U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 

Detection and Measurement of 

Land Subsidence Using Global 

Positioning System and Interferometric 

Synthetic Aperture Radar, Coachella Valley, 

California,1996–98

Prepared in cooperation with the

COACHELLA VALLEY WATER DISTRICT

01–4193

By Michelle Sneed1, Marti E. Ikehara2, D.L. Galloway3, and Falk Amelung4

Sacramento, California

2001

1

 U.S. Geological Survey, Placer Hall, 6000 J Street, Sacramento, CA 95819-6129

2

 National Geodetic Survey, 1727 30th Street, MS35, Sacramento, CA 95816

3

U.S. Geological Survey, 7801 Folsom Boulevard, Suite 325, Sacramento, CA 95826

4

Hawaii Institute of Geophysics and Planetology, University of Hawaii, 2525 Correa Road, 

Honolulu, HI 96822 7417-13
U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

For addtional information write to: Copies of this report can be purchased from:

U.S. Geological Survey

Information Services

Box 25286

Federal Center

Denver, CO 80225

The use of firm, trade, and brand names in this report is for identification purposes only and 

does not constitute endorsement by the U.S. Geological Survey.

District Chief

U.S. Geological Survey

Water Resources Division

Placer Hall, Suite 2012

6000 J Street

Sacramento, California 95819-6129
Contents III

CONTENTS

Abstract.................................................................................................................................................................................. 1

Introduction ........................................................................................................................................................................... 2

Purpose and Scope....................................................................................................................................................... 4

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

Acknowledgments ....................................................................................................................................................... 6

Mechanics of Pumping-Induced Land Subsidence ............................................................................................................... 6

Global Positioning System (GPS) Surveys............................................................................................................................ 7

GPS Methods............................................................................................................................................................... 7

Land-Subsidence Monitoring Network ............................................................................................................. 7

Determination of Ellipsoid Heights................................................................................................................... 7

1996 GPS Survey..................................................................................................................................... 7

1998 GPS Survey..................................................................................................................................... 11

GPS Results ................................................................................................................................................................. 12

Ground-Water Levels and Aquifer-System Deformation ............................................................................................ 12

Interferometric Synthetic Aperture Radar (InSAR)............................................................................................................... 16

InSAR Methods ........................................................................................................................................................... 16

InSAR Results ............................................................................................................................................................. 17

Future Monitoring.................................................................................................................................................................. 24

Summary and Conclusions .................................................................................................................................................... 24

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

FIGURES

1–3. Maps showing:

1. Coachella Valley, California, and surrounding areas, and locations of four Continuously Operating

Reference Stations ............................................................................................................................................ 3

2. Generalized geology of the Coachella Valley, California ................................................................................. 5

3. Network of Global Positioning System stations and wells used to monitor vertical changes in 

land surface and ground-water levels, respectively, in the lower Coachella Valley, California........................ 8

4. Hydrographs showing water-surface elevations for selected wells near Global Positioning System (GPS)

stations in the lower Coachella Valley, California, where vertical changes exceeded the expected GPS 

measurement error of plus or minus (±) 40 millimeters (±0.13 foot) ...................................................................... 14

5. Hydrographs showing water-surface elevations for selected wells near Global Positioning System (GPS) 

stations in the lower Coachella Valley, California, where vertical changes did not exceed the expected GPS 

measurement error of plus or minus (±40) millimeters (±0.13 foot) ....................................................................... 16

6. Amplitude image processed from SAR scenes showing land-surface features, and location of selected

production wells and pumpage for 1996–98 in Coachella Valley, California .......................................................... 18

7. Interferogram showing three areas of subsidence for May 7, 1996, to September 30, 1998, in Coachella

Valley, California...................................................................................................................................................... 20

8. Interferogram showing three areas of subsidence for 10 months during the summers of 1995, 1997, and 1998, 

Coachella Valley, California ..................................................................................................................................... 22

TABLES

1. Horizontal position, elevation change prior to 1996, ellipsoid height, and ellipsoid-height change of geodetic

network monuments, and water-level change in selected wells in the lower Coachella Valley, California............ 10
IV Contents

CONVERSION FACTORS, VERTICAL DATUM, ABBREVIATIONS, AND WELL-NUMBERING SYSTEM

CONVERSION FACTORS

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

°C = (°F – 32) / 1.8.

Vertical displacements determined by InSAR and coordinates determined by Global Positioning

system (GPS) surveying generally are reported in metric units. The industry standard for GPS usage

is that field measurements and subsequent computations, including standard error determinations,

are done in the metric system. The use of dual units in this report is intended to facilitate application

of the data by maintaining the integrity of the original units of measurement for InSAR and GPS

methods.

Vertical Datum

Sea level: In this report “sea level” refers to the National Geodetic Vertical Datum of 1929—a geodetic datum 

derived from a general adjustment of the first-order level nets of the United States and Canada, formerly called Sea Level 

Datum of 1929.

Ellipsoid heights: In this report global positioning system (GPS) measurements of ellipsoid heights are based on the 

North American Datum of 1983 (NAD83).

Abbreviations:

Caltrans California Department of Transportation

CORS Continuously Operating Reference Station

CVWD Coachella Valley Water District

ERS European Earth Remote Sensing Satellite

GPS Global Positioning System

InSAR Interferometric synthetic aperture radar

NGS National Geodetic Survey

SAR Synthetic aperture radar

SCIGN Southern California Integrated GPS Network

USGS U.S. Geological Survey

Multiply By To obtain

centimeter (cm) 0.3937 inch

cubic meter (m3) 0.0008107 acre-foot 

hectare (ha) 0.003861 square mile 

kilometer (km) 0.6214 mile (mi)

square kilometer (km2) 0.3861 square mile (mi2)

meter (m) 3.281 foot (ft)

millimeter (mm) 0.03937 inch (in.)

millimeter (mm) 0.003281 foot (ft)
Contents V

Well-Numbering System

Wells are identified and numbered according to their location in the rectangular system for the

subdivision of public lands. Identification consists of the township number, north or south; the range

number, east or west; and the section number. Each section is divided into sixteen 40-acre tracts

lettered consecutively (except I and O), beginning with “A” in the northeast corner of the section and

progressing in a sinusoidal manner to “R” in the southeast corner. Within the 40-acre tract, wells are

sequential numbered in the order they are inventoried. The final letter refers to the base line and

meridian. In California, there are three base lines and meridians; Humboldt (H), Mount Diablo (M),

and San Bernardino (S). All wells in the study area are referred to the San Bernardino base line and

meridian (S). Well numbers consist of 15 characters and follow the format 006S008E02D001S. In

this report, well numbers are abbreviated and written 6S/8E-2D1. The following diagram shows how

the number for well 6S/8E-2D1 is derived.
Abstract 1

Detection and Measurement of Land Subsidence Using 

Global Positioning System and Interferometric Synthetic 

Aperture Radar, Coachella Valley, California, 1996–98

By Michelle Sneed, Marti E. Ikehara, D.L. Galloway, and Falk Amelung

ABSTRACT

Land subsidence associated with groundwater-level declines has been recognized as a 

potential problem in Coachella Valley, California. 

Since the early 1920s, ground water has been a 

major source of agricultural, municipal, and 

domestic supply in the valley, resulting in waterlevel declines as large as 15 meters (50 feet) 

through the late 1940s. In 1949, the importation of 

Colorado River water to the lower Coachella 

Valley began, resulting in a reduction in groundwater pumping and a recovery of water levels from 

the 1950s through the 1970s. Since the late 1970s, 

the demand for water in the valley has exceeded 

the deliveries of imported surface water, again 

resulting in increased pumping and ground-waterlevel declines. 

The magnitude and temporal occurrence of 

land subsidence in the lower Coachella Valley are 

not well known; data are sparse and accuracy 

varies. Also, the area is tectonically active and has 

subsided during the past several million years, 

which further complicates interpretations of the 

data. Land-surface-elevation data have been 

collected by many agencies using various methods 

and different geographic scales; because of this, 

the −150 millimeters (−0.5 foot) of subsidence 

determined for the southern parts of the valley for 

1930–96 may have a possible error of plus or 

minus (±)90 millimeters (±0.3 foot). The location, 

extent, and magnitude of vertical land-surface 

changes from 1996 to 1998 were determined using 

Global Positioning System (GPS) and 

interferometric synthetic aperture radar (InSAR) 

methods. GPS measurements for 14 monuments in 

the lower Coachella Valley indicate that the 

vertical land-surface changes from 1996 to 1998 

ranged from −13 to −67 millimeters 

± 40 millimeters (−0.04 to −0.22 foot ±0.13 foot). 

