Final Nutrient Total Maximum Daily Load 

for Orange Lake, 

Alachua County, Florida 

Xueqing Gao and Douglas Gilbert 

Watershed Assessment Section 

Florida Department of Environmental Protection 

2600 Blair Stone Road, MS 3555 

Tallahassee, FL 32399-2400 

September 19, 2003 
Table of Contents 

Table of Contents...................................................................................... i 

List of Figures............................................................................................ iii 

List of Tables............................................................................................. iv 

Acknowledgements................................................................................... v 

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

 1.1. Purpose of Report..................................................................... 1 

 1.2. Identification of Water Body...................................................... 1 

2. Description of Standards..................................................................... 3 

3. Statement of Problem.......................................................................... 4 

4. Assessment of Sources......................................................................... 4 

4.1 Types of Sources............................................................................ 4 

4.2 Estimating Watershed Loadings of TN and TP using WMM............ 5 

 Strategy to determine loadings and assimilative capacity.................. 5 

Breakdown of sub-basins and Landuses....................................... 5 

Potential Sources of TN and TP..................................................... 7 

Estimating Watershed TN and TP .................................................. 8 

4.3 Lake Modeling using Bathtub Model.............................................. 9 

Bathtub Model................................................................................. 9 

Bathtub Data Requirements .......................................................... 10 

Calculation of Trophic State Index.................................................. 11 

Error and Variability Analysis.......................................................... 11 

4.4 TMDL Scenario Development........................................................ 12 

Available Data and Data Use ........................................................ 12 

5. Results.................................................................................................. 13 

 5.1 Data Analysis................................................................................ 13 

Historical trend of trophic status of Orange Lake........................ 13 

TN and TP Inlet and Outlet Concentrations................................... 20 

5.2 Estimating TN and TP sub-basin loadings Using WMM............... 21 

Data required for loading estimation.................................... 23 

Preparing rainfall data for WMM water quantity calibration........... 29 

WMM calibration and Simulation of Watershed Loadings............. 30 

Loading from Lochloosa Lake....................................................... 46 

Atmospheric Loading to the Lake.................................................. 46 

Summary of discharge and TN and TP loadings................................ 47

5.3 Establishing Relationship between TN and TP loading to 

the Lake and In-Lake Concentrations............................................ 49 

Data required by Bathtub model........................................................ 49 

Calibrating Bathtub Model............................................................. 51 

TSI from Model Predictions vs. TSI from Measured Data............. 54 

TP loading vs. TSI of Orange lake................................................... 54 

i
Natural Background...................................................................... 56 

6. Orange Lake TMDL........................................................................... 58 

6.1 Load Allocation............................................................................... 59 

6.2 Wasteload Allocation...................................................................... 60 

6.3 Margin of Safety............................................................................ 60 

7. Next Step............................................................................................... 60 

References............................................................................................... 62 

Appendix A ................................................................................................ 64 

ii
List of Figures 

Figure 1 General Location of Watershed.................................................. 2 

Figure 2 Orange Lake Sub-basins......................................................... 6 

Figure 3 Bathtub Conceptual Scheme...................................................... 9 

Figure 4 Location of Data Stations........................................................... 14 

Figure 5-A Lake TN Concentration vs. Volume........................................ 18 

Figure 5-B Lake TP Concentration vs. Volume........................................ 18 

Figure 5-C Lake Chla Concentration vs. Volume..................................... 19 

Figure 5-D Lake TSI vs. Volume.............................................................. 19 

Figure 6 USGS Gauging Stations and sub-basin delineation’s................ 22 

Figure 7 Location of Weather Station....................................................... 24 

Figure 8 TN Loading Camp Canal/Styx River OL subbasin.................... 32 

Figure 9 TP Loading Camp Canal/Styx River OL subbasin.................... 32 

Figure 10 TN Load and Flow from Camp Canal..................................... 33 

Figure 11 TP Load and Flow from Camp Canal..................................... 33 

Figure 12 Percent TN Loading from Newnans Lake and Camp Canal.. 34 

Figure 13 Percent TP Loading from Newnans Lake and Camp Canal.. 34 

Figure 14 Percent TN by Landuse 1997................................................ 41 

Figure 15 Percent TN by Landuse 2000................................................ 42 

Figure 16 Percent TP by Landuse 1997................................................ 43 

Figure 17 Percent TP by Landuse 2000................................................ 44 

Figure 18 TSI vs. TP Load Reductions.................................................. 55 

iii
List of Tables 

Table 1 Lake Basin Landuse and acreage................................................ 7 

Table 2 Data Types and Period of Record................................................ 12 

Table 3 Annual Average Lake TN, TP, Chla, and TSI............................... 15 

Table 4 Seasonal Variation of Lake TN, TP, Chla, and TSI...................... 15 

Table 5 Lake Annual Average Stage and Volume..................................... 16 

Table 6 Lake Quarterly Average Stage and Volume................................. 17 

Table 7 Lake Inlet and Outlet Annual Average TN Concentrations........... 20 

Table 8 Lake Inlet and Outlet Annual Average TP Concentrations........... 21 

Table 9 Annual Precipitation..................................................................... 23 

Table 10 Sub-basin Landuses and area................................................... 25 

Table 11 Percent Directly Connected Impervious Area by Landuse ........ 26 

Table 12 TN and TP Landuse based Event Mean Concentration............. 27 

Table 13 Percent suspended TN and TP by Landuse and Sub-basin...... 27 

Table 14 Camp Canal Daily discharge, TN and TP Concentration.......... 28 

Table 15 Adjusted Effective Rainfall......................................................... 30 

Table 16 Simulated Flow and Runoff to the Lake..................................... 30 

Table 17 TN and TP Loading to the Lake................................................. 31 

Table 18 TN Loads by Landuse and Subbasin (1995 – 2000)................. 35 

Table 19 TP Loads by Landuse and Subbasin (1995-2000).................... 37 

Table 20 TN and TP Loadings from Lochloosa Lake.............................. 46 

Table 21 TN and TP Atmospheric Loadings to Lake............................... 47 

Table 22 Sources of Water to the Lake................................................... 47 

Table 23 Sources of TN Loading to the Lake.......................................... 47 

Table 24 Sources of TP Loading to the Lake......................................... 48 

Table 25 Physical Characteristics of Orange Lake by Year.................. 49 

Table 26 Precipitation and Evaporation by Year...................................... 50 

Table 27 Measured TN, TP, Chla Concentrations of Lake by Year......... 50 

Table 28 Flow, TN, and TP Concentrations by Landuse.......................... 50 

Table 29 Bathtub Calibration Results ....................................................... 53 

Table 30 TSI Measured vs. Predicted....................................................... 54 

Table 31 TP Sources to Orange Lake....................................................... 56 

Table 32 TP comparision to other studies................................................. 58 

Table 33 TMDL Components.................................................................... 59 

iv
Acknowledgments 

This study could not have been accomplished without the significant 

contributions from staff within the Watershed Assessment Section. Particular 

appreciation to Barbara Donner for her contributions. All of the basin 

delineation’s, landuse aggregations, and much of the data gathering are a result 

of her efforts. Additional appreciation to Dr. TS Wu for his assistance and 

insights on modeling and to Dr. Paul Lee for his assistance in understanding the 

groundwater system. The Department also recognizes the substantial support 

and assistance provided by the St. Johns River Water Management District staff, 

particularly their contributions towards understanding the issues, history, and 

processes at work in the Orange Lake watershed. 

v
 

1. INTRODUCTION 

1.1 Purpose of Report 

This report represents the efforts to develop a nutrient TMDL for Orange Lake (Lake). 

The Lake, located in Central Florida near Gainesville (Figure 1), was verified as impaired 

by nutrients based on elevated levels of the Trophic State Index for lakes, and was 

included on the verified list of impaired waters for the Ocklawaha Basin that was adopted 

by Secretarial Order on August 26, 2002. 

According to Section 303(d) of the federal Clean Water Act (CWA) and the Florida 

Watershed Restoration Act, Chapter 403, Florida Statutes, the Florida Department of 

Environmental Protection (DEP) is required to submit on a recurring basis lists of surface 

waters that do not meet applicable water quality standards (impaired waters). The 

methodologies used by the state for the determination of impairment are established in 

Rule 62-303, Identification of Impaired waters (IWR), Florida Administrative Code (FAC). 

Once a water body or water body segment has been verified as impaired and referenced 

in the Secretarial Order Adopting the Verified List of Impaired Waters, work on 

establishment of the Total Maximum Daily Load (TMDL) begins. The TMDL process 

establishes the allowable loadings of pollutants or other quantifiable parameters for a 

waterbody based on the relationship between pollution sources and in-stream water 

quality conditions, so that states can establish water quality based controls to reduce 

pollution from both point and nonpoint sources and restore and maintain the quality of 

their water resources (USEPA, 1999) 

1.2 Identification of Water Body 

Orange Lake is located in a topographical region of the state that is known as the 

Central Lowlands (Clark et al. 1964, Latitude 290

28’03”, Longitude 82010’58”, Figure 1). 

The geology of the area is dominated by the Hawthorn formation, which is about 50 feet 

thick where Orange Lake is located (Scott 1988a). The Hawthorn formation is relatively 

impermeable and acts as a confining layer to separate the surface water from the 

influence of the Floridian Aquifer. However, a connection between the lake water and 

Floridian Aquifer exists through a sink hole system located in the southwest part of 

Orange Lake. Because the potentiometric surface of this area is about the same as or 

lower than the long-term average elevation of the lake surface (57.8 feet, Lasi 1999), the 

movement of the lake water is downward. Therefore, it is not anticipated that the 

Floridan Aquifer will have a significant direct influence on the lake water quality. 

The surface area of Orange Lake ranges from 5,000 to 14,7000 acres, depending on the 

amount of rainfall. The maximum depth of the lake under average conditions is about 12 

feet and the mean depth is approximately 5.5 feet (Deevey 1989). Major sources of 

water to the lake include: (1) interflow via Camps Canal-River Styx from Newnans Lake 

and Cross Creek from Lochloosa Lake; (2) surface runoff from the watershed; and (3) 

the direct precipitation into the lake. Water flows out of the lake through the sink hole 

system located in the southwest part of the lake and through the outlet stream, Orange 

Creek (Figure 1). 

1
 

For assessment purposes, the State of Florida has been divided into waterbody 

assessment polygons termed Waterbody Ids or WBIDs. Additional information about 

derivation and use of these WBIDs is provided in the “Documentation For the 2002 

Update to the State Of Florida’s 303(d) List” dated October 1, 2002, and GIS shapefiles 

of the WBIDs can be obtained from the following website: 

http://www.floridadep.org/water/watersheds/basin411/downloads.htm

 

Figure 1. The general location and landuse types of the Orange Lake watershed 

2
 

2. DESCRIPTION OF APPLICABLE WATER QUALITY STANDARDS AND 

CRITERIA

Orange Lake is classified as a Class III Freshwater body, with a designated use of 

recreation, propagation and maintenance of a healthy, well-balanced population of fish 

and wildlife. The Class III water quality criteria applicable to the observed impairment is 

the narrative nutrient criterion (nutrient concentrations of a body of water shall not be 

altered so as to cause an imbalance in natural populations of aquatic flora or fauna). 

Because the nutrient criterion is narrative only, a nutrient related target was needed to 

represent levels at which imbalance in flora or fauna are expected to occur. For this 

TMDL, the IWR threshold for impairment for lakes, which is based on a trophic state 

index (TSI), was used as the water quality target. 

The TSI originally developed by R. E. Carlson (1977) was calculated based on Secchi 

depth, chlorophyll concentration, and total phosphorus concentration and was used to 

describe a lake’s trophic state. Carlson’s TSI was developed based on the assumption 

that the lakes were all phosphorus limited. In Florida, because the local geology 

produced a phosphorus rich soil, nitrogen can be the sole or co-limiting factor for 

phytoplankton population in some lakes. In addition, because of the existence of darkwater lakes in the state, using Secchi depth as an index to represent lake trophic state 

can produce misleading results. Therefore, the TSI was revised to be based on 

chlorophyll a, total nitrogen, and total phosphorus concentrations. 

The Florida-specific TSI was determined based on the analysis of data from 313 Florida 

lakes. The index was adjusted so that a chlorophyll a concentration of 20 ug/L was 

equal to a TSI value of 60. A TSI of 60 was then set as the threshold for nutrient 

impairment for most lakes (for those with a color higher than 40 platinum cobalt units) 

because, generally, the phytoplankton may switch to communities dominated by bluegreen algae at chlorophyll a levels above 20 ug/L. These blue-green algae are often an 

unfavorable food source to zooplankton and many other aquatic animals. Some bluegreen algae may even produce toxins, which could be harmful to fish and other animals. 

In addition, excessive growth of phytoplankton and the subsequent death of these algae 

may consume large quantities of dissolved oxygen and result in anaerobic conditions in 

lakes, which makes conditions in the impacted lake unfavorable for fish and other 

wildlife. All of these processes may negatively impact the health and balance of native 

fauna and flora. 

Because of the amazing diversity and productivity of Florida lakes, some lakes have a 

natural background TSI that is different from 60. In recognition of this natural variation, 

the IWR allows for the use of a lower TSI (40) in very clear lakes, a higher TSI if 

paleolimnological data indicate the lake was naturally above 60, and the development of 

site-specific thresholds that better represent the levels at which nutrient impairment 

occurs. For this study, the Florida Department of Environmental Protection (DEP) used 

modeling to estimate the natural background TSI by setting land uses to natural or 

forested land, and then compared the TSI to the IWR thresholds. If the natural 

background TSI is higher than 60, then the natural background TSI will be used as the 

water quality target for the TMDL because it is unreasonable to abate the natural 

background condition. If the natural background TSI is lower than 60, then the IWR 

threshold (a TSI of 60) will be established as the target for TMDL development (since 

Orange Lake has a mean color greater than 40 platinum cobalt units, the IWR threshold 

for impairment is 60). 

3
 

3. STATEMENT OF PROBLEM 

As noted previously, the IWR threshold for nutrient impairment is an annual TSI of 

greater than 60. As shown in Table 3, several years of the verified period had annual 

TSI values greater than 60. In fact, based on summaries of the data contained in the 

DEP IWR Water Run 9.1 database (IWR-data), the long-term TSI (1989 – 2000) 

calculated from these data according to the procedures adopted in the IWR is 69, 

indicating that the high TSIs were not an anomalous event. This long-term TSI was 

based on long-term average concentrations of total phosphorus (TP), total nitrogen (TN), 

and chlorophyll a (Chla) were 46 ± 6, 1700 ± 211, and 43.8 ± 9.5 μg/L, respectively 

(Table 3). While the verified period for listing purposes is June, 1995 – December 2000, 

this report uses the period from January, 1995 through December, 2000 due to use of 

annual average values and because no data are available after 2000 in the IWRdatabase. For the verified period in this report, the TP, TN, and Chla concentrations 

were 53 ± 9, 1772 ± 363, and 59.2 ± 15.3 ug/L, respectively, and the TSI of the verified 

period was 73. 

