U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2010–5092

Prepared in cooperation with the Southwest Florida Water Management District

A Geochemical Mass-Balance Method for Base-Flow 

Separation, Upper Hillsborough River Watershed, 

West-Central Florida, 2003-2005 and 2009
Photo Credits Cover and pages i-viii, and 9 — Dan Duerr, U.S. Geological Survey (retired)
A Geochemical Mass-Balance Method for 

Base-Flow Separation, Upper Hillsborough 

River Watershed, West-Central Florida, 

2003-2005 and 2009

By G.R. Kish, C.E. Stringer, M.T. Stewart, M.C. Rains, and A.E. Torres

Prepared in cooperation with the Southwest Florida Water Management District

Scientific Investigations Report 2010–5092

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2010

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Suggested citation:

Kish, G.R., Stringer, C.E., Stewart, M.T., Rains, M.C., and Torres, A.E., 2010, A Geochemical Mass-Balance Method for 

Base-Flow Separation, Upper Hillsborough River Watershed, West-Central Florida, 2003-2005 and 2009: 

U.S. Geological Survey Scientific Investigations Report 2010–5092, 33 p..

ISBN 978-1-4113-2909-6
Acknowledgments

 The authors gratefully acknowledge the cooperation and assistance of Michael Hancock 

from the Southwest Florida Water Management District for suggestions and comments on early 

drafts of the report and Patrick Potts, Park Services Specialist, Hillsborough River State Park, 

who provided logistical support during data-collection activities.

iii
iv
Contents

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

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

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

Description of Study Area ...................................................................................................................2

Hydrogeologic Framework..................................................................................................................2

Methods of Investigation..............................................................................................................................5

Data Collection Sites............................................................................................................................6

Hydrologic Data Collection .................................................................................................................7

Water-Quality Data Collection............................................................................................................7

Laboratory Analysis............................................................................................................................12

Mass-Balance Mixing Methods.......................................................................................................12

Hydrologic Characteristics, Water Quality, and Application of Base-Flow Separation 

Techniques ......................................................................................................................................13

Precipitation, Surface-Water Discharge, and Groundwater Flow Gradients...........................13

Water Quality.......................................................................................................................................18

Geochemical Mass-Balance Methods for Base-Flow Separation ............................................18

Summary .......................................................................................................................................................31

References Cited..........................................................................................................................................31

Figures

1. Map showing study area, showing data collection sites in the upper Hillsborough 

River watershed and boundary of the Hillsborough River watershed.................................3

2. Generalized hydrostratigraphy of the study area....................................................................4

3. Photograph showing limestone outcrop of the Tampa Member of the Arcadia 

Formation in the Hillsborough River at Hillsborough River State Park. ..............................5

4. Aerial photograph showing data collection locations at the SR39 site ..............................8

5. Aerial photograph showing data collection locations at the HRSP site .............................9

6. Hydrogeologic section (A—A‘) and groundwater flow during the wet season,

(July 12, 2003) and dry season (February 11, 2005) at the SR39 site...................................11

7. Hydrogeologic section (B—B‘) and groundwater flow during the wet season,

(July 12, 2003) and dry season (February 11, 2005) at the HRSP site .................................11

8-14. Graphs showing—

8. Daily average discharge and maximum and minimum specific conductance 

during a storm event at station 2303000, Hillsborough River near Zephyrhills, 

Florida ..................................................................................................................................12

9. Daily precipitation at the SR39 and HRSP study areas from February 1, 

2003, to June 1, 2005..........................................................................................................14

10. Weekly throughfall and total precipitation at the SR39 and HRSP study 

areas from February 1, 2003, to March 30, 2005 ...........................................................14

11. Hydrographs of mean daily discharge for the SR39 and HRSP sites 

 from February 1, 2003, to April 30, 2005 ..........................................................................15

v
12. Duration curves of mean daily discharge for the SR39 site for calendar 

years 2003-2005, and the HRSP site for calendar years 2003-2009 ...........................16

13. Weekly groundwater levels and river stage at the SR39 site ....................................17

14. Weekly groundwater levels and river stage at the HRSP site ...................................17

15. Stiff diagrams of geochemical analyses for precipitation, surface water, 

and groundwater at the SR39 site...................................................................................20

16. Stiff diagrams of geochemical analyses for precipitation, surface water, and

groundwater at the HRSP site ..................................................................................................20

17-20. Graphs showing—

17. Selected median constituent concentrations in water at the SR39 and HRSP 

sites for dry and wet seasons, and for groundwater samples from Upper 

Floridan aquifer wells, 2003-2005 for calcium, dissolved organic matter, 

magnesium, and strontium...............................................................................................21

18. Strontium concentrations in surface water and the Upper Floridan aquifer 

at the SR39 site on March 19, 2003 .................................................................................22

19. Relation between 87Sr/86Sr ratio and strontium concentrations in surface 

water and the Upper Floridan aquifer at the SR39 site on March 19, 2003..............22

20. Relation between delta hydrogen-2 and oxygen-18 isotope concentrations 

in surface water at the SR39 site and HRSP site compared with local 

meteoric water for Florida................................................................................................22

21-30. Graphs showing observed storm hydrograph and discharges estimated by—

21. Geochemical mass balance for selected major ions in SR39 surface water, 

September 2003 storm ......................................................................................................24

22. Geochemical mass balance for selected major ions in SR39 surface water, 

March 2004 storm..............................................................................................................26

23. Conductivity mass balance in SR39 surface water, August to November 2004......26

24. Geochemical mass balance for selected major ions in SR39 surface water, 

January 2005 storm ...........................................................................................................27

25. Geochemical mass balance for selected major ions in SR39 surface water, 

February/March 2005 storm.............................................................................................28

26. Conductivity mass balance in HRSP surface water, July 2004..................................28

27. Conductivity mass balance in HRSP surface water, September to 

November 2004...................................................................................................................29

28. Geochemical mass balance for selected major ions in HRSP surface water, 

January 2005 storm ...........................................................................................................29

29. Geochemical mass balance for selected major ions in HRSP surface water, 

February/March 2005 storm.............................................................................................30

30. Conductivity mass balance in HRSP surface water, August and 

 September 2009..................................................................................................................30

vi
Tables

1. Surface-water data-collection sites .........................................................................................7

2. Precipitation data-collection sites...........................................................................................10

3. Groundwater data-collection sites..........................................................................................10

4. Annual, wet season, and dry season precipitation in the upper Hillsborough River

watershed for the study period and from 1976 to 2005.........................................................13

5. Summary of surface-water and groundwater quality for the Hillsborough River

sites, 2003-2005 ...........................................................................................................................19

6. Pre-event and runoff end member values used in the geochemical mass balance

method..........................................................................................................................................23

7. Model input (measured) and model results (predicted) for the sampling point at 

the highest discharge using the geochemical mass balance method for the SR39 

 site during the low intensity storm event, September 25-28, 2003 .....................................24

8. Percentage of peak and average discharge attributed to base flow determined 

 by conductivity or geochemical mass balance methods for selected constituents 

 and storm events ........................................................................................................................25

Conversion Factors, Datums, Abbreviations, and Acronyms

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2

) 2.590 square kilometer (km2) 

Flow rate

cubic foot per second (ft3

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

inches per year (in/yr) 2.54 centimeters per year (cm/yr)

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

°F=(1.8×°C)+32

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

°C=(°F-32)/1.8

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

1929 (NGVD 29).

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

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

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 25 °C).

Concentrations of chemical constituents in water are given either in milligrams per liter

(mg/L) or micrograms per liter (μg/L).

vii
Acronyms

BFI Base-flow index

CMB Conductivity mass balance

DOC Dissolved organic carbon

GMB Geochemical mass balance

HYSEP Hydrograph Separation

NWIS National Water Information System

ROMP Regional Observation and Monitor-well Program

SWFWMD Southwest Florida Water Management District

USGS U.S. Geological Survey

VSMOW Vienna Standard Mean Ocean Water

viii
A Geochemical Mass-Balance Method for Base-Flow 

Separation, Upper Hillsborough River Watershed, 

West-Central Florida, 2003-2005 and 2009

By G.R. Kish1, C.E. Stringer2, M.T. Stewart3, M.C. Rains3, and A.E. Torres1

1

 U.S. Geological Survey, Tampa, FL

2

 Bureau of Indian Affairs, Nashville, TN

3

 University of South Florida, Tampa, FL

Abstract

Geochemical mass-balance (GMB) and conductivity 

mass-balance (CMB) methods for hydrograph separation were 

used to determine the contribution of base flow to total stormflow at two sites in the upper Hillsborough River watershed in 

west-central Florida from 2003-2005 and at one site in 2009. 

The chemical and isotopic composition of streamflow and 

precipitation was measured during selected local and frontal 

low- and high-intensity storm events and compared to the 

geochemical and isotopic composition of groundwater. Input 

for the GMB method included cation, anion, and stable isotope 

concentrations of surface water and groundwater, whereas 

input for the CMB method included continuous or pointsample measurement of specific conductance.

The surface water is a calcium-bicarbonate type water, 

which closely resembles groundwater geochemically, indicating that much of the surface water in the upper Hillsborough 

River basin is derived from local groundwater discharge. This 

discharge into the Hillsborough River at State Road 39 and at 

Hillsborough River State Park becomes diluted by precipitation and runoff during the wet season, but retains the calciumbicarbonate characteristics of Upper Floridan aquifer water. 

Field conditions limited the application of the GMB 

method to low-intensity storms but the CMB method was 

applied to both low-intensity and high-intensity storms. The 

average contribution of base flow to total discharge for all 

storms ranged from 31 to 100 percent, whereas the contribution of base flow to total discharge during peak discharge 

periods ranged from less than 10 percent to 100 percent. 

Although calcium, magnesium, and silica were consistent 

markers of Upper Floridan aquifer chemistry, their use in calculating base flow by the GMB method was limited because 

the frequency of point data collected in this study was not 

sufficient to capture the complete hydrograph from pre-event 

base-flow to post-event base-flow concentrations. In this study, 

pre-event water represented somewhat diluted groundwater.

Streamflow conductivity integrates the concentrations of 

the major ions, and the logistics of acquiring specific conductance at frequent time intervals are less complicated than data 

collection, sample processing, shipment, and analysis of water 

samples in a laboratory. The acquisition of continuous specific 

conductance data reduces uncertainty associated with lessfrequently collected geochemical point data. 

Introduction

Streamflow in a natural channel consists of runoff and 

base flow, with runoff composed of overland flow and subsurface stormflow, and base flow composed of unsaturatedzone water and groundwater. Information about the base-flow 

component of streamflow aids in (1) understanding groundwater/surface-water interactions, (2) developing hydrologic 

budgets, (3) calibrating numerical models, and (4) designing 

runoff-routing strategies (Winter and others, 1998). During 

dry periods, aquatic ecosystems are maintained by groundwater discharge that sustains the flow of streams (Gebert and 

others, 2007).

In 1997, the Florida Legislature mandated that all of the 

water management districts in Florida establish minimum 

flows and levels for surface waters and aquifers to maintain 

and protect natural systems while providing water for human 

use (Southwest Florida Water Management District, 2007). 

