Problem

What does the following user notification mean? I can't change the Tc of the catchment in question. What should I do?

"Tm > .25Tp. Computation increment, Tm, is greater than 1/4 Time to Peak on Unit Hydrograph. Using a larger Tc can solve this problem."

Problem ID#: 90912

Solution

The term "Tm" refers to the Output Increment, set in your calculation options. The "Tp" term refers to the time to peak of the Unit hydrograph (not to be confused with the runoff hydrograph).

PondPack does a check to see if Tm (the time step increment) is relatively too large, based on comparing it to 1/4 of the Tp. Although seeing this notification doesn't stop the runoff hydrograph from being generated, it is an indicator that the time step may be too large, or the time of concentration may be too small. This basically means that you may be missing out on detail in the runoff hydrograph due to the time step being too large. If the Tc you entered is correct, this means that you should select a smaller Tm (time step).

If unsure on the practical effect of this in the model, consider trying a range of time steps and compare results.

Further to this, if you'd like to see details of the time to peak of the unit hydrograph, check the below reports in report builder (subsection column) :

1) Unit Hydrograph Summary - under "SCS unit hydrograph parameters", you will see the "unit peak time, Tp"

2) Unit Hydrograph coordinate - this shows the actual internal unit hydrograph used during the unit hydrograph calculations. Check the time and discharge columns and you'll see that the peak unit discharge corresponds to a time of 0.073 hours. Note that this report may not show up for you by default - you may need to go into Report Filters (button at the top of report builder), click the Edit button, and make sure this report is checked, under the "runoff hydrographs" section.

See Also

Catchment user notification about the difference between the calculated peak flow vs. interpolated peak flow being greater than 1.5%