Initial Moisture Deficit or IMD for a 100 Year SWMM 5 Simulation for Green Ampt Infiltration

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 some of the internal working of the Green Ampt Infiltration Parameters over the 100 year period.  The IMD is normally near the initial user defined value but it can go to zero during the course of the simulation (Figure 1)

Discussion:  A key internal and user defined parameter is the Green Ampt Infiltration parameter Initial Moisture Deficit.  It starts out at the user Initial Deficit and then is computed at each hydrology time step using the equation

IMD =  [ Maximum Allowable Infiltration – Current Moisture Content of the Upper Zone ] / Depth of the Upper Soil Layer

Figure 1.   Initial Moisture Deficit or IMD for a 100 Year SWMM 5 Simulation for Green Ampt Infiltration