U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2015–1075

Prepared in cooperation with the Southwest Florida Water Management District

Comparison of Evaporation at Two Central Florida Lakes, 

April 2005–November 2007

Comparison of Evaporation at Two Central 

Florida Lakes, April 2005–November 2007

By Amy Swancar

Prepared in cooperation with the Southwest Florida Water Management District

Open-File Report 2015–1075

U.S. Department of the Interior

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

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2015

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Suggested citation:

Swancar, Amy, 2015, Comparison of evaporation at two central Florida lakes, April 2005–November 2007: 

U.S. Geological Survey Open-File Report 2015–1075, 16 p., http://dx.doi.org/10.3133/ofr20151075.

ISSN 2331-1258 (online)
iii

Contents

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

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

Site Descriptions...................................................................................................................................1

Methods...........................................................................................................................................................4

Evaporation Methods...........................................................................................................................4

Water-Budget Methods.......................................................................................................................6

Results .............................................................................................................................................................6

Evaporation............................................................................................................................................6

Water Budget ......................................................................................................................................11

Rainfall, Runoff, and Pumpage ................................................................................................11

Groundwater Exchange............................................................................................................13

Discussion.....................................................................................................................................................13

Conclusions...................................................................................................................................................14

References Cited..........................................................................................................................................14

Figures

1. Map showing location of Lakes Starr and Calm in central Florida. .....................................2

2. Map showing bathymetry of Lakes Calm and Starr................................................................3

3–7. Graphs showing: 

 3. Monthly evaporation at study lakes...................................................................................7

 4. Relation between energy budget evaporation at the study lakes by thermal 

survey period..........................................................................................................................7

 5. Relations between energy budget parameters at the study lakes by thermal 

survey period........................................................................................................................10

 6. Monthly rainfall, lake stage, and net-groundwater exchange at study lakes ..........12

 7. Difference in evaporation between the two sites in relation to the difference

between change in stored heat by thermal survey period...........................................13

Tables

1. Characteristics of Lakes Calm and Starr..................................................................................2

2. Instrumentation at Lakes Calm and Starr.................................................................................5

3. Monthly water budgets for Lakes Calm and Starr..................................................................8

Appendix (available at http://dx.doi.org/10.3133/ofr20151075)

1. Energy-budget Bowen ratio evaporation at Lakes Calm and Starr by thermal

survey period ................................................................................................................. Excel file
iv

Conversion Factors

SI to Inch/Pound

Multiply By To obtain

Length

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

Area

square meter (m2

) 0.0002471 acre

hectare (ha) 0.2471 acre

Rainfall rate

millimeter per hour (mm/hr) 0.003281 foot per hour (ft/hr)

Pressure

kilopascal (kPa) 0.009869 atmosphere, standard (atm)

Mass

kilogram 2.205 pound avoirdupois (lb)

Density

kilogram per cubic meter (kg/m3

) 0.06242 pound per cubic foot (lb/ft3) 

Energy

joule (J) 2.77×10–7 kilowatthour (kWh)

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

°F=(1.8×°C)+32

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

°C=(°F–32)/1.8

Datum

Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 

(NGVD 29)

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

Abbreviations

BREB Bowen ratio energy-budget

RH relative humidity

SE standard error
Comparison of Evaporation at Two Central Florida Lakes, 

April 2005–November 2007

By Amy Swancar

Abstract

Evaporation from April 2005 through October 2007 at 

two central Florida lakes, one close to the Gulf of Mexico and 

one in the center of the peninsula, was 4.043 and 4.111 meters 

(m), respectively; evaporation for 2006 was 1.534 and 1.538 m, 

respectively. Although annual evaporation rates at the two lakes 

were similar, there were monthly differences between the two 

lakes because of changes in stored heat; the shallower Lake 

Calm (mean depth 3 m) stored less heat and exchanged heat 

more rapidly than the deeper Lake Starr (mean depth 5 m). 

Both lakes are seepage lakes (no surface-water inflows 

or outflows) that are dependent on groundwater inflow from 

their basins to offset an atmospheric deficit, because long-term 

rainfall in this area is less than evaporation. The Lake Starr 

basin, where sandy, well-drained ridges surround the lake, has 

a greater capacity to store infiltrating rain than the Lake Calm 

basin, which is flat and has poorly drained soils. The storage 

capacities of the basins affect groundwater exchange with the 

lakes. Rainfall and net groundwater exchange, which is related 

to basin characteristics, varied more between these two lakes 

than did evaporation during this study. 

Introduction

Evaporation from open water is a critical component 

of both global energy and hydrologic cycles. Fresh water is 

“distilled” from the sea by this energy-intensive process as 

water moves from the ocean to the atmosphere, with some of 

that atmospheric water reaching the land when it falls to Earth 

as rain or snow. Evaporation and transpiration from the land 

surface continue to extract from the near-surface a substantial 

percentage of the water that falls as precipitation, reducing 

recharge to surface-water and groundwater systems that are the 

main water sources for humans and other species. Therefore, it 

is important to quantify the range and variability in evaporation 

so that freshwater supplies can be accurately assessed and 

prudently managed. 

Evaporation is the largest water-loss term in many lake 

water budgets, and is especially important for seepage lakes 

that have no surface inflows or outflows. About 70 percent 

of Florida’s 7,800 lakes are seepage lakes (Schiffer, 1998). 

Energy-budget evaporation was measured at two lakes in central 

Florida, Lake Starr and Lake Calm. Lake Starr has a relatively 

long record of evaporation measurements from 1996 to 2011, 

and has been integral in a series of lake water budget and lake 

groundwater interaction studies (Lee, 2002; Sacks and others, 

1998; Sacks and others, 2014; Swancar and others, 2000; 

Swancar and Lee, 2003, Virdi and others, 2012). It would be 

useful to be able to extrapolate the longer-term evaporation 

measurements at Lake Starr to other lakes. To answer the 

question of whether evaporation measurements at Lake Starr 

can be extrapolated to other lakes, the U.S. Geological Survey 

conducted a cooperative study with the Southwest Florida Water 

Management District to collect and analyze evaporation data at 

Lake Calm from 2005 to 2007. 