Changes at seven of the monuments exceeded the 

measurement error of ±40 millimeters 

(±0.13 foot), which indicates that small amounts of 

land subsidence occurred at these monuments 

between 1996 and 1998. Some of the water levels 

measured in wells near several of these 

monuments during 1996–98 were the lowest water 

levels in the recorded histories of the wells. The 

possible relation between the stresses caused by 

historically low water levels and the measured 

vertical changes in land surface suggests that the 

preconsolidation stress of the aquifer system may 

have been exceeded during this period and that 

subsidence may be permanent. Comparisons of 

several paired monuments and wells indicated that 

the relation between short-term ground-waterlevel changes and vertical changes in land surface 

in the lower Coachella Valley is not clearly 

defined.

Results of InSAR measurements made 

between 1996 and 1998 indicate that vertical 

changes in land surface, ranging from about −20 to 

−70 millimeters ± 5–10 millimeters (−0.07 to 

−0.23 foot ± 0.02–0.03 foot), occurred in three 

areas of the Coachella Valley—near Palm Desert, 

Indian Wells, and Lake Cahuilla. The areas of 

subsidence near Palm Desert and Indian Wells 
2 Detection and Measurement of Land Subsidence Using Global Positioning System and Interferometric Synthetic Aperture Radar, Coachella Valley, CA

coincide with areas of substantial ground-water 

production during 1996–98. The Coachella Valley 

Water District reported that they had no groundwater production wells in the Lake Cahuilla area 

but that there may be private production wells in 

the area. Production from these wells or possibly 

tectonic activity may be contributing to or causing 

the subsidence. 

The geodetic network used for the GPS 

measurements described in this report covers the 

area from the Salton Sea on the south to just 

northwest of Indio. The maps processed using 

InSAR overlap the part of the geodetic network 

west of Coachella and north of Lake Cahuilla, and 

include the Palm Desert area. Both methods of 

measuring vertical land-surface changes, GPS and 

InSAR, were used to characterize vertical landsurface changes from the Palm Desert area to the 

Salton Sea. Because InSAR produces more 

spatially detailed data over large areas, it generally 

was useful where vertical land-surface changes 

were previously unrecognized, such as the Palm 

Desert and Indian Wells areas, which are north of 

the current (1998) geodetic network. Additionally, 

the detailed spatial resolution of InSAR-generated 

maps complement the coarse spatial resolution of 

the GPS network.

INTRODUCTION 

The Coachella Valley is in the northwestern 

Colorado Desert region of southern California (fig. 1). 

The valley is the northernmost extent of the Salton 

Trough, which is the landward extension of a 

ridge/transform-fault system, the East Pacific Rise, of 

the Gulf of California. Near the end of the Miocene, the 

spreading center separating the western Farallon plate 

from the eastern Pacific plate was obliquely subducted 

under the North American continent. The modern Gulf 

of California and the Salton Trough formed about 12 

million years ago when subduction ceased and an 

inland belt of east-west extension, alkali basalt 

volcanism, and subsidence and basin sedimentation 

began. The shear zone constituting the principal 

tectonic boundary between the Pacific and North 

American plates appears to have shifted about 250 km 

(155 mi) inland into the inland belt by about 6 million 

years ago beginning the formation of the modern Gulf 

of California and the Salton Trough. As the Salton 

Trough opened, it filled with sediment derived from the 

delta of the Colorado River. The river has been building 

its delta from the east into the trough since about 5 

million years ago, and sedimentation has apparently 

kept pace with subsidence (McKibben, 1993). The 

relation between subsidence that has occurred on a 

geologic time scale and the vertical land-surface 

changes measured during this study are unknown.

Ground water has been a major source of 

agricultural, municipal, and domestic water supply in 

the valley since the early 1920s. Pumping of ground 

water resulted in ground-water-level declines as large as 

15 m (50 ft) between the early 1920s and 1949. In 1949, 

the importation of Colorado River water through the 

Coachella Branch of the All-American Canal to the 

lower Coachella Valley began. As a result of the 

imported surface-water supply, pumping of ground 

water decreased in the lower Coachella Valley and, 

from the 1950s through the 1970s, water levels in some 

wells in the lower valley recovered about 15 m (50 ft). 

Since the late 1970s, however, the demand for water in 

the lower Coachella Valley has exceeded the deliveries 

of imported surface water, pumping has increased, and 

water levels have declined. Water levels in many wells 

in the lower Coachella Valley have declined more than 

15 m (50 ft) (some more than 30 m or 100 ft) since the 

late 1970s and currently (1998) are lower than 

previously recorded levels. 

Declining water levels have the potential to 

contribute to or cause land subsidence attributed to the 

compaction of aquifer systems that consist of 

unconsolidated sediments composed of a significant 

fraction of fine-grained (silts and clays) sediments. 

Ikehara and others (1997) reported as much as

−150 mm ± 90 mm (−0.5 ft ± 0.3 ft) of measured 

vertical change in land surface in the southern parts of 

the Coachella Valley during 1930–96. Land subsidence 

can disrupt surface drainage; reduce aquifer-system 

storage; cause earth fissures; and damage wells, 

buildings, roads, and utility infrastructure. Although 

land subsidence had not been documented in the 

southern parts of the Coachella Valley prior to the report 

by Ikehara and others (1997), a large earth fissure 

discovered in 1948 about 3 km (2 mi) north of Lake 

Cahuilla may have formed in response to land 

subsidence during the earlier period of ground-waterlevel declines (1920s–1949). Subsidence-related earth 
Introduction 3

Figure 1. Coachella Valley, California, and surrounding areas, and locations of four Continuously Operating Reference Stations (CORS).
4 Detection and Measurement of Land Subsidence Using Global Positioning System and Interferometric Synthetic Aperture Radar, Coachella Valley, CA

fissures have been identified in many other groundwater basins in the western United States (Holzer, 

1984).

The Coachella Valley Water District (CVWD) is 

responsible for managing the water supply for a large 

part of the Coachella Valley (fig. 1). As part of their 

water-management strategy, the CVWD plans to 

monitor changes in land surface to determine whether 

and where land subsidence may be occurring. In 1996, 

the CVWD entered into a cooperative agreement with 

the U.S. Geological Survey (USGS) to establish a 

geodetic network of monuments for monitoring land 

subsidence in the lower Coachella Valley. 

Ikehara and others (1997) documented the 

development of the monitoring network and the 

possible existence, location, and magnitude of land 

subsidence by comparing historical leveling 

measurements with Global Positioning System (GPS) 

surveying measurements for 1996. The calculated 

changes in vertical land surface between 1996 and the 

earliest measurements at monuments in the monitoring 

network do not exceed −150 mm (−0.5 ft) (Ikehara and 

others, 1997). The range of uncertainty (±90 mm or 

±0.3 ft) is large because the historical leveling surveys 

were done at different times and sometimes by different 

agencies; furthermore, the methods used had different 

standards of accuracy and the networks were of 

different geographic scales (Ikehara and others, 1997). 

Measurement uncertainty in calculations of vertical 

land-surface changes for future GPS surveys can be as 

low as ±20 mm (±0.07 ft) if the stringent procedures 

given by Zilkoski and Frakes (1996) for the 

determination of vertical coordinates by GPS surveying 

are followed. 

Purpose and Scope

This report presents the results of comparisons 

between data collected at the monuments in the 

monitoring network during GPS surveys in 1996 and in 

1998 and between ground-water-level change data for 

1996–98 and the GPS measurements. This report also 

presents spatially detailed maps of vertical land-surface 

changes between 1996 and 1998 that were generated 

using interferometric synthetic aperture radar (InSAR). 

The maps include the area near Palm Desert to the area 

near Coachella (fig. 1). For areas where InSAR showed 

land-surface displacements, ground-water production 

was examined to determine a possible relation to 

ground-water pumping.

