Abstract

A large fraction of ground water stored in the alluvial 

aquifers in the Southwest is recharged by water that percolates 

through ephemeral stream-channel deposits. The amount of 

water currently recharging many of these aquifers is insufficient 

to meet current and future demands. Improving the understanding of streambed infiltration and the subsequent redistribution 

of water within the unsaturated zone is fundamental to quantifying and forming an accurate description of streambed recharge. 

In addition, improved estimates of recharge from ephemeralstream channels will reduce uncertainties in water-budget 

components used in current ground-water models.

This chapter presents a summary of findings related to a 

focused recharge investigation along Rillito Creek in Tucson, 

Arizona. A variety of approaches used to estimate infiltration, 

percolation, and recharge fluxes are presented that provide a 

wide range of temporal- and spatial-scale measurements of 

recharge beneath Rillito Creek. The approaches discussed 

include analyses of (1) cores and cuttings for hydraulic and 

textural properties, (2) environmental tracers from the water 

extracted from the cores and cuttings, (3) seepage measurements made during sustained streamflow, (4) heat as a tracer 

and numerical simulations of the movement of heat through 

the streambed sediments, (5) water-content variations, (6) 

water-level responses to streamflow in piezometers within 

the stream channel, and (7) gravity changes in response to 

recharge events. Hydraulic properties of the materials underlying Rillito Creek were used to estimate long-term potential 

recharge rates. Seepage measurements and analyses of temperature and water content were used to estimate infiltration 

rates, and environmental tracers were used to estimate percolation rates through the thick unsaturated zone. The presence 

or lack of tritium in the water was used to determine whether 

or not water in the unsaturated zone infiltrated within the past 

40 years. Analysis of water-level and temporal-gravity data 

were used to estimate recharge volumes. Data presented in this 

chapter were collected from 1999 though 2002. Precipitation 

and streamflow during this period were less than the longterm average; however, two periods of significant streamflow 

resulted in recharge—one in the summer of 1999 and the other 

in the fall/winter of 2000.

Flux estimates of infiltration and recharge vary from 

less than 0.1 to 1.0 cubic meter per second per kilometer of 

streamflow. Recharge-flux estimates are larger than infiltration estimates. Larger recharge fluxes than infiltration fluxes 

are explained by the scale of measurements. Methods used to 

estimate recharge rates incorporate the largest volumetric and 

temporal scales and are likely to have fluxes from other nearby 

sources, such as unmeasured tributaries, whereas the methods 

used to estimate infiltration incorporate the smallest scales, 

reflecting infiltration rates at individual measurement sites.

Introduction

The city of Tucson and surrounding areas obtain most of 

their municipal, agricultural, and industrial water from ground 

water that is withdrawn from thick, alluvial-basin aquifers. 

The amount of water currently recharging the aquifers within 

the Tucson area is insufficient to meet current and future 

demands. Resultant ground-water deficits are manifested in 

water-level declines of more than 60 m since the middle of the 

20th century. These declines are largest where ground-water 

withdrawals are greatest.

The alluvial aquifers are recharged by infiltration from 

irrigation and industrial returns and by seepage losses through 

stream channels. In the Tucson area, where the climate is 

semiarid, diffuse recharge through the basin sediments from 

precipitation is considered a negligible component of total 

recharge owing to low precipitation rates and high evapotranspiration (ET) rates (Scott and others, 2000). For instance, 

annual precipitation averages 31.5 cm on the valley floor, 

and annual potential ET ranges from 90 to 190 cm (Yitayew, 

1990). Additionally, depth to ground water in the underlying 

alluvial basin can be tens of meters, providing opportunity for 

ample storage of infiltrated water. Because of these conditions, 

concentrated infiltration repeated over time, such as infiltration 

from irrigation and industrial returns, is necessary for recharge 

to occur. A large fraction of ground water stored in the alluEstimated Infiltration, Percolation, and Recharge Rates 

at the Rillito Creek Focused Recharge Investigation Site, 

Pima County, Arizona

By John P. Hoffmann, Kyle W. Blasch, Don R. Pool, Matthew A. Bailey, and James B. Callegary

USGS Professional Paper 1703—Ground-Water Recharge

in the Arid and Semiarid Southwestern United States—

Chapter H
186 Estimated Infiltration, Percolation, and Recharge Rates at the Rillito Creek Focused Recharge Investigation Site, Arizona

vial aquifer was recharged by water that percolated through 

ephemeral stream-channel deposits (Davidson, 1973; Hanson 

and Benedict, 1994). 

Rillito Creek, located in the Upper Santa Cruz Basin 

in southern Arizona (fig. 1), is typical of a large, ephemeral 

stream in the Southwest. In many basins of the Southwest, 

such as in the Upper Santa Cruz Basin, streams originating at higher elevations coalesce downstream to form 

larger ephemeral streams. Streams originating near mountain fronts typically flow over thick, alluvial valleys, lose 

hydraulic connection with the underlying aquifer, and are 

ephemeral in their lower reaches. Underlying many of 

these ephemeral streams is a coarse-grained stream-channel 

deposit that overlies a basin-fill deposit. The coarse-grained 

stream-channel deposit typically has high permeability and 

infiltration rates (Anderson and others, 1992; Hanson and 

Benedict, 1994).

Although recharge from infiltration of streamflow is 

known to occur in ephemeral-stream channels in the Southwest, such as Rillito Creek, the processes that control the 

spatial distribution and volume of infiltration that recharges 

the underlying aquifers are poorly understood. The Rillito 

Creek focused recharge investigation site was selected as one 

of six sites to study recharge processes in the Southwest (see 

chapter C) as part of the U.S. Geological Survey (USGS) 

Ground-Water Resources Program and generally is representative of ephemeral washes within the Sonoran Desert. 

Improving the understanding of streambed infiltration and 

the subsequent redistribution of water within the unsaturated 

zone is fundamental to quantifying and forming an accurate description of streambed recharge. Improved estimates 

of recharge from ephemeral stream channels will reduce 

uncertainties in water-budget components used in current 

ground-water models. In addition, recharge augmentation has 

been proposed along several reaches of ephemeral streams in 

the Tucson area, including Rillito Creek, and understanding 

processes that control recharge is important to the construction of recharge facilities.

Purpose and Scope

The purpose of this chapter is to present a summary of 

findings related to a focused recharge investigation along 

Rillito Creek, Pima County, Arizona. One of the challenges 

of quantitatively studying recharge beneath ephemeral 

streams is the need to integrate measurements made over a 

wide range of spatial and temporal scales. No single method 

of measurement or analysis can resolve the complex physical processes that contribute to infiltration, percolation, and 

recharge beneath ephemeral streams; therefore, a variety of 

approaches are presented that provide a wide range of temporal- and spatial-scale measurements of recharge beneath 

Rillito Creek.

Six approaches were used to evaluate infiltration, percolation, and recharge to the aquifer beneath Rillito Creek. 

Cores and cuttings were collected during the drilling of five 

boreholes. Laboratory measurements used to determine 

physical and hydraulic properties of these cored subsurface 

materials (Hoffmann and others, 2002) represent the smallest 

spatial scale in this investigation. The core-based data typically are on the order of several centimeters, but are scaled 

up to meters in this report. Water content extracted from the 

cores, and environmental tracers measured in these waters, 

represent a temporal scale that is a function of the thickness 

and hydraulic properties of the unsaturated zone: in general, 

these data represent a time scale of less than 2 years in this 

investigation. Seepage measurements made during sustained 

streamflow represent portions of a streamflow event and 

typically have time scales of a few hours to several days. 

Measurements of temperature and water content in vertical 

(one-dimensional) and two-dimensional profiles represent 

spatial scales that are typically less than 5 m and have temporal scales that vary from seconds to several days. Vertically 

nested piezometers were installed in the boreholes drilled 

in the stream channel to monitor water-level responses to 

streamflow. These measurements also represent a temporal scale that is a function of the thickness and hydraulic 

properties of the unsaturated zone and, in general, represents 

a time scale of weeks to several months in this investigation. Measurements of ground-water storage changes using 

temporal-gravity measurements have the largest spatial and 

temporal scales spanning several square kilometers and a 

period of record of several months to years. Data presented 

in this chapter were collected from 1999 through 2002. 

Previous Investigations

Smith (1910) probably was the first investigator to 

examine recharge along Rillito Creek. He concluded there was 

a difference in infiltration rates between the flashy, silt-laden 

summer flows, and the steady, long-duration flows of the 

winter snowmelt runoff. This conclusion was based partly on 

seasonal well hydrographs and ground-water temperature data. 

Investigators to follow, such as Schwalen and Shaw (1957) 

and Matlock (1965), also concluded that winter streamflow 

was the most effective source of recharge to the Tucson Basin. 

Burkham (1970) developed an empirical formula to estimate 

infiltration along a 15-km reach of Rillito Creek on the basis 

of streamflow losses between discharge measurement points. 

Davidson (1973) suggested that at least 90 percent of the 

amount of infiltrated water results in recharge. The remaining 

10 percent is lost to ET. Although not necessarily specific to 

Rillito Creek, the work of Wallace and Lane (1978) related 

infiltration potential to stream-channel order. Wallace and 

Lane concluded that the greatest infiltration potential occurs 

in the large-order streams because these streams contain the 

greatest volume of alluvium. Hanson and Benedict (1994) 

summarized previous estimates of recharge and developed new 

estimates on the basis of work by previous investigators and 

numerical simulation.
Introduction 187

0 10 MILES

0 20 KILOMETERS Base from U.S. Geological Survey

digital data, 1:100,000, 1982

Universal Transverse Mercator

projection, Zone 12

COCHISE COUNTY

 32°

110°30'

111°

 32°30'

 31°30'

110°30'

UNITED STATES

MEXICO

PIMA COUNTY

SANTA CRUZ COUNTY

PINAL COUNTY

PIMA COUNTY

ARIZONA

Santa Cruz

River Basin

Santa Cruz

River Basin

Study area

Rillito Creek

at naS

i R ver

Cruz

TWO-DIMENSIONAL TEMPERATURE

AND WATER-CONTENT ARRAY

AT DODGE BOULEVARD

9 METERS

COLUMNS

4 3 2 1

4 METERS

South

river bank

VERTICAL EXAGGERATION X 2

LOOKING EAST

UPSTREAM

LAND

SURFACE SRETEM 5.2

WHETSTONE

MOUNTA NI S

RINCON

MOUNTAINS

SANTA

RITA

MOUNTAINS 10

10

19

Tucson

Nogales

Figure 1. Location of study area, Pima County, Arizona.
188 Estimated Infiltration, Percolation, and Recharge Rates at the Rillito Creek Focused Recharge Investigation Site, Arizona

Figure 1.—Continued.

 32°22'30'' 111°

 32°15'

1

52'30'' 110°45'

Base from U.S. Geological Survey

digital data, 1:100,000, 1982

Universal Transverse Mercator

projection, Zone 12

Geology modified from Reynolds, 1988.

Central well field boundary modified from

Water Resources Research Center, 1999

Wash

ee r k

C

e

rd Ve

no i baS

Creek

Tanque

ta naS

a an

ad  ̃ C

del

EXPLANATION

STREAM ALLUVIUM

QUATERNARY ALLUVIUM

TERTIARY ALLUVIUM

VOLCANIC ROCK

CRYSTALLINE ROCK

CENTRAL WELL FIELD 

PRECIPITATION STATION CAMPBELL

AVENUE EXPERIMENTAL FARM

BOREHOLE AND BOREHOLE

 IDENTIFICATION

GRAVITY STATION

STREAMFLOW-GAGING STATION

 AND STATION IDENTIFICATION

VERTICAL-TEMPERATURE ARRAY

TWO-DIMENSIONAL TEMPERATURE AND

 WATER-CONTENT ARRAY

 

Or o Wash

R t li il

o

Creek

i R

ver

IDENTIFICATION OF STREAMFLOW-GAGING

STATIONS AND TEMPERATURE ARRAYS

1

2 3

4

Rillito Creek at

La Cholla Boulevard

1st Avenue verticaltemperature array

Dodge Boulevard two-dimensional

temperature and water-content array

and vertical-temperature array

Craycroft Road

verticaltemperature

array Tanque Verde

Creek

at Tucson

Rillito

Creek

at Dodge

Boulevard

Pantano Wash at

Broadway Boulevard

Dodge

Wa sh

Broadway

2

Swan

tforcyarC

daoR

daoR

allohC aL

draveluoB

Pima

1 ts eunevA 

Boulevard

draveluoB

TUCSON

MOUNTAINS

Cruz

10

SANTA CATALINA MOUNTAINS

3

4

(D-13-14)28dba

(D-13-14)26daa

(D-13-14)19bcbn

(D-13-13)16add

(D-13-13)13add

9486055

9486055

0 2 MILES

0 3 KILOMETERS

Pantano

l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l

l l l l l l l l l [ l l l l l l l l l

l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l

l l l l l l l l

l l l l l l l l

l l l l l l l l l

l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l

l

l l

l l l

l l l l l l l l 

l l l l l l l l 

9485700

9485700

9485450

(D-13-14)19bcbs
Hydrologic Setting 189

Hydrologic Setting

The climate of the study area is semiarid; annual rainfall 

averages 315 mm on the valley floor (fig. 2). There are two 

distinct seasons that account for most of the total precipitation; 

one (generally from July through September) is characterized by 

large summer convection and the other (generally from November through February) is characterized by frontal storms (fig. 2). 

The mean temperatures in January and July are 10oC and 29.6oC, 

respectively. To a lesser extent, there is a fall season of precipitation associated with tropical storms and climatic oscillations.

Rillito Creek is a tributary of the Santa Cruz River, which 

drains the Upper Santa Cruz Basin in southern Arizona. The 

Upper Santa Cruz Basin is in the Basin and Range Physiographic Province, which is characterized by block-faulted 

mountains separated by basins filled with alluvial sediments. 

The block-faulted mountains comprise Precambrian through 

Tertiary granitic, metamorphic, volcanic, and consolidated 

sedimentary rocks. The sediments that fill the basins are collectively termed alluvial basin-fill deposits and are composed 

of gravel, sand, silt, clay, and minor amounts of anhydrous 

sediments of Tertiary to Quaternary age. The basin-fill deposits generally are coarse grained along the basin margins and 

grade into finer-grained deposits and anhydrite deposits in 

the central parts of the basins. In the Upper Santa Cruz River 

Basin, thickness of the alluvium ranges from a thin veneer (a 

few meters) along the mountain fronts to as much as 3,400 m 

in the central parts of the basin (Davidson, 1973; Anderson, 

1987, 1988; Hanson and Benedict, 1994).

