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Simulated Water-Level Responses, 

Ground-Water Fluxes, and Storage 

Changes for Recharge Scenarios along 

Rillito Creek, Tucson, Arizona 

By John P. Hoffmann and S.A. Leake 

Prepared in cooperation with the ARIZONA DEPARTMENT OF WATER RESOURCES and 

the U.S. GEOLOGICAL SURVEY GROUND-WATER RESOURCES PROGRAM 

Scientific Investigations Report 2004–5286 

U.S. Department of the Interior 

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

Gale A. Norton, Secretary 

U.S. Geological Survey 

Charles G. Groat, Director 

U.S. Geological Survey, Reston, Virginia: 2005 

For sale by U.S. Geological Survey, Information Services


Box 25286, Denver Federal Center


Denver, CO 80225


Prepared by the Arizona Water Science Center, Tucson


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Suggested citation:


Hoffmann, J.P., and Leake, S.A., 2005, Simulated water-level responses, ground-water fluxes, and storage changes


for recharge scenarios along Rillito Creek, Tucson, Arizona: U.S. Geological Survey Scientific Investigations 


Report 2004–5286, 29 p.

iii 

Contents 

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


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


Purpose and scope...............................................................................................................................2


Previous investigations........................................................................................................................2


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


Physiographic setting....................................................................................................................................2


Hydrogeologic setting...................................................................................................................................4


Methods...........................................................................................................................................................4


Summary of regional ground-water flow model.......................................................................................6


Construction of local ground-water flow model.......................................................................................7


Discretization.........................................................................................................................................7


Boundary conditions ............................................................................................................................7


Simulation of recharge ........................................................................................................................8


Comparisons of water levels from local and regional models ............................................................10


Steady state.........................................................................................................................................10


Transient...............................................................................................................................................12


Simulated water levels, ground-water fluxes, storage changes, 


and evapotranspiration ................................................................................................................12


Prior to simulated recharge scenarios, 1941–2004 

Scenario A—Continued recharge at forecasted natural recharge 


.......................................................................14


Water levels................................................................................................................................14


Ground-water flux......................................................................................................................14


Storage ........................................................................................................................................16


Evapotranspiration ....................................................................................................................16


rates, 2005–2225.....................................................................................................................17


Water levels................................................................................................................................17


Ground-water flux......................................................................................................................17


Storage ........................................................................................................................................19


Evapotranspiration ....................................................................................................................19


Scenarios B, C, and D—Increased recharge rates, 2005–2225..................................................19


Water levels................................................................................................................................19


Ground-water flux......................................................................................................................25


Storage ........................................................................................................................................25


Evapotranspiration ....................................................................................................................25


Limitations of the local flow model ...........................................................................................................26


Summary and conclusions.........................................................................................................................27


References....................................................................................................................................................28

iv 

Figures 

1.	 Map showing areas of regional and local ground-water flow models, Arizona ...........3


2.	 Map showing area of local model, location of line segments, selected 


cells used for hydrographs, and the approximate water-level drawdown 


in the vicinity of the Central Well Field, Tucson, Arizona, between 


1945 and 1995 ............................................................................................................................5


3.	 Graphs showing difference in the distribution of recharge applied to the 


steady-state local model between the Recharge and Stream Packages, 


Rillito Creek, Arizona................................................................................................................8


4.	 Map showing simulated water-level altitude in layer 2 of the local model 

using the Recharge and Stream Packages along Rillito Creek, 

Tucson, Arizona. 

	 A. Water-level altitude for steady-state simulations, 1940 ...........................................9


B.	 Water-level altitude for transient simulations, 1999..................................................9


5.	 Graph showing recharge to model cells coinciding with Rillito Creek, 


Tucson, Arizona, 1941–2025..................................................................................................10


6.	 Map showing simulated water-level altitudes in layer 2 for regional and 

local models using the Stream Package along Rillito Creek, Tucson, Arizona. 

	 A. Steady-state simulation, 1940 .....................................................................................11


B.	 Transient simulation, 1999............................................................................................11


7.	 Hydrographs for selected local-model cells near Rillito Creek and the 


Central Well Field for model layers 1, 2, and 3, Tucson, Arizona, 1941–99....................12


8.	 Hydrograph for selected cells 28,38, layer 2, in the local and regional 


models, 1941–99, Rillito Creek, Arizona ..............................................................................13


9.	 Graph showing amount of streambed recharge applied to the local 


model using the Stream Package along Rillito Creek, Arizona, and 


pumpage for each scenario .................................................................................................13


10.	 Simulated hydrographs for selected cells in the local model, 


Rillito Creek, Arizona, 1941–2004 .........................................................................................14


11–14. Graphs showing:


11.	 Simulated annual ground-water flux past line segment 1 in the local 


model, Rillito Creek, Arizona, 1941–2225....................................................................15


12.	 Simulated annual ground-water flux past line segment 2 in the local 


model, Rillito Creek, Arizona, 1941–2225....................................................................15


13.	 Simulated annual change in ground-water storage in the local model, 


Rillito Creek, Arizona, 1941–2225.................................................................................16


14.	 Simulated annual loss of ground water due to evapotranspiration in 


the local model, Rillito Creek, Arizona, 1941–2225 ...................................................16


15.	 Hydrographs for selected cells in the local model, Rillito Creek, 


Arizona, 2005–2225.................................................................................................................17


16.	 Maps showing water-level altitude for scenario A in the local model for 


selected times, Rillito Creek, Arizona. 


A.	 2004 ................................................................................................................................18


B.	 2015 ................................................................................................................................18


C.	 2025 ................................................................................................................................18


D.	 2050 ................................................................................................................................18


E.	 2100 ................................................................................................................................18


F.	 2225 ................................................................................................................................18

v 

Figures—Continued 

17.	 Graphs showing water-level altitude for each scenario for cells in the 


local model, 1940–2225, Rillito Creek, Arizona. 


A.	 Scenario B .....................................................................................................................20


B.	 Scenario C .....................................................................................................................20


C.	 Scenario D .....................................................................................................................20


18.	 Graphs showing rise in water level for each scenario relative to scenario A 

for selected cells in the local model, 2000–2225, Rillito Creek, Arizona. 

A.	 Scenario B .....................................................................................................................21


B.	 Scenario C .....................................................................................................................21


C. Scenario D .....................................................................................................................21


19–21. Maps showing:


19.	 Rises in water level for scenario B relative to scenario A within 

the local model for selected times, Rillito Creek, Arizona. 

A.	 2015 ........................................................................................................................22


B.	 2025 ........................................................................................................................22


C.	 2050 ........................................................................................................................22


D.	 2100 ........................................................................................................................22


E.	 2150 ........................................................................................................................22


F.	 2225 ........................................................................................................................22


20.	 Rises in water level for scenario C relative to scenario A within 

the local model for selected times, Rillito Creek, Arizona. 

A.	 2015 ........................................................................................................................23


B.	 2025 ........................................................................................................................23


C.	 2050 ........................................................................................................................23


D.	 2100 ........................................................................................................................23


E.	 2150 ........................................................................................................................23


F.	 2225 ........................................................................................................................23


21.	 Rises in water level for scenario D relative to scenario A within 

the local model for selected times, Rillito Creek, Arizona. 

A.	 2015 ........................................................................................................................24


B.	 2025 ........................................................................................................................24


C.	 2050 ........................................................................................................................24


D.	 2100 ........................................................................................................................24


E.	 2150 ........................................................................................................................24


F.	 2225 ........................................................................................................................24

vi 

Tables 

1.	 Water-budget components in the Arizona Department of Water 

Resources’s calibrated regional ground-water flow model of 

the Tucson Active Management Area for steady-state 

simulations in 1940 ...................................................................................................................6 

2.	 Evapotranspiration within local ground-water flow model for various 

scenarios, Rillito Creek, Arizona..........................................................................................26 

Conversion Factors and Datum 

Multiply By To obtain 

Length 

foot (ft) 0.3048 meter (m) 

mile (mi) 1.609 kilometer (km) 

Area 

square mile (mi2) 2.59 square kilometers (km2) 

acre-foot (acre-ft) 1,233 cubic meter (m3) 

Flow rate 

acre-foot per year (acre-ft/yr) 1,233 cubic meter per year (m3/yr) 

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s) 

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows: 

°C=(°F-32)/1.8 

Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 

(NGVD of 1929); horizontal coordinate information is referenced to the North American Datum of 

1927 (NAD27). Altitude, as used in this report, refers to distance above or below NGVD of 1929. 
Simulated Water-Level Responses, Ground-Water Fluxes, 

and Storage Changes for Recharge Scenarios along 

Rillito Creek, Tucson, Arizona 

By John P. Hoffmann and S.A. Leake 

Abstract 

A local ground-water flow model is used to simulate four recharge scenarios along Rillito Creek in northern Tucson to 

evaluate mitigating effects on ground-water deficits and water-level declines in Tucson’s Central Well Field. The local model, 

which derives boundary conditions from a basin-scale model, spans the 12-mile reach of Rillito Creek and extends 9 miles 

south into the Central Well Field. Recharge scenarios along Rillito Creek range from 5,000 to 60,000 acre-feet per year and are 

simulated to begin in 2005 and extend through 2225 to estimate long-term changes in ground-water level, ground-water storage, 

ground-water flux, and evapotranspiration. 