4. ASSESSMENT OF SOURCES 

4.1 Types of Sources 

An important part of the TMDL analysis is the identification of source categories, source 

subcategories, or individual sources of nutrients in the Orange Lake watershed and the 

amount of pollutant loading contributed by each of these sources. Sources are broadly 

classified as either “point sources” or “nonpoint sources.” Historically, the term point 

sources has meant discharges to surface waters that typically have a continuous flow via 

a discernable, confined, and discrete conveyance, such as a pipe. Domestic and 

industrial wastewater treatment facilities (WWTFs) are examples of traditional point 

sources. In contrast, the term “nonpoint sources” was used to describe intermittent, 

rainfall driven, diffuse sources of pollution associated with everyday human activities, 

including runoff from urban land uses, runoff from agriculture, runoff from silviculture, 

runoff from mining, discharges from failing septic systems, and atmospheric deposition. 

However, the 1987 amendments to the Clean Water Act redefined certain nonpoint 

sources of pollution as point sources subject to regulation under EPA’s National 

Pollutant Discharge Elimination Program (NPDES). These nonpoint sources included 

certain urban stormwater discharges, including those from local government master 

drainage systems, construction sites over five acres, and from a wide variety of 

industries (see Appendix A for background information about the State and Federal 

Stormwater Programs). 

To be consistent with Clean Water Act definitions, the term “point source” will be used to 

describe traditional point sources (such as domestic and industrial wastewater 

discharges) AND stormwater systems requiring an NPDES stormwater permit when 

allocating pollutant load reductions required by a TMDL (see Section 6). However, the 

methodologies used to estimate nonpoint source loads do not distinguish between 

NPDES stormwater discharges and non-NPDES stormwater discharges, and as such, 

this source assessment section does not make any distinction between the two types of 

stormwater. 

4
 

4.2 Estimating TN and TP loadings using WMM 

Overall Strategy to Determine Loadings and Assimilative Capacity

The goal of the nutrient TMDL development for Orange Lake is to identify the maximum 

allowable TP and TN loadings to the lake so that the lake will meet the water quality 

standard and maintain its function and designated use as a Class III water. Specifically, 

the goal is interpreted in this study as a TSI of 60 or the natural background TSI if the 

natural background is higher than 60. 

Three steps were taken to achieve this goal. 

1. TN and TP loadings from the Orange Lake watershed were estimated using the 

Watershed Management Model (WMM). 

2. Loading estimates from the WMM were entered into the Bathtub eutrophication 

model to establish the relationship between TN and TP loadings and in-lake TN, 

TP, and Chla concentrations. The model results for in-lake TN, TP, and Chla 

were used to calculate TSI-predicted (TSI-P) for several different loading 

scenarios discussed later. 

3. The loadings to the lake were adjusted until the TSI-P calculated from the model 

results was less than 60. The TN and TP loadings that resulted in a TSI 

compliant with Section 62-303.450, FAC, were considered the nutrient TMDL for 

Orange Lake. 

Breakdown of Orange Lake watershed and landuse categories

The Orange Lake watershed drains an area of about 87,339 acres. For modeling 

purposes, the watershed was divided into two sub-basins (Figure 2). These sub-basins 

are Camps Canal – River Styx sub-basin (CCRS – area discharging directly into Camps 

Canal and River Styx), Orange Lake sub-basin (OL – area discharging directly into 

Orange Lake). 

5
 

 

" ́

" ́

" ́

" ́

" ́ Orange Creek

Camps Canal

River Styx

Cross Creek

#

Sink Hole System

Prairie Creek

CCRS

OL

3 0 3 6Miles

N

Figure 2. Sub-basins of Orange Lake watershed. CCRS and OL represent Camps Canal 

and River Styx sub-basin and Orange Lake sub-basins, respectively. 

Landuse categories in Orange Lake watershed were aggregated using the simplified 

level 1 codes tabulated in Table 1. The spatial distribution of different landuse types of 

Orange Lake watershed is demonstrated in Figure 1. 

6
 

Table 1. Classification of landuse categories of Orange Lake 

Code Landuse Acreage 

1000 Urban Open 1544 

Low density resident 5738 

Medium density resident 1248 

High density resident 25 

2000 Agriculture 29041 

3000 Rangeland 1029 

8000 Transportation, communication, and utilities 826 

4000 Forest/rural open 25490 

5000/6000 Water/Wetland 22400 

Sources assessment: potential sources of TN and TP in Orange Lake watershed

Point Sources 

There are no wastewater facilities authorized to discharge to the lake. Based on the 

information provided by EPA, none of the watershed currently lies in an area covered 

under an MS4 area. As such, there are currently no point sources authorized to 

discharge to the lake under the NPDES Program. 

Nonpoint Sources 

While there are not point sources in the watershed, TN and TP loadings to Orange Lake 

are generated from nonpoint sources. Nonpoint sources addressed in this study include 

TN and TP loadings from surface runoff, stream flow, precipitation directly on the surface 

of the lake, and the contribution from leaking septic tanks. TN and TP loadings through 

surface runoff were estimated using the Watershed Management Model (WMM) based 

on the imperviousness and event mean concentration (EMC) of TN and TP from 

different landuse types of the watershed. The spatial distribution and acreage of 

different landuse categories were identified using the St. Johns River Water 

Management District (SJRWMD) 1995 landuse coverage (scale 1:40,000) contained in 

the DEP GIS library. Methods used to estimate the TN and TP loadings from stream 

flow, precipitation directly on the surface of the lake, and the contribution from leaking 

septic tanks are described in detail in Section 5.2. 

TN and TP loadings from ground water and point source were not considered in this 

study because previous studies indicated that contribution from these sources were 

insignificant (Deevey 1988, Robison et al. 1997). 

7
 

Estimating TN and TP loading from Orange Lake watershed using WMM

WMM development was originally funded by DEP under contract to Camp Dresser and 

McKee (CDM). CDM further refined and developed the model to its present state. 

WMM is a watershed model designed to estimate annual or seasonal pollutant loadings 

from a given watershed and evaluate the effect of watershed management strategies on 

water quality (WMM User’s Manual: 1998). While the strength of the model is its 

capability to characterize pollutant loadings from nonpoint sources, such as those 

through stormwater runoff, stream baseflow, and leakage of septic tanks, the model 

handles point sources such as discharge from wastewater treatment facilities and 

combined sewer overflows (CSOs) as well. Estimation of pollution load reduction due to 

partial or full-scale implementation of on-site or regional best management practices 

(BMP) is also part of the functions of this model. The fundamental assumption of the 

model is that the stormwater runoff from any given landuse is in direct proportion to 

annual rainfall and is dictated by the portion of the landuse category that is impervious 

and the runoff coefficients of both pervious and impervious area. The governing 

equation is: 

(1) RL = [Cp + (CI – Cp) IMPL] * I 

Where: 

RL = total average annual surface runoff from land use L (in/yr); 

IMPL = fractional imperviousness of land use L; 

I = long-term average annual precipitation (in/yr); 

CP = pervious area runoff coefficient; and 

CI = impervious area runoff coefficient. 

The model estimates pollutant loadings based on nonpoint pollution loading factors 

(expressed as lbs/ac/yr) that vary by land use and the percent imperviousness 

associated with each land use. The pollution loading factor ML is computed for each 

land use L by the following equation: 

(2) ML = EMCL * RL * K 

Where: 

ML = loading factor for land use L (lbs/ac/yr); 

EMCL = event mean concentration of runoff from land use L (mg/L); EMC 

varies by land use and pollutant; 

RL = total average annual surface runoff from land use L computed 

from Equation (1) (in/yr); and 

K = 0.2266, a unit conversion constant. 

Data required for WMM application include: 

• Area of all the landuse categories and the area served by septic tanks 

• Percent impervious area of each landuse category 

• EMC for each pollutant type and landuse category 

• Percent EMC of each pollutant type that is in suspended form 

• Annual precipitation 

8
 

Calibration of WMM is usually conducted on both runoff quantity and quality. This is a 

two-step procedure since the water quality calibration is a function of the predicted runoff 

volumes. Calibration of water quantity is usually achieved through adjusting the 

pervious and impervious area runoff coefficients. Typical ranges of runoff coefficients 

are 0.05 – 0.30 for pervious area (WMM User’s Manual: 1998) and 0.85 – 1.0 for 

impervious area (Linsley and Franziani, 1979). After the water quantity calibration, water 

quality is calibrated by adjusting the pollutant delivery ratio, i.e., the percent quantity of 

pollutant in the surface runoff that is eventually delivered to the destination waterbody. 

In this study, the range of the pollutant delivery ratio was estimated using the method 

developed by Roehl (1962) that correlates the delivery ratio to watershed area. 

4.3 Establishing the relationship between TN and TP loading and in-lake TN, TP, 

and Chla concentrations using the Bathtub model 

Bathtub eutrophication model 

The Bathtub eutrophication model is a suite of empirically derived steady state models 

developed by the U. S. Army Corps of Engineering (ACOE) Waterways Experimental 

Station. The primary function of these models is to estimate nutrient concentrations and 

algal biomass resulting from different patterns of nutrient loadings. The procedures for 

selection of the appropriate model for a particular lake are described in the Users 

Manual. The empirical prediction of lake eutrophication using this approach typically can 

be described as a two-stage procedure using the following two categories of models 

(Walker 1999): 

• Nutrient balance model. This type of model relates in-lake nutrient concentration 

to external nutrient loadings, morphometry, and hydrology. 

• Eutrophication response model. This type of model describes relationships 

among eutrophication indicators within the lake, including nutrient levels, Chla, 

transparency, and hypolimnetic oxygen depletion.

Figure 3 describes the concept scheme used by Bathtub to relate external loading of 

nutrients to the in-lake nutrient concentrations and the physical, chemical, and biological 

response of the lake to the level of nutrients. 

Figure 3. Bathtub concept scheme

 Loading of nutrients 

(Flow and Concentration) 

 

Physical characters of the lake In-lake nutrient Chla, Secchi 

(Surface area and mean depth) Concentrations (TN&TP) DO 

Hydraulic characters of the lake 

 (Water residence time) 

The nutrient balance model adopted by Bathtub assumes that the net accumulation of 

nutrients in a lake is the difference between nutrient loadings into the lake from various 

9
 

sources and the nutrients carried out through outflow and losses of nutrient through 

whatever decay process occur inside lake: 

(3) Net accumulation = Inflow – Outflow – Decay 

In this study, “inflow” included TN and TP loadings from the interflow of inlet streams 

(Camps Canal and Cross Creek), stormwater surface runoff from various landuse 

categories, leakage of septic tanks, and direct atmosphere precipitation into the lake. 

Nutrient outflow was primarily through the seepage of the sink hole system and the 

outflow stream: Orange Creek. To address nutrient decay within the lake, Bathtub 

provided several alternative mass balance models depending on the inorganic/organic 

nutrient partitioning coefficient and reaction kinetics. The major pathway of decay for TN 

and TP in the model is through sedimentation to the bottom of the lake. 

Prediction of the eutrophication response by Bathtub also involves choosing through 

several alternative models depending on whether the algal communities are limited by 

phosphorus or nitrogen, or co-limited by both nutrients. Scenarios that include algal 

communities limited by light intensity or controlled by the lake flushing rate are also 

included in the suit of models. In addition, the response of chlorophyll a concentration to 

the in-lake nutrient level is characterized by two different kinetic processes: linear or 

exponential. The variety of models available in Bathtub allows the users to choose 

specific models based on the particular condition of the project lake. 

One feature offered by Bathtub is the “calibration factor.” The empirical models 

implemented in Bathtub are mathematical generalizations about lake behavior. When 

applied to data from a particular reservoir, measured data may differ from predictions by 

a factor of two or more. Such differences reflect data limitations (measurement or 

estimation errors in the average inflow and outflow concentrations), unique features of 

the particular lake (Walker 1999), and unexpected processes inherent to the lake. The 

calibration factor offered by Bathtub provides model users with a tool to calibrate the 

magnitude of lake response predicted by the empirical models. The model calibrated to 

current conditions against measured data from the lake can then be applied to predict 

changes in lake conditions likely to result from specific management scenarios under the 

condition that the calibration factor remains constant for all prediction scenarios. 

Data requirements for running Bathtub 

Data requirements for Bathtub model include: 

• Physical characteristics of the lake (surface area, mean depth, length, and mixed 

layer depth) 

• Meteorological data (precipitation and evaporation) 

• Measured water quality data (TN, TP, and Chla concentrations of the lake water, 

TN and TP concentrations in precipitation, etc.) 

• Loading data (flow and TN and TP concentrations of the flow from various 

landuse categories, inlet streams, outlet streams, and the TN and TP contribution 

from the leakage of septic tanks) 

• Coefficient of variance (CV) of all the measured data 

10
 

Calculation of Trophic State Index (TSI)

TSI was calculated using the procedures outlined in Florida’s 1996 305(b) report: 

CHLATSI = 16.8 + 14.4 × LN (CHLA)] 

TNTSI = 56 + [19.8 × LN (TN)] 

TN2TSI = 10 × [5.96 + 2.15 × LN (TN + 0.0001)] 

TPTSI = [18.6 × LN (TP × 1000)] –18.4 

TP2TS = 10 × [2.36 × LN (TP × 1000) – 2.38] 

Limiting nutrient considerations for calculating NUTRTSI: 

If TN/TP > 30 then NUTRTSI = TP2TSI

If TN/TP < 10 then NUTRTSI = TN2TSI 

If 10 < TN/TP < 30 then NUTRTSI = (TPTSI + TNTSI)/2 

TSI = (CHLATSI + NUTRTSI)/2

Error and variability analysis

The distinction between “error” and “variability” is important. Error refers to a difference 

between a measured and a predicted mean value and is usually described as: 

The absolute value of |measurement – prediction|/measurement. Variability refers to 

spatial or temporal fluctuations in measurement around the mean. Spatial variability is 

not usually included in the variability analysis of empirical modeling efforts. Empirical 

modeling variability analysis usually concentrates on those changes caused by temporal 

fluctuation. 

Variability is frequently described using the mean coefficient of variance (CV), which is 

defined as the standard error (SE) of the estimate expressed as a fraction of the 

predicted value (Walker 1999). In this study, model estimates were presented as mean 

± 1SE whenever a CV could be determined. 

