U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2012–5161

Prepared in cooperation with the St. Johns River Water Management District, 

South Florida Water Management District, and Southwest Florida Water Management District

Groundwater Flow and Water Budget in the Surficial and 

Floridan Aquifer Systems in East-Central Florida
Cover. Wekiva River, Wekiwa Springs State Park, Orange and Seminole Counties, Florida. Photograph by Alan M. Cressler, USGS. 

Used with permission.
Groundwater Flow and Water Budget in the 

Surficial and Floridan Aquifer Systems in 

East-Central Florida

By Nicasio Sepúlveda, Claire R. Tiedeman, Andrew M. O’Reilly, Jeffrey B. Davis, 

and Patrick Burger

Prepared in cooperation with the St. Johns River Water Management District, 

South Florida Water Management District, and 

Southwest Florida Water Management District

Scientific Investigations Report 2012–5161

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2012

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Suggested citation:

Sepúlveda, Nicasio, Tiedeman, C.R., O’Reilly, A.M., Davis, J.B., and Burger, Patrick, 2012, Groundwater flow and water 

budget in the surficial and Floridan aquifer systems in east-central Florida: U.S. Geological Survey Scientific 

Investigations report 2012–5161, 214 p.
iii

Acknowledgments

The authors want to express the most sincere gratitude to Wei Jin (St. Johns River Water 

Management District) for executing alternate parameter estimation runs in the St. Johns River 

Water Management District computer facilities.

The water-use data from Jill Hood (Southwest Florida Water Management District) and 

the stream network of the upper St. Johns River from Tom Jobes (St. Johns River Water 

Management District) are greatly appreciated as well as the expertise provided by hydrologists 

Brian McGurk (St. Johns River Water Management District), and Ron Basso and Mark Barcelo 

(Southwest Florida Water Management District) that helped improve the understanding of the 

flow system in east-central Florida. 

v

Contents

Acknowledgments........................................................................................................................................iii

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

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

Purpose and Scope ..............................................................................................................................2

Previous Studies ..................................................................................................................................4

Description of Study Area ...................................................................................................................4

Land Use........................................................................................................................................4

Soil Groups....................................................................................................................................7

Drainage Basins...........................................................................................................................7

Climate...........................................................................................................................................7

Rainfall..................................................................................................................................7

Evapotranspiration ...........................................................................................................13

Hydrology of the Study Area......................................................................................................................16

Surface-Water Flow System.............................................................................................................16

Groundwater Flow System................................................................................................................18

Hydrogeologic Framework.......................................................................................................18

Surficial Aquifer System..................................................................................................20

Intermediate Confining Unit and Intermediate Aquifer System................................22

Floridan Aquifer System ..................................................................................................22

The Ocala Permeable Zone....................................................................................29

The Ocala Low-Permeable Zone...........................................................................29

The Avon Park Permeable Zone............................................................................29

Middle Confining Unit I and Middle Confining Unit II ........................................36

The Lower Floridan Aquifer....................................................................................36

Chloride Distribution..................................................................................................................46

Water Use ...................................................................................................................................46

Groundwater Withdrawals..............................................................................................46

Artificial Recharge............................................................................................................50

Irrigation.............................................................................................................................51

Estimated Recharge and Evapotranspiration Rates in the Surficial Aquifer System........51

Simulation of Groundwater Flow System ................................................................................................55

Model Construction............................................................................................................................56

Spatial and Temporal Model Discretization ..........................................................................56

Initial Conditions ........................................................................................................................56

Boundary Conditions.................................................................................................................58

Flow in the Unsaturated Zone..................................................................................................63

Partitioning of Rainfall and Irrigation into Runoff and Infiltration......................................66

Representation of Hydraulic Conductivity ............................................................................67

Representation of Specific Yield and Specific Storage ......................................................74

Calibration............................................................................................................................................74

Observation Data .......................................................................................................................75

Spring Flows ......................................................................................................................75
vi

Water-Surface Altitudes at Lakes and Streamflows ..................................................76

Water Levels at Wells ......................................................................................................76

Calibration Criteria............................................................................................................80

Calibration of K Parameters for Steady-State Conditions during 1993 and 2003 ............81

Parameter Sensitivities....................................................................................................82

Preferred Value Regularization ......................................................................................83

Calibrated Hydraulic Conductivities ..............................................................................92

Calibration of Storage Parameters Using Transient Conditions During 2005–2006......101

Model Fit for 1995–2006 Transient Conditions.....................................................................101

Parameter Uncertainty ...........................................................................................................118

Simulated Potentiometric Surfaces........................................................................................................122

Simulated Groundwater Flows and Water Budget ..............................................................................124

Model Limitations.......................................................................................................................................139

Summary and Conclusions.......................................................................................................................142

References Cited........................................................................................................................................145

Appendix 1. Groundwater Withdrawals and Inflows .......................................................................150

Appendix 2. Springs ...............................................................................................................................154

Appendix 3. Lakes..................................................................................................................................159

Appendix 4. Streams..............................................................................................................................169

Appendix 5. Well Hydrographs.............................................................................................................174

Figures

1. Map showing areal extent of the East-Central Florida Transient model area and

Central Florida Coordination Area .............................................................................................3

2. Map showing areal extent and spatial distribution of grouped physiographic

regions and the Green Swamp in the study area....................................................................5

3. Map showing generalized 1995 land use in the study area...................................................6

4. Map showing areas of increased urbanization levels from 1995 to 2004 ...........................8

5. Map showing areal extent and spatial distribution of hydrologic soil groups as

classified by the Natural Resources Conservation Service..................................................9

6. Maps showing drainage basins used to delineate the drainage fields for selected

streams and lakes near Seminole and Orange Counties.....................................................10

7. Map showing basin delineation in the study area showing closed basins not

contributing runoff to the stream network .............................................................................11

8. Map showing locations of rainfall stations, measured 1995 rainfall at the stations,

and calculated rainfall from Next Generation Weather Radar data, and chart

showing monthly rainfall and potential evapotranspiration totals from 1995 to 2006 .....12

9. Map showing spatial distribution of total 1995 rainfall derived from Next

Generation Weather Radar data and the measured 1995 rainfall at the stations

shown in figure 8A......................................................................................................................14

10. Maps showing spatial distribution of potential evapotranspiration (ET) for 1995

calculated from USGS Florida Water Science Center Statewide

Evapotranspiration Database, and spatial distribution of estimated ET for 1995 ............15

11. Map showing stream-gaging stations, ungaged stream segments, segment

numbers, and stream-gaging stations used to calculate streamflows discharging

outside the East-Central Florida Transient study area .........................................................17
vii

12. Graph showing measured streamflow for St. Johns River, station 

number 02236000.........................................................................................................................18

13. Map showing lakes with water-surface altitude stations and selected 

ungaged lakes .............................................................................................................................19

14. Map showing locations of drainage wells, areal extent of drainage wells basins,

and locations of rapid infiltration basins ................................................................................20

15. Correlation chart showing the relation between stratigraphic and hydrogeologic

units in the East-Central Florida Transient study area .........................................................21

16. Map showing altitude of land surface in the East-Central Florida Transient 

study area ....................................................................................................................................23

17. Map showing thickness of the surficial aquifer system in the East-Central Florida

Transient study area...................................................................................................................24

18. Map showing hydraulic conductivity in the surficial aquifer system from selected

aquifer tests.................................................................................................................................25

19. Map showing altitude of the upper surface of the intermediate confining

unit/intermediate aquifer system in the East-Central Florida Transient study area........26

20. Map showing thickness of the intermediate confining unit/intermediate aquifer

system in the East-Central Florida Transient study area .....................................................27

21. Map showing hydraulic conductivity in the intermediate aquifer system and the

intermediate confining unit from selected aquifer tests......................................................28

22. Map showing altitude of the upper surface of the Ocala permeable zone in the

East-Central Florida Transient study area ..............................................................................30

23. Map showing thickness of the Ocala permeable zone in the East-Central Florida

Transient study area...................................................................................................................31

24. Map showing hydraulic conductivity in the Ocala permeable zone from selected

aquifer tests.................................................................................................................................32

25. Map showing estimated potentiometric surface of the Ocala permeable zone,

average 1995 conditions............................................................................................................33

26. Map showing altitude of the upper surface of the Ocala low-permeable zone in

the East-Central Florida Transient study area .......................................................................34

27. Map showing thickness of the Ocala low-permeable zone in the East-Central

Florida Transient study area......................................................................................................35

28. Map showing altitude of the upper surface of the Avon Park permeable zone in

the East-Central Florida Transient study area .......................................................................37

29. Map showing thickness of the Avon Park permeable zone in the East-Central

Florida Transient study area......................................................................................................38

30. Map showing hydraulic conductivity in the Avon Park permeable zone from

selected aquifer tests ................................................................................................................39

31. Map showing differences in average annual heads between the Ocala and Avon

Park permeable zones and between the Upper and Lower Floridan aquifers ................40

32. Map showing altitude of the upper surfaces of the middle confining units I/II in

the East-Central Florida Transient study area .......................................................................41

33. Map showing thickness of the middle confining units I/II in the East-Central

Florida Transient study area......................................................................................................42

34. Map showing altitude of the upper surface of the Lower Floridan aquifer in the

East-Central Florida Transient study area ..............................................................................43

35. Map showing thickness of the Lower Floridan aquifer in the East-Central Florida

Transient study area...................................................................................................................44

36. Map showing hydraulic conductivity in the Lower Floridan aquifer from selected

aquifer tests.................................................................................................................................45
viii

37. Map showing estimated altitude of water containing a chloride concentration of

5,000 milligrams per liter in the East-Central Florida Transient study area of the

Floridan aquifer system .............................................................................................................47

38. Map showing groundwater uses by area, and chart showing estimated average

monthly groundwater withdrawals in the East-Central Florida Transient study

area by water use.......................................................................................................................48

39. Maps showing estimated net average 1999 groundwater withdrawal rates in the

Upper Floridan aquifer and Lower Floridan aquifer..............................................................52

40. Maps showing estimated net average 2006 groundwater withdrawal rates in the

Upper Floridan aquifer and Lower Floridan aquifer..............................................................53

41. Diagrams showing the one-dimensional unsaturated-zone flow coupled with

three-dimensional groundwater flow for the shallow water table and deep 

water table...................................................................................................................................57

42. Maps showing expanded view of the groundwater flow model for the

East-Central Florida Transient study area displaying the model grid in portions of

Marion and Indian River Counties ...........................................................................................59

43. Maps showing areal extent of active areas simulated in layers 1–7 ................................60

44. Map and schematic diagram showing Lake Apopka and the corresponding lake

cells after intersection with the grid .......................................................................................61

45. Map and schematic diagram showing Peace River, some tributaries, and stream

segment numbers; and the corresponding stream cells after the intersection with

the grid..........................................................................................................................................62

46. Map showing landscape and agricultural irrigation rates used to calculate

infiltration by the Green-Ampt method for 2003.....................................................................64

47. Schematic diagram showing the implementation of flow processes in the shallow

part of the hydrologic system...................................................................................................65

48. Cross-sectional diagram showing the vertical movement of the wetting front

through the unsaturated zone and the relation between soil moisture content

variables used in the Green-Ampt infiltration method .........................................................66

49. Map showing spatial distribution of soil texture classes in the active cells of the

surficial aquifer system .............................................................................................................68

50. Diagram showing the steps used to calibrate the hydraulic conductivity of the top

soil used in the Green-Ampt infiltration method....................................................................69

51. Map showing locations of regional pilot points used for the Ocala permeable zone ....72

52. Map showing locations of spring pilot points and regional pilot points in the

northern section of the East-Central Florida Transient study area ....................................73

53. Map showing locations of wells in the surficial aquifer system and intermediate

confining unit/intermediate aquifer system for which average annual head

observations in 1999 and 2003 were used to calibrate the steady-state models ............77

54. Map showing locations of wells in the Ocala permeable zone and Ocala

low-permeable zone for which average annual head observations in 1999 and

2003 were used to calibrate the steady-state models..........................................................78

55. Map showing locations of wells in the Avon Park permeable zone, middle

confining unit I/II, and Lower Floridan aquifer for which average annual head

observations in 1999 and 2003 were used to calibrate the steady-state models ............79

56. Graphs showing composite scaled sensitivities for physical and sensitivity

parameters of the steady-state models for all observations, head observations in

layers 1 and 3, head observations in layers 5 and 7, and spring-flow observations.......84

57. Map showing preferred values of horizontal hydraulic conductivity (Kh) in the

physiographic regions used to represent Kh in the surficial aquifer system and

selected aquifer test values......................................................................................................86
ix

58. Map showing interpolation of horizontal hydraulic conductivity from preferred

values at pilot points in the intermediate confining unit/intermediate aquifer

system and selected aquifer test values ................................................................................87

59. Map showing interpolation of vertical hydraulic conductivity from preferred

values at pilot points in the intermediate confining unit/intermediate aquifer

system and selected aquifer test values ................................................................................88

60. Map showing interpolation of horizontal hydraulic conductivity from preferred

values at pilot points in the Ocala permeable zone and selected aquifer test values .......89

61. Map showing interpolation of horizontal hydraulic conductivity from preferred

values at pilot points in the Avon Park permeable zone and selected aquifer 

test values....................................................................................................................................90

62. Map showing interpolation of horizontal hydraulic conductivity from preferred

values at pilot points in the Lower Floridan aquifer and selected aquifer test values .......91

63. Map showing calibrated horizontal hydraulic conductivity of the surficial aquifer

system and selected aquifer test values ................................................................................93

64. Map showing specified vertical hydraulic conductivity of the surficial 

aquifer system ............................................................................................................................94

65. Map showing calibrated horizontal hydraulic conductivity of the intermediate

confining unit/intermediate aquifer system and selected aquifer test values.................95

66. Map showing calibrated vertical hydraulic conductivity of the intermediate

confining unit/intermediate aquifer system ...........................................................................96

67. Map showing calibrated horizontal hydraulic conductivity of the Ocala permeable

zone and selected aquifer test values ....................................................................................97

68. Map showing calibrated vertical hydraulic conductivity of the Ocala

low-permeable zone...................................................................................................................98

69. Map showing calibrated horizontal hydraulic conductivity of the Avon Park

permeable zone and selected aquifer test values ................................................................99

70. Map showing calibrated vertical hydraulic conductivity of the middle confining

units.............................................................................................................................................100

71. Map showing calibrated horizontal hydraulic conductivity of the Lower Floridan

aquifer and selected aquifer test values ..............................................................................102

72. Graph showing composite scaled sensitivities for specific yield and specific

storage parameters in the transient model, calculated using head and springflow observations from July 2005 through December 2006...............................................103

73. Graphs showing annual average head residuals versus annual average simulated

heads for 1999, 2003, and 2006................................................................................................105

74. Graphs showing weighted monthly average spring flow residuals versus monthly

average simulated flows for 1999, 2003, and 2006...............................................................105

75. Map showing head residuals for 1999 in surficial aquifer system wells, calculated

using average annual simulated and observed heads.......................................................107

76. Map showing head residuals for 1999 in Ocala permeable zone wells, calculated

using average annual simulated and observed heads.......................................................108

77. Map showing head residuals for 1999 in Avon Park permeable zone and Lower

Floridan aquifer wells, calculated using average annual simulated and 

observed heads.........................................................................................................................109

78. Map showing Head residuals for 2006 in surficial aquifer system wells, calculated

using average annual simulated and observed heads.......................................................110

79. Map showing head residuals for 2006 in Ocala permeable zone wells, calculated

using average annual simulated and observed heads.......................................................111
x

80. Map showing head residuals for 2006 in Avon Park permeable zone and Lower

Floridan aquifer wells, calculated using average annual simulated and 

observed heads.........................................................................................................................112

81. Graph showing measured and simulated flows for Rock Springs ...................................113

82. Graph showing measured and simulated flows for Wekiwa Springs..............................113

83. Graph showing measured and simulated water-surface altitude hydrographs for

Lake Monroe..............................................................................................................................114

84. Graph showing measured and simulated water-surface altitude hydrographs for

Lake Arbuckle............................................................................................................................114

85. Graph showing measured and simulated flow hydrographs for St. Johns River

station number 02236000 .........................................................................................................115

86. Graph showing measured and simulated flow hydrographs for Peace River

station number 02295637 .........................................................................................................115

87. Graph showing measured and simulated water-level hydrographs for Cocoa K..........116

88. Graph showing measured and simulated water-level hydrographs for Ridge 

Wrap H-1 ....................................................................................................................................116

89. Graph showing measured and simulated water-level hydrographs for Bithlo 2 ICU.......117

90. Graph showing measured and simulated water-level hydrographs for L-0096

Groveland Fire Tower...............................................................................................................117

91. Graph showing measured and simulated water-level hydrographs for L-0059

Crows Bluff NFS........................................................................................................................119

92. Graph showing measured and simulated water-level hydrographs for Regional

Observation and Monitor-well Program 58 Ocala...............................................................119

93. Graph showing measured and simulated water-level hydrographs for OR-47 at

Orlo Vista....................................................................................................................................120

94. Graph showing measured and simulated water-level hydrographs for Regional

Observation and Monitor-well Program deep well 101......................................................120

95. Graph showing measured and simulated water-level hydrographs for OR0614

Cocoa WF Site S zone 3...........................................................................................................121

96. Graph showing measured and simulated water-level hydrographs for OS0025 

Bull Creek APT TM2 .................................................................................................................121

97. Graph showing parameter coefficients of variation...........................................................122

98. Map showing average simulated water table in the surficial aquifer system for

1999, from the 12-year transient model.................................................................................123

99. Map showing average simulated potentiometric surface of the Ocala permeable

zone for 1999, from the 12-year transient model .................................................................125

100. Map showing average simulated potentiometric surface of the Avon Park

permeable zone for 1999, from the 12-year transient model .............................................126

101. Map showing average simulated potentiometric surface of the Lower Floridan

aquifer for 1999, from the 12-year transient model .............................................................127

102. Map showing simulated evapotranspiration rates for 1999 over active areas in 

the surficial aquifer system and in lakes, from the 12-year transient model..................130

103. Map showing simulated recharge and discharge rates over active areas of the

surficial aquifer system in 1999, from the 12-year transient model..................................131

104. Map showing simulated leakage rates between lakes and the surficial aquifer

system based on the simulated lakebed leakance, average 1999 conditions................132

105. Map showing simulated leakage rates between lakes and the intermediate

confining unit/intermediate aquifer system based on the simulated lakebed

leakance, average 1999 conditions .......................................................................................134
xi

106. Map showing simulated leakage rates between streams and the surficial aquifer

system based on the simulated streambed vertical hydraulic conductivity,

average 1999 conditions..........................................................................................................135

107. Map showing simulated vertical leakage rates between the surficial aquifer

system and the intermediate confining unit/intermediate aquifer system, average

1999 conditions..........................................................................................................................136

108. Map showing simulated vertical leakage rates between the intermediate

confining unit/intermediate aquifer system and the Ocala permeable zone,

average 1999 conditions..........................................................................................................137

109. Map showing simulated vertical leakage rates between the Ocala low-permeable

zone and Avon Park permeable zone, average 1999 conditions ......................................138

110. Map showing simulated vertical leakage rates between middle confining unit I/II

and Lower Floridan aquifer, average 1999 conditions........................................................140

Tables

1. Average groundwater withdrawals by water use category and year. .............................49

2. Annual average of inflows to the Ocala permeable zone from street drainage and

lake-level control wells by county ...........................................................................................50

3. Annual average of inflows to the surficial aquifer system from rapid infiltration

basins by county .........................................................................................................................50

4. Annual storage changes in lakes and surficial aquifer system wells from 

1995 to 2006..................................................................................................................................54

5. Estimation of groundwater flow terms for 1995–2006 in the surficial aquifer

system of the East-Central Florida Transient study area .....................................................55

6. Geographical information system coordinates of the corners of the groundwater

flow model grid............................................................................................................................58

7. Evapotranspiration extinction depths based on values from Shah and others (2007)......66

8. Wetting front capillary pressure head values used in the Green-Ampt infiltration

method based on values from Rawls and others (1983).......................................................69

9. Average calibrated hydraulic conductivity of the hydrologic soil groups used in

the Green-Ampt infiltration method.........................................................................................69

10. Model parameters defined to represent horizontal hydraulic conductivity.....................70

11. Model parameters defined to represent vertical hydraulic conductivity .........................70

12. Coefficients of variation used to calculate weights on spring-flow measurements ......76

13. Definition of sensitivity parameters.........................................................................................82

14. Estimated specific yield and specific storage values for the calibrated transient

model of 1995 to 2006 ...............................................................................................................103

15. Statistics of head residuals for the calibrated 1995–2006 transient model ....................104

16. Statistics of spring-flow residuals for the calibrated 1995–2006 transient model.........104

17. Simulated evapotranspiration and recharge rates to the surficial aquifer system

from the calibrated transient model of 1995 to 2006 ...........................................................128

18. Simulated flows entering and leaving the surficial aquifer system from the

calibrated transient model of 1995 to 2006 ...........................................................................128

19. Simulated flows entering and leaving the Ocala permeable zone from the

calibrated transient model of 1995 to 2006 ...........................................................................141

20. Simulated flows entering and leaving the Avon Park permeable zone and the

Lower Floridan aquifer from the calibrated transient model of 1995 to 2006..................141
xii

Conversion Factors and Abbreviations

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2) 2.590 square kilometer (km2) 

Volume

gallon (gal) 0.003785 cubic meter (m3) 

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

Flow rate

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

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

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

Recharge, evapotranspiration, and water exchange

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

Hydraulic conductivity

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

Leakance

foot per day per foot [(ft/d)/ft] 1 meter per day per meter

inch per year per foot [(in/yr)/ft] 83.33 millimeter per year per meter

[(mm/yr)/m]

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

°F= (1.8×°C)+32

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

°C=(°F-32)/1.8

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

(NGVD 29).

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

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

Concentrations of chemical constituents in water are given in milligrams per liter (mg/L).
xiii

Abbreviations

AFSIRS Agricultural Field Scale Irrigation Requirement Simulation

APPZ Avon Park permeable zone

CFCA Central Florida Coordination Area

CFWI Central Florida Water Initiative

DEM digital elevation model

DWRM District Wide Regional Model

ECFT East-Central Florida Transient

ET evapotranspiration

FAS Floridan aquifer system

FDEP Florida Department of Environmental Protection

GAI Green-Ampt infiltration

GHB General head boundary (MODFLOW-2005 Package)

GIS geographical information system

GOES Geostationary Operational Environmental Satellite

IAS intermediate aquifer system

ICU intermediate confining unit

LAK Lake (MODFLOW-2005 Package)

LFA Lower Floridan aquifer

MCU middle confining unit

MOR Monthly Operating Report

NEXRAD Next Generation Weather Radar

NOAA National Oceanic and Atmospheric Administration

OLPZ Ocala low-permeable zone

OMR overall mean residual

OPZ Ocala permeable zone

RIB rapid infiltration basin

RMSR root-mean-square residual

ROMP Regional Observation and Monitor-well Program

SAS surficial aquifer system

SFR Streamflow-Routing (MODFLOW-2005 Package)

SFWMD South Florida Water Management District

SJRWMD St. Johns River Water Management District

SWFWMD Southwest Florida Water Management District

UFA Upper Floridan aquifer

USGS U.S. Geological Survey

UTM Universal Transverse Mercator

UZF Unsaturated zone flow (MODFLOW-2005 Package)

Groundwater Flow and Water Budget in the Surficial and 

Floridan Aquifer Systems in East-Central Florida

By Nicasio Sepúlveda1, Claire R. Tiedeman1, Andrew M. O’Reilly1, Jeffrey B. Davis2, and Patrick Burger2

1 U.S. Geological Survey.

2 St. Johns River Water Management District.

Abstract

A numerical transient model of the surficial and Floridan 

aquifer systems in east-central Florida was developed to 

(1) increase the understanding of water exchanges between 

the surficial and the Floridan aquifer systems, (2) assess the 

recharge rates to the surficial aquifer system from infiltration 

through the unsaturated zone, and (3) obtain a simulation tool 

that could be used by water-resource managers to assess the 

impact of changes in groundwater withdrawals on spring flows 

and on the potentiometric surfaces of the hydrogeologic units 

composing the Floridan aquifer system. The hydrogeology of 

east-central Florida was evaluated and used to develop and 

calibrate the groundwater flow model, which simulates the 

regional fresh groundwater flow system.

The U.S. Geological Survey three-dimensional groundwater flow model, MODFLOW-2005, was used to simulate 

transient groundwater flow in the surficial, intermediate, and 

Floridan aquifer systems from 1995 to 2006. The East-Central 

Florida Transient model encompasses an actively simulated 

area of about 9,000 square miles. Although the model includes 

surficial processes—rainfall, irrigation, evapotranspiration 

(ET), runoff, infiltration, lake water levels, and stream water 

levels and flows—its primary purpose is to characterize and 

refine the understanding of groundwater flow in the Floridan 

aquifer system. Model-independent estimates of the partitioning of rainfall into ET, streamflow, and aquifer recharge 

are provided from a water-budget analysis of the surficial 

aquifer system. The interaction of the groundwater flow 

system with the surface environment was simulated using the 

Green-Ampt infiltration method and the MODFLOW-2005 

Unsaturated-Zone Flow, Lake, and Streamflow-Routing 

Packages. 

The model is intended to simulate the part of the groundwater system that contains freshwater. The bottom and lateral 

boundaries of the model were established at the estimated 

depths where the chloride concentration is 5,000 milligrams 

per liter in the Floridan aquifer system. Potential flow across 

the interface represented by this chloride concentration is 

simulated by the General Head Boundary Package. During 

1995 through 2006, there were no major groundwater withdrawals near the freshwater and saline-water interface, making 

the general head boundary a suitable feature to estimate flow 

through the interface.

The east-central Florida transient model was calibrated 

using the inverse parameter estimation code, PEST. Steadystate models for 1999 and 2003 were developed to estimate 

hydraulic conductivity (K) using average annual heads and 

spring flows as observations. The spatial variation of K was 

represented using zones of constant values in some layers, and 

pilot points in other layers. Estimated K values were within 

one order of magnitude of aquifer performance test data. A 

simulation of the final two years (2005–2006) of the 12-year 

model, with the K estimates from the steady-state calibration, 

was used to guide the estimation of specific yield and specific 

storage values.

The final model yielded head and spring-flow residuals 

that met the calibration criteria for the 12-year transient 

simulation. The overall mean residual for heads, defining 

residual as simulated minus measured value, was -0.04 foot. 

The overall root-mean square residual for heads was less than 

3.6 feet for each year in the 1995 to 2006 simulation period. 

The overall mean residual for spring flows was -0.3 cubic 

foot per second. The spatial distribution of head residuals was 

generally random, with some minor indications of bias.

Simulated average ET over the 1995 to 2006 period was 

34.47 inches per year, compared to the calculated average 

ET rate of 36.39 inches per year from the model-independent 

water-budget analysis. Simulated average net recharge to the 

surficial aquifer system was 3.58 inches per year, compared 

with the calculated average of 3.39 inches per year from 

the model-independent water-budget analysis. Groundwater 

withdrawals from the Floridan aquifer system averaged about 

920 million gallons per day, which is equivalent to about 

2 inches per year over the model area and slightly more than 

half of the simulated average net recharge to the surficial 

aquifer system over the same period. Annual net simulated 

recharge rates to the surficial aquifer system were less than 

the total groundwater withdrawals from the Floridan aquifer 

system only during the below-average rainfall years of 2000 

and 2006.
2 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

Introduction

The total population of Lake, Orange, Osceola, Polk, and 

Seminole Counties in east-central Florida (fig. 1) has increased 

by about 70 percent from 1990 to 2010 (Florida Office of 

Economic and Demographic Research, 2012). This growth 

has increased the demand for groundwater from the Floridan 

aquifer system (FAS), which is the primary source of water 

supply in the region. Declines in groundwater levels, spring 

flows, and water-surface altitudes at lakes, as well as increases 

in groundwater chloride concentrations, have occurred in an 

area of concern designated as the Central Florida Coordination 

Area (CFCA). The CFCA includes southern Lake County and 

all of Orange, Osceola, Polk, and Seminole Counties (fig. 1). 

Declines in groundwater levels have been reported in wells 

penetrating the Upper Floridan aquifer (UFA), the primary 

source of water for potable, industrial, and agricultural 

purposes in the CFCA. These declines have been attributed 

to groundwater withdrawals and long-term below-average 

rainfall (Spechler and Halford, 2001). More recently, a 

statistical analysis of 115 sites by Murch and Tara (2010) 

indicated that 31 of 62 wells, 4 of 6 springs, and 13 of 47 lakes 

have exhibited statistically significant (80-percent confidence 

level) declines in water levels and flows during their respective periods of record, which collectively ranged from 1941 to 

2009. A rise in groundwater levels has been observed in some 

areas, particularly in Polk County, and is attributed to a reduction in groundwater withdrawals associated with improved 

phosphate mining practices (Spechler and Kroening, 2007; 

Murch and Tara, 2010).

The cumulative effects of changes to the hydrologic 

system in the CFCA caused by natural or anthropogenic 

factors may alter the long-term balance between recharge and 

discharge. Over time, such changes could cause movement of 

the freshwater and saline-water interface. Multi-year, cyclic 

variations in rainfall may contribute to observed periods of 

generally increasing or decreasing lake levels (German and 

Adamski, 2005). Groundwater chloride concentrations have 

increased near well fields in eastern Seminole and Orange 

Counties, suggesting an upward movement of saline water that 

may have been caused by groundwater withdrawals (Spechler 

and Halford, 2001; Adamski and German, 2004). In addition to changes in rainfall or groundwater withdrawals, land 

application of reclaimed water has been shown to increase 

water-table altitudes, as well as UFA water levels (O’Reilly, 

1998; Adamski and German, 2004). The construction of 

drainage ditches to reclaim land for development could 

increase groundwater seepage to streams, as suggested by 

German and Adamski (2005) for some streams in Orange 

County exhibiting long-term increases in 7-day flow. Based on 

a conceptual model developed by Tibbals (1978), expansion of 

paved surfaces could increase recharge to the FAS if a number 

of assumptions implicit in the model are satisfied. Where the 

water table is near land surface and subject to evapotranspiration (ET), such as occurs throughout much of the CFCA, small 

changes in depth to the water table can affect the availability 

of water for recharge (Knowles and others, 2002). 

The St. Johns River Water Management District 

(SJRWMD), South Florida Water Management District 

(SFWMD), and Southwest Florida Water Management District 

(SWFWMD), manage the groundwater resources in the CFCA 

to ensure water resources are sufficient to satisfy demand 

associated with future population growth (Central Florida 

Water Initiative, 2011). To address these concerns, SJRWMD, 

SWFWMD, and SFWMD are coordinating resource management efforts within the CFCA with the creation of the Central 

Florida Water Initiative (CFWI). The initiative is a collaborative process between stakeholders to manage groundwater 

resources in the CFCA. The main goals of the CFWI are to 

develop a planning tool to evaluate water-resource management strategies in the CFCA and to develop a regional watersupply plan. 

In 2008, the U.S. Geological Survey (USGS) initiated a 

3 1/2-year study, in cooperation with SJRWMD, SFWMD, and 

SWFWMD, to determine the long-term effects on the hydrologic budget caused by changes in climate, groundwater withdrawals, and land-use practices. The CFWI has incorporated 

the USGS modeling study as part of its planning initiatives. 

To date, several groundwater flow models have been 

developed for areas within central Florida but none represent 

consistent conceptualizations of the CFCA hydrologic system. 

Water management decisions within CFCA consider the 

effects of land use and land cover change, including the development of new well fields, withdrawals from existing well 

fields, reclaimed water applications, and expansion of urban 

land use. To address study objectives, the USGS developed a 

fine-resolution, areally extensive groundwater flow model of 

the aquifer system in the CFCA and adjacent areas of eastcentral Florida (fig. 1). The model may be used to evaluate 

the effects of future groundwater withdrawal scenarios in 

the CFCA and assess the impact of changes in groundwater 

withdrawals on the potentiometric surfaces of the FAS.

Purpose and Scope

The purpose of this report is to describe the hydrologic 

flow system and water budget in east-central Florida (fig. 1) 

from 1995 to 2006, a time period associated with a wide range 

in hydrologic conditions in east-central Florida. Specifically, 

this report documents the development of the groundwater 

flow model used to (1) refine the conceptual understanding 

of the water exchanges between the surficial aquifer system 

(SAS) and the FAS and (2) improve the estimates of the 

recharge rates to the SAS from infiltration through the unsaturated zone. 

This report describes exchange of water within the SAS, 

intermediate aquifer system (IAS), and FAS, including the 

intervening confining or semiconfining units and their interaction with surface-water systems (lakes, streams, rivers, and 

springs), and external aquifer stresses including ET, rainfall, 
Introduction 3

Figure 1. Map showing areal extent of the East-Central Florida Transient (ECFT) model 

area (study area) and Central Florida Coordination Area.

7

7

7

7

7

77

7

7 7

7

7

7

77

7

7

777

7

7

SWFWMD

SWFWMD

SJRWMD

SFWMD

SJRWMD

SFWMD

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

0 10 20 MILES

0 10 20 KILOMETERS

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

Merritt

Island

East-central Florida

transient model area

St Johns

River

Bull 

Creek

C-40 Canal

Wekiva 

River

Jim Creek

River

Ocklawaha 

Wolf 

Creek

Jane Green 

Creek

Crane Creek

Cow 

Creek

Salt 

Creek

Mills

Creek

River

Econlockhatchee 

Long

Branch

Haynes 

Wolf Branch

Creek

Taylor 

Creek

Tracy 

Canal

Turkey 

Creek

St Johns 

River

Peace 

C-54

Peace River River

Reedy Creek

Kissimmee River

Shingle Creek

Palatlakaha 

River

River

River

Palatlakaha

Withlacoochee

Payne Creek

Boggy 

Creek

M Canal

Howell 

Creek

Padgett 

Branch

Fort 

Drum 

Creek

Whidden 

Creek

Creek

Snell 

Creek

Saddle 

Creek

St Johns River

Glenn 

Branch

Banana 

River

Mosquito

Lagoon

Indian 

River

Lagoon

ATLANTIC OCEAN

FLORIDA ATLANTIC OCEAN

GULF

OF 

MEXICO

East-central 

Florida transient 

model area

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY HARDEE

COUNTY

SUMTER COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

EXPLANATION

Lake included in simulation

SWFWMD

SJRWMD

SFWMD

Spring included in simulation

Water Management District boundary

Southwest Florida Water Management 

 District

South Florida Water Management District

St. Johns River Water Management 

 District

Central Florida Coordination Area inside 

 model area

7

Blue 

Spring

Alexander 

Springs

Figure 1. Areal extent of the East-Central Florida Transient (ECFT) model area (study area) and Central 

Florida Coordination Area.
4 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

well withdrawals, artificial recharge and irrigation systems. 

Also discussed are the hydrologic budget, model construction, calibration procedures, parameter sensitivity analyses, 

simulated potentiometric surfaces, analyses of vertical flow 

directions, and estimates of fluxes between model units and 

boundaries. 

The model presented herein simulates regional flow in 

the freshwater-bearing part of the groundwater flow system, 

flow between the groundwater and surface-water systems, and 

flow in the surface-water system in the CFCA. Although the 

model simulates the surficial processes of runoff, infiltration, 

water-level fluctuation in lakes and streams, and streamflow, 

emphasis is placed on simulation of the groundwater flow 

system and its interaction with the surface-water system. The 

model is not an integrated surface-water/groundwater model, 

nor is intended to model surface-water processes in detail. 

Simulation of the saline groundwater flow system or potential 

interaction between the freshwater and saline-water flow 

systems was beyond the scope of the modeling effort.

Geographical information system databases (Environmental 

Systems Research Institute, Inc., 2010) were developed to 

manage spatially distributed information about hypsography, 

hydrography, land use, soil types, hydrostratigraphy, and 

hydrogeologic properties within the study area. To achieve 

consistency of coordinate systems, geographical information 

system files used the Universal Transverse Mercator (UTM) 

projection, zone 17 of the Florida coordinate system, west 

zone (Snyder, 1983). The North American Datum of 1983 was 

used for all data generated in this study.

Previous Studies 

Several groundwater flow models have been developed 

for areas within central Florida. Models having subregional 

to regional extent were summarized by Sepúlveda (2002), 

who developed a steady-state quasi-three-dimensional model 

of the IAS and FAS for peninsular Florida using a grid cell 

size of 5,000×5,000 feet (ft). Using the Sepúlveda (2002) 

model, Environmental Simulations Inc. (2004) developed 

a steady-state, quasi-three-dimensional model of the SAS, 

IAS, and FAS that encompasses SWFWMD lands, and is 

referred to as the District Wide Regional Model (DWRM) 

version 1. This model developed into the DWRM version 

2 (Environmental Simulations Inc., 2007) transient model, 

calibrated to 1996–2002 hydrologic conditions. More 

finely discretized (2,500×2,500-ft cell size) steady-state 

quasi-three-dimensional models of the SAS and FAS were 

developed by Knowles and others (2002) covering Lake 

County and the Ocala National Forest, and by McGurk and 

Presley (2002) covering east-central Florida. The SFWMD 

used the model by McGurk and Presley (2002) to develop a 

transient quasi-three-dimensional groundwater flow model 

of the SAS and FAS for the same east-central Florida area. 

That model, the East-Central Florida Transient (ECFT) 

version 1 (David Butler, South Florida Water Management 

District, written commun., 2012), is calibrated to 1995–1999 

hydrologic conditions and uses an application designed to 

simulate wetland flows in areas where the water-table altitude 

rises above land surface. A panel of scientists evaluated the 

ECFT version 1 model objectives, flow conceptualization 

and model design, assumptions, and documentation to assess 

whether the model was a suitable tool to evaluate future 

impacts on the hydrologic system (Andersen and others, 

2007). The model described herein was developed to address, 

to the degree possible, the main recommendations of the 

review panel based on availability of data.

Description of Study Area

The simulated onshore area of the model encompasses 

about 9,000 square miles (mi2) (fig. 1). The study area spans 

approximately 112 miles (mi) on its eastern and western sides, 

approximately 92 mi on its northern and southern sides, and 

includes all but the westernmost edge of the CFCA (fig. 1). 

The land surface area simulated in the model excludes actively 

simulated lakes, as well as coastal estuaries and islands (Banana 

River, Indian River Lagoon, Mosquito Lagoon, Merritt Island, 

and other barrier islands). Land-surface altitudes range from 

NGVD 29 along the coast of the Atlantic Ocean to more than 

300 ft above NGVD 29 in Polk and Lake Counties. Numerous 

karst features, including sinkholes and springs, are present in the 

study area. Sinkholes in all stages of development are common 

throughout much of the area and range from small depressions 

to large lakes. Many of these sinkholes can be areas of high 

recharge to the underlying aquifers. The springs in the northern 

half of the study area discharge water from the UFA into rivers 

and streams that eventually flow into the Atlantic Ocean.

The physiographic delineation previously defined by White 

(1970) and based on topography was used to subdivide the study 

area into 21 distinct regions (fig. 2) and refined for this study 

based on hydrologic features. The Green Swamp (region 21, 

fig. 2) was delineated in place of the physiographic regions in 

this area because of its unique hydrogeologic nature. In addition, 

the Lake Wales Ridge was subdivided into northern (region 15), 

central (region 19), and southern (region 20) regions based on 

differences in hydrogeologic properties (fig. 2).

Land Use

Land cover and land-use areas were available for 1995, 

2000, and 2004, and generalized into eight categories based on 

detailed classifications provided by SJRWMD (Jill A. Stokes, 

St. Johns River Water Management District, written commun., 

2010). In 1995, agriculture was the most areally extensive 

land use, covering 28 percent of the study area, followed 

by wetlands (24 percent), forest (15 percent), and urban 

(14 percent) (fig. 3). The remaining 19 percent of the study 

area consisted of water (9 percent), shrubs/grass (7 percent), 

mining (3 percent), and barren land (0.4 percent) in 1995.
Introduction 5

Eastern Valley

Marion Upland

St. Johns River Offset

Geneva Hill

Osceola and Wekiva 

 Plains

Western Valley

Okeechobee Plain

Bombing Range Ridge

Sumter Upland

Lake Upland

Polk Upland

De Soto Plain

Winter Haven Ridge, 

 Lakeland Ridge, 

 Lake Henry Ridge

Mount Dora Ridge

Lake Wales 

 Ridge – North

Central Valley

DeLand Ridge

Orlando Ridge

Lake Wales 

 Ridge – Central

Lake Wales 

 Ridge – South

Green Swamp

EXPLANATION

Physiographic Regions

5

6

7

8

9

1

2

3

4 10

11

12

13

14

15

16

17

18

19

20

21

0 10 20 MILES

0 10 20 KILOMETERS

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

SEMINOLE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

BREVARD

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER COUNTY

Figure 2. Map showing areal extent and spatial distribution of grouped physiographic regions 

and the Green Swamp in the study area.

5

1

11

21

2

16

9

14

19

10

20

17

3

15

8

7

13

12

18

4

6

ATLANTIC OCEAN

East-central Florida 

transient model area

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 2. Areal extent and spatial distribution of grouped physiographic regions and the Green Swamp in 

the study area.
6 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

SEMINOLE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

BREVARD

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

HARDEE COUNTY

COUNTY

SUMTER

COUNTY

0 10 20 MILES

0 10 20 KILOMETERS

EXPLANATION

East-central Florida

transient model area

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

ATLANTIC OCEAN

Figure 3. Map showing generalized 1995 land use in the study area.

Urban

Agriculture

Shrubs/grass

Forest

Lake/water

Marsh/wetland

Barren soil

Mining

Generalized 1995 land uses in parts of the 

 surficial aquifer system

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 3. Generalized 1995 land use in the study area.
Introduction 7

Land-use data were used to infer where urbanization 

occurred and could have affected the partitioning of rainfall 

into runoff and infiltration. Urbanization levels increased in 

about 4 percent of the study area from 1995 to 2004 (fig. 4, 

Jill A. Stokes, St. Johns River Water Management District, 

written commun., 2010), ranging from a change in low-density 

to high-density population areas or from non-urban land cover 

to urban land use. Transitions from non-urban land cover 

to urban land use are particularly important because such 

changes could increase the runoff potential caused by reduced 

infiltration associated with impervious surfaces, soil compaction during development, or stormwater drainage systems. 

However, depending in part on stormwater-management 

system type, such as the use of stormwater infiltration basins 

rather than surface-water discharge, urbanization could result 

in an increased average infiltration over the study area.

Soil Groups

Soils in the study area range from having low runoff 

potential (high infiltration) to high runoff potential (low infiltration). The runoff/infiltration potential is based on hydrologic 

soil groups delineated by the Natural Resources Conservation 

Service (2011). Hydrologic soil groups are based on the intake 

of water when the soils are saturated and receive rainfall from 

long-duration storms (Schellentrager and Hurt, 1990, p. 71). 

Soils are assigned to four hydrologic groups designated by 

letters, where “A” indicates high infiltration rate, “B” indicates 

a moderate infiltration rate, “C” indicates a low infiltration rate, 

and “D” indicates a very low infiltration rate (Schellentrager and 

Hurt, 1990, p. 71). A dual designation is assigned to soils—A/D, 

B/D, and C/D—in areas where a high water table may limit infiltration; in such cases, the first letter applies to the drained condition and the second letter applies to the undrained condition. 

Soils in the study area have predominantly moderate 

infiltration rates that may be further limited by a shallow 

water table, with 56 percent of the area classified as B/D 

soils (fig. 5). The next most prevalent hydrologic soil group 

is A (19 percent); these well drained, high infiltration soils 

are commonly present in the ridge and upland physiographic 

regions (figs. 2 and 5). The third most prevalent hydrologic 

soil group is D (16 percent); these poorly drained soils generally have moderately fine to fine texture or have a layer that 

impedes downward water movement and are often associated 

with wetland areas (fig. 3 and 5). The remaining 9 percent 

of the study area consists of A/D (1 percent), B (1 percent), 

C (6 percent), and C/D (1 percent) soils. 

Drainage Basins

The study area encompasses the headwaters of the four 

major river basins of peninsular Florida, namely, the St. Johns, 

Kissimmee, Withlacoochee, and Peace River Basins (fig. 1). 

Drainage basins delineating the land surface area (watershed) 

contributing runoff to a nearby stream or lake were derived 

based on topographic divides as provided by SJRWMD. The 

area of the drainage basin for a stream depends on the length 

of the stream segment and the surrounding topography. For 

example, the basin for Soldier Creek in Seminole County 

(fig. 6A) has an area of 12.2 mi2, whereas the drainage basin 

area for Wekiva River is 152.4 mi2. The area of the drainage 

basin for a large lake can be quite extensive. For example, 

an area of 130 mi2 drains directly into Lake Tohopekaliga in 

Osceola County (fig. 6B). The delineation of drainage basins 

in an area of low relief, such as Florida, can be difficult. 

The effective drainage basin can change temporally and be 

influenced by anthropogenic changes to the surface drainage 

system or even by meteorological conditions (rainfall and 

wind patterns in flat terrains may change the direction of 

overland flow). Well-drained soils and karst terrain occur 

in some areas, and 9 percent of the study area consists of 

noncontributing areas where runoff is lost to ET or is retained 

and infiltrates in topographic depressions or closed-basin 

lakes (fig. 7).

Climate

The climate of the study area is classified as subtropical 

and is characterized by warm, normally wet summers and 

mild, dry winters. Maximum temperatures usually exceed 

90 degrees Fahrenheit (°F) during the summer, but may fall 

below freezing for several days in the winter. The mean annual 

air temperatures are 72.7 °F and 73.6 °F for the cities of 

Sanford and Bartow (locations shown on fig. 8A), respectively, 

for 1981–2010 (National Oceanic and Atmospheric Administration, 2011). In January, mean monthly air temperatures 

are 59.7 °F at Sanford and 61.8 °F at Bartow, and in July, are 

83.3 °F at Sanford and 82.8 °F at Bartow. Mean annual rainfall 

at Sanford and Bartow is 53.04 and 52.08 inches (in.), respectively, for 1981–2010 with 55 and 59 percent (station locations 

shown on fig. 8A) occurring from June through September 

(National Oceanic and Atmospheric Administration, 2011). 

During the summer and early fall, tropical storms and hurricanes can produce substantial rainfall in the area. Winter 

rainfall generally is associated with large frontal systems that 

move from the northern latitudes southward.

Rainfall

The spatial and temporal distributions of rainfall over 

the study area from 1995 to 2006 were obtained from the 

National Oceanic and Atmospheric Administration (NOAA), 

SJRWMD, SWFWMD, and SFWMD rainfall stations. Total 

measured rainfall for 1995 at 40 stations ranged from 40.97 

to 74.08 inches per year (in/yr) (fig. 8A). Because these 

rainfall values were available only at site-specific locations, 

the spatial distribution of rainfall over the study area was 

based on NOAA Next Generation Weather Radar (NEXRAD) 

data. The methodology used to calculate the initial spatial 

distribution of NEXRAD rainfall data was presented by 

Hoblit and others (2003); however, the calculated 1995 total 
8 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

SEMINOLE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

BREVARD

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

HIGHLANDS COUNTY

COUNTY

HARDEE

COUNTY

SUMTER

COUNTY

Area with significant urbanization 

 change from 1995 to 2004

East-central Florida

transient model area

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

ATLANTIC OCEAN

0 10 20 MILES

0 10 20 KILOMETERS

EXPLANATION

Figure 4. Map showing areas of increased urbanization levels from 1995 to 2004.

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 4. Areas of increased urbanization levels from 1995 to 2004.
Introduction 9

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

SEMINOLE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

BREVARD

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

HIGHLANDS COUNTY

COUNTY

HARDEE

COUNTY

SUMTER

COUNTY

EXPLANATION

A—Soil with highest 

 permeability

B—Soil with somewhat high 

 permeability

C—Soil with average 

 permeability

D—Soil with lowest permeability

A/D—Soil with highest permeability 

 under drained conditions and 

 lowest permeability when soil is 

 not drained

B/D—Soil with somewhat high 

 permeability under drained conditions

 and lowest permeability when soil is 

 not drained

C/D—Soil with average permeability 

 under drained conditions and 

 lowest permeability when soil is 

 not drained

Hydrologic Soil Groups

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

0 10 20 MILES

0 10 20 KILOMETERS

East-central Florida

transient model area

ATLANTIC OCEAN

Figure 5. Map showing areal extent and spatial distribution of hydrologic soil groups as 

classified by the Natural Resources Conservation Service.

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 5. Areal extent and spatial distribution of hydrologic soil groups as classified by the Natural Resources 

Conservation Service.
10 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

SEMINOLE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

BREVARD

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER COUNTY

A

B

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

0 10 20 MILES

0 10 20 KILOMETERS

EXPLANATION

Basin boundary

Base from U.S. Geological Survey 

digital data, Universal Transverse 

Mercator projection, zone 17

ATLANTIC OCEAN

East-central 

Florida

transient 

model area

Wekiva 

River 

basin

Lake 

Monroe 

basin

Lake 

Tohopekaliga 

basin

Soldier 

Creek 

basin

81°35' 81°30' 81°25' 81°20' 81°15' 81°10'

28°25'

28°20'

28°15'

28°10'

28°05'

81°35' 81°30' 81°25' 81°20' 81°15' 81°10'

28°50'

28°45'

28°40'

28°35'

Figure 6. Maps showing drainage basins used to delineate the drainage fields for selected streams and lakes 

near (A) Seminole and (B) Orange Counties.

Figure 6. Drainage basins used to delineate the drainage fields for selected streams and lakes near A, Seminole and B, Orange Counties.
Introduction 11

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

SEMINOLE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

BREVARD

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER

COUNTY

East-central Florida

transient model area

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

ATLANTIC OCEAN

0 10 20 MILES

0 10 20 KILOMETERS

EXPLANATION

Drainage basin boundary

Extent of closed basin

Figure 7. Map showing basin delineation in the study area showing closed basins not 

contributing runoff to the stream network.

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 7. Basin delineation in the study area showing closed basins not contributing runoff to the stream network.
12 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

0

2

4

6

8

10

12

1995 1996 1997 1998 1999 2000 2001

Year

Total inches per month

2002 2003 2004 2005 2006 2007

Bartow

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

EXPLANATION

A

B

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

0 10 20 MILES

0 10 20 KILOMETERS

ATLANTIC OCEAN

East-central Florida

transient model area

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

SEMINOLE

COUNTY

Sanford

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

BREVARD

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER COUNTY

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48.6

41.3

47.9

52.8

44.13

62.01

44.49

53.06

48.87

47.16

52.42 41.24

52.26

44.89

58.67

42.15

64.18

65.71 57.46

48.84

54.29

57.97

41.77

49.47 78.09

43.99

47.81

48.51

55.25

47.26

61.44

68.43

47.09

58.25

43.46

44.39

46.99

47.79

50.82

58

40.97

48.6

43.05

42.3

39.98

47.1

64.57

67.60

52.72

55.29

59.32

54.15 52.42

57.06

52.91

60.27

53.34

58.32 57.91

45.85

60.62

71.24

52.27

53.37

74.08

59.89

44.82

55.43

61.25

55.32

61.36

44.51

57.29

52.12

52.43

53.38

48.60

47.96

50.28

55.62

Location of rainfall station—Upper 

 number indicates measured 1995 

 rainfall at the station, lower number 

 indicates calculated rainfall for 1995 

 at the station from NEXRAD data, 

 in inches

62.01 !

64.57

EXPLANATION

Monthly rainfall

Monthly potential 

 evapotranspiration

Figure 8. Map showing (A) locations of rainfall stations, measured 1995 rainfall at the stations, and 

calculated rainfall from Next Generation Weather Radar (NEXRAD) data and chart showing (B) monthly 

rainfall and potential evapotranspiration (ET) totals from 1995 to 2006. 

Figure 8. A, Locations of rainfall stations, measured 1995 rainfall at the stations, and calculated rainfall from Next 

Generation Weather Radar (NEXRAD) data, and B, monthly rainfall and potential evapotranspiration (ET) totals from 

1995 to 2006.
Introduction 13

NEXRAD rainfall at the stations differs from the measured 

totals (fig. 8A). The 1995 rainfall totals from NEXRAD varied 

from about 40 to 78 in., ranging from a 23-in. underestimate to 

an 11-in. overestimate compared to measured totals. Changes 

in monthly rainfall from 1995 to 2006 indicate the seasonal 

pattern of higher rainfall monthly totals during the second and 

third quarters of the year and lower during the first and fourth 

quarters (fig. 8B).

To account for differences between measured and 

NEXRAD rainfall, multiplicative factors were introduced 

to make the spatially available NEXRAD rainfall values 

equal to measured rainfall. An array of multiplicative factors 

was calculated as the arithmetical ratio of the rainfall values 

shown in figure 8A, resulting in factors that ranged from about 

0.8 to 1.5 for 1995. A surface of multiplicative factors was 

generated using splines based on continuously differentiable 

quintic polynomials between any two observed points, and 

constrained at the model boundary by adding a set of unity 

values along the study area boundary. These multiplicative factor surfaces were multiplied by the corresponding 

NEXRAD daily point data to derive the final adjusted spatial 

rainfall distribution, which were summed to yield the spatial 

distribution of 1995 annual rainfall (fig. 9). Similarly, independent interpolations for the 11 years from 1996 to 2006 were 

made to adjust NEXRAD rainfall data so that they equaled 

the measured annual rainfall at the stations. The application 

of these multiplicative factor adjustments resulted in rainfall 

totals ranging from 41 to about 63 in. for 1999, and from about 

46 to 62 in. for 2003. The general spatial distribution of rainfall data from NEXRAD was retained, although adjustment 

to the daily rainfall values was made using the multiplicative 

factors to agree with measured rainfall values at the rainfall 

stations. 

