Appropriate model time-steps

Selection of the time-step for a catchment  depends on the time step of Meteorological data.

The selection of an appropriate time-step for modelling is a compromise between accuracy and run time/computing power requirements. A smaller time-step will require longer computing time, and this may be an issue, particularly when running on slower machines or using Catchment files with many nodes .

An appropriate time-step will depend not only on the Source Node or overall Catchment size, but also on the size of Treatment Nodes within the catchment. For example, running daily time-step simulations on a 2m wide grass swale of 50m length, will produce unreliable results for both the routing of flows, and the behaviour of pollutants (because the variation in both will occur at time-steps of considerably less than one day).

Therefore, selection of time-steps should be based on the size of the smallest Source Nodes and Treatment Nodes within the overall Catchment, using a simple three-step process:

1. Calculate or estimate the time of concentration (tc) of the smallest sub-catchment within the model;
2. Calculate or estimate the shortest expected detention time of proposed treatment measures; and
3. Select a time-step which is equal to or smaller than the smaller of 1 and 2.