Ensemble modeling is a process whereby multiple base models are created and simulated in a collective fashion to predict an outcome. In HEC-HMS, base models may currently consist of either a Simulation Run or a Forecast Alternative. Each base model may include different basin configurations, modeling methods, parameter sets, initial conditions, and/or boundary conditions. As long as the base models are diverse and independent, the prediction error of the base models decrease when the ensemble modeling approach is used. While the ensemble model is composed of multiple models, it acts and performs as a single model.