Evaporation in the Global Hydrological Cycle

Evaporation is the transfer of moisture from the surface to the atmosphere. The availability of soil moisture and energy to convert liquid water into vapor is the key constraint on evaporation. At catchment and site scale, there is some success in simplifying the complex physics of evaporation that arises from the interaction of surface properties such as conductance with those of the atmosphere such as moisture and radiation. We discuss the general theory of evaporation from the Penman–Monteith equation and the applications to vegetated, bare, and open water surfaces. There is some scope for understanding the regional energy controls of evaporation that leads relative stable, equilibrium-type evaporation equations, such as those of Priestly–Taylor. Globally, evaporation from the land surface to the atmosphere amounts to 71×10 3  km 3 yr −1 . Evaporation from the oceans is a far larger component of the hydrological cycle at an estimated 428×10 3  km 3 yr −1 . We briefly discuss the pan evaporation paradox and conclude that regional differences in controlling factors of evaporation (moisture vs. energy) probably are more important in explaining the trends than global dimming or brightening. Despite the considerable advances at the practical level that have been made in our ability to estimate and observe evaporation at a local level, our understanding of evaporation in the global climate system still shows significant gaps.