U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2008–5209

National Water Availability and Use Pilot Program

Methods and Indicators for Assessment of Regional 

Ground-Water Conditions in the Southwestern 

United States

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MESA

ELOY

TEMPE

TONOPAH

PHOENIX

GLENDALE

FLORENCE

CAREFREE

STANFIELD

QUEEN CREEK

CASA GRANDE

TUBAC

MARANA

TUCSON

ORO VALLEY

GREEN VALLEY

Wells With Long-Term Records

!( < 25 years

!( 25 to 50 years

!( 50 to 75 years

!( >75 years

Ground-water basins

MEXICO

UNITED STATES

MESA

ELOY

TEMPE

TONOPAH

PHOENIX

GLENDALE

FLORENCE

CAREFREE

STANFIELD

QUEEN CREEK

CASA GRANDE

TUBAC

MARANA

TUCSON

ORO VALLEY

GREEN VALLEY

Ground-water basins

MEXICO

UNITED STATES

MESA

ELOY

TEMPE

TONOPAH

PHOENIX

GLENDALE

FLORENCE

CAREFREE

STANFIELD

QUEEN CREEK

CASA GRANDE

TUBAC

MARANA

TUCSON

ORO VALLEY

GREEN VALLEY

Ground-water basins

MEXICO

UNITED STATES

MESA

ELOY

TEMPE

TONOPAH

PHOENIX

GLENDALE

FLORENCE

CAREFREE

STANFIELD

QUEEN CREEK

CASA GRANDE

TUBAC

MARANA

TUCSON

ORO VALLEY

GREEN VALLEY

Ground-water basins

MEXICO

UNITED STATES

Central Arizona Project

(CAP) Canal

Central Arizona Project

(CAP) Canal

Central Arizona Project

(CAP) Canal

Central Arizona Project

(CAP) Canal

Historical Declines in

Ground-Water Levels

75 to 125 ft

125.01 to 200 ft

200.01 to 250 ft

250.01 to 350 ft

> 350 ft

Recent Rises in 

Ground-Water Levels

50 to 100 ft

100.01 to 150 ft

150.01 to 200 ft

200.01 to 275 ft

> 275 ft

Recent Trends in

Ground-Water Levels

1997–2006

Falling

Nearly Stable

Risi

Well Location

ng
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National Water Availability and Use Pilot Program

Methods and Indicators for Assessment of Regional 

Ground-Water Conditions in the Southwestern 

United States

By Fred D Tillman, Stanley A. Leake, Marilyn E. Flynn, Jeffrey T. Cordova, Kurt T. Schonauer, 

and Jesse E. Dickinson

Scientific Investigations Report 2008–5209

U.S. Department of the Interior

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

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2008

This report and any updates to it are available online at:

http://pubs.usgs.gov/sir/2008/5209/

For product and ordering information:

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

Telephone: 1-888-ASK-USGS

For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, 

natural hazards, and the environment:

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

Telephone: 1-888-ASK-USGS

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

U.S. Government.

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

reproduce any copyrighted materials contained within this report.

Suggested citation:

Tillman, F.D, Leake, S.A., Flynn, M.E., Cordova, J.T., Schonauer, K.T., and Dickinson, J.E., 2008, Methods and indicators for 

assessment of regional ground-water conditions in the southwestern United States: U.S. Geological Survey Scientific 

Investigations Report 2008-5209, 22 p. [http://pubs.usgs.gov/sir/2008/5209/].

Produced in the Western Region, Menlo Park, California

Manuscript approved for publication, October 29, 2008

Text edited by Peter H. Stauffer

Layout by David R. Jones
iii

Contents

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

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

Acknowledgments.........................................................................................................................................2

Approach to Analyzing Ground-Water Conditions in the Southwestern United States ....................2

Ground-Water Level Data.............................................................................................................................3

Subsets of Ground-Water Conditions Information...................................................................................4

Wells Experiencing Significant Water-Level Decline or Rise .......................................................4

Recent Trends in Ground-Water Levels............................................................................................7

Determining Trends in Water-Level Data.................................................................................7

Spatial Representation of Water-Level Trend Information.................................................10

Wells With Long-Term Records........................................................................................................15

Recent Depth to Ground Water........................................................................................................16

Summary and Conclusions.........................................................................................................................19

References Cited .........................................................................................................................................22

Figures

1. Schematic representation of algorithm for VBA® program that produces 

ground-water conditions subset of wells experiencing water-level declines 

and wells experiencing water-level rises ...........................................................................................4

2. Examples of annotated hydrographs produced for wells experiencing 

water-level decline and wells experiencing water-level rise .........................................................5

3.

wells experiencing water-level decline ..............................................................................................6

4. Wells experiencing significant ground-water level decline during historical 

(pre-1997) time in the most developed basins of south-central Arizona .......................................8

5. Wells experiencing significant ground-water level rise continuing through 

recent time (post-1997) in the most developed basins of south-central Arizona .........................9

6. Schematic representation of FORTRAN algorithm for computing trends 

in ground-water level data ..................................................................................................................11

7. Examples of different subsets of water-level data during a given time 

period used for trend analyses ...........................................................................................................12

8. Screenshot of example output of trend-computation program 

with header indicating selection choices..........................................................................................14

9. Annotated hydrograph produced during the recent trends in ground-water 

levels analysis of ground-water conditions .....................................................................................14

10. Schematic representation of FORTRAN algorithm for computing modified 

Thiessen polygons of selected wells for spatial representation of water-level trends ............15

11. Trends in ground-water levels for the 10-year period from 1997 through 2006 

for the most developed basins in south-central Arizona................................................................17

12. Portion of example output in Microsoft Excel® spreadsheet of VBA® program written 

to provide information on time span and frequency of ground-water level observations .......18

13. Example of a hydrograph produced for the “Wells with Long-Term Records”

layer of ground-water conditions .......................................................................................................19

14. Wells with long-term records in the most developed basins of south-central Arizona ............20

15. Recent observations of depth to ground water (since 2004) in the most 

developed basins of south-central Arizona ......................................................................................21

®

®
iv

Conversion Factors

Multiply By To obtain

Length

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

Area

square meter (m2

) 0.0002471 acre 

square kilometer (km2

) 0.3861 square mile (mi2)

Flow rate

cubic meter per day (m3

/d) 0.0004087 cubic foot per second (ft3/s) 

cubic meter per day (m3

/d) 264.2 gallon per day (gal/d) 

cubic meter per day (m3

/d) 0.2961 gallon per minute (gal/min)

cubic meter per day (m3

/d) 0.1834 acre-foot per year

Recharge and evapotranspiration rate

meter per day (m/d) 3.281 foot per day (ft/d)

Hydraulic conductivity

meter per day (m/d) 3.281 foot per day (ft/d)

Specific storage

per meter (m-1) 0.3048 per foot (ft-1)

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 (NAVD 88).

Horizontal coordinate information is referenced to North American Datum of 1927 (NAD 27).

Altitude, as used in this report, refers to distance above the vertical datum.

Tables

1. Location of well and water-level information in input spreadsheet used for all 

programs created to analyze and present ground-water conditions..............................................3

2. Number of wells from example Idaho dataset with acceptable linear trends in 

ground-water levels for different subsets of water-level observations analyzed.......................13
Methods and Indicators for Assessment of Regional 

Ground-Water Conditions in the Southwestern 

United States

By Fred D Tillman, Stanley A. Leake, Marilyn E. Flynn, Jeffrey T. Cordova, Kurt T. Schonauer, and 

Jesse E. Dickinson

Abstract

Monitoring the status and trends in the availability of 

the Nation’s ground-water supplies is important to scientists, 

planners, water managers, and the general public. This is especially true in the semiarid to arid southwestern United States 

where rapid population growth and limited surface-water 

resources have led to increased use of ground-water supplies 

and water-level declines of several hundred feet in many aquifers. Individual well observations may only represent aquifer 

conditions in a limited area, and wells may be screened over 

single or multiple aquifers, further complicating single-well 

interpretations. Additionally, changes in ground-water conditions may involve time scales ranging from days to many 

decades, depending on the timing of recharge, soil and aquifer 

properties, and depth to the water table. The lack of an easily identifiable ground-water property indicative of current 

conditions, combined with differing time scales of water-level 

changes, makes the presentation of ground-water conditions a 

difficult task, particularly on a regional basis. One approach is 

to spatially present several indicators of ground-water conditions that address different time scales and attributes of the 

aquifer systems. This report describes several methods and 

indicators for presenting differing aspects of ground-water 

conditions using water-level observations in existing datasets. 