Based on the adjusted NEXRAD data, the average annual 

rainfall over the ECFT study area from 1995 to 2006 ranged 

from about 31 in/yr during 2000 to about 61 in/yr during 2005, 

averaging about 51 in/yr over the 12-year period. Annual 

average rainfall for 2006 was 36.5 in/yr over the ECFT study 

area, making 2000 and 2006 years with below average rainfall. 

Annual average rainfall was above average in 2002, 2004, and 

2005, exceeding 58 in/yr.

Evapotranspiration

Whereas rainfall provides the greatest input of water to 

the hydrologic system in central Florida, the greatest annual 

water loss occurs through ET. The fraction of annual precipitation returned as ET in Florida ranges from about 50 percent in 

settings of relatively deep water table, shallow rooted vegetation, and sandy soils (Sumner and Bradner, 1996) to almost 

110 percent from lakes (Swancar and others, 2000).

ET may be difficult to quantify, because of the paucity, 

cost, and labor of ET measurements (Sumner, 2006), necessitating the use of approximations. Such approximations 

commonly incorporate more easily derived potential ET 

data, which is then scaled by a factor or factors (typically 

derived empirically) to approximate ET (O’Reilly, 2007). 

The USGS Florida Water Science Center provides State-wide 

coverage, at a 2-kilometer pixel resolution, of daily potential 

ET derived from satellite and ground-based data from June 

1995 to the near-present (U.S. Geological Survey, 2012). As 

an approximation, potential ET rates for January to May 1995 

were set equal to the potential ET rates for January to May 

1996. Potential ET is computed by using the Priestley-Taylor 

equation (Sumner and Jacobs, 2005). Potential ET represents 

the rate for freely available water and thus would approximate 

evaporation from a free water surface such as a lake, river, 

or a lush green, actively growing landscape. Potential ET is 

independent of land cover type and ranged from about 47 to 

62 in/yr in 1995 (fig. 10A).

Potential ET was used to estimate ET at six sites 

across central Florida where ET measurements were available (fig. 10B). These sites cover a range of environmental 

settings, from dry areas having well drained, sandy soil with a 

relatively deep water table to wetlands having poorly drained 

soils (Sumner, 1996; Sumner, 2001; Knowles, 2005; Sumner, 

2006; O’Reilly, 2007). Using monthly data and the results 

of multilinear regressions, ET was estimated as a function of 

potential ET and rainfall, yielding potential ET coefficients 

ranging from 0.39 to 0.87 and rainfall coefficients ranging 

from 0 to 0.08. The estimated ET is strongly correlated with 

potential ET and weakly correlated with rainfall, and regression coefficients are consistent with those reported by Cheng 

and others (2011) for humid and subtropical watersheds. 

Based on land cover and water-table depth, the study 

area was subdivided into ET zones. Each zone was assigned 

an ET rate corresponding to a site, having similar physical 

characteristics for which ET was calculated. Regions 

characterized by marsh, forest, shrub, or ridge each had an 

associated site from which multiple linear regressions could 

be performed to estimate ET from potential ET and rainfall 

(figs. 3 and 10B). For example, marsh areas were assigned ET 

rates derived from the regression coefficients calculated for 

the Blue Cypress Marsh ET station, and forest areas having 

a deep water table were assigned ET rates derived from 

regression coefficients calculated for the Lyonia Preserve ET 

station (fig. 10B). Potential ET and rainfall regression coefficients could not be calculated for some land cover categories 

because of the lack of measured ET rates. These categories 

include agricultural, mining, barren soil, or urban land cover. 

Thus, ET zones are modified from the land cover distribution 

map (fig. 3) to adapt the existing data to estimate ET rates. 

Lakes were also designated as an ET zone. The ET rate at 

each lake was set equal to the potential ET at the nearest lake 

for which potential ET was estimated from the Statewide 

Evapotranspiration Database (U.S. Geological Survey, 2012). 

In total, the study area was divided into nine regions for 

which ET rates were assigned, including eight regions based 

on land-use types as identified in figure 3 and one region for 

the Lakes Wales Ridge (fig. 2). Annual ET rates estimated for 

1995 ranged from about 35 to 62 in/yr (fig.10B). The spatial 
14 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

Figure 9. Map showing spatial distribution of total 1995 rainfall derived from Next Generation 

Weather Radar (NEXRAD) data and the measured 1995 rainfall at the stations shown in 

figure 8A.

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

SEMINOLE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

BREVARD

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER COUNTY

0 10 20 MILES

0 10 20 KILOMETERS

East-central Florida

transient model area

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

ATLANTIC OCEAN

Total 1995 rainfall, in inches

42.6 to 45.0

45.1 to 47.0

47.1 to 49.0

49.1 to 51.0

51.1 to 53.0

53.1 to 55.0

55.1 to 58.0

58.1 to 60.0

60.1 to 63.0

63.1 to 66.6

EXPLANATION

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 9. Spatial distribution of total 1995 rainfall derived from Next Generation Weather Radar (NEXRAD) data 

and the measured 1995 rainfall at the stations shown in figure 8A.
Introduction 15

Figure 10. Maps showing (A) spatial distribution of potential evapotranspiration 

(ET) for 1995 calculated from USGS Florida Water Science Center Statewide 

Evapotranspiration Database and (B) spatial distribution of estimated ET for 1995.

East-central Florida

transient model area

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

0 10 20 MILES

0 10 20 KILOMETERS

ATLANTIC OCEAN

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Tiger Bay

Duda 

Farms

Lyonia Preserve

Disney 

Preserve

Lake 

Wales 

Ridge

Blue 

Cypress 

Marsh

East-central Florida

transient model area

ATLANTIC OCEAN

EXPLANATION

47.3 to 49.3

49.4 to 50.2

50.3 to 51.1

51.2 to 51.8

51.9 to 53.0

53.1 to 54.3

54.4 to 58.5

58.6 to 60.2

60.3 to 62.1

Potential 1995 

 evapotranspiration, 

 in inches

A

B

EXPLANATION

35.4 to 38.6

38.7 to 40.3

40.4 to 41.8

41.9 to 43.3

43.4 to 44.6

44.7 to 47.4

47.5 to 50.4

50.5 to 51.8

51.9 to 56.3

56.4 to 61.9

Tiger Bay

Estimated 1995 

 evapotranspiration

 rates, in inches

Evapotranspiration station 

 location and name

Evapotranspiration stations

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

SEMINOLE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

BREVARD

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER COUNTY

Figure 10. A, Spatial distribution of potential evapotranspiration (ET) for 1995 calculated from 

USGS Florida Water Science Center Statewide Evapotranspiration Database, and B, spatial 

distribution of estimated ET for 1995.
16 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

distribution of estimated ET rates for 1995 is an initial 

distribution, subject to refinements from model calibration 

because simulation of ET depends on physical features of the 

land surface such as soil permeability, changes in land cover, 

and drainage characteristics. From 1995 to 2006, the largest 

potential ET rate was 63.67 in/yr in 2000 and the smallest was 

34.25 in/yr in 2001.

Differences between the annual maximum and minimum 

estimated ET rates over the 1995–2006 study period, namely 

the range of annual estimated ET rates, were nearly constant 

between years. For example, the ET range in 1999 was about 

22 in/yr, while in 2003 this range was about 23 in/yr.

Hydrology of the Study Area

The principal hydrologic components of the study area 

are the surface-water flow system, groundwater flow system, 

and the major sources of water that enter and exit the system. 

These components are discussed within the context of 

study area hydrogeology and subsurface areas containing 

freshwater.

Surface-Water Flow System

The rivers that form the major surface-water flow system 

in the study area are the St. Johns, Palatlakaha, and Ocklawaha 

Rivers, which flow north; the Peace and Kissimmee Rivers 

(fig. 1), which flow south; the Withlacoochee River, which 

flows west; and several streams that flow east and discharge at 

locations along the eastern Atlantic coastline.

For the purpose of numerical model discretization, 

each stream segment was identified by a unique number, and 

identifiers numerically increase in the downstream direction. 

Each stream segment begins either at the most upstream point 

of a river, a lake, or the confluence of one or more streams and 

ends at another stream confluence, a lake, or outside the model 

(fig. 11). The stream discharge and water-surface altitude data 

are available from stations located along several of the stream 

segments (fig. 11). In addition, the stream discharge recorded 

at stream segments along the ECFT model boundaries provide 

estimates of flows exiting the study area and were used in the 

water-budget analysis. In total, there are 320 stream segments 

in the ECFT study area.

The highest monthly average streamflow in the time 

period from 1995 to 2006 was about 12,000 cubic feet per 

second (ft3/s), measured in October 2004 at station number 

02236000 on the St. Johns River (figs. 11 and 12). The lowest 

monthly average streamflow for the same period at this station 

was -119 ft3/s, measured in May 2002 (fig. 12). Negative discharge indicates wind effects or tidal flow of water 

upstream that reverse the flow direction when the surfacewater hydraulic gradient is small.

Measured monthly average upstream discharge of the 

Kissimmee River is generally higher than downstream during 

certain periods, suggesting well withdrawals may induce 

surface-water recharge to the aquifer. A small part of this loss 

can be attributed to streamflow leakage to the groundwater 

system; a larger fraction of this decrease could be associated 

with the operation of structures and storage of water along the 

Kissimmee River. An example of such a structure is located 

within stream segment number 253, upstream from station 

number J9202 (fig. 11). Similarly, the Peace River has several 

discharge stations with greater flows measured at upstream 

locations. In general, the Kissimmee River and the Peace 

River exhibit greater evidence of stream-aquifer interaction 

than does the St. Johns River.

Most lakes considered in this study are directly connected 

to streams or have a water-surface altitude station (fig. 13). Of 

the 351 lakes considered in this study, 287 lakes had monthly 

average water-surface altitudes measured for at least 1 month 

from 1995 to 2006. Ungaged lakes having a surface area less 

than 0.1 mi2 were not considered in the study because these 

were smaller than the area of one model cell.

Water-surface altitude at lakes at gaged stations and 

water levels at nearby wells provide an assessment of surfacewater and groundwater interactions. During the 1995–2006 

period of study, the average water-surface altitude of gaged 

lakes was lowest at the close of 2000, which was slightly 

more than 3 ft below the water-surface altitudes measured at 

the beginning of 1995. This is consistent with 2000 representing a below-average rainfall year. By the close of 2005, 

average water-surface altitudes at gaged lakes were about 

0.25 ft above the water altitudes at the beginning of 1995. 

The drought of 2006 caused average water-surface altitudes 

at lakes to drop again to about 1.5 ft below those measured at 

the beginning of 1995.

Flow occurs between the surface-water and the groundwater flow systems through areas of recharge and discharge. 

Drainage well inflows are a type of recharge to the UFA. 

Rapid infiltration basins are a means of land application of 

reclaimed water that also recharge the groundwater system. 

The calculated monthly average rate of recharge to the SAS 

from rapid infiltration basins (RIBs) during 1995 to 2006 

was 33 million gallons per day (Mgal/d). Drainage wells 

capture surface-water runoff and recharge the upper zone of 

the UFA. These drainage wells are found predominantly in 

the Orlando metropolitan area. Calculated monthly average 

recharge rates to the UFA from drainage wells during 1995 

to 2006 was 19.6 Mgal/d. The drainage areas of these wells, 

delineated based on the digital elevation model (DEM) for 

the ECFT study area, were used to identify model cells 

that contribute runoff as recharge to the UFA. These runoff 

volumes were not directed to lakes or streams and were 

mainly limited to the Orlando metropolitan area (fig. 14). The 

locations of the drainage wells were identified by Bulmer 

(2003) and RIB locations were obtained from databases of the 

SJRWMD (fig. 14).
Hydrology of the Study Area 17

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

SEMINOLE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

BREVARD

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

HARDEE COUNTY

COUNTY

SUMTER

COUNTY

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

#

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7

5

25

23

13

(299)

296

298

292 290

289

289

289

289

289

288

287

285 286 286

284

283

(281)

(274)

(267)

(267)

(265)

(264)

264

(263)

(262)

(261)

(258)

(257)

(256)

255

254

253 253

253

253 253

(253)

253

251

247

245

217

(196) (167)

(164)

65

51

48

47

43

41

37 39

36

259

258 258 239

239

238

234

233

232

229

227

223

222

222

222

221

221

221 219

218

213

213

212

210

206 205

201

203

203

199

174

174 173

173

98 97

96

95

84

82

81

76

76

75

75

74 73

268

254

194

193

189 183

179

154

153

141

139

136

129

127

121

107

101

56

52

111

87

197

20

72

35

58

295 275

88

45

83

64

241

80

244

293

53

50

46

62

294

86

150

68

168

61

112

248

249

297

49

242

79

152

71

178

143

133

149

129

181

246

183

195

171

69

117

159

276

104

113

182

57

252

170

273

282

106

77

240

192

208

166

162

198

250

176

187

177

180

66

116

270

237

236

291

191

131

8

1

3

14

6

37

12

34

9

33

40

17

2

27

19

60

15

18

10

23

89

16

28 26

22

67

11

30

204

44

99

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#

ATLANTIC OCEAN

East-central Florida 

transient model area

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

EXPLANATION

13

Stream or canal and stream 

 segment number

Stream-gaging station location 

 and stream segment number

Stream-gaging station used to 

 calculate flows leaving the 

 model area. Number between

 parentheses indicates segment

 number

#

10

# 02270000

0 10 20 MILES

0 10 20 KILOMETERS

J9202

02295637

02270000

02269520

02251210

02251000

02251500

02250700

02250500

02250030

02249518

02249007

02248510

02248060

02248053 02238500

18523784

02236000

02251767

#

Figure 11. Map showing stream-gaging stations, ungaged stream segments, segment 

numbers, and stream-gaging stations used to calculate streamflows discharging outside 

the East-Central Florida Transient (ECFT) study area.

Figure 11. Stream-gaging stations, ungaged stream segments, segment numbers, and stream-gaging stations 

used to calculate streamflows discharging outside the East-Central Florida Transient (ECFT) study area.
18 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

Figure 12. Graph showing measured streamflow for St. Johns River station 

number 02236000 in figure 11.

–2,000

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006

Measured streamflow, in cubic feet per second

Year

May 2002, –199 cubic 

feet per second

October 2004, 12,000 cubic 

feet per second

geologic characteristics, such as fracturing and dissolutionenhanced porosity or cementation. Hydrogeologic units may 

transgress lithostratigraphic boundaries, especially where 

fractures or secondary porosity enhancement features are 

present. Regional structural, depositional, and erosional trends 

may also affect the correlation between hydrogeologic and 

lithostratigraphic units. 

The designation of the hydrogeologic units listed herein 

generally follows the regional designations of Miller (1986), 

integrated with important subunits within the UFA in the 

ECFT study area as designated by Davis and Boniol (2011). 

Some units described by Miller (1986) (for example, LFA 

subunits) are not included in this hydrogeologic framework 

because they either are not present within the study area or 

were not mapped or delineated for this study. The regional 

hydrogeologic units in the ECFT study area consist of the 

SAS, the ICU and IAS, and the units within the FAS. The 

FAS comprises the UFA, the middle confining units I and II 

(MCU I/II), and the LFA. Within the UFA, defined hydrogeologic units include the Ocala permeable zone (OPZ), the Ocala 

low-permeable zone (OLPZ), and the Avon Park permeable 

zone (APPZ) (fig. 15). The upper surfaces and thicknesses of 

the hydrogeologic units were mapped using the interpretations from Miller (1986), as well as additional borehole 

geophysical data, test-hole and core descriptions, and selected 

lithologic descriptions from water management district and 

Florida Geological Survey files that have been collected since 

that publication. Hydrogeologic surfaces were identified by 

comparing published interpretations with units identified 

in nearby boreholes. Geophysical log characteristics were 

compared between boreholes, and additional well data were 

used to help refine the interpreted hydrogeologic designations.

Groundwater Flow System

Horizontal groundwater flow is controlled mainly by 

the horizontal hydraulic conductivity of the UFA and Lower 

Floridan aquifer (LFA) (fig. 15). Vertical groundwater flow is 

mainly controlled by the water levels in the SAS, the potentiometric surfaces of the UFA and LFA, and the vertical hydraulic 

conductivity of the intermediate confining unit (ICU), which 

underlies the SAS. The SAS generally is recharged by rainfall, 

irrigation, and by diffuse upward leakage in areas where 

the hydraulic head of the UFA is higher than the water-table 

altitude. The assessment of vertical flow between the UFA and 

the LFA and flow within the LFA itself could be conducted 

only at a few sites due to the paucity of LFA water-level data. 

UFA recharge or discharge occurs mostly through vertical 

ICU leakage. The vertical leakage rate to or from the UFA 

is a function of the hydraulic gradient between the SAS and 

UFA, the vertical hydraulic conductivity of the ICU, and the 

thickness of the ICU.

Hydrogeologic Framework

The hydrogeologic units that define the model layers 

have been correlated with regional lithostratigraphic units 

across the ECFT study area (fig. 15). Lithostratigraphic units 

are mappable bodies of bedded or unbedded rocks characterized on the basis of their lithologic properties or lithologic 

properties and stratigraphic position. Hydrogeologic units 

are defined by their ability to transmit or restrict flow, which 

is the result of both primary geologic characteristics, such 

as grain size distribution and composition, and secondary 

Figure 12. Measured 

streamflow for St. Johns 

River, station number 

02236000 in figure 11.
Hydrology of the Study Area 19

East-central Florida

transient model area

0 10 20 MILES

0 10 20 KILOMETERS

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

ATLANTIC OCEAN

64

226

46

154

9

38

149

35

257

15

56

52

267

201

17

251

143

219

237

259

185

214

271

268

113

161

181

169

95

12

107 105

195

246

188

171

79

19

112

261

22

120

269

141

170

187 308

175

174

6

241

184

196

285

306

48

159

76

27

265

168

205

218

16

223

43

293

138

8

177

65

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10

41

114

255

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262

238

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284

91

77

294

25

7

304

305

166

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207

274

275

292

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250

29 26

93

59

232

124 127

217

253

39

60

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240

278

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279 339

276 45

23

5

145

125

228

287

289

290 291

109

129

212 204

172

74

18

63

20

66

83

53

31

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108

236

165

221

134

298

234

176

110

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260

3 2

270

272

71

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62

51

37

89

81

49

327

283 286

162

242

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264

21

92

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90

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111 330

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318 307

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102 194

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118

87

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119

58

136 126

152

164

155

163

133

94

139

98

72

135

147

78

320

13

42

24

4

317

14

40

316

319

84

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY HARDEE

COUNTY

SUMTER COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

Figure 13. Map showing lakes with water-surface altitude stations and selected 

ungaged lakes.

EXPLANATION

Water-surface altitude

 station and lake number

Ungaged lake and number

267

68

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 13. Lakes with water-surface altitude stations and selected ungaged lakes.
20 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

Orlando

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!

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!

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!!

!

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! !

!

!

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!

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!

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!

!

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!

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! !

!

!

! !

!! ! !

!

!

!

!

!

!

!

! !

! !

!! !

!!!! !

!

!

!

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!

!

!

!

!

!

!

!

Figure 14. Map showing locations of drainage wells, areal extent of drainage wells basins, and locations of 

rapid infiltration basins.

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

SEMINOLE

COUNTY

SEMINOLE

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY OSCEOLA

COUNTY BREVARD

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER COUNTY

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Base from U.S. Geological Survey 

digital data, Universal Transverse 

Mercator projection, zone 17

ATLANTIC OCEAN

East-central 

Florida

transient 

model area

81°40' 81°20'

28°40'

28°20'

LAKE

COUNTY

ORANGE

COUNTY

POLK

COUNTY

!

0 10 20 MILES

0 10 20 KILOMETERS

Drainage well—Indicates location 

 of street-drainage well or lake-level 

 control well

Drainage basin—Indicates areal extent 

 of basin drained by drainage well

Rapid infiltration basin—Indicates 

 areal extent of spraying fields that 

 recharge the surficial aquifer system

EXPLANATION 0 5 10 MILES

0 5 10 KILOMETERS

FAS across the ECFT study area. This technique maintained 

consistency between hydrogeologic surfaces, and prevented 

unrealistic estimations of altitude or thickness in areas having 

limited well control. These procedures captured the large-scale 

geologic structures and local karst features, while honoring 

actual data values.

Surficial Aquifer System

In the study area, the unconfined to semiconfined SAS 

consists of unlithified sediments of variable permeability 

comprising Holocene undifferentiated sediments and Pliocene 

to Pleistocene formations (post-Hawthorn Group sediments). 

The sediments are primarily composed of clean sands and 

intermittent clay beds occurring mostly as discontinuous lenses. 

The lithology of the surficial sediments can vary substantially 

both vertically and laterally across short distances. The shell 

and sand beds of the Pleistocene Anastasia Formation and 

the sand beds of the Okeechobee, Nashua, and Cypresshead 

Formations compose the water-producing zones of the SAS. 

Where surficial sediments are absent due to erosion, the SAS 

is thin or non-existent at land surface. In other areas, surficial 

sediments can be greater than 100 ft thick, such as in upland 

The altitude of the top of each hydrogeologic unit was 

mapped using geostatistical estimation techniques available 

in the software ISATIS (Geovariances, 2011). A geostatistical 

analysis was performed to estimate the altitude of the top 

of the FAS and the top of the ICU, which had substantially 

more well control than the other hydrogeologic units. This 

geostatistical analysis utilized kriging and a moving geostatistics procedure to account for local variability of the data, the 

uneven spatial distribution of data points, and the long-range 

drift related to structural or depositional features. Because 

strong correlation was identified between the global trend 

of the altitude of the top of the FAS and the deeper units, a 

thickness filtering approach was used to guide the estimation 

of the deeper hydrogeologic units that had less well control. 

The thickness between the interpreted top surface of the FAS 

and each lower hydrogeologic unit was calculated at each 

sampled location. The thickness values were then used in an 

ordinary kriging process to map the thickness across the area. 

This thickness map displays a more stationary behavior than 

could be derived by kriging only the data values available for 

the top of a given hydrogeologic unit. The upper surface of 

a hydrogeologic unit was then computed by subtracting the 

mapped thickness from the mapped altitude of the top of the 

Figure 14. Locations of drainage wells, areal extent of drainage wells basins, and locations of rapid infiltration basins.
Hydrology of the Study Area 21

Figure 15. Correlation chart showing the relation between stratigraphic and hydrogeologic units in the East-Central 

Florida Transient (ECFT) study area.

Series

Holocene

and 

Pleistocene

Pliocene

Miocene

Oligocene

Late Eocene

Middle Eocene

Early Eocene

Paleocene

Lithology Stratigraphic 

unit

Sand, 

silt, 

clay, 

and shell

Undifferentiated 

sediments

An upper lithozone consists of recrystallized dolostone interbedded 

with white to tan recrystallized foraminiferal limestone. Beds of tan 

to brown to gray dolomitic limestone and dolostone are common 

and may be very impermeable unless fractured. May contain peat 

beds or other organic material. A lower dolostone lithozone may 

contain pyrite and glauconite grains

Consolidated to unconsolidated shell (mollusks) 

beds, molluskan limestone, quartz sand

Variably calcareous, shelly sand and finely 

sandy shell coquina

Unfossiliferous, variably argillaceous quartz 

sand, silt and gravel occurring in the higher 

elevation ridges

Dolostone, dolomitic limestone, and evaporites. The lower 

two-thirds consists of finely crystalline dolostone with interbedded 

anhydrite. This lower zone is significantly less porous and is areally 

extensive, forming the sub-Floridan confining unit

An upper lithozone is composed of white to grey, dolomitic, 

recrystallized, calcarenitic limestone and brown recrystallized 

dolostone. A lower lithozone consists of very hard and massive 

dolostone, with traces of glauconite, pyrite, peat and phosphate

Highly variable, clay, silt, quartz sand, shell beds, limestone, 

dolostone, chert (especially in lower section), phosphate. Intervals 

with abundant clay mineral or clay size material can be very 

impermeable. Sand and shell beds may be locally very permeable

Dolomitic, microfossiliferous calcarenitic limestone with silt-sized 

phosphate. Present eastern Indian River and southeastern Brevard 

Counties, and in localized areas of central Florida

Porous and permeable, thickly bedded, foraminiferal limestone 

containing abundant granule to pebble sized foraminifera, 

echinoids, mollusks, corals, and bryozoans. Typically includes and 

upper and lower lithozone with foraminiferal calcarenite and 

calcilutite in the lower zone and extremely fossiliferous, 

foraminiferal calciruditic limestone interbedded with fossiliferous, 

and foraminiferal calcarenitic limestone in the upper lithozone. 

Moldic porosity, well developed secondary permeability developed 

by karst processes. May be recrystallized or dominantly a lime 

mudstone with very low permeability

Hydrogeologic unit

Surficial aquifer system (Layer 1)

Intermediate confining unit or 

intermediate aquifer system 

(Layer 2)

Ocala permeable zone 

(Layer 3)

Ocala low-permeable 

zone 

(Layer 4)

Avon Park permeable zone 

(Layer 5)

Middle confining unit I 

Floridan aquifer system

Upper Floridan aquifer

Middle confining unit II 

(Layer 6)

Lower Floridan aquifer 

(Layer 7)

Sub-Floridan confining unit

Anastasia 

Formation

Nashua 

Formation

Cypresshead 

Formation

Hawthorn Group

Suwannee Limestone

Ocala Limestone

Avon Park Formation

Oldsmar Formation

Cedar Keys Formation

Figure 15. Relation between stratigraphic and hydrogeologic units in the East-Central Florida Transient (ECFT) study area.
22 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

ridge areas underlain by recent terrace deposits. The surficial 

siliciclastic sediments may be thicker than at the nearby 

mantle that infills karst collapse features.

The mapped upper surface of the SAS in the study area 

directly corresponds with land surface altitude (fig. 16), which 

ranges from 0 to 302 ft NGVD 29 in the ECFT study area. 

The thickness of the SAS in the ECFT study area ranges from 

5 to 240.6 ft (fig. 17). Across most of the study area the SAS 

is underlain by the ICU/IAS, but in some western parts of the 

study area, the SAS may directly overlie the UFA. The base of 

the SAS may be marked by a distinct clay unit or a gradational 

increase in clay or other fine-grained sediment.

Generally, groundwater within the SAS is unconfined, 

although in some areas, beds of low permeability cause semiconfined or locally confined conditions in deeper parts of the 

aquifer. Hydraulic conductivity data for the SAS were selected 

from databases of the USGS, SJRWMD, SWFWMD, and 

SFWMD. According to these data, the horizontal hydraulic 

conductivity in the SAS in the ECFT study area ranges from 

6 to 55 feet per day (ft/d) (fig. 18). McGurk and Presley 

(2002) and Sepúlveda (2002) describe the prevailing hydraulic 

gradients in the SAS, its correlation with land-surface altitude, 

and maps of the water-table altitude.

The utility of the SAS for water supply depends on 

sufficient thickness and lateral extent of permeable zones to 

provide consistent groundwater storage and transmission. The 

SAS is thickest in the south-central portion of the ECFT study 

area (fig. 17). Areas in the northwest may also be thick enough 

to provide sufficient production for specific uses. In some 

areas, localized, low permeable clay, silt, or very fine sand 

layers may limit use of the SAS. Although withdrawals from 

the SAS are greatest in Polk, Indian River, and Highlands 

Counties, the SAS is used to some extent everywhere within 

the ECFT study area.

Intermediate Confining Unit and Intermediate Aquifer 

System

The ICU/IAS comprises the largely clastic sediments 

of the Miocene Hawthorn Group that generally restrict the 

movement of water between the overlying SAS and underlying FAS (fig. 15). Generally, the ICU acts as a confining unit 

for the FAS, but where there are multiple permeable zones, 

the sequence of hydrogeologic units is referred to as the IAS. 

In this report, these units are referred to collectively as the 

ICU/IAS, and are conceptualized for simulation purposes as a 

single composite unit. 

 The top of the ICU/IAS is defined by the uppermost, 

laterally extensive, and vertically continuous sediments of 

lower permeability below the SAS or, where the surficial 

sediments are eroded, by land surface. In areas where the 

Hawthorn Group sediments are thin or absent, such as 

northern Brevard County and parts of Volusia and Lake Counties, erosional remnants of the Hawthorn Group and Pliocene 

sediments act as a semiconfining unit. In the northeastern 

part of the ECFT study area, fine-grained Pliocene-aged or 

reworked Hawthorn Group sediments above or in place of 

the Hawthorn Group are considered part of the ICU/IAS. The 

base of the ICU/IAS is at the top of the vertically continuous, 

permeable carbonate section of the FAS. In the study area, 

the altitude of the top of the ICU/IAS ranges from -136.8 to 

135.4 ft (fig. 19), and ICU/IAS thickness ranges from 3.3 to 

412.1 ft (fig. 20) 

The IAS is present throughout most of the southwestern 

part of the ECFT study area and is characterized by clastic 

sediments interbedded with carbonate rocks of Miocene age 

that generally coincide with the Hawthorn Group (fig. 21). 

Although the IAS is less permeable than the underlying UFA, 

the carbonate strata within the IAS are sufficiently permeable 

and productive to constitute a groundwater supply. Two main 

permeable zones are present in the IAS in the ECFT study area 

(Basso, 2003; Basso and Hood, 2005; Knochenmus, 2006); 

where these are present, the IAS is confined above and below 

by confining zones of the ICU.

The horizontal hydraulic conductivity of the IAS from 

aquifer tests obtained from databases of the SWFWMD ranged 

from 1 to 230 ft/d (fig. 21). The only hydraulic conductivity 

value found for the ICU (23 ft/d, fig. 21) was from an aquifer 

test performed in east-central Orange County. This production zone in the ICU is outside the extent of the IAS, and the 

hydraulic conductivity value obtained is not typical of the ICU 

and therefore not considered representative.

Floridan Aquifer System

The FAS comprises a thick sequence of layered, 

carbonate rocks of varying permeability. The limestones, dolomitic limestones, dolostones, and evaporites that compose the 

FAS were deposited in marine carbonate shelf environments, 

resulting in variable mineralogy and depositional characteristics. Post-depositional processes, including cementation and 

dissolution, as well as fracturing, karst processes and largerscale structural features, have subsequently altered the original 

physical characteristics of the sediments and resulted in highly 

variable lithology and permeability within the FAS (Miller, 

1986). Vertically and laterally extensive zones of enhanced 

porosity and permeability are present in the aquifer units and 

alternate with extensive zones of relatively lower porosity and 

permeability that compose the confining units.

The FAS generally corresponds to the Paleocene upper 

Cedar Keys Formation, the early Eocene Oldsmar Formation, 

the middle Eocene Avon Park Formation, the late Eocene 

Ocala Limestone, and the Oligocene Suwannee Limestone 

(fig. 15). The top of the FAS generally coincides with a 

distinct downward transition from the largely clastic, finegrained Miocene-aged sediments, to a permeable carbonate 

section that is vertically and laterally extensive. Throughout 

most of the study area, the uppermost surface of the FAS 

coincides with the top of the Ocala Limestone, yet where 

present, the Suwannee Limestone forms the top of the FAS. 

In localized areas of central peninsular Florida, the Suwannee 

Limestone and Ocala Limestone are absent due to erosion or 
Hydrology of the Study Area 23

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

BREVARD

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER COUNTY

SEMINOLE

COUNTY

ATLANTIC OCEAN

East-central Florida 

transient model area

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

EXPLANATION

Estimated altitude of the top of the surficial 

 aquifer system, or land surface altitude, 

 in feet NGVD 29. White areas are inactive 

 in the SAS

0 to 30.0

30.1 to 60.0

60.1 to 80.0

80.1 to 100.0

100.1 to 120.0

120.1 to 140.0

140.1 to 170.0

170.1 to 210.0

210.1 to 302.0

0 10 20 MILES

0 10 20 KILOMETERS

Figure 16. Map showing altitude of land surface (top of the surficial aquifer system) in the 

East-Central Florida Transient (ECFT) study area.

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 16. Altitude of land surface (top of the surficial aquifer system) in the East-Central Florida Transient 

(ECFT) study area.
24 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

SEMINOLE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY BREVARD

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER COUNTY

East-central Florida

transient model area

0 10 20 MILES

0 10 20 KILOMETERS

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

Figure 17. Map showing thickness of the surficial aquifer system in the East-Central Florida 

Transient (ECFT) study area.

EXPLANATION

Estimated thickness of the surficial aquifer 

 system, in feet. White areas are inactive 

 in the SAS

5.0 to 30.0

30.1 to 50.0

50.1 to 70.0

70.1 to 90.0

90.1 to 110.0

110.1 to 130.0

130.1 to 150.0

150.1 to 170.0

170.1 to 240.6

ATLANTIC OCEAN

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 17. Thickness of the surficial aquifer system in the East-Central Florida Transient (ECFT) study area.
Hydrology of the Study Area 25

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY HARDEE

COUNTY

SUMTER COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

&

&

&

&

&

&

& &

&

&

&

!

!

!

!

!

!

!

!

6

6

19

24 50

32

34

34 10

55

48

19

20

46

32

33

42

50

21

East-central Florida

transient model area

0 10 20 MILES

0 10 20 KILOMETERS

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

EXPLANATION

& 55 Location of aquifer test in the SAS 

 and hydraulic conductivity, in 

 feet per day

ATLANTIC OCEAN

Figure 18. Map showing hydraulic conductivity in the surficial aquifer system (SAS) from 

selected aquifer tests.

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 18. Hydraulic conductivity in the surficial aquifer system (SAS) from selected aquifer tests.
26 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

0 10 20 MILES

0 10 20 KILOMETERS

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

Figure 19. Map showing altitude of the upper surface of the intermediate confining 

unit/intermediate aquifer system in the East-Central Florida Transient (ECFT) 

study area.

EXPLANATION

Estimated altitude of the top of the 

 intermediate confining unit or the 

 intermediate aquifer system, in feet 

 NGVD 29

–136.8 to –80.0

–79.9 to –60.0

–59.9 to –40.0

–39.9 to –20.0

–19.9 to 0

0.1 to 20.0

20.1 to 40.0

40.1 to 80.0

80.1 to 135.4

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

ATLANTIC OCEAN

East-central Florida 

transient model area

Figure 19. Altitude of the upper surface of the intermediate confining unit/intermediate aquifer system in the 

East-Central Florida Transient (ECFT) study area.
Hydrology of the Study Area 27

0 10 20 MILES

0 10 20 KILOMETERS

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

SEMINOLE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

BREVARD

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

HARDEE COUNTY

COUNTY

SUMTER COUNTY

ATLANTIC OCEAN

East-central Florida

transient model area

EXPLANATION

3.3 to 50.0

50.1 to 90.0

90.1 to 130.0

130.1 to 170.0

Estimated thickness of the intermediate

 confining unit or intermediate aquifer 

 system, in feet

170.1 to 210.0

210.1 to 270.0

270.1 to 320.0

320.1 to 412.1

Figure 20. Map showing thickness of the intermediate confining unit/intermediate aquifer 

system in the East-Central Florida Transient (ECFT) study area.

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 20. Thickness of the intermediate confining unit/intermediate aquifer system in the East-Central Florida 

Transient (ECFT) study area.
28 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY HARDEE

COUNTY

Approximate 

eastern 

extent of the 

intermediate 

aquifer 

system

SUMTER COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

0 10 20 MILES

0 10 20 KILOMETERS

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

East-central Florida

transient model area

ATLANTIC OCEAN

!

!

! !

!

! !

!

!

! !

!

! !

EXPLANATION

Location of aquifer test in the 

 IAS or ICU and hydraulic 

 conductivity, in feet per day

! 160

23

1

1

5

2

19 81

24

12

49

35

160

230

150

Figure 21. Map showing hydraulic conductivity in the intermediate aquifer system (IAS) 

and the intermediate confining unit (ICU) from selected aquifer tests.

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 21. Hydraulic conductivity in the intermediate aquifer system (IAS) and the intermediate confining 

unit (ICU) from selected aquifer tests.
Hydrology of the Study Area 29

nondeposition, and the Avon Park Formation is the uppermost 

geologic unit of the FAS. The base of the FAS is within the 

Cedar Keys Formation and is characterized by a regionally continuous sequence of anhydrite beds interlayered 

with dolostone.

The FAS is subdivided into regionally extensive aquifers 

and confining units, including the UFA, MCU I/II, and LFA. 

The UFA is further subdivided into the regionally extensive 

OPZ, OLPZ, and APPZ (fig. 15). All of these units are 

explicitly represented in the ECFT model. The permeable 

zones of the UFA are characterized by variable porosity 

and permeability, depending on the depositional texture and 

mineralogy of the rocks and on the presence of fractures and 

secondary dissolution features. Within each hydrogeologic 

unit, local zones of contrasting permeability may be present, 

but these contrasting zones generally do not affect the regional 

hydraulic characteristics of a unit.

The configuration, dip, and thickness of the OPZ, OLPZ, 

and APPZ are influenced by regional and local structural 

features, as well as karst and erosional processes. The altitude 

of the top of each unit follows regional structures, and the 

thickness of each unit tends to be greater in structurally low 

areas and less in the structurally high areas. The subregional to 

localized highs and lows in the ECFT study area are probably erosional or karst features superimposed on the regional 

structural features. The bottom of the UFA is marked by a 

regionally extensive confining unit of lower permeability, 

corresponding either to MCU I in most of the study area or to 

MCU II in the southwestern part of the study area.

The Ocala Permeable Zone

The OPZ is a highly productive zone at the top of the 

FAS, characterized by intergranular, moldic, and vuggy 

primary porosity and well-developed secondary porosity 

(Davis and Boniol, 2011); the zone corresponds to the Ocala 

Limestone and the Suwannee Limestone (where present) 

lithostratigraphic units. The OPZ is equivalent to zone A of 

the UFA (O’Reilly and others, 2002) and the upper permeable zone of the UFA (Spechler, 2010). Zones of enhanced 

permeability are characterized by cavities, caves, and other 

dissolution features likely formed by karst processes. The OPZ 

is the primary groundwater production zone for many areas 

of Florida, except for those areas having poor water quality. 

The altitude of the top of the OPZ in the study area ranges 

from 426.2 to 85.6 ft NGVD 29 (fig. 22). The thickness of the 

OPZ ranges from 50.1 to 519 ft (fig. 23). Horizontal hydraulic 

conductivity values derived from aquifer tests selected from 

USGS, SJRWMD, SWFWMD, and SFWMD databases 

ranged from 6 to 14,000 ft/d in the OPZ and show large spatial 

variability (fig. 24).

A potentiometric surface was estimated for the OPZ 

for average 1995 conditions from monthly average heads 

(fig. 25). The data for most of the sites used to generate the 

surface include 12 average monthly head values, but about 

10 percent of the sites had fewer data. For each of these sites, 

average annual head was estimated using multiple linear 

regressions that relate the average annual head to two monthly 

average heads.

The potentiometric surface of the OPZ shows a groundwater high in Polk County that groundwater would flow radially away from. This high corresponds to a topographic high 

in the southern Green Swamp physiographic area (figs. 2 and 

16). The shape of the potentiometric mound around this high 

is oblong, creating an approximately north/south divide in the 

western part of the study area, from Marion County through 

the high in Highlands County that approximately follows the 

Lake Wales Ridge (fig. 25). Groundwater flows away from 

the divide and generally in a direction perpendicular to the 

potentiometric surface contours. The 10-ft closed depression 

contour in the northeastern area shown in figure 25 corresponds to the location of Blue Spring (table 2–1, figs. 25 and 

2–1), the spring with the greatest flow rate in the model area. 

The absence of a corresponding depression in the potentiometric surface map for Alexander Springs (figs. 1 and 25) is 

likely the result of the 5-ft contour interval, which is larger 

than the depression in local heads caused by the spring. 

Closely spaced contours in the west-central section of the 

model area reflect a transition zone in hydraulic conductivity 

(figs. 24 and 25).

The Ocala Low-Permeable Zone

The OLPZ is a zone of relatively lower permeability than 

that of the overlying OPZ and underlying APPZ, and consists 

of micritic limestone, dolomitic limestone, and dolostone 

within the Ocala Limestone and the upper Avon Park Formation. This zone is equivalent to the middle confining unit I 

separating the UFA from the APPZ as defined by Reese and 

Richardson (2008). The OLPZ is characterized by the lack of 

enhanced dissolution features or fractures that are prevalent in 

permeable units of the UFA. Although intervals of moderate 

permeability or thin cavities can be present within the OLPZ, 

this unit is not a major source of groundwater, and generally 

acts as a semiconfining zone. The term “Ocala low-permeable 

zone” used herein replaces nomenclature previously used to 

describe the Ocala semiconfining zone (Mallams and Lee, 

2005). The OLPZ has, however, been used in some recent 

SWFWMD Regional Observation and Monitor-well Program 

(ROMP) reports (LaRoche, 2007). The altitude of the OLPZ 

in the study area ranges from -929.7 to -90.3 ft NGVD 29 

(fig. 26). The thickness of the OLPZ ranges from 10 to 

715.1 ft (fig. 27).

The Avon Park Permeable Zone

The APPZ consists of thick beds of dolostone with thinner 

interbedded limestone and dolomitic limestone that are present 

within the middle to upper Avon Park Formation (O’Reilly and 

others, 2002). This hydrogeologic unit has been alternatively 

defined as zone B of the UFA (O’Reilly and others, 2002), and 

the dolostone zone (McGurk and Presley, 2002). The dolostone texture varies from poorly to moderately consolidated, 
30 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

Figure 22. Map showing altitude of the upper surface of the Ocala permeable zone (top 

of the Upper Floridan aquifer) in the East-Central Florida Transient (ECFT) study area. SUMTER COUNTY SUMTER COUNTY

ATLANTIC OCEAN

East-central Florida 

transient model area

Base from U.S. Geological Survey 

digital data, Universal Transverse 

Mercator projection, zone 17

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY HIGHLANDS

COUNTY

HARDEE

COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

EXPLANATION

Estimated altitude of the top of the Ocala 

 permeable zone, in feet NGVD 29

–426.2 to –320.0

–319.9 to –260.0

–259.9 to –200.0

–199.9 to –140.0

–139.9 to –90.0

–89.9 to –50.0

–49.9 to 0

0.1 to 85.6

0 10 20 MILES

0 10 20 KILOMETERS

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 22. Altitude of the upper surface of the Ocala permeable zone (top of the Upper Floridan aquifer) in 

the East-Central Florida Transient (ECFT) study area.
Hydrology of the Study Area 31

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

SEMINOLE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

BREVARD

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

HARDEE COUNTY

COUNTY

SUMTER

COUNTY

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

EXPLANATION

50.1 to 130.0

130.1 to 170.0

170.1 to 210.0

210.1 to 250.0

250.1 to 290.0

290.1 to 350.0

350.1 to 410.0

410.1 to 519.0

Estimated thickness of the Ocala 

 permeable zone, in feet

East-central Florida

transient model area

0 10 20 MILES

0 10 20 KILOMETERS

ATLANTIC OCEAN

Figure 23. Map showing thickness of the Ocala permeable zone in the East-Central Florida 

Transient (ECFT) study area.

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 23. Thickness of the Ocala permeable zone in the East-Central Florida Transient (ECFT) study area.
32 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY HARDEE

COUNTY

SUMTER

COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

! !

!

!

!

!

!

!

!

!

!

!

!

! !

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

! !

!

!

!

!

!

!

!

East-central Florida

transient model area

0 10 20 MILES

0 10 20 KILOMETERS

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

ATLANTIC OCEAN

190

350

100

110

230

600

190

120

260

180

240

730

740

340

110

110

320

320

980

840

730

700

320 260

240

150

280

700

310

270

140

100

100

220

390

140

270

190

220

160 110

270

7,600

2,500

1,200

14,000

6

50

38

29

29

16

14

64

53

20

23

19

18

83

73

50

35 56

44 31

46

38

47

50

45

51

22 27

16

33

7

34

55

59

44

97

63

EXPLANATION

Location of aquifer test in the Ocala 

 permeable zone and hydraulic 

 conductivity, in feet per day

! 390

9

52

Figure 24. Map showing hydraulic conductivity in the Ocala permeable zone from selected 

aquifer tests.

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 24. Hydraulic conductivity in the Ocala permeable zone from selected aquifer tests.
Hydrology of the Study Area 33

MARION

COUNTY

LAKE

COUNTY VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER

COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

45

40

50

35

55

60

65

70

75

30

25

80

20

15

85

90

10

95

110

100

105

5

115

120

125

10

60

80

65

25

85 50

45

70

55

20

75

30

50

15

35

10 !

!

! !

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

! !

!

!

! !

!

!

!

! !

!

! !

!

!

! !

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

! ! ! !

!

!

! ! ! ! !

!

!

! !

! !

!

!

! !

! ! ! ! !

!

!

! ! ! ! !

!

!

!

! !! ! !

! ! ! !

! ! !

! !

! ! ! ! !

!

!

! !

!

!

!

!

!

!

! ! ! !

! ! !

!

! !

! !

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

! !

! !

! !

!

!

! !

!

!

!

!

!

!

!

!

!

!

!

!

! !

!

!

!

!

Blue 

Spring

Alexander 

Springs

East-central Florida

transient model area

0 10 20 MILES

0 10 20 KILOMETERS

Base from U.S. Geological Survey 

digital data, Universal Transverse 

Mercator projection, zone 17

ATLANTIC OCEAN

EXPLANATION

!

85 Potentiometric surface contour—Shows altitude at which 

 water level would have stood in tightly cased wells. 

 Contour interval 5 feet. Hachures indicate depressions. 

 Altitudes are in feet NGVD 29

Groundwater divide—Groundwater flows away 

 from the divide, and perpendicular to the potentiometric 

 surface contours

Observation well

Flow direction from potentiometric high in Polk County

Figure 25. Map showing estimated potentiometric surface of the Ocala permeable zone 

(Upper Floridan aquifer), average 1995 conditions.

7

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 25. Estimated potentiometric surface of the Ocala permeable zone (Upper Floridan aquifer), average 

1995 conditions.
34 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

East-central Florida

transient model area

0 10 20 MILES

0 10 20 KILOMETERS

Base from U.S. Geological Survey 

digital data, Universal Transverse 

Mercator projection, zone 17

ATLANTIC OCEAN

EXPLANATION

–929.7 to –720.0

–719.9 to –620.0

–619.9 to –520.0

–519.9 to –420.0

Estimated altitude of the top of the Ocala 

 low-permeable zone, in feet NGVD 29

–419.9 to –320.0

–319.9 to –220.0

–219.9 to –150.0

–149.9 to –90.3

Figure 26. Map showing altitude of the upper surface of the Ocala low-permeable zone in 

the East-Central Florida Transient (ECFT) study area.

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

SUMTER COUNTY

Figure 26. Altitude of the upper surface of the Ocala low-permeable zone in the East-Central Florida 

Transient (ECFT) study area.
Hydrology of the Study Area 35

Figure 27. Map showing thickness of the Ocala low-permeable zone in the East-Central 

Florida Transient (ECFT) study area.

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

SEMINOLE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

BREVARD

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

EXPLANATION

10.1 to 40.0

40.1 to 110.0

110.1 to 180.0

180.1 to 260.0

Estimated thickness of the Ocala 

 low-permeability zone, in feet

260.1 to 360.0

360.1 to 460.0

460.1 to 560.0

560.1 to 715.1

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

0 10 20 MILES

0 10 20 KILOMETERS

East-central Florida

transient model area

ATLANTIC OCEAN

SUMTER COUNTY

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 27. Thickness of the Ocala low-permeable zone in the East-Central Florida Transient (ECFT) study area.
36 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

to dense, hard, and massive. The dolostones typically have low 

intergranular primary porosity, with high secondary porosity 

and permeability caused by fracturing and dissolution cavities. 

Zones of fractured and cavernous dolostone in the APPZ are a 

major source of water supply. The degree of hydraulic connectivity between productive zones is uncertain and dependent 

upon the presence of interconnected fractures and caverns. 

Layers of dense, unfractured dolostone provide a degree of 

confinement between the flow zones. 

 APPZ wells are commonly characterized by numerous 

abrupt borehole enlargements when drilled into carbonate rock 

affected by dissolution, fracturing, and downhole sloughing 

of rock material. Characteristic borehole geophysical data 

include an increase in gamma-ray activity associated with the 

dolostone, high resistivity except where affected by borehole 

enlargement, and erratic neutron detection activity or acoustic 

velocity suggesting substantial porosity variability. Borehole 

flowmeter data indicate the presence of large flow zones 

marked by large temperature or fluid resistivity changes (Reese 

and Richardson, 2008; O’Reilly and others, 2002). Video 

logs also show the presence of distinct fracture and cavernous 

zones. Within the study area, many of the wells utilized for 

public supply and agricultural purposes are completed into this 

zone. In the ECFT study area, the altitude of the upper surface 

of the APPZ ranges from -1,131 to -104 ft NGVD 29 (fig. 28) 

and APPZ thickness ranges from 50 to 629.4 ft (fig. 29).

Selected horizontal hydraulic conductivity values compiled 

for the APPZ ranged from 30 to 3,400 ft/d (fig. 30), a narrower 

range than that compiled for the OPZ, but this may be at least 

partly the result of fewer data. Because of the leakiness of the 

OLPZ, head differences between the OPZ and the APPZ are relatively small and range from about -3 ft in southwestern Volusia 

County, indicating an upward gradient, to almost 4 ft in eastern 

Seminole County, indicating a downward gradient (fig. 31).

Middle Confining Unit I and Middle Confining Unit II

The MCU I and MCU II are two distinct regional confining 

units that correlate with the Avon Park Formation and separate 

the UFA from the LFA. MCU I corresponds to the middle 

semiconfining unit I in the eastern half of peninsular Florida, 

and MCU II corresponds to the middle semiconfining unit II in 

west-central Florida, as designated by Miller (1986). MCU I 

consists of dolostone, micritic limestone, and dolomitic limestone, locally containing some gypsum and chert, and corresponds to the middle to lower part of the Avon Park Formation. 

This confining unit is characterized and distinguished from the 

overlying UFA and underlying LFA by secondary mineralization 

in the pore spaces and a lack of secondary porosity. MCU I can 

be identified in borehole logs by a consistent temperature and 

fluid resistivity relative to overlying and underlying permeable 

zones, suggesting intra-borehole flow and the lack of substantial 

flow zones (O’Reilly and others, 2002). MCU II is present in 

the southwestern part of the study area at a lower altitude than 

MCU I and is composed of hard, crystalline dolomitic limestone 

and gypsiferous dolomite corresponding to the middle to lower 

parts of the Avon Park Formation. MCU II is characterized 

by the presence of gypsum as beds or infilling of pore spaces, 

and by dense, unfractured dolomite that exhibits a decrease in 

formation resistivity and a sharp decrease in flow compared to 

the overlying and underlying permeable zones.

 MCU I and MCU II are offset, and the former areally 

overlaps the latter within the study area, so these units are 

considered together herein. The altitude of the upper surface 

of the MCU I/II in the study area ranges from -1,604.3 to 

-153.9 ft NGVD 29 (fig. 32) and its thickness ranges from 

115.8 to 743.7 ft (fig. 33).

The Lower Floridan Aquifer

The Lower Floridan aquifer (LFA) comprises a sequence 

of alternating zones of relatively higher and lower permeability. 

The top of the LFA is identified by a substantial increase in 

permeability relative to the overlying MCU I/II as indicated by 

borehole flow, temperature, and fluid resistivity logs (O’Reilly 

and others, 2002). The upper permeable zone of the LFA 

consists largely of dolostone and dolomitic limestone, with 

abundant fractures and dissolution features corresponding to 

the lower part of the Avon Park Formation and the upper part of 

the Oldsmar Formation. Some chert, peat, and minor amounts 

of gypsum and anhydrite may be present in lower parts of this 

zone. Enhanced permeability occurs in intervals containing 

fractured dolostone. The upper permeable zone of the LFA can 

be highly productive in some locations and is the source for a 

large number of public water-supply utilities in the study area. 

The altitude of the top of the LFA in the study area ranges 

from -1,854.1 to -435.1 ft NGVD 29 (fig. 34). The hydrogeologic framework presented herein does not differentiate 

permeable zones below the LFA upper permeable zone. The 

number of wells for which identification of the lower units is 

possible is limited and insufficient for regional mapping. The 

altitude of the base of the FAS as mapped by Miller (1986, 

plate 33) was used to estimate the thickness of the LFA in the 

study area, which ranged from 922 to 1,944 ft (fig. 35).

The limited information available concerning the 

lithology, permeability, and water-bearing characteristics 

of the LFA generally pertains to the upper permeable zone. 

Selected values of horizontal hydraulic conductivity from 

aquifer tests in the LFA, compiled from databases of the 

USGS, SJRWMD, SWFWMD, and SFWMD, ranged from 

23 to 7,700 ft/d (fig. 36).

Where average monthly head data are available for both 

the UFA and LFA, the head differences between the aquifers 

range from more than 37 ft in northeastern Polk County, indicating a downward gradient, to almost -9 ft in southwestern 

Volusia County, indicating an upward gradient. The MCU II 

has characteristically low permeability in the southwestern 

portion of the ECFT study area, and likely causes the large 

head differences between the UFA and the LFA observed near 

the boundary of the eastern extent of the MCU II (fig. 31) 

(B.E. McGurk, St. Johns River Water Management District, 

written commun., 2010).
Hydrology of the Study Area 37

Figure 28. Map showing altitude of the upper surface of the Avon Park permeable zone 

in the East-Central Florida Transient (ECFT) study area.