Recent stream-channel deposits and basin-fill deposits 

underlie Rillito Creek. The recent stream-channel deposits, consisting of fine- to coarse-grained alluvium, are about 10 m thick 

and are detritus from the surrounding mountain ranges. The 

basin-fill deposits, which underlie the stream-channel deposits, 

are regionally extensive sedimentary units that form the regional 

aquifer system. Previous investigators have divided the basinfill deposits into upper and lower basin-fill units on the basis of 

their general hydrogeologic characteristics (Pool, 1986; Hanson 

and Benedict, 1994). The upper basin-fill unit can be as much 

as 300 m thick. It consists mostly of unconsolidated to semiconsolidated 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 a few 

thousand meters thick and consists of conglomerates, gravels, 

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

1988). The lower basin-fill unit is represented by the Pantano 

Formation and the lower and middle Tinaja beds described by 

Anderson (1987, 1988).

Stream-channel infiltration is the predominant mechanism 

of recharge to the regional aquifer in the basin-fill deposits and, 

combined with contributions from other sources of recharge for 

the area, is less than the amount of water withdrawn to support the 

growing metropolitan population. As a result, water-level declines 

and related land subsidence have occurred in some areas. Depth 

to ground water immediately beneath Rillito Creek ranges from 

less than 6 m in the upper reach (near the mountain front) to 45 m 

near the Santa Cruz River (Hoffmann and others, 2002). Flow of 

ground water generally is northwestward; water-table elevations 

range from about 760 m in the southeast to 640 m in the northwest 

(Tucson Water, 2000). Ground water flows southwestward near 

the upper reach of Rillito Creek toward the major pumping center 

within the city of Tucson.

Rillito Creek has a drainage area of 2,256 km2

. It is 

ephemeral and most flows occur during the summer monsoon 

(July–September) and winter frontal storms (December–

March; fig. 2). Characteristic monsoon streamflows result 

from localized short-duration convective storms, whereas winter streamflows are produced by longer-duration frontal storms 

and accumulated snowmelt. To a lesser extent, there also is a 

fall season in which tropical storms and climatic oscillations 

often result in streamflow.

The creek has two major tributaries, Tanque Verde Creek 

and Pantano Wash; Rillito Creek begins at the confluence of 

these two tributaries. Tanque Verde Creek drains a 702 km2

 area 

from the Santa Catalina and Rincon Mountains; Pantano Wash 

drains a 1,554 km2 area between the Rincon, Santa Rita, and 

Whetstone Mountains. Several small washes divert runoff from 

the northeastern suburbs of Tucson into Rillito Creek. Rainfall 

runoff and snowmelt runoff from the Santa Catalina and Rincon 

Mountains contribute most of the flow to Rillito Creek. The 

creek flows westward to the Santa Cruz River from an elevation 

of 762 m at the confluence of Tanque Verde Creek and Pantano 

Wash to 657 m at its confluence with the Santa Cruz River. The 

creek is about 100 m wide and the channel slopes toward the 

Santa Cruz River at approximately 5.2 m/km with little variation 

in the slope. Flows in Rillito Creek typically are less than 28 

m3/s; the maximum recorded discharge was 680 m3/s during the 

1993 El Niño season (Tadayon and others, 2000). On average, 

Rillito Creek flows about 36 days per year at the streamflowgaging station Rillito Creek at Dodge Boulevard (09485700). 

The average annual flow is approximately 33.3×106 m3; about 

44 percent of the flow occurs from the summer monsoonal 

storms, whereas about 56 percent of the flow occurs from the 

winter frontal storms. 

The amount of water flowing in Rillito Creek, and therefore the amount available for recharge, is primarily related to 

precipitation frequency, distribution, and intensity, as well as 

to basin/channel runoff characteristics. The temporal distribution of flow in ephemeral streams is highly variable with 

observed decadal oscillations (Webb and Betancourt, 1992; 

Don Pool, Hydrologist, U.S. Geological Survey, written 

commun., 2003). Because of this, it is particularly difficult to 

estimate or predict recharge rates for ephemeral-stream channels on the basis of limited temporal observations. During the 

period of investigation there were two significant streamflow 

periods (fig. 3); one occurred in the summer of 1999 and the 

other in the fall of 2000 (mostly after September 30, or during water year 2001). Annual streamflow in Rillito Creek for 

the period of study was somewhat less frequent and smaller 

in volume than the long-term average (table 1). Prior to this 

study, a significant streamflow period occurred in the winter 
190 Estimated Infiltration, Percolation, and Recharge Rates at the Rillito Creek Focused Recharge Investigation Site, Arizona

and spring of 1998 (water year 1998) that was associated with 

El Niño precipitation and snowmelt. From February through 

April 1998, a total of 28.7×106 m3 flowed past the streamflowgaging station at Dodge Boulevard.

Infiltration, Percolation, and Recharge 

Rates

Physical and Hydraulic Properties of StreamChannel and Basin-Fill Deposits

In March and April 1999, five boreholes were drilled 

at four sites in the active channel of Rillito Creek (fig. 1) 

20

15

10

5

0

DEC.

30

A

40

25 50

30

20

60

10

0

JULY AUG. SEPT. OCT. NOV.

Mean annual

temperature

is 19.8 Celsius

Temperature

Precipitation

NOTE: Elevation is 710 meters

 above NGVD of 1929. Mean

 annual precipitation is 315

 millimeters

JAN. FEB. MAR. APR. MAY JUNE

TEMPERATURE, IN DEGREES CELSIUS

PRECIPITATION, IN MILLIMETERS

0

200

B

1999 2000 2001 2002

PRECIPITATION, IN MILLIMETERS

180

160

140

120

100

80

60

40

20

Figure 2. A, Monthly average temperature and precipitation, 1972–2002, at National Weather Service Station Campell Avenue 

Experimental Farm near Rillito Creek, Pima County, Arizona; B, monthly precipitation near Rillito Creek during period of study, 1999–2002.

to determine the physical and hydraulic properties of the 

stream-channel and basin-fill sediments down to about 10 

m below the water table (Hoffmann and others, 2002). Each 

borehole was drilled using the ODEX air-hammer method, 

which is also known as the under-reamer method (Driscoll, 

1986; Hammermeister and others, 1986). The ODEX method 

was used because it does not use fluids, thereby minimizing 

disturbance of the subsurface materials. At each hole, cuttings were collected about every 0.3 m. Fifty-one cores also 

were collected from these boreholes at 2- to 6-m intervals. 

The cores and cuttings were analyzed for physical properties, 

such as particle-size distribution, bulk density, particle density, 

porosity, volumetric water content, and percent saturation, and 

for hydraulic properties, such as saturated and unsaturated 

hydraulic conductivity, matric potential, and water-retention 

fitting terms. The detailed findings of these analyses are 
Infiltration, Percolation, and Recharge Rates 191

described in Hoffmann and others (2002). This section of the 

chapter focuses on the hydraulic properties of the sediments 

and their role in infiltration rates, velocity of the wetting front, 

and potential recharge.

In order for ephemeral streamflow within Rillito Creek to 

recharge the underlying aquifer, the water must first infiltrate 

into the stream-channel deposits and percolate downward 

through the underlying deposits. The ability of water to 

infiltrate and percolate through these deposits is primarily a 

function of stream discharge and hydraulic properties of the 

deposits. One-dimensional steady-state vertical flow through 

a homogeneous, isotropic medium can be described by a form 

of Darcy’s Law as:

q K z = − ( ) θ ∂ψ ( + ∂ 1) , (1)

where

q is the flux [L/T],

q is the volumetric water content,

K(h) is the hydraulic conductivity [L/T] as a 

function of the volumetric water content, 

w is the pressure head of the water phase [L], 

and

z is the vertical dimension [L].

Determination of the rate of flow requires knowledge of the 

hydraulic conductivity and saturation of the porous media, and 

the head gradient. Water continues to move within the unsaturated zone after it has infiltrated across the ground surface. 

This subsurface redistribution is described by the unsaturatedflow equation:

∂ ∂ = ∇ θ t q , (2)

with the flux, q, as defined above. Because redistribution is 

inherently transient and multidimensional, fewer simplifying 

0

1999 2000 2001 2002

DISCHARGE, IN CUBIC METERS PER SECOND

80

70

60

50

40

30

20

10

Figure 3. Mean discharge at Rillito Creek at Dodge Boulevard (streamflow-gaging station 09485700), 1999–2002, Pima County, Arizona.

Table 1. Annual streamflow measured at Rillito Creek at Dodge 

Boulevard (streamflow-gaging station 09485700), Pima County, 

Arizona, during period of study.

Water 

year1

Total 

annual 

streamflow, 

in cubic 

meters

Annual 

flow as a 

percentage 

of long-term 

annual 

streamflow

Percentage 

of annual 

streamflow that 

occurred in 

summer

Percentage 

of annual 

streamflow that 

occurred in 

winter

1999 11 × 106 33 98 2

2000 3.5 × 106 10.5 100 0

2001 19.6 × 106 59 5 95

2002 2 × 106 6 99 1

1

Water year extends from October 1 through September 30 and is designated by the calendar year in which it ends.

assumptions can be applied in the analysis of redistribution 

than can be applied to infiltration. To fully characterize subsurface redistribution, measurements of both water flux and 

changes in subsurface water storage must be made repeatedly 

throughout the unsaturated zone. 

As shown in the section titled “Temperature and Water 

Content,” vertical infiltration rates at the onset of infiltration 

were as high as 22 mm/s because of high hydraulic permeability, low antecedent water content, and resulting large 

capillary gradients. Two-dimensional flow is also evident, and 

lateral velocities were about the same as vertical velocities. 

Shortly after the onset of infiltration, however, the near-surface 

stream-channel deposits are saturated, and large capillary 

gradients decline. Flow of water becomes predominantly 

vertical, as gravity is the dominant process controlling the 
192 Estimated Infiltration, Percolation, and Recharge Rates at the Rillito Creek Focused Recharge Investigation Site, Arizona

direction of flow. From Darcy’s Law, the flow rate through the 

sediments under saturated conditions is equal to the product 

of the hydraulic conductivity and the hydraulic gradient. If 

both hydraulic conductivity and gradient are known, then the 

flow rate can be calculated. This method of flow-rate estimation can be used for saturated and unsaturated conditions. 

Assuming properties of the pore water are constant, saturated 

hydraulic conductivity (Ksat) is a constant and is related to the 

texture and structure of the sediment. Unsaturated hydraulic 

conductivity is not a constant as it decreases rapidly as water 

content decreases. As surface flow proceeds, the infiltrated 

water moves farther below the surface of the streambed, capillary forces become less significant, and the hydraulic gradient approaches unity. If a unit gradient is assumed, the rate 

of infiltration becomes equivalent to the saturated hydraulic 

conductivity of the channel deposits.

On the basis of findings from the cuttings and cores, 

the stream-channel deposits beneath Rillito Creek are coarse 

grained, typically consisting of more than 95 percent gravel 

and sand. The underlying basin-fill deposits also are sandy 

gravels or gravelly sands, but typically contain more silt and 

clay than the stream-channel deposits. 

Saturated vertical hydraulic conductivity of the deposits 

positively correlates with grain size (fig. 4). Values for the 

stream-channel deposits range from 0.3 to 2.5 m/d, whereas 

values for the basin-fill deposits tend to be less than about 

0.6 m/d and in places are as low as 0.012 m/d. For heterogeneous media, such as the deposits beneath Rillito Creek, the 

equivalent vertical hydraulic conductivity is calculated as the 

harmonic mean of the Ksat for each layer within the deposits and is always less than the arithmetic mean (Freeze and 

Cherry, 1979). Although differing at each borehole, the overall 

average equivalent hydraulic conductivity of the streamchannel deposits is 1.2 m/d; the overall average equivalent 

hydraulic conductivity of the basin-fill deposits is 0.19 m/d; 

and the equivalent hydraulic conductivity of the combined 

sediments (stream-channel and basin-fill deposits) is 0.23 m/d. 

The calculated average vertical hydraulic conductivity for the 

basin-fill sediments reported by Hoffmann and others (2002) 

includes values associated with a fine-grained unit found in a 

lower reach of Rillito Creek near the Santa Cruz River. These 

values typically are as low as 0.012 m/d and may not be representative of the hydraulic conductivity in upstream reaches. 

Excluding the hydraulic-conductivity values for the basin-fill 

sediments in the lower reaches where the fine-grained unit 

was present, the average vertical hydraulic conductivity of the 

study area is 0.3 m/d. Assuming a unit gradient, equivalent 

vertical hydraulic conductivity values are equivalent to the rate 

of infiltration and provide an estimate of potential recharge 

rates under saturated conditions.

Saturated conditions will exist only after sustained periods of streamflow infiltration at a rate that enables water to 

fully saturate the underlying sediments from the streambed to 

the aquifer. Once a saturated hydraulic connection is achieved 

between the stream and underlying aquifer, the system behaves 

as though the stream were perennial. Unsaturated hydraulicFigure 4. Relation of saturated hydraulic conductivity to sand, silt, and clay content for cores collected from boreholes drilled along 

Rillito Creek, Pima County, Arizona.

0 

100 

3 

2 

1 

R U T A S C I L U A R D Y H DE T A 

Y A D RE P SR E T E M NI ,Y T I V I T C U D N O C 

0 10 20 30 40 50 60 70 80 90 

SILT AND CLAY, IN PERCENT BY WEIGHT 

EXPLANATION

STREAM-CHANNEL DEPOSITS BASIN-FILL DEPOSITS 
Infiltration, Percolation, and Recharge Rates 193

Figure 5. Relation of volumetric water content to sand, silt, and clay content for cores collected from boreholes drilled along Rillito 

Creek, Pima County, Arizona.

conductivity values need to be considered when estimating 

potential recharge rates for conditions prior to full saturation. 

Unsaturated hydraulic conductivity of the deposits beneath 

Rillito Creek varies by several orders of magnitude as a function of water content. For water-content conditions at the time 

of core collection, the unsaturated hydraulic conductivity was 

generally two or more orders of magnitude less than the saturated hydraulic conductivity (Hoffmann and others, 2002).