Description Of Study Area

The Coachella Valley is a 105-km (65-mi) long, 

northwest-trending valley in southeastern California 

(fig. 1). The valley is about 1,040 km2 (400 mi2) 

(California Department of Water Resources, 1964) and 

includes the cities and communities of Palm Springs, 

Palm Desert, Indio, and Coachella. The valley is 

bordered by the San Jacinto and Santa Rosa Mountains 

on the west, the San Bernardino and the Little San 

Bernardino Mountains on the north, the Cottonwood 

Mountains and Mecca Hills on the east, and the Salton 

Sea on the south (fig. 1). The measurements relating to 

vertical land-surface changes described in this report 

were made only in the lower Coachella Valley, an area 

from the Salton Sea to Palm Desert (fig. 1). The 

Coachella Valley is drained primarily by the 

Whitewater River, which flows into the Whitewater 

Stormwater Channel and eventually discharges into the 

Salton Sea (fig. 1). Land-surface elevations vary from 

more than 3,050 m (10,000 ft) above sea level at the 

peaks of the surrounding mountains to more than 70 m 

(230 ft) below sea level at the Salton Sea. 

The climate of the Coachella Valley floor is arid. 

Average annual rainfall ranges from 80 mm (3.15 in.) 

on the valley floor to more than 760 mm (30 in.) on the 

crests of the mountains to the west and north of the 

valley (California Department of Water Resources, 

1964). Temperatures range from about 49°C (120°F) on 

the valley floor in the summer to below 0°C (32°F) in 

the surrounding mountains in the winter.

The Coachella Valley is filled with as much as 

3,700 m (12,000 ft) of sediments; the upper 610 m 

(2,000 ft) is defined as water-bearing deposits 

(California Department of Water Resources, 1979). In 

this report, the water-bearing deposits are referred to as 

the aquifer system, which consists of a complex 

unconsolidated to partly consolidated assemblage of 

gravel, sand, silt, and clay of alluvial and lacustrine 

origins (fig. 2). Toward the southern parts of the valley, 

sediments tend to be finer grained (contain more silt and 

clay) than sediments in the northern parts owing to the 

greater depositional distance from mountain runoff and 

the lacustrine deposition from ancient Lake Cahuilla. In 

the lower Coachella Valley, the aquifer system consists 

of a near-surface, semiperched zone that is fairly 

persistent southeast of Indio, an upper aquifer, a 

confining layer, and a lower aquifer (California 

Department of Water Resources, 1964, 1979).
Introduction 5

The lower aquifer is the most productive source 

of ground water in the lower Coachella Valley. The 

lower aquifer consists of unconsolidated and partly 

consolidated silty sands and gravels with interbeds of 

silt and clay. The top of the lower aquifer is about 

91–183 m (300–600 ft) below land surface. Available 

data indicate that the lower aquifer is at least 152 m 

(500 ft) thick and may be as much as 610 m (2,000 ft) 

thick. This thick aquifer is overlaid by a confining layer 

that is 30–60 m (100–200 ft) thick. The upper aquifer 

overlies the confining layer and is similar in lithology to 

the lower aquifer, but it is only about 45–91 m 

(150–300 ft) thick. The near-surface semiperched zone 

consists of silts, clays, and fine sands and ranges from 0 

to 30 m (0 to 100 ft) in thickness, and generally is an 

effective barrier to deep percolation (California 

Department of Water Resources, 1964, 1979).

Geologic structures in the Coachella Valley have 

a marked influence on the occurrence and movement of 

ground water. Principal structural features of Coachella 

Figure 2. Generalized geology of the Coachella Valley, California. Figure modified from Tyley (1971).
6 Detection and Measurement of Land Subsidence Using Global Positioning System and Interferometric Synthetic Aperture Radar, Coachella Valley, CA

Valley are faults and associated drag and compressional 

folds. The most notable fault system is the northwesttrending San Andreas Fault Zone, which flanks the 

eastern side of the valley (fig. 2). Although movement 

within the San Andreas Fault Zone is predominantly 

right lateral (across the fault, movement is to the right), 

vertical displacement has depressed the southwest 

block (California Department of Water Resources, 

1964).

Acknowledgments

The GPS observations for the 1998 survey were 

completed by Marti E. Ikehara, employed by the U.S. 

Geological Survey at the time of the survey, and 

surveyors Daniel C. Aguirre, Luis G. Luna, Larry D. 

Martin, and Brent S. Stewart from the CVWD. 

Successful completion of the fieldwork would not have 

been possible without the support of the CVWD Chief 

Surveyor, Tim Lytsell. The interest shown and efforts of 

these colleagues are gratefully acknowledged. We also 

acknowledge the Southern California Integrated GPS 

network (SCIGN) and its sponsors, the W.M. Keck 

Foundation, the National Aeronautics and Space 

Administration, the National Science Foundation, the 

USGS, and the Southern California Earthquake Center, 

for providing data used in this study.

MECHANICS OF PUMPING-INDUCED LAND 

SUBSIDENCE

Land subsidence is known to occur in valleys 

containing aquifer systems that are, in part, made up of 

fine-grained sediments and that have undergone 

extensive ground-water development. The pore 

structure of a sedimentary aquifer system is supported 

by a combination of the granular skeleton of the aquifer 

system and the fluid pressure of the ground water that 

fills the intergranular pore space (Meinzer, 1928). 

When ground water is withdrawn in quantities that 

result in reduced pore-fluid pressures and regional 

water-level declines, more of the weight of the 

overlying sedimentary material must be supported by 

the skeleton. Loss of the fluid-pressure support 

increases the intergranular load, or effective stress, on 

the skeleton. With a change in effective stress, the 

skeleton is subject to deformation—an increase in 

effective stress causes some degree of skeletal 

compression, and a decrease in effective stress causes 

some degree of expansion. The vertical component of 

this deformation sometimes results in irreversible 

compaction of the aquifer system and land subsidence. 

An aquifer-system skeleton that consists primarily of 

fine-grained sediments, such as silt and clay, is much 

more compressible than one that consists primarily of 

coarse-grained sediments, such as sand and gravel. 

Aquifer-system deformation is elastic 

(recoverable) if the stress imposed on the skeleton is 

smaller than the previous maximum effective stress. 

The largest historical effective stress imposed on the 

aquifer system—sometimes as a result of the lowest 

ground-water level—is called the “preconsolidation 

stress.” If a stress imposed on the skeleton is greater 

than the preconsolidation stress, the pore structure 

(granular framework) of the fine-grained sediments 

undergoes rearrangement towards a configuration more 

stable at higher stress; this rearrangement results in an 

irreversible reduction of pore volume and thus in 

inelastic (largely irreversible) compaction of the aquifer 

system. Furthermore, deformation under stresses in 

excess of the preconsolidation stress is 20 to more than 

100 times greater than deformation under stresses less 

than the preconsolidation stress (Riley, 1998).

For an aquifer-system skeleton that contains an 

appreciable thickness of fine-grained sediments, a 

significant part of the total compaction occurring may 

be residual compaction (compaction that occurs in 

confining layers while heads equilibrate with heads in 

the adjacent aquifers). Depending on the thickness and 

the vertical hydraulic diffusivity of a confining layer, 

pressure equilibration—and thus compaction—lags 

behind pressure, or head, declines in the adjacent 

aquifers. The time constant of a confining layer is the 

amount of time required to achieve approximately 93 

percent of the ultimate compaction (Riley, 1969). The 

time constant is directly proportional to the length of 

the drainage path and inversely proportional to the 

hydraulic diffusivity (the ratio of vertical hydraulic 

conductivity to specific storage) of the confining layer 

(Riley, 1969; Ireland and others, 1984). Ultimate 

compaction is the total compaction that would occur if 

a given decline in head is maintained until equilibration 

is achieved. For example, the time constants of the 

alluvial basin in the San Joaquin Valley in central 

California ranged from 5 to 1,350 years and averaged 

159 years; more than −8.5 m (−28 ft) of subsidence has 

occurred in this basin (Ireland and others, 1984). For a 

more complete description of aquifer-system 
Global Positioning System (GPS) Surveys 7

compaction, see Poland (1984), and for a review and 

selected case studies of land subsidence caused by 

aquifer-system compaction in the United States, see 

Galloway and others (1999). 

GLOBAL POSITIONING SYSTEM (GPS) SURVEYS

GPS Methods

GPS is a U.S. Department of Defense satellitebased navigation system designed to provide 

continuous worldwide positioning and navigation 

capability. GPS surveying is a method used to measure 

data from satellites and Earth-based reference stations 

to accurately determine the position and ellipsoid 

height of geodetic monuments. In 1996, the USGS 

established a land-subsidence monitoring network in 

the lower Coachella Valley to determine historical landsurface-elevation changes (Ikehara and others, 1997) 

and to establish baseline values for comparisons with 

future monitoring measurements. 