This report presents (1) a comparison between evaporation 

rates measured at two central Florida lakes, (2) a discussion 

of the factors affecting the differences between evaporation 

rates, and (3) a comparison of water budgets for the two lakes 

focusing on groundwater exchange. The report presents the 

water and energy budgets for the two lakes during the period 

from April 4, 2005, to November 2, 2007. 

Site Descriptions

Lake Calm is located in northwest Hillsborough County 

in the relatively flat-lying terrain of the Gulf Coastal Lowlands 

(White, 1970) (fig. 1). Lake Starr is located in Polk County in 

the center of the State. Lakes Calm and Starr are both mid-sized 

lakes between 40 and 60 hectares (ha) in size that were formed 

from sinkholes in the mantled-karst terrain of central Florida 

(table 1). Mantled-karst is a landscape common throughout 

Florida where carbonate rocks beneath the surface are overlain 

by clays and sands (Tihansky, 1999). As the carbonates dissolve 

over time, cavities form that eventually collapse, causing 

the overlying clay and sand to subside into the cavities. The 

surface expressions of these collapses can be wetlands, lakes, 

or sinkholes, depending on the elevation of the water table and 

the degree of subsidence. Both Lakes Calm and Starr consist 

of multiple collapse features that are evident in the bathymetry 

(fig. 2). At Lake Starr, seismic reflection also was used to 

delineate subsurface collapse features (Swancar and others, 

2000). 
2 Comparison of Evaporation at Two Central Florida Lakes, April 2005–November 2007

Lake Calm is shallower than Lake Starr (table 1), with a 

number of isolated deep holes rather than broad low areas in the 

bathymetry. Topography in the Lake Calm basin also is flatter 

and lower in elevation than in the Lake Starr basin. Lake Starr 

is located along the Lake Wales Ridge, the most prominent 

north-south trending ridge in the center of the State, which was 

formed as a relict beach shoreline when sea levels were at least 

60 meters (m) higher than modern sea level (White, 1970). 

When the water level (stage) of Lake Calm is greater than 

about 15.25 m above NGVD 29, there is potential for water 

to flow from Lake Calm to Lake Keystone through a small 

wetland and canal on the southwest side of the lake (D. Leeper, 

Southwest Florida Water Management District, written commun., 2005). Historically and through the period of record at 

Lake Calm (1965 to 2007), outflow probably only occurred 

during 1970 and 2004 (estimates based on stage record only, no 

measurements of flow are available). No outflow was observed 

during this study, even though the stage was between 15.25 

and 15.30 m for a few weeks in August 2005. Therefore, for 

the purpose of this analysis, Lake Calm was considered to be 

Gulf Coastal Lowlands

Lake Wales Ridge

EXPLANATION

Lake

Calm

Keystone

Lake

Figure 1. 

ALABAMA

GEORGIA

GULF

OF

MEXICO

ATLANTIC

OCEAN

Lake

Starr

Polk

County

Hillsborough

County

87° 86° 85° 84° 83° 82° 81° 80°

31°

29°

28°

27°

26°

25°

30°

0 20 40 60 80 100 MILES

0 50 100 KILOMETERS

Figure 1. Location of Lakes Starr and Calm in central Florida.

Table 1. Characteristics of Lakes Calm and Starr.

[USGS, U.S. Geological Survey; m, meter; NGVD 29, National Geodetic 

Vertical Datum of 1929; ha, hectare]

Descriptor Lake Calm Lake Starr

USGS station identifier 02307227 02293763

Latitude (degrees, minutes, 

seconds)

28°08'20" 27°57'15"

Longitude (degrees, minutes, 

seconds)

82°35'00" 81°35'33"

Sections 10, 11, 14, 15 14, 23

Township-Range T 27S, R17E T 29S, R 27E

Reference lake surface elevation 

(m above NGVD 29)

14.63* 31.7

Surface area (ha) at reference 

elevation 

48.16* 54.23

Maximum depth at reference 

elevation (m)

8.23* 9.75

Mean depth at reference 

elevation (m)

3.05 4.88

1:24,000 USGS quadrangle 

name

Odessa, FL Lake Wales, FL

Drainage basin area (ha) 104* 298

Maximum elevation of drainage 

divide (m above NGVD 29)

19.8 68.6

*

Data from D. Leeper (Southwest Florida Water Management District, 

written commun., 2005).

a seepage lake. Lake Starr has a relatively steep, well-defined 

topographic basin and no surface-water connections (table 1). 

Although physiographic differences can be expected to 

affect the water budgets of the two lakes because they influence 

groundwater exchange (Lee, 2002; Sacks, 2002), they might not 

have a large effect on evaporation rates. Evaporation rates are 

mostly a function of radiant energy entering the lake (sunlight 

and longwave radiation), which is a function of season and 

cloud cover (Aslyng, 1974; Finch and Hall, 2005; Tanner and 

Lemon, 1962). Lake water-budget terms are linked to the energy 

budget, however, through the change in stored energy and latent 

and advected energy fluxes (Anderson, 1954). Both water and 

energy budgets are presented and compared in this report.
Introduction 3

7.5

6.0

9.0

4.5

3.0

1.5

Lake Starr

0 500 1,000 1,500 FEET

0 100 300200 400 METERS

82°35' 82°34'45"

Base from Southwest Florida Water Management District

Lake Calm

4.5

3.0

3.0

4.5

7.5 4.5

6.0

4.5

6.0

6.0

3.0

6.0

4.5

3.0

3.0

3.0

3.0

1.5

1.5

1.5

7.5 4.5

Figure 2.

28°08'45"

28°08'30"

81°35'30" 81°35'15" 81°35'

27°57'45"

27°57'30"

27°57'15"

EXPLANATION

Bathymetric contour–Depth in meters below

 surface, adjusted to datum of 31.70 meters

 above NGVD 29. Contour interval is 1.5 meters

5

0 500 1,000 1,500 FEET

0 100 300200 400 METERS

Base from Southwest Florida Water Management District

Sheet no. 15-29-27, aerial photography with contours,

January 1987

EXPLANATION

Bathymetric contour–Depth in meters below

 surface, adjusted to datum of 14.63 meters

 above NGVD 29. Contour interval is 1.5 meters

5

4.5

Figure 2. Bathymetry of Lakes Calm and Starr.
4 Comparison of Evaporation at Two Central Florida Lakes, April 2005–November 2007

Methods 

The energy-budget method was used to calculate 

evaporation at Lakes Calm and Starr. Water-budget analyses 

incorporated data on rainfall, evaporation, and estimates of 

runoff and pumpage, which were used to calculate groundwater exchanges at these two seepage lakes. 

