Observations cannot adequately “close” budgets since too many fundamental processes are missing. Models, especially analysis models are required for many individual processes that are not really measured in any meaningful way on a continental scale. For example, soil moisture and evaporation are measured at only a few sites, although there are intensive efforts to develop satellite-derived products and evaporation and soil moisture variations are sometimes derived as a residual. High-resolution atmospheric moisture and dry static energy convergence also cannot easily be derived from radiosonde observations since knowledge of the vertical motions is also required.
Models that properly represent the many complicated atmospheric and near-surface interactions are required for overall descriptions of the budgets. Models will also be needed for eventual predictions of these water and energy processes. Therefore, different classes of models have also been compared with available observations. The comparison includes a representative global general circulation model, regional climate model, and a macroscale hydrologic model. There does appear to be a clear advantage to using a regional analysis over a global analysis or a regional simulation over a global simulation to describe the Mississippi River Basin budgets. There also appears to be some advantage to using a macroscale hydrologic model for at least the surface water budgets.