Water-Resources Investigations Report 02-4295

Prepared in cooperation with

SOUTHWEST FLORIDA WATER MANAGEMENT DISTRICT

U.S. Department of the Interior

U.S. Geological Survey

Effects of Recharge, Upper Floridan Aquifer Heads,

and Time Scale on Simulated Ground-Water Exchange

with Lake Starr, a Seepage Lake in Central Florida
Effects of Recharge, Upper Floridan 

Aquifer Heads, and Time Scale on Simulated 

Ground-Water Exchange with Lake Starr, a 

Seepage Lake in Central Florida

By Amy Swancar and Terrie M. Lee

U.S. Geological Survey

Water-Resources Investigations Report 02-4295

Prepared in cooperation with the

SOUTHWEST FLORIDA WATER MANAGEMENT DISTRICT

Tallahassee, Florida

2003
U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

Use of trade, product, or firm names in this publication is for descriptive purposes only 

and does not imply endorsement by the U.S. Geological Survey

For additional information 

write to: 

U.S. Geological Survey

2010 Levy Avenue

Tallahassee, FL 32310

Additional information about water resources in Florida is available on the internet 

at http://fl.water.usgs.gov

Copies of this report can be 

purchased from:

U.S. Geological Survey

Branch of Information Services

Box 25286

Denver, CO 80225-0286

888-ASK-USGS
Contents III

CONTENTS

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

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

Purpose and Scope ........................................................................................................................................................... 3

Acknowledgments............................................................................................................................................................ 3

Background................................................................................................................................................................................ 3

Hydrogeologic Setting ..................................................................................................................................................... 5

Water Budget Approach................................................................................................................................................... 9

Description of the Ground-Water Model ................................................................................................................................. 11

Hydrogeologic Units in the Model................................................................................................................................. 11

Surficial Aquifer System...................................................................................................................................... 11

Intermediate Confining Unit ................................................................................................................................ 14

Upper Floridan Aquifer........................................................................................................................................ 14

Breaches in the Intermediate Confining Unit....................................................................................................... 14

Method Used to Simulate Lake Starr............................................................................................................................. 16

Boundary Conditions for Steady-State Simulations ...................................................................................................... 16

Lateral Boundaries ............................................................................................................................................... 16

Recharge............................................................................................................................................................... 18

Upper Floridan Aquifer........................................................................................................................................ 18

Boundary Conditions for Transient Simulations............................................................................................................ 18

Recharge............................................................................................................................................................... 20

Upper Floridan Aquifer........................................................................................................................................ 22

Results of Steady-State Simulations........................................................................................................................................ 23

Results of Transient Simulations ............................................................................................................................................. 29

Monthly Simulations...................................................................................................................................................... 31

Weekly Simulations ....................................................................................................................................................... 35

Higher Recharge Weekly Simulation............................................................................................................................. 38

Combined Approach for Estimating Ground-Water Inflow........................................................................................... 40

Effects of Boundary Conditions and Time Scale on Ground-Water Exchange ....................................................................... 46

Recharge Boundary........................................................................................................................................................ 46

Upper Floridan Aquifer Boundary................................................................................................................................. 48

Time Scale...................................................................................................................................................................... 48

Summary and Conclusions ...................................................................................................................................................... 49

References................................................................................................................................................................................ 52

FIGURES

1. Map showing topography and outline of model area of the Lake Starr basin............................................................... 4

2. Section showing vertical head distribution along section A-A′ for representative low water-level condition, 

May 29, 1997................................................................................................................................................................. 6

3. Maps showing water-table configuration and potentiometric surface of the Upper Floridan aquifer in the 

Lake Starr basin for representative low water-level condition, May 29, 1997.............................................................. 7

4-5. Graphs showing:

4. Hydrographs of selected Upper Floridan aquifer wells and Lake Starr stage........................................................ 8

5. Monthly water-budget terms for Lake Starr, August 1996 through July 1998..................................................... 10

6. Map showing model grid and location of monitor wells............................................................................................. 12

7. Section showing vertical discretization and calibrated values of hydraulic conductivity for steady-state 

simulation along row 55 .............................................................................................................................................. 13
IV Contents

8. Map showing spatial distribution of hydraulic conductivity of the intermediate confining unit for the 

steady-state model........................................................................................................................................................15

9. Section showing model row 64 through 2PNS piezometer nest wells showing hydraulic conductivity zones 

used in steady-state simulation ....................................................................................................................................17

10-11. Maps showing:

10. Upper Floridan aquifer heads specified in steady-state simulation ......................................................................19

11. Simulated recharge zones depicting estimated time delay between rainfall and recharge at the water table.......21

12. Graph showing monthly recharge rates using three recharge methods, August 1996 through July 1998...................23

13. Map showing specified head cells for transient simulations in layer 16 of the Lake Starr basin model .....................24

14. Graph showing estimated daily heads in the Upper Floridan aquifer on the north and south sides of Lake Starr 

and weekly and monthly variation in specified head used in the ground-water flow model, August 1996 through 

July 1998......................................................................................................................................................................25

15. Map showing steady-state simulated water-table altitudes, water-table and water-level residuals, and area that 

contributes ground water to the lake, median September 1996-February 1998 conditions.........................................26

16. Graph showing comparison of simulated heads to median observed heads in the Lake Starr basin for the 

calibrated steady-state model, median September 1996-February 1998 .....................................................................27

17. Section showing particle pathlines showing direction, rates of movement, and vertical area that contributes 

ground water to the lake along model row 55..............................................................................................................30

18-28. Graphs showing:

18. Effect of three different recharge boundaries on simulated head in well WTS-1.................................................31

19. Effect of three different recharge boundaries on simulated monthly net ground-water flow to Lake Starr, 

and net ground-water flow from the water budget................................................................................................32

20. Observed and monthly simulated water levels in wells WTS-23 and WTS-24, WTS-4 and WTS-5, WTS-13 

and WTS-12, and WTS-8 and WTS-9..................................................................................................................33

21. Measured and monthly simulated stage for Lake Starr, August 1996 through July 1998 ....................................34

22. Effect of different Upper Floridan aquifer specified-head boundary conditions on simulated monthly 

net ground-water flow to Lake Starr, and net ground-water flow from the water budget ....................................34

23. Weekly observed and simulated Lake Starr stage and simulated head in well WTS-13 showing flow reversals, 

August 1996 through July 1998............................................................................................................................36

24. Weekly simulated net ground-water flow to Lake Starr, net ground-water flow from the higher recharge 

simulation, and net ground-water flow from the water budget from July 28, 1996, to August 17, 1997, and 

August 24, 1997, to July 26, 1998 ........................................................................................................................37

25. Cumulative absolute error in net ground-water flow for two simulations and the weekly lake water budget......39

26. Weekly simulated, observed, and adjusted stage of Lake Starr, and weekly simulated and observed head in 

well WTS-13.........................................................................................................................................................41

27. Weekly calculated ground-water inflow to and simulated leakage from Lake Starr during the 2-year 

study period ..........................................................................................................................................................42

28. Relation between simulated leakage from Lake Starr and monthly average head difference between 

Lake Starr and ROMP 57 Upper Floridan aquifer well, August 1996 through July 1998 ...................................45

TABLES

1. Calibrated values of hydraulic parameters used in the steady-state model .........................................................................14

2. Depth to water, land-surface altitude, and assumed time delay for recharge to reach the water table................................20

3. Steady-state fluxes in the Lake Starr model........................................................................................................................27

4. Sensitivity of steady-state model results to changes in recharge and hydraulic conductivity parameters ..........................28

5. Weekly inflows to and outflows from Lake Starr, with ground-water inflows calculated using a combined approach, 

July 20, 1996, through August 1, 1998 ...............................................................................................................................43

6. Maximum daily, weekly, and monthly average recharge rates simulated by the LEACHM model....................................46
Contents V

CONVERSION FACTORS, DATUMS, AND ABBREVIATIONS

ACRONYMS AND ABBREVIATIONS

Vertical coordinate information is referenced to the National Geodetic Vertical 

Datum of 1929 (NGVD of 1929); horizontal coordinate information is referenced 

to the North American Datum of 1927 (NAD 27).

Multiply By To obtain

inch (in.) 25.4 millimeter (mm)

inch per year (in/yr) 25.4 millimeter per year (mm/yr)

foot (ft) 0.3048 meter (m)

square foot (ft2

) 0.09290 square meter (m2)

acre 0.4047 hectare (ha)

square mile (mi2

) 2.59 square kilometer (km2)

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

cubic foot per day (ft3

/d) 0.028317 cubic meter per day (m3/d)

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

/d)

RMSE root mean squared error

K hydraulic conductivity

Kv vertical hydraulic conductivity

Kh horizontal hydraulic conductivity

r

2 coefficient of determination

USGS U.S. Geological Survey
VI Contents
Abstract 1

Effects of Recharge, Upper Floridan Aquifer Heads, and 

Time Scale on Simulated Ground-Water Exchange with 

Lake Starr, a Seepage Lake in Central Florida

By Amy Swancar and Terrie M. Lee

ABSTRACT

Lake Starr and other lakes in the mantled

karst terrain of Florida’s Central Lake District are

surrounded by a conductive surficial aquifer

system that receives highly variable recharge from

rainfall. In addition, downward leakage from these

lakes varies as heads in the underlying Upper Floridan aquifer change seasonally and with pumpage.

A saturated three-dimensional finite-difference

ground-water flow model was used to simulate the

effects of recharge, Upper Floridan aquifer heads,

and model time scale on ground-water exchange

with Lake Starr. The lake was simulated as an

active part of the model using high hydraulic

conductivity cells. Simulated ground-water flow

was compared to net ground-water flow estimated

from a rigorously derived water budget for the

2-year period August 1996-July 1998.

Calibrating saturated ground-water flow

models with monthly stress periods to a monthly

lake water budget will result in underpredicting

gross inflow to, and leakage from, ridge lakes in

Florida. Underprediction of ground-water inflow

occurs because recharge stresses and ground-water

flow responses during rainy periods are averaged

over too long a time period using monthly stress

periods. When inflow is underestimated during

calibration, leakage also is underestimated because

inflow and leakage are correlated if lake stage is

maintained over the long term. Underpredicted

leakage reduces the implied effect of ground-water

withdrawals from the Upper Floridan aquifer on

the lake. 

Calibrating the weekly simulation required

accounting for transient responses in the water

table near the lake that generated the greater range

of net ground-water flow values seen in the weekly

water budget. Calibrating to the weekly lake water

budget also required increasing the value of annual

recharge in the nearshore region well above the

initial estimate of 35 percent of the rainfall, and

increasing the hydraulic conductivity of the deposits

around and beneath the lake. 

To simulate the total ground-water inflow to

lakes, saturated-flow models of lake basins need to

account for the potential effects of rapid and efficient recharge in the surficial aquifer system closest to the lake. In this part of the basin, the ability

to accurately estimate recharge is crucial because

the water table is shallowest and the response time

between rainfall and recharge is shortest. Use of the

one-dimensional LEACHM model to simulate the

effects of the unsaturated zone on the timing and

magnitude of recharge in the nearshore improved

the simulation of peak values of ground-water

inflow to Lake Starr. Results of weekly simulations

suggest that weekly recharge can approach the

majority of weekly rainfall on the nearshore part

of the lake basin. However, even though a weekly
2 Effects of Recharge, Upper Floridan Aquifer Heads, and Time Scale on Simulated Ground-Water Exchange with Lake Starr, a 

Seepage Lake in Central Florida

simulation with higher recharge in the nearshore

was able to reproduce the extremes of groundwater exchange with the lake more accurately, it

was not consistently better at predicting net

ground-water flow within the water budget error

than a simulation with lower recharge. The more

subtle effects of rainfall and recharge on groundwater inflow to the lake were more difficult to

simulate. The use of variably saturated flow modeling, with time scales that are shorter than weekly

and finer spatial discretization, is probably necessary to understand these processes. The basin-wide

model of Lake Starr had difficulty simulating the

full spectrum of ground-water inflows observed in

the water budget because of insufficient information about recharge to ground water, and because of

practical limits on spatial and temporal discretization in a model at this scale. 

In contrast, the saturated flow model

appeared to successfully simulate the effects of

heads in the Upper Floridan aquifer on water levels

and ground-water exchange with the lake at both

weekly and monthly stress periods. Most of the

variability in lake leakage can be explained by the

average vertical head difference between the lake

and a representative Upper Floridan aquifer well.

Simulated lake leakage was correlated (r2 = 0.86)

to the average head difference between the lake and

a representative Upper Floridan aquifer well,

suggesting that leakage was largely a linear function of this head difference. 

The best estimate of weekly or monthly

ground-water inflow can be derived using the water

budget and simulated lake leakage. Ground-water

inflow is calculated as a new residual to the water

budget for Lake Starr after substituting the simulated values of lake leakage into the equation.

Using this combined approach to calculate the

ground-water inflow to Lake Starr gave gross

inflow estimates of 15,689,000 and 20,778,000

cubic feet for the first and second years of the

study, respectively (33 and 42 inches per year,

respectively). Ground-water inflow was about

42 percent of the total inflow to the lake for the

2-year time period. Lake leakage was estimated

to be 14,389,000 and 12,115,000 cubic feet per

year for the first and second years, respectively

(29 and 24 inches per year, respectively), and was

31 percent of the total outflow during the 2 years. 

The linear regression between simulated lake

leakage and average head difference between the

lake and an index well provides a method for

estimating leakage from Lake Starr beyond the

modeled time period. Leakage estimates from the

relation with head difference could be used along

with a lake water budget to estimate ground-water

inflow over any period that water budget and average

head measurements in the index well are available.

INTRODUCTION

Ground-water inflow and lake leakage can be

important components of lake water budgets (Stauffer,

1985; Crowe, 1989; Grubbs, 1995; LaBaugh and others,

1995; Sacks and others, 1998). Lake basins in the karst

terrain of central Florida are subject to highly variable

rainfall, both annually and seasonally, that affects the

interaction between lakes and ground water. Groundwater withdrawals from the deeper Upper Floridan aquifer system, which also are time-variable, affect groundwater interactions with many of these lakes. Understanding the effects of changing rainfall patterns and groundwater withdrawals on lake water budgets is a fundamental requirement for water managers who must safeguard

lake water levels and set limits on ground-water withdrawals from the underlying aquifers (Barcelo and

others, 1990; Southwest Florida Water Management

District, 1994). 

Combining ground-water flow modeling of lake

basins with detailed lake water budgets is a balanced

approach to understanding the effects of hydrogeologic

setting, climate and ground-water pumping on groundwater exchange (Lee, 1996; Lee and Swancar, 1997;

Choi and Harvey, 2000; Swancar and others, 2000;

Merritt, 2001). However, differences in the magnitude

of ground-water exchange estimated by modeling and

water budgets reveal ground-water flow processes

inadequately accounted for by the models. To date, the

U.S. Geological Survey (USGS) has used this

combined approach to investigate five lakes in mantled

karst settings in Florida (Grubbs, 1995; Lee, 1996, Lee

and Swancar, 1997; Merritt, 2001). At the two seepage

lake basins modeled in greatest detail, three-dimensional transient saturated ground-water flow models

were used to simulate ground-water fluxes to and from

the lakes. These fluxes were then compared with

detailed monthly lake water budgets. The models

underestimated ground-water inflow to the lakes for

months with large rainfall totals when compared with

the lake water budget (Grubbs, 1995; Lee, 1996).
Background 3

During the driest periods, the models underestimated

lake leakage. These models probably underestimated

ground-water inflow in part because they failed to

simulate the ground-water inflow generated by shortlived water-table mounds near the lakeshore. Rapid

water table mounding has been simulated using a more

detailed variably saturated ground-water flow model of

a small portion of the lake basin, and daily boundary

conditions (Lee, 2000). However, this process has not

been evident in saturated flow models calibrated to

monthly lake water budgets.

The weekly water budget of Lake Starr provides

evidence that short-lived, water-table mounding affects

ground-water inflow to the lake (Swancar and others,

2000). Water-table mounding was documented within

150 feet (ft) of the water’s edge of Lake Starr during

weeks with large ground-water inflows. The water

budget of Lake Starr also suggests that ground-water

pumping increased the lake leakage by lowering the

potentiometric surface of the Upper Floridan aquifer

for periods as short as weekly. If processes that occur

within a week significantly affect lake water budgets,

then calibrating flow models to weekly fluxes should

clarify relevant processes for lakes in mantled karst

terrain of central Florida. Improving our ability to

simulate short-term ground-water exchanges with Florida lakes also should improve our estimates of annual

ground-water exchanges. 

Purpose and Scope

The purpose of this study is to (1) quantify

ground-water inflow and lake leakage using a threedimensional, saturated ground-water flow model of the

Lake Starr basin, and (2) to investigate how representing the time-varying boundary conditions (recharge to

the surficial aquifer system and the potentiometric

surface of the Upper Floridan aquifer) affect simulated

ground-water fluxes. Ground-water inflow to the lake

and lake water leaking into the surficial aquifer system

(hereafter referred to as lake leakage) were simulated

over a 2-year period from August 1996 through July

1998. The two boundary conditions were represented

spatially to different levels of detail, and were represented in transient simulations using both monthly and

weekly averaged data. 

Simulated values of net ground-water flow were

compared with the more accurate net ground-water

flow calculated from a rigorously derived lake water

budget. The difference between the gross or total

ground-water inflow and the gross lake leakage for any

given time period is the net ground-water flow. The

agreement between the two estimates of net groundwater flow indicated whether or not the representation

of boundary conditions improved the simulation. 

The model described in this report is based on

a study of Lake Starr by Swancar and others (2000).

The basin hydrogeology, weekly and monthly lake

water budgets, and the occurrence of transient watertable mounds near the edge of the lake are described in

that report. This report as well as the initial report by

Swancar and others (2000) are extensions of a study by

Sacks and others (1998), who used lake water budget

and chemical and isotopic mass balance approaches to

estimate ground-water exchange at Lake Starr and nine

other central Florida lakes. All of these studies were

funded cooperatively by the USGS and the Southwest

Florida Water Management District.

Acknowledgments

The assistance of the property owners around

Lake Starr in the monitoring efforts associated with this

study was greatly appreciated. Thanks to Keith Halford

for constructing a parameter estimation ground-water

flow model on a hypothetical lake/ground-water

system, and for helpful discussions. Thanks to Dick

Yager for encouragement and assistance in using the

one-dimensional unsaturated flow model to estimate

recharge. 

BACKGROUND

Lake Starr is representative of the many lakes in

Florida’s Central Lake District, an upland area of welldrained sand ridges (Brooks, 1981). It is a 135-acre

lake (at a stage of 105 ft above NGVD of 1929) in a

topographic basin with no natural surface drainage

(fig. 1). Land and water-use in the Lake Starr basin also

are representative of the region. Homes on relatively

large (1- to 4-acre) lots surround the lake. Commercial

citrus groves are located in higher parts of the basin. A

few lakefront residents pump water directly from the

lake for irrigation of landscaping or citrus trees, but this

component of the lake water budget is estimated to be

relatively small (less than 3 inches per year (in/yr),

Swancar and others, 2000). Soils in the basin are

mostly excessively drained fine-to-medium silica sands

of the Candler series (Soil Conservation Service, 1990).
4 Effects of Recharge, Upper Floridan Aquifer Heads, and Time Scale on Simulated Ground-Water Exchange with Lake Starr, a 

Seepage Lake in Central Florida

81°36′15′′ 81°35′ 81°34′15′′ 81°36′ 45′′ 45′′ 30′′ 30′′ 15′′

27°58′

27°56′45′′

27°57′

45′′

30′′

15′′

..

.. .. ..

.. ..

.. ..

.. .. ..

..

..

..

..

..

.. .. .. ..

..

..

.. ..

..

..

..

..

.. ..

..

Base from U.S. Geological Survey

Lake Wales Quadrangle, 1:24,000, 1987

Universal Transverse Mercator projection, Zone 17

0

0

750 1,500 FEET

200 400 METERS

.. TOPOGRAPHIC BASIN BOUNDARY

EXPLANATION

OPEN WATER

OUTLINE OF MODEL AREA

210

LINE OF EQUAL LAND SURFACE ELEVATION--

 In feet above NGVD of 1929. Contour interval 10 feet.

