Estimates of Evaporation Using Stable Isotopes in Florida Bay
Peter K. Swart
University of Miami, RSMAS-MGG, Miami, FL
René M. Price
Florida International University, SERC and Dept. of Earth Sciences, Miami, FL
William K. Nuttle, Consultant, Ottawa, Ontario, Canada
Tom Lee
University of Miami, RSMAS-MPO, Miami, FL
As the amount of freshwater entering Florida Bay directly from the Everglades is only a
fractionation of amount derived from precipitation and lost through evaporation,
knowledge of the variations in these parameters is important so that anticipated changes
in the hydrological system of the Everglades can be evaluated. While precipitation can
be directly measured using a variety of methods, the study of variations in evaporation is
more enigmatic. We are investigating evaporation using a variety of different methods
including evaporation pans, energy budget-Priestly-Taylor method, vapor-flux-Dalton
Law Formula, a mass balance model of salinity, and a mass balance of the stable isotopes
of oxygen and hydrogen.
7.0
70.0
60.0
50.0
3.0
40.0
1.0
30.0
Salinity
Oxygen Isotopic Composition
5.0
-1.0
20.0
-3.0
Oxygen
10.0
Salinity
-5.0
Aug-92
Jul-93
Jul-94
Jul-95
Jun-96
Jun-97
Jun-98
Jun-99
May-00
May-01
May-02
0.0
Apr-03
Date
Figure 1:Monthly oxygen isotopic and salinity data (from FIU) for Whipray basin in Florida Bay
between October 1993 and July 2002. The trend lines through the data represent four month running
means of the data.
In this study we present the first results of a stable isotope (oxygen and hydrogen) mass
balance approach applied to Whipray basin in central Florida Bay.
This basin has a mean18O of +2 ‰ but varies seasonally between +5 and approximately
0 ‰ as a result of evaporation and dilution by runoff and rainfall. The stable H and O
isotopic composition of a closed water body increases as a result of evaporation,
eventually reaching a steady state value dependent upon the temperature, relative
humidity and the isotopic composition of the atmosphere (Figure 2). As any basin within
Florida Bay is dynamic, in order to model the evaporation of this basin using the stable
oxygen isotopic composition of any particular month we need to know the isotopic
50.0
40.0
30.0
20.0
Hydrogen
10.0
0.0
-2.0
-1.0
0.0
-10.0
1.0
2.0
3.0
4.0
5.0
-20.0
-30.0
Oxygen
Figure 2: Relationship between the oxygen and hydrogen isotopic composition of waters from
Whipray Basin between 1994 and 2003 (Figure 1) relative to the Meteoric Water Line (MWL).
Deviations below the MWL indicate evaporation. Increasing deviation from the MWL reflects
evaporation into environments of decreasing relative humidity.
composition of the inputs and estimate the approximate flux across the boundaries into
adjacent boundaries. The isotopic composition of the adjacent boundaries can be taken
as the values measured in adjacent basins, Rankin Lake to the west, Twin Key Basin to
the South, Terrapin Bay to the east, and the Everglades to the north. Employing the
equations outlined by Gonfiantini (Handbook of Environmental Isotope Geochemistry,
Fritz and Fontes, 1986), we can attempt to estimate the fraction of water lost by
evaporation (x) according to the following equation.
x  s  I / A  sB  (s  I )(1  h   ) /(s  1)(    /  )  h(a  s )
In this equation s, I, and a represent the isotopic composition of the basin, the
incoming water, and the atmospheric water vapor respectively. The terms A and B are
related to the water balance in the area and are controlled by temperature and relative
humidity, h= relative humidity, and kinetic enrichment factor. In this presentation
we present model solutions to this equation using various estimates of the isotopic
composition of the input fluid and variations in temperature, relative humidity, and the
isotopic composition of atmospheric water vapor.
QP
QE
P
Everglades
Q1O
Q4l
Q2O
S
Whipray Basin
Q2I
Q4O
2l
Q3l
? G
Q1I
Terrapin Bay
Rankin Lake
4l
E
1l
Q3O
Twin Keys Basin
3l
QG
Figure 3: Schematic mass balance of water flow into and out of Whipray Basin with
associated isotopic compositions. The values Q represent the fluxes into and out of Whipray
Basin (QP=precipitation, QE=evaporation, and QG=potential groundwater input), while the 
values represent the isotopic compositions. A mass balance can be constructed using a
combination of the fluxes, salinity, and stable O or H isotopic composition. Simplification
of the mass balance yields the equation presented in this abstract.
Peter K. Swart, University of Miami, RSMAS/MGG, 4600 Rickenbacker Causeway,
Miami, Fl, 33149, Phone: 305-361-4103, Fax: 305-361-4632.
pswart@rsmas.miami.edu, Question 1