Retrospective Review of Watershed 

Characteristics and a Framework for 

Future Research in the Sarasota Bay 

Watershed, Florida

By George R. Kish1, Arnell S. Harrison2, and Mark Alderson3

Prepared in cooperation with the

Sarasota Bay Estuary Program

Open-File Report 2007-1349

U.S. Department of the Interior

U.S. Geological Survey

1 U.S. Geological Survey, Tampa, Florida

2 U.S. Geological Survey, St. Petersburg, Florida

3 Sarasota Bay Estuary Program, Sarasota, Florida
U.S. Department of the Interior

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2008

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Suggested citation:

Kish, G.R., Harrison, A.S., and Alderson, Mark, 2008, Retrospective Review of Watershed Characteristics and 

a Framework for Future Research in the Sarasota Bay Watershed, Florida: U.S. Geological Survey Open-File 

Report 2007-1349, 49 p.
Contents

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

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

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

Historical Perspective of Sarasota Bay............................................................................................3

Sarasota Bay National Estuary Program..........................................................................................3

Acknowledgments................................................................................................................................4

Environmental Setting of the Sarasota Bay Watershed..........................................................................4

Climate....................................................................................................................................................4

Geography..............................................................................................................................................6

Soils.........................................................................................................................................................6

Water Use ..............................................................................................................................................8

Hydrogeology.........................................................................................................................................9

Upper Floridan Aquifer..............................................................................................................10

Intermediate Aquifer System...................................................................................................10

Surficial Aquifer System...........................................................................................................10

Watershed Characteristics Influencing Recharge and Discharge.....................................................20

Soil Characteristics ............................................................................................................................20

Porosity, Bulk Density, and Penetration Resistance............................................................20

Infiltration Rate...........................................................................................................................22

Water Budget ......................................................................................................................................23

Urbanization and Land Use ...............................................................................................................24

Land Practices.....................................................................................................................................24

Agriculture ..................................................................................................................................25

Construction................................................................................................................................25

Recreation...................................................................................................................................27

Infiltration and Runoff ........................................................................................................................28

Framework for Future Research in the Watershed................................................................................31

Low-Impact Development .................................................................................................................31

Pollutant Loading and Watershed Modeling..................................................................................33

Event Mean Concentrations for Pollutant Loading Models................................................38

Research Topics..................................................................................................................................39

Summary........................................................................................................................................................40

Selected References...................................................................................................................................41

Figures

1. Map showing location of the Sarasota Bay watershed in west-central Florida .................. 2 

2-6. Graphs showing:

2. Population of Sarasota County from 1920 to 2000 ..............................................................3

3. Average annual precipitation for Sarasota County from 1915 to 2003 ..........................4

4. Daily precipitation in the Sarasota Bay watershed from January 10, 2004, to 

 January 10, 2005........................................................................................................................5

iii
5. Departure from average annual precipitation in the Sarasota Bay 

 watershed from 1915 to 2003 .................................................................................................5

6. Average monthly precipitation from 1915 to 2003 and 1998 El Niño 

 precipitation in the Sarasota Bay watershed ....................................................................6

7. Map showing geomorphological features in west-central Florida ..........................................6

8. Chart showing ground-water use by aquifer in Sarasota County in 2000 ...............................8

9. Graph showing annual ground-water use in Sarasota County from 1977 to 2000.................9

10. Graph showing ground-water use for public supply and agriculture in Sarasota 

County from 1977 to 2000 ...................................................................................................................9

11. Stratigraphic, lithologic, and hydrogeologic units in west-central Florida...........................10

12. Map showing locations of geologic sections in the study area..............................................12

13-16. Geologic sections:

13. A-A’.............................................................................................................................................13

14. B-B’.............................................................................................................................................14

15. C-C’.............................................................................................................................................15

16. D-D’.............................................................................................................................................16

17-19. Maps showing generalized:

17. Thickness of the surficial aquifer system in the study area.........................................17

18. Direction of flow in the surficial aquifer system in west-central Florida..................18

19. Recharge/discharge areas of the Upper Floridan aquifer in west-central Florida.......19

20. Graph showing elevation of water levels in the surficial, intermediate, and

Floridan aquifer systems in well cluster TR 5-2 near Walton for 1993-94, 

west-central Florida..................................................................................................................21

21. Map showing generalized isopleths of chloride concentration in the surficial 

aquifer system....................................................................................................................................21

22-24. Graphs showing:

22. Relation between moisture content and soil depth: (A) Horton infiltration, 

 (B) Dunne infiltration, (C) Horton runoff, and (D) Dunne runoff................................. 23

23. Average monthly water budget for west-central Florida............................................. 24

24. Generalized relation between compaction and moisture content of an 

 average soil ............................................................................................................................. 25

25. Schematic showing potential routes of water movement........................................................30

26. Maps showing examples of seasonal variation in saturated source areas for 

a catchment in Vermont...................................................................................................................30

27. Schematics showing examples of variation in runoff with increasing impervious 

surfaces ..............................................................................................................................................34

28. Graph showing variability in selected metal concentrations for different event mean 

concentration (EMC) sampling schemes.....................................................................................39

Tables

1. Physical characteristics of Flatwoods, Depressions, and Coastal Islands soils in the 

Sarasota Bay watershed.......................................................................................................................7-8

2. Hydraulic properties of the surficial aquifer system in the vicinity of the Sarasota

Bay watershed ..........................................................................................................................................11

3. Selected physical and hydrologic properties for selected earth materials .................................22

4. Characteristics of Horton and Dunne mechanisms for infiltration and runoff.............................24

5. Bulk density of common urban settings...............................................................................................27

iv
6. Effect of bulk density on root growth for various soil textures .......................................................27

7. Soil treatments to reduce bulk density.................................................................................................27

8. Runoff curve numbers by housing density and hydrologic soil group...........................................28

9. Percent change in simulated runoff characteristics of land-cover types from 

conventional BMPs to innovative design with BMPs from a study in the Chesapeake

Bay watershed ..........................................................................................................................................32

10. Total nitrogen loads to Sarasota Bay projected for 2012 for several land management

alternatives in the Sarasota Bay watershed.......................................................................................35

11. Percentage of pollutant load contained in storm runoff for selected levels of impervious

ground cover..............................................................................................................................................38

Conversion Factors, Datums, Acronyms, and Abbreviations

Multiply By To obtain

Length

centimeters (cm) 0.3937 inch (in.)

meter (m) 3.281 foot (ft)

kilometer (km) 0.6214 mile (mi)

Area

square meter (m2) 0.0002471 acre

square centimeter (cm2) 0.001076 square foot (ft2)

square meter (m2) 10.76 square foot (ft2)

hectare (ha) 0.003861 square mile (mi2)

square kilometer (km2) 0.3861 square mile (mi2)

Volume

cubic meter (m3) 35.31 cubic foot (ft3)

Flow rate

centimeters per hour (cm/hr) 0.3937 inch per hour (in/hr)

meter per second (m/s) 3.281 foot per second (ft/s)

cubic meter per second (m3/s) 35.31 cubic foot per second (ft3/s)

liters per minute (L/min) 0.2642 gallons per minute (gal/min)

liters per second (L/s) 0.0228 million gallons per day (Mgal/d)

Mass, hydraulic gradient, and density

gram (g) 0.002205 pound (lb)

meter per kilometer (m/km) 5.27983 foot per mile (ft/mi)

grams per cubic centimeter (g/cm3) 62.4298 pounds per cubic foot (lb/ft3)

Transmissivity*

square meters per day (m2/d) 10.76 square feet per day (ft2/d)

Load – application rate

kilograms per year (kg/yr) 2.205 pounds per year (lb/yr)

kilograms per hectare per year [kg/ha/yr] 0.8921 pounds per acre per year [(lb/acre)/yr]

Temperature

Celsius (°C) °F = (1.8 × °C) + 32 Fahrenheit (°F)

Fahrenheit (°F) °C = (°F - 32)/1.8 Celsius (°C)

*Transmissivity: The standard unit for transmissivity is cubic foot per day per square foot times foot 

of aquifer thickness [(ft3/d)/ft2]ft. In this report, the mathematically reduced form, foot squared per day 

(ft2/d), is used for convenience.

v
vi

Datums

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

Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 (NGVD 29)

Acronyms and Abbreviations

BMP Best Management Practice

CCMP Comprehensive Conservation and Management Plan

DCIA Directly Connected Impervious Area

EMC Event Mean Concentration

FY&N Florida Yards and Neighborhoods Program

IMP Integrated Management Practices

LID Low-impact Development

NEXRAD Next Generation Weather Radar System

SBEP Sarasota Bay Estuary Program

SUNOM Simplified Urban Nutrient Output Model

SWUCA Southern Water Use Caution Area

SWFWMD Southwest Florida Water Management District

TMDL Total Maximum Daily Load

USEPA U.S. Environmental Protection Agency

USGS U.S. Geological Survey

μg/L micrograms per liter

mg/L milligrams per liter
Abstract

The U.S. Geological Survey, in cooperation with the 

Sarasota Bay Estuary Program, conducted a retrospective 

review of characteristics of the Sarasota Bay watershed 

in west-central Florida. This report describes watershed 

characteristics, surface- and ground-water processes, and the 

environmental setting of the Sarasota Bay watershed.

Population growth during the last 50 years is transforming 

the Sarasota Bay watershed from rural and agriculture to urban 

and suburban. The transition has resulted in land-use changes 

that influence surface- and ground-water processes in the 

watershed. Increased impervious cover decreases recharge to 

ground water and increases overland runoff and the pollutants 

carried in the runoff. Soil compaction resulting from agriculture, construction, and recreation activities also decreases 

recharge to ground water.

Conventional approaches to stormwater runoff have 

involved conveyances and large storage areas. Low-impact 

development approaches, designed to provide recharge near 

the precipitation point-of-contact, are being used increasingly 

in the watershed. 

Simple pollutant loading models applied to the Sarasota 

Bay watershed have focused on large-scale processes and 

pollutant loads determined from empirical values and mean 

event concentrations. Complex watershed models and more 

intensive data-collection programs can provide the level of 

information needed to quantify (1) the effects of lot-scale 

land practices on runoff, storage, and ground-water recharge, 

(2) dry and wet season flux of nutrients through atmospheric 

deposition, (3) changes in partitioning of water and 

contaminants as urbanization alters predevelopment rainfallrunoff relations, and (4) linkages between watershed models 

and lot-scale models to evaluate the effect of small-scale 

changes over the entire Sarasota Bay watershed. As urbanization in the Sarasota Bay watershed continues, focused research 

on water-resources issues can provide information needed by 

water-resources managers to ensure the future health of the 

watershed. 

Introduction

Comprehensive watershed planning is the key to 

successful multiple-use watershed management. Scientific 

tools available for watershed planning rely upon extensive, 

representative data collection programs that are well designed 

and well maintained. An understanding of the dynamics of 

the hydrologic system is necessary for efficient management 

of a watershed. The Sarasota Bay Estuary Program (SBEP) 

and its many partners have addressed and continue to address 

multiple-use issues to improve the health of the Sarasota 

Bay watershed in west-central Florida (fig. 1). The U.S. 

Geological Survey (USGS), in cooperation with the SBEP, 

initiated a study in 2003 in the Sarasota Bay watershed to 

provide information about the watershed characteristics that 

affect recharge to ground water and overland runoff to surface 

water. A thorough understanding of the watershed factors that 

affect the interaction between surface water and ground water 

in the Sarasota Bay watershed will enhance efficient use and 

management of the watershed. 

Retrospective Review of Watershed Characteristics 

and a Framework for Future Research in the 

Sarasota Bay Watershed, Florida

By George R. Kish1, Arnell S. Harrison2, and Mark Alderson3

1 U.S. Geological Survey, Tampa, Florida

2 U.S. Geological Survey, St. Petersburg, Florida

3 Sarasota Bay Estuary Program, Sarasota, Florida
2 Retrospective Review of Watershed Characteristics and a Framework for Future Research 

Figure 1. Location of the Sarasota Bay watershed in west-central Florida (modified from Sarasota Bay Estuary Program, 2005).

Parrish

Bradenton

Gulf

of

Mexico

Sarasota

Location

of study area

0 5 10 KILOMETERS 

0 5 10 MILES 

SARASOTA

COUNTY

CHARLOTTE

COUNTY

Myakka River

Lake

Myakka

Lower

Myakka

Lake

Venice

Sarasota

Bay

Little

Sarasota

Bay

Lemon Bay

Tampa

Bay

Charlotte

Harbor

Siesta Key

Lido Key

Longboat Key

Casey Key

Manasota Key

Manatee River

Manatee River

New

Pass

Anna

Marie

Sound

Big

Pass

Cowpen Slough

Philippi Creek

Venice

Inlet

MANATEE

COUNTY

Holmes

Beach

EXPLANATION

Sarasota Bay watershed

Myakka River
Introduction 3

Purpose and Scope

The purpose of this report is to describe the watershed 

characteristics and the environmental setting of the Sarasota 

Bay watershed as they relate to surface-water and ground-water 

interactions. A comprehensive literature review of published 

reports about the Sarasota Bay watershed and applicable 

scientific literature regarding the potential influence of recharge 

to ground water and overland runoff to surface water on 

characteristics of the Sarasota Bay watershed form the basis 

of this report. Cultural practices and their influence on soil 

compaction and infiltration are emphasized. This report presents 

watershed science concepts that will enhance future approaches 

to modeling in humid, shallow water-table areas in urbanizing 

watersheds. 

Historical Perspective of Sarasota Bay

Prior to 1800, the Sarasota area experienced little 

development or population growth. In 1842, the Armed 

Occupation Act encouraged homesteading in Florida by 

providing 160-acre parcels free to individuals who agreed to 

inhabit the land for 5 years. From 1868 to 1883, areas were 

cleared by settlers for orange groves and cattle. In 1870, the first 

settlement was established in Sarasota. Scottish settlers established a community in Sarasota in 1885 and were credited with 

building America’s first golf course in 1886. By 1898, cattle 

ranching had expanded and Sarasota cattle supplied beef for 

troops stationed in Tampa during the Spanish-American War. 

Development of the watershed remained relatively slow because 

transportation connecting Sarasota with other areas was limited 

to sailing ships or steamboats. During the period from 1900 to 

1920, the town of Sarasota was incorporated, a railroad from 

Sarasota to Tampa was built, electric service was established, 

celery farmers began shipping their crop to northern markets, an 

air strip was built, and the Manasota Lumber Company saw mill 

began operation (Sarasota County History, 2005).

At the start of the Great Florida Land Boom in 1920, the 

population of Sarasota was about 2,150 (fig. 2). With substantial increases in businesses, schools, churches, hospitals, 

farming, cattle ranching, newspapers, homes, and apartments, 

by 1930, urban and agricultural development increased 

substantially as the population grew to 12,440 (fig. 2). From 

the 1930s to the 1950s, urban development continued and military bases were established (Sarasota County History, 2005). 

From 1950 to 2000, the population of Sarasota increased from 

28,827 to 325,957 (U.S. Census Bureau, 2004). The growth in 

population brought increases in government services, housing 

developments, and infrastructure (for example, roads, utilities). 

Since the 1960s, conversion of agricultural land to urbanized 

land has continued to occur as the result of rapid population 

increases. Presently, the Sarasota Bay area is one of the most 

rapidly expanding urban areas in the Nation – primarily 

through residential and commercial development in the eastern 

part of Sarasota County.

Sarasota Bay National Estuary Program

The U.S. Congress named Sarasota Bay an estuary 

of national significance in the Water Quality Act of 1987. 

In 1995, the State of Florida and the U.S. Environmental 

Protection Agency (USEPA) approved the Comprehensive 

Conservation and Management Plan (CCMP) for Sarasota 

Bay. From 1995 to 2000, more than $200 million was 

committed to Sarasota Bay restoration. The CCMP consists 

of action plans designed to reduce nitrogen pollution from 

stormwater and wastewater, increase available habitat for fish 

and wildlife, and increase access to the bay and its resources 

(Sarasota Bay National Estuary Program, 2000).

Studies completed by the Sarasota Bay National Estuary 

Program in the early 1990s indicated that Sarasota Bay 

was more degraded than originally hypothesized (Sarasota 

Bay National Estuary Program, 2000). Nitrogen loading to 

Sarasota Bay in 1989, for example, had increased to 480 

percent above predevelopment levels. In response to efforts 

by local, State, and Federal agencies, nitrogen pollution to 

Sarasota Bay decreased 47 percent between 1989 and 2000, 

to levels approximating 254 percent above predevelopment 

levels (Sarasota Bay National Estuary Program, 2000). The 

load reduction has been achieved as a result of (1) improved 

wastewater treatment, (2) reduced volume of wastewater 

discharge due to construction of reclaimed water systems, and 

(3) constructing stormwater treatment systems. 

Recent estimates indicate that 56 percent of the remaining 

nitrogen load to Sarasota Bay is from stormwater (Sarasota 

Bay National Estuary Program, 2000). The Sarasota Bay 

community is addressing additional nitrogen reductions 

in stormwater through several integrated water-resource 

management approaches: (1) regional stormwater treatment 

systems (for example, Phillippi Creek), (2) stormwater reuse, 

and (3) regional educational programs to promote cultural 

change and water conservation such as the Florida Yards and 

Neighborhoods (FY&N) Program (Garner and others, 2001). 

0

50,000

100,000

150,000

200,000

250,000

350,000

300,000

1920 1930 1940 1950 1960 1970 1980 20001990

YEAR

POPULATION

Figure 2. Population of Sarasota County from 1920 to 2000 

(from U.S. Census Bureau, 2004).
4 Retrospective Review of Watershed Characteristics and a Framework for Future Research 

Acknowledgments

Gary Raulerson, Senior Environmental Scientist of the 

SBEP, is gratefully acknowledged for his guidance in developing this report. Dave Tomasko, formerly of the Southwest 

Florida Water Management District (SWFWMD), provided 

valuable insight and details about the effect of nitrogen loads 

on the watershed. Mark Shelby, formerly with the University 

of Florida Institute of Food and Agricultural Sciences, 

provided information about construction and landscaping 

practices.

