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

What and Why

Understanding the water budget 

of the Everglades system is crucial to 

the success of restoration and management strategies. Although the 

water budget is simple in concept, it is 

difficult to assess quantitatively. Models used to simulate changes in water 

levels and vegetation resulting from 

management strategies need to accurately simulate all components of the 

water budget.

One of the most important components of the Everglades water budget is evapotranspiration (ET). ET is 

water removed from the surface and 

soils by direct evaporation and plant 

transpiration. In South Florida, ET 

rates may exceed 40 in/yr (inches per 

year) on the average; during dry 

years, the ET could exceed rainfall 

(about 50 in/yr). Thus, most of the 

water that falls on the land surface as 

rainfall is returned to the atmosphere 

by ET. Despite the importance of ET 

in the Everglades water budget, our 

knowledge of ET is, at present, only 

semi-quantitative. Recent advances 

in instrumentation and measurement 

techniques have made it possible to 

continuously measure ET, so that an 

accurate evaluation of ET in the Everglades can be made.

 In 1995, a study to measure and 

model ET in the Everglades (fig. 1) 

was begun as part of the South Florida 

Ecosystem Program (McPherson and 

others, 1995). The principle objective 

of the study is to develop an understanding of ET within the Everglades 

drainage unit, excluding forested agricultural and brackish environments. 

To achieve this, a network of eight 

ET-measurement sites was established, representing the various types 

of hydrologic and vegetative environments. Continuous measurement of 

ET at these sites for at least a 2-year 

period (October 1995 through September 1997) will be used to develop 

regional models of ET that can be 

used to estimate ET at other times 

throughout the Everglades.

This fact sheet describes the 

basic principles of ET measurement, 

and the locations and features of the 

ET stations operated as part of this 

study. 

Figure 1. The Everglades and locations of evapotranspiration (ET) stations.

0

0

10

10

20

20

30 MILES

30 KILOMETERS

Everglades 

National Park

Lake 

Okeechobee

N

1,2

4

3 

8

6

7

5

G

ULF

OF MEXICO

ATLANTIC

OCEAN

Everglades system

Loxahatchee National

Wildlife Refuge

Canal

Everglades National Park boundary

Water Conservation Area boundary

Identifier for Water Conservation Area

Evapotranspiration measurement site

Big Cypress

National Preserve

EXPLANATION

5

Location

Map

FLORIDA

How

Methods of measuring evapotranspiration generally are divided 

into two categories: water budgeting 

and energy budgeting. The former 

method relies on accurate accounting 

of water flow into and out of a control 

area. Accurate determination of flow 

components is difficult in much of the 

Everglades due to low hydraulic gradients and sheet flow conditions. 

Therefore, water budgeting was not 

used in this study.

Regional Evaluation of 

Evapotranspiration 
Two types of stations are being 

used to monitor the components of the 

energy budget. One type, shown in 

figure 3, is used at locations where 

plant transpiration could be an important part of the total ET. This type of 

station features a pair of movable air 

temperature and humidity sensors for 

measuring ∆T and ∆e. These sensors 

are mounted on an exchange mechanism so that the sensor positions are 

reversed every 15 minutes. This 

reversal of position makes it possible 

to eliminate the effect of sensor bias 

on the difference measurements and is 

necessary to measure accurately the 

small differences in temperature and 

humidity that occur over the 3 - to - 

5-ft vertical distance. The effect of 

sensor bias is removed from the 30-

minute average by simply averaging 

the differences measured during two 

succesive 15-minute intervals.

The other type of monitoring station is much simpler in operation and 

is used where permanent areas of 

open water occur with little emergent 

vegetation, so that plant transpiration 

is an insignificant part of the total ET. 

At such open-water stations, the same 

Bowen-ratio principle is used to partition the energy flux into convective 

and evaporative components. However, the temperature and vapor pressure differences can be measured 

from water to air, rather than within 

the air. Since the water-to-air differences are much greater than differences in the air over similar distances, 

the effect of air and vapor pressure 

sensor bias is insignificant. Therefore, 

the sensor exchange mechanism is not 

required, and only one vapor pressure 

sensor is needed. Vapor pressure at 

the water surface can be calculated 

from the water temperature.

Data are recorded at 15-minute 

intervals at all of the stations used in 

vegetated areas, and at 30-minute 

intervals at the open-water stations. 

Some of the types of data recorded at 

the ET stations are not used in 

 Energy budget methods rely on 

an accurate accounting of energy to 

and from the land surface and generally assume one-dimensional energy 

flow. Because the energy needed to 

evaporate water is relatively large 

(about 580 calories per gram), ET, or 

latent heat, can represent a significant 

component of the energy budget 

(fig. 2). Net radiation (Rn) is the difference between incoming shortwave 

(solar) radiation and outgoing shortwave and longwave radiation, sensible heat (H) is the energy transported 

from the land surface by air movement (convection), latent heat (λE) is 

the energy transported away from the 

surface by evaporating water, and 

soil/water heat (G) is the heat that 

goes into changing the temperature of 

the soil or the water standing on the 

land surface. Any of these components may reverse in direction—for 

example, latent heat is added to the 

surface by condensation (dew formation), and net radiation is directed 

upward at night.