Antecedent pore water underlying Rillito Creek derives 

from streamflow infiltration and subsequent percolation. In 

this study, cores were collected in late March 1999; therefore, 

antecedent pore water was derived from streamflows prior 

to March 1999. Several flow events prior to 1999 could have 

been sources of the antecedent water. On the basis of streamflow records at the streamflow-gaging station at Dodge Boulevard, the most recent flow prior to core collection occurred 

in November 1998 and lasted for 2 days (average streamflow 

was less than 0.82 m3

/s). The last recorded flow prior to 

November 1998 occurred in the summer of 1998 and lasted 

for about 1 day. The most voluminous and long-lasting flow 

within a few years of the core collection was the sustained 

flow from February to April 1998, which was related to the 

1998 El Niño precipitation.

Volumetric water content in the unsaturated zone ranged 

from 0.02 to 0.46 at the time of core collection (fig. 5). Variability in water content primarily is controlled by differences in 

sediment texture and is positively correlated with the percentage 

of fine-grained material (fig. 5). The stream-channel deposits 

averaged 17.8 percent water content and 57.6 percent saturation; the basin-fill deposits averaged 24 percent water content 

and 69.3 percent saturation. Integrating the water content over 

the thickness of the unsaturated zone, cumulatively, the unsaturated sediments beneath Rillito Creek contained 0.5 to 12.2 m 

of water. The smallest amount of water was in the upstream 

area at the borehole ((D-13-14)26daa) near Craycroft Road 

where the unsaturated zone was about 3 m thick; the largest 

amount of water was in the downstream area at the borehole 

((D-12-14)26add) near La Cholla Boulevard where a 12-m thick 

fine-grained unit lies above the water table. The sites probably 

most representative of the unsaturated zone beneath Rillito 

Creek are in the middle reaches where the unsaturated zone was 

30 to 40 m thick and had 6.1 m of water (boreholes (D-13-

14)19bcbn, (D-13-14)19bcbs, and (D-13-14)28dba). The large 

amount of water stored in the unsaturated zone indicates that 

much of this water probably originated from the sustained flows 

prior to the summer of 1998, that the stored water is likely to 

be from several different streamflow and infiltration events, and 

that the sediments beneath Rillito Creek drain slowly.

Environmental Tracers

Environmental tracers of tritium (3H), oxygen-18 (18O), 

deuterium (2H or D), and chloride from the pore waters in the 

unsaturated and saturated zones were analyzed to evaluate spatial variations in infiltration and recharge patterns along Rillito 

Creek. Tritium is a naturally occurring radioactive isotope 

of hydrogen with a half life of 12.43 years. Large concentrations of tritium were introduced into the atmosphere beginning in 1952 as a result of the atmospheric testing of nuclear 

0.4 

0.3 

0.2 

0.1 

0 

SILT AND CL AY FRACTION, IN PERCENT BY WEIGHT 

0 10 20 30 40 50 60 70 80 90 100 

0.6 

0.5 

C I B U C NI ,T N E T N O C RE T A W CI R T E M U L O V 

R E T E M I T N E C CI B U C RE P SR E T E M I T N E C 

Local site identifier: 

 (D-13-13)16add 

 (D-13-14)19bcbn 

 (D-13-14)19bcbs 

 (D-13-14)28dba 
194 Estimated Infiltration, Percolation, and Recharge Rates at the Rillito Creek Focused Recharge Investigation Site, Arizona

Figure 6. Profile of tritium content in water collected from 

boreholes drilled along Rillito Creek, Pima County, Arizona.

60

DEPTH BELOW LAND SURFACE, IN METERS

0

10

20

30

40

50

0 2 4 6

GW GW

GW

GW

GW

GW GW

GW

GW GW

TRITIUM UNITS

8 10 12

NOTE: Samples labeled GW indicate

 ground water was pumped from

 beneath the water table

EXPLANATION

CRAYCROFT ROAD

DODGE BOULEVARD

FIRST AVENUE NORTH

FIRST AVENUE SOUTH

LA CHOLLA BOULEVARD

weapons. The input of tritium into the atmosphere related to 

nuclear weapon testing peaked at nearly 6,000 tritium units 

(TU) in 1963–64 prior to a ban on the tests. Atmospheric 

tritium concentrations returned to natural conditions by 1992 

(Dr. Chris Eastoe, geochemist, University of Arizona, written 

commun., 2003), and the concentration of tritium in precipitation today is about 5 to 7 TU. Tritium concentrations are often 

used for dating ground water and to detect events, such as the 

1963–64 peak. In this study, tritium was measured in water 

vacuum extracted from cores and analyzed by liquid scintillation with electrolytic enrichment (Thatcher and others, 1977) 

at the USGS laboratory in Menlo Park, California. Tritium was 

detected in pore water extracted from each core sample and 

ranged in concentration from 2 to 11 TU (fig. 6). The precision 

of individual measurements was ±0.3 TU. For the purposes of 

this study, waters having tritium concentrations in this range 

are interpreted as having infiltrated within the past 40 years. 

Tritium concentrations within a given profile could not be used 

to identify an event marker, such as the 1963 peak.

Given the presence of tritium in the unsaturated zone, 

the locally high vertical hydraulic conductivity values of more 

than 1 m/d in the stream-channel deposits, and depths to the 

water table of generally less than 40 m, it is likely that most 

of the pore water extracted from the cores was derived from 

runoff events during the few years prior to 1999.

Assuming negligible mixing of infiltrated waters, variations in isotopic signatures and chloride concentrations can be 

used to identify individual runoff and infiltration events. These 

unique signatures remain intact during infiltration and allow 

direct identification of runoff events as the infiltrated water 

migrates through the unsaturated profile. Sampling through 

the unsaturated zone at a point in time provides a snapshot of 

the isotopic and chemical signatures throughout the profile. 

This snapshot represents the composition of the water that 

infiltrated into the sediment over a period of time—the deepest 

water representing the beginning of the period. The downward 

rate of water movement at a particular site is calculated using 

the time elapsed between the runoff event and depth of infiltration of the water in the profile.

Oxygen and hydrogen isotopic compositions were determined for water extracted from cores by azeotropic distillation 

(Revesz and Woods, 1990) with toluene at the USGS laboratory in Reston, Virginia, using analytical methods described 

by Epstein and Mayeda (1953). Isotopic variations in water 

are expressed as a per mil ratio (d value), which is a ratio of 18O/16O and D/1

H in a sample relative to the ratio in an ocean 

water standard (Clark and Fritz, 1997). The delta symbol in 

this report is followed by the chemical symbol for the heavier 

isotope measured during isotopic analysis. Isotopic values are 

described as lighter or heavier in relation to each other. Lighter 

isotopic values are smaller or more negative per mil values, 

and heavier isotopic values are larger or more positive per mil 

values.

The source of precipitation is evaporation of seawater; 

therefore, the d18O and dD composition of precipitation is 

linearly correlated, which is known as the meteoric water line 

(MWL; Craig, 1961). Values of d18O and dD in precipitation 

vary from event to event and also seasonally. Many variables 

influence the isotopic signature of a precipitation event, 

including the origin, travel path, and duration of the storm, 

and the elevation and temperature of condensation prior to 

rainfall. In general, however, variations in isotopic signature 

are predominately temperature dependent. The cool, high-altitude precipitation events produce water having lighter isotopic 

ratios than water from the warm precipitation events. In addition, evaporation will result in d18O–dD data for the remaining water that plot to the right of the MWL. A more detailed 

discussion on variations of isotopic ratios can be found in 

Clark and Fritz (1997).

Values of d18O and dD in waters extracted from the 

unsaturated zone range from about -12 to -6 per mil, and 

-80 to -37 per mil, respectively (fig. 7). Data for many of the 

samples indicate the effect of evaporation (fig. 7). Isotopic 

compositions from the Dodge Boulevard site generally indicate the greatest amount of evaporation, and isotopic data from 

a sample collected at First Avenue North from a depth of less 

than 1 m indicated the greatest amount of evaporation for any 

sample (fig. 7). Variations in isotopic compositions of water 

from the unsaturated zone beneath Rillito Creek are, therefore, 

attributed to both changes in the isotopic signatures of the 

source water and to evaporation. Those samples that lack an 

evaporative signal indicate that percolating waters were not 

exposed to significant evaporation prior to infiltration.
Infiltration, Percolation, and Recharge Rates 195

–95

–35

–45

–55

–65

–75

–85

–14 –13 –9 –8

D18O, PER MIL

EXPLANATION

–12 –11 –10 –7 –6 –5 –4

CRAYCROFT ROAD

DODGE BOULEVARD

FIRST AVENUE NORTH

FIRST AVENUE SOUTH

LA CHOLLA BOULEVARD

–95

DDEUTERIUM, PER MIL

–35

–45

–55

–65

–75

–85

Global meteoric

water line

Evaporative

trend line

Global meteoric

water line

A

B

Figure 7. Stable isotope data of pore water collected from 

cores collected from boreholes drilled along Rillito Creek, Pima 

County, Arizona. A, unsaturated zone; B, below the water table.

Several trends were evident in the isotopic ratios of Rillito 

Creek pore water. One trend is related to the location in the 

stream reach where the samples were collected. The waterweighted mean isotopic signature of the pore water in the 

unsaturated zone generally becomes lighter in the downstream 

direction (fig. 8). The water weighted mean uses the water content of the core as a weight that is multiplied by the isotopic ratio 

of the extracted water. Pore water in the borehole near Dodge 

Boulevard ((D-13-14)28dba) had d18O and dD values of -8.0 of 

-55.0 per mil, respectively, whereas pore water downstream in 

the borehole near LaCholla ((D-13-13)16add) had d18O and dD 

values of -9.5 and -64.0 per mil, respectively. At the intermediate boreholes near First Avenue ((D-13-14)bcbn and (D-13-

14)19bcbs), d18O and dD values were -8.8 of -60.3 per mil, 

respectively. These variations are larger than the analytical precision (2-sigma values of 0.2 and 2 for d18O and dD, respectively; 

thus, in 95 percent of repeat analyses the same sample would 

result in d18O within 0.2 per mil of the originally reported value 

and the dD values would be within 2 per mil of the originally 

reported value). Evaporative effects would cause a trend opposite to the observed data; therefore, the trend is likely a function 

of the origin and season of precipitation events that resulted in 

streamflow and subsequent infiltration at the downstream sites. 

Specifically, for the time period represented by the unsaturated 

zone pore water, it is the longer duration and isotopically lighter 

winter storms that were more important contributors to infiltration in the downstream reaches than in the upstream reaches. The 

exception to this trend is at the uppermost borehole near Craycroft Road ((D-13-14)26daa) where the lightest values, a d18O of 

-10 per mil, and a dD of –65 per mil, were measured. Depth to 

water at this site, however, is commonly only a few meters below 

the streambed; therefore, water in the unsaturated zone at the 

time of core collection probably is representative of infiltration 

only from the most recent streamflows.

Another trend in the isotopic data is that water from the 

saturated zone has d18O and dD values that are consistently 

lighter than those in water from the unsaturated zone and do 

not reflect evaporation effects as do those in water from the 

Figure 8. Weighted average of stable isotope values from 

pore water collected from boreholes along Rillito Creek Pima 

County, Arizona.

–66

DDEUTERIUM, PER MIL

–52

–54

–56

–58

–60

–62

–64

–10.0 –8.5

D18O, PER MIL

–7.5

Average Upstream

Weighted average

UZ weighted average

–9.5 –9.0 –8.0

Downstream

Dodge

Boulevard

First Avenue South

First Avenue North

La Cholla Boulevard

Craycroft Road
196 Estimated Infiltration, Percolation, and Recharge Rates at the Rillito Creek Focused Recharge Investigation Site, Arizona

Figure 10. Comparison of stable isotope values in vertical profile to stable isotopic values of precipitation.

Figure 9. Hydrograph of streamflow gaging station 09485700 Rillito Creek near Dodge Boulevard (09485700) and associated stable 

isotope values determined for precipitation.

0

1997 1998 1999

–20.0

–51.67

–38.0

–64.3 –31.9

–59.15

–52.3

–47.29

–63.43

MEAN DAILY DISCHARGE,

IN CUBIC METERS PER SECOND

55

45

35

25

15

5

50

40

30

20

10

NOTE: Values shown above streamflow

 hydrograph are Adeuterium

 for precipitation at Tucson

Snowmelt,

likely to be

depleted in

deuterium

Drilling and

coring begins

1998

DEPTH BELOW LAND SURFACE, IN METERS

NOV.

OCT.

SEPT.

AUG.

JULY

JUNE

MAY

APR.

MAR.

FEB.

DDEUTERIUM FOR PRECIPITATION, PER MIL

0

5

10

15

20

25

30

35

40

45

–75 –50

DDEUTERIUM FOR VERTICAL PROFILE AT DODGE BOULEVARD, PER MIL

–35

Water table

Unsaturated zone,

pore water

–70 –65 –60 –55 –50 –45 –40 –35 –30 –70 –65 –60 –55 –45 –40

unsaturated zone (fig. 8). This lighter isotopic composition 

in the ground water from beneath the water table indicates a 

greater influence of winter and (or) higher elevation precipitation in the ground water than for the water in the unsaturated 

zone at the time of collection. The lack of an evaporative signal indicates that water that reaches the water table is exposed 

to minimal evaporation.

Oxygen-18 (d18O) and dD data for precipitation in 

the Tucson Basin collected prior to the study period were 

analyzed to define a possible isotopic input function to the 

system. Isotopic compositions for precipitation in the Tucson 

Basin that resulted in Rillito Creek streamflow indicate that 

light compositions are associated with the winter 1998 El 

Niño weather pattern, whereas compositions are varied for the 

summer precipitation events of 1997 and 1998 (fig. 9). 

A substantially heavy isotopic signal was associated with 

precipitation in April and August 1998. Assuming conservative behavior, isotopic compositions measured in the precipitation are possible event markers that might be identified in 

the unsaturated zone. In order to identify event markers in 

the unsaturated zone, vertical profiles of d18O and dD were 

compared to d18O and dD in samples of runoff (fig. 10). This 

analysis was done for the borehole near Dodge Boulevard 

because it is near the streamflow-gaging station at Dodge 
Infiltration, Percolation, and Recharge Rates 197

Boulevard (09485700). The hydrograph for this gaging station 

was used to determine timing of streamflow prior to drilling 

and coring. The trends in isotopic compositions measured 

in the pore-water cores approximately match the pattern of 

signatures in the basin precipitation from particular storms 

that occurred from February to November 1998. For instance, 

the heavy isotopic compositions in precipitation in April 

and August 1998 are possibly identified in pore water from 

depths of 6.5 and 23 m, respectively (fig. 10). Below 23 m, the 

compositions become lighter and are similar to those of the El 

Niño precipitation events about 14 months prior to the drilling 

and coring. If this interpretation is correct, water in the unsaturated zone below about 23 m is likely to have originated from 

sustained flows related to 1998 El Niño precipitation. Discrete 

pore-water sampling and minor mixing of pore water within 

the unsaturated zone can explain why the values for precipitation tend to have a wider range (-32 to -65 dD) than values for 

the unsaturated zone pore water (-40 to -61 dD). Correlation 

between the precipitation events and depths of infiltration, on 

the basis of corresponding isotopic signatures, indicates an 

average vertical linear velocity of approximately 0.049 m/d at 

the borehole near Dodge Boulevard. This velocity represents 

the downward percolation rate of water. Percolation is defined 

here as the flow of water that has infiltrated and is moving 

downward or lateral toward the water table. Note that infiltration connotes movement of water into sediments, in contrast 

to percolation, which connotes movement through sediments. 

