Quoted from: https://www.cbrfc.noaa.gov/present/rdhm/RDHM_User_Manual.pdf

Inputs to the routing techniques are runoff depths (e.g. mm/hr). The routing techniques can ingest two types of runoff: fast (surface) and slow (subsurface/ground) runoff. Within each cell, fast runoff is routed over a conceptual hillslope to a channel (Figure 3.2), and then channel inflow from the hillslopes, combined with a slow runoff component and upstream pixel outflows, is routed through a cell conceptual channel. A conceptual hillslope consists of a number of uniform hillslopes (the number of uniform hillslopes depends on the stream channel density specified for the cell and the cell area). The conceptual channel that passes water from cell-to-cell usually represents the highest order stream in a selected cell. A topographically defined cell-to-cell connectivity sequence is used to move water from upstream to downstream cells and to basin outlets (Figure 3.3). Figure 3.4 defines the SAC-SMA runoff components that are classified as slow and fast response for routing purposes.