Land-Subsidence Monitoring Network

The land-subsidence monitoring network 

consists of geodetic monuments used as GPS stations 

(fig. 3A). Geodetic monuments are markers that are 

anchored in the ground or to a structure and can be used 

repeatedly for precise measurements of horizontal or 

vertical positions (and changes), or both, for surveying. 

During the 1996 study, historical data for monuments in 

the lower Coachella Valley were compiled and 

reviewed to determine their geographic extent and the 

quality of the vertical-control data. Sources of these 

data include the National Geodetic Survey (NGS) 

(formerly the U.S. Coast and Geodetic Survey), the 

California Department of Transportation (Caltrans), the 

U.S. Bureau of Reclamation, and the CVWD (Ikehara 

and others, 1997). The geodetic monuments were 

visited at the beginning of this study to determine 

whether they had been damaged or destroyed and to 

reassess their suitability for GPS observations. 

The original subsidence monitoring network in 

the lower Coachella Valley, established in 1996, 

consisted of 17 geodetic monuments. The network was 

modified for the 1998 GPS survey because 2 of the 17 

monuments, D1299 Tie (D12T) and Caltrans 14.3 

Reset 1994 (C143), had been destroyed. To 

compensate, two others—G70 1928 (G70) and 

Caltrans 13.2 1986 (C132), which are near D1299 Tie 

and Caltrans 14.3 Reset 1994, respectively—were 

added to the network (table 1; fig. 3A). The spacing 

between the monuments meets the network design 

criterion established by Zilkoski and Frakes (1996) 

which requires that the distance between local network 

points does not exceed 10 km (6 mi).

Determination of Ellipsoid Heights

GPS measurements were made at the geodetic 

monuments to determine horizontal positions and 

ellipsoid heights. Ellipsoid height is the vertical 

coordinate relative to a geodetic reference system; the 

ellipsoid that closely approximates the Earth’s shape in 

the study area is the North American Datum of 1983 

(NAD83). To determine changes in ellipsoid heights, 

heights from successive GPS surveys are compared, 

and the differences in the ellipsoid heights are used to 

determine the existence, location, and magnitude of 

vertical land-surface changes. For this current study, 

vertical land-surface changes that occurred between 

GPS surveys done in 1996 and 1998 were computed by 

comparing the differences in ellipsoid heights 

determined at the geodetic monuments. The surveying 

methodology for the 1996 GPS survey and the ellipsoid 

heights determined from that survey were published in 

Ikehara and others (1997); that data, however, were 

reprocessed for this study because of technological 

advances in GPS data-processing software and 

methodology, and the ellipsoid heights were 

recomputed. This recomputation was necessary to 

minimize error in the computation of vertical landsurface changes between 1996 and 1998. The computed 

historical subsidence reported in Ikehara and others 

(1997) is not significantly affected by this 

recomputation. 

1996 GPS Survey

GPS measurements were made using six dualfrequency, half-wavelength P-code GPS receivers 

(Ashtech LD-XII and Ashtech MD-XII) and choke-ring 

antennas (Dorne-Margolin) at the 17 geodetic 

monuments between June 3 and 14, 1996, to determine 

horizontal positions and ellipsoid heights (Ikehara and 

others, 1997). The GPS surveying was done in 

accordance with version 4 of “Guidelines for 

Establishing GPS-Derived Ellipsoid Height” by 

Zilkoski and Frakes (1996) with two minor variations. 
Global Positioning System (GPS) Surveys 9

Figure 3.—Continued.
10 Detection and Measurement of Land Subsidence Using Global Positioning System and Interferometric Synthetic Aperture Radar, Coachella Valley, CA Table 1. Horizontal position, elevation change prior to 1996, ellipsoid height, and ellipsoid-height change of geodetic network monuments, and water-lev

lower Coachella Valley, California [Latitude and longitude are referenced to the North American Datum of 1983. Elevation-change data from Ikehara and others, (1997). Negative values for elevation chang

subsidence. Error for elevation change is plus or minus (±) 90 millimeters (±0.3 foot). Error for ellipsoid-height change is ±40 millimeters (±0.13 foot). Negative values fofoot; m, meter; mm, millimeter; —, no data]

1 Network control monument for 1996 survey. 2 Value is within measurement error indicating relative stability.3 At least one water-level measurement during 1996–98 was lower than any previously recorded water level.4 Network control monument for 1996 and 1998 survey. 5 Network control monument for 1998 survey.

Global 

Positioning 

System (GPS) 

code name

Monument

name Latitude Longitude 

Elevation change Measurement 

interval

Status of 

monument 

for 1998 

GPS survey

Ellipsoid height Ellipsoid-height change, 1996–98 Pro

(mm) (ft) 1996 (m) 1998 (m) (mm) (ft) 

DUNE 1 DUNEPORT 

Azimuth

33°44′46′′ 116°16′10′′ 2

−90 2

−0.3 1991–96 Suitable −17.349 −17.388 2

−39 2

−0.13 5

5

COCH 4 COACH 1931 33°44′25′′ 116°09′30′′ —— — Suitable 33.445 33.417 2

−28 2

−.09 5

55

R70R R70 Reset 1958 33°40′49′′ 116°10′26′′ 2

−60 2

−.2 1956–96 Suitable −54.386 −54.453 −67 −.22 6

6

5211 USBR 52.11 33°40′17′′ 116°06′43′′ 2

−90 2

−.3 1946–96 Suitable −32.677 −32.721 −44 −.14 6

66

54JA Ave 54 and Jackson 33°39′24′′ 116°13′00′′ —— — Suitable −46.351 −46.415 −64 −.21 6

6

CAHU 4 Lake Cahuilla 33°38′19′′ 116°16′25′′ —— — Suitable −30.750 −30.807 −56 −.19 

S753 S753 1945 33°38′13′′ 116°09′49′′ 2

−90 2

−.3 1945–96 Suitable −68.203 −68.253 −50 −.16 6

6

PAIN 4 Painted Canyon 33°36′43′′ 116°00′30′′ —— — Suitable 93.357 93.339 2

−18 2

−.06 

C143 Caltrans 14.3 Reset 

1994

33°35′54′′ 116°10′52′′ —— — Destroyed −72.045 — — — 6

7

C132 Caltrans 13.2 1986 33°34′59′′ 116°10′54′′ — — — New — −74.029 — — 6

7

K572 K572 1939 33°34′09′′ 116°05′42′′ −120 −.4 1939–96 Suitable −91.499 −91.543 −44 −.14 7

JOHN Johnson 33°33′03′′ 116°03′18′′ 2

−90 2

−.3 1991–96 Suitable −94.153 −94.178 2

−25 2

−.08 7

C101 4 Caltrans 10.1 1986 33°32′44′′ 116°09′16′′ 2

+30 2

+.1 1986–96 Suitable −50.368 −50.425 −56 −.18 7

77

K70 K70 1928 33°32′09′′ 116°00′21′′ −150 −.5 1928–96 Suitable −91.497 −91.525 2

−28 2

−.09 7

P572 P572 1939 33°31′32′′ 116°06′46′′ −150 −.5 1939–96 Suitable −91.274 −91.286 2

−13 2

−.04 7

SWC Stormwater Channel 33°31′27′′ 116°04′42′′ 2

−30 2

−.1 1967–96 Suitable −100.291 −100.313 2

−22 2

−.07 7

D12T 1 D1299 Tie 33°30′42′′ 115°55′11′′ —— — Destroyed −93.679 — — — 7

G70 5 G70 1928 33°30′27′′ 115°54′51′′ — — — New — −93.093 — — 7C427 Caltrans 4.27 1987 33°28′25′′ 116°06′27′′ 2

0 2.0 1987–96 Suitable — −73.356 — — 8

88
Global Positioning System (GPS) Surveys 11

The first variation nearly tripled the duration of the field 

measurements from those stipulated in the guidelines to 

compensate for using half-wavelength receivers rather 

than the full-wavelength GPS receivers (Ikehara and 

others, 1997). GPS measurements were made at the 17 

geodetic monuments on at least 2 different days and 

data were recorded for 2.5 to 3 hours during each 

observation period (Ikehara and others, 1997). Six of 

the 17 geodetic monuments also were network control 

stations; GPS measurements were made at these six 

stations on 3 additional days and data were recorded for 

about 6 hours during each observation period. The 

other variation from the guidelines was that singlebaseline, rather than multi-baseline, processing 

software was used for postprocessing. There are no 

known conclusive tests that permit an objective 

evaluation of the effect of using single-baseline, rather 

than multi-baseline, processing software (Craymer and 

Beck, 1992). Software used for the baseline and 

relative-positioning computations was GPSurvey 

version 2.30 (Trimble). 

