeast by 

the Tortolita Mountains. The surrounding mountains range in 

altitude from about 4,500 ft to 6,000 ft above sea level. The 

valley floor ranges from 1,800 ft above sea level near Picacho 

Peak to 2,600 ft near Three Points. Annual precipitation in 

Avra Valley ranges from less than 10 in. on the valley floor to 

about 12 in. in the mountains. 

The Santa Cruz River is the major surface-water drainage in the Tucson Basin and Avra Valley. Before large-scale 

ground-water pumping began in the basin, the Santa Cruz River 

was perennial in sections of the study area. As of 1998, the 

baseflow in the river is effluent and occurs in reaches below the 

three water-treatment plants located where the river intersects 

with Roger Road, Ina Road, and Tangerine Road. Natural flow 

occurs only during periods of runoff from storms. Other streams 

in the study area generally flow only in response to local precipitation and include the Rillito River and the Cañada del Oro 

Wash in Tucson Basin and Brawley Wash in Avra Valley.

Methods of Data Collection

In Tucson Basin and Avra Valley, land-surface elevation 

change is monitored approximately annually by using GPS surveys. The GPS is a United States Department of Defense satellitebased system designed to provide continuous worldwide positioning capability. The system comprises a full constellation of at least 

24 satellites that act as reference points so that a GPS receiver can 

be used to calculate accurate position information. In GPS surveying, the satellite-based system with Earth-based reference stations 

is used to determine accurately the position of geodetic monuments. The vertical positions of stations are monitored by using 

repeat GPS surveys to determine any changes in elevation.
Figure 2. Long-term elevation-change monitoring stations in the Tucson Active Management Area, south-central Arizona. 

32o

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Methods of Data Collection 5

Figure 2. Long-term elevation-change monitoring stations in the Tucson Active Management Area.
6 Land Subsidence and Aquifer-System Compaction in the Tucson Active Management Area, South-Central Arizona, 1987–2005

In the spring of 1987, the GPS was used to establish a vertical-control network for monitoring land subsidence in Tucson 

Basin and Avra Valley (fig. 2). The initial survey was done by 

using satellite receivers and antennas from the Motorola, Inc., 

Government Electronics Group. Presently, survey instruments 

are Trimble 4800 and 5700 geodetic-grade receivers. Receivers 

are placed on fixed-height tripods at most stations to minimize 

errors in antenna-height measurements (fig. 3). 

The initial GPS survey consisted of a series of measurements at bench marks at 43 sites; however, at least 11 of the 

original benchmarks had been destroyed by the time annually 

repeated surveys had begun (table 1). In 1998, approximately 

annual GPS surveys began, using as much of the original network as possible. Many new stations were added to the network 

between 1998 and 2005 to provide additional information on 

both amounts and areal distribution of land-surface elevation 

change in the growing metropolitan areas of the TAMA.

Surveys of the Tucson Basin and Avra Valley networks 

were made by using static-surveying techniques. Static 

surveying is the most precise GPS-surveying technique and 

is performed with dual-frequency receivers. A static survey 

requires the use of at least two receivers; one receiver at each 

of two (or more) stations defines the baseline. Each receiver 

logs observations simultaneously from at least four common 

satellites. Static surveying requires that observations be logged 

at each station for an extended period of time. In the TAMA, 

the GPS surveys were performed by using multiple overlapping subnetworks of three to six stations. A single base station 

was common to every subnetwork. Stations were occupied for 

a minimum of 30 minutes, and many stations were occupied 

for several hours. Generally, about 20 percent of the stations 

were reoccupied for redundancy and quality assurance. 

The GPS data were post-processed to achieve an accuracy 

of 0.8 in. or better. GPS data were downloaded and processed 

by using software that calculates differential positions between 

stations. A least-squares network adjustment was performed by 

using stations on bedrock with known positions that are assumed 

to be stable from year to year. Additionally, one to several stations within the basins with little to no historical movement also 

were used in the network adjustment. Stable, stationary stations 

included TUC, N419, POST, GUARD, and THOR in the Tucson 

Basin and H291 and PASS in Avra Valley. Resulting station positions generally were accurate to within 0.8 in. in the vertical position and 0.4 in. in the horizontal position. Results of the network 

adjustments of each annual survey were differenced to determine 

changes in station vertical position and land subsidence.

In order to directly compare data sets from year to year, 

the same set of points were used in the network adjustments. 

Using different points for network adjustments could introduce 

some error by allowing the entire network to “rotate” in order 

to fit a new set of control points. If analogous points are used 

each year for the network adjustment, the network is fixed to 

the same place each time, allowing for the best possible comparison of station positions from each successive survey. 

Surveys from 2004 and 2005 did not contain some 

control points used historically in the network adjustment, and 

therefore, could not be processed to the same level of certainty. 

To allow for a full network adjustment, control points missing 

from 2004 and 2005 data sets were imported from the 2002 

GPS data set. Bedrock control points (their positions and vectors) from past surveys can be included in more recent surveys 

if those points are stable as assumed. 

In order to compare the annual GPS surveys from 1998 

through 2005 to the original 1987 GPS survey documented in 

Schumann and Anderson (1988), it was necessary to adjust 

the original 1987 vectors to the control stations used in postprocessing the surveys from 1998 to 2005. Additionally, it 

was necessary to re-project the original survey coordinates 

and ellipsoid heights from datum WGS-72 to NAD-83. The 

original survey, National Geodetic Survey (NGS) project 

GPS082, was performed in 1987 in support of the USGS project to monitor subsidence in the Tucson Basin and Avra Valley 

(Dave Minkel, NGS State Geodesist, oral commun.). 

Dave Minkel used program ADJUST, version 4.30, for the 

adjustment. The original 1987 vector components and station 

coordinates were edited to resolve issues with the older format 

no longer supported by the program ADJUST. Two adjustments 

were performed; a free (minimally constrained) adjustment, and 

a constrained adjustment using coordinates and ellipsoid heights 

for two stations (N 419 and TUC) provided by the authors. The 

free adjustment shows the original survey results were surprisingly good considering the size of the GPS constellation and 

capability of GPS equipment in 1987 (Dave Minkel, NGS State 

Geodesist, oral commun., 2007).

In addition to the annual GPS surveys of the vertical-control 

network of benchmarks in the TAMA, a borehole-extensometer 

network was established for measuring and monitoring aquifersystem compaction and land subsidence caused by ground-water 

depletion. The network of 14 borehole extensometers was established in 1983 to monitor the rates and magnitude of aquifer-system compaction and water-level change. Seven extensometers 

are in Tucson basin and seven are in Avra Valley (fig. 4). 

Figure 3. Photograph of a typical setup of a geodetic GPS receiver 

in the Tucson Active Management Area.
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32o30’

