ON THE USE OF RADIATIVE SURFACE TEMPERATURE TO ESTIM ATE
SENSIBLE HEAT FLUX OVER SPARSE SHRUBS IN NEVADA
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).
Abstract
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 -
1- INTRODUCTION
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