 Hachures indicate depression

LOCATION OF HYDROGEOLOGIC SECTION SHOWN

 IN FIGURE 2

OLD RAILROAD GRADE

A

A'

A'

A

MASTERPIECE ROAD

STARR 

TOWER ROAD

ALT. 27

STARR AVENUE

LAKE MABEL LOOP ROAD

SOUTH LAKE

BOULEVARD

Study area

located within 

Polk County

Unnamed Lake

Lake 

Suzanne

Dinner

Lake

Lake Mabel

130

160

140

140

130

150

130

150

170

170

130

150

170

160

170

190

190

210

220

220

210

220

210

210

170

150

130

190

190

190

200

200

140

190

170

160

190

230

130

130

Lake Starr

150

110

130 120 160

WTS-4

WTS-5

1PNS-125

2PNS-156

WTS-26

Nelson

Close

WTS-19

Nelson WELL AND NAME USED IN SECTION

Hart FLRD

Figure 1. Topography and outline of model area of the Lake Starr basin.
Background 5

Near the shoreline, the soils (Myakka series) are

typically more organic and poorly drained because of

shallow depths to the water table. Topography is

controlled by sinkhole formation in this mantled karst

terrain. Up to 200 ft of surficial sands and clays overlie

a thick carbonate sequence that begins at about NGVD

of 1929. The numerous lakes and sinkholes in the

region result from collapse or subsidence of overlying

sediments into voids formed by dissolution of the

underlying limestone (Sinclair and others, 1985;

Tihansky, 1999). 

Hydrogeologic Setting

The two aquifers of interest in the study of the

lake are the surficial aquifer system and the underlying

Upper Floridan aquifer (fig. 2). The surficial aquifer

system (Southeastern Geological Society, 1986)

surrounds the lake and is comprised of unconsolidated

sand and clay sediments. The water table in the surficial aquifer system is close to land surface near the

lakeshore but is over 120 ft below land surface beneath

the highest parts of the basin. A thin and discontinuous

layer of clay, called the intermediate confining unit,

underlies the surficial aquifer system and the lake. This

unit separates the surficial aquifer system from the

confined carbonate Upper Floridan aquifer, and ranges

in thickness from 20 to 40 ft. Although breaches in the

intermediate confining unit occur throughout the basin,

they are most common on the southeast side of the

lake. Breaches in the intermediate confining unit are

formed by sinkholes that allow clays of the intermediate confining unit and sandy surficial deposits to be

transported downward along columns or “pipes” into

solution cavities in the underlying limestone aquifer.

Two breaches exist under the deeper parts of the lake

(Swancar and others, 2000). 

The Central Lake District is a recharge area for

the Upper Floridan aquifer, and water in the Lake Starr

basin flows downward from the lake and the surficial

aquifer system to the underlying Upper Floridan aquifer (Swancar and others, 2000). Recharge to the Upper

Floridan aquifer is enhanced by breaches in the intermediate confining unit. The top of the Upper Floridan

aquifer is about 30 ft above NGVD of 1929 on the west

side of the basin and slopes downward to about 20 ft

below NGVD of 1929 on the east side. Superimposed

on this sloping limestone surface are depressions due to

irregular dissolution and collapse. 

Lake Starr is a flow-through lake with respect to

the surficial aquifer system. Ground water flows into

the lake on the northwest side, whereas water flows out

of the lake into the surficial aquifer system on the

southeast side of the lake (fig. 3A). In some lakes, the

proportion of the lake’s perimeter that receives lateral

ground-water inflow from its basin varies seasonally

(Sacks and others, 1998; Metz and Sacks, 2002). At

Lake Starr, however, the lakeshore areas experiencing

ground-water inflow and outflow did not vary substantially during 3 years of data collection, except during

periods during and immediately after large rain events.

Contrary to the topography, ground water flows toward

the highest hillside in the basin on the southeast side of

the lake, which is also the direction of several conspicuous sinkholes (figs. 1 and 2). 

Short-lived water-table mounding was observed

in continuously monitored wells near the inflow and

outflow sides of Lake Starr following rainstorms

(Swancar and others, 2000). Mounds on the outflow

side of the lake briefly reversed the flow direction,

temporarily reducing lateral leakage and generating

ground-water inflow. For example, during a particularly wet week (July 27-August 2, 1997), ground-water

inflow to the lake from transient mounding could have

generated one-half the volume of positive net groundwater flow calculated from the lake water budget for

that week (Swancar and others, 2000). Flow reversals

typically lasted less than a day. The duration and size of

transient water-table mounds are related to the volume

of rain, depth to water, topography, and soil properties

and their degree of saturation.

The potentiometric surface of the Upper Floridan

aquifer (a surface analogous to the water table, but for a

confined aquifer) slopes downward from northwest to

southeast across the lake basin, making the groundwater flow direction in this aquifer similar to that in the

surficial aquifer system (fig. 3b). Heads in the Upper

Floridan aquifer rose and declined more steeply than

heads in the surficial aquifer system, and were strongly

affected by local and regional irrigation pumping for

citrus cultivation (fig. 4). The potentiometric surface of

the Upper Floridan aquifer was highest in March 1998,

following an unusually wet winter. On the west and

northwest sides of the lake basin, heads in the Upper

Floridan aquifer were very close to or higher than lake

stage from December 1997 through March 1998. As a

result, the potential was low for downward flow of

water on the northwest side of the basin during this

period. On the southeast side of the lake, heads in the
6 Effects of Recharge, Upper Floridan Aquifer Heads, and Time Scale on Simulated Ground-Water Exchange with Lake Starr, a 

Seepage Lake in Central Florida

93.15

Lake Starr

108

107

106

105

104

103

102

101

99

97

95

99

97

101

VERTICAL SCALE GREATLY EXAGGERATED

EXPLANATION

0

0 100 200 300 METERS

500 1,000 FEET

108.24

104.86

100.94

104.41

103.98

104.40

104.38

104.37

104.41

103.96

103.59

103.83

103.81

102.08

103.01

101.04(e)

water

table

LINE OF EQUAL HEAD-- In feet above NGVD of 1929. 

 Contour interval 1 and 2 feet. Dashed where approximate

HEAD-- In feet above NGVD of 1929

100

101.90

FEET

250

200

150

100

50

NGVD

of 1929

 50

 100

 150

A A' Close Well WTS-4 Well WTS-5 Well 1PNS Wells 2PNS Wells Hart FLRD Well WTS-19 Well WTS-26 Well Nelson Well

INFERRED GEOLOGIC CONTACT

INTERMEDIATE CONFINING UNIT

North South

GROUND-WATER FLOW DIRECTION

95.30

SURFICIAL

AQUIFER

SYSTEM

UPPER

FLORIDAN

AQUIFER

Figure 2. Vertical head distribution along section A-A′ for representative low water-level condition, May 29, 1997, 

(line of section shown in figure 1).
Background 7

Lake Starr

81°35′45′′ 81°34′45′′ 30′′ 15′′ 81°35′

81°35′45′′ 81°34′45′′ 30′′ 15′′ 81°35′

27°57′45′′

27°57′

30′′

15′′

27°57′45′′

27°57′

30′′

15′′

(A)

(B)

Lake Starr

100

100.45

EXPLANATION

OBSERVATION WELL-- Number is altitude of water level in well in feet above NGVD of 1929, (e) estimated value

POTENTIOMETRIC CONTOUR-- Showing altitude at which water would have stood in tightly 

 cased wells. Contour interval 2 feet. Dashed where approximate. Datum is NGVD of 1929

108

107

106

105

104

103

102

108.24

104.98

106.39

105.05

104.63

104.86

104.41

104.10

106.85

104.17

104.16

104.02

104.65

104.08

103.95

103.37

103.85

103.59 102.38

101.74

102.21

101.04 (e)

102.64

102.08

103.58

103.83

103.81

103.81

103.98

105.78

108 LINE OF EQUAL WATER-TABLE ALTITUDE-- In feet above NGVD of 1929. Contour interval 1 foot. 

 Dashed where approximate

GROUND-WATER FLOW DIRECTION

94

98

96

100

100.45 (e)

98.94

100.06 (e) 93.51 (e)

100.94

100.44

95.30

93.15

92.84

0

0

1,500 FEET

200 400 METERS

Base from U.S. Geological Survey 750

Lake Wales Quadrangle, 1:24,000, 1987

Universal Transverse Mercator projection, Zone 17

Figure 3. Maps showing (A) water table configuration and (B) potentiometric surface of the Upper Floridan aquifer 

in the Lake Starr basin for representative low water-level condition, May 29, 1997.
8 Effects of Recharge, Upper Floridan Aquifer Heads, and Time Scale on Simulated Ground-Water Exchange with Lake Starr, a 

Seepage Lake in Central Florida WATER- LEVEL ALTITUDE, IN FEET ABOVE NGVD OF 1929

1PNS-125 

LAKE STARR

HART FLRD

1996 1997 1998

9/2

10/2

11/1

12/1

12/31

1/30

3/1

3/314/305/306/297/298/289/27

10/27

11/26

12/26

1/252/24

3/264/255/256/247/24

108

107

106

105

104

103

102

101

100

99

98

97

96

95

94

93

92

Upper Floridan aquifer were always at least 5 ft lower

than the lake stage. The potentiometric surface of the

Upper Floridan aquifer was lowest in the spring and

early summer months (April, May, and June) of 1997

and 1998, when it was from 3 to 10 ft lower than the

lake stage from the west to the east side of the basin,

respectively. 

Heads in the Upper Floridan aquifer responded

differently to ground-water pumpage in different parts

of the basin (fig. 4). Heads measured in wells on the

northwest side of the lake showed steep rises and drops

in response to pumping, whereas heads measured in

wells on the southeast side of the basin showed a more

moderated response. Measured heads in the Upper

Floridan aquifer well 1PNS-125 on the northwest side

of the basin ranged from 97.13 to 106.50 ft above

NGVD of 1929, whereas heads in the Hart FLRD well

on the southeast side ranged from 93.58 to 99.22 ft

above NGVD of 1929. The difference in response is

partly due to the differences in the degree of confinement of the Upper Floridan aquifer between the northwest and southeast sides of the basin. Many sinkholes

on the southeast side of the lake, and particularly near

the Hart FLRD well (figs. 1 and 2), indicate breaches in

the intermediate confining unit that reduce confinement (Swancar and others, 2000).

On the south side of the lake, a nest of wells was

drilled in an area where collapse of overlying units into

the Upper Floridan aquifer had occurred. This nest of

wells provided a unique opportunity to study the

hydrologic character of a collapse feature. The deepest

well at the 2PNS site (2PNS-156 for 156 ft below land

surface), found sand at a depth of 46 ft below NGVD of

1929, or tens of feet below the top of the Upper Floridan aquifer in other areas of the basin (fig. 2). Sand

instead of the limestone of the Upper Floridan aquifer

at this altitude indicated collapse. The downward head

difference between the lake and this well was statistically correlated to the head in a nearby Upper Floridan

aquifer well (Hart FLRD well, see figs. 1 and 2)

(r2 (coefficient of determination) = 0.80, standard error

= 0.073 ft), indicating a connection between the sinkhole and the aquifer. The downward head difference

increased by about 0.1 ft for every 1 ft decrease in the

Figure 4. Hydrographs of selected Upper Floridan aquifer wells and Lake Starr stage 

(well locations shown on figure 1).
Background 9

Upper Floridan aquifer head. Heads in well 2PNS-156

were consistently about 6 ft higher than heads in the

Hart FLRD well, which is about 300 ft to the south of

the 2PNS well nest. This substantial head difference

indicates that even though a connection exists between

the sinkhole and the aquifer, some degree of confinement is still in place between these two points. 

Nearly all water for residential and agricultural

use in the basin comes from wells drilled into the

Upper Floridan aquifer. Irrigation of citrus groves is

the largest water use. In the 6 square miles (mi2)

surrounding Lake Starr, 14 large-capacity, deep wells

were permitted to pump ground water from the Upper

Floridan aquifer at average rates from 100,000 to

284,600 gallons per day (gal/d) per well (J. Whalen,

Southwest Florida Water Management District, written

commun., 1999). Thirty-six permits for ground-water

withdrawals averaging less than 100,000 gal/d also

existed in this area in 1999. Pumping causes head

declines of up to 15 ft in the Upper Floridan aquifer in

the lake basin for short periods, and the region has been

designated a Water-Use Caution Area by the Southwest

Florida Water Management District because of

concerns about the effects of ground water-use on both

surface- and ground-water levels (Yobbi, 1996).

Water Budget Approach

Ground-water exchanges with Lake Starr

computed using the lake water budget provide the basis

for evaluating the accuracy of simulated ground-water

flow to and from the lake. When a water budget

approach is applied in detail, the errors in estimating

net ground-water flow to the lake can be substantially

less than those inherent in modeling approaches

(Lee and Swancar, 1997). A detailed water budget was

computed for Lake Starr for a 2-year period (July 20,

1996-August 1, 1998). Results are presented in Swancar

and others (2000), and a brief summary of the water

budget approach is presented below. 

For a seepage lake with no surface-water inflows

or outflows, net ground-water flow (the difference

between ground-water inflow and outflow) can be

calculated as a residual to the lake water budget using

the equation: 

Net GW = Gi – Go = 

∆S – P + E + Q ± e∆S ±eP ± eE ± eQ (1)

where Net GW is the net ground-water flow to the lake

during a given time period, ∆S is the change in lake

volume for that time period, P is precipitation, E is

evaporation, Gi is ground-water inflow, Go is groundwater outflow or lake leakage, Q is irrigation withdrawal directly from the lake, and other terms are errors

associated with each component. All of the terms are

expressed in units of volume or as depths over the average lake surface area for the given period. Irrigation

directly from the lake (Q) is a small loss term in the lake

water budget for Lake Starr that can be of minor importance in its seasonal or annual water budgets. This term

is not relevant for some lakes.

The net ground-water flow term is positive for

periods when ground-water inflow exceeds outflow,

and negative when outflow exceeds inflow. Positive

values of net ground-water flow are sometimes referred

to as net ground-water inflow, and negative values as

net ground-water outflow. In this study, the net groundwater flow term derived from the water budget was

used to track changes in the gross ground-water fluxes

to the lake. For example, if net ground-water flow was

negative in one week, and large and positive in the

following week, the implication was that the lake

received a large amount of ground-water inflow during

the second week. Lake leakage also may have been

reduced, but generally this change is due to a large

increase in ground-water inflow associated with a

recharge event. 

As shown in equation 1, the Net GW term

includes the errors in the other water budget components. For some time periods, this error can approach

the size of the net ground-water term, making the interpretation difficult. In general, errors expressed as a

percentage increase for shorter time periods and

decrease for longer periods. Errors in monthly water

budget terms were assumed to be 5 percent for rainfall,

15 percent for evaporation, 5 percent for change in

stage, and 100 percent for direct pumping from the

lake. For weekly water budgets, rainfall error was

assumed to be 10 percent, and for annual water

budgets, evaporation error was assumed to be 10

percent (Swancar and others, 2000). The overall error

is the square root of the sum of the individual errors

squared (Winter, 1981): 

enet GW = [(e∆S)2 + (eP)2 + (eE)2 + (eQ)2]1/2 (2)

Runoff to the lake was assumed to be negligible

in the water budget. During a few intense storms,

hourly increases in stage were sometimes slightly

greater than the measured rainfall, which could be

interpreted as either runoff or transient ground-water

inflow. These discrepancies also could be due to errors
10 Effects of Recharge, Upper Floridan Aquifer Heads, and Time Scale on Simulated Ground-Water Exchange with Lake Starr, a 

Seepage Lake in Central Florida INCHES PER MONTH OVER LAKE SURFACE AREA

-20

-15

-10

-5

0

5

10

15

20

25

Aug

Sep

Oct

Nov

Dec

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

Jan

Feb

Mar

Apr

May

Jun

Jul

1996 1997 1998

NET GROUND WATER (+/-)

CHANGE IN STAGE (+/-)

PUMPING (-)

EVAPORATION (-)

RAIN (+)

in the rainfall and stage measurements. The assumption

that runoff is negligible is reasonable because basin

soils are excessively drained and most of the basin is

unpaved. Even if runoff does occur rarely, it is unlikely

that the volume of runoff from the basin exceeds the 10

percent error assumed for the measurement of rain falling directly on the lake. 

Monthly water budget terms for Lake Starr for

the 2-year period August 1996 through July 1998 are

shown in figure 5. Annual rainfall for the 2 years of

the study (August 1996-July 1997 and August 1997-

July 1998) was 50.68 and 54.04 inches, respectively.

Rainfall during both years was within 2.1 inches of the

71-year annual average (51.99 inches), and was

greater than the 30-year average (48.21 inches).

Annual evaporation was 57.08 and 55.88 inches for

the 2 years, respectively. Direct pumping from the lake

was the smallest component of the water budget,

accounting for losses of 2.3 and 2.9 inches from the

lake for the 2 years, respectively. Overall, the lake lost

water the first year (-4.9 inches) and gained water the

second year (+12.7 inches).

The annual net ground-water flow was positive

in both years of the study (3.9 inches the first year and

17.5 inches the second year), indicating that groundwater inflow was greater than lake leakage. Positive

net ground-water flow occurred mainly in months with

positive net precipitation (rainfall minus evaporation),

but also occurred in drier months that followed periods

of high rainfall (fig. 5). For example, despite low rainfall in April and May 1998, net ground-water flow

continued to be positive in response to above average

rainfall from November 1997 through March 1998.

Negative net ground-water flow typically occurred in

drier months when net precipitation was negative.

Net ground-water flow differed substantially between

the 2 years. The difference of 13.6 inches was due to

both an increase in the amount of ground-water inflow

and a decrease in lake leakage in the second year.

Ground-water inflow was higher during the second

year because half of the rainfall occurred in the winter

when evapotranspiration losses were smaller, resulting

in greater recharge to the ground-water basin. Lake

leakage was probably reduced during the second year

because the head in the Upper Floridan aquifer was

higher (Swancar and others, 2000).

Figure 5. Monthly water-budget terms for Lake Starr, August 1996 through July 1998.
Description of the Ground-Water Model 11

The water budget indicated that ground-water

exchange with Lake Starr was responsive to the shortterm fluctuations in Upper Floridan aquifer heads.

Weekly and monthly net ground-water flow was correlated to the head in the Upper Floridan aquifer. Swancar and others (2000) found that changes in weekly

average head in the Upper Floridan aquifer could

account for 59 percent of the variability of weekly net

ground-water flow to Lake Starr. Sacks and others

(1998) also inferred the potential for increased leakage

from some lakes in response to changes in Upper Floridan aquifer heads on a weekly or shorter time scale.

The daily drop in lake stage at several lakes in the

Central Lakes District (for days without rainfall) was

found to be greatest for days with air temperatures

below freezing. These were the same days when heavy

ground-water pumpage, and steep declines in Upper

Floridan aquifer heads, were most likely to occur as a

result of irrigating the local citrus crop for freeze

protection. 

The water budget analysis also indicated that

ground-water inflow to Lake Starr was highly variable

from week to week. Months with large, positive net

ground-water flow typically resulted from only 1 or 2

weeks with large amounts of net ground-water inflow.

Weeks with large net ground-water inflows typically

had one or two large daily rainfall events and showed

transient water-table mounding and flow-reversals near

the shoreline. Because daily rainfall and recharge

processes had an important effect on ground-water

inflow, the weekly and monthly net ground-water

flows were not well correlated to the weekly or

monthly total rainfall or net precipitation (Swancar and

others, 2000). 

DESCRIPTION OF THE GROUND-WATER 

MODEL

A three-dimensional finite-difference numerical

ground-water flow model of the Lake Starr basin was

constructed using the USGS model code MODFLOW

(McDonald and Harbaugh, 1988). The model simulated ground-water flow in the surficial aquifer system

and the intermediate confining unit within a 3-mi2 area

surrounding Lake Starr (fig. 1). The model also simulated Upper Floridan aquifer heads in cells where the

head was not specified. The model consisted of 98

rows and 105 columns of cells that were 100 by 100 ft

(fig. 6). The active part of the model was 78 rows by 79

columns. The outer 10 rows (on the north and south

sides) and 10 and 16 columns (on the west and east

sides, respectively) were used to test boundary conditions, and were inactive in most simulations. In addition,

the northeast corner of the model (approximately

192,000 ft2) also was inactive in most simulations (fig. 6).

The steady-state model consisted of 22 layers that

varied in thickness from 1 to 91 ft (fig. 7). On

the southeast side of the lake, the bottom of model

layer 1 was lowered so that the simulated water table

stayed within layer 1 during transient simulations.