32o

PINAL COUNTY

PIMA COUNTY

PIMA COUNTY

SANTA CRUZ

COUNTY

UNITED STATES

MEXICO

Marana

Green

Valley

Tucson

Oro

R

Valley

R

Creek

i i to ll

C

v

r uz

a

aS

t n

i

er

0 5 10 KILOMETERS

0 5 10 MILES

Base from U.S. Geological Survey

digital data, 1:100,000, 1982

Universal Transverse Mercator

projection, Zone 12

NOTE: See fig.12 for water level and extensometer-

 compaction readings

19

10

10

83

77

79

86

286

TUCSON

BASIN

AVRA

VALLEY

SC30

SC17

WR53

B76

D61

C45

WR52

AV25

TA44

TA33

TA32

AF14

AF17

Avra

Valley

extensometers

Tucson

Basin

extensometers

TA13

Methods of Data Collection 7

Figure 4. Tucson Active Management Area borehole extensometer stations.
Station NAD83 horizontal 

coordinates,

meters

Ellipsoid height, feet Elevation change 

since 1987, inches

E levation change since 

1998, inches

Ele

sin

UTME UTMN 1987 1998 1999 2000 2001 2002 2004 2005 1998 2004 2005 1999 2000 2001 2002 2000 

TUCSON BASIN

5DOR 519252 3550967 2853.5 2853.5 2853.5 2853.5 2853.5 2853.4 -- -- 0.4 -- -- -0.6 -0.2 -0.2 -1.4 0

A50A 495343 3579970 -- 2244.5 2244.4 2244.4 2244.5 2244.4 2244.5 2244.5 -- -- -- -0.7 -0.9 -0.6 -1.1 -0

B34 504329 3562633 -- 2358.6 2358.5 2358.5 2358.5 2358.4 2358.4 -- -- -- -- -0.6 -0.7 -1.3 -1.5 -0

B77A 510486 3568617 -- 2361.4 2361.5 2361.4 2361.4 2361.4 2361.4 -- -- -- -- 0.8 -0.3 -0.2 -0.3 -1

B7A 507136 3565844 -- 2398.4 2398.4 2398.3 2398.3 2398.2 -- -- -- -- -- 0.3 -1.4 -1.5 -2.6 -1

B92A 502941 3575697 -- 2404.8 2404.7 2404.7 2404.8 2404.7 2404.7 2404.7 -- -- -- -0.5 -1.0 0.5 -1.0 -0

C22A 514583 3565813 -- 2490.8 2490.8 2490.8 2490.8 2490.7 2490.7 -- -- -- -- 1.1 0.3 0.0 -0.1 -0

C317 510739 3556597 2573.1 2573.0 2573.0 2573.0 2573.0 2573.0 2572.9 -- -0.6 -2.1 -- -0.4 -0.6 0.0 -0.8 -0

C45 509759 3562889 2458.7 2458.4 2458.5 2458.4 2458.4 2458.4 2458.3 -- -3.2 -5.0 -- 0.9 0.0 -0.0 -0.6 -0

COT1 502651 3565315 -- 2409.2 2409.4 2409.4 2409.4 2409.4 2409.3 2409.0 -- -- -- 2.1 2.1 2.1 1.9 0

D4 513302 3561800 -- 2557.9 2558.0 2557.9 2557.8 2557.9 2557.8 -- -- -- -- 1.3 0.4 -0.2 0.2 -0

E9A 516983 3558461 -- 2710.9 2710.9 2710.9 2710.8 2710.9 2710.8 -- -- -- -- -0.4 -0.3 -0.9 -0.1 0

FD62 494738 3573014 2247.7 2247.7 2247.7 2247.7 2247.7 2247.6 2247.7 2247.7 0.4 0.4 0.4 0.0 0.0 0.0 -0.6 0

GP15 521344 3545062 -- 2985.4 2985.3 2985.3 2985.3 2985.3 -- -- -- -- -- -0.7 -1.4 -1.1 -0.5 -0

GUARD 523374 3536344 3338.8 3338.9 3338.9 3338.9 3338.9 3338.9 3338.9 -- 1.1 1.1 -- 0.0 0.0 0.0 0.0 0

L75 504365 3546580 2507.6 2507.7 2507.5 2507.6 2507.7 2507.6 2507.6 -- 1.3 0.5 -- -1.8 -1.3 -0.1 -1.3 0

LINE 513387 3535747 -- 2821.0 2820.9 2820.9 2820.9 2820.9 2820.8 -- -- -- -- -1.2 -1.3 -0.9 -1.4 -0

MAGNETIC 515284 3567735 -- 2452.6 2452.6 2452.7 2452.6 2452.6 -- -- -- -- -- -0.2 0.6 0.2 0.1 0

N419 501108 3564098 2281.4 2281.4 2281.4 2281.4 2281.4 2281.4 2281.4 2281.4 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0

PA4 500510 3540597 2700.8 2700.7 2700.5 2700.5 2700.6 2700.5 2700.5 -- -1.0 -4.1 -- -2.6 -2.4 -1.4 -3.3 0PHPS 521453 3561262 2801.8 2801.8 2801.8 2801.8 2801.8 2801.8 2801.8 -- -0.6 -0.4 -- 0.2 0.5 0.5 0.1 0

SC25A 513261 3540610 -- 2750.6 2750.5 2750.5 2750.5 2750.4 2750.5 -- -- -- -- -0.7 -1.1 -0.7 -1.4 -0

THOR 495638 3587453 2598.9 2598.9 2598.9 2598.9 2598.9 2598.9 2598.9 2598.9 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0

TUC 520271 3574775 2870.2 2870.2 2870.2 2870.2 2870.2 2870.2 2870.2 -- 0.0 0.0 -- 0.0 0.0 0.0 0.0 0WR142 506108 3566327 -- 2373.9 2373.9 2373.8 2373.8 2373.7 -- -- -- -- -- 0.2 -0.8 -1.4 -2.5 -1

WR175A 509387 3553709 -- 2580.2 2580.1 2580.1 2580.2 2580.1 2580.1 -- -- -- -- -1.0 -1.0 -0.0 -1.0 0WR52 504242 3568767 2299.6 2299.5 2299.5 2299.5 2299.5 2299.5 2299.5 2299.5 -0.9 -0.8 -1.2 0.4 -0.2 0.5 -0.1 -0

WR53 507443 3556554 2506.2 2506.2 2506.2 2506.2 2506.2 2506.2 2506.2 -- -0.5 -0.8 -- 0.2 0.2 0.6 0.1 0

WR56A 502734 3554784 -- 2431.4 2431.3 2431.2 2431.2 2431.1 2430.9 -- -- -- -- -0.8 -1.4 -1.6 -3.5 -0X333 503930 3549284 2496.0 -- -- 2496.0 2496.0 2495.9 -- -- -- -- -- -- -- -- -- 0

X419 505349 3560633 2406.2 2405.9 2405.9 2405.9 2405.9 2405.8 2405.7 -- -2.7 -5.0 -- 0.2 -0.2 -0.3 -1.4 -0

XAVIER 503806 3550860 2491.9 2491.8 2491.8 2491.8 2491.8 2491.7 2491.7 -- -0.4 -1.7 -- -0.9 -0.9 -0.5 -1.3 -0

Table 1. Positions and elevations of monitoring stations in the Tucson Active Management Area and changes in vertical position from 1987 to 2005.
Station NAD83 horizontal 

coordinates,

meters

Ellipsoid height, feet Elevation change 

since 1987, inches

Elevation change since 

1998, inches

Ele

sin

UTME UTMN 1987 1998 1999 2000 2001 2002 2004 2005 1998 2004 2005 1999 2000 2001 2002 200

AVRA VALLEY

171+82 AZHD 478981 3592341 1874.5 1874.4 1874.6 1874.4 1874.5 1874.5 -- -- -0.9 -- -- 2.0 -0.2 0.4 0.3 -2

1899 USGS 472267 3600476 1815.2 1815.2 1815.2 1815.1 1815.2 1815.2 -- -- -0.8 -- -- 0.0 -0.1 0.2 0.3 -06WBC 465409 3602421 1730.2 -- 1729.9 1729.9 1729.9 1730.0 -- -- -- -- -- 0.0 -- -- -- 0

AF13 473365 3584136 -- 1901.1 1901.0 1901.0 1901.0 -- -- 1899.3 -- -- -- -0.7 -0.8 -0.1 -- -0AF16A 468661 3577661 -- 1972.6 1972.6 1972.6 1972.6 1972.5 1972.6 1972.4 -- -- -- 0.4 -0.2 0.2 -0.3 -0

