Daily Evapotranspiration For Mixed Pixels:models And Applications

Evapotranspiration(ET)is the main component of energy balance and water cycle in the terrestrial surface.It is currently accepted that remote sensing is the most effective method to estimate ET in region scale.However,surface heterogeneity and temporal scale extrapolation are severe problems in estimating ET by remote sensing.The purpose of this paper is to propose a simple model for estimating ET of mixed pixels which adapted to multi-source remote sensing data inputs.On the other hand,an evaporative fraction(EF)method was used to extrapolate the instantaneous ET observed by satellite data to the daily scale.Firstly,this paper deduced a method for calculating the EF of mixed pixels based on the meaning of EF and the ideal of patch model.In this method,the EF of mixed pixels is estimated through using the EF of sub-pixels and the area fraction(AF)based on high-resolution land-cover data.At the same time,two key assumptions were proposed.Assumption 1 is that the available energy(AE)of each sub-pixel equals approximately that of any other sub-pixels in the same mixed pixel within acceptable margin of bias,and as same as the AE of the mixed pixel.Assumption 2 is that the EF of sub-pixels in the mixed pixels(EFi)can be established a bridge to the value of the EF of the pure pixels in the same class,so that the EFi can be replaced by the value of the EF of the pure pixels.For the second assumption which was the representation of EF of sub pixels,this paper discussed seven kinds of scenarios using remote sensing retrieved data and site measurement data compared.The Assumption 2 can be described that the EF of each sub-pixel in a mixed pixel equals the mean of the EF of a pure pixel/pixels which is/are as same land cover as the sub-pixel and that is/are the closest to the mixed pixel.Secondly,the model for estimating daily ET of mixed pixels can be established based on the method for calculating the EF of mixed pixels.And the model was verified and analyzed in Zhangye Oasis in the midstream of Heihe River.The data of CCD and IRS of HJ-1B was used to estimate the lumped heat fluxes which are made at the scale of 300 m after resampling the 30-m resolution datasets to 300 m resolution based on the coarser resolution of temperature.And then EF of each pixel in 300 m resolution can be calculate using the 300-m net radiation,soil heat flux,sensible heat flux and latent heat flux(LE)data.The results shows that the results of this model for mixed pixels were better than lumped ET method.