Towards a rational definition of potential evaporation

摘要

The concept of potential evaporation is defined onthe basis of the following criteria: (i) it must establish an upper limitto the evaporation process in a given environment (the term "environment"including meteorological and surface conditions), and (ii) this upper limitmust be readily calculated from measured input data. It is shown that thisupper limit is perfectly defined and is given by the Penman equation, appliedwith the corresponding meteorological data (incoming radiation and aircharacteristics measured at a reference height) and the appropriate surfacecharacteristics (albedo, roughness length, soil heat flux). Since eachsurface has its own potential evaporation, a function of its own surfacecharacteristics, it is useful to define a reference potential evaporationas a short green grass completely shading the ground. Although the potentialevaporation from a given surface is readily calculated from the Penmanequation, its physical significance or interpretation is not so straightforward,because it represents only an idealized situation, not a real one. Potentialevaporation is the evaporation from this surface, when saturated and extensiveenough to obviate any effect of local advection, under the same meteorologicalconditions. Due to the feedback effects of evaporation on air characteristics,it does not represent the "real" evaporation (i.e. the evaporation whichcould be physically observed in the real world) from such an extensivesaturated surface in these given meteorological conditions (if this saturatedsurface were substituted for an unsaturated one previously existing).From a rigorous standpoint, this calculated potential evaporation is notphysically observable. Nevertheless, an approximate representation canbe given by the evaporation from a limited saturated area, the dimensionof which depends on the height of measurement of the air characteristicsused as input in the Penman equation. If they are taken at a height of2 m (the height of the meteorological observations), the dimension of thesaturated surface in the direction of the wind ranges roughly from 50 to200 m for a short green grass completely shading the ground.