Variation of Evaporation from Florida Bay
William K. Nuttle
Consultant, Ottawa, Ontario, Canada
René M. Price
Florida International University, SERC and Dept. of Earth Sciences, Miami, FL
Peter K. Swart
University of Miami, RSMAS-MGG, Miami, FL
Any attempt to evaluate the effects of hydrologic restoration on Florida Bay’s
ecosystem must also account for the natural variation in evaporation and rainfall
over the Bay. The current goal for restoration will reduce direct runoff into the
northeast portion of the Bay by about 35 percent, based on comparison of D13R
to 95 Base scenarios. On the basis of an average year, the amount of fresh water
entering directly from the Everglades is only a fraction of rainfall and
evaporation, Figure 1. Seasonal and year-to-year variation in direct evaporation
and rainfall overshadow the influence of runoff on salinity variation – a key
characteristic that influences the distribution and composition of plant and animal
communities (Nuttle et al. 2000). Therefore, patterns of rainfall and evaporation
must be characterized well before we can evaluate the consequences of
anticipated changes in Everglades’ hydrology on the Florida Bay ecosystem.
This study characterizes variation of evaporation from Florida Bay over time and
space. Little is known about this important component of the water balance, and
the information that is available covers only a short period of time, i.e. Smith
(2000), Pratt and Smith (1999). By comparison, fairly complete rainfall records
exist for eight locations in the Bay spanning about ten years. This study employs
several methods to estimate the average rate of evaporation from Florida Bay and
characterize its variation. The Priestly-Taylor method estimates evaporation as a
component of the thermal energy budget, and the Dalton Law method estimates
evaporation as a vapor flux. Data on the energy balance and vapor flux were
collected at platforms in the west (Rabbit Key) and the east (Butternut Key).
Additional information on the spatial variation in the radiation budget was
collected during two synoptic surveys, in June 2001 and in January 2002. Finally,
a mass balance model for salinity was applied to estimate average monthly
evaporation from an analysis of salinity, rainfall, and runoff data for the period
1993 through 1999.
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Estimates of annual evaporation rates range between 125 cm to 169 cm, Table 1.
The Priestly-Taylor method resulted in rates of 169 cm/yr for Rabbit and
163cm/yr for Butternut. Rates of 155 cm/yr for Rabbit and 133 cm/yr for
Butternut were obtained using Dalton Law developed by Satori (2000). These
annual rates of evaporation are about 30% higher than the box model estimate of
125 cm/yr, bay wide.
From analysis of salinity patterns for the period 1996 through 2001, we estimate
annual evaporation to be 125 cm/yr, slightly less than combined input by rainfall
and direct runoff. Results with the box model are most sensitive to uncertainty in
estimated rainfall over the Bay, and to a lesser degree, uncertainty in estimated
average water depth and freshwater runoff.
Table 1. Summary of Monthly Rainfall and Evaporation Rates in cm.
Month
Rabbit Rabbit
Butternut Butternut
Box Model
Vapor Priestly- Vapor
PriestlyFlux
Taylor
Flux
Taylor
Jun 2001
16.8
21.01
6.22
19.98
14.81
Jul 2001
16.95
20.21
6.59*
6.11*
16.06
Aug 2001
17.33
19.96
13.77
15.8
15.80
Sep 2001
14.94
14.9
12.73
14.59
14.10
Oct 2001
12.6
16.4
12.14
12.27
11.42
Nov 2001
10.44
14.6
9.19
9.27
8.47
Dec 2001
9.25
10.3
9.18
8.46
6.05
Jan 2002
7.7
13.7
7.36
10.13
4.80
Feb 2002
9.68
11.1
9.84
10.14
5.06
Mar 2002
4.65
1.94
11.51
15.08
6.76
Apr 2002
16.4
9
17.05
20.51
9.44
May 2002
18.71
16.64
17.65
21.44
12.39
Total
155.45 169.76
133.22
163.79
125.24
*Data missing from 12 July to 7 August 2001
Evaporation varies seasonally, between 5-9 cm per month in the winter and 16-21
cm per month in the summer, coefficient of variation (c.v.) ~ 0.32. By
comparison, variation in rainfall, c.v. 0.85, is larger and dominates the seasonal
variation in net freshwater supply to the Bay’s water column, Figure 1.
The seasonal pattern of evaporation estimated both by the box model and the
Dalton Law method follows the seasonal pattern of thermal loading to the Bay’s
waters by solar radiation. This supports the hypothesis that solar radiation drives
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seasonal variation in evaporation, a finding that is consistent with the results
obtained by German (2000) in his study of evaporation in the Everglades.
Components of the thermal radiation budgets measured at the two monitoring
locations are similar, and there was little difference in monthly averaged values
between two years of radiation measurements. The synoptic surveys found
differences in albedo related to characteristics of the water column, similar to
measurements reported previously by Stumpf et al. (1999). However, it does not
appear that this contributes to significant variation in the rate of evaporation.
The rate of evaporation over the extensive, shallow banks may be different than
our estimates reported above. Neither our instrumented platforms nor the
synoptic radiation survey characterize conditions over shallow, seasonally
exposed banks. Although we expect that the radiation forcing over the banks will
be the same as elsewhere, the different characteristics of the water column
(shallow) and bottom (dense sea grass) might affect how outgoing heat flux is
partitioned between long wave radiation, conduction, and evaporation.
Figure 1: Evaporation is the largest single flux of freshwater in Florida Bay.
Direct runoff into the northeastern portion of the Bay is a small component of the
water balance, but it accounts for about a third of the net freshwater supply in late
summer. Evaporation rates from box model.
Monthly Water Flux (cm)
20
Rainfall
10
Runoff
0
Net
-10
Evaporation
-20
0
2
4
6
8
10
12
Month
William, Nuttle, 11 Craig St., Ottawa, Ontario, Canada, K1S 4B6, phone: 613233-4544, wnuttle@eco-hydrology.com, Question 1.
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