The base case for comparison of simulated water levels and flows, referred to as scenario A, uses a long-term recharge 

rate of 5,000 acre-feet per year to 2225. Scenario B, which increases the recharge along Rillito Creek by 9,500 acre-feet per 

year, has simulated water-level rises beneath Rillito Creek that range from about 53 feet to 86 feet. Water-level rises within the 

Central Well Field range from about 60 feet to 80 feet. More than half of these rises occur by 2050, and more than 95 percent 

occur by 2188. Scenario C, which increases the recharge along Rillito Creek by 16,700 acre-feet per year relative to scenario A, 

has simulated water-level rises beneath Rillito Creek that range from about 71 feet to 102 feet. Water-level rises within the Central Well Field range from about 80 feet to 95 feet. More than half of the rises occur by 2036, and more than 95 percent occur by 

2100. Scenario D, which initially increases the recharge rate by about 55,000 acre-feet per year relative to scenario A, resulted 

in simulated water levels that rise to land surface along Rillito Creek. This rise in water level resulted in rejected recharge. As 

the water table continued to rise, the area of stream-channel surface intersected by the water table increased causing continual 

decline in the recharge rate until a long-term recharge rate of about 34,000 acre-feet per year was sustained. The long-term 

recharge rate for scenario D is about 29,000 acre-feet per year greater than the long-term recharge rate for scenario A. Simulated 

long-term water-level rises beneath Rillito Creek range from about 97 feet to 131 feet, resulting in water levels near or at the 

land surface. Shallow depths to water associated with this scenario have implications for contamination owing to the presence of 

landfills within or adjacent to Rillito Creek. Water-level rises for cells within the Central Well Field range from about 96 feet to 

109 feet. More than half of the water-level rises occur by 2018 and more than 95 percent occur by 2041. 

Almost all the increased water added to the ground-water system in the recharge scenarios can be accounted for by a 

combination of increased storage near Rillito Creek, ground-water flux to the south, ground-water flux to the northwest, and 

increased discharge as evapotranspiration along Rillito Creek. The percentage of newly added water accounted for by storage 

changes is large relative to the percentage accounted for by changes in flux and evapotranspiration at the onset of each scenario; 

however, the changes in storage become smaller throughout the simulation, and the long-term component accounted for by storage is minimal. Long-term ground-water fluxes to the south increase by about 3,300, 4,840, and 7,500 acre-feet per year for scenarios B, C, and D, respectively. The percentage of increased recharge that flows south toward the Central Well Field, therefore, 

is 35, 29, and 26 percent for scenarios B, C, and D, respectively. Long-term ground-water fluxes to the northwest increase by 

about 3,100, 3,900, and 6,980 acre-feet per year for scenarios B, C, and D, respectively. The long-term percentage of increased 

recharge flowing northwestward is about 31, 25, and 21 percent for scenarios B, C, and D, respectively. Shallow ground-water 

evapotranspiration along Rillito Creek increases by about 310, 4,100, and 12,000 acre-feet per year for scenarios B, C, and D, 

respectively. The losses owing to evapotranspiration account for about 3, 25, and 41 percent of the added recharge for scenarios 

B, C, and D, respectively. 
 Simulated Water-Level Responses, Ground-Water Fluxes, and Storage Changes for Recharge Scenarios, Rillito Creek 

Introduction 

The city of Tucson and surrounding areas (fig. 1) obtain 

most of their municipal, agricultural, and industrial water 

from ground water that is withdrawn from thick alluvial 

aquifers underlying the desert basins. A large fraction of 

ground water stored in these aquifers was recharged by water 

that percolated through ephemeral stream-channel deposits 

(Matlock and Davis, 1972; Davidson, 1973; Hanson and 

Benedict, 1994). The amount of water that recharges the 

aquifers is insufficient to meet current and expected future 

demands. The projected resultant ground-water deficit, which 

is expected to grow as the population increases, is manifested 

in water-level declines of more than 200 ft since the middle 

of the twentieth century. These projected declines are largest 

in the Central Well Field where ground-water withdrawals 

are greatest. To help mitigate the deficit and associated 

water-level declines, an in-stream recharge facility has been 

proposed for the Rillito Creek channel. Rillito Creek is an 

ephemeral stream in the northern part of Tucson. A possible 

source of water for artificial recharge is Colorado River water 

that is transported from Lake Havasu and delivered to the 

Tucson area through the Central Arizona Project (CAP) canal. 

In 1999, the U.S. Geological Survey (USGS), in cooperation 

with the Arizona Department of Water Resources (ADWR), 

began an investigation to evaluate the physical characteristics 

of the shallow subsurface sediments beneath Rillito Creek 

that can influence recharge processes. Information from this 

investigation, combined with ADWR’s calibrated regional 

ground-water flow model of the Tucson Active Management 

Area (TAMA), which includes the Upper Santa Cruz Basin 

and Avra Valley (Dale Mason, hydrologist, Arizona 

Department of Water Resources, written commun., 2003), 

is used to simulate various recharge scenarios along Rillito 

Creek. These simulations are used to evaluate mitigating 

effects on ground-water deficits and water-level declines for 

various recharge scenarios. 

Purpose and Scope 

The purpose of this report is to describe numerical 

simulations used to estimate possible responses of groundwater level, ground-water flux, and selected water-budget 

terms to four recharge scenarios along Rillito Creek. Recharge 

scenarios along Rillito Creek were developed on the basis 

of consultation between the USGS and the ADWR and are 

simulated to begin in 2005 by using ADWR’s calibrated 

model input data sets that extend to 2025. The predictive 

simulations are based on ADWR’s estimate of groundwater withdrawal amounts and locations, and assume a 

constant long-term natural recharge rate along Rillito Creek 

(Dale Mason, hydrologist, Arizona Department of Water 

Resources, written commun., 2003). To estimate the longterm responses of water level, ground-water flux, and 

selected water-budget components, four recharge scenarios 

are simulated by extending the 2025 input data sets to 2225. 

The responses for three scenarios are compared to a base 

case scenario. The numerical model used to simulate the four 

recharge scenarios was created by embedding a local groundwater flow model within the calibrated ADWR regional 

transient ground-water flow model (TAMA model). The local 

model spans the 12-mi reach of Rillito Creek and extends 9 mi 

south into Tucson’s Central Well Field. In this report, waterlevel altitudes for model layer 2 are shown for all comparisons 

because layer 1 becomes unsaturated in parts of the model 

during the transient simulation. 

Previous Investigations 

Investigations conducted within the Rillito Creek area 

include geologic and hydrogeologic analyses and numerical 

ground-water modeling. Pashley (1966), Davidson (1973), 

and Anderson (1987) documented the geology and stratigraphy of the area. Hoffmann and others (2002) characterized 

the shallow deposits beneath Rillito Creek. Burkham (1970), 

Davidson (1973), and Anderson and others (1990) described 

the hydrogeology and water resources of the area. Anderson 

(1972), Hanson and Benedict (1994), and Travers and Mock 

(1984) developed ground-water flow models of the area. 

These models are of regional scale and do not specifically 

focus on effects of varying recharge along Rillito Creek. 

Acknowledgments 

Dale Mason and Lou Bota of ADWR’s Hydrology Section provided input data sets for the TAMA model. Matthew 

Weber and Denise Weiland of the TAMA ADWR office were 

instrumental in providing guidance on recharge-scenario selection and served as liaisons between the ADWR’s Hydrology 

Section and the USGS. 

Physiographic Setting 

Rillito Creek is in the northern part of the Upper 

Santa Cruz Basin and drains an area of about 900 mi2

 (fig. 1). 

The Upper Santa Cruz Basin is a north-south trending alluvial 

basin that is generally bounded by block-faulted mountains. 

The basin contains the Tucson metropolitan area, which is 

the major urban population center in southeastern Arizona. 

The Avra Valley Basin is west of the Upper Santa Cruz 

Basin and is in hydraulic connection with the Upper Santa 

Cruz Basin in the northwestern part of the study area near 

Marana, Arizona. 
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 Simulated Water-Level Responses, Ground-Water Fluxes, and Storage Changes for Recharge Scenarios, Rillito Creek 

The climate of the Upper Santa Cruz Basin is semiarid; 

rainfall ranges from 11 to 16 in. on the valley floor to as much 

as 30 in. in the surrounding mountains. In January, the mean 

daily maximum temperature is 75°F and the mean daily minimum is 50°F. In July, the mean daily maximum temperature is 

105°F and the mean daily minimum is 83°F. 

Hydrogeologic Setting 

The Upper Santa Cruz Basin is in the Basin and Range 

Physiographic Province, which is characterized by blockfaulted mountains separated by basins filled with alluvial 

sediments (Fenneman, 1931). The block-faulted mountains 

comprise Precambrian through Tertiary granitic, metamorphic, 

volcanic, and consolidated sedimentary rock. The sediments 

that fill the basins are collectively termed alluvial basin-fill 

deposits and consist of gravel, sand, silt, clay, and minor 

amounts of anhydrous evaporate sediments of Tertiary to 

Quaternary age. The basin-fill deposits are generally coarse 

grained along the basin margins and grade into finer-grained 

deposits and anhydrous evaporites in the central parts of the 

basins. The basin-fill deposits form regionally extensive sedimentary units that form the regional aquifer system. 

Rillito Creek and much of the Upper Santa Cruz Basin 

are underlain by thick basin-fill deposits that store the groundwater resources for the Tucson metropolitan area. Recharge 

to the basin-fill aquifer is less than the amount of groundwater withdrawal needed to support the growing metropolitan 

population (Water Resources Research Center, 1999). As a 

result of these ground-water withdrawals, water levels have 

declined over the long term, causing land subsidence in some 

areas. Rivers and washes that drain the basin are generally 

ephemeral, only flowing in response to local rainfall. Major 

tributaries to Rillito Creek are Tanque Verde Creek, Pantano 

Wash, and Sabino Creek (fig. 1). 

Previous investigators have divided the basin-fill deposits 

into an upper basin-fill unit and a lower basin-fill unit on the 

basis of general hydrogeologic characteristics (Pool, 1985; 

Hanson and Benedict, 1994). The upper basin-fill unit ranges 

in thickness from several hundred feet to as much as 1,000 ft. 

