1 research outputs found
Estimating daily evapotranspiration based on a model of evaporative fraction (EF) for mixed pixels
Currently, applications of remote sensing evapotranspiration (ET) products
are limited by the coarse 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 evaporative
fraction (EF) and area fraction (AF) to increase the accuracy of ET estimate
over heterogeneous land surfaces. 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 error 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 300 m
resolution. The results generated before and after making corrections were
compared and validated using site 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 2.47 MJ m−2(decreased
approximately to 0.64 from 0.99 mm) and the mean bias error (MBE) decreased
from 1.92 to 1.18 MJ m−2 (decreased from approximately 0.77 to
0.47 mm). It is concluded that EFAF can reproduce daily ET with reasonable
accuracy; can be used to produce the ET product; and can be applied to
hydrology research, precision agricultural management and monitoring natural
ecosystems in the future.</p