U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2018–3032

August 2018

Changes in Earth’s Gravity Reveal Changes in Groundwater Storage

Introduction

Newton’s Law of Universal 

Gravitation says that the acceleration 

due to gravity at any point depends on 

the surrounding mass. When the mass of 

an aquifer changes, either by recharge 

or by discharge to surface water or 

wells, the gravitational acceleration at 

the land surface also changes. Although 

this change is small, it is detectable with 

highly precise instruments. In addition 

to groundwater, gravity measurements 

are sensitive to Earth tides, barometric 

pressure changes, mass changes from 

volcanism, and other effects. Most 

of these can be accounted for, or are 

negligible, in groundwater studies. 

[Continued on page 3]

Changes in the amount of water stored in underground aquifers cause small changes in Earth’s gravitational field. The U.S. Geological Survey’s Southwest Gravity Program has developed methods for measuring terrestrial gravity 

changes with part-per-billion precision. The measurements allow scientists to map changes in groundwater storage and 

to improve models that simulate groundwater flow.

Relative-gravity meters measure differences 

in the force exerted on a mechanical spring as 

the meter is moved from place to place. These 

meters are highly portable but require extreme 

care during field surveys to minimize random drift 

and offsets in readings.

Absolute-gravity meters measure the acceleration 

of a mass falling in a vacuum, using a laser 

interferometer to measure distance and an atomic 

oscillator to measure time. They provide a direct 

measurement of the absolute force of gravity (about 

9.8 meters per second squared).

Superconducting gravity meters measure changes 

in the force required to levitate a sphere within a 

magnetic field. Because of the superconducting 

characteristic of the meters, the magnetic field is highly 

stable, and part-per-billion accuracy can be maintained 

over many years. (Inset shows detailed view.)

How is Gravity Measured?

The USGS Southwest Gravity Program uses three types of gravity meters to measure gravity change at the Earth’s surface—relative, absolute, and superconducting 

gravity meters. In addition to land-based measurements, gravity changes can also be measured by satellite. These sources provide similar data, but land-based 

measurements provide finer spatial resolution with lower accuracy than satellite measurements, which are highly accurate but provide only a single, average value 

of water-storage changes over several thousand square kilometers. (USGS photographs by Michael Landrum and Jeffrey Kennedy.)

As generalized in this image, a gravity measurement is sensitive to a cone-shaped region of the 

subsurface—as depth increases, the sensitivity to individual water molecules decreases, but 

the region of sensitivity expands. The result is that for a given height of water-storage change, 

the corresponding gravity change is the same (1 meter of water=42×10–8 meters per second 

squared), regardless of whether the water is stored near the land surface or at depth. The 

amount of gravity change is also independent of the porosity of the aquifer or soil.

Gravity meter

Unsaturated zone

Aquifer
Gravity measurements reveal storage 

changes in the aquifer underlying 

groundwater recharge basins of 

the Southern Avra Valley Storage 

and Recovery Project of the City of 

Tucson, Arizona. The map above 

shows an aerial view of three of the 

recharge basins (RB) and gravity 

change between recharge periods 

during a March to April 2013 drying 

cycle. The graphs below compare 

infiltration from the basins, gravity 

change, and groundwater-elevation 

change from spring 2012 to spring 

2013. Gravity changes were immediate 

in response to infiltration in the basin 

adjacent to each gravity meter, 

whereas groundwater levels changed 

gradually in response to infiltration in 

all nine basins at the facility, as well 

as to other hydrologic processes in 

the region. The lack of accumulated 

gravity change after 13 months of 

recharge shows that the facility is 

transmitting water efficiently through 

the unsaturated zone above the water 

table. (Modified from Kennedy and 

others, 2016a.)

