Simulated Effects of Ground-Water 

Augmentation on the Hydrology of Round 

and Halfmoon Lakes in Northwest 

Hillsborough County, Florida

By Richard M. Yager and P.A. Metz

U.S. Department of the Interior

U.S. Geological Survey

Prepared in cooperation with the

SOUTHWEST FLORIDA WATER MANAGEMENT DISTRICT

Water-Resources Investigations Report 03-4322
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

U.S. Geological Survey, Reston, Virginia: 2004

For sale by U.S. Geological Survey, Information Services

Box 25286, Denver Federal Center

Denver, CO 80225

For more information about the USGS and its products:

Telephone: 1-888-ASK-USGS

World Wide Web: http://www.usgs.gov/

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to reproduce any copyrighted materials contained within this report.
 iii

Contents

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

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

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

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

Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Background. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Physical Setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

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

Surficial Aquifer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Intermediate Confining Unit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Upper Floridan Aquifer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Ground Water and Lake Interaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Lake-Water Budgets. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Ground-Water Recharge Simulation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Unsaturated Flow Model. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Simulation Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Round Lake Basin Ground-Water Flow Simulation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Model Design. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Calibration Procedure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Simulation Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Optimum Parameter Values. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Water Levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Water Budget . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Halfmoon Lake Basin Ground-Water Flow Simulation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Model Design. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Calibration Procedure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Simulation Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Optimum Parameter Values. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Water Levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Water Budget . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Simulated Effects of Lake Augmentation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

Round Lake . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

Distribution of Lake Leakage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Potential Augmentation Schedules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

Halfmoon Lake. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

Distribution of Lake Leakage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

Potential Augmentation Schedule . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

Comparison of Lake Leakage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
iv 

Figures

1-3. Maps showing—

1. Location of study area showing Halfmoon Lake and Round Lake, Hillsborough

 County, Florida . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3

2. Potentiometric surface of the Upper Floridan aquifer and boundaries of groundwater-flow models, Hillsborough County, Florida. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6

3. Average water-table elevation, August 1996 to July 1997; and generalized geologic 

section A-A ́, showing distribution of hydraulic head and ground-water flowpaths 

near Round Lake basin during dry conditions, April 1997. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7

4. Hydrographs showing stage in Round Lake and water levels in wells RL2 and RL8, 

northeast and southwest of lake, respectively, July 1996 to February 1999. . . . . . . . . . . . . . . . . . . . . . . . . . .8

5. Map showing average water-table elevation, August 1996 to July 1997; and generalized 

geologic section B-B ́, showing distribution of hydraulic head and ground-water flowpaths 

near Halfmoon Lake basin during dry conditions, April 1997 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9

6. Hydrograph showing stage in Halfmoon Lake and water levels in wells HML2 and HML12, 

northeast and southwest of lake, respectively, July 1996 to February 1999. . . . . . . . . . . . . . . . . . . . . . . . . .10

7. Graph showing water-retention curve, relating matric potential to water content for coarsetextured and fine-textured soils specified in cases A, B, and C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

8. Schematic diagram showing soil profiles specified for shallow-rooted vegetation (cases A 

and C) and deep-rooted vegetation (case B) used in unsaturated flow model LEACHM. . . . . . . . . . . . 13

9. Bar chart showing monthly evapotranspiration and recharge, computed with unsaturated 

flow model LEACHM, for coarse-textured soil and shallow-rooted vegetation (case A) at 

Round Lake, April 1996 to March 1999. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

10. Map showing boundary conditions and hydrologic features of the ground-water-flow model 

of the Round Lake basin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

11-13. Schematic diagrams showing—

11. Model layers used to represent surficial and Upper Floridan aquifers in Round 

Lake basin. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

12A. Water table in June 1997 during average conditions, computed by transient-state 

simulation with model RL-A (high recharge), of the surficial aquifer near Round Lake, 

showing plan view. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

12B. Water table in June 1997 during average conditions, computed by transient-state 

simulation with model RL-A (high recharge), of the surficial aquifer near Round Lake, 

showing section view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

13A. Water table in March 1998 during El Niño conditions, computed by transient-state 

simulation with model RL-A (high recharge), of the surficial aquifer near Round Lake, 

showing plan view. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

13B. Water table in March 1998 during El Niño conditions, computed by transient-state 

simulation with model RL-A (high recharge), of the surficial aquifer near Round Lake, 

showing section view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

14. Hydrographs showing water levels measured in the surficial aquifer near Round Lake and 

heads, computed by transient-state simulation with model RL-A (high recharge), July 1996 

to February 1999. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23, 24

15. Graphs showing measured heads, simulated heads, and residuals, computed from 

transient-state simulation with model RL-A (high recharge), of the surficial aquifer near 

Round Lake, July 1996 to February 1999 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

16. Bar chart showing simulated water budget for the surficial aquifer in the Round Lake model area, 

July 1996 to February 1999 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .26
 v

17. Bar chart showing simulated net leakage rates from Round Lake, computed from transientstate simulation with model RL-A (high recharge), and estimated rates from monthly water 

budgets, July 1996 to February 1999. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

18. Graph showing simulated lake leakage rates and residuals, computed from transient-state 

simulation with model RL-A (high recharge), and estimated rates from monthly water 

budgets, July 1996 to February 1999. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

19. Map showing boundary conditions and hydrologic features of the ground-water-flow model 

of the Halfmoon Lake basin. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

20. Schematic diagram showing model layers used to represent surficial and Upper Floridan 

aquifers in the Halfmoon Lake basin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

21A. Schematic diagram showing water table during average conditions (July 1996), computed 

by steady-state simulation with model HML-S, of the surficial aquifer near Halfmoon Lake, 

plan view with residual error. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

21B. Schematic diagram showing water table during average conditions (July 1996), computed 

by steady-state simulation with model HML-S, of the surficial aquifer near Halfmoon Lake, 

section view. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

22. Graphs showing measured heads, simulated heads, and residuals, computed by steady-state 

simulation with model HML-S, of the surficial aquifer near Halfmoon Lake, July 1996. . . . . . . . . . . . . . 33

23.. Hydrographs showing water levels measured in the surficial aquifer near Halfmoon Lake and 

heads, computed by transient simulation with model HML-T, July 1996 to April 1998. . . . . . . . . . . . 34, 35

24. Graphs showing measured heads, simulated heads, and residuals, computed from 

transient-state simulation with model HML-T, of the surficial aquifer near Halfmoon Lake, 

July 1996 to April 1998 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

25. Hydrographs showing water levels measured in the surficial aquifer near Halfmoon Lake 

and heads, computed by transient-state simulation with model HML-T, July 1996 to 

February 1999 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

26. Bar chart showing simulated water budget for the Halfmoon Lake model area, July 1996 to 

April 1998. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

27. Bar chart showing simulated net leakage rates to and from Halfmoon Lake, computed from 

transient-state simulation with model HML-T, and estimated rates from monthly water 

budgets, July 1996 to April 1998 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

28. Graphs showing simulated leakage rates and residuals, computed from transient-state 

simulation with model HML-T, and estimated rates from monthly water budgets, 

July 1996 to April 1998 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

29A. Diagram showing extent of lake leakage from Round Lake after 150 days (perspective view), 

as computed by steady-state simulation with solute-transport model with no dispersion . . . . . . . . . . 40

29B. Diagram showing extent of lake leakage from Round Lake after 150 days (plan view), as 

computed by steady-state simulation with solute-transport model with no dispersion . . . . . . . . . . . . . 41

30. Graph showing relation between stable isotopes of oxygen-18 and deuterium in Round 

Lake and ground water sampled from wells screened in the surficial aquifer and the Upper 

Floridan aquifer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

31. Graph showing relations between Round Lake stage, heads in the Upper Floridan aquifer, 

and augmentation rate, as computed by steady-state simulation with model RL-A 

(high recharge). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .42

32. Hydrographs showing water levels measured in Round Lake, July 1996 to February 1999, 

and water levels computed by transient-state simulation by model RL-A (high recharge) 

with prescribed augmentation schedule derived for recharge under average and 

El Niño conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

33A. Graph showing extent of lake leakage after 400 days from Halfmoon Lake (perspective view), 

as computed by steady-state simulation with solute-transport model with no dispersion . . . . . . . . . . 44
vi 

33B. Graph showing extent of lake leakage after 400 days from Halfmoon Lake (plan view), as 

computed by steady-state simulation with solute-transport model with no dispersion . . . . . . . . . . . . . 45

34. Graph showing relations between Halfmoon Lake stage, heads in the Upper Floridan aquifer, 

and augmentation rate, as computed by steady-state and transient-state simulation with 

models HML-S and HML-T, respectively. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

35 Hydrographs showing water levels measured in Halfmoon Lake, July 1996 to February 1999, 

and water levels computed by transient-state simulations using augmentation rates computed 

with aquifer properties estimated by models HML-S and HML-T, with prescribed 

augmentation schedule derived for recharge under average conditions). . . . . . . . . . . . . . . . . . . . . . . . . . 47

Tables

1. Soil properties specified in unsaturated flow model LEACHM. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2. Evapotranspiration and recharge rates for northwest Hillsborough County, computed 

with unsaturated flow model LEACHM, for different types of vegetation and soils, and 

expressed on an annual basis for average conditions, El Niño, and La Niña . . . . . . . . . . . . . . . . . . . . . . . 14

3. Optimum values of aquifer properties estimated for the Round Lake model area through 

nonlinear regression in transient-state simulations, and error in predicted heads and flows . . . . . . . 17

4. Simulated water budget for the Round Lake model area during average conditions and 

El Niño conditions, modelRL-A. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

5. Optimum values of aquifer properties estimated for the Halfmoon Lake model area through 

nonlinear regression in steady-state and transient-state simulations, and error in predicted 

heads and flows. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

6. Simulated water budget for the Halfmoon Lake model area under average conditions, 

July 1996, model HML-S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
 vii

Conversion Factors, Acronyms, Abbreviations 

and Datum

Multiply By To obtain

inch (in.) 25.4 millimeter (mm)

inch per month (in/mo) 25.4 millimeter per month (mm/mo)

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

foot (ft) 0.3048 meter (m)

cubic foot (ft3) 0.02832 cubic meter (m3)

cubic foot per day (ft3/d) 0.02832 cubic meter per day (m3/d)

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

mile (mi) 1.609 kilometer (km)

square mile (mi2) 2.590 square kilometer (km2)

acre 0.4047 hectare (ha)

gallon per minute (gal/min) 0.06309 liter per second (L/s)

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

AET Actual evapotranspiration

kaPa kilopascal

MWL meteoric waterline

NADP National Atmospheric Deposition Program

R2 Coefficient of determination

SWFWMD Southwest Florida Water Management District

USGS U.S. Geological Survey

g/cm3 gram per cubic centimater

cm centimeter

cm3 cubic centimater

mo month

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

°F = (1.8 x °C) + 32

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

°C = (°F - 32) / 1.8

Vertical coordinate information is referenced to the National Vertical Datum of 1929 (NGVD 

of 1929); horizontal coordinate information is referenced to the North American Datum 

of 1927 (NAD 27).
viii 
Simulated Effects of Ground-Water Augmentation on the 

Hydrology of Round and Halfmoon Lakes in Northwestern 

Hillsborough County, Florida

By Richard M. Yager and P.A. Metz 

Abstract

Pumpage from the Upper Floridan aquifer in northwest 

Hillsborough County near Tampa, Florida, has induced 

downward leakage from the overlying surficial aquifer and 

lowered the water table in many areas. Leakage is highest where 

the confining layer separating the aquifers is breached, which is 

common beneath many of the lakes in the study area. Leakage 

of water to the Upper Floridan aquifer has lowered the water 

level in many lakes and drained many wetlands. Ground water 

from the Upper Floridan aquifer has been added (augmented) to 

some lakes in an effort to maintain lake levels, but the resulting 

lake-water chemistry and lake leakage patterns are substantially 

different from those of natural lakes. Changes in lake-water 

chemistry can cause changes in lake flora, fauna, and lake 

sediment composition, and large volumes of lake leakage are 

suspected to enhance the formation of sinkholes near the 

shoreline of augmented lakes.

The leakage rate of lake water through the surficial aquifer 

to the Upper Floridan aquifer was estimated in this study using 

ground-water-flow models developed for an augmented lake 

(Round Lake) and non-augmented lake (Halfmoon Lake). Flow 

models developed with MODFLOW were calibrated through 

nonlinear regression with UCODE to measured water levels 

and monthly net ground-water-flow rates from the lakes 

estimated from lake-water budgets. Monthly estimates of 

ground-water recharge were computed using an unsaturated 

flow model (LEACHM) that simulated daily changes in storage 

of water in the soil profile, thus estimating recharge as drainage 

to the water table.

Aquifer properties in the Round Lake model were 

estimated through transient-state simulations using two sets of 

monthly recharge rates computed during July 1996 to February 

1999, which spanned both average conditions (July 1996 

through October 1997), and an El Niño event (November 1997 

through September 1998) when the recharge rate doubled. 

Aquifer properties in the Halfmoon Lake model were estimated 

through steady-state simulations of average conditions in July 

1996. Simulated hydrographs computed by the Round and 

Halfmoon Lake models closely matched measured water-level 

fluctuations, except during El Niño, when the Halfmoon Lake 

model was unable to accurately reproduce water levels. 

Possibly, potential recharge during El Niño was diverted 

through ground-water-flow outlets that were not represented in 

the Halfmoon Lake model, or a large part of the rainfall was 

diverted into runoff before it could become recharge.

Solute transport simulations with MT3D indicate that 

leakage of lake water extended 250 to 400 feet into the surficial 

aquifer around Round Lake, and from 75 to 150 feet around 

Halfmoon Lake before flowing to the underlying Upper 

Floridan aquifer. These results are in agreement with 

concentrations of stable isotopes of oxygen-18 (δ18O) and 

deuterium (δD) in the surficial aquifer. Schedules of monthly 

augmentation rates to maintain constant stages in Round and 

Halfmoon Lakes were computed using an equation that 

accounted for changes in the Upper Floridan aquifer head and 

the deviation from the mean recharge rate. Resulting lake stages 

were nearly constant during the first half of the study, but 

increased above target lake stages during El Niño; modifying 

the computation of augmentation rates to account for the higher 

recharge rate during El Niño resulted in lake stages that were 

closer to the target lake stage.

Substantially more lake leakage flows to the Upper 

Floridan aquifer from Round Lake than from Halfmoon Lake, 

because the estimated vertical hydraulic conductivities of lake 

and confining layer sediments and breaches in the confining 

layer beneath Round Lake are much greater. Augmentation 

rates required to maintain the low guidance stages in Round 

Lake (53 feet) and Halfmoon Lake (42 feet) under average 

Upper Floridan aquifer heads are estimated as 33,850 cubic feet 

per day and 1,330 to 10,000 cubic feet per day, respectively. 

These rates equate to 26 inches per month of water applied to 

the entire surface of Round Lake and 0.34 to 2.5 inches per 

month of water applied to Halfmoon Lake.
2 Simulated Effects of Ground-Water Augmentation on the Hydrology of Round and Halfmoon Lakes

Introduction

Pumping from the Upper Floridan aquifer has lowered 

water levels by more than 10 ft from predevelopment levels in 

parts of northwest Hillsborough County and surrounding areas 

(Southwest Florida Water Management District, 1996). These 

drawdowns have induced downward leakage from the overlying surficial aquifer and lowered the water table in many areas. 

The rate of leakage is lowest in areas where the permeability of 

the confining layer that separates the two aquifers is low; leakage is highest in areas where the confining layer is breached, 

thus increasing permeability. These breaches commonly occur 

beneath the many shallow lakes present in the area and are 

typically associated with collapse structures in the underlying 

limestone. Downward leakage of water beneath the lakes has 

lowered the water level in many lakes and drained many 

wetlands. 

Ground water pumped from the Upper Floridan aquifer is 

currently (2002) added to 15 lakes in northwest Hillsborough 

County in an effort to maintain lake levels, and additional lakes 

are being considered for augmentation (Southwest Florida 

Water Management District, written commun., 2002). As water 

demands increase, many scientists, water managers, and citizens are concerned about the consequences of using ground 

water to restore lake levels. These concerns include (1) the 

chemistry of ground water is substantially different from that of 

natural lake water (Stewart and Hughes, 1974); (2) addition of 

ground water can cause changes in lake flora, fauna, and lake 

sediment composition; and (3) formation of sinkholes could be 

enhanced near augmented lakes (Metz and Sacks, 2002).

In 1995, the U.S. Geological Survey (USGS) began a 5-

year study with the Southwest Florida Water Management District (SWFWMD) to study the effects of ground-water augmentation on lakes in northwestern Hillsborough County. In the first 

part of the study, Metz and Sacks (2002) provided a detailed 

description of the hydrogeology, water budgets, and water quality of three lake basins in northwest Hillsborough County: 

Round Lake and Dosson Lake, augmented and non-augmented 

lakes, respectively, that lie in areas affected by ground-water 

withdrawals; and Halfmoon Lake, a non-augmented lake that 

lies within a 2-mi radius of three major well fields. In the second 

part of the study, described herein, ground-water-flow and 

transport models were developed to estimate the losses to 

ground water at Round and Halfmoon Lakes during the 2-year 

period July 1996 through September 1998 and to assess the success of different augmentation strategies on maintaining lake 

levels. The location of the study lakes and the boundary of the 

ground-water-flow models are shown in figure 1.

Purpose and Scope

This report describes the exchange of water between 

Round and Halfmoon Lakes and the surrounding surficial 

aquifer, and presents estimates of leakage rates of lake water 

to the ground-water flow system. The report presents 

(1) computation of ground-water recharge using an unsaturated 

flow model; (2) results of flow simulations, including groundwater budgets and changes in water levels during the 3-year 

study; (3) results of transport simulations that depict the extent 

of leakage of lake water into the surficial aquifer; and (4) results 

of simulated augmentation strategies to maintain lake levels.

Previous Investigations

Regional ground-water-flow models that included Hillsborough County were developed by Hutchinson (1984), 

Bengtsson (1987) and Hancock and Basso (1993). Several 

investigators developed ground-water-flow models of small 

Florida lakes similar to those in Hillsborough County. Grubbs 

(1995) delineated contributing areas and the vertical flow paths 

between the surficial aquifer and the Upper Floridan aquifer in 

a ground-water-flow model of the Lake Five-O basin. Lee and 

Swancar (1997) simulated ground-water flow near Lake Lucerne and concluded that the geometry of a sinkhole complex 

beneath the lake was the primary control on lake leakage. Lee 

(2000) simulated transient water-table mounds and groundwater-flow reversals near Lake Barco in Putnam County. 