Determining the ellipsoid heights of the 17 

geodetic monuments in the network involved two 

phases of relative positioning. In the first phase, the 

horizontal coordinates and ellipsoid heights for six 

Coachella Valley network control monuments—C101, 

CAHU, COCH, PAIN, D12T, and DUNE—were 

determined by processing the GPS measurements made 

at these monuments with measurements made 

simultaneously at three Continuously Operating 

Reference Stations (CORS), DHLG, PIN1, and CRFP 

in southern California (fig. 1), and by using precise 

satellite orbital data and accurate coordinates of the 

CORS produced by the NGS. The GPS observations of 

the CORS were recorded continually (at 30-second 

intervals) and archived by members of the SCIGN. The 

network control monuments were selected on the basis 

of geographic distribution; they are at the perimeters of 

the monitoring network. For the second phase, the 

positions of the six Coachella Valley network control 

monuments were considered known and held fixed, and 

the positions and ellipsoid heights for the other 

11 monuments were determined. The level of 

uncertainty for these heights is ±40 mm (±0.13 ft) at the 

95-percent confidence level. The level of uncertainty is 

given by the vertical differences, in millimeters, of 

repeat measurements. At a 95-percent confidence level, 

95 percent of repeat measurements are expected to be 

within 40 mm or less. 

1998 GPS Survey

GPS measurements were made using five dualfrequency, full-wavelength P-code GPS receivers 

(Ashtech MD-XII) and choke-ring antennas (DorneMargolin) at the 17 geodetic monuments between 

October 5 and 9, 1998, to determine horizontal 

positions and ellipsoid heights. The GPS surveying was 

done in accordance with version 4 of “Guidelines for 

Establishing GPS-Derived Ellipsoid Height” by 

Zilkoski and Frakes (1996). GPS measurements were 

made at the 17 geodetic monuments on at least 2 

different days and data were recorded for about 45 

minutes during each observation period. Five of the 17 

geodetic monuments for the 1998 GPS survey were also 

network control stations; GPS measurements were 

made at these five stations on 3 additional days and data 

were recorded for at least 4.5 hours during each 

observation period. The only variation from the 

guidelines was that single-baseline, rather than multibaseline, processing software was used for 

postprocessing. Software used for the baseline and 

relative-positioning computations was GPSurvey 

version 2.30 (Trimble). 

The determination of ellipsoid heights for the 17 

geodetic monuments in the network involved two 

phases of relative positioning. In the first phase, 

horizontal coordinates and ellipsoid heights for five 

Coachella Valley network control monuments—C101, 

CAHU, COCH, PAIN, and G70—were determined by 

processing the GPS measurements made at these 

monuments with measurements made simultaneously 

at three CORS (DHLG, PIN1, and WIDC) in southern 

California (fig. 1) and by using precise satellite orbital 

data and accurate coordinates of the CORS produced by 

the NGS. The GPS observations of the CORS were 

recorded continually (at 30-second intervals) and 

archived by members of the SCIGN. Owing to the 

destruction of one of the network control monuments, 

D12T, used in the 1996 survey, a nearby monument, 

G70, was added to the network and used as a network 

control monument for the 1998 survey. DUNE, a 

network control monument for the 1996 survey, was not 

used as a control monument for the 1998 survey 

because two nearby network control 

monuments—COCH and CAHU—provided sufficient 

control for that part of the network. For the second 

phase, the positions of the five Coachella Valley 

network control monuments were considered known 

and held fixed and the positions and ellipsoid heights 
12 Detection and Measurement of Land Subsidence Using Global Positioning System and Interferometric Synthetic Aperture Radar, Coachella Valley, CA

for the other 12 monuments were determined. The level 

of uncertainty for these ellipsoid heights is ±20 mm 

(±0.07 ft) at a 95-percent confidence level.

GPS Results

The recomputed ellipsoid heights of the 

monuments in the GPS network, which were 

reprocessed from GPS data collected in 1996, were 

compared with the ellipsoid heights of the monuments 

in the network in 1998 to determine the location and 

magnitude of vertical land-surface changes in the lower 

Coachella Valley (table 1; fig. 3B). Because of the 

destruction of two monuments during the period 

1996–98 (D12T and C143) and the poor quality (large 

discrepancies between repeat measurements) of the 

GPS data collected at monument C427 in 1996, only 14 

of the 17 ellipsoid heights determined from the 1996 

survey were suitable for comparison with the ellipsoid 

heights determined from the 1998 survey (table 1; 

fig. 3B). The ellipsoid heights determined for two new 

monuments (C132 and G70) will serve as baseline 

measurements for future surveys.

Calculated vertical land-surface changes 

between 1996 and 1998 ranged from −13 to 

−67 mm ± 40 mm (−0.04 to −0.22 ft ± 0.13 ft) at 14 

monuments (table 1, fig. 3B). The ellipsoid-height 

differences at seven of the monuments (R70R, 5211, 

54JA, CAHU, S753, K572, and C101) exceeded the 

measurement error of ±40 mm (±0.13 ft), indicating 

small amounts of land subsidence have occurred at 

these monuments between 1996 and 1998. The 

ellipsoid-height differences at the other seven 

monuments (DUNE, COCH, PAIN, JOHN, K70, P572, 

and SWC) did not exceed the measurement error of ±40 

mm (±0.13 ft), indicating relative vertical stability at 

these monuments between 1996 and 1998. Subsidence 

magnitudes generally were less at monuments in the 

southern and extreme northern parts of the network 

than at monuments in the central part. 

The ellipsoid-height differences of the seven 

monuments that exceeded the measurement error 

indicate subsidence ranging from −44 to −67 mm 

± 40 mm (−0.14 to −0.22 ft ± 0.13 ft) (table 1; fig. 3B). 

Prior to 1996, subsidence was −120 mm (−0.4 ft) at 

monument K572. However, vertical changes within 

measurement error (±90 mm) were –90 mm (–0.3 ft) at 

monuments 5211 and S753 and –60 mm (–0.2 ft) at 

monument R70R (see table 1 for the period for each 

monument). There are no historical data for monument 

54JA by which to determine subsidence prior to 1996 

(Ikehara and others, 1997). Information regarding 

vertical changes at monument CAHU prior to 1996 was 

not addressed in Ikehara and others (1997) owing to 

GPS processing procedures. 

The ellipsoid-height differences of the remaining 

seven monuments indicate vertical differences ranging 

from −13 to −39 mm ± 40 mm (−0.04 to −0.13 ft 

± 0.13 ft) (table 1, fig. 3B); all of these measurements 

are within measurement error. Prior to 1996, subsidence 

was −150 mm (−0.5 ft) at monuments K70 and P572 

(Ikehara and others, 1997). However, vertical changes 

within measurement error (±90 mm) were –90 mm 

(–0.3 ft) at monuments DUNE and JOHN and –30 mm 

(–0.1 ft) at monument SWC (table 1). Information 

regarding vertical changes at monuments COCH and 

PAIN prior to 1996 was not addressed in Ikehara and 

others (1997) owing to GPS processing procedures. 

Ground-Water Levels and Aquifer-System 

Deformation

Water-level-change data for wells near the 

monuments were compared with data for monuments 

with ellipsoid-height changes near or larger than the 

GPS measurement error; monument CAHU was 

excluded from the comparisons because water-level 

data for the nearby wells were unavailable. Results of 

these comparisons indicated that the relation between 

ground-water-level changes and subsidence between 

1996–98 in the lower Coachella Valley is not clearly 

defined (table 1, fig. 3B); stresses on the aquifer system, 

other than those caused by fluctuating ground-water 

levels (such as tectonic stresses), may be contributing to 

vertical land-surface changes. A possible relation 

between preconsolidation stresses and prevailing 

stresses during the period 1996–98 was evaluated for 

this study by examining ground-water-level changes. 