The steady-state model contained more layers than the

transient model. To reduce the running time, the

number of layers was reduced to 16 for transient simulations. For transient simulations, layers 12-14 and

15-17, which represented the deep surficial aquifer

system and the intermediate confining unit, respectively, were combined into two layers. Layers 18-20,

which represented the shallow Upper Floridan aquifer,

also were combined into one layer. Porosity of hydrogeologic units throughout the model was 0.3, specific

storage was 5 x 10-5, and specific yield of the surficial

aquifer system was 0.2 per ft.

Hydrogeologic Units in the Model

The representation of hydrogeologic units in the

model was based on geologic and geophysical logs of

wells in the basin, and sublake geology interpreted

from a seismic reflection survey of the lake (Swancar

and others, 2000). Initial estimates of hydraulic properties of different hydrogeologic units were from field

measurements or other references (Lee, 1996; Yobbi,

1996; Lee and Swancar, 1997). Values of hydraulic

parameters were adjusted within realistic ranges during

model calibration to improve the simulation of heads in

the basin and ground-water exchanges with the lake.

Calibrated values for parameters used in the steadystate simulation are listed in table 1. 

Surficial Aquifer System

The uppermost 14 layers of the steady-state

model (12 for transient) represented the surficial aquifer system. The horizontal hydraulic conductivity (Kh)

values for the surficial aquifer system were estimated

from slug tests made in wells in the basin and ranged

from 1 to 60 ft per day (ft/d). Slug tests indicated that

deeper wells had lower Kh (1-10 ft/d) compared to

shallow wells (20-60 ft/d). Hydraulic conductivity (K)

of the surficial aquifer system was assumed to decrease

with depth below land surface. Modeled Kh values

were reduced throughout the surficial aquifer system in
12 Effects of Recharge, Upper Floridan Aquifer Heads, and Time Scale on Simulated Ground-Water Exchange with Lake Starr, a 

Seepage Lake in Central Florida

MODEL OUTLINE OF LAKE STARR

AREA OF HIGHER SURFICIAL AQUIFER SYSTEM

 HYDRAULIC CONDUCTIVITY

AREA OF LOWER SURFICIAL AQUIFER SYSTEM

 HYDRAULIC CONDUCTIVITY

EXPLANATION

SURFICIAL AQUIFER SYSTEM WELL AND NAME

UPPER FLORIDAN AQUIFER WELL AND NAME

8,800

0

-1,000

 1,000

 2,000

 3,000

 4,000

 5,000

 6,000

 7,000

 8,000

-1,000 0 2,000 4,000 6,000 8,000 9,500

DISTANCE FROM ORIGIN, IN FEET

DISTANCE FROM ORIGIN, IN FEET

INACTIVE CELLS

ROW 55

Lake Starr

WTS-4

WTS-7

WTS-10

3PNS

WTS-6

WTS-5

WTS-2

WTS-3

WTS-1

WTS-25

WTS-21

WTS-20

WTS-24

1PNS-125

WTS-8

WTS-26

WTS-15

WTS-14

WTS-17

WTS-22

WTS-22

WTS-18 WTS-19

2PNS

WTS-23

Hart FLRD

Hart FLRD

WTS-13

WTS-11

WTS-12

WTS-9

N

Figure 6. Model grid and location of monitor wells.
Description of the Ground-Water Model 13

MONITOR WELL MONITOR WELL

LAKE STARR

LAKE SEDIMENTS

BREACH

INACTIVE CELLS

INACTIVE CELLS LAYERS 15 - 17 

LAYERS 18 - 22

UPPER 

FLORIDAN 

AQUIFER

LAYERS 1 - 14

SURFICIAL 

AQUIFER 

SYSTEM

EXPLANATION

WELL SCREEN

INTERMEDIATE CONFINING UNIT

9,500 8,000 6,000 4,000 2,000 0 -1,000

 100

 30 60

906030

115

NGVD

of 1929

DISTANCE FROM ORIGIN, IN FEET

SHALLOW SURFICIALAQUIFER SYSTEM IN LOWER BASINSHALLOW SURFICIALAQUIFER SYSTEM IN UPPER BASIN

DEEP SURFICIALAQUIFER SYSTEM

INTERMEDIATECONFINING UNITTHICK INTERMEDIATE CONFINING UNIT

UPPER FLORIDAN AQUIFER BREACH

LAKE STARRLAKE SEDIMENTS

(Kh = 30 ft/d; Kv = 0.15 ft/d)

(Kh = 15 ft/d; Kv = 0.075 ft/d)

(Kh = 2 ft/d; Kv = 0.1 ft/d)

(Kh = 2 ft/d; Kv = 0.2 ft/d)

(Kh = 125 ft/d; Kv = 50 ft/d)(Kh = 50,000 ft/d; Kv = 50,000 ft/d)(Kh = 0.3 ft/d; Kv = 0.015 ft/d)

(Kh = 0.25 ft/d; Kv = 0.0125 ft/d)

(Kh = 0.25 ft/d; Kv = 0.0075 ft/d)

HORIZONTAL (Kh) AND VERTICAL (Kv) HYDRAULIC CONDUCTIVITY, IN FEET PER DAY (ft/d)

LAYER FEET

1510152022

Figure 7. Vertical discretization and calibrated values of hydraulic conductivity for steady-state simulation along row 55.
14 Effects of Recharge, Upper Floridan Aquifer Heads, and Time Scale on Simulated Ground-Water Exchange with Lake Starr, a 

Seepage Lake in Central Florida

the upper basin because of the increased depth to water

and in the deepest surficial aquifer system (layers 12-14)

to reflect this relation (fig. 7). Anisotropy (the ratio

of horizontal to vertical hydraulic conductivity) was

specified as 20:1 for most of the model. However,

anisotropy was decreased to 10:1 in model cells representing breaches in the confining unit to reflect

increased vertical hydraulic conductivity (Kv) caused

by disruption of sediment layering. 

Intermediate Confining Unit

Layers 15-17 in the steady-state model (layer 13

in transient models) represented the intermediate

confining unit where this unit was present in the

subsurface, and breaches in the intermediate confining

unit where confinement was locally absent. Reducing

the intermediate confining unit from three layers to one

for transient simulations did not affect the head difference across the unit. Whether one or three layers were

simulated, this unit had a uniform thickness of 20 ft

in the model; for steady-state simulations, the K

values are specified in table 1. Where the intermediate

confining unit was represented in the model, Kh was

0.25 ft/d and Kv was 0.0125 ft/d; where breaches were

represented, Kh was 2 ft/d and Kv was 0.2 ft/d (fig. 8).

In areas of the basin where the intermediate confining

unit was thicker than 20 ft (primarily the east side of

the basin, see fig. 8), a lower Kv (0.0075 ft/d) was specified for the cells in this layer. Hydraulic properties for

the intermediate confining unit and the Upper Floridan

aquifer used in the model were similar to those used in

other flow models of these units (Lee, 1996; Yobbi,

1996; Lee and Swancar, 1997). The altitude of the

bottom of the intermediate confining unit conformed to

the top of the Upper Floridan aquifer. 

Upper Floridan Aquifer

Layers 18 through 22 represented the Upper

Floridan aquifer throughout most of the steady-state

model (layers 18-20 for transient). Because the Upper

Floridan aquifer was treated as a specified-head boundary, it could have been modeled as a single layer except

for the need to simulate a breach that extended downward into the aquifer in a small area near the lake on

the south side, surrounding the 2PNS well nest (fig. 1).

The specified-head boundary acted as a discharge zone

(sink) for the model because all of the water that enters

the basin as recharge was either lost to lake evaporation

or flow to the Upper Floridan aquifer. The Kh and Kv

values of the Upper Floridan aquifer were 125 and

50 ft/d, respectively (table 1 and fig. 7). 

Breaches in the Intermediate Confining Unit

Breaches in the intermediate confining unit were

represented in parts of the basin where sinkholes were

indicated by well logs or seismic data (fig. 8). Sinkholes

were also assumed to underlie closed topographic lows

in the land surface, including other lakes within the

modeled area. Breaches in the intermediate confining

unit were represented by areas that had hydraulic properties similar to the overlying surficial aquifer system.

The thickness of the unit, the size of the breaches, and

the hydraulic properties of the confining unit and

breaches control ground-water flow across these layers.

It is possible that a number of slightly different representations of sinkholes in the model could produce the

same leakage rate across the confining unit to the

Upper Floridan aquifer. For example, making sinkholes

either larger or leakier can increase leakage across the

layer(s) that represent the intermediate confining unit.

Table 1. Calibrated values of hydraulic parameters used in 

the steady-state model

[Kh, horizontal hydraulic conductivity; Kv, vertical hydraulic conductivity] 

Parameter Value

Recharge (inches per year)

Basin recharge 18

Lake recharge -4

Hydraulic Conductivities (feet per day) Kh Kv

Lake 50,000 50,000

Lower basin surficial aquifer system 30 1.5

Upper basin surficial aquifer system 15 0.75

Deep surficial aquifer system 2 0.1

Upper Floridan aquifer 125 50

Intermediate confining unit 0.25 0.0125

Thicker confining unit 0.25 0.0075

Breaches in confining unit 2 0.2

2PNS "wall" 1 0.02 0.001

Lake sediments 0.3 0.015

Anisotropy (ratio of horizontal to 

vertical K)

Breaches 10:1

Lake 1:1

Upper Floridan aquifer 5:2

Rest of model 20:1

1 The 2PNS “wall” simulates confining conditions on the sides of a 

collapse feature.
Description of the Ground-Water Model 15

-1,000 0 2,000 4,000 6,000 8,000 9,500

HORIZONTAL (Kh) AND VERTICAL (Kv) HYDRAULIC CONDUCTIVITY, IN FEET PER DAY (ft/d)

DISTANCE FROM ORIGIN, IN FEET

DISTANCE FROM ORIGIN, IN FEET

8,800

0

-1,000

 1,000

 2,000

 3,000

 4,000

 5,000

 6,000

 7,000

 8,000

N

EXPLANATION

THICK INTERMEDIATE 

CONFINING UNIT

INTERMEDIATE

CONFINING UNIT

BREACHES

OUTLINE OF LAKE STARR PROJECTED

 FROM LAYER 1

(Kh = 2 ft/d;

Kv = 0.2 ft/d)

(Kh = 0.25 ft/d;

Kv = 0.0075 ft/d)

(Kh = 0.25 ft/d; 

Kv = 0.0125 ft/d)

INACTIVE

CELLS

Figure 8. Spatial distribution of hydraulic conductivity of the intermediate confining unit for the steady-state model.
16 Effects of Recharge, Upper Floridan Aquifer Heads, and Time Scale on Simulated Ground-Water Exchange with Lake Starr, a 

Seepage Lake in Central Florida

A collapse feature (sinkhole) that encompasses

well nest 2PNS was represented in the model by a

vertical shaft into the Upper Floridan aquifer that was

infilled with surficial aquifer system materials (fig. 9).

To simulate the observed head difference between

2PNS-156 and the Hart FLRD well, low-conductivity

cells that retarded flow into the surrounding Upper

Floridan aquifer cells were placed around this column

of sandy material (2PNS “wall,” fig. 9). 

Method Used to Simulate Lake Starr

Lake Starr was represented as an area of active

cells in model layers 1 to 8 (fig. 6 and 7). In steadystate simulations, lake cells had a uniform K value of

50,000 ft/d, whereas in transient simulations the value

was 10,000 ft/d. Values higher than these caused problems with numeric convergence. For the lake cells, a

storage coefficient of 0.999 was specified because the

model preprocessor (Visual MODFLOW; Guiguer and

Franz, 1997) would not allow a storage coefficient of 1.

The large K of lake cells resulted in a low head gradient

across the lake. The simulated head gradient across the

lake surface was typically less than or equal to 0.05 ft

in a 1,000 ft distance, or 5x10-5. 

In transient simulations, the lake stage and

volume fluctuated in the top model layer (layer 1) in

response to changes in net recharge to the lake and to

flows to and from other parts of the model (groundwater exchanges). The lake volume in underlying

model layers was constant. Stage-volume-area relations

for the lake are given in Swancar and others (2000).

A thin layer of lake sediments was simulated beneath

the deeper parts of the lake in layer 9 of the model.

Sediments at Lake Starr are relatively thin (maximum

thickness 5 ft), and cover only about a quarter of the

lake bottom (Swancar and others, 2000). 

The lake area in the top layer was fixed at

5,900,000 square ft (ft2), an area that corresponds to an

altitude of 105.00 ft above NGVD of 1929. The error

associated with using a fixed area for the top layer of

the lake was always less than 0.05 ft of lake stage, or

0.3 percent of the average lake volume during the study

period. This calculation is based on the difference

between the simulated volume at the stage farthest

from 105.00 ft that occurred during the study (106.56 ft),

and the measured volume at that altitude. Because the

stage never changed this much within a model stress

period, the error in individual time periods would

always be less. The error associated with using a

constant surface area could be unacceptable in some

applications, especially if the surface area changed

rapidly as stage changed. These effects could be minimized by using methods described in Cheng and

Anderson (1993) or Merritt and Konikow (2000). 

Boundary Conditions for Steady-State 

Simulations

Three boundary conditions were used to characterize the ground-water flow system in the Lake Starr

basin: (1) a lateral no-flow boundary, (2) an upper

specified-flow (recharge) boundary, and (3) a lower

specified-head (Upper Floridan aquifer) boundary. 

Lateral Boundaries

Lateral model boundaries were specified as noflow boundaries in all steady-state and transient simulations. The northern, southern, and eastern boundaries

of the model coincided with lateral ground-water flow

paths, where ground water flowed along the boundary

rather than across it. In addition, the predominance of

vertical ground-water flow in the Lake Starr basin

meant that lateral boundaries could be treated as noflow boundaries if they were sufficiently far from the

lake so that horizontal ground-water flow components

at the boundary were minimal. The western model

boundary was not along a horizontal flow path based

on water-table configuration, but was in an area where

flow was predominantly vertical. Despite deviating

from the ideal conditions for a no-flow boundary on the

west side of the model, all of the lateral model boundaries were sufficiently far enough away from the lake

so that the effect of the boundary condition on groundwater flow to and from the lake was minimal. Extending the boundaries farther out on all sides by 1,000 ft

for both steady-state and transient simulations

increased net ground-water flow to the lake by less

than 5 percent. Nearly all of the water that entered the

model as recharge in the area beyond the original

model boundary flowed downward to the Upper Floridan aquifer rather than to the lake. 

Previous studies of lake basins have simulated

lateral boundaries using specified heads to represent

nearby lakes (Lee, 1996; Merritt, 2001). Representing

lakes that surround the Lake Starr topographic basin

(Dinner, Suzanne, and Mabel, see fig. 1) with specified-head boundaries in steady-state simulations did

not affect the shape of the simulated water table or

ground-water exchange with the lake. The model
Description of the Ground-Water Model 17

DISTANCE FROM ORIGIN, IN FEET

INACTIVE CELLS

INACTIVE CELLS

9,500 8,000 6,000 4,000 2,000 0 -1,000

 100

115

906030

 30 60

FEET

NGVD

of 1929

HORIZONTAL (Kh) AND VERTICAL (Kv) HYDRAULIC CONDUCTIVITY, IN FEET PER DAY (ft/d)

LAKE STARR (Kh = 50,000 ft/d; Kv = 50,000 ft/d)

LAYER

1510152022

SHALLOW SURFICIALAQUIFER SYSTEM IN LOWER BASINSHALLOW SURFICIALAQUIFER SYSTEM IN UPPER BASIN

DEEP SURFICIALAQUIFER SYSTEM

(Kh = 125 ft/d; Kv = 50 ft/d)

LAKE STARR

EXPLANATION

BREACH

2PNS "WALL" INTERMEDIATE

CONFINING UNIT 

(Kh = 30 ft/d; Kv = 1.5 ft/d)(Kh = 15 ft/d; Kv= 0.75 ft/d) (Kh = 0.02 ft/d; Kv = 0.001 ft/d)

(Kh = 2 ft/d; Kv = 0.1 ft/d)

(Kh = 2.0 ft/d; Kv = 0.2 ft/d)

2PNS NEST WELLS

(Kh = 0.25 ft/d;

 Kv = 0.0125 ft/d)

UPPER FLORIDAN AQUIFER 

THICK INTERMEDIATECONFINING UNIT 

(Kh = 0.25 ft/d; Kv= 0.0075 ft/d)

WELL SCREENS

Figure 9. Model row 64 through 2PNS piezometer nest wells showing hydraulic conductivity zones used in steady-state simulation.
18 Effects of Recharge, Upper Floridan Aquifer Heads, and Time Scale on Simulated Ground-Water Exchange with Lake Starr, a 

Seepage Lake in Central Florida

produced water-table altitudes in these parts of the

model that were similar to the observed stages of the

surrounding lakes without the use of specified heads. 

Recharge

The upper boundary of the ground-water and

lake flow system was defined by the water-table and

lake surface. Recharge to the surficial aquifer system

and the lake was represented as a specified flux to the

upper boundary of the model. The location of the upper

boundary is partially determined by the flux of water

(recharge) applied to the uppermost active cell.

Recharge to the lake was net precipitation, computed as

the difference between rainfall and evaporation for a

given time period. Rainfall and evaporation were

measured at Lake Starr using a tipping-bucket rain

gage at the edge of the lake and the energy-budget

method, respectively (Swancar and others, 2000).

For steady-state simulations, recharge to the lake was

-4 in/yr, indicating that on average, evaporation

exceeded precipitation over the lake during the study

period. The contribution of runoff from the basin to

lake recharge was assumed to be negligible. 

Steady-state simulations were calibrated to average conditions during the 2-year study, which were

assumed to be similar to long-term climatic conditions.

Annual rainfall during the study (50.68 and 54.09 inches

per year for August 1996 through July 1997 and

August 1997 through July 1998, respectively) was

close to the long-term average of 51.99 inches per year.

Initial estimates of the recharge rate to the surficial aquifer system in the steady-state simulation were

based on a chloride mass-balance between rainfall and

shallow ground water. Assuming the chloride mass is

conservative, the chloride concentrations in rain and

ground water (measured where land-use effects on

chloride concentrations are expected to be negligible)

can be used to estimate the percentage of rain that

reaches the ground-water system. Using this method,

recharge to the surficial aquifer system was estimated

to range between 28 and 37 percent of average annual

rainfall (L.A. Sacks, U.S. Geological Survey, written

commun., 1998), or between 15 and 19 in/yr. In similar

upland settings in other parts of the State, Lee (1996)

estimated recharge to be 26 percent of the annual rainfall during a drought year based on a calibrated model,

whereas Sumner (1996) found that rainfall minus

evapotranspiration was between 43 and 53 percent of

rainfall during a year with average rainfall. In steadystate simulations, average recharge to the surficial

aquifer system was assumed to be 35 percent of rainfall, or 18 in/yr, and recharge rates ranging from +25 to

+50 percent were considered in the sensitivity analysis. 

Upper Floridan Aquifer

The lower boundary of the ground-water flow

system, which represents the Upper Floridan aquifer,

was defined by a specified-head boundary. In steadystate simulations, head was specified in all of layer 22

and in the parts of layers 18 and 21 that represented the

Upper Floridan aquifer. The steady-state specifiedhead boundary was defined by a potentiometric surface

contoured to median heads observed in 10 Upper Floridan aquifer wells in the basin (fig. 10). Heads in these

wells were measured or estimated biweekly (Coffin

and Fletcher, 1998, 1999; Swancar and others, 2000).

The highest head in the steady-state potentiometric

surface occurred at the western boundary (106.50 ft),

and the lowest head was at the southeastern corner

(92.24 ft).

Boundary Conditions for Transient 

Simulations

During transient simulations, recharge and

Upper Floridan aquifer specified heads were redefined

for different blocks of time. The convention of

McDonald and Harbaugh (1988) and Anderson and

Woessner (1992, p. 206) was followed; these blocks of

time are referred to as “stress periods.” Other external

stresses on the flow system, such as pumping from

wells, were absent from the simulations. The effects of

ground-water pumpage were incorporated into the

model through the Upper Floridan aquifer boundary.

Because only boundary conditions changed with time,

a stress period in this report is synonymous with the

term “boundary stress period” used in the MODFLOW

documentation (McDonald and Harbaugh, 1988). 

Transient simulations represented variations in

recharge and head in the Upper Floridan aquifer over a

27-month period from May 1996 through July 1998.

