Coupled Vegetation and Soil Moisture Dynamics Modeling in Heterogeneous and Sloping Terrains

Two vegetation dynamic models (VDMs) were coupled to a soil–vegetation–atmosphere transfer scheme aimed at simulating water, heat, and CO 2 fluxes and to a three-dimensional unsaturated flow and heat diffusion model allowing the study of heterogeneous soils and vegetation. The VDMs differ in reproducing gross photosynthesis. One was adapted from a daily model, while the other is a more complex model, needing the continuous monitoring of CO 2 concentration and a larger number of parameters, although it is useful for assessing feedback due to an increase in CO 2 concentration. The two coupled models were validated on a half-hour time step through leaf area index (LAI), ground energy, and water flux measurements performed during the summer of 2007 and 2009 in an alfalfa ( Medicago sativa L.) field in the Mediterranean (southern Italy) and during the spring of 2001 in a predominantly C 3 grass covered field (California). Furthermore, the models were used at the field scale in numerical experiments for assessing the effects of model dimensionality on evapotranspiration, CO 2 flux, and LAI in the presence of flat heterogeneous or sloping homogeneous terrains using both the fully three-dimensional soil water flow model and a simpler model allowing only vertical flow. For the flat terrain experiment, one-dimensional modeling generally provided significant differences in the simulated quantities with respect to the three-dimensional model, even though in some cases similar results were found. For the sloping terrain, one-dimensional modeling proved quite unsuitable for analyzing slope effects, while three-dimensional modeling allowed detailed description of different degrees of slope interactions between soil moisture dynamics and surface fluxes for different slope angles.