A coupled ecohydrological-three-dimensional unsaturated flow model describing energy, H2O and CO2 fluxes

A soil–vegetation–atmosphere‐transfer (SVAT) scheme and a vegetation dynamic model (VDM) have been coupled to a three‐dimensional unsaturated flow model developed in a cellular automata (CA) environment, for reproducing the interactions between vegetation dynamics, subsurface flow processes, and water, energy and CO2 budgets. The performances of the model were verified through Leaf Area Index (LAI), ground energy and CO2 fluxes' measurements recorded by eddy covariance systems in a southern Italian site and in California (USA). The model proved to be a reliable tool for the estimation of CO2, energy and water fluxes, vegetation dynamics, and of the space–time distribution of soil moisture. Furthermore, a sensitivity analysis has been performed on 49 parameters plus five initial conditions, revealing that there are relatively few parameters significantly influencing the main output of the model. The most innovative feature of the model is that it was developed following a discrete approach based on an extension of the original CA computational paradigm, whose structure permits high computational efficiency on parallel architectures to be achieved. This allows the proposed model to be used on wide areas with a highly detailed description of the processes involved, especially the soil water diffusion dynamics, both along the vertical and the horizontal directions. A three‐dimensional test case is proposed with boundary conditions inducing a significant lateral flow. In this case the coupled model showed both the importance of a detailed modelling of the three‐dimensional water diffusion, and its potential in describing the reciprocal influences among soil water flow, water fluxes and CO2 fluxes at the surface in the case of bare soil and growing vegetation. Specifically, in the case of vegetation cover, root activity reduced the propagation of the wet front in the soil, involving percentage differences in soil moisture up to 16% after 15 days simulation. Copyright © 2010 John Wiley & Sons, Ltd.