An HEC-WAT Flood Risk Analysis (FRA) compute requires a hydrologic event generator to generate the sequence of events utilized during the FRA compute. The hydrologic event generator is implemented as an additional set of methods defined in the plugin API. Two hydrologic event generator plugins are provided by HEC to control the FRA computes:

Hydrologic event generators are given a set of seeds by HEC-WAT for the natural variability and knowledge uncertainty random variables. From the set of seeds, the plugins that implement this interface are responsible for creating a sequence of events within the time window of the analysis period. The hydrologic event generator plugins run first in the program order, and cannot appear twice in the program order for an FRA compute. There are no significant limitations to how the hydrologic event generator creates events, but the general convention is that they produce one event per year within the time window defined by the analysis period. 

The plugins that implement the hydrologic event generator methods must set the start and end time of events within the analysis period and provide the list of events to HEC-WAT to communicate to the plugins in the program order.  Since HEC-WAT iterates over the list of generated events for a lifecycle, the hydrologic event generator plugins must create events for an entire lifecycle at the beginning of a lifecycle. Hydrologic event generators generally also provide a series of boundary conditions that define the magnitude, timing, and spatial extent of the events for which it creates time windows.

A user must set up a model alternative for a hydrologic event generator plugin prior to setting up an HEC-WAT FRA simulation, because it must be selected before a simulation can be saved. See the following hydrologic event generator user manuals Hydrologic Sampler and Stochastic Data Importer) for more information.