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

SEMINOLE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

BREVARD

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

HARDEE COUNTY

COUNTY

EXPLANATION

–1,131.0 to –900.0

–899.9 to –800.0

–799.9 to –700.0

–699.9 to –600.0

Estimated altitude of the top of the Avon Park 

 permeability zone, in feet NGVD 29

–599.9 to –450.0

–449.9 to –350.0

–349.9 to –250.0

–249.9 to –103.9

Base from U.S. Geological Survey 

digital data, Universal Transverse 

Mercator projection, zone 17

East-central Florida

transient model area

0 10 20 MILES

0 10 20 KILOMETERS

ATLANTIC OCEAN

SUMTER COUNTY

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 28. Altitude of the upper surface of the Avon Park permeable zone in the East-Central Florida Transient 

(ECFT) study area.
38 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

Figure 29. Map showing thickness of the Avon Park permeable zone in the East-Central 

Florida Transient (ECFT) study area.

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

SEMINOLE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

BREVARD

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

HARDEE COUNTY

COUNTY

ATLANTIC OCEAN

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

0 10 20 MILES

0 10 20 KILOMETERS

East-central Florida

transient model area

SUMTER COUNTY

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

EXPLANATION

Estimated thickness of the Avon Park 

 permeability zone, in feet

50.0 to 100.0

100.1 to 170.0

170.1 to 240.0

240.1 to 310.0

310.1 to 380.0

380.1 to 440.0

440.1 to 500.0

500.1 to 629.4

Figure 29. Thickness of the Avon Park permeable zone in the East-Central Florida Transient (ECFT) study area.
Hydrology of the Study Area 39

SUMTER COUNTY

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

!

!

!

!

!

!

!

!

!

!

!

!

!

!

290

940

130

1,200

3,400

3,100

53

30

140

190

430

690

220

200

EXPLANATION

! 290 Location of aquifer test in the Avon 

 Park permeable zone and hydraulic 

 conductivity, in feet per day

0 10 20 MILES

0 10 20 KILOMETERS

ATLANTIC OCEAN

East-central Florida 

transient model area

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

Figure 30. Map showing hydraulic conductivity in the Avon Park permeable zone from 

selected aquifer tests.

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 30. Hydraulic conductivity in the Avon Park permeable zone from selected aquifer tests.
40 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

ATLANTIC OCEAN

East-central Florida 

transient model area

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

!

! !

!

! !

!

!

!

!

!

!

!

! !

!

!

!

!

! !

!!

! !

! !

! !

!

!

3.82

1997

0.32

1999

0.56

1996

–0.11

2000

1.85

1995

2.19

1999

–1.7

1997

1.02

1999

–2.98

2005

–2.63

2001

–0.5

1996

12.4

2003

4.32

2004

–0.65

1997

–3.33

1995

4.53

1997

0.49

1995

37.59

2006

–2.16

2005

–3.66

2001

1.7

1997

1.41

1995

2.38

1995

–2.65

1997

5.03

2006

0.98

1999

–7.35

1997

1.26

2002

0.00

2004

–8.84

1995

33.12

2001

Eastern

extent

of

middle

confining unit II

EXPLANATION

Location of UFA and LFA wells. First 

 number indicates head in the UFA 

 (either an OPZ or an APPZ well), 

 minus head in the LFA, second 

 number indicates year for which 

 average heads were calculated. 

 Head difference in feet NGVD 29

Location of OPZ and APPZ wells. First 

 number indicates head in the OPZ, 

 minus head in the APPZ, second 

 number indicates year for which 

 average heads were calculated. 

 Head difference in feet NGVD 29

!

!

0.56

1996

0.49

1995

0 10 20 MILES

0 10 20 KILOMETERS

Figure 31. Map showing differences in average annual heads between the Ocala and Avon 

Park permeable zones (OPZ and APPZ) and between the Upper and Lower Floridan aquifers 

(UFA and LFA).

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY HARDEE

COUNTY

SUMTER COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 31. Differences in average annual heads between the Ocala and Avon Park permeable zones (OPZ 

and APPZ) and between the Upper and Lower Floridan aquifers (UFA and LFA).
Hydrology of the Study Area 41

Base from U.S. Geological Survey 

digital data, Universal Transverse 

Mercator projection, zone 17

0 10 20 MILES

0 10 20 KILOMETERS

East-central Florida

transient model area

Figure 32. Map showing altitude of the upper surfaces of the middle confining units I/II 

(MCU I/II) in the East-Central Florida Transient (ECFT) study area.

EXPLANATION

Estimated altitude of the top of the middle confining 

 unit I and middle confining unit II, in feet NGVD 29

–1,604.3 to –1,350.0

–1,349.9 to –1,200.0

–1,199.9 to –1,000.0

–999.9 to –900.0

–899.9 to –700.0

–699.9 to –550.0

–549.9 to –400.0

–399.9 to –153.9

middle

confining unit II

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

SEMINOLE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

BREVARD

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

HIGHLANDS COUNTY

COUNTY

HARDEE

COUNTY

Eastern

extent

of

ATLANTIC OCEAN

SUMTER COUNTY

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 32. Altitude of the upper surfaces of the middle confining units I/II (MCU I/II) in the East-Central Florida 

Transient (ECFT) study area.
42 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

Figure 33. Map showing thickness of the middle confining units I/II (MCU I/II) in the East-Central 

Florida Transient (ECFT) study area.

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

SEMINOLE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

BREVARD

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

HARDEE COUNTY

COUNTY

EXPLANATION

Estimated thickness of middle confining 

 units I and II, in feet

115.8 to 210.0

210.1 to 270.0

270.1 to 330.0

330.1 to 400.0

400.1 to 470.0

470.1 to 540.0

540.1 to 610.0

610.1 to 743.7

Base from U.S. Geological Survey 

digital data, Universal Transverse 

Mercator projection, zone 17

0 10 20 MILES

0 10 20 KILOMETERS

East-central Florida

transient model area

ATLANTIC OCEAN

SUMTER COUNTY

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 33. Thickness of the middle confining units I/II (MCU I/II) in the East-Central Florida Transient (ECFT) 

study area.
Hydrology of the Study Area 43

Base from U.S. Geological Survey 

digital data, Universal Transverse 

Mercator projection, zone 17

0 10 20 MILES

0 10 20 KILOMETERS

East-central Florida

transient model area

Figure 34. Map showing altitude of the upper surface of the Lower Floridan aquifer in the 

East-Central Florida Transient (ECFT) study area.

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

SEMINOLE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

BREVARD

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

HARDEE COUNTY

COUNTY

EXPLANATION

–1,854.1 to –1,600.0

–1,599.9 to –1,400.0

–1,399.9 to –1,250.0

–1,249.9 to –1,100.0

–1,099.9 to –1,000.0

–999.9 to –900.0

–899.9 to –700.0

–699.9 to –435.1

Estimated altitude of the top of the Lower Floridan 

 upper permeable zone, in feet NGVD 29

ATLANTIC OCEAN

SUMTER COUNTY

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 34. Altitude of the upper surface of the Lower Floridan aquifer in the East-Central Florida Transient 

(ECFT) study area.
44 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

Base from U.S. Geological Survey 

digital data, Universal Transverse 

Mercator projection, zone 17

East-central Florida

transient model area

Figure 35. Map showing thickness of the Lower Floridan aquifer in the East-Central Florida 

Transient (ECFT) study area.

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

SEMINOLE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

BREVARD

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

921.8 to 1,150.0

1,150.1 to 1,250.0

1,250.1 to 1,350.0

1,350.1 to 1,450.0

1,450.1 to 1,550.0

1,550.1 to 1,650.0

1,650.1 to 1,800.0

1,800.1 to 1,944.3

Estimated thickness of the Lower Floridan aquifer, 

 in feet

0 10 20 MILES EXPLANATION

0 10 20 KILOMETERS

ATLANTIC OCEAN

SUMTER COUNTY

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 35. Thickness of the Lower Floridan aquifer in the East-Central Florida Transient (ECFT) study area.
Hydrology of the Study Area 45

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY !

!

!

!

!

!

!

!

!

! !

!

!

!

!

!

!

!

!

!

!

!

!

98

220

280

700 170

160

430

1,600

1,600 3,100

1,500

7,700

1,000

23

690

120

360

1,200

1,200

3,100

0 10 20 MILES

0 10 20 KILOMETERS

ATLANTIC OCEAN

East-central Florida 

transient model area

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17 280

EXPLANATION

! Location of aquifer test in the Lower

 Floridan aquifer and hydraulic 

 conductivity, in feet per day

1,700

1,400

1,200

Figure 36. Map showing hydraulic conductivity in the Lower Floridan aquifer from selected 

aquifer tests.

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 36. Hydraulic conductivity in the Lower Floridan aquifer from selected aquifer tests.
46 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

Chloride Distribution

The surface representing 5,000 milligrams per liter 

(mg/L) of chloride in groundwater, or the 5,000 mg/L isochlor, 

was estimated to determine where the freshwater/saltwater 

transition zone was likely to act as a barrier to flow because 

of vertical density differences, and thus represented the lower 

boundary of the simulated part of the groundwater system. 

Flow across the interface between the freshwater and salinewater flow systems can be assumed to be negligible where the 

transition and consequent density disparity occur abruptly and 

in the absence of any large nearby stress, such as a pumping 

well (Reilly, 2001, p. 11). The 5,000 mg/L isochlor was chosen 

as the base of the freshwater flow system in east-central 

Florida because (1) it approximately represents the boundary 

between moderately brackish water and very brackish to saline 

water (Sprinkle, 1989), and (2) the thickness of the transition 

zone between the 5,000 and 10,000 mg/L isochlors is relatively thin (McGurk and Presley, 2002). UFA water levels near 

the study-area coastlines lie above NGVD 29 and decrease 

seaward, indicating an onshore-to-offshore flow gradient that 

minimizes the movement of seawater inland (Todd, 1980).

To construct the 5,000 mg/L isochlor, a database 

containing site locations, water-sample depths, and chloride 

concentrations of groundwater samples from FAS wells was 

created from data compiled by the SJRWMD, SFWMD, 

SWFWMD, and USGS. The chloride data were used to 

estimate the altitudes in the FAS where chloride concentrations were approximately 5,000 mg/L. The SJRWMD mapped 

the altitude where the 5,000-mg/L isochlor was present in the 

FAS in east-central Florida (McGurk and Presley, 2002 and 

Sepúlveda, 2002). The altitude of the 5,000 mg/L isochlor was 

based on measured groundwater chloride concentrations and 

Sprinkle’s (1989) map showing the altitude of water having a 

10,000 mg/L chloride concentration (fig. 37). In the western 

part of the study area, chloride concentrations were extrapolated based on chloride trends in the eastern part because of 

the paucity of available data.

Water Use

Water use is categorized as (1) groundwater withdrawals 

from the SAS, IAS, OPZ, OLPZ, APPZ, MCU I/II, and the 

LFA; (2) artificial recharge systems, including drainage wells 

that directly recharge confined aquifers and rapid infiltration basins (RIBs) that recharge the SAS; and (3) irrigation, 

which recharges the SAS. The various types of groundwater 

withdrawal rates or direct water-recharge (inflows) data 

were provided for the years 1995 to 2006 by the SJRWMD, 

SWFWMD, SFWMD, and the Florida Department of Environmental Protection (FDEP); these data represent the best available data at the time of this study. The reported and estimated 

water withdrawals and inflows represent amounts for all of 

Seminole, Orange, Brevard, and Osceola Counties and parts 

of Lake, Marion, Volusia, Sumter, Polk, Indian River, Hardee, 

Highlands, Okeechobee, and St. Lucie Counties (fig. 38A). 

Groundwater Withdrawals

Groundwater withdrawals included public water supply, 

commercial or industrial, agricultural, domestic self-supply, 

and a small component from free-flowing wells. Total estimated annual average groundwater withdrawals for all uses 

for the period 1995 to 2006 ranged from 819 to 1,146 Mgal/d. 

Annual average withdrawals by water use category for all 

years were 456 Mgal/d for public water supply (49 percent 

of the total); 351 Mgal/d for agricultural use (38 percent of 

the total); 61 Mgal/d for commercial and industrial uses; 

54 Mgal/d for domestic self-supply use; and less than 

1 Mgal/d for free-flowing wells (table 1). Monthly variation 

in water use for each water use category for the period 1995 

to 2006 showed the peaks of the two largest categories (public 

supply and agricultural) generally coincided in time (fig. 38B). 

Based on consideration of well construction data, more than 

95 percent of groundwater withdrawn within the ECFT 

study area for the 12-year study period was from the FAS, 

with the following average values the other hydrogeologic 

units: 10 Mgal/d from the SAS; 13 Mgal/d from the IAS; 

722 Mgal/d from the UFA (including 3 Mgal/d from MCU); 

and 121 Mgal/d from the LFA.

Public-water-supply groundwater-withdrawal data for the 

SJRWMD and SFWMD were gathered from FDEP Monthly 

Operating Reports (MORs) for 1995 through 2006. Annual 

average public-supply water use for the study area ranged 

from 371.6 Mgal/d in 1995 to 549 Mgal/d in 2006. This 

category represents all water sent to service from the watertreatment facilities for public water supply on a monthly basis. 

Total monthly water amounts delivered from a water treatment 

plant were distributed equally to active wells that supply the 

plant. Some exceptions to this approach included assigning 

specific well distributions determined from permitting 

compliance reports submitted to the three Water Management 

Districts (WMDs) by the utilities. Water accounted as public 

water supply includes all permit holders that pump from wells 

with a casing at least 4 inches in diameter at a rate equal to 

or greater than 100,000 gallons per day (gal/d). Groundwater 

withdrawals for public water supply in the SWFWMD area 

were compiled from metered data for most production wells 

or from estimates for a few wells within the SWFWMD area. 

Public-supply water use by county shows Orange County 

having the largest withdrawal rates from 1995 to 2006 

(appendix 1, table 1–1).

In cases where groundwater withdrawals from individual 

public supply wells were unknown, each active well was 

assigned a withdrawal amount equal to the total withdrawal 

multiplied by the ratio of the well capacity to the sum of well 

capacities for all active wells in the well field. Many production wells withdraw from more than one discrete aquifer unit 

within the FAS. To apportion the groundwater withdrawals 

between hydrogeologic units within the FAS, the assigned 

groundwater withdrawal for each unit was set equal to the total 

withdrawal rate multiplied by the ratio of the transmissivity of 

the interval open to a particular aquifer to the transmissivity of 
Hydrology of the Study Area 47

0 10 20 MILES

0 10 20 KILOMETERS

ATLANTIC OCEAN

East-central Florida 

transient model area

Base from U.S. Geological Survey 

digital data, Universal Transverse 

Mercator projection, zone 17

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER

COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

EXPLANATION

Estimated altitude of water with chloride concentration 

 of 5,000 milligrams per liter in the Floridan aquifer 

 system, in feet NGVD 29

–2,200.0 to –2,100.0

–2,099.9 to –1,900.0

–1,899.9 to –1,700.0

–1,699.9 to –1,300.0

–1,299.9 to –1,000.0

–999.9 to –600.0

–599.9 to –400.0

–399.9 to –100.0

Figure 37. Map showing estimated altitude of water containing a chloride concentration of 

5,000 milligrams per liter in the East-Central Florida Transient (ECFT) study area of the 

Floridan aquifer system.

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 37. Estimated altitude of water containing a chloride concentration of 5,000 milligrams per liter in the 

East-Central Florida Transient (ECFT) study area of the Floridan aquifer system.
48 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

Figure 38. Map showing (A) Groundwater uses by area and chart showing (B) estimated 

average monthly groundwater withdrawals in the East-Central Florida Transient (ECFT) 

study area by water use.

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

HARDEE COUNTY

COUNTY SUMTER COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

0

200

400

600

800

700

500

300

100

1,000

1,200

1,400

1,300

1,100

900

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006

Year

Average monthly groundwater withdrawal, 

in million gallons per day

EXPLANATION

Public water-supply 

 area

Agricultural area

Commercial or industrial 

 area

Domestic self-supply 

 area

Free-flowing area

ATLANTIC OCEAN

0 10 20 MILES

0 10 20 KILOMETERS

Base from U.S. Geological Survey 

digital data, Universal Transverse 

Mercator projection, zone 17

EXPLANATION

82°00'

A

B

81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

East-central Florida

transient model area

Public-supply area

Domestic self-supply 

 area

Commercial or industrial 

 area

Agricultural area

Free-flowing area

Lakes

Figure 38. A, Groundwater uses by area, and B, estimated average monthly groundwater withdrawals in the 

East-Central Florida Transient (ECFT) study area by water use.
Hydrology of the Study Area 49

the entire open interval of the well. Transmissivity was calculated from the thickness of the open interval and the simulated 

hydraulic conductivity of the unit.

Groundwater withdrawals for commercial or industrial 

uses from SJRWMD and SFWMD areas were compiled from 

either FDEP MORs, or from water use reports submitted by 

well-field operators to the WMDs for 1995 through 2006. 

SWFWMD either measures or estimates groundwater withdrawals from commercial and industrial wells within their 

district. Water use for commercial or industrial uses varied 

from a high of 74.8 Mgal/d in 1995 to a low of 43.9 Mgal/d in 

2005 (table 1). Annual averages of commercial or industrial 

uses were highest in Polk County (appendix 1, table 1–2). 

Water use for each commercial entity was distributed among 

active wells based on the well radius listed in the permit. 

Secondary users for commercial or industrial purposes, which 

are supplied water from public water supply, are not included 

in the groundwater withdrawals listed in table 1 or appendix 1, 

table 1–2.

Two approaches were taken to account for agricultural 

water use. Information supplied by SWFWMD was used 

directly and consisted of either measured or estimated monthly 

pumping rates from wells at known locations. Monthly 

pumping rates from wells and locations used for agricultural 

purposes in SJRWMD and SFWMD were estimated using the 

Agricultural Field Scale Irrigation Requirement Simulation 

(AFSIRS) model (Smajstrla, 1990) and land-use data. These 

estimates were further refined with actual water-use data 

collected from the SJRWMD Benchmark Farms agricultural 

meter data program. The Benchmark Farms program consists 

of a group of farms that provide a statistical sampling of 

various agricultural commodity groups through the monitoring 

of water use at metered wells (Portier, 1988, Singleton, 1996).

For SJRWMD and SFWMD, the AFSIRS model was 

used to estimate monthly crop water requirements in response 

to climatic data for the calibration period. AFSIRS incorporates crop type, soil type, irrigation method, rainfall, and 

evapotranspiration (ET) to develop these monthly irrigation 

requirements. In order to determine the spatial extent of 

agriculture within the ECFT study area, SJRWMD developed 

data for irrigated lands within its area using land-use data 

from 1995, 1999, and 2004. The 1995 land-use data were 

used for 1995 to 1999, the 1999 land-use data were used for 

1999 to 2004, and the 2004 land-use data were used for 2004 

to 2006. Field observations were then made by SJRWMD 

to estimate the crop type and irrigation methods. Similar 

information for SFWMD was taken from land-use data, 

where available, or estimated based on similar practices in 

the region for the same land-use years. Rainfall data were 

obtained from SJRWMD NEXRAD rainfall database, and the 

ET data were obtained from the USGS (2012) as derived from 

Geostationary Operational Environmental Satellite (GOES) 

satellite data using the methodology of Mecikalski and others 

(2011). These data were compiled for each agricultural area 

and input files for the AFSIRS program were developed. The 

AFSIRS estimates of crop irrigation demands were used as 

initial values and then adjusted to better represent the average 

irrigation observed from the Benchmark Farms meter data 

(appendix 1, table 1–3). When Benchmark Farms information 

was not available, specific crop types from AFSIRS data were 

used. Data for agricultural well locations within SJRWMD and 

SFWMD were estimated based on the irrigated crop locations. 

Total groundwater withdrawals for agriculture represent the 

sum of pumping rates for all crop types or for the dominant 

crop associated with the irrigated project. All well withdrawals 

were assumed to be from the OPZ, unless otherwise specified 

in the available data. When surface water was the known 

source of irrigation water, the rates were not included in the 

agricultural water-use category. Agricultural water use ranged 

from a maximum of 528.7 Mgal/d in 2000 to a minimum of 

159.4 Mgal/d in 2005 (table 1). Annual averages of agricultural use show that Polk County has the largest groundwater 

withdrawals for each year within the ECFT study area 

(appendix 1, table 1–4). Agricultural water use has a clearly 

discernible seasonal variation (fig. 38B).

Estimated groundwater withdrawals from domestic selfsupply wells for 1995 to 2006 were provided by SJRWMD 

in the ECFT study area. Withdrawal rates were estimated 

using a geographical information system (GIS) approach that 

incorporated land use and per-capita use rate for each county. 

Residential land uses outside public-water-supply service 

areas were identified from appropriate databases for 1995 and 

Table 1. Average groundwater withdrawals by water use category and year. 

[PWS, public-water supply; AG, agricultural; CI, commercial or industrial; DSS, domestic self-supply; FFW, free-flowing well; Ave, average rate from 1995 to 

2006; all rates are in million gallons per day]

Use 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 Ave

PWS 371.6 402.6 403.2 450.6 458.7 506.6 449.0 453.9 452.5 482.8 487.0 549.0 455.6

AG 320.6 396.3 340.4 431.4 385.0 528.7 400.7 370.1 264.6 291.6 159.4 328.9 351.4

CI 74.8 72.5 69.5 71.8 64.8 60.0 56.5 58.8 57.7 48.9 43.9 53.9 61.1

DSS 49.6 49.6 49.6 49.6 49.6 49.6 49.6 49.6 49.6 49.6 75.1 75.1 53.9

FFW 2.2 1.8 1.4 1.2 1.1 0.6 0.0 0.0 0.0 0.0 0.0 0.0 0.7

Total 818.8 922.8 864.1 1,004.6 959.2 1,145.5 955.8 932.4 824.4 872.9 765.4 1,006.9 922.7
50 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

2004, and per capita use rates were multiplied by the estimated 

residential capacities to develop estimated water-use volumes 

for 1995 and 2005. Estimates for 1995 were applied to rates 

from 1995 to 2004, inclusive, and estimates for 2005 were 

applied to rates from 2005 to 2006. Domestic self-supply 

water use was estimated to be 49.6 Mgal/d from 1995 to 2004, 

and 75.1 Mgal/d for 2005 and 2006 (table 1).

Discharge rates from free-flowing wells in SJRWMD 

were estimated to decrease from 2.2 Mgal/d in 1995 to 

0.6 Mgal/d in 2000 (table 1). These estimates were obtained 

from the SJRWMD artesian well plugging program (St. Johns 

River Water Management District, 2004). There have been no 

known free-flowing wells in the SJRWMD since 2000, and 

no information was available for free-flowing wells, if any, 

outside the SJRWMD.

Artificial Recharge

Direct inflow to the OPZ occurs through 210 active 

drainage wells that are mostly concentrated in the Orlando 

metropolitan area (fig. 14). The status and locations of these 

wells were verified by Bulmer (2003). These wells, which 

are cased to the top of the OPZ and then drilled open-hole 

into the OPZ, either discharge street runoff from storm drains 

(street drainage wells) or control lake levels (lake-level control 

wells). The estimated total recharge to the OPZ through 

drainage wells in the study area ranged from a maximum of 

32.7 Mgal/d in 1997 to a minimum of 4.8 Mgal/d in 2000 

(table 2) and is based on rainfall for the delineated basins in 

figure 14 for the period 1995 to 2006, and the permeability of 

the top soils in the basins of these drainage wells. The recharge 

rates to the OPZ at drainage wells are strongly correlated to 

the amount of total rainfall for any period. The method used to 

derive the recharge rates is the same as that used to compute 

surface runoff throughout the model, which is explained later 

in this report.

RIBs provide a direct means of recharge to the SAS, and 

the SJRWMD has collected information about the locations 

of these basins and their applied rates of flow to the SAS 

through these basins. The data for RIBs were provided from 

multiple sources, including the FDEP and several utilities 

that operate these facilities. These recharge-rate data were 

reviewed for accuracy by the utility. Average monthly 

recharge flows from RIBs were calculated for the 1995 to 

2006 period. Annual average inflows to the SAS from RIBs 

in the ECFT study area showed these recharge rates were 

highest during 2005 and lowest during 1999 (table 3). An 

Table 2. Annual average of inflows to the Ocala permeable zone from street drainage and lake-level control wells by county.

[All rates are in million gallons per day]

County 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 Ave

Orange 26.5 25.4 31.1 17.2 12.0 4.6 15.5 22.2 18.6 29.4 16.0 6.1 18.7

Seminole 1.4 1.3 1.6 0.9 0.6 0.2 0.8 1.2 0.4 1.5 0.8 0.3 0.9

Total 27.9 26.7 32.7 18.1 12.6 4.8 16.3 23.4 19.0 30.9 16.8 6.4 19.6

Table 3. Annual average of inflows to the surficial aquifer system from rapid infiltration basins by county.

[Ave, average rate from 1995 to 2006; all rates are in million gallons per day]

County 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 Ave

Brevard 0.0 0.0 0.0 0.0 0.0 1.4 0.7 0.6 0.6 0.4 0.6 0.2 0.4

Lake 0.0 0.0 0.0 0.0 1.1 8.1 1.5 1.9 2.3 2.7 2.1 0.6 1.7

Orange 25.9 27.0 22.3 22.0 20.7 22.0 21.4 26.3 23.1 25.7 28.0 26.1 24.2

Osceola 2.6 3.7 4.1 2.5 2.5 2.5 2.5 2.6 4.1 2.5 3.7 2.6 3.0

Polk 0.2 1.6 0.7 2.3 1.0 1.0 1.0 0.2 0.7 1.0 1.6 0.2 1.0

Seminole 1.3 1.1 1.3 1.2 1.0 3.7 3.2 4.1 3.8 3.1 3.3 1.9 2.4

Sumter 0.0 0.0 0.0 0.0 0.0 0.1 0.1 0.1 0.1 0.0 0.0 0.0 0.0

Volusia 0.0 0.0 0.0 0.0 0.0 0.4 0.8 0.8 0.9 0.5 0.5 0.2 0.3

Total 30.0 33.4 28.4 28.0 26.3 39.2 31.2 36.6 35.6 35.9 39.8 31.8 33.0
Hydrology of the Study Area 51

additional source of recharge to the SAS was inflow from 

septic tanks, simulated as infiltration to the unsaturated zone. 

The estimated inflow from septic tanks was assumed to be 

50 percent of domestic self-supply well withdrawals.

Net average groundwater withdrawal and recharge rates 

from production wells, drainage wells, and RIBs were calculated for 1999 and 2006 to illustrate the spatial distribution of 

these rates. The spatial distribution patterns observed for the 

other years were similar. Drainage wells in the ECFT study 

area (fig. 14) are responsible for the observed net recharge 

the UFA, including both OPZ and APPZ, in central Orange 

County (figs. 14, 39A, and 40A). There is an extensive area 

of agricultural groundwater withdrawals in the UFA although 

the rates are not as large as those for many public-supply 

wells (figs. 38A, 39A, and 40A). The largest measured rates of 

groundwater withdrawal were in the Lower Floridan aquifer, 

concentrated in central Orange County, although at fewer 

wells than those with relatively large withdrawal rates in the 

UFA (figs. 39A–B, 40A–B).

Irrigation

Agricultural irrigation was applied based on the estimated 

crop needs presented in the Groundwater Withdrawals section. 

Recharge to the SAS resulting from agricultural irrigation 

rates in SJRWMD and SFWMD was applied to irrigated 

crop parcels identified in the SJRWMD and to agricultural 

lands for SFWMD based on the 1995, 2000, and 2005 data. 

Irrigation rates for the years 1996 to 1999 were applied based 

on 1995 land-use data, rates for 2000 to 2004 were applied 

based on 2000 land-use data, and rates for 2005 to 2006 were 

applied based on 2005 land-use data. Agricultural irrigation 

for SWFWMD was derived using data from metered wells 

and applied to the agricultural areas found in land-use parcels 

closest to the metered location. To account for the lack of 

100 percent irrigation efficiency, annual agricultural irrigation 

rate estimates were added to rainfall to account for potential 

recharge to the SAS.

There is no direct measurement of landscape irrigation 

from publicly supplied water, commercial or industrial water, 

or domestic self-supplied water. Landscape irrigation rates 

were estimated by using the aesthetic irrigation application rates and applying these rates to appropriate land uses. 

Average monthly water-requirement application rates were 

derived from the AFSIRS model. The AFSIRS model was 

applied for sod production under various soil characteristics 

as mapped by the Natural Resource Conservation Service for 

typical residential and commercial size parcels in each county 

of the ECFT study area. Because the AFSIRS model utilizes 

long-term climate data, these rates reflect 30-year annual 

average use rates for sod production in each of the counties based on soil type. These rates were then applied to the 

residential and commercial land-use categories (as established 

in the land-use database) within each county, independent of 

the source of irrigation water.

In order to account for changes in land use, the method 

just described was applied to the 1995, 1999, and 2004 

land-use maps provided by the WMDs. Because of the lack 

of land-use data for other years, 1995 land-use data were 

used to calculate 1995 to 1998 irrigation estimates, 1999 

land-use data were used to calculate 1999 to 2003 irrigation 

estimates, and 2004 land-use data were used to calculate 2004 

to 2006 irrigation estimates. The 30-year annual average rates 

determined by AFSIRS were used and variations in monthly 

applications were taken from the 30-year annual average 

information. Because AFSIRS calculates an amount for sod 

production purposes, these amounts were then further reduced 

to represent a range of 40 to 75 percent of the county’s total 

for public water supply, commercial industrial, and domestic 

self-supply use amounts in the ECFT study area. The monthly 

volumes were proportioned by the total available irrigation 

area, incorporating the percentage of impervious land for 

the appropriate land uses receiving irrigation. These rates, 

determined for the ECFT study area, were then incorporated 

into the model as additional rainfall to calculate runoff and 

infiltration rates.

No specific reuse irrigation rates were considered because 

of a lack of reuse irrigation data. Irrigation rates for the reuse 

category were assumed to be represented by the public water 

supply, commercial or industrial, and domestic self-supply use 

categories as described earlier, which do not identify specific 

sources. A nominal rate at which residential and commercial 

land uses are irrigated was used.

Estimated Recharge and Evapotranspiration 

Rates in the Surficial Aquifer System

An approximation of a water-balance equation that 

considers all inflows and outflows over the SAS domain can 

be used to calculate ET rates by solving for ET after estimating 

or measuring the remaining parameters in the balance equation. Rainfall to the SAS domain is an inflow and considered 

positive, whereas the total measured streamflow leaving the 

model through the lateral stream cells and the estimated ET 

rates are outflows and considered negative. The leakage rates 

between the streams and the SAS occur within the same domain 

of interest and thus are not considered flows leaving or entering 

the SAS domain. By convention, rates of flow entering the 

SAS are positive whereas those leaving the SAS are negative. 

The net groundwater basin flows to the SAS through the lateral 

boundaries and the net vertical leakage between the SAS 

and the ICU or IAS were initially estimated using a regional 

groundwater flow model (Sepúlveda, 2002). Both quantities 

were mostly negative, indicating outflows from the SAS; these 

rates apply to the entire SAS area, including lakes. Groundwater 

injections from the RIBs were inflows to the SAS. Changes in 

storage in the SAS were calculated using measured water levels 

in SAS wells and measured water-surface altitudes at lakes. 

Average rainfall is the largest inflow to the SAS domain. 
52 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida 82°00' 81°00' 81°30' 29°00' 28°30' 28°00' 27°30'

A

East-central Floridatransient model area

82°00'

B

81°00' 81°30'

29°00'28°30'28°00'27°30'

Base from U.S. Geological Survey digital data,Universal Transverse Mercator projection, zone 17

East-central Floridatransient model area

ATLANTIC OCEAN

ATLANTIC OCEAN

SUMTER COUNTY

SUMTER COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY HARDEE COUNTY

BREVARD

COUNTY

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

SEMINOLE

COUNTY

POLK

COUNTY OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

BREVARD

COUNTY

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

SEMINOLE

COUNTY

0 100 10 20 KILOMETERS

20 MILES

Figure 39. Maps showing estimated net average 1999 groundwater withdrawal rates in the (

A) Upper Floridan aquifer and (

B) Lower 

Floridan aquifer.

EXPLANATION

Groundwater withdrawals and injections,  in net average million gallons per day.  Positive numbers indicate injections or  recharge from drainage wells whereas negative numbers indicate withdrawals

–4.61 to –4.00–3.99 to –2.00–1.99 to –1.00–0.99 to –0.50–0.49 to –0.25

–0.24 to –0.10–0.09 to 00.01 to 0.100.11 to 0.250.26 to 0.76

EXPLANATION

Groundwater withdrawals and injections, in  net average million gallons per day. Positive  numbers indicate injection wells whereas negative numbers indicate withdrawals

–7.79 to –5.00–4.99 to –3.00–2.99 to –2.00–1.99 to –1.00–0.99 to –0.50

–0.49 to –0.25–0.24 to 00.01 to 0.250.26 to 0.500.51 to 1.00

Figure 39. Estimated net average 1999 groundwater withdrawal rates in the 

A, Upper Floridan aquifer, and 

B, Lower Floridan aquifer.
Hydrology of the Study Area 53

82°00' 81°00' 81°30'

29°00'28°30'28°00'27°30'

A

East-central Floridatransient model area

82°00'

B

81°00' 81°30'

29°00'28°30'28°00'27°30'

Base from U.S. Geological Survey digital data,Universal Transverse Mercator projection, zone 17

East-central Floridatransient model area

ATLANTIC OCEAN ATLANTIC OCEAN

SUMTER COUNTY

SUMTER COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

BREVARD

COUNTY

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

SEMINOLE

COUNTY

POLK

COUNTY OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY HARDEE

COUNTY

BREVARD

COUNTY

MARION

COUNTY

LAKE

COUNTY VOLUSIA

COUNTY

ORANGE

COUNTY

SEMINOLE

COUNTY

0 100 10 20 KILOMETERS

20 MILES

EXPLANATION EXPLANATION

Groundwater withdrawals and injections, in  net average million gallons per day. Positive  numbers indicate injections or recharge  from drainage wells whereas negative  numbers indicate withdrawals

–4.31 to –3.00–2.99 to –2.00–1.99 to –1.00–0.99 to –0.50–0.49 to –0.25

–0.24 to –0.10–0.09 to 00.01 to 0.100.11 to 0.250.26 to 0.30

–9.76 to –5.00–4.99 to –3.00–2.99 to –2.00–1.99 to –1.00–0.99 to –0.50

Groundwater withdrawals and injections,  in net average million gallons per day.  Positive numbers indicate injection wells  whereas negative numbers indicate  withdrawals

–0.49 to –0.25–0.24 to 00.01 to 0.250.26 to 0.500.51 to 1.00

Figure 40. Maps showing estimated net average 2006 groundwater withdrawal rates in the (A) Upper Floridan aquifer and (B) Lower Floridan aquifer. Figure 40. Estimated net average 2006 groundwater withdrawal rates in the A, Upper Floridan aquifer and B, Lower Floridan aquifer.
54 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

The water balance equation can be written as 

(1)

where 

RAIN is the total rainfall into the SAS, including 

irrigation,

SFG is the total stream flow leaving the SAS 

through gaged streams, 

SFU is the total stream flow leaving the SAS area 

through ungaged streams, 

GWBF is the net groundwater basin flow through the 

lateral boundaries of the SAS,

VLICU is the net vertical leakage between the 

underlying ICU/IAS and the SAS, 

GWSAS is the net groundwater withdrawals and 

injections to and from the SAS,

EET is the estimated ET, 

Sy is the specific yield, 

0.9387 is a coefficient equal to the percentage of the 

SAS area not covered by lakes,

0.0613 is the percentage of the SAS area covered by 

lakes, 

∆

∆

h

t

w is the change in water levels at the wells over 

a period of time Δt, and 

∆

∆

L

t

is the change in water-surface altitudes at 

lakes over a period of time Δt. 

The potential changes in percentages of the SAS covered by 

lakes during the 12-year period resulted in changes in estimated ET rates within a few tenths of an inch, which is generally smaller than the error associated with the measurements 

of the flow parameters in equation 1. The estimated ET rates 

are used to calculate recharge rates to the SAS by subtracting 

ET rates and streamflow from rainfall. 

Average annual ET and recharge rates to the SAS 

were calculated to provide guidelines for model calibration 

by allowing a comparison between annual estimated and 

simulated ET rates, and to identify the years for which the 

calculated changes in storage are smallest to select the most 

appropriate periods to approximate steady-state conditions 

without introducing significant errors to the model. During 

model calibration, it was recognized that the margin of error in 

some of the estimated rates in equation 1 imply that an exact 

match between measured and simulated ET rates and recharge 

to the SAS is not practical.

The evaluation of the right-hand side of equation 1 was 

accomplished by (1) assuming a specific yield value of 0.14; 

(2) calculating the average change in lake altitudes from the 

gaged lakes in the ECFT study area (fig. 13) for each year; 

and (3) calculating the average change in water-table altitudes 

from the wells withdrawing from the SAS. Temporal changes 

in the water-table altitude and in lake altitudes were calculated 

RAIN + + SFG + SFU GWBF V+ + LICU GWSAS

= Sy ≈ + Sy ∆

∆

∆

∆

h

t

h

t

w 0.9387 0.0613 ∆

∆

L

t

+ EET 

from monthly average data for each year from 1995 to 2006. 

All changes were calculated by subtracting the value at the 

beginning of the year from the value at the end of the year; 

negative rates thus indicate a loss in storage relative to conditions at the beginning of the year and positive rates indicate 

a gain in storage. Results from these calculations showed 

that the smallest absolute values for the right-hand side of 

equation 1 were for 1999 and 2003 (table 4). Based on the 

cumulative rates of ∆

∆

L

t , it was observed that the average 

water-surface altitudes at lakes at the end of 2006 were about 

1.5 ft lower than corresponding levels at the beginning of 

1995. Only in 2005 did average water-surface altitudes at 

lakes exceed those at the beginning of 1995 (table 4). The 

calculation of the Sy ∆

∆

h

t

terms in table 4 for the SAS results in 

the largest possible values when compared to confined units or 

aquifers because the Sy values are always larger than those for 

storage coefficients.

The calculation of each term in the left-hand side of equation 1 was accomplished by (1) using the NEXRAD rainfall 

data with the adjustment made explained in the Rainfall 

section of this report; (2) calculating the total streamflow 

measured at stream-gaging stations in figure 11 leaving the 

SAS in the ECFT study area; (3) using a regional groundwater 

flow model (Sepúlveda, 2002) to estimate the horizontal flows 

entering and leaving the SAS through the lateral boundaries 

of the SAS area and the vertical flows through the ICU or 

IAS; and (4) calculating the average groundwater withdrawals 

Table 4. Annual storage changes in lakes and surficial aquifer 

system wells from 1995 to 2006.

[Values in bold represent storage changes for steady-state model years. 

CCIS, cumulative changes in storage; refer to equation 1 for the definition of 

column headers; all rates are in inches per year]

Year

∆

∆

L

t

∆

∆

h

t

w Sy ∆

∆

h

t

CCIS

1995 –10.06 –1.39 –0.80 –10.06

1996 –2.00 –3.86 –0.63 –12.06

1997 8.66 3.88 1.04 –3.40

1998 –13.09 –4.57 –1.40 –16.49

1999 5.24 2.00 0.58 –11.25

2000 –26.47 –15.96 –3.72 –37.72

2001 13.02 6.64 1.67 –24.70

2002 14.00 4.98 1.51 –10.70

2003 –4.29 –2.31 –0.57 –14.99

2004 12.52 2.77 1.13 –2.47

2005 5.93 2.71 0.72 3.46

2006 –21.94 –7.46 –2.33 –18.48
Simulation of Groundwater Flow System 55

from the SAS. The calculation of the first six terms in equation 1 and the data in the fourth column of table 4 allows the 

estimation of EET for each year. Results from these calculations showed that EET ranged from about 27 to 44 in/yr 

(table 5) from 1995 to 2006. These results required estimating 

the SFU term, the streamflow leaving the SAS area through 

ungaged streams. The estimation of SFU was based on the 

product of the measured streamflow at stations shown in 

figure 11 (or SFG in table 5) and the ratio of the drainage area 

of ungaged streams leaving the SAS in the ECFT study area to 

the drainage area of the gaged streams leaving the SAS. The 

ratio of these two drainage areas was calculated to be 0.221 

and it was assumed to be constant for all years. In addition, 

independent measurements or estimates of VLICU and GWBF 

in equation 1 cannot be made. Initial estimates of GWBF and 

VLCIU were derived from Sepúlveda (2002), representing 

average steady-state conditions (August 1993 through July 

1994). Annual average values simulated by the ECFT model 

were used in table 5, and are similar in magnitude to those 

from Sepúlveda (2002). Values for GWBF are negligible, and 

any error can be disregarded. Values for VLICU are within 

0.4 in/yr of that (-3.6 in/yr) from Sepúlveda (2002). Because 

flows in table 5 represent an approximation of inflows and 

outflows for the SAS, the ET and recharge rates to the SAS in 

table 5 are estimates, not measured rates. 

Simulation of Groundwater Flow 

System

The conceptual model of the flow system in the ECFT 

study area (hereafter called the model area) represents the 

same area where the flow system is simulated and can be 

presented in terms of the interactions among (1) the surfacewater system represented by lakes and streams, (2) the 

unconfined (SAS), and (3) the confined groundwater system 

(FAS). Rainfall and irrigation are the sources of runoff to lakes 

and streams and of infiltration to the unsaturated zone. Infiltration is routed to the SAS through the unsaturated zone, and 

water that is not lost to ET recharges the SAS. The exchange 

of water between streams and the SAS can occur through the 

streambed, whereas interaction between the lakes and the SAS 

or ICU/IAS can occur through the lakebed because lakes are 

assumed to penetrate the entire thickness of the SAS. Water 

exchange between the unconfined SAS and the confined 

FAS occur through the confining units as upward leakage in 

discharge areas and as downward leakage in recharge areas. 

Lateral flows through the boundaries are assumed to leave or 

enter the model area in all hydrogeologic units: SAS, ICU/

IAS, OPZ, OLPZ, APPZ, MCU I/II, and LFA. Flow through 

the freshwater and saline-water interface, assumed to be 

Table 5. Estimation of groundwater flow terms for 1995–2006 in the surficial aquifer system of the East-Central Florida Transient 

(ECFT) study area.

[RAIN, rainfall plus irrigation; SFG, streamflow leaving the SAS through gaged streams; SFU, streamflow leaving the surficial aquifer system (SAS) area 

through ungaged streams; GWBF, groundwater flow leaving the study area; VLICU, net downward leakage from the SAS to the underlying intermediate 

confining unit (ICU); GWSAS, net groundwater withdrawals and injections to and from the SAS; EET, estimated ET; RCH, recharge rate to the SAS, equal to 

SFG+SFU+EET+RAIN; Ave, average rates for the 1995–2006 period; negative flows indicate outflows to the SAS whereas positive flows indicate inflows; all 

rates are in inches per year]

Year RAIN SFG SFU GWBF VLICU GWSAS EET RCH

1995 53.12 –12.08 –2.67 –0.02 –3.44 0.05 –35.77 2.60

1996 50.13 –8.46 –1.87 –0.01 –3.64 0.06 –36.84 2.96

1997 54.78 –7.20 –1.59 –0.02 –3.49 0.04 –41.49 4.50

1998 49.39 –12.61 –2.78 0.00 –3.72 0.05 –31.72 2.28

1999 49.60 –6.65 –1.47 –0.05 –3.86 0.04 –37.03 4.45

2000 31.72 –3.32 –0.73 –0.06 –4.05 0.07 –27.35 0.32

2001 50.96 –7.95 –1.75 –0.05 –3.71 0.05 –35.87 5.39

2002 60.41 –9.31 –2.05 –0.02 –3.69 0.07 –43.90 5.15

2003 53.04 –11.51 –2.54 0.00 –3.49 0.07 –36.13 2.86

2004 58.78 –11.71 –2.58 –0.01 –3.48 0.07 –39.94 4.55

2005 61.46 –14.17 –3.13 0.01 –3.46 0.09 –40.08 4.08

2006 37.14 –4.15 –0.92 –0.03 –3.90 0.08 –30.54 1.53

Ave 50.88 –9.09 –2.01 –0.02 –3.66 0.06 –36.39 3.39
56 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

located at altitudes where the 5,000-mg/L chloride isochlor is 

present, depends on the hydraulic conductivity of the interface 

between the aquifer cell and the boundary and specified heads 

along the interface.

The rainfall and irrigation rates were partitioned to 

calculate surface runoff and infiltration based on soil properties. The surface runoff was directed to streams, lakes, outside 

the model, or in the case of closed basins, back to the cell 

as infiltration, based on the drainage basins. Infiltration was 

routed through the unsaturated zone to become recharge to the 

SAS after considering ET losses in the unsaturated zone and 

at the water table if the extinction depth was below the water 

table (fig. 41). Additionally, in areas having a relatively thin 

unsaturated zone (fig. 41A) and a shallow water table, groundwater ET occurs and is withdrawn directly from the saturated 

zone whenever the extinction depth is below the water table, 

whereas in areas with a thick unsaturated zone (fig. 41B), total 

ET equals the unsaturated ET because groundwater ET is zero.

The hydrogeologic framework, conceptual groundwater 

flow system, and other hydrologic information presented 

in the previous sections were used to construct a numerical 

groundwater flow model of the SAS, ICU/IAS, OPZ, OLPZ, 

APPZ, MCU I/II, and LFA. The ECFT model simulates 

transient groundwater flow in the study area (fig. 1) for a 

period of 12 years, starting in 1995, based on the hydrologic 

and hydraulic properties derived from both steady-state and 

transient model calibration.

Model Construction

The USGS three-dimensional finite-difference groundwater flow model code MODFLOW-2005 (version 1.8) 

was used to simulate the flow system (Harbaugh, 2005). 

Interaction of the groundwater flow system with the surface 

environment (rainfall, irrigation, ET, lakes, and streams) was 

simulated using the Green-Ampt infiltration (GAI) method 

(Chow and others, 1988), Unsaturated-Zone Flow (UZF1) 

Package (Niswonger and others, 2006), Lake (LAK7) Package 

(Merritt and Konikow, 2000), and Streamflow-Routing 

(SFR2) Package (Niswonger and Prudic, 2006). Integration 

of the GAI method and the UZF1, LAK7, SFR2 Packages in 

MODFLOW-2005 ensures a water mass balance in surface 

water and groundwater.

Spatial and Temporal Model Discretization

The model area was horizontally discretized into a 

uniform grid with a cell size of 1,250 by 1,250 ft. The grid 

consists of 472 rows and 388 columns and is oriented along a 

north-south axis for simplicity, because the majority of stresses 

or boundary conditions were not oriented in any specific direction. The UTM coordinates of the model grid corners, specifying the extent of the model area, are provided in table 6. The 

resolution of the model grid is depicted in the expanded views 

of two grid corners (shown in fig. 42). Vertical discretization 

involved partitioning the model into seven layers: the SAS, 

ICU, OPZ, OLPZ, APPZ, MCU I/II, and LFA. 

Temporal discretization involved dividing the 12-year 

simulation period into 144 monthly stress periods, over each 

of which boundary conditions remain constant, and each 

stress period comprises 10 time steps that are uniform in 

duration. The simulation period begins in January 1995 and 

ends in December 2006. Average monthly water levels for 

this 12-year simulation were calculated for wells, lakes, and 

streams; average monthly flows were calculated for springs 

and streams. Average monthly groundwater withdrawals were 

calculated for all water use types.

Flow-model development included assigning or estimating specific yield, storage coefficients, and horizontal and 

vertical hydraulic conductivity for each aquifer and confining 

unit, resulting in a fully three-dimensional groundwater 

flow model. Although the IAS includes vertically stratified 

permeable zones (Basso, 2003; Basso and Hood, 2005; 

Knochenmus, 2006), which may affect local groundwater 

flow, the IAS in the southwestern corner of the model area 

(fig. 21) is simulated as a single model layer representative of 

regional flow conditions in this aquifer system.

Initial Conditions

Initial conditions for the 12-year transient model were 

equal to the head distribution solution for steady-state conditions for January 1995. Because the true head distribution is 

not known with certainty, an approximation was derived from 

a separate steady-state forward model run for January 1995. 

Boundary conditions for this run are based on average hydrologic conditions in the ECFT model area in January 1995, and 

the run uses calibrated hydraulic conductivity values discussed 

later in this report. This model-derived set of initial conditions 

eliminated the effects of head interpolation over large areas 

where few measured heads are available. There is some error 

associated with the assumption of steady-state conditions 

for January 1995, however, because actual conditions were 

transient and influenced by antecedent hydrologic conditions. 

Different initial conditions could lead to different simulated 

heads and flows at later times. To test the effect of the initial 

conditions, forward runs of the 12-year transient model were 

done with two different sets of initial conditions to determine 

the time at which simulated heads in both runs became nearly 

equal. The first set is that derived from the steady-state model 

run representing January 1995 conditions, described earlier. 

The second set of initial heads was derived from the head 

solution for a steady-state model representing annual average 

1995 conditions.

Comparison of results from the 12-year transient model 

runs with the two different sets of initial conditions showed 

that by stress period 7 (representing July 1995) the maximum 

absolute value in head difference was 0.05 ft in layer 1 and 

0.01 ft in layers 2 to 7. By the end of 1995, the maximum 

head difference in layer 1 was 0.01 ft. Because the goal was to 

calculate the number of simulated stress periods required for 
Simulation of Groundwater Flow System 57

Evapotranspiration (ET)

Overland flow

to streams and lakes

Land surface Groundwater ET

SAS

SAS

ICU/IAS

ICU/IAS

One-dimensional unsaturated flow

Shallow water table

ET extinction depth

Three-dimensional groundwater flow

A

Overland flow

to streams and lakes

Land surface

One-dimensional unsaturated flow

ET extinction depth

Deep water table

Three-dimensional groundwater flow

Evapotranspiration (ET) B

FAS

FAS

Figure 41. Diagrams showing the one-dimensional unsaturated-zone flow 

coupled with three-dimensional groundwater flow for (A) shallow water table, 

and (B) deep water table. Modified from Niswonger and others (2006).

Figure 41. One-dimensional unsaturated-zone flow coupled with three-dimensional 

groundwater flow for the A, shallow water table, and B, deep water table. Modified 

from Niswonger and others (2006).
58 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

two different initial head distributions to coincide, the effects 

of the differences in initial conditions dissipate in at most one 

year of simulation.

Boundary Conditions

The main boundary conditions imposed in the ECFT 

model are (1) head-dependent flux at lateral boundary cells 

(using the General Head Boundary (GHB) Package), (2) headdependent flux at springs, (3) head-dependent flux between 

lakes and the SAS, and between the lakes and the ICU/IAS 

through representation of vertical leakance of both lakebed 

and ICU/IAS, (4) head-dependent flux between streams and 

the SAS through the vertical hydraulic conductivity of the 

streambed, and (5) the no-flow condition imposed at the 

bottom of the LFA. 

The model simulates the freshwater flow system, and 

cells representing areas where the OPZ, OLPZ, APPZ, MCU 

I/II, or LFA (layers 3 to 7) contained water with chloride 

concentrations exceeding 5,000 mg/L were made inactive. 

This minimized potential errors introduced by simulating 

aquifer areas containing water having variable density. By 

definition, the base of the simulated freshwater flow system 

coincides with the base of the LFA (Sepúlveda, 2002), or the 

altitude at which chloride concentrations of 5,000 mg/L occur 

(fig. 37), whichever is shallower. GHB cells along the bottom 

of the simulated freshwater interface allow potential flux to 

occur across this interface. The active portions of each model 

layer were delineated by intersecting the surfaces representing 

the top of each hydrogeologic unit (figs. 22, 26, 28, 32, and 

34) with the altitude of the 5,000 mg/L chloride concentration 

isochlor (fig. 37) to generate the lateral extent of active cells in 

each layer (fig. 43). The thickness of the hydrogeologic units 

in active cells near the lateral boundary cells is thinner than 

that of active cells away from the lateral boundaries because 

the freshwater and saline-water interface is shallower near the 

lateral boundaries.

Model boundaries were assigned to approximate the 

actual groundwater flow system as accurately as possible. 

Head-dependent flow boundary conditions were simulated 

using the GHB Package and were applied to the lateral 

boundaries shown in figure 43 for the seven layers in the 

model. Simulation of the freshwater and saline-water interface 

is a simplified approximation of the physical system, which 

could be more accurately simulated by a variable-density flow 

model. The GHB Package was used to represent potential 

flow across the 5,000 mg/L chloride concentration boundary. 

The magnitude of this flow depends on the specified heads 

estimated from measured heads near the boundary cells and 

on the conductance calculated from hydraulic conductivity. 

Such conceptualization is a common approach for simulating 

the freshwater parts of the aquifer system on a regional scale. 

Specified heads used in the GHB Package were interpolated 

along the lateral boundaries based on measured water levels at 

nearby wells. Where needed, heads were interpolated based on 

hydraulic gradients between hydrogeologic units (fig. 31). The 

specified heads at the GHB cells were varied annually for the 

1995–2006 simulation period. Boundary conductance values 

used in the GHB Package were computed as the product of 

the horizontal hydraulic conductivity of the model layer for 

the cell at the lateral model boundary and the cross-sectional 

area of the cell, divided by an internode distance of 1,250 ft 

(Harbaugh, 2005). The base of the simulated flow system was 

assigned a no-flow boundary condition and coincided with the 

5,000 mg/L isochlor (fig. 37) or the base of the LFA as shown 

in Sepúlveda (2002), whichever was shallower.

