U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2008–5239

Geospatial Data to Support Analysis of WaterQuality Conditions in Basin-Fill Aquifers in the 

Southwestern United States

National Water-Quality Assessment Program

Geospatial Data to Support Analysis of 

Water-Quality Conditions in Basin-Fill 

Aquifers in the Southwestern United 

States

By Tim S. McKinney and David W. Anning

Scientific Investigations Report 2008–5239

U.S. Department of the Interior

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

KEN SALAZAR, Secretary 

U.S. Geological Survey 

Suzette M. Kimball, Acting Director 

U.S. Geological Survey, Reston, Virginia: 2009 

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Suggested citation:

McKinney, T.S., and Anning, D.W., 2009, Geospatial data to support analysis of water-quality conditions in basin-fill 

aquifers in the southwestern United States: U.S. Geological Survey Scientific Investigations Report 2008-5239, 16 p. 

Available at http:// pubs.er.usgs.gov/sir/2008/5239. 
iii

Contents

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

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

Regional Analysis ............................................................................................................................... 3

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

Data Compilation and Processing ............................................................................................................. 3

Hydrogeologic Area Boundaries ............................................................................................................... 3

Elevation ........................................................................................................................................................ 6

Geology .......................................................................................................................................................... 6

Land Use ........................................................................................................................................................ 6

Precipitation .................................................................................................................................................. 6

Population ..................................................................................................................................................... 6

Water Use ..................................................................................................................................................... 9

Public-Supply Water Use .................................................................................................................. 9

Irrigated Agriculture Water Use ....................................................................................................... 9

Spatial Statistics for Hydrogeologic Areas and Alluvial Basins ........................................................ 12

Summary ...................................................................................................................................................... 12

References Cited ........................................................................................................................................ 12

Figures

1. Map showing basin-fill aquifers in the Southwest Principal Aquifers study area ....... 2

2. Map showing basins studied as part of the National Water-Quality Assessment 

(NAWQA) program in the Southwest Principal Aquifers study area ........................... 4

3. Map showing hydrogeologic areas in the Southwest Principal Aquifers study 

area ......................................................................................................................................... 5

4. Map showing generalized surficial geology for the Southwest Principal Aquifers 

study area............................................................................................................................... 7

5. Map showing average annual precipitation data generated by the ParameterElevation Regressions on Independent Slopes Model (PRISM) for the Southwest 

Principal Aquifers study area, 1971–2000 ......................................................................... 8

6. Map showing public-supply water use in urban areas in the Southwest Principal 

Aquifers study area, 2005 .................................................................................................. 10

7. Map showing estimated agricultural irrigation in the Southwest Principal Aquifers 

study area, 2000................................................................................................................... 11

Table

1. Example of spatial statistics derived for the National Water-Quality Assessment 

(NAWQA) program case-study basins in the Southwest Principal Aquifers 

study area .............................................................................................................................13
iv

Conversion Factors and Datums 

Multiply By To obtain

Length

mile (mi) 1.609 kilometer (km)

Area

acre 4,047 square meter (m2)

acre 0.004047 square kilometer (km2)

square mile (mi2) 2.590 square kilometer (km2)

Volume

acre-foot (acre-ft) 1,233,489 liter (L)

gallon (gal) 3.785 liter (L)

Flow rate

gallon per day (gal/d) 0.003785 cubic meter per day (m3

/d)

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88). 
Geospatial Data to Support Analysis of Water-Quality 

Conditions in Basin-Fill Aquifers in the Southwestern 

United States 

By Tim S. McKinney and David W. Anning

Abstract

The Southwest Principal Aquifers study area consists of 

most of California and Nevada and parts of Utah, Arizona, 

New Mexico, and Colorado; it is about 409,000 square miles. 

The Basin-fill aquifers extend through about 201,000 square 

miles of the study area and are the primary source of water for 

cities and agricultural communities in basins in the arid and 

semiarid southwestern United States (Southwest). The demand 

on limited ground-water resources in areas in the southwestern 

United States has increased significantly. This increased 

demand underscores the importance of understanding factors 

that affect the water quality in basin-fill aquifers in the 

region, which are being studied through the U.S. Geological 

Survey’s National Water-Quality Assessment (NAWQA) 

program. As a part of this study, spatial datasets of natural and 

anthropogenic factors that may affect ground-water quality 

of the basin-fill aquifers in the southwestern United States 

were developed. These data include physical characteristics 

of the region, such as geology, elevation, and precipitation, 

as well as anthropogenic factors, including population, land 

use, and water use. Spatial statistics for the alluvial basins 

in the Southwest have been calculated using the datasets. 

This information provides a foundation for the development 

of conceptual and statistical models that relate natural and 

anthropogenic factors to ground-water quality across the 

Southwest. 

A geographic information system (GIS) was used to 

determine and illustrate the spatial distribution of these basinfill variables across the region. One hundred-meter resolution 

raster data layers that represent the spatial characteristics of 

the basins’ boundaries, drainage areas, population densities, 

land use, and water use were developed for the entire 

Southwest. 

Introduction 

The Southwest Principal Aquifers study area consists of 

most of California and Nevada and parts of Utah, Arizona, 

New Mexico, and Colorado; it is about 409,000 mi2 (square 

mile). Basin-fill aquifers are areas of alluvial basins composed 

of permeable unconsolidated deposits, extend through about 

201,000 mi2 of the study area and are the primary source of 

water for cities and agricultural communities in basins in the 

arid and semiarid southwestern United States (Southwest). 

