Environmental drivers of evapotranspiration in a shrub wetland and an upland forest in northern Wisconsin

[1] To improve our predictive understanding of daily total evapotranspiration (E_T), we quantified the differential impact of environmental drivers, radiation (Q), and vapor pressure deficit (D) in a wetland and upland forest. Latent heat fluxes were measured using eddy covariance techniques, and data from four growing seasons were used to test for (1) environmental drivers of E_T between the sites, (2) interannual differences in E_T responses to environmental drivers, and (3) changes in E_T responses to environmental drivers between the leaf expansion period and midsummer. Two simple E_T models derived from coupling theory, one radiation-based model, and another using mass transfer were used to examine the mechanisms underlying the drivers of E_T. During summer months, E_T from the wetland was driven primarily by Q, whereas it was driven by D in the upland. During the leaf expansion period in the upland forest the dominant driver was Q. E_T from the wetland was linearly related to net radiation using coupling coefficients ranging from a low of 0.3-0.6 to a high of 1.0 between early May and midsummer. Interannually, E _T from the upland forest exhibited near linear responses to D, with an effective reference canopy stomatal conductance varying from 1 to 5 mm s ^-1. The results show that E_T predictions in northern Wisconsin and other mixed wetland-upland forests need to consider both wetland and upland forest processes. Furthermore, leaf phenology effects on E _T represent a knowledge gap in our understanding of seasonal environmental drivers.