U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2013–5040

Prepared in cooperation with the Arizona Department of Water Resources

Hydrology of the Middle San Pedro Watershed, Southeastern Arizona
COVER

A reach of the San Pedro River near St. David, Arizona, shown in January 2009 (top) and in August 2008 (bottom). Both 

photographs by Candice B. Hopkins, U.S. Geological Survey.
Hydrology of the Middle San Pedro 

Watershed, Southeastern Arizona

By Jeffrey T. Cordova, Jesse E. Dickinson, Kimberly R. Beisner, Candice B. 

Hopkins, Jeffrey R. Kennedy, D.R. Pool, Edward P. Glenn, Pamela L. Nagler, and 

Blakemore E. Thomas 

Prepared in cooperation with the Arizona Department of Water Resources

Scientific Investigations Report 2013–5040

U.S. Department of the Interior

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

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2015

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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 material contained within this report.

Suggested citation:

Cordova, J.T., Dickinson, J.E., Beisner, K.R., Hopkins, C.B., Kennedy, J.R., Pool, D.R., Glenn, E.P., Nagler, P.L., and 

Thomas, B.E., 2015, Hydrology of the middle San Pedro Watershed, southeastern Arizona: U.S. Geological Survey 

Scientific Investigations Report 2013–5040, 77 p., http://dx.doi.org/10.3133/sir20135040.

http://dx.doi.org/10.3133/sir20135040

ISSN 2328-0328 (online)
 iii

Conversion Factors and Datum

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

Area

square meter (m2

) 0.0002471 acre 

square hectometer (hm2

) 2.471 acre

Volume

cubic meter (m3

) 0.0008107 acre-foot (acre-ft) 

cubic hectometer (hm3

) 810.7 acre-foot (acre-ft) 

Flow rate

cubic meter per year (m3

/yr) 0.0008107 acre-foot per year (acre-ft/yr) 

cubic hectometer per year (hm3

/yr) 810.7 acre-foot per year (acre-ft/yr)

cubic meter per day (m3

/d) 35.31 cubic foot per day (ft3/d) 

meter per year (m/yr) 3.281 foot per year ft/yr) 

millimeter per year (mm/yr) 0.03937 inch per year (in/yr) 

Radioactivity

picocurie per liter (pCi/L) 0.037 becquerel per liter (Bq/L) 

Acceleration

microGal (μ Gal)

microGal (μ Gal)

10

0.328 x 10-9

nanometer/second2 (nm/s2) 

 feet/second2 (ft/s2

)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows: °F=(1.8×°C)+32

Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1988 (NGVD 88).

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

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

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) or micrograms per liter (μg/L).
iv 

Contents

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

Acknowledgments.........................................................................................................................................1

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

Purpose and Scope ..............................................................................................................................2

Description of the Study Area ............................................................................................................2

Previous Investigations........................................................................................................................4

Climate.............................................................................................................................................................6

Surface Water ...............................................................................................................................................6

Streamflow Data ...................................................................................................................................8

Streamflow Characteristics and Trends ...........................................................................................9

Base Flow ............................................................................................................................................11

Hydrogeology................................................................................................................................................14

Hydrogeologic Framework................................................................................................................14

Groundwater........................................................................................................................................18

Long-Term Changes in Groundwater Levels in Wells near Mountain Fronts..................22

Long-Term Changes in Groundwater Levels in Unconfined Parts of the 

Lower Basin Fill ............................................................................................................23

Long-Term Changes in Groundwater Levels in Confined and Semiconfined 

Parts of the Lower Basin Fill.......................................................................................24

Long-Term Changes in Groundwater Levels near the San Pedro River...........................24

Recent Changes in Groundwater Levels near the San Pedro River .................................26

Recent Changes in Groundwater Storage ............................................................................26

Groundwater Budgets ................................................................................................................................31

Benson Subarea and Narrows-Redington Subarea Predevelopment 

Groundwater Budgets ..........................................................................................................32

Groundwater Recharge .....................................................................................................................32

Mountain-Front and Mountain-Block Recharge ...........................................................................32

Distributed-Parameter Water Balance Model......................................................................32

Spatial and Temporal Distribution of Mountain-Block and Mountain-Front 

Recharge in the Benson Subarea ...................................................................34

Spatial and Temporal Distribution of Mountain-Block and Mountain-Front 

Recharge in the Narrows-Redington Subarea ..............................................34

Mountain-Front-Recharge Estimation Using the SWAB RASA Regression Model........35

Comparison of the Recharge Estimates Between the Distributed-Parameter 

Water Balance Model and the Regression Model .................................................35

Groundwater Recharge by Seepage from the San Pedro River.................................................36

Groundwater Recharge by Seepage in the Benson Subarea............................................37

Groundwater Recharge by Seepage in the Narrows-Redington Subarea ......................37

Incidental Recharge from Irrigation ................................................................................................40

Groundwater Underflow....................................................................................................................40

Groundwater Discharge .............................................................................................................................41

Groundwater Withdrawals................................................................................................................41

Groundwater Withdrawal Data ..............................................................................................41
 v

Irrigation......................................................................................................................................41

Municipal and Industrial...........................................................................................................42

Self-Supplied Domestic Groundwater Withdrawals ...........................................................43

Evapotranspiration of Groundwater in Riparian Areas ................................................................43

ET Models Based on Vegetative Index ..................................................................................44

Application of the EVI* ET Model............................................................................................48

Changes in Groundwater Storage ...................................................................................................49

Benson Subarea Groundwater Budget...........................................................................................49

Narrows-Redington Subarea Groundwater Budget.....................................................................50

Hydrochemistry and Water Quality...........................................................................................................50

Isotopic Composition of Precipitation .............................................................................................51

Surface Water Samples.....................................................................................................................51

Groundwater and Spring Water Samples.......................................................................................52

Spatial Variability of Groundwater Chemistry................................................................................52

Water Quality.......................................................................................................................................57

Study Limitations and Considerations for Future Data Collection and Analysis...............................57

Summary and Conclusions.........................................................................................................................62

References Cited..........................................................................................................................................64

Appendix........................................................................................................................................................69

Figures

 1. Map showing the middle San Pedro Watershed in southeastern Arizona ................................3

 2. Bar graph showing average monthly precipitation at Tombstone, Arizona, 1913–2009...........6

 3. Scatter plots showing annual and seasonal precipitation at Tombstone, Arizona, 

1905–2009 .......................................................................................................................................7

 4. Bar graph of annual precipitation at Tombstone, Arizona.............................................................8

 5. Scatter plot of annual streamflow of the San Pedro River at Charleston, Arizona............ 10

 6. Bar graph showing annual streamflow ..........................................................................................11

 7. Plot of average daily streamflow throughout the year at gaging stations on the 

San Pedro River in the middle San Pedro Watershed..........................................................12

 8. Bar chart of annual peak flows at the Redington and Charleston gaging stations ................13

 9. Bar chart of annual base flow at the San Pedro River near Tombstone, Arizona...................14

10. Bar chart of estimated median base flow at the Tombstone, Arizona......................................15

11. Map showing distribution of surficial hydrogeologic units in the middle 

San Pedro Watershed................................................................................................................15

12. Hydrogeologic cross sections showing distribution of coarse-grained, interbedded, and 

fine-grained hydrogeologic units in the middle San Pedro Watershed, Arizona ............16

13. Map showing wells in the middle San Pedro Watershed for which lithologic logs 

are available ................................................................................................................................17

14. Map showing predevelopment water-level contours in 1940.....................................................19

15. Map showing wells in which water levels were measured at daily and irregular 

intervals, and absolute gravity monitoring sites....................................................................20

16. Map showing water-level contours for the period October–November 2006 .........................21

17. Plots of long-term water-level records for 12 wells .....................................................................22
vi 

18. Plots of long-term water-level records in wells perforated in the coarse lower 

basin fill under confined or semiconfined conditions ..........................................................24

19. Long-term water-level records in 10 wells near the San Pedro River, all measuring 

less than 60 m in depth, and perforated in the Holocene alluvium.........................................25

20. Plots of daily water levels in wells near the San Pedro River and streamflow 

at the gaging station San Pedro River near Tombstone, Arizona ......................................27

21. Plots of daily water levels in wells near the San Pedro River and streamflow 

at the gaging stations San Pedro River near Tombstone, Arizona ....................................28

22. Plots of daily water levels in wells near the San Pedro River and streamflow 

at the streamgaing stations San Pedro River near Benson, Arizona ................................29

23. Plots of cumulative aquifer storage change and rainfall in the Benson subarea 

for 2008 through 2010 .................................................................................................................30

24. Conceptual illustration of groundwater recharge and discharge in the middle 

San Pedro Watershed................................................................................................................33

25. Plot showing decadal average annual groundwater recharge calculated from 

the distributed-parameter water balance model ..................................................................33

26. Plot showing decadal average annual recharge in the middle San Pedro Watershed 

calculated from the Southwest Alluvial Basins-Regional Aquifer System Analysis.......35

27. Plots of annual and seasonal streamflow seepage in the Benson and NarrowsRedington subareas of the middle San Pedro River Watershed, Arizona ........................38

28. Plot showing annual groundwater withdrawals for irrigation, municipal, self-supplied 

domestic, and industrial uses in the Benson subarea for 1936–2009 ................................42

29. Plot showing annual groundwater withdrawals for irrigation and self‐supplied 

domestic uses in the Narrows‐Redington subarea for 1936–2009.....................................42

30. Maps showing seasonal distribution of the groundwater component of 

evapotranspiration .....................................................................................................................46

31. Plot showing seasonal evapotranspiration (ET) and groundwater component of 

evapotranspiration in the Benson subarea for the years 2001–2009.................................48

32. Plot showing seasonal evapotranspiration (ET) and groundwater component of 

evapotranspiration in the Narrows-Redington subarea for years 2001–2009..................48

33. Plot showing oxygen and hydrogen isotope composition in samples of precipitation 

in the middle San Pedro Watershed.....................................................................................51

34. Plot showing δ18O compositions for precipitation relative to altitude of sample collection..51

35. Plots showing δ18O and δ2H values from surface-water samples plotted on the 

local meteoric water line...........................................................................................................52

36. Plots showing fluoride and sulfate in groundwater and surface water samples 

in the middle San Pedro Watershed........................................................................................52

37. Piper diagram showing relative ionic composition of surface water and 

groundwater in the middle San Pedro Watershed. ..............................................................53

38. Plot showing δ18O and δ2H values from the San Pedro River collected in August 2008 .........53

39. Map showing locations of groundwater and surface water sampling sites ...........................54

40. Plot showing tritium values in groundwater samples from the middle San Pedro 

Watershed, plotted against δ18O...............................................................................................55

41. Plot showing calcium and strontium ratios plotted against 14C in groundwater 

samples from the middle San Pedro Watershed...................................................................55

42. Plot showing sulfur isotope ratios (δ34S) in groundwater and surface water..........................55

43. Map of wells in the middle San Pedro Watershed.......................................................................58

44. Box plots of fluoride concentration in groundwater for each aquifer type in the 

middle San Pedro Watershed...................................................................................................58
 vii

Tables

 1. Streamflow-gaging stations on the San Pedro River in and near the middle San 

Pedro Watershed, Arizona..........................................................................................................9

 2. Average annual streamflow and selected annual exceedance-level streamflows 

at gaging stations on the San Pedro River for specified periods of records....................10

 3. Base flow discharge estimates at the Tombstone streamflow-gaging station .......................13

 4. Predevelopment groundwater budgets for the Benson and Narrows-Redington 

subare...........................................................................................................................................32

 5. Distributed-parameter water-balance model calculated average annual runoff and 

recharge for 1940–2009 in the middle San Pedro Watershed and subareas..................... 34

 6. Modified average annual natural recharge for the Benson subarea, NarrowsRedington subarea, and the Middle San Pedro Watershed for 1940–2009 ......................34

 7. Description of methods for estimating monthly seepage in reaches of the San Pedro 

River, middle San Pedro Watershed, Arizona..........................................................................36

 8. Gains and losses in monthly measured streamflow between gaging stations .......................37

 9. Average daily discharge and seepage values used to develop equation for 

estimating seepage between Tombstone and Benson.............................................................39

10. Regression equations for estimating monthly seepage along the San Pedro River...............40

11. Monthly changes in seepage along the San Pedro River between Charleston 

and Tombstone. .................................................................................................................................40

12. Average estimated annual rates of evapotranspiration and the groundwater 

component of evapotranspiration for 2001–2009 ..................................................................44

13. Groundwater budget for the Benson subarea for water years 2001–2009...............................49

14. Groundwater budget for the Narrows-Redington subarea for water years 2001–2009.........50

15. Differences in physical and mineral characteristics and in constituent 

concentrations in groundwater samples................................................................................56

16. U.S. Environmental Protection Agency drinking-water-quality standards...............................57

17. Results of analyses of blank samples .............................................................................................59

18. Results of analyses of replicate samples.......................................................................................60

19. Differences in constituent concentrations between samples analyzed at the U.S. 

Geological Survey National Water Quality Laboratory and at the University of 

Arizona Department of Hydrology and Water Resources Laboratories............................61

A1. Water-quality data for precipitation, surface water, and groundwater in the middle 

San Pedro watershed collected from 2007 to February 2009..............................................70

A2. Water-quality data for groundwater in middle San Pedro Watershed collected in 

August and December 2009 ......................................................................................................76
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Hydrology of the Middle San Pedro Watershed, 

Southeastern Arizona

By Jeffrey T. Cordova, Jesse E. Dickinson, Kimberly R. Beisner, Candice B. Hopkins, Jeffrey R. Kennedy, D.R. 

Pool, Edward P. Glenn, Pamela L. Nagler, and Blakemore E. Thomas 

Abstract 

In the middle San Pedro Watershed in southeastern 

Arizona, groundwater is the primary source of water supply 

for municipal, domestic, industrial, and agricultural use. 

The watershed comprises two smaller subareas, the Benson 

subarea and the Narrows-Redington subarea. Early 21st century 

projections for heavy population growth in the watershed 

have not yet become a reality, but increased groundwater 

withdrawals could have undesired consequences—such as 

decreased base flow to the San Pedro River, and groundwaterlevel declines—that would lead to the need to deepen existing 

wells. This report describes the hydrology, hydrochemistry, 

water quality, and development of a groundwater budget for 

the middle San Pedro Watershed, focusing primarily on the 

elements of groundwater movement that could be most useful 

for the development of a groundwater model.

Precipitation data from Tombstone, Arizona, and base 

flow at the stream-gaging station on the San Pedro River at 

Charleston both show relatively dry periods during the 1960s 

through the mid-1980s and in the mid-1990s to 2009, and 

wetter periods from the mid-1980s through the mid-1990s. 

Water levels in four out of five wells near the mountain fronts 

show cyclical patterns of recharge, with rates of recharge 

greatest in the early 1980s through the mid-1990s. Three wells 

near the San Pedro River recorded their lowest levels during 

the 1950s to the mid-1960s. The water-level record from one 

well, completed in the confined part of the coarse-grained 

lower basin fill, showed a decline of approximately 21 meters.

 Annual flow of the San Pedro River, measured at the 

Charleston and Redington gages, has decreased since the 

1940s. The median annual streamflow and base flow at the 

gaging station on the river near Tombstone has decreased by 

50 percent between the periods 1968–1986 and 1997–2009. 

Estimates of streamflow infiltration along the San Pedro River 

during 1914–2009 have decreased 44 percent, with the largest 

decreases in the months June–October in the Benson subarea. 

In the Narrows-Redington subarea, streamflow infiltration has 

decreased about 65 percent during 1914–2009.

The average annual outflow (27.6 hm3/year [cubic hectometers per year]) from the Benson subarea aquifer for water 

years 2001 through 2009 exceeded the inflows (20.0hm3/yr) 

by 7.60 hm3/yr. In the Narrows-Redington subarea for the 

same period, the average annual outflow (15.7 hm3/yr) from 

the aquifer system exceeded the inflows (13.8 hm3/yr) by 

nearly 2 hm3/yr. The largest withdrawals of groundwater in 

both subareas are for irrigation; these withdrawals peaked in 

1973 and have been steadily decreasing since then. Recharge 

from streamflow infiltration exceeded recharge from the 

mountain-front and from ephemeral channels in the Benson 

subarea. In the Narrows-Redington subarea, however, recharge 

from mountain-front and ephemeral channel recharge exceeded 

recharge from streamflow infiltration. Evapotranspiration by 

phreatophytes accounts for the largest outflow of groundwater 

for both subareas—78 percent of the outflow in the NarrowsRedington subarea and 62 percent of the outflow in the Benson 

subarea. 

Precipitation, surface-water, and groundwater chemistry 

and isotope data indicated the relative age and residence time 

of groundwater, the amount of interaction between geologic 

sources and groundwater, and how recharge elevation and 

season were related to the presence of modern water. The 

bedrock aquifer receives modern recharge (<50 years old) 

that is a mix of both summer and winter precipitation; water 

in the fractured system has very little time to interact with the 

surrounding rock matrix. The unconfined basin-fill aquifer 

shows evidence of some modern recharge from multiple 

recharge seasons and altitudes; water has a long residence 

time with potential for increased water-rock interaction. The 

confined basin-fill aquifer does not receive a significant amount 

of modern recharge, and water has a long residence time with 

potential for increased water-rock interaction. The alluvial 

aquifer primarily receives modern recharge during the summer 

monsoon season by floodwater in the river.

Acknowledgments

The authors are grateful to Leslie Graser, Bill Remick, 

Tom Whitmer, Linda Stitzer, and John Fortune of the Arizona 

Department of Water Resources for their technical guidance throughout the study. The authors also wish to thank 

the many landowners in the San Pedro Basin that permitted 

land access and installation of hydrologic monitoring equipment. This study would lack much of the data essential to its 

success without their help and support. Mary McCool of the 

Community Watershed Alliance provided a forum for discussing technical aspects of the study. Apache Nitrogen Products 
2 Hydrology of the Middle San Pedro Watershed, Southeastern Arizona

shared data collected near St. David, Ariz. Barbara Clark 

and David Harris of The Nature Conservancy helped obtain 

land access for installation of monitoring equipment. Kathleen McHugh and Leo Leonhart of Hargis and Associates 

in Tucson, Ariz., provided well inventories and water levels 

for the St. David area. Mark McGowan, Daniel Weber, and 

Mark Cross of Errol L. Montgomery and Associates provided 

aquifer-test and geophysical data and helped to obtain land 

access. David Croft and Nathan Dieterich of the U.S. Bureau 

of Land Management helped collect water level data in the 

San Pedro Riparian National Conservation Area (SPRNCA). 

Christopher Eastoe and Jennifer McIntosh of the University 

of Arizona provided guidance in developing the geochemical 

model. Brandon Forbes and Jade Dickens of the U.S. Geological Survey assisted with the collection and compilation of 

water use data.

Introduction

Groundwater is the primary source of water for municipal, 

domestic, industrial, and agricultural use in the middle San 

Pedro Watershed (fig. 1), and demand for water resources in 

the watershed is projected to increase to meet the water needs 

of the growing population in the region (Barrios, 2005). To be 

effective and acceptable to all interested parties, planning and 

management of the groundwater supply needs to be guided by 

an objective and comprehensive scientific understanding of the 

groundwater system. 

Benson, which had a population of 5,042 in 2009 (Arizona 

Department of Administration Office of Employment and 

Population Statistics, 2011), is the largest city in the middle 

San Pedro Watershed. The population of the area has grown 

moderately during the past 20 years (U.S. Census Bureau, 

2006). In 2007, more than 20,000 housing units were planned 

for the Benson area, but because of the national economic 

recession, only 1,700 housing units were still planned for 

the area as of 2009 (Cochise College Center for Economic 

Research, 2007, 2009).

A wide range of interested parties is concerned about 

the future availability and sustainability of the water supply. 

Increased groundwater pumping could have several undesirable 

consequences, including loss of available groundwater storage, 

increased cost of pumping, the need to deepen existing wells 

and add new wells, impaired quality of drinking water, land 

subsidence, and damage to the riparian habitat of the San Pedro 

River. An improved understanding of the hydrologic system and 

of groundwater–surface-water interactions in the region will 

improve the capability of resource managers to optimally use 

this important resource and to minimize or mitigate the effects 

of development. This project is part of the Rural Watershed 

Initiative (RWI), a program established by the State of Arizona 

and managed by the Arizona Department of Water Resources 

(ADWR). In 2005, the U.S. Geological Survey (USGS) began 

this investigation of the hydrogeology of the middle San Pedro 

Watershed in cooperation with ADWR. 

Purpose and Scope

This is the second of three planned USGS reports that 

describe the various aspects of the hydrology of the middle San 

Pedro Watershed. The first report described the hydrogeologic 

framework (Dickinson and others, 2010a). The purpose of the 

current report is to describe the occurrence and movement of 

surface water and groundwater, to analyze the hydrochemistry 

and water quality of surface water and groundwater, and to 

estimate a water budget for the subwatersheds and aquifer 

systems in the middle San Pedro Watershed. 

This report includes the review of an extensive search 

of published and unpublished hydrologic information related 

to the middle San Pedro Watershed, including climate, 

streamflow, groundwater levels, and water-use data that 

were used to develop initial conceptual models and identify 

data needs. Climate records were analyzed to determine 

whether trends in precipitation correlate with trends in base 

flow and recharge. Streamflow records from several USGS 

gaging stations were analyzed to determine seasonal trends 

in streamflow. Water-level data from wells with more than 

50 years of record were analyzed to provide information on the 

responses of the aquifer to changes in climate and groundwater 

withdrawals. Water-use data were compiled and analyzed to 

quantify groundwater withdrawals for domestic self-supply, 

and municipal, industrial, and irrigation uses. A distributedparameter, water-balance model was used to calculate available 

water for recharge. Remote-sensing data also were used to 

calculate groundwater outflow through evapotranspiration by 

riparian vegetation along the San Pedro River.

New groundwater-level data, particularly along the 

San Pedro River, were used to delineate groundwater flow 

patterns in shallow and deep aquifer units and to document 

the groundwater-level response to streamflow events in the 

San Pedro River. Additional water-level data from 325 wells 

were collected by the ADWR in 2006 to identify changes in 

the groundwater elevations. Groundwater and surface-water 

samples were collected and analyzed to describe the water 

quality in the basin and identify areas of groundwater recharge. 

Description of the Study Area

The middle San Pedro Watershed (fig. 1) is within the 

Basin and Range Province (Fenneman, 1931) and lies in a 

north-trending structural trough that extends from south of the 

United States-Mexico border to several kilometers beyond the 

confluence of the Gila River. Although referred to herein as a 

watershed, the middle San Pedro Watershed does not technically 

conform to a watershed boundary and is more of a management 

area. The middle San Pedro Watershed comprises the northern 

portion of the Upper San Pedro Basin of Cochise County and 

the southern portion of the Lower San Pedro Basin of Cochise, 

Pima, and Graham Counties in southeastern Arizona. The Upper 

and Lower San Pedro Basins are groundwater management 

areas designated by the state of Arizona.
Introduction 3

Redington

Benson

St David

80

82

90

 

San Pedro River at 

Redington Bridge

near Redington

(09472050)

110o

15’ 110o30’

32o

32o15’

10

The

Narrows

San

R

vi er

P

de

ro

GRAHAM CO

COCHISE CO

ESI HCOC O

O

C

C A IMP

Cascabel

Pomerene

San Pedro River

near Redington

(09472000)

San Pedro River 

near Benson

(09471800)

San Pedro River

near Tombstone

(09471500)

San Pedro River 

near Benson

(09471800)

San Pedro River

near Tombstone

(09471550)

Happy

Valley

Johnny Lyon Hills

Allen

Flat

Base from U.S. Geological Survey digital data, 1:100,000, 1982

Universal Transverse Mercator projection, Zone 12

0 5 MILES

0 5 KILOMETERS

RINCON

MOUNTAINS

DRAGOON

MOUNTAINS

MOUNTAINS

WHETSTONE

LITTLE

DRAGOON

MOUNTAINS

SANTA

CATALINA

MOUNTAINS

GALIURO

MOUNTAINS

WINCHESTER

MOUNTAINS

Upper San Pedro Basin

Lower San Pedro Basin

2640

2335

2640 Elevation, in meters

USGS streamflow-gaging station

Boundaries:

City of Benson

San Pedro Riparian National Conservation Area

between the Benson and the Narrows-Redington subareas

between the Benson subarea and the Sierra Vista Subwatershed

EXPLANATION

1863

2264

1746

2350

2265

2050

2560

1910

Benson subarea

Sierra Vista Subwatershed

Benson subarea

Narrows-Redington

subarea

ARIZONA

Upper San Pedro

Basin (green)

UNITED STATES

MEXICO

Benson

Subwatershed

Sierra Vista

Subwatershed

Tucson

Gila 

River

San Pedro River

near Redington

(09472000)

Tombstone

Figure 1. Map showing the middle 

San Pedro Watershed in southeastern 

Arizona. The boundary between the 

Benson and Narrows-Redington 

subareas, which passes through the 

Narrows, coincides with the boundary 

between the Upper and Lower San 

Pedro Basins.

 The middle San Pedro Watershed covers an area of 

about 3,500 km2, is about 110 km from south to north, and 

30 to 50 km wide. The western and eastern boundaries of the 

watershed coincide with the surface-water drainage of the 

San Pedro River. The Whetstone, Rincon, and Santa Catalina 

Mountains form the western boundary of the watershed. The 

eastern boundary is formed by the Dragoon, Little Dragoon, 

and Galiuro Mountains. The highest point in the watershed is 

2,400 meters in the Rincon Mountains and the lowest point is 

860 meters at the San Pedro River outflow at the Redington 

gaging station (hereafter “gaging station” or simply “station.”

The San Pedro River flows from south to north and 

bisects the middle San Pedro Watershed. It begins near 

the USGS gaging station 09471550, San Pedro River near 

Tombstone (herein referred to as the Tombstone gaging 

station), and ends near the Redington Bridge, the location of 

USGS gaging station 094472050, San Pedro at Redington 

Bridge (herein referred to as the Redington Bridge gaging 

station). The southern boundary of the middle San Pedro 

Watershed coincides with the northern boundary of the 

Sierra Vista Subwatershed (fig. 1).The Upper and Lower San 

Pedro Basins are separated by a geologic constriction north 
4 Hydrology of the Middle San Pedro Watershed, Southeastern Arizona

of Benson, Arizona (fig. 1), near gaging station 09471800, 

San Pedro River near Benson (herein referred to as the 

Benson gaging station). This constriction is informally 

called the Narrows. Gaging station 09471000, San Pedro 

River at Charleston (herein referred to as the Charleston 

gaging station), is located about 14 km south of the southern 

boundary of the watershed. 

The middle San Pedro Watershed is further divided into 

two smaller subareas. The part of the watershed within the 

Upper San Pedro Basin is referred to as the Benson subarea 

and the part within the Lower San Pedro Basin is referred 

to as the Narrows-Redington subarea (fig.1). The Upper and 

Lower San Pedro Basins combined have been referred to as 

the San Pedro River Valley (Roeske and Werrell, 1973). 

The San Pedro River flows from its origins in Mexico 

north to the Gila River north of the middle San Pedro 

Watershed. The river is intermittent from the southern 

boundary of the middle San Pedro Watershed to St. David 

(21 km) and is mostly ephemeral from St. David to the 

northern boundary at Redington (84 km) (Dickinson and 

others, 2010a). All the tributaries to the river are ephemeral, 

incised less than 10 m, and generally flow at right angles to the 

river. The Arizona-Idaho Conservation Act of 1988 designated 

60 km of the San Pedro River from the international boundary 

north to near St. David as the San Pedro Riparian National 

Conservation Area (SPRNCA). The purpose of the designation 

is to protect and enhance this desert riparian ecosystem. The 

northernmost 14 km of the SPRNCA are within the Benson 

subarea of the middle San Pedro Watershed. 

In 2009, the population in the Benson Census County 

Division (CCD) was 12,449 (Arizona Department of 

Commerce, 2010). The Benson CCD is a subdivision of a 

county designated by the U.S. Census Bureau and coincides 

with portions of the middle San Pedro Watershed in Cochise 

County. The Benson CCD includes the City of Benson 

(population 5,042 in 2009), the unincorporated area of St. 

David, and surrounding unincorporated areas. 

Previous Investigations

The geology and hydrology of the San Pedro River 

Valley have been studied since the early 20th century. The 

hydrology of the valley has been investigated mainly to locate 

and quantify water resources, to describe the quality of water 

available for human use, and to improve understanding of how 

the hydrologic system responds to climatic and anthropogenic 

stresses. The geology has been investigated to improve the 

understanding of the geologic history and geologic structure 

as well as for mineral exploration. In the 1990s and 2000s, 

comprehensive studies by the USGS, in cooperation with 

ADWR, the Upper San Pedro Partnership, and university 

researchers, led to much of the current understanding of 

the hydrogeology of the Upper San Pedro Basin, including 

groundwater–surface-water connections and of the health of 

the riparian areas in the SPRNCA. 

Geologic investigations generally mapped and described 

the geologic structure of the San Pedro River Valley and 

proposed a geologic history for some of the major features. The 

scales of previous geologic investigations range from small, 

individual basin studies to regional studies across the state of 

Arizona (Shafiqullah and others, 1980; Scarborough and Peirce, 

1978; Drewes, 1981; Dickinson, 1991). The entrenchment 

and widening of the San Pedro River as well as the extent 

of the Holocene channel and floodplain alluvium have been 

investigated by Hereford (1993), who described the preentrenchment evolution of the San Pedro River channel from the 

international boundary to near St. David. Cook and others (2009) 

mapped the Holocene River Alluvium along the San Pedro River. 

The earliest hydrologic investigations in the San Pedro 

Valley were conducted by Lee (1905) and by Bryan and others 

(1934). Lee (1905) described the early groundwater development 

near the Benson area. Bryan and others (1934) focused on 

describing hydrologic conditions for the purpose of obtaining 

water for irrigation along the San Pedro River near St. David and 

Pomerene. They identified confined groundwater conditions and 

artesian wells in areas near those communities. The hydrologic 

conditions of the San Pedro Valley were also summarized by 

Roeske and Werrell (1973), who from 1966 to 1968 inventoried 

wells, measured water levels, and mapped irrigated fields in 

1966. They also collected samples of groundwater for analysis of 

water quality in the San Pedro Valley.

Descriptions of hydrogeologic units have focused 

primarily on the Sierra Vista Subwatershed and are provided 

by Brown and others (1966), Freethey (1982), Jahnke 

(1994), Corell and others (1996), Maddock III and Vionnet 

(1998), Pool and Coes (1999), Goode and Maddock (2000), 

Leenhouts and others (2006), and Coes and Pool (2007). The 

hydrogeologic units discussed by these authors also extend 

into or are similar to the units within the middle San Pedro 

Watershed (Dickinson and others, 2010a). Interpretations of 

the hydrogeologic framework of the previous studies were 

mainly based on data from geologic maps, descriptions of the 

aquifer lithology from driller’s logs, and rarely, aquifer tests.

The collection of geophysical data in the San Pedro 

River Valley has led to improvements and refinements to 

the hydrogeologic framework. Results of detailed airborne 

geophysical surveys of the Sierra Vista Subwatershed in the 

late 1990s (Wynn, 2006) and application of the geophysical 

survey to improve a groundwater flow model (Pool and 

Dickinson, 2007) convinced ADWR to further utilize airborne 

electromagnetic methods to explore the aquifer system in 

the middle San Pedro Watershed as well as in a number 

of other basins across Arizona. The results of the airborne 

electromagnetic surveys were used to further refine the 

hydrogeologic framework of the middle San Pedro Watershed 

(Dickinson and others, 2010a; 2010b). 

Investigations have been performed to improve the 

understanding of the groundwater flow system in the Upper 

San Pedro Basin. Pool and Coes (1999) analyzed precipitation, 

streamflow, geophysical, hydrochemical, and water-level 

data to refine the hydrogeologic framework and increase 
Introduction 5

understanding of stream-aquifer interactions and rates and 

locations of base flow depletion owing to groundwater 

withdrawals. Coes and Pool (2007) collected and analyzed 

water chemistry data and geophysical profiles from bore holes 

in ephemeral stream channels and on the basin floor to estimate 

infiltration and recharge in the Sierra Vista Subwatershed. 

Most groundwater modeling efforts in the San Pedro 

Valley have been focused on the Sierra Vista Subwatershed 

in the Upper San Pedro Basin (Freethey, 1982; Anderson and 

others, 1992; Putman and others, 1988; Vionett and Maddock, 

1992; Corell and others, 1996; Pool and Dickinson, 2007). 

Models were developed to simulate groundwater conditions 

for the entire San Pedro River Valley as well as for various 

subareas outside of the Sierra Vista Subwatershed. Anderson 

and Freethey (1995) modeled the groundwater conditions 

around the Benson subarea and the area from the Narrows to the 

confluence of the San Pedro River and the Gila River. Rovey 

(1989) modeled groundwater conditions of the entire San Pedro 

River Valley from the international boundary to Winkleman, 

Arizona. Rovey (1989) used seepage rates along the San Pedro 

River to estimate the impact of groundwater on surface flows. 

Jahnke (1994) simulated the groundwater conditions from 

Fairbank to Redington. Goode and Maddock (2000) simulated 

the groundwater conditions of the entire San Pedro River Valley, 

including the portion in Mexico for the time period 1940–1997. 

Whittier and Maddock (2004) simulated groundwater conditions 

from the Narrows to the confluence of the San Pedro and Gila 

Rivers for the period 1940–2000. 

