2. Comparison of Dual-Source Evapotranspiration Models in Estimating Potential Evaporation and Transpiration

Potential evapotranspiration (PET) is defined as the amount of ET that could occur if a sufficient water source were available, the value of which is a function of atmospheric conditions and surface vegetation distribution characteristics. However, traditional empirical PET models, such as the Hargreaves model and Priestley-Taylor model, only account for the effect of atmospheric demand on PET, whereas the impact of vegetation on PET is simply ignored. On the other hand, physical-based PET models, such as the Penman-Monteith model (refer to as the P-M model hereafter), take vegetation effect on PET into consideration. Nevertheless, the P-M model treats the land surface as a uniform layer, where the vegetation covers the land surface fully and uniformly as a “big leaf”. This simplification of vegetation treatment makes the P-M model unable to distinguish evaporation from soil (E) and transpiration from canopy (T), and therefore may not be appropriate for use in partially vegetated areas.