Understanding the Role of Initial Soil Moisture Deficit (IMD) in SWMM5's Green-Ampt Infiltration

Understanding the Role of Initial Soil Moisture Deficit (IMD) in SWMM5's Green-Ampt Infiltration

In SWMM5's Green-Ampt implementation for Storage Units, IMD plays a crucial role in determining the infiltration capacity of the soil. Let's break down its significance:

• IMD (Initial Moisture Deficit): Represents the difference between the soil's porosity (maximum water holding capacity) and its initial moisture content at the beginning of the simulation. A higher IMD indicates a drier soil, capable of absorbing more water before saturation occurs.
• IMDMax (Maximum Moisture Deficit): This is the upper limit of the IMD, effectively representing the soil's porosity minus the soil's moisture content at the wilting point. In the context of Storage Units, it is set equal to the user input for IMD. This differs from the way IMDMax is handled in subcatchments where it is constant and related to soil parameters.
• Green-Ampt Equation: SWMM5 uses the Green-Ampt equation to calculate infiltration. The equation considers factors like hydraulic conductivity, wetting front suction head, and the current moisture deficit.

The Problem with IMD = 0 in Storage Units

When you set IMD to zero in a SWMM5 Storage Unit, the following occurs:

1. IMDMax = 0: In Storage Units the IMDMax is set to the user input IMD. If IMD is 0 so is IMDMax.
2. Zero Initial Infiltration Capacity: The Green-Ampt equation in the initial time step will result in no infiltration. The soil is essentially treated as if it's already saturated at the start of the simulation. The maximum infiltration possible is limited to the hydraulic conductivity.
3. Potentially Unrealistic Runoff: Since infiltration is restricted from the beginning, more rainfall is converted to runoff. This can lead to:
   • Overestimated Peak Flows: Higher runoff volumes will contribute to larger peak flows.
   • Overestimated Runoff Volumes: The total volume of runoff will likely be higher than in reality.
   • Underestimated Storage Utilization: The Storage Unit might not fill up as much as it would in reality because less water is infiltrating into the underlying soil.

The Impact of Non-Zero IMD Values

When you use a non-zero IMD (which is more realistic for most scenarios), the infiltration behavior changes significantly:

1. Initial Infiltration Occurs: The soil has an initial capacity to absorb water.
2. Gradual Reduction in Infiltration: As the soil moisture content increases, the moisture deficit decreases, and the infiltration rate gradually reduces according to the Green-Ampt equation.
3. More Realistic Runoff: Infiltration is modeled more realistically, potentially leading to:
   • Lower Peak Flows: Some of the rainfall infiltrates, reducing the volume of runoff contributing to peak flows.
   • Lower Runoff Volumes: The total runoff volume is likely to be lower.
   • Better Storage Utilization: The Storage Unit can potentially fill up more as water infiltrates and is then available for evaporation/exfiltration between events.

Visualizing the Difference with Graphics (Conceptual)

To illustrate the impact of IMD, let's imagine two scenarios (which you would then illustrate with actual graphs from SWMM5 output):

Scenario 1: IMD = 0

• Infiltration Curve: The infiltration rate starts low (limited by hydraulic conductivity) and doesn't change much over time because there is no storage for infiltrated water.
• Runoff Hydrograph: The hydrograph will show a rapid rise and a higher peak flow, with a larger runoff volume.
• Storage Unit Depth: The depth in the storage unit will rise quickly initially, and then more slowly.

Scenario 2: IMD > 0

• Infiltration Curve: The infiltration rate starts high and gradually decreases as the soil moisture deficit is filled.
• Runoff Hydrograph: The hydrograph will show a slower rise, a lower peak flow, and a smaller runoff volume compared to the IMD = 0 case.
• Storage Unit Depth: The depth in the storage unit will initially rise less quickly due to the infiltration, potentially followed by a continued rise as the infiltration capacity is reached. The peak depth will be lower than the zero IMD case.

Example Graphs (Conceptual - To Be Generated in SWMM5)

You should generate the following graphs using SWMM5 to compare the two scenarios:

1. Infiltration Rate vs. Time: Show the infiltration rate over time for both IMD = 0 and IMD > 0.
2. Runoff Hydrograph: Compare the runoff hydrographs for both scenarios.
3. Storage Unit Depth vs Time: Compare the water depth in the Storage Unit over time for both scenarios.
4. Storage Unit Infiltration Volume vs Time Compare the cumulative infiltration volume over time for both scenarios.

Recommendations

1. Avoid IMD = 0 in Storage Units: Unless you are specifically modeling a fully saturated initial condition, using a non-zero IMD is strongly recommended for Storage Units.
2. Estimate Realistic IMD: Try to estimate a realistic IMD based on:
   • Antecedent Moisture Conditions: Consider the dryness or wetness of the soil before the simulation starts.
   • Soil Type: Different soil types have different porosity and wilting point values, which influence IMDMax.
3. Sensitivity Analysis: Perform a sensitivity analysis by running simulations with a range of IMD values to understand how sensitive your results are to this parameter.
4. Calibration: If you have observed data (e.g., flow measurements, storage unit water levels), calibrate your model by adjusting the IMD (and other parameters) to achieve a good match between simulated and observed values.

Conclusion

The Initial Soil Moisture Deficit (IMD) is a critical parameter in SWMM5's Green-Ampt infiltration model for Storage Units. Using IMD = 0 can lead to unrealistic results because it eliminates the soil's initial infiltration capacity. Employing a non-zero, physically meaningful IMD value is crucial for accurately simulating infiltration, runoff, and the overall performance of Storage Units in SWMM5. Using graphical comparisons, as outlined above, will help you visualize and understand the significant impact of IMD on your simulation results. Remember to couple these analyses with sensitivity analysis and calibration for robust modeling.