Transient simulations were prepared using monthly and

weekly stress periods to assess the sensitivity of results

to the length of the stress period. Transient simulations

were begun 3 months before the study period (August

1996-July 1998) to minimize the effect of initial conditions on the transient results. Initial head conditions for

transient simulations came from a steady-state simulation that was calibrated to ground-water levels
Description of the Ground-Water Model 19

94

106

96

98

100

104

102

106

-1,000 0 2,000 4,000 6,000 8,000 9,500

EXPLANATION

LINE OF EQUAL SPECIFIED-HEAD--

 In feet above NGVD of 1929

UPPER FLORIDAN AQUIFER WELL--

 Shows water level, in feet above NGVD of 1929, 

 (e), estimated

OUTLINE OF LAKE STARR PROJECTED 

 FROM LAYER 1

103.82

103.82

103.62 (e)

INACTIVE

CELLS

DISTANCE FROM ORIGIN, IN FEET

DISTANCE FROM ORIGIN, IN FEET

93.90

96.44

94.64

100.21

101.82 (e)

103.06

93.65 (e)

N

8,800

0

-1,000

 1,000

 2,000

 3,000

 4,000

 5,000

 6,000

 7,000

 8,000

Figure 10. Upper Floridan aquifer heads specified in steady-state simulation.
20 Effects of Recharge, Upper Floridan Aquifer Heads, and Time Scale on Simulated Ground-Water Exchange with Lake Starr, a 

Seepage Lake in Central Florida

observed in early May 1996. Weekly stress periods

corresponded to periods over which water budgets

already had been calculated (Swancar and others,

2000). These “weekly” periods were nearly all 7 days

in length, but ranged from 5 to 13 days because of a

few periods of missing record that affected the evaporation calculation.

Comparison of the weekly and monthly simulations reveals how stress-period length affects interpretation of the flow system. Shortening the stress period

from monthly to weekly greatly increases the variability in recharge because, typically, most of the rain that

falls within a month actually occurs within a few days.

This variability changes some of the underlying

presumptions in the model. Namely, the increased variability of recharge requires that the ground-water flow

system respond more quickly to the imposed stress.

Thus, the weekly model is expected to be fundamentally different from the monthly model because it

presumes a system that responds more rapidly to

stresses. 

Recharge

In transient as well as steady-state simulations,

recharge to the water table was calculated as a function

of rainfall. But for transient simulations, the timing of

recharge was lagged behind rainfall events to reflect

the traveltime through the unsaturated zone (table 2).

The basin was divided into seven zones based on the

thickness of the unsaturated zone (fig. 11). The time

delay in each zone was determined by computing the

time delay between peak lake stage and peak groundwater levels at 26 water-table wells in the basin (see

fig. 6 for well locations). An average time delay was

computed for each well, and these delays were applied

to areas of the model where the land-surface altitude

was within the same altitude range as the well (table 2).

Land-surface altitude was a fairly accurate indicator of

the unsaturated zone thickness because the water table

was relatively flat compared to the land surface.

Three methods were used to compute recharge to

the surficial aquifer system during transient simulations. To compare the results, each method was

adjusted to deliver the same total amount of recharge

over the 2-year water budget period. This amount of

recharge was initially specified as 35 percent of the

total rainfall for the 2 years of the study. In later transient simulations, total recharge in some parts of the

model was revised to as high as 86 percent of the

2-year rainfall. The three methods distributed this total

recharge differently within the 2-year timeframe.

The first method simply assumed that recharge

over the stress period was equal to a fixed percentage

(35 percent initially) of the rainfall within the stress

period. The second method accounted for evaporation

losses and intensity of rainfall by weighting larger rain

events more heavily than smaller events, so that larger

events generated more recharge. Rainfall events

beneath a certain threshold contributed no recharge,

and when daily rainfall was less than daily evaporation,

recharge was assumed to be zero. This method was

based on that of Lee (1996), and is referred to as the

“threshold” method in this report. 

Daily threshold recharge was estimated using the

equation: 

Recharge = (Total daily rainfall – monthly 

average evaporation) *WF (3)

where WF is the weighting factor and all units are in

inches per day (in/d) except for the weighting factor,

which is dimensionless. The weighting factor was

defined as follows for threshold recharge summing to

35 percent of the 2-year rainfall:

Daily net precipitation

(Inches)

Weighting factor

(unitless)

<0.50 0.10

 0.50-0.74 0.25

 0.75-0.99 0.35

 1.00-1.24 0.50

 1.25-1.39 0.60

 1.40-1.99 0.70

 2.00-2.99 0.80

<3.00 0.95

Table 2. Depth to water, land-surface altitude, 

and assumed time delay for recharge to reach 

the water table 

[<, less than; >, greater than; altitudes relative to 

National Vertical Geodetic Datum of 1929] 

Depth to

water

(feet)

Land-surface

altitude

(feet)

Time delay

(days)

0-20 <125 0

21-40 126-145 10

41-60 146-165 25

61-80 166-185 35

81-100 186-205 45

101-120 206-220 55

121-140 >220 65
Description of the Ground-Water Model 21

INACTIVE CELLS

RECHARGE 

ZONE

TIME DELAY 

(DAYS)

RECHARGE 

ZONE

TIME DELAY

 (DAYS)

EXPLANATION

DISTANCE FROM ORIGIN, IN FEET

DISTANCE FROM ORIGIN, IN FEET

0 LAKE

<125

126 -145

146 -165

166 - 185

186 - 205

206 - 220

> 220

0

10

25

35

45

55

65

ALTITUDE

RANGE (FEET)

ALTITUDE 

RANGE (FEET)

0 2,000

8,800

0

-1,000

 1,000

 2,000

 3,000

 4,000

 5,000

 6,000

 7,000

 8,000

-1,000 4,000 6,000 9,500 8,000

N

Figure 11. Simulated recharge zones depicting estimated time delay between rainfall and recharge at the water table. 
22 Effects of Recharge, Upper Floridan Aquifer Heads, and Time Scale on Simulated Ground-Water Exchange with Lake Starr, a 

Seepage Lake in Central Florida

The third method for estimating recharge used

the one-dimensional unsaturated flow model

LEACHM (Hutson and Wagenet, 1992). Daily

recharge calculated by LEACHM is based upon daily

rainfall, the physical character and moisture status of

the soils above the water table, and losses attributed to

evapotranspiration. This method performed a daily

accounting of water stored in the unsaturated zone.

Using this method, a storm occurring during a wet

period generated more recharge than a similar storm

during a drier period because available storage in the

unsaturated zone was already used up. LEACHM can

simulate both water and chemical transport, but only

the water transport part of the model was used to estimate recharge. Soil moisture accounting was

computed using the capacity or “tipping bucket”

approach of Addiscott (1977). Representative soils

(fine sand, less than 3 percent silt, no clay) and shallow-rooted vegetation (grass) were used in the initial

LEACHM model to achieve the 35 percent of total

2-year rainfall criterion. In a later test of the groundwater flow model, recharge was increased from 35 to

86 percent of 2-year rainfall by reducing the silt

content of the soil to 1 percent and eliminating the

shallow-rooted vegetation in the LEACHM model.

The one-dimensional LEACHM model

consisted of 20 vertical segments of 10 centimeters

(3.94 inches) each, for a total profile thickness of

2 meters (6.56 ft). The LEACHM model was designed

for use in areas where the water table is shallow;

however, it was adapted for use with the deeper water

table found in the Lake Starr basin by applying the

time delay. Weekly potential evapotranspiration was

estimated as a function of the open-water evaporation

calculated for Lake Starr, adjusted to fit the time scale

required for the LEACHM model (R.M. Yager, U.S.

Geological Survey, written commun., 2000). Evaporation estimates for periods before August 1996 were

assumed to be equal to the evaporation estimates for

the same week in 1997. 

Daily recharge rates calculated from the threshold and LEACHM methods were averaged over either

monthly or weekly periods for transient simulations.

The recharge flux to the model had the least variable

distribution with time when recharge was calculated as

a fixed percentage of rainfall during the stress period.

The LEACHM method generated the most variable

flux with time, and the threshold method generated

a flux distribution that fell between the other two

methods (fig. 12). 

Upper Floridan Aquifer

To represent the potentiometric surface of the

Upper Floridan aquifer in transient simulations, heads

were specified at cells located along equipotential

contour lines in the lowest Upper Floridan aquifer

layer (layer 16) (fig. 13). The head values specified for

these cells changed at each stress period. Heads were

simulated in the remaining cells of layer 16 and in the

parts of layers 14 and 15 that represented the Upper

Floridan aquifer. Model cells between specified-head

cells were active, and due to the high Kh in this layer

(125 ft/d), simulated heads between contours were a

linear interpolation of the contoured specified heads.

The results of simulating the Upper Floridan aquifer

boundary condition in this manner were virtually identical to results when heads in all of the cells in layer 16

were specified. The boundary was much simpler to

construct in this way given the number of transient

stress periods. 

The potentiometric surface of the Upper Floridan aquifer beneath Lake Starr was strongly affected

by pumping and was highly variable in time. To estimate the average head condition for each stress period,

biweekly measurements of representative wells on the

northwest and southeast sides of the lake (1PNS-125

and Hart FLRD, respectively) were first linearly correlated to hourly readings at nearby wells that had

continuous water-level recorders. Biweekly measurements at well 1PNS-125 on the northwest side of the

Lake Starr were correlated to hourly water levels in

well ROMP 58, located about 2.5 miles southwest of

the lake, and biweekly heads in the Hart FLRD well on

the southeast side of the lake were correlated to hourly

water levels in well ROMP 57A, located about

4.5 miles southeast of the lake (Coffin and Fletcher,

1998, 1999). Then, a synthetic hourly record was

generated for the two wells within the lake topographic basin, and an average head condition for the

stress period was derived from the synthetic data. The

difference between the average synthetic head over a

stress period and the 2-year average measured head at

each of these two wells was calculated. These deviations from the average head were used to define the

rise and fall of each of the contour lines that determined the lower specified-head boundary to the model

(fig. 14).
Results of Steady-State Simulations 23

0

1

2

3

4

5

6

7

Aug

Sep

Oct

Nov

Dec

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

Jan

Feb

Mar

Apr

May

Jun

Jul

35 PERCENT OF RAINFALL

THRESHOLD RECHARGE

LEACHM RECHARGE

1996 1997 1998

RECHARGE, IN INCHES PER MONTH

RESULTS OF STEADY-STATE 

SIMULATIONS 

Steady-state conditions in the Lake Starr basin

were simulated to (1) describe the long-term average

heads and fluxes in the basin, (2) to test the sensitivity

of the model to changes in input parameters, (3) to

determine areas of the basin that contributed groundwater to the lake, and (4) to establish initial conditions

for transient simulations. Two steady-state models

were constructed. One was calibrated to long-term

heads and fluxes, and the other was calibrated to May

1996 head conditions. The May 1996 steady-state

simulation was used to define initial head conditions

for transient simulations. 

For the long-term steady-state simulation, simulated heads were calibrated to median water levels

measured in 26 water-table wells and 13 piezometers

over an 18-month period (see fig. 6 for well locations,

or Swancar and others (2000) fig. 5 and table 2 for

more detail on monitor wells). Simulated steady-state

ground-water inflow to the lake also was calibrated to

an estimate of the annual average ground-water inflow

to Lake Starr calculated using an isotope mass-balance

approach along with water budget data to derive gross

ground-water inflow from the net ground-water term

(31,000 cubic ft per day (ft3/d) + 50 percent, or about

23 in/yr + 11.5 inches; Sacks and others, 1998). The

focus of the steady-state calibration was on heads,

however, as many different steady-state simulations

produced ground-water inflow that was within the error

of this estimate. 

 The steady-state simulation provided a good fit

to the median water-table configuration (fig. 15) and

median vertical head distribution in the aquifer beneath

Lake Starr. The overall root-mean-squared error (RMSE)

Figure 12. Monthly recharge rates using three recharge methods, August 1996 through July 1998.
24 Effects of Recharge, Upper Floridan Aquifer Heads, and Time Scale on Simulated Ground-Water Exchange with Lake Starr, a 

Seepage Lake in Central Florida

-1,000 0 2,000 4,000 6,000 8,000 9,500

DISTANCE FROM ORIGIN, IN FEET

DISTANCE FROM ORIGIN, IN FEET

8,800

0

 -1,000

 1,000

 2,000

 3,000

 4,000

 5,000

 6,000

 7,000

 8,000

N

INACTIVE

CELLS

EXPLANATION

SPECIFIED HEAD CELL IN LOWER (UPPER FLORIDAN AQUIFER) 

 MODEL BOUNDARY LAYER

OUTLINE OF LAKE STARR PROJECTED FROM LAYER 1

Figure 13. Specified head cells for transient simulations in layer 16 of the Lake Starr basin model.
Results of Steady-State Simulations 25

90

92

94

96

98

100

102

104

106

108

8/1

8/3110/1

10/31

12/1

12/31

1/31

3/24/25/26/27/28/29/1

10/211/112/2

1/12/13/34/35/36/37/3

DAILY NORTH

WEEKLY NORTH

MONTHLY NORTH

DAILY SOUTH

WEEKLY SOUTH

MONTHLY SOUTH

1996 1997 1998

HEAD, IN FEET ABOVE NGVD OF 1929

between simulated and median observed water levels

was less than 0.5 ft (fig. 16). The RMSE is the square

root of the sum of squares of differences between

calculated and observed heads divided by the number

of observation wells. The RMSE is an approximation

of the standard deviation, which means that two-thirds

of the errors between simulated and median observed

water levels were less than 0.5 ft. The steady-state

simulated lake stage was 104.17 ft above NGVD of

1929, 0.53 ft less than the 18-month median value of

104.70 ft. Simulated ground-water inflow and lake

leakage were calculated using the zone budget program

of Harbaugh (1990); these flows were then summed to

derive net ground-water flow to the lake. 

On a steady-state basis, 98 percent of the total

recharge within the model area ultimately flows to the

Upper Floridan aquifer specified-head cells (table 3). Of

the total recharge to the land surface of 220,400 ft3/d (18

inches per year), 215,000 ft3/d ultimately flows to the

specified-head cells. Steady-state ground-water inflow

to the lake was within the error of the inflow estimate

from the isotope mass-balance approach. Net groundwater flow to the lake was equal to the net precipitation

(rainfall minus evaporation) (table 3).

Figure 14. Estimated daily heads in the Upper Floridan aquifer on the north and south sides 

of Lake Starr and weekly and monthly variation in specified head used in the ground-water 

flow model, August 1996 through July 1998.

Table 3. Steady-state fluxes in the Lake Starr model

[Fluxes to and from lake, in inches, are relative to lake surface area 

at 105 feet above NGVD of 1929; other fluxes, in inches, are relative 

to applicable basin areas]

Flux Volume

(cubic feet)

Flux over

surface area

(inches/year)

Recharge to basin 220,400 18.0

Recharge to lake -5,400 -4.0

Ground-water inflow to lake 26,300 19.5

Lake leakage 20,900 15.5

Net ground-water flow to lake 5,400 4.0

Flow to specified head cells 215,000 6.1
26 Effects of Recharge, Upper Floridan Aquifer Heads, and Time Scale on Simulated Ground-Water Exchange with Lake Starr, a 

Seepage Lake in Central Florida

-1,000 0 2,000 4,000 6,000 8,000 9,500

DISTANCE FROM ORIGIN, IN FEET

DISTANCE FROM ORIGIN, IN FEET

109

EXPLANATION

LINE OF EQUAL SIMULATED WATER-TABLE ALTITUDE--

 In feet above NGVD of 1929

MONITOR WELL AND DIFFERENCE BETWEEN SIMULATED

 AND MEDIAN WATER LEVEL-- In feet

MODEL OUTLINE OF LAKE STARR

INACTIVE CELLS

Lake

Starr

0.30

0.46

0.74

0.12

0.46

0.14

-0.32

-0.32

-0.21

0.05

0.07

0.18

0.12

0.23

-0.56

-0.51

-0.41

-0.30

-0.36

-0.21

0.09

-0.64 -0.53

0.16 0.15

0.38

0.37

-0.34

-0.25

-0.39

109

108

107

106

105

104

110

102

101

100

99

103

109

AREA THAT CONTRIBUTES

 GROUND WATER TO THE LAKE

8,800

0

-1,000

 1,000

 2,000

 3,000

 4,000

 5,000

 6,000

 7,000

 8,000

N

Figure 15. Steady-state simulated water-table altitudes, water-table and water-level residuals, and area that 

contributes ground water to the lake, median September 1996-February 1998 conditions.
Results of Steady-State Simulations 27

101

102

103

104

105

106

107

108

109

110

101 102 103 104 105 106 107 108 109 110

SIMULATED HEAD, IN FEET ABOVE NGVD OF 1929

MEDIAN OBSERVED HEAD, IN FEET ABOVE NGVD OF 1929

Root mean squared error = 0.443

Mean error = 0.008

Mean absolute error = 0.371

1:1 ratio

Simulated heads and fluxes were most sensitive

to recharge and K values in the intermediate confining

unit (including breaches) and the surficial aquifer

system (table 4). Heads were less sensitive to hydraulic

conductivity in the surficial aquifer system, with fluxes

to and from the lake directly related to changes in this

value. Lower ranks of sensitivities (1-3) meant that the

model results (RMSE in head, lake stage, and groundwater inflow) changed more in response to changes in

the value of these parameters. Certain model results are

more sensitive to some parameters than others. For

example, intermediate confining unit K has a strong

effect on heads throughout the basin, but has less effect

on ground-water inflow to the lake, whereas the K of

the surficial aquifer system has a strong effect on

ground-water inflow but a lesser effect on heads.

It is most critical to accurately define parameters such

as recharge that have low rankings for more than one

model result when the goal is a realistic simulation of

the lake/ground-water system. The presence of organic

lake sediments in the model had little effect on simulated ground-water exchange with the lake. Because

organic sediments covered a relatively small area of the

bottom of Lake Starr, their presence or absence in the

model did not appear to inhibit ground-water exchange,

which could still occur across most of the lake bottom. 

In steady-state simulations, there is no change in

aquifer storage, and conservation of mass requires that

all of the water that comes into the model as recharge

either flows to the Upper Floridan aquifer or is lost as

lake evaporation. Most of the water that enters the Lake

Starr model as recharge flows to the Upper Floridan

Figure 16. Comparison of simulated heads to median observed heads in the Lake 

Starr basin for the calibrated steady-state model, median September 1996-February 

1998.
28 Effects of Recharge, Upper Floridan Aquifer Heads, and Time Scale on Simulated Ground-Water Exchange with Lake Starr, a 

Seepage Lake in Central Florida

aquifer specified-head boundary. For a given recharge

rate, the rate of flow across the intermediate confining

unit largely determines head conditions throughout the

surficial aquifer system. A simple parameter estimation

model of a similar hypothetical lake/ground-water

system using the MODOPTIM program by Halford

(1992) indicated that recharge and intermediate confining unit Kv were significantly correlated. Because of

this correlation, it is prudent to decide which of these

parameters is best known. Then, a value for that parameter is set, and calibration is used to determine the

lesser-known parameter. For this model, as in Lee (1996),

recharge was assumed to be known more accurately than

confining unit properties, and confining unit properties

were varied in the calibration process using a trial-anderror approach (Anderson and Woessner, 1992).

Simulation of water levels in the 2PNS well nest

was successful after the model was modified to reflect

the increased hydraulic connection between the surficial aquifer system and the breach in which the wells

were located, and after adding a vertical “barrier”

between the deeper parts of the breach and the

surrounding Upper Floridan aquifer cells. Without the

barrier, water flowed from the breach to the specifiedhead cells too rapidly to maintain the observed head

difference (about 6 ft) between the nested wells and the

nearest Upper Floridan aquifer well. Simulation of the

downward vertical gradient observed at this nest was

improved by using three layers for the intermediate

confining unit/breach cells because the model was

better able to represent the vertical head differences

seen in the nest when more layers were present. Simulated heads in the 2PNS nest were lower than observed

heads by about the same amount (0.29-0.56 ft) as the

simulated lake level was below its observed water level

(0.53 ft). The simulated downward head difference was

0.52 ft compared to the observed difference of 0.75 ft.