The indicators of ground-water conditions developed in this 

study include areas experiencing water-level decline and 

water-level rise, recent trends in ground-water levels, and current depth to ground water. The computer programs written to 

create these indicators of ground-water conditions and display 

them in an interactive geographic information systems (GIS) 

format are explained and results illustrated through analyses 

of ground-water conditions for selected alluvial basins in the 

Lower Colorado River Basin in Arizona.

Introduction

Ground water is an important resource in the southwestern United States, making up from 22 percent in Colorado to 

90 percent in New Mexico of water needs for domestic use 

(Anderson and Woolsey, 2005). Availability of deep-well 

turbine pumps and rural electricity in the mid-20th century 

allowed development of agriculture in desert basins that 

receive minimal rainfall. Adding additional stress to limited 

ground-water resources, Nevada, Arizona, Colorado, and 

Utah rank 1st through 4th, respectively, in the list of fastest 

growing states in the United States (Anderson and Woolsey, 

2005). As ground water is increasingly developed in the rapidly growing arid and semiarid southwestern United States, 

tools are needed to evaluate the effects of ground-water 

withdrawals on aquifer conditions. Knowledge of the current status of ground-water conditions is an indication of the 

availability of ground-water supplies. Monitoring changes in 

ground-water conditions allows for evaluation of the impact 

of growth, management actions, and climatic variations on 

these resources. In recent decades, nonextractive uses of 

ground water, such as maintaining baseflow to streams and 

high water levels beneath riparian vegetation, have become 

increasingly important and require investigation and evaluation of ground-water conditions. Earth fissures and land 

subsidence have occurred in some areas of the Southwest 

experiencing significant water-level declines and consequent 

aquifer compaction, adversely impacting canals and roads 

and leading to a loss of aquifer storage. Information on 

regional ground-water conditions and trends is necessary to 

evaluate the potential for such aquifer compaction. In 1984, 

the U.S. Geological Survey (USGS) published a nationwide 

map indicating areas of major water-level change (U.S. 

Geological Survey, 1984), and maps have been prepared 

by the USGS for select parts of the Lower Colorado River 

Basin and other areas of the United States (Anderson, 1995). 

Despite the need for better and updated information, however, no update of ground-water conditions has been completed that parallels the ongoing analysis of surface-water 

resources. This report describes the development of methods 

of assessment and display of ground-water conditions from 

analyses of existing information available in public databases. Several indicators of ground-water conditions, each 

describing a different aspect of the aquifer system, were 

developed. The use of these methods to present ground-water 

conditions is demonstrated in this report, and on a publicly 

available interactive Web site, using data from the most 
2 Methods and Indicators for Assessment of Regional Ground-Water Conditions in the Southwestern United States

developed alluvial basins in Arizona (Tillman and others, 

2007). Representing ground-water conditions in a broadly 

understandable manner presents challenges that are different from those for surface-water bodies. However, educating 

the public and policymakers on the status of ground-water 

resources is an increasingly important task in ensuring the 

sustainable management of this critical but largely unseen 

resource.

Acknowledgments

Development of methods and indicators of ground-water 

conditions for the alluvial basins in Arizona was funded by the 

USGS Water Availability and Use Program. Teri Davis and 

Terry Voght of the Arizona Department of Water Resources, 

and Bill Cunningham, Kevin Dennehy, and Tom Reilly of the 

USGS provided valuable technical guidance at various stages 

in the development of the methods and interactive map. Jeff 

Phillips, formerly of the USGS, was instrumental in developing ideas for presentation of data. Cheryl Partin of the USGS 

provided database support.

Approach to Analyzing Ground-Water 

Conditions in the Southwestern United 

States

Are there areas in my region that have experienced 

declining ground-water levels in the past? What areas continue to have falling water levels? Are there areas where water 

levels have started coming back up? What are the trends in 

ground-water levels over recent years? Are there wells with 

long-term records that may demonstrate decadal influences 

of changing climate and water use? What is the most recent 

observation of depth to ground water in my area? Each of 

these questions represents a potentially important spatial and 

temporal aspect of the condition of ground-water resources. 

With surface-water systems such as rivers, streams, lakes, and 

reservoirs, the condition of the water body is fairly straightforward to analyze and present and is easily understood by 

an audience with a wide range of backgrounds. Indicators of 

surface-water conditions such as stage, discharge, and days 

of water remaining can be observed on a frequent schedule 

and may easily be compared to historical observations to 

produce an indication of the current status of the system availability. River and lake levels can change rapidly due to local 

or upstream precipitation or management actions. Information on changes in these indicators may therefore need to be 

obtained on a fairly short time scale to provide water managers 

with information in order to manage reservoir releases, time 

irrigation allocations, and issue flood warnings, among other 

issues. The public may be interested in short-term indicators of 

surface-water systems in order to make decisions on waterrecreation activities such as boating, rafting, or fishing.

The frequent collection of data on ground-water levels 

and comparison to historical observations has been successful 

in understanding ground-water conditions in some areas of 

the United States (for example, see http://pa.water.usgs.gov/

potomac/; http://ma.water.usgs.gov/water/water_g.htm; http://

md.water.usgs.gov/groundwater/; http://www.sflorida.er.usgs.

gov/; http://nh.water.usgs.gov/WaterData/2008/mar08gwmap.

htm; http://groundwaterwatch.usgs.gov/). Typically, for these 

comparisons to be a meaningful indicator of ground-water 

conditions, the historical observations should vary around a 

fairly stable average, at least on a monthly or seasonal basis. 

In heavily developed areas of the arid and semiarid Southwest, however, steady declines in water levels over many 

years, combined with infrequent water-level observations, 

may make comparisons to historical averages less meaningful. An investigation of ground-water conditions in the most 

developed alluvial basins of Arizona, for example, found 

nearly 1,400 wells with records indicating at least 100 feet of 

decline during their period of record and more than 100 wells 

with greater than 300 feet of decline. Additionally, groundwater levels in the Southwest may not necessarily require such 

frequent assessment as can be provided by real-time transducer technology, both because the lag in aquifer response to 

recharge/discharge can be on the order of months to decades 

(with the important exception of riparian areas with shallow 

ground-water levels) and because it is unclear what decisions 

and actions would necessarily be required based on such shortterm information. 

Other efforts at presenting ground-water data in Arizona 

include Internet-based maps of index wells (http://sahra.

arizona.edu/wells/) and automated ground-water monitoring sites (http://arcims.azwater.gov/gwsi/Default.aspx), all 

with links to hydrographs of water-level data. These Web 

sites simply present the available well data with no attempt at 

explaining ground-water conditions in the area. New computer programs were developed that group data from existing 

databases into logical subsets, each of which tells a different 

part of the story of ground-water conditions. A new method 

for analyzing and presenting recent trends is also described 

in this report. These tools present ground-water information 

in a GIS format that allows visualization of ground-water 

conditions at differing temporal and spatial scales. Computer 

programs are described that create well hydrographs that are 

linked to well locations in many of the subsets. These custom 

hydrographs highlight pertinent observations in the waterlevel observation dataset. By presenting several interpretative indicators of ground-water conditions, each answering a 

different question about the ground-water system, we provide 

more than just water-level data and take advantage of an 

opportunity to inform the public and policymakers on the 

status and trends of ground-water conditions.
Ground-Water Level Data 3

Ground-Water Level Data

Much of the data on ground-water levels for the Nation’s 

aquifers are held in the USGS National Water Information 

System (NWIS) database of each of the 48 USGS Science 

Centers. Public access is provided via NWISWeb to these data 

that are retrieved and collected from the science centers. This 

publicly accessible database contains real-time, daily, and 

other less frequent observations of ground-water levels for 

thousands of wells throughout the country (see http://waterdata.usgs.gov/nwis/gw). However, in some areas of the United 

States, other Federal, State or local governments or private 

industries (for example, environmental consulting firms or 

mining operations) may collect water levels that ultimately 

reside in an archive that is not electronically accessible. For 

example, in the State of Arizona, most of the recent waterlevel monitoring is performed by the Arizona Department of 

Water Resources (ADWR). The repository for ADWR waterlevel observations is its own database, called the Groundwater 

Site Inventory (GWSI), which is made available to the public 

through CD-ROM. This periodic dissemination of data precludes the type of automatic access via NWISWeb or updated 

Web-based products used for the USGS Ground-Water Watch 

(http://groundwaterwatch.usgs.gov/), with the exception of the 

300 or so “active” wells in the State available in NWIS, nearly 

half of which are located in a single Arizona county (see http://

groundwaterwatch.usgs.gov/StateMaps/AZ.html). 

