To model the runoff formation, it is considered to transform the effective precipitation of a catchment area into a discharge hydrograph at the outlet node of this catchment area.

For the calculation of the runoff formation, the so called “*hydrologic model approaches”* dominates. It assumes that the discharge concentration can be decomposed into translation and retention process, where both of them can be linearly superposed.

Applying the **translation** model, it is possible to describe time delay of the runoff (discharge) streamlines in the water network and subcatchments. Lines of the same translation time, that correspond to the flow duration of a water particle from one point of the catchment area to the outlet node is referred to as *isochrones*.

On the other hand, the discharge also depends on the **retention** characteristics. Thus, the discharge concentration is also influenced by the storage capacity of the catchment and not only by the translation process. The retention capacity depends on the soil type, vegetation cover and the slope of the terrain. It particularly influences the shape of the rising limb of hydrograph.