Regional groundwater flow model of the tucson active management area Tucson, Arizona : simulation and application, modeling report no. 13

              ARIZONA DEPARTMENT OF WATER RESOURCES
REGIONAL GROUNDWATER FLOW MODEL
M O D E L I N G R E P O RT N O . 1 3
OF THE TUCSON ACTIVE MANAGEMENT AREA
TUCSON, ARIZONA: SIMULATION AND APPLICATION
BY
DALE A. MASON AND LICINIU BOTA
HYDROLOGY DIVISION
PHOENIX, ARIZONA
2006
Contents
Table of Contents
ABSTRACT ...................................................................................................................................................... VI
ACKNOWLEDGEMENTS............................................................................................................................VII
CHAPTER 1 INTRODUCTION.......................................................................................................................1
Groundwater Management Act ......................................................................................................................1
Tucson Active Management Area ..................................................................................................................1
Purpose and Objectives .................................................................................................................................1
General Description of Model Area ...............................................................................................................4
Upper Santa Cruz Sub-basin.......................................................................................................................4
Avra Valley Sub-basin...............................................................................................................................4
Previous Investigations..................................................................................................................................5
Sources of Data..............................................................................................................................................5
CHAPTER 2 REGIONAL HYDROGEOLOGIC SETTING .........................................................................7
General Overview of Regional Hydrogeology ...............................................................................................7
Structural Geology and Tectonic History .......................................................................................................7
Hydrogeology..............................................................................................................................................10
Upper basin-fill........................................................................................................................................10
Lower Basin-fill.......................................................................................................................................13
CHAPTER 3 REGIONAL GROUNDWATER FLOW SYSTEM................................................................14
Conceptual Model of the Regional Aquifer System.....................................................................................14
Aquifer System............................................................................................................................................15
Fort Lowell Formation.............................................................................................................................16
Upper Tinaja beds....................................................................................................................................17
Middle and lower Tinaja beds ..................................................................................................................17
Pantano Formation...................................................................................................................................17
Predevelopment Groundwater System .........................................................................................................17
Inflows.....................................................................................................................................................18
Outflows ..................................................................................................................................................23
Groundwater in Storage...........................................................................................................................24
Groundwater Development Period: 1941 – 1999 .........................................................................................25
Inflows.....................................................................................................................................................27
Outflows ..................................................................................................................................................31
Transient Water Level Conditions............................................................................................................34
Change in Storage....................................................................................................................................34
CHAPTER 4 NUMERICAL MODEL.............................................................................................................35
Modeling Approach.....................................................................................................................................35
Model Code .................................................................................................................................................35
Model Development ....................................................................................................................................35
Model Grid and Cell Definitions ..............................................................................................................35
Model Layer Definitions ..........................................................................................................................37
MODFLOW Packages.............................................................................................................................39
Boundary Conditions...................................................................................................................................40
Model Data Development............................................................................................................................40
Water Levels............................................................................................................................................40
Aquifer Parameters..................................................................................................................................41
Pumpage Data..........................................................................................................................................49
Evapotranspiration...................................................................................................................................52
Natural Recharge .....................................................................................................................................52
Incidental Recharge .................................................................................................................................53
Regional Groundwater Flow Model of the Tucson AMA, Simulation and Application.
i
Contents
CHAPTER 5 SIMULATION OF GROUNDWATER FLOW......................................................................55
Model Calibration Process...........................................................................................................................55
Calibration Criteria and Model Error ...........................................................................................................55
Calibration Targets ..................................................................................................................................56
Steady-State Calibration ...............................................................................................................................57
Steady-State Calibration Criteria..............................................................................................................58
Steady-State Model Results......................................................................................................................58
Transient Calibration ...................................................................................................................................67
Transient Calibration Criteria...................................................................................................................67
Transient Calibration Process...................................................................................................................67
Transient Calibration Results ...................................................................................................................68
CHAPTER 6 SENSITIVITY ANALYSIS.......................................................................................................83
Model Sensitivity.........................................................................................................................................83
Sensitivity Procedures .................................................................................................................................83
Steady-State Sensitivity Analysis .................................................................................................................83
Transient State .............................................................................................................................................84
CHAPTER 7 BASE CASE FUTURE SIMULATION ...................................................................................87
Introduction .................................................................................................................................................87
Base Case Water Demands..........................................................................................................................87
Well Pumpage Distribution ......................................................................................................................87
Municipal Water Demand ........................................................................................................................88
Agricultural Water Demand .....................................................................................................................88
Industrial Water Demand.........................................................................................................................89
Miscellaneous Water Demands ................................................................................................................90
Base Case Water Supplies ............................................................................................................................90
CAP Surface-water..................................................................................................................................90
Effluent....................................................................................................................................................92
Base Case Future Scenario Predictions ........................................................................................................93
Water Budget...........................................................................................................................................93
Water Level Changes...............................................................................................................................96
Summary ...................................................................................................................................................104
CHAPTER 8 SUMMARY ..............................................................................................................................106
Summary ...................................................................................................................................................106
Model Limitations .....................................................................................................................................106
Recommendations .....................................................................................................................................107
SELECTED REFERENCES..........................................................................................................................109
Regional Groundwater Flow Model of the Tucson AMA, Simulation and Application.
ii
Contents
APPENDIX A. MODEL WATER BUDGETS
Steady-state Water Budget……………………………………………………………………………A-1
Transient Water Budget………………………………………………………………………...…….A-2
APPENDIX B. MODEL RESIDUALS
Head Observation Weights……………………………………………………………………………B-1
Table B-1. Weighting factors for observed water levels as determined from site altitude accuracy
values in the GWSI database…………………………………………………………………………B-2
Table B-2. 1940 Steady-state Weighted Residuals……...………………………………………..….B-3
Table B-3. 1960 Weighted Residuals……………………………………………………………...…B-5
Table B-4. 1983 Weighted Residuals……………………………………………………………...…B-9
Table B-5. 1999 Weighted Residuals……………………………………………………………….B-15
APPENDIX C. SELECTED HYDROGRAPHS
Map of Hydrograph Locations………………………………………………………………………..C-1
List of Figures
Figure 1. Map showing location of study area, Tucson AMA, Arizona................................................................2
Figure 2. Map showing water level declines 1940 to 1995, in the Tucson AMA, Arizona...................................3
Figure 3. Generalized geologic cross-section of the Upper Santa Cruz sub-basin. ...............................................8
Figure 4. Generalized geologic cross-section of the Avra Valley sub-basin. ........................................................9
Figure 5. Map showing the location of known or suspected faults and areas of perched water, Tucson AMA,
Arizona. ......................................................................................................................................................11
Figure 6. Average monthly rainfall 1896-2000, Tucson, Arizona.......................................................................16
Figure 7. Estimated pumpage in the Tucson area, 1915 to 1940.........................................................................19
Figure 8. Map showing 1940 water level elevations, location of 1940 water level data, and location of
groundwater inflow and outflow, Tucson AMA, Arizona. ..........................................................................20
Figure 9. Estimated and reported pumpage in the Tucson AMA: 1940 – 1999. .................................................25
Figure 10. Map showing 1999 water level elevations, Tucson AMA, Arizona...................................................26
Figure 11. Maximum potential agricultural recharge in Tucson AMA, 1940 - 1999. .........................................28
Figure 12. Annual CAP water use in Tucson AMA, 1993 - 1999.......................................................................29
Figure 13. Estimated and reported effluent releases into the Santa Cruz River 1950 – 2000...............................30
Figure 14. Estimated and reported pumpage in the Avra Valley sub-basin of the Tucson AMA, Arizona, 1941 -
1999 ............................................................................................................................................................32
Figure 15. Estimated and reported pumpage in Upper Santa Cruz sub-basin, Tucson AMA, Arizona, 1941 -
1999. ...........................................................................................................................................................33
Figure 16. Map showing location of the cell grid and the maximum extent of the three model layers, Tucson
AMA, Arizona. ...........................................................................................................................................38
Figure 17. Map showing the distribution of hydraulic conductivity and transmissivity values (a) layer 1, (b)
layer 2, (c) layer 3, and (d) total composite model transmissivity: Tucson groundwater flow model, Tucson
AMA, Arizona ............................................................................................................................................42
Figure 18. Map showing specific yield distribution: Tucson groundwater flow model, Tucson AMA, Arizona.
(a) Layer 1, (b) Layer 2, (c) Layer 3 ..........................................................................................................46
Figure 19. Map showing the distribution of (a) layer 1 vertical leakance (Vcont) and (b) layer 2 vertical
leakance (Vcont), Tucson AMA, Arizona. ..................................................................................................50
Figure 20. Map showing measured and simulated 1940 water levels and the distribution of the steady-state
model weighted residuals, Tucson AMA, Arizona. .....................................................................................60
Figure 21. Steady-state weighted residual histogram Tucson AMA, Arizona. ...................................................61
Figure 22. Scatter plots of steady-state a) weighted observed heads vs. weighted simulated heads and b)
weighted residuals vs. unweighted observed heads. ..............................................................................62
Figure 23. Map showing steady-state boundary conditions (a) natural recharge cells and inflow/outflow
boundaries, (b) boundary conditions and discharge cells, Tucson AMA, Arizona......................................65
Figure 24. Map showing 1999 observed vs. simulated water level contours Tucson AMA, Arizona.................69
Figure 25. Histogram of 1999 weighted residuals Tucson AMA, Arizona. ........................................................71
Regional Groundwater Flow Model of the Tucson AMA, Simulation and Application.
iii
Contents
Figure 26. Scatter plots of transient a) weighted observed heads vs. weighted simulated heads and b) weighted
residuals vs. unweighted observed heads. ..................................................................................................72
Figure 27. Map showing distribution of simulated 1999 head residuals, Tucson AMA, Arizona.......................73
Figure 28. Map showing transient boundary conditions and distribution of recharge cells, Tucson AMA,
Arizona – 1999............................................................................................................................................76
Figure 29. Graph showing total effluent released and simulated effluent infiltrated into the Santa Cruz River –
1951 to 1999. ..............................................................................................................................................78
Figure 30. Map showing transient model boundary conditions and distribution of cells with pumpage cells,
Tucson AMA, Arizona – 1999.....................................................................................................................80
Figure 31. Graph showing simulated annual change-in-storage for Avra Valley sub-basin 1941 - 1999, Tucson
AMA, Arizona. ...........................................................................................................................................81
Figure 32. Graph showing simulated annual change-in-storage for USC sub-basin 1941-1999, Tucson AMA,
Arizona. ......................................................................................................................................................81
Figure 33. Projected population for Tucson AMA model area, 2000 – 2025......................................................89
Figure 34. Projected future agricultural water use and supply sources, 2000 – 2025..........................................90
Figure 35. Annual CAP surface-water recharge allocations (recharge and In Lieu use) by stress period for the
Base Case projection...................................................................................................................................93
Figure 36. Annual projected change in storage 2000 - 2025, Avra Valley sub-basin, Tucson AMA, Arizona...94
Figure 37. Annual projected change in storage 2000 - 2025, USC sub-basin, Tucson AMA, Arizona...............94
Figure 38. Projected water level changes for the Tucson AMA Tucson, Arizona: 2000 – 2025, a) change 2000 –
2005, b) change 2000 – 2010, c) change 2000 – 2015, d) change 2000 – 2020, and e) change 2000-2025.97
Figure 39. Projected depth to water in the Tucson AMA, Arizona: 2025 .........................................................102
Figure 40. Estimated annual pumpage in the central well field area for the Base Case projection. ..................103
Figure 41. Hydrograph of measured, simulated, and projected water levels for well D-14-14 16ccc in the
central well field area................................................................................................................................103
Figure 42. Hydrograph of measured, simulated, and projected water levels for well D-11-10 08ddd in the
northern Avra Valley. ...............................................................................................................................104
List of Tables
Table 1. Correlation of stratigraphic units and Tucson AMA model units to orogenic events............................12
Table 2. Average monthly precipitation totals for Tucson, Arizona 1894-2000. ................................................15
Table 3. Summary of previously published groundwater budget components 1940 to 1984. .............................21
Table 4. Conceptual steady-state groundwater budget for study area. ................................................................24
Table 5. Tucson model components. ...................................................................................................................36
Table 6. Vertical conductance values used in the Tucson AMA, Arizona. .........................................................49
Table 7. Statistical summary of steady-state model weighted residual, Tucson AMA, Arizona.........................59
Table 8. Frequency distribution of the absolute value of the steady-state weighted residuals, Tucson AMA,
Arizona. ......................................................................................................................................................59
Table 9. Comparison between 1940 conceptual water budget and steady-state simulated water budget, Tucson
AMA, Arizona. ...........................................................................................................................................64
Table 10. Statistical summary of transient model weighted head residuals .......................................................70
Table 11. Frequency distribution of the absolute value of the 1999 weighted residuals. ....................................71
Table 12. Weighted head residuals for the transient period 1941- 1999, Tucson AMA. Arizona.......................74
Table 13. Annual simulated model water budget, 1941 - 1999 ...........................................................................77
Table 14. Sensitivity analysis of the steady-state and transient models: a) head changes, and b) water budget
changes, Tucson AMA, Arizona..................................................................................................................85
Table 15. Water providers in the Tucson AMA participating in developing the Base Case projection data........88
Table 16. Permitted recharge projects in the Tucson AMA. ...............................................................................91
Table 17. Base Case water budget 200-2025 Tucson AMA, Arizona.................................................................95
Regional Groundwater Flow Model of the Tucson AMA, Simulation and Application.
iv
Abstract
Abstract
A numerical groundwater flow model of the Tucson Active Management Area (AMA) in Pinal, Pima and Santa
Cruz Counties, Arizona, was developed to simulate the regional hydrologic system during a pre-development
(steady-state) period of 1940, a developed (transient) period from 1941 to 1999, and for a projection period
from 2000 to 2025. The upper and lower basin-fill alluvium in the Tucson AMA forms a complex regional
aquifer system that is divided into 3 model layers.