AF1A 471385 3593356 -- 1812.8 1812.8 1812.8 1812.8 1812.8 1812.9 1812.7 -- -- -- 0.0 -0.1 0.2 0.3 -0

AF2 471385 3590499 -- 1822.0 1822.0 1822.0 1822.1 1822.0 1822.2 1822.0 -- -- -- 0.3 -0.2 0.8 0.1 -0

AF25A 466979 3571262 -- 2051.1 2051.1 2051.1 2051.2 2051.1 2051.2 2051.1 -- -- -- 0.2 -0.3 1.3 0.2 -0

AF35A 478616 3566732 -- 2166.8 2166.8 2166.8 2166.8 2166.8 2166.8 2166.7 -- -- -- -0.4 -0.9 -0.6 -0.7 -0

AV13A 477727 3559980 -- 2224.2 2224.2 2224.1 2224.2 2224.2 2224.3 2224.1 -- -- -- 0.4 -0.4 0.4 0.4 -0

AV25 479566 3558472 2228.3 2227.9 2227.9 2227.9 2227.9 2227.9 2228.0 2227.9 -4.2 -3.8 -4.3 0.0 -0.7 0.0 -0.4 -0

D296 482709 3554826 2330.1 2329.9 2329.9 2330.0 2330.0 2329.9 2329.9 2329.9 -2.2 -1.8 -2.6 0.5 1.0 1.2 0.3 0

F294 471394 3589144 1832.6 1832.6 1832.6 1832.6 1832.6 1832.6 -- -- -0.3 -- -- 0.2 0.3 1.1 0.4 0F424 476803 3594162 1849.7 1849.7 1849.8 1849.7 1849.8 1849.8 -- -- -0.1 -- -- 0.3 -0.4 0.1 0.5 -0H291 479609 3570295 -- 2309.3 2309.3 2309.3 2309.3 2309.3 2309.3 2309.3 -- -- -- 0.0 0.0 0.0 0.0 0

H292 476275 3576032 2017.1 2017.0 2016.9 2017.0 2016.9 2016.9 2017.0 2016.9 -1.3 -0.7 -2.4 -0.1 0.0 -0.1 -0.6 0

H301 472123 3567811 2123.2 2123.2 2123.1 2123.0 2123.2 2123.1 2123.1 2123.0 -0.7 -1.2 -2.4 -0.5 -1.5 -0.1 -0.6 -1

L94 466769 3591697 1819.1 1819.1 1819.1 1819.1 1819.1 1819.1 1819.2 1819.0 0.4 1.4 -1.4 -0.4 -0.7 -0.6 -0.5 -0

M279 463734 3611992 1699.4 1699.4 1699.3 1699.3 1699.3 -- -- -0.2 -- -- -0.4 -- -0.6 -0.5 -- MISSAZDT 476391 3551790 2400.3 2400.1 2400.1 2400.1 2400.1 2400.0 2400.1 2400.1 -3.1 -2.4 -2.9 0.5 0.4 0.8 -0.5 -0

NAVISKA2 475248 3596193 1852.5 1852.5 1852.5 1852.5 1852.5 1852.5 -- -- -0.1 -- -- 0.3 0.4 0.3 0.0 0

PASS 462945 3611845 -- 1886.1 1886.1 1886.1 1886.1 1886.1 1886.1 -- -- -- -- 0.0 0.0 0.0 0.0 0

Q279 466601 3608533 1720.3 1720.3 1720.2 1720.2 1720.2 1720.2 1720.3 1720.1 -1.1 -1.1 -3.1 -0.3 -0.3 -0.6 -0.2 0

Q94 478704 3585792 1916.6 1916.5 1916.5 1916.5 1916.6 1916.5 1916.6 1916.4 -0.8 0.6 -1.4 0.5 0.4 0.6 -0.2 -0

SA105 469731 3572810 2048.5 2048.3 2048.3 2048.2 2048.2 2048.2 2048.3 2048.1 -3.3 -3.3 -4.8 -0.2 -0.6 -0.7 -0.8 -0

TA13 471579 3605427 -- 1827.5 1827.5 1827.5 1827.6 1827.5 1827.4 0.0 -- -- -- -- -- 0

USBR938 479244 3567997 -- 2222.1 2222.1 2222.0 2222.0 2222.0 -- -- -- -- -- 0.0 -0.5 -0.1 -0.3 -0

WR16B 475937 3579485 -- 1955.7 1955.6 1955.6 1955.6 1955.6 1955.7 1955.5 -- -- -- -0.4 -0.7 -0.8 -1.1 -0

WR29A 472926 3566350 -- 2148.1 2148.1 2148.0 2148.1 2148.1 2148.1 2148.0 -- -- -- -1.0 -1.1 -0.7 -1.0 -0

Table 1. Positions and elevations of monitoring stations in the Tucson Active Management Area and changes in vertical position from 1987 to 2005—C
Data-logger Digital-shaft recorder box

Steel table

Adjusting nut

Threaded rod

Steel tape

Pressure transducer

(for water level indication)

Water surface

Counter

weights

NOTE: The extensometer pipe is isolated from the well casing.

 As the aquifer materials compact, the land surface moves 

 downward in elevation to the top of the extensometer pipe. 

10 feet of 8-inch

steel casing

Steel

casing

1 to 2 feet of

open hole

Extensometer pipe is

schedule 80 steel pipe,

11

⁄2, 2, or 3 inches or a

combination of these

Extensometer pipe rests on 

concrete plug or jetted into 

the formation to resistance

Minimum of

12 feet of 3-inch

steel pipe for

table legs set

on concrete

plug

Land surface

H-beam fulcrum arm

10 Land Subsidence and Aquifer-System Compaction in the Tucson Active Management Area, South-Central Arizona, 1987–2005

Aquifer-system compaction is measured by the borehole-extensometer pipes that extend from the land surface to 

the bottom of cased wells or test holes (fig. 5). The extensometer pipes are isolated from the well casings and are 

jetted into the formation, or are set on concrete plugs placed 

at the bottom of the well. As the aquifer materials compact, 

the land surface moves downward in relation to the top of the 

extensometer pipe. Thus, borehole extensometers measure 

compaction for the portion of the aquifer system between the 

land surface and the depth at which the bottom of the extensometer is anchored. The design and operation of borehole 

extensometers is described in detail by Schumann (1986) and 

Anderson and others (1982).

In the TAMA, the base of the extensometer pipes are 

jetted into bedrock as stated previously, or are grouted at the 

bottom of the well into less compressible alluvium. Most 

of the extensometers are anchored in alluvium at depths 

between 800 and 1,400 ft. Extensometers measure aquifersystem compaction in the depth interval of the extensometer pipe which might represent only a portion of the total 

compaction, whereas GPS surveys measure land subsidence. 

Aquifer-system compaction occurring beneath the base of 

the extensometer is not measured by the extensometer but is 

represented in the GPS-measured land subsidence (Amelung 

and others, 1999; Evans and Pool, 2000). 

Effective monitoring of the extent and rate of land subsidence is a continuous challenge owing to the cost, time, and 

resources needed to perform ground-based GPS surveys and 

maintain the borehole-extensometer network. Interferometric 

Synthetic Aperture Radar (InSAR) is a satellite technology 

that can provide high-resolution mapping of earth-surface 

topography and deformation. The radar transmits a series of 

microwave pulses and records both the amplitude and phase of 

the backscattered responses from the surface. The phase difference between two radar images (interferogram) taken at different times, contains signals associated with surface topography 

and deformation, as well as differences in the atmosphere and 

satellite position at the time of each acquisition. Isolating the 

deformation signal by applying phase corrections to satellite position and surface topography produces a differential 

interferogram in which one cycle of phase change represents 

a half-wavelength (about 1.1 in. for C-band radar) of surface 

movement in the range or line-of-sight of the radar transmitter. 

The ADWR presently is using InSAR to map sub-centimeter 

deformation in Phoenix and Tucson (Brian Conway, ADWR, 

oral commun., 2007).

Hydrogeology

Basins in the TAMA were formed as a result of crustal 

extension during the Cenozoic Basin and Range orogeny. 

The Basin and Range orogeny was accompanied by block 

faulting, the formation of a horst-and-graben terrain, and 

the accumulation of sedimentary basin fill. The Basin and 

Range orogeny transformed the landscape of the basins in the 

TAMA from an area of generally moderate relief into one of 

high relief characterized by deep structural basins bounded 

by high mountain ranges (Anderson, 1987a). 