Owing to the decrease in hydraulic conductivity with decreasing water content, the percolation rate is less than the measured saturated hydraulic conductivity.

Chloride concentrations in pore-water leachate were 

determined from drill cuttings at 0.3-m depth intervals. To 

derive the leachate, 50 mL of distilled water was mixed with 

50 g of sieved cuttings having a particle size of less than 

1 mm. The mixture was stirred and the specific conductance 

measured. An ion-specific probe was used to measure the 

chloride concentration after the specific conductance stabilized. Chloride concentrations are reported for the boreholes 

near Dodge Boulevard and near First Avenue (fig. 11).

Chloride concentrations at the borehole near La Cholla 

Boulevard between about 10 and 30 m are not presented 

because the fine-grained unit at this depth made leaching and 

sieving difficult, and therefore the results were suspect. Chloride concentrations are not presented for the borehole near 

Craycroft Road because of the shallow water table there.

Chloride concentration in runoff varies as a function 

of the precipitation location and the runoff travel path and 

surface-contact time. Chloride concentrations measured in the 

profile varied substantially through the upper 18 m at all sites 

(fig. 11). Below about 18 m, the variation in concentration 

declined considerably. On the basis of d18O and dD data discussed previously, this zone of smaller variation corresponds 

to the infiltration depth of the 1998 El Niño water. The small 

variation and low mean chloride concentration are attributed 

to infiltration from sustained streamflow having a low chloride 

concentration. In addition, the low chloride concentration indicates the water had little exposure to evaporation. The greater 

variation and higher concentration observed in post-El Niño 

pore water are likely due to mobilization of chloride from 

evaporative concentrates and dry fallout on the streambed and 

tributaries deposited between precipitation events. Calculation 

of a downward percolation rate using an event marker in the 

chloride profile at Dodge Boulevard yields an average linear 

velocity of 0.055 m/d, and corresponds closely to the rate 

calculated using the stable-isotope data. On the basis of the 

environmental tracers measured at the borehole near Dodge 

Boulevard, pore water in the unsaturated zone represents water 

that infiltrated into the sediments within the 12 to 14 months 

prior to drilling, and approximately half of the water (water 

in the deeper half of the unsaturated zone) infiltrated from the 

previous El Niño runoff event.

Multiplying the downward percolation rate by the volumetric water content results in the flux of recharge that reaches 

the water table. Using a percolation-rate estimate of 0.05 m/d, 

an average volumetric water content of 13 percent measured 

in the cores from the borehole near Dodge Boulevard borehole 

(Hoffmann and others, 2002), and a wetted perimeter of 25 m, 

results in a flux across the water table of 0.002 cubic meters 

per second per linear kilometer of streamflow (m3

/s/km).

Seepage Measurements

Seepage measurements were made at several sites 

along Rillito Creek during the El Niño-related sustained 

flows of March through April 1998 to determine infiltration 

rates for different stream reaches (Don Pool, hydrologist, 

U.S. Geological Survey, written commun., 2003). Findings 

from the 1998 seepage measurements are summarized here 

because, on the basis of stable-isotope ratios and chlorideconcentration profiles, infiltration from the sustained El Niño 

flow is likely to have provided most of the water stored in 

the unsaturated zone at the time of borehole drilling and core 

collection. Streamflow losses owing to infiltration along Rillito Creek ranged from 0.07 to 0.9 m3

/s/km and were largest 

downstream from Swan Road (fig. 1). The small streamflow losses in the upstream area were attributed to rejected 

recharge; depth to ground water upstream from Swan Road 

was near land surface after several days of sustained flow, 

whereas the depth downstream from Swan Road was typically greater than 30 m. 

Seepage measurements were made again at four 

sites along Rillito Creek during an 8-hr period on 

October 24, 2000. Unlike the 1998 seepage measurements, 

the October 2000 seepage measurements did not isolate 

streamflow-infiltration rates relative to Swan Road; however, measured streamflow-infiltration rates generally agreed 

with those measured in 1998, as they ranged from 0.09 

to 0.4 m3

/s/km and averaged 0.22 m3/s/km (fig. 12). The 

October 2000 seepage measurements were made during an 

8-hr period along a 14-km reach having an average wetted 

perimeter of about 25 m. Using a wetted perimeter of 25 m 

results in an average infiltration rate of 0.75 m/d, which is 
198 Estimated Infiltration, Percolation, and Recharge Rates at the Rillito Creek Focused Recharge Investigation Site, Arizona

Figure 11. Profile of concentration of chloride from cuttings leachate from boreholes drilled along Rillito Creek, Pima County, Arizona. DEPTH BELOW LAND SURFACE, IN METERS

0

CHLORIDE CONCENTRATION, IN MILLIGRAMS PER LITER

18

45

5

10

0

141210864 16

First Avenue South

2

15

20

25

30

35

40

DEPTH BELOW LAND SURFACE, IN METERS

0

CHLORIDE CONCENTRATION, IN MILLIGRAMS PER LITER

18

45

5

10

0

141210864 16

Dodge Boulevard

2

15

20

25

30

35

40

DEPTH BELOW LAND SURFACE, IN METERS

0 18

45

5

10

0

141210864 16

La Cholla Boulevard

2

15

20

25

30

35

40

DEPTH BELOW LAND SURFACE, IN METERS

0 18

45

5

10

0

141210864 16

First Avenue North

2

15

20

25

30

35

40

similar to the measured saturated hydraulic conductivity 

of the stream-channel deposits (1.2 m/d), but more than an 

order of magnitude greater than percolation rates determined 

by tracers. The difference between the estimated infiltration 

and percolation rates probably is due to several factors, such 

as (1) the variably saturated nature of the sediments—the 

near-surface stream-channel deposits are likely to be nearly 

saturated during streamflow and, therefore, are approaching 

their saturated hydraulic conductivity, whereas the deeper 

sediments are less than fully saturated and will, therefore, 

have a lower hydraulic conductivity; (2) some water will 

spread laterally reducing the downward flux, and (3) percolation rates determined by tracers reflect average rates 

over longer periods of time than infiltration rates, including 

periods when the creek is not flowing.

Assuming 6 m of water in storage in the unsaturated 

zone beneath the creek (as determined by volumetric watercontent measurements made on the cores; fig. 5) and a 

stream width of 100 m, the amount of pore water beneath 

every kilometer of the creek at the time of drilling and 
Infiltration, Percolation, and Recharge Rates 199

Figure 12. Seepage loses during October 8, 2000 streamflow in 

Rillito Creek, Pima County, Arizona. INFILTRATION RATE, IN CUBIC METERS PER KILOMETER

90,000

80,000

70,000

60,000

50,000 Cumulative

infiltration

40,000

30,000

20,000

Infiltration

rate

10,000

0

0

DISTANCE DOWNSTREAM FROM CRAYCROFT ROAD

TO LA CHOLLA BOULEVARD, IN KILOMETERS

5 15

0

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

10

Craycroft Road La Cholla Boulevard 

CUMULATIVE INFILTRATION, IN CUBIC METERS

coring was 6 × 105 m3. There was less water in the upper 

reach where the unsaturated zone is shallower and more 

water in the downstream reach where there is an 18-m-thick 

fine-grained unit having high water content. Assuming an 

average streamflow loss rate of 0.22 m3/s/km, about 33 days 

of streamflow infiltration would be needed to infiltrate the 

amount of water stored in the unsaturated zone. There were 

about 31 days of cumulative flow in the creek in the 12 

months prior to drilling and coring; about two-thirds of the 

flow days were associated with the El Niño snowmelt runoff 

that occurred about 1 year before the drilling and coring. 

Assuming the water in the bottom two-thirds of the unsaturated zone collected in 1999 originated from surface water 

flowing a year earlier, the average downward percolation rate 

would be about 0.04 to 0.09 m/d, which brackets the percolation rate estimated by environmental tracers. This analysis of 

seepage and cumulative streamflow duration required to provide the amount of water held in storage in the unsaturated 

zone provides an independent method of estimating the age 

of water in the unsaturated zone, estimating infiltration and 

percolation rates, and substantiates the interpretation made 

on the basis of the environmental tracers.

Temperature and Water Content

One-Dimensional Temperature Monitoring and 

Modeling

Heat can be transferred through sediments by a combination of advection and conduction. Although both advective and 

conductive heat transport occur during infiltration, advective 

heat transport is more prevalent in high water-flux settings, 

whereas conductive heat transport is more prevalent in very low 

or no water-flux conditions. For most hydrologic applications 

related to infiltration through alluvial sediments, advection is the 

primary mechanism for the transport of heat by flowing water, 

and conductive heat transport is regarded as a negligible component of heat transfer (Constantz and others, 2003).

In this study, heat as a tracer was used to estimate onedimensional vertical infiltration by inversely determining the 

vertical saturated hydraulic-conductivity profiles beneath the 

streambed. Stream-channel deposits were instrumented with 

vertical nests of thermistors at three sites along Rillito Creek 

(fig. 1). Thermographs predicted by numerical simulations 

were fitted to measured thermographs from the field by adjusting model parameters within appropriate ranges until the least 

error (best match) was found between simulated and measured 

thermographs. The three vertical-temperature arrays are buried 

in the stream-channel deposits near the boreholes (fig. 1). One 

array is near Craycroft Road in the upper reach of the study 

area; one is near Dodge Boulevard in the middle reach of 

the study area; and the other is near First Avenue also in the 

middle reach of the study area.

The simulation domains for the Rillito Creek models 

were represented numerically in one dimension, oriented 

vertically. The upper boundary and datum of the simulation 

domains was the streambed. The lower boundary was at or 

near the depth of the deepest measurements of temperature or 

pressure, or both. For the simulations, time-varying hydraulic 

and temperature potentials were defined at the upper- and 

lower-domain boundaries.

The hydraulic head at the streambed is equivalent to the 

stream stage. USGS streamflow-gaging stations provided 

measurements of stage after correction for the datum elevation. Streambed temperatures were measured by thermistors 

buried about 0.2 m below the streambed in the most active 

part of the channel. Pressure-head measurements from 

piezometers were used to define heads at the lower boundary 

of the three study sites. The lower-boundary temperatures 

for the Craycroft Road site were inferred from thermistor 

measurements near the lower-boundary depth. The lowerboundary temperatures coinciding with the water table for 

the Dodge Boulevard and First Avenue sites were approximated from ground-water temperatures measured within 5 

meters of the water table at those sites. The inverse simulations were made with a numerical coupled water-flow and 

heat-transport model, VS2DH (Healy and Ronan, 1996), that 

was linked with parameter estimation software (PEST). A 

detailed description of the theory, modeling approach, and 

findings of this investigation is documented in Bailey (2002).

Infiltration at Craycroft Road Site

Infiltration for two periods of sustained streamflow 

were modeled for the upstream-most site (near Craycroft 
200 Estimated Infiltration, Percolation, and Recharge Rates at the Rillito Creek Focused Recharge Investigation Site, Arizona

Road). The first model period extends from July 20, 1999, 

to July 27, 1999. The second modeled period extends from 

July 29, 1999, to August 2, 1999. Streamflow and stage at 

the nearby streamflow-gaging station (Tanque Verde Creek at 

Tucson) fluctuated between 2.23 and 0.33 m3/s, and between 

0.26 and 0.11 m, respectively, during the first modeled flow 

period. Streamflow decreased from 1.11 to 0.05 m3/s and 

stage decreased from 0.20 to 0.03 m during the second modeled flow period. The wetted perimeter of the streambed near 

the Craycroft Road site during these periods was about 10 m.

The measured thermograph at a depth of 1.2 m for the 

first modeled period shows a characteristic sinusoidal pattern that varies between about 24 and 26.5o

C, whereas the 

thermograph for the second modeled period is generally flat 

and varies only between 25 and 25.5oC. Model simulation 

at the Craycroft Road site approximately reproduced the 

thermograph of the observed data for both model periods 

(fig. 13). Model simulations were optimized on vertical 

saturated hydraulic conductivity. Simulated vertical saturated hydraulic-conductivity values were in general agreement with those measured in the laboratory (Bailey, 2002; 

Hoffmann and others, 2002). A physical change within the 

streambed between flow periods at this site required the 

addition of a thin surface layer having a low vertical hydraulic conductivity within the model domain. The addition of 

this surface layer resulted in a lowering of the simulated 

equivalent saturated hydraulic conductivity by four orders 

of magnitude, from about 4 m/d to 3 × 10–4 m/d. The four 

orders of magnitude change in hydraulic conductivity is too 

large to result solely from changes in water viscosity owing 

to temperature changes. Given the tranquil flow conditions 

during the first flow period, it is possible that the change 

was the deposition of a fine-grained layer at the streambed 

surface. In fact, a thin layer of fine-grained material commonly was observed at the Craycroft Road site after small 

streamflow events.

Flow in the creek typically resulted in hydraulic connection between streamflows and ground water (see section titled 

“Water-Level Responses”). Thus, vertical gradients measured at 

the vertically nested piezometer at the borehole near Craycroft 

Road enabled estimation of infiltration rates using simulated 

equivalent vertical saturated hydraulic conductivity. Vertical 

hydraulic gradients measured from these piezometers typically ranged from 0.06 to 0.2 m/m and averaged 0.1 m/m. The 

highest gradients occurred during and shortly after streamflow. 

Assuming a typical gradient during and shortly after streamflow 

(0.2 m/m) and a wetted perimeter of 10 m, estimated infiltration 

rates ranged from 0.09 m3

/s/km during the first modeled period 

to 8×10-6 m3/s/km during the second modeled period. The first 

modeled period is probably most representative of the typical 

ephemeral-streamflow conditions; the second period of streamflow modeled is probably most representative of small flows 

that occur after a layer of fine-grained deposits has been deposited. The infiltration rate of 0.09 m3

/s/km for the first modeled 

period is about half of that estimated by seepage measurements 

(average of 0.21 m3/s/km).

