U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2008–5005, Second Edition—February 2009

Prepared in cooperation with the Mobile Area Water and Sewer System

Hydrogeologic Conditions and a Firm-Yield Assessment for 

J.B. Converse Lake, Mobile County, Alabama, 1991–2006
Cover illustration: Map showing location of J.B. Converse Lake, Alabama (see figure 1).
Hydrogeologic Conditions and a Firm-Yield 

Assessment for J.B. Converse Lake, 

Mobile County, Alabama, 1991–2006

By Carl S. Carlson and Stacey A. Archfield

Prepared in cooperation with the Mobile Area Water and Sewer System

Scientific Investigations Report 2008–5005, Second Edition—February 2009

U.S. Department of the Interior

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

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2009

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Suggested citation:

Carlson, C.S., and Archfield, S.A., 2009, Hydrogeologic conditions and a firm-yield assessment for J.B. Converse Lake, 

Mobile County, Alabama, 1991–2006: U.S. Geological Survey Scientific Investigations Report 2008–5005, Second 

Edition—February 2009, 21 p.
iii

Contents

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

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

Hydrogeology of the J.B. Converse Lake Watershed..............................................................................4

Hydrologic Setting ................................................................................................................................4

Climatic Conditions......................................................................................................................5

Surface-Water Inflows ...............................................................................................................5

Reservoir Characteristics...........................................................................................................7

Reservoir Conditions: 1994 through 2006 .................................................................................7

Geologic Setting....................................................................................................................................8

Major Hydrogeologic Units ........................................................................................................8

Aquifer Properties .......................................................................................................................8

Firm-Yield Estimator Methodology and Calibration..................................................................................9

Firm-Yield Assessment of J.B. Converse Lake........................................................................................17

Model Limitations and Uncertainty...........................................................................................................18

Summary........................................................................................................................................................20

References Cited..........................................................................................................................................21

Figures

1. Map showing locations of three continuous (1991–2006) and four short-term 

(1991 or 1991–92) streamflow data-collection sites in the J.B. Converse Lake 

watershed, Mobile County, Alabama ........................................................................................2

2. Diagram showing possible sources and losses of water for a hypothetical 

drinking-water reservoir in contact with a sand and gravel aquifer...................................3

3. Map showing underlying geologic units and the locations of seven gaged 

subwatersheds, and a smaller, ungaged watershed that compose the total 

J.B. Converse Lake watershed, Mobile County, Alabama.....................................................4

4–7. Graphs showing—

 4. Monthly precipitation for 1991 through 2006 recorded at the National 

Weather Service station, Mobile airport, Alabama, in relation to normal 

monthly precipitation at this station for the period of record 1971–2000.....................5

 5. Mean monthly pan evaporation data obtained from the National Weather 

Service station at Mobile airport and converted to free-surface 

evaporation using evaporation coefficients from Farnsworth and others (1982) ......6

 6. Coefficients of relations of reservoir surface area to storage and 

reservoir stage to storage for J.B. Converse Lake and for use 

in the firm-yield estimator....................................................................................................7

 7. Average withdrawals from J.B. Converse Lake in Alabama for public supply, 

in million gallons per day, for the period 1990 through 2006...........................................8

8. Diagrams showing reservoir water level at equilibrium with the water table in 

the surrounding aquifer, response of the water table to a decrease in the 

water level of the reservoir relative to the water table, and an increase 

in the water level of the reservoir relative to the water table.............................................10
iv

Tables

1. Streamflow conditions for the 1991–2006 water years in selected 

tributaries to J.B. Converse Lake, Alabama.............................................................................6

2. Aquifer transmissivities calculated by using the Prudic (1991) equation 

and specific-capacity data and aquifer-storage coefficients for each 

of 10 wells in the area of J.B. Converse Lake, Alabama........................................................9

3. Non-unique combinations of input values for transmissivity (T), aquifer 

storage (S), aquifer width (a), and length of shoreline in contact with 

the aquifer (L) that matched the lowest measured water level (101.7 feet 

in drought-year 2000) for model-calibration simulations to determine 

a reasonable value for aquifer transmissivity, using the withdrawal 

rate of 74.13 million gallons per day in 2000 ...........................................................................16

4. Estimates of average daily firm yield for J.B. Converse Lake, Alabama, 

calculated with total surface-water flow and ground-water exchange 

with the reservoir using results of the firm-yield estimator 

model-calibration process and sensitivity analysis and values 

from table 2 for various values of transmissivity (T), aquifer 

storage (S), aquifer width (a), and length of shoreline in 

contact with the aquifer (L).......................................................................................................19

9. Graphs showing factors used in the firm-yield estimator to represent 

monthly demand for the period 1991–2006 calculated as total withdrawals by 

month divided by the average withdrawal for the corresponding year and the 

average demand for the period 1991–2006 for J.B. Converse Lake in Alabama..............12

10. Map showing variables that represent the distance from the reservoir 

shoreline to the aquifer boundary, a, and the length of the reservoir shoreline 

in contact with the aquifer, L, used for the ground-water exchange component 

of firm yield for a reservoir in contact with a sand and gravel aquifer, 

J.B. Converse Lake, Alabama...................................................................................................13

11. Graphs showing simulated and measured reservoir water levels for 

drought-year 2000 and the period 1991 through 2006 for model 

calibration simulations run to match the lowest measured drought-year 

2000 water level (101.7 ft) to estimate aquifer transmissivity for 

J.B. Converse Lake, Alabama...................................................................................................14

12. Graphs showing total measured monthly precipitation and estimated 

usable reservoir storage for the period 1991–2006 for each average 

daily firm-yield estimate of 79 million gallons per day and 

83 million gallons per day for J.B. Converse Lake, Alabama...............................................17
v

Conversion Factors and Abbreviations

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 4,047 square meter (m2

)

square mile (mi2) 2.590 square kilometer (km2) 

Volume

gallon (gal) 3.785 liter (L) 

million gallons (Mgal) 3,785 cubic meter (m3

)

cubic foot (ft3) 0.02832 cubic meter (m3) 

cubic mile (mi3) 4.168 cubic kilometer (km3) 

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

)

Flow rate

foot per day (ft/d) 0.3048 meter per day (m/d)

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

gallon per day (gal/d) 0.003785 cubic meter per day (m3

/d)

million gallons per day (Mgal/d) 0.04381 cubic meter per second (m3

/s)

Transmissivity*

foot squared per day (ft2/d) 0.09290 meter squared per day (m2/d) 

Vertical coordinate information is referenced to the North American Vertical Datum of 1929 

(NGVD 29).

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

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

*Transmissivity: The standard unit for transmissivity is cubic foot per day per square foot times 

foot of aquifer thickness [(ft3/d)/ft2]ft. In this report, the mathematically reduced form, foot 

squared per day (ft2/d), is used for convenience.

Abbreviations

a Average distance from reservoir shoreline to aquifer boundary

FYE Firm-yield estimator

GIS Geographic Information System

L Length of reservoir shoreline in contact with the aquifer

MassDEP Massachusetts Department of Environmental Protection 

MAWSS Mobile Area Water and Sewer System

Mgal Million gallons

NWIS National Water Information System

S Aquifer storage value

T Aquifer transmissivity value

USGS U.S. Geological Survey

Abstract 

J.B. Converse (Converse) Lake is the primary source of 

drinking water for the City of Mobile, Alabama. Concerns 

regarding the ability of the reservoir to meet current and future 

water demands during drought conditions have prompted 

this study. The 1991 through 2006 water years1 included a 

drought that occurred during 2000, and drought conditions 

currently (2007) are affecting the area. To assist officials of 

the Mobile Area Water and Sewer System in planning for 

future demands for drinking water in the Mobile metropolitan 

area, the firm yield for Converse Lake was estimated by the 

U.S. Geological Survey. 

The firm yield of Converse Lake was estimated using 

the Massachusetts Department of Environmental Protection’s 

firm-yield-estimator (FYE) model, which recently was refined 

by the U.S. Geological Survey. The model uses a mass-balance 

approach to determine the maximum average daily withdrawal rate that can be sustained during a period of record that 

includes a drought of record. If the reservoir is in contact with 

an aquifer, the FYE also includes routines that estimate the 

volume of ground-water and surface-water exchange between 

the aquifer and the reservoir. 

The average daily firm yield for Converse Lake was 

estimated to be 79 million gallons per day using the FYE 

routine that does not include ground-water exchange between 

the reservoir and the adjacent aquifer. Observed lake levels 

and withdrawals during the drought of 2000 indicate that 

more than 74 million gallons per day of water were withdrawn 

without complete depletion of reservoir storage. Therefore, it 

is likely that ground-water exchange with the reservoir may 

supplement available reservoir storage. If water exchange 

occurs between the aquifer and the reservoir, an increase in the 

volume of water available to the reservoir may occur during 

a drought. To quantify the potential ground-water contribution to reservoir storage, an analytical solution was applied 

to the FYE simulation of Converse Lake to estimate groundwater exchange between the reservoir and the aquifer. Aquifer 

1

 Water year is the period October 1 through September 30 and is identified 

by the year in which the period ends. For example, the 1991 water year began 

on October 1, 1990, and ended on September 30, 1991.

properties required by the FYE were estimated by model 

calibration to observed water levels that occurred during the 

drought of 2000. When ground-water exchange between the 

reservoir and adjacent aquifer is included, the average daily 

firm yield increased to 83 million gallons per day. 

The estimate of 83 million gallons per day incorporates 

both total surface-water flow and ground-water exchange 

components. This analysis indicated that direct ground-water 

interaction contributes about 5 percent of the firm yield of 

Converse Lake. However, the average daily firm yield of 

83 million gallons per day, based in part on calibrated values 

for aquifer transmissivity and storage, can be used only as a 

guideline until these aquifer properties can be defined better 

by field investigation in the Converse Lake watershed.