Accurately representing the potentiometric

surface of the Upper Floridan aquifer was essential to

successfully simulating the water-table configuration in

the steady-state model. The slope of the potentiometric

surface across the basin has a strong effect on the

water-table shape and imposes the same flow direction

in the surficial aquifer system, causing lake water to

flow out along the southeast shore despite the higher

topography on this side of the basin. Variations in the

intermediate confining unit thickness and breaches in

this unit superimpose an additional effect on the watertable shape, lowering the water table in the southern

part of the basin where the confining unit is more

dissected, and raising the water table where the confining unit is thicker. For example, to calibrate the steadystate model to water levels in the area around wells

WTS-19 and WTS-26, located in sinkholes on the

south side of the lake (see fig. 1), breaches in the intermediate confining unit were added. Otherwise, simulated heads in these wells were higher than observed.

Table 4. Sensitivity of steady-state model results to changes in recharge and hydraulic conductivity parameters

[RMSE, root mean squared error of simulated heads; %, percent; ft, feet above NGVD of 1929; ft3/d, cubic feet per day; K, hydraulic 

conductivity; SAS, surficial aquifer system; ICU, intermediate confining unit; 2PNS, 2PNS well nest; values with asterisks (*) are the 3 most 

sensitive parameters for each result] 

Parameter

Change in result for given change in parameter

RMSE (ft) 

[calibrated model 

result, 0.39]

Rank

Lake stage (ft) 

[calibrated model 

result, 104.17]

Rank

Ground-water inflow (ft3/d) 

[calibrated model 

result, 26,320]

Rank

-50% +50% -50% +50% -50% +50%

Basin recharge 2.78 2.55 *1 101.66 106.61 *1 19,730 32,920 *1

Lake recharge 0.47 0.34 7 104.01 104.34 7 27,550 25,000 5

Lower basin SAS K 0.73 0.34 5 103.64 104.45 5 21,010 29,440 *2

Upper basin SAS K 0.44 0.39 8 104.18 104.18 8.5 25,530 26,330 6

Deep SAS K 0.63 0.52 6 104.82 103.92 4 26,180 26,490 8

ICU K 1.75 1.09 *2 106.04 103.26 *2 28,750 25,510 4

Thick ICU K (east side) 0.74 0.65 4 104.61 103.91 6 26,640 26,560 7

Breach K 0.85 0.80 *3 105.05 103.59 *3 23,360 28,280 *3

2PNS "wall" K 0.39 0.40 9 104.18 104.16 8.5 26,410 26,350 9.5

Lake sediment K 0.39 0.39 10 104.17 104.17 10 26,220 26,340 9.5
Results of Transient Simulations 29

Ground-water flow paths were delineated using

the particle-tracking method of Pollock (1994) to determine areas of the basin that contributed ground-water

inflow to the lake (figs. 15 and 17). The area contributing ground water to the lake, called the ground-water

catchment, was west and north of the lake and

extended about 1,200 ft from the lake on the west side.

The steady-state ground-water catchment covered

about 16 percent of the topographic basin (116 of 736

acres). Rainfall recharging the surficial aquifer system

outside the ground-water catchment flowed downward

to the Upper Floridan aquifer rather than to the lake.

Most of the ground water flowing into the lake entered

at shallow depths along the lake bottom and came from

the parts of the basin closest to the lake. Ground-water

flow paths entered the lake at depths up to about 15 ft

below the lake surface on the west side of the lake in

the steady-state simulation. Seventy-three percent of

the ground water entering the lake from a row on the

west side (row 55, see figs. 6 and 17) originated as

recharge within 900 ft of the lakeshore. The maximum

simulated traveltime for recharge to reach the lake was

about 25 years, and the minimum was less than 20 days

for a particle starting about 30 ft from the simulated

lake shore. Most flow paths to the lake had traveltimes

that were less than 10 years. Minimum traveltimes to

reach the Upper Floridan aquifer were about 5 years.

The computed traveltime is directly related to the

assumed porosity of 0.3; smaller porosity values would

yield proportionally shorter traveltimes. 

Two additional long-term steady-state simulations were conducted to assess the possible effect of

changing heads in the Upper Floridan aquifer on

ground-water catchment size. The highest observed

Upper Floridan aquifer heads were about 3 ft higher

than the median levels, and occurred during about

10 percent of the 2-year study period. These levels are

comparable to a predevelopment condition. When the

steady-state Upper Floridan aquifer heads were raised

by 3 ft, the ground-water catchment area increased

from 16 to 20 percent of the topographic basin, groundwater inflow to the lake was 20 percent greater, and the

lake stage was 2.7 ft higher compared to the original

steady-state model. In contrast, the lowest observed

Upper Floridan aquifer heads were about 1 ft lower

than the median levels, and occurred during about

25 percent of the 2-year study period. This condition as

well as the median condition reflects the lower aquifer

heads induced by ground-water pumping for irrigation

to meet the needs of the citrus crop whenever rainfall is

insufficient for that purpose. When the steady-state

Upper Floridan aquifer heads were lowered by 1 ft, the

catchment area was reduced to 13 percent of the topographic basin, ground-water inflow was reduced by

15 percent, and lake stage was 0.51 ft lower compared

to the original steady-state model.

Short-term or seasonal variations in recharge and

Upper Floridan aquifer heads probably do not affect the

size of the contributing area as much as the long-term

average boundary conditions. Traveltimes along

ground-water flow paths to Lake Starr range from 20

days to 25 years, but flow from farther parts of the

catchment takes more than 10 years to reach the lake,

whereas short-term changes in boundary conditions

occur over periods of hours. According to Reilly and

Pollock (1995), changes in boundary conditions begin

to affect contributing areas when the time scale of the

change approaches the traveltime of ground-water flow

paths. However, variations in the recharge and Upper

Floridan aquifer boundary conditions over shorter periods of days to weeks can have a strong effect on the

volume of ground water that reaches the lake, which

will be discussed in the following section. 

To establish initial head conditions for transient

simulations, an additional steady-state simulation was

calibrated to heads measured in the Lake Starr basin

during May 1996. The RMSE between simulated and

observed heads for this simulation was 0.476 ft, and

simulated lake stage was 104.49 ft, which was 0.09 ft

less than observed. Simulated net ground-water inflow

was 17,500 ft3/d. Steady-state calibration was achieved

by setting recharge equal to 20 in/yr, lake recharge to

4 in/yr, and by specifying Upper Floridan aquifer heads

equal to levels similar to those observed during that

month. 

RESULTS OF TRANSIENT SIMULATIONS 

The objectives of transient simulations were to

quantify ground-water inflow and lake leakage and to

improve the understanding of the ground-water flow

system around central Florida seepage lakes and the

factors affecting lake/ground-water interactions.

A range of recharge rates to the surficial aquifer system

and head conditions in the Upper Floridan aquifer were

used in transient simulations over both monthly

and weekly stress periods. Hydraulic parameters specified in the transient simulations were initially

the same as steady-state simulations. Some parameters
30 Effects of Recharge, Upper Floridan Aquifer Heads, and Time Scale on Simulated Ground-Water Exchange with Lake Starr, a 

Seepage Lake in Central Florida

INACTIVE 

CELLS

INACTIVE 

CELLS

106

104

102

100

98

96

94

EXPLANATION

AREA THAT CONTRIBUTES GROUND WATER TO LAKE STARR

DISTANCE FROM ORIGIN, IN FEET

MODEL OUTLINE OF LAKE STARR

115

906030

NGVD

of 1929

 30 60

 100

-1,000 0 2,000 4,000 6,000 8,000 9,500

100 PARTICLE TRACKS-- Particles start at the water table in row 55. Arrowheads are 5 years apart LINE OF EQUAL SIMULATED HEAD-- In feet above NGVD of 1929

UPPER FLORIDAN AQUIFER 

UNSATURATED ZONE

MONITOR WELL MONITOR WELL

UNIT

CONFINING

INTERMEDIATE

FEET

SURFICIAL

AQUIFER 

SYSTEM

Figure 17. Particle pathlines showing direction, rates of movement, and vertical area that contributes ground water to the lake along model row 55.
Results of Transient Simulations 31

HEAD, IN FEET ABOVE NGVD OF 1929

105

106

107

108

109

110

111

112

1

1

8/1

8/31

10/1

0/31

12/1

2/31

1/31

3/2

4/2

5/2

6/2

7/2

8/2

9/1

10/2

11/1

12/2

1/1

2/1

3/3

4/3

5/3

6/3

7/3

OBSERVED HEAD

WTS-1

RECHARGE=35

PERCENT OF RAIN

THRESHOLD

RECHARGE

LEACHM RECHARGE

1996 1997 1998

(recharge and surficial aquifer K) were adjusted during

calibration of the transient model to improve simulation of ground-water exchange with the lake.

In general, errors in simulated heads were less than

errors in simulated net ground-water flows to the lake.

Results of transient simulations were sensitive to the

same parameters as steady-state simulations, as well as

to the specific yield of the surficial aquifer system. For

transient simulations, net ground-water flow to the lake

was computed as the sum of ground-water inflow and

lake leakage for each stress period, assuming that average net ground-water flow over the stress period was

equal to the net ground-water flow at the end of the

stress period. 

Monthly Simulations

Three recharge conditions (35 percent of rainfall,

threshold, and LEACHM) were used to simulate

monthly heads in the basin and ground-water exchange

with the lake. For each recharge condition, the Upper

Floridan aquifer lower boundary was represented by a

monthly average potentiometric surface. All of the three

recharge conditions did reasonably well simulating

heads in the basin over the 2-year study period,

although some recharge conditions did better than

others in certain parts of the basin. In general, recharge

estimated as a fixed percentage of rainfall over each

stress period generated head changes that were smaller

than observed, whereas the LEACHM method generated head changes that were larger than observed

(fig. 18). The threshold method did the best of the three

methods overall in simulating transient heads throughout the basin, but LEACHM recharge did nearly as

well. The LEACHM method is not as appropriate for

the upper basin, even though it was modified by applying a time delay, because it was designed for a shallow

water table (Hutson and Wagenet, 1992). However,

including LEACHM recharge in the model improved

the simulation of ground-water flow to and from the

lake compared to the other two recharge conditions

(fig 19). For these reasons, a combination of threshold

and LEACHM recharge methods was used in

subsequent transient simulations. The threshold

Figure 18. Effect of three different recharge boundaries on simulated head in well WTS-1 

(location of well shown in figure 6).
32 Effects of Recharge, Upper Floridan Aquifer Heads, and Time Scale on Simulated Ground-Water Exchange with Lake Starr, a 

Seepage Lake in Central Florida

-60,000

-40,000

-20,000

0

20,000

40,000

60,000

80,000

100,000

Aug

Sep

Oct

Nov

Dec

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

Jan

Feb

Mar

Apr

May

Jun

Jul

WATER BUDGET

WATER BUDGET ERROR

RECHARGE = 35 PERCENT RAIN

THRESHOLD RECHARGE

LEACHM RECHARGE

1996 1997 1998

NET GROUND-WATER FLOW, IN CUBIC FEET PER DAY

method was used to estimate recharge in the upper and

middle parts of the basin where the water table was

greater than 20 ft deep and the LEACHM recharge

method was used near the lake where the water table

was less than 20 ft deep.

Heads simulated using threshold recharge in the

upper basin and LEACHM recharge near the lake were

generally within 1 ft of the observed values (fig. 20A-D).

Simulated heads agreed best with observed heads on

the west side of the basin in the area that contributed

ground-water inflow to the lake (for example see well

WTS-23, fig. 20A). Agreement was not as close on the

southeast side of the basin where topography was highest (well WTS-8, fig 20D). Uncertainty regarding the

timing of recharge in the highest regions of the basin

probably explains this lack of agreement. 

Even with the combination of threshold and

LEACHM recharge methods, the model failed to simulate the highest lake levels and to predict peaks and

lows in net ground-water flow to the lake (figs. 21 and 22,

respectively). Simulated lake stage was consistently

lower than observed for the last half of 1996 and the

spring of 1997. Later in 1997, simulated stage was

below the peak stage that occurred in August. The large

rise and fall in stage between December 1997 and July

1998 was more accurately simulated, although the peak

lake stage in April 1998 was not achieved. 

Simulated net ground-water flow to the lake

agreed within the error bars of budget-derived estimates for only 14 of the 24 months (fig. 22). During

wet periods, the monthly simulation underpredicted net

ground-water inflow. For example, in March 1998, net

ground-water flow to the lake calculated from the

water budget was 83,300 (±13,700) ft3/d, but the simulated net flow was only 52,500 ft3/d. Accounting for

water budget errors, the simulated net ground-water

flow in March 1998 was between 54 and 75 percent of

net ground-water flow estimated from the water

budget. Simulated net ground-water flow fit the water

budget best when net ground-water flow was near zero;

periods when ground-water inflows and outflows were

balanced.

Figure 19. Effect of three different recharge boundaries on simulated monthly net ground-water 

flow to Lake Starr, and net ground-water flow from the water budget.
Results of Transient Simulations 33

113112111110109108107106105104103

8/1

8/31

10/1

10/31

12/1

12/31

1/31

3/2

4/2

5/2

6/2

7/2

8/2

9/1

10/2

11/1

12/2

1/1

2/1

3/3

4/3

5/3

6/3

7/3

WTS-23 OBSERVEDWTS-23 SIMULATEDWTS-24 OBSERVEDWTS-24 SIMULATED

1997 1998

114113112111110109108107106105104

8/1

8/31

10/1

10/31

12/1

12/31

1/31

3/2

4/2

5/2

6/2

7/2

8/2

9/1

10/2

11/1

12/2

1/1

2/1

3/3

4/3

5/3

6/3

7/3

8/1

8/31

10/1

10/31

12/1

12/31

1/31

3/2

4/2

5/2

6/2

7/2

8/2

9/1

10/2

11/1

12/2

1/1

2/1

3/3

4/3

5/3

6/3

7/3

WTS-4 OBSERVEDWTS-4 SIMULATEDWTS-5 OBSERVEDWTS-5 SIMULATED

1997 1998

111110109108107106105104103102101

8/1

8/31

10/1

10/31

12/1

12/31

1/31

3/2

4/2

5/2

6/2

7/2

8/2

9/1

10/2

11/1

12/2

1/1

2/1

3/3

4/3

5/3

6/3

7/3

WTS-13 OBSERVEDWTS-13 SIMULATEDWTS-12 OBSERVEDWTS-12 SIMULATED

1997 1998

110109108107106105104103102101100

WTS-8 OBSERVEDWTS-8 SIMULATEDWTS-9 OBSERVEDWTS-9 SIMULATED

1997

1996

1996

1996

1996 1998

ALTITUDE, IN FEET ABOVE NGVD OF 1929

ALTITUDE, IN FEET ABOVE NGVD OF 1929

(A) (B)(C) (D)

Figure 20. Observed and monthly simulated water levels in wells (A) WTS-23 and WTS-24, (B) WTS-4 and WTS-5, (C) WTS-13 and WTS-12, and 

(D) WTS-8 and WTS-9 (locations of wells shown in figure 6).
34 Effects of Recharge, Upper Floridan Aquifer Heads, and Time Scale on Simulated Ground-Water Exchange with Lake Starr, a 

Seepage Lake in Central Florida

103.5

104.0

104.5

105.0

105.5

106.0

106.5

107.0

1

1

8/1

8/31

10/1

0/31

12/1

2/31

1/31

3/2

4/2

5/2

6/2

7/2

8/2

9/1

10/2

11/1

12/2

1/1

2/1

3/3

4/3

5/3

6/3

7/3

OBSERVED LAKE STAGE

MONTHLY SIMULATED LAKE STAGE

1996 1997 1998

ALTITUDE, IN FEET ABOVE NGVD OF 1929

-60,000

-40,000

-20,000

0

20,000

40,000

60,000

80,000

100,000

Aug

Sep

Oct

Nov

Dec

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

Jan

Feb

Mar

Apr

May

Jun

Jul

WATER BUDGET

WATER BUDGET ERROR

STEADY-STATE SPECIFIED

UPPER FLORIDAN AQUIFER BOUNDARY

MONTHLY AVERAGE SPECIFIED

UPPER FLORIDAN AQUIFER HEAD BOUNDARY

1996 1997 1998

NET GROUND-WATER FLOW, IN CUBIC FEET PER DAY

Figure 21. Measured and monthly simulated stage for Lake Starr, August 1996 through 

July 1998.

Figure 22. Effect of different Upper Floridan aquifer specified-head boundary conditions on 

simulated monthly net ground-water flow to Lake Starr, and net ground-water flow from the 

water budget.
Results of Transient Simulations 35

For any given recharge condition, representing

the Upper Floridan aquifer specified-head boundary

with monthly averages instead of a steady-state potentiometric level improved the simulation of monthly

ground-water exchange and basin water levels.

To evaluate this effect, the model using the combined

threshold/LEACHM recharge condition was simulated

with the Upper Floridan aquifer lower specified-head

boundary as a steady-state condition. The results of this

model were then compared to the simulation that used

monthly average specified heads to represent the Upper

Floridan aquifer. Changes in water-table altitude were

less than observed (hydrographs were “flatter”) in the

simulation using a specified steady-state Upper Floridan aquifer lower boundary, and simulated net groundwater flow deviated more from the results of the lake

water budget (fig. 22). The amplitude of simulated net

ground-water flow was reduced when using specified

steady-state Upper Floridan aquifer heads, and net

ground-water flow was within the water budget error in

only 10 of 24 months, compared with 14 of 24 from the

simulation with monthly specified heads. When the

potentiometric surface was highest (March 1998),

ground-water inflow simulated using specified

monthly Upper Floridan aquifer heads was 52 percent

(21,300 ft3/d) greater than inflow simulated with a

steady-state boundary, and outflow was 47 percent

(8,700 ft3/d) less. As a result, net ground-water flow to

the lake was 133 percent (30,000 ft3/d) greater when

the monthly average Upper Floridan aquifer head

boundary was used. When the potentiometric surface

was lowest (June 1998), ground-water inflow simulated using specified monthly Upper Floridan aquifer

heads was 4 percent (1,200 ft3/d) less than inflow simulated using specified steady-state Upper Floridan aquifer heads, outflow was 15 percent (3,600 ft3/d) greater,

and net ground-water flow to the lake was 56 percent

(2,400 ft3/d) less. Substantial errors are introduced into

simulated monthly net ground-water flow if the lower

boundary is specified as steady state. 

Weekly Simulations

 Ground-water levels in the upper basin, where

the unsaturated zone was greater than about 40 ft thick,

did not respond to weekly changes in rainfall. Instead,

water levels in the upper basin rose and fell more gradually because of the time delay between rainfall and

recharge to the water table caused by greater thickness

of the unsaturated zone. Longer traveltimes and drainage of water stored in the unsaturated zone tended to

smooth out the effects of individual rain events on the

water table. Applying any of the recharge conditions on

a weekly basis worsened the simulation of transient

ground-water levels in the upper basin. For this reason,

recharge rates in the upper basin for weekly transient

simulations were averaged over a 4-week period, and

simulated heads in upper basin wells were consequently similar to those from monthly simulations.

Results of weekly simulations are therefore more

reflective of changes in recharge conditions in the basin

near the lake.

Weekly stress periods simulated lake stage more

accurately than monthly stress periods, primarily

because differences in rainfall and lake evaporation

were accounted for on a shorter time scale (figs. 23

and 21, respectively). The simulated rise in lake

stage during the winter of 1996-97 was less than

observed, similar to monthly simulations. But after

March 1997, the simulated changes in weekly lake

stage followed observed changes more closely than

monthly simulations. 

Unlike monthly simulations, weekly recharge

conditions simulated the occurrence of transient watertable mounds that were observed near the shoreline of

Lake Starr. For example, in the weekly simulation,

heads in the nearshore area occasionally rose above the

lake level because of large rain events (fig. 23). These

peaks in nearshore water levels reversed the groundwater flow direction on the outflow (southeast) side of

the lake (for example, near well WTS-13). Flow reversals were simulated at the same times that flow reversals were observed in continuous water-level recorders.

For example, after a series of large rainfall events the

week of July 27-August 2, 1997, the simulated head at

the end of the stress period for a cell on the southeast

side of the lake near the 2PNS well nest was 0.25 ft

higher than the lake. Observed flow reversals were

higher but more short-lived than simulated reversals;

the observed head in a nearshore well rose to 0.5 ft

above lake stage on August 2, 1997, but the flow reversal lasted only about 6 hours (Swancar and others,

2000). 

