The Internal Green Ampt Parameters, Soil Moisture and 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. 

Discussion:  Key internal parameters for Green Ampt infiltration are

FUMax is the Saturated Moisture Content of the Upper Zone (internal units of feet)

F or FTOT in the graph is the Cumulative event infiltration at start of time interval (internal units of feet)

FU is the current moisture content of upper zone (internal units of feet)

As shown in Figure 1,  IMD and Soil Moisture are related.  When the soil moisture is zero then the IMD is equal to the maximum values of IMD or IMDMax.

These three parameters are used to calculate the IMD during the simulation among other important uses

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

Or  IMD = [ FUMax – FU ] / Depth of the Upper Soil Layer

Soil Moisture = IMDMax - IMD 

Figure 1.   The Internal Green Ampt Parameters, Soil Moisture and IMD for a 100 Year SWMM 5 Simulation for Green Ampt Infiltration