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A surface energy balance was conducted to calculate the latent heat flux (lambda E) using aerodynamic methods and the Penman-Monteith (PM) method. Computations were based on gridded weather and Landsat satellite reflected and thermal data. The surface energy balance facilitated a comparison of impacts of different parameterizations and assumptions, while calculating lambda E over large areas through the use of remote sensing. The first part of the study compares the full aerodynamic method for estimating latent heat flux against the appropriately parameterized PM method with calculation of bulk surface resistance (r(s)). The second part of the study compares the appropriately parameterized PM method against the PM method, with various relaxations on parameters. This study emphasizes the use of separate aerodynamic equations (latent heat flux and sensible heat flux) against the combined Penman-Monteith equation to calculate lambda E when surface temperature (T-s) is much warmer than air temperature (T-a), as will occur under water stressed conditions. The study was conducted in southern Idaho for a 1000-km(2) area over a range of land use classes and for two Landsat satellite overpass dates. The results show discrepancies in latent heat flux (lambda E) values when the PM method is used with simplifications and relaxations, compared to the appropriately parameterized PM method and full aerodynamic method. Errors were particularly significant in areas of sparse vegetation where differences between T-s and T-a were high. The maximum RMSD between the correct PM method and simplified PM methods was about 56 W/m(2) in sparsely vegetated sagebrush desert where the same surface resistance was applied.