Earth scientists assume that evaporation depends on the gradient in water vapour concentration (Fick’s law), but this is only true in an isothermal system. Temperature gradients can impact evaporation via the Soret effect. Here we evaluate the relative magnitude of the Soret effect and find that it is at least 2 orders of magnitude smaller than classical concentration-dependent mass (“Fickian”) diffusion. This result justifies the standard practice of assuming evaporation follows Fick’s law.

Here we test a maximum evaporation theory that acknowledges the interdependence between radiation, surface temperature, and evaporation over saturated land. We show that the maximum evaporation approach recovers observed evaporation and surface temperature under non-water-limited conditions across a broad range of bio-climates. The implication is that the maximum evaporation concept can be used to predict potential evaporation that has long been a major difficulty for the hydrological community.