Evaporation Methods

The energy-budget method used to calculate evaporation 

at the two lakes was originally described by Anderson (1954) 

for Lake Hefner in Oklahoma, and has been applied throughout the world since then (dos Reis and Dias, 1998; Harbeck 

and others, 1958; Rosenberry and others, 2004; Roulet 

and Woo, 1986; Rouse and others, 2005; Sacks and others, 

1994; Sturrock and others, 1992; Wiche, 1992; Winter and 

others, 2003). This method, commonly called the Bowen ratio 

energy-budget variant or BREB, estimates the energy used for 

evaporation by quantifying energy gains and losses and the 

change in stored energy. The energy-budget equation is

 Qs–Qr+Qa–Qar–Qbs–Qe–Qh–Qw+Qv=Qx (1)

where

Qs is incoming solar radiation;

Qr is reflected solar radiation;

Qa is incoming longwave radiation;

Qar is reflected longwave radiation;

Qbs is emitted (backscattered) longwave radiation;

Qe is energy used for evaporation, or the latentheat flux;

Qh is energy conducted from the lake to the 

atmosphere as sensible heat;

Qw is energy advected from the lake to the 

atmosphere by the evaporating water;

Qv is net energy advected into the lake; and

Qx is change in stored energy (stored heat).

Heat exchange with bottom sediments is assumed to be 

negligible. The first five terms can be measured or estimated 

separately, or combined as net radiation, Qn. All Q terms are 

expressed in watts per square meter.

The evaporation rate, E, in meters per second, is

E = Qe ρw

–1λ–1, (2)

where

	 λ is latent heat of vaporization, 2.45×106 joules 

per kilogram (J kg–1) at 20 °C, 

	 ρw is the density of water, 1,000 kilograms per 

cubic meter (kg m–3) at 4 °C, and 1 joule 

(J) = 1 watt second (W s).

The term Qh

 is derived from the Bowen ratio, R, the ratio 

of Qh

 to Qe (Bowen, 1926)

 (3)

Qw is calculated from

 

 (4)

where

cw is the specific heat of water, 4,186 J kg–1 °C–1 

at 15 °C;

Te is the temperature of evaporating water 

(assumed equal to the water-surface 

temperature), in degrees Celsius; and

Tb is the base temperature, set to 0 °C. 

Substituting equations 2–4 into equation 1, and solving 

for E gives

 

The 8.64×107 multiplier is used to convert units from 

meters per second tomillimeters per day. 

For this form of the energy-budget equation (the BREB 

variant), R is calculated from vapor pressure and temperature 

differences using the following equation:

 (6)

where

To is water-surface temperature, in degrees 

Celsius;

Ta is air temperature, in degrees Celsius;

eo is saturation vapor pressure at the watersurface temperature, in kilopascals, and

ea is vapor pressure of the air, in kilopascals; and 

0.062 is the psychrometric constant, in kilopascals 

per degree Celsius.

The energy budget is calculated over a selected time 

period called a thermal survey period. Assuming that temperature profiles at a single site are representative of the whole 

lake and continuous water temperature profiles are measured 

at that site, daily thermal survey periods can be defined starting at midnight each day. However, the change in stored heat 

over a single day tends to be small, and the error in estimating 

this change is large relative to the daily difference, often as 

much as 100 percent. For this reason, thermal survey periods 

are usually defined as a week or longer, over which stored heat 

changes in the lake can be more accurately measured. Weekly 

thermal surveys were used in this analysis except for periods 

of missing data; 67 percent of thermal survey periods were a 

week in length and 87 percent were between 7 and 10 days 

Qh

=RQe.

Qw=cw ρw E(Te

–Tb),

E

Q Q Q

R c T T

n x v

w w o b

( )

.

millimeters per day =

× × ( ) − +

( ) + + −

8 64 10

1

7

ρ λ ( )  



R = 0.062 (To–Ta) / (eo–ea) ,

(5)
Methods 5

in length. Because thermal survey periods can be irregular 

in length and consequently have irregularly spaced start and 

end dates, monthly evaporation also was calculated at both 

sites for ease of presentation. For Lake Calm, daily average 

evaporation rates calculated by thermal survey period were 

aggregated to estimate monthly rates. 

Details on data collection at Lake Starr and how the 

energy budget was applied can be found in Swancar and others 

(2000). Additional analysis of up to 15 years of evaporation 

data from Lake Starr can be found in Lee and others (2014) 

and Sacks and others (2014). Advected energy is assumed 

to come only from rain, and this term is small in magnitude. 

Advected energy from groundwater at these sites also is negligible compared to other energy fluxes. Table 2 summarizes 

the instruments used at the two lakes. The main difference 

in data collection between the two sites was that a raft was 

deployed in the deepest part of Lake Starr to continuously 

measure lake water temperature at 0.3-m intervals, as well 

as air temperature and relative humidity (RH) at 2 m above 

the lake surface. At Lake Calm, air temperature and RH were 

measured over the lake from a dock on the south side of the 

lake. Lake temperature profiles at Lake Calm were measured 

in two ways: (1) manually from a boat at weekly to biweekly 

intervals at the beginning and end of the study at 0.3-m depth 

intervals, and (2) using a string of temperature sensors at 1-m 

depth intervals that was deployed in the deepest part of the 

lake from November 16, 2005, to April 9, 2007. 

Table 2. Instrumentation at Lakes Calm and Starr.

[RH, relative humidity; CSI, Campbell Scientific, Inc.]