In some areas, these aquifers play a vital ecological role in 

providing base flow to streams supporting important aquatic 

and riparian habitats. Basin-fill aquifers include four of the 

principal aquifers of the United States (U.S. Geological 

Survey, 2003b): the Basin and Range basin-fill aquifers in 

Nevada, Utah, and Arizona; the Rio Grande aquifer system 

in New Mexico and Colorado; and the Coastal Basin aquifers 

and the Central Valley aquifer system in California (fig. 1). 

The study area is delineated by these southwestern principal 

aquifers. Withdrawals from these aquifers in 2000 accounted 

for more than one quarter of the total withdrawals from all 

aquifers in the United States for irrigation, public-supply, and 

self-supplied industrial water uses (Maupin and Barber, 2005). 

Ground-water management plans have been established 

for several southwestern basins on the basis of estimated 

average annual recharge from natural—and in some cases 

artificial—sources to the aquifer system. Ground-water 

resources are fully appropriated with respect to legal rights in 

many of these basins, often at quantities much larger than the 

actual annual recharge amount. Human activities associated 

with urban and agricultural land uses can change water quality, 

and the effects can be exacerbated by stresses imposed on a 

ground-water system, such as withdrawals from wells and 

artificial recharge, which can accelerate and (or) change the 

direction of ground-water flow. Low recharge rates, long 

residence times, and slow rates of contaminant degradation 

retard cleansing of water in basin-fill aquifers; therefore, 

to optimally protect and use ground-water resources, it is 

imperative to understand the status of, trends in, and factors 

influencing ground-water quality.
2 Geospatial data to support analysis of water-quality conditions in basin-fill aquifers

River

Sevier River

Jordan Bear River

Humboldt

River

Reese River

River

Quinn

Meadow Valley Wash

White River Pahranagat Wash

Muddy

River

Colorado River

Verde River

Mojave River

Salt River

Gila River

Rio

Grande

Rio

Chama

Rio Puerco

Rio Grande

Kern River

River

Amaragosa

Owens River

San Joaquin River

Kings River

River

River

River

Tuolumne

Merced

Salinas River

San Joaquin

River

River

River

American

American River

River

N Fork

River River

River NorthYuba

Sacramento

River

Trinity

Eel

lumne Moke

r Feathe

Truckee

Russian

0 50 100 150 200 Miles0 50 100 150 200 Kilometers

G WYOMIN

UTAH

ARIZONA

CALIFORNIA

COLORADO

NEW MEXICO

40°0'35°0'

120°0' 115°0' 110°0' 105°0'

Pacific Ocean

NEVADA

IDAHO

OREGON

Basin and Range basin-fill aquifersCalifornia Coastal Basin aquifersCentral Valley aquifer systemRio Grande aquifer systemConsolidated rocks or alluvial material not 

included with principal aquifers in this study

EXPLANATION

U.S. Geological Survey digital data, 1:500,000 and 750,000 scale, 1992–1997 and 2006National Elevation Data 1:24,000 1999Albers Equal Area Conic projection, NAD 83

Study areaboundary

Study area

Figure 1. Basin-fill aquifers in the Southwest Principal Aquifers study area. 
Hydrogeologic Area Boundaries 3

Regional Analysis 

Similarities in the hydrogeology, land- and water-use 

practices, and water-quality issues affecting the basin-fill 

aquifers in the Southwest allow for a single, comprehensive 

regional analysis. Factors that commonly affect water quality 

were investigated on the basis of data and information derived 

from a thoroughly studied subset of basin-fill aquifers, and the 

relations determined from these areas can then be extrapolated 

to areas lacking extensive ground-water quality data and 

interpretive studies. Regional analysis, therefore, was a costeffective means of providing water managers with information 

on and insight into the status, trends, and factors affecting 

water quality in large, unmonitored parts of the aquifers. 

The National Water-Quality Assessment (NAWQA) 

program of the U.S. Geological Survey (USGS) has studied 

the water quality in parts of selected basin-fill aquifers used 

for domestic and public supply in 15 basins in the Southwest 

(fig. 2). Data were collected primarily to characterize the 

ground-water quality in the aquifer by determining the 

occurrence and distribution of a wide suite of constituents, 

including major and trace elements, nutrients, pesticides, and 

volatile organic compounds. Although these basin-specific 

studies resulted in a greater understanding of local conditions 

and factors affecting ground-water quality, a regional 

assessment still is needed to synthesize the findings from these 

studies into a common set of factors and themes that could be 

applied across the basin-fill aquifers in the Southwest. More 

specifically, the factors that affect the fate and transport of 

constituents from land-surface and aquifer-matrix sources 

through the shallow parts of aquifers and into the deeper parts 

of aquifers that are commonly used for drinking-water supply 

need to be better understood. 

Purpose and Scope

Regional analysis of factors affecting water quality in 

basin-fill aquifers in the Southwest requires a consistent set 

of data representing source, transport, and fate type variables, 

as well as a set of basin boundaries derived using a consistent 

set of criteria. These data, in addition to the water-quality 

data, provide a foundation for subsequent analysis through 

development of conceptual and statistical models. Such 

datasets are also valuable for other types of regional and basinscale hydrologic studies. 

The purpose of this report is to document several spatial 

datasets developed for the regional analysis of water quality 

in basin-fill aquifers in the Southwest. The datasets cover the 

entire extent of basin-fill aquifers in the Southwest Principal 

Aquifers study area (fig. 1) and were collected, processed, 

and analyzed using consistent methods. The datasets represent 

natural and anthropogenic characteristics of the land surface 

that can either be used directly to analyze factors that affect 

water quality or can be used as potential surrogates for other 

factors in such analysis. These factors include hydrogeologic 

area boundaries, elevation, surface geology, precipitation, 

land cover, population, and water-use information. 