The presence of multiple primary sources of groundwater data presents a challenge to analyzing ground-water 

conditions. The first task in the present study was the development of a method to combine databases into a single dataset 

that may then be used by other computer programs created 

to analyze and display ground-water conditions. A Visual 

Basic for Applications (VBA®) program was developed in 

Microsoft® Excel® to combine multiple ground-water datasets. 

While this initial step cannot be performed in real-time, batch 

processing of this combination of existing data and further 

data processing can be performed on a cycle that should be 

sufficient to capture most of the change in ground-water 

systems in a reasonable time frame. A dataset containing, at a 

minimum, a unique well identifier, well-location coordinates, 

and water-level observations and dates is required, with the 

location of these data in the spreadsheet specified (table 1). 

Additional well information, such as well depth and altitude of 

land surface, are useful in presenting informative hydrographs. 

All data from each of the datasets to be merged are first placed 

in a single spreadsheet, with data in predefined columns 

(table 1). The former limitation of Excel® 2003’s maximum 

of 65,536 rows of data has been greatly expanded in Excel®

2007, with 1,048,576 rows of data now allowed. The VBA®

program creates a new spreadsheet of the merged datasets, 

eliminating duplicate entries having the same site identification number, water-level observation date, and depth to ground 

water, and formats the remaining data for use in the other 

analysis programs described below. The program also eliminates water-level measurements from the combined dataset 

that are flagged as being collected during pumping conditions 

at the well (an entry of “P” in column I of the spreadsheet), 

although this function can be suppressed or additional flagged 

data eliminated by changing a code statement. The pumping observations were judged to not be indicative of natural 

ground-water conditions and were therefore not utilized in 

further analyses. 

Subsets of information on ground-water conditions are 

presented in a GIS environment, several with a link from the 

well location to an annotated hydrograph created for the layer. 

VBA® programs were written to create these annotated well 

hydrographs for each subset of data, all of which utilize a 

common approach. All of the developed hydrograph-VBA®

programs utilize the combined dataset spreadsheet and present 

all water-level observations (except observations flagged as 

pumping as explained above). Water levels that are observed 

more than one year apart are indicated with a dashed line, 

whereas observations occurring one year or less apart are indicated with a solid line. A common scale for date of observation 

and water-level depth are used for all hydrographs to aid in 

comparison of results between wells. Additional information 

is provided in the hydrograph header, including site identification number, local well name, latitude and longitude of the 

well location, the altitude of land surface at the well location, 

and the well depth (if this information is provided in the combined data spreadsheet). The date the hydrograph was created 

is presented in the footer area of the chart.

Table 1. Location of well and water-level information in input 

spreadsheet used for all programs created to analyze and present 

ground-water conditions.

[GWSI, Ground-Water Site Inventory; LSD, Land Surface Datum.]

Spreadsheet 

column

USGS GWSI 

component number Description

A C004 Source agency code

B C001 Site identification number

C C012 Station name

D C009 Latitude

E C010 Longitude

F C036 Latitude/Longitude datum

G C235 Water-level measurement date

H C237 Water-level below LSD

I C238 Water-level status

J C016 Altitude of land surface

K C022 Altitude datum

L C024 Primary use of water

M C027 Hole depth

N C028 Well depth
4 Methods and Indicators for Assessment of Regional Ground-Water Conditions in the Southwestern United States

Subsets of Ground-Water Conditions 

Information

Unlike surface-water indicators of stage or discharge, 

ground water may not have a unique identifier that supplies 

sufficient information on aquifer conditions. Differences in 

response times as well as spatial limitations of individual measurements require a different approach to analyzing, presenting, and understanding ground-water conditions. Although a 

map showing well locations containing links to hydrographs 

of water-level observations is a first-order approach to present 

the complete set of data, much additional information can be 

shown to better understand ground-water conditions. Methods were developed to present ground-water conditions using 

subsets of existing water-level observations, with the results 

of each subset designed to address a different aspect of aquifer 

conditions. The common components of all subsets of groundwater information presented here are that the analyses are 

performed on a single, combined dataset in a spreadsheet with 

specified locations (as described above) and that all results are 

presented in a GIS format in a way that promotes understanding of the aspect of ground-water conditions of interest. The 

development of each subset of information is described and 

demonstrated with examples from the most developed alluvial 

basins in the Lower Colorado River Basin in Arizona.

Wells Experiencing Significant Water-Level 

Decline or Rise

Ground-water development in many areas throughout 

the Southwest has caused sometimes dramatic water-level 

declines. Some of these water-level declines may have ceased 

during past time periods, while others continue during recent 

times. Likewise, wells in some areas may have experienced 

rises in water levels during prior time periods, only to cease 

rising more recently. Other wells may still be experiencing 

water-level rises that continue today. Thus, an important aspect 

of the history of ground-water systems might be indicated by 

answers to questions such as: Are there areas in my region that 

have experienced declining ground-water levels in the past? 

What areas continue to have falling water levels? Are there 

areas where water levels have started recovering? 

In an effort to discern areas with falling or rising water 

levels, either in the past or continuing to the present, subsets of 

existing water-level observations were developed for specific 

decline and rise criteria. Although a well may demonstrate 

several periods of rising and falling water levels throughout 

its period of record, only the maximum total decline or rise is 

presented in these layers. Creation of these indicators of groundwater conditions is facilitated by two VBA® programs. The first 

program analyzes the data record for all wells in a dataset and 

produces a spreadsheet of summary decline and rise information for all wells (fig. 1). The second program produces portable 

document format (pdf) hydrographs of wells in the spreadsheet 

Figure 1. Schematic representation of algorithm for VBA® program 

that produces ground-water conditions subset of wells experiencing 

water-level declines and wells experiencing water-level rises.

produced by the preceding program and an output file for 

projecting the selected wells in a GIS system. Producing custom 

hydrographs for many of the indicators of ground-water conditions allows for descriptive information to be added to the hydrograph that emphasizes the data used in the interpretation. For 

the water-level decline and rise conditions, observations used to 

compute the total decline and rise values are highlighted on the 

hydrograph and the values themselves are displayed (fig. 2). This 

allows those interested in further details to see the observations 

used in determining if a well is included in a declining or rising 

water-level subset in the context of all available data for the well.

DETERMINE RANGE OF DATA 

FOR CURRENT SITE 

IDENTIFICATION NUMBER

ADVANCE TO 

NEXT SITE ID 

IN DATASET

LAST SITE

ID IN DATASET?

NO

YES

FIND WATER-LEVEL 

OBSERVATION IN CURRENT 

RANGE THAT IS MAXIMUM 

DEPTH TO GROUND 

WATER FOR SITE

• SAVE OBSERVATION DATE AND DEPTH

FIND MINIMUM DEPTH TO 

GROUND WATER BEFORE 

MAXIMUM DEPTH 

• SAVE OBSERVATION DATE AND DEPTH

• COMPUTE WATER-LEVEL DECLINE

SORT DECLINE AND RISE 

WORKSHEETS, CREATING 

SUBSETS BASED ON USER 

REQUIREMENTS

DECLINE LAYER RISE LAYER

• MINIMUM NUMBER OF OBSERVATIONS

DEFINING DECLINE OR RISE

• MINIMUM WATER-LEVEL DECLINE 

OR RISE

• DATE OF MAX/MIN DEPTH

WRITE RESULTS FOR 

THIS SITE ID TO

 “DECLINE WORKSHEET”,

• SITE ID, MAX DECLINE, MIN DEPTH

DATE, MIN DEPTH, MAX DEPTH DATE, 

MAX DEPTH, NUMBER OF OBSERVATIONS

BETWEEN MAX AND MIN, ADDITIONAL 

INFORMATION FOR HYDROGRAPH

FIND MINIMUM DEPTH TO 

GROUND WATER AFTER 

MAXIMUM DEPTH 

• SAVE OBSERVATION DATE AND DEPTH

• COMPUTE WATER-LEVEL RISE

WRITE RESULTS FOR 

THIS SITE ID TO

 “RISE WORKSHEET”,

• SITE ID, MAX RISE, MAX DEPTH

DATE, MAX DEPTH, MIN DEPTH DATE, 

MIN DEPTH, NUMBER OF OBSERVATIONS

BETWEEN MAX AND MIN, ADDITIONAL 

INFORMATION FOR HYDROGRAPH
Subsets of Ground-Water Conditions Information 5

Figure 2. Examples of annotated hydrographs produced for wells experiencing water-level decline (top) and wells 

experiencing water-level rise (bottom).