The steady-state groundwater conditions indicate inflows into Tucson AMA include 34,425 acre-feet of
mountain-front recharge, 39,445 acre-feet of stream infiltration, and 24,155 acre-feet of groundwater underflow.
Steady-state outflows consisted of 59,695 acre-feet of pumpage, 17,170 acre-feet of evapotranspiration, and
21,191 acre-feet of groundwater outflow. Groundwater underflow within the Tucson AMA from the Upper
Santa Cruz (USC) sub-basin to the Avra Valley sub-basin was about 14,580 acre-feet. Transient model results
indicate a cumulative loss of 6.9 million acre-feet of water from the regional aquifer between 1941 and 1999.
Transient outflows were simulated as 15.9 million acre-feet of groundwater pumpage and natural outflows of
about 1.5 million acre-feet; simulated inflows included about 4.0 million acre-feet of incidental recharge from
agricultural and industrial sources and about 6.5 million acre-feet of natural inflows. Simulated irrigation
recharge ranged from 33 percent of total irrigation pumpage in the 1940s and 1950s, to 25 percent of pumpage
in the 1980s and 1990s.
The transient model simulated both the widespread, long-term water level declines in agricultural areas of the
northern Avra Valley sub-basin and recoveries in the area since the mid-1970s. The model also simulated the
historic overdrafting of large areas of the regional aquifer in the USC sub-basin, which has resulted in long-term
water level declines throughout much of the sub-basin during the transient period. Observed and simulated
water level recoveries in the USC sub-basin are generally limited to areas along the Santa Cruz River and its
tributaries where flood flows provided sufficient recharge to offset local pumpage.
The results of a Base Case projection simulation from 2000 to 2025 that maximized the utilization of renewable
water supplies indicates that the Tucson AMA will not achieve its goal of reaching “Safe Yield” by 2025.
However, the AMA-wide annual overdraft is projected to be between 14,000 and 20,000 acre-feet. The Avra
Valley sub-basin will have a net increase in storage during the Base Case projection of about 453,000 acre-feet
and water levels are projected to continue to recover due to extensive artificial recharge of renewable water and
projected declines in agricultural pumpage. The Upper Santa Cruz sub-basin will experience a net loss of
storage of 1,000,000 acre-feet; however, water levels are projected to rise in the City of Tucson’s central
wellfield area, T 14 S, R 14 E, for the period 2000 to 2020. The projected recovery is due to dramatically
reduced withdrawals as pumpage is shifted to recovery of renewable supplies from recharge projects. After
2020, the water level recovery in the central wellfield is projected to slow as increasing municipal demand is
satisfied by increased pumpage. Water levels in the southern areas of the basin near the Santa Cruz River are
projected to rise due to recharge projects. However, water levels are projected to decline by between 50 to 225
feet in the eastern and southeastern areas of the Tucson AMA where demand is expected to be satisfied by non-renewable
groundwater.
Regional Groundwater Flow Model of the Tucson AMA, Simulation and Application.
vi
Acknowledgements
Acknowledgements
The Department would like to thank those individuals and organizations that have provided information,
guidance, and suggestions during the development and review of this modeling project. Special recognition
goes to the City of Tucson and the U. S. Geological Survey who provided large amounts of hydrologic data
from their files used to update and improve the numerical model. Many water providers in the Tucson AMA
furnished estimates of future water use that were incorporated into the future projection model simulation.
Without their assistance and input the Base Case future projection would have been impossible to develop.
Numerous individuals have provides information, suggestions and comments, and assistance during the
development of the model. Special thanks go to the following: Stan Leake, John Hoffman, and Don Pool of the
U. S. Geological Survey; Ralph Marra and Wally Wilson of the City of Tucson; Mike Block of the Metropolitan
Water District, and Brad Prudhom of the Bureau of Reclamation for their help during model development and
comments on the model report; Ken Seasholes, Cindy Shimakosu, Ann Philips, Laura Grignano, Jeff Tannler,
Matt Weber, and those Tucson AMA staff members who worked with the various Tucson AMA water
providers on the future water use data that allowed the Department develop the Base Case future projection data
sets; and to Susan Smith and Carlos Renteria for their assistance and patience in preparing the maps and figures
for this report.
Regional Groundwater Flow Model of the Tucson AMA, Simulation and Application.
vii
Chapter 1
Chapter 1
Introduction
Groundwater Management Act
In 1980, the Arizona legislature passed the Groundwater Management Act (GMA), which created the Arizona
Department of Water Resources (ADWR) and established four Active Management Areas (AMAs) within the
state; a fifth AMA was added in 1994. The AMAs are designated for special, intensive management of
groundwater resources due to the impacts of historic groundwater withdrawals. By 1980, overdrafting of
regional aquifers within the AMAs had created water level declines of as much as 500 ft in some areas. The
goal for most AMAs is the elimination of groundwater overdrafting by achieving “safe-yield”. Safe-yield is
defined as,” a groundwater management goal which attempts to achieve, and thereafter maintain, a long-term
balance between the amount of groundwater withdrawn in an active management area and the annual amount
of natural and artificial recharge in the active management area.” To accomplish this goal, each AMA
provides a water rights-system for allocating existing water resources, requires new urban development to have
long-term, dependable water supplies, and is responsible for developing and setting water management goals so
that future water needs may be met.
Tucson Active Management Area
The Tucson Active Management Area (AMA) is one of the original management areas designated in 1980. In
1994, the southern portion of the Tucson AMA located in Santa Cruz County was split off to form the Santa
Cruz AMA. The current extent of the Tucson AMA is shown in Figure 1. The management goal of the Tucson
AMA is to achieve “safe-yield”, as defined by the GMA, by 2025. To achieve this goal the Tucson AMA has
implemented mandatory conservation requirements for agricultural, industrial, and municipal water users, and
encouraged the use of renewable surface water supplies from the Central Arizona Project (CAP) and reuse of
effluent.
By 1995, groundwater overdrafting in the Tucson AMA had lowered water levels by as much as 200 ft in Upper
Santa Cruz (USC) sub-basin and by at least 150 ft in the agricultural areas of the Avra Valley sub-basin (Figure
2). The loss of saturated aquifer thickness in central Tucson and in the northern part of Avra Valley has
resulted in land subsidence and loss of well productivity. To help Tucson AMA staff evaluate the effectiveness
of various water management alternatives in reversing these declines and achieving safe-yield, the ADWR has
developed a regional groundwater flow model of the AMA. The study began in 1996 with the assembling of
reference literature, review of past modeling efforts by the ADWR and the United States Geological Survey
(USGS), and collection of various types of hydrologic data. The model study area was selected to coincide with
several previous regional modeling studies completed in the Tucson area by the ADWR and the USGS. A
common model grid was utilized so that the information developed during previous modeling studies could be
utilized in development of the new ADWR model, and so that the results of the ADWR model could be more
easily compared to the results of previous models.
Purpose and Objectives
The purpose of this modeling effort is to produce an updated regional groundwater flow model for the Tucson
AMA by combining existing regional models developed by the USGS and the ADWR with updated modeling
capabilities and new data. The updated model used existing data from modeling studies by Anderson (1972),
Mooseburner (1972), Travers and Mock (1984), Hanson and others (1990), and Hanson and Benedict (1994).
The existing models were either one-layer or two-layer models that used older groundwater flow model
software codes. The updated model has three layers and uses the latest MODFLOW software code, well
Regional Groundwater Flow Model of the Tucson AMA, Simulation and Application.
1
^
^
#
#
Wash
River
Santa
Cruz
Brawley
Altar
Wash
Canada
del
Oro
Rillito Creek Tanque
Verde
Creek
Pantano
Wash
T o r t o l i t a M t s
S a n t a Ca t a l i n a M t s
R i n c o n M t s
S a n t a R i t a
M t s
S i e r r i t a M t s
T u c s o n M t s
P i c a c h o M t s
B a b o q u i v ari M t s
S a n L u i s M t s
T o r t i l l a Mts
S i l v e r B e l l M t s
A v r a V a l l e y S u b b a s i n
U p p e r S a n t a C r u z S u b b a s i n
TTuuccssoonn
MMaarraannaa
OOrroo VVaall lleeyy
SSaahhuuaarr ii ttaa
T8S
R8E
T20S
T12S
T10S
T14S
R12E
T18S
R10E R14E R16E
T16S
T22S
?Ï !"a$
?Ë
!"d$
Aû
?Ä
Iz
!"a$
Iz
PINAL COUNTY
PIMA COUNTY
PIMA COUNTY
SANTA CRUZ COUNTY
PIMA COUNTY
SANTA CRUZ COUNTY
B l a c k
M t n
GGrr ee eenn VVaa ll ll ee yy
TThh rr ee ee PPoo ii nn tt ss
Ina Road WPCF
Roger Road
WWTF
San Xavier Indian Reservation
^ Town
# Wastewater Treatment Facility
Road
Stream
San Xavier Indian Reservation
City Boundary
Active Model Boundary
Study Area Boundary
Township & Range
County
Subbasin Boundary
Tucson AMA Boundary
Figure 1.
Map showing location of study area,
Tucson Active Management Area (AMA), Arizona.
¨
10/01/05, \\adwrgis250s\models, p:\tucson\projects\loubotta\report\fig1sue100105.mxd
0 2.5 5 10
Miles
2
Regional Groundwater Flow Model of the Tucson AMA, Simulation and Application.
3
Regional Groundwater Flow Model of the Tucson AMA, Simulation and Application.
^
^
T o r t o l i t a M t s
S a n t a C a t a l i n a M t s
S a n t a R i t a
M t s
S i e r r i t a M t s
T u c s o n M t s
u i s M t s
S i l v e r B e l l M t s
Wash
River
Santa
Cruz
Brawley
Canada
del
Oro
Rillito Creek
Pantano
Wash
Verde
Tanque
Creek
Wash
Altar
Sabino
Creek
Rincon
Creek
-50
-100
-150
-200
-150
-50
-50
-50
-50
-150
-100
-100
-100
-50
-100
-50
-100
TTuucc ssoonn
MMaa rr aannaa
OOrroo VVaall ll ee yy
SSaahhuuaarr ii tt aa
T16S
T14S
T18S
T20S
T12S
R10E R12E R14E R16E
Aû
?Ï
Iz !"a$
!"d$
?Ë
?Ò ?Ä
!"a$
B l a c k
M t n
B
B
A
A
G r e e n V a l l e y
T h r e e P o i n t s
San Xavier Indian Reservation
Figure 2.
Map showing water level declines 1940 to 1995,
in the Tucson AMA, Arizona.
¨
10/01/05, \\adwrgis250s\models, p:\tucson\projects\loubotta\report\fig2sue100105.mxd
^ Town
1940 to 1995 Water Level Decline
Dashed where uncertain
Road
Road
Stream
San Xavier Indian Reservation
City Boundary
Active Model Boundary
Study Area Boundary
Township & Range
Tucson AMA Boundary
Source(s): ADWR, USGS, City of Tucson.
0 2.5 5 10
Miles
contour interval 50 ft.
Chapter 1
specific pumpage data from 1984 to 1999, and other hydrologic data developed since the last modeling project
was completed in 1990.
The objectives of this project are to develop a regional groundwater flow model that accurately simulates the
regional hydrologic flow regime and to accumulate updated hydrologic, geologic, and water use data. The
Tucson AMA staff and local water use managers can then use the updated model to analyze the effect of
different water supply and demand scenarios on the regional aquifer. Projecting future water levels based on
assumed water management scenarios would allow local water managers determine if the planning scenarios
help the AMA reach its goal of safe yield.
General Description of Model Area
The Tucson AMA is located in southeastern Arizona and encompasses approximately 4,000 square miles
(Figure 1). The AMA consists of two parallel north-south trending alluvial basins that are separated by block-faulted
mountains. The two alluvial basins divide the AMA into two sub-basins, the Upper Santa Cruz (USC)
sub-basin and the Avra Valley sub-basin (Figure 1). The USC sub-basin contains the Tucson metropolitan area,
which is the major urban population center in the Tucson AMA. The Avra Valley sub-basin consists of Altar
and Avra Valleys and contains a large agricultural area, which is centered in the central and northern sections of
the sub-basin around Marana, Arizona (Figure 1).
The Tucson AMA is located within the Sonoran Desert sub-province of the Basin and Range physiographic
province. The climate at the lower elevations is semiarid with sparse vegetation consisting of creosote,
mesquite, and cacti at the lowlands. Higher rainfall totals in the upper elevations of the mountains around the
Tucson AMA’s margins support larger conifers and deciduous trees such as aspens, Douglas firs, and oaks.
Annual rainfall ranges from 11 inches to 16 inches on the valley floors to as much as 30 inches in the
surrounding mountains. In January, the mean daily maximum temperature is 75o F (24o C) and the mean daily
minimum temperature is 50o F (10o C). In July, the mean daily maximum temperature is 105o F (40.5o C) and
the mean daily minimum is 83o F (28o C) (Hydrodata, 2001).
Upper Santa Cruz Sub-basin
The USC sub-basin is a large alluvial valley that slopes to the north and northwest and is underlain with thick
basin-fill deposits. The sub-basin has experienced long-term water level declines and some related land
subsidence due to past groundwater withdrawals for irrigation and municipal demands (Figure 2). The Santa
Cruz River is the main surface water drainage, entering the Tucson AMA from the south and exiting the sub-basin
between the Tucson and Tortolita Mountains (Figure 1). Throughout most of the sub-basin the Santa
Cruz is ephemeral, flowing only in response to local rainfall events. However, effluent discharges into the
riverbed from two Pima County Waste Water Treatment plants have created a perennial reach downstream from
the discharge points. During the winter months, effluent discharges are sufficient to maintain surface water
flows all the way to the Tucson AMA - Pinal AMA boundary between the Silver Bell and Picacho Mountains.