Weekly stress periods simulated a larger variability in net ground-water flow to the lake compared to

monthly stress periods (fig. 24). Simulated weekly net

ground-water flow rates ranged from –29,400 to
36 Effects of Recharge, Upper Floridan Aquifer Heads, and Time Scale on Simulated Ground-Water Exchange with Lake Starr, a 

Seepage Lake in Central Florida

103.0

103.5

104.0

104.5

105.0

105.5

106.0

106.5

107.0

OBSERVED LAKE STAGE

SIMULATED LAKE STAGE

SIMULATED HEAD IN WELL WTS-13

FLOW REVERSALS

1996 1997 1998

ALTITUDE, IN FEET ABOVE NGVD OF 1929

8/1

8/3110/1

10/31

12/1

12/31

1/31

3/24/25/26/27/28/29/1

10/211/112/2

1/12/13/34/35/36/37/3

77,400 ft3/d, whereas monthly rates ranged from

–29,800 to 52,500 ft3/d. Weekly simulated net groundwater flow was within the error of budget-derived estimates in 46 of 105 weeks. Weekly simulation increased

the peak net ground-water inflow rates compared to

monthly, but peak net ground-water outflows were

similar to monthly rates. However, the simulated

weekly net ground-water flow rates were still well

below the minimum and maximum rates derived from

a weekly water budget (–59,700 and 117,500 ft3/d,

respectively). 

Simulated net ground-water flow did not alternate from large net ground-water inflow to large net

leakage as quickly as indicated by the weekly water

budget. For example, during the fall of 1996, budgetderived net ground-water flow to the lake dropped

rapidly from 64,600 to –30,000 ft3/d, a decrease of

94,600 ft3/d, over the 5-week period October 6 through

November 10 (fig. 24A, see inset). The weekly simulated net ground-water flow fell from 30,100 to

4,900 ft3/d, a decrease of only 25,200 ft3/d, during the

same 5-week period. The weekly water budget

indicated that ground-water inflow rates were larger

than simulated rates and that net ground-water flow

rates changed more rapidly than simulated rates.

Like monthly simulations, weekly simulations

underestimated net ground-water inflow derived from

the water budget. Problems simulating ground-water

inflow rather than outflow were the primary cause of

underestimated net ground-water flow. Gross groundwater inflow should always exceed positive net

ground-water flow, yet simulated gross ground-water

inflow was commonly lower than the net flow derived

from the water budget, even when errors in the water

budget were considered. For example, simulated gross

ground-water inflow was less than budget-derived net

ground-water flow in 16 of the 31 weeks when net

ground-water flow was both positive and larger than

the water budget error. The discrepancy between the

simulated and budget-derived weekly net inflows was

greatest during weeks with large rainfall events and

large ground-water inflows to the lake (weeks of

Figure 23. Weekly observed and simulated Lake Starr stage and simulated head in well 

WTS-13 showing flow reversals, August 1996 through July 1998 (location of well shown on 

figure 6). 
Results of Transient Simulations 37

-100,000

-50,000

0

50,000

100,000

150,000

200,000

7/27/968/31/9610/5/96

11/9/96

12/14/96

1/18/972/22/973/29/97

5/3/976/7/97

7/12/978/16/97

WATER BUDGET

10/6/96 - 11/10/96

WEEKLY SIMULATION

HIGHER RECHARGE SIMULATION

WATER-BUDGET ERROR

-100,000

-50,000

0

50,000

100,000

150,000

200,000

8/23/979/27/97

11/1/97

12/6/971/10/982/14/983/21/984/25/985/30/98

7/4/98

NET GROUND-WATER FLOW, IN CUBIC FEET PER DAY 

(A)

WATER BUDGET

WEEKLY SIMULATION

HIGHER RECHARGE SIMULATION

(B)

Figure 24. Weekly simulated net ground-water flow to Lake Starr, net ground-water flow from 

the higher recharge simulation, and net ground-water flow from the water budget from (A) July 

28, 1996, to August 17, 1997, and (B) August 24, 1997, to July 26, 1998 (values plotted on the 

first day of the period).
38 Effects of Recharge, Upper Floridan Aquifer Heads, and Time Scale on Simulated Ground-Water Exchange with Lake Starr, a 

Seepage Lake in Central Florida

August 17, 1996, July 27, 1997, and July 12, 1998, in

fig. 24). This discrepancy may reflect the inability of

the numerical model to adequately simulate the influence of transient mounding on ground-water inflow.

Transient water-table mounding in nearshore wells

could have generated half of the net ground-water flow

to the lake during the week of July 27, 1997 (Swancar

and others, 2000).

The representation of specified weekly Upper

Floridan aquifer heads in the transient model had a

smaller effect on the magnitude of simulated net

ground-water flow than the representation of weekly

recharge, but the effect was still noticeable. For example,

during a 2-week period with low recharge (March 2-16,

1997), a decline in the average head in the Upper Floridan aquifer on the north side of the lake of 2.12 ft (see

fig. 14) in the second week increased the lake leakage

22 percent, from 31,300 to 38,300 ft3/d. This was the

largest week-to-week decline in the average Upper

Floridan aquifer head on this side of the lake during the

study. Ground-water inflow during the second week

also was affected by the change in the Upper Floridan

aquifer boundary condition, dropping 41 percent from

15,400 to 9,100 ft3/d. Net ground-water flow from the

lake in the second week was 85 percent (13,400 ft3/d)

lower than the previous week, falling from -15,800

to –29,200 ft3/d.

Higher Recharge Weekly Simulation

Both monthly and weekly simulations underestimated net ground-water inflows to Lake Starr during

wet periods, and underestimated peak net ground-water

outflows during dry periods. Underestimating groundwater inflow is likely due to an underestimation of

recharge during wet periods. In addition, the weekly

water budget indicated that the week-to-week changes

in ground-water inflow and lake leakage could be

considerably larger than simulated, suggesting faster

exchange of water between the lake and the surrounding aquifer.

To test these hypotheses, a weekly simulation

was run using higher recharge rates, greater Kh in the

surficial aquifer system adjacent to the lake, and

greater K in the sublake breaches. Simulations using

higher recharge rates basin-wide raised the water table

unrealistically high in the upper basin. Higher heads in

the upper basin did not substantially increase groundwater inflow to the lake because most of the groundwater inflow results from recharge occurring closer to

the lake. Therefore, recharge in the upper basin was not

increased, as heads in this area were being accurately

simulated. Instead, the higher recharge simulation

tested the effect of increasing recharge only in the area

closest to the lake. This higher recharge simulation was

evaluated for its ability to reproduce the weekly net

ground-water flow derived from the water budget. 

In the higher recharge simulation, recharge in the

zone closest to the lake was increased to 86 percent of

the 2-year rainfall total, recharge in the second recharge

zone was increased to 50 percent of rainfall, and

recharge in the upper basin was kept at 35 percent of

the 2-year rainfall total. Recharge for both of the nearshore zones was calculated using the unsaturated flow

model LEACHM. 

There is no precedent for applying recharge at

greater than 50 percent of rainfall over longer time

periods; however, this assumption allows us to test the

effect of very high recharge rates in some weeks on

ground-water flow to the lake. Recharge rates in sandy

ridge soils and their annual variability are not well

known, and may exceed the initial estimate of 35

percent of rainfall. For example, Sumner (1995)

reported that between 43 and 53 percent of the rainfall

recharged the ground-water system in a Florida ridge

setting where the depth to water was less than 10 ft and

the annual rainfall amount was near average. The

higher recharge rates used in the nearshore for this

study are consistent with results of Lee (2000), who

found that daily recharge greater than 90 percent of

rainfall is needed to generate the short-term water-table

rise in nearshore wells during individual storm events. 

In addition to increasing the nearshore recharge,

two other changes were made in the higher recharge

simulation to more closely match the variable timing

and magnitude of weekly net ground-water flow seen

in the water budget. The Kh of the surficial aquifer

system in the lower basin was increased from 40 ft/d to

60 or 80 ft/d, and larger and more conductive breaches

were placed in the intermediate confining unit beneath

the lake. The size of sublake breaches was increased

from about 10 to 18 acres, and the K of breaches was

doubled. Larger breaches below the lake are plausible,

as these features were estimated to total at least 16

acres in size from a seismic reflection survey of the

lake (Swancar and others, 2000). The higher Kh values

in the surficial aquifer in the high recharge simulation

are not far outside the range of measured values; 80 ft/d

is about 25 percent greater than the highest measured K

(64 ft/d). 
Results of Transient Simulations 39

0

2

4

6

8

10

12

14

16 8/1

8/31

10/1

10/31

12/1

12/31

1/31

3/2

4/2

5/2

6/2

7/2

8/2

9/1

10/2

11/1

12/2

1/1

2/1

3/3

4/3

5/3

6/3

7/3

WEEKLY HIGH RECHARGE

WEEKLY LOW RECHARGE

WEEKLY WATER BUDGET

1996 1998 1997

CUMULATIVE ABSOLUTE ERROR IN

NET GROUND-WATER FLOW, IN MILLION CUBIC FEET

The higher recharge simulation approximated the

peak weekly values of net ground-water flow from the

water budget more closely than the original weekly

simulation (fig. 24). Simulated net ground-water flow

was within the water-budget errors for 51 of 105 weeks.

The higher recharge simulation also was better at

producing the relatively rapid transitions from net

ground-water inflow to net ground-water outflow, as

well as the steep increases in net ground-water outflow.

The improved ability to simulate budget-derived drops

in net ground-water flow suggests that lake leakage and

the K of the sublake region were better represented in

the high recharge simulation than the original model.

For example, in the high recharge simulation, weekly

average net ground-water flow drops from 58,000

to –9,300 ft3/d during the 5-week period between October 6 and November 10, 1996. The total simulated

decline in net ground-water flow of 67,300 ft3/d from

the high recharge simulation compares more closely to

the budget-derived decline of 94,600 ft3/d than the

initial weekly simulation (25,200 ft3/d)(fig. 24A inset).

The steeper decline in simulated net ground-water flow

is governed by both the greater amount of ground-water

inflow and by the more conductive sublake geology. 

In addition to improving the representation of the

large week-to-week variations in the net ground-water

flow to Lake Starr, the higher recharge simulation was

the only transient model that approximated the cumulative net ground-water exchange calculated by the 2-year

water budget. To quantify the error in simulated groundwater exchange with Lake Starr, the absolute value of

the difference between the budget-derived and simulated net ground-water flow was calculated for each

stress period, and these differences were accumulated

for the 2-year study period. For comparison, the water

budget error was accumulated for the same period.

Comparing the cumulative absolute errors of the different simulations is more telling than comparing the total

net ground-water exchanges with the lake because a

simulation can generate the same annual net groundwater exchange as the water budget without simulating

the extremes of ground-water exchange with the lake.

The cumulative absolute error was substantially less for

the higher recharge simulation than for the lower

recharge simulation (fig. 25). For the 2-year period,

the cumulative absolute error was 17,057 ft3/d

(12,451,400 ft3 total) for the higher recharge simulation,

which was about the same as the weekly average error

Figure 25. Cumulative absolute error in net ground-water flow for two simulations 

and the weekly lake water budget.
40 Effects of Recharge, Upper Floridan Aquifer Heads, and Time Scale on Simulated Ground-Water Exchange with Lake Starr, a 

Seepage Lake in Central Florida

in the water budget of 17,072 ft3/d, and well below the

low recharge simulation error of 20,057 ft3/d

(14,641,700 ft3 total). When applied over monthly stress

periods, the higher recharge simulation also had a

substantially lower cumulative absolute error than the

lower recharge simulation and also was about the same

as the monthly water budget error. 

The higher recharge simulation did an excellent

job predicting lake stage during the first year of the

study, but simulated lake stage was higher than

observed during most of the second year (fig. 26A).

After the minor peak in lake stage in August 1997, the

simulated stage did not drop as quickly as observed

during the fall of 1997. The simulated stage remained

about 0.5 ft higher than observed for the remainder of

the period, although rises and falls were equivalent to

observed changes. An adjusted stage (simulated stage

minus 0.5 ft) also is plotted on figure 26 for comparison. An explanation for the better fit of the adjusted

stage to the lake hydrograph is that the model did well

simulating lake/ground-water interactions during most

of the study period, but generated too much groundwater inflow during the summer of 1997 (this additional simulated ground-water inflow also is evident in

figure 24). Simulated heads in nearshore wells showed

more transient peaks in the higher recharge simulation

because of the large recharge inputs to the water table

in this part of the model, and also were higher than

observed values in the second year, similar to lake

stage (fig. 26B). Peaks that were of the same magnitude as those simulated (0.5-1.0 ft) were observed in

continuous measurements of water levels in wells near

Lake Starr during this period (Swancar and others,

2000). Water-level rises up to 3 ft have been observed

in nearshore wells around lakes about 60 miles west of

Lake Starr (Metz and Sacks, 2002). 

The high recharge condition had little effect on the

catchment contributing ground-water inflow to the lake.

A steady-state simulation using the higher recharge

model K distribution and recharge estimates produced a

contributing area that was the same size as the original

long-term steady-state simulation. If higher recharge and

increased K were applied basin-wide, the size of the

contributing area would be expected to increase (Lee,

2002). In the higher recharge simulation, however,

changes in both recharge and hydraulic parameters were

made primarily close to and beneath the lake, and therefore would not affect the contributing area as much.

Offsetting the potential effects of increased K and

recharge on the catchment size is an increase in flow that

bypasses the lake through the more conductive surficial

aquifer system and sublake breaches. 

A much greater volume of ground water flows

through the lake under the higher recharge condition.

The gross ground-water inflow predicted by the higher

recharge simulation for the 2-year period was

36,501,600 ft3 (74.2 inches), 59 percent greater than

the gross inflow predicted by the lower recharge simulation for this same period of 22,965,100 ft3 (46.7

inches). Lake leakage from the higher recharge simulation was 26,503,100 ft3 (53.9 inches) over the 2-year

period, 72 percent greater than leakage from the lower

recharge simulation of 15,408,400 ft3 (31.3 inches).

Lake leakage was 14,388,500 and 12,114,600 ft3 per

year for the first and second years, respectively (29.3

and 24.6 in/yr, respectively).

In summary, the higher recharge weekly simulation provided the best agreement with the water

budget-derived estimates of ground-water exchange

with Lake Starr over the 2-year budget period. The

higher recharge simulation also was able to accurately

reproduce both peak net ground-water inflow and

outflow, and the transition between net inflow and net

outflow. However, the higher recharge simulation

generated too much inflow during the summer of 1997,

so that simulated lake stage after this point was about

0.5 ft higher than observed. 

Combined Approach for Estimating 

Ground-Water Inflow 

The results of the higher recharge weekly simulation were coupled with water budget information

from Lake Starr in a combined approach for estimating

the ground-water inflow to Lake Starr. In the higher

recharge weekly simulation, as in all of the simulations,

lake leakage was a saturated flow process where leakage was directly proportional to the hydraulic conductivity of the material around the lake and the vertical

head difference between the lake and the Upper Floridan aquifer beneath the lake. The saturated groundwater flow model can represent leakage effectively.

Ground-water inflow, in contrast to lake leakage, was a

nonlinear process affected by the highly variable

recharge boundary. This process was more difficult to

simulate. In the combined approach, simulated lake
Results of Transient Simulations 41

103.5

104.0

104.5

105.0

105.5

106.0

106.5

107.0

107.5

SIMULATED

OBSERVED

ADJUSTED

(A) LAKE STAGE

103.0

103.5

104.0

104.5

105.0

105.5

106.0

106.5

107.0

107.5

8/1

8/31

10/1

10/31

12/1

12/31

1/31

3/2

4/2

5/2

6/2

7/2

8/2

9/1

10/2

11/1

12/2

1/1

2/1

3/3

4/3

5/3

6/3

7/3

1996 1997 1998

SIMULATED-HIGHER RECHARGE

SIMULATED-LOWER RECHARGE

OBSERVED

(B) HEAD IN WELL WTS-13

ALTITUDE, IN FEET ABOVE NGVD OF 1929

Figure 26. (A) Weekly simulated, observed, and adjusted stage of Lake Starr, and (B) weekly 

simulated and observed head in well WTS-13 (adjusted lake stage is 0.5 foot less than 

simulated).
42 Effects of Recharge, Upper Floridan Aquifer Heads, and Time Scale on Simulated Ground-Water Exchange with Lake Starr, a 

Seepage Lake in Central Florida

-100,000

-50,000

50,000

100,000

150,000

8/1

8/31

10/1

10/31

12/1

12/31

1/31

3/2

4/2

5/2

6/2

7/2

8/2

9/1

10/2

11/1

12/2

1/1

2/1

3/3

4/3

5/3

6/3

7/3

Calculated Ground-Water Inflow (Gi)

from Combined Approach

Simulated Lake Leakage (Go)

1996 1997 1998

GROUND-WATER FLOW, IN CUBIC FEET PER DAY

0

leakage was substituted into the water budget equation

and ground-water inflow is calculated as the new residual term as follows:

Gi = ∆S – P + E + Q + Go ± e∆S ±eP ± eE ± eQ (4)

This method eliminates some of the problems

estimating recharge inputs, but maintains the increased

“leakiness” of the sublake ground-water flow system

that was needed to simulate the transition from net

ground-water inflows to outflows. When expressed as

linear units over the lake surface area, simulated leakage is divided by a fixed lake area, as are other simulated fluxes to and from the lake. Inflow calculated

using the combined approach is relative to a variable

surface area similar to other water budget terms. By

definition, net ground-water flow is the same as the

water budget. 

Weekly ground-water inflows calculated using

the combined approach varied over a larger range of

values than simulated lake leakage (outflows) (fig. 27

and table 5). Weekly ground-water inflow was generally inversely related to outflow. Weeks with increased

ground-water inflow were usually accompanied by

reduced outflow, with two factors contributing to this

effect. First, flow reversals associated with increased

ground-water inflow can temporarily reduce the

percentage of the perimeter experiencing lake leakage

(Lee, 2000). Secondly, and probably more importantly,

vertical lake leakage can be reduced because pumping

from the Upper Floridan aquifer for irrigation is

reduced during wet weeks, raising heads in the Upper

Floridan aquifer. During the wet season, these effects

continue for many weeks at a time. Using the combined

approach, the annual ground-water inflow was

16,327,200 and 21,033,900 ft3 for the first and second

years of the study, respectively (33.0 and 41.7 in/yr,

respectively). Ground-water inflow was larger and lake

leakage was smaller in the second year compared to the

first. As a result, net ground-water flow was 13.7

inches greater the second year (3.7 and 17.4 in/yr for

the first and second years, respectively). Note, net

ground-water values are slightly different from those

presented in Swancar and others (2000) because of

differences in the way volumes are converted to linear

units over the lake surface area.