Federal Geographic Data Committee (FGDC) compliant 

documentation includes descriptions of the original raw data 

sources, processing steps, data scales, and resulting datasets. 

Data Compilation and Processing

Several existing spatial datasets were compiled for 

this study. Datasets that represent the geology, elevation, 

hydrogeologic area (defined below), land cover, population, 

and water use were available for all or much of the study area. 

These datasets were processed to provide information that 

could be used to better understand the ground-water quality in 

basin-fill aquifers in the Southwest. 

Geographic Information System (GIS) methods were 

used for processing the regional spatial data layers. GIS raster 

data represent variables as a matrix of cells in a continuous 

space. A common set of analysis-environment settings 

for resolution and spatial extent were configured in a GIS 

before data were processed to ensure accurate cell-by-cell 

combination of the raster data. A common cell size resolution 

of 100 meters by 100 meters was specified across the study 

area. An analysis window limits the spatial extent of the data 

and snaps cell data to common cell boundaries for accurate 

processing, and the mask allows only selected cells within 

the analysis window to be used for data processing. The GIS 

vector (point, line, or polygon feature) data layers described 

in this report were converted to raster data, for example, 

rasterized, using the common analysis environment settings. 

Existing raster data were resampled (change cell size), 

reclassified, and processed as necessary to conform to the 

analysis and environment settings. The following subsections 

explain the procedures used to yield the systematic set of 

spatial statistics for each of the alluvial basins in the study 

area.

Hydrogeologic Area Boundaries

Hydrogeologic areas (HGA) are conceptualized as basins 

that generally have coincident ground-water and surfacewater basin boundaries (Anning and Konieczki, 2005). The 

HGA surrounding an alluvial basin is considered to be the 

contributing drainage area to the basin-fill aquifer in the 

alluvial basin. HGAs were used, for this study, in calculations 

of spatial statistics. HGAs for the study area were compiled 

from earlier studies of much of the region by Anning and 

Konieczki (2005) and were delineated for the California 

Coastal Basin aquifers and Central Valley aquifer system and 

the Rio Grande aquifer system in New Mexico and Colorado 

as part of this study. The resulting polygon dataset of HGAs 

in the Southwest Principal Aquifers study area consists of 425 

basins (fig. 3). 
4 Geospatial data to support analysis of water-quality conditions in basin-fill aquifers

0 50 100 150 200 Miles0 50 100 150 200 Kilometers

G WYOMIN

UTAH

ARIZONA

CALIFORNIA

COLORADO

NEW MEXICO

40°0'35°0'

120°0' 115°0' 110°0' 105°0'

Pacific Ocean

NEVADA

IDAHO

OREGON

U.S. Geological Survey digital data, 1:500,000 scale, 1992–1997National Elevation Data 1:24,000 1999Albers Equal Area Conic projection, NAD 83

Study areaboundary

Santa Ana Inland Basin

San Jacinto Basin

Las Vegas Valley

Upper Santa Cruz Basin

West Salt River Valley

Sierra Vista Subbasin

San Luis Valley

Salt Lake Valley

Spanish Springs Valley

Truckee Meadows

Eagle Valley

Carson Valley

Central Valley

Santa Ana Coastal Basin

Middle Rio Grande Basin

Figure 2. Basins studied as part of the National Water-Quality Assessment (NAWQA) program in the Southwest Principal Aquifers study area.
Hydrogeologic Area Boundaries 

5

0 50 100 150 200 Miles0 50 100 150 200 Kilometers

Hydrogeologic area 

G WYOMIN

UTAH

ARIZONA CALIFORNIA

COLORADO

NEW MEXICO

40°0'35°0'

120°0' 115°0' 110°0' 105°0'

Pacific Ocean

NEVADA

IDAHO

OREGON

EXPLANATION

U.S. Geological Survey digital data, 1:500,000 and 750,000 scale, 1992–1997 and 2006National Elevation Data 1:24,000 1999Albers Equal Area Conic projection, NAD 83

Study areaboundary

Figure 3. Hydrogeologic areas in the Southwest Principal Aquifers study area. 
6 Geospatial data to support analysis of water-quality conditions in basin-fill aquifers

HGAs were added to areas originally defined by Anning 

and Konieczki (2005). The additional HGAs were delineated 

for parts of the Southwest outside of the original extent. The 

original HGA data that were acquired from the Arizona USGS 

Water Science Center in Tucson, Arizona, were overlaid with 

the California hydrologic province and watershed boundaries 

coverage (California Interagency Watershed Mapping 

Committee, 1999) and the National Hydrography Dataset 

(NHD) stream lines (U.S. Geological Survey, 1999b) to check 

for possible conflicts. HGA boundaries were modified to 

produce a more hydrologically consistent drainage system 

for areas where the basin boundary followed a stream line or 

otherwise was not aligned with the NHD stream dataset. The 

vectors used to modify the boundaries were from the Elevation 

Derivatives for National Applications (EDNA) catchments 

coverage (U.S. Geological Survey, 2005). New HGAs for 

the study area were delineated using the same methods as the 

earlier dataset.

Elevation

Digital elevation model (DEM) raster data from the 

USGS 30-meter (1 arc-second) National Elevation Dataset 

(NED) (U.S. Geological Survey, 1999a) were used to calculate 

land-surface slope and elevation in each cell. Slope for a 

cell is calculated using the rate of change in elevation for 

each neighboring cell. First, DEM data were hydraulically 

conditioned; artificial sinks and peaks were filled or leveled 

to remove inaccuracies resulting from errors in creating the 

DEM (McCoy and others, 2001). Next, the regional 2-degreeslatitude by 6-degrees-longitude DEMs were merged into a 

single dataset for the entire study area. Finally, slope and 

elevation statistics were calculated using the merged DEM for 

each alluvial basin and HGA. 