Major tributaries to the Santa Cruz River are Pantano Wash, Rillito Creek, Tanque Verde Creek, and Cañada
del Oro (Figure 1).
Avra Valley Sub-basin
The Avra Valley sub-basin is a broad, flat alluvial valley that slopes to the north and northwest. Thick basin-fill
deposits also underlie the sub-basin. The southern part of the alluvial valley is called Altar Valley; north of
Three Points, Arizona, at about Township 16 South, Range 10 East, the valley narrows, and north of this point
is called Avra Valley. The Altar Valley section of the sub-basin is sparsely developed and is not included
within the active model boundary (Figure 1). The Avra Valley section of the sub-basin has been extensively
developed, originally for agriculture and more recently for residential purposes. Water levels in the southern
part of the sub-basin are generally stable; however, developed areas in the central and northern part of the sub-basin
have experienced long-term water level declines (Figure 2). The sub-basin has two major surface water
features, the Santa Cruz River in the north and Altar and Brawley Washes in the south. Altar Wash drains the
Altar Valley section of the sub-basin; Altar Wash is renamed Brawley Wash where it enters the Avra Valley
Regional Groundwater Flow Model of the Tucson AMA, Simulation and Application.
4
Chapter 1
part of the sub-basin. The Santa Cruz River enters the sub-basin between the Tucson and Tortolita Mountains
and flows to the northwest across the northern part of the sub-basin. Brawley Wash and the Santa Cruz River
exit the Tucson AMA into the Pinal AMA between the Silver Bell and Picacho Mountains (Figure 1).
Previous Investigations
The Tucson AMA area has been extensively studied beginning in the early 1900’s up to the present. Major
topics of investigations conducted within the two basins include geology and stratigraphy, hydrogeology, water
resources, and numerical groundwater modeling. Studies documenting geology and stratigraphy include Heindl
and White (1965), Pashley (1966), Davidson (1973), and Anderson (1987, 1988, 1989). Hydrogeology and
water resources studies include Smith (1910), Turner (1943), Turner and others (1947), Schwalen and Shaw
(1957), White, Matlock, and Schwalen (1966), Burkham (1970), Condes de la Torre (1970), Matlock and Davis
(1972), Osterkamp, (1973, 1974), Davidson (1973), Brown (1976), Hollett and Garrett (1984), Murphey and
Hedley, (1984), Cuff and Anderson (1987), Leake and Hanson (1987), Hanson (1989), Anderson, Freethey, and
Tucci (1990), Webb and Betancourt (1990), Hammett and Sicard (1996), and Pool (1999). Regional
Groundwater flow modeling investigations include Moosburner (1972), Anderson (1972), Travers and Mock
(1984), Hanson, Anderson, and Pool (1990), and Hanson and Benedict (1994).
This list is by no means an exhaustive references list for all hydrologic, geologic, or modeling studies for the
Tucson area. The studies cited above were used to develop a conceptual understanding of the regional aquifer
system in the Tucson AMA and helped in constructing the basic framework of the Tucson regional groundwater
flow model.
Sources of Data
In addition to the literature cited above there is a wide variety of hydrogeologic information available for the
area encompassed by the Tucson AMA. The information available includes water level data, well location and
construction records, estimated and measured pumpage totals, annual effluent release data, crop census data,
aquifer test results, stratigraphic interpretations and particle-size analysis derived from well cores, and data sets
from previous modeling studies. Much of this data had been gathered or developed by previous investigators
and was made available to the ADWR through the cooperation of the USGS, the City of Tucson, and Pima
County. Additional data was obtained from ADWR’s own files and databases, which contain an extensive
amount of well-related data and are maintained as part of ADWR’s regulatory and administrative
responsibilities.
ADWR maintains four databases that contain well-related information that were used in developing well
locations and pumpage values for the regional model. The GroundWater Site Inventory (GWSI) database,
State Well Registry database (called the 55 File), and Registry of Groundwater Rights (RGR) database are
active databases and were important sources of well and pumpage data used in this report. A fourth database,
the old State Land Department Well Registry, (called the 35 File), which was a precursor to the current ADWR
Well Registry, was also used in developing historic well data.
The GWSI database contains field-checked data on selected wells that have been visited by personnel from
ADWR’s Basic Data section or the USGS. Information in GWSI includes measured water levels, construction
data on selected wells, well perforation data, and well location coordinates. Water level data from the GWSI
was used in constructing water level contour maps used in the steady-state and transient model calibration. The
Well Registry database contains well completion data, well use information, well locations, and ownership
information. Well construction and location data from the Well Registry and the GWSI were used to assign
pumpage to cells and distribute pumpage by layer in the steady-state and transient model simulations.
The water rights system implemented through the GMA requires that pumpage from all large water production
wells, those wells with a capacity of over 35 gallons per minute or that irrigate more than 2 acres, be reported to
the ADWR and entered into the RGR database. Wells that irrigate less than 2 acres or have a capacity of less
than 35 gallons per minute are exempt from reporting requirements. Since 1984, all pumpage from non-exempt
Regional Groundwater Flow Model of the Tucson AMA, Simulation and Application.
5
Chapter 1
wells in the AMA’s have been reported to the RGR database. Model pumpage files for the transient model
period of 1984 to 1999 were constructed using data from the RGR pumpage files.
Information provided by the City of Tucson and the USGS were very useful in developing the ADWR model.
Pumpage estimates for the 1940 steady-state period and the transient period of 1941 to 1983 were developed
from data provided by the USGS, the City of Tucson, and from ADWR files. The City of Tucson provided
well-specific pumpage data from 1956 to 1983 from their well production files, well log data, aquifer test
results, and water level data. The USGS provided well log data that was used to develop the basic model layer
structure and vertical hydraulic conductance inputs to the model. Pumpage estimates, transmissivity and aquifer
storage distributions, and natural recharge estimates developed for the USGS regional groundwater flow models
by Mooseburner (1972), Anderson (1972), Hanson and others (1990), and Hanson and Benedict (1994) were
important sources of data used to develop the ADWR model inputs.
Travers and Mock (1984) gathered a large amount of hydrologic data during the development of the first
ADWR Tucson regional groundwater flow model completed in 1984. The Travers and Mock data included
pumpage data from the Cortaro-Marana Irrigation District (CMID), Farmers Investment Co-Operative (FICO)
and crop census and well inventory data from the University of Arizona’s (U of A) Agricultural Engineering
College. The U of A was actively gathering a wide variety of agricultural production information in the Tucson
area from the 1930s through the 1970s. The pumpage data, crop census information, and transmissivity data
gathered in the ADWR files by Travers and Mock were very useful in developing water budget information and
model data sets for the early transient period of 1941 to 1983.
Regional Groundwater Flow Model of the Tucson AMA, Simulation and Application.
6
Chapter 2
Chapter 2
Regional Hydrogeologic Setting
General Overview of Regional Hydrogeology
The Tucson AMA is in the Basin and Range physiographic province, which is characterized by block-faulted
mountains separated by basins filled with alluvial sediments. As previously discussed, the Tucson
AMA contains two separate alluvial basins, which divide the AMA into two groundwater sub-basins
(Figure 1). The block-faulted mountains are composed of Precambrian through Tertiary age granitic,
metamorphic, volcanic, and consolidated sedimentary rock. The sedimentary deposits that fill the
intervening basins are collectively termed basin-fill deposits and are of Tertiary to Quaternary age. The
basin-fill deposits are composed of volcanic deposits and unconsolidated to consolidated sediments
consisting of gravel, sand, silt, and clay with minor amounts of gypsiferous and anhydrous sediments. The
basin-fill sediments are generally coarse-grained along the basin margins, and grade into finer-grained and
evaporite deposits in the central parts of the basins. Generalized geologic cross-sections for each sub-basin
are presented in Figures 3 and 4.
Previous investigators have divided the basin-fill sediments into a lower basin-fill and an upper basin-fill
unit based on their general hydrogeologic characteristics (Davidson, 1973; Pool, 1986; Hanson and others,
1990; Hanson and Benedict, 1994). The basin-fill has also been subdivided into stratigraphic units based
on lithologic descriptions, structural relationships, and depositional history (Davidson, 1973, Pool, 1986,
Anderson, 1987, 1988, 1989). In ascending order the lower basin-fill unit has been divided into the
Pantano Formation and the lower and middle Tinaja beds, and the upper basin-fill unit has been divided
into the upper Tinaja beds, Fort Lowell Formation and surficial alluvial deposits, which include stream-channel
deposits, described by Anderson (1987, 1988, 1989) and Davidson (1973).
Structural Geology and Tectonic History
The physical landscape and sedimentary deposits of the Tucson AMA have been strongly affected by
tectonic activities during the Tertiary Period. The mid-Tertiary orogeny and the subsequent Basin and
Range disturbance during the late Tertiary combined to create the current landscape and the sedimentary
units that make up the Tucson AMA regional aquifer. The alluvial sediments deposited during these
disturbances make up the lower and upper basin-fill units.
The mid-Tertiary tectonic activity, which began about 35 million years ago, is characterized as a period of
regional uplifting, extensive sedimentation, and widespread intensive volcanism (Anderson, 1987). The
metamorphic core complex rocks that make up the Rincon, Santa Catalina, Tanque Verde, and Tortolita
Mountains were uplifted and deformed during the mid-Tertiary orogeny (Anderson, 1987). Sedimentary
rocks related to the mid-Tertiary orogeny are highly faulted, folded, and interbedded with volcanic rocks
and include conglomerates, gravels, mudstones, and evaporite deposits. The sedimentary units deposited
during and immediately after this tectonic episode include the Pantano Formation and the lower Tinaja beds
of the lower basin-fill unit.
In the Tucson area the Basin and Range disturbance began about 12 million years ago and included two
distinct periods of faulting and sedimentation (Anderson, 1987). The first episode of faulting featured
block faulting along deep-seated, high-angle normal faults that formed a landscape of deep, closed
structural troughs, called grabens, surrounded by high block faulted mountains (Anderson, 1987; Davidson,
1973). In the USC sub-basin this period of block faulting created the Santa Cruz fault and a parallel series
of faults along the north and east sides of the present day valley (Figure 5). Thousands of feet of coarse-grained
to fine-grained basin-fill sediments were deposited in the troughs by rivers flowing into the closed
Regional Groundwater Flow Model of the Tucson AMA, Simulation and Application.
7
8
Regional Groundwater Flow Model of the Tucson AMA, Simulation and Application.
Figure 3. Generalized hydrogeologic cross-section of the Upper Santa Cruz Sub-basin, Tucson AMA, Arizona.
Modified from Figure 3 Hanson and Benedict (1994)
6,000
5,000
4,000
3,000
2,000
1,000
-1,000 VERTICAL SCALE GREATLY EXAGGERATED
Basin fill-Hachured pattern
0
0 5 KILOMETERS
5 MILES
HIGH-ANGLE FAULT- Arrows indicate
relaitive movement
UPPER
ALLUVIUM
LOWER
ALLUVIUM
BEDROCK
BOUNDARY BETWEEN MODEL LAYERS
WATER TABLE
WATER- QUALITY BOUNDARY - Below
the boundary, water contains more than
500 milligrams per liter or dissolved solids
Holocene alluvium
denotes fine-grained facies
Fort Lowell Formation
Tinaja beds (Undifferentiated)
Pre-basin and range deposits-
Pantano Formation
Granitic rocks
Intrusive and sedimentary rocks
FEET Southwest
Northeast
Sierrita
Mountains
Santa Cruz River
Santa Cruz Fault
Pantano Wash
Tanque Verde Wash
Santa Catalina
Mountains
A A
9
Regional Groundwater Flow Model of the Tucson AMA, Simulation and Application.
FEET
QUATERNARY
Oligocene Pleistocene
TERTIARY
Miocene and
Pliocene
Tsu Tsu
Tsm (?)
Tos
Tsu
? ?
?
Tsm (?)
Figure 4. Generalized hydrogeologic cross-section of the Avra Valley Sub-basin, Tucson AMA, Arizona.
WATER TABLE
TINAJA BEDS, UNDIFFERENTIATED--Gravel
and conglomerate to gypsiferous and anhydritic
clayey silt and mudstone. Includes tuff beds
and interbedded volcanic flows
Ts
Tsu Upper Tinaja beds--Gravel to clayey silt
Middle Tinaja beds--Gravel and conglomerate to
gypsiferous and anhydritic clayey silt and mudstone.
Queried where uncertain
Lower Tinaja beds--Gravel and conglomerate
to clayey silt and mudstone. Queried where uncertain
PANTANO FORMATION-- Conglomerate, mudstone,
and gypsiferous sandstone, mudstone. Includes
megabreccia, tuff beds, and interbedded volcanic
flows. Queried where uncertain
Tos
G Granitic rocks
FORT LOWELL FORMATION-- Gravel to clayey silt;
also includes thin surficial alluvial deposits of late
Pleistocene and Holocene age
Qf
Qf Qf
G
Tsm
Tsl
3000
2600
2200
1800
1400
1000
600
after Anderson, 1988
? ?