Figure 27. Weekly calculated ground-water inflow to and simulated leakage from Lake Starr 

during the 2-year study period.
Results of Transient Simulations 43

Table 5. Weekly inflows to and outflows from Lake Starr, with ground-water inflows calculated using a combined 

approach, July 20, 1996, through August 1, 1998 

[All units cubic feet per day unless otherwise noted; GW, ground water; %, percent; most weeks are 7 days in length, but periods vary from 5 to 

13 days; calculated inflow set to zero if negative]

 Week begin

date

INFLOWS OUTFLOWS

Rain Calculated 

GW Inflow

GW inflow

 as % of total Evaporation Lake pumping Simulated GW outflow

GW outflow

 as % of total

07/20/96 1,423 38,470 96 119,072 3,153 30,375 20

07/28/96 189,606 42,436 18 121,042 4,787 29,073 19

08/04/96 217,090 79,158 27 99,346 0 23,917 19

08/11/96 195,302 134,681 41 103,533 0 24,825 19

08/18/96 8,263 81,343 91 112,445 1,224 28,816 20

08/25/96 46,345 104,950 69 91,825 1,224 31,832 25

09/01/96 109,278 65,761 38 102,203 1,225 27,477 21

09/08/96 130,618 76,429 37 85,157 0 24,310 22

09/15/96 74,831 88,005 54 88,015 0 24,910 22

09/20/96 39,039 64,142 62 103,749 961 26,199 20

09/29/96 84,345 73,990 47 79,044 6,174 28,540 25

10/06/96 136,180 86,487 39 90,944 0 21,859 19

10/14/96 19,655 73,610 79 68,579 7,274 27,918 27

10/20/96 0 59,169 100 70,840 8,710 33,530 30

10/27/96 2,510 52,938 95 59,374 8,683 41,102 38

11/03/96 30,057 20,379 40 83,265 2,473 44,886 34

11/10/96 0 14,754 100 87,687 6,138 44,774 32

11/17/96 0 29,360 100 40,984 8,560 50,100 50

11/24/96 56,883 23,990 30 56,532 2,439 48,168 45

12/01/96 211,048 59,976 22 43,309 0 33,950 44

12/08/96 0 54,111 100 48,818 1,229 39,492 44

12/15/96 2,485 47,808 95 70,158 8,591 42,136 35

12/22/96 0 54,906 100 18,473 8,564 45,462 63

12/29/96 0 67,589 100 29,348 8,559 47,253 55

01/05/97 47,034 33,572 42 45,092 4,886 48,177 49

01/12/97 17,307 21,320 55 62,806 0 43,811 41

01/21/97 6,887 34,252 83 18,938 8,499 50,268 65

01/26/97 19,664 43,910 69 39,046 3,639 46,959 52

02/02/97 4,907 43,229 90 31,253 4,843 46,724 56

02/09/97 21,206 25,112 54 34,725 7,249 47,576 53

02/16/97 34,269 37,700 52 54,309 0 43,535 44

02/23/97 813 12,971 94 50,144 8,422 49,679 46

03/02/97 0 18,162 100 74,811 8,380 54,258 39

03/09/97 106,960 0 0 67,926 4,764 57,048 44

03/16/97 54,780 16,696 23 82,120 0 48,498 37

03/23/97 12,821 35,604 74 69,134 3,557 51,395 41

03/30/97 0 6,623 100 109,363 8,257 55,621 32

04/06/97 42,276 19,346 31 83,756 7,034 61,588 40

04/13/97 64,899 27,702 30 87,770 0 45,879 34

04/20/97 165,221 31,446 16 72,121 2,333 39,991 35

04/27/97 67,964 47,680 41 67,195 0 40,198 37

05/04/97 0 10,372 100 100,229 7,011 50,856 32

05/11/97 37,856 23,570 38 77,574 2,503 46,954 37

05/24/97 55,476 21,800 28 104,345 1,012 42,439 29

06/01/97 41,642 4,906 11 93,927 0 40,695 30

06/08/97 275,973 46,086 14 100,500 0 28,614 22

06/15/97 52,767 57,092 52 92,145 0 33,903 27

06/22/97 121,288 57,895 32 105,364 0 33,306 24

06/29/97 89,491 22,962 20 104,240 0 32,535 24

07/06/97 208,152 44,358 18 97,390 0 24,872 20
44 Effects of Recharge, Upper Floridan Aquifer Heads, and Time Scale on Simulated Ground-Water Exchange with Lake Starr, a 

Seepage Lake in Central Florida

07/13/97 108,983 30,715 22 95,827 0 27,499 22

07/20/97 38,519 22,454 37 95,293 0 31,049 25

07/27/97 416,592 127,686 23 111,144 0 19,169 15

08/03/97 94,563 26,629 22 90,124 0 22,656 20

08/10/97 142,956 30,233 17 92,414 0 21,744 19

08/17/97 3,694 36,384 91 103,200 2,372 27,154 20

08/24/97 11,060 16,283 60 106,094 8,274 29,972 21

08/31/97 98,407 22,300 18 111,223 1,178 24,937 18

09/07/97 22,707 30,964 58 85,735 3,528 30,723 26

09/14/97 0 14,440 100 86,063 8,200 35,432 27

09/21/97 174,329 29,240 14 85,027 3,501 32,819 27

09/28/97 730 41,515 98 83,441 1,169 31,658 27

10/05/97 50,927 21,059 29 94,689 0 34,584 27

10/12/97 5,788 28,578 83 72,757 4,649 38,365 33

10/19/97 723 38,566 98 79,394 8,112 40,743 32

10/26/97 191,533 36,693 16 50,547 1,157 30,633 37

11/02/97 28,294 39,122 58 70,275 0 29,703 30

11/09/97 187,442 49,744 21 43,837 4,647 25,076 34

11/16/97 0 32,866 100 48,547 2,337 31,285 38

11/23/97 727 67,150 99 40,309 8,175 35,787 42

11/30/97 67,676 33,851 33 46,697 4,670 33,767 40

12/07/97 319,721 67,243 17 24,175 0 22,173 48

12/14/97 65,148 42,283 39 58,891 0 23,273 28

12/21/97 115,231 71,418 38 2,197 5,947 25,901 76

12/28/97 4,469 34,416 89 59,292 1,194 29,457 33

01/04/98 75,408 77,133 51 0 4,781 28,325 86

01/11/98 53,173 47,682 47 39,122 0 27,416 41

01/18/98 156,539 62,893 29 22,937 1,203 22,415 48

01/25/98 2,267 40,935 95 48,198 2,424 27,329 35

02/01/98 149,233 48,763 25 60,627 1,212 22,906 27

02/08/98 27,021 65,551 71 40,042 4,873 30,152 40

02/15/98 428,354 137,341 24 28,532 0 19,880 41

02/22/98 72,878 97,913 57 65,396 0 23,433 26

03/01/98 9,435 110,303 92 72,377 1,263 27,821 27

03/08/98 106,561 84,774 44 101,219 1,264 24,704 19

03/15/98 346,235 110,990 24 60,235 3,807 21,191 25

03/22/98 0 107,198 100 60,256 2,600 25,497 29

03/29/98 3,246 139,412 98 64,292 9,119 31,486 30

04/05/98 0 76,659 100 98,899 9,110 34,668 24

04/12/98 0 88,696 100 89,407 9,070 37,199 27

04/19/98 6,445 70,947 92 103,488 9,041 39,726 26

04/26/98 46,353 82,586 64 81,219 5,145 42,575 33

05/03/98 20,850 96,278 82 80,409 0 36,719 31

05/10/98 0 60,294 100 115,046 7,702 39,873 25

05/17/98 0 45,135 100 113,327 8,922 42,915 26

05/24/98 5,512 54,860 91 79,634 8,864 45,256 34

05/31/98 787 49,096 98 117,385 8,817 51,217 29

06/07/98 2,342 35,415 94 128,051 8,741 54,339 28

06/14/98 0 37,464 100 119,831 8,666 60,719 32

06/21/98 13,729 28,962 68 114,359 8,594 61,098 33

Table 5. Weekly inflows to and outflows from Lake Starr, with ground-water inflows calculated using a combined 

approach, July 20, 1996, through August 1, 1998 (Continued)

[All units cubic feet per day unless otherwise noted; GW, ground water; %, percent; most weeks are 7 days in length, but periods vary from 5 to 

13 days; calculated inflow set to zero if negative]

 Week begin

date

INFLOWS OUTFLOWS

Rain Calculated 

GW Inflow

GW inflow

 as % of total Evaporation Lake pumping Simulated GW outflow

GW outflow

 as % of total
Results of Transient Simulations 45

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0

-6 -4 -2 0 2 4 6

y = -0.20x - 2.22

R2 = 0.86

AVERAGE HEAD DIFFERENCE BETWEEN

LAKE STARR AND ROMP 57 WELL, IN FEET

SIMULATED LAKE LEAKAGE,

IN INCHES PER MONTH

06/28/98 5,320 32,565 86 122,047 4,875 50,916 29

07/05/98 123,979 64,297 34 103,974 4,853 44,591 29

07/12/98 281,519 108,360 28 96,297 0 29,356 23

07/19/98 96,220 57,665 37 96,993 0 30,196 24

07/26/98 5,372 43,416 89 108,895 7,391 38,987 25

Total:

August 1996-July 1998, 

in cubic feet

51,482,377 37,361,090 42 55,932,446 2,624,855 26,503,093 31

Table 5. Weekly inflows to and outflows from Lake Starr, with ground-water inflows calculated using a combined 

approach, July 20, 1996, through August 1, 1998 (Continued)

[All units cubic feet per day unless otherwise noted; GW, ground water; %, percent; most weeks are 7 days in length, but periods vary from 5 to 

13 days; calculated inflow set to zero if negative]

 Week begin

date

INFLOWS OUTFLOWS

Rain Calculated 

GW Inflow

GW inflow

 as % of total Evaporation Lake pumping Simulated GW outflow

GW outflow

 as % of total

Over the 2-year study, ground-water inflow

calculated using the combined approach was about

37,361,100 ft3 and accounted for 42 percent of the total

input of water to the lake, while 58 percent of inflow

was from rainfall (table 5). During the relatively dry

months of May and June 1998, ground-water inflow

provided over 90 percent of the total inflow to the lake.

During the study period, simulated lake leakage was

about 26,503,100 ft3 and accounted for 31 percent of

the total losses of water from the lake, while evaporation was 66 percent and direct pumping was 3 percent.

Calculated ground-water inflow and simulated lake

leakage at Lake Starr were on the high end of the range

of possible values calculated by Sacks and others

(1998) using an isotopic mass balance, and were more

than double the estimates of minimum inflow and

outflow from Swancar and others (2000). 

An adaptation of this combined approach can be

used to calculate ground-water exchange with Lake

Starr beyond the model period. Simulated leakage

values are not required; instead, a statistical regression

can be used to estimate lake leakage. This adaptation

requires that a lake water budget and continuous head

measurements in the Upper Floridan aquifer are available for the time period of interest. Simulated lake

leakage was highly correlated to the average head

difference between the lake and a continuously recording Upper Floridan aquifer well outside the basin

(ROMP 57, fig. 28). This relation allows the monthly

lake leakage to be estimated from the vertical

head difference. The predictive equation explained

86 percent of the variance in the simulated leakage,

and had a standard error of 0.23 inches, or about

11 percent of the average monthly value of lake leakage.

The remaining variability in simulated leakage that

could not be explained by the regression is probably

related to the use of a single average water level to

represent the Upper Floridan aquifer potentiometric

surface beneath the lake. In contrast to the good relation for lake leakage, calculated ground-water inflow

showed poor predictive relations with rainfall or net

precipitation (r2 = 0.12 and 0.11, respectively). 

Figure 28. Relation between simulated leakage from Lake 

Starr and monthly average head difference between Lake 

Starr and ROMP 57 Upper Floridan aquifer well, August 

1996 through July 1998. 
46 Effects of Recharge, Upper Floridan Aquifer Heads, and Time Scale on Simulated Ground-Water Exchange with Lake Starr, a 

Seepage Lake in Central Florida

Because of the correlation of leakage to the head

difference between the lake and the Upper Floridan

aquifer, lake leakage estimated by the regression can be

substituted into the water budget, and ground-water

inflow can be calculated as the new residual term to the

water budget equation. This approach was used to estimate the ground-water inflow to Lake Starr beyond the

model period in a study using isotopic tracers to understand lake/ground-water interactions (Sacks, 2002). 

EFFECTS OF BOUNDARY CONDITIONS 

AND TIME SCALE ON GROUND-WATER 

EXCHANGE

Improving the simulation of transient boundary

conditions in ground-water flow models can improve

the simulation of ground-water exchange with lakes.

Understanding how the recharge boundary affects

simulated ground-water exchange is important because

lack of information on recharge is a common model

limitation. Understanding how ground-water exchange

with Lake Starr is controlled by the lower specifiedhead boundary is important because regulated groundwater pumping largely determines the specified head.

Investigating how differences in model time scale

affect ground-water exchange with lakes makes it

possible to determine the time scale of processes that

affect lake/ground-water interactions. 

Recharge Boundary

Representing unsaturated zone processes in the

recharge boundary of the saturated flow model

improved the simulation of ground-water levels and

ground-water exchange with Lake Starr. The use of

LEACHM in the nearshore region of the basin, where

the water table was relatively shallow, provided more

realistic timing of recharge, and magnified the recharge

contribution of intense rainfall periods. 

Using the lower of two recharge estimates,

neither monthly nor weekly simulations came close to

estimating the budget-derived estimates of net groundwater flow to Lake Starr. This result was true regardless of the temporal variability of the Upper Floridan

aquifer boundary condition. However, the weekly

model, like the weekly water budget, did reveal rapid

changes in magnitude and direction (net gain or loss) of

net ground-water flow on a week-to-week basis. The

weekly simulation also was capable of generating periodic flow reversals along the outflow side of the lake. 

Calibrating the weekly simulation to the net

ground-water flow seen in the weekly water budget

provided insight into recharge processes and the hydrogeology of Lake Starr that were lacking in the monthly

simulation. The simulated and budget-derived estimates of annual net ground-water flow to Lake Starr

began to converge only when the weekly recharge estimates and the aquifer hydraulic conductivity around

the lake were substantially increased. With these

changes, the model was able to accurately simulate the

larger and more rapid changes in net ground-water flow

on a week-to-week basis. 

Despite the improved simulation of periods of

peak ground-water inflows and outflows, the higher

recharge simulation still had difficulty consistently

predicting the smaller variations in weekly net groundwater flow. The model may have been unable to consistently simulate the weekly inflows because it could not

simulate the recharge and water-table response on a

daily or shorter time scale. Averaging the daily

recharge rates calculated by LEACHM over a weeklong stress period greatly decreased the maximum

daily recharge rate applied for the week (table 6).

Table 6. Maximum daily, weekly, and monthly average recharge rates simulated by the LEACHM model

[Maximum rates are in inches per day; 35% P, soil characteristics adjusted so that LEACHM model delivers total 2-year 

recharge equal to 35 percent of rainfall; 86% P, total 2-year recharge is equal to 86 percent of rainfall; most weeks are 

7 days in length, but periods vary from 5 to 13 days] 

Month

Maximum daily rate Maximum weekly rate Maximum monthly rate

35% P 86% P 35% P 86% P 35% P 86% P

June 1996 1.30 2.58 0.46 0.82 0.17 0.36

July 1997 1.39 2.09 0.60 0.83 0.16 0.29

February 1998 1.96 2.00 0.71 0.83 0.24 0.32
Effects of Boundary Conditions and Time Scale on Ground-Water Exchange 47

Yet, variably saturated flow modeling of nearshore

mound formation in a similar setting and with daily

stress periods indicated that 80-90 percent of large

daily rainfall events had to recharge the shallow water

table to produce the observed water-table rise. These

large daily events also were responsible for generating

substantial fluxes of net ground-water inflow (Lee,

2000). Averaging daily recharge over a monthly stress

period reduced the maximum recharge rates further,

effectively masking the impact of these transient

processes. 

For a shallow water table (for example less than

6 ft), recharge from a given rainfall event could

approach the majority of the net precipitation (rainfall

minus daily evapotranspiration; Wu and others, 1996;

Lee, 2000). However, it is unlikely that the long-term

or annual average recharge is as high as 86 percent of

rainfall in the closest nearshore zone defined for the

lake basin. The fact that this amount of weekly

recharge was required to calibrate the weekly simulation to the budget-derived net ground-water flow

suggests that processes influencing ground-water

inflow on a weekly basis remain only partially represented in the model. 

To simulate the processes that generate highly

transient ground-water inflows, the model would

require even shorter stress periods and smaller grid

sizes, which should represent nearshore water-table

mounds more accurately. Simulation of transient watertable mounding near lakeshores requires relatively fine

spatial discretization and short time scales. For example, Lee (2000) simulated transient mounding using

daily stress periods and spatial discretization on the

order of tens of centimeters in a two-dimensional, variably saturated finite-element model of two lake hillsides. Nearshore cells in the current model are four

orders of magnitude larger, and this scale difference

limits the ability of the model to accurately simulated

nearshore processes. Finer temporal and spatial discretization in the nearshore could allow the model to

generate the ground-water inflow to the lake seen in the

water budget without having to use such a high longterm recharge rate. 

One possible reason for the high nearshore

recharge needed to calibrate the weekly simulation is

that runoff from the basin to the lake is assumed to be

negligible when this may not be true. Runoff could be

occurring from saturated soils near the shoreline during

intense rainfall events, but this process is probably

relatively unimportant in this basin compared to transient ground-water inflow. Runoff and the earliest

ground-water inflow both cause short-term rises in lake

stage in excess of the amount of rainfall, so the potential effect of runoff on lake stage can be difficult to

isolate. Short-term (hourly) rises in stage that were

substantially in excess of rainfall occurred only a few

times during the 2-year study during the most intense

storms (rainfall rates greater than 1 inch per hour).

Even if runoff is occurring in these few instances, its

effect on lake stage is expected to be well within the

10 percent error assumed for weekly rainfall. Washouts

or gullies that would be evidence of runoff to the lake

were not observed at the lakeshore; however, runoff to

ditches and storm drains from roads in the basin does

route water from upper parts of the basin to areas closer

to the lake, which could affect the distribution of

recharge within the basin. 

In contrast to the lack of evidence of the influence of runoff on lake stage, the influence of transient

ground-water inflow was evident during the days after

rain events. Lake stage rose or remained stable for

several days after the largest rainfall events had ended.

For example, lake stage rose 0.37 ft from March 19

to 21, 1998, in response to 4.41 inches (0.368 ft) of

rain, and continued to rise another 0.04 ft by April 1,

1998, even though no more rain fell during that period.

This rise could only be due to increased net groundwater flow to the lake. 

To determine whether increasingly shorter stress

periods could improve the prediction of ground-water

flow to the lake, the higher recharge simulation was

rerun using daily stress periods for the 10 weeks

with the highest LEACHM recharge. In this scenario,

the simulated peak ground-water inflows exceeded

those from the weekly model (245,000 compared to

144,000 ft3/d), but the total inflow generated during the

10 weeks was actually less than the weekly simulation

by a significant amount (27 percent). This difference

may be an artifact of the way flows to and from the

lake were calculated. In calculating flows to and from

the lake, flow rates at the end of the stress period were

assumed to be equal to the average flow rate during the

stress period. However, the magnitude of flow to or

from the lake increases with each time step within the

stress period, while the rate of change declines. As the

stress period length increases, the flow rate at the end

of the stress period is more likely to approach a stable

value that is representative of the average flow rate

over the stress period. For shorter stress periods (days

or weeks), the flow rate at the end of the stress period is
48 Effects of Recharge, Upper Floridan Aquifer Heads, and Time Scale on Simulated Ground-Water Exchange with Lake Starr, a 

Seepage Lake in Central Florida

less likely to have stabilized. Using this method to

calculate fluxes appears to have overestimated the

magnitude of the average flow rate during the 10 weeks

with the highest LEACHM recharge compared to a

simulation with daily stress periods. When average

flow rates for weekly stress periods were assumed to be

the average of the beginning and end fluxes (similar to

a running average), the daily simulation had 15 percent

higher inflow compared to the weekly simulation for

the same 10-week period. However, using this method

for the entire 2-year study period led to lower estimates

of peak net ground-water flow to the lake than previously calculated, and a poorer fit to the budget-derived

net ground-water flow. 

In attempting to simulate ground-water interactions with lakes in mantled karst terrain, models need

to account for the effect of nearshore recharge

processes. There continues to be a need for a better

understanding of annual and shorter-term recharge

processes for all types of hydrologic models. Future

efforts on quantifying recharge rates should consider

combining results of variably saturated flow models

with field observations similar to Lee (2000), using

geochemical tracer methods (Plummer and Friedman,

1999), and calculating recharge by subtracting evapotranspiration from rainfall (Sumner, 1996). 

Upper Floridan Aquifer Boundary

Modeling results confirm that accurately representing the Upper Floridan aquifer heads spatially and

temporally is important to successfully simulating the

ground-water flow patterns in the basin around Lake

Starr. Heads in the basin and ground-water exchange

with the lake are strongly affected by heads in the

Upper Floridan aquifer. The shape of the potentiometric surface of the Upper Floridan aquifer in the basin is

the primary control on the flow-through pattern of the

lake with respect to the surficial aquifer system, and

which areas of the basin contribute ground water to the

lake. Recharge superimposes temporary fluctuations on

the water-table shape, but Upper Floridan aquifer heads

also control the general shape of the water table.

To thoroughly understand the variability of the potentiometric surface in space and time, head measurements

in the Upper Floridan aquifer should be recorded

continuously at several locations in the lake basin.

