Hydrogeology and Simulated Effects of 

Ground-Water Withdrawals for Citrus Irrigation, 

Hardee and De Soto Counties, Florida

By P.A. Metz

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 93-4158

Prepared in cooperation with the

SOUTHWEST FLORIDA WATER MANAGEMENT DISTRICT

Tallahassee, Florida 

1995
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director

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

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

For additional information Copies of this report can be

write to: purchased from:

District Chief U.S. Geological Survey

U.S. Geological Survey Earth Science Information Center

Suite 3015 Open-File Reports Section

227 N. Bronough Street Box 25286, MS 517

Tallahassee, FL 32301 Denver Federal Center

	Denver, CO 80225
CONTENTS

Abstract ....................................................................................................................................................................^ 1

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

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

Previous Investigations ................................................................................................................................................... 2

Description of the Study Area ........................................................................................................................................ 3

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

Climate .................................................................................................................................................................^ 4

Approach ......................................................................................................................................................................... 6

Hydrogeologic Framework ....................................................................................................................................................... 6

Surficial Aquifer ............................................................................................................................................................. 9

Hydraulic Properties ............................................................................................................................................. 9

Water Table ........................................................................................................................................................... 9

Intermediate Aquifer System .......................................................................................................................................... 9

Hydraulic Properties ........................................................................................................................................... 14

Potentiometric Surface and Water-Level Fluctuations ....................................................................................... 21

Floridan Aquifer System ............................................................................................................................................... 21

Hydraulic Properties ........................................................................................................................................... 25

Potentiometric Surface and Water-Level Fluctuations ....................................................................................... 25

Water Use ............................................................................................................................................................................... 29

Simulation of Ground-Water Flow ......................................................................................................................................... 33

Boundary Conditions .................................................................................................................................................... 39

Input parameters ........................................................................................................................................................... 41

Geographical Information System Procedures ............................................................................................................. 41

Steady-State Conditions ................................................................................................................................................ 42

Steady-State Calibration ..................................................................................................................................... 44

Sensitivity Analysis ............................................................................................................................................ 47

Transient Conditions ..................................................................................................................................................... 47

Boundaries .......................................................................................................................................................... 49

Transient Calibration .......................................................................................................................................... 49

Sensitivity Analysis ............................................................................................................................................ 60

Simulated Ground-Water Budget for 1988-89 ............................................................................................................. 63

Simulated Ground-Water Flow Analysis for September 1988, May 1989, and September 1989 ................................ 64

Hypothetical Development Scenarios .................................................................................................................................... 65

May 2000 ........................................................................................................_

May 2020 ................................................................

Simulated Effects of Increased Withdrawals for Citrus Irrigation ......................................................................................... 67

Summary and Conclusions ..................................................................................................................................................... 67

Selected References ................................................................................................................................................................ 74

Appendix. Pumpage Distribution for Transient Simulations for the Intermediate Aquifer System and Upper Floridan

Aquifer, Hardee and De Soto Counties, Florida ................................................................................................................ 77

Contents III
Figure

1. Map showing location of the study and model area ..................................................................................................... 3

2. Map showing physiographic subdivisions and boundaries of study and model area .................................................. 4

3. Map showing generalized location of citrus groves in Hardee and De Soto Counties, 1988 ...................................... 5

4. Graph showing projected agricultural land use for 1990, 1995, 2000, 2010, and 2020 .............................................. 6

5. Correlation chart showing hydrogeologic framework ................................................................................................. 7

6. Map showing locations of generalized hydrogeologic sections and selected well sites .............................................. 8

7. Generalized hydrogeologic sections A-A', B-B', C-C', and D-D' ........................................................................... 9, 10

8. Map showing altitude of the water table in the surficial aquifer, September 1988 ................................................... 12

9. Graphs showing long-term water-level trends in surficial aquifer wells in or near the study area, 1978-88 ............ 13

10-18. Maps showing:

10. Areas of upward and downward leakage through the upper confining unit of the intermediate aquifer

system, September 1988 ..................................................................................................................................... 15

11. Thickness of the intermediate aquifer system ..................................................................................................... 16

12. Thickness and model-derived leakance of the upper confining unit of the intermediate aquifer system ........... 17

13. Transmissivity of the intermediate aquifer system ............................................................................................. 18

14. Areas of upward and downward leakage through the lower confining unit of the intermediate aquifer

system, September 1988 ..................................................................................................................................... 19

15. Thickness and leakance of the lower confining unit of the intermediate aquifer system ................................... 20

16. Potentiometric surface of the intermediate aquifer system, September 1988 ..................................................... 22

17. Potentiometric surface of the intermediate aquifer system, May 1989 ............................................................... 23

18. Potentiometric surface of the intermediate aquifer system, September 1989 ..................................................... 24

19. Graphs showing long-term water-level trends for selected intermediate aquifer system wells, 1970-88 ................. 25

20-24. Maps showing:

20. Thickness of the Upper Floridan aquifer ............................................................................................................ 26

21. Transmissivity of the Upper Floridan aquifer ..................................................................................................... 27

22. Potentiometric surface of the Upper Floridan aquifer, September 1988 ............................................................ 28

23. Potentiometric surface of the Upper Floridan aquifer, May 1989 ...................................................................... 30

24. Potentiometric surface of the Upper Floridan aquifer, September 1989 ............................................................ 31

25. Graphs showing long-term water-level trends for selected Upper Floridan aquifer wells, 1978-88 ......................... 32

26. Diagram showing percentage distribution of irrigation water use, by crop, Hardee and De Soto Counties, 1988 ... 33

27. Graphs showing rainfall at Arcadia, ground-water withdrawals, and irrigated citrus acreage in Hardee and

De Soto Counties, 1975-88 ........................................................................................................................................ 34

28. Map showing locations of citrus irrigation wells open to the intermediate aquifer system, 1988 ............................. 35

29. Map showing locations of citrus irrigation wells open to the Upper Floridan aquifer, 1988 .................................... 36

30. Graphs showing ground-water levels in wells open to the intermediate aquifer system and the Upper 

Floridan aquifer, irrigation pumpage at Joshua Grove, and monthly rainfall at Arcadia, September 198 Q 

through September 1989 ............................................................................................................................................ 37

31. Diagram showing conceptualized hydrogeologic representation of a north-south section of the aquifer system ..... 38

32. Diagram showing schematic cross section of the layered digital ground-water model and the hydrologic units

represented by the model layers ................................................................................................................................. 39

33. Diagram showing finite-difference grid, lateral boundary conditions, and cross section used for

sensitivity analysis ..................................................................................................................................................... 40

34. Graphs showing maximum daily water levels for selected wells open to the intermediate aquifer system and

the Upper Floridan aquifer, September 1987 through September 1988 .................................................................... 43

35. Map showing interpolated and simulated potentiometric surfaces and direction of ground-water movement in

the intermediate aquifer system, September 1988 ..................................................................................................... 45

36. Map showing interpolated and simulated potentiometric surfaces and direction of ground-water movement in

the Upper Floridan aquifer, September 1988 ............................................................................................................. 46

37. Graphs showing results of sensitivity analysis of the steady-state model parameters for the intermediate.

aquifer system and the Upper Floridan aquifer, September 1988 ............................................................................. 48

38. Graphs showing hydrographic linear template used to estimate general head boundaries for transient simulation ........ 50

39. Map showing locations of wells used for hydrographic analysis .............................................................................. 52

IV Contents
40. Graphs showing simulated and observed water levels in selected wells open to the intermediate aquifer

system and the Upper Floridan aquifer, October 1988 through September 1989 ................................................ 53-55

41-44. Maps showing:

41. Interpolated and simulated potentiometric surfaces and direction of ground-water movement in the

intermediate aquifer system. May 1989 .............................................................................................................. 56

42. Interpolated and simulated potentiometric surfaces and direction of ground-water movement in the

Upper Floridan aquifer, May 1989 ...................................................................................................................... 57

43. Interpolated and simulated potentiometric surfaces and direction of ground-water movement in the

intermediate aquifer system, September 1989 .................................................................................................... 58

44. Interpolated and simulated potentiometric surfaces and direction of ground-water movement in the

Upper Floridan aquifer, September 1989 ............................................................................................................ 59

45. Graphs showing results of sensitivity analyses of storage and pumpage parameters on the transient

calibration for the intermediate aquifer system and the Upper Floridan aquifer, May 1989 ..................................... 61

46. Graphs showing results of sensitivity analyses of model boundary conditions on the transient calibration

for the intermediate aquifer system and the Upper Floridan aquifer, May 1989 ....................................................... 62

47-51. Maps showing:

47. Locations of model grid blocks with projected increases in citrus acreage zones for the years 2000 

and 2020 ........................................................................^^^^

48. Simulated potentiometric surface of the intermediate aquifer system, May 2000, and simulated declines 

in the potentiometric surface as a result of projected increases in ground-water withdrawals for citnr 

irrigation, May 1989 to May 2000 ...................................................................................................................... 68

49. Simulated potentiometric surface of the Upper Floridan aquifer, May 2000, and simulated declines in the 

potentiometric surface as a result of projected increases in ground-water withdrawals for citrus irrigation, 

May 1989 to May 2000 ....................................................................................................................................... 69

50. Simulated potentiometric surface of the intermediate aquifer system, May 2020, and simulated declines 

in the potentiometric surface as a result of projected increases in ground-water withdrawals for 

citrus irrigation, May 1989 to May 2020 ............................................................................................................ 70

51. Simulated potentiometric surface of the Upper Floridan aquifer, May 2020, and simulated declines in the 

potentiometric surface as a result of projected increases in ground-water withdrawals for citrus irrigation, 

May 1989 to May 2020 ....................................................................................................................................... 71

52-57. Diagrams showing pumpage distribution used in transient simulations for the:

52. Intermediate aquifer system, May 1989 .............................................................................................................. 78

53. Intermediate aquifer system, May 2000 .............................................................................................................. 79

54. Intermediate aquifer system, May 2020 .............................................................................................................. 80

55. Upper Floridan aquifer, May 1989 ...................................................................................................................... 81

56. Upper Floridan aquifer, May 2000 ...................................................................................................................... 82

57. Upper Floridan aquifer, May 2020 ...................................................................................................................... 83

Table

1. Ground-water withdrawals in Hardee and De Soto Counties, by use category, 1988 ............................................... 32

2. Statistical summary of differences between observed and simulated heads for the intermediate aquifer system

and the Upper Floridan aquifer, September 1988 ...................................................................................................... 47

3. Coefficients used for determining monthly irrigation pumping rates ........................................................................ 51

4. Ground-water pumpage for public supply, industrial use, andirrigation from the intermediate aquifer system

and the Upper Floridan aquifer during the irrigation period, Septemberl988 through May 1989 ............................ 51

5. Statistical summary of differences between observed and simulated heads for the intermediate aquifer system

and the Upper Floridan aquifer, May and September 1989 ....................................................................................... 60

6. Simulated water budget for 1988-89 flow conditions ................................................................................................ 63

7. Current and projected ground-water withdrawal rates and citrus acreage for 1989, 2000, and 2020 ........................ 67

8. Simulated water budgets for May 1989, May 2000, and May 2020 flow conditions ................................................ 72

Contents V
CONVERSION FACTORS, VERTICAL DATUM, AND ACRONYMS

Multiply By To obtain

inch (in.) 25.4 millimeter

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

foot (ft) 0.3048 meter

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

foot per day per foot [(ft/d)/ft] 1.00 meter per day per meter

mile (mi) 1.609 kilometer

foot squared per day (f^/d) ^ 0.0290 meter squared per day

square mile (mi2) 2.590 square kilometer

acre 0.4047 hectare

gallon per minute (gal/min) 0.06308 liter per second

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

y The standard unit for transmissivity is cubic foot per day per square foot times foot of aquifer thickness 

[ft3/d)/ft2]ft. In this report, the mathematically reduced form, foot squared per day (fr/d), is used for 

convenience.

Sea level: In this report, "sea level" refers to the National Geodetic Vertical Datum of 1929 (NGVD of 1929) 

a geodetic datum derived from a general adjustment of the first-order level nets of the United States and Canada, 

formerly called Sea Level Datum of 1929.

Acronyms

GIS = geographical information system 

MMSP = modular model statistical processor 

RASA = Regional Aquifer System Analysis 

RMSE = root mean square error

SWFWMD = Southwest Florida Water Management District 

USGS = U.S. Geological Survey

VI Contents
Hydrogeology and Simulated Effects of Ground-Water 

Withdrawals for Citrus Irrigation, Hardee and De Solo 

Counties, Florida

By P.A. Metz

Abstract

The hydrogeology of Hardee and De Soto 

Counties in west-central Florida was evaluated, 

and a ground-water flow model was developed to 

simulate the effects of expected increases in 

ground-water withdrawals for citrus irrigation on 

the potentiometric surfaces of the intermediate 

aquifer system and the Upper Floridan aquifer. In 

1988, total citrus acreage in Hardee and De Soto 

Counties was 89,041 acres. By the year 2020, 

citrus acreage is projected to increase to 130,000 

acres.

Ground water is the major source of water 

supply in the study area, and 94 percent of the 

ground-water withdrawn in the area is used for 

irrigation purposes. The principal sources of 

ground water in the study area are the surficial 

aquifer, the intermediate aquifer system, and upper 

water-yielding units of the Floridan aquifer 

system, commonly referred to as the Upper Floridan aquifer. The surficial aquifer is a permeable 

hydrogeologic unit contiguous with land surface 

that is comprised predominately of surficial quartz 

sand deposits that generally are less than 100 feet 

thick. The intermediate aquifer system is a somewhat less permeable hydrogeologic unit that lies 

between and retards the exchange of water 

between the overlying surficial aquifer and the 

underlying Upper Floridan aquifer. Thickness of 

the intermediate aquifer system ranges from about 

200 to 500 feet and transmissivity ranges from 400

to 7,000 feet squared per day. The highly productive Upper Floridan aquifer consists of 1,200 to 

1,400 feet of solution-riddled and fractured limestone and dolomite. Transmissivity values for this 

aquifer range from 71,000 to 850,000 feet squared 

per day. Wells open to the Upper Floridan aquifer, 

the major source of water in the area, can yield as 

much as 2,500 gallons of water per minute.

The potential effects of projected increases in 

water withdrawals for citrus irrigation on groundwater heads were evaluated by the use of a quasithree-dimensional, finite-difference, ground-water 

flow model. The model was calibrated under 

steady-state conditions to simulate September 

1988 heads and under transient conditions to simulate head fluctuations between September 1988 

and September 1989. The calibrated model was 

then used to simulate hydraulic heads for tH years 

2000 and 2020 that might result from projected 

increases in pumpage for citrus irrigation.

The model simulation indicated that increased 

pumpage might be expected to result in:

 A maximum decline of more than 10 feef in the 

intermediate aquifer system at a proposed 

grove in eastern De Soto County and an 

average decline of more than 2 feet in much 

of the study area.

 An increase in downward leakage to the intermediate aquifer system from the overlying 

surficial aquifer system from 178 to 1F3 

million gallons per day.

Abrtract
A decrease in upward leakage from the intermediate aquifer system to the surficial aquifer 

from 1.58 to 1.47 million gallons per day.

A maximum decline of about 5 feet in the Upper 

Floridan aquifer at a proposed grove in eastern 

De Soto County and a decline of more than 

2 feet in much of the model area.

An increase in downward leakage to the Upper 

Floridan aquifer from the intermediate aquifer 

system from 180 to 183 million gallons per 

day.

A decrease in upward leakage from the Upper 

Floridan aquifer to the intermediate aquifer 

system from 4.32 million gallons per day in 

1989 to 3.89 million gallons per day in the year 

2,000, but an increase in upward leakage to 

5.10 million gallons per day by the year 2020, 

reflecting a change in hydraulic gradient.

this aquifer system are greater than those of the surficial aquifer, but are much less than those of the deeper 

Upper Floridan aquifer. The Upper Floridan aquifer is 

the principal source of water supply in the study area. 

Water withdrawn from the Upper Floridan aquifer is 

used for irrigation and industrial, public, arid domestic 

supply. Wells open to the Upper Floridan aquifer yield 

large quantities of freshwater; however, dissolvedsolids concentrations exceed limits fcr potable supply 

in the southern half of the study area.

As the demand for water in Hardee and De Soto 

Counties increases, additional information about the 

aquifers is needed to manage and develop the water 

supply effectively. The U.S. Geological Survey 

(USGS), in cooperation with the Southwest Florida 

Water Management District (SWFWMD), conducted 

a study from 1987 to 1990 to evaluate the effects 

of increased citrus irrigation on the ground-water 

resources of Hardee and De Soto Counties. The results 

of the study are summarized in this renort.

INTRODUCTION

Several periods of below freezing temperatures 

during the 1980's in northern and central Florida 

resulted in extensive damage to Florida's citrus crops. 

To avoid future crop damage, many citrus growers 

relocated to counties farther south, including Hardee 

and De Soto Counties. As a result, citrus acreage in 

Hardee and De Soto Counties has increased from 

77,966 acres in 1978 to 89,041 acres in 1988 (Marella, 

1992, table 13). The Southwest Florida Water Management District projects that total citrus acreage in those 

two counties will increase to more than 130,000 acres 

by the year 2020 (Taylor and others, 1990).

Ground water, the principal source of water supply 

in Hardee and De Soto Counties, is obtained from three 

aquifers in the study area: the surficial aquifer, the 

intermediate aquifer system, and the Upper Floridan 

aquifer. The surficial aquifer has limited use because of 

the low yield to wells and the potential for contamination. Water withdrawn from the surficial aquifer is used 

primarily for lawn irrigation and stock watering. The 

intermediate aquifer system is used extensively in 

some parts of Hardee and De Soto Counties as a source 

of water for irrigation and public and domestic supply. 

The yield to wells and total withdrawals of water from

Purpose and Scope

This report presents the results of a study to 

describe the hydrogeology and ground-water flow of 

the multiaquifer system in Hardee and De Soto Counties. 

A digital ground-water flow model wrs developed and 

used to simulate the effects of anticipated increased 

ground-water withdrawals for citrus irrigation. A description of the hydrogeologic framework of the study 

area is presented, the long-term water-level trends are 

defined, and the results of model simulations of possible future pumping scenarios are described with 

respect to the intermediate aquifer system and the 

Upper Floridan aquifer.

Previous Investigations

Numerous investigations have contributed to an 

understanding of the geology, hydrogeology, and 

ground-water resources of Hardee and De Soto Counties. The geology of Hardee and De Soto Counties was 

described in reports by Bergendahl (1956), Puri and 

Vernon (1964), White (1970), Wilson (1977), and Scott 

(1988). Reports presenting results of investigations by 

Wilson (1972), Wolansky and Corral (1985), Duerr and

2 Hydrogeology and Simulated Effects of Ground-Water Withdrawals for Citrus Irrigation, Hardee and De So*o Counties, Fla.
Wolansky (1986), Miller (1986), Aucott (1988), Duerr 

and others (1988), and Duerr and Enos (1991) describe 

the hydrogeology of the study area. Appraisals of the 

ground-water resources for the study area were 

included in reports by Kaufman (1967), Hutchinson 

(1978), Robertson and others (1978), Leach and Healy 

(1980), Duerr and Trommer (1982), Duerr and Sohm 

(1983), and Yobbi (1983). Reports that presented 

results of computer simulations of ground-water flow 

in or near the study area included those by Wilson 

(1977), Ryder (1982; 1985), Wilson and Gerhart 

(1982), and Tibbals (1990). A summary of the hydrology of the Floridan aquifer system, was presented by 

Johnston and Bush (1988).

Description of the Study Area

Hardee and De Soto Counties are in west-central 

Florida and have a combined area of 1,371 mi 2 . The location of Hardee and De Soto Counties is shown in figure 1 

along with the boundaries of the study area included in 

the ground-water flow model that will be discussed in 

subsequent sections of this report. The study area lies 

entirely in the midpeninsular physiographic zone 

described by White (1970) and includes parts of three 

subdivisions: the Polk Upland, the De Soto Plain, and 

the Gulf Coastal Lowlands (fig. 2). The physiographic 

subdivisions correspond approximately to several marine 

terraces or plains that were formed by the invasion of seas

Florida

HARDEE COUNTY

STUDY AREA

DE SOTO COUNTY

82°09'15" 

27 °43'57"

81°22'

26°56'30"

(POLK COUNTY

y* KILOMETERS

Figure 1. Location of the study and model area.

Introduction
MODEL 

BOUNDARY

Figure 2. Physiographic subdivisions and boundaries of study and model area. (Modified from White, 1970.)

during the Pleistocene Epoch. The inland boundary of 

each subdivision is delineated by a low scarp or break in 

slope that represents the position of a former marine 

shoreline (Wilson, 1977). The Polk Upland is a broad, 

sandy area that ranges in altitude from 100 to 245 ft above 

sea level. A large part of the study area lies within the 

gently sloping De Soto Plain and ranges in altitude from 

30 to 100 ft above sea level. The Gulf Coastal Lowlands 

subdivision, which encompasses a large part of the Peace 

River valley, consists of poorly drained, low-lying land at 

altitudes of 30 to 40 ft above sea level in central and 

southwestern De Soto County (Wilson, 1977). East of the 

study area is the Lake Wales Ridge, a major ground-water 

recharge area. Altitudes of the Lake Wales Ridge range 

from 100 to 245 ft above sea level. This long, narrow 

ridge is a series of subparallel, eroded, sandy ridges with 

intervening valleys that contain numerous lakes.

