1st The time step you use in SWMM 5 is controlled from the top by the rainfall interval (Figure 1):
1. All of your time steps should be less than the rainfall interval,
2. The hydrology time step should be less than or equal to the smallest raingage rainfall interval in your network,
3. The hydraulic time step should be less than or equal to the hydrology time step and should be based on the hydraulic needs of the your network. Short length links, pump and weirs may require a smaller maximum hydraulic time step.
2nd The report time step controls what you see in the graphics output of SWMM 5. If you see a large difference between that you see in the graphics output and the report text file it is because you have a large difference between the report time step and the average time step used during the simulation.
Solution: If there is a large discrepancy in the graphics and report text file then the best solution is to reduce the maximum time hydraulic time step so it is closer to the average time step and also to make the report time step closer to the Maximum time step (Figure 2).
Figure 1: Relationship between the rainfall, hydrology and hydraulic time steps.
Figure 2: Relationship between the minimum, average and maximum simulation time steps and the report time step.
The average step shown in the routing time step summary is a good choice for the best time step. Here is the Routing Time Step Summary in EPA SWMM 5.1.015+
Routing Time Step Summary
Minimum Time Step : 2.30 sec
Average Time Step : 23.87 sec
Maximum Time Step : 30.00 sec
Percent in Steady State : 0.00
Average Iterations per Step : 2.73
Percent Not Converging : 1.59
Time Step Frequencies :
30.00 - 3.82 sec : 97.21 %
3.82 - 0.49 sec : 2.79 %
0.49 - 0.06 sec : 0.00 %
0.06 - 0.01 sec : 0.00 %
0.01 - 0.00 sec : 0.00 %