U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 9

Estimating Ground-Water Exchange with 

Lakes Using Water-Budget and Chemical 

Mass-Balance Approaches for Ten Lakes in 

Ridge Areas of Polk and Highlands 

Counties, Florida

Prepared in cooperation with the

SOUTHWEST FLORIDA WATER MANAGEMENT DISTRICT

8-4133

By Laura A. Sacks, Amy Swancar, and Terrie M. Lee

Tallahassee, Florida

1998
U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

Thomas J. Casadevall, Acting Director

For addtional information write to: Copies of this report can be purchased from:

U.S. Geological Survey

Branch of Information Services

Box 25286

Denver, CO 80225-0286

The use of firm, trade, and brand names in this report is for identification purposes 

only and does not constitute endorsement by the U.S. Geological Survey.

District Chief

U.S. Geological Survey, WRD

Suite 3015

227 North Bronough Street

Tallahassee, FL 32301
Contents III

CONTENTS

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

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

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

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

Physical Setting ..................................................................................................................................................................... 3

Description of Study Lakes ......................................................................................................................................... 5

Ground-Water Flow Patterns Around Lakes ............................................................................................................... 12

Water-Budget Approach........................................................................................................................................................ 13

Methods ....................................................................................................................................................................... 13

Estimating Components of the Water Budget ................................................................................................... 13

Net Ground-Water Flow Results ................................................................................................................................. 16

Seepage Lakes ................................................................................................................................................... 18

Lakes with Below Average Rainfall........................................................................................................ 18

Lakes with Above Average Rainfall ....................................................................................................... 20

Surface-Water Drainage Lakes.......................................................................................................................... 22

Factors Affecting Ground-Water Exchange with Lakes ............................................................................................. 24

Lateral Ground-Water Flow .............................................................................................................................. 24

Vertical Ground-Water Flow............................................................................................................................. 26

Chemical Mass-Balance Approach ....................................................................................................................................... 27

Methods ....................................................................................................................................................................... 28

Theory................................................................................................................................................................ 28

Estimating Deuterium and Oxygen-18 in Evaporating Water................................................................. 29

Sample Collection.............................................................................................................................................. 30

General Water Quality Characteristics ........................................................................................................................ 31

Ground Water .................................................................................................................................................... 31

Lake Water ........................................................................................................................................................ 35

Chemical Mass-Balance Results ................................................................................................................................. 37

Solute Tracer...................................................................................................................................................... 37

Isotope Tracer.................................................................................................................................................... 39

Estimating Ground-Water Exchange................................................................................................................. 40

Summary and Conclusions .................................................................................................................................................... 42

References ............................................................................................................................................................................. 44

Appendix ............................................................................................................................................................................... 47

FIGURES

1. Map showing locations of the study lakes in central Florida................................................................................... 3

2. Schematic showing generalized cross section of ground-water flow and hydrogeology through a Florida 

ridge lake ................................................................................................................................................................. 4

3-7. Maps showing:

3. Locations of monitoring wells, general ground-water flow patterns, and bathymetric map of Lake 

Annie, Highlands County, and Lake George, Polk County , Florida................................................................. 7

4. Locations of monitoring wells, general ground-water flow patterns, and bathymetric map of Grassy 

Lake and Lake Hollingsworth, Polk County, Florida ........................................................................................ 8
IV Contents

5. Locations of monitoring wells, general ground-water flow patterns, and bathymetric map of 

Lake Isis and Lake Olivia, Highlands County, Florida ..................................................................................... 9

6. Locations of monitoring wells, general ground-water flow patterns, and bathymetric map of Round

Lake and Saddle Blanket Lake, Polk County, Florida....................................................................................... 10

7. Locations of monitoring wells, general ground-water flow patterns, and bathymetric map of Lake 

Starr and Swim Lake, Polk County, Florida...................................................................................................... 11

8-15. Graphs showing:

8. Relation between annual net ground-water flow and rainfall for calendar year 1996 at the study lakes.......... 17

9. Stage hydrographs and monthly net ground-water flow for seepage lakes with below average rainfall 

during 1996: Lake Isis, Lake Olivia, Saddle Blanket Lake, and Round Lake .................................................. 18

10. Stage hydrographs and monthly net ground-water flow for seepage lakes with above average rainfall 

during 1996: Swim Lake, Lake Starr, and Grassy Lake.................................................................................... 21

11. Cumulative daily rainfall and daily stage of Swim Lake, June through July 1996 ........................................... 21

12. Stage hydrographs and monthly net ground-water flow for surface-water drainage lakes: Lakes Annie, 

George, and Hollingsworth................................................................................................................................ 23

13. Monthly net ground-water flow for Grassy Lake and Lake Olivia, plotted with percent of lake perimeter 

that receives ground-water inflow, for the period October 1995 through December 1996............................... 24

14. Relation between minimum air temperature and daily change in lake stage, for days without rain between

December 1995 and March 1996, and December 1996, for Swim Lake, Lake Isis, and Lake Olivia .............. 27

15. Head in the Upper Floridan aquifer plotted with minimum air temperature, for the period December 1995 

to March 1996, for a site near Lake Isis, Highlands County, Florida................................................................ 27

16. Stiff diagrams showing chemical composition of ground-water inflow and lake water......................................... 33

17-19. Graphs showing:

17. Relation between delta deuterium and delta oxygen-18 in ground water, lake water, and rainwater..................... 34

18. Relation between computed ground-water inflow and assumed chloride concentration of ground-water 

inflow for Lake Starr and Swim Lake ..................................................................................................................... 38

19. Relation between delta deuterium and delta oxygen-18 for mean lake water and lake water computed 

theoretically using equation 5.................................................................................................................................. 42

TABLES

1. Location and physical features of the study lakes .................................................................................................. 6

2. Linear regression equations of stage-volume and stage-area relations for shallow water depths for the 

study lakes .............................................................................................................................................................. 6

3. Ground-water flow characteristics of the study lakes, October 1995 through December 1996............................. 12

4. Water-budget terms, computed net ground-water flow, uncertainty in net ground-water flow, and estimated 

ground-water outflow and inflow for the study lakes for calendar year 1996........................................................ 17

5. Summary of strength of linear relations between monthly net ground-water flow and cumulative rainfall 

and between monthly net ground-water flow and head difference between the lake and the Upper Floridan 

aquifer for the study lakes....................................................................................................................................... 19

6. Selected hydrologic and chemical parameters used for the chemical mass-balance approach .............................. 28

7. Monthly mean values used to compute the isotopic composition of evaporating water at Swim Lake using 

deuterium and assuming atmospheric moisture in isotopic equilibrium with rainwater........................................ 30

8. Range and median water-quality and isotopic data for water from the surficial aquifer system in near-lake, 

inflow wells for each lake basin ............................................................................................................................. 32

9. Mean water-quality and isotopic data and principal land use for the study lakes .................................................. 35

10. Computed ground-water inflow to the study lakes using various tracers, and sensitivity of inflow calculations 

to concentrations of chloride and sodium in ground-water inflow......................................................................... 38

11. Estimate of ground-water inflow and outflow and uncertainty using the most successful tracer .......................... 41
Contents V

Conversion Factors, Vertical Datum, Acronyms, and Additional Abbreviations

Temperature in degrees Celsius (oC) can be converted to degrees Fahrenheit (oF) as follows:

°F = (1.8 × °C) + 32

Temperature in degrees Fahrenheit (oF) can be converted to degrees Celsius (oC) as follows:

°C = (°F - 32) / 1.8

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 and Additional Abbreviations used in Report:

Multiply By To obtain

inch (in.) 2.54 centimeter

foot (ft) 0.3048 meter

mile (mi) 1.609 kilometer

acre 4,047 square meter

acre 0.4047 hectare

cubic foot per second (ft3

/s) 0.02832 cubic meter per second

inch per month (in/mo) 2.54 centimeter per month

inch per year (in/yr) 2.54 centimeter per year

Cl chloride

Gnet net ground-water flow

mg/L milligrams per liter

mL milliliter

N nitrogen

Na sodium

NWS National Weather Service

P phosphorus

r

2 coefficient of determination

So surface-water outflow

SWFWMD Southwest Florida Water Management District

USGS U.S. Geological Survey

δ delta

δ18O delta oxygen-18

δa isotopic composition of atmospheric moisture

δD delta deuterium

δE isotopic composition of evaporating water

μS/cm microsiemens per centimeter
Abstract 1

Estimating Ground-Water Exchange with Lakes Using 

Water-Budget and Chemical Mass-Balance Approaches 

for Ten Lakes in Ridge Areas of Polk and Highlands 

Counties, Florida

By Laura A. Sacks, Amy Swancar, and Terrie M. Lee

Abstract

Water budget and chemical mass-balance 

approaches were used to estimate ground-water 

exchange with 10 lakes in ridge areas of Polk and 

Highlands Counties, Florida. At each lake, heads 

were monitored in the surficial aquifer system and 

deeper Upper Floridan aquifer, lake stage and rainfall were measured continuously, and lakes and 

wells were sampled three times between October 

1995 and December 1996. 

The water-budget approach computes net 

ground-water flow (ground-water inflow minus 

outflow) as the residual of the monthly waterbudget equation. Net ground-water flow varied 

seasonally at each of the 10 lakes, and was notably 

different between lakes, illustrating short-term 

differences in ground-water fluxes. Monthly 

patterns in net ground-water flow were related to 

monthly patterns of other hydrologic variables 

such as rainfall, ground-water flow patterns, and 

head differences between the lake and the Upper 

Floridan aquifer.

The chemical mass-balance approach 

combines the water budget and solute or isotope 

mass-balance equations, and assumes steady-state 

conditions. Naturally occurring tracers that were 

analyzed for include calcium, magnesium, 

sodium, potassium, chloride, and bromide, the 

isotopes deuterium and oxygen-18. Chloride and 

sodium were the most successful solute tracers; 

however, their concentrations in ground water 

typically varied spatially, and in places were 

similar to that in lake water, limiting their sensitivity as tracers. In contrast, the isotopes were more 

robust tracers because the isotopic composition of 

ground water was relatively uniform and was 

distinctly different from the lake water. Groundwater inflow computed using the chemical massbalance method varied significantly between 

lakes, and ranged from less than 10 to more than 

150 inches per year. Both water-budget and chemical mass-balance approaches had limitations, but 

the multiple lines of evidence gained using both 

approaches improved the understanding of the role 

of ground water in the water budget of the lakes. 

INTRODUCTION

Characterizing the exchange of ground water 

with lakes is an important prerequisite to managing 

water levels and water quality in Florida lakes. Numerous lake basins are within the sand hills and ridges of 

Florida’s Central Lakes District (Brooks, 1981). Rainfall quickly percolates through the permeable sands to 

the water table, favoring ground-water flow over 

surface-water drainage. As a result, about 70 percent 

of Florida’s 7,800 lakes are seepage lakes, having no 

natural surface flow into or out of them (Palmer, 1984). 

In the absence of direct pumping and engineered stormwater drainage, the stage of seepage lakes is affected by 

the local climate (rainfall and evaporation) and by the 

exchange of ground water with the lake. Lake stage 

can also be influenced by pumping from the underlying 
2 Estimating Ground-Water Exchange with Lakes Using Water-Budget and Chemical Mass-Balance Approaches for Ten Lakes in 

Ridge Areas of Polk and Highlands Counties, Florida

Upper Floridan aquifer by inducing vertical groundwater outflow (Barcelo and others, 1990; Yobbi, 1996). 

Differences in how individual lakes respond to similar 

climatic conditions and pumping stresses is partly 

attributable to differences in their local hydrogeologic 

setting, which controls the interaction between the lake 

and adjacent ground-water system (Winter, 1976; 

Winter and Pfannkuch, 1984; Lee and Swancar, 1997).

Ground water also can have a strong influence on 

the chemical composition of lake water, even when it is 

a relatively minor part of a lake’s water budget 

(Schwartz and Gallup, 1978; Baker and others, 1988; 

Kenoyer and Anderson, 1989). Solutes that originate 

in the shallow ground water can occur naturally from 

geologic or atmospheric sources (Stauffer, 1985; Baker 

and others, 1986; Pollman and others, 1991) or can be 

derived from anthropogenic sources such as septic tank 

leachate or fertilizers (Fellows and Brezonik, 1981; 

Stauffer, 1991; Tihansky and Sacks, 1997). As a lake 

basin becomes developed, ground water can become 

enriched in major ions and nutrients because of anthropogenic sources. If this enriched ground water flows 

into the lake, it can eventually affect the water quality 

of the lake. In order to understand a lake’s nutrient or 

chemical budget, ground-water inputs and losses also 

must be quantified.

Previous basin-scale studies of seepage lakes in 

Florida indicate that the ground-water component to 

lake-water budgets can vary widely (Grubbs, 1995; 

Lee, 1996; Lee and Swancar, 1997). For example, 

ground-water inflow to two seepage lakes ranged from 

less than 20 percent to more than 75 percent of water 

inputs (Lee and Grubbs, in press). These studies 

provided detailed descriptions of ground-water flow 

patterns around lakes and quantified ground-water 

exchanges using detailed water budgets and groundwater flow models. As a result, the understanding of 

the various factors affecting ground-water exchange 

has greatly improved. The site-specific nature and 

limited time frame of these basin-scale studies, however, make it difficult to extrapolate results to the larger 

population of lakes in Florida. Contemporaneous 

studies of ground-water exchange are needed for a 

larger number of lakes. With this information we can 

begin to make comparisons between lakes and to characterize how lakes respond to hydrologic conditions 

within a region.

More simplified approaches are clearly needed 

to understand the relative importance of ground water 

in the water budget of lakes in Florida. Recent 

approaches have improved our understanding of the 

magnitude of ground-water exchange with lakes by 

combining chemical evidence of ground-water inflow 

to the lake with hydrologic data (Stauffer, 1985; 

Krabbenhoft and others, 1990; Pollman and others, 

1991). In 1995, the U.S. Geological Survey (USGS) 

began a cooperative study with the Southwest Florida 

Water Management District (SWFWMD) to better 

understand ground-water/lake interactions in ridge 

areas of Florida. This study examines lakes at three 

different scales: (1) multiple-basin (or regional) scale 

using relatively simple techniques, (2) topographic 

basin scale for use as a long term study site (Lake 

Starr), and (3) intrabasin scale to study the specific 

process of recharge on ground-water inflow and outflow. This report summarizes results from the 

multiple-basin scale study, which uses both water and 

chemical budgets to determine ground-water contributions and losses from lakes.

Purpose and Scope

This report presents estimates of the amount of 

ground water in the water budgets of 10 lakes in Polk 

and Highlands Counties based on water-budget and 

chemical mass-balance approaches. Water budgets 

were computed monthly for the 15-month study period, 

October 1995 through December 1996, with net 

ground-water flow (ground-water inflow minus outflow) computed as the residual to the water budget 

equation. Heads in the shallow surficial aquifer system 

were mapped monthly in the vicinity of each lake to 

define ground-water head gradients around the lakes 

and to delineate regions of ground-water inflow and 

outflow. The head in the deeper Upper Floridan aquifer also was measured monthly near each lake to define 

the vertical head gradient between the lake and the 

Upper Floridan aquifer.

Lakes and selected wells were sampled three 

times during the study for major ions, selected trace 

elements, and nutrients; the isotopes deuterium and 

oxygen-18 were sampled twice in lakes and selected 

wells. The chemical mass-balance method was used to 

independently compute annual ground-water inflow 

and outflow using a steady-state approach. Inorganic 

solutes and the stable isotopes of water were considered as conservative tracers. Computed ground-water 

inflow results from the various tracers are presented, 

and limitations to this approach are discussed.
Physical Setting 3

Acknowledgments

The authors thank Archbold Biological Station, 

The Florida Chapter of The Nature Conservancy, city 

of Lakeland, city of Avon Park, Highlands County, and 

numerous private land owners for access to their property and for permitting the USGS to install, monitor, 

and sample ground-water wells. Without their cooperation, this project would not have been possible. We 

also thank our local observers (Peggy Davis, Robert 

Desness, Nancy Deyrup, John Hodgkinson, David 

Little, Bal Sukhraj, and James Thornhill) for collection 

of data at the lakes. Finally, we thank USGS employees Richard Earp and Timothy O’Hare for dedicated 

and thorough data collection for this project.

PHYSICAL SETTING

The 10 study lakes are in ridge and upland areas 

of Polk and Highlands Counties (fig. 1), in an area 

characterized by numerous lakes. Polk County ranks 

fourth in the State in the number of lakes per county 

(Gant, 1993). The physical setting of the lake, 

including its hydrogeologic setting, is important in 

controlling how ground water interacts with a lake. 

Saint Cloud

Lake

Hollingsworth

Grassy

Lake

Round 

Lake Lake

Starr

Swim 

Lake

Lake

George

Saddle Blanket

Lake

Lake 

Olivia

Lake 

Isis

Lake 

Annie

28°15′

28°00′

45′

30′

27°15′

82°00′ 45′ 30′ 81°15′

HIGHLANDS COUNTY

OSCEOLA COUNTY

HARDEE COUNTY

DE SOTO COUNTY

POLK COUNTY

HILLSBOROUGH COUNTY MANATEE COUNTY

SARASOTA

COUNTY

Base from Southwest Florida Water Management District digital data, 1:250,000, 1992

Universal Transverse Mercator projection, Zone 17

Kissimmee River

Winter Haven

Ridge

Lakeland

Ridge

Lake Henry

Ridge

Lake Wales

Ridge

Bombing Range

Ridge

Peace Rive r

Horse Creek

LOCATION

OF STUDY

AREA

0 5 10 MILES

0 5 10 KILOMETERS

RIDGE AREAS 

(after White, 1970)

EXPLANATION

Figure 1. Locations of the study lakes in central Florida.
4 Estimating Ground-Water Exchange with Lakes Using Water-Budget and Chemical Mass-Balance Approaches for Ten Lakes in 

Ridge Areas of Polk and Highlands Counties, Florida

General descriptions of Florida lakes are available elsewhere (for example, Brenner and others, 1990; Schiffer, 

1998).

Much of the study area is within the Central 

Lakes District (Brooks, 1981) and includes the Lake 

Wales Ridge, Lakeland Ridge, and Winter Haven Ridge 

as defined by White (1970). In this mantled karst terrain, unconsolidated sands and clays overlie an irregular 

limestone surface. As the underlying limestone 

dissolves, overlying sands and clays subside. The 

resulting sinkholes and surface depressions often hold 

water, forming lakes. Some lakes have no surface 

drainage (seepage lakes), whereas others are connected 

to surface flow (drainage lakes), depending on the local 

hydrogeologic setting, manmade features, head gradients, and topography. Lakes can undergo multiple 

episodes of subsidence and infilling, and this can 

influence hydrogeologic units beneath the lake 

(Tihansky and others, 1996).

Hydrostratigraphic units influencing groundwater/lake interactions are the surficial aquifer system, 

intermediate confining unit, and Upper Floridan aquifer. Within the study area, the surficial aquifer system 

ranges from about 50 to 300 ft thick and is composed of 

unconsolidated sand and clay (Barr, 1992; Tihansky 

and others, 1996). These surficial deposits thicken from 

north to south and on ridges. The surficial aquifer 

system is separated from the 

underlying carbonate Upper 

Floridan aquifer by the intermediate confining unit/intermediate aquifer system. This 

unit is heterogeneous, with 

clay-rich beds of low transmissivity hydraulically separating the surficial aquifer 

system from the Upper Floridan aquifer. However, sinkholes and subsidence 

features often modify and 

breach the confining unit, 

particularly beneath lakes 

(Lee and others, 1991; Sacks 

and other, 1992a; Evans and 

others, 1994; Tihansky and 

others, 1996). The top of the 

Upper Floridan aquifer is 

about 100 ft below land surface in the northern part of 

the study area and 600 ft below land surface in the 

southern part of Highlands County (Tibbals, 1990; 

Tihansky and others, 1996).

Ground-water flow around lakes is influenced by 

the lake’s local hydrogeologic setting. A cross section 

through a typical lake is shown in figure 2. Lakes 

receive ground-water inflow from the surrounding 

surficial aquifer system. The area of influence can vary 

widely, depending upon the specific lake basin, and 

does not necessarily correspond to the topographic 

drainage area. In addition, ground water flowing 

towards a lake from the upper part of its topographic 

basin might not intercept the lake; instead, this water 

could move downward and bypass the lake (Lee, 1996). 

Ground water typically enters a lake in shallow areas 

near the shore (McBride and Pfannkuch, 1975; Fellows 

and Brezonik, 1980). Some lakes receive ground-water 

inflow around their entire perimeter. Other lakes, 

called flow-through lakes, have distinct areas of 

ground-water inflow and outflow around their 

perimeters.

Lakes "leak" or lose water as ground-water 

outflow. Ground-water outflow can have both lateral 

and vertical components, which are controlled by 

separate factors. Lateral ground-water outflow is 

driven by local and regional head gradients within the 

surficial aquifer system. The vertical component of 

















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RECHARGE

CITRUS

GROVES

SURFICIAL AQUIFER

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INTERMEDIATE CONFINING

UNIT

UPPER FLORIDAN

AQUIFER

EVAPORATION

ARROW INDICATES DIRECTION OF GROUND-WATER MOVEMENT

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TABLE

RAINFALL

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Figure 2. Generalized cross section of ground-water flow and hydrogeology through a 

Florida ridge lake (modified from Tihansky and Sacks, 1997).
Physical Setting 5

flow is driven by downward head gradients between 

the lake and the Upper Floridan aquifer, and groundwater outflow ultimately moves downward to recharge 

the deeper aquifer. Breaches in confinement beneath a 

lake can cause preferential pathways for vertical 

ground-water outflow. This vertical ground-water outflow, however, can be inhibited where lake sediments 

are thick (Cornett and others, 1989; Guyonnet, 1991;

Lee and Swancar, 1997). 

The climate in the study area is humid subtropical. Average annual rainfall is about 50 in/yr, with 

about 60 percent occurring during the summer months. 

Rainfall from winter frontal activity generally is not as 

intense as summer rainstorms. Annual average lake 

evaporation is estimated to be between 48 and 50 in/yr 

(Farnsworth and others, 1982). However, lake evaporation can be higher during periods of drought (Sacks 

and others, 1994; Lee and Swancar, 1997). Pan evaporation rates are highest between April and August. 

Evaporation from shallow lakes tend to follow this 

same trend, but evaporation from deep lakes can be 

seasonally lagged (Sacks and others, 1994).

Description of Study Lakes

A number of factors were taken into consideration during the selection of the 10 study lakes. Lakes 

with a wide range of physical features were selected to 

get as many representative lake types as possible. 

These features included surface area, depth, topographic steepness of the basin, and stage response 

during periods of drought or excess rainfall. In this 

report, lake basin is defined topographically and is 

equivalent to a surface-water drainage basin in other 

settings. Other factors considered in lake selection 

were geographic distribution in the ridge areas of Polk 

and Highlands Counties, length of stage record at the 

site, previous studies at the lake, and relatively uniform 

land use in the basin (in order to better define groundwater inflow chemistry). Another selection factor was 

the cooperation of property owners in allowing access 

to the lake for installing and monitoring wells in the 

basin.

