Desert ecosystems contain sparse vegetation cover, which is randomly distributed over the landscape. In many cases where shrubs have invaded primarily grassland ecosystems, it has caused a marked change in vegetation structure and distribution resulting in significant changes in surface roughness and the spatial distribution of bare soil and vegetation. Use of standard bulk similarity formulations with radiometric temperature observations for estimating energy fluxes over heterogeneous, sparsely vegetated surfaces are shown to be generally unreliable. More recent efforts have been in developing formulations that accommodate differences between radiometric and aerodynamic temperatures, and hence would be more applicable to such heterogeneous surfaces. Two such models developed by Norman et al. (1995) and Chehbouni et al. (1996) are described in this paper, which have potential of being implemented operationally with satellite data. These models are applied to data collected during JORNEX (JORnada EXperiments) over a mesquite dune site in the USDA-ARS Jornada Experimental Range near Las Cruces, New Mexico. This site contains complex topography and heterogeneous cover where 0.5 m tall honey mesquite (Prosopis glandulosa Torr.) vegetation grow on dunes that are 1-2 m in height and 10's of meters in width. Very little if any vegetation exists in the interspace region.

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