Relative gravity, in microgals Meters above sea sevel

Monitoring well number

300

100

200

0

Gravity change

Groundwater level (land surface is at about 700 meters elevation)

Jul Aug Sep Oct Nov Dec

2012 2013

Mar Apr May Jun Jan Feb Mar

Meters per day

0

0.1

0.2

0.3

0.4

0.5

Infiltration from filled basins

RB206

RB207

596

598

600

602

604

606

608

610

612

207

206

14

11

15

Gravity Changes Immediately in Response to Infiltration

207

RB207

RB206

206

14

11

15

111°13'30'' 111°13'15''

32°11'

32°10'45''

32°11'15''

Study area

0

0

100 200 300

500 1,000

METERS

FEET

RB207

RB206

Gravity change, in microgals

-75

+33

0

ARIZONA Pumping well

Monitoring well

Gravity station

EXPLANATION

2
This map, overlaid on a satellite image, shows aquifer-storage change 

beneath the riparian area along the Colorado River near San Luis, 

Arizona, during the first month of a 2014 experimental “pulse flow” that 

released about 102 million cubic meters (82,700 acre-feet) of water from 

Morelos Dam on the U.S.-Mexico border into the Colorado River Delta. 

Gravity-change data were used directly and to determine specific yield to 

convert measured depth-to-groundwater change to storage change (that 

is, to convert change in hydraulic head to a thickness of free-standing 

water). The gravity-derived increase in aquifer storage, 8.4×106 cubic 

meters, agreed well with the decrease in streamflow measured between 

the two discharge measurement stations shown on the map. (Modified 

from Kennedy and others, 2016b.)

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114°50' 114°49' 114°48' 

32°32'

32°30'

32°31'

EXPLANATION

! Gravity station ! Monitoring well

Riparian area boundary

Discharge measurement station

Storage change, March 20—April 21, 2014,

in meters of water

0 1 2

Basemap: Esri, Digital Globe, GeoEye, USGS 0

0

0.5

0.5

1

1

KILOMETER

MILE

DMS-4

DMS-3a

CALIFORNIA ARIZONA

MEXICO

Gulf of 

California

Study area

Colorad o Riv er

0.25 0.5 0.75 1.25 1.5 1.75 2.25 2.5

Since the 1980s, the U.S. Geological 

Survey (USGS) has developed field 

methods, software, and analytical 

procedures for using gravity data to aid in 

hydrologic investigations. Scientists with 

the USGS Southwest Gravity Program can 

measure terrestrial gravity changes with 

part-per-billion precision to map changes 

in groundwater storage and to improve 

models that simulate groundwater flow.

Depth to Groundwater Versus 

Gravity Change

Changes in both gravity and the depth 

to groundwater indicate changes in the 

amount of water stored in an aquifer. For 

depth-to-groundwater changes, determining 

the amount of storage change depends on 

the aquifer-storage coefficient, which is 

often poorly known and must be estimated 

based on assumptions about the aquifer. 

In confined aquifers (those between two 

impermeable layers), the storage coefficient 

may be very small, causing large depth-togroundwater changes even if storage change 

is small. Whereas depth-to-groundwater 

measurements indirectly indicate aquiferstorage change, gravity change is a direct 

measurement of the change in total water 

stored in an aquifer. In most cases, the 

measured gravity change, in units of 

acceleration (for example, m/s2), can 

be converted directly to a change in the 

thickness of free-standing water, regardless 

of the depth to groundwater or the aquiferstorage coefficient.

Groundwater levels are measured at 

a single point, which may or may not be 

representative of the aquifer. In contrast, 

the region of sensitivity of a gravity 

measurement is well defined by Newton’s 

Law and encompasses a large volume of 

the subsurface. A practical guideline is that 

sensitivity extends to a radius equal to about 

10 times the depth to groundwater. 

What Are the Hydrologic 

Applications?

Gravity-change data are useful 

for estimating aquifer-storage change, 

for estimating recharge in groundwater 

systems, and for estimating the aquiferstorage coefficient (Pool, 2008). 