Published reports from studies of the Tucson Basin were 

used as references for describing the hydrogeologic conditions 

within the TAMA. Alluvial deposits that accumulated in the 

structural basins can be grouped into three stratigraphic units 

of basin fill on the basis of structural relations (fig. 6). The 

three sedimentary units compose the alluvial-aquifer system 

(Davidson, 1973; Allen, 1981; Anderson, 1987a, 1987b, 1989; 

and Hanson 1989) and are correlative with the lower, middle, 

and upper hydrostratigraphic units of adjacent basins within 

the region.

The lower stratigraphic unit was deposited before and 

during the early phases of extensional tectonism associated 

with low-angle faulting and includes the Pantano Formation 

in the TAMA. The Pantano Formation consists of conglomerate, sandstone, mudstone, and gypsiferous mudstone, as well 

as megabreccia, bedded tuffs, and interbedded volcanic flows 

(Anderson, 1987a). The Pantano Formation yields small to 

moderate amounts of water to wells.

Figure 5. Diagrammatic sketch of a borehole extensometer in the 

Tucson Active Management Area.
32o

111o

111o30’

32o30’

32o

PINAL COUNTY

PIMA COUNTY

PIMA COUNTY

SANTA CRUZ

COUNTY

UNITED STATES

MEXICO

Marana

Green

Valley

Tucson

Oro

Valley

r uz

a

aS

t n

River

0 5 10 KILOMETERS

0 5 10 MILES

Base from U.S. Geological Survey

digital data, 1:100,000, 1982

Universal Transverse Mercator

projection, Zone 12

19

10

10

83

77

79

86

286

C

EXPLANATION

STREAM-CHANNEL ALLUVIUM

UPPER STRATIGRAPHIC UNIT—Surficial deposits of

 Holocene to middle Pleistocene. Includes upper Tinaja

 beds and the Fort Lowell Formation

MIDDLE STRATIGRAPHIC UNIT—Older surficial deposits

 of middle Pleistocene. Includes the lower and middle 

 Tinaja beds

LOWER STRATIGRAPHIC UNIT—Includes crystaline

 rocks and sedimentary rocks of the Pantano Formation

Geology modified from Reynolds, 1988

AVRA

VALLEY

Hydrogeology 11

Figure 6. Generalized geology of the Tucson Active Management Area.
12 Land Subsidence and Aquifer-System Compaction in the Tucson Active Management Area, South-Central Arizona, 1987–2005

The middle stratigraphic unit was deposited during the 

transition from low- to high-angle faulting and includes the 

lower and middle Tinaja beds in the TAMA. The lower Tinaja 

beds consist of gravel and conglomerate to clayey silt and 

mudstone and are hundreds of feet thick. The middle Tinaja 

beds consist of gravel conglomerate to gypsiferous and anhydritic clayey silt and mudstone and are hundreds to thousands 

of feet thick. The lower and middle Tinaja beds yield small to 

moderate amounts of water to wells (Anderson, 1987a).

The upper stratigraphic unit is relatively undisturbed 

by faulting in comparison to the older units and includes the 

upper Tinaja beds and Fort Lowell Formation in the TAMA. 

The upper Tinaja beds are gravel to clayey silt and are hundreds to thousands of feet thick. The Fort Lowell Formation 

consists of gravel to clayey silt and includes thin surficial 

alluvial deposits of late Pleistocene and Holocene age. The 

Fort Lowell Formation ranges in thickness from several feet to 

several hundreds of feet (Anderson, 1987a). The Fort Lowell 

Formation is the most permeable unit of the aquifer and yields 

moderate to large amounts of water to wells (Davidson, 1973). 

Subsequent to the accumulation of the three stratigraphic units 

of basin fill, a thin layer of alluvium was deposited along 

major drainage channels.

Ground water is replenished by mountain-front recharge 

and underflow in the TAMA (Hanson, 1989). Additional 

streamflow infiltration from effluent and floods contributes to 

recharge along the Santa Cruz River and its tributaries. The 

Santa Cruz River and ground-water outflow from the Tucson 

Basin enter Avra Valley northwest of Rillito. Additional underflow enters Avra Valley from Altar Valley to the south. Groundwater outflow from Avra Valley occurs between the Silverbell 

Mountains and Picacho Peak and enters the Eloy Basin in the 

southern part of the Lower Santa Cruz River Basin. Natural 

ground-water flow paths and head distributions have been 

altered by ground-water withdrawals (Hanson, 1989). 

In some places, continued withdrawal of ground water 

and infiltration of irrigation water have created perched zones 

of local saturation atop low-permeability deposits above 

the zone of regional saturation. Cuff and Anderson (1987) 

outlined an area of perched ground water in the north-central 

part of Avra Valley that is similar to an area in west-central 

Tucson. Perched zones, which are caused by irrigation return 

flow or artificial recharge, can increase geostatic load (stress), 

and transient-vertical gradients can result in seepage stresses 

(Hanson, 1989). Both conditions can inrease the effective 

(intergranular) stress on aquitards, potentially affecting the 

magnitude, rate and distribution of land subsidence.

Potential for Land Subsidence

Permanent land subsidence can occur in alluvial basins 

when water is removed from aquifer systems (Galloway and 

others, 1999). The geostatic loads in aquifer systems such 

as those in the TAMA are supported in part by the granular 

skeleton of the aquifer system and in part by the pore-fluid 

pressure. Under conditions of constant geostatic stress, when 

ground water is withdrawn and the pore-fluid pressure is 

reduced, the stress on the granular skeleton (effective stress) 

is increased by an equivalent amount and the aquifer system compacts, causing some lowering of the land surface. 

The compaction1 may be permanent (inelastic) or reversible 

(elastic) depending on the stress history of aquifer system. 

The magnitude of effective stress that determines whether the 

skeleton will undergo elastic or inelastic compaction is known 

as the preconsolidation stress and is approximated by the previous maximum effective stress or in terms of hydraulic head, 

the historic minimum head—critical head. Inelastic compaction occurs when the preconsolidation stress is exceeded or in 

terms of head—when head falls below the critical head. For 

equal incremental changes in effective stress, compaction in 

the elastic range of stress is typically much less than compaction in the inelastic range of stress.

Both the aquifers (sand and gravel) and aquitards (clay 

and silt) of aquifer systems deform as a result of changes in 

effective stress, but to different degrees. Elastic compaction 

occurs in both the aquifers and aquitards in the aquifer systems. For example, when ground-water levels are raised fluid 

pressure increases and effective stress decreases. Some support 

previously provided by the aquifer and (or) aquitard skeleton 

is transferred to the fluid pressure, and the skeleton expands. 

This fully recoverable deformation commonly results in 

seasonal, reversible displacements in land surface (uplift and 

subsidence) of more than 1 in. (Amelung and others, 1999; 

Galloway and others, 1999). Conversely, most permanent 

compaction and land subsidence occurs due to the inelastic 

compaction of aquitards. 

Potential for aquifer-system compaction in the TAMA 

was investigated by Anderson (1987a) and Hanson (1989). 

The Pantano Formation, lower Tinaja beds, and middle Tinaja 

beds consist largely of moderately indurated to indurated 

deposits that generally are resistant to deformation related to 

ground-water withdrawal (Anderson, 1987a). Thus, the potential for aquifer-system compaction and its effects in the TAMA 

may depend more on the character of the upper units of the 

Tinaja beds and the Fort Lowell Formation (which contain a 

higher percentage of silt and clay) than on the character of the 

lower units in the TAMA. 

In addition to the higher silt and clay content of the upper 

units, the thickness and the relation between the upper units 

and bedrock affect the potential for aquifer-system compaction 

in the TAMA. The thickness of the Fort Lowell Formation and 

the upper Tinaja beds varies throughout the TAMA as a result 

of structural deformation of the underlying rock. Generally, 

areas of greater rates of inelastic compaction are those areas 

that contain a larger percentage of silt and clay in the saturated 

zones and are associated with ground-water withdrawals. 