Flow at 22 springs in the northern part of the ECFT 

model area (fig. 1, appendix 2, table 2–1) was simulated 

using estimated spring-pool altitudes and the assumption of 

increased hydraulic conductivity near springs. A major component spring flow is the net aquifer recharge from rainfall; 

however, spring response is delayed by aquifer-matrix storage. 

Higher spring flows are common in late fall after the rainy 

season, whereas lower flows occur in late spring when rainfall 

is low. Spring flows from the UFA tend to create depressions in the potentiometric surface, the areal extent of which 

depends on the magnitude of the spring flow and aquifer 

and the confining-unit properties near the spring. The GHB 

Package was used to simulate spring flow by specifying an 

estimated head at the spring that best represented the springpool altitude and specifying a conductance value calculated 

from the horizontal hydraulic conductivity estimated by 

calibration. The estimated pool altitudes at the springs were 

varied annually for the 1995–2006 simulation period. 

Lakes were simulated as another boundary condition on 

the model. All gaged lakes in the ECFT model area, as well 

as ungaged lakes that were part of the stream network, were 

included in the simulation. Ungaged lakes smaller than a single 

grid cell were not considered in the model. The main consequence of not considering a lake in a flow simulation is that 

recharge to the SAS is simulated by routing infiltration through 

the unsaturated zone instead of simulating leakage to or from 

the unit beneath the lake through the lakebed material. As a 

conceptual simplification, a few ungaged lakes with surface 

areas up to 0.25 mi2 were not considered in the study because 

of a lack of corresponding water-surface altitude data. Ungaged 

Table 6. Geographical information system coordinates of the 

corners of the groundwater flow model grid.

[XUTM and YUTM coordinates refer to Universal Transverse Mercator 

projection, North American Datum 1983, Zone 17N, in meters (Snyder, 

1983)]

Grid corner XUTM YUTM

Upper left 401023.75 3220231.0

Upper right 548470.75 3220231.0

Lower left 401023.75 3040780.0

Lower right 548470.75 3040780.0
Simulation of Groundwater Flow System 59

EXPLANATION

0

0

5 MILES

5 KILOMETERS

2.5

2.5

0 10 20 MILES

0 10 20 KILOMETERS

ATLANTIC OCEAN

East-central 

Florida

transient 

model area

82°00'

29°05'

29°00'

82°00' 81°55' 81°50' 81°45'

27°40'

27°35'

27°30'

80°45' 80°40' 80°35'

81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

A

B

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY BREVARD

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

HARDEE COUNTY

COUNTY

SUMTER COUNTY

INDIAN RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

MARION

COUNTY

LAKE

COUNTY

SUMTER

COUNTY

Figure 42. Maps showing expanded view of the groundwater flow model for the East-Central Florida Transient 

(ECFT) study area displaying the model grid in portions of (A) Marion and (B) Indian River Counties.

Model-grid cells

Figure 42. Expanded view of the groundwater flow model for the East-Central Florida Transient (ECFT) study area displaying the model 

grid in portions of A, Marion, and B, Indian River Counties.

their represented area occupied by a lake. Initial water-surface 

altitudes at the lakes were either estimated from the DEM or 

assigned from measured values near the beginning of the simulation, namely January 1995. For ungaged lakes discharging to 

streams, the initial 1995 water-surface altitude was assigned a 

value based on the water-surface slope continuity between the 

stream and the lake. Selected lakes and lake names, with the 

number of average monthly measured water-surface altitudes 

close to the total number of stress periods (144), are shown in 

appendix 3.

The main parameters that control the changes in watersurface altitudes at the lakes are the land-surface runoff, the 

vertical hydraulic conductivity of the lakebed, the streambed 

slope and channel width of the first stream reach of the lake’s 

lakes with parts of their drainage areas lying outside the model 

area were not considered in the study because the calibration of 

fluxes at such lakes would have required an overestimation of 

runoff from the part of the drainage area within model bounds. 

Regardless of whether or not an ungaged lake was considered, 

water exchanges were considered either by simulating leakage 

through the lakebed material or by simulating recharge to the 

SAS. The water-surface altitudes at the 351 lakes included 

in the ECFT model (fig. 13) were simulated using the LAK7 

Package developed by Merritt and Konikow (2000). As is 

shown for Lake Apopka (fig. 44), lakes in figure 13 were 

intersected with the model grid to generate the lake cells, 

which are considered to be inactive cells in the SAS. Lake cells 

were, in general, those grid cells with at least 50 percent of 
60 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

Figure 43. Maps showing areal extent of active areas simulated in (

A) layer 1, the surficial aquifer system (SAS), (

B) layers 2 and 3, 

the intermediate confining unit or intermediate aquifer system (ICU/IAS) and the Ocala permeable zone (OPZ), which coincided in the extent of active areas, (

C) layer 4, the Ocala low-permeable zone (OLPZ), (

D) layer 5, the Avon Park permeable zone (APPZ), (

E) 

layer 6, the middle confining units I/II (MCU I/II), and (

F) layer 7, the Lower Floridan aquifer (LFA) in east-central Florida. Shading 

indicates active areas simulated in each layer.

SAS OPZ

ICU/IAS OLPZ

APPZ MCU I/II LFA

EXPLANATION

Active areas in each layer Inactive areas in each layer

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY HIGHLANDS COUNTY HARDEE COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY HIGHLANDS COUNTY HARDEE COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

SUMTER COUNTY

SUMTER COUNTY

SUMTER COUNTY

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY HIGHLANDS COUNTY HARDEE COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY HIGHLANDS COUNTY HARDEE COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

SUMTER COUNTY

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY HIGHLANDS COUNTY HARDEE COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

SUMTER COUNTY

SUMTER COUNTY

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

SEMINOLE

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

BREVARD

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

B

C

0 10 20 MILES0 10 20 KILOMETERS

Base from U.S. Geological Survey digital data,Universal Transverse Mercator projection, zone 17

E F

East-central Floridatransient model area ATLANTIC 

OCEAN

ATLANTIC 

OCEAN

ATLANTIC 

OCEAN

AD

East-central Floridatransient model area

East-central Floridatransient model area

East-central Floridatransient model area ATLANTIC 

OCEAN

ATLANTIC 

OCEAN

ATLANTIC 

OCEAN

East-central Floridatransient model area

East-central Floridatransient model area

29°00'28°30'28°00'27°30'

81°30' 81°00'

29°00'28°30'28°00'27°30'

81°30' 81°00'

29°00'28°30'28°00'27°30'

82°00' 82°00' 82°00' 81°30' 81°00'

29°00'28°30'28°00'27°30'

82°00' 81°30' 81°00'

29°00'28°30'28°00'27°30'

82°00' 81°30' 81°00'

29°00'28°30'28°00'27°30'

82°00' 81°30' 81°00'

Figure 43.

Areal extent of active areas simulated in 

A, layer 1, the surficial aquifer system (SAS); 

B, layers 2 and 3, the intermediate confining unit or intermediate aquifer system (ICU/IAS) and the Ocala permeable zone (OPZ), which coincided in the extent of active areas; 

C, layer 4, the Ocala low-permeable 

zone (OLPZ); 

D, layer 5, the Avon Park permeable zone (APPZ); 

E, layer 6, the middle confining units I/II (MCU I/II); and 

F, layer 7, the Lower Floridan aquifer 

(LFA) in east-central Florida. Shading indicates active areas simulated in each layer.
Simulation of Groundwater Flow System 61

Figure 44. Map and schematic diagram showing (A) Lake Apopka and (B) the corresponding lake cells after 

the intersection with the grid.

0

0

5 MILES

5 KILOMETERS

2.5

2.5

LAKE

APOPKA

LAKE

APOPKA

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

81°45' 81°40' 81°35'

28°40'

28°35'

81°45' 81°40' 81°35'

28°40'

28°35'

Base from U.S. Geological Survey 

digital data, Universal Transverse 

Mercator projection, zone 17

ATLANTIC OCEAN

East-central 

Florida

transient 

model area

MARION

COUNTY

LAKE

COUNTY

LAKE

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY ORANGE

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

BREVARD

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER COUNTY

A

B

0 10 20 MILES

0 10 20 KILOMETERS

EXPLANATION

Model-grid cells

Figure 44. A, Lake Apopka, and B, the corresponding lake cells after intersection with the grid.

the Palatlakaha River (fig. 1). As illustrated in figure 45 for 

the Peace River and some of its tributaries, the streams in 

figure 11 were intersected with the model grid to subdivide 

the 320 stream segments into cell-specific reaches within each 

segment. Initial flow at the uppermost reach of each stream 

was specified for the first three stress periods of 1995 and for 

each stress period where the uppermost reach of the stream 

is a spring. Examples of measured streamflows at selected 

stream-gaging stations are shown in appendix 4.

The parameters that determine the magnitude and the 

routing of the streamflow are the surface runoff applied to each 

stream, the stream-channel width, the stream water-surface altitude slope, the streambed slope, the thickness of the streambed, 

the vertical hydraulic conductivity of the streambed, the 

potential ET, the rainfall applied to the stream surface area, and 

outlet, the rainfall, and the potential ET at the lake surface. 

The vertical hydraulic conductivity of the lakebed and the 

difference between water-surface altitude and head determined 

the lateral flow to or from the SAS and the vertical flow to 

or from the ICU/IAS, depending on the lake location. The 

streambed slope of the first stream reach of the lake’s outlet 

was adjusted as needed during the calibration of the watersurface altitudes at the lakes.

The streamflow routing was performed using the SFR2 

Package developed by Niswonger and Prudic (2006). All 

gaged streams in the ECFT model area were included in the 

simulation. All perennial streams, whether ungaged or gaged, 

were also included in the simulation. The main streams in 

the ECFT model area, and included in the simulation, are the 

St. Johns River, the Kissimmee River, the Peace River, and 
62 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

SEMINOLE

COUNTY

ORANGE

COUNTY

POLK

COUNTY

POLK

COUNTY

POLK

OSCEOLA COUNTY

COUNTY

BREVARD

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

HARDEE COUNTY

COUNTY HARDEE

COUNTY

HARDEE

COUNTY

SUMTER COUNTY

A

B

0

0

5 MILES

5 KILOMETERS

2.5

2.5

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Base from U.S. Geological 

Survey digital data,

Universal Transverse 

Mercator projection, 

zone 17

0 10 20 MILES

0 10 20 KILOMETERS

ATLANTIC 

OCEAN

East-central 

Florida

transient 

model area

Figure 45. Map and schematic diagram showing (A) Peace River, some tributaries, and stream segment numbers 

and (B) the corresponding stream cells after the intersection with the grid.

C-54

Reedy Creek

295

294

299

297

293

298

296

81°55' 81°50' 81°45' 81°40'

81°55' 81°50' 81°45' 81°40'

27°40'

27°35'

27°30'

Peace River

Payne Creek

Max Branch

27°40'

27°30'

27°35'

EXPLANATION

Model-grid cells

Figure 45. A, Peace River, some tributaries, and stream segment numbers; and B, the corresponding stream cells after the intersection 

with the grid.

altitude slopes at streams estimated from the DEM were 

adjusted following the general rule that water-surface slopes 

are generally lower than terrain slopes.

The streambed slope was calibrated at the point where a 

lake discharges to streams based on the net flow to the lake and 

the water-surface altitude at the lake. Because of the lack of 

data for streambed thickness, all stream segments were assumed 

to have a streambed thickness of 5 ft. The vertical hydraulic 

conductivity of the streambed, under the 5-ft streambed 

thickness assumption, was set to a uniform calibrated value 

of 0.02 ft/d for all stream segments. The ratio of the vertical 

hydraulic conductivity to its streambed thickness controls the 

groundwater flux between the stream and the aquifer, and thus, 

if a streambed is actually thicker than 5 ft, then the calibrated 

vertical hydraulic conductivity should be larger than 0.02 ft/d. 

Manning’s dimensionless roughness coefficient. The roughness 

coefficient, assigned a value of 0.05 for all streams, is a 

measure of the resistance to flow in the channel. The surface 

runoff applied to each stream depended on the drainage area of 

each stream, which was estimated from the basin delineation in 

databases of the SJRWMD, SWFWMD, and SFWMD (Brian 

McGurk, St. Johns River Water Management District, written 

commun., 2008). The stream channel width was approximated 

using aerial photographs available from GIS (Environmental 

Systems Research Institute, ArcMap, version 10.0, 2010). The 

stream water-surface altitude slope was assigned the value of 

0.0001, 0.0002, or 0.0003, depending on the calculated slope 

between operating water-surface stations, where available, 

or in the absence of gaged stations, estimated from the DEM 

developed by Arthur and others (2007). The water-surface 
Simulation of Groundwater Flow System 63

The uniform value of 0.02 ft/d was modified only for stream 

segments 268 and 283 to 289 (fig. 11), where measured 

downstream flow was measured to be less than the measured 

upstream flow for specific stress periods, suggesting a strong 

stream-aquifer hydraulic connection. This was observed for 

stream-segment numbers 287 and 289 of Peace River (figs. 1 

and 11). Stream segment 268 corresponds to Wolf Branch Creek 

(figs. 1 and 11), which discharges to a sinkhole and therefore 

needs to have a greater streambed vertical hydraulic conductivity, guided by those values that reduce the residuals between 

simulated and measured streamflows. The remaining stream 

segments that had a calibrated streambed vertical hydraulic 

conductivity different from 0.02 ft/d were stream segments 283 

to 286, 288, and 290 to 299 (fig. 11).

Potential ET rates and rainfall were applied directly to 

stream-surface areas. The following flows are hydraulically 

routed using the SFR2 Package: land-surface runoff to streams 

segments, rainfall (falling on the stream surface area), and lake 

outflow to streams where it occurs. Groundwater interactions 

between the streams and the SAS are also simulated by the 

SFR2 Package.

Rainfall and irrigation at cells within closed basins was 

treated as infiltration because runoff is assumed not to drain 

out of the closed basins. Irrigation rates consisted of landscape 

and agricultural irrigation, which were added to rainfall before 

partitioning total rainfall and irrigation into surface runoff and 

infiltration. Landscape and agricultural irrigation generally was 

much smaller than rainfall. For example, total 2003 landscape 

and agricultural irrigation ranged from 0.1 to 35 in. (fig. 46), 

whereas total 2003 rainfall ranged from 46.5 to 62.3 in.

Flow in the Unsaturated Zone

Conceptually, the unsaturated zone is the link between 

infiltration, resulting from rainfall and irrigation applied at 

land surface and computed by the GAI method, and the saturated groundwater flow system (fig. 47). The effects of flow, 

ET, and storage in the unsaturated zone were simulated using 

the UZF1 Package. This Package solves the one-dimensional 

form of the Richards’ equation, which is approximated by a 

kinematic-wave equation by assuming vertical flux is only 

driven by gravitational forces (Niswonger and others, 2006). 

This formulation neglects capillary processes.

When the rainfall rate exceeds the infiltration rate, it 

results in surface runoff known as Hortonian runoff (Horton, 

1933). When the water-table altitude is at, near, or above land 

surface, rainfall becomes mostly runoff, and this type of runoff 

is referred to as Dunnian runoff (Dunne and Black, 1970). 

Hortonian runoff is not simulated by the UZF1 Package in 

this model because the maximum rainfall intensity over the 

stress period of any month was always less than the infiltration 

rate, which is the hydraulic conductivity in the unsaturated 

zone calculated from Brooks-Corey equation (Niswonger and 

others, 2006). One of the primary reasons for the application 

of the UZF1 Package, as opposed to simpler conceptualizations, is its ability to simulate the time lag incurred during 

percolation of surface infiltration through a thick unsaturated 

zone, such as occurs in many of the local ridge or upland 

physiographic regions (for example, Lake Wales Ridge and 

Marion Upland; fig. 2). Additionally, the UZF1 Package simulates groundwater seepage to land surface (sometimes referred 

to as surface leakage) when the simulated water table is near, 

at, or above land surface, which may occur in topographic 

depressions. Both groundwater seepage and Dunnian runoff 

are added instantaneously to the specified streams or lakes 

according to the drainage field (fig. 7).

Dunnian runoff is calculated by the UZF1 Package 

(Niswonger and others, 2006). The rate of infiltrated water 

is reduced by the ET simulated by the UZF1 Package in the 

unsaturated zone, ultimately resulting in the net recharge to 

the SAS. The calculation of Hortonian runoff using the GAI 

method was applied to cells draining to streams or lakes 

outside the model area, cells draining to streams, including 

stream cells, and cells draining to lakes (cell types 2, 3, 4, 

fig. 47). The GAI method was not applied to lake cells or cells 

in closed basins because rainfall over lakes goes directly to the 

water surface and closed basins are characterized by topography in which runoff does not leave the basin, causing runoff 

to become additional infiltration (cell types 1 and 5, fig. 47).

The Brooks-Corey function (Brooks and Corey, 1966) 

is used in the UZF1 Package to characterize the water 

storage and transmission properties of the unsaturated zone, 

which requires specification of the following parameters: 

saturated hydraulic conductivity, residual water contentr , 

saturated water contents , and the Brooks-Corey exponent 

(Niswonger and others, 2006). Saturated hydraulic conductivity values were set equal to a few feet per day less than the 

calibrated horizontal hydraulic conductivity of the SAS, which 

was obtained from model runs. Althoughr and ε can vary 

depending on soil type, constant average values of 0.10 and 

3.50, respectively, were used for all active cells in the SAS. In 

order to maintain water mass balance between the unsaturated 

and saturated zones, specific yield Sy of the SAS was specified to be equal to  s r − (Niswonger and others, 2006, p. 6). 

Values of Sy were estimated by model calibration, ands

values used in the UZF1 Package and those used in the GAI 

method were adjusted accordingly.

ET from the unsaturated and saturated zones was 

simulated using the UZF1 Package. The specified ET rate is 

a function of the extinction depth and the extinction water 

content, below which no ET can be removed from the unsaturated zone. The ET losses are withdrawn from the unsaturated 

zone until the water content is reduced to the extinction 

water content or the ET demand is met. If insufficient water 

is available in the unsaturated zone to meet the ET demand 

and the water table is above the extinction depth, then ET is 

removed from groundwater by the UZF1 Package using the 

same algorithm as in the MODFLOW ET Package (McDonald 

and Harbaugh, 1988). Otherwise, if the water table is below 

the extinction depth, the simulated ET is limited to that 

withdrawn until the unsaturated zone water content is reduced 

to the extinction water content. The ET rate specified in the 
64 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

Figure 46. Map showing landscape and agricultural irrigation rates used to calculate infiltration 

by the Green-Ampt method for 2003.

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

ATLANTIC OCEAN

East-central Florida 

transient model area

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

HIGHLANDS COUNTY

COUNTY

HARDEE

COUNTY SUMTER COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

0 10 20 MILES

0 10 20 KILOMETERS

EXPLANATION

0.1 to 2.0

2.1 to 5.0

5.1 to 8.0

8.1 to 10.0

10.1 to 12.0

12.1 to 15.0

15.1 to 20.0

20.1 to 25.0

25.1 to 35.0

Total 2003 landscape and agricultural 

 irrigation rates, in inches

Figure 46. Landscape and agricultural irrigation rates used to calculate infiltration by the Green-Ampt 

method for 2003.
Simulation of Groundwater Flow System 65 Figure 47. Schematic diagram showing the implementation of flow processes in the shallow part of the hydrologic system. EXPLANATION

Layer 1 SASLayer 2 ICU/IASLayer 3 OPZLayer 4 OLPZLayer 5 APPZLayer 6 MCU I/IILayer 7 LFA

Inactive SAS cell

Stream

Stream

Lake

(1 ) Closed Basin cell

(3) Stream cell or cell draining to stream

(4) Cell draining to lake

(5) Lake cell

(2) Open Basin cell not draining  to stream or lake

R + I

R + I

R + I

R + I

R

UZF1

ET

ET

ET

Infiltration

Infiltration

Infiltration

Infiltration

Infiltration

SAS

layer 1

SAS

layer 1

SAS

layer 1

ICU/IAS

layer 2

Calculation of Hortonian runoff 

Calculation of Dunnian runoff if water table isat or near land surface

Calculation of Dunnian runoff if water table isat or near land surface

Calculation of Dunnian runoff if water table isat or near land surface GAI

Calculation of Hortonian runoff

Calculation of Hortonian runoff

stream

Lake

Leakage/

discharge

Leakage/

discharge

Leakage/

recharge

Leakage/

recharge

Leakage/

recharge

Infiltration

Infiltration

LAK7

UZF1

SFR2

LAK7

GAI

Water directed into systemWater taken out of systemWater exchanged between  parts of system

MODFLOW packageSurficial aquifer systemIntermediate confining unit or  intermediate aquifer system

Green Ampt-infiltration  preprocessorEvapotranspirationRainfallRainfall plus irrigation rates  applied to cell areaActive model cellLake cell

R

ET

R+I

SAS

ICU/IAS

GAI

GAI

UZF1

ET

UZF1

SAS

layer 1

SAS

layer 1

ICU/IAS

layer 2

Closed basin

Figure 47. Implementation of flow processes in the shallow part of the hydrologic system.
66 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

UZF1 Package was set equal to the estimated ET, as described 

previously in the Evapotranspiration section; monthly values 

of estimated ET were used. Extinction water content, required 

to be betweenr ands , was set equal tor + 0 0. 1 as a result 

of model calibration. Greater values thanr + 0 0. 1for the 

extinction water content result in the underestimation of 

simulated ET; setting the extinction water content equal to

r results in numerous dry cells and an overestimation of ET. 

Extinction depth was specified based on values reported by 

Shah and others (2007) for generalized soil texture classes and 

a grass-land cover (table 7), where values are listed with the 

same accuracy published in their paper.

Partitioning of Rainfall and Irrigation into Runoff 

and Infiltration

The main variables used in the GAI method are the 

hydraulic conductivity of the top soil K; the initial water 

contenti ; the residual water contentr ; the saturated water 

contents ; the difference between saturated and initial water 

content ∆ ; and the wetting front capillary pressure head ; 

and the thickness of the wetting front L (fig. 48). Althoughr

can vary depending on soil type, a constant average value of 

0.10 was used for all active cells in the SAS. The downward 

movement of the wetting front causes increases in water 

content, ranging from the residual water contentr to the 

Table 7. Evapotranspiration extinction depths based on 

values from Shah and others (2007).

Soil texture class Extinction depth, in feet 

below land surface

Sand 4.76

Loamy sand 5.58

Sandy loam 7.55

Sandy clay loam 7.55a

Sandy clay 7.55a

Muck 7.55a

Urban 4.76b

Greater than 50 percent Water 4.76b

Wetlands 4.76b

a The value for sandy loam was used.

b

 The value for sand was used.

Wetting front

Wet zone of depth L

with constant conductivity K

Ponded water table

Rainfall and irrigation

Land surface 0

L

θr

θ ∆θ i

θi

θs

θs

θs=θi +∆θ

ho

ψ

θr

∆θ

θi

θs

ψ

Residual water contents

Saturated water contents

Initial water contents

Saturated minus initial water contents

Wetting front capillary pressure head

EXPLANATION

Figure 48. Vertical movement of the wetting front through the unsaturated zone and the relation between soil 

moisture content variables used in the Green-Ampt infiltration method.

saturated water contents (fig. 48). Thes values were derived 

from calibration of the groundwater flow model and conceptually represent the water content when the soil is effectively 

saturated. Thus,s excludes the effect of gas bubble entrapment and is less than porosity. Gas bubbles trapped beneath a 
Simulation of Groundwater Flow System 67

descending wetting front or generated by microorganisms are 

common and can effectively reduces (Fayer and Hillel, 1986; 

Faybishenko, 1995). The effects of entrapped gas have been 

observed in central Florida during laboratory wetting of sand 

and clayey-sand cores representing flooded conditions in a 

RIB (Sumner and Bradner, 1996, p. 12, 18), and during natural 

water-table fluctuations (Nachabe and others, 2004).

The following steps are used to calculate infiltration and 

Hortonian runoff from rainfall and irrigation; further details 

are provided in Chow and others (1988).

1. At time t = 0, or just before rainfall begins, it is assumed 

that the cumulative infiltration F0 is zero. The infiltration 

rate, f0 , is thus assumed to be a hypothetical maximum to 

avoid division by zero. At time t > 0, the cumulative infiltration Ft at time t and the rainfall intensity i

t

 are assumed 

to be known from the previous time step. For any time 

t > 0, the infiltration rate ft at time t is calculated from: 

f K F t

t

= + 











ψ θ∆ 1 . (2)

2. If the infiltration rate ft is less than or equal to the rainfall 

intensity i

t , then ponding occurs throughout the interval 

t t + ∆ and Ft t + ∆ is calculated from:

F F K t

F

F t t t

t t

t

+

+ = + +

+

+









 ∆  ∆ ∆ ∆ ∆

∆ ψ θ ψ θψ θ

ln ; (3)

proceed to step 5 below to calculate runoff and 

infiltration.

3. If the infiltration rate ft is greater than i

t , then no ponding occurs at the beginning of the time interval t t + ∆ and 

a tentative value F F ′

+ = + i t t t ∆ t t ∆ is calculated, deriving 

′

+ ft t ∆ from ′ Ft t +∆ using equation 2. If ′

+ ft t ∆ is less than or 

equal to i

t , then ponding occurs during interval t t + ∆ . 

The cumulative infiltration at ponding Fp is calculated 

from i

t and equation

F K

i K p i K

t

= t − < ψ θ∆

( ) , . (4)

4. The time at which ponding first occurs within time interval ∆t is calculated from ∆t F ′ = − ( ) F i p t t / . The cumulative infiltration Ft t + ∆ is calculated by replacing Ft and ∆t in 

equation 3 with Fp and ∆t′ . If ′

+ ft t ∆ is greater than i

t , then 

no ponding occurs throughout interval t + Δt, and Ft t + ∆ is 

calculated from F F i t t t +∆ = +t t ∆ .

5. Cumulative runoff is calculated by subtracting cumulative 

infiltration from cumulative rainfall. Runoff for each time 

step is calculated from cumulative runoff. Infiltration for 

each time step is calculated by subtracting runoff from 

rainfall. Current time t is then increased by Δt, and the 

process is continued at step 1 above.

Variability in antecedent moisture conditions required 

the adjustment of i for each rainfall event. Defining the 

duration of each rainfall event as the number of consecutive 

days with nonzero rainfall, total rainfall was calculated for 

each event. Values of i , calculated empirically as a variation 

between r and s , were adjusted as part of the calibration of 

measured and simulated streamflows. Rainfall events preceded 

by greater rainfall events with relatively few days in between 

had a larger i than those preceded by a large number of days 

without rainfall. The capability of the soil to drain is considered in the value of K used in equations 2 through 4; and not 

in i because the sensitivity of the GAI method toi was low 

for daily rainfall.

Values for the wetting front capillary pressure head, , 

were obtained from average values (log mean) of soil horizons 

reported by Rawls and others (1983) for each generalized 

soil texture class in the ECFT model area (fig. 49, table 8). 

Because the units of  are independent of time, values listed 

in table 8 were not changed during the calibration of infiltration and runoff rates.

The calibrated K values used in equation 3 compensate 

for using a coarse temporal resolution for rainfall data (day) 

in the model to describe rainfall/infiltration/runoff processes 

that have large intra-day variability. The calibrated hydraulic 

conductivity of the top soil used for the GAI method is different 

than the saturated hydraulic conductivity values used in the 

UZF1 Package or the hydraulic conductivity values used for the 

SAS. The smaller the time interval of the rainfall, the closer K

would be to field values as the need for parameter compensation 

of coarse temporal resolution declines. Field values for K would 

only be attained when using instantaneous rainfall. Effective 

K values were derived by matching observed groundwater 

levels, water-surface altitudes at lakes, and streamflows, thereby 

yielding proper partitioning of rainfall and irrigation into runoff 

and infiltration in accordance with these regional-scale observations. The calibration of K in the GAI method was performed 

based on the hydrologic soil groups (fig. 5), the steps shown 

in figure 50, and adhering to decreasing values from the most 

permeable soil group, A, to the least permeable, D (table 9). 

However, many factors other than Darcian-derived K values 

affect infiltration processes—such as two-phase flow (water and 

soil gas), macropore flow, and biological and chemical characteristics of the top soil—leading to the need for “composite” or 

“effective” parameter values to simulate these processes at the 

scale of a regional model.

Representation of Hydraulic Conductivity 

Several model parameters were defined to represent the 

distribution of horizontal (Kh) and vertical (Kv) hydraulic conductivity for the seven model layers (tables 10 and 11, respectively). 

Generally, Kh and Kv are represented as spatially variable 
68 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

Figure 49. Spatial distribution of soil texture classes in the active cells of the surficial aquifer system.

Figure 49. Map showing spatial distribution of soil texture classes in the active cells of the 

surficial aquifer system.

0 10 20 MILES

0 10 20 KILOMETERS

ATLANTIC OCEAN

East-central Florida 

transient model area

Base from U.S. Geological Survey 

digital data, Universal Transverse 

Mercator projection, zone 17

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

EXPLANATION

Loamy sand

Muck

Sand

Sandy clay

Sandy clay loam

Sandy loam

Urban

Greater than 

 50 percent 

 water

Wetlands

Soil classes

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY
Simulation of Groundwater Flow System 69

Table 8. Wetting front capillary pressure head values 

used in the Green-Ampt infiltration method based on 

values from Rawls and others (1983).

Soil texture class

Wetting front capillary 

pressure head, 

in feet

Sand 0.162

Loamy sand 0.201

Sandy loam 0.361

Sandy clay loam 0.717

Sandy clay 0.784

Muck 1.037a

Urban 0.162b

Greater than 50 percent Water 0.162b

Wetlands 0.292c

a The value for clay (Rawls and others, 1983) was used.

b The value for sand (Rawls and others, 1983) was used.

c

 The value for loam (Rawls and others, 1983) was used.

Change vertical K in Green-Ampt infiltration method

Calculate lake and stream runoff volumes, 

infiltration rates to the unsaturated zone from 

rain and irrigation, then simulate lake stages,

stream stages, streamflows, and groundwater levels

Evaluate errors between 

simulated and observed flows

and water levels

No

Can 

errors be

reduced?

Yes

K in Green-Ampt

infiltration method 

is calibrated

Figure 50. Diagram showing the steps used to calibrate 

the hydraulic conductivity of the top soil used in the 

Green-Ampt infiltration method.

Figure 50. Steps used to calibrate the hydraulic conductivity of 

the top soil used in the Green-Ampt infiltration method.

Table 9. Average calibrated hydraulic conductivity 

of the hydrologic soil groups used in the Green-Ampt 

infiltration method.

Hydrologic 

soil group

Average K value used 

over all active cells, 

in feet per day

A 0.085

B 0.067

C 0.055

D 0.038

A/D 0.080

B/D 0.060

C/D 0.045
70 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

Table 10. Model parameters defined to represent horizontal hydraulic conductivity.

[SAS, surficial aquifer system; ICU/IAS, intermediate confining unit and intermediate aquifer system; OPZ, Ocala permeable zone; OLPZ, Ocala low-permeable zone; APPZ, Avon Park permeable zone; MCU, middle confining units I and II; LFA, Lower Floridan aquifer; Kh, horizontal hydraulic conductivity; 

ft/d, feet per day; —, not applicable]

Model 

layer

Hydrogeologic 

unit

Method used to represent 

spatial variability in Kh

Number of

defined 

parameters

Number of

estimated 

parameters

Number of

specified 

parameters

Number of tied 

parameters

1 SAS Zones of uniform values 21 21 — —

2 ICU/IAS Regional Pilot Points 285 59 (IAS) 226 (ICU; Kh=7 ft/d) —

3 OPZ Regional Pilot Points and

Spring Pilot Points

285

96

 285

 22

—

—

—

 74

4 OLPZ Uniform Value 1 — 1; Kh=20 ft/d

5 APPZ Regional Pilot Points and

Spring Pilot Points

275

96

 275

—

—

—

—

 96

6 MCU Uniform Value 1 — 1; Kh=1 ft/d —

7 LFA Regional Pilot Points 228 228 — —

Total 1,288 890 228 170

Table 11. Model parameters defined to represent vertical hydraulic conductivity.

[SAS, surficial aquifer system; ICU/IAS, intermediate confining unit and intermediate aquifer system; OPZ, Ocala permeable zone; OLPZ, Ocala low-permeable zone; APPZ, Avon Park permeable zone; MCU, middle confining units I and II; LFA, Lower Floridan aquifer; Kh, horizontal hydraulic conductivity; 

Kv, vertical hydraulic conductivity; Direct, Kv defined; Calculated, α defined and Kv=Kh/α; α, vertical anisotropy; —, not applicable]

Model 

layer

Hydrogeologic 

unit

Method of 

defining Kv

Method used to represent 

spatial variability in Kv

Number 

of defined 

parameters

Number of 

estimated 

parameters

Number of

specified

parameters

Number of tied

parameters

1 SAS Direct Zones of uniform values 21 — 21; Kv=0.15– 

 0.95 —

2 ICU/IAS Direct Regional Pilot Points 285 285 — —

3 OPZ Calculated Uniform Value 1 — 1; α=100 —

4 OLPZ Direct Regional Uniform Value

Spring Pilot Points

1

96

 1

—

—

—

—

 96

5 APPZ Calculated Uniform Value 1 — 1; α=100 —

6 MCU Direct Zones of uniform values 2 2 — —

7 LFA Calculated Uniform Value 1 — 1; α=125 —

Total 408 288 24 96
Simulation of Groundwater Flow System 71

data available for guiding the delineation of zones in the IAS 

(layer 2), the APPZ (layer 5), and the LFA (layer 7) (figs. 21, 

30 and 36). Furthermore, in each of these hydrogeologic 

units, aquifer test data vary over short distances, which can 

confound use of these data to delineate zones of equal Kh. 

For the ICU (layer 2), there is thought to be a high degree of 

spatial variability in Kv, but again there are very few geologic 

or hydraulic test data to guide constructing zones of constant 

Kv. The occurrence of sinkholes indicates locations where the 

vertical hydraulic conductivity of the ICU may be presumed 

to be relatively large. However, for a groundwater flow model 

in the Lake Wales Ridge area of west Orange and southeast 

Lake Counties, O’Reilly (1998, p. 51) reported unsatisfactory results when attempting to calibrate ICU leakance based 

on sinkhole density inferred from land-surface depressions. 

Because of these difficulties with delineating zones, the pilotpoints approach was used for representing Kh of layers 2, 3, 5, 

and 7, and Kv of layer 2. 

For the layers in which regional-scale variability of Kh 

or Kv is represented using the pilot point method, a regularlyspaced grid of regional pilot points was placed over the 

extent of the model area in each layer. The grid of points in 

layer 3 (fig. 51) shows that the distance between pilot points 

in the row or column direction is 25 grid cells, representing a 

distance of 31,250 ft (5.9 mi). This spacing allowed an appropriate degree of variability to be estimated given the scale 

of the flow model. It produced a total of 1,358 pilot points 

for the entire model, and Kh or Kv was estimated at 1,156 

of these points. An exponential variogram, recommended by 

Doherty and others (2010), was used for the kriging procedure to interpolate Kh or Kv values for all model cells in a 

given layer: 

γ(h C ) e = − [ ] xp( / −h a) , 0 1 (5)

where  ( ) h is the variogram,C0 is the sill, h is the separation 

distance between points, and the range is given by 3a (where 

the variogram reaches 95 percent of the sill). The sill does 

not affect interpolated values. For the regional variogram, the 

value of a is denoted ar and is set to 49,213 ft (15,000 meters), 

which is about 1.6 times the pilot point separation distance. 

This is consistent with the recommendation of Doherty and 

others (2010) that for uniformly spaced pilot points, the 

value of a should be set equal to 1 to 2 times the pilot point 

separation distance. Ordinary kriging, in which the mean of 

the interpolated values is not specified a priori, is used for 

interpolation of the regional pilot points. 

An additional 288 spring pilot points were used to 

represent the distribution of a hydraulic conductivity (K) 

multiplier that allows for increased hydraulic conductivity 

near the springs because of the commonly observed state 

of enhanced dissolution of carbonate rocks near the springs 

(fig. 52). The springs emanate from the UFA, which is 

composed of the OPZ, OLPZ, and APPZ. Thus, the K multipliers are used to define Kh in the corresponding layers 3 

in layers where the calibration observations are sensitive to 

these aquifer properties and field data indicate spatial variability, and as spatially uniform in layers where the calibration 

observations are not sensitive to the aquifer properties and 

field data provide little or no information on spatial variability. 

Kh is spatially variable in the aquifers (layer 1, the IAS of 

layer 2, and layers 3, 5, and 7), and is constant in the confining 

units (layer 4, the ICU of layer 2, and layer 6). Kv is represented directly, and independently of Kh, in layers 1, 2, 4, and 

6 by defining its spatial distribution. In layers 3, 5, and 7, Kv 

is represented by defining vertical anisotropy (α = Kh/Kv); 

MODFLOW then internally calculates Kv (equal to Kh/α) 

using the spatially variable Kh for these layers. 

Sets of regional-scale pilot-point parameters are used 

to represent the regional-scale variability of Kh for layers 2, 

3, 5, and 7 (table 10), and of Kv for layer 2 (table 11). Pilot 

points are a method for estimating the spatial distribution of 

a hydraulic property that varies smoothly over all or part of 

a model layer. The inverse modeling code PEST (Doherty, 

2010a, b), which was used to estimate parameters of the 

ECFT model, includes a suite of programs that facilitates use 

of the pilot-points method (Doherty, 2008). By this method, 

pilot points are placed at some of the model cells in the region 

where a smooth parameter distribution is to be estimated. 

Parameter values at some or all pilot points are estimated 

at all iterations of the nonlinear regression, and parameter 

values at all model cells in the region are interpolated from 

the values at the pilot points. In this way, parameter values 

at all cells are estimated, but the computational expense of 

calculating parameter sensitivities is limited to the pilot point 

parameter locations. PEST supports kriging as the interpolation method, using a variogram that is specified a priori. 

Zones of constant parameter value are used to represent 

the spatial variability of Kh or Kv in some layers (tables 10 

and 11). For layer 1, zones corresponding to the 21 physiographic regions shown in figure 2 are used to represent both 

Kh and Kv, because the hydrogeologic properties of the SAS 

are thought to vary by these regions. For Kv of layer 6, two 

zones are defined, one corresponding to the region where 

MCU I is present, and the other corresponding to the region 

where MCU II is present. The Kv of the MCU II, in the 

southwestern part of the ECFT area (fig. 32), is expected to 

be substantially smaller than that of the MCU I, owing to the 

widespread occurrence of the permeability limiting gypsum 

mineralization within the MCU II.

Model results, geologic data, and aquifer test data 

supported the need to represent Kh of layers 2, 3, 5, and 7, and 

Kv of layer 2 using a spatially variable distribution instead of 

zones with uniform values. Geologic data provide only sparse 

information about the variability of rock properties within 

the hydrogeologic units represented by each of these layers. 

There are a large number of aquifer test data from the OPZ 

(model layer 3), but the variability in the Kh values from these 

tests precludes using these data to delineate zones of constant 

hydraulic conductivity (fig. 24). There are few aquifer test 
72 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

Figure 51. Map showing locations of regional pilot points used for the Ocala permeable 

zone (OPZ, layer 3). The spring name corresponding to the identifier shown is given in 

table 2–1.

! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

!

!

! !

!

!

!

! !

!

!

!

!

!

!

!

!

!

!

!

!

!

9

8

7

6

5

4

3

2

1

22

21

20

19

18

13

17

16

14

15

12

11

10

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

0 10 20 MILES

0 10 20 KILOMETERS

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17 17 !

! Regional pilot point

Spring and identifier

EXPLANATION

East-central Florida 

transient model area

ATLANTIC OCEAN

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 51. Locations of regional pilot points used for the Ocala permeable zone (OPZ, layer 3). The spring name 

corresponding to the identifier shown is given in table 2–1.
Simulation of Groundwater Flow System 73

and 5, and Kv in corresponding layer 4. A set of 96 spring 

pilot points was applied to each of these 3 layers to represent 

the K multipliers, with the pilot-point configuration identical 

in each layer (fig. 52). Simple kriging, in which the mean of 

the interpolated values is specified a priori, and an exponential 

variogram (equation 5) are used for interpolation of the K 

multipliers. The mean K multiplier value is set to 1.0 and the 

multiplier variogram parameter a, denoted as , is set to 4,921 ft 

(1,500 meters), a deliberately small value, so that the effect of 

the estimated multiplier parameters does not extend far from 

the location of the springs. 

The regional pilot points and the spring pilot points are 

used together to obtain the Kh of layer 3 according to the 

following instructions:

1. Using the Kh parameter values at the regional pilot points 

in layer 3 at the start of a PEST run or at the start of an 

optimization iteration of a PEST run, interpolate the 

regional Kh values over all model cells in layer 3 using 

the exponential variogram with range 3ar .

2. Using the K multiplier parameter values at the spring pilot 

points in layer 3 at the start of a PEST run or at the start 

of an optimization iteration of a PEST run, interpolate the 

spring K multiplier values over all model cells in layer 3 

using the exponential variogram with range 3as. Because 

the mean value of the multiplier is set to 0, the K multiplier will equal 0 at all cells that are further than 3as from 

any spring pilot point. 

Figure 52. Map showing locations of spring pilot points and regional pilot points in the northern 

section of the East-Central Florida Transient (ECFT) study area. The spring name corresponding 

to the identifier shown is given in table 2–1.

! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! !

!!! ! ! ! !!!

!

! ! ! ! ! ! ! !

!

! ! ! ! ! !

! ! ! ! !

! ! ! !

! ! ! ! ! !

!

!

! ! !

!

! ! !!! ! ! ! ! ! ! ! ! ! ! !!! ! ! !

! !

! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! !

! !! !

!

!

! ! !

!

777 7

7

77

7

7

7

7

7

7

7 7

77 7

7 7

7

7

9

8

7

6

5 4

11

3

2

1

22

21

20

19

17

15

14

18

16

13

12

10

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

SUMTER COUNTY

SEMINOLE

COUNTY

EXPLANATION

!

21 7 Upper Floridan aquifer 

 spring and spring 

 identifier

Regional pilot point

! Spring pilot point

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

81°50' 81°40' 81°30' 81°20' 81°10'

29°00'

28°50'

28°40'

0 5 10 KILOMETERS

0 5 10 MILES

Figure 52. Locations of spring pilot points and regional pilot points in the northern section of the East-Central 

Florida Transient (ECFT) study area. The spring name corresponding to the identifier shown is given in table 2–1.
74 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

3. Multiply the regional Kh values for layer 3 by the K

multiplier values for layer 3, on a cell-by-cell basis. This 

produces the Kh of layer 3.

The same procedure is used to estimate the Kh of layer 5. 

For layer 4, a uniform value of Kv applies at the regional 

scale, rather than spatially variable values (table 11). Thus, in 

step 3, this value of Kv is multiplied by the multiplier values 

at each cell of layer 4 to produce the Kv of layer 4.

These approaches to parameterizing the horizontal and 

vertical hydraulic conductivity values resulted in 1,696 defined 

parameters, including 1,288 defined Kh parameters (table 10) 

and 408 defined Kv parameters (table 11). Of these defined 

parameters, 1,178 were estimated, including 890 Kh parameters and 288 Kv parameters. The rationale for selecting the 

subset of parameters to be estimated is explained later in the 

discussion of sensitivities.

Representation of Specific Yield and Specific 

Storage

The specific yield Sy of layer 1 was parameterized in the 

same manner as Kh of layer 1, using zones corresponding to 

the physiographic regions shown in figure 2. This resulted in 

21 S

y parameters (Sy_1, Sy_2, Sy_3, ..., Sy_21). Because of 

the lack of data concerning the spatial variability of specific 

storage Ss within the other aquifers and confining units, this 

property was assumed to be uniform within each layer. One 

Ss parameter was defined for layer 2 (Ss_2) because of the 

greater clay content of the ICU compared to the deeper layers. 

A second Ss parameter (Ss_3-7) was defined to represent Ss

in layers 3, 4, 5, 6, and 7, because of the similar carbonate 

lithology of the hydrogeologic units constituting the FAS.

Calibration

The model was calibrated by estimating values of 

hydraulic conductivity K, specific yield Sy, and specific 

storage Ss parameters to match observed groundwater 

heads and spring flows within specified targets. The vertical 

hydraulic conductivity of the streambed, leakance of the 

lakebeds, hydraulic conductivity of the top soil used for the 

GAI method, and the streambed slopes were updated after 

every calibration run. The computer execution time for a 

forward simulation of the 1995–2006 transient model varied 

from about 6 to 12 hours (depending on input values) using 

a state-of-the-art computer workstation. This execution time 

was prohibitively long for the 12-year model to be used as 

the basis for parameter estimation by inverse methods. With 

the software used in this study (presented later), the minimum 

simulation time for one parameter estimation iteration is 

equal to about four times the forward simulation time, even 

if an unlimited supply of computer processors are available. 

Commonly, at least seven iterations are needed, and thus 

for an 8-hour forward run time, the minimum parameter 

estimation run time is 224 hours, or about 9 days. For the 

ECFT model, calibration involved numerous parameter 

estimation runs, because model development and calibration 

proceeded concurrently. The 9-day run time would have 

substantially impeded progress on completing the model.

Based on the aforementioned concerns, model calibration 

was achieved by iteratively (1) estimating K parameters using 

inverse modeling with steady-state approximations of the 

model that represent average annual conditions in 1999 and 

2003, then (2) estimating Sy and Ss using inverse modeling 

for the final 2 years (2005–2006) of the transient model, and 

finally (3) using model fit results of a forward simulation 

of the 12-year transient model to guide manual adjustments 

of the conceptual model and (or) boundary conditions. This 

iterative procedure repeats until parameter estimates are 

within reasonable ranges defined on the basis of independent 

information about the hydraulic and storage properties of 

the aquifers, boundary conditions do not violate known or 

estimated information about the flow system, and model fit 

targets are met for the simulation results of step 3.

The inverse modeling code PEST (Doherty, 2010a, b) 

is used in steps 1 and 2 of the model calibration approach 

to estimate ECFT model parameters. PEST uses a modified 

Gauss-Newton nonlinear regression method to estimate 

parameters that minimize a sum of squared weighted residuals 

objective function. For this study, the objective function, Φ , 

can be expressed as:

Φ =  ( ) ′ −  

 +  ( ) ′ −   = =

∑ ∑ ω μ i i i ω i

N

k k k

k

N

y y P P

d pr 2

1

2

1 (6)

where 

Nd is the number of observations, 

yi is a head or spring-flow observation, 

y′

i is the simulated equivalent of that 

observation, 

i is the weight on yi ,

Npr is the number of prior values, 

Pk is the kth prior information value, 

P′

k is the simulated value of Pk , 

k is the weight on Pk , and

 is the regularization weight factor.

The difference y y i i ′ − is the observation residual, and P P k k ′ − is 

the prior information residual. Conventionally, these residuals 

are usually defined as y y i i − ′ and P P k k − ′ , but a different 

convention is used here so that a positive residual indicates 

simulated values are too high, and a negative residual indicates 

simulated values are too low.

In PEST, observation weights are defined as

ω σ i i = 1/ (7)

wherei is the standard deviation of the observation error. 
Simulation of Groundwater Flow System 75

Similarly, prior weights are defined as

ω σ k k = 1/ (8)

where k is the standard deviation of the error in the prior 

value. Observations, prior information, and weights for the 

ECFT model are discussed later. The value of  is estimated 

by PEST if prior information is used as regularization, and 

equals 1.0 otherwise. 

Regularization was used in calibrating the ECFT model 

and is a method of stabilizing the inverse problem when aquifer 

properties are represented in the model by a larger number of 

parameters than can be estimated with the available observations. Models in which a large number of parameters are used 

to represent the variability of an aquifer property are commonly 

called “highly parameterized”. For such models, two potential 

issues are that (1) multiple combinations of such parameters 

can produce an equally good fit to the observations, and (2) the 

parameter estimation procedure might achieve a very close fit 

to the observation data at the expense of producing unrealistic 

variability in the distribution of a parameter field, for example, 

unrealistic hydraulic conductivity values. These issues also 

can arise when using small numbers of parameters in model 

calibration, but they tend to be exacerbated when using a highly 

parameterized approach. The first issue can be explored by 

evaluating parameter uncertainty, discussed later therein. To 

illustrate how PEST addresses the second issue, the objective 

function in equation 6, can be rewritten as: 

Φ Φ= + m r  Φ (9)

where

Φm i i i

i

N

y y

d

= −  ( )′  

 =

∑  2

1

is the measurement objective function 

and Φr k k k

k

N

P P

pr

= −  ( )′  

 =

∑  2

1

is the regularization objective 

function. When regularization is used, PEST incorporates a 

user-defined variable denoting the “limiting measurement 

objective function” (PHIMLIM), into the algorithm for 

minimizing the total objective function Φ . PHIMLIM is the 

smallest allowable value of the measurement objective function and is used to prevent the regression from overfitting the 

observation data. To estimate parameters when regularization 

is used, PEST minimizes Φ of equation 9 while constraining 

Φm to be greater than or equal to PHIMLIM. As part of this 

process, PEST calculates the regularization weight factor μ

(equations 6 and 9) and uses this variable to implement an 

appropriate degree of regularization strength.

If weights on the observations used in the measurement 

objective function reflect all sources of observation error as 

well as model error, then theoretically PHIMLIM equals the 

number of observations (Fienen and others, 2009). However, 

sources of error, particularly model error, can be difficult to 

quantify and observation weights consequently might not 

reflect all error components. This is the case for observation weights in the ECFT models; therefore, a trial and error 

procedure was used to determine an appropriate value of 

PHIMLIM, with the goal of selecting a value that produces an 

acceptable fit to the observation data and estimated parameter 

values that are within the reasonable ranges.

Observation Data

This section explains the determination of annual and 

monthly average spring flows, monthly average streamflows, 

and monthly average water levels from wells used as targets 

for calibration. Also discussed are the multiple linear regressions used to calculate annual water levels at wells used for 

the steady-state models. Several calibration criteria that were 

used to achieve calibration for the 12-year transient model are 

also presented. 

Spring Flows

For each of the 22 springs, average annual flows used 

to calibrate the steady-state models were calculated from 

the average monthly spring-flow measurements. The limited 

availability of spring-flow measurements potentially increases 

the error associated with computing representative average 

annual spring-flow observations. The calibration of the 

2005–2006 transient model used average monthly flows from 

July 2005 to December 2006 at the 22 springs; explanation 

for excluding the observed flows from January 2005 to June 

2005 is given later in the section Water Levels at Wells. To 

assess the fit to spring flows in the 12-year transient model, 

average monthly flows from January 1995 through December 

2006 were used. Examples of measured spring-flow hydrographs for selected springs and the locations of the selected 

springs are provided in the Model Fit section of this report 

and in appendix 2 (figs. 2–1 to 2–7). After calibration was 

completed using the GHB Package for the springs, the same 

spring conductance and spring-pool altitude data were used to 

generate the Drain Package for the 22 springs, obtaining the 

same results for the 12-year transient simulation as generated 

when simulating the springs with the GHB Package. The 

Drain Package is a practical feature to predict spring flow 

because if the simulated aquifer head drops below the pool 

altitude, then the discharge is set to zero, making this a headdependent flow boundary condition and a non-linear function 

of simulated head. The GHB Package is an efficient boundary 

condition for model calibration of spring flow when using 

parameter estimation because the head-dependent flow is a 

linear function of the simulated aquifer head and the specified 

spring-pool altitude.

Weights on spring-flow measurements for both the 

steady-state and transient calibrations were calculated 

using equation 7, expressed asi i i = × 1/ ( ) cv y , where 
76 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

i i i = × ( ) cv y and cvi is the discharge error coefficient of 

variation for spring-flow observation yi . The values for cvi

varied by spring magnitude (table 12). The product 100× cv

can be thought of as the percent error in observed discharge. 

The percent errors are somewhat smaller than the likely errors 

associated with field measurements of discharge, particularly 

for the larger magnitude flows, but were used because of the 

small number of spring-flow observations compared to the 

head observations, and the consequently smaller influence 

on the objective function. The spring-flow observations were 

matched poorly by the model when larger cv values were 

used. The total number of spring-flow observations used for 

each steady-state model was equal to the number of springs, 

which allowed all springs to be considered in the calibration 

(table 12).

Water-Surface Altitudes at Lakes and Streamflows

Monthly average water-surface altitudes calculated 

for gaged lakes (fig. 13) were used to calibrate the GAI 

parameters that determine the land-surface runoff to lakes 

and the lakebed leakance, which is defined as the hydraulic 

conductivity of the lakebed material divided by the thickness 

of the lakebed. The steps followed to achieve calibration of 

the water-surface altitudes at the lakes were to (1) adjust the 

extent of the drainage field of each lake, based on the DEM 

values, to estimate realistic surface runoff volumes to arrive 

at the lakes, (2) adjust the K value used in equation 3 for 

cells in the drainage field of each lake to achieve consistency 

between soil types and infiltration parameters, (3) calibrate 

the streambed slope at lake outlets (where it applies), and 

(4) increase or decrease lakebed leakance incrementally to 

systematically reduce the water-surface altitude residuals at 

the lakes while still simulating realistic leakage rates between 

the gaged lakes and the underlying ICU. Measured watersurface altitude hydrographs and the locations of these gaged 

lakes are shown in the Model Fit section of this report and in 

appendix 3 (figs. 3–1 to 3–19). 