The unit consists mostly of semiconsolidated to unconsolidated gravel, sands, and clayey silt, and is correlated to the 

upper Tinaja beds and the Fort Lowell Formation described 

by Anderson (1987, 1988). The lower basin-fill unit is several 

thousands of feet thick and consists of conglomerates, gravels, 

sands, silts, anhydritic clayey silts, and mudstones (Anderson, 

1988). The unit is equivalent to the Pantano Formation and the 

lower and middle Tinaja beds described by Anderson (1987, 

1988). Thickness of the basin fill ranges from a veneer along 

the mountain fronts to as much as 11,200 ft in the Upper Santa 

Cruz Basin (Davidson, 1973; Anderson, 1987, 1988; Hanson 

and Benedict, 1994). 

Depth to ground water in the basin-fill deposits beneath 

and surrounding Rillito Creek varies from less than 20 ft in 

the upper reaches of Rillito Creek, to more than 350 ft in the 

Central Well Field south of Rillito Creek (Tucson Water, 2000, 

plate 1; Hoffmann and others, 2002). Depth to ground water 

immediately beneath Rillito Creek generally ranges from 

less than 20 ft in the upper reach to 150 ft in the middle and 

lower reaches (Hoffmann and others, 2002). Flow of ground 

water is generally northwestward, and water-table altitudes 

are about 2,500 ft in the southeastern part of the study area 

and 2,100 ft in the northwestern part. Ground water along 

the upper reach of Rillito Creek locally flows southwestward 

toward the Central Well Field, and ground water along the 

middle and lower reaches locally flows northwestward 

(Tucson Water, 2000, plate 2). 

The mountains surrounding the Upper Santa Cruz Basin 

comprise Precambrian to Tertiary-aged crystalline and sedimentary rocks that generally yield little water and are not 

considered part of the regional aquifer. 

Methods 

A calibrated regional ground-water flow (TAMA) model 

(Dale Mason, hydrologist, Arizona Department of Water 

Resources, written commun., 2003) was used to provide 

boundary conditions for a local flow model centered approximately over Rillito Creek and the Central Well Field. The 

telescoping mesh-refinement program, MODTMR, was 

used to create ground-water flow model input data sets for 

MODFLOW, which is the Modular Three-Dimensional Finite 

Difference Ground-Water Flow Model (MODFLOW96) 

developed by the USGS (McDonald and Harbaugh, 1988; 

McDonald and Harbaugh, 1996). The TAMA model is used 

to define boundary conditions and model parameters for the 

smaller local model (Leake and Claar, 1999). Output from the 

TAMA model was used to construct the data sets that specify 

boundary conditions on the perimeter of the local model. 

The method is used where a detailed model is needed for an 

area that is small relative to the entire aquifer system or the 

regional model. In this case, the local model covers an area of 

108 mi2

 centered approximately over Rillito Creek (fig. 2), and 

the TAMA model has an area of 3,250 mi2

 (fig. 1). 

The purpose of this investigation was to evaluate recharge 

scenarios related to the introduction of CAP water to Rillito 

Creek; therefore, it was necessary to simulate recharge along 

Rillito Creek by using the Stream Package for MODFLOW 

(Prudic, 1989). In the TAMA model, however, recharge 

is applied through the Recharge Package (McDonald and 

Harbaugh, 1988). The TAMA model was adapted for this 

investigation by removing specified recharge and adding the 

Stream Package along cells that coincide with Rillito Creek. 
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Area of local model, location of line segments, selected cells used for hydrographs, and the approximate 

water-level drawdown in the vicinity of the Central Well Field, Tucson, Arizona, between 1945 and 1995.

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Applying recharge by using the Stream Package has advantages over applying recharge by using the Recharge Package 

because recharge in the Stream Package is calculated on the 

basis of head differences between the stream and the aquifer 

and a streambed-conductance term. Using the Stream Package allows a variety of recharge scenarios to be evaluated 

by using different streamflow values along Rillito Creek. 

Streamflow values used in the recharge scenarios range from 

5,000 acre-ft/yr, a long-term average natural recharge rate 

used in the TAMA model for cells underlying Rillito Creek, 

to 60,000 acre-ft/yr, an amount of CAP water potentially 

available for artificial recharge along Rillito Creek. When 

Figure . 

the Stream Package is used, recharge to the aquifer is ceased 

when all streamflow has leaked into the aquifer and the stream 

is dry, or when the water table reaches the land surface and 

additional recharge is rejected. 

The computer code RIVGRID (Leake and Claar, 1999) 

was used to create the MODFLOW input data sets for the 

Stream Package. RIVGRID is a program that uses the coordinates of stream paths and information such as altitude, width, 

and streambed hydraulic conductivity, thickness, and roughness to estimate streambed-conductance terms. Streambed 

altitudes were obtained from orthophotoquads having 2-ft 

contour intervals; streambed widths were estimated on the 
 Simulated Water-Level Responses, Ground-Water Fluxes, and Storage Changes for Recharge Scenarios, Rillito Creek 

basis of field measurements; streambed hydraulic conductivity 

values were varied on the basis of findings of Hoffmann and 

others (2002); and streambed roughness values were varied on 

the basis of literature citations (White, 1979). 

Steady-state and transient simulations were run on the 

TAMA and local models. The computer code TMRDIFF 

(Leake and Claar, 1999) was used to compare simulated waterlevel altitude in the local model with simulated water-level 

altitude in the regional model. Each recharge scenario was 

simulated within the TAMA model to provide flow conditions at the perimeter of the local model. Results from the 

local-model recharge scenarios were used to analyze spatial 

and temporal changes in water level, ground-water flux, and 

selected water-budget terms. 

The local model was first used to predict aquifer response 

up to 2025 for different recharge scenarios that take place 

starting in 2005. Selection of 2005 to begin varying the 

recharge for respective scenarios was arbitrary. The characteristics and boundary conditions for the local model were 

derived from the TAMA model. Hydrologic data from 1940 

to 1999 were used to calibrate the TAMA model; therefore, 

application of the local model for studies of system responses 

resulting from various recharge scenarios can be considered to be predictive up to the year 2025. This study further 

uses the local model to compute responses for an additional 

200 years, ending in 2225. Predictions for this additional 

200-year period probably are not as reliable as the predictions 

for the period prior to 2025 because of the amount of data used 

to calibrate the model. The longer-term analysis, therefore, is 

considered to be an interpretive model application. Interpretive 

models are used to evaluate the possible long-term responses 

on the basis of the most complete information on the system 

currently available. 

Summary of Regional Ground-Water 

Flow Model 

The code used to simulate regional ground-water flow in 

the ADWR’s TAMA model is the Modular Three-Dimensional 

Finite Difference Ground-Water Flow Model (MODFLOW96) 

developed by the USGS (McDonald and Harbaugh, 1988; 

McDonald and Harbaugh, 1996). The TAMA model study 

area is 3,250 mi2 and includes portions of the Upper Santa 

Cruz Basin and Avra Valley (fig. 1). The TAMA model is 

divided into a regular orthogonal grid, consisting of 130 rows 

and 100 columns, that is oriented north-south and is 50 mi east 

to west and 65 mi north to south. Each cell is 0.5 mi on a side. 

The model simulates steady-state conditions in 1940 

and transient conditions from 1941 to 2000. It is assumed 

that there was a balance between inflow to the aquifer and 

outflow from the aquifer before 1940. About 91,000 acre-ft/yr 

flows through the regional ground-water system under the 

steady-state conditions of 1940 on the basis of water-budget 

components for the TAMA model (Dale Mason, hydrologist, 

Arizona Department of Water Resources, written commun., 

2003; table 1). 

Table 1. Water-budget components in the Arizona Department 

of Water Resources’s calibrated regional ground-water flow 

model of the Tucson Active Management Area for steady-state 

simulations in 1940 

Inflow, Outflow, 

Budget component in acre-feet in acre-feet 

per year per year 

Mountain-front recharge 33,670 

Stream infiltration 38,875 

Ground-water underflow 18,485 19,930 

Evapotranspiration 11,550 

Pumpage 59,555 

Total 91,030 91,035 

Although there was ground-water development in the 

early 1900s, the balance between inflows and outflows 

probably was maintained by a loss of evapotranspiration (ET) 

from the riparian areas approximately equal to the amount 

of pumpage. The MODFLOW ET Package (McDonald and 

Harbaugh, 1988) is used to simulate ground-water discharge 

through transpiring phreatophytes or direct evaporation at 

the land surface. It simulates ET by removing ground water 

at specified cells at a specified rate depending on the depth 

to ground water. The ET rate is maximized when the water 

table is at the land surface; the rate is reduced linearly with 

depth and ceases when the water table exceeds a specified 

depth termed the extinction depth. The TAMA model uses 

an extinction depth of 25 ft at cells simulating ET, which 

predominantly occurs along the Santa Cruz River, Rillito 

Creek, and the tributaries of these channels. An ET value of 

11,550 acre-ft/yr is used in the model for the steady-state 

period. The ET rate was determined on the basis of vegetation 

type and density determined from aerial photos taken in 1936 

and 1941, and from model calibration (Hanson and Benedict, 

1994; Dale Mason, hydrologist, Arizona Department of Water 

Resources, written commun., 2003). Results from updated 

TAMA model simulations by the ADWR were provided 

following the completion of this study. The updated model 

has refined cell-centered altitudes based on 10-m digital 

elevation model coverages. These refined altitudes resulted in 

a total steady-state ET rate of 13,820 acre-ft/yr (Dale Mason, 

hydrologist, Arizona Department of Water Resources, written 

commun., 2004). 
The TAMA model simulates transient conditions from 

1941 to 1999 by using 59 stress periods of 365 days each, 

and 12 time steps in each stress period. It simulates threedimensional ground-water flow in the regional aquifer 

system by using three layers. The layers delineate the major 

water-bearing units that are identified on the basis of regional 

hydrogeologic characteristics and stratigraphy. Model layer 

1 is the saturated portion of the stream-channel alluvium and 

the Fort Lowell Formation and is simulated as an unconfined 

water-table aquifer. The Fort Lowell Formation is the most 

productive and heavily utilized water-bearing unit in the 

area. Model layer 2 consists of the upper Tinaja beds, which 

make up the next most productive unit in the modeled area. 

