Improving the Penman-Monteith evapotranspiration model based on the coupling principle of carbon and water fluxes

Evapotranspiration(ET) is an important component of the land water cycle. It includes the evaporation of the soil surface and wet leaves and the transpiration of plants. The Penman-Monteith equation is an important method used to estimate ET. The stomatal conductance of leaves or canopies in the equation is the key factor controlling the estimation accuracy. According to the coupling principle of the carbon and water cycles, a plant photosynthesis model can be used to estimate the stomatal conductance of leaves or canopies. Plant photosynthesis models can be divided into three categories:1) the big-leaf model(BL), which uses total canopy conductance;2) the two big-leaf model(TBL), which distinguishes the canopy conductance of shaded and sunlit leaves;and 3) the two-leaf model(TL), which uses stomatal conductances of shaded and sunlit leaves. Corresponding to these three types of photosynthesis models, three evapotranspiration estimation models based on different conductance calculation methods are then derived. The main difference among the three evapotranspiration models is whether or not the stomatal conductance is integrated from the leaf scale to the canopy scale. Finally, the calculated stomatal conductance is input into the Penman-Monteith equation, so as to estimate the ET. Among them, the BL model has the highest degree of aggregation. Due to the fact that the relationship between the transpiration and stomatal conductance is nonlinear, the aggregation of stomatal conductance will lead to negative bias in ET estimation. Also, as the BL model has the largest aggregation of stomatal conductance, the bias error also the greatest. The results of previous studies show that the TL evapotranspiration model causes no significant bias error in comparison with flux measurements.