Swancar and Lee (2003) developed a ground-water-flow model 

of Lake Starr in Polk County to quantify ground-water inflow 

and lake leakage.

Other investigators have contributed to understanding 

the effects of ground-water augmentation on lake hydrology. 

Stewart and Hughes (1974) concluded that ground-water 

augmentation at Round Lake increased lake leakage and 

evaporation and altered the chemistry of lake water. Belanger 

and Kirkner (1994) determined that more than 90 percent of 

augmented surface water returned to the ground-water system 

at Mountain Lake in Polk County. Metz and Sacks (2002) 

concluded from their study of Round, Dosson, and Halfmoon 

Lakes that (1) lakes underlain by a thin or highly breached, 

intermediate confining unit (such as Round Lake) will require 

more augmentation to maintain lake levels than lakes in areas 

where the intermediate confining unit is thicker or more intact; 

(2) augmented lakes are maintained at levels higher than the 

surrounding water table, causing lateral leakage from the lake 

and limiting ground-water inflow to the lake; (3) the difference 

in head between augmented lakes and the underlying Upper 

Floridan aquifer is increased by augmentation and local 

pumping, resulting in a higher rate of vertical leakage of lake 

water; and (4) the increase in ground-water circulation in the 

leakage-dominated hydrogeologic setting at Round Lake has 

made the basin more susceptible to karst activity (limestone 

dissolution, subsidence and sinkhole development).

Acknowledgments

The authors express their appreciation to members of the 

Round and Halfmoon Lake Associations, as well as many private homeowners, for making the hydrologic investigation of 

these private lakes possible by permitting the drilling of wells, 
EXPLANATION

WELL FIELDS

Hillsborough

County

STUDY

AREA

Florida

Tampa

MODELED LAKES

GROUND-WATER-FLOW

MODEL BOUNDARIES

40 AVERAGE WATER-TABLE

ALTITUDE

In feet

above NGVD of 1929,

contour interval 2 feet

(August 1996

through July 1997)—

82°34′ 33′ 32′ 31′ 30′ 82°29′

28°08′

07′

06′

05′

04′

28°03′

0 1 2 MILES

0 1 2 KILOMETERS

44

46

52

50

48

46

44

42

44

48

50

50

52

52

52

54

46

40

40

46

48

40

38

36

50

42

54

54

52

42

COSME-ODESSA

WELL FIELD

Gunn Highway

Ehrlich Road

Dale Mabry Highway

Van Dyke Road

Expressway

Veterans

Veterans

Rocky Creek

Expressway

SECTION 21

WELL FIELD

(Lake Park)

Lake

Jackson

Round

Lake

Brushy

40

38

36

42

42

44

38

46

48

36

Dosson

Lake

Simmons

Lake

Crum

Lake

Starvation

Lake Lake

Crenshaw

Saddleback

Lake

Lake

LeClare

Turkey Ford Lake

Rock

Lake

Lake

Josephine

Pretty

Lake

Lake

Armistead

Little

Halfmoon Lake

Halfmoon Lake

Creek

NORTHWEST HILLSBOROUGH

REGIONAL WELL FIELD

Base from U.S. Geological Survey digital data, 1:24,000, datum nad83

Albers Equal-Area Conic Projection,

Standard parallels 29 30 and 45 30 , central meridian -83 00 °° ° ′′ ′

Figure 1. Location of study area showing Halfmoon Lake and Round Lake, Hillsborough County, Florida.

Introduction 3
4 Simulated Effects of Ground-Water Augmentation on the Hydrology of Round and Halfmoon Lakes

the installation of hydrologic instrumentation, and sampling of 

well water. Acknowledgments are extended to the Florida 

Department of Transportation for granting permission to drill 

monitor wells on public roadways and the city of St. Petersburg 

for permitting the installation of an evaporation site at the Section 21 well field (Lake Park). Other hydrologists from the U.S. 

Geological Survey also contributed to this study: Terrie M. Lee 

and J.W. Grubbs aided in the model designs and provided 

review comments, and Keith Halford designed and analyzed the 

aquifer tests conducted at Round, Dosson, and Halfmoon 

Lakes. The authors also would like to express their gratitude to 

Michael Hancock, Southwest Florida Water Management 

District, for his technical support.

Background

The study area is located in mantled karst terrain about 

12 mi north of downtown Tampa, in northwest Hillsborough 

County, Fla. (fig. 1). This area lies within the Gulf Coastal Lowlands (White, 1970) in an area named Land-O-Lakes within the 

Tampa Plain (Brooks, 1981). The topographic setting is a relatively flat, sandy plain where elevations range from 40 to 60 ft 

above National Geodetic Vertical Datum of 1929 (NGVD of 

1929). The lakes are in a layer or mantle of sand and clay that 

blankets the Upper Floridan aquifer, an extensive and highly 

productive limestone aquifer. Surface drainage on the sandy 

plain is poorly defined, but typically flows south and westward 

toward Tampa Bay. Brushy and Rocky Creeks drain the central 

and western parts of the study area, respectively.

Sinkholes are prevalent throughout the study area and 

include stable ancient depressions as well as actively forming 

depressions. Sinkholes result from dissolution of the deeper 

limestone by infiltrating ground water and the resulting collapse 

of the overlying sand and clay into the voids created by limestone dissolution. The limestone typically is not exposed at land 

surface, but remains buried beneath the surficial deposits, creating numerous depressions at land surface. Shallow depressions 

generally contain swamps or cypress domes, typical geomorphic features in the study area, whereas deep depressions, 

extending 15 to 20 ft below land surface, are filled with water 

to form sinkhole lakes. Many of these lakes have shapes that 

reflect the sinkholes that formed them, varying from circular to 

irregular. Lakes with an irregular and elongated shoreline commonly result when many small sinkholes coalesce (Sinclair and 

others, 1985).

Physical Setting

The climate of the study area is subtropical with warm, 

wet summers and relatively mild, dry springs. Long-term 

average rainfall is 49 in/yr and annual evaporation rates are high 

(48-59 in/yr) due to high solar radiation and water temperatures 

(Metz and Sacks, 2002). During the 1998 water year (October 

1997 to September 1998), a tropical weather phenomenon, 

El Niño, produced record rainfall throughout the study area, 

resulting in about 40 in. of above-average annual rainfall (Metz 

and Sacks, 2002). Climatic conditions changed again during the 

1999 water year with the onset of a weather pattern called La 

Niña, which was characterized by drier than normal conditions 

that resulted in below-average rainfall for the year (Metz and 

Sacks, 2002).

Round Lake is a privately owned, 11-acre lake located 

about 500 ft east of the Section 21 well field (fig. 1). This lake 

is augmented and received an average of 0.14 Mgal/d of water 

from the Upper Floridan aquifer during the 3-year study, 

enough to fill the lake 5.6 times (Metz and Sacks, 2002). The 

30-acre topographic basin for Round Lake is bounded by Saddleback and Crenshaw Lakes to the east and Starvation and 

Crum Lakes to the west. Land use in this small drainage basin 

is residential with many homes located adjacent to the lake. 

There is no surface outlet from Round Lake, but a drainage 

ditch connects the northeastern side of the lake to Saddleback 

Lake, which occasionally allows overflow from that lake to 

enter Round Lake.

Round Lake has an average depth of about 9 ft at a lake 

level of 52.5 ft above NGVD of 1929 with several deep depressions (20-25 ft below the lake surface) along the northern and 

western shoreline (Metz and Sacks, 2002). Lake-bottom dredging at Round Lake was documented by Stewart and Hughes 

(1974) and later confirmed by verbal communication with local 

residents. A gelatinous brown-gray sediment from 1 to 3 ft thick 

covers much of the lake bottom, but is absent in some areas. 

Calcium carbonate has accumulated in the lake bottom 

sediments as a result of ground-water augmentation that has 

raised the alkalinity of the lake and enabled the proliferation of 

snails, such as Planorbella sp. (Brenner and Whitmore, 1999). 

Seismic-reflection data collected along 19 transects at Round 

Lake display a lack of continuity at the the surface of the 

limestone bedrock, suggesting breaches in the intermediate 

confining unit.

Halfmoon Lake is a privately owned, 33-acre lake that is 

located within an approximate 2-mi radius of three well fields: 

Section 21, Cosme-Odessa, and Northwest Hillsborough 

Regional (fig. 1). The topography of the 131-acre basin surrounding Halfmoon Lake is slightly elevated on the eastern side 

and slopes gently downward toward Rocky Creek on the western side. Land use within the Halfmoon Lake basin is mostly 

residential, but a 25-acre citrus grove is situated on the southeastern side of the lake. Several small cypress wetlands are 

present in the southern and northeastern sides of the basin. The 

northwestern side of the lake has an overflow weir structure that 

controls lake levels during periods of high rainfall. 

The average depth of Halfmoon Lake is about 10 ft at a 

lake level of 41.4 ft above NGVD of 1929 (Metz and Sacks, 

2002). A gelatinous brown-gray sediment from 1 to 4 ft thick 

covers much of the bottom of the lake, but is absent in some 

areas. Seismic-reflection data collected along 15 transects at 

Halfmoon Lake indicate possible subsidence features, based on 

the discontinuity of the limestone bedrock surface and the 

presence of dipping beds in overlying sediments.
Background 5

Hydrogeologic Setting

The geologic units underlying the study area consist of 

sand, clay, and carbonate rocks that were deposited primarily in 

a marine environment. Deposition of each formation was followed by a period of erosion that resulted in the development of 

solution cavities and surface irregularities. The principal hydrogeologic units in the study area include the surficial aquifer, the 

intermediate confining unit, and the Upper Floridan aquifer. 

The surficial aquifer is unconfined and consists of sand and 

clayey sand that is separated from the Upper Floridan aquifer by 

the clay-rich intermediate confining unit. The Upper Floridan 

aquifer is a highly productive carbonate aquifer that is an abundant water-supply source for west-central Florida.

Surficial Aquifer

The surficial aquifer consists of an upper unit of fine sand, 

a middle unit of clayey sand, and a lower unit of sandy clay. 

Contacts between these units are indistinct, but the cohesive 

nature of the sand increases with depth because of the higher 

percentage of clay. Where more than one permeable zone is 

present or where the deposits are interbedded, this unit is commonly termed the surficial aquifer system. In this report, the 

deposits are considered to form a single homogeneous aquifer, 

which is referred to as the surficial aquifer. The average thickness of surficial deposits was about 30 ft near Round Lake and 

40 ft near Halfmoon Lake (Metz and Sacks, 2002). Seismic 

reflection surveys indicate that the thickness of surficial deposits is greater beneath both lakes, suggesting subsidence of the 

limestone bedrock surface through voids and cavities beneath 

the lakes (Metz and Sacks, 2002).

The surficial aquifer is unconfined and contains a water 

table that is at a relatively shallow depth (about 0.5-10 ft below 

land surface) within the study area. Recharge to the water table 

is rapid, because the surface soils are generally permeable. 

Although wells in the surficial aquifer are not an important 

source of water supply because yields are relatively low (less 

than 5 gal/min), the surficial aquifer has a large storage capacity 

and serves as a substantial source of recharge to the underlying 

Upper Floridan aquifer. The hydraulic conductivity of the surficial aquifer ranges from 1 to 10 ft/d, based on aquifer tests conducted at Round, Dosson, and Halfmoon Lakes (P.A. Metz, 

U.S. Geological Survey, written commun., 1999).

Intermediate Confining Unit

The intermediate confining unit within the study area consists of the undifferentiated deposits of the Hawthorn Group 

(Southeastern Geological Society, 1986). The unit consists of 

dense, marine green-gray plastic clay that contains varying 

amounts of sand, chert, phosphate, organic material, and carbonate mud (Sinclair, 1974). Carr and Alverson (1959) and 

Sinclair (1974) describe the clay in the intermediate confining 

unit as a weathered residuum of the limestone in the underlying 

Tampa Member. The clay unit is variable in extent and thickness throughout the study area; Sinclair (1974) reported the 

presence of clay in 47 of 59 test holes that was as much as 20 ft 

thick and averaged about 4 ft thick. The vertical hydraulic conductivity of the intermediate confining unit was estimated as 

5.0x10-4 ft/d from aquifer tests conducted at Halfmoon Lake 

(P.A. Metz, U.S. Geological Survey, written commun., 1999).

Localized breaches in the intermediate confining unit associated with subsidence features or sinkholes create permeable 

connections between the surficial aquifer and the underlying 

Upper Floridan aquifer. The breaches appear to be sand columns that form when material from the surficial aquifer moves 

downward through the collapsed clay and into limestone cavities. The sand columns generally are several feet or less in diameter, and channel recharge from the surficial aquifer to the 

Upper Floridan aquifer (Stewart and Parker, 1992). Three new 

sinkholes were reported near the perimeter of Round Lake 

during this 3-year study (Metz and Sacks, 2002).

Upper Floridan Aquifer

The Upper Floridan aquifer is the major water supply 

within the study area. This aquifer consists of limestone and 

dolomite and contains many solution-enlarged fractures that 

commonly yield large quantities of water to wells. Transmissivities determined from aquifer tests of the Upper Floridan aquifer 

vary throughout the study area and range from 15,000 to 

70,000 ft2

/d (Wolansky and Corral, 1985; Southwest Florida 

Water Management District, 1994; and Langevin and others, 

1998). The transmissivity of the Upper Floridan aquifer ranged 

from 15,000 to 20,000 ft2/d in aquifer tests conducted at 

Halfmoon Lake (P.A. Metz, U.S. Geological Survey, written 

commun., 1999).

The Tampa Member is the uppermost geologic unit of the 

Upper Floridan aquifer in the study area and ranges from 20 to 

240 ft thick (Stewart, 1968). The Tampa Member is described 

as tan to white, commonly sandy, fossiliferous, highly weathered in places, and commonly contains clay lenses and cavities. 

Sand and clay are common within the Tampa Member far below 

the typical depth range of clastic deposits, suggesting infilling 

of material from the surficial aquifer into cavities in the limestone. Many domestic and production wells tap the Tampa 

Member in the study area and well yields range from several to 

several hundred gallons per minute (Stewart, 1968).

Ground Water and Lake Interaction

Ground-water exchange with lakes is governed by horizontal and vertical head gradients in the surrounding surficial 

aquifer. Water levels and vertical head gradients in the surficial 

aquifer in northwest Hillsborough County are controlled by 

recharge and ground-water withdrawals from the underlying 

Upper Floridan aquifer. Withdrawals at each of the three well 

fields that lie within the study area average about 10 Mgal/d. 

Pumpage from the Upper Floridan aquifer has lowered water 

levels more than 10 ft since the 1960s and created a downward 
6 Simulated Effects of Ground-Water Augmentation on the Hydrology of Round and Halfmoon Lakes

hydraulic gradient between the two aquifers. Water in the surficial aquifer generally flows downward toward pumping centers 

in the Upper Floridan aquifer through the intermediate confining unit. Surface drainage and sinkholes can influence flow 

directions locally, causing ground water to flow laterally toward 

these discharge areas.

The water table in the surficial aquifer (fig. 1) indicates 

that ground water discharges to Brushy and Rocky Creeks, the 

two largest perennial streams that traverse the study area. The 

hydraulic gradient in the Upper Floridan aquifer slopes westsouthwest (fig. 2), and a cone of depression is apparent in the 

vicinity of the Section 21 well field. Boundaries chosen for the 

ground-water-flow models of Round and Halfmoon Lakes 

approximately coincide with ground-water divides in the 

surficial aquifer.

EXPLANATION

POTENTIOMETRIC CONTOUR Shows altitude at

which water would have stood in tightly cased wells.

Contour interval 2 feet. Datum is

OBSERVATION WELLS Number is altitude of water

level in feet above NGVD of 1929

—

NGVD of 1929

—

MODELED LAKES

SECTION 21 WELL FIELD

GROUND-WATER-FLOW

MODEL BOUNDARIES

Base from U.S. Geological Survey digital data, 1:24,000, datum nad83

Albers Equal-Area Conic Projection,

Standard parallels 29 30 and 45 30 , central meridian -83 00 °° ° ′′ ′

Figure 2. Potentiometric surface of the Upper Floridan aquifer and boundaries of ground-water-flow 

models, Hillsborough County, Florida.

The water-table surface is uneven near Round Lake, 

reflecting the effects of lake augmentation and ground-water 

withdrawals from the Upper Floridan aquifer at the Section 21 

well field. The concentric pattern of water-level contours 

around Round Lake (fig. 3A) is the result of ground-water 

augmentation, which raises the lake level above the adjacent 

water table, resulting in leakage around the entire lake 

perimeter that recharges the surrounding surficial aquifer. 

This pattern was temporarily altered during El Niño when 

ground-water levels were nearly equal or slightly higher than 

lake stage (fig. 4). Ground water in the surficial aquifer near 

Round Lake flows downward to the Upper Floridan aquifer and 

southwestward toward Crum Lake and the Section 21 well field 

(fig. 3B).
52

33.32

42

A A’

42

44

46

46

54

52

52 52

50

4240

48

44 48

50

52

54

50

48

46

Water

table

EXPLANATION

ELEVATION, IN FEET ABOVE AND BELO

W

NGVD OF 1929

A

B

52

51

50

49

52

51

50 49

48

47

46

45 44 43 42 41

40

36

34

38

0 1 KILOMETER

0 1 MILE

A

A’

Sunshine Lake

Dale Mabry

Expressway

Veterans

SECTION 21

WELL FIELD

(LAKE PARK)

Dosson

Lake

Lake

Darby

Van Dyke Lake

Simmons

Lake

Lake

Jackson

Starvation Lake

Crum

Lake

Saddleback Lake

Lake Crenshaw

Round

Lake

Highway

Base from U.S. Geological Survey digital data, 1:24,000, datum NAD83

Albers Equal-Area Conic Projection,

Standard parallels 29 30 and 45 30 , central meridian -83 00 °° ° ′′ ′

38.18

49.43

33.32

43.43

36.13

49.08

49.29

48.90

50.33 49.07

44.36 46.91

50.79

50.17

70

60

50

40

30

20

10

0

-10

-20

52.08

40.50

52.24

Crum Lake

Round Lake

Saddleback

Lake

EXPLANATION

A A’

SAND (Surficial aquifer)

CLAY (Intermediate confining unit)

LIMESTONE (Upper Floridan Auifer)

LAKE SEDIMENT

HYDRAULIC HEAD VALUE—Height of box is

screened interval

EQUIPOTENTIAL LINE—Contour interval 1 and 2

feet. Dashed where inferred

GEOLOGIC CONTROL DATA

INFERRED GEOLOGIC CONTACT

GENERAL GROUND-WATER-FLOW DIRECTION

MODELED LAKE

WELL FIELD

GROUND-WATER-FLOW MODEL

BOUNDARY

AVERAGE WATER-TABLE ELEVATION

(August 1996 through July 1997)—In feet

above NGVD of 1929. Contour interval 2

feet

LOCATION OF GEOLOGIC SECTION

SURFICIAL AQUIFER WELLS

—Precipitation, pan

evaporation, humidity, temperature, and wind

velocity

RAINFALL MEASUREMENT SITE

WEATHER STATION

LAKE-STAGE OR CREEK MEASUREMENT SITE

0 500 1,000 FEET

0 100 200 300 METERS

Figure 3. (A) Average 

water-table elevation, 

August 1996 to July 1997; 

and (B) generalized geologic 

section A-A ́, showing distribution of hydraulic head and 

ground-water flowpaths 

near Round Lake basin 

during dry conditions, 

April 1997.