 

When WMM was used to simulate TN and TP loadings from surface runoff, neither error 

analysis nor variability analysis were conducted. This was because no flow data were 

available from any gauging station located within the Orange Lake watershed. In 

addition, the variability analysis within WMM required CVs for the EMC of TN and TP 

from different landuse categories and the CV for the suspended fraction of TN and TP 

from different landuse categories. Because we did not have these CVs, the variability 

analysis was not conducted using WMM. WMM simulation was conducted for all the 

years for which there were data. Model predictions for all the years were later averaged 

to calculate the long-term annual mean and CV, which are required for the error and 

variability analysis for Bathtub. 

Bathtub allows the input of the CV for both measured data and model predictions from 

WMM. Therefore, both error and variability analyses were conducted with Bathtub. To 

accomplish this, several years of measured data from the non-model variables 

(precipitation, lake volume, and evaporation) and the WMM predictions (TN, TP, and 

flow) were averaged and the mean values and CVs of these data were entered to 

Bathtub as input. 

11
 

4.4 TMDL development for Orange Lake 

Once WMM and Bathtub model calibrations were achieved, TMDL of the lake was 

developed through evaluating TSIs of the following scenarios: 

A. TSI of current condition 

B. TSI after the loadings from all human landuse categories (urban open, low, medium, 

and high density residential, agriculture and rangeland, and transportation, 

communication, and utilities) within the watershed were assessed as the landuse 

category Forest/Rural Open. This is the watershed Natural Background condition 

C. TSI after all the loadings from human landuse categories were assessed as natural 

background and the TN and TP loadings from the two upper reach lakes, i.e. 

Newnans Lake and Lochloosa Lake, through the interflow of Camps Canal-River 

Styx and Cross Creek achieved their TMDL goals. 

The scenario C was considered the natural background condition of the lake. If the TSI 

of scenario C was lower than 60, the loadings from human landuse would be allowed to 

increase and up to the final TSI of 60. If the TSI of Scenario C were higher than 60, then 

the Natural Background TSI would become the target for the TSI. 

Requirement for historical data, overall data availability and the years from which data 

were chosen for the modeling

Model calibration and simulation of this study requires that several types of data should 

have measured historical record. These data types and their availability are listed in 

Table 2. 

Table 2. Data types that are required to have historical records and the period 

these data are availability 

Data type Available time period 

Precipitation 1995 – 2000 

Stream flow at the mouth of Camps Canal 1990 –2000 

Lake stage 1989 –2000 

Stream water quality data 1995 – 2000 

Lake water quality data 1989 –2000 

Because calibration of the model requires that data from the different types be in the 

same time period, 1995 to 2000 were chosen as the years from which data were used 

for model calibration. 

12
 

5. RESULTS 

5.1 Data Analysis 

Historical trend of trophic status of Orange Lake 

Monthly TN, TP, and Chla concentrations for Orange Lake from 1989 through 2000 were 

retrieved from the IWR database. The locations of the individual stations from which 

water quality data were collected are shown in Figure 4. Analysis of the data indicated 

that the spatial variation between stations across Orange Lake is not significant. 

Therefore, data from all the stations within the lake were pooled together and treated as 

data collected from one station. 

Quarterly mean values for TN, TP, and Chla concentrations were calculated based on 

the monthly data. Quarterly TSIs were calculated based on the quarterly mean values of 

TN, TP, and Chla concentrations, and quarterly TN, TP, Chla, and TSI values were then 

used to calculate annual mean values. The long term annual average values of these 

data were calculated based on annual mean values of each year from 1989 through 

2000. The individual annual average values for the verified period were calculated 

based on the mean values from 1995 through 2000. 

The seasonal trend of TN, TP, Chla, and TSI were examined by calculating the longterm quarterly mean values based on the quarterly mean values of each year (1989 – 

2000). The quarterly means for the verified period were calculated using the data from 

1995 through 2000. The individual annual mean TN, TP, Chla, and TSI values are listed 

in Table 3 and the long-term quarterly TN, TP, Chla, and TSI results are listed in Table 

4. 

13
 

#

#

# #

#

#

# #

#

Orange Creek

Camps Canal

River Styx

Cross Creek

#

Sink Hole System

Prairie Creek

CCRS

OL

3 0 3 6 Miles

N

Figure 4. Locations of water quality stations

14
 

According to Table 3, the long-term annual average of TN, TP, and Chla concentrations 

are 1.70 ± 0.21 mg/L, 0.046 ± 0.006 mg/L, and 43.8 ± 9.5 μg/L, respectively. The long 

term TSI is 69 ± 3. The long-term average TN/TP ratio is about 39, suggesting that the 

algal communities in this lake may be limited by phosphorus. For the verified period, the 

TN, TP, and Chla concentrations are 1.77 ± 0.36 mg/L, 0.053 ± 0.009 mg/L, and 59.2 ±

15.3 μg/L, respectively. The TSI of the verified period is 73 ± 3. Based on these data, 

the lake is eutrophic and exceeded the IWR TSI threshold of 60 for lakes. 

Table 3. Annual averages of TN, TP, Chla, and TSI values of Orange Lake from 

1989 through 2000. Data represent mean ± 1SE (n=4) 

TN TP Chla TSI 

(mg/L) (mg/L) (μg/L) 

1989 1.32 ± 0.21 0.038 ± 0.003 24.0 ± 3.8 61 ± 3 

1990 2.46 ± 0.34 0.048 ± 0.008 37.2 ± 9.3 67 ± 3 

1991 2.35 ± 0.89 0.070 ± 0.020 65.0 ± 28.1 71 ± 5 

1992 1.11 ± 0.08 0.018 ±0.002 9.8 ± 2.6 43 ± 1 

1993 1.32 ± 0.04 0.035 ± 0.007 15.4 ± 3.5 57 ± 1 

1994 1.20 ± 0.02 0.030 ± 0.004 19.5 ± 1.9 58 ± 2 

1995 1.05 ± 0.03 0.026 ± 0.001 20.6 ± 3.7 49 ± 7 

1996 1.46 ± 0.15 0.042 ± 0.003 44.4 ± 9.1 66 ± 4 

1997 1.44 ± 0.12 0.038 ± 0.002 42.4 ± 5.6 66 ± 1 

1998 1.49 ± 0.10 0.057 ± 0.004 59.2 ± 10.5 68 ± 2 

1999 1.66 ± 0.15 0.065 ± 0.022 58.4 ± 13.6 70 ± 2 

2000 3.54 ± 0.85 0.089 ± 0.016 130.1 ± 48.8 82 ± 5 

Mean-L 1.70 ± 0.21 0.046 ± 0.006 43.8 ± 9.5 69 ± 3 

Mean-V 1.77 ± 0.36 0.053 ± 0.009 59.2 ± 15.2 73 ± 3 

Mean-P 1.63 ± 0.25 0.040 ± 0.007 28.5 ± 8.2 64 ± 4 

Note: 

• Mean-L represents results of record mean 

• Bolded data were annual means for the verified period. 

• Mean-V: mean values for the modified verified period (January of 1995 

through December of 2000) 

• Mean-P: mean values for the pre-verified period (1989 through 1994) 

Table 4. Seasonal variation of TN, TP, Chla, and TSI in Orange Lake 

TN TP Chla TSI 

(mg/L) (mg/L) (μg/L) 

General long-term quarterly mean 

1st quarter 1.70 ± 0.38 0.060 ± 0.012 30.9 ± 7.0 70 ± 4 

2nd quarter 1.52 ± 0.15 0.045 ± 0.005 44.2 ± 12.2 69 ± 3 

15
 

3rd quarter 1.80 ± 0.35 0.043 ± 0.008 54.3 ± 20.9 70 ± 4 

4th quarter 1.89 ± 0.40 0.045 ± 0.008 46.0 ± 15.4 69 ± 4 

Quarterly mean for the verified period 

1st quarter 1.35 ± 0.11 0.056 ± 0.015 34.5 ± 5.9 69 ± 4 

2nd quarter 1.64 ± 0.21 0.049 ±0.006 50.4 ± 11.4 71 ± 4 

3rd quarter 1.98 ± 0.55 0.053 ± 0.013 84.6 ± 32.4 75 ± 5 

4th quarter 2.12 ± 0.65 0.053 ± 0.013 67.3 ± 23.8 74 ± 4 

Data represent mean ±1SE. n equals to 12 years for the general long-term quarterly 

mean values and 6 for quarterly mean values for the verified period. 

There was a sudden increase and subsequent decrease of TN, TP, and Chla 

concentrations between 1989 and 1992. During the period from 1992 through 2000, a 

continuous increase of TN, TP, and Chla concentrations was observed. TN, TP, and 

Chla increased by 218% (from 1.11 mg/L in 1992 to 3.54 mg/L in 2000), 407% (from 

0.018 mg/L in 1992 to 0.089 mg/L in 2000), and 1224% (from 9.8 μg/L in 1992 to 130.1 

μg/L in 2000) during this time period, respectively (Table 3). At the same time, TSI 

increased by 93% from 43 in 1992 to 82 in 2000. 

Although dramatic changes of annual TN, TP, and Chla concentrations were observed 

between years, seasonal variation was not very obvious, except that the mean value of 

Chla concentration of the first quarter was significantly lower than that of the third 

quarter. The TN, TP, Chla, and TSI values of all the other quarters were not significantly 

different from each other throughout an average year (Table 4). 

To explain the annual variation, stage data of Orange Lake collected from 1989 through 

2000 were converted to lake volumes using the Elevation – Lake Volume curve of 

Orange Lake developed by the St. John River Water Management District (Robison 

1997). The annual average stage elevation and lake volume are listed in Table 5. The 

long-term quarterly average stage elevation and lake volume calculated based on data 

from 1989 through 2000 are listed in Table 6. 

Table 5. Annual average stage elevation and volume of Orange Lake. Data 

represent mean SE (n=4) 

Stage elevation Lake volume 

(feet) (acre-foot) 

1989 56.6 ± 0.4 56750 ± 4888 

1990 54.4 ± 0.4 34000 ± 4163 

1991 54.1 ± 0.8 33000 ± 7360 

1992 54.7 ± 0.4 

56 0 ± 0 3

37250 ± 3794 

1993 56.0 ± 0.3 50000 ± 2677 

1994 56.5 ± 0.3 54450 ± 3853 

1995 57.9 ± 0.2 72500 ± 2784 

1996 58.3 ± 0.1 77750 ± 946 

16
 

1997 57.6 ± 0.1 68500 ± 1893 

1998 58.7 ± 0.4 83250 ± 5329 

1999 56.9 ± 0.5 60250 ± 5406 

2000 53.1 ± 0.8 25250 ± 5836 

Mean 56.3 ± 0.5 54413 ± 4078 

Low stage elevation and lake volume for Orange Lake was observed in 1990, 1991, and 

2000. This is consistent with the explanation that the high TN, TP, and Chla 

concentrations of these years might be caused by the concentration process (Table 5). 

However, lake volume alone obviously can not explain the dynamics of TN, TP, and 

Chla concentrations in all the other years. According to Figure 5-A, B, C, and D, lake 

volume steadily increased from 1992 through 1998. TN, TP, and Chla concentrations 

also increased during the same time period and these increases could have been 

caused by the increase of TN and TP loading into the lake. 

Table 6. Quarterly average stage elevation and volume of Orange Lake. Data 

represent mean SE (n=6). 

Stage elevation Lake volume 

(feet) (acre-foot) 

1st quarter 56.6 ± 0.6 58542 ± 6454 

2nd quarter 56.2 ± 0.5 53358 ± 5910 

3rd quarter 56.0 ± 0.5 51833 ± 5476 

4th quarter 56.3 ± 0.6 55727 ± 5663 

17
 

0.000

0.500

1.000

1.500

2.000

2.500

3.000

3.500

4.000

4.500

5.000

1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000

Year

TN concentration (mg/L)

0

10000

20000

30000

40000

50000

60000

70000

80000

90000

100000

Lake Volume (acre-feet)

TN

Lake Volume

Figure 5-A TN concentration vs. volume of Orange Lake 

 

0.000

0.020

0.040

0.060

0.080

0.100

0.120

1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000

Year

TP concentration (mg/L)

0

10000

20000

30000

40000

50000

60000

70000

80000

90000

100000

Lake Volume (acre-feet)

TP

Lake Volume

Figure 5-B. TP concentration vs. volume of Orange Lake 

18
 

0.000

20.000

40.000

60.000

80.000

100.000

120.000

140.000

160.000

180.000

200.000

198919901991 199219931994 199519961997 199819992000

Year

Chla concentration (μg/L)

0

10000

20000

30000

40000

50000

60000

70000

80000

90000

100000

Lake Volume (acre-feet)

Chla

Lake Volume

Figure 5-C. Chla concentration vs. volume of Orange Lake 

0.000

10.000

20.000

30.000

40.000

50.000

60.000

70.000

80.000

90.000

100.000

1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000

Year

TSI

0

10000

20000

30000

40000

50000

60000

70000

80000

90000

100000

Lake Volume (acre-feet)

TSI

Lake Volume

Figure 5-D. TSI vs. lake volume of Orange Lake 

19
 

No clear long-term seasonal trend was found with the lake volume (Table 6). This was 

consistent with the findings that TN, TP, Chla, and TSI did not show significant variation 

between different quarters of an average year. 

TN and TP concentrations of inlet and outlet streams and the sink hole system of 

Orange Lake

Orange Lake has two major inlet streams including the Camps Canal – River Styx and 

Cross Creek. Measured TN and TP concentrations were only available for Cross Creek. 

TN and TP concentrations for Camps Canal – River Styx was estimated by dividing the 

sum of TN and TP loads of the interflow from the Prairie Creek and TN and TP load in 

the surface runoff from CCRS sub-basin into Camps Canal – River Styx system by the 

total volume of the interflow and surface run off. TN and TP loads from the surface 

runoff and the volume of the surface runoff were estimated using WMM as shown in a 

later section. 

No directly measured TN and TP concentrations were available for the two major outlets 

(the Orange Creek and the sink hole system). For modeling purpose, TN and TP 

concentrations of these outlets were considered equal to the lake concentrations. TN 

and TP concentrations for the major inlets and outlets of Orange Lake are listed in Table 

7 and Table 8, respectively. 

Table 7. Annual average TN concentration of Camps Canal – River Styx, Cross 

Creek, Orange Creek, and the sink hole system. Data represent mean ± SE. 