Parameter Lake Calm Lake Starr

Net radiation REBS Q*7.1 (2) Eppley PSP and PIR

Air temperature and RH Vaisala HMP45 CSI HMP35

Water temperature HOBO Water temp Pro or CSI type-T thermocouple CSI type-T thermocouple

Rainfall Texas electronics TE525-M Texas electronics TE525

Change in stage DAA H-310 or KPSI pressure transducer Handar 436B shaft encoder with float tape

Datalogger CR10X CR10X
6 Comparison of Evaporation at Two Central Florida Lakes, April 2005–November 2007

Water-Budget Methods

The water budget for a lake is useful for estimating the net 

groundwater seepage to or from the lake. For the seepage lakes 

discussed in this report, which have no surface-water flows, the 

water-budget equation is

delta S ± edelta S =

 P – E + Gi

– Go – Q + R ± eP ± eE ± eGi ± eGo ± eQ ± eR, (7)

where 

delta S is change in volume,

P is precipitation,

E is evaporation,

Gi is groundwater inflow,

Go is groundwater outflow,

Q is direct pumping from the lake, and

R is runoff from the basin,

and the remaining terms are errors associated with each component. P and E are typically the largest water gain and loss terms, 

respectively, for seepage lakes in Florida; however, groundwater 

fluxes may exceed P and E in the water budget of some lakes 

(Grubbs, 1995; Sacks and others, 1998). Water-budget terms 

can be expressed either as linear units over the average lake 

surface area during the water-budget period, as they were in this 

study, or as volumes.

The seepage-lake water budget can be used to estimate net 

groundwater flow. Net groundwater flow (groundwater inflow 

minus outflow) can be calculated by rearranging equation 7 to 

get

 Net GW = Gi

– Go = 

 delta S – P + E + Q – R ±edelta S ± eP ± eE ± eQ ± eR (8)

The magnitude of net groundwater flow gives an indication of 

the magnitudes of groundwater inflow and outflow, but for a 

lake that has both, the magnitude will always be less than the 

actual inflow or outflow. For example, during a period when 

net groundwater flow is positive, there is usually still outflow 

occurring, so the actual (gross) groundwater inflow is probably 

greater than the computed (net) inflow.

Results

This section will first present evaporation differences, then 

water budget differences, and finally differences in net groundwater exchange calculated as a residual to the two lake water 

budgets. Comparison of evaporation rates at these two lakes 

is most relevant in the context of their water budgets, because 

uncertainty in the large evaporative flux adds uncertainty to 

the entire budget. Supporting data (energy-budget terms and 

evaporation by thermal survey period) for the two lakes are 

included in appendix 1.

Evaporation

Overall, evaporation at Lake Calm was only 0.2 percent 

less than that of Lake Starr, totaling 4.043 m during the 

31-month study, compared to 4.111 m at Lake Starr (table 3, 

fig. 3). This small difference in evaporation is well within the 

overall error in the method, and also within the 17-percent 

standard error (SE) of the regression between evaporation 

measured at the two sites by thermal survey period (fig. 4). Error 

in energy-budget evaporation is assumed to range from 10 to 

15 percent for monthly and weekly calculations, respectively, 

and 10 percent annually (Sacks and others, 1998; Winter, 1981). 

This error range is based on accumulating errors in individual 

terms, and assumes errors may all be in the same direction (for 

example, all positive bias) (Ramette, 1981).

The energy budget incorporates a number of different 

measurements, and the evaporation calculated from equation 1 

is more sensitive to some measurements than others (Lee and 

Swancar, 1997). Evaporation on an annual timescale is most 

sensitive to incoming and emitted longwave radiation, incoming 

shortwave radiation, and water-surface temperature (Lee and 

Swancar, 1997; Sacks and others, 1994). Errors in stored heat 

add the greatest uncertainty to the estimate, especially over 

periods less than a week (Anderson, 1954; Winter and others, 

2003). Advected heat (Qv

) is generally small for seepage lakes, 

and the energy budget method is insensitive to this term in these 

settings. 

Values of measured parameters by thermal survey period 

were used to compare measurements that are inputs to the energy 

budget at the two sites (fig. 5). Air (Ta

) and water-surface (To) 

temperatures both show strong relations between the two lakes, 

with coefficient of determination (r2

) = 0.99 and SE = 2 percent 

for both. This difference also is close to the combined errors of 

the instruments used to measure temperature (0.2 to 0.3 °C, or 

1 percent on average). Both relations show a small deviation 

from a 1:1 line, however. Air and water-surface temperatures 

were higher at Lake Calm than Lake Starr in the summer and 

lower than Lake Starr in the winter (fig. 5).

Average net radiation showed a good relation between 

the two sites (r2

= 0.92, SE = 9 percent), but the average net 

radiation was 2 percent lower at Lake Calm than at Lake Starr 

(fig. 5). This difference is within the range of the combined 

errors of the instruments used to measure net radiation (6 percent 

and 3 percent for the REBS Q*7.1 and Eppley PSP and PIR 

radiometers, respectively), and less than the standard error of 

the regression. Net radiation (Qn

) is the sum of incoming and 

reflected short-wave radiation and incoming, reflected, and 

emitted longwave radiation. The comparison of radiation terms 

was confined to net radiation rather than the individual radiation 

components because only net radiation was measured at Lake 

Calm. The difference in net radiation may be due to local 

weather at the two lakes, but also may be due to the radiometers 

used at these sites. In controlled field experiments, Brotzge and 

Duchon (2000) and Cobos and Baker (2003) found that the 

REBS radiometers used at Lake Calm underestimate daytime 

net radiation by 11 and 7 percent, respectively, compared to 
Results 7

200

175

150

125

100

75

50

25

0

Evaporation, in millimeters per month

Lake Calm

Lake Starr

EXPLANATION

April June Aug. April

2005 2006 2007

Oct. Dec. Feb. June Aug. Oct. Dec. Feb. April June Aug. Oct.

Figure 3.

Figure 3. Monthly evaporation at study lakes.

y = 0.7899x + 0.9195

R2 = 0.717

8

6

4

2

0

0 2 4 6 8

Lake Calm

Lake Starr

Evaporation, in millimeters per day

Figure 4. Relation between energy budget evaporation at the study lakes by 

thermal survey period.
8 Comparison of Evaporation at Two Central Florida Lakes, April 2005–November 2007

Table 3. Monthly water budgets for Lakes Calm and Starr.

