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Discussion papers | Copyright
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 22 May 2018

Research article | 22 May 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Hydrology and Earth System Sciences (HESS).

Estimating Daily Evapotranspiration Based on a Model of Evapotranspiration Fraction (EF) for Mixed Pixels

Fugen Li1,2, Xiaozhou Xin1, Zhiqing Peng1,2, and Qinhuo Liu1,3 Fugen Li et al.
  • 1State Key Laboratory of Remote Sensing Science, Institute of Remote Sensing and Digital Earth, Beijing, 100101, China
  • 2University of Chinese Academy of Sciences, Beijing 100049, China
  • 3Joint Center for Global Change Studies (JCGCS), Beijing 100875, China

Abstract. Currently, applications of remote sensing evapotranspiration (ET) products are limited by the low resolution of satellite remote sensing data caused by land surface heterogeneities and the temporal scale extrapolation of the instantaneous latent heat flux (LE) based on satellite overpass time. This study proposes a simple but efficient model (EFAF) for estimating the daily ET of remotely sensed mixed pixels using a model of the evapotranspiration fraction (EF) and area fraction (AF). To accomplish this goal, we derive an equation for calculating the EF of mixed pixels based on two key hypotheses. Hypothesis 1 states that the available energy (AE) of each sub-pixel is approximately equal to that of any other sub-pixels in the same mixed pixel within an acceptable margin of bias and is equivalent to the AE of the mixed pixel. This approach simplifies the equation, and uncertainties and errors related to the estimated ET values are minor. Hypothesis 2 states that the EF of each sub-pixel is equal to that of the nearest pure pixel(s) of the same land cover type. This equation is designed to correct spatial scale errors for the EF of mixed pixels; it can be used to calculate daily ET from daily AE data. The model was applied to an artificial oasis located in the midstream area of the Heihe River using HJ-1B satellite data with a 300m resolution. Results generated before and after making corrections were compared and validated using sites data from eddy covariance systems. The results show that the new model can significantly improve the accuracy of daily ET estimates relative to the lumped method; the coefficient of determination (R2) increased to 0.82 from 0.62, the root mean square error (RMSE) decreased to 1.60 from 2.47MJ·m−2, and the mean bias error (MBE) decreased from 1.92 to 1.18MJ·m−2.

Fugen Li et al.
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Fugen Li et al.
Fugen Li et al.
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Latest update: 17 Oct 2018
Publications Copernicus
Short summary
This study proposes a simple but efficient model for estimating daily evapotranspiration considering heterogeneity of mixed pixels. In order to do so, an equation to calculate evapotranspiration fraction (EF) of mixed pixels was derived based on two key hypotheses. The model is easy to apply and is independent and easy to be embedded in the traditional remote sensing algorithms of heat fluxes to get daily ET.
This study proposes a simple but efficient model for estimating daily evapotranspiration...