Introduction

J.B. Converse Lake, hereafter referred to as Converse 

Lake, is a 3,600-acre tributary-storage reservoir in Mobile 

County in southwestern Alabama (fig. 1). The lake is the 

primary source of drinking water for the Mobile area and 

a popular recreational area for fishing and boating. The 

watershed is largely rural, but residential and commercial 

development is expanding westward from the City of Mobile 

(Journey and Gill, 2001). 

The Mobile Area Water and Sewer System (MAWSS) 

manages Converse Lake and the E. Morgan Stickney and 

Harry E. Myers Water Filtration Facilities. These two facilities 

treat raw water withdrawn by a pumping station on Converse Lake (fig. 1) and deliver the treated water to the City of 

Mobile. As demands for public drinking water are likely to 

increase in the future, the maximum withdrawal rate of water 

from the reservoir during severe drought conditions needs to 

be determined. 

The amount of water available for withdrawal from a 

water-supply reservoir depends on the water balance of the 

reservoir. Most reservoirs accumulate water from surface- and 

ground-water inflows and from precipitation on the reservoir surface; outflows occur by evaporation from the reservoir surface, water-supply withdrawals, and controlled and 

uncontrolled releases (fig. 2; Waldron and Archfield, 2006). 

Hydrogeologic Conditions and a Firm-Yield Assessment for 

J.B. Converse Lake, Mobile County, Alabama, 1991–2006

By Carl S. Carlson and Stacey A. Archfield
2 Hydrogeologic Conditions and a Firm-Yield Assessment for J.B. Converse Lake, Mobile County, Alabama, 1991–2006 Snow Rd.

Lott Rd.

Wilmer

Georgetown

Road

Tanner Williams Rd.

CONTINUOUS (1991-2006)

 STREAMFLOW DATA-

 COLLECTION SITE AND LABEL

1991 STREAMFLOW DATA-

 COLLECTION SITE AND LABEL 

1991-1992 STREAMFLOW

 DATA-COLLECTION SITE 

 AND LABEL

See table 1 for full station names 

LON

JUN

BIG

EXPLANATION

LOCATION OF J.B.CONVERSE LAKE WATERSHED 

IN MOBILE COUNTY, ALABAMA 

Gulf of Mexico

Pumping

Station

Figure 1. Locations of three continuous (1991–2006) and four short-term (1991 or 1991–92) 

streamflow data-collection sites in the J.B. Converse Lake watershed, Mobile County, Alabama. 
Introduction 3

Future water-supply demands could increase the likelihood 

that withdrawals may deplete available reservoir-storage 

capacity and result in supply shortfalls. Current water-supply 

demands may be met under normal climatic conditions, but a 

severe drought may reduce the reservoir’s current yield. Therefore, the reservoir’s response to drought conditions needs to 

be evaluated. A common way to evaluate reservoir response is 

by estimating the firm yield, which is the maximum yield of a 

reservoir under drought conditions, without completely depleting available reservoir storage (Waldron and Archfield, 2006). 

Withdrawals from a reservoir that are equal to or less than the 

firm yield ensure that the reservoir will meet water-supply 

demands, assuming that long-term climatic and hydrologic 

conditions remain stationary. As future demands for water 

supply increase, estimation of the firm yield provides an upper 

limit that can be compared to future demands to determine if 

the reservoir will be able to meet those additional demands in 

excess of current withdrawals. Alternatively, the firm yield can 

be compared to current reservoir withdrawals to determine if 

the withdrawals can be sustained through a drought (Archfield 

and Carlson, 2006). In response to likely increases in future 

water demand, the firm yield of Converse Lake was estimated 

by the U.S. Geological Survey (USGS), in cooperation with 

the Mobile Area Water and Sewer System (MAWSS), using 

the firm-yield estimator (FYE) (Archfield and Carlson, 2006; 

Waldron and Archfield, 2006).

Evaporation

Drinking-water

withdrawals (yield)

Other

withdrawal

Precipitation

Bedrock

Unavailable storage

Available storage

Uncontrolled

release

Required

release

Surface-water inflow

Sand and gravel

Ground-water inflow

Loss to ground water

Figure 2. Possible sources and losses of water for a hypothetical drinking-water reservoir in contact with a sand 

and gravel aquifer (Waldron and Archfield, 2006).
4 Hydrogeologic Conditions and a Firm-Yield Assessment for J.B. Converse Lake, Mobile County, Alabama, 1991–2006

This report describes the hydrogeologic conditions of the Converse Lake 

watershed; the development, calibration, 

and limitations of the FYE relative to 

the Converse Lake watershed; and average daily firm-yield estimates for Converse Lake with and without groundwater exchange between the aquifer and 

the reservoir. The simulation period for 

this analysis was 1991 through 2006, 

which included the drought of 2000. 

The FYE provides an estimate of the 

upper limit for the rate of withdrawal 

that can be pumped from the reservoir 

during a drought similar to the drought 

that occurred during 2000 without 

depleting available reservoir storage.

Hydrogeology of the 

J.B. Converse Lake 

Watershed

The Converse Lake watershed is 

west of the City of Mobile near the Alabama State line (fig. 1). Converse Lake, 

previously known as Big Creek Lake, 

was formed from the impoundment of 

Big Creek in 1952. Two geologic units 

consisting of semiconsolidated sand, 

silt, gravel, and clay underlie the Converse Lake watershed (fig. 3). 

Hydrologic Setting

Most reservoirs accumulate water 

from precipitation on the reservoir 

surface and from surface- and groundwater inflows; outflows occur by evaporation from the reservoir surface, watersupply withdrawals, and controlled and 

uncontrolled releases. The USGS collected streamflow data on seven major 

streams contributing flow to Converse 

Lake. Other data collected for the period 

July 1990 through September 2006 

consisted of monthly precipitation and 

monthly pumpage from the reservoir. 

Further data included average monthly 

evaporation rates, reservoir volume and 

area characteristics, and estimates of 

aquifer properties. These data were used 

in the FYE to estimate average daily 

firm yield for Converse Lake.

0 1 2 3 MILES

0 1 2 3 KILOMETERS

30°45'

30°52'30"

31°

88°22'30" 88°15'

 CITRONELLE FORMATION

MIOCENE SERIES

 UNDIFFERENTIATED

 OPEN WATER

UNGAGED 27.2-SQUARE-

 MILE AREA OF POTENTIAL

 RUNOFF 

BASIN BOUNDARY

MAJOR GEOLOGIC UNITS

EXPLANATION

J.B. Converse

Lake

BIG

JUN

COL

LON

CRO

BOG

HAM

Crooked

Creek

HamiltonCreek

Boggy

Branch

Long

Branch

Creek Collins

Creek

Juniper

Big

Creek

1991–2006 CONTINUOUS

 STREAMFLOW DATA-

 COLLECTION SITE

 AND LABEL 

1991 AND 1991–92 SHORTTERM STREAMFLOW DATACOLLECTION SITE AND

LABEL 

JUN

BIG

Figure 3. Underlying geologic units and the locations of seven gaged 

subwatersheds, and a smaller, ungaged watershed that compose the total 

J.B. Converse Lake watershed, Mobile County, Alabama.
Hydrogeology of the J.B. Converse Lake Watershed 5

Climatic Conditions

The climate of the Converse Lake watershed is subtropical. Precipitation occurs almost exclusively as rainfall. Several 

types of weather patterns influence the climate. The Gulf of 

Mexico produces warm, humid air masses that move inland 

and provide precipitation in the form of episodic thunderstorms. Extremely high-intensity rainfall is produced from 

large tropical storms that enter the Gulf of Mexico and move 

inland in late summer and early fall (July to September). In 

winter, the climate is further influenced by arctic fronts that 

move south from the Midwest and contribute more continuous 

precipitation and cooler temperatures (Journey and Gill, 2001). 

Precipitation data from the National Weather Service 

station at Mobile airport were used in this study (fig. 4). The 

average annual precipitation in the Converse Lake area for the 

period 1991 through 2006 was 67 inches (in.) and ranged from 

46 in. in year 2000 to 87 in. in 1998 (National Oceanic and 

Atmospheric Administration, 2007). Average monthly precipitation during this period ranged from 3.7 in. in October to about 

7.5 in. in July. Monthly precipitation exceeded 10 in. for several 

months during the 1991–2006 period. During the period 1961 

to 1990, the lowest average annual precipitation was 44 in. 

(Journey and Gill, 2001), which is similar to the lowest average 

annual value of 46 in. that occurred in year 2000 during this 

study. Therefore, the observed drought during year 2000 represents the drought of record for the purposes of this analysis.

Average monthly pan evaporation data were obtained 

from the National Weather Service station at Mobile airport 

(Farnsworth and Thompson, 1982). Pan evaporation data were 

converted to free-surface evaporation using evaporation coefficients from Farnsworth and others (1982) for input to the 

FYE (fig. 5). 

Surface-Water Inflows

Streamflow data were collected continuously from 1991 

through 2006 on three streams (Big Creek, Crooked Creek, 

and Hamilton Creek) in the Converse Lake watershed (table 1). 

Short-term continuous streamflow data were collected for 

1991 at Boggy Branch and Long Branch and for 1991–1992 at 

Collins Creek and Juniper Creek (fig. 1; table 1). 

The seven major gaged streams, covering an area of 

71.6 square miles (mi2) of the total Converse Lake watershed 

area of 104.4 mi2, contribute flow to Converse Lake (fig. 3). 

During the 1991 through 2006 water years, the greatest 

measured inflows to the reservoir were from Big Creek, 

Crooked Creek, and Hamilton Creek with average surfacewater flows of 62.0, 17.2, and 22.4 cubic feet per second 

(ft3/s), respectively (table 1). Streamflow records for the period 

1991 through 2006 for the three continuous streamflow datacollection sites were used to extend the record for the four 

short-term sites. The daily streamflow data were aggregated to 

determine total monthly surface-water flows to Converse Lake. 