[NGVD 29, National Geodetic Vertical Datum of 1929; m, meter; mm, millimeter; m2, square meter; SA, surface area: n/a, not applicable]

Month/year

Lake Calm

Stage on first day of 

month (0000 hour), 

in m above 

NGVD 29

Average 

SA, 

in m2

Change in stage as 

average volume to 

average SA,

in mm

Rain, 

in mm

Evaporation, 

in mm

Runoff, 

in mm

Net 

groundwater, 

in mm

Apr-2005 15.06 508,807 –74 86 145 10 –25

May-2005 14.98 500,475 –24 108 146 29 –15

Jun-2005 14.96 497,019 141 185 135 0 91

Jul-2005 15.10 512,818 124 189 156 30 61

Aug-2005 15.22 524,529 43 177 162 0 28

Sep-2005 15.27 528,519 –144 26 135 0 –35

Oct-2005 15.12 515,088 –107 41 116 0 –32

Nov-2005 15.02 505,072 –98 42 90 0 –50

Dec-2005 14.92 494,518 –40 57 63 0 –34

Jan-2006 14.88* 492,009 –88 22 72 0 –37

Feb-2006 14.79 486,448 48 110 75 21 –8

Mar-2006 14.84 489,524 –180 0 138 0 –42

Apr-2006 14.66 478,270 –192 13 157 0 –48

May-2006 14.47 466,424 –186 33 179 0 –39

Jun-2006 14.28 455,130 146 236 170 39 41

Jul-2006 14.43 464,004 183 243 159 21 78

Aug-2006 14.61 475,255 ** 160 159  

Sep-2006 **  ** ** 154 136 ** **

Oct-2006 14.83* 488,941 –110 26 139 12 –9

Nov-2006 14.72 482,074 –9 81 94 0 3

Dec-2006 14.71 481,503 –6 61 56 0 –10

Jan-2007 14.71 481,123 –36 53 80 0 –9

Feb-2007 14.67 478,840 –49 28 74 0 –3

Mar-2007 14.62 475,813 –128 19 119 0 –28

Apr-2007 14.49 467,914 –137 25 145 0 –16

May-2007 14.36 459,577 –253 4 167 0 –89

Jun-2007 14.10 444,401 –54 82 172 13 23

Jul-2007 14.05 441,196 –52 117 178 0 9

Aug-2007 14.00 438,169 –6 141 170 10 14

Sep-2007 13.99 437,813 –67 96 151 0 –12

Oct-2007 13.92 433,896 3 124 105 0 -16

Nov-2007 13.93 n/a n/a n/a n/a n/a n/a

Annual Totals

First year (Apr 2005-Mar 2006) n/a n/a –398 1,041 1,432 91 –99

Second year (Apr 2006-Mar 2007) n/a n/a –168 1,106 1,523 n/a n/a

Calendar year 2006 n/a n/a –174 1,137 1,534 n/a n/a

31-month Total n/a n/a n/a 2,735 4,043 n/a n/a

*

Estimated.

**Missing stage data, unable to estimate accurately because of heavy rain, water-budget terms that rely on this data are also excluded.

the Eppley radiometers used at Lake Starr. At night, however, 

these studies found that the REBS radiometer overestimated net 

radiation compared to the Eppleys. Nighttime radiation fluxes 

over lakes are particularly important because of the emitted 

longwave radiation given off by the water. 

Vapor pressure of water in air above the lake surface (ea), 

which is calculated from the measured air temperature and 

relative humidity, also shows a strong relation between the 

two sites (r2=0.99, SE = 3 percent). While ea is lower at Lake 

Calm by 5 percent on average, this difference also is close to 

the combined errors of the sensors (1 percent for temperature, 

2–3 percent for RH). 

Change in stored heat (Qx) differed between the two sites 

more than any other energy-budget term; Lake Calm had only 

about half the change in stored heat observed at Lake Starr, on 

average (fig. 5). 
Results 9

Table 3. Monthly water budgets for Lakes Calm and Starr.—Continued

[NGVD 29, National Geodetic Vertical Datum of 1929; m, meter; mm, millimeter; m2, square meter; SA, surface area: n/a, not applicable]