Environmental Setting of the Sarasota 

Bay Watershed

The Sarasota Bay watershed drains more than 200 mi2

(518 km2

) within Manatee, Sarasota, and Charlotte Counties 

in west-central Florida (fig. 1). The watershed consists of five 

major embayments—Sarasota Bay, Roberts Bay, Little Sarasota 

Bay, Blackburn Bay, and Lemon Bay—and a series of creeks 

and bayous, inlets, upland drainage and barrier islands. Phillippi 

Creek is the primary tributary in the watershed. Many reaches 

of the creeks have been converted to ditches throughout the 

Sarasota Bay watershed to facilitate drainage of the relatively 

flat, low-gradient subbasins. Inlets between the barrier islands 

exchange water between Sarasota Bay and the Gulf of Mexico. 

Sarasota Bay extends north to south about 56 mi (90 km) and 

ranges in width from 300 ft (91 m) to 4.5 mi (7.2 km).

The hydrologic system is closely linked to the unique 

environmental setting of the area. Climate, geography, soils, 

hydrogeology, land cover, land use, and urbanization all affect 

water movement within the watershed.

Climate

The climate of west-central Florida is characterized by 

long, warm, humid summers and short, mild, dry winters. 

The monthly maximum temperatures range from about 92 °F 

(33.3 °C) in July and August to 71 °F (21.7 °C) in January, 

with a mean annual maximum temperature of 82 °F (27.8 °C). 

The monthly minimum temperatures range from about 72 °F 

(22.2 °C) in August to 48 °F (8.9 °C) in January, with a 

mean annual minimum temperature of 61 °F (16.1 °C) (Soil 

Conservation Service, 1991). Average annual precipitation 

within Sarasota County between 1915 and 2003 was 52.47 in. 

(133 cm); the maximum and minimum recorded annual 

precipitation was 85.54 in. (217 cm) for 1959 and 32.77 in. 

(83 cm) for 2000, respectively (fig. 3) (Southwest Florida 

Water Management District, 2005). 

Precipitation events occur in somewhat predictable 

seasonal patterns; winter frontal storms move southeastward 

across the continental United States, whereas tropical storms 

and hurricanes move toward the west across the Atlantic 

Ocean, and local thunderstorms occur almost daily during the 

summer. Although seasonal patterns are somewhat consistent, 

the intensity and duration of individual storms are difficult to 

predict. Converging sea breezes, the proximity to the Atlantic 

high pressure system, the shape of the peninsula, and its 

subtropical latitude all provide conditions suitable for thunderstorm development during the summer months. During the 

wet season, daily and hourly precipitation varies substantially. 

Precipitation amounts for 2004 from a rainfall station in 

Sarasota County indicate that precipitation occurs almost daily 

during the rainy period from July to October, but that precipitation events are sparse from November through June (fig. 4) 

(Southwest Florida Water Management District, 2005).

West-central Florida receives about 60 to 100 precipitation 

events per year and about 20 of these events result in precipitation totals greater than 0.5 in. (1.27 cm), which typically 

generate runoff (Mark Ross, University of South Florida, 

oral commun., 2005). During the dry season, a relatively 

even rainfall pattern predominates, punctuated by occasional 

frontal storms. Annual precipitation amounts may be highly 

variable and deviations from normal precipitation may 

vary substantially (fig. 5). For example, in 1995 and 1996, 

the annual precipitation amounts in Sarasota County were 

22.35 in. (57 cm) above normal and 8.22 in. (21 cm) below 

normal, respectively (Southwest Florida Water Management 

District, 2005).

Temperature changes in the Atlantic and Pacific Oceans 

strongly influence the climate of Florida. When the temperature of the Atlantic Ocean near the Equator is higher than 

normal, changing wind patterns bring less moisture to Florida 

from the Gulf of Mexico (Henry, 1998). The El Niño/Southern 

Oscillation is a disturbance between the ocean and the 

atmosphere in the equatorial Pacific Ocean resulting in global 

shifts in weather patterns (National Oceanic and Atmospheric 

Administration, 2007). El Niño and La Niña events are the 

extreme phases of the El Niño/Southern Oscillation climate 

cycle occurring in the tropical Pacific Ocean. Normally, warm 

ocean water in the western Pacific Ocean is pushed westward 

0

10

20

30

40

50

60

70

80

90

1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010

YEAR

AVERAGE ANNUAL PRECIPITATION,

IN INCHES

Figure 3. Average annual precipitation for Sarasota County 

from 1915 to 2003 (from the Southwest Florida Water 

Management District, 2005).
Environmental Setting of the Sarasota Bay Watershed 5

by strong trade winds out of the east. Meanwhile, water 

upwelling from the cold ocean depths cools the water in the 

eastern Pacific Ocean. Every 2 to 7 years, the El Niño phase of 

the cycle occurs when the trade winds weaken and warm water 

from the western Pacific Ocean moves eastward. As the warm 

water enters the eastern Pacific Ocean, upwelling of cool, 

deep water is suppressed. El Niño events cause the following 

effects in Florida; greater than normal winter precipitation, 

more frequent, intense storms from the Gulf of Mexico, and 

less frequent hurricanes than normal years (Henry, 1998). The 

difference between precipitation during the 1998 El Niño year 

and the average monthly precipitation for the Sarasota Bay 

watershed is shown in figure 6. Precipitation was well above 

normal during January, February, and March of the El Niño 

year, followed by below normal precipitation from April to 

August (Southwest Florida Water Management District, 2005). 

La Niña is the opposite phase of the cycle. During La Niña 

years, trade winds are stronger than normal, holding back the 

eastward advance of warm, western Pacific water, while permitting cool, deep Eastern Pacific water to move westward. La Niña 

events cause lower winter rainfall in Florida than during normal 

years. The recurrence and strength of El Niño and La Niña 

events and the frequency and intensity of rainy season thunderstorms and hurricanes add complexity to precipitation patterns. 

0.5

1.0

0

1.5

2.0

2.5

3.0

3.5

4.0

1/10/20042/10/20043/10/20044/10/20045/10/20046/10/20047/10/20048/10/20049/10/2004

10/10/200411/10/200412/10/2004

1/10/2005

DATE

DAILY PRECIPITATION, IN INCHES

Figure 4. Daily precipitation in the Sarasota Bay watershed from January 10, 2004, to January 10, 2005 (from the 

Southwest Florida Water Management District, 2005).

-40

-20

0

20

40

1915 1925 1935 1945 1955 1965 1975 1985 1995 2005

YEAR

DEPARTURE FROM AVERAGE ANNUAL

PRECIPITATION, IN INCHES

Figure 5. Departure from average annual precipitation in the Sarasota Bay watershed from 1915 to 2003 

(from the Southwest Florida Water Management District, 2005).
6 Retrospective Review of Watershed Characteristics and a Framework for Future Research 

Geography 

The Sarasota Bay watershed drains northern and western 

Sarasota County and small parts of Manatee and Charlotte 

Counties (fig. 1). Most of the watershed lies within the Southern 

Gulf Coastal Lowlands, whereas the headwaters originate in the 

Desoto Plain (fig. 7) (White, 1970). Elevations within the watershed range from 0 ft along the coast to about 135 ft (41.1 m) 

above the National Geodetic Vertical Datum of 1929 (NGVD 29) 

in south-central Manatee County. The topography consists of a 

series of relict marine terraces (Campbell, 1985) and is characterized by (1) broad flatlands with many sloughs and swamps 

in lowland areas, and (2) gradually sloping scarps and terraces 

created by various Pleistocene sea-level stands in coastal areas. 

Seasonally dry upland areas consisting of palustrine forest, 

scrub-shrub, or palustrine emergent wetlands are scattered 

throughout inland areas (Knochenmus and Bowman, 1998). 

Four terraces exist within the watershed, but the scarps that 

separate them are poorly defined. From the coastline inland, the 

terraces are Pamlico (8-25 ft above NGVD 29), Talbot (25-42 ft 

above NGVD 29), Penholoway (42-70 ft above NGVD 29), and 

Wicomico (70-100 ft above NGVD 29) (Healy, 1975). 

Soils

Soils in the Sarasota Bay watershed are typically poorly 

to very poorly drained fine sands. The Soil Conservation 

Service (1991) grouped soils within the watershed into 

Flatwoods soils, Depressions soils, and Coastal Islands soils. 

Physical properties of the soil series within each soil group are 

listed in table 1.

0

2

4

6

8

10

12

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

MONTH

AVERAGE MONTHLY PRECIPITATION, IN INCHES

1998 El Niño Year

Figure 6. Average monthly precipitation from 1915 to 2003 and 1998 El Niño precipitation in the Sarasota Bay 

watershed (from Southwest Florida Water Management District, 2005).

Southern

Gulf Coastal

Lowlands

27°

82°45' 82°30' 82°15'

27°30'

27°15'

Gulf of Mexico

Tampa

Bay 

0

0

10

10

20 KILOMETERS

20 MILES

Base from U.S. Geological Survey digital data, 1:2,000,000, 1972

Albers Equal-Area Conic projection

Standard Parallels 29°30' and 45°30', central meridian -83°00'

Polk

Upland

Desoto Plain

Sarasota

County

Manatee

County

Charlotte

County

Gulf

Barrier

Chain

Figure 7. Geomorphological features in west-central Florida 

(from White, 1970).
Environmental Setting of the Sarasota Bay Watershed 7

Table 1. Physical characteristics of Flatwoods, Depressions, and Coastal Islands soils in the Sarasota Bay watershed.

 [U.S. Department of Agriculture (1983, 1991). ft, feet; in., inches; g/cm3, grams per cubic centimeter; >, greater than]

Soil

 series

Hydrologic

soil group

Depth to

high water

table (ft)

Depth 

interval

(in.)

Bulk

density

(g/cm3)

Comments

Flatwoods Soils

Eau Gallie B/D -0.5 to -1.5 Poorly drained, sandy, siliceous, from marine sediments, acidic, spodic horizon, 

nearly level

0 - 22 1.25 - 1.50

22 - 44 1.45 - 1.60

44 - 48 1.45 - 1.65

48 - 66 1.55 - 1.70 Clay content 13-31 percent

66 - 80 1.45 - 1.55

Holopaw D >2.0 to -1.0 Very poorly drained, loamy, siliceous, from marine sediments, acidic, spodic 

horizon, nearly level

0 - 50 1.35 - 1.60 

50 - 66 1.60 - 1.70 Clay content 13-28 percent

66 - 80 1.50 - 1.60

Myakka D -0.5 to -1.5 Very poorly drained, sandy, siliceous, from marine sediments, acidic, spodic 

horizon, nearly level on broad flatwoods

0 - 24 1.25 - 1.45

24 42 1.45 - 1.60

42-80 1.48 - 1.70

Pineda B/D 0.0 to -1.0 Poorly drained, loamy, sandy, nearly level on low hammocks and in broad, poorly 

defined sloughs

0 - 22 1.25 - 1.60

22 - 36 1.40 - 1.70

36 - 48 1.50 - 1.70 Clay content 10-25 percent

48 - 80 1.45 - 1.60

Pomello C -2.0 to -3.5 Moderately well drained, sandy, nearly level to gently sloping on low ridges and 

knolls on flatwoods

0-48 1.35 - 1.65 

48-80 1.45 - 1.60 

Flatwoods Soils

Delray B/D >2.0 to -1.0 Very poorly drained, loamy, sandy, from marine sediments, acidic, slope is less than 

2 percent, found in depressions

0 - 20 1.35 – 1.45

20 - 54 1.50 – 1.65

54 - 80 1.45 – 1.60

Felda D >2.0 to -1.0 Poorly to very poorly drained, from sandy and loamy marine sediments, slope from 

0 to 2 percent on low hammocks or flood plain

0 - 22 1.40 – 1.55

22 - 60 1.50 – 1.60

60 - 80 1.45 – 1.55

Floridana D 0.0 to -1.0 Very poorly drained from sandy and loamy marine sediments; nearly level; found in 

depressions

0 - 14 1.40 – 1.50

14 - 36 1.50 – 1.60

36 - 80 1.60 – 1.70

Holopaw D >2.0 to -1.0 Very poorly drained, loamy, sandy, from marine sediments, acidic, spodic horizon, 

nearly level

0 - 50 1.35 - 1.60 

50 - 66 1.60 - 1.70 Clay content 13-28 percent

66 - 80 1.50 - 1.60
8 Retrospective Review of Watershed Characteristics and a Framework for Future Research 

Flatwoods occupy about 90 percent of the Sarasota Bay 

watershed. Soils of Flatwoods are nearly level, moderately to 

very poorly drained, and sandy to loamy. Soil series in this 

group include Eau Gallie, Holopaw, Myakka, Pineda, and 

Pomello. In the upper portion of the watershed, Flatwoods 

soils commonly have a spodic horizon (layer of accumulated 

minerals) generally 2-3 ft (0.61-0.91 m) below land surface. 

In some Flatwoods soils, the spodic horizon is chiefly hardpan, 

restricting the infiltration of water and subjecting the areas to 

seasonal flooding and drying.

Depressions soils are found in the north-central portion 

of the watershed and along sloughs. Soils in this group 

typically are very poorly drained and mucky, sandy, or loamy. 

Depressions soils have been used extensively for agriculture, principally celery. Poorly drained soils throughout the 

watershed enhance depressional storage of water in swamps, 

marshes, and ponds (Soil Conservation Service, 1991). 

Coastal Islands soils are found on the barrier islands. Soils 

in this group are gently sloping, moderately to poorly drained, 

and sandy. Soil series of Coastal Islands include Beaches, 

Canaveral, and Kesson. Shell fragments are common and areas 

of very poorly drained sand are covered with a mucky layer.

Soil characteristics play an important role in the hydrologic regime of the Sarasota Bay watershed. Precipitation that 

occurs in the watershed may infiltrate directly into the soil and 

percolate to ground water, or it may flow over land toward 

depressional surface features such as swamps, ponds, and 

streams. The path taken depends upon physical and chemical 

characteristics of the soil, the intensity and duration of precipitation, slope, antecedent soil moisture conditions, depth to 

ground water, and cultural practices. 

Water Use

In 2000, total water use in Sarasota County was 46.3 

Mgal/d (2,028 L/s) with ground water accounting for 87.2 

percent or 40.37 Mgal/d (1,769 L/s) of the total. About 69.5 

percent was used for public supply, 17.3 percent for agriculture, 

12.3 percent for recreational irrigation, 0.8 percent for commercial/industrial, and 0.1 percent for domestic (Marella, 2004).

The Upper Floridan aquifer is the principal source of 

water supply in Sarasota County, accounting for 67.5 percent 

of ground-water use in 2000 (R.L. Marella, U.S. Geological 

Survey, oral commun., 2007). About 32.4 percent of the total 

amount of ground-water withdrawn was from the overlying 

intermediate aquifer system and relatively little (0.1 percent) 

ground water was withdrawn from the surficial aquifer system 

(fig. 8) (Marella, 2004). 

 Ground-water use has remained relatively constant in 

Sarasota County since 1977 (fig. 9) but ground-water use by 

category has changed substantially. In 1977, ground-water 

withdrawals were 9.58 Mgal/d (420 L/s) for public supply and 

22.65 Mgal/d (992 L/s) for agriculture. By 2000, ground-water 

withdrawals were 27.56 Mgal/d (1,207 L/s) for public supply 

and 6.88 Mgal/d (301 L/s) for agriculture (fig. 10).

Soil

 series

Hydrologic

soil group

Depth to

high water

table (ft)

Depth 

interval

(in.)

Bulk

density

(g/cm3

)

Comments

Coastal Island Soils

Beaches D 0.0 to -6.0 0 - 80 — Tide and surf-washed sand and shell fragments along the Gulf of Mexico shoreline; 

nearly level to moderately sloping

Canaveral C -1.0 to -3.0 0 - 7 1.25 - 1.50 Somewhat poorly drained to moderately well-drained; formed in thick deposits of 

sand and fine shell fragments; nearly level to gently sloping from 0 to 5 percent on 

low, dune-like ridges

Kesson D 0.0 to - 0.5 Deep, very poorly drained from marine deposits of sand and shell fragments; nearly 

level; flooded during normal high tides; upper layer of organic muck

0 - 7 0.15 - 0.35

7 - 80 1.50 - 1.65

Upper Florida

Aquifer

67.5%

Intermediate

aquifer system

32.4%

Surficial

aquifer system

0.1%

Figure 8. Ground-water use by aquifer 

in Sarasota County in 2000 (from 

Marella, 2004).

Table 1. (Continued) Physical characteristics of Flatwoods, Depressions, and Coastal Islands soils in the Sarasota Bay watershed.
Environmental Setting of the Sarasota Bay Watershed 9

Total withdrawals for public supply in Sarasota County 

are projected to increase to about 63 Mgal/d (2,760 L/s) by 

2020 (Southwest Florida Water Management District, 1992). 

The increased demand for water in the region has resulted in a 

reevaluation of the surficial aquifer system for use as irrigation 

water or for blending with other water sources (Southwest 

Florida Water Management District, 1998). The surficial 

aquifer system is closely linked to the surface-water system 

(Winter and others, 1998), supplies water to depressional 

features and wetlands, and is vulnerable to contamination from 

the surface.

Hydrogeology

Ground water is an abundant but vulnerable resource 

in Florida. About 93 percent of Florida’s population obtains 

drinking water from ground water (Berndt and others, 1998). 

In Sarasota County, more than 87 percent of the water used is 

obtained from ground-water sources and the demand for public 

supply is expected to increase by about 130 percent over the 

period 2000 to 2020 (Marella, 2004). Ground-water resources 

are vulnerable to contamination from land-surface activities 

and saltwater intrusion from over-pumping. Replenishment of 

ground-water resources depends upon the infiltration of rainfall 

into porous, sandy soils that eventually becomes part of the 

ground-water system (recharge). As urbanization continues, the 

landscape changes from naturally pervious soils to impervious 

surfaces, which results in a reduction of the volume of water 

reaching the ground-water system.