The study area is drained by the Peace River, which 

flows southward for about 70 mi from its source in Polk 

County to Charlotte Harbor in Charlotte County. Major 

tributaries that flow into the Peace River include Payne 

Creek, Charlie Creek, Joshua Creek, Horse Creek, and 

Prairie Creek (fig. 1).

Land Use

In 1988, 53 percent of the land in Hardee and 

De Soto Counties was agricultural, 31 percent was 

prairie grassland, and less than 4 percent was urban; 

the remaining 12 percent was undeveloped. Land-use 

data were obtained from the De Soto County 1987 

comprehensive plan and from LANDS AT data imagery

photographs. Agricultural land in Hardee and De Soto 

Counties is primarily in pasture, citrus groves, cropland, and nurseries. Citrus groves, which constituted 

the second largest agricultural land-use in the study 

area, covered 45,898 acres in Hardee County and 

43,143 acres in De Soto County in 1988 (Florida 

Agricultural Statistical Service, 1989). Generalized 

locations of citrus groves in Hardee and De Soto 

Counties for 1988 are shown in figure 3. Future landuse projections by the SWFWMD indicate that a significant number of citrus growers will relocate southward 

into Hardee and De Soto Counties (Taylor and others, 

1990). Citrus groves and other agricultural land-use 

projections for Hardee and De Soto Counties for 1990, 

1995, 2000, 2010, and 2020 are shown in figure 4.

Climate

The climate of the study area is subtropical humid 

and is characterized by warm, relatively wet summers 

and mild, relatively dry springs. Rainfall averages 

about 53 in. per year and varies seasonally with more 

than half the annual rainfall occurring frorr June 

through September (Palmer and Bone, 1977). Rainfall 

tends to be distributed unevenly throughout the area 

during the summer because most summer rainfall in 

Florida is derived from localized, convective thunderstorms. Winter rainfall commonly is more evenly distributed throughout the study area because it generally 

results from frontal-type air masses that move from 

north to south across the State.

Hydrogeology and Simulated Effects of Ground-Water Withdrawals for Citrus Irrigation, Hardee and De Soto Counties, Fla.
82° 09'15" 81° 22'

27° 43'57'

26° 56' 30'

HILLSBOROUGH 

COUNTY

CHARLOTTE 

COUNTY

HARDEE COUNTY 

SOTO COUNTY

^

" % HIGHLANDS 

^ COUNTY

I GLADES 

COUNTY

10 20 MILES

10 20 KILOMETERS

EXPLANATION

AREA OF CITRUS GROVES

Figure 3. Generalized location of citrus groves in Hardee and De Soto Counties, 1988. (From LANDSAT date 

imagery photos.)

Introduction 5
Approach

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60,000

50,000

40,000

30,000

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- MELONS HARDEE COUNTY ;

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2020

80,000

70,000 -

60,000

50,000

40,000

30,000 -

20,000

10,000 -

1990 1995 2000 2010 2020

Figure 4. Projected agricultural land use for 1990, 1995, 

2000, 2010, and 2020.

The hydrogeologic framework for the study area 

was defined through analyses of well logs, aquifer tests, 

and water-level measurements. Water-level trends were 

determined by reviewing historical water-level data 

from observation wells within and adjacent to the study 

area. Regional potentiometric-surface maos for the 

intermediate aquifer system and the Uppe1-- Floridan 

aquifer for September 1988, May 1989, ard September 

1989 were used to evaluate seasonal grourd-water flow 

conditions. Quantitative estimates of flow to and from 

the ground-water system were made by calibrating and 

applying a quasi-three-dimensional ground-water flow 

model. Potentiometric-surface levels for the intermediate aquifer system and Upper Floridan aquifer for the 

years 2000 and 2020 were simulated by stressing the 

calibrated model with projected pumpage for citrus 

irrigation.

The model was calibrated and tested using input 

parameters stored in a geographical information system 

(GIS) data base. Except for head values and groundwater withdrawal rates, initial input parameters for the 

model were based on data from a coarse-grid model 

developed as part of a Regional Aquifer System 

Analysis (RASA) study (Ryder, 1985). A finer resolution ground-water flow model was developed from this 

regional scale model and subsequently was used to 

simulate hypothetical pumping scenarios.

The RASA data base was updated bas^d on studies 

by subsequent investigators. Duerr and Enos (1991) 

defined the upper and lower confining units of the 

intermediate aquifer system in the study area from 

geologic logs. The results of an intermediate aquifer 

system aquifer test (Duerr and Enos, 1991) in westcentral Hardee County also was incorporated into the 

hydrogeologic data base.

HYDROGEOLOGIC FRAMEWOF"

The hydrogeologic units underlying the study area 

consist of deposits of sand, clay, marls, ard carbonates 

that were deposited in a marine environment. These 

hydrogeologic units, their equivalent stratigraphic 

units, and brief lithologic descriptions of these units are 

presented in figure 5. Wilson and Gerhart (1982) 

grouped the units into four major lithologic sequences 

of hydrologic significance. From youngest to oldest, 

these sequences are:

Hydrogeology and Simulated Effects of Ground-Water Withdrawals for Citrus Irrigation, Hardee and De Soto Counties, Fla.
Quaternary

Tertiary

Holocene 

and 

Pleistocene

Pliocene

Miocene

Oligocene

Eocene

Paleocene

iiiiiiliiiliipilllii;

Surficial sand, 

terrace sand, 

phosphorite

Undifferentiated 

deposits 1 

Tamiami Formation

Bone Valley 

§ Formation o 

U Peace River

c Formation

_o Arcadia 

 ^ Formation

(0

X Tampa Member

Suwannee Limestone

Ocala Limestone

Avon Park 

Formation

Oldsmar 

and 

Cedar Keys 

Formations

Predominantly fine 

sand; interbedded 

clay, marl, shell, 

and phosphorite.

Clayey and pebbly 

sand; clay, marl, 

shell, phosphatic.

Dolomite, sand, 

clay, and limestone, 

silty, phosphatic.

Limestone, sandy, 

phosphatic, fossiliferous; 

sand and clay in lower 

part in some areas.

Limestone, sandy 

limestone, fossiliferous.

Limestone, chalky, 

foraminiferal, dolomitic 

near bottom.

Limestone and hard 

brown dolomite; 

intergranular evaporite 

in lower part in some 

areas.

Dolomite and limestone 

with intergranular 

gypsum and anhydrite.

Iltlllillll fiiiiiiii

Sand

Clastic

Carbonate 

and clastic

Carbonate

Carbonate 

with 

evaporites

Evaporites

Surficial aquifer

Confining unit

Aquifer

Confining unit

Upper Floridan 

aquifer

Middle confning 

unit

Lower Floridan 

aquifer

iate Aquifer System

Intermed

Aquifer System Floridan 

Sub-Flo'idan 

confininc unit

Includes all or parts of Caloosahatchee Marl and Bone Valley Formation. 

Figure 5. Correlation chart showing hydrogeologic framework. (Modified from Ryder, 1985, table 1.)

1. Surficial sand deposits, generally less than 100 ft 

thick;

2. A heterogeneous clastic and carbonate section of 

interbedded limestone, dolomite, sand, clay, and 

marl generally greater than several hundred feet 

thick;

3. A carbonate section of limestone and dolomite, 

generally more than 1,000 ft thick; and

4. Carbonate rocks containing intergranular anhydrite 

and gypsum.

The first three sequences constitute distinct waterbearing units of interest to this study: the surficial 

aquifer, the intermediate aquifer system, and the Upper 

Floridan aquifer. The Upper Floridan aquifer is underlain by the fourth sequence, the middle confining unit 

of the Floridan aquifer system.

Lines of hydrogeologic section and se'ected 

well sites in Hardee and De Soto Counties are shown 

in figure 6. Variations in the distribution, thickness, 

and dip of the hydrogeologic units are deputed in 

generalized hydrogeologic sections in figure 7.

Hydrogeologic Framework
82°09'15" 81°22;

HILLSBOROUGH 

COUNTY

HARDEE COUNTY 

DE SOTO ^COUNTY 

ROMP 2

26°56'30"

0 10 20 KILOMETERS

EXPLANATION

D D' LINE OF SECTION SHOWN IN FIGURE 7

 W-9177 TEST WELL AND NUMBER

Figure 6. Locations of generalized hydrogeologic sections and selected well sites. (Modified from Duerr and Enos, 1991.) 

8 Hydrogeology and Simulated Effects of Ground-Water Withdrawals for Citrus Irrigation, Hardee and De Soto Counties, Fla.
FEET

CONFINING 

UNITS

WATER

BEARING

UNITS

FLORttMN 

AQUIFER SYSTEM

HYDROGEOLOGIC CONTACT. 

DASHED WHERE INFERRED

VERTICAL SCALE GREATLY EXAGGERA

XWATERBEARING 

UNITS N.

INTERMEDIATE 

AQUIFER 

SYSTEM

FLOHIDAN 

AQUIFER SYSTEM

HYDROGEOLOGIC CONTACT

VERTICAL SCALE GREATLY EXAGGERATED 

0 10 20 MILES

0 10 20 KILOMETERS

Figure 7. Generalized hydrogeologic sections A-A', 6-6', C-C', and D-D'. 

(Modified from Duerr and Enos, 1991. Lines of section are shown in fig. 6.)

Hydrogeologic Framework
C1

^fr 05 z CM f>j oi >*

GO A O IjJ K.. hs, {O

"~ "- ~ O ^ -- v- ,-

FEET ? ? g ^i S ? ? ? FEET

200- 

100 -

LEVEL "

100 -

200-

300 -

400-

500 -

600 -

700 -

800 -

onn .

at i £ *.i

BEARING

UNITS s.

_^ ^

SURFtc7AL-\/

INTERMEDIATE

AQUIFER

SYSTEM

~^

 HYDROGEOLOGIC

CONTACT

c 3

s.

 s

^

C 3

AQUIFER

~--^_____^

CONF

NO ^^*~

FLORIDAN

AQUIFER SYSTEM

1 1

1 / 

'INING

UNITS

-.

\

-200 

.-100

Oir- A

_SEA 

 LEVEL

-100

-200

-300

-- 400

- 500

-600

-700

-800

. . ODD

VERTICAL SCALE GREATLY EXAGGERATED 

0 10 20 MILES

10 20 KILOMETERS D'

J

FEET ; 

200 -\

100 -

SEA 

LEVEL "

100- 

200-

300-

400 --

500-

600-

700- 

800- 

onn . .

i

\ f> Ul 0

& % *$ 2 c

C* Q. «

INTERMEDIATE 

AQUIFER 

SYSTEM

-^

\ i

! s

 ^^

/

CONFINING 

UNITS

i^

° K *

* w c

» 0 C 

6 ^ 5

URFICIAll6u»FEH_

WATER 

./BEARING 

' UNITS >>^s>

^^

FLORIDAN

^

»

>

:

AQUIFER SYSTEM

YDROGEOLOGIC 

CONTACT

FEET 

r 200

L 100

SEA 

r LEVEL

-100 

-200

-300

-400

-500

-600

-700 

-800 

-Qnn

VERTICAL SCALE GREATLY EXAGGERATED

10 20 MILES

10 20 KILOMETERS

Figure 7. Generalized hydrogeologic sections A-A', B-B', C-C', and D-D'.

(Modified from Duerr and Enos, 1991. Lines of section are shown in fig. 6.)-Continued

10 Hydrogeology and Simulated Effects of Ground-Water Withdrawals for Citrus Irrigation, Hardee and De Soto Counties, Fla.
The sections indicate that the units are laterally 

continuous and generally are uniform in thickness. 

The following descriptions of the areal distribution of 

hydraulic characteristics for aquifers and confining 

units are based largely on the results of RASA groundwater flow model investigations reported by Ryder 

(1985) and the results of similar investigations 

described in a later section of this report. A range of 

characteristic values derived from both studies is 

reported herein for each respective unit.

Surficial Aquifer

The unconfined surficial aquifer is the permeable 

hydrogeologic unit contiguous with land surface. This 

aquifer is composed principally of unconsolidated to 

poorly indurated clastic deposits (Southeastern Geological Society, 1986). The surficial aquifer consists of 

predominately fine sand and interbedded clay, marl, 

shell, and phosphorite (fig. 5). More than one permeable zone may be present where these deposits are 

interbedded; where this occurs, the unit commonly is 

termed the surficial aquifer system. However, for purposes of this report, the deposits are considered to form 

a single homogeneous aquifer, which is referred to as 

the surficial aquifer.

Hydraulic Properties

The water-bearing properties of the surficial 

aquifer are largely dependent upon aquifer thickness 

and the grain-size distribution of the sediments within 

the aquifer (Wilson, 1977). Thickness of the deposits 

ranges from about 25 ft in Hardee County to about 

100 ft in northeastern De Soto County (Wolansky and 

others, 1979). Average transmissivity of the surficial 

aquifer is estimated to be 1,100 ft2/d on the basis of 

an average hydraulic conductivity of 20 ft/d and an 

average saturated thickness of 55 ft (Wilson, 1977). 

The surficial aquifer is an insignificant source of 

water supply when compared to the thicker and more 

transmissive underlying aquifers.

Water Table

During years of normal rainfall in the study area, 

the altitude of the water table in the surficial aquifer 

probably is similar to that shown for September 1988 in 

figure 8. Based on this figure, the altitude of the water 

table in the study area ranges from about 20 ft above 

sea level in southwestern De Soto County to about

125 ft above sea level in northeastern Hardee County. 

The water-table contours in figure 8 were based on field 

measurements of water levels in selected wells, on 

river and lake elevations, and on estimates brsed on 

land-surface elevations from USGS topographic maps 

(scale 1:24,000). Water levels were estimated to be at 

or a few feet below land surface in swampy zreas, at 

depths of 5 to 10 ft below land surface for the lowlands 

plain area, and 15 to 20 ft below land surface along the 

Lake Wales Ridge. Relatively moderate water-table 

gradients exist near the major stream courses, and 

gentle gradients exist in the broad interstream areas.

The water table fluctuates with seasonal rainfall 

(fig. 9). Water-table altitudes are highest during the wet 

season, June through September, and lowest during the 

dry season, October through May. Long-terrr hydrographs for wells completed in the surficial aquifer (fig. 9) 

indicate that seasonal fluctuations of 3 to 5 ft are common and that recharge from the summer rains generally 

is adequate to replenish the aquifer.

Ground-water movement in the surficial aquifer 

involves a complex interrelation between recharge, 

runoff, infiltration, discharge, and evapotranspiration. 

The surficial aquifer is recharged directly by rainfall 

that annually averages about 53 in. in the study area 

(Palmer and Bone, 1977). Most of the rain that falls in 

the study area drains to local streams or is lost to evapotranspiration. Some rainfall, however, percolates down 

through the surficial deposits and enters the surficial 

aquifer. Recharge to the surficial aquifer also includes 

some downward percolation of septic-tank effluent and 

irrigation water and by upward leakage of water from 

underlying aquifers in areas where the potentiometric 

surfaces of the intermediate aquifer system and the 

Upper Floridan aquifer are above the water table. 

Discharge from the surficial aquifer is by pumpage; 

seepage to lakes, streams, and ditches; downward 

leakage to lower aquifers where the hydraulic gradient 

is downward; and evapotranspiration from thr water 

table.

Intermediate Aquifer System

The intermediate aquifer system includes all waterbearing units (aquifers) and confining units between 

the overlying surficial aquifer and the underlying 

Upper Floridan aquifer (Duerr and others, 1988). The 

intermediate aquifer system consists of the undifferentiated deposits of Pleistocene and Pliocene age and the 

Hawthorn Group of Pliocene and Miocene age (fig. 5).

Hydrogeologic Framework 11
82°09'15" 

27° 43'57

26° 56' 30'

HILLSBOROUGH

COUNTY

10 20 MILES

10 20 KILOMETERS

EXPLANATION

 25 WATER-TABLE CONTOUR Shows altitude 

of the water-table in the surficial aquifer, 

in feet above sea level. Contour interval 

25 feet

 WELL CONTROL POINT

Figure 8. Altitude of the water table in the surficial aquifer, September 1988.

12 Hydrogeology and Simulated Effects of Ground-Water Withdrawals for Citrus Irrigation, Hardee and De Sotc Counties, Fla.
LU 

>

LU

_J

< 

LU 

</> 

LJJ

O 

CO

LJJ 

LJJ

LJJ

LJJ

_l

CC 

LJJ

145

140

135

130

80

75

70

65

40

35

30

25

DEPTH 15 FEET 

CASING 10 FEET

ROMP 40 SHALLOW WELL

1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988

DEPTH 15 FEET 

CASING 10 FEET

ROMP 26 SHALLOW WELL

1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988

DEPTH 25 FEET 

CASING 19 FEET

BIG SLOUGH SHALLOW WELL

1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988

Figure 9. Long-term water-level trends in surficial aquifer wells in or near the study area, 1978-88. (Locations of 

wells are shown in fig. 8.)

Hydrogeologic Framework 13
The intermediate aquifer system consists of at least 

three hydrogeologic units (fig. 5): a clayey and pebbly 

sand, clay, and marl upper confining unit that separates 

the uppermost water-bearing unit in the intermediate 

aquifer system from the surficial aquifer; one to three 

water-bearing units composed primarily of carbonate 

rocks, sand, and discontinuous beds of sand and clay; 

and a sandy clay or clayey sand lower confining unit 

that lies directly over the Upper Floridan aquifer 

(Ryder, 1985). The diversity in lithology of the intermediate aquifer system reflects the variety of depositional environments in west-central Florida that 

occurred during the Pliocene and Miocene Epochs. 

These environments included open-marine, shallowwater, coastal-marine, and fluvial and estuarine 

processes (Gilboy, 1985).

Hydraulic Properties

The intermediate aquifer system underlies all of 

Hardee and De Soto Counties and is hydraulically 

separated from the surficial aquifer by the upper confining unit. The upper confining unit has a low vertical 

hydraulic conductivity and, consequently, retards 

interaquifer flow. For most of the study area, however, 

the upper confining unit does transmit, or leak, water 

downward from the surficial aquifer into the intermediate aquifer system, and the system is referred to as a 

leaky-aquifer system (Wilson, 1977). Water is also 

transmitted upward through the upper confining unit 

into the surficial aquifer and Peace River where the 

hydraulic gradient is upward. Areas of upward and 

downward leakage through the upper confining unit of 

the intermediate aquifer system in September 1988 are 

shown in figure 10. The thickness of the intermediate 

aquifer system is shown in figure 11. Thickness of the 

intermediate aquifer system ranges from about 200 ft in 

northeastern Hardee County to about 500 ft in southern 

De Soto County (Duerr and Enos, 1991).

The thickness of the upper confining unit varies 

widely in the study area and ranges from less than 25 ft 

to about 265 ft (Duerr and Enos, 1991) (fig. 12). Little is 

known about the areal variations in hydraulic conductivity and hydraulic properties of the upper confining unit 

in the study area. Ryder (1985, p. 20) reported that the 

leakance (the ratio of an estimated vertical hydraulic 

conductivity of the confining unit to its thickness), as 

derived from RASA flow-model investigations, ranges 

from 3x10~3 to IxlO'5 (ft/d)/ft for the upper confining 

unit in the study area. Leakance values used in this study 

ranged from 3xlO'3 to IxlO'6 (fig. 12).

The water-bearing units of the intermediate aquifer 

system consist of limestone and dolomite. Clay beds of 

variable lateral extent and thickness can occur within 

the water-bearing units of the intermediate aquifer 

system (Duerr and Enos, 1991). Transmissivity of these 

water-bearing units in Hardee and De Soto Counties, as 

determined from aquifer tests, ranges frorr. 400 to 

7,000 ft2/d (Ryder, 1982). The highest transmissivity in 

the study area is in areas adjacent to the Peace River, 

indicating that a more active flow system exists where 

ground water moves upward into the river and 

enhances development of secondary porosity in the 

carbonate rock (Ryder, 1985). Areally disHbuted 

values of transmissivity for the intermedia*? aquifer 

system are shown in figure 13. The wide range of transmissivity values for the intermediate aquifer system 

indicates formational heterogeneity that is substantiated by geophysical logs (Hutchinson, 1978). Storage 

coefficients for the intermediate aquifer system were 

estimated to range from 2.0x10'4 to 5.0xlO"4.

The lower confining unit lies at the base of the 

intermediate aquifer system and hydraulically separates 

the water-bearing deposits of the intermediate aquifer 

system from the underlying Upper Floridan aquifer. The 

lower confining unit has a low vertical hydraulic conductivity and consequently retards interaquifer flow. For 

most of the study area, however, the lower confining unit 

does allow water to leak downward from tha, intermediate aquifer system into the Upper Floridan aquifer. In 

some areas, hydraulic gradients are such that water is 

transmitted from the Upper Floridan aquife" upward 

through the lower confining unit into the intermediate 

aquifer system. Areas of upward and downward leakage 

through the lower confining unit of the intermediate 

aquifer system in September 1988 are showr in figure 14.

