ESP is a significant portion of the NWS River Forecast System (NWSRFS) as it produces an ensemble of possible streamflow hydrographs which can be analyzed using standard statistical techniques to generate forecasts and outlooks.
ESP is run to produce an ensemble of equally likely to occur stages for each forecast point.
In its basic form, it assumes that historical meteorological data are representative of possible future conditions and uses these as an input data to hydrologic models, along with the current states of these models obtained from the forecast component of the NWSRFS.
A separate streamflow time series is simulated for each year of historical data using current conditions as the starting point for each simulation.
The streamflow time series for each year's simulation can be analyzed statistically for peak flows, minimum flows, flow volumes, etcetera, for any future time period to produce a probabilistic forecast for the streamflow variable.
This analysis can be repeated for different forecast periods and additional streamflow variables of interest.
Short-term quantitative forecasts of precipitation and temperature can be used to weight the years of simulated streamflow based on the similarity between the climatological conditions of each historical year and the current year.