Beneath the shallow soils of the Sarasota Bay watershed, 

the ground-water system consists of a series of unconsolidated 

and consolidated marine sediments. The principal hydrogeologic units underlying Sarasota Bay watershed are, in 

ascending order, the Upper Floridan aquifer, intermediate

aquifer system, and surficial aquifer system (fig. 11). 

0

10

20

30

40

50

60

70

197719781979198019811982198319841985198619871988198919901991199219931994199519961997199819992000

YEAR

GROUND-WATER USE,

IN MILLION GALLONS PER DAY

Figure 9. Annual ground-water use in Sarasota County from 1977 to 2000 (from Marella, 2004).

0

5

10

15

20

25

30

35

1975 1980 1985 1990 1995 2000 2005

YEAR

GROUND-WATER USE, IN

MILLION GALLONS PER DAY

Public Supply

Agriculture

Figure 10. Ground-water use for public supply and agriculture in Sarasota County from 1977 to 2000 (from Marella, 2004).
10 Retrospective Review of Watershed Characteristics and a Framework for Future Research 

Upper Floridan Aquifer

In the Sarasota Bay watershed, the Upper Floridan 

aquifer consists of the Suwannee Limestone, which is 

composed of limestone and dolostone with some clay and 

quartz sand. The Upper Floridan aquifer provided 27.2 Mgal/d 

(1,192 L/s) for public supply in Sarasota County in 2000 

(Marella, 2004).

Intermediate Aquifer System

The intermediate aquifer system overlies the Upper 

Floridan aquifer in the Sarasota Bay watershed. The intermediate aquifer system is a series of intercalated permeable zones 

and poorly permeable confining units consisting of sandy 

clay, clay, limestone, and dolostone of the Hawthorn Group 

(Barr, 1996). The permeable zones function regionally as a 

water-yielding hydraulic unit (Duerr and Wolansky, 1986); the 

intermediate aquifer system provided 13.1 Mgal/d (574 L/s) 

for public supply in Sarasota County in 2000 (Marella, 2004). 

Surficial Aquifer System

In 2004, less than 1 percent of the ground-water use 

in Sarasota County was obtained from the surficial aquifer 

system (Marella, 2004). Because of the increase in the demand 

for water in the region, the surficial aquifer system is being 

reevaluated as a potential source of water for irrigation and for 

blending with other sources of water (Southwest Florida Water 

Management District, 1998). Most previous hydrogeologic 

investigations in southwest Florida have been focused on 

deeper zones used for water supply; hence, information about 

the hydrologic characteristics of the surficial aquifer system 

is limited. This section summarizes the available information 

about the hydrologic characteristics of the surficial aquifer 

system.

The surficial aquifer system overlies the intermediate 

aquifer system in the Sarasota Bay watershed and consists of 

undifferentiated surficial deposits that are predominantly fineto medium-grained sand with some shell fragments, clay, and 

limestone. The surficial aquifer system is generally unconfined; 

lenses of sand, clayey sand, and limestone may contain water 

System Series Stratigraphic unit General lithology

Unconsolidated to weakly

indurated clastics

and marine deposits

Pliocene 

Miocene 

Upper

Oligocene 

Lower

Oligocene 

Undifferentiated deposits

Tamiami Formation

Peace River Formation

Hydrogeologic unit

Arcadia

Formation

Suwannee Limestone

Nocatee

Member

Tampa

Member

Fossiliferous limestone

and dolostone, some clay

and quartz sand; some traces

of phosphate near top

Fossiliferous limestone

and dolostone, quartz and

phospahtic sand, and clay

Fine to medium quartz and

phospahtic sand, clayey sand,

limestone, clay, and sand

Fossiliferous limestone and dolostone, clay, quartz

and phospahtic sand, and sandy calcarious clay

Surficial aquifer system

Permeable Zone 1

Upper

Floridan aquifer

Intermediate aquifer system

Confining unit

Confining unit

Permeable Zone 2

Permeable Zone 3

Holocene

and

Pleistocene

Quaternary

Tertiary

Figure 11. Stratigraphic, lithologic, and hydrogeologic units in west-central Florida (modified from Barr, 1996).
Environmental Setting of the Sarasota Bay Watershed 11

under confined conditions in some areas. The surficial aquifer 

system provided 0.04 Mgal/d (1.8 L/s) for public supply in 

Sarasota County in 2000 (Marella, 2004).

The average depth to the water table in the surficial 

aquifer system is generally less than 5 ft (1.5 m) in the Sarasota 

Bay watershed. In the higher elevations of the watershed, the 

water table may be greater than 5 ft below land surface; in 

areas of low topographic relief and near the coast, the water 

table can occur at land surface. Fluctuations of the water table 

are generally seasonal and vary within about a 5-ft (1.5 m) range. 

The seasonal low water table usually occurs during May or 

June, at the end of the dry season. The seasonal high water 

table usually occurs during the wet summer months. For the 

poorly drained Flatwoods soil group in the Sarasota Bay watershed, the depth to the high water table may be as little as 0.5 ft 

(0.15 m) below land surface or, in the case of the Holopaw soil 

series, the high water table may be above land surface.

Major sources of recharge to the surficial aquifer system 

in the watershed are (1) rainfall, (2) upward leakage where 

the elevation of the potentiometric surface of the intermediate 

aquifer system is higher than the water table, (3) infiltration 

of irrigation water, and (4) ground-water inflow from adjacent 

areas. Major types of discharge from the surficial aquifer 

system are (1) evapotranspiration, (2) seepage into streams, 

lakes, swamps, and canals, (3) pumping from wells, and 

(4) downward leakage where the elevation of the water table 

is higher than the potentiometric surface of the underlying 

aquifers (Duerr and Wolansky, 1986). 

The hydraulic properties of the surficial aquifer system 

vary widely because of the heterogeneous nature of the aquifer 

material (such as grain size, sorting, and compaction) and 

the thickness of the unit. The SWFWMD (1988b) compiled 

hydraulic properties from aquifer tests of wells that penetrate 

sections of the surficial aquifer system. Transmissivities 

determined from these tests varied from 267-6,000 ft2/d 

(25-557 m2/d) and storage coefficients varied from 0.05 to 

0.19 (table 2). The surficial aquifer system supplies limited 

water to wells in the Venice area and the yield of these wells 

generally is less than 50 gal/min (189 L/min) (Duerr and 

Wolansky, 1986). 

The locations of geologic sections used to characterize 

the thickness and extent of the surficial aquifer system in the 

vicinity of the Sarasota Bay watershed are shown in figure 12 

(Barr, 1996). Within the Sarasota Bay watershed, the elevation of the top of the surficial aquifer system varies from less 

than 10 ft (3 m) above NGVD 29 near the coast to about 40 ft 

(12 m) above NGVD 29 in the extreme northeastern part of the 

watershed. The surficial aquifer system is thin and contiguous 

along the coast from Manatee County to Charlotte County 

(fig. 13). About 5-10 mi (8-16 km) inland from the coastline, 

the surficial aquifer system is thin from Manatee County 

through a portion of Sarasota County, but thickens near Venice 

and southward toward Engelwood (fig. 14) and eastward from 

Venice toward the Charlotte County boundary (fig. 15). In the 

northern portion of the watershed, the thickness of the surficial 

aquifer system increases slightly near the coastline (fig. 16).

The saturated thickness of the surficial aquifer system varies 

from less than 10 ft (3 m) in the northern coastal area of the 

Sarasota Bay watershed to about 60 ft (18 m) in the southern 

area of the watershed, and is generally less than 20 ft (6 m) 

over most of the watershed (fig. 17) (Barr, 1996).

The direction of ground-water flow in the surficial aquifer 

system is from areas of higher elevation to areas of lower 

elevation (fig. 18); however, this pattern is interrupted locally 

where the aquifer discharges to streams, lakes, or low swampy 

areas. The interaction between ground water and surface water 

in coastal areas depends upon ground-water discharge from 

regional ground-water systems, local ground-water systems 

associated with scarps and terraces, evapotranspiration, runoff, 

and coastal flooding (Winter and others, 1998). Recharge to 

ground water occurs along a north-south orientation at the 

center of the peninsula, where the surficial aquifer system is 

thick and permeable, and the confining clays are thin (Purdum, 

2002). As ground water travels from the middle of the peninsula to the coastline, overlying clays and low-permeability 

sediments confine the ground water (Purdum, 2002). Areas 

of regional recharge to ground water and discharge to surface 

water in the Sarasota Bay watershed are shown in figure 19. 

In coastal areas, the Upper Floridan aquifer is under sufficient 

artesian pressure to cause ground water to flow from wells or 

springs. Generalized patterns of vertical ground-water flow in 

the Sarasota Bay watershed are upward toward the surficial 

aquifer system (Knochenmus and Bowman, 1998). Greater 

hydrostatic pressure in the Upper Floridan aquifer creates the 

potential for ground water to move upward into the surficial 

aquifer system as shown for the well cluster TR 5-2 (fig. 20), 

which is near the middle of the line of cross section B–B' 

between the Walton and Wheelright wells (see fig. 12 for 

location of well). All ground-water levels at this well cluster 

show a seasonal fluctuation. In 1994 after the rainy season 

(October), the surficial aquifer system reached a high that was 

about 4.5 ft (1.4 m) above the low occurring near the end of 

the dry season (May and June). 

Table 2. Hydraulic properties of the surficial aquifer system in 

the vicinity of the Sarasota Bay watershed.

[From Southwest Florida Water Management District (1988). ft2/d, feet 

squared per day; --, no data]

Transmissivity

(ft2/d)

Storage

coefficient Location

267 0.1 Southern Manatee County

600 0.05 Northeast Sarasota County

1,110 0.15 Central Sarasota County

1,805 0.19 Central Sarasota County

1,000 -- West-central Sarasota County

3,800 -- Southwestern Sarasota County

6,000 -- Southwestern Sarasota County
12 Retrospective Review of Watershed Characteristics and a Framework for Future Research 

0

0

5 10 MILES

5 10 KILOMETERS

26°30'

27°00'

27°30'

82°45' 82°15' 81°45'

MANATEE COUNTY

HARDEE COUNTY

DE SOTO

COUNTY

SARASOTA

COUNTY

CHARLOTTE

COUNTY

LEE COUNTY

Base from U.S. Geological Survey digital data, 1:2,000,000, 1972

Albers Equal-area Conic projection

Standard Parallels 29°30' and 45°30', central meridian -83°00'

Gulf of Mexico

Myakka River

Sarasota

Englewood

TR 3-3

A

B

C’ 

D’ 

A’

D

C

W-16999

A A’ TRACE OF HYDROGEOLOGIC SECTION

WELL AND NAME

TR 8-1

BRT 1

ROMP 22

ROMP 23-1

TR 7-1

Reagan

TR 7-2

Cowart

Ranch

Gulf Gate 

NPIW

W-9185

Bay Indies

South Venice

Sorrento Shores

France

TR 6-1

KME

Ranch 

TR 4-2

ROMP 10

MS 8

TR 5-1

Key Towers

TR 3-3

Placida

TR 5-2

IC-10

QWIP 21-467

Oaks 6

ROMP 20

TR 6-3

GIIW

Walton

Venice

East

Wheelwright

B’ 

EXPLANATION

Figure 12. Locations of geologic sections in the study area (modified from Barr, 1996).
Environmental Setting of the Sarasota Bay Watershed 13

TR 8-1 (1,2,3,4)TR 7-1 (1,2,3,4)

TR 6-1 (SECTION C-C') (1,2,3,4)GULF GATE (3)

ROMP 20 (1,2,4)

SORRENTO SHORES (2)

TR 5-1 (1,2,3,4)

BAY INDIES (2)

SOUTH VENICE (1,2,3)

TR 4-2 (SECTION D-D') (1,2,3,4)

FRANCE (2,3)

TR 3-3 (SECTION B-B') (1,2,4)

PLACIDIA (1,2)

MANATEE CO.

KEY TOWERS (2,3)

100

FEET

100

200

300

400

500

600

700

800

GIIW (1,2) LEE CO.

W-16999 (1,2,4)OAKS 6 (2,3)

SARASOTA CO.

CHARLOTTE CO.

PZ3

UFA

SAS

PZ1

PZ2

EXPLANATION

SURFICIAL AQUIFER SYSTEM

INTERMEDIATE AQUIFER SYSTEM

 Confining unit

 Permeable zone 1

 Permeable zone 2

 Permeable zone 3

UPPER FLORIDAN AQUIFER

ESTIMATED

DATA USED FOR INTERPRETATION:

(1) Lithologic log

(2) Gamma log

(3) Electric log

(4) Head

SAS

PZ1

PZ2

PZ3

UFA

VERTICAL SCALE GREATLY EXAGGERATED

0 5 10 KILOMETERS

0 5 10 MILES

NGVD

1929

A A' CHARLOTTE CO. IC-10 (2,3) TR 7-2 (1,2,4) SARASOTA CO.

Figure 13. Geologic section A–A’ (modified from Barr, 1996).
14 Retrospective Review of Watershed Characteristics and a Framework for Future Research 

Figure 14. Geologic section B–B’ (modified from Barr, 1996).

PZ3

UFA

SAS

PZ1

PZ2

REAGAN (2,3)

100

FEET

100

200

300

400

500

600

700

0 5 10 KILOMETERS

VERTICAL SCALE GREATLY EXAGGERATED 0 5 10 MILES MS 8) (2,3) MANATEE CO. SARASOTA CO. COWART RANCH (2) BRT 1 (SECTION C-C') (2) WALTON (1,2,3) TR 5-2 (1,2,3,4) WHEELWRIGHT (2,3) VENICE EAST (2,3) QWIP 21-467 (SECTION D-D') (2,3) TR 3-3 (SECTION A-A') (1,2,4)

B B'

NGVD

1929

EXPLANATION

SURFICIAL AQUIFER SYSTEM

INTERMEDIATE AQUIFER SYSTEM

 Confining unit

 Permeable zone 1

 Permeable zone 2

 Permeable zone 3

UPPER FLORIDAN AQUIFER

ESTIMATED

DATA USED FOR INTERPRETATION:

(1) Lithologic log

(2) Gamma log

(3) Electric log

(4) Head

SAS

PZ1

PZ2

PZ3

UFA
Environmental Setting of the Sarasota Bay Watershed 15

TR 6-3 (1,2,3,4)BRT 1 (SECTION B-B') (2)TR 6-1 (SECTION ROMP 23-1 (1,2)

A-A') (1,2,3,4)

ROMP 22 (1,2,3,4)

SARASOTA CO. 

KME RANCH (2)

CHARLOTTE CO.

C C'

100

FEET

NGVD

1929

100

200

300

400

500

600

0 5 10 MILES

0 5 10 KILOMETERS

VERTICAL SCALE GREATLY EXAGGERATED

PZ3

UFA

SAS

PZ1

PZ2

EXPLANATION

SURFICIAL AQUIFER SYSTEM

INTERMEDIATE AQUIFER SYSTEM

 Confining unit

 Permeable zone 1

 Permeable zone 2

 Permeable zone 3

UPPER FLORIDAN AQUIFER

ESTIMATED

DATA USED FOR INTERPRETATION:

(1) Lithologic log

(2) Gamma log

(3) Electric log

(4) Head

SAS

PZ1

PZ2

PZ3

UFA

Figure 15. Geologic section C–C’ (modified from Barr, 1996).
16 Retrospective Review of Watershed Characteristics and a Framework for Future Research SARASOTA CO. CHARLOTTE CO. TR 4- 2 (SECTION A-A') (1,2,3,4) QWIP 21- 467 (SECTION B-B') (1,2) NPIW (1,2) ROMP 10 (1,2,3)

DE SOTO CO.

CHARLOTTE CO.

W-9185 (1)

100

FEET

100

200

300

400

500

600

700 0 10 MILES

5

5

0 10 KILOMETERS

PZ3

UFA

SAS

PZ1

PZ2

VERTICAL SCALE GREATLY EXAGGERATED

D D'

NGVD

1929

EXPLANATION

SURFICIAL AQUIFER SYSTEM

INTERMEDIATE AQUIFER SYSTEM

 Confining unit

 Permeable zone 1

 Permeable zone 2

 Permeable zone 3

UPPER FLORIDAN AQUIFER

ESTIMATED

DATA USED FOR INTERPRETATION:

(1) Lithologic log

(2) Gamma log

(3) Electric log

(4) Head

SAS

PZ1

PZ2

PZ3

UFA

Figure 16. Geologic section D–D’ (modified from Barr, 1996).
Environmental Setting of the Sarasota Bay Watershed 17

MANATEE COUNTY

HARDEE COUNTY

DE SOTO

COUNTY

SARASOTA

COUNTY

CHARLOTTE

COUNTY

LEE COUNTY

30

20

20

10

30

40

50

30

40

10

10

20

10

20

30

40

50

20

3 0

20

30

40

50

60

10

30

20

20

10

Gulf of Mexico

EXPLANATION

0

0

5 10 MILES

5 10 KILOMETERS

26°30'

27°00'

27°30'

82°45' 82°15' 81°45'

30 GENERALIZED LINE OF EQUAL THICKNESS OF

THE SURFICIAL AQUIFER SYSTEM – Interval

is 10 feet. Dashed where approximate.

