Why Do I See Difference in Time Series Plot And Output Results / Analysis Summary?

A lot of times, SSA users find that there is a slight difference in the results seen in the Time Series Plots and the numbers seen within the Analysis Summary in the elements Property dialog as shown below:

Figure 1.0 Different results seen in the Time Series Plot and Analysis Summary

The Time Steps section within the Analysis Options is the key to resolve this difference seen in the results. It establishes the time step length to be used for the runoff computations, routing computations, and results reporting. The Reporting and Routing Time Step is the most important thing for network analysis.

SSA uses a default Reporting time step of 5 minutes and Routing time step of 30 seconds. In most cases users tend to use the default and define only the Start and End Dates to perform the analysis. A very important thing to note here is that the Output Report or Analysis Summary within the element properties windows shows the results based on every routing time step whereas the Time Series Plot is generated on the Reporting time steps defined. This is where the difference emanates.

The default reporting and routing time step implies that the program computes for every 30 seconds and then generates the report with 5 minutes interval. It so happens that the reporting time step misses the peak value and hence the difference. Note that Hydrodynamic Routing method requires a much smaller time step than the other flow routing methods (i.e., Kinematic Wave and Steady Flow routing methods). Typical values for Hydrodynamic Routing range from 60 seconds down to 1 second. However, routing time steps less than 1 second can be manually entered. The diagram below explains how a small enough routing time step is required to capture the peak flow value.

Figure 2.0 A small enough routing time step is required to capture the peak flow value

The easiest and simplest way to match the results seen the Time Series Plots and the Analysis Summary or Output Report is to match the Reporting and Routing Time Step within the Analysis Options window. The image below shows how, when matching the reporting and routing time step generates the same result.

Figure 3.0 Matching the Reporting and Routing time step produce the same results

I hope this piece of information help you in your Model Analysis. Keep tuning in to this site for more on Hydrology and Hydraulic topics and SSA in particular.