Rainfall and Losses One Watershed for a 100 Year Simulation with SWMM 5 Statistics at a saved time step of one hour.

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. 

Embark on a hydrological odyssey spanning a millennium with this series of blogs, anchored by the extensive 1000-year rainfall/runoff/hydraulics model available at SWMM2000. Utilizing a specialized QA/QC version of SWMM 5 that boasts enhanced graphics, these insights aim to illuminate the intricate mechanics of SWMM 5, as well as its counterparts InfoSWMM and H2oMap SWMM. 🌧️🔄🛠️

The quest for understanding isn't just academic; it's a practical exploration into how pivotal parameters influence the model's behavior, highlighting their significance or, occasionally, their redundancy. 📊🧐

Key Insight: The essence of this narrative lies in the influence of reporting time steps on statistical output. The granularity of data — whether recorded every hour or every quarter of an hour — can dramatically shape the story your model tells. Miss a beat, and you could miss a flood. ⏱️💧

Focal Point: This particular chapter delves into the interplay between rainfall and pervious surface losses over a century-long saga. The model suggests a critical truth: the finer the temporal resolution of rainfall and reporting, the sharper the picture of runoff. As demonstrated in Figure 1 and supported by SWMM5.net, pervious runoff is a rare and minimal occurrence, emerging only when rainfall intensity surpasses the rate of infiltration. This revelation underscores the delicate dance between rainfall intervals and pervious surface parameters. 🌳💦📈

Stay tuned to this blog series for more revelations from the vast timescales of hydrological phenomena, where every parameter tells a tale, and every setting shapes the flow of urban water wisdom. 🚀🌍💡

Figure 1.   Rainfall and Losses One Watershed for a 100 Year Simulation with SWMM 5 Statistics at a saved time step of one hour.