Hachures indicate areas of thinner beds

Myakka River

Base from U.S. Geological Survey digital data, 1:2,000,000, 1972

Albers Equal-area Conic projection

Standard Parallels 29°30' and 45°30', central meridian -83°00'

Figure 17. Generalized thickness of the surficial aquifer system in the study area (modified from Barr, 1996).
18 Retrospective Review of Watershed Characteristics and a Framework for Future Research 

MANATEE

COUNTY

HARDEE COUNTY

DE SOTO

COUNTY

SARASOTA

COUNTY

CHARLOTTE

COUNTY

LEE COUNTY

Gulf of Mexico

River

Myakka

20

40

10

5

5

5

110

80

60

40

100

5 10 20 30

40

80

80

60

60

20

20

10 10

20

30

20

60

40

30

10

20

40

5

10

10

10

5

5

5

0

0

5 10 MILES

5 10 KILOMETERS

26°30'

27°00'

27°30'

82°45' 82°15' 81°45'

Base from U.S. Geological Survey digital data, 1:2,000,000, 1972

Albers Equal-area Conic projection

Standard Parallels 29°30' and 45°30', central meridian -83°00'

EXPLANATION

10 WATER TABLE SURFACE CONTOUR – Shows 

elevation of water table surface. Contour 

interval is 5, 10, and 20 feet. Datum is NGVD 

1929

DIRECTION OF GROUND-WATER FLOW

Figure 18. Generalized direction of flow in the surficial aquifer system in west-central Florida (modified from 

Berndt and others, 1998 and published with permission).
Environmental Setting of the Sarasota Bay Watershed 19

0

0

5 10 MILES

5 10 KILOMETERS

26°30'

27°00'

27°30'

82°45' 82°15' 81°45'

MANATEE COUNTY

HARDEE COUNTY

DE SOTO

COUNTY

SARASOTA

COUNTY

CHARLOTTE

COUNTY

LEE COUNTY

Base from U.S. Geological Survey digital data, 1:2,000,000, 1972

Albers Equal-area Conic projection

Standard Parallels 29°30' and 45°30', central meridian -83°00'

Gulf of Mexico

Myakka River

Sarasota

Englewood

EXPLANATION

DISCHARGE AND RECHARGE RATE FOR 

THE UPPER FLORIDAN AQUIFER — In 

inches per year

Discharge less than 1

Recharge less than 1

SARASOTA BAY WATERSHED BOUNDARY

Figure 19. Generalized recharge/discharge areas of the Upper Floridan aquifer in west-central Florida (data 

from the Southwest Florida Water Management District, 2005).
20 Retrospective Review of Watershed Characteristics and a Framework for Future Research 

Maintaining the surficial aquifer system at or near 

predevelopment water levels by increasing recharge from the 

surface can minimize saltwater intrusion and stabilize the 

ground-water supply. Recharge to the ground-water system 

also reduces pollutant loading to surface water by reducing 

the volume of runoff available to transport contaminants to 

surface-water bodies. Surface water that infiltrates through 

soils and becomes part of the ground-water system receives 

natural filtration.

One of the current challenges to the Sarasota Bay area 

and other coastal communities is to maintain a balance 

between (1) removing surface-water runoff rapidly to prevent 

flooding during heavy rainfall events in shallow water-table 

environments, (2) maintaining the ground-water supply as 

demand continues to increase, and (3) reducing pollutant loads 

to surface waters to maintain the health of Sarasota Bay.

Watershed Characteristics 

Influencing Recharge and Discharge 

Recharge to ground water and overland runoff to surface 

water in the Sarasota Bay watershed are affected by soil 

characteristics (porosity, bulk density, penetration resistance, 

and infiltration rate), the water budget, urbanization and land 

use, land practices (construction, agriculture, and recreation), 

and infiltration and runoff.

Soil Characteristics

The water-holding potential and compaction of soils can 

be inferred or estimated by physical measurements of porosity, 

bulk density, penetration resistance, and infiltration rate. 

Porosity, Bulk Density, and Penetration 

Resistance

Because soil consists of both solid particles and air 

spaces (voids), the volume of the voids determines the 

maximum space available for storing water. Porosity increases 

as the volume of the voids increases with respect to the total 

volume of solids. Porosity is the ratio of the volume of the 

voids to the total volume of soil expressed as a percent:

Porosity (n) = [(V(total) – V(solids))/V(total)] × 100, (1)

where

V(total) is the total volume of soil,

V(solids) is the volume of the solids, and

V(total) – V(solids) is the volume of the voids.

8

32

10

12

14

16

18

20

22

24

26

28

30

ELEVATION OF WATER LEVEL, IN FEET ABOVE NGVD 1929

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

1993 1994

Surficial aquifer system monitor well, casing 8 feet, depth 13 feet

Intermediate aquifer system, permeable zone 2 monitor well, casing 100 

feet, depth 120 feet

Intermediate aquifer system, permeable zone 3 monitor well, casing 245 

feet, depth 265 feet

Intermediate aquifer system, permeable zone 3 monitor well, casing 360 

feet, depth 400 feet

Upper Floridan aquifer monitor well, casing 510 feet, depth 700 feet

Figure 20. Elevation of water levels in the surficial, intermediate, 

and Upper Floridan aquifer systems in well cluster TR 5-2 near 

Walton for 1993-94, west-central Florida (modified from Barr, 1996; 

see fig. 12 for location of well).

Ground-water withdrawals have lowered potentiometric 

heads in the Upper Floridan aquifer from 30-50 ft (9-15 m) 

below predevelopment levels in the upper reaches of the 

Sarasota Bay watershed (Broska and Knochenmus, 1996). 

The decline in potentiometric heads increases the potential for 

recharge from the surficial and intermediate aquifer systems to 

the Upper Floridan aquifer. Ground-water withdrawals from 

major pumping centers in coastal areas of the southwestern 

part of the watershed have caused lateral saltwater intrusion 

and upconing into both the surficial and intermediate aquifer 

systems (Barr, 1996). Regional ground-water movement, 

ground-water withdrawals, and proximity to the coast have 

produced high chloride concentrations in the Upper Floridan 

aquifer (Barlow, 2003). The chloride concentrations in the surficial aquifer system exceed 5,000 mg/L near Venice, and exceed 

1,000 mg/L near Englewood (fig. 21). A steady increase in 

saltwater intrusion has been associated with increased groundwater withdrawals, especially in the vicinity of operating well 

fields (Southwest Florida Water Management District, 1998).
Watershed Characteristics Influencing Recharge and Discharge 21

Bulk density provides an estimate of soil density and is the 

weight of a known volume of oven-dried soil divided by the 

volume of the soil expressed as grams per cubic centimeter 

(Brady, 1990):

Bulk density (Db) = dry weight of soil/volume of soil. (2)

Porosity and bulk density are closely related soil 

characteristics; as the volume of pore spaces increases, bulk 

density decreases (table 3). Bulk density measurements for 

peat typically range from 0.2-0.3 g/cm3 (12.5-18.7 lb/ft3), 

whereas the porosity of peat is about 92 percent (Schueler, 

2000a). Glacial till, compressed beneath ice sheets during 

0

0

5 KILOMETERS

5 MILES

SARASOTA CO. SARASOTA CO.

MANATEE CO.

CHARLOTTE CO.

Myakka River

Gulf of Mexico

40

33

250

27°00'

27°15'

82°30' 82°15'

40

33

180

33

206

145

145

13

14

19

13

7,000

12

58

10

32

11

68

131

224

204

186

200

79

205

68

203

54

202

56

201

3,900

159

43

199

132

139

118

142

124

Englewood Water District

well field 2

Englewood Water District

well field 3

Englewood Water District

well fields 1 and 4

Venice Gardens Utility

Venice Well Fields

Plantation Utility

Osprey

Venice

Englewood

170

7

207

38

198

4,250 196

140

15

192

130

265

100

100

250

100

1,000

5,000

250

250

100

1,000

1,000

5,000

1,000 250

100

100

100

EXPLANATION

Base from U.S. Geological Survey digital data, 1:2,000,000, 1972

Albers Equal-area Conic projection

Standard Parallels 29°30' and 45°30', central meridian -83°00'

MAJOR PUMPING CENTER — Well field 

or utility

LINE OF EQUAL CHLORIDE CONCENTRATION — In milligrams per liter. 

Concentration interval variable. Dashed 

where approximate. Hachures indicate 

areas of lesser value

WELL — Upper number is index number 

in table 3 (Barr, 1996); lower number is 

chloride concentration

Figure 21. Generalized isopleths of chloride concentration in the surficial aquifer system (modified from Barr, 1996).
22 Retrospective Review of Watershed Characteristics and a Framework for Future Research 

the last Ice Age, exhibits a bulk density range of 1.6-2.0 g/cm3

(99.9-124.9 lb/ft3) and porosity range of 10 to 20 percent 

(Fetter, 1994). Bulk densities of Flatwoods soils in the Sarasota 

Bay watershed vary from 1.25-1.70 g/cm3 (78.0-106.1 lb/ft3) 

(Soil Conservation Service, 1991). Under natural conditions, 

bulk density generally increases with depth, a consequence of 

compression by the overlying soil and the scarcity of organic 

matter and biological activity (Schueler, 2000a). Measurements 

of bulk density can be used to estimate porosity and the effect 

of porosity on the water-holding capacity, infiltration, and root 

penetration of the soil (Schueler, 2000a). 

Penetration resistance is a measure of the external force 

required to drive a probe a specific distance into the soil. 

Penetration resistance is a rapid field-based measurement 

frequently used as a surrogate for bulk density to measure 

compaction in altered soils.

Infiltration Rate

Infiltration rate is the rate at which water enters the 

soil and is related to porosity and bulk density. A doublering infiltrometer is used typically to measure infiltration 

rate in units of inches per hour or millimeters per hour (Pitt 

and others, 1999). Infiltration rate is highest for sand at 

greater than 2.0 in/hr (5.1 cm/hr) and lowest for clay at less 

than 0.1-0.2 in/hr (0.25-0.51 cm/hr) (table 3). The infiltration rate is also related to bulk density for a particular soil. 

Smith and others (2001) observed that infiltration is greater 

than 14 in/hr (36 cm/hr) when the bulk density is less than 

1.5 g/cm3 (93.6 lb/ft3), but as bulk density reaches 1.65 g/cm3

(103.0 lb/ft3), infiltration drops to less than 2 in/hr (5.1 cm/hr). 

As bulk density increases beyond 1.65 g/cm3 (103.0 lb/ft3), 

infiltration approaches zero. Infiltration is negligible on impervious surfaces (for example, sidewalks and parking lots) or 

when the rainfall exceeds the capacity of a pervious medium 

to hold or transmit water. In soils with frequent wetting and 

drying cycles, infiltration may be inhibited by a crust on the 

soil surface formed by the breakdown of soil structure by 

flowing water or raindrops (Natural Resource Conservation 

Service, 1996a). 

In dry soils, infiltration begins rapidly but approaches a 

uniform, low rate as the soil becomes saturated. This relation 

was first described by Horton in the 1930s and is referred to 

as Horton infiltration (Freeze and Cherry, 1979). The initial 

precipitation rate is equal to the infiltration rate; as rainfall 

continues, infiltration decreases but reaches a constant rate. 

The water table is assumed to be deep and does not play a role 

in the infiltration rate. Runoff occurs when the precipitation 

rate exceeds the infiltration rate.

In humid, high water-table environments, the capacity 

of a pervious medium to hold or transmit water is affected 

by the proximity of ground water to land surface. Dunne 

infiltration (also described as saturation excess runoff) is most 

often ascribed to conditions in Florida, primarily during the 

rainy season. In Dunne infiltration, a dynamic water level is 

assumed; as rainfall continues, available pore spaces are filled. 

When all pore spaces are filled, no additional infiltration can 

occur. The precipitation becomes runoff because the surface 

is virtually impervious and no additional infiltration can occur 

(fig. 22).

Table 3. Selected physical and hydrologic properties for selected earth materials.

[From Heath (1983) and Dragun (1988). ppv, percent per volume; g/cm3, grams per cubic centimeter; 

in/hr, inch per hour; –, no data; <, less than; >, greater than]

Earth material Porosity

(ppv)

Specific 

yield

(ppv)

Specific 

retention

(ppv)

Bulk density

(g/cm3)

Infiltration rate

(in/hr)

Peat 92 – – 0.2 – 0.3 –

Average soil 55 40 15 – –

Clay 50 2 48 1.0 – 1.2 <0.1 – 0.2

Sand 25 22 3 1.1 – 1.3 0.5 – >2.0

Gravel 20 19 1 – –

Limestone 20 18 2 – –

Sandstone 11 6 5 – –

Basalt 11 8 3 – –

Glacial till 10 – – 1.6 – 2.0 –

Granite 0.1 0.09 0.01 – –
Watershed Characteristics Influencing Recharge and Discharge 23

In Florida, Horton infiltration is likely to predominate in 

upland areas of Scrub and Sandhill ecosystems, typically along 

the ridge area of central Florida, whereas Dunne infiltration is 

predominant in high water-table areas, especially in discharge 

areas along or near the coastline of Sarasota Bay. Figure 22 

shows the differences between Horton and Dunne infiltration, 

and table 4 describes the conditions under which Horton or 

Dunne infiltration predominates. In west-central Florida where 

the water table is at or near land surface, Dunne infiltration 

and saturation excess runoff may occur early during a rainfall 

event. Over the course of wet and dry seasons, both Horton 

and Dunne infiltration may occur.

Water Budget

Henry (1998) estimated a simple climatic water budget 

for Tampa that is used here to represent conditions in the 

Sarasota Bay watershed. The water budget considers only 

precipitation, potential evapotranspiration, actual evapotranspiration, and soil storage. Potential evapotranspiration 

is defined as the amount of water that would be lost from the 

soil if there was always sufficient water available for plants to 

transpire at their maximum rates. Actual evapotranspiration 

is the amount of water that would be lost based on the availability of soil moisture. Henry (1998) defines a soil-water 

0

0 n

t

1

t

2

t

3

t

4

t

3

t

4

t

0

- t4

t

1

- t5

0

0

n

t

1

t

2

t

3

t

4

t

5

t

4

t

0

- t2

t

3

t

0

t

0

DEPTH

MOISTURE CONTENT 



MOISTURE CONTENT 

RATE

p

0

t

1 t0 t2 t3 t4 t1 t0 t2 t3 t5 t4

Rainfall

Infiltration

Overland

flow

TIME

KS

KS

KS

p

0

TIME

(A) (B)

(C) (D)

EXPLANATION

Theoretical start time for precipitation

Theoretical time steps as precipitation continues

Moisture content of subsurface medium

Saturated moisture content of subsurface medium

Precipitation rate

Hydraulic conductivity of saturated subsurface medium

Rainfall

Infiltration

Overland flow

Inverted zone of saturation

Water table

Figure 22. Relation between moisture content and soil depth: (A) Horton infiltration, (B) Dunne 

infiltration, (C) Horton runoff, and (D) Dunne runoff (reprinted from Freeze, 1980 and published 

with permission).
24 Retrospective Review of Watershed Characteristics and a Framework for Future Research 

deficit as the difference between potential evapotranspiration 

and actual evapotranspiration. When precipitation exceeds 

actual evapotranspiration, soil storage increases (recharge); 

when precipitation is less than actual evapotranspiration, water 

stored in soil is reduced because of evaporation from soil 

and transpiration by plants. From September until mid-May, 

precipitation is generally less than evapotranspiration and 

water needs of plants are satisfied by utilizing soil moisture 

(fig. 23). By mid-January, much of the available soil moisture 

is gone and the system runs at a deficit until the summer rains 

begin. From about mid-May until September, precipitation 

contributes the bulk of the recharge to the system.

Urbanization and Land Use

Coastal areas of the Sarasota Bay watershed are 

characterized by high-density residential, recreational, and 

commercial development, whereas inland areas are characterized by agricultural land use (Knochenmus and Bowman, 

1998). Population growth has affected the landscape by 

causing loss of habitat and natural vegetative cover, while 

simultaneously increasing the spatial extent of impervious 

surfaces (Sarasota Bay National Estuary Program, 2000). 

The increase in impervious cover alters the hydrology of 

the watershed and necessitates infrastructure changes to 

handle the increased runoff. Land practices that may affect 

hydrologic processes in the watershed include agriculture, 

construction, and recreation. 

Land Practices

Most land practices tend to compact soil and reduce 

water movement through soils, either by design or as a 

consequence of using the land. Agricultural and construction 

practices affect the infiltration of rainfall to the ground water 

on a large scale. Recreational areas, often thought to provide 

pockets of increased infiltration in urbanized areas, may 

provide limited infiltration capacity during rainfall events. 

The concept of compaction and its effect on infiltration is 

well documented in the agricultural literature and provides 

the basis for understanding the effects of construction and 

recreational activities on soil structure and infiltration. These 

concepts applied to the Sarasota Bay watershed provide a 

useful framework for understanding runoff processes and 

their effects on recharge to and discharge from the surficial 

aquifer system. 

Table 4. Characteristics of Horton and Dunne mechanisms for infiltration and runoff.

[From Knighton (1998)]

Characteristic Horton overland flow Dunne overland flow

Rainfall High rates, short duration Light to moderate rate, long duration

Infiltration Surface infiltration capacity Transmissibility in lower soil horizons

Temporal distribution Begins when rainfall intensity exceeds 

infiltration rate

Begins only when underlying soil layers 

are saturated

Spatial distribution Semi-arid, poorly drained areas Humid, high water-table areas

10

9

8

7

6

5

4

3

2

1

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

WATER DEPTH, IN INCHES

Water deficiency

Soil moisture utilization

Soil moisture recharge

EXPLANATION

Rainfall

Potential evapotranspiration

Actual evapotranspiration

Figure 23. Average monthly water budget for west-central Florida 

(modified from Henry, 1998 and published with permission).
Watershed Characteristics Influencing Recharge and Discharge 25

Agriculture

Compaction is a common problem in agricultural soils. 

Compaction may damage the soil structure and (1) restrict 

rooting depth and growth, (2) decrease infiltration of water 

and nutrients reaching the plant roots, and (3) increase runoff 

and erosion (Peet, 1995; Natural Resource Conservation 

Service, 1996a; DeJong-Hughes and others, 2001). Major 

causes of compaction in agricultural soils are raindrop impact, 

tillage, and wheel traffic. Raindrop impact causes a crust 

to form at the soil surface. The crust is commonly less than 

0.5 in. (1.27 cm) thick and can prevent the healthy propagation of seedlings (Natural Resource Conservation Service, 

1996b). Surface hoeing is generally used to break up the crust 

(DeJong-Hughes and others, 2001). Tillage is the mechanical 

moving, turning, or stirring of the soil. Historically, tilling 

has been used to incorporate fertilizers, crop residues and 

pesticides, increase infiltration, control weeds, and reduce 

erosion (Harpstead and others, 1997). The use of certain types 

of conventional tillage over a number of years causes subsurface compaction, creating tillage pans or plow pans. This 

condition can be intensified when plowing occurs during wet 

conditions (McBride and others, 1997). 

