Introduction: This set of blogs uses the 1000 year rainfall/runoff/hydraulics model that you can download at http://swmm2000.com/forum/topics/1000-year-simulation-with-rainfall-in-swmm-5 to show the inner workings of SWMM 5 and by extension InfoSWMM and H2oMap SWMM using a QA/QC version of SWMM 5 with extended graphics. I always hope that seeing the inner workings of a SWMM 5 feature helps to understand the code, sensitivity and importance of a parameter. It also helps show sometimes when a parameter is not important. In this example, we show how the statistics change when you change the reporting time step. You can only see what you report, if you only save every hour or 15 minutes then you may miss some important model results.
Discussion: In this blog we look at the Rainfall and Pervious Losses for the 100 year period:
Generally, it is better to have finer time steps for rainfall and for reporting. As you can see in Figure 1 we only get pervious runoff when the rainfall is greater than the infiltration rate which explains the findings of this blog http://www.swmm5.net/2013/12/runoff-from-one-watershed-for-100-year.html in which the pervious flow is small an infrequent. The pervious area runoff is a function not only of the pervious infiltration parameters but the rainfall time interval.
Figure 1. Rainfall and Losses One Watershed for a 100 Year Simulation with SWMM 5 Statistics at a saved time step of one hour.
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