Comparison of the Evaporative Fraction and the Priestley-Taylor α for Parameterizing Daytime Evaporation


  • Richard D. Crago


This is a comparison between the evaporative fraction EF, which is the surface latent heat flux divided by the available energy, and the Priestley-Taylor parameter α. It is shown for the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) that the hypothesis that the midday values of EF and α are the same as their daytime average values must be rejected at the 0.05 level of significance. However, correlations are high between the midday and daytime average values for both variables. Daytime average latent heat fluxes were estimated based on midday values of EF and α along with daytime average available energy and equilibrium evaporation. In addition, the relationships between soil moisture and the midday and daytime average values of EF and α were considered, along with the utility of these relationships in determining daytime total evaporation. The conclusion is that at FIFE formulations based on EF are essentially interchangeable with corresponding formulations based on α.