Streamflows on Boggy Branch and Long Branch (1991) 

and Collins and Juniper Creeks (1991–92) were recorded for 

a short period. To obtain coincident streamflow records for 

the FYE simulation, the period of record was extended using 

the record extension technique, MOVE.1 (Hirsch, 1982). 

Daily streamflow data for the three continuous-record sites 

and the four short-term record sites were obtained from the 

USGS National Water Information System (NWIS) database. 

Monthly streamflows were calculated from daily streamflow 

data for the three continuous-record sites. Monthly flows for 

the three sites and the record for each short-term site were 

used as input to a computer program written to calculate 

MOVE.1 values for each site where the record was to be 

0

5

10

15

20

25

19911992199319941995

1996199719981999200020012002

2003

200420052006

YEAR

PRECIPITATION, IN INCHES

Normal precipitation for period 

of record 1971 through 2000

Figure 4. Monthly precipitation for 1991 through 2006 recorded at the National Weather 

Service station, Mobile airport, Alabama, in relation to normal monthly precipitation at this 

station for the period of record 1971–2000.
6 Hydrogeologic Conditions and a Firm-Yield Assessment for J.B. Converse Lake, Mobile County, Alabama, 1991–2006

extended (Gregory E. Granato, U.S. Geological Survey, written commun., 2007). The MOVE.1 streamflow estimates from 

the two continuous-record sites with the best correlation (Big 

Creek and Crooked Creek) were averaged to obtain the final 

record extension for each short-term site. The average volume 

of surface-water flow to Converse Lake for the period 1991 

through 2006, calculated by record extension of the four shortterm sites, was 44.7 ft3

/s. 

Because the area of the Converse Lake watershed drained 

by these seven streams is 71.6 mi2 of the total Converse Lake 

watershed area of 104.4 mi2 (Converse Lake covers about 

5.6 mi2), that leaves a smaller, ungaged drainage area of 

potential runoff of 27.2 mi2—about 26 percent of the total Converse Lake watershed area (fig. 3). Potential average runoff from 

this 27.2-mi2 area was estimated to be 56.5 ft3/s by the drainagearea ratio method, as applied by Ries and Friesz (2000). 

Average streamflow from the three continuous-record 

sites was 101.6 ft3/s; record-extension streamflow for the four 

short-term record sites was estimated to be 44.7 ft3/s; and runoff of 56.5 ft3

/s was estimated by using a drainage-area ratio 

method for the smaller, ungaged drainage area of 27.2 mi2. 

The resulting total average streamflow to Converse Lake for 

the 1991 through 2006 water-year period was 202.9 ft3/s.

Figure 5. Mean monthly pan evaporation data obtained from the 

National Weather Service station at Mobile airport (Farnsworth and 

Thompson, 1982) and converted to free-surface evaporation using 

evaporation coefficients from Farnsworth and others (1982). 

Table 1. Streamflow conditions for the 1991–2006 water years in selected tributaries to J.B. Converse Lake, Alabama. 

[Water year is the period October 1 through September 30 and is identified by the year in which the period ends. For example, the 1991 water year began on 

October 1, 1990, and ended on September 30, 1991]

Station label 

(figs. 1 and 3)

Station 

number Station name Drainage area (square miles)

Period of 

record

Measured or estimated 

average streamflows for the 

1991–2006 water years 

(cubic feet per second)

BIG 2479945 Big Creek at County Road 63 near Wilmer, AL 31.5 1991–2006 62.0

CRO 2479980 Crooked Creek near Fairview, AL 8.08 1991–2006 17.2

HAM 2480002 Hamilton Creek at Snow Road near Semmes, AL 8.22 1991–2006 22.4

JUN 2479948 Juniper Creek at Glenwood Road near Fairview, AL 9.22 1991–1992 17.5

COL 2479950 Collins Creek 8.54 1991–1992 15.7

LON 2479955 Long Branch near Wilmer, AL 2.85 1991 5.5

BOG 2479960 Boggy Branch at County Road near Wilmer, AL 3.17 1991 6.0

Total 71.58 146.3

0

1

2

3

4

5

6

J F M A M J J A S O N D

MONTH

MEAN EVAPORATION, IN INCHES
Hydrogeology of the J.B. Converse Lake Watershed 7

Reservoir Characteristics

The volume of Converse Lake is about 52,000 acre-feet, 

or approximately 17 billion gallons at the normal operational pool level of 110 feet (ft) above NGVD 29 (City of 

Mobile, Board of Water and Sewer Commissioners, written 

commun., October 5, 1994). Converse Lake has an average 

depth of 14.4 ft at full pool, with maximum depths of more 

than 50 ft near the dam and spillway. The pump station is 

located in a small embayment below the mouth of Hamilton 

Creek (fig. 1; Journey and Gill, 2001). The embayment has 

a maximum depth of about 27 ft. The minimum lake-level 

elevation for pumps to operate is about 90 ft above NGVD 29 

(Bennie White, Mobile Area Water and Sewer System, written 

commun., 2007). 

The relations of the reservoir surface area to storage and 

stage to storage are based on reservoir bathymetry that relates 

the volume of water in available reservoir storage to reservoir 

surface area (surface area being a function of reservoir stage). 

These relations are needed to estimate total precipitation to 

and evaporation from the reservoir surface and to compute the 

ground-water contribution to reservoir storage. Data from the 

original 1952 reservoir construction drawings for Converse 

Lake bathymetry related a value of reservoir storage to a corresponding value of reservoir stage in 5-ft increments from 

90 ft to 110 ft (Les Brown, Mobile Area Water and Sewer 

System, written commun., 2008). Water storage in Converse 

Lake that is usable for public supply is the volume of water 

that corresponds to the reservoir surface elevation above 

90 ft. At the reservoir surface elevation of 90 ft, which covers 

a surface area of about 1,180 acres, the volume of water in 

reservoir storage available for public supply is zero million 

gallons (Mgal). At a pool elevation of 110 ft, which covers 

an area of about 3,590 acres, the volume available for public 

supply is about 14,770 Mgal. Points defining the relations of 

surface area to storage (fig. 6A) and stage to storage (fig. 6B) 

were plotted in a spreadsheet program to provide a continuous 

relation between reservoir surface area to storage and stage to 

storage. Piece-wise linear equations were determined to provide the best fit for these relations; lines fit to the bathymetric 

data points and the corresponding coefficients used in the FYE 

are shown in figures 6A, B. 

Reservoir Conditions: 1994 through 2006

The 1991 through 2006 water years include a drought 

that occurred during 2000. After 2000, withdrawals from 

Converse Lake decreased compared with withdrawals before 

year 2000. Although withdrawals decreased after 2000, reservoir spillage also decreased in partial response to a decrease 

in rainfall from 2000 through 2006. Because of the observed 

decreases in withdrawals and reservoir spillage, comparisons 

of before- and after-2000 conditions were made. For the following comparisons of conditions before and after 2000, a 

6-year period of comparison was used (1994 through 1999 

and 2001 through 2006). Average annual withdrawals from 

Converse Lake were 85.6 Mgal/d for the period 1994 through 

1999 and 59.37 Mgal/d for the period 2001 through 2006 

(about 30 percent lower than the withdrawals during 1994–99, 

fig. 7). Drought-year 2000 withdrawal was 74.13 Mgal/d. 

Average high-lake levels for 1994 through 1999 (109.6 ft) 

were 0.6 percent greater than for 2001 through 2006 (109.0 ft). 

The average high-lake level in 2000 was 105.4 ft. Average 

low-lake levels for 1994 through 1999 (108.7 ft) were 0.8 percent greater than for 2001 through 2006 (107.9 ft). The average low-lake level in 2000 was 104.2 ft. Average monthly rainfall recorded at the National Weather Service station at Mobile 

airport for 1994 through 1999 (5.7 in.) was 3 percent greater 

than the average monthly rainfall for 2001 through 2006 

(5.5 in.). The average monthly rainfall for 2000 was 3.8 in. 

(National Oceanic and Atmospheric Administration, 2007). 

Although these changes appear to be minor, they can have a 

large effect on reservoir spillage. Average spillage from the 

lake for 1994 through 1999 (131.7 ft3/s) was 17 percent greater 

than the average spillage for 2001 through 2006 (109.3 ft3/s). 

No spillage occurred in 2000. 

Figure 6. Coefficients of relations of (A) reservoir surface 

area to storage and (B) reservoir stage to storage for 

J.B. Converse Lake and for use in the firm-yield estimator.

RESERVOIR VOLUME (STORAGE) AVAILABLE FOR

PUBLIC SUPPLY, IN MILLION GALLONS

RESERVOIR SURFACE AREA, IN SQUARE MILES

RESERVOIR SURFACE ELEVATION (STAGE), 

IN FEET 

A.

B.

y = 0.002151x + 90.0

85

90

95

100

105

110

115

120

y = 0.000337x + 1.837500

1

2

3

4

5

6

7

8

0 2,500 5,000 7,500 10,000 12,500 15,000

y = 0.001375x + 92.041097

y = 0.000243x + 2.071414

y = 0.000959x + 95.833828

y = 0.000236x + 2.132049
8 Hydrogeologic Conditions and a Firm-Yield Assessment for J.B. Converse Lake, Mobile County, Alabama, 1991–2006

Overall, the 6-year period following the drought in 2000 

showed Converse Lake having decreases in withdrawals, average high- and low-lake levels, spillage, and rainfall. The withdrawals decreased by about 30 percent, and spillage decreased 

by 17 percent in partial response to an average annual decrease 

in precipitation of about 3 percent (2.4 in. per year). These 

results highlight the relative importance of small changes in 

precipitation on reservoir response.

