Duration is a versatile, but complicated query. Three settings are available: 1) number of days, 2) a selection of statistics to be computed for each duration in the season, and 3) a selection of statistics to be computed using the time series of computed values produced in step two.
The number of days defines a duration interval (Figure 8). Duration calculations are performed from the beginning of season to the end of season. So for the start date, HEC-EFM considers all data values within the duration interval (per setting 1), computes a statistic (per setting 2), records that value for the start date, and then advances a day and repeats the process until the end of season is reached. This process produces a statistical time series of Minimums, Medians, Maximums, User defined percentages, or Means (per setting 2) that has one value for each day of the season.
The final step in the Duration query involves selecting a single seasonal value (per setting 3) from the statistical time series. Options are provided for Minimum, Median, Maximum, User defined percentage, and Mean. This process produces a time series of seasonal results that has one value per season.
Figure provides an example of this process using an 8 day duration interval, minimums, and then maximum of the minimums.
Figure 8. Sample use of the Duration query in HEC-EFM. Computations are shown for one water year. Statistical queries (to right of graph) are shown as in the EFM interface in Figure 4.