EAPS

Title
Simplifying Evapotranspiration

Abstract
Evapotranspiration (ET) links the terrestrial water, energy and carbon cycles, but is difficult to model due to its strong  dependence on the heterogeneous land surface. It can be useful to  study simple, idealized cases to gain conceptual insight. Two approaches are presented for simplifying ET models at two different time scales. At instantaneous time scales, it is shown that the widely-used Penman-Monteith equation -- an approximate but explicit solution for ET -- is wrong in important limiting cases, which has led to widespread confusion about the behavior of ET in an idealized 'equilibrium' state. A new solution is presented that remedies this problem, without requiring additional assumptions, empiricism or computational cost. At longer time scales, it is shown that, in many regions, tight coupling between the land and atmosphere means ET is encoded in the near-surface atmospheric state. In these regions, ET can be estimated from atmospheric observations alone, without requiring information on the land surface. Since near-surface atmospheric observations have much greater spatial and temporal coverage than current measurements of ET, this opens up new opportunities for studying terrestrial ET at global scales, and how it has changed in a warming world.