Geologic Setting

The Converse Lake watershed is underlain by an undifferentiated sedimentary deposit of Miocene age (named Miocene Series undifferentiated) and by the Citronelle Formation 

of Pliocene age. These units compose the major aquifer system 

that contributes ground water to streams and lakes in the Converse Lake watershed.

Major Hydrogeologic Units

Two geologic units consisting of semiconsolidated sand, 

silt, gravel, and clay underlie the Converse Lake watershed 

(fig. 3). The two units are undifferentiated sedimentary deposits of Miocene age (named Miocene Series undifferentiated) 

overlain by the Citronelle Formation of Pliocene age. The 

units thicken to the southwest (Journey and Gill, 2001). The 

Miocene Series undifferentiated is of marine and estuarine origin and consists of laminated to thinly bedded, laterally extensive clays, sands, and sandy clays. The unit is about 3,000 ft 

thick near the coast. The Citronelle Formation is relatively thin 

(about 30 ft) in the northern part of the watershed but thickens 

toward the south to about 130 ft near the coast. The Citronelle 

Formation consists of gravelly sands and sandy clays with thin 

(5 to 15 ft thick) lenses of sandy clay and clayey sand interbedded with gravelly sand (Journey and Gill, 2001).

The Pliocene-Miocene aquifer is the major aquifer 

system contributing ground water to streams and lakes in the 

Converse Lake watershed. Although clayey sediments in the 

Miocene Series undifferentiated cause the aquifer to behave 

as if it were semiconfined at depth, the sand and gravel beds 

of the Citronelle Formation and the upper part of the Miocene 

Series are hydraulically connected to the land surface (Journey 

and Gill, 2001). For the firm-yield analysis, the properties of 

the portion of the sand and gravel aquifer within about 100 ft 

of land surface in the Converse Lake watershed are of interest. 

Aquifer Properties

Although aquifer properties, such as hydraulic conductivity, transmissivity, and storage coefficients, are largely 

unknown for the Pliocene-Miocene aquifer, estimates of 

aquifer transmissivity and storage are required for the FYE to 

estimate a firm yield that incorporates ground-water exchange 

with the reservoir (Archfield and Carlson, 2006). Specificcapacity data of unknown accuracy were available for 10 wells 

in the general area of Converse Lake. An equation developed 

by Prudic (1991) for estimating transmissivity from specificcapacity data and an aquifer storage coefficient were used to 

calculate a range of transmissivities for the 10 wells. Because 

both the FYE and the Prudic (1991) equation require an 

aquifer-storage coefficient, these values had to be estimated. 

Prudic (1991) used an aquifer-storage coefficient of 0.15 in his 

report on the Gulf Coast Regional Aquifer System. A range of 

values was used to represent accepted aquifer-storage coefficients for a confined aquifer (0.0001; Gandl, 1979) to storage 

for an unconfined aquifer (0.3; Heath, 1982). This range of 

90

80

70

60

50

40

30

20

10

0

19901991199219931994199519961997199819992000200120022003200420052006

YAD REP SNOLL AG N IOLLI M IN L A WARDHTI WEGAREVA

YEAR

Figure 7. Average withdrawals from J.B. Converse Lake in Alabama for public supply, 

in million gallons per day, for the period 1990 through 2006.
Firm-Yield Estimator Methodology and Calibration 9

aquifer-storage coefficients (0.0001 to 0.3) was used in the 

Prudic (1991) equation, and the resulting transmissivities 

from specific-capacity data for each well ranged from 6,760 to 

340 feet squared per day (ft2/d), respectively (table 2). Average 

transmissivity values for the group of 10 wells for each value 

of aquifer storage (0.0001, 0.1, 0.15, and 0.3) were 2,993, 

1,884, 1,815, and 1,704 ft2/d, respectively (table 2). This 

range of coeffiecients is similar to the transmissivity values of 

120 to 3,300 ft2/d in the Oligocene aquifer system and 4,000 to 

13,000 ft2/d in the Citronelle aquifers in Mississippi reported 

by Gandl (1979, 1982). Renken (1996) reported a transmissivity of 13,000 ft2

/d for the water-bearing units within the 

Miocene sediments in Mississippi. From a regional study 

by Grubb (1998) that included the coastal lowlands aquifer 

system, estimates of hydraulic conductivity ranged from 20 

to 50 ft/d in the general area of Converse Lake. If an effective aquifer thickness of 100 ft is assumed, a corresponding 

estimated range of transmissivity would be from 2,000 ft2/d 

to 5,000 ft2/d. Because the Citronelle Formation and Miocene 

Series undifferentiated geologic units that underlie Converse 

Lake most likely would represent an unconfined aquifer system within 100 ft of land surface, a reasonable range for coefficients of aquifer storage from estimated and reported values 

would be 0.1 to 0.3. By combining the results for transmissivity presented in table 2 and reported values discussed above, 

an initial range of paired transmissivity and aquifer-storage 

coefficients for the aquifer that underlies Converse Lake would 

be 400 ft2/d and 0.1 to 13,000 ft2/d and 0.3, respectively.

Firm-Yield Estimator Methodology 

and Calibration

The firm-yield estimator (FYE) was developed by 

the Massachusetts Department of Environmental Protection (MassDEP) for the purpose of estimating reservoir 

firm yield—the maximum yield that can be delivered by the 

reservoir, even under drought conditions, without completely 

depleting available reservoir storage (Waldron and Archfield, 

2006). Using the routine presented in Waldron and Archfield 

(2006), average daily firm yield can be estimated for a reservoir without ground-water exchange. The routine to estimate 

average daily firm yield without ground-water exchange is 

based on the FYE guidance document (Massachusetts Department of Environmental Protection, 1996) and from input data 

that include data from the surface-water regime in the form of 

total monthly surface-water flow to a reservoir (Waldron and 

Archfield, 2006). 

Water in available storage in a reservoir can be supplemented or reduced if a reservoir is in contact with an aquifer 

(fig. 8A), as a result of direct inflow from the aquifer to the 

reservoir (fig. 8B) or direct outflow of water from the reservoir 

to the aquifer (fig. 8C). The direction of exchange depends 

on the relative elevations of reservoir stage and the watertable surface (fig. 2; Archfield and Carlson, 2006). When the 

reservoir shoreline is in contact with an aquifer, as is the case 

for Converse Lake, the effect of ground-water contributions on 

firm yield can be variable. 

Table 2. Aquifer transmissivities calculated by using the Prudic 

(1991) equation and specific-capacity data and aquifer-storage 

coefficients (0.0001 to 0.3) for each of 10 wells in the area of 

J.B. Converse Lake, Alabama. 

Well 

identification

Aquifer-storage coefficient (S)

0.0001 0.1 0.15 0.3

Transmissivity (T), in feet squared per day

WF2 670 400 370 340

WF4 890 500 480 440

MOBY-07 1,420 830 800 740

MOBX-9 1,920 1,180 1,140 1,070

WF3 2,310 1,430 1,370 1,290

MOBX-01 2,360 1,520 1,470 1,390

MOBQ-01 3,050 1,880 1,800 1,680

WF1 3,900 2,550 2,470 2,340

MOBAA-07 6,650 4,230 4,080 3,830

WF5 6,760 4,320 4,170 3,920

Average 2,993 1,884 1,815 1,704
10 Hydrogeologic Conditions and a Firm-Yield Assessment for J.B. Converse Lake, Mobile County, Alabama, 1991–2006

Equilibrium

Decrease in stage

Increase in stage

A.

B.

C.

Reservoir Surface

Reservoir Surface

Reservoir Surface

Aquifer

Aquifer

Aquifer

Stage

Saturated

Unsaturated

Water table

Flow direction

EXPLANATION

Bedrock

Bedrock

Bedrock

Figure 8. (A) Reservoir water level 

at equilibrium with the water table in 

the surrounding aquifer, (B) response 

of the water table to a decrease in the 

water level of the reservoir relative to 

the water table, and (C) an increase 

in the water level of the reservoir 

relative to the water table (modified 

from Archfield and Carlson, 2006).
Firm-Yield Estimator Methodology and Calibration 11

The same routine used to estimate firm yield without 

ground-water exchange can be used to estimate firm yield with 

ground-water exchange with the addition of a term (QGWi) 

that accounts for the volume of water that moves between 

the reservoir and the aquifer. Firm yield with ground-water 

exchange is then computed by solving the monthly water balance (eq. 1) of a reservoir for a period of record that includes a 

drought of record (Archfield and Carlson, 2006):

Si = Si–1 + Awi Qri + Ari (Pi – Ei ) ± QGWi

– ai ni Qy – Qri – Qsi – Qoi , (1)

where

i is the month;

Si is the water in usable storage at the end of the 

month, in million gallons;

Si–1 is the amount of water in usable storage 

at the end of the previous month, 

in million gallons;

Awi is the contributing drainage area, 

in miles squared;2

Qri is the streamflow per mile of reservoir 

drainage, in miles;2

Ari is the area of the reservoir surface, 

in miles squared;2

Pi is the precipitation, in miles;2

Ei is the evaporation from the reservoir, 

in miles;2

QGWi is the volume of water that moves between 

the reservoir and aquifer due to the present 

change in reservoir stage during the 

month and the time varying past changes 

in reservoir stage over previous months, 

in million gallons; 

ai is the peak-use factor, dimensionless;

ni is the number of days in the month;

Qy is the yield, in million gallons per day;

Qri is the required release, in million gallons 

per month;

Qsi is the uncontrolled spill, in million gallons 

per month; and

Qoi is the withdrawal from the reservoir by other 

users, in million gallons per month.