Created on 9/25/2007

0

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Date of Measurement

Depth to Ground Water Below LSD (feet)

Site ID: 332347113094101

Well Name: B-01-09 26CCB1

Longitude: -113.162131

Latitude: 33.39642822

Altitude of Land Surface: 1115 (ft AMSL)

Well Depth: 1015 [ft]

Maximum Water-level Rise

During Period of Record = 386 [ft]

Water-level Observation

Observations > 1 Year Apart

Observations ≤ 1 Year Apart

Observations for Computing Rise

Created on 9/28/2007

0

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Date of Measurement

Depth to Ground Water Below LSD (feet)

Site ID: 325727112072601

Well Name: D-05-02 26DCC

Longitude: -112.123888

Latitude: 32.9575

Altitude of Land Surface: 1337 (ft AMSL)

Well Depth: 600 [ft]

Maximum Water-level Decline

During Period of Record = 435 [ft]

Water-level Observation

Observations > 1 Year Apart

Observations ≤ 1 Year Apart

Observations for Computing Decline
6 Methods and Indicators for Assessment of Regional Ground-Water Conditions in the Southwestern United States

The VBA® program written to produce the decline or rise 

subsets of data analyzes all observations for each well in the 

spreadsheet. For a particular well record, the maximum depth 

to ground water is first identified, followed by the minimum 

depth to ground water. For the declining water-level subset, 

the minimum depth to ground water will occur before the 

maximum depth and may be the first observation in the dataset 

for a constantly declining water table. The rising water-level 

subset contains wells with the minimum depth to water occurring after the maximum depth. The maximum depth to water 

may be the first observation in the dataset for the well. A 

worksheet is produced containing summary information for 

each well in the dataset (fig. 3). This information includes the 

site identification number, the maximum water-level decline 

or recovery, the dates of observation, and depths of minimum 

and maximum ground-water levels. Additional information 

is contained in the summary worksheet for each well that is 

utilized by the hydrograph-VBA® program to highlight the 

points of inflection on the hydrograph. The summary worksheet allows the user to see the range of water-level declines 

and rises for the dataset being analyzed, as well as the number 

of observations defining the decline or rise. On the basis of 

this information, the user can decide what subset of the decline 

or rise wells to display in the GIS. For the alluvial basins 

analyzed in the Lower Colorado River Basin in Arizona, a 

minimum water-level decline of 75 feet was used for inclusion 

in the declining water-level subset and a minimum rise of 50 

feet was used for inclusion in the rising water-level subset. For 

both the declining and rising subsets, at least four observations were required to define the rising or falling water levels. 

Screening by minimum levels of decline and rise, and by 

number of observations defining the decline and rise, produces 

a manageable number of wells in the subsets, eliminating 

wells with observations that simply oscillate. By providing 

the dates of the maximum and minimum water-level observations, the user is able to further break the subset of declining 

or rising wells into time periods of interest by displaying these 

sites separately in a GIS system. For the analyses in Arizona, 

the datasets were divided into a “historical” group with waterlevel declines or rises that occurred before 1997 and a “recent” 

set with declines or rises that continue after that date. Displaying the declines and rises in time-selected groupings allows for 

temporal information to be conveyed through the GIS system.

A second VBA® program was created that constructs annotated hydrographs for all of the wells in the summary decline 

or rise worksheets produced by the first program and writes an 

output worksheet used in displaying the wells (with links) in 

a GIS system. This program scans through the list of wells on 

Figure 3. Screenshot of example spreadsheet output from VBA® program summarizing wells experiencing water-level decline.
Subsets of Ground-Water Conditions Information 7

the summary worksheet, pulls all observations for each well 

from the original data worksheet, plots the data on x-y charts 

on individual worksheets, and puts highlight bars on each chart 

indicating which observations were used for the decline or rise 

calculations (fig. 2). A worksheet is also created that contains 

the latitude, longitude, site identification number, magnitude 

of water-level decline or rise, and a link containing the path 

and filename for each well hydrograph. This worksheet can be 

read by GIS programs as an xyz input file and used for projecting the location of wells in this layer with linked hydrographs. 

The magnitude of water-level decline or rise is used to define 

the color intensity of the symbol representing the locations of 

wells (figs. 4 and 5). An overview of locations of wells that 

have experienced declines or rises can then be seen, with further 

information available on a well hydrograph by selecting the 

well of interest with the hyperlink tool.

The visual inspection of the hydrograph of each well 

provides a quality assurance of data for the decline and rise 

subsets. Although no observations flagged as “pumping” in 

the database are used in the processing of these subsets, there 

may be minimum or maximum depth observations found that 

appear to not be indicative of local ground-water conditions 

(for example, a sharp decline or rise in water level in a short 

time period between otherwise fairly stable observations). 

A third VBA® program was written for processing the wells 

of the decline and rise subsets as an aid in the correction of 

hydrographs found to be utilizing possibly erroneous observations as determined by the user during visual inspection. This 

program requests the new maximum or minimum water-level 

observation and observation date (determined by the user from 

visual inspection of the hydrograph) and updates the summary 

worksheet, the maximum or minimum depth highlight bars, 

the computed decline or rise, and the associated output worksheet used by the GIS system.

A significant and obvious limitation of the interpretation 

provided in the declining and rising subsets is that the amount 

of decline and rise for each well can only be computed from 

available data. In many instances in the Arizona water-level 

data analyzed using the methods in this report, the minimum 

depth to ground water was the first observation in the well’s 

period of record. While water-level decline computed on 

the basis of this first observation is informative, it does not 

include any decline in water level in the area that may have 

occurred before this first recorded observation. Therefore, 

water-level declines and rises in these subsets should properly 

be termed “observed declines and rises” to distinguish these 

measured changes in water level from those that occurred 

before or after the observations in the well record. Presentation of subsets of data indicating declining or rising water 

levels does, however, illustrate which areas have been under 

stress during different time periods and which areas may be 

responding with increased water levels. By displaying these 

subsets of declining and rising water levels in a GIS format 

with linked interpretive hydrographs, information (and not 

just data) can be made available and understandable on the 

history of changes in water levels.

Recent Trends in Ground-Water Levels

Presenting subsets of wells experiencing significant 

water-level declines or rises during different time periods is an 

important component of understanding ground-water conditions. However, information on the current trends in water 

levels may be a better indicator of the current and near-future 

status of the aquifer system. A method was developed to indicate ground-water conditions by evaluating trends in groundwater levels in recent time periods and presenting these trends 

spatially to highlight regional conditions. A computer program 

was written to determine linear water-level trends in recent 

years on the basis of user-defined criteria. Wells with data 

achieving a specified goodness of linear fit are then used in a 

second program that constructs modified Thiessen polygons 

around each well and writes output files for displaying these 

polygons in a GIS system. Spatial presentation of these recent 

trends in water levels provides regional indicators of groundwater conditions and identifies areas having rising, stable, or 

falling ground-water levels.

Determining Trends in Water-Level Data

To determine trends in ground-water levels, a FORTRAN 

program was written that performs a linear regression on a 

subset of the data determined interactively by user-specified 

criteria (fig. 6). It is desirable to utilize as many wells as possible in a given area to represent trends in ground-water levels. 

By including more wells, ground-water conditions in a greater 

area can be interpreted and more confidence can be placed 

in conclusions based on similar trends in adjacent areas. 

However, only wells presenting a definitive trend should 

be considered in order to avoid misinterpretations based on 

scattered data. The trend-determination program works with a 

comma-delimited version of the well and water-level worksheet described previously (table 1). The trend-determination 

program begins by reading in all data from the worksheet. 

The user is first prompted for the start and end dates of the 

time period under investigation. The program sorts through 

the unique sites in the dataset, counts the number of wells 

with ≥2 water-level observations in the specified time period 

and reports this number to the user. The number of wells with 

less than two observations is also reported. Confirmation of 

the time period is then requested of the user, with an option 

to modify the start and/or end dates. The subset of data in the 

approved time period is saved.

Once the subset of observations in the requested time 

period has been determined, the user is prompted to select 

the data in the subset that will be used for trend analysis. 

Options for this selection are (1) using all water-level observations in the date range (fig. 7A), (2) using only seasonal 

data based on user-specified months of the year (fig. 7B), (3) 

using only observations that occur at the maximum or minimum amplitude of cyclical data (figs. 7C,D), or (4) using any 

combination of these subsets of data. Multiple subsets can be 
8 Methods and Indicators for Assessment of Regional Ground-Water Conditions in the Southwestern United States

Figure 4. Wells experiencing significant ground-water level decline during historical (pre-1997) time in the most developed basins of 

south-central Arizona. Well locations are linked to annotated hydrographs in the GIS system.

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Historical Declines in

Ground-Water Levels

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Centra

Ground-water basins

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Subsets of Ground-Water Conditions Information 9

Figure 5. Wells experiencing significant ground-water level rise continuing through recent time (post-1997) in the most developed 

basins of south-central Arizona. Well locations are linked to annotated hydrographs in the GIS system.