Month/year

Lake Starr

Stage on first day of 

month (0000 hour), 

in m above

 NGVD 29

Average

 SA, 

in m2

Change in stage as 

average volume to 

average SA,

in mm

Rain, 

in mm

Evaporation, 

in mm

Pumping 

from lake, 

in mm

Net 

groundwater, 

in mm

Apr-2005 32.50 568,959 –55 101 160 3 8

May-2005 32.45 566,330 –20 129 151 3 5

Jun-2005 32.42 565,347 371 427 126 0 70

Jul-2005 32.83 582,543 122 149 153 0 126

Aug-2005 32.96 587,751 185 247 164 2 103

Sep-2005 33.18 595,213 23 83 144 3 87

Oct-2005 33.20 596,118 160 241 136 0 55

Nov-2005 33.40 602,133 25 38 80 6 73

Dec-2005 33.43 603,039 19 38 73 8 63

Jan-2006 33.45 603,755 –42 13 69 10 24

Feb-2006 33.40 602,224 –22 62 74 9 –1

Mar-2006 33.37 601,399 –134 4 124 13 –1

Apr-2006 33.21 596,317 –162 24 161 7 –18

May-2006 33.02 589,852 –146 65 170 5 –36

Jun-2006 32.86 583,728 –74 122 170 8 –19

Jul-2006 32.78 580,489 –58 124 154 3 –26

Aug-2006 32.71 577,886 –59 152 170 1 –39

Sep-2006 32.65 575,210 –51 149 146 2 –52

Oct-2006 32.59 572,859 –198 22 145 12 –64

Nov-2006 32.38 563,357 –134 16 91 10 –48

Dec-2006 32.24 556,601 0 127 66 4 –57

Jan-2007 32.24 556,601 –94 51 79 10 –56

Feb-2007 32.14 551,711 –68 63 73 5 –53

Mar-2007 32.07 548,103 –196 16 118 10 –83

Apr-2007 31.87 537,342 –158 70 154 6 –68

May-2007 31.71 528,296 –239 43 190 6 –87

Jun-2007 31.47 514,051 –93 114 154 3 –51

Jul-2007 31.37 508,367 –48 136 165 1 –19

Aug-2007 31.32 505,402 –30 192 175 2 –45

Sep-2007 31.29 503,538 –133 59 155 1 –36

Oct-2007 31.16 495,257 –39 104 123 4 –15

Nov-2007 31.12 n/a n/a n/a n/a n/a n/a

Annual Totals

First year (Apr 2005-Mar 2006) n/a n/a 632 1,533 1,453 58 611

Second year (Apr 2006-Mar 2007) n/a n/a –1,240 932 1,543 78 –551

Calendar year 2006 n/a n/a –1,081 880 1,538 85 –337

31-month Total n/a n/a –1,349 3,182 4,111 159 -262
10 Comparison of Evaporation at Two Central Florida Lakes, April 2005–November 2007

y = 1.08665x + 2.1691

R2 = 0.993

30

25

20

15

10

4

3

2

1

0

35

30

25

20

15

250

200

150

100

50

0

0.6

0.4

0.2

0

–0.2

–0.4

–0.6

10 15 20 25 30

Lake Calm

Lake Starr

Air temperature, in degrees Celsius

y = 1.1152x + 2.6411

R2 = 0.9886

15 20 25 30 35

Lake Calm

Lake Starr

Water-surface temperature, in degrees Celsius

y = 1.0199x + 1.5653

R2 = 0.9874

0 1 2 3 4

Lake Calm

Lake Starr

Vapor pressure, in kilopascals

y = 0.8801x + 15.14

R2 = 0.9217

0 50 100 150 200 250

Lake Calm

Lake Starr

Net radiation, in watts per square meter

y = 0.519x + 0.770

R2 = 0.349

Lake Starr

Change in stored heat, in watts per square meter

y = 0.814x + 0.034

R2 = 0.876

–0.6 –0.4 –0.2 0 0.2 0.4 0.6

Lake Calm

Lake Starr

Bowen ratio

100

50

0

–50

–100

–150

–150 –100 –50 0 50 100

Lake Calm

Figure 5. Relations between energy budget parameters at the study lakes by thermal survey period.
Results 11

In addition, the relation between the two sites for this parameter 

was poor (r2 = 0.35, SE = 90 percent). Although the error in 

calculating the change in stored heat is typically high, a poor 

relation is reasonable because Lake Calm is not as large or 

deep as Lake Starr. The greater volume of water in Lake Starr 

accounts for the larger total stored heat, and, consequently, the 

larger change in stored heat (Gorham, 1964; Rouse and others, 

2005). 

The Bowen ratio, which is calculated as the ratio of the 

temperature difference to the vapor pressure difference above 

the lake surface using equation 6, is an important parameter 

that indicates how much of the net energy is partitioned to 

evaporation. Bowen ratios were similar at the two lakes during 

this study, indicating that near-surface evaporative processes 

are similar (r2

=0.88, SE = 20 percent). Negative Bowen ratios 

that occur during winter periods when the lake surface is 

cooler than the overlying air are responsible for most of the 

scatter in this relation (fig. 5). Evaporation rates calculated 

during winter periods can be problematic because of the 

effects of small temperature and vapor pressure differences 

on the Bowen ratio, which may be subject to relatively larger 

errors compared to other seasons. These higher errors are not 

detrimental to evaporation rate calculations, however, because 

evaporation rates are typically low during this time of the year. 

Water Budgets

Rainfall is the most variable water-budget term both 

spatially and temporally in Florida because of the effect 

of localized summer convective thunderstorms (Chen and 

Gerber, 1990; Sumner and Belaineh, 2005). Although evaporation is the largest loss from Florida seepage lakes, groundwater outflows are typically the next greatest in magnitude 

(Lee, 2002). Reduced lake levels caused by groundwater 

withdrawals are a problem in some areas of west-central 

Florida (Southwest Florida Water Management District, 2006). 

Determining how losses are partitioned between evaporation 

and groundwater exchanges requires accurate evaporation 

values. Although evaporation is somewhat difficult to quantify, 

groundwater fluxes are even more so, primarily because of 

potentially large errors in estimating the hydraulic properties 

of aquifers underlying lake basins. 

Rainfall, Runoff, and Pumpage

Total rainfall at Lake Starr during the study was 

3,182 millimeters (mm), with almost half of that rain falling in 

the first 12 months (table 3). Total rainfall at Lake Calm was 

lower (2,735 mm), but was more evenly distributed in time 

(fig. 6). Most of the difference in total rainfall (450 mm) during the 31-month study period was due to greater than normal 

rainfall at Lake Starr in 2005, when about 1,140 mm of rain 

fell from June through October, compared to 617 mm at Lake 

Calm. Lake Calm received more rainfall in the summer of 

2006, however, totaling 810 mm from June through October, 

compared to 570 mm at Lake Starr. Although large spatial 

variations in rainfall are characteristic of Florida’s climate, 

there is no evidence of long-term differences between rainfall 

in these two areas (Southwest Florida Water Management 

District, 2009). 

Lake Starr stage increased by more than 1 m in response 

to rainfall in the summer of 2005 and the associated net 

groundwater inflow (fig. 6). Lake Starr stage then declined 

after 2005 in response to below-normal rainfall. Lake Calm 

had less stage variation and less rise and fall, but stage in both 

lakes decreased by at least 1 m during the study period. 

Several minor water-budget components, including 

surface runoff (Lake Calm) and pumpage (Lake Starr), were 

not measured directly for the purposes of this study. For 

Lake Calm, stage increases in excess of the rainfall amount 

indicate that surface runoff from the basin can contribute 

water to the lake, but only during heavy rains (typically 

greater than 25 millimeters per hour [mm h–1]). This process 

is not evident at Lake Starr, mainly because soils in that basin 

are excessively drained (Natural Resources Conservation 

Service, 2008). In the Lake Calm basin, soils are poorly 

drained (Natural Resources Conservation Service, 2008). The 

amount of runoff at Lake Calm was estimated as the daily 

stage increase greater than 9 mm that was not accounted for 

by that day’s rainfall. Using this method, inflow from runoff 

varied from zero during most months to 39 mm in June 2006 

(table 3). Some of this increase could have been due to 

short-term groundwater inflow caused by transient water-table 

mounds (Metz and Sacks, 2002; Swancar and others, 2000). 

