Relative Controls of Vapor Pressure Deficit and Soil Water Stress on Canopy Conductance in Global Simulations by an Earth System Model
Climate change impacts soil water (SW) and vapor pressure deficit (VPD), critically influencing land-atmosphere water vapor and CO2 exchange. There is a lack of model-based global evaluation of the relative roles of the aforementioned 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 relative dominance of the limitation of Gc by water stresses caused by VPD and SW in the growing season. The empirical model is based on a multiplicative algorithm that adjusts Gc according to VPD, SW saturation, and their interaction. Our results show positive trends of VPD over land during the historical period, with large differences between simulations with/without PHS. Despite the differences, Gc is more limited by SW than VPD except in grasslands when SW saturations are above the 60th quantiles, and the fraction of grassland limited by VPD becomes small under dry soil conditions. Ignoring SW and VPD interaction may underestimate the relative role of VPD on Gc by as much as 33% in the dry SW quantile. Based on our uncalibrated results, including PHS may accentuate SW stress limitation on Gc by simulating wetter soil and higher evapotranspiration that lower VPD. Elevated CO2 reduces Gc by increasing water use efficiency and ameliorates SW stress on Gc.