<p>Evapotranspiration (ET) is an important process in the water cycle and in the land-surface energy balance. Over the last decades, remote sensing has provided valuable information to quantify ET. However, methodologies that use data from microwave passive sensors, such as "Soil Moisture Active Passive" (SMAP) mission, have been recently developed. In this work, a formulation to derive the relative evapotranspiration and ET from in situ and microwave data, is presented. The methodology is

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... on a modification of the original Komatsu (2003) equation by introducing a calibration parameter to represent the wind speed and vegetation effects and estimate the relative evapotranspiration. This new equation was used on the Bouchet's complementary relationship with the Priestley-Taylor's equation, to estimate ET at regional scales. The results were compared with observed data in the Southern Great Plains – USA (SGP) area, indicating that the new model estimated ET with a root mean square error (RMSE) of 0.88 mmd^–1 and a coefficient of determination (R² ) greater than 0.8. The calibrated model was applied in an extremely humid period in Argentinean Pampas region with results near to potential rates.</p>