Improvement of a simplified process-based model for estimating transpiration under water-limited conditions

Plant transpiration depends on environmental conditions, and soil water availability is its primary control under water deficit conditions. In this study, we improve a simplified process-based model (hereafter “BTA”) by including soil water potential (ψ_soil) to explicitly represent the dependence of plant transpiration on root-zone moisture conditions. The improved model is denoted as the BTA-ψ model. We assessed the performance of the BTA and BTA-ψ models in a subtropical monsoon climate and a Mediterranean climate with different levels of water stress. The BTA model performed reasonably in estimating daily and hourly transpiration under sufficient water conditions, but it failed during dry periods. Overall, the BTA-ψ model provided a significant improvement for estimating transpiration under a wide range of soil moisture conditions. Although both models could estimate transpiration (sap flow) at night, BTA-ψ was superior to BTA in this regard. Species differences in the calibrated parameters of both models were consistent with leaf-level photosynthetic measurements on each species, as expected given the physiological basis of these parameters. With a simplified representation of physiological regulation and reasonable performance across a range of soil moisture conditions, the BTA-ψ model provides a useful alternative to purely empirical models for modelling transpiration.