HIGH-ANGLE FAULT- Arrows indicate
relaitive movement
EXPLANATION
B B
WEST EAST
Chapter 2
basins. The internal drainage system that developed during this time deposited coarse-grained materials in
alluvial fans near the mountain-fronts and finer-grained sediments in the centers of the troughs. The fine-grained
deposits include evaporite sequences that were deposited by playas and intermittent lakes that
formed along the trough’s central axis (Davidson, 1973; Anderson, 1987). Sediments that make up the
middle Tinaja beds of the lower basin-fill unit were deposited as a result of this first episode of Basin and
Range block faulting.
About 5 million years ago, following a period of erosion, a second period of regional uplift and faulting
occurred (Davidson, 1973; Anderson, 1987). The previously deposited lower basin-fill sediments were
faulted or folded and covered by a new sequence of alluvial sediments that are several hundreds of feet
thick. Once again, coarse alluvial sediments eroded from the uplifted areas were deposited along the
margins of the basin near the uplifted areas and the finer-grained materials were deposited along the central
axis of the basins. The fine-grained sediments deposited during this tectonic event lack the evaporite
deposits found in the older, lower basin-fill deposits. The upper Tinaja beds of the upper basin-fill unit
were deposited during this last episode of Basin and Range faulting. Figure 5 shows the locations of
known or suspected faults in the Tucson AMA study area that formed during the Tertiary orogenic events.
About 1.5 to 2 million years ago the Basin and Range tectonic activity gradually diminished. As tectonic
activity ended a period of regional erosion began and the internal drainage system in the previously closed
basins evolved into one that featured through-flowing rivers. The Fort Lowell Formation, the overlying
surficial alluvium, and the current stream-channel deposits were deposited during and after the
development of the through flowing river system.
The relationship between the Tucson AMA hydrologic units, stratigraphic units and orogenic events is
presented in Table 1. For a more detailed description of the hydrologic units, stratigraphic, structural, and
geologic history of the Tucson area the reader is referred to Pashley (1966), Davidson (1973), Pool (1986,
1999), Anderson (1987,1988, 1989), and Anderson and others (1990).
Hydrogeology
As described above, the Tucson AMA contains a wide variety of igneous, metamorphic, and sedimentary
rocks and unconsolidated sedimentary material. The mountains surrounding the AMA are composed of
crystalline and sedimentary rocks that generally yield very little water and are not considered part of the
regional aquifer, and are therefore, not part of this study. The basin-fill sediments are composed of
consolidated to unconsolidated sedimentary material of Tertiary to Quaternary age. The thickness of the
basin-fill deposits range from a thin veneer along the mountain-fronts to as much as 9,000 ft thick in the
Avra Valley sub-basin and 11,200 ft thick in the USC sub-basin (Davidson, 1973; Anderson, 1987, 1988,
1989; Hanson and others, 1990; Hanson and Benedict, 1994). As described above, the basin-fill has been
divided into a lower basin-fill unit and an upper basin-fill unit based on regional hydrogeologic
characteristics, and further sub-divided into stratigraphic units based on lithology and deposition
environment by Pashley (1966), Davidson (1973), Pool (1986), and Anderson (1987, 1988, 1989). The
general characteristics of the basin-fill deposits are described below.
Upper basin-fill
The upper basin-fill unit ranges from several hundred feet to as much as 1,000 ft thick in both sub-basins.
The unit consists mostly of semi-consolidated to unconsolidated gravel, sands, and clayey silt. In the Avra
Valley sub-basin the upper basin-fill consists largely of finer grained material in the north and central parts
of the sub-basin (Mooseburner, 1972; Anderson, 1988). The upper basin-fill is generally coarser in the
southern part of Avra Valley. In the USC sub-basin the upper basin-fill is generally coarser north of
Township 13 South and finer grained throughout the rest of the sub-basin (Hanson and Benedict, 1994).
The upper basin-fill is correlated to the upper Tinaja beds, the Fort Lowell Formation, and the surficial
alluvium deposits as described by Anderson (1987, 1988, 1989) and Davidson (1973).
Regional Groundwater Flow Model of the Tucson AMA, Simulation and Application.
10
11
Regional Groundwater Flow Model of the Tucson AMA, Simulation and Application.
T o r t o l i t a M t s
S a n t a C a t a l i n a M t s
S a n t a R i t a
M t s
S i e r r i t a M t s
T u c s o n M t s
u i s M t s
S i l v e r B e l l M t s
^
^
D
D U
D U
D U
D U
U D
U D
D U
D U
D U
D U
U
U
D
U
D
D
U
U
D
D
D
D U
U
D U
U
U
D
D
D U
D U
D U
D U
D U
D U
D U
A v r a V a l l e y S u b b a s i n
U p p e r S a n t a C r u z S u b b a s i n
Wash
River
Santa
Cruz
Brawley
Canada
del
Oro
Rillito Creek
Pantano
Wash
Verde
Tanque
Creek
Wash
Altar
Sabino
Creek
Rincon
Creek
TTuucc ssoonn
MMaa rr aannaa
OOrroo VVaall ll eeyy
SSaahhuuaa rr ii tt aa
T16S
T14S
T18S
T20S
T12S
R10E R12E R14E R16E
?Ï
Aû
?Ë
!"d$
!"a$
Iz
!"a$
Iz ?Ä
G r e e n
V a l l e y
B l a c k
M t n
T h r e e
P o i n t s
San Xavier Indian Reservation
¨
10/25/05, \\adwrgis250s\models, p:\tucson\projects\loubotta\report\fig5sue100105.mxd
^ Town
Fault
Dashed where uncertain
Road
Stream
San Xavier Indian Reservation
City Boundary
Township & Range
Perched Area
Active Model Boundary
Study Area Boundary
Subbasin Boundary
Tucson AMA Boundary
Figure 5.
Map showing locations of known or suspected major geologic faults
and areas of perched water zones, Tucson AMA, Arizona.
0 2.5 5 10
Miles
Source(s): Davidson (1973) and Hollett and Garrett (1984).
Chapter 2
Table 1. Correlation of stratigraphic units and Tucson AMA model units to orogenic events.
Stratigraphic Units
Hydrologic Unit
Orogenic Events
Geologic Age
Geologic
Period
Holocene
General tectonic stability and
development of through flowing
drainage
Pleistocene
1.7 – 2.2 m.y.a.
Quaternary
Upper Basin-Fill
Second phase of Basin and
Range faulting, 5.8 m.y.a and
transition to tectonic stability by
2.2 m.y.a
Pliocene
4.9 – 5.3 m.y.a.
Basin and Range faulting
12 – 2.2 m.y.a.
Miocene
Transition from Mid-Tertiary
Orogenic event to Basin and
Range Disturbance, 24 – 12
m.y.a.
23 – 26 m.y.a.
Lower Basin-Fill
Mid-Tertiary Orogenic Event
35 - 24 m.y.a.
Oligocene
34 –38 m.y.a.
Eocene
54 – 56 m.y.a.
Surficial Alluvium
0.01 – 1.3 m.y.a
-------- unconformity ------
Fort Lowell Formation
1.3 – 2.2 m.y.a.
-------- unconformity ------
Upper Tinaja Beds
2.2 – 5.8 m.y.a.
-------- unconformity (?)----
Middle Tinaja Beds
5.8 - 12 m.y.a.
-------- unconformity (?)---
Lower Tinaja Beds
12 – 24 m.y.a
-------- unconformity ------
Pantano Formation
24 – 35 m.y.a.
-------- unconformity ------
Pre-Oligocene Igneous,
Sedimentary, and
Metamorphic Rocks
Pre-Oligocene Geologic Event
Pre-Eocene
Tertiary
After Anderson, 1987, Plate 1.
Million Years Ago – m.y.a.
Regional Groundwater Flow Model of the Tucson AMA, Simulation and Application.
12
Chapter 2
The surficial alluvial deposits are composed of gravels, sands and silty sands and include alluvial-fan, terrace
and stream-channel deposits. The surficial deposits are not hydrologically significant except for the stream-channel
deposits, which are usually referred to as the Younger Alluvium. The Younger Alluvium is very
permeable and ranges from 40 to 100 ft thick (Davidson, 1973). During pre-development times, the Younger
Alluvium was probably partially-to-fully saturated along most of the Santa Cruz River and its tributaries. By
1940, water level declines from localized groundwater pumpage had drained much of the Younger Alluvium
along the Santa Cruz River and its tributaries. However, the Younger Alluvium remains hydrological important
because it serves as a conduit for floodflow recharge that infiltrates into the underlying regional aquifer
The sediments of the Fort Lowell Formation are generally flat lying and are at most 300 ft to 400 ft thick
(Davidson, 1973; Anderson, 1988, 1989). The Fort Lowell Formation is generally unconsolidated to weakly
cemented and composed of gravel, sands and clayey silt. In the northern areas of the USC sub-basin the
sediments of the Fort Lowell Formation are coarser-grained than in the central and southern parts of the sub-basin.
In the Avra Valley sub-basin the unit is generally more coarse-grained in the southern part of the sub-basin
and finer-grained in the central and northern parts of the sub-basin.
The upper Tinaja beds are several hundred ft thick and consist of unconsolidated to slightly cemented gravels,
sands and clayey silts. In well cuttings it is hard to differentiate the contact between upper Tinaja and the Fort
Lowell Formation due to their similar lithologies. The choice of selecting a boundary between the Fort Lowell
and the upper Tinaja beds is based, in part, on changes in color, cementation, and mineralogy. In the USC sub-basin
the sediments of the upper Tinaja beds are coarsest in the northern section of the sub-basin, becoming
finer-grained in the central and southern section of the sub-basin. The upper Tinaja beds are coarser in the
central and southern parts of the Avra Valley sub-basin and grade into finer grained deposits in the northern part
of the sub-basin.
Deposition of the upper Tinaja beds occurred during the late Basin and Range faulting episode. As a result, the
upper Tinaja beds are thickest in the downthrown blocks and thinner on the upthrown blocks of the structural
basins in the USC and Avra Valley sub-basins (Figures 3 and 4). A complete sequence of upper, middle, and
lower Tinaja beds can be found in the downthrown block, whereas the middle Tinaja beds are generally missing
from the sedimentary sequence on the upthrown blocks (Anderson, 1987, 1988, 1989).
Lower Basin-fill
The lower basin-fill is several thousand feet thick and consists of conglomerates, gravels, sands, silts, anhydritic
clayey silts, and mudstones. In the Avra Valley sub-basin the lower basin-fill grades from mostly sands,
gravels, and conglomerates in the southern part of the sub-basin to anhydritic clayey silts and mudstones in the
central and northern parts of the sub-basin (Anderson, 1988; Hanson and others, 1990). The lower basin-fill is
more coarse-grained in the northern part of the USC sub-basin with finer grained deposits, including extensive
evaporite deposits, occurring in the central grabens of the USC sub-basin (Davidson, 1973; Anderson, 1989;
Hanson and Benedict, 1994). The lower basin-fill is equivalent to the Pantano Formation and the lower and
middle Tinaja beds described by Anderson (1987, 1988, 1989).
The middle and lower Tinaja beds are several hundred to several thousand feet thick and their composition
ranges from gravels and conglomerates to gypsiferous and anhydritic clayey silts, and mudstones. The
sediments of the middle and lower Tinaja beds are found in the downthrown blocks of the structural basins in
the USC sub-basin and the northern part of the Avra Valley sub-basin. The middle Tinaja sediments are
generally not present on the upthrown blocks, having been removed by erosion between periods of Basin and
Range faulting (Anderson, 1987). In the downthrown blocks the middle and lower Tinaja sediments are
generally fine-grained and can contain thick deposits of gypsiferous and anhydritic clayey silts.
The Pantano Formation consists of semiconsolidated to consolidated conglomerates, sandstones, mudstones and
gypsiferous mudstones (Davidson, 1973, Anderson, 1987, 1988, 1989). The total thickness of the Pantano
Formation is not known, but it is estimated to be several thousands of feet thick (Davidson, 1973). The unit is
usually deeply buried by overlying Tinaja beds along the central axis of the USC sub-basin in the downthrown
structural blocks. Along the basin’s margins, on the upthrown fault blocks, the Tinaja beds are much thinner,
and the Pantano Formation is closer to the surface and sometimes exposed at the surface.
Regional Groundwater Flow Model of the Tucson AMA, Simulation and Application.
13
Chapter 3
Chapter 3
Regional Groundwater Flow System
Conceptual Model of the Regional Aquifer System
The upper and lower basin-fill sediments within the Tucson AMA are saturated at depth and form the
regional aquifer system. Groundwater in the regional aquifer is generally unconfined or partially confined
to depths of about 1,000 ft (Davidson, 1973; Hanson, 1988, 1989). Localized confining conditions occur in
areas where fine-grained materials in the basin-fill sediments exist. Localized perched zones have also
been observed in the regional aquifer (Figure 5). Water level declines due to excessive groundwater
withdrawals and/or deep percolation of excess agricultural irrigation are the probable mechanisms for
creating the perched areas. For example, fine-grained layers in the basin-fill may strand existing
groundwater in areas of large water level declines, or trap irrigation recharge that is percolating through the
vadose zone. Perched areas generally occur in the central and northern parts of the Avra Valley sub-basin
and in the central and southern parts of the Upper Santa Cruz (USC) sub-basin.
Inflow into the regional aquifer system occurs as groundwater underflow from the SCAMA, mountain-front
recharge, stream infiltration, infiltration of effluent released into the bed of the Santa Cruz River, and
deep percolation of excess agricultural irrigation water. Central Arizona Project CAP surfacewater became
available in the early 1990’s and is currently being utilized by the agricultural, industrial, and municipal
sectors within the Tucson AMA. CAP water is also being recharged into the regional aquifer at artificial
recharge sites located on both the Avra Valley and USC sub-basins. Groundwater is discharged from the
regional aquifer through pumpage, evapotranspiration, and as underflow into the Pinal AMA (PAMA).
