The development of multi-scale/multi-physics models for integrating and simulating hydrologic, biogeochemical, and ecological processes is being actively pursued by the scientific community. A unified multi-scale model (UMSM) developed by scientists at DOE’s Pacific Northwest National Laboratory offers realistic potential toward this goal. Based on fundamental principles of fluid flow in pores and porous media, the UMSM has been successfully demonstrated for its applicability up to the ecosystem scale. The Disney Wilderness Preserve (DWP) site in Kissimmee, Florida, which is subject to frequent changes in soil moisture saturation, the geometry and volume of surface water bodies, and groundwater and surface water exchange, was used to demonstrate the UMSM. The UMSM can simultaneously simulate all the hydrologic phenomena in surface water and groundwater components, including inundation and draining, riverbank flow, groundwater table change, soil saturation, hydrologic interactions between groundwater and surface water, and migration of surface water and groundwater interfaces. It is a cross-scale approach that is particularly suitable to simulate coupled surface-water and groundwater flow in ecosystems with strong surface-water and groundwater interactions.