Layer 2 is simulated as a convertible aquifer that can switch 

from confined to unconfined conditions. Layer 3 contains 

all the water-bearing basin-fill deposits below the upper 

Tinaja beds. This layer includes the middle and lower Tinaja 

beds and the Pantano Formation. Layer 3 is simulated as a 

convertible aquifer that has a specified transmissivity. Layer 3 

is the most aerially extensive model layer, but represents the 

least productive water-bearing unit. The tops and bottoms of 

the three model layers are defined on the basis of previous 

geologic investigations, well logs, and earlier ground-water 

model layer definitions (Anderson, 1987, 1988; Hanson and 

Benedict, 1994). 

Boundaries for the TAMA model are along mountain 

fronts and at points of underflow into and out of the study 

area. The mountain fronts are represented as no-flow 

boundaries with specified fluxes representing mountain-front 

recharge assigned to active cells along the mountain fronts. 

During the transient simulation the inflow-outflow boundaries 

are simulated to allow the flux across a boundary to vary over 

time as the head at the boundary changes. Recharge from 

mountain-front, streamflow, and return infiltration, and inflow 

from adjacent basins are simulated by using the MODFLOW 

Recharge Package. Deep percolation of irrigation return flows, 

infiltration of effluent released into the channel of the Santa 

Cruz River, and seepage from mine tailings ponds also are, 

collectively, a large source of recharge within the regional 

model. Return flows are estimated to be a function of annual 

irrigation inefficiency and pumpage. The areal distribution 

of return-flow recharge was developed from agricultural 

pumpage and crop census reports. 

Calibration of the TAMA transient model simulation was 

based on the following criteria: no model residual (difference 

between the simulated and observed water-level altitude) 

is greater than 150 ft; 95 percent of the absolute values of 

residuals are less than 75 ft; the mean absolute error for 

head residuals is less than 30 ft; the ratio of the root mean 

square error to the overall head loss in the system is equal to 

or less than 2 percent, and; hydrographs of measured waterlevel altitudes and simulated water-level altitudes during the 

Construction of Local Ground-Water Flow Model  

simulation reasonably match. The mean absolute value for 

950 residuals is 23 ft with a standard deviation of 17.3 ft. 

The absolute value of the maximum residual is 95 ft, and the 

range is + 95 ft to – 95 ft. The ratio of the root mean square 

error to the total head change in the model is 1.15 percent. 

Results from updated TAMA model simulations by the 

ADWR were provided to the author following the completion 

of this study. In the updated model, the ADWR has converted 

the regional model code to MODFLOW-2000 (Harbaugh 

and others, 2000). The use of MODFLOW-2000 enabled the 

observed heads to be weighted on the basis of their accuracy, 

which led to improved calibration statistics (Dale Mason, 

hydrologist, Arizona Department of Water Resources, written 

commun., 2004). 

Construction of Local Ground-Water 

Flow Model 

Discretization 

Cells in the local model are 528 ft on a side and are 

orientated on the same north-south and east-west grid as cells 

in the TAMA model; therefore, there are 25 local cells within 

each TAMA cell. Local-model layering is the same as that in 

the TAMA model. 

Boundary Conditions 

MODTMR specifies the boundary condition type for 

all local-model perimeter cells, including those in inactive 

areas of the regional model. Specified-flow boundaries are 

assigned on all perimeter cells with the exception of the 

northwest-most cell of the local model, which is a specifiedhead cell. The specified flow along the local-model perimeter 

is defined by the flow crossing the local-model boundary 

from the TAMA model and is defined for each perimeter cell 

for each layer and for each time step and stress period. The 

specified head in the northwest-most model cell represents the 

head in the aquifer at that location within the TAMA model 

and is specified for each layer and each time step and stress 

period. A specified flow of 0 is assigned to inactive areas. 

The amount of recharge within the local model is the 

same as that used in the corresponding area of the TAMA 

model. Mountain-front recharge occurs in ephemeral washes 

and through alluvial fans along the fronts of the Santa Catalina Mountains upgradient from Rillito Creek. The amount 

and distribution of mountain-front recharge applied in the 

local model were maintained from the TAMA model by using 

the Recharge Package. Estimates of annual mountain-front 
 Simulated Water-Level Responses, Ground-Water Fluxes, and Storage Changes for Recharge Scenarios, Rillito Creek 

recharge for the TAMA model were developed from values 

used in the Hanson and Benedict (1994) ground-water flow 

model. Recharge from streamflow infiltration occurs in stream 

channels of the Santa Cruz River, Rillito Creek, Pantano 

Wash, and Tanque Verde Creek. The amount and distribution 

of stream-channel recharge applied in the local model were 

maintained from the TAMA model by using the Recharge 

Package and the Stream Package. The Stream Package was 

used along Rillito Creek, whereas the Recharge Package was 

!#2%
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used elsewhere. 

The amount of pumpage within the local model is the 

same as that used in the corresponding area of the TAMA 

model. In this area, ground-water pumping increases between 



n

1940 and the late 1990s. In the late 1990s several wells were 

retired as a portion of ground-water pumpage was shifted to 

Avra Valley—the adjacent basin to the west. Pumpage values 

beyond 2000 were projected to 2025 by the ADWR (Dale 

Mason, hydrologist, Arizona Department of Water Resources, 

written commun., 2003). The projected values indicate a 

gradual increase in pumpage from the early 2000s to 2025. 

Simulation of Recharge 

The TAMA model used to create boundary conditions for 

the local model simulates recharge with the Stream Package 

for cells coincident with Rillito Creek. When the Recharge 

Package is used, the total recharge is the sum of specified 

recharge for individual cells traversed by Rillito Creek. When 

the Stream Package is used, the total recharge is controlled by 

the amount of surface flow specified at the upper end of Rillito 

Creek for most scenarios discussed in this report. Although 

the amount of recharge applied by using the Stream Package is 

the same as that applied by using the Recharge Package in the 

TAMA model, the distribution of recharge is slightly different (fig. 3). The greatest differences between the two model 

simulations are in the upper reaches where the Stream Package 

simulations result in less recharge than the Recharge Package 

simulations, and in the middle reaches where Stream Package 

simulations result in more recharge than Recharge Package 

simulations. These differences are related to the distribution 

of streambed-conductance terms used in the Stream Package. 

The variation in streambed-conductance terms can be justified 

on the basis of the findings of Hoffmann and others (2002) as 

explained in the following discussion on the use of the computer program RIVGRID. 

The recharge distribution resulting from the use of the 

Stream Package agrees better with results from seepage 

measurements and microgravity surveys than with the 

distribution simulated by using the Recharge Package (Pool 

and Schmidt, 1997; Don Pool, hydrologist, U.S. Geological 

Survey, written commun., 2003). About 66 to 89 percent of 

streamflow losses during sustained flows in Rillito Creek 

occur in the middle reach. The low recharge rates in the upper 

reaches are related to the shallow depths to ground water. The 

greater depths to water in the middle reaches allow for more 

storage for the infiltrating water. 

n

n

     

2%')/.!, #/,5-. .5-"%2

Figure . Difference in the distribution of recharge applied to 

the steady-state local model between the Recharge and Stream 

Packages, Rillito Creek, Arizona. Positive values indicate more 

recharge is simulated by the Stream Package. 

Simulated water-level altitudes that result from recharge 

distributions created by the Recharge and Stream Packages are 

similar for both steady-state and transient simulations (fig. 4); 

however, small systematic differences are evident within the 

local-model area. For instance, in the steady-state simulations, 

the water-level altitude differences are greatest on the eastern 

side of the local model, and for the transient simulations differences are greatest on the western side of the local model. 

The differences are typically about 5 ft for the steady-state 

simulation and can be as much as 30 ft at the end of the 

transient period in 1999. These differences in simulated 

water-level altitudes between recharge application methods 

are considered small given the range in water-level variations 

over the model area. 

Recharge to cells that represent Rillito Creek was 

simulated as varying over time. Between 1941 and 1999, 

total recharge for model cells coinciding with Rillito Creek 

ranged from about 4,900 to 6,000 acre-ft/yr (fig. 5). Recharge 

values applied to the calibrated model were simplified to a 

step function for use in the Stream Package (fig. 5). Recharge 

in the regional model that is used to create input data sets and 

boundary conditions for the local model was applied at a rate 

of about 4,900 acre-ft/yr for 1941–1954, at a rate of about 

5,500 acre-ft/yr for 1955–1963, at a rate of about 6,000 acreft/yr for 1964–1980, and at a rate of about 4,900 acre-ft/yr for 

1981–1999. 
Construction of Local Ground-Water Flow Model  

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Figure . Simulated water-level altitude in layer 2 of the local model using the Recharge and Stream Packages along Rillito Creek, 

Tucson, Arizona. A, Water-level altitude for steady-state simulations, 1940; B, Water-level altitude for transient simulations, 1999. 
10 Simulated Water-Level Responses, Ground-Water Fluxes, and Storage Changes for Recharge Scenarios, Rillito Creek 















                 

2ECHARGE VALUES

3TEP FUNCTION USED IN

3TREAM 0ACKAGE

Figure . Recharge to model cells coinciding with Rillito Creek, Tucson, Arizona, 1941–2025. 

