U.S. Department of the Interior
U.S. Geological Survey
FS-168-96

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 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.

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.

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 (lE) 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 continuous measurement of actual ET is available. The method has been previously used at other locations in Florida (Bidlake, 1993). When using this method, R_n and G are monitored at 15 minute intervals using thermopile and thermocouple sensors. The difference between R_n and G is the energy transported upward by H and lE, but the relative magnitudes of H and lE cannot be determined from energy-budget data alone. The Bowen ratio ( B) is the ratio of H to lE. It can be shown that B is also a function of measured vertical differences of temperature and vapor pressure in the air, or

where g is a weak function of air temperature and barometric pressure (roughly a constant), DT is the temperature difference between two points at different heights above the land surface, and De 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:

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 DT and De. 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 successive 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 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 below:

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. 2).

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.

Bidlake, W.R., Woodham, W.M., and Lopez, M.A., 1993, Evapotranspiration from areas of native vegetation in west-central Florida: 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, 4p.

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For more information contact:

Edward R. German
U.S. Geological Survey
224 West Central Parkway
Suite 1006
Orlando, FL 32714
407-865-7575
FAX 407-865-6241