Infiltration at Dodge Boulevard and First Avenue sites

Infiltration also was modeled for two time periods at 

the Dodge Boulevard and First Avenue sites: the first modeled period extends from July 14, 1999, to July 17, 1999, and 

the second modeled period extends from July 25, 1999, to 

July 29, 1999. The proximity of the thermistors to the streamflow-gaging station 09485700 at Dodge Boulevard allowed 

direct use of the gaging-station measurements to define the 

hydraulic head at the upper boundary within the Dodge Boulevard model domain. These boundary conditions also were 

used for the First Avenue model domain. Discharge and stage 

for these two periods reached maximums of 254 m3/s and 2.7 

m, respectively. The wetted perimeter for these flows was 

about 10 m. An important difference between these sites and 

the site near Craycroft Road is that the depth to the water table 

near the Dodge Boulevard and First Avenue sites is greater 

than 30 m; therefore, ephemeral flow in the stream channel at 

these sites may never result in hydraulic connection between 

the stream and the aquifer.

Thermographs from a depth of 0.8 m at the Dodge 

Boulevard site for the two modeled periods have contrasting 

signals. The thermograph for the first period shows a decline 

in temperature associated with streamflow from about 29.5 

to 20.5o

C that is followed by a gradual increase to 26oC; the 

thermograph for the second period shows a rapid increase in 

temperature associated with streamflow from about 29 to 32oC 

that is followed by a gradual decrease to 27oC (fig. 14). The 

most accurate prediction of the observed thermograph for the 

first modeled time period at the Dodge Boulevard site resulted 

in an equivalent vertical saturated hydraulic conductivity of 

about 5 m/d, which is similar to, yet slightly higher than, the 

equivalent vertical saturated hydraulic conductivity simulated 

at the Craycroft Road site. Vertical hydraulic gradients were 

not measured at the Dodge Boulevard site, thus, infiltration 

rates can not be estimated; however, by assuming a vertical 

gradient of 0.2 m/m (on the basis of the measured gradient 

at the Craycroft Road site), an equivalent vertical hydraulic 

conductivity of 5 m/d, and a wetted perimeter of 10 m, the 

infiltration flux was calculated as 0.12 m3

/s/km.

To match the simulated thermograph to the measured 

thermograph for the second modeled period, a thin surface 

layer having a low-vertical hydraulic conductivity needed 

to be added during the modeled period. During the first part 

of the second simulation, the equivalent vertical saturated 

hydraulic conductivity was 0.7 mm/s, which, if sustained, 

would equate to 66 m/d. After about 3 hours of streamflow, 

the addition of the low-conductivity surface layer reduced the 

equivalent vertical hydraulic conductivity to about 0.25 m/d. 

The decrease in hydraulic conductivity during a flow event is 

consistent with the deposition and accumulation of fine sediments on the receding limb of a hydrograph. 

Model simulations for the site near First Avenue relied 

on stage data from the streamflow gaging-station 09485700 

at Dodge Boulevard (Bailey, 2002). Simulations for this site 

covered the same two time periods and resulted in equivalent 
Infiltration, Percolation, and Recharge Rates 201

Figure 13. Measured and simulated thermographs in Rillito Creek near Craycroft Road, Pima County, Arizona.

2019 21 22 23 24 25 26 27 28 29 30 31 21

TEMPERATURE, IN DEGREES CELSIUS

JULY 1999 AUGUST 1999

29

28

27

26

25

24

23

22

Measured

Simulated

NOTE: Depth of thermister 1.2 meters

Figure 14. Measured and simulated thermographs in Rillito Creek near Dodge Boulevard, Pima County, Arizona.

14 15 16

TEMPERATURE, IN DEGREES CELSIUS

JULY 1999

25 26 27 28 29

30

35

25

20

Measured

Simulated

NOTE: Depth of thermister 0.8 meters

vertical hydraulic conductivity values that were similar to 

those estimated for the site near Dodge Boulevard. In addition, 

the simulation for the second modeled period required the 

addition of a low vertical hydraulic conductivity surficial layer 

during the latter part of the period. The model simulations for 

the site near First Avenue, however, were less successful in 

matching the observed data than the simulations for the site 

near Dodge Boulevard. The inability of the simulations to 

match the magnitude and changes in the temperature measured 

at the site near First Avenue indicates the numerical model 

likely is not representing some of the infiltration processes, 

such as multidimensional flow beneath the streambed, the 

model has poorly constrained sediment and hydraulic parameters, or a combination of these factors.

Results of this investigation indicate that, under well-constrained conditions where predominantly vertical infiltration 

can be assumed, a one-dimensional inverse numerical model 

can be used to estimate infiltration rates. One-dimensional 

modeling, however, assumes lateral spreading does not occur. 

If lateral spreading does occur, the estimated rates predicted 

by one-dimensional modeling will be smaller than actual rates. 

The results also indicate that streambed hydraulic conductivity can limit infiltration and that hydraulic conductivity can 

vary significantly between and during flow events. Erosion 
202 Estimated Infiltration, Percolation, and Recharge Rates at the Rillito Creek Focused Recharge Investigation Site, Arizona

Figure 15. Water content of stream-channel sediments for duration of a streamflow event including onset and cessation, from 

within two-dimensional array in Rillito Creek near Dodge Boulevard, Pima County, Arizona.

27 28 29 30 31 1 2 3 4

WATER CONTENT, IN PERCENT BY VOLUME

JANUARY 2001 FEBRUARY 2001

42

38

34

30

26

22

18

14

Period of streamflow

Saturated

conditions

Unsaturated

conditions

1.25

0.75

0.50

1.00

2.00

2.50

1.50

1.25

0.75

0.50

1.00

2.00

2.50

1.50

NOTE: Depth of

 thermograph

 given in

 meters

and deposition associated with low-frequency, high-intensity 

ephemeral streamflow can result in large variations in the 

hydraulic conductivity of the streambed surface owing to an 

accumulation or removal of fine-grained sediments. This variability affects the cumulative infiltration, which could vary 

greatly as a function of the amount of streamflow and a function of the nature of the streamflow and the accumulation or 

removal of fine-grained sediments. Additionally, these results 

indicate that simulation of streambed infiltration should allow 

for temporal variation of the streambed hydraulic conductivity 

or should be done using short time periods to account for rapid 

changes in the streambed hydraulic conductivity.

Two-Dimensional Temperature and WaterContent Monitoring

For the purpose of measuring infiltration fluxes at the 

onset and throughout the duration of streamflow events, 

Blasch and others (2003) instrumented the stream-channel 

deposits beneath Rillito Creek near Dodge Boulevard with a 

two-dimensional vertical array of 28 paired thermocouples, 

temperature probes, and time-domain reflectometry (TDR) 

water-content probes placed perpendicular to flow (fig. 1). The 

paired probes were arranged in four columns (profiles C1, C2, 

C3, and C4 in figure 1) spaced 3 m apart. There are seven rows 

(depths) within the array at depths of about 0.50, 0.75, 1.0, 

1.25, 1.50, 2.0, and 2.5 m below the streambed. Depths of the 

probes varied by as much as 0.25 m owing to deposition and 

erosion during flow events. A near-surface temperature sensor 

also was placed adjacent to the paired two-dimensional array 

at a depth of 0.05 m. 

The highly transient conditions that exist during the onset 

of streamflow are more difficult to simulate using temperature measurements exclusively than the saturated conditions 

presented in the previous section because of the rapid changes 

in water fluxes and increased multidimensional infiltration. 

Combined water-content and temperature measurements are 

needed to simulate initial transient infiltration rates in unsaturated sediments. Additionally, infiltration rates can be estimated using wetting-front arrival times and changes in water 

content at successive TDR probes. 

Water-content data show rapid changes shortly after 

the onset of streamflow (fig. 15). Volumetric water content 

increases throughout the measured profiles from about 20 percent to 40 percent within minutes of the onset of streamflow. 

Initial infiltration rates measured as the change in volumetric 

water content over time per unit area were as high as 2 mm/s, 

which if sustained would be equivalent to 166 m/d. The high 

rates are likely to include vertical and lateral flow components. 

Temperature and water-content data for a September 2000 

event indicate that infiltration occurs in both the horizontal 

and vertical directions at the onset of streamflow (figs. 16 and 

17). Measured lateral wetting-front velocities between profiles 

were similar in magnitude to vertical velocities. The similar 

velocities measured at the onset of streamflow probably were 

due to tension gradients being much larger than the gravitational gradient. Water traveled laterally almost the entire 9-m 
Infiltration, Percolation, and Recharge Rates 203

distance between profiles within the first few minutes of the 

onset of streamflow. 

Multidimensional-flow simulations are required to 

accurately represent the full volume of water infiltrating 

into a porous, heterogeneous medium near the margins of 

the advancing wetting front where capillary flow dominates. 

Infiltration, however, is increasingly vertical near the center 

of streamflow in a homogeneous medium, and lateral flow 

diminishes as capillary gradients decline with distance from 

the boundary of the wetted perimeter. The time from the onset 

of flow required for vertical infiltration to dominate varies 

depending on streamflow conditions and the texture of the 

streambed material. For instance, small braided-ribbon flows 

over fine material may never result in predominantly vertical infiltration, whereas large bank-to-bank flows over coarse 

material may produce predominantly vertical infiltration 

beneath the streambed within minutes on the basis of the large 

wetted perimeter and conductivity of the sediments. 

A typical set of measured Rillito Creek thermographs from 

an April 2001 event and from a November 2000 event were 

simulated using a variably saturated heat and mass transport 

model, VS2DH (Healy and Ronan, 1996). The thermographs 

for the 0.5- and 2.5-m depths were used as boundary conditions, 

and the thermographs for the remaining five depths were used 

as observation points. One-dimensional models were created. 

Parameter optimization software, PEST, was used to calibrate 

thermal and hydraulic properties. Numerical simulations shown 

for data from the two-dimensional array are for a 10-m wide 

flow event starting on April 6, 2001 (fig. 18). The assumption 

of vertical one-dimensional flow was considered valid because 

temperature changes were predominantly in the vertical direction. The simulated infiltration rates varied from about 0.35 to 

0.39 m/d throughout the initial 2 days of flow (fig. 19) and average 0.37 m/d. This represents a variation in predicted infiltration 

rates of less than 10 percent among the four columns, indicating that infiltration was uniform and predominantly vertical. 

Although the simulated and measured thermographs are in 

general agreement, departures do exist. Simulated temperatures 

differ from measured temperatures for several reasons, such as 

error in the measured boundary conditions, error in hydraulic 

and thermal property assignments, or an inability of the models 

to fully represent multidimensional infiltration. 

The infiltration rate generally declines as the streamflow 

event proceeds. The declining infiltration rate is attributed to 

a declining pressure head and (or) development of a thin, finegrained surface layer. Infiltration rates continued to decline 

during the flow event and averaged 0.32 m/d for the 12 days 

measured. By converting to flux units that are comparable 

with those estimates made at the vertical temperature nests 

described above and using a wetted perimeter of 10 m, an 

average infiltration rate of 0.32 m/d equates to a flux of 0.04 

m3

/s/km. There are two important differences between these 

estimates and estimates made at the one-dimensional temperature-array sites near Craycroft Road and Dodge Boulevard. 

First, the estimated infiltration rates at the vertical-array sites 

near Craycroft Road and Dodge Boulevard are about twice 

that of the infiltration estimates made at the two-dimensional 

array. Second, the infiltration rates at the vertical-array sites 

required a low-hydraulic conductivity layer be incorporated 

into the model in the later stages of infiltration. The addition 

of a low-conductivity layer resulted in significantly reduced 

infiltration rates during the later stages of streamflow. The 

two-dimensional array was at the lowest part of the cross section, whereas the vertical nest was near a depositional fringe 

within the streambed, which may account for the deposition of 

the low-conductivity layer at the vertical nests.

An estimated sustained infiltration rate of about 0.32 m/d 

for the April 2000 streamflow event agrees with simulation 

results for a November 2000 event lasting 10 days. An average 

infiltration rate of 0.37 m/d was simulated for the November 

event. These rates show general agreement with infiltration 

rates of 0.41 to 0.50 m/d estimated by other investigators along 

Rillito Creek (Burkham, 1970; Katz, 1987). Simulations were 

also extended about 2 days beyond the periods of streamflow to 

estimate the redistribution of water in the subsurface. Redistribution rates, similar to infiltration rates, are determined from the 

elapsed time between sharp decreases in water content at each 

depth (fig. 15). After the cessation of streamflow, temperature 

measurements indicated the simulated dewatering flux was 0.33 

and 0.30 m/d for the April and November events, respectively. 

Estimated redistribution rates using water-content changes were 

0.08 and 0.1 m/d for the April and November events, respectively. Thus, redistribution rates were slightly less than infiltration fluxes during steady-state flow, and estimates of dewatering 

using water-content measurements were less than the simulated 

fluxes using temperature methods.

The variety of physical and chemical methods presented 

in this chapter (and elsewhere) are used to estimate rates of 

infiltration and percolation under steady-state conditions. For 

long-duration events (several days to months) steady-state 

conditions may be an accurate assumption, but for short-duration events (less than 24 hrs) typical of those in Rillito Creek 

and other streams in southern Arizona, infiltration fluxes that 

occur shortly after the onset of flow may differ substantially 

in magnitude from those throughout the duration of the event. 

For detailed water-budget analyses and hydrologic models 

dependent on infiltration flux estimates, the infiltration flux has 

to be estimated more accurately than is possible when using 

the assumption of steady-state conditions. Infiltration fluxes 

through the first 2 m of unsaturated sediments at the onset of 

streamflow calculated for 20 events ranged from 13 to 166 m/d. 

Variability in antecedent water content and fluid temperature 

were examined as possible factors contributing to the range in 

onset fluxes. Onset infiltration rates were inversely correlated 

to antecedent water content with a log-linear relation (fig. 20). 

Measured onset infiltration fluxes differed from those of the 

steady-state infiltration fluxes by four orders of magnitude. 

Infiltration rates after onset declined more quickly for events 

in which the antecedent water content was high; events starting with higher antecedent water content required 3.7 hours to 

achieve near steady-state conditions, whereas events with lower 

antecedent water content required 6.8 hrs. 
204 Estimated Infiltration, Percolation, and Recharge Rates at the Rillito Creek Focused Recharge Investigation Site, Arizona

Figure 16. Two-dimensional distribution of temperature during different streamflow conditions in 

Rillito Creek on September 10, 2000, near Dodge Boulevard, Pima County, Arizona. A, Thermal transport 

through conduction before the onset of streamflow; B, thermal transport through a combination of 

advection and conduction at the onset of streamflow exhibiting multidimensional flow through the 

sediments; C, combined advection and conduction thermal transport to the deeper sediments several 

hours after the onset of streamflow.