The Florida Legislature defined “minimum flow” as the “flow 

for a surface watercourse at which further withdrawals would 

be significantly harmful to the water resources or ecology of 

the area and which may provide for the protection of non-consumptive uses” (Florida Administrative Code, 2000). Determining minimum flows for surface waters is difficult if the 

base-flow contribution to streamflow is not accurately known, 

as is the case in the upper Hillsborough River.
2 A Geochemical Mass-Balance Method for Base-Flow Separation, Upper Hillsborough River Watershed

Several methods have been developed to separate 

base flow from streamflow: (1) recession-curve methods, 

(2) recursive-digital filter methods, and (3) mass-balance 

methods. During 2003–2005 and 2009, the U.S. Geological 

Survey (USGS), in cooperation with the Southwest Florida 

Water Management District (SWFWMD), conducted a study 

to assess the surface-water and groundwater contributions to 

the upper Hillsborough River watershed using geochemical 

mass-balance (GMB) methods for base-flow separation. This 

study addresses the priority water-resource issues for USGS 

scientific activities of surface-water and groundwater interactions as related to water-resource management (U.S. Geological Survey, 1999). Understanding the interaction between 

groundwater and surface water is necessary in west-central 

Florida because of the proximity of the karstified Upper 

Floridan aquifer to land surface, and because it provides 

information that can be used to evaluate the susceptibility and 

sustainability of the aquifer and surface-water resources in this 

relatively undeveloped part of the Hillsborough River watershed (Trommer and others, 2007). Because demands for water 

are projected to continue to increase in the future, an understanding of how groundwater sustains streamflow during dry 

periods will be necessary for water-resource managers to make 

informed decisions about future groundwater development. 

Purpose and Scope

This report describes the two approaches used to estimate 

the contribution of base flow to total discharge for selected 

storm events in two areas of the upper Hillsborough River 

watershed. The GMB method uses point samples of specific 

cations, anions, and stable isotopes, and the conductivity 

mass-balance (CMB) method uses both point samples of 

specific conductance and continuously-measured specific 

conductance to estimate the contribution of base flow to total 

discharge. 

Water-quality characteristics of streamflow and 

precipitation were measured during selected low- and highintensity local and frontal storm events, and compared to the 

water-quality characteristics of pre-event surface water and 

groundwater. A total of 207 water-quality samples were collected, consisting of 141 surface-water samples, 55 groundwater samples, and 11 precipitation samples. Field data were 

collected from 2003 to 2005 and during September 2009. A 

mixing model was developed to estimate the percentage of 

pre-event (base flow) and event (runoff) water composing 

the hydrograph. The mixing model, which produces a direct 

separation of base flow and runoff, was previously used by 

Stewart and others (2007) to evaluate and calibrate graphical 

hydrograph separation techniques using a CMB method. In 

this report, the mixing model was applied to point samples of 

specific conductance, major ions (such as calcium, magnesium), stable isotopes (hydrogen and oxygen) and continuous 

specific conductance for selected storm events.

Description of Study Area

The study area is located in the upper Hillsborough 

River watershed between Dade City and Hillsborough River 

State Park. The upper Hillsborough River watershed drains 

parts of eastern Pasco and Hillsborough Counties (fig. 1). 

Crystal Springs, a second magnitude spring, is located next 

to the river about 2 mi upstream from the state park. During 

dry periods, Crystal Springs is a major source of water to the 

river. A limerock mine also is located near the river, about 

2.5 mi upstream from State Road 39. Most of the river channel above the limerock mine is poorly defined and lies within 

a broad, heavily forested floodplain with extensive riverine 

wetlands and low topographic relief. The river channel below 

the mine to State Road 39 is moderately incised and the 

channel below State Road 39 to the state park is well defined 

(Lewelling, 2004).

With the exception of the urban and residential areas 

around Zephyrhills, Lakeland, and Plant City (fig. 1), the 

upper Hillsborough River watershed has not been extensively 

developed and consists primarily of heavily forested floodplains, upland forests, open grazing and rangeland, extensive 

riverine wetlands, isolated wetlands and prairies, and some 

sod- and row-crop cultivation (Trommer and others, 2007). In 

the upper Hillsborough River watershed, land-surface altitudes 

range from about 40 to 100 ft above NGVD 29. About 35 

percent of the entire upper Hillsborough River watershed is 

agricultural land (mostly pasture and rangeland), 23 percent 

is urban, 19 percent is wetlands, and 21 percent is a variety of 

undeveloped uplands (Trommer and others, 2007).

The climate in the study area is subtropical and humid, 

with an average annual temperature of 72 oF. The average 

annual precipitation from 1915 to 2005 was 53.95 in.; the 

maximum precipitation was 78.38 in., in 1959, and the minimum precipitation was 31.42 in., in 2000 (Southwest Florida 

Water Management District, 2008). The annual precipitation 

for the 30-year period from 1976 to 2005 was between 37.46 

and 76.09 in/yr, averaging about 53.24 in/yr for the entire 

watershed (Southwest Florida Water Management District, 

2008). About 60 percent of all rainfall occurs during the wet 

season from June through September during intense, localized 

thunderstorms, as well as occasional tropical storms and hurricanes. Winter frontal storms account for most of the rainfall 

from December through March. 

Hydrogeologic Framework

The upper Hillsborough River watershed is underlain by 

a thick sequence of carbonate rocks covered by unconsolidated 

surficial sediments (Wolansky and Thompson, 1987) (fig. 2). 

“The karst nature of the landscape in the watershed is due to 

irregular weathering of the limestone surface, illustrated by 

small, localized sinkholes, sinkhole lakes (primarily on the 

ridges), isolated circular wetlands, and coalescence of multiple 

sinkholes in wetlands and lakes” (Trommer and others, 2007). 
Introduction 3

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

datum NAD 83, Albers Equal-Area Conic Projection,

Standard parallels 29°30’ and 45°30’,

Central Meridian -83°00’

Withlacoochee

28°30’

15’

28°00’

82°45’ 30’ 15’ 82°00’

5 10 MILES

0 5 FLORIDA

LOCATION

OF STUDY

AREA

HERNANDO COUNTY

PASCO COUNTY

HILLSBOROUGH

COUNTY

POLK COUNTY

PINELLAS

COUNTY

SUMTER

COUNTY

New

Port

Richey

Old

Tampa

Bay

Gulf of Mexico

LAKE

COUNTY

10 KILOMETERS

0

Tarpon

Springs

Land O Lakes

Temple

Terrace

Hillsborough

River

State Park

Plant City

Lakeland

Brandon

Tampa

Clearwater

Brooksville

Dade

City

River

Cr

River

eek

River

River

Cypress

River

Alafia

Anclote

Pithlachascotee

Hillsborough

301

39

EXPLANATION

HILLSBOROUGH RIVER WATERSHED BOUNDARY

UPPER HILLSBOROUGH RIVER WATERSHED BOUNDARY

DATA-COLLECTION SITES

SR39

HRSP

Crystal Springs

Blackwater Creek

Zephyrhills



Figure 1. Study area, showing data collection sites in the upper Hillsborough River watershed and boundary of the 

Hillsborough River watershed. Aerial photos of SR39 and HRSP sites are shown on figures 4 and 5, respectively.
4 A Geochemical Mass-Balance Method for Base-Flow Separation, Upper Hillsborough River Watershed

Suwannee

Limestone

Ocala

Limestone

Avon Park

Formation

Undifferentiated

Sands and clays

Quartz sand, silty sand, clayey

sand

Clay, minor quartz sand,

phosphate, fine-grained

dolomite, residual limestone

Limestone, minor quartz sand,

phosphate, chert, clay, fine-grained

dolomite

Limestone, packstone to

grainstone, trace quartz sand,

organics, variable dolomite and

clay content, highly fossiliferous,

vuggy

Limestone, micritic, chalky, very

fine- to fine-grained, soft,

poorly indurated, trace organics,

clays and dolomite, abundant

foraminifera

Evaporites present in dolomite as

interstitial gypsum and anhydrite,

with evaporite filling pore space,

and as interbeds in the lower part

Dolomite is hard, brown, sucrosic

in texture and commonly fractured

Limestone, dolomite, and

evaporites

Limestone and dolomite interbeds

typical in upper part; deeper beds

are continuous dolomite with increasing evaporites at base

Limestone is fine-grained, tan,

packstone with

variable amounts of organic-rich

laminations near top

recrystallized

Intermediate

confining unit

Tampa/Suwannee

producing zone

Ocala

semiconfining

unit

Ocala/Avon Park

producing zone

Avon Park

producing zone

Upper Floridan aquifer

Middle confining unit

Surficial aquifer system

Hawthorn Group

Tampa

Member

of the

Arcadia

Formation

Holocene

to

Pliocene

Miocene

Oligocene

Eocene

SYSTEM STRATIGRAPHIC

UNIT

GENERAL

LITHOLOGY

HYDROGEOLOGIC

UNIT

Figure 2. Generalized hydrostratigraphy of the study area (modified from Tihansky, 2005). 

Dashed lines where inferred. 

and thickest to the south and toward the ridges” (Trommer and 

others, 2007). The surficial aquifer is not a substantial source 

of water supply in the upper Hillsborough River watershed. 

The surficial aquifer does, however, provide a source of water 

that flows to streams and recharges the Upper Floridan aquifer, 

either by downward vertical leakage through the confining 

unit or directly through breaches in the confining unit (Trommer and others, 2007). A continuous surficial aquifer does not 

extend across the entire study area because of the variability of 

the underlying confining unit (Champion and DeWitt, 2000). 

The surficial sediments pinch out in parts of the study area, 

exposing limestone in areas such as river channels where 

outcrops are present (fig. 3). 

Principal hydrogeologic units within the watershed are in 

descending order the surficial aquifer, the intermediate confining unit, and the Upper Floridan aquifer (fig. 2).

“The top of the surficial aquifer is contiguous with land 

surface and the aquifer consists of unconsolidated clastic 

sediments of quartz sand, clayey sand, and organic debris that 

range in age from Holocene to Pliocene” (Trommer and others, 2007). This unit commonly is referred to as the surficial 

aquifer system in areas where more than one permeable unit 

is present or where the deposits are interbedded (Metz and 

Sacks, 2002). In this report these deposits are considered to 

form a single aquifer referred to as the surficial aquifer, rather 

than a system. “The thickness of the aquifer is variable; generally, surficial deposits are thinnest near the stream channels 
Methods of Investigation 5

The intermediate confining unit underlies the surficial 

aquifer. This semiconfining layer consists of undifferentiated 

deposits of the Hawthorn Group (Southeastern Geological Society, 1986), a predominantly Miocene-age clay. The Hawthorn 

Group can include chert as well as carbonate mud that has been 

described as a residuum of the limestone in the Tampa Member 

of the Arcadia Formation (Sinclair, 1974). The confining unit is 

discontinuous near the Hillsborough River channel and along the 

middle and lower reaches of Blackwater Creek (fig. 1), although 

clay outcrops are present in the stream channel and along the 

banks at many other locations (Trommer and others, 2007). The 

intermediate confining unit ranges from 0 to greater than 30 ft 

thick in the study area (Southwest Florida Water Management 

District, 1996). The driller’s log for the HRSP site “Suspension 

Bridge” well indicates that surficial and/or Hawthorn Group 

materials are present to a depth of 12 ft below land surface. 

