Estimation of Daily Evapotranspiration Using Instantaneous Decoupling Coefficient From the MODIS and Field Data

Daily evapotranspiration (ET) is of great significance among various practical applications including water management, drought monitoring, and climate change study. The ET estimations from remotely sensed models are usually instantaneous values. Various upscaling methods have been developed to extrapolate the instantaneous ET to daily scale. In the applications of these methods, the accuracy of daily ET estimation relies on both the accuracy of instantaneous ET calculation and the upscaling methods. This paper used the decoupling model to estimate daily ET directly, according to the constancy of the decoupling coefficient (Ω) in the model in a diurnal cycle, without the calculation of instantaneous ET. The estimated daily ET was compared with the Eddy covariance measurements which were corrected by the Bowen ratio method to close the energy imbalances. The result from field data alone showed that the coefficient of determination (R 2 ) of daily ET estimation was 0.860, with a root-mean-square error (RMSE) of 18.2 W/m 2 , and a bias of −4.7 W/m 2 . Combining MODIS data and field data, the estimated daily ET had a R 2 of 0.860, a RMSE of 21.6 W/m 2 , and a bias of −5.0 W/m 2 . Therefore, it is feasible and effective to obtain daily ET using remote-sensing based instantaneous Ω to replace daily value in the decoupling model.