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The computer program RIVGRID was used to 

calculate streambed conductances used in the Stream 

Package. The streambed conductance (L2/T) is the hydraulic 

conductivity of the streambed multiplied by the product of the 

width of the streambed and its length divided by the thickness 

of the streambed. Hydraulic conductivity values and streambed 

widths and thicknesses were varied spatially in RIVGRID 

on the basis of findings of Hoffmann and others (2002). 

Particle-size analyses of cores and cuttings and DC electricalresistivity surveys indicated that, in general, streambed 

deposits in the upper two-thirds of Rillito Creek are coarser 

grained than the deposits in the lower third. In addition, a 

fine-grained deposit below the streambed but above the water 

table exists in the lower third of Rillito Creek. Although 

streambed material in the uppermost reach is coarser grained 

than that in the middle reach, it also is poorly sorted and has 

a larger fraction of fine-grained material. Owing to variations 

in grain-size distribution, the middle reaches have the largest 

streambed-conductance values and the lower reaches have the 

smallest values. 

MODFLOW’s Stream Package computes leakage to 

or from the stream by using Darcy’s Law as the product of 

the conductance term and the difference between the head 

in the stream and the head in the aquifer. Head in the stream 

is calculated in the Stream Package by using the Manning 

formula (Ozbilgin and Dickerman, 1984). Manning’s 

roughness coefficients used in this equation were based on 

literature citations (White, 1979). 

Comparisons of Water Levels from 

Local and Regional Models 

Steady State 

Steady-state water-level altitudes simulated by using the 

Stream Package in the local and TAMA models are in good 

agreement (fig. 6). Water levels within the local-model area 

in both the TAMA and local steady-state simulations range 

from about 2,420 ft in the southeast to about 2,160 ft in the 

northwest. The difference in water-level altitudes between the 

local model and the TAMA model for layer 2 averages about 

3 ft and is generally less than 13 ft. The largest differences 

between the local and TAMA models occur in the northeastern 

part of the local model near inactive cells and at the southern 

boundary of the local model (fig. 6). The difference in water 

levels in the northeastern part of the model area probably is 

related to boundary effects near the inactive part of the model 

in this area. This agreement provides confidence that the local 

model is a fair representation of the TAMA model and thus 

can be used to test transient simulations. 
,/#!, -/$%,

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Comparions of Water Levels from Local and Regional Models 11 

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Figure . Simulated water-level altitudes in layer 2 for regional and local models using the Stream Package along Rillito Creek, 

Tucson, Arizona. A, Steady-state simulation, 1940; B, Transient simulation, 1999. 
1 Simulated Water-Level Responses, Ground-Water Fluxes, and Storage Changes for Recharge Scenarios, Rillito Creek 

Transient 

Simulated transient water-level altitudes in the TAMA 

and local models are in good agreement (fig. 7). Transient 

water-level altitudes in the TAMA and local models were 

compared to each other in plan view and at specific 

observation cells. Dewatering of the aquifer represented by 

layer 1 caused the aquifer represented by layer 2 to become 

the predominant aquifer in the area; therefore, water-level 

altitudes for model layer 2 are shown for all comparisons. 

In addition, water-level altitudes in layer 2 can be used as an 

estimate for altitudes in layers 1 and 3; where the layers are 

saturated, water-level altitudes in the layers are similar (fig. 7). 

In general, the temporal water-level trend shown in figure 7 is 

representative of the modeled area and shows a decline in the 

water-level altitude near Rillito Creek and within the Central 

Well Field from 1941 to 1999. The difference in water-level 

altitude between the three model layers is less than 6 ft and 

generally less than 1 ft. 

The simulated water-level altitudes plotted in plan view 

allows for assessment of spatial variations in water levels, 

whereas simulated water-level altitudes through time at 

selected observation cells allows for assessment of temporal 

variation. Water-level altitudes in 1999 for both the TAMA 

and local transient simulations range from about 2,420 ft in 

the eastern part of the local model to about 2,100 ft in the 

northwestern part (fig. 6B). The difference in water-level 

altitudes between the local and TAMA models averages 

about 3 ft. The maximum differences between the local 

and TAMA models (about 30 ft) occur in the northeastern 























     

7

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part of the local model near an inactive part of the model. 

The difference in water-level altitudes in the northeastern 

part of the model area probably is related to nearby inactive 

model cells and local-model discretization. The difference 

in simulated water-level altitudes between the two models is 

considered small and is relatively constant with time (fig. 8). 

The small differences in water-level altitudes between the 

two models provide confidence that the transient local model 

is a fair representation of the transient TAMA model and thus 

can be used to test forecast simulations based on a variety of 

recharge scenarios. 

Simulated Water Levels, GroundWater Fluxes, Storage Changes, and 

Evapotranspiration 

Simulated water-level, ground-water flux, storage, and 

ET responses to four recharge scenarios are discussed in this 

section. The amount of recharge applied for each scenario, 

resulting from streambed leakage through Rillito Creek, is 

shown in figure 9. Recharge scenarios arbitrarily begin in 

2005 and are extended to 2225 to evaluate long-term effects. 

Recharge in scenario A (5,000 acre-ft/yr) represents longterm natural recharge conditions (no change in recharge rates 

relative to 1999). For this scenario, an upstream streamflow 

discharge of 5,000 acre-ft/yr is routed along Rillito Creek by 

using the Stream Package. 

,AYER  NEAR 2ILLITO #REEK

,AYER  NEAR 2ILLITO #REEK

,AYER  NEAR 2ILLITO #REEK

,AYER  NEAR #ENTRAL 7ELL &IELD

,AYER  NEAR #ENTRAL 7ELL &IELD

,AYER  NEAR #ENTRAL 7ELL &IELD

,OCAL MODEL CELL 

,OCAL MODEL CELL 

./4% 3EE FIG  FOR LOCATION OF MODEL CELL

      

Figure . Hydrographs for selected local-model cells near Rillito Creek and the Central Well Field for model layers 1, 2, and 3, 

Tucson, Arizona, 1941–99. 
Simulated Water-Levels, Ground-Water Fluxes, Storage Changes, and Evapotranspiration 1 





7

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,OCAL MODEL

2EGIONAL MODEL



















            

Figure . Hydrograph for selected cell 28,38, layer 2, in the local and regional models, 1941–99, Rillito Creek, Arizona. 

 

2%#(!2'% !,/.' 2),,)4/ #2%%+ ). !#2%
&%%4



05-0!'%

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Figure . Amount of streambed recharge applied to the local model using the Stream Package along Rillito Creek, Arizona, and 

pumpage for each scenario. 

Scenario B almost triples the amount of annual recharge by 

using a constant recharge rate of 14,500 acre-ft/yr. Scenario C 

uses a constant recharge rate of about 21,000 acre-ft/yr. For 

each of these scenarios, an upstream discharge equal to the 

total amount of recharge along Rillito Creek is routed through 

the creek by using the Stream Package. Discharge at the 

upstream-most cell starts as 5,000, 14,500, and 21,000 acre-ft 

for scenarios A, B, and C, respectively, and owing to infiltration along the course of the creek, flow gradually tapers to 

zero near the confluence with the Santa Cruz River. Scenario D starts with a recharge rate of 60,000 acre-ft/yr for the 

first 15 years and gradually tapers to about 34,000 acre-ft/yr. 

The 60,000 acre-ft/yr rate represents the maximum amount of 

CAP water available for artificial recharge along Rillito Creek. 

Simulations attempting a recharge rate of 60,000 acre-ft/yr 

could not be maintained and resulted in water being routed 

past the confluence with the Santa Cruz River. A discussion 

on simulated water-level, ground-water flux, storage, and ET 

responses that occur prior to the onset of recharge scenarios is 

provided first as an evaluation of historical trends. 
1 Simulated Water-Level Responses, Ground-Water Fluxes, and Storage Changes for Recharge Scenarios, Rillito Creek 

Prior to Simulated Recharge Scenarios, 

11–00 

Before recharge values are adjusted for 2005 for the 

respective scenarios, each simulation is run from 1941 through 

2004; recharge values used from 1941 through 1999 are based 

on values from the TAMA model. Recharge values from 2000 

to 2004 are held constant at 5,000 acre-ft/yr, which represents 

the value used along Rillito Creek in the forecasted transient 

simulation of the TAMA model. 

Water Levels 

Simulated hydrographs for selected cells (fig. 10) show 

a steady water-level decline for the period between 1941 

and 2003 associated with increased pumpage within the 

Central Well Field (fig. 9). These hydrographs show a small 

rise in water level for most of the area starting in about 2003. 

The water-level rise is related to the recent and proposed 

retirement of several Central Well Field production wells 

(Dale Mason, hydrologist, Arizona Department of Water 

Resources, written commun., 2003) that began in 1995. 

Ground-Water Flux 

Ground-water fluxes southward past line segment 1 

(fig. 11) and northwestward past line segment 2 (fig. 12) 

are shown for the transient period prior to the recharge 

scenarios and for all recharge scenarios that extend to the 

year 2225. Ground-water flux past these line segments 

















     

7

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varies with time and is related to water-level gradients and 

aquifer transmissivity. Flux is calculated by using a linear 

interpolation method described by Leake and Claar (1999). 

Line Segment 1.— Near the start of the transient 

simulation, all water flowing past line segment 1 is flowing 

northward at a rate of about 5,200 acre-ft/yr. The flux past 

line segment 1 is northward from 1941 to 1963 because of 

the general northwestward hydraulic gradient of the regional 

aquifer. During this time period, flux northward decreases 

about 260 acre-ft/yr as pumpage from the Central Well Field 

reduces the hydraulic gradient to the north. After 1963 the 

net total flux is southward toward the Central Well Field 

and continues to increase at a steady rate until it reaches a 

maximum of 7,670 acre-ft/yr in 1996. As pumpage from the 

Central Well Field declines (fig. 9), the southward flux of 

ground water also declines, and by 2004, southward flux is 

about 4,300 acre-ft/yr. 