The Upper Floridan aquifer underlies the intermediate 

confining unit. The aquifer is semiconfined throughout most of 

the study area and is unconfined in the stream valleys where 

the intermediate confining unit is discontinuous (Trommer and 

others, 2007). The Upper Floridan aquifer is a regional aquifer 

consisting of multiple layers of continuous limestone and 

dolomite, ranging in age from Eocene to Miocene. Locally, 

the aquifer includes the Avon Park Formation, the Ocala 

Limestone, the Suwannee Limestone, and the Tampa Member of the Arcadia Formation (Trommer and others, 2007). 

The Suwannee Limestone is commonly the first continuous 

limestone found in the study area below the surficial deposits 

and any remnant Hawthorn Group materials. The driller’s log 

for the HRSP site well describes the material collected from 

12 to 27 ft deep as “cement-like,” and indicates that it is the 

soft, sandy, marly upper part of the Suwannee Limestone. The 

limestone of the Tampa Member of the Arcadia Formation is 

the uppermost carbonate unit in the study area; where present, it lies close to land surface and crops out in some stream 

channels (Trommer and others, 2007).

Methods of Investigation

Several methods have been developed to separate base 

flow from streamflow (Nathan and McMahon, 1990; Lin and 

others, 2007). Stewart and others (2007) grouped these methods into three categories: recession-curve methods, recursivedigital filter methods, and mass-balance methods. The recession curve methods use the slope of the stream hydrograph 

recession to determine a basin-specific constant that represents 

the contribution of the groundwater system to total streamflow 

(Hall, 1968; Nathan and McMahon, 1990). The recession 

constant is thought to integrate many hydrologic and geologic 

characteristics of a watershed (Meyboom, 1961; Wright, 1970; 

Ford and Williams, 1989). Rorabaugh (1964) developed a 

recession curve method that was subsequently automated for 

use with available digital records of streamflow from USGS 

gaging stations (Rutledge, 1993). 

Figure 3. Limestone outcrop of the Tampa Member of the Arcadia Formation in the Hillsborough 

River at Hillsborough River State Park. Photograph by Dan Duerr, U.S. Geological Survey (retired).
6 A Geochemical Mass-Balance Method for Base-Flow Separation, Upper Hillsborough River Watershed

A second category of base-flow separation methods is 

based upon low-pass filters that separate the ‘‘low frequency’’ 

base-flow component from the ‘‘high-frequency’’ runoff or 

quick-flow component. Nathan and McMahon (1990) describe 

a recursive digital filter adapted from signal-processing theory 

where the degree of filtering is determined by adjusting a filter 

coefficient and selecting the number of passes the filter makes 

through the discharge dataset. The method provides objective and repeatable estimates of an index of base flow, but 

estimates are not based on hydrologic processes.

One type of analytical low-pass filter used for base-flow 

separation is based on the ‘‘time of cessation of runoff’’ (Pettyjohn and Henning, 1979). This type of filter uses a moving time window to find successive discharge minima on a 

hydrograph. The base-flow hydrograph is assumed to be the 

line connecting the selected minima. The degree of smoothing 

depends on the length of the time window, with longer times 

reducing the volume of base flow. Two automated base-flow 

separation procedures that use this general time-window 

method are the Base Flow Index (BFI) method (Wahl and 

Wahl, 1988) and the Hydrograph Separation (HYSEP) method 

(Sloto and Crouse, 1996). The BFI method is based on the 

Institute of Hydrology (1980) method. 

In these methods, the length of the time window is 

dependent on the formula:

N = A0.2 (1)

where N is the time, in days, after the peak discharge where 

all discharge returns to base flow, and A is the basin area, in 

square miles, above the station (Linsley and others, 1949). 

The only basin variable considered in this method is basin 

area. The cessation of surface runoff also depends on (1) other 

watershed characteristics, including directly connected impervious cover, average topographic slope, infiltration capacities, and drainage density; (2) the dynamic characteristics of 

the groundwater system; and (3) storage features, including 

riparian wetlands, depressions, banks, and floodplains. 

Low-pass filters, including base-flow recession or runoff 

cessation methods, are not well suited to hydrologic conditions 

in west-central Florida, where infiltration rates are often highly 

variable and surface slopes are gradual. For most graphical base-flow separation methods, N is typically assigned 

a time-period value of 1-5 days in equation 1, which is too 

short a time period for the poorly integrated drainage systems 

of west-central Florida (Ross and others, 1997; Stewart and 

others, 2007). 

Halford and Mayer (2000) provide several examples of 

discrepancies in estimates of base flow between groundwater discharge estimation methods and base-flow recession 

methods. They assert that because the major assumptions of 

the base-flow recession methods are commonly and grossly 

violated, streamflow records alone cannot be used to determine base flow. They recommend the use of multiple methods, 

including geochemical tracers, because the accuracy of any 

single base-flow separation method is difficult to assess.

The third category of base-flow separation methods 

consists of chemical mass-balance models that assume streamflow is composed of distinct flow components having a characteristic concentration of one or more conservative chemical 

constituents (Pinder and Jones, 1969; Fritz and others, 1976). 

Discrete or periodic streamflow samples are analyzed for 

selected, conservative constituents over a range of hydrologic 

conditions. For base-flow separations, two components are 

commonly determined: (1) pre-event water, consisting of 

unsaturated-zone water and groundwater; and (2) event water, 

consisting of overland flow and subsurface stormflow (Hooper 

and Shoemaker, 1986; Wels and others, 1991; Cey and others, 

1998). The application of three-component (pre-event water, 

precipitation, and event water) mass-balance methods is typically limited to individual storms in small headwater basins, 

usually less than 0.39 mi2, because of the required intensity 

of temporal and spatial sampling and analysis of the chemical constituents (Rice and Hornberger, 1998; Scanlon and 

others, 2001). Results from a meso-scale basin investigation 

by Uhlenbrook and others (2002) indicate that two-component 

separations may be more reasonable for larger basins than 

the small, high-relief headwater basins typically studied with 

three-component separations.

Hydrograph separation using isotopic-tracer mass-balance 

methods can provide valuable insight into processes affecting 

streamflow (Kendall and McDonnell, 1998). However, most 

isotopic hydrograph separation studies have been conducted 

in northern climates (Buttle, 1994), rather than in subtropical 

or tropical climates where evaporative losses during transit 

can be large (Gremillion and Wanielista, 2000). Isotopic and 

geochemical methods can be impractical for long-term streamflow applications. Consequently, shorter-term isotopic and 

geochemical measurements of base flow combined with longterm measurements of streamflow and specific conductance 

may be useful for selecting and calibrating graphical methods 

that can then be applied to existing streamflow records over 

longer periods. 

Data Collection Sites

Two data-collection sites on the upper Hillsborough 

River were selected: Hillsborough River at State Road 39 and 

Hillsborough River at the State Park (fig. 1), referred to herein 

as the SR39 and HRSP sites, respectively. Surface-water and 

groundwater data were collected at both sites. These sites were 

selected because of their location in the upper part of the Hillsborough River watershed and their direct connection to the 

Upper Floridan aquifer. Aquifer/stream interconnection in the 

area is indicated by groundwater hydraulic gradients toward 

streams, and potentiometric contours that bend upstream in the 

vicinity of the Hillsborough River on both wet and dry season 

potentiometric maps (Trommer and others, 2007). The gaged 

areas of the SR39 and HRSP sites cover 72.5 mi2 and 220 mi2

of the watershed, respectively. The SR39 site is about 5 mi 

upstream from the HRSP site and the drainage of the SR39 site 
Methods of Investigation 7

of a SWFWMD Regional Observation and Monitor-well 

Program well (ROMP 86A SWNN) on the north end, a USGS 

well (1-Deep) on the south end, and a USGS well (3-North) 

near the river (fig. 6). The three wells are finished in the Upper 

Floridan aquifer within the Suwannee Limestone, at depths of 

135, 112, and 19 ft below land surface, respectively (table 3). 

Transect B—B’ is located on the south side of the Hillsborough 

River at the HRSP site and consists of a shallow USGS well 

(“Suspension Bridge”) on the north end and two SWFWMD 

wells (“Parking Lot Shallow” and “Parking Lot Deep”) at 

the south end (fig. 7). The three wells are finished into the 

Upper Floridan aquifer within the Suwannee Limestone, at 

depths of 11.75, 37, and 50 ft below land surface, respectively. 

A semi-confining bed was not observed in transect B—B’. 

Monitoring wells and reference points at both transects were 

leveled to NGVD 29. Groundwater-level data were collected 

at approximately 1-week intervals. 

Water-Quality Data Collection

Water-quality samples were collected at surface-water, 

groundwater, and precipitation sites from 2003 to 2005 and 

2009. Surface-water samples were collected from the river 

channel, and groundwater samples were collected from three 

monitoring wells at each site. Bulk precipitation samples were 

collected weekly and composited monthly, surface-water 

samples were collected monthly, and groundwater samples 

were collected quarterly during this study. During storm 

events, surface-water samples were collected manually or 

using an automatic sampler at variously programmed time 

intervals in addition to the monthly samples. Non-stormwater 

surface-water samples were vertically integrated manually 

at the midpoint of the stream channel; storm surface-water 

samples were point samples collected either manually near the 

intake or at the intake of an automatic sampler. Precipitation 

samples were collected using the bulk precipitation samplers 

described earlier. An automatic sampler was established in 

the upland area near the SR39 site for collecting precipitation 

during periods of rainfall (table 2).

constitutes 33 percent of the drainage of the HRSP site. Direct 

connection between the Hillsborough River and the Upper 

Floridan aquifer may be an important factor for distinguishing base flow from runoff because of differences in chemical 

constituents. 

Hydrologic data (precipitation, throughfall, streamflow, 

and water levels) and water-quality data were collected during 

low-flow and stormflow conditions at both sites. Throughfall 

refers to the precipitation that reaches the ground after passing 

through the tree canopy. 

Hydrologic Data Collection

Continuous streamflow and specific conductance were 

measured at the SR39 and HRSP surface-water sites (table 1). 

The SR39 site (USGS identifier 02301988), was established 

solely to fulfill the purpose of this study, and recorded data 

from February 1, 2003, to April 30, 2005 (fig. 4). The HRSP 

site (USGS identifier 02303000), is a long-term streamflow 

monitoring station that has been operational since October 1939, with 70 years of record (fig. 5). Each station was 

equipped to measure river stage and surface-water specific 

conductance, as well as to relay data to the office using satellite telemetry so storm events could be monitored. A stagedischarge relation was developed for each site to calculate 

streamflow from continuous stage data (Rantz and others, 

1982). In addition, records of mean daily discharge were compiled for the Hillsborough River at the SR39 and the HRSP 

sites from February 1, 2003, to April 30, 2005. Precipitation 

and throughfall amounts were measured at each site using 

two bulk precipitation samplers: one located under the river 

margin canopy near the bank of the river, and the other located 

in an open canopy perpendicular to the river channel (table 2). 

Precipitation rates were measured weekly. 

Groundwater levels were measured in monitoring wells 

along two transects (figs. 4 and 5). Along these transects, 

hydrogeologic cross sections were constructed using data 

from previous reports and from drilling activities conducted 

as part of this study (figs. 6 and 7, table 3). Transect A—A’

crosses the Hillsborough River at the SR39 site and consists 

Table 1. Surface-water data-collection sites.