Firm-yield estimations begin with the month of April, 

which in Massachusetts is the month when a reservoir is 

most likely to be at full capacity. For most years during the 

1991–2006 period, Converse Lake typically had high lake 

levels and spillage from January through April. Therefore, 

beginning estimations of firm yield in April also applies to 

Converse Lake. Converse Lake also refilled after the drought 

2

 Precipitation, streamflow, and evaporation are in units of length and, when 

multiplied by Ari, become volumes (in cubic miles) that are converted to 

million gallons.

of record (2000). In addition to surface-water flows to a 

reservoir, the FYE includes a component that accounts for 

the interaction of a ground-water system with a reservoir, in 

the case where a reservoir is in contact with a sand and gravel 

aquifer (Archfield and Carlson, 2006). Detailed descriptions of 

the FYE routines for firm yield estimated from surface-water 

flow (ground-water exchange is zero) are found in Waldron 

and Archfield (2006); an analytical method for estimating firm 

yield that includes both surface-water flow and ground-water 

exchange components (the estimate of ground-water contribution to reservoir storage) is described in Archfield and Carlson 

(2006). For some reservoirs in contact with an aquifer, groundwater exchange can be substantial. The report by Archfield 

and Carlson (2006) includes analyses of idealized reservoir 

geometries based on characteristics of real reservoirs. Comparisons of firm-yield estimates with observed real reservoir 

flows, releases, and withdrawals in Massachusetts have not yet 

been conducted, however, and the FYE may not be applicable 

to all reservoir-aquifer systems.

The firm yield is estimated as an average daily value 

because, for each month, a demand factor is used in the FYE 

that accounts for reservoir-specific demand that varies by 

month. The demand factor is calculated as the total withdrawal by month divided by the average withdrawal for the 

corresponding year. Thus, withdrawal rates may be greater 

or less than the firm-yield value for some months of the year. 

The unique monthly demand factors for each month during 

the 1991–2006 period are shown in figure 9A for Converse 

Lake. Figure 9B shows the average monthly demand factor for 

the same period used for each year in the FYE based on a set 

of average monthly demand factors that were determined on 

the basis of the most recent withdrawal data and reflective of 

the demand pattern most likely to occur in the future. Firmyield estimates are sensitive to demand patterns (Waldron and 

Archfield, 2006). 

A variety of input data are used in the FYE routines to 

estimate an average daily firm yield of a reservoir (Archfield 

and Carlson, 2006; Waldron and Archfield, 2006). Some terms 

shown in eq. 1 are functions of other input data. For example, 

the area of the reservoir surface is computed from the relation 

of the reservoir surface area to storage. Several input variables 

are common to both FYE routines, including (1) total monthly 

surface-water flow, in million gallons per month, for all 

streams flowing into the reservoir; (2) monthly precipitation, 

in inches; (3) average monthly evaporation from the reservoir 

surface, in inches; (4) required releases from the reservoir for 

in-stream flow purposes, in million gallons per month, if any; 

(5) a unique monthly demand factor calculated by dividing the 

monthly withdrawals by the average withdrawals for the year 

(dimensionless, fig. 9B); (6) the contributing drainage area to 

the reservoir, in miles squared; (7) the maximum capacity of 

usable reservoir storage, in million gallons; (8) the bathymetry 

of the reservoir, in feet; and (9) the relation of the reservoir 

surface area to storage, in miles squared and million gallons. 

Additional input variables used by the FYE routine to estimate 

ground-water exchange are (1) a, the average distance from 
12 Hydrogeologic Conditions and a Firm-Yield Assessment for J.B. Converse Lake, Mobile County, Alabama, 1991–2006

the reservoir shoreline to the aquifer boundary (representing the size of the aquifer, in feet); (2) L, the length of the 

reservoir shoreline in contact with the aquifer (represents the 

size of the reservoir, in feet); (3) T, the aquifer transmissivity, in feet squared per day; (4) S, aquifer storage coefficient 

(dimensionless); and (5) the stage to storage relation based on 

reservoir bathymetry that relates reservoir storage to reservoir 

surface area (surface area being a function of reservoir stage), 

in million gallons and feet, respectively. 

The average distance from the reservoir shoreline to the 

aquifer boundary, a, which represents the size of the aquifer 

(fig. 10) was determined by calculating the average of (1) 103 

representative measurements (in all four directions—east, 

west, north, and south—from the reservoir) and (2) another 

set of three representative measurements (west, north, and east 

from the reservoir) perpendicular from the edge of the reservoir to the aquifer boundary. The overall average of each set 

of measurements then was calculated to be 25,771 ft, and this 

value was used in the FYE (fig. 10). From this relatively large 

value, the aquifer can be assumed to be infinite in terms of the 

FYE (Archfield and Carlson, 2006). The ground-water divide 

is assumed to coincide with the surface-water divide.

 The length of the reservoir shoreline in contact with the 

aquifer, L, represents the size of the reservoir and was determined from a geographic information system (GIS) coverage 

of Converse Lake (U.S. Geological Survey, Alabama Water 

Science Center, written commun., April 2, 2007). This length, 

measured at 266,402 ft, represents a detailed outline of the reservoir shoreline in contact with the aquifer (fig. 10). Two other 

measurements of L were made to represent simplified outlines 

of the reservoir that included incrementally fewer shoreline 

details, such as coves and embayments. These measurements 

represent predominant general shoreline segments that are 

perpendicular to the assumed major ground-water flow lines 

representing the path of potential water exchange between the 

aquifer and the reservoir (fig. 10). These simplified values of 

DEMAND FACTOR

AVERAGE DEMAND FACTOR FROM 

1991 THROUGH 2006

0.8

0.9

1.0

1.1

1.2

1.3

MONTH

0.8

0.9

1.0

1.1

1.2

J F M A M J J A S O N D

A.

B.

1991199219931994199519961997199819992000200120022003200420052006

YEAR

Figure 9. Factors used in the firm-yield estimator to represent (A) monthly demand for the 

period 1991–2006 calculated as total withdrawals by month divided by the average withdrawal 

for the corresponding year and (B) the average demand for the period 1991–2006 for 

J.B. Converse Lake in Alabama.
Firm-Yield Estimator Methodology and Calibration 13

L = 137,280 ft

Simplified shorelines

L = 84,480 ft a

a

a

a

L

L = 266,402 ft (measured shoreline)

DISTANCE FROM THE RESERVOIR 

 SHORELINE TO THE AQUIFER BOUNDARY,

a, REPRESENTING THE SIZE OF 

 THE AQUIFER, IN FEET—25,771 ft used 

 in the firm-yield estimator

LENGTH OF THE RESERVOIR 

 SHORELINE IN CONTACT WITH THE 

 AQUIFER, L, REPRESENTING THE SIZE 

 OF THE RESERVOIR, IN FEET

EXPLANATION

Figure 10. Variables that represent the distance from the reservoir shoreline to the aquifer 

boundary, a, and the length of the reservoir shoreline in contact with the aquifer, L, used for the 

ground-water exchange component of firm yield for a reservoir in contact with a sand and gravel 

aquifer, J.B. Converse Lake, Alabama. [Measured actual shoreline outline (L = 266,402 feet) and 

two simplified shoreline outlines (137,280 feet and 84,480 feet) are included.]
14 Hydrogeologic Conditions and a Firm-Yield Assessment for J.B. Converse Lake, Mobile County, Alabama, 1991–2006

L were 137,280 ft (about 48 percent less) and 84,480 ft (about 

68 percent less). The amount of Converse Lake shoreline in 

contact with the aquifer was 100 percent (fig. 10).

To further refine estimates of transmissivity and aquifer 

storage, the FYE was calibrated to match the lowest measured water level that occurred in October of drought-year 

2000. The parameters that were adjusted as part of the calibration process were transmissivity and aquifer-storage coefficients. The initial values of paired transmissivity (T) and aquifer storage (S) ranged from 400 ft2

/d and 0.1 to 13,000 ft2/d 

and 0.3, respectively. 

T and S were varied iteratively to match water-level 

data for a specified withdrawal rate. Simulations were made 

to match the lowest measured average monthly lake level 

(101.7 ft) that occurred in October 2000 (figs. 11A, B). The 

average withdrawal from Converse Lake (74.13 Mgal/d) 

for drought-year 2000 was used as the specified withdrawal 

rate, and several simulations were run in which T and S were 

adjusted until the lowest simulated lake level for year 2000 

matched the lowest measured average monthly lake level for 

year 2000 (101.7 ft). The various non-unique combinations of 

aquifer properties and other values that produced a match with 

Figure 11. Simulated and measured reservoir water levels for (A) droughtyear 2000 and (B) the period 1991 through 2006 for model calibration 

simulations run to match the lowest measured drought-year 2000 water level 

(101.7 ft) to estimate aquifer transmissivity for J.B. Converse Lake, Alabama. 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005

2006

YEAR

Monthly

precipitation,

 in inches

Measured water level

Calibrated water level, from year2000 average withdrawal of 74.13

million gallons per day 10

5

0

15

20

25

MEASURED

RESERVOIR WATER LEVEL

SIMULATED

Increased streamflow by 25 percent

Measured and estimated streamflow

Decreased streamflow by 25 percent

RESERVOIR WATER LEVEL, IN FEET

RESERVOIR WATER LEVEL, IN FEET

PRECIPITATION, IN INCHES

PRECIPITATION, IN INCHES

A.

B.

EXPLANATION

YEAR 2000

10

5

0

15

20

25 112

110

108

106

104

102

100

112

110

108

106

104

102

100

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.
Firm-Yield Estimator Methodology and Calibration 15

the lowest measured average monthly lake level for year 2000 

(October) are shown in table 3.

Although the goal of calibration was to reproduce 

drought-year 2000 water levels (fig. 11A) by using the year2000 average withdrawal rate of 74.13 Mgal/d, the overall 

match between simulated and measured water levels is good 

for most years during the 1991 through 2006 period (fig. 11B). 