The lakebed leakances were calibrated iteratively 

between parameter estimation runs. After several runs of the 

steady-state models, it was clear that there was a limited range 

of values on lakebed leakances that were sensitive to watersurface altitudes at lakes. The range of calibrated lakebed leakances was, for all but two lakes, two orders of magnitude. The 

two lakes outside the 0.002 to 0.00002 d-1 lakebed leakance 

range were lake numbers 318 and 346 (fig. 13). Wolf Branch 

Creek (fig. 1) discharges to a sinkhole simulated as lake 

number 318 in Lake County (fig. 13). The calibrated lakebed 

leakance of lake number 318 was 0.1 d-1, establishing a high 

hydraulic connection between the sinkhole and the aquifer. 

Lake number 346 in Polk County (fig. 13), which receives 

water for augmentation from a nearby well, had a calibrated 

lakebed leakance of 0.01 d-1.

Monthly streamflow averages calculated for streamgaging stations (fig. 12) were used to calibrate the GAI parameters that determine the land-surface runoff to streams and 

the vertical hydraulic conductivity of the streambed. Because 

tidal effects cannot be simulated with the SFR2 Package, flow 

residuals are expected to be larger for the stress periods where 

measured streamflows were negative. Examples of measured 

streamflow hydrographs and the locations of these stations are 

shown in the Model Fit section of this report and in appendix 4 

(figs. 4–1 to 4–9).

The UZF1 Package distributes the generated land-surface 

runoff along the streams reaches composing a stream segment, 

with no time delay because of overland routing. This implies 

that the time delay associated with routing overland runoff is 

not simulated because runoff is instantaneously distributed 

along the corresponding stream reaches of the stream segment. 

Thus, for large basins, the simulated time of a peak streamflow 

may be offset from the measured hydrograph peak depending 

on when in the month the rainfall event occurred. The time 

difference between the simulated and measured stream 

hydrograph peaks should be smaller for small basins than for 

large basins.

Water Levels at Wells

Average annual or monthly water levels were used as 

observations during parameter estimation for calibrating the 

hydraulic conductivity distributions for the steady-state conditions of 1999 and 2003 and the storativity properties for the 

transient conditions of 2005 and 2006. Average monthly water 

levels for 1995–2006 were used to quantitatively assess the fit 

of the transient model over the 12-year simulation period.

The average annual heads used to calibrate the model for 

steady-state conditions were calculated from average monthly 

data. Sets of 442 and 505 wells, respectively, were used in 

the calibration of the 1999 and 2003 steady-state models. The 

number of wells differed because of (1) variations in the availability of head measurements and (2) drilling of new wells 

between 1999 and 2003 (figs. 53 to 55). 

Table 12. Coefficients of variation used to calculate weights on 

spring-flow measurements.

[Number of spring flow observations provided for the steady-state simulation 

years to provide changes in spring magnitude; ft3/s, cubic foot per second]

Spring 

magnitude 

(ft3/s)

Coefficient 

of variation

(cv)

Number of 

1999 

springflow

observations

Number of 

2003 

springflow 

observations

0 to 3 0.2 5 5

 3 to 10 0.1 7 5

 10 to 100 0.05 8 10

>100 0.02 2 2
Simulation of Groundwater Flow System 77

Figure 53. Map showing locations of wells in the surficial aquifer system (SAS, layer 1) and 

intermediate confining unit/intermediate aquifer system (ICU/IAS, layer 2) for which average 

annual head observations in 1999 and 2003 were used to calibrate the steady-state models.

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MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

Base from U.S. Geological Survey 

digital data, Universal Transverse 

Mercator projection, zone 17 "

#

!

Head observation in SAS

 for both 1999 and 2003

Head observation in SAS 

 for 1999 only

Head observation in SAS 

 for 2003 only

"

#

!

Head observation in 

 ICU/IAS for both 1999 

 and 2003

Head observation in 

 ICU/IAS for 1999 only 

Head observation in 

 ICU/IAS for 2003 only 

EXPLANATION

East-central Florida 

transient model area

ATLANTIC OCEAN

0 10 20 MILES

0 10 20 KILOMETERS

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 53. Locations of wells in the surficial aquifer system (SAS, layer 1) and intermediate confining unit/

intermediate aquifer system (ICU/IAS, layer 2) for which average annual head observations in 1999 and 2003 

were used to calibrate the steady-state models.
78 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

Figure 54. Map showing locations of wells in the Ocala permeable zone (OPZ, layer 3) and 

Ocala low-permeable zone (OLPZ, layer 4) for which average annual head observations in 

1999 and 2003 were used to calibrate the steady-state models.

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

East-central Florida 

transient model area

0 10 20 MILES

0 10 20 KILOMETERS

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Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17 Head observation in OPZ 

 for both 1999 and 2003

Head observation in OPZ 

 for 1999 only

Head observation in OPZ 

 for 2003 only

Head observation in OLPZ 

 for both 1999 and 2003

Head observation in OLPZ 

 for 1999 only

"

#

!

"

#

EXPLANATION

ATLANTIC OCEAN

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 54. Locations of wells in the Ocala permeable zone (OPZ, layer 3) and Ocala low-permeable zone (OLPZ, 

layer 4) for which average annual head observations in 1999 and 2003 were used to calibrate the steady-state models.
Simulation of Groundwater Flow System 79

Figure 55. Map showing locations of wells in the Avon Park permeable zone (APPZ, layer 5), 

middle confining unit I/II (MCU I/II, layer 6), and Lower Floridan aquifer (LFA, layer 7) for which 

average annual head observations in 1999 and 2003 were used to calibrate the steady-state 

models. 

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MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

Head observation in APPZ 

 for both 1999 and 2003

Head observation in APPZ 

 for 1999 only

Head observation in APPZ 

 for 2003 only

"

#

!

Head observation in LFA

 for both 1999 and 2003

Head observation in 

 MCU I and II for 

 both 1999 and 2003

Head observation in LFA 

 for 2003 only

"

!

"

EXPLANATION

ATLANTIC OCEAN

East-central Florida 

transient model area

Base from U.S. Geological Survey 

digital data, Universal Transverse 

Mercator projection, zone 17

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

0 10 20 MILES

0 10 20 KILOMETERS

Figure 55. Locations of wells in the Avon Park permeable zone (APPZ, layer 5), middle confining unit I/II

(MCU I/II, layer 6), and Lower Floridan aquifer (LFA, layer 7) for which average annual head observations in 

1999 and 2003 were used to calibrate the steady-state models.
80 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

Head observations for the steady-state simulations were 

calculated at wells for which at least two average monthly 

heads were available in a given year. Average annual heads 

were first calculated at wells for which all 12 average monthly 

heads were available for the year. These average annual heads 

for a given model layer were then used to obtain dimensionless linear regression coefficients 



M1 and 



M 2 for any two 

months, M1 and M2, using the multiple linear regression:

h h h year = + β β M M M M + ε   1 1 2 2 (10)

where 

hM1 and hM 2 are the average monthly heads (in ft) at the 

 well for months M1 and M2, 

 is the regression error, and 

hyear is the annual average head at the well. 

For wells with fewer than 12 but with at least two average 

monthly heads for a given year, average annual heads were 

then estimated using equation 10. The correlation coefficient 

of the multiple linear regressions was always at least 0.98, 

implying a strong correlation between the annual head averages and the average monthly head averages calculated for 

the wells. Monthly average heads for the full 12-year period 

of the transient simulation were used to calculate measures of 

model fit, which included root-mean-square and overall mean 

residuals.

Monthly average heads from July 2005 through 

December 2006 were used to calibrate the Sy and Ss for 

transient conditions during 2005 to 2006. Before starting a 

regression run using the 2005–2006 model, the 1995–2006 

transient model was simulated with the starting Sy and Ss 

values used for the regression run. Simulated heads in the 

1995–2006 model at the end of 2004 were then used as initial 

heads for the 2005–2006 model simulation. However, even 

though the initial heads for this estimation run were consistent 

with the starting Sy and Ss values for the run, they became 

inconsistent when these values changed during the parameter 

estimation procedure. Tests discussed previously (see Initial 

Conditions section) showed that transient forward model runs 

with different initial head distributions take about 6 months 

simulation time for simulated heads and flows to become 

consistent with the model parameters. Therefore, the first 6 

months of observation data for the 2005-06 transient model 

were excluded from the model calibration. 

Error-based weighting is a recommended procedure for 

defining weights on observations used in model calibration 

(Hill and Tiedeman, 2007, p. 291). By this method, the weight 

for the i

th observation is defined as in equation 7 when using 

PEST, where the total standard deviation of observation error,

i , is computed from all components of observation error. For 

the 1999 and 2003 average annual hydraulic head observations, these components include error related to: (1) temporal 

variability in the monthly values used to compute the annual 

average hydraulic head at a well, (2) the location of the well, 

(3) the well measuring-point altitude, and (4) the depth-towater measurement. The four observation-error components 

listed were used to determine thati for 1999 head observations ranged from 0.22 to 18.3 ft, with a median of 1.5 ft, and 

that for 2003,i ranged from 0.20 to 12.4 ft, with a median 

of 0.96 ft. Error component 1, related to the temporal variability of monthly values used to calculate the annual average, 

dominates the total error in the head observations. Some types 

of model error can be included in the calculation of observation weights by equation 7 (Hill and Tiedeman, 2007, p. 300), 

but model error is difficult to quantify and was not included in 

the calculations. 

Using uniform head weights produced a better fit to 

observations from the 12-year transient simulation than did 

using K values estimated using error-based weights. Using 

the error-based weights de-emphasized average annual head 

observations at wells with substantial temporal variability. 

However, in the transient simulation, all monthly head observations are implicitly considered to have equal weights, for 

purposes of calculating the measures of model fit used in this 

study. Therefore, the final weighting strategy for calibration of 

the steady-state model was to set all head standard deviations 

of error for the steady-state model calibration to 1 ft, with the 

corresponding weights equal to 1 ft-1. This value was selected 

because 1 ft is (1) roughly equal to the median standard deviation computed from error components (1) to (4) above, and 

(2) conceptually is a reasonable error assumption for a flow 

system where the total range in observed heads is from about 

0 to 130 ft. Weights for the monthly head observations from 

July 2005 to December 2006 were specified using the same 

approach, and also were set to 1.0 ft-1.

Examples of hydrographs from wells open to hydrogeologic units corresponding to model layers 1 to 7 and the locations of these wells are shown in the Model Fit section of this 

report and in appendix 5 (figs. 5–1 to 5–73). The heads shown 

in these hydrographs for the 12-year transient model are the 

result of the calculation of monthly averages. The calculation 

of annual average heads from multiple linear regressions was 

used only to determine average annual heads for the steadystate calibrations.

Calibration Criteria

Data used to calibrate the model for steady-state conditions during 1993 and 2003 were the annual average measured 

heads at the wells, the annual average measured spring flows, 

the monthly average measured streamflows (used to calibrate 

annual average recharge rates to the SAS and riverbed vertical 

hydraulic conductivity), and the average monthly water-surface 

altitudes at lakes (used to calibrate annual average recharge 

rates to the SAS and lakebed vertical hydraulic conductivity). 

Data used to calibrate the model for transient conditions for the 

12-year simulation period were the monthly average measured 

heads at the wells, the monthly average measured spring flows, 

the monthly average streamflows, and the monthly average 

water-surface altitudes at lakes. 
Simulation of Groundwater Flow System 81

For the 12-year transient model, calibration criteria were 

established on the basis of the residuals (defined after equation 6) and the root-mean-square residual (RMSR) for monthly 

average heads at wells, spring flows, water-surface altitudes 

at lakes, and streamflows. The RMSR for a set of residuals is 

defined as RMSR y y n i i

i

n

= ( ) ′ − 



 



 =

∑ 2

1

1 2

/

/

, where y′

i and yi are 

defined following equation 6, and n is the number of residuals 

used in the calculation. 

The overall goal for heads at wells was to have a 

minimum of 50 percent of wells with a residual less than or 

equal to 2.5 ft in absolute value, a minimum of 80 percent 

of wells with a residual less than or equal to 5 ft in absolute 

value, an RMSR for all wells of less than 5 ft for each of the 

12 simulated years, and a maximum overall mean residual 

(OMR), defined as OMR = ( ) ′ − =

∑ y y n i i

i

n

/ 1

, of less than 1 ft 

in absolute value for each of the 12 simulated years. These 

calibration criteria are based on the potential sources of error 

in the head observations, which are related to the (1) accuracy 

of the calculated average annual water levels for 1999 and 

2003 used in the steady-state simulations, (2) accuracy of the 

measured water levels, and (3) model error in general. The 

combination of these factors was generalized into the calibration criteria listed above. An additional calibration criterion 

for head residuals was to achieve, as much as possible, a 

random spatial distribution of residuals.

The calibration criterion for spring flows, over the 12-year 

period of simulation, was to have an RMSR within 10 percent 

of the measured flows for spring flows larger than or equal to 

10 ft3/s, and within 20 percent for smaller springs. The calibration criterion for the water-surface altitudes at lakes was to have 

an annual RMSR, calculated over all lakes and for each of the 

12 years of simulation, of less than 3 ft. The reason for the 3-ft 

maximum RMSR at lakes is because control structures near 

lake outlets are not simulated because these structures control 

the flow leaving the lake under otherwise natural conditions and 

such flow data were not available. Examples of these control 

structures are at stream segments 178 and 253 (fig. 11). 

Stream control structures also limit the ability to simulate 

streamflows because flow downstream of a structure depends 

on the altitude of the structure. The presence of these control 

structures precluded establishing a calibration criterion for 

streamflow residuals. Therefore, the calibration criterion for 

streamflow was that the volumes under the hydrograph curves 

for the simulated and observed flows be as close as possible.

Calibration of K Parameters for Steady-State 

Conditions during 1993 and 2003

The model was used to represent annual stressed steadystate conditions during 1999 and 2003 for the purpose of estimating the distributions of hydraulic conductivity (K) values 

with PEST. Steady-state models were developed for 1993 

and 2003 because the estimated absolute value of the storage 

term of the groundwater flow equation was smallest for these 

two years (table 4), thus reducing the model error associated 

with the approximation of steady-state conditions. Modifications of the model to represent steady-state conditions were 

made with the goal of keeping the representation of boundary 

conditions as close as practical to that of the transient model. 

Adjustments included replacing the SFR2, LAK7, and UZF1 

Packages used for transient 1995–2006 conditions as follows: 

(1) using the River Package (Harbaugh, 2005) in place of the 

SFR2 Package; (2) using constant-head cells at the locations 

of lake cells in place of the LAK7 Package; and (3) using the 

Recharge and Evapotranspiration Packages (Harbaugh, 2005) 

in place of the UZF1 Package. 

For the 1999 and 2003 steady-state simulations, annual 

average water-surface altitudes at rivers and lakes were 

calculated from monthly averages. Monthly average watersurface altitudes at rivers were calculated from stream-gage 

measurements or estimated from the DEM. Similarly, watersurface altitudes at the lakes were calculated from lake-gage 

measurements or estimated from the DEM. Annual average 

heads used as general-head boundaries along the lateral 

boundaries of the model for steady-state simulations are the 

same as those used to represent 1999 and 2003 conditions for 

the transient simulation. Annual average recharge rates to the 

SAS for 1999 and 2003 were calculated from the net effects 

of infiltration, unsaturated zone ET, and groundwater seepage 

(surface leakage), all of which are simulated by the UZF1 

Package in the transient simulation. Surface leakage was 

simulated in the steady-state models by subtracting surface 

leakage from net recharge rates if these resulted in positive 

recharge rates. If these resulted in negative rates, recharge 

rates were set to zero and the negative rates were included in 

the Well Package. Annual average estimated ET rates for the 

1999 and 2003 steady-state simulations were calculated from 

the groundwater ET simulated by the UZF1 Package in the 

transient simulation. 

Hydraulic conductivities around and beneath the 

constant-head cells representing the lakes were adjusted so 

that leakance values calculated internally in the steady-state 

simulations equaled leakance values specified for the LAK7 

Package in the transient simulation. The horizontal hydraulic 

conductivities of the active cells in the SAS along the perimeter of the constant-head cells were calculated by multiplying 

the lakebed leakance in the transient simulation by half the 

width of the cell (625 ft). Additionally, the vertical hydraulic 

conductivity of the active cells in layer 2 that underlie the 

constant-head cells was calculated by multiplying the lakebed 

leakance in the transient simulation by half the thickness of 

the constant-head cell.

PEST was used with the steady-state simulations for 1999 

and 2003 to estimate the hydraulic conductivity parameters. 

Observations from multiple simulations can be used in a 

single PEST run to estimate parameters that are common to 

the simulations. Here, the representation of K in both of the 
82 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

steady-state simulations is identical to that of the 1995–2006 

transient simulation, and the same parameters related to K are 

defined in each of the simulations. In essence, the separate 

MODFLOW-2005 simulations for 1999 and for 2003 together 

constitute a single simulation with respect to PEST.

Parameter Sensitivities

Composite scaled sensitivities (css) are used to guide 

selection of parameters to be estimated by PEST. The css

for parameter bj is calculated as follows (Hill and Tiedeman, 

2007, p. 50, modified to use weight definition in PEST):

css y

b j b N i

j

j i

i

N

d

d

= ∂ ′

∂

























 =  ∑ ω 1

1 2

/

/

(11)

where ∂y b ′ ∂ i j / is the sensitivity of the simulated equivalent of 

observation y′

i with respect to parameter bj (hereafter denoted 

observation sensitivities), and  i is defined in equation 7. 

Each cssj is a measure of the information provided by all the 

observations about parameter bj . Typically, only parameters 

with large css relative to other parameters can be estimated by 

the regression. 

For the steady-state models, the large number of 

parameters related to K precludes evaluating the css for each 

parameter defined in tables 10 and 11. Instead, for purposes of 

calculating css, sets of these hydraulic conductivity parameters were lumped together using multiplier parameters, as 

defined in table 13. These multiplier parameters are denoted 

“sensitivity parameters” to distinguish them from the K 

multiplier parameters at the spring pilot points. There is one 

sensitivity parameter related to the Kh for each model layer in 

which this property is spatially variable, and one sensitivity 

parameter related to Kv for each model layer in which this 

property is spatially variable. For model layers where a 

uniform Kh parameter is defined (layers 4 and 6; table 10), 

the css is calculated for the Kh parameter. Similarly, for layers 

Table 13. Definition of sensitivity parameters.

[Kh, horizontal hydraulic conductivity; Kv, vertical hydraulic conductivity; ghb, general-head boundary]

Sensitivity 

parameter Definition

Kh1_m Factor that multiplies Kh of all cells in layer 1

Kv1_m Factor that multiplies Kv of all cells in layer 1

Kh2_m Factor that multiplies Kh of all cells in layer 2

Kv2_m Factor that multiplies Kv of all cells in layer 2

Kh3_m Factor that multiplies Kh of all cells in layer 3 

Kv4_m Factor that multiplies Kv of all cells in layer 4

Kh5_m Factor that multiplies Kh of all cells in layer 5 

Kv6_m Factor that multiplies Kv of all cells in layer 6 

Kh7_m Factor that multiplies Kh of all cells in layer 7

Ghb1c_m Factor that multiplies conductance of all ghb cells in layer 1

Ghb2c_m Factor that multiplies conductance of all ghb cells in layer 2

Ghb3c_m Factor that multiplies conductance of all ghb cells in layer 3

Ghb4c_m Factor that multiplies conductance of all ghb cells in layer 4

Ghb5c_m Factor that multiplies conductance of all ghb cells in layer 5

Ghb6c_m Factor that multiplies conductance of all ghb cells in layer 6 

Ghb7c_m Factor that multiplies conductance of all ghb cells in layer 7 

Riv_m Factor that multiplies conductance of all river cells

Rch99_m Factor that multiplies recharge rate in all cells of the 1999 steady-state model

Rch03_m Factor that multiplies recharge rate in all cells of the 2003 steady-state model 

Evtr99_m Factor that multiplies evapotranspiration rate in all cells of the 1999 steady-state model 

Evtr03_m Factor that multiplies evapotranspiration rate in all cells of the 2003 steady-state model 
Simulation of Groundwater Flow System 83

in which a uniform value of  is used to define Kv (layers 3, 

5, and 7; table 11), the css is calculated for  . Additional 

sensitivity parameters were defined that relate to recharge 

and ET rates and to the conductance specified for GHB and 

river cells (table 13).

The css calculated with all head and flow observations 

from both the 1999 and 2003 steady-state models are shown 

in figure 56A. These css are derived using the final parameter 

estimates. Among the sensitivity parameters of the hydraulic 

conductivity, the css values are largest for Kh1_m, Kv2_m, 

Kh3_m, Kh5_m, Kv6_m, and Kh7_m (fig. 56A). This set 

includes parameters related to the Kh of all aquifers and the 

Kv of most confining units. The css are very small for all 

sensitivity parameters related to the general head boundary 

and riverbed conductance. The css for the recharge and ET 

rate sensitivity parameters lie in the same range as the largest 

css values for the sensitivity parameters related to hydraulic 

conductivity. 

The css were calculated for subsets of the head and 

spring-flow observations, including observations from both 

1999 and 2003 (figs. 56B–D). For these css calculations, Nd

in equation 11 is equal to the number of observations in the 

subset. The maximum css for any observation subset with 

respect to any general-head-boundary sensitivity parameter is 

about 0.8, and the maximum css for any subset with respect 

to the riverbed sensitivity parameter is about 0.2, so these 

parameters are excluded from the figures. 

On the basis of the css magnitudes (fig. 56), all Kh 

parameters defined for layers 1, 3, 5, and 7 (table 10) and 

all Kv parameters defined for layers 2 and 6 (table 11) were 

selected for estimation by PEST. In PEST, such parameters 

are denoted “adjustable.” The Kv of layer 4 also was adjustable, so that the vertical properties of all confining units 

were allowed to be estimated. With regularization included 

as discussed in the next section, the regression method can 

accommodate adjustable parameters with small sensitivities 

through the use of prior information. In addition, for layer 2, 

Kh parameters representing the IAS (fig. 21) were adjustable, 

whereas the Kh parameter representing the ICU was specified at 7 ft/d, which is within the same order of magnitude 

of the only aquifer test value in the ICU away from the IAS 

(table 10, fig. 21). Much smaller values than 7 ft/d for Kh in 

the ICU would lead to an unrealistically large drawdown near 

this aquifer test value (fig. 21). The general-head boundaries 

and riverbed conductances were specified because of the 

very small css for their respective sensitivity parameters. The 

hydraulic conductivity parameters for the remaining hydrogeologic units, for layers 2 to 7, were specified at reasonable 

values based on field knowledge (tables 10 and 11).

The recharge and ET rate sensitivity parameters have 

large css, indicating that the simulated values at the observation locations are moderately to highly sensitive to the rates. 

However, the rates were not included as adjustable parameters. 

As discussed previously, the approach to development and 

calibration of the 1995–2006 ECFT model involved calculating recharge and ET rates by using extensive physical 

and hydrologic data, and by considering the physical and 

hydrologic processes that affect the movement of water at 

land surface and within the unsaturated zone. This procedure 

resulted in spatially variable estimates of the rates that are 

likely more realistic than could be obtained by the regression 

method used for flow-model parameter estimation. Furthermore, estimating both recharge rate and hydraulic conductivity 

parameters in model calibration is often problematic because 

of parameter correlations and non-unique solutions; therefore, 

the independent recharge and ET rate estimates were specified, 

rather than estimated, in the regression.

Parameters related to the spring pilot points are not 

included in the css graphs because the sensitivity parameters 

defined for calculating the css (table 13) apply to distributions of Kh in layers 3 and 5 and Kv of layer 4 that already 

account for the effects of the K multipliers estimated at 

these pilot points. Initial regression runs explored observation sensitivities for the K multiplier parameters. These runs 

showed that the observations do not support estimation of 

spatial variability in the K multipliers within any layer, nor do 

they support estimation of different K multipliers in layers 3, 

4, and 5. Therefore, a single K multiplier was adjustable for 

one of the pilot points associated with each spring, and the K 

multipliers at all other pilot points associated with that spring 

were “tied” to this estimate.

Preferred Value Regularization

Preferred value regularization was used for all adjustable 

parameters in the calibration of the steady-state simulations. 

By this method, the prior information that serves as regularization is of the form

log log '

10 ( ) P P k k = 10 ( ) (12)

where log10 is the base-10 logarithm and P′

k and Pk are 

defined following equation 6. In equation 12, Pk

'

is a Kh, Kv, 

or K-multiplier parameter and Pk is the preferred value of this 

parameter, also referred to as its prior value. In equation 12, 

log10 is used because all parameters were log-transformed in 

the regression. Log-transforming prevents the parameters from 

becoming negative, is consistent with substantial evidence that 

the distribution of hydraulic conductivity in aquifers tends to 

be lognormal, and can make nonlinear regression for groundwater systems behave more linearly.

The preferred values represent information about the 

parameters from field data and expert knowledge. These 

values are based on aquifer-performance test data (figs. 18, 

21, 24, 30, and 36), maps of the potentiometric surface that 

provide indirect evidence of low or high horizontal hydraulic 

conductivity (for example, fig. 25), and knowledge from 

USGS, SJRWMD, SFWMD, and SWFWMD personnel 

familiar with the hydrogeology of the flow system. In addition 
84 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

Figure 56. Graphs showing composite scaled sensitivities for physical and sensitivity parameters 

of the steady-state models for (A) all observations, (B) head observations in layers 1 and 3, (C) head 

observations in layers 5 and 7, and (D) spring-flow observations. Parameters Vani3, Vani5, and Vani7 

are the vertical anisotropy of layers 3, 5, and 7, respectively. Parameters Kh4 and Kh6 are the 

horizontal hydraulic conductivity of layers 4 and 6, respectively. Sensitivity parameters ending in “_m” 

are defined in table 13.

0

2

4

6

8

10

12

14

16

Kh1_m

Kv1_m

Kh2_m

Kv2_m

Kh3_m

Vani3

Kh4

Kv4_m

Kh5_m

Vani5

Kh6

Kv6_m

Kh7_m

Vani7

Rch99_mRch03_m

Evtr99_mEvtr03_m

Composite scaled

sensitivity of

observation group 

Composite scaled

sensitivity of

observation group 

Composite scaled

sensitivity of

observation group 

Parameter

0

2

4

6

8

10

12

14

16

Kh1_m

Kv1_m

Kh2_m

Kv2_m

Kh3_m

Vani3

Kh4

Kv4_m

Kh5_m

Vani5

Kh6

Kv6_m

Kh7_m

Vani7

Rch99_mRch03_m

Evtr99_mEvtr03_m

Parameter

0

2

4

6

8

10

12

14

16

Kh1_m

Kv1_m

Kh2_m

Kv2_m

Kh3_m

Vani3

Kh4

Kv4_m

Kh5_m

Vani5

Kh6

Kv6_m

Kh7_m

Vani7

Rch99_mRch03_m

Evtr99_mEvtr03_m

Parameter

0

2

4

6

8

10

A

B

C

D

Kh1_m

Kv1_m

Kh2_m

Kv2_m

Kh3_m

Vani3

Kh4

Kv4_m

Kh5_m

Vani5

Kh6

Kv6_m

Kh7_m

Vani7

Ghb1c_mGhb2c_mGhb3c_mGhb4c_mGhb5c_mGhb6c_mGhb7c_m

Rch99_mRch03_m

Evtr99_mEvtr03_m

Riv_m

Composite scaled

sensitivity

Parameter

EXPLANATION

1999 head observations, layer 1

2003 head observations, layer 1

1999 head observations, layer 3

2003 head observations, layer 3

1999 head observations, layer 5

2003 head observations, layer 5

1999 head observations, layer 7

2003 head observations, layer 7

EXPLANATION

EXPLANATION

1999 springflow observations

2003 springflow observations

Figure 56. Composite scaled sensitivities for physical and sensitivity parameters of the steady-state models 

for A, all observations, B, head observations in layers 1 and 3, C, head observations in layers 5 and 7, and 

D, spring-flow observations. Parameters Vani3, Vani5, and Vani7 are the vertical anisotropy of layers 3, 5, and 7, 

respectively. Parameters Kh4 and Kh6 are the horizontal hydraulic conductivity of layers 4 and 6, respectively. 

Sensitivity parameters ending in “_m” are defined in table 13.
Simulation of Groundwater Flow System 85

to being used as regularization, the preferred values are used 

as the initial values in parameter estimation runs.

Preferred value regularization adds a penalty to the 

objective function when an estimated parameter value deviates 

from its preferred value (equations 6 and 9). A parameter 

will deviate if doing so improves the total objective function

Φ , subject to the constraint that Φm remains equal to or 

larger than PHIMLIM (discussed following equation 9). An 

improvement in Φ occurs when the decrease in Φm , representing the fit to observations, is greater than the increase in

 Φr , the penalty term. For parameters to which the simulated 

values at observations are not very sensitive, the preferred 

value regularization ensures that the estimates will remain 

near values that are based on field data and expert knowledge. 

When using preferred value regularization (and an appropriate 

PHIMLIM) for a set of spatially distributed pilot point parameters representing an aquifer property, the estimated values of 

the property will change in locations where doing so improves

Φ , and will remain near the preferred values in other areas. 

Therefore, despite the large number of parameters defining the 

spatial distribution, the procedure allows deviations from the 

preferred values only when such changes are supported by the 

observation data.

Preferred values of Kh in layer 1 range from 8 to 60 ft/d 

(fig. 57). The lowest value corresponds to Green Swamp 

(fig. 2) where fine-grained sediments are abundant (Grubb and 

Rutledge, 1979). The highest values are associated with Polk 

Upland, Lake Wales Ridge, Winter Haven Ridge, Lakeland 

Ridge, and Lake Henry Ridge (fig. 2). In these areas, coarse 

sediments generally cause the water table to be relatively flat 

and deep beneath hills and upland areas (Sumner and Bradner, 

1996; O’Reilly, 1998).

Preferred values for Kh of layers 2, 3, 5, and 7 and for 

Kv of layer 2 are assigned at the pilot point locations. These 

values are then interpolated using the kriging procedure 

described previously. Preferred values of Kh in layer 2 are 

largely guided by aquifer test data (fig. 58). Away from areas 

with aquifer test data, the preferred Kh value in the IAS and 

the specified Kh value in the ICU are 7 ft/d. Preferred values 

of Kv in layer 2 are largest along Lake Wales Ridge, where the 

prevalence of sinkholes and the observation of similar waterlevel fluctuations in SAS and UFA wells suggest that the ICU 

is highly leaky in this area (fig. 59; Tihansky and others, 1996; 

Yobbi, 1996; O’Reilly, 1998). Data for the Kv of the ICU are 

limited, and the data available are highly variable; however, 

lithologic information indicates a greater prevalence of clayey, 

fine-grained, or otherwise lower permeability sediments 

composing the ICU throughout the Osceola and Wekiva Plains 

and Eastern Valley (fig. 2) where preferred values were less 

than 0.005 ft/d (fig. 59).

In layer 3, preferred values of Kh are low in Polk and 

southern Lake Counties, where a high in the potentiometric 

surface of the OPZ (fig. 25) is thought to be related to the 

presence of low Kh values (fig. 60). An area of low Kh also is 

present in the northeastern part of layer 3, because of another 

high in the potentiometric surface of the OPZ (fig. 25). This 

area in Volusia County generally coincides with an area of 

karst topography (Wyrick, 1960) where sinkhole collapses 

possibly have caused reduced permeability by the filling of 

cavities with a mixture of overlying sand and clay, which is 

consistent with aquifer test results indicating lower Kh of the 

OPZ (fig. 24). The largest preferred values of Kh in layer 3 are 

near springs because of the effect of the K multipliers, which 

have preferred values ranging from 2 to 20, as discussed 

subsequently. For the regional Kv of layer 4, the preferred 

value is 5 ft/d, because the OLPZ is a semiconfining unit 

and allows fairly good vertical hydraulic connection with 

the APPZ.

Preferred values of Kh in layer 5 (fig. 61) generally are 

larger than those in layer 3 (fig. 60). A regionally extensive 

zone of fracture- or dissolution-enhanced permeability in 

the APPZ contributes to these high Kh values. The preferred 

values for Kv of the MCU I and MCU II in layer 6 are, 

respectively, 0.05 and 1×10–4 ft/d, because of the prevalence 

of gypsum mineralization in the MCU II.

In layer 7, preferred values of Kh are small in the region 

underlying the MCU II and thought to be attributable to less 

developed secondary porosity caused by relatively limited 

groundwater leakage through the low permeability MCU II 

(fig. 62). The areas with the largest preferred values of Kh in 

layer 7 are those in which several aquifer tests yielded a Kh 

value of more than 1,000 ft/d.

Preferred values of the K multipliers at the springs 

vary according to the magnitude of spring flow. There are 

few aquifer test data in the vicinity of most springs (fig. 60); 

however, K is likely to be larger near the springs with large 

discharge rates, because of the greater state of dissolution of 

the carbonate rock, than near springs with smaller discharge. 

Accordingly, the preferred value of the K multiplier for spring 

number 3 is 20 (Blue Spring in Volusia County, figs. 1, 51); 

this spring has the largest discharge among all springs in the 

ECFT model area. The preferred value of the K multipliers 

for springs with measured discharge between 30 and 100 ft3/s 

is 10 (spring numbers 1, 16, 18, and 21, fig. 51), and that for 

springs with discharge between 9 and 29 ft3/s (spring numbers 

2, 5, 13, 17, 19, and 20, fig. 51) is 5. For the remainder of the 

springs, the preferred value of the K multiplier is 2.

Weights for the prior information were calculated by 

using an error-based approach. A reasonable range of values 

for each parameter was constructed using the same types of 

information that guided determination of the preferred values. 

This range was assumed to be a 95-percent confidence interval 

on the prior value. Using log transformation and assuming 

a normal distribution, the confidence interval was used to 

derive a standard deviation on each of the prior values. The 

weight for a prior information value was then calculated using 

equation 8. 

The lower and upper limits of the reasonable parameter 

ranges also were used as bounds on the parameter values. 

PEST does not allow estimated parameter values to lie 

outside the range formed by the user-specified upper and 

lower bounds. 
86 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

Figure 57. Map showing preferred values of horizontal hydraulic conductivity (Kh) in the 

physiographic regions used to represent Kh in the surficial aquifer system (SAS, layer 1) 

and selected aquifer test values.

!

!

! !

!

!

! !

!

!

!

!

!

!

!

!

!

!

! 6

6

19

24 50

32 34

34

10

55

48

19

20

46

32

33

42

50

21

East-central Florida

transient model area

EXPLANATION

8.0 to 10.0

10.1 to 20.0

20.1 to 30.0

30.1 to 45.0

45.1 to 60.0

Horizontal hydraulic conductivity of the 

 SAS, in feet per day

Location and hydraulic conductivity, 

 in feet per day, from aquifer test in

 the SAS

! 21

White areas are inactive in the SAS

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

ATLANTIC OCEAN

Base from U.S. Geological Survey 

digital data, Universal Transverse 

Mercator projection, zone 17

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

0 10 20 MILES

0 10 20 KILOMETERS

Figure 57. Preferred values of horizontal hydraulic conductivity (Kh) in the physiographic regions used to 

represent Kh in the surficial aquifer system (SAS, layer 1) and selected aquifer test values.
Simulation of Groundwater Flow System 87

Figure 58. Map showing interpolation of horizontal hydraulic conductivity from preferred 

values at pilot points in the intermediate confining unit/intermediate aquifer system (ICU/IAS, 

layer 2) and selected aquifer test values.

! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY HARDEE

COUNTY

SUMTER COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

23

1

1

5

2

81

19

24

12

49

35

160

230

150

!

!

! !

!

! !

!

!

! !

!

! !

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

East-central Florida

transient model area

0 10 20 MILES

0 10 20 KILOMETERS

EXPLANATION

!

Location and hydraulic conductivity, in feet 

 per day, from aquifer test in the ICU/IAS

Location of regional pilot point used in 

 parameter estimation for layer 2

! 49

Horizontal hydraulic conductivity of the 

 ICU/IAS, in feet per day

6 to 10

11 to 20

21 to 30

31 to 45

46 to 60

61 to 80

81 to 100

101 to 151

ATLANTIC OCEAN

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 58. Interpolation of horizontal hydraulic conductivity from preferred values at pilot points in the 

intermediate confining unit/intermediate aquifer system (ICU/IAS, layer 2) and selected aquifer test values.
88 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

Figure 59. Map showing interpolation of vertical hydraulic conductivity from preferred values 

at pilot points in the intermediate confining unit/intermediate aquifer system (ICU/IAS, layer 2) 

and selected aquifer test values.

! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

EXPLANATION

0.0011 to 0.0050

0.0051 to 0.0100

0.0101 to 0.0500

0.0501 to 0.1000

0.1001 to 0.5000

0.5001 to 1.0000

Location of regional pilot point used in 

 parameter estimation for layer 2

Vertical hydraulic conductivity of the 

 ICU/IAS, in feet per day

!

0 10 20 MILES

0 10 20 KILOMETERS

ATLANTIC OCEAN

East-central Florida 

transient model area

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

Figure 59. Interpolation of vertical hydraulic conductivity from preferred values at pilot points in the 

intermediate confining unit/intermediate aquifer system (ICU/IAS, layer 2) and selected aquifer test values.
Simulation of Groundwater Flow System 89

Figure 60. Map showing interpolation of horizontal hydraulic conductivity from preferred values 

at pilot points in the Ocala permeable zone (OPZ, layer 3) and selected aquifer test values.

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

! !

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

! !

! !

!!

!

!

!

!

!

!

!

!

!

! !

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

! !

!

!

!

!

!

!

!

! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

East-central Florida

MARION transient model area

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

0 10 20 MILES

0 10 20 KILOMETERS

EXPLANATION

1,901 to 2,800

2,801 to 4,000

4,001 to 9,000

9,001 to 15,400

27 to 35

36 to 60

61 to 95

96 to 160

161 to 260

Horizontal hydraulic conductivity of the OPZ, in feet per day

261 to 400

401 to 600

601 to 900

901 to 1,300

1,301 to 1,900

!

! 34 Location and hydraulic conductivity, in feet per day, from aquifer 

 test in the OPZ

Location of regional pilot point used in parameter estimation for 

 layer 3

6

50

38

29

29

16

14

64

53

20

23

19

18

83

73

50

56

35

44 31

46

38

47

50

45

51

22

27

16

33

7

34

55

59

44

97

63

9

52

190

350

100

110

230

600

190

120

260

180

240

730

740

340

110

110

320

320

980

730 840

700

320 260

240

150

280

700

310

270

140

100

100

220 390

140

270

190

220

160

110

270

7,600

2,500

1,200

14,000

ATLANTIC OCEAN

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 60. Interpolation of horizontal hydraulic conductivity from preferred values at pilot points in the 

Ocala permeable zone (OPZ, layer 3) and selected aquifer test values.
90 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

Figure 61. Map showing interpolation of horizontal hydraulic conductivity from preferred 

values at pilot points in the Avon Park permeable zone (APPZ, layer 5) and selected 

aquifer test values.

!

! !

!

!

!

!

!

!

! !

!

!

!

! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

East-central Florida

transient model area

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

EXPLANATION

!

32 to 35

36 to 60

61 to 95

96 to 160

161 to 260

! 30 Location and hydraulic conductivity, in feet per day, from aquifer 

 test in the APPZ

Location of regional pilot point used in parameter estimation for 

layer 5

Horizontal hydraulic conductivity of the APPZ, in feet per day

261 to 400

401 to 600

601 to 900

901 to 1,300

1,301 to 1,900

1,901 to 2,800

2,801 to 4,000

4,001 to 9,000

9,001 to 15,400

0 10 20 MILES

0 10 20 KILOMETERS

290

940

130

1,200

3,400

3,100

53

30

140

190

430 690

220

200

ATLANTIC OCEAN

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 61. Interpolation of horizontal hydraulic conductivity from preferred values at pilot points in the Avon 

Park permeable zone (APPZ, layer 5) and selected aquifer test values.
Simulation of Groundwater Flow System 91

Figure 62. Map showing interpolation of horizontal hydraulic conductivity from preferred 

values at pilot points in the Lower Floridan aquifer (LFA, layer 7) and selected aquifer 

test values.

!

!

!

!

!

!

!

!

!

! !

!

!

!

!

!

!

!

!

!

!

!

!

! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! !

! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! !

98

220

280

700

170

160

430

1,600

1,600 3,100

1,500

7,700

1,000

23

690

120

360

1,200

1,200

3,100

1,700

1,400

1,200

East-central Florida

transient model area

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

EXPLANATION

Location and hydraulic conductivity, in feet per day, from aquifer 

 test in the LFA

Location of regional pilot point used in parameter estimation for 

 layer 7

! 23

43

44 to 60

61 to 95

96 to 160

161 to 260

Horizontal hydraulic conductivity of the LFA, in feet per day

!

261 to 400

401 to 600

601 to 900

901 to 1,300

1,301 to 1,900

1,901 to 2,800

2,801 to 4,000

4,001 to 9,000

9,001 to 15,400

0 10 20 MILES

0 10 20 KILOMETERS

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

ATLANTIC OCEAN

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 62. Interpolation of horizontal hydraulic conductivity from preferred values at pilot points in the Lower 

Floridan aquifer (LFA, layer 7) and selected aquifer test values.
92 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

The value of PHIMLIM for the regularized parameter 

estimation was set equal to 10,000. The starting objective 

function value for the steady-state model calibration, with the 

parameter values equal to the preferred values, was 45,400. 

Trial-and-error exploration of different PHIMLIM values 

showed that a measurement objective function value of 10,000 

produced a realistic distribution of spatial variability in the Kh 

and Kv distributions governed by the pilot points approach 

and also yielded an acceptable model fit.

Calibrated Hydraulic Conductivities

PEST was used to estimate the 1,178 adjustable parameters, including Kh parameters for layers 1, 3, 5, 7, and part of 

layer 2; Kv parameters for layers 2, 4, and 6; and K multiplier 

parameters near springs (tables 9 and 10). The preferred value 

regularization and the variable PHIMLIM are important for 

keeping parameter estimates reasonable and preventing an 

unrealistically close fit to the observed values. Most Kh and 

Kv parameters do not differ significantly from the preferred 

values. For example, the estimates for only 82 of the 1,178 

adjustable parameters differ from their respective preferred 

values by more than a factor of 2, which is a relatively small 

difference for a hydraulic conductivity value observed to vary 

by orders of magnitude. Only 14 estimates, mostly for Kv in 

layer 2, differ from the preferred values by more than an order 

of magnitude. Overall, compared to the preferred parameter 

values, the estimation procedure moderately adjusted the 

parameter values in areas where there are Kh and Kv parameters to which the calibration observations are sensitive, and 

only slightly or negligibly adjusted the parameter values 

elsewhere. The estimated Kh and Kv parameters from PEST 

are referred to next as calibrated parameters.

Most calibrated values of Kh in layer 1 are not significantly different than the interpolated preferred values (figs. 57 

and 63). The largest differences are for zones corresponding 

to Mount Dora Ridge, Deland Ridge, and the southern part 

of Lake Wales Ridge, where the estimated values of Kh are 

smaller than the preferred values. The resulting Kh value for 

Mount Dora Ridge is equal to the lower bound for this parameter of 10 ft/d, indicating that lowering the value further would 

likely improve the measurement objective function. However, 

10 ft/d is considered the lowest reasonable value for the SAS 

sediments in this area. The vertical hydraulic conductivity of 

layer 1 had almost negligible css, which supported assigning 

the Kv values chosen. The Kv values were assigned to be 

proportional to the preferred values of Kh for layer 1 (fig. 64).

The calibrated values of Kh in layer 2 are essentially 

equal to the interpolated preferred values (figs. 58 and 65). 

This is expected given the small css for sensitivity parameter 

Kh2_m (fig. 56). For Kv of layer 2, calibrated values in the 

western part of the model area are substantially different than 

preferred values (figs. 59 and 66). The calibrated Kv along 

Lake Wales Ridge is as large as 1 ft/d, and is greater than 

0.1 ft/d over parts of the ridge. The calibrated Kv also is larger 

in the northern part of the ECFT area than the interpolated 

values. These areas of greater Kv generally coincide with 

more karstic terrain where sinkhole activity may have resulted 

in breaches of the ICU. Nine out of the 285 estimated layer 2 

Kv parameters are equal to the value of a parameter bound. 

Five of these parameters are located along the Lake Wales 

Ridge, and their calibrated values equal an upper bound of 0.5 

or 1.0 ft/d. Four of the parameters are equal to a lower bound 

of 0.004, 0.005, or 0.01 ft/d.

The calibrated values of Kh in layer 3 are not substantially different than the interpolated preferred values (figs. 60 

and 67). For areas away from the springs, the primary difference is in the shape of the low-K region in the west-central 

part of the domain. This is consistent with the relatively 

small css for the head observations with respect to Kh3_m, 

compared to the css for the head observations with respect to 

Kv2_m and Kh5_m (figs. 56B–C). Kh values in layer 3 differ 

relative to the interpolated preferred values to a greater degree 

near the springs, which is consistent with the large value of 

css for the spring-flow observations with respect to Kh3_m 

(fig. 56D). Of the 11 small springs with a preferred value of 2 

for the K multiplier, the calibrated value of this parameter is 

smaller at 10 springs, and for 4 of these, the calibrated value is 

the lower bound of 1; this lower bound is considered reasonable because it is unlikely that Kh is smaller at the few cells 

surrounding a spring, compared to its value further away.

The calibrated regional Kv of layer 4 is 6.5 ft/d, a slight 

difference compared to the uniform preferred value of 5 ft/d. 

Spatial variability in this layer occurs only where the K multipliers at the springs apply (fig. 68), allowing enhanced vertical 

hydraulic connection between the OPZ and APPZ. Because the 

regional Kv of layer 4 is uniform, the location and effects of the 

calibrated values of these K multipliers can be easily identified 

visually (fig. 68). For example, the calibrated K multiplier is 

1.0 at all spring locations where Kv is 6.5 ft/d on this figure. 

The largest calibrated K multiplier is 19.5 for spring number 1 

(fig. 51), the northernmost spring in the model area.

The calibrated values of Kh in layer 5 differ from their 

interpolated preferred values (figs. 61 and 69), consistent with 

the magnitude of the css values (fig. 56). There is a difference in 

the shape of the large high-K region in the east, and calibrated 

Kh values are greater within this region than the interpolated 

preferred values. The maximum calibrated Kh at the northern 

end of this region is 3,800 ft/d, and the maximum in the central 

part is about 2,800 ft/d. Similarly, Kh values are higher in the 

southwest high K region, to a maximum of about 3,800 ft/d. 

Minimum Kh values equal about 30 ft/d in the northeastern and 

the west-central parts of the model area. Two estimates of Kh 

at pilot points are equal to their bounds. The southernmost pilot 

point near the boundary between Hardee and Highlands Counties is equal to its lower bound of 500 ft/d. The pilot point in 

east-central Polk County is equal to its upper bound of 2,000 ft/d.

In layer 6, the calibrated value of Kv for the MCU I is about 

0.12 ft/d, greater than its preferred value of 0.05 ft/d (fig. 70). 

The calibrated value of Kv for the MCU II is equal to its 

preferred value of 10–4 ft/d, indicating that the observations are 

insensitive to this parameter as long as it is sufficiently small.
Simulation of Groundwater Flow System 93

Figure 63. Map showing calibrated horizontal hydraulic conductivity of the surficial aquifer 

system (SAS, layer 1) and selected aquifer test values.

6

6

19

24 50

32

34

34

10

55

48

19

20

46

32

33

42

50

21

!

!

! !

!

!

! !

!

!

!

!

!

!

!

!

!

!

!

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

Base from U.S. Geological Survey 

digital data, Universal Transverse 

Mercator projection, zone 17

East-central Florida 

transient model area

0 10 20 MILES

0 10 20 KILOMETERS

EXPLANATION

7.4 to 10.0

10.1 to 20.0

20.1 to 30.0

30.1 to 45.0

Location and hydraulic conductivity, in feet 

 per day, from aquifer test in the SAS

50 !

Horizontal hydraulic conductivity of the 

 SAS, in feet per day

White areas are inactive in the SAS

45.1 to 60.0

60.1 to 80.0

80.1 to 100.0

100.1 to 151.0

ATLANTIC OCEAN

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 63. Calibrated horizontal hydraulic conductivity of the surficial aquifer system (SAS, layer 1) and 

selected aquifer test values.
94 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

Figure 64. Map showing specified vertical hydraulic conductivity of the surficial aquifer 

system (SAS, layer 1).

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

HARDEE COUNTY

COUNTY

SUMTER COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

0 10 20 MILES

0 10 20 KILOMETERS

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17 EXPLANATION

0.15 to 0.20

0.21 to 0.40

0.41 to 0.60

0.61 to 0.80

0.81 to 0.95

White areas are inactive in 

 the SAS

Vertical hydraulic conductivity 

 of the SAS, in feet per day

East-central Florida 

transient model area

ATLANTIC OCEAN

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 64. Specified vertical hydraulic conductivity of the surficial aquifer system (SAS, layer 1).
Simulation of Groundwater Flow System 95

Figure 65. Map showing calibrated horizontal hydraulic conductivity of the intermediate 

confining unit/intermediate aquifer system (ICU/IAS, layer 2) and selected aquifer test values.

23

1

1

5

2

81 19

24

12

49

35

160

230

150

!

!

! !

!

! !

!

!

! !

!

! !

! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

0 10 20 MILES

0 10 20 KILOMETERS

EXPLANATION

3.8 to 10.0

10.1 to 20.0

20.1 to 30.0

30.1 to 45.0

!

! 12 Location and hydraulic conductivity, in feet 

 per day, from aquifer test in the ICU/IAS

Location of regional pilot point used in 

 parameter estimation for layer 2

Horizontal hydraulic conductivity of the 

 ICU/IAS, in feet per day

45.1 to 60.0

60.1 to 80.0

80.1 to 100.0

100.1 to 151.0

East-central Florida 

transient model area

ATLANTIC OCEAN

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 65. Calibrated horizontal hydraulic conductivity of the intermediate confining unit/intermediate aquifer 

system (ICU/IAS, layer 2) and selected aquifer test values.
96 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

Figure 66. Map showing calibrated vertical hydraulic conductivity of the intermediate confining 

unit/intermediate aquifer system (ICU/IAS, layer 2).

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

0 10 20 MILES

0 10 20 KILOMETERS

East-central Florida 

transient model area

EXPLANATION

0.0001 to 0.0010

0.0011 to 0.0050

0.0051 to 0.0100

0.0101 to 0.0500

0.0501 to 0.1000

0.1001 to 0.5000

0.5001 to 1.0000

Location of regional pilot point used 

 in parameter estimation for layer 2

Vertical hydraulic conductivity of 

 the ICU/IAS, in feet per day

!

ATLANTIC OCEAN

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 66. Calibrated vertical hydraulic conductivity of the intermediate confining unit/intermediate aquifer 

system (ICU/IAS, layer 2).
Simulation of Groundwater Flow System 97

Figure 67. Map showing calibrated horizontal hydraulic conductivity of the Ocala permeable 

zone (OPZ, layer 3) and selected aquifer test values.

!

! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

! !

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

! !

! !

!!

!

!

!

!

!

!

!

!

!

! !

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

! !

!

!

!

!

!

!

!

6

50

38

29

29

16

14

64

53

20

23

19

18

83

73

50

56 35

44

31

46

38

47

50

45

51

22

27

16

33

7

34

55

59

44

97

63

9

52

190

350

100

110

230

600

190 120

260

180

240

730

740

340

110

110

320

320

980

840

730

700

320 260

240

150

280

700

310

270

140

100

100

220

390

140

270

190

220

160

110

270

7,600

2,500

1,200

14,000

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

East-central Florida 

transient model area

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

EXPLANATION

26.2 to 35.0

35.1 to 60.0

60.1 to 95.0

95.1 to 160.0

160.1 to 260.0

!

! 230 Location and hydraulic conductivity, in feet per day, from 

 aquifer test in the OPZ

Location of regional pilot point used in parameter estimation 

 for layer 3

Horizontal hydraulic conductivity of the OPZ, in feet per day

260.1 to 400.0

400.1 to 600.0

600.1 to 900.0

900.1 to 1,300.0

1,300.1 to 1,900.0

1,900.1 to 2,800.0

2,800.1 to 4,000.0

4,000.1 to 9,000.0

9,000.1 to 15,400.0

0 10 20 MILES

0 10 20 KILOMETERS

ATLANTIC OCEAN

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 67. Calibrated horizontal hydraulic conductivity of the Ocala permeable zone (OPZ, layer 3) and 

selected aquifer test values.
98 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

Figure 68. Map showing calibrated vertical hydraulic conductivity of the Ocala low-permeable 

zone (OLPZ, layer 4).

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

East-central Florida 

transient model area

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

EXPLANATION

6.8 to 7.0

7.1 to 10.0

10.1 to 20.0

20.1 to 40.0

40.1 to 60.0

60.1 to 100.0

100.1 to 144.0

Vertical hydraulic conductivity of 

 the OLPZ, in feet per day

0 10 20 MILES

0 10 20 KILOMETERS

ATLANTIC OCEAN

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 68. Calibrated vertical hydraulic conductivity of the Ocala low-permeable zone (OLPZ, layer 4).
Simulation of Groundwater Flow System 99

Figure 69. Map showing calibrated horizontal hydraulic conductivity of the Avon Park permeable 

zone (APPZ, layer 5) and selected aquifer test values.

!

