Ensemble Interaction Documentation

ESP Ensemble Analysis Interface - Making Selections and Interpreting Results

The Ensemble Streamflow Prediction (ESP) Trace Analysis interface provides tremendous flexibility in deriving products and information from the available ensemble streamflow traces. It is very important that the selections and information be well understood, as potential for misinterpretation can be quite high.

ESP is a modeling component of the National Weather Service River Forecast System (NWSRFS). NWSRFS is a physically based, continuous modeling system which simulates snow pack, soil moisture, streamflow, and reservoir regulations. Hydrologic states within NWSRFS are updated regularly to insure that modeled hydrologic states are consistent with the observed hydrograph. ESP accesses the current hydrologic model states and uses historic meteorological data to create an ensemble of forecast hydrographs. These hydrographs are treated as equally likely outcomes. Statistical analysis can be performed on user defined time intervals to generate a variety of probabilistic forecasts, such as seasonal water supply volumes, daily peak flows, minimum weekly flow, etc.

NWSRFS is a flexible modeling system which can be structured to represent any hydrologic system configuration. The NWRFC utilizes this flexibility to calculate 'natural' streamflow, water supply volume, with and without climate forecast information.

ESP natural streamflow assume no reservoir regulation or diversions upstream of the forecast point. In other words, it is the streamflow that would be expected without man-made influences.

The ESP water supply forecasts were constructed to reflect the same adjustments that are being made in the regression water supply forecasts. ESP water supply forecasts do not always represent true natural streamflow conditions. The adjustments made at a particular water supply forecast point may be found at the water supply forecast web page under the page headings for each point. In many cases, ESP natural and ESP water supply forecasts will be the same.

Long-term climate forecast information can be included in any of these ESP forecasts. Future implementation may include temperature and precipitation distribution shifts utilizing seasonal forecasts from the Climate Prediction Center.

NOTE: All ensembles include ten days of deterministic temperature and precipitation forecasts. This will be reflected in the first ten days of the ensembles not having any uncertainty information. This may also cause large flucuations in forecasts between weekly runs. This is especially true if the forecast period is quite small.

Selecting an ESP Trace Location
Ensemble streamflow prediction traces are generated for every river forecast site at the Northwest River Forecast Center. The available ensembles represent natural flows and in some cases have been adjusted for man made reservoirs and diversions. Natural streamflow is flow that would have occurred without upstream water management (reservoir regulation, diversions, etc.). Natural ensemble selection is identified by the ESP-NAT at the end of the station name in the selection box. Water supply ensembles have been adjusted for some large regulation and diversions. They correspond to the official water supply forecasts at those sites. Water supply ensembles are identified in the selection window by the ESP-WS at the end of the name.

Select Output Variable and Accumulation Interval
These two selections really need to be addressed together because they are related.

Output Variable	Accumulation Period	Description
Mean	Day	Mean daily flow for each day in the analysis period
	Week	Mean weekly flow for each 7-day period, starting with the first day of the analysis period
	Month	Mean monthly flow for each calendar month fully contained within the analysis period
	Entire Period	Mean flow for the full analysis period
Minimum	Day	Minimum mean daily flow
	Week	Minimum weekly flow for each 7-day period, starting with the first day of the analysis period
	Month	Minimum monthly flow for each calendar month fully contained within the analysis period
	Entire Period	Minimum flow for the full analysis period
Maximum	Day	Maximun mean daily flow
	Week	Maximum weekly flow for each 7-day period, starting with the first day of the analysis period
	Month	Maximum monthly flow for each calendar month fully contained within the analysis period
	Entire Period	Maximum flow for the full analysis period
Sum	Day	Daily volume
	Week	Weekly volume for each 7-day period, starting with the first day of the analysis period
	Month	Monthly flow volume for each calendar month fully contained within the analysis period
	Entire Period	Total flow volume for the full analysis period

Selecting Start Date
Select the beginning date for your analysis period of interest. The default is the current date. The system will not allow you to select a date in the past. If an invalid date is selected, the default current date will be used.

Selecting End Date
Select the ending date for your analysis period of interest. Normally the ESP runs are for 13 months. Beginning in June runs extend out to end of the next water year.

Select a Plot Option and Generate
These options allow the user to generate a graphical plot. The content of the plot depends on the previous selections made in steps 1-5. The plot options are Trace, Probability, Expected Value, and Exceedance.