0

2.0

1

1.5

1.0

2.5

0.5

2.0

1.5

1.0

2.5

0.5

2.0

1.5

1.0

2.5

0.5

2 3 4

DISTANCE FROM LEFT BANK, IN METERS

TEMPERATURE, IN DEGREES CELSIUS

EXPLANATION

DEPTH BELOW STREAMBED, IN METERS

5 6 7 8 9

A. 1600 hours

B. 1620 hours 

C. 2000 hours

10 11 12 14 15 16 17 18 19 20 21 22 23 24 13
Infiltration, Percolation, and Recharge Rates 205

Figure 17. Two-dimensional plot of soil-water content during different streamflow conditions in Rillito 

Creek near Dodge Boulevard, Pima County, Arizona; A, Before the onset of streamflow, September 10, 

2000, at 1600; B, five minutes after the onset of streamflow, September 10, 2000, at 1605; C , immediately 

after the cessation of streamflow September 12, 2000; D, approximately 2 days after the cessation of 

streamflow, September 14, 2000.

2.0

1.5

1.0

2.5

0.5

2.0

1.5

1.0

2.5

0.5

2.0

1.5

1.0

2.5

0.5

DEPTH BELOW STREAMBED, IN METERS

A.

B.

C.

0

2.0

1

1.5

1.0

2.5

0.5

2 3 4

DISTANCE FROM LEFT BANK, IN METERS

WATER CONTENT, IN PERCENT BY VOLUME

EXPLANATION

5 6 7 8 9

D.

16 18 20 24 26 28 30 32 34 36 38 40 42 44 22
206 Estimated Infiltration, Percolation, and Recharge Rates at the Rillito Creek Focused Recharge Investigation Site, Arizona

6 7 8 9 10 11 12 13 14 15

TEMPERATURE, IN DEGREES CELSIUS

APRIL 2001

22

20

18

16

14

12

10

0.75

1.00

1.25

1.50

NOTE: Depth of thermograph given

 in meters. Solid line is

 measured thermograph and

 dashed line is simulated

 thermograph

Figure 18. Typical measured and simulated thermographs from two-dimensional temperature array from column 1 (see fig. 1 for 

column location) in Rillito Creek near Dodge Boulevard, Pima County, Arizona.

Figure 19. Simulated infiltration rates during period of flow at the two-dimensional temperature array near Dodge Boulevard, Pima 

County, Arizona.

The general agreement in infiltration estimates among 

these independent temperature and water-content methods 

indicates that these methods provide accurate estimates of 

infiltration. As such, vertical arrays of temperature probes 

can be located along stream reaches to estimate the potential 

for in-stream recharge and to provide guidance on citing 

recharge facilities. High infiltration rates at shallow depths, 

however, are not sufficient to ensure that water will percolate 

to deeper depths and recharge the deep aquifer at a high rate. 

Infiltration rates determined from shallow measurements 

should be considered an upper limit of the potential recharge 

rate for a particular site.

Water-Level Responses

Piezometer nests were installed in the stream-channel 

boreholes and monitored for water-level variations in response 

to streamflow. Nested piezometers included one shallow 

piezometer completed in what is usually the unsaturated zone, 

9!$2%032%4%-.)./)4!24,)&.)

!02), !02),

#OLUMN

#OLUMN

#OLUMN

#OLUMN













     
Infiltration, Percolation, and Recharge Rates 207

 ANTECEDENT WATER CONTENT (BLUE),

IN PERCENT BY VOLUME

32

0

34

30

28

26

24

22

20

18

16

INFILTRATION RATE, IN METERS PER SECOND

0.0004 0.0008 0.0012 0.0016 0.0020 

Best fit

R2 = 0.6376

y = –5.1706Ln(x)–14.376

Figure 20. Correlation of onset infiltration rate to antecedent 

water content.

near the contact between the recent stream-channel alluvium 

and the basin-fill deposits, and a pair of deep piezometers—

one near the water table and one about 10 m below the water 

table. With the exception of the piezometer near Craycroft 

Road, the shallow piezometers generally were dry except 

shortly after streamflow (fig. 21) and contained water for periods of days to several weeks. The magnitude of water-level 

variation and the period of time the shallow wells contained 

water varied. For instance, the water level in the shallow 

piezometer near Dodge Boulevard varied the least (usually less 

than 1 m), whereas the water level in the shallow piezometer 

near La Cholla Boulevard varied the most (usually between 

6 and 7 m); water-level variations in the shallow piezometer 

near First Avenue were usually between 2 and 6 m. Duration in which the water level remained above the bottom of 

the piezometer was longest in the piezometer near La Cholla 

Boulevard where saturation persisted for 6 months after the 

summer 1999 streamflow and 12 months after the winter 

2000 streamflow; saturation time was shortest at the shallow 

piezometer near Dodge Boulevard where it was typically less 

than 2 months (fig. 21). The long duration of saturation at the 

shallow piezometer near La Cholla Boulevard probably was 

due to the fine-grained unit at depth of about 12 to 30 m (Hoffmann and others, 2002). 

Water levels in each of the pairs of deep piezometers 

responded several days to weeks later than the shallow 

piezometers and generally after the shallow piezometers 

became dry. There were little to no vertical gradients measured 

in the pairs of deep piezometers. These responses suggest 

gravity flow predominates, and that no hydraulic connection 

is established between streamflow and the underlying deep 

aquifer. Given the low-permeability of the basin-fill deposits, 

relative to that of the stream-channel deposits, it also is likely 

that temporary perched conditions existed near the contact 

between these two units for a period of several days to weeks 

after cessation of streamflow. 

Water levels in the deep piezometers showed an overall 

decline during the period of investigation. Water-level declines 

between spring 1999 and summer 2002 ranged from about 2 m 

at the deep piezometer near Craycroft Road, to 8 m at the deep 

piezometer near Dodge Boulevard (fig. 21). The declines probably are related to ground-water pumpage from the basin-fill 

aquifer. Superimposed on the declines are a series of rises that 

range from about 0.5 to 3.9 m. These rises are in response to 

the two most significant streamflow periods that occurred in 

summer 1999 and fall/winter 2000. 

The initial response of the water table in the deep 

piezometers lags the onset of streamflow by about 2 weeks 

at the piezometer nests near Dodge Boulevard and near First 

Avenue, whereas the initial response of the water table at the 

piezometer nests near Craycroft Road and near La Cholla 

Boulevard is more rapid—within about a day of streamflow in 

summer 1999. Lag time for the initial response at the piezometers near Dodge Boulevard and near First Avenue is related 

to the presence of a thick unsaturated zone at these sites. The 

short lag time between the onset of streamflow and the initial 

water-table response could be related to factors such as preferential flow and uniformly high water content throughout the 

unsaturated zone (Hoffmann and others, 2002). The occurrence of preferential flow is supported by the rapid response 

of the water table to streamflow despite the presence of the 

fine-grained unit.

Timing of the water-level peak also varies with location, 

and the peak occurs several weeks to months after the onset of 

streamflow. The longest lag time for the peak occurs near La 

Cholla Boulevard where it was about 7 months. Dissipation to 

prerecharge-event water levels also required several months at 

each site (fig. 21).

The magnitude of the water-table response in the deep 

piezometers is greatest at the piezometer nests near Dodge 

Boulevard and First Avenue. The water-table response in the 

piezometer nest near Craycroft Road is reduced relative to 

the water-table responses near Dodge Boulevard and First 

Avenue. Shallow piezometers near Craycroft Road indicate 

the water table often reaches land surface during streamflow 

events; therefore, the reduced water-level response probably 

is related to rejected recharge. The magnitude of the watertable response at the piezometer nest near La Cholla is the 

smallest and is related to a smaller recharge rate related to less 

frequent and smaller streamflows near La Cholla, relative to 

the other sites, and the presence of the fine-grained layer in 

the unsaturated zone. The lack of vertical gradients between 

the middle and deep piezometers at each nest indicates flow is 

predominantly horizontal in the saturated zone, except in the 

piezometer nest near Craycroft Road where the vertical gradient averages about 0.1m/m. 

A series of recharge estimates were calculated on 

the basis of the water-level responses using an analyti-
208 Estimated Infiltration, Percolation, and Recharge Rates at the Rillito Creek Focused Recharge Investigation Site, Arizona

Figure 21. Hydrographs of selected piezometers within Rillito Creek, Pima County, Arizona.

48

1999 2000

La Cholla Boulevard,

shallow (10.0 meters)

La Cholla Boulevard,

middle (41.0 meters)

La Cholla, Boulevard,

deep (48.0 meters)

Dodge Boulevard, shallow

(10.5 meters)

Dry

Dodge Boulevard, middle

(41.0 meters)

Dodge Boulevard, deep

(52.6 meters)

First Avenue

North, shallow

(10.0 meters)

First Avenue North, middle

(35.6 meters)

First Avenue North,

deep (35.6 meters)

Craycroft

Road,

middle

(9.9 meters)

Craycroft Road,

deep (17.4 meters)

2001 2002

DEPTH TO WATER, IN METERS BELOW LAND SURFACE

–1

2

0

6

4

10

8

14

12

34

32

38

36

42

40

46

44

26

30

28

Dry

Dry

Dry

Streambed surface

Dry

Dry

Dry

Craycroft Road,

shallow

(2.0 meters)

NOTE: Completion depth of

 piezometer indicated

 in parentheses
Infiltration, Percolation, and Recharge Rates 209

cal approach developed by Moench and Kisiel (1970). In 

this analytical model, changes in water levels over time 

near an ephemeral-stream channel are assumed to reflect 

actual recharge. This is advantageous in that no information 

or assumptions regarding unsaturated-zone processes are 

required in order to make the calculations.

For any given streamflow event, a certain amount of 

water may recharge the aquifer and thereby cause water 

levels in nearby wells to rise. The water-level rise results in a 

mound that slowly dissipates normal to the line source. Subsurface sediments will act to slow the dispersal. The variation in rate and duration of recharge can thus be viewed as 

an input function, the aquifer as a filter or impulse-response 

function, and the water levels as output. The variation in the 

rate and duration of recharge can be calculated from a combination of water-level data and the impulse-response function, which is derived from parameters of aquifer geometry, 

aquifer storage, and hydraulic properties, using the groundwater flow equations. By summing these rates of recharge for 

the time period being studied, the volume of water recharged 

can be determined.

Various assumptions about the aquifer flow-system 

are required to implement the analytical model deveolpoed 

by Moench and Kisiel (1970). Flow is assumed to be one 

dimensional and horizontal. The aquifer is assumed to be 

homogeneous and isotropic, to be infinite in horizontal 

extent, and to receive recharge from a finite width source. 

Aquifer characteristics are assumed to be invariant with time 

and water-level fluctuations.

Input to the model includes temporal variations in water 

levels, channel width, distance of the well from the center of 

the stream channel, specific yield, and transmissivity. The 

output is recharge calculated as cubic meters per second 

per linear kilometer of stream channel (m3/s/km) over the 

period of record. Variations in measured water levels were 

assumed to be recharge events superimposed upon a generally declining water table. In order to find the amount of 

water recharged without the bias of the decline, the data 

were detrended (fig. 22). Detrending consisted, first, of calculating the trend in the data using a linear regression. This 

trend was then removed from the data under the assumption 

that all other deviations from a constant water level were 

caused by recharge. The period of record for the recharge 

calculations was based on the time required for the waterlevel response to return to prerecharge levels; therefore, the 

period of record includes both the rise in water level associated with recharge and the dissipation of the water level. A 

specific yield of 0.15 was used on the basis of measurements 

made by Pool and Schmidt (1997). Bounding transmissivities of 1.6 × 10-3 and 6.5 × 10-3 m2

/s estimated from Hanson 

and Benedict (1994) were used to show the uncertainty in 

the recharge values. The only difference between input data 

for the various sites was the individual water-level records 

and the distance of the well from the stream channel (to 

which the model showed little sensitivity in comparison 

with its sensitivity to transmissivity).

Recharge estimates were calculated for the sites near 

Dodge Boulevard, First Avenue, and La Cholla Boulevard for the 

recharge events of 1999 and 2000. The recharge event in 1999 

was caused by summer streamflows, whereas the recharge event 

in 2000 was due predominantly to fall and winter streamflows 

and only partly to summer flows (table 1). The site near Craycroft Road was excluded from this analysis because the presence 

of vertical water-level gradients violated the assumptions of the 

analytical model. The largest cumulative recharge estimates for 

the 1999 event at the site near First Avenue range from 3.0 ×105

to 6.0×105 m3/km, which is about two to three times larger 

than those from the other two sites (near Dodge Boulevard, 

1.7×105 to 3.4×105 m3/km; near La Cholla Boulevard, 1.3×105

to 2.5×105 m3/km; table 2). Cumulative recharge estimates for 

the 2000 recharge event were greatest at the site near Dodge 

Boulevard and decreased in the downstream direction. At the site 

near Dodge Boulevard, recharge estimates range from 7.1×105

to 1.4×106 m3/km, which are slightly larger than those estimated 

at the site near First Avenue (5.3×105 to 1.1×106 m3/km) and 

about three times larger than those estimated at the site near La 

Cholla Boulevard (2.2×105 to 4.3×105 m3/km). The recharge 

estimates for the 2000 event at the sites near First Avenue and 

near La Cholla Boulevard were about twice those for the 1999 

event at these sites and were more than five times that of the 

1999 event at the site near Dodge Boulevard. The trends revealed 

in this analysis are consistent with the fact that the winter 2000 

streamflows were more voluminous and longer in duration than 

the summer 1999 streamflows (table 1), and that water levels 

in the piezometers rose higher in response to the winter 2000 

streamflows than to the summer 1999 flows (fig. 21).

