The variability in climate and hydrology is caused by the natural and human forcings. Studies on such variability require a practical method of interactively coupling coarse-grid climate models with fine-grid hydrologic models. A fully coupled climate-hydrology model has been developed to simulate land surface and groundwater hydrology such as changes in atmospheric circulation and changes in individual lakes, rivers, groundwater tables, and wetlands under the past, current, and future climates. Various digital and remotely sensed data have been prepared for the hydrologic simulation over the North America. A new method was developed to aggregate 1-km digital elevation model (DEM) to coarse-resolution DEMs by ensuring the correct delineations of major river basins and drainage networks such as the Mississippi River Basin. The model was spun up before any simulations to eliminate the influence of initial hydrologic conditions on the simulations. Various coarse-resolution DEMs were used to drive simulations for examining the scaling effect on the hydrologic simulations. The simulated streamflow at the basin outlet under current climate conditions compares well with observed data in terms of volume and seasonal trend.