This blog describes how the German Runoff hydrology converts precipitation excess (rainfall and/or snowmelt less infiltration, evaporation, and initial abstraction) into surface runoff (overland flow). InfoSWMM is a distributed model as it allows a study area to be subdivided into any number of irregularly shaped Subcatchment areas to best capture the effect that spatial variability in rainfall, topography, drainage pathways, land cover, and soil characteristics have on runoff generation. Generation of runoff is therefore computed on a Subcatchment by Subcatchment basis with the option of having the runoff go to a node or another Subcatchment.  All of the ancillary processes possible in InfoSWMM: LID’s, 5 types of infiltration (Horton, Modified Horton, Green Ampt, Modified Green Ampt and SCS CN), groundwater, evaporation, snowmelt, 2D processes and water quality are active and used in the German Hydrology Option.

The source of the equations used in the German Runoff hydrology method Is the program
Storm.XXL (Storm XXL. was developed by InnoAqua )which can be seen at:  http://www.sieker.de/daten/download/sieker/public/manual_storm2006.pdf. It uses the a mathematical model called the storage cascade.  The storage cascade is a further development of the linear single storage. The linear storage cascade consists of several single storages that are switched to an in-line cascade. It is a linear storage equation and has two parameters (a Storage Constant in minutes) and a reservoir count that is integer and normally ranges from 1 to 5 (Figure 1).  The user sets the storage time for each reservoir and the number of reservoirs for each Subcatchment either in the DB Table for Subcatchments (Figure 2) or the Attribute Browser (Figure 3).

The numerical equations for each storage and the total runoff from the cascading storages are shown in Figures 4 and 5, respectively.  The Run Manager with German Runoff is shown in Figure 6.  The effect of varying the reservoir count and storage constant are shown in Figures 7 and 8.