Volumetric recharge rates were estimated by dividing the 

cumulative-recharge estimates by the cumulative duration of 

streamflow at each site during the period of record. Streamflow duration at the streamflow-gaging station at Dodge 

Boulevard totaled about 9 days between July 1, 1999 and 

September 30, 1999, whereas streamflow duration at streamflow-gaging station 09486055, near La Cholla Boulevard, for 

the same period totaled about 4.5 days. No streamflow-gaging 

station existed near First Avenue; however, in this analysis, a 

total of about 7 days of cumulative flow was assumed to have 

occurred at the site near First Avenue during 1999—a duration less than that at the upstream gage, at Dodge Boulevard, 

and greater than that at the downstream gage, near La Cholla 

Boulevard. Using the values of estimated cumulative-recharge 

determined using the Moench and Kisiel (1970) method and of 

cumulative-streamflow duration, recharge rates range from 0.2 

to 1.0 m3

/s/km for 1999 (table 2). During the period of June 15 

to December 31, 2000, a span that includes most of the streamflow for the second recharge period, cumulative-streamflow 

duration ranged from a high of 43 days at the streamflow-gaging 

station at Dodge Boulevard, to a low of 14 days at the streamflow-gaging station near La Cholla Boulevard. In this analysis, 

a flow duration of 30 days was assumed to have occurred at the 

site near First Avenue. Using these values, volumetric-recharge 

rates range from 0.2 to 0.4 m3

/s/km for 2000 (table 2). These 

rates are generally greater than volumetric-infiltration estimates 
210 Estimated Infiltration, Percolation, and Recharge Rates at the Rillito Creek Focused Recharge Investigation Site, Arizona

1999 2000 2001 2002

DEPTH TO WATER, IN METERS BELOW LAND SURFACE

48

38

Dodge Boulevard

Well (D-13-14)28dba

Best-fit

linear regression

39

40

41

42

43

44

45

46

47

50

49

38

28

First Avenue

Well (D-13-14)19bcbn

29

30

31

32

33

34

35

36

37

39

36

La Cholla Boulevard

Well (D-13-13)16add

37

38

39

40

41

42

43

44

EXPLANATION

MEASURED DATA DETRENDED SUMMER OF 1999 DATA DETRENDED WINTER OF 2000 DATA

Best

linear regr

-fit

ession

Best-fit

linear regression

Figure 22. Measured and detrended hydrographs of selected piezometers within Rillito Creek, Pima County, Arizona.
Infiltration, Percolation, and Recharge Rates 211

made using temperature, water-content, and seepage-loss 

methods. Because infiltrated water can be stored in the shallow subsurface it is available for subsequent evaporation and 

(or) transpiration; therefore, infiltration rates typically provide 

an upper bound for recharge rates. Estimated recharge rates 

in excess of infiltration rates indicate unaccounted sources 

of water. These unaccounted sources likely are inputs from 

adjacent tributaries, many of which drain the pediment north 

of Rillito Creek.

Temporal Gravity Measurements

Ground-water storage was monitored along two gravity 

profiles across Rillito Creek at Swan Road (5-km length) and 

First Avenue (6.4-km length). The profiles included 11 gravity 

stations that are closely spaced near Rillito Creek and at wells 

where water levels are monitored (fig. 1). Gravity changes 

along both profiles after summer 1999 show that increases in 

ground-water storage were primarily within the flood plain 

coincident with the stream alluvium (fig. 23). Gravity increases 

on the First Avenue profile were largest between these measurement dateswere at the station nearest Rillito Creek. The largest 

increase between June 199 and August 1999 was 48 microgals, 

which is equivalent to about 1.1 m of water, assuming the 

mass change occurs within a horizontal slab of infinite extent. 

Gravity increase on the Swan Road profile also was largest 

between the June 1999 and August 1999 measurements at the 

station about 0.5 km south of Rillito Creek. The greatest change 

between these measurement dates was 42 microgals (equivalent 

Table 2. Recharge estimates for sites in Rillito Creek, Pima County, Arizona, using the Moench and Kisiel (1970) analytical-model method.

[m2/sec, square meters per second; m3/km, cubic meters per kilometer; m3/s/km, cubic meters per second per kilometer]

Site location

Distance of well 

from center of Rillito Creek channel, 

in meters Recharge

1999 2000

Transmissivity = 

1.6×10-3 m2

/s 

Transmissivity = 

6.5×10-3 m2/s

Transmissivity = 

1.6×10-3 m2/s

Transmissivity = 

6.5×10-3 m2/s

Dodge Boulevard 3 Length of recharge 

period, in days1 9 43

Total, in m3

/km 1.7 × 105 3.4 × 105 7.1 × 105 1.4 × 106

Rate, in m3/s/km 0.2 0.4 0.2 0.4

First Avenue 45.7 Length of recharge 

period, in days1 7 30

Total, in m3

/km 3.0 × 105 6.0 × 105 5.3 × 105 1.1 × 106

Rate, in m3/s/km 0.5 1 0.2 0.4

La Cholla 

Boulevard

3 Length of recharge 

period, in days1 4.5 14

Total, in m3

/km 1.3 × 105 2.5 × 105 2.2 × 105 4.3 × 105

Rate, in m3/s/km 0.3 0.7 0.2 0.4

1

Refers to the cumulative time streamflow existed in Rillito Creek. Streamflow durations were calculated using data from the streamflow-gaging stations near 

Dodge and La Cholla Boulevards.

to about 1 m of water). The gravity changes and associated 

water-level rises indicate that the highly permeable stream-channel and flood-plain deposits act as a ground-water reservoir that 

readily accepts infiltrated streamflow.

Estimates of recharge through infiltration along Rillito 

Creek were made by assuming the storage changes measured 

by gravity at each profile were equivalent to recharge and by 

integrating the two-dimensional gravity change for the length 

of the creek. Similar to that shown with water-level trends, 

the overall storage-change estimates during the period of 

study steadily declined with two periods of recharge superimposed on the longer-term rate of decline (fig. 24). The 

long-term storage decline shown in figure 24 is related to the 

dissipation of a water-table mound produced in 1998, from 

sustained flows related to the El Niño precipitation, and to 

ground-water pumpage from nearby public and private wells. 

The periods of increased storage that reduce the rate of longterm declines were associated with the summer 1999 and 

fall/winter 2000 streamflows (figs. 3 and 24). The increase 

in storage for the summer 1999 period is about 7.5 × 106 m3 ; 

for the fall/winter 2000–2001 period it is about 11.1 × 106 m3

for the 14-km reach between Craycroft Road and La Cholla 

Boulevard. Although measurable, these seasonal storage 

increases are small compared to the storage increases associated with the 1998 El Niño event, which were about 5 × 107

m3 (fig. 24).

Assuming that the increases in storage measured in 

summer 1999 and fall/winter 2000–2001 were uniform 

throughout the 14-km reach between Craycroft Road and La 

Cholla Boulevard, they would equate to 5 × 105 m3/km and 
212 Estimated Infiltration, Percolation, and Recharge Rates at the Rillito Creek Focused Recharge Investigation Site, Arizona

Figure 23. Gravity changes along profiles crossing Rillito Creek, Pima County, Arizona, since December 1997. A, Swan Road; B, First Avenue.

0

20

40

–20

–40

60

GRAVITY CHANGE, IN MICROGALS

A. Swan Road profile

South North

DISTANCE, IN KILOMETERSRillito

Creek

–60

0 1 2 3 4

0

20

40

–20

–40

60

GRAVITY CHANGE, IN MICROGALS

B. First Avenue profile

South North

3

DISTANCE, IN KILOMETERS

Flood plain Rillito Creek

–60

0 1 2 4 5 6 7

Flood plain

EXPLANATION

JUNE 24, 1999

AUGUST 17, 1999

JUNE 6, 2000

JANUARY 18, 2000

NOVEMBER 2, 2000

MARCH 15, 2001

EXPLANATION

JUNE 25, 1999

AUGUST 18, 1999

JANUARY 21, 2000

JUNE 2, 2000

NOVEMBER 3, 2000

MARCH 14, 2001

NOTE: Positive values indicate

 storage gain and negative

 values indicate storage gain

NOTE: Positive values indicate

 storage gain and negative

 values indicate storage gain
Summary and Conclusions 213

Figure 24. Storage changes measured along Rillito Creek, Pima County, Arizona, from Craycroft Road to La Cholla Boulevard 

relative to a measurement made in December 1997.

–30,000,000

1998 1999 2000 2001 2002

WATER VOLUME, IN CUBIC METERS

60,000,000

50,000,000

40,000,000

30,000,000

20,000,000

10,000,000

0

–10,000,000

–20,000,000

Increased storage

resulting from

summer streamflow 

Increased storage

resulting from

winter streamflow 

Increased storage

resulting from El Niñorelated streamflow 

8 × 105 m3/km, respectively. Using 7 days of flow (as estimated 

at First Avenue; table 2) for the summer 1999 event and 30 

days of flow for the fall/winter 2000-2001 event, these recharge 

volumes equate to 0.9 m3/s/km in the 1999 event, and 0.3 m3/s/

km in the 2000–2001 event.

These rates are generally similar to those estimated using 

water-level methods, yet greater than volumetric-infiltration 

estimates made using temperature, water-content, and seepage-loss methods. As discussed previously, recharge-rate 

estimates in excess of infiltration-rate estimates indicate unaccounted sources of water. These unaccounted sources likely 

are inputs from adjacent tributaries, many of which drain the 

mountain-front area north of Rillito Creek. 

Two-dimensional simulations of the change in water 

distribution in the subsurface required to produce the change in 

gravity for the period June 24, 1999 to August 18, 1999, along 

both profiles are shown in figure 25. Simulations used GM-SYS 

software (version 4.6) developed by Northwest Geophysical Associates, Inc. The software calculates the gravity field 

response of polygonal features of variable subsurface density 

using the theory and algorithms of Talwani and others (1959), 

and Won and Bevis (1987). Simulation of the two-dimensional 

vertical polygons requires simplifying assumptions about the 

distribution of storage change in the third dimension, in this 

case along the stream channel, and at the margin of the simulated region. This application assumed the two-dimensional 

polygons of storage change extended in infinite length along 

the stream channel. Storage change in the crystalline rocks 

adjacent to the northern boundary of the aquifer was assumed 

to be insignificant. Storage change in the aquifer adjacent to the 

southern end of the profiles was assumed to extend laterally to 

an infinite distance. Each of these assumptions likely contributes little to no errors in the simulation. Available water-level 

data and gravity-derived estimates of specific yield were used 

to constrain the vertical distribution of saturated storage change 

at many gravity-station wells. Gravity changes along the First 

Avenue profile are explained by a combination of higher water 

levels and increases in water content in the unsaturated zone. A 

10-percent increase in unsaturated water content was required 

to match the observed gravity change at gravity stations near the 

channel. The resulting model (fig. 25A) resulted in simulation 

errors of less than 1 microgal at each station. 

Simulation of gravity change along the Swan Road profile could be explained by storage change within the zone of 

water-level change: no changes in water content in the unsaturated zone were required. Increases in gravity at all stations 

along the Swan Road profile during June 24, 1999, to August 

17, 1999, (fig. 25B) indicated that infiltration and recharge 

during the intervening period between gravity surveys resulted 

in storage increases within about 2 km of the stream channel. 

The greatest increases in gravity occurred at the two stations 

within about 0.5 km south of the channel. Gravity increases at 

stations farther from the channel ranged from 2 to 23 microgal. 

The resulting model (fig. 25B) resulted in simulation errors of 

generally less than 1 microgal at each station. 

Summary and Conclusions 

The amount of water currently recharging the aquifers 

within the Tucson area is insufficient to meet current and future 

demands. Resultant ground-water deficits are manifested in 

water-level declines of more than 60 m since the middle of the 

twentieth century. The accurate determination of recharge is 

critical to establishing a sustainable water budget on the basin 

scale. In semiarid regions, recharge beneath ephemeral-stream 
214 Estimated Infiltration, Percolation, and Recharge Rates at the Rillito Creek Focused Recharge Investigation Site, Arizona

0

20

40

–20

60

GRAVITY CHANGE, IN MICROGALS

A. Swan Road

South North

DISTANCE, IN KILOMETERSRillito

Creek

0 1 2 3 4

Flood plain

800

700

850

ALTITUDE ABOVE SEA LEVEL, IN METERS

Gravity change

Storage change

South North

DISTANCE, IN KILOMETERS

650

0 1 2 3 4

750

Well B77A

Well PR237

Land surface

Water level Aug. 16, 1999

Water level June 28, 1999

Simulated storage change

Specific yield=0.09

Measured Calculated

Water level Aug. 20, 1999

Figure 25. Gravity-model results for profiles crossing Rillito Creek, Pima County, Arizona. A, Swan Road; B, First Avenue.

channels typically represents a major component of the total 

recharge. Improved understanding of streambed infiltration and 

the subsequent redistribution of water within the unsaturated 

zone is fundamental to quantifying and forming an accurate 

description of streambed recharge. 

One of the challenges of quantitatively studying recharge 

beneath ephemeral-stream channels is the need to integrate 

measurements made over a wide range of spatial and temporal scales. No single method of measurement or analysis 

can resolve the complex physical processes that contribute to 

infiltration, percolation, and recharge beneath these channels; 

therefore, various approaches that provide a wide range of 

temporal and spatial scale measurements of recharge beneath 

Rillito Creek were used in this study. The approaches discussed in this chapter included analyses of cores and cuttings 

for hydraulic and textural properties, environmental tracers 

from the water extracted from the cores and cuttings, seepage measurements made during sustained streamflow, heat 

as a tracer and numerical simulations of the movement of 

heat through the streambed sediments, water-content variations within a two-dimensional array, water-level responses 

to streamflow in piezometers within the stream channel, and 

gravity changes in response to recharge. 

The amount of water flowing in Rillito Creek, and 

therefore the amount available for ground-water recharge, is 

primarily related to precipitation frequency, distribution, and 

intensity, as well as to streamflow and basin/channel runoff 

characteristics. The temporal distribution of flow in ephemeral streams is highly varied. Because of this, estimating or 

predicting recharge rates for ephemeral-stream channels on the 

basis of limited temporal observations is particularly difficult. 

This investigation extended from 1999 through most of 2002 

and represented a time of lower than average precipitation and 

streamflow on the basis of data from the previous 30 years. 

Estimates of cumulative infiltration and recharge during this 

study period may differ from long-term averages; however, 

estimates of infiltration and recharge rates for streamflow 

events during the study period can be extrapolated to a variety 

of climatic conditions.

In order for ephemeral streamflow within Rillito Creek 

to recharge the underlying aquifer, the water must first 

infiltrate into the stream-channel deposits and percolate 

downward through the underlying deposits. The ability of 

water to infiltrate and percolate through these deposits is a 

function of the availability of streamflow and the hydraulic 

properties of the deposits. Study results indicate that the ver-
Summary and Conclusions 215

Figure 25.—Continued.