Seven of the study lakes are seepage lakes; the 

other three lakes have some channelized surface-water 

inflow or outflow. Physical characteristics of each lake 

are included in table 1. The lakes range in surface area 

from 5 to 356 acres, and maximum depths range from 

6 to 65 ft. Lakes Annie and Isis are the deepest, and 

Lake Hollingsworth and Saddle Blanket Lake are the 

shallowest. Most study lakes are impacted to some 

degree by human development, and only two lakes, 

Lake Annie and Saddle Blanket Lake, are in pristine, 

undeveloped settings. One of the study lakes is 

unnamed on USGS topographic maps, but in this report 

it is referred to by its local name, Swim Lake. The 

Saddle Blanket Lake referred to in this report is the 

larger of the two lakes named together as Saddle 

Blanket Lakes; the smaller lake is referred to as the 

“adjacent pond” in this report.

Three of the seepage lakes lose water as a result 

of direct pumping and one gains water from stormwater 

inflow. Lake Isis receives stormwater inflow through a 

4-ft-diameter pipe draining a nearby highway, and 

residents with homes adjacent to Lake Isis often pump 

water directly from the lake for lawn watering. During 

dry periods, water is pumped directly out of Round 

Lake to irrigate citrus. Water was pumped out of 

Grassy Lake for 2 months of the study (November and 

December 1995), following a wet summer and fall, to 

alleviate flooding of houses built in low-lying areas. 

The three lakes that have channelized surfacewater inflow or outflow vary widely in the degree that 

the flow is engineered. Lake Annie has a natural 

outflow stream on the north shore of the lake and has 

intermittent inflow through two shallow dredged 

channels on the south side (Battoe, 1987). Surfacewater outflow from Lake George flows through a 

10-in.-diameter pipe and into a channel that also drains 

the wetland area on the southeast side of the lake. Lake 

Hollingsworth is in an urban setting. Water enters the 

lake through a network of 67 storm drains and two pipe 

connections from upgradient lakes. Surface water 

flows out of the lake through a controlled outflow 

structure to a stream on its southeast shore.

Bathymetric data were either collected or 

compiled from existing maps. From these data, stagevolume and stage-area relations were computed. 

Although stage-volume and stage-area relations were 

not linear over the entire range of lake depths, they 

were linear in the shallow water depths representing the 

range in lake stage during the study (table 2). These 

relations allowed change in lake volume to be directly 

related to change in lake stage. Similarly, volumetric 

water fluxes could be related to linear fluxes by dividing them by the average lake surface area during the 

time period. Bathymetric maps of each study lake are 

shown in figures 3 through 7.
6 Estimating Ground-Water Exchange with Lakes Using Water-Budget and Chemical Mass-Balance Approaches for Ten Lakes in 

Ridge Areas of Polk and Highlands Counties, Florida

Table 1. Location and physical features of the study lakes

[undev., undeveloped; res., residential; So, surface-water outflow; Si, surface-water inflow; int., intermittent; pump, water pumped directly from lake; St, 

stormwater inflow; irr., irrigation; --, not applicable; ft, feet; ft msl; feet above sea level]

 Lake

Station identification 

number1

County Section Township Range Principle land 

use in basin

Surface outflows or 

inflows

Annie 02270700 Highlands 6 38S 30E undev. So, Si (int.)

George 02293463 Polk 5 28S 26E citrus So (pipe)

Grassy 02294440 Polk 2 29S 25E citrus, res. pump (flooding)

Hollingsworth 02294342 Polk 30 28S 24E res. So (weir), Si

 (pipe), St

Isis 02269184 Highlands 15 33S 28E citrus, res. St, pump (lawn irr.)

Olivia 02269172 Highlands 6 33S 28E res. --

Round 02294048 Polk 13 29S 26E citrus pump (citrus irr.)

Saddle Blanket 02269168 Polk 25 32S 27E undev. --

Starr 02293763 Polk 14 29S 27E citrus, res. --

Swim 02266923 Polk 1 30S 28E citrus --

Lake1 Mean depth2 

(ft)

Maximum 

depth 

(ft)

Surface area

(acres)

Lake

perimeter

(ft)

Reference 

stage3

(ft msl)

Basin 

steepness4

(dimensionless)

Topographic

basin area

(acres)

Annie 30 65 92 7,659 110 0.0347 183

George 8 15 59 6,431 130 0.0325 189

Grassy 11 23 76 7,364 130 0.0191 318

Hollingsworth 4 6 356 14,927 131 0.0288 1,072

Isis 28 64 50 5,534 110 0.0337 590

Olivia 16 47 86 7,224 115 0.0127 564

Round 15 28 31 4,167 131 0.0445 119

Saddle Blanket 6 11 6 1,936 118 0.0273 77

Starr 17 33 134 10,753 105 0.0577 739

Swim 15 30 5 1,706 97 0.0493 57

1

Equivalent to the downstream order number.

2Lake volume divided by surface area at average stage during study.

3

Stage for which lake depth, suface area, and permimeter were computed.

4Average slope between the lake and the topographic basin divide for the four compass directions.

Table 2. Linear regression equations of stage-volume and stage-area relations for shallow water depths for the study lakes

[ft msl, feet above sea level; V, lake volume; A, lake surface area; all regressions significant to alpha level of 0.05]

Lakes

Stage range 

equation computed for 

(ft msl)

Linear regression equation to compute 

volume from lake stage1

Linear regression equation to compute 

surface area from lake stage2

Annie 3

97 - 110 V = 3.167 x 106 * stage - 2.296 x 108 A = 1.119 x 105 * stage - 8.334 x 106

George 129 - 130 V = 2.540 x 106 * stage - 3.085 x 108 A = 1.410 x 105

 * stage - 1.574 x 107

Grassy 128 - 132 V = 3.301 x 106 * stage - 3.918 x 108 A = 1.460 x 105

 * stage - 1.568 x 107

Hollingsworth 128 - 132 V = 1.341 x 107 * stage - 1.695 x 109 A = 1.373 x 106

 * stage - 1.650 x 108

Isis 107 - 113 V = 2.148 x 106 * stage - 1.764 x 108 A = 5.746 x 104

 * stage - 4.152 x 106

Olivia 113 - 117 V = 3.710 x 106 * stage - 3.654 x 108 A = 1.091 x 105

 * stage - 8.843 x 106

Round 130 - 132 V = 1.335 x 106 * stage - 1.550 x 108 A = 3.700 x 104

 * stage - 3.508 x 106

Saddle Blanket 115 - 120 V = 2.739 x 105 * stage - 3.051 x 107 A = 2.029 x 104

 * stage - 2.108 x 106

Starr 104 - 106 V = 5.855 x 106 * stage - 5.184 x 108 A = 1.920 x 105

 * stage - 1.432 x 107

Swim 95 - 99 V = 2.280 x 105 * stage - 1.888 x 107 A = 1.362 x 104

 * stage - 1.089 x 106

1

Stage in feet; volume in cubic feet.

2

Stage in feet; area in square feet.

3Greater than stage range during study, but relation generated from existing bathymetric map (Eckblad, 1974; Battoe, 1987).
Physical Setting 7











0 100 METERS

0 500 FEET

0 100 METERS

0 500 FEET

27°12′30′′

81°21′15′′ 81°21′ 81°20′45′′

27°12′15′′

Lake Annie

outflow

stream

10

20

30

40

50

60

28°04′45′′

81°45′ 45′′

28°04′30′′

81°44′30′′

Lake George

outflow

stream

15

10

5

EXPLANATION

WATER-TABLE WELL

MOST FREQUENTLY OBSERVED 

 GROUND-WATER FLOW DIRECTION

BATHYMETRIC CONTOUR - - Depth, in

 feet below lake surface (Jan. 1974),

 adjusted to datum of 110 feet above 

 sea level. Interval is 10 feet

 (after Eckblad, 1974) 

60

EXPLANATION

WATER-TABLE WELL

UPPER FLORIDAN AQUIFER WELL

MOST FREQUENTLY OBSERVED

 GROUND-WATER FLOW DIRECTION

WETLAND

BATHYMETRIC CONTOUR - - Depth, in

 feet below lake surface (Aug. 15, 1990),

 adjusted to datum of 130 feet above 

 sea level. Interval is 5 feet

 (after Southwest Florida Water

 Management District, written

 commun., 1995)



5

A

B

Note: Upper Floridan aquifer well

located 1,200 feet east of map edge

( 27°12′23′′ 83°20′26′′ )

Figure 3. Locations of monitoring wells, general ground-water flow patterns, and bathymetric map of (A) Lake Annie, 

Highlands County, and (B) Lake George, Polk County , Florida.
8 Estimating Ground-Water Exchange with Lakes Using Water-Budget and Chemical Mass-Balance Approaches for Ten Lakes in 

Ridge Areas of Polk and Highlands Counties, Florida

Figure 4. Locations of monitoring wells, general ground-water flow patterns, and bathymetric map of (A) Grassy Lake 

and (B) Lake Hollingsworth, Polk County, Florida.

0 500 FEET

0 100 METERS

0 1000 FEET

0 200 METERS

27°59′30′′

81°47′ 81°46′15′′ 45′′ 30′′

27°59′

15′′

Grassy 

Lake

Eagle Lake

Dinner Lake

EXPLANATION

WATER-TABLE WELL

MOST FREQUENTLY OBSERVED 

 GROUND-WATER FLOW DIRECTION

PART OF LAKE PERIMETER WHERE 

 GROUND-WATER FLOW DIRECTION 

 PERIODICALLY REVERSES

BATHYMETRIC CONTOUR - - Depth,

 in feet below lake surface 

 (Oct. 29,1987), adjusted to datum of

 130 feet above sea level. Interval is 5

 feet (after Southwest Florida Water 

 Management District, written

 commun., 1995)

5

5

10

10

15

20

UPPER FLORIDAN AQUIFER WELL

Lake 

Hollingsworth

Lake

Morton

Lake

Horney

28°02′15′′

28°02′

45′′

30′′

15′′

28°00′45′′

28°01′

81°57′30′′ 15′′ 81°57′ 81°56′15′′ 45′′ 30′′

Weir

Weir

Weir

EXPLANATION

WATER-TABLE WELL

MOST FREQUENTLY OBSERVED 

 GROUND-WATER FLOW DIRECTION

DIRECTION OF PIPED FLOW

BATHYMETRIC CONTOUR - - Depth,

 in feet below lake surface 

 (Apr. 15, 1992), adjusted to datum of

 131 feet above sea level. 

 Interval is 2 feet (after City of 

 Lakeland, written commun.,1995)

2

2

4

6

6

6

Overflow pipe

UPPER FLORIDAN AQUIFER WELL

A

B

outflow

stream
Physical Setting 9

Figure 5. Locations of monitoring wells, general ground-water flow patterns, and bathymetric map of (A) Lake Isis and 

(B) Lake Olivia, Highlands County, Florida.

0 500 FEET

0 100 METERS

0 500 FEET

0 100 METERS

Lake Isis

27°36′45′′

81°31′ 81°30′15′′ 45′′ 30′′

27°36′30′′

storm drain

60

30 20 10

50

50

40

40

EXPLANATION

WATER-TABLE WELL

MOST FREQUENTLY OBSERVED 

 GROUND-WATER FLOW DIRECTION

BATHYMETRIC CONTOUR - - Depth, in

feet below lake surface (Mar. 27, 1996),

adjusted to datum of 110 feet above 

sea level. Interval is 10 feet

10

81°33′ 81°32′30′′

27°38′

27°37′30′′

Lake Olivia

Lake Chilton 10

2030

40

EXPLANATION

WATER-TABLE WELL

MOST FREQUENTLY OBSERVED 

 GROUND-WATER FLOW DIRECTION

 PART OF LAKE PERIMETER WHERE

 GROUND-WATER FLOW DIRECTION 

 PERIODICALLY REVERSES

BATHYMETRIC CONTOUR - - Depth, in

feet below lake surface (Mar. 27,1996),

adjusted to datum of 115 feet above 

sea level. Interval is 10 feet

A

B

Note: Upper Floridan aquifer well with

monthly and continuously recorded

water-level measurements located 

7,400 feet east of map edge

( 27°36′15′′ 81°28′49′′ )

40

Note: Upper Floridan aquifer well with

monthly water-level measurements

located 2,500 feet north of map edge

( 27°38′45′′ 81°32′19′′ ); well with

continuously recorded water-level

measurements located 4.3 miles 

southeast of lake ( 27°36′15′′ 81°28′49′′ )
10 Estimating Ground-Water Exchange with Lakes Using Water-Budget and Chemical Mass-Balance Approaches for Ten Lakes 

in Ridge Areas of Polk and Highlands Counties, Florida

0 100 METERS

0 500 FEET

0 100 METERS

0 500 FEET

EXPLANATION

WATER-TABLE WELL

MOST FREQUENTLY OBSERVED

 GROUND-WATER FLOW DIRECTION

PART OF LAKE PERIMETER WHERE

GROUND-WATER FLOW DIRECTION

PERIODICALLY REVERSES

BATHYMETRIC CONTOUR - - Depth, in

feet below lake surface (Mar. 26, 1996),

adjusted to datum of 131 feet above sea level. 

Interval is 5 feet

UPPER FLORIDAN AQUIFER WELL

27°40′15′′

81°34′30′′ 81°34′15′′

27°40′

Saddle

Blanket

Lake Pond

EXPLANATION

WATER-TABLE WELL

MOST FREQUENTLY OBSERVED 

 GROUND-WATER FLOW DIRECTION

Lake Streety

BATHYMETRIC CONTOUR - - Depth, in

feet below lake surface (Sept. 6, 1996),

adjusted to datum of 118 feet above sea level. 

5

Interval is 5 feet

10

UPPER FLORIDAN AQUIFER WELL

27°57′45′′

27°57′30′′

81°40′45′′ 81°40′15′′ 30′′

Round

Lake

Lake

Winterset

5

101520

25

A

B

5

5

Figure 6. Locations of monitoring wells, general ground-water flow patterns, and bathymetric map of (A) Round 

Lake and (B) Saddle Blanket Lake, Polk County, Florida.
Physical Setting 11

A

B

0 1000 FEET

0 200 METERS

0 1000 FEET

0 200 METERS

27°57′45′′

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

27°57′

15′′

30′′

Lake Starr

EXPLANATION

WATER-TABLE WELL

MOST FREQUENTLY OBSERVED 

 GROUND-WATER FLOW DIRECTION

BATHYMETRIC CONTOUR - - Depth, in

feet below lake surface (Mar. 28, 1996),

adjusted to datum of 105 feet above sea 

level. Interval is 5 feet

510

30

15

20

25

UPPER FLORIDAN AQUIFER WELL-- 

Monthly water-level measurements

available. 

27°54′10′′

81°29′ 81°28′50′′

27°54′

Swim Lake

(unnamed)

Pond

EXPLANATION

WATER-TABLE WELL

MOST FREQUENTLY OBSERVED 

 GROUND-WATER FLOW DIRECTION

PART OF LAKE PERIMETER WHERE 

 GROUND-WATER FLOW DIRECTION

 PERIODICALLY REVERSED

BATHYMETRIC CONTOUR - - Depth, in

feet below lake surface (Sept. 5, 1996),

adjusted datum of 97 feet above sea level. 

Interval is 5 feet

10

10

20

30

Note: Upper Floridan aquifer well with

continuously recorded water-level

measurements located 2.2 miles south

of lake ( 27°55′07′′ 81°35′37′′ ) 

5

Figure 7. Locations of monitoring wells, general ground-water flow patterns, and bathymetric map of (A) Lake Starr 

and (B) Swim Lake, Polk County, Florida.
12 Estimating Ground-Water Exchange with Lakes Using Water-Budget and Chemical Mass-Balance Approaches for Ten Lakes 

in Ridge Areas of Polk and Highlands Counties, Florida

Ground-Water Flow Patterns 

Around Lakes

A water-level monitoring network was established in each lake’s topographic basin to collect head 

data from the ground-water system. In general, this network consisted of 10 surficial aquifer system wells, 

with 6 wells near the lake shore and 4 wells in the upper 

part of the basin. Depending on lake size and other 

characteristics, the number of wells in some basins 

varied slightly from this pattern. The monitoring wells 

consisted of 2-in.-diameter polyvinyl chloride finished 

5 to 10 ft below the water table with a 5-ft screened 

interval. At Lake Annie, wells with 4-ft screened intervals were used because a suitable network had already 

been established in a previous study (Battoe, 1987). 

Water-level data from the wells and lakes were used to 

describe temporal variations in the head distribution and 

evaluation patterns of ground-water inflow and outflow.

Water levels in wells were measured monthly 

between October 1995 and December 1996. From this 

information, water-table maps were constructed, and 

areas of ground-water inflow and outflow were delineated. Well and lake hydrographs were constructed to 

evaluate the response of ground water and lakes to 

recharge and precipitation. Daily lake stage and 

monthly ground-water levels for all sites are published 

in annual USGS data reports for southwest Florida 

(Coffin and Fletcher, 1998a, b).

Monthly water-level measurements also were 

made in an Upper Floridan aquifer well near each lake 

to monitor vertical head gradients between the lake and 

the deeper aquifer. Heads in the Upper Floridan aquifer 

can vary considerably within a given day or month due 

to changing pumping stresses in the area. Thus, 

monthly instantaneous measurements are highly dependent upon pumping conditions at the time of observation. If available, continuously recorded water levels in 

Upper Floridan aquifer wells are useful in defining 

monthly average vertical head gradients. Two wells 

with continuous recorders were available within 5 mi of 

the three study lakes: ROMP 43xx near Lakes Isis and 

Olivia and ROMP 58 near Lake Starr.

General ground-water flow patterns in the vicinity of each lake are shown in figures 3 through 7. The 

direction of flow most frequently observed (more than 

half the time) is shown in these figures; areas where 

near-shore flow reversals were observed are also 

shown. Most study lakes had distinct areas of groundwater inflow and outflow, and can be classified as flowthrough lakes. Ground-water flow directions do not 

necessarily correspond to topography. Lakes with 

surface-water outflow (Lakes Annie, George, and Hollingsworth) had lateral ground-water outflow on the side 

of the lake with surface outflow (figs. 3a, 3b, and 4b). 

Some lakes were flow through for most of the study, but 

had ground-water inflow around their entire perimeter 

following periods of high recharge (for example, 

Grassy Lake and Lake Olivia; figs. 4a and 5b). Other 

lakes had very stable ground-water flow regimes around 

the lakes, although water-table gradients changed 

seasonally in response to recharge (for example, Lakes 

Isis and Starr; figs. 5a and 7a).

The percentage of lake perimeter that exhibits 

ground-water inflow or outflow varies between lakes, 

and can vary seasonally at a given lake. For most lakes, 

between 67 and 80 percent of the perimeter can be characterized as ground-water inflow (table 3). Swim Lake 

had ground-water inflow around its entire perimeter for 

most of the study (94 percent). Saddle Blanket Lake 

had the smallest amount of its perimeter as groundwater inflow (less than 50 percent). Outflow watertable gradients sometimes slope toward the topographic 

basin divides and can be driven by lower levels in 

nearby lakes. For example, ground-water outflow is 

toward a nearby lake on the north side of Lake Isis, the 

west side of Lake Olivia, and the north and east sides of 

Saddle Blanket Lake (figs. 5a, 5b, and 6b, respectively). 

The degree of confinement to the Upper Floridan 

aquifer near a lake can also affect areas of lateral outflow. Ground-water outflow occurs on the south side of 

Lake Starr (fig. 7a), where sinkhole features probably 

indicate less confinement of the Upper Floridan aquifer. 

Ground-water flow characteristics of the study lakes are 

listed in table 3.

Table 3. Ground-water flow characteristics of the study 

lakes, October 1995 through December 1996

[gw, ground water; UFA, Upper Floridan aquifer; ft, feet]

Lake

Percent of lake

perimeter that is 

Percent of 

time 100 

percent 

gw inflow

Average head 

difference 

between lake 

and UFA (ft)

gw 

inflow1

1Extrapolated linearly between wells. Averaged over study period for lakes 

with transient water-table configurations.

gw 

outflow1

Annie 78 22 0 62

George 73 27 0 8

Grassy 87 13 44 26

Hollingsworth 80 20 0 51

Isis 72 28 0 22

Olivia 71 29 13 35

Round 80 20 0 22

Saddle Blanket 45 55 0 40

Starr 67 33 0 2

Swim 94 6 69 10
Water-Budget Approach 13

Downward head gradients exist between all lakes 

and the Upper Floridan aquifer. Ridge areas of Polk and 

Highlands Counties are important recharge areas to the 

Upper Floridan aquifer (Aucott, 1988; Tibbals, 1990; 

Yobbi, 1996). Head differences generally increase to 

the south in the study area, and average head differences 

during the study range from 2 ft at Lake Starr to 62 ft at 

Lake Annie (table 3). Head gradients are influenced by 

a number of factors, including degree of confinement, 

pumping stresses in the area, and regional flow patterns 

within the Upper Floridan aquifer.

WATER-BUDGET APPROACH

The water-budget method was used to compute 

annual (1996) and monthly (October 1995 through 

December 1996) net ground-water flow (ground-water 

inflow minus outflow) for the 10 lakes. Net groundwater flow estimates can give insight into seasonally 

important processes and allow comparisons between 

lakes. However, absolute ground-water inflow and outflow cannot be distinguished because only the net flow 

is computed.

Methods

A lake’s water budget is computed by measuring 

or estimating all of the lake’s water gains and losses, 

and measuring the corresponding change in lake volume 

over the same time period:

(1)

where ∆V is change in lake volume, P is direct precipitation, E is lake evaporation, Si

 is surface-water inflow 

and storm-water inflow, So is surface-water outflow 

and direct pumping, Gi

 is ground-water inflow, and Go

is ground-water outflow. Water budget terms can be 

expressed in volumetric units (precipitation and evaporation multiplied by the average lake surface area during 

that time), or in linear units over the given time period 

(by dividing change in lake volume and volumetric 

fluxes by lake surface area). For this report, linear units 

of inches are used, which allows for a better comparison 

between lakes of different surface areas. This also 

allows for ground-water fluxes to be more readily compared to lake-level fluctuations and rainfall, which are 

measured in linear units. However, ground-water fluxes 

∆V PE– Si So – Gi Go = + + –

are inherently more important in the water budget of a 

smaller lake because of the higher ratio of lake perimeter to surface area, compared to a larger lake (Millar, 

1971; Fellows and Brezonik, 1980).

Equation 1 can be rearranged to solve for net 

ground-water flow (Gnet, ground-water inflow minus 

ground-water outflow):

(2)

When Gnet is positive, ground-water inflow exceeds 

outflow, and this value can be considered the minimum 

amount of ground-water inflow in the lake’s water budget. Similarly, when Gnet is negative, net ground-water 

outflow occurs from the lake, and this value can be considered to be the minimum amount of ground-water 

outflow in the lake’s water budget. 

