The storm time step is determined by the format of meteorological data. For synthetic storms it is usually five to ten minutes. The hydrograph computational time step, DT, is determined from the watershed characteristics. For example:
Convolution with NASHYD requires DT < Tp (time to peak – preferably DT about 1/5 Tp)
Visual OTTHYMO will transform automatically for each sub-watershed, new storm input with the time step DT.
In routing with the VSC method, it is recommended to maintain a small-time step. Although this is not required for mathematical stability, Ponce and others recommend short time steps and the use of the Courant criterion for hydrologic routing.
The celerity is given by:
Celerity ranges from 1.1 to 1.6 times the average velocity. Using the above criterion, it is found that, for time steps used in convolution (hydrograph commands) the length cannot be very short.
For short reaches, the hydrograph should be simply ‘shifted’ in time. In comparison, routing with EXTRAN is usually conducted with time steps of 2 to 10 seconds, and gives an error message if the courant criterion is not met.