Unit: mg/L 

Inlets Outlets 

Camps CanalRiver Styx Cross Creek Orange Creek Sink Hole 

System 

1989 ---- ---- 1.32 ± 0.21 1.32 ± 0.21 

1990 ---- ---- 2.46 ± 0.34 2.46 ± 0.34 

1991 ---- ---- 2.35 ± 0.89 2.35 ± 0.89 

1992 ---- ---- 1.11 ± 0.08 1.11 ± 0.08 

1993 ---- ---- 1.32 ± 0.04 1.32 ± 0.04 

1994 ---- ---- 1.20 ± 0.02 1.20 ± 0.02 

1995 3.79 1.64 ± 0.44 1.05 ± 0.03 1.05 ± 0.03 

1996 3.34 1.87 ± 0.17 1.46 ± 0.15 1.46 ± 0.15 

1997 3.14 1.90 ± 0.38 1.44 ± 0.12 1.44 ± 0.12 

1998 2.03 1.68 ± 0.10 1.49 ± 0.10 1.49 ± 0.10 

1999 2.54 ---- 1.66 ± 0.15 1.66 ± 0.15 

2000 0.96 ---- 3.54 ± 0.85 3.54 ± 0.85 

Mean 2.70 ± 0.42 1.78 ± 0.10 1.77 ± 0.36 1.77 ± 0.36 

20
 

Table 8. Annual average TP concentration of Camps Canal – River Styx, Cross 

Creek, Orange Creek, and the sink hole system. Data represent mean ± SE. 

Unit: mg/L 

Inlets Outlets 

Camps CanalRiver Styx Cross Creek Orange Creek Sink Hole 

System 

1989 ---- ---- 0.038 ± 0.003 0.038 ± 0.003 

1990 ---- ---- 0.048 ± 0.008 0.048 ± 0.008 

1991 ---- ---- 0.070 ± 0.020 0.070 ± 0.020 

1992 ---- ---- 0.018 ± 0.002 0.018 ±0.002 

1993 ---- ---- 0.035 ± 0.007 0.035 ± 0.007 

1994 ---- ---- 0.030 ± 0.004 0.030 ± 0.004 

1995 0.193 0.055 ± 0.020 0.026 ± 0.001 0.026 ± 0.001 

1996 0.128 0.050 ± 0.008 0.042 ± 0.003 0.042 ± 0.003 

1997 0.127 0.067 ± 0.009 0.038 ± 0.002 0.038 ± 0.002 

1998 0.116 0.078 ± 0.005 0.057 ± 0.004 0.057 ± 0.004 

1999 0.171 ---- 0.065 ± 0.022 0.065 ± 0.022 

2000 0.096 ---- 0.089 ± 0.016 0.089 ± 0.016 

Mean 0.138 ± 0.015 0.062 ± 0.006 0.053 ± 0.009 0.053 ± 0.009 

The mean values listed in Table 7 and Table 8 were calculated based on the annual 

mean values of the years within the verified period (1995 through 2000). TN and TP 

concentrations of Cross Creek in 1999 and 2000 were unavailable. Therefore, the mean 

annual TN and TP concentrations of Cross Creek were calculated based on the annual 

means of 1995 through 1998. It appears that TN and TP concentrations of the two inlet 

streams were either similar to (Cross Creek) or significantly higher (Camps Canal – 

River Styx) than TN and TP concentrations of the lake, suggesting that the TN and TP 

concentrations of Orange Lake were controlled by the external loading of TN and TP. 

5.2 Estimating TN and TP loadings from CCRS and OL sub-basins using WMM 

External loadings of TN and TP from CCRS and OL sub-basins into Orange Lake were 

estimated using WMM in this study. TN and TP loadings from CCRS sub-basin 

discharge into Orange Lake primarily through Camps Canal – River Styx. However, 

surface runoff is not the only source of water discharged through the Camp Canal – 

River Styx system. Interflow from Prairie Creek conveyed a significant amount of 

outflow water from Newnans Lake into Orange Lake watershed. To estimate the total 

loadings of TN and TP through the Camps Canal – River Styx system using WMM, the 

interflow measured at the USGS gauging station 02241000 (Figure 6) was treated as a 

point source whose TN and TP concentrations were considered the same as the TN and 

TP concentrations of Prairie Creek. 

21
 

 

" ́

" ́

" ́

" ́

" ́ Orange Creek

Camps Canal

River Styx

Cross Creek

#

Sink Hole System

Prairie Creek

CCRS

OL

02241000

02241100

02242401

02242451

02242450

3 0 3 6 Miles

N

Figure 6. Locations of USGS gauging stations in Orange Lake watershed. 

22
 

Because no measured flow and TN and TP data were available for the Camps Canal – 

River Styx system, WMM calibration was not conducted. Most of the model parameters 

used to estimate the TN and TP loading through surface runoff were borrowed from the 

TMDL study carried out for the Newnans Lake watershed. These model parameters 

were also applied when estimating the TN and TP loadings from the OL sub-basins. 

 

Data required for estimating TN and TP loadings from CCRS and OL sub-basin using 

WMM

A. Rain precipitation data were collected from four weather stations in the proximity of 

Orange Lake (Figure 7). Precipitation data from none of these weather stations 

covered the entire period from 1995 through 2000. Therefore, precipitation data from 

all the weather stations were combined and treated as from one composite station. 

Whenever precipitation data were available at more than one weather station in a 

given year, composite mean was calculated and used as the precipitation of the year 

for the composite weather stations. The composite precipitation data are listed in 

Table 9. 

Table 9. Annual precipitation at Gainesville Regional Airport 

Year Annual Precipitation 

(in/year)

1995 49.99 ± 1.06 

1996 50.04 ± 1.60 

1997 58.92 ± 3.35 

1998 46.03 ± 0.23 

1999 44.03 

2000 33.12 

Note: Precipitation data were only available at one weather station in 1999 and 2000. 

23
 

#

#

#

# Orange Creek

Camps Canal

River Styx

Cross Creek

#

Sink Hole System

Prairie Creek

CCRS

OL

70442100

02611467

70041396

04710924

3 0 3 6Miles

N

Figure 7. Locations of weather stations in the proximity of Orange Lake 

24
 

B. Areas of different landuse categories in each sub-basin were obtained by 

aggregating GIS landuse coverage based on the simplified level 1 code listed in 

Table 1. Acreage of each landuse category for CCRS and OL sub-basins is listed in 

Table 10. Percent distributions of each landuse category in CCRS and OL subbasins are shown in Figure 8. 

Table 10. Area of each landuse category of CCRS and OL sub-basins 

Unit: acre 

CCRS OL 

Forest/Rural Open 10094 15342 

Urban Open 36 1494 

Agriculture 1273 27751 

Low Density Residential 281 5398 

Medium Density Residential 106 1082 

High Density Residential 0 13 

Communication and Transportation 27 799 

Rangeland 211 819 

Water/Wetlands 4976 17316 

Total 17004 70014 

Non-human landuse categories including Water/Wetland and Forest/Rural account for 

about 85% of the total acreage of CCRS sub-basin (Water/wetland 25% and Forest/rural 

60%, Figure 8). The leading human landuse category in CCRS sub-basin is Agriculture, 

which accounts for 7% of the total acreage. The second largest human landuse 

category is the Low Density Residential, which accounts for 2% of the total acreage. 

Landuse categories include Medium Density Residential, Transportation and 

communication, and Rangeland all account for 1% of the total watershed area. No High 

Density Residential exists in CCRS sub-basin. The total human landuse accounts for 

about 12% of the total acreage of the watershed, indicating that the watershed may be 

relatively undeveloped. 

Landuse pattern in OL sub-basin is different from CCRS sub-basin. While non-human 

landuse only accounts for 47% (Water/Wetland 25% and Forest/Rural 22%) of the total 

acreage, much less than in CCRS, Agricultural claims about 39% of the total watershed 

area. The second largest human landuse is the Low Density Residential, which 

accounts for 8% of the watershed. Medium Density Residential ranks the third, about 

2% of the total watershed area. Rangeland and Transportation and communication both 

occupy 1% of the watershed. Again, the High Density Residential is not important in OL 

sub-basin. The total human landuse claims about 53% of the OL sub-basin, suggesting 

a significant influence from human activities on the landuse pattern of the watershed. 

C. Percent impervious area of each landuse category is a very important parameter in 

estimating surface runoff using WMM. Non-point pollution monitoring studies 

25
 

throughout the U.S. over the past 15 years have shown that annual “per acre” 

discharges of urban stormwater pollution are positively related to the amount of 

imperviousness in the landuse (WMM User’s Manual 1998). Ideally, impervious area 

is considered as the area that does not retain water and therefore, 100% of the 

precipitation falling on the impervious area should become surface runoff. In practice, 

the runoff coefficient for impervious area typically ranges between 95 to 100%. 

Impervious runoff coefficients lower than this range were observed in the literature, 

but usually this number should not be lower than 80%. For pervious area, the runoff 

coefficient usually ranges between 10 to 20%. However, values lower than this 

range were also observed (WMM User’s Manual: 1998). In this study, 0.95 was 

used as the runoff coefficient for impervious area and 0.17 was used as the runoff 

coefficient for pervious area, and is consistent with what was observed with the 

TMDL study conducted on Newnans Lake. 

It should be noted that the impervious area percentages do not necessarily 

represent directly connected impervious area (DCIA). Using a single-family 

residence as an example, rain falls on rooftops, sidewalks, and driveways. The sum 

of these areas may represent 30% of the total lot. However, much of the rain that 

falls on the roof drains to the grass and infiltrates to the ground or runs off the 

property and thus does not run directly to the street. For WMM modeling purpose, 

whenever the area of the watershed that contributes to the surface runoff was 

considered, DCIA was used in place of impervious area. Because local values were 

not available, DCIAs used in this study were collected from literature published 

values or results from other studies (Table 11). 

Table 11 Percent direct connected impervious area for different landuse 

categories 

Landuse Categories DCIA Reference 

Forest/Rural Open 0.5% WMM User’s Manual: 1998 

Urban Open 0.5% WMM User’s Manual: 1998 

Agriculture 3.7% Brown 1995 

Low Density Residential 12.40% Brown 1995 

Medium Density Residential 18.70% Brown 1995 

High Density Residential 29.60% Brown 1995 

Communication and Transportation 36.20% Brown 1995 

Rangeland 3.7% CDM 

Water/Wetlands 30% Harper and Livingston 1999 

D. Local Event mean concentrations (EMC) of TN and TP for different landuse 

categories were not available and therefore were obtained from literature values 

(Table 12). 

26
 

Table 12. Event mean concentration of TN and TP for different landuse categories 

Unit: mg/L 

Landuse Categories TN TP Reference 

Forest/Rural Open 1.25 0.053 Harper 1992 

Urban Open 1.59 0.220 Harper 1992 

Agriculture 2.58 0.465 Harper 1992 

Low Density Residential 1.77 0.177 Harper 1992 

Medium Density Residential 2.29 0.300 Harper 1992 

High Density Residential 2.42 0.490 Harper 1992 

Communication and Transportation 2.08 0.340 Harper 1992 

Rangeland 1.25 0.053 Harper 1992 

Water/Wetlands 1.60 0.189 Harper 1992 

EMCs of TN and TP for most landuse categories were cited from a review prepared 

by Harper (1992). EMCs for Agriculture, Low Density Residential, and 

Water/Wetlands were directly provided by the review. However, EMCs for Urban 

Open, Medium Density Residential, High Density Residential, Transportation and 

Communication, and Rangeland were not directly defined in Harper’s review. 

Therefore, some extrapolations were made between the landuse categories in this 

study and the landuse categories defined by Harper’s review. Basically, the Urban 

Open area in our study was treated as the Low-Intensity Commercial area in 

Harper’s review. Medium Density Residential was treated as Single Family area; 

High Density Residential was treated as Multi-Family area; Transportation and 

Communication was treated mainly as Highway; and Rangeland was treated the 

same as Forest/rural. 

E. Not all the TN and TP are transported by the stormwater in the dissolved form. The 

percentage of the total EMC represented by TN and TP attached to suspended 

particles is allowed to be defined in WMM. Percent suspended TN and TP values 

were reported by Lasi (1999) for Orange Lake watershed and were used in this study 

(Table 13). 

 

Table 13. Percent TP and TN in suspended form for different landuse categories. 

Landuse Categories TP TN 

Forest/Rural Open 28% 6% 

Urban Open 57% 44% 

Agriculture 38% 20% 

Low Density Residential 57% 44% 

Medium Density Residential 57% 44% 

27
 

High Density Residential 57% 44% 

Communication and Transportation 57% 44% 

Rangeland 38% 20% 

Water/Wetlands 48% 77% 

F. The sediment delivery ratio determines how much TN and TP attaching to 

suspended particles will be delivered to the destination waterbody eventually. In this 

study, the range of sediment delivery ratio was estimated using the correlation 

between delivery ratio and watershed area developed by Roehl (1962). Because of 

the difference in total area of the watershed, CCSR and OL sub-basins were 

assigned different sediment delivery ratios, which were 0.18 and 0.10, respectively. 

G. To estimate the TN and TP loadings from leakage of septic tanks, WMM 

incorporates the concept of “septic tank failure loading rate” which defines the 

percent increase of TN and TP loadings. The annual failure rate reported for the 

country is 3-5 percent. Pollutant loading rates reported in the WMM Users Manual 

assume 50 gallons per capita per day usage. The mid-range of loading rates for 

failing septic tanks in the Manual is 2.0 mg/L for TP (about a 160% to 250% 

increase) and for TN is 15 mg/L (about a 140% to 200% increase). To provide a 

Margin of Safety, this study adopted the high end of the range in the Users Manual, 

which were 30 mg/L for TN and 4.0 mg/L for TP (WMM User Manual: 1998). 

Another value required by WMM to estimate the influence from leaking septic tanks 

on TN and TP loading is the “septic tank failure rate”, which defines the frequency at 

which septic tanks may fail. Studies conducted on the water quality of the 

Ocklawaha River Basin found that annual frequency of septic tank repairs was about 

0.97% (Basin Status Report 2001). For average annual conditions, it is conservative 

to assume that septic tank systems failures would be unnoticed or ignored for fives 

years before repair or replacement occurred (WMM User Manual: 1998). Therefore, 

the septic tank failure rate used in this study was calculated by multiplying repairing 

frequency (0.97%) by 5 (years) and was about 5%. 

H. To estimate the TN and TP loading discharged through Camps Canal – River Styx 

system into Orange Lake, interflow from Prairie Creek was included in the WMM 

modeling. To do this, the interflow was treated as a point source and was 

characterized by the flow data measured at the USGS gauging station 02241000 

(Figure 6) and TN and TP concentrations of Prairie Creek. The daily discharge and 

the TN and TP concentrations of the discharge are listed in Table 14. 

