How Does Green Ampt Initial Moisture Defiict in SWMM 5?

 How Does Green Ampt Initial Moisture Defiict in SWMM 5?

The Green Ampt method in SWMM 5 uses several parameters to calculate infiltration rates in pervious areas of subcatchments. Here's how these parameters relate to each other, particularly focusing on the Initial Moisture Deficit (IMD):

Key Parameters:

• Soil Moisture: This is calculated as: Soil Moisture=IMD Max−(FUMax−FU)Upper Soil Zone Depth
• • IMD Max: The maximum Initial Moisture Deficit, defined by the user as a fraction of the soil's capacity to hold water. It represents the initial state of dryness of the soil before the simulation begins.
  • FUMax: The saturated moisture content of the upper soil zone, which remains constant throughout the simulation, measured in feet.
  • FU: The current moisture content of the upper soil zone, which changes as infiltration occurs, also in feet.
  • Upper Soil Zone Depth: The depth of the soil layer where infiltration is considered, which influences how much water can be stored before reaching saturation.

Interpretation:

• Initial Moisture Deficit (IMD):
  • Definition: IMD represents how much additional water the soil can take before it becomes fully saturated. A higher IMD Max means the soil starts drier, allowing for more infiltration before saturation is reached.
  • Dynamic Change: As rain infiltrates, FU increases, reducing the soil moisture deficit. The equation above shows how this deficit is tracked during the simulation.
• Graphical Representation:
  • The graph would show how these parameters evolve during a rainfall event:
    • Soil Moisture: Starts as IMD Max and decreases as FU increases towards FUMax, indicating less available space for infiltration.
    • FU: Would rise as water infiltrates, approaching FUMax as the soil wets up.
    • IMD: As a fraction, it would decrease from its initial value towards zero as the soil becomes saturated.

Impact on Infiltration:

• High IMD Max: Suggests a very dry soil at the start, potentially allowing for more initial infiltration as there's more room for water absorption.
• Low IMD Max: Indicates the soil is closer to saturation initially, which could lead to quicker onset of runoff since less water can be absorbed.

Practical Use:

• Modeling Realistic Conditions: By setting an accurate IMD Max, users can model how different initial soil moisture conditions would affect runoff and infiltration in urban areas or natural landscapes.
• Calibration: This parameter might need calibration based on local conditions or historical data to ensure the model reflects real-world behavior accurately.

Figure 1.  How Soil Moisture changes over time.

 

Figure 2.  Soil Moisture and IMD are related – the Soil Moisture has a maximum of IMDMax.