Building a Model:Infiltration

With a working overland flow model in hand, the next step is to select an infiltration routine and assign the needed initial parameters. In selection of an infiltration routine, the source of runoff and streamflow should be a major consideration. The GA, multi-layer GA, and GAR models are only valid selections if the primary mechanism generating stream flow is Hortonian (Horton, 1933) infiltration excess runoff. Downer et al (2002a) explain the pitfalls of trying to apply the GA based methods to watersheds where the Hortonian runoff mechanisms is not dominant. In this case, the more general RE solution should be used to calculate infiltration. If a shallow water table is present, then the effects of the water table will also have to be included in the model. This will be discussed later. Conversely, if Hortonian flow is the dominate stream flow producing mechanism, solving RE will likely yield only small benefits over solving a less general GA approximation (Downer and Ogden, 2003a).

For single events the GA model or multi-layer GA model will suffice. For continuous simulations the GAR method is used. A single event model using GA may later be changed to GAR by changing the infiltration option and supplying the additional needed parameters. With a method of calculating infiltration selected, the appropriate initial parameters are assigned using the Mapping Table and a combination index map of soil-texture, land use, and vegetation. When using RE the appropriate vertical grid size is important (Downer, 2002a) and the effect of the grid size on runoff should be investigated to determine an adequate resolution. This model should also be run with the uniform rainfall event. As infiltration will tend to reduce the amount of runoff, the model with infiltration will likely run on the first attempt, and the time step can likely be increased without changing the shape of the predicted outlet hydrograph.