As discussed earlier, pumping-induced 

subsidence may be elastic (recoverable) or inelastic 

(permanent), depending on whether the effective 

stresses are greater or less than the preconsolidation 

stresses. Because the 1998 survey was done near the 

end of the heavy pumping season in the Coachella 

Valley, water levels probably were near their lowest 

level for that year and thus potential seasonal 

subsidence probably was near the maximum. If 

effective stresses at the time of the survey were less than 

preconsolidation stresses (that is, if ground-water levels 

were higher than historical lows), some of the 
Global Positioning System (GPS) Surveys 13

subsidence measured may be recoverable as water 

levels rise. Conversely, if stresses were greater than the 

preconsolidation stresses at the time of the survey, most 

of the subsidence measured probably is permanent and 

will not recover even when water levels rise. The 

amount of elastic deformation that may have occurred 

during 1996–98 cannot be determined with the small 

number of measurements of vertical land-surface 

change and ground-water-level change. For continuity 

with the 1996 study, the wells used in this report are the 

same wells that were used in the report by Ikehara and 

others (1997). 

At the regional scale, declining ground-water 

levels generally were in the northern and western parts 

of the geodetic network, and recovering water levels 

generally were in the eastern part of the network 

(fig. 3B). A comparison of current (1998) water-level 

data with historical data indicated that many of the 

wells were experiencing the lowest water levels in their 

measurement history (table 1; figs. 4 and 5). 

A small amount of vertical change, –44 mm 

± 40 mm (–0.14 ft ± 0.13 ft), was measured at 

monument 5211; however, water-level data analyzed 

for three wells near 5211 indicate that the average water 

levels for 1996–98 increased in wells 6S/8E-2D1 and 

12Q1 and decreased in well 6S/8E-2F1 (table 1; figs. 3

and 4). The annual average water levels for well 

6S/8E-2F1 for 1996, 1997, and 1998 were lower than 

any previous measurement. If the small amount of 

vertical change at monument 5211 is related to waterlevel changes in the aquifer system, then the water-level 

decreases in well 6S/8E-2F1, which is screened in both 

the upper and the lower aquifers, have a more 

significant effect on aquifer-system deformation than 

do water-level increases in wells 6S/8E-2D1 and 12Q1, 

which are screened in the upper aquifer. 

At monuments S753 and 54JA, subsidence was 

–50 mm (–0.16 ft) and –64 mm (–0.21 ft), respectively; 

however, water-level data for wells near these 

monuments indicate that the annual average water 

levels during 1996–98 increased in well 6S/8E-27C1 

and decreased in wells 6S/8E-19D1 and 6S/7E-23D3 

(table 1; figs. 3B and 4). The annual average water 

levels for well 6S/8E-19D1 for 1996, 1997, and 1998 

and for well 6S/7E-23D3 for 1997 and 1998 were lower 

than any previous measurements. If the subsidence at 

monuments S753 and 54JA is related to water-level 

changes in the aquifer system, then the water-level 

decreases in well 6S/8E-19D1, which is screened in the 

lower aquifer, and in well 6S/7E-23D3, which is 

screened in the upper aquifer, have a more significant 

effect on aquifer-system deformation than do the waterlevel increases in well 6S/8E-27C1, which is screened 

in both the upper and lower aquifers. 

Subsidence also was measured at monument 

R70R (–67 mm or –0.22 ft) and a small vertical change 

in land surface was measured at monument K572 

(–44 mm ± 40 mm or –0.14 ft ± 0.13 ft). The annual 

average water levels increased slightly during 1996–98 

for well 6S/8E-5R2 (screened in the upper aquifer), 

which is near monument R70R (table 1; figs. 3B and 4). 

Similarly, the annual average water level in well 7S/9E7H2 (screened in the upper aquifer), which is near 

monument K572, increased between 1996 and 1998 

(table 1; figs. 3B and 4). These water-level increases 

suggest that the subsidence measured at these 

monuments is not related to concurrent water-level 

increases in these wells but may have been caused by 

residual compaction—compaction that occurs in 

response to past water-level declines. Between 1965 

and 1995, the water levels in well 7S/9E-7H2 declined 

about 23 m (75 ft) but the water levels in well 

6S/8E-5R2 were fairly stable (fig. 4). Water-level data 

are not available for the lower aquifer near monuments 

K572 or R70R to help explain the observed subsidence. 

At monument C101, –56 mm (–0.18 ft) of 

subsidence was measured. Water-level data for three 

nearby wells (7S/8E-20H1, 22K1, and 28G1) indicate 

that the levels declined during 1996–98 (table 1; 

figs. 3B and 4). The annual average water levels for 

1996, 1997, and 1998 for wells 7S/8E-20H1 and 28G1, 

which are screened in the upper aquifer, were lower 

than any previous measurements, but the water levels in 

well 7S/8E-22K1, which is screened in both the upper 

and lower aquifers, had been lower prior to 1996. These 

data suggest that water-level declines in the aquifer 

system may be causing subsidence in this area.

From the preceding discussion, it is apparent that 

the relation between ground-water-level changes and 

vertical changes in land surface between 1996–98 in the 

lower Coachella Valley is not clearly defined (table 1, 

fig. 3B). At some monuments, such as C101, GPS 

measurements indicated subsidence and data from 

nearby wells indicated water-level declines, although 

these were small compared with historical changes 

(table 1; figs. 3B and 4). At other monuments, such as 

S753 and 54JA, GPS measurements indicated 

subsidence and data from nearby wells indicated that 
14 Detection and Measurement of Land Subsidence Using Global Positioning System and Interferometric Synthetic Aperture Radar, Coachella Valley, CA

water levels for the same general period increased in 

some wells adjacent to each monument and decreased 

in others (table 1; figs. 3B and 4). Changes in vertical 

position were negligible at the DUNE monument, and 

data from nearby well 5S/7E-9F1 indicate that current 

(1998) water levels were lower than historical lows, 

whereas water levels in wells 7S/9E-23N1 and 7S/9E30M1 were recovering near the K70 and SWC 

monuments, which also were stable monuments during 

1996–98 (table 1; figs. 3B and 5). Assuming that waterlevel and vertical land-surface changes are related, 

these results suggest that water-level changes in some 

thicknesses of sediments have affected the 

aquifer-system skeleton more than water-level changes 

in others. Furthermore, the lag in confining layer 

deformation that occurs in response to head changes in 

the adjacent aquifers may be confounding 

discrimination of the relation. Because wells typically 

are constructed with several hundred feet of screen to 

maximize production, water-level measurements for 

these wells reflect a large thickness of 

sediments—often for more than one aquifer—rather 

than specific water-level measurements for discrete 

intervals of sediments. Data from depth-dependent or 

aquifer-dependent monitoring wells are needed to 

better define the relation. 

Figure 4. Water-surface elevations for selected wells near Global Positioning System (GPS) stations in the lower Coachella Valley, 

California, where vertical changes exceeded the expected GPS measurement error of plus or minus (±) 40 millimeters (±0.13 foot). 

Numbers in parenthesis are GPS code names. See figure 3A for locations of GPS stations and wells.
Global Positioning System (GPS) Surveys 15

Because of the varying relations between 

geographically paired measurements of vertical landsurface and ground-water-level changes, other 

processes may be causing vertical land-surface changes 

and must be considered. The consolidation of 

sedimentary deposits on geologic time scales and (or) 

tectonic movement [subsidence of the Salton Trough or 

other movement along one or more faults in the lower 

Coachella Valley, including the San Andreas Fault Zone 

(figs. 2 and 3B)] could be causing the subsidence or it 

could be masking or amplifying the aquifer-system 

deformation (compaction or expansion) caused by 

water-level fluctuations or other processes. 

As discussed in the “Mechanics of PumpingInduced Land Subsidence” section, compaction of 

confining layers caused by ground-water-level declines 

does not occur instantaneously. Residual compaction 

may be a significant part of the total compaction, 

particularly if there is a significant thickness of silt and 

clay deposits. Residual compaction may occur for 

decades or even centuries (determined by the time 

constant) after ground-water levels cease to decline 

(Riley, 1998). In the lower Coachella Valley, 

geophysical logs of at least three wells indicate the 

existence of thick silt and clay deposits (Ikehara and 

others, 1997). Residual compaction, therefore, may be 

responsible for some subsidence during 1996–98, such 

as that at monument R70R where subsidence was 

measured and where water-level-changes in nearby 

wells were small (table 1; figs. 3B and 4). 

