PNNL

Abstract

Climate change impacts drought and vapor pressure deficit (VPD) that critically influence land-atmosphere water vapor and CO2 exchange. There is a lack of model-based global evaluation of the relative roles of the two types of water stresses in limiting canopy conductance (Gc). Using the Energy Exascale Earth System Model (E3SM), we conducted four land-atmosphere coupled simulations for the historical period with and without a plant hydraulics scheme (PHS) and under present-day and quadrupling of CO2 concentrations. By fitting an empirical model of Gc to the E3SM simulations, we evaluated the sensitivity of Gc to water stresses caused by VPD and soil water (SW) in the growing season. The empirical model is based on a multiplicative algorithm that adjusts Gc according to VPD, soil water saturation, and their interaction. Our results show positive trends of VPD over land during the historical period, with large differences between simulations with and without PHS. Despite the differences, a majority of land pixels that experience both SW and VPD stresses are in semi-arid and tropical savanna ecosystems. Gc is more sensitive to VPD than SW in less than 20% of land pixels where the empirical model provides a good fit for the simulated Gc. Ignoring SW and VPD interaction may underestimate the relative role of VPD on Gc by as much as 28%. Including PHS accentuates SW stress limitation on Gc by simulating wetter soil and higher ET that lower VPD. Elevated CO2 reduces Gc by increasing water use efficiency and ameliorates SW stress on Gc.