U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet 086-00

August 2000

Desert Basins of the Southwest

GROUND-WATER RESOURCES FOR THE FUTURE

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causing unacceptable environmental,

economic, or social consequences

(Alley and others, 1999).

Occurrence of Ground Water in the 

Southwestern Basins

Much of the landscape in the southwestern United States (fig. 1) consists

of valleys separated by mountain ranges.

The mountain ranges are composed of a

variety of rocks, many of which contain

water-filled fractures, but do not yield

large quantities of water to wells. Most

of the valleys, however, are basins

containing water-filled layers of gravel,

sand, silt, and clay. Though these

sediments are as much as 10,000 feet

thick, commonly only the upper few

thousand feet of sediments are aquifers

that contain water of good quality that

can be readily extracted by wells. Water

in these aquifers occurs in small pores

between the grains of sediment (fig. 2).

The pores are completely saturated

below the water table, but are only

partially saturated above the water table

where they cannot readily release water.

water resources while looking for

supplemental sources of water.

This fact sheet reviews basic information on ground water in the desert basins

of the Southwest. Also described are

some activities of the U.S. Geological

Survey (USGS) that are providing

scientific information for sustainable

management of ground-water resources

in the Southwest. Ground-water sustainability is defined as developing and

using ground water in a way that can be

maintained for an indefinite time without

round-water resources in the Southwest are among the most overused

in the United States. Natural recharge to

aquifers is low and pumping in many

areas has resulted in lowering of water

tables. The consequences of large-scale

removal of water from storage are

becoming increasingly evident. These

consequences include land subsidence;

loss of springs, streams, wetlands and

associated habitat; and degradation of

water quality. Water managers are now

seeking better ways of managing groundFigure 1. The desert basins in the Southwest, shown in the darker brown, extend

over parts of California, Nevada, Utah, Arizona, and New Mexico.

G

Ground water is among the Nation’s most important natural resources. It provides

drinking water to urban and rural communities, supports irrigation and industry,

sustains the flow of streams and rivers, and maintains riparian and wetland

ecosystems. In many areas of the Nation, the future sustainability of ground-water

resources is at risk from over use and contamination. Because ground-water systems

typically respond slowly to human actions, a long-term perspective is needed to

manage this valuable resource. This publication is one in a series of fact sheets that

describe ground-water-resource issues across the United States, as well as some

of the activities of the U.S. Geological Survey that provide information to help

others develop, manage, and protect ground-water resources in a sustainable manner.

Water

table

Figure 2. Ground water in the basins of

the Southwest occurs in pore spaces

between grains of gravel, sand, and clay

sediments.
Figure 4. Gonzales' well, Dome Rock Mountains, Yuma County Arizona, 1917. Groundwater resources were important to travelers in the Southwest in the early 1900's.

Wetlands

Evapotranspiration

Pumping

Recharge

Stream

Bedrock Outflow to

surface water

Aquifer

Inflow from surface water

Recharge Most recharge to the aquifers occurs

at higher elevations from rainfall and

snowmelt runoff in the mountains. Water

seeps into the aquifers through sand and

gravel near the edges of the basins under

normally dry washes that flow during

infrequent periods of runoff and by subsurface flow from water-filled fractures

beneath the mountains (fig. 3).

Ground water flows from basin edges

toward lower elevations in the central

parts of the basins. Movement of ground

water is slow, with typical velocities

ranging from tens to thousands of feet

per year. Water eventually discharges

from aquifers to springs, streams,

wetlands, playas, plants, and adjacent

basins. Because of the slow movement,

ground water can be in basins for tens of

thousands of years before discharging.

Many of the basin aquifers contain

large quantities of water. For example,

in a basin in which the upper 1,000 feet

of saturated sediments contain 15

percent removable water and having an

average width and length of 15 miles

and 25 miles respectively, the volume

of water is 36 million acre-feet. That

volume of water, if it could be practically pumped, would amount to a

2,000-year supply for a population of

100,000 having a per capita water use

of 160 gallons per day.

Ground-Water Use in the Desert 

Southwest

Ground water has long been a critical

resource to humans in the Southwest.

The earliest access to ground water was

Figure 3. Hydrology of a desert basin drained by a stream. Most recharge to the

aquifer occurs at higher elevations, which receive more rainfall than lower elevations.

