How is the Soil Saturated Conductivity Used in SWMM 5 Green-Ampt?

How Soil Saturated Conductivity is Used in SWMM 5 Green-Ampt

In SWMM 5's Green-Ampt infiltration model, the soil saturated conductivity (Ks) plays a pivotal role in determining how quickly water can infiltrate into the soil under saturated conditions. Here's how it functions:

Sensitivity of Infiltration to Soil Saturated Conductivity:

• Functionality:
  • Saturation Check: Ks is used to determine if the soil has reached saturation. If the infiltration rate (irate) exceeds Ks, it suggests the soil is saturated or near saturation.
  • Infiltration Rate Calculation: Ks directly influences the maximum rate at which water can infiltrate once the capillary suction has pulled water into the soil profile.
• Equations and Checks:
  • Low Rainfall: If irate < Ks, essentially all rainfall infiltrates, indicating that the soil can handle the incoming water without becoming saturated.
  • Saturation: When checking if soil is saturated, if the infiltration rate is equal to or greater than Ks, the soil is considered saturated, and any additional water would pond or run off.

Figure Analysis:

• Figure 1 & Figure 2: These figures would illustrate:
  • Total Infiltration Loss: As Ks decreases from 1 to 0.1 to 0.01 inches/hour:
    • Lower Ks leads to less total infiltration loss because the soil's ability to conduct water downward diminishes, meaning more water might remain on the surface or contribute to runoff.
  • Total Loss Rate:
    • With a higher Ks, the initial rate of infiltration would be higher, as the soil can conduct water away from the surface more effectively.
    • As Ks drops (e.g., from 1 to 0.01 inches/hour), the rate of infiltration decreases, potentially leading to ponding or increased surface runoff.

Sensitivity Analysis:

• Infiltration Sensitivity: The infiltration process in the Green-Ampt method is highly sensitive to Ks:
  • High Ks: Allows for rapid infiltration, reducing the risk of surface runoff even under heavy rain.
  • Low Ks: Results in slower infiltration, which can lead to more surface water, potentially causing runoff or ponding even in moderate rain events.
• Real-World Implications:
  • Urban Planning: In areas with lower Ks, stormwater management strategies might need to account for greater surface runoff, possibly requiring larger drainage systems or more permeable surfaces.
  • Calibration: Accurately determining Ks for local soils is crucial for model calibration to ensure that simulations reflect actual conditions.

By adjusting the soil saturated conductivity, SWMM 5 can model various scenarios, from well-draining soils to those with poor conductivity, aiding in the design of effective stormwater management systems tailored to specific soil characteristics.

 Figure 1.  The three parameters for Green-Ampt Infiltration in SWMM 5 

 Figure 2. The sensitivity of the total infiltration loss to the soil saturated conductivity in a continuous simulation