During the winter months some of the effluent released into the channel of the Santa Cruz River from the
Pima County Wastewater Treatment Plants exits the Tucson AMA as surface flow.
Groundwater movement within the regional aquifer is generally to the north and northwest, except in the
Cañada del Oro drainage, where groundwater moves south before entering the main part of the USC sub-basin.
Groundwater enters the USC sub-basin in the south from the Santa Cruz AMA and from the east
through the narrow gap between the Rincon and Santa Rita Mountains near Vail, Arizona. Groundwater
exits the sub-basin through the Rillito narrows between the Tucson and Tortolita Mountains, moving into
the northern part of the Avra Valley sub-basin. Groundwater in the Avra Valley sub-basin also flows to the
north-northwest from the southern source areas in Altar Valley to the northern Avra Valley where it exits
the Tucson AMA into the Pinal AMA through the gap between the Silver Bell and Picacho Mountains
(Figure 1).
Precipitation falling in the mountains and along the valley floors of the two sub-basins is the largest source
of natural inflows to the Tucson AMA regional aquifer. Water from precipitation generates mountain-front
recharge and flow events in ephemeral streams and washes along the valley floor. Numerous studies have
shown that in semi-arid and arid environments low-lying topographic areas such as ephemeral streams and
dry washes serve as preferred pathways for recharge. These streambeds and washes typically contain
highly permeable sands and gravels, which allow relatively rapid infiltration of runoff from precipitation
events. The rapid infiltration allows some water to infiltrate down past the root zone and beyond the effects
of high evaporation rates that are present along the valley floor. Very little, if any, of the precipitation that
infiltrates directly into the vadose zone away from low-lying areas is believed to recharge the regional
aquifer. Most water that infiltrates the vadose zone away from the stream channels in the lower valley floor
is absorbed by the soil and then lost through evaporation or transpired by plants.
Annual average precipitation ranges from about 11 inches along the valley floor to as much as 30 inches in
the higher elevations of the surrounding mountains (Hydrodata, 2000). Monthly precipitation totals for
lower elevations along the valley floor can range from zero to almost 8 inches (Table 2).
Regional Groundwater Flow Model of the Tucson AMA, Simulation and Application.
14
Chapter 3
Table 2. Average monthly precipitation totals for Tucson, Arizona 1894-2000.
Month
Monthly
Average
(inches)
Monthly
Minimum
(inches)
Year
Monthly
Maximum
(inches)
Year
January 0.89 0 1972 5.58 1993
February 0.85 0 1999 4.15 1905
March 0.76 0 1984 3.88 1905
April 0.38 0 1993 3.53 1905
May 0.18 0 2000 1.34 1931
June 0.27 0 1998 2.07 1938
July 2.05 0.05 1995 7.56 1984
August 2.14 0.08 1924 5.61 1935
September 1.17 0 1973 4.41 1996
October 0.76 0 1982 5.78 1983
November 0.78 0 1999 4.61 1905
December 0.99 0 1996 5.85 1914
Year 11.3 5.07 1924 24.17 1905
Data Source: Hydrodata, 2001
Precipitation occurs in southern Arizona in two distinct seasons; a summer wet season from July to late
September, referred to locally as the monsoon season, and a winter wet season from November to April (Figure
6). Beginning in late June to early July, the summer rainy season of isolated, localized thunderstorms provides
a break from the spring dry season. Moisture drawn into southern Arizona from the Gulf of California and the
Pacific Ocean combines with rising hot air to generate high-intensity, short-term thunderstorms. During the last
stages of the summer rainy season, in September and October, dissipating tropical cyclones that originate in the
Pacific Ocean off Mexico occasionally make their way into southern Arizona. The tropical cyclones generate
large regional storm events that can cause intense precipitation and occasional flooding in southern Arizona.
During the winter rainy season, from November to April, widespread low-intensity precipitation events are
generated by large-scale regional low-pressure frontal systems. Individual winter precipitation events generally
don’t produce large rainfall totals; however, under certain conditions winter storms can produce substantial
rainfall totals and severe flooding.
Aquifer System
The Tucson AMA regional aquifer system consists of the upper and lower basin-fill as previously described.
The Younger Alluvium, the Fort Lowell Formation, and upper Tinaja beds of the upper basin-fill are the most
productive units within the basin fill. Most high capacity wells that provide water for municipal, industrial, or
irrigation are completed in one or all of these units. As discussed above, the Younger Alluvium is not
considered a significant aquifer due to its limited extent and water level declines. However, it may still be
saturated in some localized areas, and it is hydrologically significant because it serves as a pathway for stream
infiltration into the regional aquifer. The middle and lower Tinaja beds and Pantano Formation of the lower
basin-fill are generally not highly productive and have not been widely developed as a source of groundwater.
This is due to several reasons, which may include depth of burial, increased consolidation, and presence of large
percentages of fine materials. Wells developed in the middle and lower Tinaja beds and Pantano Formation
generally produce only small to moderate amounts of water. However, there are areas along the basin margins
where the middle and lower Tinaja and Pantano formation are an important source of groundwater. The
crystalline and metamorphic units that make up the basement bedrock and the mountains surrounding the
Regional Groundwater Flow Model of the Tucson AMA, Simulation and Application.
15
Chapter 3
alluvial basins provide only small amounts of groundwater for local use and are not considered a part of the
Tucson AMA regional aquifer.
0
0.5
1
1.5
2
2.5
Precipitaion in Inches
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Month
Average Monthly Rainfall - Tucson 1894 - 2000
(Source: Hydrodata, 2001).
Figure 6. Average monthly rainfall 1896-2000, Tucson, Arizona.
Fort Lowell Formation
The Younger Alluvium, where it is saturated, the Fort Lowell Formation and the upper Tinaja beds forms the
most productive unit in the Tucson AMA aquifer system. Wells completed in the Fort Lowell Formation are
capable of producing 500 to 1,500 gallons per minute (Davidson, 1973; Anderson, 1988, 1989). The Fort
Lowell Formation has significant saturated thickness throughout most of the USC sub-basin and in the northern
parts of the Avra Valley sub-basin, and is considered the main regional aquifer. Groundwater in the Fort
Lowell Formation generally occurs under unconfined or water table conditions. Localized perching conditions,
caused by interbedded layers of fine-grained sediments, are known to exist in the USC sub-basin just north,
south, and east of Black Mountain, and in the northern sections of the Avra Valley sub-basin (Figure 5)
(Babcock and others, 1982; Anderson, 1988, 1989). Hydraulic conductivity and storage values for the Fort
Lowell Formation vary widely and are dependent on the particle-size distribution and degree of cementation
within the unit. Reported hydraulic conductivity values generally range from less than 5 to over 700 ft per day
and transmissivity values ranging 1,500 to 40,000 ft2 per day (Hanson and others, 1990; Hanson and Benedict,
1994). The highest conductivity and transmissivity values generally occur in the Younger Alluvium along the
streambed of the Santa Cruz River and its main tributaries. Estimates of specific yields for the upper basin-fill,
which includes the Younger Alluvium, Fort Lowell Formation and the upper Tinaja beds, generally range from
0.05 to 0.25 and average about 0.15 (Hanson and others, 1990; Hanson and Benedict, 1994).
Regional Groundwater Flow Model of the Tucson AMA, Simulation and Application.
16
Chapter 3
Upper Tinaja beds
The upper Tinaja beds and the Fort Lowell Formation form the most productive unit of the Tucson AMA
regional aquifer. The upper Tinaja beds have become a more important aquifer in areas where water level
declines have reduced the saturated thickness of the Fort Lowell Formation. Well yields and the hydrologic
properties of the upper Tinaja beds are generally similar to those of the Fort Lowell Formation. In well cuttings
it is hard to differentiate the contact between upper Tinaja and the Fort Lowell Formation due to their similar
lithologies. The choice of selecting the boundary between the Fort Lowell and the upper Tinaja beds is based in
part on color, cementation, and mineralogy rather than hydrologic parameters. Throughout much of Avra
Valley the Fort Lowell Formation is either not saturated or has a smaller saturated thickness than in the USC
sub-basin. As a result, the upper Tinaja beds, along with the middle and lower Tinaja beds, are more significant
aquifers in the Avra Valley Sub-basin. This is particularly true in the southern portions of the Avra Valley sub-basin
where the Fort Lowell Formation is unsaturated and the Tinaja beds consist of thick sequences of coarse-grained
sand deposits. In this area the Tinaja beds can be very productive and are the main water-bearing unit.
Middle and lower Tinaja beds
Wells completed in the middle and lower Tinaja beds generally produce only small to moderate amounts of
water (Davidson, 1973; Hanson and others, 1990; Hanson and Benedict, 1994). In the USC sub-basin the
presence of large amounts of fine-grained material and increased consolidation of the two units reduces their
ability to transmit large amounts of water to wells. As a result, these two units generally have not been highly
developed as a source of water in the USC sub-basin, except along the basin margins where the Fort Lowell
doesn’t exist. However, in Avra Valley the Tinaja beds are an important source of groundwater (Anderson,
1987).
Transmissivity and storage properties vary greatly in the middle and lower Tinaja beds depending on their
location and composition. Estimated hydraulic conductivity values for the lower basin-fill, which includes the
middle and lower Tinaja beds, range from 1 to over 200 ft per day and transmissivities range from 1,000 to over
40,000 ft2 per day (Davidson, 1973; Hanson and others, 1990; Hanson and Benedict, 1994). Storage properties
for the lower basin-fill are difficult to determine and are largely based on estimates from previous modeling
studies. Specific yield values are at the low end of reported estimates, probably ranging from 0.03 to 0.10.
Storage coefficients for the lower basin-fill below 1,000 ft are estimated to be about 1 x 10-4 (Davidson, 1973;
Hanson and others, 1990; Hanson and Benedict, 1994).
Pantano Formation
The Pantano Formation is capable of producing small to moderate amounts of water to wells (Davidson, 1973,
Anderson, 1987, 1988, 1989). The unit is generally not an important water-producing unit within the regional
aquifer because it is usually too deeply buried by overlying sediments and wells do not penetrate the unit. This
is especially true in the downthrown structural blocks where the Pantano Formation is overlain by thousands of
feet of sediments from the upper, middle, and lower Tinaja beds. However, near the basin margins on some of
the upthrown blocks, particularly west of the Santa Cruz Fault in the USC sub-basin where the Tinaja beds are
either missing or much thinner, the Pantano Formation and the overlying Tinaja beds combine to form the main
aquifer (Figure 3). Wells completed in the Pantano Formation in these areas can produce moderate amounts of
water (Davidson, 1973). Transmissivity and storage values for the Pantano Formation are similar to those
reported for the lower Tinaja beds by previous investigators.
Predevelopment Groundwater System
Prior to about 1900, the Tucson AMA regional aquifer system was in a state of dynamic equilibrium with the
long-term natural recharge balanced by long-term natural discharge. Groundwater withdrawals during this
period were small and limited to domestic and stock uses. Groundwater development in the Tucson AMA
began in the early 1900s when the first irrigation wells were constructed in the USC sub-basin to supplement
Regional Groundwater Flow Model of the Tucson AMA, Simulation and Application.
17
Chapter 3
surface water flows diverted from the Santa Cruz River (Schwalen and Shaw, 1957; Hanson and Benedict,
1994). Many of the early irrigation wells were drilled close to the Santa Cruz River and its tributaries because
that is where land had been cleared for farming and the water table was shallow (Schwalen and Shaw, 1957;
Davidson, 1973). Irrigated agriculture began in the Avra Valley sub-basin in the early 1920s, and by 1937
about 6,000 acres of land was in production in the area around Marana, Arizona (Andrews, 1937). Irrigation
water was supplied to these agricultural lands from wells located in the USC sub-basin and transported via
canals (White and others, 1966). High capacity irrigation wells were not drilled in the Avra Valley sub-basin
until after 1937.
There is consensus among previous investigators that the Tucson AMA regional aquifer system was still in a
state of dynamic equilibrium until about 1940 (Anderson, 1972; Moosburner, 1972; Davidson, 1973; Hanson
and others, 1990; Hanson and Benedict, 1994). Water budget and water level data support this conclusion.
Figure 7 presents estimated pumpage in the Tucson AMA area from 1915 to 1940, and indicates that prior to
about 1920, pumpage was, at most, only about 10,000 ac-ft/yr. From 1920 to 1940, pumpage was relatively
constant, averaging about 35,000 ac-ft/yr. The relatively uniform stress over that time period probably allowed
the regional aquifer system to adjust to withdrawals and maintain an approximate state of equilibrium between
inflows and outflows (Davidson, 1973, Hanson and Benedict, 1994). The balance between inflows and
outflows was probably maintained by a decrease of evapotranspiration from riparian areas approximately equal
to the amount of pumpage plus a small loss of aquifer storage from areas near pumping centers. Schwalen and
Shaw (1957) constructed hydrographs of wells with water level data available from the early 1930s through the
early 1940s that indicate water level declines were relatively small and concentrated along the Santa Cruz River
where the majority of irrigation and municipal wells were located. As a result, any loss of aquifer storage
probably affected a relatively small area of the aquifer system, mostly the Younger Alluvium, and probably did
not seriously affect the larger regional flow system.
For this modeling study the condition of the Tucson AMA regional aquifer system in 1940, is considered
generally representative of predevelopment times and is used as the steady-state period. Figure 8 is a water
level contour map developed by ADWR from water level data for the period 1939 to 1940. The map is similar
to contour maps of 1940-water levels developed by Moosburner (1972), Anderson (1972), Hanson and others
(1990), and Hanson and Benedict (1994), and represents the initial water level surface for the steady-state
period. A conceptual steady-state water budget for 1940 developed from numerous sources is discussed below
and presented in Table 4.