Trends in annual streamflow in the San Pedro River 

have been investigated to improve water budget estimates in 

the Upper San Pedro Basin. The streamflow record from the 

Charleston gage has been used in numerous investigations 

to infer hydrologic change in the Upper San Pedro Basin 

(Freethey, 1982; Putman and others, 1988; Vionnet and 

Maddock, 1992; Hereford, 1993; Corell and others, 1996; 

Sharma and others, 1997; Rojo and others, 1999; Pool and 

Coes, 1999; Thomas and Pool, 2006). The streamflow record 

at the Tombstone gage has been analyzed to determine the 

base flow component of the groundwater budget in the Upper 

San Pedro Basin (U.S. Department of the Interior, 2011). 

Kennedy and Gungle (2010) conducted a detailed analysis 

of base flow at the Tombstone gage in which they analyzed 

precipitation records, isotope samples, groundwater levels, 

vertical gradients, and the streamflow record to estimate the 

volumes and sources of annual base flow at the Tombstone 

gage. Kennedy and Gungle (2010) also discussed the 

decreasing trend in base flow at the Tombstone gage. 

Prior to 2000, most estimates of the groundwater withdrawals 

from riparian evapotranspiration (ET) were estimated using 

groundwater-flow models. The first detailed estimates of ET by 

riparian vegetation in the Upper San Pedro Basin were made using 

remote sensing data and field measurements (Goodrich and others, 

2000). Those estimates of riparian groundwater withdrawals were 

later updated by Scott and others (2006) using eddy-covariance 

techniques for various riparian vegetation types. Nagler and others 

(2005a) combined in-place measurements with remote sensing 

to estimate riparian ET in the Upper San Pedro Basin for years 

2000–2004. Scott and others (2008) used ET rates from eddy 

covariance methods to develop an empirical relation between ET 

and remote sensing data and land surface temperature to scale up 

ET rates in the Upper San Pedro Basin. 

The results of studies of water quality in the middle 

San Pedro Watershed have been included in sections of other 

reports, but have not been as prominent as the findings of 

studies of quantitative aspects of water resources. Water-quality 

studies have attempted to determine groundwater quality as 

a function of well location in the basin, well depth, aquifer 

type, geology, and land use. Roeske and Werrell (1973) found 

the chemical quality of the groundwater in the San Pedro 

River Valley to be suitable for most uses but described high 

levels of fluoride in the St. David-Benson and Mammoth 

areas. Coes and others (1999) collected water samples from 

wells between the international boundary with Mexico and 

the Benson Narrows and analyzed the samples for general 

mineral, nutrient, and trace constituents. Towne (2002) 

sampled groundwater at 63 sites in the Lower San Pedro Basin 

for analyses of inorganic constituents and volatile organic 

compounds and radiochemistry. Samples were collected from 

selected wells for analysis of radon and pesticides. 

 Results of analyses of groundwater samples from the 

San Pedro River Valley have been used to infer the movement 

and ages of groundwater. Coes and Pool (2007) collected and 

analyzed water chemistry data and geophysical profiles from 

bore holes in ephemeral-stream channels and the basin floor 

to estimate infiltration and recharge rates in the Sierra Vista 

Subwatershed. They found that as much as 19 percent of basin 

recharge occurs in washes where infiltration rates were about 

two orders of magnitude greater than those on the basin floor. 

Hopkins (2014, see also Adkins, 2009) collected water solute 

and isotope geochemistry samples from wells, the San Pedro 

River, and precipitation in the middle San Pedro Watershed 

to describe movement of groundwater and residence times. 

Hopkins (2014) found evidence for focused recharge in 

fracture systems along the mountain blocks flanking the basin 

and that winter precipitation contributes to recharge in the 

fracture system and the unconfined aquifer whereas summer 

precipitation recharges the alluvial aquifer. 

All of the previously mentioned investigations have greatly 

improved the understanding of the geology and hydrology of 

the San Pedro River Valley and have led to the development 

of a groundwater budget for the Sierra Vista Subwatershed. 

Although similarities can be drawn between the conditions in the 

Sierra Vista Subwatershed and the middle San Pedro Watershed, 

significant differences might exist between the two areas. The 

presence of a confining unit in the St. David area may limit 

groundwater flow through the middle San Pedro Watershed and 

affect groundwater quality. Riparian groundwater use has not 

been quantified in the middle San Pedro Watershed. Seasonal 

rates of available water for mountain front and mountain block 

recharge in the middle San Pedro Watershed have not been 

determined. This study aims to fill these knowledge gaps and 

develop a water budget for the middle San Pedro Watershed.
6 Hydrology of the Middle San Pedro Watershed, Southeastern Arizona

Climate

The climate in the middle San Pedro Watershed is arid 

or semiarid, and precipitation falls mainly during winter and 

summer months (fig. 2; Pool and Coes, 1999; Pool, 2005; 

Thomas and Pool, 2006). Temperature and precipitation 

generally vary with altitude, although some variations are 

attributed to local rain-shadow effects. Mean annual 

precipitation along the basin floor at Tombstone from 1913 

to 2009 is 351 mm (National Weather Service Cooperative 

Network, http://www.nws.noaa.gov/om/coop/, accessed 

January 21, 2010; Agricultural Research Service Southwest 

Watershed Research Center, http://www.tucson.ars.ag.gov/dap/, 

accessed January 22, 2010). The mean annual precipitation in 

the Rincon and Catalina Mountains is greater than 750 mm 

[based on PRISM (Parameter-elevation Regressions on 

Independent Slopes Models) data sets (PRISM, 2010)]. Mean 

monthly maximum temperatures in June and July on the basin 

floor are about 34 °C at Tombstone, 37 °C at Cascabel, and 

25 °C at Mount Lemmon in the Santa Catalina Mountains, 

which is west of the study area but has a climate similar 

to that in the Rincon Mountains. Mean monthly minimum 

temperatures during December and January are about 2 °C 

at Tombstone, –1 °C at Cascabel, and –4 to –5 °C at Mount 

Lemmon (Western Region Climate Center, http://www.wrcc.

dri.edu/summary/Climsmaz.html, accessed August 16, 2010). 

Seasonal differences in precipitation (fig. 2) result from 

several sources of moisture and different types of storms that 

transport moisture and deliver the precipitation. Winter season 

(November to February) cumulative precipitation (average of 

79 mm at Tombstone) is delivered by mid-latitude low pressure 

systems from the Pacific Ocean. Individual precipitation events 

are generally widespread, long in duration, and of low intensity. 

Dry season (March to May) cumulative precipitation averages 

26 mm at Tombstone, and is less common because frontal 

systems remain to the north of the study area. Wet season (June 

to October) cumulative precipitation (average of 246 mm at 

0

20

40

60

80

100

Jan. Feb. Mar. Apr. May June July Aug. Sep. Oct. Nov. Dec.

Winter Winter

Precipitation, in millimeters

Dry Wet

Figure 2. Bar graph showing average monthly precipitation at 

Tombstone, Arizona, 1913–2009.

Tombstone) is delivered by local convective thunderstorms of 

short duration and high intensity, which are fed by moisture 

from the Gulf of Mexico, Gulf of California, and Pacific 

Ocean. Precipitation during the latter part of the wet season can 

originate from convective thunderstorms, frontal systems, and 

dissipating tropical cyclones. Most precipitation occurs during 

the summer months (fig. 3). Residual moisture from tropical 

cyclones, in combination with monsoonal flow or with cutoff 

low-pressure systems from the Pacific Ocean, has contributed 

to the largest and most widespread floods in Arizona (Webb 

and others, 2004).

Long-term variations in climate in the southwestern 

United States (Cayan and Webb, 1992; Webb and Betancourt, 

1992; McCabe and Dettinger, 1999; Dettinger, 2005; Pool, 

2005; Hidalgo and others, 2009) can have immediate or 

long-term effects on water resources, including rates of 

groundwater recharge, streamflow, runoff in ephemeral 

channels, water use by plants in riparian areas, and human 

water use. Climate variability in southeastern Arizona and the 

frequency and magnitude of runoff-producing precipitation 

has been attributed to large-scale atmospheric patterns. 

The cumulative departure of precipitation from long-term 

averages, which represents periods of successive years of 

either greater or lower-than-average precipitation, is used 

to separate interannual and interdecadal variations into wet 

and dry climatic periods (fig. 4). Wet climatic periods are 

indicated by the rising limb and dry climatic periods by the 

falling limb of the cumulative departure curve. Some of the 

largest floods in Arizona occurred during wet periods in the 

beginning (1905 and 1916) and end of the 20th century (1983 

and 1993). A multidecadal drought occurred in the middle 

of the 20th century, and another appears to be ongoing in the 

early 21st century. The wet and dry periods also correspond 

well to the cumulative departure from the long term average 

of base flow in the San Pedro River at the Charleston gaging 

station (09471000), suggesting that variations in groundwater 

discharge as base flow in the San Pedro River also occurs at 

interannual and interdecadal time scales.

Surface Water 

Surface-water features in the middle San Pedro 

Watershed include the San Pedro River and ephemeral 

channels that originate in the surrounding mountains and drain 

into the San Pedro River. The length of the San Pedro River 

within the study area is about 105 km from the southern to 

northern boundary. The elevation of the river along that length 

drops from 1,150 m to 860 m and has an average gradient 

of 2.8m/ km. In the Benson subarea, the river is 58 km long 

and has an average gradient of 2.5 m/km. In the NarrowsRedington subarea, the river is slightly steeper with a length 

of 47 km and an average gradient of 3.2 m/km. The active 

channel of the river varies in width from several meters to 

tens of meters, and the channel is incised into the floodplain 
Surface Water 7

0

200

400

600

800

0

200

400

600

800

0

200

400

600

800

0

200

400

600

800

1900 1910 1920 1930 1940 1950

Year

1960 1970 1980 1990 2000 2010

A. Annual

B. Winter (November–February) 

Precipitation, in millimeters

C. Dry (March–May) 

D. Wet (June–October) 

Annual (total)

5–year moving average

Precipitation at Tombstone

Figure 3. Scatter plots showing annual and seasonal precipitation at Tombstone, Arizona, 1905–2009. A, 

Annual. B, Winter (November–February). C, Dry season (March–May). D, Wet season (June–October). The 

solid line is the 5-year moving average of the annual values.
8 Hydrology of the Middle San Pedro Watershed, Southeastern Arizona

from 1 to 10 m. In the Benson subarea, the river valley is 

about 40–50 km wide, the basin slopes moderately upward to 

mountain elevations of about 2,000 m on the west and 2,100 m 

on the east, and tributaries are incised. In the NarrowsRedington subarea, the river valley is about 30–40 km wide, 

the basin slopes steeply upward to mountain elevations of 

about 2,600 m on the west and 2,300 m on the east, and 

tributaries are steep and deeply incised. All tributaries to the 

river are ephemeral and ungaged and generally flow at right 

angles to the main channel of the San Pedro River.

 Flow in the San Pedro River is intermittent for 

approximately 21 km from the southern boundary near 

Tombstone to St. David. The San Pedro River is ephemeral 

between St. David and the Narrows, as well as between 

the Narrows and the northern boundary of the study area 

at Redington, except for intermittent, perennial reaches 

approximately 3.5 km north of the Narrows. 

Streamflow Data

The Charleston gaging station, approximately 18 km 

upstream (south) of the southern boundary of the middle San 

Pedro Watershed (fig. 1), has the longest streamflow record in 

the Upper San Pedro Basin (table 1). The Charleston gaging 

station was established in 1904 but was moved several times 

prior to 1936. For this study, the streamflow record at the 

gaging station is analyzed over water years 1936–2009.

The Tombstone gaging station is just north of the southern 

boundary of the watershed and measures the surface water 

entering the watershed. The streamflow records for water 

years 1968–1986 and 1997–2009 were analyzed separately, as 

the station was discontinued from 1987–1996. Accordingly, 

separate flow statistics are presented for the two periods 

during which the station was operational (table 1). The Benson 

gaging station near the Narrows records flow in the San Pedro 

Depletion

Depletion

Depletion

Accumulation

Accumulation

Accumulation

Accumulation

Accumulation

Depletion

0

200

400

600

800

1,000

1,200

1,400

1,600

Missing data Missing data

Annual cumulative departure of precipitation, in millimeters

Annual cumulative departure of base flow, in cubic hectometers

0

5

10

15

20

25

30

35

40

45

200

400

600

800

1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

Precipitation, in millimeters

50

Flood 1905Flood 1916Flood 1921Flood 1926

Flood 1941

Flood 1962

Flood 1965–66

Flood 1970

Flood 1972

Flood 1974Flood 1980Flood 1981Flood 1983Flood 1993

Drought

1932–36

Drought

1942–64

Drought

1974–77

Drought

1999–present

Annual cumulative departure of precipitation

Annual cumulative departure of baseflow

Mean annual precipitation

Wetting period

Annual precipitation

Figure 4. Bar graph of annual precipitation at Tombstone, Arizona (bottom), and plots of annual cumulative departures from the 

long-term average for precipitation (blue line) and base flow (red line) in the San Pedro River (top). The jagged dashed gray lines are 

schematic, illustrating inferred periods of aquifer accumulation and depletion, based upon the trends in the departure curves. The black 

dashed line is the mean annual precipitation.
Surface Water 9

River out of the Benson subarea into the Narrows-Redington 

subarea. The streamflow record at the Benson gage started 

in March 1966 and ended in March 1976 and starts again in 

October 2005 and ends in 2009. 

The San Pedro River near Redington (09472000) gaging 

station (herein referred to as the Redington gage and distinct 

from the currently operating Redington Bridge gage, 6 km to 

the north) measured streamflow about 8 km upstream of the 

northern boundary of the watershed from 1944 to 1997 but 

is missing data for the years 1948 and 1949. The streamflow 

measurements at the gage were affected by an upstream 

diversion for irrigation and to fill a stock pond. The first 

mention of upstream diversions is in January of 1972. Field 

notes from the Redington gage between 1972 and 1998 were 

used to identify months when water was being diverted out 

of the San Pedro River. The notes describe water in the San 

Pedro River being diverted by a “sand berm.” The diversions 

occurred year round but were most frequent during the months 

of April and May. When water was diverted, usually the 

entire flow was diverted out of the San Pedro River and the 

Redington gage would record zero discharge. The streamflow 

during the monsoon season was probably not affected by the 

streamflow diversions because the flow would likely wash 

away the sand barrier in the stream channel that was used to 

divert the flow. 

In July 1998, the Redington gaging station was 

deactivated, and a new station was constructed at the 

Redington Bridge, approximately 6 km downstream from the 

original station. The drainage area to the new station is six 

percent greater than that for the original station. The flow data 

from both gages are assumed to have similar characteristics 

and trends, and were combined into a single record for the 

period 1944–2009 for this study and report. The combined 

records of the Redington and Redington Bridge gages are 

referred to as the combined Redington gage record. The record 

at the new location is still affected by the upstream diversion. 

Streamflow Characteristics and Trends

Streamflow characteristics for the San Pedro River were 

calculated through analysis of data from five streamflow-gaging 

stations (table 2). The active stations from south to north are 

U.S. Geological Survey 

gaging-station number1 Location Period of record

Drainage 

area (square 

kilometers)

Gage datum 

(meters above 

sea level)

09471000 at Charleston 1913–2009 3,196 1,205

09471550 near Tombstone 1968–1986; 1997–2009 4,507 1,152

09471800 near Benson 1967–1976; 2006–2009 6,449 1,009

09472000 near Redington 1944–1997 7,581 896

09472050 at Redington Bridge near Redington 1998–2009 8,019 860

Table 1. Streamflow-gaging stations on the San Pedro River in and near the middle San Pedro Watershed, Arizona.

those at Charleston, near Tombstone, near Benson, and at 

Redington Bridge (fig. 1). The record for the discontinued 

station near Redington was also analyzed for this study. 

Annual streamflow statistics—average annual flow values, 

and the 10-, 50-, and 90-percent exceedance values—presented 

in this report are calculated from the period of record ending 

on September 30, 2009. The term “exceedance” refers to the 

streamflow value that is met or exceeded for a given percentage 

of the specified period of record. For example, the 50 percent 

exceedance probability (also called the median), is one of 

the most commonly used or cited percentiles; it represents 

the streamflow that is equaled or exceeded 50 percent of the 

time. The 10-percent exceedance probability characterizes 

streamflow that is equaled or exceeded 10-percent of the 

specified time period. Streamflow volumes for Tombstone were 

described for wet, dry, and winter periods. Long-term annual 

trends were determined from the Charleston and Redington 

streamflow gages. The record from the Charleston gage, which 

is just outside (south of) the middle San Pedro Watershed, was 

used for the analysis because of its long record, starting in 

1936. Base flow to the San Pedro River was estimated on the 

basis of the monthly 3-day minimum flows during the winter 

at the Charleston and Tombstone gages; this method was also 

used by Thomas and Pool (2006). 

Long-term streamflow records from the Charleston 

and combined Redington gages show a decreasing trend 

in average annual streamflow (fig. 5). Average annual flow 

at the Charleston gage was about 60 hm3/yr for the period 

1936–1940, and 32 hm3/yr for the period 2000–2009. 

Average annual streamflow at the combined Redington gage 

was 46 hm3/yr for the period 1944–1957 and 25 hm3/yr for 

the periods 2000–2009. The record at the Tombstone gage 

also shows a decrease in the average annual streamflow 

(fig. 6). The median values have decreased by 50 percent, 

from 0.34 m3/s to 0.17 m3/s, for the periods 1968–1986 and 

1997–2009, respectively (table 2). 

 For the period of record 1936–2009, the average 

annual streamflow at the Charleston gage was 1.29 m3/s 

(40.54 hm3/ yr; table 2). The average annual streamflow at 

the Tombstone gage was 1.57 m3/s (84.17 hm3/yr) for the 

early period of record, 1968–1986, and the average annual 

streamflow for the later period of record, 1997–2009, was 

0.99 m3/s (57.52 hm3/ yr; table 2). The average annual 
10 Hydrology of the Middle San Pedro Watershed, Southeastern Arizona

Streamflow-gaging station

Specified 

period of 

record

Average

annual 

streamflow 

(m3/s)

Average

annual 

streamflow 

(hm3/yr)

90th percentile 

exceedance 

streamflow

(m3/s)

50th percentile 

exceedance 

streamflow

(m3/s)

10th percentile 

exceedance 

streamflow

(m3/s)

San Pedro River at Charleston, Ariz. (09471000) 1936–2009 1.29 40.54 0.10 0.34 1.56

San Pedro River near Tombstone, Ariz. 1968–1986 1.57 84.17 0.001 0.34 2.32

(09471550) 1997–2009 0.99 57.52 0.001 0.17 1.08

San Pedro River near Benson, Ariz. 1967–1986 0.99 28.42 0.001 0.001 1.25

(09471800) 2006–2009 1.02 32.26 0.001 0.001 0.42

San Pedro River near Redington, Ariz. 

(09472000) 1944–1997 1.19 38.26 0.001 0.01 1.33

San Pedro River at Redington Bridge near 

Redington, Ariz. (09472050) 1999–2009 0.80 25.14 0.001 0.001 0.57

San Pedro at Redington combined record 

(0947200 and 09472050) 1944–2009 1.12 35.89 0.001 0.001 1.25

1

 Ephemeral streamflow; streamflow is present at least 90 percent of the specified time period

2

 Ephemeral streamflow; streamflow is present at least 50 percent of the specified time period

Table. 2 Average annual streamflow and selected annual exceedance-level streamflows at gaging stations on the San Pedro River for 

specified periods of records.

[Abbreviations: m3/s, cubic meters per second; hm3/yr, cubic hectometers per year]

19101915192019251930193519401945195019551960196519701975198019851990199520002005

0

50

100

150

200 Combined Redington streamflow gage

Charleston streamflow gage

LOESS fit to: 

Combined Redington streamflow gage data

Charleston streamflow gage data

Streamflow, in cubic hectometers 

Water year

Figure 5. Scatter plot of annual streamflow of the San Pedro River at Charleston, Arizona 

(09471000) and combined record from the gaging stations San Pedro River near Redington, Arizona 

(09472000) and San Pedro River at Redington Bridge near Redington, Arizona (09472050). The solid 

red and blue lines are LOWESS fit to the respective streamflow record (locally weighted scatterplot 

smoothing [Cleveland, 1979]).
Surface Water 11

19681969197019711972197319741975197619771978197919801981198219831984198519861997199819992000200120022003200420052006200720082009

0

50

100

150

Mean value

1968–1986

(50 hm3

)

Mean value

1997–2009

(31 hm3

)

Data not available for 

1987–1996

Winter months (Nov–Feb)

Wet months (June–Oct)

Dry months (Mar–May)

Streamflow

Water year

Streamflow, in cubic hectometers 

Figure 6. Bar graph showing annual streamflow, color-coded by season, as recorded at the gaging station San Pedro 

River near Tombstone, Arizona (09471550), for 1968–2009.

streamflow at the Benson gage was 0.99 m3/s (28.42 hm3/yr) 

for the 1967–1976 period and the average annual streamflow 

for the time period 2006–2009 was 1.02 m3/s (32.26 hm3/yr; 

table 2). For the combined period of records from 1944–2009 

the average annual streamflow of the combined Redington 

gage was 1.12 m3/s (35.89 hm3/yr; table 2).

The 90-percent, 50-percent, and 10-percent streamflow 

exceedance values were calculated for the five gages for 

their periods of record (table 2). At the Charleston gage, all 

of the exceedance percentiles were greater than zero, which 

is indicative of perennial streamflow. Ten- and 50-percent 

exceedance values equal to 0 m3/s are indicative of ephemeral 

channels. The 10-percent exceedance values for all gages were 

all greater than zero. In general, the exceedance streamflow 

values show that streamflow is nearly always present at the 

Charleston gage and decreases downstream as the water is lost 

to infiltration and evapotranspiration (ET) by phreatophytes. 

Streamflow also is lost to evaporation, and, at times, diversions. 

The average daily discharge values show the variability in 

streamflow at the Tombstone, Benson, and Redington Bridge 

gages for their periods of record (fig. 7). All three gages show 

streamflow beginning to increase in July and peaking in early 

August, then decreasing to zero discharge in October. At the 

Benson station, there is virtually no flow from October through 

July. The Tombstone and Redington Bridge gages show flashy 

streamflow events in October and November but these flows 

are smaller and of shorter duration than summer streamflow 

events. The Redington Bridge gage shows streamflow events 

occurring during mid-January through March. 

Peak discharges on the San Pedro River occur most 

often in the wet months as a result of monsoon storms. The 

peaks of record at the Charleston and combined Redington 

gages—2,775 m3/s and 2,549 m3/s, respectively—occurred 

on Sept. 28, 1926 (fig. 8). The annual peaks at the Tombstone 

and Benson gages all occurred in the wet months. The 

peak discharge has occurred during the winter months at 

the combined Redington gages for water years 1968, 1979, 

1985, 1993, 1995, and 2008. In 1995, peak discharge at the 

Charleston gage occurred during the winter months. The peak 

discharge has only once occurred during the dry months, in 

1991, at both the Redington and Charleston streamflow gages.

Base Flow 

The flow of the San Pedro River at the Tombstone gaging 

station includes both runoff and base flow. Runoff originates 

from precipitation in the surrounding mountains and basin 

and reaches the San Pedro River by overland flow and in 

tributaries. Base flow is the discharge of groundwater from 

the regional aquifer and from the shallow stream alluvium. 

The definition of base flow differs depending on the author 

and focus of the study. In this study, base flow at Tombstone 
12 Hydrology of the Middle San Pedro Watershed, Southeastern Arizona

Oct Nov Dec Jan Feb Mar

B.

A.

C.

Apr May Jun Jul Aug Sep Oct

0

10

20

30

0

10

20

30

0

10

20

30

Streamflow, in cubic meters per second

San Pedro River at Redington Bridge near Redington, Arizona

(09472050)

Period of record: 1998–2009

San Pedro River near Benson, Arizona

(09471800)

Period of record: 2006–2009

San Pedro River near Tombstone, Arizona

(09471550)

Period of record: 1998–2009

is considered the quantity of groundwater that sustains 

streamflow during periods of no precipitation. 

The rate of base flow is a function of recharge in 

the regional groundwater aquifer, rates of recharge in the 

stream alluvium aquifer (bank storage), the head gradients 

between the aquifer and stream, and the rates of groundwater 

withdrawals through both pumping from wells along the 

river and riparian ET. In reaches of the San Pedro River with 

intermittent streamflow, groundwater outflow through riparian 

ET affects the groundwater available for discharge as base 

flow. If the rate of riparian ET exceeds the rate of groundwater 

discharge as base flow, then base flow ceases. 

Base flow can be separated from runoff by using various 

automated base-flow separation computer codes. Each of these 

methods has limitations when applied to streams that go to 

zero discharge (Kennedy and Gungle, 2010). Several methods 

have been used to estimate base flow for the San Pedro River 

within the Upper San Pedro Basin, and a comprehensive 

review and compilation of those estimates for the Tombstone 

gage is presented by Kennedy and Gungle (2010; table 3). 

 Base flow at the Tombstone gage was approximated 

from the 3-day low flow, which is calculated by averaging 

the three lowest consecutive daily streamflow values over the 

period of interest. Thomas and Pool (2006) used the 3-day low 

Figure 7. Plot of 

average daily streamflow 

throughout the year at 

gaging stations on the San 

Pedro River in the middle 

San Pedro Watershed for 

the specified period of 

record.

flow as an approximation of base flow at the Charleston gage 

because the 3-day low flow is repeatable and unbiased and is 

not dependent on any assumptions. 

Base flow at the Tombstone gage was analyzed for 

the period 1968–1986 (early) and 1997–2009 (late). The 

Tombstone gaging station was not in operation for the 

intervening period. For each month, the 3-day low flow was 

calculated and converted to a monthly streamflow volume 

(in cubic hectometers). The annual base-flow volume was 

calculated by summing all of the monthly base-flow volumes 

from October through May. The summer months (June, July, 

August, and September) were excluded from the analysis 

because riparian ET uses all available groundwater in the 

vicinity of the Tombstone gage during this period. Streamflow 

during these months consists entirely of storm runoff. 

Annual base flow of the San Pedro River at the Tombstone 

gage has declined over the period of record (fig. 9). The median 

base flow for the early period was 5.31 hm3/yr and 2.74 hm3 /yr 

for the late period. The base flow during the winter months 

(November–February) showed the largest declines. The median 

winter and spring base flows each decreased by approximately 

50 percent from the early to later period. Annual base flow 

during the early period ranged from 2.70 hm3 in 1981 to 23hm3

in 1985. For the late period, annual base flow ranged from 
Surface Water 13

1915 1920 1925 1930 1935 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010

0

500

1000

1500

2000

2500

3000

0

500

1000

1500

2000

2500

3000

Annual peak discharge, in cubic meters per second

Water year

A

B

San Pedro River at Redington Bridge near Redington (09472050)

San Pedro River near Redington (0947200) 

Period of record, 1926–1997 and 1998–2009

San Pedro River at Charleston (09471000)

Period of record, 1916–2009

Winter months (Nov–Feb)

Wet months (June–Oct)

Dry months (Mar–May)

Unknown month

 Source Last time period used in 

analysis Method of estimating base flow 

 Tombstone gage 

Predevelopment Last time 

period

Freethey (1982) 1977 model 9.25 5.55

Corell and others (1996) 1990 model (final year) 11.72 7.03

Corell and others (1996) 1941–1990 model (total average) 11.72 7.76

Rojo and others (1999) 1990 10-year flow duration curves in combination 

with earlier models 

11.72 9.13

Arizona Department of Water 

Resources (2005a, 2005b) 

 1997–2003 NA - 4.01

USGS/Upper San Pedro 

Partnership (2005) 

1967–1986 7-day winter low flow and 

evapotranspiration 

 - 5.22

1996–2002

Pool and Dickinson (2007) pre-development earlier estimates with recent 

evapotranspiration 

10.48 - 

Pool and Dickinson (2007) 03/2002–03/2003 model 11.29 3.45

Dieterich (2009) 1968–85 median total discharge - 7.22

Kennedy and Gungle (2010) 1968–1986 delta filter - 7.19

1997–2009 - 3.55

1968–1986 and 1997–2009 - 6.02

This study (2011) 1968–1986 3-day minimum flow - 5.31

1997–2009 - 2.74

1968–1986 and 1997–2009 - 4.64

Table modified from Kennedy and Gungle (2010).

Table 3. Base flow discharge estimates at the Tombstone streamflow-gaging station. Values are in cubic hectometers per year. 

Figure 8. Bar chart 

of annual peak flows 

at the Redington and 

Charleston gaging 

stations, colored by 

the season during 

which greatest 

streamflow occurred.
14 Hydrology of the Middle San Pedro Watershed, Southeastern Arizona

0.99 hm3 in 1999 to 13 hm3 in 2001. The estimated monthly 

median base flow for the late periods show that for the months 

of October, November, and May, the median base flow has 

decreased to zero (fig. 10). The month of the largest base flow 

has also changed from January for the early period to February 

in the late period. 

Hydrogeology

Hydrogeologic Framework

The primary aquifer of the middle San Pedro Watershed 

comprises permeable deposits of alluvium that overlie relatively 

impermeable crystalline rocks of pre-Cambrian and Tertiary age, 

Paleozoic limestone, Mesozoic sandstone and mudstone, and 

Tertiary prebasin sediments (Pool and Coes, 1999; Dickinson 

and others, 2010a). The distribution of the surficial permeable 

deposits is shown in figure 11. The alluvial aquifer is a sequence 

of unconsolidated to moderately well consolidated alluvial 

Winter months (Nov–Feb)

Wet months (June–Oct)

Dry months (Mar–May)

Water year

Base flow, in cubic hectometers 

Base flow

196719681969197019711972197319741975197619771978197919801981198219831984198519861997199819992000200120022003200420052006200720082009

0

5

10

15

20

25

Median value

1968–1986

(5.31 hm3

)

Median value

1997–2009

(2.74 hm3

)

Data not available for 

1987–1996

Figure 9. Bar chart of annual base flow at the San Pedro River near Tombstone, Arizona, streamflow gaging station 

(09471550), color coded by season. Base flow was not calculated for the months June through September. 

sediments of Late Tertiary and Quaternary age that is greater 

than 400 m thick in the center of the basin (fig. 12; Dickinson 

and others, 2010a). Rocks of the underlying bedrock are not 

important aquifers with the exception of the Paleozoic limestone, 

which recharges water locally. Stratigraphic units in the alluvial 

aquifer that are significant for groundwater flow (in ascending 

stratigraphic order) are the Miocene–Pliocene lower basin fill, 

Pliocene–Pleistocene upper basin fill, and Holocene alluvium 

along the San Pedro River. The basin fill is divided into lower 

and upper parts on the basis of geologic logs, drill logs, and sonic 

velocity logs (Pool and Coes, 1999). Lower basin fill forms the 

primary aquifer through which most groundwater flows. The 

upper basin fill is unsaturated across most of the basin, but is 

important because water infiltrates through the unit and recharges 

the water table during periods of runoff in ephemeral channels. 

The Holocene alluvium is a narrow stringer of highly permeable 

stream alluvium that is incised into the basin fill along the major 

stream channels (Hereford, 1993; Pool and Coes, 1999; Cook and 

others, 2009). The stream alluvium is an important local aquifer 

that drains the basin fill aquifer, receives streamflow infiltration, 

and stores water that supports riparian vegetation during periods 

lacking runoff (Pool and Coes, 1999).
Hydrogeology 15

EXPLANATION

A A'

Hydrogeologic units

Confining fine-grained unit

Semiconfining interbedded unit

Coarse-grained unit

Basin fill sediments not present

Line of geologic section in figure 12

?

10

80

82

90

Redington

St David

(

 

0 5 MILES

0 5 KILOMETERS

110o

15’ 110o30’

32o

31o45’

32o15’

San

R

vi er

P

de

ro

GRAHAM CO

COCHISE CO

ESI HCOC O

O

C

C A IMP

Pomerene

B’'

B

A

A’'

Benson

Cascabel

Base from U.S. Geological Survey digital data, 1:100,000, 1982

Universal Transverse Mercator projection, Zone 12

RINCON

MOUNTAINS

DRAGOON

MOUNTAINS

MOUNTAINS

WHETSTONE

LITTLE

DRAGOON

MOUNTAINS

SANTA

CATALINA

MOUNTAINS

GALIURO

MOUNTAINS

WINCHESTER

MOUNTAINS

Figure 10. Bar chart of 

estimated median base flow 

at the Tombstone, Arizona, 

streamflow-gaging station for 

the early (1968–1986) and late 

(1997–2009) periods of record. 

Figure 11. Map showing 

distribution of surficial 

hydrogeologic units in the middle 

San Pedro Watershed, modified 

from Dickinson and others (2010a).

0

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

Oct Nov Dec Jan Feb Mar Apr May

Estimated base flow, in cubic meters per second

Median base flow

1968–1986

1997–2009
16 Hydrology of the Middle San Pedro Watershed, Southeastern Arizona

The lower basin fill is divided into coarse- and fine-grained 

hydrogeologic units on the basis of lithology and electrical 

geophysical surveys (fig. 12; Dickinson and others, 2010a; 

2010b). The coarse-grained unit of lower basin fill overlies 

bedrock, is the stratigraphically lowest water-bearing unit, and 

is present throughout most of the study area. It is commonly 

described in drill logs as conglomerate, decomposed granite, 

or sand and gravel with few silt and clay interbeds. A unit of 

fine-grained sediments within sand and gravel, interbedded 

between the fine-grained and coarse-grained units of the lower 

basin fill, is present along the outer margins of the fine-grained 

unit. The interbedded unit underlies the San Pedro River near 

St. David and extends southward towards areas of high bedrock. 

The fine-grained unit of lower basin fill includes small amounts 

of gypsum in a matrix of either silt and clay or mudstone, and 

few sand and gravel interbeds. At the basin center, a thick finegrained unit predominately consisting of silt and clay overlies 

the coarse-grained unit. 