The thickness of the lower confining unit of the 

intermediate aquifer system varies widely in the study 

area and ranges from less than 25 ft to 185 ft (Duerr and 

Enos, 1991) (fig. 15). Little is known about the areal 

variations in hydraulic conductivity and other hydraulic properties of the lower confining unit in the study 

area. Ryder (1985, p. 16) reported leakance values for 

this unit derived from RASA ground-water flow 

investigations model that range from 3xlO'4 to 7xlO'5 

(ft/d)/ft over most of the study area (fig. 15). Leakance 

values determined in this study, range from 1x10 to 

lx!0'5 (ft/cD/ft (fig. 15). Wilson (1977) reported that 

the effectiveness of this unit as a confining unit is 

variable. The variability in lithology and thickness of 

the lower confining unit result in a wide variation in the 

amount of leakage occurring through this unit.

14 Hydrogeology and Simulated Effects of Ground-Water Withdrawals for Citrus Irrigation, Hardee and De Soto Counties, Fla.
82°09'15"

27°43' 57"

81° 22'

26°56' 30"

liiiiiiii

10 20 MILES

10 20 KILOMETERS 

EXPLANATION

AREA OF DOWNWARD LEAKAGE 

AREA OF UPWARD LEAKAGE

Figure 10. Areas of upward and downward leakage through the upper confining unit of the intermediate aquife' system, 

September 1988.

Hydrogeoiogic Framework 15
82°09'15" 

27°43' 57'

26° 56' 30"

HILLSBOROUGH 

COUNTY

10 20 MILES

10 20 KILOMETERS

EXPLANATION

 500 LINE OF EQUAL THICKNESS OF 

THE INTERMEDIATE AQUIFER 

SYSTEM Interval is 100 feet

WELL CONTROL POINT 

Figure 11 . Thickness of the intermediate aquifer system. (Modified from Duerr and Enos, 1991; and Duerr and others, 1988.

16 Hydrogeology and Simulated Effects of Ground-Water Withdrawals for Citrus Irrigation, Hardee and De Soto Counties, Fla.
82°09'15" 81° 22'

HARDEE COUNTY 

SOTCTCOUNTY

X [ HIGHLANDS

><,. COUNTY

LEAKANCE OF THE LOWER CONFINING 

UNIT, IN FEET PER DAY PER FOOT

[ | I.0xicr6 to9.9xicr6 

HI - 1.0x10-5 to9.9x10'5

10'

10 20 KILOMETERS 

EXPLANATION

25 LINE OF EQUAL THICKNESS OF THE 

LOWER CONFINING UNIT OF THE 

INTERMEDIATE AQUIFER SYSTEM 

Dashed where approximately located. 

Interval 50 feet

 CONTROL POINT

Figure 12. Thickness and model-derived leakance of the upper confining unit of the intermediate aquifer system. 

(Modified from Duerr and Enos, 1991; and Ryder, 1985.)

Hydrogeologlc Framework 17
82°09'15" 81° 22'

27°43' 57"F

26°56' 30

10 20 MILES

10

TRANSMISSIVITY BASED ON MODEL 

CALIBRATION, IN FEET SQUARED 

PER DAY

20 KILOMETERS

EXPLANATION

300

400

700

1,500

3,000

4,000

7,000

10,500

 2,900 TRANSMISSIVITY DETERMINED 

FROM AQUIFER TESTS AND 

ESTIMATED FROM SPECIFIC 

CAPACITY TESTS, IN FEET 

SQUARED PER DAY

Figure 13. Transmissivity of the intermediate 

aquifer system. (Modified from Ryder, 1985.)

18 Hydrogeology and Simulated Effects of Ground-Water Withdrawals for Citrus Irrigation, Hardee and De Soto Counties, Ra.
82°09'15"

26°56' 30"

10 20 MILES

10 20 KILOMETERS

EXPLANATION

III AREAS OF DOWNWARD LEAKAGE 

H| AREAS OF UPWARD LEAKAGE

Figure 14. Areas of upward and downward leakage through the lower confining unit of the intermediate aquifer system, 

September 1988.

Hydrogeologic Framewo«-k 19
82°09'15" 81° 22'

27°43' 57"

26°56' 30"

10 20 MILES

LEAKANCE OF THE LOWER 

CONFINING UNIT, IN FEET 

PER DAY PER FOOT

HJ 1.0x 10'4 to3.0x1Q-3 

ii! 1.0x10'5 to9.9x10-5

10 20 KILOMETERS 

EXPLANATION 

 25- LINE OF EQUAL THICKNESS OF THE 

LOWER CONFINING UNIT OF THE 

INTERMEDIATE AQUIFER SYSTEM 

Interval 50 feet

THICKNESS CONTROL POINT

Figure 15. Thickness and leakance of the lower confining unit of the intermediate aquifer system. 

(Modified from Duerr and Enos, 1991; and Ryder, 1985.)

20 Hydrogeology and Simulated Effects of Ground-Water Withdrawals for Citrus Irrigation, Hardee and De Sot- Counties, Ra.
Potentiometric Surface and Water-Level 

Fluctuations

The potentiometric surface is an imaginary surface 

representing the level to which water will rise in tightly 

cased wells (Lohman and others, 1912). The potentiometric surface of the intermediate aquifer system in 

west-central Florida is mapped semiannually by the 

USGS in cooperation with the SWFWMD during periods when water levels are at their highest (September) 

and lowest (May). These maps contain potentiometric 

contours based on synoptic measurements of water 

levels in hundreds of wells open to the intermediate 

aquifer system.

The potentiometric surface of the intermediate 

aquifer system in September 1988 for Hardee and 

De Soto Counties and adjacent areas is shown in figure 

16. This potentiometric surface represents conditions 

near the end of the summer rainy season at a time when 

the aquifer is generally unstressed by irrigation pumpage. In September 1988, the potentiometric surface 

ranged from about 120 ft above sea level in northwestern Hardee County to about 40 ft above sea level in 

southwestern Hardee County and northwestern De 

Soto County (fig. 16). Barr (1989a) reported that water 

levels in September 1988 were an average of about 2 ft 

higher than the levels measured in September 1987. 

Major features of the potentiometric surface in figure 

16 are the potentiometric-surface highs in the northeastern and northwestern parts of the area and the relatively gentle hydraulic gradients throughout most of 

Hardee and De Soto Counties.

The potentiometric surface of the intermediate 

aquifer system in May 1989 is shown in figure 17. 

This potentiometric surface represents conditions near 

the end of a dry season during which extensive irrigation pumpage had occurred. In May 1989, the 

potentiometric surface ranged from about 110 ft above 

sea level in northwestern Hardee County to about 5 ft 

above sea level in southwestern Hardee County (fig. 

17). May 1989 water levels reported by Barr (1989b) 

averaged about 5 ft lower than the May 1988 levels 

reported by Lewelling (1989). This decline was the 

result of below normal rainfall and heavy seasonal 

ground-water withdrawals for irrigation. The major 

feature of the potentiometric-surface contours in figure 

17 is a closed depression in southwestern Hardee and 

northwestern De Soto Counties. The closed depression 

is the result of large ground-water withdrawals for 

agriculture.

Major changes in the potentiometric surface from 

September 1988 to May 1989 included an overall 

decline in the potentiometric surface and the development of the closed depression described earlier. A comparison of figures 16 and 17 indicates the pctentiometric 

surface declined about 10 ft along the ridge in the eastern part of the study area and about 35 ft in the area of 

the closed depression.

The potentiometric surface of the intermediate 

aquifer system in September 1989 is shown in figure 18. 

The September maps for 1988 and 1989 show similar 

configurations; however, September 1989 potentiometric levels averaged about 4 ft lower thar corresponding September 1988 levels (Knochenmus and Barr, 

1990a). Rainfall for the study area was 15 in. below 

normal for the period September 1988 to September 

1989 (Southwest Florida Water Management District, 

1989), resulting in additional demands on the groundwater resources for irrigation (Knochenmus and Barr, 

1990a).

Fluctuations and long-term trends of vater levels 

in two wells open to the intermediate aquifer system 

are shown in figure 19. The locations of these wells are 

shown in figure 18. In general, these hydrographs show 

that (1) the altitude of the potentiometric surface changes 

dramatically in response to changes in discharge and 

recharge; (2) there is a slight downward trend in water 

levels from 1970 through 1988; (3) the trends in the 

water levels for the two wells open to the intermediate 

aquifer system are similar; and (4) the average annual 

water-level fluctuations in the wells are as much as 

35 ft for the Rowell deep well and 30 ft for the Marshall 

deep well.

Floridan Aquifer System

The Floridan aquifer system, as defined by Miller 

(1986, p. 44), is a vertically continuous sequence of 

carbonate rocks of generally high permeability that are 

hydraulically connected in varying degree? and are 

characterized by permeabilities generally an order to 

several orders of magnitude greater than those rocks 

bounding the system above and below. The Floridan 

aquifer system in the study area consists of two aquifers: the Upper Floridan aquifer, which contains freshwater, and the Lower Floridan aquifer, which contains 

highly mineralized water. The Upper and Lower Floridan aquifers are separated by the middle confining unit 

(Miller, 1986). The Upper Floridan aquife^ commonly 

consists of a few highly permeable zones separated by 

less permeable zones (Johnston and Bush, 1988).

Hydrogeologic Framework 21
82°09'15" 

27° 43'571

81° 22'

26° 56' 30'

POLK COUNTY

HILLS- / 

BOROUGH 

COUNTY

10 20 MILES

10 20 KILOMETERS

EXPLANATION

40 POTENTIOMETRIC CONTOUR 

Shows altitude at which water level 

stands in tightly cased wells in the 

intermediate aquifer system. Contour 

interval 10 and 20 feet. Datum is 

sea level

WELL CONTROL POINT

Figure 16.- Potentiometric surface of the intermediate aquifer system, September 1988. 

(Modified from Barr, 1989a.)

22 Hydrogeology and Simulated Effects of Ground-Water Withdrawals for Citrus Irrigation, Hardee and De Soto Counties, Fla.
82° 09'15" 

27° 43'571

f 1° 22'

26° 56' 30"

HILLSBOROUGH 

COUNTY

HARDEE COUNTY 

DE SOTO COUNTY

, HIGHLANDS 

COU_NTY_

GLADES 

COUNTY

10 20 MILES

10 20 KILOMETERS

EXPLANATION

 30 POTENTIOMETRIC CONTOUR 

Shows altitude at which water level 

stands in tightly cased wells in the 

intermediate aquifer system. Contour 

interval 5, 10, and 20 feet. Hachures 

indicate depressions. Datum is sea 

level

 WELL CONTROL POINT

Figure 17. Potentiometric surface of the intermediate aquifer system, May 1989. 

(Modified from Barr, 1989b.)

Hydrogeologic Framework 23
82° 09'15" 

27° 43'57

26° 56' 30'

HILLSBOROUGH 

COUNTY

HARDEE COUNTY 

DE SOTO COUNTY

10 20 MILES

10 20 KILOMETERS

EXPLANATION

 40 POTENTIOMETRIC CONTOUR 

Shows altitude at which water level 

stands in tightly cased wells in the 

intermediate aquifer system. Contour 

interval 10 and 20 feet. Datum is 

sea level

WELL CONTROL POINT

Figure 18. Potentiometric surface of the intermediate aquifer system, September 1989. 

(Modified from Knochenmus and Barr, 1990a.)

24 Hydrogeology and Simulated Effects of Ground-Water Withdrawals for Citrus Irrigation, Hardee and De Soto Counties, Fla.
Throughout much of the study area, however, there is 

enough vertical interconnection between the permeable 

zones for these zones to function as a single hydrogeologic unit (Ryder, 1985). The top of the Upper Floridan 

aquifer is the horizon below which carbonate rocks 

consistently occur. The base of the Upper Floridan 

aquifer, the middle confining unit, is characterized by 

limestone with a drastically reduced permeability due 

to the presence of intergranular evaporites (Southeastern Geological Society, 1986). In the study area, the 

rocks below the middle confining unit have relatively 

low transmissivity and commonly do not contain 

freshwater (Ryder, 1985). Only the freshwater part of

72

62

52

[Ij 42 

LLJ

LLJ 32

03

LLJ

§ 22

LLJ LLJ

iiiiiiiiiir 

INTERMEDIATE AQUIFER SYSTEM

ROWELL DEEP WELL 

DEPTH 267 FEET 

CASING 30 FEET

1970 1972 1974 1976 1978 1980 1982 1984 1986

60

LLJ

LLI 50

or

LLJ I- 

< 40

30

20

10

UPPER FLORIDAN AQUIFER AND 

INTERMEDIATE AQUIFER SYSTEM

MARSHALL DEEP WELL 

DEPTH 478 FEET 

CASING 137 FEET

1970 1972 1974 1976 1978 1980 1982 1984 1986

Figure 19. Long-term water-level trends for selected 

intermediate aquifer system wells, 1970-88. (Locations 

of wells are shown in fig. 18.)

the Floridan aquifer system, the Upper Floridan aquifer, 

is of interest in this report. The base of tH freshwater 

flow system is considered the top of the middle 

confining unit.

Hydraulic Properties

The Upper Floridan aquifer, which is the most 

productive and widely used aquifer in th?. study area, 

consists of the Suwannee Limestone, the Ocala Limestone, and the Avon Park Formation (fig. 5). Thickness 

of the Upper Floridan aquifer ranges from 1,200 ft in 

northeastern Hardee County to 1,400 ft in western 

Hardee and De Soto Counties (fig. 20). The Upper 

Floridan aquifer consists of limestone and dolomite 

containing solution-enlarged fractures that commonly 

yield abundant supplies of water to well?. The most 

productive part of the aquifer generally occurs in a 

fractured dolomite section within the Avon Park 

Formation. The fractured dolomites in this unit are 

the principal sources of water to large-rapacity 

irrigation wells in the study area (Wilson and 

Gerhart, 1982).

The areal distribution of transmissiv'ty of the 

Upper Floridan aquifer, as determined from aquifer 

tests and specific capacity tests (point values) and 

results of flow model calibration is shown in figure 21. 

Transmissivities determined from aquife" tests of the 

Upper Floridan aquifer range from 70,6CO ft2/d in 

central Hardee County to 850,000 ft2/d in northeastern 

De Soto County (fig. 21). The large rang0, in transmissivities is characteristic of fractured-rock aquifers and 

could be due to variations in the number and size of 

fractures intercepted by the test well or variations in the 

extent of the aquifer penetrated by the w°ll. Storage 

coefficients for the Upper Floridan aquifer were 

estimated to range from l.OxlO"4 to 1.2xlO'4.

Potentiometric Surface and Water-Lev?I 

Fluctuations

The potentiometric surface of the Up^er Floridan 

aquifer in west-central Florida is mapped semiannually 

by the USGS in cooperation with the SMTWMD 

during periods when water levels are at their highest 

(September) and lowest (May). These maps contain 

potentiometric contours based on water levels in 

hundreds of wells open to the Upper Floridan aquifer.

The potentiometric surface of the Up^er Floridan 

aquifer in September 1988 for Hardee and De Soto 

Counties and adjacent areas is shown in figure 22.

Hydrogeologic Framework 25
82° 09'15" 81° 22'

27° 43' 57

26° 56' 30"

HILLSBOROUGH 

COUNTY

-1,200

HARDEEjCOUNTY__ 

DE SOTO COUNTY *

10 20 MILES

10 20 KILOMETERS

EXPLANATION

 1,200 LINE OF EQUAL THICKNESS OF THE UPPER 

FLORIDAN AQUIFER Interval 100 feet

 LINE SHOWING APPROXIMATE EXTENT OF THE 

MIDDLE CONFINING UNIT THAT FORMS THE 

BASE OF THE UPPER FLORIDAN AQUIFER. SOUTH 

OF THIS LINE THE FLORIDAN AQUIFER SYSTEM 

IS NOT DIFFERENTIATED INTO UPPER AND 

LOWER FLORIDAN AQUIFERS

WELL CONTROL POINT

Figure 20. Thickness of the Upper Floridan aquifer. (Modified from Miller, 1982.)

26 Hydrogeology and Simulated Effects of Ground-Water Withdrawals for Citrus Irrigation, Hardee and De Soto Counties, Fla.

82° 09'15" 

27° 43'571

81° 22'

HILLSBOROUGH 

COUNTY

HARDEE_COJJNTY 

DE SOTO COUNTY

10 20 MILES

10 20 KILOMETERS

EXPLANATION

 50 POTENTIOMETRIC CONTOUR 

Shows altitude at which water level 

stands in tightly cased wells in the 

Upper Floridan aquifer. Contour 

interval 10 feet. Hachures indicate 

depressions. Datum is sea level

 WELL CONTROL POINT 

Figure 22. Potentiometric surface of the Upper Floridan aquifer, September 1988. (Modified from Barr, 19P9b.)

28 Hydrogeology and Simulated Effects of Ground-Water Withdrawals for Citrus Irrigation, Hardee and De Soto Counties, Fla.
The potentiometric surface depicted in figure 22 represents conditions near the end of the summer rainy season 

when the aquifer generally is unstressed by irrigation 

pumping. In September 1988, the potentiometric surface 

ranged from about 80 ft above sea level in northeastern 

Hardee County to about 40 ft above sea level in southwestern Hardee County and northwestern De Soto 

County (fig. 22). Barr (1989c) reported that water levels 

in September 1988 were an average of about 2 ft higher 

than the levels measured in September 1987. Major 

features of the potentiometric surface in figure 22 are the 

potentiometric-surface highs in the northeastern part of 

the study area and the relatively gentle hydraulic gradients throughout most of Hardee and De Soto Counties.

The potentiometric surface of the Upper Floridan 

aquifer in May 1989 is shown in figure 23. This surface 

represents conditions near the end of a dry season during 

which extensive irrigation pumpage occurred. In May 

1989, the potentiometric surface ranged from about 60 ft 

above sea level in northeastern Hardee County to about 

5 ft above sea level in western Hardee County (fig. 23). 

May 1989 water levels averaged about 3 ft lower than 

the May 1988 levels reported by Lewelling (1988). This 

decline was the result of below normal rainfall and 

unusually large ground-water withdrawals for irrigation. 

A major feature of the potentiometric surface in figure 

23 is an east-west trough in southern Hardee and northern De Soto Counties caused by large ground-water 

withdrawals for irrigation.

The major change in the potentiometric surface from 

September 1988 to May 1989 is an overall decline. 

A comparison of figures 22 and 23 indicates that the 

declines in the potentiometric surface during this period 

were about 20 ft in the eastern part of Hardee County and 

about 35 to 40 ft in western Hardee County and eastern 

Manatee County.

The potentiometric surface of the Upper Floridan 

aquifer in September 1989 is shown in figure 24. This 

potentiometric surface is similar to that for September 

1988, except the potentiometric surface averaged about 

4 ft lower in September 1989 than in September 1988 

(Knochenmus and Barr, 1990b). As stated previously, 

rainfall for the study area was 15 in. below normal for 

the period September 1988 to September 1989, resulting 

in increased ground-water withdrawals for irrigation 

(Southwest Florida Water Management District, 1989).

Fluctuations and long-term trends in water levels 

for two wells open to the Upper Floridan aquifer are 

shown in figure 25. The locations of these wells are 

shown in figure 24. In general, these hydrographs show 

that (1) the altitude of the potentiometric surface changes

dramatically in response to changes in discharge and 

recharge; (2) there is a slight downward trend in water 

levels from 1978 through 1988; (3) the trends in the 

water levels for the two wells open to the Upper Floridan 

aquifer are similar; and (4) seasonal water-level fluctuations were as much as 20 ft for the ROMP 26 Avon Park 

well and 40 ft for the ROMP 31 Avon Parl well.

WATER USE

In 1988, water was withdrawn from the intermediate 

aquifer system and the Upper Floridan aquifer at a combined rate of 122 Mgal/d in Hardee and De Soto Counties for public supply, rural (self-supplied domestic), 

industrial, irrigation, and miscellaneous us^s (table 1). 

Estimated water use in the study area for 1988 is listed 

by county, category of use, and aquifer in table 1. The 

accuracy of these water-use estimates varies from category to category. For example, public-supp^ and larger 

industrial water-use estimates are usually more accurate 

because most public-supply systems and industrial facilities meter their usage, whereas agricultural and rural 

(self-supplied domestic) water-use estimates are often 

less accurate because these types of water use generally 

are not metered. For 1988, water-use estimates for agricultural withdrawals were based on irrigated crop acreage estimates from the water-use permitting files of the 

SWFWMD. Seasonal variations in agricultural water 

use were estimated based on studies of water use on 

selected benchmark farms (Duerr and Tronmer, 1982; 

Duerr and Sohm, 1983). Additional water-use data were 

obtained from Geurink (1986). Water-use permits do not 

delineate withdrawal data by aquifer; therefore, estimates of water withdrawn from the intermediate aquifer 

system and from the Upper Floridan aquifer were based 

upon well-construction data, including totd depth and 

cased interval, aquifer depth, and transmissivity.

Of the five water-use categories, irrigation 

accounted for the largest percentage of the ground 

water withdrawn. Irrigation is used extensively for crop 

production because of the unpredictable rainfall distribution and the low water-retention capacity of the 

sandy soils in the study area. In Hardee and De Soto 

Counties, 115.6 Mgal/d was withdrawn fcr irrigation 

use during 1988, of which 78.6 Mgal/d, cr 68 percent, 

was used for citrus irrigation (fig. 26). Ground water 

withdrawn for all uses, irrigated citrus acreage, and 

rainfall for 1975-88 are shown in figure 27. The quantity of ground water withdrawn for citrus irrigation 

generally has increased since 1983, even though overhead sprinklers have been replaced with rrore efficient 

microjet systems at many groves.

