The Rate of change query was originally added to HEC-EFM to look at ecological concerns connected to the recession limb of stage hydrographs. Its capabilities have since been expanded to include Rising, Falling, and Absolute rates of change for both Stage and Flow.
For all combinations of those settings (Figure 9), the Rate of change uses two parameters: 1) the threshold value for change and 2) the number of days. For each day beginning at the end of the season and working backwards in time, the actual rate of change is computed by subtracting the current flow or stage and the flow or stage that occurs at the end of the time interval defined by the number of days (i.e., if number of days is set to 4, the actual rate of change equals the current value minus the value 3 days into the future such that the total time interval being considered spans 4 days). The actual rate of change is then compared to the threshold value. If actual does not violate threshold, the rate of change is deemed acceptable, HEC-EFM moves backwards one day, and the test is repeated. This process continues until the threshold is violated or the beginning of season is reached.
If the threshold is violated, HEC-EFM selects the previous successful test (one day later than the failed test) as the seasonal result, which represents the date and conditions where rates of change became consistently acceptable for the rest of the season.
If the beginning of season is reached and passes the rate of change test, HEC-EFM selects that date and its corresponding conditions as the seasonal result.
Figure 9. Sample use of the Rate of change 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.
If the first test (at the end of season) fails, a zero value is recorded as the seasonal result, which indicates that no dates passed the rate of change test for that season.
It is also important to note that when Rate of change is selected in the HEC-EFM interface, the dropdown controls in the Duration query become gray and inoperable. This behavior is meant to communicate that the two queries are largely incompatible. In fact, the only Duration setting relevant to Rate of change is number of days. When duration is set to 1 day (as in Figure 9), the Rate of change query tests the mean daily values obtained from the flow regimes in the seasonal extract. When an integer of 2 or more is entered, HEC-EFM will compute a time series of mean values for that duration of flow or stage, depending on the setting in Rate of change, and then perform the rate of change tests for that computed series.