Hangen and others (2002) investigated infiltration 

patterns in silty loam that was subjected to conventional tillage 

(plowing and harrowing) and conservation tillage (minimal 

plowing and use of mulches). Tracers used during infiltration 

experiments showed much deeper water infiltration in the 

conservation tillage area. The study also quantified earthworm 

activity and macropores from crop stems and root channels. 

They concluded that deep percolation of water in the silty 

loam under conservation tillage was the result of an extensive 

macropore network that is often missing in conventionally 

tilled fields. Carmen (2002) investigated the effect of vertical 

load exerted by tires and vehicle speed on the compaction of 

loamy clay. At maximum tire load, bulk densities increased 

40 percent above background values. Vertical load was the 

most significant contributor to increased soil compaction. 

A secondary factor was velocity of the vehicle. As speed 

increased, compaction was reduced because of decreased 

contact duration between the tires and the soil. Leskew (2002) 

studied subsoil compaction in Ontario, Canada, as the result 

of wheel traffic from heavy agricultural equipment in clays, 

clayey loams, and silty clays. About 50 to 70 percent of the 

agricultural land in four counties was affected by moderate 

to severe subsurface compaction. Leskew (2002) estimated 

an increase in phosphorus loading to the Great Lakes of 

77,200-86,900 lb/yr (35,000-39,400 kg/yr) and increased 

soil erosion of 17 to 39 percent as the result of increased 

runoff caused by soil compaction. Meek and others (1992) 

developed a relation between infiltration, hydraulic conductivity, and bulk density for disturbed and undisturbed soils 

in a sandy loam. In disturbed soils, as bulk density increased 

from 1.6-1.8 g/cm3 (99.9-112.4 lb/ft3), infiltration decreased 

by 53 percent and hydraulic conductivity decreased by 

86 percent.

Management practices to reduce soil compaction and 

increase infiltration in agricultural fields have focused on 

adding organic matter (Peet, 1995) and deep-rooted plants 

(Rosolem and others, 2002), minimizing tilling or wheel 

traffic in wet conditions (McBride and others, 1997), controlling traffic (limiting the number of passes of heavy equipment 

and using the same paths), and reducing the axle load of heavy 

equipment (Leskew, 2002).

Construction

Research from agricultural practices has direct application 

to the movement of heavy equipment and vehicles during 

the construction of home sites. Soil compaction resulting 

from construction practices is site specific, but development 

of large tracts of land can affect both ground-water recharge 

and stormwater runoff in the Sarasota Bay watershed. Soil 

compaction is desirable for most development projects (for 

example, highways, earth dams, levees, and building pads) 

and is often mandated by code. In construction engineering, 

compaction is defined as the process of making soil more dense 

by squeezing air out of the voids to increase strength, decrease 

permeability, decrease compressibility, and to help reduce 

the effects of frost heaving (Strata, Inc., 2002). The goal of 

compaction for construction is to “densify” or reduce the void 

ratio (increase the dry weight) of the soil by fracturing grains 

and by reorienting, bending, or distorting particles. Compaction 

is accomplished by applying tampers or hammers to mechanically compact or densify the soil. Water is often added to 

attain a specific moisture content to enhance the compaction 

process. The optimal water content to attain the highest level of 

soil compaction for an average soil (maximize the dry weight 

of a soil) is shown in figure 24. For dry soils, the unit weight 

increases as water is added and the water lubricates the soil 

140

130

120

110

100

90

80

10 12 14 16 18 20

DRY UNIT WEIGHT,

IN POUNDS PER CUBIC FOOT

WATER CONTENT, IN PERCENT

Optimum water

content

Zero air voids

Figure 24. Generalized relation between compaction 

and moisture content of an average soil (modified from 

Goldsmith and others, 2001).
26 Retrospective Review of Watershed Characteristics and a Framework for Future Research 

particles, making compaction easier. When water is added 

beyond the optimum water content, the void spaces become 

filled with water and compaction ceases.

Schueler (2000b) compared the bulk densities of 

mechanically compacted soils for common urban settings 

(table 5). The upper range of bulk density for all of the activities in table 5 is greater than 1.65 g/cm3 (103.0 lb/ft3), the bulk 

density at which infiltration is very slow (Smith and others, 

2001). Table 6 provides bulk densities for a range of soil 

textures at which root growth may be affected; bulk densities ideal for root growth varied from less than 1.10 g/cm3

(68.7 lb/ft3) for clays to less than 1.60 g/cm3 (99.9 lb/ft3) 

for sands and loamy sands (Natural Resource Conservation 

Service, 1996b, 2000). Traditional large-scale land development practices may increase compaction over large areas. 

Infiltration may be restricted by compaction resulting from 

construction traffic and impervious structures (streets and 

driveways).

Regional recharge to ground water depends upon the 

pervious nature of the landscape. If pervious features are 

reduced, ground-water levels can be lowered and erosion 

and flooding can increase (Prince George’s County, 1999). 

Conventional approaches convey and manage runoff generated 

by impervious surfaces in large facilities located at the base 

of drainage areas (U.S. Environmental Protection Agency, 

2000a). Water is stored for a specific design volume and time 

to minimize flooding potential. In much of the Sarasota Bay 

watershed, the water table is close to land surface during 

certain times of the year. Home sites typically require fill to 

raise the level of land surface above the seasonal high water 

table to meet building codes (Mark Shelby, Sarasota County 

Agricultural Extension Service, oral commun., 2004). The 

fill brought to the site has undetermined soil characteristics 

and reworking of the soil alters its structure and infiltration characteristics. Determination of representative soil 

infiltration rates at home sites, therefore, is complicated by 

heterogeneous soil mixing during home construction. A study 

to evaluate soil compaction characteristics of reworked soils 

in residential neighborhoods in Sarasota County is currently 

being conducted (Sudeep Vyapari, University of Florida, oral 

commun., 2007). 

Construction at a home site typically involves large 

vehicles delivering materials, pouring concrete slabs, hoisting 

structural framework, and so forth. The tire pressure exerted 

on the soil surface by vehicles may substantially increase 

surface and subsurface soil compaction. Site grading before 

final landscaping also increases compaction. Schueler (2000b) 

examined the practice of selective grading (the practice of 

grading limited areas) and determined that selective grading 

did not reduce soil compaction. Randrup and Dralle (1997) 

evaluated the influence of different types of landscaping on 

soil compaction at 17 construction sites in Denmark. They 

observed higher levels of compaction at all of the construction sites compared with the reference sites. Landscaping had 

no effect on compaction because construction traffic caused 

compaction before turf or landscaping beds were installed. 

Because home lawns account for about 70 percent of the 

total turf area in urbanized landscapes, they represent substantial 

potential for infiltration (Schueler, 2000c). Legg and others 

(1996) deployed a rainfall simulator to observe infiltration from 

20 residential lawns in Madison, Wisconsin. They observed that 

the infiltration rate for lawns from 1 to 3 years after establishment was 0.43 in/hr (1.1 cm/hr); whereas the infiltration rate 

for lawns greater than 7 years old varied from 1.69-2.13 in/hr 

(4.3-5.4 cm/hr). Similarly, Hamilton and Waddington (1999) 

evaluated infiltration rates in an urbanized area of central 

Pennsylvania. Infiltration rates were low for 67 percent of the 

lawns evaluated, and they concluded that infiltration rates were 

related to construction activities at each of the building sites; 

the highest infiltration rate was observed at a location that was 

not excavated prior to construction. Installation of turf over a 

graded, compacted soil had little effect on reducing compaction. Schueler (2000b) reported that bulk density decreased by 

only 0.20 g/cm3 (12.5 lb/ft3) over a 20-year period for lawns 

(listed as “Time” in table 7). Peet (1995) noted that the addition 

of organic matter to the soil improves soil structure, enhances 

aeration and increases pores spaces, and promotes the penetration of water and roots into the soil. Pitt and others (1999) 

amended compacted urban soils with organic matter, and over 

several years infiltration increased by 1.5 to 10 times above the 

precompost infiltration rate.

Subsequently, Schueler (2000b) noted that only compost 

amendments and reforestation offer the potential for substantial decreases in bulk density (table 7). Rosolem and others 

(2002), however, studied a variety of plants that could be 

used as cover crops to modify soil compaction by vigorous 

root growth and development of macropores. The grasses 

Pennisetum americanum (pearl mullet) and Sorghum bicolor

(guinea-guinea sorghum) exhibited the highest root density 

and were recommended as possible cover crops for soils 

affected by compaction. Meek and others (1989) observed that 

infiltration rates under alfalfa cultivation doubled or tripled 

over a 3-year period, in both compacted and undisturbed soils. 

The alfalfa macropores allowed water to bypass the compacted 

surface layer. Shaw and Schmidt (2003) recommended 

northern climate plants with roots extending deep into the 

ground that aid infiltration by acting as pathways for the flow 

of water. Several of these plants reach west-central Florida 

through their native range:

Amorpha fruticosa (Indigo bush)

Asclepias tuberosa (Butterfly weed)

Cephalanthus occidentalis (Common buttonbush)

Eryngium yuccifolium (Rattlesnake master)

Euthamia graminifolia (Grass-leafed goldenrod)

Osmunda regalis (Royal fern)

Panicum virgatum (Switchgrass)

Pteridium aquilinum (Bracken fern)

Tradescantia ohiensis (Ohio spiderwort)

Schizachyrium scoparium (Little bluestem)

Sorghastrum nutans (Yellow indiangrass)
Watershed Characteristics Influencing Recharge and Discharge 27

water efficient landscaping regulations in 2002 that embodies 

the principles of the FY&N program (Sarasota County 

Extension Service, 2006).

Gregory (2004) evaluated how compaction affects 

infiltration rates in sandy soils of north-central Florida. Field 

experiments indicated that compaction substantially reduced 

the measured infiltration rates. Infiltration rates also were 

measured at three locations with installed pervious pavement. 

Infiltration rates on these pavements were highly variable 

depending upon the construction of the subgrade. Gregory 

(2004) concluded that substantial reductions in imperviousness at the lot scale for driveways (and parking areas) could be 

realized with appropriately engineered pervious surfaces.

Recreation 

The Sarasota Bay watershed contains many recreational 

areas including golf courses, sports fields, camping areas, 

beaches, and trails for hiking or cycling. Research from 

other areas has indicated that soil compaction can occur in 

high-use recreational areas. Subsurface soils from 3.9-7.9 

in (9.9-20.0 cm) deep along a pioneer trail in Iowa had 

substantially greater bulk densities than areas adjacent to the 

trail. The study concluded that the trail has been compacted 

for more than 150 years and that compaction persists today 

(Brevik and others, 2002). Similarly, Sharrat and others (1998) 

reported visibly evident soil compaction within wheel ruts of 

a pioneer trail used over 100 years ago in Minnesota. Bulk 

density and penetration resistance were 10 percent greater and 

infiltration was 50 percent lower within the trail than outside 

of the trail. An investigation of understory vegetation along 

a nature trail in Japan determined that substantially higher 

bulk densities and lower water content and porosities occurred 

along the nature trail than in the surrounding areas (Bhuji 

and Ohsawa, 1998). They also observed inhibited growth 

among woody shrubs and herbaceous plants, and deformed 

root growth in seedlings in the area of the compacted trail. 

Table 5. Bulk density of common urban settings.

[From Schuler (2000b). g/cm3, grams per cubic centimeter; 

%, percent]

Urban setting

Range of 

bulk density 

(g/cm3)

Urban lawns 1.50 - 1.90

Crushed rock parking lot 1.50 - 2.00

Urban fill soils 1.80 - 2.00

Athletic fields 1.80 - 2.00

Building pads (85% compaction) 1.50 - 1.80

Building pads (95% compaction) 1.60 - 2.10

Concrete pavement 2.20

Quartzite 2.65

Table 7. Soil treatments to reduce bulk density.

[From Schuler (2000b). g/cm3, grams per cubic centimeter]

Soil treatment

Change in 

bulk density 

(g/cm3)

Tilling -0.00 to -0.02

Soil loosening -0.05 to -0.15

Selective grading 0

Soil amendment (fly ash) -0.17

Soil amendment (compost) -0.25 to -0.35

Time -0.20

Reforestation -0.25 to -0.35

Table 6. Effect of bulk density on root growth for various 

soil textures.

[From U.S. Department of Agriculture (1996, 2000). g/cm3, grams 

per cubic centimeter; <, less than; >, greater than]

Soil texture

Bulk density (g/cm3)

Optimum 

for 

root 

growth

Impedes 

root 

growth

Restricts 

root 

growth 

Sands and loamy sands <1.60 1.69 >1.80

Sandy loams, loams <1.40 1.63 >1.80

Sandy clay loams, clay loams <1.40 1.60 >1.75

Silts, silt loams <1.30 1.60 >1.75

Silty loams, silty clay loam <1.10 1.55 >1.65

Sandy clays, silty clays <1.10 1.49 >1.58

Clay (>45 percent clay) <1.10 1.39 >1.47

In a similar way, the Florida Yards and Neighborhoods 

Program (FY&N) promotes appropriate plants, compost, and 

mulch to enhance infiltration and reduce runoff in neighborhood landscapes. The FY&N program was developed to 

provide special educational and outreach activities directed 

at the community for maintaining environmentally responsible landscapes. Among the objectives of the program are 

(1) reducing stormwater runoff and (2) decreasing nonpoint 

source pollution by promoting the value of mixed planting 

beds and soil amendments to conserve water and to enhance 

infiltration (Garner and others, 2001). Sarasota County enacted 
28 Retrospective Review of Watershed Characteristics and a Framework for Future Research 

In a study of the soil characteristics of an urban park in Hong 

Kong, bulk densities (greater than 1.75 g/cm3 or 109.2 lb/

ft3 in some locations), porosity, nutrient and water transmission, organic matter, pH, and cation exchange capacity were 

inadequate to maintain healthy vegetation in the park (Jim, 

1998). Jim suggested that soil assessments and soil improvements should be an integral part of park maintenance. Stowell 

(1994) determined that turf grasses used for golf greens are 

subjected to sufficient soil compaction to prevent root penetration beyond depths of 2-3 in. (5.1-7.6 cm) below land surface. 

Infiltration and Runoff

In 1979, Florida implemented a stormwater management 

goal for new developments to maintain post-development 

peak flow and pollutant loading at predevelopment levels 

(Livingston, 2004). To meet this goal, stormwater runoff 

in Florida is managed for the typical design storm through 

engineered conveyances and detention ponds sized for specific 

catchments. Detention ponds in Florida typically are wet 

detention ponds because the ground-water table is close to 

land surface for much of the year. Wet detention ponds are 

designed to store runoff during storm events and to release 

the water slowly from the pond. The Natural Resources 

Conservation Service (NRCS), formerly the Soil Conservation 

Service (SCS), runoff equation (eq. 3) is used in Florida to 

determine runoff for a particular catchment. The SCS method 

is commonly used in stormwater management because it is 

simple, yields consistent results, and is favored by regulatory 

agencies (Masek, 2002).

Runoff depth (Q) is estimated from a relation between 

precipitation (P), initial abstraction (Ia), and maximum 

potential soil retention (S):

Q = (P – Ia)2 / (P – Ia + S). (3)

Maximum potential soil retention in inches (S) is an 

empirical parameter based upon the runoff curve number (CN):

S = (1000 / CN) - 10, (4)

and initial abstraction is:

Ia = 0.2 * S. (5)

The runoff curve number, CN, is based on the relation 

between runoff, land cover, and soil condition. A runoff CN 

value that approaches 100 indicates increasing runoff. Table 8 

provides a list of runoff CN values for a range of soil and 

land-cover conditions. Columns A through D represent different 

hydrologic soil groups (HSG) according to soil infiltration rates 

and drainage (Soil Conservation Service, 1986) (refer to table 1 

for hydrologic soil group for soil series in the Sarasota Bay 

watershed). Soils in HSG “A” exhibit high infiltration rates and 

good drainage whereas soils in HSG “D” exhibit low infiltration 

rates and poor drainage. An example of a land cover and soil 

condition with a high runoff CN value (98) is a paved driveway 

(table 8). Nearly all of the precipitation that falls on a paved 

driveway becomes runoff. Soils in the Sarasota Bay watershed 

tend to be poorly drained (HSG B/D) with the seasonal high 

water table close to land surface for several months during the 

Table 8. Runoff curve numbers by housing density and hydrologic soil group.

[From U.S. Department of Agriculture (1986). CN, runoff curve number (39-98); CN=39, least 

runoff; CN=98, most runoff; HSG, hydrologic soil group (A-D); HSG A; soils with rapid infiltration; HSG D; soils with slow infiltration; %, percent; <, less than; >, greater than]

Cover type

Increasing runoff curve number (CN)

by decreasing infiltration (HSG) Soil condition

A B C D

Paved driveway 98 98 98 98 Impervious

Commercial district 89 92 94 95 85% impervious

Newly graded area 77 86 91 94 No vegetation

Housing lot (<1/8 acre) 77 85 90 92 65% impervious

Housing lot (1/4 acre) 61 75 83 87 38% impervious

Housing lot (1/2 acre) 54 70 80 85 25% impervious

Housing lot (2 acres) 46 65 77 82 65% impervious

“Poor” open lawn 68 79 86 89 50% grasses

“Good” open lawn 39 61 74 80 >75% grasses
Watershed Characteristics Influencing Recharge and Discharge 29

rainy season. Compaction may act to further reduce infiltration. 

A “good, open lawn” contains at least 75 percent grass (Soil 

Conservation Service, 1986). In an area where soils are poorly 

drained (HSG D), a Myakka soil overlain by a good, open 

lawn may exhibit a runoff CN value of 80, according to table 8. 