Geology

The geologic dataset compiled for this study is a 100-

meter (3 arc-second) raster layer consisting of 1:750,000-

scale surficial geology for California (Saucedo and others, 

2000) and 1:500,000-scale geology for parts of Nevada, 

Utah, Colorado, New Mexico, and Arizona (fig. 4). The 

1:500,000-scale dataset is a modified version of existing 

state geologic maps that were compiled and edge matched 

as part of the Southwest Regional Gap Analysis Program 

(SW ReGAP) (RS/GIS Laboratory, 2004) and state geologic 

maps of parts of Oregon (Walker and others, 2003) and Idaho 

(Johnson and Raines, 1996). The lithologic classifications 

developed for the SW ReGAP geology dataset are aeolian, 

carbonate, evaporite, metamorphic or igneous, sandstone, 

sedimentary, and Quaternary alluvium; these classifications 

were generalized for this raster dataset. For example, shale, 

siltstone, and sandstone were combined into one category 

called sedimentary formations (RS/GIS Laboratory, 2004).

Land Use

The National Land Cover Database (NLCD) dataset 

for 2001 (U.S. Geological Survey, Multi-Resolution Land 

Characteristics (MRLC) Consortium, 2003a) was used to 

represent the extent of agricultural land use and rangeland 

cover in the study area. The NLCD is a dataset produced 

by the Multi-Resolution Land Characteristics (MRLC) 

Consortium of nine federal agencies and is a 30-meter cell 

resolution raster dataset of land cover for the entire United 

States (Homer and others, 2004). The data were generated 

from Landsat 5 Thematic Mapper (TM) and Landsat 7 

Enhanced Thematic Mapper Plus (ETM+) satellite imagery in 

2001 and depict 29 classes of land-cover data. The NLCD is a 

nationally consistent, 30-meter resolution dataset representing 

natural land cover and anthropogenic land use. NLCD data 

were also used to distribute county-level irrigated agricultural 

water use to individual raster cells within the study area.

Precipitation

The Oregon State University Parameter-Elevation 

Regressions on Independent Slopes Model (PRISM) dataset 

was used to assign precipitation values to basins across the 

study area. The approach used to generate the PRISM dataset 

combined weather station and elevation data to create modeled 

monthly and annual precipitation raster layers by using 

weighted weather station climate data and linear elevation 

regressions in an iterative process. Because the available 

station data may not represent conditions for the surrounding 

locations, the linear regression was modified for each model 

cell to reflect changes in climate and elevation. Average 

annual precipitation data for 1971–2000 (PRISM Group, 

Oregon State University, 2004) (fig. 5) generated by PRISM 

were used to develop basin statistics. 

Population

LandScan global population raster data for 2005 were 

used to represent the extent of urban areas within the study 

area and to disaggregate county-level public-supply water 

use. The LandScan dataset was developed by Oak Ridge 

National Laboratory for the United States Department of 

Defense. LandScan data are produced at the U.S. Census 

Block level and modified using photographic interpretation, 

image analysis and population modeling (Oak Ridge National 

Laboratory, 2005). 
Population 

7

0 50 100 150 200 Miles0 50 100 150 200 Kilometers

G WYOMIN

UTAH

ARIZONA CALIFORNIA

COLORADO

NEW MEXICO

40°0'35°0'

120°0' 115°0' 110°0' 105°0'

Pacific Ocean

Carbonate rockMetamorphic or intrusive igneous rockQuaternary alluvium and surficial depositsSedimentary formationsVolcanicsWater

EXPLANATION

NEVADA

IDAHO

OREGON

U.S. Geological Survey digital data, 1:500,000; 1:750,000 1992–1997 and 2006Albers Equal Area Conic projection, NAD 83

Study areaboundary

Figure 4. Generalized surficial geology for the Southwest Principal Aquifers study area. 
8 Geospatial data to support analysis of water-quality conditions in basin-fill aquifers

Figure 5. Average annual precipitation data generated by the Parameter-Elevation Regressions on Independent Slopes Model (PRISM) for the Southwest Principal Aquifers study area, 1971–2000. 

0 50 100 150 200 Miles0 50 100 150 200 Kilometers

G WYOMIN

UTAH

ARIZONA CALIFORNIA

COLORADO

NEW MEXICO

40°0'35°0'

120°0' 115°0' 110°0' 105°0'

Pacific Ocean

EXPLANATION

Average annual precipitation, in inches

0 to less than 1212 to less than 2424 to less than 3636 to less than 4848 to less than 6060 to less than 7272 to less than 171

Study areaboundary

NEVADA

IDAHO

OREGON
Water Use 9

Water Use

The USGS county-level Estimated Use of Water in 

the United States in 2000 was the basis for developing the 

estimated public-supply and irrigated-agriculture water-use 

raster datasets for the Southwest Principal Aquifers study area. 

The original dataset was compiled nationally using consistent 

methods for determining county-level water use (Hutson and 

others, 2004). The USGS water-use program (http://water.usgs.

gov/watuse/) has compiled these data every 5 years since 1950 

and provides estimates of thermoelectric power, industrial, 

public supply, and agricultural water use in the Nation. 

Public-Supply Water Use

The data layer containing estimated public-supply water 

use in 2005 is a 100-meter (3-arc-second) cell resolution raster 

of disaggregated county-level public-supply water-use data 

for the study area (fig. 6). The data layer was generated from 

spatially distributed LandScan population data for 2005 (Oak 

Ridge National Laboratory, 2005), 1:100,000-scale county 

boundaries, and year-2000 tabular data for per capita water use 

by county estimated by the USGS (U.S. Geological Survey, 

2004) This dataset represents the spatial distribution of urban 

areas, population, and public-supply water use for the year 

2005 in the Southwest. 

