Subject:  How does the Infiltration Maximum Time to Drain the Upper Soil Zone Work in SWMM 5 Green Ampt?

You can use the monthly soil recovery factor (Figure 1) in SWMM 5 to change how the Infiltration Maximum Time to Drain the Upper Soil Zone (Figure 2) is computed each month during a continuous simulation. 

The  depth of the upper soil zone in the internal  SWMM 5 units of feet is calculated at the start of the simulation based on the Green Ampt Soil Saturated Hydraulic Conductivity

Upper Soil Zone Depth = 4  * (Soil Saturated Hydraulic Conductivity * 12 * 3600)^0.5 / 12

And the Upper Zone Moisture Depletion Factor  and Infiltration Maximum Time to Drain the Upper Soil Zone is calculated at each hydrology time step in SWMM 5.

Upper Zone Moisture Depletion Factor  = Upper Soil Zone Depth / 300 * 12 /3600 * Monthly Evaporation Recovery Factor

Infiltration Maximum Time to Drain the Upper Soil Zone = 6 / (100 * Upper Zone Moisture Depletion Factor  )

Figure 1.  Monthly Soil Recovery Factor

Figure 2.  Infiltration Maximum Time to Drain the Upper Soil Zone for a Subcatchment

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