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Titel |
Towards a rational definition of potential evaporation |
VerfasserIn |
J.-P. Lhommel |
Medientyp |
Artikel
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Sprache |
Englisch
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ISSN |
1027-5606
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Digitales Dokument |
URL |
Erschienen |
In: Hydrology and Earth System Sciences ; 1, no. 2 ; Nr. 1, no. 2, S.257-264 |
Datensatznummer |
250000145
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Publikation (Nr.) |
copernicus.org/hess-1-257-1997.pdf |
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Zusammenfassung |
The concept of potential evaporation is defined on
the basis of the following criteria: (i) it must establish an upper limit
to the evaporation process in a given environment (the term "environment"
including meteorological and surface conditions), and (ii) this upper limit
must be readily calculated from measured input data. It is shown that this
upper limit is perfectly defined and is given by the Penman equation, applied
with the corresponding meteorological data (incoming radiation and air
characteristics measured at a reference height) and the appropriate surface
characteristics (albedo, roughness length, soil heat flux). Since each
surface has its own potential evaporation, a function of its own surface
characteristics, it is useful to define a reference potential evaporation
as a short green grass completely shading the ground.
Although the potential
evaporation from a given surface is readily calculated from the Penman
equation, its physical significance or interpretation is not so straightforward,
because it represents only an idealized situation, not a real one. Potential
evaporation is the evaporation from this surface, when saturated and extensive
enough to obviate any effect of local advection, under the same meteorological
conditions. Due to the feedback effects of evaporation on air characteristics,
it does not represent the "real" evaporation (i.e. the evaporation which
could be physically observed in the real world) from such an extensive
saturated surface in these given meteorological conditions (if this saturated
surface were substituted for an unsaturated one previously existing).
From a rigorous standpoint, this calculated potential evaporation is not
physically observable. Nevertheless, an approximate representation can
be given by the evaporation from a limited saturated area, the dimension
of which depends on the height of measurement of the air characteristics
used as input in the Penman equation. If they are taken at a height of
2 m (the height of the meteorological observations), the dimension of the
saturated surface in the direction of the wind ranges roughly from 50 to
200 m for a short green grass completely shading the ground. |
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