Estimating stomatal conductance under drought: Parameterizing a phenomenological model and evaluating roles of the energy balance equation

• Parameters of the BWB-type model exhibit crop-specific responses to water stress. • Absence of VPD-sensitive parameterization leads to biases in the BWB-type model. • PM-based approach provides empirical function-free estimation of stomatal conductance . Accurate estimation of stomatal conductance ( g s ) is fundamental for modeling coupled photosynthesis-transpiration responses to climate change, yet current approaches face limitations under drought conditions. To predict g s under drought, the widely-used Ball-Woodrow-Berry (BWB) model relies on empirical soil moisture functions ( f w ) with uncertain validity. As a physically-based alternative, inverted Penman-Monteith (PM) equation derived from leaf energy balance principles offers a mechanistic solution. To evaluate the implicit assumption that BWB model parameters (excluding f w ) remain not altered by drought and the predictive capability of the PM-based energy balance approach, a two-year field experiment in the nationally-planted oil crops—soybean ( Glycine max L.) and oilseed rape ( Brassica napus L.) was conducted. Our results revealed that crop-specific responses in BWB parameter variations during water stress, with significant predictive errors occurring if responses of g s to vapor pressure deficit were not considered. While the PM-based energy balance approach performed well for g s of oilseed rape, it required modification for soybean to account for greater leaf temperature fluctuations (Δ T leaf ) under water-deficit relative to well-watered conditions caused by more variable water deficits. Our study demonstrated that integrating an inversed PM equation for g s with the CO 2 -diffusion law and a biochemical photosynthesis model accounting for Δ T leaf is an effective approach to predict both photosynthetic rate and g s under water stress without introducing empirical functions.