Aquifers in SWMM 5

 Aquifers in SWMM 5

Groundwater is conceptualized and represented in SWMM 5, using a two-zone approach and with a relationship between Aquifer and Groundwater data objects. Let's dive a bit deeper into these aspects:

SWMM 5's Two-Zone Groundwater Representation:

SWMM 5 simplifies the complex subsurface environment by dividing it vertically into two zones:

1. Unsaturated Zone (Vadose Zone):

   • Lies above the water table.
   • Pore spaces contain both air and water.
   • Water content varies with depth and time, depending on infiltration from the surface and drainage to the saturated zone.
   • Important for determining the amount of infiltration that eventually reaches the water table (recharge).
   • In the SWMM 5 Aquifer Editor, the Unsaturated Zone parameters are:
     • Soil Wilting Point: The moisture content of the soil below which plants can no longer extract water.
     • Soil Field Capacity: The amount of water held in the soil after excess water has drained away.
     • Soil Porosity: The volume of pore space in the soil expressed as a fraction of the total soil volume.
     • Soil Conductivity: The rate at which water can move through the soil when saturated.
     • Conductivity Slope: The rate of change of soil conductivity with moisture content.
     • Soil Tension Slope: The rate of change of soil suction with moisture content.

2. Saturated Zone:

   • Lies below the water table.
   • All pore spaces are filled with water.
   • Represents the groundwater aquifer.
   • Water can move laterally within this zone and can interact with surface water features (streams, lakes) or drainage nodes.
   • In the SWMM 5 Aquifer Editor, the Saturated Zone parameters are:
     • Soil Porosity: The volume of pore space in the soil expressed as a fraction of the total soil volume.
     • Soil Conductivity: The rate at which water can move through the soil when saturated.
     • Bottom Elevation: The elevation of the bottom of the aquifer.

Aquifer and Groundwater Data Objects:

The separation of Aquifer and Groundwater data objects is a clever way that SWMM 5 manages groundwater parameters:

• Aquifer Data Object:

  • Purpose: Defines the physical properties of the aquifer material. These properties are generally consistent across a given geological formation.
  • Parameters:
    • Porosity
    • Wilting Point
    • Field Capacity
    • Hydraulic Conductivity
    • Conductivity Slope
    • Tension Slope
    • Evaporation Fraction
    • Bottom Elevation of the Aquifer
    • And others...
  • Reusability: Importantly, a single Aquifer object can be assigned to multiple subcatchments that share the same underlying aquifer material. This avoids redundant data entry and ensures consistency.

• Groundwater Data Object:

  • Purpose: Defines the hydrological conditions and flow parameters that govern the interaction between a specific subcatchment and the underlying aquifer.

  • Parameters: The Groundwater Data connects the Subcatchment to the Aquifer,

    • Associated Aquifer: Specifies which Aquifer object applies to this subcatchment.
    • Groundwater Threshold Elevation: The groundwater elevation above which exfiltration to a node occurs.
    • Receiving Node Invert Elevation: The node to which groundwater exfiltration occurs.
    • Surface Elevation: The average ground surface elevation of the subcatchment.
    • Groundwater Flow Coefficient (A1): A parameter in the equation that calculates the rate of groundwater flow. This is a calibration parameter.
    • Groundwater Flow Exponent (B1): Another parameter in the groundwater flow equation (also a calibration parameter).
    • Surface Water Flow Coefficient (A2): A parameter that determines the amount of groundwater flow that goes to the surface water system.
    • Surface Water Flow Exponent (B2): An exponent in the surface water flow relationship.
    • Surface Water Gையம் 5 is modeled as two zones: (1) Saturated and (2) Unstaturated. The data for the Groundwater Simulation consists of physical data in an Aquifer and elevation and flow coefficient and exponent data in the Groundwater Data. The Aquifer data object can be applied to multiple Subcatchments but each Subcatchment has its own set of Groundwater data. For example, in this model all of the Subcatchments share the same Aquifer data but each Subcatchment has different elevation and flow data – the labels on the basin are the groundwater elevations. A parameter that determines how much groundwater flow goes to surface water flow when the groundwater table is above the surface water depth.
    • Lateral Groundwater Flow Equation: The flow between the aquifer and the node is often expressed as a power function: Q = A1 * (HGW - HTH)^B1 where Q is the flow, HGW is the groundwater table elevation, HTH is the threshold elevation (either fixed or the node invert), and A1 and B1 are empirical coefficients.
    • Initial Groundwater Elevation: The starting groundwater elevation for the simulation.

  • Subcatchment-Specific: Each subcatchment must have its own unique Groundwater data object because the interaction with the aquifer will vary depending on local conditions, such as the receiving node, the surface elevation, and the desired calibration parameters (A1, B1, etc.).

Example Scenario:

In the model you described, the labels on the subcatchments likely represent the initial groundwater elevations defined in their respective Groundwater data objects. Although all subcatchments share the same Aquifer object (meaning they have the same soil properties), the differences in their Groundwater data (initial elevations, flow coefficients, etc.) will lead to different groundwater responses and interactions with the drainage network during the simulation.

Benefits of this Approach:

• Efficiency: Avoids redundant data entry for aquifer properties.
• Consistency: Ensures that the same aquifer properties are used across all subcatchments that share that aquifer.
• Flexibility: Allows for subcatchment-specific calibration of groundwater flow parameters.
• Realism: More accurately represents the spatial variability of groundwater conditions within a watershed.

Conclusion:

SWMM 5's two-zone groundwater model, coupled with the distinct Aquifer and Groundwater data objects, provides a robust and flexible framework for simulating groundwater-surface water interactions. This approach allows for efficient data management, consistent representation of aquifer properties, and realistic simulation of subcatchment-specific groundwater behavior. Understanding these concepts is crucial for effectively using SWMM 5 to model watersheds where groundwater plays a significant role in the overall hydrology.