Land subsidence and aquifer-system compaction in the 

TAMA has not been as great as that in the nearby Eloy and 

Stanfield Basins because alluvial basins in the TAMA have not 

1

 In this report, the term ”compaction” refers to a decrease in thickness of 

sediments as a result of increase in vertical compressive stress. The identical 

physical process is referred to as ”consolidation” by soils engineers.
Land Subsidence and Aquifer-System Compaction 13

been pumped as extensively as the Eloy and Stanfield Basins 

where ground water has been used extensively for agriculture 

(Anning and Duet, 1994). In addition, the Fort Lowell Formation and the upper Tinaja beds do not contain as much compressible clay as the upper hydrostratigraphic units of the Eloy 

Basin do and, therefore, would not be expected to compact as 

much under similar ground-water withdrawal conditions. The 

Fort Lowell Formation and the upper Tinaja beds do, however, 

contain compressible clay layers that have compacted inelastically in response to ground-water withdrawals.

Long-term ground-water withdrawal rates in the TAMA 

that are greater than rates of inflow to the ground-water system 

have resulted in removal of water from ground-water storage 

and in water-level declines during the last several decades. 

Superimposed on the long-term water-level declines are shortterm increases in storage and water levels that occur over 

periods of months to years after occasional significant increases 

in the rate of recharge (Pool, 2005). Additionally, long-term 

water-level declines have stabilized or reversed since 2000 at 

some monitoring wells (figs. 7 and 8) in Tucson Basin (wells 

B76, C45, D61, and SC17) and Avra Valley (wells AF14, 

AF17, TA32, and TA33). These areas of water-level increase—

likely a result of decreases in ground-water withdrawal and 

redistribution of pumpage as Central Arizona Project water has 

become available for artificial recharge and municipal consumption—reduce the potential for continued land subsidence 

due to aquifer-system compaction in the TAMA.

Land Subsidence and Aquifer-System 

Compaction

Permanent subsidence, seasonal elastic deformation, and 

uplift have been observed during the period of data collection 

from 1987 to 2005. In the spring of 1987, a GPS survey was 

used to measure horizontal and vertical positions for bench 

marks at 43 sites to establish a network for monitoring land 

subsidence in Tucson Basin and Avra Valley. Between 1987 

and 2005, the original number of subsidence-monitoring 

stations was gradually increased to more than 100 stations to 

meet the need for information in the growing metropolitan 

area. Data from approximately 60 stations common to the GPS 

surveys done after 1987, in addition to the remaining stations 

from the original survey, were used to document land subsidence in the TAMA. 

Between 1987 and 1998, land subsidence was greater in 

Avra Valley than in Tucson Basin on the basis of the average 

subsidence at 31 stations that were common to the original 

GPS survey in 1987. The average subsidence in the Tucson 

Basin during the 11-year period from 1987 to 1998 was 0.5 in., 

and the maximum subsidence of 3.2 in. and 2.7 in. occurred in 

the middle of Tucson Basin at stations C45 and X419, respectively (fig. 9 and table 1). In the southern portion of Tucson 

Basin, several stations showed uplift, and two stations (L75 and 

GUARD) showed uplift in excess of 1 in. In Avra Valley, the 

average subsidence during the 11-year period was 1.2 in. Most 

of the stations in Avra Valley showed subsidence, with the greatest amounts of subsidence occurring at stations in the southern 

part of the valley. About 4 in. of subsidence occurred at station 

AV25, and three stations, D296, MISSAZDT, and SA105, 

showed more than 2 in. of subsidence (fig. 9 and table 1). 

For the approximately 60 stations that are common 

between the 1998 and 2002 GPS surveys, the data indicate that 

up to 3.5 in. of subsidence occurred in Tucson Basin, and as 

much as 1.1 in. of subsidence occurred in Avra Valley (fig. 10 

and table 1). Land subsidence was greater in Tucson Basin 

than in Avra Valley between 1998 and 2002—the average 

subsidence for the 4-year period was about 0.4 in. in Tucson 

Basin and 0.1 in. in Avra Valley. In contrast to the 11-year 

period from 1987 to 1998 where most of the subsidence in 

Tucson Basin occurred in the middle of the basin between the 

Rillito and Santa Cruz Rivers, the greatest amount of subsidence for the 4-year period between 1998 and 2002 occurred in 

the southern portion of the basin along the Santa Cruz River 

at stations WR56A (3.5 in. of subsidence) and PA4 (3.3 in. 

of subsidence). In Avra Valley, most of the stations showed 

subsidence during the 4-year period between 1998 and 2002; 

however, the magnitude of subsidence was less than 1 in. at all 

stations except WR16B, which showed 1.1 in. of subsidence.

Between 2002 and 2005, the GPS surveys were conducted largely at newer stations within the TAMA, and thus, 

fewer data points were available for comparison with the 

earlier surveys. For 2005, there were 19 stations in Avra Valley 

available for comparison with the 2002 survey, but only 6 stations in the Tucson Basin common to the 2002 survey (fig. 11 

and table 1). Of the available stations from the 2005 survey in 

the Tucson Basin, station WR52, in the northern part of Tucson Basin, had the most subsidence (0.2 in.), consistent with 

the area that showed active subsidence in previous surveys. 

In Avra Valley, more data were available for comparison with 

the 2002 survey (fig. 11 and table 1). The average subsidence 

for the 19 stations surveyed in Avra Valley between 2002 and 

2005 is about 0.7 in.; however, three stations in the northern 

part of the valley showed subsidence of more than 1 in.—2.2 in. 

at station TA13 and 1.7 in. at both station Q279 and AF16A. 

Data from station MISSAZDT in the southern part of the valley showed 3.1 in. of subsidence between 1987 and 1998 and 

another 0.5 in. of subsidence between 1998 and 2002; however, MISSAZDT showed 0.7 in. of uplift between 2002 and 

2005.

A comparison of the original 1987 GPS survey to the 

2004–05 surveys was done to determine (1) the approximate 

magnitude of subsidence during the 17- to 18-year period, 

and (2) the areas of the TAMA where the most subsidence has 

occurred (fig. 12 and table 1). From the surveys in 2004–05 

for Tucson Basin, there were 14 stations in common with 

the 1987 survey. In Avra Valley, there were 9 stations from 

the 2004–05 surveys in common with the 1987 survey. The 

2004–05 GPS-survey data were combined for the comparison 

with the original 1987 survey in order to maximize the number 

of stations for the comparison during the period of record. 
DEPTH TO WATER, IN FEET BELOW LAND SURFACE

B76

180

240

260

200

220

280

0.3

0.2

0.1

0

–0.1

0.4

0.5

C45

270

330

350

290

310

370

0.3

0.2

0.1

0

–0.1

0.4

0.5

D61

270

330

350

290

310

370

0.3

0.2

0.1

0

–0.1

0.4

0.5

SC17

180 0.5

80

140

160

100

120

0.3

0.2

0.1

0

–0.1

0.4

COMPACTION, IN FEET

1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

EXPLANATION

WATER LEVEL

COMPACTION

14 Land Subsidence and Aquifer-System Compaction in the Tucson Active Management Area, South-Central Arizona, 1987–2005

Figure 7. Water level and compaction readings at extensometers in Tucson Basin, 1989 to 2005.
DEPTH TO WATER, IN FEET BELOW LAND SURFACE

SC30

160

220

240

180

200

260

0.3

0.2

0.1

0

–0.1

0.4

0.5

COMPACTION, IN FEET

WR52

180

240

260

200

220

280

0.3

0.2

0.1

0

–0.1

0.4

0.5

WR53

100

160

180

120

140

200

1989

0.3

0.2

0.1

0

–0.1

0.4

0.5

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

EXPLANATION

WATER LEVEL

COMPACTION

Land Subsidence and Aquifer-System Compaction 15

Figure 7. Water level and compaction readings at extensometers in Tucson Basin, 1989 to 2005—Continued.
DEPTH TO WATER, IN FEET BELOW LAND SURFACE