Lake Calm is in a hydrologic setting similar to two of the lakes 

studied by Metz and Sacks (2002) where transient mounds 

were observed that lasted up to a week. Transient mounds 

that lasted more than a day would be expected to continue to 

generate stage increases in excess of rainfall. Stage increases 

in excess of associated rainfall, and thus possibly caused by 

transient groundwater inflow, were not observed at Lake Calm 

except on days with heavy rain, however. Therefore, these 

stage increases are attributed mostly to runoff, although it is 

still likely that some transient groundwater inflow also occurs. 

Swancar and others (2007) found that a few local 

property owners pump directly from Lake Starr to irrigate 

citrus and other landscape plants. Using the method developed 

during that study, direct withdrawals were estimated for the 

purposes of this study by assuming that residents would 

irrigate when rainfall did not occur during the previous 7 days. 

Estimates of direct pumping ranged from near zero to about 

13 mm per month at Lake Starr during this study (table 3). 
12 Comparison of Evaporation at Two Central Florida Lakes, April 2005–November 2007

Figure 6. Monthly rainfall, lake stage, and net-groundwater exchange at study lakes.

100

50

0

150

200

250

300

350

400

450

Rainfall, in millimeters per month

Lake Calm

Lake Starr

EXPLANATION

A

2005 2006 2007

13.5

14.0

14.5

15.0

15.5

16.0

16.5

31.0

31.5

32.0

32.5

33.0

33.5

34.0

Lake Calm stage, in meters above NVGD 1929

–100

–50

0

50

100

150

Net groundwater exchange, in millimeters per month

Lake Starr stage, in meters above NVGD 1929

Lake Calm

Lake Starr

EXPLANATION

B

Lake Calm

Lake Starr

EXPLANATION

C

Apr.

May

Apr.

June

July

Aug.

Sept.

Oct.

Nov.

Dec.

Jan.

Feb.

Mar.

May

Apr.

June

July

Aug.

Sept.

Oct.

May

June

July

Aug.

Sept.

Nov.

Dec.

Jan.

Feb.

Mar.

Oct.

Apr. 2005

Apr. 2006

June 2005

Aug. 2005

Oct. 2005

Dec. 2005

Feb. 2006

Apr. 2007

June 2006

Aug. 2006

Oct. 2006

Dec. 2006

Feb. 2007

June 2007

Aug. 2007

Oct. 2007
Discussion 13

Groundwater Exchange

Rainfall, change in lake stage, evaporation, and other 

terms were used to calculate net groundwater exchange at the 

two lakes using equation 8 (fig. 6). Month-to-month variations 

in net groundwater exchange with Lake Starr are lower than 

those with Lake Calm, probably because Lake Starr has a 

larger contributing basin, and the unconfined aquifer beneath 

the water table under the upper parts of the basin has a larger 

capacity to store infiltrating rainwater, which can recharge 

groundwater and subsequently flow to the lake. Thus, net 

groundwater remains positive at Lake Starr from May 2005 

through the end of that year in response to rain during summer 

2005, while at Lake Calm net groundwater flow was positive 

only in June, July, and August in 2005. This pattern is partly 

due to the very large rain events at Lake Starr in 2005, but the 

same short-lived effect of rainfall on groundwater exchange 

also occurred in 2006, when rainfall was almost 250 mm more 

at Lake Calm than at Lake Starr. 

The thicker unconfined aquifer and longer flow paths 

for groundwater flowing to Lake Starr also are evident in the 

relation between monthly rainfall and net groundwater flow 

at the two lakes. Comparing the relations between monthly 

net groundwater exchange and cumulative rainfall over the 

previous 1- to 6-month periods (data not shown), the relation 

at Lake Calm is strongest between net groundwater flow 

and rainfall within a given month (r2

 = 0.73) and weakens 

as longer periods of cumulative rainfall are considered as 

predictors of monthly net groundwater exchange. This pattern 

indicates that net groundwater flow to Lake Calm occurs 

primarily during the month that rain occurs, and through 

relatively short groundwater flow paths. In contrast, there is 

no relation between net groundwater exchange and rainfall 

at Lake Starr for a given month (r2 = 0.00), and the relation 

is strongest for cumulative rainfall for the previous 6 months 

(r2 = 0.54). The relation at Lake Starr can vary, however, 

depending on the period of study. Sacks and others (1998) 

reported that the strongest relation at Lake Starr was found 

between cumulative net-groundwater exchange and rainfall 

over the previous 4 months for the period from October 1995 

through December 1996. 

Discussion

On an annual timescale, the 0.8-percent difference in 

evaporation rates between the two lakes is within the error of 

the energy-budget method and is not considered significant. 

The difference is due partly to lower net radiation measured 

at Lake Calm compared to Lake Starr. Based on a sensitivity 

analysis of the energy-budget equation, a 2-percent reduction 

in net radiation is equivalent to 1 percent less evaporation 

overall, assuming this is the only difference in the energy 

budgets of the two sites. Differences in changes in heat 

storage between the two lakes do not affect annual evaporation 

because the annual change in stored heat is near zero. 

On a monthly timescale, differences in net radiation, air 

and water temperature, and the Bowen ratio between the two 

lakes are relatively small and do not translate to significantly 

different evaporation rates. When these terms are combined 

in the energy budget, positive and negative differences appear 

to partly cancel, so that differences between evaporation rates 

show no relation to any of these individual differences. 