Upper and lower basin fill boundary

Water table altitude in wells

Piezometric surface altitude in wells

Cased interval in well

Perforated interval in well

Groundwater-flow direction

Crystalline bedrock

Crystalline, sedimentary,

and limestone bedrock

0 10,000 20,000 30,000 40,000 50,000 60,000 70,000 80,000 90,000

Interbedded

Fine and coarse

The Narrows Benson subarea Narrows-Redington

subarea

700

800

900

1,000

1,100

1,200 B

B B'

Coarse

Fine

A

West

North South

East

A'

San Pedro

River

Fine and coarse

streambed

alluvium Upper basin fill

Lower basin fill

0 5,000 10,000 15,000 20,000 25,000 30,000

Distance, meters

Distance, meters

Coarse

Interbedded

fine and coarse

Fine

800

900

1,000

1,100

1,200

1,300

1,400

1,500

1,600

Altitude, meters Altitude, meters

Unconfined Semi-confined Unconfined

Crystalline bedrock

Crystalline and limestone bedrock

? ?

? ?

?

?

?

?

?

?

?

?

Mostly unconfined Semi-confined Unconfined

Figure 12. Hydrogeologic cross sections showing 

distribution of coarse-grained, interbedded, and 

fine-grained hydrogeologic units in the middle San 

Pedro Watershed, Arizona.

The aquifer system is unconfined along the basin margins 

and is confined or semi-confined in the basin center owing to 

the occurrence of the thick fine-grained unit of the lower basin 

fill (Dickinson and others, 2010a; 2010b). In the unconfined 

portion, the amount of water that can be extracted by pumping 

is controlled by the hydraulic connection between perforated 

well intervals and sediments that yield water by a lowered 

water table and drainage of pore spaces. Groundwater occurs 

under confined conditions beneath thick sequences of the 

fine-grained unit of the lower basin fill where lowering of 

the water level in wells results in little to no drainage of pore 

spaces within the thick fine-grained unit. Wells having artesian 

pressures are mostly near the San Pedro River between areas 

south of St. David to north of Pomerene (fig. 13), further 

indicating confined conditions. Near St. David and Pomerene, 

boreholes for deep wells commonly penetrated dry intervals 

of clay and silt up to 300 m thick before reaching saturated 

materials. Semi-confined conditions occur where sufficiently 
Hydrogeology 17

Redington

Benson

St David

80

82

90

110o

15’ 110o30’

32o

31o45’

32o15’

10

THE

NARROWS

GRAHAM CO

COCHISE CO

ESI H

C

O

C

O

O

C

C

A IM

P

B’'

B

A

A'

A A' Line of geologic section in figure 12

RINCON

MOUNTAINS

DRAGOON

MOUNTAINS

MOUNTAINS

WHETSTONE

LITTLE

DRAGOON

MOUNTAINS

SANTA

CATALINA

MOUNTAINS

GALIURO

MOUNTAINS

WINCHESTER

MOUNTAINS

Base from U.S. Geological Survey digital data, 1:100,000, 1982

Universal Transverse Mercator projection, Zone 12

0–20

20–60

60–100

100–200

200–300

300–815

EXPLANATION

Panel A. Wells artesian when drilled

Well depth, in meters

Pomerene

Cascabel

Pomerene

San

Pedro

River

0 5 MILES

0 5 KILOMETERS

Benson

Figure 13. Map showing wells in the middle San Pedro Watershed for which lithologic logs are available, and (inset) 

wells having artesian conditions when drilled.
18 Hydrology of the Middle San Pedro Watershed, Southeastern Arizona

thick fine-grained intervals result in delayed drainage of 

water from pore spaces in response to lowering of the water 

table. Semi-confined conditions occur at the margins of the 

intersection of the water table with the fine-grained unit of the 

lower basin fill.

Groundwater

Groundwater generally flows through the regional aquifer 

from recharge areas near the mountains toward the San Pedro 

River, where it either discharges to perennial reaches of the 

stream as base flow, is captured by pumping from wells, 

or is transpired by phreatophytes(Pool and Coes, 1999). 

Groundwater also enters the aquifer as underflow from the 

Sierra Vista Subwatershed, and leaves as underflow to the 

north of Redington into the Lower San Pedro Basin. Recharge 

near the mountains occurs by infiltration of precipitation 

through permeable rocks and fractures, and by infiltration of 

runoff in ephemeral channels. Water in the San Pedro River 

that enters from the Sierra Vista Subwatershed, as well as 

discharges to the stream as base flow in several reaches, and 

infiltrates back into the stream alluvium farther downstream 

along intermittent and ephemeral reaches. Some groundwater 

flowing toward the San Pedro River is intercepted upgradient 

from the river by wells and phreatophytes in areas of shallow 

groundwater along ephemeral streams. 

Most groundwater flows through permeable sequences 

of sand and gravel in the coarse-grained unit of the lower 

basin fill and through the stream alluvium near the San Pedro 

River. In the Benson subarea, some flow occurs in saturated 

sequences of sand, gravel, and silt and clay interbeds in the 

upper basin fill. In the Narrows-Redington subarea, flow is 

generally through the lower basin fill because most of the 

upper basin fill has been removed by erosion (Dickinson, 

2003). Groundwater flow through the Narrows between the 

subareas mostly occurs in saturated lower basin fill and stream 

alluvium that is continuous between the two subareas. In 

the Benson subarea, groundwater flows in the deep, coarsegrained unit under the thick sequences of silt and clay of the 

fine-grained unit at the basin center. Groundwater also flows 

through a shallow system of layers of sand and gravel in the 

Holocene stream alluvium. The fine-grained unit generally 

restricts vertical flow between the deep and shallow systems, 

but vertical flow does occur through discontinuous silt and 

clay layers in the interbedded unit along the margin of the finegrained unit. Groundwater from the deep system flows through 

the interbedded unit to the shallow system before discharging 

in areas near the San Pedro River. An area of vertical flow 

between the deep and shallow systems might be present along 

the San Pedro River near St. David, where the interbedded unit 

underlies the river and stream alluvium (fig.12).

Contours of water levels in 1940 (fig. 14; Freethey 

and Anderson, 1986) indicate horizontal variations in water 

levels and flow patterns in the groundwater flow system prior 

to development. The contours indicate a general pattern of 

groundwater flow from recharge areas near the mountains to 

discharge areas along the San Pedro River, some flow from 

the Upper San Pedro Basin to the Lower San Pedro Basin 

through the Narrows, and an overall northward component 

of flow. Along an east-west direction, groundwater levels are 

highest near mountain fronts, and lowest near the center of the 

basin. In the north-south direction, water levels are generally 

highest in the southern part of the Benson subarea and lowest 

in the northern part of the Narrows-Redington subarea. In 

the Benson subarea, groundwater flow from the Sierra Vista 

Subwatershed and upland areas converges toward the center of 

the basin. In the Narrows-Redington subarea, the groundwaterlevel altitude is uncertain near the mountain fronts because the 

water table is defined by fewer wells. 

Groundwater levels and flow patterns have changed since 

predevelopment conditions mainly because of interception of 

groundwater by wells that would have discharged as base flow 

and because of long-term increases in ET by phreatophytes 

along the San Pedro River. Other changes are related to 

variation in the rates and distribution of recharge in areas 

of ephemeral channels near mountain fronts, changes in the 

timing and magnitude of seasonal flows in the San Pedro River, 

and entrenchment of the San Pedro River channel. Long-term 

water-level records for frequently measured wells indicate that 

there have been historical changes in storage, recharge, and 

discharge. In areas where groundwater has been developed, 

the records indicate changes in storage due to lowering 

of the water table in unconfined parts of the aquifer, and 

reductions in pressure and artesian levels in parts of the aquifer 

having confined and semiconfined conditions. Groundwater 

withdrawals have resulted in the reduction or cessation of 

artesian flow in many wells near St. David and Pomerene. 

In areas where groundwater withdrawals are negligible, 

interannual and interdecadal water-level changes are generally 

related to long-term temporal variations in natural recharge and 

discharge. These temporal changes in recharge and discharge 

are related to climate variability, variable ET rates (Thomas and 

Pool, 2006), and lowering of stream stages because of incision 

of stream channels (Hereford, 1993). 

Changes in groundwater conditions were inferred from 

long-term water-level hydrographs selected to represent 

most of the study area (fig. 15), continuous hydrographs of 

recent water-level measurements near the San Pedro River, 

and changes in groundwater storage inferred by analyses of 

repeat absolute gravity measurements between St. David and 

the Whetstone Mountains. Long-term changes in water levels 

are evaluated by examining hydrographs from selected wells 

distributed throughout the watershed. Selection of wells for 

hydrograph analysis was based on the location of the well 

within the watershed, the availability of a well description, 

and the period of water-level record. Long-term water level 

and well construction data were obtained from the USGS 

National Water Information System (NWIS) database and 

the ADWR Groundwater Site Inventory and WELLS-55 

databases (fig. 16). Recent changes in groundwater levels, 
Hydrogeology 19

80

90

 

110o

15’ 110o30’

32o

31o45’

32o15’

10

CO

EXPLANATION

Water-level contour—shows 

altitude of the water table prior 

to development. Modified from 

Freethey and Anderson, 1986. 

Contour interval is 20 m.

Boundary of the San Pedro 

Riparian National Conservation 

Area

Nonaquifer rocks

82

San

Pedro

River

Redington

Cascabel

Pomerene

Benson

The

Narrows

GRAHAM

COCHISE CO

ESI H

C

O

C

O

O

C

C

A IM

P

Tombstone

Redington

Benson

St David

Cascabel

Pomerene

Happy

Valley

Johnny

Lyon Hills

ALLEN

FLAT

Base from U.S. Geological Survey digital data, 1:100,000, 1982

Universal Transverse Mercator projection, Zone 12

0 5 MILES

0 5 KILOMETERS

RINCON

MOUNTAINS

DRAGOON

MOUNTAINS

MOUNTAINS

WHETSTONE

LITTLE

DRAGOON

MOUNTAINS

SANTA

CATALINA

MOUNTAINS

GALIURO

MOUNTAINS

WINCHESTER

MOUNTAINS

1180

1160

1140

1120

1100

1080

1060

1040

1020

1020

1000

960

940

920

900

880

980

1200

1220

1240

1260

1280

1300

1180

1160

1140

1120

1100

1080

1060

1040

1020

1020

1000

960

940

920

900

880

980

1200

1220

1240

1260

1280

1300

Figure 14. Map showing predevelopment water-level contours in 1940, modified from Freethey and Anderson (1986).
20 Hydrology of the Middle San Pedro Watershed, Southeastern Arizona

80

82

90

110o

15’ 110o30’

31o45’

32o

32o15’

10

CO

EXPLANATION

Tank

Shallow well (<60 m depth) with near 

the San Pedro River and perforated in 

the Holocene alluvium—water levels 

measured daily 2007–2010

Shallow well (<60 m depth) near the San 

Pedro River and perforated in the 

Holocene alluvium—water levels 

measured at irregular intervals

Well near mountains perforated in the coarse 

lower basin fill or fractured bedrock—water 

levels measured at irregular intervals

Deep well (>60 m depth) perforated in the 

coarse lower basin fill under confined or 

semiconfined conditions—water levels 

measured at irregular intervals

Well perforated in the coarse lower 

basin fill under unconfined

conditions—water levels measured at 

irregular intervals

Boundary of the San Pedro Riparian National 

Conservation Area

Absolute gravity monitoring site

GRAHAM

COCHISE CO

ESI H

C

O

C

O

O

C

C

A IM

P

Happy

Valley

Allen

Flat

San

Pedro

River

Redington

Cascabel

Tombstone

Pomerene Benson

Redington

Benson

Cascabel

Pomerene

D-18-21 08CDB1

D-17-20 23ACA2,3,4,5

D-17-20 36ADB1,2

D-14-20 17DBA

D-14-20 34BBC

D-19-21 05AAB

D-19-21 09CBC D-19-21 08DAC

D-19-21 28ACD

D-11-18 27ACC1 D-11-18 27DAB1

D-13-19 22ADD1,2,3,4

D-13-20 31BCC1,2

D-15-20 10BDC1

D-15-20 10CAB2

D-15-20 21CAB1,2

D-16-20 28DAA1

D-19-21 05AAA1,2

D-19-21 21ACC2 D-19-21 21DBB1,2

D-17-21 32BAB

D-18-21 06AAB2

D-13-18 02BDD

D-13-19 12DCC

D-14-21 11BBC

D-14-22 34BDC

D-15-20 30CCC

D-17-21 05CCA

D-18-21 01DCA

D-19-21 12DBB2

D-20-20 02DDD

D-17-19 14ACA

D-18-20 06BDD

D-18-20 01DAD2

D-18-21 34BDC

D-16-20 27BBB

D-16-20 34 ACB

D-18-21 28DAA

D-17-20 24BBD

D-17-21 31CDD

D-16-20 27DCC2

D-15-20 15CDC

D-15-20 10CAB2

D-14-21 19CAD

D-13-19 25BBB

D-12-19 19ADB

Tank

BensonCA

Kartchner

Canary

Post

Two

Juan

St David

Base from U.S. Geological Survey 

digital data, 1:100,000, 1982

Universal Transverse Mercator 

projection, Zone 12

0 5 MILES

0 5 KILOMETERS

RINCON

MOUNTAINS

DRAGOON

MOUNTAINS

MOUNTAINS

WHETSTONE

LITTLE

DRAGOON

MOUNTAINS

SANTA

CATALINA

MOUNTAINS

GALIURO

MOUNTAINS

WINCHESTER

MOUNTAINS

Figure 15. Map showing wells in which water levels were measured at daily and irregular intervals, and absolute gravity 

monitoring sites.
Hydrogeology 21

Base from U.S. Geological Survey digital data, 1:100,000, 1982

Universal Transverse Mercator projection, Zone 12

0 5 MILES

0 5 KILOMETERS

RINCON

MOUNTAINS

DRAGOON

MOUNTAINS

MOUNTAINS

WHETSTONE

LITTLE

DRAGOON

MOUNTAINS

SANTA

CATALINA

MOUNTAINS

GALIURO

MOUNTAINS

WINCHESTER

MOUNTAINS

80

82

90

110o

15’ 110o30’

32o

31o45’

32o15’

10

CO

EXPLANATION

Happy

Valley

Alle

F

n

lat

COCHISE CO

PIMA CO

GRAHAM

COCHISE CO

Johnny

Lyon Hills

Redington

Cascabel

Tombstone

Benson

St David

Redington

Benson

St David

Cascabel

The

Narrows

104 0

1260 1280

900

880

1240

1060

1000

104 0

1140

1120

1300

940

920

1000

1280

920

1240

1180

1160

1100

1200

1080

1220

960

1020

980

1000

1240

1280

1200

1260

1200

1220

960

1240

1280

940

1300

1200

Water-level measured 

October–November 2006

Water-level measured 

2000–2009

Water-level contour—shows 

altitude of the water table 

October–November 2006. 

Contour interval is 20 m. 

Dashed where uncertain.

Boundary of the San Pedro 

Riparian National Conservation 

Area

Nonaquifer rocks

Figure 16. Map showing water-level contours for the period October–November 2006. Contours are constrained 

by additional water levels measured from 2000 to 2009.
22 Hydrology of the Middle San Pedro Watershed, Southeastern Arizona

from January 2007 to September 2010, were inferred from 

water levels indicated by pressure transducers installed in 

32 wells near the San Pedro River. The wells are all within 

350 m of the river, range in depth from 3 m to 75m, and are 

perforated in Holocene stream alluvium. Daily water levels 

represent the average water level obtained at 30-minute 

intervals over a 24-hour period. Changes in groundwater 

storage were inferred from absolute gravity measurements at 

7 sites between the Whetstone Mountains and St. David. 

Long-Term Changes in Groundwater Levels in 

Wells near Mountain Fronts

Groundwater levels near the mountain fronts typically 

fluctuate in response to time-varying rates of recharge. These 

fluctuations are generally of greater magnitude than the 

natural changes in wells further from the mountains that are 

completed in the regional aquifer. Large water-level changes 

occur near areas of focused recharge in ephemeral channels and 

in areas near the mountain front with thin saturated sediments. 

The changes become smaller with greater distance from 

the mountain front and recharge areas, and with increasing 

transmissivity of the aquifer owing to thickening saturated 

sediments toward the center of the basin (Pool and Coes, 1999; 

Dickinson and others, 2004).

Long-term water level records (from 1950 to 2000) for five 

wells near the mountain fronts show considerable variation and 

within cyclical patterns. The largest changes occur in wells that 

are close to major drainages. The cyclical pattern appears as a 

repeating decline in water levels that rapidly returns to a higher 

level, which suggests that the water levels increase quickly 

during periods of increased recharge and decline gradually as 

Altitude of groundwater, in meters above NGVD 29

1960 1970 1980 1990 2000 2010 1960 1970 1980 1990 2000 2010

Year

1,102

1,104

1,106

1,108

1,110

1,112

1,114

1,116

D-17-19 14ACA

1,077

1,079

1,081

1,083

1,085

1,087

1,089

1,091

D-17-21 05CCA

1,272

1,274

1,276

1,278

1,280

1,282

1,284

1,286

D-18-20 06BDD

1,152

1,154

1,156

1,158

1,160

1,162

1,164

1,166

D-18-21 01DCA

1,157

1,159

1,161

1,163

1,165

1,167

1,169

1,171

D-19-21 12DBB2

1,237

1,239

1,241

1,243

1,245

1,247

1,249

1,251

D-20-20 02DDD

A B

C D

E F

Figure 17. Plots 

of long-term waterlevel records 

for 12 wells that 

are either near 

mountain fronts 

or perforated 

in the coarse 

lower basin fill 

under unconfined 

conditions 

(continued on next 

page). Water level 

is defined as the 

altitude above sea 

level (in meters) 

using the National 

Geodetic Vertical 

Datum of 1929 

(NGVD 29). 
Hydrogeology 23

the water drains to the regional aquifer. The records of these 

mountain-front wells indicate that the cycle of the rising and 

falling pattern on these hydrographs lasts approximately 10 

years and that recharge was greatest in the early 1980s and mid1990s. The magnitudes of the declines between the periods of 

recharge range from about 5 m in well D-13-18 02BDD to 3 m in 

wellD-18-20 06BDD (fig. 17). Levels in well D-14-21 19CA

display an opposite pattern of gradual increases, separated by low 

levels between the years 1990–1996 (fig. 17). In this case, the low 

levels may reflect local groundwater withdrawals, but the large 

increases likely indicate a natural response of the water levels to 

periods of recharge. Water levels in well D-13-19 12DCC do not 

appear to vary within cyclical patterns despite the proximity of 

the well to an ephemeral channel (fig.17). Levels in well D-15-20 

30CCC decline gradually, suggesting that the water levels are not 

influenced by nearby variations in recharge (fig. 17).

Long-Term Changes in Groundwater Levels in 

Unconfined Parts of the Lower Basin Fill

In general, long-term water-level records for wells that 

are perforated in the lower basin fill in unconfined conditions 

indicate declining or constant levels from 1960 to 2010 

(fig. 17). In Allen Flat, well D-14-21 11BBC indicates little 

change in the water level, while well D-14-22 34BDC has 

an overall decline of approximately 7 m despite low water 

use in the area (Arizona Department of Water Resources, 

2005). Declines of approximately 5 m between the years 

1980 and 2008 in well D-17-19 14ACA may be related to 

groundwater withdrawals for municipal use by communities 

near Benson. The hydrographs for wells D-14-22 34BDC 

and D-17-21 05CCA indicate constant water levels or brief 

recoveries during the late 1980s and 1990s, which might be 

Altitude of groundwater, in meters above NGVD 29

1940 1950 1960 1970 1980 1990 2000 2010 1940 1950 1960 1970 1980 1990 2000 2010

Year

D-13-18 02BDD D-13-19 12DCC

D-14-21 11BBC D-14-21 19CAD

D-14-22 34BDC D-15-20 30CCC

G H

I J

K L

964

966

968

970

972

974

976

978

980

1,309

1,311

1,307

1,303

1,305

1,299

1,301

1,297

1,140

1,120

1,160

1,180

1,200

1,220

1,240

1,260

1,272

1,274

1,276

1,278

1,280

1,282

1,284

1,286

1,288

1,047

1,049

1,051

1,053

1,055

1,057

1,059

1,061

975

970

960

965

950

955

940

945

Figure 17. Plots 935

of long-term waterlevel records for 12 

wells that are either 

near mountain fronts 

or perforated in the 

coarse lower basin 

fill under unconfined 

conditions. Water 

level is defined as the 

altitude above sea 

level (in meters) using 

the National Geodetic 

Vertical Datum of 1929 

(NGVD 29).—Continued 
24 Hydrology of the Middle San Pedro Watershed, Southeastern Arizona

related to increased precipitation (fig. 5) and recharge during 

the early 1980s and 1990s that is indicated by several water 

level records for wells near the mountain fronts (fig. 17). The 

delay in these recoveries may be related to the time required 

for water from recharge events near the mountain fronts to 

reach the regional aquifer. 

Long-Term Changes in Groundwater Levels in 

Confined and Semiconfined Parts of the Lower 

Basin Fill

Water level records in four wells completed in the 

confined part of the coarse lower basin fill generally declined 

from 1944 to 2010 (fig. 18). Declines in these wells likely are 

related to reduced pressures in the confined and semiconfined 

aquifer because of groundwater withdrawals for agriculture and 

municipal use. Near St. David, levels in well D-17-21 32BAB 

declined about 13 m from 1944 to 2010. In wells D-18-20 

01DAD2 and D-18-21 34BDC, also near St. David, the levels 

decreased approximately 5 m and 7 m, respectively, from 1945 

to the mid- to late 1970s. The record for well D-18-20 01DAD2 

indicates a brief recovery of approximately 3 m from 1980 to 

1990, after which the record ends. The decline in well D-16-20 

27BBB, north of Pomerene, is approximately 21 m.

Long-Term Changes in Groundwater Levels near 

the San Pedro River

Long-term water levels in shallow wells near the San 

Pedro River have fluctuated interannually in response to 

periods of streamflow (fig. 19). Water levels from 1940 to 

2010 recovered in response to increased streamflow, and 

declined when streamflow was less frequent. Entrenchment 

of the streambed since the early 1900s likely resulted in 

some water level declines of several meters near the river, 

similar to the amount of downcutting by the river, but data are 

not available to characterize this effect on the groundwater 

system. In wells D-12-19 19ADB, D-16-20 27DCC2, D-17-21 

1,040

1,045

1,050

1,055

1,060

1,065

1,070

1,105

1,110

1,115

1,120

1,125

1,100

1,110

1,105

1,115

1,120

1,135

1,140

1,145

1,150

1,155

1945 1955 1965 1975 1985 1995 2005 1945 1955 1965 1975 1985 1995 2005

D-18-21 34BDC D-16-20 27BBB

D-17-21 32BAB D-18-20 01DAD2

Altitude of groundwater, in meters above NGVD 29

Year

A B

C D

Figure 18. Plots of long-term water-level records in wells perforated in the coarse lower basin fill under confined 

or semiconfined conditions. Water level is defined as the altitude above sea level (in meters) using the National 

Geodetic Vertical Datum of 1929 (NGVD 29).
Hydrogeology 25

Figure 19. Long-term water-level records in 10 wells near the San Pedro River, all measuring less than 60 m in depth, 

and perforated in the Holocene alluvium. Water level is defined as the altitude above sea level (in meters) using the 

National Geodetic Vertical Datum of 1929 (NGVD 29).

896

898

900

902

904

906

908

D-12-19 19ADB 946

948

950

952

954

956

958

D-13-19 25BBB

996

998

1000

1002

D-15-20 10CAB2 1,006

1,008

1,010

1,012

D-15-20 15CDC

1,051

1,053

1,055

1,057

1,059

1950 1960 1970 1980 1990 2000 2010

D-16-20 27DCC2 1,046

1,048

1,050

1,052

1,054

1950 1960 1970 1980 1990 2000 2010

D-16-20 34ACB

Altitude of groundwater, in meters above NGVD 29

A B

C D

E F

1,088

1,090

1,092

1,094

D-17-21 31CDD

1,093

1,095

1,097

1,099 D-18-21 06AAB2

1,113

1,115

1,117

1,119

1940 1950 1960 1970 1980 1990 2000 1940 1950 1960 1970 1980 1990 2000 2010

D-18-21 28DAA

1,068

1,070

1,072

1,074

D-17-20 24BBD

Altitude of groundwater,

in meters above NGVD 29

Year

G

I

H

J
26 Hydrology of the Middle San Pedro Watershed, Southeastern Arizona

31CDD, and D-18-21 06AAB2, levels were lowest during 

periods of lower precipitation and base flow in the San Pedro 

River in the 1950s to mid-1960s (fig. 5). In these same wells, 

the mean water levels are generally several meters higher 

than other wells near the San Pedro River and the interannual 

fluctuations are larger after the mid-1960s. In well D-12-19 

19ADB, between Cascabel and Redington, the fluctuations are 

as much as 10 m from the mid-1970s to the late 1980s. Higher 

water levels in wells D-13-19 25BBB, D-16-20 27DCC2, 

and D-16-20 34ACB in the 1980s may be related to periods 

of increased precipitation (fig. 5), base flow, and recharge as 

indicated by wells near the mountain fronts (fig. 17).

Recent Changes in Groundwater Levels near the 

San Pedro River

Changes in groundwater levels near the San Pedro River 

from 2007 to 2010 indicate seasonal variations in the rates of 

recharge and discharge to the river and in ET by phreatophytes 

and possibly some effects of withdrawals from wells (figs. 

20–22). Previous evaluation of water-level variations in wells 

along the San Pedro River in the Upper San Pedro Basin by 

Leenhouts and others (2006) identified an annual pattern of 

two periods of high levels and two periods of low levels. 

The low levels occur during the warm and dry periods in the 

summer during June through July and in the dry periods in the 

fall from October to November. Low levels are related to high 

phreatophyte ET rates from March to November. High water 

levels occur during the warm and wet months in the summer 

(July through September) and the cool and wet months in 

the winter (January through April). High water levels in the 

summer result from infiltration of monsoon-event streamflow 

into the alluvium, and high levels in the winter are related to 

the lack of ET by phreatophytes as well as infiltration of runoffproducing winter precipitation. In some wells in the southern 

part of the study area, water levels reach a maximum early 

in the summer and do not respond to additional streamflow 

events, which suggest that any additional potential recharge 

cannot be stored in the aquifer. 

The patterns in water-level fluctuations in wells near 

the San Pedro River in the San Pedro Riparian National 

Conservation Area (SPRNCA) generally appear in the 

hydrographs from wells in the middle San Pedro Watershed, 

but are less pronounced in downstream sites and differ in 

magnitude with time (figs. 20–21; Leenhouts and others, 2006). 

The hydrographs for wells in the southern part of the study 

area, near the SPRNCA, generally show a substantial decline in 

water levels of two or more meters from June to July, whereas 

levels in wells north of the Narrows declined two meters or 

less in this period. A large increase in water levels in all wells 

coincided with periods of relatively high flow in the San Pedro 

River from July to September, except for the summer months 

of 2009, when streamflow rates were low.

The direction of groundwater flow is determined by 

hydraulic gradient, measured by comparing water levels at 

two different horizontal or vertical locations. The downward 

movement of groundwater is indicated by a negative vertical 

gradient; the reverse is indicated by a positive vertical 

gradient. Vertical water-level gradients measured at several 

sites at which collocated wells were completed at different 

depths indicate that groundwater flow is generally downward 

and the San Pedro River is generally losing water to the 

subsurface (fig. 22). At the sites of wells D-13-20 31BCC1, 

-2 and D-13-19 22ADD1, -2, -3, and -4, the vertical gradient 

and groundwater recharge rates are largest during periods of 

summer streamflow; the gradient decreases and vertical flow 

diminishes to the lowest value in early summer, before the 

onset of summer streamflow events.

Recent Changes in Groundwater Storage 

Changes in groundwater storage can be measured by 

using the temporal-gravity method, which is described in 

Kennedy and Winester (2011). Gravity data provide a direct 

measurement of subsurface mass, as explained by Newton’s law 

of gravitation. If repeat gravity measurements are made, then 

changes in the force of gravity can reflect changes in subsurface 

mass, such as aquifer storage change (other processes that can 

cause subsurface mass change, such as active volcanism, are 

not present in the study area). If gravity measurements are 

made concurrently with water-level measurements at a well, an 

estimate of specific yield can be made. 

 A Micro-g Lacoste, Inc. A-10 absolute gravimeter was 

used to collect gravity data at seven monitoring sites in the 

middle San Pedro Basin between 2008 and 2010 (fig. 15; 

Kennedy and Winester, 2011). Five of the monitoring sites 

are near the base of the Whetstone Mountains (Two, Post, 

Canary, Benson CA, Tank), one monitoring site is midway 

between the Whetstone Mountains and the San Pedro River 

(Juan), and another is just north of the town of St. David 

(St. David) (fig. 15). Five measurements were made at most 

monitoring sites. Initial measurements were made in spring 

2008, followed by quarterly measurements during 2010. 

Gravity change was converted to aquifer storage change using 

a horizontal infinite slab approximation, by which 42 μgal is 

equivalent to about 1m of water (Telford and others, 1990).

At four of the seven monitoring sites (Tank, Juan, 

Benson CA, Post), gravity measurements (fig. 23) show 

little or no aquifer storage change between 2008 and 2010 

when measurement uncertainty is considered (about ±6 μGal, 

or 145 mm water—for details see Kennedy and Winester, 

2011). Post shows a small seasonal signal, typical of stations 

in semiarid environments where soil moisture varies. At the 

Canary monitoring site, a sizeable increase in aquifer storage 

between March 2008 and December 2009 is indicated by the 

gravity data, likely associated with recharge or storage in the 

unsaturated zone of winter rainfall during this period. Similar 

or larger amounts of rainfall during the summer in 2008, 2009, 

and 2010, however, did not result in an increase in aquifer 

storage at this or other monitoring sites. A similar but smaller 
Hydrogeology 27

0.001

0.01

0.1

1

10

100

0.001

0.01

0.1

10

100

0.001

0.01

0.1

100

1

10

1

Altitude of groundwater, in meters above NAVD 88

1

2

3

4

5

6

7

8

9

1

2

3

4

5

6

7

8

9

1

2

3

4

5

6

7

8

9

1

2

3

4

5

6

7

8

9

0.001

0.01

0.1

1

10

100

Streamflow, in cubic meters per second

Depth to groundwater below land surface, in meters 1,145

1,127

1,128

1,129

1,130

1,131

1,132

1,133

1,134

1,135

1,146

1,147

1,148

1,149

1,150

1,151

1,152

1,153 D-19-21 21DBB1 52.9 m depth

D-19-21 08DAC 3.5 m depth

D-19-21 28ACD 5.4 m depth

D-19-21 21DBB2 17.8 m depth

1,147

1,148

1,149

1,150

1,151

1,152

1,153

1,154

1,155

Streamflow at Tombstone

Streamflow at Tombstone

Streamflow at Tombstone

Streamflow at Tombstone

D-19-21 05AAA1

1,017

1,018

1,119

1,120

1,121

1,122

1,123

1,124

1,125

January

2007

July

2007

July

2008

July

2009

July

2010

January

2008

January

2009

January

2010

Figure 20. Plots of daily water levels in wells near the San Pedro River and streamflow at the gaging 

station San Pedro River near Tombstone, Arizona (09471550) from January 2007 to September 2010. Water 

level is defined as the altitude above sea level (in meters) using the North American Vertical Datum of 1988 

(NAVD 88). 
28 Hydrology of the Middle San Pedro Watershed, Southeastern Arizona

0.001

0.01

0.1

1

10

100

0.001

0.01

0.1

10

100

0.001

0.01

0.1

100

1

10

1

Altitude of groundwater, in meters above NAVD 88

D-15-20 21CAB1

D-18-21 08CDB1

D-17-20 36ADB1 

1

2

3

4

5

6

7

8

9

1

2

3

4

5

6

7

8

9

1

2

3

4

5

6

7

8

9

5

6

7

8

9

10

11

12

13

Streamflow at Tombstone

Streamflow at Tombstone

Streamflow at Benson

Streamflow at Benson 0.001

0.01

0.1

1

10

100

D-15-20 10CAB2

January

2007

July

2007

July

2008

July

2009

July

2010

January

2008

January

2009

January

2010

Streamflow, in cubic meters per second

Depth to groundwater below land surface, in meters

1,098

1,099

1,100

1,101

1,102

1,103

1,104

1,105

1,106

1,083

1,084

1,085

1,086

1,087

1,088

1,089

1,090

1,091

1,002

1,001

1,000

999

1,003

1,004

1,005

1,006

1,007

1,008

1,009

1,010

1,011

1,012

1,013

1,014

1,015

1,016

Figure 21. Plots of daily water levels in wells near the San Pedro River and streamflow at the gaging 

stations San Pedro River near Tombstone, Arizona (09471550) and San Pedro River near Benson, Arizona 

(09471800) from January 2007 to September 2010. Water level is defined as the altitude above sea level 

(in meters) using the North American Vertical Datum of 1988 (NAVD 88). 
Hydrogeology 29

0.1

100

0.001

0.01

0.1

10

100

0.001

0.01

0.1

100

1

10

1

0.001

0.01

1

10

0.001

0.01

0.1

1

10

100

Streamflow, in cubic meters per second

989

988

987

986

990

991

992

993

994

995

D-14-20 34BBC

Streamflow at Benson

Altitude of groundwater, in meters above NAVD 88

1

2

3

4

5

6

7

8

9

1

2

3

4

5

6

7

8

9

1

2

3

4

5

6

7

8

9

1

2

3

4

5

6

7

8

9

Depth to groundwater below land surface, in meters

857

859

855

853

852

854

856

858

860 D-11-18 27ACC1 28.0 m depth

D-11-18 27DAB1 21.3 m depth

Streamflow at Redington

D-13-19 22ADD4 18.9 m depth

D-13-19 22ADD3 9.1 m depth

845

847

843

841

840

842

844

846

948

Streamflow at Benson

958

960

956

954

953

955

957

959

961

D-13-20 31BCC1 16.8 m depth

D-13-20 31BCC2 9.1 m depth

Streamflow at Benson

January

2007

July

2007

July

2008

July

2009

July

2010

January

2008

January

2009

January

2010

Figure 22. Plots of daily water levels in wells near the San Pedro River and streamflow at the 

streamgaing stations San Pedro River near Benson, Arizona (09471800) and San Pedro River at Redington 

Bridge near Redington, Arizona (09472050) from January 2007 to September 2010. Water level is defined as 

the altitude above sea level (in meters) using the North American Vertical Datum of 1988 (NAVD 88). 
30 Hydrology of the Middle San Pedro Watershed, Southeastern Arizona

January 1,

2008

July 1,

2008

December 31,

2008

July 1,

2009

December 31,

2009

July 1,

2010

December 31,

2010

Cumulative rainfall

Post

Tank

Juan

Canary

St. David

Two

BensonCA

Aquifer storage change

Water table elevation

400

200

0

-200

-400

400

200

0

-200

-400

600

300

0

-300

-600

1,000

750

500

250

0

Millimeters

Water table elevation, in meters

Aquifer storage change, in millimeters of water

0

-200

-400

-600

-800

1,110

1,109

1,108

1,107

1,106

400

200

0

-200

-400

600

400

200

0

-200

400

200

0

-200

-400

Water level, linear trend line

Figure 23. Plots of cumulative aquifer storage change and rainfall in the Benson subarea for 

2008 through 2010. The graph for gravity monitoring site St. David also shows depth-to-water at a 

nearby well (D-17-21 32BAB) and a linear trend line fit to the depth-to-water data, which are used 

to calculate specific yield.
Groundwater Budgets 31

increase in gravity values, and thus in aquifer storage, occurred 

at monitoring sites Juan, Two, and Post during this same period.