Inflows
During predevelopment times inflows to the Tucson AMA regional aquifer occurred as groundwater underflow,
mountain-front recharge, and streambed infiltration from flow events along the Santa Cruz River and its’ major
tributaries. Table 3 presents previous estimates of natural recharge from studies that included part or all of the
areas included in the Tucson AMA’s groundwater basins. Previous investigators’ estimates of natural recharge
vary widely because of the varying size of the study areas and different methods employed to generate the
estimates; therefore, some recharge estimates are not directly comparable to this study’s estimates.
Mountain-Front Recharge
Mountain-front recharge occurs in streams at upper elevations of the mountains surrounding the Tucson AMA
and through alluvial fans along the mountain-fronts. Rainfall and snowmelt generate surface flows that
infiltrate into the alluvial material under the streams and washes that flow from the mountains and cross the
alluvial fans. Some water also infiltrates directly into the fans during sheet flow events (Bouwer, 1989).
Groundwater then flows into the regional aquifer system through the alluvial fans at the base of the mountains.
Estimates of mountain-front recharge in the Tucson AMA area by previous investigators are not easily
compared to this study’s estimates. Many previous study areas do not coincide with the current model area and
the assumptions used to develop past water budgets are different than those used in this study. Table 3 presents
inflow and outflow estimates from investigators whose study areas most closely match the ADWR study
boundaries. The mountain-front recharge estimates are listed by sub-basin and for the USC sub-basin range
Regional Groundwater Flow Model of the Tucson AMA, Simulation and Application.
18
Chapter 3
from 28,000 ac-ft per year to 32,000 ac-ft per year. Estimates for the Avra Valley sub-basin range from 500 ac-ft
per year to about 9,000 ac-ft per year.
The initial mountain-front recharge estimates for the ADWR model are 29,600 ac-ft/yr for the USC sub-basin
and 3,500 ac-ft per yr for the Avra Valley sub-basin for a total of 33,100 ac-ft per year (Table 3). The USC sub-basin
estimates are similar to the values developed by Anderson (1972), Davidson (1973) and Hanson and
Benedict (1994). The conceptual mountain-front recharge estimates for the Avra Valley sub-basin are based on
values developed from a water budget analysis of the sub-basin by Osterkamp (1973).
0
10,000
20,000
30,000
40,000
50,000
60,000
70,000
1915 1920 1925 1930 1935 1940
Year
Annual Pumpage in Acre-Feet
USC sub-basin Avra Valley sub-basin
(source: Anning and Duet, 1994)
Figure 7. Estimated pumpage in the Tucson area, 1915 to 1940.
Regional Groundwater Flow Model of the Tucson AMA, Simulation and Application.
19
20
Regional Groundwater Flow Model of the Tucson AMA, Simulation and Application.
T o r t o l i t a M t s
S a n t a Ca t a l i n a M t s
S a n t a R i t a
M t s
S i e r r i t a M t s
T u c s o n M t s
u i s M t s
S i l v e r B e l l M t s
1800
1800
1900
2100
2200
2200
2300
2400
2500
2600
2700
2800
2900
2900
3000
3100
2100
2800
2600
^
^
Wash
River
Santa
Cruz
Brawley
Canada
del
Oro
Rillito Creek
Pantano
Wash
Verde
Tanque
Creek
Wash
Altar
Sabino
Creek
Rincon
Creek
TTuucc ssoonn
MMaa rr aannaa
OOrroo VVaa ll ll ee yy
SSaahh uuaa rr ii tt aa
T16S
T14S
T18S
T20S
T12S
R10E R12E R14E R16E
Inflow
#
Inflow
#
Outflow
#
?Ï !"a$
!"d$
Aû
Iz
!"$a Iz ?Ä
?Ë
B l a c k
M t n
G r e e n
V a l l e y
T h r e e
P o i n t s
San Xavier Indian Reservation
¨
10\25/05, \\adwrgis250s\models, p:\tucson\projects\loubotta\report\fig8sue100105.mxd
Figure 8.
Map showing groundwater levels in 1940, locations
of 1940 water level data, and locations of groundwater
underflow, Tucson AMA, Arizona
Source(s): ADWR, Groundwater Site Inventory.
1940 Water Level Location
^ Town
1940 Measured Water Level - ft. above MSL
Road
Road
Stream
San Xavier Indian Reservation
City Boundary
Township & Range
Active Model Boundary
Study Area Boundary
Tucson AMA Boundary
Direction of groundwater flow
0 2.5 5 10
Miles
contour interval = 50 ft
21
Regional Groundwater Flow Model of the Tucson AMA, Simulation and Application.
Table 3. Summary of estimated predevelopment groundwater budget components for the Santa Cruz and Avra Valley sub-basins, Tucson AMA, Arizona. (values are in acre-feet per year)
Inflows Outflows
Mountain-Front Stream Flow Groundwater Other Sources
Time Period Source Recharge Infiltration Underflow of Recharge Total Inflow Evapotranspiration Underflow Pumpage Total Outflow
Santa Cruz sub-basin
1940 - 65 Anderson (1972) 28,000 1 19,000 1 10,000 (7,800) 1, 2 ---------- 64,800 ----------3 17,500 47,500 65,000
1940 Clifton (1981) ---------- ---------- ---------- ---------- ---------- ---------- 11,450 ---------- ----------
1940 - 84 Hanson and others (1990) ---------- ---------- ---------- ---------- ---------- ---------- 9,000 ---------- ----------
1940 Hanson and Benedict (1994) 29,840 34,020 7,500 (5,430)2, 4 ---------- 76,790 7,890 15,260 53,000 76,150
1936 –65 Osterkamp (1973) 31,900 6 63,020 6 ---------- ---------- 94,920 ---------- ---------- ---------- ----------
1936 - 63
Davidson (1973) 7 31,000 8 51,000 9 10,000 (7,800) 2 17,300 10 117,100 6,000 – 15,000 11 10,000 176,700 12 202,200
1940 Moosburner (1972) ---------- ---------- ---------- ---------- ---------- ---------- 13,000 ---------- ----------
1940 Whallon (1983) ---------- ---------- ---------- ---------- ---------- ---------- 20,100 ---------- ----------
1940 ADWR Steady-State Model 31,198 33,655 13,900 0 78,753 17,170 14,380 47,280 78,830
Avra Valley sub-basin
1940 Anderson (1972) ---------- ---------- 17,500 13 ---------- ---------- ---------- ---------- ---------- ----------
1940 Clifton (1981) 500 ---------- 3 11,450 (6,790) 14 0 18,470 0 18,470 0 18,470
1940
Freethey and Anderson (1986) 5 9,000 5,000 12,400 14 0 26,400 7,400 19,000 0 26,400
1940 Moosburner (1972) < 3,000 15 ---------- 3 13,000 (9,000) 14 0 22,000 ---------- 3 22,000 10,000 15, 16 22,000
1940 Osterkamp (1973) 7,100 14,700 ---------- 3 ---------- 3 21,800 ---------- 3 ---------- 3 ---------- 3 ---------- 3
1940 Hanson and Benedict (1994) ---------- 3 ---------- 3 15,260 ---------- 3 ---------- 3 ---------- 3 ---------- 3 ---------- 3 ---------- 3
1940 Hanson and others (1990) 0 0 9,000 (9,900)14 0 18,900 0 18,900 0 18,900
1940 Whallon (1983) ---------- 3 ---------- 3 20,100 (16,600)14 0 36,700 ---------- 3 34,700 ---------- 3 34,700
1940 ADWR Steady-State Model 3,227 5,790 14,380 (10,255)14 0 33,652 0 21,191 12000 33,191
Notes:
1. Value simulated in electric-analog model steady-state model.
2. The first value is simulated flow from the Santa Cruz AMA. If provided a second value in parenthesis is underflow simulated along the northern part of the model as underflow coming from the Canada del Oro (CDO) drainage.
3. Budget component was not estimated, simulated, or was considered negligible.
4. The Hanson and Benedict (1994) model’s southern boundary was south of Tubac, approximately 12 miles south of the southern boundary of the ADWR model.
5. Basin estimate includes Altar Valley and may not be directly comparable to other values in this report.
6. Value represents recharge for an area that is approximately the same as the Hanson and Benedict (1994) model. Recharge values for the current ADWR model study area would be less.
7. Values represent the mean values for the time period listed.
8. Value is from Anderson (1972).
9. Value is based on Burkham (1970).
10. Value represents 8,300 acre-feet per year of estimated effluent recharge and 9,000 acre-feet per year of estimated incidential recharge from industrial uses.
11. Estimated value from early 1960s.
12. Estimated pumpage for 1965.
13. Underflow from USC sub-basin, Altar Valley was not in study area.
14. The first value is simulated flow from the Upper Santa Cruz sub-basin. If provided a second value in parenthesis is simulated underflow from Altar Valley.
15. Inflow or outflow estimates reported by investigator but not used in report.
16. Pumpage estimate reported from White and others (1965).
21
Chapter 3
Stream Infiltration
Stream infiltration occurs at the lower elevations when precipitation creates flow events that infiltrate into
the normally dry beds of the Santa Cruz River and it’s tributaries. Individual flow events generated by
direct precipitation falling in the valleys are usually of short duration, especially during the summer
thunderstorm season. Some winter storms may last for several days and can generate prolonged flow
events that may produce large amounts of recharge. Flow events associated with winter storms are
believed to contribute more recharge to the regional aquifer than summer storms (Gallaher, 1979).
During predevelopment times stretches of the Santa Cruz River flowed intermittently within the study area
due to groundwater discharging from the Younger Alluvium into the riverbed. Cienegas, marshes fed by
intermittent flow, occurred near the San Xavier Mission and within the current City of Tucson boundaries
(Webb and Betancourt, 1990; Parker, 1993). However, since the early 1900s, a combination of streambed
entrenchment along the Santa Cruz River and declining water levels due to groundwater development has
impacted the river . A cycle of floods and droughts in the late 1800s and the early 1900s caused
headcutting and entrenchment of the river’s main channel through much of present day Tucson, destroying
the cienegas (Webb and Betancourt, 1990; Parker, 1993). The combination of river entrenchment and early
groundwater development, which was concentrated near the river, drained much of the Younger Alluvium.
The lowering of the local water table resulted in the regional aquifer becoming hydrologically disconnected
from the riverbed, ending natural discharge to the river. By 1940, most sections of the river channel in the
Tucson area were deeply entrenched, much of the Younger Alluvium had been dewatered, and the Santa
Cruz River flowed only in response to precipitation events.
Estimates of annual stream infiltration into the two sub-basins by previous investigators vary widely and
are not easily compared to each other or with the infiltration estimates of this study. Most investigators
study areas’ did not coincide with the current model area, and assumptions used to develop their stream
infiltration estimates may have been different than those used for the ADWR model. Estimates of annual
stream infiltration in the Santa Cruz sub-basin range from 19,000 ac-ft to about 63,000 ac-ft, and values in
Avra Valley range from 5,000 ac-ft to 14,700 ac-ft annually (Table 3). ADWR’s initial estimates of annual
stream infiltration within the study area were developed using information from Burkham (1970),
Anderson (1972), Davidson (1973), Osterkamp (1973), and Hanson and Benedict (1994). Initial estimates
for the average long-term infiltration for the steady-state period are 34,200 ac-ft per year in the USC sub-basin
and 6,000 ac-ft per year in the Avra Valley sub-basin (Table 4).
Groundwater Underflow
Underflow into the USC sub-basin occurs from the south across the Tucson AMA - Santa Cruz AMA
boundary and to the east through the bedrock gap near Vail, Arizona, where Pantano Wash enters the
Tucson AMA. Estimates of steady-state underflow crossing the southern boundary into the USC sub-basin
range from 5,600 ac-ft/yr to 10,600 ac-ft per yr (Table 3). Estimates of underflow across the eastern
boundary of the study area along the Pantano are small and were included in the stream recharge estimates
for Pantano Wash it enters the study area. In the Avra Valley sub-basin groundwater underflow moves into
Avra Valley from Altar Valley in the area of Township 16 South. Underflow into the Avra Valley portion
of the study area from Altar Valley has been estimated to range from about 6,800 ac-ft per yr to about
16,600 ac-ft per yr (Turner, 1959; Mooseburner, 1972; Brown, 1976; Whallon, 1983; Clifton, 1981;
Travers and Mock, 1984; Hanson and others, 1990). The initial estimates of steady-state groundwater
underflow into the model from Santa Cruz County and the Altar Valley are 8,600 ac-ft per year and 10,000
ac-ft per year, respectively (Table 4). The initial underflow estimate from Santa Cruz County was
calculated as the mid-point of the range of underflow estimates found in reference literature (Table 3). The
initial estimate of underflow from Altar Valley was taken from the groundwater flow model developed by
Hanson and others (1990).
Regional Groundwater Flow Model of the Tucson AMA, Simulation and Application.
22
Chapter 3
Outflows
Steady-state groundwater discharge from Tucson AMA’s regional aquifer system occurred as pumpage,
underflow, and evapotranspiration. Previous estimates of groundwater discharge may not be directly
applicable in this study due to differences between study areas and water budget assumptions. Table 3
provides a summary of outflow estimates from studies that cover all, or parts, of the study area.
Pumpage
Groundwater pumpage for agricultural, municipal, and industrial purposes were the single largest source of
withdrawals from the regional aquifer in the steady-state period. Estimates of total annual pumpage from
the Tucson AMA regional aquifer for the steady-state period, 1940, are about 61,000 ac-ft (Figure 7).