[Surface-water site locations are shown in figures 4 and 5]

Site name

Data type Period of record

Name and site ID In this 

report

Hillsborough River at Hwy 39 nr 

Crystal Springs FL (02301988) SR39 Mean daily discharge, continuous discharge, specific conductance, and temperature February 1, 2003, to April 30, 20051

Hillsborough River near 

Zephyrhills FL (02303000) HRSP

Mean daily discharge October 1939 to present

Continuous discharge, specific conductance, 

pH, and temperature February 1, 2003, to April 30, 2005

1

Mean daily discharge period of record is February 1, 2003, to May 30, 2005.
8 A Geochemical Mass-Balance Method for Base-Flow Separation, Upper Hillsborough River Watershed

Figure 4. Data collection locations at the SR39 site (transect A—A‘ shown in figure 6).

A

A’

State Road 39

H i l l s b o r o u g h

R i v e r

Romp 86A

SWNN

3 North

1 Deep

82°10’

28°11’45”

0 500 FEET

0 50 100 150 METERS

SR39

EXPLANATION

HYDROGEOLOGIC SECTION

PRECIPITATION GAGE

STREAMFLOW MONITORING

STATION AND NAME

3 North OBSERVATION WELL AND NAME

SR39

A A’
Methods of Investigation 9

Figure 5. Data collection locations at the HRSP site (transect B—B‘ shown in figure 7).

H i l l s b o r o u g h R i v e r

B

B’

Suspension

Bridge

Parking Lot Shallow

Parking Lot Deep

82°14’

0 500 FEET

0 50 100 150 METERS

28°09’ HRSP

EXPLANATION

HYDROGEOLOGIC SECTION

PRECIPITATION GAGE

STREAMFLOW MONITORING

STATION AND NAME

OBSERVATION WELL AND NAME

HRSP

B B’

Suspension

Bridge
10 A Geochemical Mass-Balance Method for Base-Flow Separation, Upper Hillsborough River Watershed

Table 2. Precipitation data-collection sites.

[All data collection sites collect weekly bulk precipitation and monthly precipitation data. Rainfall 

site locations are shown in figures 4 and 5.]

Location Exposure Canopy type

West upland of Hillsborough River downstream 

of SR39 site

Open canopy Upland

Canopy: none

Understory: pasture grass

West floodplain of Hillsborough River downstream 

of SR39 site

Throughfall River margin

Canopy: laurel oak (Quercus 

laurifolia), cabbage palm (Sabal 

palmetto)

Understory: red maple (Acer 

rubrum, wild coffee (Psychotria 

nervosa)

Upland near center of 

HRSP site

Open canopy Upland

Canopy: none

Understory: Saw palmetto (Serenoa repens); actively burned 

area

Floodplain adjacent to 

USGS gage on Hillsborough River in HRSP site

Throughfall River margin

Canopy: live oak (Quercus virginiana), laurel oak (Q. laurifolia), 

cabbage palm (S. palmetto)

Understory: red maple (A. rubrum), wild coffee (P. nervosa)

Table 3. Groundwater data-collection sites.

[Well locations are shown in figures 4 and 5. All wells extend into the Upper 

Floridan aquifer.]

USGS well identifier Well name Depth of well

 (feet)

SR39 site

281136082095602 3-North 19.0

281130082095000 1-Deep 112.0

281144082100402 ROMP 86A SWNN 135.0

HRSP site

SB-1 Suspension Bridge 11.75

280852082135601 Parking Lot Deep 50.0

280852082135602 Parking Lot Shallow 37.0
Methods of Investigation 11

Figure 6. Hydrogeologic section (A—A‘) and groundwater flow during 

the A, wet season (July 12, 2003) and B, dry season (February 11, 2005) at 

the SR39 site (modified from Trommer and others, 2007).

Figure 7. Hydrogeologic section (B—B‘) 

and groundwater flow during the A, wet 

season (July 12, 2003) and B, dry season, 

(February 11, 2005) at the HRSP site.

0 200 FEET

0 50 METERS

A, Wet season

B, Dry season

Hillsborough River

ROMP 86A

SWNN

1 Deep

SURFICIAL DEPOSITS

3 North

UPPER FLORIDAN AQUIFER

MEMBER

UNDIFFERENTIA

HAWTHORN

TED SANDS

SEMI-CONFINING BED

VERTICAL SCALE GREATLY EXAGGERATED

70

60

50

40

30

20

NORTH

ALTITUDE, IN FEET

ABOVE NGVD OF 1929

A

SOUTH

A’

54.4

55.9

56.7 59.8

56

55

57

58

59

EXPLANATION

LINE OF EQUAL HYDRAULIC HEAD – In feet above

NGVD of 1929. Contour interval is 1 foot.

GROUNDWATER FLOW DIRECTION

HYDRAULIC HEAD – In feet above NGVD of 1929

WELL – Showing altitude of top of screen

60

59.5

SEMI-CONFINING BED

VERTICAL SCALE GREATLY EXAGGERATED

62

61

60

59

59.5

SUWANNEE

LIMESTONE

70

60

50

40

30

20

NORTH

A

SOUTH

A’

SURFICIAL DEPOSITS

UPPER FLORIDAN AQUIFER

BOTTOM

OF WELL

- 66 FEET

58.9

59.1

62.2

BOTTOM

OF WELL

- 49 FEET

BOTTOM

OF WELL

-49 FEET

BOTTOM

OF WELL

-66 FEET

0 200 FEET

0 50 METERS

65

55

45

35

25

15

TAMPA

SUWANNEE LIMESTONE

GROUP

65

55

45

35

25

15

ALTITUDE, IN FEET

ABOVE NGVD OF 1929

ROMP 86A

SWNN

Hillsborough River

1 Deep

3 North

TAMPA MEMBER

HAWTHORN

GROUP

UNDIFFERENTIATED SANDS

35

36

37

38

34.4

35.7

38.6

38.5

SANDS

B B’

UNDIFFERENTIATED Suspension Bridge

Parking Lot Shallow

SANDS

UPPER

FLORIDAN

AQUIFER

SUWANNEE

LIMESTONE

B B’

50

40

30

20

10

NGVD

1929

UNDIFFERENTIATED

Parking Lot Deep

TAMPA

MEMBER

Vertical scale

greatly exaggerated

B, Dry season

A, Wet season

38.8

40.2

43.6

39

40

41

42

43

NORTH SOUTH

45

35

25

15

5

-5 0 100 FEET

0 25 METERS

Vertical scale

greatly exaggerated

0 100 FEET

0 25 METERS

50

40

30

20

10

NGVD

1929

NORTH SOUTH

45

35

25

15

5

-5

Suspension Bridge

Parking Lot Shallow

Parking Lot Deep

TAMPA

MEMBER

SUWANNEE

LIMESTONE

UPPER

FLORIDAN

AQUIFER

EXPLANATION

LINE OF EQUAL HYDRAULIC HEAD – In feet above

NGVD of 1929. Contour interval is 1 foot.

GROUNDWATER FLOW DIRECTION

HYDRAULIC HEAD – In feet above NGVD of 1929

WELL – Showing altitude of top of screen

42

43.6

43.5

ALTITUDE, IN FEET

ABOVE OR BELOW NGVD OF 1929

Hillsborough River Hillsborough River

ALTITUDE, IN FEET

ABOVE OR BELOW NGVD OF 1929
12 A Geochemical Mass-Balance Method for Base-Flow Separation, Upper Hillsborough River Watershed

d =

 −

























R R

R

sample standard

standard

×1, 000

(2)

where R is the ratio 2H/H or 18O/16O for 2H and 18O, respectively; 

Rsample refers to the isotope ratio of the environmental sample 

and Rstandard refers to the isotope ratio for Vienna Standard Mean 

Ocean Water (VSMOW) (Craig, 1961). The resulting sample 

values of delta deuterium (δ2H) and delta oxygen-18 (δ18O) 

are reported in per mil deviation relative to VSMOW and, by 

convention, the δ2H and δ18O of VSMOW are set at 0 per mil 

(Gonfiantini, 1978). Analytical precision exceeded 2.0 and 

0.2 per mil for 2H and 18O analyses, respectively.

Mass-Balance Mixing Methods

Base flow for selected low- and high-intensity storm 

hydrographs was estimated by the GMB method using major 

ion chemistry, and by the CMB method using point and continuous specific conductance of the water sampled throughout the storm. The specific ions used in the GMB method 

included calcium, magnesium, potassium, sodium, silica, 

chloride, and sulfate. 

The GMB method evaluated the mixing of base flow and 

runoff during selected low- and high-intensity storms using an 

equation originally proposed by Steele (1968) and Pinder and 

Jones (1969). Stewart and others (2007) used this approach to 

estimate base flow from specific conductance. Figure 8 provides an example of the terms used in the following equation:

 Specific conductance, temperature, pH, and dissolved 

oxygen were measured in the field using a YSI 556 multiparameter water-quality instrument. Sampling and field-processing equipment conformed to the USGS protocols for preventing sample contamination or loss (U.S. Geological Survey, 

variously dated). Quality-assurance samples (blanks and 

replicates) were collected for about 10 percent of the samples. 

Laboratory Analysis

Major cation and anion analyses and dissolved organic 

carbon analyses were conducted at the USGS National WaterQuality Laboratory, Denver, Colorado and at the University 

of South Florida Center for Water Analysis in Tampa, Florida, 

using analytical methods approved by the USGS and the U.S. 

Environmental Protection Agency (Clesceri and others, 1999). 

Bicarbonate concentrations were initially calculated from inflection-point titration values and then estimated from fixed endpoint titration values. A comparison of inflection-point titrations 

and fixed end-point titrations yielded similar values. Strontium 

isotopes were analyzed by the USGS Strontium Isotope Laboratory in Menlo Park, California, using the inductively coupled 

plasma-mass spectrometry method (Bullen and Kendall, 1998). 

Deuterium (2H) and oxygen-18 (18O) samples were 

analyzed at the USGS Stable Isotope Laboratory, in Reston, 

Virginia and the University of California at Davis, Department 

of Geology Stable Isotope Laboratory. Deuterium was analyzed using the hydrogen equilibration method (Revesz and 

Coplen, 2006), and 18O was analyzed using the carbon-dioxide 

equilibration method (Epstein and Mayeda, 1953). Isotope 

results are reported in the conventional delta notation (δ):

Figure 8. Daily average discharge and maximum and minimum specific conductance during a storm 

event at station 2303000, Hillsborough River near Zephyrhills, Florida (reprinted from Stewart and others, 

2007 and published with permission).
Hydrologic Characteristics, Water Quality, and Application of Base-Flow Separation Techniques 13

Q Q

C C

C C BF stream

stream RO

BF RO

= ( − )

( ) −



















 , (3)

where 

QBF is predicted base-flow discharge, in cubic feet per 

second; 

Qstream is measured stream discharge, in cubic feet per 

second; 

Cstream is measured specific conductance, in microsiemens 

per centimeter at 25 degrees Celsius, or ion concentration in 

milligrams per liter of discharge at any timestep, 

CRO is estimated end member concentration in runoff discharge as specific conductance in microsiemens per centimeter 

at 25 degrees Celsius, or ion concentration in milligrams per 

liter; and 

CBF is estimated end member concentration in base flow 

discharge as specific conductance in microsiemens per centimeter at 25 degrees Celsius, or ion concentration in milligrams 

per liter, just prior to the event (pre-event flow).