Gnet is a residual term, and, thus, uncertainties (or 

errors) associated with each term of the water budget 

must be assessed in order to estimate the uncertainty 

associated with the computed net ground-water flow. 

The uncertainty or error in net ground-water flow can 

be computed as:

 

(3)

where eGnet is the uncertainty or error in net groundwater flow, in inches, and e is the error or uncertainty in 

each of the water budget terms, in percent (Lee and 

Swancar, 1997). Uncertainties in budget terms can be 

from measurement error, extrapolation of regional data 

to a specific site, and uncertainties in computed or 

assumed parameters used to calculate the term (Winter, 

1981). Uncertainty values used in the error analysis 

include those estimated from the literature (Winter, 

1981), standard error of regressions (for computed 

values), and an assessment of uncertainties in calculated values. The uncertainty for Gnet calculated from 

equation 3 should be considered a maximum error 

because it assumes that all of the errors are in the same 

direction and do not cancel each other out.

Estimating Components of the Water Budget

Each component of the water budget in equation (2) was measured directly or indirectly, and a percent uncertainty was assumed. The 10 lakes were 

instrumented with a stage recorder and tipping bucket 

rain gage, which recorded stage to 0.01 ft and rainfall 

Gnet Gi Go ∆V P– E Si – So = – = + +

e

Gnet e∆V ( ) ⋅ ∆V 2 eP ( ) ⋅ P 2 eE ( ) ⋅ E 2 eSi

Si ⋅     2 e

So

So ⋅     2 = +++ +
14 Estimating Ground-Water Exchange with Lakes Using Water-Budget and Chemical Mass-Balance Approaches for Ten Lakes 

in Ridge Areas of Polk and Highlands Counties, Florida

to 0.01 in. electronically every 15 minutes. Continuous data were collected between October 1, 1995, and 

January 1, 1997. A local observer also read the lake 

stage on a staff gage and measured weekly rainfall in 

a storage rain gage. Stage measurements were converted to lake volume using stage-volume relations 

(table 2). For linear units, the change in volume for a 

given time period was normalized to the average lake 

surface area for that time period, using stage-area 

relations. A 5 percent uncertainty was assumed for 

change in volume in the water budget (Lee and Swancar, 1997).

Tipping bucket rain gages can underestimate 

rainfall, particularly during high intensity storms 

(Sacks and others, 1992a; Bidlake and Boetcher, 

1996). For each lake, rainfall totals from the storage 

and tipping bucket rain gages were compared for the 

same time interval. If the tipping bucket gage consistently read lower, a best-fit regression line through 

zero was used to correct daily rainfall for that particular gage. In cases when the rain gage malfunctioned 

and observer data were not available, estimates of 

rainfall were made based on regression relations 

between rainfall and change in lake stage. A 5 percent uncertainty in rainfall was assumed, based on 

comparisons of monthly rainfall from two rain gages 

on different sides of Lake Starr.

For two of the three lakes with surface-water 

outflows, the volume of outflow was related to lake 

stage. At Lake Annie, monthly discharge measurements were made at the outflow stream using a pointvelocity meter and standard USGS procedures (Rantz 

and others, 1982). At Lake George, volumetric measurements from the discharge pipe were made at various lake stages, and the stage at zero flow was 

determined. For these lakes, linear regressions 

between stage and discharge were generated. For 

Lake Annie, the relation was surface-water outflow 

(So) = 5.0705 * stage - 556.6 (coefficient of determination, r2

, = 0.94; standard error = 0.255 ft3/s; stage 

range 109.90 to 110.54 ft; all regressions in the report 

are significant to an alpha level of 0.05 unless otherwise noted). For Lake George, the relation was So = 

0.7855 * stage - 102.02 (r2 = 0.89; standard error = 

0.0793 ft3/s; stage range = 129.85 (no flow; surveyed 

elevation of bottom of pipe) to 130.60 ft). Surfacewater outflow was computed from the electronically 

recorded 15-minute stage data, and summed by day. 

Uncertainty in the estimate was assumed to be the 

standard error from the regression plus an assumed 

measurement error (9 percent for Lake Annie and 

7 percent for Lake George). 

For the other lake with surface-water outflow, 

Lake Hollingsworth, estimating outflow was more 

complicated. The outflow structure was replaced 

during the study (February 1996). The original structure was a rectangular sharp-crested weir, which had 

water flowing over the top. Discharge over this weir 

was computed using theoretical calculations that 

relate discharge to the geometric configuration of the 

structure, the height of the water over the structure, 

and a weir coefficient (computed as a function of the 

height of the water over the weir) (Horton, 1907; Prasuhn, 1987, p. 249). Estimates of outflow discharge 

before January 1996 were not reliable because of 

insufficient data. Because of this, water budgets for 

Lake Hollingsworth were only computed for 1996. 

The new outflow structure at Lake Hollingsworth was 

a bottom-opening sluice gate. The gate was adjusted 

occasionally during the study, and date, time, and size 

of the gate opening were recorded for each change. 

Discharge measurements were made at the outflow 

stream using a point-velocity meter and standard 

USGS procedures (Rantz and others, 1982) at least 

one time for each of the four gate settings. The 

discharge measurements were used to calibrate discharge coefficients using an equation relating discharge through the gate to the geometric 

configuration of the opening and the height of the 

water at the structure (Prasuhn, 1987, p. 252). Discharge coefficients used were 0.230, 0.633, 0.256, 

0.412, and 0.363 for 0.5-in., 1.0-in., 2.0-in. (before 

August 3, 1996), 2.0-in. (after August 29, 1996), and 

5.6-in. gate settings, respectively. Surface-water outflow was computed from the electronically recorded 

15-minute stage data, and summed by day. Assumed 

uncertainties in computed discharge were based on 

estimated uncertainties in weir or discharge coefficients.

Lake Hollingsworth also receives surfacewater inflow from upgradient Lake Horney (fig. 4b). 

This water flowed over a rectangular, sharp-crested 

weir and into a pipe network to Lake Hollingsworth. 

The stage of Lake Horney was measured weekly 

beginning in February 1996, and stage between measurements was interpolated linearly. Stage in January 

1996 was estimated based on a linear regression with 

the stage of upgradient Lake Morton, which was measured weekly. Daily discharge from Lake Horney to 

Lake Hollingsworth was estimated using a theoretical 
Water-Budget Approach 15

weir coefficient that varied as a function of the height 

of the water over the weir. The uncertainty in inflow 

estimates to Lake Hollingsworth from Lake Horney 

was assumed to be 40 percent, based on uncertainty in 

the weir coefficient and the estimated error associated 

with weekly stage measurements; for January 1996, 

the standard error of regression for estimating stage 

was also considered in the uncertainty in the inflow 

term. Most of the time, piped flow from Lake Morton 

drains directly into Lake Horney. Following high 

rainfall, however, when the stage of Lake Morton 

increases rapidly because of stormwater inflow, some 

of the outflow from Lake Morton is diverted to Lake 

Hollingsworth (B.C. Sukhraj, City of Lakeland, oral 

commun., 1997). For daily rainfall events greater 

than 2 in., it was assumed that the equivalent stormwater over the Lake Morton basin is diverted directly 

into Lake Hollingsworth. The uncertainty in inflow 

estimates to Lake Hollingsworth from Lake Morton 

was assumed to be 50 percent.

Only one other lake, Lake Annie, receives 

direct surface-water inflow. This inflow occurs intermittently in two shallow, dredged channels that drain 

stormwater and the adjacent ground water when 

water-table altitudes are high. Estimates of surfacewater inflow to Lake Annie were based on limited 

discharge measurements of flow in the western-most 

channel, field observations, and an analysis of change 

in lake stage. During the study period, significant 

flow occurred only during the fall of 1995. A 50 percent uncertainty was assumed for these estimates.

Stormwater is a significant inflow component 

to two of the study lakes, Lake Hollingsworth and 

Lake Isis. Lake Hollingsworth has 67 storm drains 

flowing into the lake (G. Medley, City of Lakeland, 

oral commun., 1997). A previous study separated the 

lake basin into subbasins and estimated stormwater 

runoff by categorizing land use and associated impervious and pervious areas in each subbasin (Dames 

and Moore, 1992). For that study, a runoff coefficient 

of 0.70 was used for directly connected impervious 

areas and a runoff coefficient of 0.05 was used for 

pervious areas. During the current study, runoff from 

the two upgradient lake basins (Lake Morton and 

Lake Horney) was not used in computing direct 

stormwater inflow to Lake Hollingsworth because 

inflow from these lakes was computed separately. 

The total area contributing to stormwater inflow was 

divided by lake surface area, and stormwater inflow 

was equivalent to about 70 percent of the rain falling 

directly on the lake. For Lake Isis, stormwater runoff 

enters the lake from a 4-ft-diameter pipe draining 

about 1 mi. of a 4-lane highway. The paved area that 

this pipe drains was estimated using aerial photographs, schematics of the storm drain plan from the 

Florida Department of Transportation, and field 

reconnaissance. Estimates of stormwater inflow from 

this paved area, with an associated runoff coefficient 

of 0.75, compared closely to an analysis of lake stage 

increase for days when total rainfall was greater than 

0.5 in. Storm-water inflow was estimated to be 

equivalent to 20 percent of rain falling directly on the 

lake. A 30 percent uncertainty was assumed for 

stormwater inflow for both lakes.

Water was pumped directly out of several of the 

study lakes. To alleviate flooding of homes in lowlying areas adjacent to Grassy Lake, water was 

pumped to nearby Eagle Lake for 2 months of the 

study (November and December 1995). Round Lake 

was pumped during dry periods to irrigate a nearby 

citrus grove. Pumping rates were measured by the 

SWFWMD using an ultrasonic flow meter (M.L. 

Phillippi, Southwest Florida Water Management 

District, written commun., 1996). Both pumps had 

hourly meters, so meter readings were related to the 

volume of water pumped. A 5 percent uncertainty in 

these estimates was assumed. At Lake Isis, lake-front 

residents pumped water out of the lake for lawn irrigation. Estimates of this pumping volume were made 

based on the number of inlet pipes in the lake, average 

lot size, and typical lawn watering habits. A 

100 percent uncertainty was assumed for this 

estimate.

Evaporation was not measured directly. 

Monthly lake evaporation was estimated from pan 

evaporation data from the National Weather Service 

(NWS) site at Lake Alfred in Polk County, multiplied 

by a pan coefficient. Pan coefficients vary monthly 

and were estimated by comparing monthly pan evaporation rates at Lake Alfred to the more accurate 

energy-budget evaporation rates from nearby Lake 

Lucerne for the 1986 water year (Lee and Swancar, 

1997). Evaporation rates at lakes with below average 

rainfall can be higher than rates at lakes with normal 

or above normal rainfall. Reduced cloud cover and 

increased solar radiation lead to warmer lake temperatures and, thus, higher evaporation rates. Rainfall 

varied significantly over the study area; lakes further 

north had above average rainfall, whereas those 

further south had below average rainfall. Much of 
16 Estimating Ground-Water Exchange with Lakes Using Water-Budget and Chemical Mass-Balance Approaches for Ten Lakes 

in Ridge Areas of Polk and Highlands Counties, Florida

this rainfall deficit was during the summer months, 

when evaporation is high. A weak but statistically 

significant relation exists between late spring and 

summer monthly pan evaporation at Lake Alfred and 

rainfall departure from normal (r2 = 0.20; standard 

error = 0.73 in., which is less than 10 percent of 

monthly pan evaporation during summer months). 

For those lakes with significantly less summer rainfall compared to Lake Alfred, the regression relation 

was used to compute monthly pan evaporation for 

specific months. This was done for June through 

August 1996 for Lake Isis, Lake Olivia, and Saddle 

Blanket Lake, and for August 1996 for Round Lake 

and Lake Annie.

Evaporation from deep lakes (mean depth 

greater than about 16 ft) is seasonally lagged 

compared to shallow lakes (Sacks and others, 1994). 

At deep Florida lakes, evaporation is lower in the 

spring because incoming energy is stored as heat in 

the deeper parts of the lake, rather than being available for evaporation. Conversely, evaporation at a 

deep lake is higher in the fall because this extra stored 

heat is being released and is available for evaporation. 

For the deepest lakes (Lakes Annie and Isis), which 

had mean depths greater than 20 ft, pan evaporation 

coefficients were adjusted lower for the winter and 

spring months and higher for the fall months to better 

reflect this lag in evaporation. Monthly temperature 

profiles at Lake Annie and Lake Isis reflect the heat 

exchange characteristics of deep, warm monomictic 

lakes (that is, they stratify from spring to late fall and 

mix in the winter). In contrast, temperature profiles 

at shallow Lake George (mean depth 8 ft) respond 

more rapidly to changes in air temperature, and the 

lake essentially behaves like an evaporation pan. 

Because seasonal lake evaporation rates are more difficult to estimate than annual rates, a 20 percent 

uncertainty was assumed for monthly evaporation 

and a 10 percent uncertainty was assumed for annual 

evaporation (Winter, 1981; Pollman and others, 

1991). 

Net Ground-Water Flow Results

Net ground-water flow varied seasonally at 

each of the 10 lakes, and was notably different 

between lakes. Monthly net ground-water flow, 

estimated uncertainty, and other components of the 

water budget are included in the appendix. Seasonal 

variability of net ground-water flow is a relative indicator of how important ground water is in the lake’s 

water budget. Lakes with more seasonal variability in 

monthly net ground-water flow are more responsive 

to seasonal changes in recharge or aquifer pumping 

stresses. In contrast, less variability in monthly net 

ground-water flow can indicate that ground water is 

not as important in the lake-water budget or that the 

lake is not as influenced by short-term hydrologic 

stresses. Lake Annie had the greatest variability 

(more than 20 in.) in the amount of monthly net 

ground-water flow, and Lake George had the least 

variability (less than 5 in.). Of the seepage lakes, 

Swim Lake and Lake Isis had the greatest variability 

in monthly net ground-water flow (16.5 and 12.8 in., 

respectively), whereas Lake Starr, Round Lake, and 

Lake Olivia had the least variability (between 5 and 

6 in.).

Annual (1996) net ground-water flow ranged 

from 180 in/yr at Lake Annie to -44 in/yr at Lake Isis 

(table 4). Except for Lake Annie, annual net groundwater flow is correlated to annual rainfall (fig. 8). 

Rainfall at the study lakes ranged from less than 40 

in/yr to more than 55 in/yr during 1996. Seepage 

lakes with below average rainfall (less than the 30-

year average at the nearest NWS site) had negative 

net ground-water flow (Lake Isis, Lake Olivia, Saddle 

Blanket Lake, and Round Lake), whereas those with 

above average rainfall had positive net ground-water 

flow (Grassy Lake, Lake Starr, and Swim Lake). All 

of the surface-water drainage lakes (Lakes Annie, 

George, and Hollingsworth) had positive net groundwater flow. Lake Annie had high positive net groundwater flow but below average rainfall for 1996; 

ground-water inflow has to be high at this lake to 

sustain the continuous surface-water outflow. 

Annual net ground-water flow would presumably be 

even higher at Lake Annie during a year with above 

average rainfall. Because net ground-water flow is so 

strongly influenced by rainfall deficit or surplus, 

annual comparisons of net ground-water flow 

between lakes are only meaningful if annual rainfall 

is similar. For example, Lakes Isis and Olivia had 

similar 1996 rainfall (about 40 in.), but net groundwater flow was more negative at Lake Isis than Lake 

Olivia (-44 and -14 in/yr, respectively). Greater 

ground-water outflow and reduced ground-water 

inflow at Lake Isis, compared to Lake Olivia, probably account for these differences. 
Water-Budget Approach 17

Although the net ground-water flow method does 

not allow ground-water inflow and outflow to be readily 

distinguished, an approximation of ground-water outflow can be derived from the month with the most negative net ground-water flow (appendix). Months with 

the most negative ground-water outflow typically have 

low rainfall. Thus, it is assumed that ground-water 

inflow is minimal and that net ground-water flow is predominantly ground-water outflow during this month. 

Further, it is assumed that ground-water outflow 

remains relatively constant throughout the year, allowing the monthly outflow estimate to be extrapolated for 

the year, but ground-water inflow is more seasonally 

variable. This assumption is based on Lee and Grubbs 

(in press) and Lee (1996), who found that monthly 

ground-water outflow varied by less than 40 percent, 

whereas monthly ground-water inflow varied by more 

than 100 percent. Ground-water pumping, however, 

can seasonally affect the downward head gradient and 

amount of ground-water outflow, which this approach 

ignores. The estimate of ground-water outflow computed in this manner can then be used with the other 

water-budget terms to estimate ground-water inflow. 

The study lakes had large differences in the 

annual estimate of ground-water inflow and outflow 

(table 4). Swim Lake and Lake Isis had the highest 

ground-water outflow (greater than 80 in/yr), and 

Grassy Lake and Lake Starr had the lowest groundwater outflow (less than 30 in/yr). Other than Lake 

Annie, which had at least 180 in/yr of ground-water 

inflow (based on net ground-water flow), Swim Lake 

had the highest ground-water inflow, followed by 

Lakes Hollingsworth and Isis (table 4). Grassy Lake, 

Lake Olivia, Round Lake, Saddle Blanket Lake, and 

Lake Starr had similarly low ground-water inflow 

estimates (between 26 and 33 in/yr).

Monthly patterns in net ground-water flow can 

be related to monthly patterns of other hydrologic 

variables. Thus, cause and effect can sometimes be 

inferred. Net ground-water flow results are presented 

for each study lake in the following sections, and 

important controlling factors on net ground-water flow 

are discussed. These sections are separated into seepage lakes and surface-water drainage lakes. The seepage-lake section is divided into two sections based on 

rainfall during the study. This facilitates comparisons 

between lakes under similar climatic conditions.

Table 4. Water-budget terms, computed net ground-water flow, uncertainty in net groundwater flow, and estimated ground-water outflow and inflow for the study lakes for calendar year 1996

[Units in inches per year, rounded to nearest inch; P, precipitation; E, evaporation; Si, surface-water inflow; So, surface-water 

outflow; ∆V, change in lake volume; Gnet, net ground-water flow (ground-water inflow minus outflow); eGnet, uncertainty or 

error in net ground-water flow; est, estimated; Go, ground-water outflow; Gi

, ground-water inflow; --, not applicable; n/a, not 

available because monthly net ground-water flow always greater than 0]

Lake P E Si So ∆V Gnet eGnet est Go

1 est Gi2

Annie 43 53 1 173 -2 180 29 n/a n/a

George 52 52 -- 37 2 39 13 n/a n/a

Grassy 55 52 -- -- 19 16 6 17 33

Hollingsworth 57 52 355 72 -2 11 20 42 53

Isis 41 55 48 53 -52 -44 7 84 40

Olivia 39 55 -- -- -29 -14 6 43 29

Round 46 53 -- 52 -17 -7 6 38 31

Saddle Blanket 38 55 -- -- -40 -23 6 49 26

Starr 55 52 -- -- 8 5 6 25 30

Swim 52 52 -- -- 6 7 6 85 92

1Most negative monthly net ground-water flow multiplied by 12, which assumes that minimal ground-water inflow

occurs during that month and that ground-water outflow is uniform during the year. 2

Computed from water budget, using estimated Go from previous column. 3Includes surface-water inflow (15 inches) and stormwater inflow (40 inches).

4

Stormwater inflow.

5

Direct pumping from lake.

1996 RAINFALL, IN INCHES PER YEAR

ANNUAL NET GROUND-WATER FLOW, 

IN INCHES PER YEAR

-50

0

50

100

150

200

35 40 45 50 55 60

Annie

Olivia

Isis

Saddle

Blanket

Round

Swim

George

Grassy

Starr

Hollingsworth

Figure 8. Relation between annual net ground-water flow 

and rainfall for calendar year 1996 at the study lakes.
18 Estimating Ground-Water Exchange with Lakes Using Water-Budget and Chemical Mass-Balance Approaches for Ten Lakes 

in Ridge Areas of Polk and Highlands Counties, Florida

Seepage Lakes

For most seepage lakes, there was a seasonal pattern of positive net ground-water flow in summer months 

(June through September, normally the wet season) and 

negative net ground-water flow (net ground-water outflow) during drier months. Seepage lakes in the southern 

part of the study area, however, had below average rainfall 

and not all of them displayed this seasonal pattern.

Lakes with Below Average Rainfall

Seepage lakes with below average rainfall in 

1996 were Lake Isis, Lake Olivia, Saddle Blanket 

Lake, and Round Lake. Monthly net ground-water 

flow and stage hydrographs for these lakes are shown 

in figure 9. The rainfall deficit was most extreme in 

northern Highlands County and southern Polk County, 

in the vicinity of Lake Isis, Lake Olivia, and Saddle 

Figure 9. Stage hydrographs and monthly net ground-water flow for seepage lakes with below average rainfall during 

1996: Lake Isis, Lake Olivia, Saddle Blanket Lake, and Round Lake.

Lake Isis

107

108

109

110

111

112

113

Lake Olivia

112

113

114

115

116

117

118

-8

-4

0

4

8

12

-8

-4

0

4

8

12

-8

-4

0

4

8

12

-8

-4

0

4

8

12

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

Lake Isis

plus uncertainty

minus uncertainty

Saddle Blanket Lake

115

116

117

118

119

120

121

LAKE STAGE, IN FEET ABOVE SEA LEVEL

10/1/95

12/16/95

3/1/96

5/16/967/31/96

10/15/9612/30/96

Round Lake

128

129

130

131

132

133

134

Lake Olivia

Saddle Blanket Lake

NET GROUND-WATER FLOW, IN INCHES PER MONTH

Round Lake 

1995 1996
Water-Budget Approach 19

Blanket Lake. These lakes also started out at a high 

stage because of a wet summer in 1995. Total rainfall 

was more than 11 in. above normal in 1995 at the nearest NWS site. Much of the 1996 rainfall deficit at these 

lakes occurred during the summer, which is typically 

the wet season. Lake Isis and Saddle Blanket Lake had 

negative net ground-water flow (net ground-water outflow) for all months except one during 1996, and as a 

result their stage dropped significantly. In contrast, 

Lake Olivia received more ground water than it lost for 

4 months between June and October 1996, and, as a 

result, its stage did not drop as dramatically.

Despite having similar amounts of rainfall, Lake 

Isis had more positive net ground-water flow during wet 

periods and more negative net ground-water flow 

during dry periods than nearby Lake Olivia and Saddle 

Blanket Lake (fig. 9). Lake Isis is a deep lake (mean 

depth of 28 ft) with a steep-sloped lake bottom. It also 

has a steeper and larger topographic basin than Lake 

Olivia and Saddle Blanket Lake. The south and west 

side of Lake Isis receives ground-water inflow, and the 

north and northeast side has ground-water outflow 

(fig. 5a). Lateral outflow is probably driven by a 10-ft 

head difference between Lake Isis and a lake 0.3 mi to 

the north. In addition to the lateral outflow, Lake Isis 

appears to be well connected to the underlying Upper 

Floridan aquifer. For example, net ground-water flow 

was more negative (net ground-water outflow) during 

months with higher head gradients between the lake and 

the Upper Floridan aquifer (difference computed 

between monthly average lake stage and monthly average head for a well with a continuous recorder). 

