Full text: Mesures physiques et signatures en télédétection

Chehbouni A^*, W.D. Nichols^, J. Qi 3, e q Njoku^, Y.H. Kerr 4 and F. Cabot 4 
1 Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, 91109, CA, USA (^Permanent 
affiliation: ORSTOM ,France), ^USGS (Carson City, Nevada), ^USDA-ARS (Tucson- 
Anzona), 4 LERTS/CNES (Toulouse-France). 
It is now recognized that accurate partitioning of available energy into 
sensible and latent heat flux is crucial to the understanding surface-atmosphere interactions. 
This issue is more complicated in and and semi-arid regions where the relative contribution 
to surface fluxes from the soil and vegetation may vary significantly throughout the day and 
throughout the season. The objective of this paper is to present a three-component model to 
estimate sensible heat flux over heterogeneous surfaces. The surface was represented with 
two adjacent compartments. The first compartment is made up of two components, shrubs 
and shaded soil, the second of open "illuminated" soil. Data collected at two different sites in 
Nevada dunng the summers of 1991 and 1992 were used to evaluate model performance. 
The results show that the present model is sufficiently general to yield satisfactory' results for 
both sites. 
Key Words Effective surface temperature, Effective resistance. Bowen ratio - 
Reliable prediction of general weather patterns depends upon our ability to 
properly represent the interaction of integrated land-surface processes and the Atmospheric 
Boundary Layer (ABL). It is thus crucial to have an accurate partitioning of the available 
energy at the surface into sensible and latent heat fluxes. This is a challenging task in the 
case of sparsely vegetated surfaces. The representation of the exchange mechanisms between 
sparsely vegetated surfaces and the atmosphere depends upon the distribution of the 
vegetation on the surface. For surfaces with sparsely and uniformly distributed vegetation, 
the exchange of heat and water with the atmosphere can be well represented by two- 
component (Two layer) models such as those developed by Choudhury and Monteith, 
(1988); Shuttleworth and Gurney, (1990); Lhomme et al. (1993). For such "uniform" 
surfaces, these models legitimately consider that the fluxes enter or leave the atmosphere 
only via the canopy. However, in arid and semi-arid regions the vegetation distribution is 
typically non-uniform, heat and mass exchange between a part of soil and the atmosphere 
may takes place with little interaction with the adjacent canopy. In this case, we think that 
such surfaces may be better represented by a multi-compartment scheme. 
The objective of this study is to present a three-component model for sensible 
heat flux estimation over sparse and non uniformly distributed shrubs in Nevada. The three 
components of the surface are shrubs, soil under-the shrubs, and open soil. These

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