Gravity measurements at the St. David monitoring site 

show a steady decline in aquifer storage of about -240 mm/

year when a linear trend is assumed (fig. 23). Water-level 

measurements at a nearby well, D-17-21 32BAB, about 700 m 

south-southeast of the gravity monitoring site, show a similar 

steady decline of about -0.9 m/year. Dividing the linear trend 

of the change in water storage by the change in water level 

gives a specific yield equal to 0.27, an average-to-high value 

for unconfined aquifers. This close correlation between gravity 

measurements and water-level change, and a relatively large 

gravity change, indicates unconfined conditions in the aquifer. 

St. David lies on the northeastern edge of fine-grained deposits 

in the center of the basin (Dickinson and others, 2010a), but 

on the basis of the present data these deposits do not appear to 

extend to the vicinity of the well and gravity monitoring site.

Gravity measurements at monitoring site Two indicate 

relatively little storage change with the exception of one low 

measurement in July 2010 (fig. 23). A likely explanation 

for the low measurement is depressed water level caused 

by pumping from one or more production wells located just 

north of the gravity station. A subsequent measurement, in 

December 2010, was nearly equal to the two measurements 

prior to the one in July, indicating that any storage change 

associated with nearby pumping was not a long-term change.

Groundwater Budgets 

A groundwater budget is based on the principle that the 

rate of change in the volume of water stored in an accounting 

unit such as an aquifer is equal to the rate at which water flows 

into the aquifer minus the rate at which water flows out of the 

aquifer (Healy and others, 2007). Water budgets provide a basis 

for assessing this rate of change, and in particular, how a natural 

or human-induced change in one part of the hydrologic cycle 

may affect other aspects of the cycle (Healy and others, 2007). 

A groundwater budget is an accounting of the inflow to, outflow 

from, and storage change in an aquifer for a select time period 

(Hollett and others, 1991). In a groundwater budget, if the 

inflows and outflows are equal, the groundwater system is said 

to be in equilibrium. If the inflows and outflows in an aquifer 

are not equal then the aquifer is in a transient condition, and 

the change in the volume of groundwater storage is manifested 

in changing water levels. Calculation of a groundwater budget 

can indicate whether aquifer storage is declining, gaining, or 

stable and can provide a better understanding of the movement 

of groundwater; this information is needed to make informed 

water-management decisions.

Groundwater budgets were developed for the Benson 

and Narrows-Redington subareas for predevelopment 

conditions (prior to 1940) and recent conditions (2001–2009). 

Groundwater inflows into the middle San Pedro Watershed 

consist of underflow from the Sierra Vista Subwatershed, 

recharge from infiltration along ephemeral channels, recharge 

from infiltration through permeable rocks and sediments 

within and near the mountain fronts as well as through 

sediments along the San Pedro River, and incidental recharge 

from irrigation. The quantity of groundwater underflow from 

the Sierra Vista Subwatershed was estimated in simulations 

made with the existing groundwater-flow model (Pool 

and Dickinson, 2007). Infiltration along the San Pedro 

River was calculated as the difference in the streamflow 

volumes at upstream and downstream gaging stations. 

Groundwater recharge was estimated by simulations made 

with a distributed-parameter water balance model (BasinCharacterization Model, BCM; Flint and Flint, 2007) and a 

regression equation between recharge and precipitation for 

alluvial basins in southern Arizona (Anderson and others, 

1992). The BCM was developed as part of a larger study 

of the processes, properties, and climate factors affecting 

groundwater recharge and runoff variability in the arid 

and semiarid southwestern United States (Flint and Flint, 

2007). Incidental recharge was estimated from groundwater 

withdrawals for irrigation and irrigation system efficiencies.

Groundwater outflows from the middle San Pedro 

Watershed include withdrawals at wells, loss through ET 

by phreatophytes, discharge to the channel of the San Pedro 

River, and underflow to the Lower San Pedro Basin north 

of Redington. Data on historical and recent groundwater 

withdrawals were compiled from previous reports and from 

municipal water company and industry records. Surface-water 

diversions at the St. David ditch and Pomerene canal were 

compiled from Putman and others (1988). Agricultural water 

use from 2006–2009 was estimated by field verification of 

crop type, area of irrigated acreages, irrigation efficiency, and 

the modified Blainey-Criddle formula as described by the 

Bureau of Reclamation (1992, appendix A). Evapotranspiration 

by phreatophytes was calculated for the period 2000 to 2009 

using remote sensing data and estimates from Freethey and 

Anderson (1986) for predevelopment conditions. Underflow to 

the Lower San Pedro Basin was determined by calculating the 

residual in the water budget of the Narrows-Redington subarea. 

The changes in groundwater storage in both subareas were 

calculated as the residual after subtracting the inflows from the 

outflows of the groundwater budget.

Groundwater budgets for water years 2001–2009 for the 

Benson and Narrows-Redington subareas are presented at the 

end of this section. This period was selected because all of 

the major components of the water budget were available for 

these years. All groundwater budget components are summed 

by water year with the exception of the water-use components, 

which are compiled by calendar year because the U.S. Census 

data on which they are based are compiled by calendar year. 

The following sections describe the methods used 

to estimate natural recharge, withdrawals from wells, and 

through ET by phreatophytes, and streamflow seepage 

components of the groundwater budget. The average 

(1940–2009) annual natural recharge component was used 

in calculating the groundwater budget to reduce anomalies 

caused by short-term variability in climate.
32 Hydrology of the Middle San Pedro Watershed, Southeastern Arizona

Benson Subarea and Narrows-Redington 

Subarea Predevelopment Groundwater Budgets

The predevelopment groundwater budgets for the 

Benson and Narrows-Redington subareas are based on 

previous work by Anderson and Freethey (1995), who 

compiled predevelopment groundwater budgets for several 

alluvial basins in Arizona. The boundaries of the Benson 

subarea are similar to their “Benson Basin” and are used 

for this report. The total annual predevelopment discharge 

from the Benson subarea is approximately 20.5 hm3/yr 

(table 4). The groundwater budget developed by Anderson 

and Freethey (1995) for the Lower San Pedro Basin extends 

from the Narrows to the confluence of the San Pedro and 

Gila Rivers. Their groundwater budget was modified for 

the boundaries of the Narrows-Redington subarea as we 

have defined it. The total annual predevelopment discharge 

from the Narrows-Redington subarea is approximately 

14.8 hm3/ yr (table 4) and it is assumed that the quantity of 

recharge as defined by Anderson and Freethey (1995) as 

“Stream Base flow Leaving (the Benson) Subarea” into the 

Narrows-Redington subarea is included in the “MountainFront and Ephemeral Channel Recharge.”

Groundwater Recharge

In the middle San Pedro Watershed, groundwater is 

recharged by infiltration and percolation of precipitation and 

snowmelt in bedrock mountains (mountain-block recharge or 

MBR), seepage of runoff within stream channels that emerge 

from the mountains and flow onto the unconsolidated basin fill 

(mountain-front recharge or MFR), infiltration of streamflow 

from the San Pedro River, northward lateral groundwater 

flow from the Sierra Vista Subwatershed to the south, and 

incidental recharge from irrigation. The contribution of 

recharge to the aquifer from infiltration and percolation of 

precipitation on the basin fill is considered minimal (Coes and 

Pool, 2007) and was not included in the groundwater budget.

Mountain-Front and Mountain-Block Recharge

 Groundwater in the middle San Pedro Watershed moves 

from recharge areas near the mountains and flows through 

the basin-fill aquifer, where it may be discharged to perennial 

reaches on the San Pedro River or is withdrawn by riparian 

ET (fig. 24). Quantifying the rates and locations of MFR 

and MBR is difficult because the water available for MFR 

and MBR is controlled by precipitation, evaporation rates, 

streamflow and runoff, and the permeability of exposed rocks 

in mountains and materials in the unsaturated zone.

The first basin-scale estimates of natural recharge in the 

San Pedro Valley were developed as part of the Southwest 

Alluvial Basins Regional-Aquifer System Analysis (SWAB 

RASA) study in the mid-1990s. As part of that study, 

groundwater budgets were developed on the basis of available 

data, and where information was lacking, a regression 

equation between the annual recharge and precipitation was 

developed (Anderson and others, 1992). These estimates of 

recharge were later incorporated into numerical groundwaterflow models (Anderson and Freethey, 1995). 

Investigations of MFR and MBR in the San Pedro Valley 

have been focused on areas in the Upper San Pedro Basin to 

the south. In the middle San Pedro Watershed, estimates of 

MFR and MBR have not been the primary focus in previous 

investigations. As part of a larger project to better understand 

the hydrology of the Sierra Vista Subwatershed, Coes and 

Pool (2005) estimated infiltration along ephemeral stream 

channels using chloride, tritium, and stable-isotope data. The 

contribution of recharge to the aquifer from infiltration and 

percolation of precipitation on the basin fill is considered 

minimal (Coes and Pool, 2005) and was not included in the 

groundwater budget.

The timing and areas of recharge were investigated by 

Hopkins (2014), who collected water samples from wells, 

the San Pedro River, and precipitation in the middle San 

Pedro Watershed to describe movement of groundwater 

and residence times using solute and isotope geochemistry. 

Hopkins found evidence that winter precipitation contributes 

to recharge in the fracture system and the unconfined aquifer, 

whereas summer precipitation recharges the alluvial aquifer. 

Distributed-Parameter Water Balance Model

The temporal and spatial distributions of potential 

recharge from MFR and MBR were estimated for the middle 

San Pedro Watershed using a distributed-parameter waterbalance model (Basin-Characterization Model, BCM; Flint 

and Flint, 2007). The BCM has been used in other recent 

studies to estimate the spatial and temporal distribution of 

Water-budget component

Benson

subarea

Narrows-Redington 

subarea

Inflow 

(hm3/yr)

Outflow 

(hm3/yr)

Inflow 

(hm3/yr)

Outflow 

(hm3/yr)

Mountain-front and 

ephemeral channel recharge

20.5 0.0 13.3 0.0

Stream baseflow leaving 

subarea

0.0 6.7 0.0 7.2

Evapotranspiration by 

phreatophytes

0.0 12.3 0.0 6.9

Groundwater underflow1 0.0 1.5 1.5 0.7

Groundwater storage 0.0 0.0 0.0 0.0

Total 20.5 20.5 14.8 14.8

1

Value from this study.

Table 4. Predevelopment groundwater budgets for the Benson 

and Narrows-Redington subareas modified from Anderson and 

Freethey (1995).

[Abbreviation: hm3/yr, cubic hectometers per year]
Groundwater Budgets 33

Precipitation 

Evapotranspiration 

Streamflow 

Infiltration 

Zone of 

lateral flow 

Zone of recharge 

Zone of 

discharge 

Zone of weathering Zone of weathering 

Mountain block Piedmont slope Valley lowlands

Infiltration and evapotranspirationInfiltration and evapotranspiration

Water table 

Basin fill 

Groundwater

divide 

Water table Groundwater

divide 

Crystalline rock Crystalline rock Limestone Limestone

Perennial reach 

on the San Pedro

River 

EXPLANATION

Evapotranspiration

Precipitation

Groundwater flow

Infiltration of in-place

recharge component

Infiltration of runoff

component

Streamflow from

runoff component

Figure 24. Conceptual illustration of groundwater recharge and discharge in the middle San Pedro Watershed.

recharge in alluvial basins in Arizona as part of groundwater 

budget and groundwater model development (Tillman and 

others, 2011; Pool and others, 2011; Garner and Truini, 2011). 

Data inputs to the BCM include precipitation, temperature, 

and geologic properties to estimate the spatial and temporal 

distribution of water available to become MFR and MBR.

The BCM calculates the monthly spatial distribution of 

available water for in-place recharge and potential runoff (in 

millimeters per month per 270-by-270-meter grid cell) using 

spatially distributed properties of soils and geologic units and 

spatial and temporal distributions of climate parameters. In-place 

recharge is water that infiltrates into the subsurface where 

precipitation occurs and represents MBR. In the middle San 

Pedro River Watershed, recharge in the mountains composed of 

nonaquifer rocks (fig. 14) is minimal, with the exception of the 

fracture zones described by Hopkins (2014). Recharge can occur 

in the mountain areas composed of the permeable limestone, 

where water from precipitation and snow melt can infiltrate, 

percolate, and flow to the basin-fill aquifer (fig. 24). Potential 

runoff originates as precipitation that does not infiltrate areas 

of nonaquifer rock and is assumed to move as surface water in 

channels in the mountain front and may infiltrate in ephemeral 

channels before reaching the San Pedro River. Potential runoff 

that enters the regional aquifer in this manner is considered 

MFR. Together, the sum of in-place recharge and potential runoff 

constitutes the potential recharge calculated from the BCM.

In previous applications of the BCM, 15 percent 

of the potential runoff was assumed to become recharge 

(Flint and Flint, 2007; Tillman and others 2011), where 

investigators considered recharge to be equal to the sum of 

the in-place recharge and 15 percent of the potential runoff. 

The 15-percent value is similar to the percent of ephemeral 

Benson subarea recharge

Benson subarea modified recharge

Narrows-Redington subarea recharge

40

35

30

25

20

15

10

5

0

1940–1949 1950–1959 1960–1969 1970–1979 1980–1989 1990–1999 2000–2009

Average annual groundwater recharge, in cubic hectometers 

EXPLANATION

Figure 25. Plot showing 

decadal average annual 

groundwater recharge 

calculated from the 

distributed-parameter 

water balance model. 

Groundwater recharge 

is equal to the sum of the 

in-place recharge and 15 

percent of the potential 

runoff. The dashed line 

represents the modified 

groundwater recharge 

value in the Benson 

subarea.
34 Hydrology of the Middle San Pedro Watershed, Southeastern Arizona

Area

Wet

Percent potential 

runoff (hm3) In-place 

recharge

Percent 

potential runoff 

recharge(hm3)

100 15 100 15

Benson subarea 0.56 0.08 0.94 1.50 1.02

Narrows-Redington 

subarea 1.63 0.24 1.47 3.10 1.71

Middle San Pedro 

Watershed (Total) 2.19 0.33 2.41 4.60 2.74

Area

Dry

Percent potential 

runoff (hm3) In-place 

recharge

Percent 

potential runoff 

recharge(hm3)

100 15 100 15

Benson subarea 0.00 0.00 0.01 0.01 0.01

Narrows-Redington 

subarea 0.48 0.07 0.59 1.07 0.66

Middle San Pedro 

Watershed (Total) 0.48 0.07 0.61 1.09 0.68

Area

Winter

Percent potential 

runoff (hm3) In-place 

recharge

Percent 

potential runoff 

recharge(hm3)

100 15 100 15

Benson subarea 1.00 0.15 1.32 2.32 1.47

Narrows-Redington 

subarea 6.76 1.01 6.27 13.03 7.28

Middle San Pedro 

Watershed (Total) 7.76 1.16 7.58 15.34 8.74

Area

Percent annual total

recharge (hm3)

100 15

Benson subarea 3.83 2.50

Narrows-Redington 

subarea 17.20 9.65

Middle San Pedro 

Watershed (Total) 21.03 12.16

Table 5. Distributed-parameter water-balance model calculated 

average annual runoff and recharge for 1940–2009 in the middle San 

Pedro Watershed and subareas. The potential recharge calculated 

using 100 percent of the potential runoff assumes that all potential 

runoff infiltrates ephemeral channels and becomes recharge.

[Total recharge equals in-place recharge plus runoff recharge; hm3, cubic 

hectometer] 

Area Annual recharge 

(hm3)

Benson subarea 7.08

Narrows-Redington subarea 9.62

Middle San Pedro Watershed (Total) 16.7

Table 6. Modified average annual natural recharge for the 

Benson subarea, Narrows-Redington subarea, and the Middle 

San Pedro Watershed for 1940–2009.

[Runoff recharge equals 15 percent of estimated total runoff. Total recharge 

equals in-place recharge plus runoff recharge; hm3, cubic hectometer] 

channel infiltration in the Sierra Vista Subwatershed 

estimated by Coes and Pool (2007). Their results suggested 

that ephemeral channel recharge accounts for about 12 to 

19 percent of the estimated average annual recharge. For 

this study, potential recharge is calculated in two ways, on 

the basis of 15- and 100-percent of potential runoff. The 

potential recharge calculated using 100 percent potential 

runoff assumes that all potential runoff infiltrates ephemeral 

channels and becomes recharge. The potential runoff is not 

routed through ephemeral channels to the San Pedro River in 

the BCM. 

Spatial and Temporal Distribution of Mountain-Block and 

Mountain-Front Recharge in the Benson Subarea 

In the Benson subarea, most potential recharge occurs in the 

Whetstone and Dragoon Mountains during the wet months, with 

lesser recharge in the winter months and least in the dry months. 

The rates of potential recharge were greatest for 1960–1969 

and 1990–1999 (fig. 25). The periods with the lowest available 

water for potential recharge were 1940–1949 and 2000–2009. 

The average annual potential recharge for the Benson subarea 

calculated using 100-percent and 15-percent runoff for 1940–2009, 

was 3.83 hm3/yr and 2.50 hm3/yr respectively (table 5). These 

potential recharge values are underestimates of potential recharge 

on the basis of previous recharge estimates for the Benson subarea 

and in comparison to the other components of the groundwater 

budget. ADWR (2005) estimated that annual recharge in the 

Benson subarea was 13.1 hm3/yr. An additional recharge constant 

of 4.58 hm3/yr was added to the average annual potential recharge 

(2.50 hm3/yr) calculated from the distributed-parameter water 

balance model. This recharge constant was estimated by balancing 

the groundwater budget without pumping or incidental recharge 

for 2001–2009. The modified distributed-parameter water-balance 

model-calculated long-term recharge value was 7.08 hm3/yr, the 

value used in the Benson subarea groundwater budget (table 6).

Spatial and Temporal Distribution of Mountain-Block 

and Mountain-Front Recharge in the Narrows-Redington 

Subarea 

In the Narrows-Redington subarea, the rates of potential 

recharge are greatest in the winter months with lesser recharge 

in the wet months and the least in the dry months. The rates 
Groundwater Budgets 35

of potential recharge are greatest in the Santa Catalina 

Mountains. The largest rate of potential recharge over a 

ten-year period was that during 1990–1999, and the driest 

decadal period was 2000–2009 (fig. 25). The average annual 

potential recharge using 100-percent and 15-percent runoff for 

the time period 1940–2009 was 17.2 hm3/yr and 9.65 hm3/yr, 

respectively (table 5). The potential recharge calculated 

from 15-percent runoff was used in the Narrows-Redington 

groundwater budget. The higher values of potential recharge 

in the Narrows-Redington subarea compared to the Benson 

subarea are consistent with the findings of Pool and others 

(2011), who found that BCM results are heavily dependent 

on altitude because of the higher precipitation rates, lower 

temperatures, and the lower rates of ET at higher altitudes than 

at lower altitudes. In general, the results of BCM simulations 

for the middle San Pedro Watershed are consistent with results 

of Tillman and others (2011) in that recharge rates are much 

greater in the Narrows-Redington subarea (Lower San Pedro 

Basin) than in the Benson subarea (Upper San Pedro Basin).

Mountain-Front-Recharge Estimation Using the 

SWAB RASA Regression Model

Groundwater recharge from MFR in the middle San 

Pedro Watershed was also calculated using a regression model 

developed by Freethey and Anderson (1986). The SWAB 

RASA Regression model may not be appropriate for the 

smaller subareas, however, and was therefore applied over the 

entire middle San Pedro Watershed. 

Annual MFR estimates made using the SWAB RASA 

regression model ranged from 13.62 hm3/yr to 26.44 hm3/yr 

Figure 26. Plot showing 

decadal average annual 

recharge in the middle 

San Pedro Watershed 

calculated from the 

Southwest Alluvial 

Basins-Regional Aquifer 

System Analysis (SWABRASA) regression 

equation and from the 

distributed-parameter 

water balance model. 

55

50

45

40

35

30

25

20

15

10

5

0

1940–1949 1950–1959 1960–1969 1970–1979 1980–1989 1990–1999 2000–2009

Average annual groundwater recharge, in cubic hectometers 

Mountain-front recharge calculated from the Southwest Alluvial 

Basins-Regional Aquifer System Analysis (SWAB-RASA)

regression equation

Groundwater recharge calculated from the modified

distributed-parameter water balance model

Groundwater recharge calculated from the distributed-parameter

water balance model (in place-recharge plus 15 percent of 

potential runoff) 

EXPLANATION 

in the middle San Pedro Watershed for the period 1940–2009. 

Annual recharge estimates from the model for the entire 

middle San Pedro Watershed averaged 19.57 hm3/yr for the 

time period 1940–2009. The estimated MFR on a decadal 

scale showed an increase during the period 1950–1999 and a 

decrease from 1980 through 2009 (fig. 26).

Comparison of the Recharge Estimates Between 

the Distributed-Parameter Water Balance Model 

and the Regression Model

Annual recharge estimates from the BCM and SWAB 

RASA regression model are compared for water years 

1940–2009. The BCM values used for the comparison include 

the sum of the in-place recharge and 15-percent of potential 

runoff. The average decadal potential recharge estimates from 

the BCM ranged from 3.49 hm3/yr, to 23.9 hm3/yr, whereas 

the average decadal values of the MFR estimated from the 

SWAB RASA regression model were higher, ranging from 

13.6 hm3/yr, to 26.4 hm3/yr,. The average annual BCMand SWAB RASA-estimated recharge was 12.2 hm3

/yr and 

19.6 hm3/yr, respectively. The BCM decadal values show 

two distinct decades of higher recharge, in 1960–1969 and 

in 1990–1999. The SWAB RASA decadal values show an 

increase in MFR starting in 1950–1959 and extending through 

1980–1989. A difference between the BCM and the SWAB 

RASA regression model is that the SWAB RASA model 

does not account for the effects of the geology and a basin 

topography, whereas the BCM does; this could be reason for 

the differences in the recharge estimates from the two models.
36 Hydrology of the Middle San Pedro Watershed, Southeastern Arizona

Groundwater Recharge by Seepage from the 

San Pedro River

Groundwater recharge by seepage is streamflow that 

infiltrates into the stream-channel sediments and then percolates 

to the water table. In stream reaches where the aquifer water table 

is higher than the stream-water surface, groundwater discharges to 

the stream and the stream gains water. In reaches where the water 

table is below the stream-water surface, the stream loses water and 

the aquifer is recharged by seepage. The amount and distribution 

of stream-aquifer exchanges are a function of magnitude and 

duration of streamflow, stream-channel morphology (width and 

depth), orientation of stream channel to down-valley subsurface 

groundwater flow, hydraulic properties of the streambed and 

underlying floodplain aquifer, transpiration from riparian 

vegetation, and depth to the water table.

Currently, the water table of the floodplain aquifer is below 

the channel of the San Pedro River throughout nearly the entire 

study area for most of the year, thus all stream-aquifer exchange 

is seepage from the river to the aquifer. Seepage of streamflow 

in the San Pedro River is from base flow to the river during the 

winter from Tombstone to near St. David, and from runoff to 

the river during the entire year, especially from thunderstorm 

runoff during the summer. The amount of groundwater recharge 

through seepage from the river channel varies considerably 

from year to year as a function of the amount of runoff in the 

main stem of the river and the amount and location of runoff in 

tributary streams. During times of large and sustained runoff, the 

river will flow continuously from Tombstone to Redington. At 

other times, the river will lose water and eventually stop flowing 

upstream of Redington. 

Groundwater seepage from the San Pedro River for 

the period 1914–2007 (table 1) was estimated from monthly 

streamflow records at five gages. The methods used to make 

these estimates are described in table 7. The differences 

in monthly flows between gages were assumed to be the 

infiltration of water (seepage) from the river channel to the 

underlying sediments. The Tombstone-to-Benson reach 

encompasses the Benson subarea and the Benson-to-Redington 

reach encompasses the Narrows-Redington subarea. Due to 

differences in the periods of records of each gage, two regression 

equations were developed to estimate monthly seepage in two 

reaches. Seepage was estimated for the concurrent periods of 

record (1968–2007) at the gaged sites (table 8) to form the 

basis for the regression equations to estimate seepage in the 

Tombstone-to-Benson and the Charleston-to-Redington reaches. 

Separate seepage estimates were calculated for total flow, low 

flow, and runoff for the concurrent periods of record.

Method for estimating monthly seepage for indicated reach

River reach

Symbol for 

streamflow 

infiltration in 

reach

Method Response variable for 

regression analysis Explanatory variables

Symbol for 

explanatory 

variables

Equation1

Charleston to 

Redington

SCR (1) Regression 

equation

Measured loss in 

runoff from Charleston 

to Redington

Measured runoff at 

Charleston

CQ SCR = a(CQ)b

Charleston to 

Tombstone

SCT (2) Change in 

median 

low flows

NA 2Median measured low 

flow at Charleston (BC) 

and Tombstone (BT)

BC, BT SCT = BT-BC

Tombstone to 

Benson

STB (3) Regression 

equation

Measured loss in 

runoff from Tombstone 

to Benson

Estimated streamflow 

infiltration from Tombstone 

to Redington

STR STB = a(STR)b

Tombstone to 

Redington

STR (4) Difference in 

estimated 

streamflow 

infiltration

NA NA NA STR = SCR-SCT

Benson to 

Redington

SBR (5) Difference 

in estimated 

streamflow 

infiltration

NA NA NA SBR = STR-STB

1

Regression equations for SCR and STB are explained in table 10.

2

Median monthly low flows for Charleston and Tombstone were calculated for the concurrent records of 1968–1986 and 1997–2007.

Table 7. Description of methods for estimating monthly seepage in reaches of the San Pedro River, middle San Pedro Watershed, Arizona.

[NA, not applicable]
Groundwater Budgets 37

Gaging-station pairs 

and river reaches1

Period of 

concurrent 

record

Gain (+) or loss (-) in monthly measured streamflow between gaging stations2

Total flow Low flow Runoff

Number 

of flows

Median 

change 

(m3/s)

Percent 

of losses

Number 

of flows

Median 

change 

(m3/s)

Percent 

of losses

Number 

of flows

Median 

change 

(m3/s)

Percent 

of losses

Charleston to Tombstone 1968–1986,

1997–2007

369 –0.03 57 244 –0.05 75 125 0.19 28

Charleston to Redington 1944–2007 739 –0.24 83 520 –0.24 93 219 –0.33 59

Tombstone to Redington 1968–1986,

1997–2007

369 –0.21 83 244 –0.17 89 125 –0.44 71

Tombstone to Benson 1967–1976,

2006–2007

141 –0.27 89 103 –0.26 98 38 –0.37 66

Benson to Redington 1967–1976,

2006–2007

153 0.00 88 112 0.00 90 41 –0.16 76

1

The gains or losses in measured streamflow for gaging-station pairs including Redington were determined by combining flows from the two gages near 

Redington (09472000, 1968-1997; 09472050, 1998-2007). 2

All months of the year were used to calculate statistics for gains and losses in flow between gaging stations.

Table 8. Gains and losses in monthly measured streamflow between gaging stations with concurrent periods of records on the 

San Pedro River, middle San Pedro Watershed, Arizona.

[m3/s, cubic meters per second]

Groundwater Recharge by Seepage in the 

Benson Subarea

Average annual seepage for 1914–2007 in the Benson 

subarea, calculated by using a regression equation, ranged 

from 5.36 hm3/yr to 29.29 hm3/yr with an average of 13.82 

hm3/yr (fig. 27). The regression equation is based on 36 pairs 

of calculated seepage between Tombstone and Benson as 

the response variable and the calculated seepage between 

Tombstone and Redington as the explanatory variable 

(table 9). The regression equation has an R2 of 0.86 and an 

average standard error of 32 percent (table 10). The longterm annual seepage in the Benson subarea appears to have 

decreased at a rapid and variable rate during the last 30 years. 

Average seepage during the wet months for the periods 1914–

1930 and 1997–2009 has decreased by 50 percent. Most of the 

seepage still occurs during the wet season, with progressively 

lesser amounts during the winter and dry seasons.

Groundwater Recharge by Seepage in the 

Narrows-Redington Subarea

Groundwater recharge by seepage in the NarrowsRedington subarea was estimated by subtracting the seepage 

from the Tombstone-Benson reach from the seepage from 

the Tombstone to Redington reach. The seepage from the 

Tombstone-Benson reach was estimated from a regression 

equation based on 121 pairs of calculated seepage from the 

Tombstone-Redington reach as the explanatory variable and 

calculated seepage from the Tombstone-Benson reach as 

the response variable. The seepage from the Tombstone-toRedington reach was estimated by subtracting the calculated 

seepage from the Charleston-Redington reach from the 

calculated seepage from the Charleston-Tombstone reach. 

The regression equation developed to calculate the monthly 

seepage between Charleston and Redington has an R2 of 

0.51 and an average standard error of 52 percent (table 10). 

A spline ratio was necessary to make a smooth transition 

between base-flow seepage and the lower regression 

estimates of seepage. All Charleston flows less than 0.40 m3/s 

were assumed to infiltrate into the aquifer. For streamflow at 

Charleston greater than 0.85 m3/s, infiltration was computed 

using the regression equation. For streamflows at Charleston 

between 0.40 m3/s and 0.85 m3/s, infiltration was estimated 

using a spline ratio. Groundwater seepage in the Charleston 

and Tombstone reach was estimated from the median change 

in low flows (table 11).

Average annual seepage for 1914–2007 in the NarrowsRedington subarea calculated from the difference between 

Tombstone-Benson seepage from the estimated TombstoneRedington seepage ranged from 0.00 m3

/yr to 10.65 m3 /yr 

(fig.27). The average annual seepage for 1914–2007 wa

2.84 m3/yr. In the Narrows-Redington subarea annual seepage 

appears to have decreased at a slow rate or almost leveled out 

during the last 30 years. Seepage has declined by about 65 

percent for wet, dry, and winter seasons. Most annual seepage 

still occurs during the wet season, with progressively lesser 

amounts during the winter and the dry seasons.
38 Hydrology of the Middle San Pedro Watershed, Southeastern Arizona

0

5

10

15

20

25

30

0

5

10

15

0

5

10

15

0

5

10

15

1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010

Annual seepage Tombstone-Benson reach

EXPLANATION

Annual seepage Benson-Redington reach

5-year moving average of annual seepage

from Tombstone-Benson reach 

5-year moving average of annual seepage 

from Benson-Redington reach 

A. Annual 

B. Winter

C. Spring

D. Wet

Streamflow seepage in, cubic hectometers

Figure 27. Plots of annual and seasonal streamflow seepage in the Benson and Narrows-Redington subareas 

of the middle San Pedro River Watershed, Arizona.
Groundwater Budgets 39

Month Year

Number of daily 

values used to 

calculate average 

discharge and 

seepage

Average daily discharge, 

in cubic meters per second

Average daily seepage, 

in cubic meters per second

Tombstone Benson Redington TombstoneRedington

TombstoneBensonBensonRedington

July 1966 4 8.11 6.70 4.81 3.31 1.42 1.89

July 1966 2 20.27 15.98 6.33 13.95 4.29 9.66

August 1966 4 1.36 0.65 0.45 0.91 0.71 0.20

July 1967 12 8.53 6.18 2.62 5.90 2.34 3.56

August 1967 5 2.46 1.74 0.33 2.13 0.72 1.41

August 1968 2 2.35 1.13 0.08 2.27 1.22 1.05

July 1969 4 9.44 4.76 0.45 8.98 4.68 4.30

August 1969 3 12.82 8.41 5.29 7.53 4.41 3.12

August 1970 3 2.37 1.08 0.45 1.92 1.29 0.62

August 1970 6 19.35 13.15 8.79 10.55 6.20 4.36

July 1971 6 20.99 10.20 5.57 15.41 10.78 4.63

August 1971 3 16.97 1.71 1.35 15.62 15.26 0.36

September 1971 3 3.23 0.59 0.02 3.21 2.64 0.57

September 1971 6 5.30 0.97 0.58 4.72 4.33 0.40

July 1972 4 8.73 3.23 1.67 7.06 5.50 1.56

August 1972 3 4.21 3.12 1.79 2.42 1.09 1.33

September 1972 2 4.93 2.36 1.78 3.14 2.56 0.58

September 1972 4 5.48 3.10 1.47 4.01 2.38 1.63

March 1973 2 0.99 0.00 0.00 0.99 0.99 0.00

July 1973 7 4.28 2.25 0.97 3.31 2.03 1.29

August 1973 4 1.17 0.32 0.08 1.08 0.85 0.23

July 1974 3 25.02 7.85 3.36 21.66 17.17 4.49

August 1974 6 25.65 18.60 2.58 23.07 7.05 16.02

July 1975 9 8.78 4.98 3.21 5.57 3.79 1.77

August 1975 3 2.78 1.23 0.59 2.18 1.55 0.64

September 1975 3 22.80 17.33 15.49 7.31 5.47 1.84

January 1976 4 1.30 0.00 0.00 1.30 1.30 0.00

July 1976 5 13.11 8.05 4.38 8.73 5.06 3.67

August 1976 4 5.23 1.07 0.23 5.00 4.16 0.84

September 1976 3 4.68 1.43 1.04 3.64 3.25 0.40

August 2006 3 10.13 6.61 5.03 5.10 3.52 1.58

September 2006 9 6.22 2.27 0.63 5.59 3.95 1.64

October 2006 3 1.26 0.52 0.04 1.22 0.75 0.48

July 2007 4 6.15 1.41 1.22 4.93 4.74 0.19

August 2007 8 11.22 8.34 4.73 6.49 2.89 3.60

September 2007 5 2.96 0.74 0.18 2.78 2.23 0.55

Table 9. Average daily discharge and seepage values used to develop equation for estimating seepage between Tombstone and Benson.
40 Hydrology of the Middle San Pedro Watershed, Southeastern Arizona

Month

Median change in low flow (seepage) 

between Charleston and Tombstone, in 

cubic meters per second1

1914–1986 1987–2007

January 0.028 0.000

February 0.000 –0.028

March 0.000 0.000

April 0.000 –0.028

May –0.028 –0.057

June –0.057 –0.057

July –0.028 –0.028

August –0.028 –0.028

September –0.028 –0.028

October –0.085 –0.085

November –0.057 –0.085

December –0.028 –0.057

Annual –0.026 –0.040

1

Positive value is gain and negative value is loss of seepage.