AF14

200

260

280

220

240

300

0.3

0.2

0.1

0

–0.1

0.4

0.5

AF17

300

360

380

320

340

400

0.3

0.2

0.1

0

–0.1

0.4

0.5

AF25

320

380

400

340

360

420

0.3

0.2

0.1

0

–0.1

0.4

0.5

TA13

300 0.5

200

260

280

220

240

0.3

0.2

0.1

0

–0.1

0.4

COMPACTION, IN FEET

1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

EXPLANATION

WATER LEVEL

COMPACTION

16 Land Subsidence and Aquifer-System Compaction in the Tucson Active Management Area, South-Central Arizona, 1987–2005

Figure 8. Water level and compaction readings at extensometers in Avra Valley, 1989 to 2005.
DEPTH TO WATER, IN FEET BELOW LAND SURFACE

TA32

300

360

380

320

340

400

0.3

0.2

0.1

0

–0.1

0.4

0.5

COMPACTION, IN FEET

TA33

300

360

380

320

340

400

0.3

0.2

0.1

0

–0.1

0.4

0.5

TA44

380

440

460

400

420

480

1989

0.3

0.2

0.1

0

–0.1

0.4

0.5

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

EXPLANATION

WATER LEVEL

COMPACTION

Land Subsidence and Aquifer-System Compaction 17

Figure 8. Water level and compaction readings at extensometers in Avra Valley, 1989 to 2005—Continued.
PINAL COUNTY

PIMA COUNTY

Tucson

Marana Oro

Valley

Green Valley

110o45’

0 5 MILES

0 5 KILOMETERS

111o15’ 111o

32o30’

15’

32o

Base from U.S. Geological Survey

digital data, 1:100,000, 1982

Universal Transverse Mercator

projection, Zone 12

Sabino

Creek

R

Creek

i ito ll

Wash

Braw ey l

Santa

re iv R

Cruz

0 to 1

0 to –1

EXPLANATION

ELEVATION CHANGE FROM 1987–98, IN INCHES:

BENCHMARK

–3 to –4

–4 to – 5

–1 to –2

–2 to –3

BOUNDARY OF TUCSON

ACTIVE MANAGEMENT

AREA

Q279

TA13

1899_USGS

NAVISKA2

L94

D296

AV25

MISSAZDT

PA4

L75

XAVIER

WR52

FD62

THOR

C45

WR53

W419

C317

GUARD

5DOR

PHPS

H292

SA105

H301

F294

Q94

F424

171_82_AZHD

AVRA

VALLEY

18 Land Subsidence and Aquifer-System Compaction in the Tucson Active Management Area, South-Central Arizona, 1987–2005

Figure 9. Land-surface elevation change in the Tucson Active Management Area from 1987 to 1998.
Tucson

Marana

110o45’

0 5 MILES

0 5 KILOMETERS

111o15’ 111o

32o30’

15’

32o

Base from U.S. Geological Survey

digital data, 1:100,000, 1982

Universal Transverse Mercator

projection, Zone 12

Sabino

Creek

0 to 1

0 to –1

EXPLANATION

ELEVATION CHANGE FROM 1998–2002, IN INCHES:

BENCHMARK

–1 to –2 –3 to –4

–2 to –3

BOUNDARY OF TUCSON

ACTIVE MANAGEMENT

AREA

PINAL COUNTY

PIMA COUNTY

Oro

Valley

Green Valley

R

Creek

i ito ll

Wash

Braw ey l

Santa

re iv R

Cruz

Q279

TA13

1899_USGS

NAVISKA2

L94

AF1A

AF16A

AF25A

AV13A

AV25

D296 MISSAZDT

LINE

SC25A PA4

XAVIER

L75

WR56A

WR53

WR142

B92A

A50A

THOR

WR142

B77A

B7A

C317

C22A

PHPS

D4

X419

B34

GUARD

GP15

USBR938

AF35A

H292

SA105

H301

WR16B

WR29A

AF2

F294

Q94

F424

171_82_AZHD

Magnetic

FD62

AVRA

VALLEY

Land Subsidence and Aquifer-System Compaction 19

Figure 10. Land-surface elevation change in the Tucson Active Management Area from 1998 to 2002.
Tucson ?

?

Marana

110o45’

0 5 MILES

0 5 KILOMETERS

111o15’ 111o

32o30’

15’

32o

Base from U.S. Geological Survey

digital data, 1:100,000, 1982

Universal Transverse Mercator

projection, Zone 12

Sabino

Creek

0 to 1

0 to –1

EXPLANATION

ELEVATION CHANGE FROM 2002–2005, IN INCHES:

BENCHMARK

–1 to –2

–2 to –3

BOUNDARY OF TUCSON

ACTIVE MANAGEMENT

AREA

PINAL COUNTY

PIMA COUNTY

Oro

Valley

Green Valley

R

Creek

i ito ll

Wash

Braw ey l

Santa

re iv R

Cruz

Q279

TA13

L94

AF1A

AF2

AF16A

AF25A

AV13A

AV25

D296

MISSAZDT

WR52

B92A

FD62

A50A

THOR

AF35A

H292

SA105

H301

WR16B

WR29A

Q94 AVRA

VALLEY

20 Land Subsidence and Aquifer-System Compaction in the Tucson Active Management Area, South-Central Arizona, 1987–2005

Figure 11. Land-surface elevation change in the Tucson Active Management Area from 2002 to 2005.
Marana

110o45’

0 5 MILES

0 5 KILOMETERS

111o15’ 111o

32o30’

15’

32o

Base from U.S. Geological Survey

digital data, 1:100,000, 1982

Universal Transverse Mercator

projection, Zone 12

Sabino

Creek

0 to 1

0 to –1

EXPLANATION

ELEVATION CHANGE FROM 1987–2004 AND 2005, IN INCHES:

BENCHMARK

–3 to –4

–4 to – 5

–1 to –2

–2 to –3

BOUNDARY OF TUCSON

ACTIVE MANAGEMENT

AREA

PINAL COUNTY

PIMA COUNTY

Oro

Valley

Green Valley

R

Creek

i ito ll

Wash

Braw ey l

Santa

re iv R

Cruz

Tucson

Q279

TA13

L94

AV25

D296

MISSAZDT

WR52

PHPS

GUARD

PA4

L75

WR56A

C317

WR53

D4 X419

FD62

THOR

H292

SA105

H301

A

Q94 VRA

VALLEY

Land Subsidence and Aquifer-System Compaction 21

Figure 12. Land-surface elevation change in the Tucson Active Management Area from 1987 to 2004 and 2005.
22 Land Subsidence and Aquifer-System Compaction in the Tucson Active Management Area, South-Central Arizona, 1987–2005

 Between 1987 and 2004–05, land subsidence was 

greater in Avra Valley than in Tucson Basin on the basis of the 

average subsidence for the stations that were common to the 

original 1987 GPS survey. The average total subsidence during 

the 17- to 18-year period was about 1.3 in. in the Tucson Basin 

and about 2.8 in. in Avra Valley. Three stations in the Tucson 

Basin displayed subsidence greater than 4 in. for the 17- to 

18-year period—5 in. at stations C45 and X419 and 4.1 in. at 

station PA4. In Avra Valley, two stations showed subsidence 

greater than 4 in. for the 17- to 18-year period greater than 4 

in.—4.3 in. at station AV25 and 4.8 in. at station SA105. 