W^ter Use 29
82°09'15" 

27° 43'571

81° 22'

26° 56' 30'

HILLSBOROUGH 

COUNTY

HARDEE COUNTY_ 

DE SOTO COUNTY

GLADES 

COUNTY

10 20 MILES

10 20 KILOMETERS

EXPLANATION

4Q POTENTIOMETRIC CONTOUR 

Shows altitude at which water level 

stands in tightly cased wells in the 

Upper Floridan aquifer. Contour 

interval 10 feet. Datum is sea level

WELL CONTROL POINT

Figure 23. Potentiometric surface of the Upper Floridan aquifer, May 1989. (Modified from Barr, 1989.)

30 Hydrogeology and Simulated Effects of Ground-Water Withdrawals for Citrus Irrigation, Hardee and De Soto Counties, Fla.
82°09'15" 81° 22'

27° 43' 57

26° 56' 30"

POLK COUNTY

HILLSBOROUGH 

OUNTY

ROMP 31 

AVON PARK 

WELL

HARDEE COUNTY _ ^.

DE SOTO COUNTY

ROMP 26 

AVON PARK . 

WELL

HIGHLANDS 

COUNTY

GLADES 

COUNTY

CHARLOTTE 

COUNTY

10 20 MILES

10 20 KILOMETERS

EXPLANATION

 40 POTENTIOMETRIC CONTOUR 

Shows altitude at which water level 

stands in tightly cased wells in the 

Upper Floridan aquifer. Contour 

interval 10 feet. Datum is sea level

WELL CONTROL POINT

Figure 24. Potentiometric surface of the Upper Floridan aquifer, September 1989. 

(Modified from Knochenmus and Barr, 1990b.)

Water Use 31
LU 

>

LU

LU 

CO

LLJ

O 

CO

LLJ 

LLJ

60

50

40

30

20

10

70

ROMP 31 AVON PARK WELL 

DEPTH 1,152 FEET 

CASING 460 FEET

1978 1980 1982 1984 1986

UJ 60 

LLJ

£ 50

40

30

20

ROMP 26 AVON PARK WELL 

DEPTH 1,320 FEET 

CASING 580 FEET

1978 1980 1982 1984 1986

Figure 25. Long-term water-level trends for selected Upper 

Floridan aquifer wells, 1978-88. (Locations of wells are shown 

in fig. 24.)

Of the 122 Mgal/d withdrawn in 1988 for all five 

water-use categories, 13 percent was from the intermediate aquifer system and 87 percent was from the 

Upper Floridan aquifer. Little water is withdrawn from 

the surficial aquifer in Hardee and De Soto Counties, 

and water use from this aquifer was considered to be 

zero for this study. On the basis of SWFWMD wateruse permits, 302 irrigation wells are open to the intermediate aquifer system in Hardee and De Soto counties 

and parts of adjacent counties, and, of those wells, 258 

are used for citrus irrigation (fig. 28). Wells open to the 

intermediate aquifer system generally yield less than 

300 gal/min (Wilson, 1977).

Table 1 . Ground-water withdrawals in Hardee and De Soto 

Counties, by use category, 1988

[All values are in million gallons per day. Modified from Sorensen 

and others, 1990]

Category Hardee De Sot Total

Public supply

Intermediate aquifer system

Upper Floridan aquifer

Total

Rural

Intermediate aquifer system

Upper Floridan aquifer

Total

Industrial

Intermediate aquifer system

Upper Floridan aquifer

Total

Irrigation

Intermediate aquifer system

Upper Floridan aquifer

Total

Miscellaneous

Intermediate aquifer system

Upper Floridan aquifer

Total

Total (all uses)

Intermediate aquifer system

Upper Floridan aquifer

Total

0

1.4

1.4

2.0

0

2.0

0

.2

0.2

6.6

59.2

65.8

.2

0

0.2

8.8

60.8

69.8

0.8

0

0.8

1.6

0

1.6

0

0

0

4.9

44.9

49.8

.1

.1

0.2

7.4

45.0

52.4

0.8

1.4

2.2

3.6

0

3.6

0

.2

0.2

11.5

104.1

115.6

.3

.1

0.4

16.2

105.8

122.0

The principal source of ground-water supply in the 

study area is the highly productive Upper Floridan aquifer. On the basis of SWFWMD water-use permits, 1,036 

irrigation wells are open to the Upper Floridan aquifer in 

Hardee and De Soto Counties and parts of adjacent 

counties. Of these wells, 799 are used for citrus irrigation 

(fig. 29). Upper Floridan aquifer wells can yield as much 

as 2,500 gal/min and are commonly 10 to 16 in. in diameter (Duerr and Enos, 1991). Although many wells are 

open to the Upper Floridan aquifer, it was estimated that 

only 10 percent of the irrigation wells are open only to 

this unit; the remaining 90 percent are open to both the 

intermediate aquifer system and the Upper Floridan 

aquifer.

Ground-water withdrawals for irrigation vary seasonally as a result of variations in temperature and precipitation. There are two irrigation seasons each year: a 

fall season from October through December and a 

winter-spring season from January through May. There 

generally are little or no withdrawals for irrigation 

during the rainy season of June through mid-September.

32 Hydrogeology and Simulated Effects of Ground-Water Withdrawals for Citrus Irrigation, Hardee and De Soto Counties, Fia.
EXPLANATION

lO CITRUS

^ VEGETABLES

D MELONS

 NURSERY/SOD

Figure 26 (above). Percentage distribution of irrigation water 

use, by crop, Hardee and De Soto Counties, 1988. (Modified 

from Sorenson and others, 1990.)

The interrelation between rainfall, irrigation pumpage, and water levels for 1988-89 is apparent from a 

comparison of the graphs shown in figure 30. The September water levels in this figure represent conditions 

near the end of the summer rainy season when both the 

intermediate aquifer system and the Upper Floridan 

aquifer are usually unstressed by irrigation pumpage. 

The steep downward trend in ground-water levels 

during the winter and spring is due to low rainfall and 

increased withdrawals for irrigation. The water levels 

are lowest in May, but recover rapidly with the onset of 

summer rains and the decrease in withdrawals for 

irrigation.

Figure 27 (at right). Rainfall at Arcadia, ground-water 

withdrawals, and irrigated citrus acreage in Hardee 

and De Soto Counties, 1975-88.

_JC0

Q

CC

LU

JL

< W

ro

rz

£Q_j _j

2

?

CC

CCLU HOC

oo°5QjZ

O

CC

CC

25 | 1 1 | 1 |

20 | | | | 1 ! |

15 | | | | |

10 | | | | \ |

5 | 1 | 1 ; |

o *-'*-*-* -Z-&-Z260 - ^ CITRUS

ED AGRICULTUR/

240 ~ D MUNICIPAL AN 

INDUSTRIAL 

220 -

200 -

_

180 - "

120 - n -

100 ~

80 -

IRRIGATED CITRUS

80,000

70,000

60,000 % pi

50,000 1 | 1 1 | ; 1

40,000 \ | I | | i |

30,000 i | I 1 1 ;i |

20,000 | | | 1 | |

10,000 11 I | | 1 i 1

1 il 1 1 i 1 i

1 1 1 1 i 1 i

1 1 1 1 : 1 i

1 1 1 i i i ' 

1 i 1 1 1 1 £

u. n -

D ~

~

- - 

-

n

-

I

I

1 i !

j \ \

mmmm 1 1 j

ACREAGE g

" il 1 1

? £ s i S ;'

x V. = : S S j

?i s ?! il * 5> i

x: |x £ J:-: :'::] Jx :

: : ^ : ': S ijij -S :

G) Q) O) O) O)

Water Use 33
82° 09'15" 81° 22'

27° 43'57

26° 56' 30"

HILLSBOROUGH 

COUNTY

CHARLOTTE 

COUNTY

HARDEE^COUNTY^ 

DE~SOTO COUNTY

10 20 MILES

10 20 KILOMETERS 

EXPLANATION

CITRUS IRRIGATION WELLS

Figure 28. Locations of citrus irrigation wells open to the intermediate aquifer system, 1988.

34 Hydrogeology and Simulated Effects of Ground-Water Withdrawals for Citrus Irrigation, Hardee and De Soto Counties, Fla.
82°09'15M 

27° 43' 57'

81° 22'

26° 56' 30'

HARDEE COUNTY.' ''

*'

. / > vv- .-i: ?/..v..J'

'."< '^ ."".:. ::/:.

 

{ . . . . .- . t

< ». r

 . ... . % . .

. «M T

:-/ \^fsfJ ' ^-&&f

CHARLOTTE 

COUNTY

 .. HIGHLAN HIGHLANDS

* ** /i COUNTY

GLADES 

COUNTY

10 20 MILES

10 20 KILOMETERS

EXPLANATION

 CITRUS IRRIGATION WELLS

Figure 29. Locations of citrus irrigation wells open to the Upper Floridan aquifer, 1988.

Water Use 35
INTERMEDIATE AQUIFER SYSTEM ROMP 26 HAWTHORN WELL

DEPTH 180 FEET 

CASING 140 FEET

UPPER FLORIDAN AQUIFER ROMP 26 AVON PARK WELL 

DEPTH 1,320 FEET 

CASING 580 FEET

S O N D J F

1988

IRRIGATION PUMPING 

AT JOSHUA GROVE

i i i i i

RAINFALL AT ARCADIA

O N

1988

M

1989

Figure 30. Ground-water levels in wells open to the intermediate aquifer system and the Upper Floridan 

aquifer, irrigation pumpage at Joshua Grove, and monthly rainfall at Arcadia, September 1988 through 

September 1989. (See fig. 1 for site location.)

36 Hydrogeology and Simulated Effects of Ground-Water Withdrawals for Citrus Irrigation, Hardee and De Soto Counties, Fla.
SIMULATION OF GROUNDWATER FLOW

The preceding analysis of the hydrogeologic 

framework formed the basis for developing a conceptual model of ground-water flow. A generalized northsouth hydrogeologic representation of the aquifer system is illustrated in figure 31. Procedures in the conceptualization include developing an understanding of the 

ground-water system in terms of external and internal 

geometry (the geologic framework), material and fluid 

parameters (transmissivity and hydraulic gradients), 

and physical and hydraulic boundaries. This conceptual model of how the ground-water system functions 

was then used to develop a numerical ground-water 

flow model.

For simulation purposes, the designated hydrogeologic units in the study area are represented by three 

model layers corresponding to the surficial aquifer 

(layer 1), the intermediate aquifer system (layer 2), and 

the Upper Floridan aquifer (layer 3) (fig. 32). Layer 1, 

representing the surficial aquifer, is represented in the 

model by a distribution of specified heads that correspond to water-table elevations of the surficial aquifer 

during a specified period. These heads remain constant 

(do not change) during simulation representing infinite 

sources or sinks to ground-water flow. Layer 1, herein, 

is termed a source-sink layer. Layers 2 and 3 are termed 

active layers. The modeling approach used in this study 

does not account for changes in storage or horizontal 

flow in confining units. Accordingly, confining units 

between the surficial aquifer and the intermediate aquifer system and between the intermediate aquifer system 

and the Upper Floridan aquifer are represented by confining unit leakance distributions, and only vertical 

flow is simulated, representing leakage between aquifers. Vertical flow within aquifers in the study area is 

considered negligible, and only horizontal flow is simulated within active model layers.

The USGS modular ground-water flow model 

(McDonald and Harbaugh, 1984) was used to simulate 

ground-water flow in the intermediate aquifer system 

and the Upper Floridan aquifer. The model uses a 

finite-difference method in which partial-differential 

equations that describe ground-water flow are solved

numerically. The model is termed quasi-tl ree-dimensional as it computes two-dimensional (x,y) flow in the 

horizontal (x,y) plane of each model layer and onedimensional vertical (z) flow across confining beds.

The model area covers 2,592 mi2 and, in addition to 

Hardee and De Soto Counties, includes parts of Sarasota, Manatee, Hillsborough, Polk, Highlc nds Glades, 

and Charlotte Counties (fig. 1). The model area is subdivided into a finite-difference, block-centered grid of 

47 rows and 46 columns (fig. 33). Each of the 2,162 

grid blocks is 5,390 ft wide (1 minute of latitude) by 

6,050 ft long (1 minute of longitude). Because of the 

configuration of the model boundaries, ground-water 

flow is actively simulated only at 2,030 grid blocks, 

representing a surface area of about 2,374 mi2.

Boundary Conditions

Boundary conditions simulated in this model are an 

upper specified-head boundary that represents the 

water table of the surficial aquifer, a no-flow boundary 

at the base of freshwater flow that represents the top of 

the middle confining unit of the Floridan aquifer system, and a lateral no-flow boundary oriented along the 

Lake Wales Ridge in the northeastern part of the study 

area that represents a hydraulic divide in both the intermediate aquifer system and the Upper Floridan aquifer 

(figs. 16-18,22-24). Other lateral boundaries for model 

layers 2 and 3 are all general head boundaries.

The upper boundary, the water table in the surficial 

aquifer, was assigned a specified head because the seasonal fluctuation of the water table throughout most of 

the study area generally is small (in the range of 2-7 ft). 

There is no-long term change (fig. 9), indicating that 

the summer rains generally are adequate to replenish 

winter storage depletions of the surficial aquifer. Small 

fluctuations in the water table are assumed to be controlled by loss of water to evapotranspiraticn and drainage to streams. Pumping from underlying aquifers is 

assumed to have little effect on the water table on a 

regional scale because of the low leakance of the upper 

confining unit of the intermediate aquifer system and 

the large storage capacity of the surficial aauifer.

Simulation of Ground-Wat^r Flow 37
NORTH

FEET 

200-1

SEA LEVEL UPPER CONFINING UNIT

INTERMEDIATE AQUIFER SYSTEM

 -^ 

ER CONFINING UNIT

400 

600 

800 

1,000 

1,200 

1,400

1,600 

1,800 

2,000

o o o

UPPER FLORIDAN AQUIFER

TOP OF HIGHLY PERMEABLE ZONE

TOP OF INTERGRANULAR EVAPORITES

MID-FLORIDAN CONFINING UNIT

SCUTH

FEE" 

 200

 SEA LEVEL

 200

 400

- 600

 800

 1,000

 1,200

 1,400

 1,600

1,800

-2,000

Figure 31. Conceptualized hydrogeologic representation of a north-south section of the aquifer system. 

(Modified from Ryder, 1985.)

38 Hydrogeology and Simulated Effects of Ground-Water Withdrawals for Citrus Irrigation, Hardee and De Soto Counties, Fla.
HYDRAULIC GRADIENTS

Intermediate aquifer system 

potentiometric surface

Surficial aquifer water-table altitude

Upper Floridan aquifer 

potentiometric surface

PUMPED

WELL 

MODEL LAYERS HYDROLOGIC UNITS

BOUNDARY EAD 

GENERAL H

LAYER 1 Casing 

Specified head boundary \| SURFICIAL AQUIFER

ill i

\

J

r

 ^ c

LAYER 2(active)

.^k.

1 UPPER

)pen hole

w ^

t=t= r

LAYERS (active)

^

I

E

=

-

-

*

CONFINING UNIT

-^

L

INTERMEDIATE AQUIFER 

SYSTEM

^ 

LOWER 

 \

i

CONFINING UNIT 

i 

^

k

-^ 

UPPER FLORIDAN AQUIFER

^ ^^-DIRECTION OF FLOW

NO-FLOW BOUNDARY

NO-FLOW BOUNDARY

Figure 32. Schematic cross section of the layered digital ground-water model and the 

hydrologic units represented by the model layers.

Simulation of Ground-Water Flow 39
10 15

COLUMNS

20 25

C/D 

O

DC 25

47

10 20 KILOMETERS

EXPLANATION

GENERAL HEAD BOUNDARY

INACTIVE MODEL CELLS

LOCATION OF CROSS-SECTION USED FOR SENSITIVITY ANALYSIS 

USED FOR SENSITIVITY ANALYSIS

NO-FLOW BOUNDARY

Figure 33. Finite-difference grid, lateral boundary conditions, and cross section used for sensitivity analysis.

40 Hydrogeology and Simulated Effects of Ground-Water Withdrawals for Citrus Irrigation, Hardee and De Soto Counties, Fla.
A no-flow boundary was used along the northeastern lateral boundary. This lateral boundary coincides 

with a ground-water divide in the vicinity of the Lake 

Wales Ridge (fig. 2) and is approximately 12 mi east of 

Hardee and De Soto Counties. Because recharge at the 

ridge is high (3-20 in/yr) (Tibbals, 1990), it is expected 

that seasonal shifts in the ground-water divide as a 

result of pumping and climatic stress will be minimal.

The remaining lateral boundaries in layers 2 and 3 

are represented by a general head boundary. This feature 

of the model allows water to enter or leave the system 

at rates that are dependent upon the aquifer properties 

and head gradients near the boundary as follows:

= C(H1-H2), (1)

where

Q is the flow rate into (+) or out (-) of the model area 

across the boundary, in cubic feet per day;

C is the lateral conductance term, in feet squared per day;

HI is the controlling head outside the model boundary, 

in feet; and

H2 is the model-simulated head in boundary grid block, 

in feet.

During initial calibration, these lateral boundaries 

were designated as specified-head boundaries to determine model-computer flow rates to and from each 

specified-head boundary cell. The computed flow rates 

(Q) and head values (H2) in each lateral boundary cell 

were then assigned a controlling head (HI) value. The 

H1 value was based on heads from the September 1988 

potentiometric-surface maps (Barr, 1989a,c) along a 

flow pathline at a distance of 6 mi from the model 

boundary. A conductance term (C) for each specifiedhead boundary cell was calculated using equation 1. 

After the conductance term was calculated, the specified-head lateral boundary condition in the intermediate aquifer system and the Upper Floridan aquifer was 

converted to a general-head boundary condition. 

Model simulations were made to determine whether 

flow rates computed at specified-head boundary cells 

matched flow rates (Q) computed using the general 

head boundary conditions; negligible error was 

detected.

Input Parameters

Input data necessary to initiate model simulation 

consist of the altitude of the water table in the surficial 

aquifer; leakance values for both the upper and lower 

confining units of the intermediate aquifer system; and 

the hydraulic heads, transmissivities, storage coefficients, lateral boundary cell heads and related conductance values at general head boundaries, and pumping 

rates for both the intermediate aquifer system and the 

Upper Floridan aquifer.

Geographical Information System 

Procedures

An important component of this study was the 

interface of a GIS with the modular model. GIS is a 

computerized data base that allows spatial data analysis 

with a display of the data. Spatial data analysis allows 

location, shape, and relations among features to be 

analyzed graphically. The ARC/INFO GIS system was 

used as an aid in the input data-base design and in the 

construction, calibration, and presentation of the model 

input and simulation results. Computer programs 

developed by D.O. Winkless and J.M. Kernodle 

(J.M. Kernodle, U.S. Geological Survey, written commun., 1988) were used to create the model grid and 

data arrays, to populate the data base, and to analyze 

the data.

The model grid was generated with row and 

column numbers as well as an x- and y-coordinate 

system. This feature allowed data to be added to the 

spatial data base and linked to each grid cell in the 

model area. Model input parameters were created as 

maps, or "coverages," in the GIS data base. Coverages 

were overlain with the model grid, and geographic 

locations and hydraulic features were assigned to the 

rows and columns of the model grid.

Water levels were the basis for hydraulic head data 

coded in the model. The water-table map fcr September 

1988 (fig. 8) was used in conjunction with GIS to 

assign head values for grid nodes in active layer 1 of the 

model. Input data for the hydraulic heads in active 

layers 2 and 3 were the potentiometric surface of the 

intermediate aquifer system and the Uppe~ Floridan 

aquifer shown in figures 16 through 18 and 22 through 

24. The potentiometric-surface maps for September 

1988, May 1989, and September 1989 in these figures

Simulation of Ground-Wal<?r Flow 41
were entered into the GIS data base. Coverages were 

generated from digitized potentiometric-surface contour lines. Head values were assigned for each model 

cell by interpolation between contour line using GIS 

commands. Coverages were then checked for accuracy 

by plotting automated data at the same scale as the 

original map.

Transmissivity data for both the intermediate aquifer system and the Upper Floridan aquifer were entered 

directly from the RASA model (Ryder, 1985). Because 

this model represents a finer resolution ground-water 

flow model than that developed for the regional scale 

RASA study, input values were changed slightly along 

transmissivity boundaries to smooth transitions 

between zones.

Initial leakance values of the upper and lower confining units of the intermediate aquifer system were 

based on estimates from Ryder (1985). Additional data 

pertaining to the thickness of the upper and lower confining units reported by Duerr and Enos (1991) were 

used to refine the leakance values reported by Ryder 

(1985).

Storage-coefficient arrays for the intermediate 

aquifer system and the Upper Floridan aquifer were 

constructed on the basis of aquifer thickness. Thickness 

maps for both aquifers were integrated with the GIS. 

The storage coefficient arrays for the intermediate 

aquifer system and the Upper Floridan aquifer were 

then determined by multiplying an estimated average 

specific storage of l.OxlCT6 ft" 1 times the permeable 

aquifer thickness (Lohman, 1979).