Recreational areas with a good, open lawn, such as ball fields, 

trails and golf courses may have a higher than expected runoff 

CN value if these areas have been compacted by frequent foot 

or vehicle traffic. The degree of compaction and the influence 

of compaction on infiltration and runoff are unknown in the 

Sarasota Bay watershed.

Land cover is based on land-use categories described 

by the Soil Conservation Service (1986) and each land-use 

category has an assigned rainfall-runoff CN. The CN values 

are based on infiltration results from previous literature. 

Subsequent land practices may substantially alter infiltration 

characteristics and the runoff CN value originally assigned 

(Schueler, 2000a). In urban settings, soil is treated as a single 

soil type (urban soil), although infiltration characteristics 

vary dramatically in these soils and infiltration cannot be 

characterized adequately without site-specific measurements. Recognizing that urban soil characteristics are poorly 

understood or documented, the NRCS conducted an extensive 

research program to characterize urban soils in New York City. 

Infiltration rates in these urban soils varied substantially from 

site to site, prompting a new soil classification scheme for 

urban soils by the International Committee on Anthropogenic 

Soils (2003).

Hawkins and others (2002) determined that the value of 

0.2 in eq. 5 was not corroborated for humid or arid environments using a least-squares fitting routine. In other words, 

the initial abstraction, Ia, is not fixed at 0.2 of the maximum 

potential soil retention, S, for all soil types. The study found 

that a value of 0.05 resulted in a better fit for data collected 

from more than 300 watersheds.

Golding (1997) challenged the use of the SCS method in 

Florida because of subjectivity in determining the runoff CN 

value. The runoff CN method for computing infiltration losses 

from rainfall using rainfall-runoff coefficients was developed 

to address deep water-table conditions in the Midwest; 

however, the method produces erroneous runoff estimates in 

shallow water-table environments (Masek, 2002). Research 

from several subbasins in west-central Florida indicates that 

widely used rainfall-runoff models produce large errors in 

predicting runoff, and adjusting runoff CN does not improve 

predictions substantially (Trommer and others, 1996a,b). 

The runoff CN values are based upon Horton infiltration where substantial depth to the water table, substantial 

pore space for water storage, and constant rainfall intensity 

(rainfall rate) is assumed. Initially, the infiltration rate equals 

the rainfall rate. The infiltration rate decreases gradually to the 

hydraulic conductivity of the saturated zone as the soil column 

becomes partially saturated. With a constant rainfall rate, 

the gradual decrease in infiltration rate is accompanied by a 

gradual increase in the runoff rate. The mechanism, therefore, 

assumes that rate differences control runoff.

In Florida, Horton infiltration is likely to be limited to 

either upland areas of Scrub and Sandhill ecosystems, typically along the ridge areas of central Florida, or lowland areas 

only after extended periods of drought. In the headwaters area 

of the Sarasota Bay watershed, upland areas occupy a small 

portion of the DeSoto Plain where dry Flatwood and Sandhill 

ecosystems are present. In high water-table areas, especially 

in discharge areas along the Southern Gulf Coastal Lowlands, 

runoff processes are dominated by Dunne infiltration and by 

lateral subsurface return flow. Dunne infiltration assumes 

a dynamic water level; as rainfall continues, available pore 

spaces are filled and the water level continues to rise. No 

runoff occurs until the pore spaces are filled. When soil saturation is reached, the soil acts as an impervious surface and all 

of the precipitation becomes runoff.

Nachabe and others (2004) observed that rapid infiltration during intense rainfall events may trap air in soil pore 

spaces. In sandy soils, water can infiltrate pore spaces 

quickly, preventing air in pore spaces from escaping to the 

atmosphere. Where substantial trapped air exists in the soil 

profile, air counterflow and compression resist the downward 

flow of water (Wangemann and others, 2000). Taboada and 

others (2001) observed that pore air volume increased to 35 

percent of the total soil volume during soil wetting but air 

did not escape at the soil surface. Consequently, hydraulic 

conductivity is reduced and runoff volumes may increase 

(Charbeneau, 2000). 

Nachabe and others (2004) modified the equation for 

determining maximum potential soil retention (S) by developing an equation for soil water storage capacity (SWSC) as a 

function of water-table depth. Although detailed soil moisture 

data are needed to determine SWSC, Nachabe and others 

(2004) assert that soil storage capacity will be overestimated 

without an estimate based upon water-table depth.

Variations in topography, antecedent soil water content, 

soil moisture storage capacity, and rainfall intensity influence 

the spatial extent of areas where saturation excess runoff will 

occur. Hernandez and others (2003) indicated that these saturated areas, called variable source areas (VSAs), produce most 

of the runoff in shallow water-table environments. In areas 

with convex hillsides, runoff may consist of shallow subsurface 

flow (unsaturated throughflow) that discharges along the banks 

of the stream channel (fig. 25). In depressional storage areas 

(for example, wet detention ponds), contributing source areas 

may be larger than anticipated because of return flow. VSAs 

typically expand during the wet season and shrink during 

the dry season. Figure 26 depicts the variation in prestorm 

saturated source areas in March, April, and August 1973 for a 

small watershed in Vermont (Dunne and Leopold, 1978). The 

percentage of the saturated catchment area (As) varies from 

51 percent in March to 15 percent in August. The expansion 

and contraction of VSAs define the hydroperiod in forested 

wetlands, which is an important consideration for ecological 

function in wetlands (Kautz, 1998). 
30 Retrospective Review of Watershed Characteristics and a Framework for Future Research 

Accurate estimation of VSAs is particularly important for 

the design capacity of wet detention ponds in Florida. Schueler 

(2000d) explained that the performance of wet detention ponds 

designed to serve large catchment areas can be compromised 

by ground water entering the detention pond. Ground water 

may serve to dilute total nitrogen in wet detention ponds, 

but stormwater load removal may be decreased. Harper and 

others (2002) observed dry detention treatment in the Florida 

Keys to evaluate hydraulic performance and nutrient removal 

efficiency. Field observation indicated that lateral groundwater flow moved into the pond before actual runoff began. 

The concave depression formed by the dry detention pond 

decreased hydraulic performance and reduced nutrient removal 

efficiency.

Harper and Baker (2003a) evaluated alternative 

stormwater treatment options for achieving predevelopment 

pollutant loads in developed areas of southwest Florida. They 

concluded that wet detention in high water-table areas of 

Florida was an appropriate option because of efficient runoff 

storage capacity, ease of maintaining, and reasonable pollutant 

removal. Runoff is held for a designated period of time and 

is released slowly to ground water, surface water, or the 

atmosphere. Wet detention is the preferred option for runoff 

storage in Florida, but designs that incorporate modifications 

to the SCS method suggested by Nachabe and others (2003) 

may exhibit better hydraulic performance than conventional 

designs. The average antecedent dry periods between precipitation events in central and south Florida are 5.3 days during 

the dry season and 1.7 days during the wet season. Harper and 

Baker (2003a) indicated that nitrogen removal from a typical 

wet detention pond would achieve 70 percent removal after 

150 days and 50 percent removal after 20 days. To achieve the 

removal efficiency goal of no net increase in loading (predevelopment loading conditions), options include wet detention 

systems in series, wastewater treatment approaches designed 

for stormwater, and reducing the volume of stormwater. 

Figure 25. Potential routes of water movement (modified from Federal 

Interagency Stream Restoration Working Group, 1998).

1,200

1,100

1,000

1,200

1,100

1,000

1,200

1,100

1,000

1,200

1,100

1,000

1,200

1,100

1,000

0 1,000 2,000 FEET

0 300 600 METERS

EXPLANATION

Saturated area (As)

75% of area saturated

50% of area saturated

Stream channel

Watershed boundary

Elevation, in feet 

 above NGVD 29

August 25, 1973

(As=15%)

March 28, 1973

(As=24.5%)

April 21, 1973

(As=19.5%)

March 21, 1973

(As=51%)

March 23, 1973

(As=36.5%)

N

Figure 26. Examples of seasonal variation in saturated source 

areas for a catchment in Vermont (reprinted from Dunn and 

Leopold, 1978 and published with permission).

Litter

layer

Dunne

saturated

overland

flow

Throughflow

Ground-water flow

Zone of

percolation

Water table

Precipitation

EXPLANATION

DIRECTION OF FLOW

Horton overland flow
Framework for Future Research in the Watershed 31

With urbanization, the hydrologic function of natural 

processes (depressional storage in swamps, cypress domes, 

and rapid infiltration in forested areas) is shifted toward 

designed engineering processes. Urban land-use practices tend 

to increase runoff and the likelihood of flooding by decreasing 

the perviousness of the land surface. Stormwater planning 

in west-central Florida makes use of traditional engineered 

structures to convey and store stormwater runoff and, with 

increasing frequency, incorporates lot-scale approaches that 

retain runoff close to rainfall point-of-impact. Engineered 

processes have been developed to address basin time-of-travel 

while attempting to retain precipitation close to where it falls 

on the land surface. These processes, designed and evaluated 

in other parts of the United States, are being applied to westcentral Florida to enhance traditional stormwater management 

and retain runoff near its source.

Framework for Future Research in the 

Watershed

As urbanization in the Sarasota Bay watershed continues, 

focused research on water-resources issues will continue to 

provide information needed by water-resources managers 

to ensure the future health of the watershed. Conventional 

approaches are used to convey and manage runoff from developed areas to large detention or treatment areas. Finer-scale 

approaches to address sustainability, restoration, and low-impact 

development (portions of subdivisions, lots, impaired parcels, 

and so forth) are being used more frequently than in the past.

Low-Impact Development

Several approaches have been developed in other regions 

of the Nation to enhance infiltration and reduce runoff by 

modifying land practices. Prince George’s County (1999) in 

Maryland developed Low-Impact Development (LID) guidelines to mimic the predevelopment site hydrology by using 

design features that store, infiltrate, evaporate, and detain 

runoff. These methods were designed to reduce runoff and 

ensure adequate ground-water recharge. In 2004, the Tampa 

Bay Regional Planning Council, Tampa Bay Estuary Program, 

and Florida Department of Environmental Protection co-sponsored a workshop on the application of LID approaches for 

Florida. Unanimous consensus at the workshop called for 

conducting research to determine what LID approaches would 

be best suited for Florida (Tampa Bay Regional Planning 

Council and others, 2004).

In an effort to restore predevelopment hydrologic 

processes, LID is designed to control stormwater volume at its 

source in discrete units throughout the watershed. Called the 

“distributed control approach,” LID emphasizes materials such 

as native plants, soil, and gravel that can be integrated easily 

into the landscape to create distributed “microcontrol” systems 

to complement regional conveyance structures. LID limits 

clearing, grading, and construction easements, minimizes and 

disconnects impervious areas, avoids development in areas 

with high infiltration rates, and maintains existing topography 

and drainage divides. LID is not designed to replace conventional stormwater management approaches but rather attempts 

to enhance the efficiency of the conventional systems already 

in place.

The USEPA guidance on best management practices 

(BMPs) for stormwater states that preventing polluted runoff 

is less costly than treating it (U.S. Environmental Protection 

Agency, 2002). Swales, filter strips, and vegetated biofilters 

are recommended by the USEPA to keep runoff at its point-ofcontact with the land surface (U.S. Environmental Protection 

Agency, 2004a, 2004b). Previously, the USEPA described 

LID practices applicable to retaining runoff close to its source 

including vegetative roof covers (green roofs), bioretention, 

grass swales, and permeable pavement (U.S. Environmental 

Protection Agency, 2000a).

Green roofs have been incorporated into building designs 

infrequently in Florida. Climatic conditions most favorable for 

green roofs are in areas that receive steady, light rainfall over 

an extended rainy season, typical of the northwestern United 

States. Conditions where the soil dries out completely for part 

of the year and where intense rainfall exceeds the soil storage 

capacity of green roofs for another part of the year present 

challenges to maintaining a green roof. However, experimental 

approaches on the campus of the University of Central Florida 

in Orlando (University of Central Florida, 2005) and at the 

Museum of Science and Industry in Tampa (W. Ostrenko, 

Museum of Science and Industry, oral commun., 2005) are 

being used to adapt green roof technologies for Florida. A new 

library planned for Sarasota County will serve as a pilot for 

the construction of a green roof (Whitt, 2006).

To increase infiltration and reduce runoff using “microcontrol” strategies, Prince George’s County (1999) developed 

microscale techniques called Integrated Management Practices 

(IMPs) and procedures, and criteria for selecting and designing 

the IMPs. Specific IMPs suggested by Prince George’s County 

include: bioretention beds, dry wells, filter buffer strips, grassy 

swales, rain barrels, cisterns, and infiltration trenches.

Bioretention manages runoff by using an engineered 

planting bed and plant materials within a shallow depression to filter and retain stormwater. The bed is prepared 

by using in-situ soil with an infiltration rate greater than 

0.5 in/hr (1.27 cm/hr) to a depth of 4 ft (1.2 m). Soil mixtures 

containing sand, loamy sand, and sandy loam with less than 10 

percent clay are ideal for bioretention. Amending the soil with 

compost, although not specifically recommended by the IMPs, 

would increase biological activity and decrease bulk density. 

Pitt and others (1999) amended compacted urban soils with 

organic matter, and over several years, infiltration increased 

by 1.5 to 10 times the precompost infiltration rate. Because 

the water table is usually less than 5 ft (1.5 m) below land 

surface along the coastline, bioretention beds may have limited 

applicability to the Sarasota Bay watershed. 
32 Retrospective Review of Watershed Characteristics and a Framework for Future Research 

Dry wells, cisterns, and rain barrels are designed to 

capture runoff from roofs and permit the slow percolation 

of runoff to ground water. Dry wells may be appropriate for 

regions with deep water tables, but water-table conditions 

in the Sarasota Bay watershed would preclude their use. 

Additionally, these devices could inadvertently serve as 

conduits for movement of contaminants from land surface 

to ground water. Rain barrels and above-ground cisterns, 

advocated by the FY&N in Sarasota and Manatee Counties, 

are being used with increasing frequency as microscale detention devices for reuse of the collected water in the landscape 

(Garner and others, 2001).

Filter buffer strips, grassy swales, and infiltration trenches 

are vegetated areas of ground cover, shrubs, and trees designed 

to divert runoff to infiltration and restore infiltration rates to 

predevelopment natural ground cover levels. Grassy swales 

have been used extensively in the Sarasota Bay watershed and 

in most other areas of Florida as integral parts of stormwater 

management programs (E. Livingston, Florida Department of 

Environmental Protection, oral commun., 2004). 

Rushton (2002) studied a low-impact design for reducing 

runoff in the parking lot of the Florida Aquarium in Tampa, 

Florida. A conventional asphalt system and a porous pavement 

system were installed side-by-side within the parking lot with 

drainage leading to separate, internally drained swales. The 

swales were directed to a series of wet detention ponds. The 

porous pavement system stored nearly all (greater than 99 

percent) of the runoff generated during the study and pollutant 

removal efficiency was about 99 percent.

Caraco and others (1998) evaluated stormwater runoff, 

stormwater infiltration, and nutrient output from conventional 

and innovative site development plans for medium-density 

residential, low-density residential, retail shopping, and 

commercial office park sites in the Chesapeake Bay watershed. The conventional site design for each site was modified to incorporate narrow streets, small parking lots, open 

space options, short driveways, and open-channel drainage. 

The modified site characteristics were incorporated into the 

innovative redesign. Subsequently, a lot-scale model, the 

Simplified Urban Nutrient Output Model (SUNOM), was used 

to simulate nutrient export and runoff/infiltration characteristics for conventional and innovative site designs. SUNOM 

nutrient export rates were adjusted to include mean removal 

efficiencies of stormwater BMPs. The simulation showed that 

changes in site characteristics, principally impervious cover, 

resulted in less runoff, more infiltration, reduced nitrogen 

output, and reduced phosphorus output for medium-density 

residential, low-density residential, and commercial office 

park sites. Stormwater infiltration did not increase for the 

retail shopping site. Table 9 shows the projected changes in 

runoff characteristics for the four scenarios. Although the 

SUNOM model was applied to conditions in the Chesapeake 

Bay watershed, the principle of simulating changes in impervious cover to lot-scale nutrient runoff is applicable to the 

Sarasota Bay watershed.

Cheng and others (2004) conducted field monitoring to 

compare nitrogen output and hydrology from conventional and 

LID site designs in two small watersheds in Prince George’s 

County, Maryland. The conventional design used a curb, gutter, 

and pipe stormwater conveyance system; whereas the LID 

design used grassy swales and bioretention areas. After 2 years 

of monitoring, total nitrogen loads decreased by 2.7 percent 

and nitrate-nitrite nitrogen loads decreased by 34.8 percent at 

the LID site compared with the conventional site. Average peak 

flow for the 2-year period at the LID site was 56 percent less 

than at the conventional site. Although basin characteristics 

were not described, expected streamflow velocities would 

probably be considerably higher than the velocities encountered in the Sarasota Bay watershed and the marked decreases 

in average peak flow and nitrate-nitrite nitrogen loads would 

likely be lower in the Sarasota Bay watershed because of the 

slower stream velocities and low relief.

The hydrologic analysis guidance for Prince George’s 

County (1999) compared the calculation of a conventional 

runoff CN with a “custom-made LID” runoff CN. A composite 

runoff CN was achieved by segregating a 1-acre (0.4 ha) 

Table 9. Percent change in simulated runoff characteristics of landcover types from conventional BMPs to innovative design with BMPs 

from a study in the Chesapeake Bay watershed.

[From Caraco and others (1998). BMPs, best management practices]

Characteristic

Medium 

density 

residential 

housing

Low 

density 

residential 

housing

Shopping 

center Office park

Impervious cover -24 -35 -18 -22

Stormwater runoff -25 -23 -17 -21

Stormwater infiltration 55 12 -2 42

Nitrogen output -45 -46 -42 -45

Phosphorus output -60 -50 -46 -47
Framework for Future Research in the Watershed 33

residential lot into directly connected and unconnected 

impervious surfaces, open space, and wooded areas. The 

CNs obtained for the residential lot were: conventional (68), 

composite (63), and predevelopment (55). The composite curve 

number was subsequently used to determine detention storage 

requirements for the residential lot. Because watershed characteristics are considerably different in the Sarasota Bay watershed than in Prince George’s County, Maryland, the approach 

for developing a custom-made LID runoff CN might not be 

applicable to conditions in west-central Florida. Developing 

lot-specific custom runoff CNs, however, would provide an 

improved understanding of runoff characteristics for Florida. 