Table 11. Monthly changes in seepage along the 

San Pedro River between Charleston and Tombstone. 

Seepage reach

Number

of flow 

pairs1

Regression equation for estimating monthly seepage, in cubic meters per second

Equation Intercept

(a) 

Coefficient 

(b) R-squared

Standard error

Positive Negative Average

Charleston to Redington 121 SCR = a(CQ)b 0.579 0.560 0.51 65 –39 52

Tombstone to Benson 36 STB = a(STR)b 0.751 0.897 0.86 37 –27 32

1

Flow pairs are measured monthly flows at Charleston and Redington and at Tombstone and Benson. Seepage values for the regression analysis were estimated by subtracting flows at the two gaging stations in the reach.

Table 10. Regression equations for estimating monthly seepage along the San Pedro River from Charleston to Redington and from 

Tombstone to Benson, middle San Pedro Watershed, Arizona.

[SCR, monthly seepage from Charleston to Redington, in cubic meters per second; CQ, monthly discharge at Charleston, in cubic meters per second; STB, 

monthly seepage from Tombstone to Benson, in cubic meters per second; STR, monthly seepage from Tombstone to Redington, in cubic meters per second; 

(STR equals estimated seepage from Charleston to Redington minus seepage from Charleston to Tombstone; see table 2)]

Incidental Recharge from Irrigation

Another component of the groundwater budget is 

incidental recharge from irrigation. It is assumed for this study 

that water applied to a crop in excess of the crop’s consumptive 

use becomes incidental groundwater recharge, and applied water 

lost to evaporation is negligible. Irrigation efficiency, defined 

as the percentage of the crop consumptive water requirement 

divided by the total quantity of water withdrawn for irrigation 

(Tadayon, 2005), was used to calculate the maximum potential 

recharge from irrigation water use. The incidental recharge 

value presented in this section is calculated from the total water 

applied to the crop, from either groundwater or surface-water 

sources. This volume of water is not measured directly. 

The 2007–2009 incidental recharge values were estimated 

from data collected in field verification surveys each growing 

season as described in the upcoming section “Groundwater 

Withdrawals.” The groundwater withdrawals and surface-water 

diversions were summed by each subarea and then multiplied 

by an inefficiency coefficient (1 minus the irrigation efficiency). 

This value represents the maximum quantity of water that 

is available for recharge from irrigation. An inefficiency 

coefficient of 0.25 was used in calculations for the period 1985–

2009, and a value of 0.30 was used for years prior to 1985. 

As would be expected, the trends in incidental recharge 

from irrigation follow the trends in total irrigation withdrawals 

and diversions in each subarea. Volumes of recharge from 

incidental recharge from irrigation are generally greater in the 

Benson subarea. Incidental recharge from irrigation in this 

subarea was greatest in 1973, when total incidental recharge 

volumes were about 14.0 hm3. In 2009, the incidental recharge 

from irrigation in the Benson subarea and the NarrowsRedington subarea were 0.90 hm3

 and 1.36 hm3, respectively. 

Groundwater Underflow

The mountains flanking the alluvial basin are composed of 

nonaquifer rocks that generally restrict groundwater movement 

from adjacent basins to the east and west of the middle San Pedro 

Watershed. Groundwater enters the aquifer as underflow through 

the aquifer from the Sierra Vista Subwatershed and leaves as 

underflow to the north of Redington into the Lower San Pedro 

Basin. Groundwater also flows between the subareas through the 

Narrows; this mostly occurs within the saturated lower basin fill 

and stream alluvium that is continuous between the two basins. 

Groundwater underflow cannot be measured directly so alternative 

methods such as Darcy’s Law calculations, groundwater model 

simulations, or a water-balance approach must be used to estimate 

this component of the groundwater budget. 

The volume of groundwater underflow from the Sierra 

Vista Subwatershed was estimated on the basis of the findings 

of previous investigations. The most recent estimate of 
Groundwater Discharge 41

groundwater underflow from the Sierra Vista Subwatershed is 

from the groundwater flow model created by Pool and Dickinson 

(2007). Their simulated value of underflow was 1.48 hm3/yr for 

the period March 2002 to March 2003. ADWR (2005) calculated 

0.54 hm3/yr of underflow based on model results from Corell and 

others (1996). Anderson and Freethey (1995) also estimated 0.49 

hm3/yr based on a groundwater model. The value of 1.48 hm3/

yr was used in the Benson subarea groundwater budget to be 

consistent with the Pool and Dickinson (2007) groundwater model 

and was assumed to be constant for the entire period 2001–2009. 

The volume of groundwater underflow out of the Benson subarea 

to the Narrows-Redington subarea also was estimated on the 

basis of previous investigations. A geologic map of the area by 

Drewes (1974) suggests the gap in the bedrock is 200–300 feet 

wide. Freethey and Anderson (1986) estimate about 0.37 hm3/yr 

of groundwater underflow based on the simulation of groundwater 

flow in the Benson subarea. In the same study, they also simulated 

the groundwater flow into the entire Lower San Pedro Basin 

and estimated 2.10 hm3/yr of groundwater underflow. ADWR 

(2005) estimated 0.25 hm3/yr using a flow net analysis based on 

information from pumping tests near the Narrows. In this report, 

underflow between the two subareas for the groundwater budget 

is estimated to be 1.48 hm3/yr based on the budget described by 

Anderson and Freethey (1995).

Groundwater underflow out of the watershed was 

calculated for this investigation on the basis of balancing 

the groundwater budget for the Narrows-Redington subarea. 

Previous investigations have developed water budgets for the 

Lower San Pedro Basin, which extends from the Narrows 

to the confluence of the Gila and San Pedro Rivers. The 

groundwater underflow out of the Narrows-Redington subarea 

was calculated on the basis of the groundwater budget 

developed for 2001–2009. 

A preliminary groundwater budget for the NarrowsRedington subarea was developed without consideration of the 

groundwater withdrawals for irrigation and incidental recharge. 

This approach is based on the assumption that the inflows to the 

aquifer would approximate the outflows from the aquifer in the 

absence of groundwater withdrawals and incidental recharge. 

A single groundwater underflow term was then added to the 

groundwater budget to make the average groundwater budget 

residual equal to zero. Groundwater outflow from the NarrowsRedington subarea was calculated to be 4.54 hm3

/yr on the basis 

of balancing the groundwater budget. This value is based on 

the groundwater budget in the Narrows-Redington subarea for 

water years 2001–2009. 

Groundwater Discharge

Discharge out of the middle San Pedro Watershed aquifer 

system occurs through groundwater withdrawals from wells 

for municipal, domestic, and industrial, and irrigation uses 

and from riparian ET along the San Pedro River. Groundwater 

also discharges as underflow near the northern boundary of the 

watershed as described in the previous section. 

Groundwater Withdrawals

The demand for groundwater in the middle San Pedro 

Watershed includes withdrawals for municipal, domestic, 

industrial, and irrigation use in the Benson subarea and 

domestic and agricultural use in the Narrows-Redington 

subarea. Groundwater withdrawals in the middle San Pedro 

Watershed have been dominated by withdrawals for irrigation 

of fields along the San Pedro River. Groundwater withdrawals 

increased for all uses around 1940 with the widespread 

adoption of high-powered pumps (Pool and Dickinson, 2007). 

Groundwater Withdrawal Data 

Annual groundwater withdrawals were estimated from 

1940 to 2009 on the basis of historical data from previous 

studies. Municipal withdrawals for the City of Benson for 

1940, 1950, 1960, and 1970 were estimated on the basis of 

data from the U.S. Census. After 1980, municipal groundwater 

withdrawals were estimated from records of metered annual 

deliveries from 13 municipal water providers in the Benson 

subarea. Annual groundwater withdrawals for industrial use 

were obtained from annual metered reports of groundwater 

withdrawals beginning in 1966. Withdrawals for irrigation were 

based on irrigated acreages for 1936, 1941, 1953, 1966, 1969, 

and 1970, which were estimated from Roeske and Werrell 

(1973). The irrigated acreages for 1956, 1971, and 1978 were 

estimated from aerial photographs downloaded from the U.S. 

Geological Survey Seamless Data Warehouse (http://seamless.

usgs.gov/, accessed September 10, 2010). The irrigated acreage 

for 1965 was estimated from the Cropland Atlas of Arizona 

by Mayes (1974). The irrigated acreages between 1985–2005 

were estimated at 5-year intervals from data downloaded from 

the U.S. Geological Survey National Water-Use Program 

website (http://water.usgs.gov/watuse/, accessed September 

15, 2010). The 2006 irrigated acreage was estimated from 

Google Earth Imagery. Irrigated acreages for 2007 through 

2009 were estimated from field surveys in both the Benson 

and the Narrows-Redington subareas. The acreage irrigated 

from surface-water diversions from the San Pedro River were 

obtained from the ADWR (1994). The self-supplied domestic 

population from 1940 through 2000 was estimated from the 

U.S. Census data and from intercensal estimates by the Arizona 

Department of Administration Office of Employment and 

Population Statistics (2011) for 2001–2009.

Irrigation

Historically, surface water diverted from the San Pedro 

River was the primary source of water for irrigation in the 

middle San Pedro Watershed. Flow in the San Pedro River, 

however, did not always occur at convenient times or arrive in 

convenient quantities. For example, diversions and canals were 

often destroyed by high streamflow events and frequently this 

runoff water contained high sediment loads that filled canals and 
42 Hydrology of the Middle San Pedro Watershed, Southeastern Arizona

ditches (ADWR, 1994). Diversions from the San Pedro 

River for irrigation purposes worked best during low-flow 

or base-flow conditions. Thus, since the 1940s, groundwater has been the preferred source of irrigation water in the 

middle San Pedro Watershed.

Groundwater withdrawals and (or) surface-water 

diversions for irrigation are based on the estimated total 

irrigated acreage in the middle San Pedro Watershed. These 

estimates of irrigated acreage in the watershed were then 

multiplied by the consumptive water requirement of the 

particular crop. The consumptive water requirements for 

crops are based on, but not limited to, monthly precipitation and temperature, crop type, and crop harvest and plant 

dates. The consumptive water requirements for crops were 

calculated for 2007–2009 from field surveys of irrigated 

acreage in the middle San Pedro Watershed. The consumptive water requirement for the crop was calculated using 

the modified Blaney-Criddle method as described by the 

Bureau of Reclamation (1992, appendix A) and the irrigation efficiency was based on the type of irrigation system. 

Fields irrigated with a surface drip system were assigned 

an efficiency between 80 and 90 percent whereas flood 

irrigated fields were assigned an efficiency ranging from 

less than 50 percent to 75 percent (Saeid Tadayon, U.S. 

Geological Survey, oral commun., 2009). 

The consumptive crop water requirements for the 

irrigated fields in the watershed for years 2007, 2008, 

and 2009 were 1.30 m, 1.25 m, and 1.32 m, respectively. 

For years 1985–2006, detailed information about irrigation systems in the middle San Pedro area was limited, 

and the average consumptive water requirement for the 

years 2007–2009 was used. For all years prior to 1985, a 

consumptive water requirement of 1.37 m was used, which 

assumes all crops were flood irrigated either by pumping 

from wells or diverting water from the San Pedro River. 

 Groundwater withdrawals for irrigation exceeded 

all other withdrawals for 1940–2009. Trends in annual 

groundwater withdrawals for irrigation in the Benson and 

the Narrows-Redington subareas are similar. In 2009, the 

withdrawals in the Benson and Narrows-Redington subareas were 2.59 hm3

 and 3.70 hm3, respectively. Withdrawals for irrigation in the Benson subarea increased from 

13.6hm3 in 1936 to 25.3 hm3 in 1966 (fig. 28) and peaked 

at 27.8 hm3 in 1973. Groundwater withdrawals for irrigation in the Benson subarea have decreased from 11.8 hm3

in 1985 to 3.7 hm3 in 2009 (fig. 28). Groundwater withdrawals for irrigation in the Narrows-Redington subarea 

increased from 4.56 hm3 in 1936 to 13.9 hm3 in 1966 (fig. 

29) and peaked at 15.4 hm3 in 1973 and also decreased 

from 1985 to 2009. 

Municipal and Industrial

The City of Benson is the largest municipal provider 

of water in the study area. Groundwater withdrawals by 

the city for the years 1940, 1950, 1960, and 1970 were 

Figure 28. Plot showing annual groundwater withdrawals for irrigation, municipal, self-supplied domestic, and 

industrial uses in the Benson subarea for 1936–2009.

2.5 30

Industrial

25 2.0

1930 1940 1950 1960 1970 1980 1990 2000 2010

Municipal

Self-supplied

domestic 

20

1.5

Irrigation

15

1.0

5

10

0.5

0 0

Groundwater withdrawals for irrigation,

in cubic hectometers

Groundwater withdrawals for municipal, industrial,

and self-supplied domestic uses, in cubic hectometers 

0.25 20

1930 1940 1950 1960 1970 1980 1990 2000 2010

15

0.20

 Self-supplied domestic 

0.15

Irrigation 

10

0.10

5

0.05

0 0

Self-supplied domestic groundwater

withdrawals, in cubic hectometers

Groundwater withdrawals for irrigation,

in cubic hectometers

Figure 29. Plot showing annual groundwater 

withdrawals for irrigation and self‐supplied domestic 

uses in the Narrows‐Redington subarea for 1936–2009. 

estimated on the basis of U.S. Census data along with a per capita 

water use of 570 liters per person per day (150 gallons per capita 

per day, gpcd). This per capita water use value was estimated 

from the average per capita water use values from the City of 

Benson for years 2006–2008, which were obtained from the 

Community Water Systems 2008 Annual Water Use Reporting 

Summary (http://www.azwater.gov/azdwr/StatewidePlanning/

Drought/documents/LSP.pdf, accessed August 16, 2010). 

The estimated groundwater withdrawals for municipal use 

(estimated from the U.S. Census population) in the Benson subarea 

increased from about 0.20 hm3 in 1940 to 0.74 hm3 in 1970 (fig. 28). 

After 1980, the withdrawals were estimated from records of metered 

annual deliveries from 13 municipal water providers and increased 

from 0.74 hm3 in 1980 to about 2.1 hm3 for the period 1998–2009 
Groundwater Discharge 43

(fig. 28). Industrial groundwater withdrawals increased from about 

0.50 hm3 in 1966 to a peak of about 0.90 hm3 in 1980, decreased 

to about 0.40 hm3 in 1985, and then increased again to about 

0.70 hm3 in 1990 (fig. 28). In 2009, the industrial groundwater 

withdrawals were 0.40 hm3 (fig. 28). 

Self-Supplied Domestic Groundwater 

Withdrawals

Households in the middle San Pedro Watershed that do 

not receive water from municipal or private water suppliers 

are considered self-supplied domestic users. These households 

use water from domestic wells and are most likely outside of 

the service area of municipal water suppliers. Domestic selfsupplied groundwater withdrawals in the middle San Pedro 

Watershed can be divided into “inside” and “outside” uses of 

water. The inside portion of domestic self-supplied groundwater 

withdrawals includes water for household uses such as drinking, 

food preparation, washing of clothes and dishes, bathing, and 

flushing of toilets. The outside portion of the withdrawals is 

used for watering of lawns and (or) small pastures and (or) 

small gardens, or for evaporative cooling and the watering of 

small numbers of livestock. 

Estimates of self-supplied domestic groundwater withdrawals 

have been based on the product of the estimated per capita water 

use and the self-supplied domestic population (Blasch and others, 

2006; Tadayon, 2005). The self-supplied domestic population 

was estimated by calculating the difference between the entire 

population of the middle San Pedro Watershed and the population 

served by municipal water suppliers and then multiplying this 

figure by a per capita water use value. Previous estimates of the 

per capita water use in the Upper San Pedro Basin range from 

570 liters per day per person (150 gpcd) estimated by the ADWR 

(1994) to 1900 liters per day per person (500 gallons gpcd) by the 

ADWR (2005). Both of these per capita water use values account 

for inside and outside water use. 

The inside and outside components of self-supplied domestic 

groundwater withdrawals in the middle San Pedro Watershed were 

estimated separately using U.S. Census population data. The inside 

and outside component are both estimated from the self-supplied 

domestic populations. The inside component was estimated using 

the self-supplied domestic population in the middle San Pedro area 

while the outside component was estimated using the approximate 

number of households with domestic wells. The U.S. Census 

data for years 1940, 1950, 1960, 1970, 1980, 1990, and 2000 

were used to estimate the domestic self-supplied population and 

households with domestic wells. The intercensal estimates by the 

Arizona Department of Administration Office of Employment and 

Population Statistics (2011) were used to estimate the domestic 

self-supplied population for years 2001 through 2009. The number 

of households with domestic wells was calculated by dividing 

the self-supplied domestic population by the average number of 

persons per household in Cochise County as obtained from U.S. 

Census data. 

Detailed monthly metered withdrawals from domestic wells 

over the one-year period from 2009–2010 were used to separate 

the inside and outside per capita water use values (Barbara Clark, 

The Nature Conservancy, written commun., 2010). From these 

records, the per capita water use for the month of March was 

identified as the per capita water use representative of inside use. 

During this month, it was assumed that there was no watering of 

the grass or garden and no evaporative cooler use. The outside use 

was calculated by subtracting the annual cumulative volume of 

inside domestic water use from the total volume of the domestic 

groundwater withdrawals; the remaining volume of water is 

considered outside domestic use. The self-supplied domestic 

population in the Narrows-Redington subarea was estimated from 

the number of households identified from 2010 aerial photos. 

Domestic self-supplied groundwater withdrawals were 

generally larger in the Benson subarea than in the NarrowsRedington subarea. Domestic self-supplied groundwater 

withdrawals in the Benson subarea ranged from 0.98 hm3/yr 

to 2.09 hm3/yr (fig. 28). Domestic self-supplied groundwater 

withdrawals in the Benson subarea increased by the greatest 

amount between 1990 and 2000. The average annual self-supplied 

domestic groundwater withdrawals for 2001–2009 in the Benson 

subarea was 1.93 hm3/yr. In the Narrows-Redington subarea, 

domestic self-supplied groundwater withdrawals ranged from 

0.13 hm3/yr to 0.20 hm3/yr (fig. 29). The average annual selfsupplied domestic groundwater withdrawals for 2001–2009 in the 

Narrows-Redington subarea was 0.20 hm3/yr. 

Evapotranspiration of Groundwater in Riparian 

Areas

Groundwater is removed by evapotranspiration (ET) 

from the saturated and unsaturated alluvium in riparian areas 

along the San Pedro River. The ET from groundwater and 

surface water has two components: direct evaporation of 

water from wet soil or canopies following a rain event, and 

plant transpiration. In the arid and semi-arid southwestern 

United States, surface soils are normally dry. Hence, 

evaporation from the soil is a relatively minor component of 

total ET in arid and semiarid regions.

The rates of ET have been studied extensively in the 

Upper San Pedro Basin since the early 2000s. The first detailed 

estimates of ET by riparian vegetation in the Upper San 

Pedro Basin were made using remote sensing data and field 

measurements (Goodrich and others, 2000). These estimates 

of riparian groundwater withdrawals were later updated by 

Scott and others (2006) using an eddy-covariance technique for 

various riparian vegetation types. Nagler and others (2005a) 

combined in-place measurements with remote sensing to 

estimate riparian ET in the Upper San Pedro Basin for years 

2000–2004. Scott and others (2008) used ET rates from eddy 

covariance methods to develop an empirical relationship 

between ET and remote sensing data and land surface 

temperature to scale up ET rates in the Upper San Pedro Basin.

In this study, ET is calculated by vegetation index 

(VI) methods to obtain basin-wide temporal and spatially 

distributed ET rates. VI is useful for calculating ET from 

phreatophyte communities in arid and semiarid regions 
44 Hydrology of the Middle San Pedro Watershed, Southeastern Arizona

because ET is dominated by the plant transpiration 

component (Scott and others, 2008). The VI models used 

to estimate ET (reviewed in Glenn and others, 2007; Kalma 

and others, 2008) are based on the observation that foliage 

density on the ground, as measured by satellite, is often 

strongly correlated with ET (Glenn and others, 2010), but 

VI methods must be combined with meteorological data to 

calculate atmospheric water demand and the energy available 

to evaporate water. Furthermore, the algorithms used to 

calculate ET in VI models need to be regressed against 

ground measurements of ET in the biome of interest to 

develop empirical relationships between ET, meteorological 

data, and VIs (Glenn and others, 2010). Thus, the accuracy 

of the VI model for ET is constrained by the accuracy of the 

ground measurements of ET by which they are calibrated and 

by the degree of correlation between foliage density and ET. 

The spatial distribution of riparian vegetation along the 

San Pedro River within the middle San Pedro Watershed were 

identified from the Arizona Gap Analysis Project map (hereinafter referred to as AzGap; Halverson and others, 2001; Bennett and others, 2004). The AzGap map shows the distribution 

of vegetation types that were classified from satellite imagery 

and field verification. The vegetation groupings within areas 

defined by the AzGap map were divided into three different groupings on the basis of the mean Enhanced Vegetative 

Index (EVI) data of the single month of June for 2000 through 

2009. These vegetation groupings are sparse grass and shrubs, 

mesquite woodlands, and cottonwood-willow (table 12). Irrigated fields along the San Pedro River were excluded from the 

analysis because they are irrigated with pumped groundwater. 

ET Models Based on Vegetative Index

Several VI methods for estimating ET have been developed for phreatophyte communities. Groeneveld and others 

(2007) used the Normalized Difference Vegetation Index 

(NDVI) approach and single, summer Landsat images to 

estimate ET for a wide variety of phreatophyte communities 

in the western United States. The VI approach was modified 

for the present study by using the Enhanced Vegetative Index 

(EVI) products (Huete and others, 2002) from the Moderate Resolution Imaging Spectrometer (MODIS) sensors on 

the Terra satellite (obtained from http://daac.ornl.gov/MODIS/

modis.shtml), instead of the NDVI products from Landsat 

imagery. MODIS products commonly are used in phenological 

and change-detection studies, which require stable vegetation 

index values over time (Glenn and others, 2010). MODIS EVI 

products consistently give better predictions of ET than MODIS 

NDVI products (Glenn and others, 2010) which support their use 

in this study. EVI was scaled between bare soil and maximum 

vegetation for this study using values for EVImin and EVImax 

from a previous study (Nagler and others, 2005b). The scaled 

value of EVI is referred to as EVI*. Potential ET was calculated 

by the Blaney-Criddle (BC) method (Blaney and Criddle,1964). 

The ET model for the middle San Pedro Watershed was 

evaluated by comparison of ET rates for agricultural crops calculated by the ET model to ET rates calculated by crop coefficient 

methods, which were obtained from The Arizona Meteorological Network (AZMET) (http://ag.arizona.edu/azmet/, accessed 

December 13, 2010). Despite a reasonable fit of predicted ET 

to measured ET values for riparian vegetation (R2 = 0.91) using 

a linear relation between EVI* and ET, ET is overestimated 

for center-pivot-irrigated grain and corn crops in the nearby 

Wilcox Basin to the east. An exponential rise-to-a-max exponential function between EVI* and ET, however, resulted in a 

reasonable fit for mixed areas of agricultural fields and riparian 

vegetation (Nagler and others, 2005b). 

Rainfall averaged 244 mm/year over the period 2000–2009. 

The groundwater component of ET (ETgw) is estimated by subtracting in-place precipitation (from PRISM, 2010) from the total 

ET (Scott and others, 2008), assuming that the remaining amount 

of water is derived from groundwater and runoff from precipitation is negligible. Most of the San Pedro River in the study area is 

ephemeral, and some surface water becomes available to phreatophytes only during short-term flow events. Surface water, however, is a source of water to phreatophytes in intermittent reaches. 

Temperature and precipitation data are required for ET 

calculations. For the EVI* ET calibration equations, the mean 

daily air temperature (Tmean) was measured at the individual 

covariance flux tower sites used by Scott and others (2008). For 

projecting over the entire length of the river, three sources of 

Tmean were available: from spatially and temporally distributed 

maximum and minimum daily temperature values (Tmax and 

Tmin,respectively) from PRISM (http://www.prism.oregonstate.

Vegetation

groupings

Area, in

hectares

 ET ETgw

mm/yr hm3/yr mm/yr hm3/yr

Sparse grass and shrubs 836 333 2.78 142 1.19

Mesquite woodlands 1769 612 10.8 379 6.71

Cottonwood-willow 1494 856 12.8 609 9.1

Sparse grass and shrubs 588 401 2.36 199 1.17

Mesquite woodlands 963 670 6.46 435 4.20

Cottonwood-willow 981 952 9.34 700 6.86

Table 12. Average estimated annual rates of evapotranspiration (ET) and 

the groundwater component of evapotranspiration (ETgw) for 2001–2009.

[mm/yr, millimeters per year; hm3/yr, cubic hectometers per year]
Groundwater Discharge 45

Figure 30. Maps showing seasonal distribution of the groundwater component of evapotranspiration 

(ETgw) in the middle San Pedro Watershed for (A) wet, (B) dry, and (C) winter seasons (continued on next 

two pages).

Redington

St David

80

82

 

0 5 MILES

0 5 KILOMETERS

110o

15’ 110o30’

32o

31o45’

32o15’

10

San Pedro

Riparian National

Conservation Area

Tombstone

Benson

90

San Pedro River at 

Redington Bridge

near Redington

(09472050)

San Pedro River

near Redington

(09472000)

San Pedro River

near Tombstone

(09471500)

San Pedro River at 

Redington Bridge

near Redington

(09472050)

San Pedro River

near Redington

(09472000)

San Pedro River

near Tombstone

(09471500)

SANTA

CATALINA

MOUNTAINS

Cascabel

Pomerene

San Pedro River 

near Benson

(09471800)

San Pedro River 

near Benson

(09471800)

San

Pedro

River

A

Base from U.S. Geological Survey digital data, 1:100,000, 1982

Universal Transverse Mercator projection, Zone 12

EXPLANATION

Wet season average daily phreatophyte 

evapotranspiration rate for water years 

2000–2009 (estimated from EVI* ET model, 

in mm per day)

Irrigated area

<0.2

0.25–0.5

0.5–0.75

0.75–1.0

1.0–1.5

1.5–2.0

2.0–2.5

2.5–3.0

3.0–4.0

>4.0

ARIZONA

UNITED STATES

MEXICO

Middle San Pedro

Watershed

Kansas

Settlement

Tucson

WHETSTONE

MOUNTAINS

GALIURO MOUNTAINS

DRAGOON MOUNTAINS

GRAHAM COUNTY 

COCHISE COUNTY

PIMA COUNTY

COCHISE COUNTY

RINCON

MOUNTAINS
46 Hydrology of the Middle San Pedro Watershed, Southeastern Arizona

WHETSTONE

MOUNTAINS

GALIURO MOUNTAINS

St David

80

82

110o

15’ 110o30’

32o

31o45’

32o15’

10

DRAGOON MOUNTAINS

San pedro

Riparian national

Conservation area

GRAHAM COUNTY 

COCHISE COUNTY

PIMA COUNTY

COCHISE COUNTY

0 5 MILE

0 5 KILOMETER

Tombstone

Benson

90

Redington

San Pedro River at 

Redington Bridge

near Redington

(09472050)

San Pedro River

near Redington

(09472000)

San Pedro River 

near Benson

(09471800)

San Pedro River

near Tombstone

(09471500)

San Pedro River at 

Redington Bridge

near Redington

(09472050)

San Pedro River

near Redington

(09472000)

San Pedro River 

near Benson

(09471800)

San Pedro River

near Tombstone

(09471500)

RINCON

MOUNTAINS

SANTA

CATALINA

MOUNTAINS

Cascabel

Pomerene

Base from U.S. Geological Survey digital data, 1:100,000, 1982

Universal Transverse Mercator projection, Zone 12

EXPLANATION

Dry season average daily phreatophyte 

evapotranspiration rate for water years 

2000–2009 (estimated from EVI* ET model, 

in mm per day)

Irrigated area

<0.2

0.25–0.5

0.5–0.75

0.75–1.0

1.0–1.5

1.5–2.0

2.0–2.5

2.5–3.0

3.0–4.0

>4.0

ARIZONA

UNITED STATES

MEXICO

Middle San Pedro

Watershed

Kansas

Settlement

Tucson

San

Pedro

River

B

Figure 30. Maps showing seasonal distribution of the groundwater component of evapotranspiration 

(ETgw) in the middle San Pedro Watershed for(A) wet, (B) dry, and (C) winter seasons.—Continued
Groundwater Discharge 47

St David

80

82

 

0 5 MILES

0 5 KILOMETERS

110o

15’ 110o30’

32o

31o45’

32o15’

10

San Pedro

Riparian National

Conservation Area

Tombstone

Redington

Benson

90

San Pedro River at 

Redington Bridge

near Redington

(09472050)

San Pedro River

near Redington

(09472000)

San Pedro River

near Tombstone

(09471500)

San Pedro River at 

Redington Bridge

near Redington

(09472050)

San Pedro River

near Redington

(09472000)

San Pedro River 

near Benson

(09471800)

San Pedro River

near Tombstone

(09471500)

Cascabel

Pomerene

WHETSTONE

MOUNTAINS

GALIURO MOUNTAINS

DRAGOON MOUNTAINS

GRAHAM COUNTY 

COCHISE COUNTY

PIMA COUNTY

COCHISE COUNTY

RINCON

MOUNTAINS

C

Base from U.S. Geological Survey digital data, 1:100,000, 1982

Universal Transverse Mercator projection, Zone 12

EXPLANATION

Winter average daily phreatophyte 

evapotranspiration rate for water years 

2000–2009 (estimated from EVI* ET model, 

in mm per day)

Irrigated area

<0.2

0.25–0.5

0.5–0.75

0.75–1.0

1.0–1.5

1.5–2.0

2.0–2.5

2.5–3.0

3.0–4.0

>4.0

ARIZONA

UNITED STATES

MEXICO

Middle San Pedro

Watershed

Kansas

Settlement

Tucson

SANTA

CATALINA

MOUNTAINS

Figure 30. Maps showing seasonal distribution of the groundwater component of evapotranspiration 

(ETgw) in the middle San Pedro Watershed for (A) wet, (B) dry, and (C) winter seasons.—Continued
48 Hydrology of the Middle San Pedro Watershed, Southeastern Arizona

edu); from the AZMET Kansas Settlement station which 

has a climate similar to the middle San Pedro area; and from 

MODIS nighttime land surface temperature (LST) satellite 

imagery. While all three sources of Tmean data could be used 

interchangeably, Tmean was obtained from the PRISM website 

because of the availability of the data for the time period 

2000–2009 and full spatial distribution. Gridded precipitation 

data were also obtained from the PRISM Website. 

Application of the EVI* ET Model

Annual ETgw values for water years 2001–2009 were 

calculated for both subareas and included in the respective 

groundwater budgets (fig. 30). The EVI* ET model is 

applied for riparian areas along the San Pedro River to obtain 

temporally and spatially distributed ETgw values from 2000–

2009. ETgw is calculated for the total riparian area defined by 

the AzGap map that coincides with the riparian area and the 

floodplain along the San Pedro River. The application methods 

are based on those used by Nagler and others (2005a, b) for 

riparian areas along the Colorado River and by Tillman and 

others (2011) for riparian and agricultural areas in basins in 

southern Arizona. The MODIS data processing and EVI* 

calculation was done using Esri’s ArcGIS software.

