Depression Storage

I am modeling a small (<1acre) pervious area and running continuous simulations of ~ 2yrs to check the impact of various depression storage values.  Based on the SWMM manual and other sources I assumed the total runoff and outflow would be reduced proportionally when I increased depression storage from 0.1 inch to 0.2 inch.  Instead I found the total runoff increased ~20%.  Has anyone else seen this happen?


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  • Yes, you can send your model to me at

  • Can you send me the model to ?

    • Mr. Robert Can you help me please ? I difficult to reduce my node floaded / junction. I used storage unit and LID for reducing floaded link.

      May i send my model to you please ?

      Thamks before

  • This is a simple example I gave to a student to look at the effect of depression storage.  Evaporation is not being modeled.  There are no LIDS.  The land area is 0% impervious with a CN of 61.  All runoff is being routed to the outlet.  Interestingly, infiltration for the 0.1 inch DStor model is much higher and accounts for the difference in runoff.  In the DStor=0.1 in model, Infil = 67.109 inches and runoff = 21.309 inches.  For the DStor = 0.2 inches, infiltration = 62.977 inches and runoff = 25.436 inches.  Since all parameters in both models are identical except for depression storage, do you have any thoughts on what might cause reduced infiltration and increased runoff when DStor in increased?

  • Hello Robert,  How is the evaporation and infiltation losses changing - is there a loss of infiltration volume along with the increase in runoff?  Are you modeling LID's ?   Where are you routing the runoff?  What is the  percentage of impervious area?

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