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<p>This study compares four different hypotheses regarding the nature of the Priestley-Taylor parameter α. They are: 1) α is a universal constant; 2) the Bowen ratio (<em>H</em>/<em>LE</em>, where <em>H</em> is the sensible and <em>LE</em> is the latent heat flux) for equilibrium (i.e. saturated air column near the surface) evaporation is a constant times the Bowen ratio at minimal advection (Andreas et al., 2013); 3) minimal advection over a wet surface corresponds to a particular relative humidity value, and 4) α is a constant fraction of the difference from the minimum value of one to the maximum value of α proposed by Priestley and Taylor (1972). Formulas for α are developed for the last three hypotheses. Weather, radiation and surface energy flux data from 171 FLUXNET eddy covariance stations were used. The condition <em>LE</em><sub>ref</sub>/<em>LE</em><sub>p</sub>>0.90, was taken as the criterion for nearly-saturated conditions (where <em>LE</em><sub>ref</sub> is the reference and <em>LE</em><sub>p</sub> is the apparent potential evaporation rate from the Penman (1948) equation). Daily and monthly average data from the sites were obtained. All formulations for α include one model parameter which is optimized such that the root mean square error of the target variable was minimized. For each model, separate optimizations were done for predictions of the target variables α, wet surface evaporation (α multiplied by equilibrium evaporation rate) and actual evaporation (the latter using a highly-successful version of the complementary relationship of evaporation). Overall, the second and fourth hypotheses received the best support from the data.</p>