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TUBAC

MARANA

TUCSON

ORO VALLEY

MESA

ELOY

TEMPE

TONOPAH

PHOENIX

GLENDALE

FLORENCE

CAREFREE

STANFIELD

QUEEN CREEK

CASA

GRANDE

GREEN VALLEY

Recent Rises in 

Ground-Water Levels

!( 50 to 100 ft

!( 100.01 to 150 ft

!( 150.01 to 200 ft

!( 200.01 to 275 ft

!( > 275 ft

Central Arizona Project 

(CAP) Canal

0 20 40 Kilometers 10

0 6 12 Miles

114° W

36° N

34°

32°

112° 110°

Yuma

Tucson

Phoenix

Prescott

Flagstaff

A R I Z O N A

Area 

Enlarged

Ground-water basins

MEXICO

UNITED STATES
10 Methods and Indicators for Assessment of Regional Ground-Water Conditions in the Southwestern United States

created by choosing more than one option. The purpose of 

providing multiple options for the selection of subsets of data 

is to compute an acceptable linear regression that accurately 

describes the trend of water levels for as many wells as possible. For infrequently measured water levels, utilizing all 

data provides the most information and the best possibility 

of identifying a trend in water-level observations. However, 

data that are collected more frequently may benefit from 

analyses of seasonal (specified months) or cyclical (maximum and minimum amplitude) subsets of data (see table 2 

for results using example Idaho dataset). If the seasonal data 

option is selected, the user is prompted to select the months 

in which observations are to be analyzed. The user is then 

informed of the number of wells with at least two observations in the given date range in the user-specified months 

and the number of wells with less than two observations. The 

user is asked to confirm this choice of months or given the 

opportunity to modify them. Subsets of the water-level data 

are saved to appropriate arrays.

Upon choosing the types of data in the date range for 

trend analysis, the user is prompted for a minimum number of 

water-level observations for the well to be used in the analysis. Information is reported to the user on the number of wells 

for each case that have at least the user-specified minimum 

number of observations and also the number of wells for 

other values of minimum observations down to two. The user 

may choose a new minimum or confirm the existing choice. 

Selecting wells by minimum number of valid measurements 

allows the user to maximize confidence in the resulting trend 

information while providing information on tradeoffs between 

minimum observations and number of wells for which a trend 

will be computed. For instance, it may be desirable to have at 

least 10 water-level observations in a 20-year period for trend 

analysis, but if there are 3 times the number of wells with at 

least 5 observations then a compromise may be acceptable. 

The user may then elect to “bracket” the start and/or end dates 

of the trend analysis. Bracketing requires that at least one 

water-level observation be within a user-specified number of 

months after the start date and/or before the end date for the 

well to be given further consideration in the trend analysis. 

The use of bracketing ensures that trends are computed for 

wells with water-level observations that span the time period 

under investigation and are not computed for wells with water 

levels that are grouped around a single point in time.

At this point in the trend-computation algorithm, 

subset(s) of water-level data have been produced through a 

process of choosing date range, type of data (all data, seasonal, 

maximum amplitude, and/or minimum amplitude), minimum 

number of observations, and possibly start and/or end bracketing. Linear trends are computed on all remaining subsets of 

data with further screening of data based on user-specified 

coefficient of determination (R2

) of the linear fit. To aid in 

selecting an appropriate minimum R2 for screening, the user is 

provided with information on the number of remaining wells 

with linear trends having R2≥0.25, 0.50, and 0.75 for each 

of the cases chosen previously. The user is prompted for a 

minimum coefficient of determination, the program reports the 

number of wells with linear trends meeting this criterion for 

each subset of data, and the user is asked to confirm or change 

this minimum R2

 value. Once a minimum R2 value is selected, 

the user decides which group will be chosen for output (all 

data, seasonal, maximum amplitude, and/or minimum amplitude, or any combination of these subsets). The program will 

output all of the water-level information and well data for each 

of the cases chosen. If a well appears in more than one subset 

of data, information on the well from the subset producing the 

highest coefficient of determination is outputted.

Two output files are produced by the trend-determination program. The first is an input file used by the program 

described in the next section that creates modified Thiessen 

polygons to represent spatially the trends in ground-water 

conditions. The header of this file contains all selection information, including date range, bracketing information, seasonal 

screening information, minimum number of observations, and 

minimum goodness of linear fit (fig. 8). This header aids in 

keeping track of multiple output files for different datasets. 

Below this header are data for each of the wells in the final 

selection, including trend information of slope, y-intercept, 

and R2

 of the linear trend of the data; the number of observations used in the trend computation; the name of the subset of 

data output; and well information of latitude and longitude of 

the well location, site identification number, well name, well 

altitude, hole depth, and well depth. The second file outputted 

by this program contains trend information for each selected 

well along with all water-level observations and dates used in 

determining the trend. This file is used by a separate VBA®

program that creates hydrographs and highlights the observations used for trend analysis (fig. 9). Creation of hydrographs 

for the wells used in the trend analyses is useful for linking with the spatial representation of the trend information 

described in the next section. Visual inspection of hydrographs 

should be conducted to identify data that should be deleted 

from the Thiessen input file.

Spatial Representation of Water-Level Trend 

Information

Spatial representation of linear-trend point information 

provides a regional picture of trends in ground-water conditions. The method developed for this tool involves spatial 

extrapolation of the trend information using the concept of 

Thiessen (or Voronoi) polygons. Thiessen polygons have been 

used to analyze spatially distributed data in ecology (individual space per plant or animal), meteorology (areal rainfall 

estimations from rain gauges), and business (delineation of the 

marketshed of retail or service nodes), as well as for analysis 

in other fields (for example, see Mumm, 2005, or Okabe and 

others, 2000). Briefly, a region of influence for a well is computed with Thiessen polygons by constructing perpendicular 

bisectors between the well and all other wells in the dataset. 

The intersections of these bisectors form potential vertices of 
Subsets of Ground-Water Conditions Information 11

Figure 6. Schematic representation of FORTRAN algorithm for computing trends in ground-water level data.

PROMPT USER FOR 

START AND END DATES 

FOR TREND ANALYSIS 

REPORT TO USER NUMBER OF WELLS 

WITH ≥ 2 AND NUMBER WITH < 2 WATER 

LEVEL OBSERVATIONS IN DATE RANGE

PROMPT USER FOR 

MINIMUM NUMBER OF 

WATER LEVEL OBSERVATIONS

REPORT TO USER NUMBER OF WELLS 

WITH ≥ USER SPECIFIED OBSERVATIONS. 

ALSO REPORT NUMBER OF WELLS FOR 

OTHER INTEGER CHOICES OF MINIMUM 

OBSERVATIONS DOWN TO 2

PROMPT USER FOR 

MINIMUM R2 FOR LINEAR TREND

REPORT NUMBER OF WELLS WITH 

TRENDS HAVING R2 ≥ 0.25, 0.5 AND 

0.75 FOR EACH SUBSET OF DATA

REPORT NUMBER OF WELLS WITH 

TRENDS HAVING R2 ≥ USER VALUE FOR 

EACH SUBSET OF DATA

PROMPT USER FOR NUMBER 

OF MONTHS AFTER START DATE 

FOR AT LEAST 1 OBSERVATION

REPORT NUMBER OF WELLS WITH AT 

LEAST 1 OBSERVATION IN START BRACKET

AND NUMBER OF WELLS WITH NONE

PROMPT USER FOR NUMBER 

OF MONTHS BEFORE END DATE 

FOR AT LEAST 1 OBSERVATION

REPORT NUMBER OF WELLS WITH ≥ 1 

OBSERVATION IN BOTH START AND END 

BRACKETS AND NUMBER WITH NONE

IS DATE RANGE 

ACCEPTABLE?

BRACKET 

START AND/OR 

END DATES?

ARE MONTHS 

ACCEPTABLE?

YES

YES

YES

YES

YES

YES

NO

IS MIN R2

ACCEPTABLE?

YES

NO

NO

WHAT DATA IN DATE RANGE

SHOULD BE USED FOR

TREND ANALYSIS? • ALL DATA

• SEASONAL (MONTHLY) DATA

• MAXIMUM AMPLITUDE DATA

• MINIMUM AMPLITUDE DATA

• ANY COMBINATION

OUTPUT TRENDS FOR WHICH 

DATA SUBSET? WRITE OUTPUT FILES FOR

CREATING THIESSEN 

POLYGONS AND DATA FOR

CREATING HYDROGRAPHS

• ALL DATA

• SEASONAL (MONTHLY) DATA

• MAXIMUM AMPLITUDE DATA

• MINIMUM AMPLITUDE DATA

• ANY COMBINATION

IF SEASONAL OPTION IS 

CHOSEN, PROMPT USER FOR 

MONTHS IN WHICH DATA 

ARE TO BE CONSIDERED

IS START 

BRACKET 

ACCEPTABLE?

