A network of nine sites provides evapotranspiration (ET) and related meteorological data at locations in the Everglades representative of the natural Everglades system in terms of plant communities, duration of water inundation, and geographic extent. Eight of the nine sites have been in operation since December 1995; the ninth site was added in January 1997 to expand coverage in drier parts of the Everglades. Site locations and other details including study objectives, methodology, and timeframe are described in a previous publication (German, 1996).

Daily ET values have been estimated for five of the nine sites through March 1997 and annual ET values have been compared among the sites. One site is in a slough that has sparse, emergent vegetative and lily-pad coverage. Three sites are in perennially wet sawgrass prairies; one of the three sites (P33) has relatively sparse sawgrass coverage and the other two sites have sawgrass coverage of medium height and density. The fifth site (Old Inghram Highway) has a relatively sparse coverage of rushes and is dry for several months each year. Preliminary estimates of ET at the five sites for 1996 ranged from 54.4 in. at the slough station to 42.1 in. at the Old Inghram Highway site. Among the perennially wet sites, ET ranged from 48.0 to 52.3 in. during 1996. The low ET determined at the relatively dry Old Inghram Highway site indicates that water level probably is more important than vegetative cover type in determining ET. Locations where water levels are always above land surface probably will have ET rates that are largely a function of solar energy input and not greatly affected by type or density of vegetative cover.

Net solar radiation is the most significant term in the energy budget and, thus, in the determination of ET. However, at times, the heat stored in water can contribute significantly to ET for a few hours or days. This generally occurs during the passing of cold fronts, when heat stored in the surface water is released partially as latent heat. The presence of surface water also can moderate the transformation of solar energy to latent heat (ET) by storing energy during the day (as the water is heated) and releasing the energy at night, partly by evaporation, as the water cools. An example of the effect on ET from the passing of a cold front is evident in the data from site P33. The net solar radiation at site P33 on February 4, 1996, averaged 86.4 watts/m2. The surface water, in cooling from a mean temperature of 23.7 C on February 3 to a mean temperature of 19.5 C on February 4, released heat energy at an average rate of 140 watts/m2. The net result of this water cooling was to boost the evaporation rate from 0.11 in. on February 3 to 0.21 in. on February 4. Further cooling on February 5 contributed to an ET of 0.16 in. The next day, water cooling was nearly complete and ET decreased to a more seasonal 0.09 in. An example of the moderating effect of surface water on ET in a daily timeframe is illustrated by data from the P33 site in July 1996. At this site, an average of 47 percent of ET occurred during the 5-hour period from 1000 to 1500 during July 1996, whereas an average of 70 percent of the daily net solar radiation occurred during the same time period.

Use of the Priestley-Taylor model of ET was investigated for the purpose of filling in periods of missing ET data at existing sites. The Priestley-Taylor model expresses ET as a function of temperature-dependent atmospheric properties, available energy, and a semi-empirical coefficient (Priestley-Taylor coefficient). The theoretical Priestley-Taylor coefficient is 1.26 for a free water surface or a dense well-watered plant canopy; sites with lower water availability will have lower coefficients.

The Priestley-Taylor model was fitted to daily ET for the 1996 calendar year for the five sites where preliminary estimates of daily ET have been made. The model explains at least 90 percent of the variation in daily ET at each site. The Priestley-Taylor coefficient ranged from 0.97 to 1.03 at the three perennially wet sawgrass prairie sites, was 0.80 at the partially dry site (Old Ingraham Highway) and was 1.19 at the slough site. The difference between the coefficients at the vegetated sites and the open-water site (slough) indicates that the presence of aquatic vegetation tends to decrease ET relative to rates that occur at less-vegetated, open-water sites for a selected level of net radiation. However, net radiation at a given solar intensity appears to be lower at the open-water site than at the perennially wet vegetated sites, possibly because of a greater amount of reflection and long-wave radiation from the water surface. The net effect of the higher Priestly-Taylor coefficient and lower net radiation at the open-water site may be to make ET at the open-water site the same as that occurring at some perennially wet, vegetated sites.

REFERENCE

German, E.R., 1996, Regional evaluation of evapotranspiration in the Everglades: U.S. Geological Survey Fact Sheet FS-168-96, 4 p.

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