The underestimation of reservoir water levels from mid-2001 

to mid-2002 may reflect (in terms of the monthly demand 

factor, fig. 9A) that real withdrawal rates during this period 

(fig. 7) decreased substantially from the year-2000 value of 

74.13 Mgal/d. This match shows that the FYE is capable of 

reproducing measured reservoir water levels given a set of 

aquifer properties. Other deviations of simulated water levels 

from measured reservoir water levels may be a result of controlled spills, which were not simulated. Validation from field 

study and measurement of these aquifer properties, specifically T and S, could produce a more accurate result.

The most reasonable set of values for use in the FYE 

from the calibration process was determined to be a equals 

25,771 ft; S equals 0.15; L equals 137,280 ft; and the actual 

measured and estimated total surface-water flow to Converse 

Lake (average of 202.9 ft3/s). The transmissivity value of 

650 ft2/d resulted from specifying the aforementioned values 

in the calibration process (table 3). If the depth of the aquifer 

in hydrologic contact with Converse Lake is assumed to be 

100 ft and T is 650 ft2/d, the corresponding value of hydraulic 

conductivity is 6.5 ft/d.

Because in the calibration process T and S are nonunique parameters that depend on a, L, and total surface-water 

flow, a sensitivity analysis was conducted using the known 

uncertainty in these values to determine a reasonable range 

of T and S for use in the firm-yield analysis. This sensitivity 

analysis used total monthly surface-water flows to the lake 

for 1991 through 2006, the unique monthly demand factor for 

withdrawals from Converse Lake (fig. 9A) for each month in 

1991 through 2006, the drought year-2000 average withdrawal 

of 74.13 Mgal/d specified in the calibration process, and other 

input variables. The aquifer storage coefficient used in the 

calibration process for most simulations was 0.15 (Prudic, 

1991). Calibration simulations also were run with adjustments 

made to S (values of 0.15 and 0.25), the length of shoreline 

in contact with the aquifer, L, (values of 84,480, 137,280, and 

266,402 ft) and increasing and decreasing total surface-water 

flow by 25 percent to determine how changes in these values 

would affect estimated T. 

The value for aquifer storage (0.15) was that used by 

Prudic (1991) to estimate T for an aquifer surrounding a well 

less than 150 ft in depth. In this report, the depth of the aquifer 

in hydrologic contact with Converse Lake was estimated to be 

100 ft. Therefore, an S value of 0.15 could be applicable to the 

aquifer in the Converse Lake watershed. Archfield and Carlson 

(2006) indicate that the equations used in the FYE provide a 

reasonable approximation of numerically simulated flow rates 

between a reservoir and aquifer (within ±10 percent) when the 

ratio of a to L is between approximately 0.5 and 2.3, regardless of the shape of the reservoir shoreline in contact with the 

aquifer. In this report a ratio of 0.19—when a is 25,771 ft and

L is 137,280 ft—is the closest result to that ratio range for the 

combinations of a and L used. Also, an L value of 266,402 ft 

is likely to be too large from too much shoreline detail, and a 

value of 84,480 ft may be too small from over-simplification 

of shoreline detail.

Surface-water flow is an important component of the 

firm-yield analysis. Although total surface-water flow to Converse Lake was either measured or estimated using accepted 

techniques (Hirsch, 1982; Ries and Friesz, 2000), error bars 

can be associated with measured and estimated surface-water 

flow values. Therefore, surface-water flow to Converse Lake 

was increased and decreased by 25 percent to determine how T 

and, consequently, firm yield could be affected. When L varied 

from 84,480 ft to 266,402 ft (table 3), and monthly surfacewater flow totals decreased by 25 percent, T varied from 4,850 

to 82,500 ft2/d; when surface-water flow totals increased by 

25 percent, T was set at 0.0001 ft2/d (essentially no exchange 

of ground water with the reservoir) (table 3). When surfacewater flow totals were decreased by 25 percent, most firmyield-estimator solutions did not converge; therefore, these 

combinations of input values may not be realistic (table 3). 

When surface-water flow totals were increased by 25 percent, 

the measured water level of 101.7 ft was not matched. Even 

with a T of 0.0001, the lowest calculated water level was 

105.8 ft (fig. 11A); therefore, an increase in total surface-water 

flow of 25 percent may not be realistic (table 3, fig. 11A). The 

closest match to year-2000 water levels was the result of the 

calibration simulations that used the original total measured 

plus estimated surface-water flow that averaged 202.9 ft3/s 

(fig. 11A).

The values of T that resulted from all simulations run 

with combinations of other realistic variables (table 3) that 

matched the measured drought-year 2000 lowest average 

monthly lake level were consistent with T estimated from 

the Prudic (1991) equation (table 2). Therefore, the resulting 

overall range of T was 170 to 6,760 ft2/d. With the assumption 

that measured and estimated surface-water flow are reasonable 

(202.9 ft3/s), a calibrated value for T is 650 ft2/d. This range of 

T in conjunction with the assumed uncertainties in S, L, and 

total surface-water flow were used to assess the uncertainty in 

model predictions of firm yield for Converse Lake.
16 Hydrogeologic Conditions and a Firm-Yield Assessment for J.B. Converse Lake, Mobile County, Alabama, 1991–2006 Table 3. Non-unique combinations of input values for transmissivity (T), aquifer storage (S), aquifer width (a), and length of shoreline in contact with the aquifer (L) that matched the lowest measured water level (101.7 feet in drought-year 2000) for model-calibration simulations to determine a reasonable value for aquifer transmissivity, using the withdrawal rate of 74.13 million gallons per day in 2000. Also shown are corresponding estimates of average daily firm yield with and without ground-water exchange calculated by using total surface-water flow to the reservoir, and for plus or minus 25 percent of total surface-water flow to J.B. Converse Lake, Alabama. [Mgal/d, million gallons per day; NC, firm-yield-estimator solution did not converge; NWL, water level estimated by the firm-yield estimator did not match measured water level of 101.7 feet] Measured and estimated total surface-water flow Decreased total surface-water flow by 25 percent Increased total surface-water flow by 25 percent Transmissiv- ity, in feet squared per day 170 650 1,740 8,100 31,050 82,500 4,850 27,800 49,100 0.0001 0.0001 0.0001 0.0001 0.0001 0.0001 Aquifer width, a, in feet 25,771 25,771 25,771 25,771 25,771 25,771 25,771 25,771 25,771 25,771 25,771 25,771 25,771 25,771 25,771 Aquifer storage coeffi- cient 0.15 0.15 0.15 0.15 0.15 0.15 0.25 0.25 0.25 0.15 0.15 0.15 0.25 0.25 0.25 Length of shoreline in contact with the aquifer, 

L, 

in feet

266,402 137,280 84,480 266,402 137,280 84,480 266,402 137,280 84,480 266,402 137,280 84,480 266,402 137,280 84,480

Firm yield 

with groundwater exchange, in Mgal/d

NC 83 83 NC NC NC 92 NC NC NWL NWL NWL NWL NWL NWL

Firm yield 

without groundwater exchange, in Mgal/d

79 62 95
Firm-Yield Assessment of J.B. Converse Lake 17

Firm-Yield Assessment of 

J.B. Converse Lake

Once all input variables required for the FYE were 

determined, the average daily firm yield was estimated for 

Converse Lake in two different ways: (1) with total surfacewater flow to the reservoir without ground-water exchange and 

(2) with total surface-water flow to the reservoir with groundwater exchange using the range of T and S values determined 

from model calibration. 

The FYE routine (Waldron and Archfield, 2006) to 

estimate the average daily firm yield of a reservoir from input 

data that included data from the surface-water regime—total 

surface-water flow to the reservoir—was used to estimate 

an average daily firm yield for Converse Lake of 79 Mgal/d 

(table 3). This value represents a conservative estimate 

because it is based on measured plus estimated surface-water 

flows to Converse Lake.

The firm-yield estimate incorporating the effects of 

ground-water and surface-water exchange based on the 

best available estimates of a (equals 25,771 ft), L (equals 

137,280 ft), S (equals 0.15), T (equals 650 ft2/d), and 

measured and estimated average surface-water flow of 

202.9 ft3/s resulted in an average daily firm yield of 83 Mgal/d. 

The aquifer properties (T and S) used in this analysis were 

derived from the model calibration process; if future studies 

are conducted to better estimate T and S, the degree of interaction between the ground-water system and the reservoir can be 

determined more accurately. 

Usable reservoir storage for the period April 1991 

through March 2006 resulting from the estimated firm-yield 

values of 79 and 83 Mgal/d, are shown in figures 12A, B, 

respectively. These graphs (figs. 12A, B) represent available 

reservoir storage for a withdrawal rate equal to the respective 

average daily firm-yield value applied as a constant withdrawal rate throughout the April 1991 through March 2006 

period to Converse Lake. Precipitation for the period is shown 

on each graph. Additionally, the graph in figure 12B shows the 

simulated water flow into the reservoir or into the aquifer that 

resulted from the firm-yield analyses that included groundwater exchange with the reservoir.

The graph (fig. 12A) of simulated usable reservoir storage resulting from the firm-yield estimate of 79 Mgal/d that 

Figure 12. Total measured monthly precipitation and estimated usable reservoir storage for the period 

1991–2006 for each average daily firm-yield estimate of (A) 79 million gallons per day and (B) 83 million 

gallons per day for J.B. Converse Lake, Alabama.

Firm yield = 99 million gallons per day, 

with total surface-water flow to reservoir 

and ground-water exchange

USABLE RESERVOIR STORAGE, IN MILLION GALLONS

 WATER FLOW INTO THE RESERVOIR (+) OR

WATER FLOW INTO THE AQUIFER (-)

SIMULATED BY THE FIRM-YIELD ESTIMATOR,

IN MILLION GALLONS PER DAY

Monthly

precipitation,

 in inches

Monthly

precipitation,

 in inches

Firm yield = 79 million gallons per day, 

with total surface-water flow to reservoir 

Firm yield = 83 million gallons per day

with total surface-water flow to reservoir 

and ground-water exchange,

A.

B.