ARE START 

AND END BRACKETS 

ACCEPTABLE?

ARE MINIMUM

OBSERVATIONS 

ACCEPTABLE?

REPORT TO USER NUMBER OF WELLS 

WITH ≥ 2 AND <2 OBSERVATIONS IN 

DATE RANGE IN CHOSEN MONTHS

NO

NO

NO

NO
12 Methods and Indicators for Assessment of Regional Ground-Water Conditions in the Southwestern United States

Figure 7. Examples of different subsets of water-level data during a given time period used for 

trend analyses including (A) all data, (B) seasonal data (here January and February), (C) minimum 

amplitude data, and (D) maximum amplitude data.

12/31/96 05/15/98 09/27/99 02/08/01 06/23/02 11/05/03 03/19/05

Date of measurement

Depth to ground water, in feet

160

165

170

175

180

185

190

150

155

160

165

170

175

225

230

235

240

245

250

0

5

10

15

20

25

A

B

C

D

Observations used for trend analysis

Best fit linear trend of observations used

Water-level observations
Subsets of Ground-Water Conditions Information 13

the polygon, with final vertices being ones closest to the well 

that form a closed loop. Thiessen polygons are constructed so 

that any location within a polygon is nearer to that polygon’s 

interior well than to any other well.

A second FORTRAN program was written to construct 

modified Thiessen polygons for spatial representation of 

ground-water trends (fig. 10). To construct the polygons 

around trend wells, a maximum distance of representation 

for a well is first required. The maximum-distance boundary limits the distance to which the recent trend information 

will be applied. Spatial data for wells selected by the trend 

program are then read into arrays. Geographic information for 

the wells (latitude and longitude in decimal degrees) is translated into a local coordinate system with the southernmost and 

westernmost well arbitrarily chosen as the datum. Geographic 

coordinates are translated to local distances in meters from this 

origin using the great-circle representation of the Earth (see 

http://www.nationalatlas.gov/articles/mapping/a_projections.

html). Having local coordinates in meters allows for easy comparison of a vertex distance from a well with the user-specified 

maximum radius of influence.

After translating coordinates, the program computes 

Thiessen polygon vertices by selecting the nearest intersections of all perpendicular bisectors between a well and all 

other wells in the domain. Additional vertices are added along 

each edge to create a smoother appearance during application 

of the maximum distance of representation. If a vertex lies 

further from a well than the user-specified maximum distance 

of representation, then the vertex is brought to the maximum 

distance along the line between the well and the vertex, thus 

creating a modified Thiessen polygon. Once all vertices are 

computed for all wells, coordinates are translated back to the 

geographic system, sorted in a clockwise direction, formatted 

for use with GIS tools, and saved to a polygon coordinates 

Table 2. Number of wells from example Idaho dataset with 

acceptable linear trends in ground-water levels for different 

subsets of water-level observations analyzed. 

[All subsets in this example required a minimum of three observations in the 

time period from January 1, 1997, to December 31, 2006, and a minimum 

goodness of linear fit of 0.50.]

file. An additional file is written that contains trend information, well information, and the file path to the hydrograph with 

data for each well used to create each polygon. This file is 

joined with the Thiessen polygon shapefile in the GIS environment as an attribute table. This permits GIS display options, 

such as categorization of trend areas by shading polygons, 

as well as hyperlinking between the trend polygons and well 

hydrographs, allowing users to see additional details of waterlevel observations.

Application of the analysis and display of water-level 

trends is demonstrated on data for areas in Arizona with the 

most developed ground-water systems using the following 

criteria: a time period of interest from January 1, 1997, to 

December 31, 2006 (the most recent 10-year period at the 

time); no bracketing of start or end times; a minimum of 

three observations required during the period of interest; and 

a goodness of linear fit (R2

) of at least 0.75 (fig. 11). Well 

hydrographs were visually inspected before the inclusion of 

a well in the trend presentation in order to ensure that the 

computed trend qualitatively represented the trend in recent 

water levels. Modified Thiessen polygons were constructed 

for the resulting subset of wells using a maximum radius of 

representation of 5 km. Three trend categories were chosen for presentation in this figure: areas with water levels 

declining at a rate of more than 1 foot per year were labeled 

“falling”; wells with water-level trends between -1 and +1 

foot per year were labeled “nearly stable”; and wells with 

water levels rising at a rate of more than 1 foot per year were 

labeled “rising”.

While the method of computing and presenting trends 

in ground-water conditions was demonstrated for a recent 

10-year period, other time periods of interest can easily be 

analyzed as well. For example, there may be interest in trends 

in water levels since the enactment of pertinent legislation or 

management decisions, or in trends since the onset of some 

climatic change such as a drought or increase in rainfall. 

Indices of falling, stable, or rising water levels used in the 

demonstration of the method are easy to understand by the 

general public, but other discretizations of trends could also be 

presented. For example, highlighting areas whose water levels 

are falling at a rate faster than some critical value might identify regions approaching acute water challenges. Analyzing 

water-level observations that occur during certain months of 

the year (the seasonal selection) or observations of maximum 

or minimum water levels in a cyclical period of record allows 

flexibility to answer questions such as: What is the trend in 

winter water levels? or What is the trend of water levels during the most extreme dry conditions each year?

Presenting ground-water conditions in terms of recent 

trends may not indicate conditions relative to a time before 

development, but it does allow analysis of wells without 

extensive historical records. An analysis of recent trends 

allows for determination of current status and near-future 

projections and would be an appropriate index for areas that 

may not have predevelopment data. In areas that have distinct 

multiple aquifers with depth, trend analyses will need to be 

Subset of water-level observations 

Number of wells in dataset 

with acceptable linear 

trends in water levels

All available observations 339

Seasonal (January and February 

observations) 268

Maximum amplitude observations 399

Minimum amplitude observations 388

Best linear fit of all subsets 616
14 Methods and Indicators for Assessment of Regional Ground-Water Conditions in the Southwestern United States

Figure 8. Screenshot of example output of trend-computation program with header indicating selection choices.

Figure 9. Annotated hydrograph produced during the recent trends in ground-water levels analysis of ground-water conditions.

50 -111.8177 42.62408 -1.27 14.7 0.98 7 4.23727E+14 09S 40E 20CBC1 556.8 -99999 52.5 Max

Created on 10/5/2007

0

50

100

150

200

250

300

350

400

450

500

550

600

Dec

1926

Dec

1931

Dec

1936

Dec

1941

Dec

1946

Dec

1951

Dec

1956

Dec

1961

Dec

1966

Dec

1971

Dec

1976

Dec

1981

Dec

1986

Dec

1991

Dec

1996

Dec

2001

Dec

2006

Date of Measurement

Depth to Ground Water Below LSD (feet)

Site ID: 324035111412301

Well Name: D-08-06 35DDD2

Longitude: -111.688333

Latitude: 32.675555

Altitude of Land Surface: 1542 [ft AMSL]

Well Depth: 770 [ft]

 Recent Trend = 2.3 [ft/year]

Water-level Observation

Observations > 1 Year Apart

Observations ≤ 1 Year Apart

Recent 10-year Linear Trend
Subsets of Ground-Water Conditions Information 15

Figure 10. Schematic representation of FORTRAN 

algorithm for computing modified Thiessen polygons 

of selected wells for spatial representation of waterlevel trends.

performed on groups of wells that screen individual hydrogeologic units. Limitations of the method stem from issues 

with the areal representation of the trend from each well. The 

selection of a maximum distance of representation for a well 

is left to the user’s judgment of hydrologic conditions in the 

area of interest. However, the maximum distance is often only 

utilized to define the trend area for isolated wells or wells at 

the perimeter of clusters. Additionally, the trend method does 

not limit trend areas where they might intersect non-aquifer 

materials such as exposed bedrock of mountains. This could 

be rectified by clipping the trend layer using a coverage of 

geologic materials in a GIS system. 

Wells With Long-Term Records

Long-term changes in ground-water levels may be caused 

by both human and climate-induced influences. In areas where 

ground water has been heavily developed for agricultural use, 

consistent water-level declines may be seen over decades. 

