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Scientific Investigations Report 2011–5212

Prepared in cooperation with the South Florida Water Management District as part of the U.S. Geological Survey Greater Everglades Priority Ecosystems Science Program

Evapotranspiration over Spatially Extensive Plant Communities in the Big Cypress National Preserve, Southern Florida, 2007–2010

By W. Barclay Shoemaker, Christian D. Lopez, and Michael J. Duever

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Evapotranspiration (ET) was quantified over plant communities within the Big Cypress National Preserve (BCNP) using the eddy covariance method for a period of 3 years from October 2007 to September 2010. Plant communities selected for study included Pine Upland, Wet Prairie, Marsh, Cypress Swamp, and Dwarf Cypress. These plant communities are spatially extensive in southern Florida, and thus, the ET measurements described herein can be applied to other humid subtropical locations such as the Everglades.

The 3-year mean annual ET was about 1,000, 1,050, 1,100, 930, and 900 mm (millimeters) at the Dwarf Cypress, Wet Prairie, Cypress Swamp, Pine Upland, and Marsh sites, respectively. Spatial differences in annual ET were considerable due to the recovery of the Marsh site from extensive forest fire and drought conditions. Temporal variability in annual ET was relatively small at sites that were well watered (Dwarf Cypress, Wet Prairie, Cypress Swamp, Pine Upland) over the 3-year study. In other words, locations that were well watered appeared to have similar annual ET rates.

Diurnal water-level variability was observed in response to ET and was less at flooded sites than at dry sites. For example, surface-water levels declined about 1.5 mm in response to ET at the flooded Cypress Swamp site during July 18-22, 2008 and declined about 10 mm in response to ET at the flooded Dwarf Cypress site from April 18-27, 2008. Specific yield was computed using ET estimates and diurnal water-level variability measured at the dry Pine Upland site as a check on the accuracy of the eddy covariance method. Water levels repeatedly dropped about 15 mm on average in response to ET at the Pine Upland site from April 27 to May 4, 2008. ET was about 3 mm on each of these days, resulting in a reasonable estimate for specific yield of 0.2 for the sandy soils at the Pine Upland site. 

Monthly ET estimates exhibited seasonal variation. ET was generally greatest between March to October when solar radiation was relatively large, and least from November to February when solar radiation was small. Monthly ET was greatest in the spring and summer at the Cypress Swamp site, reaching rates as large as 140 mm. The large ET rates at this site coincide with the most active period of cypress growth during late spring and early summer. Cypress trees begin to senesce in late summer reducing transpiration.

Net radiation and available energy explained most of the variability in ET observed at all five sites. Mean annual and monthly net radiation varied among the sites in response to cloud cover and the albedo of the land surface and plant community. Net radiation was greatest at the Cypress Swamp site, averaging about 130 W/m2 (watts per square meter) during the 3-year study. Net radiation was generally less at the Dwarf Cypress site, averaging about 115 W/m2 over 3 years. The Dwarf Cypress site apparently has the largest albedo, which likely is due to the sparse canopy and a highly reflective, calcareous, periphyton-covered land surface. Furthermore, mean annual net radiation was least in the first year of the study, which likely was due to greater cloud cover during a relatively wet year. In contrast, net radiation was greatest in the second year of the study, which likely was due to less cloud cover during a relatively dry year.

First posted December 13, 2011

For additional information contact:
Barclay Shoemaker
USGS Florida Water Science Center
7500 SW 36th Street
Davie, Florida 33314
Phone: 954-377-5956

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Suggested citation:

Shoemaker, W.B., and Lopez, C.D., and Duever, Michael, 2011, Evapotranspiration over spatially extensive plant communities in the Big Cypress National Preserve, southern Florida, 2007–2010: U.S. Geological Survey Scientific Investigations Report 2011–5212, 46 p.




Purpose and Scope

Description of Study Area

Plant Communities and Vegetation Classification

Previous Studies

Methods for Measurement of Evapotranspiration

Conceptualization of the Surface-Energy Budget

Eddy Covariance Method

Source Area and Site Selection

Instrumentation and Site Maintenance

Priestley-Taylor Evapotranspiration Models

Quantifying Evapotranspiration in the Big Cypress National Preserve

Application of Evapotranspiration Models

Data Availability for Model Calibration

Error Statistics and Model Efficiency

Factors Limiting Evapotranspiration Rates

Model Bias

Annual Water- and Energy-Budget Calculations


Net Radiation


Air Temperature

Latent-Heat Flux

Sensible-Heat Flux

Bowen Ratio


Evaporative Fraction

Available Water

Energy-Budget Closure

Monthly Water- and Energy-Budget Calculations



Available Water

Soil Volumetric Water Content

Net Radiation

Air Temperature

Water and Energy Budget Calculations

Diurnal Water-Level Variability

Available Energy

Seasonality in Evapotranspiration

Spatial and Temporal Variability in Annual Evapotranspiration

Variations in Surface-Energy Fluxes during an Extreme Cold Front


References Cited

Appendix 1

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