The Ensemble Forecast Processor (EFP) is a plugin designed to compute statistical metrics from ensemble flow time series at specified watershed locations and use those metrics to inform other model alternatives within a forecast. For instance, an HEC-ResSim alternative can incorporate a metric from the EFP—such as a percentile-based flow volume—into a reservoir rule to guide release decisions. The EFP supports several statistical computations over a defined time window, including minimum, maximum, average, and percentile volumes.

This plugin is highly flexible and can be inserted at various points within a forecast's program order, enabling a wide range of applications. One common use case is Forecast-Informed Reservoir Operations (FIRO). In this scenario, forecasted inflows from the ensemble are processed by the EFP to generate a metric—such as the 75th percentile three-day forecasted volume—which can then influence decision rules in the HEC-ResSim alternative to guide reservoir operations.

Before running a forecast, you must:

  1. Create a new program order that includes the EFP.
  2. Develop an EFP alternative.
  3. Assign it a model alternative key.
  4. Set up a new ensemble forecast run.

The sections below outline the input and output settings that can be configured when setting up a new EFP alternative.

Input Settings

The Input Settings for an EFP alternative are organized into two tabs: LookBack and Forecast. These tabs allow you to specify locations and enter time series data used by the EFP during its computations. Data from both tabs can be linked within the Model Linking Editor.

LookBack Tab

Use the LookBack Tab to add locations that correspond to time series data within the Lookback Period of the Forecast Time Window. You only need to enter data in this table if:

  • You have selected the Ensemble Member with Closest Volume to Metric Compute option, and
  • You want to merge observed time series data from the Lookback period with the ensemble member selected for the Forecast period.

For more details, refer to the documentation on Ensemble Member that Contains the Volume Closest to Each Metric.

Ensemble Forecast Processor Alternative Editor - LookBack Tab

Forecast Tab

The Forecast Tab is required for any EFP compute. You can use it to define ensemble collection pathnames used in calculating statistical metrics. Each entry must point to an ensemble collection in the HEC-DSS file, whether from an extract list or generated by a previous program in the forecast sequence.

Ensemble Forecast Processor Alternative Editor - Forecast Tab

Output Settings

The Output Settings tab defines the outputs generated by the EFP.

Locations to Compute Metrics Tab

This tab lets you define where ensemble metrics should be calculated and what input data to use during the Lookback and Forecast periods. Add a row for each location where metrics should be computed, as specified in the Cumulative Volume Scalar Decision Metrics tab.

Ensemble Forecast Processor Alternative Editor - Output Settings - Locations to Compute Metrics Tab

Cumulative Volume Scalar Decision Metrics Tab

This tab allows you to define the specific ensemble metrics to calculate for each forecast. Multiple metrics can be computed in a single alternative. 

In the example below a total of 14 metrics are calculated at each computation location. The Cumulative Volume for 1 day and 5 days will be computed, along with the following metric for each duration:

  • The minimum value
  • The maximum value
  • The 20%, 50%, 75%, and 99% percentiles
  • The average value

Multiple Metrics Selected from the Cumulative Volume Scalar Decision Metrics Dialog

Run the Ensemble Member that Contains the Volume Closest to Each Metric

This optional compute step, enabled via the checkbox on the Cumulative Volume Scalar Decision Metrics tab, finds the ensemble member whose cumulative volume is closest to each computed metric.

For each compute location and metric:

  • The EFP identifies the ensemble member with the closest cumulative volume to the metric.
  • If the Lookback record is different from the Forecast record, the returned ensemble member is merged with the Lookback record.
  • If the Lookback record is null or the same as the Forecast record, the entire ensemble time series from Lookback Start to Forecast End is returned.
  • An additional HEC-DSS record is created that contains a single value at the time of forecast that represents the ensemble member collection ID that was returned in the compute.  

In the below example, the cumulative volume over 5 days for each ensemble member would be computed, and then the maximum of those volumes would be returned as the calculated metric. The following is a list of outputs (C Parts) generated by the EFP if the parameter selected in the Locations tab was flow. 

  • FLOW-CUMULATIVE(5.0DAY),MAX-Metric
  • CUMULATIVE(5.0DAY),MAX 
  • FLOW-CUMULATIVE(5.0DAY),MAX-EnsembleTS - ensemble member with the closest volume to the metric over 5 days from the start of forecast would be returned as a timeseries, and merged with the record in the Lookback Period
  • FLOW-CUMULATIVE(5.0DAY),MAX-EnsembleID - contains a single value at the time of forecast containing which ensemble member was returned in the compute.

The MAX-Metric and MAX-EnsembleTS records are available to programs later in the Forecast Run to use as inputs (via the Model Linking Editor). 

Closest Volume to Metric Compute Selected from the Cumulative Volume Scalar Decision Metrics Dialog

Additional Resources

For step-by-step instructions on how to set up a new EFP alternative, refer to this guide.