Background 7
WATER LEVEL, IN FEET ABOVE NGVD OF 1929

58

56

54

52

50

48

46

44

42

Round Lake

RL2, northeast of lake

RL8, southwest of lake

J ASOND J FMA

1996

M J J ASOND J FMA

1997

M J J ASOND JF

1998 1999

El Niño

Figure 4. Stage in Round Lake and water levels in wells RL2 and RL8, northeast and southwest of lake, 

respectively, July 1996 to February 1999. (Well locations shown on fig. 14.)

8 Simulated Effects of Ground-Water Augmentation on the Hydrology of Round and Halfmoon Lakes

The water-table surface near Halfmoon Lake, indicates a 

steady westward gradient from a local high east of the lake 

westward to Rocky Creek (fig. 5A). Ground water flows into 

Halfmoon Lake along its eastern and northern shores. During 

dry periods, the lake recharged the water table on the western 

and southwestern sides of the lake, where the water table was 

slightly lower than the lake and dipped southwestward toward 

Rocky Creek (fig. 6). During wet periods, the water-table on the 

western and southwestern shores was higher than the lake and 

ground-water inflow occurred around the entire lake perimeter. 

Ground water in the surficial aquifer flowed downward to the 

Upper Floridan aquifer and southwestward toward Rocky 

Creek (fig. 5B).

Lake-Water Budgets

Monthly water budgets were computed for each lake 

during the 3-year period (June 1996 to May 1999) using the 

following equation (Metz and Sacks, 2002):

∆S PE– A Si So – Gi Go = ++ + – , (1)

where 

∆S is the change in lake volume for a given time period;

P is precipitation;

E is evaporation;

A is total lake augmentation;

Si is surface-water inflow;

So is surface-water outflow or direct pumping from the 

lake;

Gi is ground-water inflow; and 

Go is ground-water outflow (or lake leakage). 

All of the terms in equation 1 can be measured or estimated 

directly with the exception of the ground-water-flow terms. The 

net ground-water flow Gnet was computed by rearranging equation 1 as:

Gnet Gi Go – ∆S P– E A– Si – S = = + + o , (2)

where Gnet is positive when ground-water inflow exceeds 

ground-water outflow, resulting in net ground-water inflow. 

Because Gnet is computed as a residual to the rest of the budget, 

the net ground-water-flow term incorporates all of the errors or 

uncertainties in the other water-budget terms.
0 1 MILE

0 1 KILOMETER

B

B'

Halfmoon Lake

40.72

B B'

A

B

50

48

46

51

40

42

46

44

48

40

38

36

42

42

46

48

36

38

44

50

0 500 1,000 FEET

0 100 200 300 METERS

(Open interval 49 to 102 feet

below land surface)

Rocky

Creek

Water table

27.61

27.30

40.29 40.79 40.63

40.73

40.41

40.40

45.28

41.30

40.75

41.35

23.86

32.11

41.95 40

41 42

43

44 45

40 36

28

40

32

24

Veterans

Brushy Creek

Rocky Creek

Halfmoon

Lake

Little Halfmoon

Lake

Rock

Lake

Pretty

Lake

Lake

Josephine

Lake

LeClare

Lake

Armistead

Expressway

Ehrlich Road

52

B B’

EXPLANATION

Base from U.S. Geological Survey digital data, 1:24,000, datum NAD83

Albers Equal-Area Conic Projection,

Standard parallels 29 30 and 45 30 , central meridian -83 00 °° ° ′′ ′

MODELED LAKE

WELL FIELD

GROUND-WATER-FLOW MODEL BOUNDARY

AVERAGE WATER-TABLE ELEVATION (August

1996 through July 1997)—In feet above NGVD

of 1929. Contour interval 2 feet

LOCATION OF GEOLOGIC SECTION

SURFICIAL AQUIFER WELLS

RAINFALL MEASUREMENT SITE

LAKE-STAGE OR CREEK

MEASUREMENT SITE

ELEVATION, IN FEET ABOVE AND BELO

W

NGVD OF 1929

60

50

40

30

20

10

0

-10

-20

40.29

42

EXPLANATION

SAND (Surficial aquifer)

CLAY (Intermediate confining unit)

LIMESTONE (Upper Floridan Aquifer)

LAKE SEDIMENT

HYDRAULIC HEAD VALUE—Height of box is

screened interval

EQUIPOTENTIAL LINE—Contour interval 1 and 4

feet. Dashed where inferred

GEOLOGIC CONTROL DATA

INFERRED GEOLOGIC CONTACT

GENERAL GROUND-WATER-FLOW DIRECTION

Figure 5. (A) Average water-table elevation, August 

1996 to July 1997; and 

(B) generalized geologic section B-B ́, showing distribution of hydraulic head and 

ground-water flowpaths near 

Halfmoon Lake basin during 

dry conditions, April 1997.

Background 9
M J J A SONDJ FMA

1996

M J J A SOND J FMA

1997

M J J A SONDJ FM A

1998 1999

WATER LEVEL, IN FEET ABOVE NGVD OF 1929

52

50

48

46

44

42

40

38

Halfmoon Lake

lateral leakage from lake

toward HML12

HML2, northeast of lake

HML12, southwest of lake

El Niño

M

Figure 6. Stage in Halfmoon Lake and water levels in wells HML2 and HML12, northeast and southwest 

of lake, respectively, July 1996 to February 1999. (Well locations shown on fig. 19.)

10 Simulated Effects of Ground-Water Augmentation on the Hydrology of Round and Halfmoon Lakes

Monthly net ground-water flows computed from lakewater budgets by Metz and Sacks (2002) were used to calibrate 

the ground-water-flow models of Round and Halfmoon Lakes 

discussed in this report. The budgets were prepared using daily 

precipitation measured at several gages within the study area 

and lake evaporation estimated from energy-budget data collected at Lake Starr, 60 mi to the east (Swancar and Lee, 2003). 

Lake-surface areas and volumes were calculated using data 

from continuous stage recorders and stage-volume relations 

developed for each lake. 

Net ground-water flow from Round Lake was negative 

(leakage exceeded ground-water inflow) for all but 1 month 

during the study (December 1997; Metz and Sacks, 2002). 

Net ground-water losses during the 3-year period totaled 

19.5x106 ft3 (496 in.) of water based on a lake-surface area 

of 10.8 acres. The mean net leakage rate was 18.5 in/mo 

during average conditions (June 1996 to October 1997) and 

10.4 in/mo during El Niño (November 1997 to September 

1998). An estimated 4.7x105 ft3 (12 in.) of water overflowed 

from Saddleback Lake to Round Lake through a drainage ditch 

during December 1997.

In contrast, net ground-water flow from Halfmoon Lake 

was negative in only 16 months of the 3-year period. During 

these months, Halfmoon Lake experienced flow-through conditions with ground water entering the lake along its northern and 

eastern shores and lake water leaking along the southern and 

western shores. Ground-water discharged to the lake around its 

entire perimeter during 10 months of the study: during summer 

(July-September) when evaporation from the lake surface was 

high; and during El Niño, when recharge to the surficial aquifer 

was high. During the remaining 10 months, the estimated net 

ground-water flow was less than the error in the calculation, 

indicating that outflows from the lake nearly balanced inflows. 

The mean net leakage rate from Halfmoon Lake was 2.5 in/mo 

during average conditions, and mean rates of net ground-water 

inflow to the lake were 3.3 in/mo during summer months and 

5.9 in/mo during El Niño. An estimated 6.2x106

 ft3 (42.6 in.) 

of water was pumped from the lake to alleviate flooding during 

February and March 1998, and 2.3x106 ft3 (17.4 in.) of lake 

water was discharged through a high-water weir during September and October 1998.
Ground-Water Recharge Simulation 11

Ground-Water Recharge Simulation

Recharge to the surficial aquifer has been estimated on an 

annual basis in other studies in central Florida using groundwater chemistry and water-balance methods. Lee (1996) estimated recharge as 30 percent of rainfall at Lake Barco (110 mi 

northeast of the study area) by comparing chloride concentrations measured in ground water and atmospheric deposition. A 

similar value is obtained for northwest Hillsborough County by 

comparing the average chloride concentration in ground water 

near Round Lake with atmospheric deposition at the Verna well 

field and Chassahowitzka National Wildlife Refuge (50 mi 

southeast and 40 mi north of the study area, respectively), two 

sites that are part of the National Atmospheric Deposition Program (NADP). Chloride concentrations at RL11, a well apparently unaffected by chloride from septic discharge, averaged 

2.8 mg/L in 1996 and 1997 (Metz and Sacks, 2002). Volumeweighted mean chloride concentration in atmospheric deposition at the two NADP sites multiplied by dry deposition was 

0.91 mg/L during this period (http://nadp.sws.uiuc.edu/, 

accessed April 3, 2002). Comparing the chloride concentration 

in ground water to atmospheric deposition indicates that chloride in the surficial aquifer was enriched by a factor of 3.1. This 

indicates that about 67 percent of rainfall is lost to evapotranspiration, resulting in a recharge rate of 16 in/yr.

Recharge rates specified in other simulations of groundwater flow in Hillsborough County include 25 in/yr by Hutchinson (1984), 15 to 22 in/yr by Bengtsson (1987), and 10 to 

30 in/yr by Hancock and Basso (1993). Transient simulations of 

ground-water flow constructed for this study required the use of 

monthly time periods, however, so monthly estimates of 

ground-water recharge were computed from daily simulations 

of the unsaturated zone that represented storage of water and 

drainage to the water table.

Unsaturated Flow Model

The unsaturated flow model LEACHM (Hutson and 

Wagenet, 1992) was used to compute daily recharge from the 

unsaturated zone to the surficial aquifer. The LEACHM model 

represents the unsaturated zone (soil profile) as a series of 

equally spaced layers, and computes the water content in the 

profile on a daily basis using specified rates of precipitation and 

potential evapotranspiration. Recharge is computed as the sum 

of daily drainage from the bottom of the soil profile. Overland 

runoff from precipitation is neglected in the simulations based 

on the lack of developed drainage in much of the study areas.

Water movement through the soil profile is computed by 

LEACHM using a capacity model described by Addiscott 

(1977) and Hutson and Wagenet (1992). An alternate computation method based on the Darcian Richards’ equation was not 

used in this study because the data needed to specify additional 

soil properties were not available. The capacity model uses 

specified soil properties (texture, organic carbon content and 

bulk density) and an equation by Campbell (1974) to compute a 

water-retention curve that relates water content to matric potential for each soil layer. Matric potentials corresponding to moisture contents representing field capacity (Wf), wilting point 

(Wwp), and the division (Wd) between mobile (WM) and immobile (WI) water are specified and used to compute water fluxes 

through the soil profile. Precipitation is applied to the top soil 

layer, and water in excess of field capacity (WM - Wf) is transferred to the underlying layer. This procedure is repeated for 

each succeeding layer until either the water content is less than 

field capacity, or the lowest layer is reached, at which point 

drainage occurs.

On days without precipitation, the actual evapotranspiration is summed by computing surface evaporation and transpiration using a specified factor that represents the percentage of 

crop cover. Surface evaporation is extracted from the upper 2 in. 

of the soil profile, but limited to a rate less than a maximum 

based on the soil matric potential and hydraulic conductivity 

(Hutson and Wagenet, 1992). Transpiration removes water 

from throughout a specified root zone according to factors representing the root density in each layer, but water contents 

cannot be depleted below the plant wilting point Wwp. After 

evapotranspiration has been accounted for, the remaining 

mobile water in the soil profile (WM - WI) is redistributed by 

moving water from each layer upward or downward to equilibrate hydraulic heads throughout the profile according to the 

following method: 

(1) The hydraulic head hi in layer i is:

hi ψi zi = – , (3)

where ψ is negative matric potential [L], and z is depth below 

land surface [L]. The model calculates the volume of water Vw

required to move upward or downward so that the vertical 

hydraulic gradient between adjacent layers is zero, that is:

ψi ψi + 1 – = ∆z , (4)

where ∆z is the difference in depth [L]. 

(2) Half of the water Vw/2 is redistributed to each adjacent 

layer, thereby adjusting the hydraulic head profile toward 

equilibrium for a period of several days after rainfall.

Simulation Results

Three cases with different combinations of vegetation and 

soil texture were considered in estimating recharge with 

LEACHM. In each case, the unsaturated zone was represented 

by a 6.6-ft-long soil profile containing 20 4-in.-thick layers 

above a fixed water-table boundary. Evapotranspiration was 

represented from the unsaturated zone, but not from the water 

table. Simulations used monthly potential evaporation estimated from an energy balance computed for Lake Starr, 60 mi 

east of the study area (Swancar and Lee, 2003). Potential evapotranspiration was reduced by a factor of 8 percent to account for 

differences in temperature recorded at the Section 21 well field 
12 Simulated Effects of Ground-Water Augmentation on the Hydrology of Round and Halfmoon Lakes

and Lake Starr (Metz and Sacks, 2002). Recharge in northwest 

Hillsborough County was estimated using daily precipitation 

recorded at the Section 21 well field and Round Lake. An alternative set of recharge estimates also was computed for the same 

three cases using the precipitation measured near Halfmoon 

Lake by averaging daily precipitation recorded at the Section 21 

well field and the COSME well field, and adjusting this average 

using weekly precipitation recorded at Halfmoon Lake from 

December 1997 to March 1999.

Soils within the study area are spodisols of the Myakka 

series (sandy, siliceous, hyperthermic Aeric Haplaquods) common to poorly drained areas in the Coastal Plain of the southeastern United States (Bidlake and Boetcher, 1997). The soil 

texture within the profile was assumed uniform, with the exception of the top layer where an organic-carbon mass fraction of 

3 percent was specified. A relatively coarse soil texture was 

specified in cases A and B with a silt mass fraction of 5 percent 

and a bulk density of 1.6 g/cm3

, whereas values specified in 

case C (8 percent and 1.4 g/cm3

, respectively) represented a 

fine-textured soil (table 1). Water-retention curves (fig. 7) 

MATRIC POTENTIAL, IN KILOPASCALS

immobile water

Coarse-textured soil (case A and B)

Fine-textured soil (case C)

field capacity, Wf

Wd wilting point,

Wwp

0.01 0.1 1 10 100 1,000 2,000

mobile water

0.50

0.45

0.40

0.35

0.30

0.25

0.20

0.15

0.10

0.05

0

WATER CONTENT, IN CUBIC

CENTIMETERS WATER PER CUBIC CENTIMETERS SOIL

Figure 7. Water-retention curve, relating matric potential to water content for coarse-textured and fine-textured soils specified in cases A, B, and C.

were 

computed by LEACHM using matric potentials of -5 kPa for 

field capacity, -1,500 kPa for wilting point, and -200 kPa for the 

division between mobile and immobile water. Water contents at 

saturation were about 0.4 cm3

/cm3 for the coarse-textured soil 

(cases A and B) and 0.47 cm3

/cm3 for the fine-textured soil 

(case C). 

Table 1. Soil properties specified in unsaturated flow model 

LEACHM

[Shading indicates change in specified property] 

Soil property Case A Case B Case C

Organic carbon content, in percent1 3 3 3

Silt content, in percent 5 5 8

Bulk density, grams per cubic 

centimeter

1.6 1.6 1.4

Water content at saturation, in percent 40 40 47

Matric potential, in kilopascals

Field capacity -5 -5 -5

Division between mobile and 

immobile water

-200 -200 -200

Wilting point -1,500 -1,500 -1,500

Vegetation shallow 

rooted

deep 

rooted

shallow 

rooted

1Top layer only.

Vegetation in the study areas includes grass, trees, and 

ornamental shrubbery common to residential areas in central 

Florida. An 8-in.-thick root zone was specified in cases A and 

C to represent shallow-rooted vegetation (such as grass), and a 

48-in.-thick root zone was used in case B to represent deeprooted vegetation (such as saw palmetto) (fig. 8). A permanent 

vegetative cover of 80 percent was specified in all three 

simulations.
DEPTH , IN FEET BELOW LAND SURFACE

ROOT ZONE

MODELED

SOIL LAYER

WATER TABLE

A. Cases A & C:

shallow-rooted

vegetation

0

3.3

6.6

B. Case B:

deep-rooted

vegetation

ROOT

ZONE

0

3.3

6.6

Figure 8. Soil profiles specified for shallow-rooted vegetation (cases A and C) and deep-rooted vegetation (case B) 

used in unsaturated flow model LEACHM.

Ground-Water Recharge Simulation 13

Daily recharge was computed by LEACHM from March 

1996 through March 1999 and accumulated monthly for use in 

the ground-water-flow models. Recharge computed for the 

first month was not considered in the analysis of simulation 

results, because the initial water content of the soil profile was 

unknown and specified as 0.2 at the beginning of unsaturated 

flow simulations. Rates of actual evapotranspiration (AET) 

and recharge, computed on an annual basis by LEACHM 

(table 2), were summarized for the following three periods by 

converting the monthly rates to equivalent annual rates: 

average conditions (April 1996 to October 1997), El Niño 

(November 1997 to September 1998), and La Niña (October 

1998 to March 1999).

