Furthermore, PCSWMM's graph panel objective functions (statistics) are computed on-the-fly on the plotted time series and thus also affected by the choice of reporting time step.
A large reporting time step may miss the extremes computed by the SWMM5 engine (peaks and valleys in the time series). As the output file time series is used for plotting computed time series in the Graph panel and Profile panel, and for time series based dynamic rendering in the Map panel, the choice of reporting time step is important. The reporting time step is used by SWMM5 when writing computed time series to the SWMM5 output file. if two consecutive times in the rainfall time series are less than the specified time interval). The later versions of EPA SWMM5 check the user-entered raingage time interval and provide warnings if it is obviously incorrect (i.e. However since you are allowed to skip time steps of no rain, it is not reliable for the program to determine the actual time interval, and thus it is up to the user to set the time interval for each raingage object. Rainfall input time series for SWMM needs to be measured over a fixed interval (e.g. PCSWMM's time series editing tools can easily convert rainfall time series between preceding interval and following interval values.Ī common error is not setting the Raingage object's Time Interval to match the time series data used by the Raingage object. This isn't a big concern for 5 minute data, but will throw off the time of peak for longer time interval (say 1 hour data) considerably. So if your user-defined or time series object rainfall data is from a fixed interval data logger, you should convert it to following interval value. NCDC 3240/3260, or AES, etc.) the program knows that the time series values apply to the preceding value instead (i.e. the most common methods of input). If the rainfall time series is read from an external file with a known format (i.e. This applies when SWMM interprets time series from either time series objects or user-defined external time series data files (i.e. That means that if the rainfall value has a time stamp of 2pm and the time interval is 15 minutes, the program assumes the rainfall value (depth or intensity) is constant from 2:00pm to 2:15pm. The SWMM5 engine assumes that each rainfall value is constant over the following time interval for user-entered time series. This is great for model calibration/validation against historical events.
Hurricane Hazel in Ontario), you also have the option of applying an areal reduction factor for large catchments. PCSWMM Professional can retrieve and process NEXRAD radar rainfall, which generates individual subcatchment hyetographs (through area-weighting) at a 5 minute interval, and "ground-truth's" it against raingage data. There are a number of tools in PCSWMM to help create a useful continuous rainfall record: PCSWMM's rainfall disaggregation tool can generate 15 minute data from continuous hourly data and PCSWMM's time series editor can also stitch multiple rainfall records together to make a longer data set. For design storms, PCSWMM's design storm creator tool can create common design storm distributions at the time interval of your choice.
You can look further afield for continuous data, as this isn't typically used to calibrate a model, rather it is used for inference, and thus the record simply needs to be plausible (i.e. Long term rainfall records for continuous simulation are normally 15 minute or hourly. For typical SWMM applications, 5 minute data is optimal, 15 minute data may be acceptable, and hourly data is usually a compromise (as the response time of the smallest subcatchment is usually much faster). The raingage time interval is used by SWMM when reading/interpreting the input rainfall time series and is usually somewhere between 1 minute and 1 day. Please let me know if I have missed any considerations. I've listed the main SWMM5 time steps, and provided some of my thoughts on each. A couple of recent technical support questions inspired this post on selecting reasonable values for the various user-defined time steps required by SWMM5 when setting up a model.