Results are presented as a percentage of the total storm 

discharge attributed to base flow by using the following 

equation:

Q

Q

BF

stream

BF











 × = 100 ( ) %

, (4)

where %BF is percentage of total stormflow discharge composed of base flow. Total storm base-flow contributions are 

reported as an average percentage, because each constituent 

value gives a unique estimate of base flow throughout a storm.

Hydrologic Characteristics, Water 

Quality, and Application of Base-Flow 

Separation Techniques

Precipitation and discharge amounts, groundwater levels, 

water quality, and the application of conductance and GMB 

methods for determining base flow are described herein. The 

water-quality section discusses differences between precipitation, groundwater, and streamflow quality. The GMB method 

section includes results for each storm and an example showing how to calculate base flow with the GMB method.

Precipitation, Surface-Water Discharge, and 

Groundwater Flow Gradients

Dry season rainfall (October-May) was substantially 

above average during the first year of the study (2003), 

slightly below average for 2004, and below average for 2005. 

Wet season rainfall (June-September) was slightly above average for 2003, substantially above average for 2004, and about 

average for 2005 (table 4).

 Daily precipitation was similar at both sites with the 

exception of June 17, 2003, when 0.87 and 4.3 in. were 

recorded at the SR39 and HRSP sites, respectively (fig. 9). 

Daily and weekly composite precipitation amounts also were 

similar at the SR39 and HRSP sites (fig. 10). Monthly composite open-canopy precipitation and throughfall amounts 

differed by less than 1 percent, on average, at each site over 

the study period.

Year or statistic

Annual wet 

season rainfall 

(June-Sept.)

Departure from 

30-year wet 

season annual 

mean

Annual dry 

season rainfall 

(Oct.-May)

Departure 

from 30-year 

dry season 

annual mean

Annual 

rainfall

Departure from 

30-year annual 

mean

Study period (2003-2005)

2003 33.47 2.75 34.05 11.53 67.52 14.28

2004 49.26 18.54 21.06 -1.46 70.32 17.08

2005 30.54 -0.18 18.32 -4.20 48.86 -4.38

30-year period (1976-2005)

30-year annual 

minimum 22.59 10.29 37.46

30-year annual 

mean 30.72 22.52 53.24

30-year annual 

maximum 49.26 44.34 76.09

Table 4. Annual, wet season, and dry season precipitation in the upper Hillsborough River watershed for the study period 

and from 1976 to 2005.
14 A Geochemical Mass-Balance Method for Base-Flow Separation, Upper Hillsborough River Watershed

Figure 9. Daily precipitation at the SR39 and HRSP study areas from February 1, 

2003, to June 1, 2005.

Figure 10. Weekly throughfall and total precipitation at the SR39 and HRSP 

study areas from February 1, 2003, to March 30, 2005.

0

2

4

6

8

10

12

SR39 Site

HRSP Site

14

16

18

PRECIPITATION, IN INCHES

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

0

2

4

6

8

10

12

14

16

18

PRECIPITATION, IN INCHES

2003 2004 2005

THROUGHFALL

RAINFALL

EXPLANATION

0

2

4

6

8

PRECIPITATION, IN INCHES

HRSP Site

SR39 Site

2003 2004 2005

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

0

2

4

6

8

PRECIPITATION, IN INCHES
Hydrologic Characteristics, Water Quality, and Application of Base-Flow Separation Techniques 15

characteristics of the watershed upstream from the station. 

A flat slope indicates floodplain storage or a high base-flow 

component, whereas a steeper slope indicates less storage or 

less sustained base flow (Searcy, 1959). Discharge-duration 

curves were developed for the SR39 site for the 2003-2005 

calendar years and for the HRSP site for the 2003-2009 

calendar years, to determine the discharge duration for the 

sampled storms. Stormwater samples for the SR39 and HRSP 

sites were collected when mean daily discharges were equaled 

or exceeded between 1 and 85 percent of the time (fig. 12). 

The low slope of the discharge-duration curves for both sites 

indicates floodplain storage or a high base-flow component.

Groundwater levels measured weekly at the six Upper 

Floridan aquifer wells in transects A and B increased with 

depth and were generally higher than river stage, indicating the 

potential for upward movement of groundwater into the shallower zones of the Upper Floridan aquifer, the surficial aquifer, 

and surface streams in the study area (figs. 13 and 14, table 3). 

Head gradients in the Upper Floridan aquifer were consistently 

upward and toward the river during both wet and dry seasons 

(figs. 6 and 7) except there was little upward gradient between 

the deep and shallow parking lot wells at the HRSP site. 

Records of mean daily discharge were compiled for the 

Hillsborough River at the SR39 and the HRSP sites from February 1, 2003, to May 30, 2005. The magnitude and seasonal 

patterns of mean daily discharge, in cubic feet per second, 

varied widely over the study period. A comparison of the 

hydrographs indicates similar patterns at the two sites, but substantially different magnitudes during individual storm events 

(fig. 11). The HRSP site has a substantially greater drainage 

area, which accounts for the greater mean annual discharge 

and peak discharges at that site. These patterns were expected 

to be similar because the portion of the watershed at the SR39 

site is contained in the larger watershed of the HRSP site.

At the SR39 site, storms that occurred in March 2003, 

September 2003, March 2004, August-November 2004, January 2005, and February 2005 were selected for analysis. At the 

HRSP site, storms that occurred in July 2004, August-November 2004, January 2005, February 2005, and September 2009 

were selected for analysis.

Discharge-duration curves are cumulative frequency 

curves showing the percentage of time that the daily mean 

discharge of a stream equals or exceeds a given value during a specific time period. The shape of the curve reflects the 

Figure 11. Hydrographs of mean daily discharge for the SR39 and HRSP sites from February 1, 2003, to 

April 30, 2005.
16 A Geochemical Mass-Balance Method for Base-Flow Separation, Upper Hillsborough River Watershed

Figure 12. Duration curves of mean daily discharge for A, the SR39 site for calendar years 

2003-2005, and B, the HRSP site for calendar years 2003-2009.

10

100

1,000

10,000

1

10

100

1,000

10,000

1

DISCHARGE, IN

CUBIC FEET PER SECOND

0 20 40 60 80 100

DISCHARGE, IN

CUBIC FEET PER SECOND

10 30 50 70 90

PERCENTAGE OF TIME DISCHARGE

WAS EQUALED OR EXCEEDED

A, SR39 Site

B, HRSP Site

HILLSBOROUGH RIVER – Near

Crystal Springs, 2003-05

SAMPLED STORM

EXPLANATION

HILLSBOROUGH RIVER – Near

Zephyrills, 2003-09

SAMPLED STORM

EXPLANATION
Hydrologic Characteristics, Water Quality, and Application of Base-Flow Separation Techniques 17

50

55

60

65

ROMP86A SWNN

1 Deep

3 North

River stage

WATER LEVEL ALTITUDE,

IN FEET ABOVE NGVD OF 1929

F

2003

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

2004 2005

38

42

46

34

44

40

36

32

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

WATER LEVEL ALTITUDE,

IN FEET ABOVE NGVD OF 1929

2004 2005

River stage

Suspension

bridge well

Parking lot

deep well

Parking lot

shallow

well

Figure 13. Weekly groundwater levels and river stage at the SR39 site (well locations shown in figure 4).

Figure 14. Weekly groundwater levels and river stage at the HRSP site (well locations shown 

in figure 5).
18 A Geochemical Mass-Balance Method for Base-Flow Separation, Upper Hillsborough River Watershed

Water Quality

 The median concentrations of magnesium, potassium, 

chloride, bicarbonate plus carbonate, silica, dissolved organic 

carbon, and strontium in groundwater were similar between 

study sites, and all samples were calcium-bicarbonate water 

types (table 5). The Upper Floridan aquifer is close to land 

surface and the rock formation crops out in the stream channels in the upper part of the watershed. Because of localized 

karst features, the interconnection between the surficial aquifer 

and the Upper Floridan aquifer may be substantial (Trommer 

and others, 2007). The depth of wells completed in the Upper 

Floridan aquifer in this study ranged from 11.75 to 135 ft 

below land surface. 

Stiff diagrams were used to identify water types for precipitation, shallow and deep groundwater, and surface-water 

samples for the wet and dry seasons. Again, the surface water 

and groundwater samples collected at the SR39 and HRSP 

sites were calcium-bicarbonate water types, indicative of 

groundwater discharge through limestone. Hillsborough River 

samples collected at both sites during high- and low-flow periods resembled slightly diluted groundwater samples, indicating that most of the surface water originated from groundwater 

(figs. 15 and 16).

Although median concentrations of constituents in 

groundwater samples collected for this study did not vary 

widely, seasonal water-quality differences were present. Bar 

graphs for selected constituents are provided in figure 17 

to illustrate the similarities and differences in the median 

concentrations of major ions. Water-quality samples from the 

Hillsborough River and groundwater at the SR39 and HRSP 

sites during the wet and dry seasons are compared in each of 

the bar graphs. 

Median calcium concentrations in surface water were 

similar for the wet season at the SR39 site (62.5 mg/L), the dry 

season at the SR39 site (66.3 mg/L), and the dry season at the 

HRSP site (60.3 mg/L) (fig. 17A, table 5). The median calcium 

concentrations were substantially lower for the wet season at 

the HRSP site (37.4 mg/L). The median calcium concentrations of groundwater samples for the SR39 and HRSP sites 

were 90 and 116 mg/L, respectively. Calcium and carbonate 

are the major constituents of limestone in the Upper Floridan 

aquifer, and these ions are dissolved in groundwater as it flows 

through, and interacts with, the rock matrix.

The median dissolved organic carbon (DOC) concentrations in surface water differed substantially between seasons, 

but were similar between sites; specifically, median DOC 

concentrations at the SR39 and HRSP sites were 6.6 and 

4.2 mg/L, respectively, during the dry season and 23.5 and 

22.0 mg/L, respectively, during the wet season (fig. 17B, 

table 5). The median concentrations of DOC in groundwater at 

the SR39 and HRSP sites were 2.2 and 2.3 mg/L, respectively. 

During the wet season, DOC is released to surface waters 

when the channels and floodplains are hydraulically connected 

during high-flow periods.

Bar graphs for magnesium (fig. 17C) and strontium 

(fig. 17D) show that the median for each constituent was similar for surface-water and groundwater samples (approximately 

2-5 mg/L for magnesium, and 100-300 μg/L for strontium). 

On March 19, 2003, groundwater samples collected 

from wells at the SR39 site also were analyzed for strontium 

and strontium isotopes. The strontium concentrations in 

the 1-Deep well and the 3-North well at the SR39 site were 

816 μg/L and 181 μg/L, respectively, whereas the median 

strontium concentration in the river at State Road 39 was 

86 μg/L (fig. 18). The 87Sr/86Sr ratio in a water sample from the 

1-Deep well was 0.70787, which is within the range of values 

for Eocene-age seawater (Hess and others, 1986; Howarth and 

McArthur, 1997), and the 87Sr/86Sr ratio from the water sample 

from the 3-North well was 0.7080, which is within the range 

of values for Oligocene-age seawater (fig. 19). 

