Ensemble Computing was added to HEC-ResSim to support reservoir operations modeling using ensemble streamflow forecasts produced by NOAA's Extended Streamflow Prediction (ESP) of the National Weather Service River Forecast System (NWSRFS) and/or its successor, the Community Hydrologic Prediction System (CHPS). The ensemble computing capability can, however, be used with data that comes from other sources.
An ensemble streamflow forecast for a single location represents a set of possible streamflow hydrographs that could occur at that location over the forecast period. A full ensemble forecast for a watershed should be a correlated set of ensemble inflow forecasts for all inflow locations in the basin. This means that the first hydrograph in the ensemble forecast for a headwater location in the basin was generated with the same set of model parameters and boundary conditions as for the first hydrograph at the downstream-most location (and all points in between).
A standard ResSim alternative can be defined to simulate only one member (or trace) of an ensemble forecast set. Thus, if an ensemble forecast contained 30 members, then you would need to define 30 standard ResSim alternatives to simulate a given operation, one for each member of the ensemble forecast. This is very time consuming to setup and cumbersome to manage and analyze. However, with a new Ensemble Alternative, you only need to create one alternative to simulate through one or more members in an ensemble forecast. And, ensemble results from an Ensemble Alternative can be visualized together. "ResSim Ensemble Alternatives" of this section will describe how to define and use Ensemble Alternatives in ResSim.
But, before you can jump into creating and running Ensemble Alternatives, you will need to "collect" your ensemble hydrographs into a new HEC-DSS data representation called a Collection. Collections are defined using an F-part naming convention that identifies each member of the collection with a numeric string, followed by a separator after which the normal F-part conventions apply for identifying the dataset. The collection member number (or ID) are how you can reference specific ensemble members to be computed in the Alternative Editor. "HEC-DSS Collections" describes HEC-DSS Collections and how to create and edit them for use in ResSim Ensemble Alternatives.
