Technical Abstract: Assessing the project efficiency of irrigation systems and water use efficiency of crops over large irrigated areas requires daily or seasonal evapotranspiration (ET) maps. Mapping ET or latent heat flux (LE) can be achieved spatially for land surfaces using remote sensing inputs such as surface reflectance and radiometric surface temperature (Ts). The energy balance (EB) equation requires net radiation (Rn), soil heat flux (G), and sensible heat flux (H) to derive LE as a residual. Although Rn and G can be estimated with acceptable accuracy, H may be under estimated when Ts is used rather than the surface aerodynamic temperature (To) in the aerodynamic resistance equation. The value of To cannot be measured directly because it varies with atmospheric forcing resulting from radiation, wind speed, and air temperature, and with variable surface conditions. In this study, To for dryland cotton in the Texas High Plains and irrigated alfalfa in Colorado was determined using Ts, air temperature, leaf area index, and surface aerodynamic resistance, and ET measurements obtained with large weighing lysimeters. The specific performance of the modeled To for cotton and alfalfa was assessed using an aerodynamic profile method. Results indicated that the To model based on aerodynamic resistance better agreed (0.2 plus or minus 1 deg C) with measured To values using the aerodynamic profile method.