Changes in water management practices, such as the importation of surface water from areas outside the basin, may also 

affect aquifers and contribute to stabilization or increases in 

ground-water levels over time. Wells that tap aquifers in direct 

hydraulic connection with perennial or intermittent streams 

or rivers may record changes in water levels that indicate the 

effects of these surface-water systems and their associated 

riparian vegetation. Observations of water levels in aquifers in remote, undeveloped areas may indicate the system 

response to climate changes. In each of these cases as well as 

many others, it may be advantageous to see evidence of the 

condition of ground water over as long a period of time as is 

possible in an area of interest. To aid in the understanding of 

long-term changes in ground-water conditions, an indicator of 

ground-water conditions for wells with long-term records was 

created. Creation of this indicator is facilitated by the use of 

two VBA® programs written to provide summary information 

on the periods of records for wells in a dataset and to produce 

hydrographs and an output file for use in a GIS system based 

on user-specified criteria of period length and number of 

observations.

Although a subset of all data from all wells could be 

created for any basin of interest, it is important to balance 

the level of effort of producing this indicator subset (and 

quality-assuring the resulting hydrographs) with the additional 

information that may or may not be gained. With a focus on 

the long-term conditions of water levels in a basin, it is desirable to have the subset contain many wells that have long 

records with datasets as complete as possible. Depending on 

the history of development for a basin, the time period that 

constitutes “long term” may vary. A program was written that 

analyzes the period of record for each well in a dataset and 

presents summary information on the tradeoffs between the 

record start date, record end date, and number of observations in a record versus the number of wells that would meet 

these criteria. The program creates a summary worksheet that 

contains each unique site identification number in the dataset 

PROMPT USER FOR 

MAXIMUM DISTANCE OF 

REPRESENTATION FOR 

A WELL 

WRITE OUTPUT FILES FOR 

POLYGON VERTICES 

AND ATTRIBUTES

READ SCREENED-IN WELL

INFORMATION FROM TREND

OUTPUT FILE

CONVERT GEOGRAPHIC 

COORDINATES OF WELLS TO LOCAL

METER COORDINATES USING

‘GREAT CIRCLE’ TRANSFORMATION

COMPUTE THIESSEN 

POLYGONS FOR EACH WELL

IF VERTEX OF POLYGON LIES

OUTSIDE MAXIMUM RADIUS OF

INFLUENCE, BRING IN VERTEX

TO MAX DISTANCE ALONG 

WELL-VERTEX LINE

CONVERT LOCAL METER 

COORDINATES OF POLYGON 

VERTICES TO 

GEOGRAPHIC COORDINATES
16 Methods and Indicators for Assessment of Regional Ground-Water Conditions in the Southwestern United States

along with the date of the site’s first and last observation and 

the total number of water-level observations for the well. 

This list of period-of-record information for all wells in the 

dataset is then summarized by the program in matrices. For 

several values of minimum number of observations, a matrix 

is created comparing the number of wells with certain combinations of start/end observation dates. A two-dimensional 

surface chart graphically presents the same information (fig. 

12). From the matrix and chart summary information, the 

user can decide on what choice of record start date, most 

recent observation, and minimum number of observations 

best represents the long-term conditions of the basin of 

interest and can see how many wells will be represented by 

these choices. A second VBA® program creates hydrographs 

and an input file for the GIS system based on user-defined 

minimum number of observations, earliest observation cutoff 

date, and latest observation cutoff date. This program scans 

the dataset for wells that meet these criteria, produces graphs 

of the water-level observations over time (fig. 13), and creates a worksheet containing the longitude, latitude, site identification number, and well name of each well. Also included 

in the output worksheet are the number of observations in the 

well record, the start and end dates of the observations, and 

the link to the hydrograph for the well. This worksheet can 

be input into a GIS system as an xyz input file to display the 

location of these wells. The duration of the period of record 

of water-level observations for the wells is used to color each 

well location (fig. 14). Locations of wells with long-term 

records can then be visualized on basin maps with hydrograph and well information available by selecting the well of 

interest with the hyperlink tool.

While the aforementioned programs are capable of analyzing very large datasets (such as those for an entire State) 

for information on long-term ground-water records, it may 

be beneficial to analyze simultaneously water-level data from 

smaller groups of basins that have experienced similar historical development patterns. As an example, for some areas in 

southern Arizona records extending back 25 to 35 years may 

be sufficient to capture the human-induced changes in water 

levels, while other areas may need records extending back to 

pre-World War II time periods or beyond. For the purpose of 

calibrating ground-water flow models, this analysis and output 

may also be useful in selecting wells that span a time period of 

interest and have been frequently measured.

Recent Depth to Ground Water

A simple yet informative subset of information on 

ground-water conditions is provided by visualization of the 

recent depth to ground water in wells. In areas not serviced 

by water providers, homeowners, realtors, and potential home 

and land buyers have an interest in knowing the depth from 

land surface to ground water in their area. Additionally, water 

providers, farmers, the mining industry, and others might want 

to know how the depth to ground water varies in their local 

area and regionally. A VBA® program was written that utilizes 

existing water-level datasets to produce an input file to display 

in a GIS system the location of wells and the most recent 

depth to ground water based on a user-defined “recent” date. 

To illustrate the results of this program, an interactive map 

service was developed for Arizona that shows this information 

in a graphical format (http://montezuma.wr.usgs.gov/website/

azgwconditions/).

The definition of what is considered a recent water-level 

observation may vary in different regions. Ideally, water 

levels in all wells would be measured very frequently and 

the most recent observation, subject to some quality assurance standards, would be selected. Because wells in some 

areas are observed more frequently than in other areas, 

however, a balance must be achieved between presenting as 

many wells as possible and the elapsed time since the most 

recent observation for these wells. The inclusion of more 

wells helps maximize the area showing the locations of the 

recent-observation wells (and minimize the potential for 

users to erroneously extrapolate existing data to areas where 

there is no current information). Using observations that are 

too out of date in order to include as many wells as possible, 

however, may introduce errors in understanding current conditions. The VBA® program developed to create the subset of 

wells for the current depth to ground water first analyzes all 

water-level observations for each well in a dataset and provides the user with summary information to aid in selecting 

the recent-observation cutoff date. The total number of wells 

with at least one water-level observation after each year from 

2000 through 2007 is presented in a dialog box. The user is 

then prompted for the recent-observation cutoff date, and a 

worksheet is produced containing the latitude, longitude, site 

identification number, well depth, most recent observation 

date, and most recent water-level depth for wells measured 

on or after this date. This worksheet can be input as an xyz 

file into a GIS system for displaying the location of wells 

with recent observations. Coloration of the well location by 

groups of depth to ground water provides a visual overview 

of current ground-water conditions (fig 15). A small pop-up 

window can also be produced that displays the site identification number, most recent water-level observation and 

date, and well depth (if available) for wells selected with the 

hyperlink tool.

Possible limitations to information provided in the 

indicator of recent depth to ground water result from the 

impracticability of quality-assuring all recent observations 

and the potential for misinterpretation of the data presented. 

Owing to the large number of wells included in the recent 

depth to ground water subset, it is not feasible to view the 

most recent water-level observation for each well in the 

context of other well data to ensure that it is reasonable. For 

example, there are more than 6,000 wells with a water-level 

observation on or after January 1, 2004, in the alluvial basin 

study in Arizona. In order to utilize only measurements that 

were indicative of natural ground-water levels, any observation flagged as dry, injection, nearby injection, plugged, 
Subsets of Ground-Water Conditions Information 17

 

Figure 11. Trends in ground-water levels for the 10-year period from 1997 through 2006 for the most developed basins in south-central 

Arizona. Trend areas are linked to annotated hydrographs in the GIS system.

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!

Yuma

Tucson

Phoenix

Prescott

Flagstaff

A R I Z O N A

114° W

36° N

34°

32°

112° 110°

Area 

Enlarged

MESA

ELOY

TEMPE

TONOPAH

PHOENIX

GLENDALE

FLORENCE

CAREFREE

STANFIELD

QUEEN CREEK

CASA

GRANDE

0 20 40 Kilometers 10

0 6 12 Miles

TUBAC

MARANA

TUCSON

ORO VALLEY

GREEN VALLEY

Ground-water basins

MEXICO

UNITED STATES
18 Methods and Indicators for Assessment of Regional Ground-Water Conditions in the Southwestern United States

Figure 12. Portion of example output in Microsoft® Excel® spreadsheet of VBA® program written to provide information on time span and 

frequency of ground-water level observations to aid in selecting wells for inclusion in “Wells with Long-Term Records” layer.
Summary and Conclusions 19

obstructed, pumping, recently pumping, nearby pumping, 

nearby recently pumping, or well destroyed was not used 

in the layer. At times, however, data are input incorrectly 

into databases, either through measurement error or simply 

mistaken entry (for example, 201 ft when 20.1 was meant). 

