The phenomena that are encapsulated in the Onsager heat of transport at a liquid-vapor interface are modeled in terms of processes occurring in the capillary-wave zone at the liquid surface. The effect of localized heat release in the outer part of the capillary-wave zone is in the right direction but is too small to account for the observations. A more successful approach considers the effect of a temperature gradient on the effective barrier height for escape from the liquid. The results imply that evaporating molecules must surmount a free-energy barrier rather than an enthalpy barrier and that the detailed dynamics of barrier crossing play an important role.