LandScan population density raster data was derived 

by dividing the LandScan population raster data by a raster 

area dataset (eq 1). The population density raster was then 

resampled to a common cell resolution of 100 meters. For 

the purposes of this study, urban areas are defined as areas 

with more than 386 persons/km2 and were selected from 

the population density raster. The new urban population 

density raster was combined with a rasterized county Federal 

Information Processing Standard (FIPS) polygon dataset (U.S. 

Census Bureau, 2000), which yielded a new raster dataset in 

which each cell contained urban population density and county 

FIPS codes. USGS water-use data were joined to the urban 

population density grid using the FIPS codes. 

LandScan population / raster area = 

population density, in persons per km2 (1)

At the time of processing (2007), LandScan data were 

available only for the year 2005, and USGS water-use data for 

2005 had not been finalized. For this study, per capita rates of 

water use were assumed to remain unchanged from 2000 to 

2005. Therefore, 2000 water-use data and 2005 population data 

were combined. 

Because of a lack of regional public-supply water-use 

spatial data, public-supply water use was disaggregated over 

urban areas using county per capita public-supply water-use 

rates. 

County public-supply water use / Population in county (2)

Only urban areas were assumed to use public-supply 

water. County per capita water-use rates in gallons/day/person 

were multiplied by the population in each cell to calculate 

public-supply water use by cell in gallons per day (fig. 6). 

urban population density (persons per km2) x .01 = 

population per 100-meter cell (3)

Population and public-supply water-use estimates 

were compared with available estimates from various 

municipalities. Although most year-2005 water-use totals 

for metropolitan areas were not available at the time of 

comparison, per capita and total use data were available for 

earlier time periods. 

Irrigated Agriculture Water Use

The estimated agricultural irrigation dataset in 2000 was 

created as a 100-meter (3-arc-second) cell resolution raster 

dataset from three sources for the nearest year available: 

1:100,000-scale county boundary data, 30-meter NLCD raster 

data for 2001, and USGS county-based Estimated Use of 

Water in the United States in 2000 tabular data (USGS, 2004). 

The dataset represents regional-scale estimated irrigated 

agriculture acreages and water-use quantities (fig. 7). 

The first step in creating the estimated irrigation dataset 

for 2000 was to select cultivated crops and pasture/hay, 

represented by raster values 81 and 82, respectively, from the 

NLCD 2001 dataset and resample to create a new 100-meter 

resolution agricultural land-use raster dataset. The analysis 

environment settings were set for extent, mask, and cell size of 

the NLCD agricultural land use dataset to limit processing to 

the areas of NLCD agriculture. The county boundary polygon 

data set (U.S. Census Bureau, 2000) was then rasterized within 

this analysis environment using the county FIPS code and 

Map Algebra (McCoy and others, 2001) to create a 100-meter 

resolution raster dataset of county FIPS codes for agricultural 

areas represented in the NLCD. Tabular USGS county-based 

water-use data (U.S. Geological Survey, 2004) were joined to 

the NLCD agricultural land use-FIPS raster by the FIPS code. 

By joining water-use tabular data to the agricultural land-use 

raster data, new attribute fields could be calculated on the 

basis of the joined table fields. Dividing irrigated acreages and 

gallons withdrawn for each county by the cell count in that 

county yielded per cell values of irrigated acreages and gallons 

used per cell per day. Finally, irrigated agricultural attributes 

were multiplied by 100 and rounded to the nearest integer 

to reduce file size and to facilitate additional raster analysis. 

Many GIS analysis tools require integer input data. Floating 

point data results in larger file sizes that require more hard 

disk space and memory. 

For this study, all agricultural crops in the Southwest 

were assumed to be irrigated. USGS county-based wateruse data (U.S. Geological Survey, 2004), reports irrigated 

acreages from the U.S. Department of Agriculture Census 

of Agriculture (U.S. Department of Agriculture, 1997). Data 
10 Geospatial data to support analysis of water-quality conditions in basin-fill aquifers Reno Salt Lake City Sacramento Fresno Las Vegas Flagstaff

Phoenix

Albuquerque

Santa Fe

San JoseMetro area

San Francisco

Tucson 0 50 100 150 200 Miles

0 50 100 150 200 Kilometers

Study areaboundary

G WYOMIN

UTAH

ARIZONA CALIFORNIA

COLORADO

NEW MEXICO

40°0'35°0'

120°0' 115°0' 110°0' 105°0'

Pacific Ocean

Reno

Salt Lake

City

Sacramento

Fresno

Los AngelosMetro area

Las Vegas

Flagstaff

Phoenix

Albuquerque

Santa Fe

San Diego

Metro area

San JoseMetro area

San Francisco

Tucson

NEVADA

IDAHO

OREGON

U.S. Geological Survey digital data, 1:100,000 scale, 2000, 2003, 2006National Elevation Data 1:24,000 1999Albers Equal Area Conic projection, NAD 83

EXPLANATION

Public-supply water use, in 

gallons per day

Less than 3,0003,000 to less than 6,0006,000 to less than 10,00010,000 to less than 23,00023,000 to 97,000

Figure 6. Public-supply water use in urban areas in the Southwest Principal Aquifers study area, 2005. 
Water Use 11

0 50 100 150 200 Miles0 50 100 150 200 Kilometers

G WYOMIN

UTAH

ARIZONA CALIFORNIA

COLORADO

NEW MEXICO

40°0'35°0'

120°0' 115°0' 110°0' 105°0'

Pacific Ocean

NEVADA

IDAHO

OREGON

U.S. Geological Survey digital data, 1:100,000 scale, 2001, 2003, 2006National Elevation Data 1:24,000 1999Albers Equal Area Conic projection, NAD 83

