To date, applications have recognized the importance of change of hydrologic processes or input model parameters over time. Modern GIS have capabilities of traditional two-dimensional (2D) GIS to perform spatial analysis as well as the ability to handle and visualize third dimension (such as depth) and time as a fourth dimension (Fisher, 1993). Three-dimensional (3D) GIS are suitable for many applications in hydrology such as predictive hydrogeological modeling (Raper, 1989). Three-dimensional GIS lend themselves to the iterative process of modeling as well as the evolutionary nature of site characterization and remediation. Although most current 3D GIS provide some solutions for complex subsurface processes, they are still at the visualization stage rather than true modeling or interpretation. No one system yet meets all the needs of an ideal modeling environment, hence integration between multiple systems is desired. Also, four-dimensional (4D) GIS do not adequately represent the temporal dimension (Langran, 1992) because no GIS currently adequately handles chronology. We typically illustrate the effects of temporal change as slices of time for discrete intervals, but we need to show dynamic change over continuous time. Although many GIS can generate a 3D diagram, no commercial system has 3D geometry and topology such that disparate databases can be integrated in three dimensions as well as they are in two dimensions. The ultimate solution would be able to handle change in time as well as change in space. An ideal GIS handling time as a fourth dimension (4D GIS) will have chronology treated much like topology; before and after taking on the same importance as left and right in 2D space or above and below in 3D space. Such 4D GIS would be of immense value for a number of research areas in hydrology including soil moisture modeling, groundwater modeling, etc. because of their inherent four-dimensional nature.
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