CASC2D solves the equations of conservation of mass, energy and linear momentum to estimate watershed runoff for a given rainfall input. The overland flow routing formulation is based on an explicit 2-dimensional finite difference (FD) technique. An explicit FD technique is used for the routing of 1-dimensional channel flow. To understand the method by which the CASC2D model conducts its calculations, it is important to conceptualize the two dimensional spatial nature and resolution of the given watershed data. The explicit numerical solution is achieved by segmenting the entire catchment area into equal square elements, which are then assigned parameters relating to soil infiltration characteristics, roughness coefficients, and soil erosion parameters (Spah 2000). As each parameter is defined, it is assumed to be uniform throughout the cell area with the actual value assigned to a central nodal point.