Ground-water inflow paths at Lake Isis are probably 

relatively long and deep, as evidenced by the best relation between monthly net ground-water flow being with 

4 months cumulative rainfall (rainfall for the month 

when net ground-water flow was computed plus the 

three preceding months; to maximize the period of 

record, rainfall before the study was estimated from the 

nearest NWS site) (table 5). The higher net groundwater flow during the early, wet part of the study indicates that Lake Isis is capable of receiving significant 

amounts of ground-water inflow. However, the higher 

amount of ground-water outflow creates a “threshold” 

that recharge must exceed before the lake level can rise.

In contrast, ground water appears to be less significant in Lake Olivia’s water budget than in Lake 

Isis’ budget. This is illustrated by comparing the stage 

hydrographs and monthly net ground-water flow at 

Lakes Isis and Olivia during the study (fig. 9 and 

appendix). The stage of Lake Olivia rose less than 

Lake Isis’s stage during the early, wet part of the study 

(October to November 1995). During this period, net 

ground-water flow at Lake Olivia was only 23 percent 

of that at Lake Isis. Similarly, Lake Olivia’s stage 

dropped less than Lake Isis’s stage during the dry 

period of 1996 (January to May 1996), during which 

time Lake Isis lost 2.5 times more water to net groundwater outflow (negative net ground-water flow) than 

Lake Olivia did. Lake Olivia is shallower (mean 

depth 16 ft), has a flatter topographic basin, and has 

shallower depths to the water table in its basin than 

Lake Isis. The water-table configuration around Lake 

Olivia varied depending upon rainfall conditions. 

Most of the time, the lake was a flow-through lake, 

with outflow toward a nearby lake (Lake Chilton, 

which is about 3 ft lower) and sometimes toward the 

north (fig. 5b). However, following periods of high 

recharge, water-table mounding caused flow reversals, and the entire perimeter contributed groundTable 5. Summary of strength of linear relations between 

monthly net ground-water flow and cumulative rainfall and 

between monthly net ground-water flow and head difference 

between the lake and the Upper Floridan aquifer for the 

study lakes

[No., number; diff; difference; inst. mst, instantaneous measurements (one 

reading per month); cont. mst, continuously recorded measurements; r2

, 

coefficient of determination; all regressions significant to alpha level of 

0.05 unless otherwise noted; --, well with continuously recorded head data 

not available within 5 miles of lake] 

 Lake

No. months 

cumulative 

rainfall1

1Includes month when net ground-water flow computed; rainfall for months 

before the data collection period from nearest National Weather Service 

site; relations listed are strongest of those tested (1 to 6 months).

Cumulative 

rainfall r2

2Using rainfall collected at site only (September 1995 through December 

1996).

Head diff. 

inst. mst r2

Head diff. 

cont. mst r2

Annie 4 0.78 0.46 --

George 2

5 2.78 .29 --

Grassy 32

3

For months not affected by pumping (October 1995, and May through 

December 1996).

3

.87 .50 --

Hollingsworth 4 .47 4.03

4

Regression not statistically significant at alpha level of 0.05.

--

Isis 4 .77 .57 0.76

Olivia 2 .81 4.19 .63

Round 2 .68 4.20 --

Saddle Blanket 2 .69 4.37 --

Starr 4 .71 4.28 .81

Swim 1 .62 4.13 --
20 Estimating Ground-Water Exchange with Lakes Using Water-Budget and Chemical Mass-Balance Approaches for Ten Lakes 

in Ridge Areas of Polk and Highlands Counties, Florida

water inflow to the lake. During these times, net 

ground-water flow was greatest. Ground-water 

inflow paths at Lake Olivia are relatively short, shown 

by a good linear relation between monthly net groundwater flow and 2 months cumulative rainfall (table 5). 

Lake Olivia also might not be as influenced by shortterm stresses in the deeper aquifer as Lake Isis. The 

linear relation between monthly net ground-water 

flow and the head difference between the lake and the 

Upper Floridan aquifer was not as strong at Lake 

Olivia (r2 = 0.63,) compared to at Lake Isis (r2 = 0.76; 

using the same Upper Floridan aquifer well with continuously recorded head data; table 5).

Saddle Blanket Lake had the lowest rainfall of 

the study lakes (about 38 in. for 1996). Monthly net 

ground-water flow was typically between the amounts 

calculated for Lakes Isis and Olivia (fig. 9). Saddle 

Blanket Lake is shallow (mean depth 6 ft) and much 

smaller (6 acres) than Lakes Isis and Olivia (50 and 

86 acres, respectively). Saddle Blanket Lake received 

significant amounts of ground-water inflow during the 

early, wet part of the study, but during the dry period, 

ground-water outflow dominated the ground-water 

component of its budget. Ground-water inflow paths 

are relatively short, as seen by the best relation between 

net ground-water flow being with 2 months cumulative 

rainfall (table 5). Much of this ground-water outflow 

was probably lateral. The water table was lower than 

the lake around much of its perimeter, and inflow 

occurred only along the southern perimeter of the lake 

(fig. 6b). Lateral ground-water outflow is driven by 

head gradients toward the adjacent pond (about 1.5 ft 

lower than Saddle Blanket Lake), Lake Streety (0.3 mi 

north and 14 ft lower than Saddle Blanket Lake), and a 

nearby wetland to the northwest. During most of the 

study, change in stage at Saddle Blanket Lake paralleled that at Lake Olivia. However, during the latter 

part of 1996, when direct rainfall and evaporation were 

very similar at both lakes, Saddle Blanket Lake’s stage 

dropped more rapidly than Lake Olivia’s, indicating 

that it lost more water through ground-water outflow 

than Lake Olivia. During this period, the stage at the 

adjacent pond dropped to an extremely low level 

because of prolonged direct pumping for citrus irrigation. This lower pond stage probably induced more lateral outflow from Saddle Blanket Lake during this 

period, as indicated by a steeper slope in the water table 

near the northeast side of the lake.

Net ground-water flow at Round Lake was less 

variable than at Lake Isis and Saddle Blanket Lake 

(fig. 9). Round Lake showed a distinct seasonal 

response of net ground-water inflow during the wet 

season (June through September) and net outflow 

during the dry season. This lake is farther to the north 

and had slightly more rainfall than the other lakes with 

below average rainfall. Net ground-water flow is best 

related to 2 months cumulative rainfall (table 5), 

indicating that ground-water inflow paths to Round 

Lake are relatively short. The water table is depressed 

adjacent to the east side of the lake, indicating that 

ground-water outflow is occurring into the surficial 

aquifer system. This pattern occurs even though head 

gradients in the upper part of the basin are toward the 

lake (fig. 6a). This outflow might indicate an area of 

preferential downward flow, intercepting flow from the 

upper basin that would otherwise have entered the lake. 

The water-table gradient in the near-shore area sometimes reversed from outflow to inflow conditions. 

These reversals in gradient probably are in response to 

water-table mounding near the lake following recharge 

events. Surficial sediments contained more clay at this 

lake basin than at other sites, which also could influence heads in the shallow surficial aquifer system. 

Round Lake is occasionally pumped for citrus irrigation. Pumping was very low during the study (less than 

1.0 in/mo) and did not appear to affect net groundwater flow.

Lakes with Above Average Rainfall

Swim Lake, Lake Starr, and Grassy Lake had 

rainfall in 1996 exceeding the 30-year mean at the nearest NWS site, and, as a result, annual net ground-water 

flow was positive (fig. 8). Monthly net ground-water 

flow at these lakes generally was more positive than at 

the rainfall deficit lakes, particularly during the wet season. However, significant differences in monthly net 

ground-water flow occurred between lakes (fig. 10).

Swim Lake had the widest range of monthly net 

ground-water flow of any of the seepage lakes. This 

lake is small (about 5 acres) and relatively deep for its 

size (mean depth 15 ft). Higher net ground-water flow 

at Swim Lake is partially related to the larger ratio of 

lake perimeter to surface area at this lake, compared to 

larger lakes. During wet months, the lake received significant amounts of net ground-water inflow (fig. 10). 

Conversely, during dry months, ground-water outflow 

exceeded inflow. Ground-water inflow paths are short 

because net ground-water flow is best related to rainfall 

in the same month, rather than a cumulative response 

(table 5). During June 1996, when almost 12 in. of rain 

fell, lake stage increased by almost 16 in. This is direct 

evidence for large amounts of ground-water inflow 

following this high rainfall event (fig. 11). The water 
Water-Budget Approach 21

Swim Lake

94

95

96

97

98

99

100

Swim Lake

-8

-4

0

4

8

12

plus uncertainty

minus uncertainty

LAKE STAGE, IN FEET ABOVE SEA LEVEL

Lake Starr

102

103

104

105

106

107

108

NET GROUND-WATER FLOW, IN INCHES PER MONTH

Lake Starr

-8

-4

0

4

8

12

10/1/95

12/16/95

3/1/96

5/16/967/31/96

10/15/96

12/30/96

Grassy Lake

127

128

129

130

131

132

133

Grassy Lake

-8

-4

0

4

8

12

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

1995 1996

Figure 10. Stage hydrographs and monthly net ground-water flow for seepage lakes with above average rainfall during 

1996: Swim Lake, Lake Starr, and Grassy Lake.

table was higher than the lake around its entire 

perimeter for most of the study (fig. 7b). However, lateral ground-water outflow did occur at the southwest 

side of the lake during dry periods. Other than during 

these dry periods, ground-water outflow is dominated 

by vertical, rather than lateral, ground-water outflow; 

thus, outflow probably is controlled by head differences between the lake and the Upper Floridan aquifer. 

Swim Lake is within an extensive citrus growing area, 

where heads in the Upper Floridan aquifer are expected 

to be influenced by short-term pumping stresses. No 

statistically significant relation was found between net 

ground-water flow and the head difference between the 

lake and the Upper Floridan aquifer; however, instantaneous monthly head measurements in the Upper 

Floridan aquifer might not have been representative of 

average monthly head values, particularly because of 

the intensity of ground-water pumping in the area. 

CUMULATIVE RAINFALL, 

IN FEET

LAKE STAGE ,

IN FEET ABOVE SEA LEVEL

6/1/96

6/11/966/21/96

7/1/96

7/11/967/21/967/31/96

0

1

2

96

97

98

lake stage

cumulative rainfall

Figure 11. Cumulative daily rainfall and daily stage of 

Swim Lake, June through July 1996.
22 Estimating Ground-Water Exchange with Lakes Using Water-Budget and Chemical Mass-Balance Approaches for Ten Lakes 

in Ridge Areas of Polk and Highlands Counties, Florida

Monthly net ground-water flow did not vary as 

much at Lake Starr as it did at Swim Lake (fig. 10). 

The seasonal pattern of net ground-water flow is very 

pronounced, with the highest net ground-water flow at 

the end of the wet season, compared to the more flashy 

nature of Swim Lake. Lake Starr is a flow-through 

lake, with ground-water inflow occurring on its north 

and west sides, and ground-water outflow occurring on 

its south/southeast side (fig. 7a). Sinkholes are present 

on the outflow side of the lake, which indicates that the 

south side of the lake might have less confinement and 

a better connection to the Upper Floridan aquifer. Lake 

Starr is the largest seepage lake (134 acres) and has the 

steepest basin (table 1). Ground-water inflow paths are 

probably long, as evidenced by monthly net groundwater flow being best related to 4 months of cumulative 

rainfall (table 5), resulting in the seasonal pattern of net 

ground-water flow. Net ground-water flow is also 

related to the difference between monthly average lake 

stage and head in the Upper Floridan aquifer at a well 

with a continuous recorder (r2

 = 0.81; table 5). 

Grassy Lake had positive net ground-water flow 

for most of the study (fig. 10). Lake stage was high in 

the early part of the study following above average 

rainfall during the summer and fall of 1995. As a 

consequence, several homes built in low-lying areas 

were flooded, and the lake was lowered by pumping 

into an adjacent lake at a rate of about 20 in/mo during 

November and December 1995. This pumping induced 

additional ground-water inflow (as indicated by high 

positive net ground-water flow) during the pumped 

period, as well as during the following 4 months 

(January through April). During much of the study, the 

lake had a minor outflow head gradient on its 

west/northwest perimeter (fig. 4a). However, inflow 

occurred around the entire perimeter during the period 

influenced by pumping and following a month of high 

recharge (September 1996) when net ground-water 

flow was significantly higher than during previous or 

following months. Grassy Lake has the shallowest 

topographic basin of the study lakes, and transient 

water-table mounds can develop following periods of 

high recharge. This mounding can contribute significant amounts of ground-water inflow to the lake. 

Ground-water inflow paths to Grassy Lake are 

relatively short, as evidenced by a good linear relation 

between monthly net ground-water flow and 2 months 

cumulative rainfall (table 5). The lake also probably 

does not have a large ground-water outflow component, indicated by less negative net ground-water flow 

compared to other seepage lakes during dry periods.

Surface-Water Drainage Lakes

Lakes with surface-water drainage tend to have 

more positive net ground-water flow than the seepage 

lakes (fig. 12). They also tend to have less stage change 

than seepage lakes because surface-water outflow 

moderates the stage fluctuations. The three study lakes 

with surface-water drainage are Lakes Annie, George, 

and Hollingsworth.

Lake Annie by far had the largest amount of 

positive net ground-water flow of all the study lakes 

(fig. 12). This is consistent with very high estimates of 

ground-water inflow (345 in/yr) from an earlier study 

of the lake (Battoe, 1987). Large amounts of ground 

water must flow into the lake to sustain the continuously flowing outflow stream. Lake Annie is a deep 

lake (mean depth of 30 ft), and, thus, may intercept 

deeper ground-water flow paths. Because the lake is in 

a flow-through setting with respect to the surficial aquifer system, ground-water inflow must be very high in 

the area of inflow. Lateral ground-water outflow 

occurs on the north side of the lake, which coincides 

with the area of surface-water outflow (fig. 3a). Net 

ground-water inflow occurred during all months, 

although there was a distinct seasonal pattern of higher 

net inflow during the wet season and lower net inflow 

during the dry season. Ground-water inflow paths are 

relatively long, indicated by a good linear relation 

between monthly net ground-water flow and 4 months 

cumulative rainfall (table 5). 

Lake George also always had positive net 

ground-water flow throughout the study, although it 

was about 20 percent of the net ground-water flow at 

Lake Annie (normalized to lake surface area; fig. 12). 

Surface-water outflow at Lake George is constrained to 

a pipe in an artificially bermed area, which then flows 

into a small stream through a wetland (fig. 3b). Lake 

George is a flow-through lake, and ground-water outflow occurs in the area of surface-water outflow. The 

lake is shallow (mean depth of 8 ft), but the basin is 

relatively steep. Ground-water inflow paths are probably long because monthly net ground-water flow is best 

related to 5 months of cumulative rainfall (table 5). 

Thus, net ground-water flow at Lake George lags 

behind the period of most intense rainfall and is seasonal in nature, with the highest net ground-water flow 

occurring at the end of the wet season, and the lowest 
Water-Budget Approach 23

net ground-water flow occurring at the end of the dry 

season.

Lake Hollingsworth is the largest and most engineered of the study lakes. An engineered lake is more 

hydraulically complex because it no longer responds 

only to natural hydrologic influences. In addition, 

uncertainties in net ground-water flow are high because 

surface-water flows are relatively difficult to quantify 

(fig. 12 and appendix). Surface-water outflow at Lake 

Hollingsworth is controlled by an adjustable gate 

structure, and so stage fluctuations and surface-water 

outflow rates are not natural. When the outflow gate 

was opened to its widest setting (during August 1996), 

surface-water discharge was about four times higher 

than for the previous month. The resultant stage decline 

apparently induced additional ground-water inflow, and 

net ground-water flow was almost six times higher than 

Lake Annie

107

108

109

110

111

112

113

Lake Annie

0

5

10

15

20

25

30

35

40

(note: y-axis scale different from other plots)

Lake George 

127

128

129

130

131

132

133

Lake George

-8

-4

0

4

8

12

plus uncertainty

minus uncertainty

10/1/95

12/16/95

3/1/96

5/16/967/31/96

10/15/9612/30/96

Lake Hollingsworth

129

130

131

132

133

134

135

Lake Hollingsworth

-8

-4

0

4

8

12

LAKE STAGE, IN FEET ABOVE SEA LEVEL

NET GROUND-WATER FLOW, IN INCHES PER MONTH

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

1995 1996

Figure 12. Stage hydrographs and monthly net ground-water flow for surface-water drainage lakes: Lakes Annie, 

George, and Hollingsworth.

during the previous month. Ground-water inflow paths 

to Lake Hollingsworth are probably relatively long, as 

the best relation was between monthly net groundwater flow and 4 months cumulative rainfall (table 5). 

Clay-rich sediments in the upper part of the basin maintain both the water table and upgradient lakes at higher 

elevations than Lake Hollingsworth. For example, the 

water table at the well 1,800 ft north-northwest of the 

lake was 39 ft higher than the lake, and the stage of 

upgradient Lake Morton (0.6 mi north of Lake Hollingsworth) was 47 ft higher than Lake Hollingsworth 

(fig. 4b). Thus, downward recharge through the surficial 

aquifer system apparently is limited in this area, and 

most flow in the shallow surficial aquifer system is 

probably lateral toward Lake Hollingsworth. Under 

these conditions, the potential contributing groundwater area to Lake Hollingsworth is large. An earlier 
24 Estimating Ground-Water Exchange with Lakes Using Water-Budget and Chemical Mass-Balance Approaches for Ten Lakes 

in Ridge Areas of Polk and Highlands Counties, Florida

study of Lake Hollingsworth estimated that groundwater inflow to the lake was very high (about 240 in/yr; 

Romie, 1994).

Factors Affecting Ground-Water 

Exchange with Lakes

Numerous factors can affect ground-water 

exchange with lakes. These factors include recharge, 

artificial lowering of lake stage, pumping stresses from 

the deeper Upper Floridan aquifer, and physical features such as lake depth, basin steepness, and degree of 

confinement between the lake and the Upper Floridan 

aquifer. These factors can be classified into those that 

influence lateral ground-water flow and those that 

influence vertical ground-water flow. 

Lateral Ground-Water Flow

Head gradients between the lake and the adjacent 

surficial aquifer system influence both lateral groundwater inflow and outflow. Where the water table is 

higher than the lake, ground-water inflow can occur. If 

the gradient increases, either due to natural recharge or 

artificial pumping of a lake, ground-water inflow can 

increase. Both near-shore and upper-basin recharge 

processes can influence ground-water inflow. The 

relative importance of these processes, however, 

varies between lakes.

The near-shore process of water-table mounding 

occurs when the water table rises rapidly in response to 

high recharge. This is more likely to occur in nearshore areas where the unsaturated zone is thin and 

recharge is efficient. Transient mounds that form on 

the outflow perimeter of a lake can temporarily reverse 

the direction of ground-water flow. As a result, 

ground-water outflow areas become ground-water 

inflow areas. This, in turn, can increase ground-water 

inflow and reduce lateral ground-water outflow. Of the 

study lakes, those in the flattest topographic settings, 

Grassy Lake and Lake Olivia (table 1), are most likely 

to experience near-shore flow reversals. Ground-water 

inflow to these lakes tends to be dominated by relatively short, rapid inflow paths. For example, at Grassy 

Lake almost 10 in. of rain fell during September 1996, 

and ground-water inflow occurred around the entire 

lake perimeter. As a result, net ground-water flow was 

much higher during that month compared to previous 

or following months (fig. 13). Similarly, months with 

the highest net ground-water flow at Lake Olivia corresponded to those with the highest rainfall, when 

ground-water inflow occurred over 80 to 100 percent of 

the lake perimeter (fig. 13). Other studies of Florida 

lakes have observed transient mounding only within 

75 ft of the lake margin (Lee and Swancar, 1997; Lee, 

1996). This process might be underrepresented in our 

study because near-shore wells typically were not 

installed this close to the lake, and because below average rainfall limited ground-water recharge at some of 

the study lakes. Monthly measurements of water levels 

in wells also limited the number of observations for 

which transient events could be observed.

Other lakes have a longer response time before 

recharge influences ground-water inflow (for example, 

Lakes Annie, Isis, and Starr; table 5). These lakes, 

which are typically in steeper topographic basins, have 

longer and possibly deeper inflow paths. Upper parts 

of the basin are probably important contributing areas 

of ground-water inflow to the lakes. In a steeper basin, 

depth to the water table is greater, and it takes longer 

for recharge to affect the water table. Thus, for these 

lakes, increases in net ground-water flow typically 

occur over a succession of 3 to 4 months, instead of 

within the month of greatest rainfall. For example, at 

Lake Starr the greatest net ground-water flow occurred 

Grassy Lake

-2

-1

0

1

2

3

4

5

6

7

55

60

65

70

75

80

85

90

95

100

PERIMETER GROUND-WATER INFLOW, IN PERCENT

net groundwater flow

perimeter

ground-water

inflow

NET GROUND-WATER FLOW, IN INCHES PER MONTH

Lake Olivia

-4

-3

-2

-1

0

1

2

3

30

40

50

60

70

80

90

100

ONDJ FMAMJ J ASOND

1995 1996

period of direct 

pumping from lake

Figure 13. Monthly net ground-water flow for Grassy Lake 

and Lake Olivia, plotted with percent of lake perimeter that 

receives ground-water inflow, for the period October 1995 

through December 1996.
Water-Budget Approach 25

in September 1996, at the end of the wet season, even 

though rainfall was lower that month than the previous 

3 months (fig. 10).

Artificial lowering the level of a lake can quickly 

change the lateral head gradients between the lake and 

the adjacent surficial aquifer system. When a lake is 

lowered faster than the surrounding surficial aquifer 

system, the increased head gradient induces additional 

ground-water inflow. A lake can be artificially lowered by direct pumping or by increasing outflow from 

a control structure on an engineered lake. Grassy Lake 

was lowered by direct pumping from the lake (November and December 1995) in response to flooding of 

near-shore homes. Because the lake was lowered faster 

than the water table, the water-table gradient toward 

the lake increased during this period, which, in turn, 

induced significant amounts of ground-water inflow. 

This is illustrated by ground-water inflow occurring 

around the entire lake perimeter and net ground-water 

flow increasing, not only during the 2 months of pumping, but also during the following 4 months (fig. 13). 

More than half the equivalent volume of water directly 

pumped from the lake recharged the lake as groundwater inflow during and following the pumping period. 

Thus, direct pumping to alleviate near-shore flooding 

around a lake is not particularly efficient because 

induced ground-water inflow replaces some of the 

volume of water pumped from the lake.