Ground water flows to the central part of the basin where it may discharge to springs,

wetlands, streams, or evapotranspiration. Ground-water pumping has the local effect

of removing water from storage in the aquifer and the eventual effect of reducing

outflow to surface-water features and riparian plants.

Figure 5. Annual ground-water withdrawals in Arizona, 1915–90.

2000

1910

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2

3

4

5

6

at springs and in stream channels where

it seeps to the surface. Later access was

through shallow dug wells (fig. 4), lowcapacity drilled wells, and finally,

high-capacity large-diameter drilled

wells with turbine pumps. The highcapacity deep-well turbine pumps came

into widespread use in the mid 1940’s

and resulted in large increases in

ground-water use in desert basins. For

example, ground-water use in Arizona

more than doubled from 1945–53,

when turbine pumps were installed for

irrigation and public supply (fig. 5).

Ground-water use in some parts of

the Southwest has declined as projects

to import surface water from distant

sources have been implemented. In

many areas of the Southwest, however,

surface-water supplies are limited and

ground water remains a critical

resource.

Consequences of Ground-Water 

Use

Ground-water use has the positive

aspect of providing water for human

needs including public supply, agriculture, and industry. On the other hand,

ground-water use in areas where

recharge is limited can have negative

consequences. Concerns where withdrawals exceed recharge include loss of

available ground-water supply, land

subsidence, degradation of water

quality, and loss of riparian habitat.
Loss of Available Supply

Any withdrawal of ground water

results in removal of water from storage.

Large-scale withdrawals in many areas

in the Southwest have resulted in widespread lowering of water tables (fig. 6).

The amount of the lowering reflects a

quantity of ground water that is no

longer available. Several other aspects of

lowered water tables affect future water

availability. First, lowering of water

tables results in increased costs to lift

water a greater distance. For some water

uses such as agriculture, pumping costs

from deep aquifers could be

prohibitively high. Second, lowered

water tables can result in loss of well

productivity. In most basins, shallower

sediments are less compacted and more

readily release water to wells than

deeper sediments. Lowered water tables

can result in the need to drill more and

deeper wells to maintain a desired rate of

ground-water withdrawal.

Land Subsidence

Land subsidence is the sinking of the

land surface. A major cause of land

subsidence in the Southwest is slow

drainage of water from the clay and silt

sediments in or next to aquifers

(Galloway and others, 1999). As water

levels in the aquifers decline, the slow

drainage from the clay and silt layers

cause them to compact, and the land

surface to be lowered. Uneven land

subsidence can change the slope of the

land surface and can cause earth fissures

(fig. 7), resulting in damage to buildings,

pipelines, canals, drainage ditches, roads,

railroads, dams, and bridges. Some

locations in the Southwest having significant amounts of land subsidence are:

Location Subsidence, in feet

Lancaster, California 6

Near Mendota, California 29

Las Vegas, Nevada 6

Eloy, Arizona 15

West of Phoenix, Arizona 18

Degradation of Water Quality

Many basins in the Southwest contain the highest quality water at

shallower depths. Initial development

of ground-water resources commonly

focuses on first pumping the water of

highest quality from the shallowest part

of the aquifer. Continued pumping then

can result in degradation of water

quality over time as more water of

lower quality is pumped from deeper

parts of aquifers.

Loss of Riparian Habitat

As shown in figure 3, under natural

conditions most recharge in basins

occurs at higher elevations, resulting in

ground-water flow toward discharge

points near the center of the basins.

Many of the discharge points are

springs, streams, and wetlands. The

discharge of ground water to the

surface sustains abundant plant and

animal life in otherwise harsh environments. When ground water is withdrawn by humans, the immediate effect

is the removal of water from storage in

the area of the withdrawal. However,

after continued withdrawal, the effects

in an aquifer spread from the area of

withdrawal and decrease the amount of

discharge from the aquifer (Winter and

others, 1998). Ground-water withdrawals in excess of the amount of

discharge can result in the loss of an

entire riparian system. For example, a

mesquite bosque was present along the

Santa Cruz River south of Tucson,

Arizona, in 1942 (fig. 8 on following

page, the photograph to the left). By

1989, the large riparian trees were

gone (fig. 8, the photograph to the

right). Ground-water levels in the area

declined by more than 100 feet in that

period, resulting in a loss of the

ground water that had sustained the

riparian plants.

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Figure 6. Locations in the basins of southern California, Nevada, Utah, Arizona, and

New Mexico where significant ground-water level declines have been measured. In

some areas, water levels have recovered in response to reduction in pumping and

increased recharge efforts.