To determine the distribution and magnitude of 

pumpage, a GIS data base of wells was created from the 

water-use permit files of the SWFWMD. Those files 

included descriptions of well locations, well depths, 

casing depths, well use, total permitted acres for each 

well, permitted average pumping rate, and projected 

citrus acreage. Other sources of information used to 

create the coverage were Sorensen and others (1990) 

for metered pumpage and Taylor and others (1990) for 

projected citrus acreage.

The relational data-base management capabilities 

of the GIS were utilized to determine the following 

model input data arrays: (1) total pumpage from multiple wells in each grid cell; (2) cumulative pumping 

rates for citrus and other agricultural uses; (3) distributions of wells for each aquifer based on correlation of 

aquifer thickness maps and well depths; (4) pumping 

rates from each aquifer where wells were open to

multiple layers; (5) variations in pumping rates on the 

basis of monthly benchmark farm data; (6) locations of 

future wells for citrus irrigation on the basis of existing 

land use; (7) pumping rates for proposed citrus irrigation wells; and (8) areas of municipal, industrial, and 

agricultural land uses.

Steady-State Conditions

The simulation of steady-state conditions is a 

useful adjunct to model calibration and is frequently 

used to evaluate initial descriptions of transmissivity 

and leakance arrays and the suitability of boundary 

conditions. However, the selection of a period of time 

that is representative of steady-state conditions is difficult, particularly where aquifers are stressed by pumping. In the study area, the high diffusivity of the Upper 

Floridan aquifer, the lack of direct aquifer recharge 

from rainfall to the intermediate aquifer system and the 

Upper Floridan aquifer, and the lack of direct aquifer 

continuity with streams tend to support a relatively 

rapid achievement of equilibrium following the cessation or initiation of pumping. On the other hand, the 

relatively low diffusivity of the intermediate aquifer 

system and the low leakance of confining units tend to 

bring into question the development of short-term equilibrium conditions. Because of the low confining unit 

leakance, vertical leakance across the defined confining 

units in the study area probably never reaches a steadystate condition during any month of the annual cycle of 

pumping and rainfall previously described.

For modeling purposes, an initial calibration to 

steady-state conditions should be based on long-term 

average descriptions of head and stress. Such descriptions for the study area are highly uncertair because the 

distribution and rates of agricultural withdrawals are 

generally unknown. Accordingly, for this study, a 

quasi-steady-state condition was defined rt the end of 

the rainy season when agricultural pumpage is zero and 

water-level hydrographs show little regioral change in 

head. September 1988 was selected for the quasisteady-state simulation period because pumpage was 

known with reasonable certainty and the Hads were 

not changing appreciably during that period (fig. 34). 

Principal stresses on the aquifer during this time were 

withdrawals from industrial and municipal supply 

wells, which are known with reasonable certainty.

42 Hydrogeology and Simulated Effects of Ground-Water Withdrawals for Citrus Irrigation, Hardee and De Soto Counties, Fla.
60

UJ 50

-"< 

^ 40

_IUJ

5 CD 30

20

I I I t T I I I 1

ROMP 31 HAWTHORN WELL 

INTERMEDIATE AQUIFER SYSTEM 

DEPTH 350 FEET, CASED TO 130 FEET

ROMP 31 AVON PARK WELL 

UPPER FLORIDAN AQUIFER

DEPTH 1,152 FEET, CASED TO 460 FEET

i i i i i i i i i i i i

 ILLJ

SONDJFMAMJJAS

1987 1988

50

40

30

20

10

i i r i i i i r

ROMP 32 AVON PARK WELL 

UPPER FLORIDAN AQUIFER 

DEPTH 1,215 FEET, CASED TO 909 FEET

SONDJFMAMJJAS

1987 1988

LU

>

55

45

35

25

15

Till I I I I I I I I 

EDGEVILLE DEEP WELL 

UPPER FLORIDAN AQUIFER 

DEPTH 600 FEET, CASED TO 480 FEET

i i i i i i i

65

55

UJ[J 45

_IUJ 

(T>

SONDJFMAMJJAS

1987 1988

35

25

i r i T ^111 i i > r 

TROPICAL RIVER GROVES DEEP WELL 

UPPER FLORIDAN AQUIFER 

DEPTH 698 FEET, CASED TO 137 FEET

i i i i i i i i

SONDJFMAMJJAS

1987 1988

UJd

^£ w

_IUJ

70

60

50

40

30

i i r i T i i i i 

ROMP16OCALAWELL 

UPPER FLORIDAN AQUIFER 

DEPTH 942 FEET, CASED TO 757 FEET

i i i i

1-

Cu

_IUJ

or£

60

50

40

30

20

i i T i i i i r \ \ \ r 

' ROMP 26 AVON PARK WELL 

UPPER FLORIDAN AQUIFER 

DEPTH 1,320 FEET, CASED TO 580 FEET

ROMP 26 HAWTHORN WELL

INTERMEDIATE AQUIFER SYSTEM 

DEPTH 180 FEET, CASED TO 140 FEET

i i i i i i i i i i i i

SONDJFMAMJJAS

1987 1988

SONDJFMAMJJAS 

1987 1988

Figure 34. Maximum daily water levels for selected wells open to the intermediate aquifer system and the Upper 

Floridan aquifer, September 1987 through September 1988. (Locations of wells are shown in fig. 39.)

Simulation of Ground-Water Flow 43
Detailed pumpage records for Florida's largest citrus 

grove, a 42-mi grove in northeastern De Soto County, 

indicated that no pumpage for citrus irrigation occurred 

from August 1 through September 25, 1988. Field 

observations indicated no citrus irrigation occurred 

September 18-21, 1988, the time period when waterlevel measurements were collected for use in preparing 

the potentiometric-surface maps for the intermediate 

aquifer system and the Upper Floridan aquifer.

The flow model was initially calibrated to September 1988 conditions. Following this calibration, simulated September 1988 conditions were used as initial 

conditions for subsequent transient simulations. Final 

model calibration was achieved when quasi-steadystate and transient conditions were simulated within 

predetermined limits of accuracy using duplicate arrays 

of transmissivity and confining unit leakance.

Steady-State Calibration

Two procedures were used in the calibration 

process: (1) analysis of residuals; and (2) conversion 

of model output to contour maps. Residuals are the 

differences between the observed and the simulated 

heads at observation well sites. A negative residual 

occurs when the simulated heads are greater than the 

observed heads, and a positive residual occurs when 

the simulated heads are less than the observed heads. 

An "error" criterion of 10 ft was established for nodes 

where head residuals were determined by comparing 

simulated results to interpolated values from potentiometric-surface maps. An error criterion of 6 ft was used 

for nodes where heads were determined from observation well measurements.

Statistical analysis of model residuals involved the 

use of a statistical processor for analyzing simulations 

made using the model (Scott, 1990). The modular 

model statistical processor (MMSP) provided the 

capabilities to calculate descriptive statistics, such as 

root mean square error (RMSE), for the error analysis 

of simulated and interpolated water levels. The RMSE, 

used to judge the goodness of fit, is given by

RMSE - ( hs- ho)~/n (2)

i=l

where

n is the number of observations;

hs is the simulated head, in feet; and

h0 is the observed or interpolated head, in feet.

Another technique used in the calibration process 

was the conversion of simulated model output to contour maps using GIS. Simulated contour maps of heads

were compared to published maps by Ba~r (1989a,b, 

c,d) and Knochenmus and Barr (1990a,b). Areal distribution of discharge and recharge were compared to 

published maps by Ryder (1985), Aucott (1988), and 

Tibbals (1990).

Interpolated and simulated potentiom^tric surfaces 

for the intermediate aquifer system and the Upper 

Floridan aquifer is shown in figures 35 and 36, respectively. An error analysis for the model calibration of the 

intermediate aquifer system and Upper Floridan aquifer is presented in table 2. The residuals were analyzed 

for all 2,030 active nodes of the intermediate aquifer 

system and the Upper Floridan aquifer. Based on the 

interpolated potentiometric data shown for September 

1988 (fig. 35), the standard deviation about the -1.0-ft 

mean of the residuals for the intermediate aquifer 

system was 3.2 ft, which indicates that tha> modelsimulated heads for the intermediate aquifer system 

match the interpolated heads within a range of 4.2 ft 

above to 2.2 ft below at about 67 percent of the nodes. 

Similarly, the simulated potentiometric surface for the 

Upper Floridan aquifer matched the interpolated September 1988 surface (fig. 36) at 67 percent of the nodes 

within a range of 4.4 ft above to 3.6 ft below on the 

basis of a standard deviation of 4.0 ft about a residual 

mean of -0.4 ft. Maximum differences between the 

interpolated September 1988 potentiometric-surface 

values and the simulated values ranged from +10.0 to 

-10.2 ft for the intermediate aquifer system and +13.9 

to -8.2 ft for the Upper Floridan aquifer.

A comparison between the observed and simulated 

water levels for wells open to the intermediate aquifer 

system and the Upper Floridan aquifer also was used to 

demonstrate calibration of the steady-state model. 

Residuals were computed for 48 nodes of the intermediate aquifer system and 64 nodes of the Upper Floridan aquifer that correspond to observation well 

locations. Based on September 1988 measurements, 

the standard deviation about the 0.69-ft mean of the 

residuals for the intermediate aquifer system was 1.5 ft, 

which indicates that the model-simulated heads for the 

intermediate aquifer system match the observed heads 

within a range of 2.2 ft above to 0.8 ft below at about 

67 percent of the nodes. Similarly, the model-simulated 

water levels for the Upper Floridan aquifer matched the 

observed September 1988 water levels at 67 percent of 

the nodes within a range of 1.3 ft above to 2.1 ft below 

on the basis of a standard deviation of 1.72 ft and a 

residual mean of -0.41 ft. Maximum differences 

between the observed September 1988 water levels and 

the simulated values ranged from +4.2 to -5.6 ft for the 

intermediate aquifer system and from +5.9 to -4.5 ft for 

the Upper Floridan aquifer.

44 Hydrogeology and Simulated Effects of Ground-Water Withdrawals for Citrus Irrigation, Hardee and De Soto Counties, Fla.
82°09'15" 

27° 43' 57

26° 56' 30'

POLK COUNTY

HILLSBOROUGH 

COUNTY \

~^ .N

Model 

Boundary

-/-/ / i i i i

/ ///////

/ / / / S*^ / / / I \ \ V \ 

^ / / V / /**>^ ' I \ .\'/N« ^

HIGHLANDS 

COUNTY

- CHARLOTTE 

COUNTY

10 20 MILES

40 

40 

0 10 20 KILOMETERS 

EXPLANATION

POTENTIOMETRIC CONTOUR 

Shows altitude at which water level would have stood in tightly cased wells open to 

the intermediate aquifer system. Contour intervals 10 and 20 feet. Datum is sea level

SIMULATED POTENTIOMETRIC CONTOURS 

OF THE INTERMEDIATE AQUIFER SYSTEM

FLOW VECTOR Shows direction of ground-water 

flow. Length of vector is proportional to velocity

Figure 35. Interpolated (modified from Barr, 1989a) and simulated potentiometric surfaces and direction of ground-water 

movement in the intermediate aquifer system, September 1988.

Simulation of Ground-Water Flow 45
82°09'15" 81° 22'

27

HILLSBOROUGH 

COUNTY

HIGHLANDS^ 

COUNTY \ \ \ \ \

10 20 MILES

40 

40 

0 10 20 KILOMETERS 

EXPLANATION

POTENTIOMETRIC CONTOUR 

Shows altitude at which water level stands in tightly cased wells in the 

Upper Floridan aquifer. Contour interval 10 feet. Datum is sea level

SIMULATED POTENTIOMETRIC 

CONTOURS OF THE UPPER 

FLORIDAN AQUIFER

FLOW VECTOR Shows direction of ground-water flow. 

Length of vector directly proportional to velocity

Figure 36. Interpolated (modified from Barr, 1989c) and simulated potentiometric surfaces and direction of 

ground-water movement in the Upper Floridan aquifer, September 1988.

46 Hydrogeology and Simulated Effects of Ground-Water Withdrawals for Citrus Irrigation, Hardee and De Soto Counties, Fla.
Table 2. Statistical summary of differences between observed and simulated heads for the 

intermediate aquifer system and the Upper Floridan aquifer, September 1988

Statistics

Model-simulated heads

Intermediate Upper Floridan 

aquifer system aquifer

Observed heads

Intermediate Upper Floridan 

aquifer system aquifer

Number of active nodes

Number of individual wells

Maximum range in residuals (feet)

Arithmetic mean of residuals (feet)

Absolute mean of residuals (feet)

Standard deviation of residuals (feet)

Mean deviation

Median

Variance

Sum of absolute value of residuals (feet)

Root mean square error

2,030

10.0 to -10.2

-1.04

2.61

3.27

2.48

- .76

10.71

-2,122.58

3.44

2,030

13.9 to -8.2

- .37

2.97

4.00

2.91

- .66

16.03

-762.75

4.02

48

4.2 to -5.6

.69

1.14

1.51

1.00

.51

2.28

32.97

4.06

64

5.9 to -4.5

- .41

1.36

1.72

1.32

.33

2.95

26.17

3.92

Sensitivity Analysis

An analysis was made to determine the sensitivity 

of the calibrated model to changes in input parameters. 

Transmissivity and leakance were varied one at a time, 

over a reasonable range that might exist, and changes 

in the simulated heads were observed. These tests 

describe the relative importance of these input parameters to model simulation results.

Results of sensitivity tests for the intermediate 

aquifer system and the Upper Floridan aquifer are 

shown in figure 37. Cross sections along row 23 and 

extending from column 1 through column 44 were constructed to show the effects of the parameter changes 

on aquifer heads. A summary of residuals due to 

changes in parameters for all 2,030 active nodes 

also are listed in figure 37. The results of the analysis 

indicate that:

(1) The model is relatively insensitive to doubling or 

halving transmissivity of the intermediate aquifer 

system and the Upper Floridan aquifer, because 

the residuals are not substantially larger than 

those in the calibrated model.

(2) The model is sensitive to tenfold changes in the 

vertical hydraulic conductivity of the upper and 

lower confining units of the intermediate aquifer 

system. Residuals are substantially larger than 

those in the calibration run. The hydrographic 

section for the intermediate aquifer system in 

figure 37 shows that the departure of computed 

heads from observed heads is greatest for the 

model run with increased leakance.

Transient Conditions

The purpose of the transient simulation was to 

determine the effects of ground-water pumping on storage and ground-water flow directions in the intermediate aquifer system and the Upper Floridan aquifer. Two 

simulation periods were selected and result? were compared to potentiometric-surface maps. The first period 

corresponds to a 236-day irrigation season; the second 

to a 128-day wet-season recovery period. The 236-day 

irrigation season was simulated using nine monthly 

stress periods from the beginning of the fall irrigation 

season, September 1988, to the approximate end of the 

spring irrigation season, May 1989. The 128-day wetseason recovery period was simulated by removing the 

irrigation pumpage and simulating monthly conditions 

from June 1989 through September 1989. Simulated 

monthly water levels were compared to observed water 

levels for individual wells in the model area.

Seasonal fluctuations of the potentiome'ric surface 

during a 1-year period for six wells in the model area 

are shown in figure 34. The graphs show that the potentiometric surface may undergo several cycle? of decline 

and rise during the year, but, generally, the surface is 

highest in autumn and lowest in spring. Long-term 

hydrographs repeat this yearly cycle (figs. 19 and 25). 

The Upper Floridan aquifer in the region of a 42-mi 

citrus grove in northeastern De Soto County was 

modeled by Wilson (1972), who determined that, when 

a multiyear, hypothetical pumping schedule was simulated, the decline in potentiometric surface was about 

the same at the end of the spring pumping period each 

year and that the potentiometric surface recovered to

Simulation of Ground-Water Flow 47
o 

K

I LLJ

Pa

I5X

LLJ CD

Q. 

LLJ 

Q

.» 2 X TRANSMISSIVITY 

»«« 0.5 X TRANSMISSIVITY 

0.1 XLEAKANCE

 10 XLEAKANCE

 CALIBRATION

ROW 23 

INTERMEDIATE AQUIFER SYSTEM

15 10 15 20 25 30 35 40 44

COLUMN

INTERMEDIATE AQUIFER SYSTEM 

Residuals (2,030 grid b'ocks)

Parameter 

and 

multiplier

calibration 

2.0 X transmissivity 

0.5 X transmissivity 

0.1 Xleakance1 

10 X leakance1

Maximum 

negative 

residual 

(ft)

-10.2 

-11.1 

-9.9 

-6.4 

-36.5

Maximum 

positive 

residu-"' 

(ft)

10.0 

13.4 

15.1 

49.2 

17.8

Mean of 

absolute 

residual 

(ft)

2.6 

2.7 

2.7 

7.2 

10.3

1 Leakance for the upper confining unit of the 

Intermediate aquifer system

O

o:

LL

LUCQ

LLJ 

Q

20

15

10

l= 5

-5

-10

-15

-20

 2XTRANSMISSIVITY 

««w 0.5 X TRANSMISSIVITY 

 0.1 X LEAKANCE

 10 XLEAKANCE

 CALIBRATION

ROW 23 

UPPER FLORIDAN AQUIFER

10 15 20 25 30 35 40 44

COLUMN

UPPER FLORIDAN AQUIFER 

Residuals (2,030 grid blocks)

Parameter Maximum 

and negative 

multiplier residual 

(ft)

calibration 

2.0 X transmissivity 

0.5 X transmissivity 

0.1 Xleakance1 

lOXIeakance1

-8.3 

-7.5 

-8.9 

-4.9 

-10.8

Maximum 

positive 

residua 1 

(ft)

13.9 

19.2 

16.2 

24.1 

10.1

Mean of 

absolute 

residual 

(ft)

2.9 

3.3 

2.8 

3.3 

3.7

1 Leakance for the lower confining urt of the 

Intermediate aquifer system

Figure 37. Results of sensitivity analysis of the steady-state model parameters for the intermediate aquifer system 

and the Upper Floridan aquifer, September 1988.

48 Hydrogeology and Simulated Effects of Ground-Water Withdrawals for Citrus Irrigation, Hardee and De Sot" CXunties, Ra.
near prepumping conditions by September. Because of 

the cyclic nature of this trend and because the potentiometric surface recovers to near prepumping conditions at the end of the irrigation season, a transient 

calibration can be obtained by analyzing 1 year of 

water-level fluctuations.

An ideal test of the applicability of the model 

would be to run the model through a series of year-long 

simulation periods that, collectively, would span the 

length of the observed long-term record of water levels. 

However, data on areal distributions of pumping are 

poor, and long-term pumping data for agriculture are 

too sparse to consider this approach.

Boundaries

The initial boundary conditions for the transient 

calibration were the same as those assigned to the intermediate aquifer system and the Upper Floridan aquifer 

for the steady-state calibration. These boundaries were 

deemed suitable for the September 1988 starting conditions in the transient calibration; however, the assumption of a "constant" external head (HI) used in the 

computation of flow across the general head boundaries 

was known to be invalid for successive stress periods. 

Observed water levels fluctuated as much as 30 ft 

between September 1988 and May 1989 in parts of 

Sarasota, Manatee, and Polk Counties (fig. 38). Accordingly, boundary head values (HI) were adjusted during 

each stress period for each active layer to account for 

these changes and to reflect as closely as possible the 

hydrographic trends of nearby wells. These trends were 

approximated by two linear segments during the transient calibration period; the declining trend from September 1988 to May 1989, and the rising trend from 

May 1989 to September 1989 (fig. 38). End points for 

each trend segment were estimated using potentiometricsurface maps for September 1988, May 1989, and 

September 1989. Rates of head change were based on 

the slopes of trend lines shown in figure 38.

The specified heads representing the water table of 

the surficial aquifer throughout the model area during 

the irrigation season (October 1988 through May 1989) 

were assumed to average 3 ft lower than the heads in 

September 1988 and were adjusted accordingly. Similarly, the specified heads during the nonirrigation 

season (June 1989 through September 1989) were 

assumed to average 1 ft lower than the head in September 1988. These adjustments were based on 1988-89 

hydrographs of water levels in observation wells open 

to the surficial aquifer (fig. 9).

Transient Calibration

The equilibrium conditions simulated during the 

steady-state calibration were used as the initial condition for the transient simulation. Input parameters for 

the transient-model calibration were the same as those 

used for the calibrated steady-state model with the 

addition of arrays representing storativity and pumpage distributions for the intermediate aquifer system 

and the Upper Floridan aquifer. Storage coefficients 

were adjusted during transient calibration for layers 2 

and 3 to minimize differences between simulated and 

observed water levels. Storage coefficients were 

adjusted within a range determined from previous 

reports by Ryder (1982; 1985), Wilson and Gerhart 

(1982), and Tibbals (1990).

Irrigation pumpage arrays were developed using 

data from a benchmark farms program (Duerr and 

Trommer, 1982) to determine the monthly pumping 

rates for citrus, vegetables, melons, nurseries, and 

pasture. Coefficients were determined for each crop 

type on the basis of seasonal irrigation use and were 

applied to the monthly pumping rates (tabl^ 3). The 

pumping rates for agricultural use, industrial use, and 

public supply for the period September 1988 through 

May 1989 for the model area are listed in table 4.

