Estimating Evapotranspiration Using Quantitative Parameters Derived From Remote Sensing

Remote sensing provides an approach to monitor land surface energy and water balance over a large area simultaneously, which is very important and useful in researches and applications in global climate change, hydrology, ecology and agriculture etc. In this paper, methodologies and models on remote sensing of land surface evapotranspiration were investigated in details and validated using field and remotely sensed data, several novel concepts and models were proposed as the center part of this paper, and spatial as well as temporal scale problems were discussed in the second half of the paper.Single-layer model is convenient to apply but the unclear relationship between radiative and aerodynamic temperatures is still a bottleneck in this field. A new method was developed to derive reliable surface heat fluxes from radiative temperature viewed from arbitrary zenith angle. Aerodynamic and radiative temperatures are connected through a so-called Optimum Component Fraction (OCF) parameter-The fraction of vegetation in the field of view when the two temperatures are equivalent in oblique viewing. Heat fluxes estimated from radiative temperature by this model is more accurate than other regular corrective methods.Two-layer model has been proposed for many years but was difficult to apply in remote sensing because component temperature were unavailable in traditional thermal sensors. A new airborne multi-angular thermal sensor system and retrieved soil and canopy temperatures were used to solve two-layer model, and the simulated heat fluxes show much better accuracy than the results from one-layer model especially above dry surfaces. And this is the first full application of two-layer model in remote sensing. From separated evaporation and transpiration through the model some important field parameters can bederived, such as canopy resistance, CO2 flux and crop water use efficiency. A simplified two-layer model was also presented in case of that only radiative temperature is available.A very much different fluxes model was suggested for remote sensing estimation, which takes account of the effects of advections in vertically or horizontally anisothermal vegetations. The inter-exchange of heat can result in lower total sensible heat flux and higher total latent heat flux of the considered areas. This method is designed technically for the simulations of pixel, is a try of new generation effluxes model.Scaling-up of patch model is necessary in the calculation of surface energy fluxes and evapotranspiration from remote sensing data. The simulation error of two-layer model caused by sub-pixel heterogeneity and discontinuity of surface geometry and physics were investigated using a number of data experiments. It is shown that the error could be rather remarkable in some extreme situations and could be neglected in the others. The variance of parameters inside pixel, contexture of the pixel and the surface wind speed are the controlling factors of the scaling error. AMTIS and ASTER images were scaled up using pixel aggregation algorithm to find scaling error of surface flux estimation. The results show that the largest error appears at the interface of different coverage types, and the error show much complexity because of the discontinuity of algorithms at these boundaries. Additionally, the contamination of building, highway and other ground information also add some error in the estimation of land surface evapotranspiration.To derive the accumulated daily evapotranspiration from remotely sensed instantaneous evaporation rate is a key step to use this kind of information in other domains. So-called Simplified Methods and Self-Preservation Methods were introduced and compared, and a Self-Preservation Method was validated using field and remote sensing data. The estimated daily evapotranspiration is mostly affected by diurnal variations of surface wind speed and overpass of clouds, and more detailed and intensive research works are to be carried out to obtain more reliable daily water evaporation loses.