Improving the Jarvis-type model with modified temperature and radiation functions for sap flow simulations

Since Professor Paul G. Jarvis proposed a scheme in 1976 that relates stomatal conductance with environmental variables, there have been numerous studies exploring the relationships for estimating transpiration (E_c). Vapor pressure deficit (D) and solar radiation (R) are dominant environmental factors influencing E_c, whilst air temperature (T) is deemed important but often neglected in model applications since D and T are exponentially correlated. Thus, it is uncertain whether to construct the Jarvis-type E_c models with both T and D included will improve model performance. Meanwhile, it is worth mentioning that most Jarvis-type models cannot simulate nocturnal sap flow which has been observed across a wide range of species and climates. Therefore, this study was firstly aimed at developing a generalized temperature stress function and testing its role in the Jarvis-type model, and further improving the model by modifying a widely used radiation function for nocturnal sap flow simulations. The results show that inclusion of a T-stress function for hourly sap flow simulations can avoid overestimation of daily peaks, and the modified Jarvis-type model was able to capture the nocturnal sap flow. These improvements revive the model for ecohydrological applications in a future climate where enhanced temperature effects and increasing nocturnal transpiration resulting from rising nighttime vapor pressure deficits are likely.