An improved plant hydraulics model for the land component of the CliMA Earth System Model

We present the governing equations of the new plant hydraulics component of the land model of the Climate Modeling Alliance (CliMA) Earth System Model, and test results. Current plant hydraulic models used to model water fluxes and water content at the single tree scale and at the grid-scale for land modeling and ESM purposes present limitations (eg. their steady-state assumption). We rederived the equations for representing water fluxes and water content as a function of time and position from Darcy's law, demonstrating where the equations come from, and presented solutions to each of the current limitations. The features of this model include its 1) non-steady-state assumption, 2) representation of gravitational potential, 3) representation of water along the flow path instead of as a function of height, 4) use of a state variable which allows for the expansion of water-holding capacity beyond porosity, 5) ability to extract water at different soil depths, 6) low-complexity (few parameters), 7) modularity (can increase the resolution of the tree from 2 compartments to n compartments), and 8) its ability to model fluxes in the positive and negative directions. Although most of these fixes were introduced separately in other works, we combine them for an expected overall improved performance. We tested the model in coupled mode with the CliMA soil model at a test site in Ozark, Missouri, with prescribed transpiration and precipitation, using meteorological data from the AmeriFlux Missouri Ozark site. We present modeled soil water content and leaf water potential as a function of position in the flow path and time against measurements of these two variables. Future work includes coupling this plant hydraulics model to a transpiration scheme, and comparing transpiration fluxes from the CliMA land model to those modeled by other land models.