The Volume Frequency analysis allows the user to fit a family of probability distributions to multiple annual maximum series of volume-durations. Only regular interval flow data may be analyzed within this analysis. Several different analytical distributions | fitting methods can be used, including Log Normal | Product Moments and Log Pearson Type III | Expected Moments. Parameters of the fit distributions (for example, standard deviation and skew) can be manually altered from their computed values to adhere to expected trends across the various durations of interest. Additionally, the user can also graphically fit empirical distributions to the volume-duration data.

Create a new Volume Frequency Analysis

  • Create a new Volume Frequency analysis and name it "Aquashicola_EMA".
  • Select the Aquashicola daily flow data set within the Data Set drop down menu.

The data set will be named "Aquashicola Creek-Palmerton, PA-FLOW".

  • On the General tab:
    • Ensure that the Use Log Transform, Analyze Maxima, and Water Year radio buttons are selected.
    • Click the End Date check box within the Extract Time Window Modification panel.  Enter an end date of 30Sep2024.

      Entering an End Date of 30Sep2024 ensures that only data before this date will be extracted and made available for further analysis.

    • The General tab should resemble the following:
      Aquashicola_EMA General Tab
  • Move to the Options tab. Take note of the options that are available within this tab. However, leave contents of this tab unchanged.
  • Move to the Duration Table tab and take note of the volume-duration data contained within each column. Click the Extract Volume-Duration Data button.

    Be sure to click the Extract Volume-Duration Data button to update the information on this page.

  • On the Analytical | Settings tab, select the Log Pearson III | EMA distribution | fitting method choice within the Distribution drop down menu. Leave all other options within this tab unchanged.
  • Move to the Analytical | EMA Data tab. This is similar to the tab that was previously shown within the Bulletin 17 editor, but there is a sub-tab for each X-day duration. Perception thresholds and flow ranges can be entered for each duration within these additional tabs.

Compute and Visualize the Results

  • Click the Compute button at the bottom of the Volume Frequency editor.
    • A window informing you of any volume-duration data that is out of order will be shown. Click OK to close this window.
    • A window informing you of any events that occur near the beginning of the water year will be shown. Click OK to close this window.
    • Finally, a window informing you that the computations are complete should appear. Click OK to close this window.
  • Click the Plot Analytical Curve at the bottom of the Volume Frequency editor to view the results. A window similar to the figure below should appear.

Aquashicola_EMA Plot

  • Notice that there are 12 data sets plotted within this window. Click on each data set in the legend to better identify which data set is with each legend item in the plot.
  • When you are done, close the plot and move to the Tabular Results tab. Take note of the tabular information that is contained within this tab including the computed curves and distribution parameters for each duration as well as details of the selected data set.
  • Click the View Report button to open the report.
  • Within the report window, take note of the expanded information that can only be found within the report file. When you are finished scrolling through the report, close the window by clicking the "X" button in the upper right corner of the window.
  • Click OK to close the Volume Frequency editor.
  • Click the Save button (  ) to save the study.

Continue to Task 6. Import Data using an HEC-DSSVue Plugin.