Computed AET rates for the coarse-textured soil 

under average conditions ranged from 35.7 in/yr for shallowrooted vegetation (case A) to 42.9 in/yr for deep-rooted 

vegetation (case B). These rates compared favorably with the 

rate (40 in/yr) estimated by Bidlake and others (1996) for dry 

prairie vegetation at the T. Mabry Carlton, Jr., Memorial 

Reserve in Sarasota County using an energy-budget method. 

Computed AET rates during El Niño were less and ranged from 

31.4 to 36.2 in/yr, because there were more rainy days and less 

evaporation during this time.
Table 2. Evapotranspiration and recharge rates for northwest 

Hillsborough County, computed with unsaturated flow model 

LEACHM, for different types of vegetation and soils, and expressed on an annual basis for average conditions, El Niño, and 

La Niña 

[Units are in inches per year]

Annual rate Evapotranspiration Recharge

Case A: shallow-rooted vegetation in coarse-textured soil

Average 35.7 20.2

El Niño 31.4 52.3

La Niña 19.3 1.5

Case B: deep-rooted vegetation in coarse-textured soil

Average 42.9 12.8

El Niño 36.2 47.4

La Niña 23.6 1.8

Case C: shallow-rooted vegetation in fine-textured soil

Average 38.5 16.2

El Niño 33.7 49.9

La Niña 21.2 1.6

14 Simulated Effects of Ground-Water Augmentation on the Hydrology of Round and Halfmoon Lakes

Computed recharge rates for average conditions 

ranged from 12.8 in/yr for deep-rooted vegetation (case B) to 

20.2 in/yr for shallow-rooted vegetation (case A), or from 22 to 

35 percent of precipitation. These results compare favorably 

with the recharge rate of 16 in/yr estimated from the comparison 

of chloride concentrations in ground water and atmospheric 

deposition, and with the previously reported range of rates (10-

30 in/yr) used in other modeling studies of Hillsborough 

County. Computed recharge rates for the two vegetation types 

during El Niño were greater (47.4 in/yr for deep-rooted vegetation and 52.3 in/yr for shallow-rooted vegetation) and amounted 

to about 60 percent of the precipitation. Specifying a fine-textured soil (case C) reduced average recharge under shallowrooted vegetation to 16.2 in/yr, because the field capacity of the 

soil was greater than in the case for the coarse-textured soil, and 

more of the precipitation that infiltrated the soil profile was 

retained and eventually lost to evapotranspiration. Recharge 

rates using precipitation recorded near Halfmoon Lake were 

similar to those computed using precipitation near Round Lake, 

and ranged from 14.8 to 22.1 in/yr for average conditions and 

48.3 to 52.3 in/yr during El Niño.

The distribution of recharge computed for all three cases is 

highly variable and indicates that little recharge occurred from 

April 1996 to March 1997 when soil water content was generally less than field capacity as a result of limited precipitation 

(fig. 9). Recharge increased from April 1997 to September 1998 

when the soil water content greatly exceeded field capacity, 

allowing most of the water to drain through the soil profile during the largest rainfall events. Almost no recharge occurred 

when the soil profile dried during La Niña (October 1998 to 

March 1999).

AM J J A S OND JF MA M J J A SOND J FMAM J J A SOND J F M

1996 1997 1998 1999

recharge

actual evapotranspiration

average conditions El Niño La Niña

18

16

14

12

10

8

6

4

2

0

RATE, IN INCHES PER MONTH

Figure 9. Monthly evapotranspiration and recharge, computed with unsaturated flow model LEACHM, for coarse-textured soil and shallow-rooted vegetation (case A) at Round Lake, April 1996 to March 1999.
Round Lake Basin Ground-Water Flow Simulation 15

Round Lake Basin Ground-Water

Flow Simulation

Ground-water flow in the Round and Halfmoon Lakes 

study area was simulated using the three-dimensional, finitedifference model MODFLOW (McDonald and Harbaugh, 

1988) to represent flow conditions during a 32-month period 

from July 1996 to February 1999. The flow models were calibrated to measured water levels and ground-water exchange 

rates based on lake-water budgets by using a nonlinear regression method in the program UCODE (Poeter and Hill, 1998). 

The effects of different augmentation scenarios discussed were 

assessed using both steady- and transient-state simulations.

Ground-water-flow models developed for the Round Lake 

basin included steady-state simulations of conditions in July 

1996 and transient-state simulations during July 1996 to February 1999. Steady-state simulations provided initial conditions 

for the transient-state simulations and were used to depict flow 

paths and travel times of lake leakage to the Upper Floridan 

aquifer. Transient-state simulations were used to estimate aquifer properties with two sets of monthly recharge rates computed 

with the unsaturated flow model LEACHM.

Model Design

The modeled area covers 0.4 mi2 and is bounded by the 

four lakes surrounding Round Lake (fig. 10). Ground-water 

flow is represented in 10 model layers with the surficial aquifer 

represented in model layers 1 through 8 (fig. 11). The saturated 

thickness of the top model layer (1) varies from 1 to 16 ft, 

because the layer is unconfined and the water table fluctuates 

during the 32-month simulation period. Parts of model layer 1 

are dewatered (become dry) during conditions of low recharge 

in the simulation, so that model layer 2 becomes unconfined in 

some areas as well. As a result, the saturated thickness of model 

layer 2 fluctuates from 1 to 5 ft. The dry areas later resaturated 

during conditions of high recharge in the simulation through the 

rewetting capability provided in MODFLOW (Harbaugh and 

MacDonald, 1996). The remaining six layers representing the 

surficial aquifer are confined with a constant thickness of 5 ft. 

The intermediate confining unit and the Upper Floridan aquifer 

are represented by model layers 9 and 10, respectively, with 

specified thicknesses of 4 and 10 ft. The thickness specified for 

the Upper Floridan aquifer is arbitrary and has no affect on 

model simulations. Breaches in the confining layer beneath 

Round Lake (fig. 10), inferred from seismic reflection surveys, 

are represented by zones of higher vertical hydraulic 

conductivity in layer 9. Each of the 10 model layers is divided 

into a uniformly spaced grid of 75 ft with 49 rows and 54 columns that contains 1,971 active cells.

Specified flux boundaries are assigned in the uppermost 

model layer to represent recharge from the unsaturated zone to 

the surficial aquifer. Two sets of monthly recharge rates, computed by the unsaturated flow model LEACHM, were used in 

the MODFLOW simulations: high recharge rates in model 

RL-A (case A, coarse-textured soil with shallow-rooted vegetation); and low recharge rates in model RL-B (case B, coarsetextured soil with deep-rooted vegetation). Specified flux 

boundaries are also assigned in model layer 1 to represent 

(1) ground-water augmentation of Round Lake, based on 

monthly reported pumpage rates from the Upper Floridan aquifer; and (2) the net effects of rainfall and evaporation on Round 

Lake, based on the difference between measured precipitation 

at the Section 21 well field and the evaporation estimated from 

an energy budget computed for Lake Starr.

Specified-head boundaries that vary monthly are assigned 

to represent other inflows and outflows of water to and from the 

surficial aquifer. Ground-water flow to and from the four lakes 

that surround Round Lake (fig. 10) is represented in model layers 1, 2 and 3 based on monthly stage measurements made in the 

lakes. Finally, vertical leakage through the intermediate confining unit to the Upper Floridan aquifer is represented by assigning specified heads throughout model layer 10. Because of the 

large transmissivity of the Upper Floridan aquifer, leakage from 

the surficial aquifer was assumed to have little effect on the 

head in the Upper Floridan aquifer within the modeled area. The 

temporal distribution of head in model layer 10 was computed 

by interpolation among monthly water levels measured in 

19 wells in the Upper Floridan aquifer. Lateral boundaries of 

the modeled area coincide with streamlines interpreted from the 

regional distribution of hydraulic head in the surficial aquifer 

(see fig. 3A) and are specified as no-flow in all layers, with the 

exception of underflow (lateral ground-water flow) toward the 

municipal well field southwest of Round Lake, which is represented by specified heads in model layers 4 to 8.

Calibration Procedure

Parameter values representing aquifer properties were 

adjusted through transient-state simulations to produce a model 

that approximated water levels measured from July 1996 to 

February 1999, and net ground-water flow to and from Round 

Lake estimated from monthly water budgets computed for the 

same period. A steady-state simulation representing conditions 

in July 1996 at the beginning of the simulation period provided 

initial conditions for the transient-state simulation. The transient-state simulation included 32 monthly periods to represent 

changes in recharge rates and augmentation well pumpage used 

to maintain the water level in Round Lake.

Model parameters were estimated through a nonlinear 

regression method (Cooley and Naff, 1990) that minimized 

differences between measured and computed heads and flows. 

Initial estimates were specified for seven parameters (table 3), 

including five that were estimated through nonlinear regression. 

The other two parameters (vertical hydraulic conductivity and 

specific storage of the surficial aquifer) were too insensitive 

to estimate through the regression. Model cells representing 

open water in Round Lake in model layers 1 to 3 were 

assigned a large horizontal and vertical hydraulic conductivity 
EXPLANATION

C C' LINE OF SECTION SHOWN ON FIG. 11

5 10 15 20 25 30 35 40 45 50

49

5

10

15

20

25

30

35

40

45

Crenshaw Lake

Saddleback Lake

Starvation Lake

Crum Lake

(layer 2)

underflow to well field (layers 4 to 8)

COLUMNS

ROWS

C C'

1 54

1

ACTIVE CELL

INACTIVE CELL

SPECIFIED HEAD

ROUND LAKE

AUGMENTATION WELL

BREACH IN CONFINING

UNIT (layer 9)

0 0.25 0.5 MILE

0 0.25 0.5 KILOMETER

N

Figure 10. Boundary conditions and hydrologic features of the ground-water-flow model of the Round Lake basin.

16 Simulated Effects of Ground-Water Augmentation on the Hydrology of Round and Halfmoon Lakes
C C'

land surface model layer

1

2

3

4

5

6

7

8

9

10

surficial aquifer

EXPLANATION

Vertical exaggeration X 60

60

50

40

30

20

10

ALTITUDE, IN FEET ABOVE NGVD OF 1929

6

ACTIVE CELL

INACTIVE CELL

SPECIFIED HEAD

INTERMEDIATE

CONFINING UNIT

BREACH IN

CONFINING UNIT

LAKE SEDIMENT

ROUND LAKE

Round

Lake

intermediate

confining unit

Upper Floridan aquifer

Figure 11. Model layers used to represent surficial and Upper Floridan aquifers in Round Lake basin.

Table 3. Optimum values of aquifer properties estimated for the Round Lake model area through nonlinear regression in transient-state 

simulations, and error in predicted heads and flows

[Shading indicates estimated parameters. ft/d, feet per day; ft2, feet squared; ft, feet]

Parameter

Model 

RL-A:

high recharge

Approximate individual

95-percent

confidence interval

Coefficient of 

variation 

(percent)

Model 

RL-B:

low recharge

Aquifer property

Horizontal hydraulic conductivity of surficial aquifer, ft/d 12 11 - 13 1

2.2

1

Reported for log-transformed parameter.

12

Vertical hydraulic conductivity, ft/d

 Surficial aquifer 2 2

 Lake sediment2

2

Lake sediment, intermediate confining unit and breaches specified as isotropic.

30.15 30.15

 Intermediate confining unit2 6 × 10-3 5.6 × 10-4 - 6.4 × 10-4 0.5 5.6 × 10-3

 Breaches in intermediate confining unit2 0.24 0.16 - 0.34 6.7 0.19

Storage—all units

 Specific yield 0.23 0.21 -0.24 3.5 0.21

 Specific storage, ft-1 1.2 × 10-6 1.2 × 10-6

Model error

Sum of squared errors in heads and flows, ft2 725 769

Standard error, ft

 Heads 1.05 1.10

 Flows 1.90 1.74

3Effective value computed from Darcy’s Law, no statistics reported.

Round Lake Basin Ground-Water Flow Simulation 17
18 Simulated Effects of Ground-Water Augmentation on the Hydrology of Round and Halfmoon Lakes

(10,000 ft/d) to allow the water level to fluctuate during the 

transient simulation while maintaining a near-zero hydraulic 

gradient throughout the part of the model area representing the 

lake. Hydraulic conductivity of lake sediments, assumed to be 

isotropic, was estimated by the regression for model cells 

underlying the open-water cells (fig. 11). Simulated flow 

between the lake and the surficial aquifer occurred through cells 

representing aquifer material and through cells representing 

lake sediments. The effective hydraulic conductivity controlling the rate of flow was, therefore, a function of the hydraulic 

conductivities specified for both the aquifer material and lake 

sediments. This effective value was computed using Darcy’s 

law and the results of steady-state simulations, as described in 

the next section.

Results of transient-state simulations were compared with 

554 water levels measured in 18 surficial aquifer wells and in 

Round Lake during the simulated period, and 32 estimates of 

net ground-water flow rates from monthly water budgets. Heads 

computed by simulation were interpolated spatially and temporally to correspond to well locations and measurement times. 

Heads for wells with screens that spanned multiple model layers 

were computed near the midpoint of the well screen. Observations were weighted in the regression according to procedures 

in Hill (1992) to account for the different units associated with 

head measurements (42.6-57.5 ft) and flow measurements 

(3,200-32,200 ft3/d), so weighted residuals are computed in 

constant units of feet. The weights were adjusted so that the 

residual in the head and flow measurements (difference 

between observed and computed values) were the same order of 

magnitude. 

Simulation Results

Optimum Parameter Values

The parameter values estimated by nonlinear regression 

for the five aquifer properties are listed in table 3, together with 

their approximate individual confidence intervals and coefficients of variation. The confidence intervals were computed 

based on the method described in Hill (1992) under the assumption that the model is correct and linear in the vicinity of the 

optimum set of values, and the parameter values are normally 

distributed. The residuals are normally distributed based on the 

R2

N statistic defined by Hill (1992), but the model is not effectively linear based on the modified Beale’s measure (Cooley 

and Naff, 1990; Hill, 1994). The reported statistics are only 

approximate, therefore, because the model does not meet all the 

required assumptions, but they indicate qualitatively the relative reliability of the optimum parameter values.

The coefficients of variation for four of the parameter values estimated by nonlinear regression with model RL-A range 

from 0.5 to 7 percent, indicating that the model is sensitive to 

the estimated parameters and that the optimum values are accurately estimated. An inverse correlation was apparent between 

the parameters representing the vertical hydraulic conductivity 

of the intermediate confining unit and the breaches within it 

(r2 = -0.91), but the regression was well conditioned and converged to optimum values for these parameters despite the correlation. Optimum parameter values obtained by regression 

with model RL-B fall within the approximate 95-percent confidence intervals associated with the values estimated with model 

RL-A.

The estimated hydraulic conductivity of the surficial aquifer is 12 ft/d, about double the value estimated from the pumping test at Round Lake (5 ft/d), and the estimated specific yield 

was 0.23. The vertical hydraulic conductivity of the surficial 

aquifer (2 ft/d) was based on the geometric mean hydraulic conductivity (6.5 ft/d) and vertical anisotropy (3.3) estimated from 

pumping test results, because the model sensitivity to the 

parameter was too small to include in the regression. The vertical hydraulic conductivities of the intermediate confining unit 

and the breaches within it were estimated as 6.0x10-3 and 0.24 

ft/d, respectively. The vertical hydraulic conductivities (Kv)

were computed from the corresponding leakance (L) using the 

following equation:

L

Kv

b = ----- , (5)

where the thickness (b) of the intermediate confining unit was 

assumed to be 4 ft from drilling logs near Round Lake. If the 

average thickness of the intermediate confining unit was more 

than 4 ft, then the estimated value of vertical hydraulic conductivity would be proportionately greater. Intermediate confining 

unit sediments and lake sediments are specified as isotropic in 

models RL-A and RL-B.

The effective hydraulic conductivity of the lake sediments 

was estimated from the results of steady-state simulations representing conditions in July 1996. The effective value (k) was 

computed from Darcy’s law:

Q kiA = , (6)

where 

Q is flow rate [L3/T]; 

i is hydraulic gradient [dimensionless]; and 

A is the lake-surface area [L2/T].

The average hydraulic gradient was determined by inspection from hydraulic heads computed from the simulation, and 

the flow rate from the lake to the surficial aquifer was summed 

using the program ZONEBUDGET (Harbaugh, 1990). 

Estimated effective hydraulic conductivity of lake sediments 

with either the high or the low recharge condition were both 

about 0.15 ft/d. 
Round Lake Basin Ground-Water Flow Simulation 19

Water Levels

Water levels in the surficial aquifer in June 1997 were near 

minimum for the 32-month simulation period. Heads computed 

by transient-state simulation with model RL-A (fig. 12A)

44

44

42

52

EXPLANATION

C C'

C C'

41

43

45

46

49

50

46

48

49 50

48

50

47

47

45

46.0

52.0

50.6

51.5

39.6

LAKE—Number is specified stage, in feet above

NGVD of 1929, except Round Lake, which is

simulated stage

ACTIVE CELL

Crenshaw Lake

Saddleback Lake

Starvation Lake

Crum Lake

Round Lake

SIMULATED WATER TABLE—Contour interval

1 foot. Datum is NGVD of 1929

LINE OF SECTION SHOWN ON FIGURE 12B

OBSERVATION WELL

0 0.25 0.5 MILE

0 0.25 0.5 KILOMETER

A N

Figure 12A. Water table in June 1997 during average conditions, computed by transient-state simulation with model RLA (high recharge), of the surficial aquifer near Round Lake: plan view.

 indicated that in June 1997 ground water flowed in a southwest 

direction from Saddleback and Crenshaw Lakes toward the 

cone of depression created by the municipal well field west of 

the study area. Augmentation water maintained the stage of 

Round Lake about 2 ft above the adjacent water table, and 

simulated leakage from the lake occurred around the entire 

perimeter. Lake stage was about 5 ft above the sloping watertable surface from Saddleback Lake to Crum Lake (fig. 12B). 