Plot - Trace
This plot is often refered to as the 'spaghetti' plot. It displays all of the traces together on the same plot. This plot does not contain explicit probabilistic information but does show the variability of the forecast data. The Y-axis is the selected hydrologic parameter and the X-axis is date of the analysis selected at forecast time. The trace color code represents the year. Traces will show the streamflow time series for each calibration year over the selected analysis period. Data will be aggregated for the interval selected. For example, if you select Mean and Day, you will get the mean daily flows each day of the analysis period.

Plot - Probability Histogram
The probability histogram plot depicts exceedance probability ranges associated with values of a hydrologic output variable. The plot includes a series of stacked bars, one for each analysis interval within an analysis window. Each segment of a stacked bar represents an exceedance probability range and is plotted against the associated values of the hydrologic output variable. The example below is a weekly mean flow at a forecast point. Probability plots allow you to display the statistical probability distribution of the traces shown above. Instead of showing the individual time series, the time series are collected, analyzed, and fit with a distribution. From this distribution, ranges of exceedance probabilities are sampled and displayed. Currently, the NWRFC is fitting an empirical distribution for all analyses on this website. Further refinement of this technique may be appropriate in the future.

Users are advised to consider these probabilities as estimates as they do not represent the uncertainty associated with model errors and current model states. The uncertainty exhibited by the traces is strictly a function of the uncertainty associated with future weather and climate. For long-range seasonal volumes, the uncertainty associated with future weather and climate is so great that the model and model state errors are normally dwarfed. For nearer term forecast horizons (weeks), the uncertainly associated model and model states may be significant leading to a higher level of confidence than is appropriate. Work is currently underway to more appropriately address and include all significant sources of uncertainty.

If you select Mean and Week, you will get the probability distribution for each week in 5 colored regions. The colored regions show the range of exceedance probability which can be resolved by date and flow level. From below, there is a ~90% chance that the flow will exceed ~41,000 cfs during the week of starting May 21st.

Plot - Expected Value
This line bar plot represents the mean, standard deviation, and max/min values of the probability distribution. There is a bar for each selected time interval in the total forecast analysis window. The bars can represent any hydrologic parameter available in the ensemble forecast program. The expected value plot simply shows the probability information in a different way. Inferences related to the selection of Mean, Minimum, Maximum, and Summation can be taken from the examples related to the plots. Aside from looking different, the most important distinction is that there is no assumption or fitting of a theoretical probability distribution. Instead, the distribution mean, standard deviation, and maximum and minimum values are plotted. This is an informative plot if you are interested in quickly displaying the ensemble means. It is also helpful if you are interested in the extreme values present in the data set. Extreme values may influence, but are not explicitly shown on the probability plots.

Plot - Exceedance
The exceedance probability plot presents the results of a frequency analysis of a given output variable for a speified time window. Values of the output variable and/or the fitted probability distribution are plotted against the exceedance probability. Values of the output variable and/or the fitted probability distribution are plotted against the exceedance probability. The exceedance probability is based on the empirical distribution. The degree to which the fitted distribution fits the data values affects the degree of confidence one should place on the probability estimates. When generating probability plots or tables, its always a good idea to check the exceedance plot to make sure that the assumed distribution is a reasonable fit.

Select a Table Option and Generate
This selection option allows the user to generate a table instead of plots. Often, it is difficult to read specific numbers off of a plot.

# ESP Forecast Information 

# 

# Analysis Period: 4/1/2007 06 - 9/30/2007 24 (PST) 

# Forecast Parameters: River Flow (Mean) - (CFS) 

# 

# Forecast Interval: 1 Month 

# Forecast Point: PUDDING R AT AURORA at PUDDING R AT AURORA 

# 

# PUDDING R AT AURORA at PUDDING         0.90         0.75         0.50         0.25         0.10 Units    

# -------------------------------------------------------------------------------------------------------- 

  04/01/2007 - 04/30/2007              828.35      1111.37      1551.07      2089.02      2375.86 (CFS)    

  05/01/2007 - 05/31/2007              468.23       519.51       891.26      1167.18      1602.45 (CFS)    

  06/01/2007 - 06/30/2007              285.94       316.17       406.59       517.30       735.19 (CFS)    

  07/01/2007 - 07/31/2007               90.18       133.53       184.27       231.69       316.47 (CFS)    

  08/01/2007 - 08/31/2007                7.07        27.36        57.65        88.11       128.17 (CFS)    

  09/01/2007 - 09/30/2007               16.84        66.41       107.27       143.82       232.45 (CFS)