Table 14. Daily discharge and TN and TP concentrations in the discharge of 

Camps Canal at the USGS gauging station 02241000 

Daily Discharge TN TP 

(MGD) (mg/L) (mg/L) 

1995 17.48 4.69 0.223 

1996 17.63 4.19 0.138 

1997 15.09 4.44 0.143 

1998 63.91 2.09 0.117 

28
 

1999 3.91 4.74 0.242 

2000 0.00 4.03 0.172 

Preparing rainfall data for WMM water quantity calibration

As it has been discussed in the Method section, WMM uses Equation (1) to estimate the 

surface runoff from precipitation data. Equation (1) assumes that the amount of surface 

runoff is in direct proportion to precipitation, which implies that all the rainfall precipitation 

to some extent contributes to the surface runoff. This assumption, however, is an 

oversimplification of the ambient condition, in which a certain amount of rainfall is 

retained by soil and never contributes to the surface runoff. In other words, when the 

precipitation value is lower than a certain threshold, no surface runoff will occur

(Viessman, et al. 1989). This can be described using the following equation: 

(4) Q = k*(P – P0) 

Where, 

Q is the surface runoff produced by a given amount of annual precipitation 

k is equivalent to [Cp + (CI – Cp) IMPL] of Equation (1), which is the runoff 

coefficient of landuse category L. 

P is the annual precipitation 

P0 is the base precipitation value below which Q is zero. 

Equation (4) can be rewritten as: 

(5) Q = k*P – k*P0

Comparing Equation (5) to Equation (1), which is 

(1) R = [Cp + (CI – Cp) IMPL] * I 

It is obvious that Equation (1) fails to take into account –k*P0, which is the portion of 

rainfall that will never contribute to surface runoff. Therefore, using Equation (1), WMM 

may overestimate the surface runoff, especially for dry years during which the majority or 

even all of the rainfall is retained in the watershed and very little or even no surface 

runoff will be produced. 

Ideally, P0 could be estimated by plotting the surface runoff part of the stream flow 

against the amount of rain precipitation. The typical stream flow usually has four 

component elements: (1) direct surface runoff, (2) interflow, (3) ground water or 

baseflow, and (4) channel precipitation (Viessman et al. 1989). Because Camps Canal 

is a small stream, channel precipitation could be considered insignificant. Baseflow was 

not a significant portion of the stream flow in Orange Lake watershed (Lasi 1999). 

Therefore, the surface runoff part of the stream flow at the USGS gauging station 

02241000 could be considered as the difference between the total stream flow and the 

interflow from Prairie Creek. The interflow from Prairie Creek typically accounts for 59% 

of the total flow in Prairie Creek. The other 41% of the Prairie Creek flow go to Paynes 

Prairie. When the interflow from Prairie Creek was compared to the total flow measured 

at the USGS gauging station 02241000, no difference was observed. In other words, 

29
 

most part of the flow measured at the gauging station was the interflow from Prairie 

Creek. Surface runoff into the stream section where the gauging station is located is 

insignificant. This made it impossible to estimate the k and P0 using the surface runoff – 

rain fall correlation. Therefore, P0 used in this study was the result created with the 

TMDL study of Newnans Lake, which is 32.6 inches/year. Before the actual measured 

precipitation was used for WMM simulation, this value (32.6 inches/year) was subtracted 

from the original precipitation observations to created a set of “adjusted precipitation 

values” (Table 15), which were equivalent to I in Equation (1). 

Table 15. Adjusted annual precipitation calculated based on P0 

Unit: inches/year 

Measured annual precipitation Adjusted annual precipitation 

1995 49.99 17.83 

1996 50.04 17.88 

1997 58.92 26.76 

1998 46.03 13.87 

1999 44.03 11.87 

2000 33.12 0.96 

WMM simulation for surface runoff and TN and TP loading from CCRS and OL subbasins

By using the measured data and model parameters discussed above, WMM simulation 

was conducted to estimate the total discharge (including the interflow and surface runoff) 

and TN and TP loading through the Camps Canal – River Styx system and the surface 

runoff and TN and TP loading from OL sub-basin. The total discharge through the 

Camps Canal – River Styx system and surface runoff from the OL sub-basin are listed in 

Table 16. The TN and TP loading through Camps Canal – River Styx system and 

surface runoff from OL sub-basin are listed in Table 17. 

 

Table 16. Estimated annual flow of Camps Canal – River Styx system and surface 

runoff from OL sub-basin. 

Unit: Acre-foot/year 

Discharge through Camps Canal – 

River Styx system Surface runoff from OL sub-basin 

1995 25804 26385 

1996 25990 26459 

1997 26245 39594 

1998 76431 20519 

1999 8524 17561 

2000 335 1420 

30
 

Table 17. TN and TP loadings from Camps Canal – River Styx system and OL subbasin 

Unit: lbs/year 

Camps Canal – River Styx system OL sub-basin 

TN TP TN TP 

1995 266261 13512 90459 11934 

1996 236296 9054 90712 11967 

1997 224401 9030 138314 17907 

1998 422666 24058 70349 9280 

1999 58861 3139 60305 7942 

2000 875 88 4870 642 

TN and TP loadings from OL sub-basin follow the trend of precipitation (Figure 8 and 9). 

The highest TN and TP loadings appeared in 1997, the year of the highest rainfall. When 

the measured rainfall reached the lowest in 2000 and the effective rainfall became 

almost 0, TN and TP from OL sub-basin also reached their lowest values. TN and TP 

loadings through the Camps Canal – River Styx system do not appear to follow the trend 

of rainfall. In contrast, the loadings follow the trend of the flow measured at the mouth of 

Camps canal (USGS gauging station 02241000) very well (Figure 10 and 11). This 

suggests that TN and TP loadings through the Camps Canal – River Styx system is 

significantly influenced by the TN and TP loadings from the Newnans Lake through 

Prairie Creek. In fact, except for year 2000, during which the flow from Prairie Creek 

became 0, TN and TP loadings from Newnans Lake account for more than 70% of the 

total TN and TP loadings through the Camps Canal – River Styx system (Figure 12 and 

13). This indicates that, to control the eutrophication of Orange Lake, TN and TP 

loading from Newnans Lake should be considered as a major source of TN and TP. 

TN and TP loadings from different landuse categories are listed in Tables 18 and 19. 

Figures 14, 15, 16, and 17 show the percent contribution of TN and TP from different 

landuse categories in the wettest year (1997) and the driest year (2000), respectively. 

Leakage of septic tanks is not classical landuse categories. It is included here to offer a 

complete picture of sources of TN and TP loadings. Using these graphs, we intended to 

show how the amount of annual precipitation influences the relative importance of TN 

and TP contribution from different point and nonpoint sources. 

31
 

32

0

50000

100000

150000

200000

250000

300000

350000

400000

450000

1995 1996 1997 1998 1999 2000

TN Loading (lbs/year)

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

Precipiation (in/year)

CCRS OL Precipitation Adjusted Precipitation

Figure 8. Contribution of TN loading through the Camps Canal – River Styx 

system and the OL sub-basin. 

0

5000

10000

15000

20000

25000

1995 1996 1997 1998 1999 2000

TP loading (lbs/year)

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

Precipiation (in/year)

CCRS OL Precipitation Adjusted Precipitation

Figure 9. Contribution of TP loading through the Camps Canal – River Styx 

system and the OL sub-basin. 
 

0

50000

100000

150000

200000

250000

300000

350000

400000

450000

1995 1996 1997 1998 1999 2000

TN loading (lb/year)

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

Flow at the mouth of Camps Cana

(MGD)

CCRS Flow at the mouth of Camps Canal

Figure 10. TN loading through the Camps Canal – River Styx system vs. the flow 

at the mouth of Camps Canal (measured at USGS gauging station 02241000) 

0

5000

10000

15000

20000

25000

1995 1996 1997 1998 1999 2000

TP loading (lb/year)

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

Flow at the mouth of Camps Cana

(MGD)

CCRS Flow at the mouth of Camps Canal

Figure 11. TP loading through the Camps Canal – River Styx system vs. the flow 

at the mouth of Camps Canal (measured at USGS gauging station 02241000)

33
 

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

1995 1996 1997 1998 1999 2000

Percent TN contribution

from New nans Lake through Prairie Creek from CCRS subbasin

Figure 12. Percent contribution of TN loading from Newnans Lake and CCRS 

sub-basin in the total TN loading through the Camps Canal – River Styx system. 

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

1995 1996 1997 1998 1999 2000

Percent TP contribution

from New nans Lake through Prairie Creek from CCRS subbasin

Figure 13. Percent contribution of TP loading from Newnans Lake and CCRS 

sub-basin in the total TN loading through the Camps Canal – River Styx system.

34
 

Table 18. Contribution of TN from different landuse categories in 1995 

Units: lbs/year 

CCRS sub-basin OL sub-basin 

Forest/rural open 8410 12715 

Urban open 26 1004 

Agriculture 2215 47408 

Low density residential 341 6184 

Medium density residential 196 1899 

High density residential 0 31 

Transportation/communication 66 1824 

Rangeland 178 678 

Water/wetland 4760 13766 

Septic tank 68 4951 

Table 18(continued) Contribution of TN from different landuse categories in 1996 

Units: lbs/year 

CCRS sub-basin OL sub-basin 

Forest/rural open 8434 12750 

Urban open 26 1006 

Agriculture 2221 47541 

Low density residential 342 6202 

Medium density residential 197 1904 

High density residential 0 31 

Transportation/communication 66 1829 

Rangeland 178 680 

Water/wetland 4773 13804 

Septic tank 58 4964 

Table 18(continued) Contribution of TN from different landuse categories in 1997 

Units: lbs/year 

CCRS sub-basin OL sub-basin 

Forest/rural open 12620 19080 

35
 

Urban open 39 1506 

Agriculture 3324 71141 

Low density residential 512 9280 

Medium density residential 295 2849 

High density residential 0 46 

Transportation/communication 99 2737 

Rangeland 267 1017 

Water/wetland 7143 20657 

Septic tank 102 10000 

Table 18(continued) Contribution of TN from different landuse categories in 1998 

Units: lbs/year 

CCRS sub-basin OL sub-basin 

Forest/rural open 6541 9888 

Urban open 20 780 

Agriculture 1723 36869 

Low density residential 266 4809 

Medium density residential 153 1477 

High density residential 0 24 

Transportation/communication 51 1419 

Rangeland 138 527 

Water/wetland 3702 10705 

Septic tank 73 3851 

Table 18(continued) Contribution of TN from different landuse categories in 1999 

Units: lbs/year 

CCRS sub-basin OL sub-basin 

Forest/rural open 5597 8462 

Urban open 17 668 

Agriculture 1474 31552 

Low density residential 227 4116 

Medium density residential 131 1264 

36
 

High density residential 0 21 

Transportation/communication 44 1214 

Rangeland 118 451 

Water/wetland 3168 9162 

Septic tank 45 3396 

Table 18(continued) Contribution of TN from different landuse categories in 2000 

Units: lbs/year 

CCRS sub-basin OL sub-basin 

Forest/rural open 453 684 

Urban open 1 54 

Agriculture 119 2552 

Low density residential 18 333 

Medium density residential 11 102 

High density residential 0 2 

Transportation/communication 4 98 

Rangeland 10 37 

Water/wetland 256 741 

Septic tank 3 267 

Table 19. Contribution of TP from different landuse categories in 1995 

Units: lbs/year 

CCRS sub-basin OL sub-basin 

Forest/rural open 291 430 

Urban open 3 113 

Agriculture 329 6852 

Low density residential 29 502 

Medium density residential 22 202 

High density residential 0 5 

Transportation/communication 9 242 

Rangeland 6 23 

37
 

Water/wetland 940 3066 

Septic tank 7 500 

Table 19(continued). Contribution of TP from different landuse categories in 1996 

Units: lbs/year 

CCRS sub-basin OL sub-basin 

Forest/rural open 292 431 

Urban open 3 113 

Agriculture 330 6872 

Low density residential 29 503 

Medium density residential 22 202 

High density residential 0 5 

Transportation/communication 9 243 

Rangeland 6 23 

Water/wetland 943 3074 

Septic tank 8 501 

Table 19. Contribution of TP from different landuse categories in 1997 

Units: lbs/year 

CCRS sub-basin OL sub-basin 

Forest/rural open 437 645 

Urban open 5 169 

Agriculture 493 10283 

Low density residential 43 753 

Medium density residential 32 303 

High density residential 0 8 

Transportation/communication 14 363 

Rangeland 9 35 

Water/wetland 1411 4600 

Septic tank 11 749 

38
 

Table 19(continued). Contribution of TP from different landuse categories in 1998 

Units: lbs/year 

CCRS sub-basin OL sub-basin 

Forest/rural open 227 334 

Urban open 2 88 

Agriculture 256 5329 

Low density residential 22 390 

Medium density residential 17 157 

High density residential 0 4 

Transportation/communication 7 188 

Rangeland 5 18 

Water/wetland 731 2384 

Septic tank 6 388 

Table 19. Contribution of TP from different landuse categories in 1999 

Units: lbs/year 

CCRS sub-basin OL sub-basin 

Forest/rural open 194 286 

Urban open 2 75 

Agriculture 219 4561 

Low density residential 19 334 

Medium density residential 14 134 

High density residential 0 3 

Transportation/communication 6 161 

Rangeland 4 15 

Water/wetland 626 2040 

Septic tank 5 332 

Table 19(continued). Contribution of TP from different landuse categories in 2000 

Units: lbs/year 

CCRS sub-basin OL sub-basin 

Forest/rural open 16 23 

39
 

Urban open 0 6 

Agriculture 18 367 

Low density residential 2 27 

Medium density residential 1 11 

High density residential 0 0 

Transportation/communication 1 13 

Rangeland 0 1 

Water/wetland 51 165 

Septic tank 0 27 

40
 

A

Forest/Rural Open

51.7%

Water/Wetlands

29.3%

Septic tank

0.4%

Rangeland

1.1%

Transport and 

communication

0.4%

High density 

residential

0.0%

M edium density 

residential 

1.2%

Low density 

residential

2.1%

Agricultural

13.6%

Urban Open

0.2%

B

Forest/Rural Open

13.8%

Urban Open

1.1%

Agricultural

51.4%

Septic tank

7.2%

Transportation and 

communication

2.0%

Rangeland

0.7%

Water/Wetlands

14.9%

M edium density 

residential 

2.1%

High density 

residential

0.0%

Low density 

residential

6.7%

Figure 14. Percent contribution of TN from different landuse categories in 1997. A: 

CCRS sub-basin; B: OL sub-basin. 