At Lake Hollingsworth, the outflow gate was 

opened wider during one of the study months (August 

1996), which lowered the lake level rapidly in a manner 

similar to direct pumping (fig. 12). Because of the 

increased lateral head gradient toward the lake, net 

ground-water flow to the lake increased during this 

period of high surface-water outflow. A volume equivalent to one-third of the water that left the lake as additional surface-water outflow entered the lake as 

increased ground-water inflow during this month. As a 

result of the additional ground-water inflow, lake stage 

decline was reduced.

In areas where the water table is lower than the 

lake, lateral ground-water outflow occurs. All of the 

study lakes had an area of the lake perimeter where the 

water table was lower than the lake sometime during 

the study. Some lakes had very stable outflow head 

gradients, whereas others had reversals in flow due to 

transient water-table mounding. The lateral outflow 

head gradient from a lake can increase in response to 

low recharge, downgradient hydraulic stresses in the 

system, or short-term increases in lake stage due to 

surface-water inflow or augmentation.

Water-table gradients away from a lake can be 

controlled by a lower stage in a downgradient lake or 

by areas where confinement of the underlying aquifer 

is reduced. Several of the flow-through study lakes had 

lateral ground-water outflow that appeared to be controlled by water-table gradients between the study lake 

and a downgradient lake (for example, Lake Isis, Lake 

Olivia, and Saddle Blanket Lake). Hydraulic stresses 

on a downgradient lake can change these gradients and 

affect lateral ground-water outflow. For example, at 

Saddle Blanket Lake water from the adjacent pond was 

pumped for citrus grove irrigation. The extreme lowering of the pond’s stage between September and 

December 1996 caused a steeper slope in the water 

table away from Saddle Blanket Lake toward the pond. 

This, in turn, probably increased lateral ground-water 

outflow from Saddle Blanket Lake, indicated by the 

large negative net ground-water flow during November 

and December 1996 (fig. 9).

Lateral ground-water outflow also can be controlled by reduced confinement between the surficial 

aquifer system and Upper Floridan aquifer in areas 

peripheral to a lake (Sacks and others, 1992a; Evans 

and others, 1994; Lee, 1996). Reduced confinement 

can provide a more efficient route for downward flow 

to the Upper Floridan aquifer. Increased downward 

flow reduces the water-table elevation, thereby causing 

lateral ground-water outflow in these areas. This might 

be the case at Round Lake and Lake Starr. On the east 

side of Round Lake, flow is towards the lake in the 

upper basin, yet this flow does not intercept the lake 

because the near-lake well is in an area of ground-water 

outflow (fig. 6a). It is likely that this is an area of 

reduced confinement, allowing a route for preferential 

downward flow. Hydrostratigraphic data were not 

collected for this study, and, thus, the degree of confinement at Round Lake can only be inferred. However, at Lake Starr, which is the focus of a more 

detailed water-budget study, considerable data exist on 

confinement characteristics in the basin. Ground-water 

inflow occurs on the side of the basin with good confinement between the surficial aquifer system and the 

Upper Floridan aquifer, whereas lateral ground-water 

outflow occurs on the south side of the lake, which is a 

less confined area with several sinkholes. 

Finally, lateral ground-water outflow can be 

induced if a lake is at an artificially high level. This was 

not the case for any of our study lakes, but it has been 
26 Estimating Ground-Water Exchange with Lakes Using Water-Budget and Chemical Mass-Balance Approaches for Ten Lakes 

in Ridge Areas of Polk and Highlands Counties, Florida

noted in other studies in Florida (Stewart and Hughes, 

1974; Belanger and Kirkner, 1994). Augmenting the 

level of a lake can cause it to “mound” above the 

surrounding water table, decreasing or eliminating 

ground-water inflow and increasing lateral groundwater outflow (K.A. Stelman, U.S. Geological Survey, 

written commun., 1997).

The depth and size of a lake are other physical 

characteristics that can affect the lateral exchange of 

water between a lake and the ground water. A deeper 

lake has the potential to receive more ground-water 

inflow than a shallow lake because the deeper lake 

intercepts a larger part of the surficial aquifer system 

(Lee and Grubbs, in press). Lakes Annie and Isis are 

the deepest of the study lakes (table 1). Lake Annie had 

the highest net ground-water inflow of all the study 

lakes (table 4), and Lake Isis had high net ground-water 

inflow during the early wet part of the study (fig. 9 and 

appendix). For circular lakes, a smaller lake has a 

larger ratio of lake perimeter to surface area than a 

larger lake. Thus, a small lake has the potential to have 

more lateral ground-water inflow relative to its surface 

area than a large lake (Millar, 1971; Fellows and 

Brezonik, 1980). However, the influence of lake size 

varies between lakes and is dependent on lake depth 

and hydrogeologic setting. This variability is exemplified by the differences in net ground-water flow for the 

two smallest study lakes, Swim Lake and Saddle 

Blanket Lake (which both have surface areas less than 

10 acres). Swim Lake had a large amount of groundwater exchange, whereas Saddle Blanket Lake had 

minimal ground-water inflow, and ground-water 

exchange was dominated by ground-water outflow 

(figs. 9 and 10; appendix).

Vertical Ground-Water Flow

A downward head gradient exists between all of 

the study lakes and the Upper Floridan aquifer, indicating a potential for downward ground-water outflow to 

the deeper aquifer. Thus, the deeper aquifer never contributes ground-water inflow to the study lakes, but it 

does influence vertical ground-water outflow. In contrast to lateral ground-water outflow described in the 

preceding section, this vertical outflow is controlled by 

the head difference between the lake and the deeper 

aquifer and by the physical characteristics of the sublake geology. For example, a deeper lake can lose 

more water through vertical ground-water outflow 

because it occupies a deeper subsidence feature, and, 

thus, is closer to the influence of the Upper Floridan 

aquifer. Thickness of lake sediments and disruption in 

sublake confining units also influence vertical groundwater outflow.

Pumping from the Upper Floridan aquifer can 

influence the short-term downward head difference 

between a lake and the aquifer. For this reason, a single 

monthly aquifer head measurement can greatly misrepresent overall monthly conditions. Hydraulic conditions are more accurately represented by computing the 

difference between monthly average lake stage and 

monthly average head in the deeper aquifer (when a 

well is available with continuously recorded head 

data). For example, at Lake Starr there was a much 

better relation between monthly net ground-water flow 

and the head difference computed from continuously 

recorded water levels in an Upper Floridan aquifer well 

outside of the basin (r2 = 0.81), compared to the difference computed from a monthly water-level measurement in a well near the lake (r2

 = 0.28, which is not a 

statistically significant regression to alpha level of 

0.05) (table 5). A continuously monitored Upper 

Floridan aquifer well was in the general proximity (less 

than 5 mi) of Lakes Isis, Olivia, and Starr. These lakes 

had much better linear relations between monthly net 

ground-water flow and the vertical head difference 

(average r2 = 0.73) compared to the seven lakes with 

only instantaneous head measurements (average r2

 = 

0.28 and many of these regressions were not statistically significant to alpha level of 0.05) (table 5). Thus, 

results can be inconclusive for lakes without a nearby 

Upper Floridan aquifer well with a continuous 

recorder.

During months with larger downward head 

differences, net ground-water flow can be more negative. Interpreting these data, however, is complicated 

because pumping from the deeper aquifer and rainfall 

are inversely related. For example, during the dry season, increased pumping for citrus-grove irrigation 

occurs at the same time that recharge and ground-water 

inflow are low. During the wet season, there is less 

pumping for irrigation and ground-water inflow is 

higher. Net ground-water flow results reflect the combined effects that climate and pumping have on lateral 

and vertical exchange of ground water; thus, results 

cannot readily distinguish between these correlated 

inflow and outflow terms.

More direct evidence of the effects of pumping 

from the Upper Floridan aquifer on ground-water outflow from a lake is seen during cold periods, when 

citrus growers pump large amounts of water from the 
Chemical Mass-Balance Approach 27

aquifer to protect their crops from frost damage. At 

Swim Lake, which is a small lake surrounded by citrus 

agriculture, lake stage declined significantly during 

these cold periods (fig. 14). It is likely that these 

above-normal declines in lake stage were caused by 

ground-water outflow induced by increased withdrawals from the Upper Floridan aquifer. However, head 

gradients between the lake and the Upper Floridan 

aquifer during these cold periods could not be quantified because a deep well with a continuous recorder 

was not available near Swim Lake.

Similar observations of lake stage declines 

during cold periods were made at other study lakes, 

although they were less pronounced than at Swim Lake 

(fig. 14). For example, stage declines at Lakes Isis and 

Olivia were typically greater during periods when a 

freeze was possible than at other times. Lake Isis has 

citrus agriculture only in its upper basin, and Lake 

Olivia does not have any citrus agriculture in its basin. 

Thus, regional-scale effects of pumping on heads in the 

Upper Floridan aquifer are probably being observed. 

This is supported by daily head data from an Upper 

Floridan aquifer well near Lake Isis, where head 

declines of as much as 11 ft were observed during cold 

periods (fig. 15).

CHEMICAL MASS-BALANCE APPROACH

The chemical mass-balance approach was used 

to estimate ground-water inflow and outflow for the 

10 study lakes. This is in contrast to the water-budget 

approach discussed in the previous section, which only 

allowed the net ground-water flow to be computed. In 

addition, when using steady-state assumptions, the 

chemical mass-balance approach computes long-term 

estimates of ground-water inflow and outflow. In contrast, the water-budget approach computes short-term 

flow estimates over the time frame considered (for 

example, monthly or during the study year) but cannot 

be used to infer the longer term or steady-state nature 

of net ground-water flow.

Figure 14. Relation between minimum air temperature and 

daily change in lake stage, for days without rain between 

December 1995 and March 1996, and December 1996, for 

Swim Lake, Lake Isis, and Lake Olivia.

Swim Lake

25

30

35

40

45

50

55

60

65

-0.14 -0.12 -0.10 -0.08 -0.06 -0.04 -0.02 0.00 0.02 0.04

MINIMUM AIR TEMPERATURE, IN DEGREES FAHRENHEIT 1

freeze possible

freeze 

possible

freeze 

possible

Lake Isis 

25

30

35

40

45

50

55

60

65

-0.06 -0.05 -0.04 -0.03 -0.02 -0.01 0

Lake Olivia

25

30

35

40

45

50

55

60

65

-0.06 -0.05 -0.04 -0.03 -0.02 -0.01 0

DAILY CHANGE IN LAKE STAGE, IN FEET

1 Air temperature from nearest National Weather Service site.

2 Air temperature from following day.

2

2

78

80

82

84

86

88

90

92

12/1/95

12/21/95

1/10/961/30/962/19/963/10/963/30/9620

30

40

50

60

70

80

90

head

temperature

1Head data from ROMP43xx (Southwest Florida Water Management 

 District, written commun.,1997).

 2Temperature data from National Weather Service site Avon Park 2W. 

HEAD IN UPPER FLORIDAN AQUIFER,

IN FEET ABOVE SEA LEVEL

1

MINIMUM AIR TEMPERATURE,

IN DEGREES FAHRENHEIT

2

Figure 15. Head in the Upper Floridan aquifer plotted with 

minimum air temperature, for the period December 1995 to 

March 1996, for a site near Lake Isis, Highlands County, 

Florida.
28 Estimating Ground-Water Exchange with Lakes Using Water-Budget and Chemical Mass-Balance Approaches for Ten Lakes 

in Ridge Areas of Polk and Highlands Counties, Florida

Methods

The approach used to compute ground-water 

inflow relies on the use of a naturally occurring 

tracer. This tracer can be a conservative inorganic 

solute, such as magnesium, chloride, or calcium (for 

example, Stauffer, 1985; Pollman and others, 1991; 

Krabbenhoft and Webster, 1995). Alternatively, the 

conservative tracer can be the isotope of the water 

molecule itself (for example, Dinçer, 1968; Turner 

and others, 1984; Krabbenhoft and others, 1990). 

Krabbenhoft and others (1994) discuss the use of 

both isotope and solute tracers in computing groundwater fluxes to lakes.

Theory

The chemical mass-balance approach uses a 

naturally occurring tracer, and combines the water 

budget equation (1) with the solute or isotope massbalance equation below:

(4)

where ∆ is change over a specified time period of the 

mass of the tracer in the lake (V, lake volume, times 

CL, the concentration of the tracer in the lake) and C is 

the concentration of the tracer in precipitation (P), 

evaporation (E), surface-water inflow (Si

), and groundwater inflow (Gi

); the tracer concentration of surfacewater outflow (So) and ground-water outflow (Go) are 

assumed to equal that of the lake. The tracer is 

assumed to be conservative; that is, it is not significantly involved in any geochemical or biological reactions that significantly alter its concentration. Steadystate assumptions also require that the chemical composition of the lake is representative of longer-term 

hydrologic conditions than just the conditions during 

the study (Pollman and others, 1991; Pollman and Lee, 

1993). The water budget equation (1) can be rearranged to solve for Go and substituted into equation (4). 

This equation can then be rearranged to solve for Gi

, 

resulting in the following equation, which eliminates 

Go (and associated errors in its estimate) from this 

approach:

(5)

Ground-water outflow can then be calculated by 

solving the steady-state water budget equation (1).

Precipitation (P) used in equation 5 was the 

10-year (1987-96) average rainfall from the NWS 

site closest to each lake (table 6), and evaporation 

(E) was the 10-year average pan evaporation from 

the Lake Alfred NWS site, adjusted using a pan 

coefficient of 0.70 (53.1 in.). Surface-water inflow 

(Si

) (for Lakes Annie and Hollingsworth) was 

assumed to equal annual values measured during 

the water-budget study period (table 4). Stormwater inflow for Lakes Hollingsworth and Isis was 

estimated as a percentage of the 10-year average 

rainfall using methods described in the waterbudget section. Surface-water outflow cancels out 

when equations (1) and (4) are combined and, thus, 

does not need to be defined in equation (5).

 

∆ VCL ( ) PCP ECE – SiCSi

SoCL – GiCGi

GoCL = + + –

Gi

PCP ECE – SiCSi P E– Si ( ) + CL + –

CL CGi ( ) – = ----------------------------------------------------------------------------------------

Table 6. Selected hydrologic and chemical parameters 

used for the chemical mass-balance approach (equation 5)

[in/yr, inches per year; δE, isotopic composition of evaporating water;

δD, delta deuterium; δ18O, delta oxygen-18; VWM, volume-weighted 

mean; Cl, chloride; Na, sodium; δa, isotopic composition of atmospheric 

moisture]

Lake P1 (in/yr)

1Average for 1987-96 from nearest National Weather Service rainfall site.

δE for

δD2

(per mil)

2Using δa assuming isotopic equilibrium with rainfall.

δE for

δD3

(per mil)

3Using δa back-calculated from Lake Starr water budget using Cl as a 

tracer.

δE for 

δ18O3

(per mil)

Annie 51.4 -52.0 -47.4 -10.31

George 53.1 -35.6 -29.2 -6.07

Grassy 51.5 -32.6 -26.0 -5.37

Hollingsworth 54.8 -23.8 -17.3 -4.61

Isis 48.9 -52.6 -49.3 -10.25

Olivia 48.9 -34.4 -29.4 -6.77

Round 51.5 -22.3 -16.1 -3.44

Saddle Blanket 48.9 -14.6 -14.6 0.16

Starr 49.4 -30.8 -24.3 -4.64

Swim 49.4 -59.9 -53.7 -10.98

Parameters the same for each lake:

VWM rainfall concentration: for Cl = 0.82 mg/L4

, for Na = 

0.45 mg/L4

, for δD = -20.6 per mil, for δ18O = -3.89 per mil

4From Pollman and Canfield (1991) and Baker (1991).

 

δa back-calculated using Lake Starr water budget using Cl as a 

tracer: for δD = -105.3 per mil, for δ18O = -17.95 per mil
Chemical Mass-Balance Approach 29

Equation 5 was solved using different solutes 

(chloride, sodium, calcium, magnesium, potassium, 

and bromide, for selected lakes only) and the stable 

isotopes of the water molecule (deuterium and oxygen-18). Concentrations of tracers were measured in 

ground-water and lake-water samples collected during the study. Solute tracer concentrations in rainfall 

were estimated from 5-year volume-weighted mean 

concentrations at a site in Highlands County (Pollman and Canfield, 1991) and were adjusted for dry 

deposition based on Baker (1991) (table 6). The 

ratios of deuterium and oxygen-18 in rainfall and 

stormwater inflow were estimated as the volumeweighted mean concentration of these isotopes in 

rainwater collected at Saddle Blanket Lake. Solute 

concentrations in stormwater inflow (Lake Isis and 

Hollingsworth only) were estimated from several 

point samplings during storm events. For a solute 

tracer, the concentration of the solute in evaporating 

water (CE) is essentially zero, and the term ECE can 

be omitted from equation 5. However, the isotopic 

composition of evaporating water is not zero, and this 

term needs to be included when deuterium and oxygen-18 are used as tracers.

Estimating Deuterium and Oxygen-18 in

Evaporating Water

The greatest challenge to using deuterium (δD) 

and oxygen-18 (δ18O) as conservative tracers is 

defining the isotopic composition of evaporating 

water (Krabbenhoft and others, 1990). Isotope ratios 

are defined in delta (δ) notation normalized to Vienna 

Standard Mean Ocean Water, and are expressed in 

units of per mil. Following nomenclature from 

Krabbenhoft and others (1990) and originally derived 

by Craig and Gordon (1965), the isotopic composition of evaporating water (δE) can be defined as:

(6)

where δ is the δD or δ18O composition of evaporating 

water (E), lake water (L), and atmospheric moisture 

(a); α′ is the equilibrium isotope fractionation factor 

at the temperature of the air-water interface (which is 

equivalent to 1/α defined by Majoube, 1971, and is a 

δE

α′δL hδa – – ε

( ) 1 – 0.001 h + ∆ε = -------------------------------------------

function of the average water-surface temperature of 

the lake); h is relative humidity normalized to average lake water-surface temperature, expressed as a 

fraction; ε is the total fractionation factor, expressed 

in per mil, and is equal to 1000 (1 - α′) + ∆ε; and ∆ε

is the kinetic fractionation factor, expressed in per 

mil (estimated as 12.5 (1 - h) for δD and 14.2 (1 - h) 

for δ18O; Gonfiantini, 1986). Variables α′, h, ε, and 

∆ε are all functions of climatic variables of air temperature, lake water-surface temperature, and humidity. Assuming these are the same at all of the lakes 

(which is probably a reasonable assumption because 

of their geographic proximity; Krabbenhoft and 

others, 1994), δE can be solved as a function of δL

(which was measured) and δa (which was not measured).

Besides direct measurement of δa (Krabbenhoft and others, 1990), which was beyond the scope 

of this study, δa can be estimated by either assuming 

it is in isotopic equilibrium with rainwater, which 

was collected at one of the study lakes, or by backcalculating δa from a lake with a “known” water budget. Both approaches were attempted for this study. 

When back-calculating δa, the ground-water inflow 

to Lake Starr computed using chloride as a tracer was 

used. Values are included in table 6. Ideally, 

ground-water inflow also would be independently 

corroborated using a more accurate method like 

three-dimensional, ground-water flow modeling.

When δa was assumed to be in equilibrium 

with rainwater, the isotopic composition of evaporating water (δE) was estimated monthly using equation 

(6). From these monthly values, an annual volumeweighted mean δE was computed for each lake. An 

example for Swim Lake is shown in table 7. The 

volume-weighted mean δE was very similar to that 

computed using annual average climatic variables. 

For δ18O, there was an average of 3 percent difference between δE computed from monthly values and 

from annual mean values; for δD, the average difference was 14 percent, which is probably related to the 

poorer analytical precision for δD (2 per mil) compared to δ18O (0.2 per mil). Thus, it may be sufficient to compute δE for Florida lakes from annual 

average data, rather than from monthly values, 

because the seasonal climatic variability is relatively 

low compared to more temperate regions.
30 Estimating Ground-Water Exchange with Lakes Using Water-Budget and Chemical Mass-Balance Approaches for Ten Lakes 

in Ridge Areas of Polk and Highlands Counties, Florida

 

Sample Collection

Ground water and lake water were sampled three 

times during the study. The three samplings were 

designed to define seasonal and annual variability in 

water quality. The sampling times were at the end of 

the 1995 wet season (October to early December), at 

the end of the 1996 dry season (April to May), and at 

the end of the 1996 wet season (October to November). 

During the first sampling, all monitoring wells finished 

in the shallow part of the surficial aquifer system were 

sampled. During the following samplings, only nearlake wells were sampled. It was assumed that these 

near-lake wells intercept the ground water entering the 

lake. For the last sampling, wells sampled at most 

lakes were near-lake wells with ground-water inflow 

head gradients (that is, with the most frequently 

observed flow direction toward the lake, figs. 3 through 

12); for Grassy Lake, Lake Olivia, Round Lake and 

Lake Starr, wells with ground-water outflow gradients 

were also sampled. Results were primarily used to 

quantify the range of concentrations in ground-water 

inflow.

Samples were collected from wells after the 

volume of water in the casing was purged at least three 

times, and after field measurements of temperature, 

pH, and specific conductance stabilized. Dissolved 

oxygen and acid neutralizing capacity (alkalinity) were 

measured in the field. Dissolved oxygen was measured 

with a field meter in a flow-through chamber to eliminate exposure to the atmosphere. Acid neutralizing 

capacity was determined by titration with sulfuric acid, 

using the Gran titration method (Stumm and Morgan, 

1981). Samples were collected with a peristaltic pump, 

except when depth to water exceeded the pump capacity; in those cases, a bailer was used to purge and 

sample the wells.