EXPLANATION

More than 200

100 to 200

50 to 100

WATER-LEVEL DECLINE,

IN FEET:

Figure 7. Earth fissure related to land

subsidence, south-central Arizona.
Southwest Ground-Water 

Resources Project

The U.S. Geological Survey has a

program for continuing evaluation of the

ground-water resources of the United

States. Part of this program is the study of

major ground-water issues in different

regions of the country. In the Southwest,

the major issue under investigation is the

interaction of ground water and surface

water. The study will provide water

managers with better information on

interaction of ground water and surface

water and relations of that interaction to

sustainability of ground-water resources.

The project is focused on the desert basins

of the Southwest shown on figure 1.

Major study components include (1)

regional synthesis; (2) effects of climate

variability on recharge to and discharge

from ground-water systems; (3) effects of

development of ground-water resources

on sustainability of riparian systems; (4)

development of improved methods of

estimating recharge to desert basins; and

(5) development of improved methods of

simulating interaction of ground water and

surface water in arid and semi-arid regions.

Regional Synthesis

Hydrologic and geologic data are

collected by federal, state, and local

agencies to monitor water supply and

hydrologic conditions. Compilation of

that information and evaluation of longterm data sets are assisting in characterization of areas in terms of the

interactions of ground water and surface

water, the potential for future overdraft,

and the impact on riparian systems in the

Southwest. Land-use data also will be

used to analyze the relation between landuse changes and hydrologic conditions.

Climate Variability and Ground Water

This study is quantifying the

responses of different aquifers to

climate variations at various time

scales. For selected areas in the

Southwest, time series of precipitation,

streamflow, ground-water levels and

discharge, and tree-ring thicknesses

will be analyzed by various mathematical methods to quantify the

relations between climate variations

and ground-water resource availability.

Ground-Water Development and Riparian 

Systems

The effect of ground-water availability on riparian ecosystems is being

studied at three sites in riparian habitats

in California, Arizona, and New Mexico.

The study is using a multidisciplinary

approach centered on at least six

riparian tree species to determine the

sustainability of riparian systems with

respect to ground-water and surfacewater development in the Southwest.

Improved Methods of Estimating 

Recharge

The quantity of ground-water

recharge to alluvial basins in the

Southwest is one of the least known

factors in the water budget. Several

studies are currently being conducted to

assist in quantifying recharge estimates.

A network of four sites along streams in

California, Nevada, Arizona, and New

Mexico has been established to evaluate

methods to estimate recharge. The

methods include: geochemical analysis,

thermal tracing, Darcian flow

measurements, geophysical surveys, and

channel loss.

Figure 8. Views of the Santa Cruz River looking south from the same location on Martinez Hill, south of Tucson, Arizona. In the

1942 photograph to the left, the flood plain is covered with a mesquite bosque, valued for its wild bird population. In the 1989

photograph to the right, the bosque no longer exists.

Improved Methods of Simulating 

Interaction of Ground Water and

Surface Water 

Existing computer programs can be

used to simulate interaction of ground

water and surface water, but the methods

used are not ideal for streams that go dry

frequently or that are underlain by a

thick unsaturated zone. For use in the

desert Southwest and other arid regions,

the Southwest Ground-Water Resources

Project is developing better methods of

simulating loss of water from ephemeral

streams and movement of that water to

the underlying aquifer.

Obtaining More Information on GroundWater Resources in the Southwest 

In addition to the Southwest GroundWater Project, District Offices of the

U.S. Geological Survey in California,

Nevada, Utah, Arizona, and New

Mexico collect data and conduct studies

on ground-water resources in each state.

Links to pages on the World Wide Web

with information on District programs

and the Southwest Ground-Water

Resources Project can be found at site

http://water.usgs.gov/ogw/

Alley, W.M., Reilly, T.E., and Franke, O.L., 1999,

Sustainability of ground-water resources: U.S.

Geological Survey Circular 1186, 79 p.

Galloway, D.L. Jones, D.R., and Ingebritsen, S.E.,

1999, Land subsidence in the United States:

U.S. Geological Survey Circular 1182, 177 p.

Winter, T.C., Harvey, J.W., Franke, O.L., and

Alley, W.M., 1998, Ground water and surface

water— a single resource: U.S. Geological

Survey Circular 1139, 79 p.

References

—S.A. Leake, A.D. Konieczki, J.A.H. Rees