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

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.