Figure 4.—Continued.
16 Detection and Measurement of Land Subsidence Using Global Positioning System and Interferometric Synthetic Aperture Radar, Coachella Valley, CA

INTERFEROMETRIC SYNTHETIC APERTURE 

RADAR (INSAR)

InSAR Methods

InSAR is a powerful technique that uses 

differences in reflected radar signals acquired at 

different times to measure ground-surface deformation. 

This technique has been used to investigate 

deformation resulting from earthquakes (Massonnet 

and others, 1993), volcanoes (Massonnet and others, 

1995), and land subsidence (Massonnet and others, 

1997; Fielding and others, 1998; Galloway and others, 

1998; Amelung and others, 1999; Hoffmann and others, 

2001). Four differential interferograms were developed 

for this study using synthetic aperture radar (SAR) 

scenes (Track 127, Frame 2925) acquired for a part of 

Coachella Valley by the European Earth RemoteSensing (ERS) satellites. Using a three-pass method 

(Zebker and others, 1994), two SAR scenes that had 

nearly identical acquisition geometries (orbital 

perpendicular baseline = 43 m or 141 ft) were 

combined to form one “change” interferogram with a 

temporal baseline of 876 days from May 7, 1996, to 

September 30, 1998 (figs. 6 and 7). Additionally, three 

pairs of SAR scenes were combined to form three 

change interferograms with temporal baselines of about 

5, 2, and 3 months for the summers of 1995, 1997, and 

1998, respectively. These three interferograms were 

combined (“stacked”) to form one change 

interferogram representing about 10 summer months of 

temporally discontinuous data (fig. 8). 

The amplitude component of a change 

interferogram (fig. 6) shows the intensity of the radar 

backscatter and reveals features of the land surface such 

as mountains, roads, drainage ways, and engineered 

structures. The phase component (figs. 7 and 8) of a 

change interferogram contains information about the 

coherent displacements imaged by the radar, as well as 

information about the topography. The topography was 

removed using a 1-day interferogram formed by 

combining the May 7, 1996, scene with a third scene 

acquired May 8, 1996; however, all of the topographic 

component could not be removed for regions of steep 

topography (shown as the black regions in figures 7 and 

8). 

For landscapes with more or less stable radar 

reflectors (such as buildings or other engineered 

structures, or undisturbed rocks and ground surfaces) 

over a period of time, it is possible to make highprecision measurements (±5–10 mm or ±0.02–0.03 ft) 

of the changes in the position, or the displacements, of 

the reflectors caused by deforming aquifer systems 

(Galloway and others, 1998; 2000). The displacements 

are computed from the phase change at each point in the 

interferogram. The phase change is determined by 

subtracting or “interfering” two radar scans made of the 

same area at different times (Galloway and others, 

2000). 

Because the phase of the radar echo is 

proportional to the distance traveled by the pulse, any 

motion of the ground surface between the two SAR 

scenes causes a phase difference in the interferogram. 

Figure 5. Water-surface elevations for selected wells near Global 

Positioning System (GPS) stations in the lower Coachella Valley, 

California, where vertical changes did not exceed the expected GPS 

measurement error of plus or minus (±) 40 millimeters (±0.13 foot). 

Numbers in parenthesis are GPS code names. See figure 3A for 

locations of GPS stations and wells.
Interferometric Synthetic Aperture Radar (InSAR) 17

Phase differences also can be caused by propagation 

delays of the radar signal, such as delays owing to the 

variable water vapor content in the atmosphere (Zebker 

and others, 1997). Atmospheric artifacts are not evident 

in the change interferograms (figs. 7 and 8); therefore, 

the coherent phase difference was attributed to range 

displacements of the ground surface, which we 

assumed are vertical. On the phase images (figs. 7 and 

8), an area of coherent displacements—for example, an 

area of subsidence—is shown by color fringes that 

define a shape (see trace of subsidence signal in box 1 

in figure 7). More color fringes indicate more change; 

for figures 7 and 8, each color cycle—for example, blue 

to blue—indicates 28 mm (0.09 ft) of change in the 

range component. The direction of 

change—subsidence or uplift—is indicated by the 

color progression of the fringes toward the center of the 

polygon, which is defined in a circular scale on each 

interferogram. 

For landscapes without substantial stable radar 

reflectors, such as agricultural land where farming 

practices disturb the ground surface, high-precision 

measurements over fairly long periods cannot be made 

using InSAR. The phase image will not show the color 

fringes that define a shape, but instead, the colors will 

have an incoherent speckled pattern, as does the image 

of the area south of Coachella in figure 7. Developing 

images from shorter temporal baselines (fig. 8), 

however, often will increase image coherency owing to 

potentially fewer ground-water disturbances during a 

shorter time period.

InSAR Results

The InSAR-generated maps of the Coachella 

Valley for the period May 7, 1996, to September 30, 

1998, (fig. 7) and for 10 summer months during 1995, 

1997, and 1998 (fig. 8) show at least three areas of 

coherent signals indicating subsidence. These signals 

are located in the Palm Desert area, the Indian Wells 

area, and near Lake Cahuilla. For the following 

discussion, subsidence magnitudes and areal extents 

are taken from the interferogram representing 1996–98 

(fig. 7) and not the interferogram representing 10 

months of temporally discontinuous data (fig. 8).

The geographically largest and most prominent 

subsidence signal is located in the northwestern part of 

Palm Desert (box 1 in figs. 6, 7, and 8). The signal is 

approximately bounded by Country Club Drive on the 

north, Fred Waring Drive on the south, Highway 111 

and the San Jacinto and Santa Rosa Mountains on the 

west, and Portola Avenue on the east (fig. 6). The San 

Jacinto and Santa Rosa Mountains (consolidated rock) 

may act as a barrier to subsidence farther to the 

southwest (fig. 2), but there is no ready explanation 

regarding control of the extent of subsidence to the 

north and east. The signal is about 4 km (2.5 mi) in 

length in the northwest direction, about 3.2 km (2 mi) 

long in the northeast direction, and about 13 km2 (5 mi2

) 

(fig. 7). The signal can be separated into two parts based 

on magnitude and shape. The largest part of the 

signal—about –70 mm (–0.23 ft) of subsidence from 

the edge to the center of the signal—is in the 

southwestern part of the entire signal; it is circular and 

about 1.6 km (1 mi) in diameter (fig. 7). North and east 

of the circular signal is a larger (areal extent) and more 

irregularly shaped signal that indicates a maximum of 

about –30 mm (–0.1 ft) of subsidence (fig. 7). The 

10-month interferogram also shows this subsidence 

signal (fig. 8). 

The area of the aforementioned subsidence signal 

coincides with an area of substantial ground-water 

development. Within about 1.6 km (1 mi) of the 

subsidence signal are more than 70 production wells (in 

and near box 1 in fig. 6) that produced about 

170,000 acre-ft (Steve Robbins, Coachella Valley Water 

District, written commun., 2000) during 1996–98. The 

coincident geographic area of the subsidence signal and 

the production wells strongly suggests a relation 

between subsidence and ground-water withdrawals. 

The second area of subsidence is about 5 km 

(3 mi) east-southeast of the aforementioned subsidence 

signal (box 2 in figs. 6, 7, and 8). This area consists of 

two distinct subsidence bowls separated by a part of a 

golf course in Indian Wells. The subsidence bowl on the 

west is approximately bounded by Highway 111 on the 

north, the San Jacinto and Santa Rosa Mountains on the 

south, El Dorado Drive on the west, and a golf course 

on the east (fig. 6). The eastern subsidence bowl is 

approximately bounded by Highway 111 on the north, 

the San Jacinto and Santa Rosa Mountains on the south, 

a golf course on the west, and Manitou Drive on the east 

(fig. 6). As is suggested for the first subsidence signal 

discussed in this section, the San Jacinto and Santa 

Rosa Mountains may act as a barrier to extension of the 

subsidence bowls farther to the south (fig. 2), but there 

is no ready explanation regarding control of the extent 

of subsidence to the north, west, or east. The subsidence 

bowls are nearly equivalent in area (about 0.5 km2

 or 
18 Detection and Measurement of Land Subsidence Using Global Positioning System and Interferometric Synthetic Aperture Radar, Coachella Valley, CA

Figure 6. Amplitude image processed from SAR scenes showing land-surface features, and location of selected production wells and 

pumpage for 1996–98 in Coachella Valley, California. Boxes indicate areas of subsidence (see figs. 7 and 8).
Interferometric Synthetic Aperture Radar (InSAR) 19