Line Segment 2.— Near the start of the transient 

simulation, all water flowing past line segment 2 flows 

northwestward at a rate of about 6,400 acre-ft/yr. The 

net total flux is northwestward during the entire transient 

simulated period because of the hydraulic gradient of the 

regional aquifer; however, there is a decline in flux from 

6,000 acre-ft/yr in 1970 to about 1,320 acre-ft/yr in 2004 

(fig. 12). The relative contribution of ground-water flux varies 

with time; however, layer 1 provides most of the flux across 

line segment 2 from 1941 to 1996, and layer 2 provides most 

of the flux after 1996. 







 



./4% 3EE FIG  FOR CELL LOCATION

       

Figure 10. Simulated hydrographs for selected cells in the local model, Rillito Creek, Arizona, 1941–2004. 
Simulated Water Levels, Ground-Water Fluxes, Storage Changes, and Evapotranspiration 1 

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Figure 11. Simulated annual ground-water flux past line segment 1 in the local model, Rillito Creek, Arizona, 1941–2225. 

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Figure 1. Simulated annual ground-water flux past line segment 2 in the local model, Rillito Creek, Arizona, 1941–2225. 
1 Simulated Water-Level Responses, Ground-Water Fluxes, and Storage Changes for Recharge Scenarios, Rillito Creek 

Storage Evapotranspiration 

The loss of ground water from storage in the local-model Ground-water discharge as ET from the local-model 

area is about 3,800 acre-ft/yr in 1941 and steadily declines to area steadily increased until 1995 owing to the increasing less than 500 acre-ft/yr by 1957 owing to a falling water table 

pumpage rates within the modeled area during this period (fig. 14). Discharge of shallow ground water as ET continues 

(fig. 13). As pumpage is reduced, water levels rise, resulting in to decline gradually from 1957 to 1985; from 1985 to 2004 

positive storage changes. discharge is nearly constant at about 200 acre-ft/yr. 

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Figure 1. Simulated annual loss of ground water due to evapotranspiration in the local model, Rillito Creek, Arizona, 1941–2225. 
Simulated Water Levels, Ground-Water Fluxes, Storage Changes, and Evapotranspiration 1 

Scenario A—Continued Recharge at Forecasted 

Natural Recharge Rates, 00– 

Water-level altitudes, ground-water fluxes, storage 

changes, and ET are discussed for scenario A in absolute 

terms. Scenario A is considered the base-case scenario to 

which all other scenarios are compared. 

Water Levels 

Simulated hydrographs for specific cells within the local 

model that represent sites near recharge areas and within 

the Central Well Field (fig. 15) show a general increase in 

water-level altitude for most of the area. Cells distant from 

the Central Well Field, however (for example, cells 18,13; 

and 28,38; in fig. 15), show a small but continuous decline in 

water-level altitude. The overall increase in water-level altitude 

is a continuation of the increase that began around 2003 and 

is related to the recent and proposed retirement of several 

Central Well Field production wells (Dale Mason, hydrologist, 

Arizona Department of Water Resources, written commun., 

2003) that began in 1995. Water-level altitude maps are 

shown for the end of 2004, 2015, 2025, 2050, 2100, and 2225 

(fig. 16). These times were chosen to show altitudes at specific 

times of interest and to show areal changes that are related to 

the transient response of the hydrographs shown in figure 15. 

For instance, the initial water-level altitude map for the end 

of 2004 represents the water level of the ground-water system 













    

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immediately prior to the recharge scenarios; 2025 represents 

the final year of ADRW’s projected pumpage estimates and 

the year that the TAMA is mandated to reach “Safe Yield” 

(Arizona Department of Water Resources, 2004); and 2225 

represents the end of all scenario simulations. Other times are 

shown to represent changing water-level altitudes over time. 

Ground-Water Flux 

Line Segment 1.—The southward hydraulic gradient 

across line segment 1 is reduced owing to the general decline 

in pumpage from the Central Well Field and associated 

water-level changes; thus, southward flux of ground water 

declines from 3,400 acre-ft/yr at the end of 2004 to about 

2,000 acre-ft/yr by 2015 (fig. 11). By 2053 the flux across line 

segment 1 becomes negative, indicating the flux is northward. 

After about 2100, the net flux northward is fairly constant, 

ranging from about 500 to 750 acre-ft/yr (fig. 11). The amount 

of ground-water flux past line segment 1 for each layer varies 

with time; however, layer 2 provides most of the flux across 

line segment 1 except during 2024–2057 when the amount of 

flux in layer 1 is greater. Although the net flux for all layers 

becomes northward after 2053, the direction of ground-water 

flux in layer 1 is southward for the duration of the simulation 

and ranges from about 200 to 500 acre-ft/yr. 













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Figure 1. Hydrographs for selected cells in the local model, Rillito Creek, Arizona, 2005–2225. 
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1 Simulated Water-Level Responses, Ground-Water Fluxes, and Storage Changes for Recharge Scenarios, Rillito Creek 

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Figure 1. Water-level altitude for scenario A in the local model for selected times, Rillito Creek, Arizona. A, 2004; B, 2015; C, 2025; 

D, 2050; E, 2100; F, 2225. 
Simulated Water levels, Ground-Water Fluxes, Storage Changes, and Evapotranspiration 1 

Line segment 2.— The hydraulic gradient of the regional 

aquifer near line segment 2 is to the northwest, causing the 

net total flux past line segment 2 to be northwestward during 

the entire transient simulation; however, there is a decline in 

flux from 1,230 acre-ft/yr in 2004 to about 335 acre-ft/yr in 

2025 (fig. 12). The decline in flux predominantly comes from 

a reduction in flow from layers 1 and 2. The relative contribution of ground-water flux for each layer varies with time; however, layer 2 provides most of the flux across line segment 2. 

After 2027, layer 1 provides the least amount of flux at a rate 

less than 60 acre-ft/yr. 

Storage 

The component of ground water going into storage for 

all model layers during the simulation for scenario A rapidly 

changes from the end of 2004 to 2025 owing to changing 

pumping rates (fig. 13). The initial decrease in storage from 

2004 to 2006 is due to increases in pumpage during the same 

time period. A large positive shift in storage occurs in 2007 

owing to a reduction of ground-water pumpage. Subsequent 

forecasted pumpage increases from 2008 to 2025 (fig. 9) cause 

a decline of ground-water storage. The component of ground 

water going into storage continues to decline gradually until 

2149 when storage changes become slightly negative and there 

is a small net loss of storage to the ground-water system. 

Evapotranspiration 

The rate of shallow ground-water discharge as ET during 

scenario A is constant at 191 acre-ft/yr (fig. 14). Scenario A 

does not result in a change in ET during the period of 

simulation because water levels near riparian areas do not 

change within the upper 25 ft of the subsurface. A constant 

141 acre-ft/yr of ET, or 74 percent of the total ET from 

the local model, occurs from a zone of cells surrounding 

Rillito Creek. 

Scenarios B, C, and D—Increased Recharge 

Rates, 00– 

Most of the increased water added to the ground-water 

system in scenarios B, C, and D, relative to scenario A, 

can be accounted for by ground-water flux southward past 

line segment 1, ground-water flux northwestward past line 

segment 2, increased storage in the zone of cells surrounding 

Rillito Creek, or as increased ground-water discharge as ET 

in the zone of cells surrounding Rillito Creek. The effects of 

recharge scenarios B, C, and D on water levels, ground-water 

flux, storage, and ET are examined by describing differences 

in simulated hydrographs, water-level altitude contours, 

ground-water fluxes, storage, and losses to ET relative to 

scenario A. All water-level comparisons are made for model 

layer 2. Ground-water fluxes are shown as composite totals 

for all layers. Storage and ET changes are shown for a zone of 

cells that bound Rillito Creek. All comparisons begin in 2005, 

when recharge rates are changed, and extend to 2225. 

Water Levels 

Recharge in scenario B exceeds recharge in scenario 

A by about 9,500 acre-ft/yr (fig. 9). Simulated water levels 

rise to within about 50 ft of predevelopment conditions near 

the Central Well Field and rise by as much as 86 ft by 2225 

relative to altitudes from scenario A (figs. 17, 18, and 19). 

These rises continue throughout the transient simulation; 

the rate of rise is about 0.1 ft/yr between 2224 and 2225 for 

most cells, indicating the differences would likely continue to 

increase if the simulations were extended beyond 2225. As of 

2225, water-level rises beneath Rillito Creek range from about 

53 ft in the upper reach near Craycroft Road to 86 ft near 

First Avenue relative to scenario A. Water-level rises for cells 

within the Central Well Field range from about 60 to 80 ft. 

More than half of the rises occur by 2050; at least 95 percent 

of the rises occur by 2188. 

Recharge in scenario C exceeds recharge in scenario A, 

by about 16,700 acre-ft/yr (fig. 9). Simulated water levels rise 

to within a few feet of predevelopment conditions and rise as 

much as 102 ft by 2225 relative to scenario A (figs. 17, 18, 

and 20). The rate of water-level rise for scenario C is greater 

than that for scenario B, but unlike scenario B, scenario C 

water levels nearly stabilize, indicating only minimal waterlevel rises would occur beyond 2225. At the end of the 

transient simulation, water levels beneath Rillito Creek rise 

relative to scenario A and range from about 71 ft in the upper 

reach near Craycroft Road to 102 ft near La Cañada Avenue. 

Water-level rises for cells within the Central Well Field range 

from about 80 to 95 ft. More than half of the rises occur by 

2036; at least 95 percent of the rises occur by 2100. 

