It is well known that the spatial variability of the terrain strongly affects storm runoff and non-point source pollution processes. While representing a watershed by a small number of lumped parameters has the advantage of simplifying the hydrologic modeling, it might miss some local process that affects the overall response of the system. In this research, a general methodology for modeling storm runoff and non-point source pollution that accounts for the spatial variability of the terrain has been developed, and the necessary tools for producing hydrographs and pollutographs have been prepared.
The model developed here is a generalization of the unit hydrograph model, and can be used to route water on the surface as well as in the subsurface, provided that the systems are assumed to be linear. The model also allows the user to consider time- and space-varying rainfall, thus relaxing some of the basic assumptions of the unit hydrograph. The heterogeneity of the terrain is considered by subdividing the study area into flow-elements, each of them with specific physical characteristics. The flow-elements are connected so that a spanning tree is formed and short-circuits are avoided. For each flow-element, a different flow pattern is assumed, as well as a different runoff production and pollutant mobilization.
This research is one of the first attempts made towards modeling the horizontal motion of water and pollutants in non-homogeneous areas, within GIS. This means that GIS has been used as a modeling tool itself and not only as a link between the heterogeneous terrain and an existing lumped non-GIS model.
Although definite conclusions can be drawn only after extensive testing of the methodology, it has been observed that the handling of spatially distributed data by Arc/Info proved to be adequate; that the routing model is an improvement on the currently used unit hydrograph; and that the input generation model still has considerable room for improvement, especially with regard to pollutant mobilization.
As a result of this research, flow routing and pollutant transport modeling within GIS is viable, which allows one to account for the heterogeneity of the terrain in a more consistent way.