Variability in the change in stored heat (Qx

) appears to 

control most of the month-to-month difference in evaporation 

rates for the two lakes. Almost 70 percent of the difference 

between short-term lake evaporation at the two lakes can be 

explained by the difference in Qx (fig. 7). Sacks and others 

(1994) reported a similar result, whereby the difference in 

capacity to store and release heat was the most significant 

factor influencing the difference in evaporation rates between 

two lakes. The magnitude of the difference was greater at the 

two lakes studied by Sacks and others (1994), Lakes Barco 

and Five-O, than between Lakes Calm and Starr, because Lake 

Five-O is three times deeper than Lake Barco (average depths 

9.45 and 3.05 m, respectively). A noticeable lag in evaporation 

in the spring at Lake Five-O compared to Lake Barco was 

due to energy going into storage. The opposite was true in the 

fall, when evaporation was higher at the deeper lake as energy 

was released from storage. This seasonality is not evident at 

Lakes Calm and Starr (fig. 3), perhaps because the difference 

in mean depth between these lakes is not as great (3.05 and 

4.88 m, respectively) as it was for the lakes in Sacks and 

others (1994). 

3.00

2.50

2.00

1.50

1.00

0.50

0

0.50

–1.00

–1.50

–2.00

y = 0.022x + 0.058

R2 = 0.723

–10 40 90 140

Lake Starr – Lake Calm evaporation, in millimeters per day

Lake Starr – Lake Calm change in stored heat (Qx), in watts per square meter

Figure 7. Difference in evaporation between the two sites in 

relation to the difference between change in stored heat by 

thermal survey period.
14 Comparison of Evaporation at Two Central Florida Lakes, April 2005–November 2007

The changes in stored heat also may be responsible for 

the differences in surface-water temperature at Lakes Calm 

and Starr. As the lakes store and release heat, the energy 

exchange affects the surface temperature. The pattern of 

differences at Lake Calm and Starr is consistent with this 

process; water-surface temperatures rise more slowly in the 

first part of the year at Lake Starr, when the lake is warming, 

compared to Lake Calm. This slower temperature rise reflects 

energy going into storage. In the second half of the year when 

the lakes are cooling, water-surface temperatures drop more 

slowly at Lake Starr, reflecting energy lost from storage. 

Differences in meteorological conditions that might 

be due to the difference in location of the two lakes are not 

evident from this study. Because Lake Calm is the closest to 

the Gulf of Mexico, it was hypothesized that air and water 

temperatures might be moderated by the Gulf, and that this 

might lead to reduced evaporation, or at least reduced monthto-month variability in evaporation. Neither of these effects 

was observed in the data from these two sites beyond potential 

measurement errors. Another difference that might be related 

to location is rainfall, which is generally lower closer to the 

Gulf because convective summer thunderstorms build as the 

air masses move inland. Lower rainfall at Lake Calm may 

have been partially attributable to this effect, at least in the 

short term. Solar radiation and net radiation should be higher 

closer to the coast because of reduced cloud cover, but solar 

radiation at Lake Calm was, on average, within a few percent 

of Lake Starr and net radiation was 7 percent lower. Even 

though Lake Calm is closer to the Gulf than Lake Starr, it is 

about 20 kilometers (km) inland, so coastal effects may not 

have been as large as hypothesized. 

There may be limitations to this analysis because of 

differences in instrumentation between lakes. Even though 

standard instruments were used and calibrated before and after 

data collection, substantial differences are sometimes observed 

between sensors, particularly those measuring radiation 

(Blonquist and others, 2009; Brotzge and Duchon, 2000; 

Cobos and Baker, 2003). The lack of measurements over the 

center of Lake Calm may have obscured differences in air 

temperature and relative humidity between that site and Lake 

Starr. Most differences in meteorological parameters between 

the two sites were within, or comparable to, the magnitude of 

the measurement errors or the scatter in the relations between 

the variables. 

Conclusions

Differences in short-term evaporation rates between 

the two lakes were mostly due to differences in the change 

in stored heat; the deeper lake (Lake Starr) had about twice 

the change in stored heat as the shallower lake (Lake Calm). 

Annual evaporation from the two lakes was essentially the 

same, however, and the 0.8-percent difference was well within 

the error of the energy-budget method. 

Seasonal evaporation rates measured at Lake Starr should 

be directly comparable to those of other lakes in central 

Florida having similar depths, within an assumed 10-percent 

error in monthly evaporation for the energy-budget method. 

Comparison to shallower lakes in the same region that do 

not store and release heat energy through the seasons may 

generate a small bias during spring and fall, but annual rates 

will likely be of similar magnitude. Differences in air temperature, lake temperature, relative humidity, net radiation, and 

evaporation between the two lakes were all within 10 percent, 

about the same scale as measurement errors and less than the 

standard errors of relations between variables. 

Although the temporal rainfall distribution differed 

between the two lakes and there was about 540 millimeters 

less rain at Lake Calm compared to Lake Starr, stage at both 

lakes decreased by at least 1 meter during the course of the 

study (April 2005 through October 2007). The Lake Starr 

basin, which contains well-drained sand ridges that can store 

water during the rainy season, contributes water to the lake 

through groundwater exchange for up to 6 months after the 

rainfall occurs. Rainfall in the Lake Calm basin generates 

groundwater inflow to the lake that contributes to the lake 

volume primarily during the same month that it occurred. The 

Lake Calm basin does not have the same capacity to store 

shallow groundwater as the Lake Starr basin, and during heavy 

rainfall, runoff is observed at Lake Calm but not at Lake Starr. 

Net groundwater exchange at these two lakes is primarily 

controlled by basin characteristics. 

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Winter, T.C., Buso, D.C., Rosenberry, D.O., Likens, G.E., 

Sturrock, A.M., Jr., and Mau, D.P., 2003, Evaporation 

determined by the energy-budget method for Mirror Lake, 

New Hampshire: Limnology and Oceanography, v. 48, 

p. 995–1009. 
Manuscript approved April 14, 2015

For further information about this publication contact:

Director 

Caribbean-Florida Water Science Center

4446 Pet Lane, Suite 108

Lutz, FL 33559

(813) 498-5000

Or visit the Carribean-Florida Water Science Center

website at 

fl.water.usgs.gov

Prepared by the Raleigh Publishing Service Center
Swancar—Comparison of Evaporation at Two Central Florida Lakes, April 2005–November 2007—Open-File Report 2015–1075

ISSN 2331-1258 (online)

http://dx.doi.org/10.3133/ofr20151075