Scenario D results in simulated water-level altitude 

increases of as much as 131 ft relative to scenario A (figs. 17, 

18, and 21). The water-level response associated with 

scenario D, however, differs from that in other scenarios 

because of a varying recharge rate used in scenario D. Initially, 

recharge occurs at a rate of 60,000 acre-ft/yr (an increase 

of about 55,000 acre-ft/yr relative to scenario A); however, 

this recharge rate results in water-level rises beneath Rillito 

Creek that reach land surface. As a result, the aquifer can 

no longer accept recharge, and surface water is transported 

downstream. Rising water levels beneath Rillito Creek cause 

recharge to be rejected after 2020. Recharge rates begin to 

decline in 2021 and settle at a rate of about 34,000 acre-ft/yr 

(an increase of about 29,000 acre-ft/yr relative to scenario A) 

by 2120. A long-term recharge rate of 34,000 acre-ft per year 

or greater, therefore, is likely to result in water levels that 

reach land surface and cause rejected recharge. At the end of 

the transient simulation in 2225, water levels beneath Rillito 

Creek stabilize; water-level rises relative to scenario A range 

from about 97 ft in the upper reach near Craycroft Road to 

131 ft near La Cañada Avenue. Water-level rises for cells 

within the Central Well Field range from about 96 to 109 ft. 

More than half of the rises occur by 2018; at least 95 percent 

of the rises occur by 2041. The high water levels associated 

with scenario D could pose environmental issues owing to the 

presence of near-surface contaminant sources, such as landfills 

near and adjacent to Rillito Creek. 
0 Simulated Water-Level Responses, Ground-Water Fluxes, and Storage Changes for Recharge Scenarios, Rillito Creek 









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Figure 1. Water-level altitude for each scenario for cells in the local model, 1940–2225, Rillito Creek, Arizona. A, Scenario B; 

B, Scenario C; C, Scenario D. 
Simulated Water Levels, Ground-Water Fluxes, Storage Changes, and Evapotranspiration 1 









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Figure 1. Rise in water level for each scenario relative to scenario A for selected cells in the local model, 2000–2225, Rillito Creek, 

Arizona. A, Scenario B; B, Scenario C; C, Scenario D. 
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 Simulated Water-Level Responses, Ground-Water Fluxes, and Storage Changes for Recharge Scenarios, Rillito Creek 

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Figure 1. Rises in water level for scenario B relative to scenario A within the local model for selected times, Rillito Creek, Arizona. 

A, 2015; B, 2025; C, 2050; D, 2100; E, 2150; F, 2225. 
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Figure 0. Rises in water level for scenario C relative to scenario A within the local model for selected times, Rillito Creek, Arizona. 

A, 2015; B, 2025; C, 2050; D, 2100; E, 2150; F, 2225. 
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Figure 1. Rises in water level for scenario D relative to scenario A within the local model for selected times, Rillito Creek, Arizona. 

A, 2015; B, 2025; C, 2050; D, 2100; E, 2150; F, 2225. 
Simulated Water Levels, Ground-Water Fluxes, Storage Changes, and Evapotranspiration  

Ground-Water Flux 

Scenario B results in long-term increases of about 

3,300 acre-ft/yr in ground-water flux southward past line 

segment 1 (fig. 11) relative to scenario A. The increase in flux 

occurs as soon as the recharge scenario starts, and it reaches 

a maximum in 2010 at 4,500 acre-ft/yr. After 2010, the rate 

of southward flux decreases owing to a decrease in hydraulic 

gradient to the south. By 2225, the increase in the southward 

flux tapers to about 3,300 acre-ft/yr. 

Scenario B also results in long-term increases of about 

3,100 acre-ft/yr in ground-water flux northwestward past 

line segment 2, (fig. 12) relative to scenario A. Initially, the 

increase is small; however, the flux gradually increases, 

reaching a maximum of 3,100 acre-ft/yr in 2225. The increase 

in flux northwestward is due to an increase in hydraulic 

gradient to the northwest. 

A maximum of about 47 percent of the added recharge 

in scenario B flows southward across line segment 1. The 

maximum flux occurs in year 2016, and the amount of water 

flowing southward gradually decreases to about 35 percent 

of the increased recharge by 2150. Southward flux continues 

to decline until the end of the transient simulation. The 

percentage of increased recharge that flows northwestward 

past line segment 2 is low at the onset of the recharge scenario 

but gradually increases to about 31 percent in 2225 and 

continues to increase at the end of the transient simulation. 

Scenario C results in long-term increases of about 

4,840 acre-ft/yr in ground-water flux southward past line 

segment 1 (fig. 11), relative to scenario A. The increase in 

flux occurs as soon as the recharge scenario starts and reaches 

a maximum of 8,000 acre-ft/yr in 2015. After 2015, the rate 

of southward flux decreases owing to a decrease in hydraulic 

gradient to the south, and by 2225, the long-term southward 

flux tapers to about 4,840 acre-ft/yr (fig. 11). 

Scenario C also results in long-term increases of about 

3,900 acre-ft/yr in ground-water flux northwestward past 

line segment 2 (fig. 12), relative to scenario A. Initially, the 

increase is small; however, the flux reaches a maximum 

of about 3,960 acre-ft/yr in 2131. The increase in flux 

northwestward is due to an increase in hydraulic gradient to 

the northwest. 

A maximum of about 48 percent of the added recharge 

in scenario C flows southward across line segment 1. The 

amount of water flowing southward gradually decreases to 

about 29 percent of the increased recharge. The percentage 

of increased recharge that flows northwestward past line 

segment 2 is low at the onset of the recharge scenario 

but gradually increases to about 25 percent in 2118 and 

continues to show small increases until the end of the 

transient simulation. 

The long-term increase in recharge for scenario D relative 

to scenario A is 29,000 acre-ft/yr; however, initial increases 

are about 55,000 acre-ft/yr. Scenario D results in long-term 

increases of about 7,500 acre-ft/yr in ground-water flux 

southward past line segment 1 (fig. 11), relative to scenario A. 

The changing recharge rate of scenario D causes the initial 

increase in flux to reach a maximum of about 29,000 acreft/yr in 2013. After 2013, the rate of southward flux decreases 

owing to a decrease in hydraulic gradient to the south and to 

a reduction in recharge after 2020. By 2225, the southward 

flux has tapered to about 7,500 acre-ft/yr, which is about 

26 percent of the increase in recharge. 

Scenario D also results in long-term increases of about 

6,000 acre-ft/yr in ground-water flux northwestward past 

line segment 2 (fig. 12), relative to scenario A. Initially 

(2005–2007), the increase is small; however, the flux 

increases, reaching a maximum relative to scenario A of 

6,980 acre-ft/yr in 2019. The increase in flux northwestward 

is due to an increase in hydraulic gradient to the northwest. 

There is a gradual decrease in northwestward flux after 2030 

owing to the reduced recharge rate. About 21 percent of the 

long-term increased recharge applied in scenario D flows 

northwestward past line segment 2. 

Storage 

Storage change related to each recharge scenario is 

calculated by summing the storage budget terms for a zone 

of cells that bound Rillito Creek (fig. 2). The program 

Zonebudget (Harbaugh, 1990) was used to calculate the 

budget terms for this zone. 

The largest storage changes for scenarios B, C, and D 

relative to scenario A occur early in the simulations, and by 

2015, the changes begin to decrease for all scenarios (fig. 13). 

Change in storage accounts for more than 50 percent of the 

newly added recharge early in the recharge scenario but 

declines to near zero percent by the end of the simulation 

(fig. 13). 

Evapotranspiration 

Removal of shallow ground water through ET varies for 

each scenario (fig. 14). Increases in ground-water ET rates 

are a function of depth to water and areal distribution of the 

shallow ground water. ET rates increase the earliest and the 

most for scenario D and the latest and the least for scenario B. 

The ET rate for scenario A is 191 acre-ft/yr, and there is no 

rate change between 2004 and 2225. Losses to ET primarily 

occur in two zones of cells—one zone surrounding the Santa 

Cruz River and the other surrounding Rillito Creek. The 

fraction of the total ET that occurs within the zone of cells 

surrounding Rillito Creek varies among the scenarios (table 2). 
 Simulated Water-Level Responses, Ground-Water Fluxes, and Storage Changes for Recharge Scenarios, Rillito Creek 

Table . Evapotranspiration within local ground-water 

flow model for various scenarios, Rillito Creek, Arizona 

Total for all 

cells within Percentage of 

the local Cells along evapotranspiration 

Scenario model Rillito Creek along Rillito Creek 

A 191 141 74 

B 1,397 454 32 

C 7,324 4,260 58 

D 13,355 12,100 91 

Relative to the amount of added recharge for each scenario, 

increased ET accounts for 3, 25 and 41 percent for scenarios 

B, C, and D, respectively (fig. 14). 

Limitations of the Local Flow Model 

The local-model data sets are constructed with the 

program MODTMR by using information contained in the 

regional-model data sets. The TAMA model data sets are grid 

dependent; that is each cell is associated with specific properties. All local cells overlying TAMA model cells contain 

properties consistent with those of the TAMA model cells. 

Location information is not contained in grid-dependent 

TAMA models to precisely locate sources and sinks in the 

local-model grid. 

The accuracy of the local model is dependent on the 

accuracy of the TAMA model. All finite difference flow models, including the TAMA model, are numerical simplifications 

of a complex natural system. The numerical model is based on 

a conceptualization of the natural surface- and ground-water 

flow system. Inherent in the conceptualization is the assumption that all sources of flow and stress on the natural system 

are represented in the numerical model and are accurately 

known. In most cases, however, all sources of flow and stress 

on the natural system can never be fully accounted. 