Average annual ETgw and ET for 2001–2009 in the 

Benson subarea ranged from 14.9 hm3/yr to 18.8 hm3/yr and 

22.9 hm3/yr to 32.6 hm3/yr respectively (fig. 31). The annual 

ETgw varied about 13 percent of the annual average. This result 

suggests there is little annual variation in ETgw regardless of 

the annual precipitation. Annual ETgw has decreased from 18.7 

hm3 in 2006 to 14.9 hm3 in 2009, which is the lowest annual 

value for 2001–2009. Within a year, the rates of ETgw are the 

greatest during the wet periods (summer monsoon season) 

followed by the dry periods (spring and fall) and least during 

the winter. The average annual rates of ETgw by vegetation 

grouping in descending order are cottonwood-willow, mesquite 

woodlands, and sparse grass and shrubs (table 12). 

Average annual ETgw and ET for 2001–2009 in the 

Narrows-Redington subarea ranged from 10.2 hm3/yr to 

14.4 hm3/yr and 14.6 hm3/yr to 22.3 hm3/yr respectively 

(fig.32). The annual ETgw varied about 21 percent of the annua

average. Compared to the Benson subarea, there is more annual 

variation in ETgw in the Narrows-Redington subarea. Annual 

ETgw has decreased from 14.4 hm3 in 2006 to 9.7 hm3 in 2009, 

which is the lowest annual value for 2001–2009. The average 

annual rates of ETgw by vegetation grouping in descending 

order are cottonwood-willow, mesquite woodlands, and sparse 

grass and shrubs (table 12).

Sources of error in the fitted coefficients in the ET 

model are the measurements of ET at the eddy covariance 

flux towers. Eddy covariance flux tower data are subject to 

an energy closure error of 10–30 percent when ET results 

are used in a surface-energy-balance equation (Scott, 2010). 

ET values are usually increased in magnitude to force energy 

closure, and that procedure was used for the data used in this 

study (Scott, 2010). The cause of the energy closure and what 

Figure 31. Plot showing seasonal evapotranspiration (ET) 

and groundwater component of evapotranspiration (ETgw) in 

the Benson subarea for the years 2001–2009. 

Figure 32. Plot showing seasonal evapotranspiration (ET) 

and groundwater component of evapotranspiration (ETgw) in 

the Narrows-Redington subarea for years 2001–2009. 

0

5

10

15

20

25

30

35

2001 2002 2003 2004 2005 2006 2007 2008 2009

Evapotranspiration, in cubic hectometers

ETgw for wet months (June–Oct)

ETgw for dry months (Mar–May)

ETgw for winter months (Nov–Feb)

Total annual ET

EXPLANATION 

ETgw for wet months (June–Oct)

ETgw for dry months (Mar–May)

ETgw for winter months (Nov–Feb)

Total annual ET

EXPLANATION 

0

5

10

15

20

25

30

35

2001 2002 2003 2004 2005 2006 2007 2008 2009

Evapotranspiration, in cubic hectometers
Groundwater Discharge 49

The annual groundwater storage component for the Narrows-Redington subarea was negative for water years 2001 to 

2009. The change in groundwater storage ranged from –0.51hm3

in 2001 to –4.34 hm3 in 2002. The average annual change in 

groundwater storage was –2.00 hm3/yr. 

Benson Subarea Groundwater Budget

The average annual inflow to the Benson subarea ranged 

from 28.3 hm3 in 2001 to 15.6 hm3 in 2009 and the average for 

water years 2001–2009 was 20.0 hm3/yr (table 13). Groundwater 

recharge from mountain-front and mountain-block and streamflow seepage account for 35 percent and 48 percent, respectively, 

of the total recharge for water years 2001 to 2009. The longterm natural recharge and groundwater underflow terms were 

assumed to be constant in the groundwater budget. Variations 

in total annual recharge are from fluctuations of streamflow 

infiltration from the San Pedro River and changes in incidental 

recharge from irrigation. The annual incidental recharge ranged 

from 1.23 hm3 in 2007 to 2.34 hm3 in 2002 (table 13). The average annual incidental recharge for 2001 to 2009 was 1.74 hm3

/yr 

(table 13) and represents 9-percent of all recharge components.

The average annual outflow for water years 2001–2009 

from the Benson subarea aquifer system ranged from 24.5 hm3

in 2009 to 30.4 hm3 for 2002 (table 13) and the average annual 

discharge was 27.6 hm3/yr (table 13). Natural discharge from 

ETgw comprises 62 percent of the total discharge for water years 

to do about it is unclear (Scott, 2010); in the present case, ET 

values corrected for energy closure were 22 percent higher 

than uncorrected values, and represents a source of error or 

uncertainty in the present ET estimates.

Changes in Groundwater Storage

Under natural conditions, inflows may not equal outflows 

during short-term climate variations; however over longer periods 

under natural conditions, total inflow to and total outflow from an 

aquifer are approximately equal and net changes in groundwater 

storage are minimal. Following development of groundwater 

resources, inflows to the aquifer may not equal the outflows 

because of changes in land use and (or) changes in groundwater 

withdrawals. The source of groundwater withdrawn for pumping is increased recharge, decreased discharge such as reduction in base flow to the San Pedro River, reduction in ETgw, or 

removal from groundwater storage, or a combination of the three. 

A decrease in groundwater storage contributes to groundwaterlevel declines. That is, groundwater levels decline if the rates of 

discharge exceed the rates of recharge to the aquifer. 

The annual groundwater-storage component or water-budget 

balance for the Benson subarea was negative for water years 

2001 to 2009; outflows in the Benson subarea exceeded inflows. 

The groundwater storage component ranged in value from 

–1.77 hm3 in 2001 to –12.9 hm3 in 2002. The average annual 

change in groundwater storage was –7.70 hm3/yr. 

Water-budget component 2001 2002 2003 2004 2005 2006 2007 2008 2009 Average 

2001–2009

Average 

percent of total 

inflow and 

outflow

Inflow (recharge) hm3/yr

Mountain-front and mountain-block recharge 7.08 7.08 7.08 7.08 7.08 7.08 7.08 7.08 7.08 7.08 35

Incidental recharge from irrigation 2.22 2.34 1.97 1.85 1.73 1.60 1.23 1.36 1.36 1.74 9

Groundwater underflow 1.48 1.48 1.48 1.48 1.48 1.48 1.48 1.48 1.48 1.48 8

Streamflow seepage 17.5 6.5 5.4 6.2 10.3 13.3 11.8 10.3 5.6 9.66 48

Ouflow (discharge) hm3/yr

Evapotranspiration by phreatophytes 18.8 18.8 16.4 15.7 15.1 18.7 18.4 16.2 14.9 17.0 62

Withdrawals Irrigation 5.30 5.80 4.32 5.67 5.18 4.69 3.58 3.58 3.70 4.65 17

Self-supplied domestic 1.71 1.78 1.85 1.92 2.01 2.04 2.03 2.09 1.99 1.93 7

Public supply 2.22 2.10 2.22 2.10 2.22 2.10 2.22 1.97 2.10 2.14 8

Industrial 0.56 0.43 0.31 0.31 0.43 0.43 0.49 0.49 0.37 0.42 1

Groundwater underflow 1.48 1.48 1.48 1.48 1.48 1.48 1.48 1.48 1.48 1.48 5

Baseflow leaving subarea 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0

Totals

Total inflow (recharge) 28.3 17.4 15.9 16.6 20.6 23.5 21.6 20.2 15.6 20.0

Total outflow (discharge) 30.1 30.4 26.5 27.2 26.5 29.5 28.2 25.9 24.5 27.6

Change in groundwater storage (residual) -1.77 -12.9 -10.7 -10.5 -5.85 -5.98 -6.59 -5.68 -8.96 -7.7

Table 13. Groundwater budget for the Benson subarea for water years 2001–2009.

[hm3/yr, cubic hectometers per year]
50 Hydrology of the Middle San Pedro Watershed, Southeastern Arizona

Groundwater-budget component 2001 2002 2003 2004 2005 2006 2007 2008 2009 Average 

2001–2009

Average percent 

of total inflow 

and outflow

Inflow (recharge) hm3/yr

Mountain-front and mountain-block 

recharge

9.62 9.62 9.62 9.62 9.62 9.62 9.62 9.62 9.62 9.62 70

Incidental recharge from irrigation 0.93 0.80 0.99 0.93 0.86 0.80 0.86 0.80 0.86 0.87 6

Groundwater underflow 1.48 1.48 1.48 1.48 1.48 1.48 1.48 1.48 1.48 1.48 11

Streamflow seepage 4.47 0.06 0.16 0.80 2.56 4.22 1.86 2.06 0.00 1.8 13

Outflow (discharge) hm3/yr

Evapotranspiration by phreatophytes 13.3 13.1 11.7 10.2 12.3 14.4 13.1 12.3 9.7 12.2 78

Withdrawals Irrigation 2.84 2.34 3.08 2.84 2.59 2.34 2.59 2.47 2.59 2.63 17

Self-supplied domestic 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 1

Groundwater underflow 0.67 0.67 0.67 0.67 0.67 0.67 0.67 0.67 0.67 0.67 4

Baseflowleaving subarea 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Totals

Total inflow (recharge) 16.5 12.0 12.2 12.8 14.5 16.1 13.8 14.0 12.0 13.8

Total outflow (discharge) 17.0 16.3 15.6 13.9 15.8 17.6 16.6 15.6 13.2 15.7

Change in groundwater storage 

(residual)

-0.52 -4.34 -3.39 -1.04 -1.24 -1.49 -2.75 -1.66 -1.21 -2.0

Table 14. Groundwater budget for the Narrows-Redington subarea for water years 2001–2009. 

[Abbreviation: hm3/yr, cubic hectometers per year]

2001 to 2009. Groundwater withdrawals for irrigation comprise 

17 percent of the total discharge for water years 2001 to 2009. 

The average groundwater withdrawals for irrigation, self-supplied domestic, public supply, and industrial combined account 

for about 33 percent of groundwater withdrawals for water years 

2001–2009. Irrigation alone accounted for 17 percent of all 

groundwater withdrawals for water years 2001–2009. 

The change in groundwater storage in the Benson subarea 

reflects the relation between the inflows and the outflows to the 

aquifer system. For all years of the groundwater budget, the 

outflows exceed the inflows and the average change in groundwater storage was –7.70 hm3

/yr (table 13). The estimated change 

in groundwater storage showed the greatest decline from years 

2001 to 2002 and increased (became less negative) in 2005. 

Narrows-Redington Subarea Groundwater 

Budget

The average annual inflow to the Narrows-Redington 

subarea ranged from 16.5 hm3 in 2001 to 12.0 hm3 in water years 

2002 and 2009 (table 14) and the average annual natural inflow 

for 2001–2009 is 13.8 hm3/yr. Mountain-front and mountainblock recharge and streamflow seepage account for 70 percent 

and 13 percent, respectively, of the total annual recharge for 

water years 2001 to 2009. Variations in the natural recharge are 

from recharge from streamflow infiltration from the San Pedro 

River. Incidental recharge from irrigation comprises about 6 percent of the total recharge for water years 2001 to 2009.

The average annual outflow for water years 2001–2009 

from the Narrows-Redington subarea aquifer system ranged 

from 13.2 hm3 in 2009 to 17.6 hm3 in 2006 (table 14) and the 

average annual natural outflow for water years 2001–2009 was 

15.7 hm3/yr. The groundwater withdrawals for irrigation do not 

show much variation and range from 2.34 hm3/yr to 3.08 hm3/yr 

(table 14). The average annual groundwater withdrawals for 

irrigation for water years 2001 to 2009 was 2.63 hm3/yr and represents 17 percent of the total withdrawals. Discharge from ETgw

is 78 percent and groundwater underflow is 4 percent of the total 

outflow for water years 2001 to 2009. 

The change in groundwater storage in the Narrows-Redington subarea reflects the relation between the inflows and the 

outflows to the aquifer system. For all years of the groundwater 

budget the outflows exceed the inflows and the average change in 

groundwater storage was –2.00 hm3/yr (table 14). From 2001 to 

2002, the change in storage became more negative and from 2002 

to 2004 the change in storage became less negative. From 2004 to 

2007 the change in storage became more negative and the change 

in groundwater storage has been less negative since 2007. 

Hydrochemistry and Water Quality

The water chemistry of the middle San Pedro Watershed 

was investigated to better define groundwater flow paths and 

to identify sources of recharge to the aquifer systems (Hopkins 

and others, 2014). Groundwater and surface-water samples were 

analyzed for water temperature, pH, specific conductance, anions, 
Hydrochemistry and Water Quality 51

-70

-60

-50

-40

-16.0 -15.0 -14.0 -13.0 -12.0 -11.0 -10.0 -9.0 -8.0 -7.0 -110

-100

-90

-80

Summer 

Precipitation δ2H, in per mill (VSMOW)

δ180, in per mill (VSMOW)

Winter

Local meteoric water line 

Figure 33. Plot showing oxygen (δ18O) and hydrogen (δ2H) 

isotope composition in samples of precipitation in the middle San 

Pedro Watershed plotted in relation to the local meteoric water 

line (LMWL; Baillie, 2005). Isotopic compositions are calculated 

in per mil relative to Vienna Standard Mean Ocean Water 

(VSMOW).

1,800

2,000

2,200

2,400

2,600

2,800

Altitude, in meters above sea level

1,000

1,200

1,400

1,600

Altitude of δ18O sample

-16.0 -15.0 -14.0 -13.0 -12.0 -11.0 -10.0 -9.0 -8.0 -7.0 -6.0

δ180, in per mil (VSMOW)

Figure 34. Plot showing δ18O compositions for precipitation 

relative to altitude of sample collection. Isotopic compositions are 

calculated in per mil relative to Vienna Standard Mean Ocean 

Water (VSMOW).

cations, and a suite of stable isotopes including carbon, sulfur, 

oxygen, and hydrogen. Precipitation samples were collected 

from various elevations in the watershed and were analyzed for 

oxygen and hydrogen isotopes. Surface-water samples were 

collected multiple times to identify any seasonal trends in the 

chemical composition of the surface water. Samples were collected and analyzed following standard USGS protocols from 

the USGS National Field Manual for the Collection of Water 

Quality Data (U.S. Geological Survey, variously dated); samples 

for which exceptions to these protocols were made are noted in 

appendix A.

Isotopic Composition of Precipitation

Seven precipitation samples were collected during the 

years 2008 and 2009. These samples were collected during summer and winter precipitation events. Five-gallon precipitation 

buckets were left out for the duration of the storm season and 

water was sampled at the end of seasonal rainfall events. The 

top of the bucket was covered by a screen to prevent human or 

animal interference with the sample and a layer of mineral oil 

was added to the bottom of the bucket to ensure that collected 

rainwater would not evaporate. The elevation of bucket locations ranged from the San Pedro Basin floor (1,109m) to varied 

steps along the surrounding mountain ranges (1,665, 1,688, and 

2,583 m) to account for influences of elevation on the isotopic 

composition of precipitation.

Oxygen and hydrogen isotopes of precipitation were plotted on the Local Meteoric Water Line (LMWL), developed from 

Baillie (2005; fig. 33). These samples show variation in isotopic 

values due to both altitude and seasonal effects. In order to 

differentiate between seasonal and altitude effects, δ18O values 

were plotted against altitude (fig. 34) to help discern seasonal 

trends. These seasonal trends show that winter precipitation is 

the main contributor of the more depleted δ18O values of precipitation. The difference between winter and summer samples 

is smallest (0.3 per mil) at 1,109m and increases with elevation 

and has a seasonal difference of 5.6per mil at 2,583 m (fig. 34)

Surface Water Samples

Seventeen surface-water samples were collected from the 

San Pedro River from seven separate collection sites at different 

times of the year. Oxygen and hydrogen isotopic composition 

of river samples are within the range of summer precipitation 

values for upper San Pedro River samples while middle San 

Pedro River samples are similar or more depleted than the 

summer precipitation values (fig. 35). Winter precipitation 

isotopic values are at least 1 per mil depleted compared with 

the San Pedro River samples. Based on isotopic composition, 

summer precipitation seems to be a vital component of recharge 

to the San Pedro River (fig. 35). Surface-water samples had 

relatively low sulfate-to-chloride ratios (2.76 to 8.73), similar to 

river-sample values in the upper San Pedro Watershed (fig. 36; 

Baillie, 2005). Generally, river water was Ca-HCO3 (calciumbicarbonate) type water (fig. 37).

Six surface-water samples were collected along the San 

Pedro River from Charleston through Cascabel following a 

monsoon storm-flow event in August 2008. Oxygen and hydrogen isotope ratios became increasingly enriched with distance 

downstream (fig. 38). Progressive enrichment trends are parallel 

to the local meteoric water line, except for the R5 sample (fig. 

38), which plots farther to the right of the LMWL, indicating 

that evaporation has occurred. The overall trend indicates input 

from a more enriched source, which is often related to lowerelevation precipitation. All river samples collected during the 

storm were more enriched in oxygen and hydrogen isotope 
52 Hydrology of the Middle San Pedro Watershed, Southeastern Arizona

Figure 35. Plots showing 

δ18O and δ2H values 

from surface-water 

samples plotted on the 

local meteoric water line 

(LMWL; Baillie, 2005). 

Isotopic compositions are 

calculated in per mil (‰) 

relative to Vienna Standard 

Mean Ocean Water 

(VSMOW).

Figure 36. Plots showing 

fluoride and sulfate in 

groundwater and surface 

water samples in the middle 

San Pedro Watershed relative 

to the U.S. Environmental 

Protection Agency’s drinkingwater standards Primary 

Maximum Contaminant Level 

and Secondary Maximum 

Contaminant Level. Isotopic 

composition of sulfur are 

calculated in per mil relative 

to the Canyon Diablo Troilite 

(CDT). 0 10 20 30 40 50 60 70

Basin fill groundwater

San Pedro River within 

the upper San Pedro Basin

Bedrock groundwater

Basin fill/bedrock 

groundwater

San Pedro River within 

the middle San Pedro 

Watershed

Confined groundwater

Alluvium groundwater

EXPLANATION

SO4/Cl (mg/L/mg/L)

Fluoride (mg/L)

3

4

5

6

7

8

0

1

2

Primary maximum 

contaminant level = 4 mg/L

Secondary maximum 

contaminant level = 2 mg/L

-70

-60

-50

-40

-30

-100

-14.0 -13.0 -12.0 -11.0 -10.0 -9.0 -8.0 -7.0 -6.0 -5.0

-90

-80

δ180, in per mil (VSMOW)

δ2H, in per mil (VSMOW)

Local meteoric water line

Alluvium groundwater

Basin fill groundwater

San Pedro River within

the upper San Pedro Basin

Bedrock groundwater

Basin fill/bedrock groundwater

San Pedro River within the

middle San Pedro Watershed

Confined groundwater

Summer precipitation

Winter precipitation

EXPLANATION

composition than the precipitation samples for that season. The 

precipitation samples shown on figure 38 were a composite of 

all storms for the summer season of 2008, however, and do not 

provide a signature for particular isolated storms.

Groundwater and Spring Water Samples

Seventy-eight groundwater samples were collected from 

municipal supply wells, private domestic wells, and springs. 

Two out of six springs that were visited for sampling were 

flowing: Mackenzie Spring and 4R Spring. 

Spatial Variability of Groundwater Chemistry

Wells were classified into aquifer systems based upon their 

location and the hydrogeologic unit into which they were drilled, 

and water zones were identified based on their unique chemical 

signatures (fig. 39). Driller’s logs and screened depth intervals 

provided information about the geology of each well. Bedrock 

aquifer systems are identified by wells screened in bedrock—

limestone, sandstone, or granite; basin-fill systems have wells 

located in the unconfined basin fill, above the confining unit; 

confined basin fill systems have wells located in the lower basin 

fill, underneath confining unit; and alluvium aquifer systems 

have wells located in the shallow alluvial aquifer along the river. 

Dickinson and others (2010a) describe the aquifer units in detail. 

Major ion proportions (primarily differences in bicarbonate 

and calcium) in the groundwater varied widely across sampling 

sites (fig. 37). Fluoride concentration also varied spatially and 

was useful in discriminating between different water zones.

In addition to information gained through solute chemistry, solute ratios (calcium/strontium and sulfate/chloride) were 

utilized to better constrain groundwater geochemical reactions 
Hydrochemistry and Water Quality 53

Figure 37. Piper diagram showing relative ionic composition of surface 

water and groundwater in the middle San Pedro Watershed.

δ18O, in per mil (VSMOW)

δ2H, in per mil (VSMOW)

Precipitation

San Pedro River within the upper San Pedro Basin

San Pedro River within the middle San Pedro Watershed

Sampling location along the San Pedro River 

Local meteoric water line

-40

-30

-20

-10.0 -9.0 -8.0 -7.0 -6.0 -5.0

-70

-60

-50

P1

P3

R1 R2 R4 R5

R6

R7

R1

Figure 38. Plot showing δ18O and δ2H values from the San Pedro River 

collected in August 2008. Precipitation and surface-water sampling 

locations are shown in figure 39. Isotopic compositions are calculated 

relative to Vienna Standard Mean Ocean Water (VSMOW).

20

40

60

80

20

40

60

80

20

40

60

80

20

40

60

80

Calcium Chloride

Calcium + magnesium

Sulfate + chloride

Magnesium

Sulfate

Carbonate + bicarbonate

Sodium + potassium

100 80 60 40 20 0 0 20 40 60 80 100

0

100 100

0 0

100

Groundwater well 

sampled in August 

and December, 2009

Groundwater well 

sampled from 2007–

February, 2009

San Pedro River

EXPLANATION

Percent of milliequivalents per liter

0

20 20

80 80

along flowpaths and between aquifer systems. 

The calcium/strontium ratio offers information 

regarding residence time of groundwater and the 

amount of interaction between geologic sources 

and groundwater, whereas the sulfate/chloride ratio 

provides information pertaining to the presence 

and type of evaporites, while excluding the effects 

of evaporation. Smaller values of calcium/strontium ratios generally indicate longer groundwater 

residence time: as waters become supersaturated 

in calcite with increased water-rock interaction, 

the mineral calcite (CaCO3) may precipitate while 

strontium remains in solution. Sulfate/chloride 

ratios are highly variable in this system and are 

controlled by several processes such as reducing 

(decrease in oxidation state) groundwater environments, which would decrease the sulfate/chloride 

ratio, or a change in the input of sulfate to the system over time from anthropogenic activities such 

as mining (Baillie, 2005). 

Radiogenic isotopes, such as radioactive 14C (half-life 5,730 years) and tritium (3

H, 

half-life 12.32 years) are useful for determining relative groundwater residence times over 

short (<50years) to long (up to ~30,000 to 

50,000 years) timescales (Clark and Fritz, 1997). 

Tritium values were plotted versus δ18O to 

understand how recharge elevation and season is 

related to presence of modern water. Generally, 

samples with δ18O more depleted than -10 per 

mil (high elevation winter precipitation) were 

associated with tritium values less than 1 TU, 

and for more enriched samples tritium values 

ranged between <0.6 and 6.9 TU (fig. 40). Carbon-14 has also been utilized to estimate average 

recharge rates to regional aquifer systems to 

verify numerical flow models (Zhu, 2000).

The bedrock aquifer receives modern 

recharge, as indicated by a majority of samples 

containing detectable amounts of tritium; only 3 

of 14 samples analyzed for tritium are below the 

detection limit (fig. 40). Seven wells analyzed 

for 14C have moderate values with an average 

of 48.8percent modern carbon (pmc), and high 

calcium/strontium average value of 167 (fig. 41). 

Oxygen and hydrogen isotopes from the bedrock 

aquifer have the most negative average isotopic 

signatures -64 per mil δ2H and -9.1 per mil δ18O 

(fig. 35) and indicate that recharge to the bedrock aquifer is a mix of both summer and winter 

precipitation. Low sulfate/chloride average ratio 

(2.7) and low average fluoride concentrations 

(1.54 mg/L) indicate that waters in the fracture 

system have had very little time to interact with the 

surrounding rock matrix (fig.36). Sulfur isotope

(δ34S average 8.3 per mil, relative to the Canyon 
54 Hydrology of the Middle San Pedro Watershed, Southeastern Arizona

Redington

0

Base from U.S. Geological Survey digital data, 1:100,000, 1982

Universal Transverse Mercator projection, Zone 12

0 5 MILES

0 5 KILOMETERS

GRAHAM COUNTY

COCHISE COUNTY

PIMA COUNTY

COCHISE COUNTY

Tombstone

SANTA

CATALINA MOUNTAINS

Cascabel

WHETSTONE

MOUNTAINS

GALIURO MOUNTAINS

WINCHESTER

MOUNTAINS

DRAGOON MOUNTAINS

P1

P4

P3

BF8

BF7

B15

B11

B14

B3

B21

B19

B22

R5

R1

R2

R7

R4

CBF6

(BF, B)1

CBF12

CBF11

CBF10

CBF2

CBF21

CBF24

CBF23

BF18

32°15’

32°30’

110°30’ 110°15’ 110°00’

31°45’

32°00’

CBF31

BF11

BF16

B20

BF13

BF14

A8

A5

CBF8, P2

CBF7

A1

A4

A3A2

B16

BF9

B18

B2

BF6

B8

B17

BF12

B5

B13

B4

B10

B9

BF10

CBF19

BF15

BF4 BF2

BF3

CBF18

CBF17

CBF16

CBF5, CBF3, CBF4

CBF13

CBF9

CBF14

CBF15

CBF20 CBF22

CBF25

B12

(BF, B)2

A6

A7

CBF30

CBF29

CBF28

CBF27, B7 CBF26

CBF33

CBF34

BF17

CBF32

R3

R6

B6

BF5

BF1

B1

CBF1

A 

B

BF

CBF

R

P

BF2

BF4

 

R1

-alluvium

- bedrock

- basin fill

- confined basin fill

- river

- precipitation

- well sampled in 

 Aug and Dec, 2009

- site sampled from 

 2007–Feb, 2009

- San Pedro River 

 sampled on Aug 2008

Wells with numbered 

site designation

Figure 39. Map showing locations of groundwater and surface water sampling sites in the middle San Pedro 

Watershed and the designated hydrogeologic unit for the sampled well. 
Hydrochemistry and Water Quality 55

60

80

100

120

0

20

40

14C (percent modern carbon)

0 100 200 300 400 500 600

Ca/Sr (mg/L/mg/L)

 Groundwater types

Basin fill

Bedrock

Confined

3

4

5

6

7

Tritium units

0

-12 -11 -10 -9 -8 -7 -6

1

2

δ18O, in per mil (VSMOW)

Modern rain water

tritium >0.6 tu

Pre-bomb rainwater

 Groundwater types

Basin fill

Bedrock

Basin fill/bedrock

Confined

Alluvium

0.50

0.75

1.00

0

Ca/(Ca+S0

0.25

4) (mg/L/mg/L)

-15 -10 -5 0 5 10 15

δ34S (S04), in per mil (CDT)

Groundwater types

 Basin fill

 Bedrock

 Basin fill/bedrock

 Confined

 Alluvium

San Pedro River 

within the:

 Upper San Pedro 

 Basin

 Middle San Pedro 

 Watershed

Diablo Troilite, CDT) in this system suggest that sources of sulfate to the bedrock aquifer are rain water and gypsum dissolution 

(fig. 42). Precipitation measured in the Tucson Basin had a δ34S 

value of 4.9 per mil (Gu, 2005), while Permian gypsum from the 

Epitaph Formation in the Winchester and Dragoon Mountains 

had δ34S11.7 per mil (Burtell, 1989).

Water in the basin fill aquifer shows some evidence of 

modern recharge, as only 4 of 12 samples analyzed for tritium 

are below the detection limit (fig. 40). Five wells analyzed for 14C 

have moderate average values (48.5 pmc; fig. 41). The samples 

with 14C values lower than 35 pmc also have low calcium/strontium ratios (32.5 to 54.9, indicating long residence time and 

the potential for increased water-rock interaction (fig. 41). The 

sulfate/chloride ratios (average 6.12) were moderate to low for 

the basin-fill aquifer and only a few samples exceeded the EPA 

secondary drinking-water standard (SMCL) for fluoride of 2 

mg/L (average 1.2 mg/L; fig. 36). Average oxygen and hydrogen 

isotope values for the basin-fill aquifer (–6,152 per mil δ2H and 

-8.87 per mil δ18O) plot between the bedrock and confined basin 

fill values (fig. 35) and suggest that recharge to the basin fill aquifer is from multiple seasons and altitudes. Sulfur isotopes (-8.4 

to 13.3 per mil δ34S CDT) suggest that sources of sulfate in this 

system are a mixture of Permian gypsum and rainwater (fig. 42).

Analyses of water samples from the confined basin-fill 

aquifer do not indicate a significant amount of modern recharge, 

because 6 of the 13 samples analyzed for tritium are at or below 

the detection limit while only 3 of the 13 samples have tritium 

values greater than1 tritium unit, TU (fig. 40). Carbon-14 values 

in 16 of the 17 samples were less than 30 pmc and averaged 

16.1 pmc compared to only one sample (CBF 32) which was at 

95.9 pmc and also had the greatest tritium value (4.9 TU) from 

the confined basin-fill aquifer samples (figs. 40 and 41). Water 

sampled from the confined aquifer exhibit low Ca/Sr ratios 

(average 34.4), indicating long residence time and the potential 

for increased water-rock interaction (fig. 41). Further evidence 

for increased water-rock interaction is seen in high sulfate/chloride ratios (average 9.8) and higher amounts of fluoride (average 2.7 mg/L) (fig.

36). Average oxygen and hydrogen isotope

(–58 per mil δ2H and –8.4 per mil δ18O) suggest that recharge to 

the confined aquifer is from multiple seasons and altitudes (fig. 

35). Sulfur isotopes (–10.7 to 10.7 per mil δ34S CDT) suggest 

that sources of sulfate in this system are a mixture of Permian 

gypsum and rainwater (fig. 42).

The alluvial aquifer is primarily recharged by modern 

water, as suggested by relatively high tritium values (2.5 to 

3.3TU) in all but one sample (<0.5 TU; fig. 40). Averag

oxygen and hydrogen isotopic values (–54 per mil δ2H and 

–7.7 per mil δ18O) suggest that the alluvial aquifer is recharged 

primarily during the summer monsoon season by floodwater in 

the river (fig. 35). The alluvial aquifer samples generally have 

moderate average ion ratios: calcium/strontium (72.3), sulfate/

chloride (9.1), and all but one sample (5.97 mg/L) contained 

low levels of fluoride (0.49 to 2.1 mg/L). Sulfur isotopes (2.6 

to 9.3 per mil CDT) help to constrain the source of sulfate in 

the alluvial aquifer; river water contributes most of the sulfate 

to the near-stream alluvial system (fig. 42). 

Figure 40. Plot showing tritium values in groundwater samples 

from the middle San Pedro Watershed, plotted against δ18O. Isotopic 

ratios are presented as a per mil deviation from Vienna Standard 

Mean Ocean Water (VSMOW).

Figure 41. Plot showing calcium and strontium ratios plotted 

against 14C in groundwater samples from the middle San Pedro 

Watershed.