Monthly

precipitation,

 in inches

Monthly

precipitation,

 in inches

Usable reservoir storage

Water flow into the reservoir or into the aquifer

Usable reservoir storage

1991199219931994199519961997199819992000200120022003200420052006

YEAR

10

5

0

15

20

25

PRECIPITATION, IN INCHES

-400

-300

-200

-100

0

100

200

300

400

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

16,000

 83

T = 650 feet squared per day and S= 0.15 

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

16,000
18 Hydrogeologic Conditions and a Firm-Yield Assessment for J.B. Converse Lake, Mobile County, Alabama, 1991–2006

included surface-water flows to the reservoir (no ground-water 

exchange) indicates that the reservoir nearly failed in November 2002. Similarly, the graph (fig. 12B) that represents simulated usable reservoir storage for firm yield with surface-water 

flows and ground-water exchange (T = 650 ft2/d and S = 0.15) 

also indicates that reservoir near-failure occurred in November 2002. These results indicate that Converse Lake is largely 

affected by the amount of surface-water inflow. 

Simulations of the reservoir showed that the reservoir 

refilled to capacity after 2002 when no interaction between the 

aquifer and reservoir was simulated (fig. 12A) and also when 

ground-water exchange was included (fig. 12B). Simulated 

water flow into the reservoir or into the aquifer from the firmyield analysis (fig. 12B) indicates that water began to flow 

into the aquifer during 2002 at a comparatively greater rate 

and then tailed off gradually in an asymptotic manner until the 

beginning of 2006. This pattern could indicate that the aquifer 

was still recovering from its relative depletion from movement 

of ground water to the reservoir during and after drought-year 

2000, by continued accumulation of precipitation recharge 

and from inflow of water to the aquifer from the reservoir 

(fig. 12B). The amount of water available in the aquifer by 

2001 as supplemental inflow to the reservoir may have been 

lower than that available prior to drought-year 2000. Measured 

data for ground-water levels in wells near Converse Lake 

could provide information to confirm or refute this possibility. 

Months when total monthly precipitation exceeded 10 in. may 

have reduced the time needed to refill the reservoir and could 

be important to aquifer recharge. A potential lag in aquifer 

recovery time may affect the degree of ground-water interaction with the reservoir. 

The measured withdrawal of 74.13 Mgal/d from Converse Lake during drought-year 2000 provides data that support 

the idea of a system in which ground water interacts with the 

reservoir. No spillage from the reservoir occurred in year 2000, 

however, and because withdrawals from Converse Lake were 

lower after 2000, it is unclear how withdrawal rates prior to 

2000 may have affected water levels and the volume of usable 

reservoir storage in Converse Lake. After the 2000 drought 

(2001 through 2006), withdrawals were about 30 percent 

lower, rainfall was about 3 percent lower, average high-lake 

level was about 0.6 percent lower, and average low-lake level 

was about 0.8 percent lower than during the period 1994–99; 

average spillage from the reservoir was about 17 percent lower. 

Although withdrawals declined substantially compared with 

pre-drought withdrawals, lower spillage from Converse Lake 

occurred after year 2000. This result may also indicate that 

ground-water levels in the surrounding aquifer may take several years to recover from a drought, such as occurred in 2000.

Model Limitations and Uncertainty

Although estimates of firm yield are based on measured and estimated parameters as input to the FYE, each 

of the parameters has a level of uncertainty that can affect 

model results. The uncertainties associated with each of these 

parameters are as follows: a (17,982 to 31,637 ft); L (84,480 to 

266,402 ft); S (0.1 to 0.3); T (170 to 6,760 ft2/d); and plus or 

minus 25 percent in total measured and estimated surfacewater flow to Converse Lake. Simulations were conducted 

based on these ranges of values to provide a range in estimated firm yield given the limitations of the input parameters 

(table 4A). Although total surface-water flows were varied 

by 25 percent, the results listed in the “Firm-Yield Estimator 

Methodology and Calibration” section of this report show that 

an increase or decrease of 25 percent in total surface-water 

flow into Converse Lake may not be realistic for this analysis. 

The best available estimates of a (equals 25,771 ft), L (equals 

137,280 ft), S (equals 0.15), T (equals 650 ft2/d), and measured and estimated average surface-water flow (202.9 ft3

/s) 

were used for the respective values in the sensitivity analysis 

when one or more of the other values varied. Average daily 

firm yield estimated from the best available estimates of these 

properties was 83 Mgal/d, and with overall changes in these 

properties, firm yield ranged from 80 to 98 Mgal/d (table 4A). 

The value for a, the aquifer size based on the average 

distance from the reservoir shoreline to aquifer boundary, was 

measured. Although this value was measured and the best 

representative value for a (25,771 ft) was used in the firmyield estimation, uncertainty is associated with the measurement because of the wide range of distances from Converse 

Lake to the aquifer boundary. For example, the representative 

distance on the west side of Converse Lake is about 6,000 ft; 

to the north, about 64,000 ft; to the east, about 24,000 ft; and 

to the south, about 5,000 ft (fig. 10). The range of a values 

examined in this report was 17,982 to 31,637 ft. By lowering a

by 30 percent and increasing a by 23 percent for S values (0.1 

to 0.3) and T values up to 13,000 ft2/d, firm yield changed by 

less than about 1 percent. For the FYE, this indicates that the 

aquifer can be assumed to be infinite.

The results of the sensitivity analysis of varying the 

parameters L, S, and T are given in table 4A. Varying L

from 84,480 to 266,402 ft caused average daily firm yield 

to decrease by about 2 percent (81 Mgal/d). Varying S from 

0.1 to 0.3 caused firm yield to decrease by about 1 percent 

(82 Mgal/d). Varying T from 170 to 1,740 ft2/d caused firm 

yield to decrease by about 2 percent (81 Mgal/d). In the results 

of the sensitivity analysis listed above, the firm-yield estimator did not converge on a solution when the best available 

estimates were combined with values at the higher end of the 
Model Limitations and Uncertainty 19

Table 4. Estimates of average daily firm yield for J.B. Converse Lake, Alabama, calculated with total surface-water flow and groundwater exchange with the reservoir using results of (A) the firm-yield estimator model-calibration process and sensitivity analysis and 

(B) values from table 2 for various values of transmissivity (T), aquifer storage (S), aquifer width (a), and length of shoreline in contact 

with the aquifer (L). 

[NC, Firm-yield-estimator solution did not converge; —, Firm yield not calculated]

A. Measured and estimated total 

surface-water flow

B. Measured and estimated total 

surface-water flow with aquifer storage and 

average transmissivity values from table 2

Transmissivity, in feet squared 

per day

650 650 650 — — — —

Aquifer width, a, in feet 25,771 25,771 25,771 — — — —

Aquifer storage coefficient 0.15 0.15 0.15 — — — —

Vary L Length of shoreline in contact 

with the aquifer, L, in feet

84,480 137,280 266,402 — — — —

Firm yield with groundwater exchange81 83 NC — — — —

Transmissivity, in feet squared 

per day

650 650 650 650 650 650 650

Aquifer width, a, in feet 25,771 25,771 25,771 25,771 25,771 25,771 25,771

Vary S Aquifer storage coefficient 0.1 0.15 0.3 0.0001 0.1 0.15 0.3

Length of shoreline in contact 

with the aquifer, L, in feet

137,280 137,280 137,280 137,280 137,280 137,280 137,280

Firm yield with groundwater exchange82 83 NC 79 82 83 NC

Vary T Transmissivity, in feet squared 

per day

170 650 1,740 2,993 1,884 1,815 1,704

Aquifer width, a, in feet 25,771 25,771 25,771 25,771 25,771 25,771 25,771

Aquifer storage coefficient 0.15 0.15 0.15 0.15 0.15 0.15 0.15

Length of shoreline in contact 

with the aquifer, L, in feet

137,280 137,280 137,280 137,280 137,280 137,280 137,280

Firm yield with groundwater exchange81 83 NC NC 86 NC NC

Vary T Transmissivity, in feet squared 

per day

170 650 1,740 2,993 1,884 1,815 1,704

Aquifer width, a, in feet 25,771 25,771 25,771 25,771 25,771 25,771 25,771

Vary S Aquifer storage coefficient 0.1 0.15 0.3 0.0001 0.1 0.15 0.3

Length of shoreline in contact 

with the aquifer, L, in feet

137,280 137,280 137,280 137,280 137,280 137,280 137,280

Firm yield with groundwater exchange80 83 89 79 NC NC NC

Vary T Transmissivity, in feet squared 

per day

170 650 1,740 — — — —

Aquifer width, a, in feet 25,771 25,771 25,771 — — — —

Vary S Aquifer storage coefficient 0.1 0.15 0.3 — — — —

Vary L Length of shoreline in contact 

with the aquifer, L, in feet

84,480 137,280 266,402 — — — —

Firm yield with groundwater exchange80 83 98 — — — —
20 Hydrogeologic Conditions and a Firm-Yield Assessment for J.B. Converse Lake, Mobile County, Alabama, 1991–2006

sensitivity ranges. This may indicate that those particular combinations of input parameters are not realistic (table 4A). 

When the final sensitivity analysis (table 4B) was calculated using the best available estimates of a (equals 25,771 ft), 

L (equals 137,280 ft), S (equals 0.15), measured and estimated 

average surface-water flow (202.9 ft3/s), and a T range from 

650 to 2,993 ft2/d (the highest average T from table 2), average 

daily firm yield was again calculated at 83 Mgal/d; however, 

the overall sensitivity range narrowed (from that shown in 

table 4A) to be 79 to 86 Mgal/d (table 4B). 

