Evapotranspiration from a cypress and pine forest subjected to natural fires, Volusia County, Florida, 1998-99

Daily values of evapotranspiration from a watershed in Volusia County, Florida, were estimated for a 2-year peri od (January 1998 through December 1999) by using an energy-budget variant of the eddy correlati on method and a PriestleyTaylor model. The waters hed consisted primarily of pine flatwood uplands interspersed within cypress wetlands. A drought-induced fire in spring 1998 burned about 40 percent of the watershed, most of which was subsequently logged. The model reproduced the 449 measured values of evapotranspiration reasonably well (r 2=0.90) over a wide range of seasona l and surface-cover conditions. Annual evapotranspiration from the watershed was estimated to be 916 millimeters (36 inches) for 1998 and 1,070 millimeters (42 inches) for 1999. Evapotranspiration declined from near potential rates in the wet conditions of January 1998 to less than 50 percent of potential evapotranspiration after the fire and at the peak of the drought in June 1998. After the drought ended in early July 1998 and water le v ls returned to near land-surface, evapotranspiration increased sharply; however, the evapotranspiration rate was only about 60 percent of the poten ial rate in the burned areas, compared to about 90 percent of the potential rate in the unburned areas. This discrepancy can be explained as a result of fire damage to vegetation. Beginning in sp ring 1999, evapotranspiration from burned areas increased sharply relative to unburned areas, some times exceeding unburned evapotranspiration by almost 100 percent. Possible explanations for the dramatic increase in evapotranspiration from burned areas could include phenological chang es associated with mat uration or seasonality of plants that emerged after the fire or successional ch anges in composition of plant community within burned areas. Variations in daily evapotranspiration are primarily the result of variations in surface cove net radiation, photosyntheti cally active radiation, air temperature, and water-table depth. A water budget for the watershed supports the validity o the daily measurements and estimates of evapo transpiration. A water bu dget constructed using independent estimates of average rates of rainf runoff, and deep leakage, as well as evapotranspiration, was consistent with n 3.8 percent. An alternative water budget constructed using evapotra piration estimated by the standard eddy correlation method was consistent only within 9.1 percent. This result indicates th at the standard eddy correlation method is not as accurate as the energy-budget variant.