One important simplification of the TAMA and local 

models is that layer 1 is represented by the hydraulic properties of the Fort Lowell Formation. This simplification is 

probably adequate for most uses of the flow model. When 

recharge rates along Rillito Creek are significantly high, 

however, the water level rises to near the land surface and 

into the stream-channel deposits. Hoffmann and others (2002) 

show that the hydraulic conductivity of the stream-channel 

deposits, which are generally about 40 ft thick, is about twice 

that of the underlying basin-fill deposits. A model simulation 

in which an additional layer is used to represent the streamchannel deposits would result in somewhat differing water 

levels, ground-water fluxes, and storage changes. The larger 

hydraulic conductivity of the stream-channel deposits would 

likely result in lowered water levels. In addition, owing to the 

northwestward orientation of the Rillito Creek stream channel, 

ground-water fluxes northwestward are likely to increase relative to fluxes southward if a larger conductivity layer is used 

for the shallow stream-channel deposits. 

Another important simplification of the TAMA and local 

models is that all routed water either recharges to the aquifer 

or flows out of the stream reach. The full amount of water 

recharges instantaneously and no time is needed for water 

to flow through the unsaturated zone. Furthermore, no water 

is lost to storage in the unsaturated zone or to ET prior to 

recharge. Consequently, in order for recharge to occur at the 

rates discussed in the various recharge simulations, the amount 

of water has to increase to account for actual storage and ET 

loss. In cases where there is no rejected recharge, the actual 

recharge rates would be less than the specified discharge to 

the stream if ET in the unsaturated zone was accounted for in 

the model. The recharge rates used in the simulations are thus 

considered maximum recharge rates. 

Another simplification of the model is that streambed 

conductance is assumed to be constant during each stress 

period. Streambed conductance, which is a function of vertical 

hydraulic conductivity of the streambed, is likely to change 

during and between streamflow or recharge events. Bailey 

(2002) showed variations of vertical hydraulic conductivity 

of as much as four orders of magnitude during or between 

streamflows in Rillito Creek. 

Simulation of ET was made on the basis of vegetation 

type and density observed on aerial photographs taken in 

1936 and 1941. An extinction depth of 25 ft was assigned 

on the basis of rooting depths for mesquite and cottonwood. 

These phreatophytes, although still in existence within the 

study area, are likely to have a reduced density owing to 

human developments. A reduced ET rate due to decreased 

vegetation density is not simulated in the TAMA or local 

model. A shallower rooting depth appropriate for current vegetation also was not used. In addition, the surface area used to 

estimate the location of phreatophytes and calculate ET rates 

is made on the basis of the TAMA model cell assignments. 

Each TAMA model cell is 2,680 ft on a side and comprises 

25 local-model cells. Given that Rillito Creek is about 300 ft 

wide, the surface area represented by cells with ET assignments is larger than the actual surface area of Rillito Creek. 

Cell assignments between the TAMA and local models were 

made to maintain consistency between water-budget components in the two models. 

The local model was not calibrated, but calibration of 

the TAMA model was assumed to apply in general to the 

local model. The TAMA model used to create boundary conditions for the local model, however, simulates recharge with the 

Stream Package for cells coincident with Rillito Creek. The 

period of calibration for the TAMA model extended from 1940 

to 1999. 

Forcasted simulations of the ADWR model through 2025 

are based on a predicted pumpage distribution and a constant 

recharge rate. Subsequent forecasted simulations extended to 

2225 maintain the predicted pumpage distribution of 2025 and 

assumed a constant recharge rate for each simulation. A con-
stant pumpage distribution from 2025 to 2225 is improbable, 

as is a constant recharge rate. 

Given the simplifications and associated limitations of the 

model, results presented here cannot be considered predictive. 

Rather, these results are interpretive and are best suited for 

evaluating system responses to stress. 

Summary and Conclusions 

The amount of water currently recharging the aquifers 

within the Tucson area is insufficient to meet current and projected future demands on the ground-water flow system. Currently, resultant ground-water deficits are manifested in waterlevel declines of more than 200 ft since the middle of the 

twentieth century. These declines are largest where groundwater withdrawals are highest—generally in association with 

Tucson’s Central Well Field. To help mitigate the deficit and 

associated water-level declines, an in-stream recharge facility 

has been proposed in Rillito Creek, an ephemeral stream on 

the northern side of Tucson. Numerical simulations of a local 

ground-water flow model were used to evaluate mitigating 

effects on ground-water deficits and water-level declines for 

various recharge scenarios within Rillito Creek. 

The local model was nested within a regional (TAMA) 

ground-water flow model to provide boundary conditions. 

The local flow model was centered approximately over Rillito 

Creek and Tucson’s Central Well Field. The purpose of this 

investigation was to evaluate recharge scenarios related to the 

introduction of CAP water to Rillito Creek; therefore, recharge 

was simulated in the local model by using the Stream Package. Applying recharge by using the Stream Package had 

the advantage over applying recharge by using the Recharge 

Package because recharge in the former is calculated on 

the basis of head differences between the stream and the 

aquifer and a streambed-conductance term. In addition, the 

Stream Package enabled a variety of recharge scenarios to be 

evaluated by using differing streamflow values along Rillito 

Creek. Although the amount of recharge applied by using 

the two packages was the same, the distribution of recharge 

was slightly different along Rillito Creek. Simulations that 

included the Stream Package resulted in less recharge in 

the upper reach and more recharge in the middle reach than 

simulations that included the Recharge Package. The recharge 

distribution simulated by the Stream Package is in better agreement with results from seepage measurements and 

microgravity surveys than the distribution simulated by the 

Recharge Package. The Stream Package terms also can be 

justified on the basis of textural and geophysical data collected 

along the creek. The different recharge distributions resulted 

in only slightly different water levels. The streamflow values 

routed with the Stream Package for the recharge scenarios 

included 5,000, 14,500, 21,000, and 60,000 acre-ft/yr. A longterm average recharge rate used in the TAMA model for cells 

underlying Rillito Creek was represented by 5,000 acre-ft/yr, 

Summary and Conclusions  

and 60,000 acre-ft/yr represented the maximum amount of 

CAP water available for artificial recharge along Rillito Creek. 

Simulated steady-state and transient water-level altitudes 

in the TAMA and local models are in good agreement. This 

agreement provided confidence that the local model represents 

the TAMA model and thus could be used to evaluate forecast 

simulations based on a variety of recharge scenarios. 

Simulated water levels for recharge scenarios B, C, and 

D rose relative to water levels for scenario A. Water levels 

throughout the local modeled area rose by as much as 131 ft. 

Most of the increased water added to the ground-water 

system in the scenarios was accounted for by a combination 

of increased storage near Rillito Creek, ground-water flux 

southward, ground-water flux northwestward, and increased 

discharge as ET along Rillito Creek. The percentage of newly 

added water attributed to storage change is large relative to the 

percentage attributed to changes in ground-water flux and ET 

at the onset of each scenario; however, the change in storage 

become smaller throughout the simulation, and the long-term 

storage change is minimal. 

For scenario B, which increased the recharge along 

Rillito Creek by about 9,500 acre-ft/yr, simulated water-level 

rises beneath Rillito Creek ranged from about 53 to 86 ft relative to scenario A. Water-level rises for cells within Tucson’s 

Central Well Field ranged from about 60 to 80 ft. More than 

half of the rises occurred by 2050; at least 95 percent of the 

rises occurred by 2188. Long-term ground-water flux to the 

south and northwest increased by about 3,300 and 3,100 acreft/yr, respectively. About 35 and 31 percent of the long-term 

increased recharge flowed southward and northwestward, 

respectively. ET along Rillito Creek increased from 191 to 

454 acre-ft/yr. The increase in ET represents about 3 percent 

of the increased recharge. 

For scenario C, which increased the recharge along 

Rillito Creek by about 16,700 acre-ft/yr, simulated waterlevel rises beneath Rillito Creek ranged from about 71 to 

102 ft relative to scenario A. Water-level rises for cells within 

Tucson’s Central Well Field ranged from about 80 to 95 ft. 

More than half of the rises occurred by 2036; at least 95 percent of the rises occurred by 2100. Long-term ground-water 

flux to the south and northwest increased by about 4,840 and 

3,900 acre-ft/yr, respectively. About 29 and 25 percent of the 

long-term increased recharge flowed southward and northwestward, respectively. ET increased from 141 acre-ft/yr to 

about 4,260 acre-ft/yr along Rillito Creek. The increase in ET 

represents about 25 percent of the increased recharge. 

Scenario D, which initially increased the recharge rate 

by about 55,000 acre-ft/yr relative to scenario A, resulted in 

simulated water levels that rose to land surface along Rillito 

Creek. This rise in water level resulted in rejected recharge. 

As the water table rose, the recharge rate continued to decline 

until a long-term rate of about 34,000 acre-ft/yr along Rillito 

Creek was sustained—an increase of about 29,000 acre-ft/yr 

relative to scenario A. Simulated long-term water-level rises 

beneath Rillito Creek ranged from about 97 to 131 ft, causing 

water levels to be near or at the land surface. Shallow water 
 Simulated Water-Level Responses, Ground-Water Fluxes, and Storage Changes for Recharge Scenarios, Rillito Creek 

levels associated with this scenario could pose environmental 

problems owing to the presence of landfills adjacent to Rillito 

Creek. Water-level rises for cells within the Central Well Field 

ranged from about 96 to 109 ft. More than half of the waterlevel rises occurred by 2018; at least 95 percent of the waterlevel rises occurred by 2041. Long-term ground-water flux to 

the south and northwest increased by about 7,500 acre-ft/yr 

and 6,000 acre-ft/yr, respectively. About 26 and 21 percent 

of the long-term increased recharge flowed southward and 

northwestward, respectively. ET increased about 12,000 acreft/yr along Rillito Creek, or about 41 percent of the long-term 

increased recharge. 

Results beyond 2025 are considered interpretive owing to 

the simplification of the model used for the scenarios presented. The longer-term analysis, therefore, is considered to 

be an interpretive-model application in which results represent 

possible long-term responses that are predicated on the most 

complete information currently available. 

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