! !

!

!

!

!

!

!

! !

!

!

!

! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

290

940

130

1,200

3,400

3,100

53

30

140

190

430 690

220

200

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY HARDEE

COUNTY

SUMTER COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

EXPLANATION

Location and hydraulic conductivity, in feet per day, from aquifer 

 test in the APPZ

Location of regional pilot point used in parameter estimation for 

 layer 5

30 !

Horizontal hydraulic conductivity of the APPZ, in feet per day

!

28.9 to 35.0

35.1 to 60.0

60.1 to 95.0

95.1 to 160.0

160.1 to 260.0

260.1 to 400.0

400.1 to 600.0

600.1 to 900.0

900.1 to 1,300.0

1,300.1 to 1,900.0

1,900.1 to 2,800.0

2,800.1 to 4,000.0

4,000.1 to 9,000.0

9,000.1 to 15,400.0

0 10 20 MILES

0 10 20 KILOMETERS

East-central Florida 

transient model area

ATLANTIC OCEAN

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 69. Calibrated horizontal hydraulic conductivity of the Avon Park permeable zone (APPZ, layer 5) and 

selected aquifer test values.
100 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

Figure 70. Map showing calibrated vertical hydraulic conductivity of the middle confining 

units (MCU I/II, layer 6).

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

East-central Florida 

transient model area

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

0 10 20 MILES

0 10 20 KILOMETERS

EXPLANATION

0.0001 (MCU II)

0.1190 (MCU I)

Vertical hydraulic conductivity of 

 the MCU I/II, in feet per day

ATLANTIC OCEAN

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 70. Calibrated vertical hydraulic conductivity of the middle confining units (MCU I/II, layer 6).
Simulation of Groundwater Flow System 101

Calibrated values of Kh in layer 7 differ little compared 

to the preferred values (figs. 62 and 71). The minimum and 

maximum calibrated values of Kh are 33 ft/d and 1,900 ft/d, 

respectively.

Calibration of Storage Parameters Using 

Transient Conditions During 2005–2006

The model was used to estimate specific yield (Sy) 

of layer 1 and the specific storage (Ss) of layers 2 to 7 by 

fitting model output to measured heads and spring flows 

for transient conditions from July 2005 to December 2006. 

Only 2 years were simulated to calibrate storage parameters 

because the simulation time was prohibitively long for the 

necessary multiple iterations of the 12-year transient simulation required for regression of the parameters by PEST. In 

the transient regression simulations, the Kh and Kv distributions were fixed at the values estimated using the steadystate simulations. Simulated heads in the 12-year transient 

model at the end of 2004 were used as initial heads for the 

2-year simulation.

Simulated equivalents of the transient measured heads are 

highly nonlinear with respect to Sy, as heads increase substantially with lower specific yield values. This is illustrated by 

differences of calculated css values for different Sy values 

(fig. 72). For each css calculation, the Sy parameter values 

for all zones were the same (0.11, 0.15, or 0.24). For most 

parameters, the css values are about 5 to 10 times larger when 

calculated using an Sy value of 0.11 than when calculated 

using 0.15 (fig. 72). The nonlinearity of heads is milder, but 

still evident, for Sy in the range of 0.15 to 0.24. The values of 

Ss_2 and Ss_3-7 in all css calculations were 3 10 6 × − ft-1 and

2 10 6 × − ft-1, respectively. Values of css calculated at different 

Ss values showed that the simulated equivalents of the 

observations vary in a more linear manner with respect to Ss 

parameter values compared with Sy parameter values. 

The relative magnitudes of the css among the 21 Sy 

parameters are similar for each of the three sets of Sy parameter values (fig. 72). For an Sy value of 0.15, parameters with 

the largest css are Sy_15, Sy_19, and Sy_20. These Sy parameters are associated with, respectively, the north, central, and 

south parts of Lake Wales Ridge. These large css for specific 

yield can be explained by the fact that the water table is below 

the ET extinction depth throughout most of the Lake Wales 

Ridge (fig. 2). Groundwater ET reduces water-table fluctuations because (1) as Sy decreases, the water table rises, and 

in turn, increases groundwater ET and limits the magnitude 

of the water-table fluctuations, and (2) as Sy increases, the 

water table declines, and in turn, decreases groundwater ET 

and limits the water-table decline. Because the water table is 

below the extinction depth throughout most of the Lake Wales 

Ridge, the corresponding absence of ET causes Sy to affect 

water-table fluctuations to a greater degree in this region than 

in other physiographic regions. Parameters with very small css

values include Sy_3, Sy_6, Sy_12, and Ss_2. 

The strategy for estimating Sy and Ss was to specify the 

values of the least sensitive parameters and then designate 

the remainder of the parameters with prior information. 

Parameters Sy_3, Sy_6, and Sy_12 were each set to 0.15, and 

Ss_2 was set to 3×10–6 ft-1. Because of the nonlinearity with 

respect to Sy, the use of prior information helped stabilize the 

regression. If a parameter became insensitive during a regression simulation, its estimated value remained close to the prior 

value, which was 0.15 for all Sy parameters. This value is 

considered reasonable for the sandy sediments that compose 

the SAS. PEST’s regularization capability of a highly parameterized approach was not used for the calibration to transient 

conditions because the storage properties are represented 

using 21 Sy zones and two Ss parameters. The value of μ was 

set to 1.0 in the objective function (equation 6) because prior 

information was used and regularization was not used. 

During regression simulations, Sy tended to either stay 

near the prior value or decrease to the lower bound, which was 

initially set to 0.11. When Sy was as low as 0.11, this caused 

higher simulated heads than observed at many wells and 

lower simulated ET rates relative to the estimated minimum 

rate. To constrain model calibration to reasonable parameter 

values, the simulated ET rate generally was maintained, as 

much as possible, to be at least half the sum of rainfall and 

irrigation for the cell. Because of the tendency to estimate 

lower Sy values than was acceptable, the lower bound for all 

Sy parameters was increased to 0.14. Estimated specific yield 

parameters that remained similar to the prior value were set to 

0.15 in the calibrated model, including those for physiographic 

regions 1, 2, 4, 7, 8, 9, 16, and 17, as well as regions 3, 6, and 

12, which were not adjusted (fig. 2). Parameters with an estimated value equal to the lower bound of 0.14 included regions 

5, 10, 11, 13, 14, 15, and 18 through 21 (fig. 2, table 14). The 

estimated value of Ss_3-7 tended to become larger than that of 

Ss_2 during the regression runs. However, because the sediments in layers 3 through 7 are less compressible than those in 

layer 2, it is expected that Ss_3-7 would be smaller than Ss_2. 

Accordingly, Ss_2 was set to 3×10–6 ft-1 and Ss_3-7 was set to 

2×10–6 ft-1 in the calibrated model (table 14).

Model Fit for 1995–2006 Transient Conditions

Following calibration of hydraulic conductivity parameters for the steady-state 1993 and 2003 conditions, and 

calibration of storage parameters for 2005–2006 transient 

conditions, the estimated values were used as input for a 

forward simulation of transient conditions from 1995 to 2006. 

Simulated heads and spring flows from this forward simulation were used to assess model fit to the observed heads and 

spring flows.

The model fit satisfies all the calibration criteria for 

groundwater heads presented earlier. For each year simulated, 

almost 60 percent of the wells have an absolute value of head 

residual less than or equal to 2.5 ft, more than 85 percent of 

the wells have an absolute value of head residual less than or 

equal to 5 ft, the absolute value of the overall mean residual 
102 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

Figure 71. Map showing calibrated horizontal hydraulic conductivity of the Lower Floridan 

aquifer (LFA, layer 7) and selected aquifer test values.

!

!

!

!

!

!

!

!

!

! !

!

!

!

!

!

!

!

!

!

!

!

!

! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! !

! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! !

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

EXPLANATION

33.4 to 35.0

35.1 to 60.0

60.1 to 95.0

95.1 to 160.0

160.1 to 260.0

Location and hydraulic conductivity, in feet per day, from 

 aquifer test in the LFA

Location of regional pilot point used in parameter estimation for 

 layer 7

! 23

Horizontal hydraulic conductivity of the LFA, in feet per day

!

260.1 to 400.0

400.1 to 600.0

600.1 to 900.0

900.1 to 1,300.0

1,300.1 to 1,900.0

1,900.1 to 2,800.0

2,800.1 to 4,000.0

4,000.1 to 9,000.0

9,000.1 to 15,400.0

0 10 20 MILES

0 10 20 KILOMETERS

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

East-central Florida 

transient model area

98

220

280

170 700

160

430

1,600

1,600 3,100

1,500

7,700

1,000

23

690

120

360

1,200

1,200

3,100

1,700

1,400

1,200 ATLANTIC OCEAN

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 71. Calibrated horizontal hydraulic conductivity of the Lower Floridan aquifer (LFA, layer 7) and 

selected aquifer test values.
Simulation of Groundwater Flow System 103

Figure 72. Graph showing composite scaled sensitivities for specific yield and specific storage 

parameters in the transient model, calculated using head and spring flow observations from 

July 2005 through December 2006.

0.01

0.10

1

10

100

Sy_1Sy_2Sy_3Sy_4Sy_5Sy_6Sy_7Sy_8Sy_9

Sy_10Sy_11Sy_12Sy_13Sy_14Sy_15Sy_16Sy_17Sy_18Sy_19Sy_20Sy_21

Ss_2

Ss_3-7

Composite scaled sensitivity

Parameter

EXPLANATION

Sy = 0.11

Sy = 0.15

Sy = 0.24

Table 14. Estimated specific yield and specific storage values 

for the calibrated transient model of 1995 to 2006.

[Sy, specific yield; Ss, specific storage; PHYSR, physiographic region 

number shown in figure 2; Layer, layer number to which value was applied; 

—, not applicable]

Value PHYSR Layer

Sy

0.14 5, 10, 11, 13, 14, 15, 18, 19, 

20, 21

1

0.15 1, 2, 3, 4, 6, 7, 8, 9, 12, 16, 17 1

Ss

3×10–6 ft–1 — 2

2×10–6 ft–1 — 3, 4, 5, 6, 7

(OMR) is less than 1 ft, and the RMSR is less than 3.6 ft 

(table 15). For the entire simulation period of 1995 to 2006, 

the OMR is -0.04 ft and the RMSR is about 3.4 ft.

The transient 1995–2006 simulation results also satisfy 

the calibration criteria for spring flows. For each spring, 

average simulated flow and average observed flow were 

calculated over the entire simulated period. Residuals calculated for these averages are less than 10 percent of average 

observed flow for all springs (table 16). The spring with one 

of the highest residuals was spring number 8, which is also 

the spring with the lowest average observed flow among 

all springs (table 16). The overall mean residual between 

measured and simulated spring flow is about -0.3 ft3/s.

In addition to satisfying the calibration criteria related 

to statistical measures calculated from the residuals, it is 

important that the weighted residuals be randomly distributed 

(Hill and Tiedeman, 2007). Randomness is evaluated using 

weighted residuals from selected years of the 12-year model, 

using two types of figures. First, on graphs of weighted 

residuals versus simulated values, randomness is indicated by 

an even spread of positive and negative weighted residuals for 

all simulated values. Second, on maps of weighted residuals 

displayed geographically, randomness is indicated by a visual 

mix of positive and negative residuals distributed randomly 

throughout the model area. Nonrandom weighted residuals 

indicate model bias and possible model error. A typical indicator of bias is a cluster of weighted residuals in a geographic 

area that are large and identical in sign. 

Graphs of unweighted residuals of average annual heads 

(with all weights equal to 1.0) and spring flow weighted 

residuals versus monthly simulated flows are shown for 1999, 

2003, and 2006 (figs. 73 and 74). These selected years include 

the 2 years for which steady-state conditions were calibrated 

by PEST, as well as the final year of the 12-year simulation. 

For 1999, the head residuals are evenly spread above and 

below the horizontal grid line corresponding to a residual 

value of zero (fig. 73A). For 2003 and 2006, head residuals 

each exhibit slight bias (fig. 73C), because there are more 

Figure 72. Composite scaled sensitivities for specific yield and specific storage parameters in the transient model, 

calculated using head and spring-flow observations from July 2005 through December 2006.
104 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

Table 15. Statistics of head residuals for the calibrated 1995–2006 transient model.

[NW, number of wells; NE2.5, number of wells with absolute value of residual not exceeding 2.5 feet (ft); PNE2.5, percent of NW in NE2.5 category; NE5, 

number of wells with absolute value of residual not exceeding 5 ft; PNE5, percent of NW in NE5 category; LT-5, number of wells with residuals less than 

–5 ft; PLT-5, percent of NW in LT-5 category; GT5, number of wells with residuals greater than 5 ft; PGT5, percent of NW in GT5 category; OMR, overall 

mean residual, in feet; RMSR, root-mean-square residual, in feet]

Year NW NE2.5 PNE2.5 NE5 PNE5 LT-5 PLT-5 GT5 PGT5 OMR RMSR

1995 379 243 64.1 331 87.3 24 6.3 24 6.3 0.04 3.56

1996 413 255 61.7 362 87.7 31 7.5 20 4.8 –0.25 3.59

1997 408 257 63.0 354 86.8 39 9.6 15 3.7 –0.32 3.51

1998 436 278 63.8 375 86.0 44 10.1 17 3.9 –0.66 3.51

1999 468 295 63.0 404 86.3 31 6.6 33 7.1 0.14 3.38

2000 434 258 59.4 374 86.2 24 5.5 36 8.3 0.51 3.48

2001 474 283 59.7 412 86.9 21 4.4 41 8.6 0.59 3.58

2002 476 305 64.1 412 86.6 38 8.0 26 5.5 –0.26 3.45

2003 519 338 65.1 459 88.4 35 6.7 25 4.8 0.17 3.22

2004 522 336 64.4 457 87.5 34 6.5 31 5.9 0.12 3.25

2005 523 338 64.6 461 88.1 49 9.4 13 2.5 –0.41 3.27

2006 529 307 58.0 458 86.6 45 8.5 26 4.9 –0.13 3.46

1995–2006 –0.04 3.43

Table 16. Statistics of spring-flow residuals for the calibrated 1995–2006 transient model.

[SN, spring number, refer to table 2–1 for spring name; SF, average simulated spring flow from 1995 to 2006, in ft3/s; AF, average observed spring flow from 

1995 to 2006 in ft3/s; RES, residual equal to difference between average simulated and average observed flow, in ft3/s; PD, percent difference between average 

simulated and average observed flows, calculated relative to average observed flow; OMR, overall mean residual; Ave, average PD]

SN SF AF RES PD SN SF AF RES PD

1 103.18 103.02 0.16 0.16 13 16.00 16.04 –0.04 –0.25

2 26.16 26.85 –0.69 –2.57 14 5.47 5.61 –0.14 –2.50

3 152.08 153.49 –1.41 –0.92 15 5.40 5.61 –0.21 –3.74

4 2.71 2.77 –0.06 –2.17 16 55.58 56.13 –0.55 –0.98

5 10.74 10.93 –0.19 –1.74 17 17.93 18.78 –0.85 –4.53

6 0.82 0.83 –0.01 –1.20 18 33.43 34.43 –1.00 –2.90

7 1.39 1.31 0.08 6.11 19 12.94 13.44 –0.50 –3.72

8 0.75 0.81 –0.06 –7.41 20 19.63 19.65 –0.02 –0.10

9 9.66 9.82 –0.16 –1.63 21 63.63 64.41 –0.78 –1.21

10 1.75 1.71 0.04 2.34 22 1.95 1.97 –0.02 –1.02

11 3.41 3.45 –0.04 –1.16 OMR –0.29

12 8.14 8.09 0.05 0.62 Ave –1.39
Simulation of Groundwater Flow System 105

Figure 73. Annual average head residuals versus annual 

average simulated heads for A, 1999, B, 2003, and C, 2006. All 

values are calculated using the 12-year model.

Figure 74. Weighted monthly average spring flow residuals 

versus monthly average simulated flows for A, 1999, B, 2003, and 

C, 2006. All values are calculated using the 12-year model.

springs. For simulated flows of about 8 to 60 ft3/s in 2006, 

there are more negative weighted residuals less than -2 than 

positive weighted residuals greater than 2 (fig. 74C). The 

majority of the weighted residuals less than -2 are for springs 

2, 5, 16, and 17 (fig. 51). In addition, the cluster of negative 

weighted residuals for spring 3, at a simulated flow of about 

145 ft3/s, is an outlier on fig. 74C. Springs 2, 3, 5, 16, and 17 

are not concentrated in a specific area (fig. 51), and all but 

spring 2 are relatively near other springs, suggesting that any 

model error that is causing a bias in simulated flow at these 

springs for 2006 is relatively localized.

Average annual head residuals are displayed geographically for the four model layers corresponding to aquifers (the 

SAS, OPZ, APPZ, and LFA), for 1999 and 2006. These years 

represent 1 year for which a steady-state model was calibrated and the final year of the 12-year simulation. For 1999, 

negative than positive residuals for simulated heads of about 

70 to 110 ft. Within this range, the majority of residuals less 

than -5 ft lie along the Lake Wales Ridge and in western Polk 

and Lake Counties.

For the spring-flow observations in 1999 and 2003, 

weighted residuals are evenly distributed about a value of 

zero, except for those associated with flows that are less than 

1 ft3/s (fig. 74A–B), which occur for springs 6, 7 (1999 only), 

and 8 (fig. 51). This bias is not a great concern because of the 

small magnitudes of flows and unweighted residuals at these 

A. 1999

B. 2003

C. 2006

–16

–12

–8

–4

0

4

8

12

16

Weighted residual –16

–12

–8

–4

0

4

8

12

16

Weighted residual

1999

–16

–12

–8

–4

0

4

8

12

16

0.1 1.0 10.0 100.0

Weighted residual

Simulated spring flow, in cubic feet per second

2006

2003

EXPLANATION

Annual average flow 

 at a spring

Residual, in feet Residual, in feet Residual, in feet

A. 1999

B. 2003

C. 2006

Simulated head, in feetNGVD 29

0 50 100 150 –25

–20

–15

–10

-5

0

5

10

15

20

25

–25

–20

–15

–10

–5

0

5

10

15

20

25

–25

–20

–15

–10

–5

0

5

10

15

20

25

EXPLANATION

Annual average head 

 at a well
106 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

the head residuals in the SAS appear randomly distributed 

over much of the model area (fig. 75). An exception is in 

central Polk County, in the central and southern parts of the 

Lake Wales Ridge (figs. 2 and 75), where there is a cluster 

of negative residuals. Large simulated surface leakage rates 

along the eastern edge of Lake Wales Ridge are the result 

of relatively large differences in hydraulic conductivity and 

land-surface altitude between the Lake Wales Ridge and the 

Osceola and Wekiva Plains (figs. 2, 16, and 63). Large surface 

leakage rates reduce the recharge rates to layer 1 and thus 

the potential for larger leakage rates to the underlying layers 

representing aquifers. These reductions cause the aquifer 

heads to be underestimated, making adjustments to reduce 

residuals in this area difficult while using realistic K values, 

recharge, and ET rates.

In the underlying OPZ, the head residuals for 1999 are 

mostly randomly distributed, except in western Lake County 

where many residuals are negative and large in magnitude 

(fig. 76). At these locations, the residuals for 1999 steady-state 

conditions are randomly distributed. Therefore, it is likely that 

differences in boundary conditions between the steady-state 

and transient simulations produce differences in the residuals, 

causing annual average simulated heads in 1999 to be low 

for the 1995–2006 transient simulation. This result points 

out a limitation of the calibration approach, in that parameter 

estimates that produce a good fit for approximated steady-state 

conditions do not necessarily produce the same level of fit for 

transient conditions. As this discussion and the subsequent 

analyses of residuals show, this problem tends to be limited to 

local areas within the model.

In both the APPZ and LFA, there are few head observations 

(fig. 77), which makes assessing randomness more difficult 

because of the small sample size. In the APPZ simulated 

average annual heads are low at three wells in the south-central 

part of the domain; average annual heads also are low at these 

wells for 1999 steady-state conditions. Sensitivities from the 

1999 steady-state simulation show that for several pilot points 

near the two southernmost wells for which residuals are in the 

lowest range, increasing Kh will increase the head at one well 

and decrease head at the other well. Therefore, one reason for 

the poor fit at both wells is that the regression cannot improve fit 

at one well without worsening it at the other. In the LFA, most 

residuals are between -2.5 and 2.5 ft. (fig. 77).

Comparison of 2006 average annual head residuals in 

each aquifer with those for 1999 shows that at most sites for 

which observations exist in both of these years, the model fit 

showed small differences in residuals between 1999 and 2006 

(figs. 75 to 80). In addition, average annual head residuals that 

can be calculated only in 2006 tend to be a mix of positive and 

negative values. For the average annual head residuals that can 

be calculated only in 2006, there are slightly more residuals 

that are negative and of large magnitude than there are large 

positive residuals. However, the distributions of all average 

annual head residuals in 2006 do not show any additional 

areas of clusters of same-signed residuals, compared to those 

identified for 1999.

The ECFT model was used to generate simulated springflow hydrographs from 1995 to 2006, which is illustrated 

with Rock Springs (spring number 16) and Wekiwa Springs 

(or spring number 21) (fig. 52, table 2–1). Rock Springs and 

Wekiwa Springs, which had respective average measured 

flows of 56 and 64 ft3/s over the simulation period, had overall 

mean flow residuals of less than 1 ft3/s in absolute value 

(table 16). Simulated spring flow was greater during wet years 

than during dry years (figs. 81 and 82, table 5). Additional 

spring-flow hydrographs are shown in appendix 2.

The ECFT model was also used to generate simulated 

water-surface altitude hydrographs at lakes from 1995 to 2006, 

which are shown for Lake Monroe (lake number 38) and Lake 

Arbuckle (lake number 271) (fig. 13). Lakes Monroe and 

Arbuckle, which had respective average water-surface altitudes 

of 2.4 and 53.6 ft over the simulation period, had overall mean 

water-surface altitude residuals of about 0.06 and -0.28 ft, 

indicating a lack of bias in the simulated residuals. Fluctuations 

in water-surface altitudes over relatively short periods (figs. 83 

and 84) were simulated. Additional water-surface altitude 

hydrographs for lakes are shown in appendix 3.

The capability of the model to simulate streamflows 

at stream-gaging stations was assessed by using data from 

St. Johns River station number 02236000 (stream segment 

number 164, fig. 11) and Peace River station number 

02295637 (segment number 299, fig. 11). Respective average 

measured streamflows at these two stations (figs. 85 and 86) 

were 3,190 and 706 ft3/s based on average monthly measurements from 1995 to 2006. The respective overall mean flow 

residuals at these two stations were about -270 and 8 ft3/s 

over the simulation period, indicating a small bias in the 

flow residuals. The calibration of streamflows was limited to 

reducing, as much as possible, the differences between total 

simulated and measured water volumes over the simulation 

period. Additional streamflow hydrographs at stream-gaging 

stations are shown in appendix 4.

Measured and simulated well hydrographs in the SAS, 

IAS, OPZ, APPZ, and LFA were compared to assess the 

model fit over the entire simulation period. Well numbers 2 

and 13, in figure 5–1, named Cocoa K and Ridge Wrap H-1, 

are SAS wells with respective average measured water levels 

of 57.51 and 104.47 ft from 1995 to 2006 (figs. 87 and 88). 

The respective overall mean residuals at these two wells were 

-0.19 and 0.31 ft, indicating a lack of bias in the residuals. 

These hydrographs show measured seasonal rises and declines 

in the water table. Additional hydrographs from SAS wells are 

shown in figures 5–2 to 5–16 in appendix 5. 

Well numbers 1 and 4, in figure 5–17 (appendix 5), 

named Bithlo 2 ICU and L-0096 Groveland Fire Tower, are 

ICU wells with respective average measured heads of 47.43 

and 81.85 ft for the entire 1995 to 2006 simulation period 

(figs. 89 and 90). The respective overall mean residuals at 

these two wells were 0.21 and -1.63 ft, suggesting a small 

bias in the water-table altitude residuals. Additional hydrographs from ICU wells are shown in figures 5–18 to 5–25 in 

appendix 5. 
Simulation of Groundwater Flow System 107

Figure 75. Map showing 1999 head residuals in surficial aquifer system (SAS, layer 1) wells, 

calculated using average annual simulated (from the 12-year model) and observed heads.

! !!! !!

!

! ! !! !

! ! ! ! !

! !

!

!

! ! ! ! ! !

!

! !

!

!

! !

!

!

! ! !

! ! !

! !! ! !

! ! ! ! ! !

! !

!

!

! !

! ! ! !

! ! !!! ! ! ! !!!! !! !

! ! ! !

! ! ! ! ! ! !

!

! !!!

!

!

! ! !

!

!

! ! !

!

! !! !!

!! ! ! ! !

! !

! ! !! !

!

! ! !

!

!

!

!

! !

!

! !

! ! !

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

0 10 20 MILES

0 10 20 KILOMETERS

East-central Florida 

transient model area

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

–9.4 to –5.0

–4.9 to –2.5

–2.4 to 2.5

2.6 to 5.0

5.1 to 9.0

Residuals at wells, simulated 

 minus measured water level 

 at SAS, in feet

Green Swamp area

EXPLANATION

ATLANTIC OCEAN

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 75. Head residuals for 1999 in surficial aquifer system (SAS, layer 1) wells, calculated using average 

annual simulated (from the 12-year model) and observed heads.
108 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

Figure 76. Map showing 1999 head residuals in Ocala permeable zone (OPZ, layer 3) wells, 

calculated using average annual simulated (from the 12-year model) and observed heads.

! ! ! ! ! !

! ! !

! ! ! ! ! ! ! ! ! ! !

!

! ! ! ! ! ! ! ! !

! ! ! !

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! ! !! !! ! !

! !! ! ! ! ! ! ! !! ! !

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! ! ! ! ! ! ! ! !! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! !! ! ! !!! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

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! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! !

! ! ! ! ! ! !

! !

0 10 20 MILES

0 10 20 KILOMETERS

EXPLANATION

Residuals at wells, simulated 

 minus measured head at 

 OPZ, in feet 

Green Swamp area

–8.4 to –5.0

–4.9 to –2.5

–2.4 to 2.5

2.6 to 5.0

5.1 to 8.2

!

!

!

!

!

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

East-central Florida 

transient model area

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

ATLANTIC OCEAN

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 76. Head residuals for 1999 in Ocala permeable zone (OPZ, layer 3) wells, calculated using average 

annual simulated (from the 12-year model) and observed heads.
Simulation of Groundwater Flow System 109

!

!

!

!

!

! !

!

!

! !

!

!

! ! !

!

!

! !

! !

!

!

!

!

!

!

!

!

!

!

!

!

!

! !

!

!

!

#

#

#

#

# #

#

#

##

#

#

0 10 20 MILES

0 10 20 KILOMETERS

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

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ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

#

#

#

Residuals at wells, simulated 

 minus measured head at 

 APPZ or LFA, in feet 

LFA well

–1.8 to 2.5

2.6 to 5.0

5.1 to 5.3

!

!

!

!

!

APPZ well

–10.1 to –5.0

–4.9 to –2.5

–2.4 to 2.5

2.6 to 5.0

5.1 to 8.5

EXPLANATION

East-central Florida 

transient model area

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

ATLANTIC OCEAN

Figure 77. Map showing 1999 head residuals in Avon Park permeable zone (APPZ, layer 5) 

and Lower Floridan aquifer (LFA, layer 7) wells, calculated using average annual simulated 

(from the 12-year model) and observed heads.

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 77. Head residuals for 1999 in Avon Park permeable zone (APPZ, layer 5) and Lower Floridan aquifer 

(LFA, layer 7) wells, calculated using average annual simulated (from the 12-year model) and observed heads.
110 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

! ! !

! !!

!

! ! !

! ! !! ! !

! ! ! ! !

! ! ! ! ! !

! !

!

! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !

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!

!! ! ! !

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! ! ! ! ! ! ! ! !! ! ! ! !! !!

! ! !! ! !!!! !

! ! ! ! !

!

! ! ! ! ! !

! ! ! ! ! !

! !!! ! !

0 10 20 MILES

0 10 20 KILOMETERS

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

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ORANGE

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POLK

COUNTY

OSCEOLA

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RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

East-central Florida 

transient model area

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

!

!

!

!

!

–10.9 to –5.0

–4.9 to –2.5

–2.4 to 2.5

2.6 to 5.0

5.1 to 8.0

Residuals at wells, simulated 

 minus measured water 

 level at SAS, in feet 

Green Swamp area

EXPLANATION

ATLANTIC OCEAN

Figure 78. Map showing 2006 head residuals in surficial aquifer system (SAS, layer 1) wells, 

calculated using average annual simulated (from the 12-year model) and observed heads.

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 78. Head residuals for 2006 in surficial aquifer system (SAS, layer 1) wells, calculated using average 

annual simulated (from the 12-year model) and observed heads.
Simulation of Groundwater Flow System 111

! ! ! ! !

!

! ! !

! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! ! ! ! ! ! ! ! !

! ! !

! ! ! ! ! ! ! ! ! ! ! !

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! ! ! ! ! ! ! ! ! ! ! ! ! ! !! ! ! ! ! ! ! ! ! !

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! ! ! !

!!

!! ! ! ! ! ! ! ! ! ! ! ! ! !

! ! ! ! !

! ! ! !

! ! ! ! ! ! ! ! !

0 10 20 MILES

0 10 20 KILOMETERS

MARION

COUNTY

LAKE

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VOLUSIA

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ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

!

!

!

!

!

–9.3 to –5.0

–4.9 to –2.5

–2.4 to 2.5

2.6 to 5.0

5.1 to 6.2

Residuals at wells, simulated 

 minus measured head at 

 OPZ, in feet

Green Swamp area

EXPLANATION

East-central Florida 

transient model area

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

ATLANTIC OCEAN

Figure 79. Map showing 2006 head residuals in Ocala permeable zone (OPZ, layer 3) wells, 

calculated using average annual simulated (from the 12-year model) and observed heads.

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 79. Head residuals for 2006 in Ocala permeable zone (OPZ, layer 3) wells, calculated using average 

annual simulated (from the 12-year model) and observed heads.
112 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

! !

!

!

! !

!

! !

! !

! !

! !

! !

!

!

!

!! !

!

!

! ! !

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! ! !

!

!

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!

!

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#

#

# #

#

#

#

# #

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#

# ##

#

#

# #

#

#

0 10 20 MILES

0 10 20 KILOMETERS

MARION

COUNTY

LAKE

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ORANGE

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INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

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COUNTY

SUMTER COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

East-central Florida 

transient model area

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

#

#

#

#

#

–7.9 to –5.0

–4.9 to –2.5

–2.4 to 2.5

2.6 to 5.0

5.1 to 7.8

Residuals at wells, simulated minus 

 measured head at APPZ or LFA, in feet

!

!

!

!

!

–10.4 to –5.0

–4.9 to –2.5

–2.4 to 2.5

2.6 to 5.0

5.1 to 8.1

LFA well APPZ well

EXPLANATION

ATLANTIC OCEAN

Figure 80. Map showing 2006 head residuals in Avon Park permeable zone (APPZ, layer 5) 

and Lower Floridan aquifer (LFA, layer 7) wells, calculated using average annual simulated 

(from the 12-year model) and observed heads.

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 80. Head residuals for 2006 in Avon Park permeable zone (APPZ, layer 5) and Lower Floridan aquifer 

(LFA, layer 7) wells, calculated using average annual simulated (from the 12-year model) and observed heads.
Simulation of Groundwater Flow System 113

Figure 81. Graph showing measured and simulated flows for Rock Springs, spring 

number 16 in figure 51.

30

35

40

45

50

55

60

65

70

75

80

Spring flow, in cubic feet per second

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006

Year

EXPLANATION

Measured 

Simulated 

40

45

50

55

60

65

70

75

80

85

Spring flow, in cubic feet per second

Year

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006

EXPLANATION

Measured 

Simulated 

Figure 81. Measured and simulated flows for Rock Springs, spring number 16 in figure 51.

Figure 82. Measured and simulated flows for Wekiwa Springs, spring number 21 in figure 51.
114 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

–2

0

2

4

6

8

Year

Water-surface altitude, in feet NGVD 29

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006

Figure 83. Graph showing measured and simulated water-surface altitude hydrographs 

for Lake Monroe, lake number 38 in figure 13.

48

50

52

54

56

58

60

Year

Water-surface altitude, in feet NGVD 29

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006

Figure 83. Measured and simulated water-surface altitude hydrographs for Lake Monroe, lake 

number 38 in figure 13.

Figure 84. Measured and simulated water-surface altitude hydrographs for Lake Arbuckle, lake 

number 271 in figure 13.
Simulation of Groundwater Flow System 115

–2,000

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

Streamflow, in cubic feet per second

Year

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006

Figure 85. Graph showing measured and simulated flow hydrographs for St. Johns 

River station number 02236000 in figure 11.

0

1,000

2,000

3,000

4,000

5,000

6,000

Streamflow, in cubic feet per second

Year

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006

Figure 85. Measured and simulated flow hydrographs for St. Johns River station number 02236000

in figure 11.

Figure 86. Measured and simulated flow hydrographs for Peace River station number 02295637 in 

figure 11.
116 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

Figure 87. Measured and simulated water-level hydrographs for Cocoa K, well number 2 in 

figure 5–1.

Figure 88. Measured and simulated water-level hydrographs for Ridge Wrap H-1, well number 13 

in figure 5–1.

52

54

56

58

60

62

Year

Water level, in feet NGVD 29

Figure 87. Graph showing measured and simulated water-level hydrographs for 

Cocoa K, well number 2 in figure 5-1. 

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006

Water level, in feet NGVD 29

Year

94

98

102

106

110

114

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006
Simulation of Groundwater Flow System 117

EXPLANATION

Measured 

Simulated 

40

44

48

52

56

60

Water level, in feet NGVD 29

Year

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006

Figure 89. Graph showing measured and simulated water-level hydrographs for 

Bithlo 2 ICU, well number 1 in figure 5–17. 

72

76

80

84

88

92

Water level, in feet NGVD 29

Year

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006

Figure 89. Measured and simulated water-level hydrographs for Bithlo 2 ICU, well number 1 in 

figure 5–17.

Figure 90. Measured and simulated water-level hydrographs for L-0096 Groveland Fire Tower, well 

number 4 in figure 5–17.
118 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

previous paragraph), the sill (C0, equation 5) values of the variograms must be specified. The sill is the variance of the property 

that is interpolated using the variogram, and is the value of the 

vertical axis at which the variogram approaches a maximum. 

Whereas the value of the sill does not affect the interpolated 

values, it is used to calculate parameter uncertainty. In the 

context of calculating uncertainty using PREDUNC6, the sill 

reflects knowledge available prior to model calibration. In the 

variograms for Kh of layers 2, 3, 5, and 7, the sill is set to 0.68, 

0.39, 0.37, and 0.34, respectively. These values are the variances 

of log10(K) calculated using aquifer test data from the respective 

layers (figs. 18, 21, 24, 30, and 36). For Kv of layer 2, no such 

field data are available. Because this property is highly variable, 

the variance was calculated assuming Kv of layer 2 varies over 

four orders of magnitude, which is consistent with the estimated 

values of this property that range from about 0.0001 to 1.0 ft/d. 

This produces a log10(K) variance of 1.0. 

PREDUNC6 computes standard deviations as the measure 

of parameter uncertainty. These standard deviations are used to 

compute coefficients of variation that are not log-transformed. 

Coefficients of variation are dimensionless, and express 

uncertainty in a manner that can be compared among different 

parameters. The coefficient of variation cvbj for parameter bj is 

calculated as cv b b b j j j =  / , where bj is the standard deviation 

of bj. A cv of 1.0 indicates the parameter standard deviation is 

the same magnitude as its estimated value. 

Figure 97 shows the range of cvbj values calculated for 

the Kh of zones in layer 1, the Kh or Kv of pilot point parameters by layer, and the K multipliers at the spring pilot points. 

The cvbj values for estimated parameters not displayed on this 

figure are as follows: Kv of layer 4, 0.51; Kv of the MCU I 

in layer 6, 0.08; and Kv of the MCU II in layer 6, 0.31. The 

coefficients of variation are best evaluated in a relative sense 

for comparing uncertainty across model layers. Coefficients 

of variation for layers 4 and 6 and figure 97 show that the 

uncertainty is smallest for Kh of layer 1, Kv of layer 6, and the 

K multipliers, and largest for Kh and Kv of layer 2.

The relatively small uncertainty for Kh of layer 1 reflects 

that (1) zonation is used to represent this property and (2) the 

reasonable range of values for Kh of the SAS is narrower 

than that for other hydrogeologic units. The relatively large 

uncertainty for Kh of layer 2 reflects the small sensitivities 

of simulated values to this property (fig. 56). The uncertainty 

for individual pilot points representing Kv of layer 2 is 

highly variable, as shown by the wide range spanned by the 

minimum and maximum values on figure 97. This wide range 

partly reflects the spatial variability of the observation sensitivities to this property. The majority of the cvbj values for 

Kv of layer 2 are relatively large compared to those for other 

layers, because the ranges of reasonable values, and the a 

priori variability, for Kv parameters in layer 2 are larger than 

those for other hydrogeologic units. For Kh of layers 3 and 

5, the sill values are quite similar, and sensitivities to Kh of 

layer 5 tend to be larger than those to Kh of layer 3 (fig. 56). 

Well numbers 6 and 20, in figure 5–26 (appendix 5), 

named L-0059 Crows Bluff NFS and ROMP 58 Ocala, are 

OPZ wells with respective average measured heads of 17.35 

and 97.96 ft for the entire 1995 to 2006 simulation period 

(figs. 91 and 92). The respective overall mean residuals at 

these two wells were 0.18 and -0.75 ft, suggesting a negligible 

bias in head residuals. Additional hydrographs from OPZ 

wells are shown in figures 5–27 to 5–50 in appendix 5. Hydrographs for the OLPZ are shown in figures 5–52 and 5–53.

Well numbers 1 and 5, in figure 5–54 (appendix 5), 

named OR-47at Orlo Vista and ROMP deep well 101, are 

APPZ wells with respective average measured heads of 58.95 

and 97.46 ft for the entire 1995 to 2006 simulation period 

(figs. 93 and 94). The respective overall mean residuals at 

these two wells were -0.03 and 0.64 ft, suggesting a negligible 

bias in the head residuals. Additional hydrographs from APPZ 

wells are shown in figures 5–55 to 5–60 in appendix 5. Hydrographs from the MCU I are shown in figures 5–62 to 5–64.

Well numbers 3 and 7, in figure 5–65 (appendix 5), 

named OR0614 Cocoa WF Site S zone 3 and OS0025 Bull 

Creek APT TM2, are LFA wells with respective average 

measured heads of 36.24 and 41.51 ft from 1995 to 2006 

(figs. 95 and 96). The respective overall mean residuals at 

these two wells were 0.99 and 0.28 ft, suggesting a small bias 

in the head residuals. Additional hydrographs from LFA wells 

are shown in figures 5–66 to 5–73 in appendix 5. 

Parameter Uncertainty

Groundwater models commonly are developed and 

calibrated for purposes of making predictions, such as 

drawdown under conditions of increased pumping. In general, 

prediction uncertainty is of greater interest and importance 

than parameter uncertainty. This report, however, does not 

present predictions using the ECFT model. Therefore, parameter uncertainty, which is a component in the calculation of 

prediction uncertainty, is used to assess model uncertainty. 

Uncertainty in the K values estimated for the ECFT 

model is calculated using a linear method that considers three 

types of information about the parameters: (1) information 

about the spatial variability of Kh, Kv, or the K multiplier 

in layers where pilot points are used, which is contained in 

the variograms (equation 5) used for interpolation of the K 

estimates at the pilot points; (2) information about the ranges 

of reasonable values for the parameters, which is quantified 

by the weights on the prior information, and (3) information 

from the calibration observations, which is represented by 

the observation sensitivities and weights. The PEST application PREDUNC6 (Doherty, 2010b) enables these sources of 

information to be included in the calculation of parameter 

uncertainty. PREDUNC6 is designed to calculate prediction 

uncertainty, but can be used to calculate parameter uncertainty 

by defining the model parameters as the predictions. 

The types of information just described in (2) and (3) of 

the previous paragraph have been discussed previously. To 

include information about spatial variability (item 1 in the 
Simulation of Groundwater Flow System 119

Figure 91. Measured and simulated water-level hydrographs for L-0059 Crows Bluff NFS, well 

number 6 in figure 5–26.

Figure 92. Measured and simulated water-level hydrographs for Regional Observation and 

Monitor-well Program 58 Ocala, well number 20 in figure 5–26.

10

14

18

22

26

30

Water level, in feet NGVD 29

Year

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006

Figure 91. Graph showing measured and simulated water-level hydrographs for 

L-0059 Crows Bluff NFS, well number 6 in figure 5–26. 

84

88

92

96

100

104

108

Water level, in feet NGVD 29

Year

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006
120 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

Figure 93. Graph showing measured and simulated water-level hydrographs for 

OR-47 at Orlo Vista, well number 1 in figure 5–54. 

46

50

54

58

62

66

70

74

Water level, in feet NGVD 29

Year

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006

90

94

98

102

106

110

Water level, in feet NGVD 29

Year

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006

Figure 93. Measured and simulated water-level hydrographs for OR-47 at Orlo Vista, well 

number 1 in figure 5–54.

Figure 94. Measured and simulated water-level hydrographs for Regional Observation and 

Monitor-well Program deep well 101, well number 5 in figure 5–54.
Simulation of Groundwater Flow System 121

Figure 95. Measured and simulated water-level hydrographs for OR0614 Cocoa WF Site S zone 3, 

well number 4 in Figure 5–65. Refer to appendix 5 for figure 5–65.

Figure 96. Measured and simulated water-level hydrographs for OS0025 Bull Creek APT TM2, well 

number 7 in Figure 5–65. Refer to appendix 5 for figure 5–65.

Figure 95. Graph showing measured and simulated water-level hydrographs for 

OR0614 Cocoa WF Site S zone 3, well number 4 in Figure 5–65. Refer to 

appendix 5 for figure 5–65.

30

34

38

42

46

50

Water level, in feet NGVD 29

Year

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006

32

36

40

44

48

52

Water level, in feet NGVD 29

Year

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006
122 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

The slightly larger cvbj for Kh of layer 5 than for Kh of 

layer 3 (fig. 97), as indicated by the larger values defining the 

interquartile range, is likely because of the generally wider 

ranges of reasonable values, and consequent smaller weights 

on prior information, for Kh of layer 5 compared to layer 3. 

Uncertainties for Kh of layer 7 are somewhat larger than those 

for layers 3 and 5, reflecting smaller sensitivities compared to 

layer 5, and wider ranges of reasonable values compared to 

layer 3. 

It is important to assess the K estimates for the ECFT 

model in the context of the parameter uncertainty shown in 

figure 97. This uncertainty means that although the estimates 

presented herein are reasonable in the sense of not violating 

known or inferred information about the hydrogeologic 

properties of the model flow system, and produce a fit to the 

observations that meets the calibration criteria, it is highly 

likely that there are other distributions of model parameters 

or variations in the existing estimates that would also be 

reasonable and would fit the calibration data equally well. 

The existence of such alternative parameter distributions is 

especially likely when highly parameterized methods are used 

for representing flow system properties. Nonlinear uncertainty 

methods, such as Monte Carlo simulations, can be used to 

explore such alternative parameter distributions, but were 

beyond the scope of work for this study.

Simulated Potentiometric Surfaces

Simulated potentiometric surfaces for 1999, generated 

from the simulated heads from the 12-year transient simulation, are used to identify regional flow features. Simulated 

potentiometric surfaces for other years have the same general 

flow features present in the 1999 potentiometric surfaces. 

The average simulated water-table altitude for 1999, which is 

represented by the simulated head in layer 1 (SAS), generated 

from the 12-year transient model, reflects low water-table 

altitudes along the St. Johns River and extending to the eastern 

valley, along the Kissimmee River, and along the Peace River 

(figs. 1, 2, and 98). As expected, the altitude of the simulated 

water table was highest in the ridges (physiographic regions 

8, 13, 14, 15, and 17 through 20) and uplands (regions 2 and 9 

through 11, figs. 2 and 98).

The decline in the simulated water-table altitude from 

the eastern edge of the Lake Wales Ridge to the western edge 

of the Osceola and Wekiva Plains generates large horizontal 

hydraulic gradients in the SAS, given the magnitude of the 

water-level differences and the areal proximity between these 

two regions (figs. 2 and 98). The simulated west-to-east lateral 

flow in the SAS becomes surface leakage when such flow 

intercepts land surface along areas of the Osceola and Wekiva 

Plains. Surface leakage is also simulated elsewhere wherever 

0

0.5

1.0

1.5

2.0

2.5

Kh Layer 1 Kh Layer 2 Kv Layer 2 Kh Layer 3 Kh Layer 5 Kh Layer 7 Kmultiplier

Parameter coefficient of variation

Hydraulic conductivity parameter

Figure 97. Graph showing parameter coefficients of variation. The data summarized by 

the box plots are Kh of zones in layer 1; Kh or Kv estimates for regional pilot points in 

layers 2, 3, 5, and 7; and K multiplier estimates for the spring pilot points.

Maximum value

75 percentile

25 percentile

Minimum value

Coefficients of variation 

 of each parameter

EXPLANATION

Figure 97. Parameter coefficients of variation. The data summarized by the box plots are Kh of zones in 

layer 1; Kh or Kv estimates for regional pilot points in layers 2, 3, 5, and 7; and K multiplier estimates for the 

spring pilot points.
Simulation of Groundwater Flow System 123

East-central Florida 

transient model area

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

0 10 20 MILES

0 10 20 KILOMETERS

Figure 98. Map showing average simulated water table in the surficial aquifer system 

(SAS, layer 1) for 1999, from the 12-year transient model.

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

HARDEE COUNTY

COUNTY

SUMTER COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

EXPLANATION

–2.0 to 0

0.1 to 5.0

5.1 to 10.0

10.1 to 15.0

15.1 to 20.0

70.1 to 80.0

80.1 to 100.0

100.1 to 120.0

120.1 to 150.0

150.1 to 182.6

20.1 to 30.0

30.1 to 40.0

40.1 to 50.0

50.1 to 60.0

60.1 to 70.0

Simulated water levels in the SAS, in feet NGVD 29

Inactive areas in the SAS

ATLANTIC OCEAN

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 98. Average simulated water table in the surficial aquifer system (SAS, layer 1) for 1999, from the 

12-year transient model.
124 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

similar horizontal hydraulic gradients exist in the SAS and 

flow intercepts land surface.

The simulated potentiometric surface of layer 3, which 

represents average annual conditions for the OPZ during 

1999, reflects the main general features of the potentiometric 

surface of the OPZ for 1995 developed from observed water 

levels (figs. 25 and 99). The potentiometric-surface highs 

in north-central Polk County and in central Volusia County 

were reasonably well simulated, as was the configuration of 

the decreasing heads away from these potentiometric highs. 

The potentiometric-surface depressions along the St. Johns 

River are also present in the simulated potentiometric surface 

for layer 3 (OPZ, fig. 99). The depressions in the simulated 

potentiometric surface of layer 3 (OPZ) in the northern 

section of the model, along the St. Johns River, are the result 

of flow from Blue Spring leaving the OPZ. This feature is 

also present in the observed potentiometric surface of layer 3 

(OPZ, fig. 25).

The differences in the simulated potentiometric surfaces 

between layers 3 and 5 (OPZ and APPZ) are generally 

small except in the northeastern section of the model area, 

explained in part by the differences in the extent of active 

areas between the two layers (figs. 99 and 100). A much larger 

calibrated Kh for layer 5 than for layer 3 along the southeastern part of the model area (figs. 67 and 69) could suggest 

a large resulting simulated hydraulic gradient between these 

units; however, the low simulated and measured hydraulic 

differences between these simulated potentiometric surfaces 

can be explained by the large Kv of layer 4, representing the 

OLPZ (fig. 68) compared, for example, to layer 2, representing the ICU (fig. 66).

The simulated potentiometric surface of layer 7, representing the LFA, reflects heads nearly 40 ft lower in the 

southwestern part of the model area (fig. 101) compared to 

the corresponding heads in layer 5, representing the APPZ; 

these differences were simulated but also are observed 

between the LFA and APPZ (fig. 31). Differences in simulated 

heads between the APPZ and the LFA (fig. 31), where MCU 

I is present, are smaller than in areas where these aquifers 

are separated by MCU II. The large vertical head differences between the two aquifers in the southwestern part of 

the model area are explained by the low permeability of the 

intervening MCU II (figs. 31 and 70). The dominant eastward 

flow component in the LFA along most of the eastern edge 

of the LFA is explained by the combination of (1) relatively 

greater hydraulic conductivity of the LFA in western and 

central Orange County (fig. 71), (2) a relatively permeable 

overlying MCU I unit, compared to the MCU II unit to the 

west, and (3) the induced flow caused by the large groundwater withdrawals that take place in the LFA in Orange 

County (figs. 39B and 40B). Although heads in the eastern 

edge of the LFA mimic the heads in the APPZ with differences of only several feet, heads throughout the southwestern 

part of the LFA are at least 20 ft below the corresponding 

heads in the APPZ.

Simulated Groundwater Flows and 

Water Budget

The calculation of simulated-flow terms in the unsaturated zone and in layer 1 allows the comparison between the 

simulated ET and recharge rates to layer 1 and those rates 

derived from both measured and estimated flows (table 5). 

This comparison is accomplished by using the cell-by-cell 

flow data generated from the 12-year transient model for 

layer 1 (representing the SAS). Simulated net recharge rates 

to the SAS (NRCH, table 17) and estimated net recharge to 

the SAS (RCH, table 5) differed by less than 1.2 in/yr for 

each simulated year. These differences in flow terms from 

one year to another between simulated and estimated rates are 

within the inaccuracies of estimated flows in the water-budget 

analysis of the SAS. The calibration process required the 

adjustment of the hydraulic conductivity of the soil used in 

the Green-Ampt infiltration (GAI) method to determine the 

infiltration rates to the unsaturated zone and the adjustment 

of the regression coefficients used to estimate actual ET rates 

from potential ET rates. Each one of these parameters was 

systematically adjusted to either increase or decrease recharge 

rates to the SAS to match the rates listed in table 5. These 

modifications to the recharge parameters preceded every PEST 

simulation of the steady-state models for 1999 and 2003. 

The adjustment of parameters that reduce the differences 

between simulated and estimated recharge rates to the SAS 

was limited to using only realistic parameter values. Surface 

leakage (ASL, table 17) is the simulated groundwater seepage 

in the SAS reduced by the difference between potential ET 

and simulated ET at closed basin cells (fig. 7). ET losses in 

the unsaturated zone (UZET, table 17) were increased by the 

same difference before net recharge rates were calculated. 

In addition, ASL was further reduced by the flows at cells 

draining to streams or to lakes because those flows are part 

of the total streamflow (STFL, table 17). Equivalently, ASL 

in table 17 refers to groundwater discharge (in the form of 

seepage) in closed basin cells or in cells draining to streams 

or lakes outside the model. Average net simulated recharge to 

the SAS for the 12-year period of simulation was 3.58 in/yr 

(NRCH, table 17).

The simulated changes in storage in the SAS (STO, 

table 18) show years 2000 and 2006 as those with the largest 

changes in storage, coinciding with being the driest periods of 

the 12 years simulated. Net leakage from the SAS to streams 

was at least five times greater than the net leakage from the 

SAS to the lakes (STRS and LKSP, table 18). The simulated 

net leakage between the SAS and ICU/IAS (FLN, table 18) 

was used as an estimate for net downward leakage from the 

SAS to the underlying ICU/IAS (VLICU, table 5) because of 

the lack of measured data to complete the estimation of ET 

rates and recharge rates for the SAS. The two flows, VLICU 

and FLN, are equivalent.

The differences between the estimated and simulated ET 

rates (EET, table 5; SET, table 17) and between estimated and 
Simulated Groundwater Flows and Water Budget 125

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER

COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

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

l l l l l l l l

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

l l l l l l l l

40

45

35

50

55

60

30

65

70

75

25

20

15

80

80

85

10

90

95

5

105

110

100

115

120

125

130

70

45

5

20

65

15

70

70

10

10

55

50

30

15

75

25

35

60

90

95

0 10 20 MILES

0 10 20 KILOMETERS 50 Potentiometric surface contour—Shows 

 altitude at which water level would have 

 stood in tightly cased wells. Hachures 

 indicate depressions. Contour interval 

 5 feet. Contours are shown in active areas 

 as in figure 43B. Datum is NGVD 29

EXPLANATION

East-central Florida 

transient model area

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

ATLANTIC OCEAN

Figure 99. Map showing average simulated potentiometric surface of the Ocala permeable 

zone (OPZ, layer 3) for 1999, from the 12-year transient model.

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 99. Average simulated potentiometric surface of the Ocala permeable zone (OPZ, layer 3) for 1999, 

from the 12-year transient model.
126 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

SEMINOLE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER 

COUNTY

BREVARD

COUNTY

45

50

40

55

60

35

65

70

75

30

25

20

80

85

90 95

15

110

100

105

115

10

120

125

130

50

15

30

70

25

55

80

60

15

75

65

20

30

25

50

35

20

15

0 10 20 MILES

0 10 20 KILOMETERS

Potentiometric surface contour—Shows 

 altitude at which water level would have 

 stood in tightly cased wells. Hachures 

 indicate depressions. Contour interval 

 5 feet. Contours are shown in active areas 

 as in figure 43D. Datum is NGVD 29

80

EXPLANATION

East-central Florida 

transient model area

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

ATLANTIC OCEAN

Figure 100. Map showing average simulated potentiometric surface of the Avon Park 

permeable zone (APPZ, layer 5) for 1999, from the 12-year transient model.