Estimated withdrawals in the USC sub-basin were relatively consistent from 1915 until 1919, averaging
about 8,000 ac-ft per year. Withdrawals increased in 1920, and ranged from 30,000 ac-ft per year to 45,000
ac-ft per year until 1930 (Figure 7). Withdrawals declined slightly in the early 1930s, to less than 20,000
ac-ft per year before increasing to about 60,000 ac-ft per year in 1940. The initial steady-state pumpage
estimate for the USC sub-basin is 49,600 ac-ft (Table 4). The amount and general distribution of USC
pumpage comes from previous modeling studies by Anderson (1972), Travers and Mock (1984), and
Hanson and Benedict (1994). The initial pumpage estimates for the Avra Valley sub-basin for 1940 is
12,000 ac-ft and comes from the modeling report by Hanson and others (1990).
Underflow
Groundwater underflow exits the Tucson AMA aquifer through the gap between the Silverbell and Picacho
Mountains. Underflow between the sub-basins moves to the northwest from the USC sub-basin into the
Avra Valley sub-basin through the Rillito narrows between the Tucson and Tortolita Mountains. Previous
investigator’s estimates of groundwater underflow through the Silverbell and Picacho Mountains gap range
from 18,670 ac-ft per year to 34,500 ac-ft per year (Table 3). The conceptual steady-state estimate of
annual underflow leaving the Tucson AMA was originally set at 24,500 ac-ft. This estimate was later
revised to 22,500 ac-ft based on estimates of mountain-front, stream infiltration, and groundwater
underflow into the Avra Valley sub-basin. Estimates of underflow from the USC sub-basin into the Avra
Valley sub-basin range from 3,000 to 20,100 ac-ft per year (Table 3). The conceptual steady-state
underflow from the USC sub-basin to Avra Valley was set at 15,000 ac-ft per year.
Evapotranspiration
Estimates of evapotranspiration for the Tucson AMA area vary widely in previous investigations. Annual
evapotranspiration estimates for predevelopment times in the USC sub-basin range from 15,000 ac-ft to
55,700 ac-ft (Table 3). Prior to the 1890s, water levels in the USC sub-basin along the Santa Cruz River
and its major tributaries were shallow enough to support extensive mesquite bosques and cienigas (Bryon,
1922; Schwalen and Shaw, 1957; Parker, 1993). During that time stream infiltration along the Santa Cruz
River and its tributaries was probably in balance with evapotranspiration and surface water outflow
(Davidson, 1973; Hanson and Benedict, 1994). By 1940, the combination of streambed entrenchment,
lowering of the water table near the Santa Cruz River due to groundwater withdrawals, and development in
the floodplain had significantly reduced the areal extent of the remaining bosques and associated
evapotranspiration. Davidson (1973) estimated evapotranspiration to be between 6,000 and 15,500 ac-ft
per year in 1965. Hanson and Benedict (1994) simulated steady-state evapotranspiration at 7,890 ac-ft per
year. The conceptual estimate for the Tucson AMA model steady-state ET was rounded up to 8,000 ac-ft
per year.
There are no published estimates of evapotranspiration available for the Avra Valley sub-basin. Previous
investigators have either assumed that evapotranspiration in Avra Valley was negligible, or did not estimate
that component of their water budgets. Andrews (1937) reported the depth to water in the northern part of
Regional Groundwater Flow Model of the Tucson AMA, Simulation and Application.
23
Chapter 3
Avra Valley along the Santa Cruz River was about 150 feet below land surface, which is too deep to
support riparian vegetation. For the purposes of this study, the predevelopment water table in Avra Valley
was assumed to be too deep to support riparian vegetation; therefore, evapotranspiration was not considered
a component in the Avra Valley sub-basin water budget.
Table 4. Conceptual steady-state groundwater budget for study area.
Upper Santa Cruz
Sub-Basin
Ac-ft/yr.
Avra Valley
Sub-Basin
Ac-ft/yr.
AMA Totals
Ac-ft/yr.
Inflows
Mountain-Front Recharge 29,600 Mountain-Front Recharge 3,500 33,100
Stream Infiltration 34,200 Stream Infiltration 6,000 40,200
Underflow from Underflow from
Santa Cruz AMA 8,600 Altar Valley 10,000
Pantano 200 USC sub-basin 1 15,000
Total Underflow 8,800 Total 25,000 18,800 1
Total Inflows 72,600 Total Inflows 34,500 92,100
Outflows
Pumpage 49,600 Pumpage 12,000 61,600
Evapotranspiration 8,000 Evapotranspiration 0 8,000
Underflow to Avra Valley 1 15,000 Underflow 22,500 22,500
Total Outflows 72,600 Total Outflows 34,500 92,100
In – Out 0 In - Out 0 0
1. Underflow from the USC sub-basin to the Avra Valley sub-basin is internal to the study area and is not included in the AMA totals
calculation.
Groundwater in Storage
Estimates of groundwater in storage for the Tucson AMA regional aquifer during predevelopment times
vary depending on assumptions regarding depth to bedrock and aquifer specific yield values. Groundwater
storage estimates range from about 68 million ac-ft to about 76 million ac-ft. ADWR (1999a) estimated
that total groundwater storage to a depth of 1,200 feet below land surface during predevelopment was about
70 million ac-ft. Groundwater in storage in the USC sub-basin to 1,000 feet below land surface during pre-development
time was estimated at about 52 million ac-ft (Davidson, 1973; Hanson and Benedict, 1994).
There are no published estimates of groundwater in storage for the Altar Valley section of the Avra Valley
sub-basin due to a lack of data. However, estimates of groundwater in storage to a depth of 1,000 feet in
the Avra Valley section of the sub-basin range from about 16.5 to 24 million ac-ft (White and others, 1966;
Hanson and others, 1990).
Regional Groundwater Flow Model of the Tucson AMA, Simulation and Application.
24
Chapter 3
Groundwater Development Period: 1941 – 1999
The period from 1941 to 1999 was selected as the groundwater development period for this modeling
study. During this period increasing groundwater demands far in excess of natural recharge put the
regional aquifer into an overdraft condition. Annual estimated pumpage in the Tucson AMA rose from
about 60,000 ac-ft in 1941 to about 490,000 ac-ft in the mid-1970s at (Figure 9). Since the mid-1970s,
annual groundwater withdrawals have generally declined to approximately 265,000 ac-ft in 1999.
Groundwater development in the Tucson AMA has altered the predevelopment flow system. Figure 10
shows the Tucson AMA groundwater level map for 1999. Municipal withdrawals from the City of
Tucson’s central well field, located in T 14 S, R 14 E, have created a large cone of depression in the central
part of the USC sub-basin under central Tucson (Figure 10). A smaller elongated cone of depression has
formed in the Sahuarita-Green Valley area due to agricultural, industrial, and municipal withdrawals
(Figure 10). Heavy agricultural withdrawals in the northern part of the Avra Valley sub-basin between
Marana and the Tucson AMA - Pinal AMA boundary have created widespread water level declines and
decreased the groundwater flow gradient (Figure 10).
0
50
100
150
200
250
300
350
400
450
500
1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995
Year
Pumpage (thousands of acre-feet)
Agricultural Municipal Industral
Estimated Pumpage Reported Pumpage
1984
(source: Anning and Duet, 1994; ADWR Registry of Groundwater Rights)
Figure 9. Estimated and reported pumpage in the Tucson AMA: 1940 – 1999.
Initially most groundwater in the Tucson AMA was used for irrigation, but by the mid-1970s, irrigation
withdrawals began declining as farms were retired for their water rights and municipal and industrial
demands increased to meet population growth. By the mid-1980s, agriculture and municipal water use
were about equal, with each accounting for about 40 percent of the total groundwater withdrawn. Industrial
Regional Groundwater Flow Model of the Tucson AMA, Simulation and Application.
25
26
Regional Groundwater Flow Model of the Tucson AMA, Simulation and Application.
T o r t o l i t a M t s
S a n t a Ca t a l i n a M t s
S a n t a R i t a
M t s
S i e r r i t a M t s
T u c s o n M t s
u i s M t s
S i l v e r B e l l M t s
Wash
River
Santa
Cruz
Brawley
Canada
del
Oro
Rillito Creek
Pantano
Wash
Verde
Tanque
Creek
Wash
Altar
Sabino
Creek
Rincon
Creek
^
^
TTuucc ssoonn
MMaa rraannaa
OOrroo VVaall ll ee yy
SSaahhuuaa rr ii tt aa
2450
2500
2400
2200
2700
1700
2800
2900
1800
2000
2000
2100
2100
3000
3000
1650
2300
2800
1900
T16S
T14S
T18S
T20S
T12S
R10E R12E R14E R16E
?Ä
Iz
Aû
?Ï
!"d$
Iz
?Ë
!"a$
!"a$
B l a c k
M t n
G r e e n
V a l l e y
T h r e e
P o i n t s
San Xavier Indian Reservation
¨
10/01/05, \\adwrgis250s\models, p:\tucson\projects\loubotta\report\fig10sue100105.mxd
Figure 10.
Map showing groundwater levels in fall 1999 -
spring 2000 and locations of 1999 - 2000 water
level data, Tucson AMA, Arizona
0 2.5 5 10
Miles
Source(s): ADWR, Groundwater Site Inventory.
1999 - 2000 Water Level Location
^ Town
1999 - 2000 Measured Water Level - ft. above MSL
Road
Road
Stream
San Xavier Indian Reservation
City Boundary
Township & Range
Active Model Boundary
Study Area Boundary
Tucson AMA Boundary
Direction of groundwater flow
contour interval = 50 ft
Chapter 3
use made up the remaining 20 percent (ADWR, 2000). By the late 1990s, municipal use surpassed
agricultural use and accounted for 50 percent of all groundwater withdrawals. Agricultural use had
declined to only about 30 percent of total withdrawals with industrial remaining at 20 percent of total
pumpage (ADWR, 2000).
Inflows
Some components of inflow into the Tucson AMA regional aquifer have changed and new recharge
components have been added as a result of groundwater development in the regional aquifer. Water level
declines under streams and rivers have created a deeper vadose zone and increased potential storage for
stream flow infiltration. Groundwater withdrawals have changed water level gradients across inflow and
outflow boundaries, either increasing or decreasing underflow volumes into or out of the Tucson AMA.
New sources of recharge have been created by groundwater development. Incidental recharge from deep
percolation of excess agricultural irrigation, infiltration of effluent released into the channel of the Santa
Cruz River, and seepage from mine tailing ponds exceeded natural recharge by the mid-1960s. These new
recharge sources will continue to be major sources of recharge in the Tucson AMA in the future. Artificial
recharge projects that are on-line or are in the planning and permitting stages that will utilize CAP water
and effluent will increase the importance of incidental recharge in the future. Sources of inflow to the
Tucson AMA regional aquifer for the groundwater development period are discussed below.
Natural Recharge
Steady-state mountain-front recharge estimates represent long-term average annual recharge from
precipitation in the mountains and is the only natural recharge component that is assumed to have not
changed significantly during the developed period. Stream infiltration and underflow into the study area
have changed during the post-development period.
Stream Infiltration
The long-term annual stream infiltration distribution developed for the steady-state period is representative
for the period from 1941 to 1958. Average stream infiltration values are believed to have increased after
1959, along Rillito Creek and for the Santa Cruz River north of its confluence with the Rillito Creek.
Hanson and Benedict (1994) increased stream infiltration values in their model after 1959 based on an
analysis of stream flow by Webb and Betancourt (1990), and recharge investigations by Gallaher (1979)
and Keith (1981). Gallaher (1979) studied stable isotopes from groundwater in the Tucson basin and
determined that winter storms contribute more recharge to the regional aquifer than summer storms. Webb
and Betancourt (1990) determined that there was a change in the dominant regional storm-types from
summer monsoonal storms to fall-winter cyclonic storms after 1959. Their analysis also suggests an
increase in winter precipitation and runoff since 1959. Webb and Betancourt’s (1990) work is in agreement
with the work of Keith (1981), which also indicated an increase in winter stream flows since 1960, and that
more recharge occurs in the winter along drainages that originate in the mountains than in the summer. To
maintain consistency between the Hanson and Benedict (1984) and the ADWR update, ADWR stream
infiltration values were increased for areas along the Santa Cruz River, Rillito Creek, and Tanque Verde
Creek for 1958 to 1999.
Groundwater Underflow
Groundwater underflow across the Santa Cruz AMA – Tucson AMA boundary has changed due to water
level fluctuations in the southern part of the Tucson AMA and the northern part of the Santa Cruz AMA.
Water level declines and recoveries in the boundary area have altered the predevelopment water table
gradient and affected the groundwater flux across the study’s southern boundary. Water levels in the
southern part of Avra Valley sub-basin along the study area boundary have changed little since
predevelopment times so there probably has been no significant change in underflow across that boundary.
The potential change in groundwater underflow along the Santa Cruz AMA – Tucson AMA boundary and
Regional Groundwater Flow Model of the Tucson AMA, Simulation and Application.
27
Chapter 3
how the groundwater flow model simulates those changes during the developed period are discussed in
Chapter 4.
Incidental and Artificial Recharge
For the purposes of this report, incidental recharge is defined as water that recharges the regional aquifer
during the course of its use for agricultural, industrial, or municipal purposes. This includes water that is
recharged as a result of irrigation activities, wastewater effluent that is released into the Santa Cruz River or
used to irrigate crops and turf facilities, and water infiltrating from mine tailings ponds. Artificial recharge
is defined as water that is recharged to the regional aquifer by direct, managed, or in lieu recharge projects
permitted by the ADWR.