The accuracy of the transient calibration was 

evaluated by comparing simulated potentiometric 

levels with observed water levels on interpolated water 

levels from May 1989 and September 1989 potentiometric-surface maps for the intermediate aquifer system 

and the Upper Floridan aquifer. Simulated heads over 

time at specific grid blocks also were compared with 

hydrographs for wells in corresponding locations 

(fig. 39). Simulated water-levels for 16 observation 

wells were compared to water levels collected during 

the period September 1988 through September 1989 

(fig. 40). Particularly good comparisons were noted 

for the ROMP 30 well open to the intermediate aquifer 

system and the ROMP 32 well open to the Upper 

Floridan aquifer. The most notable deviations between 

observed water levels and simulated hydrographs were 

for the Rowell deep well open to the intermediate 

aquifer system and the ROMP 30 well open to the 

Upper Floridan aquifer. The comparisons indicated 

that the simulated heads were higher than the observed 

water levels at these sites. Pumpage in theee areas is 

not accurately defined and possibly accounts for these 

differences.

Simulation of Ground-Water Flow 49
LU 

>

LJJ

LJJ 

CO

LJJ

75

65

55

45

35

25

50

I I I I I I I I I I I I

ROMP 45 HAWTHORN WELL NORTH BOUNDARY

OBSERVED 

INTERPOLATED

INTERMEDIATE AQUIFER SYSTEM 

DEPTH 192 FEET 

CASING 110 FEET

S O N D 

1988

JFMAMJ JAS 

1989

O 40 

m

UJ 30 

LJJ

LJJ 

>

LJJ

_l

cc

LJJ

20

10

I I I I T

EDGEVILLE WELL WEST BOUNDARY

OBSERVED 

INTERPOLATED

UPPER FLORIDAN AQUIFER 

DEPTH 600 FEET 

CASING 487 FEET

i i i i i i i i

S O N D 

1988

JFMAMJ JAS 

1969

30

20

10

-10

-20

-30

i i i i i i r i 11 i i 

PUNTA GORDA HEIGHTS WELL SOUTH BOUNDARY

OBSERVED 

INTERPOLATED

- INTERMEDIATE AQUIFER SYSTEM 

DEPTH 125 FEET 

CASING 84 FEET

S O N D 

1988

JFMAMJ JAS 

1969

75

65

55

45

35

25

25

15

ROMP 45 AVON PARK WELL NORTH BOUNDARY -

OBSERVED 

INTERPOLATED

UPPER FLORIDAN AQUIFER 

DEPTH 757 FEET 

CASING 680 FEET

S O N D 

1988

JFMAMJJAS 

1989

-5

-15

-25

90

80

[ i i i r \ 

KIBLER DEEP WELL WEST BOUNDARY

OBSERVED 

INTERPOLATED

UPPER FLORIDAN AQUIFER V 

DEPTH 1,123 FEET 

CASING 208 FEET

S O N D 

1988

i i i

JFMAMJJAS 

1989

50

40

i^ ir rr T r i i i i i 

. ROMP 23 FLORIDAN WELL EAST BOUNDARY

OBSERVED 

INTERPOLATED

UPPER FLORIDAN AQUIFER 

DEPTH 1,385 FEET 

CASING 585 FEET

S O N D 

1988

i i i i

JFMAMJJAS 

1989

Figure 38. Hydrographic linear template used to estimate general head boundaries for transient simulation. 

(Locations of wells are shown in fig. 39.)

50 Hydrogeology and Simulated Effects of Ground-Water Withdrawals for Citrus Irrigation, Hardee and De Soto Counties, Ra.
Table 3. Coefficients used for determining monthly irrigation 

pumping rates

Crop

Month

September 

October

November

December

January 

February 

March

April 

May

Nursery 

and sod

0.60 

.59

.85

2.06

1.54 

.76 

1.12

.98 

1.24

Citrus

0.39 

.76

.67

1.15

1.09 

1.05 

1.33

1.31 

2.03

Vegetables

2.38 

4.19

3.05

1.36

1.18 

2.26

2.53

2.68 

3.00

Melons

0 

0

0

0

.53 

2.33 

2.82

1.58 

3.29

Table 4. Ground-water pumpage for public supply, industrial use, and irrigation 

from the intermediate aquifer system and the Upper Floridan aquifer during the 

irrigation period, September 1988 through May 1989 

[All values are in million gallons per day]

Date

1988

September

October

November

December

1989

January

February

March

April

May

1988

September

October

November

December

1989

January

February

March

April

May

Public Industr 

supply use

inl

Nursery 

and sod

Crop irrigation

Citrus Vegetables Melons

Total

Intermediate aquifer system

0.85

.85

.85

.85

.85

.85

.85

.85

.85

0

0

0

0

0

0

0

0

0

1.42

1.40

2.30

4.80

3.73

1.80

1.39

1.12

1.20

3.40

6.89

8.62

3.25

13.44

14.47

13.44

14.57

25.60

5.20

7.67

6.68

2.98

1.09

.85

1.39

.44

.22

0

0

0

0

0.12

.53

.47

.23

.42

10.9

16.8

18.4

11.9

19.2

18.5

17.5

17.2

28.3

Upper Floridan aquifer

10.30

10.30

10.30

10.30

10.30

10.30

10.30

10.30

10.30

39.40

39.40

39.40

39.40

39.40

39.40

39.40

39.40

39.40

4.11

4.03

5.86

4.00

0.80

5.20

3.99

3.59

3.39

25.10

50.10

62.60

87.70

97.70

105.20

97.70

105.20

170.30

78.20

78.20

78.20

44.70

16.40

13.10

9.96

6.57

3.29

0

0

0

0

0

4.08

3.64

1.82

3.27

157.1

182.0

196.4

196.1

174.6

177.3

165.0

166.9

230.0

The interpolated and simulated potentiometric 

surfaces for the intermediate aquifer system and the 

Upper Floridan aquifer for May 1989 are shown in 

figures 41 and 42, respectively. These potentiometric 

surfaces represent conditions near the end of a long dry 

season during which maximum pumping for irrigation 

occurred. The interpolated and simulated potentiometric - 

surface contours compare reasonably well for both the 

intermediate aquifer system and the Upper Floridan 

aquifer, except for the western part of the model area. 

The poor correlation between the interpolated and 

simulated heads in this area is possibly the result of 

inaccurately determined pumping rates.

The transient model was used to simulate the 

effects of decreased pumpage during a 12F~day recovery period on the potentiometric surface. / 11 model 

input parameters were the same as those used for the 

irrigation calibration period, except that irrigation 

pumpage was removed. The interpolated ard simulated 

potentiometric surfaces for the intermediate aquifer 

system and the Upper Floridan aquifer for September 

1989 are shown in figures 43 and 44, respectively. The 

map of the simulated potentiometric surface of the 

intermediate aquifer system for September 1989 compares reasonably well with the map of the interpolated 

potentiometric surface, except in the northeastern part

Simulation of Ground-Wat^r Flow 51
82°09'15" 

27° 43' 57'

81° 22'

26° 56' 30"

HILLSBOROUGH 

COUNTY

ROMP 45 AVON PARK POLK COUNTY WELL

ROMP 45 HAWTHORN 

WELL Q 

4.5 MILES

ROMP 40 

AVON PARK 

WELL 

ROMP 40 

HAWTHORN WELL

Rowell 

deep well

O

Kibler 

deep well 

5.2 miles ROMP 30 

TAMPA WELL ROMP 32 

AVON PARK 

WELL ROMP 30 

AVON 

PARK 

WELL

ROMP 31 

AVON PARK 

WELL 

ROMP 31 

HAWTHORN 

WELL

HARDEE COUNTY

Marshall DE SOTO COUNTY

O 

° Edgeville 

deep well Tropical 

River Groves 

deep well

ROMP 26 

OCALA WELL 

ROMP 26 

HAWTHORN 

WELL

ROMP 28 

FLORIDAN 

WELL 8.3 MILES

 ROMP 15 

AVON 

PARK 

ROMP 16 WELL

AVON PARK 

WELL 

ROMP 16 

HAWTHORN 

WELL

HIGHLANDS 

COUNTY

CHARLOTTE 

COUNTY

GLADES 

Punta Gorda COUNTY

Heights well 

6.6 miles

10 20 MILES

10 20 KILOMETERS

EXPLANATION

O INTERMEDIATE AQUIFER WELL 

 UPPER FLORIDAN WELL

Figure 39. Locations of wells used for hydrographic analysis.

52 Hydrogeology and Simulated Effects of Ground-Water Withdrawals for Citrus Irrigation, Hardee and De Soto Counties, Fla.
LU

O 

CO

UJ

cc

UJ

70

60

50

40

30

20

I 1 I 1 I

OBSERVED WATER LEVELROMP 16 WELL 

UPPER FLORIDAN AQUIFER

SIMULATED WATER LEVEL-- 

ROW 34, COLUMN 23 

UPPER FLORIDAN AQUIFER 

i i i

O N D

1988

M A M J J

1989

LU

O 

CO

£

cc

UJ

55

45

35

25

15

I I I I I I

SIMULATED WATER LEVEL - 

ROW 18, COLUMN 15 

UPPER FLORIDAN AQUIFER

OBSERVED WATER LEVELROMP 31 AVON PARK WELL 

UPPER FLORIDAN AQUIFER

O N D

1988

I I I I

A M J

1989

A S

UJ

O 

m

UJ

Z UJ

DC

UJ

50

40

30

20

10

i \ I T i i i r

SIMULATED WATER LEVEL-- 

ROW 6, COLUMN 6 

UPPER FLORIDAN AQUIFER

OBSERVED WATER LEVEL 

ROMP 40 AVON PARK WELL 

UPPER FLORIDAN AQUIFER

O N D

1988

i i i i i i

A M J

1989

UJ

>

O

CO

<

H

UJ . LU '

LI uj ^ > Z ~ uj _J

-i" <f

UJ m 

CC

UJ 1-

1

/u

60

50

40 

30

20

i i i i i i i i i i i

OBSERVED WATER LEVEL 

ROMP 15 AVON PARK WELL

UPPER FLORIDAN AQUIFER

""'* . '*' -

>"SSO % --.x''''^^^^~^""'"^"~^x. P^*"^

^^^"""' -. ^*S'- -^___^ Jf

"'X ^\vv

' SIMULATED WATER LEVEL

ROW 32, CLOUMN 30

UPPER FLORIDAN AQUIFER

ii i i i i i i i i

ONDJFMAMJJAS

1988 1989

UJ

O 

m

UJ

Z.UJ

70

60

50

UJ Jjj 40 

uj w

DC

UJ 30

20

iriti i i i i r

SIMULATED WATER LEVEL-- 

ROW 25, COLUMN 21 

UPPER FLORIDAN AQUIFER

OBSERVED WATER LEVELMARSHALL DEEP WELL 

UPPER FLORIDAN AQUIFER

O N D

1988

i i i i

A M J J

1989

UJ

O 

m

UJ

60

50

_

UJ 40 

jjj

UJ

cc

UJ

UJ 30

20

10

i i i r i i i i i r

SIMULATED WATER LEVELROW 17, COLUMN 22 

\ UPPER FLORIDAN AQUIFER

OBSERVED WATER LEVEL 

ROMP 30 AVON PARK WELL 

UPPER FLORIDAN AQUIFER

O N

i i i i i i

1988 1989

Figure 40. Simulated and observed water levels in selected wells open to the intermediate aquifer system and 

the Upper Floridan aquifer, October 1988 through September 1989.

Simulation of Ground-Water Flow 53
LLJ

O 

00

45

35

LLJ

UJ

LU

15

-5

n i i i i i r

SIMULATED WATER LEVEL -- 

ROW 17 COLUMN 6 

UPPER FLORIDAN AQUIFER

OBSERVED WATER 

LEVEL 

ROMP 32 AVON PARK WELL 

UPPER FLORIDAN AQUIFER

I i I i I i

O N D

1988

I I I I I

J F M A M J J

1989

A S

70 

LU

O

3 60

LU 

LU

50

LU

<

UJ 40

cc

LU 30

20

\ i i i r

OBSERVED WATER LEVEL -

MADDOX WELL

UPPER FLORIDAN AQUIFER

SIMULATED WATER LEVEL --- 

ROW 6 COLUMN 18 

UPPER FLORIDAN AQUIFER

I i I i i I I i i I I

O N D J

1988

F M A M J J

1989

A S

LU

O 

00 

<

LU 

LU

70

60

50

LU

DC 

LU 30

20

i i i i i i i i i i r

OBSERVED WATER LEVEL 

ROMP 26 AVON PARK WELL 

UPPER FLORIDAN AQUIFER

SIMULATED WATER 

LEVEL ------

ROW 27 COLUMN 20 

UPPER FLORIDAN AQUIFER

J___I i i i i i i i i

ONDJFMAMJJAS

1988 1989

LU

O 

00

LU 

LU

LU

LU

DC 

LU

50

40

30

20

10

i i i i r

OBSERVED WATER LEVEL - 

EDGEVILLE DEEP WELL 

UPPER FLORIDAN AQUIFER

SIMULATED WATER %. 

h LEVEL \,.--'' 

ROW 26 COLUMN 3 

UPPER FLORIDAN AQUIFER

J___I i i i i i i

ONDJFMAMJJAS

1988 1989

LU

> O

8

65 

55 

W 45

LU

LU 35 

25 

15

i \ i i i i i i i i r

OBSERVED WATER LEVEL 

AMAX WELL

UPPER FLORIDAN AQUIFER

SIMULATED WATER\ 

LEVEL --" "-..

ROW 29 COLUMN 9 \ / 

UPPER FLORIDAN AQUIFER

i i i i i i i i i i i

ONDljFMAMJJAS

1988 1989

Figure .40. Simulated and observed water levels in selected wells open to the intermediate aquifer system anc' 

the Upper Floridan aquifer, October 1988 through September 1989-Continued.

54 Hydrogeology and Simulated Effects of Ground-Water Withdrawals for Citrus Irrigation, Hardee and De Soto Counties, Fla.
LLJ 

>

O

CO

70

60

Sw 50 

z Lu

40

CE

LLJ

I

30

20

\ I I I I I I I

OBSERVED WATER LEVEL 

ROMP 26 HAWTHORN WELL 

INTERMEDIATE AQUIFER SYSTEM

SIMULATED WATER LEVEL *" 

ROW 27, COLUMN 20 

INTERMEDIATE AQUIFER SYSTEM

O N D

1988

M A M J J

1989

LLJ

O 

CO

UJ

CE 

LLJ

70

60

50

40

30

20

i i i i i \ \r

SIMULATED WATER LEVEL 

ROW 13, COLUMN 24 

INTERMEDIATE AQUIFER SYSTEM

.OBSERVED WATER

LEVEL 

ROWELL DEEP WELL 

INTERMEDIATE AQUIFER SYSTEM

O N D

1988

i i i i

A M J J A

1989

UJ

O

CO

UJ

CE

LLJ

I

70

60

50

40

30

20

\ i i i

SIMULATED WATER LEVEL 

ROW 17, COLUMN 22 

INTERMEDIATE AQUIFER SYSTEM

OBSERVED WATER LEVEL 

ROMP 30 TAMPA WELL 

INTERMEDIATE AQUIFER SYSTEM"

O N D

1988

i i i i

A M J

1989

LLJ

O 

CO

CE 

LLJ

I

60

50

40

30

20

10

r i i r i i i r 

SIMULATED WATER LEVEL 

ROW 18, COLUMN 15 

INTERMEDIATE AQUIFER SYSTEM

OBSERVED WATER LEVELROMP 31 HAWTHORN WELL 

INTERMEDIATE AQUIFER SYSTEM

i i i i r i i i

ONDJFMAMJJAS

1988 1989

LLJ 

>

O

CO

LL

140

130

120

LU 110

£ 1 °0

90

\ i \ i

OBSERVED WATER LEVEL 

ROMP 40 HAWTHORN WELL 

INTERMEDIATE AQUIFER SYSTEM

SIMULATED WATER LEVEL ----- 

ROW 6, COLUMN 6 

INTERMEDIATE AQUIFER SYSTEM

i i i i i i i

ONDJFMAMJJAS

1988 1989

Figure 40. Simulated and observed water levels in selected wells open to the intermediate aquifer system and 

the Upper Floridan aquifer, October 1988 through September 1989-Continued.

Simulation of Ground-Water Flow 55
82°09'15" 

27° 43'57

26° 56' 30"

HILLS6OROUGH

COUNTY

" / GLADES ' ' *~ *" *"' r COUNTY ' '

10 20 MILES

10 20 KILOMETERS

 40 

__ 40--

EXPLANATION

POTENTIOMETRIC CONTOUR Shows altitude at which water level would have stood in 

tightly cased wells open to the intermediate aquifer system. Contour intervals 5, 10, and 20 

feet. Hachures indicate depressions. Datum is sea level

SIMULATED POTENTIOMETRIC CONTOURS OF THE INTERMEDIATE 

AQUIFER SYSTEM

FLOW VECTOR Shows direction of ground-water flow. Length of 

vector is directly proportional to velocity

Figure 41. Interpolated (modified from Barr, 1989b) and simulated potentiometric surfaces and direction of 

ground-water movement in the intermediate aquifer system, May 1989.

56 Hydrogeology and Simulated Effects of Ground-Water Withdrawals for Citrus Irrigation, Hardee and De Soto Counties, Fla.
82° 09'15"

POLK COUNTY

Model 

Boundary

HARDEE COUNTY

DE SOTO COUNTY

2

- * < «- *-u < 

* * « * I* *

o\\\\N <\\\\

\\\\\\\\\\\\\\\\\

\\\\\\\\\\\\\\ VV\ \\

\\\\\ \ \ \ \J^A<V\\\\\\\\\\

HIGHLANDS _

CHARLOTTE . \ 

COUNTY \\\\\\\\\ t n \ \ \ \ n 1 1

26° 56' 30" f t\ \ \ \ M f It f

10 20 MILES

10 20 KILOMETERS

EXPLANATION

40 POTENTIOMETRIC CONTOUR Shows altitude at which water level would have stood in 

tightly cased wells open to the Upper Floridan aquifer. Contour intervals 5 and 10 

feet. Datum is sea level

40 SIMULATED POTENTIOMETRIC CONTOURS OF THE UPPER 

FLORIDAN AQUIFER

FLOW VECTOR Shows direction of ground-water flow. Length of 

"~ vector is directly proportional to velocity

Figure 42. Interpolated (modified from Barr, 1989d) and simulated potentiometric surfaces and direction of 

ground-water movement in the Upper Floridan aquifer, May 1989.

Simulation of Ground-Water Ffow 57
82°09'15" 

27° 43' 57

26° 56' 30"

POLK COUNTY

HILLSBOROUGH 

COUNTY

////://;// /;//////

~- -\£

HARDEE COUNTY

DE SOO COUNTY

S S / / 

S S / /

s s / /

S S / / /

10 20 MILES

10 20 KILOMETERS

EXPLANATION

POTENTIOMETRIC CONTOUR Shows altitude at which water level would have stood in 

tightly cased wells open to the intermediate aquifer system. Contour intervals 10 and 20 

feet Datum is sea level

 40 SIMULATED POTENTIOMETRIC CONTOURS OF THE INTERMEDIATE 

AQUIFER SYSTEM

- FLOW VECTOR Shows direction of ground-water flow. Length of vector 

is directly proportional to velocity

Figure 43. Interpolated (modified from Knochenmus and Barr, 1990a) and simulated potentiometric surfaces and 

direction of ground-water movement in the intermediate aquifer system, September 1989.

58 Hydrogeology and Simulated Effects of Ground-Water Withdrawals for Citrus Irrigation, Hardee and De Soto Counties, Fla.
82°09'15" £1°22'

Model 

Boundary

/ / / / / / / 

///\////

HARDEE COUNTY / / /

I 1 I \ \ 

HIGHLANDS \ 

COUNTY "/7

GLADES 

COUNTY

M I I I I I

26° 56' 30"

10 20 MILES

10 20 KILOMETERS

EXPLANATION

40 POTENTIOMETRIC CONTOUR Shows altitude at which water level would have stood in 

would have stood in tightly cased wells open to the Upper Floridan aquifer. Contour interva' 

10 feet. Datum is sea level

40 SIMULATED POTENTIOMETRIC CONTOURS OF THE UPPER 

FLORIDAN AQUIFER

 FLOW VECTOR Shows direction of ground-water flow. Length of vector 

is directly proportional to velocity

Figure 44. Interpolated (modified from Knochenmus and Barr, 1990b) and simulated potentiometric surfaces 

and direction of ground-water movement in the Upper Floridan aquifer, September 1989.

Simulation of Ground-Water Flow 59
Table 5. Statistical summary of differences between observed and simulated heads for the 

intermediate aquifer system and the Upper Floridan aquifer, May and September 1989

May 1989 September 1989

Statistical parameter Intermediate Upper Floridan 

aquifer system aquifer

Intermediate Upper Flcridan 

aquifer system aquifer

Number of individual wells

Maximum range in residuals (feet)

Arithmetic mean of residuals (feet)

Standard deviation of residuals (feet)

Median

Variance

Sum of absolute value of residuals (feet)

Root mean square error

47

32.71 to -28.17

0.45

12.32

1.56

151.80

21.05

3.07

48

I8.19to-14.12

5.07

7.40

5.83

54.79

243.54

35.15

48

15.20 to -20.74

-1.38

7.44

-1.48

55.41

-66.34

9.58

56

10.40 to -24.88

3.99

5.97

-3.68

35.65

-223.95

29.92

of the model area where a new control well was 

introduced for this map. The map of the September 

1989 simulated potentiometric surface of the Upper 

Floridan aquifer compares reasonably well with the 

map of the interpolated potentiometric surface, except 

for the western part of the model area where pumping 

rates could be inaccurate.

