Representation of plant hydraulics in the NOAH-MP Land Surface Model: Model development and multiscale evaluation

Plants are expected to face increasing water stress under future climate change. Most land surface models, including Noah-MP, employ an idealized "big-leaf" concept to regulate water and carbon fluxes in response to soil moisture stress through empirical soil hydraulics schemes (SHSs). However, such schemes have been shown to cause significant uncertainties in carbon and water simulations. In this paper, we present a novel plant hydraulics scheme (PHS) for Noah-MP (hereafter, Noah-MP-PHS), which employs a big-tree rather than big-leaf concept, wherein the whole-plant hydraulic strategy is considered, including root-level soil water acquisition, stem-level hydraulic conductance and capacitance, and leaf-level anisohydricity and hydraulic capacitance. Evaluated against plot-level observations from a mature, mixed hardwood forest at the University of Michigan Biological Station and compared with the default Noah-MP, Noah-MP-PHS better represents plant water stress and improves water and carbon simulations, especially during periods of dry soil conditions. Noah-MP-PHS also improves the asymmetrical diel simulation of gross primary production under low soil moisture conditions. Noah-MP-PHS is able to reproduce different patterns of transpiration, stem water storage and root water uptake during a 2-week dry-down period for two species with contrasting plant hydraulic behaviors, i.e., the "cavitation risk-averse" red maple and the "cavitation risk-prone" red oak. Sensitivity experiments with plant hydraulic capacitance show that the stem water storage enables nocturnal plant water recharge, affects plant water use efficiency, and provides an important buffer to relieve xylem hydraulic stress during dry soil conditions.