 The Bowen-ratio energy budget 

method (Bowen, 1926) was selected 

for use in the Everglades because relatively inexpensive instrumentation for 

Rn H λE

Net radiation Sensible heat Latent heat

Soil/water heat

Rn The energy budget: - G = H + λE

Sawgrass Water or 

ground level

G

Figure 2. Energy budget during daytime heating.

continuous measurement of actual ET 

is available. The method has been previously used at other locations in 

Florida (Bidlake, 1993). When using 

this method, Rn and G are monitored 

at 15-minute intervals using thermopile and thermocouple sensors. The 

difference between Rn and G is the 

energy transported upward by H and 

λE, but the relative magnitudes of H 

and λE cannot be determined from 

energy-budget data alone. The 

Bowen ratio ( Β) is the ratio of H to 

λE. It can be shown that Β is also a 

function of measured vertical differences of temperature and vapor pressure in the air, or 

Β = γ x ∆T / ∆e

where γ is a weak function of air temperature and barometric pressure 

(roughly a constant), ∆T is the temperature difference between two 

points at different heights above the 

land surface, and ∆e is the vapor pressure difference between the same two 

points. The energy budget can then 

be written in terms of the Bowen ratio 

to solve directly for latent heat, as:

λE = (Rn - G) / (1 - Β).
determination of ET but are needed in 

models of ET, or in general interpretation of the data. Multiple sensors are 

used for soil data because of the heterogeneous nature of soils, and the average of the soil heat fluxes is used in the 

energy balance. A summary of the 

types of data that are recorded is given 

in the table to the right of this column.

Where

The chief consideration for 

selecting the ET sites was to have a 

network of sites representative of the 

ecosystems of the Everglades system 

in terms of plant communities, duration of water inundation, and geographic coverage. Other factors were 

security and logistics. Sites in areas 

that are open to hunting and air boating were located in relatively remote 

locations and not on major air boat 

trails. In the following list, map numbers correspond to numbers on the 

location map (fig. 1).

Figure 3. Evapotranspiration (ET) site 7.

Data type

Number of sensors at each site

Vegetated sites Open-water sites

Rainfall 1 1

Wind direction 1 1

Wind velocity 1 1

Incoming solar radiation 1 1

Net solar radiation 2 1

Water level 1 1

Soil temperature 3 0

Soil heat flux 3 0

Water temperature 3 3

Air temperature 2 1

Moisture content of air 2 1

Moisture content of soil 3 0
Fact Sheet FS-168 –96 July 1996

Map number Site name Area1

1

Enr, Everglades Nutrient Removal project area; Lox, Loxahatchee National Wildlife Refuge; WCA, Water Conservation Area; 

Enp, Everglades National Park

Community Lat-long Comments

1 Enr - cattail Enr Cattails 263910 0802432 Never dry

2 Enr - open Enr Open water 263740 0802612 Never dry

3 Site 7 Lox Open water 263120 0802013 Never dry

4 F4 WCA2 Dense sawgrass 261900 0802307 Dry part of some years

5 Camp23 WCA3 Medium sawgrass 261541 0804356 Dry part of most years

6 Nesrs3 Enp Medium sawgrass 254450 0803007 Never dry

7 P33 Enp Sparse sawgrass 253655 0804211 Never dry

8 Old Inghram Hw Enp Sparse rushes 252112 0803807 Dry part of every year

When

The total length of the ET evaluation project is 

5 years. The project began in fiscal year (FY) 1995 with 

site selection and equipment installation. Data collection will continue through FY 1996 and 1997. Development of site models and a regional model of ET will 

proceed through FY 1998. The study will end in FY 

1999.

References

Bidlake, W.R., Woodham, W.M., and Lopez, M.A., 1993, 

Evapotranspiration from areas of native vegetation in 

west-central Flordia: U.S. Geological Survey Open-File 

Report 93-415, 35 p.

Bowen, I.S., 1926, The ratio of heat losses by conduction and 

by evaporation from any water surface: Physical 

Review, 2nd series, v. 27, no. 6, p. 779-787.

McPherson, B.F., Higer, A.L., Gerould, Sarah, and Kantrowitz, I.H., 1995, South Florida ecosystem program of the 

U.S. Geological Survey, FS-134-95, 4 p.

For more information on this and related studies, contact:

Edward R. German John Vecchioli, District Chief

U.S.Geological Survey U.S. Geological Survey

224 W. Central Parkway 227 N. Bronough St.

Suite 1006 Suite 3015

Altamonte Springs, FL 32714 Tallahassee, FL 32301

(407) 865-7575 (904) 942-9500

egerman@usgs.gov jvecchio@usgs.gov

—By Edward R. German