Gregory (2004) investigated lot-scale infiltration in 

areas of mixed forest and residential construction near 

Gainesville, Florida. Soils in these areas are sandy, loamy 

Paleudults (acidic, humid-region soils typically forming 

under pine forests [Buol and others, 1973]). A lot-scale 

model was developed to evaluate the effectiveness of several 

BMPs for reducing stormwater runoff by enhancing infiltration. Treatments modeled were (1) a conventional lot, (2) a 

predevelopment, high infiltration lot, (3) a pervious driveway, 

and (4) an engineered infiltration structure with runoff routed 

to it. Results indicate that predevelopment infiltration and an 

engineered infiltration structure resulted in a 50- to 92-percent 

reduction of lot-generated runoff. Gregory concluded that lotscale infiltration could be an effective method for managing 

stormwater near the precipitation point-of-contact. Gregory’s 

model has direct application to lot-scale efforts to improve 

infiltration in the Sarasota Bay watershed and may be a 

promising approach for evaluating infiltration using FY&N 

landscaping guidelines. 

To reduce runoff, LID uses the hydrologic cycle as a 

design element to balance pervious and impervious cover. 

Figure 27 generalizes this balance showing the relative 

amounts of water moving to ground water and runoff with 

increasing percentages of impervious cover. In areas with 

natural ground cover, up to 50 percent of precipitation infiltrates into the ground water and 10 percent becomes runoff. 

As the percentage of impervious surface increases, either as 

impervious cover or compacted soils, infiltration decreases. 

In urban areas, impervious surfaces typically cover 75 to 100 

percent of the land surface. Infiltration in these areas typically 

decreases to 15 percent whereas runoff increases to 55 percent 

(Federal Interagency Stream Restoration Working Group, 

1998). In 14 watersheds located in west-central Florida, the 

average runoff for natural watersheds was 27 percent and the 

average runoff for urban watersheds was 41 percent (Trommer 

and others, 1996a). 

Livingston (2003) indicated that stormwater volume 

must be reduced if post-development loading is not to exceed 

predevelopment loading. He suggested nonstructural controls, 

including LID, because structural controls alone will not 

protect the health of aquatic ecosystems. LID is not a replacement for conventional stormwater management, but instead, 

enhances stormwater management at or near the point at 

which rainfall reaches land surface. 

Pollutant Loading and Watershed Modeling

Restoring the balance between predevelopment runoff 

and recharge assumes that runoff will be reduced to enhance 

recharge to ground water. Because ground water is the 

principal source of water in the Sarasota Bay watershed, 

enhancing recharge would help to maintain the ground-water 

supply. Restoring this balance is also important to reducing 

pollutants loadings in runoff that reach surface streams and 

Sarasota Bay.

In 1992, a point/nonpoint source pollution assessment of 

Sarasota County was conducted to evaluate nutrient inputs to 

Sarasota Bay (Camp Dresser & McKee, Inc., 1992). Pollutant 

loading rates were determined from available concentration and streamflow data; surface runoff was characterized 

by using event mean concentration data developed through 

the National Urban Runoff Program (U.S. Environmental 

Protection Agency, 1983). Available literature data for 17 

land-use types were used in place of local monitoring for 

nonpoint loading factors including phosphorus, nitrogen, zinc, 

and lead.

Infiltration decreases and runoff increases as the amount 

of directly connected impervious area (DCIA) increases. 

The percentage of DCIA in the watershed was estimated for 

each urban land use from the National Urban Runoff Program 

(U.S. Environmental Protection Agency, 1983). Specific 

runoff loading estimates were made for golf courses, septic 

areas, canal communities, point sources, and rainfall. Wet- and 

dry-season results indicated that 60 percent of the annual 

loading occurred during the wet season (June-September), and 

40 percent occurred during the dry season (October-May). 

The average annual loading of total nitrogen to the watershed 

was primarily from surface runoff (46 percent) and rainfall 

(27 percent). Baseflow, septic systems, and point sources each 

accounted for less that 10 percent of the total nitrogen loads. 

Loadings and flows were simulated as steady input values 

from the average annual results generated by a spreadsheet 

model using the WASP4 model (Southwest Florida Water 

Management District, 1992).

From the estimates for the Sarasota Bay watershed, the 

Sarasota Bay National Estuary Program projected annual 

loads to Sarasota Bay for total phosphorus, total nitrogen, 

lead, and zinc for a 20-year period from 1992 to 2012 (Heyl, 

1992). The average annual total nitrogen load estimates were 

933,750 lb/yr and 1,145,730 lb/yr (423,542 kg/yr and 519,694 

kg/yr), for 1992 and 2012 respectively. Several management 

alternatives and land-use conditions were evaluated that could 

potentially affect the 20-year pollutant loading projections 

(table 10). Five of 14 build-out alternatives (build-out is an 

estimate of the total number of units that can potentially be 

built under land-use management plans) resulted in lower total 

nitrogen loads than the 1992 estimates; these five alternatives 

assumed the implementation of the Sarasota County advanced 

wastewater plan. The 30/70 (30 percent medium density and 

70 percent open space) residential cluster development concept 

was determined to be the best alternative for reduction of 
34 Retrospective Review of Watershed Characteristics and a Framework for Future Research 

total nitrogen loads. At build-out, the 30/70 residential cluster 

concept results in a projected reduction of 89,490 lb/yr (40,592 

kg/yr) of total nitrogen from the 1992 levels (Heyl, 1992). 

The 1992 pollutant loading model relied on several 

simplifying assumptions because local, site-specific data 

were sparse.

● Total nitrogen loading from precipitation was 

distributed evenly over the 52-mi2 (135 km2) 

Sarasota Bay surface area. The loadings were based 

upon the total nitrogen concentration determined 

for Tampa Bay of 0.82 mg/L and an average 

precipitation of 54.6 in. (137 cm) or 337,000 lb/yr 

(152,860 kg/yr). 

● Total nitrogen concentrations in runoff were 

estimated mostly from literature values for Florida 

from the 1983 National Urban Runoff Program.

● Runoff coefficients were estimated from the 

Soil Conservation Service (1986) hydrologic 

soil types and land-use groupings. 

The simplifying assumptions used in the 1992 pollutant 

loading model permitted long-range projections using estimation techniques developed for broad, regional applications. 

Recent research has generated site-specific data, remotesensing data, and Geographic Information System (GIS) 

methods to account for variability that had formerly been 

40% evapotranspiration

10%

runoff

25% deep

infiltration

25% shallow

infiltration

21% shallow

infiltration

20% shallow

infiltration

10% shallow

infiltration

21% deep

infiltration

15% deep

infiltration

5% deep

infiltration

20%

runoff

30%

runoff

55%

runoff

38% evapotranspiration

(C) 35% evapotranspiration (D)

(A) (B)

30% evapotranspiration

Natural ground cover 10%-20% impervious surface

35%-50% impervious surface 75%-100% impervious surface

Figure 27. Examples of variation in runoff with increasing impervious surfaces: (a) natural ground cover, (b) 10-20 percent impervious cover, 

(c) 35-50 percent impervious cover, and (d) 75-100 percent impervious cover (from Federal Interagency Stream Restoration Working Group, 1998).
Framework for Future Research in the Watershed 35

limited to basinwide, evenly distributed estimates. Algorithms 

for predicting near-realtime precipitation from NEXRAD 

data have been developed and verified (Neary and others, 

2004), and have been used in watershed modeling (Berne 

and others, 2005). 

In the 1992 pollutant loading model, rainfall concentrations of nutrients from Tampa Bay were used to represent 

rainfall concentrations for the Sarasota Bay watershed 

resulting in overestimated atmospheric deposition of nutrients 

to the Sarasota Bay watershed (Southwest Florida Water 

Management District, 2002). Wet and dry atmospheric 

deposition of total nitrogen to the Sarasota Bay watershed was 

slightly more than half of the atmospheric deposition reported 

by Poor and others (2001) for Tampa Bay, or about 3.92 lb/

acre/yr (4.4 kg/ha/yr) total nitrogen (N. Poor, University of 

South Florida, oral commun., 2004). Alexander and others 

(2001) used a spatially referenced regression watershed model 

(SPARROW) to predict the mean wet deposition of nitrogen to 

the Sarasota Bay watershed. They determined the wet deposition of total nitrogen to be 1.36 lb/acre/yr (1.53 kg/ha/yr) or 

slightly less than 40 percent of the total nitrogen deposition. 

Similar research by Poor and others (2001) in Tampa Bay 

indicated that wet deposition of total nitrogen in Tampa Bay 

accounted for 56 percent of the total nitrogen deposition 

for the period 1996 to 1999. The study also determined that 

a net flux of nitrogen from the water to the atmosphere, as 

ammonium, occurred during one summer when Tampa Bay 

warmed to 82.4 °F (28 °C). The 1992 pollutant loading model 

predicted atmospheric deposition as the second largest source 

of nitrogen to the Sarasota Bay watershed. NEXRAD data and 

nitrogen speciation/flux measurements can be used to provide 

a better means of understanding the variability of this substantial source of nitrogen to the Sarasota Bay watershed. 

The soil storage capacity variable used in the 1992 

pollutant loading model was based on the runoff numbers from 

the SCS method. Modifying watershed models to account for 

soil water storage capacity as a function of water-table depth 

will likely enhance the accuracy of runoff projections. The 

SWFWMD (2002) recommended updating the 1992 Sarasota 

Bay watershed pollutant loading model predictions by 

comparing the projections with current pollutant loading data. 

Since the early 1990s, estimates of pollutant loads have 

been improved by comprehensive watershed models linking 

load estimates to dynamic processes within the watershed. 

In 1999, the USEPA recommended the use of watershed 

loading models to evaluate the effect of land-use practices 

on pollutant loading to water bodies (U.S. Environmental 

Protection Agency, 1999). Models were categorized as simple 

or complex, and the advantages and disadvantages of each of 

the groups were described. 

Simple models, such as the watershed spreadsheet 

model, were recommended for gross, empirical estimates 

of pollutant loads when reliable data are relatively sparse. 

The 1992 Sarasota Bay watershed pollutant loading model is 

Table 10. Total nitrogen loads to Sarasota Bay projected for 2012 for several land management 

alternatives in the Sarasota Bay watershed.

[From Heyl (1992). AWT, Advanced wastewater treatment; BMPs, best management practices; lb/yr, pounds per 

year; build-out, estimate of the total number of units that potentially can be built under the land-use management 

plans; %, percent]

Total nitrogen 

load 

(lb/yr)

Land management alternative1

933,750 1991 estimate

1,145,730 Uncontrolled build-out; no best management practices (BMPs); no advanced wastewater 

treatment (AWT)

1,064,620 Wet detention BMPs, as required by law; no AWT

914,860 Wet detention BMPs, as required by law and AWT

888,490 1-acre residential lot (80% pervious); commercial (40% pervious) plus BMPs and AWT

873,010 Residential cluster development (50% medium density-50% open space, 90% pervious); 

commercial (40% pervious) plus BMPs and AWT

855,360 2-acre residential lot (90% pervious); commercial (40% pervious) plus BMPs and AWT

844,260 Residential cluster development (30% medium density-70% open space, 90% pervious); 

commercial (40% pervious) plus BMPs and AWT

 1Assumes all future development conforms to these land management alternatives.
36 Retrospective Review of Watershed Characteristics and a Framework for Future Research 

an example of this type of model. These models are of limited 

value for determining loads on a seasonal or finer scale and in 

evaluating the effect of control measures (U.S. Environmental 

Protection Agency, 1999).

Complex models utilize the current understanding of 

watershed processes affecting pollutant sources and sinks. 

Algorithms in complex models more closely simulate physical 

processes (infiltration, runoff, pollutant accumulation, 

ground-water/surface-water interaction) (U.S. Environmental 

Protection Agency, 1999). These models (1) can provide information on source loadings from specific parts of the watershed, (2) can predict the effect of different control practices, 

and (3) have greater spatial and temporal resolution. Complex 

models are appropriate for load estimates to: (1) provide 

explicit analysis of runoff and pollutant transport, (2) evaluate 

short-term processes lasting days or hours, and (3) optimize 

potential control scenarios. Predictions of variable flows and 

water-quality affected processes at numerous points within 

the watershed may be achieved using complex models (U.S. 

Environmental Protection Agency, 1999). Watershed models 

categorized into this group include:

● BASINS (Better Assessment Science Integrating 

Point and Nonpoint Sources)

● HSPF (Hydrologic Simulation Program—Fortran)

● SWAT (Soil and Water Assessment Tool)

● SWMM (Storm Water Management Model)

● SPARROW (SPAtially Referenced Regressions On 

Watershed Attributes)

BASINS is a multipurpose environmental analysis 

system developed to assist regional and local agencies with 

watershed and water quality-based evaluations by integrating 

data on water quality and quantity, land uses, and point and 

nonpoint source loading, thus providing the ability to perform 

preliminary assessments of watersheds (U.S. Environmental 

Protection Agency, 1999). BASINS consists of a series of 

interrelated components including: (1) data extraction tools for 

national databases; (2) assessment tools (TARGET, ASSESS, 

and Data Mining) for large and small-scale watershed 

characterization; (3) utilities for classifying digital elevation 

models, land use, soils, and water-quality observations; (4) an 

instream water-quality model, QUAL2E; (5) two watershed 

loading and transport models, Hydrological Simulation 

Program—Fortran (HSPF) and Soil and Water Assessment 

Tool (SWAT); (6) PLOAD, a simplified GIS-based model that 

estimates nonpoint pollutant loads on an annual average basis; 

(7) the Kinematic Runoff and Erosion Model (KINEROS), 

an event oriented, physically based model that may be used 

to determine the effects of various artificial features such as 

urban developments, small detention reservoirs, or lined channels on flood hydrographs and sediment yield; (8) Rosgen’s 

Bank Erosion Hazard Index, which has been incorporated in 

the pollutant loading model as PLOAD-BEHI; (9) AQUATOX, 

which receives and automatically formats output from HPSF 

or SWAT to integrate watershed analysis with the likely effects 

on the aquatic biota in receiving waters; and (10) a Parameter 

Estimation (PEST) tool in WinHSPF (Windows version of 

HSPF) that automates the model calibration process and 

allows users to quantify the uncertainty associated with 

specific model predictions. 

The HSPF model is used to calculate pollutant load 

and transport from complex watersheds to receiving waters 

(U.S. Environmental Protection Agency, 1999). HSPF 

provides capabilities for continuous and storm-event simulation. The model output includes a time series of the runoff 

flow rate, sediment load, nutrient and pesticide concentrations, 

and water quantity and quality at any location in the watershed. The Chesapeake Bay Program has used HSPF to model 

total watershed contributions of flow, sediment, and nutrients 

to the tidal region of the bay. Wicklein and Schiffer (2002) 

simulated hydrology and water quality of runoff using the 

HSPF model in central Florida. To characterize runoff, they 

examined six land-use types (agriculture, rangeland, forest, 

wetlands, rapid infiltration basins, and urban) for a complex 

stream system in the Reedy Creek Improvement District. 

For the period from 1990 to 1995, the model was calibrated 

for two subwatersheds and runoff was simulated for the 

remaining subwatersheds. Simulated time series for total phosphorus, phosphate, ammonia nitrogen, and nitrate nitrogen 

generally agreed with periodic data for the two subwatersheds. 

Simulation of hydrology and water quality of runoff for future 

land-use scenarios was then projected for 2008. The land-use 

scenarios assumed a decrease in forested areas by 50 percent 

and an increase in impervious areas by 300 percent. Simulated 

nutrient concentrations did not change substantially, but simulated loads for all constituents increased by 10 percent; nitratenitrogen loads increased by 17 percent. The HSPF model may 

also be used in conjunction with ground-water flow models to 

simulate surface-water and ground-water interactions. 

SWAT is a watershed-scale model developed to predict 

the effect of land management practices on water, sediment, 

and agricultural chemical yields in large complex watersheds 

with varying soils, land use, and management conditions over 

long periods of time (Neitsch and others, 2005). Data inputs 

include weather, surface runoff, return flow, percolation, 

evapotranspiration, transmission losses, pond and reservoir 

storage, crop growth and irrigation, ground-water flow, reach 

routing, nutrient and pesticide loading, and water transfer. 

The model is physically based and does not rely on regression 

relations between input and output variables. This feature 

permits modeling of watersheds with minimal monitoring data 

(Neitsch and others, 2005). 

The SWMM model simulates overland water quantity 

and quality produced by storms in urban watersheds for a 

wide range of watershed processes (U.S. Environmental 

Protection Agency, 1999). Model components include rainfall 

and runoff, water-quality analysis, and point-source inputs. 

Either continuous or storm-event simulation is possible, 

with variable and user-specified time steps (wet and dry 
Framework for Future Research in the Watershed 37

weather periods). Input data requirements include rainfall 

hyetographs, antecedent conditions, land use, topography, soil 

characteristics, dry-weather flow, hydraulic inputs (gutters or 

pipes), pollutant accumulation and wash-off parameters, and 

hydraulic and kinetic parameters. Model output includes time 

series of flow, stage, and constituent concentrations at any 

location in the watershed. 