Aquifer-storage change can 

be estimated both qualitatively and 

quantitatively. For example, the gravitychange map on page 2 shows that 

storage changes are different to the east 

and west of the groundwater-recharge 

3
ISSN 2327-6916 (print)

ISSN 2327-6932 (online)

https://doi.org/10.3133/fs20183032

facility. This qualitative interpretation 

is possible because the gravity 

measurement’s region of sensitivity 

is well known and unaffected by 

subsurface properties. With other 

geophysical methods, such as those 

that depend on the electromagnetic 

or seismic properties of the Earth, the 

region of sensitivity changes based 

on the properties of the subsurface. 

Quantitative measurements of 

aquifer-storage change are possible 

by converting gravity change to an 

equivalent thickness of water (see 

map on page 3).

Recharge in groundwater 

systems is often unknown and must 

can be estimated if gravity and depthto-groundwater data are collected at the 

same location (see graph below).

The unique relation between 

aquifer-storage change and gravity 

change (regardless of depth to 

groundwater or storage coefficient) 

and the well-known, unchanging 

region of measurement sensitivity 

make gravity methods a powerful 

tool for hydrogeologic investigations. 

From a practical standpoint, gravity 

measurements are completely 

noninvasive and have none of the 

permitting requirements or potential 

for contamination that exist when 

drilling groundwater wells.

References Cited

Kennedy, J.R., Ferré, T.P.A., and 

Creutzfeldt, B., 2016a, Timelapse gravity data for monitoring 

and modeling artificial recharge 

through a thick unsaturated zone: 

Water Resources Research, 

v. 52, p. 7244–7261, https://doi.

org/10.1002/2016WR018770.

Kennedy, J.R., Rodríguez-Burgueño, 

J.E., and Ramírez-Hernández, J.,

2016b, Groundwater response to the

2014 pulse flow in the Colorado River

Delta: Ecological Engineering, v. 106,

p. 715–724, https://doi.org/10.1016/j.

ecoleng.2016.10.072.

be estimated as a residual component of 

the groundwater budget after accounting 

for other inflows and outflows in a 

system. However, with enough gravity 

measurements to enable interpolation, 

recharge can be estimated directly from 

changes in gravity (Pool and Anderson, 

2008).

The aquifer-storage coefficient 

(specific yield in an unconfined aquifer) 

is the relation between change in the 

volume of water per unit area (storage, 

expressed in units of length) and depth-togroundwater change. It is nearly equivalent 

to porosity in an unconfined aquifer. 

Because storage change can be estimated 

from gravity data, the storage coefficient 

Pool, D.R., 2008, The utility of gravity 

and water-level monitoring at 

alluvial aquifer wells in southern 

Arizona: Geophysics, v. 73, no. 6, 

p. WA49–WA59, https://doi.

org/10.1190/1.2980395.

Pool, D.R., and Anderson, M., 2008, 

Ground-water storage change and land 

subsidence in Tucson Basin and Avra 

Valley, southeastern Arizona, 1998–

2002: U.S. Geological Survey Scientific 

Investigations Report 2007–5275, 34 p., 

https://pubs.usgs.gov/sir/2007/5275/.

Jeffrey R. Kennedy

Edited by James W. Hendley II

Layout by Cory D. Hurd

As this graph shows, gravity data can be used to 

calculate specific yield in an unconfined aquifer. The 

specific yield is the slope of a best-fit line relating the 

change in storage (volume of water per unit area) as 

estimated by the change in gravity to the change in 

groundwater level (U.S. Geological Survey data from 

Mesilla Valley, New Mexico).

For more information contact:

azgravity@usgs.gov

or

USGS Arizona Water Science Center 

Director at dc_az@usgs.gov

https://www.usgs.gov/centers/az-water

https://answers.usgs.gov/

1-888-ASK-USGS (1-888-275-8747)

More information on the USGS 

Southwest Gravity Program (including 

a complete bibliography) is also 

available at: 

https://go.usa.gov/xqBnQ

Jan 1, 2016

Apr 4, 2016

Jan 31, 2017 Jun 10, 2016

Jun 10, 2017

Specific yield=0.22

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

-1 0 1 2

Change in water storage, in meters

(from gravity data)

Change in groundwater level, in meters

4