Evapotranspiration is the combined process whereby water is lost from the soil surface by evaporation and from the plants by transpiration. It is not only a critical component in the hydrological cycle but also a main part of energy balance because liquid water absorbs large amount of heat when it transfers into water vapour. Therefore, it is very important to give an accurate estimate of evapotranspiration in water resource management and climate change studies. The objective of this research presented in this paper is to estimate spatial and temporal distribution of actual ET and quantify the actual ET in Heihe River Basin using remote sensing technology, combining with ground observations. This study has been carried out using the Moderate-resolution Imaging Spectroradiometer (MODIS) images and meteorological data for achieving the objective. The physically based Surface Energy Balance System (SEBS) model was applied for assessing the spatial and temporal distribution of actual ET. Based on the SEBS output of evaporative fraction, a time series of actual ET is generated by means of Fourier series analysis method, which provided the quantity of actual ET in the study area. However, another method of generating time series of actual ET was also carried out by applying Fourier series analysis method to the SEBS output of discrete evapotranspiration values. Both methods were analyzed in different aspects. Finally, the limitations and recommendations for further study were addressed.
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