How Does Horton Infiltration Work in SWMM 5?

This sketch summarizes what happens in a SWMM 5 Subcatchment for Horton Infiltration during a storm event:

1.       The event starts out with the potential infiltration rate at the maximum infiltration rate but
2.      Decay starts happening right away and the potential infiltration rate starts decreasing until it reaches the minimum infiltration rate (assuming the storm last long enough),
3.      The actual infiltration rate is the minimum of the rainfall rate or the potential infiltration rate,
4.      Using the Huff distributions for rainfall the runoff does not start happening until the rainfall rate exceeds the potential infiltration rate in these models
5.      The runoff ceases after the rainfall rate becomes less than the current potential infiltration rate later in the storm,
6.      The maximum infiltration volume for Horton caps the storm event infiltration at 10 mm in this example, the infiltration will cease when the cumulative infiltration reaches 10 mm.
7.      Horton Iniltration is a five parameter method
a.      Maximum infiltration rate in mm/hour
b.      Minimum infiltration rate in mm/hour
c.       Decay rate for the change from maximum to minimum infiltration rate
d.      Regeneration rate for the change from minimum to maximum infiltration rate after the storm event ends and
e.      A maximum infiltration volume per storm event in millimeters


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