Components in the Time series specifications section of HEC-EFM include Percent exceedance and three controls for managing the period of record assessed for each relationship (Water year range, Individual water year, and Relationship-defined water year).
The Percent exceedance query offers a choice of either Flow frequency or Flow duration. Flow duration should not be confused with the Duration query described earlier in this chapter. In fact, when Flow duration is selected in the HEC-EFM interface, the dropdown controls in the Duration query become gray and inoperable. As with Rate of change, the only Duration setting relevant to Flow duration is Number of days. When duration is set to 1 day, HEC-EFM generates a flow duration curve using mean daily values obtained from the flow regimes in the seasonal extract and then interpolates to obtain the flow that corresponds to the user-defined percentage. When an integer of 2 or more is entered, HEC-EFM computes a time series of mean flow values for that duration, uses those values to generate a flow duration curve, and then interpolates to obtain the flow that is equaled or exceeded for the user-defined percentage of time. The resulting value would be the statistical result.
When Flow frequency is selected (Figure 10), HEC-EFM ranks the seasonal results (computed via the Season, Duration, and Rate of change queries) and interpolates to obtain the flow (or stage, if Rate of change is being used to investigate stage dynamics) that is equaled or exceeded for the user-defined percentage of years. The resulting value would be the statistical result.
Figure 10. Sample use of the Time series specifications for a relationship with Season and Duration queries. The statistical result is the flow meeting the parameters in those queries that is equaled or exceeded in 25% of years (Flow frequency). Statistical queries (to right of graph) are shown as in the EFM interface in Figure 4.
This ranking and interpolation process is required only when seasonal results are computed for multiple years, whether for a whole flow regime as defined on the Properties tab or via the Water year range control (Figure 10).
Both the Individual water year and the Relationship-defined water year controls limit analyses to a single water year. For the Individual water year, the user specifies which water year to assess. For Relationship-defined water year, the user selects a separate relationship that is of interest, and HEC-EFM determines which of its seasonal results (and corresponding historical water year) most closely equals its statistical result. That historical water year is then used when assessing the current relationship. In both cases, as there is only one seasonal result, it is also reported as the statistical result.