A statistical summary of differences between 

observed and simulated heads for the intermediate 

aquifer system and the Upper Floridan aquifer for May 

and September 1989 are listed in table 5. Residuals 

were computed for 47 nodes of the intermediate aquifer 

system and 48 nodes for the Upper Floridan aquifer 

that correspond to the locations of observation wells for 

May 1989. Based on the observations in May 1989, the 

mean difference between observed and simulated surfaces was 0.45 ft and the RMSE was 3.07 ft for the 

intermediate aquifer system. The mean and RMSE for 

the Upper Floridan aquifer was 5.07 ft and 35.15 ft, 

respectively. Residuals were computed for 48 nodes of 

the intermediate aquifer system and for 56 nodes of the 

Upper Floridan aquifer that correspond to the locations 

of observation wells for September 1989. Based on the 

observed surfaces for September 1989, the mean difference between observed and simulated surfaces was 

-1.38 ft and the RMSE was 9.58 ft for the intermediate 

aquifer system. The mean and RMSE for the Upper 

Floridan aquifer was 3.99 ft and 29.92 ft, respectively.

Sensitivity Analysis

Sensitivity tests were run to determine the relative 

importance of pumping rates, storage coefficients, and 

boundary conditions in the calibration of the transient 

model. Transmissivity and leakance arrays used in 

transient simulations were the same arrays used in the 

steady-state model calibration. Results of sensitivity

tests of the calibrated transient model fcr the intermediate aquifer system and the Upper Floridan aquifer are 

shown in figure 45 for row 23, columns 1 through 44.

Pumping rates were changed by first doubling the 

pumping rate and then reducing the rate by half. 

Results of sensitivity tests indicate that ? change in 

pumping rates significantly affects transient model 

simulations for both aquifers. Figure 45 shows that the 

departure of observed heads from computed heads for 

May 1989 could be reduced along row 23 by increasing 

withdrawal rates in the model.

Storage coefficients were changed by a factor of 10 

for each aquifer. Results of the sensitivity test for this 

change for the intermediate aquifer system indicate that 

the model is relatively sensitive to increases and relatively insensitive to decreases in storage coefficients. 

Results of the sensitivity test for the Upper Floridan 

aquifer indicate that changes in storage coefficients had 

little effect on the aquifer, which, in turn, indicates that 

only a small amount of the water withdrawn is released 

from aquifer storage.

The effects of varying boundary conditions for 

both aquifers are shown in figure 46. Raising and lowering the water table by 3 ft throughout the model area 

resulted in a corresponding rise and drop in the simulated heads, averaging less than 1 ft for both aquifers. 

The model was tested to estimate the range in error that 

could result from selecting specified-head and no-flow 

lateral boundary conditions in the model rather than the 

general head boundary conditions that were used for 

calibration. Changing the general-head boundaries to 

specified-head boundaries resulted in a rnad change of 

approximately 20 ft at the eastern and western boundaries for both aquifers. Changing the general-head 

boundary to a no-flow boundary resulted in a maximum 

head decline of about 10 ft at the eastern boundary and 

a rise of about 30 ft along the western boundary for

60 Hydrogeology and Simulated Effects of Ground-Water Withdrawals for Citrus Irrigation, Hardee and De Soto Counties, Fla.
LU 

LU

LU 

V)

O

LU 

CO

CC 

O 

LU

O 

CO

LU 

LU

LU

LU

_J

CC SEA 

W LEVEL

ROW 23 

INTERMEDIATE AQUIFER SYSTEM

CALIBRATION 

10 X STORAGE 

0.1 X STORAGE

 - 2XPUMPAGE

 05X PUMPAGE

OBSERVED HEADS

20 25 

COLUMN

111

> 

LU

LU 

V)

I

LU 

CO 

CC 

O

LU

O 

CO

LU 

LU 

LL

LU 

LU

tr

LU

60

50

40

20

10

SEA 

LEVEL

ROW 23 

UPPER FLORIDAN AQUIFER

^ CALBRATION

 10 x STORAGE

............. o 1 X STORAGE

_._._.. 2X PUMPAGE

 05X PUMPAGE

 OBSERVED HEADS

i I i i I I i i

10 15 20 25 

COLUMN

30 35 40

Figure 45. Results of sensitivity analyses of storage and pumpage parameters on the transient calibration 

for the intermediate aquifer system and the Upper Floridan aquifer, May 1989.

Simulation of Ground-Water Flow 61
UJ

>

UJ

CO

o

_l 

UJ 

CD

cc 

o

UJ

O 

CD

UJ 

UJ

UJ 

UJ

cc

LU

I

UJ 

LU

UJ 

CO

O

UJ 

CD

CC

o

UJ

o

CD

UJ 

UJ 

UUJ

UJ

_l

CC 

UJ

I

90

70

60

40

30

10

SEA 

LEVEL

20 -

30

90

80

70

60

50

40

30

20

10

SEA 

LEVEL

10

20

30

ROW 23 

INTERMEDIATE AQUIFER SYSTEM

 CALIBRATION

SPECIFIED HEAD BOUNDARY 

....... NO ROW BOUNDARY

._.- SURFICIAL HEADS + 3 0 FT 

..... SURFICIALHEADS-30FT

..... GENERAL HEAD BOUNDARY-10 OFT

 GENERAL HEAD BOUNDARY+ 10 OFT 

OBSERVED HEADS

10 15 20 25- 

COLUMN

30 35 40 44

ROW 23 

UPPER FLORIDAN AQUIFER

	CALIBRATION

 SPECIFIED HEAD BOUNDARY

........... NO FLOW BOUNDARY

,._._.- SURFICIALHEADS + 30FT

......... SURFICIALHEADS-30FT

...... GENERAL HEAD BOUNDARY-10 OFT ~

 - GENERALHEAD BOUNDARY+ 100 FT

 OBSERVED HEADS

10 15 20 25 

COLUMN

30 35 40

Figure 46. Results of sensitivity analyses of model boundary conditions on the transient calibration for the 

intermediate aquifer system and the Upper Floridan aquifer, May 1989.

62 Hydrogeology and Simulated Effects of Ground-Water Withdrawals for Citrus Irrigation, Hardee and De Soto Counties, Fla.
Table 6. Simulated water budget for 1988-89 flow conditions 

[Mgal/d, million gallons per day]

Steady-state model 

calibration, 

Parameter September 1988

Mgal/d

Intermediate aquifer system

From storage

Boundary flow (in)

Downward leakage from the surficial aquifer

Upward leakage from the Upper Floridan aquifer

Total inflow

Into storage

Boundary flow (out)

Downward leakage to the Upper Floridan aquifer

Upward leakage to the surficial aquifer

Pumpage

Total outflow

Upper Floridan aquifer

From storage

Boundary flow (in)

Downward leakage from the intermediate aquifer system

Total inflow

Into storage

Boundary flow (out)

Upward leakage to the intermediate aquifer system

Pumpage

Total outflow

0

3

100

5

108

0

4

98

4

1

108

0

88

98

186

0

112

5

69

186

Percent

0

3

93

4

100

0

4

91

4

<1

100

0

47

53

100

0

60

3

37

100

Transient model 

calibration, 

May 1989

Mgal/d

27

4

178

o

211

0

1

180

2

28

211

47

171

180

398

0

157

4

237

398

Percent

13

2

84

1

100

0

1

85

1

13

100

12

43

45

100

0

39

1

60

100

Transient model 

calibration, 

September 1989

Mgal/d

1

3

99

4

107

0

9

93

4

1

107

2

80

93

175

0

100

6

69

175

Percent

1

3

92

4

100

0

8

87

4

1

100

1

46

53

100

0

57

4

69

100

both aquifers. These extreme head changes resulted 

from a lack of inflow at the potentiome trie-surface high 

along the eastern boundary and a buildup at the western 

boundary where ground water could not flow laterally 

out of the model area.

The model was tested to estimate the range in error 

that would result from raising and lowering the generalhead boundary (HI) by 10 ft (fig. 46). When this test 

was performed, the heads showed corresponding 

increases or decreases of approximately 10 ft from the 

calibrated heads. This test indicates that the model 

calibration is sensitive to small changes in assigned 

general-head boundaries.

Simulated Ground-Water Budget for 1988-89

The sources and amounts of ground-water inflow 

to and outflow from the intermediate aquifer system 

and the Upper Floridan aquifer in the model area for 

1988-89 are listed in table 6. Volumetric-balance computations used to compute the water budget given in 

table 6 are based on the September 1988 steady-state

and the May 1989 and September 1989 transientcalibration periods. The components of ground-water 

inflow were derived from aquifer storage, cross-boundary 

flow, and downward leakage through the upper and 

lower confining units of the intermediate aquifer system. Ground-water outflows were derived from aquifer 

storage, pumpage, upward leakage through the upper 

and lower confining units of the intermediate aquifer 

system, and cross-boundary flow.

Total inflow and outflow through the aquifers during 

the September 1988 steady-state model run was 108 

Mgal/d for the intermediate aquifer system and 186 

Mgal/d for the Upper Floridan aquifer. Results of the 

water-budget analysis for the intermediate acuifer system indicate that 93 percent of total inflow was downward leakage from the surficial aquifer, 4 percent was 

upward leakage from the Upper Floridan aquifer, and 3 

percent was from cross-boundary flow. Outflow from 

the intermediate aquifer system consisted of 91 percent 

downward leakage into the Upper Floridan anuifer, 

4 percent upward leakage into the surficial aquifer, 

4 percent cross-boundary flow, and less than 1 percent

Simulation of Ground-Water Flow 63
pumpage. Results of the water-budget analysis for the 

Upper Floridan aquifer during this same period indicate that 53 percent of inflow was derived from downward leakage from the intermediate aquifer system and 

47 percent was from cross-boundary flow. Outflow 

from the Upper Floridan aquifer was 60 percent crossboundary flow, 37 percent pumpage, and 3 percent 

upward leakage into the intermediate aquifer system. 

The amount of downward leakage (representing the 

natural recharge within the model area) was equivalent 

to 0.85 in/yr. This recharge value is comparable to 

those determined in other studies (Wilson and Gerhart, 

1982; Ryder, 1985; Aucott, 1988).

Total inflow and outflow through the aquifers during 

the pumping period of May 1989 was 211 Mgal/d for the 

intermediate aquifer system and 398 Mgal/d for the 

Upper Floridan aquifer. Results of the simulated waterbudget analysis for the intermediate aquifer system indicate that 84 percent of inflow was derived from downward leakage from the surficial aquifer, 13 percent from 

aquifer storage, 2 percent from cross-boundary flow, and 

1 percent from upward flow from the Upper Floridan 

aquifer. Outflow from the intermediate aquifer system 

was 85 percent downward leakage through the lower 

confining unit, 13 percent pumpage, 1 percent crossboundary flow, and 1 percent upward leakage to the surficial aquifer. Results of the water-budget analysis for the 

Upper Floridan aquifer during this same period indicate 

that 45 percent of inflow to the aquifer was from downward leakage, 43 percent was from cross-boundary flow, 

and 12 percent was from aquifer storage. Outflow from 

the aquifer was 60 percent pumpage, 39 percent crossboundary flow, and 1 percent upward leakage to the 

intermediate aquifer system.

Total inflow and outflow through the aquifers 

during the nonirrigation period of September 1989 was 

107 Mgal/d for the intermediate aquifer system and 175 

Mgal/d for the Upper Floridan aquifer. Results of the 

water-budget analysis for the intermediate aquifer system indicate that 92 percent of total inflow was downward leakage from the surficial aquifer, 4 percent was 

upward leakage from the Upper Floridan aquifer, 3 percent was from cross-boundary flow, and 1 percent was 

from aquifer storage. Outflow from the intermediate 

aquifer system consisted of 87 percent downward leakage into the Upper Floridan aquifer, 4 percent upward 

leakage into the surficial aquifer, 8 percent crossboundary flow, and less than 1 percent pumpage. 

Results of the water-budget analysis for the Upper 

Floridan aquifer during this same period indicate that 

46 percent of inflow was derived from downward

leakage from the intermediate aquifer system, 53 

percent was from cross-boundary flow, and 1 percent 

was from aquifer storage. Outflow from the Upper 

Floridan aquifer was 57 percent from cross-boundary 

flow, 39 percent from pumpage, and 4 percent from 

upward leakage to the intermediate aquifer system.

The water-budget analysis results indicate that most 

of the pumpage discharged from the Upper Floridan 

aquifer under heavy pumping conditions in May 1989 

was derived about equally from boundary inflow and 

downward leakage. Pumpage from the intermediate 

aquifer system was derived primarily from downward 

leakage from the surficial aquifer. The relatively low 

transmissivity of the intermediate aquifer system limits 

the amount of water that is derived from boundary 

inflow.

Simulated Ground-Water Flow Analysis 

for September 1988, May 1989, and! 

September 1989

The generalized directions and relative darcian 

velocities of the simulated ground- water fow system are 

illustrated in figures 35 and 36 and figures 41 through 44. 

The flow-vector diagrams were used to illustrate the 

results of the ground-water simulation and the magnitude and direction of water movement. The magnitude 

and direction of flow were calculated from the cell-bycell flow terms in the modular model using the MMSP 

program by Scott (1990). The darcian ve^city for 

ground water is the rate of discharge of ground water per 

unit area of porous medium measured at right angles to 

the direction of flow (Lohman and others, 1972). Each 

vector point represents the direction of flow indicated by 

the x-y gradients and the length of the vector is proportional to the darcian velocity.

The generalized directions and relative darcian 

velocities of ground-water flow in the intermediate 

aquifer system for September 1988, May 1989, and September 1989 are shown in figures 35,41, and 43; respectively. Regional ground-water flow is fron areas of high 

potential to areas of low potential. Two areas of high 

potential in the model area are the Polk Upland and the 

Lake Wales Ridge (fig. 2). Regional flow-line directions 

are south to southwest toward the coast, and localized 

flow-line directions are toward the Peace River and 

toward local pumping centers. The regional groundwater flow paths are similar for all time periods, except 

in areas where ground-water pumpage increases in May. 

Darcian velocities are lowest at the Lake Wales Ridge 

and the Polk Upland and are highest along the Peace 

River where flow is toward the river.

64 Hydrogeology and Simulated Effects of Ground-Water Withdrawals for Citrus Irrigation, Hardee and De Soto Counties, Fla.
The generalized directions and relative darcian 

velocities of ground-water flow in the Upper Floridan 

aquifer for September 1988, May 1989, and September 

1989 are shown in figures 36, 42, and 44; respectively. 

As indicated by the flow vectors, regional groundwater flow is from areas of high potential to areas of 

low potential. The high potentiometric surface along 

the Lake Wales Ridge indicates that this area is a 

recharge area. Regionally, the flow vectors are oriented 

toward the west or southwest toward southern Hillsborough and Manatee Counties where large ground-water 

withdrawals for irrigation have resulted in a perennial 

cone of depression in the potentiometric surface. The 

regional ground-water flow paths were toward this 

cone of depression during September 1988 and September 1989. In May 1989, flow vectors were primarily 

oriented toward local pumping centers. Darcian velocities in these figures are relatively uniform for the 

Upper Floridan aquifer throughout the model area 

except at pumping centers where the darcian velocities 

are slightly higher and along the Lake Wales Ridge 

where the darcian velocities are slightly lower.

HYPOTHETICAL DEVELOPMENT 

SCENARIOS

The calibrated transient model was used to 

simulate changes in the potentiometric surfaces of the 

intermediate aquifer system and the Upper Floridan 

aquifer that might result from increased ground-water 

withdrawals for citrus irrigation for the years 2000 and 

2020. Only the effects of projected increases in withdrawals for citrus irrigation were evaluated for this 

investigation. Withdrawals for public, industrial, and 

other agricultural uses were assumed to remain 

constant in future pumping scenarios.

Model runs were made to simulate the irrigation 

season (September through May). Because the September potentiometric surface recovers to nearly the 

same level each year, the model-simulation runs for projected withdrawals used starting heads based on the calibrated September 1988 potentiometric surface. Nine 

monthly stress periods were used to simulate existing 

and projected seasonal pumping rates for the irrigation 

period. Projected ground-water withdrawal rates for 

citrus irrigation were determined by multiplying the 

projected increase in acreage used for citrus by an average historical application rate of 11 in/yr for citrus drip 

irrigation (Duerr and Trommer, 1982; Taylor and others, 

1990). The projected citrus irrigation pumping rates 

were added to the transient model pumping arrays.

The areal distribution of the projected new citrus 

acreage was selected on the basis of the proximity of 

existing citrus groves to vacant land or to existing 

melon and pasture acreage (Mark Hammond, Southwest Florida Water Management District, written 

commun., 1990). The projected increase in citrus acreage is widely dispersed for the year 2000, but included 

a 15-mi area in eastern De Soto County representing a 

large new grove for the year 2020. The locations of 

these projected citrus groves in the study area are 

shown in figure 47. The current and projected citrus 

acreage and pumping rates used in the model simulations for 1989, 2000, and 2020 are presented in table 7.

Results of the transient model runs with increased 

pumpage for citrus irrigation are presented a? a series 

of contour maps that show the potentiometric surface 

for each aquifer for the end of the simulation periods, 

which represent May 2000 and May 2020. Results also 

are presented as a series of change maps that show the 

amount of decline between the May 1989 calibration 

period and May 2000 and between May 1989 and May 

2020. September was not illustrated in the change-map 

series because the potentiometric surface recovered to 

September 1989 simulated heads after pumpage was 

removed for each successive recovery period. Distribution of pumpage from the intermediate aquifer system 

and the Upper Floridan aquifer used in the simulations 

for May 1989, May 2000, and May 2020 are shown in 

appendix I.

May 2000

The simulated potentiometric surfaces of the intermediate aquifer system and the Upper Floridan aquifer 

for May 2000 and the departure from May 1989 heads 

are shown in figures 48 and 49, respectively. These 

simulations are the result of increasing pumpage for the 

1988-89 water year by 11 percent in the intermediate 

aquifer system and 10 percent in the Upper Floridan 

aquifer.

For the intermediate aquifer system, a maximum 

decline of 3 ft was simulated in three localized areas, 

and an average decline of more than 1.5 ft was simulated for most of the model area (fig. 48). For the Upper 

Floridan aquifer, a maximum decline of about 4 ft was 

simulated in a 1-mi area in Hardee County, and an 

average decline of about 1.5 ft was simulated in the 

two-county area (fig. 49).

Hypothetical Development Scenarios 65
82°09'15" 81° 22'

27°43' 57"

26°56' 30"

POLK COUNTY

HILLSBOROUGH 

COUNTY

CHARLOTTE 

COUNTY

10 20 MILES

10 20 KILOMETERS 

EXPLANATION

2000 GRID BLOCK With increased 

ground-water withdrawals for 

2020 citrus irrigation

Figure 47. Locations of model grid blocks with projected increases in citrus acreage zones for the years 2COO and 2020.

66 Hydrogeology and Simulated Effects of Ground-Water Withdrawals for Citrus Irrigation, Hardee and De Sotc Counties, Ra.
Table 7. Current and projected ground-water withdrawal 

rates and citrus acreage for 1989, 2000, and 2020

Parameter

Ground-water withdrawals, 

in million gallons per day

Hardee 

County

De Soto 

County Total

1989

Citrus acreage 43,143 45,898 89,041

Source of withdrawals

Intermediate aquifer system 4 48

Upper Floridan aquifer 34 35 69

Total 38 39 77

2000

Citrus acreage 59,047 63,518 122,565

Source of withdrawals

Intermediate aquifer system 5 5 10

Upper Floridan aquifer 47 51 98

Total 52 56 108

2020

Citrus acreage 59,047 75,377 134,424

Source of withdrawals

Intermediate aquifer system 5 6 11

Upper Floridan aquifer 47 60 107

Total 52 66 118

May 2020

The simulated potentiometric surfaces of the intermediate aquifer system and the Upper Floridan aquifer 

for May 2020 and the departure from May 1989 heads 

are shown in figures 50 and 51, respectively. These 

simulations are the result of increasing pumpage for the 

1988-89 water year by 25 percent in the intermediate 

aquifer system and 21 percent in the Upper Floridan 

aquifer.

For the intermediate aquifer system, a maximum 

decline of more than 10 ft was simulated in eastern 

De Soto County, and an average decline of more than 

2 ft was simulated for much of the model area (fig. 50). 

For the Upper Floridan aquifer, a maximum decline of 

about 5 ft was simulated in eastern De Soto County, and 

a decline of more than 2 ft was simulated for much of 

the model area (fig. 51). The largest declines occurred 

at the postulated 15-mi citrus grove.

SIMULATED EFFECTS OF INCREASED 

WITHDRAWALS FOR CITRUS IRRIGATION

The computed rates of upward and downward 

leakage, lateral inflow and outflow, pumpage, and 

water released from storage during May 1989 and 

under conditions of increased citrus irrigation projected 

for May 2000 and May 2020 are listed in table 8.

The projected increase in ground-water withdrawals 

would alter the flow system from that which existed in 

1988-89, but not to a great degree. The simulations 

were used to investigate the potential effects of 

hypothetical development between May 19°9 and 

May 2000 and between May 1989 and May 2020. 

