National Lakeshore

For most wetlands precipitation and evapotranspiration arethe major components of water gain and loss. However, studies of the hydrology of wetlands largely ignore evaporation, orcalculate itby difference or some very simple measure. As part of an integrated study of the hydrology and ecology of wetlands in the Indiana Dunes National Lakeshore, evaporation was measured irectly as its energy equivalent, he latent heat flux, using eddy correlation techniques for a 10 day period in June 1994. In addition, data were collected on the other surface energy balance fluxes (net all-wave radiation and sensible heat flux), and ancillary meteorological variables (wind speed and direction, temperature, pressure, relative humidity, solar radiation and water depth). Overall, latent heat flux dissipated 48 % of the available radiant energy, storage heat flux 35%, and sensible heat flux 17%. A simple hysteresis model for the storage heat flux was developed which performed extremely well. Proximity to Lake Michigan resulted in evaporation rates close to the equilibrium rate (average Priestley-Taylor t~ = 1.035), which were affected strongly by net all-wave radiation. Three com-monly used models of evaporation, Penman, Priestley-Taylor (a = 1.26) and equilibrium (or = 1.0) are evaluated. The relative success of the equilibrium model, with its limited data requirements, offers great potential for longer-term odeling of water and energy exchanges in this type of wetland environment. 1