41
 

A

Forest/Rural Open

51.8%

Septic tank

0.3%

Water/Wetlands

29.3%

High density 

residential

0.0%

M edium density 

residential 

1.2%

Transportation and 

communication

0.4%

Rangeland

1.1%

Low density 

residential

2.1%

Agricultural

13.6%

Urban Open

0.2%

B

Forest/Rural Open

14.1%

Agricultural

52.4%

Low density 

residential

6.8%

M edium density 

residential 

2.1%

High density 

residential

0.0%

Transportation and 

communication

2.0%

Rangeland

0.7%

Water/Wetlands

15.2%

Septic tank

5.5%

Urban Open

1.1%

Figure 15. Percent contribution of TN from different landuse categories in 2000. A: 

CCRS sub-basin; B: OL sub-basin.

42
 

A

Forest/Rural Open

17.8%

Water/Wetlands

57.5%

Septic tank

0.4%

Urban Open

0.2%

Agricultural

20.1%

Low density 

residential

1.7%

M edium density 

residential 

1.3%

High density 

residential

Transport and 0.0%

communication

0.6%

Rangeland

0.4%

B

Agricultural

57.4%

Septic tank

4.2%

Forest/Rural Open

3.6% Urban Open

0.9%

Low density 

residential

4.2%

High density 

residential

0.0%

M edium density 

residential 

1.7%

Water/Wetlands

25.7%

Rangeland

0.2%

Transportation and 

communication

2.0%

Figure 16. Percent contribution of TP from different landuse categories in 1997. A: 

CCRS sub-basin; B: OL sub-basin.

43
 

A

Forest/Rural Open

17.9%

Septic tank

0.0% Urban Open

0.2%

Agricultural

20.2%

Low density 

residential

1.8%

Rangeland

0.4%

Transportation and 

communication

0.6%

M edium density 

residential 

1.3%

High density 

residential

0.0%

Water/Wetlands

57.7%

B

Agricultural

57.4%

M edium density 

residential 

1.7%

Transportation and 

communication

2.0%

Rangeland

0.2%

High density 

residential

0.0%

Low density 

residential

4.2%

Water/Wetlands

25.7%

Forest/Rural Open

3.6%

Urban Open

0.9%

Septic tank

4.2%

 

Figure 17. Percent contribution of TP from different landuse categories in 2000. A: 

CCRS sub-basin; B: OL sub-basin. 

44
 

Because no point source of significant scale exists in either CCRS or OL sub-basin, no 

significant difference was found for the percent contribution of TN and TP from different 

landuse categories between the wet year (1977) and dry year (2000) from these two 

sub-basins. However, the percent contribution of TN and TP from different landuse 

categories in CCRS or OL sub-basins are very different. 

In 1997, the leading TN contributor in CCRS sub-basin is the Forest/Rural open landuse 

category, which alone accounts for more than 51% of the TN loading from the entire 

sub-basin (Figure 14). Water/Wetland ranks second, accounting for about 29% of TN 

loading from the sub-basin. Contributions from these two landuse categories (nonhuman landuse categories) composed of about 81% of the total TN loading from the 

CCRS sub-basin. The largest human landuse categories from the sub-basin is 

Agriculture, which accounts for about 14% of the total TN loading. This is followed by 

Low density residential and medium density residential, which accounts for 2.1% and 

1.2% of the total TN loading from the sub-basin. In addition, Rangeland claims 1.1% of 

the total TN loading. TN contribution from Septic tank only accounts for about 0.3%, 

which is insignificant in the total TN loading. Non-human landuse categories are 

responsible for about 19% of TN loading from the sub-basin. 

In the OL sub-basin, the landuse category that contributes the largest percentage of TN 

is Agriculture, which accounts for 52.4% of the total TN loading from the sub-basin 

(Figure 15). TN contribution from non-human landuse categories only makes up 29.3% 

of the total TN loading (Forest/Rural open 14.1% and Water/Wetlands 15.2%). The 

second largest TN contributor of human landuse category is the residential area, which 

accounts for 8.9% of the total TN loading (Low density residential 6.8% and Medium 

density residential 2.1%). Contribution from septic tank leakage is also higher in OL 

sub-basin than the CCRS sub-basin, which represents 5.5% of the total TN loading. 

Other human landuse categories contribute about 3.8% of the TN. This includes 2.0% 

from Transportation and communication, 1.1% from Urban open area, and 0.7% from 

Rangeland. The human landuse categories contribute about 70.7% of the TN loading 

created in OL sub-basin. 

The amount of annual precipitation does not significantly influence the percent TN 

contributions from different landuse categories. Although the exact percent TN 

contributions from different landuse categories change slightly between 1997 and 2000, 

the overall distribution of TN contribution in 2000 follows the pattern of 1997 (Figure 14-

15). 

The percent contribution of TP loading appears differ from that of TN loading. In 1997, 

the top TP contributor in CCRS sub-basin is Water/wetlands, which alone contributes 

57.5% of the total TP loading. Forest/rural open contributes about 17.8%. This makes 

the total TP contribution from non-human landuse categories 75.3% of the total TP 

loading created in CCRS sub-basin. The leading TP contributor of human landuse 

category in CCRS is Agriculture, which accounts for 20.1% of the TP loading. This is 

followed by the residential area that contributes about 3.0% (Low density residential 

1.7% and Medium density residential 1.3%). Transportation and communication, 

Rangeland, Septic tank, and Urban open contribute 0.6%, 0.4%, 0.4%, and 0.2%, 

respectively (Figure 16). 

Agriculture is the leading TP contributor in OL sub-basin, which alone represents 57.4% 

of the total TP loading created in OL sub-basin (Figure 17). Non-human landuse 

45
 

categories contribute 29.3% of the TP (Water/wetland 25.7% and Forest/rural open 

3.6%). Residential area contribute another 5.9% (Low density residential 4.2% and 

Medium density residential 1.7%). Contribution from septic tank is about 4.2%. The 

remaining portion of the TP loading is from Transportation and communication, Urban 

open, and Rangeland categories, which represent 2.0%, 0.9%, and 0.2% of the TP 

loading created in OL sub-basin, respectively. 

Once again, percent TP contribution from different landuse categories in 2000 follows 

that of 1997 (Figure 16, 17). 

TN and TP loading from Lochloosa Lake through Cross Creek

Because no measured flow data were available for Cross Creek, the annual flow for 

Cross Creek was calculated in this study by multiplying the WMM simulated annual flow 

of River Styx at the mouth to Orange Lake by 0.89 – a long-term average ratio between 

the simulated annul flow of Cross Creek and the simulated annual flow of River Styx. 

This ratio was calculated by comparing the simulated annual flow of Cross Creek with 

the annual flow of River Styx (1942 through 1991) simulated using the Streamflow 

Synthesis and Reservoir Regulation (SSARR) model by St. Johns River Water 

Management District (Robison, et al. 1997). Measured TN and TP concentrations were 

only available for the period from 1995 through 1998. The TN and TP annual loadings 

from Lochloosa Lake through the Cross Creek were calculated by multiplying the annual 

flow of the stream by the TN and TP concentrations. The resultant TN and TP loadings 

are listed in Table 20. 

Table 20. TN and TP annual loadings from Lochloosa Lake through Cross Creek 

Year Annual Flow TN 

Concentration 

TP 

Concentration 

TN annual 

loading 

TP annual 

loading 

(acre-feet) (mg/L) (mg/L) (lbs/year) (lbs/year) 

1995 17425 1.64 0.055 77727 2620 

1996 17575 1.87 0.050 89345 2369 

1997 15044 1.90 0.067 77703 2722 

1998 63716 1.68 0.078 290265 13472 

 

 

Atmospheric loading of TN and TP into Orange Lake

One source of TN and TP loading to Orange Lake that was not considered by the WMM 

was the TN and TP falling directly onto the surface of Orange Lake through precipitation. 

In this study, atmospheric loading of TN and TP was calculated by multiplying the 

amount of precipitation directly falling on to the lake surface (calculated by multiplying 

annual precipitation by surface area of the lake) by the TN and TP concentration of the 

rainfall. Because no TN and TP concentration of the rainfall was found for the project 

area, published values were adopted in this study, which were 0.1 mg/L and 0.05 mg/L 

for TN and TP respectively (Stites, et al 2001). Calculated annual TN and TP loadings 

are tabulated in Table 21. 

46
 

Table 21. Atmospheric loading of TN and TP into Orange Lake 

Units: lbs/year 

TN TP 

1995 15493 7747 

1996 15509 7754 

1997 18260 9130 

1998 14264 7132 

1999 13645 6822 

2000 10264 5132 

Mean 14572 7286 

SE 1078 539 

CV 7% 7% 

Summary of discharge and TN and TP loadings from different sources

Water discharge and TN and TP loadings from different sources including from Newnans 

Lake through Camps Canal, from Lochloosa Lake through Cross Creek, CCRS subbasin, OL sub-basin, and precipitation are listed in Table 22, 23, and 24. 

Table 22. Discharge of water from different sources 

Units: acre-feet/year 

1995 1996 1997 1998 1999 2000

From Camps Canal 19580 19748 16905 71591 4382 0

From Cross Creek 17425 17575 15044 63716 3900 0

CCRS 6224 6242 9340 4840 4142 335

OL 26385 26459 39594 20519 17561 1420

Precipitation 57092 57149 67288 52565 50281 37822

Tributary total 37005 37323 31949 135307 8282 0

Watershed 32609 32701 48934 25360 21703 1755

Total 126707 127173 148171 213232 80266 39577

Table 23. TN loading from different sources 

Units: lbs/year 

1995 1996 1997 1998 1999 2000

From Camps Canal 250000 220000 200000 410000 56551 0

47
 

From Cross Creek 77727 89345 77703 290265 ---- ----

CCRS 16261 16296 24401 12666 10822 875

OL 90459 90712 138314 70349 60305 4870

Precipitation 15493 15509 18260 14264 13645 10264

Tributary total 327727 309345 277703 700265 ---- ----

Watershed 106719 107008 162714 83015 ---- ----

Total 449939 431861 458678 797544 ---- ----

Table 24. TP loading from different sources 

Units: lbs/year 

1995 1996 1997 1998 1999 2000

From Camps Canal 11877 7413 6576 22786 2885 0

From Cross Creek 2620 2369 2722 13472 ---- ----

CCRS 1635 1641 2454 1272 1089 88

OL 11934 11967 17907 9280 7942 642

Precipitation 7747 7754 9130 7132 6822 5132

Tributary total 14497 9782 9298 36258 ---- ----

Watershed 13569 13608 20361 10552 ---- ----

Total 35813 31144 38789 53943 ---- ----

The total TN loading to Orange Lake ranges from 431861 lbs/year in 1996 to 797544 

lbs/year in 1998 (Table 23). Most TN comes from two tributary streams, which 

contribute from 61% to 88% of the total TN loading. Of the two streams, TN loading 

from Camps Canal appears more important than Cross Creek. While Cross Creek 

contribute from 17% to 36% of the total TN loading, percent TN contribution from Camps 

Canal ranges from 44% to 56%. This results from the high TN concentration in 

Newnans Lake. Watershed (CCRS and OL sub-basins) contributes from 10% to 35% of 

the TN loading, which, when compared to the TN contribution from tributary streams, is 

less important. TN loading through precipitation is not the most important source, It only 

ranges from 2% to 4%. 

The total TP loading to Orange Lake ranges from 31144 to 53943 lbs/year (Table 24). 

Contributions from tributary streams and watershed are comparable, which account for 

24% to 67% and 20% to 52% of the total TP loading, respectively. The majority TP 

loading through tributary streams is carried into the from Newnans Lake through Camps 

Canal, which alone makes up from 17% to 42% of the TP loading. Of the TP loading 

contributed by the watershed, loading from OL sub-basin always dominates, which 

ranges from 17% to 46% of the total TP loading to Orange Lake. TP loading through the 

direct precipitation on the lake surface is also significant. It represents from 13% to 25% 

of the TP into the lake. 

48
 

In summary, the majority of TN loading into Orange Lake appears to be carried by 

tributary streams, especially from Newnans Lake through Camps Canal. This is 

consistent with the fact that Newnans Lake has a high TN concentration. TP loading 

from tributary streams and watershed appears to be equally important. When loading 

control is considered, decreasing the TN concentration in Newnans Lake should be a 

priority area to target for TN control, and reducing loading from both watershed and 

tributary should receive equal emphasis. 

5.3 Establishing the relationship between TN and TP loading and in-lake TN, TP, 

and Chla concentrations 

Data required for calibrating the Bathtub eutrophication model

The relationship between TN and TP loading and the in-lake TN and TP concentrations 

was established through fitting the Bathtub predictions with the measured TN and TP 

concentrations of the lake. To calibrate the model, the following data were required: 

1. Physical characteristics of the lake (surface area, mean depth, and mixed layer 

depth) 

2. Meteorological data (precipitation and evaporation) 

3. Measured water quality data (TN, TP, and Chla concentrations of the lake water) 

4. Loading data (flow and TN and TP concentrations of the flow from various point 

and nonpoint sources) 

Because Bathtub allows both error and variability analysis, whenever there were 

historical data, long-term average and coefficient of variance (CV) of the average were 

calculated and entered into the model as input. All the data that were required for model 

calibration are listed in Tables 25 through 28. 

Table 25. Physical characteristics of Orange Lake 

Lake surface area Mean depth Mixed layer depth 

(km2

) (m) (m) 

1995 53.0 1.69 1.69 

1996 55.5 1.73 1.73 

1997 50.7 1.67 1.67 

1998 57.8 1.77 1.77 

1999 46.3 1.60 1.60 

2000 27.6 1.09 1.09 

Mean 48.5 1.59 1.59 

SE 4.5 0.10 0.10 

CV 9% 6% 6% 

49
 

Note: Because Orange Lake is a shallow lake with a relatively large surface area, wind 

mixing is strong and thermal stratification does not form in the summer. Therefore, the 

“Mixed layer depth” was assumed equal to the mean depth of the lake. 