The computed vertical distribution of head indicates predominantly downward flow beneath Round Lake toward the Upper 

Floridan aquifer. Heads in the surficial aquifer ranged from 52 

ft near Round Lake to 40 ft near Crum Lake, whereas heads 

specified in the Upper Floridan aquifer ranged from 40 ft near 

Crenshaw Lake to 33 ft near Crum Lake. Heads computed by 

transient-state simulation with model RL-B display similar flow 

patterns of June 1997.
20 Simulated Effects of Ground-Water Augmentation on the Hydrology of Round and Halfmoon Lakes

Water levels in the surficial aquifer in March 1998 were 

near maximum during El Niño conditions, and the computed 

head distribution (fig. 13A) 

land surface

Height of lake stage

above sloping

water table (5 feet)

C C'

42

EXPLANATION

Vertical exaggeration X 60

60

50

40

30

20

10

ALTITUDE, IN FEET ABOVE NGVD OF 1929

6

ACTIVE CELL

INACTIVE CELL

DRY CELL

SPECIFIED HEAD

ROUND LAKE

INTERMEDIATE

CONFINING UNIT

BREACH IN

CONFINING UNIT

LAKE SEDIMENT

SIMULATED HYDRAULIC

HEAD Contour interval 1

foot. Datum is NGVD of 1929

SIMULATED WATER TABLE

—

43

44

45

46

47

48

49

48

49

48

50

50

47

46

49

49 50

51

51

46

Round

Lake

B

Figure 12B. Water table in June 1997 during average conditions, computed by transient-state 

simulation with model RL-A (high recharge), of the surficial aquifer near Round Lake: section 

view.

indicates a much flatter, southwestly gradient (1.5x10-3) than the computed gradient for June 

1997 (9.2x10-3). The stage in Round Lake (55 ft) was near the 

altitude of the adjacent water table, and the vertical gradient 

beneath the lake remained downward toward the Upper Floridan aquifer (fig. 13B). Heads in the aquifer system during 

March 1998 generally were higher than in June 1997, ranging 

from 55 ft near Round Lake to 53 ft near Crum Lake (fig. 13B), 

whereas heads specified in the Upper Floridan aquifer ranged 

from 44 to 37 ft.

Hydrographs of simulated water levels (fig. 14) indicate 

that model RL-A closely reproduces measured water-level fluctuations in response to changes in recharge and pumpage during 

the 32-month simulation period. Hydrographs in figure 14A-D 

depict water levels in Round Lake and three wells to the south 

and west screened in the upper half of the surficial aquifer, 

whereas hydrographs in figure 14E-H depict water levels in 

four wells screened at four different depths northeast of the 

lake. Simulated water levels are underpredicted by as much as 

1 ft during average conditions (July 1996 to October 1997), and 

overpredicted by as much as 1.5 ft during La Niña conditions 

(September 1998 to February 1999) (for example, fig. 14A).

This bias in model results is apparent in the distribution of 

model residuals (difference between measured and simulated 

heads), but the residual plots indicate that heads computed with 

model RL-A generally match the measured water levels quite 

well (fig. 15). The standard error in computed heads was 1.05 ft 

and the range of measured heads was 43 to 57 ft. The standard 

error in heads computed with model RL-B was slightly greater 

(1.1 ft), and computed head distributions were nearly equivalent 

to those computed with model RL-A.
54

55 55

EXPLANATION

C C'

C C'

53

56

56

55 55.5

55

55

55.5

54.5 53.5

53.7

53.0

56.5

55.1

55.0

LAKE—Number is specified stage, in feet above

NGVD of 1929, except Round Lake, which is

simulated stage

ACTIVE CELL

Crenshaw Lake

Saddleback Lake

Starvation Lake

Crum Lake

Round Lake

SIMULATED WATER TABLE—Contour interval

0.5 foot. Datum is NGVD of 1929

LINE OF SECTION SHOWN ON FIGURE 13B

OBSERVATION WELL

0 0.25 0.5 MILE

0 0.25 0.5 KILOMETER

A N

Figure 13A. Water table in March 1998 during El Niño conditions, computed by transient-state simulation with model 

RL-A (high recharge), of the surficial aquifer near Round Lake: plan view.

Round Lake Basin Ground-Water Flow Simulation 21
C C'

land surface

EXPLANATION

Vertical exaggeration X 60

60

50

40

30

20

10

ALTITUDE, IN FEET ABOVE NGVD OF 1929

6

ACTIVE CELL

INACTIVE CELL

SPECIFIED HEAD

ROUND LAKE

INTERMEDIATE

CONFINING UNIT

BREACH IN

CONFINING UNIT

LAKE SEDIMENT

SIMULATED HYDRAULIC

HEAD Contour interval 1

foot. Datum is NGVD of 1929

SIMULATED WATER TABLE

—

54

54

54

53 53

52

51

55

51

Round

Lake

53

55

B

Figure 13B. Water table in March 1998 during El Niño conditions, computed by transient-state 

simulation with model RL-A (high recharge), of the surficial aquifer near Round Lake: section 

view.

22 Simulated Effects of Ground-Water Augmentation on the Hydrology of Round and Halfmoon Lakes
El Niño

El Niño

El Niño

El Niño

El Niño

El Niño

El Niño

G. Well RL1 H. Well RL23 El Niño

D. Well RL8

F. Well RL2

B. Well RL4

C. Well RL9

E. Well RL3

A. Round Lake

measured

simulated

1996 1997 1998 1999

60

56

52

48

44

40

WATER LEVEL, IN FEET ABOVE NGVD 1929

J ASOND J FMAMJ J ASOND J FMAMJ J ASOND J F

measured

simulated

1996 1997 1998 1999

60

56

52

48

44

40 J ASOND J FMAMJ J ASOND J FMAMJ J ASOND J F

measured

simulated

1996 1997 1998 1999

60

56

52

48

44

40 J ASOND J FMAMJ J ASOND J FMAMJ J ASOND J F

measured

simulated

1996 1997 1998 1999

60

56

52

48

44

40 J ASOND J FMAMJ J ASOND J FMAMJ J ASOND J F

measured

simulated

1996 1997 1998 1999

60

56

52

48

44

40 J ASOND J FMAMJ J ASOND J FMAMJ J ASOND J F

measured

simulated

1996 1997 1998 1999

60

56

52

48

44

40 J ASOND J FMAMJ J ASOND J FMAMJ J ASOND J F

measured

simulated

1996 1997 1998 1999

60

56

52

48

44

40 J ASOND J FMAMJ J ASOND J FMAMJ J ASOND J F

measured

simulated

1996 1997 1998 1999

60

56

52

48

44

40 J ASOND J FMAMJ J ASOND J FMAMJ J ASOND J F

Figure 14 . Water levels measured in the surficial aquifer near Round Lake and heads, computed by transient-state simulation 

with model RL-A (high recharge), July 1996 to February 1999: (A) Round Lake; (B) well RL4, (C) well RL9, (D) well RL8, (E) well RL3, 

(F) well RL2, (G) well RL1, (H) RL 23. (Location of wells and screened intervals on page 24.)

Round Lake Basin Ground-Water Flow Simulation 23
RL4

RL3

RL2

RL1

RL23

RL8 RL9

60

55

50

45

40

35

30

ALTITUDE, IN FEET ABO

25

VE NGVD OF 1929

land surface

vertical exaggeration X 20

D D'

Round Lake

RL4

RL9

RL8

RL3,2,1 & 23

D'

D

Figure 14. (Continued) Water levels measured in the surficial aquifer near Round Lake and heads, computed by 

transient-state simulation with model RL-A (high recharge), July 1996 to February 1999: location of wells and screened 

intervals.

40 42 44 46 48 50 52 54 56 58

SIMULATED WATER LEVEL, IN FEET

58

56

54

52

50

48

46

44

42

MEASURED WATER LEVEL, IN FEET

5

4

3

2

1

0

-1

-2

-3

-4

WEIGHTED RESIDUAL, IN FEET

underpredicted overpredicted

A

bias

B

40 42 44 46 48 50 52 54 56 58

SIMULATED WATER LEVEL, IN FEET

Figure 15. Measured heads, simulated heads, and residuals, 

computed from transient-state simulation with model RL-A (high 

recharge), of the surficial aquifer near Round Lake, July 1996 to 

February 1999: (A) measured and simulated values; (B) simulated 

values and residuals.

24 Simulated Effects of Ground-Water Augmentation on the Hydrology of Round and Halfmoon Lakes

Water Budget

Simulated water budgets for the entire modeled 

area reflect the dramatic increase in recharge from average 

to El Niño conditions (table 4). During average conditions 

(July 1996 to October 1997), the mean recharge to the 

surficial aquifer was about 1.5 in/mo (3.6 cm/mo) 

and accounted for 36 percent of the computed inflows, 

whereas leakage from surrounding lakes (30 percent) and 

augmentation from the Upper Floridan aquifer 

(22 percent) accounted for most of the remainder. 

Recharge more than doubled to 3.3 in/mo (8.1 cm/mo) 

during El Niño conditions (November 1997 to September 

1998), and accounted for 61 percent of computed inflows. 

Downward leakage to the Upper Floridan aquifer 

accounted for most of the outflow from the model area 

during both periods, with about 10 percent of leakage 

passing through breaches in the intermediate confining 

unit. The variability in recharge is evident in monthly 

budgets computed for the simulated period (fig. 16). Most 

of the recharge was temporarily stored in the surficial 

aquifer then released during succeeding months.

Net leakage (ground-water outflow) rates from 

Round Lake computed with model RL-A are similar to 

those estimated from monthly water budgets computed for 

Round Lake (fig. 17). Residuals generally are larger for 

months with lower leakage rates (fig. 18), which are not as 

accurately estimated from the monthly water budgets as 

are the larger rates. The standard error in flow rates was 

1.9 ft for model RL-A and 1.74 ft for model RL-B.
Table 4. Simulated water budget for the Round Lake model area during average conditions and El Niño conditions, 

model RL-A

[Flow rates are in thousands of cubic feet per day; <, less than]

Inflow Discharge

Source Rate Percentage 

of total Location Rate Percentage of total

Average (July 1996 to October 1997)

Recharge 42.6 36 Net precipitation from Round Lake 0.2 < 1

Storage 14.1 12 Storage 25.7 22

Leakage from surrounding lakes 35.2 30 Underflow to municipal wells < 0.1 < 1

Augmentation well 25 22 Downward leakage

Intermediate confining unit 83.8 72

Breaches in confining unit 7.2 6

TOTAL 116.9 100 TOTAL 116.9 100

El Niño (November 1997 to September 1998)

Recharge 95.5 61 Net precipitation from Round Lake 0.2 < 1

Storage 32.1 20 Storage 47.6 30

Leakage from surrounding lakes 22.9 15 Underflow to municipal wells < 0.1 < 1

Augmentation well 7 4 Downward leakage

Intermediate confining unit 102.7 66

Breaches in confining unit 7 4

TOTAL 157.5 100 TOTAL 157.5 100

Round Lake Basin Ground-Water Flow Simulation 25
J ASOND J FMAMJ J ASOND J FMAMJ J ASOND J F

J ASOND J FMAMJ J ASOND J FMAMJ J ASOND J F

400

350

300

250

200

150

100

50

0

INFLOWS

recharge

augmentation

leakage from

surrounding lakes

storage

INFLOW, IN 1,000 CUBIC FEET PER DAY

1996 1997 1998 1999

1996 1997 1998 1999

0

-50

-100

-150

-200

-250

-300

-350

-400

OUTFLOW, IN 1,000 CUBIC FEET PER DAY

OUTFLOWS

storage

leakage to

Upper Floridan Aquifer

evaporation

average conditions El Niño La Niña

average conditions El Niño La Niña

Figure 16. Simulated water budget for the surficial aquifer in the Round Lake model area, July 1996 to February 1999.

26 Simulated Effects of Ground-Water Augmentation on the Hydrology of Round and Halfmoon Lakes
average conditions El Niño La Niña

estimated from water budget

(Metz and Sacks, 2002, table 4)

simulated

Error in water budget estimate

J ASOND J FMAMJ J ASOND J FMAMJ J ASOND J F

1996 1997 1998 1999

0

5

10

15

20

25

30

35

NET LEAKAGE, IN 1,000 CUBIC FEET PER DAY

6

5

4

3

2

1

0

-1

-2

-3

-4

WEIGHTED RESIDUAL,

IN CUBIC FEET PER DAY

underpredicted overpredicted

B

A

0 10 20 30 40

SIMULATED LEAKAGE, IN 1,000 CUBIC FEET PER DAY

40

35

30

25

20

15

10

5

0

MEASURED LEAKAGE, IN 1,000

CUBIC FEET PER DAY

0 10 20 30 40

SIMULATED LEAKAGE, IN 1,000 CUBIC FEET PER DAY

Figure 17. Simulated net leakage rates 

from Round Lake, computed from transient-state simulation with model RL-A 

(high recharge), and estimated rates from 

monthly water budgets, July 1996 to 

February 1999.

Figure 18. Simulated lake leakage rates and residuals, 

computed from transient-state simulation with model RL-A 

(high recharge), and estimated rates from monthly water 

budgets, July 1996 to February 1999: (A) measured and simulated values; (B) simulated values and residuals.

Round Lake Basin Ground-Water Flow Simulation 27
28 Simulated Effects of Ground-Water Augmentation on the Hydrology of Round and Halfmoon Lakes

Halfmoon Lake Basin Ground-Water Flow 

Simulation

The ground-water-flow models developed for the Halfmoon Lake basin included steady-state simulations of conditions in July 1996 and transient-state simulations for part of the 

3-year period (July 1996 to February 1999). Steady-state simulations provided initial conditions for the transient-state simulations and were used to estimate aquifer properties, because transient-state simulations were unable to accurately reproduce the 

observed changes in water levels. Steady-state simulations also 

were used to depict flow paths and travel times of lake leakage 

to the Upper Floridan aquifer. Transient-state simulations were 

used to estimate an alternate set of aquifer properties using measured data from the first 22 months of the study (July 1996 to 

April 1998) and the lower set of monthly recharge rates (case B, 

table 2) computed with the unsaturated flow model LEACHM. 

Model Design

The modeled area covers 1.3 mi2 bounded to the west by 

Rocky Creek and extending to the north and east to Lake 

LeClare (fig. 19). Ground-water flow is represented in eight 

model layers with the surficial aquifer represented by model 

layers 1 to 6 (fig. 20). The saturated thickness of the top model 

layer varies from 1 to 17 ft, because the layer is unconfined and 

the water table fluctuates during the 22-month simulation 

period. The remaining five layers representing the surficial 

aquifer are confined with a constant thickness of 5 ft. The 

intermediate confining unit and the Upper Floridan aquifer are 

represented by model layers 7 and 8, respectively, each with a 

specified thickness of 5 ft. The thickness specified for the Upper 

Floridan aquifer is arbitrary and has no effect on model 

simulations. Breaches in the confining layer beneath Halfmoon 

Lake, inferred from seismic reflection surveys, are represented 

by zones of higher vertical hydraulic conductivity in layer 8. 

Each of the eight model layers is divided into a uniformly 

spaced grid of 75 ft with 98 rows and 86 columns that contains 

6,529 active cells.

Specified flux boundaries are assigned in model layer 1 to 

represent recharge from the unsaturated zone to the surficial 

aquifer. The set of lower monthly recharge rates computed by 

the unsaturated flow model LEACHM (case B, coarse-textured 

soil with deep-rooted vegetation) better represent conditions in 

July 1996, because a higher proportion of the modeled area is 

covered by deep-rooted vegetation than in the Round Lake area. 

Specified flux boundaries also are assigned in model layer 1 to 

represent evaporation from Halfmoon Lake, based on the difference between average measured precipitation at several locations (Halfmoon Lake, and the Section 21 and Cosme well 

fields), and evaporation estimated from an energy budget computed for Lake Starr.

Specified-head boundaries that vary monthly are assigned 

to represent other inflows and outflows of water to and from the 

surficial aquifer. Leakage from Lake LeClare is represented in 

model layers 1 to 3 based on the measured lake stage. Vertical 

leakage through the intermediate confining unit to the Upper 

Floridan aquifer is represented by assigning specified heads 

throughout model layer 8. The temporal distribution of head in 

model layer 8 was computed by interpolating monthly water 

levels measured in the Upper Floridan aquifer. Ground-water 

discharge to Rocky Creek is represented through a head-dependent flux (drain) boundary using monthly stream stages estimated from daily stages measured in Rocky Creek at a gaging 

station near Gunn Highway. The drain conductance was specified larger than the adjacent aquifer so the boundary functioned 

as a specified head that allowed outflow from the modeled area, 

but no inflow. Other lateral boundaries of the modeled area 

coincide with streamlines interpreted from the regional distribution of hydraulic head (see fig. 5) and are specified as no-flow 

in all layers.

Two zones were delineated in both the surficial aquifer and 

the intermediate confining layer in an attempt to better reproduce the observed water-table configuration in July 1996 and 

water-level fluctuations during the study (fig. 19). The fluctuation in measured water levels was about 4 to 5 ft in wells located 

more than 400 ft to the north and east of Halfmoon Lake (for 

example, wells HML-5 and HML-16), whereas the range was 6 

to 7 ft in wells surrounding the lake and to the south and west 

(for example, HML-9 and HML-12). Separate hydraulic-conductivity values were estimated for each of these zones through 

nonlinear regression.

Calibration Procedure

Parameter values representing aquifer properties were 

adjusted through steady-state simulations to produce a model 

that approximated water levels measured in July 1996. Transient-state simulations were unable to accurately reproduce 

water levels measured during El Niño, so a shorter (22-month) 

simulation that approximated water levels and net ground-water 

flow from July 1996 to April 1998 was used to estimate an alternate set of aquifer properties. 

Model parameters were estimated through the same nonlinear regression method used for the Round Lake model. Initial 

estimates were specified for nine parameters (table 5), six of 

which were estimated through nonlinear regression. The other 

three parameters (vertical hydraulic conductivity, specific storage, and specific yield of the surficial aquifer) were too insensitive to estimate through the regression. Model cells representing 

open water in Halfmoon and Little Halfmoon Lakes and an 

unnamed lake were assigned a large horizontal and vertical 

hydraulic conductivity (10,000 ft/d) in model layers 1 to 3, as in 

Round Lake simulations. The effective hydraulic-conductivity 

value of lake sediments was computed using Darcy’s law and 

the results of steady-state simulations, as described previously.
HML19

E E’

ROWS

98

5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

85

90

95

1

COLUMNS

1 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 86

Lake

LeClare

Little

Halfmoon

Lake

Halfmoon Lake

Rocky Creek

HML9

HML12

HML5

HML2

HML16

EXPLANATION

INACTIVE CELL E E’

LOW PERMEABILITY ZONE

HIGH PERMEABILITY ZONE

SPECIFIED HEAD

DRAIN CELL (Rocky Creek)

LAKE CELL

BREACH IN CONFINING

UNIT (layer 7)

LINE OF SECTION SHOWN

ON FIG. 20

OBSERVATION WELL AND

NAME

0 0.25 0.5 MILE

0 0.25 0.5 KILOMETER

N

HML4

HML1

Figure 19. Boundary conditions and hydrologic features of the ground-water-flow model of the Halfmoon Lake basin.