The results indicated that major effects of increased 

withdrawals include:

 A maximum decline of more than 10 ft in the potentiometric surface of the intermediate aquifer system at a 

projected grove in eastern De Soto County and a decline 

of more than 2 ft in the potentiometric surface of this 

aquifer system throughout much of the study area.

 An increase in downward leakage from the overlying 

surficial aquifer system to the intermediate anuifer 

system from 178 to 183 Mgal/d.

 A decrease in upward leakage from the intermediate 

aquifer system to the surficial aquifer from 1.58 to 

1.47 Mgal/d.

 A maximum decline of about 5 ft in the potentiometric 

surface of the Upper Floridan aquifer at a projected 

grove in eastern De Soto County and a decline of more 

than 2 ft in the potentiometric surface of this aquifer 

throughout much of the study area.

 An increase in downward leakage from the intermediate 

aquifer system to the Upper Floridan aquifer from 180 

to 183 Mgal/d.

 A decrease in upward leakage from the Upper Floridan 

aquifer to the intermediate aquifer system from 4.32 

Mgal/d in May 1989 to 3.89 million gallons per day, in 

May 2000 but an increase in upward leakage to 5.10 

Mgal/d between May 1989 and May 2020, reflecting a 

change in hydrologic gradient.

SUMMARY AND CONCLUSIONS

The hydrogeology of Hardee and De Sotc Counties 

in west-central Florida was evaluated to estimate 

changes in the potentiometric surfaces of the intermediate aquifer system and the Upper Floridan aquifer as 

a result of expected increases in ground-water withdrawals for citrus irrigation for the years 20CO and 

2020. Citrus groves constitute the second largest landuse category in Hardee and De Soto Counties and citrus acreage in these counties is expected to increase 

because of the favorable climatic conditions. Total 

citrus acreage in Hardee and De Soto Counties in 1988 

was 89,041 acres, and it is projected that citrus acreage 

will increase to about 130,000 acres by the year 2020.

Summary and Conclusions 67
A. 82°09'15" 

27°43' 57"

SIMULATED POTENTIOMETRIC 

SURFACE

81°22'

26°56' 30"

10 20 MILES

10 20 KILOMETERS

82°09'15

27°43' 57'

DECLINE IN POTENTIOMETRIC 

SURFACE

81°22'

26°56' 30"

GLADES 

COUNTY

10 20 MILES

0 10 20 KILOMETERS 

EXPLANATION

 40 POTENTIOMETRIC CONTOUR 

Shows altitude at which water level 

would have stood in tightly cased 

wells open to the intermediate 

aquifer system, May 2000. Contour 

intervals 10 and 20 feet. Datum is sea level

LINE OF EQUAL CHANGE IN 

POTENTIOMETRIC SURFACE 

Shows amount of decline in the 

potentiometric surface between the 

observed May 1989 level and the 

simulated May 2000 level. Contour 

interval 1 foot

Figure 48. Simulated potentiometric surface of the intermediate aquifer system, May 2000, and simulated declines in the 

potentiometric surface as a result of projected increases in ground-water withdrawals for citrus irrigation, May 1989 to 

May 2000.

68 Hydrogeology and Simulated Effects of Ground-Water Withdrawals for Citrus Irrigation, Hardee and De Sof Counties, Fla.
A. SIMULATED POTENTIOMETRIC 81 22'

26°56' 30"

10 20 MILES

10 20 KILOMETERS

B- 82°09'15" 

27C43' 57'

DECLINE IN POTENTIOMETRIC 

SURFACE

81°22'

26°56' 30"

10 20 MILES

10 20 KILOMETERS

EXPLANATION

 40 POTENTIOMETRIC CONTOUR 

Shows altitude at which water levd 

would have stood in tightly cased 

wells open to the Upper Floridan 

aquifer, May 2000. Contour interval 

is 10 feet. Datum is sea level

1 LINE OF EQUAL CHANGE IN 

POTENTIOMETRIC SURFACE 

Shows amount of decline in the 

potentiometric surface between the 

observed May 1989 level and the 

simulated May 2000 level. Contour 

intervals are 1 and 5 feet

Figure 49. Simulated potentiometric surface of the Upper Floridan aquifer, May 2000, and simulated declines in the 

potentiometric surface as a result of projected increases in ground-water withdrawals for citrus irrigation, May 1989 to 

May 2000.

Summary and Conclusions 69
A. 82°09'15" 

27°43' 57"

SIMULATED POTENTIOMETRIC 

SURFACE

81 C 22'

26°56' 30"

I POLKCOUI 

HILLS- Y 

BOROUGH

10 20 MILES

10 20 KILOMETERS

82°09'15" 

27°43' 57"

DECLINE IN POTENTIOMETRIC 

SURFACE

81° 22'

26°56' 30"

10 20 KILOMETERS

EXPLANATION

 40 POTENTIOMETRIC CONTOUR 

Shows albtude at which water leveJ 

would have stood in tightly cased 

wells open to the intermediate 

aquifer system, May 2020. Contour 

intervals are 10 and 20 feet. Datum 

is sea level

 1 LINE OF EQUAL CHANGE IN 

POTENTIOMETRIC SURFACE 

Shows amount of decline in the 

potentiometric surface between the 

observed May 1989 level and the 

simulated May 2020 level. Contour 

intervals are 1 and 5 feet

Figure 50. Simulated potentiometric surface of the intermediate aquifer system, May 2020, and simulated declines in the 

potentiometric surface as a result of projected increases in ground-water withdrawals for citrus irrigation, May 1989 to 

May 2020.

70 Hydrogeology and Simulated Effects of Ground-Water Withdrawals for Citrus Irrigation, Hardee and De Soto Counties, Fla.
A. 82°09'15" 

27°43' 57'

SIMULATED POTENTIOMETRIC 

SURFACE

81°22'

26°56'30"

82°09'15" 

27°43'57'

20 KILOMETERS

DECLINE IN POTENTIOMETRIC 

SURFACE

81°22'

26°56' 30"

HILLS. 

BOROUGH 

OUNTY '

GLADES 

COUNTY

10 20 MILES

10 20 KILOMETERS

EXPLANATION

 40 POTENTIOMETRIC CONTOUR 

Shows altitude at which water level 

would have stood in tightly cased 

wells open to the Upper Floridan 

Aquifer, May 2020. Contour interval 

10 feet Datum is sea level

LINE OF EQUAL CHANGE IN 

POTENTIOMETRIC SURFACE 

Shows amount of decline in the 

potentiometric surface between the 

observed May 1989 level and the 

simulated May 2020 level. Contour 

interval is 1 foot

Figure 51. Simulated potentiometric surface of the Upper Floridan aquifer, May 2020, and simulated declines in the 

potentiometric surface as a result of projected increases in ground-water withdrawals for citrus irrigation, May 1989 to 

May 2020.

Summary and Conclusions 71
Table 8. Simulated water budgets for May 1989, May 2000, and May 2020 flow conditions 

[Mgal/d, million gallons per day]

Parameter

Intermediate aquifer system

From storage

Boundary flow (in)

Downward leakage from the surficial aquifer

Upward leakage from the Upper Floridan aquifer

Total inflow

Into storage

Boundary flow (out)

Downward leakage to the Upper Floridan aquifer

Upward leakage to the surficial aquifer

Pumpage

Total outflow

Upper Floridan aquifer

From storage

Boundary flow (in)

Downward leakage from the intermediate aquifer system

Total inflow

Into storage

Boundary flow (out)

Upward leakage to the intermediate aquifer system

Pumpage

Total outflow

Transient model 

calibration, 

May 1989

Mgal/d

27

4

178

2

211

0

1

180

2

28

211

47

171

180

398

0

157

4

237

398

Percent

13

2

84

1

100

0

1

85

1

13

100

12

43

45

100

0

39

1

60

100

Transient model 

calibration, 

May 2000

Mgal/d

27

5

182

4

218

0

2

183

2

31

218

33

199

183

415

0

150

4

261

415

Percent

12

2

84

2

100

0

1

84

1

14

100

8

48

44

100

0

36

1

63

100

Transient model 

calibration. 

May 2020

Mgal/d

26

7

183

5

221

0

1

183

2

35

221

33

205

183

421

0

130

5

286

421

Percent

12

3

83

2

100

0

<1

83

1

16

100

8

49

43

100

0

31

1

68

100

Fresh ground water is obtained from three principal 

aquifers in the study area: the surficial aquifer, the 

intermediate aquifer system, and the highly productive 

Upper Floridan aquifer. The surficial aquifer is composed predominantly of quartz sand deposits that generally are less than 100 ft thick. Ground water in the 

surficial aquifer is unconfmed, and the water table in 

this aquifer fluctuates about 2 to 7 ft seasonally. The 

surficial aquifer is recharged by rainfall.

The intermediate aquifer system lies beneath the 

surficial aquifer and is composed of the rocks and clastic deposits of the Hawthorn Group. The intermediate 

aquifer system generally is 200 to 500 ft thick and contains a permeable unit composed of interbedded limestone and dolomite. Transmissivities of the 

intermediate aquifer system range from about 400 to 

7,000 ft2/d and storage coefficients range from about 

2.0xlO~4 to 5.0xlO-4 .

The intermediate aquifer system has an upper and 

a lower confining unit. The upper confining unit ranges 

in thickness from less than 25 to about 265 ft and consists of dolomite, sand, clay, silt, and phosphorite. As 

determined by model calibration, leakance for the

upper confining unit in the study area ranges from 

S.OxlO'3 to l.OxlO"6 (ft/d)/ft. The lower confining unit 

ranges in thickness from less than 25 to about 185 ft 

and consists of sand and clay, sand, and phosphorite. 

As determined by model calibration, leakance of the 

lower confining unit ranges from 1.0x10 to 1.0x10 

(ft/d) ft.

The potentiometric surface of the intermediate 

aquifer system fluctuates seasonally; the highest levels 

occur in September, and the lowest levels occur in May. 

The potentiometric surface in September 1988 ranged 

from 120 ft above sea level in northweste-n Hardee 

County to 40 ft above sea level in southwestern Hardee 

County and northwestern De Soto Count)'. The potentiometric surface ranged from about 110 ft above sea 

level in northwestern Hardee County to 5 ft above sea 

level in southwestern Hardee County in May 1989 at 

the end of the irrigation season. Regional ground-water 

flow in the intermediate aquifer system generally is 

south to southwest from the Polk Upland and the Lake 

Wales Ridge toward the Gulf of Mexico; locally, 

ground-water flow is toward the Peace River and 

pumping wells.

72 Hydrogeology and Simulated Effects of Ground-Water Withdrawals for Citrus Irrigation, Hardee and De Soto Counties, Fla.
The intermediate aquifer system is the second largest source of water supply in the study area. In 1988, 

ground-water withdrawals from this aquifer system 

averaged 16 Mgal/d and were primarily for irrigation 

and public supply. Several hundred wells are open to 

this aquifer system, and it is a valuable source of water 

in the southern part of the study area where the highly 

productive Upper Floridan aquifer contains mineralized water.

The highly productive Upper Floridan aquifer consists of fractured and solution-riddled carbonate rocks. 

The Upper Floridan aquifer includes all or parts of the 

Suwannee Limestone, the Ocala Limestone, and the 

Avon Park Formation. The top of the Upper Floridan 

aquifer is the horizon below which carbonate rocks 

consistently occur. The base of the Upper Floridan 

aquifer, the middle confining unit of the Floridan aquifer system, is the first persistently occurring, intergranular evaporite in the carbonate rocks. Thickness of the 

Upper Floridan aquifer ranges from 1,200 to 1,400 ft in 

the study area. Transmissivity values for this aquifer 

range from 70,600 to 850,000 ft2/d, and storage coeffi-

-4 v4 cients range from 1.0x10"^ to 1.2x10 .

The potentiometric surface of the Upper Floridan 

aquifer fluctuates seasonally; the highest levels occur 

in September and the lowest levels occur in May. In 

September 1988, the altitude of the potentiometric surface of this aquifer ranged from about 80 ft above sea 

level in northeastern Hardee County to 40 ft above sea 

level in southwestern Hardee County and northwestern 

De Soto County. In May 1989 at the end of the irrigation season, the altitude of the potentiometric surface of 

the Upper Floridan ranged from 60 ft above sea level in 

northeastern Hardee County to about 5 ft above sea 

level in western Hardee County. Regional groundwater flow generally is toward the west or southwest 

from the Lake Wales Ridge toward the Gulf of Mexico 

and toward large pumping centers in Hillsborough and 

Manatee Counties. Locally, ground-water flow is 

toward pumping wells.

The Upper Floridan aquifer is the major source of 

water supply in the study area; wells open to this aquifer can yield more than 2,500 gal/min. Thousands of 

wells are open to this aquifer and are used for irrigation, 

industrial, domestic, and public supply. Ground-water 

withdrawals from the Upper Floridan aquifer in the 

study area in 1988 averaged 106 Mgal/d, mostly for 

agricultural, public supply, and industrial use.

A quasi-three-dimensional ground-wate~ flow 

model was used to compute hydraulic head changes in 

response to changes in projected pumping rr*es in the 

intermediate aquifer system and the Upper Floridan 

aquifer. A steady-state model was calibrated to better 

define the hydrogeologic parameters of the rauifer 

system and to serve as the initial point for subsequent 

transient simulations. A steady-state calibration was 

achieved by systematically varying transmissivity and 

leakance until model simulations approximated field 

conditions. Principal stresses on the aquifer system in 

September 1988, at the end of the wet season, were 

withdrawals for industrial and municipal supplies. 

Irrigation pumpage was assumed to be zero for the 

calibration period.

A transient simulation was performed to determine 

the effects of municipal, industrial, and agricultural 

pumping on the potentiometric surfaces of the intermediate aquifer system and the Upper Floridan aquifer. 

The transient model was considered calibrated when 

simulated hydrographs for the period from S eptember

1988 to September 1989 were in reasonable agreement 

with observed hydrographs and when simulated May

1989 and September 1989 potentiometric surfaces 

were in reasonable agreement with the previously 

mapped surfaces for the intermediate aquifer system 

and the Upper Floridan aquifer.

Transient-model analyses were used to simulate 

the change in the potentiometric surfaces of the intermediate aquifer system and the Upper Florida aquifer 

for the years 2000 and 2020 that might resul^ from 

projected ground-water withdrawals for citnis irrigation. Simulation results indicated that the projected 

increase in ground-water withdrawals woul<? alter the 

flow system from that observed in 1988, but not to a 

great degree. The principal effects of hypothetical 

development are;

 A maximum decline of more than 10 ft in the potentiometric surface of the intermediate aquifer syster1 at a projected grove in eastern De Soto County and an average 

decline of more than 2 ft in the potentiometric surface of 

this aquifer throughout much of the study area.

 An increase in downward leakage from the ove-lying surficial aquifer system to the intermediate aquifer system 

from 178 to 183 Mgal/d.

 A decrease in upward leakage from the intermediate aquifer system to the surficial aquifer from 1.58 to 1.47 

Mgal/d.

Summary and Conclusions 73
A maximum decline of about 5 ft in the potentiometric 

surface of the Upper Floridan aquifer at a projected 

grove in eastern De Soto County and a decline of more 

than 2 ft in the potentiometric surface of this aquifer 

throughout much of the study area.

An increase in downward leakage from the intermediate 

aquifer system to the Upper Floridan aquifer from 180 

to!83Mgal/d.

A decrease in upward leakage from the Upper Floridan 

aquifer to the intermediate aquifer system from 4.32 

Mgal/d in May 1989 to 3.89 Mgal/d in May 2000, but an 

increase in upward leakage to 5.10 Mgal/d between May 

1989 and May 2020, reflecting a change in hydrologic 

gradient.

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discharge from the Floridian aquifer system in Florida: 

U.S. Geological Survey Water-Resources Investigations Report 88-4057, 1 sheet.

Barr, G.L., 1989a, Potentiometric surface of the intermediate aquifer, west-central Florida, September 1988: U.S. 

Geological Survey Open-File Report 89-36, 1 sheet.

 1989b, Potentiometric surface of the intermediate aquifer, west-central Florida, May 1989: U.S. Geological 

Survey Open-File Report 89-394, 1 sheet.

 1989c, Potentiometric surface of the Upper Floridan 

aquifer, west-central Florida, September 1988: U.S. 

Geological Survey Open-File Report 88-730, 1 sheet. 

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aquifer, west-central Florida, May 1989: U.S. Geological Survey Open-File Report 89-393, 1 sheet.

Bergendahl, M.H., 1956, Stratigraphy of parts of De Soto 

and Hardee Counties, Florida: U.S. Geological Survey 

Bulletin 1030-B, p. 65-97.

Duerr, A.D., and Enos, G.M., 1991, Hydrogeology of the 

intermediate aquifer system and Upper Floridan aquifer, Hardee and De Soto Counties, Florida: U.S. Geological Survey Water-Resources Investigations Report 

90-4104, 46 p.

Duerr, A.D., Hunn, J.D., Lewelling, B.R., and Trommer, 

J.T., 1988, Geohydrology and 1985 water withdrawals 

of the aquifer systems in southwest Florida, with 

emphasis on the intermediate aquifer system: U.S. Geological Survey Water-Resources Investigations Report 

87-4259, 119 p.

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southwest Florida, 1981, and summary of water use 

1970, 1975, and 1977-81: U.S. Geological Survey 

Open-File Report 83-45, 75 p.

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program-a method for estimating irrigation water use

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Geurink, J.S., 1986, Agricultural water us? permitting 

procedures: Brooksville, Southwest Florida Water 

Management District Report.

Gilboy, A.E., 1985, Hydrogeology of the Southwest Florida 

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Florida Water Management District Report, 18 p.

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G.W., 1983, Irrigation in Florida agriculture in the '80s: 

Gainesville, University of Florida/TFAS, Florida Cooperative Extension Service, Bulletin 196.

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Floridan aquifer system, west-central Florida: U.S. 

Geological Survey Water-Resources Investigations 

Report 89-4142, 13 p.

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resources and management alternatives in the upper 

Peace and eastern Alafia River basins, Florida: U.S. 

Geological Survey Water-Resources Investigations 

77-124, 57 p.

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76 Hydrogeology and Simulated Effects of Ground-Water Withdrawals for Citrus Irrigation, Hardee and De Soto Counties, Fla.
Appendix

Pumpage Distribution for Transient Simulations for the

Intermediate Aquifer System and Upper Floridan

Aquifer, Hardee and De Soto Counties, Florida

Appendix 77
10 15

COLUMNS

20 25 30 35 40

47

EXPLANATION

PUMPAGE, IN MILLION GALLONS PER DAY

| GREATER THAN 0.50 

| 0.11 -0.50

g o.o6-o.io

Q 0.01 -0.05

LESS THAN 0.01

Figure 52. Pumpage distribution used in transient simulations for the intermediate aquifer system, May 1989. 

78 Hydrogeoiogy and Simulated Effects of Ground-Water Withdrawals for Citrus Irrigation, Hardee and De Soto Counties, Fla.
10

COLUMNS

20 25 30 35 40 46

47 1 i | i | { n | { | , | | | | | | j ^ |

EXPLANATION 

PUMPAGE, IN MILLION GALLONS PER DAY

 GREATER THAN 0.50

 0.11 -0.50

 0.06-0.10

D 0.01 -0.05

D LESS THAN 0.01

Figure 53. Pumpage distribution used in transient simulations for the intermediate aquifer system, May 2000.

Appendix 79
COLUMNS

20 25

HILLSBOROUGH 

COUNTY

HARDEE COUNTY 

DE SOTO COUNT

47

EXPLANATION

PUMPAGE, IN MILLION GALLONS PER DAY

 GREATER THAN 0.50

 0.11 -0.50

 0.06-0.10

G 0.01 -0.05

D LESS THAN 0.01

Figure 54. Pumpage distribution used in transient simulations for the intermediate aquifer system, May 2020. 

80 Hydrogeology and Simulated Effects of Ground-Water Withdrawals for Citrus Irrigation, Hardee and De Soto Counties, Fla.
10 15

COLUMNS

20 25 30

o 

a:

HILLSBOROUGH 

COUNTY

47

EXPLANATION

PUMPAGE, IN MILLION GALLONS PER DAY

| GREATER THAN 1 

| .60-1 

| .11 -.59

n -05-.10

n LESS THAN .05 

Figure 55. Pumpage distribution used in transient simulations for the Upper Floridan aquifer, May 1989.

Appendix 81
10 15

COLUMNS

20 25 30 35 40

47

EXPLANATION

PUMPAGE, IN MILLION GALLONS PER DAY

| GREATER THAN 1

 .60-1

| .11 -.59

G .05-.10

G LESS THAN .05

Figure 56. Pumpage distribution used in transient simulations for the Upper Floridan aquifer, May 2000. 

82 Hydrogeology and Simulated Effects of Ground-Water Withdrawals for Citrus Irrigation, Hardee and De So*o Counties, Fla.
10 15

COLUMNS

20 25 30

o 

a:

40

47

35 40

HILLSBOROUGH 

COUNTY

EXPLANATION

PUMPAGE, IN MILLION GALLONS PER DAY

| GREATER THAN 1 

 .60-1 

| .11 -.59

n -05-.10

LESS THAN 0.5

Figure 57. Pumpage distribution used in transient simulations for the Upper Floridan aquifer, May 2020.

Appendix 83