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 100. Average simulated potentiometric surface of the Avon Park permeable zone (APPZ, layer 5) for 

1999, from the 12-year transient model.
Simulated Groundwater Flows and Water Budget 127

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

40

45

50

35

55

60

65

70

75

30

25

20

80

15

30

45

20

55

30

45

35

30

45

50

25

25

0 10 20 MILES

0 10 20 KILOMETERS

EXPLANATION

55 Potentiometric surface contour—Shows 

 altitude at which water level would have 

 stood in tightly cased wells. Contour 

 intervals is feet. Contours are shown in 

 active areas as in figure 43F. Datum is 

 NGVD 29

East-central Florida 

transient model area

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

ATLANTIC OCEAN

Figure 101. Map showing average simulated potentiometric surface of the Lower Floridan 

aquifer (LFA, layer 7) for 1999, from the 12-year transient model.

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 101. Average simulated potentiometric surface of the Lower Floridan aquifer (LFA, layer 7) for 1999, 

from the 12-year transient model.
128 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

Table 17. Simulated evapotranspiration (ET) and recharge rates to the surficial aquifer system (SAS, layer 1) from the calibrated 

transient model of 1995 to 2006.

[R, rainfall plus irrigation; LET, simulated ET at the lakes; STET, simulated ET at the streams; UZET, simulated ET in the unsaturated zone; GWET, simulated 

ET in the SAS; SET, total simulated ET, equal to LET+STET+UZET+GWET; STFL, streamflow leaving the model area; ASL, adjusted surface leakage after 

meeting potential ET in closed basins; NRCH, net recharge to the SAS, equal to R+SET+STFL+ASL; Ave, average rate for the 1995–2006 period; all rates 

are in inches per year; conversion to ft3/s should use the combined 160,651 cells between 150,651 active cells in the SAS and 10,000 lake cells; negative flows 

indicate outflows to the model area, whereas positive flows indicate inflows]

Year R LET STET UZET GWET SET STFL ASL NRCH

1995 53.12 –3.78 –0.02 –12.39 –20.53 –36.72 –11.48 –1.50 3.42

1996 50.13 –3.79 –0.02 –11.79 –19.82 –35.42 –10.50 –1.34 2.87

1997 54.78 –3.77 –0.02 –15.46 –17.66 –36.91 –11.29 –1.11 5.47

1998 49.39 –3.77 –0.02 –10.89 –17.73 –32.41 –13.49 –1.55 1.94

1999 49.60 –3.80 –0.02 –12.73 –18.38 –34.93 –8.54 –1.07 5.06

2000 31.72 –3.86 –0.02 –9.23 –13.29 –26.40 –4.87 –0.53 –0.08

2001 50.96 –3.57 –0.02 –12.24 –17.84 –33.67 –10.35 –1.14 5.80

2002 60.41 –3.67 –0.02 –12.46 –21.56 –37.71 –14.69 –1.71 6.30

2003 53.04 –3.51 –0.02 –9.16 –22.49 –35.18 –12.97 –2.04 2.85

2004 58.78 –3.64 –0.02 –13.06 –20.36 –37.08 –16.12 –1.77 3.81

2005 61.46 –3.65 –0.02 –10.95 –23.46 –38.08 –16.43 –2.37 4.58

2006 37.14 –3.91 –0.02 –9.39 –15.86 –29.18 –6.31 –0.82 0.83

Ave 50.88 –3.73 –0.02 –11.64 –19.08 –34.47 –11.42 –1.41 3.58

Table 18. Simulated flows entering and leaving the surficial aquifer system (SAS, layer 1) from the calibrated transient model of 1995 

to 2006.

[SAS, surficial aquifer system; STO, amount of water from storage to/from the SAS; FLN, net downward flow to the ICU/IAS; GWW, net flow to the SAS from 

Rapid Infiltration Basins (RIBs) and wells; GHBF, net flow through the lateral boundaries of the model; GWET, simulated ET from the SAS; UZRCH, recharge 

from the unsaturated zone, before reducing it by GWET; SL, surface leakage, unreduced in closed basins; STRS, stream seepage; LKSP, lake seepage; Ave, 

average rate for the 1995–2006 period; all rates are in inches per year; negative flows indicate outflows to the SAS area,whereas positive flows indicate inflows]

Year STO FLN GWW GHBF GWET UZRCH SL STRS LKSP

1995 0.68 –3.44 0.05 –0.02 –20.53 28.89 –5.55 –0.07 –0.01

1996 1.58 –3.64 0.06 –0.01 –19.82 27.53 –5.61 –0.08 –0.01

1997 –0.56 –3.49 0.04 –0.02 –17.66 26.61 –4.84 –0.07 –0.01

1998 1.81 –3.72 0.05 0.00 –17.73 26.59 –6.93 –0.06 –0.01

1999 0.18 –3.86 0.04 –0.05 –18.38 27.50 –5.34 –0.08 –0.01

2000 2.83 –4.05 0.07 –0.06 –13.29 17.68 –3.06 –0.11 –0.01

2001 –0.78 –3.71 0.05 –0.05 –17.84 27.58 –5.17 –0.07 –0.01

2002 –1.11 –3.69 0.07 –0.02 –21.56 33.62 –7.24 –0.06 –0.01

2003 1.62 –3.49 0.07 0.00 –22.49 32.52 –8.15 –0.07 –0.01

2004 0.97 –3.48 0.07 –0.01 –20.36 29.63 –6.76 –0.05 –0.01

2005 0.97 –3.46 0.09 0.01 –23.46 34.49 –8.58 –0.05 –0.01

2006 1.85 –3.90 0.08 –0.03 –15.86 22.25 –4.27 –0.11 –0.01

Ave 0.84 –3.66 0.06 –0.02 –19.08 27.90 –5.96 –0.07 –0.01
Simulated Groundwater Flows and Water Budget 129

103). These discharge rates correspond to the surface leakage 

simulated by the UZF1, where relatively large topographic/

altitude gradients in the land-surface altitude occur, allowing 

lateral discharge from layer 1.

Average simulated net recharge for 1999 was almost 

twice that for 2003, although total rainfall was greater in 1999 

than in 2003 (RCH, table 5; NRCH, table 17). The waterbudget analysis presented in table 5 supports the simulation 

finding that one year with higher average rainfall had less 

average net recharge than another year with lower average 

rainfall. Measured streamflow for 1999 was about half the 

measured streamflow for 2003 (SFG+SFU values, table 5). 

Simulated streamflow in 1999 also was lower than that in 

2003 (STFL, table 17). During wetter years, antecedent soil 

moisture is larger between rainfall events, resulting in greater 

runoff than during drier years. The year that preceded 2003 

was wet, whereas the year that preceded 1999 was dry. Both 

measured and simulated streamflow during 2003 were greater 

than during 1999, which justifies why both estimated and 

simulated annual average recharge rates over the SAS were 

greater during 1999 than in 2003. This observation holds when 

the annual average recharge rates are calculated over the entire 

SAS. Recharge rates to the SAS in the ridges and uplands 

(fig. 2) were higher in 2003 than in 1999, but discharge zones 

in 2003 were more areally extensive than in 1999, which 

resulted in a lower average annual recharge rate for 2003 than 

for 1999 over all active cells in the SAS.

Calibrated lakebed leakance for lakes ranged from 

0.00002 to 0.1 d-1 over the ECFT model area. Lakebed leakance affects the flow between layer 1, which represents the 

SAS, and layer 2, which represents the ICU/IAS. Lakebed 

leakance specified for lake cells along the perimeter of each 

lake and the difference between the water-table altitude and 

the water-surface altitude at the lake determined the lateral 

leakage rate between layer 1 and the lake. The average lateral 

leakage rate among cells with negative leakage rates (where 

flow is from layer 1 to the lakes) was -0.59 in/yr, the average 

lateral leakage rate among cells with positive leakage rates 

was 0.33 in/yr, and the overall average lateral leakage rate 

was -0.36 in/yr. When all 5,361 perimeter cells around lakes 

were considered, 3,360 lateral leakage rates were less than 

-0.1 in/yr, 704 rates were greater than 0.1 in/yr, and 1,297 rates 

were between -0.1 and 0.1 in/yr. The largest simulated lateral 

leakage rate to a lake from layer 1, 10.31 in/yr for 1999, was 

at lake number 263 in Polk County (figs. 13 and 104). The 

largest simulated lateral leakage rate to layer 1 from a lake, 

12.32 in/yr for 1999, was simulated at lake number 318 in 

northeastern Lake County (fig. 13), where Wolf Branch Creek 

(fig. 1), represented by stream segment 268 in figure 11, 

discharges its flow (fig. 104).

The Lake Package in MODFLOW assumes that each lake 

is characterized by only a single value of lakebed leakance. 

Lakebed leakance specified for lake cells and the difference 

between the water-surface altitude at the lake and the head in 

layer 2, which represents the ICU/IAS, determined the simulated vertical leakage rate between the lake and layer 2. The 

simulated recharge rates (RCH, table 5; NRCH, table 17) in 

the model area were the largest for the years 1997 and 2002. 

The parameters that determine the simulated ET rates were the 

same parameters used to modify the recharge rates to the SAS. 

Differences between simulated and estimated ET and recharge 

rates could be explained by the fact that the UZF1 Package 

simulates only ET from the subsurface and not direct evaporation of water at the canopy or at land surface. This simulation 

assumption could result in greater errors particularly during 

years when rainfall events result in extensive surface ponding 

causing actual ET rates to increase. If the estimated ET rates 

are increased by increasing the multiplicative factors applied 

to potential ET rates, then simulated recharge rates actually 

decrease. Thus, to avoid underestimating the water table, the 

simulated ET rates are mostly slightly lower than the estimated 

ET rates in table 5 for the SAS.

The simulated ET rates for 1999 were lowest in the 

ridges and upland physiographic regions and highest in lakes, 

in and near wetlands, and in areas where the water table 

is near land surface (figs. 98 and 102). Potential ET rates 

were applied at lakes, explaining the reason for the highest 

simulated ET rates. Low simulated ET rates in the ridges and 

uplands can be explained by the fact that ET losses at land 

surface (from surface-water bodies) are accounted for in the 

Lake and Streamflow Routing Packages of MODFLOW-2005 

(Harbaugh, 2005), but not in the UZF1 Package.

The specified ET rates input to the UZF1 Package were 

generally greater than the simulated ET rates. These differences are mostly explained by the fact that simulated ET in 

the unsaturated zone, as calculated by the UZF1 Package, is 

limited by available water. This is particularly true during dry 

months when there is insufficient water in the unsaturated 

zone for evaporation or if the water table is below the extinction depth. The UZF1 Package simulates ET from specified 

rates, which are calculated by adjustments to the regression 

coefficients (less than 1) to potential ET rates from the USGS 

Florida Water Science Center Statewide Evapotranspiration Database (U.S. Geological Survey, 2012). Because the 

simulated ET rates (fig. 102) account only for ET from the 

unsaturated zone and from groundwater, they should be less 

than actual ET rates that account for ET losses from the 

canopy and land surface.

Simulated annual net recharge rates to layer 1, representing the SAS, in 1999 (fig. 103) were calculated by 

subtracting the simulated groundwater ET rates and the 

simulated surface leakage from the simulated recharge to the 

unsaturated zone. Recharge rates were greatest in areas where 

the sum of rainfall (fig. 9) and landscape and agricultural irrigation rates (fig. 46) were greatest. Recharge rates to layer 1 

do not consider the inflow to the SAS from rapid infiltration 

basins (RIBs), because such rates were simulated as injection 

wells, and thus, were simulated in the Well Package. The 

ridge areas had larger recharge rates to the SAS than any other 

physiographic region. Large discharge rates in layer 1 were 

simulated along the eastern edge of the Lake Wales Ridge and 

the western edge of the Osceola and Wekiva Plains (figs. 2 and 
130 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

East-central Florida 

transient model area

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

ATLANTIC OCEAN

EXPLANATION

17.21 to 20.00

20.01 to 25.00

25.01 to 30.00

30.01 to 35.00

35.01 to 40.00

40.01 to 45.00

45.01 to 50.00

50.01 to 55.00

55.01 to 62.19

Simulated evapotranspiration for 1999, in 

 inches per year. Inactive areas in the 

 SAS indicated in white

0 10 20 MILES

0 10 20 KILOMETERS

Figure 102. Map showing simulated evapotranspiration rates for 1999 over active areas in 

the surficial aquifer system (SAS, layer 1) and in lakes, from the 12-year transient model.

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 102. Simulated evapotranspiration rates for 1999 over active areas in the surficial aquifer system 

(SAS, layer 1) and in lakes, from the 12-year transient model.
Simulated Groundwater Flows and Water Budget 131

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

EXPLANATION

–532.2 to –75.0

–74.9 to –30.0

–29.9 to –20.0

–19.9 to –5.0

–4.9 to 0

0.1 to 5.0

5.1 to 10.0

10.1 to 15.0

15.1 to 20.0

20.1 to 25.0

25.1 to 30.0

30.1 to 35.0

35.1 to 40.0

40.1 to 48.6

Simulated net recharge rates to the SAS for 1999, in inches per 

 year. Negative rates indicate discharge. Inactive areas in the 

 SAS indicated in white

0 10 20 MILES

0 10 20 KILOMETERS

East-central Florida 

transient model area

ATLANTIC OCEAN

Figure 103. Map showing simulated recharge and discharge rates over active areas of the 

surficial aquifer system (SAS, layer 1) in 1999, from the 12-year transient model.

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 103. Simulated recharge and discharge rates over active areas of the surficial aquifer system (SAS, 

layer 1) in 1999, from the 12-year transient model.
132 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

Figure 104. Simulated leakage rates between lakes and the surficial aquifer system (SAS, layer 1) based on the 

simulated lakebed leakance, average 1999 conditions Figure 104. Map showing simulated leakage rates between lakes and the surficial aquifer .

system (SAS, layer 1) based on the simulated lakebed leakance, average 1999 conditions.

EXPLANATION

–10.4 to –10.0

–9.9 to –5.0

–4.9 to –2.0

–1.9 to 0

0.1 to 2.0

2.1 to 5.0

5.1 to 12.4

Simulated leakage rate between lakes 

 and SAS for 1999, in inches per year. 

 Negative rates indicate flow is from 

 SAS to lakes; positive rates indicate 

 recharge to the SAS

Streams

0 10 20 MILES

0 10 20 KILOMETERS

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

East-central Florida 

transient model area

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

ATLANTIC OCEAN

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'
Simulated Groundwater Flows and Water Budget 133

In recharge areas of the SAS along the Kissimmee River 

and Peace River, the water table is generally higher than the 

water-surface altitude at the stream, which is the basis for the 

simulated direction of the flow.

Calibrated vertical hydraulic conductivity in layer 2, 

which represents the ICU/IAS, was the highest along the ridge 

physiographic regions (figs. 2 and 66), which resulted in the 

largest simulated downward vertical leakage rates between 

layer 1 and layer 2 for 1999 (fig. 107). Large upward leakage 

rates to layer 1 from layer 2, exceeding 45 in/yr, are simulated 

along the eastern perimeter of the Lake Wales Ridge, probably 

the result of cumulative lateral flow that leaks through areas 

of layer 2 that are relatively thin and permeable. The physiographic regions west of Lakes Wales Ridge, such as the De Soto 

Plain and the Polk Upland, and to the east, such as the Osceola 

Plain (figs. 2 and 107), show downward flow from layer 1; one 

exception is near the west-central part of the model area, where 

upward flow is simulated in some areas of the Green Swamp 

(figs. 2 and 107). Along the eastern edge of the ridges, and 

particularly along transitions in both the land-surface altitude 

and hydraulic conductivity of layer 1, flow to layer 1 from 

layer 2 was simulated along relatively narrow zones. Flow to 

layer 1 from layer 2 was also simulated along the boundary of 

the Lake Wales Ridge and the Green Swamp, where similar 

differences in land-surface altitude occur. The largest downward 

leakage rates to layer 2 were simulated in areas where artificial 

recharge occurs through RIBs (figs. 14 and 107). Leakage rates 

from layer 1 to layer 2 simulated for 1999 were not more than 

5 in/yr in large sections of the Osceola Plain, De Soto Plain, and 

Polk Upland (figs. 2 and 107).

The simulated leakage-rate pattern between layer 2, 

representing the ICU/IAS, and layer 3, representing the OPZ, 

is similar to that between layer 1 and layer 2 (figs. 107 and 

108). Upward flow occurs along most of the Eastern Valley, 

the St. Johns River, and the eastern edge of Lake Wales Ridge, 

whereas downward flow occurs along the ridges and uplands 

(figs. 2 and 108). Throughout most of the Osceola Plain, 

simulated leakage rates between the ICU and OPZ do not 

exceed 5 in/yr (figs. 2 and 108) because the calibrated vertical 

hydraulic conductivity of layer 2 is low in this physiographic 

region (fig. 66). The largest downward leakage rates to layer 3 

were simulated in areas where artificial recharge occurs 

through RIBs (figs. 14 and 108).

The simulated leakage rates between the ICU/IAS 

and OPZ for 1999 are substantially larger in the ridges and 

uplands than in the neighboring valleys and plains; this is 

also the case for the simulated leakage rates between the 

OLPZ and the APPZ for the same year (figs. 108 and 109). 

The exceptions are (1) the large upward leakage rates near 

springs (figs. 1 and 109), (2) the downward leakage to layer 5 

(APPZ) caused by the recharge to layer 3 from drainage wells 

(figs. 14 and 109), (3) the downward leakage to layer 5 caused 

by the downward leakage from the RIBS to layer 3, and 

(4) the upward flow from layer 5 induced by the pumping of 

agricultural and public-supply wells that pump from layer 3 

(figs. 39A and 109). 

average vertical leakage rate among cells with negative leakage 

rates (where flow is from layer 2 to the lakes) was -2.21 in/yr, 

the average vertical leakage rate among cells with positive 

leakage rates was 2.59 in/yr, and the overall average vertical 

leakage was 0.34 in/yr. When all 10,000 lake cells were considered, 4,120 vertical leakage rates were less than -0.1 in/yr, 5,160 

rates were greater than 0.1 in/yr, and 720 rates were between 

-0.1 and 0.1 in/yr. The largest vertical leakage rate to a lake 

from layer 2, 19.2 in/yr, was simulated at lake number 56 in 

northeastern Seminole County (figs. 13 and 105). The largest 

vertical leakage rate to layer 2 from a lake, 438.9 in/yr, was 

simulated at the only lake cell corresponding to lake number 

318. The next largest vertical leakage rates to layer 2 from a 

lake, which ranged from 44.06 to 34.82 in/yr, were simulated in 

the six lake cells that compose lake number 346 in central Polk 

County (figs. 13 and 105); the only lake in the ECFT model area 

with simulated augmentation from groundwater.

Calibrated vertical hydraulic conductivity values for 

streambeds were 0.08 ft/d for stream reaches of segment 

numbers 268, 287 and 289 (fig. 11), 0.05 ft/d for stream 

reaches of segment number 180 of the Palatlakaha River and 

the remaining stream segments of the Peace River and its 

tributaries (fig. 1), and 0.02 ft/d for all other stream segments. 

The average simulated leakage rate through streambeds, 

among stream cells with negative leakage rates (where flow 

is discharge from layer 1 to the streams) was -4.24 in/yr; the 

average leakage rate through streambeds, among stream cells 

with positive leakage rates (representing recharge from the 

stream to layer 1) was 1.71 in/yr; and the overall average 

leakage rate was -2.60 in/yr. This overall average represents 

a discharge of 0.011 ft3/s per stream cell from layer 1 to the 

streams or a total net discharge of 53.72 ft3/s over all stream 

cells. When all 5,004 stream cells were considered, there were 

3,467 leakage rates less than -0.1 in/yr, 1,226 rates greater than 

0.1 in/yr, and 311 rates between -0.1 and 0.1 in/yr. Simulated 

leakage rates through the streambeds ranged from a maximum 

discharge from layer 1 to a stream of about 66 in/yr, to a 

maximum recharge from a stream to layer 1 of about 30 in/yr 

(fig. 106). Most stream segments composing the Peace River 

(figs. 1 and 106) simulated discharge from layer 1 to the stream. 

The largest simulated leakage rates to layer 1 from streams 

were simulated at stream segments 52, 56, 57, 59, and 61 of 

the St. Johns River, which are upstream from lake number 

55 (figs. 1, 11, 13, and 106). The largest discharge rates to 

streams from layer 1 were simulated at stream segment 253 of 

the Kissimmee River and at stream segments 289, 293, 297, 

and 299 of the Peace River (figs. 1, 11, and 106). The direction of leakage (discharge compared to recharge) is generally 

governed by the simulated altitude of the water table relative 

to the water-surface altitude of the stream. Combined groundwater withdrawals from the Upper Floridan aquifer (figs. 1, 

39A, and 40A) reduce upward leakage to the SAS, causing 

a decline in the water table that may consequently increase 

leakage from the stream to the SAS. This may be the reason 

for the simulation of leakage from the stream to the SAS along 

stream segments 52, 56, 57, 59, and 61 of the St. Johns River. 
134 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

EXPLANATION

Simulated leakage rate between lakes 

 and ICU or IAS for 1999, in inches per 

 year. Negative rates indicate flow is 

 from ICU or IAS to lakes; positive rates 

 indicate recharge to the ICU or IAS

Streams

–19.2 to –15.0

–14.9 to –10.0

–9.9 to –5.0

–4.9 to –1.0

–0.9 to 0

0.1 to 5.0

5.1 to 10.0

10.1 to 50.0

50.1 to 438.9

0 10 20 MILES

0 10 20 KILOMETERS

East-central Florida 

transient model area

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

ATLANTIC OCEAN

Figure 105. Map showing simulated leakage rates between lakes and the intermediate 

confining unit/intermediate aquifer system (ICU/IAS, layer 2) based on the simulated 

lakebed leakance, average 1999 conditions.

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 105. Simulated leakage rates between lakes and the intermediate confining unit/intermediate aquifer 

system (ICU/IAS, layer 2) based on the simulated lakebed leakance, average 1999 conditions.
Simulated Groundwater Flows and Water Budget 135

EXPLANATION

Simulated leakage rate between streams 

 and SAS for 1999, in inches per year. 

 Negative rates indicate flow is from 

 SAS to streams; positive rates indicate 

 recharge to the SAS

Lakes

–65.6 to –25.0

–24.9 to –20.0

–19.9 to –10.0

–9.9 to –5.0

–4.9 to 0

0.1 to 5.0

5.1 to 10.0

10.1 to 20.0

20.1 to 25.0

25.1 to 29.9

0 10 20 MILES

0 10 20 KILOMETERS

East-central Florida 

transient model area

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

ATLANTIC OCEAN

Figure 106. Map showing simulated leakage rates between streams and the surficial aquifer 

system (SAS, layer 1) based on the simulated streambed vertical hydraulic conductivity, 

average 1999 conditions.

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 106. Simulated leakage rates between streams and the surficial aquifer system (SAS, layer 1) based on 

the simulated streambed vertical hydraulic conductivity, average 1999 conditions.
136 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

EXPLANATION

–170.2 to –45.0

–44.9 to –35.0

–34.9 to –25.0

–24.9 to –10.0

–9.9 to –5.0

–4.9 to 0

0.1 to 5.0

5.1 to 10.0

10.1 to 25.0

25.1 to 35.0

35.1 to 45.0

45.1 to 177.3

Simulated vertical leakage rate between the SAS and the ICU or IAS 

 for 1999, in inches per year. Negative rates indicate upward flow, 

 from the ICU or IAS to the SAS; positive rates indicate downward 

 flow, from the SAS to the ICU or IAS. White areas are inactive in 

 the SAS

0 10 20 MILES

0 10 20 KILOMETERS

East-central Florida 

transient model area

ATLANTIC OCEAN

Figure 107. Map showing simulated vertical leakage rates between the surficial aquifer 

system (SAS, layer 1) and the intermediate confining unit/intermediate aquifer system 

(ICU/IAS, layer 2), average 1999 conditions.

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY HARDEE

COUNTY SUMTER COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 107. Simulated vertical leakage rates between the surficial aquifer system (SAS, layer 1) and the 

intermediate confining unit/intermediate aquifer system (ICU/IAS, layer 2), average 1999 conditions.
Simulated Groundwater Flows and Water Budget 137

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

EXPLANATION

–113.7 to –50.0

–49.9 to –25.0

–24.9 to –10.0

–9.9 to –5.0

–4.9 to 0

0.1 to 5.0

5.1 to 10.0

10.1 to 25.0

25.1 to 50.0

50.1 to 307.6

Simulated vertical leakage rate between the 

 ICU or IAS and the OPZ for 1999, in inches 

 per year. Negative rates indicate upward 

 from the OPZ to the ICU or IAS; positive rates 

 indicate downward flow from the ICU or IAS 

 to the OPZ

0 10 20 MILES

0 10 20 KILOMETERS

East-central Florida 

transient model area

ATLANTIC OCEAN

Figure 108. Map showing simulated vertical leakage rates between the intermediate confining 

unit/intermediate aquifer system (ICU/IAS, layer 2) and the Ocala permeable zone (OPZ, layer 3), 

average 1999 conditions.

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

HARDEE COUNTY

COUNTY

SUMTER COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 108. Simulated vertical leakage rates between the intermediate confining unit/intermediate aquifer 

system (ICU/IAS, layer 2) and the Ocala permeable zone (OPZ, layer 3), average 1999 conditions.
138 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

EXPLANATION

–5,794.6 to –2,500.0

–2,499.9 to –1,000.0

–999.9 to –250.0

–249.9 to –100.0

–99.9 to –25.0

–24.9 to –5.0

–4.9 to 0

0.1 to 5.0

5.1 to 25.0

25.1 to 92.7

Simulated vertical leakage rate between 

 the OLPZ and the APPZ for 1999, in inches 

 per year. Negative rates indicate upward 

 flow from the APPZ to the OLPZ; positive 

 rates indicate downward flow from the 

 OLPZ to the APPZ. White areas are inactive 

 in the APPZ

0 10 20 MILES

0 10 20 KILOMETERS

East-central Florida 

transient model area

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

ATLANTIC OCEAN

Figure 109. Map showing simulated vertical leakage rates between the Ocala low-permeable 

zone (OLPZ, layer 4) and Avon Park permeable zone (APPZ, layer 5), average 1999 conditions.

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

HARDEE COUNTY

COUNTY

SUMTER COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 109. Simulated vertical leakage rates between the Ocala low-permeable zone (OLPZ, layer 4) and Avon 

Park permeable zone (APPZ, layer 5), average 1999 conditions.
Model Limitations 139

Average groundwater withdrawals from layers 3, 5, and 7 

(OPZ, APPZ, and LFA respectively), over the 12-year period 

of simulation, were 1.22, 0.43, and 0.35 in/yr, respectively. 

The total of these withdrawals, 2 in/yr, was less than the 

3.58 in/yr simulated average net recharge to layer 1 over the 

same period. Annual net simulated recharge to layer 1 was 

less than the total withdrawals from layers 3, 5, and 7 only 

during the dry years of 2000 and 2006 (tables 17, 19, and 20). 

The annual net simulated recharge to layer 1 was -0.08 in. (net 

loss) for 2000, and 0.83 in. for 2006 (table 17). Combined 

groundwater withdrawals from layers 3, 5, and 7 totaled 

2.55 in. in 2000 and 2.22 in. in 2006.

Model Limitations

Groundwater flow simulations generally are based on 

conceptual models that are simplified representations of 

complex, heterogeneous groundwater flow systems. The lack 

of sufficient measurements to fully describe the spatial and 

temporal variability of hydrologic conditions and the spatial 

variability of hydraulic properties throughout the model area 

often necessitate these simplifications. Assumptions about 

hydraulic properties such as isotropy, spatial uniformity, 

and the absence of preferential flow zones are examples of 

simplified representations that can be sources of error in 

a groundwater flow model. Other examples of simplified 

representations are the use of a numerical model that yields 

inherent simplifications such as constant hydraulic properties, boundary values, and hydraulic head within each grid 

cell. These simplifications produce limitations on the model's 

simulation capabilities. It is important that these limitations 

are considered during application of the model.

The feasibility of the ECFT model to represent the 

hydrologic system during the simulation period is limited 

by the assumptions and simplifications associated with the 

(1) conceptualization of the flow system, (2) data available 

to represent the physical properties of the system, as well as 

observations of system conditions such as the measured heads, 

spring flows, water-surface altitudes at lakes, and streamflows, 

(3) temporal range of hydrologic and anthropogenic stresses 

and the spatial scale of the model grid, and (4) equations used 

to simulate components of the flow system. The ECFT model 

was shown to match springflow, streamflow, lake water levels 

and aquifer water levels over the wide range of hydrologic 

stresses and groundwater withdrawal conditions for monthly 

stress periods from 1995 to 2006, which included dry, wet, and 

average hydrologic conditions. Nonetheless, it may be inappropriate to use the model with hydrologic conditions outside 

of the range tested or to impose groundwater withdrawals 

greater than those used in the simulation or near the boundaries of the model area.

Simplifying the actual flow system with the selected 

conceptualizations does not invalidate model results, although 

model results should be interpreted at scales larger than the 

representative grid cell. The model is finely discretized for a 

The distribution of leakage between layers 6 and 7 is 

characterized by a region of very low rates of downward 

leakage in the southwestern part of the simulated area 

(fig. 110), which corresponds to areas where MCU II is present 

(fig. 32) and where a low value of Kv is assigned to layer 6 

(fig. 70). The greatest downward leakage rates, from layer 6 

(MCU I) to layer 7 (LFA), were simulated in the northwestern 

part of the model area and in the northeastern part of the 

eastern extent of the MCU II (figs. 32 and 110). Leakage rates 

were relatively small in most of the remaining areas, except 

along the southern and eastern boundaries of the model where 

simulated layers 6 and 7 are relatively thin as a result of the 

presence of the 5,000 mg/L isochlor.

Total flows leaving the simulated system from layers 3, 

5, and 7 (representing the OPZ, APPZ, and LFA respectively) 

through the lateral boundaries were less than 0.7 in/yr for each 

of the 12 years of simulation (GHB3, table 19; GHB5 and 

GHB7, table 20). The rates corresponding to flux out of the 

model area from layers 3, 5 and 7 were 49, 131, and 304 ft3/s, 

respectively. Average flow leaving the lateral boundaries of the 

model in the LFA was more than 2 times greater than the flow 

leaving the lateral boundaries of the model in the APPZ. About 

90 percent of the total flow through the lateral boundaries 

of the LFA is along the northwestern corner of the model, 

where additional head data from the LFA could better define 

the specified heads for the GHB cells. Among the lateral 

boundary cells of the LFA, the northwestern corner of model 

had the highest calibrated hydraulic conductivity (fig. 71). 

Groundwater withdrawals from layer 3, which were greater 

for each of the 12 years than the combined withdrawals from 

the layers 5 and 7, induce greater net vertical flow to layer 3 

(FLN3, table 19) than the combined net vertical flows to 

layers 5 and 7 (FLN5 and FLN7, table 20). 

Simulated flows through the general-head boundaries 

allow the calculation of the outward flow through a wedge 

in the northwestern corner of the model (first 25 rows and 

first 25 columns in fig. 42A), and totaled about 14 ft3/s for 

1999 in layer 3, which represents the OPZ. Most of the flow 

during 1999 along these cells was simulated as flow out of 

the model. Outward flow through the northwestern corner 

of the model area totaled about 64 ft3/s for 1999 in layer 5, 

which represents the APPZ. Total combined flow through 

the specified northwestern corner of the model area from 

layer 3 and layer 5 was 78 ft3/s. Silver Springs is located 

about 40 mi northwest of the northwestern corner of the 

model area. Estimated average annual flow to Silver Springs 

for 1999 was about 700 ft3/s (Sepúlveda, 2009). Because the 

drainage area of Silver Springs is almost radially symmetric, 

and because the ECFT model covers about one eighth of 

Silver Springs’ drainage area, the outward flow simulated by 

the ECFT model should not exceed one eighth of the total 

flow to Silver Springs, or 87.5 ft3/s. The combined layer 3 

and layer 5 (OPZ and APPZ) outward flow simulated by the 

ECFT model through the northwestern corner of the model 

area does not exceed the reasonable limit of flow contribution 

to Silver Springs.
140 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

EXPLANATION

–43.9 to –5.0

–4.9 to –2.0

–1.9 to 0

0.1 to 2.0

2.1 to 5.0

5.1 to 10.0

10.1 to 15.0

15.1 to 21.2

Simulated vertical leakage rate between 

 the MCU I/II and the LFA for 1999, in 

 inches per year. Negative rates indicate 

 upward flow from the LFA to the MCU I/II; 

 positive rates indicate downward flow 

 from the MCU I/II to the LFA. White areas 

 are inactive in the LFA

0 10 20 MILES

0 10 20 KILOMETERS

East-central Florida 

transient model area

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

ATLANTIC OCEAN

Figure 110. Map showing simulated vertical leakage rates between middle confining unit I/II 

(MCU I/II, layer 6) and Lower Floridan aquifer (LFA, layer 7), average 1999 conditions.

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER COUNTY

BREVARD

COUNTY

SEMINOLE

COUNTY

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

Figure 110. Simulated vertical leakage rates between middle confining unit I/II (MCU I/II, layer 6) and Lower 

Floridan aquifer (LFA, layer 7), average 1999 conditions.
Model Limitations 141

Table 19. Simulated flows entering and leaving the Ocala permeable zone from the 

calibrated transient model of 1995 to 2006.

[STO3, amount of water from storage to/from the Ocala permeable zone (OPZ); FLN3, net vertical flow 

in the OPZ; GW3, groundwater withdrawals from the OPZ; GHB3, net flow through the lateral boundaries of the OPZ; DRN3, total spring flow leaving the OPZ; Ave, average rate for the 1995–2006 period; 

all rates are in inches per year; conversion to cubic feet per second should use the 169,552 active cells in 

the OPZ; negative flows indicate outflows in layer 3, whereas positive flows indicate inflows to layer 3]

Year STO3 FLN3 GW3 GHB3 DRN3

1995 0.00 1.81 –1.09 0.08 –0.80

1996 0.00 2.20 –1.28 –0.06 –0.86

1997 0.00 2.03 –1.15 –0.10 –0.78

1998 0.00 2.30 –1.41 –0.06 –0.83

1999 0.00 2.17 –1.31 –0.10 –0.76

2000 0.01 2.42 –1.67 –0.08 –0.68

2001 –0.01 2.12 –1.31 –0.11 –0.69

2002 0.00 2.10 –1.24 –0.11 –0.75

2003 0.00 1.94 –1.01 –0.11 –0.82

2004 0.00 1.92 –1.04 –0.07 –0.81

2005 0.00 1.80 –0.83 –0.10 –0.87

2006 0.01 2.15 –1.28 –0.09 –0.79

Ave 0.00 2.08 –1.22 –0.07 –0.79

Table 20. Simulated flows entering and leaving the Avon Park permeable zone and the Lower Floridan aquifer from the calibrated 

transient model of 1995 to 2006.

[STO5, amount of water from storage to/from the Avon Park permeable zone (APPZ); FLN5, net vertical flow in the APPZ; GW5, groundwater withdrawals 

from the APPZ; GHB5, net flow through the lateral boundaries of the APPZ; STO7, amount of water from storage to/from the Lower Floridan aquifer (LFA); 

FLN7, net vertical flow in the LFA; GW7, groundwater withdrawals from the LFA; GHB7, net flow through the lateral boundaries of the LFA; Ave, average 

rate for the 1995–2006 period; all rates are in inches per year; conversion to ft3/s should use the 159,139 active cells in the APPZ, and 129,441 active cells in 

the LFA; negative flows indicate outflows in layers 5 and 7, whereas positive flows indicate inflows to layers 5 and 7]

Year STO5 FLN5 GW5 GHB5 STO7 FLN7 GW7 GHB7

1995 0.00 0.70 –0.37 –0.33 0.00 0.82 –0.27 –0.55

1996 0.01 0.52 –0.39 –0.14 0.02 0.86 –0.29 –0.59

1997 0.00 0.51 –0.37 –0.14 –0.01 0.90 –0.30 –0.59

1998 0.01 0.49 –0.41 –0.09 0.02 0.91 –0.36 –0.57

1999 0.00 0.70 –0.41 –0.29 0.00 0.99 –0.38 –0.61

2000 0.01 0.64 –0.46 –0.19 0.04 1.00 –0.42 –0.62

2001 –0.01 0.64 –0.41 –0.22 –0.02 0.93 –0.35 –0.56

2002 –0.01 0.68 –0.42 –0.25 –0.03 0.86 –0.35 –0.48

2003 0.00 0.58 –0.43 –0.15 –0.01 0.91 –0.35 –0.55

2004 0.00 0.58 –0.46 –0.12 0.01 0.95 –0.38 –0.58

2005 0.00 0.67 –0.46 –0.21 –0.01 0.97 –0.38 –0.58

2006 0.01 0.78 –0.52 –0.27 0.04 0.97 –0.42 –0.59

Ave 0.00 0.63 –0.43 –0.20 0.00 0.92 –0.35 –0.57
142 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

regional-scale model, with uniform cell size of 1,250 ft per 

side (0.056 mi2). The groundwater flow equation solved by the 

model is based on Darcy’s Law and the continuity equation 

for flow derived from the principle of mass conservation along 

with the assumption that fluid properties are constant in time 

and space. This equation is valid for groundwater flow conditions in which the velocity of groundwater is low and flow 

is laminar. In karstic terrains, however, it is possible for flow 

through caverns and solution channels to be turbulent. Consequently, the groundwater flow equation is not valid for the 

entire FAS. It was assumed herein that laminar flow prevailed 

throughout the model area; therefore, if laminar flow is not a 

valid assumption in parts of the model area, then the accuracy 

of model results would be compromised in such areas.

The simulation of the SAS as a single layer may be an 

additional model limitation because this assumption places the 

altitude of the bottom of the lakebed at the top of the underlying layer. This is generally not the case for lakes in or near 

the ridges. A study by O’Reilly (1998) concluded that using 

three layers to simulate the SAS and placing all lakes in the 

top layer, including the ones in the ridges, did not substantially 

change the horizontal flow component in the SAS. However, 

only modification of the ECFT model with enhanced vertical 

discretization of the SAS, where the lakes are placed in the 

upper layer, would identify the true impact of this model 

conceptualization on SAS flowpaths.

The assumption of uniform heads throughout the vertical 

extent of each grid cell is another possible source of model 

error. Vertical hydraulic gradients between the SAS and OPZ 

suggest that the value used for simulated heads in the ICU/

IAS could be within the range of heads observed throughout 

the thickness of the ICU/IAS. Simulation of the observed 

hydraulic gradients within the ICU would require additional 

layering of this hydrogeologic unit.

The calibrated hydraulic conductivity obtained in this 

study for the ICU/IAS, OPZ, OLPZ, APPZ, MCU I/II, and 

LFA may vary from values in previously published local 

groundwater flow models. Final hydraulic conductivity values 

were constrained based on available aquifer test data and 

the expertise of the authors and water management district 

personnel. Although the calibrated hydraulic property maps 

are within acceptable ranges, it is highly likely that there 

are other distributions of hydraulic properties that would be 

reasonable and also fit the calibration criteria. Uncertainty 

analysis of this set of parameters indicates that the horizontal 

and vertical hydraulic conductivities of model layer 2 (which 

represents the ICU/IAS) are the most uncertain hydraulic 

properties given the current state of information and observations. Additionally, the lack of data for the MCU I/II and 

the LFA precludes a reliable estimation of their respective 

hydraulic conductivity distributions. The model simulates the 

decline in heads from the APPZ to the LFA in areas where the 

MCU II is present. However, additional hydraulic data for the 

MCU I could be used to improve the simulation of the upward 

head gradient between the APPZ and MCU I near Wekiwa 

Springs (spring number 21, fig. 51).

Areas in the IAS, OPZ, APPZ, or LFA where groundwater 

withdrawals were minimal may not be as well calibrated 

and may require changes to horizontal or vertical hydraulic 

conductivity as future aquifer stresses from increased groundwater withdrawals are applied and future observations and 

data are collected. The hydraulic properties were calibrated to 

conditions for specific stress periods, and model results may 

be incorrect if the system is stressed in ways outside of the 

conditions for which this model was calibrated. 

The general-head boundaries specified along the perimeter of active cells were based on the assumption of minimal 

groundwater flow across the estimated surface beneath which 

the FAS contains water having chloride concentrations greater 

than 5,000 mg/L. The use of general-head boundaries at the 

lateral extent of the freshwater and saline-water interface to 

simulate flows and levels in central Florida will not affect the 

model results substantially as long as (1) conditions in the 

coastal areas are not changed and (2) the assumption holds 

that the freshwater and saline-water interface can be approximated as constant from 1995 to 2006. The estimated surface 

shown in figure 37 can be updated as more water-quality data 

become available. In addition, interaction of groundwater 

flow with the freshwater and saline-water interface in the LFA 

might be understood better as the potentiometric surface of 

the LFA becomes better defined by more head measurements 

as additional wells are drilled. The assumption of a stationary 

freshwater and saline-water interface also could introduce 

error in the model if groundwater withdrawals or lack of 

recharge induce movement of this interface.

Errors in initial conditions were shown to be far less 

than the accuracy of water-level observations after about a 

6-month transient simulation period. Thus, any predictive runs 

for periods after 2006 that begin with the current 1995–2006 

transient stress periods would not be affected by errors in 

initial conditions.

The transient model presented herein addresses the 

surface-water and groundwater interactions with a systematic and process-oriented approach in the ECFT area, using 

appropriate modeling tools available at the time. By providing 

estimates of hydraulic parameters, this model can be used to 

generate initial estimates of the hydraulic properties as well as 

boundary conditions needed for more localized studies.

Summary and Conclusions

A seven-layer, finite-difference, transient groundwater 

flow model of the surficial aquifer system (SAS), intermediate 

confining unit (ICU) or intermediate aquifer system (IAS), 

and Floridan aquifer system (FAS) in east-central Florida 

was developed and calibrated to increase the understanding 

of groundwater flow in the SAS and FAS; assess the recharge 

rates to the SAS from infiltration through the unsaturated 

zone, and obtain a simulation tool that could assess the impact 

of changes in groundwater withdrawals on the potentiometric 
Summary and Conclusions 143

surfaces and flow in the FAS. The east-central Florida transient (ECFT) model area covers about 9,000 square miles, 

extending from about 4 miles west of the Lake-Sumter 

County line to the eastern shoreline of Florida and extending 

from northern Lake County to about 10 miles south of the 

Polk-Highland County line. The area includes more than 350 

lakes and a network of major rivers and tributary streams. 

The model simulates groundwater flow from January 1995 to 

December 2006.

Hydrologic processes at land surface and in the unsaturated 

zone were simulated by using the Green-Ampt infiltration 

(GAI) method together with MODFLOW-2005. The GAI 

method was used to partition rainfall into runoff and infiltration, which provided surface-runoff estimates as input for the 

Lake (LAK7) and Streamflow-Routing (SFR2) Packages and 

infiltration to the Unsaturated Zone Flow (UZF1) Package. The 

combined usage of the GAI and these packages enabled routing 

of infiltration through the unsaturated zone, and thus the simulation of recharge rates to the SAS. This approach more closely 

represents the physical processes compared to the methods used 

to calculate recharge rates in previous modeling efforts.

The hydrogeologic framework of the study area includes 

sediments that form the SAS; the less permeable clay and 

carbonate rocks that form the ICU, as well as the correlative 

permeable carbonate rocks that form the IAS in the southwestern part of the model area; and a thick sequence of 

carbonate rocks of variable permeability that form the FAS. A 

uniformly spaced grid composed of 472 rows and 388 columns 

of 1,250-foot square cells was used to discretize the hydraulic 

properties of the SAS (layer 1), the ICU/IAS (layer 2), the 

Ocala permeable zone (OPZ, layer 3), the Ocala low-permeable 

zone (OLPZ, layer 4), the Avon Park permeable zone (APPZ, 

layer 5), the middle confining unit I/II (MCU I/II, layer 6), and 

the Lower Floridan aquifer (LFA, layer 7).

The regional horizontal hydraulic conductivity (Kh) was 

represented by zones in layer 1 (SAS); by a grid of pilot points 

uniformly spaced 25 model cells apart in the permeable area 

of layer 2 (representing the IAS), as well as in layers 3, 5, 

and 7; and by a uniform value in layers 4 and 6. The regional 

vertical hydraulic conductivity (Kv) was represented by zones 

in layers 1 and 6, by pilot points in layer 2, by a uniform Kv 

value in layer 4, and by a uniform value of vertical anisotropy 

in layers 3, 5, and 7. A set of irregularly spaced pilot points 

near springs also was used to calculate multiplicative factors 

that increase the Kh of layers 3 and 5 and the Kv of layer 4 to 

account for enhanced dissolution of rocks. 

The ECFT model was calibrated by using PEST, an inverse 

modeling code that estimates model parameters by nonlinear 

regression with an objective function defined as the sum of 

squared weighted head and spring-flow residuals. The duration 

of a forward run for the 12-year model (more than 6 hours) 

required an iterative approach that included steady-state and 

transient simulations for subsets of the entire simulation period. 

Steady-state simulations for 1999 and 2003 were used to estimate 

hydraulic conductivity (K) parameters. Calibration observations 

included average annual heads and spring flows for 1999 and 

2003. All head weights were specified as 1 feet-1 (ft-1); springflow weights varied in proportion to the magnitude of flow. 

On the basis of sensitivity analysis, the Kh of layers 1, 3, 

5, and 7, the Kh of the IAS in layer 2, and the Kv of layers 2, 

4, and 6, were estimated by PEST. Estimated K distributions are reasonable compared to aquifer performance-test 

data, geologic information, and expert knowledge. The Kh 

of layer 1 ranged from about 7 to 71 feet per day (ft/d); the 

Kh of layer 2 ranged from about 4 to 150 ft/d; the regional 

Kh of layer 3, excluding the effects of the spring multiplicative factors, ranged from 26 to 1,440 ft/d; the regional Kh 

of layer 5 ranged from 29 to 3,860 ft/d; and the regional Kh 

of layer 7 ranged from 33 to 1,930 ft/d. Including the effects 

of the multiplicative factors near the springs, the maximum 

Kh values of layer 3 and layer 5 were 8,720 and 15,400 ft/d, 

respectively. The Kv of layer 2 ranged from 0.0001 to 1.0 ft/d, 

the regional Kv of layer 4 was 6.5 ft/d, and the Kv values 

for the two zones in layer 6 were 0.12 and 0.0001 ft/d. The 

multiplicative factors near the springs increased the maximum 

Kv of layer 4 from the regional value of 6.5 ft/d to 140 ft/d.

A transient simulation of the final 2 years (2005–2006) 

of the 12-year study period, incorporating the K values 

estimated from the steady-state simulations, was used with 

PEST to guide the estimation of specific yield and specific 

storage values. Values for specific yield were represented by 

zones corresponding to physiographic regions. Two specific 

storage parameters were defined, one for layer 2 and another 

for layers 3 to 7. Calibration observations included monthly 

average heads and flows. Nonlinear hydraulic processes 

confounded the estimation of the specific yield parameters 

for all zones, which resulted in setting values to either 0.14 or 

0.15. The specific storage was set to 3 10 6 × − ft-1 for layer 2 and

2 10 6 × − ft-1 for layers 3 to 7.

The 12-year transient simulation yielded head and springflow residuals that met the calibration criteria for the 12-year 

transient simulation. The overall mean residual for heads (with 

residual defined as simulated minus measured value) was 

-0.04 ft. The overall root-mean square residual for heads was 

less than 3.6 feet (ft) for each year of the 1995–2006 simulation period. The overall mean residual for spring flows was 

about -0.3 cubic feet per second (ft3/s). The spatial distribution 

of SAS head residuals in 1999 and 2006 shows a slight bias 

toward negative values along the Lake Wales Ridge. The 

distribution of OPZ head residuals in 2006 shows a bias 

towards negative values in the northwestern part of the model 

where simulated heads are generally lower than observed 

heads. Such bias is not present in the northwestern part of the 

model area for the 1999 residuals. 

A comparison between the simulated water-budget 

components for the SAS and values derived from a waterbudget analysis based on measured and estimated flows showed 

that the model can simulate recharge within 1 to 2 inches per 

year (in/yr) and evapotranspiration (ET) rates within several 

inches per year of independently estimated values. 
144 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

Simulated ET rates ranged from an average of about 

26 inches (in.) for the dry year of 2000 to about 38 in. for 

the wet year of 2005. Average ET over the simulated 12-year 

period was 34.47 in/yr, which accounts for 3.73 in. from lakes, 

0.02 in. from streams, 11.64 in. from the unsaturated zone, and 

19.08 in. from groundwater. The calculated average ET rate 

from a model-independent water-budget analysis over the same 

12-year period was 36.4 in/yr. Simulated net recharge rates to 

the SAS ranged from a discharge of 0.08 in. for the dry year 

of 2000 to a net recharge of 6.30 in. for 2002. The average net 

recharge rate to the SAS over the simulated 12-year period 

was about 3.6 in/yr, compared with the calculated average of 

3.4 in/yr from the model-independent water-budget analysis. 

Groundwater withdrawals from the FAS over the 12-year 

period of simulation averaged 2 in/yr, which is less than the 

3.58 in/yr simulated as average net recharge to layer 1 (SAS) 

over the same period. Annual net simulated recharge rates to 

layer 1 and net leakage rates to layer 3 (OPZ) were less than 

the total groundwater withdrawals from the layers representing 

the FAS only during the dry years of 2000 and 2006. The 

annual net simulated recharge to layer 1 was -0.08 in. (a net 

loss) for 2000 and 0.83 in. for 2006. Combined groundwater 

withdrawals from layers 3, 5, and 7 totaled 2.55 in. for 2000 

and 2.22 in. for 2006. A direct comparison of net groundwater 

withdrawals and net recharge rates for the 12-year simulation 

period generally indicates a global positive net recharge in the 

ECFT area. However, a detailed local model would be needed 

to accurately assess the local effects of pumping.

The groundwater flow model presented here represents 

improvements from previous modeling efforts in the study 

area by (1) expanding the areas of active cells in the southwestern part of the model area, (2) fully representing threedimensional flow by explicitly simulating the storativity and 

hydraulic conductivity for all confining units, (3) simulating 

the water-surface altitudes at lakes using the LAK7 Package, 

(4) routing streamflow using the SFR2 Package, (5) simulating 

the temporal distribution of recharge to the SAS using the GAI 

method and UZF1 Package, (6) using inverse modeling and 

head and spring-flow observations to estimate K parameters, 

and (7) using reliable estimates of rates from a model-independent water budget to compare against simulated recharge 

and ET rates. With these improvements, the numerical model 

more closely represents the physical processes governing 

groundwater flow and groundwater/surface-water interaction 

compared to previous models, and will help facilitate the 

management of the water resources in east-central Florida.

The calculation of the monthly infiltration and surface 

runoff rates from daily rainfall and irrigation rates, using the GAI 

method, proved to be a reliable tool for partitioning rainfall into 

infiltration and surface runoff. Dunnian runoff was calculated by 

the UZF1 Package using monthly stress periods and Hortonian 

runoff was calculated by the GAI method using daily rainfall 

data to calculate monthly infiltration and surface-runoff rates.

The simulated water exchanges between lakes and the 

SAS, between lakes and the ICU, and between streams and the 

SAS agreed within a few inches per year with corresponding 

estimated flows determined from a model-independent waterbudget analysis. The simulated partitioning of rainfall and irrigation into surface runoff and infiltration yielded an acceptable 

match between measured and simulated water-table altitudes, 

water-surface altitudes at lakes, and streamflow rates. The 

agreement between the flows simulated by the model and 

flows calculated from the model-independent method affirms 

the advantages of the approach used in developing the model.

In addition to the regular set of pilot points, extra pilot 

points were added near springs to better simulate spring flow, 

and doing so proved to be an efficient method for reducing 

flow residuals. The additional set of densely distributed pilot 

points around the springs allowed the assignment of greater 

hydraulic conductivity near springs, a feature that could not 

have been achieved with the regularly spaced set of pilot 

points. The calibrated conductances at the spring cells were 

derived using the GHB Package. A Drain Package generated 

with the calibrated heads and conductances used in the GHB 

Package caused no changes when compared to the GHB 

Package for the 12-year transient model.

The small vertical head differences between the OPZ 

and the APPZ were simulated reliably well using the general 

knowledge that the APPZ is much more permeable than the 

OPZ and the OLPZ is a confining unit with a larger vertical 

hydraulic conductivity than that of the ICU and MCU I or II. 

These conditions allow the upward leakage to be induced from 

the APPZ to the OPZ through a relatively permeable OLPZ 

whenever large groundwater withdrawals occur in the OPZ. 

The main components of groundwater flow in the LFA 

are towards the east and northeast. The higher hydraulic 

conductivity and larger groundwater withdrawals in Orange 

County induce lateral flow towards this area. The larger 

simulated flows leaving the model from the lateral boundaries 

occurred in the LFA, which could be based on the lack of 

head data in the LFA along these boundaries. As more head 

data along the lateral boundaries of the LFA become available, particularly along the eastern boundary, a more refined 

estimate of the flows leaving the model area could be made.

