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| Titel |
Examination of evaporative fraction diurnal behaviour using a soil-vegetation model coupled with a mixed-layer model |
| VerfasserIn |
J.-P. Lhomme, E. Elguero |
| 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 ; 3, no. 2 ; Nr. 3, no. 2, S.259-270 |
| Datensatznummer |
250000914
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| Publikation (Nr.) |
 copernicus.org/hess-3-259-1999.pdf |
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| Zusammenfassung |
| In many experimental conditions, the evaporative fraction, defined as the
ratio between evaporation and available energy, has been found stable during
daylight hours. This constancy is investigated over fully covering vegetation by
means of a land surface scheme coupled with a mixed-layer model, which accounts
for entrainment of overlying air. The evaporation rate follows the Penman-Monteith
equation and the surface resistance is given by a Jarvis type parameterization
involving solar radiation, saturation deficit and leaf water potential. The
diurnal course of the evaporative fraction is examined, together with the
influence of environmental factors (soil water availability, solar radiation
input, wind velocity, saturation deficit above the well-mixed layer). In
conditions of fair weather, the curves representing the diurnal course of the
evaporative fraction have a typical concave-up shape. Around midday (solar
time) these curves appear as relatively constant, but always lower that the
daytime mean value. Evaporative fraction decreases when soil water decreases or
when solar energy increases. An increment of saturation deficit above the
mixed-layer provokes only a slight increase of evaporative fraction, and wind
velocity has almost no effect. The possibility of estimation daytime
evaporation from daytime available energy multiplied by the
evaporative fraction at a single time of the day is also investigated. It
appears that it is possible to obtain fairly good estimates of daytime
evaporation by choosing adequately the time of the measurement of the
evaporative fraction. The central hours of the day, and preferably about 3 hr
before or after noon, are the most appropriate to provide good estimates. The
estimation appears also to be much better when soil water availability (or
evaporation) is high than when it is low. |
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