EXPLANATION

Irrigated agricultural water use in 

annual acre-feet per acre

0 to 2.702.71 to 3.643.65 to 4.784.79 to 7.357.36 to 10.29

Study areaboundary

Figure 7. Estimated agricultural irrigation in the Southwest Principal Aquifers study area, 2000. 
12 Geospatial data to support analysis of water-quality conditions in basin-fill aquifers

from the Census of Agriculture for irrigated acreage provide a 

more accurate report of agricultural land use than calculations 

made from individual NLCD raster cells and were used in the 

analysis for calculating irrigated acres. Water use and irrigated 

acres were distributed evenly over all NLCD agricultural land 

in a county rather than by individual fields. Thus, artificial 

discrete water-use boundaries are present at county lines 

as an artifact of the disaggregation process. Additionally, 

with this process, water may be distributed to some areas 

where agricultural land is misclassified by NLCD or is not 

in fact irrigated. Because information on the method type of 

irrigation, such as flood or sprinkler, was not consistently 

available for the entire Southwest, no attempt was made to 

distribute irrigation water use according to irrigation method. 

Spatial accuracy of the irrigation dataset is assumed 

to be approximately the same as that of the original NLCD 

cultivated land cover. Published NLCD accuracy estimates are 

81 percent error of omission, absent when predicted, and 79 

percent error of commission, predicted, but absent (Homer and 

others, 2004). 

Spatial Statistics for Hydrogeologic 

Areas and Alluvial Basins

Spatial analysis of the surficial geology, elevation, land 

cover, population, precipitation, and water use data layers 

yielded a dataset of spatial statistics for the HGAs and alluvial 

basins in the Southwest Principal Aquifer study area (fig. 1,

table 1). Calculating zonal statistics involves combining a 

zone layer (for example, an alluvial basin) with one or more 

value layers (for example, geology, elevation, land cover, 

population, precipitation, and water use) to calculate statistics 

for an individual zone based on the layer value in that zone. 

Layer values for these statistics are available for all of the 

alluvial basins in the study area (425 basins). Alluvial basins 

can be compared to determine whether they vary across a 

large or small range in individual variables or combinations 

of variables. Data for NAWQA case-study basins of the 

Southwest Principal Aquifer study area were combined with 

the geospatial datasets to create the dataset shown in table 

1. Spatial analysis of these variables highlights areas where 

natural and human influenced factors are similar.

Summary

Basin-fill aquifers of the arid and semiarid Southwest 

are an important source of water for many urban and 

agricultural communities. High withdrawals combined with 

human influence stemming from urban and agricultural land 

use increases the potential for water quality in basin-fill 

aquifers to be adversely affected. Regional effects on water 

quality are being studied by the U.S. Geological Survey’s 

National Water-Quality Assessment (NAWQA) program. 

Spatial datasets of natural and anthropogenic factors that may 

affect ground-water quality in the basin-fill aquifers of the 

southwestern United States have been developed as part of this 

study. These datasets include physical characteristics of the 

region, such as geology, elevation, and precipitation, as well 

as anthropogenic factors, including population, land use, and 

water use. Spatial statistics for the alluvial-fill basins in the 

Southwest are calculated from these datasets. This information 

provides a foundation for evaluating factors affecting groundwater quality across the Southwest through the development 

of conceptual and statistical models. 

A geographic information system (GIS) has been used to 

determine and illustrate the spatial distribution of these basinfill variables across the region. One-hundred-meter raster data 

layers representing the spatial characteristics of the basins’ 

boundaries, drainage areas, population density, land use, and 

water use have been developed for the entire Southwest. 

References Cited

Anning, D.W., and Konieczki, A.D., 2005, Classification of 

hydrogeologic areas and hydrogeologic flow systems in 

the Basin and Range physiographic province, southwestern 

United States: U.S. Geological Survey Professional Paper 

1702, 44 p., 2 sheets.

California Interagency Watershed Mapping Committee, 2000, 

The California Spatial Information Library, California 

Interagency Watershed Map of 1999 (CalWater 2.2.1), 

accessed March 17, 2006, at http://gis.ca.gov/casil/

hydrologic/watersheds/calwater.

Homer, C., Huang, C., Yang, L., Wylie, B., and Coan, M., 

2004, Development of a 2001 national landcover database 

for the United States: Photogrammetric Engineering and 

Remote Sensing, v. 70, no. 7, p. 829–840.

Hutson, S. S., Barber, N. L., Kenny, J. F., Linsey, K. S., 

Lumia, D. S., and Maupin, M. A., 2004, Estimated use of 

water in the United States in 2000: U.S. Geological Survey 

Circular 1268, 46 p. (Also available at http://pubs.usgs.gov/

circ/2004/circ1268).

Johnson, B.R., and Raines, G.L., 1996, Digital representation 

of the Idaho state geologic map: a contribution to the 

Interior Columbia River Basin Ecosystem Management 

Project: U.S. Geological Survey Open-File Report 95-690, 

scale 1:500,000.

Maupin, M.A., and Barber, N.L., 2005, estimated withdrawals 

from principal aquifers in the United States, 2000: U.S. 

Geological Survey Circular 1279, 46 p.
References Cited 13

McCoy, J., Johnston, K., Kopp, S., Borup, B., and Willison, 

J., 2001, Using ArcGIS TM Spatial Analyst: Redlands, 

California, Environmental Systems Research Institute, 232 

p.

Oak Ridge National Laboratory, 2005, LandScanTM Global 

Population Database, accessed September 2006, at http://

www.ornl.gov/landscan/.