Before the lakes were sampled, lateral and vertical stratification were evaluated by measuring profiles 

of temperature, specific conductance, and dissolved 

oxygen in about five locations in the lake. Most lakes 

did not show lateral or vertical stratification in specific 

conductance, indicating that the lakes were chemically 

well mixed for major solutes. The two deepest lakes 

(Lake Annie and Isis) occasionally had subtle vertical 

Table 7. Monthly mean values used to compute the isotopic composition of evaporating water at Swim Lake 

using deuterium and assuming atmospheric moisture in isotopic equilibrium with rainwater

[h, relative humidity; temp., temperature; deg. C, degrees Celcius; α′, equilibrium fractionation factor; ∆ε, kinetic fractionation factor; ε, 

total fractionation factor; εδp-δa, equilibrium fractionation factor between δP and δa; δ isotopic composition of rainwater (P), atmospheric 

moisture (a), lake water (L), and evaporating water (E); E, evaporation]

Monthyear

h1,2 Air temp.1

(deg C.) α′ ∆ε ε δP

3 ε

δp-δa

4 δa5 δL6 δE7 E8

Jan-96 0.673 14.7 0.91599 4.09 88.1 -18.8 91.7 -110.5 -6.45 -59.4 2.3

Feb-96 0.696 15.1 0.91648 3.80 87.3 -11.8 91.1 -102.9 -6.43 -70.1 2.4

Mar-96 0.625 16.3 0.91773 4.68 87.0 -38.8 89.6 -128.4 -6.42 -33.0 4.0

Apr-96 0.635 20.4 0.92184 4.56 82.7 -14.1 84.8 -98.9 -6.40 -69.9 5.3

May-96 0.627 24.9 0.92628 4.66 78.4 -13.1 79.6 -92.6 -6.42 -69.5 5.4

Jun-96 0.634 25.8 0.92705 4.58 77.5 -23.2 78.7 -101.9 -6.43 -51.0 5.4

Jul-96 0.657 27.5 0.92863 4.29 75.7 -18.5 76.9 -95.4 -6.45 -54.7 6.2

Aug-96 0.632 26.5 0.92777 4.59 76.8 -1.7 77.9 -79.6 -6.47 -87.4 5.3

Sep-96 0.646 26.2 0.92748 4.42 76.9 -21.1 78.2 -99.3 -6.48 -52.5 6.2

Oct-96 0.672 23.2 0.92459 4.10 79.5 -29.5 81.6 -111.1 -6.50 -32.7 4.1

Nov-96 0.632 19.4 0.92086 4.60 83.7 -17.6 85.9 -103.5 -6.48 -65.1 3.5

Dec-96 0.694 16.8 0.91820 3.83 85.6 -5.7 89.1 -94.8 -6.47 -83.2 2.3

Average 0.652 21.4 0.92287 4.35 81.5 -20.7 83.6 -104.3 -6.45 -55.2 9

-59.9

1

From Lake Starr (July through December); for January through June, data from Tampa International Airport, adjusted with linear 

regression equation computed for overlapping data. 2

Normalized to lake water-surface temperature: vapor pressure of air divided by saturation vapor pressure at lake water-surface temperature. 3Averaged between months when sample overlapped months; for January, estimated from average for December, February, and 

March samples. 4

Computed as 1000 (α - 1), where α = 1/α′. 5Assumed to be in isotopic equilibrium with rainwater (δa = δP - εδp-δa). 6

Extrapolated linearly between April and October samplings.

7Computed using equation 6.

8

From the 1996 water budget.

9 Volume-weighted δE, in per mil, computed by summing monthly δE times E, and then dividing by total E for year.
Chemical Mass-Balance Approach 31

stratification of specific conductance in the deepest 

10 ft, but this represented a very small part of the total 

lake volume. Thus, one representative sample was 

collected at all lakes. Samples were collected from a 

depth of 3 to 4 ft using a peristaltic pump.

Ground-water and lake-water samples were 

collected for major ions, selected trace constituents, 

and dissolved nutrients. Samples were filtered through 

a 0.45 micrometer in-line filter for anion, cation and 

nutrient analysis. Cation and trace metal samples were 

collected in acid-washed bottles and acidified with 

1 mL of 70 percent nitric acid. Nutrient samples were 

chilled immediately. Samples were analyzed by the 

USGS laboratory in Ocala, Florida. Analyte-free 

deionized water was run through sampling equipment 

for field blanks. Most solutes were below the laboratory detection limit in the blanks; solutes that were 

detected were always at or near the detection limit and 

were far below concentrations in the ground water or 

lakes. About 10 percent of the samples were collected 

in duplicate to better define analytical precision.

During the spring and fall 1996, deuterium and 

oxygen-18 samples were collected from each lake and 

from at least two ground-water inflow wells in each 

lake basin. For three lake basins (Grassy Lake, Round 

Lake, and Lake Starr), all near-lake wells were sampled 

for δD and δ18O. Unfiltered samples were collected in 

glass bottles with polyseal caps for isotope analysis. 

Isotope samples were analyzed by the USGS Isotope 

Fractionation Laboratory in Reston, Virginia.

Tracer concentrations in surface-water inflow to 

Lake Hollingsworth and in stormwater inflow to Lakes 

Hollingsworth and Isis needed to be estimated. During 

the spring and fall 1996 samplings, water was collected 

at the outflow structure of Lake Horney, which drains 

directly into Lake Hollingsworth through a pipe 

network. Point stormwater inflow samples were 

collected for Lakes Hollingsworth and Isis. For Lake 

Hollingsworth, three storm drains were sampled during 

2 different storm events. For Lake Isis, three storm 

events were sampled. Whole samples were collected, 

immediately chilled, and sent to the laboratory for 

filtration, preservation, and analysis.

Rainwater was collected for δD and δ18O analysis 

from Saddle Blanket Lake. A modified storage rain 

gage was used as a sample collector, with the funnel 

emptying directly into a sample bottle. A plastic ball 

was placed inside the funnel so that water from the bottle 

was sealed from the atmosphere except during rain 

events. This method has proven successful in preventing 

evaporation and subsequent isotopic enrichment of rainwater samples (Scholl and others, 1995). A mesh screen 

was used over the funnel to keep out large debris and to 

contain the plastic ball during high intensity rain events. 

The sampler was mounted about 5 ft above the ground. 

Samples were collected approximately once a month for 

about a year. Selected rainfall samples were also 

analyzed for bromide to evaluate its use as a tracer.

General Water Quality Characteristics

Before using the chemical mass-balance 

approach, general characteristics of ground-water and 

lake-water quality were examined. These data are useful 

in defining spatial variability in ground-water inflow 

concentration, and in illustrating the influence of ground 

water on lake-water quality. Complete analytical data 

are available in the USGS annual data report for southwest Florida (Coffin and Fletcher, 1998a, b).

Ground Water

Land use significantly influences the groundwater quality in the shallow surficial aquifer system. 

The presence of sandy soils combined with high 

recharge rates can readily transport fertilizers and other 

solutes applied at land surface. This can result in 

enrichment of many ions over background concentrations. The study area is characterized by heavy citrus 

agriculture and increasing amounts of residential 

development. Undeveloped lake basins are very rare. 

Land use is a significant factor when applying the 

chemical mass-balance method because major-ion 

concentrations in ground water can be quite variable in 

a developed basin. Ranges of selected constituents in 

ground water are included in table 8, and the composition of median ground-water inflow for each lake basin 

is illustrated by stiff diagrams (fig. 16).

Two of the study lakes were in undeveloped 

basins (Lake Annie and Saddle Blanket Lake). In these 

basins, ground water from the shallow part of the surficial aquifer system was relatively dilute (specific conductance typically less than 100 μS/cm) and slightly 

acidic (pH values typically less than 5). Generally, 

sodium and chloride were the dominant ions in ground 

water, and calcium, magnesium, and sulfate were 

depleted relative to evaporative concentration of atmospheric deposition (wet plus dry deposition). Nitrate 

concentrations were negligible (Tihansky and Sacks, 

1997).
Ridge Areas of Polk and Highlands Counties, Florida

32 Estimating Ground-Water Exchange with Lakes Using Water-Budget and Chemical Mass-Balance Approaches for Ten Lakes in Table 8. Range and median water-quality and isotopic data for water from the surficial aquifer system in near-lake, inflow wells for each 

[All units in milligrams per liter unless noted; no., number of; sp. cond., specific conductance, μS/cm, microsiemens per centimeter at 25 degrees Celsius; min, minimum 

value; med, medium value; ANC, acid neutralizing capacity; δD, delta deuterium; δ18O, delta oxygen-18; <, less than; N, nitrogen; CaCO3, calcium carbonate] Lake No. wells1 Sp. cond., field

(μS/cm)

Field pH (standard

units)

Calcium, 

dissolved

Magnesium, 

dissolved

Sodium,

dissolved

min max med min max med min max med min max med min max med m

Annie 2 51 72 62 3.93 4.29 4.11 0.09 0.20 0.15 0.2 1.0 0.6 1.7 3.9 3.5 <0George 4 195 386 270 4.03 6.90 4.63 14 20 18 5.0 13 8.5 2.8 6.9 5.5 10 Grassy 5 101 551 186 3.91 6.57 5.49 5.0 30 16 1.4 33 3.6 1.1 18 4.3 1Hollingsworth 6 117 312 179 4.40 6.52 5.07 2.8 56 4.8 1.0 5.0 3.6 1.8 28 7.0 0Isis 4 75 282 139 4.39 6.17 5.27 3.3 16 8.3 2.0 9.0 3.4 2.3 12 4.8 0Olivia 4 34 264 98 4.85 6.37 5.43 2.5 20 8.7 0.2 2.6 1.6 0.1 27 4.7 <0Round 4 275 597 370 4.12 4.55 4.26 12 44 25 6.0 29 16 3.5 11 5.7 8Saddle Blanket 2 59 126 93 3.65 4.53 4.19 0.05 2.6 1.0 0.2 0.8 0.5 1.8 8.7 6.1 0Starr 5 33 385 153 4.19 6.87 4.91 2.4 26 9.7 0.6 13 1.8 0.9 18 2.8 0Swim 4 261 478 308 4.23 4.81 4.48 19 32 22 5.0 16 8.4 2.4 9.6 4.8 13 

Lake Chloride, dissolved Sulfate, dissolved ANC, total, as CaCO3 Nitrate, dissolved as N

δD21

(per mil)

1Number of wells used to define ground-water inflow; near-lake wells with inflow head gradients more than half the time (figs. 3-12); each well sampled three times (fall 

1996).

2Isotopes only sampled twice; two near-lake inflow wells sampled at each lake basin except Grassy Lake, Round Lake, and Lake Starr, where all near-lake inflow wells w

min max med min max med min max med min max med min max med mi

Annie 3.4 7.1 6.6 1.2 7.2 3.3 -6.9 -3.3 -4.2 <0.02 <0.02 <0.02 -24.5 -19.5 -22.5 -4.George 14 29 19 12 58 39 -6.0 14 1.0 7.6 24 12 -17.9 -10.4 -12.0 -3.Grassy 2.4 55 14 6.8 35 23 -7.3 62 12 <0.02 37 3.5 -23.2 -11.3 -18.4 -4.0Hollingsworth 2.6 62 16 0.7 28 5.7 -1.6 141 3.2 <0.02 4.7 1.6 -16.7 -13.8 -16.0 -3.Isis 6.1 25 12 7.1 40 20 -2.0 47 3.1 0.14 11 1.6 -23.9 -16.5 -20.8 -4.Olivia 1.1 48 7.3 <0.2 22 9.5 5.1 35 11 <0.02 1.4 <0.02 -23.1 -18.9 -22.7 -4.4Round 22 44 32 0.3 100 38 -4.9 1.4 -3.1 11 36 23 -21.2 -4.6 -14.1 -3.Saddle Blanket 3.7 21 9.3 <0.2 15 4.5 -13 -0.3 -3.9 <0.02 <0.02 <0.02 -26.7 -16.8 -20.0 -4.Starr 1.5 78 5.8 2.4 55 18 -5.1 60 0.5 0.45 12 1.2 -24.6 -17.5 -22.6 -4.Swim 5.8 18 9.0 33 81 43 -4.3 -0.54 -2.8 16 28 21 -21.0 -12.7 -18.2 -3.
Chemical Mass-Balance Approach 33

Figure 16. Chemical composition of ground-water inflow and lake water.

MEDIAN

 GROUND-WATER

 INFLOW

21 012

CATIONS ANIONS

 MILLIEQUIVALENTS PER LITER

sodium chloride

bicarbonate

sulfate

nitrate

calcium

magnesium

potassium

MEAN 

LAKE 

WATER

21012

CATIONS ANIONS

 MILLIEQUIVALENTS PER LITER

Lake

 (dominant land use in basin):

Lake Annie

(undeveloped)

Saddle Blanket Lake

(undeveloped)

Lake George

(citrus)

Round Lake

(citrus)

Swim Lake

(citrus)

Grassy Lake

(mixed)

Lake Isis 

(mixed)

Lake Starr 

(mixed)

Lake Hollingsworth 

(residential)

Lake Olivia 

(residential)
34 Estimating Ground-Water Exchange with Lakes Using Water-Budget and Chemical Mass-Balance Approaches for Ten Lakes 

in Ridge Areas of Polk and Highlands Counties, Florida

Waters from the shallow surficial aquifer system 

in areas of citrus agriculture had a distinct chemical 

composition (table 8 and fig. 16). These ground waters 

were highly enriched in calcium, magnesium, potassium, sulfate, and nitrate, as a result of fertilizer applications and possibly irrigation water. Chloride was 

sometimes, but not always, enriched because chloride 

salts are not uniformly applied to all groves. The 

amount of solute enrichment varied widely and is probably related to differences in grove management practices and the well locations relative to groves and 

ground-water flow paths. Specific conductance was 

typically high (greater than 250 μS/cm) because of solute enrichment from fertilizers and possibly irrigation 

water, and pH values were typically low (less than 5). 

It is interesting to note that the water is not buffered by 

the addition of calcium and magnesium, probably 

because nitrification reactions (whereby ammonia 

from fertilizers is converted to nitrate) produces H+

ions (Stumm and Morgan, 1981). Nitrate concentrations in ground water were almost always greater than 

10 mg/L as nitrogen (N), and were greater than 

50 mg/L in water from several wells in upper parts of 

lake basins (Tihansky and Sacks, 1997).

Ground-water composition in residential areas or 

areas of mixed land use was highly variable and probably depended on a number of factors, such as homeowner and agricultural fertilizer practices, the presence 

of septic tanks, and upgradient land use. Enriched 

sodium and chloride concentrations probably are 

related to septic-tank leachate (Cogger and others, 

1988; Alhajjar and others, 1989; Alhajjar and others, 

1990). Acid neutralizing capacity and pH values were 

sometimes higher than in undeveloped and citrusgrowing areas. Nitrate concentrations typically were 

much lower than in citrus-growing areas, but in places 

were above background levels (table 8). 

In areas of the basin that received groundwater outflow from the lake (determined from head 

measurements), ground water typically had sodium 

and chloride concentrations more similar to the lake 

water than the surrounding ground water. However, 

this was not always the case, probably because 

recharge between the lake and the well site can dilute 

the lake water, and land-use practices (such as citrus 

agriculture) can overwhelm the lake-water signature. Ground-water outflow was sampled for δD and 

δ18O at only three lake basins (Grassy Lake, Round 

Lake, and Lake Starr). At two of these, lake water 

could be distinguished in the ground water because 

of enrichment of δD and δ18O (fig. 17). Similar isotopic enrichment has been noted in areas of groundy = 6.18x + 2.81

y = 4.51x - 0.41

-40

-30

-20

-10

0

10

20

-7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5

DELTA OXYGEN-18, IN PER MIL

DELTA DEUTERIUM, IN PER MIL

ground-water inflow

ground-water outflow

lake water, spring

lake water, fall

rainwater

local meteoric water line

(n = 11)

evaporation trend line

(n = 20)

(line connects 

spring and fall samples 

at same lake)

Figure 17. Relation between delta deuterium and delta oxygen-18 in ground water, lake water, and 

rainwater.
Chemical Mass-Balance Approach 35

water outflow in other studies (Krabbenhoft and others, 1990; Sacks and others, 1992b; Katz and others, 

1995). In contrast, ground-water inflow not in contact with lake water had isotopically lighter values 

and plots near the meteoric water line (fig. 17).

When using the chemical mass-balance 

approach (equation 5), the tracer concentration of 

ground-water inflow to each lake has to be estimated. Major-ion concentrations in ground water 

typically varied both temporally and spatially within 

a lake basin. In undeveloped lake basins, spatial and 

temporal variability were similar. However, in the 

other lake basins, spatial variability was always 

more than twice that of the temporal variability, and 

solute concentrations could vary spatially by more 

than 100 percent in a given lake basin. The groundwater tracer concentration used in equation 5 was the 

median concentration of water from near-shore wells 

with a ground-water inflow gradient for more than 

half the time (table 8). Sensitivity of the calculations 

to uncertainties in the ground-water inflow tracer 

concentration are discussed in a later section. In 

contrast to solute tracers, δD and δ18O ratios in 

ground-water inflow were more uniform.

Lake Water

Lakes in ridge areas of Florida are naturally 

acidic due to low base cation concentrations and low 

acid neutralizing capacity in water from the surficial 

aquifer system (Canfield, 1983; Brenner and others, 

1990; Pollman and Canfield, 1991). The study lakes 

in pristine areas (Lake Annie and Saddle Blanket 

Lake) had the lowest pH values (less than 6) 

(table 9). These lakes were dilute (specific conductance generally less than 100 μS/cm) and dominated 

by sodium and chloride ions (table 9 and fig. 16). 

Calcium and sulfate concentrations were depleted 

relative to atmospheric deposition.

 

Table 9. Mean water-quality and isotopic data and principal land use for the study lakes

[All units in milligrams per liter unless noted; undev, undeveloped; res., residential; sp. cond., specific conductance; μS/cm, microsiemens per centimeter at 

25 degrees Celsius; ANC, acid neutralizing capacity; δD, delta deuterium; δ18O, delta oxygen-18; <, less than; >, greater than; N, nitrogen; CaCO3, calcium 

carbonate; ft, feet]

Lake

Principal 

land use in 

basin

Sp. cond., lab

(μS/cm)

Field pH

(standard

units)

Calcium, 

dissolved

Magnesium,

dissolved

Sodium,

dissolved

Potassium,

dissolved

Annie undev. 42 5.57 1.7 0.7 3.6 0.7

George citrus 354 8.10 28 14 6.8 18

Grassy citrus, res. 172 7.75 9.5 7.3 8.8 4.8

Hollingsworth res. 148 9.52 16 2.6 5.7 0.7

Isis citrus, res. 216 7.10 14 9.6 5.3 6.6

Olivia res. 81 7.10 4.0 2.0 6.1 1.8

Round citrus 370 7.97 25 16 11 18

Saddle Blanket undev. 105 4.13 1.5 1.0 7.3 0.5

Starr citrus, res. 177 7.51 10 6.8 8.3 6.2

Swim citrus 321 7.88 28 11 6.4 15

Chloride,

dissolved

Sulfate,

dissolved

ANC,

 total, as

 CaCO3

Nitrate,

dissolved

 as N1

Secchi

 depth (ft)

δD2

(per mil)

δ18O2

(per mil)

Annie 6.9 5.4 -0.34 <0.02 11 -4.1 -0.80

George 24 67 53 0.07 6 2.9 0.71

Grassy 19 13 35 <0.02 2 4.1 0.96

Hollingsworth 12 5.9 42 <0.02 0.7 7.5 1.23

Isis 14 42 16 3.1 17 -4.8 -0.78

Olivia 8.6 13 6.9 0.02 10 2.8 0.46

Round 34 50 68 <0.02 5 7.9 1.65

Saddle Blanket 14 12 -3.8 <0.02 > 9 10.8 2.93

Starr 15 30 28 <0.02 9 4.8 1.22

Swim 12 52 50 6.9 15 -6.5 -1.04

1To compute mean, less than values set to half the detection limit.

2Isotopes only sampled twice.
36 Estimating Ground-Water Exchange with Lakes Using Water-Budget and Chemical Mass-Balance Approaches for Ten Lakes 

in Ridge Areas of Polk and Highlands Counties, Florida

The chemical composition of lakes in basins 

dominated by citrus agriculture is markedly different 

than lakes in pristine areas. These lakes (Lake George, 

Round Lake, and Swim Lake) had pH values greater 

than 7 and specific conductance greater than 

300 μS/cm. Major-ion concentrations were also 

distinctly different than concentrations in pristine 

lakes, with significant enrichment over background 

levels of calcium, magnesium, potassium, sulfate, 

bicarbonate, and sometimes chloride (depending upon 

grove management practices) (table 9 and fig. 16). 

This ion enrichment is consistent with other studies of 

lakes in citrus-impacted basins in Florida (Pfischner, 

1968; Stauffer, 1991). Ion enrichment is direct 

evidence of ground water’s influence on lake water 

quality. The increased loading of base cations from the 

ground water can increase the acid neutralizing capacity of a lake (Stauffer, 1991). These lakes also probably have greater loading of nitrate from ground water 

(table 8), and denitrification reactions and biological 

assimilation of nitrate during photosynthesis can 

increase lake acid neutralizing capacity and pH 

(Stumm and Morgan, 1981).

Lakes in basins with residential or mixed land 

use typically had specific conductance values between 

those in pristine and citrus-agriculture basins (less than 

250 μS/cm). Values of pH were usually greater than 

7. Lakes with mixed land use that have some citrus 

agriculture in the basin show enrichment of major ions 

similar to lakes in citrus areas, but the enrichment is 

less pronounced (fig. 16). At Lakes Starr and Isis, most 

of the citrus is in upper parts of the basins. Because 

water quality at these lakes reflects some enrichment 

from citrus land use, long ground-water flow paths 

from upper parts of their basins are probably intercepting the lake. 

Two of the study lakes (Lakes Isis and Swim) 

had significant free nitrate in the water column (greater 

than 2 mg/L as N). The other lakes had nitrate concentrations less than, or very near, the laboratory detection 

limit (0.01 mg/L as N). These lakes with high nitrate 

concentrations also had the greatest clarity based on 

secchi depths (average of 17 ft at Lake Isis and 15 ft at 

Swim Lake), and fit into a category of lakes in 

Highlands County described by Stauffer (1991) as 

“clearwater citrus ponds.” The presence of nitrate and 

the high clarity indicate that another nutrient might be 

limiting the biological activity in the lake. Orthophosphate concentrations in ground-water inflow was lower 

at these lakes (less than 0.01 mg/L as phosphorus, P) 

than at the other eight lakes (average of 0.18 mg/L as 

P), and indicates that phosphorous might be a limiting 

nutrient in Lake Isis and Swim Lake. Orthophosphate 

concentrations were less than the laboratory detection 

limit (0.01 mg/L as P) in all lakes. Net ground-water 

flow results indicated that ground water is a significant 

part of the water budgets of Lakes Isis and Swim, and 

this probably results in higher nitrate loading to these 

lakes compared to lakes with less ground-water inflow. 

At Swim Lake, ground water from near-shore wells 

(presumed to represent ground-water inflow) had a 

median concentration of 21 mg/L as N, and the lake is 

entirely surrounded by citrus groves. However, at Lake 

Isis, the median nitrate concentration in ground water 

from near-shore inflow wells was much lower 

(1.6 mg/L as N). Some of the ground-water inflow to 

Lake Isis could originate in upper parts of the basin in 

areas of citrus agriculture, where nitrate concentrations 

probably are higher. This water could then be transported to the lake along deeper flow paths, passing 

below the shallower near-shore wells.

In the chemical mass-balance calculations (equation 5), the mean solute concentration in the lake 

(table 9) was used to compute ground-water inflow. 

Major-ion concentrations in the lakes were much more 

uniform between samplings than in the ground water. 

Variability around the mean concentration was typically less than 10 percent. For lakes with historical 

water-quality data, solute concentrations during the 

study were generally consistent with data from previous samplings. 

The isotopic composition of lake water differs 

from that of rainwater and ground water because 

evaporation enriches the lake water in δD and δ18O. 