Agricultural Recharge
Water applied to crops that is not utilized by the plant for consumptive use, lost to evaporation, or held by
the soil, percolates below the plant root zone and is termed agricultural recharge. Through deep percolation
the excess water eventually reaches the water table and recharges the regional aquifer. For the Tucson
AMA model, the maximum potential agricultural recharge was estimated to be equal to the average annual
irrigation inefficiency (1 minus the average irrigation efficiency) multiplied by the total annual water
applied for irrigation (Corell and Corkhill, 1994). The total annual water applied to agricultural crops or
turf facilities (parks and golf courses) includes pumped groundwater, CAP surface water, and effluent.
The estimated average irrigation efficiency of the Tucson AMA has ranged from a low of 65 percent to a
high of 75 percent during the developed period. Irrigation efficiencies were estimated to be only 65 percent
during the early part of groundwater development in the 1940s through 1960s. Low efficiency values were
due to a number of factors; which include poor field preparation, over application of water, and poor water
conservation practices. Irrigation efficiencies improved in the 1970’s and 1980’s with the advent of laser
leveling of fields, the implementation of better water management and farming practices, and the economic
pressure of rising pumping costs. The estimated annual maximum potential agricultural recharge available
for the developed period is presented in Figure 11.
0
2 0 ,00 0
4 0 ,00 0
6 0 ,00 0
8 0 ,00 0
1 0 0 ,00 0
1 2 0 ,00 0
1941 1 945 1 949 1 953 1 957 19 61 1965 1969 1973 1977 1 981 1 985 1 989 19 93 1997
Y ear
Recharge in Acre-Feet
U pper S an ta C ruz M ax im um A g R echarge A vra V alley M aximum A g R echarge
Figure 11. Maximum potential agricultural recharge in Tucson AMA, 1940 - 1999.
Regional Groundwater Flow Model of the Tucson AMA, Simulation and Application.
28
Chapter 3
Agricultural recharge became a major source of recharge to the regional aquifer by the early 1950s. The
estimated annual irrigation recharge has ranged from 60,000 to 80,000 ac-ft during the 1950s and 1960s,
peaking in the late 1970’s at over 90,000 ac-ft. Irrigation recharge decreased in the 1980’s due to increased
irrigation efficiency and decreasing agricultural production brought on by increasing agricultural
production costs and urbanization of former farmlands. Since the early 1980s, the volume of annual
agricultural recharge has been estimated at between 20,000 and 40,000 ac-ft.
Artificial Recharge
Prior to 1993, pumped groundwater was the main source of water in Tucson AMA. CAP surface water was
introduced in 1993 and is being utilized in several ways. The largest amount of CAP water is applied for
either agricultural irrigation, or as artificial recharge. A small amount is directly used by the industrial
sector. Figure 12 shows the annual volume of CAP surface water being utilized in the Tucson AMA. The
CAP water is applied for irrigation either directly, in which case no future water credits are earned, or as in
lieu water. In lieu water use is managed through the ADWR Groundwater Saving Facility (GSF) program.
The GSF program allows agricultural customers to apply CAP water in lieu of pumping groundwater, for
which they receive recharge credits that can be withdrawn at a future time. CAP water is also directly
recharged into the aquifer at artificial recharge projects called Underground Storage Facilities (USFs).
USFs recharge and store water that will be recovered in the future as the need arises.
0
10,000
20,000
30,000
40,000
50,000
60,000
1993 1994 1995 1996 1997 1998 1999
Year
Acre-Feet per Year
Underground Storage Facilities
Groundwater Savings Facilities
Source: ADWR Registry of Groundwater Rights
Figure 12. Annual CAP water use in Tucson AMA, 1993 - 1999.
Effluent Recharge
Effluent from wastewater treatment plants has been used for irrigation in the Tucson area since the early
1900s (Schladweiler, 2001). From 1917 to 1950, effluent, including raw sewage, was used to irrigate
various city farmlands located within or near the city boundaries (Schladweiler, 2001). Effluent releases
into the Santa Cruz River began in 1951 from the then just completed Roger Road Waste Water Treatment
Plant (WWTP) (Figure 1). Between 1951 and 1956 effluent was diverted both to the river and to farms
(Esposito and Thurnbald, 1981; Pima Association of Governments, 1983). From 1956 to 1969 most
effluent produced by the Rogers Road WWTP was delivered to farms and little if any, was released directly
to the riverbed. However, the farms redirected unused effluent back to the river when they could not utilize
it, so there was an unknown amount of effluent recharge occurring through the riverbed during this time
Regional Groundwater Flow Model of the Tucson AMA, Simulation and Application.
29
Chapter 3
(Esposito and Thurnbald, 1981; Pima Association of Governments, 1983). Due to water quality concerns,
direct use of effluent for irrigation was discontinued in 1969, and the Rogers Road WWTP effluent was
discharged into the Santa Cruz River.
In 1977, the Ina Road Water Pollution Control Facility (WPCF) became operational and began releasing
effluent into the Santa Cruz River (Figure 1). Also in 1977, the Cortaro-Marana Irrigation District (CMID)
began receiving secondary treated effluent for irrigation (Bookman-Edmonston, 1978). CMID has
continued to receive effluent under contract from the Pima County Wastewater Management Department
(PCWMD). The PCWMD and the City of Tucson have also developed a reclaimed water distribution
system that supplies effluent to some turf facilities (parks, golf courses, and cemeteries) within Tucson
AMA.
Data on effluent releases into the Santa Cruz River bed was provided by PCWMD from 1978 to 1999
(Glenn Petersen, Pima County, personal communications, 2002). Release data from 1950 to 1978 was
developed from water quality studies done by the Pima Association of Governments (PAG) (Esposito and
Thurnbald, 1981; Pima Association of Governments, 1983). Effluent releases have increased from an
initial level of about 800 ac-ft/yr in 1951 to over 50,000 ac-ft/yr in the late 1990s. The releases have
increased the amount of stream infiltration recharged into the regional aquifer and have created a relatively
consistent surface water outflow component out of the Tucson AMA during the winter months. Figure 13
shows the measured and estimated effluent releases from 1951 to 2000 and represents the maximum
potential recharge available due to effluent releases.
0
1 0 ,0 0 0
2 0 ,0 0 0
3 0 ,0 0 0
4 0 ,0 0 0
5 0 ,0 0 0
6 0 ,0 0 0
1 9 5 0 1 9 5 5 1 9 6 0 1 9 6 5 1 9 7 0 1 9 7 5 1 9 8 0 1 9 8 5 1 9 9 0 1 9 9 5 2 0 0 0
Y ea r
Releases, in Acre-Feet
Sources: (Esposito and Thurnbald, 1981; PAG, 1983; Glenn Petersen, Pima County Wastewater Management Department personal
communications, 2002)
Figure 13. Estimated and reported effluent releases into the Santa Cruz River 1950 – 2000
Regional Groundwater Flow Model of the Tucson AMA, Simulation and Application.
30
Chapter 3
Mine Tailings Pond Recharge
Copper mining began in the late-1950s, along the eastern flanks of the Sierrita Mountains. Large volumes
of water are used in the mining and milling the copper ore. Some water is returned to the aquifer through
seepage from tailing ponds. For the period 1952 to 1984, estimates of mine withdrawals and incidental
recharge from tailings ponds developed by Traverse and Mock (1984) and Hanson and Benedict (1994)
were used as initial estimates in this study. Well specific pumpage reported to the ADWR ROGR system
was used to develop withdrawal and recharge volumes for the period 1984 to 1999.
Outflows
The major sources of outflow from the Tucson AMA regional aquifer during the developed period were
pumpage, groundwater underflow, and evapotranspiration. Between 1940 and the mid-1970s, annual
groundwater pumpage in the Tucson AMA increased from about 60,000 ac-ft/yr to over 470,000 ac-ft/yr
(Figure 9). The annual pumpage volume has far exceeded annual recharge since the mid-1940s, even
accounting for increased incidental recharge from irrigation, mining activities and effluent releases.
Groundwater underflow and evapotranspiration by riparian vegetation along the Santa Cruz River and its
major tributaries have been affected by the large overdrafts during the development period (1941 – 1999).
Groundwater flux leaving the Tucson AMA and evapotranspiration have both decreased from
predevelopment levels due to water level declines related to the long-term overdraft of the aquifer.
Pumpage
The distribution and amount of annual pumpage prior to the mid-to-late 1960s is not well known. During
the period of 1941 to 1960 few detailed records exist regarding the distribution and volume of individual
well pumpage. Previous investigators (Anderson, 1972) estimated annual pumpage from 1940 to the early
1960s using power consumption records and crop distribution surveys. Beginning in the 1960s, more water
users began keeping detailed withdrawal records for individual wells, so more is known about the amount
and distribution of pumpage. However, there is some uncertainty in the pumpage estimates developed
during this time period, which are still largely based on energy consumption and crop consumptive use
data, rather than metered water usage.
Starting in 1984, the location and amount of pumpage for high-capacity wells in the Tucson AMA has been
available. The GMA requires all non-exempt well owners to report well-specific annual pumpage to the
ADWR. A brief discussion of historical pumpage during the development period (1941 – 1999) for each
sub-basin is presented below.
Avra Valley sub-basin
Historically, about 95 percent of groundwater withdrawals have been used for agricultural irrigation in the
Avra Valley sub-basin with the remaining 5 percent used by the municipal and industrial sectors. The
dominance of irrigation use has changed in the last 20 to 30 years. During and following Word War II farm
acreage increased dramatically and by the early to mid-1950s agricultural development reached a maximum
with about 30,000 acres in production (White and others, 1965). The number of wells drilled to supply the
increasing water demand also increased so that by 1954, more than 100 irrigation wells were pumping
groundwater in the sub-basin (White and others, 1965). Although farm acreage peaked in the 1950s,
groundwater withdrawals continued to increase until the mid-1970s due to water application practices and
cropping schedules such as double cropping. Figure 14 shows the estimated and reported pumpage for the
Avra Valley sub-basin from 1941 to 1999.
Regional Groundwater Flow Model of the Tucson AMA, Simulation and Application.
31
Chapter 3
0
50
100
150
200
250
1941 1946 1951 1956 1961 1966 1971 1976 1981 1986 1991 1996
Year
Volume (thousands acre-feet)
Agricultural Municipal Industral
Figure 14. Estimated and reported pumpage in the Avra Valley sub-basin of the Tucson AMA, Arizona,
1941 - 1999
Annual groundwater withdrawals in the sub-basin have declined from a high of about 230,000 ac-ft in
1976, and have averaged about 55,000 ac-ft/yr since 1985. Groundwater withdrawals have declined
significantly since the mid-1970s due to several factors. In the early 1970s the City of Tucson began
purchasing and retiring farmland in Avra Valley, preserving the groundwater for future municipal use to
meet its growing demand. A weakened farm economy and the urbanization of agricultural lands around the
town of Marana have also contributed to a shift of water use from the agricultural sector to the municipal
and industrial sectors in the sub-basin. In 1999, agriculture, industrial, and municipal use accounted for 48
percent, 47 percent , and 5 percent of water withdrawals in the Avra Valley sub-basin, respectively
(ADWR, 2000).
Upper Santa Cruz sub-basin
Agricultural pumpage accounted for 80 to 90 percent of the total pumpage in the USC sub-basin until the
mid-1950s (Figure 15). Since the mid-1950’s the percentage of municipal and industrial pumpage has
increased and the percentage of agricultural pumpage has decreased. The decline in agricultural
withdrawals in the USC sub-basin reflects the shift in water use from farming to supplying municipal and
industrial water to the growing population of the Tucson area.
Groundwater withdrawals in the USC sub-basin tripled from about 50,000 ac-ft/yr to over 170,000 ac-ft/yr
from 1941 to the mid-1950s. Annual groundwater pumpage generally increased from the mid-1950s to the
mid-1970s, peaking in 1976 at over 270,000 ac-ft/yr. Since 1976, groundwater withdrawals have generally
Regional Groundwater Flow Model of the Tucson AMA, Simulation and Application.
32
Chapter 3
declined, but from 1985 to the present have averaged just over 200,000 ac-ft/yr. Water use by sector for
1999 in the USC sub-basin was municipal 58 percent, agriculture 14 percent, and industrial 28 percent.
0
50
100
150
200
250
300
1941 1946 1951 1956 1961 1966 1971 1976 1981 1986 1991 1996
Year
Volume (thousands acre-feet)
Agricultural Municipal Industral
Figure 15. Estimated and reported groundwater pumpage in Upper Santa Cruz sub-basin, Tucson AMA,
Arizona, 1941 - 1999.
Groundwater Underflow
Wide-spread water level declines in the northern part of the Avra Valley sub-basin from the early 1940s to
the mid-1970s ranged from 50 to as much as 200 feet in some areas. These declines have reduced water
level gradients and saturated thicknesses in the regional aquifer in the northern part of the sub-basin at the
Tucson AMA – Pinal AMA boundary. The decrease in saturated thickness and gradient near the boundary
reduced groundwater underflow leaving the Tucson AMA. Flow net analysis using historic water level
data was used to estimate how much groundwater underflow may have been reduced. The flow net
analysis indicated that groundwater underflow leaving the Tucson AMA may have been reduced by as
much as 10,000 ac-ft/yr from the predevelopment (steady-state) flux of 22,500 ac-ft/yr.
Evapotranspiration
Evapotranspiration (ET) from riparian vegetation has generally declined during the developed period as
water levels have dropped along the Santa Cruz River and its tributaries. Hanson and Benedict (1994)
Regional Groundwater Flow Model of the Tucson AMA, Simulation and Application.
33
Chapter 3
simulated a decrease in ET in the Tucson basin of about 5,500 ac-ft/yr, from a steady-state (1940) volume
of 7,850 ac-ft/yr to 2,400 ac-ft/yr by 1986.
Transient Water Level Conditions
Water level declines during the groundwater development period (1941 – 1999) have had a large impact on
the Tucson AMA r