The feasibility of this model to represent the hydrologic 

system during the simulation period is limited by the assumptions 

and simplifications associated with the conceptualization of the 

flow system; the data available to represent the physical properties 

of the system as well as observations of system conditions such 

as measured heads, spring flows, water-surface altitudes at lakes, 

and streamflows; the temporal and spatial scale of the model; and 

the equations used to simulate components of the flow system. 

The ECFT model was shown to adequately match springflow, 

streamflow, lake water levels and aquifer water levels over the 

wide range of hydrologic stresses and groundwater withdrawal 

conditions for monthly stress periods from 1995 to 2006, which 

included dry, wet, and average hydrologic conditions. It may 

be inappropriate to use the model with hydrologic conditions 

outside of the range tested, however, or to impose groundwater 

withdrawals greater than those used in the simulation or near the 

boundaries of the model area.
References Cited 145

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Field Scale Irrigation Requirements Simulation (AFSIRS) 

model—Version 5.5: St. Johns River Water Management 

District Special Publication SJ2008–SP17, 251 p.

Snyder, J.P., 1983, Map projections used by the U.S. Geological Survey: U.S. Geological Survey Bulletin B-1532, 313 p.

Spechler, R.M., and Halford, K.J., 2001, Hydrogeology, water 

quality, and simulated effects of ground-water withdrawals from the Floridan aquifer system, Seminole County and 

vicinity, Florida: U.S. Geological Survey Water-Resources 

Investigations Report 01–4182, 116 p.

Spechler, R.M., and Kroening, S.E., 2007, Hydrology of Polk 

County, Florida: U.S. Geological Survey Scientific Investigations Report 2006–5320, 114 p.

Spechler, R.M., 2010, Hydrogeology and groundwater quality of Highlands County, Florida: U.S. Geological Survey 

Scientific Investigations Report 2010–5097, 84 p.

Sprinkle, C.L., 1989, Geochemistry of the Floridan aquifer 

system in Florida and in parts of Georgia, South Carolina, 

and Alabama: U.S. Geological Survey Professional Paper 

1403–I, 105 p.

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ground-water exchange at Lake Starr, Polk County, Florida: 

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the Floridan aquifer in east-central Florida—A conceptual 

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148 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

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geologic structure from high-resolution seismic-reflection 

data from four sinkhole lakes in the Lake Wales Ridge, 

central Florida: U.S. Geological Survey Open-File Report 

96–224, 71 p. 

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York, John Wiley, 535 p.

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evapotranspiration information and data [cited May 2012] 

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

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164 p.

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County, Florida: Tallahassee, Florida Geological Survey 

Report of Investigations 22, 65 p.

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flow in Lake Wales Ridge and adjacent areas of central 

Florida: U.S. Geological Survey Water-Resources Investigations Report 94–4254, 82 p.
Appendixes 1–5
150 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

Appendix 1. Groundwater Withdrawals and Inflows

Table 1–1. Annual average of groundwater withdrawals from public-water supply wells by aquifer and by county.—Continued

[Counties: Bre, Brevard; Lak, Lake; Mar, Marion; Ora, Orange; Osc, Osceola; Pol, Polk; Sem, Seminole; Sum, Sumter; Vol, Volusia; Tot, total withdrawals for 

the aquifer; aquifers: SAS, surficial aquifer system; INT, intermediate confining unit or intermediate aquifer system; OPZ, Ocala permeable zone; OLPZ, Ocala 

low-permeable zone; APPZ, Avon Park permeable zone; MCU, middle confining units I and II; LFA, Lower Floridan aquifer; ALL, inclusion of all aquifers; 

Ave, average rate from 1995 to 2006; all rates are in million gallons per day]

Use 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 Ave

SAS-Bre 5.4 5.3 4.8 5.0 5.2 5.8 5.8 5.9 6.1 6.1 5.4 5.8 5.6

SAS-Lak 0.2 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.4 0.3

SAS-Ora 1.8 1.5 3.6 2.3 2.1 2.1 2.2 2.4 2.2 2.4 2.3 1.7 2.2

SAS-Pol 0.2 0.1 0.3 0.3 0.6 0.9 0.7 0.7 0.8 0.7 0.5 0.7 0.5

SAS-Tot 7.6 7.2 9.0 7.9 8.2 9.1 9.0 9.3 9.4 9.5 8.5 8.6 8.6

INT-Bre 5.5 5.1 3.7 4.3 3.8 4.1 4.2 4.4 4.7 4.9 4.8 4.8 4.5

INT-Lak 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.1 0.1 0.1 0.0 0.0 0.0

INT-Mar 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

INT-Ora 0.3 0.3 0.7 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.3 0.3 0.4

INT-Osc 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

INT-Pol 2.0 2.1 2.1 2.0 2.2 2.3 1.9 2.4 2.3 2.2 2.8 3.2 2.3

INT-Sem 0.1 0.3 0.3 0.4 0.4 0.2 0.2 0.2 0.2 0.2 0.2 0.0 0.2

INT-Vol 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

INT-Tot 7.9 7.8 6.8 7.1 6.8 7.0 6.8 7.5 7.7 7.8 8.1 8.3 7.5

OPZ-Bre 0.8 0.9 0.7 0.8 0.7 0.7 1.1 1.3 1.3 1.4 1.3 1.4 1.0

OPZ-Lak 10.5 11.5 11.3 13.2 14.0 15.2 13.3 13.2 13.0 13.3 12.9 15.6 13.1

OPZ-Mar 0.7 0.8 0.9 1.3 1.6 1.9 1.8 1.8 1.9 2.2 2.2 2.9 1.7

OPZ-Ora 15.0 18.6 19.1 19.9 18.0 20.3 17.6 18.8 17.9 17.5 18.5 19.5 18.4

OPZ-Osc 3.0 3.3 3.5 3.6 3.6 4.3 4.1 3.8 3.6 4.0 4.3 4.9 3.8

OPZ-Pol 28.4 30.3 30.6 31.3 32.9 35.3 32.3 34.3 32.4 34.2 37.5 40.6 33.3

OPZ-Sem 21.5 22.7 23.1 27.7 28.2 31.6 27.4 26.2 25.1 26.6 24.7 28.6 26.1

OPZ-Sum 1.2 1.6 1.8 3.0 3.7 4.3 5.5 6.4 6.2 6.4 5.6 8.6 4.5

OPZ-Vol 21.1 23.4 23.8 26.2 25.7 28.2 25.1 27.2 28.0 28.3 26.3 30.7 26.2

OPZ-Tot 102.2 113.1 114.8 127.0 128.4 141.8 128.2 133.0 129.4 133.9 133.3 152.8 128.2

OLPZ-Bre 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.1 0.1 0.1 0.1 0.0

OLPZ-Lak 1.3 1.3 1.5 1.7 1.8 2.2 1.8 1.7 1.7 2.0 2.1 2.7 1.8

OLPZ-Mar 0.2 0.2 0.2 0.5 0.5 0.5 0.5 0.6 0.6 0.6 0.5 0.6 0.5

OLPZ-Ora 1.5 1.8 1.5 1.3 1.3 1.3 1.2 1.2 1.1 1.1 1.1 1.2 1.3

OLPZ-Osc 0.5 0.6 0.6 0.6 0.7 0.8 0.8 0.7 0.7 0.8 0.8 0.9 0.7

OLPZ-Pol 10.1 11.1 11.9 12.2 12.4 13.0 11.9 10.3 9.2 9.1 9.7 11.0 11.0

OLPZ-Sem 3.8 3.9 4.0 4.9 5.5 5.5 4.2 4.4 3.7 3.7 3.9 4.4 4.3

OLPZ-Sum 0.0 0.0 0.0 0.0 0.0 0.1 0.1 0.1 0.2 0.2 0.2 0.2 0.1

OLPZ-Vol 0.0 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1

OLPZ-Tot 17.4 19.0 19.8 21.3 22.3 23.5 20.6 19.2 17.4 17.7 18.5 21.2 19.8

APPZ-Bre 6.4 8.2 7.6 8.2 7.6 6.6 6.1 7.9 8.3 8.9 8.4 9.7 7.8
Appendix 1. Groundwater Withdrawals and Inflows 151

Table 1–1. Annual average of groundwater withdrawals from public-water supply wells by aquifer and by county.—Continued

[Counties: Bre, Brevard; Lak, Lake; Mar, Marion; Ora, Orange; Osc, Osceola; Pol, Polk; Sem, Seminole; Sum, Sumter; Vol, Volusia; Tot, total withdrawals for 

the aquifer; aquifers: SAS, surficial aquifer system; INT, intermediate confining unit or intermediate aquifer system; OPZ, Ocala permeable zone; OLPZ, Ocala 

low-permeable zone; APPZ, Avon Park permeable zone; MCU, middle confining units I and II; LFA, Lower Floridan aquifer; ALL, inclusion of all aquifers; 

Ave, average rate from 1995 to 2006; all rates are in million gallons per day]

Use 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 Ave

APPZ-Lak 10.4 11.6 11.8 13.6 14.6 16.7 14.9 16.5 17.0 20.0 20.5 24.4 16.0

APPZ-Mar 0.6 0.7 0.7 1.0 1.4 1.6 1.5 1.5 1.8 2.2 2.1 2.8 1.5

APPZ-Ora 63.0 71.2 60.5 63.4 61.8 69.1 63.4 61.8 62.3 68.8 69.0 72.9 65.6

APPZ-Osc 13.5 14.1 15.7 19.9 22.0 25.9 22.8 22.6 22.6 25.3 25.3 28.7 21.5

APPZ-Pol 19.8 20.6 21.1 22.2 22.1 24.2 21.2 21.6 21.8 22.8 24.7 31.7 22.8

APPZ-Sem 21.4 21.2 21.5 23.6 23.7 27.0 22.3 22.3 23.2 25.1 25.2 28.7 23.8

APPZ-Sum 0.4 0.4 0.8 0.9 0.9 1.5 2.6 2.2 2.4 2.9 3.0 3.9 1.8

APPZ-Vol 4.2 5.0 4.9 5.2 5.1 6.1 5.6 5.8 5.5 5.8 6.2 7.4 5.6

APPZ-Tot 139.7 153.0 144.6 158.0 159.2 178.7 160.4 162.2 164.9 181.8 184.4 210.2 166.4

MCU-Lak 1.6 1.5 1.6 2.0 2.1 2.3 1.6 1.5 1.4 1.6 1.6 1.8 1.7

MCU-Mar 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

MCU-Ora 0.9 2.0 1.0 0.3 0.1 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.4

MCU-Osc 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

MCU-Pol 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

MCU-Sem 0.3 0.3 0.3 0.4 0.5 0.5 0.4 0.5 0.6 0.6 0.5 0.5 0.5

MCU-Sum 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

MCU-Tot 2.8 3.8 2.9 2.7 2.7 2.8 2.0 2.0 2.0 2.2 2.1 2.3 2.5

LFA-Lak 1.0 1.3 1.4 1.5 1.5 1.6 1.3 1.3 1.2 1.3 1.3 3.3 1.5

LFA-Ora 88.5 93.4 99.7 120.3 125.1 136.8 115.7 115.1 115.1 121.4 124.9 132.4 115.7

LFA-Osc 0.1 0.2 0.2 0.3 0.5 0.5 0.5 0.5 0.6 0.6 0.6 0.7 0.4

LFA-Pol 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

LFA-Sem 2.3 2.1 2.1 2.2 2.2 2.7 2.4 2.4 2.4 2.9 2.6 2.5 2.4

LFA-Sum 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.9 1.8 0.2 4.7 0.6

LFA-Tot 91.9 97.0 103.4 124.3 129.3 141.6 119.9 119.3 120.2 128.0 129.6 143.6 120.7

ALL-Tot 369.5 400.9 401.3 448.3 456.9 504.5 446.9 452.5 451.0 480.9 484.5 547.0 453.7
152 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

Table 1–2. Annual average of groundwater withdrawals from commercial or industrial wells by aquifer and by county.

[Counties: Bre, Brevard; Har, Hardee; Hig, Highlands; Lak, Lake; Mar, Marion; Oke, Okeechobee; Ora, Orange; Osc, Osceola; Pol, Polk; Sum, Sumter; Vol, 

Volusia; Tot, total withdrawals for the aquifer; aquifers: SAS, surficial aquifer system; INT, intermediate confining unit or intermediate aquifer system; OPZ, 

Ocala permeable zone; OLPZ, Ocala low-permeable zone; APPZ, Avon Park permeable zone; MCU, middle confining units I and II; LFA, Lower Floridan 

aquifer; ALL, inclusion of all aquifers; Ave, average rate from 1995 to 2006; all rates are in million gallons per day]

Use 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 Ave

SAS-Bre 0.3 0.3 0.3 0.5 0.4 0.5 0.5 0.4 0.3 0.3 0.1 0.1 0.3

SAS-Hig 0.4 0.4 0.3 0.3 0.3 0.1 0.2 0.1 0.1 0.1 0.0 0.0 0.2

SAS-Lak 0.1 0.1 0.1 0.1 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.0 0.0

SAS-Ora 0.0 0.3 0.6 0.6 0.7 0.5 0.5 0.5 0.6 0.5 0.4 0.0 0.4

SAS-Pol 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

INT-Bre 1.3 0.9 0.9 0.7 0.6 0.5 0.2 0.2 0.2 0.1 0.0 0.1 0.5

INT-Har 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

INT-Lak 0.1 0.1 0.1 0.1 0.1 0.0 0.0 0.0 0.4 0.5 0.5 0.5 0.2

INT-Ora 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

INT-Osc 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

INT-Pol 4.4 5.1 4.1 4.7 4.2 4.6 4.5 6.5 4.8 4.8 3.9 5.6 4.8

OPZ-Bre 0.8 0.7 1.0 0.8 0.9 1.3 1.0 1.4 1.6 1.2 1.0 1.3 1.1

OPZ-Hig 0.2 0.2 0.2 0.2 0.2 0.1 0.1 0.1 0.1 0.1 0.0 0.0 0.1

OPZ-Lak 3.5 3.7 4.3 4.7 4.6 2.6 0.9 0.6 0.7 0.6 0.9 1.1 2.4

OPZ-Mar 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

OPZ-Oke 0.2 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

OPZ-Ora 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

OPZ-Pol 26.1 24.5 25.4 24.6 24.4 22.1 21.8 23.6 21.3 19.7 16.5 19.4 22.5

OPZ-Sum 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

OPZ-Vol 0.1 0.0 0.0 0.0 0.1 0.1 0.5 0.1 0.3 0.1 0.1 0.1 0.1

OLPZ-Hig 0.4 0.4 0.3 0.3 0.3 0.1 0.2 0.1 0.0 0.1 0.1 0.1 0.2

OLPZ-Lak 0.1 0.1 0.1 0.2 0.2 0.1 0.1 0.0 0.3 0.1 0.3 0.2 0.2

OLPZ-Ora 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

OLPZ-Pol 30.8 29.7 26.0 28.1 22.1 21.7 20.2 19.1 20.7 15.0 14.4 18.4 22.2

APPZ-Hig 0.1 0.2 0.1 0.1 0.1 0.0 0.1 0.0 0.0 0.1 0.0 0.0 0.1

APPZ-Lak 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.4 0.5 0.5 0.6 0.5 0.6

APPZ-Ora 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

APPZ-Pol 3.1 3.1 2.9 3.0 3.0 2.8 2.9 3.5 3.8 3.3 3.2 4.3 3.2

APPZ-Vol 0.1 0.1 0.1 0.1 0.2 0.2 0.1 0.1 0.0 0.0 0.0 0.0 0.1

MCU-Lak 0.5 0.5 0.5 0.7 0.7 0.7 0.8 0.9 0.6 0.6 0.5 0.6 0.6

MCU-Ora 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

LFA-Lak 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

LFA-Ora 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.3 0.0

LFA-Vol 0.2 0.1 0.1 0.1 0.2 0.3 0.2 0.1 0.0 0.1 0.1 0.1 0.1

ALL-Tot 73.4 71.1 68.0 70.5 63.9 58.9 55.4 57.7 56.3 47.8 42.7 52.7 59.9
Appendix 1. Groundwater Withdrawals and Inflows 153

Table 1–3. Summary of Benchmark Farms data used to estimate 

groundwater withdrawals for agricultural irrigation.

[Average irrigation is in inches per month]

Crop type Number of 

projects

Average 

irrigation

Citrus 10,095 0.98

Flat wood citrus 4,851 0.78

Leather leaf 4,699 3.36

Nursery 5 2.72

Fern 942 1.17

Potatoes 5,077 1.45

Sod 1,068 1.11

Table 1–4. Annual average of groundwater withdrawals from agricultural wells by county.

[Ave, average rate from 1995 to 2006; all rates are in million gallons per day]

Use 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 Ave

Brevard 30.4 36.3 34.3 39.4 36.2 33.8 21.6 20.7 12.7 17.1 10.3 17.1 25.8

Hardee 24.2 29.2 29.3 31.6 33.4 38.1 35.0 33.2 21.1 24.0 14.0 27.5 28.4

Highlands 16.2 22.9 20.4 23.9 22.9 29.9 23.0 22.3 17.6 18.2 12.7 23.0 21.1

Indian River 36.3 51.6 38.4 51.5 52.2 97.3 58.1 60.5 41.5 40.2 16.6 43.7 49.0

Lake 27.8 31.2 25.2 38.8 28.5 41.3 41.2 26.9 27.5 22.8 11.1 27.7 29.2

Marion 2.2 3.1 2.5 3.5 3.0 2.6 2.9 1.9 2.1 1.5 0.6 1.9 2.3

Okeechobee 7.2 9.8 7.8 9.2 3.6 5.8 3.8 2.9 2.4 5.8 4.0 9.2 6.0

Orange 30.9 27.9 21.9 44.9 23.9 46.5 30.5 28.8 14.5 23.6 6.6 14.6 26.2

Osceola 11.4 15.6 13.3 16.4 13.8 22.3 15.0 13.2 10.7 11.1 6.7 15.8 13.8

Polk 92.9 116.5 110.5 121.0 123.5 153.8 128.6 121.1 83.0 93.3 60.5 109.4 109.5

Seminole 4.7 4.9 4.2 5.8 5.0 6.6 3.8 3.7 3.1 3.7 1.1 2.1 4.1

St. Lucie 21.9 30.9 20.2 30.1 24.2 29.9 23.5 20.4 18.3 19.4 9.7 27.3 23.0

Sumter 1.2 1.5 1.2 1.4 1.4 2.0 1.4 1.3 1.3 1.3 0.8 1.4 1.4

Volusia 13.3 14.9 11.3 14.0 13.4 18.8 12.5 13.4 8.8 9.6 4.9 8.3 11.9

Total 320.6 396.3 340.5 431.5 385.0 528.7 400.9 370.3 264.6 291.6 159.6 329.0 351.6
154 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

Appendix 2. Springs

Table 2–1. List of names and spring numbers in the East-Central Florida Transient (ECFT) model area.

[SN, Spring number; Row, Col, model grid row and column of spring; XUTM, YUTM, horizontal and vertical coordinates in NAD 

83 Universal Transverse Mercator, zone 17N, in meters]

SN Spring name Row Col XUTM YUTM

1 Alexander Spring 9 114 443965 3217088

2 Apopka (Gourdneck) Spring 159 86 433454 3160198

3 Blue Spring (Volusia County) 48 174 466913 3202228

4 Blue Springs (Lake County) 106 49 419175 3180423

5 Bugg Spring 104 30 411986 3180866

6 Camp La-no-che Springs 47 123 447399 3202657

7 Clifton Springs 120 200 476767 3174832

8 Droty Springs 83 130 450209 3189042

9 Gemini Springs 73 181 469694 3192839

10 Green Spring 73 197 475746 3192739

11 Holiday Springs 108 51 420141 3179484

12 Island Spring in Wekiva River 84 154 459428 3188456

13 Messant Spring 75 133 451299 3192153

14 Miami Springs 117 147 456754 3175969

15 Palm Springs 123 160 461624 3173844

16 Rock Springs 104 132 451035 3181095

17 Sanlando Springs 123 159 461352 3173581

18 Seminole Spring 79 126 448509 3190664

19 Starbuck Spring 121 161 461795 3174471

20 Wekiva Falls Resort 

(flowing 14-inch borehole) 93 153 458944 3185333

21 Wekiwa Springs 117 143 455056 3176192

22 Witherington Springs 111 135 452210 3178330
Appendix 2. Springs 155

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

9

5

3

2

1

21

17

16

8

7

6

4

22

20

19

18

15

14

13

12

11

10

81°50' 81°40' 81°30' 81°20'

29°00'

28°50'

28°40'

17 ! Spring number and location 

 for which simulated and 

 measured flow hydrographs 

 are shown

7 ! Location of spring and 

 spring number

EXPLANATION

Lake Apopka

Lake Harris

Lake 

Griffin

Lake Dora

Lake 

Eustis

5

5

0 10 MILES

0 10 KILOMETERS

LAKE

COUNTY

ORANGE

COUNTY

SEMINOLE

COUNTY

MARION

COUNTY

VOLUSIA

COUNTY

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

Figure 2–1. Selected springs for which simulated and measured spring-flow hydrographs are shown.
156 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

70

80

90

100

110

120

130

140

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Spring flow, in cubic feet per second

Year

10

15

20

25

30

35

40

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Spring flow, in cubic feet per second

Year

EXPLANATION

Measured 

Simulated 

EXPLANATION

Measured 

Simulated 

Figure 2–2. Measured and simulated flows for Alexander Spring, spring number 1 in figure 2–1.

Figure 2–3. Measured and simulated flows for Apopka (Gourdneck) Spring, spring number 2 in figure 2–1.
Appendix 2. Springs 157

100

110

120

130

140

150

160

170

180

190

200

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Spring flow, in cubic feet per second

Year

0

2

4

6

8

10

12

14

16

18

20

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Spring flow, in cubic feet per second

Year

EXPLANATION

Measured 

Simulated 

EXPLANATION

Measured 

Simulated 

Figure 2–4. Measured and simulated flows for Blue Spring, spring number 3 in figure 2–1.

Figure 2–5. Measured and simulated flows for Bugg Spring, spring number 5 in figure 2–1.
158 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

EXPLANATION

Measured 

Simulated 

EXPLANATION

Measured 

Simulated 

0

2

4

6

8

10

12

14

16

18

20

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Spring flow, in cubic feet per second

Year

5

10

15

20

25

30

35

40

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Spring flow, in cubic feet per second

Year

Figure 2–6. Measured and simulated flows for Gemini Springs, spring number 9 in figure 2–1.

Figure 2–7. Measured and simulated flows for Sanlando Spring, spring number 17 in figure 2–1.
Appendix 3. Lakes 159

64

226

46

154

38

201

251

143

237

219

271

271

181

169

95

195

171

19

79

293

349

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

SEMINOLE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

BREVARD

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER

COUNTY

EXPLANATION

Lake number and lake for which simulated 

 and measured water-surface altitude

 hydrographs are shown

Lake

Streams

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

ATLANTIC OCEAN

0 10 20 MILES

0 10 20 KILOMETERS

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

East-central Florida

transient model area

Figure 3–1. Selected lakes for which simulated and measured water-surface altitude hydrographs are shown.

Appendix 3. Lakes
160 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

25

30

35

40

45

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

Water-surface altitude, in feet NGVD 29

58

60

62

64

66

68

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

Water-surface altitude, in feet NGVD 29

EXPLANATION

Measured 

Simulated 

EXPLANATION

Measured 

Simulated 

Figure 3–2. Measured and simulated water-surface altitude hydrographs for Lake Norris, lake number 19 in 

figure 3–1.

Figure 3–3. Measured and simulated water-surface altitude hydrographs for Lake Harris, lake number 46 in 

figure 3–1.
Appendix 3. Lakes 161

60

62

64

66

68

70

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

Water-surface altitude, in feet NGVD 29

12

14

16

18

20

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

Water-surface altitude, in feet NGVD 29

EXPLANATION

Measured 

Simulated 

EXPLANATION

Measured 

Simulated 

Figure 3–4. Measured and simulated water-surface altitude hydrographs for Lake Apopka, lake number 64 in 

figure 3–1.

Figure 3–5. Measured and simulated water-surface altitude hydrographs for South Lake, lake number 79 in 

figure 3–1.
162 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

88

90

92

94

96

98

100

102

Water-surface altitude, in feet NGVD 29

6

8

10

12

14

16

18

20

Water-surface altitude, in feet NGVD 29

EXPLANATION

EXPLANATION

Measured 

Simulated 

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

Figure 3–6. Measured and simulated water-surface altitude hydrographs for Lake Minneola, lake number 95 

in figure 3–1.

Figure 3–7. Measured and simulated water-surface altitude hydrographs for Lake Poinsett, lake number 143 

in figure 3–1.
Appendix 3. Lakes 163

48

50

52

54

56

58

60

Water-surface altitude, in feet NGVD 29

10

12

14

16

18

20

22

24

Water-surface altitude, in feet NGVD 29

EXPLANATION

Measured 

Simulated 

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

Figure 3–8. Measured and simulated water-surface altitude hydrographs for Lake Tohopekaliga, lake 

number 154 in figure 3–1.

Figure 3–9. Measured and simulated water-surface altitude hydrographs for Lake Washington, lake 

number 169 in figure 3–1.
164 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

56

58

60

62

64

66

Water-surface altitude, in feet NGVD 29

64

66

68

70

Water-surface altitude, in feet NGVD 29

EXPLANATION

Measured 

Simulated 

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

Figure 3–10. Measured and simulated water-surface altitude hydrographs for Lake Gentry, lake number 171 

in figure 3–1.

Figure 3–11. Measured and simulated water-surface altitude hydrographs for Lake Marion, lake number 181 

in figure 3–1.
Appendix 3. Lakes 165

114

116

118

120

122

124

126

Water-surface altitude, in feet NGVD 29

48

50

52

54

56

58

Water-surface altitude, in feet NGVD 29

EXPLANATION

Measured 

Simulated 

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

Figure 3–12. Measured and simulated water-surface altitude hydrographs for Lake Hamilton, lake number 195 

in figure 3–1.

Figure 3–13. Measured and simulated water-surface altitude hydrographs for Lake Hatchineha, lake 

number 201 in figure 3–1.
166 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

94

96

98

100

102

104

Water-surface altitude, in feet NGVD 29

46

48

50

52

54

56

Water-surface altitude, in feet NGVD 29

EXPLANATION

Measured 

Simulated 

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

Figure 3–14. Measured and simulated water-surface altitude hydrographs for Lake Hancock, lake 

number 219 in figure 3–1.

Figure 3–15. Measured and simulated water-surface altitude hydrographs for Lake Kissimmee, lake 

number 226 in figure 3–1.
Appendix 3. Lakes 167

50

52

54

56

58

Water-surface altitude, in feet NGVD 29

54

56

58

60

62

64

66

Water-surface altitude, in feet NGVD 29

EXPLANATION

Measured 

Simulated 

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

Figure 3–16. Measured and simulated water-surface altitude hydrographs for Lake Rosalie, lake number 237 

in figure 3–1.

Figure 3–17. Measured and simulated water-surface altitude hydrographs for Lake Marian, lake number 251 

in figure 3–1.
168 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

102

104

106

108

110

112

96

98

100

102

104

106

108

110

112

114

Water-surface altitude, in feet NGVD 29 Water-surface altitude, in feet NGVD 29

EXPLANATION

Measured 

Simulated 

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

Figure 3–18. Measured and simulated water-surface altitude hydrographs for Lake Sebring, lake number 293 

in figure 3–1.

Figure 3–19. Measured and simulated water-surface altitude hydrographs for Lake Wales, lake number 349 

in figure 3–1.
Appendix 4. Streams 169

Appendix 4. Streams

#

#

#

#

#

#

# #

#

#

# #

#

#

#

#

# #

#

#

#

#

# #

# #

# #

#

#

## #

#

# #

#

#

# ##

#

#

#

#

#

#

#

##

##

#

#

##

#

#

# ##

#

#

#

#

#

# #

#

#

#

#

#

#

#

#

#

#

#

#

##

#

#

#

#

#

###

##

#

#

#

#

#

#

#

#

#

# #

# # # #

#

#

##

#

#

# #

#

#

# #

#

#

#

#

#

#

#

J9202

02295637

02251767

02263800

02237293

02236000

02235000

02234384 02234000

02233500

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

SEMINOLE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

BREVARD

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER

COUNTY

EXPLANATION

02295637# Stream gaging station number and 

 location for which simulated and 

 measured flow hydrographs 

 are shown

# Stream gagin staion location 

Lake

Streams

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

ATLANTIC OCEAN

0 10 20 MILES

0 10 20 KILOMETERS

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

East-central Florida

transient model area

Figure 4–1. Selected stream-gaging stations for which simulated and measured flow hydrographs are shown.
170 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

0

400

800

1,200

1,600

2,000

2,400

2,800

Streamflow, in cubic feet per second

0

2,000

4,000

6,000

8,000

10,000

12,000

Streamflow, in cubic feet per second

EXPLANATION

Measured 

Simulated 

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

Figure 4–2. Measured and simulated flow hydrographs for Econlockhatchee River station number 02233500 in 

figure 4–1.

Figure 4–3. Measured and simulated flow hydrographs for St. Johns River above Lake Harney station 

number 02234000 in figure 4–1.
Appendix 4. Streams 171

0

10

20

30

40

50

60

70

80

90

Streamflow, in cubic feet per second 100

200

300

400

500

600

700

800

900

Streamflow, in cubic feet per second

EXPLANATION

Measured 

Simulated 

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

Figure 4–4. Measured and simulated flow hydrographs for Soldier Creek station number 02234384 in 

figure 4–1.

Figure 4–5. Measured and simulated flow hydrographs for Wekiva River station number 02235000 in 

figure 4–1.
172 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

100

0

200

300

400

500

600

700

800

Streamflow, in cubic feet per second

0

100

200

300

400

500

600

700

800

Streamflow, in cubic feet per second

EXPLANATION

Measured 

Simulated 

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

Figure 4–6. Measured and simulated flow hydrographs for Palatlakaha River station number 02237293 in 

figure 4–1.

Figure 4–7. Measured and simulated flow hydrographs for Shingle Creek station number 02263800 in 

figure 4–1.
Appendix 4. Streams 173

0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

8,000

Streamflow, in cubic feet per second

0

200

400

600

800

1,000

Streamflow, in cubic feet per second

EXPLANATION

Measured 

Simulated 

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

Figure 4–8. Measured and simulated flow hydrographs for Kissimmee River station identification J9202 in 

figure 4–1.

Figure 4–9. Measured and simulated flow hydrographs for Fellsmere Canal station number 02251767 in 

figure 4–1.
174 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

Appendix 5. Well Hydrographs

5

1

2

8

4

6

7

3

9

17

10

11

12

16

15

14

13

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

! !

! !! !

! ! ! ! !

!

!

!

!

!

!

!

!

!

!

!

!

!

! !

!

!

!

!

!

!!

!

!

!

!

!

!

!

!

!

! !

!

!

! !

!

!

!

!

!!

!

!

!

!

!

!

!

! !

!!

!!!!

! !!

!

! !

! !

! ! !

! !

! ! !!! !

! ! !!!!! ! !

!

!!

!

! !!!

!!

!!!

!! !

!

!

!

! ! !

!

!

! ! !

! ! ! !! ! !

!

!

! ! ! !

!

! !

!

!

! !

!

!

!!!

! !

!

!

!

!

!

!

!

!

! !

!

! !

!

!

!

! !

!

!

!

!

!

!

!

!

!

!

!!

!

!!!

!!

!

!

!

!

!

! !

!

!

!

!

!

!

! !

!

!

!!

!

!

!

!

!

! !

!

!

!

!

!

!

!

!

!

!

!

!

!!

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

SEMINOLE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

BREVARD

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER

COUNTY

EXPLANATION

SAS well number and location for which 

 simulated and measured water-level 

 hydrographs are shown

SAS well

Lake

Streams

!

13 !

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

ATLANTIC OCEAN

0 10 20 MILES

0 10 20 KILOMETERS

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

East-central Florida

transient model area

Figure 5–1. Selected surficial aquifer system (SAS) wells for which simulated and measured 

water-level hydrographs are shown.

Figure 5–1. Selected surficial aquifer system (SAS) wells for which simulated and measured water-level hydrographs 

are shown.
Appendix 5. Well Hydrographs 175

66

68

70

72

74

76

88

90

92

94

96

98

100

102

Water level, in feet NGVD 29

Water level, in feet NGVD 29

EXPLANATION

Measured 

Simulated 

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

Figure 5–2. Measured and simulated water-level hydrographs for All1w2, well number 1 in figure 5–1.

Figure 5–3. Measured and simulated water-level hydrographs for Green Swamp 6 Upland, well 

number 3 in figure 5–1.
176 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

24

26

28

30

32

34

44

46

48

50

52

54

Water level, in feet NGVD 29

Water level, in feet NGVD 29

EXPLANATION

Measured 

Simulated 

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

Figure 5–4. Measured and simulated water-level hydrographs for IR0025, well number 4 in figure 5–1.

Figure 5–5. Measured and simulated water-level hydrographs for KREFFM, well number 5 in figure 5–1.
Appendix 5. Well Hydrographs 177

112

114

116

118

120

122

124

126

128

52

54

56

58

60

62

64

Water level, in feet NGVD 29

Water level, in feet NGVD 29

EXPLANATION

Measured 

Simulated 

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

Figure 5–6. Measured and simulated water-level hydrographs for L–0050, well number 6 in figure 5–1.

Figure 5–7. Measured and simulated water-level hydrographs for L–0703 Carrot Barn, well number 7 

in figure 5–1.
178 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

126

130

134

138

142

54

56

58

60

62

64

Water level, in feet NGVD 29

Water level, in feet NGVD 29

EXPLANATION

Measured 

Simulated 

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

Figure 5–8. Measured and simulated water-level hydrographs for Lake Alfred shallow, well number 8 in 

figure 5–1.

Figure 5–9. Measured and simulated water-level hydrographs for OK–3 G, well number 9 in figure 5–1.
Appendix 5. Well Hydrographs 179

26

28

30

32

34

36

38

74

76

78

80

82

84

Water level, in feet NGVD 29

Water level, in feet NGVD 29

EXPLANATION

Measured 

Simulated 

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

Figure 5–10. Measured and simulated water-level hydrographs for OR0650 Rock Springs, well number 10 in 

figure 5–1.

Figure 5–11. Measured and simulated water-level hydrographs for OR0665 Alafaya Trail WTP, well number 11 

in figure 5–1.
180 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

64

66

68

70

72

74

76

88

92

96

100

104

108

112

116

Water level, in feet NGVD 29

Water level, in feet NGVD 29

EXPLANATION

Measured 

Simulated 

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

Figure 5–12. Measured and simulated water-level hydrographs for OS0232 Campbell Ranch, well number 12 

in figure 5–1.

Figure 5–13. Measured and simulated water-level hydrographs for Ridge Wrap VC-1, well number 14 in 

figure 5–1.
Appendix 5. Well Hydrographs 181

12

15

18

21

24

46

50

54

58

62

Water level, in feet NGVD 29

Water level, in feet NGVD 29

EXPLANATION

Measured 

Simulated 

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

Figure 5–14. Measured and simulated water-level hydrographs for S-0266 at Osceola Landfill, well number 15 

in figure 5–1.

Figure 5–15. Measured and simulated water-level hydrographs for V-1062 at Lawrence Farms, well number 16 

in figure 5–1.
182 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

60

62

64

66

68

70

Water level, in feet NGVD 29

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

Figure 5–16. Measured and simulated water-level hydrographs for WR8 GW1, well number 17 in figure 5–1.
Appendix 5. Well Hydrographs 183

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

! !

!

!

! !

!

!

! !

!

!

!

! !

!

!

!

!

!

! !

!

!

!

!

!

!

!

!

!

!

!

! !

!

!

!

!

! !

!

!

!

! !

!

!

! !

!

!

!

!

!

!

!

!

! 6

7

3

8

5

4

2

!2

9

1

10 MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

SEMINOLE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

BREVARD

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER

COUNTY

EXPLANATION

ICU/IAS well number and location for 

 which simulated and measured 

 water-level hydrographs are shown

ICU/IAS well

Lake

Streams

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

ATLANTIC OCEAN

0 10 20 MILES

0 10 20 KILOMETERS

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

East-central Florida

transient model area

Figure 5–17. Selected intermediate confining unit/intermediate aquifer system (ICU/IAS) wells for which simulated 

and measured water-level hydrographs are shown.
184 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

24

27

30

33

36

42

46

50

54

58

62

Water level, in feet NGVD 29

Water level, in feet NGVD 29

EXPLANATION

Measured 

Simulated 

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

Figure 5–18. Measured and simulated water-level hydrographs for IR0956 Delta Farms, well number 2 in 

figure 5–17.

Figure 5–19. Measured and simulated water-level hydrographs for KRFNND, well number 3 in figure 5–17.
Appendix 5. Well Hydrographs 185

24

26

28

30

32

34

36

38

40

118

122

126

130

134

138

Water level, in feet NGVD 29

Water level, in feet NGVD 29

EXPLANATION

Measured 

Simulated 

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

Figure 5–20. Measured and simulated water-level hydrographs for OR0651 Rock Springs intermediate, well 

number 5 in figure 5–17.

Figure 5–21. Measured and simulated water-level hydrographs for PO0001 Thornhill Ranch deep, well 

number 6 in figure 5–17.
186 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

110

114

118

122

126

130

52

56

60

64

68

Water level, in feet NGVD 29

Water level, in feet NGVD 29

EXPLANATION

Measured 

Simulated 

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

Figure 5–22. Measured and simulated water-level hydrographs for Tennorock Road well, well number 7 in 

figure 5–17.

Figure 5–23. Measured and simulated water-level hydrographs for Toho16 W2, well number 8 in figure 5–17.
Appendix 5. Well Hydrographs 187

8

12

16

20

24

36

40

44

48

52

Water level, in feet NGVD 29

Water level, in feet NGVD 29

EXPLANATION

Measured 

Simulated 

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

Figure 5–24. Measured and simulated water-level hydrographs for V-0166 Took Farm, well number 9 in 

figure 5–17. 

Figure 5–25. Measured and simulated water-level hydrographs for V-0812 Lake Daugharty, well number 10 in 

figure 5–17.
188 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

3

8

9

7

5

2

6

1

4

13

17

11

10 24

19

18

20

21

23

16

12

26

25

22

14

15

! !

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

! !

!

!

! ! !

! !

!

!

!

! !

!

!

! ! !

!

! ! !

! !

!

!

!

!

!

!

!

!

!

! ! !

!

!

!

!

!

!

! !

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

! !

!

!

!

!

!

!

!

!

!

!

!

!

! ! ! ! !

!

! ! !

!

!

! ! ! ! ! ! !

! !!! ! ! ! ! ! !

! ! !

! ! ! ! !

!

! ! ! ! ! !

!

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! ! ! !!! !

! ! ! ! ! !

! ! ! ! ! ! !

! ! ! ! ! !

!

!

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!

! ! ! ! !

! !

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! ! ! ! !

! ! !

! ! ! ! !

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

! !

!

! !

! !

!

!

! !

! !

!

!

!

!

!

!

!

!

!

!

!

!

!

!

! !

! !

!

!

!

!

!

!

! !

!

!

!

!

!

!

!

!

!

!

!

!

! !

!

!

!

!

!

!

!

!

!

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

SEMINOLE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

BREVARD

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER

COUNTY

EXPLANATION

OPZ well number and location for which 

 simulated and measured water-level 

 hydrographs are shown

OPZ well

Lake

Streams

! 13

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

ATLANTIC OCEAN

0 10 20 MILES

0 10 20 KILOMETERS

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

East-central Florida

transient model area

Figure 5–26. Selected Ocala permeable zone (OPZ) wells for which simulated and measured water-level 

hydrographs are shown.
Appendix 5. Well Hydrographs 189

12

16

20

24

28

14

18

22

26

30

34

Water level, in feet NGVD 29

Water level, in feet NGVD 29

EXPLANATION

Measured 

Simulated 

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

Figure 5–27. Measured and simulated water-level hydrographs for BR1526 at Seminole Ranch, well number 1 

in figure 5–26.

Figure 5–28. Measured and simulated water-level hydrographs for BR1558 at Kennedy Middle School, well 

number 2 in figure 5–26.
190 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

30

34

38

42

46

50

86

90

94

98

102

106

Water level, in feet NGVD 29

Water level, in feet NGVD 29

EXPLANATION

Measured 

Simulated 

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

Figure 5–29. Measured and simulated water-level hydrographs for IR0921 Morrison at Fellsmere Farms, 

well number 3 in figure 5–26.

Figure 5–30. Measured and simulated water-level hydrographs for L–0053 at Lake Louisa State Park, well 

number 4 in figure 5–26.
Appendix 5. Well Hydrographs 191

100

104

108

112

116

120

10

14

18

22

26

30

Water level, in feet NGVD 29

Water level, in feet NGVD 29

EXPLANATION

Measured 

Simulated 

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

Figure 5–31. Measured and simulated water-level hydrographs for L-0057 Eva Deep, well number 5 in 

figure 5–26.

Figure 5–32. Measured and simulated water-level hydrographs for L-0066 at Alexander Springs, well 

number 7 in figure 5–26.
192 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

36

40

44

48

52

56

60

64

68

120

124

128

132

136

140

Water level, in feet NGVD 29

Water level, in feet NGVD 29

EXPLANATION

Measured 

Simulated 

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

Figure 5–33. Measured and simulated water-level hydrographs for Lake Adair 10 Shallow, well number 8 in 

figure 5–26.

Figure 5–34. Measured and simulated water-level hydrographs for Lake Alfred deep, well number 9 in 

figure 5–26.
Appendix 5. Well Hydrographs 193

100

104

108

112

116

120

54

58

62

66

70

74

Water level, in feet NGVD 29

Water level, in feet NGVD 29

EXPLANATION

Measured 

Simulated 

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

Figure 5–35. Measured and simulated water-level hydrographs for Lake Oliver deep, well number 10 

in figure 5–26.

Figure 5–36. Measured and simulated water-level hydrographs for Lake Yale Groves, well number 11 in 

figure 5–26.
194 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

40

44

48

52

56

60

34

38

42

46

50

54

Water level, in feet NGVD 29

Water level, in feet NGVD 29

EXPLANATION

Measured 

Simulated 

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

Figure 5–37. Measured and simulated water-level hydrographs for M-0445 Tiger Den Replacement, well 

number 12 in figure 5–26.

Figure 5–38. Measured and simulated water-level hydrographs for OK0001, well number 13 in figure 5–26.
Appendix 5. Well Hydrographs 195

24

28

32

36

40

44

34

38

42

46

50

54

Water level, in feet NGVD 29

Water level, in feet NGVD 29

EXPLANATION

Measured 

Simulated 

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

Figure 5–39. Measured and simulated water-level hydrographs for OR0265 Cocoa F, well number 14 in 

figure 5–26.

Figure 5–40. Measured and simulated water-level hydrographs for OS0019 Bull Creek Loop Rd SW, well 

number 15 in figure 5–26.
196 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

34

38

42

46

50

54

50

54

58

62

66

70

Water level, in feet NGVD 29

Water level, in feet NGVD 29

EXPLANATION

Measured 

Simulated 

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

Figure 5–41. Measured and simulated water-level hydrographs for OS0231 Campbell Ranch, well number 16 

in figure 5–26.

Figure 5–42. Measured and simulated water-level hydrographs for Reedy Creek OSF-11, well number 17 in 

figure 5–26.
Appendix 5. Well Hydrographs 197

40

44

48

52

56

60

30

34

38

42

46

50

Water level, in feet NGVD 29

Water level, in feet NGVD 29

EXPLANATION

Measured 

Simulated 

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

Figure 5–43. Measured and simulated water-level hydrographs for OSF–4 Joe Overstreet, well number 18 in 

figure 5–26.

Figure 5–44. Measured and simulated water-level hydrographs for OSF-93 TH-9 Nova Road 532 West, well 

number 19 in figure 5–26.
198 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

60

65

70

75

80

85

90

95

100

24

28

32

36

40

44

Water level, in feet NGVD 29

Water level, in feet NGVD 29

EXPLANATION

Measured 

Simulated 

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

Figure 5–45. Measured and simulated water-level hydrographs for Regional Observation and Monitor-well 

Program CL-3 Floridan, well number 21 in figure 5–26.

Figure 5–46. Measured and simulated water-level hydrographs for S-1193, well number 22 in figure 5–26.
Appendix 5. Well Hydrographs 199

24

28

32

36

40

44

48

48

52

56

60

64

68

72

Water level, in feet NGVD 29

Water level, in feet NGVD 29

EXPLANATION

Measured 

Simulated 

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

Figure 5–47. Measured and simulated water-level hydrographs for S-1408, well number 23 in figure 5–26.

Figure 5–48. Measured and simulated water-level hydrographs for Sea World Drive replacement, well 

number 24 in figure 5–26.
200 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

4

8

12

16

20

24

12

16

20

24

28

Water level, in feet NGVD 29

Water level, in feet NGVD 29

EXPLANATION

Measured 

Simulated 

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

Figure 5–49. Measured and simulated water-level hydrographs for V-0198 at Lake Ashby Tower, well 

number 25 in figure 5–26.

Figure 5–50. Measured and simulated water-level hydrographs for V-0818 Osteen Well, well number 26 in 

figure 5–26.
Appendix 5. Well Hydrographs 201

!

!

!

!

!

!

!

!

1

2

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

SEMINOLE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

BREVARD

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER

COUNTY

EXPLANATION

OLPZ well number and location for 

 which simulated and measured 

 water-level hydrographs are shown

OLPZ well

Lake

Streams

!

1

!

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

ATLANTIC OCEAN

0 10 20 MILES

0 10 20 KILOMETERS

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

East-central Florida

transient model area

Figure 5–51. Selected Ocala low-permeable zone (OLPZ) wells for which simulated and measured water-level 

hydrographs are shown.
202 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

34

38

42

46

50

54

40

44

48

52

56

60

Water level, in feet NGVD 29

Water level, in feet NGVD 29

EXPLANATION

Measured 

Simulated 

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

Figure 5–52. Measured and simulated water-level hydrographs for Ashton Forestry Tower, well number 1 

in figure 5–51.

Figure 5–53. Measured and simulated water-level hydrographs for L-0704 Eustis Spray Field at Eustis, well 

number 2 in figure 5–51.
Appendix 5. Well Hydrographs 203

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

! !

!

!

!

!

!

!

!

!

! !

!

!

! !

! !

!

!

!

!

! ! !

!

!

! !

!

!

!

1

5

4

8

6

2

7

3

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

SEMINOLE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

BREVARD

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER

COUNTY

EXPLANATION

APPZ well number and location for 

 which simulated and measured 

 water-level hydrographs are shown

APPZ well

Lake

Streams

!

3!

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

ATLANTIC OCEAN

0 10 20 MILES

0 10 20 KILOMETERS

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

East-central Florida

transient model area

Figure 5–54. Selected Avon Park permeable zone (APPZ) wells for which simulated and measured water-level 

hydrographs are shown.
204 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

24

28

32

36

40

44

34

38

42

46

50

54

Water level, in feet NGVD 29

Water level, in feet NGVD 29

EXPLANATION

Measured 

Simulated 

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

Figure 5–55. Measured and simulated water-level hydrographs for OR0673 Cocoa Well Field Site 13T, well 

number 2 in figure 5–54.

Figure 5–56. Measured and simulated water-level hydrographs for OS0022 Bull Creek APT Site 1, well 

number 3 in figure 5–54.
Appendix 5. Well Hydrographs 205

26

30

34

38

42

46

60

65

70

75

80

85

90

95

100

Water level, in feet NGVD 29

Water level, in feet NGVD 29

EXPLANATION

Measured 

Simulated 

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

Figure 5–57. Measured and simulated water-level hydrographs for Regional Observation and Monitor-Well 

Program 55 Floridan, well number 4 in figure 5–54.

Figure 5–58. Measured and simulated water-level hydrographs for S-1407 Lake Mary Disposal, well number 6 

in figure 5–54.
206 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

8

10

12

14

16

18

20

22

24

16

20

24

28

32

36

Water level, in feet NGVD 29

Water level, in feet NGVD 29

EXPLANATION

Measured 

Simulated 

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

Figure 5–59. Measured and simulated water-level hydrographs for V-0801, well number 7 in figure 5–54.

Figure 5–60. Measured and simulated water-level hydrographs for V-1098 Shunz Road, well number 8 in 

figure 5–54.
Appendix 5. Well Hydrographs 207

!

!

!

!

!

!

!

!

! 3

1

2

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

SEMINOLE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

BREVARD

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER

COUNTY

EXPLANATION

MCU I well number and location for 

 which simulated and measured 

 water-level hydrographs are shown

MCU I well

Lake

Streams

!1

!

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

ATLANTIC OCEAN

0 10 20 MILES

0 10 20 KILOMETERS

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

East-central Florida

transient model area

Figure 5–61. Selected middle confining unit I (MCU I) wells for which simulated and measured 

water-level hydrographs are shown.

Figure 5–61. Selected middle confining unit I (MCU I) wells for which simulated and measured water-level 

hydrographs are shown.
208 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

30

34

38

42

46

50

54

26

30

34

38

42

46

Water level, in feet NGVD 29

Water level, in feet NGVD 29

EXPLANATION

Measured 

Simulated 

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

Figure 5–62. Measured and simulated water-level hydrographs for OR0675 Cocoa Well Field Site 13T, 

well number 1 in figure 5–61.

Figure 5–63. Measured and simulated water-level hydrographs for S-1257 Citrus Road Winter Springs, well 

number 2 in figure 5–61.
Appendix 5. Well Hydrographs 209

24

28

32

36

40

44

Water level, in feet NGVD 29

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

Figure 5–64. Measured and simulated water-level hydrographs for V-0081, well number 3 in figure 5–61.
210 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

8

6

3

5

4

9

7

1

2

10!

!

!

!

7!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

! !

!

!

!

!

!

!

!

!

!

!

!

MARION

COUNTY

LAKE

COUNTY

VOLUSIA

COUNTY

ORANGE

COUNTY

SEMINOLE

COUNTY

POLK

COUNTY

OSCEOLA

COUNTY

BREVARD

COUNTY

INDIAN

RIVER

COUNTY

ST. LUCIE

COUNTY

OKEECHOBEE

COUNTY

HIGHLANDS

COUNTY

HARDEE

COUNTY

SUMTER

COUNTY

EXPLANATION

LFA well number and location for which 

 simulated and measured water-level 

 hydrographs are shown

LFA well

Lake

Streams

82°00' 81°30' 81°00'

29°00'

28°00'

28°30'

27°30'

ATLANTIC OCEAN

0 10 20 MILES

0 10 20 KILOMETERS

Base from U.S. Geological Survey digital data,

Universal Transverse Mercator projection, zone 17

East-central Florida

transient model area

Eastern

extent

of

middle

confining unit II

Figure 5–65. Selected Lower Floridan aquifer (LFA) wells for which simulated and measured water-level hydrographs 

are shown.
Appendix 5. Well Hydrographs 211

50

55

60

65

70

70

74

78

82

86

90

Water level, in feet NGVD 29

Water level, in feet NGVD 29

EXPLANATION

Measured 

Simulated 

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

Figure 5–66. Measured and simulated water-level hydrographs for L-0599 Carrot Barn, well number 1 in 

figure 5–65.

Figure 5–67. Measured and simulated water-level hydrographs for L-0729 Keene Lake, well number 2 in 

figure 5–65.
212 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

28

32

36

40

44

48

34

38

42

46

50

54

58

62

Water level, in feet NGVD 29

Water level, in feet NGVD 29

EXPLANATION

Measured 

Simulated 

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

Figure 5–68. Measured and simulated water-level hydrographs for OR0009 Lake Adair 9, well number 3 in 

figure 5–65.

Figure 5–69. Measured and simulated water-level hydrographs for OR0618 Long Branch, well number 5 in 

figure 5–65.
Appendix 5. Well Hydrographs 213

40

44

48

52

56

60

40

44

48

52

56

60

Water level, in feet NGVD 29

Water level, in feet NGVD 29

EXPLANATION

Measured 

Simulated 

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

Figure 5–70. Measured and simulated water-level hydrographs for OR0794 Plymouth Tower, well number 6 in 

figure 5–65.

Figure 5–71. Measured and simulated water-level hydrographs for OSF-82L, well number 8 in figure 5–65.
214 Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida

Water level, in feet NGVD 29

Water level, in feet NGVD 29

26

30

34

38

42

46

12

16

20

24

28

32

EXPLANATION

Measured 

Simulated 

EXPLANATION

Measured 

Simulated 

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007

Year

Figure 5–72. Measured and simulated water-level hydrographs for S-1078 Oviedo WTP, well number 9 in 

figure 5–65.

Figure 5–73. Measured and simulated water-level hydrographs for V-0774 Galaxy Middle School, well 

number 10 in figure 5–65.
Manuscript approved July 24, 2012

For more information about this publication, contact:

 Nick Sepúlveda, Research Hydrologist

 USGS Florida Water Science Center

 12703 Research Parkway

 Orlando, FL 32826

 phone: (407) 803-5528 

 FAX: (407) 803-5501

 e-mail: nsepul@usgs.gov

Or visit the Florida Water Science Center Web site at: 

http://fl.water.usgs.gov

Prepared by the Raleigh Publishing Service Center

 Edited by Mike G. Deacon

 Illustrations by Kimberly A. Swidarski 

 Layout by Mike G. Deacon

A PDF version of this publication is available online at

http://pubs.usgs.gov/sir/2012/5161/
Sepúlveda and others—Groundwater Flow and Water Budget in the Surficial and Floridan Aquifer Systems in East-Central Florida—Scientific Investigations Report 2012–5161