The 2H and 18O analyses for precipitation, groundwater, 

and surface-water samples were obtained at both sites throughout the study. Little difference was noted in the isotopic signature between rainfall and throughfall at either site, because 

both rainfall and throughfall isotopes plot near the local 

meteoric water line, as determined by Sacks (2002) (fig. 20). 

At both sites, 2H and 18O values for groundwater and surface 

water were offset to the right of the local meteoric water line. 

These samples plotted along the local evaporation trend line, 

described by Sacks (2002), indicating that groundwater and 

surface water were influenced by evaporation. The proximity 

of these samples to the local evaporation trend line suggests 

that these waters are composed of rainwater that has undergone some evaporation. The similar isotopic signatures of the 

groundwater and surface water further illustrate the prevalence 

of groundwater in the Hillsborough River, both during regular 

flows and throughout storm events. 

Isotope samples from the HRSP site are closer to the 

meteoric water line than samples from the SR39 site. This difference indicates that water at the HRSP site had undergone less 

enrichment (that is, less evaporation) than water at the SR39 

site. The discharge at SR39 is smaller and there is less tree cover 

over the river upstream from this site compared to the HRSP 

site, which may account for the greater evaporation at SR39.

Geochemical Mass-Balance Methods for 

Base-Flow Separation

The GMB and CMB methods for hydrograph separation 

were used to determine the contribution of base flow to total 

stormflow. The CMB method is a subset of a broader group of 

chemical mass balance methods. Input for the GMB method 

may include cations, anions, and stable isotopes, whereas 

input for the CMB method includes continuous or point 

samples of specific conductance. In this study, both GMB and 

CMB methods were used to estimate base flow. Field conditions limited the application of the GMB method to lowintensity storms, however, the CMB method was applied to 

low-intensity and high-intensity storms.
Hydrologic Characteristics, Water Quality, and Application of Base-Flow Separation Techniques 19

Constituent Median Min Max N Lower 

quartile

Upper 

quartile Median Min Max N Lower quartile

Upper 

quartile

Surface water at SR39 site, wet season Surface water at SR39 site, dry season

Ca 62.5 19.0 70.0 68 53.8 65.0 66.3 28.2 79.5 63 59.0 74.5

Mg 3.85 1.7 4.40 68 3.55 4.1 4.29 2.47 5.52 63 3.90 4.46

Na 6.5 3.6 7.2 68 6.175 6.9 7.50 5.15 9.00 63 7.27 7.75

K 0.6 0.5 2.5 68 0.5 1.125 1.10 0.47 2.00 63 0.63 1.44

Cl 10 6.7 12.00 67 9.7 10 11.76 9.20 15.57 63 11.28 13.00

SO4 16 2.3 20 67 14 19 12.60 2.77 20.00 63 9.60 15.46

Alk 157 43 177 52 151 164 158 74 423 44 147 179.25

F 0.2 0.2 0.2 3 0.2 0.2 0.20 0.19 0.22 5 0.20 0.20

SiO2 7.2 4.7 10 68 6.9 7.7 7.40 3.20 9.45 49 6.99 7.70

Br 0.07 0.05 0.31 59 0.06 0.08 0.08 0.04 0.16 26 0.06 0.10

DOC 23.5 14 35 6 22.25 29.7 6.60 2.50 28.20 47 5.45 8.75

Sr 150 39 160 68 110 160 150 65 170 49 110 154

Surface water at HRSP site, wet season Surface water at HRSP site, dry season

Ca 37.4 23 50 7 29.5 40.5 60.34 36.90 67.40 48 55.25 64.13

Mg 3.5 2.4 4.5 7 2.985 4.15 4.70 3.30 6.70 48 4.50 4.83

Na 7.9 5.2 16 7 6.99 10.75 11.88 5.40 23.72 48 9.98 12.81

K 3.3 2.3 4.8 7 2.355 3.7 2.28 0.40 4.70 48 2.03 2.73

Cl 11.3 9.2 20 7 10.3 16 17.56 10.00 32.68 46 16.00 19.00

SO4 16 4.7 26 7 5.64 17.5 15.35 6.71 32.84 46 14.00 17.15

Alk 66.5 53 80 2 59.75 73.25 152 94 193 31 133 157

F 0.3 0.2 0.3 5 0.26 0.3 0.16 0.10 0.20 6 0.12 0.20

SiO2 9.1 6.5 10 7 7.75 9.34 9.60 6.70 11.40 37 9.20 9.80

Br 0.14 0.05 0.23 2 0.095 0.185 0.09 0.04 0.11 17 0.07 0.10

DOC 22 6.8 30 7 17.5 24.45 4.20 0.80 19.00 31 3.15 5.05

Sr 130 64 250 7 118 190 286 147 380 37 240 293

Upper Floridan aquifer at SR39 site Upper Floridan aquifer at HRSP site

Ca 90.0 65.0 111 26 75.5 98.0 116 107 122 7 111 120

Mg 2.20 2.00 9.50 26 2.10 2.49 1.80 1.30 2.00 7 1.75 1.91

Na 7.60 4.70 10.7 26 6.98 8.25 4.50 1.1 5.79 7 3.45 5.47

K 0.30 0.20 2.05 26 0.20 0.40 0.20 0.20 0.3 4 0.20 0.23

Cl 13.0 0.30 19.0 26 8.23 14.0 9.30 0.2 11.8 7 5.30 10.8

SO4 2.60 0.26 23 26 0.85 12.8 5.50 3.30 6.20 5 4.90 5.70

Alk 231 200 262 14 225 235 283 274 286 3 279 285

F 0.12 <0.10 0.20 12 0.10 0.18

SiO2 12 11 20 26 11.0 12.7 14.0 12.5 18.0 5 12.8 18.0

Br 0.10 0.10 0.12 12 0.10 0.10

DOC 2.2 1.5 6.2 26 2.00 2.85 2.30 1.40 6.40 7 1.95 5.80

Sr 170 100 876 26 160 196 237 170 295 5 170 295

Table 5. Summary of surface-water and groundwater quality for the Hillsborough River sites, 2003-2005.

[Locations of SR39 and HRSP sites are shown in figure 1. Constituents reported as milligrams per liter unless otherwise noted; Ca, calcium, Mg, magnesium; 

Na, sodium; K, potassium; Cl, chloride; SO4, sulfate; Alk, alkalinity, as CaCO3; F, fluoride; SiO2, silica; Br, bromide; DOC, dissolved organic carbon; Sr, strontium, in micrograms per liter; Min, minimum; Max, maximum; N, number of samples; <, less than]
20 A Geochemical Mass-Balance Method for Base-Flow Separation, Upper Hillsborough River Watershed

Figure 15. Stiff diagrams of geochemical analyses for 

precipitation, surface water, and groundwater at the SR39 site.

Figure 16. Stiff diagrams of geochemical analyses for 

precipitation, surface water, and groundwater at the HRSP site.

6

Precipitation

Surface-water (high flow)

at SR39 site

Groundwater (3-North well)

4 2 0 2 4 6

Chloride

Sulfate

Calcium

Sodium plus potassium

Magnesium

Bicarbonate plus carbonate

CONCENTRATION, IN

MILLIEQUIVALENTS PER LITER

CATIONS ANIONS

EXPLANATION

Surface-water (low flow)

at SR39 site

Precipitation

Groundwater

(Suspension Bridge well)

6 4 2 0 2 4 6

CONCENTRATION, IN

MILLIEQUIVALENTS PER LITER

Chloride

Sulfate

Calcium

Sodium plus potassium

Magnesium

Bicarbonate plus carbonate

CATIONS ANIONS

EXPLANATION

Surface-water (high flow)

at HRSP site

Surface-water (low flow)

at HRSP site
Hydrologic Characteristics, Water Quality, and Application of Base-Flow Separation Techniques 21

0

SR39 HRSP

SURFACE WATER

SR39

DRY

SEASON

SR39

WET

SEASON

HRSP

DRY

SEASON

HRSP

WET

SEASON

UPPER FLORIDAN

AQUIFER

20

40

60

80

100

120

140

MEDIAN CALCIUM CONCENTRATION, IN MILLIGRAMS PER LITER

A

0

SR39 HRSP

UPPER FLORIDAN

AQUIFER

5

10

15

20

25

B

0

UPPER FLORIDAN

AQUIFER

10

20

30

40

50

MEDIAN MAGNESIUM CONCENTRATION, IN MILLIGRAMS PER LITER

C

UPPER FLORIDAN

AQUIFER

MEDIAN STRONTIUM CONCENTRATION, IN MICROGRAMS PER LITER

D

MEDIAN DISSOLVED ORGANIC CARBON CONCENTRATION,

IN MILLIGRAMS PER LITER

0

100

200

300

SURFACE WATER

SR39

DRY

SEASON

SR39

WET

SEASON

HRSP

DRY

SEASON

HRSP

WET

SEASON

SURFACE WATER SURFACE WATER

SR39 HRSP SR39

DRY

SEASON

HRSP

DRY

SEASON

HRSP

WET

SEASON

SR39

WET

SEASON

SR39 HRSP SR39

DRY

SEASON

HRSP

DRY

SEASON

HRSP

WET

SEASON

SR39

WET

SEASON

50

150

250

Figure 17. Selected median constituent concentrations in water at the SR39 and HRSP sites for dry and wet 

seasons, and for groundwater samples from Upper Floridan aquifer wells, 2003-2005 for A, calcium, B, dissolved 

organic matter, C, magnesium, and D, strontium.
22 A Geochemical Mass-Balance Method for Base-Flow Separation, Upper Hillsborough River Watershed

0

300

600

900

STRONTIUM CONCENTRATION,

IN MICROGRAMS PER LITER

1 DEEP

WELL

3 NORTH

WELL

SITE

SR39

SURFACE WATER

3 North well

1 Deep well

SR39 surface water

Miocene Seawater

Oligocene Seawater

0.7078

0.7079

0.7080

0.7081

0.7082

0.7083

0.7084

STRONTIUM-87/STRONTIUM-86 RATIO

0 200 400 600 800 1,000

STRONTIUM CONCENTRATION,

IN MICROGRAMS PER LITER

-60

-50

-40

-30

-20

-10

0

10

-10 -8 -6 -4 -2 0

DELTA HYDROGEN-2, PER MIL

DELTAOXYGEN-18, PER MIL

LOCAL METEORIC WATER LINE – Sacks, 2002

LOCAL EVAPORATION LINE – Sacks, 2002

OPEN RAINFALL

THROUGHFALL

HILLSBOROUGH RIVER

GROUNDWATER

EXPLANATION

B, HRSP

-50

-40

-30

-20

-10

0

10

DELTA HYDROGEN-2, PER MIL

A, SR39

Figure 18. Strontium concentrations in surface water and 

the Upper Floridan aquifer at the SR39 site on March 19, 2003.

Figure 19. Relation between 87Sr/86Sr ratio and strontium 

concentrations in surface water and the Upper Floridan 

aquifer at the SR39 site on March 19, 2003 (seawater 

boundaries from Howarth and McArthur, 1997)

Figure 20. Relation between delta hydrogen-2 and 

oxygen-18 isotope concentrations in surface water at the A, 

SR39 site, and B, HRSP site, compared with local meteoric 

water for Florida.
Hydrologic Characteristics, Water Quality, and Application of Base-Flow Separation Techniques 23

The CMB method described by Stewart and others (2007) 

is based upon four assumptions:

• Base-flow conductivity remains constant. Shortterm observations of conductivity remain relatively 

constant even when soil conditions change abruptly 

from very dry to saturated. 