Table 26. Precipitation and evaporation 

Unit: m/year 

Precipitation Evaporation 

1995 1.270 1.329 

1996 1.271 1.331 

1997 1.497 1.567 

1998 1.169 1.224 

1999 1.118 1.171 

2000 0.841 0.881 

Mean 1.194 1.250 

SE 0.088 0.092 

CV 7% 7% 

Table 27. Measured TN, TP, and Chla concentrations of Orange Lake 

Unit: ppb 

TN TP Chla 

1995 1048 26 20.58 

1996 1458 42 44.42 

1997 1440 38 42.40 

1998 1485 57 59.26 

1999 1658 65 58.45 

2000 3541 89 130.12 

Mean 1772 53 59 

SE 363 9 15 

CV 21% 17% 26% 

Table 28. Flow and TN and TP concentrations of different sources 

Flow TN TP 

Mean SE CV Mean SE CV Mean SE CV 

(hm3/yr) (ppb) (ppb) 

50
 

Forest/Rural Open 6.76 1.58 23% 1181.63 0.00 0% 40.35 0.00 0% 

Urban Open 0.41 0.10 23% 957.18 0.00 0% 107.38 0.00 0% 

Agricultural 8.82 2.06 23% 2127.32 0.00 0% 307.84 0.00 0% 

Low density 

residential 2.31 0.54 23% 1065.82 0.00 0% 86.60 0.00 0% 

Medium density 

residential 0.57 0.13 23% 1382.15 0.00 0% 147.32 0.00 0% 

High density 

residential 0.01 0.00 23% 1452.97 0.00 0% 238.63 0.00 0% 

Transportation, 

Communications, 

and Utilities 

0.57 0.13 23% 1251.27 0.00 0% 166.09 0.00 0% 

Rangeland 0.31 0.07 23% 1034.61 0.00 0% 35.34 0.00 0% 

Water/Wetlands 13.76 3.22 23% 508.97 0.00 0% 110.05 0.00 0% 

Camps Canal 27.18 12.96 48% 3998.92 487.81 12% 172.66 25.03 15% 

Cross Creek 24.19 11.50 48% 1774.79 66.23 4% 62.42 6.26 10% 

Note: 

a) Bathtub does not allow direct input of loading. Therefore, data presented here are 

flow and the TN and TP concentrations of the flow. 

b) Flows for each source presented here are calculated by aggregating individual flows 

from the two sub-basins (CCRS and OL sub-basin) and then averaging throughout 

the period from 1995 through 2000. 

c) TN and TP concentrations presented here for each source were calculated by adding 

TN and TP loadings from the two sub-basins (CCRS and OL sub-basin), dividing the 

sum by the total flow over the two sub-basins, and then averaging throughout the 

period from 1995 through 2000. 

Calibrating the Bathtub eutrophication model

To calibrate the model, each source of TN and TP was designated as an independent 

tributary. Flow and TN and TP concentrations of the flow were defined for each tributary 

as listed in Table 28. The loading from septic tanks is not defined in Table 28 because 

in Bathtub, septic tanks are characterized differently from regular point and nonpoint 

sources (because septic tanks in the model do not produce additional flow). Instead of 

being defined by flow and pollutant concentration of the flow, loading from septic tank 

leakage is defined by an areal loading rate calculated based on dividing WMM predicted 

total TN and TP loading by the area of the lake surface, which are 0.119 (SE = 0.033, 

CV = 28%) mg/m2

/d and 0.011 (SE = 0.003, CV = 23%) mg/m2/d, respectively. 

Bathtub provides alternative models for estimating the influence of sedimentation on the 

in-lake TN and TP concentrations. The settling velocity model was chosen for both TN 

and TP in this study. This model assumes that the sedimentation of TN and TP is in 

first-order kinetics and should linearly correlate with the in-lake TN and TP 

concentration. The model also assumes that the sedimentation is influenced by the 

depth of the lake. The deeper the lake, the slower the sedimentation. This model fit the 

condition of Orange Lake because the lake is relatively shallow and large in surface 

51
 

area. Continued wind mixing prevents the lake from forming thermal stratification, a 

process that prevents the particles from being re-suspended once settled down to the 

bottom. Continued wind mixing through the entire water column also reduces particle 

settling rate by continuously bringing the settled particle back in to water column. These 

processes could produce a relatively low settling rate in Orange Lake. Other 

sedimentation models provided by Bathtub assume second-order kinetics, which fit 

reasonably well with lakes that form thermal stratification during the summer, but could 

overestimate the sedimentation of Orange Lake, and in turn cause underestimation of 

the in-lake TN and TP concentration. 

Among the eutrophication responding models provided by Bathtub, the model created by 

Jones and Bachman (1976) was adopted in this study to calculate the chla 

concentration. The primary reason for choosing this model was because it assumes that 

phytoplankton communities are phosphorus limited, which is consistent with the 

observation in Orange Lake that the long-term average TN/TP ratio over the verified 

period is 35, an indication of phosphorus limitation. The model takes the following form: 

B = CB * 0.081 * P1.48 

Where, B is the chlorophyll a concentration, CB is a model calibration factor (discussed 

in a later section), and P is the phosphorus concentration. 

Several conditions need to be satisfied to use this model: (1) the non-algal turbidity (a) of 

the lake has to be smaller than 0.4 m-1; (2) the ratio between the concentration of 

inorganic nitrogen and the concentration of inorganic phosphorus has to be greater than 

7; (3) (N – 150)/P has to be greater than 12, where N is the TN concentration, P is the 

TP concentration; and (4) the flushing rate (Fs) has to be smaller than 25/year. 

The non-algal turbidity (a) is calculated using the following equation: 

 a = 1/s – 0.025 * B 

where, s is the Secchi depth, and B is the chlorophyll a concentration. For Orange Lake, 

the long-term Secchi depth over the verified period is about 0.71m. The long-term 

average chlorophyll a concentration over the same period is 59 ppb. The non-algal 

turbidity calculated from these numbers is about 0, which is obviously smaller than 0.4 

m-1. 

The long-term average concentration of inorganic nitrogen (including nitrate, nitrite, and 

ammonium) over the verified period is 181 ppb. Inorganic orthophosphate is rarely 

measured in Orange Lake. Only scattered data were found for 1991 and 1997, which 

ranged from 4 – 12 ppb. Even using the highest inorganic phosphorus concentration (12 

ppb), the ratio between the concentration of inorganic nitrogen and orthophosphate is 

still about 15, which is greater than 7. The long-term TN and TP concentration over the 

verified period are 1772 ppb and 53 ppb, respectively. This makes (N – 150)/P equal to 

30, which is greater than 12. 

The flushing rate of Orange Lake is calculated using the following equation: 

 Fs = (inflow – evaporation)/lake volume 

The long-term annual inflow (including discharge from tributary, watershed, and 

precipitation directly onto the surface of the lake) over the verified period is about 140 

cubic hectometers per year (hm3

/year) and the long-term annual average evaporation is 

about 60 hm3

/year. The average lake volume of Orange Lake is about 77 hm3. This 

makes the flushing rate of the lake about 1.06/ year, which is less than 25. 

52
 

Above analyses indicate that Orange Lake satisfies the assumptions of the Jones and 

Bachman model and therefore it was adopted in this study. Using this model, a 

reasonable fit between predicted and measured Chla concentration was achieved. 

 

Calibration factors were applied to fit TN and TP predictions to the measured data. Two 

calibration methods are provided by Bathtub for phosphorus and nitrogen: Method 0 

calibrates decay rates and Method 1 calibrates concentration. In the first case, the 

calibration factors are applied to estimated sedimentation rates in computing nutrient 

balances. In the second case, the factors are applied to estimated concentrations. In 

Method 0, it is assumed that the error is attributed primarily to the sedimentation model. 

In Method 1, the error source is unspecified (some combination of input error and 

sedimentation model error). The latter may be used when predicted nutrient profiles are 

insensitive to errors in predicted sedimentation rate because the mass balance is 

dominated by inflow and outflow terms (low hydraulic residence times) (Walker 1999). In 

this study, because wind mixing could significantly lower the sedimentation rate over the 

default sedimentation rate (based on data from lakes that form thermal stratification), it 

was assumed that the error of the model predictions came mainly from the error 

associated with the sedimentation model. Therefore, Method 0 was adopted in this 

study to calibrate the decay rate due to the sedimentation. Typical calibration factors for 

TN and TP recommended by the Bathtub User’s manual are 0.5 – 2.0 for TN and 0.33 – 

3 for TP. In this study, 0.9 and 3 were adopted for calibrating TN and TP, respectively. 

A calibration factor of 2 was applied to chlorophyll a concentration. Results of model 

calibration are listed in Table 29. 

Table 29. Bathtub calibration results 

Measured Estimated Error T statistics 

Mean CV Mean CV 1 2 3 

TP (mg/l) 0.053 0.17 0.056 0.29 5% -0.28 -0.18 -0.14 

TN (mg/l) 1.77 0.21 1.75 0.20 1% 0.05 0.05 0.04 

Chla (μg/l) 59.0 0.26 57.1 0.49 3% 0.12 0.09 0.06 

Bathtub provides for statistical comparisons between observed and predicted 

concentrations. These are computed using three alternative measures of error: 

observed error only, T(1); error typical of model development data set, T(2); and 

observed and predicted error, T(3). Tests of model applicability are normally based 

upon T(2) and T(3). If their absolute values exceed 2 for the comparison of areaweighted mean concentrations, there is less than a 5% chance that nutrient 

sedimentation dynamics in the reservoir are typical of those in the model development 

data set. This assumes that input conditions have been specified in an unbiased 

manner. Once an appropriate sedimentation model is selected, T(1) can be used as a 

basis for deciding whether calibration is appropriate. If the absolute value of T(1) 

exceeds 2, then there is less than a 5% chance that the observed and predicted means 

are equal, given the error in the observed mean (Walker 1999). In this case, no T value 

for TN, TP and Chla predictions was higher than 2. Therefore, the assumption that 

model predictions equal the measured data was not rejected and the model was 

considered calibrated properly. 

53
 

Comparing the TSI calculated from Bathtub predictions and the TSI based on measured 

data

The TSI was calculated based on Bathtub estimated TN, TP, and Chla concentrations. 

Because the mean values of model predictions for TN, TP, and Chla all have associated 

CV values estimated by Bathtub (Table 29). TSIs calculated based on model estimated 

TN, TP, and Chla include the mean, the maximum possible value, and the minimum 

possible value (Table 30). 

Table 30. TSI calculated based on model predicted and measured TN, TP, and Chla 

concentrations 

TSI calculated 

based on model predictions 

TSI calculated 

based on measured data 

Mean 73 73 

Maximum 76 79 

Minimum 68 64 

According to Table 30, the range of TSIs calculated based on model estimated TN, TP, 

and Chla appeared to overlap with the mean ± 1 SE of the TSIs calculated based on 

measured data. Therefore, the former was considered proper estimates of the latter. 

Relationship between TP loading reduction and TSI of Orange Lake and the nutrient 

TMDL for the lake

Using the calibrated Bathtub model, alternative phosphorus and chlorophyll a

concentrations can be predicted for different phosphorus loadings. The TP and 

chlorophyll a concentrations at different TP loadings can then be used to calculate TSI 

based on the procedure described in the method section. A relationship between the TP 

loading reduction and TSI of Orange Lake can thus be developed (Figure 16). 

54
 

0

10

20

30

40

50

60

70

80

90

100

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

Percent TP loading reduction

TSI

Figure 18. Relationship between TSI of Orange Lake and the TP loading reduction. 

According to Figure 16, to achieve the goal of a TSI of 60, the TP loading has to be 

reduced by about 45% to 50% from the current level (about 12,650 Kg/year). 

TP loadings from several sources can be controlled to achieve the 50% reduction. 

These sources including the loading from Newnans Lake through the Camps Canal, the 

loading from Lochloosa Lake through Cross Creek, and the TP loading from the Orange 

Lake watershed. 

Based on a concurrent nutrient TMDL study being conducted on Newnans Lake, the 

lake currently has a high TP concentration that is caused by both the external loading 

from the watershed and internal nutrient recycling process. The long-term annual 

average TP concentration measured at the outlet stream (Prairie Creek) of Newnans 

Lake over the verified period is about 0.172 mg/L. After the internal nutrient recycling is 

eliminated and the external loading is reduced to the Newnans Lake TMDL level, the inlake TP concentration in Newnans Lake would become 0.068 mg/L, which would 

reduced the TP loading from Newnans Lake into Orange Lake from 4693 kg/year to 

1848 kg/year. This step alone would reduced the total TP loading into Orange Lake 

from 12,650 kg/year to 9806 kg/year, a 22% reduction in TP loading. 

Further reduction of TP loading into Orange Lake can be achieved by controlling the 

loading from Lochloosa Lake through Cross Creek. The current TP concentration 

measured at Cross Creek is about 0.062 mg/L. When a 50% TP loading reduction is 

achieved for Lochloosa Lake, the TP concentration of Lochloosa Lake would decrease 

to 0.045 mg/L (Baniukiewicz, et al. 2003). This would reduce the TP loading from 

Lochloosa Lake into Orange Lake from 1510 kg/year to 1089 kg/year, which would result 

in about 3% of the total TP loading into Orange Lake. 

Landuse management would be another important approach to control the TP loading 

into Orange Lake. When the loading from all the watershed area occupied by human 

55
 

landuse categories is assessed as loadings from Forest/Rural open, the TP loading from 

the watershed into Orange Lake would be reduced from the current 5114 kg/year to 

about 2358 kg/year, which would result in a 22% reduction of TP loading into Orange 

Lake. 

Natural Background for Orange Lake Watershed and Newnans and Lochloosa Lakes at 

their respective TMDL loadings

Natural Background was evaluated as if the TMDL goals for Newnans Lake and 

Lochloosa Lake are achieved and the loadings from all the human landuse categories in 

Orange Lake watershed are assessed as loadings from Forest/Rural. Under these 

conditions, the reduction of TP loading into Orange Lake would be 22% (from Newnans 

Lake) + 3% (from Lochloosa Creek) + 22% (from Orange Lake watershed) = 47%, which 

would result in an in-lake TP concentration of about 0.028 mg/L and a chlorophyll a

concentration of about 21.9 μg/L. This corresponds to an annual average TP loading of 

6615 kg/year. The TSI calculated based on these TP and chlorophyll a concentrations 

would be 58. 

Final TMDL Calculation

As the natural background TSI was less than 60, the target TSI for the TMDL is 60. A 

45% TP loading reduction cuts the TP loading into Orange Lake back from the current 

12650 kg/year to 6923 kg/year. The 6923 kg/year for TP is considered the TMDL for 

Orange Lake. At an annual average loading for TP of 6923 kg/year, the in-lake 

concentration of TP and Chla should be 0.031 mg/L and 24.1 μg/L respectively. 

A long-term TSI of 73 over the verified period put Orange Lake on the nutrient impaired 

list. Based on the TN/TP ratio of the lake that is greater than 30, phytoplankton 

community of the lake are appeared to be phosphorus limited and control of 

eutrophication of the lake should focus on controlling the availability of phosphorus to the 

phytoplankton communities. 

Several sources of TP loading were identified in this study. These include loading from 

Newnans Lake through Camps Canal, loading from Lochloosa Lake through Cross 

Creek, loading from the Orange Lake watershed, and loading through the atmospheric 

precipitation. A detailed average annual TP loading (over the verified period from 1995 

through 2000) from different sources are summarized in Table 31. 

