The rainfall-runoff models in DRAINS simulate particular storm events, rather than extended and continuous time series of rainfalls and runoffs that might contain many events. With an event model it is necessary to specify antecedent conditions, particularly the wetness of the catchment prior to the event. During the storm, the model must describe the catchment becoming wetter.
In the other type of rainfall-runoff model, a continuous model, it is necessary to provide soil moisture and groundwater storages to model changes in soil moisture levels and interflow, the water flowing out of the partly-saturated soil layers located above the water table, forming seepages or springs. These models can describe both the wetting and the drying out of soils.
DRAINS only models events. The MUSIC model from eWater, used for stormwater quality modelling, treatment and water-sensitive design, operates as a continuous model.