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. Currently, applications of remote sensing evapotranspiration (ET) productsare limited by the coarse resolution of satellite remote sensing data causedby land surface heterogeneities and the temporal-scale extrapolation of theinstantaneous latent heat flux (LE) based on satellite overpass time. Thisstudy proposes a simple but efficient model (EFAF) for estimating the dailyET of remotely sensed mixed pixels using a model of the evaporativefraction (EF) and area fraction (AF) to increase the accuracy of ET estimateover heterogeneous land surfaces. To accomplish this goal, we derive anequation for calculating the EF of mixed pixels based on two key hypotheses.Hypothesis 1 states that the available energy (AE) of each sub-pixel isapproximately equal to that of any other sub-pixels in the same mixed pixelwithin an acceptable margin of error and is equivalent to the AE of the mixedpixel. This approach simplifies the equation, and uncertainties and errorsrelated to the estimated ET values are minor. Hypothesis 2 states that the EFof each sub-pixel is equal to that of the nearest pure pixel(s) of the sameland cover type. This equation is designed to correct spatial-scale errorsfor the EF of mixed pixels; it can be used to calculate daily ET from dailyAE data. The model was applied to an artificial oasis located in themidstream area of the Heihe River using HJ-1B satellite data with a 300 mresolution. The results generated before and after making corrections werecompared and validated using site data from eddy covariance systems. Theresults show that the new model can significantly improve the accuracy ofdaily ET estimates relative to the lumped method; the coefficient ofdetermination ( R2 ) increased to 0.82 from 0.62, the root mean squareerror (RMSE) decreased to 1.60 from 2.47 MJ m −2 (decreasedapproximately to 0.64 from 0.99 mm) and the mean bias error (MBE) decreasedfrom 1.92 to 1.18 MJ m −2 (decreased from approximately 0.77 to0.47 mm). It is concluded that EFAF can reproduce daily ET with reasonableaccuracy; can be used to produce the ET product; and can be applied tohydrology research, precision agricultural management and monitoring naturalecosystems in the future.

Estimating daily evapotranspiration based on a model of evaporative fraction (EF) for mixed pixels