Table 31. Summary of TP contribution from different sources 

Average 

Annual TP 

loading 

(kg/year) 

Percent of total 

TP loading 

Percent in TP 

loading from 

the watershed 

Percent in TP 

loading from 

human landuse 

categories 

Forest/Rural 

open 

273 2.2% 5.3% ---- 

Water/Wetland 1514 12.0% 29.6% ---- 

56
 

Urban open 44 0.3% 0.9% 1.3% 

Agriculture 2715 21.5% 53.1% 81.6% 

Rangeland 11 0.1% 0.2% 6.0% 

Low density 

residential 199 1.6% 3.9% 2.5% 

Medium density 

residential 84 0.7% 1.6% 0.1% 

High density 

residential 2 0.0% 0.0% 2.8% 

Transportation 

and 

communication 

95 0.7% 1.9% 0.2% 

Septic tank 177 1.4% 3.5% 5.3% 

Watershed total 5114 40.4% 100% ---- 

Camps Canal 4692 37.1% ---- ---- 

Cross Creek 1509 11.9% ---- ---- 

Precipitation 1334 10.5% ---- ---- 

Total 12651 100% ---- ---- 

Contributions of TP from different sources are obviously different. Of the total TP 

loading into Orange Lake, Newnans Lake (through Camps Canal) and Orange Lake 

watershed contribute 40.4% and 37.1%, respectively (Table 31). These two sources 

make up about 77.5% of the total TP loading. When TP loading reduction is concerned, 

these two sources should be the focus. To control the TP loading from Orange Lake 

watershed, reductions in loadings from Agricultural landuse must be evaluated because 

it alone contributes 53.1% of the total TP loading into Orange Lake and accounts for 

81.6% of the total TP loading from all the human landuse categories. 

Nutrient loading into Orange Lake was also studied by Lasi using EUTROMOD (1999). 

Comparison of the median TP loading estimates from Lasi’ study and the results from 

this study are listed in Table 32. It appears that the total TP loading from Lasi’s study is 

about 53% higher than the TP loading estimates from this study. Based on Table 32, 

the major difference comes from the difference between the TP loadings from the 

Orange Lake watershed. Loadings from tributaries and precipitation are very similar in 

both studies. The 

57
 

Table 32. Comparison of the TP loading from Lasi’s study to the TP loading 

estimates from this study. 

Units: kg/year 

From Lasi’s study From this study 

Loading from tributaries 6914 6201 

Loading from watershed 18058 5114 

Precipitation 1778 1334 

Total 27078 12649 

divergence could be attributed to the type of rainfall used in estimating the TP loading. 

In Lasi’s study, original total rainfall measurements were used, assuming that whatever 

the rainfall was, there would be some surface runoff. This assumption was not included 

for this study because in a nutrient TMDL study conducted in Newnans Lake watershed, 

the observed surface runoff became zero when the annual rainfall was lower than 32.6 

in/year. Therefore, using the procedure described in the previous section, 32.6 in/year 

was subtracted from the original rainfall measurement before the precipitation data were 

used for TP loading estimation. Consider that the long-term annual average rainfall over 

the period that this project covers (1995 through 2000) is 47.0 in/year, the adjusted 

rainfall used in this study after subtracting 32.6 in/year is 14.4 in/year, which is about 

69% lower than the original measured rainfall. This difference could account for the 

divergence between the TP loading estimate from Lasi’s study and that from this study. 

Water budget calibration using adjusted rainfall was conducted in the nutrient TMDL 

study conducted for Newnans Lake watershed. The amount of surface runoff estimated 

using adjusted rainfall was only 6% different from the measured amount of water, 

indicating that using adjust rainfall should not cause underestimation of TP loading. No 

water budget calibration was conducted in this study because no flow data were 

available. However, as Newnans Lake and Orange Lake are located in the same large 

watershed and geological characters and soil type of both watersheds are similar, it 

would be reasonable to assert that the rainfall-runoff relationship which applies to 

Newnans Lake should also apply to Orange Lake. 

6.0 Determination of TMDL 

The objective of a TMDL is to provide a basis for allocating acceptable loads among all 

of the known pollutant sources in a watershed so that appropriate control measures can 

be implemented and water quality standards achieved. A TMDL is expressed as the 

sum of all point source loads (Waste Load Allocations), nonpoint source loads (Load 

Allocations), and an appropriate margin of safety (MOS), which takes into account any 

uncertainty concerning the relationship between effluent limitations and water quality: 

TMDL = ∑ WLAs + ∑ LAs + MOS

As mentioned in Section 4.1, the WLA is broken out into separate subcategories for 

wastewater discharges and stormwater discharges regulated under the NPDES 

Program: 

 

58
 

TMDL ≅ ∑ WLAswastewater + ∑ WLAsNPDES Stormwater + ∑ LAs + MOS 

It should be noted that the various components of the TMDL equation may not sum up to 

the value of the TMDL because a) the WLA for NPDES stormwater is typically based on 

the percent reduction needed for nonpoint sources and is accounted for within the LA, 

and b) TMDL components can be expressed in different terms [for example, the WLA for 

stormwater is typically expressed as a percent reduction and the WLA for wastewater is 

typically expressed as a mass per day]. 

WLAs for stormwater discharges are typically expressed as “percent reduction” because 

it is very difficult to quantify the loads from MS4s (given the numerous discharge points) 

and to distinguish loads from MS4s from other nonpoint sources (given the nature of 

stormwater transport). The permitting of stormwater discharges is also different than the 

permitting of most wastewater point sources. Because stormwater discharges cannot be 

centrally collected, monitored and treated, they are not subject to the same types of 

effluent limitations as wastewater facilities, and instead are required to meet a 

performance standard of providing treatment to the “maximum extent practical” through 

the implementation of Best Management Practices. 

This approach is consistent with federal regulations [40 CFR § 130.2(I)], which state that 

TMDLs can be expressed in terms of mass per time (e.g. pounds per day), toxicity, or 

other appropriate measure. The TP nutrient TMDL for Orange Lake (Table 33) is 

expressed in terms of pounds per year and/or percent reduction. 

As mentioned previously, the IWR thresholds for nutrient impairment in lakes was used 

as the water quality target for the lake. Rule 62-303.352(1), FAC, specifies for lakes with 

an average color of 60 or greater that the annual average TSI should be 60 or less, 

unless paleolimnological information indicates the annual average natural TSI of the lake 

was greater than 60. The model estimate for the annual average TSI at natural 

background for the Orange Lake watershed and with Newnans and Lochloosa Lakes at 

the TMDL loading was 58. The resulting in-lake TP and Chla concentrations were 

predicted to be 0.028 mg/L and 21.9 ug/L respectively. Because the natural background 

TSI was less than 60 (58) the TMDL was calculated for a TSI of 60. 

Table 33 TMDL Components 

WLA 

WBID Parameter Wastewater 

(lbs/year) 

NPDES 

Stormwater 

LA 

(lbs/year) MOS TMDL (lbs/year) 

Percent 

Reduction 

2749A TP None NA 15,262 Implicit 15,262 45 

6.1 Load Allocation 

The allowable LA is 15,262 lbs/year for TP. This corresponds to reductions from the 

existing loadings of 45 percent for TP. At an annual average loading for TP of 15,262 

lbs/year, the in-lake concentration of TP and Chla should be 0.031 mg/L and 24.1 μg/L 

respectively. It should be noted that the LA includes loading from stormwater discharges 

59
 

regulated by the Department and the Water Management Districts that are not part of 

the NPDES Stormwater Program (see Appendix A). 

6.2 WasteLoad Allocation

NPDES Stormwater Discharges

As noted in Sections 4 and 6.1, load from stormwater discharges permitted under the 

NPDES Stormwater Program are placed in the WLA, rather than the LA. This includes 

loads from municipal separate storm sewer systems (MS4). However, based on the 

information provided by EPA, no MS4 area was found overlapping the Orange Lake 

watershed and no stormwater loads were assigned to the WLA. 

NPDES Wastewater Discharges

There are no known NPDES point source discharges within the subject watershed. 

6.3 Margin of Safety

An implicit margin of safety exists due to conservative assumptions used in the modeling 

process. Additionally, the estimates of septic tank failures were set to the maximum 

values instead of the mean values. 

The Department recognizes that the absolute value of these loading numbers may be 

significantly different from the absolute loads calculated by other models, based on 

analysis using data from other sources, use of different assumptions, and/or differing 

interpretation of the results of other researchers. However, the Department is very 

confident in the load reductions required to return the lake to a healthy condition and the 

estimated concentrations of TP (0.031 mg/L), and Chla (24.1 .ug/L) that would be 

expected in a healthy Orange Lake. 

7. NEXT STEPS: IMPLEMENTATION PLAN DEVELOPMENT AND BEYOND 

Following adoption of this TMDL by rule, the next step in the TMDL process is to develop 

an implementation plan for the TMDL, which will be a component of the Basin 

Management Action Plan for the Orange Lake Basin. This document will be developed 

in cooperation with local stakeholders and will attempt to reach consensus on more 

detailed allocations and on how load reductions will be accomplished. 

The Basin Management Action Plan (B-MAP) will include: 

• Appropriate allocations among the affected parties. 

• A description of the load reduction activities to be undertaken. 

• Timetables for project implementation and completion. 

• Funding mechanisms that may be utilized. 

• Any applicable signed agreements. 

• Local ordinances defining actions to be taken or prohibited. 

• Local water quality standards, permits, or load limitation agreements. 

• Monitoring and follow-up measures. 

60
 

It should be noted that TMDL development and implementation is an iterative process, 

and this TMDL will be re-evaluated during the BMAP development process and 

subsequent Watershed Management cycles. The Department acknowledges the 

uncertainty associated with TMDL development and allocation, particularly in estimates 

of nonpoint source loads and allocations for NPDES stormwater discharges, and fully 

expects that it may be further refined or revised over time. If any changes in the 

estimate of the assimilative capacity AND/OR allocation between point and nonpoint 

sources are required, the rule adopting this TMDL will be revised, thereby providing a 

point of entry for interested parties. 

61
 

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Water Management District. 

Brown, M. T. 1995. South Dade Watershed Project. University of Miami/SWFWMD 

Canfield, D. E. Jr. 1981. Chemical and trophic state characteristics of lakes in relation to 

regional geology. Final report submitted to Coop. Fish and Wildlife Research 

Unit, University of Florida: 444 pp. 

CDM. 1990. Point and non-point source loading assessment, phase I. Report to 

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Clark, W., R. Musgrove, C. Menke, and J. Cagle. 1964. Interim report on the water 

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Copeland, R., T. Gardner, J. Craft, W. Schmidt, C. M. Browner, H. L. Rhodes, and C. C. 

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framework. Department of Environmental Regulation. 

Deevey, E. S. 1988. Estimation of downward leakage from Florida lakes. Limnology and 

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Gido, K. B. 2002. Interspecific comparisons and potential importance of nutrient 

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Gottgens, J. F. and C. L. Montague. 1987. Orange, Lochloosa and Newnans Lakes, a 

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Harper, H. H. 1992. Estimation of stormwater loading rate parameters for Central and 

South Florida. Environmental Research & Design. Orlando, Florida. 

Harper, H. H. 1994. Stormwater loading rate parameters for central and south Florida. 

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Harper, H. H. and E. H. Livingston. 1999. Everything you always wanted to know about 

stormwater management practices but were afraid to ask. Biennial Stormwater 

Research Conference, Tampa, Florida. 

Holly, J. B. 1976. Stratigraphy and sedimentary history of Newnans lake. M. S. thesis, 

university of Florida: 102 p. 

James, R. T. and V. J. Bierman Jr. 1995. A preliminary modeling analysis of water 

quality in lake Okeechobee, Florida: Calibration results. Water Research 29: 

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Jones, J. R. and R. W. Bachman. 1976. Prediction of phosphorus and chlorophyll levels 

in lakes. Journal of Water Pollution Control Federation 48, 217-2182. 

Lasi, M. A. 1999. An external nutrient budget for Orange Lake effect of sinkhole seepage 

on in-lake nitrogen and phosphorus stores. St. Johns River Water Management 

District, Palatka, Florida 

Linsley, R. U., and J. B. Franzini. 1997. Water Resources Engineering, McGrow-Hill 

Book Co. 

Nordlie, F. G. 1976. Plankton communities of three central Florida lakes. Hydrobiologia 

48 (1): 65-78 

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Appendix A 

In 1982, Florida became the first state in the country to implement statewide regulations 

to address the issue of nonpoint source pollution by requiring new development and 

redevelopment to treat stormwater before it is discharged. The Stormwater Rule, as 

authorized in Chapter 403, Florida Statutes (F.S.), was established as a technologybased program that relies upon the implementation of BMPs that are designed to 

achieve a specific level of treatment (i.e., performance standards) as set forth in Chapter 

62-40, Florida Administrative Code (F.A.C.). 

The rule requires Water Management Districts (WMDs) to establish stormwater pollutant 

load reduction goals (PLRGs) and adopt them as part of a SWIM plan, other watershed 

plan, or rule. Stormwater PLRGs are a major component of the load allocation part of a 

TMDL. To date, stormwater PLRGs have been established for Tampa Bay, Lake 

Thonotosassa, Winter Haven Chain of Lakes, the Everglades, Lake Okeechobee, and 

Lake Apopka. No PLRG has been developed for Newnans Lake at the time this study 

was conducted.

In 1987, the U.S. Congress established section 402(p) as part of the Federal Clean 

Water Act Reauthorization. This section of the law amended the scope of the federal 

NPDES to designate certain stormwater discharges as “point sources” of pollution. 

These stormwater discharges include certain discharges that are associated with 

industrial activities designated by specific Standard Industrial Classification (SIC) codes, 

construction sites disturbing five or more acres of land, and master drainage systems of 

local governments with a population above 100,000 [which are better known as 

“municipal separate storm sewer systems” (MS4s)]. However, because the master 

drainage systems of most local governments in Florida are interconnected, EPA has 

implemented Phase 1 of the MS4 permitting program on a county-wide basis, which 

brings in all cities (incorporated areas), Chapter 298 urban water control districts, and 

the DOT (Department of Transportation) throughout the 15 counties meeting the 

population criteria. 

An important difference between the federal and the state stormwater permitting 

programs is that the federal program covers both new and existing discharges while the 

state program focuses on new discharges. Additionally, Phase 2 of the NPDES 

stormwater permitting program will expand the need for these permits to construction 

sites between one and five acres, and to local governments with as few as 10,000 

people. These revised rules require that these additional activities obtain permits by 

2003. While these urban stormwater discharges are now technically referred to as “point 

sources” for the purpose of regulation, they are still diffuse sources of pollution that can 

not be easily collected and treated by a central treatment facility similar to other point 

sources of pollution, such as domestic and industrial wastewater discharges. The DEP 

recently accepted delegation from EPA for the stormwater part of the NPDES program. 

It should be noted that most MS4 permits issued in Florida include a re-opener clause 

that allows permit revisions to implement TMDLs once they are formally adopted by rule. 

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