Runoff theory partitions the hydrologic response of a catchment into direct runoff (surface runoff, precipitation on the stream, and prompt/rapid interflow, which reach the outlet quickly) and base flow (delayed interflow and groundwater contribution, which reach the outlet slowly, often over months to years), and the storm hydrograph shape—rising limb, peak, crest segment, falling/recession limb—is governed by catchment characteristics such as basin shape, size, slope, land use, vegetation, and drainage density, as well as the time of concentration (the maximum travel time from any point in the catchment to the outlet). Separating base flow from a storm hydrograph (via straight-line or curve-extension methods) isolates the direct runoff hydrograph attributable to effective rainfall, and under the simplifying assumptions of time invariance and linearity, this relationship underlies unit hydrograph theory, in which the direct runoff hydrograph produced by a unit depth of effective rainfall over a given duration can be used to predict runoff from arbitrary rainfall patterns.
Runoff theory partitions the hydrologic response of a catchment into direct runoff (surface runoff, precipitation on the stream, and prompt/rapid interflow, which reach the outlet quickly) and base f…