Heads in the Upper Floridan aquifer largely

control the amount of water that leaks from the lake to

the ground-water flow system. Simulated lake leakage

was related to head in the Upper Floridan aquifer and the

downward head difference between the lake and the

Upper Floridan aquifer. Most of the response of the lake

stage to pumping stresses can be represented by monthly

average Upper Floridan aquifer heads, but lake stage

also responds to stresses on a weekly time scale. Even

pumping that lowers heads for several days is capable of

affecting lake stage within the same week that the drawdown occurs. For example, pumpage principally for

freeze protection of citrus crops caused the weekly average level of the Upper Floridan aquifer to drop 7 ft from

the week of December 24-30, 2000, to the week of

December 31-January 6, 2001 (data on file in Tampa

USGS database for ROMP 57 recorder well). On a daily

basis, heads ranged from 5-10 ft lower than the average

of the previous week. Using the regression relation

between simulated lake leakage and the head difference

between the lake and the Upper Floridan aquifer, this

weekly average head difference was capable of increasing outflows from Lake Starr by 53 percent, from 38,000

to 59,500 ft3/d. This is equivalent to an additional

decrease in stage of about 0.02 ft over the week.

Time Scale 

As in previous modeling studies, the use of

monthly stress periods at Lake Starr adequately simulated the observed head conditions in the surrounding

aquifers. The monthly simulation could not, however,

predict the extremes in the monthly net ground-water

exchange, even when the recharge and Upper Floridan

aquifer boundaries were represented in greater spatial

and temporal detail than in previous studies. During the

wettest months of the 2-year study period, the lake

received more ground-water inflow than the model

predicted. During the driest months, the lake leaked

more than the model predicted. 

Lake Starr is the first lake in Florida with a

detailed lake water budget on a weekly timeframe for a

2-year period. The weekly water budget provided the

basis for calibrating a numerical model of weekly

ground-water exchange with Lake Starr. Simulating the

net ground-water exchange on a weekly basis,

however, required constructing a substantially different

model than the monthly model. Compared to the

monthly model, the weekly model required calibrating

to four times as many values of net ground-water flow.
Summary and Conclusions 49

In addition, peak weekly net ground-water flow was

41 to 128 percent higher than peak monthly flow.

The weekly water budget reveals that much of the

ground-water inflow during a month can occur in a

single week. Further, the rapid rate of decline in net

ground-water inflow following a peak is not apparent

at a monthly time scale. Weekly simulations required

that lake interaction with the surrounding ground-water

flow system be much more dynamic than implied by

the monthly simulation. To agree with the budgetderived net ground-water flow, recharge and gross

ground-water flow rates had to be higher, and the

sublake geology and confining unit more leaky, than

initial simulations. 

Model stress periods and water budget time periods are inextricably linked, as accurate lake water

budgets still provide the standard against which simulations are interpreted. Some inflows probably occur

on time scales less than weekly, for example, those

resulting from transient recharge events. The weekly

water budget of Lake Starr substantiates their importance (Swancar and others, 2000). Currently, it is difficult to get interpretable lake water budgets on a shorter

timeframe than weekly, because the error in daily water

budget components becomes too large. At the same

time, simulating ground-water exchanges that are

evident from weekly lake water budgets is a sufficient

challenge for the near future. 

One consequence of using monthly or longer

time scales in lake basin models is the underestimation

of ground-water inflow to the lake, and underestimating inflow leads to underestimating lake leakage.

At Lake Starr, leakage was underestimated because the

K below the lake was underestimated. The higher K

values needed to calibrate to weekly net ground-water

fluxes lead to a revised understanding of the effects of

pumpage on the lake and the surrounding water table.

If management decisions are based on models that

underestimate these effects because of the model time

scale, lakes may not be sufficiently safeguarded from

the effects of short-term drawdown in the Upper Floridan aquifer. Unintended declines in surface-water

levels are the result. Effects of ground-water pumpage

on the lake would also be underestimated through the

use of generalized or averaged annual specified heads

to represent the Upper Floridan aquifer. 

SUMMARY AND CONCLUSIONS

Numerical modeling can be used as a tool to

understand the effects of hydrogeologic setting, climate

and ground-water pumping on ground-water exchange

with lakes. A useful approach is to combine groundwater flow modeling of lake basins with detailed lake

water budgets. Differences in the magnitude of groundwater exchange estimated by modeling and water

budgets can reveal ground-water flow processes unaccounted for by the models. The purpose of this study

was to quantify ground-water inflow and lake leakage

using a three-dimensional, numerical model of the

Lake Starr basin, and to investigate how representing

the time-varying boundary conditions (recharge to the

surficial aquifer system and the potentiometric surface

of the Upper Floridan aquifer) by monthly and weekly

averages affected the simulated ground-water fluxes.

Ground-water inflow to the lake and lake leakage were

simulated over a 2-year period from August 1996

through July 1998. 

Simulated values of net ground-water flow were

compared with the more accurate net ground-water

flow calculated from a rigorously derived lake water

budget. Net ground-water flow is the difference

between the gross or total ground-water inflow and

lake leakage for any given time period. In addition to

calibration to measured water levels in wells, the agreement between the two estimates of net ground-water

flow indicated whether or not the representation of

boundary conditions improved the simulation. A finitedifference ground-water flow model of the Lake Starr

basin was constructed using the USGS model code

MODFLOW. The model simulated ground-water flow

in the surficial aquifer system and the intermediate

confining unit within a 3-mi2 area surrounding Lake

Starr. The lake was represented as an area of active

cells with a very high hydraulic conductivity (K) of

10,000-50,000 ft/d. 

The steady-state simulation provided a good fit

to the median water-table configuration and median

vertical head distribution in the aquifer beneath Lake

Starr, with the root mean squared error between simulated and median observed water levels of less than

0.5 ft. Simulated heads and fluxes were most sensitive

to recharge, specified heads in the Upper Floridan aquifer lower boundary, and K values in the intermediate

confining unit. Heads were less sensitive to K in the

surficial aquifer system, but fluxes to and from the lake
50 Effects of Recharge, Upper Floridan Aquifer Heads, and Time Scale on Simulated Ground-Water Exchange with Lake Starr, a 

Seepage Lake in Central Florida

were directly related to changes in this value. The head

distribution of the lower boundary had a strong effect

on the water-table shape, imposing a northwest to

southeast flow direction on the surficial aquifer system,

which caused lake water to flow into the surficial aquifer system along the southeast shore despite the higher

topography on this side of the basin. The steady-state

area contributing ground water to the lake, called the

ground-water catchment, was west and north of the

lake and extended about 1,200 ft from the lake on the

west side. The ground-water catchment covered about

16 percent of the topographic basin. 

In transient simulations, a combination of two

methods for estimating recharge (threshold and

LEACHM) produced the best agreement to both heads

and fluxes. Heads were generally within 1 ft of the

observed values. However, monthly simulations with

total recharge set to 35 percent of 2-year rainfall failed

to simulate the highest lake levels and to predict peaks

and lows in net ground-water flow to the lake. Monthly

simulated net ground-water flow to the lake agreed

within the error bars of budget-derived estimates for

only 14 of 24 months. During wet periods, the monthly

simulations tended to greatly underpredict net groundwater inflow. Representing the Upper Floridan aquifer

specified-head boundary with monthly averages

instead of a steady-state potentiometric surface

improved the simulation of monthly ground-water

exchange and basin water levels. Use of a steady-state

boundary to represent the Upper Floridan aquifer

reduced the agreement with budget-derived fluxes to

only 10 of 24 months and added substantial errors to

monthly estimates of net ground-water exchange. 

Weekly stress periods simulated lake stage more

accurately and showed a larger variability in net

ground-water flow to the lake compared to monthly

stress periods. Unlike monthly stress periods, the transient water-table mounds that were observed near Lake

Starr were simulated when recharge conditions were

applied weekly. Weekly simulation increased the peak

net ground-water inflow rates compared to monthly

simulation, but peak net ground-water outflows were

similar to monthly rates. The minimum and maximum

simulated weekly net ground-water flow rates,

however, were still well below the rates derived from

the weekly water budget. Simulated net ground-water

flow did not alternate from large net ground-water

inflow to large net leakage as quickly as indicated

by the weekly water budget. The representation of

specified weekly Upper Floridan aquifer heads in the

transient model had a smaller effect on the magnitude

of simulated net ground-water flow than the representation of weekly recharge, but the effect was noticeable.

A weekly simulation also was run using higher

recharge rates, greater Kh in the surficial aquifer system

adjacent to the lake, and greater K in the sublake

region. The higher recharge simulation matched the

peak weekly values of net ground-water flow from the

water budget more closely than the original weekly

simulation. The higher recharge simulation was better

at producing the relatively rapid transitions from net

ground-water inflow to net ground-water outflow, as

well as the steep declines in net ground-water flow that

were seen in the water budget. The improved simulation of net ground-water flow suggests that lake leakage and the K of the sublake region were better

represented in the high recharge simulation than in the

original model. A much greater volume of ground

water flowed through the lake under the higher

recharge condition. The gross ground-water inflow

predicted by the higher recharge simulation for the

2-year period was 59 percent greater and lake leakage

was 72 percent greater than predicted by the lower

recharge simulation. 

Even though the higher recharge simulation was

an improvement over previous simulations, particularly

with respect to its ability to reproduce the extremes of

net ground-water flow, this simulation did not predict

net ground-water flow within the water budget error for

a greater number of weeks than the low recharge simulation. This was because of insufficient information

about the more subtle effects of rainfall and recharge

on ground-water inflow, and practical limits on spatial

and temporal discretization in a model at this scale. In

contrast, the saturated flow model appeared to successfully simulate the effects of heads in the Upper Floridan aquifer on water levels and ground-water exchange

with the lake at both weekly and monthly stress periods. The majority of the variability in lake leakage can

be explained by the average vertical head difference

between the lake and a representative Upper Floridan

aquifer well. Simulated lake leakage was highly correlated to the average head difference between the lake

and an Upper Floridan aquifer well, suggesting that

leakage was largely a linear function of this head

difference. 
Summary and Conclusions 51

The results of the higher recharge weekly simulation were coupled with water budget information

from Lake Starr in a combined approach for estimating

the ground-water inflow to Lake Starr. In the combined

approach, simulated lake leakage was substituted into

the water budget equation and ground-water inflow

was calculated as the new residual term. This method

eliminates some of the problems estimating recharge

inputs, but maintains the increased “leakiness” of the

sublake ground-water flow system that was needed to

simulate the transition from peak net ground-water

inflows to outflows. Using this approach, ground-water

inflow accounted for 42 percent of the total input of

water to the lake over the 2-year study, and rainfall

accounted for 58 percent of water inputs. Simulated

lake leakage for the 2 years accounted for 31 percent of

total water losses, while evaporation was 66 percent

and direct pumping was 3 percent. An adaptation of the

combined approach can be used to calculate groundwater exchange with Lake Starr beyond the model

period using a regression to predict lake leakage. This

adaptation requires that a lake water budget and head

measurements in the Upper Floridan aquifer are available for the time period of interest. 

This study illustrates how calibrating saturated

ground-water flow models with monthly stress periods

to a monthly lake water budget results in underpredicting gross inflow to, and leakage from, ridge lakes in

Florida. Recharge stresses and ground-water flow

responses during rainy periods are averaged over too

long a time period using monthly stress periods. Calibrating the weekly simulation required accounting for

transient responses in the water table near the lake that

generated the greater range of net ground-water flow

values seen in the weekly lake water budget. 

To simulate the total ground-water inflow to

lakes, saturated-flow models of lake basins need to

account for the potential effects of rapid and efficient

recharge in the surficial aquifer system closest to the

lake. In this part of the basin, the ability to accurately

estimate recharge is crucial because the water table is

shallowest, the response time between rainfall and

recharge is shortest, and daily recharge may approach

the daily rainfall amount. Use of the one-dimensional

unsaturated model LEACHM to simulate the effects of

the unsaturated zone on the timing and magnitude of

the recharge in the nearshore region of the model

improved the simulation of peak values of groundwater inflow to Lake Starr. The use of variably saturated

flow modeling, with time scales that are shorter than

weekly and finer spatial discretization, is probably

necessary to understand the subtle effects of rainfall

and recharge on ground-water inflow processes.

Underprediction of ground-water inflow to lakes due to

inaccurate simulation of nearshore processes can

reduce the implied effect of ground-water withdrawals

from the Upper Floridan aquifer on the lake level. 
52 Effects of Recharge, Upper Floridan Aquifer Heads, and Time Scale on Simulated Ground-Water Exchange with Lake Starr, a 

Seepage Lake in Central Florida

REFERENCES

Addiscott, T.M., 1977, A simple computer model for leaching in structured soils: Journal of Soil Science, v. 28,

p. 554-563.

Anderson, M.P., and Woessner, W.W., 1992, Applied

groundwater modeling---Simulation of flow and advective transport: San Diego, California, Academic Press,

Inc., 381 p. 

Barcelo, M.D., Slonena, D.L., Camp, S.L., and Watson, J.D.,

1990, Ridge II, a hydrologic investigation of the Lake

Wales Ridge: Brooksville, Fla., Southwest Florida

Water Management District, 121 p.

Brooks, H.K., 1981, Guide to the physiographic provinces of

Florida: Gainesville, Florida Cooperative Extension

Service, Institute of Food and Agricultural Sciences,

University of Florida, 11 p.

Cheng, Xiangxue, and Anderson, M.P., 1993, Numerical

simulation of ground-water interaction with lakes allowing for fluctuating lake levels: Ground Water, v. 31,

no. 6, p. 929-933. 

Choi, Jungyill, and Harvey, J.W., 2000, Quantifying timevarying ground-water discharge and recharge in wetlands of the northern Floridan Everglades: Wetlands,

v. 20, no. 3, p. 500-511.

Coffin, J.E., and Fletcher, W.L., 1998, Water resources data

for Florida, water year 1997: Southwest Florida ground

water, chap. 3B, in U.S. Geological Survey Water-Data

Report FL-97-3B.

____ 1999, Water resources data for Florida, water year

1998: Southwest Florida ground water, chap. 3B, in U.S.

Geological Survey Water-Data Report FL-98-3B.

Crowe, A.S., 1989, Numerical model for determining

groundwater contribution to hydrologic budget of lakes:

Volume 1 of 3, Abstracts, 28th International Geologic

Congress, Washington, D.C., July 9-19, 1989, p. 1-344.

Grubbs, J.W., 1995, Evaluation of ground-water flow and

hydrologic budget for Lake Five-O, a seepage lake in

northwest Florida: U.S. Geological Survey WaterResources Investigations Report 94-4145, 42 p.

Guiguer, Nilson, and Franz, Thomas, 1997, Users manual,

Visual MODFLOW: Waterloo, Ontario, Canada, Waterloo Hydrologic, Inc., 319 p.

Halford, K.J., 1992, Incorporating reservoir characteristics

for automatic history matching: Baton Rouge, Louisiana

State University Ph.D. dissertation, 150 p. 

Harbaugh, A.W., 1990, A computer program for calculating

subregional water budgets using results from the U.S.

Geological Survey modular three-dimensional finitedifference ground-water flow model: U.S. Geological

Survey Open-File Report 90-392, 23 p.

Hutson, J.L., and Wagenet, R.J., 1992, LEACHM: Leaching

Estimation And Chemistry Model: A process based

model of water and solute movement transformations,

plant uptake and chemical reactions in the unsaturated

zone, Continuum Vol. 2, version 3: Ithaca, N.Y., Water

Resources Institute, Cornell University.

LaBaugh, J.W., Rosenberry, D.O., and Winter, T.C., 1995,

Groundwater contribution to the water and chemical

budgets of Williams Lake, Minnesota, 1980-1991:

Canadian Journal of Fisheries and Aquatic Science,

v. 52, p. 754-767. 

Lee, T.M., 1996, Hydrogeologic controls on the groundwater

interactions with an acidic lake in karst terrain, Lake

Barco, Florida: Water Resources Research, v. 32, no. 4,

p. 831-844.

____ 2000, Effects of near-shore recharge on groundwater

interactions with a lake in mantled karst terrain, Water

Resources Research, v. 36, no. 8, p. 2167-2182.

____ 2002, Factors affecting ground-water exchange and

catchment size for Florida lakes in mantled karst terrain:

U.S. Geological Survey Water-Resources Investigations Report 02-4033, 54 p.

Lee, T.M., and Amy Swancar, 1997, Influence of evaporation, ground water, and uncertainty in the hydrologic

budget of Lake Lucerne, a seepage lake in Polk County,

Florida: U.S. Geological Survey Water-Supply Paper

2439, 61 p.

McDonald, M.G., and Harbaugh, A.W., 1988, A modular

three-dimensional finite-difference ground-water flow

model: U.S. Geological Survey Techniques of WaterResources Investigations, book 6, chapter A1. 

Merritt, M.L., 2001, Simulation of the interaction of karstic

Lakes Magnolia and Brooklyn with the Upper Floridan

aquifer, Southwestern Clay County, Florida: U.S. Geological Survey Water-Resources Investigations Report

00-4204, 62 p.

Merritt, M.L., and Konikow, L.F., 2000, Documentation of a

computer program to simulate lake-aquifer interaction

using the MODFLOW ground-water flow model and

the MOC3D solute-transport model: U.S. Geological

Survey Water-Resources Investigations Report 00-4167,

146 p.

Metz, P.A., and Sacks, L.A., 2002, Comparison of the hydrogeology and water quality of a ground-water augmented

lake with two non-augmented lakes in northwest Hillsborough County, Florida: U.S. Geological Survey

Water-Resources Investigations Report 02-4032, 74 p.

Plummer, L.N., and Freidman, L.C., 1999, Tracing and dating young ground water: U.S. Geological Survey Fact

Sheet 134-99, 4 p.

Pollock, D.W., 1994, User’s guide for MODPATH/MODPATH-PLOT, version 3: A particle tracking post-processing package for MODFLOW, the U.S. Geological

Survey finite-difference ground-water flow model: U.S.

Geological Survey Open-File Report 94-464.
References 53

Reilly, T.E., and Pollock, D.W., 1995, Effect of seasonal and

long-term changes in stress on sources of water to wells:

U.S. Geological Survey Water-Supply Paper 2445, 25 p.

Sacks, L.A., 2002, Estimating ground-water inflow to lakes

in central Florida using the isotope mass-balance

approach: U.S. Geological Survey Water-Resources

Investigations Report 02-4192, 54 p.

Sacks, L.A., Swancar, Amy, and Lee, T.M., 1998, Estimating

ground-water interaction with lakes using water-budget

and chemical mass-balance approaches for ten lakes in

ridge areas of Polk and Highlands Counties, Florida:

U.S. Geological Survey Water-Resources Investigations

Report 98-4135, 51 p.

Sinclair, W.C., Stewart, J.W., Knutilla, R.L., Gilboy, A.E.,

and Miller, R.L., 1985, Types, features and occurrence

of sinkholes in the karst of west-central Florida: U.S.

Geological Survey Water-Resources Investigations

Report 85-4126, 81 p.

Soil Conservation Service, 1990, Soil survey of Polk

County, Florida: U.S. Department of Agriculture,

235 p., 154 plates. 

Southeastern Geological Society, 1986, Hydrogeological

units of Florida: Florida Geological Survey Special Publication no. 28, 9 p.

Southwest Florida Water Management District, 1994, Report

of the subcommittee for assessment of the hydrologic

effects of ground-water withdrawals on surface-water

resources; Brooksville, Fla., Southwest Florida Water

Management District. 

Stauffer, R.E., 1985, Use of solute tracers released by weathering to estimate groundwater inflow to seepage lakes:

Environmental Science and Technology, v. 19, p. 405-411.

Sumner, D.M., 1996, Evapotranspiration from successional

vegetation in a deforested area of the Lake Wales Ridge,

Florida: U.S. Geological Survey Water-Resources

Investigations Report 96-4244, 37 p.

Swancar, Amy, Lee, T.M., and O’Hare, T.M., 2000, Hydrogeologic setting, water budget, and preliminary analysis

of ground-water exchange with Lake Starr, a seepage

lake in Polk County Florida: U.S. Geological Survey

Water-Resources Investigations Report 00-4030, 66 p.

Tihansky, A.B., 1999, Sinkholes, west-central Florida, in

Galloway, Devin, Jones, D.R., and Ingebritsen, S.E.,

eds., Land subsidence in the United States: U.S. Geological Survey Circular 1182, p. 121-140.

Winter, T.C., 1981, Uncertainties in estimating the water balance of lakes; Water Resources Bulletin, v. 17, p. 82-115.

Wu, J., Zhang, R., and Yang, J., 1996, Analysis of rainfallrecharge relationships: Journal of Hydrology, v. 177,

no. 1-2, p. 143-160.

Yobbi, D.K., 1996, Analysis and simulation of ground-water

flow in Lake Wales Ridge and adjacent areas of central

Florida: U.S. Geological Survey Water-Resources

Investigations Report 94-4254, 82 p.