• Runoff conductivity remains constant. Although 

surface runoff conductivity increases from the 

groundwater divide to the stream with increased contact time with the soil, conductivity measurements 

from one storm event to the next at the same location 

yield consistent results. Regardless of whether or not 

a single storm event occurred after an extended dry 

period, or frequent runoff events occurred during the 

wet season, surface runoff conductivity remained 

relatively constant at a given location.

• Streamflow conductivity at lowest flows (pre-event 

flow) is used as an estimate of base-flow conductivity for the CMB method, because this value represents the weighted average conductivity of all base 

flow upstream from the measurement. 

• Streamflow conductivity at highest flows is used 

as an estimate of runoff conductivity for the CMB 

method, because this value represents the weightedaverage conductivity of all surface runoff influx 

upstream from the measurement. 

From these assumptions, Stewart and others (2007) provide a 

framework for applying the CMB method for annual estimates 

of base flow from mean daily data. The CMB method relies on 

measured values of stream conductivity during periods of very 

high flows and very low flows to estimate runoff and baseflow conductivity, respectively. It is reasonable to use lowflow values to estimate base-flow conductivity, as there is little 

or no surface runoff during extended periods of low flow. 

If a short period of record of stream conductivity that 

does not include many high-flow events is used for calibrating 

the CMB method for an annual estimate of base flow, it is possible that a value of runoff conductivity will be selected that 

is higher than the actual long-term runoff conductivity. When 

this conductivity value is applied to a longer discharge record 

that does contains higher flows than the period for which 

specific conductance data are available, base-flow contributions may be underestimated. Similarly, if the stream conductivity does not reach a maximum (assumed to be 100 percent 

groundwater) before the next storm event, base-flow contributions may be overestimated. For this reason, short records 

of mean daily stream specific conductance data that do not 

contain substantial periods of high flows may not produce an 

accurate calibration when using the CMB method. Estimates 

of base flow when using the GMB method over the duration 

of individual storms may improve base-flow estimates by 

providing storm-specific base-flow estimates.

The GMB and CMB methods were applied to five storms 

at the SR39 site (September 2003, March 2004, AugustNovember 2004, January 2005, and February 2005), four 

storms at the HRSP site (July 2004, September-October 2004, 

January 2005, and February 2005), and one high-intensity 

storm at the HRSP site (September 2009). The CMB method 

alone was applied to high-intensity storms at the SR39 site 

(March 2003, August-November 2003) and the HRSP site 

(July 2004, August-November 2004, and September 2009) 

because geochemical data were unavailable.

The GMB and CMB methods use two end-members, 

concentration of pre-event flow (base flow) and concentration 

of peak discharge, to estimate the base-flow contribution to total 

discharge during storm events. The GMB/CMB results were 

determined from calcium and magnesium concentrations and 

specific conductance. Other cations, anions, and isotopes did not 

exhibit substantial enough differences between pre-event and/

or peak runoff so that they could be used as end members for 

the GMB method. Pre-event values and peak discharge values 

for selected constituents were determined for the period of study 

(table 6). These values were used in the GMB model to predict 

the percentage of stormflow attributable to pre-event flow for 

the peak flow and average flow of each storm event. 

Base-flow discharge computed by the CMB/GMB 

methods from the storm data is expressed as a percentage of 

the measured discharge. The first storm sampling period at 

the SR39 site was from September 25 to September 28, 2003. 

Over this 3-day period, 0.74 in. of rainfall caused stream discharge to increase from 28 to 50 ft3

/s at the SR39 site (fig. 21). 

Table 7 shows the input to the model (measured values) and 

the calculated results (estimated base-flow end members) for 

the sampling point associated with the highest discharge during this storm. The model also calculated the QBF (base-flow 

discharge estimated from specific conductance or constituent 

concentration) and %BF (percentage of discharge attributed to 

base flow) determined by conductivity or geochemical mass 

balance methods for each sampling point during the course of 

the storm. Table 8 provides a summary of the percentage of 

peak and average discharge attributed to base flow for selected 

constituents and storm events.

Table 6. Pre-event and runoff end member values used in the 

geochemical mass balance method.

[Pre-event value assumed to be 100 percent base flow. Runoff value assumed 

to be 100 percent runoff; μS/cm at 25oC, microsiemens per centimeter at 

25 degrees Celsius; mg/L, milligram per liter]

Constituent Units Pre-event Runoff

Specific 

conductance μS/cm at 25 o

C 350 100

Calcium mg/L 52.7 16.0

Magnesium mg/L 4.4 2.1
24 A Geochemical Mass-Balance Method for Base-Flow Separation, Upper Hillsborough River Watershed

20

30

40

50

60

0

10

DISCHARGE, IN CUBIC FEET PER SECOND

SEPTEMBER 25 SEPTEMBER 26 SEPTEMBER 27

2003

CONTINUOUS-MEASURED DISCHARGE – At SR39 site

BASE-FLOW DISCHARGE – Estimated from continuous specific conductance

BASE-FLOW DISCHARGE – Estimated from sampled specific conductance

BASE-FLOW DISCHARGE – Estimated from calcium concentration

BASE-FLOW DISCHARGE – Estimated from magnesium concentration

EXPLANATION

Table 7. Model input (measured) and model results (predicted) for the 

sampling point at the highest discharge using the geochemical mass balance 

method for the SR39 site during the low intensity storm event, September 

25-28, 2003.

[Cstream, measured concentration of calcium in surface water; mg/L, milligrams per liter; 

CRO, predicted end member concentration of calcium in runoff discharge; CBF, predicted 

end member concentration of calcium in surface water in base flow dicharge; Qstream, stream 

discharge; ft3/s, cubic feet per second; QBF, predicted base flow discharge; ft3/s, cubic feet 

per second; Percent BF, predicted percentage of total stormflow discharge composed of 

base flow]

Parameter Value

Model input

CStream (measured) 51 mg/L 

QStream (measured) 50 ft3

/s

Model results

QBF (predicted base flow) 47 ft3

/s

PercentBF (predicted percent base flow) 95%

CRO (estimated end member) 16 mg/L 

CBF (estimated end member) 53 mg/L

Figure 21. Observed storm hydrograph and discharges estimated by geochemical mass balance for selected major 

ions in SR39 surface water, September 2003 storm.
Hydrologic Characteristics, Water Quality, and Application of Base-Flow Separation Techniques 25

Using calcium as an example, the QBF and %BF for the 

September 2003 peak stormflow at the SR39 sites were solved 

for as follows:

Q Q C C

C C BF stream

stream RO

BF RO

= −

−





 



 = ( ) −

( ) −





 ( 

50

51 16

53 16 ×



 = 47 3 3 . ft /s

, 

and

% . BF

BF . %

stream

Q

Q = × × 100 = = 47 350

100 94 6

(5)

Base-flow (pre-event) estimates based on sampled specific conductance, calcium concentration, magnesium concentration, and continuous specific conductance behaved similarly 

throughout the storm (fig. 21). Base flow at the SR39 site, 

estimated from sampled specific conductance and continuous 

specific conductance and expressed as a percentage of discharge, composed 100 percent of peak flow and 100 percent of 

average flow during the storm (table 8). Base flow estimated 

from calcium and magnesium concentrations was slightly and 

substantially lower, respectively, composing 95 and 52 percent 

for peak discharge and 91 and 46 percent of the average 

discharge during the storm.

The second storm sampling period at the SR39 site was 

from March 16 to March 23, 2004. Stream discharge ranged 

from 21 to 51 ft3/s over this period, in response to 1.7 in. of 

precipitation. Base flow estimated from sampled specific 

conductance, continuous specific conductance, and calcium 

concentration, expressed as a percentage discharge, composed 94 to 100 percent of peak flow and 92 to 99 percent of 

average flow during the storm (fig. 22 and table 8). Base flow 

estimated from magnesium concentration was lower, composing 65 percent of peak discharge and 85 percent of average 

discharge during the storm.

The next storm sampling period at the SR39 site was 

from August to November 2004 during an active hurricane 

season. Discharge ranged from 20 to 1,459 ft3/s over this 

4-month period, and daily rainfall exceeded 2 in. on several 

days, causing periods of substantial stormwater-runoff. Base 

flow estimated from continuous specific conductance composed less than 10 percent of the peak flow during these runoff 

events and 32 percent of the average base flow during this 

period (fig. 23 and table 8).

Table 8. Percentage of peak and average discharge attributed to base flow determined by conductivity or geochemical mass balance 

methods for selected constituents and storm events.

[Location of SR39 and HRSP sites are shown in figure 1. CMB, conductivity mass balance method; GMB, geochemical mass balance method; nd, not 

determined; <, less than]

Site Storm event

Percentage of base flow in peak and average discharge 

determined by CMB and GMB methods

Determined from

continuous specific 

conductance

Determined from 

sampled specific conductance

Determined from calcium

concentration

 Determined from magnesium concentration

Peak Average Peak Average Peak Average Peak Average

SR39 September 2003 100 100 100 100 94.6 90.9 52.2 45.7

SR39 March 2004 94.0 92.4 93.6 93.7 100 98.7 65.2 85.3

SR39 August-November 2004 <10.0 31.9 nd nd nd nd nd nd

SR39 January 2005 100 100 100 100 100 100 100 100

SR39 February 2005 100 100 100 100 100 100 100 98.6

HRSP July 2004 50.8 87.9 nd nd nd nd nd nd

HRSP September-November 2004 <10.0 36.4 nd nd nd nd nd nd

HRSP January 2005 100 100 100 100 100 100 100 100

HRSP February 2005 100 100 100 100 51.8 92.0 36.5 89.1

HRSP September 2009 nd nd <10.0 44.7 nd nd nd nd
26 A Geochemical Mass-Balance Method for Base-Flow Separation, Upper Hillsborough River Watershed DISCHARGE, I

N CUBIC FEET PER SECOND

0

10

20

30

40

50

60

15 16 17 18 19 20 21 22 23 24

MARCH, 2004

CONTINUOUS-MEASURED DISCHARGE – At SR39 site

BASE-FLOW DISCHARGE – Estimated from continuous specific conductance

BASE-FLOW DISCHARGE – Estimated from sampled specific conductance

BASE-FLOW DISCHARGE – Estimated from calcium concentration

BASE-FLOW DISCHARGE – Estimated from magnesium concentration

EXPLANATION

500

1,000

1,500

0

DISCHARGE, IN CUBIC FEET PER SECOND

AUGUST SEPTEMBER OCTOBER NOVEMBER

2004

CONTINUOUS-MEASURED

DISCHARGE – At SR39 site

BASE-FLOW DISCHARGE –

Estimated from continuous

specific conductance

EXPLANATION

Figure 22. Observed storm hydrograph and discharges estimated by geochemical mass balance for selected major ions in 

SR39 surface water, March 2004 storm.