On the basis of the GPS-survey data, the area with the 

greatest magnitude of subsidence in Tucson Basin is the 

northern portion of the basin bounded by the Rillito and Santa 

Cruz rivers, and in the southwestern portion of the basin 

along the Santa Cruz River. In Avra Valley, GPS-survey data 

indicate that the greatest magnitude of subsidence occurred in 

the middle of the basin near station SA105 and in the southern portion of the basin near station AV25. All Avra Valley 

stations showed cumulative subsidence for the 17- to 18-year 

period. In the Tucson Basin, station GUARD in the southeastern portion of the basin, showed a cumulative uplift of 1.1 in.; 

and station FD62, near the confluence of the Rillito and Santa 

Cruz rivers, showed a cumulative uplift of 0.4 in.

The GPS-survey data also indicate that there are several 

areas within the TAMA where the measured subsidence is 

recoverable, an indication that the history of stress on the 

granular skeleton of the aquifer system in these areas is within 

the elastic range of compaction (i.e. the preconsolidation stress 

has not been exceeded). Reversible compaction has occurred 

at stations within Tucson Basin during the periods of comparison between 1987 and 2004–2005. Station L75 showed 

uplift of 1.3 in. between 1987 and 1998, subsidence of 1.3 in. 

between 1998 and 2002, followed by 0.5 in. of uplift between 

2002 and 2004 (figs. 9–11 and table 1). Station FD62, near 

the confluence of the Rillito and Santa Cruz rivers, showed 

0.4 in. of uplift between 1987 and 1998, subsidence of 0.6 in. 

between 1998 and 2002, followed by 0.6 in. of uplift between 

2002 and 2005 (figs. 9–11 and table 1).

In Avra Valley, there also are areas where reversible 

compaction has occurred at stations during the periods of comparison between 1987 and 2004–2005. Station D296 showed 

subsidence of 2.2 in. between 1987 and 1998, uplift of 1.3 in. 

between 1998 and 2002, followed by 0.8 in. of subsidence 

between 2002 and 2005 (figs. 9–11 and table 1). In the northern portion of Avra Valley, several stations showed subsidence 

of 0 to 1 in. between 1987 and 1998, a period of uplift from 

1998 to 2002, followed by another period of subsidence of 0 to 

2 in. from 2002 to 2005 (figs. 9–11 and table 1).

Results from the aquifer-compaction monitoring at the 

network of 14 borehole extensometers, seven in Tucson Basin 

and seven in Avra Valley, are displayed in figures 7-8 and 

table 2. The extensometers provide a continuous record of 

water level and aquifer-system compaction for the part of the 

aquifer system penetrated by each well. At the seven extensometers in Tucson Basin, aquifer-system compaction from 

1989 and 2005 ranged from 0.7 in. to 4.3 in., while in Avra 

Valley, aquifer-system compaction for the same period ranged 

from 0.1 in. at TA13, to 1.7 in. at AF17 (table 2). Additionally, 

the extensometer at AF14 measured about –0.6 in. of compaction for the 1989 to 2005 period.

In Tucson Basin, the greatest cumulative aquifer-system 

compaction occurred in the northern portion of the basin 

at extensometers B76, C45, and D61 (4.3, 3.2, and 2.3 in., 

respectively). Cumulative water-level change at these stations 

for the same period was –18.8, –42.53, and –41.6 ft., respectively. These results agree with data from the GPS surveys, 

which indicate the greatest magnitude of subsidence in Tucson 

Basin is in the northern portion of the basin between the Rillito 

and Santa Cruz Rivers. 

The extensometer at C45 also is a station measured annually by using GPS-survey methods. For about the same period 

(1987 to 2005), GPS survey data indicated that station C45 

had a cumulative subsidence of 5 in., versus 3.2 in. of aquifersystem compaction measured by the extensometer from 1989 

to 2005. Thus, the data indicate that most of the subsidence 

occurring in the vicinity of station C45 is due to aquifer-system compaction within the zone measured by the extensometer. As noted previously, aquifer-system compaction measured 

at borehole extensometers generally is less than the subsidence 

measured by repeated GPS surveys for the same time period 

because extensometers measure compaction between the land 

surface and the depth at which the bottom of the extensometer 

is anchored, and repeated GPS surveys measure total land subsidence due to fluid withdrawal throughout the entire thickness 

of the aquifer system, including any portion that is below the 

level of the extensometer. 

As noted previously, aquifer-system compaction at the 

seven extensometers in Avra Valley ranged from 0.1 in. at 

station TA13 to 1.7 in. at station AF17 for the period between 

1989 and 2005 (table 2). The extensometer at station AF14 

measured a cumulative compaction of about –0.6 in. for the 

period of record. Most of this occurred from 1997 to 2005 during a corresponding increase in water level of more than 70 ft. 

At station AV25 in Avra Valley, the aquifer-system 

compaction measured by the extensometer from 1989 to 2005 

was 0.9 in. During the period of record there was a water-level 

decline of 55 ft. GPS-survey data from 1987 to 2005 indicate 

that more than1989 and 2005 4 in. of subsidence has occurred 

at station AV25. Thus, the data indicate that most of the 

subsidence occurring in the vicinity of station AV25 is due to 

aquifer-system compaction below the depth measured by the 

extensometer. 

A review of the time-series records of the extensometer 

data also show evidence of residual compaction due to delayed 

drainage of aquitards following a net increase in water level in 

the surrounding aquifer system. The equilibrium of hydraulic 

heads in the aquitards of an aquifer system typically lag head 

changes in the surrounding aquifer because of the low vertical 

hydraulic conductivity of the fine-grained silts and clays that 

make up the aquitards (Hoffmann and others, 2003). In Tucson 

Basin, the time-series data for the extensometer at well SC17 
PINAL COUNTY

PIMA COUNTY

Tucson

Marana

Oro

Valley

111o15’ 111o 110o45’

32o30’

15’

32o

Base from U.S. Geological Survey

digital data, 1:100,000, 1982

Universal Transverse Mercator

projection, Zone 12

Sabino

Creek

EXPLANATION

BOUNDARY OF TUCSON

ACTIVE MANAGEMENT

AREA

0 5 MILES

0 5 KILOMETERS

–1.1 0 1.1

Uplift

Area of uplift

Area of uplift

Area of

subsidence

Subsidence

APPROXIMATE ELEVATION CHANGE , IN INCHES:

Green Valley

R

Creek

i ito ll

Wash

Braw ey l

Santa

re iv R

Cruz

AVRA

VALLEY

InSAR data © ESA 2003-2006

Land Subsidence and Aquifer-System Compaction 23

Figure 13. Land-surface elevation change in the Tucson Active Management Area based on a 3-year, 

8-month interferogram, February 2003 to October 2006.
Name Latitude Longitude

Compaction data Water-level data

Start date End date

Land surface

compaction (ft) Total 

(in)

Start date End date

 Depth-to-water 

(ft)

Delta

Start End Start End Water level 

(ft)