Figure 42. Plot showing sulfur isotope ratios (δ34S) in groundwater 

and surface water, plotted against calcium bicarbonate relative 

to calcium over calcium plus sulfate ratio in the middle San Pedro 

Watershed. Isotopic compositions are calculated as a per mil deviation 

from the Canyon Diablo Troilite (CDT).
56 Hydrology of the Middle San Pedro Watershed, Southeastern Arizona

Station number 314115110205601 314441110311501 314507110050101 314953110124201 315520110130501 315712110205401 320457110204101 320705110182001

Station name D-20-20 18CCC D-19-18 33AAA2 D-19-22 27ACC D-18-21 33BBB D-17-21 29DCA D-17-20 18BBB1 D-15-20 31CCB D-15-20 21BDA

Differences in: Percent Concentration Percent Concentration Percent Concentration Percent Concentration Percent Concentration Percent Concentration Percent Concentration Percent Concentration

Physical characteristics

Temperature 

(°C)

0.0 0.0 1.5 0.3 1.2 0.3 -4.9 -1.0 -2.6 -0.6 5.4 1.4 -0.4 -0.1 -0.4 -0.1

Dissolved 

oxygen (mg/L)

11 0.5 -19 -0.9 1.3 0.1 -50 -0.4 31 1.8 14 0.8 2.2 0.1 5.8 0.3

pH (standard 

units)

0.0 0.0 -1.4 -0.1 1.3 0.1 1.4 0.1 1.2 0.1 0.0 0.0 1.2 0.1 0.0 0.0

General mineral characteristics

Alkalinity 

(mg/L)

4.0 7 -0.5 -1 5.4 7 3.8 9 1.6 2 -2.7 -5 -- -- 2.7 4

Dissolved solids 

(mg/L)

0.5 2 -1.0 -3 0.5 1 -8.1 -31 0.0 0 3.0 7 -7.7 -26 12 30

Specific 

conductance 

(μs/cm)

-0.3 -2 -2.9 -14 -6.1 -19 -6.6 -39 -0.7 -2 -3.8 -14 -14 -70 1.4 5

Bicarbonate 

(mg/L)

4.2 9 -0.4 -1 5.7 9 3.9 11 2.6 4 -0.9 -2 -- -- 2.2 4

Hardness (mg/L) 0.0 0 0.0 0 -9.1 -10 -5.0 -10 -29 -11 0.0 0 -21 -5 2.1 2

Noncarbonate 

hardness (mg/L)

-9.1 -10 -- -- -- -- -- -- -- -- -- -- -- -- -- --

Mineral constituents, in mg/L

Calcium 0.7 0.6 -1.0 -0.6 -1.3 -0.5 -6.2 -3.7 -28 -3.9 1.3 0.5 -24 -2.2 5.3 1.3

Magnesium 0.5 0.1 2.8 0.4 -12 -0.6 -13 -1.6 -49 -0.4 -6.2 -0.7 -23 -0.1 -2.4 -0.2

Sodium -1.9 -0.2 2.7 0.5 17 3.4 -12 -6.1 7.7 3.4 -0.9 -0.2 -5.2 -4.9 -5.3 -2.0

Potassium -4.3 -0.1 -1.3 0.0 2.2 0.05 -6.5 -0.2 -10 -0.2 -12 -0.2 -15 -0.2 0.0 0.0

Chloride 0.5 0.0 -1.1 -0.2 -20 -2.0 -17 -1.4 -3.1 -0.1 -0.5 -0.02 5.9 0.2 2.2 0.2

Sulfate 4.0 5 -1.0 -0.1 11 1 -60 -41 -2.6 -0.1 -9.6 -0.4 -35 -36 25 7

Fluoride -33 -0.2 13 0.1 14 0.1 -9.6 -0.1 -23 -0.5 12 0.1 -15 -0.3 -12 -0.2

Silica (SiO2) -1.2 -0.3 2.4 0.6 3.5 1.1 -0.8 -0.2 1.7 0.5 6.7 2.3 2.6 0.5 1.9 0.5

Nutrient constituents, in mg/L

Nitrate plus 

Nitrite (N)

6.0 0.1 -4.0 -0.1 -23 -0.9 -- -- -- -- -13 -0.1 -- -- -- --

Orthophosphate -- -- -- -- 26 0.01 16 0.02 -- -- -- -- -- -- -- --

Orthophosphate (P) -- -- -- -- 29 0.00 17 0.01 -- -- -- -- -- -- -- --

Phosphorus -- -- -- -- -- -- 67 0.02 -- -- -- -- -- -- -- --

Trace constituents, in μg/L

Arsenic 14 0.5 29 0.4 -18 -1.7 5.4 0.4 -122 -0.6 3.2 0.1 12 4.0 9.1 0.4

Barium -29 -8 -1.4 -5 -17 -4 -15 -18 -25 -20 -69 -91 -27 -4 -10 -3

Bromide -43 -0.03 5.0 0.01 -19 -0.03 -22 -0.02 -25 -0.01 -20 -0.01 0.0 0.0 0.0 0.0

Chromium -552 -2.5 -230 -2.1 -- -- -- -- -25 -0.6 -- -- -48 -2.9 -20 -0.5

Iron -- -- -- -- -- -- -- -- -20 -1.0 -- -- -467 -28 -67 -4.0

Lead -178 -0.6 -- -- -- -- -- -- -- -- -770 -1.8 -- -- -- --

Manganese -- -- -- -- -- -- -- -- -275 -2.2 -- -- -400 -0.8 -300 -1.5

Molybdenum -25 -0.4 -- -- -5.3 -0.1 -25 -0.6 -25 -0.4 23 0.3 3.2 0.2 4.8 0.4

Nickel -239 -1.4 -30 -0.7 -- -- -488 -1.7 -- -- -- -- -- -- -- --

Uranium -15 -0.3 16 0.2 -2.0 -0.1 27 2.2 8.5 1.2 -4.6 -0.2 19 8.1 -2.4 -0.1

Zinc -2754 -132 9.4 2.8 -- -- -900 -18 -70 -12 -- -- -- -- -64 -14

Table 15. Differences in physical and mineral characteristics and in constituent concentrations in groundwater samples collected in the middle San 

Pedro Watershed in 1996 and 2009.

[°C, degrees Celsius; μg/L, micrograms per liter; mg/L, milligrams per liter; μS/cm, microsiemens per centimeter]
Study Limitations and Considerations for Future Data Collection and Analysis 57

Constituent U.S. Environmental Protection Agency 1

Primary MCL Secondary MCL

Aluminum -- 50-200

Antimony 6 --

Arsenic 10 --

Barium 2,000 --

Beryllium 4 --

Cadmium 5 --

Chloride (mg/L) -- 250

Chromium 100 --

Copper 1,300 1,000

Fluoride (mg/L) 4 2

Iron -- 300

Lead 15 --

Manganese -- 50

Nitrate (mg/L) 10 --

Nitrite (mg/L) 1 --

pH -- 6.5–8.5

Selenium 50 --

Silver -- 100

Sulfate (mg/L) -- 250

Thallium 2 --

Total Dissolved Solids (mg/L) -- 500

Uranium 30 --

Zinc -- 5,000

1

Drinking-water-quality standards (U.S. Environment Protection Agency, 2007)

Table 16. U.S. Environmental Protection Agency drinking-waterquality standards.

[Constituents are dissolved and are reported in micrograms per liter unless 

otherwise noted; MCL, Maximum Contaminant Level; mg/L, milligrams per 

liter; --, no standard]

Two samples from the confined aquifer contain low 14C 

values (13.1 and 20.2 pmc), but elevated tritium (1.8 and 

2.6TU; figs. 40 and 41). These two samples (CBF14 an

CBF16) are located near a gaining portion of the river by St. 

David, which may indicate an upgradient input of younger 

water. In addition, one well in the alluvial aquifer (A1) demonstrates tritium below the detection limit and high fluoride 

concentration and is located near a gaining portion of the river. 

These anomalous wells may indicate mixing between the confined aquifer and the alluvial aquifer.

Water Quality

Eight wells sampled in 1996 by Coes and others (1999) 

were resampled in 2009 using the same sampling protocols 

and laboratory analytical methods. The values of the majority 

of the analytes did not change substantially (table 15). Several 

constituents exceeded the drinking-water-quality standards 

determined by the U.S. Environmental Protection Agency (EPA; 

U.S. Environmental Protection Agency, 2007; table 16). Distribution of water quality exceedances are presented in figure 43. 

Fluoride concentrations in the confined basin fill aquifer were 

substantially greater than the other aquifers (fig.44). Arseni

and fluoride are contaminants of concern in the San Pedro 

Watershed and are common in concentrations above the EPA 

drinking-water-quality standards in southwestern alluvial basins 

(Robertson, 1991).

Quality-assurance samples (blank and replicate samples) 

were collected during sampling, and the results are presented in 

tables 17 and 18, respectively. The blank collected in 2009 contained only one constituent (lead) with a detectable value. The 

value was between the method detection level of 0.03 and the 

method reporting level of 0.06 and was given a value of E0.059 

(table 17); the E stands for estimated. The blanks collected 

in 2008 contained detectable amounts of chloride (2.4 and 

2.5 mg/L), nitrate (1.2 mg/L), potassium (0.09 mg/L), strontium 

(0.85 mg/L), and sulfate (2.4 and 2.9 mg/L) (table 17). The 

blank detections may indicate potential bias in environmental sample values, especially for environmental samples with 

concentrations less than 10 times the blank concentration. Some 

environmental sample concentrations of chloride, nitrate, and 

sulfate are within the 10 times blank concentration range and 

these values should be used with caution. 

Replicate sample results are presented in table 18. The 

replicate collected in 2009 at site BF1 was very similar (less 

than15percent different) compared to the environmental sample 

for all constituents analyzed except for zinc, which was 94 

percent different. Replicate samples collected in 2008 at sites 

B9, CBF15, and CBF22 (table 18) were very similar (less than 

15 percent different) compared to the environmental sample 

for most constituents analyzed except for fluoride at 21 percent 

(CBF22) and potassium at 34 percent different (B9). The absolute concentration difference between the environmental and 

replicate sample for these two samples was small for fluoride 

(0.12 mg/L) and potassium (0.8 mg/L). Eight samples were 

sent to both the U.S. Geological Survey National Water Quality Laboratory and the University of Arizona Department of 

Hydrology and Water Resources Laboratory. Values calculated 

by the two laboratories differed by more than 15 percent for 

several constituents: calcium, magnesium, sodium, potassium, 

chloride, sulfate, fluoride, silica, strontium and bromide (table 

19). Large-percentage differences for fluoride and bromide are 

due to low concentrations and therefore have small magnitudes 

of actual concentration differences (table 19).

Study Limitations and Considerations 

for Future Data Collection and Analysis

The hydrologic investigation described in this report 

represents the most comprehensive baseline set of available 

hydrologic and geochemical data in the middle San Pedro 

Watershed to date. The data collected describe the climate, 
58 Hydrology of the Middle San Pedro Watershed, Southeastern Arizona

Figure 43. Map of wells in the middle San Pedro Watershed at which 

U.S. Environmental Protection Agency (EPA) drinking-water standards 

were exceeded in samples collected from 2007 to 2009. EPA drinking-water 

standards are listed in table 16.

Figure 44. Box plots of fluoride 

concentration in groundwater for each aquifer 

type in the middle San Pedro Watershed. The 

dashed lines represent the U.S. Environmental 

Protection Agency’s drinking-water standards: 

Primary Maximum Contaminant Level (PMCL) 

and Secondary Maximum Contaminant Level 

(SMCL).

Alluvium Bedrock Basin fill Confined

basin fill

Aquifer type

EXPLANATION

0

8

Fluoride (mg/L)

75th percentile

Median

25th percentile

95th percentile

5th percentile 

Maximum value

Minimum value

4

2

1

3

5

6

7

(12)

(20)

(23)

(36)

(20) Number of observations

PMCL 4 mg/L

SMCL 2 mg/L

Redington

80

82

90

110o

15’ 110o30’

32o

31o45’

32o15’

10

GRAHAM CO

COCHISE CO

ESI HCOC O

O

C

C A IMP

Tombstone

St David

Cascabel

Pomerene

San

Pedro

EXPLANATION

Well water quality sample

Well sample dates

CBF - confined basin fill

R - river

P - precipitation

August and December, 2009

2007–February, 2009

B3 (F)

B19 (NO3)

(BF, B)1 (F)

(BF, B)2 (F)

CBF2 (F , pH)

CBF21 (F)

BF18 (SO4)

B8 (NO3)

B5 (F, NO3)

B4 (F)

B13 (NO3)

B9 (F)

BF10 (pH)

BF4 (pH)

BF3 (F, NO3, SO4)

BF2 (F, NO3, SO4)

CBF34 (SO4)

CBF9 (F)

CBF13 (F)

C18 (F, SO4)

CBF23 (As)

CBF32 (SO4)

CBF31 (F, NO3, SO4)

CBF30 (As, F, U)

CBF29 (F, SO4)

CBF15 (F), B6 (F)

CBF10 (F)

CBF14 (F), CBF17 (F)

A5 (F, NO3, SO4), CBF20 (F), CBF19 (F)

CBF8 (F, NO3)

A1 (F)

CBF6 (F)

CBF3 (pH), CBF4 (F), CBF5 (F)

A4 (NO3) CBF16 (F, SO4)

Base from U.S. Geological Survey digital data, 1:100,000, 1982

Universal Transverse Mercator projection, Zone 12

0 5 MILES

0 5 KILOMETERS

RINCON

MOUNTAINS

DRAGOON

MOUNTAINS

MOUNTAINS

WHETSTONE

LITTLE

DRAGOON

MOUNTAINS

SANTA

CATALINA

MOUNTAINS

GALIURO

MOUNTAINS

WINCHESTER

MOUNTAINS

BF2

BF4

A - alluvium

B - bedrock

BF - basin fill

As - Arsenic >10 mg/L

F - Fluoride >4 mg/L

F - Fluoride >2 mg/L 

NO3 - Nitrate >10 mg/L

SO4 - Sulfate >250 mg/L

U - Uranium >30 μg/L

pH - <6.5 or >8.5 
Study Limitations and Considerations for Future Data Collection and Analysis 59

Station ID 315712110205401 315547110025301 315434110064501

Station name (BF, B)1 B9 BF9

Sample date 08/07/2009 3/16/08 6/16/08

Sample time 9:40

Constituent Units

Aluminum, dissolved μg/L <4

Ammonia, dissolved mg/L <0.02

Antimony, dissolved μg/L <0.04

Arsenic, dissolved μg/L <0.06

Barium, dissolved μg/L <0.4

Beryllium, dissolved μg/L <0.02

Boron, dissolved μg/L <5 <5

Bromide, dissolved mg/L <0.02 <0.01 <0.01

Cadmium, dissolved μg/L <0.02

Calcium, dissolved mg/L <0.02 <0.1 <0.1

Chloride, dissolved mg/L <0.12 2.5 2.4

Cobalt, dissolved μg/L <0.02

Copper, dissolved μg/L <1

Chromium, dissolved μg/L <0.12

Dissolved solids mg/L <10

Fluoride, dissolved mg/L <0.08 <0.05 <0.05

Iron, dissolved μg/L <4

Lead, water, dissolved μg/L E0.059

Magnesium, dissolved mg/L <0.012 <0.1 <0.1

Manganese, dissolved μg/L <0.2

Molybdenum, dissolved μg/L <0.02

Nickel, dissolved μg/L <0.12

Nitrate as N, dissolved mg/L <0.04 <0.1 1.2

Nitrite, dissolved mg/L <0.002

Nitrogen, dissolved mg/L <0.1

Orthophosphate, dissolved mg/L <0.008

pH (lab) 7.9

Phosphorus, dissolved mg/L <0.006

Potassium, dissolved mg/L <0.06 <0.1 0.09

Dissolved Solids mg/L <10

Selenium, dissolved μg/L <0.06

Silica, dissolved mg/L <0.02

Silver, dissolved μg/L <0.008

Sodium, dissolved mg/L <0.12 <0.1 <0.1

Specific conductance (lab) μS/cm <5

Strontium, dissolved μg/L <5 0.85

Sulfate, dissolved mg/L <0.18 2.4 2.9

Uranium, dissolved μg/L <0.006

Zinc, dissolved μg/L <2

Table 17. Results of analyses of blank samples. Samples collected in 2008 were analyzed by 

University of Arizona Department of Hydrology and Water Resources Laboratory. The sample 

collected in 2009 was analyzed by the U.S. Geological Survey National Water Quality Laboratory.

[μg/L, micrograms per liter; mg/L, milligrams per liter; μS/cm, microsiemens per centimeter]
60 Hydrology of the Middle San Pedro Watershed, Southeastern Arizona

Station ID 314115110205601 315955110183101 315547110025301 315525110141001

Station name BF1 CBF22 B9 CBF15

Sample date 8/11/09 2/10/08 3/16/08 6/13/08

Sample time 14:30 14:35 10:00 10:05 14:30 14:35 15:00 15:05

Constituent Units

Alkalinity as CaCO3 mg/L 177 178 183 178 121 135 120 120

Aluminum, dissolved μg/L <4 <4

Ammonia, dissolved mg/L <0.02 <0.02

Antimony, dissolved μg/L 0.057 0.061

Arsenic, dissolved μg/L 3.5 3.7

Barium, dissolved μg/L 28.3 28.5

Beryllium, dissolved μg/L <0.02 <0.02

Boron, dissolved μg/L 40 34 65 72

Bromide, dissolved mg/L 0.07 0.08 0.07 0.06 0.16 0.16 0.11 0.12

Cadmium, dissolved μg/L <0.02 <0.02

Calcium, dissolved mg/L 82.6 82.8 38 38 21 22 8.3 8.3

Chloride, dissolved mg/L 6.4 6.4 5.6 5.6 18 16 5.8 5.8

Cobalt, dissolved μg/L 0.14 0.14

Copper, dissolved μg/L <1 E0.57

Chromium, dissolved μg/L 0.46 0.48

Dissolved solids mg/L 405 412

Fluoride, dissolved mg/L 0.45 0.49 0.59 0.47 7.7 7.4 5.6 5.6

Iron, dissolved μg/L <4 <4

Lead, water, dissolved μg/L 0.36 0.37

Magnesium, dissolved mg/L 20.1 20.2 6.8 7.0 0.51 0.53 0.78 0.76

Manganese, dissolved μg/L <0.2 <0.2

Molybdenum, dissolved μg/L 1.6 1.7

Nickel, dissolved μg/L 0.59 0.62

Nitrate as N, dissolved mg/L 1.2 1.2 1.9 2.0 1.1 1.0 3.0 3.1

Nitrite, dissolved mg/L <0.002 <0.002

Nitrogen, dissolved mg/L 1.1 1.2

Orthophosphate, dissolved mg/L 0.01 0.01

pH (lab) 7.5 7.5

Phosphorus, dissolved mg/L <0.006 <0.006

Potassium, dissolved mg/L 2.1 2.1 3.0 3.0 1.4 2.2 2.3 2.3

Selenium, dissolved μg/L 1.3 1.3

Silica, dissolved mg/L 26 26

Silver, dissolved μg/L <0.008 <0.008

Sodium, dissolved mg/L 11 11 43 44 104 105 62 62

Specific conductance (lab) μS/cm 606 605

Strontium, dissolved μg/L 913 942 454 459 939 964

Sulfate, dissolved mg/L 125 125 17 16 116 112 18 18

Uranium, dissolved μg/L 1.7 1.8

Zinc, dissolved μg/L 4.8 9.3

Table 18. Results of analyses of replicate samples. Samples analyzed at the University of Arizona Department of Hydrology and 

Water Resources Laboratory and the U.S. Geological Survey National Water Quality Laboratory.

[μg/L, micrograms per liter; mg/L, milligrams per liter; μS/cm, microsiemens per centimeter]
Study Limitations and Considerations for Future Data Collection and Analysis 61

surface-water conditions, groundwater conditions, 

hydrochemistry and water quality, and further the 

understanding of the movement of groundwater in the middle 

San Pedro Watershed. The development of the middle San 

Pedro Watershed groundwater budget quantifies the current 

relationship between inflows and outflows from the aquifer 

system. The results of this investigation also helped identify 

data gaps and limitations of previous models used for 

representing hydrologic processes in the watershed.

The distributed-parameter water-balance model 

underestimated natural recharge in the Benson subarea on 

the basis of previous investigations and in comparison to 

the other groundwater budget components. This result is 

probably a consequence of the model’s limited representation 

of streamflow infiltration in ephemeral channels as well as 

the model’s use of the monthly calculated potential ET value. 

The collection of additional streamflow and precipitation 

data from ephemeral channels in the Benson subarea could 

reduce uncertainty in the model results and improve the 

understanding of recharge processes in the subarea. 

In this study, ETgw is the largest groundwater outflow 

component from both subareas and the uncertainty in the 

ETgw estimates arise because ETgw is difficult to measure 

directly at the watershed scale. The ETgw estimates could 

be overestimated by as much as 22 percent due to an 

energy closure error. Even if compensated for the energy 

Percent (%) difference

Station 

name Station ID Sample date pH Calcium (mg/L)

Magnesium 

(mg/L)

Sodium 

(mg/L)

Potassium 

(mg/L)

Chloride 

(mg/L)

Sulfate 

(mg/L)

Fluoride 

(mg/L)

Silica 

(mg/L)

Strontium 

(μg/L)

Bromide 

(mg/L)

Alkalinity 

(mg/L)

A7 321727110230201 1/10/09 0% 24% 26% -37% -4% 40% -6% 18% -43% -7% 88% -2%

CBF24 320024110175401 6/18/08 2% 15% 9% 8% 0% 22% -7% 20% -47% -7% 194% 13%

A8 322658110292101 1/10/09 -1% 20% 12% -32% -4% 4% -3% 7% -46% -12% 49% -5%

A6 321542110205301 1/10/09 3% 9% 13% -17% -17% 4% 6% 4% -50% 0% 6% 6%

BF11 315504110030501 11/8/08 8% 8% 2% -39% -10% 10% -2% 3% -53% -18% 21% -3%

BF15 315745110133001 11/8/08 10% 3% 8% -10% 11% 48% 34% 4% -50% 3% 40% 6%

B10 315551110250101 5/14/08 2% 8% 24% 3% 1% -3% 0% 6% -47% 0% 50% 3%

A3 315454110135101 6/13/08 3% 4% 6% 8% 139% -9% 0% 16% -44% -9% 57% -3%

Concentration difference

Station 

name Station ID Sample date pH Calcium (mg/L)

Magnesium 

(mg/L)

Sodium 

(mg/L)

Potassium 

(mg/L)

Chloride 

(mg/L)

Sulfate 

(mg/L)

Fluoride 

(mg/L)

Silica 

(mg/L)

Strontium 

(μg/L)

Bromide 

(mg/L)

Alkalinity 

(mg/L)

A7 321727110230201 1/10/09 0.02 19 2.9 8.7 0.16 1.6 1.6 0.09 9.9 41 0.04 6.3

CBF24 320024110175401 6/18/08 0.17 4.4 0.50 4.3 0.00 0.99 2.5 0.09 15 70 0.08 23

A8 322658110292101 1/10/09 0.07 8.1 0.86 12 0.17 0.46 1.1 0.06 8.3 43 0.04 9.4

A6 321542110205301 1/10/09 0.21 6.3 2.3 17 1.1 0.91 10 0.06 11 4.4 0.01 16

BF11 315504110030501 11/8/08 0.65 7.1 0.14 5.3 0.09 0.72 0.59 0.04 16 39 0.02 7.3

BF15 315745110133001 11/8/08 0.77 0.76 0.36 2.5 0.19 1.7 1.7 0.07 14 24 0.02 7.1

B10 315551110250101 5/14/08 0.14 4.5 1.8 0.32 0.01 0.46 0.00 0.04 11 0.12 0.06 4.6

A3 315454110135101 6/13/08 0.22 4.7 1.4 7.2 1.0 1.2 0.20 0.24 22 168 0.08 14

Table 19. Differences in constituent concentrations between samples analyzed at the U.S. Geological Survey National Water Quality 

Laboratory and at the University of Arizona Department of Hydrology and Water Resources Laboratories.

[mg/L, milligrams per liter; μg/L, micrograms per liter]
62 Hydrology of the Middle San Pedro Watershed, Southeastern Arizona

closure error, the ETgw estimates would still be the largest 

groundwater outflow component from both subareas. The 

methodology used in this investigation utilized remote 

sensing data at 16-day intervals combined with direct 

measurements of ET from flux towers in the Upper San 

Pedro Basin. The establishment of flux towers in the middle 

San Pedro Watershed could be used to verify the calculated 

ETgw rates and improve the estimates of consumptive use 

of irrigated crops. The collection of precipitation data 

from riparian areas could also reduce the uncertainty in the 

estimated rates. Stable isotope collection could also be used 

to distinguish the source of water for ETgw and further reduce 

uncertainty.

Estimates of groundwater underflow into and out of 

the two subareas also have high uncertainties. Additional 

hydrogeologic data such as the saturated thickness, hydraulic 

conductivity, and storage values collected near the upstream 

and downstream ends of the watershed would help quantify 

the rates of groundwater underflow. These data could then be 

incorporated into a flow model for simulation. The underflow 

into the Benson subarea from the Sierra Vista Subwatershed 

has been estimated as part of other investigations and might 

have less uncertainty. The uncertainty of the underflow at the 

Narrows could also be reduced with additional hydrologic 

information from that area, including more sampling of the 

wells to the north and south of the Narrows to better define 

groundwater sources and map the spatial distribution of 

groundwater underflow. Additional well sampling at the other 

underflow locations could be helpful in constraining those 

values, although underflow would be best estimated with a 

groundwater flow model. The amount of water that leaves the 

watershed as underflow is highly uncertain, primarily because 

of the arbitrary downstream boundary of the watershed. 

Previous investigators have focused on the area from the 

Narrows to the confluence of the Gila River. 

The uncertainty in water-use estimates arises from the 

methods used to estimate the groundwater withdrawals. 

The 1991–2009 estimates of the groundwater withdrawals 

for municipal and industrial uses are derived from values 

supplied by the water providers and industrial operations and 

have a higher degree of accuracy. Irrigation data estimated 

for 2000–2009 is based on verification of the crops planted 

and irrigation systems. The irrigation data could be improved 

if surface-water diversions from the San Pedro River were 

regularly measured. Total irrigation withdrawals include both 

surface-water and groundwater withdrawals; having a value 

for surface-water withdrawals would improve the estimation 

of actual groundwater withdrawals. Water-use estimates for 

before 1980 were calculated based on U.S. Census data. In 

the Benson subarea a portion of the population resides near 

Benson on a seasonal basis, and this may not be reflected in 

the U.S. Census data. 

The self-supplied domestic per capita water use was 

estimated with a limited data set and more water use data 

is needed to reduce the uncertainty in that information. The 

self-supplied domestic population is not accurately known 

in both subareas. The self-supplied domestic groundwater 

withdrawals also were estimated based on the assumption that 

each home irrigated at least one acre of a combination of grass 

and garden. In total, additional withdrawal information and 

population refinements are needed to reduce the uncertainty 

in these groundwater budget components. The incidental 

recharge value also has some uncertainty because it is 

primarily based on the irrigation withdrawals. Additional 

investigations and information such as the infiltration rates in 

the fields are needed to adequately estimate this term.

More sampling of surface water, groundwater, 

and precipitation is needed to improve the conceptual 

understanding of groundwater flow paths between recharge 

and discharge areas. Focused sampling of groundwater 

from the confined aquifer, specifically in areas where results 

of this study indicate mixing with the floodplain aquifer, 

would be especially useful. Age dating of groundwater along 

an east-west transect from bedrock wells in the mountain 

block towards the San Pedro River would provide a better 

understanding of groundwater residence time. Sampling wells 

in the floodplain aquifer after a large flood event and analysis 

for major ions and stable isotopes in relation to groundwater 

level changes would provide valuable information about 

riverbank storage. The collection of additional samples at 

multiple times each year could also be used to determine any 

seasonal trends in the movement of water in the watershed. 

Summary and Conclusions

This report is the first comprehensive hydrologic analysis 

of the middle San Pedro Watershed. Examining existing 

data and collection of new data have greatly improved the 

understanding of the movement of water in the middle San 

Pedro Watershed. The hydrogeologic units and groundwater 

and surface water and the development of groundwater 

budgets for both subareas are described in this report. These 

groundwater budgets show the relative magnitudes of the 

inflow and outflow components and can serve as the baseline 

for comparison to any future hydrologic changes in the area. 

Important findings in this study include the identification of 

wetting and drying periods in the watershed, the decrease in 

annual streamflow in the San Pedro River, long-term changes 

in groundwater levels throughout the watershed and base 

flow at the streamgaging station at Charleston, and annual 

estimates of the groundwater component of (ETgw) in both 

subareas. 

The analysis of streamflow, water levels from wells, 

and long-term precipitation at Tombstone gives an indication 

of how the groundwater system in the middle San Pedro 

Watershed responds to long-term variations in climate. On 

the basis of precipitation and base flow at Charleston, drying 

periods occurred from 1950–1980 and from 1995 to the 

present and a distinct wetting period occurred in the mid1980s through the mid-1990s. Water levels from five wells 
Summary and Conclusions 63

near the mountain front and wells in the unconfined parts of 

the lower basin fill show periods of recharge also consistent 

with the wetting periods in the mid-1980s and mid-1990s. 

Water levels in three wells near the San Pedro River showed 

the lowest water levels in the 1950s to mid-1960s and two 

wells showed increases in water levels in the 1980s. 

The comparison of groundwater budgets developed 

for predevelopment and 2001–2009 time periods gives an 

indication of how the groundwater system has changed due 

to development in the middle San Pedro Watershed. The 

predevelopment outflow from the Benson subarea and the 

Narrows-Redington subareas of the watershed are 20.5 hm3/yr 

and 14.8 hm3/yr, respectively. The largest outflow components 

in the Benson subarea are ETgw followed by stream base 

flow. In the Narrows-Redington subarea, the largest outflow 

component is stream base flow followed by ETgw. 

For the years 2001–2009, in the absence of groundwater 

pumping for irrigation, public supply, industrial, and selfsupplied domestic uses, the groundwater outflows from the 

Benson and Narrows-Redington subareas are 18.5 hm3/y and 

12.9 hm3/yr, respectively. The largest outflow component for 

both subareas for 2001–2009 was ETgw. Base flow leaving 

both subareas as of 2009 is now zero as groundwater is 

discharged only in select reaches of the San Pedro River and 

only for part of the year. It must be noted when comparing the 

groundwater budgets that the period 2001–2009 is drier than 

the long-term average and this could be one reason why the 

predevelopment outflows are greater than the natural outflows 

for 2001–2009. Another possible reason for the decrease in 

the natural outflows is increasing ETgw in the middle San 

Pedro Watershed, which will reduce the overall groundwater 

available for natural outflow. 

For 2001–2009, the average annual groundwater 

withdrawal for water use (excluding ETgw) was 9.1 hm3/ yr and 

2.8 hm3/yr in the Benson and Narrows-Redington subareas, 

respectively. The outflows exceeded the inflows for both 

subareas for the period. The largest groundwater withdrawal 

in both subareas is for irrigation, although irrigation use 

has been steadily decreasing since 1973, when it peaked at 

43.17 hm3. In 2009, irrigation withdrawals had declined to 

6.29 hm3. The Benson subarea is the most populated part of 

the watershed and groundwater withdrawals could be one 

reason for the decline in water levels. 

The use of EVI data to quantify and calculate 

the temporal and spatial distribution of ET has greatly 

improved the conceptual understanding of how ET impacts 

the hydrology of the watershed. The ET is calculated on 

a 16-day interval and ETgw is the largest groundwater 

outflow component for 2001–2009. The average annual 

groundwater withdrawals for ETgw for 2001–2009 make up 

about 62 percent of the total groundwater withdrawals in the 

Benson subarea and 78 percent in the Narrows-Redington 

subarea. 

Mountain-front recharge (MFR) and mountain-block 

recharge (MBR), and groundwater recharge by seepage from 

the San Pedro River are the primary sources of recharge in 

both subareas. In the Benson subarea, the average annual 

recharge from seepage is greater than recharge from MFR 

and MBR, while in the Narrows-Redington subarea, average 

annual MFR and MBR is greater than seepage. Long-term 

streamflow records from the Charleston and Redington 

gaging stations show declines in annual flow. Mean annual 

streamflow has also decreased at the Tombstone gaging 

station. As expected, the estimated annual seepage has 

decreased in both subareas but seepage has decreased the 

most during the wet months. 

For most years in both subareas, seepage correlates 

with ETgw. In both subareas, seepage and ETgw have been 

decreasing since 2006 and the highest rates for both ETgw and 

seepage occurred in 2001 and 2006. In the Benson subarea, 

ETgw stayed almost constant at least one year after years of 

high streamflow seepage: ETgw stayed almost constant in 2001 

and 2002 and then again 2006 and 2007. In the NarrowsRedington subarea, ETgw remained almost constant for 2001 

and 2002 but not for 2006 and 2007 despite high seepage in 

2006. In 2001, there was seepage during wet, dry, and winter 

periods in the Narrows Redington and Benson subareas. In 

2006, there was only seepage during the wet months. These 

results suggest that during years with high streamflow, 

enough water is held in storage in the local streambed aquifer 

to sustain the same levels of ETgw for up to one year. 

 Hydrochemical analysis of water samples from wells 

in the confined basin fill show relatively low levels of tritium 

and 14C, suggesting little evidence of modern recharge. In 

contrast, hydrochemical analysis of water samples from wells 

in the unconfined basin fill show evidence of modern recharge 

on the basis of tritium and 14C. There are some indications 

that mixing of water between the confined and unconfined 

aquifer occurs in wells near Saint David. Water-quality 

parameters have not changed significantly since 1996–1997. 

Fluoride levels are greater in the confined wells than in the 

unconfined wells in the middle San Pedro Watershed.

The groundwater-flow system in the middle San Pedro 

Watershed continues to respond to variations in climate and 

increased groundwater withdrawals. Understanding of the 

complex mechanisms controlling the movement of water 

from recharge areas to discharge areas is simplified with the 

groundwater budgets. The groundwater budgets can only 

be used to infer the relative magnitude of the components 

for previous time periods, however, and cannot be used to 

predict the local effects on the groundwater and surfacewater systems from withdrawals at specific wells. The 

data collected as part of this study can be used to develop 

a groundwater flow model that could be used to further 

understand the complex groundwater flow system.