Figure 23. Observed storm hydrograph and discharges estimated by conductivity mass balance in 

SR39 surface water, August to November 2004.
Hydrologic Characteristics, Water Quality, and Application of Base-Flow Separation Techniques 27

In January 2005, the SR39 site was sampled from 

January 13 to January 28. The discharge ranged from 16 to 

22 ft3/s, in response to 0.86 in. of precipitation. Base flow 

estimated from sampled specific conductance, continuous 

specific conductance, calcium concentration, and magnesium 

concentration was 100 percent of peak flow and 100 percent of 

average flow during the storm (fig. 24 and table 8). 

During the storm event between February 24 and 

March 4, 2005, at the SR39 site, discharge increased from 12 

to 38 ft3/s in response to 2.13 in. of precipitation. Base flow 

estimated from sampled specific conductance, continuous specific conductance, and calcium concentration was 100 percent 

of peak flow and 100 percent of average flow during the 

storm (fig. 25 and table 8). Base flow estimated from magnesium concentration was 100 percent of peak discharge and 

99 percent of average discharge during the storm.

At the HRSP site, the first stormflow lasted from July 16 

to July 30, 2004. Stream discharge ranged from 83 to 551 ft3/s 

over the 15-day period, in response to 13.8 in. of precipitation. 

Base flow estimated from continuous specific conductance 

was 51 percent of peak flow and 88 percent of average flow 

during the storm (fig. 26 and table 8).

The next storm sampling period at the HRSP site was 

from September to November 2004 during an active hurricane season. Discharge ranged from 188 to 6,410 ft3

/s over 

this period and peaked on September 7, 2004, after Hurricane 

Frances deposited 7 in. of precipitation. Several instances of 

daily rainfall exceeding 2 in. resulted in periods of substantial 

stormwater runoff events. Base flow estimated from continuous specific conductance was less than 10 percent of peak 

flow during three runoff events, and 36 percent of average 

discharge during this period (fig. 27 and table 8).

In January 2005, the HRSP site was sampled between 

January 13 to January 28. The discharge ranged from 110 to 

190 ft3/s over this period in response to 0.8 in. of precipitation. 

Base flow estimated from continuous specific conductance 

was 100 percent of peak flow and 100 percent of average flow 

during the storm (fig. 28 and table 8). Base flow estimated 

from sampled specific conductance, calcium and magnesium 

concentrations, and sampled discharge was 100 percent of 

peak flow and 100 percent of average flow during the storm 

but the sampling frequency did not adequately represent the 

continuous storm hydrograph.

During a 9-day storm event between February 24 and 

March 4, 2005, discharge at the HRSP site increased from 81 

to 162 ft3/s in response to 2.08 in. of precipitation. Base flow 

estimated from sampled specific conductance and continuous specific conductance was 100 percent of peak flow and 

100 percent of average flow during the storm. Base flow 

estimated from calcium was 52 percent of peak flow and 

92 percent of average discharge during the storm (fig. 29 and 

table 8). Base flow estimated from magnesium concentration was 36 percent of peak flow and 89 percent of average 

discharge during the storm.

During the period from August 13 to September 9, 2009, 

discharge at the HRSP site ranged from 111 to 1,580 ft3/s, in 

response to 10.17 in. of precipitation. Base flow estimated 

from sampled specific conductance was less than 10 percent 

of peak flow and 45 percent of average flow during the storm 

(fig. 30 and table 8).

20

25

DISCHARGE, IN CUBIC FEET PER SECOND

13 15 20 23 26

CONTINUOUS-MEASURED DISCHARGE – At SR39 site

BASE-FLOW DISCHARGE – Estimated from continuous specific conductance

BASE-FLOW DISCHARGE – Estimated from sampled specific conductance,

calcium concentration, and magnesium concentration

14 19 22 25 29

JANUARY, 2005

EXPLANATION

10

15

16 17 18 21 24 27 28

Figure 24. Observed storm hydrograph and discharges estimated by geochemical mass balance for selected 

major ions in SR39 surface water, January 2005 storm.
28 A Geochemical Mass-Balance Method for Base-Flow Separation, Upper Hillsborough River Watershed

10

20

30

40

DISCHARGE, IN CUBIC FEET PER SECOND

24 26 28 2 4

FEBRUARY MARCH

25 27 1 3 5

2005

5

15

25

35

CONTINUOUS-MEASURED DISCHARGE – At SR39 site

BASE-FLOW DISCHARGE – Estimated from continuous

specific conductance

BASE-FLOW DISCHARGE – Estimated from sampled specific conductance

and calcium concentration

BASE-FLOW DISCHARGE – Estimated from magnesium concentration

EXPLANATION

100

200

300

600

DISCHARGE, IN CUBIC FEET PER SECOND

CONTINUOUS-MEASURED DISCHARGE – At HRSP site

BASE-FLOW DISCHARGE – Estimated from continuous

specific conductance

5 25

JULY, 2004

EXPLANATION

5

10 15 20 30

500

400

Figure 25. Observed storm hydrograph and discharges estimated by geochemical mass balance for selected 

major ions in SR39 surface water, February/March 2005 storm.

Figure 26. Observed storm hydrograph and discharges estimated by conductivity mass balance in HRSP 

surface water, July 2004.
Hydrologic Characteristics, Water Quality, and Application of Base-Flow Separation Techniques 29

6,000

8,000

DISCHARGE, IN CUBIC FEET PER SECOND

4 15 30 10 30

SEPTEMBER

CONTINUOUS-MEASURED DISCHARGE – At HRSP site

BASE-FLOW DISCHARGE – Estimated from continuous specific conductance

10 20 5 20 3

2004

EXPLANATION

0

4,000

25 15 25

OCTOBER NOV

2,000

180

220

DISCHARGE, IN CUBIC FEET PER SECOND

13 15 18 20 24

JANUARY, 2005

14 16 19 22 25

100

140

17 21 23

120

CONTINUOUS-MEASURED DISCHARGE – At HRSP site

BASE-FLOW DISCHARGE – Estimated from continuous specific conductance

BASE-FLOW DISCHARGE – Estimated from sampled specific conductance

BASE-FLOW DISCHARGE – Estimated from calcium and

magnesium concentrations

EXPLANATION

160

200

26 27 28

Figure 27. Observed storm hydrograph and discharges estimated by conductivity mass balance in HRSP surface 

water, September to November 2004.

Figure 28. Observed storm hydrograph and discharges estimated by geochemical mass balance for selected 

major ions in HRSP surface water, January 2005 storm.
30 A Geochemical Mass-Balance Method for Base-Flow Separation, Upper Hillsborough River Watershed

100

180

DISCHARGE, IN CUBIC FEET PER SECOND

24 26 1 3

2005

25 27 2

0

60

28

20

80

140

4

40

120

160

FEBRUARY MARCH

CONTINUOUS-MEASURED DISCHARGE – At SR39 site

BASE-FLOW DISCHARGE – Estimated from continuous specific conductance

BASE-FLOW DISCHARGE – Estimated from sampled specific conductance

BASE-FLOW DISCHARGE – Estimated from calcium concentration

BASE-FLOW DISCHARGE – Estimated from magnesium concentration

EXPLANATION

1,000

DISCHARGE, IN CUBIC FEET PER SECOND

10

2009

0

600

200

800

1,400

400

1,200

1,600

AUGUST SEPTEMBER

POINT DISCHARGE – At HRSP site

BASE-FLOW DISCHARGE – Estimated from sampled

specific conductance

EXPLANATION

1,800

15 20 25 30 1 5 10

Figure 29. Observed storm hydrograph and discharges estimated by geochemical mass balance for selected 

major ions in HRSP surface water, February/March 2005 storm.

Figure 30. Observed storm hydrograph and discharges estimated by conductivity mass balance in HRSP surface 

water, August and September 2009.
31 A Geochemical Mass-Balance Method for Base-Flow Separation, Upper Hillsborough River WatershedReferences Cited 31

Calcium, magnesium, and silica were used as end 

members for the GMB method, but their use in calculating base flow by the GMB method was limited because the 

frequency of samples was not sufficient to capture the complete hydrograph from pre-event flow to post-event base-flow 

concentrations. Hydrograph recession curves that are typical of hydrologic conditions in Florida often did not reach a 

minimum before the next storm event, and pre-event base-flow 

constituent concentrations represented a mixture of runoff and 

base flow. 

The contribution of base flow to total discharge for all 

storms at both sites ranged from 31 to 100 percent, whereas 

the contribution of base flow to total discharge during peak 

discharge ranged from less than 10 to 100 percent (table 8). 

Pre-event base-flow constituent concentrations were lower 

than the peak runoff constituent concentrations for four storm 

events at the SR39 site (September 2003, March 2004, January 2005, and February 2005) and for two storm events at 

the HRSP site (January 2005 and February 2005). The model 

assumption that the pre-event base-flow concentrations are 

greater than peak runoff concentration was violated for these 

storm events. This may suggest that during these smaller storm 

events, the ratio of groundwater to runoff increased, so that 

peak flow had higher concentrations than pre-event flow. 

Summary 

The CMB and GMB methods were used to estimate the 

contribution of base flow to discharge in the upper part of the 

Hillsborough River watershed. Input for the GMB method 

included cations, anions, and stable isotopes, whereas input 

for the CMB method included continuous or point samples of 

specific conductance. 

The chemical and isotopic composition of streamflow and 

precipitation was measured during selected local and frontal 

storm events and compared to the chemical and isotopic composition of groundwater. Daily and weekly precipitation at two 

sites in the upper Hillsborough River watershed, one a subbasin 

of the other, followed similar patterns, and surface-water and 

groundwater hydrographs mimicked the precipitation patterns. 

Surface water in the watershed study area is a calciumbicarbonate type that closely resembles groundwater geochemically, indicating that much of the surface water in the upper 

Hillsborough River basin is derived from local discharge of 

groundwater. Discharge at the SR39 site and at the HRSP site 

becomes diluted by precipitation and runoff during the wet 

season, but retains the calcium-bicarbonate characteristic of 

Upper Floridan aquifer water. The 2H and 18O isotopes at both 

sites also were similar for groundwater and surface water, 

indicating groundwater was the principal source of streamflow.

Field conditions limited the application of the GMB 

method to low-intensity storms, but the CMB method was 

applied to both low-intensity and high-intensity storms. 

The average contribution of base flow to total discharge 

for all storms ranged from 31 to 100 percent, whereas the 

contribution of base flow to total discharge during peak discharge ranged from less than 10 percent to 100 percent. 

Calcium, magnesium, and silica are consistent markers 

of Upper Floridan aquifer chemistry, but their use in calculating base flow by the GMB method was limited because the 

frequency of the point data collected in this study was not 

sufficient to capture the complete hydrograph from pre-event 

base flow to post-event base-flow concentrations. In this study, 

pre-event water represents somewhat diluted groundwater.

Streamflow conductivity integrates the concentrations of 

the major ions, and the logistics of acquiring specific conductance at frequent time intervals are less complicated than data 

collection, sample processing, shipment, and analysis of water 

samples in a laboratory. The acquisition of continuous specific 

conductance data reduces uncertainty associated with less 

frequently collected geochemical point data. 

Total discharge at the upper Hillsborough River sites 

during low-intensity storm events consisted primarily of base 

flow. During high-intensity storms, total discharge was substantially diluted with runoff water so that less than 10 percent 

of the total discharge originated from groundwater. 

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Kish and others— A Geochemical Mass-Balance Method for Base-Flow Separation, Upper Hillsborough River Watershed, West-Central Florida, 2003-2005 and 2009—SIR 2010-5092