This chapter describes the models that simulate the process of direct runoff of excess precipitation on a watershed. This process refers to the "transformation" of precipitation excess into point runoff. The program provides two options for these transform methods:
- Empirical models (also referred to as system theoretic models). These are the "traditional" unit hydrograph models. The system theoretic models attempt to establish a causal linkage between runoff and excess precipitation without detailed consideration of the internal processes. The equations and the parameters of the model have limited physical significance. Instead, they are selected through optimization of some goodness-of-fit criterion.
- A conceptual model. The conceptual models included in the program are the Kinematic Wave model of overland flow and the Two-Dimensional (2D) Diffusion Wave model. They represent, to the extent possible, all physical mechanisms that govern the movement of the excess precipitation over the watershed land surface (and in small collector channels in the watershed, in the case of the Kinematic Wave transform).