Figure 6. —Continued.
20 Detection and Measurement of Land Subsidence Using Global Positioning System and Interferometric Synthetic Aperture Radar, Coachella Valley, CA

Figure 7. Interferogram showing three areas of subsidence for May 7, 1996, to September 30, 1998, in Coachella Valley, California. Boxes 

indicate areas of subsidence (mm, millimeter).
Interferometric Synthetic Aperture Radar (InSAR) 21

Figure 7.—Continued.
Figure 8. Interferogram showing three areas of subsidence for 10 months during the summers of 1995, 1997, and 1998, Coachella Valley, 

California. (mm, millimeter)

Whitewater River

Palm Desert area

33 40'

45'

1

N

33 50'

116 30'

25'

116 20'

10

111

0

7

-14

- 7

14

21

- 21

Subsidence

Uplift

displacement,

in mm 

22 Detection and Measurement of Land Subsidence Using Global Positioning System and Interferometric Synthetic Aperture Radar, Coachella Valley, CA
Interferometric Synthetic Aperture Radar (InSAR) 23

Figure 8.—Continued.
24 Detection and Measurement of Land Subsidence Using Global Positioning System and Interferometric Synthetic Aperture Radar, Coachella Valley, CA

0.2 mi2) and magnitude (about −40 mm or −0.13 ft) in 

the areas of maximum signal (fig. 7). The bowl to the 

west has a very coherent signal and is nearly concentric 

with a slight northwest elongation. The bowl to the east 

also has a substantially coherent signal, but it is less 

concentric than the signal to the proximate west, 

showing an east-west elongation (fig. 7). The 10-month 

interferogram also shows this subsidence signal (fig. 8).

The area showing the pair of subsidence bowls 

coincides with an area of significant ground-water 

development. Within about 1.6 km (1 mi) of the 

subsidence signals, a total of about 50,000 acre-ft of 

water was pumped from more than 20 production wells 

during 1996–98 (Steve Robbins, Coachella Valley 

Water District, written commun., 2000) (in and near 

box 2 in fig. 6). The coincident geographic area of the 

subsidence signal and production wells strongly 

suggests a relation between subsidence and groundwater withdrawals. 

The third area of subsidence is about 1.6 km 

(1 mi) southeast of Lake Cahuilla (box 3 in figs. 6, 7, 

and 8). The subsidence signal is elongate in the 

northeast direction, is about 0.5 km2 (0.2 mi2

), and has 

a maximum subsidence value of about −20 mm 

(−0.07 ft) (box 3 in fig. 7). The signal from the 

interferogram for May 7, 1996, to September 30, 1998, 

is a coherent and spatially closed signal which indicates 

that this area is subsiding; the 10-month interferogram 

(fig. 8) shows that this signal probably extends farther 

to the north than is shown in figure 7. The CVWD 

reported that there are no CVWD production wells in 

the vicinity of this subsidence signal (Steve Robbins, 

Coachella Valley Water District, written commun., 

2000). Visual examination of the area revealed that the 

area is rather barren and surrounded by date orchards. 

There may be non-CVWD wells in and near this area, 

but their location and production volumes are 

unknown. Future investigations in this area are 

necessary to determine the cause or causes of this 

subsidence signal.

The 10-month interferogram shows that other 

areas in the Coachella Valley may be subsiding (fig. 8). 

These areas generally are small and the amount of 

subsidence is small. Future investigations using 

interferometry may reveal additional areas of 

subsidence.

FUTURE MONITORING

Continued monitoring in the lower Coachella 

valley is warranted because, in some areas, groundwater levels continue to decline and therefore the small 

amounts of pumping-induced land subsidence 

documented by this study are likely to increase. 

Because the measured amounts of vertical land-surface 

changes during 1996–98 were small compared to the 

expected GPS measurement error, future GPS surveys 

could be done less frequently than biannually. An 

interval of 3 to 5 years between surveys probably would 

be appropriate to detect any measurable changes in the 

vertical position of monuments significantly larger than 

the expected GPS measurement error. Meanwhile, 

spatially detailed InSAR-derived maps of ground 

displacements could be processed annually (depending 

on availability of data). Detection of small changes 

(±5 mm or ±0.02 ft) in the vertical position of land 

surface is possible and has been achieved elsewhere 

(Hoffmann and others, 2001). Multiple InSAR maps 

processed for different periods within a single year may 

be useful in estimating the elastic component of vertical 

deformation. The GPS network could be modified for 

future land-based surveys to include areas where the 

InSAR maps indicate previously unrecognized areas of 

vertical land-surface changes, such as the Palm Desert 

and Indian Wells areas. Data from future monitoring of 

the modified GPS network and information from the 

InSAR maps could be used to provide ground truth for 

the more spatially detailed and potentially higher 

resolution InSAR measurements.

SUMMARY AND CONCLUSIONS

In the Coachella Valley, ground water has been a 

major source of agricultural, municipal, and domestic 

water supplies since the early 1920s. Ground-water 

levels declined throughout the valley from the 1920s 

until 1949, the year that the lower Coachella Valley 

began importing surface water. The importation of 

water resulted in decreased pumping and a recovering 

of water levels in some areas. In the 1970s, the demand 

for water in the lower Coachella Valley exceeded the 

deliveries of imported surface water, and ground-water 

levels began declining again. These observed waterlevel declines have the potential to induce new or 

renewed land subsidence in the Coachella Valley. 

Results of a previous study by the U.S. Geological 

Survey (Ikehara and others, 1997) indicate that land 
References Cited 25

subsidence may have been as much as about −150 mm 

(−0.5 ft) in the southern parts of the valley during about 

1930–96.

The location, extent, and magnitude of vertical 

land-surface changes during 1996–98 were determined 

using Global Positioning System (GPS) and 

interferometric synthetic aperture radar (InSAR) 

techniques. A GPS survey was done in the lower 

Coachella Valley, and InSAR was applied to the Palm 

Desert and the lower Coachella Valley. InSAR 

generally was useful in areas where vertical landsurface changes were previously unrecognized.

GPS results revealed that small amounts of 

subsidence occurred in some parts of the lower 

Coachella Valley during 1996–98. GPS measurements 

for 14 monuments in the GPS network located in the 

lower Coachella Valley indicated vertical land-surface 

changes, ranging from −13 to −67 mm ± 40 mm (−0.04 

to −0.22 ft ± 0.13 ft), over the 2-year period. The 

vertical land-surface changes at seven of the 

monuments exceeded the measurement error of ±40 

mm (±0.13 ft), indicating that small amounts of land 

subsidence occurred at these monuments between 1996 

and 1998. In wells near several of these monuments, 

water levels during 1996–98 were the lowest in their 

measurement history. Although the magnitude of 

subsidence is small, the relation between subsidence 

and record-low water levels in many of the wells 

measured suggests that the preconsolidation stress of 

the aquifer system may have been exceeded during this 

period and that compaction may be permanent in these 

areas. In some areas, the observed subsidence may have 

been caused by residual compaction—compaction that 

occurs in response to past water-level declines. The 

relation between the ground-water-level changes and 

vertical changes in land surface during 1996–98 is not 

clearly defined. Data from depth-dependent or aquiferdependent monitoring wells are needed to better define 

the relation.

Interferograms, which were developed using 

InSAR, revealed that land subsidence occurred in at 

least three areas of the Coachella Valley during 

1996–98, ranging in magnitude from about −20 to −70 

mm ± 5–10 mm (–0.07 to –0.23 ft ±0.02–0.03 ft). These 

areas are near Palm Desert, Indian Wells, and Lake 

Cahuilla. The subsidence areas were the largest in 

geographic extent and in magnitude in the Palm Desert 

and Indian Wells areas. Both of these areas correspond 

to areas of significant ground-water development, as 

indicated by the number of production wells and the 

volumes of water produced during 1996–98. The 

subsidence near Lake Cahuilla does not seem to be 

associated with CVWD ground-water production. 

Future studies are needed to determine the cause of this 

subsidence.

This study supports part of the watermanagement strategy of CVWD to monitor changes in 

land surface to determine whether and where land 

subsidence may be occurring. Continued monitoring in 

Coachella Valley is needed because ground-water levels 

are continuing to decline—some are lower than any 

previously recorded levels—and land subsidence is 

continuing as indicated by the small amounts of land 

subsidence documented during this study.

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