Halfmoon Lake Basin Ground-Water Flow Simulation 29
30 Simulated Effects of Ground-Water Augmentation on the Hydrology of Round and Halfmoon Lakes

Table 5. Optimum values of aquifer properties estimated for the Halfmoon Lake model area through nonlinear regression in steadystate and transient-state simulations, and error in predicted heads and flows

[Shading indicates estimated parameters; ft/d, feet per day; ft, feet; ft2, feet squared; NA, not applicable]

Parameter

Model 

HML-S:

steady-state

Approximate 

individual 

95-percent

confidence interval

Coefficient 

of 

variation1

(percent)

1

Reported for log-transformed parameter.

Model 

HML-T:

transient

Coefficient 

of 

variation1

(percent)

Aquifer property

Horizontal hydraulic conductivity of surficial aquifer, ft/d

Low permeability zone 3.9 2.3 - 6.9 20 6.1 6

High permeability zone 17.5 9.4 - 33 10 17.5

Vertical hydraulic conductivity, ft/d

Surficial aquifer 2 2

Lake sediment2

2

Lake sediment, intermediate confining layer and breaches specified as isotropic.

3

2.6 × 10-3

3

Effective value computed from Darcy’s law.

3

3.9 × 10-3

Intermediate confining unit2

Beneath low permeability zone 7.5 × 10-4 6.5 × 10-4 - 8.5 × 10-4 0.8 1.3 × 10-3 0.6

Beneath high permeability zone 5.5 × 10-4 5 × 10-4 - 6 × 10-4 0.5 1.2 × 10-3 0.4

Breaches in intermediate confining unit2 2.2 × 10-2 4.85 × 10-3 - 1.1 × 10-1 14 4.6 × 10-3 9

Storage—all units

Specific yield NA 0.27

Specific storage, ft-1 NA 1.2 × 10-6

Model error

Sum of squared errors in heads and flows, ft2 8.7 1,531

Standard error, ft

Heads 0.6 1.4

Flows 0.2 4.3

ALTITUDE, IN FEET ABOVE NGVD OF 1929

land surface

50

45

40

35

30

25

20

15

10

5

0

Vertical exaggeration X 60

E E’ Rocky Creek

Halfmoon Lake

EXPLANATION

INACTIVE CELL

LOW PERMEABILITY CELL

HIGH PERMEABILITY CELL

SPECIFIED HEAD

ROCKY CREEK

HALFMOON LAKE

INTERMEDIATE CONFINING UNIT

BREACH IN CONFINING UNIT

LAKE SEDIMENT

model layer

1

2

3

4

5

6

7

8

surficial aquifer

intermediate

confining unit

Upper Floridan

aquifer

Figure 20. Model layers used to represent surficial and Upper Floridan aquifers in the Halfmoon Lake basin.
Halfmoon Lake Basin Ground-Water Flow Simulation 31

Results of steady-state simulations were compared with 

water levels measured in 25 wells and Halfmoon Lake, and 

baseflow to Rocky Creek at the model boundary estimated from 

the relation between baseflow and drainage area computed for 

the gaging station at Gunn Highway (fig. 1). Results of transient-state simulations were compared with 566 water levels 

measured at 26 surficial aquifer wells and in Halfmoon Lake 

during the 22-month period, and monthly net ground-water 

flow rates estimated from water budgets. Observations were 

weighted in the regression such that the residual in the head and 

flow measurements (difference between observed and computed values) were the same order of magnitude, as in Round 

Lake simulations.

Simulation Results

Optimum Parameter Values

Parameter values estimated by nonlinear regression for six 

of the seven aquifer properties in the steady-state simulation 

(model HML-S) are listed in table 5. The residuals are normally 

distributed based on the R2

N statistic defined by Hill (1992), 

but the model is not effectively linear, based on the modified 

Beale’s measure (Cooley and Naff, 1990; Hill, 1994). The 

reported statistics are only approximate, therefore, because the 

model does not meet all the required assumptions, but they 

indicate qualitatively the relative reliability of the optimum 

parameter values.

Coefficients of variation for five of the parameter values, 

estimated by nonlinear regression with model HML-S, range 

from 0.5 to 20 percent, indicating that the optimum values are 

not as accurately estimated as were parameters for the Round 

Lake model (0.5-7 percent, table 3). No correlation was apparent among the estimated parameters. The model is most sensitive to the parameter values estimated for the vertical hydraulic 

conductivities of the intermediate confining unit, and there is a 

small difference in the values estimated for the low and high 

conductivity zones. In contrast, there is a significant difference 

in the estimated values for the horizontal hydraulic conductivities for the zones, indicating that a difference in hydraulic 

conductivity is a possible cause of the differences in measured 

water-table fluctuations. The vertical hydraulic-conductivity 

value estimated for breaches in the intermediate confining 

unit is about 34 times the value estimated for the remainder 

of the unit.

Optimum parameter values obtained by regression with 

the transient-state simulation (model HML-T) for the surficial 

aquifer and lake sediments are similar to those estimated with 

the steady-state simulation (table 5). The largest differences in 

the values estimated by the two regressions are in the vertical 

hydraulic conductivities of the intermediate confining unit. 

Regression results for model HML-T suggest that the intermediate confining unit is more permeable than indicated by model 

HML-S, and that breaches in the confining unit are only four 

times more permeable than the remainder of the unit.

Estimated hydraulic conductivity of the surficial aquifer 

ranged from 4 to 6 ft/d for the zone surrounding Halfmoon Lake 

and extending south and west toward Rocky Creek, which is 

less than one-third the value that was estimated for the higher 

permeability zone north and east of the lake (18 ft/d). A value 

for the higher permeability zone could not be estimated with 

model HML-T, because the model was relatively insensitive to 

values greater than 18 ft/d. The sharp contrast in estimated 

hydraulic-conductivity values produced a relatively flat hydraulic gradient from Lake LeClare to Little Halfmoon Lake that 

steepened sharply near the shore of Halfmoon Lake, closely 

reproducing measured water levels in July 1996. 

Vertical hydraulic conductivity of the surficial aquifer 

(2.4 ft/d) was specified from values used in the Round Lake 

model and computed from the estimated leakance using equation 5. Intermediate confining unit sediments and lake sediments are specified as isotropic in both models HML-S and 

HML-T.

The effective hydraulic conductivity of the lake sediments 

was estimated from the results of steady-state simulations of a 

hypothetical scenario based on conditions in July 1996. An 

augmentation rate of 6,000 ft3

/d was specified to the lake in the 

simulations to ensure that outflow from the lake occurred 

around the entire perimeter, thereby allowing an accurate calculation of the magnitude of flow. The effective value was computed from Darcy’s law (eq. 6), as discussed previously for the 

Round Lake simulations. Estimated values for models HML-S 

and HML-T were 2.6x10-3 and 3.9x10-3 ft/d, respectively.

Water Levels

Water levels in the surficial aquifer in July 1996 were near 

average for the 3-year period. Heads computed by steady-state 

simulation with model HML-S (fig. 21A) indicate groundwater flowed southwest from Lake LeClare toward Halfmoon 

Lake and Rocky Creek. Breaches in the confining unit diverted 

ground-water flow beneath Halfmoon Lake, and the computed 

vertical distribution of head (fig. 21B) indicates predominantly 

downward flow beneath the lake toward the Upper Floridan 

aquifer. Simulated hydraulic heads indicate that water in the 

surficial aquifer flows toward Halfmoon Lake, reflecting both 

the effect of breaches in the confining layer beneath the lake and 

the net loss of water from the lake surface through evaporation 

during average conditions. Residual plots indicate that heads 

computed with model HML-S generally match the measured 

water levels reasonably well (figs. 21A and 22A), but there is 

some bias in model results, as heads are generally overpredicted 

by up to 1 ft. The standard error in computed heads was 0.6 ft 

and the range of measured heads was 42 to 50 ft.

Hydrographs of simulated water levels (fig. 23) indicate 

that model HML-T generally reproduces measured water-level 

fluctuations during average conditions (July 1996 to October 

1997), but cannot accurately reproduce the rise in water levels 

during El Niño (starting November 1997), especially in areas 

more than 400 ft from the lake. Simulated water levels in 
40

0.04

0.05

0.27

0.19

0.39

0.34

-1.35

-0.03

-1.15 -1.17

-0.56

-1.24

-0.91

-1.67

-0.80 -1.02

-0.66

E E’

Little

Halfmoon

Lake

Halfmoon Lake

Rocky Creek

EXPLANATION

INACTIVE CELL E E’

LOW PERMEABILITY ZONE

HIGH PERMEABILITY ZONE

LAKE CELL

SPECIFIED HEAD

DRAIN CELL (Rocky Creek)

SIMULATED WATER

TABLE—Contour interval 1

foot. Datum is NGVD of 1929

LINE OF SECTION SHOWN

ON FIG. 21B

MONITORING WELL AND

WATER-LEVEL RESIDUAL,

IN FEET—

,

blue numbers are

under predicted red numbers

are over predicted

41

42 39

38

43

44 45

46

47

48

49

44

45

46

42

44

48

47

49

50

44 -1.17

0.34

Lake

LeClare

0 0.25 0.5 MILE

0 0.25 0.5 KILOMETER

A N

Figure 21A. Water table during average conditions (July 1996), computed by steady-state simulation with model 

HML-S, of the surficial aquifer near Halfmoon Lake: plan view with residual error.

32 Simulated Effects of Ground-Water Augmentation on the Hydrology of Round and Halfmoon Lakes
ALTITUDE, IN FEET ABOVE NGVD OF 1929

land surface

50

45

40

35

30

25

20

15

10

5

0

Vertical exaggeration X 60

E E’ 48 47 46 45 44 43 42 40414238 4343 42 41

Halfmoon Lake

Rocky Creek

EXPLANATION

INACTIVE CELL

LOW PERMEABILITY CELL

HIGH PERMEABILITY CELL

SPECIFIED HEAD

ROCKY CREEK

HALFMOON LAKE

INTERMEDIATE CONFINING

UNIT

BREACH IN CONFINING

UNIT

LAKE SEDIMENT

SIMULATED HYDRAULIC

HEAD Contour interval 1

foot. Datum is NGVD of 1929

SIMULATED WATER TABLE

—

48

B

Figure 21B. Water table during average conditions 

(July 1996), computed by steady-state simulation with 

model HML-S, of the surficial aquifer near Halfmoon 

Lake: section view.

WEIGHTED RESIDUAL, IN FEET

2

1

0

-1

-2

underpredicted overpredicted

B

SIMULATED WATER LEVEL, IN FEET

42 44 46 48 50

MEASURED WATER LEVEL, IN FEET

50

48

46

44

42

A

SIMULATED WATER LEVEL, IN FEET

42 44 46 48 50

Figure 22. Measured heads, simulated heads, and residuals, 

computed by steady-state simulation with model HML-S, of the 

surficial aquifer near Halfmoon Lake, July 1996: (A) measured 

and simulated values; (B) simulated values and residuals.

Halfmoon Lake Basin Ground-Water Flow Simulation 33
El Niño

El Niño

El Niño

El Niño

El Niño

El Niño

El Niño

El Niño

measured

simulated

WATER LEVEL, IN FEET ABOVE NGVD OF 1929

measured

simulated

measured

simulated

measured

simulated

measured

simulated

measured

simulated

measured

simulated

measured

simulated

A. Halfmoon Lake B. Well HML17

C. Well HML9 D. Well HML7

E. Well HML1 F. Well HML12

G. Well HML4 H. Well HML3

52

50

48

46

44

42

40

38

1996 1997 1998

52

50

48

46

44

42

40

38

1996 1997 1998

52

50

48

46

44

42

40

38

1996 1997 1998

52

50

48

46

44

42

40

38

1996 1997 1998

52

50

48

46

44

42

40

38

1996 1997 1998

52

50

48

46

44

42

40

38

1996 1997 1998

52

50

48

46

44

42

40

38

1996 1997 1998

52

50

48

46

44

42

40

38

1996 1997 1998

J A SOND J FM AM J J A SOND J FM A

J A SOND J FM AM J J A SOND J FM A

J A SOND J FM AM J J A SOND J FM A

J A SOND J FM AM J J A SOND J FM A

J A SOND J FM AM J J A SOND J FM A

J A SOND J FM AM J J A SOND J FM A

J A SOND J FM AM J J A SOND J FM A

J A SOND J FM AM J J A SOND J FM A

Figure 23. Water levels measured in the surficial aquifer near Halfmoon Lake and heads, computed by transient simulation with 

model HML-T, July 1996 to April 1998: (A) Halfmoon Lake; (B) well HML17; (C) well HML9; (D) well HML7; (E) well HML1; (F) well 

HML12; (G) well HML4; (H) well HML3. (Location of wells and screened intervals on page 35.)

34 Simulated Effects of Ground-Water Augmentation on the Hydrology of Round and Halfmoon Lakes
HML4 & 3

HML1

HML7

HML9

HML17

Little

Halfmoon

Lake

Halfmoon

Lake HML12

E'

E

HML17 HML9

HML3

HML12

HML7

HML1

HML4

50

45

40

35

30

25

ALTITUDE, IN FEET

ABOVE NGVD OF 1929

land surface

vertical exaggeration X 20

Little

Halfmoon

Lake

Halfmoon Lake

E E'

Figure 23. (Continued) Water levels measured in the surficial aquifer near Halfmoon Lake and heads, computed by transient 

simulation with model HML-T, July 1996 to April 1998: location of wells and screened intervals.

Halfmoon Lake Basin Ground-Water Flow Simulation 35

Halfmoon Lake display a dampened response to changes in 

recharge during the 22-month period--lake stage is overpredicted by about 1 ft during the summer of 1997 and underpredicted by about 2 ft in December 1997 shortly after the beginning of El Niño. Residual plots for model HML-T (fig. 24) 

A

53

51

49

47

45

43

41

39

MEASURED WATER LEVEL, IN FEET

39 41 43 45 47 49 51 53

SIMULATED WATER LEVEL, IN FEET

6

4

2

0

-2

-4

-6

underpredicted

WEIGHTED RESIDU

overpredicted

AL, IN FEET

B

39 41 43 45 47 49 51 53

SIMULATED WATER LEVEL, IN FEET

Figure 24. Measured heads, simulated heads, and residuals, 

computed from transient-state simulation with model HML-T, of 

the surficial aquifer near Halfmoon Lake, July 1996 to April 1998: 

(A) measured and simulated values; (B) simulated values and 

residuals.

indicate absolute errors as much as 4 ft in heads greater than 

45 ft that were measured in areas upgradient from Halfmoon 

Lake. The standard error in computed heads was 1.4 ft and the 

range of measured heads was 39 to 53 ft.

Discrepancies between measured and simulated hydrographs could result from the monthly recharge rates specified in 

the simulation, possibly reflecting differences in rainfall 

received at Halfmoon Lake and at the daily precipitation gages 

(the Section 21 and Cosme well fields) used to compute 

recharge. Comparison of simulated and measured hydrographs 

for the 32-month period from July 1996 to February 1999 

(fig. 25), however, indicates that discrepancies probably result 

from another source of model error. Simulated water levels for 

Halfmoon Lake (fig. 25A) match the measured levels reasonably well, with the exception of peak lake stages during El Niño 

and the decline in stage during La Niña. Moreover, simulated 

water levels for wells near the lake (for example, HML4, 

fig. 25B) agree closely with measured levels. Simulated water 

levels for well HML1 900 ft south of the lake (fig. 25C), however, are overpredicted by as much as 10 ft during El Niño, a 

pattern that also is apparent for several other wells north and 

east of the lake. Repeated attempts were made to improve the 

match in these areas during model calibration by testing different sets of parameter values, including additional breaches in 

the confining layer and changing the boundaries of hydraulic 

conductivity zones specified in the model. None of these 

attempts, however, were able to duplicate aquifer conditions 

during both average conditions and El Niño.
J ASOND J FMAMJ J ASOND J FMAMJ J ASOND J F

1996 1997 1998 1999

52

50

48

46

44

42

40

38

WATER LEVEL, IN FEET ABOVE NGVD OF 1929

J ASOND J FMAMJ J ASOND J FMAMJ J ASOND J F

1996 1997 1998 1999

52

50

48

46

44

42

40

38

El Niño

C. Well HML1

B. Well HML4

measured

simulated

J ASOND J FMAMJ J ASOND J FMAMJ J ASOND J F

1996 1997 1998 1999

A. Halfmoon Lake

52

50

48

46

44

42

40

38

measured

simulated

measured

simulated

El Niño

El Niño

Figure 25. Water levels measured in the surficial aquifer near 

Halfmoon Lake and heads, computed by transient-state simulation with model HML-T, July 1996 to February 1999: (A) Halfmoon 

Lake; (B) well HML4; (C) well HML1. (Well locations shown on 

figure 19.)

36 Simulated Effects of Ground-Water Augmentation on the Hydrology of Round and Halfmoon Lakes

Specified heads at the boundary representing the Upper 

Floridan aquifer largely control water levels in the surficial 

aquifer. Near the lake, breaches in the confining unit enhance 

the hydraulic connection between the aquifers, and simulated 

water levels for wells located in this area more closely match 

the measured levels. In areas distant from the lake, the 

simulated hydraulic connection between the aquifers through 

the intermediate confining layer is less permeable. The leakance 

value of the confining unit for these areas was adjusted in model 

simulations to produce water levels that matched those 

observed during the relatively dry conditions. The subsequent 

rise in water levels observed during El Niño was much less than 

simulated, suggesting that a large part of the rainfall was 

diverted through overland runoff to drainage channels before it 

could become recharge--this possibility was not considered in 

the estimation of recharge through unsaturated flow simulations 

with LEACHM.

Water Budget

The water budget for the entire modeled area computed by 

steady-state simulation (model HML-S) for average conditions 

in July 1996 (table 6) indicates that recharge was the principal 

source of inflow and that discharge to the Upper Floridan 

aquifer accounted for most of the outflow (88 percent). About 

5 percent of the discharge flowed through breaches in the 

intermediate confining unit and about 11 percent flowed to 

Rocky Creek. The computed ground-water discharge to Rocky 

Creek (9,700 ft3/d) agrees with the estimated baseflow 

(10,000 ft3/d). The variability in recharge is evident in monthly 

basin budgets computed for the simulated period (fig. 26). Most 

of the recharge was temporarily stored in the surficial aquifer 

and then released during succeeding months, as in simulations 

for Round Lake.

