PNNL

Abstract

Plant transpiration plays a critical role in global water and energy cycles, requiring better process understanding as climate change intensifies drought stress and alters plant responses. Most hydrological models such as the widely-used SWAT lack representation of plant hydraulics, the mechanistic processes controlling plant water regulation and transpiration. We developed SWAT-PHS by integrating a plant hydraulics scheme (PHS) into SWAT model, enabling explicit simulation of root water uptake, sap flow, storages and transpiration at sub-daily timescales for watershed-scale modeling. Applied to the Hanjiang River Basin, SWAT-PHS improved streamflow simulation performance and particularly better captured low-flow conditions. The model can simulate reasonable plant water dynamics, including diurnal transpiration patterns and drought responses showing declining transpiration flux, hydraulic buffering through stem water storage, and depth-dependent root water uptake strategies. Sensitivity analysis shows that SWAT-PHS captured mechanistic relationships between plant hydraulic traits and transpiration, with root distribution and stem capacitance positively affecting annual transpiration while vulnerability parameters showed negative effects. This work provides a pathway for improving hydrologic modeling and water resource management by better representing plant water regulation under climate change and expected intensifying water stress conditions.