An Ensemble Alternative is a new type of alternative available in ResSim. As described in the introduction of this Appendix, it was created to facilitate reservoir operations modeling using ensemble forecasted inflows. So, while a standard ResSim alternative represents only one correlated set of inflow hydrographs for the basin and produces one set of outflow hydrographs, an ensemble alternative represents multiple correlated sets of inflow hydrographs and produces the associated multiple correlated outflow hydrographs.
Ensemble inflows and the associated ensemble results utilize a new HEC-DSS time-series data group called a "collection". Collections are described in "HEC-DSS Collections". Collections can contain time series dataset that have varying time windows. However, collections that represent ensemble forecasts must contain only time series datasets with the same time window (or that span a common time window).
An ensemble alternative is essentially the same as a standard alternative except that instead of representing a single run and computing through a single set of inflow time-series as a standard alternative does, an ensemble alternative represents several runs, one for each selected member of the ensemble inflow datasets. ensemble set of inflow time-series. To do that, an ensemble alternative must use at least one collection dataset in place of a single time-series for any time-series boundary condition (e.g., inflow) expected by the alternative.