Abstract: A series of processes of exchange of water and energy are contained in SPAC. This paper mainly deals with the processes and the determination of evapotranspiration, which includes mainly: a comprehensive SPAC model, description and determination of the parametric equations of evapotransipiration, and estimation of actual evapotranspiration from field. Based on the measured data obtained from Qihe County, Shandong Province, the models of evapotranspiration are developed and the fairly results in comparison with measurements are obtained.