EVAPORATION OF THE OKEFENOKEE SWAMP: CALCULATIONS BASED ON STABLE ISOTOPE MASS BALANCE

Evaporation from terrestrial environments is modeled to account for 14% of the global water flux to the atmosphere (Schlesinger, 1997). Large, open bodies of freshwater are of particular interest because their evaporation affects not only regional hydrological cycles, but also regional weather phenomena (Gat et al., 1994). Stable isotope abundances of D/H and ¹⁸O in water have been measured and modeled at the regional scale for the entire U.S. (Bowen and Wilkinson, 2002) and the values are thought to reflect temperature, relative humidity and kinetic isotopic effects during evaporation (Kendall and Coplen, 2001). In order to constrain evaporation from the Okefenokee Swamp, the second largest wetland in the U.S. (1727² km), we collected 62 water samples from sites spanning 14 kilometers of the main canal of the swamp. Samples were taken every 400 meters during travel from the East Entrance to the Canal Run Shelter. The sampling route was designed to represent a transect across the deepest water portion of the swamp (36 samples), streams flowing into the canal (14 samples) and shallower areas connected to the canal (i.e., ponds and prairies; 15-45 cm deep; 12 samples).

Stable isotopic analyses showed that both the oxygen and the hydrogen isotope composition increased markedly downstream from the entrance of the canal to the farthest point sampled. Within the canal the δ¹⁸O values ranged between -2.1‰ and +2.3‰ and δD values ranged between -13‰ and +6‰. The highest δ¹⁸O and δD values of the transect (δ¹⁸O = +4.5‰; δD = +14‰) occurred approximately 4.5 km from the east entrance to the canal, within the Mizell Prairie. Oxygen and hydrogen isotope compositions of our water samples comprise a line that deviates clearly from the GMWL. The slope of the line is 4.3 (n = 61; R² = 0.99), and is considerably less than the slope of the GWML (= 8). The low slope value implies that all waters sampled are significantly evaporated relative to meteoric water. We discuss time-independent calculations of evaporation, that can be made using our stable isotope data in conjunction with a mass-balance model and several assumed inputs, including the composition and mass of inflow, outflow and precipitation. We then compare the magnitude of our evaporation estimates with previous estimates, based on a conceptual hydrological model (Brook and Hyatt, 1985).