The SPARROW model incorporates a statistical modeling 

approach that retains spatial referencing for illustrating predictions and for relating upstream nutrient sources to downstream 

nutrient loads (Smith and others, 1997). SPARROW is based 

on a digital stream-network data set that is composed of 

stream segments (reaches) that are attributed with travel time 

and connectivity information. Drainage-basin boundaries are 

defined for each stream reach in the network data set through 

the use of a digital elevation model (Preston and Brakebill, 

1999). The application of SPARROW for watershed assessment offers three principal features. First, the statistical basis 

of SPARROW provides an objective means of identifying 

relations between stream-water quality and environmental 

factors such as contaminant sources in the watershed and 

land-surface characteristics that affect contaminant delivery to 

streams (Alexander and others, 2002). Second, SPARROW’s 

spatially detailed network and travel-time data provide a 

means of estimating instream loss rates. These loss rates 

allow upstream watershed factors to be related to downstream 

loads in a more integrated manner than previously possible, 

and allow the simultaneous evaluation of many factors that 

affect loads including storage, denitrification, interbasin 

transfer, and ground-water/surface-water exchange (Alexander 

and others, 2002). Third, SPARROW provides a means of 

retaining detailed spatial information about all environmental 

factors considered in the regression model. Because the regression models are linked to spatial information, predictions 

and subsequent analytical results can be illustrated through 

detailed maps that provide information about nutrient loading 

at detailed spatial scales. 

McMahon and Roessler (2002) used the SPARROW 

model to develop total nitrogen inputs and total nitrogen 

delivery and a total nitrogen budget for the Neuse River 

basin in North Carolina. SPARROW estimated nitrogen yield 

within 25 percent of the observed values at most of the 44 

monitoring stations used to calibrate the model. Observed 

values consisted of water-quality data collected both by 

the USGS and the State of North Carolina. Soil drainage 

characteristics (specifically, ground-water recharge) and 

channel transport factors (aquatic processes in streams and 

reservoirs) both substantially influence the transport of total 

nitrogen at the reach and whole-basin scale. Total nitrogen 

losses associated with in-stream processes occurred at a rate 

of about 8 percent per mi (5 percent per km) in streams with a 

mean annual discharge less than 37 ft3/s (1.04 m3/s); losses in 

streams with greater discharge occurred at a rate of 0.3 percent 

per mi (2 percent per km). SPARROW included statistical 

uncertainty in nitrogen delivery estimates and the estimates 

were used to map the spatial probability distribution of stream 

reaches. McMahon and Roessler (2002) noted the flexibility of 

SPARROW to accept multiple model specifications that aid in 

the understanding of complex terrestrial-aquatic processes. 

Sprague and others (2000) estimated nitrogen loads 

using observed concentration and streamflow data with the 

USGS ESTIMATOR model—a log-linear regression model 

that uses time, flow, and season terms to predict daily nutrient 

concentrations (Cohn and others, 1989). They obtained flowadjusted concentrations to remove the bias of flow from the 

load estimates and applied the Kendall-Theil test to evaluate 

trends in nutrient loads. Sprague and others (2000) then used 

the Chesapeake Bay Watershed Model (WSM) to quantify 

nitrogen sources for 1995 and 1998 scenarios. The WSM 

is spatially and temporally variable and based on the HSPF 

model (Donigian, and others, 1994). Results from the WSM 

were used as input for the SPARROW model to determine the 

primary factors affecting the observed nutrient trends. Sprague 

and others (2000) found that cultural changes in nutrient 

sources and natural variations in streamflow were the major 

factors affecting the trends in nitrogen loads. 

Revising the 1992 pollutant loading model predictions, as 

the SWFWMD has suggested, could involve the application of 

a detailed model (such as HSPF) to determine the spatial and 

temporal details of pollutant fate and transport and the application of SPARROW to assess the contribution of pollutant 

loads to the watershed by stream reaches. The accuracy of the 

multiple model approach could be refined by incorporating 

lot-scale models into the HSPF domain to elicit lot-scale 

inputs to pollutant loads in the watershed.

Jones, Edmund and Associates, Inc. (2006) developed 

a spatially integrated model for pollutant loading estimates 

(SIMPLE) to estimate pollutant loading for the Sarasota Bay 

watershed. SIMPLE is based upon previously developed 

models by Camp Dresser & McKee, Inc. (1992) and Harper 

and Baker (2003b) and includes functionality to permit user 

control of constituents, rainfall, and BMPs through an ArcMap 

interface. Baseflow contributions are inferred by data inspection 

and an average baseflow contribution factor of 0.0012 ft3/acre 

(0.07 m3/ha) was computed for the watershed. Pollutant loads 

were computed by multiplying runoff volumes by event mean 

concentrations (EMCs), similar to the procedures used by 

Camp Dresser & McKee, Inc. (1992) and Harper and Baker 

(2003b). Pollutant loads were estimated from the EMCs 

presented in Camp Dresser & McKee, Inc. (1992). Additional 

pollutant load estimates for (1) wetlands BOD (biochemical 

oxygen demand), TSS (total suspended solids), TP (total 

phosphorus), and TN (total nitrogen), (2) lead and zinc, (3) oil 

and grease, and (4) fecal coliform bacteria were obtained from 

other studies (Jones, Edmund and Associates, Inc., 2006). 

The consistency of the estimation methods used in the 

1992 and 2006 studies enabled a temporal comparison of 

the increase or decrease in pollutant loads reaching Sarasota 

Bay. In a complimentary approach, detailed field studies offer 

the possibility of measuring further reductions in pollutant 

loads caused by relatively small changes in watershed practices. The methods originally suggested by the USEPA for 
38 Retrospective Review of Watershed Characteristics and a Framework for Future Research 

determining EMCs emphasized generalizing the EMCs to 

be applicable to many geographic areas (U.S. Environmental 

Protection Agency, 1983). The literature values determined in 

the National Urban Runoff Program were applied to watersheds to identify large-scale changes in pollutant loads with 

minimal data collection. Since 1983, changes in sampling 

methods and analysis have resulted in improved strategies for 

determining EMCs.

Event Mean Concentrations for Pollutant Loading 

Models

One way to characterize constituent concentrations and 

stormwater loads in streams and rivers involves using the EMC 

approach originally suggested by the USEPA through the 

Urban Stormwater Management Program (U.S. Environmental 

Protection Agency, 1983). Event mean concentrations have 

been used extensively to estimate runoff constituent loads to 

surface-water bodies; however, some EMCs are derived from 

literature values whereas others are derived from sparse data 

or limited storms (U.S. Environmental Protection Agency, 

1999). Stormwater sampling strategies for characterizing the 

concentrations of constituents in runoff include grab, peak 

discharge, first-flush, composite, random, volume-weighted 

and flow-weighted sampling.

Chang and others (1990) sampled the first 0.5 in. 

(1.27 cm) of runoff for water-quality constituents to evaluate 

the first flush of stormwater runoff to streams. They found 

that although the first flush concentrations were elevated, the 

first 0.5 in. (1.27 cm) of runoff did not carry the majority of 

the storm load. As the amount of storm precipitation and the 

percent impervious cover increased, the percentage of constituent load in the first flush of runoff decreased (table 11). 

Precipitation falling on small, impervious catchments causes 

rapid changes in streamflow, dissolved constituents, and 

suspended solids and, therefore, a large number of samples 

are required to adequately characterize the stormwater loads 

(Breautt and Granato, 2000).

Stenstrom and others (2002) evaluated sampling errors 

resulting from the number of stormwater samples collected 

over the hydrograph during a typical storm. They found that 

a smaller percentage of error occurred with equal-volume 

or equal-rainfall sampling intervals than with equal-timing 

intervals or random timing. Error percentage also decreased 

as sample size increased. Automatic samplers were preferred 

over grab samples except in the case where sample contamination from carry-over is a concern.

Kayhanian (2002) studied the variability of EMCs for 

metal concentrations in stormwater as the result of sampling 

method (flow-weighted composite, first flush, all grabs, peak, 

Table 11. Percentage of pollutant load contained in storm runoff for selected levels of 

impervious ground cover.

[Data from Chang and others (1990); in., inches; BOD, biochemical oxygen demand; COD, chemical oxygen 

demand; NH3, ammonia nitrogen; NO2+NO3, nitrite plus nitrate nitrogen; TSS, total suspended solids]

Pollutant

Percentage of pollutant load

First 0.5 in. 

of runoff

Last 0.25 in. 

of runoff

First 0.5 in. 

of runoff

Last 0.75 in. 

of runoff

First 0.5 in. 

of runoff

Last 0.75 in. 

of runoff

30 percent

impervious cover1 50 percent impervious cover2 90 percent impervious cover3

BOD 68 32 53 47 33 67

COD 77 23 57 43 48 52

NO2+NO3 62 38 56 44 40 60

NH3 71 29 58 42 30 70

TSS 81 19 61 39 41 59

Copper 67 33 42 58 31 69

Fecal coliform bacteria 80 20 44 56 36 64

10.75 in. average maximum runoff. 

21.25 in. average maximum runoff.

32.25 in. average maximum runoff.
Framework for Future Research in the Watershed 39

end, and random). Concentrations for copper, lead, and zinc 

showed considerable variability based upon the sampling 

method used (fig. 28), and the variability had a substantial 

impact on the calculation of annual loads. Othmer and Berger 

(2002) evaluated sampling methods for EMCs for solids, 

organic carbon, hydrocarbons, metals, and nutrients. They 

concluded that substantial variability between sampling 

methods indicated the need for establishing criteria for the 

collection of representative samples.

● Determining the effect of lot-scale land practices 

on runoff, storage, and ground-water recharge 

including:

○ Compaction by construction traffic patterns 

and grading.

○ Retaining the original soil structure including 

organic and leaching horizons.

○ Infiltration specifications for fill used to raise 

the elevation in areas beyond the house foundation.

○ Infiltration for mixed-species planting beds in 

lot-scale landscapes.

○ Water and chemical needs for establishing and 

maintaining mixed species planting beds.

○ Engineered bioretention planting beds for 

reducing nutrient runoff.

○ Use of pervious materials (for sidewalks and 

driveways), disconnected impervious areas, 

swales, and other infiltration enhancements to 

reduce runoff.

○ Reducing the energy of runoff to streams 

to minimize transport and resuspension of 

contaminants (such as metals, pathogens, and 

endocrine disrupting compounds) from urban 

areas.

● Quantifying dry season flux of nutrients from 

atmospheric deposition to accurately determine 

atmospheric contributions.

● Quantifying changes in partitioning of water 

(infiltration, runoff, deposition, resuspension) and 

contaminants as urbanization alters predevelopment 

rainfall-runoff relations.

● Determining the variability in runoff pollutant 

concentrations and loads transported by streams.

● Using remote sensing and GIS to quantify spatial 

variability in watershed characteristics (such as soil 

moisture and precipitation) over large areas.

● Refining lot-scale hydrologic and hydraulic models.

● Evaluating sample-collection strategies to provide 

detailed environmental data for determining nonpoint 

inputs and loads.

● Developing watershed models that account for 

spatial variability, dynamic infiltration, and lot-scale 

hydrology.

● Developing the linkage between watershed models 

and lot-scale models to evaluate the effect of smallscale changes over the entire Sarasota Bay watershed.

Figure 28. Variability in selected metal concentrations for different 

event mean concentration (EMC) sampling schemes (reprinted from 

Kayhanian, 2002 and published with permission).

400

350

300

250

200

150

100

50

0

CONCENTRATION, IN

MICROGRAMS PER LITER

COPPER (Cu) LEAD (Pb) ZINC (Zn)

Composite EMC

First flush EMC

All grabs EMC

Peak EMC

End EMC

Random EMC

Soller and others (2005) observed that pollutant concentrations in the first part of the wet season were up to 20 times 

higher than late season concentrations. They investigated 

pollutant loads from the first seasonal storm flows as an 

estimate of annual pollutant loads. Seasonal first flush did 

not consistently produce peak concentrations for total metals, 

dissolved metals, and anions because of variability in total 

flow, antecedent dry days, and season (Soller and others, 

2005). EMCs from highway runoff decreased as total storm 

runoff increased, and increased as the number of dry antecedent 

days before a storm increased (Kim and others, 2005). 

Continuous monitoring using in-stream sensors has been 

used as a surrogate to estimate pollutant concentrations and 

address the sample size errors described by Soller and others 

(2005). Regression equations for turbidity determined from 

nutrient and bacterial concentrations provided reliable estimates of nutrient and bacterial loads from continuous turbidity 

measurements (Rasmussen and others, 2005). 

Research Topics

Management strategies for multiple uses of the water 

resources of the Sarasota Bay watershed will depend on 

continued research to understand the dynamics of the 

watershed’s hydrology. Water movement and use within the 

watershed is complex and understanding water movement 

is paramount to refining strategies for balanced use of the 

resource. Important topics to consider for future research 

include:
40 Retrospective Review of Watershed Characteristics and a Framework for Future Research 

Summary

Comprehensive watershed planning is the key to 

successful multiple-use watershed management. Scientific 

tools available for watershed planning rely upon extensive and 

representative data-collection programs that are well designed 

and well maintained. The Comprehensive Conservation 

and Management Plan developed by the SBEP provides the 

framework for long-term management of stormwater and 

wastewater, maintenance and restoration of habitats, and 

access to the natural resources of the Sarasota Bay watershed. 

This report summarizes characteristics of the Sarasota Bay 

watershed and factors influencing recharge to ground water 

and overland runoff to surface water in the watershed.

The Sarasota Bay watershed drains more than 200 mi2

(518 km2) within Manatee, Sarasota, and Charlotte Counties 

and empties into several embayments before flowing into the 

Gulf of Mexico. Climate, geography, soils, hydrogeology, land 

cover, land use, and urbanization all affect water movement 

within the watershed. The climate of west-central Florida 

is characterized by long, warm, humid summers and short, 

mild, dry winters. Precipitation is seasonal and nearly 60 

percent of annual precipitation occurs during the wet season, 

from June through September. Average annual precipitation 

within Sarasota County between 1915 and 2003 was 52.47 in. 

(133 cm); the maximum and minimum recorded annual 

precipitation was 85.54 in. (217 cm) for 1959 and 32.77 in. 

(83 cm) for 2000, respectively.

The Sarasota Bay watershed lies within the Southern 

Gulf Coastal Lowlands where soils are poorly drained and the 

water table is near land surface. As the population of westcentral Florida continues to grow, pasture and forested lands 

have been converted to urban or suburban use. The literature 

reviewed indicates that land conversion increases the impervious cover, which alters the hydrology of the watershed by 

increasing runoff and decreasing infiltration. Most land-use 

practices tend to compact soil and reduce water infiltration, either by design or as a consequence of using the land. 

Impervious cover and compaction disrupt the natural process 

of recharge to the surficial aquifer system and increase runoff. 

Enhancing recharge benefits the watershed by replenishing 

the ground-water system, encouraging natural filtration, and 

reducing the transport of pollutants in runoff. 

Land practices that (1) reduce the removal of native 

soil horizons, (2) limit vehicular traffic during home site 

construction, (3) promote deeply rooted landscaping plants, 

and (4) add organic matter to the soil may enhance infiltration near the precipitation point-of-contact. Infiltration in the 

watershed does not occur exclusively by Horton infiltration, 

the infiltration mechanism employed by most rainfall-runoff 

models. Without accounting for Dunne infiltration, common in 

humid, high water-table areas, runoff CNs and runoff estimates may yield inaccurate results. 

As urbanization in the Sarasota Bay watershed continues, 

conventional approaches used to convey and manage runoff to 

large detention or treatment areas are being augmented with 

integrated management practices to address sustainability, 

restoration, and low-impact development. Integrated management practices used or planned in the watershed include 

vegetative roof covers, bioretention, grass swales, and permeable pavement. These practices have been shown to reduce 

runoff and pollutant concentration in runoff and to increase 

infiltration.

Restoring the balance between predevelopment runoff 

to surface water and recharge to ground water assumes that 

runoff will be reduced to enhance ground-water recharge. 

Reducing pollutant loadings in runoff that reach surface 

streams and the Sarasota Bay is an important goal of the 

SBEP. Annual pollutant loadings to Sarasota Bay were 

estimated in 1992 and 2006 from simple pollutant loading 

models based on sparse and empirical data. Simple models are 

useful for annual estimates, but they are of limited value for 

determining loads on a seasonal or finer scale for evaluating 

the effect of control measures. 

Complex models can provide information about 

(1) source loadings from different parts of the watershed, 

(2) effects of control measures, and (3) processes occurring 

at greater spatial and temporal resolution. Available complex 

watershed models include: BASINS (Better Assessment 

Science Integrating Point and Nonpoint Sources), HSPF 

(Hydrologic Simulation Program—Fortran), SWAT (Soil and 

Water Assessment Tool), SWMM (Storm Water Management 

Model), and SPARROW (SPAtially Referenced Regressions 

On Watershed Attributes).

Improved methods for determining model inputs for event 

mean concentrations are available. Automated, dischargeweighted sampling produced fewer errors than random grab, 

seasonal first-flush, and hydrograph peak sampling strategies. 

Finer scale spatial and temporal data and modeling approaches 

have the potential to elicit detailed information about the 

watershed characteristics influencing (1) recharge to ground 

water and (2) discharge by overland runoff and pollutant 

loading to surface water in the Sarasota Bay watershed.
Selected References 41

Selected References

Acker, W.L., 1974, Basic procedures for soil sampling and core 

drilling: Scranton, Pa., Acker Drill Co., 246 p. 

Alexander, R.B., Johnes, P.J., Boyer, E.W., and Smith, R.A., 2002, 

A comparison of models for estimating the riverine export of 

nitrogen from large watersheds: Biogeochemistry, v. 57/58, 

p. 295-339. 

Alexander, R.B., Smith, R.A., Schwarz, G.E., Preston, S.D., Brakesbill, J.W., Srinivasan, R., and Pacheco, P.A., 2001, Atmospheric 

nitrogen flux from the watersheds of major estuaries of the United 

States: An application of the SPARROW watershed model, in

Valigura, R., and others, eds., Nitrogen Loading in Coastal Water 

Bodies: An Atmospheric Perspective, American Geophysical 

Union Monograph, v. 57, p. 119-170. 

American Forests, 2000, Urban Ecosystem Analysis for the Houston 

Gulf Coast Region: U.S. Department of Agriculture Report, 11 p. 

American Forests, 2000, Local Ecosystem Analysis for Garland, 

Texas: U.S. Department of Agriculture Report, 8 p. 

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