Table 6. Simulated water budget for the Halfmoon Lake model 

area under average conditions, July 1996, model HML-S

[Flow rates are in thousands of cubic feet per day; <, less than] 

Inflow Discharge

Source Rate

Percentage of 

total

Location Rate

Percentage of 

total

Recharge 85.4 99 Net precipitation 

from Halfmoon 

Lake

0.3 < 1

Leakage 

from Lake 

LeClare

0.9 1 Rocky Creek 9.7 11

Downward leakage

Intermediate confining unit72.2 83

Breaches in confining unit4.1 5

TOTAL 86.3 100 TOTAL 86.3 100
average conditions El Niño

J ASOND J FMAMJ J ASOND J FMA

1996 1997 1998

1,500

1,000

500

0

INFLOW, IN 1,000 CUBIC FEET PER DAY

0

500

1,000

1,500

OUTFLOW, IN 1,000 CUBIC FEET PER DAY

J ASOND J FMAMJ J ASOND J FMA

1996 1997 1998

INFLOWS

recharge

storage

NOTE: leakage from Lake LeClare less than 3 percent

OUTFLOWS

storage

leakage to Upper Floridan aquifer

evaporation

Rocky Creek

average conditions El Niño

Figure 26. Simulated water budget for the Halfmoon Lake model area, July 1996 to April 1998.

Halfmoon Lake Basin Ground-Water Flow Simulation 37
38 Simulated Effects of Ground-Water Augmentation on the Hydrology of Round and Halfmoon Lakes

Net ground-water flows to and from 

Halfmoon Lake computed by transient-state simulation with model 

HML-T (fig. 27)

average conditions El Niño

FLOW

estimated from water budget

(Metz and Sacks, 2002,

table 3)

simulated

Error in water budget estimate

Error = 135,000

NET LEAKAGE, IN 1,000 CUBIC FEET PER DAY

OUTFLO

W INFLOW

90

80

70

60

50

40

30

20

10

0

-10

-20

-30

-40 J ASOND J FMAMJ J ASOND J FMA

1996 1997 1998

Figure 27. Simulated net leakage rates to and from Halfmoon Lake, computed from 

transient-state simulation with model HML-T, and estimated rates from monthly water 

budgets, July 1996 to April 1998.

 reproduce the general pattern of inflows and outflows to 

and from the lake as seen in the water 

budget. Simulated flows are underpredicted, however, particularly during El Niño when overland flow (not 

accounted for in the model) could 

have accounted for some of the 

inflow to the lake. Residuals display 

wide scatter, but no consistent pattern 

of bias is apparent (fig. 28), and the 

standard error in flow rates was 4.3 ft.

Simulated Effects of 

Lake Augmentation

The calibrated models developed for 

the Round Lake and Halfmoon Lake 

study areas were used in a series of 

subsequent simulations to delineate 

the extent of lake leakage into the 

surficial aquifer and to assess the 

effects of potential augmentation 

schedules on the maintenance of lake 

stages. Leakage of lake water was 

mapped in solute transport simulations of steady-state conditions under 

average recharge rates. Alternative 

augmentation schedules were computed from the relations between lake 

stages, augmentation rates, and Upper 

Floridan aquifer heads identified in 

steady-state simulations, and then 

applied in transient-state simulations 

of the study period.

Round Lake

The stage in Round Lake typically is 

maintained 5 ft above the sloping 

water-table surface by augmenting 

the lake with water pumped from the 

Upper Floridan aquifer. Downward 

leakage of lake water was simulated 

with the solute-transport model 

MT3D (Zheng and Wang, 1998) to 

delineate the extent of lake leakage in 

the surficial aquifer. Augmentation 

rates were adjusted monthly to 

account for seasonal variation in 
Simulated Effects of Lake Augmentation 39

recharge and changing water levels in the Upper Floridan 

aquifer to maintain a constant stage in Round Lake. A schedule 

of monthly augmentation rates, derived from the results of 

steady-state flow simulations, was applied in the transient-state 

simulation from July 1996 to February 1999 to assess the 

potential for using a prescribed augmentation schedule to 

maintain a constant lake stage.

B

10

8

6

4

2

0

-2

-4

-6

-8

-10

WEIGHTED RESIDUAL,

IN CUBIC FEET PER DAY

underpredicted overpredicted

-100 -80 -60 -40 -20 0 20 40

SIMULATED LEAKAGE, IN 1,000 CUBIC FEET PER DAY

-100 -80 -60 -40 -20 0 20 40

SIMULATED LEAKAGE, IN 1,000 CUBIC FEET PER DAY

A

40

20

0

-20

-40

-60

-80

-100

MEASURED LEAKAGE,

IN 1,000 CUBIC FEET PER DAY

Figure 28. Simulated leakage rates and residuals, computed from 

transient-state simulation with model HML-T, and estimated rates 

from monthly water budgets, July 1996 to April 1998: (A) measured 

and simulated values, (B) simulated values and residuals.

Distribution of Lake Leakage

The solute-transport model was constructed to simulate 

advection and dispersion of leakage from Round Lake during 

1 year under average conditions observed from July 1996 to 

October 1997. The model used the hydraulic-head distribution 

computed by a steady-state flow simulation with model RL-A 

(mean recharge rate of 20 in/yr). Water in Round Lake was represented by a constant concentration boundary, and water 

pumped from the Upper Floridan aquifer was represented by a 

constant flux boundary with a mean augmentation rate of 

26,000 ft3

/d (135 gal/min). An arbitrary concentration of 

100 mg/L was specified at both these boundaries so that simulated concentrations are equivalent to the percentage of lake 

water within a model cell. The numerical solution for the advective-dispersion equation was obtained using the method of characteristics (Zheng and Wang, 1998) to minimize the effects of 

numerical dispersion on model results.

Transport simulations considered dispersivity values of 0 

and 50 ft. Leakage of lake water reached the intermediate confining unit about 150 days after the start of the simulation of 

advective transport in which zero dispersion was specified 

(fig. 29A). Most of the leakage exited from the perimeter of the 

lake and did not mix with water directly beneath the lake 

sediments until later in the simulation. The concentration 

distribution reached steady state after about 180 days when lake 

leakage extended about 250 ft from the lakeshore (fig. 29B).
EXPLANATION

100 50 1

LAKE WATER PERCENT

A N

Figure 29A. Extent of lake leakage from Round Lake after 150 days (perspective view), as computed by steady-state 

simulation with solute-transport model with no dispersion.

40 Simulated Effects of Ground-Water Augmentation on the Hydrology of Round and Halfmoon Lakes

Lake leakage did not reach several wells that surround the 

lake, a result that is in good agreement with data presented in 

Metz and Sacks (2002) on concentrations of stable isotopes 

measured in ground water from the surficial aquifer in the fall 

of 1997. Stable isotope concentrations of oxygen-18 (δ18O) and 

deuterium (δD) in ground water that is unaffected by lake leakage plot along the meteoric waterline (MWL), whereas δ18O 

and δD concentrations in waters containing lake leakage plot 

along a mixing line extending between Round Lake water and 

the MWL (fig. 30). Stable-isotope concentrations of Round 

Lake water are enriched in δ18O as a result of evaporation and 

plot below the MWL (Metz and Sacks, 2002). Three of the five 

wells where lake water was not detected (RL8, RL11, and 

RL12) are located beyond the extent of lake leakage, as delineated by the transport simulation (fig. 29B). The transport simulation indicates that one of the other wells, RL9 located about 

100 ft west of the lake, contains more lake water (30 percent) 

than indicated by the stable-isotope concentrations (10 percent). 

Water sampled from the remaining well, RL3 located less than 

100 ft from the lake, plots along the MWL and probably reflects 

recent recharge from precipitation.

Specifying longitudinal and transverse dispersivity values 

of 50 ft increased the extent of lake leakage and decreased the 

travel time to the intermediate confining unit. Lake water 

breached the intermediate confining unit within about 30 days, 

and a steady-state concentration distribution was reached after 

about 100 days when lake water extended about 400 ft from the 

lakeshore. Results of the transport simulation with dispersion 

also were consistent with the stable-isotope analyses, but lake 

water nearly reached wells RL8, RL11, and RL12, indicating 

that larger values of dispersivity would simulate lake leakage 

beyond areas where isotopic data indicated lake leakage.

Inspection of the distribution of lake leakage in steadystate simulations of average conditions using ZONEBUDGET 

indicated that 31 percent of the total leakage (26,000 ft3

/d) 

flowed horizontally through the shallow perimeter of the lake in 

model layer 1, whereas the remainder flowed vertically through 

the lake bottom represented by deeper model layers. A similar 

analysis of the transient simulation results with model RL-A 

indicates that a nearly equal percentage (29 percent) of the total 

leakage (29,700 ft3/d) flowed through the lake perimeter in June 

1997. Only 2 percent of the total leakage (6,000 ft3

/d) flowed 

through the lake perimeter in March 1998, however, when the 

lateral gradients within 500 ft of the lake (1.1x10-3, fig. 13A) 

were much smaller than in June 1997 (1.3x10-2, fig. 12A). 

Potential Augmentation Schedules

Relations between Round Lake stage, heads in the Upper 

Floridan aquifer, and augmentation rates were defined for average conditions (July 1996 to October 1997) using a series of 

steady-state simulations with ground-water-flow model RL-A 

(mean recharge rate of 20 in/yr). The simulations considered 

the range in Upper Floridan aquifer heads measured during the 
EXPLANATION

100 50 1

LAKE WATER PERCENT

RL11

RL12

RL4,5,6

RL1,2,3,22,23

RL8

LAKE

ACTIVE CELL

OBSERVATION WELL AND NUMBER

Crenshaw Lake

Saddleback Lake

Starvation Lake

Crum Lake

RL9

RL8

0 0.25 0.5 MILE

0 0.25 0.5 KILOMETER

B N

Figure 29B. Extent of lake leakage from Round Lake after 150 days (plan view), as computed by steady-state 

simulation with solute-transport model with no dispersion.

Simulated Effects of Lake Augmentation 41
EXPLANATION

Round Lake

Upper Floridan aquifer

surficial aquifer:

affected by lake leakage

not affected by lake leakage

RL3

RL11

RL12

RL8

RL9

RL2

RL6

RL1

RL22

RL23

RL4,5

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

DELTA O-18, IN PER MIL

15

10

5

0

-5

-10

-15

-20

-25

-30

-35

mixing line

DELTA D, IN PER MIL

Meteoric water line

Figure 30. Relation between stable isotopes of oxygen-18 and deuterium in Round Lake and ground water sampled from 

wells screened in the surficial aquifer and the Upper Floridan aquifer.

42 Simulated Effects of Ground-Water Augmentation on the Hydrology of Round and Halfmoon Lakes

study (34-44 ft) and augmentation rates that ranged from 0 to 

50,000 ft3/d (260 gal/min). The simulated Round Lake stage is 

proportional to both the Upper Floridan aquifer head and the 

augmentation rate, suggesting that under constant recharge conditions, the lake stage can be maintained by adjusting the augmentation rate to account for changing water levels in the Upper 

Floridan aquifer. For example, a lake stage of 52 ft can be 

attained with an Upper Floridan aquifer head of 38 ft by an augmentation rate of 30,000 ft3/d (160 gal/min) under average 

recharge conditions (fig. 31).

Round Lake stage

50

52

54

0 2 4 6 8 10

34 44 36 38 40 42

WATER LEVEL IN UPPER FLORIDAN AQUIFER,

IN FEET ABOVE NGVD OF 1929

AUGMENTATION RATE,

IN 1,000 CUBIC FEET PER DAY

50

40

30

20

10

0

DIFFERENCE ( h) BETWEEN UPPER FLORIDAN AQUIFER

HEAD AND MINIMUM HEAD (34), IN FEET

∆

Figure 31. Relations between Round Lake stage, heads in the Upper Floridan aquifer, and augmentation rate, as computed by 

steady-state simulation with model RL-A (high recharge).

 The augmentation rate (Q)

required to maintain a specified lake stage can be computed 

from the following equation:

Q bm = + ∆h , (7)

where

b is the ordinate intercept on figure 31 that corresponds 

to the target lake stage, in cubic feet per day;

m is -2,250 ft3/d per foot, the average slope of the 

relations shown in fig. 31; and

∆h is the height of the Upper Floridan head above the 

minimum level measured during the study (34 ft).
Simulated Effects of Lake Augmentation 43

Monthly augmentation rates were computed using the following equation to maintain a constant lake stage of 52 ft during 

transient-state simulation during July 1996 to February 1999:

Qi b m ∆hi 1 Ri – 1 Ra = + [ ] + ( ) – ⁄ , (8)

where

Qi is the augmentation rate in month i, in cubic feet per 

day;

b is 38,850 ft3/d;

∆hi is the height of the Upper Floridan head above 34 ft in 

the current month, in feet;

Ri-1 is the monthly recharge rate in the previous month, in 

feet per day; and

Ra is the average monthly recharge rate, 0.00458 ft/d 

(20 in/yr).

The first term in square brackets in equation 8 adjusts the 

augmentation rate for the Upper Floridan aquifer head. The second term is an empirical expression that accounts for deviation 

in monthly recharge from the mean recharge rate used to derive 

the relations shown in fig. 31. The augmentation rates required 

to attain other desired lake stages can be computed by substituting in equation 8 the appropriate values for b derived from 

inspection of fig. 31. The m-values associated with different 

lake stages vary slightly, but are nearly constant throughout the 

range of Upper Floridan aquifer heads considered.

Transient-state simulations conducted with model RL-A 

using augmentation rates Qi computed with equation 8, produced stages ranging between 52 and 53 ft in Round Lake during average conditions (July 1996 to October 1997), but yielded 

stages that were greater than 54 ft after the start of El Niño in 

November 1997, when the recharge rate more than doubled to 

50 in/yr (fig. 32).

El Niño

measured

no augmentation

simulated lake level with augmentation

rates computed for the following

recharge conditions:

average conditions (20 in/yr)

El Niño conditions (50 in/yr)

J ASOND J FMAMJ J ASOND J FMAMJ J ASOND JF

1996 1997 1998 1999

60

58

56

54

52

50

48

46

44

42

40

WATER LEVEL, IN FEET ABOVE NGVD OF 1929

Figure 32. Water levels measured in Round Lake, July 1996 to February 1999, and water levels computed by transientstate simulation by model RL-A (high recharge) with prescribed augmentation schedule derived for recharge under average and El Niño conditions (20 and 50 inches per year, respectively).

 An additional transient-state simulation 

was run in which Qi values for November 1997 to February 

1998 were computed using m and b values derived from steadystate simulations of El Niño conditions (mean recharge rate of 

50 in/yr). In this simulation, Round Lake stage only exceeded 

54 ft when recharge was highest in February and March 1998 

(fig. 32). The simulated volume of water pumped from the 

Upper Floridan aquifer (564,000 ft3

) nearly equaled the actual 

pumped volume (554,000 ft3). A third simulation, in which no 

augmentation was applied to Round Lake during the entire 

study, predicted that lake stage would have fallen below 45 ft 

during 1997, but would have recovered to nearly 50 ft during 

El Niño.
44 Simulated Effects of Ground-Water Augmentation on the Hydrology of Round and Halfmoon Lakes

These results indicate that a transient pumping schedule 

could be implemented to maintain the lake stage within about 

1 ft of a predetermined elevation. The selection of the 52-ft 

stage in the simulation was arbitrary and slightly below the 

Round Lake guidance stage (53.26 ft) recommended by the 

SWFWMD (Michael Hancock, oral commun., 2002). The augmentation rate could be computed from the relations depicted in 

fig. 31 and monthly measurements of head in the Upper Floridan aquifer. Monthly augmentation rates computed using equation 8 would require estimates of monthly recharge. These 

recharge estimates could be computed from measurements of 

daily precipitation using the unsaturated flow model LEACHM, 

but might be difficult on a real-time basis. Simulation of 

El Niño conditions indicates that substantial changes in climatic 

conditions that cause increased precipitation would also have to 

be accounted for in the computation of augmentation rates.

An attempt was made to simulate a fluctuating stage in 

Round Lake to mimic natural conditions by varying the augmentation rates on a seasonal basis, but the transient simulation 

was unable to reproduce the desired pattern of smoothly raising 

and lowering the lake stage over the 2.5-ft range specified by 

the SWFWMD. The model-derived dependence of lake stage 

on Upper Floridan head is affected by recharge, as noted previously, and the method used to compute augmentation rates 

accounts for changes in recharge in an empirical manner. The 

method does reasonably well in computing rates to maintain a 

constant lake stage, but is not accurate enough to reproduce 

more complicated patterns, such as seasonal fluctuations.

Halfmoon Lake

Steady-state simulations of average conditions in July 

1996 also were used to delineate the extent of lake leakage in 

the surficial aquifer around Halfmoon Lake, and to devise a 

schedule of monthly augmentation rates to maintain lake stage 

at a constant prescribed level. Leakage of lake water was simulated with the solute-transport model MT3D (Zheng and Wang, 

1998) as in the Round Lake model. Monthly augmentation 

rates, adjusted to correct for seasonal variation in recharge and 

changing water levels in the Upper Floridan aquifer, were 

applied in transient-state simulations during July 1996 to 

February 1999.

Distribution of Lake Leakage

The solute-transport model simulated advection and dispersion of leakage from Halfmoon Lake for 2 years under average conditions using the hydraulic-head distribution computed 

for July 1996 by a steady-state flow simulation with model 

HML-S. Water in Halfmoon Lake was represented by an arbitrary constant concentration of 100 mg/L, as in the Round Lake 

model, so that simulated concentrations were equivalent to the 

percentage of lake water within a model cell. 

Transport simulations considered dispersivity values 

ranging from 0 to 50 ft. Leakage of lake water breached the 

intermediate confining unit about 400 days after the start of the 

simulation in which zero dispersion was specified (fig. 33A,B).

EXPLANATION

100 50 1

LAKE WATER PERCENT

A

N

Figure 33A. Extent of lake leakage after 400 days from Halfmoon Lake (perspective view), as computed by steady-state 

simulation with solute-transport model with no dispersion.