Estimates of firm yield with no ground-water exchange 

were determined by varying total surface-water flow by plus or 

minus 25 percent. Firm yield with no ground-water exchange 

was 62 Mgal/d (22 percent lower) when surface-water flow 

decreased and 95 Mgal/d (20 percent higher) when surfacewater flow increased (table 3). The firm-yield estimator did 

converge on a solution when no ground-water exchange and 

varying total surface-water flow by plus or minus 25 percent 

were simulated; however, the results listed in the “Firm-Yield 

Estimator Methodology and Calibration” section of this 

report show that an increase or decrease of 25 percent in total 

surface-water flow into Converse Lake may not be realistic 

for this analysis. The average daily firm yield of 79 Mgal/d 

(table 3) is, therefore, a reasonable estimate for average 

daily firm yield with total surface-water flow to the reservoir 

because the best available estimate for total surface-water flow 

was used.

One additional point of uncertainty could be the value 

for total usable reservoir capacity. For example, if total usable 

reservoir capacity were reduced by about 65 percent, to 

5,214 Mgal, as a result of sedimentation in the reservoir or 

by another cause, and if the associated relations of reservoir 

surface area to storage and reservoir stage to storage were 

correspondingly adjusted, average daily firm-yield results 

would change greatly, and the calibrated value for T would be 

24,500 ft2/d. If the values listed above were used as input to 

the firm-yield estimator, average daily firm yield with groundwater exchange would be 99 Mgal/d with an uncertainty range 

of 77 to 132 Mgal/d, and average daily firm yield using total 

surface-water flow to the reservoir without ground-water 

exchange would be 63 Mgal/d (with an uncertainty range of 

52 to 74 Mgal/d). These estimates of average daily firm yield 

would seem to be reasonable given the assumptions made 

about values of the input data to the firm-yield estimator. This 

example underscores the importance of accurate measured 

values for total usable reservoir capacity and the corresponding relations of reservoir surface area to storage and reservoir 

stage to storage. A study by Juracek (2006) indicated that 

decreases in reservoir water-storage capacity due to sedimentation ranged from less than 10 percent to 40 percent for certain reservoirs in Kansas. Results presented by Stricklin (2001) 

for Lake Tuscaloosa, Alabama, show that sedimentation in that 

lake is a dynamic process where maximum sediment deposition changes over time. When combining the results of the 

sensitivity test on total usable reservoir capacity described 

above, the age of the bathymetric data for Converse Lake, and 

results from other reservoir-sedimentation studies, use of the 

most accurate and up-to-date measured reservoir bathymetric 

data for the whole reservoir volume could substantially change 

the value of estimated firm yield for Converse Lake. 

Summary

The estimate of average daily firm yield for a reservoir 

that is used for public supply can be used as an upper limit for 

withdrawals from the reservoir. Withdrawals from a reservoir 

that are equal to or less than the estimated firm yield ensure 

that the reservoir will meet these demands, assuming the reservoir continues to be affected by climatic and hydrologic conditions similar to those used in determining the estimated firm 

yield. As future demands for water supply increase, estimation 

of the firm yield provides an upper limit that can be compared 

to future demands to determine if the reservoir will be able to 

meet those additional demands in excess of current withdrawals. Alternatively, the firm yield can be compared to present 

reservoir withdrawals to determine if present withdrawals 

could be sustained through a drought (Archfield and Carlson, 

2006). To assist officials of the Mobile Area Water and Sewer 

System in planning for future drinking-water demands in the 

service area, the average daily firm yield for Converse Lake 

was estimated by using two methods—(1) using total surfacewater flow to the reservoir without ground-water exchange 

and (2) using total surface-water flow to the reservoir with 

ground-water exchange and T and S values determined from 

model calibration.

The results presented in this report include a comprehensive estimation of firm yield values for Converse 

Lake that were dependent on the specific input parameters 

used. Although many variables were measured, some were 

estimated. Average daily firm yield estimated by the first 

method—using total surface-water flow to the reservoir without ground-water exchange—was 79 Mgal/d, and firm yield 

estimated by the second method—using total surface-water 

flow to the reservoir with ground-water exchange and T and 

S values determined from model calibration—was 83 Mgal/d. 

The difference in these firm-yield values indicates that groundwater interaction with Converse Lake may account for about 

5 percent of the firm yield. However, aquifer properties that 

would describe more accurately this hydrologic interaction of the ground-water aquifer with the reservoir are not 

fully known. 

When firm yield was estimated using total surfacewater flows without ground-water exchange (79 Mgal/d), the 

result was about 8 percent lower than average withdrawals 

(85.60 Mgal/d) during 1994 through 1999 and about 25 percent greater than withdrawals (59.37 Mgal/d) during 2001 

through 2006—the periods before and after, respectively, the 

drought of 2000. The firm yield of 79 Mgal/d represents a conservative estimate because it does not include ground-water 

exchange with Converse Lake and the adjacent aquifer.
References Cited 21

Results of this study indicate that ground water can be 

a component of inflow to a reservoir in contact with a sand 

and gravel aquifer, but the degree to which it is important to 

Converse Lake is still uncertain. Also, following a drought, 

ground-water levels may take several years to recover to predrought levels. Given the role that ground-water exchange 

likely plays in the firm yield of Converse Lake, long-term 

monitoring of ground-water levels around the reservoir could 

provide useful information about the ground-water system surrounding Converse Lake. In addition, the firm-yield value may 

change substantially if, in future studies, up-to-date reservoir 

bathymetric data are used, as well as more accurate aquiferproperty values of transmissivity and storage estimated from 

field-measurement techniques rather than from the limited data 

available for this study, or if a detailed numerical ground-water 

flow model of the Converse Lake watershed is developed.

References Cited

Archfield, S.A., and Carlson, C.S., 2006, Ground-water contributions to reservoir storage and the effect on estimates of 

firm yield for reservoirs in Massachusetts: U.S. Geological 

Survey Scientific Investigations Report 2006–5045, 27 p.

Farnsworth, R.K., and Thompson, E.S., 1982, Mean monthly, 

seasonal, and annual pan evaporation for the United States: 

Washington, D.C., National Oceanic and Atmospheric 

Administration Technical Report NWS 34, 82 p. 

Farnsworth, R.K., Thompson, E.S., and Peck, E.L., 1982, 

Evaporation atlas for the contiguous 48 United States: 

Washington, D.C., National Oceanic and Atmospheric 

Administration Technical Report NWS 33.

Gandl, L.A., 1979, The Oligocene aquifer system in Mississippi: U.S. Geological Survey Water-Resources Investigations Report 79–28, 1 pl.

Gandl, L.A., 1982, Characterization of aquifers designated as 

potential drinking-water sources in Mississippi: U.S. Geological Survey Open-File Report 81–550, 30 p.

Grubb, H.F., 1998, Summary of hydrology of the regional 

aquifer systems, South-Central United States: U.S. Geological Survey Professional Paper 1416–A, 61 p.

Heath, R.C., 1982, Basic ground-water hydrology: 

U.S. Geological Survey Water-Supply Paper 2220, 86 p.

Hirsch, R.M., 1982, A comparison of four streamflow record 

extension techniques: Water Resources Research, vol. 18, 

no. 4, p. 1081–1088.

Journey, C.A., and Gill, A.C., 2001, Assessment of waterquality conditions in the J.B. Converse Lake watershed, 

Mobile County, Alabama, 1990–1998: U.S. Geological Survey Water-Resources Investigations Report 01–4225, 131 p.

Juracek, K.E., 2006, Sedimentation in Kansas reservoirs 

[abs.], in Program of 23rd Annual Water and the Future 

of Kansas Conference, March 16, 2006, Topeka, Kansas: 

Manhattan, Kansas, Kansas State Research and Extention 

[unnumbered page].

Massachusetts Department of Environmental Protection, 1996, 

Estimating the firm yield of a surface-water reservoir-supply 

system in Massachusetts—A guidance document, Version 

1.0: Massachusetts Department of Environmental Protection, Office of Watershed Management [variously paged].

National Oceanic and Atmospheric Administration, 2007, 

Record of climatological observations for Mobile Regional 

Airport station 015478: National Climatic Data Center; 

accessed online April 19, 2007, at http://cdo.ncdc.noaa.gov/

dly/DLY

Prudic, D.E., 1991, Estimates of hydraulic conductivity from 

aquifer-test analyses and specific-capacity data, Gulf Coast 

Regional Aquifer Systems, south-central United States: 

U.S. Geological Survey Water-Resources Investigations 

Report 90–4121, 38 p.

Renken, R.A., 1996, Hydrogeology of the southeastern 

Coastal Plain aquifer system in Mississippi, Alabama, 

Georgia, and South Carolina: U.S. Geological Survey 

Professional Paper 1410–B, 101 p.

Ries, K.G., III, and Friesz, P.J., 2000, Methods for estimating low-flow statistics for Massachusetts streams: 

U.S. Geological Survey Water-Resources Investigations 

Report 00–4135, 81 p.

Stricklin, V.E., 2001, Sedimentation profiles in Lake Tuscaloosa, Alabama, 2000: U.S. Geological Survey Open-File 

Report 01–317, 19 p.

Waldron, M.C., and Archfield, S.A., 2006, Factors affecting firm yield and the estimation of firm yield for selected 

streamflow-dominated drinking-water supply reservoirs 

in Massachusetts: U.S. Geological Survey Scientific 

Investigations Report 2006–5044, 39 p.

Prepared by:

USGS Publishing Network

Raleigh Publishing Service Center

3916 Sunset Ridge Road

Raleigh, NC 27607

For additional information regarding this publication, contact:

Carl S. Carlson, Hydrologist

USGS Massachusetts–Rhode Island Water Science Center

10 Bearfoot Road

Northborough, MA 01532

phone: 508–490–5047

email: cscarlso@usgs.gov

Or visit the USGS Alabama Water Science Center website at:

http://al.water.usgs.gov/
Carlson and Archfield—Hydrogeologic Conditions for J.B. Converse Lake, Mobile County, Alabama, 1991–2006—Scientific Investigations Report 2008–5005, Second Edition—February 2009

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