RS/GIS Laboratory, 2004, Southwest region geology data 

(1:500,000 scale): College of Natural Resources, Utah State 

University, accessed January 2006, at http://earth.gis.usu.

edu/swgap/geology.html

Saucedo, G. L., Bedford, B. R., Raines, G. L., Miller, R. J., 

and Wentworth, C. M., 2000, Modified from California 

Division of Mines and Geology, CD-ROM 2000-007, GIS 

data for the geologic map of California

PRISM Group, Oregon State University, 2004, Precipitationelevation Regression on Independent Slopes Model 

(PRISM), Oregon State University, accessed on April 2005, 

at http://www.ocs.oregonstate.edu/prism/. 

U.S. Census Bureau, 2000, 1:100,000-scale Census 2000 

county and county equivalent areas cartographic boundary 

files, accessed December 2007, at http://www.census.gov/

geo/www/cob/co2000.html. 

U.S. Department of Agriculture, National Agricultural 

Statistics Service, 1997, The census of agriculture. 

(Also available at http://www.agcensus.usda.gov/

Publications/1997/index.asp) 

U.S. Geological Survey, 1999a, National Elevation Dataset, 

accessed January 2006, at http://ned.usgs.gov/. 

U.S. Geological Survey, 1999b: U.S. Geological Survey web 

site, accessed March 17, 2006, at http://edna.usgs.gov/. 

U.S. Geological Survey, Multi-Resolution Land 

Characteristics (MRLC) Consortium, 2003a, National Land 

Cover Database 2001, accessed February 2007, at http://

www.mrlc.gov/.

U.S. Geological Survey, 2003b, Principal aquifers of the 48 

conterminous United States, Hawaii, Puerto Rico, and the 

U.S. Virgin Islands: U.S. Geological Survey National Atlas: 

U.S. Geological Survey web site, accessed October 2005, at 

http://www.nationalatlas.gov/mld/aquifrp.html. 

U.S. Geological Survey, 2004, Estimated use of water in the 

United States; county-level data for 2000: U.S. Geological 

Survey web site, accessed online [date] at http://water.usgs.

gov/watuse/data/2000/index.html

U.S. Geological Survey, 2005, Elevation derivatives for 

national applications (EDNA): U.S. Geological Survey web 

site, accessed March 2006, at http://edna.usgs.gov/.

Walker, G.W., MacLeod, N.S., Miller, R.J., Raines, G.L., 

and Connors, K.A., 2003, Spatial digital database for the 

geologic map of Oregon: U.S. Geological Survey Open-File 

Report 2003-67, scale 1:500,000.

The results of this study are linked below:

Estimated Irrigated Agricultural Water Use In 2000

http://water.usgs.gov/lookup/getspatial?sir2008-5239_swpa_agir_wu_2000 

Aluvial basin statistics of the Southwest Principal Aquifers (SWPA) study

http://water.usgs.gov/lookup/getspatial?sir2008-5239_swpa_basin_stats 

Surficial geology of the Southwest Principal Aquifers (SWPA) study

http://water.usgs.gov/lookup/getspatial?sir2008-5239_swpa_geo 

Hydrogeologic Areas of the Southwest Principal Aquifers (SWPA) study (sw_hga08)

http://water.usgs.gov/lookup/getspatial?sir2008-5239_swpa_hga 

Estimated Public Supply Water Use of the Southwest Principal Aquifers (SWPA) study in 2005

http://water.usgs.gov/lookup/getspatial?sir2008-5239_swpa_public_supply_wu_2005 
14 Geospatial data to support analysis of water-quality conditions in basin-fill aquifers Table 1. Example of spatial statistics derived for the National Water-Quality Assessment (NAWQA) program case-study basins in the Southwest Principal Aquifers study area. Variable Units Arizona Nevada West Salt River Valley Sierra Vista Subbasin Upper Santa Cruz Basin Truckee Meadows Carson Valley Eagle Valley (Carson City area) Spanish Springs Valley Las Vegas Valley Physiographic province Name Basin and Range Basin and Range Basin and Range Cascade- Sierra Mountains / Basin and Range Cascade- Sierra Mountains / Basin and Range Cascade- Sierra Mountains / Basin and Range Basin and Range Basin and Range Alluvial basin area square miles 1,040 1,131 1,465 94 162 23 29 922 Maximum alluvial basin elevation feet 1,817 5,094 4,264 5,606 5,329 4,967 4,797 5,548 Minimum alluvial basin elevation feet 771 3,320 2,057 4,370 4,623 4,577 4,419 1,250 Hydrogeologic basin area acres 920,440 1,168,528 1,621,660 162,008 580,840 40,530 49,445 1,013,393 Maximum hydrogeologic basin elevation feet 4,363 9,419 9,144 10,725 11,247 9,180 7,087 11,850 Minimum hydrogeologic basin elevation feet 771 3,307 2,011 4,380 4,623 4,577 4,419 1,224 Average precipitation on alluvial basin per unit area, 1971–2000 feet/year 0.75 1.23 1.28 0.87 0.88 0.84 0.83 0.51 Total average precipitation across entire alluvial basin, 1971–2000 acre-feet/year 505,156 903,148 1,213,112 53,279 92,616 12,619 15,743 306,338 Average precipitation in hydrogeologic basin per unit area, 1971–2000 feet/year 0.78 1.35 1.42 1.34 2.32 1.03 0.93 0.63 Total average precipitation across entire hydrogeologic basin, 1971–2000 acre-feet/year 715,259 1,572,785 2,308,620 216,757 1,348,698 41,710 45,739 641,749 Percent irrigated agricultural land in alluvial basin1 percent 13 1 1 3 43 7 5 0 Percent present day urban land in study area2 percent 33 2 15 55 6 57 19 29 Total water use in alluvial basin (public supply and irrigated agriculture) acre-feet/year 979,964 49,093 209,203 84,766 167,745 20,419 7,091 592,130 Ground-water use in alluvial basin (public supply and irrigated agriculture) acre-feet/year 465,499 48,456 200,630 22,893 17,266 10,359 1,571 76,773 Surface-water use in alluvial basin (public supply and 