0

20

40

–20

60

GRAVITY CHANGE, IN MICROGALS

B. First Avenue

South North

Gravity change

Land surface

Land surface

Well A-1B Well A-54

Well

B-91

Well

B-92

Well A-18

Measured Calculated

Flood plain Rillito Creek

–40

DISTANCE, IN KILOMETERS

700

600

850

ALTITUDE ABOVE SEA LEVEL, IN METERS

Storage change

South North

DISTANCE, IN KILOMETERS

650

650

ALTITUDE ABOVE SEA LEVEL, IN METERS

650

625

675

600

700

0 1 2 3 4 5 6 7

3

DISTANCE, IN KILOMETERS

2.75 3.00 3.25 3.50 3.75 4.00 3.25

0 1 2 4 5 6 7

Well WR-52 Well First

Avenue

North

Saturated-storage gain

Saturated-storage gain Specific yield=0.27

Specific yield=0.28

Unsaturated-storage gain

Change in water content=0.10

Saturatedstorage loss

Specific yield=0.28
216 Estimated Infiltration, Percolation, and Recharge Rates at the Rillito Creek Focused Recharge Investigation Site, Arizona

tical hydraulic conductivity of the stream-channel deposits 

ranges from 0.3 to 2.5 m/d, whereas hydraulic conductivity of the the basin-fill deposits ranges from 0.012 to 0.61 

m/d. For heterogeneous media, such as the deposits beneath 

Rillito Creek, the equivalent vertical hydraulic conductivity is calculated as the harmonic mean of the Ksat for each 

textural layer within the deposits. Although differing at each 

borehole, the overall average equivalent saturated hydraulic 

conductivity of the stream-channel deposits is 1.2 m/d; the 

overall average equivalent saturated hydraulic conductivity of the basin-fill deposits is 0.19 m/d; and the equivalent 

hydraulic conductivity of the stream-channel and basin-fill 

deposits together is about 0.23 m/d (table 3). Assuming no 

preferential flow occurs and unit gradient conditions, these 

equivalent vertical hydraulic conductivity values provide an 

estimate of long-term potential recharge rates under saturated 

conditions. To convert these values into potential recharge 

volumes, assumptions of flow duration and average wetted perimeter and length must be made. For example, using 

an annual cumulative flow duration of 36 days per year for 

Rillito Creek, an average wetted perimeter of 20 m and a 

wetted length of 20 km, yields an annual volumetric recharge 

of 4.3 × 106 m3/y, which is about two-thirds of the commonly 

reported long-term average recharge of 6 × 106 m3/y (Hanson 

and Benedict, 1994).

Environmental tracers were used to evaluate spatial 

variations in infiltration and recharge patterns along Rillito 

Creek and estimate percolation rates through the unsaturated 

zone. Tritium was detected in pore water extracted from all 

core samples and ranged from 2 to 11 TU, indicating that 

water in the unsaturated zone infiltrated during the past 40 

years. Given the presence of tritium in the unsaturated zone, 

the locally high vertical hydraulic conductivity values greater 

than 1 m/d in the stream-channel deposits, and a depth to the 

water table of generally less than 40 m, it is likely that most 

of the pore water extracted from the cores infiltrated during 

runoff events in the past few years. Variations in isotopic 

compositions of ground water beneath Rillito Creek are 

attributed to changes in the compositions of the source water 

and to evaporation. The lack of an evaporative signal for 

some samples indicates that percolating water was exposed 

to minimal evaporation.

Isotopic ratios measured in the pore water during the 

study are representative of the isotopic ratios in the infiltrating waters of the recent past. Isotopic ratios in Rillito Creek 

pore water become lighter in the downstream direction indicating that for the time period represented by the unsaturated-zone pore water, the longer duration and isotopically 

lighter winter storms were more important contributors to 

infiltration in the downstream reaches than in the upstream 

reaches. The trends in isotopic ratios measured in the 

pore-water cores approximately match the trends in ratios 

in the basin precipitation from particular storms during the 

year prior to drilling. Correlation between the precipitation events and depths of infiltration, on the basis of corresponding isotopic signatures, indicates an average verticalpercolation rate of approximately 0.049 m/d at the Dodge 

Boulevard borehole site (table 3). Chloride concentrations 

in pore-water leachate determined from drill cuttings varied 

substantially through the upper 18 m at all sites. Below 

about 18 m, variation in chloride concentration declined 

considerably. This zone of smaller variation is interpreted as 

corresponding to the infiltration depth of the 1998 El Niño 

water. The low variability and low mean value of chloride 

concentration is attributed to a constant supply of runoff 

having a low chloride concentration. In addition, the low 

chloride concentration indicates the water had little exposure to evaporation. The higher variability and concentration 

observed in post-El Niño pore water above 18 m likely is 

due to mobilization of chloride from evaporative concentrates and dry fallout deposited between precipitation events 

that resulted in runoff along a variety of surface-water flow 

paths. Calculation of a downward percolation rate using 

the El Niño event marker in the chloride profile yields an 

average percolation rate of 0.055 m/d (table 3). Although 

the estimates of recharge determined by hydraulic properties and environmental tracers required the assumption of 

no preferential flow, the water-level responses measured in 

the deep piezometers indicate that factors such as preferential flow likely influence recharge rates. Estimates made 

using these techniques, therefore, are likely representative of 

minimum values.

Infiltration rates are typically assumed to provide an 

upper bound for recharge rates. Infiltration rates were estimated using seepage-loss, temperature, and water-content 

methods. Seepage measurements made at several sites along 

Rillito Creek during the El Niño-related sustained flows of 

March through April 1998 and again on October 24, 2000, 

indicate that streamflow losses owing to infiltration along 

Rillito Creek ranged from 0.07 to 0.9 m3

/s/km and averaged 

0.21 m3/s/km (table 3). Streamflow losses were smallest in 

the upstream reaches. The losses in these reaches were attributed to a shallow water table and rejected recharge. Using a 

wetted perimeter of 25 m, which was the average width of 

the flow that occurred during October 2000, the calculated 

average infiltration rate is 0.7 m/d, (table 3) which is similar to the measured saturated hydraulic conductivity of the 

stream-channel deposits but more than an order of magnitude 

greater than percolation rates determined by tracers. The 

difference between the estimated infiltration and percolation rates probably is due to the variably saturated nature of 

the sediments. The near-surface stream-channel deposits are 

likely to be nearly saturated during streamflow and therefore are approaching their saturated hydraulic conductivity, 

whereas the deeper sediments are less saturated and will 

therefore have a lower hydraulic conductivity.

Infiltration-rate estimates made using temperature and 

water-content methods in this study are one dimensional; 

information on stream width and length are required to estimate volumetric rates from the infiltration estimates. Model 

simulations using streambed-temperature data indicate 

the likelihood of a thin surface layer having a low vertical 
Summary and Conclusions 217

hydraulic conductivity at the site near Craycroft Road. The 

addition of this surface layer to the model domain resulted 

in a lowering of the simulated equivalent saturated hydraulic 

conductivity by four orders of magnitude from about 4 m/d 

to 3 × 10-4 m/d. The four orders of magnitude change is too 

large to result solely from changes in water viscosity owing 

to temperature changes. Given the tranquil flow conditions 

during the first modeled flow period near Craycroft Road, 

it is possible that the change resulted from the deposition 

of a fine-grained layer of sediment on the streambed surface. In fact, a thin layer of fine-grained material commonly 

was observed at the sites after small streamflows. Vertical 

hydraulic gradients measured in the nested piezometers 

at the site near Craycroft Road allowed for estimation of 

infiltration rates using simulated equivalent vertical saturated hydraulic conductivity. Vertical hydraulic gradients 

during and shortly after streamflow were typically 0.1 m/m. 

Estimated infiltration rates ranged from 0.09 m3

/s/km during the period probably most representative of ephemeralstreamflow conditions. The infiltration rate of 0.09 m3

/s/

km (table 3) is about half of that estimated using seepagemeasurement data. The difference between these methods is 

primarily the wetted-perimeter value used in the calculation. 

The seepage-measurement estimates used a wetted perimeter 

of 25 m that was based on measured stream width, whereas 

a wetted perimeter of 10 m was used for flows during the 

period modeled with the temperature method because flows 

during this period were smaller than those during the seepage 

measurements. If hydraulic conductivity and vertical gradient 

are assumed not to change with increasing wetted perimeter 

and these rates are extrapolated to a wetted perimeter of 25 

m, infiltration rates estimated with the temperature method 

are 0.23 m3/s/km—virtually the same that measured by the 

seepage-loss method.

Two-dimensional arrays of temperature and watercontent sensors indicated that water-content measurements 

enable better estimates of rapid-infiltration rates associated with the onset of streamflow. Infiltration rates within 

the first few minutes after the onset of streamflow were as 

large as 166 m/d; however, saturation within the relatively 

homogeneous stream-channel deposits of Rillito Creek was 

established in less than 10 minutes and subsequent infiltration rates declined significantly. Simplified one-dimensional 

model simulations used to estimate infiltration as soon as the 

sediments were saturated indicate infiltration rates declined 

as streamflow duration increased and averaged 0.32 m/d for 

the 12-day event in April 2000 and 0.37 m/d for the 10-day 

event in November (table 3). The declining infiltration rate is 

attributed to a declining pressure head and (or) development 

of a thin fine-grained surface layer. On the basis of a wetted 

perimeter of 10 m, an average infiltration rate of 0.32 m/d 

equates to a flux of 0.04 m3

/s/km (table 3).

Water levels in the stream-channel piezometers showed 

an overall decline during the period of investigation in relation to long-term records. The water-level decline probably is 

related to ground-water pumpage from the basin-fill deposits. 

Superimposed on the water-level decline is a series of waterlevel rises that range from about 0.5 to 3.9 m. These rises 

were in response to the two periods of greatest streamflow 

occurring in the summer of 1999 and the fall/winter of 2000. 

The water-level responses to streamflow were followed by 

gradual water-level dissipation. 

Water-level responses were analyzed by using an analytical 

model to simulate cumulative recharge for the water-level rises 

measured in 1999 and 2000. The largest cumulative-recharge 

estimates for the 1999 event were for a piezometer site near the 

middle of the study reach and range from 3.0×105 to 6.0×105

m3

/km. This range is about two to three times as large as that 

for the other two sites. Estimates of recharge for 2000 were 

about two- to five-times that of the estimates for 1999. The 

trends revealed in this analysis are consistent with streamflow 

volumes and duration, and the magnitude of water-level changes 

that occurred in the piezometers in response to the streamflow. 

Cumulative-recharge estimates for the 2000 recharge event were 

greatest at the upstream-most site and decreased in the downstream direction. Recharge estimates for the upstream-most site 

range from 7.1×105 to 1.4 ×106 m3/km; these values are slightly 

larger than those estimated for the middle-reach site and about 

three times greater than that estimated for the site farthest downstream. Recharge rates, estimated by dividing the cumulativerecharge estimates by the cumulative duration of streamflow, 

resulted in rates that range from 0.2 to 1.0 m3/s/km for 1999 and 

from 0.2 to 0.4 m3/s/km for 2000 (table 3).

Gravity methods used to estimate recharge through 

infiltration along Rillito Creek provide values similar to those 

made by using the water-level method. Both gravity- and 

water level-derived estimates, however, are higher than the 

infiltration-rate estimates made by using seepage, temperature, or water-content change methods. Typically, infiltrationrate estimates provide an upper bound for recharge-rate 

estimates as some of the infiltrated water is stored in the 

shallow subsurface and used by vegetation, or is subsequently 

evaporated. The relatively high estimates of recharge determined by the gravity and water-level methods compared to 

estimates of infiltration determined by use of the seepage, 

temperature, and water-content methods probably is due to 

recharge from ungaged tributaries. The ungaged tributaries 

provide additional wetted perimeter and channel length not 

accounted for in the recharge estimates made by using the 

seepage, temperature, and water-content methods.
218 Estimated Infiltration, Percolation, and Recharge Rates at the Rillito Creek Focused Recharge Investigation Site, Arizona

Table 3. Summary of methods used and estimated rates of infiltration, percolation, and recharge along Rillito Creek, Pima County, 

Tucson, Arizona.

[m/d, meters per day; m3/s, cubic meters per second; m3/yr, cubic meters per year]

Method

One-dimensional 

infiltration rate, 

m/d

Vertical 

percolation rate, 

m/d

Volumetric 

rate, m3

/s per 

kilometer of 

streamflow 

(wetted 

perimeter 

of 25 meters 

is used for 

temperaturemethod 

estimates)

Potential 

annual 

average 

recharge; 

assumes 

36 days of 

flow in the 

20 kilometer reach, 

m3

/yr Comments

Physical: equivalent saturated 

hydraulic 

conductivity

0.23 Not 

calculated

Not 

calculated

4.1×106 From Darcy’s Law the flow rate through the sediments 

under saturated conditions is equal to the product of 

the hydraulic conductivity and the hydraulic gradient. 

This method uses the average vertical equivalent saturated hydraulic conductivity of the combined streamchannel and basin-fill deposits multiplied by a unit 

gradient to estimate a recharge rate in meters per day.

Stable-isotope 

profiles

Not 

calculated

0.049 Not 

calculated

Not 

calculated

Method uses isotopic signatures associated with specific 

streamflow events and correlates pore water at depth 

with timing of introduction of water to unsaturated 

zone. 

Chloride profiles Not 

calculated

0.055 Not 

calculated

Not 

calculated

Method uses chloride concentrations associated with 

streamflow seasons and correlates pore water at 

depth with season (timing) water was introduced to 

unsaturated zone. 

Seepage losses 0.75 Not 

calculated

0.07 to 0.9; 

average of 

0.21

13.7×106 Method uses differences in streamflow measurements to 

calculate streamflow losses. Seepage losses represent 

infiltration rates in cubic meters per kilometer per 

second of streamflow. The one-dimensional rate 

(0.75 meter per day) was calculated by using the 

average (0.21 cubic meters per kilometer per second). 

One-dimensional 

temperature 

modeling

0.8 to 1.0 Not

calculated

0.23 14.3×106 Rates shown are representative of infiltration rates prior 

to the reduced rates simulated to occur owing to the 

accumulation of fine sediments on the streambed surface. One-dimensional rates are based on a modeled 

hydraulic conductivity of 4 to 5 meters per day and a 

measured vertical hydraulic gradient of 0.2. 

Temperature data 

from two-dimensional trench

0.32 (April 2000)

0.37 (November 

2001)

Not 

calculated

0.09 5.8 ×106 Temperature modeling results in vertical infiltration 

rates of 0.32 to 0.37 meters per day; drainage-rate 

measurements after cessation of streamflow result in 

a vertical velocity of 0.46 meters per day. 

Water level Not calculated Not 

calculated

0.2 to 1.0 for 

1999;

0.2 to 0.4 for 

2000–2001

Not 

calculated

Method uses an analytical solution to simulate recharge 

on the basis of a hydrograph response. Volumetric rates represent recharge rates for the period of 

streamflow in respective years.

Gravity integrated 

from Craycroft 

Road to 

La Cholla 

Boulevard

Not calculated Not 

calculated

0.8 for 1999; 

0.3 for 

2000–2001

Not 

calculated

Method uses ground-water storage changes measured at 

Swan Road and First Avenue, and extents the storage 

changes upstream to Craycroft Road and downstream 

to La Cholla Boulevard. An average flow duration 

listed in table 2 is used to estimate the rate.
References Cited 219

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