Data from the lakes plot along an evaporation line that 

is offset from the meteoric water line (fig. 17). Lakes 

with little ground-water exchange are more highly 

evaporated, and should plot farther along the evaporation line; in contrast, lakes with high rates of groundwater exchange should plot closer to the meteoric water 

line (Krabbenhoft and others, 1994). The trend line for 

rainwater collected for this study had a lesser slope 

(6.2) than the global meteoric water line (slope of 8). It 

is possible that some of the rainwater samples were 

exposed to evaporation during collection. However, 

the trend line through the ground-water inflow samples 

had a very similar slope (6.0), suggesting that the local 

meteoric water line might indeed be different in central 

Florida than the global line. The lakes typically were 

more enriched in δD and δ18O during the fall, follow-
Chemical Mass-Balance Approach 37

ing the summer months when evaporation rates are 

highest, than during the spring (fig. 17). Lake Hollingsworth is the exception, where lighter δD and δ18O 

values in the fall are probably indicative of dilution 

from stormwater inflow (which is about 70 percent of 

rainfall) during the wet season. The average of the two 

lake samplings for stable isotopes was assumed to be 

representative of annual average conditions. Longterm data, however, were not available on the isotopic 

composition of the lakes, and whether these two samples are representative of long-term conditions is not 

known.

Chemical Mass-Balance Results

The following sections describe ground-water 

fluxes computed using solutes and stable isotopes of 

water (δD and δ18O). Ground-water inflow was calculated using equation 5 for the various tracers. The 

inflow estimates were then used to compute groundwater outflow as the residual to the steady-state water 

budget equation (1).

Solute Tracer

Ground-water inflow was computed using several different solutes: chloride, sodium, magnesium, 

calcium, potassium, and bromide (for selected lake 

basins). For a given lake, estimates of ground-water 

inflow typically were considerably different for different tracers, which is not unusual when using multiple 

solute tracers (for example, LaBaugh and others, 1997; 

Katz and others, in press). In some cases, computed 

ground-water inflow or outflow was less than zero, 

which is an invalid numerical solution. About half the 

calculations using magnesium, calcium, and potassium 

gave invalid numerical solutions, indicating that these 

solutes are probably not conservative. Chloride (Cl) 

and sodium (Na) were the most successful tracers, 

based on the more frequent occurrence of valid solutions.

Bromide was sampled at several lakes to investigate its usefulness as a tracer because it is generally 

considered to be conservative. However, concentrations of bromide were too low in the lakes and rainfall 

(typically less than the laboratory detection limit of 

0.001 mg/L) to be used as a tracer for this study. Bromide concentrations in ground water typically were 

above detection limits. However, they were spatially 

variable in a given lake basin, like the major ions. 

Thus, even if analytical detection limits were 

improved, bromide is probably not a better tracer than 

chloride or sodium.

Ground-water inflow results using Cl and Na are 

shown in table 10. The lake with the highest computed 

ground-water inflow was Lake Annie, followed by 

Lake Hollingsworth (about 240 and 110 in/yr, respectively, using the average of Cl and Na tracers). The 

high inflow computed for these lakes is relatively consistent with, but slightly lower than, estimates from 

previous studies, which estimated that ground-water 

inflow was 345 in/yr at Lake Annie (Battoe, 1987) and 

about 240 in/yr at Lake Hollingsworth (Romie, 1994). 

High ground-water inflow at Lake Annie is also consistent with net ground-water flow results (table 4). Lakes 

with the lowest computed ground-water inflow using 

Cl as a tracer were Grassy Lake and Lake Starr, and 

using Na as a tracer were Grassy Lake, Lake Isis, and 

Round Lake (table 10).

 Invalid numerical solutions for Lake George 

and Lake Isis using Cl as a tracer might have been a 

result of not accurately defining the tracer concentration in ground-water inflow. Computed ground-water 

outflow was less than zero for Lake George using both 

Cl and Na, and computed inflow for this lake was 

significantly lower than 1996 net ground-water flow 

(tables 5 and 9). These results indicate that solute 

concentrations in average ground-water inflow are 

probably higher than the median concentrations used in 

these calculations; higher ground-water inflow and 

positive ground-water outflow were computed using 

the higher range of Cl and Na concentrations measured 

in ground-water inflow (table 8). Alternatively, 

surface-water outflow during 1996 might have been 

significantly greater than steady-state conditions (even 

after considering the uncertainty in the 1996 estimate).

For Lake Isis, the negative ground-water inflow 

and outflow computed using Cl as a tracer implies that 

the median Cl concentration from near-lake inflow 

wells was not representative of ground water flowing 

into the lake. In order to arrive at a valid numerical 

solution, the Cl concentration of ground-water inflow 

would need to be higher, but still within the range of 

measured ground-water inflow concentrations. Significant amounts of ground-water inflow might originate 

in upper parts of the Lake Isis basin, where citrus 

agricultural practices can contribute Cl to the ground 

water. This water might then follow deeper flow paths 

that intercept the lake at points deeper than the nearlake wells.
38 Estimating Ground-Water Exchange with Lakes Using Water-Budget and Chemical Mass-Balance Approaches for Ten Lakes 

in Ridge Areas of Polk and Highlands Counties, Florida

 

The biggest uncertainty in applying the chemical 

mass-balance method is in defining the solute concentration of ground water inflow. As discussed earlier, Cl 

and Na concentrations can vary widely in a given lake 

basin. This uncertainty in the inflow concentration is 

even more significant when the solute concentration in 

the lake water is similar to that in the ground water 

because the denominator in equation 5 becomes very 

small (Stauffer, 1985). This occurs when ground-water 

inflow is high, resulting in lake water more similar in 

composition to ground water than to evaporated rainwater. In this situation, small uncertainties in the tracer 

concentration of ground-water inflow can result in 

large changes in the computed ground-water inflow. 

For example, figure 18 shows the relation between 

computed ground-water inflow and the assumed value 

of Cl concentration in ground-water inflow for two 

lakes. The median Cl concentration of ground-water 

inflow is bounded by a 25 percent range of uncertainty. 

For Lake Starr, which has ground-water and lake-water 

Cl concentrations that are distinctly different, a 25 percent increase in the ground-water Cl concentration 

results in a 19 percent increase in the computed inflow. 

In contrast, Swim Lake has a Cl concentration more 

similar to that of ground water, and a 25 percent 

increase in the Cl concentration of ground-water inflow 

causes a 300 percent increase in computed groundwater inflow. The range of ground-water inflow, given 

a 25 percent uncertainty in the median Cl and Na concentrations of ground-water inflow, is shown for all the 

lakes in table 10.

If a transient formulation of the chemical massbalance approach is to be used to estimate groundTable 10. Computed ground-water inflow to the study lakes using various tracers, and sensitivity of inflow calculations 

to concentrations of chloride and sodium in ground-water inflow

[Units in inches per year; gw, ground water; conc., concentration; %, percent; δD, delta deuterium, δ18O,

delta oxygen-18; δa, isotopic composition of atmospheric moisture; n/a, numerically invalid solution; Cl, chloride]

Computed ground-water inflow

Using chloride Using sodium Using δD Using δ18O

Lake

median gw 

inflow 

conc.

minus - plus 

25 % of 

median conc.

median gw 

inflow 

conc.

minus - plus 

25 % of 

median conc.

assuming δa in 

equilibrium 

with rainwater

δa backcalculated from 

Starr Cl 

results

δa backcalculated from Starr 

Cl results

Annie 180 128 - n/a 293 130 - n/a 190 178 1100

George 1

8 14 - 51 119 19 - n/a 54 231 233

Grassy 15 9 - 49 8 7 - 11 30 15 19

Hollingsworth 90 n/a - 45 123 n/a - 52 n/a n/a n/a

Isis n/a n/a - 61 8 2 - n/a 101 90 95

Olivia 57 24 - n/a 33 18 - 166 33 22 37

Round 48 10 - n/a 8 6 - 11 6 n/a n/a

Saddle Blanket 21 14 - 42 42 19 - n/a n/a n/a n/a

Starr 11 9 - 13 10 9 - 11 23 311 3

11

Swim 29 16 - 122 29 17 - 116 183 155 160

1Negative ground-water outflow computed, which cannot be explained by uncertainty in 1996 surface-water ouflow estimate.

2

Negative ground-water outflow computed, but within uncertainty of 1996 surface-water outflow estimate.

3

Not independently computed.

Swim Lake

0

20

40

60

80

100

120

140

160

180

200

4 5 6 7 8 9 10 11 12

uncertainty in computed ground-water inflow,

assuming 25 percent uncertainty 

in chloride concentration of

median minus

25 percent

median chloride

concentration of

ground water

median plus

25 percent

Lake Starr

0

10

20

30

40

50

60

70

80

90

100

2 4 6 8 10 12 14

CHLORIDE CONCENTRATION OF GROUND-WATER 

INFLOW, IN MILLIGRAMS PER LITER

CHLORIDE CONCENTRATION OF GROUND-WATER 

INFLOW, IN MILLIGRAMS PER LITER

COMPUTED GROUND-WATER INFLOW, IN INCHES PER YEAR

uncertainty in computed ground-water inflow,

assuming 25 percent uncertainty in chloride

concentration of ground-water inflow

median minus

 25 percent

median plus

25 percent

median chloride

concentration of

ground water

ground-water inflow

Figure 18. Relation between computed ground-water 

inflow and assumed chloride concentration of groundwater inflow for Lake Starr and Swim Lake.
Chemical Mass-Balance Approach 39

water inflow on a short-term basis (for example, 

monthly or semiannually), more detailed data collection would be needed. The data needed include more 

accurate quantification of the solute concentration of 

ground water seeping into the lake, better analytical 

precision in lake solute concentrations (so that change 

in mass can be computed more accurately), sampling of 

rainfall at each lake, and a better estimate of short-term 

lake evaporation rates. A transient formulation of the 

chemical mass-balance approach is even more sensitive to uncertainties in terms such as the solute concentration of ground-water inflow, than is the steady-state 

approach (Pollman and Lee, 1993).

Isotope Tracer

Ground-water inflow was also computed using 

the stable isotopes of water (δD and δ18O). Some of 

the biggest drawbacks to using a solute tracer are 

avoided when using these isotopes. For example, the 

isotopic composition of ground-water inflow is much 

more uniform than the solute concentrations. In 

addition, there is always a big difference between the 

isotopic composition of ground-water inflow and lake 

water because evaporation enriches the lake water in 

heavier isotopes. Therefore, relatively large values are 

always in the denominator of equation 5, which makes 

the solution much more stable. The biggest difficulty 

in using the isotope method is the need for estimating 

the isotopic composition of evaporating water, which is 

not necessary when using a solute tracer.

For this study, the isotopic composition of atmospheric moisture (δa) was estimated two ways for both 

deuterium and oxygen-18. The first method assumed 

that δa was in isotopic equilibrium with rainwater 

(table 7). The second method back-calculated δa, using 

ground-water inflow estimated for Lake Starr using Cl 

as a tracer; this estimate of ground-water inflow for 

Lake Starr was relatively insensitive to the assumed 

ground-water inflow concentration (fig. 18 and 

table 10).

Ground-water inflow estimated from these two 

methods using δD as a tracer were relatively consistent 

(table 10), although the inflow estimates using δa backcalculated from the Lake Starr water budget were 

slightly lower. In contrast, when using δ18O and 

assuming that δa was in isotopic equilibrium with rainwater, unrealistically high ground-water inflow 

(greater than 100 in/yr) was computed for most of the 

lakes; therefore, results are not presented in table 10. 

These high computed inflow rates probably indicate 

that δ18O content of atmospheric moisture is not in 

isotopic equilibrium with rainwater, and that additional 

kinetic fractionation occurs. A more accurate definition of δa (for example, from direct data collection, 

which was beyond the scope of this study) could 

resolve this discrepancy. Ground-water inflow computed using δ18O as a tracer, with δa back-calculated 

from the Lake Starr results, was very similar to results 

using δD (table 10).

Ground-water inflow computed with isotope 

tracers was highest for Swim Lake and Lake Isis (about 

170 and 95 in/yr, respectively, using the average of the 

isotope tracers in table 10). These high inflow rates are 

supported by net ground-water flow results. Swim 

Lake and Lake Isis both had some months with very 

positive or very negative net ground-water flow, indicating that ground water is a significant part of their 

water budgets (figs. 9 and 10). Further, annual groundwater inflow estimated using the water-budget 

approach was highest for Swim Lake and third highest 

for Lake Isis (table 4). The presence of nitrate in the 

water column of these two lakes (table 9) could also be 

evidence that ground-water inflow is high, resulting in 

very high nitrate loading rates from the ground water. 

The high ground-water inflow rates computed using 

δD and δ18O are not consistent with results using Cl 

and Na as tracers (table 10). This is probably due to 

large uncertainties in the Cl and Na inflow estimates 

because concentrations in lake water and ground water 

were similar (table 10). 

Ground-water inflow calculated at Lake George 

using isotope tracers was consistent with net groundwater flow results (tables 5 and 10), and probably is 

between 30 and 60 in/yr. Although negative groundwater outflow was calculated for the isotope tracers 

when δa was back-calculated from the Lake Starr water 

budget, ground-water outflow was positive when 

surface-water outflow was reduced to within the range 

of uncertainty in this estimate. When Cl or Na tracers 

were used, ground-water inflow was too low, causing 

negative ground-water outflow to be computed. Cl and 

Na concentrations in water from wells at Lake George 

might not be representative of all ground-water inflow 

sources.

 For four of the study lakes (Grassy Lake, Lake 

Olivia, Round Lake, and Lake Starr), ground-water 

inflow computed using isotope tracers was relatively 

consistent with inflow computed using either Cl or Na 

as tracers (table 10). Ground-water inflow at Lake 

Olivia is probably slightly greater (between 20 and 
40 Estimating Ground-Water Exchange with Lakes Using Water-Budget and Chemical Mass-Balance Approaches for Ten Lakes 

in Ridge Areas of Polk and Highlands Counties, Florida

40 in/yr) than at Grassy Lake and Lake Starr (between 

10 and 30 in/yr). Ground-water inflow to Round Lake 

is probably lowest (less than 10 in/yr). For Round 

Lake, results using δD, assuming δa is isotopic equilibrium with rainwater, were consistent with results using 

Na as a tracer (table 10). Ground-water inflow 

computed using Cl as a tracer, however, was much 

higher. Cl was enriched to varying degrees in the 

ground water in this citrus-agricultural basin because 

of fertilizer application. Na was not as enriched and, 

therefore, was probably the better tracer. Results for 

Round Lake, using δa back-calculated from the Lake 

Starr water budget, resulted in negative ground-water 

inflow and outflow. However, slight changes in δa

resulted in positive, but very low, ground-water inflow. 

(For example, a 3 percent decrease in δa will cause 

inflow to increase to 6 in/yr). 

Valid results were not obtained using δD and 

δ18O for three of the study lakes (Lake Annie, Lake 

Hollingsworth, and Saddle Blanket Lake). Results 

using solute tracers indicated that ground-water inflow 

was a very significant part of the water budgets of 

Lakes Annie and Hollingsworth (about 240 and 

110 in/yr at Lake Annie and Lake Hollingsworth, 

respectively, using the average of Cl and Na tracer 

results). For Lake Annie, ground-water inflow calculated from isotope tracers was too low, causing negative ground-water outflow to be computed. For Lake 

Hollingsworth, negative ground-water inflow and outflow were computed. The only term in equations 5 and 

6 that can account for these low computed values is δa, 

which would need to be at least 20 percent greater for 

positive ground-water inflow and outflow to be 

computed. Lakes Annie and Hollingsworth are farther 

away from the other study lakes (fig. 1), and could be 

exposed to atmospheres with different isotopic compositions. Lake Annie is the farthest south of the study 

lakes, and is probably influenced by a more tropical 

south Florida climate. Swart and others (1989) 

determined that δa of deuterium in Florida Bay in south 

Florida was -75 per mil; using this δa value for Lake 

Annie results in a ground-water inflow estimate 

(240 in/yr) that is consistent with the average inflow 

estimate using Cl and Na (table 10). Lake Hollingsworth is farthest west, and closer to the Gulf of Mexico 

than the other lakes. It might be influenced by coastal 

atmospheric moisture. For future studies, atmospheric 

moisture or rainfall should be collected in several locations to assess the geographic variability in δa.

For Saddle Blanket Lake, negative ground-water 

inflow and outflow were computed using the isotope 

tracers (table 10). The period sampled for this small, 

shallow lake might not have been representative of 

steady-state conditions. Between the spring and fall 

1996 samplings, 28 percent of the lake volume was lost 

due to evaporation and ground-water outflow. This did 

not significantly affect the solute concentrations in the 

lake, but δD and δ18O values were significantly 

enriched between the first and second samplings 

because of fractionation due to evaporation. The 

isotopic composition of the lake (δL) is an important 

term in estimating the isotopic composition of evaporating water (δE) in equation 6. If the lake’s isotopic 

composition from only the spring sampling is used to 

estimate δE, computed ground-water inflow is 16 in/yr. 

Thus, the isotopic composition of Saddle Blanket Lake 

in the fall of 1996 might not have reflected steady-state 

conditions, which particularly affects the δE computed 

using equation 6.

Estimating Ground-Water Exchange

Reasonable estimates of ground-water inflow 

were made for seven of the study lakes using deuterium, assuming that δa is in isotopic equilibrium with 

rainwater (table 11). The isotope tracers were more 

robust than the solute tracers because there was always 

a distinct difference between the isotopic composition 

of lake water and ground water, and because the isotopic composition of ground water was more spatially 

uniform than concentrations of solutes in the shallow 

ground water. Thus, results for these seven lakes were 

assumed to be good estimates of annual ground-water 

inflow (table 11). Ground-water outflow computed 

from the inflow estimates are also shown in table 11. 

Uncertainties in these inflow and outflow values were 

computed by estimating an uncertainty associated with 

each term used in equation 5 (for inflow) or equation 1 

(for outflow), following methods described by Lee and 

Swancar (1997). For the three lakes where the isotope 

approach did not work because of difficulties defining 

δE (Lake Annie, Lake Hollingsworth, and Saddle 

Blanket Lake), the best estimate for ground-water 

inflow and outflow were assumed to be the average 

values computed using Cl and Na as tracers (table 11). 
Chemical Mass-Balance Approach 41

Ground-water flow results using the best tracer 

from the chemical mass-balance approach (table 11) 

were typically similar in magnitude to those computed 

using the water-budget approach (table 4). For example, Lake Annie, Lake Hollingsworth, Lake Isis, and 

Swim Lake had the most ground-water inflow using 

either method. Inflow and outflow estimates for these 

lakes, however, were lower for the water-budget 

approach, compared to the chemical mass-balance 

approach. The water-budget approach underestimated 

ground-water inflow and outflow for lakes with large 

amounts of ground-water exchange because it was 

assumed that the month with the most negative net 

ground-water flow had predominantly ground-water 

outflow and had minimal ground-water inflow. For 

lakes with large amounts of ground-water exchange, 

ground-water inflow is probably always significant. 

The water-budget approach, however, might produce a 

good approximation for lakes with lesser amounts of 

ground-water exchange. Ground-water inflow and outflow computed using the water budget approach were 

within the uncertainty bounds of values computed using 

the chemical mass-balance approach for most lakes 

(tables 4 and 11). Other differences in ground-water 

exchange between the two approaches are probably 

related to differences in hydrologic stresses during the 

1996 water-budget period or seasonality of groundwater outflow. For example, lakes with below normal 

rainfall during 1996 could have had lower ground-water 

inflow because of reduced recharge and higher groundwater outflow because of greater ground-water pumpTable 11. Estimate of ground-water inflow and outflow and 

uncertainty using the most successful tracer

[units in inches per year, rounded to two significant figures; ground-water 

inflow estimated using deuterium as tracer and assuming atmospheric moisture in isotopic equilibrium with rainwater unless otherwise noted; Gi

, 

ground-water inflow; eGi, uncertainty or error in ground-water inflow estimate; Go, ground-water outflow; eGo, uncertainty in ground-water outflow 

estimate] 

 Lake Gi eGi Go eGo

Annie 1240 120 66 120

George 54 33 17 36

Grassy 30 16 28 17

Hollingsworth 1110

1Average of Cl and Na tracers, error assumed to be 50 percent.

55 92 60

Isis 100 37 100 38

Olivia 33 18 29 19

Round 6 10 2 12

Saddle Blanket 130 15 26 16

Starr 23 12 19 13

Swim 180 61 180 61

ing for irrigation, compared to the 10-year average conditions used in the chemical mass-balance approach.

Differences in ground-water exchange between 

the 10 study lakes illustrate the wide spectrum of 

ground-water/lake interactions for lakes in ridge settings of Florida. Ground-water inflow and outflow 

ranged over an order of magnitude at the study lakes, 

from less than 10 to greater than 150 in/yr. For seepage 

lakes, ground-water outflow is very similar to groundwater inflow on a long-term basis because long-term 

average precipitation and evaporation rates are very 

similar in the study area. Thus, a seepage lake with a 

large ground-water inflow component also has a correspondingly large ground-water outflow component. 

Extended periods of below normal rainfall can reduce 

recharge and ground-water inflow, resulting in large 

stage reductions because net precipitation (precipitation 

minus evaporation) and ground-water inflow decline 

but outflow does not. Outflow can actually increase 

during these periods because increased ground-water 

pumping can increase vertical ground-water outflow, 

and possibly because a greater part of the lake perimeter 

experiences lateral ground-water outflow. The opposite is true for wet periods, and the stage of lakes with 

large ground-water exchange can oscillate more widely 

between wet and dry years than the stage of lakes with 

lesser amounts of ground-water exchange.

Differences in stage fluctuations are inherently 

related to the ground-water exchange characteristics at 

individual lakes, which, in turn, are related to the lake’s 

physical and hydrogeologic setting. Physical characteristics of the lake are important in controlling how much 

interaction the lake has with the ground-water system. 

For example, the deepest study lakes (Lakes Annie and 

Isis) had high rates of ground-water inflow. Results 

from this study indicate that lakes can be classified 

according to their amount of ground-water exchange; 

however, the number of lakes studied was insufficient 

to establish that physical characteristics of the lakes can 

be used to predict how much a lake’s stage would 

respond to climatic extremes or increased ground-water 

pumping. 

The isotopic composition of lakes in a similar 

geographic area can provide valuable information 

about the relative amount of ground-water inflow that a 

lake receives (Krabbenhoft and others, 1994). Equation 

5 was rearranged to calculate CL (the isotopic composition of the lake), assuming different amounts of groundwater inflow. This was done using parameters from one 

of the study lakes (Lake Starr) as an example, in a manner referred to as the "index lake method" (Krabbenhoft 
42 Estimating Ground-Water Exchange with Lakes Using Water-Budget and Chemical Mass-Balance Approaches for Ten Lakes 

in Ridge Areas of Polk and Highlands Counties, Florida

and others, 1994); results were plotted along with the 

mean isotopic composition of the other study lakes 

(fig. 19). Lakes become isotopically enriched (more 

positive) through evaporation; thus, a more positive 

isotopic composition indicates that evaporation is a 

greater part of the water budget of the lake. The 

isotopic composition of lakes plot within different 

ranges of calculated ground-water inflow. For example, the composition of Lake Isis and Swim Lake are 

isotopically depleted and plot in the high range of 

ground-water inflow (greater than 100 in/yr). These 

lakes have a greater amount of ground-water exchange 

compared to evaporation. In contrast, the composition 

of Round Lake is isotopically enriched and plots at the 

low range of computed ground-water inflow (fig. 19), 

indicating that evaporation is a greater part of its water 

budget and ground-water exchange is less important. 