irrigated agriculture)

acre-feet/year 503,525 536 4,559 60,144 150,326 9,686 5,443 502,064

Public-supply water use in alluvial basin (from USGS water-use compilation for 2000)

acre-feet/year 477,586 5,005 174,989 75,328 7,037 16,295 3,323 578,836

Irrigated agriculture water use in alluvial basin acre-feet/year 491,437 43,987 30,200 7,709 160,555 3,750 3,691 0Amount of water applied to irrigated agricultural land feet/year 5.62 4.14 4.32 3.60 3.56 3.35 3.60 0Population per unit area of alluvial basin (population density)

persons/square mile

1,813 68 605 2,461 223 2,056 497 1,738

Population in alluvial basin persons 1,885,915 76,587 885,677 232,129 36,164 47,964 14,619 1,603,169Population in hydrogeologic basin persons 1,971,786 79,808 914,822 318,921 41,376 53,639 20,416 1,662,7211 Values for irrigated agricultural land use acreage are from Census of Agriculture data (United States Department of Agriculture, 1997).2 Values for urban land use acreage are derived from LandScan population data (Oak Ridge National Laboratory, 2005)
References Cited 15

Variable Units

Utah California Colorado New Mexico

Salt Lake Valley

Central Valley Santa Ana 

Inland Basin

Santa Ana 

Coastal 

Basin

San Jacinto 

Basin

San Luis 

Valley

Middle Rio 

Grande Basin Sacramento 

Valley

San Joaquin 

Valley

Physiographic province Name

Basin and Range Pacific 

Border

Pacific 

Border

Pacific 

Border

Pacific 

Border

Pacific 

Border

Southern 

Rocky 

Mountains

Basin and 

Range / 

Colorado 

Plateaus

Alluvial basin area square miles 417 6,170 13,752 655 340 240 4,889 2,886Maximum alluvial basin elevation feet 5,396 661 846 2,611 411 2,400 8,887 6,232Minimum alluvial basin elevation feet 4,203 -13 -16 486 0 1,234 5,833 4,665Hydrogeologic basin area acres 474,150 10,454,153 20,805,320 949,950 380,493 509,510 6,322,943 2,819,390Maximum hydrogeologic basin elevation feet 11,224 10,325 14,327 11,355 5,663 10,751 14,232 10,663Minimum hydrogeologic basin elevation feet 4,199 -13 -16 486 -3 1,234 5,833 4,665Average precipitation on alluvial basin per unit area, 1971–2000 feet/year 1.33 1.82 0.89 1.37 1.10 1.14 0.91 0.88Total average precipitation across entire alluvial basin, 1971–2000 acre-feet/year 358,756 7,255,584 7,913,067 582,232 242,852 253,636 2,891,234 1,650,382Average precipitation in hydrogeologic basin per unit area, 1971–2000 feet/year 1.87 3.21 1.74 1.83 1.19 1.33 1.47 0.99Total average precipitation across entire hydrogeologic basin, 1971–2000 acre-feet/year 886,079 33,512,251 36,149,307 1,737,840 452,794 678,134 9,266,871 2,804,933Percent irrigated agricultural land in alluvial basin1 percent 7 45 62 8 5 38 20 2Percent present day urban land in alluvial basin2 percent 48 8 5 60 91 33 0 6Total water use in alluvial basin (public supply and irrigated agriculture) acre-feet/year 352,676 7,077,471 17,953,539 664,364 490,463 372,135 1,640,182 443,897Ground-water use in alluvial basin (public supply and irrigated agriculture) acre-feet/year 92,747 2,259,879 8,406,027 405,065 255,518 101,534 764,393 148,902Surface-water use in alluvial basin (public supply and irrigated agriculture) acre-feet/year 253,351 4,805,634 9,528,212 247,718 224,563 270,601 875,855 292,042Public-supply water use in alluvial basin acre-feet/year 286,826 538,788 816,614 504,469 452,060 103,699 3,479 129,481Irrigated agriculture water use in alluvial basin acre-feet/year 59,272 6,526,724 17,119,625 148,314 28,020 268,436 1,636,769 311,463Amount of water applied to irrigated agricultural land feet/year 3.21 3.71 3.14 4.47 2.63 4.59 2.63 7.69Population per unit area of alluvial basin (population density)

persons/square mile 2,260 370 273 3,161 7,001 1,602 15 262

Population in alluvial basin persons 943,535 2,285,641 3,752,586 2,071,764 2,379,037 384,555 71,082 756,560Population in hydrogeologic basin persons 952,014 2,746,214 4,078,694 2,329,954 2,582,390 519,380 75,949 770,6091 Values for irrigated agricultural land use acreage are from Census of Agriculture data (U.S. Department of Agriculture, 1997)2 Values for urban land use acreage are derived from LandScan population data (Oak Ridge National Laboratory, 2005)

Table 1. Example of spatial statistics derived for the National Water-Quality Assessment (NAWQA) program case-study basins in the Southwest Principal Aquifers study area.—Continued 
16 Geospatial data to support analysis of water-quality conditions in basin-fill aquifers

McKinney and Anning—Geospatial Data to Support Analysis of Water-Quality Conditions in Basin-Fill Aquifers in the Southwestern United States—Scientific 

Investigations Report 2008–5239