So how does the ESP technique work? You begin by having the hydrologic model save the model states at a particular time. By comparing the model results to observed conditions, the model states can be adjusted to reflect the current conditions. Once saved, a model state acts as a starting point for future simulations. Since observed precipitation and temperature data haven’t happened yet, multiple scenarios using historic or forecast data can be used as input to the model. From each set of inputs, a possible streamflow hydrograph can be generated. Taken together, an ensemble of possible hydrographs are produced.
Once several scenario hydrographs are calculated, the results can be used to generate probabilistic streamflow values.