The concentration distribution reached steady state after about 
Simulated Effects of Lake Augmentation 45

2 years when lake leakage extended about 75 ft from the lakeshore (fig. 33B). Specifying longitudinal and transverse dispersivity values of 50 ft increased the extent of lake leakage and 

decreased the travel time to the intermediate confining unit. 

Lake water breached the intermediate confining unit within 

about 40 days, and a steady-state concentration distribution was 

reached after about 300 days, when lake water extended less 

than 150 ft from the lakeshore.

HML26

HML11 ROWS

25

30

35

40

45

50

55

60

65

70

75

COLUMNS

15 20 25 30 35 40 45 50 55 60 65 70

Little

Halfmoon

Lake

Halfmoon

Lake

Rocky Creek

HML9

EXPLANATION

LOW PERMEABILITY ZONE

HIGH PERMEABILITY ZONE

DRAIN CELL (Rocky Creek)

LAKE CELL

OBSERVATION WELL AND NAME

79

HML24

HML4

HML10

HML29

HML28

HML25

HML27

HML7

100 50 1

LAKE WATER PERCENT

0 0.25 0.5 MILE

0 0.25 0.5 KILOMETER

N

HML2

HML5

HML11

B

Figure 33B. Extent of 

lake leakage after 400 

days from Halfmoon Lake 

(plan view), as computed 

by steady-state simulation with solute-transport 

model with no dispersion.

Results of the transport simulations generally were consistent with the stable isotope analyses of δ18O and δD in ground 

water near Halfmoon Lake. Metz and Sacks (2002) reported no 

evidence of lake leakage in stable-isotope concentrations in 

water sampled from wells along the lakeshore, suggesting that 

most of the lake leakage flows downward through the lake 

bottom. Solute transport simulations also indicate limited lateral movement of lake leakage, with less than 20 percent lake 

water computed for model cells adjacent to Halfmoon Lake. 

The predicted lateral movement of lake leakage is partially an 

artifact of the model design, because lake sediments are only 

represented beneath the lake in the flow model. The relatively 

coarse vertical discretization (5 ft) does not allow accurate representation of the sloping bottom near the perimeter of the lake. 
46 Simulated Effects of Ground-Water Augmentation on the Hydrology of Round and Halfmoon Lakes

Specifying additional model layers to better represent the lake 

bathymetry caused solver convergence problems, as a substantial part of the modeled area was dewatered during transient 

simulation. As a result, computed flow paths exit horizontally 

from the lake, rather than downward as expected under actual 

conditions.

Inspection of the distribution of ground-water exchange 

with Halfmoon Lake in the steady-state simulation using 

ZONEBUDGET indicated slightly more inflow to the lake 

(3,120 ft3

/d) than outflow (2,920 ft3/d), because evaporation 

from the lake surface makes the lake a sink (discharge area) 

under average conditions. Nearly all the ground water discharging to the lake (96 percent) flowed though the shallow lake 

perimeter in model layer 1; the remainder flowed through 

deeper model layers representing the lake bottom. About 

17 percent of the leakage from the lake flowed though the lake 

perimeter, while the remainder flowed through the lake bottom.

Potential Augmentation Schedule

Relations between Halfmoon Lake stage, heads in the 

Upper Floridan aquifer, and augmentation rates were defined 

for average conditions in July 1996 using steady-state simulations of ground-water flow with model HML-S and a recharge 

rate of 13 in/yr. The simulations considered the range in Upper 

Floridan aquifer heads measured during the study (24-34 ft) 

and augmentation rates that ranged from 0 to 15,000 ft3/d 

(80 gal/min). The simulated Halfmoon Lake stage is proportional to both the Upper Floridan aquifer head and the augmentation rate, as in Round Lake, suggesting that under constant 

recharge conditions, fluctuations in lake stage can be minimized 

by adjusting the augmentation rate to correct for changing water 

levels in the Upper Floridan aquifer (fig. 34). An alternate set of 

steady-state simulations were conducted using aquifer properties estimated with the transient model HML-T. The augmentation rates required to maintain a specified lake stage under these 

conditions are greater than predicted with model HML-S 

(fig. 34), because the conductance of the intermediate confining 

unit is higher in these simulations.

Two sets of monthly augmentation rates were computed 

using equation 8 and the relations shown in figure 34 to maintain the Halfmoon Lake guidance stage of 43 ft (Michael 

Hancock, oral commun., 2002) during transient-state simulations from July 1996 to February 1999. Both sets of augmentation rates produced relatively constant lake stages during average conditions (July 1996 to October 1997), but lake stages 

diverged from the target level during El Niño when recharge 

rates increased dramatically (fig. 35), as in the Round Lake simulations. Predicted lake stages of both sets of augmentation 

rates were within 1 ft of the target stage, but slightly lower for 

the set computed with aquifer properties estimated by model 

HML-S (steady-state simulation), and slightly higher with 

properties estimated with model HML-T (transient-state simulation). Simulated volumes of water pumped from the Upper 

Floridan aquifer for 16-months (July 1996 to October 1997) 

were 37,000 ft3

 and 180,000 ft3 for rates computed with model 

HML-S and HML-T, respectively. Additional water-level data 

documenting the response of lake stage and ground-water levels 

to lake augmentation are required to improve the estimates of 

the volume of water needed to maintain Halfmoon Lake stage 

for the observed range in Upper Floridan aquifer heads. Additional simulations of Halfmoon Lake during El Niño and 

La Niña were unwarranted due to the inability of the models to 

accurately reproduce water levels during these periods.

Comparison of Lake Leakage

Results of steady-state model simulations indicate substantially more lake leakage flows to the Upper Floridan aquifer 

from Round Lake than from Halfmoon Lake. The estimated 

vertical hydraulic conductivities (Kv) of lake and intermediate 

confining unit sediments and breaches in the confining unit are 

much larger for the Round Lake area than for the Halfmoon 

Lake area, resulting in larger leakage rates. The estimated Kv 

of lake sediments in Round Lake (0.2 ft/d) is about 

70 times greater than the values estimated for Halfmoon 

Lake (3x10-3 ft/d). Similarly, Kv values estimated for the confining unit and breaches in the unit are about 10 times greater at 

Round Lake (6x10-3 and 0.2 ft/d, respectively) than Halfmoon 

Lake (6x10-4 and 0.02 ft/d, respectively). The relative permeability of breaches in the confining unit is about 30 times greater 

than the Kv value of the rest of the confining unit in both models, 

although the ratio was estimated to be 10 times smaller by 

regression with transient-state simulations of Halfmoon Lake.

Steady-state simulations of average conditions in June 

1997 indicate that about 30 percent of the 26,000 ft3/d of the 

water pumped into Round Lake leaks through the shallow lake 

perimeter as a result of the steep lateral hydraulic gradients 

(1.3x10-2) surrounding the lake. Under similar conditions at 

non-augmented Halfmoon Lake, only 17 percent of the total 

lake leakage of 2,920 ft3/d flows through the lake perimeter, as 

lateral gradients surrounding the lake (5.6x10-4) are much lower 

than those near Round Lake. Nearly all (96 percent) of the 

ground water entering Halfmoon Lake (3,120 ft3/d), however, 

flows through the shallow lake perimeter.

As a result of the higher lateral and vertical leakage losses 

from Round Lake, much higher augmentation rates are required 

to maintain a given lake stage. For example, steady-state simulations for July 1996 indicate that an augmentation rate of 

33,850 ft3

/d is required to maintain a stage of 53 ft in Round 

Lake (near the low guidance limit) at an Upper Floridan aquifer 

head of 38 ft. A comparable stage near the low guidance level 

of 41.4 ft in Halfmoon Lake requires a rate of 1,330 to 

10,000 ft3

/d under an Upper Floridan aquifer head of 28 ft. 

These augmentation rates are equivalent to about 26 in/mo of 

water applied to the entire surface of Round Lake, as opposed 

to 0.34 to 2.5 in/mo applied to Halfmoon Lake. 
40

42

44

Halfmoon Lake

stage

Model

HML-S HML-T

WATER LEVEL IN UPPER FLORIDAN AQUIFER,

IN FEET ABOVE NGVD OF 1929

AUGMENTATION RATE, IN 1,000 CUBIC FEET PER DAY

15

10

5

0

24 26 28 30 32 34

El Niño

measured

simulated lake level with augmentation

rates computed with the following

models:

HML-S, steady-state calibration

HML-T, transient calibration

J ASOND J FMAMJ J ASOND J FMAMJ J ASOND JF

1996 1997 1998 1999

55

53

51

49

47

45

43

41

39

37

35

WATER LEVEL, IN FEET ABOVE NGVD OF 1929

Figure 34. Relations between Halfmoon Lake 

stage, heads in the Upper Floridan aquifer, and 

augmentation rate, as computed by steadystate and transient-state simulation with 

models HML-S and HML-T, respectively.

Figure 35. Water levels measured in Halfmoon Lake, July 1996 to February 1999, and water levels computed by transient-state simulations using augmentation rates computed with aquifer properties estimated by models HML-S and 

HML-T, with prescribed augmentation schedule derived for recharge under average conditions (13 inches per year).

Simulated Effects of Lake Augmentation 47
48 Simulated Effects of Ground-Water Augmentation on the Hydrology of Round and Halfmoon Lakes

Summary

Pumping from the Upper Floridan aquifer has lowered 

water levels and induced downward leakage from the overlying 

surficial aquifer, lowering the water table and lake levels in 

some parts of northwestern Hillsborough County. A detailed 

description of the hydrogeology and water quality of three lake 

basins in northwest Hillsborough County was provided in a previous study wherein lake-water budgets were used to estimate 

net ground-water flow to the lakes (Metz and Sacks, 2002). 

Based on the results of the previous study, numerical simulations were prepared to quantify hydraulic parameters that 

control the flow system, and to estimate the lake augmentation 

rates required to maintain lake levels during periods of belowaverage precipitation.

Ground-water-flow models were developed with MODFLOW for the Round Lake and Halfmoon Lake study areas to 

represent flow conditions during the 32 months from July 1996 

to February 1999. Monthly estimates of ground-water recharge 

in the study areas were computed by an unsaturated flow model 

(LEACHM) that represented daily changes in storage of water 

in the soil profile and drainage to the water table. The groundwater-flow models were calibrated through nonlinear regression with UCODE to measured water levels, and monthly net 

ground-water-flow rates from the lakes were estimated from 

lake-water budgets. The calibrated models were used in a series 

of subsequent simulations to delineate the extent of lake leakage 

into the surficial aquifer and to assess the effects of potential 

augmentation schedules on maintaining lake stages.

Recharge through the soil profile was computed as daily 

drainage with the LEACHM model using specified rates of precipitation and potential evaporation. Three cases with different 

combinations of vegetation and soil texture were considered in 

unsaturated zone simulations: shallow-rooted (grass) and deeprooted (saw palmetto) vegetation with a coarse-textured soil, 

and shallow-rooted vegetation with a fine-textured soil.

Computed actual evapotranspiration (AET) rates for the 

coarse-textured soil under average conditions (April 1996 to 

October 1997) were 36 and 43 in/yr for shallow-rooted and 

deep-rooted vegetation, respectively, and compared favorably 

with the rate of 40 in/yr, which was estimated previously for dry 

prairie vegetation in Sarasota County using an energy-budget 

method. Computed AET rates for El Niño (November 1997 to 

September 1998) were less (31-36 in /yr), because there were 

more rainy days during the period. Computed recharge rates for 

average conditions at Round Lake were 20 in/yr for shallowrooted vegetation and 13 in/yr for deep-rooted vegetation, and 

compare favorably with the rate of 15 in/yr that was estimated 

from the comparison of chloride concentrations in ground water 

and atmospheric deposition. Computed recharge rates during 

El Niño were greater (52 and 47 in/yr, respectively), and 

amounted to about 60 percent of the precipitation.

The Round Lake model was calibrated through transientstate simulations to estimate aquifer properties with two sets 

of monthly recharge rates computed by LEACHM. The 

coefficients of variation for the estimated values ranged from 

0.5 to 7 percent, indicating that the values were accurately 

estimated. Hydrographs of simulated water levels indicate that 

the Round Lake model closely reproduced measured waterlevel fluctuations in response to changes in recharge and 

pumpage during the 32-month simulation period; the standard 

error in heads was 1.1 ft. Computed water budgets for the entire 

model area reflected the dramatic increase in recharge from 

average conditions, when recharge totaled about 1.5 in/mo, 

to El Niño conditions, when recharge more than doubled to 

3.3 in/mo. Most of the recharge was temporarily stored and 

then released during succeeding months. Leakage to the Upper 

Floridan aquifer accounted for most of the outflow from the 

model during average and El Niño periods (78 and 70 percent, 

respectively), with a relatively small proportion of this leakage 

passing through breaches in the intermediate confining unit (8 

and 6 percent, respectively).

The Halfmoon Lake model was calibrated through steadystate simulations to produce a model that approximated water 

levels measured in July 1996. Transient-state simulations were 

unable to accurately reproduce water levels measured during 

El Niño, so a shorter (22-month) simulation period representing 

water levels and lake leakage rates from July 1996 to April 1998 

was used to estimate an alternate set of parameter values. Coefficients of variation for five of the estimated parameter values 

ranged from 0.5 to 20 percent, indicating that the optimum 

values are not as accurately estimated as were parameters for 

the Round Lake model.

Hydraulic heads for the Halfmoon Lake model, which 

were computed with the steady-state simulations, match water 

levels measured in July 1996 quite well, and the standard error 

in heads was 0.6 ft. The computed steady-state water budget for 

the entire model area indicates that discharge to the Upper Floridan aquifer accounted for most of the ground-water outflow 

(88 percent, including about 5 percent to breaches in the intermediate confining unit), and nearly all of the remaining ground 

water discharged to Rocky Creek. Hydraulic heads computed 

with transient-state simulations generally reproduced measured 

water-level fluctuations during July 1996 to October 1997, but 

did not accurately reproduce the abrupt rise in water levels during El Niño. The discrepancy between measured and simulated 

hydrographs results from an unknown source of model error. 

Simulated water levels for Halfmoon Lake and wells near the 

lake match the measured levels reasonably well, but simulated 

water levels for wells more than 400 ft north and east of the lake 

are overpredicted by as much as 10 ft during El Niño. The poor 

match during El Niño suggests that a large part of the rainfall 

was diverted, possibly through overland runoff, before it could 

become recharge. Runoff was assumed to be negligible in the 

LEACHM estimates of recharge.

Solute transport simulations with MT3D were used to 

delineate the extent of lake leakage from Round and Halfmoon 

Lakes into the surficial aquifer under steady-state conditions 

with average recharge rates. Leakage of water from Round Lake 

breached the intermediate confining unit in about 130 days 

when zero dispersion was specified, and the concentration 
References 49

distribution reached steady-state after about 150 days when lake 

leakage extended about 250 ft from the lakeshore. Specifying 

longitudinal and transverse dispersivity values of 50 ft 

increased the extent of lake leakage (400 ft) and decreased the 

travel time to the intermediate confining unit (30 days). Results 

using zero dispersivity values of 50 ft were both in good agreement with concentrations of stable isotopes of oxygen-18 

(δ18O) and deuterium (δD) that were measured in water from 

the surficial aquifer.

Leakage of water from Halfmoon Lake breached the intermediate confining unit in about 400 days with zero dispersion 

and a steady-state concentration distribution was reached after 

about 2 years when lake leakage extended about 75 ft from the 

lakeshore. Increasing the longitudinal and transverse dispersivity values to 50 ft increased the extent of lake leakage to 150 ft 

and decreased the travel time to the intermediate confining unit 

from 400 to 40 days. These results are generally consistent with 

the stable-isotope concentrations in ground water near Halfmoon Lake, which suggest that most of the lake leakage flows 

downward through the lake bottom.

Results of steady-state simulations indicate that lake stage 

is proportional to both the Upper Floridan aquifer head and the 

augmentation rate at Round Lake, indicating that fluctuations in 

lake stage can be minimized by adjusting the augmentation rate 

to account for changing water levels in the Upper Floridan aquifer under constant recharge conditions. Monthly augmentation 

rates were computed using an equation that accounted for 

changes in the Upper Floridan aquifer head and the deviation 

from the mean recharge rate on a monthly basis to maintain a 

constant lake stage. 

In transient-state simulations of Round Lake, computed 

augmentation rates produced stages that were close to the target 

lake stage of 52 ft during average conditions (July 1996 to 

October 1997), but yielded stages that were greater than 54 ft 

after the start of El Niño in November 1997. When the equation 

was modified to account for the higher recharge rate during 

El Niño, the stage only exceeded 54 ft when recharge was 

highest in February and March 1998. Two sets of monthly 

augmentation rates were computed for Halfmoon Lake using 

parameter estimates obtained with both steady- and transientstate simulations. Both sets of augmentation rates produced a 

relatively constant lake stage near 43 ft during average 

conditions (July 1996 to October 1997), but the computed stage 

diverged from the target lake stage during El Niño, as in the 

Round Lake simulations.

Model simulations indicate that substantially more lake 

leakage flows to the Upper Floridan aquifer from Round Lake 

than from Halfmoon Lake, because the estimated vertical 

hydraulic conductivities (Kv) of Round Lake and the confining 

layer sediments and breaches in the confining layer are much 

larger. The estimated Kv of lake sediments in Round Lake is 

about 70 times greater than the value estimated for Halfmoon 

Lake, and the Kv values estimated for the confining layer and 

breaches in the layer are 10 times higher. Steady-state simulations indicate that about 30 percent of the leakage at Round 

Lake flows through the shallow lake perimeter, whereas only 

17 percent of the leakage at Halfmoon Lake flows through the 

lake perimeter. This is because the lateral hydraulic gradients 

are steeper at Round Lake (1.3x10-2) than at Halfmoon Lake 

(5.6x10-4).

As a result of the higher lateral and vertical leakage 

losses from Round Lake, the augmentation rate required to 

maintain the low guidance level in Round Lake (53 ft) under 

average Upper Floridan aquifer heads (38 ft) is estimated as 

33,850 ft3

/d, whereas only 1,330 to 10,000 ft3/d is required to 

maintain a comparable level in Halfmoon Lake (42 ft) under 

similar conditions (Upper Floridan aquifer head of 28 ft). These 

augmentation rates are the equivalent of 26 in/mo of water 

applied to the entire surface of Round Lake, as opposed to 0.34 

to 2.5 in/mo applied to Halfmoon Lake. 

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50 Simulated Effects of Ground-Water Augmentation on the Hydrology of Round and Halfmoon Lakes

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