The Flow Duration: Analyze seasons feature enables a hybrid flow duration - flow frequency analysis for relationships that also use Time series specifications - Percent exceedance - Flow duration and Reverse lookup options. Users are required to enter a Percent exceedance value (Figure 17).
Figure 17. The Flow Duration: Analyze seasons feature is available via the Relationships tab. It requires the user to specify the % exceedance to compute and report.
The compute process begins by applying Season and Duration criteria - as done for basic flow duration reverse lookup relationships - though this is where the basic and hybrid approaches diverge. The basic approach would then rank all daily values from all seasons in the period of record and computes the percentage of time the reverse lookup value is equaled or exceeded. The hybrid approach considers each season independently. Daily values within each season are ranked and used to compute the percentage of time the reverse lookup value is equaled or exceeded. This is repeated for each season in the period of record to compute a set of seasonal results. Seasonal results are then ranked and the % exceedance value associated with the Flow Duration: Analyze seasons feature is used to compute a corresponding percentage of time for the whole period of record (Figure 18).
Figure 18. Seasonal flow duration analyses use flow duration and flow frequency concepts to gain insights about seasonal variability.
This nested approach supports consideration of season to season variability in flow duration statistics. Similar functionality is provided via the Duration - User defined percentages control, though the User defined percentages option can be applied only as part of Time series specifications - Percent exceedance - Flow frequency relationships. These hybrid approaches are among the most involved statistical analyses performed by HEC-EFM. Ordered flows and corresponding plotting position values are output to help clarify the compute sequence. The whole progression of output follows: 1) seasonal extract, 2) ordered values, 3) plotting positions, 4) seasonal results – per the Flow duration percent exceedance criterion, 5) a ranked set of seasonal results from #4, and 6) the statistical result - per the flow frequency Percent exceedance value specified by the user as part of the Flow Duration: Analyze seasons settings.