For Lake Annie, Lake Hollingsworth, and Saddle Blanket Lake, the isotope approach did not work because of 

difficulties defining δE. The isotopic composition of 

these three lakes plot in ranges of less ground-water 

inflow (or off the scale, in the case of Saddle Blanket 

Lake) than would be expected based on results in 

table 11 (fig. 19). For Lake Hollingsworth, the isotopic 

composition of surface-water inflows are also not 

accounted for using this approach. Interestingly, the 

composition of Lake Annie is isotopically depleted 

enough to still plot in the high range of ground-water 

inflow (greater than 100 in/yr), even though actual 

ground-water inflow (table 11) is even greater than 

where its isotopic composition plots on figure 19. 

These results reveal that the isotope approach is 

a promising method for evaluating the ground-water 

flow regime for numerous lakes in geographic proximity in Florida. The isotopes δD and δ18O were used as 

tracers for this study in a reconnaissance manner. This 

is the first study to use this approach to quantify 

ground-water inflow to Florida lakes. Limitations exist 

in applying this approach because the lack of long-term 

data on the isotopic composition of lake water, atmospheric moisture, and rainwater. More information on 

spatial variability of the isotopic composition of rainwater and atmospheric moisture, and of climatic variables would also help define geographic variability and 

appropriate geographic range when applying this 

method in Florida. 

SUMMARY AND CONCLUSIONS

Lakes can vary in their stage response to climatic 

conditions because of differences in the amount of 

ground water in their water budgets. Quantifying 

ground-water fluxes independently at an individual 

lake is time-consuming and costly, and results often 

cannot be extrapolated to a larger population of lakes in 

the region because of differences in hydrogeologic settings. Simpler techniques were used to understand 

ground-water exchange with Florida lakes on a more 

regional scale. Water budget and chemical massbalance approaches were used to estimate groundwater exchange with 10 lakes in ridge areas of Polk and 

Highlands Counties. At each of the lakes, heads were 

monitored in the surficial aquifer system and deeper 

Upper Floridan aquifer, lake stage and rainfall were 

measured continuously, and lakes and wells were 

sampled three times between October 1995 and 

December 1996. 

The study lakes were all in flow-through settings 

with respect to the surficial aquifer system during some 

of the study period. Ground water enters a lake along 

part of its perimeter, and the lake loses water to the 

aquifer along another part of its perimeter. Groundwater flow patterns do not necessarily follow topography. Several lakes exhibited ground-water flow reversals due to transient water-table mounding following 

periods of high recharge. Heads in the surficial aquifer 

system and lake stages were consistently higher than 

those in the underlying Upper Floridan aquifer, indicating a consistent potential for downward flow from the 

shallow system to the Upper Floridan aquifer.

The water-budget approach computes net 

ground-water flow (ground-water inflow minus outflow) as the residual of the monthly water-budget equation. Net ground-water flow varied seasonally at each 

-12

-8

-4

0

4

8

12

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0

DELTA OXYGEN-18, IN PER MIL

DELTA DEUTERIUM, IN PER MIL

Mean isotopic composition of 

 lake water

Swim

Annie

Isis

Olivia

George

Saddle Blanket

Hollingsworth Round

Starr Grassy

200

150

100

50

0 inches

 per year

9080

7060

40

30

20

10

250

Computed isotopic composition 

 of lake water, using equation 5 

 and data from Lake Starr,

 assuming 0 to 250 inches per 

 year of ground-water inflow

Figure 19. Relation between delta deuterium and delta 

oxygen-18 for mean lake water and lake water computed 

theoretically using equation 5.
Summary and Conclusions 43

of the 10 lakes and was notably different between 

lakes, illustrating short-term differences in groundwater fluxes. Net ground-water inflow typically 

occurred during the wet season, and net ground-water 

outflow typically occurred during the dry season. On 

an annual basis, seepage lakes experiencing below 

average rainfall had net ground-water outflow, whereas 

those with above average rainfall had net ground-water 

inflow. Estimates of ground-water inflow and outflow 

were made by assuming that the month with the most 

negative net ground-water flow had predominantly 

ground-water outflow and that this outflow was relatively constant throughout the year. Using this 

approach, the study lakes exhibited large differences in 

the annual estimate of ground-water inflow and outflow, which varied by more than a factor of three 

between lakes.

Monthly patterns in net ground-water flow were 

related to monthly patterns of other hydrologic variables, such as rainfall, ground-water flow patterns, and 

head differences between the lake and the Upper 

Floridan aquifer. Important factors influencing 

ground-water flow are lateral head gradients between 

the lake and surficial aquifer system and vertical head 

gradients between the lake and the Upper Floridan 

aquifer. Lakes in shallow topographic basins are more 

susceptible to transient water-table mounding following rainy periods, which increases net ground-water 

flow. Lakes in steeper topographic basins have more 

delayed responses to recharge, and net ground-water 

flow responds more seasonally. Artificial lowering of 

a lake’s stage can induce additional ground-water 

inflow and affect ground-water flow patterns around 

the lake. Vertical ground-water outflow is influenced 

by the vertical head difference between the lake and the 

Upper Floridan aquifer. Much stronger relations were 

found between net ground-water flow and this head 

difference when computed using an Upper Floridan 

well with continuously recorded head data, compared 

to a well with single monthly head measurements. 

More direct evidence of the effects of pumping from 

the Upper Floridan aquifer on ground-water outflow 

was illustrated by large daily declines in lake stage that 

occurred when agricultural pumping was at a maximum to protect crops during potential freezes.

The chemical mass-balance approach combines 

the water-budget and solute or isotope mass-balance 

equations, and assumes steady-state conditions. The 

tracers were naturally occurring solutes and the stable 

isotopes of water (deuterium and oxygen-18). In order 

to use this technique, the chemical composition of the 

ground water and lake water were defined. Groundwater and lake-water quality were influenced by land 

use. Ground water and lake water in undeveloped areas 

were dilute and dominated by sodium and chloride 

ions. Ground water beneath citrus groves was enriched 

in most major ions and nitrate because of fertilizer use; 

this ground water subsequently enriches major-ion 

concentrations in lakes dominated by citrus agriculture. 

Ground-water composition in residential areas or areas 

of mixed land use was highly variable and probably 

depends on local fertilizer use, the presence of septic 

tanks, and upgradient land use; lakes in residential or 

mixed land use areas were less enriched in major ions 

than basins dominated by citrus agriculture.

Estimates of ground-water inflow computed 

using the chemical mass-balance approach varied 

depending on the tracer used. Chloride and sodium 

were the most successful solute tracers. However, 

chloride and sodium concentrations in ground water 

typically varied spatially, and in places were similar to 

that in lake water, limiting their sensitivity as tracers. 

In contrast, the isotopes were more robust tracers 

because the isotopic composition of ground water was 

relatively uniform and was distinctly different from the 

composition of lake water. Ground-water inflow computed using isotope tracers is sensitive to the isotopic 

composition of atmospheric moisture, and isotope tracers could be used more accurately if this term were 

measured.

Ground-water inflow varied significantly 

between lakes. It ranged from less than 10 to more than 

150 in/yr at the 10 lakes. These results underscore how 

variable ground water can be in the water budget of 

Florida lakes and the unique nature of ground-water 

exchange at individual lakes. Both net ground-water 

flow and chemical mass-balance approaches had limitations, but the multiple lines of evidence helped better 

explain ground water’s role in lake-water budgets. 

Although the lakes encompass a range of sizes and 

shapes that are representative of the Central Lakes 

District, 10 lakes were an insufficient number to establish regional classifications of lakes based on readily 

identifiable features. The isotope mass-balance 

approach is a promising method that could be applied 

to a larger number of lakes in the region to classify 

ground-water exchange for a larger population of lakes. 

This could help water managers and residents understand the impacts of climatic variability and aquifer 

pumping on lake water resources. 
44 Estimating Ground-Water Exchange with Lakes Using Water-Budget and Chemical Mass-Balance Approaches for Ten Lakes 

in Ridge Areas of Polk and Highlands Counties, Florida

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Appendix
48 Estimating Ground-Water Exchange with Lakes Using Water-Budget and Chemical Mass-Balance Approaches for Ten Lakes 

in Ridge Areas of Polk and Highlands Counties, Florida

Appendix. Monthly water-budget terms, computed net ground-water flow, and estimated 

uncertainty in net ground-water flow for the study lakes between October 1995 and December 

1996

[Units in inches per month; P, precipitation; E, evaporation; Si, surface-water inflow; So, surface-water outflow; ∆V, change in 

lake volume; Gnet, net ground-water flow (ground-water inflow minus outflow); eGnet, uncertainty or error in net ground-water 

flow; --, not applicable; <, less than]

Month Year P E Si So ∆V Gnet eGnet

Lake Annie:

October 1995 7.7 6.7 8.0 37.7 0.5 29.2 5.8

November 1995 0.3 4.8 1.0 21.8 -3.9 21.4 3.0

December 1995 0.6 3.8 -- 15.3 -0.6 17.9 2.6

January 1996 1.9 1.5 -- 17.4 0.0 17.0 2.6

February 1996 1.5 1.5 -- 12.8 -0.9 11.9 2.3

March 1996 5.3 2.8 -- 12.7 2.7 12.9 2.4

April 1996 1.2 3.9 -- 14.3 -2.7 14.3 2.5

May 1996 8.2 4.6 -- 12.4 3.8 12.6 2.5

June 1996 7.2 4.6 -- 23.5 -1.1 19.8 3.1

July 1996 6.2 5.4 0.5 22.8 -1.8 19.7 3.1

August 1996 4.4 6.3 < 0.1 19.0 0.0 20.9 3.0

September 1996 1.7 7.0 -- 12.1 -1.4 16.0 2.7

October 1996 3.6 6.3 -- 13.2 0.7 16.6 2.7

November 1996 0.3 5.3 -- 6.3 -1.7 9.6 2.3

December 1996 1.6 4.1 -- 6.1 0.2 8.8 2.3

Lake George:

October 1995 5.9 4.3 -- 5.4 -0.5 3.3 1.4

November 1995 1.2 3.1 -- 3.6 -2.1 3.4 1.2

December 1995 0.5 2.1 -- 2.1 -1.4 2.3 1.1

January 1996 5.8 2.3 -- 4.4 1.8 2.7 1.2

February 1996 3.3 2.4 -- 2.9 -0.2 1.8 1.1

March 1996 6.5 4.0 -- 2.9 2.1 2.5 1.3

April 1996 1.3 5.3 -- 2.7 -3.5 3.2 1.4

May 1996 2.3 5.4 -- 0.5 -2.4 1.2 1.5

June 1996 7.1 5.4 -- 0.9 3.6 2.8 1.5

July 1996 3.9 6.2 -- 3.0 -2.1 3.2 1.6

August 1996 7.7 5.3 -- 2.9 3.2 3.7 1.5

September 1996 5.3 6.2 -- 3.9 0.5 5.3 1.6

October 1996 4.5 4.1 -- 5.2 0.8 5.6 1.3

November 1996 0.6 3.5 -- 3.5 -2.6 3.8 1.2

December 1996 4.1 2.3 -- 4.1 1.0 3.3 1.1

Grassy Lake1:

October 1995 7.2 4.3 -- -- 4.4 1.5 1.0

November 1995 3.7 3.1 -- 19.6 -12.9 6.1 1.3

December 1995 0.6 2.1 -- 20.9 -19.0 3.4 1.5

January 1996 6.0 2.3 -- -- 9.3 5.6 0.7

February 1996 2.7 2.4 -- -- 2.5 2.2 0.5

March 1996 7.7 4.0 -- -- 6.3 2.6 0.9

April 1996 1.8 5.3 -- -- -1.0 2.5 1.1

May 1996 2.8 5.4 -- -- -2.9 -0.3 1.1

June 1996 6.3 5.4 -- -- 1.3 0.4 1.1

July 1996 5.2 6.2 -- -- -0.2 0.8 1.3

August 1996 6.4 5.3 -- -- 1.8 0.7 1.1

September 1996 9.9 6.2 -- -- 6.4 2.7 1.4

October 1996 3.2 4.1 -- -- 0.4 1.3 0.8

November 1996 0.7 3.5 -- -- -4.2 -1.4 0.7

December 1996 2.2 2.3 -- -- -1.0 -0.9 0.5
Appendix 49

Lake Hollingsworth2:

January 1996 5.4 2.3 6.7 8.0 0.6 -1.2 3.2

February 1996 2.8 2.4 2.4 4.1 -1.8 -0.5 1.4

March 1996 7.4 4.0 6.3 4.6 1.6 -3.5 2.2

April 1996 2.2 5.3 1.9 4.3 -4.7 0.8 1.7

May 1996 4.1 5.4 3.6 4.0 -1.8 -0.1 1.8

June 1996 7.5 5.4 6.3 4.0 2.6 -1.8 2.3

July 1996 5.5 6.2 5.1 4.5 0.8 0.9 2.2

August 1996 7.3 5.3 8.7 18.1 -2.1 5.3 3.5

September 1996 7.5 6.2 6.6 6.5 6.0 4.6 2.3

October 1996 4.0 4.1 4.3 6.1 -0.2 1.7 1.5

November 1996 1.1 3.5 0.8 5.4 -3.9 3.1 0.9

December 1996 2.5 2.3 2.1 2.6 1.1 1.4 0.8

Lake Isis3:

October 1995 7.8 6.7 1.6 0.1 8.4 5.8 1.5

November 1995 2.4 4.8 0.5 0.2 0.2 2.3 1.0

December 1995 0.6 3.8 0.1 0.2 -6.8 -3.5 0.9

January 1996 3.6 1.5 0.7 0.2 -1.9 -4.5 0.5

February 1996 1.2 1.5 0.2 0.2 -7.3 -7.0 0.5

March 1996 4.8 2.8 1.0 0.2 -2.7 -5.5 0.7

April 1996 1.7 3.9 0.3 0.2 -6.9 -4.8 0.9

May 1996 5.4 4.6 1.1 0.2 -5.1 -6.8 1.1

June 1996 5.8 5.4 1.2 0.2 -1.2 -2.6 1.2

July 1996 1.9 6.0 0.4 0.3 -6.7 -2.7 1.3

August 1996 4.6 6.2 0.9 0.2 -3.2 -2.3 1.5

September 1996 5.7 7.0 1.1 0.2 -1.0 -0.6 1.5

October 1996 4.4 6.3 0.9 0.2 0.2 1.4 1.3

November 1996 0.3 5.3 0.1 0.3 -11.5 -6.3 1.2

December 1996 1.9 4.1 0.4 0.2 -4.4 -2.4 0.9

Lake Olivia:

October 1995 6.8 4.3 -- -- 4.7 2.2 1.0

November 1995 2.0 3.1 -- -- -1.4 -0.3 0.6

December 1995 1.2 2.1 -- -- -4.5 -3.6 0.5

January 1996 3.7 2.3 -- -- 0.2 -1.2 0.5

February 1996 1.1 2.4 -- -- -4.8 -3.5 0.5

March 1996 3.8 4.0 -- -- -2.6 -2.4 0.8

April 1996 1.4 5.3 -- -- -6.2 -2.3 1.1

May 1996 5.1 5.4 -- -- -2.4 -2.1 1.1

June 1996 6.1 6.2 -- -- 1.8 1.9 1.3

July 1996 2.9 6.8 -- -- -2.8 1.1 1.4

August 1996 3.6 6.3 -- -- -2.9 -0.2 1.3

September 1996 4.3 6.2 -- -- -1.6 0.3 1.3

October 1996 5.2 4.1 -- -- 1.8 0.7 0.9

November 1996 0.5 3.5 -- -- -6.4 -3.4 0.8

December 1996 1.6 2.3 -- -- -3.4 -2.7 0.5

Appendix. Monthly water-budget terms, computed net ground-water flow, and estimated 

uncertainty in net ground-water flow for the study lakes between October 1995 and December 

1996 (Continued)

[Units in inches per month; P, precipitation; E, evaporation; Si, surface-water inflow; So, surface-water outflow; ∆V, change in 

lake volume; Gnet, net ground-water flow (ground-water inflow minus outflow); eGnet, uncertainty or error in net ground-water 

flow; --, not applicable; <, less than]

Month Year P E Si So ∆V Gnet eGnet
50 Estimating Ground-Water Exchange with Lakes Using Water-Budget and Chemical Mass-Balance Approaches for Ten Lakes 

in Ridge Areas of Polk and Highlands Counties, Florida

Round Lake1:

October 1995 5.1 4.3 -- -- 2.4 1.6 0.9

November 1995 3.3 3.1 -- -- 0.9 0.7 0.6

December 1995 0.8 2.1 -- -- -3.7 -2.4 0.5

January 1996 5.0 2.3 -- -- 3.3 0.6 0.5

February 1996 1.2 2.4 -- -- -3.5 -2.3 0.6

March 1996 6.0 4.0 -- -- 1.1 -0.9 1.0

April 1996 0.8 5.3 -- 0.8 -5.9 -0.6 1.1

May 1996 3.4 5.4 -- 0.8 -5.2 -2.4 1.1

June 1996 9.4 5.4 -- -- 4.9 0.9 1.2

July 1996 4.2 6.2 -- 0.3 -0.2 2.1 1.3

August 1996 4.2 6.3 -- -- -1.0 1.1 1.3

September 1996 6.2 6.2 -- -- 1.2 1.2 1.3

October 1996 1.9 4.1 -- -- -3.9 -1.7 0.8

November 1996 0.8 3.5 -- 0.5 -6.4 -3.2 0.8

December 1996 2.8 2.3 -- -- -1.5 -2.0 0.5

Saddle Blanket Lake:

October 1995 6.0 4.3 -- -- 6.6 4.9 1.0

November 1995 1.2 3.1 -- -- -2.7 -0.8 0.6

December 1995 0.9 2.1 -- -- -3.8 -2.6 0.5

January 1996 3.7 2.3 -- -- 0.9 -0.5 0.5

February 1996 1.3 2.4 -- -- -4.1 -3.0 0.5

March 1996 5.0 4.0 -- -- -1.5 -2.5 0.9

April 1996 1.7 5.3 -- -- -5.9 -2.3 1.1

May 1996 5.1 5.4 -- -- -3.3 -3.0 1.1

June 1996 6.0 6.2 -- -- 0.2 0.4 1.3

July 1996 2.8 6.8 -- -- -4.6 -0.6 1.4

August 1996 2.0 6.5 -- -- -6.1 -1.6 1.3

September 1996 3.7 6.2 -- -- -4.4 -1.9 1.3

October 1996 4.3 4.1 -- -- -0.3 -0.5 0.8

November 1996 0.7 3.5 -- -- -6.5 -3.7 0.8

December 1996 1.7 2.3 -- -- -4.7 -4.1 0.5

Lake Starr:

October 1995 3.5 4.3 -- -- 1.7 2.5 0.9

November 1995 2.8 3.1 -- -- 2.2 2.5 0.7

December 1995 0.5 2.1 -- -- -1.6 0.0 0.4

January 1996 5.0 2.3 -- -- 3.6 0.9 0.5

February 1996 0.6 2.4 -- -- -3.5 -1.7 0.5

March 1996 7.2 4.0 -- -- 2.9 -0.3 0.9

April 1996 1.1 5.3 -- -- -4.8 -0.6 1.1

May 1996 1.9 5.4 -- -- -5.6 -2.1 1.1

June 1996 11.7 5.4 -- -- 8.1 1.8 1.3

July 1996 5.5 6.2 -- -- 0.6 1.3 1.3

August 1996 9.2 5.3 -- -- 6.8 2.9 1.2

September 1996 4.8 6.2 -- -- 1.9 3.3 1.3

October 1996 3.5 4.1 -- -- 0.8 1.4 0.8

November 1996 1.3 3.5 -- -- -4.1 -1.9 0.7

December 1996 3.0 2.3 -- -- 0.8 0.1 0.5

Appendix. Monthly water-budget terms, computed net ground-water flow, and estimated 

uncertainty in net ground-water flow for the study lakes between October 1995 and December 

1996 (Continued)

[Units in inches per month; P, precipitation; E, evaporation; Si, surface-water inflow; So, surface-water outflow; ∆V, change in 

lake volume; Gnet, net ground-water flow (ground-water inflow minus outflow); eGnet, uncertainty or error in net ground-water 

flow; --, not applicable; <, less than]

Month Year P E Si So ∆V Gnet eGnet
Appendix 51

Swim Lake:

October 1995 4.1 4.3 -- -- 1.2 1.4 0.9

November 1995 2.9 3.1 -- -- -2.5 -2.3 0.7

December 1995 0.6 2.1 -- -- -8.6 -7.1 0.6

January 1996 5.0 2.3 -- -- 5.6 2.9 0.6

February 1996 0.6 2.4 -- -- -6.6 -4.8 0.6

March 1996 6.8 4.0 -- -- 0.7 -2.1 0.9

April 1996 3.8 5.3 -- -- 2.1 3.6 1.1

May 1996 3.9 5.4 -- -- -0.1 1.4 1.1

June 1996 11.7 5.4 -- -- 15.7 9.4 1.5

July 1996 1.7 6.2 -- -- -2.6 1.9 1.3

August 1996 8.0 5.3 -- -- 6.2 3.5 1.2

September 1996 4.2 6.2 -- -- -2.0 0.0 1.3

October 1996 2.2 4.1 -- -- -7.0 -5.1 0.9

November 1996 0.7 3.5 -- -- -5.0 -2.2 0.7

December 1996 3.0 2.3 -- -- -1.1 -1.8 0.5

1For Grassy Lake and Round Lake, So is direct pumping from lake. 2For Lake Hollingsworth, Si

 is stormwater inflow plus surface-water inflow. 3

For Lake Isis, Si is stormwater inflow and So is direct pumping from lake.

Appendix. Monthly water-budget terms, computed net ground-water flow, and estimated 

uncertainty in net ground-water flow for the study lakes between October 1995 and December 

1996 (Continued)

[Units in inches per month; P, precipitation; E, evaporation; Si, surface-water inflow; So, surface-water outflow; ∆V, change in 

lake volume; Gnet, net ground-water flow (ground-water inflow minus outflow); eGnet, uncertainty or error in net ground-water 

flow; --, not applicable; <, less than]

Month Year P E Si So ∆V Gnet eGnet
52 Estimating Ground-Water Exchange with Lakes Using Water-Budget and Chemical Mass-Balance Approaches for Ten Lakes 

in Ridge Areas of Polk and Highlands Counties, Florida