The monitoring network established in the middle San 

Pedro Watershed is a collaborative effort of the Arizona 

Department of Water Resources and the U.S. Geological 

Survey. Continued collection of EVI, streamflow, waterlevel, and water-use data will continue to improve the 

understanding of hydrology of the middle San Pedro 

Watershed. 
64 Hydrology of the Middle San Pedro Watershed, Southeastern Arizona

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Appendix
70 Hydrology of the Middle San Pedro Watershed, Southeastern Arizona Exceptions to standard protocols

Station

Sample 

date

Temperature, °C

Specific 

conductance,

μS/cm

pH

Calcium

Magnesium

Sodium

Potassium

Chloride

Nitrate

Sulfate

Fluoride

Bromide

Number Name Field Lab Field Lab mg/L 

5, 6 315712110205401 (BF, B)1A 9/15/2007 25.3 120 7.4 43.7 12.4 16.8 2.2 5.0 3.11 3.9 2.3 0.08 

5, 6 315655110205201 (BF, B)2 9/15/2007 25.5 400 7.3 45.1 11.1 17.3 2.3 7.3 3.67 4.1 2.4 0.09 

5, 6 315216110122801 A1 9/15/2007 24.8 330 7.6 57.6 6.4 25.2 1.9 4.6 1.61 44.2 6.0 0.08 

5 315444110135001 A2 6/13/2008 19.7 677 7.0 135 25.0 91.2 0.68 13.6 7.81 131 1.5 0.20 

5 315454110135101 A3 6/13/2008 19.4 764 1040 7.0 7.2 123 23.7 85.2 0.74 14.5 5.93 132 1.5 0.14 

5 315640110160901 A4 5/29/2008 20.6 923 7.2 108 19.5 114 3.2 19.1 30.6 206 2.0 0.41 

2, 3, 5 315810110162401 A5 6/18/2008 20.5 1756 7.1 93.6 47.8 387.9 1.6 39.5 31.7 637 2.1 0.61 

5, 6 321542110205301 A6 2/15/2009 19.4 866 894 7.8 8.0 72.2 17.9 101 6.8 21.8 1.13 172 1.4 0.19 

5, 6 321727110230201 A7 2/15/2009 22.3 548 567 7.6 7.6 77.8 11.1 23.8 4.1 3.9 4.25 25.1 0.49 0.05 

3, 5, 6 322658110292101 A8 2/15/2009 20.6 511 443 7.8 7.7 40.1 7.0 37.2 3.9 10.5 3.92 30.8 0.87 0.08 

1, 5 314514110032601 B2 1/13/2009 9.9 329 7.9 33.3 17.4 14.2 3.5 5.9 6.92 12.5 0.71 0.09 

2, 5, 6 314958110211501 B3 9/15/2007 22.4 519 7.2 63.9 13.2 17.1 1.3 11.2 3.55 24.4 2.0 0.11 

3, 5 315218110212401 B4 4/14/2008 22.4 476 7.3 35.5 8.7 70.7 1.5 23.9 2.43 17.0 3.8 0.32 

1 315309110214601 B5 4/14/2008 22.7 545 7.4 61.2 17.6 46.1 1.0 54.2 17.8 12.6 2.8 0.55 

5, 6 315528110222001 B7 11/3/2007 24.1 304 8.3 45.3 9.3 9.9 1.3 9.1 6.71 7.2 0.72 0.14 

1, 5 315544110252301 B8 4/14/2008 23.0 652 7.3 70.2 27.5 26.2 1.9 31.8 15.1 14.2 1.4 0.26 

5 315547110025301 B9 3/16/2008 20.6 628 8.2 21.4 0.51 104 1.4 17.7 1.11 116 7.7 0.16 

5 315551110250101 B10 5/14/2008 21.3 416 398 7.5 7.6 60.1 7.6 11.1 1.4 14.6 8.75 11.6 0.55 0.12 

3 315655110024301 B11 6/20/2008 20.0 591 7.4 110 20.8 12.0 2.8 11.2 7.71 108 0.76 0.23 

5, 6 315656110195301 B12 9/15/2007 26.4 180 7.3 36.1 15.2 19.7 1.9 5.0 2.52 9.7 1.0 0.08 

5 315719110265701 B13 5/30/2008 22.8 406 6.7 57.1 10.7 25.3 1.7 12.0 11.1 8.7 1.1 0.17 

5 315736110250301 B14 2/10/2008 23.8 729 7.8 22.4 18.2 82.9 3.0 6.7 0.08 26.6 1.5 0.09 

3, 4, 5 315738110020501 B15 6/20/2008 18.8 236 7.0 145 29.0 19.1 1.5 12.5 3.39 151 0.70 0.25 

315946110211701 B16 3/17/2008 22.1 211 7.6 40.0 11.9 22.6 1.4 10.0 6.56 7.9 1.0 0.17 

4, 5 320148110090701 B17 6/16/2008 22.7 376 7.4 75.8 14.0 28.8 2.1 10.2 6.31 29.2 0.86 0.17 

5, 6 320200110022101 B18 12/13/2008 26.0 436 7.7 51.4 19.3 10.9 2.0 9.3 6.25 22.0 1.0 0.13 

2, 5, 6 320209110282401 B19 11/3/2007 20.7 222 8.0 39.5 3.9 9.2 1.3 6.0 10.1 9.3 1.0 0.09 

320316110223301 B20 2/2/2008 7.8 38.7 6.6 14.4 1.9 10.2 7.38 5.6 0.56 0.14 

320327110255901 B21 1/15/2008 7.9 221 7.2 19.8 4.4 23.5 1.4 9.0 1.51 35.1 0.48 0.12 

1, 5, 6 320422110265601 B22 11/3/2007 20.5 190 7.8 13.9 3.4 29.9 1.3 10.0 2.46 5.7 1.1 0.11 

5 314245110204901 BF2 11/2/2008 23.9 1100 7.3 242 79.6 7.9 2.8 29.6 11.9 645 2.1 0.01 

5 314300110203001 BF3 11/2/2008 23.5 1080 7.5 256 81.7 7.9 3.0 29.8 11.7 711 2.1 0.01 

5 314318110132701 BF4 11/22/2008 21.0 323 8.7 1.8 0.12 64.6 0.46 6.4 1.75 22.8 0.58 0.07 

3, 5 315305110053101 BF6 4/26/2008 22.4 326 7.6 51.5 4.2 12.3 1.5 5.5 4.15 4.9 1.5 0.13 

4, 5 315315110060501 BF7 4/26/2008 24.0 321 7.9 55.6 4.5 11.9 1.2 8.9 3.95 5.7 1.1 0.14 

Table A1. Water-quality data for precipitation, surface water, and groundwater in the middle San Pedro watershed collected from 2007 to 

February 2009. 

[all major and trace elements are dissolved. °C, degrees Celsius; μS/cm, microsiemens per centimeter; mg/L, milligrams per liter; N, nitrogen; P, phosphorus; o/oo, per mil; TU, 

tritium units; pmc, percent modern carbon]
Appendix 71

Station 

name

Alkalinity

Hardness

Aluminum

Barium

Boron

Iron

Lithium

Silica as SiO2, mg/L

Strontium, μg/L

δ2H δ18O

14C, pmc

Tritium, TU

DIC, mg/L

δ13C DIC

δ34S SO4

as CaCO3, mg/L μg/L H2O (‰) Percent (%)

(BF, B)1A 168 17.0 348 -54 -7.7 <0.3 46.7 -6.2 11.0

(BF, B)2 171 16.7 232 -57 -8.2 <0.4 41.8 -6.6 8.7

A1 160 18.0 1016 -51 -7.4 <0.6 38.3 -10.1 5.5

A2 434 27.9 1746 -51 -7.0 2.7 108.6 -7.9 3.3

A3 424 410 E 0.8 175 E 5 54 50.1 1870 -52 -7.0 2.5 108.7 -10.3 2.6

A4 300 14.5 2570 -52 -7.2 2.6 72.7 -9.1 5.4

A5 501 18.1 3463 -50 -6.8 2.9 119.5 -9.1 7.0

A6 287 260 133 322 99 22.8 1210 -58 -8.7 3.3 63.0 -10.3 6.6

A7 273 240 48 < 4 25 22.7 570 -62 -9.2 68.9 -11.8 6.0

A8 168 130 52 160 48 17.9 354 -53 -7.9 40.2 -9.6 9.3

B2 151 19.4 389 38.7 38.5 -5.5 5.6

B3 194 11.7 138 -66 -9.4 2.4 44.5 -8.0 3.8

B4 219 460 -71 -10.0 0.9 49.9 -9.7 11.7

B5 202 313 -63 -8.8 58.7 0.5 46.8 -8.9 7.6

B7 140 10.6 142 -60 -8.7 38.7 <0.6 32.2 -10.0 9.0

B8 248 462 -63 -9.0 56.1 1.3 -9.3 6.9

B9 121 454 -75 -10.6 23.64 <0.5 28.4 -7.3 11.2

B10 171 180 < 1.6 63.5 33 E 4 12 22.9 156 -63 -9.0 2.7 40.3 -8.2 5.4

B11 240 16.3 450 -63 -8.8 55.4 -7.6 6.1

B12 188 16.1 535 -58 -8.7 <0.4 42.5 -6.1 10.5

B13 180 13.7 296 -59 -8.2 1.3 47.0 -6.9 10.5

B14 258 9.6 648 -70 -10.0 60.5 -5.9 13.9

B15 326 12.5 1263 -64 -9.1 78.0 -9.3 6.2

B16 170 365 -58 -8.3 29.93 1.1 37.6 11.4

B17 235 11.2 254 -62 -8.8 53.2 -11.2 9.0

B18 171 9.6 957 -62 -9.1 45.5 -4.9 11.3

B19 96 15.9 197 -74 -10.8 1.0 24.1 4.8

B20 116 14.8 387 -60 -8.6 0.8 32.8 -8.0 10.5

B21 52 25.2 213 -58 -8.4 95.9 5.1 15.2 3.0

B22 85 32.2 137 -67 -9.3 0.8 24.6 7.1

BF2 139 13.3 4539 -60 -8.8 16.8 33.0 -3.2 13.3

BF3 142 13.8 1157 -61 -9.0 31.8 -3.3 13.3

BF4 117 11.5 33 -82 -11.5 7.7 <0.7 28.9 -6.8 7.8

BF6 149 13.5 144 -60 -8.7 <0.3 33.7 -7.3 9.2

BF7 151 15.6 155 -62 -9.1 <0.4 35.0 -6.4

Table A1. Water-quality data for precipitation, surface water, and groundwater in the middle San Pedro watershed collected from 2007 to 

February 2009. 

[all major and trace elements are dissolved. °C, degrees Celsius; μS/cm, microsiemens per centimeter; mg/L, milligrams per liter; N, nitrogen; P, phosphorus; o/oo, per mil; TU, 

tritium units; pmc, percent modern carbon]
72 Hydrology of the Middle San Pedro Watershed, Southeastern Arizona Exceptions to standard protocols

Station

Sample 

date

Temperature, °C

Specific 

conductance,

μS/cm

pH

Calcium

Magnesium

Sodium

Potassium

Chloride

Nitrate

Sulfate

Fluoride

Bromide

Number Name Field Lab Field Lab mg/L 

5 315337110052901 BF8 5/14/2008 23.2 330 8.0 51.3 4.1 12.7 1.0 6.0 4.05 7.5 1.0 0.13 

1, 4 315434110064501 BF9 6/16/2008 30.5 263 7.8 49.7 5.8 11.0 1.3 5.2 3.58 5.1 1.1 0.13 

4, 5 315459110035201 BF10 6/10/2008 25.9 414 8.6 86.7 7.4 12.1 0.93 7.2 2.04 38.7 1.2 0.16 

1, 5, 6 315504110030501 BF11 11/15/2008 15.1 180 484 7.2 7.8 88.7 7.0 13.6 0.89 7.0 1.91 38.1 1.3 0.12 

5 315541110042701 BF12 4/26/2008 25.0 377 8.2 51.0 9.5 15.9 1.3 10.4 7.64 13.2 1.5 0.12 

5 315650110233801 BF13 6/10/2008 27.1 314 7.8 17.0 7.3 53.4 2.5 11.0 9.19 6.7 1.0 0.19 

5 315724110215601 BF14 6/9/2008 27.2 309 7.9 49.2 7.4 17.4 2.0 8.6 6.50 8.8 0.71 0.14 

5, 6 315745110133001 BF15 11/5/2008 22.2 103 267 7.3 8.1 26.5 4.7 25.4 1.8 3.6 2.75 5.1 1.7 0.04 

1, 5 320327110222901 BF16 2/2/2008 20.3 318 7.3 25.9 2.9 16.7 2.5 6.6 3.84 11.0 0.29 0.11 

5 320656110182801 BF17 1/10/2008 20.4 977 7.3 88.5 19.3 50.4 4.4 6.4 165 0.71 1.0 

1, 4, 5 321216110183601 BF18 6/19/2008 24.1 1553 7.2 89.1 49.6 237 2.6 118 6.59 269 2.0 0.96 

3, 4, 5 315005110122001 CBF2 6/17/2008 24.0 167 8.6 7.2 <0.1 35.0 1.2 6.2 8.03 2.9 7.4 0.12 

1, 5 315116110120601 CBF3 11/22/2008 19.1 191 8.9 2.5 <0.1 30.3 1.1 4.0 2.64 4.4 1.6 0.05 

3, 5 315116110121801 CBF4 11/22/2008 27.2 325 6.9 3.3 <0.1 67.8 1.1 6.0 2.49 31.4 3.3 0.05 

5 315116110123401 CBF5 11/22/2008 25.6 223 8.3 3.5 <0.1 46.9 1.2 4.5 2.60 5.4 5.8 0.06 

3, 5 315203110131001 CBF6 6/17/2008 26.8 300 8.0 1.2 <0.1 68.5 1.0 5.5 2.89 6.9 2.4 0.10 

5 315216110123001 CBF7 6/19/2008 26.9 198 8.1 17.5 0.39 27.3 1.8 3.8 2.77 4.0 1.8 0.10 

5 315235110124201 CBF8 6/10/2008 26.5 618 7.0 64.3 9.3 73.2 1.4 45.3 13.1 188 6.1 0.81 

5 315414110152301 CBF9 5/29/2008 23.9 388 7.6 41.7 12.4 36.4 2.5 4.7 2.29 10.8 2.8 0.11 

5, 6 315426110124501 CBF10 9/15/2007 22.7 200 7.7 29.5 2.0 20.1 2.1 4.6 1.76 4.0 3.3 0.08 

5 315429110124501 CBF11 6/19/2008 24.6 250 7.7 34.2 3.2 21.9 1.8 4.9 2.60 5.0 1.1 0.12 

5 315454110124701 CBF12 6/19/2008 24.2 243 7.7 32.7 5.2 19.5 1.7 4.8 2.91 4.9 1.1 0.12 

2, 3, 5 315456110151201 CBF13 1/11/2009 13.3 486 7.7 35.6 6.4 62.3 3.4 10.7 2.41 51.9 4.1 0.07 

5 315513110151101 CBF14 5/29/2008 23.2 677 7.9 48.5 9.1 115 3.9 29.8 2.50 168 4.4 0.16 

5 315525110141001 CBF15 6/13/2008 26.8 241 8.4 8.3 0.78 61.7 2.3 5.8 3.03 18.0 5.6 0.11 

5 315533110154901 CBF16 5/30/2008 27.0 1409 7.2 291 15.9 53.3 3.2 12.2 5.82 631 3.5 0.28 

2 315556110162301 CBF17 5/30/2008 27.6 422 7.4 38.5 10.6 41.6 2.4 4.8 2.43 2.4 3.8 0.09 

4, 5 315611110173101 CBF18 6/17/2008 30.5 451 8.0 48.9 12.4 47.5 2.4 4.8 2.40 70.8 1.1 0.12 

1, 4, 5 315757110144301 CBF19 6/18/2008 24.9 239 8.2 12.3 0.76 53.0 2.2 4.9 3.00 7.5 3.7 0.11 

5 315809110153801 CBF20 6/18/2008 32.4 429 7.9 24.2 4.1 69.6 3.3 5.4 2.72 21.2 3.6 0.11 

5 315942110162501 CBF21 6/18/2008 29.5 358 7.7 27.3 4.3 62.3 3.3 5.0 2.43 13.1 3.4 0.11 

4, 5 315955110183101 CBF22 2/10/2008 22.3 517 7.5 37.8 6.8 43.3 3.0 5.6 1.92 17.2 0.59 0.07 

5 320003110172501 CBF23A 6/18/2008 29.8 680 7.6 34.8 8.8 44.5 2.5 5.4 2.84 13.7 0.46 0.46 

5 320024110175401 CBF24 6/18/2008 31.9 417 415 7.6 7.8 29.1 5.8 53.5 2.5 4.5 3.14 37.8 0.47 0.04 

5 320309110194201 CBF25 1/9/2008 25.8 633 8.1 17.1 1.8 84.3 2.2 5.1 2.71 69.3 0.70 0.09 

4, 5 320412110210201 CBF26 6/9/2008 29.1 1367 6.9 126 3.0 260 2.0 12.4 5.65 735 2.6 0.30 

Table A1. Water-quality data for precipitation, surface water, and groundwater in the middle San Pedro watershed collected from 2007 to 

February 2009.—Continued
Appendix 73

Station 

name

Alkalinity

Hardness

Aluminum

Barium

Boron

Iron

Lithium

Silica as SiO2, mg/L

Strontium, μg/L

δ2H δ18O

14C, pmc

Tritium, TU

DIC, mg/L

δ13C DIC

δ34S SO4

as CaCO3, mg/L μg/L H2O (‰) Percent (%)

BF8 151 15.5 155 -62 -8.9 78.8 0.9 33.0 -7.5 7.9

BF9 155 14.3 168 -62 -9.2 4.4 -8.3 8.9

BF10 200 14.2 234 -60 -8.7 6.9 48.1 -9.1 3.8

BF11 206 250 12 5 4 29.6 221 -61 -8.8 105.4 3.7 50.9 -10.0 3.0

BF12 157 16.2 331 -60 -8.4 1.3 35.0 -3.8 9.5

BF13 148 20.7 405 -67 -9.2 34.6 -5.8 -8.4

BF14 158 15.3 163 -57 -8.1 35.2 -7.9 11.9

BF15 133 86 38 34 56 27.8 815 -56 -8.1 33.7 <0.6 29.3 -7.5 9.5

BF16 87 13.6 355 -55 -8.0 3.0 29.6 -12.0 4.9

BF17 195 9.3 1260 -52 -7.7 4.1 55.3 -8.2

BF18 401 20.5 1448 -61 -8.4 3.6 95.4 -9.0 9.3

CBF2 68 11.3 353 -54 -7.7 12.58 12.1 -8.9 9.0

CBF3 55 9.6 -49 -7.4 20.3 20.3 -8.6 9.6

CBF4 108 9.7 316 -55 -8.2 21.4 -7.0 9.1

CBF5 91 9.2 902 -49 -7.6 19.6 -9.5 8.8

CBF6 141 9.5 213 -57 -8.1 10.5 <0.7 24.6 -5.8 10.1

CBF7 102 13.3 421 -51 -7.2 18.62 22.4 -8.5 10.0

CBF8 49 7.1 1802 -54 -7.8 <0.6 8.4 -5.2 6.9

CBF9 199 15.6 1498 -55 -8.0 18.70 0.9 44.9 -7.2 10.6

CBF10 107 14.5 571 -56 -8.1 1.2 0.8 28.3 -7.7 7.8

CBF11 129 15.8 715 -56 -8.0 29.8 -8.3 8.9

CBF12 132 17.5 633 -56 -7.9 29.3 -8.2 9.4

CBF13 196 11.4 1375 -60 -8.9 <0.6 43.6 -6.9 7.5

CBF14 174 11.3 1710 -61 -8.9 13.1 1.8 39.0 -5.6 8.6

CBF15 120 9.4 939 -53 -7.8 21.68 27.8 -7.5 6.0

CBF16 147 16.1 5082 -55 -8.0 20.2 2.6 40.0 -6.1 8.1

CBF17 184 17.9 1082 -56 -8.3 47.5 -5.9 9.4

CBF18 181 17.4 1288 -57 -8.2 17.29 42.2 -5.5 -8.2

CBF19 123 12.2 435 -54 -7.8 22.9 27.8 -7.8 -10.7

CBF20 192 12.0 918 -58 -8.3 41.6 -2.9 9.2

CBF21 190 13.6 894 -57 -8.4 17.1 42.6 -5.7 9.5

CBF22 183 17.8 913 -59 -8.6 27.3 <0.7 42.7 -6.4 9.3

CBF23A 199 17.9 960 -57 -8.3 25.3 43.3 -6.2 8.1

CBF24 198 97 E 1.0 65 15 77 31.1 967 -58 -8.3 41.5 -6.7 9.0

CBF25 132 13.0 794 -64 -9.0 8.0 <0.5 32.0 -6.0 10.3

CBF26 113 9.3 2880 -75 -10.4 -7.4 10.0

Table A1. Water-quality data for precipitation, surface water, and groundwater in the middle San Pedro watershed collected from 2007 to 

February 2009.—Continued
74 Hydrology of the Middle San Pedro Watershed, Southeastern Arizona Exceptions to standard protocols

Station

Sample 

date

Temperature, °C

Specific 

conductance,

μS/cm

pH

Calcium

Magnesium

Sodium

Potassium

Chloride

Nitrate

Sulfate

Fluoride

Bromide

Number Name Field Lab Field Lab mg/L 

1, 2, 5 320420110211801 CBF27 2/2/2008 24.7 473 8.3 10.3 <0.1 48.6 2.2 4.8 2.21 31.1 0.72 0.06 

3, 5 320439110205501 CBF28 1/11/2009 27.8 382 8.1 9.7 0.28 82.9 2.0 5.3 3.03 42.8 0.73 0.07 

1, 5 320446110205201 CBF29 2/2/2008 11.6 870 8.5 40.8 1.3 176 1.6 12.1 5.84 360 3.4 0.31 

3, 5 320543110183201 CBF31 1/11/2009 18.4 988 7.4 95.8 24.5 102 2.7 40.3 15.0 273 2.8 0.01 

1, 3, 4, 

5

320642110181101 CBF32 6/19/2008 28.2 1120 7.2 120 24.8 125 2.5 27.2 7.27 299 1.3 0.54 

4, 5 320708110182401 CBF34 1/10/2008 19.7 1174 8.5 43.6 5.5 191 2.1 7.4 1.16 343 1.6 0.12 

09471000 R1A 9/15/2007 26.0 190 8.0 56.3 10.1 21.9 3.8 6.6 0.66 22.7 1.0 0.09 

09471000 R1B 10/22/2007 15.8 513 8.4 55.5 14.1 45.0 2.8 8.7 1.25 24.0 0.68 0.14 

09471000 R1C 8/1/2008 28.2 292 8.3 56.4 8.7 30.1 4.2 7.1 1.56 25.7 0.44 0.13 

314323110113701 R2A 8/1/2008 32.4 296 8.5 51.1 7.7 27.2 4.8 6.5 1.70 23.2 0.42 0.13 

314323110113701 R2B 11/2/2008 20.5 375 8.4 67.2 13.8 29.3 3.5 9.5 1.10 32.1 0.61 0.12 

314323110113701 R2C 1/12/2009 12.2 507 8.3 60.9 13.9 30.7 2.5 9.3 1.09 32.9 0.61 0.12 

314738110130601 R3 1/13/2009 12.9 453 8.3 59.6 13.1 31.1 2.7 9.2 1.09 35.1 0.59 0.11 

315232110131001 R4A 2/10/2008 6.9 72.0 12.9 54.6 4.4 8.7 66.2 1.3 0.12 

315232110131001 R4B 8/1/2008 34.3 212 8.4 52.6 7.2 26.2 5.7 6.2 1.58 32.0 0.48 0.12 

315232110131001 R4C 12/13/2008 8.1 70.6 15.2 37.4 2.9 9.0 1.08 68.2 1.4 0.11 

315232110131001 R4D 1/11/2009 16.7 546 7.5 69.4 13.8 36.4 3.5 9.5 1.12 60.1 1.2 0.11 

315754110164301 R5A 9/7/2007 59.3 9.4 35.7 5.0 9.5 2.56 44.8 0.43 0.14 

315754110164301 R5B 2/24/2008 8.2 66.1 15.3 79.2 3.7 11.8 7.71 103 1.8 0.19 

315754110164301 R5C 8/2/2008 33.1 240 8.5 53.8 7.7 26.4 5.9 6.3 3.16 37.5 0.54 0.09 

09471840 R6A 8/15/2008 8.2 40.7 6.6 23.0 3.7 6.0 3.94 35.8 0.55 0.11 

09471840 R6B 9/11/2008 30.1 5.9 44.6 4.1 

09471820 R7 8/15/2008 7.8 39.4 6.5 21.7 2.7 5.4 3.86 34.5 0.69 0.10 

322500110433201 P1A 4/22/2008 

322500110433201 P1B 12/1/2008 

315230110130701 P2A 5/15/2008 

315230110130701 P2B 7/15/2009 

315939110002101 P3A 5/15/2008 

315939110002101 P3B 10/11/2008 

314944110233201 P4 5/15/2008 

Exceptions to standard protocols

1. Sample collected after pressure tank

2. Sample collected after water piped away from well head

3. Sample collected in bucket

4. Sample collected before three well volumes were purged

5. Sample collected before stabilization of field parameters

6. Field parameter meters not calibrated

Table A1. Water-quality data for precipitation, surface water, and groundwater in the middle San Pedro watershed collected from 2007 to 

February 2009.—Continued
Appendix 75

Station 

name

Alkalinity

Hardness

Aluminum

Barium

Boron

Iron

Lithium

Silica as SiO2, mg/L

Strontium, μg/L

δ2H δ18O

14C, pmc

Tritium, TU

DIC, mg/L

δ13C DIC

δ34S SO4

as CaCO3, mg/L μg/L H2O (‰) Percent (%)

CBF27 86 9.2 351 -70 -10.2 0.9 -5.9 10.7

CBF28 160 9.4 240 -62 -9.1 <0.5 34.8 -5.8 10.6

CBF29 25 10.6 983 -85 -11.8 3.6 0.8 -7.3 10.1

CBF31 262 11.6 1362 -50 -7.4 63.0 -9.3 5.9

CBF32 260 14.1 1634 -54 -7.5 95.89 4.9 60.2 -9.2 5.1

CBF34 124 7.7 1437 -65 -9.4 31.4 9.8

R1A 186 10.5 431 -57 -8.1 45.8 -8.9

R1B 240 12.3 551 -58 -7.9 60.7 6.0

R1C 203 10.7 372 -58 -8.0

R2A 179 9.5 323 -56 -7.8

R2B 247 10.6 499 -55 -7.6 53.2 6.9

R2C 239 10.5 451 -55 -7.7 54.4

R3 243 10.0 511 53.9 -6.3

R4A 242 12.2 1190 -52 -7.1 55.6 -8.3 4.8

R4B 176 9.1 429 -55 -7.6

R4C 242 11.6 1242 -52 -7.3 57.1 -8.2

R4D 250 11.4 1039 -52 -7.3

R5A 191 8.1 882

R5B 239 11.2 1429 -41 -6.2

R5C 181 9.0 524 -54 -7.1

R6A 118 7.0 462 -42 -6.1

R6B 6.2 513 -31 -4.9

R7 116 6.6 454 -35 -5.2

P1A -100 -15.0

P1B -60 -9.4

P2A -52 -7.6

P2B -55 -7.9

P3A -75 -10.6

P3B -56 -8.2

P4 -75 -10.5

Table A1. Water-quality data for precipitation, surface water, and groundwater in the middle San Pedro watershed collected from 2007 to 

February 2009.—Continued
76 Hydrology of the Middle San Pedro Watershed, Southeastern Arizona

Table A2. Water-quality data for groundwater in middle San Pedro Watershed collected in August and December 2009.

Station number Station 

Name

Sample 

Date

Temperature, 

°C

Specific

conductance,

in μS/cm

pH Dissolved 

oxygen, 

mg/L

Barometric 

pressure, 

mmHg

Turbidity, 

NTU

Dissolved 

solids,

dried at 180 °C, 

Field Lab Field Lab mg/L

315712110205401 (BF, B)1B 8/7/2009 25.8 369 374 7.5 7.6 5.8 662 0.2 234

314441110311501 B1 12/16/2009 19.9 484 503 7.2 7.6 4.7 646 3.1 295

315520110130501 B6 8/5/2009 23.5 267 270 8.1 8.2 5.8 670 22 173

314115110205601 BF1 8/11/2009 25.9 603 606 7.2 7.5 4.7 656 0.2 405

314507110050101 BF5 8/5/2009 25.8 314 330 7.7 7.8 7.6 654 0.7 211

314953110124201 CBF1 8/4/2009 20.3 594 605 7.4 7.5 0.8 667 0.3 385

320003110172501 CBF23B 8/12/2009 29.9 384 389 7.5 7.7 4.9 676 0.3 245

320457110204101 CBF30 8/6/2009 27.6 488 505 8.4 8.5 4.6 676 0.5 339

320705110182001 CBF33 8/6/2009 27.5 369 378 7.7 7.9 5.2 677 3 247

Major ions

Station number Station 

Name

Sample 

Date

Calcium Magnesium Sodium Potassium Chloride Sulfate Fluoride Bicarbonate Alkalinity Hardness

in mg/L as CaCO3, mg/L

315712110205401 (BF, B)1B 8/7/2009 39.5 11.3 21.8 1.97 4.28 3.74 0.91 222 182 150

314441110311501 B1 12/16/2009 59.4 14.4 18.5 3.06 18.8 9.6 0.46 255 209 210

315520110130501 B6 8/5/2009 14.1 0.74 44.4 1.81 3.2 2.34 2.19 153 126 38

314115110205601 BF1 8/11/2009 82.6 20.1 10.8 2.11 6.43 125 0.45 216 177 290

314507110050101 BF5 8/5/2009 37.5 4.99 20.4 2.25 9.96 7.56 0.35 158 129 110

314953110124201 CBF1 8/4/2009 59.3 12.4 48.9 3.1 8.31 68.8 0.73 285 234 200

320003110172501 CBF23B 8/12/2009 29.4 7.32 41.4 2.37 4.39 16.8 0.34 222 182 100

320457110204101 CBF30 8/6/2009 8.85 0.39 95.1 1.04 3.4 104 2.09 149 122 24

320705110182001 CBF33 8/6/2009 24.3 8.89 38 2.4 7.26 29.3 1.7 180 148 97

Nutrients

Station number Station 

Name

Sample 

Date

Ammonia Nitrite Nitrate plus 

nitrite, mg/L 

as N

Total 

nitrogen Orthophosphate Phosphorus Orthophosphate

mg/L as N mg/L mg/L as P

315712110205401 (BF, B)1B 8/7/2009 < 0.020 < 0.002 0.78 0.74 0.039 < 0.006 0.013

314441110311501 B1 12/16/2009 < 0.020 < 0.002 2.5 2.46 0.051 0.008 0.017

315520110130501 B6 8/5/2009 E 0.012 < 0.002 0.54 0.53 0.038 < 0.006 0.012

314115110205601 BF1 8/11/2009 < 0.020 < 0.002 1.17 1.14 0.037 < 0.006 0.012

314507110050101 BF5 8/5/2009 < 0.020 < 0.002 3.91 3.85 0.042 E 0.003 0.014

314953110124201 CBF1 8/4/2009 E 0.010 < 0.002 0.18 0.2 0.109 0.03 0.036

320003110172501 CBF23B 8/12/2009 < 0.020 < 0.002 0.5 0.51 0.05 < 0.006 0.016

320457110204101 CBF30 8/6/2009 < 0.020 < 0.002 0.45 0.47 0.037 < 0.006 0.012

320705110182001 CBF33 8/6/2009 < 0.020 < 0.002 0.6 0.58 0.035 < 0.006 0.011

[All major and trace elements are dissolved. °C, degrees Celsius; mmHg, millimeters of mercury; μS/cm, microsiemens per centimeter; mg/L, milligrams per 

liter; NTU, nephlometric turbidity units; N, nitrogen; P, phosphorus]
Appendix 77

Menlo Park Publishing Service Center, California

Manuscript approved for publication April 12, 2013

Edited by Larry B. Slack and Claire M. Landowski

Design and layout by Jeanne S. DiLeo

Trace elements

Station number Station 

Name

Sample 

Date

Aluminum Antimony Arsenic Barium Beryllium Bromide Cadmium Chromium Cobalt Copper

μg/L

315712110205401 (BF, B)1B 8/7/2009 < 4.0 E 0.03 3.1 132 E 0.01 0.05 < 0.02 0.5 0.04 < 1.0

314441110311501 B1 12/16/2009 < 3.4 < 0.05 1.4 348 < 0.01 0.2 E 0.01 0.91 0.27 < 1.0

315520110130501 B6 8/5/2009 < 4.0 E 0.03 0.45 79 < 0.02 0.04 < 0.02 2.4 0.02 < 1.0

314115110205601 BF1 8/11/2009 < 4.0 0.06 3.5 28 < 0.02 0.07 < 0.02 0.46 0.14 < 1.0

314507110050101 BF5 8/5/2009 < 4.0 0.08 9.3 23 < 0.02 0.16 < 0.02 0.24 0.09 < 1.0

314953110124201 CBF1 8/4/2009 < 4.0 0.08 7.4 117 < 0.02 0.09 E 0.01 0.44 0.07 < 1.0

320003110172501 CBF23B 8/12/2009 6.8 0.04 23 350 E 0.01 0.04 < 0.02 0.94 0.05 < 1.0

320457110204101 CBF30 8/6/2009 E 3.4 0.11 34 15 < 0.02 0.04 E 0.02 6.1 E 0.01 < 1.0

320705110182001 CBF33 8/6/2009 E 3.3 E 0.04 4.4 30 < 0.02 0.06 0.02 2.5 0.03 < 1.0

Trace elements

Station number Station 

Name

Sample 

Date

Iron Lead Manganese Molybdenum Nickel Selenium Silver Uranium Zinc Silica 

as SiO2, 

mg/L μg/L

315712110205401 (BF, B)1B 8/7/2009 < 4 0.23 < 0.2 1.3 0.16 0.36 < 0.008 4.78 E 1.1 34.3

314441110311501 B1 12/16/2009 9 0.07 1.2 0.9 2.3 0.56 < 0.010 1.19 29.8 24.6

315520110130501 B6 8/5/2009 5 0.1 0.8 1.6 0.27 0.44 < 0.008 14.2 17.1 29.5

314115110205601 BF1 8/11/2009 < 4 0.36 < 0.2 1.6 0.59 1.3 < 0.008 1.74 4.8 25.7

314507110050101 BF5 8/5/2009 < 4 E 0.04 < 0.2 1.9 0.16 0.82 < 0.008 2.94 < 2.0 31.1

314953110124201 CBF1 8/4/2009 < 4 0.32 E 0.1 2.4 0.34 0.33 < 0.008 8.22 2 24.8

320003110172501 CBF23B 8/12/2009 < 4 0.62 E 0.1 3.9 0.21 0.61 < 0.008 4.01 20.3 32.9

320457110204101 CBF30 8/6/2009 6 < 0.06 0.2 6.2 E 0.07 0.49 < 0.008 42.1 8.8 19.5

320705110182001 CBF33 8/6/2009 6 0.4 0.5 8.4 E 0.11 0.54 < 0.008 5.86 21.9 26.5

Table A2. Water-quality data for groundwater in middle San Pedro Watershed collected in August and December 2009.—Continued 

 
Cordova and others—Hydrology of the Middle San Pedro Watershed, Southeastern Arizona—Scientific Investigations Report 2013–5040

http://dx.doi.org/10.3133/sir20135040

ISSN 2328-0328 (online)