Avra Valley

AF14 32.394 -111.283 1/1/1989 9/30/2005 0.008 -0.056 -0.576 6/5/1989 9/30/2005 286.20 213.80 72.40

AF17 32.394 -111.322 1/1/1989 9/30/2005 0.039 0.181 1.704 9/28/1989 9/30/2005 345.50 327.00 18.50

AV25 32.162 -111.216 1/1/1989 9/30/2005 0.032 0.105 0.876 8/8/1989 9/30/2005 351.67 406.80 -55.13

TA13 32.587 -111.303 4/6/1989 9/30/2005 0.000 0.010 0.120 7/6/1989 9/30/2005 240.20 244.30 -4.10

TA32 32.349 -111.223 4/16/1989 9/30/2005 0.000 0.070 0.840 3/16/1989 9/30/2005 351.50 338.10 13.40

TA33 32.263 -111.244 4/6/1989 9/30/2005 0.000 0.005 0.060 3/24/1989 9/30/2005 358.60 339.60 19.00

TA44 32.174 -111.168 5/4/1989 7/19/2004 0.000 0.023 0.276 4/5/1989 9/30/2005 400.60 442.00 -41.40

Tucson Basin

B76 32.183 -110.943 1/1/1989 9/30/2005 0.070 0.426 4.272 7/7/1989 9/30/2005 208.40 227.20 -18.80

C45 32.202 -110.896 1/1/1989 9/30/2005 0.094 0.357 3.156 7/7/1989 9/30/2005 278.97 321.50 -42.53

D61 32.199 -110.888 1/1/1989 9/30/2005 0.051 0.241 2.280 7/7/1989 9/30/2005 291.90 333.50 -41.60

SC17 32.094 -110.960 1/1/1989 9/30/2005 0.084 0.197 1.356 1/5/1989 9/30/2005 113.14 97.71 15.43

SC30 31.986 -110.902 1/1/1989 9/30/2005 0.065 0.204 1.668 8/8/1989 9/30/2005 213.20 220.80 -7.60

WR52 32.255 -110.955 1/1/1989 9/30/2005 0.035 0.126 1.092 7/7/1989 9/30/2005 204.68 251.50 -46.82

WR53 32.146 -110.920 1/1/1989 9/30/2005 0.008 0.065 0.684 7/7/1989 9/30/2005 146.85 152.70 -5.85

24 Land Subsidence and Aquifer-System Compaction in the Tucson Active Management Area, South-Central Arizona, 1987–2005

Table 2. Aquifer-system compaction and water-level data for the Tucson Active Management Area, 1989-2005.

shows continued compaction during a net water level rise of 

about 20 ft for the period between 2000 and 2005 (fig. 7). 

Similarly in Avra Valley, the time-series data for the extensometer at well AF17 shows continued compaction during a 

net water level rise of about 15 ft for the period between 1998 

and 2005 (fig. 8).

InSAR interferograms are a powerful mapping tool in 

the assessment and monitoring of subsidence. The method 

has been used successfully to measure and map subsidence 

and uplift of the earth’s surface (as small as a a few tenths of 

an inch) caused by aquifer-system compaction (Bawden and 

others, 2003, Galloway and others, 2000, Galloway and Hoffmann, 2007). In Arizona, the ADWR has added InSAR to its 

program of subsidence monitoring with repeated GPS surveys 

in Phoenix and Tucson with encouraging results (Brian Conway, ADWR, oral commun., 2007). Figure 13, provided by the 

ADWR, is a portion of an interferogram from February 2003 

to October 2006 that shows the same area in the TAMA where 

subsidence is monitored by using the network of GPS stations 

and borehole extensometers. 

To read the interferogram, count the number of InSAR 

fringes between two points on the interferogram, where one 

fringe is one complete color cycle (that is, blue, red, yellow, 

green, and blue). Then multiply the number of fringes by 1.1 in. 

and determine if the ground moved closer (uplift) or further 

away (subsidence) by matching how the colors change on 

the InSAR scale bar. For the area in the northern part of the 

Tucson Basin between the Rillito and Santa Cruz rivers, there 

are about 1.5 fringes or 1.65 in. of subsidence indicated by 

the interferogram. This northern area includes the GPS station 

C45. Between 2002 and 2004, the GPS surveys at station C45 

show a subsidence of 1.2 in., indicating a good correlation 

between the GPS-survey and InSAR data. InSAR technology has the potential to enhance the capability to assess and 

monitor subsidence and allow for better understanding of the 

seasonal elastic deformation in the TAMA.

Summary

The U.S. Geological Survey monitors land subsidence 

and aquifer-system compaction caused by ground-water depletion in the Tucson Basin and Avra Valley. In southern Arizona, 

ground-water pumping in excess of natural recharge is the 

primary cause of aquifer-system compaction and associated 

land subsidence. Improved scientific understanding, detection, 

and monitoring of aquifer-system compaction and land subsidence could prove useful for stakeholders concerned with the 

sustainable development of land and water resources within 

the TAMA. 

Compaction of sediments within the saturated ground-water 

system is the focus of this report; however, near-surface compaction or expansion of soils from wetting and drying has been documented as contributing to subsidence in the TAMA, particularly 

in areas that are underlain by soils with a high clay content.

In the spring of 1987, GPS-survey methods were used to 

measure horizontal and vertical positions for bench marks at 

43 sites to establish a network for monitoring land subsidence 

in the TAMA. The average subsidence in Tucson Basin during 
References Cited 25

the 11-year period from 1987 to 1998 was 0.5 in., and the 

maximum subsidence of 3.2 in. occurred in the northern part 

of the basin at station C45. In the southern portion of Tucson 

Basin, several stations showed uplift, and two stations (L75 

and GUARD) showed uplift in excess of 1.0 in. In Avra Valley, 

the average subsidence during the 11-year period was 1.2 in., 

and the maximum subsidence of 4.0 in. occurred at station 

AV25 in the southern part of the valley.

A comparison of the GPS surveys between 1998 and 

2002 indicates that up to 3.5 in. of subsidence occurred in 

Tucson Basin, and as much as 1.1 in. of subsidence occurred 

in Avra Valley. Between 2002 and 2005, the maximum subsidence of 0.2 in occurred in Tucson Basin at station WR52 in the 

northern part of the basin. In Avra Valley, the average subsidence for the 3-year period between 2002 and 2005 was about 

0.7 in.; however, three stations showed subsidence of more 

than 1.0 in. in the northern part of the valley. 

Between 1987 and 2004–05, land subsidence was greater 

in Avra Valley than in Tucson Basin on the basis of the average 

subsidence for the stations that were common to the original 

1987 GPS survey. The average total subsidence during the 17- to 

18-year period was about 1.3 in. in Tucson Basin and about 2.8 

in. in Avra Valley. Three stations in Tucson Basin displayed subsidence greater than 4.0 in. for the period—5.0 in. at stations C45 

and X419 and 4.1 in. at station PA4. In Avra Valley, two stations 

showed subsidence for the 17- to 18-year period greater than 4.0 

in.—4.3 in. at station AV25 and 4.8 in. at station SA105. 

Results from aquifer-compaction monitoring at borehole 

extensometers in Tucson Basin show that aquifer-system 

compaction ranged from 0.7 in. to 4.3 in. between 1989 and 

2005. The greatest cumulative aquifer-system compaction in 

Tucson Basin, 4.3, 3.2, and 2.3 in., occurred in the northern 

part of the basin at extensometers B76, C45, and D61, respectively. Cumulative water-level change at these stations for the 

1989–2005 period was –18.8, –42.53, and –41.6 ft., respectively. These results are in agreement with the area shown by 

the GPS surveys to have the greatest magnitude of subsidence 

in Tucson Basin. 

At the seven extensometers in Avra Valley, aquifersystem compaction ranged from 0.1 in. at TA13, to 1.7 in. at 

AF17 between 1989 and 2005. The extensometer at AF14 

measured a cumulative uplift of about 0.6 in. for the period of 

record. Most of the uplift occurred from 1997 to 2005 during 

a corresponding increase in water level of more than 70 ft. At 

AV25 in Avra Valley, the aquifer-system compaction measured 

by the extensometer from 1989 to 2005 was less than 0.9 in. 

During the period of record there was a water-level decline 

of 55 ft. The GPS surveys between 1987 and 2005 show that 

more than 4.0 in. of subsidence has occurred at AV25. The 

data indicate that most of the subsidence occurring in the 

vicinity of AV25 is due to aquifer-system compaction below 

the portion of the aquifer measured by the extensometer.

For the area in the northern part of Tucson Basin 

between the Rillito and Santa Cruz rivers, an InSAR interferogram indicates that about 1.65 in. of subsidence occurred 

between 2003 and 2006. Between 2002 and 2004, the GPS 

station at C45 in the same northern part of the basin shows a 

subsidence of 1.2 in., indicating a good correlation between 

the GPS and the InSAR data. InSAR is a tool that promises to 

enhance the capability to assess and monitor subsidence and 

allow for a better understanding of the seasonal elastic deformation in the TAMA.

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