These data may or may not be indicated with any of the 

flags described above and therefore may present misleading 

information about the depth to ground water. Additionally, 

depth to ground water in a well, especially near mountain 

pediments, may represent only local perched conditions. 

Attempts to infer depth to ground water some distance from 

these perched wells may lead to erroneous conclusions.

Summary and Conclusions

Ground water is the source of drinking water for 50 

percent of the population in the United States and as much as 

90 percent of the population in rural areas, especially in the 

southwestern United States (Anderson and Woolsey, 2005). 

The dependence of domestic use and irrigation upon ground 

water makes it a highly valuable natural resource. Unlike 

surface-water indicators such as stage or discharge, groundwater conditions may be more difficult to assess and present. 

Individual ground-water levels in wells are an illustrative 

measure of an aquifer system, but these point data do not give 

a good sense of the regional scale of conditions. Additionally, 

changes in ground-water conditions may involve time scales 

ranging from days to many years, depending on recharge, soil 

properties, and depth to the water table. The lack of an easily 

identifiable ground-water property indicative of current conditions and inclusive of differing time scales makes the presentation of ground-water conditions a challenging task, particularly on a regional basis. Publicly available tools utilizing easy 

to understand ground-water information are needed to improve 

the value of existing datasets. Methods were developed to 

explore ways of utilizing existing water-level databases for the 

analysis and presentation of ground-water conditions. Computer software was written to use existing water-level data and 

aid in creating different indicators of ground-water conditions to display in a GIS system. Several subsets of information covering different time periods and different aspects of 

Figure 13. Example of a hydrograph produced for the “Wells with Long-Term Records” layer of ground-water conditions. 

Created on 9/28/2007

0

50

100

150

200

250

300

350

400

450

500

550

600

Dec

1926

Dec

1931

Dec

1936

Dec

1941

Dec

1946

Dec

1951

Dec

1956

Dec

1961

Dec

1966

Dec

1971

Dec

1976

Dec

1981

Dec

1986

Dec

1991

Dec

1996

Dec

2001

Dec

2006

Date of Measurement

Depth to Ground Water Below LSD (feet)

Site ID: 321407110560801

Well Name: D-14-14 08BAA

Longitude: -110.936201

Latitude: 32.23535369

Altitude of Land Surface: 2472 [ft AMSL]

Well Depth: 449 [ft]

Water-level Observation

Observations > 1 Year Apart

Observations ≤ 1 Year Apart
20 Methods and Indicators for Assessment of Regional Ground-Water Conditions in the Southwestern United States

Figure 14. Wells with long-term records in the most developed basins of south-central Arizona. Well locations are linked to 

annotated hydrographs in the GIS system.

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Central Arizona Project 

(CAP) Canal

Yuma

Tucson

Phoenix

Prescott

Flagstaff

A R I Z O N A

114° W

36° N

34°

32°

112° 110°

Area 

Enlarged

MESA

ELOY

TEMPE

TONOPAH

PHOENIX

GLENDALE

FLORENCE

CAREFREE

STANFIELD

QUEEN CREEK

CASA GRANDE

0 20 40 Kilometers 10

0 6 12 Miles

TUBAC

MARANA

TUCSON

ORO VALLEY

GREEN VALLEY

Wells With Long-Term Records

!( < 25 years

!( 25 to 50 years

!( 50 to 75 years

!( >75 years

Ground-water basins

MEXICO

UNITED STATES
Summary and Conclusions 21

Figure 15. Recent observations of depth to ground water (since 2004) in the most developed basins of south-central Arizona.

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Yuma

Tucson

Phoenix

Prescott

Flagstaff

A R I Z O N A

114° W

36° N

34°

32°

112° 110°

Area 

Enlarged

MESA

ELOY

TEMPE

TONOPAH

PHOENIX

GLENDALE

FLORENCE

CAREFREE

STANFIELD

QUEEN CREEK

CASA GRANDE

0 20 40 Kilometers 10

0 6 12 Miles

TUBAC

MARANA

TUCSON

ORO VALLEY

GREEN VALLEY

Central Arizona Project

(CAP) Canal

Recent Depth to 

Ground Water

(since 2004)

!( 0 to 50 ft

!( 50.01 to 100 ft

!( 100.01 to 200 ft

!( 200.01 to 400 ft

!( > 400 ft

Ground-water basins

MEXICO

UNITED STATES
22 Methods and Indicators for Assessment of Regional Ground-Water Conditions in the Southwestern United States

ground-water conditions are described and examples presented 

in a GIS system using data from the most developed areas of 

Arizona. Subsets of wells with water-level declines give an 

indication of how aquifer systems have been depleted, both 

historically (before 1997) and continuing until present (since 

1997). Layers of wells with water-level rises indicate areas 

that may have recovered, in either past or current times. Longterm well records present information over the longest period 

available for the basins and may exhibit effects of development, management, and climate. A new method of recent trend 

analysis provides a depiction of areas whose water levels 

appear to be improving, worsening, or holding steady on the 

basis of the most recent 10-year record. Trend analyses take 

into account anthropogenic and climatic impacts on aquifers 

and allow insight into systems where lengthy historical records 

are unavailable. A simple but informative analysis and presentation of recent depth to ground water in wells provides easy 

to understand information on local aquifer conditions. The 

use of consistent coloration across basins in the GIS system to 

identify levels of information for each subset of wells provides a visual overview of the region. The intensity of colored 

symbols can be used to highlight areas that are under stress or 

responding to management actions. For most indicators, additional details are provided through annotated hydrographs that 

are hyperlinked to well locations in the GIS system. This presents an opportunity to highlight aspects of a well’s hydrograph 

that provide evidence of the interpretation being emphasized 

in the indicator of ground-water conditions. 

The methods presented highlight the importance of 

frequent and consistent collection of ground-water level 

data (also, see Taylor and Alley, 2001). More information is 

available from frequently collected datasets, such as seasonal 

trends, than is available from more sparsely collected data. 

Additionally, it is important for ground-water levels to be 

monitored in rural areas in which significant development 

is planned in order to have a sufficient baseline from which 

to compare future impacts on the aquifer system. Groundwater levels are one good measure of the health of an aquifer 

system. Publicly available tools that are easy to understand are 

needed to make use of these valuable datasets. Application on 

a regional scale of the methods and indicators developed here 

would provide a consistent picture of ground-water conditions 

across the southwestern United States that is currently lacking. 

Regionally consistent indicators of ground-water conditions 

are of growing importance in an area where rapidly increasing 

population and potential climate change put increased stress 

on this valuable but unseen resource. Though estimates of 

recharge and ground-water extractions, and forecasts based on 

ground-water models are useful, ultimately only “facts under 

the ground” are evidence that ground-water resources are 

being managed in a sustainable manner.

References Cited

Anderson, Mark T., and Woolsey, Lloyd H., Jr., 2005, Water 

availability for the Western United States—key scientific 

challenges: U.S. Geological Survey Circular 1261, 85 p. 

[http://pubs.er.usgs.gov/usgspubs/cir/cir1261, accessed 

October 28, 2008].

Anderson, T.W., 1995, Summary of the Southwest Alluvial 

Basins, Regional Aquifer-System Analysis, south-central 

Arizona and parts of adjacent States: U.S. Geological Survey Professional Paper 1406-A, 33 p.

Mumm, M., 2005, Voronoi diagrams: Montana Mathematics 

Enthusiast, v. 1, no. 2, p. 44–55.

Okabe, A., Boots, B., Sugihara, K., and Chiu, S.N., 2000, 

Spatial tessellations; concepts and applications of Voronoi 

diagrams (2nd ed.) Hoboken, N.J., John Wiley, 696 p.

Taylor, C.J., and Alley, W.M., 2001, Ground-water-level 

monitoring and the importance of long-term water-level 

data: U.S. Geological Survey Circular 1217, 68 p. [http://

pubs.er.usgs.gov/usgspubs/cir/cir1217, accessed October 

28, 2008].

Tillman, Fred D., Leake, Stanley A., Flynn, Marilyn E., Cordova, Jeffrey T., and Schonauer, Kurt T., 2007, An online 

interactive map service for displaying ground-water conditions in Arizona: U.S. Geological Survey Open-File Report 

2007-1436, 16 p. [http://pubs.usgs.gov/of/2007/1436/, 

accessed October 28, 2008].

U.S. Geological Survey, 1984, National water summary 1983 

—hydrologic events and issues: U.S. Geological Survey 

Water-Supply Paper 2250, 243 p. [http://pubs.er.usgs.gov/

usgspubs/wsp/wsp2250, accessed October 28, 2008].
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Tillman and others—Methods and Indicators for Assessment of Regional Ground-Water Conditions—Scientific Investigations Report 2008–5209